Magnon gap excitations selectively coupled to phonon modes have been studied in ${\mathrm{FePSe}}_3$ layered antiferromagnet with magneto-Raman-scattering experiments performed at different temperatures. The bare magnon excitation in this material has been found to be split (by $\approx 1.2{\mathrm{cm}}^{-1}$) into two components each being selectively coupled to one of the two degenerate, nearby phonon modes. Lifting the degeneracy of the fundamental magnon mode points out toward the biaxial character of the ${\mathrm{FePSe}}_3$ antiferromagnet, with an additional in-plane anisotropy complementing much stronger, out-of-plane anisotropy. Moreover, the tunability, with magnetic field and temperature, of the phonon- versus the magnon-like character of the observed coupled modes has been demonstrated.

Dzyaloshinskii-Moriya interaction (DMI), the antisymmetric exchange interaction in noncentrosymmetric magnets, is a key ingredient in forming and manipulating the magnetic skyrmion, a promising candidate for next-generation data storage. With the development of non-Hermitian topological phases, the role of DMI has been extended to a broader stage recently. The non-Hermitian skin effect (NHSE) of magnons has been theoretically predicted to take place in magnetic systems with DMI and dissipation. Though exceptional band structures in van der Waals (vdW) magnetic structures have been observed, the NHSE of magnons has not been identified yet. Here we fabricate $\mathrm{W}{\mathrm{Se}}_2/\mathrm{F}{\mathrm{e}}_3\mathrm{Ge}{\mathrm{Te}}_2$ vdW heterostructure and observe the edge-enhanced negative magnetoresistance (NMR). The larger NMR at the sample's edges is attributed to the stronger electron-magnon scattering at the boundaries, indicating the edge accumulation of magnons. The aggregation of magnons towards sample boundaries is consistent with the magnonic NHSE, which is favored by the interface DMI and magnetic dissipation inherently in the heterojunction. Our work should be valuable for understanding the non-Hermitian topology in magnetic systems.

A comparative resonant inelastic x-ray scattering (RIXS) study of three well-known Kitaev materials is presented: $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3, {\mathrm{Na}}_2{\mathrm{IrO}}_3$, and $\alpha \text{−}{\mathrm{RuCl}}_3$. Despite similar low-energy physics, these materials show distinct electronic properties, such as the large difference in the size of the charge gap. The RIXS spectra of the spin-orbit exciton for these materials show remarkably similar three-peak features, including sharp low energy peak (peak A) as well as transitions between $j_{\text{eff}}=1/2$ and $j_{\text{eff}}=3/2$ states. Comparison of experimental spectra with cluster calculations reveals that the observed three-peak structure reflects the significant role that nonlocal physics plays in the electronic structure of these materials. In particular, the low-energy peak A arises from a holon-doublon pair rather than a conventional particle-hole exciton as proposed earlier. Our study suggests that while spin-orbit assisted Mott insulator is still the best description for these materials, electron itinerancy cannot be ignored when formulating low-energy Hamiltonian of these materials.

In this work, we study the generalization of decohered average symmetry-protected topological phases to open quantum systems with a combination of subsystem symmetries and global symmetries. In particular, we provide examples of two types of intrinsic average higher-order topological phases with average subsystem symmetries. A classification scheme for these phases based on generalized anomaly cancellation criteria of average symmetry is also discussed.

One of the key applications of AdS/CFT correspondence is the duality it dictates between the entanglement entropy of anti–de Sitter (AdS) black holes and lower-dimensional conformal field theories (CFTs). Here we employ a square lattice of fermions with inhomogeneous tunneling couplings that simulate the effect rotationally symmetric three-dimensional (3D) black holes have on Dirac fields. When applied to 3D Banados-Teitelboim-Zanelli (BTZ) black holes we identify the parametric regime where the theoretically predicted two-dimensional CFT faithfully describes the black hole entanglement entropy. With the help of the universal simulator, we further demonstrate that a large family of 3D black holes exhibit the same ground-state entanglement entropy behavior as the BTZ black hole. The simplicity of our simulator enables direct numerical investigation of a wide variety of 3D black holes and the possibility to experimentally realize it with optical lattice technology.

