The atomic scale origin of the invar effect and the large magnetovolume effect in Fe-Ni alloys is revisited by investigating the Ni-content dependence of bulk modulus $B_0$ of Fe-Fe, Fe-Ni, and Ni-Ni atomic pairs using reverse Monte Carlo modeling with a dataset of extended x-ray absorption fine structure and powder x-ray diffraction under high pressure. To this end, the results of the ${\mathrm{Fe}}_{65}{\mathrm{Ni}}_{35}$ invar alloy were compared with those of Ni-rich ferromagnetic ${\mathrm{Fe}}_{55}{\mathrm{Ni}}_{45}$ and ${\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80}$ alloys. The elongation of Fe-Fe pairs compared to the bond lengths of Fe-Ni and Ni-Ni pairs is commonly observed in the ferromagnetic states of all Fe-Ni alloys. The $B_0$ value of each atomic pair is comparable to the value of bulk $B_0$ when the ferromagnetic phase is stable under high pressure, while the rapid shrinkage of the elongating Fe-Fe pairs results in a smaller value of $B_0$ at the destabilized ferromagnetic region. Therefore, the magnetovolume effect in Fe-Ni alloys is interpreted as a result of the elongating Fe-Fe pairs and their population in the alloy. The elongation and subsequent shrinkage of Fe-Fe pairs at the destabilization of the ferromagnetic order plays a crucial role for initiating the invar effect.

Trillium lattices, in which magnetic ions form a three-dimensional chiral network of corner-sharing equilateral triangular motifs, offer a prominent platform to explore exotic quantum states. In this work, we report ground-state properties of the $S=5/2$ trillium lattice compound ${\mathrm{K}}_2\mathrm{FeSn}{\left({\mathrm{PO}}_4\right)}_3$ through thermodynamic, electron spin resonance (ESR), and muon spin relaxation ($\mu \mathrm{SR}$) experiments. Thermodynamic and ESR measurements reveal the two-step evolution of magnetic correlations across $T^{*}=11\mathrm{K}$, which results from an interplay between dominant antiferromagnetic Heisenberg interactions and subleading interactions. Below $T^{*}$, dc and ac magnetic susceptibilities indicate weak magnetic ordering at $T_{\mathrm{N}}\approx 2\mathrm{K}$ under low fields, which is suppressed for ${\mu }_{0}H\ge 2$ T, consistent with a power-law dependence of magnetic specific heat at low temperatures. $\mu \mathrm{SR}$ experiments confirm the dominance of persistent spin dynamics and the absence of conventional spin freezing, supporting the subtle nature of weak magnetic ordering coexisting with strong spin fluctuations. These findings underscore the potential for realizing a classical spin-liquid ground state with exotic excitations in high-spin trillium lattice systems.

We investigated pressure-induced electronic and structural transitions in the quasi-one-dimensional superconducting materials ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_3\mathrm{I}$ using transport measurements and Raman spectroscopy up to 110 GPa. At ambient pressure, ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_3\mathrm{I}$ exhibits a rare coexistence of weak ferromagnetism, charge density wave, and superconducting (SC) states. Applying pressure suppresses the SC-I phase above 3 GPa, while a distinct high-pressure superconducting state (SC-II) emerges at 6.7 GPa with a maximum $T_c$ onset of ∼9.3 K and persisting up to 110 GPa. Raman evidence of structural transition at 5.6 GPa coincides with SC-II emergence, suggesting a direct link between lattice rearrangement and the emergence of SC-II. Our findings establish ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_3\mathrm{I}$ as a unique platform to study the interplay between structure and SC in low-dimensional superconductors.

We extend the previously defined many-body marker for two-dimensional ${\mathbb{Z}}_2$ topological insulators [I. Gilardoni et al., Phys. Rev. B 106, L161106 (2022)] to distinguish trivial, weak, and strong topological insulators in three dimensions, in the presence of inversion symmetry. The marker is written in terms of ground-state expectation values of position operators and can be employed to detect topological phases of interacting systems beyond mean-field approximations, e.g., within quantum Monte Carlo techniques. Here we show that the correct results of the noninteracting limit are reproduced by the many-body marker.

