The waveguide-coupled plasmonic nanocavity is known to be a deep subwavelength platform that combines on-chip compatibility with a strong light-matter interaction at the resonance of local field enhancement; however, the resonant local field enhancement provides no magneto-optical activity. We conceptually investigate the deep subwavelength flow-resonant modes of the waveguide-coupled plasmonic nanocavity at which the local field enhancement deviates from the strongest and, instead, the power flow reaches its maximum. These flow-resonant modes exhibit resonant enhancements of magneto-optical activity and the inverse Faraday effect. Frequency-selective enhancement of light-magnetization interaction at the flow-resonant modes allows us to propose multichannel all-optical writing and reading of magnetic bits in nanophotonic integrated circuits. Our findings would open the way towards on-chip deep subwavelength magneto-optical devices and opto-magnetic recording with a great potential for high-speed on-chip memory.

Generalizing a previously developed analytical model of metal-enhanced fluorescence to the case of the strong coupling between a quantum emitter and a plasmonic nanoantenna, we study the fluorescence quenching in the strong coupling regime. When the nanoantenna is a simple Ag sphere and the quantum emitter approaches to its surface the fluorescence turns suppressed (both dipole and quadrupole moments of the system vanish) in the whole spectral range. However, if the nanoantenna is a plasmonic dimer with a tiny gap between two plasmonic nanoparticles, and the coupling grows due to the increase of the emitter dipole moment, the fluorescence quenching never occurs. This unexpected result explains why the nanolaser regime can be achieved with these nanoantennas, whereas a simple nanosphere coupled to quantum emitters can be a spaser.

Resonant inelastic x-ray scattering (RIXS) is increasingly used to quantify vibronic interactions in materials. In the case of periodic systems, this is most often done through fitting experimental results to a parameterized but exact analytical solution of a simple Holstein Hamiltonian that consists of a single electronic level coupled linearly to a single Einstein vibrational mode. Working within this standard framework, we consider the impact of minor generalizations of this model, namely, introducing a second Einstein oscillator and allowing the curvature of the excited-state potential energy surface to differ from that of the ground-state potential energy surface. We find that dynamics occurring in the RIXS intermediate (electronically excited) state considerably alter the quantitative interpretation of the phonon features observed in the RIXS final state. This complicates the use of the single mode model when multiple phonon modes are active. Our generalized model may in principle be substituted in this case, though we find that accurate quantitative results rely on knowledge of the excited-state potential energy surface, which typically is not known.

We theoretically investigate the Fulde-Ferrell-Larkin-Ovchinnikov state by using the microscopic quasiclassical Eilenberger equation. The Pauli paramagnetic effects and the orbital depairing effects due to vortices are treated on an equal footing for three-dimensional spherical Fermi surface model and $s$-wave pairing. The field evolution of the Larkin-Ovchinnikov (LO) state is studied in detail, such as the $H\text{−}T$ phase diagram, spatial structures of the order parameter, the paramagnetic moment, and the internal field. Field dependences of various thermodynamic quantities—the paramagnetic moment, entropy, and the zero-energy density of states—are calculated. Those quantities are shown to start quickly growing upon entering the LO state. We also evaluate the wavelength of the LO modulation, the flux line lattice form factors for small-angle neutron scattering, and the NMR spectra to facilitate the identification of the LO state. Two cases of strong and intermediate Pauli paramagnetic effect are studied comparatively. The possibility of the LO phase in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, ${\mathrm{CeCoIn}}_5$, ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$, and the organic superconductors is critically examined and crucial experiments to identify it are proposed.

A temperature-dependent approach involving Green-Kubo equilibrium atomic and spin dynamics (GKEASD) is reported to assess phonon and magnon thermal transport processes accounting for phonon-magnon interactions. Using body-centered cubic (BCC) iron as a case study, GKEASD successfully reproduces its characteristic temperature-dependent spin and lattice thermal conductivities. The nonelectronic thermal conductivity, i.e., the sum of phonon and magnon thermal conductivities, calculated using GKEASD for BCC Fe, agrees well with experimental measurements. Spectral energy analysis reveals that high-frequency phonon-magnon scattering rates are one order of magnitude larger than those at low frequencies due to energy scattering conservation rules and high densities of states. Higher temperatures further accentuate this phenomenon. This new framework fills existing gaps in simulating thermal transport across the ferro- to paramagnetic transition. Future application of this methodology to phonon- and magnon-dominant insulators and semiconductors will enhance understanding of emerging thermoelectric, spin caloritronic, and superconducting materials.

