Crystal field (CF) excitations in the ground multiplet ${}^3{H}_6$ of $\mathrm{T}{\mathrm{m}}^{3+}$ ions were investigated using terahertz transmission spectra of magnetoelectric (ME) $\mathrm{TmA}{\mathrm{l}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ and $\mathrm{T}{\mathrm{m}}_{0.05}\mathrm{Y}{\mathrm{b}}_{0.1}{\mathrm{Y}}_{0.85}\mathrm{A}{\mathrm{l}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$. These excitations were identified as mainly magnetic dipole transitions from the ground singlet $A_1$ to the next excited doublet $E$, split by the CF of the $D_3$ symmetry. The fine structure of the modes was resolved at low temperatures. It manifested differently in lightly doped and in pure Tm borates, consistent with different distortions of the local CF with $D_3$ symmetry. Strong natural optical activity was observed near the CF transitions resulting in a polarization plane rotation up to 25°. The optical activity is quantitatively described by contributions from magnetic and electric dipole transitions to dynamic ME susceptibility and considering the classification of local distortions.

The magnetic anisotropy energy (MAE) in the honeycomb lattice $\mathrm{Cr}{\mathrm{I}}_3$ has attracted much attention due to the fact that it is crucial for the stability of two-dimensional (2D) long-range magnetism. Numerous calculations and experiments demonstrate that $\mathrm{Cr}{\mathrm{I}}_3$ exhibits Kitaev interaction, which is typically linked to edge-sharing octahedral coordination. However, significant lattice distortions in $\mathrm{Cr}{\mathrm{I}}_3$ disrupt the ideal octahedral geometry, leading to additional off-diagonal interactions beyond the conventional Kitaev term. Here we perform a systematic first-principles analysis of the MAE in monolayer $\mathrm{Cr}{\mathrm{I}}_3$ and unambiguously quantify the symmetric off-diagonal exchange ${{\mathrm{\Gamma }}^{\prime }}_1$ in the ferromagnetic (FM) state, a term largely ignored in previous $J\text{−}K\text{−}\mathrm{\Gamma }\text{−}{A}_{\mathbf{k}}$ models. We using a combined energy mapping and angular-dependent MAE approach to determine that $\mathrm{Cr}{\mathrm{I}}_3$ indeed manifests a multineighbor $J\text{−}{K}_1\text{−}{{\mathrm{\Gamma }}^{\prime }}_1\text{−}{K}_2\text{−}{A}_{\mathbf{k}}$ model. Moreover, the ${{\mathrm{\Gamma }}^{\prime }}_1$ contribution in the FM phase exceeds all other anisotropic terms, and meanwhile the single-ion anisotropy $A_{\mathbf{k}}$ largely compensates the remaining bond-dependent contributions. Therefore, ${{\mathrm{\Gamma }}^{\prime }}_1$ not only provides the principal stabilization of 2D FM but also sets the out-of-plane easy axis. Finally, we analyze the strain dependence of interaction parameters and find that lattice expansion amplifies the Kitaev-related terms, highlighting a tunable pathway for spin-orbit coupling enhancement in 2D magnets. Our methodology leverages continuous variation MAE principles to enable precise decomposition of magnetic parameters and rigorously demonstrate the interactions.

Controlling nonequilibrium dynamics in quantum materials requires ultrafast probes with spectral selectivity. We report femtosecond reflectivity measurements on the cuprate superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ using free-electron laser extreme-ultraviolet (23.5–177 eV) and near-infrared (1.5 eV) pump pulses. EUV pulses access deep electronic states, while NIR light excites valence-band transitions. Despite these distinct channels, both schemes produce nearly identical dynamics: above $T_c$, excitations relax through fast (100–300 fs) and slower (1–5 ps) channels; below $T_c$, a delayed component signals quasiparticle recombination and condensate recovery. We find that when electronic excitations are involved, the ultrafast response is governed mainly by absorbed energy rather than by the microscopic nature of the excitation. In contrast, bosonic driving in the THz or midinfrared produces qualitatively different dynamics. By demonstrating that EUV excitation of a correlated superconductor yields macroscopic dynamics converging with those from optical pumping, this work defines a new experimental paradigm: FEL pulses at core-level energies provide a powerful means to probe and control nonequilibrium electronic states in quantum materials on their intrinsic femtosecond timescales. This establishes FEL-based EUV pumping as a new capability for ultrafast materials science, opening routes toward soft x-ray and attosecond studies of correlated dynamics.