The real-space variation quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC) are used to calculate the energy bands and the effective mass of the paramagnetic and ferromagnetic two-dimensional uniform electron liquid (2D-UEL). The many-body finite-size errors are minimized by performing simulations for three system sizes with the number of electrons, $N=146$, 218, and 302, for paramagnetic and $N=91$, 139, and 151 for ferromagnetic systems. We consider 2D-UEL to be within the density range $1\le r_s\le 10$. The VMC and DMC results predict that the effective mass $m^{*}$ of the paramagnetic 2D-UEL at high density $r_s=1$ is very close to 1, suggesting that the effective mass renormalization due to electron-electron interaction is negligible. We find that $m^{*}$ of the paramagnetic 2D-UEL obtained by the VMC and DMC methods increases by $r_s$ but with different slopes. Our VMC and DMC results for ferromagnetic 2D-UEL indicate that $m^{*}$ decreases rapidly by reducing the density due to the strong suppression of the electron-electron interaction.

Rare-earth compounds $R\text{CoSi}$ exhibit distinct structural behaviors depending on whether $R$ is a light, middle, or heavy rare-earth element. Notably, CeCoSi exhibits a structural phase transition under pressure, where the transition pressure increases with temperature. Experimental evidence indicates a direct correlation between this transition and the delocalization of $\text{Ce-4}f$ electrons. In this work, we systematically study the evolution of the electronic structure of CeCoSi with temperature and pressure using density functional theory (DFT) + dynamical mean field theory (DMFT). Our results reveal the Kondo metal behavior of CeCoSi at ambient pressure, with hybridization between $\mathrm{Ce}\text{−}4f$ and $\mathrm{Co}\text{−}3d$. Under high pressure, CeCoSi exhibits extremely strong hybridization of $\mathrm{Ce}\text{−}4f$ and $\mathrm{Co}\text{−}3d$ even at high temperatures, while the effective mass and occupation number of $\mathrm{Ce}\text{−}4f$ are significantly reduced. This indicates that pressure causes CeCoSi to transform from a Kondo metal to a mixed-valence state. The quantum transition pressure $P_{\mathrm{Q}}$, though slightly higher than the experimental structural transition pressure $P_{\mathrm{S}}$, shows identical temperature dependence. Notably, the mixed-valence state exhibits significantly enhanced hybridization strength compared to the Kondo metal state. Our results suggest that the valence instability of $\mathrm{Ce}\text{−}4f$ by pressure is the cause of the structural phase transition, possibly by inducing electron density redistribution that destabilizes the lattice.

We derive a $t\text{−}J$ model applicable to strongly correlated two-orbital systems including bilayer nickelate superconductors. Using the Schrieffer-Wolff transformation, we exclude the doubly occupied states, which raise the onsite Coulomb energy, and derive the resulting spin interactions from the two-orbital Hubbard model. We also introduce effective interactions attributed to the interorbital Coulomb interaction. To adapt the effective model to bilayer nickelates that exhibit high-temperature superconductivity, we quantitatively evaluate the strengths of the spin interactions based on the hopping parameters in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. Considering the evaluated effective interactions, we propose a simplified $t\text{−}J$ model for bilayer nickelate superconductors.

We investigate the action of a noninvertible symmetry on spin chains whose topological lines are labeled by representations of the four-dimensional Taft algebra. The main peculiarity of this symmetry is the existence of junctions between distinct indecomposable lines. Sacrificing Hermiticity, we construct several symmetric, frustration-free, gapped Hamiltonians with real spectra and analyze their ground-state subspaces. Our study reveals two intriguing phenomena. First, we identify a smooth path of gapped symmetric Hamiltonians whose ground states transform inequivalently under the symmetry. Second, we find a model where a product state and the so-called $W$ state spontaneously break the symmetry, and propose an explanation for the indistinguishability of these two states in the infinite-volume limit in terms of the symmetry category.

Symmetry classification is crucial in understanding universal properties of quantum matter. Recently, the scope of symmetry classification has been extended to open quantum systems governed by the Lindblad master equation. However, the classifications of Lindbladians and steady states remain largely separate, because the former requires the non-Hermitian classification framework, while the latter relies on the classification scheme for Hermitian matrices. In this paper we build connections between symmetry classes of quadratic Lindbladians and their steady states, despite their different classification frameworks. We classify the full matrix representation of generic quadratic Lindbladians with particle conservation, showing they fall into 27 non-Hermitian symmetry classes. Among these, 22 classes lead to an infinite-temperature steady state. The remaining five classes have one-to-one correspondence with five steady-state Hermitian symmetry classes. Numerical simulations of random Lindbladian dynamics confirm the convergence to the correct steady-state symmetry classes at long time.

