Effects of a bias electric current have been theoretically investigated in a spin-triplet superconducting ring in a magnetic field. Based on the Ginzburg-Landau theory, we show that the bias current can stabilize a half-quantum-flux (HQF) state via couplings to the Zeeman field and the dipole-type spin-orbit interaction, the latter effect becoming active when the field is tilted from the ring axis. The emergence of the HQF state is reflected as a field-induced half-quantum shift in the Little-Parks (LP) oscillation in the critical current. Possible relevance to recent LP experiments is also discussed.

The many-body localization (MBL) proximity effect is an intriguing phenomenon where a thermal bath localizes due to the interaction with a disordered system. The interplay of thermal and nonergodic behavior in these systems gives rise to a rich phase diagram, whose exploration is an active field of research. In this paper, we study a bosonic Hubbard model featuring two particle species representing the bath and the disordered system. Using state-of-the-art numerical techniques, we investigate the dynamics of the model in different regimes, based on which we obtain a tentative phase diagram as a function of coupling strength and bath size. When the bath is composed of a single particle, we observe clear signatures of a transition from an MBL proximity effect to a delocalized phase. Increasing the bath size, however, its thermalizing effect becomes stronger and eventually the whole system delocalizes in the range of moderate interaction strengths studied. In this regime, we characterize particle transport, revealing diffusive behavior of the originally localized bosons.

Exploring the relationship between magnetism and band topology has become a popular research topic in condensed matter physics. In this work, we investigate this interplay in a two-dimensional (2D) metal-organic framework (MOF) ${\mathrm{Mn}}_3{\left({\mathrm{C}}_6{\mathrm{O}}_6\right)}_2$. We discover that this MOF has a “soft” ferromagnetic property, making it possible to control the topological phases by adjusting the magnetization direction. In its ground state, the MOF has Weyl points precisely located at the Fermi level and only in one spin channel, which means it can be described as an ideal, fully spin-polarized 2D Weyl semimetal. We determine that the stability of these Weyl points is protected by the vertical mirror symmetries. Particularly, the Weyl point can be preserved even with considering the spin-orbital coupling, with an in-plane magnetization direction parallel to the mirror normal vector. By altering the in-plane magnetization away from the normal vector of the vertical mirror symmetry, we demonstrate that the Weyl point is gapped, transformed into the quantum anomalous Hall (QAH) phase, resulting in a chiral edge state at the boundary. We calculate a Chern number to capture the main feature of the QAH, which indicates quantized Hall conductance. Furthermore, by rotating the magnetization direction in the plane, we discover that the Weyl point is a critical point where the QAH changes the sign of the Chern number, along with inversing the propagating direction of the chiral edge state. Overall, our findings highlight ${\mathrm{Mn}}_3{\left({\mathrm{C}}_6{\mathrm{O}}_6\right)}_2$ as a promising material candidate for studying the interplay between topology and magnetism, Weyl semimetal, and QAH phases.

The pursuit of efficient spin polarization in quaternary Heusler alloys with the general formula $X{X}^{\prime }YZ$ (where $X, X^{\prime }$, and $Y$ are transition metals and $Z$ is a $p$-block element) has been a subject of significant scientific interest. While previous studies showed that isoelectronic substitution of a $4d$ element in place of a $3d$ element in quaternary Heusler alloys improves the half-metallic ferromagnetic characteristics, our research on the quaternary Heusler alloy NiRuMnSn reveals a strikingly different scenario. In this study, we present a detailed structural analysis of the material using x-ray absorption fine structure and neutron diffraction techniques which confirms the formation of a single-phase compound with 50:50 site disorder between Ni and Ru atoms at $4c$ and $4d$ sites. Contrary to expectations, our density functional theory calculations suggest a considerable decrease in spin polarization even in the ordered structure. Additionally, we report on the compound's exceptional behavior, displaying a rare reentrant spin-glass property below $\sim 60$ K, a unique and intriguing feature for quaternary Heusler-type compounds.

Superconductivity in doped quantum paramagnets has been a subject of long theoretical inquiry. In this work, we report a density matrix renormalization group study of lightly doped $t\text{−}J$ models on the finite-width square lattice (doped hole densities $\delta =1/12$ and 1/8) with parameters for which previous studies have suggested that the undoped system in 2D is either a quantum spin liquid or a valence bond crystal. Our studies are performed on cylinders with width up to 8. Ground-state correlations are found to be nearly identical for the “doped quantum spin liquid” and “doped valence bond crystal.” Upon increasing the cylinder widths from 4 to 8, we observed a significant strengthening of the quasi-long-range superconducting correlations and a dramatic suppression of any “competing” charge density wave order. Extrapolating from the observed behavior of the width eight cylinders, we speculate that the system has a nodeless $d$-wave superconducting ground state in the 2D limit.

