Zircon-type ${R\mathrm{VO}}_4$ compounds (where R = rare-earth atom) constitute a family of ternary oxides which are abundant in both nature and the industrial field. Their structural systematics upon compression has been widely studied. According to the observed systematics, the high-pressure crystal structure is determined by the rare-earth cation size. Nonetheless, the mechanisms involved in the phase transitions remain relatively unexplored. Thus, we gathered the experimental data reported in the literature and compared it to our ab initio calculations, including not only enthalpy, but also the frequency of lattice modes and the value of elastic constants. Our study reveals systematic trends, where high-pressure phases appear only if they are thermodynamically stable and are always preceded by a mechanical or dynamical instability, which counteracts the effects of kinetic barriers in the zircon-to-monazite or -scheelite phase transition, respectively.

Unlike most pure materials, liquid water and silicon exhibit a decrease in density upon solidification, a behavior with important implications for natural and industrial processes. In this work, we propose a modification to previous vapor-liquid-solid phase-field crystal (PFC) models that allows the simulation of such anomalous density changes upon solidification. We describe the differences between the original and modified PFC models' phase diagrams and energy landscapes, specifically highlighting the equilibrium properties of the anomalous density model. The model is used to simulate a deeply quenched liquid droplet undergoing solidification under the assumption of two-time dynamics to allow for rapid elastic relaxation. Overpressurization-induced cavitation and damped oscillator behavior are observed within the simulated droplet, in agreement with recent experiments on supercooled water droplets.

It is possible for $\langle 110\rangle$ tilt grain boundaries (GBs) in bcc metals to perform a conservative displacement by the glide of intrinsic GB line defects, namely, disconnections. The paper presents the characteristic of these defects in $\left\{112\right\}$ and $\left\{332\right\}$ twin boundaries, their vicinal GBs, and the $\left\{116\right\}$ GB studied by molecular dynamics simulation. The sources of disconnections and their interaction with the other GB dislocations are described together with their role in the shear-coupled GB migration. The absence of gliding disconnections in the $\left\{111\right\}$ GB impedes the shear-coupled GB migration, but two pure shuffles inside the coincident site lattice unit facilitate the transformation of the $\left\{111\right\}$ GB into $\left\{110\right\}/\left\{001\right\}$ or $\left\{110\right\}/\left\{112\right\}$ facets in regions of concentration of stresses.

Most metallurgical properties, e.g., dislocation propagation, precipitate formation, can only be fully understood atomistically but most phenomena and quantities of interest cannot be measured experimentally. Accurate simulation methods are essential but first-principles density functional theory (DFT) is prohibitively expensive while empirical interatomic potentials are rarely sufficiently accurate for alloys. Machine learning (ML) is emerging as an approach to create computationally-efficient atomistic potentials achieving near-DFT accuracy. Building on recent work on binary Al-Cu and ternary Al-Mg-Si, here a family of neural network potentials (NNPs) for Al alloys of Al-Cu-Mg and Al-Cu-Mg-Zn is developed and assessed using the Behler-Parinello formulation. Training of the potentials uses a robust set of metallurgically-relevant structures including intermetallic phases, stacking faults, solute/solute and solute/stacking fault interactions, solute clusters, and matrix/precipitate interfaces. The accuracy of these NNPs is then demonstrated across a comprehensive set of properties derived from the training set structures and, moreover, many important structures not represented in the training set such as the generalized stacking fault energy (GSFE) surface of the critical S-phase precipitate in Al-Mg-Cu, and antisite and vacancy formation energies for Al-Cu-Mg intermetallics. The broader Al-Cu-Mg-Zn NNPs also have high accuracy for subtle properties such as Cu substitutional energies in the ${\eta }^{\prime }$ and T phases and formation energies of small Al-Zn-Mg clusters. Together with earlier results, this paper shows how increasingly complex multicomponent alloy potentials can be systematically developed by expanding a training database, leading to a comprehensive set of potentials for a broad alloy family, demonstrated here for the technological Al-2xxx, Al-5xxx, and Al-7xxx Al alloys.

