Higher-order topology realizes topologically robust corner modes as a manifestation of nontriviality. We theoretically propose non-Hermitian skin effects which stem from the second-order topology of chiral-symmetric Hermitian systems. It is found that the skin modes are localized at the corners. We demonstrate two types of second-order topological skin effects by two-dimensional intrinsic and extrinsic second-order topology. The intrinsic second-order topological skin effect is characterized topologically by bulk inversion symmetry as well as chiral symmetry. Meanwhile, the extrinsic second-order topological skin effect occurs from the topological correspondence between the edges and corners. We show that non-Hermitian skin modes emerge by using a relationship between second-order and conventional first-order topology.

Solids with topologically robust electronic states exhibit unusual electronic and optical properties that do not exist in other materials. A particularly interesting example is chiral charge pumping, the so-called chiral anomaly, in recently discovered topological Weyl semimetals, where simultaneous application of parallel DC electric and magnetic fields creates an imbalance in the number of carriers of opposite topological charge (chirality). Here, using time-resolved terahertz measurements on the Weyl semimetal TaAs in a magnetic field, we optically interrogate the chiral anomaly by dynamically pumping the chiral charges and monitoring their subsequent relaxation of the nonequilibrium state. Theory based on Boltzmann transport shows that the observed effects originate from an optical nonlinearity in the chiral charge pumping process. Our measurements reveal that the nonequilibrium chiral excitation relaxation time is much greater than 1 ns. The observation of terahertz-controlled chiral carriers with long coherence times and topological protection suggests the application of Weyl semimetals for quantum optoelectronic technology.

We show that spin-orbit coupling (SOC) plays Janus-faced roles on the orbitally selective Mott transitions in a three-orbital Hubbard model with crystal field splitting at a specific filling of $2/3$, which is a minimal Hamiltonian for ruthenates. While the SOC favors the metallic state due to enhancement of orbital hybridization at smaller on-site Coulomb repulsions, it stabilizes the Mott-insulating state ascribed to the lifting of orbital degeneracies and enhancement of band polarizations at larger electronic interaction. Moreover, an orbitally selective non-Fermi liquid (OSNFL), where breakdown and retention of the Fermi liquid coexist in different orbitals, emerges between the orbitally selective Mott phase and the Fermi-liquid state. This novel state can be used to account for the exotic metallic behavior observed in $4d$ materials, such as ${\mathrm{Ca}}_{1.8}{\mathrm{Sr}}_{0.2}{\mathrm{RuO}}_4, {\mathrm{Ba}}_2{\mathrm{RuO}}_4$ under strain, and ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ under uniaxial pressure. We propose that orbitally selective Kondo breakdown may account for the OSNFL.

We perform time-resolved exact diagonalization of the Hubbard model with time-dependent hoppings on small clusters of up to 12 sites. Here, the time dependence originates from a classic electromagnetic pulse, which mimics the impact of a photon. We investigate the behavior of the double occupation and spectral function after the pulse for different cluster geometries and onsite potentials. We find impact ionization in all studied geometries except for one-dimensional chains. Adding next-nearest-neighbor hopping to the model leads to a significant enhancement of impact ionization, as does disorder and geometric frustration of a triangular lattice.