We report results of a study of strongly correlated $d$-wave superconducting slabs within a $t$-J model solved using the statistically consistent Gutzwiller approach. For different dopings, we model slabs cut at ${45}^{\circ }$ relative to the main crystallographic $ab$ axes, i.e., [110] orientation, and conclude that quasiparticle charge is drawn to the edges. Thus, the correlations are locally strengthened, and a region near the edge is closer to the Mott- insulating state with reduced hopping amplitudes on the links. Superconductivity is locally weakened near the edges in the correlated state, and the spectral weight of zero-energy Andreev bound states is substantially reduced compared to weak coupling theory. The renormalization of the edge states leaves no room for an extended $s$-wave component to form for hole dopings ranging from strongly underdoped to slightly overdoped.

We present a probe for the first-order transition for layered vortex matter: A second step in the screening of an ac field that is independent of the frequency and amplitude of the excitation. This second step is observed in the intermediate temperature and field ranges where detecting the jump in induction associated with the transition is rather elusive with standard magnetometry techniques. We observe this second step following an experimental protocol where the screening of a locally generated ac field is remotely detected in another region of the sample. The coincidence of the typical temperature of the second step in direct and remote measurements strongly supports that this feature is a probe of the first-order transition. This nonlocal effect detected at distances of thousands of vortex lattice spacings away indicates that a very efficient mechanism propagates the change in rigidity of the structure from the more (liquid) to the less (solid) symmetric vortex phases.

Quantum impurities give rise to rich physical phenomena, with some exhibiting critical behavior described by conformal field theories (CFTs) in the low-energy limit. In parallel, party-time ($\mathcal{PT}$) symmetric non-Hermitian systems host exceptional points (EPs) at criticality, leading to exotic features governed by nonunitary CFTs. Here, we establish a connection between non-Hermitian impurities and CFTs by demonstrating that the critical properties of a $(1+1)$-dimensional free-fermion chain with central charge $c=1$ can be drastically altered by the presence of a local non-Hermitian impurity. Through a systematic analysis of entanglement/Rényi entropy, the finite-size scaling of the many-body spectrum, and fidelity susceptibility, we identify that this impurity-induced non-Hermitian criticality is characterized by a nonunitary CFT with central charge $c=-2$. Furthermore, we find that these nonunitary critical properties exhibit strong sensitivity to boundary conditions.

With x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific electronic structure and the magnetic interactions of the double perovskite compounds ${\mathrm{PrCo}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$ and ${\mathrm{LaCo}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$ were studied. In both compounds, Co and Ni depart from their nominal valence state of $+3$, implying a charge transfer from Ni to Co. XMCD measurements clearly reveal a ferromagnetic behavior of Co and Ni in both samples. The substitution of La by Pr is isovalent but leads to a partial spin-state transition of ${\mathrm{Co}}^{3+}$ from high spin to low spin, resulting in a significant decrease in the magnetic moment of Co. Furthermore, a magnetic moment is found on the Pr site as well, with the Pr spin moment oriented antiparallel to those of Co and Ni. This antiferromagnetic coupling is induced by the superexchange interaction between the Pr $4f$ and Co/Ni $3d$ states.

We investigate the emergence of correlated electron phases in rhombohedral $N$-layer graphene due to two-valley Coulomb interactions within a low-energy $k·p$ framework. Analytical expressions for Lindhard susceptibilities in intra- and intervalley channels are derived, and the critical temperatures for phase transitions are estimated using both the random phase approximation (RPA) and the parquet approximation (PA). Within RPA, only Stoner and intervalley coherent (IVC) phases are supported, while the PA reveals a richer phase structure including particle-particle (PP) channel instabilities. We establish a general scaling law for the critical temperature with respect to layer number $N$, highlighting an upper bound as $N\to \infty$, and demonstrate a nonmonotonic decrease of the critical temperature with increasing chemical potential. The PA uncovers the role of interaction symmetry: $SU\left(4\right)$-symmetric interactions favor intervalley Stoner order in the density channel, whereas $SU\left(2\right)\times SU\left(2\right)$-symmetric interactions permit a broader set of phases. A crossover in the dominant instability occurs in the particle-hole channel at a critical layer number, suggesting the emergence of magnetic or IVC phases in thicker systems. We also identify conditions under which pair-density wave (PDW) order could form in the PP channel, though its physical realization may be constrained.

Twist engineering in van der Waals heterostructures has unlocked a range of novel physical phenomena, yet its influence on lattice thermal transport—particularly in heterostructures—remains underexplored. In this study, we employ a neuroevolution potential (NEP) based atomistic framework, combining homogeneous nonequilibrium molecular dynamics (HNEMD) and the Boltzmann transport equation, to investigate twist-angle–dependent thermal conductivity in graphene–hexagonal boron nitride bilayers. Our simulations reveal a pronounced nonmonotonic trend, with thermal conductivity peaking at a twist angle of 17.9°, corresponding to a remarkable 125% enhancement over the untwisted $\mathit{AA}$-stacked configuration. This behavior starkly contrasts with twisted homostructures, where introducing twist typically suppresses thermal conductivity due to enhanced phonon scattering. Mode-resolved analysis attributes this anomalous enhancement to in-plane basal phonons, the transport of which is promoted by reduced interlayer coupling and suppressed anharmonicity at intermediate angles. Additionally, the NEP-based HNEMD approach demonstrates exceptional computational efficiency and predictive accuracy, enabling large-scale simulations of moiré supercells beyond the reach of traditional first-principles methods. These findings reveal a twist-enabled mechanism for thermal transport enhancement in two-dimensional heterostructures, offering opportunities for phonon engineering and nanoscale thermal management.

