In recent years, great progress has been made in the use of machine-learning algorithms to develop interatomic potential models. Machine-learned potential models are typically orders of magnitude faster than density functional theory but also orders of magnitude slower than physics-derived models such as the embedded atom method (EAM). In our previous work, we used symbolic regression to develop fast, accurate, and transferrable interatomic potential models for copper with functional forms that resemble those of the EAM. To determine the extent to which the success of these forms was specific to copper, here, we explore the generalizability of these models to other face-centered cubic transition metals and analyze their out-of-sample performance on several material properties. We found that these forms work particularly well on elements that are chemically similar to copper. When compared with optimized Sutton-Chen models, which have similar complexity, the functional forms discovered using symbolic regression perform better across all elements considered, except gold, where they have a similar performance. They perform similarly to a moderately more complex embedded atom form on properties on which they were trained, and they are more accurate on average on other properties. We attribute this improved generalized accuracy to the relative simplicity of the models discovered using symbolic regression. The genetic programming models are found to outperform other models from the literature $\sim 50\%$ of the time in a variety of property predictions, with $\sim \frac{1}{10}\mathrm{th}$ the model complexity on average. We discuss the implications of these results to the broader application of symbolic regression to the development of potentials and highlight how models discovered for one element can be used to seed searches for different elements.

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 ${\mathrm{Pr}}_{0.92}{\mathrm{Ca}}_{0.08}{\mathrm{NiO}}_{2.75}$ single crystals obtained via a topotactic reduction of the perovskite phase ${\mathrm{Pr}}_{0.92}{\mathrm{Ca}}_{0.08}{\mathrm{NiO}}_3$, using ${\mathrm{CaH}}_2$ 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.

The exceptional properties of MAB phases (where M = transition metal, A = aluminum, and B = boron) make them a promising option for various applications. However, their usage in most fields requires an understanding of the defect formation and migration processes, which can aid in designing durable and functional materials. Through density functional theory calculations, we investigated the stability and mobility of the most prominent point and extended defects in MAlB (where M = Mo and W) and ${\mathrm{N}}_2{\mathrm{AlB}}_2$ (where N = Cr and Fe) MAB phases under different chemical conditions. Our findings indicate that ${\mathrm{V}}_{\mathrm{B}}/{\mathrm{N}}_{\mathrm{Al}}$ is the easiest defect to form under M/N-rich conditions. We observed that vacancy and antisite defects form more easily than interstitial ones in all studied systems. We also investigated different extended structural defects and stacking faults and revealed the stability of relevant compositional defects and tilt/rotational boundaries in MAlB and ${\mathrm{N}}_2{\mathrm{AlB}}_2$. Our results suggest that these systems can likely be synthesized by tuning the experimental conditions. We additionally observed grain boundary formation in ${\mathrm{N}}_2{\mathrm{AlB}}_2$ and compared our findings with relevant experiments.

The two-dimensional electron gas residing in shallow InAs quantum wells coupled to epitaxial aluminum is a widely utilized platform for exploration of topological superconductivity. Strong spin-orbit coupling, a large effective $g$ factor, and control over proximity-induced superconductivity are important attributes. Disorder in shallow semiconductor structures plays a crucial role for the stability of putative topological phases in hybrid structures. We report on the transport properties of 2DEGs residing 10 nm below the surface in shallow InAs quantum wells in which mobility may exceed 100 000 ${\mathrm{cm}}^2/\mathrm{V}$ s at 2DEG density $n_{2\mathrm{DEG}}\le 1\times {10}^{12}{\mathrm{cm}}^{-2}$ at low temperature.

