Metal halide perovskites (MHPs) have emerged as highly promising materials for optoelectronic applications, with all-inorganic MHPs presenting enhanced stability compared to their hybrid counterparts. Here, we investigate the atomic dynamics and structural fluctuations in single crystals of $\mathrm{CsSnBr}{}_3$ and $\mathrm{CsPbBr}{}_3$ through systematic inelastic neutron scattering (INS) measurements as a function of temperature. Our experiments are compared with first-principle simulations, augmented with large-scale molecular dynamics modeling, based on machine-learned neural network potentials. Through both INS and simulations, we find quasi-elastic diffuse rods in reciprocal space in both compounds, originating from fluctuating planar domains featuring correlated tilts of Br octahedron. The diffuse rods exhibit a slow, overdamped dynamic process, modulated across $\mathit{Q}$ space, reflecting the strong lattice anharmonicity of the inorganic framework. We do not find evidence for dynamic off-centering of the ${\mathrm{Sn}}^{2+}$ ions besides phonon vibrations at the center of the Br octahedron. These results offer valuable insights into the unusual anharmonic atomic dynamics and intricate correlated structural distortions in MHPs, which will be critical for rationalizing and further tailoring their thermal and optoelectronic properties.

Hexagonal boron nitride (hBN) hosts luminescent defects possessing spin qualities compatible with quantum sensing protocols at room temperature. Vacancies, in particular, are readily obtained via exposure to high-energy ion beams. While the defect creation mechanism via such irradiation is well understood, the occurrence rate of optically active negatively charged vacancies (${\mathrm{V}}_{\mathrm{B}}^{-}$) is still an open question. In this work, we exploit focused helium ions to systematically create optically active vacancy defects in hBN flakes at varying densities. By comparing the density-dependent spin splitting measured by magnetic resonance to calculations based on a microscopic charge model, in which we introduce a correction term due to a constant background charge, we are able to quantify the number of ${\mathrm{V}}_{\mathrm{B}}^{-}$ defects created by the ion irradiation. We find a lower bound for the fraction (0.2%) of all vacancies in the optically active, negatively charged state. Our results provide a protocol for measuring the creation efficiency of ${\mathrm{V}}_{\mathrm{B}}^{-}$, which is necessary for understanding and optimizing luminescent centers in hBN.

The application of density functional theory to materials in the warm dense matter regime has motivated the development of exchange-correlation functionals which incorporate proper, explicit temperature dependence. Previous work has yielded fully thermal exchange-correlation free energy functionals at the local density approximation and generalized gradient approximation (GGA) levels of refinement. Recently, an additive thermal correction scheme was utilized to construct a meta-GGA exchange-correlation (XC) functional in which thermal effects are treated at the GGA level. The $f\mathrm{TSCAN}$ free-energy XC functional presented here includes thermal effects through the meta-GGA level in the context of the SCAN (strongly constrained and appropriately normed) ground-state functional. The $f\mathrm{TSCAN}$ functional provides generality while achieving similar performance to a thermal GGA functional at high temperatures, e.g., pressures within 1% of path integral Monte Carlo simulations of warm dense hydrogen and a significant improvement over ground-state functionals. At low temperatures, $f\mathrm{TSCAN}$ demonstrates improvements in accuracy relative to lower-level and deorbitalized functionals, indicating that calculations using $f\mathrm{TSCAN}$ may be expected to perform well across experimentally relevant densities and pressures.

The hallmark feature of polymorphic systems is their ability to assemble into many possible structures at the same thermodynamic state. Designer polymorphic materials can in principle be engineered via programmable self-assembly, but the robustness of the assembly process depends on dynamical factors that are poorly understood. Here we predict a failure mode for the growth of multicomponent polymorphic materials in which dynamical coexistence occurs between ordered and disordered assembly trajectories. We show that this transition is preceded by the formation of a steady-state disordered wetting layer, suggesting a nonequilibrium analogy to premelting phenomena at equilibrium. This dynamical phase transition is likely to occur in a variety of systems and may fundamentally limit the complexity of polymorphic materials that can be designed through programmable self-assembly.

