Searching for a new scheme to control the antiferromagnetic (AFM) domain wall is one of the most important issues for AFM spintronic devices. In this work, we study theoretically the domain wall motion of an AFM nanowire, driven by the axial anisotropy gradient generated by an external electric field and an electrocontrol of AFM domain wall motion in the merit of ultralow energy loss is demonstrated. The domain wall velocity depending on the anisotropy gradient magnitude and intrinsic material properties is simulated based on the Landau-Lifshitz-Gilbert equation and also deduced using the energy dissipation theorem. It is shown that the domain wall moves at a nearly constant speed for the small anisotropy gradient, and this motion is accelerated for the large gradient due to the enlarged domain wall width. While the domain wall mobility is independent of the lattice dimension and types of the domain wall, it can be enhanced by the Dzyaloshinskii-Moriya interaction. In addition, the physical mechanism for much faster AFM wall dynamics than ferromagnetic wall dynamics is qualitatively explained. This work unveils a promising strategy for controlling the AFM domain walls, benefiting the future of AFM spintronic applications.

Based on the dynamics of diagonal entropy (DE), we provide a nonequilibrium method to study the properties of many-body localization (MBL) transition including the critical point and the universality class. By systematically studying the dynamical behaviors of DE in the fully explored Heisenberg spin chain with quasiperiodic field, we demonstrate the DE method can efficiently detect the transition point $W_c$ between the thermal and MBL phase. We further use the method to study the MBL transition in the isotropic $XX$-ladder model, showing $W_c\sim 8.05$. We also widely explore the $XX$-ladder model with various parameters. Our results indicate that the MBL transition in the Heisenberg model and the $XX$-ladder model belong to distinct universality classes according to the obvious difference between the scaling exponents. These results can be tested in ongoing quantum simulation experiments with larger qubit numbers, since the diagonal elements of the density matrix directly yielding the DE can be easily obtained by repeatedly running single-shot measurements.

Paramagnetic spin systems with spontaneously broken spatial symmetries, such as valence bond solid phases, can host topological defects carrying nontrivial quantum numbers, which enables the paradigm of deconfined quantum criticality. In this work, we study the properties of topological defects in valence plaquette solid (VPS) phases on square and cubic lattices. We show that the defects of the VPS order parameter, in addition to possessing nontrivial quantum numbers, have fracton mobility constraints deep in the VPS phase, which has been overlooked previously. The spinon inside a single vortex cannot move freely in any direction, while a dipolar pair of vortices with spinon pairs can only move perpendicularly to its dipole moment. These mobility constraints, while they persist, can potentially inhibit the condensation of vortices and preclude a continuous transition from the VPS to the Néel antiferromagnet. Instead, the VPS melting transition can be driven by proliferation of spinon dipoles. For example, we argue that a two-dimensional VPS can melt into a stable gapless phase in the form of an algebraic bond liquid with algebraic correlations and long-range entanglement. Such a bond liquid phase yields a concrete example of the elusive two-dimensional Bose metal with symmetry fractionalization. We also study the three-dimensional valence plaquette and valence cube ordered phase, and demonstrate that the topological defects therein also have fractonic dynamics. Possible nearby phases after melting the valence plaquettes or cubes are also discussed.

Multiple-$Q$ spin textures, such as magnetic bubble and skyrmion lattices, have been observed in a large family of magnets. These spin textures can be driven into motion by external stimuli. The motion of spin textures affects the electronic states. Here we show that to correctly describe the electronic dynamics, the momentum space needs to be transcended to higher dimensions by including the ancillary dimensions associated with phason modes of the translational motion of the spin textures. The electronic states have nontrivial topology characterized by the first and second Chern numbers in the high-dimensional hybrid momentum space. This gives rise to an anomalous electric charge transport due to the motion of spin textures. By deforming the spin textures, a nonlinear response associated with the second Chern number can be induced and results in the piezoelectricity. The charge transport is derived from the semiclassical equation of motion of electrons that depends on the Berry curvature in the hybrid momentum space. Our results suggest that the motion of multiple-$Q$ spin textures has significant effects on the electronic dynamics and provides a platform to explore high-dimensional topological physics.

Artificial neural networks have been successfully incorporated into the variational Monte Carlo method (VMC) to study quantum many-body systems. However, there have been few systematic studies exploring quantum many-body physics using deep neural networks (DNNs), despite the tremendous success enjoyed by DNNs in many other areas in recent years. One main challenge of implementing DNNs in VMC is the inefficiency of optimizing such networks with a large number of parameters. We introduce an importance sampling gradient optimization (ISGO) algorithm, which significantly improves the computational speed of training DNNs by VMC. We design an efficient convolutional DNN architecture to compute the ground state of a one-dimensional SU($N$) spin chain. Our numerical results of the ground-state energies with up to 16 layers of DNNs show excellent agreement with the Bethe ansatz exact solution. Furthermore, we also calculate the loop correlation function using the wave function obtained. Our work demonstrates the feasibility and advantages of applying DNNs to numerical quantum many-body calculations.

