We use an electron beam to induce and directly observe dislocation glide in strained GaAs thin films on silicon substrates at room temperature using electron channeling contrast imaging (ECCI). Plastic behavior in this brittle material is kinetically facilitated by a lowering of the Peierls barrier for glide due to recombination of cathodogenerated electron-hole pairs at dislocations. The high residual strain in the GaAs film provides the overall driving force for motion. The dynamics of a variety of previously inaccessible dislocation interactions are studied by tracking individual threading dislocations with high spatial and temporal resolution. The ECCI technique has an inherently high resolution and was conducted in a convenient scanning electron microscope environment. The results and methods described in this Rapid Communication provide insight into dislocation-filtering processes and failure mechanisms in technologically important III-V devices on silicon, as well as other mismatched heteroepitaxial systems.

The growing interest of modern technologies in oxide materials is in part due to the possibility of their doping and/or mixing, which often leads to artificial compounds with tunable properties, such as the gap width or the band edge positions. However, engineering of mixed oxide materials requires a better fundamental understanding of their structural, electronic, and ordering properties, well beyond low doping levels, and with reference to their parent materials. In the present work, a series of mixed $M{M}^{\prime }{\mathrm{O}}_3$ oxides $(M,M^{\prime }=\text{Al},\text{Ti},\text{V},\text{Cr},\text{Fe})$ in three corundum-type structures—ilmenite, ${\mathrm{LiNbO}}_3$, and C—is studied by means of first-principles DFT+$U$ simulations. We find that regardless of the precise atomic structure, the local structural and electronic characteristics of most of the compounds are very close to those of their parent corundum $M_2{\mathrm{O}}_3$ oxides. The two noticeable exceptions are ${\mathrm{TiVO}}_3$ and ${\mathrm{TiFeO}}_3$, for which the structural, electronic, and magnetic characteristics are consistent with a mixing-induced change of the cation oxidation states. We show that this actual oxidoreduction process can be rationalized by analyzing the relative band edge positions of the parent oxides. The formation energies of these mixed oxides correlate well with the experimental evidence and, within a mean-field approximation, allow predicting the thermodynamics of solid solutions $M_{2-2x}{M}_{2x}^{\prime }{\mathrm{O}}_3$ at finite temperature.

${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ (CZTS) is a promising thin-film photovoltaic absorber material consisting of earth-abundant and nontoxic elements, though the efficiency achieved in actual devices is well below the theoretical one. Cation disorder has been proposed as a possible explanation; it is, however, still badly understood. A recent investigation based on high-resolution neutron diffraction experiments by Bosson et al. [J. Mater. Chem. A 5, 16672 (2017)] contradicts previous reports and indicates Cu-Zn disorder, not just in the $2c$ and $2d$ Wyckoff sites but also in the $2a$ site of Cu. To clear up this confusion on the exact nature of the disorder, we employ a ternary cluster expansion in combination with Monte Carlo simulations to analyze the phase characteristics of CZTS while undergoing order-disorder transitions. Our calculations show that, in excellent agreement with experiments, a low-temperature phase transition takes place close to 545 K and that the nature of the corresponding disorder is the same as the one recently observed experimentally by Bosson et al.

Monolayer ${\mathrm{MoS}}_2$ has emerged as an interesting material for nanoelectronic and optoelectronic devices. The effect of substrate screening and defects on the electronic structure of ${\mathrm{MoS}}_2$ are important considerations in the design of such devices. We find a giant renormalization to the free-standing quasiparticle band gap in the presence of metallic substrates, in agreement with recent scanning tunneling spectroscopy and photoluminescence experiments. Our sulfur vacancy defect calculations using the density functional theory plus GW formalism, reveal two charge transition levels (CTLs) in the pristine band gap of ${\mathrm{MoS}}_2$. The $(0/-1)$ CTL is significantly renormalized with the choice of substrate, with respect to the pristine valence band maximum (VBM). The $(+1/0)$ level, on the other hand, is pinned 100 meV above the pristine VBM for the different substrates. This opens up a pathway to effectively engineer defect charge transition levels in two-dimensional materials through the choice of substrate.

