arXiv daily: Materials Science

arXiv daily: Materials Science (cond-mat.mtrl-sci)

1.Two-dimensional metal halide perovskites and their heterostructures: from synthesis to applications

Authors:Athanasia Kostopoulou, Ioannis Konidakis, Emmanuel Stratakis

Abstract: Size- and shape- dependent unique properties of the metal halide perovskite nanocrystals make them promising building blocks for constructing various electronic and optoelectronic devices. These unique properties together with their easy colloidal synthesis render them efficient nanoscale functional components for multiple applications ranging from light emission devices to energy conversion and storage devices. Recently, two-dimensional (2D) metal halide perovskites in the form of nanosheets (NSs) or nanoplatelets (NPls) are being intensively studied due to their promising 2D geometry which is more compatible with the conventional electronic and optoelectronic device structures where film-like components are employed. In particular, 2D perovskites exhibit unique thickness-dependent properties due to the strong quantum confinement effect, while enabling the bandgap tuning in a wide spectral range. In this review the synthesis procedures of 2D perovskite nanostructures will be summarized, while the application-related properties together with the corresponding applications will be extensively discussed. In addition, perovskite nanocrystals/2D material heterostructures will be reviewed in detail. Finally, the wide application range of the 2D perovskite-based structures developed to date, including pure perovskites and their heterostructures, will be presented while the improved synergetic properties of the multifunctional materials will be discussed in a comprehensive way.

2.Anisotropic Rheology and Friction of Suspended Graphene

Authors:Andrea Mescola, Andrea Silva, Ali Khosravi, Andrea Vanossi, Erio Tosatti, Sergio Valeri, Guido Paolicelli

Abstract: Graphene is a powerful membrane prototype for both applications and fundamental research. Rheological phenomena including indentation, twisting, and wrinkling in deposited and suspended graphene are actively investigated to unravel the mechanical laws at the nanoscale. Most studies focused on isotropic set-ups, while realistic graphene membranes are often subject to strongly anisotropic constraints, with important consequences for the rheology, strain, indentation, and friction in engineering conditions.

3.Slidephononics: Tailoring Thermal Transport Properties by van der Waals Sliding

Authors:Linfeng Yu, Chen Shen*, Guangzhao Qin, Hongbin Zhang

Abstract: By interlayer sliding in van der Waals (vdW) materials, the switching electric polarization of ultrathin ferroelectric materials leads to the widely studied slidetronics. In this work, we report that such sliding can further tailor anharmonic effects and hence thermal transport properties due to the changed intrinsic coupling between atomic layers. And we propose an unprecedented concept dubbed as slidephononics, where the phonons and associated physical properties can be controlled by varying the intrinsic stacking configurations of slidetronic vdW materials. Based on the state-of-the-art first-principles calculations, it is demonstrated that the thermal conductivity of boron nitride (BN) bilayers can be significantly modulated (by up to four times) along the sliding pathways. Detailed analysis reveals that the variation of thermal conductivities can be attributed to the tunable (de-)coupling of the out-of-plane acoustic phonon branches with the other phonon modes, which is induced by the interlayer charge transfer. Such strongly modulated thermal conductivity via interlayer sliding in vdW materials paves the way to engineer thermal management materials in emerging vdW electronic devices, which would shed light on future studies of slidephononics.

4.Quantifying physical insights cooperatively with exhaustive search for Bayesian spectroscopy of X-ray photoelectron spectra

Authors:Hiroyuki Kumazoe, Kazunori Iwamitsu, Masaki Imamura, Kazutoshi Takahashi, Yoh-ichi Mototake, Masato Okada, Ichiro Akai

Abstract: We analyzed the X-ray photoemission spectra (XPS) of carbon 1s states in graphene and oxygen-intercalated graphene grown on SiC(0001) using Bayesian spectroscopy. To realize highly accurate spectral decomposition of the XPS spectra, we proposed a framework for discovering physical constraints from the absence of prior quantified physical knowledge, in which we designed the prior probabilities based on the found constraints and the physically required conditions. This suppresses the exchange of peak components during replica exchange Monte Carlo iterations and makes possible to decompose XPS in the case where a reliable structure model or a presumable number of components is not known. As a result, we have successfully decomposed XPS of one monolayer (1ML), two monolayers (2ML), and quasi-freestanding 2ML (qfs-2ML) graphene samples deposited on SiC substrates with the meV order precision of the binding energy, in which the posterior probability distributions of the binding energies were obtained distinguishably between the different components of buffer layer even though they are observed as hump and shoulder structures because of their overlapping.

5.Reconstruction, rumpling, and Dirac states at the (001) surface of a topological crystalline insulator Pb1-xSnxSe

Authors:A. Łusakowski, P. Bogusławski, T. Story

Abstract: Equilibrium atomic configuration and electronic structure of the (001) surface of IV-VI semiconductors PbTe, PbSe, SnTe and SnSe, is studied using the density functional theory (DFT) methods. At surfaces of all those compounds, the displacements of ions from their perfect lattice sites reveal two features characteristic of the rock salt crystals. First, the ionic displacements occur only along the direction perpendicular to the surface, and they exhibit the rumpling effect, i.e., the vertical shifts of cations and anions differ. Second, the interlayer spacing of the first few monolayers at the surface oscillates. Our results are in good agreement with the previous X-ray experimental data and theoretical results where available. They also are consistent with the presence of two {110} mirror planes at the (001) surface of the rock salt. One the other hand, experiments preformed for the topological Pb$_{1-x}$Sn$_x$ Se alloy indicate breaking of the mirror symmetry due to a large 0.3 {\AA} relative displacement of the cation and anion sublattices at the surface, which induces the opening of the gap of the Dirac cones. Our results for Pb$_{1-x}$Sn$_x$Se including the simulated STM images, are in contradiction with these findings, since surface reconstructions with broken symmetry are never the ground state configurations. The impact of the theoretically determined surface configurations and of the chemical disorder on the surface states is analyzed.

6.A New Approach to Measure Fundamental Microstructural Influences on the Magnetic Properties of Electrical Steel using a Miniaturized Single Sheet Tester

Authors:Nora Leuning, Martin Heller, Markus Jaeger, Sandra Korte-Kerzel, Kay Hameyer

Abstract: Magnetic properties of electrical steel are usually measured on Single Sheet Testers, Epstein frames or ring cores. Due to the geometric dimensions and measurement principles of these standardized setups, the fundamental microstructural influences on the magnetic behavior, e.g., deformation structures, crystal orientation or grain boundaries, are difficult to separate and quantify. In this paper, a miniaturized Single Sheet Tester is presented that allows the characterization of industrial steel sheets as well as from in size limited single, bi- and oligocrystals starting from samples with dimensions of 10x22 mm. Thereby, the measurement of global magnetic properties is coupled with microstructural analysis methods to allow the investigation of micro scale magnetic effects. An effect of grain orientation, grain boundaries and deformation structures has already been identified with the presented experimental setup. In addition, a correction function is introduced to allow quantitative comparisons between differently sized Single Sheet Testers. This approach is not limited to the presented Single Sheet Tester geometry, but applicable for the comparison of results of differently sized Single Sheet Testers. The results of the miniaturized Single Sheet Tester were validated on five industrial electrical steel grades. Furthermore, first results of differently oriented single crystals as well as measurements on grain-oriented electrical steel are shown to prove the additional value of the miniaturized Single Sheet Tester geometry.

7.Stress evolution in plastically deformed austenitic and ferritic steels determined using angle- and energy-dispersive diffraction

Authors:M. Marciszko-Wiackowska, A. Baczmanski, Ch. Braham, M. Watroba, S. Wronski, R. Wawszczak, G. Gonzalez, P. Kot, M. Klaus, Ch. Genzel

Abstract: In the presented research, the intergranular elastic interaction and the second-order plastic incompatibility stress in textured ferritic and austenitic steels were investigated by means of diffraction. The lattice strains were measured inside the samples by the multiple reflection method using high energy X-rays diffraction during uniaxial in situ tensile tests. Comparing experiment with various models of intergranular interaction, it was found that the Eshelby-Kr\"oner model correctly approximates the X-ray stress factors (XSFs) for different reflections hkl and scattering vector orientations. The verified XSFs were used to investigate the evolution of the first and second-order stresses in both austenitic and ferritic steels. It was shown that considering only the elastic anisotropy, the non-linearity of $\sin^2{\psi}$ plots cannot be explained by crystallographic texture. Therefore, a more advanced method based on elastic-plastic self-consistent modeling (EPSC) is required for the analysis. Using such methodology the non-linearities of $\cos^2{\phi}$ plots were explained, and the evolutions of the first and second-order stresses were determined. It was found that plastic deformation of about 1- 2% can completely exchange the state of second-order plastic incompatibility stresses.

8.Large effective magnetic fields from chiral phonons in rare-earth halides

Authors:Jiaming Luo, Tong Lin, Junjie Zhang, Xiaotong Chen, Elizabeth R. Blackert, Rui Xu, Boris I. Yakobson, Hanyu Zhu

Abstract: Time-reversal symmetry (TRS) is pivotal for materials optical, magnetic, topological, and transport properties. Chiral phonons, characterized by atoms rotating unidirectionally around their equilibrium positions, generate dynamic lattice structures that break TRS. Here we report that coherent chiral phonons, driven by circularly polarized terahertz light pulses, can polarize the paramagnetic spins in CeF3 like a quasi-static magnetic field on the order of 1 Tesla. Through time-resolved Faraday rotation and Kerr ellipticity, we found the transient magnetization is only excited by pulses resonant with phonons, proportional to the angular momentum of the phonons, and growing with magnetic susceptibility at cryogenic temperatures, as expected from the spin-phonon coupling model. The time-dependent effective magnetic field quantitatively agrees with that calculated from phonon dynamics. Our results may open a new route to directly investigate mode-specific spin-phonon interaction in ultrafast magnetism, energy-efficient spintronics, and non-equilibrium phases of matter with broken TRS.

1.Full-scale field-free spin-orbit switching of the CoPt layer grown on vicinal substrates

Authors:Luo Yongming, Liang Mengfan, Feng Zhongshu, Chen Haoran, Jiang Nan, Chen Jianhui, Yuan Mingyue, Zhang Jingcang, Cheng Yifeng, Sun Lu, Bai Ru, Miao Xiaohe, Wang Ningning, Wu Yizheng, Che Renchao

Abstract: A simple, reliable and field-free spin orbit torque (SOT)-induced magnetization switching is a key ingredient for the development of the electrical controllable spintronic devices. Recently, the SOT induced deterministic switching of the CoPt single layer has attracts a lot of interests, as it could simplifies the structure and add new flexibility in the design of SOT devices, compared with the Ferromagnet/Heavy metal bilayer counterparts. Unfortunately, under the field-free switching strategies used nowadays, the switching of the CoPt layer is often partial, which sets a major obstacle for the practical applications. In this study, by growing the CoPt on vicinal substrates, we could achieve the full-scale (100% switching ratio) field-free switching of the CoPt layer. We demonstrate that when grown on vicinal substrates, the magnetic easy axis of the CoPt could be tilted from the normal direction of the film plane; the strength of Dzyaloshinskii Moriya interaction (DMI) would be also be tuned as well. Micromagnetic simulation further reveal that the field-free switching stems from tilted magnetic anisotropy induced by the vicinal substrate, while the enhancement of DMI help reducing the critical switching current. In addition, we also found that the vicinal substrates could also enhance the SOT efficiency. With such simple structure, full-scale switching, tunable DMI and SOT efficiency, our results provide a new knob for the design SOT-MRAM and future spintronic devices.

2.Influence of swift heavy ion irradiation on structure and morphology of La0.25Pr0.375Ca0.375MnO3 perovskite film

Authors:Harsh Bhatt, Yogesh Kumar, R. B. Tokas, A. P. Singh, Fouran Singh, Surendra Singh

Abstract: The effects of Ag15+ (120 MeV) swift heavy ion irradiation on the structural and morphological properties of epitaxial La0.25Pr0.375Ca0.375MnO3 (LPCMO) thin films was investigated by x-ray scattering and atomic force microscopy (AFM) techniques. LPCMO films of thickness ~ 280 {\AA} were irradiated with an Ag15+ ion beam at different fluences of 1E11, 5E11, and 1E12 ions/cm2. XRD results suggested the development of the tensile stress along the out-of-plane direction of the LPCMO film upon ion irradiation which increased on increasing the ion fluence. The morphology of the film also modified with the irradiation and an increase in the fluence of the ion beam enhanced the in-plane height-height correlation length scale (grain size) with a loss of the fractal behaviour.

3.Note on Angular Momentum of Phonons in Chiral Crystals

Authors:Akihito Kato, Jun-ichiro Kishine

Abstract: Phonon angular momentum in chiral materials has been widely studied in spintronics and condensed matter physics. In chiral crystals, this is not the conserved quantity in contrast to the pseudo-angular momentum. To highlight this point and to understand the behavior of the angular momentum, we reexamine phonon dispersion theory based on the irreducible representation of helix and show the distinction of these angular momentum is originated from chirality.

4.Unraveling Femtosecond Spin and Charge Dynamics with EUV T-MOKE Spectroscopy

Authors:Henrike Probst, Christina Möller, Maren Schumacher, Thomas Brede, John Kay Dewhurst, Marcel Reutzel, Daniel Steil, Sangeeta Sharma, G. S. Matthijs Jansen, Stefan Mathias

Abstract: The magneto-optical Kerr effect (MOKE) in the extreme ultraviolet (EUV) regime has helped to elucidate some of the key processes that lead to the manipulation of magnetism on ultrafast timescales. However, as we show in this paper, the recently introduced spectrally-resolved analysis of such data can lead to surprising experimental observations, which might cause misinterpretations. Therefore, an extended analysis of the EUV magneto-optics is necessary. Via experimental determination of the dielectric tensor, we find here that the non-equilibrium excitation in an ultrafast magnetization experiment can cause a rotation of the off-diagonal element of the dielectric tensor in the complex plane. In direct consequence, the commonly analyzed magneto-optic asymmetry may show time-dependent behaviour that is not directly connected to the magnetic properties of the sample. We showcase such critical observations for the case of ultrafast magnetization dynamics in Ni, and give guidelines for the future analysis of spectrally-resolved magneto-optical data and its comparison with theory.

5.Verification of ultrafast spin transfer effects in FeNi alloys

Authors:Christina Möller, Henrike Probst, G. S. Matthijs Jansen, Maren Schumacher, Mariana Brede, John Kay Dewhurst, Marcel Reutzel, Daniel Steil, Sangeeta Sharma, Stefan Mathias

Abstract: The optical intersite spin transfer (OISTR) effect was recently verified in Fe$_{50}$Ni$_{50}$ using magneto-optical Kerr measurements in the extreme ultraviolet range. However, one of the main experimental signatures analyzed in this work, namely a magnetic moment increase at a specific energy in Ni, was subsequently found also in pure Ni, where no transfer from one element to another is possible. Hence, it is a much-discussed issue whether OISTR in FeNi alloys is real and whether it can be verified experimentally or not. Here, we present a comparative study of spin transfer in Fe$_{50}$Ni$_{50}$, Fe$_{19}$Ni$_{81}$ and pure Ni. We conclusively show that an increase in the magneto-optical signal is indeed insufficient to verify OISTR. However, we also show how an extended data analysis overcomes this problem and allows to unambiguously identify spin transfer effects. Concomitantly, our work solves the long-standing riddle about the origin of delayed demagnetization behavior of Ni in FeNi alloys.

6.Topotactically induced oxygen vacancy order in nickelate single crystals

Authors:Yu-Mi Wu, Pascal Puphal, Hangoo Lee, Jürgen Nuss, Masahiko Isobe, Bernhard Keimer, Matthias Hepting, Y. Eren Suyolcu, Peter A. van Aken

Abstract: The strong structure-property coupling in rare-earth nickelates has spurred the realization of new quantum phases in rapid succession. Recently, topotactic transformations have provided a new platform for the controlled creation of oxygen vacancies and, therewith, for the exploitation of such coupling in nickelates. Here, we report the emergence of oxygen vacancy ordering in Pr$_{0.92}$Ca$_{0.08}$NiO$_{2.75}$ single crystals obtained via a topotactic reduction of the perovskite phase Pr$_{0.92}$Ca$_{0.08}$NiO$_{3}$, using CaH$_2$ as the reducing agent. We unveil a brownmillerite-like ordering pattern of the vacancies by high-resolution scanning transmission electron microscopy, with Ni ions in alternating square-pyramidal and octahedral coordination along the pseudocubic [100] direction. Furthermore, we find that the crystal structure acquires a high level of internal strain, where wavelike modulations of polyhedral tilts and rotations accommodate the large distortions around the vacancy sites. Our results suggest that atomic-resolution electron microscopy is a powerful method to locally resolve unconventional crystal structures that result from the topotactic transformation of complex oxide materials.

7.Crack arrest markings in stress corrosion cracking of 7xxx aluminium alloys: insights into active hydrogen embrittlement mechanisms

Authors:Martí López Freixes, Xuyang Zhou, Raquel Aymerich-Armengol, Miquel Vega-Paredes, Lionel Peguet, Timothy Warner, Baptiste Gault

Abstract: Crack growth in stress corrosion cracking (SCC) in 7xxx Al alloys is an intermittent process, which generates successive crack arrest markings (CAMs) visible on the fracture surface. It is conjectured that H is generated at the crack tip during crack arrest, which then facilitates crack advancement through hydrogen embrittlement. Here, nanoscale imaging by 4D-scanning-transmission electron microscopy and atom probe tomography show that CAMs are produced by oxidation at the arrested crack tip, matrix precipitates dissolve and solute diffuse towards the growing CAM. Substantial homogenous residual strain remains underneath the fracture surface, indicative of non-localized plastic activity. Our study suggests that H induces crack propagation through decohesion.

8.Adsorption of CO and NO molecules on pristine, vacancy defected and doped graphene-like GaN monolayer: A first-principles study

Authors:Han-Fei Li, Si-Qi Li, Guo-Xiang Chen

Abstract: In order to study the novel gas detection or sensing applications of gallium nitride monolayer (GaN-ML), we mainly focused on the structural, energetic, electronic and magnetic properties of toxic gas molecules (CO, NO) adsorbed on pristine, single vacancy (N-vacancy, Ga-vacancy) defected, and metals (Al, Fe, Pd and Pt) doped GaN-ML using density functional theory (DFT-D2 method) in this work. The calculations demonstrate that pristine GaN-ML is extremely insensitive to CO and NO together with the existence of a weak physisorption nature due to small adsorption energy, charge transfer, and long adsorption distance. It is found that both N-vacancy defected GaN-ML and Fe-doped GaN-ML can significantly increase the adsorption energy and charge transfer for CO. The CO adsorption induces the metallic characteristics of N-vacancy GaN-ML to be converted to the half-metallic characteristics together with 100% spin polarization, and it also drastically changes the magnetic moment, implying that N-vacancy GaN-ML exhibits excellent sensitivity to CO. However, Fe-doped GaN-ML is not conducive to CO detection. Moreover, N-vacancy defected and Pt-doped GaN-ML greatly improve the adsorption ability for NO compared to other substrates, and the presence of stronger orbital hybridization suggests that the interaction between them is chemisorption. Therefore, N-vacancy defected GaN-ML and Pt-doped GaN-ML can serve as potential materials in future NO sensing devices.

9.Accurate and efficient treatment of spin-orbit coupling via second variation employing local orbitals

Authors:Cecilia Vona, Sven Lubeck, Hannah Kleine, Andris Gulans, Claudia Draxl

Abstract: A new method is presented that allows for efficient evaluation of spin-orbit coupling (SOC) in density-functional theory calculations. In the so-called second-variational scheme, where Kohn-Sham functions obtained in a scalar-relativistic calculation are employed as a new basis for the spin-orbit-coupled problem, we introduce a rich set of local orbitals as additional basis functions. Also relativistic local orbitals can be used. The method is implemented in the all-electron full-potential code \exciting. We show that, for materials with strong SOC effects, this approach can reduce the overall basis-set size and thus computational costs tremendously.

10.Solid-Lubrication Performance of Ti$_3$C$_2$T$_x$ -- Effect of Tribo-Chemistry and Exfoliation

Authors:Andreas Rosenkranz, Bo Wang, Dario Zambrano, Javier Marqués Henríquez, Jose Y. Aguilar-Hurtado, Edoardo Marquis, Paolo Restuccia, Brian C. Wyatt, M. Clelia Righi, Babak Anasori

Abstract: Multi-layer Ti$_3$C$_2$T$_x$ coatings have demonstrated an outstanding wear performance with excellent durability due to beneficial tribo-layers formed. However, the involved formation processes dependent on the tribological conditions and coating thickness are yet to be fully explored. Therefore, we spray-coated Ti$_3$C$_2$T$_x$ multi-layer particles onto stainless steel substrates to create coatings with two different thicknesses and tested their solid lubrication performance with different normal loads (100 and 200 mN) and sliding frequencies (1 and 2.4 Hz) using linear-reciprocating ball-on-disk tribometry. We demonstrate that MXenes' tribological performance depends on their initial state (delaminated few-layer vs. multi-layer particles), coating thickness and sliding velocity. Specifically, the best behavior is observed for thinner multi-layer coatings tested at the lower frequency. In contrast, coatings made of delaminated few-layer MXene are not as effective as their multi-layer counterparts. Our high-resolution interface characterization by transmission electron microscopy revealed unambiguous differences regarding the uniformity and chemistry of the formed tribo-layers as well as the degree of tribo-induced MXenes' exfoliation. Atomistic insights into the exfoliation process and molecular dynamic simulations quantitatively backed up our experimental results regarding coating thickness and velocity dependency. This ultimately demonstrates that MXenes' tribological performance is governed by the underlying tribo-chemistry and their exfoliation ability during rubbing.

11.Extraordinary Tunneling Magnetoresistance in Antiferromagnetic Tunnel Junctions with Antiperovskite Electrodes

Authors:Gautam Gurung, Ding-Fu Shao, Evgeny Y. Tsymbal

Abstract: Recent theoretical predictions and experimental demonstrations of a large tunneling magnetoresistance (TMR) effect in antiferromagnetic (AFM) tunnel junctions (AFMTJs) offer a new paradigm for information technologies where the AFM N\`eel vector serves as a state variable. A large TMR is beneficial for the applications. Here, we predict the emergence of an extraordinary TMR (ETMR) effect in AFMTJs utilizing noncollinear AFM antiperovskite XNMn$_{3}$ (X = Ga, Sn,...) electrodes and a perovskite oxide ATiO$_{3}$ (A = Sr, Ba,...) barrier layer. The ETMR effect stems from the perfectly spin-polarized electronic states in the AFM antiperovskites that can efficiently tunnel through the low-decay-rate evanescent states of the perovskite oxide while preserving their spin state. Using an GaNMn$_{3}$/SrTiO$_{3}$/GaNMn$_{3}$ (001) AFMTJ as a representative example, we demonstrate a giant TMR ratio exceeding $10^{4}$% and originating from the ETMR effect. These results are promising for the efficient detection and control of the N\`eel vector in AFM spintronics.

1.Insight into the electronic structure of the centrosymmetric skyrmion magnet GdRu$_2$Si$_2$

Authors:S. V. Eremeev, D. Glazkova, G. Poelchen, A. Kraiker, K. Ali, A. V. Tarasov, S. Schulz, K. Kliemt, E. V. Chulkov, V. S. Stolyarov, A. Ernst, C. Krellner, D. Yu. Usachov, D. V. Vyalikh

Abstract: The discovery of a square magnetic-skyrmion lattice in GdRu$_2$Si$_2$, with the smallest so far found skyrmion diameter and without a geometrically frustrated lattice, has attracted significant attention, particularly for potential applications in memory devices and quantum computing. In this work, we present a comprehensive study of surface and bulk electronic structures of GdRu$_2$Si$_2$ by utilizing momentum-resolved photoemission (ARPES) measurements and first-principles calculations. We show how the electronic structure evolves during the antiferromagnetic transition when a peculiar helical order of 4$f$ magnetic moments within the Gd layers sets in. A nice agreement of the ARPES-derived electronic structure with the calculated one has allowed us to characterize the features of the Fermi surface (FS), unveil the nested region along the $k_z$ at the corner of the 3D FS, and reveal their orbital compositions. Our findings suggest that the Ruderman-Kittel-Kasuya-Yosida interaction plays a decisive role in stabilizing the spiral-like order of Gd 4$f$ moments responsible for the skyrmion physics in GdRu$_2$Si$_2$. Our results provide a deeper understanding of electronic and magnetic properties of this material, which is crucial for predicting and developing novel skyrmion-based devices.

2.Zero-Point Quantum Diffusion of Proton in Hydrogen-rich Superconductor $LaH_{10}$

Authors:Xuejian Qin, Hongyu Wu, Guyong Shi, Chao Zhang, Peiheng Jiang, Zhicheng Zhong

Abstract: $LaH_{10}$, as a member of hydrogen-rich superconductors, has a superconducting critical temperature of 250 K at high pressures, which exhibits the possibility of solving the long-term goal of room temperature superconductivity. Considering the extreme pressure and low mass of hydrogen, the nuclear quantum effects in $LaH_{10}$ should be significant and have an impact on its various physical properties. Here, we adopt the method combines deep-potential (DP) and quantum thermal bath (QTB), which was verified to be able to account for quantum effects in high-accuracy large-scale molecular dynamics simulations. Our method can actually reproduce pressure-temperature phase diagrams of $LaH_{10}$ consistent with experimental and theoretical results. After incorporating quantum effects, the quantum fluctuation driven diffusion of proton is found even in the absence of thermal fluctuation near 0 K. The high mobility of proton is found to be compared to liquid, yet the structure of $LaH_{10}$ is still rigid. These results would greatly enrich our vision to study quantum behavior of hydrogen-rich superconductors.

3.Theory of magnetic field-stabilized compact skyrmions in thin film ferromagnets

Authors:Anne Bernand-Mantel, Sarah Barnova, Anaïs Fondet, Cyrill B. Muratov, Theresa M. Simon

Abstract: We present a micromagnetic theory of compact magnetic skyrmions under applied magnetic field that accounts for the full dipolar energy and the interfacial Dzyaloshinskii-Moryia interaction (DMI) in the thin film regime. Asymptotic analysis is used to derive analytical formulas for the parametric dependence of the skyrmion size and rotation angle, as well as the energy barriers for collapse and bursting, two processes that lead to a finite skyrmion lifetime. We demonstrate the existence of a new regime at low DMI, in which the skyrmion is stabilized by a combination of non-local dipolar interaction and a magnetic field applied parallel to its core, and discuss the conditions for an experimental realization of such field-stabilized skyrmions.

4.Unraveling the interlayer and intralayer coupling in two-dimensional layered MoS$_2$ by X-ray absorption spectroscopy and ab initio molecular dynamics simulations

Authors:Inga Pudza, Dmitry Bocharov, Andris Anspoks, Matthias Krack, Aleksandr Kalinko, Edmund Welter, Alexei Kuzmin

Abstract: Understanding interlayer and intralayer coupling in two-dimensional layered materials (2DLMs) has fundamental and technological importance for their large-scale production, engineering heterostructures, and development of flexible and transparent electronics. At the same time, the quantification of weak interlayer interactions in 2DMLs is a challenging task, especially, from the experimental point of view. Herein, we demonstrate that the use of X-ray absorption spectroscopy in combination with reverse Monte Carlo (RMC) and ab initio molecular dynamics (AIMD) simulations can provide useful information on both interlayer and intralayer coupling in 2DLM 2H$_c$-MoS$_2$. The analysis of the low-temperature (10-300 K) Mo K-edge extended X-ray absorption fine structure (EXAFS) using RMC simulations allows for obtaining information on the means-squared relative displacements $\sigma^2$ for nearest and distant Mo-S and Mo-Mo atom pairs. This information allowed us further to determine the strength of the interlayer and intralayer interactions in terms of the characteristic Einstein frequencies $\omega_E$ and the effective force constants $\kappa$ for the nearest ten coordination shells around molybdenum. The studied temperature range was extended up to 1200 K employing AIMD simulations which were validated at 300 K using the EXAFS data. Both RMC and AIMD results provide evidence of the reduction of correlation in thermal motion between distant atoms and suggest strong anisotropy of atom thermal vibrations within the plane of the layers and in the orthogonal direction.

5.Accelerating the electronic-structure calculation of magnetic systems by equivariant neural networks

Authors:Yang Zhong, Binhua Zhang, Hongyu Yu, Xingao Gong, Hongjun Xiang

Abstract: Complex spin-spin interactions in magnets can often lead to magnetic superlattices with complex local magnetic arrangements, and many of the magnetic superlattices have been found to possess non-trivial topological electronic properties. Due to the huge size and complex magnetic moment arrangement of the magnetic superlattices, it is a great challenge to perform a direct DFT calculation on them. In this work, an equivariant deep learning framework is designed to accelerate the electronic calculation of magnetic systems by exploiting both the equivariant constraints of the magnetic Hamiltonian matrix and the physical rules of spin-spin interactions. This framework can bypass the costly self-consistent iterations and build a direct mapping from a magnetic configuration to the ab initio Hamiltonian matrix. After training on the magnets with random magnetic configurations, our model achieved high accuracy on the test structures outside the training set, such as spin spiral and non-collinear antiferromagnetic configurations. The trained model is also used to predict the energy bands of a skyrmion configuration of NiBrI containing thousands of atoms, showing the high efficiency of our model on large magnetic superlattices.

6.The orbital angular momentum of helical electrons and its implication on spin selectivity at chiral-achiral interfaces

Authors:Xiaoming Wang, Yeming Xian, Yanfa Yan

Abstract: Quantum numbers identify and differentiate between quantum states of a quantum system. The azimuthal or orbital angular momentum quantum numbers characterize wave functions that are invariant under discrete rotations. For a chiral system with screw symmetry, the rotational invariance is broken and the Bloch orbital angular momentum is generally not acknowledged. Here, we show that the wave functions of Bloch electrons in such a chiral system, denoted as helical electrons, are helical or vortex waves and, therefore, have well-defined orbital angular momentum along the screw axis. The collinear orbital-momentum locking imposed by the screw-induced orbital helicity leads to the orbital-selective transport as illustrated by tight-binding model calculations. We verify the helical states and orbital selectivity for two real chiral materials, namely, peptide helix and trigonal Se, by first-principles band structure calculations. Finally, we show that the interconversion between the orbital angular momentum and spin angular momentum at the chiral-achiral interfaces together with the orbital selectivity of the propagating helical electrons provide a fundamental principle for the chiral induced spin selectivity. Our understandings of the screw symmetry induced orbital angular momentum in chiral materials and the interplay between orbital angular momentum and spin angular momentum at the chiral-achiral interface paves a way for designing orbitronics and spintronics with chiral materials.

7.First-Principles Property Assessment of Hybrid Formate Perovskites

Authors:Abduljelili Popoola, Partha Sarathi Ghosh, Maggie Kingsland, Ravi Kashikar, Derrick DeTellem, Yixuan Xu, Shengqian Ma, Sarath Witanachchi, Sergey Lisenkov, Inna Ponomareva

Abstract: Hybrid organic inorganic formate perovskites, AB(HCOO)$_3$, is a large family of compounds which exhibit variety of phase transitions and diverse properties. Some examples include (anti)ferroelectricity, ferroelasticity, (anti)ferromagnetism, and multiferroism. While many properties of these materials have already been characterized, we are not aware of any study that focuses on comprehensive property assessment of a large number of formate perovskites. Comparison of the materials property within the family is challenging due to systematic errors attributed to different techniques or the lack of data. For example, complete piezoelectric, dielectric and elastic tensors are not available. In this work, we utilize first-principles density functional theory based simulations to overcome these challenges and to report structural, mechanical, dielectric, piezoelectric, and ferroelectric properties for 29 formate perovskites. We find that these materials exhibit elastic stiffness in the range 0.5 to 127.0 GPa , highly anisotropic linear compressibility, including zero and even negative values; dielectric constants in the range 0.1 to 102.1; highly anisotropic piezoelectric response with the longitudinal values in the range 1.18 to 21.12 pC/N, and spontaneous polarizations in the range 0.2 to 7.8 $\mu$C/cm$^2$. Furthermore, we propose and computationally characterize a few formate perovskites, which have not been reported yet.

1.Database mining and first-principles assessment of organic proton-transfer ferroelectrics

Authors:Seyedmojtaba Seyedraoufi, Elin Dypvik Sødahl, Carl Henrik Görbitz, Kristian Berland

Abstract: In organic proton-transfer ferroelectrics (OPTFe), molecules are linked together in a hydrogen-bonded network and proton transfer (PT) between molecules is the dominant mechanism of ferroelectric switching. Their fast switching frequencies make them attractive alternatives to conventional ceramic ferroelectrics, which contain rare and/or toxic elements, and require high processing temperatures. In this study, we mined the Cambridge Structural Database for potential OPTFes, uncovering all previously reported compounds, both tautomers and co-crystals, in addition to seven new candidate tautomers. The mining was based on identifying polar crystal structures with pseudo center-of-symmetry and viable PT paths. The spontaneous polarization and PT barriers were assessed using density functional theory.

2.CrTe$_2$ as a two-dimensional material for topological magnetism in complex heterobilayers

Authors:Nihad Abuawwad, Manuel dos Santos Dias, Hazem Abusara, Samir Lounis

Abstract: The discovery of two-dimensional (2D) van der Waals magnetic materials and their heterostructures provided an exciting platform for emerging phenomena with intriguing implications in information technology. Here, based on a multiscale modelling approach that combines first-principles calculations and a Heisenberg model, we demonstrate that interfacing a CrTe$_2$ layer with various Te-based layers enables the control of the magnetic exchange and Dzyaloshinskii-Moriya interactions as well as the magnetic anisotropy energy of the whole heterobilayer, and thereby the emergence of topological magnetic phases such as skyrmions and antiferromagnetic Neel merons. The latter are novel particles in the world of topological magnetism since they arise in a frustrated Neel magnetic environment and manifest as multiples of intertwined hexamer-textures. Our findings pave a promising road for proximity-induced engineering of both ferromagnetic and long-sought antiferromagnetic chiral objects in the very same 2D material, which is appealing for new information technology devices employing quantum materials.

3.Microstructure quality control of steels using deep learning

Authors:Ali Riza Durmaz, Sai Teja Potu, Daniel Romich, Johannes Möller, Ralf Nützel

Abstract: In quality control, microstructures are investigated rigorously to ensure structural integrity, exclude the presence of critical volume defects, and validate the formation of the target microstructure. For quenched, hierarchically-structured steels, the morphology of the bainitic and martensitic microstructures are of major concern to guarantee the reliability of the material under service conditions. Therefore, industries conduct small sample-size inspections of materials cross-sections through metallographers to validate the needle morphology of such microstructures. We demonstrate round-robin test results revealing that this visual grading is afflicted by pronounced subjectivity despite the thorough training of personnel. Instead, we propose a deep learning image classification approach that distinguishes steels based on their microstructure type and classifies their needle length alluding to the ISO 643 grain size assessment standard. This classification approach facilitates the reliable, objective, and automated classification of hierarchically structured steels. Specifically, an accuracy of 96% and roughly 91% is attained for the distinction of martensite/bainite subtypes and needle length, respectively. This is achieved on an image dataset that contains significant variance and labeling noise as it is acquired over more than ten years from multiple plants, alloys, etchant applications, and light optical microscopes by many metallographers (raters). Interpretability analysis gives insights into the decision-making of these models and allows for estimating their generalization capability.

4.Bulk conducting states of intrinsically doped Bi$_2$Se$_3$

Authors:Rodrigo T. Paulino, Marcos A. Avila

Abstract: With a large band gap and a single Dirac cone responsible for the topological surface states, Bi2Se3 is widely regarded as a prototypical 3D topological insulator. Further applications of the bulk material has, however, been hindered by inherent structural defects that donate electrons and make the bulk conductive. Consequently, controlling these defects is of great interest for future technological applications, and while past literature has focused on adding external doping elements to the mixture, a complete study on undoped Bi2Se3 was still lacking. In this work, we use the self-flux method to obtain high-quality Bi2Se3 single-crystals in the entire concentration range available on the phase-diagram for the technique. By combining basic structural characterization with measurements of the resistivity, Hall effect and Shubnikov-de Haas (SdH) quantum oscillations, the effects of these impurities on the bulk transport are investigated in samples with electron densities ranging from 10^17 cm^-3 to 10^19 cm^-3, from Se-rich to Bi-rich mixtures, respectively, evidencing the transition into a degenerate semiconductor regime. We find that electron-donor impurities, likely Se vacancies, unavoidably shift the Fermi level up to 200 meV inside the conduction band. Other impurities, like interstitial Bi and Se, are shown to play a significant role as scattering centres, specially at low temperatures and in the decoherence of the SdH oscillations. Previous open questions on Bi2Se3, such as the upturn in resistivity below 30 K, the different scattering times in transport and quantum oscillations, and the presence of additional low mobility bands, are addressed. The results outlined here provide a concise picture on the bulk conducting states in flux-grown Bi2Se3 single crystals, enabling better control of the structural defects and electronic properties.

5.Unraveling the Catalytic Effect of Hydrogen Adsorption on Pt Nanoparticle Shape-Change

Authors:Cameron J. Owen, Nicholas Marcella, Yu Xie, Jonathan Vandermause, Anatoly I. Frenkel, Ralph G. Nuzzo, Boris Kozinsky

Abstract: The activity of metal catalysts depends sensitively on dynamic structural changes that occur during operating conditions. The mechanistic understanding underlying such transformations in small Pt nanoparticles (NPs) of $\sim1-5$ nm in diameter, commonly used in hydrogenation reactions, is currently far from complete. In this study, we investigate the structural evolution of Pt NPs in the presence of hydrogen using reactive molecular dynamics (MD) simulations and X-ray spectroscopy measurements. To gain atomistic insights into adsorbate-induced structural transformation phenomena, we employ a combination of MD based on first-principles machine-learned force fields with extended X-ray absorption fine structure (EXAFS) measurements. Simulations and experiments provide complementary information, mutual validation, and interpretation. We obtain atomic-level mechanistic insights into `order-disorder' structural transformations exhibited by highly dispersed heterogeneous Pt catalysts upon exposure to hydrogen. We report the emergence of previously unknown candidate structures in the small Pt NP limit, where exposure to hydrogen leads to the appearance of a `quasi-icosahedral' intermediate symmetry, followed by the formation of `rosettes' on the NP surface. Hydrogen adsorption is found to catalyze these shape transitions by lowering their temperatures and increasing the apparent rates, revealing the dual catalytic and dynamic nature of interaction between nanoparticle and adsorbate. Our study also offers a new pathway for deciphering the reversible evolution of catalyst structure resulting from the chemisorption of reactive species, enabling the determination of active sites and improved interpretation of experimental results with atomic resolution.

6.Improving the reliability of machine learned potentials for modeling inhomogenous liquids

Authors:Kamron Fazel, Nima Karimitari, Tanooj Shah, Christopher Sutton, Ravishankar Sundararaman

Abstract: The atomic-scale response of inhomogeneous fluids at interfaces and surrounding solute particles plays a critical role in governing chemical, electrochemical and biological processes at such interfaces. Classical molecular dynamics simulations have been applied extensively to simulate the response of inhomogeneous fluids directly, and as inputs to classical density functional theory, but are limited by the accuracy of the underlying empirical force fields. Here, we deploy neural network potentials (NNPs) trained to \emph{ab initio} simulations to accurately predict the inhomogeneous response of two widely different fluids: liquid water and molten NaCl. Although the advantages of NNPs is that they can be readily trained to model complex systems, one limitation in modeling the inhomogeneous response of liquids is the need for including the correct configurations of the system in the training data. Therefore, first we establish protocols, based on molecular dynamics simulations in external atomic potentials, to sufficiently sample the correct configurations of inhomogeneous fluids. We show that NNPs trained to inhomogeneous fluid configurations can predict several properties such as the density response, surface tension and size-dependent cavitation free energies in water and molten NaCl corresponding to \emph{ab initio} interactions, more accurately than with empirical force fields. This work therefore provides a first demonstration and framework for extracting the response of inhomogeneous fluids from first principles for classical density-functional treatment of fluids free from empirical potentials.

1.Monofluorinated Ether Electrolyte with Acetal Backbone for High-Performance Lithium Metal Batteries

Authors:Elizabeth Zhang, Yuelang Chen, Zhiao Yu, Yi Cui, Zhenan Bao

Abstract: High degree of fluorination for ether electrolytes has resulted in improved cycling stability of lithium metal batteries (LMBs) due to stable SEI formation and good oxidative stability. However, the sluggish ion transport and environmental concerns of high fluorination degree drives the need to develop less fluorinated structures. Here, we introduce bis(2-fluoroethoxy)methane (F2DEM) which features monofluorination of the acetal backbone. High coulombic efficiency (CE) and stable long-term cycling in Li||Cu half cells can be achieved with F2DEM even under fast Li metal plating conditions. The performance of F2DEM is further compared with diethoxymethane (DEM) and 2-[2-(2,2-Difluoroethoxy)ethoxy]-1,1,1-Trifluoroethane (F5DEE). The structural similarity of DEM allows us to better probe the effects of monofluorination, while F5DEE is chosen as the one of the best performing LMB electrolytes for reference. The monofluorine substitution provides improved oxidation stability compared to non-fluorinated DEM, as demonstrated in the linear sweep voltammetry (LSV) and voltage holding experiments in Li||Pt and Li||Al cells. Higher ionic conductivity compared to F5DEE is also observed due to the decreased degree of fluorination. Furthermore, 1.75 M lithium bis(fluorosulfonyl)imide (LiFSI) / F2DEM displays significantly lower overpotential compared with the two reference electrolytes, which improves energy efficiency and enables its application in high-rate conditions. Comparative studies of F2DEM with DEM and F5DEE in anode-free (LiFePO4) LFP pouch cells and high-loading LFP coin cells with 20 {\mu}m excess Li further show improved capacity retention of F2DEM electrolyte.

2.High-Entropy Anti-Perovskites with Negative Thermal Expansion Behavior

Authors:Xiuliang Yuan, Bing Wang, Ying Sun, Huaiming Guo, Kewen Shi, Sihao Deng, Lunhua He, Huiqing Lu, Hong Zhang, Shengdi Xu, Yi Du, Shengqi Chu, Weichang Hao, Cong Wang

Abstract: The negative thermal expansion (NTE) material is counterintuitive due to its typical feature of volume contraction on heating, which can act as the thermal-expansion compensators to counteract the normal positive thermal expansion. A wide temperature range of NTE behavior is desired, whereas the performance optimization by traditional doping strategy has reached its upper limit. In this paper, the unique sluggish characteristic in high entropy materials is proposed to broaden the NTE temperature range by relaxing the sharp phase transition in Mn-based anti-perovskite nitride. We propose an empirical screening method to synthesis the high-entropy anti-perovskite (HEAP). A remarkable NTE behavior (up to {\Delta}T = 235 K, 5 K < T < 240 K) with the coefficient of thermal expansion (CTE) of - 4.7 ppm/K has been observed in typical HEAP Mn3Cu0.2Zn0.2Ga0.2Ge0.2Mn0.2N, whose working temperature range is far wider than that of traditional low-entropy doping system. The wide temperature range of phase separation due to sluggish phase transition is responsible for the broadened NTE behavior in HEAP. Our demonstration provides a unique paradigm in the broadening of the NTE temperature range for phase transition induced NTE materials through entropy engineering.

3.Accelerating Optimal Elemental Configuration Search in Crystal using Ising Machine

Authors:Kazuhide Ichikawa, Satoru Ohuchi, Koki Ueno, Tomoyasu Yokoyama

Abstract: This research demonstrates that Ising machines can effectively solve optimal elemental configuration searches in crystals, with Au-Cu alloys serving as an example. The energy function is derived using the cluster expansion method in the form of a QUBO function, enabling efficient problem-solving via Ising machines. We have successfully obtained reasonable solutions for crystal structures consisting of over 10,000 atoms. Notably, we have also obtained plausible solutions for optimization problems with constrained solutions, such as situations where the composition ratio of atomic species is predetermined. These findings suggest that Ising machines can be valuable tools for addressing materials science challenges.

4.Implementation of the SCAN Exchange-Correlation Functional with Numerical Atomic Orbitals

Authors:Renxi Liu, Daye Zheng, Xinyuan Liang, Xinguo Ren, Mohan Chen, Wenfei Li

Abstract: Kohn-Sham density functional theory (DFT) is nowadays widely used for electronic structure theory simulations, and the accuracy and efficiency of DFT rely on approximations of the exchange-correlation functional. By inclusion of the kinetic energy density $\tau$, the meta-generalized-gradient approximation (meta-GGA) family of functionals achieves better accuracy and flexibility while retaining the efficiency of semi-local functionals. The SCAN meta-GGA functional has been proven to yield accurate results for solid and molecular systems. We implement meta-GGA functionals with both numerical atomic orbitals and plane wave basis in the ABACUS package. Apart from the exchange-correlation potential, we also discuss the evaluation of force and stress. To validate our implementation, we perform finite-difference tests and convergence tests with the SCAN meta-GGA functional. We further test water hexamers, weakly interacting molecules of the S22 dataset, as well as 13 semiconductors. The results show satisfactory agreements with previous calculations and available experimental values.

5.Sliding and Pinning in Structurally Lubric 2D Material Interfaces

Authors:Jin Wang, Ali Khosravi, Andrea Vanossi, Erio Tosatti

Abstract: A plethora of two-dimensional (2D) materials entered the physics and engineering scene in the last two decades. Their robust, membrane-like sheet permit -- mostly require -- deposition, giving rise to solid-solid dry interfaces whose bodily mobility, pinning, and general tribological properties under shear stress are currently being understood and controlled, experimentally and theoretically. In this Colloquium we use simulation case studies of twisted graphene system as a prototype workhorse tool to demonstrate and discuss the general picture of 2D material interface sliding. First, we highlight the crucial mechanical difference, often overlooked, between small and large incommensurabilities, corresponding e.g., to small and large twist angles in graphene interfaces. In both cases, focusing on flat, structurally lubric, "superlubric" geometries, we elucidate and review the generally separate scaling with area of static friction in pinned states and of kinetic friction during sliding, tangled as they are with the effects of velocity, temperature, load, and defects. Including the role of island boundaries and of elasticity, and corroborating when possible the existing case-by-case results in literature beyond graphene, the overall picture proposed is meant for general 2D material interfaces, that are of importance for the physics and technology of existing and future bilayer and multilayer systems.

6.On the photovoltaic effect asymmetry in ferroelectrics

Authors:S. Semak, V. Kapustianyk, Yu. Eliyashevskyy, O. Bovgyra, M. Kovalenko, U. Mostovoi, B. Doudin, B. Kundys

Abstract: Despite symmetrical polarization, the magnitude of a light-induced voltage is known to be asymmetric with respect to poling sign in many photovoltaic (PV) ferroelectrics (FEs). This asymmetry remains unclear and is often attributed to extrinsic effects. We show here for the first time that such an asymmetry can be intrinsic, steaming from the superposition of asymmetries of internal FE bias and electro-piezo-strictive deformation. This hypothesis is confirmed by the observed decrease of PV asymmetry for smaller FE bias. Moreover, the both PV effect and remanent polarization are found to increase under vacuum-induced expansion and to decrease for gas-induced compression, with tens percents tunability. The change in cations positions under pressure is analysed through the first-principle density functional theory calculations. The reported properties provide key insight for FE-based solar elements optimization.

7.Power Spectral Density Analysis and Correlation of Growth and Morphology of Ni Films on Si Substate

Authors:Harsh Bhatt, Lavanya Negi

Abstract: Ni thin films grown by thermal evaporation and sputtering under different deposition conditions are characterized for structural and morphological properties using X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques. XRD results suggested the growth of polycrystalline fcc Ni phase for all the samples. Morphological characteristics of the films were compared by analysing AFM data for root mean square roughness, height-height correlation function and power spectral density (PSD) measurements. Applying fractal and k-correlation fitting models to the PSD data, different morphological parameters are quantified. The study suggested that Ni films grown at higher substrate temperature (~ 150 oC) by thermal evaporation and at low Ar pressure (~ 0.2 Pa) by sputtering techniques yielded films of small surface roughness with Brownian fractal self-affine surfaces.

8.Near-Atomic Scale Perspective on the Oxidation of Ti3C2Tx MXenes: Insights from Atom Probe Tomography

Authors:Mathias Krämer, Bar Favelukis, Ayman A. El-Zoka, Maxim Sokol, Brian A. Rosen, Noam Eliaz, Se-Ho Kim, Baptiste Gault

Abstract: MXenes are a family of 2D transition metal carbides and nitrides with remarkable properties and great potential for energy storage and catalysis applications. However, their oxidation behavior is not yet fully understood, and there are still open questions regarding the spatial distribution and precise quantification of surface terminations, intercalated ions, and possible uncontrolled impurities incorporated during synthesis and processing. Here, atom probe tomography analysis of as-synthesized Ti3C2Tx MXenes reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as unetched Al. Following oxidation of the colloidal solution of MXenes, it is observed that the alkalies enriched in TiO2 nanowires. Although these elements are tolerated through the incorporation by wet chemical synthesis, they are often overlooked when the activity of these materials is considered, particularly during catalytic testing. This work demonstrates how the capability of atom probe tomography to image these elements in 3D at the near-atomic scale can help to better understand the activity and degradation of MXenes, in order to guide their synthesis for superior functional properties.

9.Cellular automata inspired multistable origami metamaterials for mechanical learning

Authors:Zuolin Liu, Hongbin Fang, Jian Xu, Kon-Well Wang

Abstract: Recent advances in multistable metamaterials reveal a link between structural configuration transition and Boolean logic, heralding a new generation of computationally capable intelligent materials. To enable higher-level computation, existing computational frameworks require the integration of large-scale networked logic gates, which places demanding requirements on the fabrication of materials counterparts and the propagation of signals. Inspired by cellular automata, we propose a novel computational framework based on multistable origami metamaterials by incorporating reservoir computing, which can accomplish high-level computation tasks without the need to construct a logic gate network. This approach thus eleimates the demanding requirements for fabrication of materials and signal propagation when constructing large-scale networks for high-level computation in conventional mechano-logic. Using the multistable stacked Miura-origami metamaterial as a validation platform, digit recognition is successfully implemented through experiments by a single actuator. Moreover, complex tasks, such as handwriting recognition and 5-bit memory tasks, are also shown to be feasible with the new computation framework. Our research represents a significant advancement in developing a new generation of intelligent materials with advanced computational capabilities. With continued research and development, these materials could have a transformative impact on a wide range of fields, from computational science to material mechano-intelligence technology and beyond.

10.Chemical state analysis assisted combinatorial exploration of the Zn-Ta-N phase space and synthesis of wurtzite Zn2TaN3

Authors:Siarhei Zhuk Empa - Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland, Alexander Wieczorek Empa - Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland, Amit Sharma Empa - Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun, Switzerland, Jyotish Patidar Empa - Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland, Kerstin Thorwarth Empa - Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland, Johann Michler Empa - Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun, Switzerland, Sebastian Siol Empa - Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland

Abstract: The discovery of new functional materials is one of the key challenges in materials science. Combinatorial high-throughput approaches using reactive sputtering are commonly employed to screen unexplored phase spaces. During reactive combinatorial deposition the process conditions are rarely optimized, which can lead to poor crystallinity of the thin films. In addition, sputtering at shallow deposition angles can lead to off-axis preferential orientation of the grains. This can make the results from a conventional structural phase screening ambiguous. Here we perform a combinatorial screening of the Zn-Ta-N phase space with the aim to synthesize the novel semiconductor Zn2TaN3. While the results of the XRD phase screening are inconclusive, including chemical state analysis mapping in our workflow allows us to see a very clear discontinuity in the evolution of the Ta binding environment. This is indicative of the formation of a new ternary phase. In additional experiments, we isolate the material and perform a detailed characterization confirming the formation of single phase WZ-Zn2TaN3. Besides the formation of the new ternary nitride, we map the functional properties of ZnxTa1-xN and report previously unreported clean chemical state analysis for Zn3N2, TaN and Zn2TaN3. Overall, the results of this study showcase common challenges in high-throughput materials screening and highlight the merit of employing characterization techniques sensitive towards changes in the materials' short-range order and chemical state.

11.Enhancing interfacial thermal conductance of Si/PVDF by strengthening atomic couplings

Authors:Zhicheng Zong, Shichen Deng, Yangjun Qin, Xiao Wan, Jiahong Zhan, Dengke Ma, Nuo Yang

Abstract: The thermal transport across inorganic/organic interfaces attracts interest for both academic and industry due to its widely applications in flexible electronics etc. Here, the interfacial thermal conductance of inorganic/organic interfaces consisting of silicon and polyvinylidene fluoride is systematically investigated by molecular dynamics simulations. Interestingly, it is demonstrated that a modified silicon surface with hydroxyl groups can drastically enhance the conductance by 698%. These results are elucidated based on interfacial couplings and lattice dynamics insights. This study not only provides feasible strategies to effectively modulate the interfacial thermal conductance of inorganic/organic interfaces but also deepens the understanding of the fundamental physics underlying phonon transport across interfaces.

12.Investigating densification during sintering with molecular dynamics and phase-field simulations

Authors:Marco Seiz, Henrik Hierl, Britta Nestler

Abstract: The resulting microstructure after the sintering process determines many materials properties of interest. In order to understand the microstructural evolution, simulations are often employed. One such simulation method is the phase-field method, which has garnered much interest in recent decades. However, the method lacks a complete model for sintering, as previous works could show unphysical effects and the inability to reach representative volume elements. Thus the present paper aims to close this gap by employing molecular dynamics and determining rules of motion which can be translated to a phase-field model. The resulting phase-field model is shown to be representative starting from particle counts between 97 and 262 and contains the qualitative correct dependence of sintering rate on particle size.

13.Atom-by-atom design of metal oxide catalysts for the oxygen evolution reaction with machine learning

Authors:Jaclyn R. Lunger, Jessica Karaguesian, Hoje Chun, Jiayu Peng, Yitong Tseo, Chung Hsuan Shan, Byungchan Han, Yang Shao-Horn, Rafael Gomez-Bombarelli

Abstract: Green hydrogen production is crucial for a sustainable future, but current catalysts for the oxygen evolution reaction (OER) suffer from slow kinetics, despite many efforts to produce optimal designs, particularly through the calculation of descriptors for activity. In this study, we develop a dataset of density functional theory calculations of bulk and surface perovskite oxides, and adsorption energies of OER intermediates, which includes compositions up to quaternary and facets up to (555). We demonstrate that per-site properties of perovskite oxides such as Bader charge or band center can be tuned through element substitution and faceting, and develop a machine learning model that accurately predicts these properties directly from the local chemical environment. We leverage these per-site properties to identify promising perovskites with high theoretical OER activity. The identified design principles and promising new materials provide a roadmap for closing the gap between current artificial catalysts and biological enzymes.

14.Hardness and fracture toughness models by symbolic regression

Authors:Jinbin Zhao, Peitao Liu, Jiantao Wang, Jiangxu Li, Haiyang Niu, Yan Sun, Junlin Li, Xing-Qiu Chen

Abstract: Superhard materials with good fracture toughness have found wide industrial applications, which necessitates the development of accurate hardness and fracture toughness models for efficient materials design. Although several macroscopic models have been proposed, they are mostly semiempirical based on prior knowledge or assumptions, and obtained by fitting limited experimental data. Here, through an unbiased and explanatory symbolic regression technique, we built a macroscopic hardness model and fracture toughness model, which only require shear and bulk moduli as inputs. The developed hardness model was trained on an extended dataset, which not only includes cubic systems, but also contains non-cubic systems with anisotropic elastic properties. The obtained models turned out to be simple, accurate, and transferable. Moreover, we assessed the performance of three popular deep learning models for predicting bulk and shear moduli, and found that the crystal graph convolutional neural network and crystal explainable property predictor perform almost equally well, both better than the atomistic line graph neural network. By combining the machine-learned bulk and shear moduli with the hardness and fracture toughness prediction models, potential superhard materials with good fracture toughness can be efficiently screened out through high-throughput calculations.

1.Strain Relaxation in Core-Shell Pt-Co Catalyst Nanoparticles

Authors:Elliot Padgett, Megan E. Holtz, Anusorn Kongkanand, David A. Muller

Abstract: Surface strain plays a key role in enhancing the activity of Pt-alloy nanoparticle oxygen reduction catalysts. However, the details of strain effects in real fuel cell catalysts are not well-understood, in part due to a lack of strain characterization techniques that are suitable for complex supported nanoparticle catalysts. This work investigates these effects using strain mapping with nanobeam electron diffraction and a continuum elastic model of strain in simple core-shell particles. We find that surface strain is relaxed both by lattice defects at the core-shell interface and by relaxation across particle shells caused by Poisson expansion in the spherical geometry. The continuum elastic model finds that in the absence of lattice dislocations, geometric relaxation results in a surface strain that scales with the average composition of the particle, regardless of the shell thickness. We investigate the impact of these strain effects on catalytic activity for a series of Pt-Co catalysts treated to vary their shell thickness and core-shell lattice mismatch. For catalysts with the thinnest shells, the activity is consistent with an Arrhenius dependence on the surface strain expected for coherent strain in dislocation-free particles, while catalysts with thicker shells showed greater activity losses indicating strain relaxation caused by dislocations as well.

2.Phenomenological model of lithium-ion battery formation cycling and aging

Authors:Andrew Weng, Everardo Olide, Iaroslav Kovalchuk, Jason B. Siegel, Anna Stefanopoulou

Abstract: This work proposes a semi-empirical model for the SEI growth process during the early stages of lithium-ion battery formation cycling and aging. By combining a full-cell model which tracks half-cell equilibrium potentials, a zero-dimensional model of SEI growth kinetics, and a semi-empirical description of macroscopic cell expansion, the resulting model replicated experimental trends measured on a 2.5 Ah pouch cell, including the first-cycle efficiency, cell thickness changes, and electrolyte reduction peaks during the first charge dQ/dV signal. This work also introduces an SEI growth boosting formalism which enables a unified description of SEI growth during both formation cycling and aging. The model further provides a homogenized representation of multi-component SEI reactions which enables the study of both solvent and additive consumption during formation. This work bridges the gap between electrochemical descriptions of SEI growth and applications towards industrial battery manufacturing technology where battery formation is an essential but time-consuming final step. We envision that the formation model can further be used to predict the impact of formation protocols and electrolyte systems on SEI passivation and resulting battery longevity.

3.Ferromagnetic interlayer coupling in CrSBr crystals irradiated by ions

Authors:Fangchao Long, Mahdi Ghorbani-Asl, Kseniia Mosina, Yi Li, Kaiman Lin, Fabian Ganss, René Hübner, Zdenek Sofer, Florian Dirnberger, Akashdeep Kamra, Arkady V. Krasheninnikov, Slawomir Prucnal, Manfred Helm, Shengqiang Zhou

Abstract: Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggest that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

4.Terahertz emission from transient currents and coherent phonons in layered MoSe$_2$ and WSe$_2$

Authors:Jessica Afalla, Joselito Muldera, Semmi Takamizawa, Takumi Fukuda, Keiji Ueno, Masahiko Tani, Muneaki Hase

Abstract: Terahertz (THz) time-domain emission spectroscopy was performed on layered 2H-MoSe2 and 2H-WSe2. The THz emission shows an initial cycle attributed to surge currents and is followed by oscillations attributed to coherent interlayer phonon modes. To obtain the frequencies of the interlayer vibrations, analysis of the THz emission waveforms were performed, separating the two contributions to the total waveform. Results of the fitting show several vibrational modes in the range of 5.87 to 32.75 cm-1 for the samples, attributed to infrared-active interlayer shear and breathing modes. This study demonstrates that THz emission spectroscopy provides a means of observing these low frequency vibrational modes in layered materials.

5.The effect of compression shock heating in collision welding

Authors:Georg C. Ganzenmüller, Sebastian Hütter, Martin Reder, Andreas Prahs, Daniel Schneider, Britta Nestler, Thorsten Halle, Stefan Hiermaier

Abstract: This work discusses the origin of temperature rise during the collision welding process. The different physical irreversible and reversible mechanisms which act as heat sources are described: isentropic compression work, shock dissipation, plasticity, and phase transitions. The temperature increase due to these effects is quantified in a continuum mechanics approach, and compared to predictions of atomistic molecular dynamics simulations. Focusing on a single impact scenario of 1100 aluminium at 700 m/s, our results indicate that shock heating and plastic work only effect a temperature rise of 100 K, and that the effects of phase change are not significant. This temperature rise cannot explain welding. In consequence, the relevant mechanism which effects bonding in collision welding must be due to the jet, which is only formed at oblique impact angles.

6.Local field of spin-spin interactions in the nuclear spin system of n-GaAs

Authors:V. M. Litvyak, R. V. Cherbunin, V. K. Kalevich, K. V. Kavokin

Abstract: At low lattice temperatures the nuclear spins in a solid form a closed thermodynamic system that is well isolated from the lattice. Thermodynamic properties of the nuclear spin system are characterized by the local field of spin-spin interactions, which determines its heat capacity and the minimal achievable nuclear spin temperature in demagnetization experiments. We report the results of measurement of the local field for the nuclear spin system in GaAs, which is a model material for semiconductor spintronics. The choice of the structure, a weakly doped GaAs epitaxial layer with weak residual deformations, and of the measurement method, the adiabatic demagnetization of optically cooled nuclear spins, allowed us to refine the value of nuclear spin-spin local field, which turned out to be two times less than one previously obtained. Our experimental results are confirmed by calculations, which take into account dipole-dipole and indirect (pseudodipolar and exchange) nuclear spin interactions.

7.Temperature dependent second-order Raman scattering in CuI

Authors:Ron Hildebrandt, Michael Seifert, Janine George, Steffen Blaurock, Silvana Botti, Harald Krautscheid, Marius Grundmann, Chris Sturm

Abstract: We report second-order Raman scattering spectra of copper iodide bulk single crystals aside from the fundamental TO and LO mode. The spectral shape was reproduced by a 2-phonon density of states calculated by DFT. Characteristic multi-phonon features were identified and assigned to combination, overtone and difference modes. In this way, the energy of acoustic zone-boundary phonons was determined. The temperature dependence of those modes and the fundamental optical phonons was analyzed by means of phonon-phonon interactions and lattice expansion effects up to room temperature. Processes related to the mode energy shift and width were identified for phonons at high symmetry points. The shifts due to lattice expansion are in accordance with the predictions by DFT in quasi-harmonic approximation using PBEsol functional.

8.High temperature decomposition and age hardening of single-phase wurtzite Ti$_{1-x}$Al$_{x}$N thin films grown by cathodic arc deposition

Authors:J. Salamania, F. Bock, L. J. S. Johnson, F. Tasnádi, K. M. Calamba Kwick, A. F. Farhadizaeh, I. A. Abrikosov, L. Rogström, M. Odén

Abstract: We investigated the high temperature decomposition behavior of wurtzite phase Ti$_{1-x}$Al$_{x}$N films using experimental methods and first-principles calculations. Single phase metastable wurtzite Ti$_{1-x}$Al$_{x}$N (x = 0.65, 0.75, 085 and 0.95) solid solution films were grown by cathodic arc deposition using low duty cycle pulsed substrate-bias voltage. First-principles calculated elastic constants of the wurtzite Ti$_{1-x}$Al$_{x}$N phase show a strong dependence on alloy composition. The predicted phase diagram shows a miscibility gap with an unstable region. High resolution scanning transmission electron microscopy and chemical mapping demonstrate decomposition of the films after high temperature annealing (950$^{\circ}$C), which resulted in nanoscale chemical compositional modulations containing Ti-rich and Al-rich regions with coherent or semi coherent interfaces. This spinodal decomposition of the wurtzite film causes age hardening of 1-2 GPa.

9.Ion irradiation-induced sinking of Ag nanocubes into substrates

Authors:Shiva Choupanian, Wolfhard Moeller, Martin Seyring, Claudia Pacholski, Elke Wendler, Andreas Undisz, Carsten Ronning

Abstract: Ion irradiation can cause burrowing of nanoparticles in substrates, strongly depending on the material properties and irradiation parameters. In this study, we demonstrate that the sinking process can be accomplished with ion irradiation of cube-shaped Ag nanoparticles on top of silicon; how ion channeling affects the sinking rate; and underline the importance of the amorphous state of the substrate upon ion irradiation. Based on our experimental findings, the sinking process is described as being driven by capillary forces enabled by ion-induced plastic flow of the substrate.

10.Aging and passivation of magnetic properties in Co/Gd bilayers

Authors:T. J. Kools, Y. L. W. van Hees, K. Poissonnier, P. Li, B. Barcones Campo, M. A. Verheijen, B. Koopmans, R. Lavrijsen

Abstract: Synthetic ferrimagnets based on Co and Gd bear promise for directly bridging the gap between volatile information in the photonic domain and non-volatile information in the magnetic domain, without the need for any intermediary electronic conversion. Specifically, these systems exhibit strong spin-orbit torque effects, fast domain wall motion and single-pulse all-optical switching of the magnetization. An important open challenge to bring these materials to the brink of applications is to achieve long-term stability of their magnetic properties. In this work, we address the time-evolution of the magnetic moment and compensation temperature of magnetron sputter grown Pt/Co/Gd trilayers with various capping layers. Over the course of three months, the net magnetic moment and compensation temperature change significantly, which we attribute to quenching of the Gd magnetization. We identify that intermixing of the capping layer and Gd is primarily responsible for this effect, which can be alleviated by choosing nitrides for capping as long as reduction of nitride to oxide is properly addressed. In short, this work provides an overview of the relevant aging effects that should be taken into account when designing synthetic ferrimagnets based on Co and Gd for spintronic applications.

11.Search for magnetoelectric monopole response in Cr$_2$O$_3$ powder

Authors:Syed Q. A. Shah, Ather Mahmood, Arun Parthasarathy, Christian Binek

Abstract: Powder samples have been suggested as a pathway to fabricate isotropic magnetoelectric (ME) materials which effectively only have a pseudoscalar or monopole ME response. We demonstrate that random distribution of ME grains alone does not warrant isotropic ME response because the activation of a non-vanishing ME response requires a ME field cooling protocol which tends to induce preferred axes. We investigate the evolution of ME susceptibility in powder chromia samples for various ME field cooling protocols both theoretically and experimentally. In particular, we work out the theoretical expressions for ME susceptibility for powder Chromia in the framework of statistical mechanics where Boltzmann factors weigh the orientation of the N\'eel vector relative to the local orientation of the c-axis of a grain. Previous approximations oversimplified the thermodynamic nature of the annealing process giving rise to misleading conclusions on the role of the magnitude of the applied product of electric and magnetic fields on the ME response. In accordance with our refined theory, a strong dependence of the functional form of $\alpha$ vs. $T$ of Chromia powders on the ME field cooling protocol is observed. It shows that Chromia powder is not generically an isotropic ME effective medium but provides a pathway to realize the elusive isotropic ME response.

