Several works have recently addressed the emergence of exceptional points (EPs), i.e., degeneracies of non-Hermitian Hamiltonians, in the long-wavelength dynamics of coupled magnetic systems. Here, by focusing on the driven magnetization dynamics of a van der Waals ferromagnetic bilayer, we show that exceptional points can appear over extended portions of the first Brillouin zone as well. Furthermore, we demonstrate that the effective non-Hermitian magnon Hamiltonian, whose eigenvalues are purely real or come in complex conjugate pairs, respects an unusual wavevector-dependent pseudo-Hermiticity. Finally, for both armchair and zigzag nanoribbon geometries, we discuss both the complex and purely real spectra of the topological edge states and their experimental implications.

We investigate the influence of surface states on the nonlinear Hall response driven by the Berry curvature dipole in non-centrosymmetric time-reversal invariant Weyl semimetals. To do so, we perform a tomography of the Berry curvature dipole in a slab system using a minimal two-band model. We find that in the type-I phase, the nonlinear Hall response is not particularly sensitive to the presence of Fermi arcs or other trivial surface states. However, in the type-II phase, we find that these surface states, be they topologically trivial or not, contribute substantially to the Berry curvature dipole, leading to a strong thickness dependence of the nonlinear Hall response. This feature depends on the nature of the surface states and, henceforth, on the slab geometry adopted. In order to assess the validity of this scenario for realistic systems, we performed Berry curvature dipole calculations by first principles on the ${\mathrm{WTe}}_2$, confirming the dramatic impact of surface states for selected slab geometries. Our results suggest that surface states, being topological or not, can contribute much more efficiently to the nonlinear Hall response than bulk states. This prediction is not limited to topological semimetals and should apply to topologically trivial noncentrosymmetric materials and heterostructures, paving the way to interfacial engineering of the nonlinear Hall effect.

Kagome-lattice materials have attracted tremendous interest due to the broad prospect for seeking superconductivity, quantum spin liquid states, and topological electronic structures. Among them, the transition-metal kagome lattices are high-profile objects for the combination of topological properties, rich magnetism, and multiple-orbital physics. Here we report an inelastic neutron scattering study on the spin dynamics of a kagome-lattice antiferromagnetic metal ${\mathrm{Fe}}_{0.89}{\mathrm{Co}}_{0.11}\mathrm{Sn}$. Although the magnetic excitations can be observed up to $\sim 250$ meV, well-defined spin waves are only identified below $\sim 90$ meV and can be modeled using Heisenberg exchange with ferromagnetic in-plane nearest-neighbor coupling $J_1$, in-plane next-nearest-neighbor coupling $J_2$, and antiferromagnetic (AFM) interlayer coupling $J_c$ under linear spin-wave theory. Above $\sim 90$ meV, the spin waves enter the itinerant Stoner continuum and become highly damped particle-hole excitations. At the $K$ point of the Brillouin zone, we reveal a possible band crossing of the spin wave, which indicates a potential Dirac magnon. Our results uncover the evolution of the spin excitations from the planar AFM state to the axial AFM state in ${\mathrm{Fe}}_{0.89}{\mathrm{Co}}_{0.11}\mathrm{Sn}$, solve the magnetic Hamiltonian for both states, and confirm the significant influence of the itinerant magnetism on the spin excitations.

The optical conductivity encodes the current response to a time dependent electric field. We develop a theory of the optical conductivity $\sigma \left(\omega \right)$ in the presence of a dc supercurrent. Current induced optical response is prohibited when current is conserved, an illustrative example of which is Galilean invariant systems. However, we show that lattice effects give rise to a pronounced current dependent peak in $\sigma \left(\omega \right)$ at the gap edge $\omega =2\mathrm{\Delta }$, which diverges in the clean limit. We demonstrate this in a model of a multiband superconductor. Our theory predicts the current induced peak in $\sigma \left(\omega \right)$ to scale quadratically with the super-current density, as was recently reported in experiments on NbN by Nakamura et al. [Phys. Rev. Lett. 122, 257001 (2019)]. This provides the potential for a new mechanism for direct activation of the Higgs mode with light.

Light-induced ferroelectric domain wall motion turns out to be a promising phenomenon to develop new photocontrolled devices. However, the physical origin of this light-matter coupling when material is irradiated with visible light remains unclear. Here, a phenomenological model predicting the motion of charged domain walls (CDWs) is developed. The photoinduced electronic reconstruction mechanism is proposed as the primary absorption mechanism, leading to a linear dependence for the polarization perturbation with the light intensity. Domain wall motion is then driven by the energetic difference between domains in a CDW array, such that the macroscopic polarization can be easily tuned.