A small dielectric object with positive permittivity may resonate when the free-space wavelength is large in comparison with the object dimensions if the permittivity is sufficiently high. We show that these resonances are described by the magnetoquasistatic approximation of the Maxwell's equations in which the normal component of the displacement current density field vanishes on the surface of the particle. They are associated to values of permittivities and frequencies for which source-free quasistatic magnetic fields exist, which are connected to the eigenvalues of a magnetostatic integral operator. We present the general physical properties of magnetoquasistatic resonances in dielectrics with arbitrary shape. They arise from the interplay between the polarization energy stored in the dielectric and the energy stored in the magnetic field. Our findings improve the understanding of resonances in high-permittivity dielectric objects and provide a powerful tool that greatly simplifies the analysis and design of high-index resonators.

The full coherent control of hybridized systems such as strongly coupled cavity-magnon states is a crucial step to enable future information processing technologies. Thus, it is particularly interesting to engineer deliberate control mechanisms such as the full control of the coupling strength which can act as a measure for coherent information exchange. In this work, we employ cavity resonator spectroscopy to demonstrate the complete control of the coupling strength of hybridized cavity-magnon states. For this, we use two driving microwave inputs which can be tuned at will. For these inputs, both the relative phase $\varphi$ and relative amplitude ratio ${\delta }_0$ can be independently controlled. We demonstrate that for specific quadratures between both tones we can increase the coupling strength, close the anticrossing gap, and enter a regime of level merging. At the transition, the absolute cavity signal is modified by $30\mathrm{dB}$ and we observe an additional linewidth decrease of $13\%$ at resonance level merging. This kind of control over the coupling, and hence linewidth, opens an avenue to enable or suppress an exchange of information and bridges the gap between quantum information and spintronics applications.

Electric, thermal, and thermoelectric transport in correlated electron systems probe different aspects of the many-body dynamics, and thus provide complementary information. These are well studied in the low- and high-temperature limits, while the experimentally important intermediate regime, in which elastic and inelastic scattering are both important, is less understood. To fill this gap, we provide comprehensive solutions of the Boltzmann equation in the presence of an electric field and a temperature gradient for two different cases: First, when electron-electron collisions are treated within the relaxation-time approximation while the full momentum dependence of electron-impurity scattering is included and, second, when the electron-impurity scattering is momentum independent, but the electron-electron collisions give rise to a momentum-dependent inelastic scattering rate of the Fermi-liquid type. We find that for Fermi-liquid as well as for Coulomb interactions, both methods give the same results for the leading temperature dependence of the transport coefficients. Moreover, the inelastic relaxation rate enters the electric conductivity and the Seebeck coefficient only when the momentum dependence of the electron-impurity collisions, analytical or nonanalytical, is included. Specifically, we show that inelastic processes only mildly affect the electric conductivity, but can generate a nonmonotonic dependence of the Seebeck coefficient on temperature and even a change of sign. Thermal conductivity, by contrast, always depends on the inelastic scattering rate even for a constant elastic relaxation rate.

As a signature of symmetry-breaking processes, the generation and annihilation of topological defects (domain walls, strings, etc.) are of great interest in condensed matter physics and cosmology. Here we propose a distinctive self-organization process through phase transitions, in which all the generated topological defects are dimensionality reduced and self-annihilated. In crystalline solids, such a unique mechanism allows a perfect single crystal after plastic deformation, which originates from the coupling of different types of broken symmetries.

We numerically investigate the properties of the quasihole excitations above the bosonic fractional Chern insulator state at filling $\nu =1/2$, in the specific case of the Harper-Hofstadter Hamiltonian with hard-core interactions. For this purpose, we employ a tree tensor network technique, which allows us to study systems with up to $N=18$ particles on a $16\times 16$ lattice and experiencing an additional harmonic confinement. First, we observe the quantization of the quasihole charge at fractional values and its robustness against the shape and strength of the impurity potentials used to create and localize such excitations. Then, we numerically characterize quasihole anyonic statistics by applying a discretized version of the relation connecting the statistics of quasiholes in the lowest Landau level to the depletions they create in the density profile [E. Macaluso et al., Phys. Rev. Lett. 123, 266801 (2019)]. Our results give a direct proof of the anyonic statistics for quasiholes of fractional Chern insulators, starting from a realistic Hamiltonian. Moreover, they provide strong indications that this property can be experimentally probed through local density measurements, making our scheme readily applicable in state-of-the-art experiments with ultracold atoms and superconducting qubits.

