We show how nuclear quantum effects shape the thermal and mechanical behavior of cubic silicon carbide (3C–SiC) under extreme conditions using path-integral Monte Carlo simulations. Our results indicate that quantum fluctuations account for over $80\%$ of vibrational energy at cryogenic temperatures and persist above $800\mathrm{K}$ under pressure. Quantum softening lowers the bulk modulus by up to $6\mathrm{GPa}$ and induces negative thermal expansion at high pressure. Simulations reproduce experimental data within $0.5\%$, demonstrating that path-integral Monte Carlo simulation is a robust tool for modeling covalent materials where classical methods fail.

We present a comprehensive investigation of light-induced phase transitions and strain in two-dimensional $\mathrm{NbO}{X}_2$ ($X=\mathrm{Cl}$, Br, I) using first-principles calculations. In particular, we identify a light-induced ferroelectric-to-paraelectric phase transition in these 2D systems. Furthermore, we demonstrate the possibility of inducing an antiferroelectric-to-paraelectric transition under illumination. Additionally, we find that these 2D systems exhibit significant photostrictive behavior, adding a new functionality to their already notable optical properties. The ability to control and manipulate ferroelectric order in these nanoscale materials through external stimuli, such as light, holds considerable promise for the development of next-generation electronic and optoelectronic devices.

The $Ln\mathrm{SbTe}$ ($Ln=$ lanthanides) family is well known for hosting a plethora of intriguing characteristics stemming from its crystalline symmetry, magnetic structure, $4f$ electronic correlation, and spin-orbit coupling (SOC) phenomena. In this work, we investigate the nodal-line semimetal candidate ErSbTe through transport and thermodynamic measurements as well as angle-resolved photoemission spectroscopy (ARPES) complemented by first-principles calculations. Temperature-dependent susceptibility and resistivity measurements reveal a paramagnetic to antiferromagnetic transition at 1.94 K and a secondary anomaly at 1.75 K. The zero field resistivity measurement does not show the characteristic humplike feature, commonly observed in the other $Ln\mathrm{SbTe}$ materials. Our ARPES measurements show a diamond-shaped Fermi surface and symmetry-enforced crossings along the $\overline{\mathrm{\Gamma }}\text{−}\overline{\text{X}}$ direction as well as a momentum-dependent gap peaking along the $\overline{\mathrm{\Gamma }}\text{−}\overline{\text{M}}$ direction, consistent with our first principles calculations. This study demonstrates the critical role of SOC in shaping the electronic structure of ErSbTe.

Topological defects in ferroic materials are attracting significant attention due to their role as a platform for distinctive physical phenomena and their potential applications in electronic devices. Bismuth ferrite stands out as a well-known multiferroic material, exhibiting both ferroelectric and antiferromagnetic properties at room temperature. While magnetic topological defects in bismuth ferrite begin to be well documented, the influence of cycloids with different propagation directions on the magnetic structure at ferroelectric domain walls remains relatively unexplored. In this study, we employ a first-principles-based effective Hamiltonian method in conjunction with Monte Carlo simulations to predict the formation of distinct magnetic textures at ferroelectric domain walls. An analysis of the Pontryagin charge density reveals them to be localized Bloch points, merons, and homogeneous Pontryagin charge-density tubes, which hold promise for future usage in electronic devices.

Early transition metals form a variety of compounds with carbon, nitrogen, and oxygen that exhibit favorable structural and functional properties. Many of these compounds adopt the rocksalt crystal structure, consisting of an FCC metal sublattice that accommodates C, N, or O atoms within octahedrally coordinated interstitial sites. Interstitial compounds often tolerate high concentrations of vacancies on the interstitial sublattice, with important consequences for diffusion. To determine the factors that affect diffusion in an interstitial compound, we performed a first-principles statistical mechanics study of nitrogen diffusion in nitrogen deficient ${\mathrm{ZrN}}_y$, a prototypical rocksalt interstitial compound. Nitrogen hops between octahedral sites of the FCC Zr sublattice of ${\mathrm{ZrN}}_y$ are predicted to pass through intermediate tetrahedral sites. This leads to a strong dependence of the migration barrier on the local nitrogen concentration, favoring hops into vacancy clusters. Kinetic Monte Carlo simulations predict that nitrogen hops occur primarily into triple vacancies in ${\mathrm{ZrN}}_y$ and that this mechanism leads to highly correlated diffusion. The reliance on triple vacancy hops as a mechanism of nitrogen diffusion also results in a strong dependence of the diffusion coefficient on the nitrogen concentration of the compound. The qualitative predictions of this work have important implications for diffusion in the large family of transition metal carbides, nitrides, and oxides that adopt the rocksalt crystal structure.