The tuning of the magnetic phase, chemical potential, and structure is crucial to observe diverse exotic topological quantum states in $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_m$ $\left(m=0–3\right)$. Here we show a ferromagnetic (FM) phase with a chiral crystal structure in $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$, obtained via tuning the growth conditions and Sb concentration. Unlike previously reported $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$, which exhibits FM transitions only at high Sb doping levels, our samples show FM transitions $(T_{\mathrm{C}}=13.5\mathrm{K})$ at 15%–27% doping levels. Furthermore, our single-crystal x-ray-diffraction structure refinements find Sb doping leads to a chiral structure with the space group of $P3$, contrasted with the centrosymmetric $P\overline{3}m1$ crystal structure of the parent compound ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$. Through angle-resolved photoemission spectroscopy measurements, we also demonstrated that the nontrivial band topology is preserved in the Sb-doped FM samples. Given that the nontrivial band topology of this system remains robust for low Sb doping levels, our success in making FM $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$ with $x=0.15$, 0.175, 0.2, and 0.27 paves the way for realizing the predicted topological quantum states, such as the axion insulator and Weyl semimetals. Additionally, we also observed magnetic glassy behavior in both antiferromagnetic ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ and FM $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$ samples, which we believe originates from cluster spin-glass phases coexisting with long-range antiferromagnetic/FM orders. We have also discussed how the antisite Mn ions impact the interlayer magnetic coupling and how FM interlayer coupling is stabilized in this system.

Weyl semimetals were experimentally discovered as a new quantum phase of matter that exhibits topologically protected states characterized by separated Weyl points. Similar to other topological materials, the research of Landau level transitions can provide abundant information on Fermi surfaces. Extensive experimental efforts on low-temperature transport and optical properties have been dedicated to investigating the nontrivial topology structure. However, there are some theoretically predicted unconventional Landau level transitions in Weyl semimetals that have not been observed experimentally. And, their contribution to the electromagnetic response of Weyl semimetal remains elusive. Here, we report the magneto-optical study of Landau quantization and selection rules in Weyl semimetal NbP under Voigt geometry. By changing the direction of the electric vector of the incoming light under Voigt and Faraday geometry, abundant Landau level transition modes which contain $\mathrm{\Delta }=0$, ±1, ±2 selection rules are obtained. The richness of the optical spectra, particularly the ones from the Voigt geometry, allows us to determine the location of Fermi energy and its evolution with magnetic fields. We further extract the transitions between Landau levels and Fermi energy and deduce the Landau index. Our results reveal the track of Fermi energy under varying magnetic fields based on the Landau level transitions under Voigt geometry and suggest that a combination of the magneto-optical spectra under Voigt and Faraday geometry can effectively determine the key properties of topological materials that are otherwise inaccessible.

${\mathrm{Sm}}_2{\mathrm{Fe}}_{17}$ has long been known as a potential high-performance magnet whose deficiencies—planar anisotropy and lower-than-optimal $T_c$—can be remedied by nitrogen addition, but which presents synthesis difficulties. Herein we apply first-principles calculations to search for alternative low-cost, high-performance permanent magnets in this family, by exploring simultaneous Fe and Al substitution. Specifically, the goal is to improve properties of ${\mathrm{Sm}}_2{\mathrm{Fe}}_{14}{\mathrm{Al}}_3$ easy-plane magnet at the stoichiometric composition. Density functional theory calculations were executed for three series of compounds, i.e., ${\mathrm{Sm}}_2{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_{14}{\mathrm{Al}}_3$, ${\mathrm{Sm}}_2{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_{15}{\mathrm{Al}}_2$, and ${\mathrm{Sm}}_2{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_{16}\mathrm{Al}$. We find that substitution of Fe with 12–18 of Co in $\%{\mathrm{Sm}}_2{\mathrm{Fe}}_{14}{\mathrm{Al}}_3$ modifies the magnetic anisotropy type from easy plane to easy axis with a substantial anisotropy of 7.1 MJ/${\mathrm{m}}^3$. We also demonstrate that the largest part of magnetic anisotropy is introduced by $4f$ Sm atom electrons. Thus the rotation of magnetic moment orientation from $\langle 1\overline{1}0\rangle$ to $\langle 111\rangle$ is followed by an increase of the occupied $4f$ state number and, as a result, the orbital part of the magnetic moment of one of the Sm atoms. This increase of the occupied $4f$ state number at an energy $\sim -$4.3 eV results in a significant reduction of band structure energy. The substitution of Fe by Co does not significantly reduce the magnetization of the compound and keeps it slightly above 1 T. This combination of magnetic anisotropy and magnetization makes the compound a promising candidate for a permanent magnet.