Type-II multiferroic materials, in which ferroelectric polarization is induced by inversion nonsymmetric magnetic order, promise new and highly efficient multifunctional applications based on the mutual control of magnetic and electric properties. Although this phenomenon has to date been limited to low temperatures, here we report a giant pressure dependence of the multiferroic critical temperature in ${\mathrm{CuBr}}_2$. At 4.5 GPa, $T_{\mathrm{C}}$ is enhanced from 73.5 to 162 K, to our knowledge the highest value yet reported for a nonoxide type-II multiferroic. This growth shows no sign of saturating and the dielectric loss remains small under these high pressures. We establish the structure under pressure and demonstrate a 60% increase in the two-magnon Raman energy scale up to 3.6 GPa. First-principles structural and magnetic energy calculations provide a quantitative explanation in terms of dramatically pressure-enhanced interactions between ${\mathrm{CuBr}}_2$ chains. These large, pressure-tuned magnetic interactions motivate structural control in cuprous halides as a route to applied high-temperature multiferroicity.

Motivated by the dramatic success of realizing cavity exciton-polariton condensation in experiment we consider the formation of polaritons from cavity photons and the amplitude or Higgs mode of a superconductor. Enabled by the recently predicted and observed supercurrent-induced linear coupling between these excitations and light, we find that hybridization between Higgs excitations in a disordered quasi-2D superconductor and resonant cavity photons can occur, forming Higgs-polariton states. This provides the potential for a new means to manipulate the superconducting state as well as the potential for novel photonic cavity circuit elements.

The power factor of a thermoelectric device is a measure of the heat-to-energy conversion efficiency in nanoscopic devices. Yet, even as interest in low-dimensional thermoelectric materials has increased, experimental research on what influences the power factor in these systems is scarce. Here, we present a detailed thermoelectric study of graphene quantum dot devices. We show that spin degeneracy of the quantum dot states has a significant impact on the zero-bias conductance of the device and leads to an increase of the power factor. Conversely, we demonstrate that nonideal heat exchange within the leads can suppress the power factor near the charge degeneracy point and nontrivially influences its temperature dependence.

Hall viscosity, also known as the Lorentz shear modulus, has been proposed as a topological property of a quantum Hall fluid. Using a recent formulation of the composite fermion theory on the torus, we evaluate the Hall viscosities for a large number of fractional quantum Hall states at filling factors of the form $\nu =n/(2pn\pm 1)$, where $n$ and $p$ are integers, from the explicit wave functions for these states. The calculated Hall viscosities ${\eta }^A$ agree with the expression ${\eta }^A=(\hslash /4)\mathcal{S}\rho$, where $\rho$ is the density and $\mathcal{S}=2p\pm n$ is the “shift” in the spherical geometry. We discuss the role of modular covariance of the wave functions, projection of the center-of-mass momentum, and also of the lowest-Landau-level projection. Finally, we show that the Hall viscosity for $\nu =\frac{n}{2pn+1}$ may be derived analytically from the microscopic wave functions, provided that the overall normalization factor satisfies a certain behavior in the thermodynamic limit. This derivation should be applicable to a class of states in the parton construction, which are products of integer quantum Hall states with magnetic fields pointing in the same direction.

A great number of studies about monolayer transition metal dichalcogenides in recent years have revealed that this kind of compound has many metastable phases with unique physical properties, not just a $1H$ phase. Here, we focus on the $1T^{″}$ phase, already existing in the experiments, and systematically investigate the electronic structures and transport properties of ${MX}_2(M$ = Mo, W; $X$ = S, Se, Te) using first-principles calculations with Boltzmann transport theory. It is found that only three molybdenum compounds have a small direct band gap at the $K$ point, which derives from the distortion of octahedral coordination $\mathrm{[}\mathrm{Mo}{X}_6]$. Among these three cases, the hole carrier mobility of ${\mathrm{MoSe}}_2$ is estimated to be as high as $690{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ at room temperature, far higher than that in the other two $\mathrm{Mo}{X}_2$. For this reason, the combination of the modest carrier effective mass and weak electron-phonon coupling leads to the outstanding transport performance of ${\mathrm{MoSe}}_2$. The Seebeck coefficient of ${\mathrm{MoSe}}_2$ is also evaluated to be as high as $\sim 300\mu \mathrm{V}/\mathrm{K}$ at room temperature. Due to the temperature-dependent mobility of $T^{-1.9}$ and higher Seebeck coefficient at low temperature, the highest thermoelectric power factor of ${\mathrm{MoSe}}_2$ is $10.2\times {10}^{-3}\mathrm{W}/{\mathrm{mK}}^2$ at 200 K. More importantly, ${\mathrm{MoSe}}_2$ has a large thermoelectric power factor with a value of $\sim 6.0\times {10}^{-3}\mathrm{W}/{\mathrm{mK}}^2$ in the temperature range from 100 to 500 K. The present results suggest that $1T^{″}{\mathrm{MoSe}}_2$ has high-performance carrier transport and is an excellent candidate for thermoelectric material.