The effect of epitaxial compressive and tensile strains on the physical properties of multiferroic ${\mathrm{Sr}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{MnO}}_3$ films is investigated via the use of ab initio calculations. Striking phenomena are predicted. Examples include (1) the existence of a previously overlooked strain-induced low-symmetry phase bridging the tetragonal (compressively strained) and orthorhombic (tensile-strained) ferroelectric structures, which exhibits a very large piezoelectric response; (2) a first-order magnetic transition within the tetragonal polar state, which is accompanied by remarkable changes in polarization and out-of-plane lattice constant; (3) the existence of a state that is both ferromagnetic and ferroelectric with a large polarization and axial ratio at large enough compressive strain; and (4) the possibility to induce a structural phase transition between states of different symmetries (e.g., between tetragonal and monoclinic, or between orthorhombic and monoclinic) by applying a magnetic field, which constitutes an original magnetoelectric effect. Experiments are called for to confirm such predictions.

Titanium can dissolve unusually high concentrations of interstitial elements such as carbon and nitrogen to form a rich variety of compounds with beneficial structural and functional properties. A first-principles statistical mechanics study was performed to predict phase stability in the Ti-C and Ti-N binaries. Density functional theory calculations were combined with the cluster-expansion approach to determine ground-state carbon-vacancy and nitrogen-vacancy orderings over the octahedral sites of hcp and fcc Ti. A large number of vacancy-ordered rocksalt phases were found to be stable at low temperature. Monte Carlo simulations showed that the ordered rocksalts transform to a disordered rocksalt that can tolerate high vacancy concentrations at intermediate to high temperatures. Clear trends in phase stability, rooted in electronic structure, are revealed upon a comparison of the calculated Ti-C and Ti-N phase diagrams with the Ti-O phase diagrams from a previous study [N. S. H. Gunda et al., Phys. Rev. Mater. 2, 033604 (2018)].

$\left\{10\overline{1}2\right\}\langle \overline{1}011\rangle$ twin is a prominent deformation mode in hexagonal close-packed materials. It is experimentally observed that twin interfaces are not flat entities, but characterized by kink pairs (KPs) of diverse heights. It has been shown that these kinks or facets delimit basal and prismatic planes. The nature of the defects constituting the facets prescribes their properties, in terms of stability and mobility, which relates to twin growth. In this work, we examine the basic features of such kinks in $\alpha$-Ti from an atomistic modeling viewpoint. We analyze the response of the system with KPs varying in width and height upon normal and shear stresses and under pure bending conditions. We show that bending indeed modifies the interaction energy between kinks, which raises further questions about the nature of the defects. We calculate the nucleation and migration energy barrier for the twin depending on the applied shear stress, resulting in small values, which implies that small thermal energy suffices to activate twin growth. We observe a crossover in the stable height of the KP depending on the applied stress and its width: the wider the KP the higher the most stable. We have developed a twin thickening model that accounts for the thermal KP nucleation and the propagation of the kinks. We show how the model compares satisfactorily with molecular dynamics (MD) simulations. Finally, we have developed a kinetic Monte Carlo methodology to study twin growth, with much less computational burden than MD, that is able to explore the growth rate under a broader set of external conditions.

The lack of reliable methods for identifying descriptors—the sets of parameters capturing the underlying mechanisms of a material's property—is one of the key factors hindering efficient materials development. Here, we propose a systematic approach for discovering descriptors for materials' properties, within the framework of compressed-sensing-based dimensionality reduction. The sure independence screening and sparsifying operator (SISSO) tackles immense and correlated features spaces, and converges to the optimal solution from a combination of features relevant to the materials' property of interest. In addition, SISSO gives stable results also with small training sets. The methodology is benchmarked with the quantitative prediction of the ground-state enthalpies of octet binary materials (using ab initio data) and applied to the showcase example of predicting the metal/insulator classification of binaries (with experimental data). Accurate, predictive models are found in both cases. For the metal-insulator classification model, the predictive capability is tested beyond the training data: It rediscovers the available pressure-induced insulator-to-metal transitions and it allows for the prediction of yet unknown transition candidates, ripe for experimental validation. As a step forward with respect to previous model-identification methods, SISSO can become an effective tool for automatic materials development.

