We propose the entanglement dipole polarization to describe the topological quadrupole phase. The quadrupole moment can be regarded as a pair of the dipole moment, in which the total dipole moment is canceled. The entanglement polarization, we propose, is useful to detect such a constituent dipole polarization. We first introduce partitions of sites in the unit cell and divide the system into two subsystems. Then, introducing an entanglement Hamiltonian by tracing out one of the subsystems partly, we compute the dipole polarization of the occupied states associated with the entanglement Hamiltonian, which is referred to as the entanglement polarization. Although the total dipole polarization is vanishing, those of the subsystems can be finite. The entanglement dipole polarization is quantized by reflection symmetries. We also introduce the entanglement polarization of the edge states, which reveals that the edge states themselves are gapped and topologically nontrivial. Therefore, such edge states yield the zero energy edge states if the system has boundaries. This is the origin of the corner states.

We report on a systematic excitation-density-dependent all-optical femtosecond time-resolved study of the spin density wave state in iron-based superconductors. The destruction and recovery dynamics are measured by means of the standard and a multipulse pump-probe technique. The experimental data are analyzed and interpreted in the framework of an extended three-temperature model. The analysis suggests that the optical-phonon energy relaxation plays an important role in the recovery of almost exclusively electronically driven spin density wave order.

We reconsider the long-explored problem that the magnitude of the measured Casimir force between an Au sphere and an indium tin oxide (ITO) film decreases significantly after the UV treatment with no respective changes in the ITO dielectric permittivity required by the Lifshitz theory. Two plausible resolutions of this conundrum are discussed: the phase transition of an ITO film from metallic to dielectric state and the modification of a film surface under the action of UV light. To exclude the latter option, we propose an improvement in the experimental scheme by adding a graphene sheet on top of an ITO film. The formalism is developed allowing precise calculation of the Casimir force between an Au sphere and a graphene sheet on top of ITO film deposited on a quartz substrate. In doing so Au, ITO, and quartz are described by the frequency-dependent dielectric permittivities and real graphene sheet with nonzero mass-gap parameter and chemical potential by the polarization tensor at nonzero temperature. Numerical computations performed both before and after the phase transition resulting from the UV treatment show that the presence of graphene leads to only a minor decrease in the drop of the Casimir force which remains quite measurable. At the same time, in the presence of graphene the guess that an observed drop originates from the modification of an ITO surface by the UV light breaks down. Similar results are obtained for the configuration of two parallel plates consisting of a graphene sheet, an ITO film, and a quartz substrate. The proposed experiments involving additional graphene sheets may help in resolution of the problems arising in application of the Lifshitz theory to real materials.

Monolayers of transition-metal dichalcogenides (TMDCs) exhibit an exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. High densities of excited charge carriers in the various band-structure valleys cause strong many-body renormalizations that influence both the electronic properties and the optical response of the material. We investigate electronic and optical properties of the typical monolayer TMDCs ${\mathrm{MoS}}_2, {\mathrm{MoSe}}_2, {\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$ in the presence of excited carriers by solving semiconductor Bloch equations on the full Brillouin zone. With increasing carrier density, we systematically find a reduction of the exciton binding energies due to Coulomb screening and Pauli blocking. Together with excitation-induced band-gap shrinkage this leads to red-shifts of excitonic resonances up to the dissociation of excitons. As a central result, we predict for all investigated monolayer TMDCs that the $\mathrm{\Sigma }$ valley shifts stronger than the $K$ valley. Two of the materials undergo a transition from direct to indirect band gaps under carrier excitation similar to well-known strain-induced effects. Our findings have strong implications for the filling of conduction-band valleys with excited carriers and are relevant to transport and optical applications as well as the emergence of phonon-driven superconductivity.

We predict the emergence of the valley Edelstein effect (VEE), which is an electric-field-induced spin-polarization effect, in gated monolayer transition-metal dichalcogenides (MTMDs). We found an unconventional valley-dependent response in which the spin polarization is parallel to the applied electric field with opposite spin polarization generated by opposite valleys. This is in sharp contrast to the conventional Edelstein effect in which the induced spin polarization is perpendicular to the applied electric field. We identify the origin of VEE as combined effects of conventional Edelstein effect and valley-dependent Berry curvatures induced by coexisting Rashba and Ising spin-orbit couplings in gated MTMDs. Experimental schemes to detect the VEE are also considered.