12.Topologically-forced electro-modulation of friction

Authors:Yu Zhang, Di Jin, Ran Tivony, Nir Kampf, Jacob Klein

Abstract: Controlling the friction between sliding surfaces via their electric potential (electro-modulation) is a long-standing tribological goal. Phospholipid assemblies, whether as continuous bilayers or as close-packed vesicles (liposomes), form highly-lubricious surface boundary layers in aqueous media, via the hydration lubrication mechanism at the lipid-lipid interfaces, with friction coefficients {\mu}(= [force to slide]/load) down to 10-4, thus offering scope for large friction changes. Here we show that the friction between two such lipid-coated surfaces can be massively modulated through very small potentials applied to one of them, changing reversibly by up to 200-fold or more. Atomistic simulations indicate that this arises from (fully reversible) electroporation of the lipid bilayers under the potential-driven inter-surface electric fields. The porated topology of the bilayers leads to increased dehydration-induced attraction between the headgroups of opposing bilayers; at the same time, the porated bilayer structures may bridge the gap between the sliding surfaces. These effects act in parallel to modulate the friction by topologically-forcing the slip plane to pass through the intra-bilayer acyl tail interface, for which {\mu}{\approx}0.1. This enables facile, fully-reversible electro-modulation of the friction, with a dynamic range up to some 2 orders of magnitude larger than achieved to date.

13.Driving skyrmions in flow regime in synthetic ferrimagnets

Authors:Sougata Mallick, Yanis Sassi, Nicholas Figueiredo Prestes, Sachin Krishnia, Fernando Gallego, Thibaud Denneulin, Sophie Collin, Karim Bouzehouane, André Thiaville, Rafal E. Dunin-Borkowski, Vincent Jeudy, Albert Fert, Nicolas Reyren, Vincent Cros

Abstract: Despite significant advances in the last decade regarding the room temperature stabilization of skyrmions or their current induced dynamics, the impact of local material inhomogeneities still remains an important issue that impedes to reach the regime of steady state motion of these spin textures. Here, we study the spin-torque driven motion of skyrmions in synthetic ferrimagnetic multilayers with the aim of achieving high mobility and reduced skyrmion Hall effect. We consider Pt|Co|Tb multilayers of various thicknesses with antiferromagnetic coupling between the Co and Tb magnetization. The increase of Tb thickness in the multilayers allows to reduce the total magnetic moment and increases the spin-orbit torques allowing to reach velocities up to 400 m.s-1 for skyrmions with diameters of about 160 nm. We demonstrate that due to reduced skyrmion Hall effect, combined with the edge repulsion of the magnetic track making the skyrmions moving along the track without any transverse deflection. Further, by comparing the field-induced domain wall motion and current-induced skyrmion motion, we demonstrate that the skyrmions at the largest current densities present all the characteristics of a dynamical flow regime.

14.Tuning thermoelectric properties of Sb$_2$Te$_3$-AgSbTe$_2$ nanocomposite thin film -- synergy of band engineering and heat transport modulation

Authors:Abhishek Ghosh, Khushboo Agarwal, Sergio Gonzalez Munoz, Prashant Bisht, Chandan K Vishwakarma, Narinder Kaur, Mujeeb Ahmad, Per Erik Vullum, Branson D. Belle, Rajendra Singh, O. V. Kolosov, Bodh Raj Mehta

Abstract: The present study demonstrates a large enhancement in the Seebeck coefficient and ultralow thermal conductivity (TE) in Sb$_2$Te$_3$-AgSbTe$_2$ nanocomposite thin film. The addition of Ag leads to the in-situ formation of AgSbTe$_2$ secondary phase nanoaggregates in the Sb$_2$Te$_3$ matrix during the growth resulting in a large Seebeck coefficient and reduction of the thermal conductivity. A series of samples with different amounts of minor AgSbTe$_2$ phases are prepared to optimize the TE performance of Sb$_2$Te$_3$ thin films. Based on the experimental and theoretical evidence, it is concluded that a small concentration of Ag promotes the band flattening and induces a sharp resonate-like state deep inside the valence band of Sb$_2$Te$_3$, concurrently modifying the density of states (DOS) of the composite sample. In addition, the electrical potential barrier introduced by the band offset between the host TE matrix and the secondary phases promotes strong energy-dependent carrier scattering in the composite sample, which is also responsible for enhanced TE performance. A contemporary approach based on scanning thermal microscopy is performed to experimentally obtain thermal conductivity values of both the in-plane and cross-plane directions, showing a reduced in-plane thermal conductivity value by ~ 58% upon incorporating the AgSbTe$_2$ phase in the Sb$_2$Te$_3$ matrix. Benefitting from the synergistic manipulation of electrical and thermal transport, a large ZT value of 2.2 is achieved at 375 K. The present study indicates the importance of a combined effect of band structure modification and energy-dependent charge carrier scattering along with reduced thermal conductivity for enhancing TE properties.

1.Spin current generation from an epitaxial tungsten dioxide WO_2

Authors:Kohei Ueda, Hayato Fujii, Takanori Kida, Masayuki Hagiwara, Jobu Matsuno

Abstract: We report on efficient spin current generation at room temperature in rutile type WO_2 grown on Al_2O_3(0001) substrate. The optimal WO_2 film has (010)-oriented monoclinically distorted rutile structure with metallic conductivity due to 5d^2 electrons, as characterized by x-ray diffraction, electronic transport, and x-ray photoelectron spectroscopy. By conducting harmonic Hall measurement in Ni_{81}Fe_{19}/WO_2 bilayer, we estimate two symmetries of the spin-orbit torque (SOT), i.e., dampinglike (DL) and fieldlike ones to find that the former is larger than the latter. By comparison with the Ni_{81}Fe_{19}/W control sample, the observed DL SOT efficiency \xi_{DL} of WO_2 (+0.174) is about two thirds of that of W (-0.281) in magnitude, with a striking difference in their signs. The magnitude of the \xi_{DL} of WO_2 exhibits comparable value to those of widely reported Pt and Ta, and Ir oxide IrO_2. The positive sign of the \xi_{DL} of WO_2 can be explained by the preceding theoretical study based on the 4d oxides. These results highlight that the epitaxial WO_2 offers a great opportunity of rutile oxides with spintronic functionalities, leading to future spin-orbit torque-controlled devices.

2.Pressure-Constant Monte Carlo Simulation of Solid CO2 Phase I up to 10 GPa using Kihara Potential Model

Authors:Koji Kobashi

Abstract: Solid CO2 phase I was studied using the pressure-constant NPT Monte Carlo simulation and the Kihara core potential in the temperature range below 194 K and the pressure range below 10 GPa. At a pressure of 1 bar, the temperature dependence of the calculated lattice constant agreed reasonably well with experiment. It was found that the random distribution of molecular orientations due to temperature gave a significant contribution to the increase in the lattice constant. At high pressure, the pressure dependence of the lattice constant also agreed well with experiment.

3.Essential L-Amino Acid-Functionalized Graphene Oxide for Liquid Crystalline Phase Formation

Authors:H. Gharagulyan, Y. Melikyan, V. Hayrapetyan, Kh. Kirakosyan, D. A. Ghazaryan, M. Yeranosyan

Abstract: The colloidal 2D materials based on graphene and its modifications are of great interest when it comes to forming LC phases. These LC phases allow controlling the orientational order of colloidal particles, paving the way for the efficient processing of modified graphene with anisotropic properties. Here, we present the peculiarities of AA functionalization of GO, along with the formation of its LC phase and orientational behavior in an external magnetic field. We discuss the influence of pH on the GOLC, ultimately showing its pH-dependent behavior for GO-AA complexes. In addition, we observe different GO morphology changes due to the presence of AA functional groups, namely L-cysteine dimerization on the GO platform. The pH dependency of AA-functionalized LC phase of GO is examined for the first time. We believe that our studies will open new possibilities for applications in bionanotechnologies due to self-assembling properties of LCs and magnificent properties of GO.

4.Large spin splitting and piezoelectricity in a two-dimensional topological insulator Al$_2$SbBi with double-layer honeycomb structure

Authors:D. Q. Fang, H. Zhang, D. W. Wang

Abstract: Two-dimensional materials provide remarkable platforms to uncover intriguing quantum phenomena and develop nanoscale devices of versatile applications. Recently, AlSb in the double-layer honeycomb (DLHC) structure was successfully synthesized exhibiting a semiconducting nature [ACS Nano 15, 8184 (2021)], which corroborates the preceding theoretical predictions and stimulates the exploration of new robust DLHC materials. In this work, we propose a Janus DLHC monolayer Al$_2$SbBi, the dynamical, thermal, and mechanical stabilities of which are confirmed by first-principles calculations. Monolayer Al$_2$SbBi is found to be a nontrivial topological insulator with a gap of about 0.2 eV, which presents large spin splitting and peculiar spin texture in the valence bands. Furthermore, due to the absence of inversion symmetry, monolayer Al$_2$SbBi exhibits piezoelectricity and the piezoelectric strain coefficients d$_{11}$ and d$_{31}$ are calculated to be 7.97 pm/V and 0.33 pm/V, respectively, which are comparable to and even larger than those of many piezoelectric materials. Our study suggests that monolayer Al$_2$SbBi has potential applications in spintronic and piezoelectric devices.

5.Novel Electronic Structure of Nitrogen-Doped Lutetium Hydrides

Authors:Adam Denchfield, Hyowon Park, Russell J. Hemley

Abstract: First-principles density functional theory (DFT) calculations of Lu-H-N compounds reveal low-energy configurations of Fm$\overline{3}$m Lu$_{8}$H$_{23-x}$N structures that exhibit novel electronic properties such as flat bands, sharply peaked densities of states (van Hove singularities, vHs), and intersecting Dirac cones near the Fermi energy (E$_F$). These N-doped LuH$_3$-based structures also exhibit an interconnected metallic hydrogen network, which is a common feature of high-T$_c$ hydride superconductors. Electronic property systematics give estimates of T$_c$ for optimally ordered structures that are well above the critical temperatures predicted for structures considered previously. The vHs and flat bands near E$_F$ are enhanced in DFT+U calculations, implying strong correlation physics should also be considered for first-principles studies of these materials. These results provide a basis for understanding the novel electronic properties observed for nitrogen-doped lutetium hydride.

6.Point Defects in Two-Dimensional RuCl3

Authors:Wenqi Yang, Linghan Zhu, Yan Lu, Erik Henriksen, Li Yang

Abstract: Defects are crucial in determining a variety of material properties especially in low dimensions. In this work, we study point defects in monolayer alpha-phase Ruthenium (III) chloride (alpha-RuCl3), a promising candidate to realize quantum spin liquid with nearly degenerate magnetic states. Our first-principles simulations reveal that Cl vacancies, Ru vacancies, and oxygen substitutional defects are the most energetically stable point defects. Besides, these point defects break the magnetic degeneracy: Cl vacancies and oxygen substitutional defects energetically favor the zigzag-antiferromagnetic configuration while Ru vacancies favor the ferromagnetic configuration, shedding light on understanding the observed magnetic structures and further defect engineering of magnetism in monolayer {\alpha}-RuCl3. We further calculated their electronic structures and optical absorption spectra. The polarization symmetry of optical responses provides a convenient signature to identify the point defect types and long-range magnetic orders.

7.Light-induced weak ferromagnetism through nonlinear magnonic rectification

Authors:Tom Kahana, Dominik M. Juraschek

Abstract: Rectification describes the generation of a quasistatic component from an oscillating field, such as an electric polarization in optical rectification, or a structural distortion in nonlinear phononic rectification. Here, we present a third fundamental process for magnetization, in which spin precession is rectified along the coordinates of a nonlinearly driven magnon mode in an antiferromagnet. We demonstrate theoretically that a quasistatic magnetization can be induced by transient spin canting in response to the coherent excitation of a chiral phonon mode that produces an effective magnetic field for the spins. This mechanism, which we call nonlinear magnonic rectification, is generally applicable to magnetic systems that exhibit degenerate chiral phonon modes. Our result serves as an example of light-induced weak ferromagnetism and provides a promising avenue to creating nonequilibrium spin configurations.

8.Local step-flow dynamics in thin film growth with desorption

Authors:Xiaozhi Zhang, Jeffrey G. Ulbrandt, Peco Myint, Andrei Fluerasu, Lutz Wiegart, Yugang Zhang, Christie Nelson, Karl F. Ludwig, Randall L. Headrick

Abstract: Desorption of deposited species plays a role in determining the evolution of surface morphology during crystal growth when the desorption time constant is short compared to the time to diffuse to a defect site, step edge or kink. However, experiments to directly test the predictions of these effects are lacking. Novel techniques such as \emph{in-situ} coherent X-ray scattering can provide significant new information. Herein we present X-ray Photon Correlation Spectroscopy (XPCS) measurements during diindenoperylene (DIP) vapor deposition on thermally oxidized silicon surfaces. DIP forms a nearly complete two-dimensional first layer over the range of temperatures studied (40 - 120 $^{\circ}$C), followed by mounded growth during subsequent deposition. Local step flow within mounds was observed, and we find that there was a terrace-length-dependent behavior of the step edge dynamics. This led to unstable growth with rapid roughening ($\beta>0.5$) and deviation from a symmetric error-function-like height profile. At high temperatures, the grooves between the mounds tend to close up leading to nearly flat polycrystalline films. Numerical analysis based on a 1 + 1 dimensional model suggests that terrace-length dependent desorption of deposited ad-molecules is an essential cause of the step dynamics, and it influences the morphology evolution.

1.Ferroelectricity driven-resistive switching and Schottky barrier modulation at CoPt/MgZnO interface for non-volatile memories

Authors:Mohamed Belmoubarik International Iberian Nanotechnology Laboratory, INL, Av. Mestre José Veiga s/n, Braga, Portugal Department of Electronic Engineering, Tohoku University, Sendai 890-8579, Japan, Muftah Al-Mahdawi Center for Science and Innovation in Spintronics Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan, George Machado Jr. International Iberian Nanotechnology Laboratory, INL, Av. Mestre José Veiga s/n, Braga, Portugal, Tomohiro Nozaki Department of Electronic Engineering, Tohoku University, Sendai 890-8579, Japan, Cláudia Coelho University of Minho, Campus de Azurém, Guimarães, Portugal, Masashi Sahashi Department of Electronic Engineering, Tohoku University, Sendai 890-8579, Japan, Weng Kung Peng International Iberian Nanotechnology Laboratory, INL, Av. Mestre José Veiga s/n, Braga, Portugal Songshan Lake Materials Laboratory, 523-808, Dongguan, China

Abstract: Ferroelectric memristors have attracted much attention as a type of nonvolatile resistance switching memories in neuromorphic computing, image recognition, and information storage. Their resistance switching mechanisms have been studied several times in perovskite and complicated materials systems. It was interpreted as the modulation of carrier transport by polarization control over Schottky barriers. Here, we experimentally report the isothermal resistive switching across a CoPt/MgZnO Schottky barrier using a simple binary semiconductor. The crystal and texture properties showed high-quality and single-crystal Co$_{0.30}$Pt$_{0.70}$/Mg$_{0.20}$Zn$_{0.80}$O hetero-junctions. The resistive switching was examined by an electric-field cooling method that exhibited a ferroelectric T$_C$ of MgZnO close to the bulk value. The resistive switching across CoPt/MgZnO Schottky barrier was accompanied by a change in the Schottky barrier height of 26.5 meV due to an interfacial charge increase and/or orbital hybridization induced reversal of MgZnO polarization. The magnitude of the reversed polarization was estimated to be a reasonable value of 3.0 (8.25) $\mu$ C/cm$^2$ at 300 K (2 K). These findings demonstrated the utilities of CoPt/MgZnO interface as a potential candidate for ferroelectric memristors and can be extended to probe the resistive switching of other hexagonal ferroelectric materials.

2.The mechanism of the Silicon irradiation synergistic effect explained by multiscale simulations of Monte Carlo and excited-state first-principle calculations

Authors:Zeng-hui Yang, Yang Liu, Ning An, Xingyu Chen

Abstract: Neutron and $\gamma$-ray irradiation damages to transistors are found to be non-additive, and this is denoted as the irradiation synergistic effect (ISE). Its mechanism is not well-understood. The recent defect-based model [ACS Appl. Electron. Mater. 2, 3783 (2020)] for Silicon bipolar junction transistors (BJT) achieve quantitative agreement with experiments, but it remains phenomenological and its assumptions on the defect reactions are unverified. Going beyond the phenomenological model requires directly representing the effect of $\gamma$-ray irradiation in first-principles calculations, which is not feasible previously. In this work, we examine the defect-based model of the ISE by developing a multiscale method for the simulation of the $\gamma$-ray irradiation, where the $\gamma$-ray-induced electronic excitations are treated explicitly in excited-state first-principles calculations. We find the calculations agree with experiments, and the effect of the $\gamma$-ray-induced excitation is significantly different from the effects of defect charge state and temperature. We propose a diffusion-based qualitative explanation of the mechanism of positive/negative ISE in NPN/PNP BJTs in the end.

3.Topotactic Transition: A Promising Opportunity for Creating New Oxides

Authors:Ziang Meng, Han Yan, Peixin Qin, Xiaorong Zhou, Xiaoning Wang, Hongyu Chen, Li Liu, Zhiqi Liu

Abstract: Topotactic transition is a structural phase change in a matrix crystal lattice mediated by the ordered loss/gain and rearrangement of atoms, leading to unusual coordination environments and metal atoms with rare valent states. As early as in 1990s, low temperature hydride reduction was utilized to realize the topotactic transition. Since then, topological transformations have been developed via multiple approaches. Especially, the recent discovery of the Ni-based superconductivity in infinite-layer nickelates has greatly boosted the topotactic transition mean to synthesizing new oxides for exploring exotic functional properties. In this review, we have provided a detailed and generalized introduction to oxygen-related topotactic transition. The main body of our review include four parts: the structure-facilitated effects, the mechanism of the topotactic transition, some examples of topotactic transition methods adopted in different metal oxides (V, Mn, Fe, Co, Ni) and the related applications. This work is to provide timely and thorough strategies to successfully realize topotactic transitions for researchers who are eager to create new oxide phases or new oxide materials with desired functions.

4.Effect of solute atoms segregation on Al grain boundary properties by First-principles study

Authors:Xuan Zhang, Liang Zhang, Zhihui Zhang, Xiaoxu Huang

Abstract: First-principles calculations were carried out to study the segregation behavior of Mg, and Cu and their effect on the energy and mechanical properties of different Al grain boundaries (GBs). Four symmetrical tilt GBs were selected for study, namely {\Sigma}5[001](210) GB, {\Sigma}5[001](310) GB, {\Sigma}9[110](221) GB, and {\Sigma}11[110](332) GB. The results show that both Mg and Cu have a segregation tendency at the GBs, and the segregation tendency of Cu is stronger than Mg. Mg is prone to form substitutional segregation at the GBs, but Cu is more likely to segregate at the interstitial sites. The segregation of Mg and Cu can reduce GB energy, and the GB energy continues to decrease with the increase of the segregation concentration. First-principles calculation tensile test shows that the segregation of Mg has a negative effect on the strength of GBs, and the GB strength decreases with the increase of the Mg concentration, while the GB strength was gradually enhanced with the increase of the Cu concentration. The strength of {\Sigma}5(210) GB and {\Sigma}9(221) GB are more sensitive to the segregation of solute atoms than the other two GBs. By calculating the charge density and the density of states of the pristine and the segregated GBs, it was found that the segregation of Mg caused charge depletion and structure expansion at the GBs, while the segregation of Cu increases the charge density of GBs and form new bonds with the surrounding Al atoms. The results provide useful information for improving the mechanical properties of materials by using the concept of GB segregation engineering.

5.Giant enhancement of exciton radiative lifetime by ferroelectric polarization: The case of monolayer TiOCl$_2$

Authors:Hongwei Qu, Yuanchang Li

Abstract: Exciton binding energy and lifetime are the two most important parameters controlling exciton dynamics, and the general consensus is that the larger the former the larger the latter. However our first-principles study of monolayer ferroelectric TiOCl$_2$ shows that this is not always the case. We find that ferroelectric polarization tends to weaken exciton binding but enhance exciton lifetime. This stems from the different effects of the induced built-in electric field and structural distortion by the spontaneous polarization: the former always destabilizes or even dissociates the exciton while the latter leads to a relaxation of the selection rule and activates excitons that are otherwise not optically active. Their combined effect leads to a halving of the exciton binding energy but a substantial increase in lifetime by 40 times. Our results deepen the understanding of the interaction of light with ferroelectric materials and provide new insights into the use of ferroelectricity to control exciton dynamics.

6.Correlative Theoretical and Experimental Study of the Polycarbonate | X Interfacial Bond Formation (X = AlN, TiN, TiAlN) during Magnetron Sputtering

Authors:Lena Patterer, Pavel Ondračka, Dimitri Bogdanovski, Stanislav Mráz, Soheil Karimi Aghda, Peter J. Pöllmann, Yu-Ping Chien, Jochen M. Schneider

Abstract: To understand the interfacial bond formation between polycarbonate (PC) and magnetron-sputtered metal nitride thin films, PC | X interfaces (X = AlN, TiN, TiAlN) are comparatively investigated by ab initio simulations as well as X-ray photoelectron spectroscopy. The simulations predict significant differences at the interface, as N and Ti form bonds with all functional groups of the polymer, while Al reacts selectively only with the carbonate group of pristine PC. In good agreement with simulations, experimental data reveal that the PC | AlN and the PC | TiAlN interfaces are mainly defined by interfacial C-N bonds, whereas for PC | TiN, the interface formation is also characterized by numerous C-Ti and (C-O)-Ti bonds. Bond strength calculations combined with the measured interfacial bond density indicate the strongest interface for PC | TiAlN followed by PC | AlN, whereas the weakest is predicted for PC | TiN due to its lower density of strong interfacial C-N bonds. This study shows that the employed computational strategy enables prediction of the interfacial bond formation between PC and metal nitrides and that it is reasonable to assume that the research strategy proposed herein can be readily adapted to other organic | inorganic interfaces.

7.First-principles study of solute atoms segregation in Al Σ5(210) metastable grain boundaries

Authors:Zhihui Zhang, Liang Zhang, Xuan Zhang, Xiaoxu Huang

Abstract: Grain boundary (GB) segregation of solute atoms plays an important role in the microstructure and macroscopic mechanical properties of materials. The study of GB segregation of solute atoms using computational simulation has become one of the hot spots in recent years. However, most studies mainly focus on ground-state GB structures with the lowest energy, and the impact of GB metastability with higher energy on solute segregation remains poorly understood. In this work, the first-principles method based on the density functional theory was adopted to investigate the effect of solute atoms Mg and Cu segregation on ground-state {\Sigma}5(210) GB (GB-I) and metastable GBs(GB-II, GB-III) in Al. GB energy, segregation energy, and theoretical tensile strength of Mg and Cu segregation at three GBs were calculated. The results show that both Mg and Cu have a large driving force to segregate to Al GBs, which reduces the GB energy and improves improve GB stability. The segregation of Mg and Cu on GB-III induces the transformation of the GB structural unit and the GB structural phase transformations. For the above three GBs, Cu segregation increases the theoretical tensile strength of GBs to varying degrees. The segregation of Mg would reduce the resistance of GB-I and GB-II, but enhances the strength of GB-III. The effect of solute atoms segregation on the mechanical properties of GBs was investigated by charge density distribution and density of states.

8.Phase-field modelling of failure in ceramics with multiscale porosity

Authors:Riccardo Cavuoto Department of Structures for Engineering and Architecture, University of Naples "Federico II", Naples, Italy, Pietro Lenarda IMT School for Advanced Studies Lucca, Piazza San Francesco 19, 55100 Lucca, Italy, Anna Tampieri Institute of Science and Technology for Ceramics, National Research Council, Via Granarolo 64, 48018 Faenza, Italy, Davide Bigoni Instabilities Lab, University of Trento, Via Mesiano 77, Trento, 38123 Italy, Marco Paggi IMT School for Advanced Studies Lucca, Piazza San Francesco 19, 55100 Lucca, Italy

Abstract: Many stiff biological materials exhibiting outstanding compressive strength/weight ratio are characterized by high porosity, spanning different size-scales, typical examples being bone and wood. A successful bio-mimicking of these materials is provided by a recently-obtained apatite, directly produced through a biomorphic transformation of natural wood and thus inheriting its highly hierarchical structure. This unique apatite (but also wood and bone) is characterized by two major distinct populations of differently-sized cylindrical voids, a porosity shown in the present paper to influence failure, both in terms of damage growth and fracture nucleation and propagation. This statement follows from failure analysis, developed through in-silico generation of artificial samples (reproducing the two-scale porosity of the material) and subsequent finite element modelling of damage, implemented with phase-field treatment for fracture growth. It is found that small voids promote damage nucleation and enhance bridging of macro-pores by micro-crack formation, while macro-pores influence the overall material response and drive the propagation of large fractures. Our results explain the important role of multiscale porosity characterizing stiff biological materials and lead to a new design paradigm, by introducing an in-silico tool to implement bio-mimicking in new artificial materials with brittle behaviour, such as carbide or ceramic foams.

9.Accounting for Quantum Effects in Atomistic Spin Dynamics

Authors:Marco Berritta, Stefano Scali, Federico Cerisola, Janet Anders

Abstract: Atomistic spin dynamics (ASD) is a standard tool to model the magnetization dynamics of a variety of materials. The fundamental dynamical model underlying ASD is entirely classical. In this letter, we present two approaches to effectively incorporate quantum effects into ASD simulations, thus enhancing their low temperature predictions. The first allows to simulate the magnetic behavior of a quantum spin system by solving the equations of motions of a classical spin system at an effective temperature. This effective temperature is determined a priori from the microscopic properties of the system. The second approach is based on a semi-classical model where classical spins interact with an environment with a quantum-like power spectrum. The parameters that characterize this model can be calculated ab initio or extracted from experiments. This semi-classical model quantitatively reproduces the low-temperature behavior of a magnetic system, thus accounting for the quantum mechanical aspects of its dynamics. The methods presented here can be readily implemented in current ASD simulations with no additional complexity cost.

10.Direct visualization of the charge transfer in Graphene/$α$-RuCl$_3$ heterostructure

Authors:Antonio Rossi, Riccardo Dettori, Cameron Johnson, Jesse Balgley, John C. Thomas, Luca Francaviglia, Andreas K. Schmid, Kenji Watanabe, Takashi Taniguchi, Matthew Cothrine, David G. Mandrus, Chris Jozwiak, Aaron Bostwick, Erik A. Henriksen, Alexander Weber-Bargioni, Eli Rotenberg

Abstract: We investigate the electronic properties of a graphene and $\alpha$-ruthenium trichloride (hereafter RuCl$_3$) heterostructure, using a combination of experimental and theoretical techniques. RuCl$_3$ is a Mott insulator and a Kitaev material, and its combination with graphene has gained increasing attention due to its potential applicability in novel electronic and optoelectronic devices. By using a combination of spatially resolved photoemission spectroscopy, low energy electron microscopy, and density functional theory (DFT) calculations we are able to provide a first direct visualization of the massive charge transfer from graphene to RuCl$_3$, which can modify the electronic properties of both materials, leading to novel electronic phenomena at their interface. The electronic band structure is compared to DFT calculations that confirm the occurrence of a Mott transition for RuCl$_3$. Finally, a measurement of spatially resolved work function allows for a direct estimate of the interface dipole between graphene and RuCl$_3$. The strong coupling between graphene and RuCl$_3$ could lead to new ways of manipulating electronic properties of two-dimensional lateral heterojunction. Understanding the electronic properties of this structure is pivotal for designing next generation low-power opto-electronics devices.

11.One-particle and excitonic band structure in cubic Boron Arsenide

Authors:Swagata Acharya, Dimitar Pashov, Mikhail I Katsnelson, Mark van Schilfgaarde

Abstract: Cubic BAs has received recent attention for its large electron and hole mobilities and large thermal conductivity. This is a rare and much desired combination in semiconductor industry: commercial semiconductors typically have high electron mobilities, or hole mobilities, or large thermal conductivities, but not all of them together. Here we report predictions from an advanced self-consistent many body perturbative theory and show that with respect to one-particle properties, BAs is strikingly similar to Si. There are some important differences, notably there is an unusually small variation in the valence band masses . With respect to two-particle properties, significant differences with Si appear. We report the excitonic spectrum for both q=0 and finite q, and show that while the direct gap in cubic BAs is about 4 eV, dark excitons can be observed down to about $\sim$1.5 eV, which may play a crucial role in application of BAs in optoelectronics.

12.How to verify the precision of density-functional-theory implementations via reproducible and universal workflows

Authors:Emanuele Bosoni, Louis Beal, Marnik Bercx, Peter Blaha, Stefan Blügel, Jens Bröder, Martin Callsen, Stefaan Cottenier, Augustin Degomme, Vladimir Dikan, Kristjan Eimre, Espen Flage-Larsen, Marco Fornari, Alberto Garcia, Luigi Genovese, Matteo Giantomassi, Sebastiaan P. Huber, Henning Janssen, Georg Kastlunger, Matthias Krack, Georg Kresse, Thomas D. Kühne, Kurt Lejaeghere, Georg K. H. Madsen, Martijn Marsman, Nicola Marzari, Gregor Michalicek, Hossein Mirhosseini, Tiziano M. A. Müller, Guido Petretto, Chris J. Pickard, Samuel Poncé, Gian-Marco Rignanese, Oleg Rubel, Thomas Ruh, Michael Sluydts, Danny E. P. Vanpoucke, Sudarshan Vijay, Michael Wolloch, Daniel Wortmann, Aliaksandr V. Yakutovich, Jusong Yu, Austin Zadoks, Bonan Zhu, Giovanni Pizzi

Abstract: In the past decades many density-functional theory methods and codes adopting periodic boundary conditions have been developed and are now extensively used in condensed matter physics and materials science research. Only in 2016, however, their precision (i.e., to which extent properties computed with different codes agree among each other) was systematically assessed on elemental crystals: a first crucial step to evaluate the reliability of such computations. We discuss here general recommendations for verification studies aiming at further testing precision and transferability of density-functional-theory computational approaches and codes. We illustrate such recommendations using a greatly expanded protocol covering the whole periodic table from Z=1 to 96 and characterizing 10 prototypical cubic compounds for each element: 4 unaries and 6 oxides, spanning a wide range of coordination numbers and oxidation states. The primary outcome is a reference dataset of 960 equations of state cross-checked between two all-electron codes, then used to verify and improve nine pseudopotential-based approaches. Such effort is facilitated by deploying AiiDA common workflows that perform automatic input parameter selection, provide identical input/output interfaces across codes, and ensure full reproducibility. Finally, we discuss the extent to which the current results for total energies can be reused for different goals (e.g., obtaining formation energies).

1.Rules of plastic strain-induced phase transformations and nanostructure evolution under high-pressure and severe plastic flow

Authors:Feng Lin, Valery Levitas, Krishan Pandey, Sorb Yesudhas, Changyong Park

Abstract: Rough diamond anvils (rough-DA) are introduced to intensify all occurring processes during an in-situ study of heterogeneous compression of strongly pre-deformed Zr in diamond anvil cell (DAC). Crystallite size and dislocation density of Zr are getting pressure-, plastic strain tensor- and strain-path-independent during {\alpha}-{\omega} phase transformation (PT) and depend solely on the volume fraction of {\omega}-Zr. Rough-DA produce a steady nanostructure in {\alpha}-Zr with lower crystallite size and larger dislocation density than smooth-DA, leading to a two-time reduction in a minimum pressure for {\alpha}-{\omega} PT to a record value 0.67 GPa. The kinetics of strain-induced PT unexpectedly depends on time.

2.Bandgap manipulation of hBN by alloying with aluminum: absorption properties of hexagonal BAlN

Authors:Jakub Iwański, Mateusz Tokarczyk, Aleksandra K. Dąbrowska, Jan Pawłowski, Piotr Tatarczak, Johannes Binder, Andrzej Wysmołek

Abstract: The versatile range of applications for two-dimensional (2D) materials has encouraged scientists to further engineer the properties of these materials. This is often accomplished by stacking layered materials into more complex van der Waals heterostructures. A much less popular but technologically promising approach is the alloying of 2D materials with different element compositions. In this work, we demonstrate a first step in manipulating the hBN bandgap in terms of its width and indirect/direct character of the optical transitions. We present a set of aluminum alloyed hexagonal boron nitride (hBAlN) samples that were grown by metal organic vapor phase epitaxy (MOVPE) on 2-inch sapphire substrates with different aluminum concentration. Importantly, the obtained samples revealed a sp$^2$-bonded crystal structure. Optical absorption experiments disclosed two strong peaks in the excitonic spectral range with absorption coefficient $\alpha \sim 10^6$ cm$^{-1}$. Their energies correspond very well with the energies of indirect and direct bandgap transitions in hBN. However, they are slightly redshifted. This observation is in agreement with predictions that alloying with Al leads to a decrease of the bandgap energy. The observation of two absorption peaks can be explained in terms of mixing electronic states in the K and M conduction band valleys, which leads to a significant enhancement of the absorption coefficient for indirect transitions.

3.Structure and properties of the films based on ternary transition metal borides: theory and experiment

Authors:A. A. Onoprienko, V. I. Ivashchenko, V. I. Shevchenko

Abstract: The review presents the results of theoretical and experimental studies of the structure, bonding between atoms, mechanical properties, thermal stability, and oxidation and corrosion resistance of films based on ternary transition metal borides.

4.Heesch Weyl Fermions in inadmissible chiral antiferromagnets

Authors:Xue-Jian Gao, Zi-Ting Sun, Ruo-Peng Yu, Xing-Yao Guo, K. T. Law

Abstract: Symmetry is a crucial factor in determining the topological properties of materials. In nonmagnetic chiral crystals, the existence of the Kramers Weyl fermions reveals the topological nature of the Kramers degeneracy at time-reversal-invariant momenta (TRIMs). However, it is not clear whether Weyl nodes can also be pinned at points of symmetry in magnetic materials where the time-reversal is spontaneously broken. In this study, we introduce a new type of Weyl fermions, called Heesch Weyl fermions (HWFs), which are stabilized and pinned at points of symmetry by the Heesch groups in inadmissible chiral antiferromagnets. The emergence of HWFs is fundamentally different from that of Kramers Weyl fermions, as it does not rely on any anti-unitary symmetry $\mathcal{A}$ that satisfies $\mathcal{A}^2=-1$. Importantly, the emergence of HWFs is closely related to the antiferromagnetic order, as they are generally obscured by nodal lines in the parent nonmagnetic state. Using group theory analysis, we classify all the magnetic little co-groups of momenta where Heesch Weyl nodes are enforced and pinned by symmetry. With the guidance of this classification and first-principles calculations, we identify antiferromagnetic (AFM) materials such as YMnO$_3$ and Mn$_3$IrGe as candidate hosts for the AFM-order-induced HWFs.We also explore novel properties of Heesch Weyl antiferromagnets, such as nonlinear anomalous Hall effects and axial movement of Heesch Weyl nodes. Our findings shed new light on the role of symmetry in determining and stabilizing topological properties in magnetic materials, and open up new avenues for the design and exploration of topological materials.

5.Computational study of structural, elastic, electronic, phonon dispersion relation and thermodynamic properties of orthorhombic CaZrS$_3$ for optoelectronic applications

Authors:M. D. Kassa, N. G. Debelo, M. M. Woldemariam

Abstract: Chalcogenide perovskites offer superior thermal and aqueous stability as well as a benign elemental composition compared to organic halide perovskites for optoelectronic applications. In this study, the structural, electrical, elastic, phonon dispersion, and thermodynamic features of the orthorhombic phase of chalcogenide perovskite CaZrS$_3$ (space group Pnma) were examined by first principles calculations utilizing the plane wave pseudopotentials (PW-PPs) in generalized gradient approximations (GGA). The ground state properties such as lattice parameters, unit cell volume, bulk modulus, and its derivative were calculated and are in a good agreement with existing findings. The mechanical properties such as bulk modulus, shear modulus, Young's modulus and elastic anisotropy were calculated from the obtained elastic constants. The ratio of bulk modulus to shear modulus confirms that the orthorhombic phase of CaZrS$_3$ is a ductile material. The absence of negative frequencies in phonon dispersion curve and the phonon density of states give an indication that the structure is dynamically stable. Finally, thermodynamic parameters such as free energy, entropy, and heat capacity were calculated with variation in temperature. The estimated findings follow the same pattern as previous efforts.

6.Electronic structure and X-ray magnetic circular dichroism in the MAX phases T$_2$AlC (T=Ti and Cr) from first principles

Authors:L. V. Bekenov, S. V. Moklyak, B. F. Zhuravlev, Yu. N. Kucherenko, V. N. Antonov

Abstract: We study the electronic and magnetic properties of T$_2$AlC (T=Ti and Cr) compounds in the density-functional theory using the generalized gradient approximation (GGA) with consideration of strong Coulomb correlations (GGA+$U$) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital (LMTO) band-structure method. The X-ray absorption spectra and X-ray magnetic circular dichroism (XMCD) at the Cr $L_{2,3}$ and Cr, Ti, and C $K$ edges were investigated theoretically. The calculated results are in good agreement with experimental data. The effect of the electric quadrupole $E_2$ and magnetic dipole $M_1$ transitions at the Cr $K$ edge has been investigated.

7.Pressure driven Weyl-topological insulator phase transition in Weyl semimetal SrSi$_{2}$

Authors:Aditya Shende, Shivendra Kumar Gupta, Ashish Kore, Poorva Singh

Abstract: Using DFT-based first-principles calculations, we demonstrate the tuning of the electronic structure of Weyl semimetal SrSi$_{2}$ via external uniaxial strain. The uniaxial strain facilitates the opening of bandgap along $\Gamma$-X direction and subsequent band inversion between Si $p$ and Sr $d$ orbitals. Z$_{2}$ invariants and surface states reveal conclusively that SrSi$_{2}$ under uniaxial strain is a strong topological insulator. Hence, uniaxial strain drives the semimetallic SrSi$_{2}$ into fully gapped topological insulating state depicting a semimetal to topological insulator phase transition. Our results highlight the suitability of uniaxial strain to gain control over the topological phase transitions and topological states in SrSi$_{2}$.

8.Experimental Evidence for Defect Tolerance in Pb-Halide Perovskites

Authors:Naga Prathibha Jasti Bar Ilan University Weizmann Institute of Science, Igal Levine Helmholtz-Zentrum Berlin, Yishay Feldman Weizmann Institute of Science, Sigalit Aharon Weizmann Institute of Science, David Cahen Bar Ilan University Weizmann Institute of Science

Abstract: The term defect tolerance (DT) is used often to rationalize the exceptional optoelectronic properties of Halide Perovskites, HaPs, and their devices. Even though DT lacked direct experimental evidence, it became fact in the field. DT in semiconductors implies tolerance to structural defects without the electrical and optical effects (e.g., traps), associated with such defects. We present first direct experimental evidence for DT in Pb HaPs by comparing the structural quality of 2D, 2D_3D, and 3D Pb HaP crystals with their optoelectronic characteristics using high sensitivity methods. Importantly, we get information from the material bulk, because we sample at least a few 100 nm, up to several micrometer, from the sample surface, which allows assessing intrinsic bulk (and not only surface) properties of HaPs. The results point to DT in 3D, to a lesser extent in 2D_3D, but not in 2D Pb HaPs. We ascribe such dimension dependent DT to the higher number of (near)neighboring species, available to compensate for structural defect effects in the 3D than in the 2D HaP crystals. Overall, our data provide an experimental basis to rationalize DT in Pb HaPs. These experiments and findings can guide the search for, and design of other materials with DT.

9.Photogalvanic effect induced charge and spin photocurrent in group-V monolayer systems

Authors:Li-Wen Zhang, Ya-Qing Yang, Jun Chen, Lei Zhang

Abstract: Photogalvanic effect (PGE) occurs in materials with non-centrosymmetric structures when irradiated by linearly or circularly polarized light. Here, using non-equilibrium Green's function combined with density functional theory (NEGF-DFT), we investigated the linear photogalvanic effect (LPGE) in monolayers of group-V elements (As, Sb, and Bi) by first-principles calculations. First, by designing a two-probe structure based on the group-V elements, we found a giant anisotropy photoresponse of As between the armchair and zigzag directions. Then, we analyzed Sb and Bi's charge and spin photocurrent characteristics when considering the spin-orbit coupling (SOC) effect. It is found that when the polarization direction of linearly polarized light is parallel or perpendicular to the transport direction ($\theta$ = $0^ \circ$ or $90^ \circ$), the spin up and spin down photoresponse in the armchair direction has the same magnitude and direction, leading to the generation of net charge current. However, in the zigzag direction, the spin up and spin down photoresponse have the same magnitude with opposite directions, leading to the generation of pure spin current. Furthermore, it is understood by analyzing the bulk spin photovoltaic (BSPV) coefficient from the symmetry point of view. Finally, we found that the net charge current generated in the armchair direction and the pure spin current generated in the zigzag direction can be further tuned with the increase of the material's buckling height $|h|$. Our results highlight that these group-V monolayers are promising candidates for novel functional materials, which will provide a broad prospect for the realization of ultrathin ferroelectric devices in optoelectronics due to their spontaneous polarization characteristics and high Curie temperature.

10.Optimizing Experimental Parameters for Orbital Mapping

Authors:Manuel Ederer, Stefan Löffler

Abstract: A new material characterization technique is emerging for the transmission electron microscope (TEM). Using electron energy-loss spectroscopy, real space mappings of the underlying electronic transitions in the sample, so called orbital maps, can be produced. Thus, unprecedented insight into the electronic orbitals responsible for most of the electrical, magnetic and optical properties of bulk materials can be gained. However, the incredibly demanding requirements on spatial as well as spectral resolution paired with the low signal-to-noise ratio severely limits the day-to-day use of this new technique. With the use of simulations, we strive to alleviate these challenges as much as possible by identifying optimal experimental parameters. In this manner, we investigate representative examples of a transition metal oxide, a material consisting entirely of light elements, and an interface between two different materials to find and compare acceptable ranges for sample thickness, acceleration voltage and electron dose for a scanning probe as well as for parallel illumination.

11.Revealing the bonding nature and electronic structure of early transition metal dihydrides

Authors:Curran Kalha, Laura E. Ratcliff, Giorgio Colombi, Christoph Schlueter, Bernard Dam, Andrei Gloskovskii, Tien-Lin Lee, Pardeep K. Thakur, Prajna Bhatt, Yujiang Zhu, Jürg Osterwalder, Francesco Offi, Giancarlo Panaccione, Anna Regoutz

Abstract: Hydrogen as a fuel plays a crucial role in driving the transition to net zero greenhouse gas emissions. To realise its potential, obtaining a means of efficient storage is paramount. One solution is using metal hydrides, owing to their good thermodynamical absorption properties and effective hydrogen storage. Although metal hydrides appear simple compared to many other energy materials, understanding the electronic structure and chemical environment of hydrogen within them remains a key challenge. This work presents a new analytical pathway to explore these aspects in technologically relevant systems using Hard X-ray Photoelectron Spectroscopy (HAXPES) on thin films of two prototypical metal dihydrides: YH$_{2-\delta}$ and TiH$_{2-\delta}$. By taking advantage of the tunability of synchrotron radiation, a non-destructive depth profile of the chemical states is obtained using core level spectra. Combining experimental valence band spectra collected at varying photon energies with theoretical insights from density functional theory (DFT) calculations, a description of the bonding nature and the role of d versus sp contributions to states near the Fermi energy are provided. Moreover, a reliable determination of the enthalpy of formation is proposed by using experimental values of the energy position of metal s band features close to the Fermi energy in the HAXPES valence band spectra.

12.Transport regimes for exciton-polaritons in disordered microcavities

Authors:A. N. Osipov, I. V. Iorsh, A. V. Yulin, I. A. Shelykh

Abstract: Light-matter coupling in a planar optical cavity substantially modifies the transport regimes in the system in presence of a short range excitonic disorder. Basing on Master equation for a resonantly coupled exciton-photon system, and treating disorder scattering in the Born-Markov approximation we demonstrate the onset of ballistic and diffusive transport regimes in the limits of weak and strong disorder respectively. We show that transport parameters governing the crossover between these two regimes strongly depend on the parameters characterizing light-matter coupling, in particular Rabi energy and detuning between excitonic and photonic modes. The presented theory agrees with recent experimental data on transport in disordered organic microcavities.

13.Delving into the anisotropic interlayer exchange in bilayer CrI$_3$

Authors:Srdjan Stavrić, Paolo Barone, Silvia Picozzi

Abstract: Bilayer CrI$_3$ attracted much attention owing to peculiar switching between the layered ferromagnetic and antiferromagnetic order upon stacking alternation. This finding pointed out the importance of the apparently small interlayer exchange, yet, existing literature addresses only its isotropic part. To fill this gap, we combine the density functional theory with Hamiltonian modeling to examine the anisotropic interlayer exchange in bilayer CrI$_3$ - Dzyaloshinskii-Moriya (DMI) and the Kitaev interaction (KI). We develop and apply a novel computational procedure that yields the off-diagonal exchange matrix elements with $\mu$eV accuracy. Inspecting two types of bilayer stacking, we found a weak interlayer KI and much stronger DMI between the sublattices of monoclinic bilayer and their complete absence in rhombohedral bilayer. We show how these anisotropic interactions depend on the interlayer distance, stacking sequence, and the spin-orbit coupling strength and suggest the dominant superexchange processes at play. In addition, we demonstrate that the single-ion anisotropy largely depends on stacking, increasing by 50% from monoclinic to rhombohedral structure. Remarkably, our findings prove that iodines, owing to their spatially extended 5p orbitals featuring strong spin-orbit coupling, are extremely efficient in mediating DMI across the van der Waals gap in two-dimensional magnetic heterostructures. Given that similar findings were previously demonstrated only in metallic multilayers where the DMI shows a much longer range, our study gives promise that the chiral control of spin textures can be achieved in two-dimensional semiconducting magnetic bilayers whose ligands feature strong spin-orbit coupling.

14.Distinguishing erbium dopants in Y$_2$O$_3$ by site symmetry: \textit{ ab initio} theory of two spin-photon interfaces

Authors:Churna Bhandari, Cüneyt Şahin, Durga Paudyal, Michael E. FlattÃ\c{opyright}

Abstract: We present a first-principles study of defect formation and electronic structure of erbium (Er)-doped yttria (Y$_2$O$_3$). This is an emerging material for spin-photon interfaces in quantum information science due to the narrow linewidth optical emission from Er dopants at standard telecommunication wavelengths and their potential for quantum memories. We calculate formation energies of neutral, negatively, and positively charged Er dopants and find the configuration to be the most stable, consistent with experiment. Of the two substitutional sites of Er for Y, the $C_2$ and $C_{3i}$, we identify the former (with lower site symmetry) as possessing the lowest formation energy. The electronic properties are calculated using the Perdew-Burke-Ernzerhof (PBE) functional along with the Hubbard $U$ parameter {\color{black} and spin-orbit coupling (SOC)}, which yields a $\sim$ 6 $\mu_B$ orbital and a $\sim$ 3 $\mu_B$ spin magnetic moment, and 11 electrons in the Er $4f$ shell, confirming the formation of charge-neutral Er$^{3+}$. This standard density functional theory (DFT) approach underestimates the band gap of the host and lacks a first-principles justification for $U$. To overcome these issues we performed screened hybrid functional (HSE) calculations, including a negative $U$ for the $4f$ orbitals, with mixing ($\alpha$) and screening ($w$) parameters. These produced robust electronic features with slight modifications in the band gap and the $4f$ splittings depending on the choice of tuning parameters. We also computed the many-particle electronic excitation energies and compared them with experimental values from photoluminescence.

15.Influence of orientational disorder in the adsorbent on the structure and dynamics of the adsorbate: MD simulations of SO$_2$ in ZSM-22

Authors:I. Dhiman, Sadique Vellamarthodika, Siddharth Gautam

Abstract: Structural and dynamical behavior of SO$_2$ molecules within ZSM22 is studied using MD simulations, to understand the influence of orientational disorder (OD) and intercrystalline spacing in ZSM22 as a function of adsorbate loading. Addition of inter-crystalline space provides connectivity of isolated pores in ZSM22 and is shown to suppress both translational and rotational motion of SO$_2$. We infer that geometry and dimensionality of the connecting space is an important factor in determining the effects of pore connectivity on the adsorbed species behavior. As a function of OD, decrease in self diffusion coefficient of SO$_2$ in ZSM22 is observed. An increase in rotational correlation time t and a decrease in libration angle with OD is observed, due to the restriction imposed on the orientational freedom of the adsorbate by an increase in OD. The behavior of SO$_2$ result from an interplay of guest-host interactions and the dimensionality and confinement geometry.

16.Modeling of experimentally observed topological defects inside bulk polycrystals

Authors:Siddharth Singh, He Liu, Rajat Arora, Robert M. Suter, Amit Acharya

Abstract: A rigorous methodology is developed for computing elastic fields generated by experimentally observed defect structures within grains in a polycrystal that has undergone tensile extension. An example application is made using a near-field High Energy X-ray Diffraction Microscope measurement of a zirconium sample that underwent $13.6\%$ tensile extension from an initially well-annealed state. (Sub)grain boundary features are identified with apparent disclination line defects in them. The elastic fields of these features identified from the experiment are calculated.

17.Optically controlling the competition between spin flips and intersite spin transfer in a Heusler half-metal on sub-100 fs timescales

Authors:Sinéad A. Ryan, Peter C. Johnsen, Mohamed F. Elhanoty, Anya Grafov, Na Li, Anna Delin, Anastasios Markou, Edouard Lesne, Claudia Felser, Olle Eriksson, Henry C. Kapteyn, Oscar Grånäs, Margaret M. Murnane

Abstract: The direct manipulation of spins via light may provide a path toward ultrafast energy-efficient devices. However, distinguishing the microscopic processes that can occur during ultrafast laser excitation in magnetic alloys is challenging. Here, we combine the element-specificity of extreme ultraviolet high harmonic probes with time-dependent density functional theory to disentangle the competition between three ultrafast processes that occur in Co2MnGa: same-site Co-Co spin transfer, intersite Co-Mn spin transfer, and ultrafast spin flips mediated by spin-orbit coupling. By measuring the dynamic magnetic asymmetry across the entire M-edges of the two magnetic sublattices involved, we uncover the relative dominance of these processes at different probe energy regions and times during the laser pulse. The theoretical calculations demonstrate that pump-induced changes of magnetic asymmetry do not necessarily scale linearly with changes of the magnetic moment. The combined theoretical and experimental approach presented here enables a comprehensive microscopic interpretation of laser-induced magnetization dynamics on time scales shorter than 100 fs.

18.First Principles Study of Photocatalytic Water Splitting by M$_1$M$_2$CO$_2$ (M$_1$ = Zr,Hf; M$_2$ = Hf,Ti,Sc) MXenes

Authors:Sima Rastegar, Alireza Rastkar Ebrahimzadeh, Jaber Jahanbin Sardroodi

Abstract: Using density functional theory (DFT), we investigated the structural, electronic and optical properties of functionalized and doped MXenes such as M$_1$M$_2$CO$_2$ (M$_1$ = Zr,Hf; M$_2$ = Hf,Ti,Sc). This study aimed to find a suitable photocatalyst that would work well in the water splitting process. Among the calculated nanostructures, MXenes ZrHfCO$_2$ and ZrTiCO$_2$ were chosen as the suitable photocatalysts for the water splitting process. The calculated value of the band gaps with the GGA-PBE functional was 1.08(0.79) eV for the ZrHfCO$_2$ (ZrTiCO$_2$) monolayer. Also, the band gaps for these monolayers with the HSE06 hybrid functional were 1.86 and 1.57 eV, respectively. These MXenes' optical properties, such as complex dielectric function, refractive index, extinction coefficient, and reflectivity, were also investigated. The results showed that these monolayers had good absorption in the visible and ultraviolet regions. Additionally, we discovered that ZrHfCO$_2$ and ZrTiCO$_2$ MXenes could be used for the water splitting process by calculating the photocatalytic properties. Meanwhile, the results showed that the monolayers of M$_1$M$_2$CO$_2$ could be promising candidates for photocatalytic, solar energy, and optoelectronic applications.

19.Disorder-driven localization and electron interactions in Bi$_x$TeI thin films

Authors:Paul Corbae, Nicolai Taufertshöfer, Ellis Kennedy, Mary Scott, Frances Hellman

Abstract: Strong disorder has a crucial effect on the electronic structure in quantum materials by increasing localization, interactions, and modifying the density of states. Bi$_x$TeI films grown at room temperature and \SI{230}{K} exhibit dramatic magnetotransport effects due to disorder, localization and electron correlation effects, including a MIT at a composition that depends on growth temperature. The increased disorder caused by growth at 230K causes the conductivity to decrease by several orders of magnitude, for several compositions of Bi$_x$TeI. The transition from metal to insulator with decreasing composition $x$ is accompanied by a decrease in the dephasing length which leads to the disappearance of the weak-antilocalization effect. Electron-electron interactions cause low temperature conductivity corrections on the metallic side and Efros-Shklovskii (ES) variable range hopping on the insulating side, effects which are absent in single crystalline Bi$_x$TeI. The observation of a tunable metal-insulator transition and the associated strong localization and quantum effects in Bi$_x$TeI shows the possibility of tuning spin transport in quantum materials via disorder.

1.Detection of Non-uniformity in Parameters for Magnetic Domain Pattern Generation by Machine Learning

Authors:Naoya Mamada, Masaichiro Mizumaki, Ichiro Akai, Toru Aonishi

Abstract: We attempt to estimate the spatial distribution of heterogeneous physical parameters involved in the formation of magnetic domain patterns of polycrystalline thin films by using convolutional neural networks. We propose a method to obtain a spatial map of physical parameters by estimating the parameters from patterns within a small subregion window of the full magnetic domain and subsequently shifting this window. To enhance the accuracy of parameter estimation in such subregions, we employ employ large-scale models utilized for natural image classification and exploit the benefits of pretraining. Using a model with high estimation accuracy on these subregions, we conduct inference on simulation data featuring spatially varying parameters and demonstrate the capability to detect such parameter variations.

2.Piezomagnetic Properties in Altermagnetic MnTe

Authors:Takuya Aoyama, Kenya Ohgushi

Abstract: We examined the piezomagnetic effect in an antiferromagnet composed of MnTe, which is a candidate material for altermagnetism with a high critical temperature. We observed that the magnetization develops with the application of stress and revealed that the piezomagnetic coefficient Q is 1.38$\times10^{-8}$ ${\mu}$B/MPa at 300 K. The poling-field dependence of magnetization indicates that the antiferromagnetic domain can be controlled using the piezomagnetic effect. We demonstrate that the piezomagnetic effect is suitable for detecting and controlling the broken time reversal symmetry in altermagnets.

3.Resonance of Domain Wall in a Ferromagnetic Nanostrip: Relation Between Distortion and Velocity

Authors:Ganghwi Kim, Dae-Han Jung, Hee-Sung Han, Ki-Suk Lee

Abstract: The resonance of the magnetic domain wall under the applied field amplifies its velocity compared to the one-dimensional model. To quantify the amplification, we define the distortion variation rate of the domain wall that can represent how fast and severely the wall shape is variated. Introducing that rate gives a way to bring the resonance into the one-dimensional domain wall dynamics model. We obtain the dissipated energy and domain wall velocity amplification by calculating the distortion variation rate. The relationship between velocity and distortion variation rate agrees well with micromagnetic simulation.

4.Structure prediction and characterization of CuI-based ternary $p$-type transparent conductors

Authors:Michael Seifert Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena and European Theoretical Spectroscopy Facility, Tomáš Rauch Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena and European Theoretical Spectroscopy Facility, Miguel A. L. Marques Research Center Future Energy Materials and Systems of the University Alliance Ruhr, Faculty of Mechanical Engineering, Ruhr University Bochum, Silvana Botti Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena and European Theoretical Spectroscopy Facility Research Center Future Energy Materials and Systems, Faculty of Physics and Astronomy, Ruhr Universität Bochum

Abstract: Zincblende copper iodide has attracted significant interest as a potential material for transparent electronics, thanks to its exceptional light transmission capabilities in the visible range and remarkable hole conductivity. However, remaining challenges hinder the utilization of copper iodide's unique properties in real-world applications. To address this, chalcogen doping has emerged as a viable approach to enhance the hole concentration in copper iodide. In search of further strategies to improve and tune the electronic properties of this transparent semiconductor, we investigate the ternary phase diagram of copper and iodine with sulphur or selenium by performing structure prediction calculations using the minima hopping method. As a result, we find 11 structures located on or near the convex hull, 9 of which are unreported. Based on our band structure calculations, it appears that sulphur and selenium are promising candidates for achieving ternary semiconductors suitable as $p$-type transparent conducting materials. Additionally, our study reveals the presence of unreported phases that exhibit intriguing topological properties. These findings broaden the scope of potential applications for these ternary systems, highlighting the possibility of harnessing their unique electronic characteristics in diverse electronic devices and systems.

5.Generalised Framework for Controlling and Understanding Ion Dynamics with Passivated Lead Halide Perovskites

Authors:Tomi K. Baikie, Philip Calado, Krzysztof Galkowski, Zahra Andaji-Garmaroudi, Yi-Chun Chin, Joel Luke, Charlie Henderson, Tom Dunlop, James McGettrick, Ji-Seon Kim, Akshay Rao, Jenny Nelson, Samuel D. Stranks, Piers R. B. Barnes

Abstract: Metal halide perovskite solar cells have gained widespread attention due to their high efficiency and high defect tolerance. The absorbing perovskite layer is as a mixed electron-ion conductor that supports high rates of ion and charge transport at room temperature, but the migration of mobile defects can lead to degradation pathways. We combine experimental observations and drift-diffusion modelling to demonstrate a new framework to interpret surface photovoltage (SPV) measurements in perovskite systems and mixed electronic ionic conductors more generally. We conclude that the SPV in mixed electronic ionic conductors can be understood in terms of the change in electric potential at the surface associated with changes in the net charge within the semiconductor system. We show that by modifying the interfaces of perovskite bilayers, we may control defect migration behaviour throughout the perovskite bulk. Our new framework for SPV has broad implications for developing strategies to improve the stability of perovskite devices by controlling defect accumulation at interfaces. More generally, in mixed electronic conductors our framework provides new insights into the behaviour of mobile defects and their interaction with photoinduced charges, which are foundational to physical mechanisms in memristivity, logic, impedance, sensors and energy storage.

6.Investigating strain between phase-segregated domains in Cu-deficient CuInP2S6

Authors:Rahul Rao, Ryan Selhorst, Jie Jiang, Benjamin S. Conner, Ryan Siebenaller, Emmanuel Rowe, Andrea Giordano, Ruth Pachter, Michael A. Susner

Abstract: CuInP2S6 (CIPS) is an emerging layered ferroelectric material with a TC above room temperature. When synthesized with Cu deficiencies (i.e., Cu1-xIn1+x/3P2S6), the material segregates into CIPS and In4/3P2S6 (IPS) self-assembled heterostructures within the same single crystal. This segregation results in significant in-plane and out-of-plane strains between the CIPS and IPS phases as the volume fraction of CIPS (IPS) domains shrink (grow) with decreasing Cu fraction. Here, we synthesized CIPS with varying amounts of Cu (x = 0, 0.2, 0.3, 0.4, 0.5, 0.7, 0.8 and 1) and measured the strains between the CIPS and IPS phases through the evolution of the respective Raman, infrared, and optical reflectance spectra. Density functional theory calculations revealed vibrational modes unique to the CIPS and IPS phases, which can be used to distinguish between the two phases through two-dimensional Raman mapping. A comparison of the composition-dependent frequencies and intensities of the CIPS and IPS Raman peaks showed interesting trends with decreasing CIPS phase fraction (i.e., Cu/In ratio). Our data reveal red- and blue-shifted Raman and infrared peak frequencies that we correlate to lattice strains arising from the segregation of the material into CIPS and IPS chemical domains. The strain is highest for a Cu/In ratio of 0.33 (Cu0.4In1.2P2S6), which we attribute to equal and opposite strains exerted by the CIPS and IPS phases on each other. In addition, bandgaps extracted from the optical reflectance spectra revealed a decrease in values, with the lowest value (~ 2.3 eV) for Cu0.4In1.2P2S6.

7.ZnO nanowires grown on Al2O3-ZnAl2O4 nanostructure using solid-vapor mechanism

Authors:Wiktoria Zajkowska, Jakub Turczynski, Boguslawa Kurowska, Henryk Teisseyre, Krzysztof Fronc, Jerzy Dabrowski, Slawomir Kret

Abstract: We present Al2O3-ZnAl2O4-ZnO nanostructure, which could be a prominent candidate for optoelectronics, mechanical and sensing applications. While ZnO and ZnAl2O4 composites are mostly synthesized by sol-gel technique, we propose a solid-vapor growth mechanism. To produce Al2O3-ZnAl2O4-ZnO nanostructure, we conduct ZnO:C powder heating resulting in ZnO nanowires (NWs) growth on sapphire substrate and ZnAl2O4 spinel layer at the interface. The nanostructure was examined with Scanning Electron Microscopy (SEM) method. Focused Ion Beam (FIB) technique enabled us to prepare a lamella for Transmission Electron Microscopy (TEM) imaging. TEM examination revealed high crystallographic quality of both spinel and NW structure. Epitaxial relationships of Al2O3-ZnAl2O4 and ZnAl2O4-ZnO are given.

8.Non-collinear Magnetic Atomic Cluster Expansion for Iron

Authors:Matteo Rinaldi, Matous Mrovec, Anton Bochkarev, Yury Lysogorskiy, Ralf Drautz

Abstract: The Atomic Cluster Expansion (ACE) provides a formally complete basis for the local atomic environment. ACE is not limited to representing energies as a function of atomic positions and chemical species, but can be generalized to vectorial or tensorial properties and to incorporate further degrees of freedom (DOF). This is crucial for magnetic materials with potential energy surfaces that depend on atomic positions and atomic magnetic moments simultaneously. In this work, we employ the ACE formalism to develop a non-collinear magnetic ACE parametrization for the prototypical magnetic element Fe. The model is trained on a broad range of collinear and non-collinear magnetic structures calculated using spin density functional theory. We demonstrate that the non-collinear magnetic ACE is able to reproduce not only ground state properties of various magnetic phases of Fe but also the magnetic and lattice excitations that are essential for a correct description of the finite temperature behavior and properties of crystal defects.

9.Topological surface states hybridized with bulk states of Bi-doped PbSb2Te4 revealed in quasiparticle interference

Authors:Yuya Hattori, Keisuke Sagisaka, Shunsuke Yoshizawa, Yuki Tokumoto, Keiichi Edagawa

Abstract: Topological surface states of Bi-doped PbSb2Te4 [Pb(Bi0.20Sb0.80)2Te4] are investigated through analyses of quasiparticle interference (QPI) patterns observed by scanning tunneling microscopy. Interpretation of the experimental QPI patterns in the reciprocal space is achieved by numerical QPI simulations using two types of surface density of states produced by density functional theory calculations or a kp surface state model. We found that the Dirac point (DP) of the surface state appears in the bulk band gap of this material and, with the energy being away from the DP, the isoenergy contour of the surface state is substantially deformed or separated into segments due to hybridization with bulk electronic states. These findings provide a more accurate picture of topological surface states, especially at energies away from the DP, providing valuable insight into the electronic properties of topological insulators.

10.Dirac half-semimetallicity and antiferromagnetism in graphene nanoribbon/hexagonal boron nitride heterojunctions

Authors:Nikita V. Tepliakov, Ruize Ma, Johannes Lischner, Efthimios Kaxiras, Arash A. Mostofi, Michele Pizzochero

Abstract: Half-metals have been envisioned as active components in spintronic devices by virtue of their completely spin-polarized electrical currents. Actual materials hosting half-metallic phases, however, remain scarce. Here, we predict that recently fabricated heterojunctions of zigzag nanoribbons embedded in two-dimensional hexagonal boron nitride are half-semimetallic, featuring fully spin-polarized Dirac points at the Fermi level. The half-semimetallicity originates from the transfer of charges from hexagonal boron nitride to the embedded graphene nanoribbon. These charges give rise to opposite energy shifts of the states residing at the two edges while preserving their intrinsic antiferromagnetic exchange coupling. Upon doping, an antiferromagnetic-to-ferrimagnetic phase transition occurs in these heterojunctions, with the sign of the excess charge controlling the spatial localization of the net magnetic moments. Our findings demonstrate that such heterojunctions realize tunable one-dimensional conducting channels of spin-polarized Dirac fermions that are seamlessly integrated into a two-dimensional insulator, thus holding promise for the development of carbon-based spintronics.

11.Bending-induced isostructural transitions in ultrathin layers of van der Waals ferrielectrics

Authors:Anna N. Morozovska, Eugene A. Eliseev, Yongtao Liu, Kyle P. Kelley, Ayana Ghosh, Ying Liu, Jinyuan Yao, Nicholas V. Morozovsky, Andrei L Kholkin, Yulian M. Vysochanskii, Sergei V. Kalinin

Abstract: Using Landau-Ginzburg-Devonshire (LGD) phenomenological approach we analyze the bending-induced re-distribution of electric polarization and field, elastic stresses and strains inside ultrathin layers of van der Waals ferrielectrics. We consider a CuInP2S6 (CIPS) thin layer with fixed edges and suspended central part, the bending of which is induced by external forces. The unique aspect of CIPS is the existence of two ferrielectric states, FI1 and FI2, corresponding to big and small polarization values, which arise due to the specific four-well potential of the eighth-order LGD functional. When the CIPS layer is flat, the single-domain FI1 state is stable in the central part of the layer, and the FI2 states are stable near the fixed edges. With an increase of the layer bending below the critical value, the sizes of the FI2 states near the fixed edges decreases, and the size of the FI1 region increases. When the bending exceeds the critical value, the edge FI2 states disappear being substituted by the FI1 state, but they appear abruptly near the inflection regions and expand as the bending increases. The bending-induced isostructural FI1-FI2 transition is specific for the bended van der Waals ferrielectrics described by the eighth (or higher) order LGD functional with consideration of linear and nonlinear electrostriction couplings. The isostructural transition, which is revealed in the vicinity of room temperature, can significantly reduce the coercive voltage of ferroelectric polarization reversal in CIPS nanoflakes, allowing for the curvature-engineering control of various flexible nanodevices.

