We study the hydrodynamic viscous electronic transport in a two-dimensional sample separated into two semi-infinite planes by a one-dimensional infinite barrier. The semi-infinite planes are electrically connected via the finite-size slit in the barrier. We calculate the current through the slit assuming finite voltage drop between the planes and neglecting disorder-induced Ohmic resistance, so dissipation and resistance are purely viscosity induced. We find that the only solution to the Stokes equation in this geometry, which yields a finite dissipation at finite resistance (and, hence, is not self-contradictory), is the one that fulfills both the no-stress and no-slip boundary conditions simultaneously. As a remarkable consequence, the obtained velocity profile satisfies the so-called partial-slip (Maxwell) boundary condition for any value of the slip length, which drops out from all final results. We also calculate the electronic temperature profile for the small and large heat conductivity and find asymmetric (with respect to the barrier) temperature patterns in the former case.

Skyrmions in reduced dimensions such as thin layers and interfaces are of both fundamental and technological importance. In these systems, itinerant electrons are often present together with the Rashba and Dresselhaus spin-orbit coupling (SOC). Here, we show that an itinerant electron in the presence of these interactions can nucleate the skyrmion state, even when the standard Dzyaloshinskii-Moriya interaction (DMI) is absent, and the electron can become self-trapped in the skyrmion core, forming the “skyrmionic polaron” (SkP). The formation of the SkP is investigated from a continuum model of the electron, exchange coupled to the lattice spins, by solving the appropriate Euler-Lagrange equations. The skyrmion (antiskyrmion) texture is favored by the Rashba (Dresselhaus) SOC, with the binding energy increasing quadratically with the strength of the interaction. In contrast, if the skyrmion is already formed due to a nonzero DMI, the electron is delocalized and avoids the skyrmion core until the strength of the Rashba or Dresselhaus SOC exceeds a critical value. Below this critical value, the electron is not bound to the skyrmion core, the polaron does not form, and the electron has little effect on the skyrmion state. Our work envisions the possibility of manipulating the skyrmion state in device applications by altering the strength of the Rashba or Dresselhaus interactions, e.g., by an external electric field.

We present a proposal to produce bipartite and tripartite entanglement in a hybrid magnon cavity QED system. Two macroscopic yttrium iron garnet (YIG) spheres are coupled to a single-mode microwave cavity, where the cavity photons are generated via a two-photon process induced by a strong pump field. Using mean-field theory, we show that nonlinear pumping can result in strong bipartite entanglement between the cavity photon and magnon under two conditions, i.e., ${\delta }_c{\delta }_m=2g^2$ and ${\delta }_c=-{\delta }_m$. For the latter one, we also show the possibility for producing bipartite entanglement between two magnon modes as well as tripartite entanglement among three modes. Combining these two conditions, we further derive a third condition, i.e., ${\delta }_m^2-{\varphi }^2+2g^2=0$, where tripartite entanglement can be achieved when two magnon modes have different resonant frequencies.

We present the exact solution for a two-chain quantum spin model with zigzag interactions (or a single chain with nearest-neighbor and next-nearest-neighbor couplings). Despite the geometrical frustration of the model, the integrability permits us to find an infinite number of integrals of motion using the algebraic Bethe ansatz. Exact results for the ground state energy and elementary excitations are obtained.

Magnetic skyrmions are topological quasiparticles of great interest for data storage applications because of their small size, high stability, and ease of manipulation via electric current. Antiferromagnetic (AF) skyrmions, with new features and huge benefits (ultrasmall skyrmion sizes, no transverse deflection and efficient manipulation), have recently become the subject of intense focus. Here we show that a spontaneous antiferromagnetic skyrmion/antiskyrmion lattice (AF-SkL/ASkL) emerges in the classical Heisenberg antiferromagnet on the triangular-lattice under magnetic fields, taking only exchange interactions up to third nearest neighbors ($J_1\text{−}{J}_2\text{−}{J}_3$). By means of the Luttinger-Tisza approximation and large-scale Monte Carlo simulations (combining parallel-tempering and over-relaxation with the Metropolis algorithm), we present a rich $J_2\text{−}{J}_3$ magnetic phase diagram including exotic multiple-$q$ phases, degenerate states, and a spontaneous AF-SkL/ASkL lattice at intermediate magnetic fields. In addition, we show that at zero magnetic field, exotic spin-liquid states with ringlike degeneracy emerge at intermediate temperatures, which are broken by thermal fluctuations selecting different multiple-$q$ states. These findings greatly enrich the research on antiferromagnetic skyrmions in centrosymmetric materials or lattices including relatively weak Dzyaloshinskii-Moriya interaction.