Two-dimensional (2D) $A$-type antiferromagnetic van der Waals (vdW) materials with either intralayer ferromagnetic and/or intralayer antiferromagnetic coupling have sparked great interest due to their potential applications in spintronics. By first-principles design we predict that a heterostructure formed by sandwiching the 2D $A$-type antiferromagnet, $2\mathrm{H}\text{−}\mathrm{V}{\mathrm{Se}}_2$, between the 2D out-of-plane ferroelectric, ${\mathrm{Sc}}_2{\mathrm{CO}}_2$, presents a semimetal band structure with a half-metallic conduction band. The mechanism giving rise to this peculiar structure cooperatively originates from the strong built-in electric field of the double-layer ferroelectric ${\mathrm{Sc}}_2{\mathrm{CO}}_2$, and from the charge transfer selectively occurring only at one interface. Based on the so-designed ${\mathrm{Sc}}_2{\mathrm{CO}}_2\text{/}\mathrm{V}{\mathrm{Se}}_2$ multiferroic heterostructure, two concepts for nonvolatile memory devices are proposed, in which two states (“1” and “0”) are realized by switching the polarization direction of the ferroelectric layers. These results demonstrate that the combination of 2D ferroelectrics and $A$-type antiferromagnetic vdW materials provides not only a fascinating way to achieve half-metallicity in 2D materials, but also a route to the design of new types of nonvolatile ferroelectric memory devices.

The magnetic long-range ordering states of van der Waals magnets at low temperatures have gained much interest, yet their high-temperature paramagnetic states remain largely unexplored. In this paper, we apply the disordered local moment picture for describing the paramagnetic states of two-dimensional magnetic semiconductors. We calculate the electronic structure of paramagnetic phases of ${\mathrm{CrI}}_3,{\mathrm{CrSiTe}}_3$, and ${\mathrm{NiPS}}_3$ using the density functional theory with static Hubbard corrections. The semiconducting electronic structures are successfully reproduced without any inclusion of dynamical correlation effects. The local electronic structure of magnetic ions in the paramagnetic phase resembles those in the magnetically ordered phase. The band structures of the paramagnetic phases are also analyzed. The itinerant ferromagnet ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ is also briefly discussed where the disordered local moment picture is not applicable due to the coexistence of itinerant and local magnetism.

We study the ultrafast demagnetization of Ni/NiMn and Co/NiMn ferromagnetic/antiferromagnetic bilayer systems after excitation by a laser pulse. We probe the ferromagnetic order of Ni and Co using magnetic circular dichroism in time-resolved pump-probe resonant x-ray reflectivity. Tuning the sample temperature across the antiferromagnetic ordering temperature of the NiMn layer allows us to investigate effects induced by the magnetic order of the latter. The presence of antiferromagnetic order in NiMn speeds up the demagnetization of the ferromagnetic layer, which is attributed to bidirectional laser-induced superdiffusive spin currents between the ferromagnetic and the antiferromagnetic layer.

The spin Nernst effect (SNE) and anomalous Nernst effect (ANE) convert a heat current generated by a temperature gradient to a spin current in nonmagnetic metals and ferromagnets, respectively. We present a diffusive spin transport theory of the combination of these thermoelectric effects in ferromagnet (F)/nonmagnet (N) heterostructures and their respective output signals. In a F/N bilayer, in the presence of an in-plane temperature gradient, electrical voltages are induced by the SNE in N and the ANE in the F layers, via the inverse spin Hall effect in the N. The analytical expression of the thermally driven spin Hall magnetoresistance (TSMR) output is derived, which captures both the local SNE contribution in the N and the nonlocal mixing contribution due to the ANE in the adjacent F. Interestingly, the SNE and ANE give additive contributions to the transverse TSMR voltages but subtract from each other for the longitudinal component. This anisotropic response suggests a possible means to extract the individual SNE and ANE contributions, as well as spin-dependent polarization parameters. In a F1/N/F2 trilayer, both SNE and ANE contribute to spin torques on the free F2 layer. We analyze the ferromagnetic resonance (FMR) of the free layer and show that both thermoelectric effects contribute additively to the FMR magnitude and linewidth for the considered magnetization configuration. Finally, we show that TSMR can provide a more sensitive experimental detection of SNE and ANE down to a threshold thermal gradient of the order of ${10}^{-6}$ K/nm.

