In this paper, we look at four generalizations of the one-dimensional Aubry-André-Harper (AAH) model which possess mobility edges. We map out a phase diagram in terms of population imbalance and look at the system size dependence of the steady-state imbalance. We find nonmonotonic behavior of imbalance with the system parameters, which contradicts the idea that the relaxation of an initial imbalance is fixed only by the ratio of the number of extended states to the number of localized states. We propose that there exist dimensionless parameters, which depend on the fraction of single-particle localized states, single-particle extended states, and the mean participation ratio of these states. These ingredients fully control the imbalance in the long time limit and we present numerical evidence of this claim. Among the four models considered, three of them have interesting duality relations and their locations of mobility edges are known. One of the models (next-nearest-neighbor hopping) has no known duality but a mobility edge exists and the model has been experimentally realized. Our findings are an important step forward to understanding nonequilibrium phenomena in a family of interesting models with incommensurate potentials.

Within the framework of the mean-field approximation, a coherently excited two-dimensional system of weakly repulsive bosons is predicted to show a giant loop scattering when the rotational symmetry is reduced. The considered process combines (i) the parametric decay of the driven condensate into different $k$ states and (ii) their massive backscattering owing to spontaneous synchronization of several four-wave mixing channels. The hybridization of the direct and inverse scattering processes, which are different and thus do not balance each other, makes the condensate oscillate under constant one-mode excitation. In particular, the amplitude of a polariton condensate excited by a resonant electromagnetic wave in a uniform polygonal GaAs-based microcavity is expected to oscillate in the subterahertz frequency domain.

Measurements of thermal diffusivity in several insulators have been shown to reach a Planckian bound on thermal transport that can be thought of as the limit of validity of semiclassical phonon scattering. Beyond this regime, the heat transport must be understood in terms of incoherent motion of the atoms under strongly anharmonic interactions. In this work, we propose a model for heat transport in a strongly anharmonic system where the thermal diffusivity can be lower than the Planckian thermal diffusivity bound. Similar to the materials that exhibit thermal diffusivity close to this bound, our scenario involves complex unit cells with incoherent intra-cell dynamics. We derive a general formalism to compute thermal conductivity in such cases with anharmonic intra-cell dynamics coupled to nearly harmonic inter-cell coupling. Through direct numerical simulation of the nonlinear unit-cell motion, we explicitly show that our model allows sub-Planckian thermal diffusivity. We find that the propagator of the acoustic phonons becomes incoherent throughout most of the Brillouin zone in this limit. We expect these features to apply to more realistic models of complex insulators showing sub-Planckian thermal diffusivity, suggesting a multispecies generalization of the thermal diffusivity bound that is similar to the viscosity bound in fluids.

We put forth an ab initio framework to calculate local moment magnetic interaction parameters, renormalized to treat both the lattice and magnetic systems as a function of temperature $T$. For bcc Fe, magnetic and lattice thermal disorders act in opposition, the former strengthening the Heisenberg-like interactions, while the latter decreasing them. Below $T_C$, $J$ stays nearly independent of $T$, while around and above $T_C$, it exhibits a sharp decrease. This remarkable behavior reflects an intricate spin-lattice coupling and its evolution with $T$, in which magnetic interactions and interatomic bonds are each renormalized by the other. This finding is consistent with magnetization data and with the observed softening of magnon and phonon modes at high temperatures. Magnetization as well as magnon and phonon mode softening are discussed.

