In strongly correlated electronic systems, several novel physical properties are induced by the orbital degree of freedom. In particular, orbital degeneracy near the Fermi level leads to spontaneous symmetry breaking, such as the nematic state in FeSe and the orbital ordering in several perovskite systems. Here, the novel layered perovskite material ${\mathrm{CsVF}}_4$, with a $3d^2$ electronic configuration, was systematically studied using density-functional theory and a multiorbital Hubbard model within the Hatree-Fock approximation. Our results show that ${\mathrm{CsVF}}_4$ should be magnetic, with a G-type antiferromagnetic arrangement in the $ab$ plane and weak antiferromagnetic exchange along the $c$ axis, in agreement with experimental results. Driven by the Jahn-Teller distortion in the ${\mathrm{VF}}_6$ octahedra that shorten the $c$ axis, the system displays an interesting electron occupancy $d_{xy}^1{\left(d_{xz}{d}_{yz}\right)}^1$ corresponding to the lower nondegenerate $d_{xy}$ orbital being half-filled and the other two degenerate $d_{yz}$ and $d_{xz}$ orbitals sharing one electron per site. We show that this degeneracy is broken and a novel $d_{yz}$/$d_{xz}$ staggered orbital pattern is here predicted by both the first-principles and Hubbard model calculations. This orbital ordering is driven by the electronic instability associated with degeneracy removal to lower the energy.

The coupling between the organic ${\mathrm{CH}}_3\mathrm{N}{{\mathrm{H}}_3}^{+}$ cations and inorganic perovskite $\mathrm{Pb}{{\mathrm{Br}}_3}^{\text{–}}$ framework in a large single crystal of $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)\mathrm{Pb}{\mathrm{Br}}_3$ weighing 13 g was studied using neutron diffraction and inelastic neutron scattering. Two lattice incommensurate (ICM) phases were found; one at higher temperatures, marked $\mathrm{IC}{\mathrm{M}}_{\mathrm{HT}}$, appeared between 150 and 134 K. The second one, marked $\mathrm{IC}{\mathrm{M}}_{\mathrm{LT}}$, developed below 143 K and remained at 75 K. The transition from the $\mathrm{IC}{\mathrm{M}}_{\mathrm{LT}}$ to $\mathrm{IC}{\mathrm{M}}_{\mathrm{HT}}$ phase upon warming gave rise to extremely large lattice shrinking, followed by extremely large lattice expansion of the tetragonal basal plane of the $\mathrm{Pb}{\mathrm{Br}}_3$ lattice. There was a progressive decrease in the width of the diffuse peak at the Bragg position from the $\mathrm{Pb}{{\mathrm{Br}}_3}^{\text{–}}$ lattice upon warming, which can be described using a critical exponent to show complete ordering of the atoms into a $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)\mathrm{Pb}{\mathrm{Br}}_3$ lattice at 200 K. $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right)\mathrm{Pb}{\mathrm{Br}}_3$ exhibits six definitive acousticlike phonon branches at 75 K. The six branches reconstruct into two at 200 K, with the frequencies of both the transverse and longitudinal modes greatly enhanced. The asymmetric structure of the ${\mathrm{CH}}_3{\mathrm{NH}}_3$ ions together with the indirect interactions between the ${\mathrm{CH}}_3{\mathrm{NH}}_3$ ions mediated through the Br ions are used to understand the observed behaviors.

Here we report a pressure-induced reemergence of superconductivity in a recently discovered superconductor ${\mathrm{K}}_2{\mathrm{Mo}}_3{\mathrm{As}}_3$, which is an experimental case observed in quasi-one-dimensional superconductors. We find that, after full suppression of the ambient-pressure superconducting (SC-I) state at 8.7 GPa, an intermediary nonsuperconducting state sets in and prevails to the pressure up to 18.2 GPa, however, above this pressure a different superconducting (SC-II) state appears unexpectedly. High pressure x-ray diffraction measurements demonstrate that the pressure-induced dramatic change of the lattice parameter $c$ contributes mainly to the emergence of the SC-II state. Combined with the theoretical calculations on band structure, our results suggest that the reemergence of superconductivity is associated with the change of the complicated interplay among different orbital electrons, driven by the pressure-induced anisotropic change of the lattice.