Topological Weyl semimetals (WSMs), a new quantum state of matter, provide the realization of relativistic Weyl fermions in solid-state physics. The appearance of the Weyl nodes will demonstrate novel transport phenomena related to the chiral anomaly. Theoretical prediction hints that the giant planar Hall effect (PHE) can be induced by the chiral anomaly in the WSMs. In this work, we performed PHE measurements to probe chiral charge in type-II WSM $T_d\text{−}\mathrm{MoT}{\mathrm{e}}_2$. Robust transverse voltage can be acquired when the magnetic field and electric current is coplanar. Subsequently, the amplitude of intrinsic PHE increased sharply at $T=50\mathrm{K}$ with the decreasing temperature. The increasing trend in the temperature dependence is attributed to coupling effect between the chiral charge and the electric one, which drives the system into intermediate-coupling states at higher field. Our work confirms that the PHE measurement is a practical transport tool to probe chiral charge pumping effect in the WSMs systems.

We present a microscopic theory of a photon drag effect that appears in a Bose-Einstein condensate of neutral particles, considering indirect excitons in a double quantum well nanostructure under the action of a polarized electromagnetic field. It is shown that the dynamical polarization of excitons results in a resonant behavior of the exciton photon drag flux when the frequency of light is close to the gap between two energy levels of internal exciton motion. Specifically, we consider the ground and first excited energy states characterized by the angular momentum difference $\pm 1$, and thus, the helicity of light matters. We show that the resulting drag current is caused by both Bose-condensed particles and the particles in the excited states. As a result, the total current represents a superposition of thresholdlike and resonant contributions—a property which can be used in frequency-selective photodetection.

The layer-based random-phase approximation is further developed to investigate electronic excitations in trilayer ABC-stacked graphene. All layer-dependent atomic interactions and Coulomb interactions are included in the dynamic charge screening. There exist rich and unique (momentum, frequency)-excitation phase diagrams, in which the complex single-particle excitations and five kinds of plasmon modes are dominated by unusual energy bands and doping carrier densities. The latter frequently experience significant Landau damping due to the former, leading to coexistence or destruction in the energy-loss spectra. Specifically, the dispersion of the only acoustic plasmon in the pristine case is dramatically changed from linear to square root, even at very low doping.

${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is considered a promising excitonic insulator (EI) candidate, in which the EI state is due to Coulomb interaction between electrons and holes. In ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, the role of phonons is significant since the EI transition is accompanied by an orthorhombic-to-monoclinic structural transition at $T_{\mathrm{s}}=328\mathrm{K}$. Our inelastic x-ray scattering measurements reveal that the transverse acoustic mode exhibits strong softening just above $T_{\mathrm{s}}$. Furthermore, the transverse optical mode exhibits linewidth broadening caused by strong electron–optical-phonon coupling above $T_{\mathrm{s}}$. Density functional theory calculations indicate that the coupled optical modes arise due to the vibration of Ta ions. Consequently, in the monoclinic phase, the coupled optical modes are “frozen” such that Ta and Se approach each other, forming atomic-displacement-type electric dipoles that are obviously different from a Peierls transition or Jahn–Teller distortion. The characteristic of electronic toroidal moment formation by the antiferroelectric arrangements of the electric dipoles forms the common feature between ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ and $1T-\mathrm{TiS}{\mathrm{e}}_2$.

We reconsider the problem of a surface-state spectrum in type-I Dirac semimetals. We find that the surface states, despite being gapped, always form branches terminating at Dirac points. Furthermore, we consider evolution of the surface states in the case when rotational symmetry is broken, and as a result, Dirac points are gapped. We find that, in this case, a special role is played by mirror symmetry relative to the plane connecting the Dirac points. When it is present, the resulting gapped state is a topological crystalline insulator in which the surface spectrum can contain either one or three Dirac points, two of which are protected solely by the mirror symmetry. The Dirac semimetal can be viewed as a topological phase transition between two phases with different mirror Chern numbers.