We report a dynamical quantum phase transition portrait in the alternating field transverse XY spin chain with Dzyaloshinskii-Moriya interaction by investigating singularities in the Loschmidt echo and the corresponding rate function after a sudden quench of system parameters. Unlike the Ising model, the analysis of the Loschmidt echo yields nonuniformly spaced transition times in this model. Comparative study between the equilibrium and the dynamical quantum phase transitions in this case reveals that there are quenches where one occurs without the other and reveals the regimes where they coexist. However, such transitions happen only when quenching is performed across at least a single gapless or critical line. Contrary to equilibrium phase transitions, bipartite entanglement measures do not turn out to be useful for the detection, while multipartite entanglement emerges as a good identifier of this transition when the quench is done from a disordered phase of this model.

We present muon spin relaxation ($\mu \mathrm{SR}$) measurements of the $S=3/2$ undistorted triangular lattice established in ${\mathrm{RbAg}}_2\mathrm{Cr}{\left[{\mathrm{VO}}_4\right]}_2$. The zero- (ZF) and longitudinal-field $\mu \mathrm{SR}$ spectra evidence the absence of spin freezing and long-range magnetic ordering down to $T=25$ mK, supporting the formation of a dynamic ground state. Noticeably, we observe an anomalous temperature dependence of the ZF muon spin relaxation rate ${\lambda }_{Z}F\left(T\right)$, featuring a decrease below $T=20$ K. This suggests the alteration of the dominant relaxation mechanism by the development of short-range magnetic correlations. A subsequent leveling off of ${\lambda }_{Z}F\left(T\right)$ below $T=2$ K indicates persistent spin dynamics and reveals the presence of exotic magnetic excitations. The field dependence of the muon spin relaxation rate at $T=25$ mK is well described by a diffusive spin transport model with algebraic spin-spin correlations. The suppressed long-range order and the peculiar temperature-dependent behavior of ${\lambda }_{Z}F\left(T\right)$ will be discussed in terms of the exchange interaction between ${\mathrm{Cr}}^{3+}$ moments via nonmagnetic ${\left[{\mathrm{VO}}_4\right]}^{3-}$ entities. In the title compound, the degeneracy of the $t_{2g}$-orbital set is not lifted by a space group symmetry reduction or subject to significant anisotropy resulting from spin-orbit coupling.

We report high-resolution laser-based angle-resolved photoemission spectroscopy measurements on a newly discovered iron-based superconductor, ${\mathrm{KCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ ($T_C=33.5$ K), which consists of stacking FeAs blocks with two FeAs layers separated by insulating ${\mathrm{Ca}}_2{\mathrm{F}}_2$ blocks. Bilayer splitting effect is observed for the first time in iron-based superconductors that gives rise to a total of five holelike Fermi surface sheets observed around the Brillouin zone center. Band structure calculations reproduce the observed bilayer splitting by identifying interlayer interorbital interaction between the two FeAs layers within one FeAs block. All the holelike pockets around the zone center exhibit Fermi surface-dependent and nodeless superconducting gap. The gap functions with short-range antiferromagnetic fluctuations are proposed and the gap symmetry can be well understood when the interlayer pairing is considered. The particularly strong interlayer pairing is observed for one of the bands. Our observations provide key information on the role of interlayer coupling and the interlayer pairing in generating superconductivity in iron-based superconductors.