Understanding the timescales associated with relaxation to equilibrium in closed quantum many-body systems is one of the central focuses in the study of their nonequilibrium dynamics. At late times, these relaxation processes exhibit universal behavior, emerging from the inherent randomness of chaotic Hamiltonians. In this work, we investigate a disordered spin-$1/2$ XX ladder—an experimentally realizable model known for its diffusive dynamics—to explore the connection between transport properties and spectral measures derived solely from the energy levels of the system via these relaxation timescales. We begin by analyzing the spectral form factor, which yields the time when the system begins to follow the random matrix theory (RMT) statistics, known as the RMT time. We then determine the Thouless times—the average times for a local excitation to diffuse across the entire finite system—through the linear-response theory for both spin and energy transport. Our numerical results confirm that the RMT time scales quadratically with system size and upper bounds the Thouless times. Interestingly, we also find that, unlike other nonintegrable models, spin diffusion proceeds faster than energy diffusion.

We report systematic torque magnetometry measurements to investigate the electronic properties of the newly discovered kagome compound ${\mathrm{CaTi}}_3{\mathrm{Bi}}_4$. Electrical transport, magnetic susceptibility, and thermal measurements reveal no evidence of a magnetic ground state in this material. Torque data obtained in magnetic fields up to 41.5 T exhibit clear de Haas–van Alphen (dHvA) oscillations, with nine distinct frequencies ranging from 13 to 6164 T. Angular-dependent dHvA measurements show that, with the exception of the lowest frequency (13 T), all observed frequencies nearly follow a $1/cos\theta$ dependence, where $\theta$ is the angle between the crystallographic $c$ axis and the magnetic field direction. This behavior is characteristic of quasi-two-dimensional Fermi-surface sheets with nearly circular cross sections. To further elucidate the electronic structure, we performed density functional theory (DFT) calculations of the band structure and Fermi surface. The calculated bands reveal the presence of multiple Dirac points (DP), flat bands (FB), and van Hove singularities (VHS) near the Fermi level. The resulting Fermi surface consists of several quasi-two-dimensional cylindrical sheets, consistent with the experimentally observed $1/cos\theta$ dependence. Notably, the theoretical dHvA frequencies, derived from extremal Fermi-surface cross-sectional areas, agree well with the experimental values and reproduce their angular dependence. Remarkably, all experimentally observed frequencies are captured by the DFT predictions. Pressure-dependent calculations up to 10 GPa show that the electronic features—DP, FB, and VHS—evolve systematically with pressure. In particular, the VHS shifts closer to the Fermi level, demonstrating that pressure acts as an effective tuning parameter in this material. These combined experimental and theoretical results provide a comprehensive understanding of the electronic structure of ${\mathrm{CaTi}}_3{\mathrm{Bi}}_4$ and demonstrate how pressure can be used to tune its key electronic features, offering valuable guidance for exploring related kagome materials.

Recently plasmon excitations in bilayer lattice systems were studied extensively in the weak-coupling regime. Unlike single-layer systems, these bilayers exhibit two distinct modes, ${\omega }_{\pm }$, which show characteristic dependences upon the momentum and hopping integrals along the $z$ direction. To apply them to cuprates, strong correlation effects should be considered, but a comprehensive analysis has not yet been investigated. In this work, we present a strong-coupling theory to analyze the charge dynamics of a bilayer system, utilizing the $t\text{−}J\text{−}V$ model, which includes the long-range Coulomb interaction, $V$, on a lattice. Although our theoretical framework is fundamentally different from the weak-coupling approach, we find that resulting plasmon excitations are similar to those of a weak-coupling theory. A key distinction is that our strong-coupling framework reveals a noticeable suppression of particle-hole excitations, which allows the plasmon modes to remain well defined over a wider region of momentum. We suggest that the experimentally reported plasmon excitations in Y-based cuprates can be described by the ${\omega }_{-}$ mode, although we call for more systematic experiments to verify this.

