In layered van der Waals materials, the stacking of layers plays a crucial role in their electronic states. To investigate the effect of stacking of layers on the electronic states, we utilized the stacking of two kinds of $\sqrt{13}\times \sqrt{13}$ domains, which are rotated $27.8{}^{\circ }$ relative to each other, in Nb-substituted $1T\text{−}\mathrm{Ta}{\mathrm{Se}}_2$. We found the appearance of a two-dimensional superstructure of the Star of David (SoD) lattice. The SoDs in the superstructure show three different electronic states due to the different stackings of the SoDs between the top surface and the layer below. This stacking of $\sqrt{13}\times \sqrt{13}$ domains provides insights into the effects of stacking on electronic structure and has the potential to produce unique electronic states.

We study the response of a weakly damped vibrational mode of a nanostring resonator to a moderately strong resonant driving force. Because of the geometry of the experiment, the studied flexural vibrations lack inversion symmetry. As we show, this leads to a nontrivial dependence of the vibration amplitude on the force parameters. For a comparatively weak force, the response has the familiar Duffing form, but for a somewhat stronger force, it becomes significantly different. Concurrently there emerge vibrations at twice the drive frequency, a signature of the broken symmetry. Their amplitude and phase allow us to establish the cubic nonlinearity of the potential of the mode as the mechanism responsible for both observations. The developed theory goes beyond the standard rotating-wave approximation. It quantitatively describes the experiment and allows us to determine the nonlinearity parameters.

We study the effect of electron spill-out and of nonlocality on the propagation of light inside a gap between two semi-infinite metallic regions. We compare the predictions of a local response model taking into account only the spill-out, to the predictions of a quantum hydrodynamic model able to take both phenomena into account. We show that only the latter is able to correctly retrieve the correct limit when the gap closes, while the local model suffers from undesirable features (divergence of the fields, overestimation of the losses). Finally, we show that, to a certain extent, the correct results can be retrieved using a simple local approach without spill-out or a conventional Thomas-Fermi approximation, but considering an effective gap width.

Recent optical second-harmonic generation experiments observed unexpected broken spatial symmetries in the undoped spin-orbit Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, leading to intensive debates on the nature of its ground state. We propose that it is a canted antiferromagnetism (CAF) with a hidden order of circulating staggered pseudospin current. Symmetry analysis shows that a proper $c$-axis stacking of the CAF and the pseudospin current lead to a magnetoelectric coexistence state that breaks the twofold rotation, inversion, and time-reversal symmetries, consistent with experimental observations. We construct a three-dimensional Hubbard model with spin-orbit coupling and structural distortion for the five localized $5d$ Wannier orbitals centered at Ir sites and demonstrate the microscopic realization of the desired coexistence state in a wide range of band parameters via a combination of self-consistent Hartree-Fock and variational calculations.

Metallic quantum states coined strange metal (SM), with robust linear-$T$ resistivity, have been widely observed in many quantum materials under strong electron correlations, ranging from high-$T_c$ cuprate superconductors and organic superconductors to twisted multilayer graphene and ${\mathrm{MoTe}}_2/{\mathrm{WSe}}_2$ superlattices. Despite decades of intensive studies, the mystery of SM still defies any sensible theoretical explanation and has been a key puzzle in modern condensed matter physics. Here, we solve a doped Mott insulator model called an Ising-Kondo lattice, which includes static spin fluctuation. With Monte Carlo simulation, the Ising-Kondo lattice unambiguously exhibits SM phenomena accompanied with quantum critical scaling in observables, e.g., resistivity, susceptibility, and specific heat. A closer look at SM reveals the breakdown of Landau's Fermi liquid theory without any symmetry breaking, i.e., the violation of Luttinger theorem. Our paper reveals that the long-overlooked static fluctuations in literature may play an essential role in non-Fermi liquid behaviors in correlated electron systems.

The energy and wave function of a harmonically confined two-electron system coupled to light is calculated by separating the wave functions of the relative and center-of-mass (CM) motions. The relative motion wave function has a known quasianalytical solution. The light only couples to the CM variable and the coupled equation can be solved analytically. The approach works for any coupling strength. Examples of wave functions of light-matter hybrid states are presented.

