Superscattering is usually described as a scattering cross section, induced by degenerate resonances, that exceeds the single-channel limit of a subwavelength scatterer. It has important applications in sensing, bioimaging, and emissions amplification. The quality factor $(Q$ factor) of superscattering plays a significant role in the performance of these practical applications, yet it has not been extensively investigated. Here, we used a spoof plasmonic spiral structure on a deep-subwavelength scale to achieve multifrequency superscattering with a high $Q$ factor. Based on eigenmode analysis, we found that those eigenmodes supported in the spoof plasmonic spiral structure not only had very high $Q$ factors, but also exhibited natural degeneracy at high spiral degrees. We also discuss the influence of structural variation and practical material loss of the spiral structure on the performance of the superscattering. Results indicated that multifrequency, high-$Q$ superscattering was refractory to the structural variation and practical material loss of the spiral structure. Our work should anticipate future sensitive and versatile optical devices based on high-$Q$ superscattering from the microwave to the terahertz wave band.

We theoretically study the spontaneous formation of the quantum anomalous Hall effect in a graphene system of spin-bifurcated exciton-polariton condensates under nonresonant pumping. We demonstrate that, depending on the parameters of the structure, such as intensity of the pump and coupling strength between condensates, the system shows a rich variety of macroscopic magnetic ordering, including analogs of ferromagnetic, antiferromagnetic, and resonant valence bond phases. Transitions between these magnetic polarized phases are associated with dramatic reshaping of the spectrum of the system connected with spontaneous appearance of topological order.

The discovery of two-dimensional (2D) ferroic materials has stimulated substantial efforts in developing emergent functionalities by synthesizing van der Waals (vdW) heterostructures, and one promising effect is the nonvolatile electrical control of magnetism in magnetoelectric (ME) heterostructures consisting of coupled 2D vdW ferromagnetic and ferroelectric layers. In this paper, it is proposed that the asymmetric interfacial coupling in such heterostructures may seriously distort the ferroelectric double-well potential, thus destabilizing the ferroelectricity. We investigate this consequence in $\mathrm{F}{\mathrm{e}}_3\mathrm{GeT}{\mathrm{e}}_2/\alpha -\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ vdW heterostructure using the first-principles calculations. It is revealed that one of the two potential wells for ferroelectric monolayer $\alpha -\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ is suppressed by the asymmetric interfacial coupling between electric polarization and the built-in electric field induced by intrinsic charge transfer, while the ferroelectric bistability can be recovered when the $\alpha -\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ layer is thicker than three unit cells. Therefore, this work presents an alternative mechanism of critical thickness for the 2D ferroelectricity in ME vdW heterostructures.

Understanding correlation effects in topological phases of matter is at the forefront of current research in condensed matter physics. Here we try to clarify some subtleties in studying topological behaviors of interacting Weyl semimetals. It is well known that there exist two topological invariants defined to identify their topological character. One is the many-body Chern number, which can be directly linked to the Hall conductivity and thus to the two-particle correlations. The other is the topological index constructed from the single-particle Green's functions. Because the information on Green's functions is achieved more easily than the many-body wave functions, usually only the topological index is employed in the literature. However, the approach based on the single-particle Green's function can break down in the strongly correlated phase. For illustration, an exactly solvable two-orbital model with momentum-local two-body interactions is discussed, in which both topological invariants can be calculated analytically. We find that the topological index calculated from the Green's function formalism can be nonzero even for a nontopological strongly correlated phase with a vanishing many-body Chern number. In addition, we stress that the physical surface states implied by nonzero many-body Chern numbers should be the edge modes of particle-hole collective excitations, rather than those of a quasiparticle nature derived from the Green's function formalism. Our observations thus demonstrate the limitation of the validity of the Green's function formalism in the investigations of interacting topological materials.

The discovery of fracton states of matter opens up an exciting, largely unexplored field of many-body physics. Certain fracton states' similarity to gravity is an intriguing property. In an earlier work [Phys. Rev. B 99, 155126 (2019)], we have demonstrated that a simple fracton model in anti-de Sitter space satisfies several major holographic properties. In this follow-up paper, we study the eight-vertex model dual to the original model. The dual model has the advantage of illuminating the mutual information and subsystem charges pictorially, which helps to reveal its connections to various other topics in the study of holography and fracton phases. At zero temperature, the dual eight-vertex model is a discrete realization of the bit-thread model, a powerful tool developed to visualize holography. The bit-thread picture combined with subsystem charges can give a quantitative account of the isometry between the bulk and the boundary at finite energy, which is also a key issue for holography. The black hole microscopic degrees of freedom can be identified in this picture, which turn out to be encoded nonlocally on the horizon. The eight-vertex model proves to be a very helpful venue to improve our understanding of the hyperbolic fracton model as a toy model of holography.