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate ${\mathrm{Li}}_2\mathrm{MnGe}{\mathrm{O}}_4$ were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the $Pmn{2}_1$ space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector $k=\left(1/21/21/2\right)$ has been found below $T_N\approx 8\mathrm{K}$. Symmetry analysis revealed the model of the ground state spin structure within the $C_{a}c$ (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of $4.9{\mu }_{\mathrm{B}}/{\mathrm{Mn}}^{2+}$ at 1.7 K, which corresponds to the saturated value for the high-spin configuration $S=5/2$. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above $T_N$. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in ${\mathrm{Li}}_2\mathrm{MnGe}{\mathrm{O}}_4$, which is symmetry consistent on a small scale with the long-range magnetic state below $T_N$. The reconstructed radial spin-pair correlation function $S\left(0\right)S\left(r\right)$ displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at $r\approx 12\AA$ at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below $T_N$. As a result, an exhaustive picture of the gradual formation of magnetic ordering in ${\mathrm{Li}}_2\mathrm{MnGe}{\mathrm{O}}_4$ is presented.

Polar metals are defined by the coexistence of metallicity and polar crystal structure. They have potential applications in nonlinear optics, ferroelectric devices, and quantum devices. Meanwhile, ferroelectric-ferromagnetic (FE-FM) multiferroics display simultaneous ferroelectricity and ferromagnetism, leading to new technologies in information storage. It is exceedingly scarce that metallicity, ferroelectricity, and ferromagnetism can coexist simultaneously in a material. Here, using the results from hybrid density-functional theory calculations, we report metallic FE-FM multiferroics in strain-engineered epitaxial $\mathrm{EuTi}{\mathrm{O}}_3$ with H doping. The emergence of the magnetism in polar metals provides a degree of freedom to control these materials in applications. The underlying mechanism for the coexistence of metallicity, ferroelectricity, and ferromagnetism is discussed. The ferromagnetism in metallic $\mathrm{EuTi}{\mathrm{O}}_{3-x}{\mathrm{H}}_x$ is explained by the Ruderman-Kittel-Kasuya-Yosida interaction, which agrees with experiments. The coexistence of metallicity and ferroelectricity is allowed because the electrons at the Fermi level are weakly coupled to the ferroelectric distortion. Our results suggest that the combined effect of strain and doping is responsible for achieving $\mathrm{EuTi}{\mathrm{O}}_3$-based metallic multiferroics and may provide a way for obtaining metallic multiferroics in other materials.

The interface between a ferromagnet (FM) or antiferromagnet (AFM) and a heavy metal (HM) results in an antisymmetric exchange interaction known as the interfacial Dzyaloshinskii-Moriya interaction (iDMI) which favors noncollinear spin configurations. The iDMI is responsible for stabilizing noncollinear spin textures such as skyrmions in materials with bulk inversion symmetry. Interfacial DMI values have been previously determined theoretically and experimentally for FM/HM interfaces, and, in this work, they are calculated for the metallic AFM MnPt and the insulating AFM NiO. The heavy metals considered are W, Re, and Au. Values for the iDMI, exchange, and anisotropy constants are determined for different AFM and HM thicknesses. The iDMI values of the MnPt heterolayers are comparable to those of the common FM materials, and those of NiO are lower. In few-layer films of (001) MnPt, the high spin orbit coupling of the Pt layers can give rise to a small DMI in the absence of a HM layer.

