Excitonic effects are important in many low-dimensional materials due to a reduced dielectric screening which generally enhances exciton binding strengths. For the important class of two-dimensional layered materials, we examine monolayers of GaN, $\mathrm{Mo}{\mathrm{S}}_2$, and MoSSe as well as heterojunctions composed of two layers: GaN and MoSSe. Employing many-body perturbation theory for the quasiparticle band structure (GW theory) and solving the Bethe-Salpeter equation for the excitons, we systematically study the optical properties of these systems. We also include spin-orbit coupling in our analysis and elucidate its influence on the frontier orbitals. It is found that the lowest four excitons in MoSSe monolayers appear at about 1.65 eV and have a binding energy of about 0.67 eV. These excitons are either red-shifted or blue-shifted when forming heterojunctions, depending on the polarization of the junction. These properties, as well as the binding strength and spatial localization (interlayer versus intralayer) of the excitons are systematically correlated with the system's polarization and dielectric strength, opening the way to tune optical properties in such systems by varying polarization and dielectric strength.

The adsorption of ${\mathrm{O}}_2$ on oxygen-vacancies (${\mathrm{V}}_{\mathrm{O}}$) defects can form highly reactive electrophilic adsorbed oxygen species, such as $O_2^{-},O_2^{2-},or{O}^{-}$. The formation of superoxo $\left(O_2^{-}\right)$ species has been detected on many oxides, but the detection of peroxo $\left(O_2^{2-}\right)$ species, a stronger oxidant then superoxo, has remained somewhat elusive. Here, we report, with direct spectrofluorometric and x-ray photoelectron spectroscopic evidence, the presence of a high concentration $\left(0.58\mu \mathrm{M}.{\mathrm{mg}}_{\mathrm{cat}}^{-1}\right)$ of oxygen-rich peroxo-type moieties on substoichiometric ${\mathrm{V}}_2{\mathrm{O}}_5$, an important industrial catalyst for many oxidation and oxidative dehydrogenation processes. The concentration of peroxo increases with the ${\mathrm{V}}_{\mathrm{O}}$ concentration in the ${\mathrm{V}}_2{\mathrm{O}}_5$ lattice. Comparative studies on reduced $\mathrm{Mo}{\mathrm{O}}_3$, another oxidative catalyst, show it to be about 10 times less active for peroxo generation. The driving force for the peroxo formation may be due to the ‘surface transfer doping’ phenomenon in which adsorbed atmospheric ${\mathrm{O}}_2$ and ${\mathrm{H}}_2\mathrm{O}$ at the vacancy sites drives an electrochemical reaction to form peroxide. The reported results have important implications for a broad range of applications, such as catalysis, batteries, supercapacitors, fuel cells, and sensors, where physisorbed oxygen moieties on semiconducting oxide surfaces drive the energy conversion processes.

The structural evolution of Sm substituted $\mathrm{BiFe}{\mathrm{O}}_3$ is studied by total x-ray scattering and structure modeling. It is shown that the crystal structure changes from polar to antipolar and then to nonpolar when the Sm to Bi ratio in the material approaches 20% and 40%, respectively, with no intermixing between the structure types. The evolution is driven by lattice strain induced by the difference in the size of Sm and Bi atoms, leading to changes in the pattern of octahedral tilts and Bi off-centering, which, in turn, induce changes in the multiferroic properties. The substitution ratio at which the different structure types emerge appears to be tied up with the average radius of the atomic species occupying the Bi sites in the perovskite lattice and volume occupied by a formula unit, rendering both quantities useful predictor variables for guiding computational searches for substituted $\mathrm{BiFe}{\mathrm{O}}_3$ multiferroics with improved functional properties.