The quasiharmonic approximation is the most common method for modeling the specific heat of solids; however, it fails to capture the effects of intrinsic anharmonicity. In this study, we introduce the “elastic softening approximation,” an alternative approach to modeling intrinsic anharmonic effects on thermodynamic quantities, which is grounded in Wallace's thermodynamic framework that tracks entropy changes resulting from the continuous change (e.g., softening) of phonons as a function of temperature. A key finding of our study is a correlation between Poisson's ratio and the differential rate of phonon softening at finite frequencies, compared to lower frequencies relevant to elastic moduli measurements. We observe that elemental solids such as $\alpha$-Be, diamond, Al, Cu, In, W, Au, and Pb, which span a wide range of Poisson's ratios and exhibit varying degrees of intrinsic anharmonicity, consistently follow this trend. When applied to $\alpha$-U, $\alpha$-Pu, and $\delta$-Pu, our method reveals unusually large anharmonic phonon contributions at elevated temperatures across all three light actinide metals. These findings are attributed to the unique combination of enhanced covalency and softer elastic moduli inherent in the actinides, potentially influenced by their $5f$-electron bonding.

Nanoporous materials are expected to be game changers in the energy and environmental crisis, with central applications in fluid adsorption, separation, and catalysis. Yet, besides the abundant literature on fluids nanoconfined in their extreme porosity, the acoustic behavior of nanoporous solids subjected to fluid adsorption remains to be explored. In fact, while sound testing is a routine method used to probe fluid-filled porous materials—typically for geoscience applications, the molecular origin of the acoustic signature of fluid confinement at the nanoscale has yet to be unveiled. In particular, a molecular formalism describing how nanoconfinement and surface interactions affect longitudinal and transverse wave propagation is still missing. Here, using different fluids $({\mathrm{CH}}_4,$ ${\mathrm{CO}}_2)$ in a prototypical nanoporous material (zeolite), we conduct a molecular simulation study to identify the acoustic modes (velocity and attenuation) from the system's dynamic structure factor. First, by varying the fluid nature and temperature as well as the fluid-solid interaction strength, we decipher the parameters that drive acoustic propagation and derive a simple kinematic model that predicts accurately the linear decay observed in the sound velocity upon increasing the fluid mass density (regardless of the fluid description level, e.g., flexible versus rigid, coarse-grained versus all-atom, etc.). Second, from the broadening of the acoustic modes in the dynamic structure factor, we show that attenuation increases with the fluid adsorbed amount and with the solid-fluid interaction strength due to phonon scattering at the fluid-solid interface. We establish that all data can be quantitatively rationalized by considering the change in the phonon lifetime through an additional relaxation time arising from the interaction between fluid molecules and the zeolite lattice. This simple stochastic model, which provides a robust framework to describe the acoustics of fluid-solid composites based on physical quantities amenable to simple experiments, offers new perspectives for the design of novel applications involving the acoustics of fluids in interaction with nanoporous solids.

In recent years, as the attention in spintronics expands from collinear to noncollinear magnetism, the interlayer Dzyaloshinskii-Moriya interaction (IL-DMI) has been proven capable of mediate long-range chiral spin textures between magnetic layers. Here, we experimentally confirm that analogous to the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, where indirect coupling oscillates between parallel and antiparallel states with spacer thickness, a damped oscillation in IL-DMI strength and direction is observed. Specifically, an oscillation period of $0.96\pm 0.01\mathrm{nm}$ is obtained when mediated by a Ru spacer, close to the 1.15 nm reported for conventional RKKY while well fitted by theoretical predictions. In contrast, no oscillation is observed when IL-DMI is mediated by a Pt spacer, consistent with the negligible RKKY coupling across Pt. This work experimentally confirms the physical origin of IL-DMI as stemming from the RKKY mechanism, while also providing a tuning knob in controlling IL-DMI's polarity and strength by adjusting spacer thickness, beneficial to the development of future three-dimensional spintronics memory devices.

