Materials Science (cond-mat.mtrl-sci)

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} $

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.