Two-dimensional antiferromagnets are promising materials for spintronics. The van der Waals antiferromagnet ${\mathrm{NiPS}}_3$ has attracted extensive interest due to its ultranarrow exciton feature which is closely linked with the magnetic ordering. Here, we use time-resolved terahertz spectroscopy to investigate photoexcited carriers in ${\mathrm{NiPS}}_3$. We identify the onset of interband transitions and estimate the exciton dissociation energy from the excitation wavelength and fluence dependence of the transient spectral weight. Our results provide key insights to quantify the exciton characteristics and validate the band structure for ${\mathrm{NiPS}}_3$.

The effect of polaron formation on the ground state of excitons, trions, and biexcitons confined in ${\mathrm{CsPbBr}}_3$ nanocrystals is studied in the framework of effective mass Hamiltonians, using a Haken-like (Bajaj) potential for carrier-phonon coupling. The binding energy of trions agrees well with that observed in experiments, with position-dependent dielectric screening playing a significant role. For biexcitons, however, neither polaronic effects, nor dielectric confinement, nor electronic correlations—here accounted for with a variational quantum Monte Carlo method—suffice to explain the large binding energies reported by single nanocrystal spectroscopy experiments. This result reinforces the hypothesis that biexcitons polarize the perovskite lattice differently from excitons and trions.

Quantum phase transitions in Kondo-lattice compounds generally arise from the competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida exchange interaction. Magnetic field is a convenient control parameter of the competition and can smoothly suppress the magnetic ordering to zero temperature at a quantum critical point (QCP), where singular electronic properties beyond Fermi liquid often emerge, reflecting the enhanced quantum critical fluctuations. Here, we provide experimental evidence that the antiferromagnetic compound ${\mathrm{YbNi}}_4\mathrm{Cd}$, characterized by a weak Kondo coupling, can be tuned to QCP by applying a small field of less than 1 T. This tuning results in, however, no typical non-Fermi liquid behavior. Strong damping of quantum critical fluctuations in Zeeman field, which is comparable to the Kondo effect in this compound, is evidenced by low-temperature magnetic, specific-heat, and magnetocaloric effect measurements. Our findings are discussed as generic quantum critical characteristics of weak-coupling Kondo lattices, where the Kondo temperature is comparable to or smaller than the magnetic ordering temperature.

Correlations between the energies of a system's spectrum are one of the defining features of quantum chaos. They can be probed using the spectral form factor (SFF). We investigate how each spectral distance contributes in building this two-point correlation function. Specifically, starting from the spectral distribution of $k\mathrm{th}$-neighbor-level spacing $\left(k\mathrm{nLS}\right)$, we provide analytical expressions for the $k\mathrm{th}$-neighbor spectral form factor $\left(k\mathrm{nSFF}\right)$. We do so for the three Gaussian random matrix ensembles and the “Poisson” ensemble of uncorrelated energy levels. We study the properties of the $k\mathrm{nSFF}$, namely its minimum value and the time at which this minimum is reached, as well as the energy spacing with the deepest $k\mathrm{nSFF}$. This allows us to quantify the contribution of each individual $k\mathrm{nLS}$ to the SFF ramp, which is a characteristic feature of quantum chaos. In particular, we show how the onset of the ramp, characterized either by the dip or the Thouless time, shifts to shorter times as contributions from longer-range spectral distance are included. Interestingly, the even and odd neighbors contribute quite distinctively, the first being the most important to built the ramp. They respectively yield a resonance or antiresonance in the ramp. All of our analytical results are tested against numerical realizations of random matrices. We complete our analysis and show how the introduced tools help characterize the spectral properties of a physical many-body system by looking at the interacting XXZ Heisenberg model with local on-site disorder that allows transitioning between the chaotic and integrable regimes.

We study the influence of external electric and Zeeman fields on the Josephson effect in a planar superconductor/altermagnet/superconductor junction. Remarkably, we find that the current-phase relation (CPR) can be forward or backward skewed due to a pronounced second harmonic term. It decisively depends on the altermagnetic field strength. This second harmonic can be measured directly using double SQUID devices. The CPR skewness can be effectively manipulated by electric gating. Moreover, we identify two additional impacts of external electric and magnetic fields on the Josephson current: (i) Fields can induce 0-$\pi$ transitions. (ii) Fields can substantially enhance the critical current. This enhancement is surprising since supercurrents are typically suppressed by magnetic fields.

