Triangular-lattice compounds serve as a prototypical example of geometrically frustrated systems and can host a variety of quantum states, among which quantum spin liquids (QSLs) are particularly intriguing. However, structural disorder is often present in real QSL candidates and it may drive the system into a spin-glass state. Here, we have successfully grown the single crystals of a new triangular-lattice rare-earth compound ${\mathrm{K}}_2\mathrm{CsYb}$(${\mathrm{PO}}_4{)}_2$. X-ray diffractions confirm high phase purity with no detectable structural disorder. The dc magnetic susceptibility measurements reveal dominant magnetic anisotropy, with a stronger response in the $ab$ plane than along the $c$ axis. Notably, no phase transition or spin freezing is observed, as further corroborated by ac susceptibility measurements down to 400 mK. Moreover, specific heat measurements down to 50 mK show no hint of long-range magnetic order either, but only an anomaly that can be well fitted by a two-level Schottky function due to the splitting of the Kramers doublet ground state. Collectively, these findings point to a spin disordered ground state consistent with QSL behaviors. To gain deeper insight into the underlying physics, first-principles calculations based on density functional theory are carried out, suggesting a strong spin-orbit entangled ground state characterized by an effective spin $J_{\mathrm{eff}}$ = 1/2. Our work establishes ${\mathrm{K}}_2\mathrm{CsYb}$(${\mathrm{PO}}_4{)}_2$ as a pristine platform for exploring novel quantum states in a perfect triangular-lattice system.

Measuring entanglement entropy in interacting, multipartite systems remains a significant experimental challenge. We address this challenge by developing a protocol to measure von Neumann entropy (VNE) and mutual information in quantum transport systems with both many-body interactions and multiple subsystems. Our analysis indicates that the vital connection between VNE and two-point correlation functions persists under these realistic conditions. We provide detailed derivations showing that this relation holds for weakly interacting systems via perturbation theory and for systems near a stable saddle point via the Hubbard-Stratonovich transformation. The measurement is shown to be feasible for systems with boundary interactions and, critically, for bulk-interacting systems subject to a quantum quench of their internal couplings. Our work provides a pathway to experimentally quantify entanglement in complex interacting systems and establishes mutual information as an experimentally accessible indicator for system-environment entanglement.

Neural quantum states (NQSs) are a class of variational wave functions parametrized by neural networks to study quantum many-body systems. In this work, we propose SineKAN, a NQS ansatz based on Kolmogorov-Arnold networks, to represent quantum mechanical wave functions as nested univariate functions. We show that a SineKAN wave function with learnable sinusoidal activation functions can capture the ground state energies, fidelities, and various correlation functions of the one-dimensional transverse-field Ising model, anisotropic Heisenberg model, and antiferromagnetic $J_1-J_2$ model with different chain lengths. Beyond one dimension, we further demonstrate that SineKAN, augmented with a minimal reflection symmetry (rSineKAN), accurately describes the ground states of square-lattice systems, including the transverse-field Ising model and the antiferromagnetic Heisenberg model. In our study of the $J_1-J_2$ model with $L=100$ sites, we find that the SineKAN model outperforms several previously explored neural quantum state ansätze, including restricted Boltzmann machines, long short-term memory models, and multilayer perceptrons, aka feed forward neural networks, when compared to the results obtained from the density matrix renormalization group algorithm. We find that SineKAN models can be trained to high precision and accuracy with minimal computational costs.

In recent years, there has been a growing interest in non-Hermitian phenomena in low-symmetry conductors, particularly optical gain driven by electro-optic effects. Conventional semiclassical treatments typically attribute these effects to nonlinear interactions associated with the anomalous velocity of Bloch electrons. Here, we present a phenomenological microscopic model that not only recovers these anomalous-velocity contributions, but also incorporates interband effects that become significant at higher frequencies. Our model captures a wide range of nonlinear interactions while remaining consistent with passivity and microscopic time reversibility. Using this broader framework, we study the nonlinear interactions between free and bound electrons as an alternative mechanism for optical gain. We show that, under non-equilibrium conditions in low-symmetry conductors, the linearized electromagnetic response can exhibit both nonreciprocity and gain, even without anomalous velocity contributions. Finally, we analyze the stability of electrically biased systems and highlight potential applications such as optical isolators and traveling-wave amplifiers.

