Low dimensional quantum materials have been extensively studied within the physics community, among which $\mathrm{C}{\mathrm{o}}^{2+}$ is one of the most popular magnetic ions to be embedded into the structural motif. $\mathrm{CoN}{\mathrm{b}}_2{\mathrm{O}}_6$ and $\mathrm{CoT}{\mathrm{a}}_2{\mathrm{O}}_6$ both have adopted an effective spin-1/2 state, with the former attracting enduring interest while the latter being rarely investigated. This difference can be attributed to their distinct crystal structures, not only in crystal systems, but more important, how the $\mathrm{Co}{\mathrm{O}}_6$ octahedra are connected. We successfully merge these two compounds together and map out the doping limits. We find out Ta and Nb dopants modify the properties of the parent compounds in subtle but different ways and construct a magnetic phase diagram of the $\mathrm{Co}{\left(\mathrm{N}{\mathrm{b}}_x\mathrm{T}{\mathrm{a}}_{1-x}\right)}_2{\mathrm{O}}_6$ $\left(0\le x\le 1\right)$ series. In addition, our results suggest that Ta-doping may pave a way to strengthen the lower-dimensional quantum phenomena in $\mathrm{CoN}{\mathrm{b}}_2{\mathrm{O}}_6$ before its crystal structure starts to melt down.

We present a machine-learning-guided first-principles study of rare-earth-free magnetic materials in the Fe-Co-S ternary system. A crystal-graph convolutional neural network (CGCNN) fine-tuned through three successive generations of iterative retraining, combined with adaptive genetic algorithm (AGA) structure searches, rapidly screens more than 300 000 substitutional structures and down-selects approximately 1400 candidates for density-functional-theory (DFT) validation. Spin-polarized DFT screening followed by phonon calculations identifies two dynamically stable tetragonal candidates, ${\mathrm{Fe}}_{17}{\mathrm{Co}}_4{\mathrm{S}}_4$ ($I4/m$) and ${\mathrm{Fe}}_2{\mathrm{Co}}_9{\mathrm{S}}_2$ ($I\overline{4}m2$), with saturation polarization $J_s\ge 1\mathrm{T}$ and energies approximately 0.10 eV/atom above the convex hull. These compounds emerge as the leading synthesis targets. Notably, ${\mathrm{Fe}}_{17}{\mathrm{Co}}_4{\mathrm{S}}_4$ exhibits $E_{\mathrm{hull}}=0.107$ eV/atom, $J_s=1.88\mathrm{T}$, and magnetocrystalline anisotropy $K_1=0.71 {\mathrm{MJ}/\mathrm{m}}^3$. We also identify eight near-hull phases ($E_{\mathrm{hull}}<0.01$ eV/atom) and independently rediscover the electrochemically prominent spinel ${\mathrm{FeCo}}_2{\mathrm{S}}_4$, providing an internal validation of the search strategy. Compared with our prior Fe-Co-C/Si/P studies, Fe-Co-S exhibits distinct stability and magnetization regimes consistent with the larger sulfur anion. The combination of machine learning with quantum simulations narrows synthesis targets for Fe-rich sulfides with high $J_s$ and moderate $K_1$, which are competitive with hard ferrites.

We present temperature-dependent Raman spectra of semimetallic $1{\mathrm{T}\text{−}\mathrm{TiSe}}_{2-\delta }$ single crystals from 10 to 300 K using a 532 nm excitation laser, probing the charge density wave (CDW) transition at 170 K. The first-order $E_g$ and $A_{1g}$ phonon modes exhibit asymmetric lineshapes above 170 K, which are described by the Breit–Wigner–Fano (BWF) function and become Lorentzian below 134 K. The Bose–Einstein anharmonic model reveals anomalies in the $E_g$ phonon peak position near the CDW transition, whereas the $A_{1g}$ mode follows conventional anharmonic behavior. Moreover, we observe six low-frequency Raman peaks between 84 and $123{\mathrm{cm}}^{-1}$, as well as one Raman peak at $295{\mathrm{cm}}^{-1}$. By comparing these features with the one-phonon and two-phonon density of states obtained from the first-principles calculations, we assign the low-frequency modes to defect-induced double-resonance Raman peaks and the $295{\mathrm{cm}}^{-1}$ mode to a two-phonon double-resonance Raman peak. The first-principles calculations also indicate phonon softening near the M and L points in the Brillouin zone, suggesting the lattice instability associated with the CDW transition. Our results provide Raman fingerprints of the CDW transition and highlight the role of electron–phonon interactions in semimetallic $1{\mathrm{T}\text{−}\mathrm{TiSe}}_{2-\delta }$.

