Double perovskite compounds containing $5d$ transition metal elements have been extensively studied as platforms for multipolar order phenomena stemming from spin-orbit-entangled $5d$ electrons. In this study, we examine the interplay between crystal structure, multipolar order, and magnetic order in solid solutions of double perovskites with the $5d^1$ electronic configuration: $\mathrm{B}{\mathrm{a}}_2\mathrm{CdRe}{\mathrm{O}}_6$ and $\mathrm{B}{\mathrm{a}}_2\mathrm{CaRe}{\mathrm{O}}_6$, which exhibit distinct electronic orders. The substitution of larger $\mathrm{C}{\mathrm{a}}^{2+}$ ions for $\mathrm{C}{\mathrm{d}}^{2+}$ in $\mathrm{B}{\mathrm{a}}_2\mathrm{CdRe}{\mathrm{O}}_6$, systematically increases the lattice constant with increasing the amount of substitution $x$. Although the spin-orbit-entangled $J=3/2$ state remains intact upon substitution, both the quadrupolar order below $T_{\mathrm{q}}=25$ K and the canted antiferromagnetic (AFM) order below $T_{\mathrm{m}}=12$ K in $\mathrm{B}{\mathrm{a}}_2\mathrm{CdRe}{\mathrm{O}}_6$ are progressively suppressed as $x$ increases. Magnetization measurements reveal that the canted AFM order is suppressed at $x=0.6$, transitioning to a colinear AFM order, while the quadrupolar order persists up to $x=0.9$. The experimental electronic phase diagram, summarizing the dependence of electronic orders on lattice constants, aligns well with the theoretical phase diagram considering electric quadrupolar interactions [G. Chen et al., Phys. Rev. B 82, 174440 (2010)]. This correspondence confirms that chemical pressure induced by substitution effectively tunes the interaction between $5d$ electrons. The results highlight the potential of chemical pressure to modulate multipolar interactions, paving the way for novel multipolar properties in $5d$ electron systems.

The interplay between topology and superconductivity in quantum materials presents opportunities for exploring novel quantum phenomena. In this study, we investigate the topological properties and superconductivity of the nonsymmorphic chiral superconductor ${\mathrm{NbGe}}_2$ using high-resolution angle-resolved photoemission spectroscopy (ARPES), transport measurements, and ab initio calculations. The ARPES data revealed exotic chiral surface states on the $\left(100\right)$ surface originating from the inherent chiral crystal structure. Our ab initio calculations suggest that ${\mathrm{NbGe}}_2$'s electronic structure may include elusive Weyl fermions, highlighting its potential for topological superconductivity. Furthermore, we uncovered the signatures of van Hove singularities, which may enhance many-body interactions. Additionally, transport measurements demonstrated that ${\mathrm{NbGe}}_2$ exhibits superconductivity below $2\mathrm{K}$. Overall, our comprehensive results provide strong experimental evidence that ${\mathrm{NbGe}}_2$ is a promising platform for investigating the interplay between nontrivial band topology, possible Weyl fermions, vHSs, and superconductivity in chiral quantum materials.

To overcome the limitations of the conventional Von Neumann architecture, the inspiration from the mammalian brain has led to the development of nanoscale neuromorphic networks. In the present research, the synthesis of copper nanoparticle (NP) networks, produced using gas phase condensation based on high-pressure magnetron sputtering, is shown. To be the constituents of electrically percolating networks that exhibit complex, neuromorphic-like, spiking behavior at very low potentials in the mV range, satisfying well the requirement of low energy consumption, crystalline copper NPs with a necessary core-shell copper-copper oxide structure were synthesized. Characterization of the NPs using both scanning electron microscopy and scanning transmission electron microscopy revealed pristine shape, size, and density control of copper NPs, including their oxide shells and potential aggregation of the NPs. Furthermore, a comparison between previous results on molybdenum NP networks and this work is made to reveal the working principles behind copper NP networks. Performing and analyzing long-term electrical measurements did reveal unwanted characteristics and difficulties of this, and potentially previously researched, percolating NP networks. Our results show the switching dynamics of copper NP networks but also reveal several difficulties behind percolating NP networks for realistic neuromorphic systems.

