We numerically study out-of-equilibrium dynamics in a family of Heisenberg models with $1/r^6$ power-law interactions and positional disorder. Using the semiclassical discrete truncated Wigner approximation (dTWA) method, we investigate the time evolution of the magnetization and ensemble-averaged single-spin purity for a strongly disordered system after initializing the system in an out-of-equilibrium state. We find that both quantities display robust glassy behavior for almost any value of the anisotropy parameter of the Heisenberg Hamiltonian. Furthermore, a systematic analysis allows us to quantitatively show that, for all the scenarios considered, the stretch power lies close to the one analytically obtained in the Ising limit. This indicates that glassy relaxation behavior occurs widely in disordered quantum spin systems, independent of the particular symmetries and integrability of the Hamiltonian.

A sufficiently large parallel magnetic field will generate staggered supercurrent loops and a superfluid density wave in two weakly linked superconducting (SC) ultrathin films, resulting in an inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The SC order parameter of such an FFLO state is characterized by Bloch wave functions, called the “Bloch SC state.” The staggered supercurrent loops form an array of Josephson vortex-antivortex pairs, instead of the usual Josephson vortex lattice. Enclosing a unit cell of the array, the London's fluxoid is quantized as ${\mathrm{\Phi }}^{\prime }={\mathrm{\Phi }}_0=hc/2e$, while the net orbital magnetization caused by the staggered supercurrent is zero. Meanwhile, a small parallel magnetic field gives rise to an Fulde-Ferrell (FF) state that has uniform superfluid density. The phase transition between the Bloch SC state and the FF state belongs to the universality class of two-dimensional commensurate-incommensurate transitions. An analytical solution in terms of Jacobian elliptic functions is found to be an excellent approximation to the Bloch SC order parameter.

It is generally believed that non-Hermiticity can transform Weyl semimetals into Weyl-exceptional-ring semimetals. However, this belief is from the systems without skin effect. We investigate the non-Hermitian Weyl semimental and its Floquet engineering in a system with skin effect, which breaks the bulk-boundary correspondence in its Hermitian counterpart. It is found in both the static and periodically driven cases that the skin effect makes this general belief no longer valid. We discover that exotic non-Hermitian topological matters, e.g., a composite phase of Weyl semimetal and topological insulator with the coexisting Fermi arc and chiral boundary states, a widely tunable Hall conductivity with multiple quantized plateaus, and a Weyl semimetal with anomalous Fermi arcs formed by the crossing of gapped bound state, can be generated by the Floquet engineering. Revealing the leading role of the skin effect in determining the feature of a semimental, our result supplies a useful way to artificially synthesize exotic non-Hermitian Weyl semimetals by periodic driving.

This Research Letter studies the evolution under external compression of the wrinkling modes of single- and few-layer graphene or another two-dimensional atomic crystal embedded in or placed on a compliant matrix, or on a dense fluid in a gravitational field. An analytical model based on the nonlinear bending elasticity of the graphene is developed, which shows that the compressive surface stress causes spatial localization of the extended sinusoidal wrinkling mode with a solitonlike envelope, with the localization length decreasing with overcritical external strain. The parameters of the extended sinusoidal wrinkling mode are determined from the conditions of the anomalous softening of the flexural surface acoustic wave with predominant out-of-plane and suppressed in-plane surface displacements, propagating along the graphene in a compliant anisotropic matrix. Self-localization of the wrinkling modes finally results in the formation of strongly localized modes with approximately one-period sinusoidal profiles and external-strain-independent wavelengths. Self-localization of the wrinkling modes is governed by the derived Ginzburg-Landau-type nonlinear envelope-function equation with a negative dispersive term, which we relate with the graphene-nonlinearity-induced repulsion between soft flexural surface acoustic waves with negative effective mass. One- and two-dimensional wrinkling patterns and two types of strongly localized graphene wrinkling modes with different symmetry are described.