We report resonant inelastic x-ray scattering (RIXS) measurements on the prototypical hexagonal boron nitride ($h$-BN) layered compound. The RIXS results at the B and N $K$ edges have been combined with electron energy loss spectroscopy (EELS) experiments and ab initio calculations within the framework of the Bethe-Salpeter equation of many-body perturbation theory. By means of this tight interplay of different spectroscopies, the lowest longitudinal exciton of $h$-BN has been identified. Moreover, its qualitatively different dispersions along the $\mathrm{\Gamma }\mathrm{K}$ and the $\mathrm{\Gamma }\mathrm{M}$ directions of the Brillouin zone have been determined. Our study advocates soft x-ray RIXS and EELS to be a promising combination to investigate electronic excitations in materials.

The effect of an additional quasiresonant drive on the dynamics of the ring-shaped incoherently pumped polariton condensates carrying angular momentum (vorticity) is studied theoretically. Numerical simulations of the 2D and 1D Gross-Pitaevskii equations show that the difference of the topological charges (vorticities) $\mathrm{\Delta }n$ of the condensate and the quasiresonant coherent drive plays a crucial role in the synchronization dynamics. It is shown that in an axially symmetric system, synchronization can only occur if $\left|\mathrm{\Delta }n\right|=0$, whereas in the other cases the phase of the condensate cannot be locked to the phase of the coherent drive. To explain this effect observed in the numerical simulations a perturbation theory is developed. The theory shows that the phase slip between the condensate and the coherent drive can be understood in terms of the motion of $2\pi$ kinks. It is shown that the breaking of the axial symmetry can stop the motion of the kinks, allowing the phase locking of the condensate to the coherent drive.

In the ferrimagnetic semiconductor ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$, a colossal magnetoresistance (CMR) is observed only when a magnetic field is applied along the magnetic hard axis ($\mathbf{H}\parallel c$). This phenomenon suggests an unconventional CMR mechanism potentially driven by the interplay between magnetism, topological band structure, and/or chiral orbital currents (COC). By comparing electrical resistance measurements using continuous direct currents and pulse currents, we found that the current-induced insulator-metal transition, supporting the COC-driven CMR mechanism, is likely a consequence of Joule heating effects. First-principles calculations reveal a pronounced band-gap reduction upon tilting the magnetic moments toward the $c$ axis, accompanied by increased carrier concentration and Fermi velocity. Combining spin-orientation-dependent electronic structure with Boltzmann transport theory, the calculated electrical resistance closely reproduces the CMR observed experimentally. These findings suggest that the CMR in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ stems primarily from band-gap reduction induced by partial polarization of magnetic moments along the magnetic hard axis.

We find a strongly enhanced entanglement within the pseudogap regime of the Hubbard model. This entanglement is estimated from the quantum Fisher information and, avoiding the ill-conditioned analytical continuation, the quantum variance. Both are lower bounds for the actual entanglement and can be calculated from the (antiferromagnetic) susceptibility, obtained here with the dynamical vertex approximation. Our results qualitatively agree with experimental neutron scattering experiments for various cuprates. Theory predicts a $ln(1/T)$ divergence of the entanglement for low temperatures $T$, which is however cut off by the onset of superconductivity.

Silicon carbide is widely used in electronics, ceramics, and renewable energy due to its exceptional hardness and resistance. In this study, we investigate the effects of hydrostatic and uniaxial pressure (both compressive and tensile) on the structural and electronic properties of $3C$-SiC. Our analysis is based on atomistic molecular dynamics (MD) simulations using an efficient tight-binding Hamiltonian, whose accuracy is validated against density functional theory calculations. Moreover, to account for nuclear quantum effects, we employ path-integral MD simulations. Our results show significant changes in the direct electronic gap as a function of temperature and pressure, with a renormalization of $\sim 80$ meV due to zero-point motion. Under hydrostatic tensile pressure, the direct band gap $E_{\mathrm{\Gamma }}$ vanishes at the mechanical stability limit of the material (spinodal point, where the bulk modulus $B\to 0$). For uniaxial pressure, we observe instabilities (Young's modulus $Y\to 0$) at $\sim 90$ GPa for both tension and compression, where $E_{\mathrm{\Gamma }}\to 0$. Additionally, we analyze the pressure dependence of the internal energy, lattice parameter, and bond length, along with their finite-temperature fluctuations, which exhibit anomalies near the instability points.