The recently discovered nickelate superconductor with an infinite $\mathrm{Ni}{\mathrm{O}}_2$ layer is synthesized via topochemical reduction with hydride reductant, which selectively removes apical oxygens from the perovskite precursor phase. While this thermodynamic process plays a crucial role in the materialization of the superconducting nickelate, optimizing the process is challenging due to the difficulty of controlling the selective disconnection and diffusion of oxygen in the perovskite phase. Here we demonstrate a significant improvement in the superconducting properties of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$ by tuning the topochemical reduction temperature. By gradually increasing the reduction temperature, the ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$ films exhibit a transformation from the insulating state into the superconducting state, reaching a maximum onset of superconducting transition temperature $T_{\mathrm{c}}^{\mathrm{onset}}$ of $\sim 14$ K at $x=0.20$. Moreover, the normal state metallicity is drastically enhanced despite the slight variation in the crystal structure, implying that the electrical conduction in $\mathrm{Ni}{\mathrm{O}}_2$ planes is likely a sensitive parameter to optimize the reduction state. The tunings of the reduced state in ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_{2+\delta }$ films with various Sr content $x$ also corroborate that the optimal reduction temperature for inducing superconductivity increases with increasing $x$. The systematic optimization of reduction temperature results in an expansion of superconducting dome in temperature versus $x$ phase diagram with higher $T_{\mathrm{c}}^{\mathrm{onset}}$ and wider $x$ region $(0.12\le x\le 0.28)$ compared to the previously reported diagram. Our findings provide a comprehensive understanding of the topochemical reduction process and its role in the electronic transport properties, leading to significant implications for the synthesis of low-valent nickelate superconductors.

MAX phases are a unique class of atomically layered ternary ceramics that deform plastically at room temperature owing to highly mobile basal dislocations (BDs). To understand and control the ductility of these materials, it is crucial to study the core structures and mobilities of BDs. In this study we developed a machine-learning-based spectral neighbor analysis potential (SNAP) to perform atomistic simulations of edge, screw, and mixed BDs in a ${\mathrm{Ti}}_3{\mathrm{SiC}}_2$ MAX phase. The SNAP was trained on density functional theory (DFT)-calculated data. The SNAP calculations demonstrate that the BD core structure exhibits significant asymmetry that is dependent on the position of the weakly bonded Si layer relative to the $\mathrm{Ti}\left(4f\right)–\mathrm{Si}$ slip plane. The dislocation core either splits into Shockley partials or remains compact, depending on whether compressive or tensile stresses act on the Si layer. This asymmetry in the BD core structure agrees well with DFT results. Differential displacement and Nye tensor distribution analyses reveal that undissociated BD cores are centered on Si layers and spread over adjacent parallel basal planes. Additionally, they are three orders of magnitude less mobile than partial BDs. The Peierls stresses of partial edge BDs ($\sim 71$ MPa) are closer to those of metals than those of ceramics, suggesting that edge BDs play a key role in the incipient plasticity of ${\mathrm{Ti}}_3{\mathrm{SiC}}_2$ MAX phases at low temperatures. The findings of this study contribute to a deeper understanding of the complex behavior of BDs at the atomic level and provide theoretical support for elucidating the unique deformation modes of crystals with atomically layered structures owing to the motion of BDs.

An artificial neural network (ANN) potential for Al, trained with density-functional-theory (DFT) data, is constructed to accurately predict lattice vibrational properties and thermodynamics of grain boundaries (GBs) in Al. The ANN potential is demonstrated to accurately predict not only atomic structures and energetics of the GBs at 0 K but also partial phonon densities of states and vibrational entropies, even for GBs absent in the training data sets. In addition, their total potential energies and atomic forces by DFT at elevated temperatures up to 800 K can also be well reproduced by molecular dynamics with the ANN potential. In contrast, a modified embedded atom method (MEAM) potential shows larger errors in phonon frequencies and atomic forces for atoms at GBs, as well as in the bulk, than the ANN potential. The MEAM potential is thus likely to be inadequate to quantitatively predict thermodynamic properties of GBs, particularly at high temperature. The present ANN potential is also applied to systematically examine thermodynamic stability of asymmetric tilt GBs. It is predicted that for the Σ9 system, the GB free-energy profile as a function of inclination angle exhibits a cusp at elevated temperatures, due to its larger vibrational entropies of asymmetric tilt GBs than those of Σ9 symmetric tilt GBs.