We study a kagomelike spin-$\frac{1}{2}$ Heisenberg ladder with competing ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions. Using the density-matrix renormalization group based calculations, we obtain the ground-state phase diagram as a function of the ratio between the FM and AFM exchange interactions. Five different phases exist. Three of them are spin-polarized phases: an FM phase and two kinds of ferrimagnetic (FR) phases (referred to as FR1 and FR2 phases). The spontaneous magnetization per site is $m=\frac{1}{2}, \frac{1}{3}$, and $\frac{1}{6}$ in the FM, FR1, and FR2 phases, respectively. This can be understood from the fact that an effective spin-1 Heisenberg chain formed by the upper and lower leg spins has a three-step fractional quantization of the magnetization per site as $m=1, \frac{1}{2}$, and 0. In particular, an anomalous “intermediate” state $m=\frac{1}{2}$ of the effective spin-1 chain with the reduced Hilbert space of a spin from three to two dimensional is highly unexpected in the context of conventional spin-1 physics. Thus, surprisingly, the effective spin-1 chain behaves like a spin-$\frac{1}{2}$ chain with SU(2) symmetry. The remaining two phases are spin-singlet phases with translational symmetry breaking in the presence of valence bond formations. One of them is octamer-singlet phase with a spontaneous octamerization long-range order of the system, and the other is period-4 phase characterized by the magnetic superstructure with a period of four structural unit cells. In these spin-singlet phases, we find the coexistence of valence bond structure and gapless chain. Although this may be emerged through the order-by-disorder mechanism, there can be few examples of such a coexistence.

Muon spin rotation/relaxation measurements were performed to investigate magnetic properties of $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ thin films on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$. A drastic decrease of the initial asymmetry was observed together with the peak structure in the temperature dependence of the relaxation rate of muon spins almost at the same temperature where kink anomalies were observed in the temperature-dependent resistivity. With increasing S content, the anomaly temperature increased and the magnetic fluctuations at the lowest temperature were suppressed. These results show that the S substitution induces magnetism at low temperatures in $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ thin films. Although the behaviors of the magnetic and nematic phases in FeSe films toward chemical pressure by S substitution are similar to those for bulk FeSe toward hydrostatic pressure, the behavior of $T_{\mathrm{c}}$ is significantly different between these systems. Our results demonstrate that the detailed comparative investigation among physical and chemical pressure effects is essentially important to understand the interplay of the magnetism, the nematicity, and the superconductivity in iron chalcogenides.

Motivated by the recent findings of unconventional superconductivity in $\mathrm{Co}{\mathrm{Si}}_2/\mathrm{Ti}{\mathrm{Si}}_2$ heterostructures, we study the effect of interface-induced Rashba spin-orbit coupling on the conductance of a three-terminal T-shaped superconducting device. We calculate the differential conductance for this device within the quasiclassical formalism that includes the mixing of triplet-singlet pairing due to the Rashba spin-orbit coupling. We discuss our result in light of the conductance spectra reported by S.-P. Chiu et al., arXiv:2012.13679 for $\mathrm{Co}{\mathrm{Si}}_2/\mathrm{Ti}{\mathrm{Si}}_2$ heterostructures.

We study the effects of anharmonicity on the physics of the Holstein model, which describes the coupling of itinerant fermions and localized quantum phonons, by introducing a quartic term in the phonon potential energy. We find that the presence of this anharmonic term reduces the extent of the charge density wave (CDW) phase at half-filling as well as the transition temperature to this phase. Doping away from half-filling, we observe a first-order phase transition between the CDW and a homogeneous phase which is also present in the harmonic model. In addition, we study the evolution of the superconducting susceptibility in the doped region and show that anharmonicity can enhance the superconducting response.

The spin-orbit interaction of two-dimensional (2D) electrons in semiconductor quantum wells gives rise to a variety of interesting transport and optical spin-dependent effects. In the GaAs/AlGaAs type heterosystems, this interaction consists of the isotropic Bychkov-Rashba term, which is absent in symmetric wells, and the anisotropic Dresselhaus term, reflecting the lattice symmetry. It is well-known that the first term can be controlled by electric fields in the growth direction: external or internal, induced by a charge density of 2D electrons. In this work we reveal that the 2D electron charge can substantially affect also the Dresselhaus interaction in symmetric quantum wells. Within the one-band electron Hamiltonian containing, together with the bulk Dresselhaus interaction, the two contributions to the Dresselhaus term from the quantum well interfaces, we show that the internal electric field from the 2D electron charge density can substantially renormalize the anisotropic spin-orbit interaction of 2D electrons. This effect may be important in quantitative studies of spin-dependent phenomena in quantum wells.

