First-principles computations are the driving force behind numerous discoveries of hydride-based superconductors, mostly at high pressures, during the last decade. Machine-learning (ML) approaches can further accelerate the future discoveries if their reliability can be improved. The main challenge of current ML approaches, typically aiming at predicting the critical temperature $T_{\mathrm{c}}$ of a solid from its chemical composition and target pressure, is that the correlations to be learned are deeply hidden, indirect, and uncertain. In this paper, we show that predicting superconductivity at any pressure from the atomic structure is sustainable and reliable. For a demonstration, we curated a diverse data set of 584 atomic structures for which $\lambda$ and ${\omega }_{log}$, two parameters of the electron-phonon interactions, were computed. We then trained some ML models to predict $\lambda$ and ${\omega }_{log}$, from which $T_{\mathrm{c}}$ can be computed in a postprocessing manner. The models were validated and used to identify two possible superconductors whose $T_{\mathrm{c}}\simeq 10–15$ K at zero pressure. Interestingly, these materials have been synthesized and studied in some other contexts. In summary, the proposed ML approach enables a pathway to directly transfer what can be learned from the high-pressure atomic-level details that correlate with high-$T_{\mathrm{c}}$ superconductivity to zero pressure. Going forward, this strategy will be improved to better contribute to the discovery of new superconductors.

The discovery of high-$T_c$ conventional superconductivity in high-pressure hydrides has helped establish computational methods as a formidable tool to guide material discoveries in a field traditionally dominated by serendipitous experimental search. This paves the way to an ever-increasing use of data-driven approaches to the study and design of superconductors. In this work, we propose a new adaptive method to generate meaningful datasets of superconductors, based on element substitution into a small set of representative structural templates, generated by crystal structure prediction methods—adapted high-throughput approach. Our approach realizes an optimal compromise between structural variety and computational efficiency and can be easily generalized to other elements and compositions. As a first application, we apply it to binary hydrides at high pressure, realizing a database of 880 hypothetical structures, characterized with a set of electronic, vibrational, and chemical descriptors. In our Superhydra Database, 139 structures are superconducting according to the McMillan-Allen-Dynes approximation. Studying the distribution of $T_c$ and other properties across the database with advanced statistical and visualization techniques, we are able to obtain comprehensive material maps of the phase space of binary hydrides. The Superhydra database can be thought as a first step of a generalized effort to map conventional superconductivity.

Layered perovskites viz. Ruddlesden-Popper (RP) compounds have attracted a significant research interest lately due to their superior electrocatalytic activities, on top of their rich physics. Although the versatility of their compositional space is the key to tune their stability and applicability, its complexity also makes the understanding and design of new functional RP-based materials difficult. Herein by parameterizing a cluster expansion model Hamiltonian with density-functional inputs, we have successfully scanned over the complex configurational space of an RP phase, taking ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{Ni}}_{1-y}{\mathrm{Fe}}_y{\mathrm{O}}_{4\pm \delta }$ as an example. This surrogate model is then used to predict the energies of unknown compositions and performing Metropolis Monte Carlo sampling for finding the ground-state structures. By analyzing these stable configurations at different $x, y$'s, and $\delta$=0, 0.125, we found that apical vacancies in the rock-salt layer are more favorable due to lesser steric forces but changes in $A$-site oxidation state due to Sr substitution can facilitate equatorial O-vacancy formation even in the perovskite layers. Our results uncover the structural evolution of RP oxides with increasing $A$-site (Sr) and $B$-site (Fe) substitutions. We find that a critical Sr concentration is required to stabilize Fe in these systems. With increasing Sr concentration, the $A$-site charge state changes gradually from ${\mathrm{La}}^{+3}$ to ${\mathrm{Sr}}^{+2}$, which facilitates both O-vacancy formation and stabilization of the $B$-site dopants. The findings further highlight the effects of configurational changes for the same composition on electronic structure. Thus, our work clearly demonstrates the importance of exploring configurational space of these complex oxides when using electronic-structure features as descriptors of their catalytic and other functional properties.

