Stacking faults and other topological defects in ferroics can have a significant influence on the electronic and mechanical properties of the material. Here, regular stacking faults in the tetragonal tungsten bronze material ${\mathrm{Sr}}_2{\mathrm{NaNb}}_5{\mathrm{O}}_{15}$ are investigated through transmission electron microscopy, symmetry mode analysis, and machine-learned force-field calculations. It is shown that the faults, with a fault vector of $\frac{1}{4}{\left[\overline{2}12\right]}_o$, annihilate in sets of four in the material, owing to the ¼ unit cell displacement along the b-axis. The four resulting domains emerge as four possible directions of the ${\mathrm{S}}_3$ order parameter, related to ${\mathrm{NbO}}_6$ octahedral tilts in the material. Force-field calculations reveal that the stacking faults are likely placed at positions where the octahedra in neighboring domains have similar magnitudes of rotation, and that the estimated stacking fault energy is 46 $\mathrm{mJ}/{\mathrm{m}}^2$. The investigation shows that the stacking faults have a local effect on the in-plane polar mode present in the structure, and therefore could affect the ferroelectric properties.

Sodium niobate ($\mathrm{NaNb}{\mathrm{O}}_3$, NN), as a rare room-temperature antiferroelectric material, exhibits significant applications in important fields such as pulse energy storage and multistate nonvolatile memories. However, pristine NN generally suffers from an irreversible field-induced phase transition, which prevents the observation of the characteristic double hysteresis loops of antiferroelectrics. Previous findings have revealed that its unstable antiferroelectric structure is strongly correlated with the occupancy instability of $\mathrm{N}{\mathrm{a}}^{+}$ ions under external fields. $\mathrm{N}{\mathrm{a}}_{1-3x}\mathrm{L}{\mathrm{a}}_x\mathrm{Nb}{\mathrm{O}}_3$ system is constructed in this work via doping the Na site with $\mathrm{L}{\mathrm{a}}^{3+}$, which differs obviously from $\mathrm{N}{\mathrm{a}}^{+}$ in both mass and electronic structure. This strategy aims to realize a progressive evolution from irreversible to reversible phase transition under electric fields, by controlling the doping concentration of $\mathrm{L}{\mathrm{a}}^{3+}$. Based on the precise identification of Raman spectroscopy for phonon vibrations, the in situ Raman spectra under varying electric field and temperature are measured to conduct a detailed insight into the evolution pathways of multiple crucial phonons in samples. Accordingly, the modulation of $\mathrm{L}{\mathrm{a}}^{3+}$ on the reversibility of field-induced phase transition could be effectively deciphered. This work reinterprets the unique field-induced phase transition in NN-based materials from the perspective of phonon evolution, which lays an important research foundation for the design and development of NN-based devices with multiple transitions between different electrical ordering states.

High-quality InAs quantum wells grown on InP are a promising platform for topological quantum information processing due to their large $g\text{−}\mathrm{factor}$, strong Rashba spin-orbit interaction, and their compatibility with in situ deposited superconductors. In this work, we investigate InAs/InGaAs quantum wells grown on InP (001) wafers, focusing on how the layer structure and strain influence the electronic properties and surface morphology. By combining quantum transport measurements with atomic force microscopy, we show that the layer design predominantly affects the mobility anisotropy, which aligns well with the surface morphology. Surface characterization further reveals the mechanism of quantum well collapse when the layer thickness exceeds the strain limit. In addition, transport measurements demonstrate that quantum confinement has a clear impact on band nonparabolicity.

Carbide particles act as effective hydrogen traps that can mitigate hydrogen embrittlement in steels, with their trapping efficacy governed by the binding energy of hydrogen at atomic-scale sites within the carbide and at the carbide/matrix interface. Within the carbide, carbon vacancies are established trap sites, while at the carbide/matrix interface, multiple distinct trap sites may form depending upon the interface character. State-of-the-art experimental methods cannot attribute binding energies to individual sites unambiguously, particularly at the carbide/matrix interface, where distinct trap sites may exhibit similar binding energies. Density functional theory (DFT) calculations have partially addressed this limitation, but their computational cost prohibits accurate treatment of semicoherent and incoherent interfaces, for which hydrogen binding energies remain poorly characterised. In this work, we develop a DFT-accurate machine-learning Fe–TiC–H interatomic potential that enables simulations of hydrogen energetics and kinetics at various TiC/Fe interfaces in the Baker–Nutting orientation. Predictions of structural, dynamic and energetic properties of both bulk phases (i.e., bcc Fe and rocksalt TiC) and the coherent interface with and without hydrogen are in good agreement with DFT. We apply the potential to examine the atomic structure and hydrogen trapping at semicoherent and incoherent interfaces. We find that the binding energy of hydrogen at misfit dislocations is similar to that at coherent interfaces, with a difference of no more than 0.1 eV. However, the misfit strain field reduces carbon vacancy formation energies at the interface, where hydrogen can be trapped with similar binding energies as that in a bulk C vacancy (approximately –1 eV). Lower vacancy formation energies are also found at the incoherent interface, where hydrogen binding energies span a broader range, depending sensitively on the local atomic environment. Overall, the results enable interpretation of experimental observations of hydrogen segregation around semicoherent and incoherent TiC interfaces.

An electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM) study of the semiconductor system $2\mathrm{PbS}+{\mathrm{NaSbS}}_2$ $\left({\mathrm{NaPb}}_2{\mathrm{SbS}}_4\right)$ reveals strikingly complex, nanostructural properties despite its simple NaCl-type framework. We uncover the coexistence of commensurate and incommensurate cation ordering, nanoscale defect architectures, and quasiperiodic arrays of antiphase boundaries. In many areas, the dominant observed structure type corresponds to the NaCl type, with varying lattice parameters, denoting inhomogeneities in composition. In other areas, superstructure reflections were observed in the ED patterns. The HRTEM images reveal the existence of different nanocrystals, with long-range ordering of the cations. Three different commensurate modulated structures (superstructures) of the NaCl type of structure were observed. All the nanocrystals with ordering were observed to grow endotaxially within the matrix. Moreover, the local arrangement of several defects (orientation variants and antiphase boundaries) in a quasiperiodic way leads to the local formation of incommensurate modulated structures. Finally, in some cases, diffuse scattering intensity was observed in the ED patterns. The HRTEM images, together with the ED results, reveal a transition state between the phases with long- and short-range order. These findings clarify the structural behavior of this complex semiconductor and motivate further investigation of its structural and physical properties.

Physical properties in the normal and superconducting (SC) state are investigated with ${}^{125}\mathrm{Te}$-nuclear magnetic resonance (NMR) measurements in a quasicrystal ${\left({\mathrm{Ta}}_{0.95}{\mathrm{Cu}}_{0.05}\right)}_{1.6}\mathrm{Te}$, which was a recently discovered superconductor with the SC transition temperature $T_{\mathrm{c}}=0.94 \mathrm{K}$. The nuclear spin-lattice relaxation rate $1/T_1$ shows a coherence peak just below $T_{\mathrm{c}}$, followed by an exponential decrease down to 0.1 K. The overall temperature dependence of $1/T_1$ is in good agreement with an $s$-wave SC model with an SC gap slightly smaller than the BCS value. However, the coherence peak is unusually small, which may be attributable to a reduced Bogoliubov peak theoretically predicted for quasicrystals. Furthermore, ${}^{125}\mathrm{Te}\text{−}\mathrm{NMR}$ spectra show marginal shift and almost no linewidth broadening below $T_{\mathrm{c}}$, revealing unusual superconducting responses in this quasicrystal.

Metal Phthalocyanine-based materials are known for their stability and are widely used in flexible organic electronics. The study presented here investigates the impact of substrate temperature on the growth to control the crystallinity of the deposited copper phthalocyanine (CuPc) thin films on silicon substrates and its effect on the charge transport in $\mathrm{Au}/\mathrm{CuPc}/n$-Si heterostructures. Current-voltage $\left(I\text{−}V\right)$ characterization of the heterostructures reveals the appearance of multiple transport mechanisms, ranging from direct and Fowler-Nordheim tunneling at lower temperatures to Poole-Frenkel conduction at higher temperatures, depending on the crystallinity of the deposited CuPc films. Resonant tunneling observed in the amorphous thin film suggests the existence of a $\pi$-electron bridge parallel to the vertical charge transport path. Such orientation of CuPc molecules is important for efficient charge extraction in organic solar cells.

The thermophysical properties of liquid uranium-zirconium (U,Zr) mixtures play a crucial role in understanding the early stages of nuclear accident scenarios in pressurized-water reactors, as well as in other reactor designs. In this study, atomistic modeling leveraging ab initio calculations and a machine-learning interatomic potential framework are employed to investigate the static and dynamic properties of liquid (U,Zr) mixtures. More specifically, a spectral neighbor analysis potential trained on ab initio configurations is used to model the system, and its predictions are systematically compared and analyzed with experimental data from the literature and ab initio results. The model is then used to analyze an increase of the viscosity predicted for intermediate liquid (U,Zr) compositions, and its relationship to the internal structure of the mixtures. These insights contribute to the refinement of nuclear fuel models, enhance the accuracy of in-vessel corium retention simulations related to the mitigation of severe accident scenarios, and emphasize the possibility of using a combined ab initio and machine-learning interatomic potential approach to compute properties of such complex mixtures.