The plasmon frequency in standard electron gases with a parabolic single-particle dispersion is a purely classical quantity that is not sensitive to electron interactions or the equation of state. We demonstrate that this canonical result no longer holds for plasmons in three-dimensional semimetals, which can thus be used to probe many-body effects in these systems. In particular, we show that the plasmon frequency in an external magnetic field displays quantum oscillations, which is not the case for the electron gas. Using the random phase approximation, results are presented for the magnetoplasmon dispersion and the loss function in Dirac semimetals. We include a full discussion of the loss function in a magnetic field as a function of the direction of propagation with respect to the magnetic field direction and discuss the transition from large magnetic fields to the low-field limit.

An increasing number of low carrier density materials exhibit a surprisingly large transport mean-free path due to inefficient momentum relaxation. Consequently, charge transport in these systems is markedly non-Ohmic but rather ballistic or hydrodynamic, features which can be explored by driving current through narrow channels. Using a kinetic equation approach, we theoretically investigate how a nonquantizing magnetic field discerns ballistic and hydrodynamic transport, in particular in the spatial dependence of the transverse electric field $E_y$: we find that $E_y$ is locally enhanced when the flow exhibits a sharp directional anisotropy in the nonequilibrium density. As a consequence, at weak magnetic fields, the curvature of $E_y$ has opposite signs in the ballistic and hydrodynamic regimes. Moreover, we find a robust signature of the onset of nonlocal correlations in the form of distinctive peaks of the transverse field, which are accessible by local measurements. Our results demonstrate that a purely hydrodynamic approach is insufficient in the Gurzhi regime once a magnetic field is introduced.

This work presents measurements of laser-induced solid-solid phase transitions in Co nanoparticles, using time-resolved wide-angle x-ray scattering with <100-ps time resolution. A comparison of the experimental results with scattering simulations indicates a phase transition from the as-synthesized $\epsilon$ phase into a mixed phase of 2/3 hcp and 1/3 fcc character during the first 100 ps. The measured scattering signal observed at the shortest investigated time delay of 20 ps has less scattering intensity at the dominant fcc/hcp Bragg peak positions compared to the difference signal acquired at 100 ps. This suggests the formation of a partially disordered solid intermediate phase on the sub-100-ps timescale.

We construct a model describing the response of a hybrid system where the electromagnetic field—in particular, surface plasmon polaritons—couples strongly with electronic excitations of atoms or molecules. Our approach is based on the input-output theory of quantum optics, and in particular it takes into account the thermal and quantum vibrations of the molecules. The latter is described within the $P\left(E\right)$ theory analogous to that used in the theory of dynamical Coulomb blockade. As a result, we are able to include the effect of the molecular Stokes shift on the strongly coupled response of the system. Our model then accounts for the asymmetric emission from upper and lower polariton modes. It also allows for an accurate description of the partial decoherence of the light emission from the strongly coupled system. Our results can be readily used to connect the response of the hybrid modes to the emission and fluorescence properties of the individual molecules, and thus are relevant in understanding any utilization of such systems, such as coherent light harvesting.