We develop a microscopic description of an electron-doped two-dimensional semiconductor embedded in a microcavity. Specifically, we investigate the interactions between exciton polaritons and electrons for the case where the interactions between charges are strongly screened and the system is spin polarized. As a starting point, we obtain an analytic expression for the exciton-polariton wave function, and we relate the microscopic parameters of the light-matter system to experimentally measurable quantities, such as the Rabi coupling and the cavity photon frequency. We then derive the polariton-electron interaction within the standard Born approximation and compare it with the exact polariton-electron scattering $T$ matrix that we obtain from a diagrammatic approach that has proven highly successful in the context of nuclear physics and ultracold atomic gases. In particular, we show that the Born approximation provides an upper bound on the polariton-electron coupling strength at vanishing momentum. Using our exact microscopic calculation, we demonstrate that polariton-electron scattering can be strongly enhanced compared to the exciton-electron case, which is the opposite of that expected from the Born approximation. We furthermore expose a resonancelike peak at scattering momenta near the polariton inflection point, whose size is set by the strength of the light-matter coupling. Our results arise from the non-Galilean nature of the polariton system and should thus be applicable to a range of semiconductor microcavities such as GaAs quantum wells and atomically thin materials.

We extend the damped Zaremba-Kohn model (dZK) for long-range dispersion interaction between a molecule and a planar surface [J. Tao, H. Tang, A. Patra, P. Bhattarai, and J. P. Perdew, Phys. Rev. B 97, 165403 (2018)] to molecules adsorbed on a curved cylindrical surface, and employ this extended model as an additive correction to the semilocal density functionals PBE (Perdew-Burke-Ernzerhof) and SCAN (strongly constrained and appropriately normed). The resulting PBE+vdW (van der Waals)-dZK and SCAN+vdW-dZK are applied to two systems, $\mathrm{N}{\mathrm{H}}_3$ and $\mathrm{N}{\mathrm{O}}_2$ molecules adsorbed on a single-wall carbon nanotube (CNT), for calculations of binding energies and equilibrium distances. For comparison, the results from vdW nonlocal functionals, such as SCAN+rVV10 and PBE+rVV10, are also presented. The binding energies from PBE+rVV10 (Vydrov and Van Voorhis), SCAN+rVV10, PBE+vdW-dZK, and SCAN+vdW-dZK are about 70–115 meV for the system of $\mathrm{CNT}+\mathrm{N}{\mathrm{H}}_3$ and 300–500 meV for the system of $\mathrm{CNT}+\mathrm{N}{\mathrm{O}}_2$. The results from PBE+vdW-dZK and SCAN+vdW-dZK are closer to each other than those from PBE+rVV10 and SCAN+rVV10 are. The relatively closer results from PBE+vdW-dZK and SCAN+vdW-dZK indicate the consistency of our developed vdW−dZK model for cylindrical surfaces. All methods, including PBE, SCAN, PBE+rVV10, SCAN+rVV10, PBE+vdW-dZK, and SCAN+vdW-dZK, give approximately the same binding energy differences between two adsorption configurations (types I and II) for the two systems. This implies that the two adsorption sites have approximately the same adsorption stability. The exponent of the vdW interaction power law from our vdW-dZK model for the two systems is about 0 at short distance, largely due to the damping factor, and tends slowly to −4 to −4.5 at distances $D$ about 20–50 Å. At even larger distances, the vdW power-law exponent approaches −5. This feature is very similar to the one calculated with random-phase approximation and renormalization group approaches, supporting the applicability of our methods. Our developed vdW-dZK method provides a highly efficient and reliable method for large systems with cylindrical surfaces, such as vdW interactions with nanotubes.