The effects of surface and bulk scattering on electronic and spin transport in aluminum-based ultrathin films are predicted using the Landauer-Büttiker formalism and a recursive Green's function technique. The effects of surface roughness, grain boundaries, vacancies, and surface reconstruction on resistivity, spin diffusion length, and Elliott-Yafet constant $\beta$ are investigated for a 3.6-nm-thickness Al film. It is demonstrated that for a thin sputtered film, point vacancies are the dominant contribution to the momentum relaxation, and spin relaxation is dominated by the combined effect of surface reconstruction and point vacancies, which yields a reasonable spin diffusion length and the Elliot-Yafet constant. Calculations reveal that the presence of surface corrugations results in a clear departure from Matthiessen's rule and the Elliott-Yafet prediction of $\beta$, as shown by introducing random surface corrugations. It is also found that spin diffusion length induced by surface roughness is proportional to the inverse square root of the ratio between root mean square height $\delta h$ and lateral correlation length $\xi$ of a given rough surface, i.e., ${\left(\delta \xi \right)}^{-1/2}$ as opposed to ${\left(\delta \xi \right)}^{-1}$ as is the mean free path; this can be attributed to the interference of extended surface features.

We analyze the effect of a strong perpendicular magnetic field on an intersubband transition in a disordered doped quantum well strongly coupled to an optical cavity. The magnetic field changes the lineshape of the intersubband optical transition due to the interface roughness of the quantum well from a Lorentzian to a Gaussian one. In this regime, a novel form of magnetic-field-induced cavity protection sets in, which strongly reduces the polariton linewidth to the cavity contribution only. Implications of our results for fundamental studies of nonlinear polariton dynamics and for technological applications to polariton lasers are finally highlighted.

We study real-time correlators $\langle \mathcal{O}(\mathbf{x},t)\mathcal{O}(0,0)\rangle$ of local operators in chaotic quantum many-body systems. These correlators show universal structure at late times, determined by the geometry of the dominant operator-space Feynman trajectories for the evolving operator $\mathcal{O}(\mathit{x},t)$. These trajectories, which involve the operator contracting to a point at both the initial and final time, are qualitatively different from those that dominate the out-of-time-order correlation function. In the absence of conservation laws, local correlations decay exponentially: $\langle \mathcal{O}(\mathbf{x},t)\mathcal{O}(0,0)\rangle \sim exp(-s_{\text{eq}}r\left(\mathit{v}\right)t)$, where $\mathit{v}=\mathit{x}/t$ defines a ray in spacetime, and $r\left(\mathit{v}\right)$ is a rate function associated with this ray. We express $r\left(\mathit{v}\right)$ in terms of cost functions for various spacetime structures. In $1+1\mathrm{D}$ the operator histories can exhibit a phase transition at a critical value $v_c$ of the ray velocity, which leads to a singular behavior in $r\left(\mathit{v}\right)$. At low velocities, the dominant Feynman histories are “fat”: The operator grows to a size of order $t^{\alpha }$ (with $\alpha =1/2$ in the simplest case) before contracting to a point again. At high velocities the trajectories are “thin”: The operator always remains of order-one size. In a Haar-random quantum circuit, this transition maps to a simple binding transition for a pair of random walks, which represent the left and right spatial boundaries of the operator. In higher-dimensional systems, thin trajectories always dominate. We discuss the circumstances in which the butterfly velocity $v_B$ can be deduced from a time-ordered two-point function, rather than the out-of-time ordered correlator. In the random circuit, correlators may also be computed in the framework of an effective Ising-like statistical mechanics model: we describe this calculation, as well as a special feature of the weights in the case of a $1+1\mathrm{D}$ Haar-random brickwork circuit. The present paper addresses lattice models, but also suggests the possibility of morphological phase transitions for real-time Feynman diagrams in continuum quantum field theories.

We present a study combining total-scattering powder neutron diffraction, reverse Monte Carlo simulations, and magnetic pair-distribution function analysis to deduce both the static and dynamic short-range magnetic spin correlations in two compounds of the rare-earth strontium oxides: ${\mathrm{SrGd}}_2{\mathrm{O}}_4$ and ${\mathrm{SrNd}}_2{\mathrm{O}}_4$. Both compounds exhibit a distorted honeycomb lattice which forms a set of zigzag ladders along the crystallographic $c$ axis of space group $Pnam$. Each set consists of two one-dimensional chains separated by diagonal rungs forming triangles, thus inducing a large degree of geometrical frustration due to the antiferromagnetic exchange between the magnetic ions. Significant magnetic diffuse scattering was observed well above the respective Néel temperatures and analyzed with reverse Monte Carlo techniques in reciprocal space. As a complementary analysis in real space we have derived the magnetic pair-distribution function for both compounds as a function of temperature. Our results clearly indicate that the individual zigzag ladders begin ordering well above $T_{\mathrm{N}}$ due to the dominating nearest and next-nearest interactions, long before interladder spin correlations become significant only slightly above $T_{\mathrm{N}}$. Additionally, for ${\mathrm{SrNd}}_2{\mathrm{O}}_4$ we find some qualitative differences between the short-range spin correlations above $T_{\mathrm{N}}$ and those observed in the ordered state.

