A periodic arrangement of two-dimensional (2D) conducting planes is known to host a (bulk) plasmon dispersion that interpolates between the typical, gapped behavior of three-dimensional (3D) metals and a gapless, acoustic regime as a function of the out-of-plane wave vector. The semi-infinite system—the configuration relevant to electron energy loss spectroscopy (EELS) in a reflection geometry, as in high-resolution EELS (HREELS)—is known to host a surface plasmon that ceases to propagate below a cutoff wave vector. As the f-sum rule requires a finite response whether there exist sharp excitations, we demonstrate that what remains in the surface loss function—the material response probed by HREELS—is the contribution from the (bulk) plasmon of the infinite system. We provide a one-to-one mapping between the plasmon continuum and the spectral weight in the surface loss function. In light of this result, we suggest that HREELS be considered a long-wavelength probe of the plasmon continuum in layered materials.

We study the topology of the Floquet states and time-averaged optical conductivity of the lattice model of a thin topological insulator subject to a circularly polarized light using the extended Kubo formalism. Two driving regimes, the off-resonant and on-resonant, and two models for the occupation of the Floquet states, the ideal and mean-energy occupation, are considered. In the ideal occupation the real part of DC optical Hall conductivity is shown to be quantized while it is not quantized for the mean-energy distribution. The optical transitions in the Floquet band structure depend strongly on the occupation and also the optical weight which consequently affect all components of optical conductivity. At high frequency regime we present an analytical calculation of the effective Hamiltonian and also its phase diagram which depends on the tunneling energy between two surfaces. The topology of the system shows rich phases when it is irradiated by a weak on-resonant drive giving rise to emergence of anomalous edge states.

Conventionally a mobility edge (ME) marks a critical energy that separates two different transport zones where all states are extended and localized, respectively. Here we propose a quasiperiodic spin-orbit coupled lattice model with experimental feasibility to realize a quantum phase with three coexisting energy-dependent zones, i.e., the extended, critical, and localized zones, and uncover the underlying generic mechanism for the occurrence of this quantum phase. Accordingly, this phase exhibits types of MEs which separate the extended states from critical ones and the localized states from critical ones, respectively. We introduce the diagnostic quantities to characterize and distinguish the different zones and show that the predicted phase can be detected by measuring the fractal dimension or conductivities. The experimental realization is also proposed and studied. This work extends the concept of ME and enriches the quantum phases in disordered systems, which sheds light on searching for localization and critical phenomena with transport and thermoelectric effects.

We discover superconductivity (SC) in ${\mathrm{LaAgSb}}_2$ at ambient pressure and its close correlation with a charge density wave (CDW) under pressure. The superconducting transition temperature ($T_c$) exhibits a sharp peak at a CDW critical pressure of 3.2 GPa. We demonstrate that the carriers inhabiting the Sb-square net are crucial not only in the formation of CDW but also in SC for their relatively strong electron-phonon coupling (EPC). Furthermore, theoretical EPC strength in pristine ${\mathrm{LaAgSb}}_2$ cannot explain the observed peak with $T_c\sim 1$ K, which indicates that an additional mechanism reinforces SC only around the CDW critical pressure.

Despite many efforts, we still lack a clear picture of how heavy electrons emerge and develop on the Kondo lattice. Here we introduce a key concept named the hybridization bond phase and propose a scenario based on phase correlation to address this issue. The bond phase is a gauge-invariant quantity combining two on-site hybridization fields mediated by intersite magnetic correlations. Its probabilistic distribution decays exponentially with site distance, from which a characteristic length scale can be extracted to describe the spatial correlation of Kondo hybridizations. Our calculations show that this correlation length grows logarithmically with lowering temperature at large Kondo coupling, and reveals a precursor pseudogap state with short-range phase correlation before long-range phase coherence is developed to form the Kondo insulating (or heavy electron) state at low temperatures. This provides a potential microscopic explanation of the two-stage hybridization proposed by recent pump-probe experiments and the logarithmic scaling in the phenomenological two-fluid model. Our work offers a theoretical framework to describe the phase-related physics in Kondo lattice systems.

We report on the terahertz measurements of a GaAs membrane with a grating of strips formed lithographically in a two-dimensional high-mobility electron system. The designed metasurface demonstrates a highly anisotropic response. For the incident radiation polarization along the strips, a plasma edge is present in the transmission through the structure. Its frequency can be controlled by engineering the geometrical parameters of the metasurface. When the excitation polarization is perpendicular to the strips, no plasmonic response is observed apart from the transmission governed by the Fabry-Pérot function.