The random-field spin-1/2 XXZ chains, and the corresponding Anderson insulators of spinless fermions with density-density interaction, have been intensively studied in the context of many-body localization. However, we recently argued [B. Krajewski et al., Phys. Rev. Lett. 129, 260601 (2022)] that the two-body density-density interaction in these models is not generic since only a small fraction of this interaction represents a true local perturbation to the Anderson insulator. Here we study ergodicity of strongly disordered Anderson insulator chains, choosing other forms of the two-body interaction for which the strength of the true perturbation is of the same order of magnitude as the bare two-body interaction. Focusing on the strong-interaction regime, numerical results for the level statistics and the eigenstate thermalization hypothesis are consistent with emergence of ergodicity at arbitrary strong disorder.

We investigate the one-magnon dynamics of the antiferromagnetic delta chain as a paradigmatic example of tunable equilibration. Depending on the ratio of nearest and next-nearest exchange interactions the spin system exhibits a flat band in one-magnon space—in this case equilibration happens only partially, whereas it appears to be complete with dispersive bands as generally expected for generic Hamiltonians. We provide analytical as well as numerical insight into the phenomenon.

Nonreciprocal transport, an exciting quantum physical phenomenon arising from symmetry breaking, can be significantly magnified in the superconducting state. Recent progresses have demonstrated it as a unique probe of the pairing mechanisms in symmetry-breaking superconductors (SCs). Here, we report the nonreciprocal charge transport of the superconductivity in the Bi/Ni bilayer, a $p$-wave SC candidate. The nonreciprocal transport signal is dramatically enhanced under in-plane magnetic field, but vanishes under perpendicular magnetic field. Interestingly, the nonreciprocal transport is magnified in the temperature regime that can be associated with notable vortex motion. Our results provide compelling evidence for the Rashba superconductivity in the Bi/Ni bilayer with in-plane spin splitting, a precursor for the $p$-wave SC, which might hold promise for future topological quantum computing applications.

The $S=\frac{3}{2}$ Kitaev honeycomb model (KHM) is unique among the spin-$S$ Kitaev models due to a massive ground-state quasidegeneracy that hampered previous numerical and analytical studies. In a recent work [Jin et al., Nat. Commun. 13, 3813 (2022)], we showed how an SO(6) Majorana parton mean-field theory of the $S=\frac{3}{2}$ isotropic KHM explains the anomalous features of this Kitaev spin liquid (KSL) in terms of an emergent low-energy Majorana flat band. Away from the isotropic limit, the $S=\frac{3}{2}$ KSL generally displays a quadrupolar order with gapped or gapless Majorana excitations, features that were quantitatively confirmed by density-matrix renormalization group simulations. In this paper, we explore the connection between the $S=\frac{3}{2}$ KHM with Kugel-Khomskii models and discover exactly soluble examples for the latter. We perform a symmetry analysis for the variational parton mean-field Ansätze in the spin and orbital basis for different quantum liquid phases of the $S=\frac{3}{2}$ KHM. Finally, we investigate a proposed time-reversal symmetry-breaking spin liquid induced by [111] single-ion anisotropy and elucidate its topological properties as well as experimental signatures, e.g., an unquantized thermal Hall response.

The stability and dynamics of almost strong zero and $\pi$ modes in weakly nonintegrable Floquet spin chains are investigated. Such modes can also be viewed as localized Majorana modes at the edge of a topological superconductor. Perturbation theory in the strength of integrability breaking interaction $J_z$ is employed to estimate the decay rates of these modes, and compared to decay rates obtained from exact diagonalization. The structure of the perturbation theory and thus the lifetime of the modes is governed by the conservation of quasienergy modulo $2\pi /T$, where $T$ is the period of the Floquet system. If there are $4n-1$ bulk excitations whose quasienergies add up to zero (or $\pi /T$ for a $\pi$ mode), one obtains a decay channel of the Majorana mode with a decay rate proportional to $J_z^{2n}$. Thus the lifetime is sensitively controlled by the width of the single-particle Floquet bands. For regimes where the decay rates are quadratic in $J_z$, an analytic expression for the decay rate in terms of an infinite temperature autocorrelation function of the integrable model is derived, and shown to agree well with exact diagonalization.