Electrically manipulating domain wall (DW) is central in the field of spintronics. Magnetic Weyl semimetal (WSM) offers an additional knob for electric manipulation of DW by utilizing the topological DW bound states. Using magnetic WSM material ${\mathrm{EuB}}_6$ as a prototype system, we investigate its DW bound states combining first-principles calculations and domain imaging via magnetic force microscopy (MFM). The MFM measurements reveal that domains with magnetization aligned along three [100] directions dominate over others under a small magnetic field along the [001] direction. Accordingly, first-principles calculations are performed based on experimentally determined DW type, which show a robust DW bound state featuring a spectral distribution over a large portion of the DW Brillouin zone. Such DW bound states should result in a significant amount of localized charges which can be used to drive DW motion via the electrostatic forces exerted on the localized DW charges.

The competition between fractionalized spin-liquid states and magnetically ordered phases is an important paradigm in frustrated magnetism. Spin-orbit coupled Mott insulators with Ising-like magnetic anisotropies, such as Kitaev materials, are a particularly rich playground to explore this competition. In this work, we use effective field theory methods to show that a direct quantum phase transition can occur in two-dimensional (2D) Ising spin systems between a topologically ordered chiral spin liquid and a phase with magnetic long-range order. Such a transition can be protected by lattice symmetries and is described by a theory of massless Majorana fields coupled to non-Abelian $\text{SO}\left(N\right)$ gauge fields with a Chern-Simons term. We further show that Euclidean Majorana zero modes bound to ${\mathbb{Z}}_2$ monopole-instantons in the emergent non-Abelian gauge field are key to understanding spontaneous symmetry breaking in the ordered phase.

A metal plate array (MPA) is the complementary structure of a metal hole array and forms spoof surface plasmon polaritons (SSPPs) supported by parallel plate modes in a unit cell. A ring dipole array, which is a periodic structure designed by providing an aperture in the conductive part of the unit cell of an MPA, has the potential to produce bound states in the continuum (BICs). In this paper, the transformation from SSPPs to BICs is confirmed using these two structures by broadening the aperture in the unit cell. Making use of this phenomenon, control of the skin depth of surface modes is numerically demonstrated while retaining the resonant frequency, and the fitting values of the plasmonic skin depth are in good agreement with theoretical values.

In electronics systems, the Weyl points can be classified into three types based on the geometry of the Fermi surface, and each type exhibits various unique and intriguing phenomena. While the type-I and type-II Weyl points have been achieved in both spinful and spinless systems, the realization of type-III Weyl points remains challenging, and has not been reported in artificial periodic systems. Here, we report the experimental observation of the type-III Weyl points in a sonic crystal. Remarkably, a single pair of type-III Weyl points is observed as the only band crossings in a frequency range, experimentally disproving a common belief in the field, namely, the minimal number of Weyl points in nonmagnetic systems is four. The consistency between experimental results and theoretical predictions confirms the existence of type-III Weyl points, noncontractible Fermi arc surface states, and chiral edge states. Our work not only fills the gap of the type-III Weyl point in sonic crystal but also stimulates related researches in other systems, such as photonic, mechanical, and cold-atom systems.