12.A new class of carbon stabilized austenitic steels resistant to hydrogen embrittlement

Authors:Heena Khanchandani, Stefan Zeiler, Lucas Strobel, Mathias Goeken, Peter Felfer

Abstract: High strength steels are susceptible to H-induced failure, which is typically caused by the presence of diffusible H in the microstructure. The diffusivity of H in austenitic steels with fcc crystal structure is slow. The austenitic steels are hence preferred for applications in the hydrogen-containing atmospheres. However, the fcc structure of austenitic steels is often stabilized by the addition of Ni, Mn or N, which are relatively expensive alloying elements to use. Austenite can kinetically also be stabilized by using C. Here, we present an approach applied to a commercial cold work tool steel, where we use C to fully stabilize the fcc phase. This results in a microstructure consisting of only austenite and an M7C3 carbide. An exposure to H by cathodic hydrogen charging exhibited no significant influence on the strength and ductility of the C stabilized austenitic steel. While this material is only a prototype based on an existing alloy of different purpose, it shows the potential for low-cost H-resistant steels based on C stabilized austenite.

13.Transport phenomena of TiCoSb: Defects induced modification in structure and density of states

Authors:S. Mahakal, Diptasikha Das, Pintu Singha, Aritra Banerjee, S. Chatterjee, Santanu K. Maiti, S. Assa Aravindh, K. Malik

Abstract: TiCoSb1+x (x=0.0, 0.01, 0.02, 0.03, 0.04, 0.06) samples have been synthesized, employing solid state reaction method followed by arc menting. Theoretical calculations, using Density Functional Theory (DFT) have been performed to estimate band structure and density of states (DOS). Further, energitic calculations, using first principle have been carried out to reveal the formation energy for vacancy, interstitial, anti-site defects. Detail structural calculation, employing Rietveld refinement reveals the presence of embedded phases, vacancy and interstitial atom, which is also supported by the theoretical calculations. Lattice strain, crystalline size and dislocation density have been estimated by Williamson-Hall and modified Williamson-Hall methods. Thermal variation of resistivity [\r{ho}(T)] and thermopower [S(T)] have been explained using Mott equation and density of states (DOS) modification near the Fermi surface due to Co vancancy and embedded phases. Figure of merit (ZT) has been calculated and 4 to 5 times higher ZT for TiCoSb than earlier reported value is obtained at room temperature.

14.Machine Learning Prediction of Critical Cooling Rate for Metallic Glasses From Expanded Datasets and Elemental Features

Authors:Benjamin T. Afflerbach, Carter Francis, Lane E. Schultz, Janine Spethson, Vanessa Meschke, Elliot Strand, Logan Ward, John H. Perepezko, Dan Thoma, Paul M. Voyles, Izabela Szlufarska, Dane Morgan

Abstract: We use a random forest model to predict the critical cooling rate (RC) for glass formation of various alloys from features of their constituent elements. The random forest model was trained on a database that integrates multiple sources of direct and indirect RC data for metallic glasses to expand the directly measured RC database of less than 100 values to a training set of over 2,000 values. The model error on 5-fold cross validation is 0.66 orders of magnitude in K/s. The error on leave out one group cross validation on alloy system groups is 0.59 log units in K/s when the target alloy constituents appear more than 500 times in training data. Using this model, we make predictions for the set of compositions with melt-spun glasses in the database, and for the full set of quaternary alloys that have constituents which appear more than 500 times in training data. These predictions identify a number of potential new bulk metallic glass (BMG) systems for future study, but the model is most useful for identification of alloy systems likely to contain good glass formers, rather than detailed discovery of bulk glass composition regions within known glassy systems.

1.Spontaneous spin selectivity and linear magnetoelectric effect in chiral molecules

Authors:Kouta Kondou, Shinji Miwa, Daigo Miyajima

Abstract: Chirality-induced spin selectivity (CISS) has been extensively studied over the past two decades. While current-induced spin polarization in chiral molecules is widely recognized as the fundamental principle of the CISS, only a few studies have been reported on bias-current-free CISS, where there is no bias electric current in chiral molecules. In this paper, we discuss the microscopic origin of bias-free CISS using chiral molecule/ferromagnet bilayer systems. Recent studies on the chirality-induced exchange bias and current-in-plane magnetoresistance (CIP-MR) effects indicate that chiral molecules possess thermally driven broken-time-reversal symmetry at the interface, which induces bias-current-free CISS, i.e. a spontaneous effective magnetic field in the system. We also discuss the possibility of the linear magnetoelectric effect of chiral molecules at the interface and its potential impact on the observed CISS phenomena.

2.Roughness-induced magnetic decoupling at organic-inorganic interface

Authors:Hiroki Ono, Yoshitaka Umeda, Kaito Yoshida, Kenzaburo Tsutsui, Kohei Yamamoto, Osamu Ishiyama, Hiroshi Iwayama, Eiken Nakamura, Toshihiko Yokoyama, Masaki Mizuguchi, Toshio Miyamachi

Abstract: We have investigated structural, electronic and magnetic properties of H$_2$Pc on Fe$_2$N/Fe using low-energy electron diffraction and soft x-ray absorption spectroscopy/x-ray magnetic circular dichroism. Element specific magnetization curves reveal that the magnetic coupling with H$_2$Pc enhances the perpendicular magnetic anisotropy of Fe$_2$N/Fe at the H$_2$Pc coverage of 1 molecular layer. However, adding two and three molecular layers of H$_2$Pc reverts the shape of magnetization curve back to the initial state before H$_2$Pc deposition. We successfully link appearance and disappearance of the magnetic coupling at the H$_2$Pc-Fe$_2$N/Fe interface with the change of hybridization strength at N sites accompanied by the increase in the H$_2$Pc coverage.

3.Preferential bond formation and interstitial/vacancy annihilation rate drive atomic clustering in gallium ion sputtered compound materials

Authors:Zhenyu Ma, Xin Zhang, Pu Liu, Yong Deng, Wenyu Hu, Longqing Chen, Jun Zhu, Sen Chen, Zhengshang Wang, Yuechun Shi, Jian Ma, Xiaoyi Wang, Yang Qiu, Kun Zhang, Xudong Cui, Thomas Walther

Abstract: The investigation of chemical reactions during the ion irradiation is a frontier for the study of the ion-material interaction. In order to derive the contribution of bond formation to chemistry of ion produced nanoclusters, the valence electron energy loss spectroscopy (VEELS) was exploited to investigate the Ga$^+$ ion damage in Al$_2$O$_3$, InP and InGaAs, where each target material has been shown to yield different process for altering the clustering of recoil atoms: metallic Ga, metallic In and InGaP clusters in Al$_2$O$_3$, InP and InGaAs respectively. Supporting simulations based on Monte Carlo and crystal orbital Hamiltonianindicate that the chemical constitution of cascade induced nano-precipitates is a result of a competition between interstitial/vacancy consumption rate and preferential bond formation.

4.Exploring energy landscapes of charge multipoles using constrained density functional theory

Authors:Luca Schaufelberger, Maximilian E. Merkel, Aria Mansouri Tehrani, Nicola A. Spaldin, Claude Ederer

Abstract: We present a method to constrain local charge multipoles within density-functional theory. Such multipoles quantify the anisotropy of the local charge distribution around atomic sites and can indicate potential hidden orders. Our method allows selective control of specific multipoles, facilitating a quantitative exploration of the energetic landscape outside of local minima. Thus, it enables a clear distinction between electronically and structurally driven instabilities. We demonstrate the effectiveness of this method by applying it to charge quadrupoles in the prototypical orbitally ordered material KCuF$_3$. We quantify intersite multipole-multipole interactions as well as the energy-lowering related to the formation of an isolated local quadrupole. We also map out the energy as a function of the size of the local quadrupole moment around its local minimum, enabling quantification of multipole fluctuations around their equilibrium value. Finally, we study charge quadrupoles in the solid solution KCu$_{1-x}$Zn$_x$F$_3$ to characterize the behavior across the tetragonal-to-cubic transition. Our method provides a powerful tool for studying symmetry breaking in materials with coupled electronic and structural instabilities and potentially hidden orders.

5.Laser-based angle-resolved photoemission spectroscopy with micrometer spatial resolution and detection of three-dimensional spin vector

Authors:Takuma Iwata, T. Kousa, Y. Nishioka, K. Ohwada, Kenta Kuroda, H. Iwasawa, M. Arita, S. Kumar, A. Kimura, K. Miyamoto, T. Okuda

Abstract: We have developed a state-of-the-art apparatus for laser-based spin- and angle-resolved photoemission spectroscopy with micrometer spatial resolution (micro-SARPES). This equipment is achieved through the combination of a high-resolution photoelectron spectrometer, a 6-eV laser with high photon flux that is focused down to a few micrometers, a high-precision sample stage control system, and a double very-low-energy-electron-diffraction spin detector. The setup achieves an energy resolution of 1.5 (5.5) meV without (with) the spin detection mode, compatible with a spatial resolution better than 10 micrometers. This enables us to probe both spatially-resolved electronic structures and vector information of spin polarization in three dimensions. The performance of micro-SARPES apparatus is demonstrated by presenting ARPES and SARPES results from topological insulators and Au photolithography patterns on a Si (001) substrate.

6.Energy storage properties of ferroelectric nanocomposites

Authors:Zhijun Jiang, Zhenlong Zhang, Sergei Prokhorenko, Yousra Nahas, Sergey Prosandeev, Laurent Bellaiche

Abstract: An atomistic effective Hamiltonian technique is used to investigate the finite-temperature energy storage properties of a ferroelectric nanocomposite consisting of an array of BaTiO$_{3}$ nanowires embedded in a SrTiO$_{3}$ matrix, for electric field applied along the long axis of the nanowires. We find that the energy density \textit{versus} temperature curve adopts a nonlinear, mostly temperature-independent response when the system exhibits phases possessing an out-of-plane polarization and vortices while the energy density more linearly increases with temperature when the nanocomposite either only possesses vortices (and thus no spontaneous polarization) or is in a paraelectric and paratoroidic phase for its equilibrium state. Ultrahigh energy density up to $\simeq$140 J/cm$^{3}$ and an ideal 100% efficiency are also predicted in this nanocomposite. A phenomenological model, involving a coupling between polarization and toroidal moment, is further proposed to interpret these energy density results.

7.Three-dimensional atomic positions and local chemical order of medium- and high-entropy alloys

Authors:Saman Moniri, Yao Yang, Yakun Yuan, Jihan Zhou, Long Yang, Fan Zhu, Yuxuan Liao, Yonggang Yao, Liangbing Hu, Peter Ercius, Jun Ding, Jianwei Miao

Abstract: Medium- and high-entropy alloys (M/HEAs) mix multiple principal elements with near-equiatomic composition and represent a paradigm-shift strategy for designing new materials for metallurgy, catalysis, and other fields. One of the core hypotheses of M/HEAs is lattice distortion. However, experimentally determining the 3D local lattice distortion in M/HEAs remains a challenge. Additionally, the presumed random elemental mixing in M/HEAs has been questioned by atomistic simulations, energy dispersive x-ray spectroscopy (EDS), and electron diffraction, which suggest the existence of local chemical order in M/HEAs. However, the 3D local chemical order has eluded direct experimental observation since the EDS elemental maps integrate the composition of atomic columns along the zone axes, and the diffuse reflections/streaks in electron diffraction of M/HEAs may originate from planar defects. Here, we determine the 3D atomic positions of M/HEA nanocrystals using atomic electron tomography, and quantitatively characterize the local lattice distortion, strain tensor, twin boundaries, dislocation cores, and chemical short-range order (CSRO) with unprecedented 3D detail. We find that the local lattice distortion and strain tensor in the HEAs are larger and more heterogeneous than in the MEAs. We observe CSRO-mediated twinning in the MEAs. that is, twinning occurs in energetically unfavoured CSRO regions but not in energetically favoured CSRO ones. This observation confirms the atomistic simulation results of the bulk CrCoNi MEA and represents the first experimental evidence of correlating local chemical order with structural defects in any material system. We expect that this work will not only expand our fundamental understanding of this important class of materials, but also could provide the foundation for tailoring M/HEA properties through lattice distortion and local chemical order.

8.Tunable Resins with PDMS-like Elastic Modulus for Stereolithographic 3D-printing of Multimaterial Microfluidic Actuators

Authors:Alireza Ahmadianyazdi, Isaac J. Miller, Albert Folch

Abstract: Stereolithographic 3D-printing (SLA) permits facile fabrication of high-precision microfluidic and lab-on-a-chip devices. SLA photopolymers often yield parts with low mechanical compliancy in sharp contrast to elastomers such as poly (dimethyl siloxane) (PDMS). On the other hand, SLA-printable elastomers with soft mechanical properties do not fulfill the distinct requirements for a highly manufacturable resin in microfluidics (e.g., high-resolution printability, transparency, low-viscosity). These limitations restrict our ability to SLA-print efficient microfluidic actuators containing dynamic, movable elements. Here we introduce low-viscous photopolymer resins based on a tunable blend of poly(ethylene glycol) diacrylate (PEGDA, Mw~258) and poly (ethylene glycol methyl ether) methacrylate (PEGMEMA, Mw~300) monomers. In these blends, which we term PEGDA-co-PEGMEMA, tuning the PEGMEMA-to-PEGDA ratio alters the elastic modulus of the printed plastics by ~400-fold, reaching that of PDMS. Through the addition of PEGMEMA, moreover, PEGDA-co-PEGMEMA retains desirable properties of highly manufacturable PEGDA such as low viscosity, solvent compatibility, cytocompatibility and low drug absorptivity. With PEGDA-co-PEGMEMA, we SLA-printed drastically enhanced fluidic actuators including microvalves, micropumps, and microregulators with a hybrid structure containing a flexible PEGDA-co-PEGMEMA membrane within a rigid PEGDA housing.

9.Disentangling stress and curvature effects in layered 2D ferroelectric CuInP2S6

Authors:Yongtao Liu, Anna N. Morozovska, Ayana Ghosh, Kyle P. Kelley, Eugene A. Eliseev, Jinyuan Yao, Ying Liu, Sergei V. Kalinin

Abstract: Nanoscale ferroelectric 2D materials offer unique opportunity to investigate curvature and strain effects on materials functionalities. Among these, CuInP2S6 (CIPS) has attracted tremendous research interest in recent years due to combination of room temperature ferroelectricity, scalability to a few layers thickness, and unique ferrielectric properties due to coexistence of 2 polar sublattices. Here, we explore the local curvature and strain effect on the polarization in CIPS via piezoresponse force microscopy and spectroscopy. To explain the observed behaviors and decouple the curvature and strain effects in 2D CIPS, we introduce finite element Landau-Ginzburg-Devonshire model. The results show that bending induces ferrielectric domains in CIPS, and the polarization-voltage hysteresis loops differ in bending and non-bending regions. Our simulation indicates that the flexoelectric effect can affect local polarization hysteresis. These studies open a novel pathway for the fabrication of curvature-engineered nanoelectronic devices.

1.Partial Hydrogenation of N-heteropentacene: Impact on molecular packing and electronic structure

Authors:Yutaro Ono, Ryohei Tsuruta, Tomohiro Nobeyama, Kazuki Matsui, Masahiro Sasaki, Makoto Tadokoro, Yasuo Nakayama, Yoichi Yamada

Abstract: Four-nitrogen-containing 5,6,13,14-Tetraazapentacene (BTANC) has attracted attention as a new n-type organic semiconductor with a rigid crystalline phase due to intermolecular CH-N hydrogen bonding. However, in the thin film transistor of BTANC, poor carrier transport properties and low stability in the ambient condition have been reported so far; thus further refining and understanding of the thin film of BTANC will be required. Here, by means of carefully-controlled vacuum deposition of BTANC in the narrow window of temperature avoiding impurity sublimation and thermal degradation of molecules, we produced a well-defined monolayer on Cu(111) for molecular-level investigations. Synchrotron photoemission of the monolayer revealed a noticeable alteration of the chemical state of N atoms, which is unexpected for the pure BTANC molecule. In addition, molecular imaging of the monolayer by scanning tunneling microscope (STM) revealed that the molecular packing structure in the monolayer significantly differed from that in the single crystal of BTANC. These observations can be interpreted as a result of the partial hydrogenation of N atoms in BTANC and the emergence of the NH-N type intermolecular hydrogen bonding in the monolayer. These findings will provide a general remark and strategy to control the molecular packing structure and electronic property in the molecular films of the nitrogen-containing acenes, by means of controlled hydrogenation.

2.Thermal expansion anisotropy of the Fe23Mo16 and Fe7Mo6 Mu-phases predicted from first-principles calculations

Authors:Dmitry Vasilyev

Abstract: The intermetallic Mn-phase, which precipitates in steels and superalloys, can noticeably soften the mechanical properties of their matrix. Despite the importance of developing superalloys and steels, the thermodynamic properties and directions of thermal expansion of the Mu-phase are still poorly studied. In this work, the thermal expansion paths, elastic, thermal and thermodynamic properties of the Fe23Mo16 and Fe7Mo6 Mu-phases have been studied using first-principles based quasi-harmonic Debye-Gruneisen approach. A method allowing avoids differentiation in many variables is used. The free energies consisting of the electronic, vibrational and magnetic energy contributions, calculated along different paths of thermal expansions were compared between themselves. A path with the least free energy was chosen as the trajectory of thermal expansion. Negative thermal expansion of the Fe7Mo6 compound was predicted, while the Fe23Mo16 has a conventional thermal expansion and negative TEC in the parameter c. The thermal expansions of both these compounds are not isotropic. The elastic constants, modulus, heat capacities, Curie and Debye temperatures were predicted. The obtained results satisfactorily agree with the available experimental data. Physical factors affecting the stability of Fe23Mo16 and Fe7Mo6 have been studied. The paper presents an essential feature of thermal expansions of the Mu-phase of the Fe-Mo system, which can provide an insight into future developments.

3.Lifetime of coexisting sub-10 nm zero-field skyrmions and antiskyrmions

Authors:Moritz A. Goerzen, Stephan von Malottki, Sebastian Meyer, Pavel F. Bessarab, Stefan Heinze

Abstract: Magnetic skyrmions have raised high hopes for future spintronic devices. For many applications it would be of great advantage to have more than one metastable particle-like texture available. The coexistence of skyrmions and antiskyrmions has been proposed in inversion symmetric magnets with exchange frustration. However, so far only model systems have been studied and the lifetime of coexisting metastable topological spin structures has not been obtained. Here, we predict that skyrmions and antiskyrmions with diameters below 10 nm can coexist at zero magnetic field in a Rh/Co bilayer on the Ir(111) surface -- an experimentally feasible system. We show that the lifetimes of metastable skyrmions and antiskyrmions in the ferromagnetic ground state are above one hour for temperatures up to 75 K and 48 K, respectively. The entropic contribution to the nucleation and annihilation rates differs for skyrmions and antiskyrmions. This opens the route to thermally activated creation of coexisting skyrmions and antiskyrmions in frustrated magnets with Dzyaloshinskii-Moriya interaction.

4.Transferable screened range-separated hybrid functionals for electronic and optical properties of van der Waals materials

Authors:María Camarasa-Gómez, Ashwin Ramasubramaniam, Jeffrey B. Neaton, Leeor Kronik

Abstract: The accurate description of electronic properties and optical absorption spectra is a long-standing challenge for density functional theory. Recently, the introduction of screened range-separated hybrid (SRSH) functionals for solid-state materials has allowed for the calculation of fundamental band gaps and optical absorption spectra that are in very good agreement with many-body perturbation theory. However, since solid-state SRSH functionals are typically tuned to reproduce the properties of bulk phases, their transferability to low-dimensional structures, which experience substantially different screening than in the bulk, remains an open question. In this work, we explore the transferability of SRSH functionals to several prototypical van der Waals materials, including transition-metal sulfides and selenides, indium selenide, black phosphorus, and hexagonal boron nitride. Considering the bulk and a monolayer of these materials as limiting cases, we show that the parameters of the SRSH functional can be determined systematically, using only the band-edge quasiparticle energies of these extremal structural phases as fitting targets. The resulting SRSH functionals can describe both electronic bandstructures and optical absorption spectra with accuracy comparable to more demanding ab initio many-body perturbation theory (GW and Bethe-Salpeter equation) approaches. Selected examples also demonstrate that the SRSH parameters, obtained from the bulk and monolayer reference structures, display good accuracy for bandstructures and optical spectra of bilayers, indicating a degree of transferability that is independent of the fitting procedure.

5.Enhanced piezoelectric response at nanoscale vortex structures in ferroelectrics

Authors:Xiaowen Shi, Nimish Prashant Nazirkar, Ravi Kashikar, Dmitry Karpov, Shola Folarin, Zachary Barringer, Skye Williams, Boris Kiefer, Ross Harder, Wonsuk Cha, Ruihao Yuan, Zhen Liu, Dezhen Xue, Turab Lookman, Inna Ponomareva, Edwin Fohtung

Abstract: The piezoelectric response is a measure of the sensitivity of a material's polarization to stress or its strain to an applied field. Using in-operando x-ray Bragg coherent diffraction imaging, we observe that topological vortices are the source of a five-fold enhancement of the piezoelectric response near the vortex core. The vortices form where several low symmetry ferroelectric phases and phase boundaries coalesce. Unlike bulk ferroelectric solid solutions in which a large piezoelectric response is associated with coexisting phases in the proximity of the triple point, the largest responses for pure BaTiO3 at the nanoscale are in spatial regions of extremely small spontaneous polarization at vortex cores. The response decays inversely with polarization away from the vortex, analogous to the behavior in bulk ceramics as the cation compositions are varied away from the triple point. We use first-principles-based molecular dynamics to augment our observations, and our results suggest that nanoscale piezoelectric materials with large piezoelectric response can be designed within a parameter space governed by vortex cores. Our findings have implications for the development of next-generation nanoscale piezoelectric materials.

1.DFT + U Study of structural, electronic, optical and magnetic properties of LiFePO4 Cathode materials for Lithium-Ion batteries

Authors:A. K. Wabeto, K. N. Nigussa, L. D. Deja

Abstract: In this study, we have employed a DFT+U calculation using quantum-espresso (QE) code to investigate the structural, electronic, optical, and magnetic properties of LiFePO$\rm_{4}$ cathode material for Li-ion batteries. Crystals of LiFePO$\rm_{4}$ and related materials have recently received a lot of attention due to their very promising use as cathodes in rechargeable lithium-ion batteries. The structural optimization was performed and the equilibrium parameters such as the lattice constants, and the bulk modulus are calculated using QE code and found to be a=4.76 {\AA}, b=6.00 {\AA}, c=10.28 {\AA}, B=90.2 GPa, respectively. The projected density of states (PDOS) for the LiFePO$\rm_{4}$ material is remarkably similar to experimental results in literature showing the transition metal $3d$ states forming narrow bands above the O $2p$ band. The results of the various spin configurations suggested that the ferromagnetic configuration can serve as a useful approximation for studying the general features of these systems. In the absence of Li, the majority spin transition metal $3d$ states are well-hybridized with the O 2p band in FePO$\rm_{4}$. The result obtained with a DFT + U showed that LiFePO4 is direct band gap materials with a band gap of 3.82 eV, which is within the range of the experimental values. The PDOS analyses show qualitative information about the crystal field splitting and bond hybridization and help rationalize the understanding of the structural, electronic, optical, and magnetic properties of the LiFePO$\rm_{4}$ as a novel cathode material. On the basis of the predicted optical absorbance, reflection, refractive index, and energy loss function, LiFePO$\rm_{4}$ is demonstrated to be viable and cost-effective, which is very suitable as a cathode material for Li-ion battery.

2.Multi-component low and high entropy metallic coatings synthesized by pulsed magnetron sputtering

Authors:Grzegorz W. Strzelecki, Katarzyna Nowakowska-Langier, Katarzyna Mulewska, Maciej Zielinski, Anna Kosinska, Sebastian Okrasa, Magdalena Wilczopolska, Rafal Chodun, Bartosz Wicher, Robert Mirowski, Krzysztof Zdunek

Abstract: This paper presents the findings of the synthesis of multicomponent (Al, W, Ni, Ti, Nb) alloy coatings from mosaic targets. For the study, a pulsed magnetron sputtering method was employed under different plasma generation conditions: modulation frequency (10 Hz and 1000 Hz), and power (600 W and 1000 W). The processes achieved two types of alloy coatings, high entropy and classical alloys. After the deposition processes, scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy techniques were employed to find the morphology, thickness, and chemical and phase compositions of the coatings. Nanohardness and its related parameters, namely H3.Er2, H.E, and 1.Er2H ratios, were measured. An annealing treatment was performed to estimate the stability range for the selected coatings. The results indicated the formation of as-deposited coatings exhibiting an amorphous structure as a single-phase solid solution. The process parameters had an influence on the resulting morphology-a dense and homogenous as well as a columnar morphology, was obtained. The study compared the properties of high-entropy alloy (HEA) coatings and classical alloy coatings concerning their structure and chemical and phase composition. It was found that the change of frequency modulation and the post-annealing process contributed to the increase in the hardness of the material in the case of HEA coatings.

3.As-based ternary Janus monolayers for efficient thermoelectric and photocatalytic applications

Authors:Poonam Chauhan, Jaspreet Singh, Ashok Kumar

Abstract: Highly efficient and sustainable resources of energy are of great demand today to combat with environmental pollution and the energy crisis. In this work, we have examined the novel 2D Janus AsTeX (X = Cl, Br and I) monolayers using first-principles calculations and explore their potential energy conversion applications. We have demonstrated the thermal, energetic, dynamic and mechanical stability of AsTeX (X = Cl, Br, and I) monolayers. Janus AsTeX (X = Cl, Br and I) monolayers are indirect bandgap semiconductors with high carrier mobilities and excellent visible light optical absorption. Our findings demonstrate that the Janus AsTeCl and AsTeBr monolayers exhibits low lattice thermal conductivity and excellent electronic transport properties obtained using semi-classical Boltzmann transport theory including various scattering mechanism. Additionally, the redox potential of water is adequately engulfed by the band alignments of the AsTeCl and AsTeBr monolayers. The water splitting process under illumination can proceeds spontaneously on Janus AsTeBr monolayer, while a minimal low external potential (0.26-0.29 eV) is required to trigger water splitting process on Janus AsTeCl monolayer. A more than 10% STH efficiency of these monolayers indicate their potential practical applications in the commercial production of hydrogen. Thus, our study demonstrates that these monolayers can show potential applications in energy conversion fields.

4.Fabrication of soft bio-spintronic devices for probing the CISS effect

Authors:Ritu Gupta, Hariharan V. Chinnasamy, Dipak Sahu, Saravanan Matheshwaran, Chanchal Sow, Prakash Chandra Mondal

Abstract: Bio-spinterfaces present numerous opportunities to study spintronics across the biomolecules attached to (ferro)magnetic electrodes. While it offers various exciting phenomena to investigate, it's simultaneously challenging to make stable bio-spinterfaces, as biomolecules are sensitive to many factors that it encounters during thin-film growth to device fabrication. The chirality-induced spin-selectivity (CISS) effect is an exciting discovery demonstrating an understanding that a specific electron's spin (either UP or DOWN) passes through a chiral molecule. The present work utilizes Ustilago maydis Rvb2 protein, an ATP-dependent DNA helicase (also known as Reptin) for the fabrication of bio-spintronic devices to investigate spin-selective electron transport through protein. Ferromagnetic materials are well-known for showing spin-polarization, which many chiral and biomolecules can mimic. We report spin-selective electron transmission through Rvb2 that exhibits 30% spin polarization at a low bias (+ 0.5 V) in a device configuration, Ni/Rvb2 protein/ITO measured under two different magnetic configurations. Our findings demonstrate that biomolecules can be put in circuit components without any expensive vacuum deposition for the top contact. Thus, it holds a remarkable potential to advance spin-selective electron transport in other biomolecules such as proteins, and peptides for biomedical applications.

5.Emergent room-temperature ferroelectricity in spark-plasma sintered DyCrO$_3$ and LaCrO$_3$

Authors:Suryakanta Mishra, Keerthana, Krishna Rudrapal, Biswajit Jana, Kazi Parvez Islam, Archna Sagdeo, Ayan Roy Chaudhuri, Venimadhav Adyam, Debraj Choudhury

Abstract: Identification of novel multiferroic materials with high-ordering temperatures remains at the forefront of condensed matter physics research. In this regard, the antiferromagnetic RCrO$_3$ compounds (like GdCrO$_3$) constitute a promising class of multiferroic compounds, which, however, mostly become ferroelectric concomitant with the antiferromagnetic ordering much below room-temperature, arising from a subtle competition between the ferroelectric off-centering mode and a non-polar antiferrodistortive rotation mode that inhibits ferroelectricity. Recently, room-temperature ferroelectricity of structural origin, arising from off-centering displacements of R and Cr ions, has been identified in spark-plasma sintered GdCrO$_3$ [Suryakanta Mishra et al., Phys. Rev. B 104, L180101 (2021)]. Interestingly, some of the experimentally observed non-ferroelectric RCrO$_3$ compounds have been theoretically predicted to host similar ferroelectric instabilities. Here, we have identified two such non-ferroelectric RCrO3 compounds, one DyCrO$_3$ (which is reported as a quantum paraelectric) and another LaCrO$_3$ (which is paraelectric), and using a modified synthesis protocol involving spark-plasma-sintering (SPS), we have been successful in engineering an intrinsic room-temperature ferroelectricity in the paramagnetic state, driven by noncentrosymmetric structural phase in both SPS sintered DyCrO$_3$ and LaCrO$_3$, in contrast to room-temperature paraelectricity in solid-state synthesized DyCrO$_3$ and LaCrO$_3$. While the ferroelectricity in SPS-prepared DyCrO$_3$ and LaCrO$_3$ is stable at room-temperature, it undergoes an irreversible transition from a ferroelectric (Pna2$_1$) phase to a paraelectric (Pbnm) phase at 440 K. Significantly, SPS-sintered LaCrO$_3$, which undergoes antiferromagnetic ordering at 290 K, emerges as a promising near room-temperature multiferroic material.

6.Photocatalytic Properties of Anisotropic $β$-PtX$_2$ (X= S, Se) and Janus $β$-PtSSe monolayers

Authors:Pooja Jamdagni, Ashok Kumar, Sunita Srivastava, Ravindra Pandey, K. Tankeshwar

Abstract: The highly efficient photocatalytic water splitting to produce clean energy requires novel semiconductor materials to achieve high solar-to-hydrogen energy conversion efficiency. Herein, the photocatalytic properties of anisotropic $\beta$-PtX$_2$ (X=S, Se) and Janus $\beta$-PtSSe monolayers are investigated based on density functional theory. Small cleavage energy for \{beta}-PtS2 (0.44 J/m2) and $\beta$-PtSe$_2$ (0.40 J/m$^2$) endorses the possibility of their mechanical exfoliation from respective layered bulk material. The calculated results find \{beta}-PtX2 monolayers to have an appropriate bandgap (~1.8-2.6 eV) enclosing the water redox potential, light absorption coefficients (~104 cm$^{-1}$), and excitons binding energy (~0.5-0.7 eV), which facilitates excellent visible-light driven photocatalytic performance. Remarkably, an inherent structural anisotropy leads to the anisotropic and high carrier mobility (up to ~5 x 10$^3$ cm$^2$ V$^{-1}$ S$^{-1}$) leading to fast transport of photogenerated carriers. Notably, the small required external potential to derive hydrogen evolution reaction and oxygen evolution reaction processes with an excellent solar-to-hydrogen energy conversion efficiency of $\beta$-PtSe$_2$ (~16%) and $\beta$-PtSSe (~18%) makes them promising candidates for solar water splitting applications.

7.First Principles Study of 2D Ring-Te and its Electrical Contact with Topological Dirac Semimetal

Authors:Jaspreet Singh, Ashok Kumar

Abstract: In recent years, researchers have manifested their interest in the two-dimensional (2D) mono-elemental materials of group-VI elements because of their excellent optoelectronic, photovoltaic and thermoelectric properties. Despite the intensive recent research efforts, there is still a possibility of novel 2D allotropes of these elements due to their multivalency nature. Here, we have predicted a novel 2D allotrope of tellurium (ring-Te) using density functional theory. Its stability is confirmed by phonon and ab-initio molecular dynamics simulations. The ring-Te has an indirect band gap of 0.69 eV (1.16 eV) at PBE (HSE06) level of theories and undergoes an indirect-direct band gap transition under the tensile strain. The higher carrier mobility of holes (~103cm$^2$V$^{-1}$s$^{-1}$), good UV-visible light absorption ability and low exciton binding (~0.35 eV) of ring-Te gives rise to its potential applications in optoelectronic devices. Further, the electrical contact of ring-Te with topological Dirac semimetal (sq-Te) under the influence of electric field shows that the Schottky barriers and contact types can undergo transition from p-type to n-type Schottky contact and then to ohmic contact at higher electric field. Our study provides an insight into the physics of designing high-performance electrical coupled devices composed of 2D semiconductors and topological semimetals.

8.Janus $β$-Te$_2$X (X = S, Se) Monolayers for Efficient Excitonic Solar Cells and Photocatalytic Water Splitting

Authors:Jaspreet Singh, Ashok Kumar

Abstract: Highly efficient, environmental friendly and renewable sources of energy are of great need today to combat with increasing energy demands and environmental pollution. In this work, we have investigated the novel 2D allotropes i.e., $\beta$-Te$_2$X (X = S, Se) using first-principles calculations and study their potential applications in light harvesting devices. Both the monolayers possess to have the high stability and semiconducting nature with an indirect band gap. The high carrier mobilities and excellent optical absorption of these monolayers make them potential candidates for solar conversion applications. We have proposed the type-II heterojunction solar cells and calculated their power conversion efficiencies (PCEs). The small conduction band offset and appropriate band gap of donor material in case of $\beta$-Te$_2$S(S-Side)/$\alpha$-Te$_2$S(Te-Side) heterojunction results in the PCE of ~ 21%. In addition to that, the band alignments of these monolayers properly engulf the redox potentials of the water. The overpotentials required to trigger the hydrogen reduction (HER) and water oxidation (OER) half reactions reveal that HER and OER preferred the acidic and neutral mediums, respectively. The calculated solar-to-hydrogen (STH) efficiencies of $\beta$-Te$_2$S ($\beta$-Te$_2$Se) monolayers come out to be ~ 13 % (~12 %), respectively, which implies their practical applications in water splitting. Thus, our work provides strong evidence regarding the potential applications of these materials in the field of light harvesting devices.

9.Structural Dynamics Descriptors for Metal Halide Perovskites

Authors:Xia Liang, Johan Klarbring, William Baldwin, Zhenzhu Li, Gábor Csányi, Aron Walsh

Abstract: Metal halide perovskites have shown extraordinary performance in solar energy conversion. They have been classified as "soft semiconductors" due to their flexible corner-sharing octahedral networks and polymorphous nature. Understanding the local and average structures continues to be challenging for both modelling and experiments. Here, we report the quantitative analysis of structural dynamics in time and space from molecular dynamics simulations of perovskite crystals. The descriptors cover a wide variety of properties, including octahedral tilting and distortion, local lattice parameters, molecular orientations, as well as the spatial correlation of these properties. To validate our methods, we have trained a machine learning force field (MLFF) for methylammonium lead bromide (CH$_3$NH$_3$PbBr$_3$) using an on-the-fly training approach with Gaussian process regression. The known stable phases are reproduced and we find an additional symmetry-breaking effect in the cubic and tetragonal phases close to the phase transition temperature. To test the implementation for large trajectories, we also apply it to 69,120 atom simulations for CsPbI$_3$ based on an MLFF developed using the atomic cluster expansion formalism. The structural dynamics descriptors and Python toolkit are general to perovskites and readily transferable to more complex compositions.

10.Revisiting the magnetic structure of Holmium at high pressure: a neutron diffraction study

Authors:M. Pardo-Sainz, F. Cova, J. A. Rodríguez-Velamazán, I. Puente-Orench, Y. Kousaka, M. Mito, J. Campo

Abstract: Low-temperature neutron diffraction experiments at P = 8 GPa have been conducted to investigate the magnetic structures of metallic Holmium at high pressures by employing a long d-spacing highflux diffractometer and a Paris-Edinburgh press cell inside a cryostat. We find that at P = 8 GPa and T = 5 K, no nuclear symmetry change is observed, keeping therefore the hexagonal closed packed (hcp) symmetry at high pressure. Our neutron diffraction data confirm that the ferromagnetic state does not exist. The magnetic structure corresponding to the helimagnetic order, which survives down to 5 K, is fully described by the magnetic superspace group formalism. These results are consistent with those previously published using magnetization experiments.

11.Epsilon-near-zero regime as the key to ultrafast control of functional properties of solids

Authors:Maarten Kwaaitaal, Daniel G Lourens, Carl S. Davies, Andrei Kirilyuk

Abstract: Strong light-matter interaction constitutes the bedrock of all photonic applications, empowering material elements with the ability to create and mediate interactions of light with light. Amidst the quest to identify new agents facilitating such efficient light-matter interactions, a class of promising materials have emerged featuring highly unusual properties deriving from their dielectric constant {\epsilon} being equal, or at least very close, to zero. Works so far have shown that the enhanced nonlinear optical effects displayed in this 'epsilon-near-zero' (ENZ) regime makes it possible to create ultrafast albeit transient optical switches. An outstanding question, however, relates to whether one could use the amplification of light-matter interactions at the ENZ conditions to achieve permanent switching. Here, we demonstrate that an ultrafast excitation under ENZ conditions can induce permanent all-optical reversal of ferroelectric polarization between different stable states. Our reliance on ENZ conditions that naturally emerge from the solid's ionic lattice, rather than specific material properties, suggests that the demonstrated mechanism of reversal is truly universal, being capable of permanently switching order parameters in a wide variety of systems.

12.On the validity of the bipolaron model for undoped and AlCl4- doped PEDOT

Authors:Ben Craig, Peter Townsend, Chris Kriton-Skylaris, Carlos Ponce de Leon, Denis Kramer

Abstract: The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most highly researched materials, yet electronic structure investigations of conducting polymers are still uncommon. The bipolaron model has traditionally been the dominant attempt to explain the electronic structure of PEDOT. Though recent theoretical studies have begun to move away from this model, some aspects remain commonplace, such as the concepts of bipolarons or polaron pairs. In this work, we use density functional theory to investigate the electronic structure of undoped and AlCl4- doped PEDOT oligomers. By considering the influence of oligomer length, oxidation or doping level and spin state, we find no evidence for self-localisation of positive charges in PEDOT as predicted by the bipolaron model. Instead, we find that a single or twin peak structural distortion can occur at any oxidation or doping level. Rather than representing bipolarons or polaron pairs, these are electron distributions driven by a range of factors, which also disproves the concept of polaron pairs. Localisation of distortions does occur in the doped case, although distortions can span an arbitrary number of nearby anions. Furthermore, conductivity in conducting polymers has been experimentally observed to reduce at very high doping levels. We show that at high anion concentrations, the non-bonding orbitals of the anions cluster below the HOMO-LUMO gap and begin to mix into the HOMO of the overall system. We propose that this mixing of highly localised anionic orbitals into the HOMO reduces the conductivity of the polymer and contributes to the reduced conductivity previously observed.

13.Superior ferroelectricity and nonlinear optical response in a hybrid germanium iodide hexagonal perovskite

Authors:Kun Ding, Haoshen Ye, Changyuan Su, Yu-An Xiong, Guowei Du, Yu-Meng You, Zhi-Xu Zhang, Shuai Dong, Yi Zhang, Da-Wei Fu

Abstract: Abundant chemical diversity and structural tunability make organic-inorganic hybrid perovskites (OIHPs) a rich ore for ferroelectrics. However, compared with their inorganic counterparts such as BaTiO$_3$, their ferroelectric key properties, including large spontaneous polarization ($P_s$), low coercive field ($E_c$), and strong second harmonic generation (SHG) response, have long been great challenges, which hinder their commercial applications. Here, a quasi-one-dimensional OIHP DMAGeI$_3$ (DMA=Dimethylamine) is reported, with notable ferroelectric attributes at room temperature: a large $P_s$ of 24.14 $\mu$C/cm$^2$ (on a par with BaTiO$_3$), a low $E_c$ below 2.2 kV/cm, and the strongest SHG intensity in OIHP family (about 12 times of KH$_2$PO$_4$ (KDP)). Revealed by the first-principles calculations, its large $P_s$ originates from the synergistic effects of the stereochemically active $4s^2$ lone pair of Ge$^{2+}$ and the ordering of organic cations, and its low kinetic energy barrier of small DMA cations results in a low $E_c$. Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites.

14.Metal halide thermoelectrics: prediction of high-performance CsCu2I3

Authors:Jong Woong Park, Young-Kwang Jung, Aron Walsh

Abstract: Thermoelectric devices can directly convert waste heat into electricity, which makes them an important clean energy technology. The underlying materials performance can be evaluated by the dimensionless figure of merit ZT. Metal halides are attractive candidates due to their chemical flexibility and ease of processing; however, the maximum ZT realized (ZT = 0.15) falls far below the level needed for commercialization (ZT > 1). Using a first-principles procedure we assess the thermoelectric potential of copper halide CsCu2I3, which features 1D Cu-I connectivity. The n-type crystal is predicted to exhibit a maximum ZT of 2.2 at 600 K along the b-axis. The strong phonon anharmonicity of this system is shown by locally stable non-centrosymmetric Amm2 structures that are averaged to form the observed centrosymmetric Cmcm space group. Our work provides insights into the structure-property relations in metal halide thermoelectrics and suggests a path forward to engineer higher-performance heat-to-electricity conversion.

15.Antiferromagnetic-ferromagnetic homostructures with Dirac magnons in van der Waals magnet CrI$_3$

Authors:John A. Schneeloch, Luke Daemen, Despina Louca

Abstract: Van der Waals (vdW) Dirac magnon system CrI$_3$, a potential host of topological edge magnons, orders ferromagnetically (FM) (T$_C=61$ K) in the bulk, but antiferromagnetic (AFM) order has been observed in nanometer thick flakes, attributed to monoclinic (M) type stacking. We report neutron scattering measurements on a powder sample where the usual transition to the rhombohedral (R) phase was inhibited for a majority of the structure. Elastic measurements (and the opening of a hysteresis in magnetization data on a pressed pellet) showed that an AFM transition is clearly present below $\sim$50 K, coexisting with the R-type FM order. Inelastic measurements showed a decrease in magnon energy compared to the R phase, consistent with a smaller interlayer magnetic coupling in M-type stacking. A gap remains at the Dirac point, suggesting that the same nontrivial magnon topology reported for the R phase may be present in the M phase as well.

16.$ $Machine Learning Moment Tensor Potential for Modelling Dislocation and Fracture in L1$_0$-TiAl and D0$_{19}$-Ti$_3$Al Alloys

Authors:Ji Qi, Z. H. Aitken, Qingxiang Pei, Anne Marie Z. Tan, Yunxing Zuo, M. H. Jhon, S. S. Quek, T. Wen, Zhaoxuan Wu, Shyue Ping Ong

Abstract: $ $Dual-phase $\gamma$-TiAl and $\alpha_2$-Ti$_{3}$Al alloys exhibit high strength and creep resistance at high temperatures. However, they suffer from low tensile ductility and fracture toughness at room temperature. Experimental studies show unusual plastic behaviour associated with ordinary and superdislocations, making it necessary to gain a detailed understanding on their core properties in individual phases and at the two-phase interfaces. Unfortunately, extended superdislocation cores are widely dissociated beyond the length scales practical for routine first-principles density-functional theory (DFT) calculations, while extant interatomic potentials are not quantitatively accurate to reveal mechanistic origins of the unusual core-related behaviour in either phases. Here, we develop a highly-accurate moment tensor potential (MTP) for the binary Ti-Al alloy system using a DFT dataset covering a broad range of intermetallic and solid solution structures. The optimized MTP is rigorously benchmarked against both previous and new DFT calculations, and unlike existing potentials, is shown to possess outstanding accuracy in nearly all tested mechanical properties, including lattice parameters, elastic constants, surface energies, and generalized stacking fault energies (GSFE) in both phases. The utility of the MTP is further demonstrated by producing dislocation core structures largely consistent with expectations from DFT-GSFE and experimental observations. The new MTP opens the path to realistic modelling and simulations of bulk lattice and defect properties relevant to the plastic deformation and fracture processes in $\gamma$-TiAl and $\alpha_2$-Ti$_{3}$Al dual-phase alloys.

17.Recent progress in the JARVIS infrastructure for next-generation data-driven materials design

Authors:Daniel Wines, Ramya Gurunathan, Kevin F. Garrity, Brian DeCost, Adam J. Biacchi, Francesca Tavazza, Kamal Choudhary

Abstract: The Joint Automated Repository for Various Integrated Simulations (JARVIS) infrastructure at the National Institute of Standards and Technology (NIST) is a large-scale collection of curated datasets and tools with more than 80000 materials and millions of properties. JARVIS uses a combination of electronic structure, artificial intelligence (AI), advanced computation and experimental methods to accelerate materials design. Here we report some of the new features that were recently included in the infrastructure such as: 1) doubling the number of materials in the database since its first release, 2) including more accurate electronic structure methods such as Quantum Monte Carlo, 3) including graph neural network-based materials design, 4) development of unified force-field, 5) development of a universal tight-binding model, 6) addition of computer-vision tools for advanced microscopy applications, 7) development of a natural language processing tool for text-generation and analysis, 8) debuting a large-scale benchmarking endeavor, 9) including quantum computing algorithms for solids, 10) integrating several experimental datasets and 11) staging several community engagement and outreach events. New classes of materials, properties, and workflows added to the database include superconductors, two-dimensional (2D) magnets, magnetic topological materials, metal-organic frameworks, defects, and interface systems. The rich and reliable datasets, tools, documentation, and tutorials make JARVIS a unique platform for modern materials design. JARVIS ensures openness of data and tools to enhance reproducibility and transparency and to promote a healthy and collaborative scientific environment.

1.Observation and enhancement of room temperature bilinear magnetoelectric resistance in sputtered topological semimetal Pt3Sn

Authors:Fan Yihong, Cresswell Zach, Yang Yifei, Jiang Wei, Lv Yang, Peterson Thomas, Zhang Delin, Liu Jinming, Low Tony, Wang Jian-ping

Abstract: Topological semimetal materials have become a research hotspot due to their intrinsic strong spin-orbit coupling which leads to large charge-to-spin conversion efficiency and novel transport behaviors. In this work, we have observed a bilinear magnetoelectric resistance (BMER) of up to 0.1 nm2A-1Oe-1 in a singlelayer of sputtered semimetal Pt3Sn at room temperature. Different from previous observations, the value of BMER in sputtered Pt3Sn does not change out-of-plane due to the polycrystalline nature of Pt3Sn. The observation of BMER provides strong evidence of the existence of spin-momentum locking in the sputtered polycrystalline Pt3Sn. By adding an adjacent CoFeB magnetic layer, the BMER value of this bilayer system is doubled compared to the single Pt3Sn layer. This work broadens the material system in BMER study, which paves the way for the characterization of topological states and applications for spin memory and logic devices.

2.An empirical potential for simulating hydrogen isotope retention in highly irradiated tungsten

Authors:Daniel R. Mason, Duc Nguyen-Manh, Victor W. Lindblad, Fredric G. Granberg, Mikhail Yu. Lavrentiev

Abstract: We describe the parameterization of a tungsten-hydrogen empirical potential designed for use with large-scale molecular dynamics simulations of highly irradiated tungsten containing hydrogen isotope atoms, and report test results. Particular attention has been paid to getting good elastic properties, including the relaxation volumes of small defect clusters, and to the interaction energy between hydrogen isotopes and typical irradiation-induced defects in tungsten. We conclude that the energy ordering of defects changes with the ratio of H atoms to point defects, indicating that this potential is suitable for exploring mechanisms of trap mutation, including vacancy loop to plate-like void transformations.

3.Spontaneous Electric Polarization in Graphene Polytypes

Authors:Simon Salleh Atri, Wei Cao, Bar Alon, Nirmal Roy, Maayan Vizner Stern, Vladimir Falko, Moshe Goldstein, Leeor Kronik, Michael Urbakh, Oded Hod, Moshe Ben Shalom

Abstract: A crystalline solid is a periodic sequence of identical cells, each containing one or more atoms. If the constituting unit cell is not centrosymmetric, charge may distribute unevenly between the atoms, resulting in internal electric polarization. This effect serves as the basis for numerous ferroelectric, piezoelectric, and pyroelectric phenomena. In nearly all polar materials, including multilayered van der Waals stacks that were recently found to exhibit interfacial polarization, inversion symmetry is broken by having two or more atomic species within the unit cell. Here, we show that even elemental crystals, consisting of one type of atom, and composed of non-polar centrosymmetric layers, exhibit electric polarization if arranged in an appropriate three-dimensional architecture. This concept is demonstrated here for inversion and mirror asymmetric mixed-stacking tetra-layer polytypes of non-polar graphene sheets. Furthermore, we find that the room temperature out-of-plane electric polarization increases with external electrostatic doping, rather than decreases owing to screening. Using first-principles calculations, as well as tight-binding modeling, we unveil the origin of polytype-induced polarization and its dependence on doping. Extension of this idea to graphene multilayers suggests that solely by lateral shifts of constituent monolayers one can obtain multiple meta-stable interlayer stacking sequences that may allow for even larger electrical polarization.

4.Mixed-Stacking Few-Layer Graphene as an Elemental Weak Ferroelectric Material

Authors:Aitor Garcia-Ruiz, Vladimir Enaldiev, Andrew McEllistrim, Vladimir I. Fal'ko

Abstract: Ferroelectricity (Valasek, J. Phys. Rev. 1921, 17, 475) - a spontaneous formation of electric polarisation - is a solid state phenomenon, usually, associated with ionic compounds or complex materials. Here we show that, atypically for elemental solids, few-layer graphenes can host an equilibrium out-of-plane electric polarisation, switchable by sliding the constituent graphene sheets. The systems hosting such effect include mixed-stacking tetralayers and thicker (5-9 layers) rhombohedral graphitic films with a twin boundary in the middle of a flake. The predicted electric polarisation would also appear in marginally (small-angle) twisted few-layer flakes, where lattice reconstruction would give rise to networks of mesoscale domains with alternating value and sign of out-of-plane polarisation.

5.Predictions and Measurements of Thermal Conductivity of Ceramic Materials at High Temperature

Authors:Zherui Han, Zixin Xiong, William T. Riffe, Hunter B. Schonfeld, Mauricio Segovia, Jiawei Song, Haiyan Wang, Xianfan Xu, Patrick E. Hopkins, Amy Marconnet, Xiulin Ruan

Abstract: The lattice thermal conductivity ($\kappa$) of two ceramic materials, cerium dioxide (CeO$_2$) and magnesium oxide (MgO), is computed up to 1500 K using first principles and the phonon Boltzmann Transport Equation (PBTE) and compared to time-domain thermoreflectance (TDTR) measurements up to 800 K. Phonon renormalization and the four-phonon effect, along with high temperature thermal expansion, are integrated in our \textit{ab initio} molecular dynamics (AIMD) calculations. This is done by first relaxing structures and then fitting to a set of effective force constants employed in a temperature-dependent effective potential (TDEP) method. Both three-phonon and four-phonon scattering rates are computed based on these effective force constants. Our calculated thermal conductivities from the PBTE solver agree well with literature and our TDTR measurements. Other predicted thermal properties including thermal expansion, frequency shift, and phonon linewidth also compare well with available experimental data. Our results show that high temperature softens phonon frequency and reduces four-phonon scattering strength in both ceramics. Compared to MgO, we find that CeO$_2$ has weaker four-phonon effect and renormalization greatly reduces its four-phonon scattering rates.

6.Prediction of NMR, X-ray and Mössbauer experimental results for amorphous Li-Si alloys using a novel DFTB model

Authors:Francisco Fernandez, Manuel Otero, Ma. Belén Oviedo, Daniel E. Barraco, S. Alexis Paz, Ezequiel P. M. Leiva

Abstract: Silicon anodes hold great promise for next-generation Li-ion batteries. The main obstacle to exploiting their high performance is the challenge of linking experimental observations to atomic structures due to the amorphous nature of Li-Si alloys. We unveil the atomistic-scale structures of amorphous Li-Si using our recently developed density functional tight-binding model. Our claim is supported by the successful reproduction of experimental X-ray pair distribution functions, NMR and M\"ossbauer spectra using simple nearest neighbors models. The predicted structures are publicly available.

7.Ultrashort Pulse Laser Annealing of Amorphous Atomic Layer Deposited MoS$_2$ Films

Authors:Malte J. M. J. Becher, Julia Jagosz, Rahel-Manuela Neubieser, Jan-Lucas Wree, Anjana Devi, Marvin Michel, Claudia Bock, Evgeny L. Gurevich, Andreas Ostendorf

Abstract: Thin films of molybendum disulfide grown via thermal atomic layer deposition at low temperatures, suitable for temperature sensible substrates, can be amorphous. To avoid a high temperature post treatment of the whole sample, which can cause thermal degradation of the substrate or other layers, a ultrashort pulse (usp) laser-induced transformation to crystalline layers is one of the most promising routes. In this paper we report the crystallization of amorphous MoS$_2$ layers processed with ultrashort laser pulses. The amorphous MoS$_2$ films were deposited by atomic layer deposition (ALD) and exposed to picosecond laser pulses ($\lambda = 532$ nm). The crystallization and the influence of the processing parameters on the film morphology were analyzed in detail by Raman spectroscopy and scanning electron microscopy. Furthermore, a transition of amorphous MoS$_2$ by laser annealing to self-organized patterns is demonstrated and a possible process mechanism for the ultrashort pulse laser annealing is discussed. Finally, the usp laser annealed films were compared to thermally and continuous wave (cw) laser annealed samples.

8.Unfaulting mechanisms of Frank loops in fluorite oxides

Authors:Miaomiao Jin, Jilang Miao, Yongfeng Zhang, Marat Khafizov, Kaustubh K. Bawane, Boopathy Kombaiah, David H. Hurley

Abstract: Unfaulting of Frank loops in irradiated fluoride oxides are of significance to microstructural evolution. However, the mechanisms have not been directly observed. To this end, we utilize molecular dynamics to reveal the atomistic details related to the unfaulting process of interstitial Frank loop in ThO$_2$, which involve nucleation of single or multiple Shockley partial pairs at the loop circumference. The unfaulting is achieved via a synchronous shear of the partial pairs to remove the extrinsic stacking fault in the cation sublattice and the intrinsic stacking fault in the anion sublattice. The strong oxygen motion at the dislocation core may reduce the activation barriers of dislocation nucleation and migration. These findings provide a fundamental understanding of the transformation of faulted loops in irradiated ThO$_2$, and could be transferable to other fluorite systems.

1.Cathodoluminescence spectroscopy of monolayer hexagonal boron nitride

Authors:K. Shima, T. S. Cheng, C. J. Mellor, P. H. Beton, C. Elias, P. Valvin, B. Gil, G. Cassabois, S. V. Novikov, S. F. Chichibu

Abstract: Cathodoluminescence (CL) spectroscopy is a powerful technique for studying emission properties of optoelectronic materials because CL is free from excitable bandgap limits and from ambiguous signals due to simple light scattering and resonant Raman scattering potentially involved in the photoluminescence (PL) spectra. However, direct CL measurements of atomically thin two-dimensional materials, such as transition metal dichalcogenides and hexagonal boron nitride (hBN), have been difficult due to the small excitation volume that interacts with high-energy electron beams (e-beams). Herein, distinct CL signals from a monolayer hBN, namely mBN, epitaxial film grown on a highly oriented pyrolytic graphite substrate are shown by using a home-made CL system capable of large-area and surface-sensitive excitation by an e-beam. The spatially resolved CL spectra at 13 K exhibited a predominant 5.5-eV emission band, which has been ascribed to originate from multilayered aggregates of hBN, markedly at thicker areas formed on the step edges of the substrate. Conversely, a faint peak at 6.04 eV was routinely observed from atomically flat areas. Since the energy agreed with the PL peak of 6.05 eV at 10 K that has been assigned as being due to the recombination of phonon-assisted direct excitons of mBN by Elias et al. [Nat. Commun. 10, 2639 (2019)], the CL peak at 6.04 eV is attributed to originate from the mBN epilayer. The CL results support the transition from indirect bandgap in bulk hBN to direct bandgap in mBN, in analogy with molybdenum disulfide. The results also encourage to elucidate emission properties of other low-dimensional materials with reduced excitation volumes by using the present CL configuration.

2.Competition between spin-glass and antiferromagnetic states in Tsai-type 1/1 and 2/1 quasicrystal 3 approximants

Authors:Farid Labib, Hiroyuki Takakura, Asuka Ishikawa, Ryuji Tamura

Abstract: Systematic research was performed to investigate magnetic properties of the Tsai-type Ga-Pd-RE (RE = Gd, Tb, Dy, and Ho) systems, where both 1/1 and 2/1 quasicrystal approximants (ACs) are attainable at the same compositions as thermodynamical stable phases. Most of the samples exhibited spin-glass (SG)-like freezing behavior at low temperatures except Ga-Pd-Tb 2/1 AC and Ga-Pd-Ho 1/1 AC. The former showcased antiferromagnetic order at 5.78 K while the latter did not show any anomaly down to 1.8 K. Furthermore, 2/1 ACs were noticed to be less frustrated than their corresponding 1/1 ACs presumably due to the disorder-free environment in the nearest neighbors of the rare earth sites that form a network of distorted octahedron in the 2/1 ACs. The spin dynamic in SG samples was also characterized by means of ac magnetic susceptibility measurements. The results evidenced a weak response of the freezing temperatures to the measurement frequency in the Heisenberg systems, i.e., Gd-contained ACs, in contrast to the non-Heisenberg systems, i.e., Tb, Dy and Ho-contained ACs, where significant dependency is noticed for the latter. The spin-glass samples were further examined by fitting their freezing temperatures to the Vogel-Fulcher law.

3.A first-principles machine-learning force field for heterogeneous ice nucleation on microcline feldspar

Authors:Pablo M. Piaggi, Annabella Selloni, Athanassios Z. Panagiotopoulos, Roberto Car, Pablo G. Debenedetti

Abstract: The formation of ice in the atmosphere affects precipitation and cloud properties, and plays a key role in the climate of our planet. Although ice can form directly from liquid water at deeply supercooled conditions, the presence of foreign particles can aid ice formation at much warmer temperatures. Over the past decade, experiments have highlighted the remarkable efficiency of feldspar minerals as ice nuclei compared to other particles present in the atmosphere. However, the exact mechanism of ice formation on feldspar surfaces has yet to be fully understood. Here, we develop a first-principles machine-learning model for the potential energy surface aimed at studying ice nucleation at microcline feldspar surfaces. The model is able to reproduce with high fidelity the energies and forces derived from density-functional theory (DFT) based on the SCAN exchange and correlation functional. We apply the machine-learning force field to study different fully-hydroxylated terminations of the (100), (010), and (001) surfaces of microcline exposed to vacuum. Our calculations suggest that terminations that do not minimize the number of broken bonds are preferred in vacuum. We also study the structure of supercooled liquid water in contact with microcline surfaces, and find that water density correlations extend up to around 1 nm from the surfaces. Finally, we show that the force field maintains a high accuracy during the simulation of ice formation at microcline surfaces, even for large systems of around 30,000 atoms. Future work will be directed towards the calculation of nucleation free energy barriers and rates using the force field developed herein, and understanding the role of different microcline surfaces on ice nucleation.

4.Elementary mechanisms of shear-coupled grain boundary migration for different complexions of a copper tilt grain boundary

Authors:Swetha Pemma, Rebecca Janisch, Gerhard Dehm, Tobias Brink

Abstract: The migration of grain boundaries leads to grain growth in polycrystals and is one mechanism of grain-boundary-mediated plasticity, especially in metallic nanocrystals. This migration is due to the movement of dislocation-like defects, called disconnections, which couple to externally applied shear stresses. Here, we investigate a $\Sigma$19b symmetric tilt grain boundary without pre-existing defects using atomistic computer simulations with classical potentials. This specific grain boundary exhibits two different atomic structures with different microscopic degrees of freedom (complexions), called ``domino'' and ``pearl'' complexion. We show that the grain boundary migration is affected by both the formation energy of a disconnection dipole and the Peierls-like barrier required to move the disconnections. For the pearl complexion, the latter is much higher, leading to a high stress required for grain boundary migration at low temperatures. However, in absolute values, the Peierls barrier is low and can be overcome by thermal energy even at room temperature. Since the domino complexion has higher disconnection formation energies, it is more resistant to migration at room temperature and above.

5.Dislocation correlations and the continuum dynamics of the weak line bundle ensemble

Authors:Joseph Pierre Anderson, Anter El-Azab

Abstract: Progress toward a first-principles theory of plasticity and work-hardening is currently impeded by an insufficient picture of dislocation kinetics (the dynamic effect of driving forces in a given dislocation theory). This is because present methods ignore the short-range interaction of dislocations. This work presents a kinetic theory of continuum dislocation dynamics in a vector density framework which takes into account the short-range interactions by means of suitably defined correlation functions. The weak line bundle ensemble of dislocations is defined, whereby the treatment of dislocations by a vector density is justified. It is then found by direct averaging of the dislocation transport equation that additional driving forces arise which are dependent on the dislocation correlation functions. A combination of spatial coarse-graining and statistical averaging of discrete dislocation systems are used to evaluate the various classes of tensorial dislocation correlations which arise in the kinetic theory. A novel, chiral classification of slip system interactions in FCC is used to define proper and improper rotations by which correlation functions corresponding to the six traditional interaction classifications can be evaluated. The full set of dislocation correlations are evaluated from discrete data. Only the self-correlations (for densities of like slip system) are found to be highly anisotropic. All correlation functions are found to decay within 2-4 times the coarse-graining distance. One type of interaction (coplanar correlations) are found to be negligible. Implications of the evaluated correlation functions for the future of vector density continuum dislocation dynamics are discussed, especially in terms of additional correlation driving forces and a gesture towards a coarse-grained dislocation mobility.

1.Unconventional anomalous Hall effect in epitaxially stabilized orthorhombic Ru$^{3+}$ perovskite thin films

Authors:L. -F. Zhang, T. C. Fujita, Y. Masutake, M. Kawamura, T. Arima, H. Kumigashira, M. Tokunaga, M. Kawasaki

Abstract: Complex oxides are mesmerizing material systems to realize multiple physical properties and functionalities by integrating different elements in a single compound. However, owing to the chemical instability, not all the combinations of elements can be materialized despite the intriguing potential expected from their magnetic and electronic properties. In this study, we demonstrate an epitaxial stabilization of orthorhombic Ru$^{3+}$ perovskite oxides: LaRuO$_3$ and NdRuO$_3$, and their magnetotransport properties that reflect the difference between non-magnetic La$^{3+}$ and magnetic Nd$^{3+}$. Above all, an unconventional anomalous Hall effect accompanied by an inflection point in magnetoresistance is observed around 1.3 T below 1 K for NdRuO$_3$, which is ascribed to topological Hall effect possibly due to a non-coplanar spin texture on Nd$^{3+}$ sublattice. These studies not only serve a new testbed for the interplay between spin-orbit coupling and Coulomb interaction but also open a new avenue to explore topological emergent phenomena in well-studied perovskite oxides.

2.Influence of inter-layer interactions and external stimuli on MX$_2$ (M= Mo/W, X= S/Se) heterobilayers

Authors:Ravina Beniwal, M. Suman Kalyan, Nicolas Leconte, Jeil Jung, Bala Murali Krishna Mariserla, S. Appalakondaiah

Abstract: Understanding the inter-layer interactions in transition metal dichalcogenides (TMDs) based heterostructures plays a vital role owing to the symmetry of the structure, bandgap nature, and excitonic effects. In this present work, we have studied the structural and electronic properties of MX$_2$ (M= Mo/W, X= S/Se) heterobilayers using first-principles calculations based on density functional theory. Unlike the traditional homobilayers of TMDs, these heterobilayers result in broken inversion symmetry and alter their point group from D$_{3d}$ $\rightarrow$ C$_{3v}$. From the calculated Raman spectra of these heterobilayers, we have observed that the shear and layer breathing modes (LBM) at lower frequencies ($<$ 50 cm$^{-1}$), arise due to the interlayer interactions between the different monolayers. We have simulated the electronic properties using the G$_0$W$_0$ method and perceived the nature of the band gap which mainly depends on the chalcogen atoms. Our results clearly indicate that the band gap is of direct nature for hetero bilayers with different chalcogen atoms and indirect nature for the same chalcogen based heterobilayers, with a band gap range between 1.4 to 1.7 eV. The exciton states of these materials are calculated with the Bethe-Salpeter equation (BSE) and found that the binding energies of inter-layer exciton are of the order of $\sim$ 250 meV, which makes them useful for infrared optoelectronic applications. We have also examined the electronic properties under the effect of minimal strain and twist for different chalcogen-based TMDs, and it shows the band gap tunability from direct to indirect due to interlayer interactions.

3.Metastability and topology in the magnetic topological insulator MnBi$_{2}$Te$_{4}$

Authors:Jeonghwan Ahn, Seoung-Hun Kang, Mina Yoon, Panchapakesan Ganesh, Jaron T. Krogel

Abstract: We study the effect of stacking faults on the topological properties of the magnetic topological insulator MnBi$_{2}$Te$_{4}$ (MBT) using density functional theory calculations and the Hubbard $U$ being tuned with many-body diffusion Monte Carlo techniques. We show that a modest deviation from the equilibrium interlayer distance leads to a topological phase transition from a non-trivial to a trivial topology, suggesting that tuning the interlayer coupling by adjusting the interlayer distance alone can lead to different topological phases. Interestingly, due to the locally increased interlayer distance of the top layer, a metastable stacking fault in MBT leads to a nearly gapless state at the topmost layer due to charge redistribution as the topmost layer recedes. We further find evidence of spin-momentum locking in the surface state along with a weak preservation of the band inversion in the near gapless state, which is indicative of the non-trivial topological surface states for the metastable stacking fault. Our findings provide a possible explanation for reconciling the long-standing puzzle of gapped and gapless states on MBT surfaces.