The interplay between magnetism and band topology is a focus of current research on magnetic topological systems. Based on first-principle calculations and symmetry analysis, we reveal that multiple intriguing topological states can be realized in a single system EuAgAs, controlled by the magnetic ordering. The material is a Dirac semimetal in the paramagnetic state, with a pair of accidental Dirac points. Under different magnetic configurations, the Dirac points can evolve into magnetic triply degenerate points, magnetic linear and double Weyl points, or being gapped out and making the system a topological mirror semimetal characterized by mirror Chern numbers. The change in bulk topology is also manifested in the surface states, including the surface Fermi arcs and surface Dirac cones. In addition, the antiferromagnetic states also feature a nontrivial ${\mathbb{Z}}_4$ index, implying a higher order topology. These results deepen our understanding of magnetic topological states and provide different perspectives for spintronic applications.

Magnetic multilayers are promising tunable systems for hosting magnetic skyrmions at/above room temperature. Revealing their intriguing switching mechanisms and associated inherent electrical responses are prerequisites for developing skyrmionic devices. In this Letter, we theoretically demonstrate the annihilation of single skyrmions occurring through a multilayer structure, which is mediated by the hopping dynamics of topological hedgehog singularities known as Bloch points. The emerging intralayer dynamics of Bloch points are dominated by the Dzyaloshinskii-Moriya interaction, and their propagation can give rise to solenoidal emergent electric fields in the vicinity. Moreover, as the topology of spin textures can dominate their emergent magnetic properties, we show that the Bloch-point hopping through the multilayer will modulate the associated topological Hall response, with the magnitude proportional to the effective topological charge. We also investigate the thermodynamic stability of these states regarding the layer-dependent magnetic properties. This study casts light on the emergent electromagnetic signatures of skyrmion-based spintronics, rooted in magnetic-multilayer systems.

We report on time-resolved linear and nonlinear terahertz spectroscopy of the two-band superconductor $\mathrm{Mg}{\mathrm{B}}_2$ with a superconducting transition temperature $T_c\approx 36$ K. Third-harmonic generation (THG) is observed below $T_c$ by driving the system with intense narrow-band THz pulses. For the pump-pulse frequencies $f=0.3$, 0.4, and 0.5 THz, the temperature-dependent evolution of the THG signals exhibits a resonance maximum at the temperatures with the resonance conditions $2f=2{\mathrm{\Delta }}_{\pi }\left(T\right)$ fulfilled, for the dirty-limit superconducting gap $2{\mathrm{\Delta }}_{\pi }$. In contrast, for $f=0.6$ and 0.7 THz with $2f>2{\mathrm{\Delta }}_{\pi }(T\to 0)=1.03$ THz, the THG intensity increases monotonically with decreasing temperature. Moreover, for $2f<2{\mathrm{\Delta }}_{\pi }(T\to 0)$ the THG is found nearly isotropic with respect to the pump-pulse polarization. These results suggest a predominant contribution of the driven Higgs amplitude mode of the dirty-limit $\pi$-band superconducting gap, pointing to the importance of scattering for observation of the Higgs mode in superconductors.

In this Letter, we study three-dimensional non-Hermitian higher-order Dirac semimetals (NHHODSMs). Our focus is on $C_4$-symmetric non-Hermitian systems where we investigate inversion ($\mathcal{I}$) or time-reversal ($\mathcal{T}$) symmetric models of NHHODSMs having real bulk spectra. We show that they exhibit the striking property that the bulk and surfaces are anti-$\mathcal{P}\mathcal{T}$ and $\mathcal{P}\mathcal{T}$ symmetric, respectively, and so belong to two different topological classes realizing a non-Hermitian topological phase which we call a hybrid-$\mathcal{P}\mathcal{T}$ topological phase. Interestingly, while the bulk spectrum is still fully real, we find that exceptional Fermi rings (EFRs) appear connecting the two Dirac nodes on the surface. This provides a route to probe and utilize both the bulk Dirac physics and exceptional rings/points on equal footing. Moreover, particularly for $\mathcal{T}$-NHHODSMs, we also find real hinge arcs connecting the surface EFRs. We show that this higher-order topology can be characterized using a biorthogonal real-space formula of the quadrupole moment. Furthermore, by applying Hermitian $C_4$-symmetric perturbations, we discover various phases, particularly (i) an intrinsic $\mathcal{I}$-NHHODSM having hinge arcs and surface exceptional Fermi arcs, and (ii) a $\mathcal{T}$-symmetric skin-topological HODSM which possesses both topological and skin hinge modes. The interplay between non-Hermition and higher-order topology in this Letter paves the way toward uncovering rich phenomena and hybrid functionality that can be readily realized in experiment.