A central challenge in high-throughput density functional theory (HT-DFT) calculations is selecting a combination of input parameters and postprocessing techniques that can be used across all materials classes, while also managing accuracy-cost tradeoffs. To investigate the effects of these parameter choices, we consolidate three large HT-DFT databases: Automatic-FLOW (AFLOW), the Materials Project (MP), and the Open Quantum Materials Database (OQMD), and compare reported properties across each pair of databases for materials calculated using the same initial crystal structure. We find that HT-DFT formation energies and volumes are generally more reproducible than band gaps and total magnetizations; for instance, a notable fraction of records disagree on whether a material is metallic (up to 7%) or magnetic (up to 15%). The variance between calculated properties is as high as 0.105 eV/atom (median relative absolute difference, or MRAD, of 6%) for formation energy, 0.65 ${\AA }^3/\text{atom}$ (MRAD of 4%) for volume, 0.21 eV (MRAD of 9%) for band gap, and $0.15{\mu }_{\mathrm{B}}/\text{formula unit}$ (MRAD of 8%) for total magnetization, comparable to the differences between DFT and experiment. We trace some of the larger discrepancies to choices involving pseudopotentials, the $\mathrm{DFT}+U$ formalism, and elemental reference states, and argue that further standardization of HT-DFT would be beneficial to reproducibility.

Half-Heusler compounds with 18 valence electrons per unit cell are well-known nonmagnetic semiconductors. Employing first-principles electronic band-structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas or hole gas at the interface. A qualitative analysis of the origin of the spin polarization at the interfaces suggests that strong correlations are responsible. For the cases of magnetic interfaces where half-metallicity is also present, we propose a modified Slater-Pauling rule similar to the one for bulk half-metallic half-Heusler compounds. Additionally, we calculate exchange parameters, Curie temperatures, and magnetic anisotropy energies for magnetic interfaces. Our study, along with recent experimental evidence confirming the presence of two-dimensional electron gas at CoTiSb/NiTiSn heterojunctions, may motivate future efforts to explore and realize device applications using these heterojunctions.

Recent observation of quantum transport phenomena of Weyl fermions has brought much attention to $4d$ ferromagnetic perovskite ${\mathrm{SrRuO}}_3$ as a magnetic Weyl semimetal. Additionally, the hump-like Hall effect anomaly, which might have a topological origin, has been reported for this material. Here, we show that the emergence of such phenomena is governed by the degree of scattering determined by the defect density (Ru-deficiency- and/or interface-driven-defect scattering) and measurement temperature (phonon scattering), where the former is controlled by varying the growth conditions of the ${\mathrm{SrRuO}}_3$ films in molecular beam epitaxy as well as the film thickness. The resulting electronic transport properties can be classified into three categories: clean, intermediate, and dirty regimes. The transport of Weyl fermions emerges in the clean regime, whereas that of topologically trivial conduction electrons in the ferromagnetic metal state prevails in the intermediate and dirty regimes. In the clean and intermediate regimes, anomalous Hall resistivity obeys a scaling law incorporating the intrinsic Karplus-Luttinger and extrinsic side-jump mechanisms. The hump-like Hall effect anomaly is observed only in the dirty regime, which is contrary to the scaling law between anomalous Hall resistivity and longitudinal resistivity. Hence, we conclude that this anomaly is not inherent to the material and does not have a topological origin. We also provide defect- and temperature-dependent transport phase diagrams of stoichiometric ${\mathrm{SrRuO}}_3$ and Ru-deficient ${\mathrm{SrRu}}_{0.7}{\mathrm{O}}_3$ where the appearance of Weyl fermions and hump-like Hall effect anomaly is mapped. These diagrams may serve as a guideline for designing $\mathrm{Sr}{\mathrm{Ru}}_{1-x}{\mathrm{O}}_3$-based spintronic and topological electronic devices.