It has recently been established that open quantum systems may exhibit a strong spectral sensitivity to boundary conditions, known as the non-Hermitian/Liouvillian skin effect (NHSE/LSE), making the topological properties of the system boundary-condition sensitive. In this Letter, we ask the query: Can topological phase transitions of open quantum systems, captured by open boundary conditioned invariants, be observed in the dynamics of a system under periodic boundary conditions, even in the presence of NHSE/LSE? We affirmatively respond to this question, by considering the quench dynamics of entanglement spectrum in a periodic open quantum fermionic system. We demonstrate that the entanglement spectrum exhibits zero-crossings only when this periodic system is quenched from a topologically trivial to nontrivial phase, defined from the spectrum under open boundary conditions, even in systems featuring LSE. Our results reveal that non-Hermitian topological phases leave a distinctive imprint on the unconditional dynamics within a subsystem of fermionic systems.

We calculate the frequency-dependent mobility of the Holstein polaron in one dimension near the adiabatic limit using the method based on dynamical quantum typicality, as well as the quantum-classical method. The agreement between fully quantum and quantum-classical solutions is very good. The most prominent feature is the appearance of a zero-frequency peak in the mobility, in addition to the displaced peak associated to the precursors of Anderson localization. The zero-frequency peak cannot be obtained within the phenomenological transient localization approach, which is often used in a semiquantitative description of charge transport in quasi-one-dimensional organic semiconductors.

Searching for highly coherent degrees of freedom in noisy solid-state environments is a major challenge in condensed matter. In disordered dipolar systems, such as magnetically doped insulators, compact clusters of two-level systems (TLS) have recently been shown to have significantly longer coherence times than typical single TLS. Coupling weakly to their environment, they sense and probe its many-body dynamics through the induced dephasing. However, it has remained an open question whether further mechanisms exist that protect the coherence of such solid-state qubits. Here, we show that symmetric clusters of a few TLS couple even more weakly to their surroundings, making them highly sensitive quantum sensors of slow many-body dynamics. Furthermore, we explore their use as qubits for quantum information storage, detailing the techniques required for their preparation and manipulation. Our findings elucidate the role of symmetry in enhancing quantum coherence in disordered and noisy systems, opening a route toward a sensitive experimental probe of many-body quasilocalization dynamics as well as the development of quantum technologies in solid-state systems.

Spin-space groups (SSGs), including the traditional space groups (SGs) and magnetic space groups (MSGs) as subsets, describe the complete symmetries of magnetic materials with weak spin-orbit coupling (SOC). In the present work, we systematically study the irreducible representations (irreps) of SSGs by focusing on the projective irreps of the little cogroup $L\left(k\right)$ of any momentum point $\mathit{k}$. We analyze the factor systems of $L\left(k\right)$, and then reduce the projective regular representation of $L\left(k\right)$ into a direct sum of irreps using the Hamiltonian approach. Especially, for collinear SSGs which contain continuous spin rotation operations, we adopt discrete subgroups to effectively capture their characteristics. Furthermore, we apply the representation theory of SSGs to study the band structure of electrons and magnons in magnetic materials. After identifying the SSG symmetry group, we extract relevant irreps and determine the $k·p$ models. As an example, we illustrate how our approach works for the material ${\mathrm{Mn}}_3\mathrm{Sn}$. Degeneracies facilitated by SSG symmetry are observed, underscoring the effectiveness of application in material analysis. The SSG recognition and representation code is uploaded to GitHub, and the information of irreps of all SSGs is also available in the online Database. Our work provides a practical toolkit for exploring the intricate symmetries of magnetic materials and paves the way for future advances in materials science.

To gain deeper insight into the complex, stable, and robust configurations of magnetic textures, topological characterization has proven essential. In particular, while the skyrmion number is a well-established topological invariant for two-dimensional magnetic textures, the Hopf index serves as a key topological descriptor for three-dimensional magnetic structures. In this paper, we present and compare various methods for numerically calculating the Hopf index, provide implementations, and offer a detailed analysis of their accuracy and computational efficiency. Additionally, we identify and address common pitfalls and challenges associated with the numerical computation of the Hopf index, offering insights for improving the robustness of these techniques.