Strain engineering has emerged as a strategic approach for modulating the piezoelectric and ferroelectric properties of thin films. We investigated local piezoelectric properties in clamped and freestanding epitaxial $\mathrm{BaTi}{\mathrm{O}}_3$ films via piezoresponse force microscopy. The freestanding membrane exhibits a $\sim 300\%$ increase in the effective piezoelectric coefficient compared to the clamped films, driven by increased nonlinearity. Quantitative analysis reveals about fourfold increase in the nonlinearity parameter, originating from enhanced domain wall motion enabled by strain relaxation after removal of substrate-induced clamping. Our molecular dynamics simulations further support about threefold increase in domain-wall velocity in the freestanding film. Additionally, under optical excitation, the membrane exhibits a photoinduced enhancement of the piezoelectric response arising from coupled photostriction effect and light-driven domain wall motion. Density functional theory calculations reveal $\sim 2\%$ decrease in the out-of-plane lattice parameter of the freestanding membrane under optical excitation, consistent with the observed photoinduced deformation. However, the corresponding change in the clamped film ceases to exist. These results identify substrate clamping as a critical limitation to piezoelectric performance and establish freestanding ferroelectric membranes as promising platforms for low-power electromechanical and optoelectronic devices.

Spin-pumping experiments in superconductor–ferromagnet heterostructures, which probe spin-sinking by the superconductor, have revealed a variety of complex behaviors. Most experimental studies have focused on conventional $s$-wave superconductors combined with metallic or insulating ferromagnets. Here, we study a $d$-wave superconductor paired with a half-metallic ferromagnet in epitaxial $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-\delta }/\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ heterostructures with two crystalline orientations: one in which YBCO is $c$-axis oriented and the other in which YBCO grows along the (103) direction. Using ferromagnetic resonance (FMR), we probe the temperature-dependent Gilbert damping coefficient α. For (103) heterostructures, α(T) initially decreases below $T_c$, but then increases at lower temperatures, eventually exceeding normal-state levels. This behavior can be understood in terms of the opening of the superconducting gap and spin transport via nodal quasiparticles, whose contribution is enhanced when the $\mathit{ab}$ plane of YBCO is exposed at the interface. In stark contrast, $c$-axis heterostructures exhibit a pronounced enhancement of α(T) below $T_{\mathrm{C}}$, peaking at $\sim 0.65-0.7T_{\mathrm{C}}$ before decaying. This anomaly suggests the dominance of interfacial Andreev bound states, arising from a locally suppressed superconducting order parameter because of proximity effects with the half-metallic LSMO.