We show that the magnetic properties of high-entropy alloys (HEAs) can be captured by ab initio calculations within the coherent potential approximation, where the atomic details of the high-entropy mixing are considered as a uniform effective medium. This is demonstrated using the face-centered-cubic ($A1$) phase of $\text{MnFeCoNiCu}$ and the tetragonal ($L{1}_0$) phase of $\text{(MnFeCoNiCu)Pt}$ by comparing the density functional theory (DFT) results with the experimental values. Working within the primitive unit cell of the underlying lattice and its corresponding Brillouin zone, we show that DFT can capture the smooth profile of magnetic properties such as the magnetic moment, the Curie temperature, and the magnetic anisotropy, using only a sparse set of sampling points in the vast compositional space. The smooth profiles given by DFT indeed follow the experimental trend, demonstrating the promising potential of using machine learning to explore the magnetic properties of HEAs, by establishing reasonably large data sets with high-throughput DFT calculations.

We report on the impact of compressive strain on (100)-oriented $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ epitaxial thin films grown on partially strain-relaxed monoclinic $\theta \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ buffer layers by oxygen-radical-assisted pulsed-laser deposition. Reflection high-energy electron diffraction was used to monitor the in-plane lattice spacing during the deposition. This observation revealed that the tensile strain in the $\theta \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ buffer layer on the (100) $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ substrate gradually relaxed by the formation of misfit dislocations, whereas the $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ epilayer adapted the lattice spacing of the $\theta \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ buffer layer up to −3% lattice mismatch along the $b$-axis direction. The widening of the band gap of $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ with increasing compressive strain was observed from both reflection electron energy-loss spectroscopy and polarized optical reflectance measurements. Experimental results are supported by results from density functional theory calculations.

Breathing pyrochlores are a unique class of materials characterized by a three-dimensional lattice of corner-sharing tetrahedra. However, unlike conventional pyrochlores where all tetrahedra are identical in size, the breathing pyrochlore lattice is composed of alternating large and small tetrahedra. Experimental realizations of the breathing pyrochlore lattice are rare but they do occur in $A$-site-ordered spinels, as in the prototype materials ${\mathrm{LiGaCr}}_4{\mathrm{O}}_8$ and ${\mathrm{LiInCr}}_4{\mathrm{O}}_8$. In this work, we demonstrate that Cr cannot be straightforwardly substituted with other magnetic transition metals while retaining the breathing pyrochlore structure. To explain this observation, we perform density functional theory (DFT) calculations to investigate the formation and stability of ${\mathrm{LiGaCr}}_4{\mathrm{O}}_8$ and ${\mathrm{LiInCr}}_4{\mathrm{O}}_8$, focusing on the energy scales associated with $A$- and $B$-site orderings as well as the magnetic exchange interactions of Cr ions. We identify the strong octahedral site preference of ${\mathrm{Cr}}^{3+}$ as a key factor in protecting the structural integrity of the pyrochlore sublattice, which in turn enables the breathing distortion to proceed. We also demonstrate that the charge order between the $A$ (Li) and $A^{\prime }$ sites (Ga or In) is maintained at all temperatures up to decomposition. Furthermore, while the magnetic exchange interactions constitute a relatively small energy scale and therefore do not play a fundamental role in the structural stability of ${\mathrm{LiGaCr}}_4{\mathrm{O}}_8$ and ${\mathrm{LiInCr}}_4{\mathrm{O}}_8$, magnetism may play a critical role in setting the magnitude of the tetrahedral distortion. Ultimately, we conclude that the scarcity of breathing pyrochlores is a consequence of the stringent requirement on site ordering for their formation.

In this paper we present a systematic investigation into the relative stability of hydrogen isotopes within body-centered-cubic (bcc) Fe using the density functional theory (DFT) method with zero-point energy and elastic corrections. We focus on hydrogen interactions in the bulk, trapping in monovacancies and adsorption on clean low-index surfaces. Hydrogen interactions are further considered with Cr in the dilute limit within the Fe matrix. We show that all hydrogen isotopes are highly mobile in the bulk and reside in tetrahedral interstitials, but the diffusivity decreses with mass. Protium migrates via the octahedral site whilst tritium rotates into an adjacent tetrahedral site. The activation barrier for both pathways is degenerate for deuterium which becomes the most mobile isotope at fusion operating temperatures. However, anharmonicity has not been considered. Hydrogen is repelled by Cr which tends to increase migration barriers by up to 20 meV. Trapping energies of deuterium in a mono-vacancy closely match desorption stages from published ion-beam implantation-annealing experiments. The binding of the heavier isotopes is more exothermic than protium. However, dissolution energies between isotopes is typically within 10 meV such that we do not expect practical kinetic isotope effects at operating temperatures. Cr substitutions near a vacancy change the relative incremental binding energies and push hydrogen away from nearoctahedral sites. The binding energy to a Cr–vacancy complex is lower for 1–3 hydrogen atoms relative to a vacancy in the absence of Cr but sincrease as the complex becomes saturated. The binding energy of heavier hydrogen isotopes to both (100) and (110) surfaceis  lower than for protium. Subsurface Cr is shown to change the preferred hollow adsorption site to a bridge-site absorption on a (100) surface, increasing the binding energy by as much as 50%. Finally, we present elastic dipole tensors and relaxation volumes of the hydrogen-implanted systems, which can be used to represent source terms in continuum models of irradiated materials.