Strain applied to a condensed-matter system can be used to engineer its excitation spectrum via artificial gauge fields or it may tune the system through transitions between different phases. Here we demonstrate that strain tuning of the ground state of otherwise highly degenerate frustrated systems can induce novel phases, both ordered and disordered. For the classical Heisenberg antiferromagnet on the kagome lattice, we show that weak triaxial strain reduces the degeneracies of the system, leading to a classical spin liquid with noncoplanar configurations, while stronger strain drives the system into a highly unconventional state which displays signatures of both spin-glass behavior and magnetic long-range order. We provide experimentally testable predictions for the magnetic structure factor, characterize the ground-state degeneracies and the excitation spectrum, and analyze the influence of sample shape and boundaries. Our paper opens the way to strain engineering of highly frustrated magnets.

We study the ordering properties of the isotropic RKKY Heisenberg model on the two-dimensional (2D) triangular lattice by extensive Monte Carlo simulations to get insights into the chiral-degenerate skyrmion crystal (SkX) of metallic magnets. Our Hamiltonian contains only the spin-quadratic RKKY interaction derived from the spherical Fermi surface, containing neither the nesting nor the many-body interaction. The SkX phase is stabilized under applied fields where the frustration associated with the oscillating nature of the RKKY interaction and the emergent many-body interactions generated by thermal fluctuations play important roles. Replica symmetry breaking, reported in our recent paper on the 3D RKKY model [K. Mitsumoto and H. Kawamura, Phys. Rev. B 104, 184432 (2021)], turns out to be absent in the present 2D model. Implications to the SkX formation mechanism are discussed.

We present a simple two-dimensional model in which the lattice degrees of freedom mediate the interactions between magnetic moments and electric dipoles. This model reproduces basic features, such as a sudden electric polarization switch-off when a magnetic field is applied and the ubiquitous dimerized distortion patterns and magnetic $\uparrow \uparrow \downarrow \downarrow$ ordering, observed in several multiferroic materials of different composition. The list includes E-type manganites, $R\mathrm{Mn}{\mathrm{O}}_3$, nickelates such as $\mathrm{Y}\mathrm{Ni}{\mathrm{O}}_3$, and other materials under strain, such as $\mathrm{Tb}\mathrm{Mn}{\mathrm{O}}_3$. In spite of its simplicity, the model presented here captures the essence of the origin of multiferroicity in a large class of type-II multiferroics.

The surface magnon polaritons (SMPs) present in thin film antiferromagnet semiconductors can be extremely confined, having most of their energy distributed within the magnetic medium. For extremely thin films these SMPs can therefore be well described using a quasimagnetostatic treatment. In this work Berreman's $4\times 4$ transfer matrix method (TMM) is used to study the SMPs supported by thicker films, beyond the magnetostatic approximation. Focus is placed on the antiferromagnet semiconductor ${\mathrm{MnF}}_2$ for which attenuated total reflection measurements are modeled, probing the hyperbolic dispersion of the medium. The dispersion relations from both the TMM and the analytical quasimagnetostatic approximation are compared. For thicker films, the coupling efficiencies into the SMP energy channels are analyzed as a function of air gap distances. The TMM is used to probe the SMP dispersion for realistic experimental configurations with modifications including type of substrate, film thickness, and crystal axis orientation. Rich phenomena are observed such as strong SMP-waveguide mode coupling and SMPs with negative refraction.

The electronic structure of ${\mathrm{ThPd}}_2{\mathrm{Al}}_3$, which is isostructural to the heavy fermion superconductor ${\mathrm{UPd}}_2{\mathrm{Al}}_3$, was investigated by photoelectron spectroscopy. The band structure and Fermi surfaces of ${\mathrm{ThPd}}_2{\mathrm{Al}}_3$ were obtained by angle-resolved photoelectron spectroscopy (ARPES), and the results were well explained by the band-structure calculation based on the local density approximation. The comparison between the ARPES spectra and the band-structure calculation suggests that the Fermi surface of ${\mathrm{ThPd}}_2{\mathrm{Al}}_3$ mainly consists of the $\mathrm{Al}3p$ and $\mathrm{Th}6d$ states with a minor contribution from the $\mathrm{Pd}4d$ states. The comparison of the band structures between ${\mathrm{ThPd}}_2{\mathrm{Al}}_3$ and ${\mathrm{UPd}}_2{\mathrm{Al}}_3$ argues that the $\mathrm{U}5f$ states form Fermi surfaces in ${\mathrm{UPd}}_2{\mathrm{Al}}_3$ through hybridization with the $\mathrm{Al}3p$ state in the $\mathrm{Al}$ layer, suggesting that the Fermi surface of ${\mathrm{UPd}}_2{\mathrm{Al}}_3$ has a strong three-dimensional nature.