It has been shown that illumination could have significant effects on the dopability in semiconductors. However, a general understanding on these effects is still lacking. In this paper, we present a self-consistent scheme to study the doping properties in semiconductors under illumination, and unravel a general picture: the excess carriers induced by illumination can substantially suppress the formation of compensating defects and enhance the carrier mobility as well as the density of the majority carrier, because the illumination leads to an asymmetric Fermi level shift. This is exemplified by a prototype Mg doped GaN system upon exposure to light and is consistent with available experimental observations. Our work provides a fundamental understanding of the physical process occurring in semiconductors upon illumination, and paves the way to overcome the doping bottleneck via nonequilibrium techniques in semiconductors.

The Anderson transition in random graphs has raised great interest, partly out of the hope that its analogy with the many-body localization (MBL) transition might lead to a better understanding of this hotly debated phenomenon. Unlike the latter, many results for random graphs are now well established, in particular, the existence and precise value of a critical disorder separating a localized from an ergodic delocalized phase. However, the renormalization group flow and the nature of the transition are not well understood. In turn, recent works on the MBL transition have made the remarkable prediction that the flow is of Kosterlitz-Thouless type. In this paper, we show that the Anderson transition on graphs displays the same type of flow. Our work attests to the importance of rare branches along which wave functions have a much larger localization length ${\xi }_{\parallel }$ than the one in the transverse direction ${\xi }_{\perp }$. Importantly, these two lengths have different critical behaviors: ${\xi }_{\parallel }$ diverges with a critical exponent ${\nu }_{\parallel }=1$, while ${\xi }_{\perp }$ reaches a finite universal value ${\xi }_{\perp }^c$ at the transition point $W_c$. Indeed, ${\xi }_{\perp }^{-1}\approx {{\xi }_{\perp }^c}^{-1}+{\xi }^{-1}$, with $\xi \sim {(W-W_c)}^{-{\nu }_{\perp }}$ associated with a new critical exponent ${\nu }_{\perp }=\frac{1}{2}$, where $exp\left(\xi \right)$ controls finite-size effects. The delocalized phase inherits the strongly nonergodic properties of the critical regime at short scales, but is ergodic at large scales, with a unique critical exponent $\nu =\frac{1}{2}$. This shows a very strong analogy with the MBL transition: the behavior of ${\xi }_{\perp }$ is identical to that recently predicted for the typical localization length of MBL in a phenomenological renormalization group flow. We demonstrate these important properties for a small-world complex network model and show the universality of our results by considering different network parameters and different key observables of Anderson localization.

We have performed a combined experimental and theoretical investigation of the vibrational properties of ${\mathrm{Ni}}_2\mathrm{Mn}\mathrm{Sn}$ Heusler alloys. Sn-partial vibrational density of states (VDOS) of ${}^{119}\mathrm{Sn}$ was measured by nuclear resonant inelastic x-ray scattering at temperatures of 15 and 300 K, while magnetism and local environment of Sn was resolved by ${}^{119}\mathrm{Sn}$ Mössbauer spectroscopy. Using density functional theory, we associate the peaks in the VDOS with particular features in the element-resolved phonon dispersion of $\mathrm{L}{2}_1$ ordered ${\mathrm{Ni}}_2\mathrm{Mn}\mathrm{Sn}$. The good agreement between theory and experiment in the low-energy region provides the evidence that the inversion of optical modes at $\mathrm{\Gamma }$ involving the displacement of Ni and the heavier main group element atoms, which was predicted previously for other Ni-Mn-based Heusler compounds, is also a characteristic property of ${\mathrm{Ni}}_2\mathrm{Mn}\mathrm{Sn}$. Introducing different types of magnetic and antisite disorder in our calculations results in a distinctive redistribution and broadening of the Sn-VDOS, suggesting that considering partial disorder further improves the agreement with the experiment in particular at the highest phonon energies.