Single-phase multiferroic materials with large polarization and magnetization at room temperature are highly desirable but still rather rare. In this study, tetragonal-like $\mathrm{BiF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{o}}_{0.5}{\mathrm{O}}_3$ films have been synthesized and systematically characterized. For its normal state, both the highly polar structure and ferrimagnetic net magnetization are evidenced in experimental measurements at room temperature and further confirmed using first-principle calculations. Furthermore, an abnormal state driven by voltage with lattice compression along the $c$ axis (shorter for $\sim 20\%)$ is observed, rendering very rare giant piezoelectric response in multiferroics. The magnetoelectric coupling has been confirmed by the increasing saturated magnetization from 52.47 to $163.60\mathrm{emu}/\mathrm{c}{\mathrm{m}}^3$ after the application of electric field, which is due to the spin-state transition of $\mathrm{C}{\mathrm{o}}^{3+}$ ions.

First-principles calculations are conducted on two magnetic brownmillerites, namely, ${\mathrm{Ca}}_2{\mathrm{Co}}_2{\mathrm{O}}_5$ and ${\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_5$. Both systems possess a previously overlooked polar $Pmc{2}_1$ phase that is low in energy. This ferroelectric state is even predicted to be the ground state of ${\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_5$, which therefore renders such material multiferroic. Strikingly, the ferroelectricity associated with this $Pmc{2}_1$ phase involves an original energetic coupling that is linear in the polar mode, quadratic in another distortion, and linear in a third mode. Such ferroelectricity is therefore of a novel type, since it differs from previously reported proper, improper, hybrid improper, and triggered kinds of ferroelectricity.

Engineering of emergent properties at oxide interfaces is an exciting route towards realizing oxide electronics. Such properties are often the result of a balance between cooperating and competing mechanisms in the materials which are difficult to decouple. In this paper, we address the interplay between an antiferromagnetic spin axis and the occurrence of magnetic reconstructions at the (111)-oriented $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3/\mathrm{LaFe}{\mathrm{O}}_3$ interface. We report a critical $\mathrm{LaFe}{\mathrm{O}}_3$ thickness where we observed a magnetic interface reconstruction with a net switchable Fe moment only for thicknesses less than or equal to 16 $d_{111}$ layers of $\mathrm{LaFe}{\mathrm{O}}_3$. A change from an out-of-plane to in-plane antiferromagnetic spin axis is found at the same critical thickness. We ascribe the interfacial moment to a successively decreased out-of-plane canting of the antiferromagnetic spin axis towards the in-plane $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ magnetization at the interface. This points towards the importance of the local antiferromagnetic order interacting with concurrent structural reconstructions to establish a magnetically reconstructed interface.

The coil-to-globule transition of poly($N$-isopropylacrylamide) (pNIPAm) in water is generally believed to be driven by hydrophobic interaction between the isopropyl groups of its side chains. However, it is still unclear how dehydration and critical fluctuations of the polymer chains are correlated. Here, we use small- and wide-angle x-ray scattering and dielectric relaxation spectroscopy to cover a wide range of the relevant length and time scales, enabling us to grasp an overall picture of this phase transition. We find that the hydration number of pNIPAm decreases only moderately with temperature up to about 6 K below its spinodal temperature $T_{\mathrm{S}}$, but then drops steeply on approaching $T_{\mathrm{S}}$. This rapid dehydration is coupled to a mean-field-like power-law divergence of the correlation length $\xi$, representing fluctuations of the density order parameter. Real-space decoding of an observed interference peak reveals partial-globule formation even far below $T_{\mathrm{S}}$ and demonstrates that the polymer-rich phase above $T_{\mathrm{S}}$ can be understood as a high-density assembly of the microglobules. Strikingly, condensation of the microglobules and the divergence of $\xi$ do not run parallel. Instead, the condensation occurs only above $T_{\mathrm{S}}$ and is completed about 6 K above $T_{\mathrm{S}}$. The local number density of the microglobules, exhibiting a steplike increase just above $T_{\mathrm{S}}$, should be identified as an additional microscopic order parameter governing the phase transition of pNIPAm.

In our density functional theory study we systematically explored how the interaction between magnetic and nonmagnetic transition metal dichalcogenide (TMD) monolayers and a magnetic Fe/Ir(111) electrode reciprocally modifies their electronic and magnetic properties. From an electronic point of view, all investigated semiconductor TMDs become metallic upon adsorption on the Fe/Ir(111) substrate related to a rather strong TMD-Fe/Ir(111) hybridization. On the other hand, from a magnetic point of view, the TMD-surface hybridization leads to a spin-imbalanced electronic structure of such hybrid systems with system-specific features. Additionally, the calculated exchange coupling constants for the Fe surface atoms evidenced a stronger magnetic interaction upon TMD adsorption as compared to the case of the clean surface. This observation indicates the presence of a magnetic hardening of the exchange coupling constants similarly to the case of molecular systems on magnetic surfaces, which emphasizes the generality of this adsorbate-induced effect on magnetic substrates.