The sustainability and capacity of macroscopic deformation by polycrystalline metals and metallic alloys is controlled by the propagation of dislocation-mediated slip through grains. In this paper, the interaction of a pileup of $1/2\langle 111\rangle$ dislocations with the $\left\{332\right\}$ tilt grain boundary (GB) is studied as a function of temperature in three bcc metals: iron (Fe), chromium (Cr), and tungsten (W). The interaction results in the transformation of the crystal dislocation into GB dislocations. The $\left\{332\right\}$ tilt GB absorbs the crystal dislocations of the pileup, neither the transmission nor reflection of dislocations was observed. The reaction product at the GB is determined by the crystallography of the GB and the features of the crystal dislocations involved, specifically, the orientation of the Burgers vector and the glide plane of the dislocation. In general, the decomposition results in the formation of a sessile GB dislocation with a riser that facets the GB and several elementary disconnections that glide away. In some cases, the riser increases its length with the number of dislocations absorbed and a new asymmetrical grain boundary of $\left\{112\right\}/\left\{110\right\}$ type is created. For a given external shear stress, the number of dislocations absorbed depends on the orientation of the Burgers vector, glide plane of the pileup, and material.

An insulating-metallic transition mediated by substitutional atoms is predicted in a series of two-dimensional carbon-based structures. Introducing Si atoms in selected sites of tetrahexcarbon [B. Ram and H. Mizuseki, Carbon 137, 266 (2018)] according to rational chemical rules, metallicity by trivial band inversion without a band gap opening is induced. Additional substitution of the remaining C atoms by BN dimers introduces no changes in the metallic properties. A series of isomorphous two-dimensional materials with isoelectronic structures derived by exchanging group IV elements exhibiting various band gaps is obtained. Dynamical stability is verified with a phonon analysis and beyond the harmonic approximation with molecular dynamics up to room temperature. The semimetallic compounds have well-nested pockets of carriers and are good candidates for the formation of an excitonic insulator.

We investigate the Bloch flat bands in twisted bilayers from nonpolar to polar two-dimensional semiconductors using first-principles calculations and density functional based tight-binding simulations. First, to delineate the underlying mechanism of the formation of the flat bands, we rely on a tight-binding model of modified graphene where a bias between the A-B sublattice of the hexagonal lattice is introduced. By analyzing the evolution of the valence and conduction band edges of the bilayer of the modified graphene with different stacking patterns, a mechanism attributed to the splitting of the defect-like band edge states induced by different stacking patterns is revealed. The magic angle mechanism is no longer needed. Next, guided by the revealed mechanism, we predict the formation of flat bands in twisted bilayers of a series of two-dimensional systems from nonpolar to polar semiconductors. Our finding has important implications for exploring the flat band physics in low dimensions.

$\mathrm{Th}{\mathrm{Mn}}_{12}$ nitrides are good candidates for high performance permanent magnets. However, one of the remaining challenges is to transfer the good properties of the powder into a useful bulk magnet. Thus, understanding the denitrogenation process of this phase is of key importance. In this study, we investigate the magnetic and structural stability of the ${({\mathrm{Nd}}_{0.75},{\mathrm{Pr}}_{0.25})}_{1.2}{\mathrm{Fe}}_{10.5}{\mathrm{Mo}}_{1.5}{\mathrm{N}}_x$ compound ($x=0$ and 0.85) as function of temperature by means of neutron powder diffraction. Thermal dependence of the lattice parameters, formation of $\alpha$-(Fe,Mo), as well as the nitrogen content in the nitrides are investigated by heating the compounds up to 1010 K. The decomposition takes place mainly via the formation of the $\alpha$-(Fe,Mo) phase, which starts at around 900 K, whereas the nitrogen remains stable in the lattice. Additionally, we show that the magnetic properties of the nitrides [$M\left(4\text{T}\right)=90$ ${\mathrm{Am}}^2$/kg and $H_c=0.55$ T] are maintained after the thermal treatments up to 900 K. This study demonstrates that the $\mathrm{Th}{\mathrm{Mn}}_{12}$ nitrides with the Mo stabilizing element offer good prospects for a bulk magnet provided an adequate processing route is found.

