Eigenstate thermalization refers to the property that an energy eigenstate of a many-body system is indistinguishable from a thermal equilibrium ensemble at the same energy as far as expectation values of local observables are concerned. In systems with degeneracies, the choice of an energy eigenbasis is not unique and the fraction of basis states exhibiting eigenstate thermalization can vary. We present a simple example where this fraction vanishes in the thermodynamic limit for one basis choice, but remains nonzero for another choice. In other words, the weak eigenstate thermalization hypothesis is satisfied in the first, but violated in the second basis. We furthermore prove that degeneracies must abound whenever a system is simultaneously symmetric under spatial translations and reflection. Finally, we derive general bounds on how strongly eigenstate thermalization may depend on the choice of the basis, and we reveal some interesting implications regarding the temporal relaxation properties of such systems.

In the study of open quantum systems it is often useful to treat mixed states as pure states of a fictitious doubled system. In this work we explore the opposite approach: mapping isolated bilayer systems to open monolayer systems. Specifically, we show that arbitrary bilayer Hamiltonians possessing an antiunitary layer exchange symmetry, and subject to a constraint on the sign of interlayer couplings, can be mapped to Lindbladians on a monolayer system with some of the jump operators postselected on a fixed outcome (“monitored”). Low-energy states of the bilayer Hamiltonian then correspond to late-time states of the monolayer dynamics. Simulating the latter by quantum trajectory methods has the potential of substantially reducing the computational cost of estimating low-energy observables in the bilayer Hamiltonian by effectively halving the system size. The overhead due to sampling quantum trajectories can be controlled by a suitable importance sampling scheme. We show that, when the quantum trajectories exhibit free fermion dynamics, our approach reduces to the auxiliary field quantum Monte Carlo (AFQMC) method. This provides a physically transparent interpretation of the AFQMC sign-free criteria in terms of properties of quantum dynamics. Finally, we benchmark our approach on the 1D quantum Ashkin-Teller model.

Antiferromagnetic (AFM) materials show significant potential for spintronic applications because of their fast spin dynamics, magnetic stability, and uncompensated magnetic moments, which can be leveraged for information storage. In this study, we investigate long-range coupling between single-crystalline ${\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}$ (FeMn) layers separated by a Cu spacer in fully epitaxial 8-monolayer (ML) FeMn/Cu/6-ML FeMn/Co/Fe/Cu(001) heterostructures. We observe a periodic modulation of the induced perpendicular magnetic anisotropy as a function of Cu spacer thickness, with a period of approximately 5.5–6 ML. In contrast, control samples [8-ML FeMn/Cu/6-ML FeMn/Co/Cu(001)], designed with in-plane magnetization and interfacial coupling, show a similar modulation phase but a weaker variation in coupling strength. These findings provide clear evidence of: (1) long-range exchange coupling between spatially separated AFM FeMn layers across the Cu spacer, and (2) a periodic modulation of long-range AFM ordering in the FeMn films, driven by this interlayer coupling as a function of Cu spacer thickness, where this behavior is primarily governed by the out-of-plane uncompensated moments of the FeMn layers, stemming from their $3Q$-type noncollinear AFM spin structure. Our results emphasize the crucial role of AFM spin structure in mediating long-range magnetic interactions, offering valuable insights for designing next-generation AFM/normal-metal-based spintronic heterostructures.

The discovery of high-temperature superconductors remains a central challenge in materials science. Hydrogen-rich compounds are among the most promising candidates, as they can exhibit phonon-mediated superconductivity at elevated critical temperatures, though their stabilization typically requires extreme pressures. Here, we report the identification of ${\mathrm{YSbH}}_6$ as a promising superconductor by a multistage high-throughput screening on ternary A15-type hydrides, followed by a high-throughput computational search of the Y-Sb-H system, accelerated by ephemeral data derived potentials. The cubic $Pm\overline{3} {\mathrm{YSbH}}_6$ phase exhibits a predicted critical temperature of 118 K at 50 GPa, among the highest $T_{\mathrm{c}}$ reported to date for an A15 hydride at this pressure. Thermodynamic analysis shows that ${\mathrm{YSbH}}_6$ lies $\sim 100\mathrm{meV}/$ atom above the convex hull at 50 GPa, but only 26 meV/atom above the hull at 120 GPa, suggesting possible metastability and synthesis at similar high-pressure conditions. The phase is dynamically stable over a wide pressure range (20–120 GPa), displays kinetic stability at 50 GPa, and elastic stability at 20 and 50 GPa, key ingredients for long-lived metastable behavior at moderate pressures. These results highlight ${\mathrm{YSbH}}_6$ as a benchmark case illustrating the balance between high-$T_{\mathrm{c}}$ performance and limited thermodynamic stability in ternary hydrides, and underscore the importance of combined dynamic, thermodynamic, kinetic, and elastic stability analyses for guiding experimental synthesis of metastable superconductors.