Low magnetic damping and high group velocity of spin waves (SWs) or magnons are two crucial parameters for functional magnonic devices. Magnonics research on signal processing and wave-based computation at GHz frequencies focused on the artificial ferrimagnetic garnet ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG) so far. We report on spin wave spectroscopy studies performed on the natural mineral hematite ($\alpha {\text{-Fe}}_2{\text{O}}_3$), which is a canted antiferromagnet. By means of broadband GHz spectroscopy and inelastic light scattering, we determine a damping coefficient of $1.1\times {10}^{-5}$ and magnon group velocities of a few 10 km/s, respectively, at room temperature. Covering a large regime of wave vectors up to $k\approx 24\mathrm{rad}/\mu \mathrm{m}$, we find the exchange stiffness length to be relatively short and only about 1 Å. In a small magnetic field of 30 mT, the decay length of SWs is estimated to be 1.1 cm similar to the best YIG. Still, inelastic light scattering provides surprisingly broad and partly asymmetric resonance peaks. Their characteristic shape is induced by the large group velocities, low damping, and distribution of incident angles inside the laser beam. Our results promote hematite as an alternative and sustainable basis for magnonic devices with fast speeds and low losses based on a stable natural mineral.

The electronic and magnetic properties in $AB{\mathrm{O}}_3$ perovskite oxides are extremely sensitive to lattice structure, but also to dimensionality, such as the thickness in thin film-form. Here, we report the thickness-dependent electromagnetic properties of ultrathin epitaxially stabilized ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x\mathrm{Ru}{\mathrm{O}}_3$ ($x=0.08$, 0.2) thin films on a $\mathrm{SrTi}{\mathrm{O}}_3$ (001) substrate. The results reveal that the barium doping $(0.08\le x\le 0.20)$ reduces $\mathrm{Ru}{\mathrm{O}}_6$ orthorhombic distortions existing in $\mathrm{SrRu}{\mathrm{O}}_3$ and induces a tetragonal distortion, as evidenced by out-of-plane lattice expansion. A metal-to-insulator transition, accompanied by a ferromagnetic to nonmagnetic transition occurs with reducing film thickness from 10 to 3 unit cells for both $x=0.08$ and 0.2, regardless of the doping level. The results suggest that the effects of compositional vacancies and surface/interface contributions introduced via dimensional confinement are more dominant than $A$-site chemical disorder or structural distortion for the loss of metallicity and ferromagnetism in ultra-thin epitaxial films.

Controlling the uniaxial magnetic anisotropy is of practical interest for a wide variety of applications. We study ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ single films grown on various crystalline orientations of ${\mathrm{LiNbO}}_3$ substrates and on oxidized silicon. We identify the annealing conditions that are appropriate to induce or suppress in-plane uniaxial anisotropy. Anisotropy fields can be increased by annealing up to 11 mT when using substrates with anisotropic surfaces. They can be decreased to below 1 mT when using isotropic surfaces. In the first case, the observed increase of the anisotropy originates from the biaxial strain in the film caused by the anisotropic thermal contraction of the substrate when back at room temperature after strain relaxation during annealing. In the second case, anisotropy is progressively removed by applying successive orthogonal fields that are assumed to progressively suppress any chemical ordering within the magnetic film. The method can be applied to CoFeB/Ru/CoFeB synthetic antiferromagnets, but the tuning of the anisotropy comes with a decrease of the interlayer exchange coupling and a drastic change in the exchange stiffness.