4.Wang tiles enable combinatorial design and robot-assisted manufacturing of modular mechanical metamaterials

Authors:Martin Doškář Faculty of Civil Engineering, Czech Technical University in Prague, Michael Somr Faculty of Civil Engineering, Czech Technical University in Prague, Radim Hlůžek Faculty of Civil Engineering, Czech Technical University in Prague, Jan Havelka Faculty of Civil Engineering, Czech Technical University in Prague, Jan Novák Faculty of Civil Engineering, Czech Technical University in Prague, Jan Zeman Faculty of Civil Engineering, Czech Technical University in Prague

Abstract: In this paper, we introduce a novel design paradigm for modular architectured materials that allows for spatially nonuniform designs from a handful of building blocks, which can be robotically assembled for efficient and scalable production. The traditional, design-limiting periodicity in material design is overcome by utilizing Wang tiles to achieve compatibility among building blocks. We illustrate our approach with the design and manufacturing of an L-shaped domain inspired by a scissor-like soft gripper, whose internal module distribution was optimized to achieve an extreme tilt of a tip of the gripper's jaw when the handle part was uniformly compressed. The geometry of individual modules was built on a 3$\times$3 grid of elliptical holes with varying semi-axes ratios and alternating orientations. We optimized the distribution of the modules within the L-shaped domain using an enumeration approach combined with a factorial search strategy. To address the challenge of seamless interface connections in modular manufacturing, we produced the final designs by casting silicone rubber into modular molds automatically assembled by a robotic arm. The predicted performance was validated experimentally using a custom-built, open-hardware test rig, Thymos, supplemented with digital image correlation measurements. Our study demonstrates the potential for enhancing the mechanical performance of architectured materials by incorporating nonuniform modular designs and efficient robot-assisted manufacturing.

5.Ambient and high-pressure electrical transport and structural investigations of magnetic Weyl semimetal PrAlGe

Authors:U. Dutta, P. Král, M. Míšek, B. Joseph, J. Kaštil

Abstract: We present ambient and high-pressure electrical transport and structural properties of recently discovered magnetic Weyl semimetal PrAlGe. Electrical resistivity at ambient pressure shows an anomaly at $T_C$ = 15.1 K related to the ferromagnetic transition. Anomalous Hall effect (AHE) is observed below $T_C$. We observe a 1.4 K/GPa increase of $T_C$ with pressure, resulting in $T_C$ $\approx$ 47 K at 23.0 GPa. Strong competition between Lorentz force and spin-scattering mechanisms suppressed by magnetic field is deduced from the magnetoresistance measurements under pressure. As in the ambient pressure case, the AHE is found to be present below $T_C$ up to the highest applied pressure. We observe a clear anomaly in the pressure dependence of $T_C$, magnetoresistance and Hall effect at 12.5 GPa suggesting the occurrence of a pressure-induced electronic transition at this pressure. X-ray diffraction (XRD) experiment under pressure revealed the lattice structure to be stable up to $\sim$19.6 GPa with the absence of any symmetry changing structural phase transition from the initial $I4_1md$ structure. Careful analysis of the pressure dependent XRD data reveal an isostructural transition near 11 GPa. Observed isostructural transition may be related to the pressure-induced electronic transition deduced from the magnetoresistance and Hall effect data.

6.Procedures for assessing the stability of proposed topological materials

Authors:Jeonghwan Ahn, Seoung-Hun Kang, Mao-Hua Du, Mina Yoon, Jaron T. Krogel, Fernando A. Reboredo

Abstract: We investigate the stability of MnPb$_{2}$Bi$_{2}$Te$_{6}$ (MPBT), which is predicted to be a magnetic topological insulator (TI), using density functional theory calculations. Our analysis includes various measures such as enthalpies of formation, Helmholtz free energies, defect formation energies, and dynamical stability. Our thermodynamic analysis shows that the phonon contribution to the energy gain from finite temperature is estimated to be less than 10~meV/atom, which may not be sufficient to stabilize MPBT at high temperatures, even with the most favorable reactions starting from binaries. While MPBT is generally robust against the formation of various defects, we find that anti-site defect formation of $\text{Mn}_{\text{Pb}}$ is the most likely to occur, with corresponding energy less than 60~meV. This can be attributed to the significant energy cost from compressive strain at the PbTe layer. Our findings suggest that MPBT is on the brink of stability in terms of thermodynamics and defect formation, underscoring the importance of conducting systematic analyses of the stability of proposed TIs, including MPBT, for their practical utilization. This study offers valuable insights into the design and synthesis of desirable magnetic TI materials with robust stabilities.

7.Atomic structure of the unique antiferromagnetic 2/1 quasicrystal approximant

Authors:Farid Labib, Hiroyuki Takakura, Asuka Ishikawa, Ryuji Tamura

Abstract: The atomic structure of the recently discovered antiferromagnetic Ga50Pd35.5Tb14.5 2/1 approximant to quasicrystal with the space group of Pa-3(No. 205), a = 23.1449(0) angstrom was determined by means of a single crystal X-ray diffraction. The refined structure model revealed two main building units, namely, a Tsai-type rhombic triacontahedron (RTH) cluster with three concentric inner shells and an acute rhombohedron filling the gaps in between the RTH clusters. One of the interesting findings was a very low number of chemically mixed sites in the structure, which amount to only 7.40 % of the all the atomic sites within an RTH cluster. In particular, a disorder-free environment was noticed within a nearest neighbor of an isolated Tb3+ ion, which is presumably one of the main contributors in enhancing antiferromagnetic order in the present compound. The second significant finding was the observance of an orientationally ordered trigonal pyramid-like unit with a height of 4.2441(7) angstrom at the center of the RTH cluster, which has never been observed in Tsai-type compounds before. Such unit is noticed to bring structural distortion to outer shells, in particular, to the surrounding dodecahedron cage being another possible contributor of the antiferromagnetic order establishment in the present compound. The results, therefore, are suggestive of a possible link between chemical/positional order and the antiferromagnetic order establishment.

8.Unconventional phonon spectra and obstructed edge phonon modes

Authors:Ruihan Zhang, Haohao Sheng, Junze Deng, Zhilong Yang, Zhijun Wang

Abstract: Based on the elementary band representations (eBR), some topologically trivial materials are classified as unconventional ones (obstructed atomic limit), where the eBR decomposition of electronic states is not consistent with the atomic valence-electron band representations. In the work, we identify that the unconventional nature can also exist in phonon spectra, where the eBR decomposition of the phonon modes is not consistent with atomic vibration band representations (aBR). The unconventionality has two types: type I is on an empty site; type II is on an atom site with non-atomic vibration orbitals. Our detailed calculations show that black phosphorus (BP) and 1H-MoSe$_2$ have unconventional both phonon spectra and electronic band structures. The BP has the type-I unconventional phonon spectrum, while 1H-MoSe$_2$ has the type-II one. The obstructed phonon modes are obtained for two types of unconventional phonon spectra.

9.Ultra-low lattice thermal conductivity in tungsten-based scheelite ceramics

Authors:Hicham Ait Laasri GREMAN, Université de Tours - CNRS - INSA Centre Val de Loire - UMR7347, France, Eliane Bsaibess GREMAN, Université de Tours - CNRS - INSA Centre Val de Loire - UMR7347, France, Fabian Delorme GREMAN, Université de Tours - CNRS - INSA Centre Val de Loire - UMR7347, France, Guillaume F. Nataf GREMAN, Université de Tours - CNRS - INSA Centre Val de Loire - UMR7347, France, Fabien Giovannelli GREMAN, Université de Tours - CNRS - INSA Centre Val de Loire - UMR7347, France

Abstract: $BaWO_{4}$, $Ce_{2/3}\square_{1/3}WO_{4}$ and $La_{2/3}\square_{1/3}WO_{4}$ polycrystalline ceramics were synthesized by conventional solid-state reaction route. The effect of cation-deficiency on the crystallographic structure, microstructure and thermal properties of these scheelite-type compounds were investigated. X-ray diffraction was used to identify the single-phase scheelite structure of the studied ceramics. Scanning Electron Microscopy technique has revealed a homogenous and dense microstructure with a few micro-cracks. The thermal conductivity of $BaWO_{4}$ scheelite decreases from $1.3\pm0.2$ to $1.0\pm0.1 W m^{-1} K^{-1}$ in the range 373 K - 673 K. The cation-deficient scheelites $Ce_{2/3}\square_{1/3}WO_{4}$ and $La_{2/3}\square_{1/3}WO_{4}$ ceramics display an ultra-low thermal conductivity of $0.3\pm0.04 W m^{-1} K^{-1}$ and $0.2\pm0.03 W m^{-1} K^{-1}$ at 673 K, respectively. These materials exhibit among the lowest known values of thermal conductivity in crystalline oxides, in this temperature range. Therefore, they appear as very attractive for thermal barrier coating and thermoelectric applications.

10.A Mini Review on The Application of Nanomaterials in Forensic Science

Authors:Aaromal Venugopal, Vanshika Seth, Shreya Subhash Naik, Sreya Valappil, Aman Verma, Shalini Rajan, Pranav Vilas Shetgaonkar, Akshita Sinha, Sandeep Munjal

Abstract: Herein, we report a minireview to give a brief introduction of applications of nanomaterials in the field of forensic science. The materials that have their size in nanoscale (1 - 100 nm) comes under the category of nanomaterials. Nanomaterials possess various applications in different fields like cosmetic production, medical, photoconductivity etc. because of their physio-chemical, electrical and magnetic properties. Due to the different characteristic property that nanomaterials have, they are widely employed in diverse domains. In various fields of forensic science such as fingerprints, toxicology, medicine, serology, nanomaterials are being used extensively. Large surface area to volume ratio of the materials in nano-regime makes the nanomaterials suitable for all these application with high efficiency. This review article briefs about the nanomaterials, their advantages and their novel applications in various fields, focusing especially in the field of forensic science. The basic idea of different areas of forensic science such as development of fingerprints, detection of drugs, estimating the time since death, analysis of GSR, detection of various explosives and for the extraction of DNA etc. has also been provided.

1.On recrystallization nucleation in pure aluminum

Authors:Adam Morawiec

Abstract: Static recrystallization is an important aspect of metal processing. The initial stage of recrystallization - nucleation of new grains - determines its later stages. The accepted mechanisms of recrystallization nucleation are based on the assumption that embryos with orientations of the nuclei preexist in the deformed matrix. However, this standard picture seems to be incomplete. There are indications that, in some cases, the deformed matrix has no orientations observed in recrystallized material. Therefore a mechanism of early stage recrystallization without preexisting embryos is considered. Since the recrystallization growth shows strong orientation selection, and the recrystallization front is believed to migrate through collective shuffling of atoms, it is postulated that the shuffling mechanism responsible for oriented growth of a recrystallizing grain extends to the very beginning of the grain's existence, i.e., that a new orientation can be created via rearrangement of atoms in a strained region. The postulate explains the formation of new orientations, and it has the potential to significantly change the understanding of the early phase of recrystallization.

2.Synthesis and scintillation properties of some dense X-ray phosphors

Authors:C Dujardin UCBL, A. Garcia-Murillo, C. Pedrini, C. Madej, C. Goutaudier, A. Koch, A. G. Petrosyan, M. J. Weber

Abstract: Many ultra-dense lutetium or gadolinium based compounds doped with Eu 3+ have been prepared. This paper reports on the major scintillation performances of these compounds. One of them (Lu 2 O 3 :Eu) is particularly promising and have been deposited on a screen. Performances of such a screen are presented.

3.Ultrasound cavitation and exfoliation dynamics of 2D materials re-vealed in operando by X-ray free electron laser megahertz imaging

Authors:Kang Xiang, Shi Huang, Hongyuan Song, Vasilii Bazhenov, Valerio Bellucci, Sarlota Birnsteinova, Raphael de Wijn, Jayanath C. P. Koliyadu, Faisal H. M. Koua, Adam Round, Ekaterina Round, Abhisakh Sarma, Tokushi Sato, Marcin Sikorski, Yuhe Zhang, Eleni Myrto Asimakopoulou, Pablo Villanueva-Perez, Kyriakos Porfyrakis, Iakovos Tzanakis, Dmitry G. Eskin, Nicole Grobert, Adrian Mancuso, Richard Bean, Patrik Vagovic, Jiawei Mi

Abstract: Ultrasonic liquid phase exfoliation is a promising method for the production of 2D layered materials. A large number of studies have been made in investigating the underlying ultrasound exfoliation mechanisms. Due to the experimental challenge in capturing in real-time the highly transient and dynamic phenomena in sub-microsecond time scale and micrometer length scale at the same time, most theories reported to date are still under intensive debate. Here, we report the exciting new findings from the first scheduled user experiment using the free electron laser MHz X-ray Microscopy at the SPB/SFX instrument of the European X-ray Free-Electron Laser. The ultra-short X-ray pulse (~25 fs) and the unique pulse train time structure allow us to reveal fully the ultrasound cavitation dynamics, including the nucleation, growth, implosion dynamics of cavitation bubbles at different wave amplitudes. Using ma-chine-learning image processing, the instance of bubble cloud shock wave emission, their periodic impact onto the graphite materials and the cyclic fatigue exfoliation mechanism in multi-time scale from sub-microsecond to tens of minutes were all quantified and elucidated in this research.

4.In-plane thermal diffusivity determination using beam-offset frequency-domain thermoreflectance with a one-dimensional optical heat source

Authors:Kai Xu, Jiali Guo, Grazia Raciti, Alejandro R. Goni, M. Isabel Alonso, Xavier Borrise, Ilaria Zardo, Mariano Campoy-Quiles, Juan Sebastian Reparaz

Abstract: We present an innovative contactless method suitable to study in-plane thermal transport based on beam-offset frequency-domain thermoreflectance using a one-dimensional heat source with uniform power distribution. Using a one-dimensional heat source provides a number of advantages as compared to point-like heat sources, as typically used in time- and frequency-domain thermoreflectance experiments, just to name a few: (i) it leads to a slower spatial decay of the temperature field in the direction perpendicular to the line-shaped heat source, allowing to probe the temperature field at larger distances from the heater, hence, enhancing the sensitivity to in-plane thermal transport; (ii) the frequency range of interest is typically < 100 kHz. This rather low frequency range is convenient regarding the cost of the required excitation laser system but, most importantly, it allows the study of materials without the presence of a metallic transducer with almost no influence of the finite optical penetration depth of the pump and probe beams on the thermal phase lag, which arises from the large thermal penetration depth imposed by the used frequency range. We also show that for the case of a harmonic thermal excitation source, the phase lag between the thermal excitation and thermal response of the sample exhibits a linear dependence with their spatial offset, where the slope is proportional to the inverse of the thermal diffusivity of the material. We demonstrate the applicability of this method to the cases of: (i) suspended thin films of Si and PDPP4T, (ii) Bi bulk samples, and (iii) Si, glass, and highly-oriented pyrollitic graphite (HOPG) bulk samples with a thin metallic transducer. Finally, we also show that it is possible to study in-plane heat transport on substrates with rather low thermal diffusivity, e.g., glass, even using a metallic transducer.

5.Quantum Monte Carlo study of Doppler broadening of positron annihilation radiation in semiconductors and insulators

Authors:Kristoffer Simula, Jan Härkönen, Iuliia Zhelezova, Neil Drummond, Filip Tuomisto, Ilja Makkonen

Abstract: The measurement of the momentum distribution of positron annihilation radiation is a powerful method to detect and identify open-volume defects in crystalline solids. The Doppler broadening of the 511 keV line of the $2\gamma$ electron-positron annihilation event reflects the momentum density of annihilating pairs and local electron momenta at positron annihilation sites. It can provide information on the chemical surroundings of vacancies, such as the impurity atoms around them. Accurate methods based on first-principles calculations are crucial for interpreting measured Doppler spectra. In this work we will validate such a method based on variational quantum Monte Carlo by benchmarking results in aluminium nitride and silicon against experimental data measured from defect-free reference samples. The method directly models electron-positron correlations using variational wave functions. We achieve better agreement with experiments for our test set than conventional state-of-the-art methods. We show that normalized Doppler broadening spectra calculated with quantum Monte Carlo converge rapidly as a function of simulation cell size, and backflow transformations have only a minor effect. This makes the method robust and practical to support positron-based spectroscopies.

6.Zeta potential and nanodiamond self assembly assisted diamond growth on lithium niobate and lithium tantalate single crystal

Authors:Soumen Mandal, Karsten Arts, David Morgan, Zhuohui Chen, Oliver A. Williams

Abstract: This study focuses on the self-assembly and subsequent diamond growth on SiO$_2$ buffered lithium niobate (LiNbO$_3$) and lithium tantalate (LiTaO$_3$) single crystals. The zeta-potential of LNO and LTO single crystal were measured as a function of pH. They were found to be negative in the pH range 3.5-9.5. The isoelectric point for LNO was found to be at pH $\sim$ 2.91 and that of LTO to be at pH $\sim$ 3.20. X-ray photoelectron spectroscopy performed on the surfaces show presence of oxygen groups which may be responsible for the negative zeta potential of the crystals. Self-assembly of nanodiamond particles on LTO and LNO, using nanodiamond colloid, were studied. As expected, high nanodiamond density was seen when self-assembly was done using a positively charged nanodiamond particles. Diamond growth was attempted on the nanodiamond coated substrates but they were found to be unsuitable for direct growth due to disintegration of substrates in diamond growth conditions.. A $\sim$100nm thick silicon dioxide layer was deposited on the crystals, followed by nanodiamond self assembly and diamond growth. Thin diamond films were successfully grown on both coated crystals. The diamond quality was analysed by Raman spectroscopy and atomic force microscopy.

7.Order parameter dynamics in Mn$_3$Sn driven by DC and pulsed spin-orbit torques

Authors:Ankit Shukla, Siyuan Qian, Shaloo Rakheja

Abstract: We numerically investigate and develop analytic models for both the DC and pulsed spin-orbit-torque (SOT)-driven response of order parameter in single-domain Mn$_3$Sn, which is a metallic antiferromagnet with an anti-chiral 120$^\circ$ spin structure. We show that DC currents above a critical threshold can excite oscillatory dynamics of the order parameter in the gigahertz to terahertz frequency spectrum. Detailed models of the oscillation frequency versus input current are developed and found to be in excellent agreement with the numerical simulations of the dynamics. In the case of pulsed excitation, the magnetization can be switched from one stable state to any of the other five stable states in the Kagome plane by tuning the duration or the amplitude of the current pulse. Precise functional forms of the final switched state versus the input current are derived, offering crucial insights into the switching dynamics of Mn$_3$Sn. The readout of the magnetic state can be carried out via either the anomalous Hall effect, or the recently demonstrated tunneling magnetoresistance in an all-Mn$_3$Sn junction. We also discuss possible disturbance of the magnetic order due to heating that may occur if the sample is subject to large currents. Operating the device in pulsed mode or using low DC currents reduces the peak temperature rise in the sample due to Joule heating. Our predictive modeling and simulation results can be used by both theorists and experimentalists to explore the interplay of SOT and the order dynamics in Mn$_3$Sn, and to further benchmark the device performance.

8.Band-filling-controlled magnetism from transition metal intercalation in $N_{1/3}$NbS$_2$ revealed with first-principles calculations

Authors:Z. Hawkhead, T. J. Hicken, N. P. Bentley, B. M. Huddart, S. J. Clark, T. Lancaster

Abstract: We present a first-principles study of the effect of 3$d$ transition metal intercalation on the magnetic properties of the 2H-NbS$_2$ system, using spin-resolved density functional theory calculations to investigate the electronic structure of $N_{1/3}$NbS$_2$ ($N$ = Ti, V, Cr, Mn, Fe, Co, Ni). We are able to accurately determine the magnetic moments and crystal field splitting, and find that the magnetic properties of the materials are determined by a mechanism based on filling rigid bands with electrons from the intercalant. We predict the dominant magnetic interaction of these materials by considering Fermi surface nesting, finding agreement with experiment where data are available.

9.Pressure-Induced Phase Transition Versus Amorphization in Hybrid Methylammonium Lead Bromide Perovskite

Authors:Akun Liang, Robin Turnbull, Catalin Popescu, Ismael Fernandez-Guillen, Rafael Abargues, Pablo P. Boix, Daniel Errandonea

Abstract: The crystal structure of CH3NH3PbBr3 perovskite has been investigated under high-pressure by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH3NH3PbBr3 (Pm-3m->Im-3->Pmn21), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies which reported amorphization of CH3NH3PbBr3 between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH3NH3PbBr3 remains crystalline up to 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn21, but the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments and visual observations and by the fact that changes induced by pressure up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy which is discussed using the structural information obtained from X-ray diffraction.

10.Comparative Electronic Structures of the Chiral Helimagnets Cr1/3NbS2 and Cr1/3TaS2

Authors:Lilia S. Xie, Oscar Gonzalez, Kejun Li, Matteo Michiardi, Sergey Gorovikov, Sae Hee Ryu, Shannon S. Fender, Marta Zonno, Na Hyun Jo, Sergey Zhdanovich, Chris Jozwiak, Aaron Bostwick, Samra Husremovic, Matthew P. Erodici, Cameron Mollazadeh, Andrea Damascelli, Eli Rotenberg, Yuan Ping, D. Kwabena Bediako

Abstract: Magnetic materials with noncollinear spin textures are promising for spintronic applications. To realize practical devices, control over the length and energy scales of such spin textures is imperative. The chiral helimagnets Cr1/3NbS2 and Cr1/3TaS2 exhibit analogous magnetic phase diagrams with different real-space periodicities and field dependence, positioning them as model systems for studying the relative strengths of the microscopic mechanisms giving rise to exotic spin textures. Here, we carry out a comparative study of the electronic structures of Cr1/3NbS2 and Cr1/3TaS2 using angle-resolved photoemission spectroscopy and density functional theory calculations. We show that bands in Cr1/3TaS2 are more dispersive than their counterparts in Cr1/3NbS2 and connect this result to bonding and orbital overlap in these materials. We also unambiguously distinguish exchange splitting from surface termination effects by studying the dependence of their photoemission spectra on polarization, temperature, and beam size. We find strong evidence that hybridization between intercalant and host lattice electronic states mediates the magnetic exchange interactions in these materials, suggesting that band engineering is a route toward tuning their spin textures. Overall, these results underscore how the modular nature of intercalated transition metal dichalcogenides translates variation in composition and electronic structure to complex magnetism.

11.Atomistic mechanisms underlying the maximum in diffusivity in doped Li$_7$La$_3$Zr$_2$O$_{12}$

Authors:Juan C. Verduzco, Ernesto E. Marinero, Alejandro Strachan

Abstract: Doped lithium lanthanum zirconium oxide (LLZO) is a promising class of solid electrolytes for lithium-ion batteries due to their good electrochemical stability and compatibility with Li metal anodes. Ionic diffusivity in these ceramics is known to occur via correlated, vacancy mediated, jumps of Li+ between alternating tetrahedral and octahedral sites. Aliovalent doping at the Zr-site increases the concentration of vacancies in the Li+ sublattice and cation diffusivity, but such an increase is universally followed by a decrease for Li+ concentration lower than 6.3 - 6.5 Li molar content. Molecular dynamics simulations based on density functional theory show that the maximum in diffusivity originates from competing effects between the increased vacancy concentration and the increasing occupancy of the low-energy tetrahedral sites by Li+, which increases the overall activation energy associated with diffusion. For the relatively high temperatures of our simulations, Li+ concentration plays a dominant role in transport as compared to dopant chemistry.

1.On automatic determination of quasicrystal orientations by indexing of detected reflections

Authors:Adam Morawiec

Abstract: Automatic crystal orientation determination and orientation mapping are important tools for research on polycrystalline materials. The most common methods of automatic orientation determination rely on detecting and indexing individual diffraction reflections. These methods, however, have not been used for orientation mapping of quasicrystalline materials. The paper describes necessary changes to existing software designed for orientation determination of periodic crystals so that it can be applied to quasicrystals. The changes are implemented in one of such programs. The functioning of the modified program is illustrated by an example orientation map of an icosahedral polycrystal.

2.Ab-initio investigation of the physical properties of BaAgAs Dirac semimetal and its possible thermo-mechanical and optoelectronic applications

Authors:A. S. M. Muhasin Reza, S. H. Naqib

Abstract: BaAgAs is a ternary Dirac semimetal which can be tuned across a number of topological orders. In this study we have investigated the bulk physical properties of BaAgAs using density functional theory based computations. Most of the results presented in this work are novel. The optimized structural parameters are in good agreement with previous results. The elastic constants indicate that BaAgAs is mechanically stable and brittle in nature. The compound is moderately hard and possesses fair degree of machinability. There is significant mechanical/elastic anisotropy in BaAgAs. The Debye temperature of the compound is medium and the phonon thermal conductivity and melting temperature are moderate as well. The bonding character is mixed with notable covalent contribution. The electronic band structure calculations reveal clear semimetallic behavior with a Dirac node at the Fermi level. BaAgAs has a small ellipsoidal Fermi surface centered at the G-point of the Brillouin zone. The phonon dispersion curves show dynamical stability. There is a clear phonon band gap between the acoustic and the optical branches. The energy dependent optical constants conform to the band structure calculations. The compound is an efficient absorber of the ultraviolet light and has potential to be used as an anti-reflection coating. Optical anisotropy of BaAgAs is moderate. The computed repulsive Coulomb pseudopotential is low indicating that the electronic correlations in this compound are not strong.

3.A Digital Twin to overcome long-time challenges in Photovoltaics

Authors:Larry Lüer, Marius Peters, Ana Sunčana Smith, Eva Dorschky, Bjoern M. Eskofier, Frauke Liers, Jörg Franke, Martin Sjarov, Mathias Brossog, Dirk Guldi, Andreas Maier, Christoph J. Brabec

Abstract: The recent successes of emerging photovoltaics (PV) such as organic and perovskite solar cells are largely driven by innovations in material science. However, closing the gap to commercialization still requires significant innovation to match contradicting requirements such as performance, longevity and recyclability. The rate of innovation, as of today, is limited by a lack of design principles linking chemical motifs to functional microscopic structures, and by an incapacity to experimentally access microscopic structures from investigating macroscopic device properties. In this work, we envision a layout of a Digital Twin for PV materials aimed at removing both limitations. The layout combines machine learning approaches, as performed in materials acceleration platforms (MAPs), with mathematical models derived from the underlying physics and digital twin concepts from the engineering world. This layout will allow using high-throughput (HT) experimentation in MAPs to improve the parametrization of quantum chemical and solid-state models. In turn, the improved and generalized models can be used to obtain the crucial structural parameters from HT data. HT experimentation will thus yield a detailed understanding of generally valid structure-property relationships, enabling inverse molecular design, that is, predicting the optimal chemical structure and process conditions to build PV devices satisfying a multitude of requirements at the same time. After motivating our proposed layout of the digital twin with causal relationships in material science, we discuss the current state of the enabling technologies, already being able to yield insight from HT data today. We identify open challenges with respect to the multiscale nature of PV materials and the needed volume and diversity of data, and mention promising approaches to address these challenges.

4.Disorder and cavity evolution in single-crystalline Ge during implantation of Sb ions monitored in-situ by spectroscopic ellipsometry

Authors:Tivadar Lohner, Attila Nemeth, Zsolt Zolnai, Benjamin Kalas, Alekszej Romanenko, Nguyen Quoc Khanh, Edit Szilagyi, Endre Kotai, Emil Agocs, Zsolt Toth, Judit Budai, Peter Petrik, Miklos Fried, Istvan Barsony, Jozsef Gyulai

Abstract: Ion implantation has been a key technology for the controlled surface modification of materials in microelectronics and generally, for tribology, biocompatibility, corrosion resistance and many more. To form shallow junctions in Ge is a challenging task. In this work the formation and accumulation of shallow damage profiles was studied by in-situ spectroscopic ellipsometry (SE) for the accurate tracking and evaluation of void and damage fractions in crystalline Ge during implantation of 200-keV Sb ions with a total fluence up to 1E16 cm-2 and an ion flux of 2.1E12 cm-2 s-1. The consecutive stages of damage accumulation were identified using optical multi-layer models with quantitative parameters of the thickness of modified layers as well as the volume fractions of amorphized material and voids. The effective size of damaged zones formed from ion tracks initiated by individual bombarding ions can be estimated by numerical simulation compared with the dynamics of damage profiles measured by ion beam analysis and ellipsometry. According to our observations, the formation of initial partial disorder was followed by complete amorphization and void formation occurring at the fluence of about 1E15 cm-2, leading to a high volume fraction of voids and a modified layer thickness of approx. 200 nm by the end of the irradiation process. This agrees with the results of numerical simulations and complementary scanning electron microscopy (SEM) measurements. In addition, we found a quasi-periodic time dependent behavior of amorphization and void formation represented by alternating accelerations and decelerations of different reorganization processes, respectively.

1.Consequences and control of multi-scale (dis)order in chiral magnetic textures

Authors:Berit H. Goodge, Oscar Gonzalez, Lilia S. Xie, D. Kwabena Bediako

Abstract: Transition metal-intercalated transition metal dichalcogenides (TMDs) are promising platforms for next-generation spintronic devices based on their wide range of electronic and magnetic phases, which can be tuned by varying the host lattice or the identity of the intercalant, along with its stoichiometry and spatial order. Some of these compounds host a chiral magnetic phase in which the helical winding of magnetic moments propagates along a high-symmetry crystalline axis. Previous studies have demonstrated that variation in intercalant concentrations can have a dramatic impact on the formation of chiral domains and ensemble magnetic properties. However, a systematic and comprehensive study of how atomic-scale order and disorder impacts collective magnetic behavior are so far lacking. Here, we leverage a combination of imaging modes in the (scanning) transmission electron microscope (S/TEM) to directly probe (dis)order across multiple length scales and show how subtle changes in the atomic lattice can be leveraged to tune the mesoscale spin textures and bulk magnetic response, with direct implications for the fundamental understanding and technological implementation of such compounds.

2.Carrier-Density Control of the Quantum-Confined 1$T$-TiSe$_2$ Charge-Density-Wave

Authors:T. Jaouen, A. Pulkkinen, M. Rumo, G. Kremer, B. Salzmann, C. W. Nicholson, M. -L. Mottas, E. Giannini, S. Tricot, P. Schieffer, B. Hildebrand, C. Monney

Abstract: Using angle-resolved photoemission spectroscopy, combined with first principle and coupled self-consistent Poisson-Schr\"odinger calculations, we demonstrate that potassium (K) atoms adsorbed on the low-temperature phase of 1$T$-TiSe$_2$ induce the creation of a two-dimensional electron gas (2DEG) and quantum confinement of its charge-density-wave (CDW) at the surface. By further changing the K coverage, we tune the carrier-density within the 2DEG that allows us to nullify, at the surface, the electronic energy gain due to exciton condensation in the CDW phase while preserving a long-range structural order. Our study constitutes a prime example of a controlled exciton-related many-body quantum state in reduced dimensionality by alkali-metal dosing.

3.Interfacial two-dimensional oxide enhances photocatalytic activity of graphene/titania via electronic structure modification

Authors:Dario De Angelis, Francesco Presel, Naila Jabeen, Luca Bignardi, Daniel Lizzit, Paolo Lacovig, Silvano Lizzit, Tiziano Montini, Paolo Fornasiero, Dario Alfè, Alessandro Baraldi

Abstract: A two-dimensional layer of oxide reveals itself as a essential element to drive the photocatalytic activity in a nanostructured hybrid material, which combines high-quality epitaxial graphene and titanium dioxide nanoparticles. In particular, it has been revealed that the addition of a 2D Ti oxide layer sandwiched between graphene and metal induces a p-doping of graphene and a consistent shift in the Ti d states. These modifications induced by the interfacial oxide layer induce a reduction of the probability of charge carrier recombination and enhance the photocatalytic activity of the heterostructure. This is indicative of a capital role played by thin oxide films in fine-tuning the properties of heterostructures based on graphene and pave the way to new combinations of graphene/oxides for photocatalysis-oriented applications.

4.Electron density control in tungsten diselenide monolayers via photochlorination

Authors:E. Katsipoulaki, G. Vailakis, I. Demeridou, D. Karfaridis, P. Patsalas, K. Watanabe, T. Taniguchi, I. Paradisanos, G. Kopidakis, G. Kioseoglou, E. Stratakis

Abstract: Modulation of the Fermi level using an ultraviolet (UV)-assisted photochemical method is demonstrated in tungsten diselenide monolayers. Systematic shifts and relative intensities between charged and neutral exciton species indicate a progressive and controllable decrease of the electron density and switch tungsten diselenide from n-type to a p-type semiconductor. The presence of chlorine in the 2D crystal shifts the Fermi level closer to the valence band while the effect can be only partially reversible via continuous wave laser rastering process. The presence of chlorine species in the lattice is validated by X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations predict that adsorption of chlorine on the selenium vacancy sites leads to p-type doping. The results of our study indicate that photochemical techniques have the potential to enhance the performance of various 2D materials, making them suitable for potential applications in optoelectronics.

5.Reactive pulsed direct current magnetron sputtering deposition of semiconducting yttrium oxide thin film in ultralow oxygen atmosphere: A spectroscopic and structural investigation of growth dynamics

Authors:H. Arslan Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia, I. Aulika Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia, A. Sarakovskis Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia, L. Bikse Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia, M. Zubkins Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia, A. Azarov Department of Physics, Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway, J. Gabrusenoks Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia, J. Purans Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia

Abstract: An experimental investigation was conducted to explore spectroscopic and structural characterization of semiconducting yttrium oxide thin film deposited at 623 K (+/- 5K) utilizing reactive pulsed direct current magnetron sputtering. Based on the results obtained from both x-ray diffraction and transmission electron microscope measurements, yttrium monoxide is very likely formed in the transition region between {\beta}-Y2O3 and {\alpha}-Y2O3, and accompanied by the crystalline Y2O3. Resulting from either the low energy separation between 4d and 5s orbitals and/or different spin states of the corresponding orbitals' sublevels, the stability of monoxide is most presumably self-limited by the size of the crystal in thermodynamic terms. This behavior develops a distortion in the structure of the crystal compared to the metal oxide cubic structure and it also effectuates the arrangement in nanocrystalline/amorphous phase. In addition to this, spectroscopic ellipsometry denotes that the semiconducting yttrium oxide has the dominant, mostly amorphous, formation character over crystalline Y2O3. Our purpose, by means of the current findings, is to advance the understanding of formation kinetics/conditions of yttrium with an unusual valency (2+).

6.Polarization superposition of room-temperature polariton condensation

Authors:Yuta Moriyama, Takaya Inukai, Tsukasa Hirao, Yusuke Ueda, Shun Takahashi, Kenichi Yamashita

Abstract: A methodology for forming a qubit state is essential for quantum applications of room temperature polaritons. While polarization degree of freedom is expected as a possible means for this purpose, the coupling of linearly polarized polariton condensed states has been still a challenging issue. In this study, we show a polarization superposition of a polariton condensed states in an all-inorganic perovskite microcavity at room temperature. We realized the energy resonance of the two orthogonally polarized polariton modes with the same number of antinodes by exploiting the blue shift of the polariton condensed state. The polarization coupling between the condensed states results in a polarization switching in the polariton lasing emission. The orthorhombic crystal structure of the perovskite active layer and/or a slight off-axis orientation of the perovskite crystal axis from the normal direction of microcavity plane enable the interaction between the two orthogonally polarized states. These observations demonstrate a great promise of polariton as a room temperature qubit technology.

7.MAPbBr3 monocrystals under electron beam radiolysis and degradation revealed by cathodoluminescence spectroscopy

Authors:Yu. O. Kulanchikov, P. S. Vergeles, K. Konstantinova, A. R. Ishteev, D. S. Muratov, E. E. Yakimov, E. B. Yakimov, D. S. Saranin

Abstract: Study of the local optical properties using electron beam (e-beam) can provide a valuable information concerning the inspection of the materials quality, the presence of the different phase inclusions and defects. Halide perovskites have been shown to be highly sensitive to external stress conditions like ambient atmosphere, light, and heat. In this paper, the cathodoluminescence (CL) spectroscopy has been exploited to carry out the investigation of CH3NH3PbBr3 monocrystals under low energy electron beam irradiation. The CL spectra exhibited strong transformation with the increase of the irradiation dose and significant shifts of the peak maximums from 2.23 eV to >2.5 eV. Utilizing a larger e-beam energy (>20 keV) was found to be preferable to slow down the dynamics of the decomposition and corrosion. The mechanisms of the changes in MAPbBr3 properties after e-beam exposure and correlation to the in-depth distribution of deposited energy were discussed.

8.Vibrations and tunneling of strained nanoribbons at finite temperature

Authors:Paul Z. Hanakata, Sourav S. Bhabesh, David Yllanes, David R. Nelson, Mark J. Bowick

Abstract: Crystalline sheets (e.g., graphene and transition metal dichalcogenides) liberated from a substrate are a paradigm for materials at criticality because flexural phonons can fluctuate into the third dimension. Although studies of static critical behaviors (e.g., the scale-dependent elastic constants) are plentiful, investigations of dynamics remain limited. Here, we use molecular dynamics to study the time dependence of the midpoint (the height center-of-mass) of doubly clamped nanoribbons, as prototypical graphene resonators, under a wide range of temperature and strain conditions. By treating the ribbon midpoint as a Brownian particle confined to a nonlinear potential (which assumes a double-well shape beyond the buckling transition), we formulate an effective theory describing the ribbon's tunneling rate across the two wells and its oscillations inside a given well. We find that, for nanoribbbons compressed above the Euler buckling point and thermalized above a temperature at which the non-linear effects due to thermal fluctuations become significant, the exponential term (the ratio between energy barrier and temperature) depends only on the geometry, but not the temperature, unlike the usual Arrhenius behavior. Moreover, we find that the natural oscillation time for small strain shows a non-trivial scaling $\tau_{\rm o}\sim L_0^{\,z}T^{-\eta/4}$, with $L_0$ being the ribbon length, $z=2-\eta/2$ being the dynamic critical exponent, $\eta=0.8$ being the scaling exponent describing scale-dependent elastic constants, and $T$ being the temperature. These unusual scale- and temperature-dependent dynamics thus exhibit dynamic criticality and could be exploited in the development of graphene-based nanoactuators.

9.Thermodynamic properties for metal oxides from first-principles

Authors:Joakim Brorsson, Ivana Staničić, Jonatan Gastaldi, Tobias Mattison, Anders Hellman

Abstract: In this study, an efficient first-principles approach for calculating the thermodynamic properties of mixed metal oxides at high temperatures is demonstrated. More precisely, this procedure combines density functional theory and harmonic phonon calculations with tabulated thermochemical data to predict the heat capacity, formation energy, and entropy of important metal oxides. Alloy cluster expansions are, moreover, employed to represent phases that display chemical ordering as well as to calculate the configurational contribution to the specific heat capacity. The methodology can, therefore, be applied to compounds with vacancies and variable site occupancies. Results are, moreover, presented for a number of systems of high practical relevance: Fe-K-Ti-O, K-Mn-O, and Ca-Mn-O. In the case of ilmenite (FeTiO3), the agreement with experimental measurements is exceptionally good. When the generated data is used in multi-phase thermodynamic calculations to represent materials for which experimental data is not available, the predicted phase-diagrams for the K-Mn-O and K-Ti-O systems change dramatically. The demonstrated methodology is highly useful for obtaining approximate values on key thermodynamic properties in cases where experimental data is hard to obtain, inaccurate or missing.

10.Designing Mixed-Metal Electrocatalyst Systems for Photoelectrochemical Dinitrogen Activation

Authors:Manpreet Kaur, Marc Walker, Steven Hindmarsh, Charlotte Bolt, Stephen York, Yisong Han, Martin R. Lees, Katharina Brinkert

Abstract: Efficient artificial photosynthesis systems are currently realized as catalyst- and surfacefunctionalized photovoltaic tandem- and triple-junction devices, enabling photoelectrochemical (PEC) water oxidation while simultaneously recycling CO2 and generating hydrogen as a solar fuel for storable renewable energy. Although PEC systems also bear advantages for the activation of dinitrogen - such as a high system tunability with respect to the electrocatalyst integration and a directly controllable electron flux to the anchoring catalyst through the adjustability of incoming irradiation - only a few PEC devices have been developed and investigated for this purpose. We have developed a series of photoelectrodeposition procedures to deposit mixed-metal electrocatalyst nanostructures directly on the semiconductor surface for light-assisted dinitrogen activation. These electrocatalyst compositions containing Co, Mo and Ru in different atomic ratios follow previously made recommendations of metal compositions for dinitrogen reduction and exhibit different physical properties. XPS studies of the photoelectrode surfaces reveal that our electrocatalyst films are to a large degree nitrogen-free after their fabrication, which is generally difficult to achieve with traditional magnetron sputtering or e-beam evaporation techniques. Initial chronoamperometric measurements of the p-InP photoelectrode coated with the Co-Mo alloy electrocatalyst show higher photocurrent densities in the presence of N2(g) than in the presence of Ar at -0.09 V vs RHE. Indications of successful dinitrogen activation have also been found in consecutive XPS studies, where both, N 1s and Mo 3d spectra, reveal evidence of nitrogen-metal interactions.

11.Atomic-layer controlled THz Spintronic emission from Epitaxially grown Two dimensional PtSe$_2$/ferromagnet heterostructures

Authors:K. Abdukayumov, M. Mičica, F. Ibrahim, C. Vergnaud, A. Marty, J. -Y. Veuillen, P. Mallet, I. Gomes de Moraes, D. Dosenovic, A. Wright, J. Tignon, J. Mangeney, A. Ouerghi, V. Renard, F. Mesple, F. Bonell, H. Okuno, M. Chshiev, J. -M. George, H. Jaffrès, S. Dhillon, M. Jamet

Abstract: Terahertz (THz) Spintronic emitters based on ferromagnetic/metal junctions have become an important technology for the THz range, offering powerful and ultra-large spectral bandwidths. These developments have driven recent investigations of two-dimensional (2D) materials for new THz spintronic concepts. 2D materials, such as transition metal dichalcogenides (TMDs), are ideal platforms for SCC as they possess strong spin-orbit coupling (SOC) and reduced crystal symmetries. Moreover, SCC and the resulting THz emission can be tuned with the number of layers, electric field or strain. Here, epitaxially grown 1T-PtSe$_2$ and sputtered Ferromagnet (FM) heterostructures are presented as a novel THz emitter where the 1T crystal symmetry and strong SOC favor SCC. High quality of as-grown PtSe$_2$ layers is demonstrated and further FM deposition leaves the PtSe$_2$ unaffected, as evidenced with extensive characterization. Through this atomic growth control, the unique thickness dependent electronic structure of PtSe$_2$ allows the control of the THz emission by SCC. Indeed, we demonstrate the transition from the inverse Rashba-Edelstein effect in one monolayer to the inverse spin Hall effect in multilayers. This band structure flexibility makes PtSe$_2$ an ideal candidate as a THz spintronic 2D material and to explore the underlying mechanisms and engineering of the SCC for THz emission.

12.Multiple Types of Unconventional Quasiparticles in Chiral Crystal CsBe$_2$F$_5$

Authors:Xin-Yue Kang, Jin-Yang Li, Si Li

Abstract: Recently, unconventional topological quasiparticles have been attracting significant research interest in condensed matter physics. Here, based on first-principles calculations and symmetry analysis, we reveal the coexistence of multiple types of interesting unconventional topological quasiparticles in the phonon spectrum of chiral crystal CsBe$_2$F$_5$. Specifically, we identified eight entangled phonon bands in CsBe$_2$F$_5$, which gave rise to various unconventional topological quasiparticles, including the spin-1 Weyl point, the charge-2 Dirac point, the nodal surface, and the novel hourglass nodal loop. We demonstrate that these unconventional topological quasiparticles are protected by crystal symmetry. We show that there are two large Fermi arcs connecting projections of the bulk spin-1 Weyl point and charge-2 Dirac point on the (001) surface and across the entire surface Brillouin zone (BZ). Our work not only elucidate the intriguing topological properties of chiral crystals but also provides an excellent material platform for exploring the fascinating physics associated with multiple types of unconventional topological quasiparticles.

13.\emph{Ab initio} calculations of structural stability, thermodynamic and elastic properties of Ni, Pd, Rh, and Ir at high pressures

Authors:N. A. Smirnov

Abstract: The paper presents results of a comprehensive study from first principles into the properties of Ni, Pd, Rh, and Ir crystals under pressure. We calculated elastic constants, phonon spectra, isotherms, Hugoniots, sound velocities, relative structural stability, and phase diagrams. It is shown that in nickel and palladium under high pressures ($>$0.14 TPa) and temperatures ($>$4 kK), the body-centered cubic structure is thermodynamically most stable instead of the face-centered cubic one. Calculated results suggest that nickel under Earth-core conditions ($P$$\sim$0.3 TPa, $T$$\sim$6 kK) have a bcc structure. No structural changes were found to occur in Rh and Ir under pressures to 1 TPa at least. The paper also provides estimations for the pressure and temperature at which the metals of interest begin to melt under shock compression.

14.On radiation-free velocities of edge dislocations in arbitrary slip systems of anisotropic crystals

Authors:Daniel N. Blaschke, Ta Duong, Michael J. Demkowicz

Abstract: Transonic defect motion is of interest for high strain-rate plastic deformation as well as for crack propagation. Ever since Eshelby's 1949 prediction in the isotropic limit of a 'radiation-free' transonic velocity $v_\text{RF}=\sqrt{2}c_{\textrm{T}}$, where shock waves are absent, there has been speculation about the significance of radiation-free velocities for defect mobility. Here, we argue that they do not play any significant role in dislocation dynamics in metals, based on comparing theoretical predictions of radiation-free velocities for transonic edge dislocations with molecular dynamics simulations for two face-centered cubic (FCC) metals: Cu, which has no radiation-free states, and Ag, which does.

15.Local structure, thermodynamics, and melting curve of boron phosphide at high pressures by deep learning-driven ab initio simulations

Authors:N. M. Chtchelkatchev, R. E. Ryltsev, M. V. Magnitskaya, S. M. Gorbunov, K. A. Cherednichenko, V. L. Solozhenko, V. V. Brazhkin

Abstract: Boron phosphide (BP) is a (super)hard semiconductor constituted of light elements, which is promising for high demand applications at extreme conditions. The behavior of BP at high temperatures and pressures is of special interest but is also poorly understood because both experimental and conventional ab initio methods are restricted to studying refractory covalent materials. The use of machine learning interatomic potentials is a revolutionary trend that gives a unique opportunity for high-temperature study of materials with ab initio accuracy. We develop a deep machine learning potential (DP) for accurate atomistic simulations of solid and liquid phases of BP as well as their transformations near the melting line. Our DP provides quantitative agreement with experimental and ab initio molecular dynamics data for structural and dynamic properties. DP-based simulations reveal that at ambient pressure tetrahedrally bonded cubic BP crystal melts into an open structure consisting of two interpenetrating sub-networks of boron and phosphorous with different structures. Structure transformations of BP melts under compressing are reflected by the evolution of low-pressure tetrahedral coordination to high-pressure octahedral coordination. The main contributions to structural changes at low pressures are made by the evolution of medium-range order in B-subnetwork and at high pressures by the change of short-range order in P-sub-network. Such transformations exhibit an anomalous behavior of structural characteristics in the range of 12--15 GPa. Analysis of the results obtained raise open issues in developing machine learning potentials for covalent materials and stimulate further experimental and theoretical studies of melting behavior in BP.

16.Using Pressure to Unravel the Structure-Dynamic-Disorder Relationship in Metal Halide Perovskites

Authors:Kai Xu, Luis Pérez-Fidalgo, Bethan L. Charles, Mark T. Weller, M. Isabel Alonso, Alejandro R. Goñi

Abstract: The exceptional optoelectronic properties of metal halide perovskites (MHPs) are presumed to arise, at least in part, from the peculiar interplay between the inorganic metal-halide sublattice and the atomic or molecular cations enclosed in the cage voids. The latter can exhibit a roto-translative dynamics, which is shown here to be at the origin of the structural behavior of MHPs as a function of temperature, pressure and composition. The application of high hydrostatic pressure allows for unraveling the nature of the interaction between both sublattices, characterized by the simultaneous action of hydrogen bonding and steric hindrance. In particular, we find that under the conditions of unleashed cation dynamics, the key factor that determines the structural stability of MHPs is the repulsive steric interaction rather than hydrogen bonding. Taking as example the results from pressure and temperature-dependent photoluminescence and Raman experiments on MAPbBr$_3$ but also considering the pertinent MHP literature, we provide a general picture about the relationship between the crystal structure and the presence or absence of cationic dynamic disorder. The reason for the structural sequences observed in MHPs with increasing temperature, pressure, A-site cation size or decreasing halide ionic radius is found principally in the strengthening of the dynamic steric interaction with the increase of the dynamic disorder. In this way, we have deepened our fundamental understanding of MHPs; knowledge that could be coined to improve performance in future optoelectronic devices based on this promising class of semiconductors.

17.Structural Anisotropy in Sb Thin Films

Authors:Pradip Adhikari, Anuradha Wijesinghe, Anjali Rathore, Timothy Jinsoo Yoo, Gyehyeon Kim, Hyoungtaek Lee, Sinchul Yeom, Alessandro R. Mazza, Changhee Sohn, Hyeong-Ryeol Park, Mina Yoon, Matthew Brahlek, Honggyu Kim, Joon Sue Lee

Abstract: Sb thin films have attracted wide interests due to their tunable band structure, topological phases, and remarkable electronic properties. We successfully grow epitaxial Sb thin films on a closely lattice-matched GaSb(001) surface by molecular beam epitaxy. We find a novel anisotropic directional dependence of their structural, morphological, and electronic properties. The origin of the anisotropic features is elucidated using first-principles density functional theory (DFT) calculations. The growth regime of crystalline and amorphous Sb thin films was determined by mapping the surface reconstruction phase diagram of the GaSb(001) surface under Sb$_2$ flux, with confirmation of structural characterizations. Crystalline Sb thin films show a rhombohedral crystal structure along the rhombohedral (104) surface orientation parallel to the cubic (001) surface orientation of the GaSb substrate. At this coherent interface, Sb atoms are aligned with the GaSb lattice along the [1-10] crystallographic direction but are not aligned well along the [110] crystallographic direction, which results in anisotropic features in reflection high-energy electron diffraction patterns, surface morphology, and transport properties. Our DFT calculations show that the anisotropic features originate from the GaSb surface, where Sb atoms align with the Ga and Sb atoms on the reconstructed surface. The formation energy calculations confirm that the stability of the experimentally observed structures. Our results provide optimal film growth conditions for further studies of novel properties of Bi$_{1-x}$Sb$_x$ thin films with similar lattice parameters and an identical crystal structure as well as functional heterostructures of them with III-V semiconductor layers along the (001) surface orientation, supported by a theoretical understanding of the anisotropic film orientation.

1.Multi-Hill Strategy in Metadynamics for Interstitial Diffusion in Crystals

Authors:Kazuaki Toyoura

Abstract: We propose an efficient and general strategy of metadynamics (MetaD) for investigating interstitial diffusion in a crystal by exploiting crystallographic symmetry. Assuming complete ignorance of the diffusion phenomenon of interest, the three-dimensional coordinates of the interstitial atom with the periodic boundaries are chosen as the collective variables (CVs). Multiple potential hills are simultaneously deposited at all crystallographically-equivalent positions on the free energy surface (FES) defined in the CV space. As a result, the proposed multi-hill strategy highly accelerates atomic jumps in comparison with the single-hill strategy in the conventional MetaD. The key features are that the FES estimated from the final bias potential is exactly satisfied with the symmetry of the host crystal and that all elementary processes of interstitial diffusion are obtained by the single MetaD simulation without any prior knowledge on the diffusion mechanism. The high efficiency and efficacy of the multi-hill strategy are demonstrated, taking the proton diffusion in barium zirconate with the cubic perovskite structure as a model case.

2.The local alloy order in GeSn/Ge epitaxial layer

Authors:Agnieszka Anna Corley-Wiciak, Shunda Chen, Omar Concepción, Marvin Hartwig Zoellner, Detlev Grützmacher, Dan Buca, Tianshu Li, Giovanni Capellini, Davide Spirito

Abstract: The local ordering of alloys directly influences their electronic and optical properties. In this work, the atomic arrangement in optoelectronic-grade GeSn epitaxial layers featuring a Sn content in the 5-14% range is investigated. By using polarization-dependent Raman spectroscopy and density functional theory calculations, different local environments for Ge atoms, induced by the Sn atoms and their corresponding distortion of the atomic bond, are identified, giving rise to two spectral features at different energies. Furthermore, all the other observed vibrational modes are associated with a combination of Ge and Sn displacement. This analysis provides a valuable framework for advancing the understanding of the vibrational properties in (Si)GeSn alloys, particularly with regard to the impact of local ordering of the different atomic species.

3.Symmetry and nonlinearity of spin wave resonance excited by focused surface acoustic waves

Authors:Piyush J. Shah, Derek A. Bas, Abbass Hamadeh, Michael Wolf, Andrew Franson, Michael Newburger, Philipp Pirro, Mathias Weiler, Michael R. Page

Abstract: The use of a complex ferromagnetic system to manipulate GHz surface acoustic waves is a rich current topic under investigation, but the high-power nonlinear regime is under-explored. We introduce focused surface acoustic waves, which provide a way to access this regime with modest equipment. Symmetry of the magneto-acoustic interaction can be tuned by interdigitated transducer design which can introduce additional strain components. Here, we compare the impact of focused acoustic waves versus standard unidirectional acoustic waves in significantly enhancing the magnon-phonon coupling behavior. Analytical simulation results based on modified Landau-Lifshitz-Gilbert theory show good agreement with experimental findings. We also report nonlinear input power dependence of the transmission through the device. This experimental observation is supported by the micromagnetic simulation using mumax3 to model the nonlinear dependence. These results pave the way for extending the understanding and design of acoustic wave devices for exploration of acoustically driven spin wave resonance physics.

4.Foray into the topology of poly-bi-[8]-annulenylene

Authors:Varadharajan Muruganandam, Manas Sajjan, Sabre Kais

Abstract: Analyzing phase transitions using the inherent geometrical attributes of a system has garnered enormous interest over the past few decades. The usual candidate often used for investigation is graphene -- the most celebrated material among the family of tri co-ordinated graphed lattices. We show in this report that other inhabitants of the family demonstrate equally admirable structural and functional properties that at its core are controlled by their topology. Two interesting members of the family are Cylooctatrene(COT) and COT-based polymer: poly-bi-[8]-annulenylene both in one and two dimensions that have been investigated by polymer chemists over a period of 50 years for its possible application in batteries exploiting its conducting properties. A single COT unit is demonstrated herein to exhibit topological solitons at sites of a broken bond similar to an open one-dimensional Su-Schrieffer-Heeger (SSH) chain. We observe that Poly-bi-[8]-annulenylene in 1D mimics two coupled SSH chains in the weak coupling limit thereby showing the presence of topological edge modes. In the strong coupling limit, we investigate the different parameter values of our system for which we observe zero energy modes. Further, the application of an external magnetic field and its effects on the band-flattening of the energy bands has also been studied. In 2D, poly-bi-[8]-annulenylene forms a square-octagon lattice which upon breaking time-reversal symmetry goes into a topological phase forming noise-resilient edge modes. We hope our analysis would pave the way for synthesizing such topological materials and exploiting their properties for promising applications in optoelectronics, photovoltaics, and renewable energy sources.

5.Bridging Borophene and Metal Surfaces: Structural, Electronic, and Electron Transport Properties

Authors:Wanderlã L. Scopel, F. Crasto de Lima, Pedro H. Souza, José E. Padilha, Roberto H. Miwa

Abstract: Currently, solid interfaces composed of two-dimensional materials (2D) in contact with metal surfaces (m-surf) have been the subject of intense research, where the borophene bilayer (BBL) has been considered a prominent material for the development of electronic devices based on 2D platforms. In this work, we present a theoretical study of the energetic, structural, and electronic properties of the BBL/m-surf interface, with m-surf = Ag, Au, and Al (111) surfaces, and the electronic transport properties of BBL channels connected to the BBL/m-surf top contacts. We find that the bottom-most BBL layer becomes metalized, due to the orbital hybridization with the metal surface states, resulting in BBL/m-surf ohmic contacts, meanwhile, the inner and top-most boron layers kept their semiconducting character. The net charge transfers reveal that BBL has become $n$-type ($p$-type) doped for m-surf = Ag, and Al (= Au). A thorough structural characterization of the BBL/m-surf interface, using a series of simulations of the X-ray photoelectron spectra, shows that the formation of BBL/m-surf interface is characterized by a redshift of the B-$1s$ spectra. Further electronic transport results revealed the emergence of a Schottky barrier between 0.1 and 0.2\,eV between the BBL/m-surf contact and the BBL channels. We believe that our findings are timely, bringing important contributions to the applicability of borophene bilayers for developing 2D electronic devices.

1.Origin of the exotic electronic states in antiferromagnetic NdSb

Authors:Peng Li, Tongrui Li, Sen Liao, Zhipeng Cao, Rui Xu, Yuzhe Wang, Jianghao Yao, Shengtao Cui, Zhe Sun, Yilin Wang, Xiangang Wan, Juan Jiang, Donglai Feng

Abstract: Using angle resolved photoemission spectroscopy measurements and first principle calculations, we report that the possible unconventional 2q antiferromagnetic (AFM) order in NdSb can induce unusual modulation on its electronic structure. The obvious extra bands observed in the AFM phase of NdSb are well reproduced by theoretical calculations, in which the Fermi-arc-like structures and sharp extra bands are originated from the in-gap surface states. However, they are demonstrated to be topological trivial. By tuning the chemical potential, the AFM phase of NdSb would go through a topological phase transition, realizing a magnetic topological insulator phase. Hence, our study sheds new light on the rare earth monopnictides for searching unusual AFM structure and the potential of intrinsic magnetic topological materials.

2.Surface characterisation of template-synthesised multi-walled carbon nanotubes

Authors:M. -R. Babaa LCSM, E. Mcrae LCSM, Sandrine Delpeux CRMD, J. Ghanbaja LCSM, F. Valsaque LCSM, F. Béguin CRMD

Abstract: Physisorption studies and transmission electron microscopy have been used to characterise multi-walled carbon nanotubes (MWNTs) made by a template-synthesis technique. Microscopic investigations revealed formation of 'branched nanotubes' with significant irregularities in diameters and with structural defects on the external surfaces of the tubes. Krypton adsorption isotherms at 77 K were recorded; comparison of these isotherms with those obtained under the same conditions on well defined MWNTs made by the catalytic chemical vapour deposition (CCVD) technique is discussed in the light of the sample morphologies. The effect of annealing on the crystallinity of the surface is reported.

3.FEL stochastic spectroscopy revealing silicon bond softening dynamics

Authors:Dario De Angelis, Emiliano Principi, Filippo Bencivenga, Daniele Fausti, Laura Foglia, Yishay Klein, Michele Manfredda, Riccardo Mincigrucci, Angela Montanaro, Emanuele Pedersoli, Jacopo Stefano Pelli Cresi, Giovanni Perosa, Kevin C. Prince, Elia Razzoli, Sharon Shwartz, Alberto Simoncig, Simone Spampinati, Cristian Svetina, Jakub Szlachetko, Alok Tripathi, Ivan A. Vartanyants, Marco Zangrando, Flavio Capotondi

Abstract: Time-resolved X-ray Emission/Absorption Spectroscopy (Tr-XES/XAS) is an informative experimental tool sensitive to electronic dynamics in materials, widely exploited in diverse research fields. Typically, Tr-XES/XAS requires X-ray pulses with both a narrow bandwidth and sub-picosecond pulse duration, a combination that in principle finds its optimum with Fourier transform-limited pulses. In this work, we explore an alternative xperimental approach, capable of simultaneously retrieving information about unoccupied (XAS) and occupied (XES) states from the stochastic fluctuations of broadband extreme ultraviolet pulses of a free-electron laser. We used this method, in combination with singular value decomposition and Tikhonov regularization procedures, to determine the XAS/XES response from a crystalline silicon sample at the L2,3-edge, with an energy resolution of a few tens of meV. Finally, we combined this spectroscopic method with a pump-probe approach to measure structural and electronic dynamics of a silicon membrane. Tr-XAS/XES data obtained after photoexcitation with an optical laser pulse at 390 nm allowed us to observe perturbations of the band structure, which are compatible with the formation of the predicted precursor state of a non-thermal solid-liquid phase transition associated with a bond softening phenomenon.

4.On Crystal-Structure Matches in Solid-Solid Phase Transitions

Authors:Fang-Cheng Wang, Qi-Jun Ye, Yu-Cheng Zhu, Xin-Zheng Li

Abstract: The exploration of solid-solid phase transition (SSPT) suffers from the uncertainty of how two crystal structures match. We devised a theoretical framework to describe and classify crystal-structure matches (CSM). Such description fully exploits the translational and rotational symmetries and is independent of the choice of supercells. This is enabled by the use of the Hermite normal form, an analog of reduced echelon form for integer matrices. With its help, exhausting all CSMs is made possible, which goes beyond the conventional optimization schemes. As a demonstration, our enumeration algorithm unveils the long-sought concerted mechanisms in the martensitic transformation of steel accounting for the most commonly observed Kurdjumov-Sachs (KS) orientation relationship (OR) and the Nishiyama-Wassermann OR. Especially, the predominance of KS OR is explained. Given the unprecedented comprehensiveness and efficiency, our enumeration scheme provide a promising strategy for SSPT mechanism research.

5.Coaxial Ion Source : pressure dependence of gas flow and field ion emission

Authors:Djouher Bedrane CINaM, Arnaud Houel CINaM, Anne Delobbe CINaM, Mehdi Lagaize CINaM, Philippe Dumas CINaM, Stéphane Veesler CINaM, E. Salançon CINaM

Abstract: We investigated the pressure dependence of gas flow and field ion intensity of a coaxial ion source operating at room temperature over a wide pressure range, testing various gases and ionisation voltages. Flow conductance measurements taking into account the different gases' viscosity and molecular mass consistently exhibit a generic pattern. Three different flow regimes appear with increasing upstream pressure. Since the coaxial ion source supplies the gas locally, very near the apex of the tip where ionisation occurs, large ionisation currents can be obtained without degrading the propagation conditions of the beam. Compared with field ionisation in a partial pressure chamber, using the coaxial ion source increases the ion current a hundredfold for the same residual low pressure. We also show that the gas flow regime does not impact ionisation yield. Although a fuller characterisation remains to be performed, brightness reaches 3 x 10 11 A/m 2 /sr at 12kV extracting voltage. a)

6.Photo-Induced CO Desorption Dominates over Oxidation on Different O+CO Covered Ru(0001) Surfaces

Authors:Auguste Tetenoire, J. Iñaki Juaristi, Maite Alducin

Abstract: The photo-induced desorption and oxidation of CO on Ru(0001) is simulated using ab initio molecular dynamics with electronic friction that accounts for the non-equilibrated excited electrons and phonons. Different (O,CO) coverages are considered, the experimental room temperature coverage consisting in 0.5ML-O+0.25ML- CO (low coverage), the saturation coverage achieved experimentally at low temperatures (0.5ML-O+0.375ML-CO, intermediate coverage), and the equally mixed monolayer that is stable according to our calculations but not experimentally observed yet (0.5ML-O+0.5ML-CO, high coverage). The results of our simulations for the three coverages are consistent with femtosecond laser experiments showing that the CO photo-desorption largely dominates over CO photo-oxidation. These results cannot be explained in terms of the distinct activation energies calculated for the relaxed surfaces. Different (dynamical) factors such as the coupling to the laser-excited electrons and, more importantly, the interadsorbate energy exchange and the strong surface distortions induced in the more crowded surfaces are fundamental to understand the competition between these two processes under the extremely non-equilibrated conditions created by the laser.

7.Harnessing the magnetic proximity effect: induced spin polarization in Ni/Si interfaces

Authors:Simone Laterza, Antonio Caretta, Richa Bhardwaj, Paolo Moras, Nicola Zema, Roberto Flammini, Marco Malvestuto

Abstract: The investigation of the properties of metal-semiconductor interfaces has gained significant attention due to the unique features that emerge from the combination of both metal and semiconductor attributes. In this report, the magnetic properties of Ni/Si interfaces utilizing X-ray magnetic circular dichroism (XMCD) spectroscopy at the Ni and Si edges have been studied. This approach allows to distinguish unambiguously the local magnetism on Ni and Si via individual core-level excitations. Two samples with different semiconductor dopings were investigated using both total electron yield (TEY) and reflectivity configurations. The experimental results uncovered magnetization at equilibrium in both the metallic layer and in the proximal layer of the semiconductor substrate, implying the presence of induced spin polarization in Si at equilibrium, possibly arising from the depletion layer region. These results hold significant value in the field of spintronics, as similar systems have been demonstrated to generate spin injection through optical medium, opening a new pathway for next generation nonvolatile high speed devices.

8.Turing Pattern and Chemical Medium-Range Order of Metallic glasses

Authors:Song Ling Liu Institute of Physics, Chinese Academy of Sciences, Beijing, China School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, China, Xin Yu Luo Institute of Physics, Chinese Academy of Sciences, Beijing, China School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, China, Jing Shan Cao Institute of Physics, Chinese Academy of Sciences, Beijing, China School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, China, Zhao Yuan Liu Shandong Computer Science Center, Bei Bei Xu Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China, Yong Hao Sun Institute of Physics, Chinese Academy of Sciences, Beijing, China School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, China Songshan Lake Materials Lab, Dongguan, Guangdong, China, Weihua Wang Institute of Physics, Chinese Academy of Sciences, Beijing, China School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, China Songshan Lake Materials Lab, Dongguan, Guangdong, China

Abstract: The formation of bulk metallic glass requires the constituent elements to have a negative heat of mixing but has no restrictions on its magnitude. An understanding of this issue is lacking due to the absence of a valid method for describing chemical ordering of metallic glasses. For example, the radial distribution function is ineffective in identifying the elemental preferences of packed atoms. Here, we show that using molecular-dynamics simulation, the chemical medium-range ordering of liquid alloys can be evaluated from persistent homology. This inherently arising chemical medium-range order in metallic glasses is exclusively regulated by the activation and inhibition of the constituent components, making the topology of metallic glasses a Turing pattern. The connecting schemes of atoms of the same species form three distinct regions, reflecting different correlations at the short and medium length scales, while the difference in the schemes corresponds to chemical ordering. By changing the elemental types, it is demonstrated that the chemical medium-range order strongly depends on the relative depth of the interatomic-potential wells. The study separates metallic glasses from crystals under the condition of negative heat of mixing by emphasizing their fundamental difference in interatomic potentials.