Chalcogenide materials are investigated for various applications because of their tunable properties. In this work, we compare the Cu-induced microstructural evolution of epitaxial and polycrystalline $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ thin films using advanced transmission electron microscopy. Cu sputtering onto epitaxial (001) $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ at room temperature results in Cu intercalation into the layered structure. This expands the $c$-lattice parameter, particularly the van der Waals-like gaps, while largely preserving the layered framework. Chemical analysis reveals Cu enrichment at grain boundaries, which act as preferential diffusion pathways, followed by anisotropic in-plane Cu redistribution. Local Cu enrichment promotes transformation into layered Cu–Sb–Te phases with Sb/Te antisite defects, structurally related to hexagonal triple-layered $\mathrm{C}{\mathrm{u}}_7{\left(\mathrm{S}{\mathrm{b}}_{0.4}\mathrm{T}{\mathrm{e}}_{0.6}\right)}_4$ and trigonal double-layered $\mathrm{C}{\mathrm{u}}_7\mathrm{T}{\mathrm{e}}_4$ structures. The transformation proceeds through local reconstruction of the Te sublattice, involving changes in the stacking sequence and in-plane shifts of Te planes. Resistive switching of a Cu/(001) $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$/(111) Si memory stack further promotes Cu redistribution and conversion into layered Cu–Sb–Te structures. In contrast, polycrystalline $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ transforms into tetragonal $\mathrm{C}{\mathrm{u}}_{3-x}\mathrm{T}{\mathrm{e}}_2$-related and $\mathrm{C}{\mathrm{u}}_7{\left(\mathrm{S}{\mathrm{b}}_{0.4}\mathrm{T}{\mathrm{e}}_{0.6}\right)}_4$-type structures upon Cu sputtering. Additional Pt deposition onto the Cu layer promotes the growth of large $\mathrm{C}{\mathrm{u}}_3{\left(\mathrm{S}{\mathrm{b}}_{0.4}\mathrm{T}{\mathrm{e}}_{0.6}\right)}_2$-type grains, structurally related to rickardite $\mathrm{C}{\mathrm{u}}_{3-x}\mathrm{T}{\mathrm{e}}_2$ but containing Sb/Te antisite defects. These results show that Cu-induced phase formation in $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ is influenced by Cu diffusion pathways, the initial $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ microstructure, and thermal activation, which together affect the local Cu distribution and resulting phase evolution. The findings provide insight into the structural factors governing Cu–Sb–Te phase formation and are of potential relevance to the design of chalcogenide thin films for thermoelectric and memory applications.

High-entropy alloys (HEAs) are materials which utilize the entropy of mixing to encourage immiscible elements to form single-phase solid solutions. These materials have shown superior structural and mechanical properties, especially at high temperatures, but there is increasing interest in their potential in functional applications such as memory storage and energy devices. In the current work, we investigate the magnetic properties of HEAs and proto-HEAs ${\mathrm{Al}}_x\mathrm{CoCrFeNi}$ ($0\le x\le 2$). These materials form a single phase at $x<1$ but phase separate at higher concentrations. Interestingly, with phase separation, the magnetic moment and Curie temperature increase with the inclusion of the nonmagnetic aluminum. The phase separation and the resulting magnetic evolution are investigated at elevated temperatures with scanning transmission electron microscopy and small-angle neutron scattering. These complementary techniques provide a comprehensive understanding of both the compositional and magnetic phase separation. First-order reversal curve analysis is used to correlate the temperature-dependent phase separation with the magnetic coupling and reversal behavior.

Ion migration in $\mathrm{W}{\mathrm{O}}_3$ is a critical process for various technological applications, such as in batteries, electrochromic devices, and energy-efficient brain-inspired computing devices. In this study, we investigate the migration mechanisms of ${\mathrm{H}}^{+}, \mathrm{L}{\mathrm{i}}^{+}$, and $\mathrm{M}{\mathrm{g}}^{2+}$ ions in monoclinic $\mathrm{W}{\mathrm{O}}_3$, and how energy barriers are affected by the presence of electron polarons and by lattice strain. Our approach in calculating the migration paths and barriers is based on density functional theory methods. The results show that the presence of polarons leads to association effects and lattice deformations that increase ion migration barriers. Therefore, the consideration of polarons is critical to accurately predict activation energies of ion migration. We further show that lattice strain modulates ion migration barriers, however, the impact of strain depends on the migrating ion. For protons that are embedded in the oxygen ion electronic shells and hop from donor to acceptor oxygens, compressive lattice strain accelerates migration by reducing the donor-acceptor distance. In contrast, the migration barriers of larger ions decrease with tensile lattice strain that increases the free space for the ion in the transition state. These insights into the effects of polarons and lattice strain are important for understanding and tuning the properties of $\mathrm{W}{\mathrm{O}}_3$ when aiming for optimized device characteristics.