Strong coupling of semiconductor spin qubits to superconducting microwave resonators was recently demonstrated [X. Mi et al., Nature 555, 599 (2018); A. J. Landig et al., Nature 560, 179 (2018); N. Samkharadze et al., Science 359, 1123 (2018); T. Cubaynes et al., NPJ Quant. Inf. 5, 47 (2019)]. These breakthroughs pave the way for quantum information processing that combines the long coherence times of solid-state spin qubits with the long-distance connectivity, fast control, and fast high-fidelity quantum-non-demolition readout of existing superconducting qubit implementations. Here we theoretically analyze and optimize the dispersive readout of a single spin in a semiconductor double quantum dot (DQD) coupled to a microwave resonator via its electric dipole moment. The strong spin-photon coupling arises from the motion of the electron spin in a local magnetic field gradient. We calculate the signal-to-noise ratio (SNR) of the readout accounting for both Purcell spin relaxation and spin relaxation arising from intrinsic electric noise within the semiconductor. We express the maximum achievable SNR in terms of the cooperativity associated with these two dissipation processes. We find that while the cooperativity increases with the strength of the dipole coupling between the DQD and the resonator, it does not depend on the strength of the magnetic field gradient. We then optimize the SNR as a function of experimentally tunable DQD parameters. We identify wide regions of parameter space where the unwanted backaction of the resonator photons on the qubit is small. Moreover, we find that the coupling of the resonator to other DQD transitions can enhance the SNR by at least a factor of two, a “straddling” effect [J. Koch et al., Phys. Rev. A 76, 042319 (2007)] that occurs only at nonzero energy detuning of the DQD double-well potential. We estimate that with current technology, single-shot readout fidelities in the range 82–95% can be achieved within a few $\mu \mathrm{s}$ of readout time without requiring the use of Purcell filters.

We examine the directional characteristics of emission from a dipole on hyperbolic metamaterial (HMM) and we show the possibility of chiral emission which is strongly dependent on the polarization of the dipole. Using rigorous solutions of the Maxwell equations, we present analytical results on chirality which depends on the excitation of different evanescent modes in the HMM, some of which are easily accessible in current experiments. The origin of chirality lies in the near field interference of emission arising from parallel and perpendicular components of the dipole polarization especially if the two polarizations are out of phase by $\pi /2$. The dependence of chirality on the distance of the dipole from the HMM is obtained. The truly unidirectional emission is possible for some specific complex elliptic polarization of the dipole. Our analysis is based on the Green's function for the multilayered HMM. We also present results on chiral emission from emitters on metal surfaces. These results provide opportunities for controlling the excitation of the surface waves and can have potential applications in sensing molecules on HMM surfaces.

An effective Hamiltonian is derived to describe the conduction band of monolayer black phosphorus (phosphorene) in the presence of spin-orbit coupling and external electric field. The envelope function approximation along with symmetry arguments and Lowdin partitioning are utilized to derive extrinsic spin-orbit coupling. The resulting spin splitting appears in fourth-order perturbation terms and is shown to be linear in both the magnitude of the external electric field and the strength of the atomic spin-orbit coupling, similar to the Bychkov-Rashba expression but with an in-plane anisotropy. The anisotropy depends on the coupling between conduction band and other bands both close and distant in energy. The spin relaxation of conduction electrons is then calculated within the Dyakonov-Perel mechanism where momentum scattering randomizes the polarization of a spin ensemble. We show how the anisotropic Fermi contour and the anisotropic extrinsic spin splitting contribute to the anisotropy of spin-relaxation time. Scattering centers in the substrate are considered to be charged impurities with a screened Coulomb potential.

We present rigorous results for the $\mathrm{SU}\left(n\right)$ Fermi-Hubbard model on the railroad-trestle lattice. We first study the model with a flat band at the bottom of the single-particle spectrum and prove that the ground states exhibit $\mathrm{SU}\left(n\right)$ ferromagnetism when the total fermion number is the same as the number of unit cells. We then perturb the model by adding extra hopping terms and make the flat band dispersive. Under the same filling condition, it is proved that the ground states of the perturbed model remain $\mathrm{SU}\left(n\right)$ ferromagnetic when the bottom band is nearly flat. This is the first rigorous example of the ferromagnetism in nonsingular $\mathrm{SU}\left(n\right)$ Hubbard models in which both the single-particle density of states and the on-site repulsive interaction are finite.

High-harmonic generation (HHG), a typical nonlinear optical effect, has been actively studied in electron systems such as semiconductors and superconductors. As a natural extension, we theoretically study HHG from electric polarization, spin current, and magnetization in magnetic insulators under terahertz or gigahertz electromagnetic waves. We use simple one-dimensional spin-chain models with or without multiferroic coupling between spins and the electric polarization, and study the dynamics of the spin chain coupled to an external ac electric or magnetic field. We map spin chains to two-band fermions and invoke an analogy of semiconductors and superconductors. With a quantum master equation and Lindblad approximation, we compute the time evolution of the electric polarization, spin current, and magnetization, showing that they exhibit clear harmonic peaks. We also show that the even-order HHG by magnetization dynamics can be controlled by static magnetic fields in a wide class of magnetic insulators. We propose experimental setups to observe these HHGs, and estimate the required strength of the ac electric field $E_0$ for detection as $E_0\sim 100$ kV/cm–1 MV/cm, which corresponds to the magnetic field $B_0\sim 0.1$ T–1 T. The estimated strength would be relevant also for experimental realizations of other theoretically proposed nonlinear optical effects in magnetic insulators such as Floquet engineering of magnets.

