Materials Science (cond-mat.mtrl-sci)

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.