A new triangular-lattice intermetallic compound ${\mathrm{Gd}}_2{\mathrm{Ir}}_{0.97}{\mathrm{Si}}_{2.97}$ was successfully synthesized as single phase by deliberately introducing vacancies. Theoretical analysis suggests that the ground state is competing with several low-energy spin configurations due to magnetic frustration on a nearly ideal triangular lattice. Despite a number of competing magnetic states, the compound exhibits long-range antiferromagnetic order at 16 K, a long-range ferrimagnetic transition at 6.5 K, and a reentrant cluster-glass transition below $T_f$ $\sim$ 3 K. The complex magnetism in the compound could be correlated with competing antiferromagnetic and ferrimagnetic structures predicted theoretically.

We revisit the well-known theoretical phase diagram for a two-dimensional chiral magnet with an easy-axis uniaxial anisotropy and an applied magnetic field. A particular emphasis is given to merons—quanta of the phase transition between the cycloid and the hexagonal skyrmion lattice, which possess fractional topological charges $\pm 1/2$. A localized state of two coupled merons with opposite topological charges exhibits the nonreciprocal current-driven dynamics: it experiences no skyrmion Hall effect, but due to the asymmetry of the attracting intermeron potential gives rise to asymmetric current-velocity characteristics. We also address the problem of skyrmion ordering into hexagonal and square skyrmion crystals. In an entire field range, the square lattice turns out a saddle point state between differently oriented hexagonal lattices. Merons separating elongated skyrmions were also shown to form a local energy minimum by forming a hexagonal lattice shadowing the ordinary lattice of skyrmions. In addition, we focus on the anisotropy-driven transformations of modulated phases and their isolated counterparts, which exist as detached branches of solutions on both sides from the critical anisotropy value. Moreover, a family of isolated skyrmions with both rotational senses can be found near this anisotropy value. Our results are useful from both the fundamental point of view, since they complement previous findings, and from the point of view of applications, since they provide guidelines for developing meron-based spintronic devices.

We propose and investigate a new platform for the realization of Majorana zero modes in a thin-film heterostructure composed of an easy-plane ferromagnet and a superconductor with spin-orbit coupling. The system can support an energetically favorable bound state comprising a magnetic vortex and a superconducting vortex. We show that a hybrid vortex thus created can host a robust zero-energy Majorana bound state at its core over a wide range of parameters, with its partner zero mode located at the boundary of a disk-shaped topological region. We identify a novel mechanism underlying the formation of the topological phase that, remarkably, relies on the orbital effect of the magnetization field and not on the usual Zeeman effect. The in-plane components of magnetization couple to electrons as a gauge potential with nonzero curl, thus creating an emergent magnetic field responsible for the gapped topologically nontrivial region surrounding the vortex core. Our construction allows the mobility of magnetic vortices to be imposed on the Majorana zero mode at the core of the superconducting vortex. In addition, the system shows a rich interplay between magnetism and superconductivity which might aid in developing future devices and technologies.

The recently discovered kagome metal $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) with intertwined superconductivity and charge density wave (CDW) order has attracted considerable interest. Here we investigate the evolution of CDW and superconductivity in $\mathrm{Cs}{\left({\mathrm{V}}_{1-x}{\mathrm{Cr}}_x\right)}_3{\mathrm{Sb}}_5$, where chromium substitution for vanadium introduces extra electron carriers. Our experiments reveal that the CDW order is gradually suppressed under chromium doping, which can be understood from the band structure modifications induced by electron doping. This highlights the importance of the van Hove singularities near the $M$ and $L$ points for CDW instability. The anomalous Hall effect that is coupled with CDW in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ only appears in lightly doped $\mathrm{Cs}{\left({\mathrm{V}}_{1-x}{\mathrm{Cr}}_x\right)}_3{\mathrm{Sb}}_5$ and vanishes at $x\ge 0.06$ when the CDW order remains relatively strong. Unlike hole- and isovalent-doped $A{\mathrm{V}}_3{\mathrm{Sb}}_5$, where superconductivity is enhanced upon suppression of CDW, superconductivity is rapidly suppressed under chromium doping, pointing to a conventional $s$-wave pairing.