The two-dimensional transition metal dichalcogenide ${\mathrm{PdTe}}_2$ has recently attracted much attention due to its phase coexistence of a type-II Dirac semimetal and type-I superconductivity. Here we report a 67% enhancement of the superconducting transition temperature in 1T-PdSeTe in comparison to that of ${\mathrm{PdTe}}_2$ through partial substitution of Te atoms by Se. The superconductivity has been unambiguously confirmed by the magnetization, resistivity, and specific heat measurements. 1T-PdSeTe shows type-II superconductivity with large anisotropy and a nonbulk superconductivity nature with a volume fraction of $\approx 20\%$ estimated from magnetic and heat capacity measurements. 1T-PdSeTe expands the family of superconducting transition metal dichalcogenides and thus provides additional insights for understanding superconductivity and topological physics in the 1T-${\mathrm{PdTe}}_2$ system

Halide perovskite nanocrystals are suitable materials for photonic devices because their highly efficient luminescence can be tuned over a wide wavelength range by changing the nanocrystal composition and size. Here, we report on the high-order harmonic generation in a solution-processed perovskite ${\mathrm{CsPbBr}}_3$ nanocrystal film that is excited by a strong midinfrared laser. We observe harmonics up to the 13th order, which is ultraviolet light well above the band-gap energy. By using elliptically polarized laser light, we analyze the influence of the sample structure on the intensity of the 5th harmonic. It is also found that the randomness in the orientation of the nanocrystals in the film induces a reduction in the harmonic intensities due to the interference among the harmonics emitted from nanocrystals with different phases. Our observation of high-order harmonics from nanocrystal films opens a way towards the development of an intensity modulator that can be tuned simply by changing the excitation ellipticity.

Hydriding and dehydriding transitions in bulk and nanocrystalline binary metal hydrides were studied using the Pd-H model system by measuring pressure-composition isotherms with in situ x-ray diffractometry. Nanocrystalline Pd showed a smaller pressure hysteresis, solvus hysteresis, and hysteresis in lattice parameter, compared to bulk Pd. The time-dependence of pressure equilibration was measured after dosing with aliquots of hydrogen, giving equilibration times that were much faster in the single-phase regions than in the two-phase plateaus. In the broad two-phase plateaus, the pressure relaxations were exponential functions of time. An explanation of hysteresis is developed that is based on a dissipative potential barrier that impedes the motion of the interface due to interactions between lattice defects and the two-phase interface. The exponential pressure relaxations and hysteresis are consistent for this mechanism. For a simple model of the pinning potential, the potential barrier maximum is an order of magnitude less than typical grain boundary energies. These pinning effects are substantially different in the nanocrystalline Pd, suggesting differences in the hydriding mechanism.