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 setups, while realistic graphene membranes are often subject to strongly anisotropic constraints, with important consequences for the rheology, strain, indentation, and friction in engineering conditions. Graphene in particular is recognized as the thinnest solid lubricant material and a large amount of work has been dedicated to understanding the fundamentals mechanisms of this effect and to unravel parameters relevant to its technological development. Here, we experimentally show how graphene's frictional response to an external indenter is severely altered by conditions of anisotropic suspension, specifically when graphene is clamped across a long and narrow groove. Results show that the friction coefficient is significant when the tip is sliding parallel to the groove while becoming ultralow in the orthogonal direction. While the experimental data suggest that––rather unexpectedly––prestrain of the graphene sheet as a result of clamping is negligible, the key to understanding the underlying mechanism is provided by simulations. The paramount mechanism is provided by the extra anisotropic strain induced from indentation under anisotropic constraints, which in turn produces an anisotropic stiffening of the graphene. While the focus of this work is on graphene, we believe our experimental protocol and the physical mechanism uncovered by our model can be applicable to other 2D membranelike materials.

Methods to improve the fracture toughness of oxide glasses are needed since low fracture toughness is a major bottleneck for their applications. To overcome this, it is critically important to investigate the effect of both short- and medium-range structural features on fracture toughness. Recent work reported a record-high fracture toughness for a bulk lithium aluminoborate glass subjected to hot compression. Here, we further explore the structural origin of this high fracture toughness by subjecting different alkali aluminoborate glasses to hot compression. Through a combination of x-ray total scattering experiments and atomistic simulations, we find that hot compression causes significant changes to both the short- and medium-range order structure of the glasses, e.g., increased coordination numbers (CNs) of network forming species and decreased average size of ring-type structures. To this end, we reveal positive correlations between the pressure-induced increase in fracture toughness and (i) the increase in average CN of network forming species and (ii) the area of the first sharp diffraction peak in the structure factor. Our study thus improves the understanding of which structural features benefit intrinsic toughening of oxide glasses.

A semisupervised machine learning method for the discovery of structure-spectrum relationships is developed and then demonstrated using the specific example of interpreting x-ray absorption near-edge structure (XANES) spectra. This method constructs a one-to-one mapping between individual structure descriptors and spectral trends. Specifically, an adversarial autoencoder is augmented with a rank constraint (RankAAE). The RankAAE methodology produces a continuous and interpretable latent space, where each dimension can track an individual structure descriptor. As a part of this process, the model provides a robust and quantitative measure of the structure-spectrum relationship by decoupling intertwined spectral contributions from multiple structural characteristics. This makes it ideal for spectral interpretation and the discovery of descriptors. The capability of this procedure is showcased by considering five local structure descriptors and a database of >50 000 simulated XANES spectra across eight first-row transition metal oxide families. The resulting structure-spectrum relationships not only reproduce known trends in the literature but also reveal unintuitive ones that are visually indiscernible in large datasets. The results suggest that the RankAAE methodology has great potential to assist researchers in interpreting complex scientific data, testing physical hypotheses, and revealing patterns that extend scientific insight.

Achieving large-scale growth of two-dimensional (2D) ferromagnetic materials with high Curie temperature $T_{\mathrm{C}}$ and perpendicular magnetic anisotropy (PMA) is highly desirable for the development of ultracompact magnetic sensors and magnetic memories. In this context, van der Waals (vdW) ${\mathrm{Cr}}_2{\mathrm{Te}}_3$ appears to be a promising candidate. Bulk ${\mathrm{Cr}}_2{\mathrm{Te}}_3$ exhibits strong PMA and a $T_{\mathrm{C}}$ of 180 K. Moreover, both PMA and $T_{\mathrm{C}}$ might be adjusted in ultrathin films by engineering composition or strain or applying an electric field. In this work, we demonstrate the molecular beam epitaxy (MBE) growth of vdW heterostructures of five-monolayer quasifreestanding ${\mathrm{Cr}}_2{\mathrm{Te}}_3$ on three classes of 2D materials: graphene (semimetal), ${\mathrm{WSe}}_2$ (semiconductor), and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (topological insulator). By combining structural and chemical analysis down to the atomic level with ab initio calculations, we confirm the single-crystalline character of ${\mathrm{Cr}}_2{\mathrm{Te}}_3$ films on the 2D materials with sharp vdW interfaces. They all exhibit PMA and $T_{\mathrm{C}}$ close to the bulk ${\mathrm{Cr}}_2{\mathrm{Te}}_3$ value of 180 K. Ab initio calculations confirm this PMA and show how its strength depends on strain. Finally, Hall measurements reveal a strong anomalous Hall effect, which changes sign at a given temperature. We theoretically explain this effect by a sign change of the Berry phase close to the Fermi level. This transition temperature depends on the 2D material in proximity, notably as a consequence of charge transfer. MBE-grown ${\mathrm{Cr}}_2{\mathrm{Te}}_3$/2D material bilayers constitute model systems for the further development of spintronic devices combining PMA, large spin-orbit coupling, and sharp vdW interface.