Solid titanium trisulfide ($\mathrm{Ti}{\mathrm{S}}_3$), a layered semiconducting material, has attracted considerable attention in recent times because of its robust anisotropic optical and electronic properties, which opened tremendous application possibilities. In a recent experiment, it was observed that $\mathrm{Ti}{\mathrm{S}}_3$ undergoes a semiconductor-to-metal transition near 22 GPa accompanying an isostructural transition. However, the mechanism of semiconductor-metal transition, within the same crystal symmetry is not fully understood, which is vital for its device applications as well as understanding the relative role of lattice and electronic effects on MIT. By applying density functional theory (DFT)-based methods and orbital-based bonding descriptors, we show that $\mathrm{Ti}{\mathrm{S}}_3$ develops multicenter interactions under pressure because of charge transfer from $3s^2$ lone electron pair (LEP) to $pp{\sigma }^{*}$-antibonding state of S–S dimer creating a structural instability. Finally, at a critical pressure interchange of primary and secondary S–S bonds causes an isostructural phase transition, whereas reduction of Ti charge transfer to bridging S atom causes metallization.

The recently synthesized layered electride ${\mathrm{Gd}}_2\mathrm{C}$ exhibits an in-plane easy magnetization axis (EMA) in samples exfoliated from the bulk. However, a detailed understanding of the underlying mechanisms of this magnetic anisotropy has been lacking. Our first-principles DFT $+ U$ calculations reveal that the EMA is determined by the interplay of magnetocrystalline anisotropy, driven by spin-orbit coupling, and shape anisotropy, stemming from dipole-dipole interactions, with their relative contributions varying as a function of film thickness. The magnetocrystalline anisotropy energy, which favors the out-of-plane direction, decreases sharply with increasing thickness, while the shape anisotropy energy, favoring the in-plane direction, declines more gradually due to its long-range nature. Consequently, the monolayer displays an out-of-plane EMA, while thicker films exhibit an in-plane EMA, consistent with experimental observations, and the EMA reverts to the out-of-plane direction in the bulk. Our analysis indicates that interorbital couplings in the Gd $4f$ orbitals contribute significantly to the magnetocrystalline anisotropy energies, with both positive and negative components that largely cancel each other out, highlighting the Gd $5p$ orbitals as the dominant factor. The magnetocrystalline anisotropy from the Gd $5p$ orbitals is markedly enhanced in the monolayer compared to the bulk, primarily due to band shifts caused by charge redistribution at the monolayer surface. These findings offer a comprehensive understanding of the thickness-dependent magnetic anisotropy in ${\mathrm{Gd}}_2\mathrm{C}$ and underscore its potential in nanoscale magnetic devices, where shape anisotropy and surface effects play a crucial role.

Despite the 60-year history of research on band magnetism in MnSi, the field remains a vibrant area of study. This area is still of great interest although the physics of weak itinerant magnetism is complicated because of small magnetic moments and an uncertain role of local interactions. This work presents Rh-doped MnSi compounds in which a high-spin (HS) state of Mn magnetic moments has been detected in ${}^{55}\mathrm{Mn}$ nuclear magnetic resonance (NMR) measurements. The doping of MnSi with Rh results in a transition to the HS state for ${\mathrm{Mn}}_{1-x}{\mathrm{Rh}}_x\mathrm{Si}$ at $x=x_c\approx 0.025$ with two Mn magnetic moments $\approx 1.3$ and $2.2{\mu }_B$, which are ordered just below 200 K. This transition occurs only in part of the Mn atoms, while the other Mn atoms remain in a low-spin (LS) state. Concurrently, the Dzyaloshinsky-Moriya (DM) interaction for LS helical states of Mn moments is preserved up to $x\approx 0.13$. Furthermore, variations in the Rh concentration result in discernible alterations in the magnetic field-temperature phase diagrams. In this case, it was observed that the temperature range of existence of the A phase, host skyrmion lattice, was markedly increased in presence of Rh doping, up to $x=0.025$ at least. Small-angle neutron scattering has evidenced the existence of a skyrmion lattice in the helicoidal magnetic phase of ${\mathrm{Mn}}_{0.98}{\mathrm{Rh}}_{0.02}\mathrm{Si}$. The Rh-doped MnSi compound thus demonstrates the coexistence of the HS and LS states of Mn. Our DFT calculations has indicated that this behavior can only be the case when Rh occupies not only Mn but also Si positions in the MnSi compound. Furthermore, our findings indicate that Ir doping of MnSi does not result in the formation of a high-temperature phase, but rather in the suppression of the DM interaction. Although Rh and Ir belong to the same column of Mendeleev's periodic table, they exhibit disparate behaviors upon MnSi doping.