Increased electrical resistance from grain boundary phases (complexions) limit the performance of many thermoelectric materials. A promising strategy for mitigating the excess boundary resistance in polycrystalline materials is altering the composition at a grain boundary by causing a complexion transition. In NbFeSb, increasing the Ti content has been shown to result in Ti-rich boundaries that effectively eliminate boundary resistance and make Ti-doped NbFeSb one of the best $p$-type thermoelectric materials in the medium temperature range. In this study, we have developed a model based on the average band offset between the grain boundaries and grain to identify complexion transitions from resistive, insulating boundaries to metallic boundaries. This model has been applied to NbFeSb, where a transition occurs at a Ti chemical potential with an approximate composition of ${\mathrm{Nb}}_{0.9}{\mathrm{Ti}}_{0.10}\mathrm{FeSb}$. This approach is an interesting method to evaluate the behavior of grain boundaries in thermoelectric materials and can be applied to any material with thermoelectric transport data and grain size measurements.

Vanadium in silicon carbide is a promising spin photon interface candidate with optical transitions in the telecom range and a long lived electron spin, hosted in an advanced semiconductor platform. In this detailed investigation of the defect's 16-dimensional ground state spin manifold at millikelvin temperatures, a wide range of previously unreported transitions are observed which are accurately described using a theoretical model that includes strain. Using a superconducting microcoil we achieve Rabi frequencies exceeding $20\mathrm{MHz}$ and perform the first coherent manipulation of a direct hyperfine transition. These insights further underscore the defect's potential for strain engineering and sensing, as well as for fault-tolerant qudit encoding.

Chiral multifold fermions in solids exhibit unique band structures and topological properties, making them ideal for exploring fundamental physical phenomena related to nontrivial topology, chirality, and symmetry breaking. However, the challenge of obtaining clean, flat surfaces through cleavage has hindered the investigation of their unique electronic states. In this study, we utilize high-resolution angle-resolved photoemission spectroscopy and density functional theory calculations to investigate the low-energy electronic structure of the cleavable single-crystal PdSbSe. Our combined experimental and theoretical analysis reveals the presence of multifold-degenerate fermions within this chiral crystal. We also observe multiple chiral Fermi arc surface states and spin-splitting behavior in the associated bulk bands. These findings provide unique insights into chiral, multifold fermionic states in easily cleavable crystals and offer a robust platform for further research into their unique electronic properties and potential applications in novel electronic devices.

We report the measurement, using angle-resolved photoemission spectroscopy, of the metallic electronic structure of the hole-doped thermoelectric oxide ${\mathrm{CuRh}}_{0.9}{\mathrm{Mg}}_{0.1}{\mathrm{O}}_2$. The material is found to have a “pudding mold” type band structure, with a nearly flat band edge located near the Fermi level, which is thought to be the origin of the thermoelectric behavior of this material. The experimental data match the density functional theory of the undoped parent compound, simply corrected by a rigid shift of the bands. Transport calculations based on the observed band structure yield a Seebeck coefficient of $\sim 200\mu \mathrm{V}/\mathrm{K}$ for the undoped parent material, consistent with experimental measurements. Our results show that ${\mathrm{CuRhO}}_2$ is a textbook example of how pure band-structural effects can result in a large thermoelectric figure of merit, demonstrating that flat band edges in oxides are a realistic route for the efficient conversion of thermal energy.