We present a comprehensive theory of the magnetic phases in monolayer nickel-based Janus dihalides (NiIBr, NiICl, NiBrCl) through a combination of first-principles calculations and atomistic simulations. Phonon band structure calculations and finite-temperature molecular dynamics simulations show that Ni-dihalide Janus monolayers are stable. The parameters of the interacting spin Hamiltonian extracted from ab initio calculations show that nickel-dihalide Janus monolayers exhibit varying degrees of magnetic frustration together with a large Dzyaloshinskii-Moriya interaction due to their inherent inversion symmetry breaking. The atomistic simulations reveal a delicate interplay between these two competing magnetic interactions in giving rise to skyrmion formation.

Weyl semimetal (WSM), a novel state of quantum matter, hosts Weyl fermions as emergent quasiparticles resulting from the breaking of either inversion or time-reversal symmetry. Magnetic WSMs that arise from broken time-reversal symmetry provide an exceptional platform to understand the interplay between magnetic order and Weyl physics, but only a few WSMs have been realized. Here, we identify CeAlSi as a new noncentrosymmetric magnetic WSM via angle-resolved photoemission spectroscopy (ARPES) and first-principles, density-functional theory based calculations. Our surface-sensitive vacuum ultraviolet ARPES data confirm the presence of surface Fermi arcs as the conclusive evidence for the existence of the Weyl semimetallic state in CeAlSi. We also observe bulk Weyl cones at the Fermi arc terminations in CeAlSi using bulk-sensitive soft-x-ray ARPES measurements. These results implicate CeAlSi as a unique platform for investigating exotic quantum phenomena resulting from the interaction of topology and magnetism.

We investigate structural changes in Si islands deposited at room temperature on epitaxial few-layer graphene on SiC(0001) during subsequent annealing. Annealing causes the Si islands move on epitaxial graphene thicker than monolayer with monolayer-deep trenches left behind. In contrast, Si islands do not form trenches on monolayer graphene, but the Si atoms are intercalated to form quasifreestanding bilayer graphene. The islands which terminated their motion are made of SiC. During annealing, Si islands are carbonized into SiC islands by incorporating carbon atoms from graphene, resulting in the etching of graphene. The carbonization reaction of Si is a driving force of the motion of the Si islands.

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. ${\mathrm{SnSe}}_2$ 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 measurements of the electric and thermoelectric properties of ${\mathrm{SnSe}}_2$ flakes. For a 30-μm-thick ${\mathrm{SnSe}}_2$ flake at room temperature, we measure electron mobility of 40 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, a carrier density of $4\times {10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$, a Seebeck coefficient $S\approx -400\mu \mathrm{V}/\mathrm{K}$, and thermoelectric power factor $S^2\sigma \approx 0.35\mathrm{mW}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$. 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 impurities 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 ${\mathrm{SnSe}}_2$ 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.