The key optical and electronic functionalities in condensed materials are determined by the band filling of electrons and their Coulomb correlations. To directly capture how many-body correlations influence the correlated electron dynamics under the driving of laser pulses, the comprehensible quasiparticle picture is desired to distinguish the ultrafast creation and annihilation of doublon-holon pairs in attosecond timescale. Here, we adopt the one-dimensional non-half-filling Hubbard model to study the high-harmonic generation (HHG) in correlated electron systems. As the value of Hubbard $U$ increases, the cutoff energy and spectral features in HHG spectra have been revealed and their scaling laws are explained by the characteristic energy gaps obtained from the electronic bands under the quasiparticle picture. The adjustable degree of band filling in correlated systems paves a way to enhance high-harmonic emissions by regulating the creation of doublon-holon pairs. The complex temporal profile of HHG is elucidated in this work by the light-induced evolution of quasiparticle bands in correlated materials. In addition, the dephasing of the HHG spectrum caused by the vibration-induced disorder or lattice temperature is affirmed in the correlated electron systems and this dephasing process is attributed to quasiparticle-lattice scattering during the doublons and holons propagating among the disordered lattice sites.

Recent experiments reported that the magnetic field can drive the Lifshitz transition and one-dimensional (1D) Weyl nodes in the quantum limit of three-dimensional pentatellurides, as they own low carrier densities and can achieve the extreme quantum limit at a low magnetic field. In this paper, we will investigate the conditions for the existence of the 1D Weyl nodes and their dc transport properties. We find that in the strong topological insulator (TI) phase of ${\mathrm{ZrTe}}_5$, the formation of the Weyl nodes depends heavily on the carrier density; while in the weak TI phase of ${\mathrm{HfTe}}_5$, the Weyl nodes are more likely to appear. These behaviors are attributed to the fact that in the strong and weak TI phases, the zeroth Landau levels exhibit opposite evolutions with the magnetic field. Moreover, the signatures of the critical fields that characterize the distinct behaviors of the system can be directly captured in the conductivities.

We investigate the physical properties of $G_2$-symmetric integrable chains with local degrees of freedom in the fundamental representation; given the typical connection between integrability and critical points, we test the model's properties against a hypothesis of conformal-invariant long-distance behavior. Leveraging an embedding between the $G_2$ exceptional Lie algebra and $\mathrm{SU}\left(2\right)$-symmetric chains with local spin-3 representations, we perform numerical analyses via exact diagonalization (ED) targeted at specific spin sectors, as well as via non-Abelian density-matrix renormalization group . A basic study of the momentum-resolved ED spectrum suggests the low-energy system is effectively described by a ${\left(G_2\right)}_1$ Wess-Zumino-Witten (WZW) theory, but we find challenges in further numerical characterization of conformal data. The study and control of the phenomenology of this model may have implications for the development of accessible models for Fibonacci anyons.

We present a systematic study of the nonlinear thermal Hall responses in bosonic systems using the quantum kinetic theory framework. We demonstrate the existence of an intrinsic nonlinear boson thermal current, arising from the quantum metric, which is a wavefunction dependent band geometric quantity. In contrast to the nonlinear Drude and nonlinear anomalous Hall contributions, the intrinsic nonlinear thermal conductivity is independent of the scattering timescale. We demonstrate the dominance of this intrinsic thermal Hall response in topological magnons in a two-dimensional ferromagnetic honeycomb lattice without Dzyaloshinskii-Moriya interaction. Our findings highlight the significance of band geometry-induced nonlinear thermal transport and motivate experimental probe of the intrinsic nonlinear thermal Hall response with implications for quantum magnonics.

Schwinger boson mean-field theory (SBMFT) is a nonperturbative approach which treats ordered and disordered phases of magnetic systems on equal footing. We leverage its versatility to evaluate the spin correlators which determine thermally induced spin transport the spin Seebeck effect (SSE) in Heisenberg ferromagnets (FMs) and antiferromagnets (AFMs), at arbitrary temperatures. In SBMFT, the spin current $J_s$ is made up of particle-hole-like excitations which carry integral spin angular momentum. Well below the ordering temperature, $J_s$ is dominated by a magnonic contribution, reproducing the behavior of a dilute-magnon gas. Near the transition temperature, an additional, paramagneticlike contribution becomes significant. In the AFM, the two contributions come with opposite signs, resulting in a signature, rapid inversion of the spin Seebeck coefficient as a function of temperature. Ultimately, at high temperatures, the low-field behavior of the paramagnetic SSE reduces to Curie-Weiss physics. An analysis based on our theory confirms that in recent experiments on gadolinium gallium garnet, the low-field spin Seebeck coefficient $\mathcal{S}\left(T\right)\propto \chi \left(T\right)$, the spin susceptibility, down to the Curie-Weiss temperature. At lower temperatures in the disordered phase, our theory shows a deviation of $\mathcal{S}\left(T\right)$ relative to $\chi \left(T\right)$ in both FMs and AFMs, which increases with decreasing temperature and arises due to a paramagnetic liquid phase in our theory. These results demonstrate that the SSE can be a probe of the short-ranged magnetic correlations in disordered correlated spin systems and spin liquids.