The geometric structure of quantum states plays a fundamental role in determining the intrinsic dynamics of electrons in solids. In this work, we study the geometric origin of orbital angular momentum and its transport in a general two-band electronic system. Without assuming any symmetry or dimensional constraints, we show that the orbital Berry curvature, which governs the orbital Hall effect, can be universally expressed as the product of the band energy and the square of the Berry curvature. This highlights the central role of Berry curvature in engineering orbital Hall responses. We also discuss the applicability of our framework by analyzing a realistic model. Our findings underscore the geometric universality of itinerant intrinsic orbital dynamics.

$\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$, a representative member of the quaternary diamondlike compound family, exhibits intrinsic Dirac states and promising thermoelectric performance due to its intrinsically low thermal conductivity. However, the physical mechanisms underlying its ultralow κ remains poorly understood. Here, we successfully control the concentration of $\mathrm{C}{\mathrm{u}}_{\mathrm{Hg}}$ antisite defects (ASDs) in $\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$ single crystals by tuning the growth conditions. The two types of $\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$ crystal samples are found to have very different $\mathrm{C}{\mathrm{u}}_{\mathrm{Hg}}$ ASDs concentrations using the single-crystal x-ray structure refinements. Thermal conductivity measurements at room temperature show that the sample with a high concentration of ASDs exhibits an ultralow lattice thermal conductivity of only $0.44\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, nearly half of that of the low-defect sample. First-principles calculations for the two $\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$ crystals with and without ASDs are in good agreement with the experimental data, confirming the pivotal role of $\mathrm{C}{\mathrm{u}}_{\mathrm{Hg}}$ ASDs in suppressing lattice thermal conductivity. These findings provide direct insight into the structure–property relationship in $\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$ and demonstrate that defect engineering is an effective strategy to tailor thermal transport in quaternary diamondlike semiconductors, with implications for enhanced thermoelectric and photovoltaic performance.

This study investigates the low-temperature vibrational and magnetic properties, along with the high-pressure structural and vibrational behavior, of the honeycomb magnetoelectric ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$. Raman spectroscopy and magnetic susceptibility measurements reveal pronounced spin-phonon coupling up to $\sim 250$ K (${\mathrm{T}}_{\mathrm{sro}}$), accompanied by local symmetry breaking linked to magnetic short-range order. High-pressure Raman spectroscopy and synchrotron radiation x-ray diffraction identify three isostructural phase transitions at $\sim 3, \sim 5.5$, and $\sim 7.7$ GPa, followed by two long-range crystal symmetry transitions at $\sim 13$ and $\sim 18.4$ GPa. The first symmetry transition distorts the P-3c1 phase partially into P2/c symmetry, while the second transition introduces an additional monoclinic C2/c phase. These structural transitions align well with our density functional theory based calculations. Partial density of states and Bader charge analyses further reveal charge redistribution between Co and O atoms as a key factor driving the onset of long-range structural transition under high pressure. The stronger renormalization of Raman modes at low temperatures, combined with the lower-pressure structural phase transitions and distinct symmetry sequence compared to the related compound ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$, reflects the enhanced spin-orbit coupling introduced by Ta substitution. Notably, anomalies in Raman mode linewidths and the appearance of new modes $[\omega$(1) and $\omega$(2)] in the 3–7.7 GPa pressure range resemble the low-temperature magnetic behavior, indicating persistence of potential short-range interactions at high pressures. These findings highlight the impact of heavier $5d$-Ta compared to $4d$-Nb on magnetolattice coupling and pressure-induced phenomena in $A_4{B}_2{\mathrm{O}}_9$ ($A$ = Fe, Co, Mn and $B$ = Nb, Ta) systems, offering valuable insights for future research on these materials.

The Nernst effect is a versatile phenomenon relevant for energy harvesting, magnetic sensing, probing band topology, and charge-neutral excitations. The planar Nernst effect (PNE) generates an in-plane voltage transverse to an applied temperature gradient under an in-plane magnetic field. Conventional Berry curvature (BC)-induced PNE is absent in two-dimensional (2D) systems, as the out-of-plane BC does not couple to the in-plane electron velocity. Here, we challenge this notion by demonstrating a distinct PNE in quasi-2D materials (2DPNE). We show that the 2DPNE originates from previously overlooked planar components of BC and orbital magnetic moment, arising from interlayer tunneling in multilayered 2D systems. We comprehensively analyze the band-geometric origin and crystalline symmetry constraints on 2DPNE responses. We illustrate its experimental feasibility in strained bilayer graphene. Our findings significantly expand the theoretical understanding of PNEs, providing a clear pathway for next-generation magnetic sensing and energy-harvesting applications.