Epitaxial stabilization of thermodynamically metastable phases and advances in atomic control of complex oxide thin-film growth can be used effectively for realizing novel phenomena and as an alternative for bulk synthesis under extreme thermodynamic conditions. Here, we investigate infinite layer (IL) based cuprate superlattices, where 7–8 unit cells of ${\mathrm{Sr}}_{0.6}{\mathrm{Ca}}_{0.4}\mathrm{Cu}{\mathrm{O}}_2$ (SCCO) are sandwiched between ultrathin spacer layers of $\mathrm{SrTi}{\mathrm{O}}_3$ (STO), $\mathrm{SrRu}{\mathrm{O}}_3$, or $\mathrm{BaCu}{\mathrm{O}}_2$ (BCO) and only observe superconductivity in the pure [SCCO/BCO] superlattice (SL) without spacer layers. Apparently, the insertion of an additional STO spacer layer in the latter SL prevents the occurrence of superconductivity. The observed superconductivity in [SCCO/BCO] SL is discussed in terms of a structural model involving the interplay between the $\mathrm{Cu}{\mathrm{O}}_2$ plane and the CuO chain similar to the bulk $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ superconductor. The structural origin was found by the identification of a metastable IL-$\mathrm{BaCu}{\mathrm{O}}_2$ variant, which deviates highly from its parent bulk crystal structure and exhibits a relatively larger out-of-plane lattice parameter (around $7\AA$) when sandwiched with SCCO in the form of [SCCO/BCO] SL. However, this variant is absent when STO spacer layers are introduced between SCCO and BCO layers. X-ray absorption spectra of the Cu $L$ edge for BCO exhibits a slightly higher energy satellite peak as compared to the $3d^{9}L$ Zhang-Rice character observed in SCCO. This result indicates the existence of contrasting plane and chain-type Cu-O blocks in SCCO and BCO, respectively, which is further corroborated using annular bright field scanning transmission electron microscopy imaging. This work unravels an unexpected structure of BCO which helps in realizing superconductivity in [SCCO/BCO] SL and provides a wider perspective in the growth and design of cuprate-based hybrid structures.

Wurtzite Boron Nitride ($w\mathrm{BN}$) is a wide band gap BN polymorph with unique mechanical properties such as hardness and stiffness. Initially synthesized in 1963 by transforming hexagonal BN ($h\mathrm{BN}$) under high temperature and pressure conditions, $w\mathrm{BN}$ can now be stabilized at atmospheric pressure to obtain high-quality samples. Our first-principles study investigates the electronic, vibrational, and optical properties of $w\mathrm{BN}$ across a broad range of pressures. We account for the electron-hole interaction in the optical response, revealing that this effect is crucial to interpret the available experimental spectra. We also calculate the coupling between excitons and phonons and provide a phonon-assisted emission spectrum, centered around 6.02 eV. Our results hold significant importance for the potential application of $w\mathrm{BN}$ as a dielectric material in BN-based technologies, especially in optoelectronics and harsh environments. We also expect that our prediction could be verified in the future, and it could aid in the identification of $w\mathrm{BN}$ through cathodoluminescence experiments.

We investigate the behavior of amorphous silicon under hydrostatic compression using molecular simulations. During compression, amorphous silicon undergoes a transformation from a low-density to a high-density structure. Depending on the temperature and the compression rate, the transformation occurs in a pressure range between 6 and 16 GPa. Ensemble-averaged density and elastic constants change discontinuously across the transition. Densification of individual glassy samples occurs through a series of discrete plastic events, each of which is accompanied by a vanishing shear modulus. This is the signature of a series of elastic instabilities, similar to shear transformation zones observed during shear yielding of glasses. We compare the structure obtained during compression with a quasiequilibrium form of amorphous silicon obtained by quenching a melt at constant pressure. This gives structures identical to compression at low and high pressure, but the transition between low- and high-pressure structures occurs gradually rather than discontinuously. Our observations indicate that the polyamorphic transition has the characteristics of a yield transition that occurs under compression instead of shear.