We broaden the scope of quantum field theory by introducing a general class of discrete gauge theories that realize either topological order or fracton behavior across dimensions. We start from translation-invariant systems endowed with unconventional charge-conservation laws, which we term exponential polynomial symmetries. Gauging these symmetries yields ${\mathbb{Z}}_N$ gauge theories in 2D that exhibit topological order whose quasiparticles have constrained mobility and whose ground-state degeneracy shows ultraviolet (UV) dependence. These features are reminiscent of spatial symmetry-enriched topological order, wherein quasiparticle excitations transform nontrivially under lattice translations. We further propose a Chern-Simons variant that produces non-CSS stabilizer codes and outline a framework for exponentially symmetric subsystem SPT phases. Finally, we extend this gauging procedure to 3D, obtaining variants of fracton topological order.

We report proximate quantum spin liquid behavior in ${\mathrm{K}}_2{\mathrm{Co}}_2$(${\mathrm{SO}}_4{)}_3$ with the magnetic ${\mathrm{Co}}^{2+}$ ions embedded on a highly frustrated three-dimensional double trillium lattice. Single-crystal and high-resolution synchrotron powder x-ray diffraction experiments reveal a structural phase transition at $T_{\mathrm{t}}\simeq 125\mathrm{K}$ from high-temperature cubic to low-temperature monoclinic phase with the threefold superstructure. Magnetization and heat capacity consistently show the formation of the $J_{\mathrm{eff}}=1/2$ state of ${\mathrm{Co}}^{2+}$ below 50 K. In zero field, ${\mathrm{K}}_2{\mathrm{Co}}_2$(${\mathrm{SO}}_4{)}_3$ shows signatures of static magnetic order formed below $T^{*}\simeq 0.6\mathrm{K}$, but muon spin relaxation experiments reveal a large fluctuating component that persists down to at least 50 mK, reminiscent of quantum spin liquid (QSL). Static order is completely suppressed in the small magnetic field of $\sim 1\mathrm{T}$, and low-temperature heat capacity demonstrates the $T^2$ behavior above this field, another fingerprint of QSL. Ab initio calculations show a competition of several antiferromagnetic couplings that render ${\mathrm{K}}_2{\mathrm{Co}}_2$(${\mathrm{SO}}_4{)}_3$ a promising pseudospin-$\frac{1}{2}$ material for studying quantum magnetism in the double trillium lattice geometry.

Although ferroelectric materials show great promise for nonvolatile memory applications, conventional bulk ferroelectrics suffer from performance degradation as device dimensions are reduced to the nanoscale, owing to increased polarization-reversal cycles and sensitivity to defects. Interestingly, two-dimensional sliding ferroelectrics, featuring atomic-scale thickness and a unique interlayer-sliding polarization switching mechanism, offer unique opportunities for device miniaturization. However, whether unavoidable common local point vacancy defects and adsorption of ambient gas molecules impact their performance as in traditional bulk ferroelectrics remain unclear. Herein, we conduct a systematic investigation into the effects of local point vacancy defects and adsorption of ambient gas molecules (${\mathrm{O}}_2, {\mathrm{CO}}_2, {\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{H}}_2$) on representative two-dimensional sliding ferroelectric materials, including h-BN, 3R-${\mathrm{MoS}}_2$, γ-InSe, and ${\mathrm{WGe}}_2{\mathrm{N}}_4$. We find that the influence of local symmetry breaking on polarization fluctuations is minimal (within $\pm 10\%$). Specifically, the polarization switching in sliding ferroelectrics occurs through the relative sliding of atomic layers—without breaking or reconstructing intralayer chemical bonds—which contrasts with conventional bulk ferroelectrics that rely on intralayer ionic displacement. Moreover, even under local symmetry breaking, the switching energy barrier remains low (on the order of meV per atom). This work paves the way for designing next-generation miniaturized ferroelectric devices with superior reliability and defect resilience.

A comprehensive theoretical study of MnS is conducted using linear response constrained density functional theory to determine the Hubbard $U$ parameter, structure searching to predict high-pressure phases, and first-principles calculations to investigate phase transition behavior and electronic structure. To validate the predicted high-pressure phases, theoretical x-ray diffraction patterns are compared with experimental data from the literature. Theoretical Raman and infrared spectra are also provided. Our findings predict a previously unreported hexagonal phase of MnS, revise its phase transition sequence, and elucidate the underlying physical mechanisms. With increasing pressure, MnS undergoes sequential phase transitions from the $Fm\overline{3}m$ phase to the $P{6}_3/mmc$ and further to the $Pnma$ structure, with the latter transition accompanied by Jahn-Teller distortion, spin crossover, and negative charge transfer. Both $P{6}_3/mmc$ and $Pnma$ phases exhibit antiferromagnetic order with prominent altermagnetic characteristics. These findings elucidate the lattice structures, electronic structures, and altermagnetism of MnS under high pressure, establishing essential theoretical foundations for altermagnetic materials.