9.Magneto-optical Properties of Reduced Titania Probed by First-principles Calculations: Polarons

Authors:C. Echeverria-Arrondo, H. Raebiger, J. Perez-Conde, C. Gomez-Polo, A. Ayuela

Abstract: The magneto-optical properties of titanium dioxide systems are related to the presence of impurity states in the band gap due to oxygen vacancies. To understand about the interplay between localized electrons and structural distortions at the vacancy sites and the magneto-optical properties, we employ a self-interaction corrected density functional theory method to calculate bulk and small nanoparticles of rutile, anatase, and brookite titania. Our computations reveal bipolaron configurations associated to an oxygen vacancy with optical transition levels in the band gap. The ground state for these bipolarons is a spin-triplet state in bulk rutile TiO2 and also in the nanoparticles independently of the crystal phase, a result which may support the idea of oxygen vacancies as a source of magnetism in this material. The ground state for bipolarons in bulk anatase TiO2 is however a spin-singlet state, different from the spin-triplet configuration reported in a previous work based on hybrid functionals.

10.Coexistence of multiple interfacial states at heterogeneous solid/liquid interface

Authors:Jiaojiao Liu, Hongtao Liang, Jinfu Li, Brian B. Laird, and Yang Y

Abstract: The growing trend towards engineering interfacial complexion (or phase) transitions has been seen in the grain boundary and solid surface systems.Meanwhile, little attention has been paid to the chemically heterogeneous solid/liquid interfaces. In this work, novel in-plane multi-interfacial states coexist within the Cu(111)/Pb(l) interface at a temperature just above the Pb freezing point is uncovered using atomistic simulations.Four monolayer interfacial states, i.e., two CuPb alloy liquids and two pre-freezing Pb solids, are observed coexisting within two interfacial layers sandwiched between the bulk solid Cu and bulk liquid Pb. Through computing the spatial variations of various properties along the direction normal to the in-plane solid-liquid boundary lines for both interfacial layers, a rich and varied picture depicting the inhomogeneity and anisotropy in the mechanical, thermodynamical, and dynamical properties is presented. The bulk values extracted from the in-plane profiles suggest that each interfacial state examined has distinct equilibrium values from each other and significantly deviates from those of the bulk solid and liquid phases, and indicate that the complexion (or phase) diagrams for the Cu(111)/Pb(l) interface bears a resemblance to that of the eutectic binary alloy systems, instead of the monotectic phase diagram for the bulk CuPb alloy. The reported data could support the development of interfacial complexion (or phase) diagrams and interfacial phase rules and provide a new guide for regulating heterogeneous nucleation and wetting processes.

11.Muonium state exchange dynamics in n-type Gallium Arsenide

Authors:K. Yokoyama, J. S. Lord, P. W. Mengyan, M. R. Goeks, R. L. Lichti

Abstract: Muonium (Mu), a pseudo-isotope atom of hydrogen with a positively charged muon at the place of the proton, can form in a wide range of semiconductor materials. They can appear in different states, depending on their charge state and microscopic site within a crystal lattice. After the Mu formation, they undergo interactions with free charge carriers, electronic spins, and other Mu sites, and form a dynamic network of state exchange. We identified the model of Mu dynamics in n-type Gallium Arsenide using the density matrix simulation and photoexcited muon spin spectroscopy technique. Fitting to the dark and illuminated $\mu$SR data provided transition rates between Mu states, which in turn showed the underlying mechanism of the $\mu$SR time spectra. Deduced capture/scattering cross sections of the Mu states reflected the microscopic dynamics of Mu. Illumination studies enable us to measure interactions between Mu and generated minority carriers, which are unavailable in dark measurements. The methodology we developed in this study can be applied to other semiconductor systems for a deeper microscopic understanding of the Mu state exchange dynamics.

12.Atomic-scale Mapping Unravel Anisotropic Dissolution Behaviors of Gibbsite Nanosheets

Authors:Xiaoxu Li, Qing Guo, Yatong Zhao, Ping Chen, Benjamin A Legg, Lili Liu, Chang Liu, Qian Chen, Zheming Wang, James J. De Yoreo, Carolyn I Pearce, Aurora E. Clark, Kevin M. Rosso, Xin Zhang

Abstract: This study examines the anisotropic dissolution of the basal plane gibbsite ({\gamma}-Al(OH)3) nanoplates in sodium hydroxide solution using in situ atomic force microscopy (AFM) and density functional theory (DFT) calculations. In the surface-reaction controlled regime, in situ AFM measurements reveal anisotropic dissolution of hillocks and etch pits on the gibbsite basal plane, with preferred dissolution directions alternating between layers. The mirror-symmetric pattern of dissolution preference between adjacent gibbsite aluminum hydroxide sheet, observed along the crystallographic a-c plane, results from the matching symmetry between the structures of the adjacent (001) and (002) crystal planes. Consequently, the overall dissolution rate of gibbsite nanoplates exhibits crystallographic a-c plane symmetry, as the rate of parallel steps is governed by the slower ones. DFT calculations suggest that the anisotropic dissolution is partially due to the orientation and strength of Al-OH-Al linkages pair within gibbsite surface structure. These findings offer a comprehensive understanding of anisotropic dissolution behavior of gibbsite and illuminate the mechanisms behind preferential dissolution.

13.Energetic contributions to deformation twinning in magnesium

Authors:Enver Kapan, Sertan Alkan, C. Cahit Aydıner, Jeremy K. Mason

Abstract: Modeling deformation twin nucleation in magnesium has proven to be a challenging task. In particular, the absence of a heterogeneous twin nucleation model which provides accurate energetic descriptions for twin-related structures belies a need to more deeply understand twin energetics. To address this problem, molecular dynamics simulations are performed to follow the energetic evolution of $\{10\overline{1}2\}$ tension twin embryos nucleating from an asymmetrically-tilted grain boundary. The line, surface and volumetric terms associated with twin nucleation are identified. A micromechanical model is proposed where the stress field around the twin nucleus is estimated using the Eshelby formalism, and the contributions of the various twin-related structures to the total energy of the twin are evaluated. The reduction in the grain boundary energy arising from the change in character of the prior grain boundary is found to be able to offset the energy costs of the other interfaces. The defect structures bounding the stacking faults that form inside the twin are also found to possibly have significant energetic contributions. These results suggest that both of these effects could be critical considerations when predicting twin nucleation sites in magnesium.

14.Deep learning and genetic algorithm framework for tailoring mechanical properties via inverse microstructure optimization

Authors:Xiao Shang, Zhiying Liu, Jiahui Zhang, Tianyi Lyu, Yu Zou

Abstract: Materials-by-design has been historically challenging due to complex process-microstructure-property relations. Conventional analytical or simulation-based approaches suffer from low accuracy or long computational time and poor transferability, further limiting their applications in solving the inverse material design problem. Here, we establish a deep learning- and genetic algorithm-based framework that combines forward prediction and inverse exploration. Our framework provides an end-to-end solution for microstructure optimization to achieve application-specific mechanical properties of materials. In this study, we select the widely used Ti-6Al-4V to demonstrate the effectiveness of this framework by tailoring its microstructure to achieve various yield strength and elastic modulus across a large design space, while minimizing the stress concentration factor. Compared with conventional methods, our framework is efficient, versatile, and readily transferrable to other materials and properties. Paired with additive manufacturing's potential in controlling local microstructural features, our method has far-reaching potential for accelerating the development of application-specific, high-performing materials.

1.Interfacial origin of unconventional spin-orbit torque in Py/$γ-$IrMn$_{3}$

Authors:Akash Kumar, Pankhuri Gupta, Niru Chowdhury, Kacho Imtiyaz Ali Khan, Utkarsh Shashank, Surbhi Gupta, Yasuhiro Fukuma, Sujeet Chaudhary, Pranaba Kishor Muduli

Abstract: Angle-resolved spin-torque ferromagnetic resonance measurements are carried out in heterostructures consisting of Py (Ni$_{81}$Fe$_{19}$) and a noncollinear antiferromagnetic quantum material $\gamma-$IrMn$_{3}$. The structural characterization reveals that $\gamma-$IrMn$_{3}$ is polycrystalline in nature. A large exchange bias of 158~Oe is found in Py/$\gamma-$IrMn$_{3}$ at room temperature, while $\gamma-$IrMn$_{3}$/Py and Py/Cu/$\gamma-$IrMn$_{3}$ exhibited no exchange bias. Regardless of the exchange bias and stacking sequence, we observe a substantial unconventional out-of-plane anti-damping torque when $\gamma-$IrMn$_{3}$ is in direct contact with Py. The magnitude of the out-of-plane spin-orbit torque efficiency is found to be twice as large as the in-plane spin-orbit torque efficiency. The unconventional spin-orbit torque vanishes when a Cu spacer is introduced between Py and $\gamma-$IrMn$_{3}$, indicating that the unconventional spin-orbit torque in this system originates at the interface. These findings are important for realizing efficient antiferromagnet-based spintronic devices via interfacial engineering.

2.Charge density wave induced anomalous Hall effect in kagome ScV6Sn6

Authors:Changjiang Yi, Xiaolong Feng, Premakumar Yanda, Subhajit Roychowdhury, Claudia Felser, Chandra Shekhar

Abstract: Compounds with kagome lattice structure are known to exhibit Dirac cones, flat bands, and van Hove singularities, which host a number of versatile quantum phenomena, including an unusual anomalous Hall conductivity (AHC) in Co3Sn2S2 and AV3Sb5. Inspired by the intriguing properties of these compounds, we investigate the temperature-dependent electromagnetic properties of ScV6Sn6, a non-magnetic charge density wave (CDW) compound. We found AHC of the order of 104 with a 20 % of anomalous Hall angle, which are fully consistent with the CDW phase and disappear above the CDW transition. Moreover, only the topological Fermi surfaces containing cone and van Hove singularity are typically active and exhibit the Shubnikov de-Haas oscillations, contrary to those displayed by AV3Sb5, with an average of 70-fold increase in the momentum conserving time in the CDW phase. Combining these interesting physical properties with the CDW phase, ScV6Sn6 presents a unique material example of the versatile HfFe6Ge6 family and provides various promising opportunities to explore the series further.

3.Sub-100 nm β-Ga2O3 MOSFET with 100 GHz fMAX and >100 V breakdown

Authors:Chinmoy Nath Saha, Abhishek Vaidya, A F M Anhar Uddin Bhuiyan, Lingyu Meng, Hongping Zhao, Uttam Singisetti

Abstract: This letter reports a highly scaled 90 nm gate length beta-Ga2O3 T-gate MOSFET with no current collapse and record power gain cut off frequency (fMAX). The epitaxial stack of 60 nm thin channel MOSFET was grown by Molecular Beam Epitaxy (MBE) and highly doped (n++) contact regrowth was carried out by Metal Organic Chemical Vapour Deposition (MOCVD) in the source/drain region. Maximum on current (IDS, MAX) of 160 mA/mm and transconductance (gm) around 36 mS/mm was measured at VDS= 10 V for LSD= 1.5 micrometer channel length. Transconductance is limited by higher channel sheet resistance (Rsheet). We observed no current collapse for both drain and gate lag measurement even at higher VDG,Q quiescent bias points. This is the first report of Ga2O3 FET showing no current collapse without any external passivation. Breakdown voltage around 125 V was reported for LGD= 1.2 micrometer. We extracted 27 GHz current gain cut off frequency (fT) and 100 GHz fMAX for 20 V drain bias. fMAX value mentioned here is the highest for Ga2O3 and the first demonstration of 100 GHz operation. fT. VBR product of 3.375 THz.V has been calculated which is comparable with state-of-art GaN HEMT. This letter suggests that Ga2O3 can be a suitable candidate for X-band application.

4.Photo-accelerated hot carrier transfer at MoS2/WS2:a first-principles study

Authors:Zhi-Guo Tao, Guo-Jun Zhu, Weibin Chu, Xin-Gao Gong, Ji-Hui Yang

Abstract: Charge transfer in type-II heterostructures plays important roles in determining device performance for photovoltaic and photocatalytic applications. However, current theoretical studies of charge transfer process don't consider the effects of operating conditions such as illuminations and yield systemically larger interlayer transfer time of hot electrons in MoS2/WS2 compared to experimental results. Here in this work, we propose a general picture that, illumination can induce interfacial dipoles in type-II heterostructures, which can accelerate hot carrier transfer by reducing the energy difference between the electronic states in separate materials and enhancing the nonadiabatic couplings. Using the first-principles calculations and the ab-initio nonadiabatic molecular dynamics, we demonstrate this picture using MoS2/WS2 as a prototype. The calculated characteristic time for the interlayer transfer (60 fs) and the overall relaxation (700 fs) processes of hot electrons is in good agreement with the experiments. We further find that illumination mainly affects the ultrafast interlayer transfer process but has little effects on the relatively slow intralayer relaxation process. Therefore, the overall relaxation process of hot electrons has a saturated time with increased illumination strengths. The illumination-accelerated charge transfer is expected to universally exist in type-II heterostructures.

5.Large magnetocaloric effect in the kagome ferromagnet Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$

Authors:Akshata Magar, Somesh K, Vikram Singh, J. J. Abraham, Y. Senyk, A. Alfonsov, B. Büchner, V. Kataev, A. A. Tsirlin, R. Nath

Abstract: Single-crystal growth, magnetic properties, and magnetocaloric effect of the $S = 3/2$ kagome ferromagnet Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ (trigonal, space group: $P\bar{3}c1$) are reported. Magnetization data suggest dominant ferromagnetic intra-plane coupling with a weak anisotropy and the onset of ferromagnetic ordering at $T_{\rm C} \simeq 2.6$ K. Microscopic analysis reveals a very small ratio of interlayer to intralayer ferromagnetic couplings ($J_{\perp}/J \simeq 0.02$). Electron spin resonance data suggest the presence of short-range correlations above $T_{\rm C}$ and confirms quasi-two-dimensional character of the spin system. A large magnetocaloric effect characterized by isothermal entropy change of $-\Delta S_{\rm m}\simeq 31$ J kg$^{-1}$ K$^{-1}$ and adiabatic temperature change of $-\Delta T_{\rm ad}\simeq 9$ K upon a field sweep of 7 T is observed around $T_{\rm C}$. This leads to a large relative cooling power of $RCP \simeq 284$ J kg$^{-1}$. The large magnetocaloric effect, together with negligible hysteresis render Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ a promising material for magnetic refrigeration at low temperatures. The magnetocrystalline anisotropy constant $K \simeq -7.42 \times 10^4$ erg cm$^{-3}$ implies that the compound is an easy-plane type ferromagnet with the hard axis normal to the $ab$-plane, consistent with the magnetization data.

6.AutoEIS: automated Bayesian model selection and analysis for electrochemical impedance spectroscopy

Authors:Runze Zhang, Robert Black, Debashish Sur, Parisa Karimi, Kangming Li, Brian DeCost, John Scully, Jason Hattrick-Simpers

Abstract: Electrochemical Impedance Spectroscopy (EIS) is a powerful tool for electrochemical analysis; however, its data can be challenging to interpret. Here, we introduce a new open-source tool named AutoEIS that assists EIS analysis by automatically proposing statistically plausible equivalent circuit models (ECMs). AutoEIS does this without requiring an exhaustive mechanistic understanding of the electrochemical systems. We demonstrate the generalizability of AutoEIS by using it to analyze EIS datasets from three distinct electrochemical systems, including thin-film oxygen evolution reaction (OER) electrocatalysis, corrosion of self-healing multi-principal components alloys, and a carbon dioxide reduction electrolyzer device. In each case, AutoEIS identified competitive or in some cases superior ECMs to those recommended by experts and provided statistical indicators of the preferred solution. The results demonstrated AutoEIS's capability to facilitate EIS analysis without expert labels while diminishing user bias in a high-throughput manner. AutoEIS provides a generalized automated approach to facilitate EIS analysis spanning a broad suite of electrochemical applications with minimal prior knowledge of the system required. This tool holds great potential in improving the efficiency, accuracy, and ease of EIS analysis and thus creates an avenue to the widespread use of EIS in accelerating the development of new electrochemical materials and devices.

1.Rich structural polymorphism of monolayer C60 from cluster rotation

Authors:Xueao Li, Fan Zhang, Xuefei Wang, Weiwei Gao, Jijun Zhao

Abstract: The recent experimental fabrication of monolayer and few-layer C60 polymers paves the way for synthesizing two-dimensional cluster-assembled materials. Compared to atoms with the SO(3) symmetry, clusters as superatoms (e.g., C60) have an additional rotational degree of freedom, greatly enriching the phase spaces of superatom-assembled materials. Using first-principles calculations, we find the energy barriers of cluster rotation in quasi-tetragonal monolayer C60 structures are rather low (about 10 meV/atom). The small rotational energy barriers lead to a series of tetragonal C60 polymorphs with energies that are close to the experimental quasi-tetragonal (expt-qT) phase. Similarly, several dynamically stable quasi-hexagonal monolayer C60 structures are found to have energies within 7 meV/atom above the experimental quasi-hexagonal phase. Our calculations demonstrate photo-excited electron-hole pairs and electrostatic doping of electrons can effectively modulate the relative energies of quasi-tetragonal C60 polymorphs. Particularly, the unstable monolayer expt-qT phase becomes dynamically stable when it is electrostatically doped with electrons. In contrast, the relative energies between different quasi-hexagonal polymorphs are insensitive to electrostatic doping of electrons.

2.Investigating particle size effects on NMR spectra of ions diffusing in porous carbons through a mesoscopic model

Authors:Anagha Sasikumar, Céline Merlet

Abstract: Characterizing ion adsorption and diffusion in porous carbons is essential to understand the performance of such materials in a range of key technologies such as energy storage and capacitive deionisation. Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful technique to get insights in these systems thanks to its ability to distinguish between bulk and adsorbed species and to its sensitivity to dynamic phenomena. Nevertheless, a clear interpretation of the experimental results is sometimes rendered difficult by the various factors affecting NMR spectra. A mesoscopic model to predict NMR spectra of ions diffusing in carbon particles is adapted to include dynamic exchange between the intra-particle space and the bulk electrolyte surrounding the particle. A systematic study of the particle size effect on the NMR spectra for different distributions of magnetic environments in the porous carbons is conducted. The model demonstrates the importance of considering a range of magnetic environments, instead of a single chemical shift value corresponding to adsorbed species, and of including a range of exchange rates (between in and out of the particle), instead of a single timescale, to predict realistic NMR spectra. Depending on the pore size distribution of the carbon particle and the ratio between bulk and adsorbed species, both the NMR linewidth and peak positions can be largely influenced by the particle size.

3.High-Resolution Scanning Tunneling Microscope and its Adaptation for Local Thermopower Measurements in 2D Materials

Authors:Jose D. Bermúdez-Perez, Edwin Herrera-Vasco, Javier Casas-Salgado, Hector A. Castelblanco, Karen Vega-Bustos, Oscar L. Herrera-Sandoval, Hermann Suderow, Paula Giralgo-Gallo, Jose A. Galvis

Abstract: We present the design, fabrication and discuss the performance of a new combined high-resolution Scanning Tunneling and thermopower Microscope (STM/SThEM). We also describe the development of the electronic control, the user interface, the vacuum system, and arrangements to reduce acoustical noise and vibrations. We demonstrate the microscope performance with atomic-resolution topographic images of Highly oriented pyrolytic graphite (HOPG) and local thermopower measurements in the semimetal Bi2Te3 sample. Our system offers a tool to investigate the relationship between electronic structure and thermoelectric properties at the nanoscale.

4.Minority-spin conducting states in Fe substituted pyrite CoS$_2$

Authors:Anustup Mukherjee, Alaska Subedi

Abstract: There has been a longstanding debate whether the pyrite CoS$_2$ or its alloys with FeS$_2$ are half metallic. We argue using first principles calculations that there is a finite occupation of minority-spin states at the Fermi level throughout the series Co$_{1-x}$Fe$_x$S$_2$. Although the exchange-correlation functional influences the specifics of the electronic structure, we observe a similar trend with increasing Fe concentration in both LDA and GGA calculations. Specifically, even as band filling is decreased through Fe substitution, the lowest-lying conduction band in the minority-spin channel broadens such that these states keep getting lowered relative to the Fermi level, which is contrary to the expectations from a rigid band picture. Furthermore, the exchange splitting decreases as more Co atoms are replaced by Fe, and this again brings the minority-spin states closer to the Fermi level. These two mechanisms, in conjunction with the experimental observation that minority-spin bands cross the Fermi level in stoichiometric CoS$_2$, indicate that minority-spin charge carriers will always be present in Co$_{1-x}$Fe$_x$S$_2$.

5.Atomic Cluster Expansion for a General-Purpose Interatomic Potential of Magnesium

Authors:Eslam Ibrahim, Yury Lysogorskiy, Matous Mrovec, Ralf Drautz

Abstract: We present a general-purpose parameterization of the atomic cluster expansion (ACE) for magnesium. The ACE shows outstanding transferability over a broad range of atomic environments and captures physical properties of bulk as well as defective Mg phases in excellent agreement with reference first-principles calculations. We demonstrate the computational efficiency and the predictive power of ACE by calculating properties of extended defects and by evaluating the P-T phase diagram covering temperatures up to 3000 K and pressures up to 80 GPa. We compare the ACE predictions with those of other interatomic potentials, including the embedded-atom method, an angular-dependent potential, and a recently developed neural network potential. The comparison reveals that ACE is the only model that is able to predict correctly the phase diagram in close agreement with experimental observations.

6.X-ray Magnetic Circular Dichroism in Altermagnetic $α$-MnTe

Authors:A. Hariki, T. Yamaguchi, D. Kriegner, K. W. Edmonds, P. Wadley, S. S. Dhesi, G. Springholz, L. Šmejkal, K. Výborný, T. Jungwirth, J. Kuneš

Abstract: Altermagnetism is a recently identified magnetic symmetry class combining characteristics of conventional collinear ferromagnets and antiferromagnets, that were regarded as mutually exclusive, and enabling phenomena and functionalities unparalleled in either of the two traditional elementary magnetic classes. In this work we use symmetry and ab initio theory to explore X-ray magnetic circular dichroism (XMCD) in the altermagnetic class. Our results highlight the distinct phenomenology in altermagnets of this time-reversal symmetry breaking response, and its potential utility for element-specific spectroscopy and microscopy in altermagnets. As a representative material for our XMCD study we choose $\alpha$-MnTe with the compensated antiparallel magnetic order in which an anomalous Hall effect has been already demonstrated both in theory and experiment. The predicted magnitude of XMCD lies well within the resolution of existing experimental techniques.

7.Enhanced hydrogen-gas permeation through rippled graphene

Authors:Wenqi Xiong, Weiqing Zhou, Pengzhan Sun, Shengjun Yuan

Abstract: The penetration of atomic hydrogen through defect-free graphene was generally predicted to have a barrier of at least several eV, which is much higher than the 1 eV barrier measured for hydrogen-gas permeation through pristine graphene membranes. Herein, our density functional theory calculations show that ripples, which are ubiquitous in atomically thin crystals and mostly overlooked in the previous simulations, can significantly reduce the barriers for all steps constituting the mechanism of hydrogen-gas permeation through graphene membranes, including dissociation of hydrogen molecules, reconstruction of the dissociated hydrogen atoms and their flipping across graphene. Especially, the flipping barrier of hydrogen atoms from a cluster configuration is found to decrease rapidly down to <1 eV with increasing ripples' curvature. The estimated hydrogen permeation rates by fully considering the distribution of ripples with all realistic curvatures and the major reaction steps that occurred on them are quite close to the experimental measurements. Our work provides insights into the fundamental understanding of hydrogen-gas permeation through graphene membranes and emphasizes the importance of nanoscale non-flatness (ripples) in explaining many surface and transport phenomena (for example, functionalization, corrosion and separation) in graphene and other two-dimensional materials.

8.Machine learning for accelerated bandgap prediction in strain-engineered quaternary III-V semiconductors

Authors:Badal Mondal, Julia Westermayr, Ralf Tonner-Zech

Abstract: Quaternary III-V semiconductors are one of the major promising material classes in optoelectronics. The bandgap and its character, direct or indirect, are the most important fundamental properties determining the performance and characteristics of optoelectronic devices. Experimental approaches screening a large range of possible combinations of III- and V-elements with variations in composition and strain are impractical for every target application. We present a combination of accurate first-principles calculations and machine learning based approaches to predict the properties of the bandgap for quaternary III-V semiconductors. By learning bandgap magnitudes and their nature at density functional theory accuracy based solely on the composition and strain features of the materials as an input, we develop a computationally efficient yet highly accurate machine learning approach that can be applied to a large number of compositions and strain values. This allows for a computationally efficient prediction of a vast range of materials under different strains, offering the possibility for virtual screening of multinary III-V materials for optoelectronic applications.

9.Study the effect of scratching depth and ceramic-metal ratio on the scratch behavior of NbC/Nb Ceramic/Metal nano-laminates using molecular dynamics simulation and machine learning

Authors:Md Mesbah Uddin

Abstract: The tribological characteristics of NbC/Nb ceramic/metal nano-laminate (CMNLs) coatings were studied using molecular dynamics atomistic simulations of nano-indentation and nano-scratching by penetrating and moving a spherical indenter into the models. The effect of individual metallic or ceramic layer thickness and penetration depth on the scratching behavior of the NbC/Nb nanolaminates were investigated. The results showed generally the individual metallic and ceramic layer thickness play a significant role. However, some punctures were seen on the top ceramic layer of some model which can significantly after the scratching behavior by reducing the effect the individual metallic and ceramic layer thickness on the scratching behavior. The reason for being punctured models is the thickness of the top ceramic layer is too low that the indenter can easily puncture the ceramic layer instead of pushing the atoms of ceramic. The least thickness that can resist the indenter can be defined as a critical thickness which is dependent on the indenter size and penetration depth. In the later part of this paper, machine learning has been employed to reduce the computational cost and it is shown that the machine learning based model can predict the friction coefficient with R- squared value 0.958.

1.Ultra-high-density double-atom catalyst with spin moment as activity descriptor for oxygen reduction reaction

Authors:Peng Lv, Wenjing Lv, Donghai Wu, Gang Tang, Xunwang Yan, Zhansheng Lu, Dongwei Ma

Abstract: One of the great challenges facing atomically dispersed catalysts, including single-atom catalyst (SAC) and double-atom catalyst (DAC) is their ultra-low metal loading (typically less than 5 wt%), basically limiting the practical catalytic application, such as oxygen reduction reaction (ORR) crucial to hydrogen fuel cell and metal-air battery. Although some important progresses have been achieved on ultra-high-density (UHD) SACs, the reports on UHD-DACs with stable uniform dispersion is still lacking. Herein, based on the experimentally synthesized M2N6 motif (M = Sc-Zn), we theoretically demonstrated the existence of the UHD-DACs with the metal loading > 40 wt%, which were confirmed by systematic analysis of dynamic, thermal, mechanical, thermodynamic, and electrochemical stabilities. Furthermore, ORR activities of the UHD-DACs are comparable with or even better than those of the experimentally synthesized low-density (LD) counterparts, and the Fe2N6 and Co2N6 UHD-DACs locate at the peak of the activity volcano with ultra-low overpotentials of 0.31 and 0.33 V, respectively. Finally, spin magnetic moment of active center is found to be a catalytic descriptor for ORR on the DACs. Our work will stimulate the experimental exploration of the ultra-high-density DACs and provides the novel insight into the relationship between ORR activity of the DACs and their spin states.

2.Accelerated Screening of Ternary Chalcogenides for High-Performance Optoelectronic Materials

Authors:Chen Shen, Tianshu Li, Yixuan Zhang, Teng Long, Nuno Miguel Fortunato, Fei Liang, Mian Dai, Jiahong Shen, Chris Wolverton, Hongbin Zhang

Abstract: Chalcogenides, which refer to chalcogen anions, have attracted considerable attention in multiple fields of applications, such as optoelectronics, thermoelectrics, transparent contacts, and thin film transistors. In comparison to oxide counterparts, chalcogenides have demonstrated higher mobility and \textit{p}-type dopability, owing to larger orbital overlaps between metal-X covalent chemical bondings and higher-energy valence bands derived by p-orbitals. Despite the potential of chalcogenides, the number of successfully synthesized compounds remains relatively low compared to oxides, suggesting the presence of numerous unexplored chalcogenides with fascinating physical characteristics. In this study, we implemented a systematic high-throughput screening process combined with first-principles calculations on ternary chalcogenides using 34 crystal structure prototypes. We generated a computational material database containing over 400,000 compounds by exploiting the ion-substitution approach at different atomic sites with elements in the periodic table. The thermodynamic stabilities of the candidates were validated using the chalcogenides included in the Open Quantum Materials Database. Moreover, we trained a model based on Crystal Graph Convolutional Neural Networks to predict the thermodynamic stability of novel materials. Furthermore, we theoretically evaluated the electronic structures of the stable candidates using accurate hybrid functionals. A series of in-depth characteristics, including the carrier effective masses, electronic configuration, and photovoltaic conversion efficiency, was also investigated. Our work provides useful guidance for further experimental research in the synthesis and characterization of such chalcogenides as promising candidates, as well as charting the stability and optoelectronic performance of ternary chalcogenides.

3.Influence of high pressure on the remarkable itinerant electron behaviour in Y$_{0.7}$Er$_{0.3}$Fe$_2$D$_{4.2}$ compounds

Authors:Z. Arnold, O. Isnard, V. Paul-Boncour

Abstract: Monoclinic Y$_{0.7}$Er$_{0.3}$Fe$_2$D$_{4.2}$ compound exhibits unusual magnetic properties with different field induced magnetic transitions. The deuteride is ferrimagnetic at low temperature and the Er and Fe sublattices present magnetic transitions at different temperatures. The Er moments are ordered below T$_{Er}$=55 K, whereas the Fe moments remain ferromagnetically coupled up to T$_{M0}$ = 66 K. At T$_{M0}$ the Fe moments display a sharp ferromagnetic-antiferromagnetic transition (FM-AFM) through an itinerant electron metamagnetic (IEM) behaviour very sensitive to any volume change. Y$_{0.7}$Er$_{0.3}$Fe$_2$D$_{4.2}$ becomes paramagnetic above T$_N$=125 K. The pressure dependence of T$_{Er}$ and T$_{M0}$ have been extracted from magnetic measurements under hydrostatic pressure up to 0.49 GPa. Both temperatures decrease linearly upon applied pressure with dT$_{Er}$/dP=-126 and dTM0/dP=-140 K.GPa$^{-1}$ for a field of B=0.03 T. Both magnetic Er and ferromagnetic Fe order disappear at P=0.44(4) GPa. However, under a larger applied field B=5 T, dT$_{M0}$/dP=-156 K.GPa$^{-1}$ whereas dT$_{Er}$/dP=-134 K.GPa$^{-1}$ showing a weaker sensitivity to pressure and magnetic field. At 2 K the decrease of the saturation magnetization under pressure can be attributed to a reduction of the mean Er moment due to canting and/or crystal field effect. Above T$_{M0}$ the magnetization curves display a metamagnetic behaviour from AFM to FM state, which is also very sensitive to the applied pressure. The transition field B$_{trans}$, which increases linearly upon heating, is shifted to lower temperature upon applied pressure with dT=-17 K between 0 and 0.11 GPa. These results show a strong decoupling of the Er and Fe magnetic sublattices versus temperature, applied field and pressure.

4.Structural, Vibrational, and Electronic Behavior of Two GaGeTe Polymorphs under compression

Authors:Enrico Bandiello, Samuel Gallego-Parra, Akun Liang, J. A. Sans, Vanesa Cuenca-Gotor, Estelina Lora da Silva, Rosario Vilaplana, Plácida. Rodríguez-Hernández, Alfonso Muñoz, Daniel Diaz-Anichtchenko, Catalin Popescu, Frederico Gil Alabarse, Carlos Rudamas, Čestmír Drašar, Alfredo Segura, Daniel Errandonea, F. J. Manjón

Abstract: GaGeTe is a layered topological semimetal that has been recently found to exist in at least two different polytypes, $\alpha$-GaGeTe ($R\bar{3}m$) and $\beta$-GaGeTe ($P6_3 mc$). Here we report a joint experimental and theoretical study of the structural, vibrational, and electronic properties of these two polytypes at high pressure. Both polytypes show anisotropic compressibility and two phase transitions, above 7 and 15 GPa, respectively, as confirmed by XRD and Raman spectroscopy measurements. Although the nature of the high-pressure phases is not confirmed, comparison with other chalcogenides and total-energy calculations allow us to propose possible high-pressure phases for both polytypes with an increase in coordination for Ga and Ge atoms from 4 to 6. In particular, the simplification of the X-ray patterns for both polytypes above 15 GPa suggests a transition to a structure of relatively higher symmetry than the original one. This result is consistent with the rocksalt-like high-pressure phases observed in parent III-VI semiconductors, such as GaTe, GaSe, and InSe. Pressure-induced amorphization is observed upon pressure release. The electronic band structures of $\alpha$-GaGeTe and $\beta$-GaGeTe and their pressure dependence also show similarities to III-VI semiconductors, thus suggesting that the germanene-like sublayer induces a semimetallic character in both GaGeTe polytypes. Above 3 GPa, both polytypes lose their topological features, due to the opening of the direct band gap, while the reduction of the interlayer space increases the thermal conductivity at high pressure.

5.Study of novel properties of graphene-ZnO heterojunction interface using density functional theory

Authors:H. D. Etea, K. N. Nigussa

Abstract: Studies of the structural, electronic, and optical characteristics of the interfaces between graphene and ZnO polar surfaces is carried out using first-principles simulations. At the interface, a strong van der Waals force is present, and because of the different work functions of graphene and ZnO, charge transfer takes place. Graphene's superior conductivity is not impacted by its interaction with ZnO, since its Dirac point is unaffected despite its adsorption on ZnO. In hybrid systems, excited electrons with energies between 0 and 3 eV (above Fermi energy) are primarily accumulated on graphene. The calculations offer a theoretical justification for the successful operation of graphene / ZnO hybrid materials as photocatalysts and solar cells. ZnO semiconductor is found to be a suitable material with modest band gap, ($\sim$ 3 eV), having high transparency in visible region and a high optical conductivity.

6.A Hybrid-DFT Study of Intrinsic Point Defects in $MX_2$ ($M$=Mo, W; $X$=S, Se) Monolayers

Authors:Alaa Akkoush, Yair Litman, Mariana Rossi

Abstract: Defects can strongly influence the electronic, optical and mechanical properties of 2D materials, making defect stability under different thermodynamic conditions crucial for material-property engineering. In this paper, we present an account of the structural and electronic characteristics of point defects in monolayer transition metal dichalcogenides $MX_2$ with $M$=Mo/W and $X$= S/Se, calculated with density-functional theory using the hybrid HSE06 exchange correlation functional including many-body dispersion corrections. For the simulation of charged defects, we employ a charge compensation scheme based on the virtual crystal approximation (VCA). We relate the stability and the electronic structure of charged vacancy defects in monolayer MoS$_2$ to an explicit calculation of the S monovacancy in MoS$_2$ supported on Au(111), and find convincing indication that the defect is negatively charged. Moreover, we show that the finite-temperature vibrational contributions to the free energy of defect formation can change the stability transition between adatoms and monovacancies by 300--400 K. Finally, we probe defect vibrational properties by calculating a tip-enhanced Raman scattering image of a vibrational mode of a MoS$_2$ cluster with and without an S monovacancy.

7.Magnetic properties of Nd6Fe13Cu single crystals

Authors:Jianing Liu, Ruiwen Xie, Alex Aubert, Lukas Schäfer, Hongbin Zhang, Oliver Gutfleisch, Konstantin Skokov

Abstract: The understanding of coercivity mechanism in high performance Nd-Fe-B permanent magnets relies on the analysis of the magnetic properties of all phases present in the magnets. By adding Cu in such compounds, a new Nd6Fe13Cu grain boundary phase is formed, however, the magnetic properties of this phase and its role in the magnetic decoupling of the matrix Nd2Fe14B grains are still insufficiently studied. In this work, we have grown Nd6Fe13Cu single crystals by the reactive flux method and studied their magnetic properties in detail. It is observed that below the N\'eel temperature (TN = 410 K), the Nd6Fe13Cu is antiferromagnetic in zero magnetic field; whereas when a magnetic field is applied along the a-axis, a spin-flop transition occurs at approx. 6 T, indicating a strong competition between antiferromagnetic and ferromagnetic interactions in two Nd layers below and above the Cu layers. Our atomistic spin dynamics simulation confirms that an increase in temperature and/or magnetic field can significantly change the antiferromagnetic coupling between the two Nd layers below and above the Cu layers, which, in turn, is the reason for the observed spin-flop transition. These results suggest that the role of antiferromagnetic Nd6Fe13Cu grain boundary phase in the coercivity enhancement of Nd-Fe-B-Cu magnets is more complex than previously thought, mainly due to the competition between its antiferro- and ferro-magnetic exchange interactions.

8.Bridging Theory with Experiment: Digital Twins and Deep Learning Segmentation of Defects in Monolayer MX2 Phases

Authors:Addis S. Fuhr, Panchapakesan Ganesh, Rama K. Vasudevan, Bobby G. Sumpter

Abstract: Developing methods to understand and control defect formation in nanomaterials offers a promising route for materials discovery. Monolayer MX2 phases represent a particularly compelling case for defect engineering of nanomaterials due to the large variability in their physical properties as different defects are introduced into their structure. However, effective identification and quantification of defects remains a challenge even as high-throughput scanning tunneling electron microscopy (STEM) methods improve. This study highlights the benefits of employing first principles calculations to produce digital twins for training deep learning segmentation models for defect identification in monolayer MX2 phases. Around 600 defect structures were obtained using density functional theory calculations, with each monolayer MX2 structure being subjected to multislice simulations for the purpose of generating the digital twins. Several deep learning segmentation architectures were trained on this dataset, and their performances evaluated under a variety of conditions such as recognizing defects in the presence of unidentified impurities, beam damage, grain boundaries, and with reduced image quality from low electron doses. This digital twin approach allows benchmarking different deep learning architectures on a theory dataset, which enables the study of defect classification under a broad array of finely controlled conditions. It thus opens the door to resolving the underpinning physical reasons for model shortcomings, and potentially chart paths forward for automated discovery of materials defect phases in experiments.

9.Ultrahigh oxygen ion mobility in ferroelectric hafnia

Authors:Liyang Ma, Jing Wu, Tianyuan Zhu, Yiwei Huang, Qiyang Lu, Shi Liu

Abstract: Ferroelectrics and ionic conductors are important functional materials, each supporting a plethora of applications in information and energy technology. The underlying physics governing their functional properties is ionic motion, and yet studies of ferroelectrics and ionic conductors are often considered separate fields. Based on first-principles calculations and deep-learning-assisted large-scale molecular dynamics (MD) simulations, we report ferroelectric-switching-promoted oxygen ion transport in HfO2, a wide-band-gap insulator with both ferroelectricity and ionic conductivity. Applying a unidirectional bias can activate multiple switching pathways in ferroelectric HfO2, leading to polar-antipolar phase cycling that appears to contradict classical electrodynamics. This apparent conflict is resolved by the geometric-quantum-phase nature of electric polarization that carries no definite direction. Our MD simulations demonstrate bias-driven successive ferroelectric transitions facilitate ultrahigh oxygen ion mobility at moderate temperatures, highlighting the potential of combining ferroelectricity and ionic conductivity for the development of advanced materials and technologies.

10.Quantifying the magnetic interactions governing chiral spin textures using deep neural networks

Authors:Jian Feng Kong, Yuhua Ren, M. S. Nicholas Tey, Pin Ho, Khoong Hong Khoo, Xiaoye Chen, Anjan Soumyanarayanan

Abstract: The interplay of magnetic interactions in chiral multilayer films gives rise to nanoscale topological spin textures, which form attractive elements for next-generation computing. Quantifying these interactions requires several specialized, time-consuming, and resource-intensive experimental techniques. Imaging of ambient domain configurations presents a promising avenue for high-throughput extraction of the parent magnetic interactions. Here we present a machine learning-based approach to determine the key interactions -- symmetric exchange, chiral exchange, and anisotropy -- governing chiral domain phenomenology in multilayers. Our convolutional neural network model, trained and validated on over 10,000 domain images, achieved $R^2 > 0.85$ in predicting the parameters and independently learned physical interdependencies between them. When applied to microscopy data acquired across samples, our model-predicted parameter trends are consistent with independent experimental measurements. These results establish ML-driven techniques as valuable, high-throughput complements to conventional determination of magnetic interactions, and serve to accelerate materials and device development for nanoscale electronics.

11.Effective rectification of THz electromagnetic fields in a ferrimagnetic iron garnet

Authors:T. G. H. Blank, E. A. Mashkovich, K. A. Grishunin, C. Schippers, M. V. Logunov, B. Koopmans, A. K. Zvezdin, A. V. Kimel

Abstract: It is found that single-cycle THz electromagnetic fields efficiently excite a GHz spin resonance mode in ferrimagnetic Tm$_3$Fe$_5$O$_{12}$, despite the near absence of GHz spectral components in the exciting THz pulse. By analyzing how the efficiency of excitation depends on the orientation and strength of the THz electric field, we show that it can be explained in terms of the nonlinear THz inverse Cotton-Mouton effect. Here, the THz electric field gets effectively rectified and acts on the ferrimagnetic spins as a uni-polar effective magnetic field pulse. This interpretation is confirmed by a theoretical model based on the phenomenological analysis of the effective magnetic field, combined with the equations of motion derived from the effective Lagrangian for a ferrimagnet. Moreover, by using the outcome of two-dimensional THz spectroscopy, we conjecture a quantum-mechanical interpretation of the observed effect in terms of stimulated Raman scattering of THz photons by the crystal-field split f-f electronic transitions of Tm$^{3+}$.

12.Accelerating GW calculations through machine learned dielectric matrices

Authors:Mario G. Zauchner, Andrew Horsfield, Johannes Lischner

Abstract: The GW approach produces highly accurate quasiparticle energies, but its application to large systems is computationally challenging, which can be largely attributed to the difficulty in computing the inverse dielectric matrix. To address this challenge, we develop a machine learning approach to efficiently predict density-density response functions (DDRF) in materials. For this, an atomic decomposition of the DDRF is introduced as well as the neighbourhood density-matrix descriptor both of which transform in the same way under rotations. The resulting DDRFs are then used to evaluate quasiparticle energies via the GW approach. This technique is called the ML-GW approach. To assess the accuracy of this method, we apply it to hydrogenated silicon clusters and find that it reliably reproduces HOMO-LUMO gaps and quasiparticle energy levels. The accuracy of the predictions deteriorates when the approach is applied to larger clusters than those included in the training set. These advances pave the way towards GW calculations of complex systems, such as disordered materials, liquids, interfaces and nanoparticles.

1.Substitution of Lead with Tin Suppresses Ionic Transport in Halide Perovskite Optoelectronics

Authors:Krishanu Dey University of Cambridge, United Kingdom, Dibyajyoti Ghosh Indian Institute of Technology Delhi, India, Matthew Pilot University of Bath, United Kingdom, Samuel R Pering Loughborough University, United Kingdom, Bart Roose University of Cambridge, United Kingdom, Priyanka Deswal Indian Institute of Technology Delhi, India, Satyaprasad P Senanayak National Institute of Science Education and Research, India, Petra J Cameron University of Bath, United Kingdom, M Saiful Islam University of Oxford, United Kingdom, Samuel D Stranks University of Cambridge, United Kingdom

Abstract: Despite the rapid rise in the performance of a variety of perovskite optoelectronic devices with vertical charge transport, the effects of ion migration remain a common and longstanding Achilles heel limiting the long-term operational stability of lead halide perovskite devices. However, there is still limited understanding of the impact of tin (Sn) substitution on the ion dynamics of lead (Pb) halide perovskites. Here, we employ scan-rate-dependent current-voltage measurements on Pb and mixed Pb-Sn perovskite solar cells to show that short circuit current losses at lower scan rates, which can be traced to the presence of mobile ions, are present in both kinds of perovskites. To understand the kinetics of ion migration, we carry out scan-rate-dependent hysteresis analyses and temperature-dependent impedance spectroscopy measurements, which demonstrate suppressed ion migration in Pb-Sn devices compared to their Pb-only analogues. By linking these experimental observations to first-principles calculations on mixed Pb-Sn perovskites, we reveal the key role played by Sn vacancies in increasing the iodide ion migration barrier due to local structural distortions. These results highlight the beneficial effect of Sn substitution in mitigating undesirable ion migration in halide perovskites, with potential implications for future device development.

2.Band degeneracy, resonant level formation and low thermal conductivity in dilute In and Ga co-doped thermoelectric compound SnTe

Authors:Gaurav Jamwal, Ankit Kumar, Mohd Warish, Shruti Chakravarty, Saravanan Muthiah, Asokan Kandasami, Asad Niazi

Abstract: We report the effect of co-doping of In and Ga at low concentrations on the structural, electronic, and thermoelectric properties of SnTe based compositions $Sn_{1.03-2x}In_{x}Ga_{x}Te$ (x = 0, 0.01, 0.02, 0.04) prepared by the solid-state route and spark plasma sintering (SPS). All compositions formed in the fcc structure (Fm-3m) with no other impurity phase. The optical band gap increased with the co-doping, indicative of band convergence effects. First principle electronic structure calculations showed band convergence and the formation of resonant levels, due to Ga and In doping respectively. The carrier concentration increased on hole-doping by In and Ga ions while carrier mobility decreased due to impurity scattering. The resistivity increased with temperature, indicative of the degenerate semiconducting character of the compounds. The Seebeck coefficient of the doped samples increased linearly with temperature, reaching 85 - 95 ${\mu}$V/K at 783 K. Thermal conductivity decreased sharply with co-doping, and the lattice thermal conductivity dropped to 0.42 W$m^{-1}$ $K^{-1}$ above 750 K. The enhanced power factor and low lattice thermal conductivity on doping resulted in a maximum figure of merit ZT = 0.34 at 773 K, twice that of the pristine SnTe.

3.Disentangling nucleation and domain growth during a laser-induced phase transition

Authors:Maximilian Mattern, Jasmin Jarecki, Jon Anders Arregi, Vojtěch Uhlíř, Matthias Rössle, Matias Bargheer

Abstract: We use ultrafast x-ray diffraction and the time-resolved polar magneto-optical Kerr effect to study the laser-induced metamagnetic phase transition in two FeRh films with thicknesses below and above the optical penetration depth. In the thin film, we identify an intrinsic 8 ps timescale for the lightinduced nucleation of ferromagnetic domains in the antiferromagnetic material that is substantially slower than the speed of sound. For the inhomogeneously excited thicker film, we additionally identify kinetics of out-of-plane domain growth mediated by near-equilibrium heat transport, which we experimentally verify by comparing Kerr effect experiments in front- and backside excitation geometry.

4.Adaptative Diffraction Image Registration for 4D-STEM to optimize ACOM Pattern Matching

Authors:Nicolas Folastre, Junhao Cao, Gozde Oney, Sunkyu Park, Arash Jamali, Christian Masquelier, Laurence Croguennec, Muriel Veron, Edgar F. Rauch, Arnaud Demortière

Abstract: The technique known as 4D-STEM has recently emerged as a powerful tool for the local characterization of crystalline structures in materials, such as cathode materials for Li-ion batteries or perovskite materials for photovoltaics. However, the use of new detectors optimized for electron diffraction patterns and other advanced techniques requires constant adaptation of methodologies to address the challenges associated with crystalline materials. In this study, we present a novel image processing method to improve pattern matching in the determination of crystalline orientations and phases. Our approach uses sub-pixelar adaptative image processing to register and reconstruct electron diffraction signals in large 4D-STEM datasets. By using adaptive prominence and linear filters such as mean and gaussian blur, we are able to improve the quality of the diffraction pattern registration. The resulting data compression rate of 103 is well-suited for the era of big data and provides a significant enhancement in the performance of the entire ACOM data processing method. Our approach is evaluated using dedicated metrics, which demonstrate a high improvement in phase recognition. Our results demonstrate that this data preparation method not only enhances the quality of the resulting image but also boosts the confidence level in the analysis of the outcomes related to determining crystal orientation and phase. Additionally, it mitigates the impact of user bias that may occur during the application of the method through the manipulation of parameters.

5.From Early Theories of Dzyaloshinskii-Moriya Interactions in Metallic Systems to Today's Novel Roads

Authors:Albert Fert, Mairbek Chshiev, André Thiaville, Hongxin Yang

Abstract: Since the early 1960's, the discovery of Dzyaloshinskii-Moriya interaction (DMI) helped to explain the physical mechanisms behind certain magnetic phenomena, such as net moment in antiferromagnets, or enhanced anisotropy field from heavy metals impurity in dilute Cu:Mn alloy. Since the researchers unveil the key role that DMI plays in stabilizing chiral Neel type magnetic domain wall and magnetic skyrmions, the studies on DMI have received growing interest. Governed by spin-orbit coupling (SOC) and various types of inversion symmetry breaking (ISB) in magnetic systems, DMI drives the forming of distinct morphologies of magnetic skyrmions. Our aim is to briefly introduce the research history of DMI and its significance in the field of modern spintronics.

6.Quantitative three-dimensional local order analysis of nanomaterials through electron diffraction

Authors:Ella Mara Schmidt, Paul Benjamin Klar, Yasar Krysiak, Petr Svora, Andrew L. Goodwin, Lukas Palatinus

Abstract: Structure-property relationships in ordered materials have long been a core principle in materials design. However, the intentional introduction of disorder into materials provides structural flexibility and thus access to material properties that are not attainable in conventional, ordered materials. To understand disorder-property relationships, the disorder - i.e., the local ordering principles - must be quantified. Correlated disorder can be probed experimentally by diffuse scattering. The analysis is notoriously difficult, especially if only powder samples are available. Here, we combine the advantages of three-dimensional electron diffraction - a method that allows single crystal diffraction measurements on sub-micron sized crystals - and three-dimensional difference pair distribution function analysis (3D-{\Delta}PDF) to address this problem. 3D-{\Delta}PDFs visualise and quantify local deviations from the average structure and enable a straightforward interpretation of the single crystal diffuse scattering data in terms of a three-dimensional local order model. Comparison of the 3D-{\Delta}PDF from electron diffraction data with those obtained from neutron and x-ray experiments of yttria-stabilized zirconia demonstrates the reliability of the newly proposed approach.

7.Crystal Structure and Phonon Density of States of FeSi up to 120 GPa

Authors:Ravhi S Kumar, Han Liu, Quan Li, Yuming Xiao, Paul Chow, Yue Meng, Michael Y. Hu, Ercan Alp, Russell Hemley, Changfeng Chen, Andrew L Cornelius, Zachary Fisk

Abstract: The strongly correlated material FeSi exhibits several unusual thermal, magnetic, and structural properties under varying pressure-temperature (P-T) conditions. It is a potential thermoelectric alloy and a materials of several geological implications as a possible constituent at the Earth's core mantle boundary (CMB). The phase transition behavior and lattice dynamics of FeSi under different P-T conditions remain elusive. A previous theoretical work predicted a pressure induced B20-B2 transition at ambient temperature, yet the transition is only observed at high P-T conditions in the experiments. Furthermore, the closing of the electronic gap due to a dramatic renormalization of the electronic structure and phonon anomalies has been reported based on density function calculations. In this study we have performed high pressure powder diffraction and Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements up to 120 GPa to understand the phase stability and the lattice dynamics. Our study shows evidence for a nonhydrostatic stress induced B20-B2 transition in FeSi around 36 GPa for the first time. The Fe partial phonon density of states (PDOS) and thermal parameters were derived from NRIXS up to 120 GPa with the density function theoretical (DFT) calculations. These calculations further predict and are consistent with pressure-induced metallization and band gap closing around 12 GPa.

8.Fabrication of bulk delta-phase Zirconium Hydride from Zircaloy-4 for use as moderators in microreactors

Authors:D. Parkison, M. A. Tunes, T. J. Nizolek, T. A. Saleh, P. Hosemann, C. A. Kohnert

Abstract: The fabrication of bulk delta-phase Zirconium Hydride ($\delta$-ZrH) using Zircaloy-4 as a precursor is herein reported. Characterization using electron-microscopy methods indicate that the fabricated material is of a single-phase. Sn-rich segregation zones have been observed to form as a direct result of the hydriding process. These findings experimentally validate previous \textit{ab initio} calculations on the influence H incorporation in Zircaloy-4 constitutional elements such as Sn, Fe and Cr. The effect of hydriding and Sn segregation on pre-existing Zr(Fe,Cr)$_{2}$ Laves phases is also evaluated. Major implications on the development of moderators for use in microreactors within the nuclear industry are discussed.

1.Size dependent etching of nanodiamond seeds in the early stages of CVD diamond growth

Authors:R. Salerno, B. Pede, M. Mastellone, V. Serpente, V. Valentini, A. Bellucci, D. M. Trucchi, F. Domenici, M. Tomellini, R. Polini

Abstract: We present an experimental study on the etching of detonation nanodiamond (DND) seeds during typical microwave chemical vapor deposition (MWCVD) conditions leading to ultra-thin diamond film formation, which is fundamental for many technological applications. The temporal evolution of the surface density of seeds on Si(100) substrate has been assessed by scanning electron microscopy (SEM). The resulting kinetics have been explained in the framework of a model based on the effect of particle size, according to the Young-Laplace equation, on both chemical potential of carbon atoms in DND and activation energy of reaction. We found that seeds with size smaller than a critical radius, r*, are etched away while those greater than r* can grow. Finally, the model allows to estimate the rate coefficients for growth and etching from the experimental kinetics.

2.The Premartensite and Martensite in Fe50Rh50 System

Authors:Esmaeil Adabifiroozjaei Advanced Electron Microscopy, Institute of Material Science, Technical University of Darmstadt, 64287 Darmstadt, Germany, Fernando Maccari Functional Materials, Institute of Materials Science, Technical University Darmstadt, 64287 Darmstadt, Germany, Lukas Schaefer Functional Materials, Institute of Materials Science, Technical University Darmstadt, 64287 Darmstadt, Germany, Tianshu Jiang Advanced Electron Microscopy, Institute of Material Science, Technical University of Darmstadt, 64287 Darmstadt, Germany, Oscar Recalde-Benitez Advanced Electron Microscopy, Institute of Material Science, Technical University of Darmstadt, 64287 Darmstadt, Germany, Alisa Chirkova Functional Materials, Institute of Materials Science, Technical University Darmstadt, 64287 Darmstadt, Germany Bielefeld Institute for Applied Materials Research, Bielefeld University of Applied Sciences, D33619 Bielefeld, Germany, Navid Shayanfar Functional Materials, Institute of Materials Science, Technical University Darmstadt, 64287 Darmstadt, Germany, Imants Dirba Functional Materials, Institute of Materials Science, Technical University Darmstadt, 64287 Darmstadt, Germany, Nagaarjhuna A Kani Advanced Electron Microscopy, Institute of Material Science, Technical University of Darmstadt, 64287 Darmstadt, Germany Division of Research with Neutrons and Muons, Paul Scherrer Institute, Switzerland, Olga Shuleshova The Leibniz Institute for Solid State and Materials Research, 01069, Dresden, Germany, Robert Winkler Advanced Electron Microscopy, Institute of Material Science, Technical University of Darmstadt, 64287 Darmstadt, Germany, Alexander Zintler Advanced Electron Microscopy, Institute of Material Science, Technical University of Darmstadt, 64287 Darmstadt, Germany Karlsruhe Institute of Technology, Laboratory for Electron Microscopy, Ziyuan Rao Max-Planck-Institut fuer Eisenforschung, Duesseldorf 40237, Germany, Lukas Pfeuffer Functional Materials, Institute of Materials Science, Technical University Darmstadt, 64287 Darmstadt, Germany, Andras Kovacs Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons and Peter Grunberg Institute, Forschungszentrum Juelich, Julich 52425, Germany, Rafal E Dunin-Borkowski Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons and Peter Grunberg Institute, Forschungszentrum Juelich, Julich 52425, Germany, Konstantin Skokov Functional Materials, Institute of Materials Science, Technical University Darmstadt, 64287 Darmstadt, Germany, Baptiste Gault Max-Planck-Institut fuer Eisenforschung, Duesseldorf 40237, Germany Department of Materials, Royal School of Mines, Imperial College London, London, SW7 2AZ, UK, Markus Gruner Faculty of Physics and Center for Nanointegration Duisburg-Essen, Oliver Gutfleisch Functional Materials, Institute of Materials Science, Technical University Darmstadt, 64287 Darmstadt, Germany, Leopoldo Molina-Luna Advanced Electron Microscopy, Institute of Material Science, Technical University of Darmstadt, 64287 Darmstadt, Germany

Abstract: Metallic/intermetalic materials with BCC structures hold an intrinsic instability due to phonon softening along [110] dirrection, causing BCC to lower-symmetry phases transformation when the BCC structures are thermally or mechanically stressed. Fe50Rh50 binary system is one of the exceptional BCC structures (ordered-B2) that has not been yet showing such transformation upon application of thermal stress, although mechanical deformation results in B2 to disordered FCC (gamma) and L10 phases transformation. Here, a comprehensive transmission electron microscopy (TEM) study is conducted on thermally-stressed samples of Fe50Rh50 aged at water and liquid nitrogen from 1150 degree C and 1250 degree C. The results show that, samples quenched from 1150 degree C into water and liquid nitrogen show presence of 1/4{110} and 1/2{110} satellite reflections, the latter of which is expected from phonon dispersion curves obtained by density functional theory calculation. Therefore, it is believed that Fe50Rh50 maintains the B2 structure that is in premartensite state. Once Fe50Rh50 is quenched from 1250 degree C into liquid nitrogen, formation of two short-range ordered tetragonal phases with various c/a ratios (~1.15 and 1.4) is observed in line with phases formed from mechanically deformed (30%) sample. According to our observations, an accurate atomistic shear model ({110}<1-10>) is presented that describes the martensitic transformation of B2 to these tetragonal phases. These findings offer implications useful for understanding of magnetic and physical characteristics of metallic/intermetallic materials.

3.Exploring the interrelation between Urbach energy and Fano-type asymmetric Raman line shape in GO-hBN nanocomposites

Authors:Vidyotma Yadav, Tanuja mohanty

Abstract: This paper reports a combination of experimental and theoretical approaches to find a significant correlation between Urbach energy (Eu) and asymmetry parameter (q) of Raman mode in GO-hBN nanocomposite. The experiment involves liquid phase exfoliation synthesis of hexagonal boron nitride (hBN) and GO-hBN nanocomposite with varying hBN and graphene oxide (GO) ratios. We study the electron-phonon interaction (EPI) strength in the nanocomposites via UV-Vis absorption and Raman spectroscopic analysis. The induced disorders in the nanocomposites due to varying compositions of hBN and GO have been quantified by Eu. Simultaneously, the EPI contribution to the induced disorders is measured via UV-Vis absorption spectra and represented as Ee-p. The EPI impact is also observed in Raman phonon modes and quantified as an asymmetry parameter (q). The inverse of the asymmetry parameter provides electron-phonon interaction strength (Ee-p), i.e. Ee-p ~ 1/|q|. A lower value of q indicates more substantial interference between electronic and phononic scattering in the nanocomposites, thus justifying the presence of more disorders, which has also been quantified by Eu. A linear relationship has been observed between Eu and the proportional parameter (k), where k involves both asymmetry parameter q and intensity of specific Raman mode (I).

4.Stress and heat flux via automatic differentiation

Authors:Marcel F. Langer, J. Thorben Frank, Florian Knoop

Abstract: Machine-learning potentials provide computationally efficient and accurate approximations of the Born-Oppenheimer potential energy surface. This potential determines many materials properties and simulation techniques usually require its gradients, in particular forces and stress for molecular dynamics, and heat flux for thermal transport properties. Recently developed potentials feature high body order and can include equivariant semi-local interactions through message-passing mechanisms. Due to their complex functional forms, they rely on automatic differentiation (AD), overcoming the need for manual implementations or finite-difference schemes to evaluate gradients. This study demonstrates a unified AD approach to obtain forces, stress, and heat flux for such potentials, and provides a model-independent implementation. The method is tested on the Lennard-Jones potential, and then applied to predict cohesive properties and thermal conductivity of tin selenide using an equivariant message-passing neural network potential.

5.Theory of polarization textures in crystal supercells

Authors:Daniel Bennett, Wojciech J. Jankowski, Gaurav Chaudhary, Efthimios Kaxiras, Robert-Jan Slager

Abstract: Recently, topologically nontrivial polarization textures have been predicted and observed in nanoscale systems. While these polarization textures are interesting and promising in terms of applications, their topology in general is yet to be fully understood. For example, the relation between topological polarization structures and band topology has not been explored, and polar domain structures are typically considered in topologically trivial systems. In particular, the local polarization in a crystal supercell is not well-defined, and typically calculated using approximations which do not satisfy gauge invariance. Furthermore, local polarization in supercells is typically approximated using calculations involving smaller unit cells, meaning the connection to the electronic structure of the supercell is lost. In this work, we propose a definition of local polarization which is gauge invariant and can be calculated directly from a supercell without approximations. We show using first-principles calculations for commensurate bilayer hexagonal boron nitride that our expressions for local polarization give the correct result at the unit cell level, which is a first approximation to the local polarization in a moir\'e superlattice. We also illustrate using an effective model that the local polarization can be directly calculated in real space. Finally, we discuss the relation between polarization and band topology, for which it is essential to have a correct definition of polarization textures.

6.Crystal Thermal Transport in Altermagnetic RuO2

Authors:Xiaodong Zhou, Wanxiang Feng, Run-Wu Zhang, Libor Smejkal, Jairo Sinova, Yuriy Mokrousov, Yugui Yao

Abstract: We demonstrate the emergence of a pronounced thermal transport in the recently discovered class of magnetic materials-altermagents. From symmetry arguments and first principles calculations performed for the showcase altermagnet, RuO2, we uncover that crystal Nernst and crystal thermal Hall effects in this material are very large and strongly anisotropic with respect to the Neel vector. We find the large crystal thermal transport to originate from three sources of Berry's curvature in momentum space: the pseudo-nodal surfaces, the Weyl fermions due to crossings between well-separated bands, and the spin-flip ladder transitions, defined by transitions among very weakly spin-split states of similar dispersion crossing the Fermi surface. Moreover, we reveal that the anomalous thermal and electrical transport coefficients in RuO2 are linked by an extended Wiedemann-Franz law in a temperature range much wider than expected for conventional magnets. Our results suggest that altermagnets may assume a leading role in realizing concepts in spincaloritronics not achievable with ferromagnets or antiferromagnets.

7.On-Surface Carbon Nitride Growth from Polymerization of 2,5,8-Triazido-s-heptazine

Authors:Matthias Krinninger, Nicolas Bock, Sebastian Kaiser, Johanna Plansky, Tobias Bruhm, Felix Haag, Francesco Allegretti, Ueli Heiz, Klaus Koehler, Barbara A. J. Lechner, Friedrich Esch

Abstract: Carbon nitrides have recently come into focus for photo- and thermal catalysis, both as support materials for metal nanoparticles as well as photocatalysts themselves. While many approaches for the synthesis of three-dimensional carbon nitride materials are available, only top-down approaches by exfoliation of powders lead to thin film flakes of this inherently two-dimensional material. Here, we describe an in situ on-surface synthesis of monolayer 2D carbon nitride films, as a first step towards precise combination with other 2D materials. Starting with a single monomer precursor, we show that 2,5,8-triazido-s-heptazine (TAH) can be evaporated intact, deposited on a single crystalline Au(111) or graphite support, and activated via azide decomposition and subsequent coupling to form a covalent polyheptazine network. We demonstrate that the activation can occur in three pathways, via electrons (X-ray illumination), photons (UV illumination) and thermally. Our work paves the way to coat materials with extended carbon nitride networks which are, as we show, stable under ambient conditions.

8.Many-body phonon-ion conduction in solid electrolyte driven by THz modes

Authors:Kim H. Pham, Kiarash Gordiz, Jonathan M. Michelsen, Hanzhe Liu, Daniele Vivona, Yang Shao-Horn, Asegun Henry, Kimberly A. See, Scott K. Cushing

Abstract: Superionic conductivity, or conductivity that rivals or exceeds those of the liquid phase (0.01 S/cm), is predicted to be possible through couplings between the mobile ion and the phonon vibrations of the crystal lattice. However, few experimental techniques have directly probed the many-body phonon modes that enable superionic conductivity. In this work, we develop a laser-driven impedance technique to measure the relative contribution of specific phonon modes to ion migration in a solid-state electrolyte Li0.5La0.5TiO3 (LLTO). Through ab initio calculations, we find that a few collective phonon-ion modes, mostly TiO6 rocking modes in the <6 Terahertz (THz) range, provide more than 40% of the energy required for the ion hop in the LLTO lattice. Next, we experimentally measure the change in impedance of LLTO using electrochemical impedance spectroscopy (EIS) while driving the TiO6 octahedral rocking modes with THz radiation. In agreement with our ab initio theoretical calculations and molecular dynamics simulations, experimentally exciting these rocking modes decreases the measured impedance ten-fold compared to exciting the acoustic and optical phonon bath at similar populations or heating LLTO. The decreased impedance also persists longer than the driving light. Our newly developed experiments can quantify phonon-ion coupling in multiple classes of ion conductors and suggest the potential for metastable states for opto-ionic materials.