Quantum spin liquids (QSLs) are an exotic state of matter that is subject to extensive research. However, the relationship between the ubiquitous disorder and the QSL behaviors is still unclear. Here, by performing comparative experimental studies on two kagomé-lattice QSL candidates, ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$ and ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$, which are isostructural to each other but with strong and weak structural disorder, respectively, we show unambiguously that the disorder can induce spin-liquid-like features. In particular, both compounds show dominant antiferromagnetic interactions with a Curie-Weiss temperature of $-17.4$ and $-28.7$ K for ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$ and ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$, respectively, but remain disordered down to about 0.05 K. Specific-heat results suggest the presence of gapless magnetic excitations characterized by a residual linear term. Magnetic excitation spectra obtained by inelastic neutron scattering (INS) at low temperatures display broad continua. All these observations are consistent with those of a QSL. However, we find in ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$, which has strong disorder resulting from the random mixing of the magnetic ${\mathrm{Tm}}^{3+}$ and nonmagnetic ${\mathrm{Zn}}^{2+}$, that the low-energy magnetic excitations observed in the specific-heat and INS measurements are substantially enhanced compared to those of ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$, which has much less disorder. We believe that the effective spins of the ${\mathrm{Tm}}^{3+}$ ions in the ${\mathrm{Zn}}^{2+}/{\mathrm{Mg}}^{2+}$ sites give rise to the low-energy magnetic excitations, and the amount of the occupancy determines the excitation strength. These results provide direct evidence of the mimicry of a QSL caused by disorder.

The rhombohedral $\mathrm{BiFe}{\mathrm{O}}_3$ (BFO), as a star multiferroics, has been of particular interest, but yet it exhibits zero magnetization, which limits its practical application. Remarkably, net magnetization and magnetoelectric (ME) coupling was reported in orthorhombic BFO at room temperature in a recent experiment. However, the underlying mechanism of its ME coupling has not been fully understood. Here, we systematically investigated magnetic structures and ME coupling of the orthorhombic BFO using density functional theory and symmetry analysis. We find it can host large electric polarization and weak ferromagnetism simultaneously, the possible coupling between them is $\mathit{\omega }·\left(\mathit{L}\times \mathit{M}\right)$. More interestingly, its polarization $P$ and magnetization $M$ can be switched simultaneously under an electric field, and it shows a linear ME effect when an external magnetic field is applied. Thus, our study provides a substantially deeper understanding of the ME effect of the orthorhombic BFO and confirm it is a multiferroic with the linear ME coupling.

The elementary excitations from conventional magnetic ordered states, such as ferromagnets and antiferromagnets, are magnons. Here, we elaborate a case where the well-defined magnons are completely absent and the spin excitation spectra exhibit an entire continuum in the itinerant edge ferromagnetic state of graphene arising from the flat-band edge electronic states. Based on further studies of the entanglement entropy and a finite-size analysis, we show that the continuum other than the Stoner part results from the spin-1/2 spinons deconfined from magnons. The magnetically ordered edge state without magnonic excitations but a spinon continuum is ascribed to a ferromagnetic Luttinger liquid. The investigation is carried out by using the numerical exact diagonalization method with a projection of the interacting Hamiltonian onto the flat band.

Continuous variable entanglement between magnon modes in Heisenberg antiferromagnets with Dzyaloshinskii-Moriya (DM) interaction is examined. Different bosonic modes are identified, which allows us to establish a hierarchy of magnon entanglement. We argue that entanglement between magnon modes is determined by a simple lattice-specific parameter, together with the ratio of the strengths of the DM and Heisenberg exchange interactions, and that magnon entanglement can be detected by means of quantum homodyne techniques. As an illustration of the relevance of our findings for possible entanglement experiments in the solid state, a typical antiferromagnet with the perovskite crystal structure is considered, and it is shown that long wave length magnon modes have a maximal degree of entanglement.

For the unconventional organic superconductor $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ [BETS = bis(ethylenedithio) tetraselenafulvalene], the importance of both spin and charge degrees of freedom has been discussed based on the broadening of the NMR linewidth due to charge disproportionation and the occurrence of a spin-density-wave phase near the superconducting phase. NMR with the nuclear spin of $1/2$, previously used for studying organic conductors, is an effective method for revealing electronic states microscopically; however, it cannot distinguish between charge and spin anomalies. To resolve this problem, in this paper, we performed ${}^{69,71}\mathrm{Ga}$-NMR measurements, which enabled us to study both charge and spin dynamics using different gyromagnetic ratios and quadrupole moments between two isotopes. The spin-lattice relaxation rate is dominated by electric-field gradient fluctuations originating from molecular dynamics above 150 K and, below this temperature, it is dominated by spin fluctuations derived from the $\pi$ electrons of BETS layers. This change in the relaxation mechanism is considered to be due to the development of interactions between ${\mathrm{GaCl}}_4$ ions and BETS layers upon freezing of the molecular motion by cooling. Below 150 K, the contribution of spin fluctuations monotonically increases, and no increase in the charge fluctuation was observed, suggesting that the spin degree of freedom plays a major role in low-temperature physical properties. Our findings will aid theoretical studies on superconducting properties.