We present a combined x-ray diffraction and transmission electron microscopy study of superconducting FeSe/FeTe multilayers grown by molecular beam epitaxy on ${\mathrm{SrTiO}}_3\left(001\right)$ substrates. While x-ray diffraction confirms perfect in-plane epitaxial alignment between FeSe, FeTe, and the substrate, scanning transmission electron microscopy reveals a surprising lack of atomic registry at the $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ interface. Instead of adapting to the substrate lattice, FeSe retains its own in-plane lattice spacing. A periodic lateral shift between the atomic positions of FeSe and ${\mathrm{SrTiO}}_3$ is observed, with a registry recurrence length that matches the lattice mismatch determined by x-ray diffraction. No misfit dislocations or other relaxation features are detected at the interface. This coexistence of directional alignment and registry-free growth suggests an unconventional regime of epitaxy in which crystallographic orientation is maintained without atomic matching. The findings offer insight into strain accommodation in layered systems and may have implications for interface engineering in Fe-based superconductors.

Crystal structure prediction (CSP) is crucial for identifying stable crystal structures in given systems and is a prerequisite for computational atomistic simulations. Recent advances in neural network potentials (NNPs) have reduced the computational cost of CSP. However, searching for stable crystal structures across the entire composition space in multicomponent systems remains a significant challenge. Here, we propose an improvement of a genetic algorithm (GA)-based CSP method using a universal NNP designed to efficiently expand convex hull volumes while preserving the diversity of crystal structures. Our hull-informed filtering and elitist-selection procedures incorporate an aging mechanism that prioritizes recently improved compositions. We also employ niching to prevent convergence to a small set of stoichiometries, thereby preserving a diverse, high-quality population. Our evaluation shows that the present method outperforms the symmetry-aware random structure generation and existing CSP methods, achieving a larger convex hull with fewer trials. We demonstrated that our approach, combined with the developed universal NNP (PFP), can accurately reproduce and explore phase diagrams obtained through DFT calculations; this indicates the validity of PFP across a wide range of crystal structures and element combinations. This study, which integrates a universal NNP with a GA-based CSP method, highlights the promise of these methods in materials discovery.

Understanding the structure–property relationships in layered transition-metal carbides or nitrides, known as MXenes, is of critical importance for their rational design, synthesis, and application. However, the vast chemical and structural diversity of MXenes, stemming from their wide range of M and X elements, surface terminations, and different atomic coordination environments, makes it challenging to clearly understand these structure–property relationships. In this work, we perform first-principles density functional theory (DFT) calculations and molecular dynamics (MD) simulations to comprehensively investigate the stability and a variety of physical properties of MXenes with different coordination environments. Using Ti- and Mo-based carbide MXenes as model systems, energetic calculations reveal that Ti-based MXenes are most stable in octahedral coordination, whereas Mo-based MXenes preferentially adopt prismatic coordination. This fundamental difference in preferred atomic coordination gives rise to markedly distinct properties between these two systems as a function of the fraction of octahedral and prismatic sites. For instance, the in-plane stiffness of Ti-based MXenes increases as octahedral coordination becomes dominant, but it decreases in the Mo-based MXenes under the same conditions. Additional stability analyses based on mechanical, lattice-dynamical, and temperature-dependent thermodynamic properties demonstrate that many metastable MXenes not only satisfy the strict stability criteria but can also undergo phase transitions among different structures and even become stabilized at elevated temperatures. Although surface terminations, such as F and O atoms, do not alter the energetic ordering or the overall stiffness trends among stable and metastable MXenes, they influence other material properties. For instance, O termination can induce semiconducting behavior in both stable and metastable ${\mathrm{Ti}}_2{\mathrm{CO}}_2$ MXenes. This study significantly advances the fundamental understanding of structure–property relationships in MXenes and provides valuable guidance for developing coordination-based design principles to precisely engineer MXenes with improved properties.

Spin-orbit torque (SOT) magnetoresistive random-access memory (MRAM), a nonvolatile memory technology, exhibits promising prospects for advancing memory devices. However, it still faces pivotal challenges in achieving high SOT efficiency and realizing deterministic field-free switching in perpendicular magnetization SOT devices. Herein, we propose an effective strategy to realize high-efficiency field-free switching with an ultralow critical current density $(J_{\mathrm{c}}=3.9$ $\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2)$ by doping copper (Cu) into the topological insulator (TI) ${\mathrm{Bi}}_{0.9}{\mathrm{Sb}}_{0.1}$ (BiSb). The cosputtered Cu-BiSb alloy film achieves a high conventional transverse spin polarization efficiency $({\xi }_{\mathrm{y},\mathrm{DL}}=0.53)$. In contrast to undoped BiSb, it exhibits a significant fieldlike torque induced by spin currents with out-of-plane spin polarization $({\xi }_{\mathrm{z},\mathrm{FL}}=0.55)$. This unconventional spin polarization enables deterministic field-free switching of perpendicular magnetization with as low as $J_{\mathrm{c}}\sim 4$ $\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2$ in Cu-BiSb/Pt/Co/Pt heterostructures. Our work provides critical support for the development of TI-based, high-performance, and integrable SOT-MRAM devices.