Magnetic frustration can suppress conventional magnetic ordering and give rise to exotic magnetic states and emergent functionalities. In this study, we investigate the ferrorotational compound $\mathrm{RbFe}{\left(\mathrm{Se}{\mathrm{O}}_4\right)}_2$ with strong magnetic frustration. Our results reveal a helical magnetic structure emerging below the Néel temperature $T_N\sim 1.57$ K. Magnetic diffuse scattering above $T_N$ confirms short-range correlations. Magnetoelectric measurements demonstrate magnetoelectric coupling behavior below $T_N$. Monte Carlo simulations reproduce the experimental χ-T and M-H curves and help extract effective exchange interactions. These findings position $\mathrm{RbFe}{\left(\mathrm{Se}{\mathrm{O}}_4\right)}_2$ as a promising platform for studying the interplay between spin magnitude, dimensionality, frustration, magnetoelectric coupling, and exploring the crossover between classical and quantum magnetic regimes.

Using first-principles calculations we investigated the electronic structure, magnetic properties, and structural stability of ternary Co-Mn-Fe alloys in their disordered body-centered cubic structures. The robustness of the presence of the spin-polarized ${\mathrm{\Delta }}_1$ band at the Fermi level is confirmed for a wide range of chemical compositions, which is beneficial in obtaining a large tunneling magnetoresistance ratio when utilized as the electrode materials for a MgO-based magnetic tunnel junction (MTJ). Very high ferromagnetic transition temperature ($>1000\mathrm{K}$) evaluated on the basis of a mean-field approximation is reported here, which will effectively reduce the thermal fluctuation of the spin moments at the interface with MgO. A remarkably low value of Gilbert damping constant ($\sim {10}^{-4}$) has been obtained in this study. Furthermore, the Co-Mn-Fe alloys possess a sizably large perpendicular magnetic anisotropy when they are tetragonally distorted. These findings support the viability of the Co-Mn-Fe alloys as a potential candidate for the electrode materials in the MgO-based MTJ.

Point defects and impurities are highly effective mechanisms for manipulating thermal conductivity, primarily because they significantly enhance phonon scattering. However, accurately computing thermal conductivities in defect-laden systems using density functional theory is computationally expensive, leading to a scarcity of theoretical simulations. Here we tackle the case of the widely studied double half-Heusler compound ${\mathrm{TiFe}}_x{\mathrm{Ni}}_{1-x}\mathrm{Sb}$, using first principles and Green's function methods to calculate the phonon-defect scattering rates. Existing predictions for the lattice thermal conductivity of pristine ${\mathrm{TiFe}}_{0.5}{\mathrm{Ni}}_{0.5}\mathrm{Sb}$ are significantly higher than measurements in actual samples. In contrast, we achieve excellent agreement with experimental points for Fe-rich systems with 1.16% ${\mathrm{Fe}}_{\mathrm{Ni}}$ substitutions and Ni-rich systems with 0.87% ${\mathrm{Ni}}_{\mathrm{int}}$ interstitial defects. We provide detailed results for three types of defects and assess their contributions to the behavior of specific compositions. The data suggests that Hall concentrations can significantly overestimate defect concentrations. These results highlight the predictive capabilities of ab initio phonon transport modeling and its importance in understanding and quantifying defects in semiconductors.