The heterointerface between topological insulators (TIs) and magnetic materials provides a crucial platform for investigating exotic electronic and magnetic phases, with implications for both fundamental studies and potential applications. A key challenge is determining the spatial extent of the magnetic perturbation across the interface. In this study we grew $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ films with mixed thicknesses of five and six quintuple layers (QLs) on a thick ferromagnetic $\mathrm{C}{\mathrm{r}}_2\mathrm{T}{\mathrm{e}}_3$ film. Chemical analysis indicates gradual Cr diffusion across the interface into $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$, reaching up to the fourth QL, while the fifth and sixth QLs remain free of Cr. Despite the absence of direct Cr incorporation, quasiparticle interference imaging using spectroscopic scanning tunneling microscopy reveals strong backscattering on the fifth QL surface. This suggests an intrinsic magnetic proximity effect, which breaks time-reversal symmetry without magnetic doping, extending into the fifth QL. Although variations may exist across different systems, the large magnetic perturbation length scale observed here is a valuable guiding principle for engineering exotic electronic and magnetic states at heterointerfaces between TIs and magnetic materials.

A systematic study of atomic dynamics and thermal properties of the family of layered chalcogenide compounds $M\mathrm{Cr}{X}_2$ $(M=\mathrm{Ag},\mathrm{Cu};X=\mathrm{S},\mathrm{Se})$ was performed, including neutron and x-ray scattering, thermal characterization, and first-principles simulations. In all compounds, we observe a breakdown of specific phonon modes across the superionic phase transition, for phonons whose eigenvectors exhibit large contributions of mobile ions. In particular, the nondispersive portions of transverse acoustic (TA) branches at short-wavelengths and the low-energy optical phonons with large contributions from ${\mathrm{Ag}}^{+}$ or ${\mathrm{Cu}}^{+}$ become severely damped in the superionic phase. However, well-defined quasiparticles persist in the superionic state for long-wavelength TA phonons. In the case of ${\mathrm{AgCrS}}_2$, the coupling of lattice dynamics with its antiferromagnetic transition was also investigated. The magnetic ordering couples with the monoclinic–rhombohedral structural transition, and the ${\mathrm{Cr}}^{3+}$ spin arrangement strongly affects the phonon dispersions. We qualitatively reproduce the magnetic and nuclear components of the INS measurement for antiferromagnetic ${\mathrm{AgCrS}}_2$ by combining models of spin-waves and spin-polarized first-principles phonon simulations. Quasielastic magnetic fluctuations persist in the paramagnetic phase up to high temperature, but are clearly distinguished from the nuclear component through their momentum dependence. Finally, we report measurements of the thermal properties of the selenide compounds and find good agreement with our DFT simulations.

We investigate and compare the electronic structure of ${\mathrm{OsO}}_2$ with the extensively studied ${\mathrm{RuO}}_2$. Calculations show that ${\mathrm{OsO}}_2$ exhibits antiferromagnetism and spin splitting driven by crystal symmetry, a characteristic of altermagnetism and similar to ${\mathrm{RuO}}_2$. Examination of the Fermi surface, with and without spin-orbit coupling, reveals that ${\mathrm{OsO}}_2$ has lower Fermi group velocities compared to ${\mathrm{RuO}}_2$, suggesting limited charge carrier mobility in ${\mathrm{OsO}}_2$. While this characteristic may constrain its performance in high-speed electronic transport applications, it may also enhance stability for spin-based information storage in spintronics. Additionally, comparison of the vibrational properties of these rutile oxide systems demonstrates dynamical stability, typical mass dependent behaviors, and favorable agreement with measured Raman data. The calculated phonon density of states for ${\mathrm{RuO}}_2$ also agrees with our neutron scattering data. These observations substantiate the implications of the electronic and vibrational behaviors in ${\mathrm{OsO}}_2$ and ${\mathrm{RuO}}_2$, encouraging further investigation of their potential for emerging technological applications.