Over the past decade, there has been steady research on superconducting junctions with a ferromagnet as the weak link and where triplet correlations can transport supercurrents over a substantial distances. Of particular interest are half-metallic ferromagnets, in which only one spin band is present, so that, presumably, the induced supercurrent is fully spin polarized. We have earlier reported on a study of triplet transport in planar ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3\left(\mathrm{LSMO}\right)$ nanostrip Josephson junctions with NbTi superconducting contacts, where we found high values for the supercurrents, and large junction lengths (up to 1.3 µm). Here we extend that work by studying the dependence of the critical current $I_c$ on the length of the nanostrip between the contacts and the width of the strip. All junctions show strong supercurrents, but we do not observe simple systematics. Apparently, the fabrication process does not allow sufficient control over some of its parameters. To gain more insight in the mechanism for triplet generation at the LSMO/NbTi interface, we also studied the effect of Pt as an interlayer between the LSMO and the NbTi. For this, we etched a NbTi/Pt electrode structure on a full film of LSMO. The results are highly promising, showing sharp superconducting transitions and zero-resistance states being reached at an electrode distance of 2 µm, with indications that larger distances should be feasible.

First-principles studies were performed on two Mn-based ferrimagnetic Heusler compounds with $L{2}_1$ and $B2$ structures, that is, ${\mathrm{Mn}}_2\mathrm{V}\mathit{Z}$ ($\mathit{Z}=\mathrm{Al}$ or Ga). The aim was to investigate their magnetic properties, electronic structures, and spin-resolved longitudinal conductivity at both zero and elevated temperatures. Density functional theory and functional integral theory were used. This approach incorporates transverse spin fluctuations through a disordered local moment method and the coherent potential approximation. In most cases, the calculated theoretical Curie temperatures were lower than the experimental values. Alloys with a $B2$ structure exhibit higher Curie temperatures compared to compounds with an $L{2}_1$ structure. Calculations of the temperature dependence of the density of states (DOS) indicate that the half-metallic electronic structure collapses due to self-energy effects that reflect spin disorder at nonzero temperature. In other words, transverse spin fluctuations are renormalized in the electronic structure. However, the spin-resolved longitudinal conductivities demonstrated an improved spin polarization, particularly for ${\mathrm{Mn}}_2\mathrm{VGa}$ with an $L{2}_1$ structure. The competition between the half-metal-to-normal-metal transitions, which are caused by a modification of the DOS, and scattering coming from spin-disorder explains this behavior of conductivities. Both of these effects are induced by transverse spin fluctuations. Additionally, the results show that the spin polarizations defined in terms of the conductivities and of the DOS do not follow the same trends at $T>0$. Their temperature dependencies are not the same, particularly for the $L{2}_1$ structures, although they are related to each other. Finally, the total energy landscape of the paramagnetic state was calculated using the fixed spin moment method to investigate the strength of the longitudinal spin fluctuations. These results suggest that the alloys may exhibit strong longitudinal spin fluctuations.

The discovery of filling-controlled phase modulations in correlated systems opens an emerging paradigm to unlock unique electromagnetic states and physical phenomena, transcending traditional paradigms. Here, correlated $\mathrm{V}{\mathrm{O}}_2$ (B) is selected as a model system to realize multiple electronic phase modulations through ionic evolution, leveraging the inherent ${\mathrm{V}}_4{\mathrm{O}}_{10}$-type double-layered structure and thermodynamic metastability. The introduction of electron carriers into the $\mathrm{V}-3d$ orbital of metastable $\mathrm{V}{\mathrm{O}}_2$ (B), as driven by ${\mathrm{W}}^{6+}$ substitution, oxygen deficiency, or protonation, triggers the carrier delocalization, giving rise to sequential electronic phase modulations, beyond well-established M1 and R phases of $\mathrm{V}{\mathrm{O}}_2$. Of particular note is the synergistic interplay between oxygen defects and interstitial protons in cooperatively driving electronic state evolutions in $\mathrm{V}{\mathrm{O}}_2$ (B) through band-filling regulation, enabling the robust control over the energy landscape in a reversible pathway. Utilizing synchrotron-related spectroscopy techniques and theoretical calculations, we reveal that the band filling in low-energy ${\pi }^{*}$ orbital of $\mathrm{V}{\mathrm{O}}_2$ (B) through electron doping governs electronic phase modulations, delivering a unified physical picture. Our findings not only demonstrate a powerful tuning knob for adjusting correlated electronic states in metastable layered-structure materials, fostering exotic physical functionalities and phenomena but also extend the horizons in materials designs for iontronic device applications.