We study hard-core bosons on the honeycomb lattice in the presence of anisotropic nearest-neighbor repulsive interactions. Using a quantum Monte Carlo (QMC) technique, we extract the phase diagram of the model in terms of the filling and the anisotropy. At half-filling we find a dimer insulator phase near maximum anisotropy that is characterized by a finite topological entanglement entropy $ln\left(2\right)/2$, indicative of a fractional quantum Hall state for bosons. We identify the presence of edge states and derive a QMC-based method to extract and verify their chirality. Remarkably, this phase arises in the absence of magnetic flux and without explicit lattice frustration.

The measurement of hydrogen-induced changes in the electronic structure of transition metal oxides by x-ray photoelectron spectroscopy is a challenging endeavor, since no photoelectron can be unambiguously assigned to hydrogen. The H-induced electronic structure changes in tungsten trioxide have been known for more than 100 years but are still controversially debated. The controversy stems from the difficulty in disentangling effects due to hydrogenation from the effects of oxygen deficiencies. Using a membrane approach to x-ray photoelectron spectroscopy, in combination with tunable synchrotron radiation, we measure simultaneously core levels and the valence band up to a hydrogen pressure of 1000 mbar. Upon hydrogenation, the intensities of the ${\mathrm{W}}^{5+}$ core level and a state close to the Fermi level increase following the pressure-composition isotherm curve of bulk ${\mathrm{H}}_x{\mathrm{WO}}_3$. Combining experimental data and density functional theory, the description of the hydrogen-induced coloration by a proton polaron model is corroborated. Although hydrogen is the origin of the electronic structure changes near the Fermi edge, the valence band edge is now dominated by tungsten orbitals instead of oxygen as is the case for the pristine oxide, having wider implications for its use as a (photoelectrochemical) catalyst.

We present a theoretical study of the onset of electric polarization close to a surface in magnetic materials and in thin films and multilayers. We consider two different paths that lead to the onset of multiferroic behavior at the boundary in materials that are bulk collinear ferromagnets (or even antiferromagnets). These two paths are distinguished by the presence or absence of a surface-induced Dzyaloshinskii-Moriya interaction, which can be taken into account through Lifshitz invariants in the free energy of the system. Experimental consequences are discussed in light of the developed theory.

Spin pumping consists in the injection of spin currents into a nonmagnetic material due to the precession of an adjacent ferromagnet. In addition to the pumping of spin the precession always leads to pumping of heat, but in the presence of spin-orbital entanglement it also leads to a charge current. We investigate the pumping of charge, spin, and heat in a device where a superconductor and a quantum spin Hall insulator are in proximity contact with a ferromagnetic insulator. We show that the device supports two robust operation regimes arising from topological effects. In one regime, the pumped charge, spin, and heat are quantized and related to each other due to a topological winding number of the reflection coefficient in the scattering matrix formalism, translating to a Chern number in the case of Hamiltonian formalism. In the second regime, a Majorana zero mode switches off the pumping of currents owing to the topologically protected perfect Andreev reflection. We show that the interplay of these two topological effects can be utilized so that the device operates as a robust charge, spin, and heat transistor.

The families of high-temperature superconductors recently welcomed a new member: hole-doped nickelate ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ with a $\sim 15\mathrm{K}$ transition temperature. To understand its emergent low-energy behaviors and experimental properties, an immediate key question is whether the superconducting hole carriers reside in oxygen as in the cuprates or in nickel as in most nickelates. We answer this crucial question via a $(\mathrm{LDA}+U)+\mathrm{ED}$ scheme: deriving an effective interacting Hamiltonian of the hole carriers from density functional $\mathrm{LDA}+U$ calculation and studying its local many-body states via exact diagonalization. Surprisingly, distinct from the expected ${\mathrm{Ni}}^{2+}$ spin-triplet state found in most nickelates, the local ground state of two holes is actually a Ni-O spin-singlet state with the second hole greatly residing in oxygen. The emerged eV-scale model therefore resembles that of the cuprates, advocating further systematic experimental comparisons. Tracing the microscopic origin of this unexpected result to the lack of apical oxygen in this material, we proposed a route to increase superconducting temperature and a possible quantum phase transition absent in the cuprates.