Single-crystal europium iron garnet (EuIG) thin films were epitaxially grown on gadolinium gallium garnet (GGG)(001) substrates using off-axis sputtering and showed strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, particle-free surface with a root-mean-square roughness as low as 0.1 nm, as observed by atomic force microscopy. High-resolution x-ray diffraction analysis and reciprocal space maps showed pseudomorphic film growth, a very smooth film/substrate interface, excellent film crystallinity with a rocking curve of 0.012° (ω scans), and an in-plane compressive strain without relaxation. In addition, spherical aberration-corrected scanning transmission electron microscopy showed an atomically abrupt interface between the EuIG film and GGG. The saturation magnetization $\left(M_s\right)$ and coercive field $\left(H_c\right)$ were measured using a vibrating sample magnetometer. The square-shaped out-of-plane M-H loops in conjunction with angle-dependent x-ray magnetic dichroism demonstrated the PMA in the films. The spin Hall magnetoresistance on Pt/EuIG samples was measured to obtain the PMA field strength $\left(H_{\perp }\right)$, which increases from 4.21 to 18.87 kOe with the increasing Eu/Fe ratio and in-plane compressive strain. We also measured spin transport in the Pt/EuIG bilayer structure and directly obtained the real part of spin mixing conductance to be $3.48\times {10}^{14}{\mathrm{\Omega }}^{\text{–}1}{\mathrm{m}}^{\text{–}2}$. We demonstrated current-induced magnetization switching with a low critical switching current density of $3.5\times {10}^6\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$, showing excellent potential for low-dissipation spintronic devices.

Magnetic multilayers are a rich class of material systems with numerous highly tunable physical parameters that determine both their magnetic and electronic properties. Here, we present a comprehensive experimental study of the system Pt/Co/Mn, which extends the group of $\mathrm{Pt}/\mathrm{Co}/X$ ($X=$ metal) multilayers that have been investigated thus far. We demonstrate that an increasing Co layer thickness changes the magnetic anisotropy from out of plane to in plane, whereas the deposition of thicker Mn layers leads to a decrease in the saturation magnetization. Temperature-dependent magnetometry measurements reinforce the hypothesis of antiferromagnetic coupling at the Co/Mn interfaces being responsible for the observed Mn thickness dependence of the magnetization reversal. Moreover, magneto-optical imaging experiments indicate systematic changes in magnetic domain patterns as a function of the Co and Mn layer thickness, suggesting the existence of bubblelike domains—potentially even magnetic skyrmions—in the case of sufficiently thick Mn layers, which are expected to contribute to a sizable Dzyaloshinskii-Moriya interaction in the multilayer stacks. We identify Pt/Co/Mn as a highly complex multilayer system with strong potential for further fundamental studies and possible applications.

A commonly used procedure for computing the properties of defects in crystalline materials is to consider a large supercell that includes the defect of interest. This is a straightforward technique as standard energy band codes can be used for such computations. For neutral defects, the only impediment of such an approach is to avoid defect-defect interactions between adjoining cells. However, this procedure can be complex if the defect of interest is charged as the system at large contains Coulombic divergences. Moreover, some have recently argued that the conventional definition of formation energies for charged defects cannot be reconciled with statistical mechanics. Here, we focus on an alternative approach. We consider large nanocrystals wherein a charged defect can be placed. Since the system is confined, a charged defect within the nanocrystal does not result in a Coulombic divergence. The chief impediment is computational, i.e., while no defect-defect or Coulombic divergences are present, the nanocrystal must be sufficiently large to allow the system to properly replicate a bulklike configuration. With the development of new algorithms and hardware advances, computations for systems of sufficient size to address this issue are feasible. In particular, we solve the Kohn-Sham equation in real space using pseudopotential-density-functional theory for large silicon nanocrystals, which contain thousands of atoms. We focus on (i) the screening of a point charge and (ii) the formation of a charged vacancy in hydrogen-terminated silicon nanocrystals. This approach allows us to examine the role of quantum confinement in addition to exploring the bulk limit. Comparisons to other methods confirm the viability of this approach.

Materials exhibiting a drastic change in electrical resistivity depending on magnetic-field direction is of great interest and is rarely seen in real systems. We report the huge anisotropic magnetoresistance ratio exceeding 7500 at low-magnetic field of 0.4 T at 2 K in the layered magnetic semiconductor ${\mathrm{CeTe}}_{1.83}{\mathrm{Sb}}_{0.17}$ with extremely low carrier density in charge-density-wave state. We found that the electrical resistivity is governed only by the out-of-plane component of magnetization and steeply decreases nearly four orders of magnitude in the magnetic field along the $c$ axis. In contrast, it is quite insensitive to the in-plane component of magnetization and huge anisotropy is preserved even in a much higher magnetic field.