We investigate the electronic state of Ni-substituted ${\mathrm{CeCo}}_{1-x}{\mathrm{Ni}}_x{\mathrm{In}}_5$ by nuclear quadrupole and magnetic resonance (NQR/NMR) techniques. The heavy fermion superconductivity below $T_{\mathrm{c}}=2.3$ K for $x=0$ is suppressed by Ni substitutions, and $T_{\mathrm{c}}$ reaches zero for $x=0.25$. The ${}^{115}\mathrm{In}$ NQR spectra for $x=0.125$ and 0.25 can be explained by simulating the electrical field gradient that is calculated for a virtual supercell with density functional theory. The spin-lattice relaxation rate $1/T_1$ indicates that Ni substitution weakens antiferromagnetic correlations that are not localized near the substituent but instead are uniform in space. The temperature ($T$) dependence of ${\left(T_{1}T\right)}^{-1}$ for $x=0.25$ shows a maximum around $T_{\mathrm{g}}=2$ K and ${\left(T_{1}T\right)}^{-1}$ decreases toward almost zero when temperature is further reduced as if a gap might be opening in the magnetic excitation spectrum; however, the magnetic specific heat and the static magnetic susceptibility evolve smoothly through $T_{\mathrm{g}}$ with a $-lnT$ dependence. The peculiar $T$ dependence of ${\left(T_{1}T\right)}^{-1}$ and non-Fermi-liquid specific heat and susceptibility can be interpreted in a unified way by assuming nested antiferromagnetic spin fluctuations in a quasi-two-dimensional electronic system.

Quantum anomalous Hall (QAH) insulators host many exotic topological properties including the gapped bulk states and the dissipationless chiral edge states, which have both fundamental scientific importance and the potential application in future electronic devices. Inspired by the ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBT) family, we design a new class of compounds $X{Z}_2{T}_4$ ($X=\mathrm{V}$, Mn, Ni; $Z=\mathrm{Ga}$, In; $T=\mathrm{Se}$, Te) by substituting Bi of group VA with $Z$ of group IIIA. Based on density functional theory calculations, we find that the $Z$ atoms favor the $Z{T}_4$ tetrahedra in $X{Z}_2{T}_4$ rather than the ${\mathrm{BiTe}}_6$-like octahedral geometry in MBT. After investigating the magnetic and electronic structures of 36 materials in the $X{Z}_2{T}_4$ family, we pick out the $\alpha \text{−}{\mathrm{VGa}}_2{\mathrm{Te}}_4$ monolayer with tunable topological properties, which can transit from a trivial gapless ferromagnetic semimetal into a QAH insulator with a 10-meV global band gap. Our studies on $\alpha \text{−}{\mathrm{VGa}}_2{\mathrm{Te}}_4$ provide a practical example to explore more QAH insulators beyond the MBT family.

We propose the concept of two-dimensional (2D) non-Abelian topological insulators which can explain the energy distributions of the edge states and corner states in systems with parity-time symmetry. From the viewpoint of non-Abelian band topology, we establish the constraints on the 2D Zak phase and polarization. We demonstrate that the corner states in some 2D systems can be explained as the boundary mode of the one-dimensional edge states arising from the multiband non-Abelian topology of the system. We also propose the use of the off-diagonal Berry phase as complementary information to assist the prediction of edge states in non-Abelian topological insulators. Our work provides an alternative approach to study edge and corner modes and this idea can be extended to three-dimensional systems.

We numerically study a $\frac{5}{2}$ fractional quantum Hall system with even number of electrons using the exact diagonalization where both the strong Landau-level (LL) mixing and a finite width of the quantum well have been considered and adapted into a screened Coulomb interaction. With the principal component analysis, we are able to recognize a compressible-incompressible phase transition in the parameter space made of the magnetic field and the quantum well width by the competition between the first two leading components of the ground-state wave functions, which is consistent with the low-lying spectral feature and previous works in the odd-electron system. In addition, the presence of the subdominant third component suggests an incompressible transition occurring as the LL mixing strength grows into a certain parameter region associated with the ZnO experiments. We further investigate the strongly LL-mixed phase in this emerging region with the Hall viscosity, wave function overlaps, and the entanglement spectra. Results show it can be well described as a particle-hole symmetrized Pfaffian state with the dual topological properties of the Pfaffian and the anti-Pfaffian states.