Negative thermal expansion (NTE) materials with framework structures are of considerable interest due to their ability to mitigate thermal stress and enhance dimensional stability in composite systems. However, broader applications remain limited by the relatively small coefficients of negative thermal expansion (CNTE) and narrow operational temperature windows available in most materials. Here, we employ an interpretable machine-learning framework based on Shapley additive explanations to investigate the structure–property relationships associated with CNTE and the NTE temperature range. By analyzing physical and structural descriptors for a broad range of framework NTE materials, we identify mean electronegativity and electronegativity range, mass density, porosity, lattice parameters, and space-group symmetry as the main factors associated with these two properties. A symmetry-derived proxy descriptor, wfi_proxy, defined by the number of symmetry operations normalized by the number of atoms in the chemical formula, is also included in the CNTE analysis. We identify thresholdlike behavior in electronegativity and mass density, and show that their trends are nonmonotonic and may depend on other structural descriptors. Notably, the descriptors that favor large CNTE often act in the opposite direction for the NTE temperature range, revealing a trade-off between NTE magnitude and temperature window. We refer to this relationship as the approach–approach conflict. These findings extend our earlier correlation-based analysis and provide structure–property trends that may guide the discovery and design of framework NTE materials.

Eu triangular lattice layer compounds described by the general formula $\mathrm{Eu}{A}_2{X}_2$ have attracted growing attention due to a wide range of layered crystal structures and corresponding magnetic topological phases, arising from the coexistence of large Eu magnetic moments and energetically inverted $A$ and $X$ bands. Among the $\mathrm{Eu}{A}_2{X}_2$ family, ${\mathrm{EuIn}}_2{\mathrm{Sb}}_2$ has been predicted to host a robust axion insulator and higher-order topological insulator states by stronger spin-orbit coupling but still needs to be experimentally realized, including the structural identification. Here, we report the epitaxial stabilization of ${\mathrm{EuIn}}_2{\mathrm{Sb}}_2$ by adopting the molecular beam epitaxy technique. Structural characterization reveals that ${\mathrm{EuIn}}_2{\mathrm{Sb}}_2$ films are based on a unique In-on-In stacking, forming a different trilayer structure distinct from previously predicted structures in the $\mathrm{Eu}{A}_2{X}_2$ family. In addition to the in-plane antiferromagnetic ordering at $T_{\text{N}}=10.5\mathrm{K}$, the stronger spin-orbit coupling and enhanced In-In hybridization in ${\mathrm{EuIn}}_2{\mathrm{Sb}}_2$ make it an ideal candidate of axion insulator and higher-order topological insulator phases. These results establish ${\mathrm{EuIn}}_2{\mathrm{Sb}}_2$ as a member of the $\mathrm{Eu}{A}_2{X}_2$ family and offer an advanced platform for exploring these magnetic topological phases.

Post-transition metal and metalloid chalcogenides can have a wide range of unique and valuable properties. ${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_3$ (BST) is part of a family of materials that exhibit multiple functionalities, including thermoelectricity, superconductivity, and electronic topological transitions at high pressure. We have studied the phase transition sequence and equation of state of BST under pressures up to 54 GPa at ambient temperature and 32.5 GPa in the superconducting states using x-ray diffraction. At ambient conditions, BST crystallizes in a hexagonal $R\overline{3}m$ structure and undergoes three structural phase transitions, forming monoclinic C2/m, C2/c, and cubic $\mathit{Im}\overline{3}m$ structures, at 11, 16, and 25 GPa, respectively. Low-temperature diffraction experiments reveal that the superconducting states of BST share the same structures as their room-temperature counterparts. Although the x-ray diffraction clearly indicates that the electronic charge density distribution of the highest-pressure phase has $\mathit{Im}\overline{3}\mathit{m}$ space group symmetry, the symmetry of the underlying atomic lattice depends on the extent of order-disorder of the nuclei. Candidate partially ordered lower symmetry structures of the high-pressure phase are examined in the DFT calculations.