Layered nickelates have been studied extensively over the last three decades due to their structural similarities to the high-$T_c$ superconducting cuprates. Using reactive oxide molecular beam epitaxy (MBE), we synthesize ${\mathrm{Nd}}_{2-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_4$ thin films for $x=0-1.4$ to probe the properties and electronic structure as a function of hole doping. The samples with lower doping show semiconducting behavior across the temperatures probed with an onset of metallic conductivity at $x=1.4$. We also present polarization-dependent O $K$ and Ni $L_{2,3}$ x-ray absorption spectra to track the evolution of the oxygen-nickel hybridization, distribution of holes between O $2p$ and Ni $3d$ states and the nickel oxidation state across the series. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal a Fermi surface that comprises a cupratelike hole pocket of $d_{x^2-y^2}$ character with an additional electron pocket of $d_{3z^2-r^2}$ character at $\mathrm{\Gamma }$. The emergence of a quasiparticle peak at the Fermi vector for $x=1.4$ corroborates the insulator-to-metal transition at $x\sim 1$. Finally, observe a fully two-dimensional Fermi surface with no momentum-dependent pseudogap, in contrast to measurements of the related bulk compound, ${\mathrm{Eu}}_{0.9}{\mathrm{Sr}}_{1.1}{\mathrm{NiO}}_4$.

We consider the process of precipitation in binary alloys in the presence of mechanical deformation. It is commonly observed that mechanical deformation prior to or during precipitation leads to microstructure with excess defects, which allows for enhanced precipitate nucleation and growth rates [1–3]. To investigate this phenomenon, we employ a two-dimensional phase-field crystal alloy model endowed with a temperature dependent mobility, making it capable of recovering isothermal transformation (TTT) diagrams with a characteristic inflection point (nose) about a critical temperature. We examine the variation in the timescale of precipitation and its connection to the timescale of the applied deformation, focusing on the roles of atomic defects in the processes involved. Our results indicate that precipitation is initially delayed through application of a deformation until a critical strain is achieved, beyond which precipitation proceeds more rapidly, assisted by plastic deformation such as grain boundary serration or dislocation nucleation. We show that the evolution of the precipitated fraction, $f\left(t\right)$, departs from classical Avrami behavior. Specifically, $df/dt$ develops two peaks indicative of a “plateau”-like inflection in $f\left(t\right)$, signaling the transition to defect-assisted precipitate nucleation. We analyze these plateaus as a function of the deformation rate and demonstrate that they exhibit a discontinuous bifurcation as the timescale of applied deformation is increased. These findings are compared to and found to be consistent with experiments.

We synthesize topological crystal insulator SnTe thin films with different patterns of polygon holes, polygon networks, and semiregular arrays of square holes. Magnetoconductivity on the patterned films shows that despite the presence of the holes, weak antilocalization (WAL) effects are present at all measured temperatures, from 2  to 10 K. Via fitting with the Hikami-Larkin-Nagaoka equation, a larger number of WAL channels at all temperatures in the patterned films compared to a flat continuous film was observed, suggesting the suppression of bulk conduction by the presence of the patterns. In the films patterned with square holes, the coherence length is limited to 200 nm at low temperatures (2  to 5 K), consistent with diffuse scattering of topologically protected surface state carriers to the hole boundaries. Our findings reveal opportunities to tune topological properties with nanopatterning.

${\mathrm{AuSn}}_4$ is an example of a compound that exhibits topological properties. Recent XRD measurements reveal an ambiguous nature of the crystal structure, as it can be realized with either Aea2 or Ccca symmetry. Motivated by this, we analyze the dynamical stability of the compound. We discuss the role of van der Waals (vdW) corrections within the ab initio calculation. Interestingly, our main result indicates that ${\mathrm{AuSn}}_4$ can be unstable with both Aea2 and Ccca symmetries, due to the soft modes in the phonon spectra. From the soft mode analyses, we find a dynamically stable Pc structure. This structure always has smaller energy than Aea2 or Ccca crystal and it stays independent of the vdW correction. We also show the comparison of theoretical electronic properties with the experimental ARPES measurements. Our findings may be valuable in the future investigations of ${\mathrm{AuSn}}_4$-like compounds.