We theoretically investigate the light response of a two-dimensional material with a finite Berry curvature dipole under a static temperature gradient. Based on the semiclassical Boltzmann equation, we derive the frequency-dependent non-Hermitian optical conductivity tensor. For an electron system in ${\mathrm{Cl}}_2{\mathrm{Rh}}_2{\mathrm{S}}_2–\mathrm{GeS}$ heterostructure described by a gapped merging double-Dirac-cone model, we demonstrate the absorption gain of THz light exceeding $-10\%$, which can be tuned by the amplitude and orientation of the temperature gradient, the light frequency, and the chemical potential. An analytical expression for dissipation power is presented. The optical amplification arising from thermally driven steady states offers a path to Berry curvature engineering and thermal control of optoelectronic functionalities.

Density functional theory is used to investigate the effect of biaxial strain on the structural, electronic and magnetic properties of [111]-oriented ${\mathrm{NdNiO}}_3$, as a representative of the rare-earth perovskites that undergo metal-to-insulator transitions. We find that this constraint on the system induces unique structural phase transitions not previously observed under the well-studied bulk or [001]-oriented strained systems. We also report unique electronic behavior, including amplification of the electronic band gap with tensile strain, and insulating, charge-ordered phases with nonorthorhombic tilt patterns. To provide clarity to the trends we observe, we also investigate the coupling between the breathing mode and strain, where we observe certain strains to directly favor and disfavor the creation of the breathing mode (and thus the associated charge-ordering). The amplification of the band gap with strain is understood in terms of a cooperative coupling between the elastic constraint and octahedral breathing, which expands on the previously reported triggered mechanism mediated by octahedral tilting.

The interplay between the superconducting and normal states is essential to understanding the underlying mechanisms of high-temperature superconductivity. Here, photoinduced changes in conductivity of an optimally doped $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-\delta }$ superconducting film at temperatures well below the critical temperature ($T_c$) are examined using transient terahertz spectroscopy. The pump-fluence-dependent experiment unveils that the excitations of superfluid and normal fluid lead to distinct nonequilibrium conductivities and recovery dynamics. The selective excitation of the normal fluid below $T_c$ exhibits exceptionally slower recovery dynamics with a bimolecular decay pattern, and the temperature dependence of its recovery time indicates a characteristic divergence at a temperature substantially below $T_c$. The dynamic behaviors of the normal fluid are dramatically different from those in the normal metals. These anomalous dynamic behaviors may suggest the existence of an energy gap for the normal state that displays remarkable likeness to the short-range pair-density-wave state. Our finding sheds light on understanding the dynamic interplay between the coexisting superconducting and nonsuperconducting pairs in cuprate superconductors.

We investigate the six-state clock universality of the Ising model on the kagome lattice, considering antiferromagnetic nearest-neighbor (NN) and ferromagnetic next-nearest-neighbor (NNN) interactions. Our comprehensive study employs three approaches: the level-spectroscopy method, Monte Carlo simulations, and a machine-learning phase classification technique. In this system, we observe two Berezinskii-Kosterlitz-Thouless (BKT) transitions. We present a phase diagram consisting of three phases: the low-temperature ordered phase with sublattice magnetizations, the intermediate BKT phase, and the high-temperature disordered phase, as a function of the ratio of the NNN interaction to the NN interaction. We verify the six-state clock universality through the machine-learning study, which uses data from the six-state clock model on the kagome lattice for training.