Hierarchical microstructures are often invoked to explain the high resilience and fracture toughness of biological materials such as bone and nacre. Biomimetic material models inspired by such hierarchical biomaterials face the obvious challenge of capturing their inherent multiscale complexity, both in experiments and in simulations. To study the influence of hierarchical microstructure on fracture properties, we propose a large-scale three-dimensional hierarchical beam-element simulation framework, in which we generalize the constitutive framework of Timoshenko beam elasticity and maximum distortion energy theory failure criteria to the complex case of hierarchical networks of up to six self-similar hierarchical levels, consisting of approximately 5 million elements. We perform a statistical study of stress-strain relationships and fracture surface morphologies and conclude that hierarchical systems are capable of arresting crack propagation, an ability that reduces their sensitivity to preexisting damage and enhances their fault tolerance compared to reference fibrous materials without microstructural hierarchy.

Nonmagnetic topological semimetals that combine chirality in real and momentum spaces host unconventional multifold fermions and exhibit exotic electronic and optical properties endowed by their topologically nontrivial electronic band structure. Although the synthesis of nonmagnetic chiral single crystals with a noncentrosymmetric cubic B20 structure is well established, their heteroepitaxial growth in crystalline thin films remains a notable challenge. In this study, we present the structural, magnetic, and electrical magnetotransport properties of 24- and 51-nm-thick films of a B20-RhSi stoichiometric compound grown by magnetron sputtering. RhSi crystalline thin films on Si (111) single-crystal substrates exhibit a preferred (111) orientation with twin domains. The RhSi films display a nonmagnetic ground state, and their electrical resistivity demonstrates a clear and nonsaturating metallic behavior from 300 to 5 K. Magnetotransport measurements reveal that hole-type carriers dominate the Hall response with multiband contributions to electronic transport in the system. The good agreement with the Bloch-Grüneisen model and our first-principles calculations confirms that temperature-dependent electrical resistivity is governed by electron-phonon scattering. The ability to grow textured-epitaxial thin films of nonmagnetic B20 chiral topological semimetals is an important step toward accessing and controlling their remarkable topological surface states for designing chiraltronic devices with novel optoelectronic or spintronic functionalities.

Chemical compression effects on the ferroic orders in La substituted $\mathrm{BiFe}{\mathrm{O}}_3$ are studied by atomic pair distribution function analysis and structure modeling. While rhombohedral $\mathrm{BiFe}{\mathrm{O}}_3$ exhibits ferroelectricity and antiferromagnetism (AF) with a cycloidal Fe spin arrangement leading to zero magnetization, La substituted $\mathrm{BiFe}{\mathrm{O}}_3$ exhibits a reduced lattice polarization, strengthened AF order, and an apparently suppressed spin cycloid leading to nonzero magnetization. The observed changes in the ferroic orders arise from the chemical compression induced changes in the octahedral rotation and ferroelectric displacement of Bi atoms, which appear as lattice degrees of freedom entangling the electronic and magnetic orders in $\mathrm{BiFe}{\mathrm{O}}_3$.

The locally noncentrosymmetric superconductor SrPtAs is proposed to host a topological chiral $d$-wave state, but experimental reports have been limited to polycrystalline samples. Here we report the synthesis of single crystalline SrPtAs grown from Pb flux. SrPtAs crystallizes in the hexagonal space group $P{6}_3/mmc$ with lattice parameters $a=4.2445\left(4\right)$ Å and $c=8.9513\left(18\right)$ Å. Magnetic susceptibility and electrical resistivity measurements reveal a superconducting transition at $T_c\sim 2.2$ K, in agreement with previous reports on polycrystalline samples. Surprisingly, heat capacity data show only a small bulk transition at 0.7 K. We discuss the possible origins of the discrepancy between the various measurements.