Tungsten (W) is a refractory body-centered-cubic (bcc) metal with exceptional high-temperature strength and an extremely high melting point. However, its ductile-to-brittle transition temperature (DBTT) occurs above room temperature, significantly restricting its formability and, consequently, its range of applications. One approach to modifying the plastic behavior of W at low temperatures is alloying it with rhenium (Re), which enhances its ductility, though the underlying dislocation mechanisms responsible for this improvement are not fully understood. In this work, the phase field dislocation dynamics model is extended to treat {112}$\langle 111\rangle$ dislocation dynamics in a bcc crystal and applied to elucidate the effect of Re additions on dislocation mechanisms in W. The model is used to simulate thermally activated glide of long, initially screw and edge dislocations in W and W-20%Re for a range of elevated temperatures that includes their DBTT. We show that for both materials and at all temperatures, the critical resolved shear stress (CRSS) for {112} glide is higher than for {110} glide and the CRSS for screw dislocations is higher than edge dislocations. The CRSS is higher in the {112} antitwinning (AT) direction than {112} twinning (T) direction up to a certain temperature, at which they become nearly equal. The {112}T/AT equivalence temperature is lower for W-20Re than pure W. We find that Re increases the activation enthalpy for screw dislocation glide for the {110} mode, but decreases it for the {112} mode. Together, these findings suggest that Re additions promote slip activity from being primarily {110} slip to both {110} and {112}T/AT slip.

We assess the capability of recently developed foundational atomistic models (FAMs) to simulate iron alloys under the extreme pressures and temperatures of Earth's core. Static equations of state for hexagonal close-packed (hcp) and body-centered cubic (bcc) iron, computed using 17 FAMs, are benchmarked against ab initio calculations. Two representative models, MatterSim and MACE, are further evaluated for their ability to reproduce phonon spectra, liquid structure, and melting relations of iron at core conditions. While both models capture several key properties, MACE substantially overestimates the stability of bcc iron and fails to correctly describe the stability of hcp iron. Their performance is also examined for binary liquids, superionic phases, and a seven-component Fe–Ni–Si–S–O–H–C liquid. Although these FAMs were not explicitly trained on data from core conditions, they can reproduce several structural and dynamical properties across a wide range of compositions. However, none of the tested models consistently reproduces all first-principles benchmarks. By analyzing the origins of these discrepancies, we identify several limitations of current FAMs, particularly the lack of an explicit treatment of thermal electronic excitations, which significantly affect phase stability and thermodynamic properties under core conditions. We further discuss directions for improving FAMs to enable predictive simulations of core-forming materials under extreme conditions.

Half-metallic Heusler compounds are of significant interest for spintronics. For device fabrication, compounds that can be epitaxially grown on III-V semiconductors are particularly attractive. We present a first-principles investigation of four Co-based and two Ni-based Heusler compounds that are lattice-matched to InAs. The results of density functional theory (DFT) using semilocal and hybrid functionals are compared with quasiparticle self-consistent $GW$ ($\mathrm{QP}GW$). We also assess DFT with machine-learned Hubbard $U$ corrections, as proposed in M. Yu et al. [npj Comput. Mater. 6, 180 (2020)]. Here, the $U$ values that yield the closest agreement with the $\mathrm{QP}GW$ band structure are determined using an updated Bayesian optimization (BO) objective function that considers the atomic magnetic moments in addition to the band structure. We find that $\mathrm{DFT}+U$(BO) can adequately reproduce the key $\mathrm{QP}GW$ features in most cases. Our results reveal a strong method dependence of the degree of spin polarization at the Fermi level and, in some cases, even the dominant spin channel (majority or minority). Of the materials studied here, ${\mathrm{Co}}_2\mathrm{TiSn}$ and ${\mathrm{Co}}_2\mathrm{ZrAl}$ are the most likely to be half-metals.