${\mathrm{HfO}}_2$-based ferroelectrics have emerged as promising materials for advanced nanoelectronics, with their robust polarization and silicon compatibility making them ideal for high-density, nonvolatile memory applications. Oxygen vacancies, particularly in positively charged states, are suggested to profoundly impact the polymorphism kinetics and phase stability of hafnia, thereby affecting its ferroelectric behavior. The electronic structures of pristine and oxygen-deficient hafnia polymorph have been extensively studied using density functional theory, primarily employing (semi-)local exchange-correlation functionals. However, these methods often underestimate band gaps and may not accurately capture the localized nature of $d$ electrons. In this work, we investigate hafnia in various phases using $\mathrm{DFT}+U+V$, with on-site $U$ and intersite $V$ Hubbard parameters computed self-consistently via the pseudohybrid Hubbard density functional, ACBN0, and its extended version eACBN0. We find that the self-consistent $\mathrm{DFT}+U$ method provides comparable accuracy to the computationally more expensive Heyd-Scuseria-Ernzerhof hybrid density functional in predicting relative thermodynamic stability, band gaps, and density of states. Furthermore, it is a cost-effective approach for estimating the formation energies of oxygen vacancies. Additionally, we demonstrate that environmentally dependent Hubbard parameters serve as useful indicators for analyzing bond strengths and electronic structures in real space.

Ferroelectric materials have promising applications in solar-energy conversion and electro-optic devices. The internal gradient fields produced by the macroscopic polarization may improve electronic and transport semiconducting properties. However, ferroelectrics tend to display relatively large band gaps and hence low solar-energy conversion efficiencies. In this work, we explore materials with an intrinsic inverse relation between band gap and polarization in a single ferroelectric phase. Ferroelectrics with an inverse band gap versus polarization relation are characterized by low density of states contribution at the conduction states and negligible orbital hybridization at the valence states. We use high-throughput and first principles methods to find 15 ferroelectric materials with an inverse band gap versus polarization relation in the Materials Project database. Our work provides a new pathway to design small-band gap large-polarization ferroelectrics, by simultaneously tailoring the band gap and polarization of ferroelectrics with an inverse relation through an external tuning parameter.

Understanding phonon transport properties in defect-laden AlN is important for their device applications. Here, we construct a machine-learning potential to describe phonon transport with ab initio accuracy in pristine and defect-laden AlN, following the template of Behler-Parrinello-type neural network potentials (NNPs) but extending them to consider multiple charge states of defects. The high accuracy of our NNP in predicting second- and third-order interatomic force constants is demonstrated through calculations of phonon bands, three-phonon anharmonic, phonon-isotope and phonon-defect scattering rates, and thermal conductivities. In particular, our NNP accurately describes the difference in phonon-related properties among various native defects and among different charge states of the defects. They reveal that the phonon-defect scattering rates induced by ${\mathrm{V}}_{\mathrm{N}}^{3+}$ are the largest, followed by ${\mathrm{V}}_{\mathrm{Al}}^{3-}$, and that ${\mathrm{V}}_{\mathrm{N}}^{1+}$ is the least effective scatterer. This is further confirmed by the magnitude of the respective depressions of the thermal conductivity of AlN. Our findings reveal the significance of the contribution from structural distortions induced by defects to the elastic scattering rates. The present work shows the usefulness of our NNP scheme to cost-efficiently study phonon transport in partially disordered crystalline phases containing charged defects.

This paper combines molecular dynamics, kinetic Monte Carlo, and density functional theory (DFT) methods to calculate the diffusion coefficients of oxygen vacancies and interstitials based on the migration of point defects on the lattice in ${\mathrm{UO}}_{2\pm x},{\mathrm{PuO}}_{2\pm x}$, and (U, Pu) ${\mathrm{O}}_{2\pm x}$ systems. The valence and electronic structures of oxygen defects in the ${\mathrm{MO}}_{2\pm x}$ system are considered, the configurations of oxygen vacancies and interstitials were identified using the DFT and lattice dynamics methods, and the influence of cation valence change on oxygen diffusion were analyzed quantitatively. The chemical and self-diffusion coefficients in the different stoichiometric ${\mathrm{MO}}_{2\pm x}$ systems were obtained on the basis of the defect chemistry theory. In addition, the oxygen diffusion model of the ${\mathrm{MO}}_{2\pm x}$ system, considering the effects of cation valence change due to local charge conservation effect, is constructed at the mechanistic level, and the relationships between the diffusion coefficients and the O/M ratio, temperature, Pu concentration, and oxygen partial pressure were analyzed. The findings of this paper provide reference parameters for macrobehavioral analyses of mixed-oxide and ${\mathrm{UO}}_2$ fuels, such as constituent redistribution and fuel restructuring. The method in this paper also provides a fresh perspective for the simulation and calculation of oxygen point defect behavior in oxide systems with variable valence cations.