1.Subcycle control of valley-selective excitation via dynamical Franz-Keldysh effect in WSe$_2$ monolayer

Authors:Shunsuke Yamada, Kazuhiro Yabana, Tomohito Otobe

Abstract: This study performed first-principles calculations based on the time-dependent density functional theory to control the valley degree of freedom relating to the dynamical Franz-Keldysh effect (DFKE) in a monolayer of transition metal dichalcogenide. By mimicking the attosecond transient absorption spectroscopy, we performed numerical pump-probe experiments to observe DFKE around the $K$ or $K'$ valley in WSe$_2$ monolayer with a linearly-polarized pump field and a circularly-polarized probe pulse. We found that the circularly-polarized probe pulse with a given helicity can selectively observe the transient conductivity modulated by DFKE in each valley. The transient conductivity and excitation probability around each valley oscillate with the pump field frequency $\Omega$. The phases of the $\Omega$ oscillation for the $K$ and $K'$ valleys are opposite to each other. Furthermore, the pump-driven DFKE alters the absorption rate of WSe$_2$ monolayer and yields the valley-dependent $\Omega$ oscillation of the electron excitation induced by the pump plus probe field. With a simplified two-band model, we identified the $\Omega$ oscillation of the off-diagonal conductivity caused by the band asymmetry around the valleys as the physical mechanism responsible for the valley-selective DFKE.

2.Perpendicular magnetic anisotropy of an ultrathin Fe layer grown on NiO(001)

Authors:Soki Kobayashi, Hiroki Koizumi, Hideto Yanagihara, Jun Okabayashi, Takahiro Kondo, Takahide Kubota, Koki Takanashi, Yoshiaki Sonobe

Abstract: The magnetic anisotropy and magnetic interactions at the interface between Fe and NiO(001) were investigated. Depending on the growth conditions of the NiO(001) layers and the post-annealing temperature, the preferential magnetization direction of the ultrathin Fe layer grown on a NiO(001) layer changed from in-plane to a direction perpendicular to the film plane. The lattice constant of the NiO(001) layers parallel to the growth direction increased with O$_2$ flow rate, while that parallel to the in-plane were locked onto the MgO(001) substrate regardless of the growth conditions of the NiO layers. Moreover, perpendicular magnetization was observed only when the NiO layer was grown with O$_2$ flow rates higher than 2.0 sccm corresponding to oxygen-rich NiO. X-ray magnetic circular dichroism measurements revealed an enhancement in anisotropic orbital magnetic moments similar to the origin of perpendicular magnetic anisotropy at the Fe/MgO(001) interface. The interfacial magnetic anisotropy energies were 0.93 and 1.02 mJ/m$^2$ at room temperature and at 100 K, respectively, indicating less temperature dependence. In contrast, the coercivity $H_c$ exhibited a significant temperature dependence. Although no signature of exchange bias or unidirectional loop shift was observed, $H_c$ was strongly dependent on the NiO layer thickness, indicating that the exchange interaction at the interface between the ferromagnetic and antiferromagnetic layers was not negligible, despite the NiO(001) being a spin-compensated surface.

3.Crystalline Phase Effects on the Nonlinear Optical Response of MoS2 and WS2 Nanosheets

Authors:Michalis Stavrou, Nikolaos Chazapis, Eleni Nikoli, Raul Arenal, Nikos Tagmatarchis, Stelios Couris

Abstract: In the present work, some MoS2 and WS2 nanosheets were prepared and characterized. Depending on the preparation procedures, trigonal prismatic (2H) or octahedral (1T) coordination of the metal atoms were obtained, exhibiting metallic (1T) or semiconducting (2H) character. Both MoS2 and WS2 nanosheets were found exhibiting large nonlinear optical (NLO) response, strongly dependent on their metallic (1T) or semiconducting (2H) character. So, the semiconducting character 2H-MoS2 and 2H-WS2 exhibit positive nonlinear absorption and strong self-focusing, while their metallic character counterparts exhibit strong negative nonlinear absorption and important self-defocusing. In addition, the semiconducting MoS2 and WS2 were found exhibiting important and very broadband optical limiting action extended from 450 to 1750 nm. So, by selecting the crystalline phase of the nanosheets, i.e., their semiconduction/metallic character, their NLO response can be greatly modulated. The results of the present work demonstrate unambiguously that the control of the crystalline phase of MoS2 and WS2 provides an efficient strategy for 2D nanostructures with custom made NLO properties for specific optoelectronic and photonic applications.

4.Molybdenum diselenide-manganese porphyrin bifunctional electrocatalyst for hydrogen evolution reaction and selective hydrogen peroxide production

Authors:Antonia Kagkoura, Christina Stangel, Raul Arenal, Nikos Tagmatarchis

Abstract: Electrochemical reactions for hydrogen and hydrogen peroxide production are essential for energy conversion to diminish energy crisis, but still lack efficient electrocatalysts. Development of non\-noble metal bifunctional electrocatalysts for hydrogen evolution and 2e oxygen reduction reaction to ease reaction kinetics is a challenging task. Integration of single components by employing easy strategies provides a key\-step towards the realization of highly active electrocatalysts. In this vein, MoSe2 owns catalytic active sites and high specific surface area but suffers from insufficient conductivity and high catalytic performance that noble\-metals provide. Herein, MoSe2 was used as a platform for the incorporation of manganese metallated porphyrin. The developed hybrid, namely MoSe2\-MnP, by the initial metal\-ligand coordination and the subsequent grafting with MnP was fully characterized and electrochemically assessed. The bifunctional electrocatalyst lowered the overpotential toward hydrogen evolution, improved reaction kinetics and charge transfer processes and was extremely stable after 10000 ongoing cycles. Simultaneously, rotating ring disk electrode analysis showed that oxygen reduction proceeds through the 2e pathway for the selective production of hydrogen peroxide with a high yield of 97 percent. The new facile modification route can be applied in diverse transition metal dichalcogenides and will help the development of new advanced functional materials.

5.Discovery and construction of surface kagome electronic states induced by p-d electronic hybridization

Authors:Li Huang, Xianghua Kong, Qi Zheng, Yuqing Xing, Hui Chen, Yan Li, Zhixin Hu, Shiyu Zhu, Jingsi Qiao, Yu-Yang Zhang, Haixia Cheng, Zhihai Cheng, Xianggang Qiu, Enke Liu, Hechang Lei, Xiao Lin, Ziqiang Wang, Haitao Yang, Wei Ji, Hong-Jun Gao

Abstract: Kagome-lattice materials possess attractive properties for quantum computing applications, but their synthesis remains challenging. Herein, we show surface kagome electronic states (SKESs) on a Sn-terminated triangular Co3Sn2S2 surface, which are imprinted by vertical p-d electronic hybridization between the surface Sn (subsurface S) atoms and the buried Co kagome lattice network in the Co3Sn layer under the surface. Owing to the subsequent lateral hybridization of the Sn and S atoms in a corner-sharing manner, the kagome symmetry and topological electronic properties of the Co3Sn layer is proximate to the Sn surface. The SKESs and both hybridizations were verified via qPlus non-contact atomic force microscopy (nc-AFM) and density functional theory calculations. The construction of SKESs with tunable properties can be achieved by the atomic substitution of surface Sn (subsurface S) with other group III-V elements (Se or Te), which was demonstrated theoretically. This work exhibits the powerful capacity of nc-AFM in characterizing localized topological states and reveals the strategy for synthesis of large-area transition-metal-based kagome lattice materials using conventional surface deposition techniques.

6.Accelerating microstructure modelling via machine learning: a new method combining Autoencoder and ConvLSTM

Authors:Owais Ahmad, Naveen Kumar, Rajdip Mukherjee, Somnath Bhowmick

Abstract: Phase-field modeling is an elegant and versatile computation tool to predict microstructure evolution in materials in the mesoscale regime. However, these simulations require rigorous numerical solutions of differential equations, which are accurate but computationally expensive. To overcome this difficulty, we combine two popular machine learning techniques, autoencoder and convolutional long short-term memory (ConvLSTM), to accelerate the study of microstructural evolution without compromising the resolution of the microstructural representation. After training with phase-field generated microstructures of ten known compositions, the model can accurately predict the microstructure for the future nth frames based on previous m frames for an unknown composition. Replacing n phase-field steps with machine-learned microstructures can significantly accelerate the in silico study of microstructure evolution.

1.Ab-initio study of short-range ordering in vanadium-based disordered rocksalt structures

Authors:Zinab Jadidi, Julia H. Yang, Tina Chen, Luis Barroso-Luque, Gerbrand Ceder

Abstract: Disordered rocksalt Li-excess (DRX) compounds are attractive new cathode materials for Li-ion batteries as they contain resource-abundant metals and do not require the use of cobalt or nickel. Understanding the delithiation process and cation short-range ordering (SRO) in DRX compounds is essential to improving these promising cathode materials. Herein, we use first-principles calculations along with the cluster-expansion approach to model the disorder in DRX Li2-xVO3, 0 < x < 1. We discuss the SRO of Li in tetrahedral and octahedral sites, and the order in which Li delithiates and V oxidizes with respect to local environments. We reveal that the number of nearest-neighbor V dictates the order of delithiation from octahedral sites and that V are oxidized in a manner that minimizes the electrostatic interactions among V. Our results provide valuable insight for tailoring the performance of V-based DRX cathode materials in general by controlling the SRO features that reduce energy density.

2.High harmonic interferometer:For probing sub-laser-cycle electron dynamics in solids

Authors:K. Uchida, K. Tanaka

Abstract: High harmonic emissions from crystalline solids contain rich information on the dynamics of electrons driven by intense infrared laser fields and have been intensively studied owing to their potential use as a probe of microscopic electronic structures. Especially, the ability to measure the temporal response of high harmonics may allow us to investigate electron dynamics directly in quantum materials. However, this most essential aspect of high harmonic emissions has been challenging to measure. Here, we propose a simple solution for this problem: a high harmonic interferometer, where high harmonics are generated in each of the path of a Mach-Zehnder interferometer and an interferogram of them is captured. The high harmonic interferometer allows us to achieve a relative time resolution between the target and reference high harmonics of less than 150 attoseconds, which is fine enough to track sub-cycle dynamics of electrons in solids. By using high harmonic interferometrer, we succeeded in capturing the real time dynamics of Floquet states in WSe2, whose indirect signature had so far been caught only by time-averaged measurement. Our simple technique will open a door to attosecond electron dynamics in solids.

3.Investigation and field effect tuning of thermoelectric properties of SnSe2 flakes

Authors:I. Pallecchi, F. Caglieris, M. Ceccardi, N. Manca, D. Marre', L. Repetto, M. Schott, D. I. Bilc, S. Chaitoglou, A. Dimoulas, M. J. Verstraete

Abstract: The family of Van der Waals dichalcogenides (VdWDs) includes a large number of compositions and phases, exhibiting varied properties and functionalities. They have opened up a novel electronics of two-dimensional materials, characterized by higher integration and interfaces which are atomically sharper and cleaner than conventional electronics. Among these functionalities, some VdWDs possess remarkable thermoelectric properties. SnSe2 has been identified as a promising thermoelectric material on the basis of its estimated electronic and transport properties. In this work we carry out experimental meas-urements of the electric and thermoelectric properties of SnSe2 flakes. For a 30 micron thick SnSe2 flake at room temperature, we measure electron mobility of 40 cm^2 V^-1 s^-1, a carrier density of 4 x 10^18 cm^-3, a Seebeck coefficient S around -400 microV/K and thermoelectric power factor around 0.35 mW m^-1 K^-2. The comparison of experimental results with theoretical calculations shows fair agreement and indicates that the dominant carrier scattering mechanisms are polar optical phonons at room temperature and ionized im-purities below 50 K. In order to explore possible improvement of the thermoelectric properties, we carry out reversible electrostatic doping on a thinner flake, in a field effect setup. On this 75 nm thick SnSe2 flake, we measure a field effect variation of the Seebeck coefficient of up to 290 % at low temperature, and a corresponding variation of the thermoelectric power factor of up to 1050 %. We find that the power factor increases with the depletion of n-type charge carriers. Field effect control of thermoelectric transport opens perspectives for boosting energy harvesting and novel switching technologies based on two-dimensional materials.

4.Glass-like Cross-plane Thermal Conductivity of Kagome Metals RbV3Sb5 and CsV3Sb5

Authors:Yu Pang, Jinjin Liu, Zeyu Xiang, Xuanhui Fan, Jie Zhu, Zhiwei Wang, Yugui Yao, Xin Qian, Ronggui Yang

Abstract: This work reports the thermal conductivity of RbV3Sb5 and CsV3Sb5 with three-dimensional charge density wave phase transitions from 80 K to 400 K measured by pump-probe thermoreflectance techniques. The in-plane (basal plane) thermal conductivities are found moderate, i.e., 12 W/mK of RbV3Sb5 and 8.8 W/mK of CsV3Sb5 at 300 K. Low cross-plane (stacking direction) thermal conductivities are observed, with 0.72 W/mK of RbV3Sb5 and 0.49 W/mK of CsV3Sb5 at 300 K. A unique glass-like temperature dependence in the cross-plane thermal conductivity is observed, which decreases monotonically even lower than the Cahill-Pohl limit as the temperature decreases below the phase transition point TCDW. This temperature dependence is found to obey the hopping transport picture. In addition, a peak in cross-plane thermal conductivity is observed at TCDW as a fingerprint of the modulated structural distortion along the stacking direction.

5.Multilayer metamaterials with mixed ferromagnetic domain core and antiferromagnetic domain wall structure

Authors:Ruslan Salikhov, Fabian Samad, Sebastian Schneider, Darius Pohl, Bernd Rellinghaus, Benny Böhm, Rico Ehrler, Jürgen Lindner, Nikolai S. Kiselev, Olav Hellwig

Abstract: Magnetic nano-objects possess great potential for more efficient data processing, storage and neuromorphic type of applications. Using high perpendicular magnetic anisotropy synthetic antiferromagnets in the form of multilayer-based metamaterials we purposely reduce the antiferromagnetic (AF) interlayer exchange energy below the out-of-plane demagnetization energy, which controls the magnetic domain formation. As we show via macroscopic magnetometry as well as microscopic Lorentz transmission electron microscopy, in this unusual magnetic energy regime, it becomes possible to stabilize nanometer scale stripe and bubble textures consisting of ferromagnetic (FM) out-of-plane domain cores separated by AF in-plane Bloch-type domain walls. This unique coexistence of mixed FM/AF order on the nanometer scale opens so far unexplored perspectives in the architecture of magnetic domain landscapes as well as the design and functionality of individual magnetic textures, such as bubble domains with alternating chirality.

6.Anisotropic Electronic Structure of the Two-Dimensional Electron Gas at the AlOx/KTaO3(110) interface

Authors:E. A. Martínez, J. Dai, M. Tallarida, N. M. Nemes, F. Y. Bruno

Abstract: Oxide-based two-dimensional electron gases (2DEGs) have generated significant interest due to their potential for discovering novel physical properties. Among these, 2DEGs formed in KTaO3 stand out due to the recently discovered crystal face-dependent superconductivity and large Rashba splitting, both of which hold potential for future oxide electronics devices. In this work, angle-resolved photoemission spectroscopy is used to study the electronic structure of the 2DEG formed at the (110) surface of KTaO3 after deposition of a thin Al layer. Our experiments revealed a remarkable anisotropy in the orbital character of the electron-like dispersive bands, which form a Fermi surface consisting of two elliptical contours with their major axes perpendicular to each other. The measured electronic structure is used to constrain the modeling parameters of self-consistent tight-binding slab calculations of the band structure. In these calculations, an anisotropic Rashba splitting is found with a value as large as 4 meV at the Fermi level along the [-110] crystallographic direction. This large unconventional and anisotropic Rashba splitting is rationalized based on the orbital angular momentum formulation. These findings provide insights into the interpretation of spin-orbitronics experiments and help to constrain models for superconductivity in the KTO(110)-2DEG system.

7.First-principles Prediction of Potential Candidate Materials MCu$_3$X$_4$ (M = V, Nb, Ta; X = S, Se, Te) for Neuromorphic Computing

Authors:Baoxing Zhai, Ruiqing Cheng, Tianxing Wang, Li Liu, Lei Yin, Yao Wen, Hao Wang, Sheng Chang, Jun He

Abstract: Inspired by the neuro-synaptic frameworks in the human brain, neuromorphic computing is expected to overcome the bottleneck of traditional von-Neumann architecture and be used in artificial intelligence. Here, we predict a class of potential candidate materials, MCu$_3$X$_4$ (M = V, Nb, Ta; X = S, Se, Te), for neuromorphic computing applications through first-principles calculations based on density functional theory. We find that when MCu$_3$X$_4$ are inserted with Li atom, the systems would transform from semiconductors to metals due to the considerable electron filling [~0.8 electrons per formula unit (f.u.)] and still maintain well structural stability. Meanwhile, the inserted Li atom also has a low diffusion barrier (~0.6 eV/f.u.), which ensures the feasibility to control the insertion/extraction of Li by gate voltage. These results establish that the system can achieve the reversible switching between two stable memory states, i.e., high/low resistance state, indicating that it could potentially be used to design synaptic transistor to enable neuromorphic computing. Our work provides inspiration for advancing the search of candidate materials related to neuromorphic computing from the perspective of theoretical calculations.

8.Competing signatures of intersite and interlayer spin transfer in the ultrafast magnetization dynamics

Authors:Simon Häuser, Sebastian T. Weber, Christopher Seibel, Marius Weber, Laura Scheuer, Martin Anstett, Gregor Zinke, Philipp Pirro, Burkard Hillebrands, Hans C. Schneider, Bärbel Rethfeld, Benjamin Stadtmüller, Martin Aeschlimann

Abstract: Optically driven intersite and interlayer spin transfer are individually known as the fastest processes for manipulating the spin order of magnetic materials on the sub 100 fs time scale. However, their competing influence on the ultrafast magnetization dynamics remains unexplored. In our work, we show that optically induced intersite spin transfer (also known as OISTR) dominates the ultrafast magnetization dynamics of ferromagnetic alloys such as Permalloy (Ni80Fe20) only in the absence of interlayer spin transfer into a substrate. Once interlayer spin transfer is possible, the influence of OISTR is significantly reduced and interlayer spin transfer dominates the ultrafast magnetization dynamics. This provides a new approach to control the magnetization dynamics of alloys on extremely short time scales by fine-tuning the interlayer spin transfer.

9.Towards engineering the perfect defect in high-performing permanent magnets

Authors:S. Giron, N. Polin, E. Adabifiroozjaei, Y. Yang, A. Kovács, T. P. Almeida, D. Ohmer, K. Üstüner, M. Katter, I. A. Radulov, R. E. Dunin-Borkowski, M. Farle, K. Durst, H. Zhang, L. Alff, K. Ollefs, B. -X. Xu, O. Gutfleisch, L. Molina-Luna, B. Gault, K. P. Skokov

Abstract: Permanent magnets draw their properties from a complex interplay, across multiple length scales, of the composition and distribution of their constituting phases, that act as building blocks, each with their associated intrinsic properties. Gaining a fundamental understanding of these interactions is hence key to decipher the origins of their magnetic performance and facilitate the engineering of better-performing magnets, through unlocking the design of the "perfect defects" for ultimate pinning of magnetic domains. Here, we deployed advanced multiscale microscopy and microanalysis on a bulk Sm2(CoFeCuZr)17 pinning-type high-performance magnet with outstanding thermal and chemical stability. Making use of regions with different chemical compositions, we showcase how both a change in the composition and distribution of copper, along with the atomic arrangements enforce the pinning of magnetic domains, as imaged by nanoscale magnetic induction mapping. Micromagnetic simulations bridge the scales to provide an understanding of how these peculiarities of micro- and nanostructure change the hard magnetic behaviour of Sm2(CoFeCuZr)17 magnets. Unveiling the origins of the reduced coercivity allows us to propose an atomic-scale defect and chemistry manipulation strategy to define ways toward future hard magnets.

10.Controllable Topological Insulator Phases in Litharge-phase InBi Monolayer

Authors:Zhenyao Fang, Andrew M. Rappe

Abstract: Despite recent advances of layered square-net topological material models that possess ideal semimetallic electronic structures and promising potential in material applications, the identification of experimentally accessible two-dimensional square-net materials with related topological properties has proven challenging. Due to the highly tunable physical and topological properties of III-V semiconductors, we revisit the class of III-V materials and observe that the litharge-phase InBi is a layered square-net material and can be exfoliated into the InBi monolayer. We present a comprehensive first-principles study of the energy landscape of the InBi monolayer. We identify a paraelastic phase and three ferroelastic phases and study their topological properties. Specifically, we show that the paraelastic InBi monolayer is a trivial insulator due to the orbital-ordering-induced band inversion occurring between states with the same parity. Substituting one Bi atom per cell with another V-group element (N, P, As) or applying an electric field that breaks the inversion symmetry and changes the orbital onsite energy, the paraelastic InBi monolayer can be driven into the topological insulator phase. Furthermore, one of the ferroelastic phases of pure InBi, which can be obtained by gently straining the paraelastic phase, also possesses such topological insulating properties. These results provide several experimentally accessible routes to tune the nontrivial topology in the InBi monolayer, including creating heterostructures with piezoelectric or ferroelectric substrates and applying mechanical strain, making the InBi monolayer an ideal platform to study the interplay of reduced dimensionality, square-net chemical bonding networks, and band topology.

11.Interplay of electron-magnon scattering and spin-orbit induced electronic spin-flip fcattering in a two-band Stoner model

Authors:Félix Dusabirane, Kai Leckron, Baerbel Rethfeld, Hans Christian Schneider

Abstract: Magnons are one of the carriers of angular momentum that are involved in the ultrafast magnetization dynamics in ferromagnets, but their contribution to the electronic dynamics and their interplay with other scattering process that occur during ultrafast demagnetization has not yet been studied in the framework of a microscopic dynamical model. The present paper presents such an investigation of electronic scattering dynamics in itinerant ferromagnets at the level of Boltzmann scattering integrals for the magnon distributions and spin-dependent electron distributions. In addition to electron-magnon scattering, we include spin-conserving and effective Elliott-Yafet like spin-flip electron-electron scattering processes and the influence of phonons. In our model system, the creation or annihilation of magnons leads to transitions between two spin-split electronic bands with energy and momentum conservation. Due to the presence of spin-orbit coupling, Coulomb scattering transitions between these bands are also possible, and we describe them on an equal footing in terms of Boltzmann scattering integrals. For an instantaneous carrier excitation process we analyze the influence of both interaction processes on the magnon and spin-dependent electron dynamics, and show that their interplay gives rise to an efficient creation of magnons at higher energies and wave vectors accompanied by only a small increase of the electronic spin polarization. These results present a microscopic dynamical scenario that shows how non-equilibrium magnons may dominate the magnetic response of a ferromagnet on ultrafast timescales.

1.Thin film growth of the Weyl semimetal NbAs

Authors:Wilson Yanez, Yu-Sheng Huang, Supriya Ghosh, Saurav Islam, Emma Steinebronn, Anthony Richardella, K. Andre Mkhoyan, Nitin Samarth

Abstract: We report the synthesis and characterization of thin films of the Weyl semimetal NbAs grown on GaAs (100) and GaAs (111)B substrates. By choosing the appropriate substrate, we can stabilize the growth of NbAs in the (001) and (100) directions. We combine x-ray characterization with high-angle annular dark field scanning transmission electron microscopy to understand both the macroscopic and microscopic structure of the NbAs thin films. We show that these films are textured with domains that are tens of nanometers in size and that, on a macroscopic scale, are mostly aligned to a single crystalline direction. Finally, we describe electrical transport measurements that reveal similar behavior in films grown in both crystalline directions, namely carrier densities of $\sim 10^{21} - 10^{22} $

2.Giant Bulk Electro-photovoltaic Effect in Hetero-nodal-line Systems

Authors:Xiao Jiang, Lei Kang, Jianfeng Wang, Bing Huang

Abstract: Realization of giant and continuously tunable second-order photocurrent is desired for many nonlinear optical (NLO) and optoelectronic applications, which remains to be a great challenge. Here, based on a simple two-band model, we propose a concept of bulk electro-photovoltaic effect, that is, an out-of-plane external electric-field ($E_{ext}$) can continuously tune in-plane shift current along with its sign flip in a hetero-nodal-line (HNL) system. While strong linear optical transition around the nodal-loop may potentially generate giant shift current, an $E_{ext}$ can effectively control the radius of nodal-loop, which can continuously modulate the shift-vector components inside and outside nodal-loop holding opposite signs. This concept has been demonstrated in the HNL HSnN/MoS$_2$ system using first-principles calculations. The HSnN/MoS$_2$ hetero-bilayer not only can produce giant shift current with 1~2 magnitude order larger than other reported systems, but also can realize a giant bulk electro-photovoltaic effect. Our finding opens new routes to create and manipulate giant NLO responses in 2D materials.

3.Comparison of Raman imaging assessment methods in phase determination and stress analysis of zirconium oxide layer

Authors:K. Suchorab, M. Gaweda, L. Kurpaska

Abstract: This work describes Raman imaging and its data evaluation methods by using the softwares original features: built-in fitting function and K-means cluster analysis KMC followed by fitting in an external environment. For the first time, these methods were compared in terms of their principles, limitations, versatility, and process duration. The performed analysis showed the indispensability of Raman imaging in terms of phase distribution, phase content calculation, and stress determination. Zirconium oxide formed on different zirconium alloys under various oxidation conditions was selected as an exemplary material for this analysis. The reason for the material choice is that it is an excellent example of the application of this type of Raman analysis since both phase distribution and stress analysis in zirconium oxide are of crucial importance for the development of zirconium alloys, especially for nuclear applications. The juxtaposition of the results showed advantages and limitations of both procedures allowing a definition of the criteria for selecting the evaluation method for different applications.

4.Room Temperature Ferrimagnetism, Magnetodielectric and Exchange Bias Effect in CoFeRhO$_4$

Authors:P. Mohanty, N. Sharma, D. Singh, Y. Breard, D. Pelloquin, S. Marik, R. P. Singh

Abstract: Geometrically frustrated structures combined with competing exchange interactions that have different magnitudes are known ingredients for achieving exotic properties. Herein, we studied detailed structural, magnetic, thermal (specific heat), magneto-dielectric, and magnetic exchange bias properties of a mixed 3d - 4d spinel oxide with composition CoFeRhO$_4$. Detailed magnetization, heat capacity, and neutron powder diffraction studies (NPD) highlight long-range ferrimagnetic ordering with an onset at 355 K. The magnetic structure is established using a ferrimagnetic model (collinear-type) that has a propagation vector k = 0, 0, 0. The magneto-dielectric effect appears below the magnetic ordering temperature, and the exchange bias (EB) effect is observed in field cooled (FC) conditions below 355 K. The magneto-dielectric coupling in CoFeRhO$_4$ originates due to the frustration in the structure, collinear ferrimagnetic ordering, and uncompensated magnetic moments. The unidirectional anisotropy resulting from the uncompensated magnetic moments causes the room-temperature exchange bias effect. Remarkably, the appearance of technologically important properties (ferromagnetism, magnetodielectric effect, and EB) at room temperature in CoFeRhO$_4$ indicates its potential use in sensors or spintronics.

5.Magnetism and exchange bias properties in Ba$_{2}$ScRuO$_{6}$

Authors:Prachi Mohanty, Sourav Marik, R. P. Singh

Abstract: This paper presents structural, detailed magnetic, and exchange bias studies in polycrystalline Ba$_{2}$ScRuO$_{6}$ synthesized at ambient pressure. In contrast to its strontium analogue, this material crystallizes in a 6L hexagonal structure with the space group P$\overline{3}$m1. The Rietveld refinement using the room-temperature powder X-ray diffraction pattern suggests a Ru-Sc disorder in the structure. The temperature variation of the dc-electrical resistivity highlights a semiconducting behaviour with the electron conduction corresponding to the Mott 3D-VRH model. Detailed magnetization measurements show that Ba$_{2}$ScRuO$_{6}$ develops antiferromagnetic ordering at T$_{N}$ $\approx$ 9 K. Interestingly, below 9 K (T$_{N}$), the field cooled (FC) magnetic field variation of the magnetization curves highlights exchange bias effect in the sample. The exchange bias field reaches a maximum value of 1.24 kOe at 2 K. The exchange bias effect below the magnetic ordering temperature can be attributed to inhomogeneous magnetic correlations owing to the disorder in the structure.

6.Optical Properties and Electronic Structures of Intrinsic Gapped Metals: Inverse Materials Design Principles for Transparent Conductors

Authors:Muhammad Rizwan Khan, Harshan Reddy Gopidi, Oleksandr I. Malyi

Abstract: Traditional solid-state physics has long correlated the optical properties of materials with their electronic structures. However, recent discoveries of intrinsic gapped metals have challenged this classical view. Gapped metals possess electronic properties distinct from both metals and insulators, with a large concentration of free carriers without any intentional doping and an internal band gap. This unique electronic structure makes gapped metals potentially superior to materials designed by intentional doping of the wide band gap insulators. Despite their promising applications, such as transparent conductors, designing gapped metals for specific purposes remains challenging due to the lack of understanding of the correlation between their electronic band structures and optical properties. This study focuses on representative examples of gapped metals and demonstrates the cases of (i) gapped metals (e.g., CaN2) with strong intraband absorption in the visible range, (ii) gapped metals (e.g., SrNbO3) with strong interband absorption in the visible range, (iii) gapped metals (e.g., Sr5Nb5O17) that are potential transparent conductors. We explore the complexity of identifying potential gapped metals for transparent conductors and propose inverse materials design principles for discovering new-generation transparent conductors.

7.Defect emission and its dipole orientation in layered ternary Znln2S4 semiconductor

Authors:Rui Wang, Quan Liu, Sheng Dai, Chao-Ming Liu, Yue Liu, Zhao-Yuan Sun, Hui Li, Chang-Jin Zhang, Han Wang, Cheng-Yan Xu, Wen-Zhu Shao, Alfred J. Meixner, Dai Zhang, Yang Li, Liang Zhen

Abstract: Defect engineering is promising to tailor the physical properties of two-dimensional (2D) semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors have not been clarified. In this work, we thoroughly studied the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2S4 via versatile spectroscopic tools in combination with theoretical calculations. It has been demonstrated that the Zn-In anti-structural defects induce the formation of a series of donor and acceptor levels inside the bandgap, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair (DAP) in Znln2S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2S4 was directly revealed by back focal plane (BFP) imagining, where it presents obviously in-plane dipole orientation within a dozen layers thickness of Znln2S4. These unique features of defects in Znln2S4 including extrinsic absorption, rich recombination paths, gate tunability and in-plane dipole orientation will definitely benefit to the advanced orientation-functional optoelectronic applications.

8.Defects drive the tribocharging strength of PTFE

Authors:A. Ciniero, G. Fatti, M. Marsili, D. Dini, M. C. Righi

Abstract: If polytetrafluoroethylene (PTFE), commonly known as Teflon, is put into contact and rubbed against another material, almost surely it will be more effective than its counterpart in collecting negative charges. This simple, basic property is captured by the so called triboelectric series, where PTFE ranks extremely high, and that qualitatively orders materials in terms of their ability to electrostatically charge upon contact and rubbing. However, while classifying materials, the series does not provide an explanation of their triboelectric strength, besides a loose correlation with the workfunction. Indeed, despite being an extremely familiar process, known from centuries, tribocharging is still elusive and not fully understood. In this work we employ density functional theory to look for the origin of PTFE tribocharging strength. We study how charge transfers when pristine or defective PTFE is put in contact with different clean and oxidised metals. Our results show the important role played by defects in enhancing charge transfer. Interestingly and unexpectedly our results show that negatively charged chains are more stable than neutral ones, if slightly bent. Indeed deformations can be easily promoted in polymers as PTFE, especially in tribological contacts. These results suggest that, in designing materials in view of their triboelectric properties, the characteristics of their defects could be a performance determining factor.

9.Intercalation of metal into transition metal dichalcogenides in molten salts

Authors:Lin Gao, Mian Li, Binjie Hu, Qing Huang

Abstract: Van der Waals (vdW) layered materials have drawn tremendous interests due to their unique properties. Atom intercalation in the vdW gap of layered materials can tune their electronic structure and generate unexpected properties. Here we report a chemical-scissor mediated method that enables metal intercalation into transition metal dichalcogenides (TMDCs) in molten salts. By using this approach, various guest metal atoms (Mn, Fe, Co, Ni, Cu, and Ag) were intercalated into various TMDCs hosts (such as TiS2, NbS2, TaS2, TiSe2, NbSe2, TaSe2 and Ti0.5V0.5S2). The structure of the intercalated compound and intercalation mechanism was investigated. The results indicate that the vdW gap and valence state of TMDCs can be modified through metal intercalation, and the intercalation behavior is dictated by the electron work function. Such a chemical-scissor mediated intercalation provides an approach to tune the physical and chemical properties of TMDCs, which may open an avenue in functional application ranging from energy conversion to electronics.

10.Infrared spectra in amorphous alumina: a combined ab initio and experimental study

Authors:L. Giacomazzi, N. S. Shcheblanov, M. E. Povarnitsyn, Y. Li, A. Mavrič, B. Zupančič, J. Grdadolnik, A. Pasquarello

Abstract: We present a combined study based on experimental measurements of infrared (IR) dielectric function and first-principles calculations of IR spectra and vibrational density of states (VDOS) of amorphous alumina (am-Al$_2$O$_3$). In particular, we show that the main features of the imaginary part of the dielectric function $\epsilon_2(\omega)$ at $\sim$380 and 630 cm$^{-1}$ are related to the motions of threefold coordinated oxygen atoms, which are the vast majority of oxygen atoms in am-Al$_2$O$_3$. Our analysis provides an alternative point of view with respect to an earlier suggested assignment of the vibrational modes, which relates them to the stretching and bending vibrational modes of AlO$_{n}$ ($n=$ 4, 5, and 6) polyhedra. Our assignment is based on the additive decomposition of the VDOS and $\epsilon_2(\omega)$ spectra, which shows that: (i) the band at $\sim$380 cm$^{-1}$ features oxygen motions occurring in a direction normal to the plane defined by the three nearest-neighbor aluminum atoms, i.e. out-of-plane motions of oxygen atoms; (ii) Al--O stretching vibrations (i.e. in-plane motions of oxygen atoms) appear at frequencies above $\sim$500 cm$^{-1}$, which characterize the vibrational modes underlying the band at $\sim$630 cm$^{-1}$. Aluminum and fourfold coordinated oxygen atoms contribute uniformly to the VDOS and $\epsilon_2(\omega)$ spectra in the frequency region $\sim$350--650 cm$^{-1}$ without causing specific features. Our numerical results are in good agreement with the previous and presently obtained experimental data on the IR dielectric function of am-Al$_2$O$_3$ films. Finally, we show that the IR spectrum can be modeled by assuming isotropic Born charges for aluminum atoms and fourfold coordinated oxygen atoms, while requiring the use of three parameters, defined in a local reference frame, for the anisotropic Born charges of threefold coordinated oxygen atoms.

11.Dislocation Pinning in Helium-Implanted Tungsten: A Molecular Dynamics Study

Authors:Suchandrima Das, Andrea Sand, Felix Hofmann

Abstract: The interaction of edge dislocation with helium-implantation-induced defects in tungsten is investigated using molecular dynamics. Following prior investigations, we consider defects with two helium ions in a vacancy with a self-interstitial bound to it (He2V-SIA). Our observations suggest 3-10 He2V-SIA cluster together, with their pinning strength on glide dislocations increasing with size. For all cluster sizes, the dislocation bows around the cluster, until it gets unpinned, carrying the SIAs with it and leaving behind a helium-vacancy complex and newly created vacancies in its wake. The remnant helium-vacancy complex has little pinning effect, highlighting the defect-clearing process. A total solute hardening force for a distribution of clusters of different sizes, induced by 3000 appm of helium, is found to be approximately 700 MPa. This is in good agreement with the corresponding value of 750 MPa estimated in a previously developed crystal plasticity model simulating the deformation behaviour of the helium-implanted tungsten.

12.Se Nano-Powder Conversion into Lubricious 2D Selenide Layers by Tribochemical Reactions

Authors:Philipp G. Grützmacher, Michele Cutini, Edoardo Marquis, Manel Rodríguez Ripoll, Helmut Riedl, Philip Kutrowatz, Stefan Bug, Chia-Jui Hsu, Johannes Bernardi, M. Clelia Righi, Carsten Gachot, Ali Erdemir

Abstract: Transition metal dichalcogenide (TMD) coatings have attracted enormous scientific and industrial interest due to their outstanding tribological behavior. The paradigmatic example is MoS2, even though selenides and tellurides have demonstrated superior tribological properties. Here, we describe an innovative in-operando conversion of Se nano-powders into lubricious 2D selenides by sprinkling them onto sliding metallic surfaces coated with Mo and W thin films. Advanced material characterization confirms the tribochemical formation of a thin tribofilm containing selenides, reducing the coefficient of friction down to below 0.1 in ambient air, levels typically reached using fully formulated oils. Ab initio molecular dynamics simulations under tribological conditions reveal the atomistic mechanisms that result in shear-induced synthesis of selenide monolayers from nano-powders. The use of Se nano-powder provides thermal stability and prevents outgassing in vacuum environments. Additionally, the high reactivity of the Se nano-powder with the transition metal coating in the conditions prevailing in the contact interface yields highly reproducible results, making it particularly suitable for the replenishment of sliding components with solid lubricants, avoiding the long-lasting problem of TMD-lubricity degradation caused by environmental molecules. The suggested straightforward approach demonstrates an unconventional and smart way to synthesize TMDs in-operando and exploit their friction- and wear reducing impact.

13.Coherent magnetization dynamics in strongly quenched Ni thin films

Authors:Akira Lentfert, Anulekha De, Laura Scheuer, Benjamin Stadtmüller, Georg von Freymann, Martin Aeschlimann, Philipp Pirro

Abstract: The remagnetization process after ultrafast demagnetization can be described by relaxation mechanisms between the spin, electron, and lattice reservoirs. Thereby, collective spin excitations in form of spin waves and their angular momentum transfer play an important role on the longer timescales. In this work, we address the question whether the strength of demagnetization affects the coherency and the phase of the excited spin waves. We present a study of coherent magnetization dynamics in thin nickel films after ultrafast demagnetization using the all-optical, time-resolved magneto-optical Kerr-effect (tr-MOKE) technique. The largest coherent oscillation amplitude was observed for strongly quenched systems, showing the conservation of coherency for demagnetizations of up to 90%. Moreover, the phase of the excited spin-waves increases with pump power, indicating a delayed start of the precession during the remagnetization.

14.Modeling diffusion in ionic, crystalline solids with internal stress gradients

Authors:Benjamin L. Hess, Jay J. Ague

Abstract: Intracrystalline diffusion is an invaluable tool for estimating timescales of geological events. Diffusion is typically modeled using gradients in chemical potential. However, chemical potential is derived for constant pressure and temperature conditions and therefore cannot be used to model diffusion when pressure is not constant. Internal stress variations in minerals create gradients in strain energy which will drive diffusion. Consequently, it is necessary to have a method that incorporates stress variations into diffusion models. We derive a flux expression that allows diffusion to be modeled in ionic, crystalline solids under arbitrary stress states. Our derivation utilizes gradients in a thermodynamic potential called relative chemical potential which quantifies changes in free energy due to the exchanges of constituents on lattice sites under arbitrary stress conditions. We apply our derivation to the common quaternary garnet solid solution almandine-pyrope-grossular-spessartine. The rates and directions of divalent cation diffusion in response to stress are determined by endmember molar volume or lattice parameters, elastic moduli, and non-ideal activity interaction parameters. Our results predict that internal stress variations of one hundred MPa or more are required to shift garnet compositions by at least a few hundredths of a mole fraction. Mineral inclusions in garnet present a potential environment to test and apply our stress-driven diffusion approach, as stress variations ranging from hundreds of MPa to GPa-level are observed or predicted around such inclusions. The ability to model stress-induced diffusion may provide new information about the magnitudes of both intracrystalline stresses and the timescales during which they occurred, imparting a better understanding of large-scale tectono-metamorphic processes.

15.Xsorb: a software for identifying the most stable adsorption configuration and energy of a molecule on a crystal surface

Authors:Enrico Pedretti, Paolo Restuccia, M. Clelia Righi

Abstract: Molecular adsorption is the first important step of many surface-mediated chemical processes, from catalysis to tribology. This phenomenon is controlled by physical/chemical interactions, which can be accurately described by first principles calculations. In recent years, several computational tools have been developed to study molecular adsorption based on high throughput/automatized approaches. However, these tools can sometimes be over-sophisticated for non-expert users. In this work, we present Xsorb, a Python-based code that automatically generates adsorption configurations, guides the user in the identification the most relevant ones, which are then fully optimized. The code relies on well-established Python libraries, and on an open source package for density functional theory calculations. We show the program capabilities through an example consisting of a hydrocarbon molecule, 1-hexene, adsorbed over the (110) surface of iron. The presented computational tool will help users, even non-expert, to easily identify the most stable adsorption configuration of complex molecules on substrates and obtain accurate adsorption geometries and energies.

16.TribChem: a Software for the First-principles, High-Throughput Study of Solid Interfaces and their Tribological properties

Authors:Gabriele Losi, Omar Chehaimi, M. Clelia Righi

Abstract: High throughput first-principles calculations, based on solving the quantum mechanical many-body problem for hundreds of materials in parallel, have been successfully applied to advance many materials-based technologies, from batteries to hydrogen storage. However, this approach has not yet been adopted to systematically study solid-solid interfaces and their tribological properties. To this aim, we developed TribChem, an advanced software based on the FireWorks platform, which is here presented. TribChem is constructed in a modular way, allowing for the separate calculation of bulk, surface, and interface properties. At present the calculated interfacial properties include adhesion, shear strength, and charge redistribution. Further properties can be easily added due to the general structure of the main workflow.

1.Robustness of the intrinsic anomalous Hall effect in Fe3GeTe2 to a uniaxial strain

Authors:Mijin Lim, Byeonghyeon Choi, Minjae Ghim, Je-Geun Park, Hyun-Woo Lee

Abstract: Fe3GeTe2 (FGT), a ferromagnetic van der Waals topological nodal line semimetal, has recently been studied. Using first-principles calculations and symmetry analysis, we investigate the effect of a uniaxial tensile strain on the nodal line and the resultant intrinsic anomalous Hall effect (AHE). Our results reveal their robustness to the in-plane strain. Moreover, the intrinsic AHE remains robust even for artificial adjustment of the atomic positions introduced to break the crystalline symmetries of FGT. When the spin-orbit coupling is absent, the nodal line degeneracy remains intact as long as the inversion symmetry or the two-fold screw symmetry is maintained, which reveal that the nodal line may emerge much more easily than previously predicted. This strong robustness is surprising and disagrees with the previous experimental report [Y. Wang et al., Adv. Mater. 32, 2004533 (2020)], which reports that a uniaxial strain of less than 1 % of the in-plane lattice constant can double the anomalous Hall resistance. This discrepancy implies that the present understanding of the AHE in FGT is incomplete. The possible origins of this discrepancy are discussed.

2.Metalloporphyrins on Oxygen-Passivated Iron: Conformation and Order Beyond the First Layer

Authors:David Maximilian Janas, Andreas Windischbacher, Mira Sophie Arndt, Michael Gutnikov, Lasse Sternemann, David Gutnikov, Till Willershausen, Jonah Elias Nitschke, Karl Schiller, Daniel Baranowski, Vitaliy Feyer, Iulia Cojocariu, Khush Dave, Peter Puschnig, Matija Stupar, Stefano Ponzoni, Mirko Cinchetti, Giovanni Zamborlini

Abstract: On-surface metal porphyrins can undergo electronic and conformational changes that play a crucial role in determining the chemical reactivity of the molecular layer. Therefore, understanding those properties is pivotal for the design and implementation of organic-based devices. Here, by means of photoemission orbital tomography supported by density functional theory calculations, we investigate the electronic and geometrical structure of two metallated tetraphenyl porphyrins (MTPPs), namely ZnTPP and NiTPP, adsorbed on the oxygen-passivated Fe(100)-p(1x1)O surface. Both molecules weakly interact with the surface as no charge transfer is observed. In the case of ZnTPP our data correspond to those of moderately distorted molecules, while NiTPP exhibits a severe saddle-shape deformation. From additional experiments on NiTPP multilayer films, we conclude that this distortion is a consequence of the interaction with the substrate, as the NiTPP macrocycle of the second layer turns out to be flat. We further find that distortions in the MTPP macrocycle are accompanied by an increasing energy gap between the highest occupied molecular orbitals (HOMO and HOMO-1). Our results demonstrate that photoemission orbital tomography can simultaneously probe the energy level alignment, the azimuthal orientation, and the adsorption geometry of complex aromatic molecules even in the multilayer regime.

3.Giant segregation transition as origin of liquid metal embrittlement in the Fe-Zn system

Authors:Reza Darvishi Kamachali, Theophilus Wallis, Yuki Ikeda, Ujjal Saikia, Ali Ahmadian, Christian H. Liebscher, Tilmann Hickel, Robert Maaß

Abstract: A giant Zn segregation transition is revealed using CALPHAD-integrated density-based modelling of Zn segregation into Fe grain boundaries (GBs). The results show that above a threshold of only a few atomic percent Zn in the alloy, a substantial amount of up to 60 at.% Zn can segregate to the GB. We also found that the amount of segregation significantly increases with decreasing temperature, while the required Zn content in the alloy for triggering the segregation transition decreases. Direct evidence of this Zn segregation transition is obtained using high-resolution scanning transmission electron microscopy. We trace the origin of the segregation transition and its temperature dependence back to the low cohesive energy of Zn and a miscibility gap in Fe-Zn GB, arising from the magnetic ordering effect, which is demonstrated by ab initio calculations. We show that the massive Zn segregation resulting from the segregation transition greatly assists with liquid wetting and reduces the work of separation along the GB. These findings reveal the fundamental origin of GB weakening and therefore liquid metal embrittlement in the Fe-Zn system.

4.Impact of capping agent on structural and optical properties of ZnS nanoparticles

Authors:Samiran Mandal, Sk Irsad Ali, Subhamay Pramanik, Atis Chandra Mandal

Abstract: Nanocrystalline samples of pristine capped and uncapped zinc sulphide were synthesized via the sol-gel technique. The nanocrystallinity of the samples were confirmed by the X-ray diffraction technique, where size of the particle size decreases with the increasing of mol. concentration (x = 0.00, 0.02, 0.03, 0.04 Mol). of capping agent sodium dodecyle sulphate. The obtained crystallite sizes were found to be in the range 4.6 nm to 2.7 nm respectively. The optical band gaps of the samples were estimated by using ultra-violet visible spectroscopic techniques and the band gap values were in the range 3.8 eV to 4.4 eV. All the samples showed quantum confinement behavior compared to bulk sample. Fluorescence (FL) spectra showed three emission peaks at the emission wavelengths around 434 nm, 520 nm, 545 nm, 628 nm, and 694 nm. The FL intensities were proportional to the concentration of capping agent.

5.Assessing the potential of perfect screw dislocations in SiC for solid-state quantum technologies

Authors:Daniel Barragan-Yani, Ludger Wirtz

Abstract: Although point defects in solids are one of the most promising physical systems to build functioning qubits, it remains challenging to position them in a deterministic array and to integrate them into large networks. By means of advanced ab initio calculations we show that undissociated screw dislocations in cubic 3C-SiC, and their associated strain fields, could be used to create a deterministic pattern of relevant point defects. Specifically, we present a detailed analysis of the formation energies and electronic structure of the divacancy in 3C-SiC when located in the vicinity of this type of dislocations. Our results show that the divacancy is strongly attracted towards specific and equivalent sites inside the core of the screw dislocations, and would form a one-dimensional arrays along them. Furthermore, we show that the same strain that attracts the divacancy allows the modulation of the position of its electronic states and of its charge transition levels. In the case of the neutral divacancy, we find that these modulations result in the loss of its potential as a qubit. However, these same modulations could transform defects with no potential as qubits when located in bulk, into promising defects when located inside the core of the screw dislocations. Since dislocations are still mostly perceived as harmful defects, our findings represent a technological leap as they show that dislocations can be used as active building blocks in future defect-based quantum computers.

6.One-dimensional electronic structure of phosphorene chains

Authors:Maxim Krivenkov, Maryam Sajedi, Dmitry Marchenko, Evangelos Golias, Matthias Muntwiler, Oliver Rader, Andrei Varykhalov

Abstract: Phosphorene, a 2D allotrope of phosphorus, is technologically very appealing because of its semiconducting properties and narrow band gap. Further reduction of the phosphorene dimensionality may spawn exotic properties of its electronic structure, including lateral quantum confinement and topological edge states. Phosphorene atomic chains self-assembled on Ag(111) have recently been characterized structurally but were found by angle-resolved photoemission (ARPES) to be electronically 2D. We show that these chains, although aligned equiprobably to three <$1 \bar{1} 0$> directions of the Ag(111) surface, can be characterized by ARPES because the three rotational variants are separated in the angular domain. The dispersion of the phosphorus band measured along and perpendicular to the chains reveals pronounced electronic confinement resulting in a 1D band, flat and dispersionless perpendicular to the chain direction in momentum space. Our density functional theory calculations reproduce the 1D band for the experimentally determined structure of P/Ag(111). We predict a semiconductor-to-metal phase transition upon increasing the density of the chain array so that a 2D structure would be metallic.

7.Lessons from the harmonic oscillator -- a reconciliation of the Frequency-Resolved Frozen Phonon Multislice Method with other theoretical approaches

Authors:Paul M. Zeiger, Juri Barthel, Leslie J. Allen, Ján Rusz

Abstract: We compare the Frequency-Resolved Frozen Phonon Multislice (FRFPMS) method, introduced in Phys. Rev. Lett. 124, 025501 (2020), with other theoretical approaches used to account for the inelastic scattering of high energy electrons, namely the first-order Born approximation and the quantum excitation of phonons model. We show, that these theories lead to similar expressions for the single inelastically scattered intensity as a function of momentum transfer for an anisotropic quantum harmonic oscillator in a weak phase object approximation of the scattered waves, except for a too small smearing of the scattering potential by the effective Debye-Waller factor (DWF) in the FRFPMS method. We propose that this issue can be fixed by including an explicit DWF smearing into the potential and demonstrate numerically, that in any realistic situation, a FRFPMS approach revised in this way, correctly accounts for the single inelastically scattered intensity and the correct elastic scattering intensity. Furthermore our simulations illustrate that the only requirement for such a revised FRFPMS method is the smallness of mean squared displacements for all atomic species in all frequency bins. The analytical considerations for the FRFPMS method also explain the $1/\omega^2$-scaling of FRFPMS spectra observed in Phys. Rev. B 104, 104301 (2021) by the use of classical statistics in the molecular dynamics simulation. Moreover, we find that the FRFPMS method inherently adds the contributions of phonon loss and gain within each frequency bin. Both of these issues related to the frequency-scaling can be fixed by a system-independent post-processing step.

8.Direct observation of Néel-type skyrmions and domain walls in a ferrimagnetic thin film via scanning transmission X-ray microscopy

Authors:Chen Luo, Kai Chen, Victor Ukleev, Sebastian Wintz, Markus Weigand, Karel Prokeš, Florin Radu

Abstract: Isolated magnetic skyrmions are stable, topologically protected spin textures that are at the forefront of research interests today due to their potential applications in information technology. A distinct class of skyrmion hosts are rare earth - transition metal (RE-TM) ferrimagnetic materials. To date, the nature and the control of basic traits of skyrmions in these materials are not fully understood. We show that for an archetypal ferrimagnetic material that exhibits strong perpendicular anisotropy, the ferrimagnetic skyrmion size can be tuned by external magnetic fields. Moreover, by taking advantage of the high spatial resolution of scanning transmission X-ray microscopy (STXM) and utilizing a large x-ray magnetic linear dichroism (XMLD) contrast that occurs naturally at the RE resonant edges, we resolve the nature of the magnetic domain walls of ferrimagnetic skyrmions. We demonstrate that through this method one can easily discriminate between Bloch and N\'eel type domain walls for each individual skyrmion. For all isolated ferrimagnetic skyrmions, we observe that the domain walls are of N\'eel-type. This key information is corroborated with results of micromagnetic simulations and allows us to conclude on the nature of the Dzyaloshinskii-Moriya interaction (DMI) which concurs to the stabilisation of skyrmions in ferrimagnetic systems. Establishing that an intrinsic DMI occurs in RE-TM materials will also be beneficial towards a deeper understanding of chiral spin texture control in ferrimagnetic materials.

1.Magnetocaloric effect and its electric-field regulation in CrI$_3$/metal heterostructure

Authors:Weiwei He, Ziming Tang, Qihua Gong, Min Yi, Wanlin Guo

Abstract: The extraordinary properties of a heterostructure by stacking atom-thick van der Waals (vdW) magnets have been extensively studied. However, the magnetocaloric effect (MCE) of heterostructures that are based on monolayer magnets remains to be explored. Herein, we deliberate MCE of vdW heterostructure composed of a monolayer CrI$_3$ and metal atomic layers (Ag, Hf, Au, and Pb). It is revealed that heterostructure engineering by introducing metal substrate can improve MCE of CrI$_3$, particularly boosting relative cooling power to 471.72 $\mu$Jm$^{-2}$ and adiabatic temperature change to 2.1 K at 5 T for CrI$_3$/Hf. This improved MCE is ascribed to the enhancement of magnetic moment and intralayer exchange coupling in CrI$_3$ due to the CrI$_3$/metal heterointerface induced charge transfer. Electric field is further found to tune MCE of CrI$_3$ in heterostructures and could shift the peak temperature by around 10 K in CrI$_3$/Hf, thus manipulating the working temperature window of MCE. The discovered electric-field and substrate regulated MCE in CrI$_3$/metal heterostructure opens new avenues for low-dimensional magnetic refrigeration.

2.Ferroelectric domain wall clusters in barium titanate

Authors:Chris Halcrow, Egor Babaev

Abstract: We study ferroelectric domain walls in barium titanate. We search for structurally nontrivial, so-called non-Ising domain walls, where the Polarisation is non-zero along the entire wall. Our approach enables us to find solutions for domain walls in any orientation, and the existence and energy of these walls depend on their particular orientation. We find that, across all phases of the material, there are orientations where the non-Ising walls have lower energy than Ising walls. The most interesting property of these domain walls is their non-monotonic interaction forces, allowing them to form stable domain-wall clusters rather than following standard behavior where domain walls annihilate or repel each other. We found the required external electric field to create the non-Ising configurations. Besides theoretical interest, this unconventional property of domain walls makes them a good candidate for memory application.

3.Magnetization Switching in van der Waals Systems by Spin-Orbit Torque

Authors:Xin Lin, Lijun Zhu

Abstract: Electrical switching of magnetization via spin-orbit torque (SOT) is of great potential in fast, dense, energy-efficient nonvolatile magnetic memory and logic technologies. Recently, enormous efforts have been stimulated to investigate switching of perpendicular magnetization in van der Waals systems that have unique, strong tunability and spin-orbit coupling effect compared to conventional metals. In this review, we first give a brief, generalized introduction to the spin-orbit torque and van der Waals materials. We will then discuss the recent advances in magnetization switching by the spin current generated from van der Waals materials and summary the progress in the switching of Van der Waals magnetization by the spin current.

4.Imperfectly coordinated water molecules pave the way for homogeneous ice nucleation

Authors:Mingyi Chen, Lin Tan, Han Wang, Linfeng Zhang, Haiyang Niu

Abstract: Water freezing is ubiquitous on Earth, affecting many areas from biology to climate science and aviation technology. Probing the atomic structure in the homogeneous ice nucleation process from scratch is of great value but still experimentally unachievable. Theoretical simulations have found that ice originates from the low-mobile region with increasing abundance and persistence of tetrahedrally coordinated water molecules. However, a detailed microscopic picture of how the disordered hydrogen-bond network rearranges itself into an ordered network is still unclear. In this work, we use a deep neural network (DNN) model to "learn" the interatomic potential energy from quantum mechanical data, thereby allowing for large-scale and long molecular dynamics (MD) simulations with ab initio accuracy. The nucleation mechanism and dynamics at atomic resolution, represented by a total of 36 $\mu$s-long MD trajectories, are deeply affected by the structural and dynamical heterogeneity in supercooled water. We find that imperfectly coordinated (IC) water molecules with high mobility pave the way for hydrogen-bond network rearrangement, leading to the growth or shrinkage of the ice nucleus. The hydrogen-bond network formed by perfectly coordinated (PC) molecules stabilizes the nucleus, thus preventing it from vanishing and growing. Consequently, ice is born through competition and cooperation between IC and PC molecules. We anticipate that our picture of the microscopic mechanism of ice nucleation will provide new insights into many properties of water and other relevant materials.

5.Effects of C and B microalloying additions on the microstructure and processability of René 41 Ni-based superalloy

Authors:Wai Fung Wilson Tse

Abstract: Ren\'e 41 is a cast and wrought Ni-based superalloy with high yield strength and stress-rupture properties contrasted with poor processability. The aim of this thesis is to systematically investigate the influence of C and B microalloying additions on processability of Ren\'e 41. The first approach is an experimental one using hot compression testing and material characterisation. A second approach using machine learning methodology was also used to provide linkage for the experimental observations with industrial Ren\'e 41 materials based on ultrasonic defects and chemical composition. Three Ren\'e 41 variants with nominal, high C, and high B compositions were industrially fabricated and homogenized to be used in this study. The resultant flow stresses from hot compression testing were used to model hyperbolic sine constitutive equations. The activation energy for hot deformation was found to be 757, 728, and 697 kJmol-1 for the nominal, high B, and high C Ren\'e 41 variants respectively. Finite element method simulations based on the obtained flow curves found that effective plastic strain varied considerably through the sample geometry. Quantitative analysis via electron back-scatted diffraction found that while the three Ren\'e 41 variants have nearly identical recrystallised grain size, high C contain 64 vol.% recrystallised fractions compared to that of the nominal variant with 31 vol.% at the same deformation condition.

6.X-ray absorption spectroscopy of oligothiophene crystals from ab initio many-body theory

Authors:Konstantin Lion, Caterina Cocchi, Claudia Draxl

Abstract: We present an x-ray absorption spectroscopy study from the carbon $K$, sulfur $K$, and sulfur $L_{2,3}$ edges of crystalline oligothiophenes of varying length, i.e. bithiophene (2T), quaterthiophene (4T), and sexithiophene (6T), performed from first principles by means of all-electron density-functional theory and many-body perturbation theory. A comprehensive assignment of all relevant spectral features is performed based on the electronic structure and the character of the target conduction states. The inclusion of electron-hole effects leads to significant redistribution of oscillator strengths and to strongly bound excitons with binding energies ranging from 1.5 to 4.5 eV. When going from 2T to 6T, exciton binding energies decrease by up to 1 eV, which we attribute to the reduction of the average Coulomb attraction with increasing oligomer length. These high values are significantly larger than their counterparts in the optical excitations of these systems and indicative of their localization on the respective molecules. For the same reason, local-field effects which typically dominate the optical absorption of organic crystals, turn out to play only a negligible role at all edges. We identify two sets of carbon atoms, i.e. with or without sulfur bonding, which exhibit distinct features at the C $K$-edge. The sulfur atoms, on the other hand, yield similar contributions in the S, $K$, and $L_{2,3}$ edge spectra. Our results show excellent agreement with available experimental data.

7.Hardness Descriptor Derived from Symbolic Regression

Authors:Christian Tantardini, Hayk A. Zakaryan, Zhong-Kang Han, Sergey V. Levchenko, Alexander G. Kvashnin

Abstract: Hard and superhard materials are critical components in numerous industrial applications required for sustainable development. However, discovering new materials with high hardness is challenging, because hardness is a complex and multiscale property with a non-trivial connection to atomic properties of the material. Here, we present a low-dimensional physical descriptor for Vickers hardness derived from symbolic-regression artificial intelligence approach to data analysis. The descriptor is a mathematical combination of materials' properties that can be much easier evaluated than hardness itself via the atomistic simulations and it is therefore suitable for a high-throughput screening. The developed artificial intelligence model was trained on the experimental values of hardness and then high-throughput screening were performed among 635 compounds including binary, ternary, and quaternary transition-metal borides, carbides, nitrides, carbonitrides, carboborides, and boronitrides to find the optimal superhard material. The proposed descriptor is an analytic formula, which is physically interpretable, allowing us to get an insight into the multiscale relationship between atomic structure (i.e., micro) and hardness (i.e., macro). In details, we have found that the hardness is proportional to the Voigt-averaged bulk modulus and inversely proportional to the Poisson's ratio and Reuss-averaged shear modulus. Results of high-throughput search showed the possible way of tuning hardness of existing materials by making mixtures with harder, but metastable structures (e.g., metastable VN, TaN, ReN$_2$, Cr$_3$N$_4$, and ZrB$_6$ possess high hardness).

8.Surface passivation of FAPbI3-rich perovskite with caesium iodide outperforms bulk incorporation

Authors:Thomas P. Baumeler, Essa A. Alharbi, George Kakavelakis, George C. Fish, Mubarak T. Aldosari, Miqad S. Albishi, Lukas Pfeifer, Brian I. Carlsen, Jun-Ho Yum, Abdullah S. Alharbi, Mounir D. Mensi, Jing Gao, Felix T. Eickemeyer, Kevin Sivula, Jacques-Edouard Moser, Shaik M. Zakeeruddin, Michael Graetzel

Abstract: Metal halide perovskites (MHPs) have shown an incredible rise in efficiency, reaching as high as 25.7%, which now competes with traditional photovoltaic technologies. Herein, we excluded CsX and RbX, the most commonly used cations to stabilize FAPbI3, from the bulk of perovskite thin films and applied them on the surface, as passivation agents. Extensive device optimization led to a power conversion efficiency (PCE) of 24.1% with a high fill factor (FF) of 82.2% upon passivation with CsI. We investigated in-depth the effect of CsI passivation on structural and optoelectronic properties using X-ray diffraction (XRD), angle resolved X-ray photoelectron spectroscopy (ARXPS), Kelvin Probe Force (KPFM) microscopy, time-resolved photoluminescence (TRPL), photoluminescence quantum yield (PLQY) and electroabsorption spectroscopy (TREAS). Furthermore, passivated devices exhibit enhanced operational stability, with optimized passivation with CsI leading to a retention of ~90% of initial PCE under 1 Sun illumination with maximum power point tracking for 600 h.

9.Defect-enhanced diffusion of magnetic skyrmions

Authors:Philipp Rieger, Markus Weißenhofer, Ulrich Nowak

Abstract: Defects, i.e. inhomogeneities of the underlying lattice, are ubiquitous in magnetic materials and can have a crucial impact on their applicability in spintronic devices. For magnetic skyrmions, localized and topologically non-trivial spin textures, they give rise to a spatially inhomogeneous energy landscape and can lead to pinning, resulting in an exponentially increased dwell time at certain positions and typically a strongly reduced mobility. Using atomistic spin dynamics simulations, we reveal that under certain conditions defects can instead enhance thermal diffusion of ferromagnetic skyrmions. By comparing with results for the diffusion of antiferromagnetic skyrmions and using a quasi-particle description based on the Thiele equation, we demonstrate that this surprising finding can be traced back to the partial lifting of the impact of the topologigal gyrocoupling, which governs the dynamics of ferromagnetic skyrmions in the absence of defects.

10.Evolution of ferroelectricity with annealing temperature and thickness in sputter deposited undoped HfO$_2$ on silicon

Authors:Md Hanif Ali, Adityanarayan Pandey, Rowtu Srinu, Paritosh Meihar, Shubham Patil, Sandip Lashkare, Udayan Ganguly

Abstract: Ferroelectricity in sputtered undoped-HfO$_2$ is attractive for composition control for low power and non-volatile memory and logic applications. Unlike doped HfO$_2$, evolution of ferroelectricity with annealing and film thickness effect in sputter deposited undoped HfO$_2$ on Si is not yet reported. In present study, we have demonstrated the impact of post metallization annealing temperature and film thickness on ferroelectric properties in dopant-free sputtered HfO$_2$ on Si-substrate. A rich correlation of polarization with phase, lattice constant, and crystallite size and interface reaction is observed. First, anneal temperature shows o-phase saturation beyond 600 oC followed by interface reaction beyond 700 oC to show an optimal temperature window on 600-700 oC. Second, thickness study at the optimal temperature window shows an alluring o-phase crystallite scaling with thickness till a critical thickness of 20 nm indicating that the films are completely o-phase. However, the lattice constants (volume) are high in the 15-20 nm thickness range which correlates with the enhanced value of 2Pr. Beyond 20 nm, crystallite scaling with thickness saturates with the correlated appearance of m-phase and reduction in 2Pr. The optimal thickness-temperature window range of 15-20 nm films annealed at 600-700 oC show 2Pr of ~35.5 micro-C/cm$^2$ is comparable to state-of-the-art. The robust wakeup-free endurance of ~$10^$8 cycles showcased in the promising temperature-thickness window has been identified systematically for non-volatile memory applications.

11.Simulations of Magnetization Reversal in FM/AFM Bilayers With THz Frequency Pulses

Authors:Joel Hirst, Sergiu Ruta, Jerome Jackson, Thomas Ostler

Abstract: It is widely known that antiferromagnets (AFMs) display a high frequency response in the terahertz (THz) range, which opens up the possibility for ultrafast control of their magnetization for next generation data storage and processing applications. However, because the magnetization of the different sublattices cancel, their state is notoriously difficult to read. One way to overcome this is to couple AFMs to ferromagnets - whose state is trivially read via magneto-resistance sensors. Here we present conditions, using theoretical modelling, that it is possible to switch the magnetization of an AFM/FM bilayer using THz frequency pulses with moderate field amplitude and short durations, achievable in experiments. Consistent switching is observed in the phase diagrams for an order of magnitude increase in the interface coupling and a tripling in the thickness of the FM layer. We demonstrate a range of reversal paths that arise due to the combination of precession in the materials and the THz-induced fields. Our analysis demonstrates that the AFM drives the switching and results in a much higher frequency dynamics in the FM due to the exchange coupling at the interface. The switching is shown to be robust over a broad range of temperatures relevant for device applications.

1.Machine learning for predicting fatigue properties of additively manufactured materials

Authors:Min Yi, Ming Xue, Peihong Cong, Yang Song, Haiyang Zhang, Lingfeng Wang, Liucheng Zhou, Yinghong Li, Wanlin Guo

Abstract: Fatigue properties of additively manufactured (AM) materials depend on many factors such as AM processing parameter, microstructure, residual stress, surface roughness, porosities, post-treatments, etc. Their evaluation inevitably requires these factors combined as many as possible, thus resulting in low efficiency and high cost. In recent years, their assessment by leveraging the power of machine learning (ML) has gained increasing attentions. Here, we present a comprehensive overview on the state-of-the-art progress of applying ML strategies to predict fatigue properties of AM materials, as well as their dependence on AM processing and post-processing parameters such as laser power, scanning speed, layer height, hatch distance, built direction, post-heat temperature, etc. A few attempts in employing feedforward neural network (FNN), convolutional neural network (CNN), adaptive network-based fuzzy system (ANFS), support vector machine (SVM) and random forest (RF) to predict fatigue life and RF to predict fatigue crack growth rate are summarized. The ML models for predicting AM materials' fatigue properties are found intrinsically similar to the commonly used ones, but are modified to involve AM features. Finally, an outlook for challenges (i.e., small dataset, multifarious features, overfitting, low interpretability, unable extension from AM material data to structure life) and potential solutions for the ML prediction of AM materials' fatigue properties is provided.