We present a combined single-crystal x-ray diffuse scattering and ab initio molecular dynamics study of lead telluride, PbTe. Well-known for its thermoelectric and narrow-gap semiconducting properties, PbTe recently achieved further notoriety following the report of an unusual off-centering of the lead atoms, accompanied by a local symmetry breaking, on heating. This observation, which was named emphanisis, ignited considerable controversy regarding the details of the underlying local structure and the appropriate interpretation of the total scattering experiments. In this study, we identify an unusual correlated local dipole formation extending over several unit cells with an associated local reduction of the cubic symmetry in both our x-ray diffuse scattering measurements and our molecular dynamics simulations. Importantly, when averaged spatially or temporally, the most probable positions for the ions are at the centers of their coordination polyhedra. Our results therefore clarify the nature of the local symmetry breaking, and reveal the source of the earlier controversy regarding the existence or absence of off-centering. Finally, we provide an interpretation of the behavior in terms of coupled soft optical and acoustic modes which is linked also to the high thermoelectric performance of PbTe.

In this work we developed a practical and general modeling approach for thermal conductivity of metals and metal alloys that integrates ab initio and semiempirical physics-based models to maximize the strengths of both techniques. The approach supports creation of highly accurate, mechanistic, and extensible thermal conductivity modeling of alloys. The model was demonstrated on α-U and U-rich U-Zr and U-Mo alloys, which are potential fuels for advanced nuclear reactors. The safe use of U-based fuels requires quantitative understanding of thermal transport characteristics of the fuel. The model incorporated both phonon and electron contributions, displayed good agreement with experimental data over a wide temperature range, and provided insight into the different physical factors that govern the thermal conductivity under different temperatures. This model is general enough to incorporate more complex effects like additional alloying species, defects, transmutation products, and noble gas bubbles to predict the behavior of complex metallic alloys like U-alloy fuel systems under burnup.

Based on an anisotropy function with the anisotropic strength determined via atomistic calculations, the mechanism of the three-dimensional (3D) growth pattern formation of magnesium alloy dendrite is investigated by performing phase-field simulations with the parallel adaptive-mesh-refinement algorithm. It is found that the 3D morphological transition of the $\alpha$-Mg dendrite is dependent on the growth parameters, including the partition coefficient, the anisotropic strength, and the supercooling during solidification. The $\alpha$-Mg dendrite exhibits growth tendency along both the basal and nonbasal directions, but the dendritic growth tendency along the basal direction is weaker. Consequently, the 3D morphology of the $\alpha$-Mg dendrite would transform from an 18-primary-branch pattern to a 12-primary-branch pattern if the local growth driving force on the basal plane is insufficient. During dendrite growth, the solute concentration increases as the distance from the dendritic nucleus center increases, and reaches the local maximum at the solid/liquid interface, beyond which it decreases before reaching a constant value. The simulation results agree well with those found in experiments and the existing solidification theory.

Molecular dynamics simulations of diamond-cubic silicon and carbon under combined shear and compression show the formation of an amorphous solid with liquidlike structure at room temperature. Consistent with the opposite density changes of the two crystals upon melting, the amorphous material is denser than the crystal in silicon and less dense than the crystal in carbon. As a result, its rate of formation is enhanced by pressure in silicon but suppressed in carbon. These results are particularly unexpected for silicon, whose amorphous structure is supposed to be liquidlike only when hydrostatically compressed above the polyamorphic transition pressure ($\sim 14\mathrm{GPa}$). Below this pressure, amorphous silicon is expected to have a low-density structure with density close to that of the diamond-cubic crystal. Our simulations show that this polyamorphic transition disappears under shear and high-density, liquidlike amorphous silicon with metallic ductility forms even at low pressure. These results are potentially transferable to other diamond-cubic crystals, like germanium and ice ${\mathrm{I}}_{\mathrm{h}}$, and provide insights into nonequilibrium materials transformations that govern friction and wear in tribological systems.