We present an all-electron, periodic $G_{\text{0}}{W}_{\text{0}}$ implementation within the numerical atomic orbital (NAO) basis framework. A localized variant of the resolution-of-the-identity (RI) approximation is employed to significantly reduce the computational cost of evaluating and storing the two-electron Coulomb repulsion integrals. We demonstrate that the error arising from localized RI approximation can be reduced to an insignificant level by enhancing the set of auxiliary basis functions, used to expand the products of two single-particle NAOs. An efficient algorithm is introduced to deal with the Coulomb singularity in the Brillouin zone sampling that is suitable for the NAO framework. We perform systematic convergence tests and identify a set of computational parameters, which can serve as the default choice for most practical purposes. Benchmark calculations are carried out for a set of prototypical semiconductors and insulators, and compared to independent reference values obtained from an independent $G_{\text{0}}{W}_{\text{0}}$ implementation based on linearized augmented plane waves (LAPWs) plus high-energy localized orbitals (HLOs) basis set, as well as experimental results. With a moderate (FHI-aims tier 2) NAO basis set, our $G_{\text{0}}{W}_{\text{0}}$ calculations produce band gaps that typically lie in between the standard LAPW and the $\mathrm{LAPW}+\mathrm{HLO}$ results. Complementing tier 2 with highly localized Slater-type orbitals (STOs), we find that the obtained band gaps show an overall convergence towards the $\mathrm{LAPW}+\mathrm{HLO}$ results. The algorithms and techniques developed in this work pave the way for efficient implementations of correlated methods within the NAO framework.

The layered delafossite ${\mathrm{PdCoO}}_2$ has been predicted to be one of very few materials with a thermopower that is highly anisotropic and switches sign between different crystallographic directions. These properties are of interest for various applications, but have been difficult to verify because sufficiently large crystals have not been available. We report measurements of the high-temperature electrical resistivity, thermal conductivity, and thermopower of phase-pure ${\mathrm{PdCoO}}_2$ powder compacts prepared by a highly Pd-efficient synthesis route. While the electronic transport of the polycrystalline samples is dominated by that of the Pd planes, the thermopower exhibits a well-defined deviation from the in-plane character at temperatures above $600\mathrm{K}$, which is indicative of opposing trends in the Seebeck coefficients within and perpendicular to the delafossite layers. The experimental data are consistently described by a combination of effective-medium models based on the main axes transport quantities. The results support the predicted ambipolar thermopower anisotropy in ${\mathrm{PdCoO}}_2$.

The chalcogenide glasses have always drawn attention in photonics for being the most suitable materials for applications in the infrared region. Although their use in integrated photonics is promising, there are yet many challenges to integrating optical components in nano- and microscale. The dewetting of thin films is a mostly unexplored and attractive alternative to produce self-assembled structures in solid substrates. Chalcogenide glasses are among the few materials to present dewetting below 300 °C. We report the thermal-induced dewetting of ${\mathrm{As}}_x{\mathrm{Se}}_{100\text{–}x}$ thin films, deposited by electron beam. The activation energy for dewetting is obtained by analyzing the kinetics of dewetting at different temperatures. This energy is greatly affected by the number of homopolar bonds and the glass dimensionality. As a consequence, the higher activation energy is found in the composition with fewer degrees of freedom $\left({\mathrm{As}}_{40}{\mathrm{Se}}_{60}\right)$. The rupture mechanism and the size of the droplets are also greatly affected by the glass composition. This study provides an insight on how to control and use dewetting as an alternative route for nano- and microfabrication in photonics.