An intrinsic feature of disordered and out-of-equilibrium materials, such as glasses, is the dependence of their properties on their history. An important example is rheological memory, in which disordered solids obtain properties based on their deformation history. Here, we employ x-ray photon correlation spectroscopy with in situ rheometry to characterize memory formation in a nanocolloidal soft glass due to cyclic shear. During a cycle, particles undergo irreversible displacements composed of a combination of shear-induced diffusion and heterogeneous, residual strain fields. At lower shear amplitudes, the displacements resemble a random walk in which the directions in each cycle are independent of those in preceding cycles, while at high amplitude, the irreversible displacements in consecutive cycles become correlated. The magnitudes of the displacements decrease with each cycle before reaching a steady state where the microstructure has been trained to achieve enhanced reversibility even at shear amplitudes well above yielding and despite the presence of thermal fluctuations. At amplitudes below and near yielding, these decreases are monotonic, while well above yielding, they are nonmonotonic, suggesting evidence of shear banding. Accompanying this microstructural training are corresponding decreases in the dissipation during each cycle and the magnitude of the residual stress toward steady-state values. Memory of the training is revealed by measurements in which the amplitude of the shear is changed after steady state is reached. The magnitude of the particle displacements, as well as the dissipation and the change in residual stress, vary nonmonotonically with the new shear amplitude, having minima near the training amplitude, thereby revealing correlated microscopic and macroscopic signatures of memory.

Resolving the local structure motifs that characterize phase evolution as a function of composition is a key challenge in structure characterization of complex materials. In this study, we combine first-principles simulations and x-ray absorption near-edge structures (XANES) analysis to gain insights into the structure evolution revealed by measurements across a combinatorial zinc titanate thin film, which was grown with smoothly varying composition over a wide range of the Ti:Zn ratio. Specifically, we propose a cluster blind-signal-separation (cBSS) method for XANES spectral analysis based on a library of the structures and spectra of representative local motifs. In addition to motifs from zinc titanate crystals, two types of Ti-defect models constructed in this study are key to the understanding of the structure characteristics in the Zn-rich region. The cBSS method makes use of both spectral clustering of the simulated site-XANES spectra library and the BSS procedure to construct high-fidelity spectral basis functions from an experimental spectral sequence. The method provides a rigorous measure of the spectral sensitivity and basis completeness. The results of the XANES analysis are corroborated with other experimental modalities, including x-ray diffraction and spectroscopic ellipsometry, to validate the cBSS method. The calculated motif weights resulting from fitting the XANES spectra with the cBSS basis probe the atomic structure characteristics of both crystalline and amorphous phases as a function of the Ti/Zn composition. The insights of the local structure motif evolution are pivotal to the understanding of the nonmonotonic trend in the optical gap, which may lead to potential applications through tuning the optical properties of zinc titanate. The workflow of the XANES spectral analysis developed in this work can be generalized to construct the structure-property relationship in a broad material space.

The physical properties of single-crystalline ${\mathrm{Eu}}_2{\mathrm{CuZn}}_2{\mathrm{P}}_3$ were inspected via thermodynamic, transport, and neutron diffraction measurements. ${\mathrm{Eu}}_2{\mathrm{CuZn}}_2{\mathrm{P}}_3$ is composed of EuCuP and ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ motifs stacked in a one-to-one ratio. The magnetic response of ${\mathrm{Eu}}_2{\mathrm{CuZn}}_2{\mathrm{P}}_3$ is similar to that of ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ in its A-type antiferromagnetic (semiconducting) state. A Néel temperature of $T_N=40.3\mathrm{K}$ is obtained from the specific heat capacity. Single-crystal neutron diffraction reveals a (001) magnetic propagation vector, with primarily in-plane moments coupled along [001] in an up-up-down-down pattern. This spin structure retains the ferromagnetic EuCuP motifs and antiferromagnetic coupling across the ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ motifs. The electrical resistivity is characterized by metallic behavior above 100 K, an increase in $\rho$ upon cooling below $\approx 75\mathrm{K}$, and a large negative magnetoresistance below $T_{\mathrm{N}}$. First principles calculations evidence a narrow band gap for antiferromagnetic ordering and semimetallic behavior for the ferromagnetic state. This is consistent with the observed effect of magnetic field on the electrical resistivity. Hall effect measurements reveal a field-induced, negative anomalous Hall effect that increases on cooling down to 2 K. Also, an unconventional contribution to the Hall effect dominates the low-field Hall signal, particularly near $T_{\mathrm{N}}$, and is similar in appearance to a topological Hall effect. However, further work is necessary to understand if any of the transport properties have topological origins or if magnetism-induced modifications to the Fermi surface induce these unusual Hall effect signals in this and related materials.