We use magnetic force microscopy to study spatial uniformity of magnetization of epitaxially grown $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ thin films. Compared to films which exhibit no quantum anomalous Hall (QAH) effect, films with QAH effect are observed to have more spatial uniformity of magnetization with larger domain size. The domain evolution upon magnetic field sweeping indicates that the magnetic domains or the spatial nonuniformity of magnetization originate from the strong pinning of the inherent sample inhomogeneity. A direct correlation between the Hall resistivity ${\rho }_{yx}$ and the domain size has been established by analyzing a series of thin films with and without QAH effect. Our observation shows that one has to suppress the spatial nonuniformity of magnetization to allow the Hall resistivity ${\rho }_{yx}$ to be quantized. The fact that a sizable longitudinal resistivity ${\rho }_{xx}$ remains even for the QAH sample suggests a quantized Hall insulator scenario. Our work provides important insights to the understanding of the quantization mechanism and the dissipation of the QAH state in $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ system.

In this study, a novel material from the rare-earth based breathing pyrochlore family, ${\mathrm{Ba}}_3{\mathrm{Tm}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$, was successfully synthesized. Powder x-ray diffraction and high-resolution powder neutron diffraction confirmed phase purity and the $F\overline{4}3m$ breathing pyrochlore crystal structure, while thermogravimetric analysis revealed incongruent melting behavior compared to its counterpart, ${\mathrm{Ba}}_3{\mathrm{Yb}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$. High-quality single crystals of ${\mathrm{Ba}}_3{\mathrm{Tm}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$ were grown using the traveling solvent floating zone technique and assessed using Laue x-ray diffraction and single crystal x-ray diffraction. Thermodynamic characterization indicated paramagnetic behavior down to 0.05 K, and inelastic neutron scattering measurements identified distinct dispersionless crystal electric field energy bands, with the fitted crystal electric field model predicting a single-ion singlet ground state and an energy gap of $\sim 9\mathrm{meV}$ separating it from the first excited (singlet) state. Additional low-energy excitation studies on single crystals revealed dispersionless bands at 0.8 and 1 meV. Computed phonon dispersions from first-principles calculations ruled out phonons as the origin of these modes, further illustrating the puzzling and unique properties of ${\mathrm{Ba}}_3{\mathrm{Tm}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$.

Hf and Zr nitrides are promising compounds for many technologically important areas, including high-temperature structural applications, quantum computing, and solar and optical applications. This article reports on a comprehensive first-principles statistical mechanics study of phase stability in the Hf-N and Zr-N binary systems. A high solubility of nitrogen in the hcp forms of Hf and Zr is predicted. The rocksalt forms of HfN and ZrN can also tolerate a high degree of off-stoichiometry through the introduction of nitrogen and metal vacancies. The Hf-N binary favors a family of stacking faulted parent crystal structures at intermediate nitrogen concentrations that host a unique form of short-range order among nitrogen interstitials and vacancies. These phases can accommodate some degree of configurational entropy and remain ordered to temperatures as high as 1200 K.

Half-Heusler compounds are known for their various compositions and multifunctional properties including topological phases. In this paper, we investigate the topological classification of this class of materials based on the ordering of the ${\mathrm{\Gamma }}_6$ and ${\mathrm{\Gamma }}_8$ states, and a previously overlooked ${\mathrm{\Gamma }}_6^{*}$ state, during an adiabatic expansion process. Using first-principles calculations based on density functional theory, we observed that the nontrivial topology is governed by a three-band mechanism. We provide a simple model derived from the $k·p$ Hamiltonian that interprets the topological phase in half-Heusler systems. Additionally, we explore the robustness of the topological phase under tension and a different perspective on the topological nature of half-Heusler compounds.