We demonstrate an approach for the selective modulation of the $A_{1\mathrm{g}}$ and ${}^1{E}_{\mathrm{g}}$ optical phonon amplitudes in bismuth thin films. Combining polarization-sensitive excitation with controlled pump-pulse delays, this is shown to enable the targeted manipulation of the reflectivity signals associated with distinct phonon modes. Employing an electro-optic detection scheme reveals that the observed changes arise from selective amplification or suppression of specific symmetry components within the detected signal. This combined excitation and detection strategy is broadly applicable to other material systems and may facilitate pathways for ultrafast phononic information processing and nanoscale thermal management, critical for next-generation optoelectronic technologies.

Periodically driven systems provide unique opportunities to investigate the dynamics of topological excitations far from equilibrium. In this paper, we report a time-crystalline soliton (TCS) state in a driven-dissipative ${\varphi }^4$ model. This state exhibits spontaneous breaking of discrete time-translational symmetry while simultaneously displaying spatial soliton behavior. During time evolution, the soliton pattern periodically oscillates between kink and antikink configurations. We further study TCS dynamics under noise, demonstrating that soliton random walk can induce a dynamical transition between two distinct $Z_2$ symmetry-breaking time-crystalline phases in time domain. Finally, we examine the annihilation of two spatially separated TCSs under noise. Importantly, in contrast to the confined behavior of time-crystalline monopoles reported in Yue and Cai [Phys. Rev. Lett. 131, 056502 (2023)], the dynamics of time-crystalline solitons is deconfined despite the nonequilibrium nature of our model: the statistically averaged annihilation time scales as a power law with the solitons' initial separation.

We theoretically study terahertz pump-probe experiments in $h\mathrm{BN}$ from the perspective of Floquet optical materials and photonic time crystals. The anisotropic nature of this material results in a large separation of frequencies of in-plane and out-of-plane optical phonons and leads to several branches of phonon-polariton excitations. We consider a slab of finite thickness pumped at the frequency of middle polaritons, around 25 THz. We theoretically analyze properties of the pump induced state and identify several interesting features, including possible light amplification and parametric instability at 2 THz. We relate these features to resonant pair scattering processes of polaritons excited by the pump pulse. We show that amplification/instability frequency ranges depend on both the pump intensity and the slab thickness. We point out that this setup can be utilized to strongly enhance optical nonlinearities at terahertz frequencies. Richness and tunability of this system suggest that it holds promise for practical applications in terahertz technology.

Excitation and control of antiferromagnetic magnon modes lie at the heart of coherent antiferromagnetic spintronics. Here, we propose a topological surface magnon-polariton as a new approach in the prototypical magnonic material hematite. We show that in an insulating canted antiferromagnet, where strong-coupled magnon-photon modes can be achieved using on-chip microwave layouts, a surface magnon-polariton mode exists in the gap of the bulk magnon-photon bands. The emergence of surface magnon-polariton mode is further attributed to the nontrivial topology of bulk magnon-photon bands. Magnon-photon coupling enhances the Berry curvature near the anticrossing points, leading to a topological bulk Chern band associated with the surface magnon-polaritons. Our work provides a general principle for the utilization of optomagnetic properties in antiferromagnets, with an illustration of its experimental feasibility and wide generality as manifested in hematite.