We report experimental studies of a series of $\mathrm{BaF}{\mathrm{e}}_2{\mathrm{S}}_{3-x}\mathrm{S}{\mathrm{e}}_x$ ($0\le x\le 3$) single crystals and powder specimens using x-ray diffraction, neutron-diffraction, muon-spin-relaxation, and electrical transport measurements. A structural transformation from $Cmcm$ ($\mathrm{BaF}{\mathrm{e}}_2{\mathrm{S}}_3$) to $Pnma$ ($\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_3$) was identified around $x=0.7-1$. Neutron-diffraction measurements on the samples with $x=0.2$, 0.4, and 0.7 reveal that the Néel temperature of the stripe antiferromagnetic order is gradually suppressed from ∼120 to 85 K, while the magnitude of the ordered $\mathrm{F}{\mathrm{e}}^{2+}$ moments shows very little variation. Similarly, the block antiferromagnetic order in $\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_3$ remains robust for $1.5\le x\le 3$ with negligible variation in the ordered moment and a slight decrease of the Néel temperature from 250 K ($x=3$) to 225 K ($x=1.5$). The sample with $x=1$ near the $Cmcm$ and $Pnma$ border shows coexisting, two-dimensional, short-range stripe- and block-type antiferromagnetic correlations. The system remains insulating for all $x$, but the thermal activation gap shows an abrupt increase when traversing the boundary from the $Cmcm$ stripe phase to the $Pnma$ block phase. The results demonstrate that the crystal structure, magnetic order, and electronic properties are strongly coupled in the $\mathrm{BaF}{\mathrm{e}}_2{\mathrm{S}}_{3-x}\mathrm{S}{\mathrm{e}}_x$ system.

An electromagnetic illusion technique for free-standing objects for far-field observation using passive lossless metasurfaces is presented. Over a closed surface, the electric and magnetic surface currents needed for generating the scattered fields associated with a given physical scatterer are obtained using the surface equivalence principle. The closed surface defines an impenetrable metasurface characterized by surface impedance. A set of auxiliary evanescent surface waves are introduced and optimized over the entire surface to make the metasurface pointwise passive and lossless. Example 2D designs in TE polarization show that desired scattered fields are established immediately away from the illusion device surface. The illusion metasurfaces may be realized as tensor gradient metasurfaces.

Simple intersections between one-dimensional channels can act as coherent beam splitters for noninteracting electrons. Here we examine how coherent splitting at such intersections is affected by interparticle interactions, in the special case of an intersection of topological edge states. We derive an effective impurity model which represents the edge-state intersection within Luttinger liquid theory at low energy. For Luttinger $K=1/2$, we compute the exact time-dependent expectation values of the charge density as well as the density-density correlation functions. In general, a single incoming charge density wave packet will split into four outgoing wave packets with transmission and reflection coefficients depending on the strengths of the tunneling processes between the wires at the junction. We find that when multiple charge density wave packets from different directions pass through the intersection at the same time, reflection and splitting of the packets depend on the relative phases of the waves. Active use of this phase-dependent splitting of wave packets may make Luttinger interferometry possible. We also find that coherent incident packets generally suffer partial decoherence from the intersection, with some of their initially coherent signal being transferred into correlated quantum noise. In an extreme case, four incident coherent wave packets can be transformed entirely into density-density correlations, with the charge density itself having a zero expectation value everywhere in the final state.

Trapping electrons in quantum dots and controlling their quantum states is crucial for converting semiconductor structures into bits of quantum information processing. Here, we study single- and two-particle states in electrostatically induced quantum dots in gapped bilayer graphene (BLG), where the electron's valley states appear in a pair with their spin quantum number. Moreover, the features of BLG's electronic structure, such as the three minivalleys around each valley, and the corresponding orbital magnetic moment, modify the characteristics of the dot states. In the weakly gapped case, the single-particle level scheme is that of an almost quadratic band. For a sufficiently strong gap, an additional threefold “minivalley degeneracy” emerges. Based on these single-particle states, we analyze the orbital, spin, and valley states of two interacting electrons in the dot. We find spin and valley singlets and triplet states depending on the BLG and dot parameters, as well as two-particle interaction strength and external magnetic field. The multiplicity and the spin/valley configuration of the single- and two-particle states are essential for using these degrees of freedom in quantum transport and quantum computing with BLG quantum dot devices.

In the study of dispersion forces, nonretarded, retarded, and thermal asymptotes with their distinct scaling laws are regarded as cornerstone results governing interactions at different separations. Here, we show that when particles interact in a medium, the influence of retardation is qualitatively different, making it necessary to consider the nonmonotonous potential in full. We discuss different regimes for several cases and find an anomalous behavior of the retarded asymptote. It can change sign and lead to a trapping potential.