Fragile topology is firstly demonstrated in acoustic crystals and then a realistic scheme is proposed to manipulate the transport of acoustic topological edge states (ATESs), i.e., by coupling them with side acoustic cavities. We find that single-mode cavities can completely flip the ATES pseudospin to form a perfect reflection, as long as their resonant frequencies fall into the topological band gap. The perfect reflection of the ATESs is protected by the fragile topology, which is proved by the one-dimensional topological waveguide-cavity transport theory. This fragile topologically protected perfect reflection is immune to the conventional defects (such as bending and disorder) and provides a realistic paradigm for manipulating the ATES transport. As examples, two potential applications, i.e., distance sensors and acoustic switches, are proposed based on the perfect reflection.

The real-time dynamics of equal-site correlation functions is studied for one-dimensional spin models with quenched disorder. Focusing on infinite temperature, we present a comparison between the dynamics of models with different quantum numbers $S=\frac{1}{2},1,\frac{3}{2}$, as well as of chains consisting of classical spins. Based on this comparison as well as by analyzing the statistics of energy-level spacings, we show that the putative many-body localization transition is shifted to considerably stronger values of disorder for increasing $S$. In this context, we introduce an effective disorder strength $W_{\text{eff}}$, which provides a mapping between the dynamics for different spin quantum numbers. For small $W_{\text{eff}}$, we show that the real-time correlations become essentially independent of $S$, and are moreover very well captured by the dynamics of classical spins. Especially for $S=\frac{3}{2}$, the agreement between quantum and classical dynamics is remarkably observed even for very strong values of disorder. This behavior also reflects itself in the corresponding spectral functions, which are obtained via a Fourier transform from the time to the frequency domain. As an aside, we also comment on the self-averaging properties of the correlation function at weak and strong disorder. Our work sheds light on the correspondence between quantum and classical dynamics at high temperatures and extends our understanding of the dynamics in disordered spin chains beyond the well-studied case of $S=\frac{1}{2}$.

Coupling of excitations leads to intriguing effects on the spectra of materials. We propose a cumulant formulation for neutral electronic excitations which opens the way to describe effects such as double plasmon satellites or exciton-exciton coupling. Our approach starts from the $GW+\mathrm{Bethe}$-Salpeter approximation to many-body perturbation theory which is based on a quasiparticle picture, and it adds coupling of excitations through a consistent inclusion of dynamically screened interactions. This requires one to consider scattering contributions that are usually neglected. The result is formulated in a way that highlights essential physics, that can be implemented as a postprocessing tool in first-principles codes, and that suggests which kind of materials and measurements should exhibit strong effects. This is illustrated using a model.

The integer quantum Hall (QH) effects characterized by topologically quantized and nondissipative transport are caused by an electrically insulating incompressible phase that prevents backscattering between chiral metallic channels. We probed the incompressible area susceptible to the breakdown of topological protection using a scanning-gate technique incorporating nonequilibrium transport. The obtained pattern revealed the filling-factor- ($\nu$-) dependent evolution of the microscopic incompressible structures located along the edge and in the bulk region. We found that these specific structures, respectively, attributed to the incompressible edge strip and bulk localization, show good agreement in terms of $\nu$-dependent evolution with a calculation of the equilibrium QH incompressible phases, indicating the robustness of the QH incompressible phases under the nonequilibrium condition. Further, we found that the $\nu$ dependency of the incompressible patterns is, in turn, destroyed by a large imposed current during the deep QH effect breakdown. These results demonstrate the ability of our method to image the microscopic transport properties of a topological two-dimensional system.

Current-induced magnetization switching (CIMS) via the spin-orbit torque is technologically important for applications of spintronic devices. Existing experimental studies are focused on $5d$ heavy metals with large spin-orbit coupling but theoretical studies indicate that $3d$ or $4d$ light metals with weak spin-orbit coupling may also generate sizable torque through the strong orbital Hall effect. However, CIMS has seldom been experimentally demonstrated using $3d$ or $4d$ light metals. Here, we report sizable current-induced torque and a robust CIMS in weakly spin-orbit coupled Zr based perpendicular magnetized multilayers. Current-induced effective torque field changes its sign as the Zr layer thickness varies, indicating the competition of multiple effects. Possible roles of the orbital Hall effect in the competition are discussed. Our findings widen the material scope of the CIMS and imply possible applications of the orbital Hall effect.