Many-body localization is a striking mechanism that prevents interacting quantum systems from thermalizing. The absence of thermalization behavior manifests itself, for example, in a remanence of local particle number configurations, a quantity that is robust over a parameter range. Local particle numbers are directly accessible in programmable quantum simulators, in systems of cold atoms, even in two spatial dimensions. Yet, the classical simulation aimed at building trust in quantum simulations is highly challenging. In this work, we present a comprehensive tensor network simulation of a many-body localized systems in two spatial dimensions using a variant of an infinite projected entangled pair states algorithm. The required translational invariance can be restored by implementing the disorder into an auxiliary spin system, providing an exact disorder average under dynamics. We can quantitatively assess signatures of many-body localization for the infinite system: Our methods are powerful enough to provide crude dynamical estimates for the transition between localized and ergodic phases. Interestingly, in this setting of finitely many disorder values, which we also compare with simulations involving noninteracting fermions and for which we discuss the emergent physics, localization emerges in the interacting regime, for which we provide an intuitive argument, while Anderson localization is absent.

We apply the boson exchange parquet solver for dual fermions to the half-filled Hubbard model on a square lattice at small interaction. Our results establish that, in this regime, nonlocal vertex corrections play an important role in the formation of the pseudogap. Namely, in comparison to the simpler ladder approximation, these additional vertex corrections included in the parquet equations enhance the coupling of spin fluctuations with the quasiparticles. The pseudogap thus opens already at a higher temperature, in quantitative agreement with the numerically exact diagrammatic Monte Carlo. The representation of the parquet diagrams in terms of boson exchange facilitates large lattice sizes and gives rise to an unbiased fluctuation diagnostic of the self-energy, which does not rely on the Fierz ambiguity. The fluctuation diagnostic implies that nodal and antinodal fermions are affected equally by spin fluctuations with the exact commensurate nesting vector $(\pi ,\pi )$. However, the antinode couples more efficiently to incommensurate fluctuations than the node, leading to the nodal/antinodal dichotomy. We corroborate this finding in terms of a spin-fermion-like calculation.

Magneto-optical studies of Weyl semimetals have been proposed as a versatile tool for observing low-energy Weyl fermions in candidate materials including the chiral Landau level. However, previous theoretical results have been restricted to the linearized regime around the Weyl node and are at odds with experimental findings. Here, we derive a closed form expression for the magneto-optical conductivity of generic Weyl semimetals in the presence of an external magnetic field aligned with the tilt of the spectrum. The systems are taken to have linear dispersion in two directions, while the tilting direction can consist of any arbitrary continuously differentiable function. This general calculation is then used to analytically evaluate the magneto-optical conductivity of Weyl semimetals expanded to cubic order in momentum. In particular, systems with arbitrary tilt, as well as systems hosting trivial Fermi pockets are investigated. The higher-order terms in momentum close the Fermi pockets in the type-II regime, removing the need for unphysical cutoffs when evaluating the magneto-optical conductivity. Crucially, the ability to take into account closed over-tilted and additional trivial Fermi pockets allows us to treat model systems closer to actual materials and we propose a simple explanation why the presence of parasitic trivial Fermi pockets can mask the characteristic signature of Weyl fermions in magneto-optical conductivity measurements.