Despite having the fascinating physical, electronic, and optical properties of two-dimensional (2D) crystals of ${\mathrm{MoS}}_2$, $h$-BN, and InSe, none of them solely meet all the desired criteria required for high efficiency optoelectronic devices, such as a suitable band gap with very high carrier mobility, a moderate excitonic lifetime, a desirable bending modulus, environmental stability against air and water, etc. Herein, we demonstrate that these fundamental limitations can easily be overcome by building a van der Waals heterostructure (vdW-HS) of monolayer InSe either with single-layer ${\mathrm{MoS}}_2$ or $h$-BN. Our first-principles calculations suggest that compared to individual monolayers, the examined $\mathrm{InSe}/{\mathrm{MoS}}_2$ and $\mathrm{InSe}/h$-BN vdW-HSs are not only thermodynamically and mechanically more robust but also possess improved electronic and optical properties, which can be particularly useful for solar harvesting devices. Importantly, through a systematic study, we elucidate that the band gap and its nature can largely be modulated ($\sim 0.1–1.6$ eV, indirect $\rightleftharpoons$ direct, type I $\rightleftharpoons$ type II) for both the examined heterobilayers by applying mechanical strain and transverse electric field. Even more interestingly, we further show that with such bilayer heterostructures it is possible to get electron and hole mobility almost in the same order of magnitude (${10}^3$–${10}^4{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$), either naturally or by applying small biaxial strain.

We analyze the friction force exerted on a small probe particle sliding over an atomic-scale surface by means of a Green-Kubo relation and classical molecular dynamics simulations. We find that, on the atomic scale, the friction tensor can drastically vary as a function of position and sliding direction. The Green-Kubo relation yields this positional and directional dependence from equilibrium simulations of the time-dependent covariance of force acting on the probe. We find, unexpectedly, that the positional and directional dependence of energy dissipation is related to the (much simpler) static force covariance, especially in the limit where the probe only mildly perturbs the surface particles. In contrast, the (free-)energy landscape experienced by the probe is in general not a good indicator of local dissipation. We also discuss optimization strategies making use of the locally and directionally resolved friction tensor. This enables us to find optimal sliding paths and velocity protocols, e.g., minimizing energy dissipation, between two points on the surface in a given time.

We establish the correspondence between the fractional charge bound to a vortex in a textured lattice and the relevant bulk band topology in two-dimensional (2D) topological crystalline insulators. As a representative example, we consider the Kekule textured graphene whose bulk band topology is characterized by a 2D ${\mathbb{Z}}_2$ topological invariant ${\nu }_{2\mathrm{D}}$ protected by inversion symmetry. The fractional charge localized at a vortex in the Kekule texture is shown to be related to the change in the bulk topological invariant ${\nu }_{2\mathrm{D}}$ around the vortex, as in the case of the Su-Schriefer-Heeger model in which the fractional charge localized at a domain wall is related to the change in the bulk charge polarization between degenerate ground states. We show that the effective three-dimensional (3D) Hamiltonian, where the angle $\theta$ around a vortex in Kekule-textured graphene is a third coordinate, describes a 3D axion insulator with a quantized magnetoelectric polarization. The spectral flow during the adiabatic variation of $\theta$ corresponds to the chiral hinge modes of an axion insulator and determines the accumulated charge localized at the vortex, which is half-quantized when chiral symmetry exists. When chiral symmetry is absent, electric charge localized at the vortex is no longer quantized, but the vortex always carries a half-quantized Wannier charge as long as inversion symmetry exists. For the cases when magnetoelectric polarization is quantized due to the presence of symmetry that reverses the space-time orientation, we classify all possible topological crystalline insulators whose vortex defect carries a fractional charge.

The electronic and superconducting properties associated with the topologically nontrivial bands in Weyl semimetals have recently attracted much attention. We report the microscopic properties of the type-I Weyl semimetal TaAs measured by ${}^{75}\mathrm{As}$ nuclear magnetic (quadrupole) resonance under zero and elevated magnetic fields over a wide temperature range up to 500 K. The magnetic susceptibility measured by the Knight shift $K$ is found to be negative at low magnetic fields and has a strong field ($B$) dependence as $lnB$ at $T=1.56\mathrm{K}$. Such nonlinear field-dependent magnetization can be well accounted for by Landau diamagnetism arising from the three-dimensional linearly dispersed bands, and thus is a fingerprint of topological semimetals. We further study the low-energy excitations by the spin-lattice relaxation rate $1/T_1$. At zero field and $30\le T\le 250\mathrm{K}$, $1/T_{1}T$ shows a $T^2$ variation due to Weyl-node excitations. At $B\sim 13\mathrm{T}$, $1/T_{1}T$ exhibits the same $T$ dependence but with a smaller value, scaling with $K^2\propto T^2$, which indicates that the Korringa relation also holds for a Weyl semimetal. Analysis of the Korringa ratio reveals that the energy range of the linear bands is about 250 K in TaAs.