The motion of electron wave packets in the Dirac semimetals $A_3\text{Bi}$ ($A=$ Na,K,Rb) is studied in a semiclassical approximation. Because of the twofold degeneracy of the Dirac points and a momentum-dependent gap term in the low-energy Hamiltonian, the associated Berry curvature is non-Abelian. In the presence of background electromagnetic fields, such a Berry curvature leads to a splitting of trajectories for the wave packets that originate from different Dirac points and chiral sectors. The nature of the splitting strongly depends on the background fields as well as the initial chiral composition of the wave packets. In parallel electric and magnetic fields, while a well-pronounced valley splitting is achieved for any chirality composition, the chirality separation takes place predominantly for the initially polarized states. On the other hand, in perpendicular electric and magnetic fields, there are clear deviations from the conventional Abelian trajectories, albeit without a well-pronounced valley splitting.

Atomistic calculations of passivated nanostructures remain difficult due to the computational demands related to the high number of atoms to be typically considered. The empirical pseudopotential method (EPM) offers a good alternative in this sense, but finding trustable pseudopotentials for passivants in this method is still elusive. Following the idea of extracting nonspherically symmetric potentials from density functional theory (DFT) calculations, hydrogen pseudopotentials for silicon passivation are derived here and used to calculate the electronic states of low-dimensional structures within the EPM scheme. The single-particle Schrödinger equation is solved with the ensuing pseudopotentials for slabs with surfaces on the (111), (110), and (100) planes, as well as for passivated quantum dots and wires of different size. In all cases, the band gap is traced as a function of the sample size, showing good convergence towards the bulk value. For the slabs, the surface local density of states is also calculated and compared successfully to experiments. The derivation of the nonspherical pseudopotentials is based on an analytic formulation of the crystal potential and its connection to a series of DFT calculations, resulting in reliable, highly transferable and first-principles based passivant pseudopotentials to be used with the EPM.

Non-Abelian anyons can exist as pointlike particles in two-dimensional systems and have particle exchange statistics which are neither bosonic nor fermionic. Like in spin systems, the role of fusion (Heisenberg-like) interactions between anyons has been well studied. However, unlike our understanding of the role of bosonic and fermionic statistics in the formation of different quantum phases of matter, little is known concerning the effect of non-Abelian braid statistics. We explore this physics using an anyonic Hubbard model on a two-legged ladder which includes braiding and nearest-neighbor Heisenberg interactions among anyons. We study two of the most prominent non-Abelian anyon models: the Fibonacci and Ising types. We discover rich phase diagrams for both anyon models and show the different roles of their fusion and braid statistics.

Topological phononic crystals are artificial periodic structures that can support nontrivial acoustic topological bands, and their topological properties are linked to the existence of topological edge modes. Most previous studies have been focused on the topological edge modes in Bragg gaps, which are induced by lattice scattering. While local resonant gaps would be of great use in subwavelength control of acoustic waves, whether it is possible to achieve topological interface states in local resonant gaps is a question. In this paper, we study the topological properties of subwavelength bands in a local resonant acoustic system and elaborate the band-structure evolution using a spring-mass model. Our acoustic structure can produce three band gaps in the subwavelength region: one originates from the local resonance of unit cell and the other two stem from band folding. It is found that the topological interface states can only exist in the band-folding-induced band gaps, but never appear in the local resonant band gap. In addition, the numerical simulation in a practical system perfectly agrees with the theoretical results. Our study provides an effective approach of producing robust acoustic topological interface states in the subwavelength region.

We study spin-2 deformed-AKLT models on the square lattice, specifically a two-parameter family of $O\left(2\right)$-symmetric ground-state wave functions as defined by Niggemann, Klümper, and Zittartz, who found previously that the phase diagram consists of a Néel-ordered phase and a disordered phase which contains the AKLT point. Using tensor-network methods, we not only confirm the Néel phase but also find an XY phase with quasi-long-range order and a region adjacent to it, within the AKLT phase, with very large correlation length, and investigate the consequences of a perfectly factorizable point at the corner of that phase.