The majority of the iron-based superconductors (FeSCs) exhibit a two-dimensional square lattice structure. Recent reports of pressure-induced superconductivity in the spin-ladder system, ${\mathrm{BaFe}}_2{X}_3(X=\text{S}$, Se), introduce a quasi-one-dimensional prototype and an insulating parent compound to the FeSCs. Here we report x-ray, neutron diffraction, and muon spin relaxation experiments on ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ under hydrostatic pressure to investigate its magnetic and structural properties across the pressure-temperature phase diagram. A structural phase transition was found at a pressure of 3.7(3) GPa. Neutron diffraction measurements at 6.8(3) GPa and 120 K show that the block magnetism persists even at these high pressures. A steady increase and then fast drop of the magnetic transition temperature $T_{\mathrm{N}}$ and greatly reduced moment above the pressure $P_s$ indicate potentially rich and competing phases close to the superconducting phase in this ladder system.

We explore an unusual type of quantum matter that can be realized by qubits having different physical origins. Interactions in this matter are described by essentially different coupling operators for all qubits. We show that the simplest such models, which can be realized with localized states in Dirac materials, satisfy integrability conditions that we use to describe pseudospin dynamics in a linearly time-dependent magnetic field. Generalizing to an arbitrary number of qubits, we construct a spin Hamiltonian, which we call the $\gamma$-magnet. This system does not conserve polarization of any spin and the net spin polarization. Nevertheless, for arbitrarily strong interactions, nonadiabatic dynamics, and any initial eigenstate, we find that quantum interference suppresses spin flips. This behavior resembles many-body localization but occurs in phase space of many spins rather than real space. This effect may not have a counterpart in classical physics and can be a signature of a new type of spin ordering, which is different from both disordered spin glasses and ordered phases of spin lattices.

A comparative study of ${\mathrm{PbTiO}}_3, {\mathrm{PbZrO}}_3$, and the solid solution ${\mathrm{PbZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$ is performed on the soft-mode lattice dynamics within the first Brillouin zone. We consider the six unique B-site orderings for ${\mathrm{PbZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$ representable within the $2\times 2\times 2$ primitive perovskite supercell as well as the virtual crystal approximation (VCA) to extract the phonon dispersion relations of a high-symmetry cubic-constrained form using density functional perturbation theory. We find that the most unstable modes in the rock-salt ordered structure and the VCA, like pure ${\mathrm{PbZrO}}_3$, are antiferrodistortive (AFD) while lower symmetry arrangements are dominated by $\mathrm{\Gamma }$-point ferroelectric (FE) instabilities like pure ${\mathrm{PbTiO}}_3$. Despite similarities in the phonon dispersion relations between the rock-salt ordered supercell and the VCA, the character of modes at high symmetry points are found to be different. In particular, the ${\mathrm{a}}^0{\mathrm{a}}^0{\mathrm{c}}^{-}$ and ${\mathrm{a}}^0{\mathrm{a}}^0{\mathrm{c}}^{+}$ AFD instabilities of the rock-salt ordering are replaced with ${\mathrm{a}}^{-}{\mathrm{b}}^{-}{\mathrm{c}}^{-}$ and ${\mathrm{a}}^{+}{\mathrm{b}}^{+}{\mathrm{c}}^{+}$ instabilities within the VCA. Such a rotation pattern is not seen in any of the supercell-based calculations, thus serving as a quantitative example of the inability of the method to accurately represent local structural distortions. Single modes are found exhibiting dual order parameters. At the zone center, some arrangements show mixed FE and antipolar soft modes (due to Pb motion transverse to the polar axis), and at long wavelengths all arrangements have soft modes of a mixed antipolar and AFD character. These are described with direct analysis of the eigendisplacements.