We fabricated two types of cobalt-ferrite (001) thin films, insulative Fe-rich cobalt-ferrite ${\mathrm{Co}}_x{\mathrm{Fe}}_{3-x}{\mathrm{O}}_{4+\delta }$ (I-CFO) and conductive Fe-rich cobalt-ferrite ${\mathrm{Co}}_y{\mathrm{Fe}}_{3-y}{\mathrm{O}}_4$ (C-CFO), with perpendicular magnetic anisotropy (PMA) on MgO (001) substrates. Although the stoichiometric cobalt ferrite is known as an insulating material, it is found that the conductivity of Fe-rich cobalt ferrites can be controlled by changing the source materials and deposition conditions in the pulsed laser deposition technique. The I-CFO and C-CFO films exhibit PMA through the in-plane lattice distortion. We investigated the Fe-ion-specific valence states in both I-CFO and C-CFO films by Mössbauer spectroscopy and x-ray magnetic circular dichroism and found that the difference in conductivity corresponds to the abundance ratio of the ${\mathrm{Fe}}^{2+}$ state at the octahedral $B$-site $\left(O_{\mathrm{h}}\right)$ in the inverse spinel structure. Furthermore, first-principles calculations reproduce the changes in the density of states at the Fermi level depending on the cation vacancies at the $B$-site, which explains the difference in conductivity between I-CFO and C-CFO.

We develop the theoretical framework of nonequilibrium ultrafast photonics in monolayer quantum spin Hall insulators supporting a multitude of topological states. In these materials, ubiquitous strong light-matter interactions in the femtosecond scale lead to nonadiabatic quantum dynamics, resulting in topology-dependent nonlinear optoelectronic transport phenomena. We investigate the mechanism driving topological Dirac fermions interacting with strong ultrashort light pulses and uncover various experimentally accessible physical quantities that encode fingerprints of the quantum material's topological electronic state from the high-harmonic generated spectrum. Our work sets the theoretical cornerstones to realize the full potential of time-resolved harmonic spectroscopy for understanding nonequilibrium processes in quantum topological systems and identifying topological invariants in two-dimensional quantum spin Hall solid state systems.

In Laves phases, synchroshear is the dominant basal slip mechanism. It is accomplished by the glide of synchro-Shockley dislocations. However, the atomic-scale mechanisms of motion of such zonal dislocations are still not well understood. In this paper, using atomistic simulations, two $30{}^{\circ }$ synchro-Shockley dislocations with different Burgers vectors and core structures and energies are identified. We demonstrate that nucleation and propagation of kink pairs is the energetically favorable mechanism for the motion of the synchro-Shockley dislocation (partial I). Vacancy hopping and interstitial shuffling are identified as two key mechanisms related to kink propagation, and we investigated how vacancies and antisite defects assist kink nucleation and propagation, which is crucial for kink mobility. Additionally, we identified a mechanism of nonsequential atomic shuffling for the motion of the synchro-Shockley dislocation (partial II). These findings provide insights into the dependency on temperature and chemical composition of plastic deformation induced by zonal dislocations in Laves phases and the many related topologically close-packed phases.

Herein we investigate the spectra of crystal structures and their metastability for elemental carbon and the entire set of group-IV monocarbides (SiC, GeC, and SnC) using the ab initio random structure sampling. All of the known structures of elemental C and SiC are correctly identified as high probability in the random sampling and the predicted metastabilities of polymorphs are consistent with the existing knowledge. Furthermore, we find the same four structure types, tetrahedrally bonded zincblende, wurtzite, and rhombohedral-$R\overline{3}m$, as well as octahedrally coordinated rocksalt at high pressure, as the only relevant polymorphs for all studied carbides. However, the rocksalt structure in GeC is shown to be dynamically unstable, contrary to SiC and SnC. Additionally, all GeC and SnC structures are found to have positive enthalpies of formation and are therefore unstable relative to decomposition. The tetrahedrally bonded polymorphs, especially the SnC ones, are predicted to have very high intrinsic (phonon-limited) electron mobilities comparable with the best known semiconductors. Additionally, wurtzite SnC is lattice matched to InN suggesting epitaxial growth as a possible avenue to realize wurtzite SnC.

Antiferromagnetic domain formations and behaviors are long-standing implicit questions in magnetism and are becoming more important in recently emerging antiferromagnetic spintronics. In this paper, we experimentally investigate and numerically simulate evolutions of antiferromagnetic domains in NiO of a bulk single crystal under magnetic fields. With appropriate magnetic and crystallographic potential energy considerations, we extend a micromagnetic framework with a simulation algorithm which reproduces well the domain patterns and furnishes detailed understanding of the domain evolutions. The present work provides an important theoretical methodology to predict and understand behaviors of micromagnetic structures in antiferromagnets.