Positronium beam formation and manipulation are required in several fundamental experiments. Efficient positron/positronium conversion in transmission configuration would offer important geometrical advantages over the reflection one for these applications. A novel type of transmission positron/positronium converters, which consists of silicon membranes with pass-through nanochannels, was produced and tested. The amount of forward emitted positronium was studied as a function of the thickness of the membranes and the nanochannel size. A maximum of, at least, $(16\pm 4)\%$ of positrons implanted in $(3.5\pm 0.5)-\mu \mathrm{m}$-thick membrane with a nanochannel size of 5–8 nm were found to be forward emitted as positronium. A similar maximum amount of, at least, $(16\pm 5)\%$, was found to be emitted from a membrane $(7.7\pm 1.3)-\mu \mathrm{m}$-thick with a nanochannel size of 7–10 nm. A preliminary evaluation shows that the maximum amount of forward emitted positronium with the entire kinetic energy distribution below 1 eV is, at least, 9% of the positrons implanted in the $(3.5\pm 0.5)-\mu \mathrm{m}$-thick membrane.

We investigate the effect of ellipticity ratio of the polarized radiation field on optoelectronic properties of a two-dimensional (2D) semi-Dirac (SD) system. The optical conductivity is calculated within the energy balance equation approach derived from the semiclassical Boltzmann equation. We find that there exists the anisotropic optical absorption induced via both the intra- and interband electronic transition channels in the perpendicular $xx$ and $yy$ directions. Furthermore, we examine the effects of the ellipticity ratio, the temperature, the carrier density, and the band-gap parameter on the optical conductivity of the 2D SD system placed in transverse and vertical directions, respectively. It is shown that the ellipticity ratio, temperature, carrier density, and band-gap parameter can play the important roles in tuning the strength, peak position, and shape of the optical conductivity spectrum. The results obtained from this study indicate that the 2D SD system can be a promising anisotropic and tunable optical and optoelectronic material for applications in innovative 2D optical and optoelectronic devices, which are active in the infrared and terahertz bandwidths.

We investigate the continuous variation of electronic states from a Fermi liquid to a quantum spin liquid induced by chemical substitution in the organic dimer-Mott system of $\kappa -{\left[{\left(\text{BEDSe-}\mathrm{TTF}\right)}_x{\left(\text{BEDT-}\mathrm{TTF}\right)}_{1-x}\right]}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\text{Br}$. Electrical transport measurements reveal that the mixing of BEDSe-TTF into the BEDT-TTF layers induces a quantum Mott transition around $x=0.10$. Although a charge gap disappears at this point, magnetic susceptibility and heat capacity measurements indicate that fermionic low-energy excitations remain in the insulating salts, suggesting that fermionic spin excitations persist. We propose that the transition can be classified into a genuine Mott transition.

We study the hydrodynamic transport of electrons in a Weyl semimetal in a strong magnetic field. Impurity scattering in a Weyl semimetal with two Weyl nodes is strongly anisotropic as a function of the direction of the field and is significantly suppressed if the field is perpendicular to the separation between the nodes in momentum space. This allows for convenient access to the hydrodynamic regime of transport, in which electron scattering is dominated by interactions rather than by impurities. In a strong magnetic field, electrons move predominantly parallel to the direction of the field, and the flow of the electron liquid in a Weyl-semimetal junction resembles the Poiseuille flow of a liquid in a pipe. We compute the viscosity of the Weyl liquid microscopically and find that it weakly depends on the magnetic field and has the temperature dependence $\eta \left(T\right)\propto T^2$. The hydrodynamic flow of the Weyl liquid can be generated by a temperature gradient. The hydrodynamic regime in a Weyl-semimetal junction can be probed via the thermal conductance $G_q(B,T)\propto B^{2}T$ of the junction.