We clarify the pairing mechanism of high-$T_c$ superconductivity in bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure by employing the static auxiliary field Monte Carlo approach to simulate a minimal effective model that contains local $d_{z^2}$ interlayer spin singlets and metallic $d_{x^2-y^2}$ bands. Superconductivity is induced when the local spin singlet pairs are mobilized and attain long-distance phase coherence by hybridization with the metallic bands. When projected onto realistic Fermi surfaces, it yields a nodeless $s$-wave gap on the $\gamma$ Fermi surface, and extended $s$-wave gaps of the same (opposite) sign on the $\alpha$ ($\beta$) Fermi surface due to its bonding (antibonding) character, with nodes or gap minima along the diagonal direction of the two-dimensional Brillouin zone. We find a dual role of the hybridization that not only induces global phase coherence but also competes with the spin singlet formation. This leads to a tentative phase diagram where $T_c$ varies nonmonotonically with the hybridization, in good correspondence with experimental observations. A roughly linear relation is obtained for realistic hopping and hybridization parameters: $T_c\approx 0.04-0.05J$, where $J$ is the interlayer superexchange interaction. We emphasize the peculiar tunability of the bilayer structure and propose that $T_c$ may be further enhanced by hole doping or applying uniaxial pressure along the $c$ axis on superconducting ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. Our work provides reliable numerical evidence for the pairing mechanism of high-$T_c$ superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ and points out a potential route to achieve even higher $T_c$.

The robust transport of quantized particles in gap systems through adiabatic cyclic evolution corresponds to dynamical versions of topological insulators, which have recently emerged as a thriving topic. Until now, these connections were thought to be limited to gap systems. Here, we report a mechanism for corner state transfer in a gapless system, which arises as a synthetic higher-order Weyl semimetal. This is realized in the phononic version of a breathing kagome lattice, which is stacked layer by layer with weak interlayer couplings in the $z$ direction, mimicking the time axis. We observed the corner state transfer, which hosts Weyl points and hinge states in synthetic three-dimensional (two-dimensional lattice$+$one-dimensional time) space. Our proposed corner states periodically undergo two topologically nontrivial phases along the time axis, resulting in the transport of the corner states, which corresponds to the switching of the two hinge states. Moreover, we experimentally demonstrated that the transport process is robust against defects. Our results provide insight into studying topological phases in synthetic space as well as an effective approach for manipulating acoustic waves.

The electrical control of a material's conductivity is at the heart of modern electronics. Conventionally, this control is achieved by tuning the density of mobile charge carriers. A completely different approach is possible in Mott insulators such as ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$, where an insulator-to-metal transition (IMT) can be induced by a weak electric field or current. While the driving force of the IMT is poorly understood, it has been thought to be a breakdown of the Mott state. Using in operando angle-resolved photoemission spectroscopy, we show that this is not the case: The current-induced conductivity is caused by the formation of in-gap states with only a minor reorganization of the Mott state. Electronic structure calculations show that these in-gap states form at the boundaries of structural domains that emerge during the IMT. At such boundaries, the overall gap is drastically reduced, even if the structural difference between the domains is small and the individual domains retain their Mott character. The inhomogeneity of the sample is thus key to understanding the IMT, as it leads to a nonequilibrium semimetallic state that forms at the interface of Mott domains.

Exotic electric polarization-related phenomena have recently been reported in layered van der Waals material such as ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ and its derivatives, but the physical origin for such behaviors, especially the formation of quadrupole-well ferroelectric states and the anomalous polarization switching between these states, has not been clearly understood. Here, we provide a simple theoretical explanation using group theory analysis, highlighting the role of local symmetry-determined orbital interactions between copper ions and surrounding ligands. Based on this theory, we predict a surprising effect: carrier doping, which is commonly believed to suppress electric polarization, can lead to enhanced polarization in ${\mathrm{CuInP}}_2{\mathrm{S}}_6$. Our first-principles simulations confirm this unusual effect and further show that an elevated hole doping level can cause electric polarization to give way to emerging ferromagnetism. Our results not only pave a way to realize ferroelectric metals, but also broaden the scope of magnetoelectric coupling mechanisms and may help enrich the potential applications of layered ferroelectric materials in the future.