The altermagnetic state combines broken time-reversal symmetry with compensated magnetic order to generate spin-split bands at zero net magnetization and offering promising spintronic functionalities. In this work, we use first-principles calculations to demonstrate deterministic control of valley and spin states in two-dimensional altermagnetic $\mathrm{F}{\mathrm{e}}_2\mathrm{Mo}{\mathrm{S}}_2{X}_2$ ($X=\mathrm{S}$, Se, Te). Monolayer $\mathrm{F}{\mathrm{e}}_2\mathrm{Mo}{\mathrm{S}}_2\mathrm{S}{\mathrm{e}}_2$ exhibits giant strain tunable valley polarization (58 meV at 4% tensile), while $\mathrm{A}{\mathrm{A}}^1-\mathrm{stacked}$ bilayer $\mathrm{F}{\mathrm{e}}_2\mathrm{Mo}{\mathrm{S}}_2\mathrm{S}{\mathrm{e}}_2$ show electrically programmable spin splitting (105 meV and 133 meV in X/Y valleys at $E_z=0.1\mathrm{V}/\AA$). Spin-flip switching, enabled by magnetic moment reversal and detected via magneto-optical Kerr contrast, emerges as a distinct phenomenon in bilayer $\mathrm{F}{\mathrm{e}}_2\mathrm{Mo}{\mathrm{S}}_2\mathrm{S}{\mathrm{e}}_2$. These findings establish two-dimensional altermagnetic $\mathrm{F}{\mathrm{e}}_2\mathrm{Mo}{\mathrm{S}}_2{X}_2$ ($X=\mathrm{S}$, Se, Te) as a platform for valleytronics, with design principles applicable to ultra-low-power spintronic devices.

Heterostructures made of alternating metallic layers of ${\mathrm{Nd}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{NiO}}_3$ and layers displaying a metal-to-insulator transition (such as ${\mathrm{SmNiO}}_3$) have been grown varying the period of the superlattices on top of ${\mathrm{LaAlO}}_3$ substrates. Exploiting the phase boundary cost found between metallic and insulating phases of nickelates, it is shown that a metallic layer can be turned into an insulating one by means of heterostructuring in an intermediate thickness regime in which the single layers retain their metallic behavior.

We adapt the minima hopping method (MHM) to the problem of interfacial structure prediction and apply it to study a canonical problem, the tilt grain boundaries in ${\mathrm{SrTiO}}_3$. Our method employs a hybrid approach by first exploring the potential energy surface (PES) of different grain boundary samplings with an empirical force field, among which the fifteen candidates with lower energies are then refined using ab initio density functional theory (DFT) calculations. During the exploratory stage, we bias the search using a local order parameter to primarily sample various reconstructions in the vicinity of the interface, while preserving the crystallinity of the bulk regions. We further enhance the search by incorporating initial structures with rigid body displacements to account for translational variations between bulk phases, enabling the MHM to effectively generate both stoichiometric and nonstoichiometric ${\mathrm{SrTiO}}_3 \mathrm{\Sigma }3$(111)[110] and $\mathrm{\Sigma }3$(112)[110] grain boundaries. From an algorithmic standpoint, MHM outperforms earlier studies based on genetic algorithms (GA) by identifying more stable interfacial structures of several ${\mathrm{SrTiO}}_3$ grain boundaries. The performance of the present implementation of the MHM approach is primarily limited by exploring an approximate description of the PES with a rather simple Buckingham potential. This limitation leads to variations in performance when compared to approaches utilizing more advanced surrogate PES models, such as direct DFT-PES sampling or GA with the embedded atom method (EAM). Despite the present limitations, the MHM approach is able to yield interfacial structures with comparable or lower interfacial energies in specific cases, such as $\mathrm{\Sigma }3$(111)[110] $\mathrm{\Gamma }=1,\pm 0.5$ and $\mathrm{\Sigma }3$(112)[110] $\mathrm{\Gamma }=\pm 1,-2$, underscoring the robustness of the MHM approach even with a simple approximation of the DFT PES. The MHM interfacial structure prediction method thus offers an efficient approach to understanding the grain boundaries and heterointerfaces at the atomic scale, providing an important prerequisite for effective materials design.

Nitrogen-vacancy (NV) color centers in 4H-SiC have gained prominence in quantum technology as room-temperature controllable near-infrared single-photon sources and quantum bits, with various ion irradiation techniques employed for their fabrication; among these methods, hydrogen (H) ion irradiation has emerged as a particularly attractive approach because of its precision and minimal lattice damage. However, the influence of H on the photoluminescence (PL) properties of NV centers remains largely unexplored. In this work, we systematically investigate the effects of H irradiations using first-principles calculations. Our results reveal the dual roles of H in 4H-SiC: on the one hand, H can passivate $\mathrm{N}{\mathrm{V}}^{-}$ by inevitably forming $\mathrm{NV}{\mathrm{H}}_{\mathrm{n}}$ complexes; on the other hand, H can form new color centers $\mathrm{NV}{\mathrm{H}}^{-}\left(\mathrm{S}=\frac{1}{2}\right)$ with significantly lower zero-phonon line (ZPL) energies than $\mathrm{N}{\mathrm{V}}^{-}$, exhibiting PLs with no overlap with those of $\mathrm{N}{\mathrm{V}}^{-}$ in the range 1150–1450 nm. Our results not only reveal the underlying mechanism for the experimentally observed PL decay in $\mathrm{N}{\mathrm{V}}^{-}$ ensemble prepared via hydrogen irradiation but also suggest $\mathrm{NV}{\mathrm{H}}^{-}$ as a promising infrared single-photon source (SPS) in the IR-B region. We further elucidate the distinct PL characteristics of $\mathrm{N}{\mathrm{V}}^{-}$ and $\mathrm{NV}{\mathrm{H}}^{-}$ arising from their atomic configuration differences, providing detailed theoretical interpretations for experimental observations.