We predict a twisted skyrmion structure at the boundary of two antiparallel magnetic domains stabilized by a (synthetic or crystal) antiferromagnetic coupling on the border. Through this novel state, skyrmions with opposite polarities can be freely switched between each other by spin-polarized electric currents. We therefore propose the concept of double-track skyrmion racetrack memories and logic gates where the binary data are represented by skyrmions with different polarities. The dynamics of the skyrmion polarity reversal is theoretically studied. Using the method of image, we derive the analytical formula of the repulsive potential when a normal skyrmion approaches the domain boundary. A harmonic attractive potential is obtained for the twisted skyrmion across the border. Micromagnetic simulations compare well with analytical results. The method of image skyrmion proposed in this work can be used to deal with a large class of skyrmion-boundary interaction problems.

When quasiparticles in a BCS superconductor recombine into Cooper pairs, phonons are emitted within a narrow band of energies above the pairing energy at $2\mathrm{\Delta }$. We show that a phonon band gap restricting the escape of recombination phonons from a superconductor can increase the quasiparticle recombination lifetime by more than an order of magnitude. A phonon band gap can be realized and matched to the recombination energy with a phononic crystal, a periodically patterned dielectric membrane. We discuss in detail the nonequilibrium quasiparticle and phonon distributions that arise in a superconductor due to a phonon band gap and a pair-breaking photon signal. Although intrinsically a nonequilibrium effect, the lifetime enhancement in the small-signal regime is remarkably similar to an estimate from an equilibrium formulation. The equilibrium estimate closely follows $exp({\mathrm{\Omega }}_{bg}/k_B{T}_b)$, where ${\mathrm{\Omega }}_{bg}$ is the phonon band gap energy bandwidth above $2\mathrm{\Delta }$ and $T_b$ is the phonon bath temperature of the coupled electron-phonon system. We discuss the impact of a phononic band gap on the performance of a superconducting circuit element and propose a microwave resonator to measure the enhancement in the quasiparticle lifetime.

The absence of a realistic polarization screening scenario at the pivotal ferroelectric-metal interface impedes the widespread application of low-dimensional ferroelectric heterostructures. Employing quantitative atom-resolved (scanning) transmission electron microscopy and first-principles calculations, we report that structural and chemical reconstruction universally lowers symmetry of the ferroelectric-metal interface. Irrespective of structural and strain mismatch, chemical termination and diffusion, polar catastrophe, and electrode type, the polarization screening is executed by a flexible polarization rotation at several-unit-cell-thick interfaces. By combining nanoscale and atomic-scale microscopy investigations, our ex situ electric-field biasing experiments reveal that the monoclinically distorted interfaces may act as seeds to nucleate new domains during the polarization switching process. These findings suggest that the long-standing fatigue issue is expected to be overcome by interface modification engineering at the monolayer scale.

We present ${}^{77}\mathrm{Se}$-NMR measurements on ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ samples with sulfur content $x=0\%$, 9%, 15%, and $29\%$. Twinned nematic domains are observed in the NMR spectrum for all samples except $x=29\%$. The NMR spin-lattice relaxation rate shows that antiferromagnetic (AFM) fluctuations are initially enhanced between $x=0\%$ and $x=9\%$, but are strongly suppressed for higher $x$ values. The observed behavior of the AFM fluctuations parallels the superconducting transition temperature $T_{\mathrm{c}}$ in these materials, providing strong evidence for the primary importance of AFM fluctuations for superconductivity, despite the presence of nematic quantum criticality in the ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ system.

We construct a theory of the electrodynamic response of ferroelectric thin films containing periodic domain textures (PDTs) with ${180}^{\circ }$ polarization-oriented domains. We demonstrate that a depolarization field induced by PDTs gives rise to the negative capacitance of the ferroelectric film. We derive frequency-dependent dielectric permittivity related to the PDT dynamics across the entire frequency range. We find the resonance mode of domain oscillations in the terahertz (THz) spectral band and the singular points in the phase of the reflected THz beam that are intimately related to the negative capacitance. Our findings provide a material platform for the THz negative-capacitance-based optics of ferroelectric films and for epsilon-near-zero plasmonic THz metamaterials.