Nematic aerogel (nAG) supports so-called polar phase in liquid ${}^3\mathrm{He}$. The experiments [Dmitriev et al., JETP Lett. 112, 780 (2020)] showed that the onset of polar phase inside the nAG is accompanied by emergence of a sound wave with frequency quickly growing with cooling down from transition temperature and reaching a plateau. To describe this behavior, we start by calculating the elastic properties of the dry nematic AG that appear to depend only on Young's modulus of the parent material (e.g., mullite), the volume fraction of the solid phase $\psi (=5\%)$ and the aspect ratio of the representative volume of nAG. The elastic constants are then used to solve elastohydrodynamic equations for various sound vibrations of nAG filled with ${}^3\mathrm{He}$. The (isotropic) first sound and anisotropic second sound in the polar phase are strongly hybridized with fourth sound and standard elastic modes in nAG. The hybrid second and the transverse fourth sound start with zero velocity at the transition, similar to pure ${}^3\mathrm{He}$, and quickly grow with lowering temperature until they hit the sample finite size cutoff.

The attractive Hubbard model plays a paradigmatic role in the study of superconductivity (superfluidity) and has become directly realizable in ultracold atom experiments on optical lattices. However, the critical temperatures, $T_c$'s, remain lower than the lowest temperatures currently achievable in experiments. Here, we explore a possible route to enhance $T_c$ by introducing an additional next-nearest neighbor (NNN) hopping, $t^{\prime }$, in a two-dimensional square lattice. We perform sign problem-free determinant quantum Monte Carlo simulations to compute response functions such as pairing correlation functions, superfluid density, and uniform spin susceptibility. Our results show that a judicious choice of $t^{\prime }$ can increase $T_c$ by up to $50\%$ compared to the case with only nearest neighbor hopping. In contrast, the preformed pairs' temperature scale, named pairing temperature, $T_p$, decreases with increasing $|t^{\prime }/t|$, which should represent a reduction of the pseudogap region, favoring a more BCS-like behavior at intermediate coupling. We further analyze the interacting density of states to characterize the transition from a pseudogap regime to a fully gapped superconducting state. These findings suggest that NNN hopping could be a viable route to increase $T_c$ to values closer to experimentally accessible temperature scales.

Non-Fermi liquids in dimensions $d>2$ remain poorly understood, particularly when relevant perturbations destabilize them. A prominent example is the overscreened Kondo fixed points in impurity models, and their one-dimensional analogs known as chirally stabilized fixed points. While such models can potentially host non-Abelian anyons, their extension to genuinely higher-dimensional, topologically ordered states has lacked a controlled framework. Here we develop a Wilsonian operator-product-expansion renormalization-group framework and use it to construct higher-dimensional analogs of overscreened fixed points in arbitrary dimension $d\le 4$. We show that these models are governed by a nontrivial conformal fixed point and uncover a mechanism by which relevant perturbations destroy this criticality: symmetry-breaking masses generate nonperturbative corrections that drive the system away from the conformal point toward strong coupling, triggering dynamic mass generation. This provides a controlled pathway from a non-Fermi-liquid metal to a gapped phase with a rich order-parameter manifold.

A generic question in the field of ultrafast dynamics is concerned with the relaxation dynamics and the subsequent thermalization of optically excited charge carriers. Among several possible relaxation channels available in a solid-state system, we focus on the coupling to magnetic excitations. In this paper, we study the real-time dynamics of a paradigmatic model, the Kondo lattice model in one dimension. We conduct a comprehensive study of the relaxation processes by evaluating the spin polarization of the conduction electron, the local spin-spin correlation between localized and conduction electrons, and the electronic momentum distribution. While in the well-studied cases of one or two charge carriers in a ferromagnetic background, no thermalization occurs, we demonstrate that the stationary state is compatible with thermalization if either the electronic filling is finite or the magnetic background is in the singlet sector. Our real-time simulations using the time-dependent Lanczos method are corroborated by a direct comparison with finite-temperature expectation values and an analysis of the spectrum in terms of the gap ratio.

We analytically study the interaction between a single confined spin with a four-level $\mathrm{\Pi }$ configuration in an optical cavity and a single photon. The spin's ground and excited manifolds are each split into two levels coupled by circularly polarized optical transitions. Starting from the spin in an excited state, we analyze the spontaneous emission of the photon, which leaves the spin and photon entangled, and then follow the subsequent time evolution of the system under delayed feedback. We examine how photon reabsorption depends on energy splittings in both the ground and excited spin states, and how the feedback delay influences the degree of spin-photon entanglement. Finally, we identify the conditions that maximize this entanglement after photon reabsorption and re-emission.

