We consider the nonequilibrium behavior of a central spin system where the central spin is periodically reset to its ground state. The quantum-mechanical evolution under this effectively dissipative dynamics is described by a discrete-time quantum map. Despite its simplicity this problem shows surprisingly complex dynamical features. In particular, we identify several metastable time-crystal resonances. Here the system does not relax rapidly to a stationary state but undergoes long-lived oscillations with a period that is an integer multiple of the reset period. At these resonances the evolution becomes restricted to a low-dimensional state space within which the system undergoes a periodic motion. Generalizing the theory of metastability in open quantum systems, we develop an effective description for the evolution within this long-lived metastable subspace and show that in the long-time limit a nonequilibrium stationary state is approached. Our study links to timely questions concerning emergent collective behavior in the “prethermal” stage of a dissipative quantum many-body evolution and may establish an intriguing link to the phenomenon of quantum synchronization.

The modern quantum theory of magnetism in solids is getting commonly derived using Green's function formalism. The popularity draws itself from remarkable opportunities to capture the microscopic landscape of exchange interactions, starting from a tight-binding representation of the electronic structure. Indeed, the conventional method of infinitesimal spin rotations, considered in terms of local force theorem, opens vast prospects of investigations regarding the magnetic environment, as well as pairwise atomic couplings. However, this theoretical concept practically is not devoid of intrinsic inconsistencies. In particular, naturally expected correspondence between single and pairwise infinitesimal spin rotations is being numerically revealed to diverge. In this work, we elaborate on this question on the model example and canonical case of body-centered-cubic (bcc) iron bulk crystal. Our analytical derivations discovered the principal preference of onsite magnetic precursors if the compositions of individual atomic interactions are in focus. The problem of extremely slow or even absent spatial convergence while considering metallic compounds was solved by developed technique, based on reciprocal space framework. Using fundamental Fourier-transform-inspired interconnection between this technique and traditional spatial representation, we shed light on symmetry breaking in bcc Fe on the level of orbitally decomposed total exchange surrounding.

The search for triplet superconductivity has been pursued intensively in a broad field of material science and quantum information for decades. Nevertheless, these novel states remain rare. Within a simplified effective three-orbital model, we reveal a spin triplet pairing in doped ${\mathrm{MoS}}_2$ by employing both the finite-temperature determinant quantum Monte Carlo approach and the ground-state constrained-phase quantum Monte Carlo method. In a wide filling region of $\langle n\rangle =0.60$–0.80 around charge neutrality $\langle n\rangle =2/3$, the $f$-wave pairing dominates over other symmetries. The pairing susceptibility strongly increases as the on-site Coulomb interaction increases, and it is insensitive to spin-orbit coupling.

We theoretically investigate asymmetric two-junction SQUIDs with different current-phase relations in the two Josephson junctions, involving higher Josephson harmonics. Our main focus is on the “minimal model” with one junction in the SQUID loop possessing the sinusoidal current-phase relation and the other one featuring additional second harmonic. The current-voltage characteristic (CVC) turns out to be asymmetric, $I(-V)\ne -I\left(V\right)$. The asymmetry is due to the presence of the second harmonic and depends on the magnetic flux through the interferometer loop, vanishing only at special values of the flux such as integer or half-integer in the units of the flux quantum. The system thus demonstrates the flux-tunable Josephson diode effect (JDE), the simplest manifestations of which is the direction dependence of the critical current. We analyze asymmetry of the overall $I\left(V\right)$ shape both in the absence and in the presence of external ac irradiation. In the voltage-source case of external signal, the CVC demonstrates the Shapiro spikes. The integer spikes are asymmetric (manifestation of the JDE) while the half-integer spikes remain symmetric. In the current-source case, the CVC demonstrates the Shapiro steps. The JDE manifests itself in asymmetry of the overall CVC shape, including integer and half-integer steps.