The optimization of atomic structures plays a pivotal role in understanding and designing materials with desired properties. However, conventional computational methods often struggle with the formidable task of navigating the vast potential energy surface, especially in high-dimensional spaces with numerous local minima. Recent advancements in machine learning-driven surrogate models offer a promising avenue for alleviating this computational burden. In this study, we propose anapproach that combines the strengths of universal machine learning potentials with a Bayesian approach using Gaussian processes. By using the machine learning potentials as priors for the Gaussian process, the Gaussian process has to learn only the difference between the machine learning potential and the target energy surface calculated for example by density functional theory. This turns out to improve the speed by which the global optimal structure is identified across diverse systems for a well-behaved machine learning potential. The approach is tested on periodic bulk materials, surface structures, and a cluster.

We report measurements of the low-temperature ($5\mathrm{K}\le T\le 300\mathrm{K}$) thermal conductivity ($\kappa$) of single-crystal CuO along its main symmetry directions. The influence of the antiferromagnetic phase transitions ($T_{N1}\simeq 213\mathrm{K}$, $T_{N2}\simeq 230\mathrm{K}$) on $\kappa$ is modest. The application of an electric field ($\lesssim 6\mathrm{k}\mathrm{V}/\mathrm{cm}$) in the multiferroic regime ($T_{N1}\le T\le T_{N2}$) was explored but found to produce no measurable effect on $\kappa$ attributable to a manipulation of magnetic domains. In the collinear ground state, there is considerable anisotropy of $\kappa$, with substantially larger values along ($\left[10\overline{1}\right]$) and transverse to ($\left[101\right]$) the CuO chains for which superexchange interactions are strongest. Calculations of the lattice thermal conductivity employing the measured anisotropic phonon dispersions satisfactorily reproduce the conductivities in all but the $\left[10\overline{1}\right]$ and $\left[101\right]$ directions, suggesting a substantial magnon contribution (${\kappa }_m$) along the latter. A model employing the measured spin-wave dispersions and anisotropic spin gap produces reasonable agreement with the inferred ${\kappa }_m\left(T\right)$.

Dislocation engineering has the potential to open new avenues toward the exploration and modification of the properties of quantum materials. Strontium titanate ($\mathrm{SrTi}{\mathrm{O}}_3$, STO) and potassium tantalate ($\mathrm{KTa}{\mathrm{O}}_3$, KTO) are incipient ferroelectrics that show metallization and superconductivity at extremely low charge-carrier concentrations and have been the subject of resurgent interest. These materials also exhibit remarkable ambient-temperature ductility, and thus represent exceptional platforms for studies of the effects of deformation-induced dislocation structures on electronic properties. Recent work on plastically deformed STO revealed an enhancement of the superconducting transition temperature and the emergence of local ferroelectricity and magnetism near self-organized dislocation walls. Here we present a comprehensive structural analysis of plastically deformed STO and KTO, employing specially designed strain cells, diffuse neutron and x-ray scattering, Raman scattering, and nuclear magnetic resonance (NMR). Diffuse scattering and NMR provide insight into the dislocation configurations and densities and their dependence on strain. As in the prior work on STO, Raman scattering reveals evidence for local ferroelectric order near dislocation walls in plastically deformed KTO. Our findings provide valuable information about the self-organized defect structures in both materials, and they position KTO as a second model system in which to explore the associated emergent physics.

In x-ray photoelectron spectroscopy (XPS), identifying the origin of peaks in the spectrum can be guided by theory calculations. With density functional theory (DFT), using pseudopotentials, one can obtain the difference in photoelectron energy for electrons originating from atoms of different environments, for example surface and bulk atoms, and thus model the surface core level shift (SCLS) energies. The focus in this work is to prepare for calculations of magnesium (Mg) $2p$ SCLSs in material systems where dispersion interactions play a role, primarily in Mg surface degradation and adsorption of molecules as relevant, for example, in degradable bioimplants. For SCLS DFT calculations in metallic surfaces, the state of the photoelectron must be treated as a core state. In the case of Mg, standard pseudopotentials treat the $2p$ state as a valence state, not a core state. We therefore need Mg pseudopotentials with the $2p$ electrons in the core, leaving only two electrons for the valence region (the $3s$ electron); Two-valence electron pseudopotentials are not common, because DFT calculations of Mg-containing materials usually are better or more easily described using 10 valence electrons. In this work, new two-electron Mg pseudopotentials are therefore created for use in dispersion-inclusive DFT calculations. To our knowledge, no such two-electron Mg pseudopotentials exist, proven to work well with the nonlocal, dispersion-inclusive, exchange-correlation functional vdW-DF-cx or similar functionals. We create a number of two-electron Mg pseudopotentials and their $2p$-hole partners, and for four of the most promising we assess their performance in vdW-DF-cx. We provide results for Mg and MgO bulk phases and for the Mg(0001) surface energy, structure, and SCLS, and where possible we compare with results that we obtain by using conventional 10-electron Mg pseudopotentials, all-electron calculations, and with experiments. This work not only reports and tests the specific conditions for creating $2p$ Mg results, it will also, we believe, be of help in creating other similar pseudopotentials to aid the analysis of XPS spectra in other materials.