2.Gate-tunable ferromagnetism in a van der Waals magnetic semimetal

Authors:Hideki Matsuoka, Shun Kajihara, Yue Wang, Yoshihiro Iwasa, Masaki Nakano

Abstract: Magnetic semimetals form an attractive class of materials because of the non-trivial contributions of itinerant electrons to magnetism. Due to their relatively low-carrier-density nature, a doping level of those materials could be largely tuned by a gating technique. Here we demonstrate gate-tunable ferromagnetism in an emergent van der Waals magnetic semimetal Cr3Te4 based on an ion-gating technique. Upon doping electrons into the system, the Curie temperature (TC) sharply increases, approaching near to room temperature, then decreases to some extent. Interestingly, this non-monotonous variation of TC accompanies the switching of the magnetic anisotropy. Furthermore, such evolutions of TC and anisotropy occur synchronously with the sigh changes of the ordinary and anomalous Hall effects. Those results clearly elucidate that the magnetism in Cr3Te4 should be governed by its semimetallic band nature, where the band crossing points play a crucial role both for the magneto-transport properties and magnetism itself.

3.Epitaxial monolayers of magnetic 2D semiconductor FeBr$_{2}$ grown on Au(111)

Authors:S. E. Hadjadj, C. González-Orellana, J. Lawrence, D. Bikaljević, M. Peña-Díaz, P. Gargiani, L. Aballe, J. Naumann, M. Á. Niño, M. Foerster, S. Ruiz-Gómez, S. Thakur, I. Kumberg, J. Taylor, J. Hayes, J. Torres, C. Luo, F. Radu, D. G. de Oteyza, W. Kuch, J. I. Pascual, C. Rogero, M. Ilyn

Abstract: Magnetic two-dimensional (2D) semiconductors have attracted a lot of attention because modern preparation techniques are capable of providing single crystal films of these materials with precise control of thickness down to the single-layer limit. It opens up a way to study rich variety of electronic and magnetic phenomena with promising routes towards potential applications. We have investigated the initial stages of epitaxial growth of the magnetic van der Waals semiconductor FeBr\textsubscript{2} on a single-crystal Au(111) substrate by means of low-temperature scanning tunneling microscopy, low-energy electron diffraction, x-ray photoemission spectroscopy, low-energy electron emission microscopy and x-ray photoemission electron microscopy. Magnetic properties of the one- and two-layer thick films were measured via x-ray absorption spectroscopy/x-ray magnetic circular dichroism. Our findings show a striking difference in the magnetic behaviour of the single layer of FeBr\textsubscript{2} and its bulk counterpart, which can be attributed to the modifications in the crystal structure due to the interaction with the substrate.

4.Absolute radiation tolerance of amorphous alumina coatings at room temperature

Authors:A. Zaborowska, Ł. Kurpaska, M. Clozel, E. J. Olivier, J. H. O'Connell, M. Vanazzi, F. Di Fonzo, A. Azarov, I. Jóźwik, M. Frelek-Kozak, R. Diduszko, J. H. Neethling, J. Jagielski

Abstract: In this study structural and mechanical properties of a 1 um thick Al2O3 coating, deposited on 316L stainless steel by Pulsed Laser Deposition (PLD), subjected to high energy ion irradiation were assessed. Mechanical properties of pristine and ion-modified specimens were investigated using the nanoindentation technique. A comprehensive characterization combining Transmission Electron Microscopy and Grazing-Incidence X-ray Diffraction provided deep insight into the structure of the tested material at the nano- and micro- scale. Variation in the local atomic ordering of the irradiated zone at different doses was investigated using a reduced distribution function analysis obtained from electron diffraction data. Findings from nanoindentation measurements revealed a slight reduction in hardness of all irradiated layers. At the same time TEM examination indicated that the irradiated layer remained amorphous over the whole dpa range. No evidence of crystallization, void formation or element segregation was observed up to the highest implanted dose. Reported mechanical and structural findings were critically compared with each other pointing to the conclusion that under given irradiation conditions, over the whole range of doses used, alumina coatings exhibit excellent radiation resistance. Obtained data strongly suggest that investigated material may be considered as a promising candidate for next-generation nuclear reactors, especially LFR-type, where high corrosion protection is one of the highest prerogatives to be met.

5.High versus low energy ion irradiation impact on functional properties of PLD-grown alumina coatin

Authors:A. Zaborowska, Ł. Kurpaska, E. Wyszkowska, A. Azarov, M. Turek, A. Kosińska, M. Frelek-Kozak, J. Jagielski

Abstract: It is well known that ion irradiation can be successfully used to reproduce microstructural features triggered by neutron irradiation. Even though the irradiation process brings many benefits, it is also associated with several drawbacks. For example, the penetration depth of the ion in the material is very limited. This is particularly important for energies below MeV, ultimately reducing the number of available irradiation facilities. In addition to that, extracting information exclusively from the modified volume may be challenging. Therefore, extreme caution must be taken when interpreting obtained data. Our work aims to compare the findings of nanomechanical studies already conducted separately on thin amorphous ceramic coatings irradiated with ions of different energies, hence layers of different thicknesses. In this work, we show that in some instances, the 10% rule may be obeyed. In order to prove our finding, we compared results obtained for ion irradiated (with two energies: 0.25 and 1.2 MeV up to 25dpa) alumina coating system. Mechanical properties of pristine and ion-irradiated specimens were studied by nanoindentation technique. Interestingly, the qualitative relationship between nanohardness and irradiation damage level is very similar, regardless of the energy used. The presented work proves that for some materials (e.g., hard coatings), the qualitative assessment of the mechanical changes using nanoindentation might be feasible even for shallow implantation depths.

6.Emission limited logarithmic and power law transients in pump-probe spectroscopy of perovskites

Authors:Pradeep R. Nair

Abstract: Optical pump-probe techniques like absorption spectroscopy and microwave conductivity are widely used to characterize the carrier dynamics in perovskites for optoelectronic applications. In contrast to the prevalent assumption of exponentials, here we predict the possibility of trap emission limited logarithmic and power-law transients. These predictions are validated by detailed numerical simulations and well supported by several experimental reports from recent literature. Interestingly, these findings indicate the need to revisit the existing schemes which rely on simplified rate equations and exponential decays to estimate the recombination parameters from pump-probe spectroscopy. Accordingly, we suggest appropriate methodologies to back extract parameters related to trap distribution from such non-exponential transients. Indeed, the insights shared in this manuscript could fundamentally impact the usage and interpretation of transient spectroscopy for emerging materials for optoelectronic applications.

7.Unconventional Ferroelectricity in Violation with Neumann's Principle

Authors:Junyi Ji, Guoliang Yu, Changsong Xu, H. J. Xiang

Abstract: The physical properties of crystals are governed by their symmetry according to Neumann's principle. However, we present a case that contradicts this principle wherein the polarization is not invariant under its symmetry. We term this phenomenon as unconventional ferroelectricity in violation of Neumann's principle (UFVNP). Our group theory analysis reveals that 33 symmorphic space groups have the potential for UFVNP, with 26 of these symmorphic space groups belonging to non-polar groups. Notably, the polarization component in UFVNP materials is quantized. Our theory can explain the experimentally proven in-plane polarization of the monolayer {\alpha}-In2Se3, which has C3v symmetry. Additionally, we employ first-principles calculations to demonstrate the existence of UFVNP in Td phase AgBr, which was not initially anticipated to exhibit polarization. Thus, UFVNP plays an integral role in characterizing and exploring the possible applications of ferroelectrics, significantly expanding the range of available materials for study.

8.Bilayer Stacking Ferrovalley without Breaking Time-Reversal Symmetry

Authors:Guoliang Yu, Junyi Ji, Changsong Xu, H. J. Xiang

Abstract: Non-volatile manipulation of valley polarization in solids has long been desired for valleytronics applications but remains challenging. Here, we propose a novel strategy for non-volatile manipulating valleys through bilayer stacking, which enables spontaneous valley polarization without breaking time-reversal symmetry. We call this noval physics as bilayer stacking ferrovalley (BSFV). The group theory analysis reveals that the two-dimensional (2D) valley materials with hexagonal and square lattices can host BSFV. By searching the 2D material database, we discovered 14 monolayer 2D materials with direct gaps that are candidates for realizing BSFV. Further first-principles calculations demonstrate that BSFV exists in RhCl3 and InI bilayers. The bilayer stacking breaks their three- and four-fold rotation symmetry, resulting in 39 and 326 meV valley polarization, respectively. More interestingly, the valley polarization in our systems can be switched by interlayer sliding. Our study opens up a new direction for designing ferrovalley materials and thus greatly enriches the platform for the research of valleytronics.

9.Machine learning modeling of the atomic structure and physical properties of alkali and alkaline-earth aluminosilicate glasses and melts

Authors:Charles Le Losq, Barbara Baldoni

Abstract: The first version of the machine learning greybox model i-Melt was trained to predict latent and observed properties of K$_2$O-Na$_2$O-Al$_2$O$_3$-SiO$_2$ melts and glasses. Here, we extend the model compositional range, which now allows accurate predictions of properties for glass-forming melts in the CaO-MgO-K$_2$O-Na$_2$O-Al$_2$O$_3$-SiO$_2$ system, including melt viscosity (accuracy equal or better than 0.4 log$_{10}$ Pa$\cdot$s in the 10$^{-1}$-10$^{15}$ log$_{10}$ Pa$\cdot$s range), configurational entropy at glass transition ($\leq$ 1 J mol$^{-1}$ K$^{-1}$), liquidus ($\leq$ 60 K) and glass transition ($\leq$ 16 K) temperatures, heat capacity ($\leq$ 3 \%) as well as glass density ($\leq$ 0.02 g cm$^{-3}$), optical refractive index ($\leq$ 0.006), Abbe number ($\leq$ 4), elastic modulus ($\leq$ 6 GPa), coefficient of thermal expansion ($\leq$ 1.1 10$^{-6}$ K$^{-1}$) and Raman spectra ($\leq$ 25 \%). Uncertainties on predictions also are now provided. The model offers new possibilities to explore how melt/glass properties change with composition and atomic structure.

10.Atomic resolution interface structure and vertical current injection in highly uniform $MoS_{2}$ heterojunctions with bulk GaN

Authors:F. Giannazzo, S. E. Panasci, E. Schilirò, G. Greco, F. Roccaforte, G. Sfuncia, G. Nicotra, M. Cannas, S. Agnello, E. Frayssinet, Y. Cordier, A. Michon, A. Koos, B. Pécz

Abstract: The integration of two-dimensional $MoS_{2}$ with $GaN$ recently attracted significant interest for future electronic/optoelectronic applications. However, the reported studies have been mainly carried out using heteroepitaxial $GaN$ templates on sapphire substrates, whereas the growth of $MoS_{2}$ on low-dislocation-density bulk GaN can be strategic for the realization of truly vertical devices. In this paper, we report the growth of ultrathin $MoS_{2}$ films, mostly composed by single-layers ($1L$), onto homoepitaxial $n-GaN$ on $n^{+}$ bulk substrates by sulfurization of a pre-deposited $MoO_{x}$ film. Highly uniform and conformal coverage of the $GaN$ surface was demonstrated by atomic force microscopy, while very low tensile strain (0.05%) and a significant $p^{+}$-type doping ($4.5 \times 10^{12} cm^{-2}$) of $1L-MoS_{2}$ was evaluated by Raman mapping. Atomic resolution structural and compositional analyses by aberration-corrected electron microscopy revealed a nearly-ideal van der Waals interface between $MoS_{2}$ and the $Ga$-terminated $GaN$ crystal, where only the topmost $Ga$ atoms are affected by oxidation. Furthermore, the relevant lattice parameters of the $MoS_{2}/GaN$ heterojunction, such as the van der Waals gap, were measured with high precision. Finally, the vertical current injection across this 2D/3D heterojunction has been investigated by nanoscale current-voltage analyses performed by conductive atomic force microscopy, showing a rectifying behavior with an average turn-on voltage $V_{on}=1.7 V$ under forward bias, consistent with the expected band alignment at the interface between $p^{+}$ doped $1L-MoS_{2}$ and $n-GaN$.

11.Non-linear optics at twist interfaces in h-BN/SiC heterostructures

Authors:Abhijit Biswas, Rui Xu, Gustavo A. Alvarez, Jin Zhang, Joyce Christiansen-Salameh, Anand B. Puthirath, Kory Burns, Jordan A. Hachtel, Tao Li, Sathvik Ajay Iyengar, Tia Gray, Chenxi Li, Xiang Zhang, Harikishan Kannan, Jacob Elkins, Tymofii S. Pieshkov, Robert Vajtai, A. Glen Birdwell, Mahesh R. Neupane, Elias J. Garratt, Tony Ivanov, Bradford B. Pate, Yuji Zhao, Hanyu Zhu, Zhiting Tian, Angel Rubio, Pulickel M. Ajayan

Abstract: Understanding the emergent electronic structure in twisted atomically thin layers has led to the exciting field of twistronics. However, practical applications of such systems are challenging since the specific angular correlations between the layers must be precisely controlled and the layers have to be single crystalline with uniform atomic ordering. Here, we suggest an alternative, simple and scalable approach where nanocrystalline two-dimensional (2D) film on three-dimensional (3D) substrates yield twisted-interface-dependent properties. Ultrawide-bandgap hexagonal boron nitride (h-BN) thin films are directly grown on high in-plane lattice mismatched wide-bandgap silicon carbide (4H-SiC) substrates to explore the twist-dependent structure-property correlations. Concurrently, nanocrystalline h-BN thin film shows strong non-linear second-harmonic generation and ultra-low cross-plane thermal conductivity at room temperature, which are attributed to the twisted domain edges between van der Waals stacked nanocrystals with random in-plane orientations. First-principles calculations based on time-dependent density functional theory manifest strong even-order optical nonlinearity in twisted h-BN layers. Our work unveils that directly deposited 2D nanocrystalline thin film on 3D substrates could provide easily accessible twist-interfaces, therefore enabling a simple and scalable approach to utilize the 2D-twistronics integrated in 3D material devices for next-generation nanotechnology.

1.Calculation of Carrier Doping-Induced Half-Metallicity, and Transformation of Easy Axis in Two-Dimensional MSi2N4 (M = Cr, Mn, Fe, and Co) Monolayers

Authors:Ziyuan An, Linhui Lv, Ya Sū, Yanyan Jiang, Zhaohong Guan

Abstract: We study the stability, electrical properties, and magnetic properties of MSi2N4 (M = Cr, Mn, Fe, and Co) monolayers based on the density functional theory.

2.Efficient GW calculations via the interpolation of the screened interaction in momentum and frequency space: The case of graphene

Authors:Alberto Guandalini, Dario A. Leon, Pino D'Amico, Claudia Cardoso, Andrea Ferretti, Daniele Varsano

Abstract: The calculation of the GW self-energy may be a computational challenge due to the double convolution integrals over frequency and transferred momentum. In this work, we combine the recently developed multipole approximation (MPA) with the W-av method. MPA accurately approximates full-frequency response functions using a small number of poles, while W-av improves the convergence with respect to the Brillouin zone (BZ) sampling in 2D materials. The combination of these techniques is applied to obtain an accurate G0W0 QP band structure of graphene. The screened interaction of graphene shows a complex low-energy frequency dependence, that is poorly described with plasmon pole approximations (PPA), and a sharp q dependence of the dynamical dielectric function over momentum transfer, making standard BZ integration techniques inefficient. Within the present development, we compare the calculated QP band structure of graphene finding an excellent agreement with angle resolved photoemission spectroscopy (ARPES) measurements.

3.First-Principles Modeling of Equilibration Dynamics of Hyperthermal Products of Surface Reactions Using Scalable Neural Network Potential

Authors:Qidong Lin, Bin Jiang

Abstract: Equilibration dynamics of hot oxygen atoms following O2 dissociation on Pd(100) and Pd(111) surfaces are investigated by molecular dynamics simulations based on a scalable neural network potential enabling first-principles description of O2 and O interacting with variable Pd supercells. We find that to accurately describe the equilibration dynamics after dissociation, the simulation cell length necessarily exceeds twice the maximum distance of equilibrated oxygen adsorbates. By analyzing hundreds of trajectories with appropriate initial sampling, the measured distance distribution of equilibrated atom pairs on Pd(111) is well reproduced. However, our results on Pd(100) suggest that the ballistic motion of hot atoms predicted previously is a rare event under ideal conditions, while initial molecular orientation and surface thermal fluctuation could significantly affect the overall post-dissociation dynamics. On both surfaces, dissociated oxygen atoms remain primarily locate their nascent positions and then randomly cross bridge sites nearby.

4.Comparison of Matlantis and VASP bulk formation and surface energies in metal hydrides, carbides, nitrides, oxides, and sulfides

Authors:Shinya Mine, Takashi Toyao, Ken-ichi Shimizu, Yoyo Hinuma

Abstract: Generic neural network potentials without forcing users to train potentials could result in significantly acceleration of total energy calculations. Takamoto et al. [Nat. Commun. (2022), 13, 2991] developed such a deep neural network potential (NNP) and made it available in their Matlantis package. We compared the Matlantis bulk formation, surface, and surface O vacancy formation energies of metal hydrides, carbides, nitrides, oxides, and sulfides with our previously calculated VASP values obtained from first-principles with the PBEsol(+U) functional. Matlantis bulk formation energies were consistently ~0.1 eV/atom larger and the surface energies were typically ~10 meV/{\AA}^2 smaller than the VASP counterpart. Surface O vacancy formation energies were generally underestimated within ~0.8 eV. These results suggest that Matlantis energies could serve as a relatively good descriptor of the VASP bulk formation and surface energies.

5.Exploiting the close-to-Dirac point shift of Fermi level in Sb2Te3/Bi2Te3 topological insulator heterostructure for spin-charge conversion

Authors:E. Longo, L. Locatelli, P. Tsipas, A. Lintzeris, A. Dimoulas, M. Fanciulli, M. Longo, R. Mantovan

Abstract: Properly tuning the Fermi level position in topological insulators is of vital importance to tailor their spin-polarized electronic transport and to improve the efficiency of any functional device based on them. Here we report the full in situ Metal Organic Chemical Vapor Deposition (MOCVD) and study of a highly crystalline Bi2Te3/Sb2Te3 topological insulator heterostructure on top of large area (4'') Si(111) substrates. The bottom Sb2Te3 layer serves as an ideal seed layer for the growth of highly crystalline Bi2Te3 on top, also inducing a remarkable shift of the Fermi level to place it very close to the Dirac point, as visualized by angle-resolved photoemission spectroscopy. In order to exploit such ideal topologically-protected surface states, we fabricate the simple spin-charge converter Si(111)/Sb2Te3/Bi2Te3/Au/Co/Au and spin-charge conversion (SCC) is probed by spin pumping ferromagnetic resonance. A large SCC is measured at room temperature, which is interpreted within the inverse Edelstein effect (IEE), thus resulting in a conversion efficiency lambda_IEE of 0.44 nm. Our results demonstrate the successful tuning of the surface Fermi level of Bi2Te3 when grown on top of Sb2Te3 with a full in situ MOCVD process, which is highly interesting in view of its future technology transfer.

6.Vibrational dynamics of CO on Pd(111) in and out of thermal equilibrium

Authors:Bombín Raúl, S. Muzas Alberto, Novko Dino, Juaristi J. Iñaki, Alducin Maite

Abstract: Using many-body perturbation theory and density functional perturbation theory, we study the vibrational spectra of the internal stretch (IS) mode of CO on Pd(111) for the bridge and hollow adsorption structures that are experimentally identified at 0.5~ML coverage. Our theoretical treatment allows us to determine the temperature dependence of the IS vibrational spectra under thermal conditions as well as the time evolution of the non-equilibrium transient spectra induced by femtosecond laser pulses. Under thermal conditions (i.e., for equal electronic $T_e$ and phononic $T_l$ temperatures), the calculated lifetimes at 10-150~K are mostly due to nonadiabatic couplings (NC), i.e., first-order electronic excitations. As temperature increases, also the contribution of the second-order electron mediated phonon-phonon couplings (EMPPC) progressively increases from 25\% at low temperatures to 50\% at 300~K. Our calculations for the laser-induced non-equilibrium conditions comprise experimental absorbed fluences of 6-130~J/m$^2$. For fluences for which $T_e>$2000~K, the transient vibrational spectra are characterized by two different regimes that follow the distinct time-evolution of $T_e$ and $T_l$ and are respectively dominated by NC and EMPPC processes. At lower fluences, the initial fast regime becomes progressively negligible as $T_e$ decreases and only the steady second regime remains visible. Qualitatively, all these spectral properties are common to the both adsorption structures studied here.

7.Ab initio calculation for electronic structure and optical property of tungsten carbide in a TiCN-based cermet for solar thermal applications

Authors:Shota Hayakawa, Toshiharu Chono, Kosuke Watanabe, Shoya Kawano, Kazuma Nakamura, Koji Miyazaki

Abstract: We present an ab initio calculation to understand electronic structures and optical properties of a tungsten carbide WC being a major component of a TiCN-based cermet. We found that the WC has a fairly low-energy plasma excitation $\sim$0.6 eV (2 $\mu$m) and therefore can be a good constituent of a solar selective absorber. The evaluated figure of merit for photothermal conversion is prominently high compared to those of the other materials included in the TiCN-based cermet. The imaginary part of the dielectric function is considerably small around the zero point of the real part of the dielectric function, corresponding to the plasma excitation energy. Therefore, a clear plasma edge appeared, ensuring the high performance of the WC as the solar absorber.

8.Whats special about Y6; the working mechanism of neat Y6 organic solar cell

Authors:Elifnaz Saglamkaya, Artem Musiienko, Mohammad Saeed Shadabroo, Bowen Sun, Sreelakshmi Chandrabose, Giulia Lo Gerfo M, Niek F van Hulst, Dieter Neher, Safa Shoaee

Abstract: Non-fullerene acceptors (NFA) have delivered advance in bulk heterojunction organic solar cell efficiencies, with the significant milestone of 20% now in sight. However, these materials challenge the accepted wisdom of how organic solar cells work. In this work we present neat Y6 device with efficiency above 4.5%. We thoroughly investigate mechanisms of charge generation and recombination as well as transport in order to understand what is special about Y6. Our data suggest Y6 generates bulk free charges, with ambipolar mobility, which can be extracted in the presence of transport layers

9.Glass fracture surface energy calculated from crystal structure and bond-energy data

Authors:Marco Holzer, Tina Waurischk, Janine George, Robert Maaß, Ralf Müller

Abstract: We present a novel method to predict the fracture surface energy of oxide glasses, {\gamma}, using readily available crystallographic structure data of their isochemical crystal and tabled diatomic chemical bond energies, D0. The method assumes that {\gamma} equals the fracture surface energy of the most likely cleavage plane of the crystal. Calculated values were in excellent agreement with those calculated from measured glass density and D0 in an earlier work. This finding demonstrates a remarkable equivalence between crystal cleavage planes and glass fracture surfaces.

10.Optical properties of MoSe$_2$ monolayer implanted with ultra-low energy Cr ions

Authors:Minh N. Bui Peter Grünberg Institute 9 Department of Physics, RWTH Aachen University, Aachen, Germany, Stefan Rost Peter Grünberg Institute 1 Department of Physics, RWTH Aachen University, Aachen, Germany, Manuel Auge II. Institute of Physics, University of Göttingen, Göttingen, Germany, Lanqing Zhou Peter Grünberg Institute 9 Department of Physics, RWTH Aachen University, Aachen, Germany, Christoph Friedrich Peter Grünberg Institute 1, Stefan Blügel Peter Grünberg Institute 1 Department of Physics, RWTH Aachen University, Aachen, Germany, Silvan Kretschmer Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany, Arkady V. Krasheninnikov Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany Department of Applied Physics, Aalto University School of Science, Aalto, Finland, Kenji Watanabe Research Center for Functional Materials, National Institute for Materials Science, Namiki, Tsukuba, Japan, Takashi Taniguchi International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Namiki, Tsukuba, Japan, Hans C. Hofsäss II. Institute of Physics, University of Göttingen, Göttingen, Germany, Detlev Grützmacher Peter Grünberg Institute 9 Department of Physics, RWTH Aachen University, Aachen, Germany, Beata E. Kardynał Peter Grünberg Institute 9 Department of Physics, RWTH Aachen University, Aachen, Germany

Abstract: The paper explores the optical properties of an exfoliated MoSe$_2$ monolayer implanted with Cr$^+$ ions, accelerated to 25 eV. Photoluminescence of the implanted MoSe$_2$ reveals an emission line from Cr-related defects that is present only under weak electron doping. Unlike band-to-band transition, the Cr-introduced emission is characterised by non-zero activation energy, long lifetimes, and weak response to the magnetic field. To rationalise the experimental results and get insights into the atomic structure of the defects, we modelled the Cr-ion irradiation process using ab-initio molecular dynamics simulations followed by the electronic structure calculations of the system with defects. The experimental and theoretical results suggest that the recombination of electrons on the acceptors, which could be introduced by the Cr implantation-induced defects, with the valence band holes is the most likely origin of the low energy emission. Our results demonstrate the potential of low-energy ion implantation as a tool to tailor the properties of 2D materials by doping.

11.Conversion of La$_2$Ti$_2$O$_7$ to LaTiO$_2$N via Ammonolysis: An ab-initio Investigation

Authors:Chiara Ricca, Tristan Blandenier, Valérie Werner, Xing Wang, Simone Pokrant, Ulrich Aschauer

Abstract: Perovskite oxynitrides are, due to their reduced band gap compared to oxides, promising materials for photocatalytic applications. They are most commonly synthesized from {110} layered Carpy-Galy (A$_2$B$_2$O$_7$}) perovskites via thermal ammonolysis, i.e. the exposure to a flow of ammonia at elevated temperature. The conversion of the layered oxide to the non-layered oxynitride must involve a complex combination of nitrogen incorporation, oxygen removal and ultimately structural transition by elimination of the interlayer shear plane. Despite the process being commonly used, little is known about the microscopic mechanisms and hence factors that could ease the conversion. Here we aim to derive such insights via density functional theory calculations of the defect chemistry of the oxide and the oxynitride as well as the oxide's surface chemistry. Our results point to the crucial role of surface oxygen vacancies in forming clusters of NH$_3$ decomposition products and in incorporating N, most favorably substitutionally at the anion site. N then spontaneously diffuses away from the surface, more easily parallel to the surface and in interlayer regions, while diffusion perpendicular to the interlayer plane is somewhat slower. Once incorporation and diffusion lead to a local N concentration of about 70% of the stoichiometric oxynitride composition, the nitridated oxide spontaneously transforms to a nitrogen-deficient oxynitride.

12.Self-consistent homogenization approach for polycrystals within second gradient elasticity

Authors:Yury Solyaev

Abstract: In this paper, we propose a generalized variant of Kr\"oner's self-consistent scheme for evaluation of the effective standard and gradient elastic moduli of polycrystalline materials within Mindlin-Toupin second-gradient elasticity theory. Assuming random orientation of crystallites (grains) we use an extended Eshelby's equivalent inclusion method and mapping conditions between the prescribed linear distribution of macro-strain and corresponding micro-scale field variables averaged over the volume and all possible orientations of single grain. It is found that developed self-consistent scheme predicts the absence of strong gradient effects at the macro-scale level for the model of spherical grains. However, for the more general shape of the grains, considered approach allows to obtain a set of non-linear relations for determination of all effective standard and gradient elastic moduli of polycrystals.

13.In-situ study and modeling of the reaction kinetics during molecular beam epitaxy of GeO2 and its etching by Ge

Authors:Wenshan Chen, Kingsley Egbo, Hans Tornatzky, Manfred Ramsteiner, Markus Wagner, Elias Kluth, Martin Feneberg, Rüdiger Goldhahn, Oliver Bierwagen

Abstract: Rutile GeO2 has been predicted to be an ultra-wide bandgap semiconductor suitable for future power electronics devices while quartz-like GeO2 shows piezoelectric properties. To explore these crystalline phases for application and fundamental materials investigations, molecular beam epitaxy (MBE) is a well-suited thin film growth technique. In this study, we investigate the reaction kinetics of GeO2 during plasma-assisted MBE using elemental Ge and plasma-activated oxygen fluxes. The growth rate as a function of oxygen flux is measured in-situ by laser reflectometry at different growth temperatures. A flux of the suboxide GeO desorbing off the growth surface is identified and quantified in-situ by the line-of-sight quadrupole mass spectrometry. Our measurements reveal that the suboxide formation and desorption limits the growth rate under metal-rich or high temperature growth conditions, and leads to etching of the grown GeO2 layer under Ge flux in the absence of oxygen. The quantitative results fit the sub-compound mediated reaction model, indicating the intermediate formation of the suboxide at the growth front. This model is further utilized to delineate the GeO2-growth window in terms of oxygen-flux and substrate temperature. Our study can serve as a guidance for the thin film synthesis of GeO2 and defect-free mesa etching in future GeO2-device processing.

14.What is missing in autonomous discovery: Open challenges for the community

Authors:Phillip M. Maffettone, Pascal Friederich, Sterling G. Baird, Ben Blaiszik, Keith A. Brown, Stuart I. Campbell, Orion A. Cohen, Tantum Collins, Rebecca L. Davis, Ian T. Foster, Navid Haghmoradi, Mark Hereld, Howie Joress, Nicole Jung, Ha-Kyung Kwon, Gabriella Pizzuto, Jacob Rintamaki, Casper Steinmann, Luca Torresi, Shijing Sun

Abstract: Self-driving labs (SDLs) leverage combinations of artificial intelligence, automation, and advanced computing to accelerate scientific discovery. The promise of this field has given rise to a rich community of passionate scientists, engineers, and social scientists, as evidenced by the development of the Acceleration Consortium and recent Accelerate Conference. Despite its strengths, this rapidly developing field presents numerous opportunities for growth, challenges to overcome, and potential risks of which to remain aware. This community perspective builds on a discourse instantiated during the first Accelerate Conference, and looks to the future of self-driving labs with a tempered optimism. Incorporating input from academia, government, and industry, we briefly describe the current status of self-driving labs, then turn our attention to barriers, opportunities, and a vision for what is possible. Our field is delivering solutions in technology and infrastructure, artificial intelligence and knowledge generation, and education and workforce development. In the spirit of community, we intend for this work to foster discussion and drive best practices as our field grows.

1.Spectroscopic studies on phosphate-modified silicon oxycarbide-based amorphous materials

Authors:Magdalena Gawęda, Piotr Jeleń, Maciej Bik, Magdalena Szumera, Zbigniew Olejniczak, Maciej Sitarz

Abstract: Vibrational spectroscopy is the most effective, efficient and informative method of structural analysis of amorphous materials with silica matrix and, therefore, an indispensable tool for examining silicon oxycarbide-based amorphous materials (SiOC). The subject of this work is a description of the modification process of SiOC glasses with phosphate ions based on the structural examination including mainly Infrared and Raman Spectroscopy. They were obtained as polymer-derived ceramics based on ladder-like silsesquioxanes synthesised via the sol-gel method. With the high phosphate's volatility, it was decided to introduce the co-doping ions to create [AlPO4] and [BPO4] stable structural units. As a result, several samples from the SiPOC, SiPAlOC and SiPBOC systems were obtained with various quantities of the modifiers. All samples underwent a detailed structural evaluation of both polymer precursors and ceramics after high-temperature treatment with Fourier-transformed infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD) and magic angle spinning nuclear magnetic resonance (MAS-NMR). Obtained results proved the efficient preparation of desired materials that exhibit structural parameters similar to the unmodified one. They were X-ray-amorphous with no phase separation and crystallisation. Spectroscopic measurements confirmed the presence of the crucial Si-C bond and how modifying ions are incorporated into the SiOC network. It was also possible to characterise the turbostratic free carbon phase. The modification was aimed to improve the bioperformance of the materials in the context of their future application as bioactive coatings on metallic implants.

2.Surface and in-depth structural changes in nuclear graphite irradiated with noble gases described with Raman imaging

Authors:Magdalena Gawęda, Magdalena Wilczopolska, Kinga Suchorab, Małgorzata Frelek-Kozak, Łukasz Kurpaska, Jacek Jagielski

Abstract: 4th Generation high-temperature gas-cooled nuclear reactors (HTGR) are regarded as possible sources of industrial heat in Poland and Europe, allowing for a substantial reduction of the dependency on gas and coal import. It is mainly due to their safety of use, reliability and economy in a current energetic crisis. In this work, graphite, as a primary construction material and neutron moderator in HTGR, was evaluated before and after ion irradiation since its properties depend on the material's structure and purity. Commercial graphite materials (IG-110, NBG-17) and the laboratory's in-home material were chosen for the exemplary samples. The structural damage in HTGR was simulated with energetic Ar+ and He+ ions with fluencies from 1E12 to 2E17 ion/cm2. Raman imaging was chosen to assess radiation damage build-up: the crystallites' evolution, occurrence and types of defects. The recorded evolution showed stronger disordering of the material with heavier Ar+ ions than with He+.

3.Noncollinear DFT+$U$ and Hubbard parameters with fully-relativistic ultrasoft pseudopotentials

Authors:Luca Binci, Nicola Marzari

Abstract: The magnetic, noncollinear parametrization of Dudarev's DFT+$U$ method is generalized to fully-relativistic ultrasoft pseudopotentials. We present the definition of the DFT+$U$ total energy functional, and the calculation of forces and stresses in the case of orthogonalized atomic orbitals defining the localised Hubbard manifold, where additional contributions arising from the derivative of the inverse square root of the overlap matrix appear. We further extend the perturbative calculation of the Hubbard $U$ parameters within density-functional perturbation theory to the noncollinear relativistic case, by exploiting an existing and recently developed theoretical approach that takes advantage of the time-reversal operator to solve a second Sternheimer equation. We validate and apply the new scheme by studying the electronic structure and the thermodynamics of the binary compounds EuX (where X = O, S, Se, Te is a chalcogen atom), as representative simple crystals, and of the pyrochlore Cd$_2$Os$_2$O$_7$, representative of a more structurally complex oxide.

4.Extending intergranular normal-stress distributions using symmetries of isotropic linear-elastic polycrystalline materials

Authors:S. El Shawish

Abstract: Intergranular normal stresses (INS) are critical in the initiation and evolution of grain boundary damage in polycrystalline materials. To model the effects of such microstructural damage on a macroscopic scale, knowledge of INS is usually required statistically at each representative volume element subjected to various loading conditions. However, calculating INS distributions for different stress states can be time-consuming. This study proposes a new method to extend existing INS distributions to arbitrary loading conditions using the symmetries of isotropic linear-elastic polycrystalline materials. The method relies on a fact that INS distributions can be accurately reproduced from the first (typically) ten statistical moments, which depend trivially on just two deviatoric-stress invariants and a few material invariants due to assumed isotropy and linearity of the polycrystalline model. While these material invariants are complex averages, they can be extracted numerically from a few existing INS distributions and tabulated for later use. Practically, only two such INS distributions at properly selected loadings are required to provide all relevant material invariants for the first 11 statistical moments, which can then be used to reconstruct the INS distribution for arbitrary loading conditions. The proposed approach is demonstrated to be accurate and feasible for an arbitrarily selected linear-elastic material under various loading conditions.

5.An extreme value statistics model of heterogeneous ice nucleation for quantifying the stability of supercooled aqueous systems

Authors:Anthony N. Consiglio, Yu Ouyang, Matthew J. Powell-Palm, Boris Rubinsky

Abstract: The propensity of water to remain in a metastable liquid state at temperatures below its equilibrium melting point holds significant potential for cryopreserving biological material such as tissues and organs. The benefits conferred are a direct result of progressively reducing metabolic expenditure due to colder temperatures while simultaneously avoiding the irreversible damage caused by the crystallization of ice. Unfortunately, the freezing of water in bulk systems of clinical relevance is dominated by random heterogeneous nucleation initiated by uncharacterized trace impurities, and the marked unpredictability of this behavior has prevented implementation of supercooling outside of controlled laboratory settings and in volumes larger than a few milliliters. Here, we develop a statistical model that jointly captures both the inherent stochastic nature of nucleation using conventional Poisson statistics as well as the random variability of heterogeneous nucleation catalysis through bivariate extreme value statistics. Individually, these two classes of models cannot account for both the time-dependent nature of nucleation and the sample-to-sample variability associated with heterogeneous catalysis, and traditional extreme value models have only considered variation of the characteristic nucleation temperature. We conduct a series of constant cooling rate and isothermal nucleation experiments with physiological saline solutions and leverage the statistical model to evaluate the natural variability of kinetic and thermodynamic nucleation parameters. By quantifying freezing probability as a function of temperature, supercooled duration, and system volume, while accounting for nucleation site variability, this study also provides a basis for the rational design of stable supercooled biopreservation protocols.

1.Probing Phonon dynamics and Electron-Phonon Coupling by High Harmonic Generation in Solids

Authors:Shi-Qi Hu, Hui Zhao, Xin-Bao Liu, Da-Qiang Chen, Sheng Meng

Abstract: Acting as a highly nonlinear response to the strong laser field, high harmonic generation (HHG) naturally contains the fingerprints of atomic and electronic properties of materials. Electronic properties of a solid such as band structure and topology can thus be probed, while the phonon dynamics during HHG are often neglected. Here we show that by exploiting the effects of phonon deformation on HHG, the intrinsic phonon information can be deciphered and direct probing of band- and mode-resolved electron-phonon couplings (EPC) of photoexcited materials is possible. Considering HHG spectroscopy can be vacuum free and unrestricted to electron occupation, this work suggests HHG is promising for all-optical characterization of EPC in solids, especially for gapped quantum states or materials under high pressure.

2.Room-temperature antiferromagnetic CrSe monolayer with tunable metal-insulator transition in ferroelectric heterostructures

Authors:Xiao-Sheng Ni, Yue-Yu Zhang, Dao-Xin Yao, Yusheng Hou

Abstract: Recently, there has been a rapidly growing interest in two-dimensional (2D) transition metal chalcogenide monolayers (MLs) due to their unique magnetic and electronic properties. By using an evolutionary algorithm and first-principles calculations, we report the discovery of a previously unexplored, chemically, energetically, and thermodynamically stable 2D antiferromagnetic (AFM) CrSe ML with a N\'eel temperature higher than room temperature. Remarkably, we predict an electric field-controllable metal-insulator transition (MIT) in a van der Waals (vdW) heterostructure comprised of CrSe ML and ferroelectric Sc2CO2. This tunable transition in CrSe/Sc2CO2 heterostructure is attributed to the change in the band alignment between CrSe and Sc2CO2 caused by the ferroelectric polarization reversal in Sc2CO2. Our findings suggest that 2D AFM CrSe ML has important potential applications in AFM spintronics, particularly in the gate voltage conducting channel.

3.Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3D Nanoscale Strain Mapping

Authors:Kieran W. P. Orr, Jiecheng Diao, Muhammad Naufal Lintangpradipto, Darren J. Batey, Affan N. Iqbal, Simon Kahmann, Kyle Frohna, Milos Dubajic, Szymon J. Zelewski, Alice E. Dearle, Thomas A. Selby, Peng Li, Tiarnan A. S. Doherty, Stephan Hofmann, Osman M. Bakr, Ian K. Robinson, Samuel D. Stranks

Abstract: In recent years, halide perovskite materials have been used to make high performance solar cell and light-emitting devices. However, material defects still limit device performance and stability. Here, we use synchrotron-based Bragg Coherent Diffraction Imaging to visualise nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. We find significant strain heterogeneity within MAPbBr$_{3}$ (MA = CH$_{3}$NH$_{3}^{+}$) crystals in spite of their high optoelectronic quality, and identify both $\langle$100$\rangle$ and $\langle$110$\rangle$ edge dislocations through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, we uncover dramatic light-induced dislocation migration across hundreds of nanometres. Further, by selectively studying crystals that are damaged by the X-ray beam, we correlate large dislocation densities and increased nanoscale strains with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. Our results demonstrate the dynamic nature of extended defects and strain in halide perovskites and their direct impact on device performance and operational stability.

4.Screening Spin Lattice Interaction Using Deep Learning Approach

Authors:Teng Yang, Zefeng Cai, Zhengtao Huang, Wenlong Tang, Ruosong Shi, Andy Godfrey, Hanxing Liu, Yuanhua Lin, Ce-Wen Nan, LinFeng Zhang, Han Wang, Ben Xu

Abstract: Atomic simulations hold significant value in clarifying crucial matters such as phase transitions and energy transport in materials science. Their success stems from the presence of potential energy functions capable of accurately depicting the relationship between system energy and lattice changes. In magnetic materials, two atomic scale degrees of freedom come into play: the lattice and the magnetic moment. Nonetheless, precisely portraying the interaction energy and its impact on lattice and spin-driving forces, such as atomic force and magnetic torque, remains a formidable task in the computational domain. Consequently, there is no atomic-scale approach capable of elucidating the evolution of lattice and spin at the same time in magnetic materials. Addressing this knowledge deficit, we present DeepSPIN, a versatile approach that generates high-precision predictive models of energy, atomic forces, and magnetic torque in magnetic systems. This is achieved by integrating first-principles calculations of magnetic excited states with advanced deep learning techniques via active learning. We thoroughly explore the methodology, accuracy, and scalability of our proposed model in this paper. Our technique adeptly connects first-principles computations and atomic-scale simulations of magnetic materials. This synergy presents opportunities to utilize these calculations in devising and tackling theoretical and practical obstacles concerning magnetic materials.

5.Stochastic theory of ferroelectric domain structure formation dominated by quenched disorder

Authors:Olga Y. Mazur, Leonid I. Stefanovich, Yuri A. Genenko

Abstract: A self-consistent stochastic model of domain structure formation in a uniaxial ferroelectric, quenched from a high-temperature paraelectric phase to a low-temperature ferroelectric phase, is developed with an account of the applied electric field and the feedback effect via local depolarization fields. Both polarization and field components are considered as Gauss random variables. A system of integro-differential equations for correlation functions of all involved variables is derived and solved analytically and numerically. Phase diagram in terms of the average value and dispersion of polarization reveals different possible equilibrium states and available final single-domain and multi-domain states. The time-dependent evolution of the average polarization and dispersion discloses a bifurcation behavior and the temperature-dependent value of the electric field, deciding between the single-domain and multi-domain final states, which can be interpreted as the coercive field. Analytical and numerical results for the time-dependent correlation length and correlation functions exhibit plausible agreement with available experimental data.

6.Transferable Interatomic Potentials for Aluminum from Ambient Conditions to Warm Dense Matter

Authors:Sandeep Kumar, Hossein Tahmasbi, Kushal Ramakrishna, Mani Lokamani, Svetoslav Nikolov, Julien Tranchida, Mitchell A. Wood, Attila Cangi

Abstract: We present a study on the transport and materials properties of aluminum spanning from ambient to warm dense matter conditions using a machine-learned interatomic potential (ML-IAP). Prior research has utilized ML-IAPs to simulate phenomena in warm dense matter, but these potentials have often been calibrated for a narrow range of temperature and pressures. In contrast, we train a single ML-IAP over a wide range of temperatures, using density functional theory molecular dynamics (DFT-MD) data. Our approach overcomes computational limitations of DFT-MD simulations, enabling us to study transport and materials properties of matter at higher temperatures and longer time scales. We demonstrate the ML-IAP transferability across a wide range of temperatures using molecular-dynamics (MD) by examining the thermal conductivity, diffusion coefficient, viscosity, sound velocity, and ion-ion structure factor of aluminum up to about 60,000 K, where we find good agreement with previous theoretical data.

7.Activated and quantum creep of the charge-density waves in magnetic field in {\it o}-TaS$_3$

Authors:I. A. Cohn, S. V. Zaitsev-Zotov

Abstract: We demonstrate that magnetoresistance of creeping charge-density waves in the quasi-one dimensional conductor {\it o}-TaS$_3$ changes its character from a negative parabolic at $T\gtrsim 10$ K where it obeys $1/T^2$ law to a weakly temperature dependent negative nearly linear one at lower temperatures. The dominant contribution into the negative parabolic magnetoresistance comes from magnetic field induced splitting of the CDW order parameter. The linear magnetoresistance arises due to CDW quantum interference similar to the scenario of negative linear magnetoresistance in single-electron systems.

1.Transient non-collinear magnetic state for all-optical magnetization switching

Authors:Sergii Parchenko, Antoni Frej, Hiroki Ueda, Robert Carley, Laurent Mercadier, Natalia Gerasimova, Giuseppe Mercurio, Justine Schlappa, Alexander Yaroslavtsev, Naman Agarwal, Rafael Gort, Andreas Scherz, Anatoly Zvezdin, Andrzej Stupakiewicz, Urs Staub

Abstract: Resonant absorption of a photon by bound electrons in a solid can promote an electron to another orbital state or transfer it to a neighboring atomic site. Such a transition in a magnetically ordered material could affect the magnetic order. While this process is an obvious road map for optical control of magnetization, experimental demonstration of such a process remains challenging. Exciting a significant fraction of magnetic ions requires a very intense incoming light beam, as orbital resonances are often weak compared to above-band-gap excitations. In the latter case, a sizeable reduction of the magnetization occurs as the absorbed energy increases the spin temperature, masking the non-thermal optical effects. Here, using ultrafast x-ray spectroscopy, we were able to resolve changes in the magnetization state induced by resonant absorption of infrared photons in Co-doped yttrium iron garnet, with negligible thermal effects. We found that the optical excitation of the Co ions affects the two distinct magnetic Fe sublattices differently, resulting in a transient non-collinear magnetic state. The present results indicate that the all-optical magnetization switching most likely occurs due to the creation of a transient, non-collinear magnetic state followed by coherent spin rotations of the Fe moments.

2.Electron irradiation: from test to material tayloring

Authors:A. Alessi, O. Cavani, R. Grasset, H. -J. Drouhin, V. I. Safarov, M. Konczykowski

Abstract: In this article, we report some examples of how high-energy electron irradiation can be used as a tool for shaping material properties turning the generation of point-defects into an advantage beyond the presumed degradation of the properties. Such an approach is radically different from what often occurs when irradiation is used as a test for radiation hard materials or devices degradation in harsh environments. We illustrate the potential of this emerging technique by results obtained on two families of materials, namely semiconductors and superconductors.

3.Predicting the Charge Density Response in Metal Electrodes

Authors:Andrea Grisafi, Augustin Bussy, Rodolphe Vuilleumier

Abstract: The computational study of energy storage and conversion processes call for simulation techniques that can reproduce the electronic response of metal electrodes under electric fields. Despite recent advancements in machine-learning methods applied to electronic-structure properties, predicting the non-local behaviour of the charge density in electronic conductors remains a major open challenge. We combine long-range and equivariant kernel methods to predict the Kohn-Sham electron density of metal electrodes decomposed on an atom-centered basis. By taking slabs of gold as an example, we show that including long-range correlations into the learning model is essential to accurately reproduce the charge density and potential in bare electrodes of increasing size. A finite-field extension of the method is then introduced, which allows us to predict the charge transfer and the electrostatic potential drop induced by the application of an external electric field. Finally, we demonstrate the capability of the method to extrapolate the non-local electronic polarization generated by the interaction with an ionic species for electrodes of arbitrary thickness. Our study represents an important step forward in the accurate simulation of energy materials that include metallic interfaces.

4.High quality and wafer-scale cubic silicon carbide single crystals

Authors:Guobin Wang, Da Sheng, Yunfan Yang, Hui Li, Congcong Chai, Zhenkai Xie, Wenjun Wang, Jian-gang Guo, Xiaolong Chen

Abstract: Silicon carbide (SiC) is an important semiconductor material for fabricating power electronic devices that exhibit higher switch frequency, lower energy loss and substantial reduction both in size and weight in comparison with its Si-based counterparts1-4. Currently, most devices, such as metal-oxide-semiconductor field effect transistors, which are core devices used in electric vehicles, photovoltaic industry and other applications, are fabricated on a hexagonal polytype 4H-SiC because of its commercial availability5. Cubic silicon carbide (3C-SiC), the only cubic polytype, has a moderate band gap of 2.36 eV at room-temperature, but a superior mobility and thermal conduction than 4H-SiC4,6-11. Moreover, the much lower concentration of interfacial traps between insulating oxide gate and 3C-SiC helps fabricate reliable and long-life devices7-10,12-14. The growth of 3C-SiC crystals, however, has remained a challenge up to now despite of decades-long efforts by researchers because of its easy transformation into other polytypes during growth15-19, limiting the 3C-SiC based devices. Here, we report that 3C-SiC can be made thermodynamically favored from nucleation to growth on a 4H-SiC substrate by top-seeded solution growth technique(TSSG), beyond what's expected by classic nucleation theory. This enables the steady growth of quality and large sized 3C-SiC crystals (2~4-inch in diameter and 4.0~10.0 mm in thickness) sustainable. Our findings broaden the mechanism of hetero-seed crystal growth and provide a feasible route to mass production of 3C-SiC crystals,offering new opportunities to develop power electronic devices potentially with better performances than those based on 4H-SiC.

5.Controllable Strain-driven Topological Phase Transition and Dominant Surface State Transport in High-Quality HfTe5 Samples

Authors:Jinyu Liu, Yinong Zhou, Sebastian Yepez Rodriguez, Matthew A. Delmont, Robert A. Welser, Nicholas Sirica, Kaleb McClure, Paolo Vilmercati, Joseph W. Ziller, Norman Mannella, Javier D. Sanchez-Yamagishi, Michael T. Pettes, Ruqian Wu, Luis A. Jauregui

Abstract: Controlling materials to create and tune topological phases of matter could potentially be used to explore new phases of topological quantum matter and to create novel devices where the carriers are topologically protected. It has been demonstrated that a trivial insulator can be converted into a topological state by modulating the spin-orbit interaction or the crystal lattice. However, there are limited methods to controllably and efficiently tune the crystal lattice and at the same time perform electronic measurements at cryogenic temperatures. Here, we use large controllable strain to demonstrate the topological phase transition from a weak topological insulator phase to a strong topological insulator phase in high-quality HfTe5 samples. After applying high strain to HfTe5 and converting it into a strong topological insulator, we found that the sample's resistivity increased by more than two orders of magnitude (24,000%) and that the electronic transport is dominated by the topological surface states at cryogenic temperatures. Our findings show that HfTe5 is an ideal material for engineering topological properties, and it could be generalized to study topological phase transitions in van der Waals materials and heterostructures. These results can pave the way to create novel devices with applications ranging from spintronics to fault-tolerant topologically protected quantum computers.

6.Single-test evaluation of directional elastic properties of anisotropic structured materials

Authors:Jagannadh Boddapati, Moritz Flaschel, Siddhant Kumar, Laura De Lorenzis, Chiara Daraio

Abstract: When the elastic properties of structured materials become direction-dependent, the number of their descriptors increases. For example, in two-dimensions, the anisotropic behavior of materials is described by up to 6 independent elastic stiffness parameters, as opposed to only 2 needed for isotropic materials. Such high number of parameters expands the design space of structured materials and leads to unusual phenomena, such as materials that can shear under uniaxial compression. However, an increased number of properties descriptors and the coupling between shear and normal deformations render the experimental evaluation of material properties more challenging. In this paper, we propose a methodology based on the virtual fields method to identify six separate stiffness tensor parameters of two-dimensional anisotropic structured materials using just one tension test, thus eliminating the need for multiple experiments, as it is typical in traditional methods. The approach requires no stress data and uses full-field displacement data and global force data. We show the accuracy of our method using synthetic data generated from finite element simulations as well as experimental data from additively manufactured specimens

7.Lattice deformation at the sub-micron scale: X-ray nanobeam measurements of elastic strain in electron shuttling devices

Authors:C. Corley-Wiciak IHP - Leibniz-Institut für innovative Mikroelektronik, Frankfurt, M. H. Zoellner IHP - Leibniz-Institut für innovative Mikroelektronik, Frankfurt, I. Zaitsev IHP - Leibniz-Institut für innovative Mikroelektronik, Frankfurt, K. Anand IHP - Leibniz-Institut für innovative Mikroelektronik, Frankfurt, E. Zatterin ESRF - European Synchrotron Radiation Facility, Grenoble, France, Y. Yamamoto IHP - Leibniz-Institut für innovative Mikroelektronik, Frankfurt, A. A. Corley-Wiciak IHP - Leibniz-Institut für innovative Mikroelektronik, Frankfurt, F. Reichmann IHP - Leibniz-Institut für innovative Mikroelektronik, Frankfurt, W. Langheinrich Infineon Technologies Dresden GmbH und Co.KG, Dresden, Germany, L. R. Schreiber JARA-FIT Institute for Quantum Information, Forschungszentrum Jülich and RWTH Aachen University, Germany, C. L. Manganelli IHP - Leibniz-Institut für innovative Mikroelektronik, Frankfurt, M. Virgilio Department of Physics Enrico Fermi, Universita di Pisa, Pisa, Italy, C. Richter IKZ - Leibniz -Institut für Kristallzüchtung, Berlin, Germany, G. Capellini IHP - Leibniz-Institut für innovative Mikroelektronik, Frankfurt Dipartimento di Scienze, Universita Roma Tre, Roma, Italy

Abstract: The lattice strain induced by metallic electrodes can impair the functionality of advanced quantum devices operating with electron or hole spins. Here we investigate the deformation induced by CMOS-manufactured titanium nitride electrodes on the lattice of a buried, 10 nm-thick Si/SiGe Quantum Well by means of nanobeam Scanning X-ray Diffraction Microscopy. We were able to measure TiN electrode-induced local modulations of the strain tensor components in the range of $2 - 8 \times 10^{-4}$ with ~60 nm lateral resolution. We have evaluated that these strain fluctuations are reflected into local modulations of the potential of the conduction band minimum larger than 2 meV, which is close to the orbital energy of an electrostatic quantum dot. We observe that the sign of the strain modulations at a given depth of the quantum well layer depends on the lateral dimensions of the electrodes. Since our work explores the impact of device geometry on the strain-induced energy landscape, it enables further optimization of the design of scaled CMOS-processed quantum devices.

1.Morphology of buried interfaces in ion-assisted magnetron sputter deposited 11B4C-containing Ni/Ti multilayer neutron optics investigated by grazing incidence small angle scattering

Authors:Sjoerd Broekhuijsen, Naureen Ghafoor, Mattias Schwartzkopf, Anton Zubayer, Jens Birch, Fredrik Eriksson

Abstract: The achieved interface width in multilayers is crucial for the performance of different optical components used in neutron beamlines. In this work we investigate how different growth conditions affect the interface morphology of Ni/Ti based multilayers, giving a crucial insight into the optimization of multilayer growth. Specifically, the effects of incorporating low-neutron-absorbing 11B4C into Ni/Ti multilayers have been investigated, as well as the effects of different ion assistance schemes. Coupled fits on combined X-ray and neutron reflectivity data reveal achieved interface widths in the multilayers of 2.7 {\AA} using multilayers where the growth parameters were optimized to the found conditions.

2.Atomistic Wear Mechanisms in Diamond: Effects of Surface Orientation, Stress, and Interaction with Adsorbed Molecules

Authors:Huong T. T. Ta, Nam V. Tran, M. C. Righi

Abstract: Despite its unrivaled hardness, diamond can be severely worn during the interaction with others, even softer materials. In this work, we calculate from first-principles the energy and forces necessary to induce the atomistic wear of diamond, and compare them for different surface orientations and passivation by oxygen, hydrogen, and water fragments. The primary mechanism of wear is identified as the detachment of carbon chains. This is particularly true for oxidized diamond and diamond interacting with silica. A very interesting result concerns the role of stress, which reveals that compressive stresses can highly favor wear, making it even energetically favorable.

3.Ionization Potentials and Fundamental Gaps in Atomic Systems from the Ensemble-DFT Approach

Authors:Sharon Lavie, Yuli Goshen, Eli Kraisler

Abstract: Calculations in Kohn-Sham density functional theory crucially rely on high-quality approximations for the exchange-correlation (xc) functional. Standard local and semi-local approximations fail to predict the ionization potential (IP) and the fundamental gap, departing from the Kohn-Sham orbital energies, due to the deviation of the total energy from piecewise-linearity and the absence of the derivative discontinuity. The ensemble generalization procedure introduced in Phys. Rev. Lett. 110, 126403 (2013) restores, to a large extent, these features in any approximate xc functional and improves its ability to predict the IP and the fundamental gap with negligible additional computational effort. In this work we perform an extensive study of atoms and first ions across the Periodic Table, generalizing the local spin-density and the Perdew-Burke-Ernzerhof approximations. By applying the ensemble generalization to a variety of systems, with s-, p- and d-character, we assess the accuracy of the method and identify important trends. In particular, we find that the accuracy of our approach heavily depends on the character of the frontier orbitals: when d-orbitals are involved, the performance is far less accurate. Possible sources of error are discussed and ways for further improvement are outlined.

4.Mechanical behavior of ion-irradiated ODS RAF steels strengthened with different types of refractory oxides

Authors:M. Frelek-Kozak, Ł. Kurpaska, K. Mulewska, M. Zieliński, R. Diduszko, A. Kosińska, D. Kalita, W. Chromiński, M. Turek, K. Kaszyca, A. Zaborowska, J. Jagielski

Abstract: In the present work, authors focused on verifying structural and mechanical properties of Oxide Dispersed Strengthening (ODS) steels strengthened by three different types of refractory oxides submitted to ion-irradiation. Three materials strengthened with Y2O3 or Al2O3 or ZrO2 were produced by mechanical alloying and Spark Plasma Sintering technique. Specimens have been submitted to high energy Ar-ion irradiation at room temperature with three fluences. This procedure allowed to generate strongly damaged zone with a thickness of 230nm. SEM/EBSD and TEM observations, GIXRD analysis, and nanoindentation tests have been included in examination of modified layers. Investigation revealed alteration of structural and mechanical features as a result of Ar-irradiation. Obtained results showed a strong correlation between the strengthening oxide and materials' behavior under radiation damage. It has been proved that below 1x1015ions/cm2 mechanical properties in the modified layer of all materials are very similar. Reported behavior may be related to the efficient annealing of the radiation defect process. Above this limit, significant differences between the materials are visible. It is believed that described phenomenon is directly related to the presence of the structural features and their capacity to act as defect sinks. Consequently, type of dominant mechanisms occurring in modified layer is proposed.

5.Development of Nb-GaAs based superconductor semiconductor hybrid platform by combining in-situ dc magnetron sputtering and molecular beam epitaxy

Authors:Clemens Todt, Sjoerd Telkamp, Filip Krizek, Christian Reichl, Mihai Gabureac, Rüdiger Schott, Erik Cheah, Peng Zeng, Thomas Weber, Arnold Müller, Christof Vockenhuber, Mohsen Bahrami Panah, Andreas Wallraff, Werner Wegscheider

Abstract: We present Nb thin films deposited in-situ on GaAs by combining molecular beam epitaxy and magnetron sputtering within an ultra-high vacuum cluster. Nb films deposited at varying power, and a reference film from a commercial system, are compared. The results show clear variation between the in-situ and ex-situ deposition which we relate to differences in magnetron sputtering conditions and chamber geometry. The Nb films have critical temperatures of around $9 \textrm{K}$. and critical perpendicular magnetic fields of up to $B_{c2} = 1.4 \textrm{T}$ at $4.2 \textrm{K}$. From STEM images of the GaAs-Nb interface we find the formation of an amorphous interlayer between the GaAs and the Nb for both the ex-situ and in-situ deposited material.

6.Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2

Authors:Sikandar Aftab, Muhammad Arslan Shehzad, Muhammad Salman Ajmal, Fahmid Kabir, Muhammad Zahir Iqbal

Abstract: In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional material (2D), MoTe2, caused by phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semi-metallic 1T-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 meV to 63 meV when phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semi-metallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 uA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30 %, respectively. Our findings unveil distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials, and a novel approach to reach high efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

7.Evolution of dislocation loops in irradiated α-Uranium: An atomistically-informed cluster dynamics investigation

Authors:Sanjoy Kumar Mazumder, Tiankai Yao, Anter El-Azab

Abstract: An atomistically informed mean field cluster dynamics model has been presented to investigate the nucleation and growth of defect loops in irradiated {\alpha}-U. TEM analysis of neutron irradiated {\alpha}-U shows the evolution of SIA and vacancy loops on (010) and (100) crystallographic planes respectively, resulting in an anisotropic swelling of the face-centered orthorhombic crystal. The accumulation of such loops, on irradiation, has been closely estimated using the cluster dynamics model. Parameters of the model, namely, the binding energy of point defects, i.e., Ui and VU, to SIA and vacancy loops respectively and the diffusivity of point defects govern the energetics and kinetics of the defect clustering phenomenon. We have studied the crystallography of defect loops and computed the binding energy of point defects to such loops using an angular dependent EAM potential in classical MD simulations. Using bond-boost hyperdynamics in LAMMPS, the anisotropic diffusion of Ui and VU in {\alpha}-U has been investigated. The mechanisms of point defect diffusion and the associated migration energies have also been reported and compared with previous DFT studies. Our CD model uses the computed parameters, within their error ranges, to predict the population of defect clusters with a dose-rate and temperature similar to the neutron irradiation experiments. The predictions show an accumulation of small sized vacancy loops along with a population of large and growing SIA loops which closely corresponds to the TEM observations.

8.Ferroelastic twin angles at the surface of CaTiO$_\mathrm{3}$ quantified by PhotoEmission Electron Microscopy

Authors:G. Magagnin, C. Lubin, M. Escher, N. Weber, L. Tortech, N. Barrett

Abstract: We use photoemission electron microscopy to measure the ferroelastic twin wall angles at the surface of CaTiO$_\mathrm{3}$(001) and deduce the strain ordering. We analyze the angular dependence of the photoelectron emission from different domain surfaces, each with their own characteristic tilt angle in the factory roof-like topography. By considering the surface topography as a field perturbation, the offset in the photoemission threshold can be directly related to the tilt angles. With knowledge of the symmetry allowed twin walls we quantify twin topography between 179.1{\deg} to 180.8{\deg}.

9.Exciton band structure of V$_2$O$_5$

Authors:Vitaly Gorelov, Lucia Reining, Matteo Gatti

Abstract: Excitonic effects due to the correlation of electrons and holes in excited states of matter dominate the optical spectra of many interesting materials. They are usually studied in the long-wavelength limit. Here we investigate excitons at non-vanishing momentum transfer, corresponding to shorter wavelengths. We calculate the exciton dispersion in the prototypical layered oxide V$_2$O$_5$ by solving the Bethe-Salpeter equation of many-body perturbation theory. We discuss the change of excitation energy and intensity as a function of wavevector for bright and dark excitons, respectively, and we analyze the origin of the excitons along their dispersion. We highlight the important role of the electron-hole exchange with its impact on the exciton dispersion, the singlet-triplet splitting and the difference between the imaginary part of the macroscopic dielectric function and the loss function.

10.Pressure-control of non-ferroelastic ferroelectric domains in ErMnO3

Authors:O. W. Sandvik, A. M. Müller, H. W. Ånes, M. Zahn, J. He, M. Fiebig, Th. Lottermoser, T. Rojac, D. Meier, J. Schultheiß

Abstract: Mechanical pressure controls the structural, electric, and magnetic order in solid state systems, allowing to tailor and improve their physical properties. A well-established example is ferroelastic ferroelectrics, where the coupling between pressure and the primary symmetry breaking order parameter enables hysteretic switching of the strain state and ferroelectric domain engineering. Here, we study the pressure-driven response in a non-ferroelastic ferroelectric, ErMnO3, where the classical stress-strain coupling is absent, and the domain formation is governed by creation-annihilation processes of topological defects. By annealing ErMnO3 polycrystals under variable pressures in the MPa-regime, we transform non-ferroelastic vortex-like domains into stripe-like domains. The width of the stripe-like domains is determined by the applied pressure as we confirm by three-dimensional phase field simulations, showing that pressure leads to highly oriented layer-like periodic domains. Our work demonstrates the possibility to utilize mechanical pressure for domain engineering in non-ferroelastic ferroelectrics, providing a processing-accessible lever to control their dielectric, electromechanical, and piezoelectric response.

11.Ab-initio Simulations of Coherent Phonon-Induced Pumping of Carriers in ZrTe$_5$

Authors:Tao Jiang, Peter P. Orth, Liang Luo, Lin-Lin Wang, Feng Zhang, Cai-Zhuang Wang, Jin Zhao, Kai-Ming Ho, Jigang Wang, Yong-Xin Yao

Abstract: Laser-driven coherent phonons can act as modulated strain fields and modify the adiabatic ground state topology of quantum materials. We use time-dependent first-principles and effective model calculations to simulate the effect of a strong terahertz electric field on electronic carriers in the topological insulator ZrTe$_5$. We show that a coherent $A_\text{1g}$ Raman mode modulation can effectively pump carriers across the band gap, even though the phonon energy is about an order of magnitude smaller than the equilibrium band gap. We reveal the microscopic mechanism of this effect which occurs via Landau-Zener-St\"uckelberg tunneling of Bloch electrons in a narrow region in the Brillouin zone center where the transient energy gap closes when the system switches from strong to weak topological insulator. The quantum dynamics simulation results are in excellent agreement with recent pump-probe experiments in ZrTe$_5$ at low temperature.

12.Phase Stability of Hexagonal/cubic Boron Nitride Nanocomposites

Authors:Abhijit Biswas, Rui Xu, Joyce Christiansen-Salameh, Eugene Jeong, Gustavo A. Alvarez, Chenxi Li, Anand B. Puthirath, Bin Gao, Arushi Garg, Tia Gray, Harikishan Kannan, Xiang Zhang, Jacob Elkins, Tymofii S. Pieshkov, Robert Vajtai, A. Glen Birdwell, Mahesh R. Neupane, Bradford B. Pate, Tony Ivanov, Elias J. Garratt, Pengcheng Dai, Hanyu Zhu, Zhiting Tian, Pulickel M. Ajayan

Abstract: Boron nitride (BN) is an exceptional material and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the co-existence of two phases can lead to strong non-linear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN grain sizes governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorphs based nanocomposites with desirable properties for optoelectronics and thermal energy management applications.