We study non-Hermitian higher-order Weyl semimetals (NHHOWSMs) possessing real spectra and having inversion $\mathcal{I}$ ($\mathcal{I}$-NHHOWSM) or time-reversal symmetry $\mathcal{T}$ ($\mathcal{T}$-NHHOWSM). When the reality of bulk spectra is lost, the NHHOWSMs exhibit various configurations of surface Fermi arcs and exceptional Fermi rings (EFRs), providing a setup to investigate them on an equal footing. The EFRs only appear in the region between second-order Weyl nodes. We also discover Weyl nodes originating from non-Hermiticity, called non-Hermitian Weyl nodes (NHWNs). Remarkably, we find $\mathcal{T}$-NHHOWSMs which host only second-order NHWNs, having both surface and hinge Fermi arcs protected by the quantized biorthogonal Chern number and quadrupole moment, respectively. We call this intrinsically non-Hermitian phase exceptional HOWSM. In contrast to ordinary Weyl nodes, the NHWNs can instantly deform to line nodes, forming a monopole comet. The NHWNs also show exceptional tilt rigidity, which is a strong resistance towards titling due to attachment to exceptional structures. This phenomenon can be a promising experimental knob. Finally, we reveal the exceptional stability of Fermi arcs called exceptional helicity. Surface Fermi arcs having opposite chirality can live on the same surface without gapping out each other due to the complex nature of the spectrum. Our work motivates an immediate experimental realization of NHHOWSMs.

The thermal, acoustic, and dielectric properties of glasses below 1 K are dictated by the interaction of two-level systems (TLS) with strain and electric fields. In a previous paper, we proposed a modified TLS model to quantitatively account for the universally small phonon scattering in glasses at low temperatures. A key ingredient of this model was a wide distribution of couplings between TLS and phonons, contrary to the standard model which assumes a single averaged value is sufficient. In this paper, we expand on this view and include couplings to strain as well as electric fields. We then compare our theoretical results to measurements obtained using superconducting qubits. We find that the predictions of the modified TLS model are more consistent with experiments than those of the standard model. For the distribution of couplings between TLS and the strain field, there is a better agreement with experiments if we include a random distribution of local strains. Such a distribution of local strains is consistent with those found from molecular dynamics simulations.

We derive semiclassical equations of motion for an accelerated wave packet in a two-band system. We show that these equations can be formulated in terms of the static band geometry described by the quantum metric. We consider the specific cases of the Rashba Hamiltonian with and without a Zeeman term. The semiclassical trajectories are in full agreement with the ones found by solving the Schrödinger equation. This formalism successfully describes the adiabatic limit and the anomalous Hall effect traditionally attributed to Berry curvature. It also describes the opposite limit of coherent band superposition, giving rise to a spatially oscillating Zitterbewegung motion, and all intermediate cases. At $k=0$, such a wave packet exhibits a circular trajectory in real space, with its radius given by the square root of the quantum metric. This quantity appears as a universal length scale, providing a geometrical origin of the Compton wavelength. The quantum metric semiclassical approach could be extended to an arbitrary number of bands.

We discuss all the characteristics of Yu-Shiba-Rusinov states for clusters of impurities with classical magnetic moments in a superconducting substrate with $s$-wave symmetry. We consider the effect of the multiorbital structure of the impurities and the effect of the crystal field splitting. We solve the problem exactly and calculate the subgap Green's function, which has poles at the energies of the Shiba states and defines the local density of states associated with their wave functions. For the case of impurities sufficiently separated, we derive an effective Hamiltonian to describe the hybridization mediated by the substrate. We analyze the main features of the spectrum and the spectral density of the subgap excitations for impurities in dimer configurations with different relative orientations of the magnetic moments. We also illustrate how the same formalism applies for the solution of a trimer with frustration in the orientation of the magnetic moments.