Pair localization in one-dimensional quasicrystals with nearest-neighbor hopping is independent of whether short-range interactions are repulsive or attractive. We numerically demonstrate that this symmetry is broken when the hopping follows a power law $1/r^{\alpha }$. In particular, for repulsively bound states, we find that the critical quasiperiodicity that signals the transition to localization is always bounded by the standard Aubry-André critical point, whereas attractively bound dimers get localized at larger quasiperiodic modulations as the range of the hopping increases. Extensive numerical calculations establish the contrasting nature of the pair energy gap for repulsive and attractive interactions, as well as the behavior of the algebraic localization of the pairs as a function of quasiperiodicity, interaction strength, and power-law hops. The results here discussed are of direct relevance to the study of the quantum dynamics of systems with power-law couplings.

Hydrogen vibration excitations of fluorite-type $\mathrm{Zr}{\mathrm{H}}_{1.8}$ and $\mathrm{Ti}{\mathrm{H}}_{1.84}$ were investigated at pressures up to 21 and 4 GPa, respectively, by incoherent inelastic neutron scattering experiments. The excitations of both the samples were well described by quantum harmonic oscillator (QHO) over the entire pressure region of this study. The first excitation energies increased with increasing pressure, as described by the equations $E_1\left(\mathrm{meV}\right)=141.4\left(2\right)+1.02\left(2\right)P$ (GPa) and $E_1\left(\mathrm{meV}\right)=149.4\left(1\right)+1.21\left(8\right)P$ (GPa) for $\mathrm{Zr}{\mathrm{H}}_{1.8}$ and $\mathrm{Ti}{\mathrm{H}}_{1.84}$, respectively. Coupling with pressure dependence of lattice parameters determined by diffraction experiments, the relations between metal-hydrogen distance $\left(d_{\mathrm{M}\text{−}\mathrm{H}}\right)$ and $E_1$ of $\mathrm{Zr}{\mathrm{H}}_{1.8}$ and $\mathrm{Ti}{\mathrm{H}}_{1.84}$ at high pressures are found to be well described by the equations $E_1\left(\mathrm{meV}\right)=1.62\left(9\right)\times {10}^3{d}_{\mathrm{M}\text{−}\mathrm{H}}^{-3.32\left(7\right)}\left(\AA \right)$ and $E_1 \left(\mathrm{meV}\right)=1.47\left(21\right)\times {10}^3{d}_{\mathrm{M}\text{−}\mathrm{H}}^{-3.5\left(2\right)}\left(\AA \right)$, respectively. The slopes of these curves are very steep compared to the previously reported trend in various fluorite-type metal hydrides at ambient pressure, suggesting that pressure and chemical substitution affect the hydrogen vibration excitations differently. The hydrogen wave function spreading estimated from the $E_1$ value assuming the QHO model showed the preferential shrinkage of the wave function to the tetrahedral sites, suggesting that the local potential field for a hydrogen atom shrinks more intensively than the tetrahedral site. The preferential shrinkage of the hydrogen wave function and the steep rise in the $E_1$ at a small $d_{\mathrm{M}\text{−}\mathrm{H}}$ under pressure are likely caused by the rigid metal ion core compared to hydrogen atoms and the resulting confinement of the hydrogen atom in the narrower potential field at high pressures.

We investigated the Jahn-Teller phonon modes in ${\mathrm{LaMnO}}_3$ film grown on an orthorhombic (110)-oriented ${\mathrm{GdScO}}_3$ substrate using Raman spectroscopy, which enabled us to verify the presence of the cooperative Jahn-Teller distortion in the ultrathin limit. A characteristic $B_{1g}$ phonon mode with preserved symmetry was observed down to the two-unit-cell thickness. This finding suggests the absence of a film-thickness-driven critical transition of the cooperative Jahn-Teller distortion and the corresponding orbital order when the two-dimensional Jahn-Teller-distortion plane is designed to be parallel to the substrate by misfit strain.

We report a detailed experimental and theoretical study on the effect of hydrostatic pressure on various structural and magnetic aspects of the layered honeycomb antiferromagent $\alpha \text{−}{\mathrm{RuCl}}_3$. Through measurements of the magnetic susceptibility $\chi$ performed under almost ideal hydrostatic-pressure conditions by using helium as a pressure-transmitting medium, we find that the phase transition to zigzag-type antiferromagnetic order at $T_N$ = 7.3 K can be rapidly suppressed to about 6.1 K at a weak pressure of about 94 MPa. A further suppression of $T_N$ with increasing pressure is impeded, however, due to the occurrence of a pressure-induced structural transition at $p\ge$ 104 MPa, accompanied by a strong dimerization of Ru-Ru bonds, which gives rise to a collapse of the magnetic susceptibility. Whereas the dimerization transition is strongly first order, the magnetic transition under varying pressure and magnetic field also reveals indications for a weakly first-order transition. We assign this observation to a strong magnetoelastic coupling in this system. Measurements of $\chi$ under varying pressure in the paramagnetic regime ($T>T_N$) and before dimerization ($p<$ 100 MPa) reveal a considerable increase of $\chi$ with pressure. These experimental observations are consistent with the results of ab initio density functional theory (DFT) calculations on the pressure-dependent structure of $\alpha \text{−}{\mathrm{RuCl}}_3$ and the corresponding pressure-dependent magnetic model. We find that pressure strengthens the nearest-neighbor Heisenberg $J$ and off-diagonal anisotropic $\mathrm{\Gamma }$ coupling and simultaneously weakens the Kitaev $K$ and anisotropic ${\mathrm{\Gamma }}^{\prime }$ coupling. Comparative susceptibility measurements on a second crystal showing two consecutive magnetic transitions instead of one, indicating the influence of stacking faults, reveal that by the application of different temperature-pressure protocols the effect of these stacking faults can be temporarily overcome.