The electric Hall effect (EHE) is a newly identified Hall effect characterized by a perpendicular electric field inducing a transverse charge current in two-dimensional (2D) systems. Here, we propose a spin and valley version of EHE in ballistic tunnel junctions. We demonstrate that the transverse spin and valley currents can be generated in an all-in-one tunnel junction based on a buckled 2D hexagonal material in response to a perpendicular electric field, referred to as the electric spin and valley tunneling Hall effects, respectively. These effects arise from the perpendicular-electric-field-induced backreflection phase of electrons in the junction spacer, independent of Berry curvature. The valley Hall conductance exhibits an odd response to the perpendicular electric field, whereas the spin Hall conductance shows an even one. The predicted effects can further enable the transverse separation of a pair of pure spin-valley-locked states with full spin-valley polarization while preserving time-reversal symmetry, as manifested by equal spin and valley Hall angles. Our findings present a mechanism for realizing the spin and valley Hall effects and provide a route to the full electric-field manipulation of spin and valley degrees of freedom, with significant potential for future applications in spintronics and valleytronics.

Growing research focuses on chiral quasibound states in the continuum (q-BICs) due to urgent needs in left-/right-circularly polarized (LCP/RCP) detection and absorption devices. Conventional chiral q-BICs suffer from quickly decaying quality factors (Q factors), making robust high-Q responses critical. Here, we present a planar chiral metasurface absorber with dynamically tunable chirality, enabled by a periodically arranged sixfold-symmetric double-skeleton-fan (DSF) structure and backed by a metallic mirror. Introducing gap perturbations folds guided modes above the light cone via Brillouin zone folding (BZF), creating robust high-Q BZF-BICs with topological singularities. Breaking in-plane ${\mathrm{C}}_2$ symmetry transforms BZF-BICs into high-Q q-BZF-BICs and generates a pair of C points, achieving strong extrinsic absorption circular dichroism (absorption $\mathrm{CD}=-0.96$, $Q>{10}^7$) and near-perfect RCP absorptance (0.997) verified by temporal coupled-mode theory. Breaking $z$ symmetry reveals a BZF-accidental BIC (BZF-A-BIC) on the guided resonance band, directly enabling high-Q intrinsic chiral absorptance (absorption $\mathrm{CD}=-0.87$, $Q>{10}^6$). Notably, we demonstrate dynamically tunable intrinsic chirality with sign reversal (absorption ${\mathrm{CD}}_{\text{max}}=0.91$ and tunable range ${\mathrm{\Delta }}_{\text{CD}}=1.78$) using phase-change material ${\mathrm{Sb}}_2{\mathrm{Se}}_3$, showcasing flexibility for chiral sensing, lasing, and imaging in future.

The non-Hermitian skin effect is a unique feature of non-Hermitian systems, in which an extensive number of boundary modes appear under open boundary conditions. It has been determined that this effect originates from intrinsic non-Hermitian topology protected by spatial symmetry or particle-hole symmetry. Here, we reveal another kind of skin effect which is protected by custodial chiral symmetry. Custodial chiral symmetry is common in the standard model of particle physics. Recent investigations have shown that such symmetry is also present in a classical Su-Schrieffer-Heeger electrical circuit with memory. We extend this discussion to non-Hermitian systems. It is shown that the skin effect can appear through the introduction of memristors that break the conventional chiral symmetry while giving rise to an emergent custodial chiral symmetry. This skin effect protected by custodial chiral symmetry, along with the associated topological edge states, enables tunable non-Hermitian topological behavior and exhibits strong robustness against perturbations.

The giant magnetoresistance with a huge hysteresis is observed in the organic metal $\kappa$-(BEDT-TTF)${}_2{\mathrm{Hg}\left(\mathrm{SCN}\right)}_2\mathrm{Br}$ at low temperature in a pressure interval around $3\mathrm{kbar}$ of a width $\sim 1\mathrm{kbar}$. The hysteretic magnetoresistance is isotropic with respect to the direction of magnetic field, which excludes the orbital effect of magnetic field as its origin. The observed temperature and magnetic-field dependence of this hysteresis and of its relaxation time indicates the strong influence of the spin-glass state on magnetoresistance. Although a quantitative theory of this effect, originating from strong electronic correlations, requires complex numerical calculations, we suggest its explanation and a simple model which qualitatively describes the observed magnetoresistance behavior and shows a strong charge-spin entanglement. The proposed effect suggests another class of extreme magnetoresistance mechanisms.