Krein-Hermitian Hamiltonians, i.e., Hamiltonians Hermitian with respect to an indefinite inner product, have emerged as an important class of non-Hermitian Hamiltonians in physics, encompassing both single-particle bosonic Bogoliubov–de Gennes (BdG) Hamiltonians and so-called “$PT$-symmetric” non-Hermitian Hamiltonians. In particular, they have attracted considerable scrutiny owing to the recent surge in interest for boson topology. Motivated by these developments, we formulate a perturbative Krein-unitary Schrieffer-Wolff transformation for finite-size dynamically stable Krein-Hermitian Hamiltonians, yielding an effective Hamiltonian for a subspace of interest. The effective Hamiltonian is Krein-Hermitian and, for sufficiently small perturbations, also dynamically stable. As an application, we use this transformation to justify codimension-based analyses of band touchings in bosonic BdG Hamiltonians, which complement topological characterization. We use this simple approach based on symmetry and codimension to revisit known topological magnon band touchings in several materials of recent interest.

We utilize a hybrid quantum-classical equation of motion approach to investigate the spin dynamics and spin-transfer torque in a spin valve under bias voltage. We show that the interplay between localized classical magnetic moments and conduction electrons induces a complex effective exchange coupling between the magnetic layers. This leads to a declination of magnetizations from layers' anisotropy axes even in equilibrium. Introducing a finite bias voltage triggers spin currents and related spin-transfer torques which further tilt the magnetizations and govern the relaxation processes of the spin dynamics. Analyzing different scenarios of the applied bias voltage, we show that symmetric and asymmetric voltage drops can lead to relaxation times of the spin dynamics that differ by several orders of magnitude at comparable charge currents. In both cases, we observe resonant features, where the relaxation is boosted whenever the chemical potential of the leads matches the maxima in the density of the states of the spin-valve electrons.

Spin splitting induced in a conventional superconductor weakens superconductivity by destroying spin-singlet and creating spin-triplet Cooper pairs. We demonstrate theoretically that such an effect is also caused by an adjacent compensated antiferromagnet, which yields no net spin splitting. We find that the antiferromagnet produces Néel triplet Cooper pairs, whose pairing amplitude oscillates rapidly in space similar to the antiferromagnet's spin. The emergence of these unconventional Cooper pairs reduces the singlet pairs' amplitude, thereby lowering the superconducting critical temperature. We develop a quasiclassical Green's functions description of the system employing a two-sublattice framework. It successfully captures the rapid oscillations in the Cooper pairs' amplitude at the lattice spacing scale as well as their smooth variation on the larger coherence length scale. Employing the theoretical framework thus developed, we investigate this Néel proximity effect in a superconductor/antiferromagnet bilayer as a function of interfacial exchange, disorder, and chemical potential, finding rich physics. Our findings also offer insights into experiments which have found a larger than expected suppression of superconductivity by an adjacent antiferromagnet.

We have investigated the Cu-derived electronic structure of iridium spinel compound ${\mathrm{CuIr}}_2{\mathrm{S}}_4$ with a metal-insulator transition (MIT) at $T_{\mathrm{MIT}}\sim 230$ K by means of high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS) and Cu $K\alpha$ resonant x-ray emission spectroscopy (RXES) around the Cu $K$ absorption edge. The Cu $K$ HERFD-XAS spectrum in the metallic phase exhibits a peak in the pre-edge region at an incident photon energy of $h{\nu }_{\mathrm{in}}=8981$ eV. The peak is ascribed to the Cu $1s–3d$ quadrupole transition and is abruptly shifted to higher energy by about 0.5 eV on going from metallic to insulating phases. The energy shift of the unoccupied Cu $3d$ states is related to the insulator gap formation of ${\mathrm{CuIr}}_2{\mathrm{S}}_4$ via the hybridization with the Ir $5d$–S $3p$ bands. A hysteresis behavior is observed in the Cu $1s–3d$ peak intensity between cooling and heating processes around $T_{\mathrm{MIT}}$, consistent with those observed in the transport properties. The abrupt change in the unoccupied Cu $3d$ states across MIT is also clearly detected in the Cu $K\alpha$ RXES spectra.

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.