Solid solution strengthening is a common method used in physical metallurgy to increase the strength of metals. However, it is also possible for solute atoms to reduce the strength of metals, known as the solid solution softening effect. In this paper, atomistic simulations were carried out using molecular dynamics and Monte Carlo simulations to explore the softening phenomenon in single crystalline metal nanowires (MNWs) of different alloy systems. It was found that, for single crystalline MNWs, softening is more prominent than strengthening when solute atoms are introduced, which contrasts with the solid solution strengthening that is usually observed in bulk metals. The reduction of unstable stacking fault energy, increase in atomic size misfit, and solute clustering are responsible for this phenomenon, as they facilitate the surface dislocation nucleation in the alloyed nanowires. Additionally, while the nanowire diameter, orientation, surface segregation, and chemical short-range ordering all influence the yield strength, they do not alter the overall softening trend. It is assumed that the softening mechanisms uncovered in this paper are applicable to metallic structures whose yielding is determined by dislocation nucleation.

Kagome magnets are expected to feature emergent properties due to the interplay among geometry, magnetism, electronic correlation, and band topology. The magnetism and topological electronic states can be tuned via the rare-earth engineering in ${R\mathrm{V}}_6{\mathrm{Sn}}_6$ kagome metals, where $R$ is a rare-earth element. Herein, we present the synthesis and characterization of ${\mathrm{SmV}}_6{\mathrm{Sn}}_6$, a metal with two-dimensional kagome nets of vanadium and a frustrated triangular Sm lattice. Some of the Sm atoms are shifted from the normal $R$ positions by $c/2$ along the $c$ axis. Magnetic measurements reveal obvious anisotropy, where the easy magnetic axis is within the $ab$ plane. Electronic transport shows multiband behaviors below 200 K. Density functional theory calculations find that the electronic structure of ${\mathrm{SmV}}_6{\mathrm{Sn}}_6$ hosts flat bands, Dirac cones, and saddle points arising from the V $3d$ electrons near the Fermi level. No evidence for the existence of a charge density wave or magnetic order down to 2 K can be observed. Thus, ${\mathrm{SmV}}_6{\mathrm{Sn}}_6$ can be viewed as a modest disordered derivative of the ${R\mathrm{V}}_6{\mathrm{Sn}}_6$ structure, in which the disordered rare-earth ions can suppress the magnetic order and charge density wave in the ${R\mathrm{V}}_6{\mathrm{Sn}}_6$ kagome family.

We report on time-resolved optical and terahertz ultrafast spectroscopy of charge-carrier dynamics in the room-temperature antiferromagnetic semiconductor $\alpha$-MnTe. By optically pumping the system with 1.55 eV photons at room temperature, we excite charge carriers in the conduction band through the indirect band gap and investigate the dynamical response of the nonequilibrium states using optical as well as terahertz transmission probes. Three relaxation processes are revealed by their characteristic relaxation times of the order of 1, 10, and 100 ps, whose exact values are a function of the pump fluence. For high pump fluences a nonlinear dependence on the pump fluence is observed both in the optical and terahertz probes.

Controlling the composition of ternary III-V and III-nitride nanomaterials such as vertical nanowires, horizontal nanowires, nanosheets, and nanomembranes grown by different epitaxy techniques is essential for band gap engineering and fabrication of nanoheterostructures with tunable properties. Herein, we investigate the diffusion-induced growth process of III-V ternary materials in different geometries including planar layers, nanomembranes, and horizontal and vertical nanowires grown by selective area epitaxy or with a catalyst droplet on top and derive a rather general equation connecting the composition of ternary solid with the composition of vapor. The form of this vapor-solid distribution remains identical for a wide range of geometries, while the coefficients entering the equation contain thermodynamic factors, kinetic constants of the material transport, and geometrical parameters of the growth template. General properties of the vapor-solid distribution are investigated with respect to material constants, growth condition, and geometry, including the interplay of thermodynamics and growth kinetics leading to the suppression of the miscibility gaps in InGaAs and InGaN systems. A good correlation of the model with the data on the compositions of InGaAs, InGaP, and AlGaAs materials grown by different methods is demonstrated. Overall, these results give a simple analytical tool for understanding the compositional trends and compositional tuning of III-V ternary nanostructures, which should work equally well for Si-Ge and II-VI material systems.