In the rapidly evolving field of quantum materials, compounds hosting exotic electronic and topological states have attracted considerable attention owing to their potential applications in spintronics and quantum technologies. Using comprehensive first-principles calculations, we investigate the superconducting equiatomic compounds MoNiP and WNiP, which crystallize in a noncentrosymmetric distorted kagome structure. Our calculations reveal multiple symmetry-protected topological features, including mirror-symmetry-protected nodal lines near the Fermi level and rotation- and mirror-symmetry-protected triple-point (TP) fermions. Notably, while the TP-induced surface states in MoNiP are subtle, the stronger spin-orbit coupling in WNiP drives the emergence of highly distinct and robust topological surface states. The calculated Fermi surfaces exhibit pronounced nesting characteristics, and the real and imaginary parts of the bare Lindhard susceptibility show corresponding finite-$q$ enhancements at the same wave vectors. This nesting suggests a possible charge-density-wave (CDW) tendency, which is further supported by phonon softening at related wave vectors, with estimated electron-phonon coupling constants of $\lambda \approx 0.80$ and 0.59 for MoNiP and WNiP, respectively. Furthermore, both compounds exhibit sizable spin Hall conductivity driven by enhanced spin Berry curvature, with WNiP exhibiting a larger value of approximately $676(\hslash /e)\mathrm{S}{\mathrm{cm}}^{-1}$ at the Fermi level compared with $241(\hslash /e)\mathrm{S}{\mathrm{cm}}^{-1}$ for MoNiP. Phonon calculations also reveal topological phononic nodal lines together with triple-point bosons. Ultimately, the coexistence of experimentally reported superconductivity, possible CDW tendencies, and robust TP surface states provides a promising platform for investigating possible topological superconducting phenomena, establishing MoNiP and WNiP as ideal candidates for exploring complex correlated quantum phenomena.

Anisotropic superconductivity accompanied by spontaneous symmetry breaking provides key insights into unconventional pairing. Recently, the noble metal alloy ${AB}_4$ has attracted great interest due to the coexistence of superconductivity, topological band structures, and strong spin-orbit interaction. However, the nature of the superconductivity remains elusive. Here, we report signatures of quasi-two-dimensional superconductivity with in-plane anisotropy in a tetragonal ${\mathrm{PtPb}}_4$ crystal, which crystallizes in a geometrically frustrated Shastry-Sutherland (SS) lattice with a unique nonsymmorphic symmetry. Through polar angular-dependent transport measurements, we uncover pronounced out-of-plane anisotropy in the superconducting state, accompanied by evidence of a Berezinskii-Kosterlitz-Thouless transition, indicative of quasi-two-dimensional superconducting behavior. Furthermore, azimuthal angular-dependent measurements reveal a striking in-plane twofold symmetry in both the superconductivity and upper critical field, which disappears in the normal state, suggesting a possible unconventional pairing. Our findings establish a promising platform for further exploring exotic superconducting states such as nematic or topological superconductivity.

$\mathrm{LaCo}{\mathrm{O}}_3$ is a prototypical perovskite oxide, exhibiting simultaneous $\sim 500\text{−}\mathrm{K}$ insulator-metal and spin-state transitions, which have been intensively studied. As is often the case, understanding which of the various coupled degrees of freedom primarily drives the insulator-metal transition (IMT) in $\mathrm{LaCo}{\mathrm{O}}_3$ has proven challenging. We show here that heteroepitaxial strain in thin-film $\mathrm{LaCo}{\mathrm{O}}_3$ can be used to entirely decouple the high-temperature spin-state and insulator-metal transitions, providing important insight. Specifically, the IMT is found to occur independent of the strain-tuned spin state, establishing that it is the IMT in bulk $\mathrm{LaCo}{\mathrm{O}}_3$ that fundamentally drives the high-temperature spin-state transition, not the inverse. The strain dependence of the IMT temperature in thin-film $\mathrm{LaCo}{\mathrm{O}}_3$ is then shown to be remarkably consistent with the trend versus cell volume across the bulk $\mathrm{RCo}{\mathrm{O}}_3$ series ($\mathrm{R}=\mathrm{rare}$-earth). Density-functional theory results on bulk $\mathrm{RCo}{\mathrm{O}}_3$ and biaxially strained $\mathrm{LaCo}{\mathrm{O}}_3$ support interpretation of these results in terms of trends in electronic bandwidths and orbital degeneracy.