Magnetic nanoparticles in suspensions provide fascinating model systems to study field-induced effects. Their response to external fields also opens up promising new applications, e.g., in hyperthermia. Despite significant research efforts, several basic questions regarding the influence of external fields on the magnetization dynamics are still open. Here we revisit the classical model of a suspended magnetic nanoparticle with combined internal and Brownian dynamics in the presence of an external field and discuss the field-dependent longitudinal relaxation. While internal and Brownian dynamics are independent in the field-free case, the coupling of both processes when an external field is present leads to richer and more complicated behavior. Using a highly efficient and accurate solver to the underlying Fokker-Planck equation allows us to study a broad parameter range. We identify different dynamical regimes and study their respective properties. In particular, we discuss corrections to the popular rigid-dipole approximation which are captured in terms of a simplified diffusion-jump model in the Brownian-dominated regime with rare Néel relaxation events. In addition, we discover a regime with surprising mode-coupling effects for magnetically soft nanoparticles. We explain our findings with the help of a perturbation theory, showing that in this regime the magnetization relaxation at late times is slaved by the slow Brownian motion of the nanoparticle. We discuss consequences of these findings such as the discrepancy of the longest relaxation time and the inverse frequency of the loss peak of the magnetic susceptibility.

Single-layer molybdenum disulfide ($\mathrm{SL}{\mathrm{MoS}}_2$) is a promising candidate in next-generation flexible electronics owing to its atomic thickness and excellent semiconductor property. Bending stiffness is an essential property for assessing the deformation and function of flexible devices, but its accurate evaluation remains a challenge. In this study, a spring-driven bending method is proposed to predict the bending stiffness of $\mathrm{SL}{\mathrm{MoS}}_2$ based on molecular dynamics simulation. This method can effectively constrain $\mathrm{SL}{\mathrm{MoS}}_2$ to arbitrary curvatures without the limitation of characteristic size. The results indicate that the bending property of the small-size $\mathrm{SL}{\mathrm{MoS}}_2$ is highly size and chirality dependent, but that of the large-size $\mathrm{SL}{\mathrm{MoS}}_2$ is isotropic. The bending stiffness of $\mathrm{SL}{\mathrm{MoS}}_2$ with different characteristic sizes (1–50 nm) ranges 4.88–8.87 eV, which is ascribed to the competition from different regions of $\mathrm{SL}{\mathrm{MoS}}_2$ with different bending resistances. Furthermore, a theoretical formula is established to estimate the size- and chirality-dependent bending stiffness of $\mathrm{SL}{\mathrm{MoS}}_2$. The research findings provide a computational method for evaluating the bending property of ultrathin two-dimensional (2D) materials, which offers a valuable strategy for designing the 2D materials-based flexible components with adjustable bending performance.

We study tunnel junctions consisting of a two-dimensional ferroelectric material sandwiched between graphene electrodes. We formulate a theory for the interplay of the polarization and induced free charges in such devices, taking into account quantum capacitance effects. We predict a gate-sensitive voltage difference across the polar domains, which can be measured using electrostatic force microscopy. Incorporating this electrostatic theory in the tunneling current-voltage characteristics, we identify a resonance peak associated with aligned Dirac cones as a highly sensitive probe of the polarization. This opens the way for device applications with few atom-thick polar layers acting as readable ultra-high-density memory.

Many-body systems subject to a high-frequency drive can show intriguing thermalization behavior. Prior to heating to a featureless infinite-temperature state, these systems can spend an exponentially long time in prethermal phases characterized by various kinds of order. Here, we uncover the rich nonequilibrium phase diagram of a driven frustrated two-dimensional Ising magnet with competing short-range ferromagnetic and long-range dipolar interactions. We show that the ordered stripe and nematic phases, which appear in equilibrium as a function of temperature, underpin subsequent prethermal phases in a new multistep heating process en route towards the ultimate heat death. We discuss implications for experiments on ferromagnetic thin films and other driving induced phenomena in frustrated magnets.