Inversion symmetry-breaking-driven charge generation in ferroelectrics upon visible light absorption has become a focal point for light-harvesting applications. Although several light-induced phenomena have been proposed to explain photovoltaic effects in many ferroelectric thin-film systems, photovoltaic mechanisms in ${\mathrm{Bi}}_2{\mathrm{FeCrO}}_6$ (BFCO) films remain elusive. Here, by combining advanced scanning probe techniques capable of locally assessing light-induced charge generation with macroscopic photovoltaic and spectroscopic experiments, we reveal that the photovoltaic effect in this system is strongly influenced by abrupt discontinuities in electronic band structure at the BFCO/substrate interface. In particular, BFCO/Nb-doped ${\mathrm{SrTiO}}_3$ is found to exhibit an interfacial built-in electric field that promotes a dominant p-n junction-like photovoltaic effect, whereas $\mathrm{BFCO}/{\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ shows no interfacial built-in field, demonstrating instead a bulk photovoltaic effect. These findings suggest that an interfacial electric field can significantly reduce bulk photovoltaic effects. In addition, strong sensitivity to ferroelectric polarization direction is observed, offering a promising approach to controlling photoelectric responses. This study provides clearer insights into the photovoltaic properties of BFCO thin films, suggesting potential applications not only in solar energy harvesting but also in switch-type optoelectronic devices.

Topology and superconductivity, two distinct phenomena, offer unique insight into quantum properties and their applications in quantum technologies, spintronics, and sustainable energy technologies. Tin (Sn) plays a pivotal role here as an element because of its two structural phases, α-Sn exhibiting topological characteristics and β-Sn showing superconductivity. Here, we demonstrate precise control of these phases in Sn thin films using molecular beam epitaxy with systematically varied lattice parameters of the buffer layer. The Sn films exhibit either β-Sn or α-Sn phases as the buffer layer's lattice constant varies from $6.10\AA$ to $6.48\AA$, spanning the range from GaSb (like InAs) to InSb. The crystal structures of α- and β-Sn films are characterized by x-ray diffraction and confirmed by Raman spectroscopy and scanning transmission electron microscopy. Atomic force microscopy validates the smooth, continuous surface morphology. Electrical transport measurements further verify the phases: resistance drop near 3.7 K for β-Sn superconductivity and Shubnikov-de Haas oscillations for α-Sn topological characteristics. Density functional theory shows that α-Sn is stable under tensile strain and β-Sn under compressive strain, aligning well with experimental findings. Hence, this study introduces a platform controlling Sn phases through lattice engineering, enabling innovative applications in quantum technologies and beyond.

Aluminum oxide (alumina, ${\mathrm{Al}}_2{\mathrm{O}}_3$) exists in various structures and has broad industrial applications. While the crystal structure of $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ is well-established, those of transitional aluminas remain highly debated. In this study, we propose a universal machine learning interatomic potential (MLIP) for aluminas, trained using the neuroevolution potential (NEP) approach. The dataset is constructed through iterative training and farthest point sampling, ensuring the generation of the most representative configurations for an exhaustive sampling of the potential energy surface. The accuracy and generality of the potential are validated through simulations under a wide range of conditions, including high temperatures and pressures. A phase diagram is presented that includes both transitional aluminas and $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ based on the NEP. We also successfully extrapolate the phase diagram of aluminas under extreme conditions ([0, 4000] K and [0, 200] GPa ranges of temperature and pressure, respectively), while maintaining high accuracy in describing their properties under more moderate conditions. Furthermore, combined with our developed structure search workflow, the NEP provides an evaluation of existing $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ structure models. The NEP developed in this work enables highly accurate dynamic simulations of various aluminas on larger scales and longer timescales, while also offering new insights into the study of transitional aluminas structures.