The search for energy-efficient and environmentally friendly cooling technologies is a key driver for the development of magnetic refrigeration based on the magnetocaloric effect (MCE). This phenomenon arises from the interplay between magnetic and lattice degrees of freedom that is strong in certain materials, leading to a change in temperature upon application or removal of a magnetic field. Here we explore in detail an emerging material, ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{NiSi}}_{0.95}{\mathrm{Al}}_{0.05}$, with an exceptionally large isothermal entropy at room temperature. By combining experimental and theoretical methods we outline the microscopic mechanism behind the large MCE in this material. It is demonstrated that the competition between the ${\mathrm{Ni}}_2\mathrm{In}$-type hexagonal phase and the TiNiSi-type orthorhombic phase, that coexist in this system, combined with the distinctly different magnetic properties of these phases, is a key parameter for the functionality of this material for magnetic cooling.

The hardness of materials plays an important role in material design. There are numerous experimental methods for measuring the hardness of materials, but theoretical prediction of hardness is challenging. By studying the correlation between hardness and the elastic properties of materials, namely shear and bulk moduli, the pressure derivative of the bulk modulus, we have constructed a simple and physically intuitive hardness model. By introducing the spatial variation of the shear modulus, it is possible to predict the hardness anisotropy of materials to define the minimum and maximum values of hardness possessed by a given material. Furthermore, by using the equation of states to define the pressure derivative of the bulk modulus, it is possible to determine the temperature dependence of hardness for given materials. All quantities in the model can be obtained directly from accurate first-principles calculations or from experiments, making it suitable for practical applications.

We present a scalable machine-learning (ML) force-field model for the adiabatic dynamics of cooperative Jahn-Teller (JT) systems. Large-scale dynamical simulations of the JT model also shed light on the orbital ordering dynamics in colossal magnetoresistance manganites. The JT effect in these materials describes the distortion of local oxygen octahedra driven by a coupling to the orbital degrees of freedom of $e_g$ electrons. An effective electron-mediated interaction between the local JT modes leads to a structural transition and the emergence of long-range orbital order at low temperatures. Assuming the principle of locality, a deep-learning neural-network model is developed to accurately and efficiently predict the electron-induced forces that drive the dynamical evolution of JT phonons. A group-theoretical method is utilized to develop a descriptor that incorporates the combined orbital and lattice symmetry into the ML model. Large-scale Langevin dynamics simulations, enabled by the ML force-field models, are performed to investigate the coarsening dynamics of the composite JT distortion and orbital order after a thermal quench. The late-stage coarsening of orbital domains exhibits pronounced freezing behaviors, which are likely related to the unusual morphology of the domain structures. Our paper highlights a promising avenue for multiscale dynamical modeling of correlated electron systems.

We propose an $s{p}^2$ and $s{p}^3$ hybridized carbon allotropes based on the allotropes previously considered (C20-sc, C21-sc, and C22-sc) and investigate physical properties of these allotropes in the framework of the density functional theory. The new allotrope named “C21-${\mathrm{sc}}^{\prime }$” is found to exhibit a magnetic moment in its equilibrium state. Furthermore, it is found that these allotropes with substitutional boron doping are expected to show superconductivity, e.g., $T_c=23.4\mathrm{K}$ for C21-${\mathrm{sc}}^{\prime }$ with 19 at. % B doping. Interestingly, these results correspond very well with experimental results of so-called Q-carbon, suggesting that these allotropes could represent the main local geometries of Q-carbon.