New materials discovery is the driving force for progress in solid state physics and chemistry. Here we solve the crystal structure and comprehensively study physical properties of ZnVSb in the polycrystalline form. Synchrotron x-ray diffraction reveals that the compound attains a layered ZrSiS-type structure ($P4$/nmm, $a$ = 4.09021(2) Å, $c$ = 6.42027(4) Å). The unit cell is composed of a 2D vanadium network separated by Zn-Sb blocks that are slightly distorted from the ideal cubic arrangement. A considerable amount of vacancies were observed on the vanadium and antimony positions; the experimental composition is ${\mathrm{ZnV}}_{0.91}{\mathrm{Sb}}_{0.96}$. Low-temperature x-ray diffraction shows very subtle discontinuity in the lattice parameters around 175 K. Bonding V-V distance is below the critical separation of 2.97 Å known from the literature, which allows for V-V orbital overlap and subsequent metallic conductivity. From ab initio calculations, we found that ZnVSb is a pseudogap material with an expected dominant vanadium contribution to the density of states at the Fermi level. The energy difference between the antiferromagnetic and nonordered magnetic configurations is rather small (0.34 eV/f.u.). X-ray photoelectron spectroscopy fully corroborates the results of the band structure calculations. Magnetic susceptibility uncovered that, in ZnVSb, itinerant charge carriers coexist with a small, localized magnetic moment of ca. 0.25 ${\mu }_{\mathrm{B}}$. The itinerant electrons arise from the ordered part of the vanadium lattice. Fractional localization, in turn, was attributed to V atoms neighboring vacancies, which hinder full V-V orbital overlap. Furthermore, the susceptibility and electrical resistivity showed a large hysteresis between 120 K and 160 K. The effect is not sensitive to magnetic fields up to 9 T. Curie-Weiss fitting revealed that the amount of itinerant charge carriers in ZnVSb drops with decreasing temperature below 160 K, which is accompanied by a concurrent rise in the localized magnetic moment. This observation together with the overall shape of the hysteresis in the resistivity allows for suggesting a plausible origin of the effect as a charge-transfer metal-insulator transition. Ab initio phonon calculations show the formation of a collective phonon mode at 2.8 THz (134 K). The experimental heat capacity reflected this feature by a boson peak with Einstein temperature of 116 K. Analysis of the heat capacity with both an ab initio perspective and Debye-Einstein model revealed a sizable anharmonic contribution to heat capacity, in line with disordered nature of the material. Further investigation of the electron and phonon properties for ZnVSb is likely to provide valuable insight into the relation between structural disorder and the physical properties of transition-metal-bearing compounds.

We propose a data assimilation method for evaluating the finite-temperature magnetization of a permanent magnet over a high-dimensional composition space. Based on a general framework for constructing a predictor from two data sets including missing values, a practical scheme for magnetic materials is formulated in which a small number of experimental data in limited composition space are integrated with a larger number of first-principles calculation data. We apply the scheme to ${\left({\mathrm{Nd}}_{1-\alpha -\beta -\gamma }{\mathrm{Pr}}_{\alpha }{\mathrm{La}}_{\beta }{\mathrm{Ce}}_{\gamma }\right)}_2{\left({\mathrm{Fe}}_{1-\delta -\zeta }{\mathrm{Co}}_{\delta }{\mathrm{Ni}}_{\zeta }\right)}_{14}\mathrm{B}$. The magnetization in the whole $(\alpha ,\beta ,\gamma ,\delta ,\zeta )$ space at arbitrary temperature is obtained. It is shown that the Co doping does not enhance the magnetization at low temperatures, whereas the magnetization increases with increasing $\delta$ above 320 K.

MnPdGa, a compound crystallizing in the ${\mathrm{Ni}}_2\mathrm{In}$ structure, is a material displaying magnetic ordering near room temperature and is a potential ambient-temperature magnetocaloric. Screening based on electronic structure calculations suggest that MnPdGa may exhibit a high magnetocaloric figure of merit due to its strong magnetostructural coupling. Here we report the preparation of MnPdGa and employ high–resolution synchrotron x-ray diffraction to confirm its hexagonal ${\mathrm{Ni}}_2\mathrm{In}$-type structure. The zero-field ground state is shown to be a conical spin-wave state, defined by a long-range modulation of the conventional conical antiferromagnet structure. Near the Curie temperature, the measurements carried out here coupled with electronic structure calculations suggest that a fully ferromagnetic state can form at elevated temperatures under an applied field. A peak magnetocaloric entropy change $\mathrm{\Delta }{S}_M=-3.54\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ ($30.1\mathrm{mJ}{\mathrm{cm}}^{-3}{\mathrm{K}}^{-1}$) is measured at $T_C=315\mathrm{K}$ at an applied field $H=5\mathrm{T}$. The exchange-driven, nontrivial magnetic structure found in MnPdGa is compared with the somewhat better-studied MnPtGa on the basis of electronic structure calculations.