A two-dimensional honeycomb-structured magnet ${\mathrm{Ni}}_2{\mathrm{Te}}_3{\mathrm{O}}_8$ was synthesized, characterized, and comprehensively investigated for its intriguing physical properties. DC magnetization, specific heat, and neutron diffraction revealed a long-range commensurate antiferromagnetic ordering at $T_{\mathrm{N}}\sim 35\mathrm{K}$ with a propagation vector $\mathbf{k}=\left(100\right)$. The magnetic sublattice comprises stacking distorted honeycomb layers along the $a$ axis. The ${\mathrm{Ni}}^{2+}$ spins on the honeycomb lattice are essentially pointing out of the layers and are antiferromagnetically coupled to the neighboring spins. Temperature ($T$) and magnetic field ($H$) dependent dielectric measurements indicated an apparent anomaly near $T_{\mathrm{N}}$, accompanied by a weak magnetodielectric effect. Raman mode renormalization and lattice anomalies near $T_{\mathrm{N}}$ demonstrated spin-lattice coupling through magnetoelastic and spin-phonon interactions. These findings highlight the fascinating interplay between spin, charge, and phonon degrees of freedom in ${\mathrm{Ni}}_2{\mathrm{Te}}_3{\mathrm{O}}_8$.

We propose a physical picture based on the wormhole effect of the path-integral formulation to explain the mechanism of the entanglement spectrum (ES), such that our picture not only explains the topological state with bulk-edge correspondence of the energy spectrum and ES (the Li and Haldane conjecture), but is generically applicable to other systems independent of their topological properties. We point out it is ultimately the relative strength of bulk energy gap (multiplied with inverse temperature $\beta =1/T$) with respect to the edge energy gap that determines the behavior of the low-lying ES of the system. Depending on the circumstances, the ES can resemble the energy spectrum of the virtual edge, but can also represent that of the virtual bulk. We design models both in one and two dimensions to successfully demonstrate the bulklike low-lying ES at finite temperatures, in addition to the edgelike case conjectured by Li and Haldane at zero temperature. Our results support the generality of viewing the ES as the wormhole effect in the path integral and the different temperature dependence for the edge and bulk of ES.

Ultrafast optical spectroscopy was used to investigate the ultrafast quasiparticle dynamics of antiferromagnetic Kondo lattice ${\mathrm{CeAuSb}}_2$ as a function of temperature and fluence. Our results reveal (i) the opening of a narrow hybridization gap ($\mathrm{\Delta }\sim 4.5$ meV) near the Fermi level below the coherence temperature $T^{*}\approx 100$ K, (ii) the coupling of coherent phonon modes with Kondo effect and magnetic orders, leading to the frequencies anomaly at the characteristic temperatures, and (iii) a possible photoinduced nonthermal phase transition. Our observations thus shed light on the hybridization dynamics and magnetic orders in heavy fermion systems.

Phantom helix states are a special set of degenerate eigenstates of the XXZ Heisenberg model which lie in the energy levels around zero energy and are bidirectionally equal. In this work, we study the helix state in the XXZ Heisenberg model with the Dzyaloshinskii-Moriya interaction (DMI). We show exactly that only the helix states in one direction remain unchanged in the presence of resonant DMI. Based on the Holstein-Primakoff transformation, the quantum spin model is mapped to a boson model, which allows us to understand the underlying mechanism. Furthermore, it also indicates that such phantom states can be separated from the spectrum by the strong DMI to enhance the robustness of the states. We demonstrate the dynamic formation processes of unidirectional phantom helix states by numerical simulations. The results indicate that the DMI, as expected, acts as a filter with high efficiency.

Entanglement entropy of typical quantum states, also known as the Page curve, plays an important role in quantum many-body systems and quantum gravity. However, little has hitherto been understood about the role of symmetry in quantum entanglement. Here, we establish the comprehensive classification of typical quantum entanglement for free fermions, or equivalently the quadratic Sachdev-Ye-Kitaev model with symmetry, on the basis of the tenfold fundamental symmetry classes of time reversal, charge conjugation, and chiral transformation. Through both analytical and numerical calculations of random matrix theory, we show that the volume-law contribution to average entanglement entropy is robust and remains unaffected by symmetry. Conversely, we uncover that the constant terms of the average and variance of entanglement entropy yield tenfold universal values unique to each symmetry class. These constant terms originate from the combination of a global scaling of the entanglement spectrum due to time-reversal symmetry and a singular peak at the center of the entanglement spectrum due to chiral or particle-hole symmetry. Our work elucidates the interplay of symmetry and entanglement in quantum physics and provides a characterization of symmetry-enriched quantum chaos.