Magnetic skyrmions (skyrmions hereafter) are magnetization configurations, whose topological robustness and nanoscale size have led to speculation that they could find use as a next-generation information carrier. Skyrmions have been observed in magnetic multilayer materials that are thin compared to the radius of a skyrmion, and chiral cubic single crystals that can be far larger than any characteristic skyrmion scale. In these single crystals, one would expect that skyrmions could exhibit interesting three-dimensional (3D) characteristics. Here, the symmetry of the micromagnetic free energy is investigated. This symmetry permits a complex 3D modulation of a skyrmion string, which we show to be a requirement of a skyrmion coexisting with the conical state. We discuss the implications of this modulation with respect to Thiele's equation and interskyrmion interactions. Further to this internal modulation, we study theoretically and show experimentally that the strings themselves must contort towards the surfaces of their confining crystals.

Recently, the electrical switching of antiferromagnetic (AFM) order has been intensively investigated because of its application potential in data storage technology. Herein, we report the current switching of the AFM Néel vector in epitaxial Pd/CoO films as a function of temperature. Using combined measurements of Hall resistance (HR) and x-ray magnetic linear dichroism (XMLD) below and above the AFM Néel temperature, we unambiguously identified both magnetic and nonmagnetic contributions to the current-induced HR change. Through magnetic field-induced HR measurements, we quantitatively determined the percentage of current-induced CoO spin switching. Further, we showed that the thermal effect dominated the CoO magnetic switching more in samples with a thinner Pd layer and that samples with a thicker CoO layer required higher thermal activation for current-induced magnetic switching. These results provide a clear and comprehensive picture of current-induced AFM spin switching across the AFM Néel temperature.

According to the Bardeen-Cooper-Schrieffer theory, superconductivity usually needs well-defined Fermi surface(s) with strong electron-phonon coupling and moderate quasiparticle density of states. A kagome lattice can host flat bands and topological Dirac bands; meanwhile, due to the parallel Fermi surfaces and saddle points, many interesting orders are expected. Here, we report the observation of superconductivity by pressurizing a kagome compound ${\mathrm{Rb}}_2{\mathrm{Pd}}_3{\mathrm{Se}}_4$ using a diamond-anvil-cell. The parent compound shows an insulating behavior; however, it gradually becomes metallic and turns to a superconducting state when high pressure is applied. High-pressure synchrotron measurements show that there is no structural transition occurring during this process. The density-functional-theory calculations illustrate that the insulating behavior of the parent phase is due to the crystalline field splitting of the partial $\mathrm{Pd}\text{−}4d{t}_{\text{2g}}$ bands and the Se-derivative $4p$ band. However, the threshold of metallicity and superconductivity are reached when the Lifshitz transition occurs, leading to the emergence of a tiny Fermi surface at the $\mathrm{\Gamma }$ point. Our results point to an unconventional superconductivity and shed light on understanding the electronic evolution of a kagome material.

The microscopic origin of the electric Dzyaloshinskii-Moriya interaction (eDMI) is unveiled and discussed by analytical analysis and first-principles based calculations. As similar to the magnetic Dzyaloshinskii-Moriya interaction (mDMI), eDMI also originates from the electron-mediated effect and more specifically from certain electron hoppings that are being activated due to certain local inversion-symmetry breaking. However, the eDMI energy is found to be at least a third-order interaction in atomic displacements instead of bilinear in magnetic dipole moments for mDMI. Furthermore, the eDMI energy form is presented, and we find that novel electrical topological defects (namely, chiral electric bobbers) can arise from this eDMI. Thus unraveling the microscopic origin of eDMI has the potential to lead to, and explain, the discovery of novel polar topological phases.

Topological invariants in the band structure, such as Chern numbers, are important for the understanding and classification of the topological properties of matter and dictate the occurrence of exotic behaviors, yet their direct spectroscopic determination has been largely limited to electronic bands. Here, we use inelastic neutron scattering in conjunction with ab initio calculations to identify a variety of topological phonon band crossings in MnSi and CoSi single crystals. We find a distinct relation between the Chern numbers of a band-crossing node and the scattering intensity modulation in momentum space around the node. Given sufficiently high resolution, our method can be used to determine arbitrarily large Chern numbers of topological phonon band-crossing nodes.