A layered perovskite-type oxide ${\mathrm{Ba}}_2{\mathrm{RhO}}_4$ was synthesized by a high-pressure technique with the support of convex-hull calculations. The crystal and electronic structure were studied by both experimental and computational tools. Structural refinements for powder x-ray diffraction data showed that ${\mathrm{Ba}}_2{\mathrm{RhO}}_4$ crystallizes in a ${\mathrm{K}}_2{\mathrm{NiF}}_4$-type structure, isostructural to ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. Magnetic, resistivity, and specific-heat measurements for polycrystalline samples of ${\mathrm{Ba}}_2{\mathrm{RhO}}_4$ indicate that the system can be characterized as a correlated metal. Despite the close similarity to its ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ counterpart in the electronic specific-heat coefficient and the Wilson ratio, ${\mathrm{Ba}}_2{\mathrm{RhO}}_4$ shows no signature of superconductivity down to 0.16 K. Whereas the Fermi surface topology has reminiscent pieces of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, an electronlike $e_{\mathrm{g}}-\left(d_{x^2-y^2}\right)$ band descends below the Fermi level, making this compound unique also as a metallic counterpart of the spin-orbit coupled Mott insulator ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$.

Superelastic Ti-Ni-based shape memory alloys are promising elastocaloric materials for solid-state refrigeration. In this paper, we studied the mechanical properties and elastocaloric effect of a severely cold rolled Ti-44Ni-5Cu-1Al (at.%) alloy composed of nanograins $(\sim 5\text{–}16\mathrm{nm})$. The alloy exhibits partial stress-induced martensitic transformation with a large quasilinear reversible strain of $\sim 3.4\%$ and a small energy dissipation of $\sim 1.6\mathrm{MJ}/{\mathrm{m}}^3$ when a maximum stress of 1 GPa is applied. The temperature decrease induced by adiabatic removal of a stress of 1 GPa is of about −5 K, which is mainly due to the reverse transition occurring during unloading. The effective working temperature window of elastocaloric effect larger than 200 K. The coefficient of performance of this alloy reaches ∼13, which is higher than values reported so far for Ti-Ni alloys.

Kinetic Monte Carlo models parameterized by first principles calculations are widely used to simulate atomic diffusion. However, accurately predicting the activation energies for diffusion in substitutional alloys remains challenging due to the wide variety of local environments that may exist around the diffusing atom. We address this challenge using a cluster expansion model that explicitly includes a sublattice of sites representing transition states and assess its accuracy in comparison with other models, such as the broken bond model and a model related to Marcus theory, by modeling vacancy-mediated diffusion in Pt-Ni nanoparticles. We find that the prediction error of the cluster expansion is similar to that of other models for small training sets, but with larger training sets the cluster expansion has a significantly lower prediction error than the other models with comparable execution speed. Of the simpler models, the model related to Marcus theory yields predictions of nanoparticle evolution that are most similar to those of the cluster expansion, and a weighted average of the two approaches has the lowest prediction error for activation energies across all training set sizes.

The sluggish diffusion of strontium vacancies in the perovskite-oxide ${\mathrm{SrTiO}}_3$ was studied by means of classical metadynamics simulations. Two different mechanisms were examined: the migration of an isolated strontium vacancy and the migration of a strontium vacancy in a defect associate with an oxygen vacancy. Combining activation Gibbs energies of migration with the appropriate temperature-dependent pre-exponential terms, we obtained diffusivities of strontium vacancies by the two mechanisms for the temperature range $1000\le T/K\le 2000$. Comparisons with experimental data (where available) yield excellent agreement, both in terms of the effective activation enthalpy of migration as well as the absolute rate of diffusion. In this way we demonstrate the ability of classical metadynamics simulations to predict diffusion coefficients quantitatively for extremely slow-moving defects in oxides, and we highlight the complexity of cation diffusion in perovskite materials.