High-spin systems, like those that incorporate rare-earth $4f$ elements, are increasingly relevant in many fields. Although research on such systems is sparse, the large Hilbert spaces they occupy are promising for many applications. In this work, we examine a one-dimensional linear array of europium atoms on a Au(111) surface and study their electronic and magnetic excitations. Ab initio calculations using vasp with the Perdew-Burke-Ernzerhof (PBE) functional $+ U$ are employed to study the structure. We find Eu atoms have a net charge when on gold, consistent with a net magnetic moment of $\simeq 3.5{\mu }_B$. Examining various spin-projection configurations, we can evaluate first- and second-neighbor exchange energies in an isotropic Heisenberg model between spin-$\frac{7}{2}$ moments to obtain $J_1\approx -1.2\mathrm{K}$ and $J_2\approx 0.2\mathrm{K}$ for a relaxed-chain atomic separation of $a\approx 5$ Å. These parameters are used to obtain the full spin excitation spectrum of a physically realizable four-atom chain. The large $|J_1|/J_2$ ratio results in a highly degenerate ferromagnetic ground state that is split by a significant easy plane single-ion anisotropy of 0.6 K. Spin-flip excitations are calculated to extract differential conductance profiles such as those obtained by scanning tunneling microscopy techniques. We uncover interesting behavior of local spin excitations, especially as we track their dispersion with applied magnetic fields.

Recent advances in realizing ferroelectric and superconducting two-dimensional heterobilayers provide appealing platforms for exploring the interplay between ferroelectricity and superconductivity, which is not only crucial for understanding the superconducting mechanism but also important for designing next-generation superconducting devices. Based on first-principles calculations, we demonstrate that an ${\mathrm{IrTe}}_2$ monolayer can be stabilized on a ferroelectric ${\mathrm{In}}_2{\mathrm{Se}}_3$ monolayer via interlayer coupling. The superconducting transition temperature of the ${\mathrm{IrTe}}_2/{\mathrm{In}}_2{\mathrm{Se}}_3$ heterobilayer is substantially enhanced from that of bulk ${\mathrm{IrTe}}_2$ mainly due to enhanced interlayer coupling, supplemented by the increase in the density of states at the Fermi level and phonon softening; the latter is further tied to Fermi surface nesting. Our calculations show that superconductivity is dominant over several typical competing orders, including charge density wave, magnetism, and nematicity. Moreover, we find that the band topology of ${\mathrm{IrTe}}_2/{\mathrm{In}}_2{\mathrm{Se}}_3$ can be switched between trivial and nontrivial by reversing the ferroelectric polarization of the ${\mathrm{In}}_2{\mathrm{Se}}_3$ substrate. By further substituting Ir with Pd, the topological edge states can be tuned close to the Fermi level, making ${\mathrm{IrTe}}_2/{\mathrm{In}}_2{\mathrm{Se}}_3$ a potential candidate for realizing topological superconductivity. Our work provides a realistic system that can simultaneously harbor ferroelectricity, superconductivity, and nontrivial band topology, paving the way for integrating multiple applications, such as superconducting field transistors, topological quantum computing, and tunable superconducting diodes, in a single system.

The ferroelectric phase transitions described by Landau-Devonshire's phenomenological theory can be statistically simulated using the effective Hamiltonian $\left(H_{\mathrm{eff}}\right)$ based on first-principles calculations. In a Monte Carlo (MC) simulation, we observe a statistical singularity energy caused by a subtle difference between the macroscopic phenomenological model and the microscopic statistical model in the mathematical space of the phase transition point. This statistical singularity energy is measured according to the parameters of the $H_{\mathrm{eff}}$ model and introduced by the transition probability of the MC algorithm, which greatly improves the accuracy of the prediction of the phase transition temperatures and restores the true fluctuations near the phase transition point. For the cubic-tetragonal phase transition for $\mathrm{BaTi}{\mathrm{O}}_3$ and $\mathrm{PbTi}{\mathrm{O}}_3$, we obtain a good estimation of the thermal hysteresis corresponding to the experiments. We further reproduce the difference of the phase transitions between heating and cooling sequences, which is verified by the experimental observations.

The local magnetic susceptibility of the spin-triplet superconductor ${\mathrm{UTe}}_2$ has been investigated by positive muon (${\mu }^{+}$) Knight shift measurements in the normal state. Three distinct ${\mu }^{+}$ Knight shift components are observed for a magnetic field applied parallel to the $c$ axis. Two of these exhibit a breakdown in the linear relationship with the bulk magnetic susceptibility ($\chi$) below a temperature $T^{*}\sim 30\mathrm{K}$, which points to a gradual emergence of a correlated Kondo liquid. Below $T_{\mathrm{r}}\sim 12\mathrm{K}$ linearity is gradually restored, indicating relocalization of the Kondo liquid quasiparticles. The third Knight shift component is two orders of magnitude larger, and despite the $c$-axis alignment of the external field, scales with the $a$ axis $\chi$ above $T_{\mathrm{r}}\sim 12\mathrm{K}$. We conjecture that this component is associated with magnetic clusters and the change in the temperature dependence of all three Knight shift components below $T_{\mathrm{r}}$ is associated with a change in magnetic correlations. Our findings indicate that prior to the onset of superconductivity the development of the itinerant heavy-electron fluid is halted by a gradual development of local U $5f$-moment fluctuations.