In systems with linear electron-phonon interaction (EPI), bound states of polarons, or bipolarons, form by gaining energy from the lattice deformation. The quadratic EPI case is fundamentally different: bipolarons form because electrons lose less energy when the total charge density is “compacted.” As the coupling constant is increased, the bipolarons first appear as extended (but finite radius) soliton-type states. They subsequently decrease in radius until their size reaches the interatomic scale. We present the first numerically exact solution of the bipolaron problem from quadratic EPI in the presence of both on-site Hubbard and long-range Coulomb repulsion, and compute estimates of the largest superconducting transition temperature within the bipolaron mechanism. We find that $T_c/\mathrm{\Omega }$ ratios, where $\mathrm{\Omega }$ is the optical phonon frequency, can be several times larger than what one may expect from the linear EPI provided the phonon frequency is increased by orders of magnitude on occupied sites. Electron-electron repulsion can be tolerated at the expense of stronger EPI and the most detrimental effect comes from the Coulomb potential because it easily eliminates extended soliton states.

We report here the observation of robust thermoremanent magnetization (TRM) in Fe-intercalated ${\mathrm{TaS}}_2$ single-crystal ${\mathrm{Fe}}_x{\mathrm{TaS}}_2$, which develops on cooling under a moderate magnetic field of 500 Oe or higher. The thermoremanence is found to exist in a zero magnetic field up to a temperature as high as 160 K, which is well above the ferromagnetic Curie point ($T_C=90\mathrm{K}$) of the sample. The TRM in this compound can be traced back to the strong Ising character and the quasi-two-dimensional nature of the spins. The analysis of the magnetization data indicates the presence of short-range magnetic correlation in the form of a Griffiths singularity above $T_C$ to a temperature as high as $2T_C$. The TRM is further manifested in the form of a large anomalous Hall effect (AHE) in zero magnetic field. In contrast to TRM, which exists well above $T_C$, the thermoremanent Hall effect vanishes as soon as the sample is heated above $T_C$. This may be an indication that a long-range ordered phase is essential for the emergence of AHE.

In this comprehensive study, we present results of bulk measurements (magnetization, specific heat, ac susceptibility, thermal expansion, and magnetostriction) combined with local methods such as nuclear magnetic resonance (${}^{23}\mathrm{Na}$ NMR) and electron spin resonance (ESR) and simulations (McPhase) on polycrystalline and single-crystalline ${\mathrm{NaGdS}}_2$ samples. The rare-earth delafossite ${\mathrm{NaGdS}}_2$ is a triangular-lattice magnet with $S=7/2$ spin-only ${\mathrm{Gd}}^{3+}$ moments with suppressed single-ion anisotropy. In our study, we estimate that ${\mathrm{NaGdS}}_2$ has a weak antiferromagnetic exchange $(J_H/k_B\approx 52\mathrm{mK})$ and signs of long-range magnetic order are absent down to lowest temperature. However, indications of short range magnetic order are found below 180 mK in the ac susceptibility and thermal expansion. Our results indicate an interplay of Heisenberg-type and dipolar exchange. Due to the large moment of the ${\mathrm{Gd}}^{3+}$ ions, one expects a strong impact of the dipolar coupling in ${\mathrm{NaGdS}}_2$, in contrast to the related ${\mathrm{NaYbS}}_2$. ESR and ${}^{23}\mathrm{Na}$ NMR measurements, indeed, indicate the formation of short-range ferromagnetic correlations. ${\mathrm{NaGdS}}_2$ appears to be a rare system, in which magnetic order is suppressed by a competition between Heisenberg and dipolar interactions.

When noncollinear spin textures are driven by current, an emergent electric field arises due to the emergent electromagnetic induction. So far, this phenomenon has been reported in several materials, manifesting the current-nonlinear imaginary part of the complex impedance. Recently, Furuta et al. proposed a time-varying temperature increase due to Joule heating as an alternative explanation for these current-nonlinear complex impedances [Furuta et al., Phys. Rev. B 110, 174402 (2024)]. In this Comment, we demonstrate that the experimentally observed nonlinear complex impedances in helimagnets cannot be explained by the time-varying temperature increase. Instead, these dependencies of the imaginary part of the nonlinear impedance are consistent with the expected behavior in the theory of emergent electromagnetic induction.