We present a complete set of chemo-structural descriptors to significantly extend the applicability of machine learning (ML) in material screening and mapping the energy landscape for multicomponent systems. These descriptors allow differentiating between structural prototypes, which is not possible using the commonly used chemical-only descriptors. Specifically, we demonstrate that the combination of pairwise radial, nearest-neighbor, bond-angle, dihedral-angle, and core-charge distributions plays an important role in predicting formation energies, band gaps, static refractive indices, magnetic properties, and modulus of elasticity for three-dimensional materials as well as exfoliation energies of two-dimensional (2D)-layered materials. The training data consist of 24 549 bulk and 616 monolayer materials taken from the JARVIS-DFT database. We obtained very accurate ML models using a gradient-boosting algorithm. Then we use the trained models to discover exfoliable 2D-layered materials satisfying specific property requirements. Additionally, we integrate our formation-energy ML model with a genetic algorithm for structure search to verify if the ML model reproduces the density-functional-theory convex hull. This verification establishes a more stringent evaluation metric for the ML model than what is commonly used in data sciences. Our learned model is publicly available on the JARVIS-ML website (https://www.ctcms.nist.gov/jarvisml), property predictions of generalized materials.

Due to outstanding room temperature electron mobility, the wide-gap perovskite semiconductor $\mathrm{BaSn}{\mathrm{O}}_3$ is of high current interest. Although $n$ doping with Sb and O vacancies has been reported, most work has focused solely on La doping. Here we report bulk single crystals of $\mathrm{B}{\mathrm{a}}_{1-x}{R}_x\mathrm{Sn}{\mathrm{O}}_{3-\delta }$ with $R=\mathrm{La},\mathrm{Pr}$, and Nd, as well as unintentionally doped $\mathrm{BaSn}{\mathrm{O}}_{3-\delta }$, thus exploring new rare earth (magnetic) dopants in addition to O vacancy doping. Consistent with recent results on epitaxial films, O vacancies are shown capable of generating mid-${10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$ Hall electron densities, with single crystal mobilities $\sim 100\text{–}150\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Despite apparent solubility limits below $\sim 0.5\mathrm{at}.\%$, Pr and Nd are also shown to be effective $n$ dopants, yielding Hall electron densities $>1\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$, and ambient and low temperature mobilities up to 175 and $430\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively. In contrast to the La-doped case, clear paramagnetism occurs with Pr and Nd doping, allowing for direct estimates of dopant concentrations for quantitative comparison with Hall densities. We show that dopant and Hall densities can be approximately reconciled, but only after accounting for O vacancy doping. Specific heat measurements were also performed, confirming the $\mathrm{BaSn}{\mathrm{O}}_3$ Debye temperature, and revealing electronic contributions roughly consistent with reported effective masses. Interestingly, and likely related to crystalline electric field effects, Pr-doped $\mathrm{BaSn}{\mathrm{O}}_3$ exhibits large deviations from simple Curie-Weiss susceptibility, and a pronounced Schottky anomaly, which we analyze in detail. These results provide significant insight into doping in $\mathrm{BaSn}{\mathrm{O}}_3$, establishing new rare earth magnetic dopants, clarifying the role of O vacancies, and determining dopant concentrations and solubility limits.

Large and persistent photoconductivity (LPPC) in semiconductors is due to the trapping of photogenerated minority carriers at crystal defects. Theory has suggested that anion vacancies in II-VI semiconductors are responsible for LPPC due to negative-$U$ behavior, whereby two minority carriers become kinetically trapped by lattice relaxation following photoexcitation. By performing a detailed analysis of photoconductivity in CdS, we provide experimental support for this negative-$U$ model of LPPC. We also show that LPPC is correlated with sulfur deficiency. We use this understanding to vary the photoconductivity of CdS films over nine orders of magnitude, and vary the LPPC characteristic decay time from seconds to ${10}^4\mathrm{s}$ by controlling the activities of $\mathrm{C}{\mathrm{d}}^{2+}$ and ${\mathrm{S}}^{2-}$ ions during chemical bath deposition. We suggest a screening method to identify other materials with long-lived, nonequilibrium, photoexcited states based on the results of ground-state calculations of atomic rearrangements following defect redox reactions, with a conceptual connection to polaron formation.