Understanding the nature and dynamics of material defects in superconducting circuits is of paramount importance for improving qubit coherence and parameter stability and much needed for implementing large-scale quantum computing. Here we present measurements on individual highly coherent environmental two-level systems (TLS). We trace the spectral diffusion of specific TLS and demonstrate that it originates from the TLS coupling to a small number of low energy incoherent fluctuators. From the analysis of these fluctuations, we access the relevant parameters of low energy fluctuators: dipole moments, switching energies, and, more importantly, interaction energies. Our approach opens up the possibility of deducing the macroscopic observables in amorphous glassy media from direct measurements of local fluctuator dynamics at the microscopic level—a route towards substantiating commonly accepted, but so far phenomenological, models for the decohering environment.

We study a recently introduced and exactly solvable mean-field model for the density of vibrational states $\mathcal{D}\left(\omega \right)$ of a structurally disordered system. The model is formulated as a collection of disordered anharmonic oscillators, with random stiffness $\kappa$ drawn from a distribution $p\left(\kappa \right)$, subjected to a constant field $h$ and interacting bilinearly with a coupling of strength $J$. We investigate the vibrational properties of its ground state at zero temperature. When $p\left(\kappa \right)$ is gapped, the emergent $\mathcal{D}\left(\omega \right)$ is also gapped, for small $J$. Upon increasing $J$, the gap vanishes on a critical line in the $(h,J)$ phase diagram, whereupon replica symmetry is broken. At small $h$, the form of this pseudogap is quadratic, $\mathcal{D}\left(\omega \right)\sim {\omega }^2$, and its modes are delocalized, as expected from previously investigated mean-field spin glass models. However, we determine that for large enough $h$, a quartic pseudogap $\mathcal{D}\left(\omega \right)\sim {\omega }^4$, populated by localized modes, emerges, the two regimes being separated by a special point on the critical line. We thus uncover that mean-field disordered systems can generically display both a quadratic-delocalized and a quartic-localized spectrum at the glass transition.

We study correlations, transport, and chaos in a Heisenberg magnet as a classical model many-body system. By varying temperature and dimensionality, we can tune between settings with and without symmetry breaking and accompanying collective modes or quasiparticles (spin waves) which in the limit of low temperatures become increasingly long-lived. Changing the sign of the exchange interaction from a ferro- to an antiferromagnetic one varies the spin-wave spectrum, and hence the low-energy spectral density. We analyze both conventional and out-of-time-ordered spin correlators (decorrelators) to track the spreading of a spatiotemporally localized perturbation—the wing beat of the butterfly—as well as transport coefficients and Lyapunov exponents. We identify a number of qualitatively different regimes. Trivially, at $T=0$, there is no dynamics at all. In the limit of low temperature, $T=0^{+}$, integrability emerges, with infinitely long-lived magnons; here the wave packet created by the perturbation propagates ballistically, yielding a light cone at the spin-wave velocity which thus subsumes the butterfly velocity; inside the light cone, a pattern characteristic of the free spin-wave spectrum is visible at short times. On top of this, residual interactions (nonlinearities in the equations of motion) lead to spin-wave lifetimes which, while divergent in this limit, remain finite at any nonzero $T$. At the longest times, this leads to a standard chaotic regime; for this regime, we show that the Lyapunov exponent is simply proportional to the (inverse) spin-wave lifetime. Visibly strikingly, between this and the short-time integrable regimes, a scarred regime emerges: Here, the decorrelator is spatiotemporally highly nonuniform, being dominated by rare and random scattering events seeding secondary light cones. As the spin correlation length decreases with increasing $T$, the distinction between these regimes disappears and at high temperature the previously studied chaotic paramagnetic regime emerges. For this, we elucidate how, somewhat counterintuitively, the ballistic butterfly velocity arises from a diffusive spin dynamics.