${\mathrm{Li}}_2{\mathrm{MnO}}_3$ is considered as one of the candidates for Li-rich cathode material of next-generation lithium-ion batteries because of its high energy density. Due to the synthesis environments and migration of Mn in the charging/discharging cycles, Mn/Li disordered mixing in Li/transition-metal layers of ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ is frequently observed and breaks the uniformity of the original structure. Nevertheless, the role of Mn/Li disordered mixing in ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ has not been well understood, preventing the rational design of better Li-rich Mn-based cathode materials. In this work, we study the energetic and electronic properties involving local Mn/Li disordered mixing in ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ through first-principles calculations. A Mn/Li disordered mixing model is created, in which the Li-rich and Mn-rich regions are utilized to describe the structural inhomogeneity. The results indicate that O ions in the Li-rich area are of more isolated $2p$ states, which show higher energy in the electronic structure. Hence, O ions in the Li-rich region possess a higher electrochemical activity and suffer more severe oxidation during the subsequent charging process, which eventually leads to the earlier destabilization of O ions. The Li-rich region facilitates the diffusion of Li in the Li layer, but the close-packed arrangement of Mn in the Mn-rich region impedes the interlayer and intralayer diffusion of Li near it. Furthermore, the misplaced Mn ion in the Mn-rich region is more prone to diffuse into the Li layer. The identification of the role of Mn/Li disordered mixing in ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ provides a further understanding for the better design of Li-rich Mn-based cathode materials.

In this paper, we investigate the clustering model and glass-forming ability (GFA) in response to variant compositions and cooling rates. The structural units are obtained by molecular dynamics and hierarchical clustering. In the clustering model, a nesting relationship from small- to large-scale units, i.e., from short- to medium-range order, is observed during the hierarchical clustering. Furthermore, the packing efficiency of short-range units is applied to depict the GFA, and the result suggests that the packing efficiency is not strictly increased with respect to cooling rate. A positive correlation between a high packing efficiency and a large skewness is found, which validates that alloys with the stronger GFA have a more disordered structure.

The T center in silicon is a well-known carbon-based color center that has been recently considered for quantum technology applications. Using first-principles computations, we show that the excited state is formed by a defect-bound exciton made of a localized defect state occupied by an electron to which a hole is bound. The localized state is of strong carbon $p$ character and reminiscent of the localization of the unpaired electron in the ethyl radical molecule. The radiative lifetime for the defect-bound exciton is calculated to be on the order of microseconds, much longer than for other quantum defects such as the nitrogen vacancy center in diamond and in agreement with experiments. The longer lifetime is associated with the small transition dipole moment as a result of the very different nature of the localized and delocalized states forming the defect-bound exciton. Finally, we use first-principles calculations to assess the stability of the T center. We find the T center to be stable against decomposition into simpler defects when keeping the stoichiometry fixed. However, we identify that the T center is easily prone to (de)hydrogenation and so requires very precise annealing conditions (temperature and atmosphere) to be efficiently formed.

Ferromagnetic insulators (FMIs) are crucial for sensing and storage, but they are currently rare. The magnetism of the widely studied ${\mathrm{CrI}}_3$ and ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ is layer dependent; thus, obtaining FMIs from these materials is challenging because the film thickness must be carefully controlled. Based on the generalized gradient approximation (GGA), $\mathrm{GGA}+U$, and HSE06 hybrid functional calculations, this work predicts a new structural phase of ${\mathrm{Cr}}_2{\mathrm{S}}_3$ $\left(\gamma \text{−}{\mathrm{Cr}}_2{\mathrm{S}}_3\right)$ and further predicts it as an intrinsic and layer-independent FMI. The material is dynamically metastable. It crystallizes a rhombohedral lattice, stacking the ${\mathrm{Cr}}_2{\mathrm{S}}_3$ quintuple layers (QLs) along the [111] direction through van der Waals interactions. The in-plane lattice constant is 3.4 Å, and each Cr atom carries a magnetic moment of $3.0{\mu }_B$. $\gamma \text{−}{\mathrm{Cr}}_2{\mathrm{S}}_3$ behaves as an intrinsic FMI, irrespective of the layer number, but its indirect energy gap can be tuned from 1.23 to 1.97 eV by changing the film thickness according to the HSE06 calculations. In-plane compressive strain facilitates the ferromagnetism and decreases the energy gap, while tensile strain leads to antiferromagnetism and decreases the energy gap. In the 1QL case, 0.5% in-plane tensile strain causes the ferromagnetic-antiferromagnetic phase transition. The origin of the ferromagnetism and the strain-induced magnetic transition of $\gamma \text{−}{\mathrm{Cr}}_2{\mathrm{S}}_3$ are discussed and attributed to the dominant ferromagnetic part of the superexchange.