Explainable machine learning can help to discover new physical relationships for material properties. To understand the material properties that govern the activation energy for oxygen diffusion in perovskites and pyrochlores, we build a database of experimental activation energies and apply a grouping algorithm to the material property features. These features are then used to fit seven different machine learning models. An ensemble consensus determines that the most important features for predicting the activation energy are the ionicity of the A-site bond and the partial pressure of oxygen for perovskites. For pyrochlores, the two most important features are the A-site $s$ valence electron count and the B-site electronegativity. The most important features are all constructed using the weighted averages of elemental metal properties, despite weighted averages of the constituent binary oxides being included in our feature set. This is surprising because the material properties of the constituent oxides are more similar to the experimentally measured properties of perovskites and pyrochlores than the features of the metals that are chosen. The easy-to-measure features identified in this work enable rapid screening for new materials with fast oxide-ion diffusivity.

With the growing importance of hydrogen (H) as a future clean energy source, high-entropy alloy (HEA) materials have attracted increasing attention in the areas of H storage, transport, and embrittlement resistance. The great tailorability of elemental composition in HEAs holds the promise of an unprecedented potential for tuning H solubility, diffusivity, and trapping with a target property. In this study, we formulate CrMoNbV HEAs by mixing elements with a high and low H affinity and calculate the H solution energy at all the 768 distinct tetrahedral interstitial sites that are available in a 128-atom unit cell via density functional theory, thereby obtaining the full energy landscapes of H interstitials for 12 different elemental compositions. The results are systematically analyzed from the perspective of the relationship between global and local composition with respect to the H solution energy. A significant influence of the composition on the H solution energy landscape is observed, predicting that the mean H solution energy is tunable by “global” composition while the energy variance is affected by the residual strain of the HEAs caused by different atomic sizes. Large fluctuations in the H solution energy over the same “local” composition are observed, which suggests that the prediction of H solution energy solely in terms of local atomic descriptors requires a cautious approach. We also show that among all the constituent elements, the Mo content has a notable impact on tuning the distribution of H solution energy with a wide range of the mean and variance.

Hexagonal ${\mathrm{TbInO}}_3$ exhibits spin liquid behavior at low temperatures and improper ferroelectricity at room temperature, and as such may host novel electronic and magnetic states upon carrier doping. This paper presents density functional theory (DFT) calculations of the electronic, dielectric, and defect properties of ${\mathrm{TbInO}}_3$. We study ${\mathrm{Ti}}^{4+}$ and ${\mathrm{Ca}}^{2+}$ as substitutional dopants replacing ${\mathrm{In}}^{3+}$ and ${\mathrm{Tb}}^{3+}$ to introduce electron- and hole-doping, respectively. Point defect calculations reveal that ${\mathrm{Ti}}^{4+}$ dopants introduce shallow defect states near the conduction band minimum, suggesting the possibility of $n$-type conductivity, whereas ${\mathrm{Ca}}^{2+}$ dopants result in deep in-gap states. We also analyze changes to structural properties with relatively large (8%–16%) Ca and Ti concentrations. We utilize molecular beam epitaxy (MBE) to synthesize epitaxial thin films of ${\mathrm{TbInO}}_3$ doped with ${\mathrm{Ti}}^{4+}$ and ${\mathrm{Ca}}^{2+}$, and use scanning transmission electron microscopy (STEM) imaging to show that the experimental films corroborate the doping-induced structural changes found with DFT. However, all samples remain electrically insulating, which we attribute to the localization of added carriers on the Tb cation. Finally, we propose charge-transfer doping as an alternative strategy to induce conductivity in ${\mathrm{TbInO}}_3$, and identify several possible substrates to achieve this.