A quasi-1D superconductor with an odd number of Fermi surfaces is expected to exhibit a nondegenerate Majorana bound state at the Fermi level at its boundary with an insulator (where the latter could be an actual insulator material or vacuum, for a terminated sample). Previous explicit theoretical demonstrations of this property were done for specific microscopic models of the bulk Hamiltonian and, most importantly, of the boundary. In this work, we theoretically demonstrate that this property holds for the whole class of systems, using the symmetry-based formalism of low-energy continuum models and general boundary conditions. We derive the general form of the Bogoliubov–de Gennes low-energy Hamiltonian that is subject only to charge-conjugation symmetry ${\mathcal{C}}_{+}$ of the type ${\mathcal{C}}_{+}^2=+1$ and a few minimal assumptions. Crucially, we also derive the most general form of the boundary conditions describing the boundary with an insulator, subject only to the fundamental principle of the probability-current conservation and ${\mathcal{C}}_{+}$ symmetry. Such normal-reflection boundary conditions do not contain scattering between electrons and holes. We find that for an odd number of Fermi surfaces a Majorana bound state always exists as long as the bulk is in the gapped superconducting state, irrespective of the parameters of the bulk Hamiltonian and boundary conditions. Importantly, our general model includes a possible Fermi-point mismatch, when the two Fermi points are not at exactly opposite momenta, which disfavors superconductivity. We find that the Fermi-point mismatch does not have a direct destructive effect on the Majorana bound state, in the sense that once the bulk gap is opened the bound state is always present.

Being a lithophile element at ambient pressure, magnesium has been long believed to be immiscible with iron. Authors of a recent study showed that pressure turns magnesium into a siderophile element and can produce unconventional Fe-Mg compounds [Gao et al., New J. Chem. 43, 17403 (2019)]. Here, we extend the investigation to exoplanetary pressure conditions using an adaptive genetic algorithm-based variable-composition structural prediction approach. We identify several Fe-Mg phases up to 3 TPa at 0 K. Our cluster alignment analysis reveals that most of the predicted Fe-Mg compounds prefer a body-centered cubic packing motif at terapascal pressures. In this paper, we provide a more comprehensive structure database to support future investigations of the high-pressure structural behavior of Fe-Mg and ternary, quaternary, etc. compounds involving these elements.

In this work we apply methods for describing three-dimensional images to the problem of encoding atomic environments in a way that is invariant to rotations, translations, and permutations of the atoms and, crucially, can be decoded back into the original environment modulo global orientation without the need for training a model. From the point of view of decoding, the descriptor is optimally complete and can be extended to arbitrary order, allowing for a systematic convergence of the fidelity of the description. In experiments on molecules ranging from 3 to 29 atoms in size, we demonstrate that positions can be decoded with a 97% success rate and positions plus species with a 70% rate of success, rising to 95% if a second fingerprint is used. In all cases, consistent recovery is observed for molecules with 17 or fewer atoms. Additionally, we evaluate the descriptor's performance in predicting the energies and forces of bulk Ni, Cu, Li, Mo, Si, and Ge by means of a neural network model trained on DFT data. When comparing to six machine learning interaction potential methods that use various descriptors and regression schemes, our descriptor is found to be competitive, in several cases outperforming well established methods. The combined ability to both decode and make property predictions from a representation that does not need to be learned lays the foundations for a novel way of building generative models that are tasked with solving the inverse problem of predicting atomic arrangements that are statistically likely to have certain desired properties.

We study numerically the nonequilibrium glass formation and depinning transition of a system of two-dimensional cluster-forming monodisperse particles in the presence of pinning disorder. The pairwise interaction potential is nonmonotonic and is motivated by the intervortex forces in type-1.5 superconductors but also applies to a variety of other systems. Such systems can form cluster glasses due to the intervortex interactions following a thermal quench, without underlying disorder. We study the effects of vortex pinning in these systems. We find that a small density of pinning centers of moderate depth has a limited effect on vortex glass formation, i.e., formation of vortex glasses is dominated by intervortex interactions. At higher densities, pinning can significantly affect glass formation. The cluster glass depinning, under a constant driving force, is found to be plastic, with features distinct from non-cluster-forming systems such as clusters merging and breaking. We find that, in general, vortices with cluster-forming interaction forces can exhibit stronger pinning effects than regular vortices.

We propose realization of non-Abelian topological superconductivity in two-dimensional quasicrystals by the same mechanism as in crystalline counterparts. Specifically, we study a two-dimensional electron gas in Penrose and Ammann-Beenker quasicrystals with Rashba spin-orbit coupling, perpendicular Zeeman magnetic field, and conventional $s$-wave superconductivity. We find that topological superconductivity with broken time-reversal symmetry is realized in both Penrose and Ammann-Beenker quasicrystals at low filling, where the Bott index is unity. The topological nature of this phase is confirmed by the existence of a zero-energy surface bound state and the chiral propagation of a wave packet projected onto the midgap bound state along the surfaces. Furthermore, we confirm the existence of a single Majorana zero mode each in a vortex at the center of the system and along the surfaces, signifying the non-Abelian character of the system when the Bott index is unity.