The diffusionless Burgers-Bain phase transition from a hexagonal close-packed (hcp) arrangement to a cuboidal lattice (face-centered cubic, fcc, and body-centered cubic, bcc) is analyzed in great detail for Lennard-Jones (LJ) solids. From the lattice vectors of an underlying bilattice smoothly connecting these phases, we are able to express the corresponding lattice sums for inverse power potentials in terms of fast converging Bessel function expansions, resulting in an efficient evaluation to computer accuracy for cohesive energies. From the kissing hard-sphere limit we derive exact analytical expressions for the lattice parameters varying along the minimum energy path of the phase transition. This simple model suggests that the Burgers-Bain transformation of a LJ solid requires a minimum of four lattice parameters, $(a,\alpha ,\beta ,\gamma =c/a)$, describing the change in the base lattice lengths $a$ and $c$, the shear force acting on the hexagonal base plane through a parameter $\alpha$, the sliding force of the middle layer in the original hexagonal packing arrangement through a single parameter $\beta$, and the cuboidal transformation through a parameter $\gamma =c/a$. This choice results in a smooth transformation through a two-step process: $\mathrm{hcp}\to \mathrm{fcc}\to \mathrm{bcc}$. However, a further extension of the parameter space including an additional slide parameter for the middle layer, one suddenly observes a distinct symmetry-breaking effect along the $\mathrm{hcp}\to \mathrm{fcc}$ transition path with a bifurcation point appearing joining the original Burgers with the Bain path of the $\mathrm{bcc}\to \mathrm{fcc}$ cuboidal transition. Furthermore, for soft LJ potentials the bcc phase appears as a local minimum along the Burgers $\mathrm{hcp}\to \mathrm{fcc}$ path with two transition states to either the hcp or fcc phase. The underlying topology of the Burgers-Bain phase transition also incorporates the rhombohedral distortion of the bcc phase, which is analyzed in detail. As a first application of our formalism, we discuss solid argon and compare the LJ results with variable-cell nudge elastic band optimizations using density functional theory. We find that the activation energy for the $\mathrm{hcp}\to \mathrm{fcc}$ transformation is highly sensitive to the density functional applied, and that dispersion corrections are important as expected for weakly interacting systems.

The transition metal dichalcogenide ${\mathrm{IrTe}}_2$ displays a remarkable series of first-order phase transitions below room temperature, involving lattice displacements as large as 20% of the initial bond length. This is nowadays understood as the result of strong electron-phonon coupling leading to the formation of local multicenter dimers that arrange themselves into one-dimensional stripes. In this work, we study the out-of-equilibrium dynamics of these dimers and track the time evolution of their population following an infrared photoexcitation using free-electron lased-based time-resolved x-ray photoemission spectroscopy. First, we observe that the dissolution of dimers is driven by the transfer of energy from the electronic subsystem to the lattice subsystem, in agreement with previous studies. Second, we observe a surprisingly fast relaxation of the dimer population on the timescale of a few picoseconds. By comparing our results to published ultrafast electron diffraction and angle-resolved photoemission spectroscopy data, we reveal that the long-range order needs tens of picoseconds to recover, while the local dimer distortion recovers on a short timescale of a few picoseconds.

The spin-boson (SB) model is a standard prototype for quantum dissipation, which we generalize in this work, to explore the dissipative effects on a one-dimensional spin-orbit (SO) coupled particle in the presence of a subohmic bath. We analyze this model by extending the well-known variational polaron approach, revealing a localization transition accompanied by an intriguing change in the spectrum, for which the doubly degenerate minima evolves to a single minimum at zero momentum as the system-bath coupling increases. For a translational invariant system with conserved momentum, a continuous magnetization transition occurs, whereas the ground state changes discontinuously. We further investigate the transition of the ground state in the presence of harmonic confinement, which effectively models a quantum-dot-like nanostructure under the influence of the environment. In both scenarios, the entanglement entropy of the spin sector can serve as a marker for these transitions. Interestingly, for the trapped system, a catlike superposition state corresponds to maximum entanglement entropy below the transition, highlighting the relevance of the present model for studying the effect of decoherence on intraparticle entanglement in the context of quantum information processing.