When a conduction electron couples with a non-coplanar localized magnetic moment, the real-space Berry curvature is exerted to cause the geometrical Hall effect, which is not simply proportional to the magnetization. So far, it has been identified in the case mostly where the non-coplanar magnetic order is present on the sublattice of conduction electrons. Here, we demonstrate that the geometrical Hall effect shows up even without long-range magnetic order of conduction electrons, as induced by non-coplanar exchange fields from the localized magnetic moments, in hole-doped pyrochlore molybdates (${\mathrm{Tb}}_{1-x}{\mathrm{Ca}}_x{)}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$. We find that the geometrical Hall effect is markedly anisotropic with respect to the applied magnetic field direction, which is in good accordance with the field-dependent magnitude and sign change of the real-space scalar spin chirality of local Tb moments. These results may facilitate the understanding of emergent electromagnetic responses induced by the Kondo-like coupling between conduction electrons and local spins in a broad material class.

Ultrasound velocity measurements of the pseudocubic spin-crossover cobaltite ${\mathrm{LaCoO}}_3$ and the lightly Ni-substituted $\mathrm{La}\left({\mathrm{Co}}_{0.99}{\mathrm{Ni}}_{0.01}\right){\mathrm{O}}_3$ reveal two types of symmetry-resolved elastic anomaly in the insulating paramagnetic state that are commonly observed in these compounds. The temperature dependence of the bulk modulus exhibits Curie-type softening upon cooling below 300 K down to $\sim 70$ K, indicating the presence of isostructural lattice instability arising from orbital fluctuations. The temperature dependence of the tetragonal and trigonal shear moduli exhibits unusual hardening upon cooling below 300 K, indicating the occurrence of elasticity crossover arising from the spin crossover. The present study also reveals that the isostructural lattice instability in ${\mathrm{LaCoO}}_3$ is sensitively suppressed with the Ni substitution, indicating the suppression of orbital fluctuations with the light Ni substitution. This Ni substitution effect in ${\mathrm{LaCoO}}_3$ can be explained on the basis that the isostructural lattice instability arises from the coupling of the lattice to the Co spin state fluctuating between the high-spin state and the intermediate-spin state.

The Eilenberger-Larkin-Ovchinnikov-Eliashberg quasiclassical theory of superconductivity is a powerful method enabling studies of a wide range of equilibrium and nonequilibrium phenomena in conventional and unconventional superconductors. We introduce here a finite element method, based on a discontinuous Galerkin approach, to self-consistently solve the underlying transport equations for general device geometries, arbitrary mean free path, and symmetry of the superconducting order parameter. We present results on (i) the influence of scalar impurity scattering on phase crystals in $d$-wave superconducting grains at low temperatures and (ii) the current flow and focusing in $d$-wave superconducting weak links, modeling recent experimental realizations of grooved high-temperature superconducting Dayem bridges. The high adaptability of this finite element method for quasiclassical theory paves the way for future investigations of superconducting devices and new physical phenomena in unconventional superconductors.

In $4d/5d$ transition-metal systems, many interesting physical properties arise from the interplay of bandwidth, electronic correlations, and spin-orbit interactions. Here, using ab initio density functional theory, we systematically study the double-perovskite-like system ${\mathrm{K}}_2\mathrm{Os}{X}_6 (X=\mathrm{F},\mathrm{Cl},\text{and}\mathrm{Br})$ with a $5d^4$ electronic configuration. Our main result is that the $J=0$ nonmagnetic insulating state develops in this system, induced by strong spin-orbit coupling (SOC). Specifically, the well-separated $\mathrm{Os}{X}_6$ octahedra lead to the cubic crystal-field limit and result in dramatically decreasing hoppings among nearest neighbor Os-Os sites. In this case, the three degenerate $t_{2g}$ orbitals are reconstructed into two “effective” $j_{\mathrm{eff}}$ ($j_{\mathrm{eff}}=1/2$ and $j_{\mathrm{eff}}=3/2$ states) states separated by the strong SOC, opening a gap with four electrons occupying the $j_{\mathrm{eff}}=3/2$ orbitals. Furthermore, the hybridization between the Os $5d$ orbitals and the $X (X=\mathrm{F},\mathrm{Cl},\text{and}\mathrm{Br}) p$ orbitals increases from F to Br, leading the electrons in ${\mathrm{K}}_2{\mathrm{OsF}}_6$ to be more localized than in ${\mathrm{K}}_2{\mathrm{OsCl}}_6$ and ${\mathrm{K}}_2{\mathrm{OsBr}}_6$, resulting in a smaller bandwidth for ${\mathrm{K}}_2{\mathrm{OsF}}_6$ than in the Cl- or Br-cases. Our results provide guidance to experimentalists and theorists working on this interesting family of osmium halides.