Nonequilibrium properties of quantum materials present many intriguing properties, among them athermal behavior, which violates the eigenstate thermalization hypothesis. Such behavior has primarily been observed in disordered systems. More recently, experimental and theoretical evidence for athermal eigenstates, known as “quantum scars,” has emerged in nonintegrable disorder-free models in one dimension with constrained dynamics. In this Rapid Communication, we show the existence of quantum scar eigenstates and investigate their dynamical properties in many simple two-body Hamiltonians with “staggered” interactions, involving ferromagnetic and antiferromagnetic motifs, in arbitrary dimensions. These magnetic models include simple modifications of widely studied ones (e.g., the XXZ model) on a variety of frustrated and unfrustrated lattices. We demonstrate our ideas by focusing on the two-dimensional frustrated spin-1/2 kagome antiferromagnet, which was previously shown to harbor a special exactly solvable point with “three-coloring” ground states in its phase diagram. For appropriately chosen initial product states—for example, those which correspond to any state of valid three-colors—we show the presence of robust quantum revivals, which survive the addition of anisotropic terms. We also suggest avenues for future experiments which may see this effect in real materials.

Plasmon-polariton modes in two-dimensional electron gases have a dual field-matter nature that endows them with unusual properties when electrical conductivity exceeds a certain threshold set by the speed of light. In this regime plasmons display an interesting relation with tachyons, the hypothetical faster-than-light particles. While not directly observable, tachyons directly impact properties of plasmon modes. Namely, in the “tachyon” regime, plasmon resonances remain sharp even when the carrier collision rate $\gamma$ exceeds plasmon resonance frequency. Resonances feature a recurrent behavior as $\gamma$ increases, first broadening and then narrowing and acquiring asymmetric non-Lorentzian line shapes with power-law tails extending into the tachyon continuum $\omega >ck$. This unusual behavior can be linked to the properties of tachyon poles located beneath $\omega >ck$ branch cuts in the complex $\omega$ plane: as $\gamma$ grows, tachyon poles approach the light cone and hybridize with plasmons. Narrow resonances persisting for $\gamma >\omega$, along with the unusual density and temperature dependence of resonance frequencies, provide clear signatures of the tachyon regime.

Titanium dioxide (${\mathrm{TiO}}_2$) presents a long-standing challenge for approximate Kohn-Sham density functional theory (KS-DFT), as well as to its Hubbard-corrected extension, $\text{DFT+}\mathit{\text{U}}$. We find that a previously proposed extension of first-principles $\text{DFT+}\mathit{\text{U}}$ to incorporate a Hund's $J$ correction, termed $\text{DFT+}\mathit{\text{U}}\text{+}\mathit{\text{J}}$, in combination with parameters calculated using a recently proposed linear-response theory, predicts fundamental band gaps that are accurate to well within the experimental uncertainty in rutile and anatase ${\mathrm{TiO}}_2$. Our approach builds upon established findings that Hubbard correction of both the titanium $3d$ and oxygen $2p$ subspaces in ${\mathrm{TiO}}_2$, symbolically giving $\text{DFT+}{\mathit{\text{U}}}^{d,p}$, is necessary to achieve acceptable band gaps using $\text{DFT+}\mathit{\text{U}}$. This requirement remains when the first-principles Hund's $J$ is included. We also find that the calculated gap depends on the correlated subspace definition even when using subspace-specific first-principles $U$ and $J$ parameters. Using the simplest reasonable correlated subspace definition and underlying functional, the local density approximation, we show that high accuracy results from using a relatively uncomplicated form of the $\text{DFT+}\mathit{\text{U}}\text{+}\mathit{\text{J}}$ functional. For closed-shell systems such as ${\mathrm{TiO}}_2$, we describe how various $\text{DFT+}\mathit{\text{U}}\text{+}\mathit{\text{J}}$ functionals reduce to $\text{DFT+}\mathit{\text{U}}$ with suitably modified parameters, so that reliable band gaps can be calculated for rutile and anatase with no modifications to a conventional $\text{DFT+}\mathit{\text{U}}$ code.