In this paper, we study a previously unexplored class of topological states protected by hidden chiral symmetries that are local, that is, that protect against any off-diagonal disorder. We derive their related topological invariant, and show that these previously unidentified symmetries can act together with standard chiral symmetries to increase the protection of the end modes, using the Creutz ladder as an example. Finally, thanks to local hidden symmetries, we show that the diamond necklace chain can have three different types of topological end modes: strong, weak, or square-root, with some of the states inheriting their topology from others.

Weak ergodicity breaking, particularly through quantum many-body scars (QMBS), has become a significant focus in many-body physics. Krylov state complexity quantifies the spread of quantum states within the Krylov basis and serves as a powerful diagnostic for analyzing nonergodic dynamics. In this work, we study spin-one XXZ magnets and reveal nonergodic behavior tied to QMBS. For the XY model, the nematic Néel state exhibits periodic revivals in Krylov complexity. In the generic XXZ model, we identify spin helix states as weakly ergodicity-breaking states, characterized by low entanglement and nonthermal dynamics. Across different scenarios, the Lanczos coefficients for scarred states display an elliptical pattern, reflecting a hidden SU(2) algebra that enables analytical results for Krylov complexity and fidelity. These findings, which exemplify the rare capability to characterize QMBS analytically, are feasible with current experimental techniques and offer deep insights into the nonergodic dynamics of interacting quantum systems.

Chemical order-disorder transitions commonly exist in alloys, which serve as a degree of freedom to tune alloy properties in addition to chemical composition. The dependence of alloy properties on the order parameter is thus key for optimized alloy design. Conventionally, the dependence of alloy properties on a single order parameter has been well established. In complex alloys, there might exist multiple chemical orders; the general dependence of alloy properties on multiple chemical orders is, however, still unclear, which significantly limits the sophisticated design of complex alloys. Here, we theoretically derive the generalized dependence of alloy properties on multiple chemical orders, and validate our derived formula by explicit calculations based on first principles and a machine-learning force field for a prototypical complex alloy of ${\mathrm{GaInAs}}_2$, which is known for the existence of multiple chemical orders. Our study lays the foundation for the precise control of alloy performance via manipulating chemical orders.

The Berry phase-related nontrivial electronic band geometries can significantly influence bulk and edge plasmons, resulting in their nonreciprocal propagation and opening unique opportunities for plasmonics. In the present paper, we extend the hydrodynamic framework to describe the scattering of plasmons in a Dirac electron fluid off a circular region with an induced nonzero anomalous Hall response, i.e., a Berry flux target. We demonstrate that the scattering has a giant asymmetry or skewness and exhibits a series of resonances. The latter appears due to a chiral nontopological trapped mode circulating the target. We discuss possible experimental realizations, including the surface of a topological insulator film and graphene irradiated by the circularly polarized beam.

The unique band structure in quantum spin Hall insulators (QSHIs) is predicted to induce strong nonlinear optical rectification, which is crucial for solar energy conversion and infrared detection. However, the inherent requirement of breaking inversion symmetry for nonlinear optical effects and the scarcity of suitable candidate materials have hindered the exploration of nonlinear optical responses in QSHIs. The synthesis approach involving two-dimensional (2D) Janus materials offers a route to induce inversion symmetry breaking without significantly altering the original band structures. This provides an effective strategy for designing materials with strong nonlinear rectification properties based on QSHIs. Taking advantage of this approach, we designed six Janus structures based on the experimentally synthesized QSHI ${\mathrm{Si}}_2{\mathrm{Te}}_2$. Although the topological nature of these structure configurations is disrupted, the high joint density of states and the large interband dipole matrix elements are maintained, which actually govern the enhancement of optical response in QSHIs. Remarkably, these noncentrosymmetric Janus materials exhibit large shift currents exceeding 1000 $\mu \text{A}\text{/}{\mathrm{V}}^2$ in the infrared spectrum and 240 $\mu \text{A/}{\mathrm{V}}^2$ in the visible spectrum. This paper highlights the potential of 2D Janus materials for applications in both infrared detection and energy harvesting.