${\mathrm{Cu}}_2\mathrm{Se}$ and ${\mathrm{Cu}}_2\mathrm{S}$ are excellent model systems of superionic conductors with large diffusion coefficients that have been reported to exhibit different solidlike and liquidlike Cu-ion diffusion. In this paper, we clarify the atomic dynamics of these compounds with temperature-dependent ab initio molecular dynamics (AIMD) simulations and inelastic neutron scattering experiments. Using the dynamical structure factor and Van Hove correlation function, we interrogate the jump time, hopping length distribution, and associated diffusion coefficients. In cubic ${\mathrm{Cu}}_2\mathrm{Se}$ at 500 K, we find solidlike diffusion with Cu jump lengths matching well the first-neighbor Cu-Cu distance of ∼3 Å in the crystal, and clearly defined optic phonons involving Cu vibrations. Above 700 K, the jump-length distribution becomes a broad maximum centered around 4 Å, spanning the first and second neighbor lattice distances, and a concurrent broadening of the Cu-phonon density of states. Further, above 900 K, the Cu diffusion becomes close to liquidlike, with distributions of Cu atoms continuously connecting crystal sites, while the vibrational modes involving Cu motions are highly damped, though still not fully overdamped as in a liquid. At low temperatures, the solidlike diffusion is consistent with previous x-ray diffraction and quasielastic neutron scattering experiments, while the higher-temperature observation of the liquidlike diffusion is in agreement with previous AIMD simulations. We also report AIMD simulations in ${\mathrm{Cu}}_2\mathrm{S}$ in the hexagonal and cubic superionic phases, and observe nearly liquidlike diffusion above ∼500 K. The calculated ionic conductivity is in fair agreement with reported experimental values.

The composition and magnetic dependent interfacial energy in Cu-Co immiscible alloys is investigated within a coherent interface model using ab initio calculations. We translate the composition dependence of the interfacial energy to the temperature dependence considering the variations of the equilibrium compositions of precipitate and matrix with respect to temperature. The obtained results are in reasonable agreement with those obtained by experiments and thermodynamic calculations. Reviewing the experimental methods for determining the interfacial energy based on kinetic models for precipitate nucleation and coarsening, as well the thermodynamic models based on broken-bond models, we point out that the temperature effect on the interfacial energy in the above models is primarily due to the composition change of the interface. The present work emphasizes the effort to understand the meaning of the speciously same quantity in different methods.

We employ a combined experimental-theoretical study of the first- and second-order Raman modes of monoclinic $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$. Gallium oxide has attracted considerable interest due to its deep-UV band gap paired with a high critical field strength, offering promising applications in power electronics. A crucial prerequisite for the future development of ${\mathrm{Ga}}_2{\mathrm{O}}_3$-based devices is a detailed understanding of the lattice vibrations, i.e., phonons, as they govern important material properties such as elasticity, thermal conductivity, temperature-dependence of the band gap, or free-carrier transport. Polarized micro-Raman spectroscopy measurements on the (010) and ($\overline{2}01$) planes of $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ single crystals enable the determination of the phonon frequencies of all 15 first-order and more than 40 second-order Raman modes. The experimental results are correlated with calculations of the mode frequencies, the phonon dispersion relation, and the phonon density of states using density functional perturbation theory. By applying a group-theoretical analysis, we are able to distinguish between overtones and combinational modes and identify the high-symmetry points in the Brillouin zone that contribute to the observed second-order modes. Based on this information, we demonstrate the simultaneous determination of Raman, IR, and acoustic phonons in $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ by second-order Raman spectroscopy.

We present a first-principles investigation of the electronic and optical properties of hybrid organic-inorganic interfaces consisting of ${\mathrm{MoS}}_2$ monolayer and the $\pi$-conjugate molecules pyrene and pyridine. For both hybrid systems, the quasiparticle band structure obtained from the $G_0{W}_0$ approximation shows—in contrast to density-functional theory—level alignment of type I, owing to the mutual dynamical screening of the interface constituents. Ab initio calculations of the absorption spectrum based on the Bethe-Salpeter equation reveal besides intralayer excitons on the ${\mathrm{MoS}}_2$ side, hybrid as well as charge-transfer excitons at the interface. These findings indicate that hybrid systems consisting of semiconducting transition-metal dichalcogenides and organic $\pi$-conjugate molecules can host a rich variety of optical excitations and thus provide a promising venue to explore many-body interactions and exciton physics in low dimensionality.