1.Electronic properties of 2{\it H}-stacking bilayer MoS$_2$ measured by terahertz time-domain spectroscopy

Authors:Xingjia Cheng, Wen Xu, Hua Wen, Jing Zhang, Heng Zhang, Haowen Li, Qingqing Chen

Abstract: Bilayer (BL) molybdenum disulfide (MoS$_2$) is one of the most important electronic structures not only in valleytronics but also in realizing twistronic systems on the basis of the topological mosaics in Moir\'e superlattices. In this work, BL MoS$_2$ on sapphire substrate with 2$H$-stacking structure is fabricated. We apply the terahertz (THz) time-domain spectroscopy (TDS) for examining the basic optoelectronic properties of this kind of BL MoS$_2$. The optical conductivity of BL MoS$_2$ is obtained in temperature regime from 80 to 280 K. Through fitting the experimental data with the theoretical formula, the key sample parameters of BL MoS$_2$ can be determined, such as the electron density, the electronic relaxation time and the electronic localization factor. The temperature dependence of these parameters is examined and analyzed. We find that, similar to monolayer (ML) MoS$_2$, BL MoS$_2$ with 2$H$-stacking can respond strongly to THz radiation field and show semiconductor-like optoelectronic features. The theoretical calculations using density functional theory (DFT) can help us to further understand why the THz optoelectronic properties of BL MoS$_2$ differ from those observed for ML MoS$_2$. The results obtained from this study indicate that the THz TDS can be applied suitably to study the optoelectronic properties of BL MoS$_2$ based twistronic systems for novel applications as optical and optoelectronic materials and devices.

2.Mapping the complex evolution of ferroelastic/ferroelectric domain patterns in epitaxially strained PbTiO3 heterostructures

Authors:Céline Lichtensteiger, Marios Hadjimichael, Edoardo Zatterin, Chia-Ping Su, Iaroslav Gaponenko, Ludovica Tovaglieri, Patrycja Paruch, Alexandre Gloter, Jean-Marc Triscone

Abstract: We study the complex ferroelastic/ferroelectric domain structure in the prototypical ferroelectric PbTiO3 epitaxially strained on (110)o-oriented DyScO3 substrates, using a combination of atomic force microscopy, laboratory and synchrotron x-ray diffraction and high resolution scanning transmission electron microscopy. We observe that the anisotropic strain imposed by the orthorhombic substrate creates a large asymmetry in the domain configuration, with domain walls macroscopically aligned along one of the two in-plane directions. We show that the periodicity as a function of film thickness deviates from the Kittel law. As the ferroelectric film thickness increases, we find that the domain configuration evolves from flux-closure to a/c-phase, with a larger scale arrangement of domains into superdomains.

3.Recent advances in La2NiMnO6 Double Perovskites for various applications; Challenges and opportunities

Authors:Suresh Chandra Baral Department of Physics, Indian Institute of Technology Indore, Indore, 453552, India, P. Maneesha Department of Physics, Indian Institute of Technology Indore, Indore, 453552, India, E. G. Rini Department of Physics, Indian Institute of Technology Indore, Indore, 453552, India, Somaditya Sen Department of Physics, Indian Institute of Technology Indore, Indore, 453552, India

Abstract: Double perovskites R2NiMnO6 (R= Rare earth element) (RNMO) are a significant class of materials owing to their Multifunctional properties with structural modifications. In particular, multifunctional double perovskite oxides La2NiMnO6 (LNMO) which possess both electric and magnetic orderings, chemical flexibility, versatility, and indispensable properties like high ferromagnetic curie temperature, high absorption rates, dielectrics, etc. have drawn a lot of attention due their rich physics and diverse applications in various technology. This justifies the intense research in this class of materials, and the keen interest they are subject to both the fundamental and practical side. In view of the demands of this material in lead-free perovskite solar cells, photocatalytic degradation of organic dyes, clean hydrogen production, electric tuneable devices, Fuel cells, gas sensing, and Biomedical applications, there is a need for an overview of all the literature so far, the ongoing research and the future prospective. This review summarised all the Physical and Structural Properties of LNMO such as electric, magnetic, catalytic, and dielectric properties with their underlying mechanisms. This review article provides insight into the scope of studies in LNMO material for exploring unexposed properties in new material research and to identify areas of future investigation of the materials in the double perovskite family.

4.Electrical transport properties driven by unique bonding configuration in gamma-GeSe

Authors:Jeongsu Jang, Joonho Kim, Dongchul Sung, Jong Hyuk Kim, Joong-Eon Jung, Sol Lee, Jinsub Park, Chaewoon Lee, Heesun Bae, Seongil Im, Kibog Park, Young Jai Choi, Suklyun Hong, Kwanpyo Kim

Abstract: Group-IV monochalcogenides have recently shown great potential for their thermoelectric, ferroelectric, and other intriguing properties. The electrical properties of group-IV monochalcogenides exhibit a strong dependence on the chalcogen type. For example, GeTe exhibits high doping concentration, whereas S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we investigate the electrical and thermoelectric properties of gamma-GeSe, a recently identified polymorph of GeSe. gamma-GeSe exhibits high electrical conductivity (~106 S/m) and a relatively low Seebeck coefficient (9.4 uV/K at room temperature) owing to its high p-doping level (5x1021 cm-3), which is in stark contrast to other known GeSe polymorphs. Elemental analysis and first-principles calculations confirm that the abundant formation of Ge vacancies leads to the high p-doping concentration. The magnetoresistance measurements also reveal weak-antilocalization because of spin-orbit coupling in the crystal. Our results demonstrate that gamma-GeSe is a unique polymorph in which the modified local bonding configuration leads to substantially different physical properties.

5.Tuning the lattice thermal conductivity in van-der-Waals structures through rotational (dis)ordering

Authors:Fredrik Eriksson, Erik Fransson, Christopher Linderälv, Zheyong Fan, Paul Erhart

Abstract: It has recently been demonstrated that MoS2 with irregular interlayer rotations can achieve an extreme anisotropy in the lattice thermal conductivity (LTC), which is for example of interest for applications in waste heat management in integrated circuits. Here, we show by atomic scale simulations based on machine-learned potentials that this principle extends to other two-dimensional materials including C and BN. In all three materials introducing rotational disorder drives the through-plane LTC to the glass limit, while the in-plane LTC remains almost unchanged compared to the ideal bulk materials. We demonstrate that the ultralow through-plane LTC is connected to the collapse of their transverse acoustic modes in the through-plane direction. Furthermore, we find that the twist angle in periodic moir\'e structures representing rotational order provides an efficient means for tuning the through-plane LTC that operates for all chemistries considered here. The minimal through-plane LTC is obtained for angles between 1 and 4 degree depending on the material, with the biggest effect in MoS2. The angular dependence is correlated with the degree of stacking disorder in the materials, which in turn is connected to the slip surface. This provides a simple descriptor for predicting the optimal conditions at which the LTC is expected to become minimal.

6.Reactive single-step hot-pressing and magnetocaloric performance of polycrystalline Fe$_2$Al$_{1.15-x}$B$_2$Ge$_x$Ga$_x$ ($x=0, 0.05$) MAB phases

Authors:Benedikt Beckmann, Tarek A. El-Melegy, David Koch, Ulf Wiedwald, Michael Farle, Fernando Maccari, Joshua Snyder, Konstantin P. Skokov, Michel W. Barsoum, Oliver Gutfleisch

Abstract: Reactive single-step hot-pressing at 1473 K and 35 MPa for 4 h produces dense, bulk, near single-phase, low-cost and low-criticality Fe$_2$Al$_{1.15}$B$_2$ and Fe$_2$Al$_{1.1}$B$_2$Ge$_{0.05}$Ga$_{0.05}$ MAB samples, showing a second-order magnetic phase transition with favorable magnetocaloric properties around room temperature. The magnetic as well as magnetocaloric properties can be tailored upon Ge and Ga doping, leading to an increase of Curie temperature $T_C$ and spontaneous magnetization $m_S$. The maximum isothermal entropy change $\Delta s_{T,max}$ of hot-pressed Fe$_2$Al$_{1.15}$B$_2$ in magnetic field changes of 2 and 5 T amounts to 2.5 and 5 J(kgK)$^{-1}$ at 287.5 K and increases by Ge and Ga addition to 3.1 and 6.2 J(kgK)$^{-1}$ at 306.5 K, respectively. The directly measured maximum adiabatic temperature change $\Delta T_{ad,max}$ is improved by the composition modification from 0.9 to 1.1 K in magnetic field changes of 1.93 T. Overall, we demonstrate that hot-pressing provides a much faster, more scalable and processing cost reducing alternative compared to conventional synthesis routes to produce heat exchangers for magnetic cooling devices. Therefore, our criticality assessment shows that hot-pressed Fe-based MAB phases provide a promising compromise of material and processing cost, criticality and magnetocaloric performance, demonstrating the potential for low-cost and low-criticality magnetocaloric applications around room temperature.

7.Recent progress in thermoelectric MXene-based structures versus other 2D materials

Authors:Subrahmanyam Bandaru, Agnieszka M. Jastrzębska, Magdalena Birowska

Abstract: Thermoelectricity is a next-generation solution for efficient waste heat management. Although various thermoelectric materials exist, there is still a lot of scope for advancement. Recently, two-dimensional (2D) materials, including MXenes, showed promise as thermoelectric materials. The progress of MXenes as magnificent thermoelectric materials is very well established in the form of a tailor-made review. MXenes outstanding thermoelectric activity comes from a unique band structure created from its atomically thin layers and the defective surface of the external layers of atoms. Furthermore, the variety of MXenes chemical composition and MXenes-based nanostructures facilitates the research path based on energy band engineering, optimization, carrier concentration and mobility. The thermoelectric efficiency of MXenes has been mapped over the landscape of other 2D and traditional thermoelectric materials. Meanwhile, MBenes, the latest family member of the flatland, exhibits an incredible diversity of structures with additional crystal symmetries. Owing to the orthorhombic crystal structure, an in-plane structural anisotropy, and hence, the in-plane dependent thermoelectric properties are plausible. As a future prospective, certain strategies that can enhance the thermoelectric performance of MBenes have been presented. In addition, few insights and challenges that have to be considered to overcome the limitations in the thermoelectric field have been debated.

8.Three-dimensional Van Hove singularity with divergent density of states in a topological magnet

Authors:Wenbin Wu, Zeping Shi, Mykhaylo Ozerov, Yuhan Du, Yuxiang Wang, Xiao-Sheng Ni, Xianghao Meng, Chunhui Pan, Haifeng Pan, Zhenrong Sun, Run Yang, Yang Xu, Yusheng Hou, Zhongbo Yan, Cheng Zhang, Hai-Zhou Lu, Junhao Chu, Xiang Yuan

Abstract: Arising from the extreme or saddle point in electronic bands, Van Hove singularity (VHS) strongly enhances the electronic correlation in its vicinity and leads to various new states of matter such as density wave and unconventional superconductivity. In contrast to the divergent density of states (DOS) in one and two dimensions, the VHS is generally non-divergent in three dimension (3D). Here we report the observation of divergent 3D VHS in a topological magnet EuCd2As2 by magneto-infrared spectroscopy. The divergent 3D VHS is generated by substantially tuning the energy dispersion and momentum distribution of the Weyl bands. Applying the external magnetic field allows effective control of exchange interaction between itinerant electrons and local magnetic moments. As a result, the 3D Weyl bands are found to shift continuously with the canting of magnetic moment, which leads to the gradual increase of Fermi velocity. Above the critical field Bc ~ 0.6 T, the formation of a divergent 3D VHS is evidenced by the abrupt spectral emergence of the inter-band transitions. This type of VHS allows in situ tunability by external magnetic fields with adjustable energy position, DOS and even its presence. The experimental spectrum and the emergence of 3D VHS can be quantitatively described by a two-band minimal model of Weyl semimetal with the exchange interaction. The deduced model predicts three additional optical transitions and their energy crossings, which are quantitatively verified by the magneto-near-infrared spectrum. Our results pave the way to exploring divergent VHS in the 3D systems with strong tunability and provide a platform to uncover the coordination between electronic correlation and the topological phase.

9.Epitaxial ferroelectric hafnia stabilized by symmetry constraints

Authors:Tianyuan Zhu, Shiqing Deng, Shi Liu

Abstract: Ferroelectric memories experienced a revival in the last decade due to the discovery of ferroelectricity in HfO$_2$-based nanometer-thick thin films. These films exhibit exceptional silicon compatibility, overcoming the scaling and integration obstacles that impeded perovskite ferroelectrics' use in high-density integrated circuits. The exact phase responsible for ferroelectricity in hafnia films remains debated with no single factor identified that could stabilize the ferroelectric phase thermodynamically. Here, supported by density functional theory (DFT) high-throughput (HT) calculations that screen a broad range of epitaxial conditions, we demonstrate conclusively that specific epitaxial conditions achievable with common substrates such as yttria-stabilized zirconia (YSZ) and SrTiO$_3$ can favor the polar Pca2$_1$ phase thermodynamically over other polar phases such as R3m and Pmn2$_1$ and nonpolar P2$_1$/c phase. The substrate's symmetry constraint-induced shear strain is crucial for the preference of Pca2$_1$. The strain-stability phase diagrams resolve experiment-theory discrepancies and can guide the improvement of ferroelectric properties of epitaxial hafnia thin films.

10.Electric-field-induced non-ergodic relaxor to ferroelectric transition in BiFeO3-xSrTiO3 ceramics

Authors:Leonardo Oliveira, Jeppe Ormstrup, Marta Majkut, Maja Makarovic, Tadej Rojac, Julian Walker, Hugh Simons

Abstract: While BiFeO3-based solid solutions show great promise for applications in energy conversion and storage, realizing this promise necessitates understanding the structure-property relationship in particular pertaining to the relaxor-like characteristics often exhibited by solid solutions with polar-to-non-polar morphotropic phase boundaries. To this end, we investigated the role of the compositionally-driven relaxor state in (100-x)BiFeO3-xSrTiO3 [BFO-xSTO], via in situ synchrotron X-ray diffraction under bipolar electric-field cycling. The electric-field induced changes to the crystal structure, phase fraction and domain textures were monitored via the {111}pc, {200}pc, and 1/2{311}pc Bragg peaks. The dynamics of the intensities and positions of the (111) and (11-1) reflections reveal an initial non-ergodic regime followed by long-range ferroelectric ordering after extended poling cycles. The increased degree of random multi-site occupation in BFO-42STO compared to BFO-35STO is correlated with an increase of the critical electric field needed to induce the non-ergodic-to-ferroelectric transition, and a decrease in the degree of domain reorientation. Although both compositions show an irreversible transition to a long-range ferroelectric state, our results suggest that the weaker ferroelectric response in BFO-42STO is related to an increase in ergodicity. This, in turn, serves to guide the development of BFO-based systems into promising platform for further property engineering towards specific capacitor applications.

11.Escape from the Second Dimension: A Topological Distinction Between Edge and Screw Dislocations

Authors:Paul G. Severino, Randall D. Kamien

Abstract: Volterra's definition of dislocations in crystals distinguishes edge and screw defects geometrically, according to whether the Burgers vector is perpendicular or parallel to the defect. Here, we demonstrate a distinction between screw and edge dislocations that enables a unified, purely topological means of classification. Our construction relies on the construction of real or virtual disclination-line pairs at the core of the dislocation in a smectic and can be generalized to crystals with triply-periodic order. The connection between topology and geometry is exploited.

1.High-performance descriptor for magnetic materials:Accurate discrimination of magnetic symmetries

Authors:Michi-To Suzuki, Takuya Nomoto, Eiaki V. Morooka, Yuki Yanagi, Hiroaki Kusunose

Abstract: The magnetic structure is crucial in determining the physical properties inherent in magnetic compounds. We present an adequate descriptor for magnetic structure with proper magnetic symmetry and high discrimination performance, which does not depend on artificial choices for coordinate origin, axis, and magnetic unit cell in crystal. We extend the formalism called ``smooth overlap of atomic positions'' (SOAP) providing a numerical representation of atomic configurations to that of magnetic moment configurations. We introduce the descriptor in terms of the vector spherical harmonics to describe a magnetic moment configuration and partial spectra from the expansion coefficients. We discuss that the lowest order partial spectrum is insufficient to discriminate the magnetic structures with different magnetic anisotropy, and a higher order partial spectrum is required in general to characterize detailed magnetic structures on the same atomic configuration. We then introduce the fourth-order partial spectrum and evaluate the discrimination performance for different magnetic structures, mainly focusing on the difference in magnetic symmetry. The modified partial spectra that are defined not to reflect the difference of magnetic anisotropy are also useful in evaluating magnetic structures obtained from first-principles calculations without spin-orbit coupling. We apply the present method to the symmetry-classified magnetic structures for the crystals of Mn$_3$Ir and Mn$_3$Sn, which are known to exhibit anomalous transport under the antiferromagnetic order, and examine the discrimination performance of the descriptor for different magnetic structures on the same crystal.

2.Morphological transitions in the patterning of the crystalline Ge(001) surface induced by ion irradiation

Authors:Denise J. Erb, Daniel A. Pearson, Tomáš Škereň, Martin Engler, R. Mark Bradley, Stefan Facsko

Abstract: We investigate the morphologies of the Ge(001) surface that are produced by bombardment with a normally incident, broad argon ion beam at sample temperatures above the recrystallization temperature. Two previously-observed kinds of topographies are seen, i.e., patterns consisting of upright and inverted rectangular pyramids, as well as patterns composed of shallow, isotropic basins. In addition, we observe the formation of an unexpected third type of pattern for intermediate values of the temperature, ion energy and ion flux. In this type of transitional morphology, isolated peaks with rectangular cross sections stand above a landscape of shallow, rounded basins. We also extend past theoretical work to include a second order correction term that comes from the curvature dependence of the sputter yield. For a range of parameter values, the resulting continuum model of the surface dynamics produces patterns that are remarkably similar to the transitional morphologies we observe in our experiments. The formation of the isolated peaks is the result of a term that is not ordinarily included in the equation of motion, a second order correction to the curvature dependence of the sputter yield.

3.Capacitance characterization of Graphene/n-Si Schottky junction solar cell with MOS capacitor

Authors:Masahiro Teraoka, Yuzuki Ono, Hojun Im

Abstract: We have demonstrated a simple and accurate method for characterizing the capacitance of Graphene/n-Si Schottky junction solar cells (GSSCs) which embed the metal-oxide-semiconductor (MOS) capacitor. We measured two types of GSSCs, one with thermal annealing treatments (w-a) and one without (wo-a). It was found that the wo-a GSSC exhibits a two-step feature in the phase versus forward bias voltage relationship, which may be attributed to the presence of polymethyl methacrylate residues. By considering the capacitance of the MOS capacitor (Cmos) and its standard deviation, we successfully obtained the capacitance of the Schottky junction (CSch), and evaluated meaningful built-in potentials (Schottky barrier heights) which are 0.51V (0.78eV) and 0.47V (0.75eV) for the w-a and wo-a GSSCs, respectively, by the Mott-Schottky analysis. We also briefly discuss the relationship between CSch and the Nyquist and Bode plots, finding that the RC time constant decreases due to the subtraction of Cmos.

4.Self-doping effect in confined copper selenide semiconducting quantum dots for efficient photoelectrocatalytic oxygen evolution

Authors:Jie Ren, Chenya Zhao, Lanshan He, Congcong Wu, Wenting Jia, Shengwen Xu, Daojian Ye, Weiyang Xu, Fujin Huang, Hang Zhou, Chengwu Zou, Ce Hu, Ting Yu, Xingfang Luo, Cailei Yuan

Abstract: Self-doping can not only suppress the photogenerated charge recombination of semiconducting quantum dots by self-introducing trapping states within the bandgap, but also provide high-density catalytic active sites as the consequence of abundant non-saturated bonds associated with the defects. Here, we successfully prepared semiconducting copper selenide (CuSe) confined quantum dots with abundant vacancies and systematically investigated their photoelectrochemical characteristics. Photoluminescence characterizations reveal that the presence of vacancies reduces the emission intensity dramatically, indicating a low recombination rate of photogenerated charge carriers due to the self-introduced trapping states within the bandgap. In addition, the ultra-low charge transfer resistance measured by electrochemical impedance spectroscopy implies the efficient charge transfer of CuSe semiconducting quantum dots-based photoelectrocatalysts, which is guaranteed by the high conductivity of their confined structure as revealed by room-temperature electrical transport measurements. Such high conductivity and low photogenerated charge carriers recombination rate, combined with high-density active sites and confined structure, guaranteeing the remarkable photoelectrocatalytic performance and stability as manifested by photoelectrocatalysis characterizations. This work promotes the development of semiconducting quantum dots-based photoelectrocatalysis and demonstrates CuSe semiconducting quantum confined catalysts as an advanced photoelectrocatalysts for oxygen evolution reaction.

5.Ab initio insights into the interaction mechanisms between H$_2$, H$_2$O, and O$_2$ molecules with diamond surfaces

Authors:Nam V. Tran, M. C. Righi

Abstract: Diamond displays outstanding chemical, physical, and tribological properties, making it attractive for numerous applications ranging from biomedicine to tribology. However, the reaction of the materials with molecules present in the air, such as oxygen, hydrogen, and water, could significantly change the electronic and tribological properties of the films. In this study, we performed several density functional theory calculations to construct a database for the adsorption energies and dissociation barriers of these molecules on the most relevant diamond surfaces, including C(111), C(001), and C(110). The adsorption configurations, reaction paths, activation energies, and their influence on the structure of diamond surfaces are discussed. The results indicate that there is a strong correlation between adsorption energy and surface energy. Moreover, we found that the dissociation processes of oxygen molecules on these diamond surfaces can significantly alter the surface morphology and may affect the tribological properties of diamond films. These findings can help to advance the development and optimization of devices and antiwear coatings based on diamond.

6.Element-Specific Study of Magnetic Anisotropy and Hardening in SmCo$_{5-x}$Cu$_{x}$ Thin Films

Authors:Georgia Gkouzia, Damian Günzing, Ruiwen Xie, Teresa Weßels, András Kovács, Alpha T. N Diaye, Márton Major, J. P. Palakkal, Rafal E. Dunin-Borkowski, Heiko Wende, Hongbin Zhang, Katharina Ollefs, Lambert Alff

Abstract: This work investigates the effect of copper substitution on the magnetic properties of SmCo$_{5}$ thin films synthesized by molecular beam epitaxy. A series of thin films with varying concentrations of Cu were grown under otherwise identical conditions to disentangle structural and compositional effects on the magnetic behavior. The combined experimental and theoretical studies show that Cu substitution at the Co$_{3g}$ sites not only stabilizes the formation of the SmCo$_{5}$ structure but enhances magnetic anisotropy and coercivity. Density functional theory calculations indicate that Sm(Co$_4$Cu$_{3g}$)$_5$ possesses a higher single-ion anisotropy as compared to pure SmCo$_{5}$. In addition, X-ray magnetic circular dichroism reveals that Cu substitution causes an increasing decoupling of the Sm 4\textit{f} and Co 3\textit{d} moments. Scanning transmission electron microscopy confirms predominantly SmCo$_{5}$ phase formation and reveals nanoscale inhomogeneities in the Cu and Co distribution. Our study based on thin film model systems and advanced characterization as well as modeling reveals novel aspects of the complex interplay of intrinsic and extrinsic contributions to magnetic hysteresis in rare earth-based magnets, \textit{i.e.} the combination of increased intrinsic anisotropy due to Cu substitution and the extrinsic effect of inhomogeneous elemental distribution of Cu and Co.

7.Importance of the catalytic effect of the substrate in the functionality of lubricant additives: the case of MoDTC

Authors:Stefan Peeters, Catherine Charrin, Isabelle Duron, Sophie Loehlé, Benoit Thiebaut, M. C. Righi

Abstract: Molybdenum dithiocarbamates (MoDTCs) are lubricant additives very efficient in reducing the friction of steel and they are employed in a number of industrial applications. The functionality of these additives is ruled by the chemical interactions occurring at the buried sliding interface, which are of key importance for the improvement of the lubrication performance. Yet, these tribochemical processes are very difficult to monitor in real time. Ab initio molecular dynamics simulations are the ideal tool to shed light into such a complicated reactivity. In this work we perform ab initio simulations, both in static and tribological conditions, to understand the effect of surface oxidation on the tribochemical reactivity of MoDTC and we find that when the surfaces are covered by oxygen, the first dissociative steps of the additives are significantly hindered. Our preliminary tribological tests on oxidized steel discs support these results. Bare metallic surfaces are necessary for a stable adsorption of the additives, their quick decomposition, and the formation of a durable MoS$_2$ tribolayer. This work demonstrates the importance of the catalytic role of the substrate and confirms the full capability of the computational protocol in the pursuit of materials and compounds more efficient in reducing friction.

8.Aromatic molecules as sustainable lubricants explored by ab initio simulations

Authors:Stefan Peeters, Gabriele Losi, Sophie Loehlé, M. C. Righi

Abstract: In the pursuit of sustainable lubricant materials, the conversion of common organic molecules into graphitic material has been recently shown to effectively reduce friction of metallic interfaces. Aromatic molecules are perfect candidates due to their inertness and possibility to form carbon-based tribofilms. Among many promising possibilities, we selected a group of common aromatic compounds and we investigated their capability to reduce the adhesion of iron interface. Ab initio molecular dynamic simulations of the sliding interface show that hypericin, a component of St. John's wort, effectively separates the mating iron surfaces better than graphene. This phenomenon is due to the size of the molecule, the reactivity of the moieties at its edges and the possibility to stack several of these structures that can easily slide on top of each other. The decomposition of the lateral groups of hypericin observed in the dynamic simulations suggests that the clustering of several molecules is possible, offering innovative paths to lubricate sliding contacts with compounds not typically employed in tribology.

9.Probing magnetic ordering in air stable iron-rich van der Waals minerals

Authors:Muhammad Zubair Khan MCL, Oleg E. Peil MCL, Apoorva Sharma MCL, Oleksandr Selyshchev MCL, Sergio Valencia MCL, Florian Kronast MCL, Maik Zimmermann MCL, Muhammad Awais Aslam MCL, Johann G. Raith MCL, Christian Teichert MCL, Dietrich R. T. Zahn MCL, Georgeta Salvan MCL, Aleksandar Matković MCL, Chair of Physics MCL, Department Physics MCL, Mechanics MCL, Electrical engineering MCL, Montanuniversität Leoben MCL, 8700 MCL, Leoben MCL, Austria. MCL, Materials Center Leoben Forschung GmbH MCL, 8700 MAIN, Leoben MAIN, Austria. MAIN, Semiconductor Physics MAIN, Chemnitz University of Technology MAIN, D-09107 MAIN, Chemnitz MAIN, Germany. MAIN, Department of Spin MAIN, Topology in Quantum Materials MAIN, Helmholtz-Zentrum Berlin MAIN, Albert-Einstein-Str. 15 MAIN, D-12489 MAIN, Berlin MAIN, Germany. MAIN, Chair of Resource Mineralogy MAIN, Montanuniversität Leoben MAIN, 8700 MAIN, Leoben MAIN, Austria. MAIN, Centre for Materials MAIN, Architecture MAIN, Integration of Nanomembranes MAIN, Chemnitz University of Technology, 09126, Chemnitz, Germany

Abstract: In the rapidly expanding field of two-dimensional materials, magnetic monolayers show great promise for the future applications in nanoelectronics, data storage, and sensing. The research in intrinsically magnetic two-dimensional materials mainly focuses on synthetic iodide and telluride based compounds, which inherently suffer from the lack of ambient stability. So far, naturally occurring layered magnetic materials have been vastly overlooked. These minerals offer a unique opportunity to explore air-stable complex layered systems with high concentration of local moment bearing ions. We demonstrate magnetic ordering in iron-rich two-dimensional phyllosilicates, focusing on mineral species of minnesotaite, annite, and biotite. These are naturally occurring van der Waals magnetic materials which integrate local moment baring ions of iron via magnesium/aluminium substitution in their octahedral sites. Due to self-inherent capping by silicate/aluminate tetrahedral groups, ultra-thin layers are air-stable. Chemical characterization, quantitative elemental analysis, and iron oxidation states were determined via Raman spectroscopy, wavelength disperse X-ray spectroscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. Superconducting quantum interference device magnetometry measurements were performed to examine the magnetic ordering. These layered materials exhibit paramagnetic or superparamagnetic characteristics at room temperature. At low temperature ferrimagnetic or antiferromagnetic ordering occurs, with the critical ordering temperature of 38.7 K for minnesotaite, 36.1 K for annite, and 4.9 K for biotite. In-field magnetic force microscopy on iron bearing phyllosilicates confirmed the paramagnetic response at room temperature, present down to monolayers.

10.Adhesion, Friction and Tribochemical Reactions at the Diamond-Silica Interface

Authors:Michele Cutini, Gaia Forghieri, Mauro Ferrario, Maria Clelia Righi

Abstract: Diamond-based coatings are employed in several technological applications, for their outstanding mechanical properties, biocompatibility, and chemical stability. Of significant relevance is the interface with silicon oxide, where phenomena of adhesion, friction, and wear can affect drastically the performance of the coating. Here we monitor such phenomena in real-time by performing massive ab initio molecular dynamics simulations in tribological conditions. We take into account many relevant factors that can play a role, i.e. the diamond surface orientation and reconstruction, silanol density, as well as, the type and concentration of passivating species. The large systems size and the long simulations time, put our work at the frontier of what can be currently done with fully ab initio molecular dynamics. The results of our work point to full hydrogenation as an effective way to reduce both friction and wear for all diamond surfaces, while graphitization is competitive only on the (111) surface. Overall we expect that our observations will be useful to improve technological applications where the silica-diamond interface plays a key role. Moreover, we demonstrate that realistic and accurate in silico experiments are feasible nowadays exploiting HPC resources and HPC optimized software, paving the way to a more general understanding of the relationship between surface chemistry and nanoscale-tribology.

11.Macroscopic polarization from nonlinear gradient couplings

Authors:Massimiliano Stengel

Abstract: We show that a lattice mode of arbitrary symmetry induces a well-defined macroscopic polarization at first order in the momentum and second order in the amplitude. We identify a symmetric flexoelectric-like contribution, which is sensitive to both the electrical and mechanical boundary conditions, and an antisymmetric Dzialoshinskii-Moriya-like term, which is unaffected by either. We develop the first-principles methodology to compute the relevant coupling tensors in an arbitrary crystal, which we illustrate with the example of the antiferrodistortive order parameter in SrTiO$_3$.

12.Direct experimental evidence of tunable charge transfer at the $LaNiO_{3}/CaMnO_{3}$ ferromagnetic interface

Authors:J. R. Paudel, M. Terilli, T. -C. Wu, J. D. Grassi, A. M. Derrico, R. K. Sah, M. Kareev, C. Klewe, P. Shafer, A. Gloskovskii, C. Schlueter, V. N. Strocov, J. Chakhalian, A. X. Gray

Abstract: Interfacial charge transfer in oxide heterostructures gives rise to a rich variety of electronic and magnetic phenomena. Designing heterostructures where one of the thin-film components exhibits a metal-insulator transition opens a promising avenue for controlling such phenomena both statically and dynamically. In this letter, we utilize a combination of depth-resolved soft X-ray standing-wave and hard X-ray photoelectron spectroscopies in conjunction with polarization-dependent X-ray absorption spectroscopy to investigate the effects of the metal-insulator transition in $LaNiO_{3}$ on the electronic and magnetic states at the $LaNiO_{3}/CaMnO_{3}$ interface. We report on a direct observation of the reduced effective valence state of the interfacial Mn cations in the metallic superlattice with an above-critical $LaNiO_{3}$ thickness (6 u.c.) due to the leakage of itinerant Ni 3d $e_{g}$ electrons into the interfacial $CaMnO_{3}$ layer. Conversely, in an insulating superlattice with a below-critical $LaNiO_{3}$ thickness of 2 u.c., a homogeneous effective valence state of Mn is observed throughout the $CaMnO_{3}$ layers due to the blockage of charge transfer across the interface. The ability to switch and tune interfacial charge transfer enables precise control of the emergent ferromagnetic state at the $LaNiO_{3}/CaMnO_{3}$ interface and, thus, has far-reaching consequences on the future strategies for the design of next-generation spintronic devices.

1.Effect of Environmental Screening and Strain on Optoelectronic Properties of Two-Dimensional Quantum Defects

Authors:Shimin Zhang, Kejun Li, Chunhao Guo, Yuan Ping

Abstract: Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control of SPEs requires in-depth understanding of their optoelectronic properties. However, how the surrounding environment of host materials, including number of layers, substrates, and strain, influences SPEs has not been fully understood. In this work, we study the dielectric screening effect due to the number of layers and substrates, and the strain effect on the optical properties of carbon dimer and nitrogen vacancy defects in hBN from first-principles many-body perturbation theory. We report that the environmental screening causes lowering of the GW gap and exciton binding energy, leading to nearly constant optical excitation energy and exciton radiative lifetime. We explain the results with an analytical model starting from the BSE Hamiltonian with Wannier basis. We also show that optical properties of quantum defects are largely tunable by strain with highly anisotropic response, in good agreement with experimental measurements. Our work clarifies the effect of environmental screening and strain on optoelectronic properties of quantum defects in two-dimensional insulators, facilitating future applications of SPEs and spin qubits in low-dimensional systems.

2.Janus monolayer TaNF: a new ferrovalley material with large valley splitting and tunable magnetic properties

Authors:Guibo Zheng, Shuixian Qu, Wenzhe Zhou, Fangping Ouyang

Abstract: Materials with large intrinsic valley splitting and high Curie temperature are a huge advantage for studying valleytronics and practical applications. In this work, using first-principles calculations, a new Janus TaNF monolayer is predicted to exhibit excellent piezoelectric properties and intrinsic valley splitting, resulting from the spontaneous spin polarization, the spatial inversion symmetry breaking and strong spin-orbit coupling (SOC). TaNF is also a potential two-dimensional (2D) magnetic material due to its high Curie temperature and huge magnetic anisotropy energy. The effective control of the band gap of TaNF can be achieved by biaxial strain, which can transform TaNF monolayer from semiconductor to semi-metal. The magnitude of valley splitting at the CBM can be effectively tuned by biaxial strain due to the changes of orbital composition at the valleys. The magnetic anisotropy energy (MAE) can be manipulated by changing the energy and occupation (unoccupation) states of d orbital compositions through biaxial strain. In addition, Curie temperature reaches 373 K under only -3% biaxial strain, indicating that Janus TaNF monolayer can be used at high temperatures for spintronic and valleytronic devices.

3.An improved grand-potential phase-field model of solid-state sintering for many particles

Authors:Marco Seiz, Henrik Hierl, Britta Nestler

Abstract: Understanding the microstuctural evolution during the sintering process is of high relevance as it is a key part in many industrial manufacturing processes. Simulations are one avenue to achieve this understanding, especially field-resolved methods such as the phase-field method. Recent papers have shown several weaknesses in the most common phase-field model of sintering, which the present paper aims to ameliorate. The observed weaknesses are shortly recounted, followed by presenting model variations aiming to remove these deficiencies. The models are tested in the classical two-particle geometry, with the most promising model being run on large-scale three-dimensional packings to determine representative volume elements. A densification that is strongly dependent on the packing size is observed, which suggests that the model requires further improvement.

4.Exploring the Interfacial Coupling between Graphene and Antiferromagnetic Insulator MnPSe$_3$

Authors:Xin Yi, Qiao Chen, Kexin Wang, Yuanyang Yu, Yi Yan, Xin Jiang, Chengyu Yan, Shun Wang

Abstract: Interfacial coupling between graphene and other 2D materials can give rise to intriguing physical phenomena. In particular, several theoretical studies predict that the interplay between graphene and an antiferromagnetic insulator could lead to the emergence of quantum anomalous Hall phases. However, such phases have not been observed experimentally yet, and further experimental studies are needed to reveal the interaction between graphene and antiferromagnetic insulators. Here, we report the study in heterostructures composed of graphene and the antiferromagnetic insulator MnPSe$_3$. It is found that the MnPSe$_3$ has little impact on the quantum Hall phases apart from doping graphene via interfacial charge transfer. However, the magnetic order can contribute indirectly via process like Kondo effect, as evidenced by the observed minimum in the temperature-resistance curve between 20-40 K, far below the N\'eel temperature (70 K).

5.Non-Speckle-based DVC for Measuring Large Deformations in Homogeneous Solids using Laboratory X-ray CT

Authors:Zifan Wang, Akshay Joshi, Angkur Jyoti Dipanka Shaikeea, Vikram Susdhir Deshpande

Abstract: X-ray computed tomography (XCT) has become a reliable metrology tool for measuring internal flaws and other microstructural features in engineering materials. However, tracking of material points to measure three-dimensional (3D) deformations has hitherto relied on either artificially adding tracer particles (speckles) or exploiting inherent microstructural features such as inclusions. This has greatly limited the spatial resolution and magnitude of the deformation measurements. Here we report a novel Flux Enhanced Tomography for Correlation (FETC) technique that leverages the inherent inhomogeneities within nominally homogeneous engineering polymers to track 3D material point displacements without recourse to artificial speckles or microstructural features such as inclusions. The FETC is then combined with a Eulerian/Lagrangian transformation in a multi-step Digital Volume Correlation (DVC) methodology to measure all nine components of the deformation gradient within the volume of complex specimens undergoing extreme deformations. FETC is a powerful technique that greatly expands the capabilities of laboratory-based XCT to provide amongst other things the inputs required for data-driven constitutive modelling approaches.

6.Electrical Characteristics of in situ Mg-doped beta-Ga2O3 Current-Blocking Layer for Vertical Devices

Authors:Sudipto Saha, Lingyu Meng, A F M Anhar Uddin Bhuiyan, Ankit Sharma, Chinmoy Nath Saha, Hongping Zhao, Uttam Singisetti

Abstract: The lack of p-type doping has impeded the development of vertical gallium oxide (Ga2O3) devices. Current blocking layers (CBL) using implanted deep acceptors has been used to demonstrate vertical devices. This paper presents the first demonstration of in situ Mg-doped beta-Ga2O3 CBLs grown using metalorganic chemical vapor deposition. Device structures were designed with in-situ Mg doped layers with varied targeted Mg doping concentrations, which were calibrated by quantitative secondary ion mass spectroscopy (SIMS). The effectiveness of the CBL is characterized using temperature dependent current-voltage measurements using n-Mg-doped-n structures, providing crucial insight into the underlying mechanisms. To further validate the experimental results, a TCAD simulation is performed and the electrically active effective doping is found to be dependent on the Mg-doping density, offering a new perspective on the optimization of CBL performance. Breakdown measurements show a 3.4 MV/cm field strength. This study represents a significant step forward in the development of Ga2O3-based devices and paves the way for future advancements in this exciting field.

7.Machine-Learning Recognition of Dzyaloshinskii-Moriya Interaction from Magnetometry

Authors:Bradley J. Fugetta, Zhijie Chen, Dhritiman Bhattacharay, Kun Yue, Kai Liu, Amy Y. Liu, Gen Yin

Abstract: The Dzyaloshinskii-Moriya interaction (DMI), which is the antisymmetric part of the exchange interaction between neighboring local spins, winds the spin manifold and can stabilize non-trivial topological spin textures. Since topology is a robust information carrier, characterization techniques that can extract the DMI magnitude are important for the discovery and optimization of spintronic materials. Existing experimental techniques for quantitative determination of DMI, such as high-resolution magnetic imaging of spin textures and measurement of magnon or transport properties, are time consuming and require specialized instrumentation. Here we show that a convolutional neural network can extract the DMI magnitude from minor hysteresis loops, or magnetic `fingerprints', of a material. These hysteresis loops are readily available by conventional magnetometry measurements. This provides a convenient tool to investigate topological spin textures for next-generation information processing.

1.Deep learning of experimental electrochemistry for battery cathodes across diverse compositions

Authors:Peichen Zhong, Bowen Deng, Tanjin He, Zhengyan Lun, Gerbrand Ceder

Abstract: Artificial intelligence (AI) has emerged as a powerful tool in the discovery and optimization of novel battery materials. However, the adoption of AI in battery cathode representation and discovery is still limited due to the complexity of optimizing multiple performance properties and the scarcity of high-fidelity data. In this study, we present a comprehensive machine-learning model (DRXNet) for battery informatics and demonstrate the application in discovery and optimization of disordered rocksalt (DRX) cathode materials. We have compiled the electrochemistry data of DRX cathodes over the past five years, resulting in a dataset of more than 30,000 discharge voltage profiles with 14 different metal species. Learning from this extensive dataset, our DRXNet model can automatically capture critical features in the cycling curves of DRX cathodes under various conditions. Illustratively, the model gives rational predictions of the discharge capacity for diverse compositions in the Li--Mn--O--F chemical space and high-entropy systems. As a universal model trained on diverse chemistries, our approach offers a data-driven solution to facilitate the rapid identification of novel cathode materials, accelerating the development of next-generation batteries for carbon neutralization.

2.Bulk Electronic Structure of Ni2MnGa studied by Density Functional Theory and Hard X-ray Photoelectron Spectroscopy

Authors:Joydipto Bhattacharya, Pampa Sadhukhan, Shuvam Sarkar, Vipin Kumar Singh, Andrei Gloskovskii, Sudipta Roy Barman, Aparna Chakrabarti

Abstract: A combined study employing density functional theory (DFT) using the experimentally determined modulated structures and bulk-sensitive hard x-ray photoelectron spectroscopy on single-crystalline Ni$_2$MnGa is presented in this work. For the aforementioned modulated structures, all of the characteristic features in the experimental valence band (VB) are in excellent agreement with the theoretical VB calculated from DFT, evincing that it is the true representation of Ni$_2$MnGa in the martensite phase. We establish the existence of a charge density wave (CDW) state in the martensite phase from the shape of the VB near $E_F$ that shows a transfer of spectral weight in excellent agreement with DFT. Furthermore, presence of a pseudogap is established by fitting the near $E_F$ region with a power law function predicted theoretically for the CDW phase. Thus, the present work emphasizes that the atomic modulation plays an important role in hosting the CDW phase in bulk stoichiometric Ni$_2$MnGa.

3.Anticorrelated Photoluminescence and Free Charge Generation Proves Field-Assisted Exciton Dissociation in Low-Offset PM6:Y5 Organic Solar Cells

Authors:Manasi Pranav, Thomas Hultzsch, Artem Musiienko, Bowen Sun, Atul Shukla, Frank Jaiser, Safa Shoaee, Dieter Neher

Abstract: Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high performance donor-acceptor systems. Here, we probe the origin of field-dependent free charge generation and photoluminescence in non-fullerene acceptor (NFA) based organic solar cells using the polymer PM6 and NFA Y5 - a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artefacts from non-geminate recombination of photogenerated- and dark charge carriers near the electrodes. We then correlate free charge generation with steady state photoluminescence intensity, and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low offset system is entirely controlled by the field dependent exciton dissociation into charge transfer states.

4.Band nonlinearity-enabled manipulation of Dirac nodes, Weyl cones, and valleytronics with intense linearly polarized light

Authors:Ofer Neufeld, Hannes Hübener, Gregor Jotzu, Umberto De Giovannini, Angel Rubio

Abstract: We study monochromatic linearly-polarized laser-induced band structure modifications in material systems with valley (graphene and hexagonal-Boron-Nitride), and topological (Dirac and Weyl semimetals), properties. We find that for Dirac-like linearly-dispersing bands, the laser dressing effectively moves the Dirac nodes away from their original position by up to ~10% of the Brillouin zone (opening a large pseudo-gap in their original position). The direction of the movement can be fully controlled by rotating the laser polarization axis. We prove that this effect originates from band nonlinearities away from the Dirac nodes (without which the effect completely vanishes, and which are often neglected). We demonstrate that this physical mechanism is applicable beyond two-dimensional Dirac semimetals, and can move the positions of the valley minima in hexagonal materials to tune valley selectivity, split and move Weyl cones in higher-order Weyl semimetals, and merge Dirac nodes in three-dimensional topological Dirac semimetals. The model results are validated with ab-initio time-dependent density functional theory calculations. Our results directly affect theoretical and experimental efforts for exploring light-dressed electronic-structure, suggesting that one can benefit from band nonlinearity for tailoring material properties. They also highlight the importance of describing the full band structure in nonlinear optical phenomena in solids.

5.Design of a low-velocity impact framework for evaluating space-grade materials

Authors:Vineet Dawara, Ashok Bajantri, Harish Singh Dhami, SVS Narayana Murty, Koushik Viswanathan

Abstract: Material deformation and failure under impact loading is a subject of active investigation in space science and often requires very specialized equipment for testing. In this work, we present the design, operational analysis and application of a low-velocity ($\sim 100$ m/s) projectile impact framework for evaluating the deformation and failure of space-grade materials. The system is designed to be modular and easily adaptable to various test geometries, while enabling accurate quantitative evaluation of plastic flow. Using coupled numerical methods and experimental techniques, we first establish an operating procedure for the system. Following this, its performance in two complementary impact configurations is demonstrated using numerical and experimental analysis. In the first, a Taylor impact test is performed for predicting the deformed shape of a cylindrical projectile impinging on a rigid substrate. In the second, deformation of a plate struck by a rigid projectile is evaluated. In both cases, physics-based models are used to interpret the resulting fields. We present a discussion of how the system may be used both for material property estimation (e.g., dynamic yield strength) as well as for failure evaluation (e.g., perforation and fracture) in the same projectile impact configuration.

6.Collinear Rashba-Edelstein effect in non-magnetic chiral materials

Authors:Karma Tenzin, Arunesh Roy, Frank T. Cerasoli, Anooja Jayaraj, Marco Buongiorno Nardelli, Jagoda Sławińska

Abstract: Efficient generation and manipulation of spin signals in a given material without invoking external magnetism remain one of the challenges in spintronics. The spin Hall effect (SHE) and Rashba-Edelstein effect (REE) are well-known mechanisms to electrically generate spin accumulation in materials with strong spin-orbit coupling (SOC), but the exact role of the strength and type of SOC, especially in crystals with low symmetry, has yet to be explained. In this study, we investigate REE in two different families of non-magnetic chiral materials, elemental semiconductors (Te and Se) and semimetallic disilicides (TaSi$_2$ and NbSi$_2$), using an approach based on density functional theory (DFT). By analyzing spin textures across the full Brillouin zones and comparing them with REE magnitudes calculated as a function of chemical potential, we link specific features in the electronic structure with the efficiency of the induced spin accumulation. Our findings show that magnitudes of REE can be increased by: (i) the presence of purely radial (Weyl-type) spin texture manifesting as the parallel spin-momentum locking, (ii) high spin polarization of bands along one specific crystallographic direction, (iii) low band velocities. By comparing materials possessing the same crystal structures, but different strengths of SOC, we conclude that larger SOC may indirectly contribute to the enhancement of REE. It yields greater spin-splitting of bands along specific crystallographic directions, which prevents canceling the contributions from the oppositely spin-polarized bands over wider energy regions and helps maintain larger REE magnitudes. We believe that these results will be useful for designing spintronics devices and may aid further computational studies searching for efficient REE in materials with different symmetries and SOC strengths.

7.Adsorption tuning of polarity and magnetism in AgCr2S4 monolayer

Authors:Ranran Li, Yu Wang, Ning Ding, Shuai Dong, Ming An

Abstract: As a recent successfully exfoliated non van der Waals layered material, AgCrS2 has received a lot of attentions. Motivated by its structure related magnetic and ferroelectric behavior, a theoretical study on its exfoliated monolayer AgCr2S4 has been carried out in the present work. Based on density functional theory, the ground state and magnetic order of monolayer AgCr2S4 have been determined. The centrosymmetry emerges upon two-dimensional confinement and thus eliminates the bulk polarity. Moreover, two-dimensional ferromagnetism appears in the CrS2 layer of AgCr2S4 and can persist up to room temperature. The surface adsorption has also been taken into consideration, which shows a nonmonotonic effect on the ionic conductivity through ion displacement of the interlayer Ag, but has little impact on the layered magnetic structure.

8.Micromagnetics and Multiscale Hysteresis Simulations of Permanent Magnets

Authors:Yangyiwei Yang, Patrick Kühn, Mozhdeh Fathidoost, Bai-Xiang Xu

Abstract: Confronting the unveiled sophisticated multiscale structural and physical characteristics of hysteresis simulation of permanent magnets, notably samarium-cobalt (Sm-Co) alloy, a novel scheme is proposed linking physics-based micromagnetics on the nanostructure level and magnetostatic homogenization on the mesoscale polycrystal level. Thereby the micromagnetics-informed surrogate hysteron is the key to bridge the scales of nanostructure and polycrystal structure. This hysteron can readily emulate the local magnetization reversal with the nanoscale mechanisms considered, such as nucleation of domains, and domain wall migration and pinning. The overall hysteresis, based on a sintered Sm-Co polycrystal, considering both mesoscale and nanoscale characteristics, is simulated and discussed.

9.Transient translation symmetry breaking via quartic-order negative light-phononcoupling at Brillouin zone boundary in KTaO${}_{3}$

Authors:Adrián Gómez Pueyo, Alaska Subedi

Abstract: KTaO${}_{3}$ presents a rich hyper-Raman spectrum originating from two-phonon processes at the Brillouin zone boundary, indicating the possibility of driving these phonon modes using intense midinfrared laser sources. We obtained the coupling of light to the highest-frequency longitudinal optic phonon mode $Q_{\rm{HY}}$ at the $X$ $(0,0, \frac{1}{2})$ point by first principles calculations of the total energy as a function of the phonon coordinate $Q_{\rm{HY}}$ and electric field $E$. We find that the energy curve as a function of $Q_{\rm{HY}}$ softens for finite values of electric field, indicating the presence of $Q_{\rm{HY}}^2 E^2$ nonlinearity with negative coupling coefficient. We studied the feasibility of utilizing this nonlinearity to transiently break the translation symmetry of the material by making the $Q_{\rm{HY}}$ mode unstable with an intense midinfrared pump pulse. We also considered the possibility that nonlinear phonon-phonon couplings can excite the lowest-frequency phonon coordinates $Q_{\rm{LZ}}$ and $Q_{\rm{LX}}$ at $X$ when the $Q_{\rm{HY}}$ mode is externally driven. The nonlinear phonon-phonon couplings were also obtained from first principles via total-energy calculations as a function of the phonon coordinates, and these were used to construct the coupled classical equations of motion for the phonon coordinates in the presence of an external pump term on $Q_{\rm{HY}}$. We numerically solved them for a range of pump frequencies and amplitudes and found three regimes where the translation symmetry is broken: i) rectification of the lowest-frequency coordinates due to large amplitude oscillation of the $Q_{\rm{HY}}$ coordinate about its equilibrium position, ii) rectification of only the $Q_{\rm{HY}}$ coordinate without displaced oscillations of the lowest-frequency coordinates, and iii) rectification of all three coordinates.

10.Additive manufacturing of Ni-Mn-Sn shape memory Heusler alloy -- Microstructure and magnetic properties from powder to printed parts

Authors:Franziska Scheibel, Christian Lauhoff, Philipp Krooß, Stefan Riegg, Niklas Sommer, David Koch, Konrad Opelt, Heiner Gutte, Olena Volkova, Stefan Böhm, Thomas Niendorf, Oliver Gutfleisch

Abstract: Ni-Mn-based Heusler alloys like Ni-Mn-Sn show an elastocaloric as well as magnetocaloric effect during the magneto-structural phase transition, making this material interesting for solid-state cooling application. Material processing by additive manufacturing can overcome difficulties related to machinability of the alloys, caused by their intrinsic brittleness. Since the magnetic properties and transition temperature are highly sensitive to the chemical composition, it is essential to understand and monitoring these properties over the entire processing chain. In the present work the microstructural and magnetic properties from gas-atomized powder to post-processed Ni-Mn-Sn alloy are investigated. Direct energy deposition was used for processing, promoting the evolution of a polycrystalline microstructure being characterized by elongated grains along the building direction. A complete and sharp martensitic transformation can be achieved after applying a subsequent heat treatment at 1173 K for 24 h. The Mn-evaporation of 1.3 at. % and the formation of Mn-oxide during DED-processing lead to an increase of the transition temperature of 45 K and a decrease of magnetization, clearly pointing at the necessity of controlling the composition, oxygen partial pressure and magnetic properties over the entire processing chain.

1.Predicted T-XY (X$\neq$Y=P, As and Sb) monolayer with intrinsic persistent spin helix and large piezoelectric response

Authors:San-Dong Guo, Xu-Kun Feng, Dong Huang, Shaobo Chen, Yee Sin Ang

Abstract: The persistent spin helix (PSH) is robust against spin-independent scattering and renders an extremely long spin lifetime, which can improve the performance of potential spintronic devices. To achieve the PSH, a unidirectional spin configuration is required in the momentum space. Here, T-XY (X$\neq$Y=P, As and Sb) monolayers with dynamical, mechanical and thermal stabilities are predicted to intrinsically possess PSH. Due to the $C_{2\upsilon}$ point-group symmetry, a unidirectional spin configuration is preserved in the out-of-plane direction for both conduction and valence bands around the high-symmetry $\Gamma$ point. That is, the expectation value of the spin $S$ only has the out-of-plane component $S_z$. The application of an out-of-plane external electric field can induce in-plane components $S_x$ and $S_y$, thus offering a promising platform for the on-off logical functionality of spin devices. T-XY (X$\neq$Y=P, As and Sb) monolayers are determined to be excellent two-dimensional (2D) piezoelectric materials. The in-plane piezoelectric coefficient $d_{11}$ (absolute value) of T-SbP is 226.15 pm/V, which is larger than that reported for most 2D materials, providing possibility of tuning spin-splitting of PSH by in-plane electric field induced with a uniaxial in-plane strain through piezoelectric effect. Our work reveals a new family of T-phase 2D materials, which could provide promising applications in spintronic and piezoelectric devices.

2.Interaction of in-plane Drude carrier with c-axis phonon in $\rm PdCoO_2$

Authors:Dongmin Seo, Gihyeon Ahn, Gaurab Rimal, Seunghyun Khim, Suk Bum Chung, A. P. Mackenzie, Seongshik Oh, S. J. Moon, Eunjip Choi

Abstract: We performed polarized reflection and transmission measurements on the layered conducting oxide $\rm PdCoO_2$ thin films. For the ab-plane, an optical peak near $\Omega$ $\approx$ 750 cm$^{-1}$ drives the scattering rate $\gamma^{*}(\omega)$ and effective mass $m^{*}(\omega)$ of the Drude carrier to increase and decrease respectively for $\omega$ $\geqq$ $\Omega$. For the c-axis, a longitudinal optical phonon (LO) is present at $\Omega$ as evidenced by a peak in the loss function Im[$-1/\varepsilon_{c}(\omega)$]. Further polarized measurements in different light propagation (q) and electric field (E) configurations indicate that the Peak at $\Omega$ results from an electron-phonon coupling of the ab-plane carrier with the c-LO phonon, which leads to the frequency-dependent $\gamma^{*}(\omega)$ and $m^{*}(\omega)$. This unusual interaction was previously reported in high-temperature superconductors (HTSC) between a non-Drude, mid-infrared band and a c-LO. On the contrary, it is the Drude carrier that couples in $\rm PdCoO_2$. The coupling between the ab-plane Drude carrier and c-LO suggests that the c-LO phonon may play a significant role in the characteristic ab-plane electronic properties of $\rm PdCoO_2$ including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.

3.Electrical tuning of robust layered antiferromagnetism in MXene monolayer

Authors:Xinyu Yang, Ning Ding, Jun Chen, Ziwen Wang, Ming An, Shuai Dong

Abstract: A-type antiferromagnetism, with an in-plane ferromagnetic order and the interlayer antiferromagnetic coupling, owns inborn advantages for electrical manipulations but is naturally rare in real materials except in those artificial antiferromagnetic heterostructures. Here, a robust layered antiferromagnetism with a high N\'eel temperature is predicted in a MXene Cr$_2$CCl$_2$ monolayer, which provides an ideal platform as a magnetoelectric field effect transistor. Based on first-principles calculations, we demonstrate that an electric field can induce the band splitting between spin-up and spin-down channels. Although no net magnetization is generated, the inversion symmetry between the lower Cr layer and the upper Cr layer is broken via electronic cloud distortions. Moreover, this electric field can be replaced by a proximate ferroelectric layer for nonvolatility. The magneto-optic Kerr effect can be used to detect this magnetoelectricity, even if it is a collinear antiferromagnet with zero magnetization.

4.Gas-to-nanotextile: high-performance materials from floating 1D nanoparticles

Authors:Isabel Gómez-Palos, Miguel Vazquez-Pufleau, Richard S Schäufele, Anastasiia Mikhalchan, Afshin Pendashteh, Álvaro Ridruejo, Juan J. Vilatela

Abstract: Suspended in the gas phase, 1D inorganic nanoparticles (nanotubes and nanowires) grow to hundreds of microns in a second and can be thus directly assembled into freestanding network materials. The corresponding process continuously transforms gas precursors into aerosols into aerogels into macroscopic nanotextiles. By enabling the assembly of very high aspect ratio nanoparticles, this processing route has translated into high-performance structural materials, transparent conductors and battery anodes, amongst other embodiments. This paper reviews progress in the application of such manufacturing process to nanotubes and nanowires. It analyses 1D nanoparticle growth through floating catalyst chemical vapour deposition (FCCVD), in terms of reaction selectivity, scalability and its inherently ultra-fast growth rates (107-108 atoms per second) up to 1000 times faster than for substrate CVD. We summarise emerging descriptions of the formation of aerogels through percolation theory and multi-scale models for the collision and aggregation of 1D nanoparticles. The paper shows that macroscopic ensembles of 1D nanoparticles resemble textiles in their porous network structure, high flexibility and damage-tolerance. Their bulk properties depend strongly on inter-particle properties and are dominated by alignment and volume fraction. Selected examples of nanotextiles that surpass granular and monolithic materials include structural fibres with polymer-like toughness, transparent conductors, and slurry-free composite electrodes for energy storage.

5.Recipe for single-pair-Weyl-points phonons carrying the same chiral charges

Authors:Guangqian Ding, Chengwu Xie, Jingbo Bai, Zhenxiang Cheng, Xiaotian Wang, Weikang Wu

Abstract: Recently, Wang et al. [Phys. Rev. B, 106, 195129 (2022)] challenged a widely held belief in the field of Weyl physics, demonstrating that single-pair-Weyl-points (SP-WPs) can exist in nonmagnetic spinless systems, contrary to previous assumptions that they could only exist in magnetic systems. Wang et al. observed that the SP-WPs with opposite and even chiral charges (i.e., |C| = 2 or 4) could also exist in nonmagnetic spinless systems. In this Letter, we present a novel finding in which SP-WPs have a partner, namely a charged nodal surface, in nonmagnetic spinless systems. In contrast to previous observations, we show that the SP-WPs can have uneven chiral charges (i.e., |C| = 1). We identify 6 (out of 230) space groups (SGs) that contain such SP-WPs by searching the encyclopedia of emergent particles in three-dimensional crystals. Our finds were confirmed through the phonon spectra of two specific materials Zr3O (with SG 182) and NaPH2NO3 (with SG 173). This discovery broadens the range of materials that can host SP-WPs and applies to other nonmagnetic spinless crystals.

6.Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains

Authors:William Baldwin, Xia Liang, Johan Klarbring, Milos Dubajic, David Dell'Angelo, Christopher Sutton, Claudia Caddeo, Samuel D. Stranks, Alessandro Mattoni, Aron Walsh, Gábor Csányi

Abstract: Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behaviour. In the higher temperature, higher symmetry phases of these materials, several complex structural features have been observed. The local structure can differ greatly from the average structure and there is evidence that dynamic two-dimensional structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI$_3$ is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, we reveal that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and we report the length and timescales of the motion. Finally, we investigate and visualise the spatial arrangement of these features and their interactions, providing a comprehensive picture of local structure in the higher symmetry phases.

7.Low-Frequency Electronic Noise in the Aluminum Gallium Oxide Schottky Barrier Diodes

Authors:Subhajit Ghosh, Dinusha Herath Mudiyanselage, Sergey Rumyantsev, Yuji Zhao, Houqiang Fu, Stephen Goodnick, Robert Nemanich, Alexander A. Balandin

Abstract: We report on the low-frequency electronic noise in (Al$_x$Ga$_{1-x}$)$_2$O$_3$ Schottky barrier diodes. The noise spectral density reveals 1/f dependence, characteristic of the flicker noise, with superimposed Lorentzian bulges at the intermediate current levels (f is the frequency). The normalized noise spectral density in such diodes was determined to be on the order of 10$^{-12}$ cm$^2$/Hz (f=10 Hz) at 1 A/cm$^2$ current density. At the intermediate current regime, we observed the random telegraph signal noise, correlated with the appearance of Lorentzian bulges in the noise spectrum. The random telegraph signal noise was attributed to the defects near the Schottky barrier. The defects can affect the local electric field and the potential barrier, and correspondingly, impact the electric current. The obtained results help to understand noise in Schottky barrier diodes made of ultra-wide-band-gap semiconductors and can be used for the material and device quality assessment.

8.A data-driven framework for structure-property correlation in ordered and disordered cellular metamaterials

Authors:Shengzhi Luan, Enze Chen, Joel John, Stavros Gaitanaros

Abstract: Cellular solids and micro-lattices are a class of lightweight architected materials that have been established for their unique mechanical, thermal, and acoustic properties. It has been shown that by tuning material architecture, a combination of topology and solid(s) distribution, one can design new material systems, also known as metamaterials, with superior performance compared to conventional monolithic solids. Despite the continuously growing complexity of synthesized microstructures, mainly enabled by developments in additive manufacturing, correlating their morphological characteristics to the resulting material properties has not advanced equally. This work aims to develop a systematic data-driven framework that is capable of identifying all key microstructural characteristics and evaluating their effect on a target material property. The framework relies on integrating virtual structure generation and quantification algorithms with interpretable surrogate models. The effectiveness of the proposed approach is demonstrated by analyzing the effective stiffness of a broad class of two-dimensional (2D) cellular metamaterials with varying topological disorder. The results reveal the complex manner in which well-known stiffness contributors, including nodal connectivity, cooperate with often-overlooked microstructural features such as strut orientation, to determine macroscopic material behavior. We further re-examine Maxwell's criteria regarding the rigidity of frame structures, as they pertain to the effective stiffness of cellular solids and showcase microstructures that violate them. This framework can be used for structure-property correlation in different classes of metamaterials as well as the discovery of novel architectures with tailored combinations of material properties.

9.Consistent optical and electrical determination of carrier concentrations for the accurate modeling of the transport properties of n-type Ge

Authors:José Menéndez, Chi Xu, John Kouvetakis

Abstract: A consistent methodology is presented to extract carrier concentrations in n-type Ge from measurements of the infrared dielectric function and the Hall effect. In the case of the optical measurements, usually carried out using spectroscopic ellipsometry, the carrier concentration is affected by the doping dependence of the conductivity effective mass, which is computed using a model of the electronic density of states that accounts for non-parabolicity and is fit to electronic structure calculations. Carrier concentrations obtained from Hall measurements require a knowledge of the Hall factor, which is arbitrarily set equal to unit in most practical applications. We have calculated the Hall factor for n-Ge using a model that accounts for scattering with phonons and with ionized impurities. We show that determinations of the carrier concentration n using our computed effective mass and Hall factor virtually eliminates any systematic discrepancy between the two types of measurement. We then use these results to compute majority carrier mobilities from measured resistivity values, to compare with measurements of minority carrier mobilities, and to fit empirical expressions to the doping dependence of the mobilities that can be used to model Ge devices.

10.Model-free characterization of topological edge and corner states in mechanical networks

Authors:Marcelo Guzman, Xiaofei Guo, Corentin Coulais, David Carpentier, Denis Bartolo

Abstract: Topological materials can host edge and corner states that are protected from disorder and material imperfections. In particular, the topological edge states of mechanical structures present unmatched opportunities for achieving robust responses in wave guiding, sensing, computation, and filtering. However, determining whether a mechanical structure is topologically nontrivial and features topologically-protected modes has hitherto relied on theoretical models. This strong requirement has limited the experimental and practical significance of topological mechanics to laboratory demonstrations. Here, we introduce and validate an experimental method to detect the topologically protected zero modes of mechanical structures without resorting to any modeling step. Our practical method is based on a simple electrostatic analogy: topological zero modes are akin to electric charges. To detect them, we identify elementary mechanical molecules and measure their chiral polarization, a recently introduced marker of topology in chiral phases. Topological zero modes are then identified as singularities of the polarization field. Our method readily applies to any mechanical structure and effectively detects the edge and corner states of regular and higher-order topological insulators. Our findings extend the reach of chiral topological phases beyond designer materials, and allow their direct experimental investigation.

11.Spin-phonon interactions and magnetoelectric coupling in Co$_4$$B_2$O$_9$ ($B$ = Nb, Ta)

Authors:K. Park, J. Kim, S. Choi, S. Fan, C. Kim, D. G. Oh, N. Lee, S. -W. Cheong, V. Kiryukhin, Y. J. Choi, D. Vanderbilt, J. H. Lee, J. L. Musfeldt

Abstract: In order to explore the consequences of spin-orbit coupling on spin-phonon interactions in a set of chemically-similar mixed metal oxides, we measured the infrared vibrational properties of Co$_4B_2$O$_9$ ($B$ = Nb, Ta) as a function of temperature and compared our findings with lattice dynamics calculations and several different models of spin-phonon coupling. Frequency vs. temperature trends for the Co$^{2+}$ shearing mode near 150 cm$^{-1}$ reveal significant shifts across the magnetic ordering temperature that are especially large in relative terms. Bringing these results together and accounting for noncollinearity, we obtain spin-phonon coupling constants of -3.4 and -4.3 cm$^{-1}$ for Co$_4$Nb$_2$O$_9$ and the Ta analog, respectively. Analysis reveals that these coupling constants derive from interlayer (rather than intralayer) exchange interactions and that the interlayer interactions contain competing antiferromagnetic and ferromagnetic contributions. At the same time, beyond-Heisenberg terms are minimized due to fortuitous symmetry considerations, different than most other 4$d$- and 5$d$-containing oxides. Comparison with other contemporary oxides shows that spin-phonon coupling in this family of materials is among the strongest ever reported, suggesting an origin for magnetoelectric coupling.