Two-dimensional band dispersion of $\left(2\times 2\right)$ superstructure with Bi grown on Ag(111), which has been proposed as an ultraflat hexagonal bismuthene, is investigated using angle-resolved photoemission spectroscopy (ARPES). The $\left(2\times 2\right)$-Bi superstructure can be grown on the Ag(111) surface at low temperatures; it transforms into a surface alloy with a $\left(\sqrt{3}\times \sqrt{3}\right)$ superstructure at 300 K. ARPES measurements are in agreement with the band structure of ultraflat bismuthene in previous reports. The band structure of $\left(2\times 2\right)$-Bi surface remains unchanged with decreasing Ag layer thickness, indicating the limited penetration of Bi $p$ orbitals into the Ag layer.

We group materials into five symmetry classes and determine in which of these classes phonons carry angular momentum in the Brillouin zone, away from a high-symmetry point, line, or plane. In some materials phonons acquire angular momentum via the forces induced by relative displacements of atoms out of their equilibrium positions. However, for other materials, such as ferromagnetic iron, phonon angular momentum arises from the forces induced by relative velocities of atoms. These effects are driven by the spin-orbit interaction.

The ability to readily switch between optical pulling and pushing forces on micro-objects is both important and fascinating for arbitrary optical manipulations. The bipolar optical force (namely, optical pulling versus optical pushing) on an object can be achieved generally by elaborately tailoring illuminating optical fields and/or surrounding media. Here, we propose an approach to achieve the temperature-reconfigured bipolar optical force on a dipolar nanoparticle. The basic idea is demonstrated on a particle immersed in a Bessel beam and made of material exhibiting a temperature-induced reversible transition, such as vanadium dioxide. Numerical results based on the decomposed force expressions indicate that the optical pulling force originates from the recoil force dominated by the coupling of electric and magnetic dipoles simultaneously excited on the particle, while the optical pushing force comes mostly from the interception force, which surpasses the recoil force when the excitation of the magnetic dipole is suppressed. Our dynamical simulations show that the nanoparticle can be transversely trapped stably in the vicinity of maximum light intensity while longitudinally switchable between the pulling against and pushing along light propagation. The bipolar characteristic does not require any change of the manipulating light or ambient material, opening interesting possibilities in flexible optical manipulations.

We use spin-lattice dynamics simulations to study the possibility of modeling the magnetic hysteresis behavior of a ferromagnetic material. The temporal evolution of the magnetic and mechanical degrees of freedom is obtained through a set of two coupled Langevin equations. Hysteresis loops are calculated for different angles between the external field and the magnetocrystalline anisotropy axes. The influence of several relevant parameters is studied, including the field frequency, magnetic damping, magnetic anisotropy (magnitude and type), magnetic exchange, and system size. The role played by a moving lattice is also discussed. For a perfect bulk ferromagnetic system we find that, at low temperatures, the exchange and lattice dynamics barely affect the loops, while the field frequency and magnetic damping have a large effect on it. The influence of the anisotropy magnitude and symmetry are found to follow the expected behavior. We show that a careful choice of simulation parameters allows for an excellent agreement between the spin-lattice dynamics measurements and the paradigmatic Stoner-Wohlfarth model. Furthermore, we extend this analysis to intermediate and high temperatures for the perfect bulk system and for spherical nanoparticles, with and without defects, reaching values close to the Curie temperature. In this temperature range, we find that lattice dynamics has a greater role on the magnetic behavior, especially in the evolution of the defective samples. This study opens the possibility for more accurate inclusion of lattice defects and thermal effects in hysteresis simulations.

Spin excitations that soften near the onset of magnetic order have long been known to act as “paramagnon” pairing glue that can drive superconductivity. Recent findings of superconductivity in graphene bilayers and trilayers, occurring in the proximity of different itinerant ordered phases polarized in isospin (spin and valley), have motivated us to conduct a comprehensive study of an isospin extension of the paramagnon pairing mechanism in the vicinity of different isospin orders. In each case, we identify a soft mode, associated with the order-parameter fluctuations, that mediates pairing interaction. The pairing interaction is proportional to the susceptibility of the corresponding spin/valley order parameter given diagrammatically by summation of the contributions most strongly divergent at the onset of the isospin orders. This interaction is not always attractive, but if it is, it gives rise to an enhancement of superconducting $T_c$ in an appropriate pairing channel. In the cases when the pairing interaction is attractive, it leads to the formation of a superconducting state which can be either spin triplet and valley singlet or vice versa, depending on the specific isospin order type. These findings demonstrate that the occurrence of superconductivity in the vicinity of an itinerant magnetic phase is a generic phenomenon, closely mirroring experimental observations.