We benchmark the recently proposed projection method [M. Sánchez Sánchez et al., Phys. Rev. B 111, 205133 (2025)] for magic-angle twisted bilayer graphene (MATBG) across various symmetry-breaking phases at charge neutrality. The flat-band projected solutions agree well with the full tight binding, with band structures and total energies differing by only a few meV. The projection to the flat bands is justified, owing to the increased gap in the remote bands in the normal state. Moreover, we employ a set of order parameters that allows us to visualize the wave functions locally in real space and quantify the breaking of various symmetries in the correlated phases. These order parameters are suitable for characterizing MATBG and generic honeycomb systems.

We study the boundary physics of topologially trivial Mott insulators, representing the boundary by three coupled Hubbard chains in a linear confining potential. In the Hartree-Fock approximation, the ground state at and slightly off the particle-hole symmetric point remains insulating even at large slopes of the confining potential. By contrast, accounting for quantum fluctuations and correlations through a combination of bosonization and renormalization group (RG) methods, we find that there is always a gapless dipole mode, but it does not contribute to a finite compressibility. Moreover, when increasing the slope of the confining potential at half filling, we find a quantum phase transition between an insulating and a metallic state, indicating the formation of a soft edge state of non-topological origin. Away from half filling, a similar transition takes place.

Non-Hermitian skin effects (NHSEs) have recently been extensively studied at the single-particle level. When two-particle interactions become dominant, novel non-Hermitian phenomena can emerge. In this work, we propose an experimentally accessible mechanism to induce and control NHSEs in interacting and reciprocal dissipative systems. We consider both one-dimensional (1D) and 2D Bose-Hubbard lattices subject to staggered two-particle loss combined with synthetic magnetic flux and long-range hopping. When the two-particle loss is small, the bound eigenstates (e.g., doublons and triplons) are all localized at the same boundary due to the interplay between the magnetic flux and staggered two-particle loss. In contrast, for strong two-particle loss, the skin-mode localization direction of the bound particles is unexpectedly reversed. This reversal stems from the combined effect of the staggered two-particle loss, synthetic magnetic flux, and long-range hopping, through which virtual second-order and third-order hopping processes induce effectively strong nonreciprocal hopping of doublons. Our results open up a new avenue for exploring novel non-Hermitian phenomena in interacting systems.

Friction forces are conventionally modeled via semiclassical theories that associate energy dissipation with newtonian motion on corrugated interface potentials. This consolidated approach is challenged at the nanoscale by observation of nearly unimpeded water flow in narrow carbon nanotubes (CNTs), in spite of nonvanishing energy corrugations. Here we go beyond the standard newtonian perspective, adopting a quantum mechanical description of ${}^4\mathrm{He}$ flow through narrow CNTs. Building upon our Bloch-wave dynamics [Phys. Rev. Lett. 131, 206301 (2023)] we explore realistic flow conditions, including non-negligible interface interactions, finite temperatures, and imperfect CNTs. At $T$ = 0 K we found that ${}^4\mathrm{He}$ waves can propagate through ideally periodic, corrugated interface potentials with no friction: below a critical velocity regulated by interface corrugations, energy loss by emission of plasmon and phonon quanta is forbidden. Introducing realistic impurities/defects one still finds very large mean free paths that can exceed the $\mu \mathrm{m}$ scale, while thermal phonons and plasmons yield even lower scattering rates. This establishes the unexpected emergence of ballistic wavelike transport in narrow CNTs within realistic nanoscale devices, and demonstrates the intrinsic quantumness of nanoscale interfaces.

We study the instabilities to the conformal critical point of an exactly solvable family of Gross-Neveu-Yukawa models. Using conformal field theory techniques, we construct the zero-temperature phase diagram and identify the superconducting and charge neutral ordered phases that destabilize the critical point. Both instabilities appear only when the fermions are strongly renormalized, above a critical anomalous dimension. A higher fermion anomalous dimension also raises the critical degree of time-reversal-symmetry breaking required to suppress superconductivity, indicating that pairing becomes more robust with stronger renormalization.