There are several strategies to discover new superconductors. Growing new materials and applying high pressures have been the classic ways since superconductivity was found. Also, chemical processing, such as annealing, is another way to induce superconductivity in a nonsuperconducting material. Here, we show chemical processing effects in the nonsuperconducting material, sulfur-substituted FeTe. It has been known that superconductivity in S-substituted FeTe is induced by ${\mathrm{O}}_2$ annealing. We revealed that hydrochloric acid etching and vacuum annealing for ${\mathrm{O}}_2$-annealed samples made the quality of superconductivity higher by several physical property measurements. Furthermore, we visualized the superconducting regions by a magneto-optical imaging technique, indicating that the superconductivity in the processed sample was bulk. In this sample we confirmed that the concentration of excess iron was reduced compared to that in the as-grown state. These results provide an important route to bulk superconductivity in S-substituted FeTe and its related iron-based compounds.

Pressure-induced phase transformations are widely observed in crystalline materials, attracting interest across diverse fields from superconductors to pharmaceuticals. However, studying the mechanisms of these transformations in situ, with sufficient spatial and temporal resolution, remains a challenge. Here, we investigate the influence of pressure on the behavior of 16 crystal structure types that have been shown to self-assemble in molecular dynamics simulations using isotropic, pairwise interaction potentials. We reproduce several experimentally observed solid–solid phase transitions, and we discover four new crystal structures that constitute intermediate phases along the transition pathways. A variety of transformation behaviors as a function of pressure are observed, and we investigate the connection between different high-pressure behaviors and the structural parameters of the underlying system. Our study finds that all systems approach an average coordination number of approximately 12 at high pressure, irrespective of the structural characteristics (coordination number, symmetry, or complexity) of their zero-pressure phases, due to the isotropic interactions between particles.

The atomic cluster expansion (ACE) efficiently parametrizes complex energy surfaces of pure elements and alloys. Due to the local nature of the many-body basis, ACE is inherently local or semilocal for graph ACE. Here, we employ descriptor-constrained density-functional theory (DFT) for parametrizing ACE with charge or other degrees of freedom, thereby transfering the variational property of the density functional to ACE. The descriptors can be of scalar, vectorial, or tensorial nature. From the simplest case of scalar atomic descriptors we directly obtain charge-dependent ACE with long-range electrostatic interactions between variable charges. We observe that the variational properties of the charges greatly help in training, avoiding the need for charge-constrained DFT calculations.

We develop a quantum simulation-based approach for studying properties of strongly correlated magnetic materials at increasing scale. We consider a paradigmatic example of a quantum spin liquid (QSL) state hosted by the honeycomb Kitaev model and use a trainable, symmetry-guided ansatz for preparing its ground state. Applying the tools of quantum subspace expansion (QSE), Hamiltonian operator approximation, and overlap measurements, we simulate the QSL at zero temperature and finite magnetic field, thus moving outside of the symmetric subspace. Next, we implement a protocol for quantum subspace expansion-based measurement of spin-spin correlation functions. Finally, we perform QSE-based simulation of the dynamical structure factor obtained from Green's functions of the finite field Kitaev model. Our results show that quantum simulators offer an insight into quasiparticle properties of strongly correlated magnets and can become a valuable tool for studying material science.

Rare-earth diantimonides display intriguing ground states often associated with structural order, which can be manipulated in thin-film geometries. In this study, we report epitaxial synthesis of one such compound, ${\mathrm{YbSb}}_2$, on III-V substrates using molecular-beam epitaxy. The synthesized thin films exhibit large, nonsaturating, linear magnetoresistance across a wide magnetic field range. Additionally, they demonstrate superconducting properties, with a critical temperature of $\approx 1.025$ K and a critical field of $\approx 83.85$ Oe, consistent with the reports in bulk single crystals. While ${\mathrm{YbSb}}_2$ has been classified as a type-I superconductor in its bulk form, our findings provide evidence of a mixed state in the epitaxial thin films. This work demonstrates the realization of an epitaxial superconductor-semiconductor hybrid system and paves the way for controlling the electronic ground state in rare-earth diantimonides through thin-film engineering.