Graphene's near-field radiative heat transfer is determined from its electrical conductivity, which is commonly modeled using the local (wave-vector independent) Kubo and Drude formulas. In this paper, we analyze the nonlocality of the graphene electrical conductivity using the Lindhard model combined with the Mermin relaxation time approximation. We also study how the variation of the electrical conductivity with the wave vector affects near-field radiative conductance between two graphene sheets separated by a vacuum gap. It is shown that the variation of the electrical conductivity with the wave vector, $k_{\rho }$, is appreciable for $k_{\rho }\mathrm{s}$ greater than $100k_0$, where $k_0$ is the magnitude of the wave vector in the free space. The Kubo model is obtained by assuming $k_{\rho }\to 0$, and thus is not valid for $k_{\rho }>100k_0$. The Kubo electrical conductivity provides an accurate estimation of the spectral radiative conductance between two graphene sheets except for around the surface-plasmon-polariton frequency of graphene and at separation gaps smaller than 20 nm where there is a non-negligible contribution from electromagnetic modes with $k_{\rho }>100k_0$ to the radiative conductance. The Drude formula proves to be inaccurate for modeling the electrical conductivity and radiative conductance of graphene except for at temperatures much below the Fermi temperature and frequencies much smaller than $\frac{2{\mu }_c}{\hslash }$, where ${\mu }_c$ and $\hslash$ are the chemical potential and reduced Planck's constant, respectively. It is also shown that the electronic scattering processes should be considered in the Lindhard model properly, such that the local electron number is conserved. A simple substitution of $\omega$ by $\omega +i\gamma$ ($\omega , i$, and $\gamma$ being the angular frequency, imaginary unit, and scattering rate, respectively) in the collisionless Lindhard model does not satisfy the conservation of the local electron number and results in significant errors in computing the electrical conductivity and radiative conductance of graphene.

The stability of a discrete time crystal against thermal fluctuations has been studied numerically by solving a stochastic Landau-Lifshitz-Gilbert equation of a periodically driven classical system composed of interacting spins, each of which couples to a thermal bath. It is shown that in the thermodynamic limit, even though the long-range temporal crystalline order is stable at low temperature, it is melting above a critical temperature, at which the system experiences a nonequilibrium phase transition. The critical behaviors of the continuous phase transition have been systematically investigated, and it is shown that despite the genuine nonequilibrium feature of such a periodically driven system, its critical properties fall into the three-dimensional Ising universality class with a dynamical exponent ($z=2$) identical to that in the critical dynamics of a kinetic Ising model without driving.

A system of one-dimensional electrons interacting via a short-range potential described by Hubbard model is considered in the regime of strong coupling using the Bethe ansatz approach. We study its momentum distribution function at zero temperature and find one additional singularity at the $3k_{\mathrm{F}}$ point. We identify that the second singularity is of the same Luttinger liquid type as the low-energy one at $k_{\mathrm{F}}$. By calculating the spectral function simultaneously, we show that the second Luttinger liquid at $3k_{\mathrm{F}}$ is formed by charge modes only, unlike the known one around $k_{\mathrm{F}}$ consisting of both spin and charge modes. This result reveals the ability of the spin-charge separation effect to split the Fermi point of free electrons into two, demonstrating its robustness beyond the low-energy limit of Luttinger liquid where it was originally found.

An unusual fine spectrum of very narrow lines ($<1\mathrm{meV}$ overall) is observed in resonance Raman scattering from a CdTe quantum well under normal light incidence, when the laser excitation is about one longitudinal optical (LO) phonon energy higher than the quantum well exciton ground state. The visibility of this fine spectrum is improved by sample doping which reduces the quantum yield of the exciton photoluminescence. The appearance of the four sideway lines of the fine spectrum is explained by the Raman scattering on the combination of LO and longitudinal acoustic (LA) [transverse acoustic (TA)] phonons with the $e1hh1\left(2s\right)$ exciton serving as an intermediate state. The central line is due to elastic scattering of the $2s$ exciton to the hot $1s$-exciton state on a static random potential of the heterostructure followed by emission of a LO phonon.