We investigate the nonequilibrium dynamics of complex Sachdev-Ye-Kitaev (SYK) models in the $q\to \infty$ limit, where $q/2$ denotes the order of the random Dirac fermion interaction. We extend previous results by Eberlein et al. [Phys. Rev. B 96, 205123 (2017)] to show that a single SYK $q\to \infty$ Hamiltonian for $t\ge 0$ is a perfect thermalizer in the sense that the local Green's function is instantaneously thermal. The only memories of the quantum state for $t<0$ are its charge density and its energy density at $t=0$. Our result is valid for all quantum states amenable to a $1/q$ expansion, which are generated from an equilibrium SYK state in the asymptotic past and acted upon by an arbitrary combination of time-dependent SYK Hamiltonians for $t<0$. Importantly, this implies that a single SYK $q\to \infty$ Hamiltonian is a perfect thermalizer even for nonequilibrium states generated in this manner.

The dominating superconducting pairing symmetry of the kagome-lattice Hubbard model is investigated using the determinant quantum Monte Carlo method. The superconducting instability may occur when doping the correlated insulators formed by the Hubbard interaction near the Dirac filling, and the possible superconducting state exhibits an electron-hole asymmetry. Among the pairing symmetries allowed, we demonstrate that the dominating channel is $d$ wave in the hole-doped case. This opens the possibility of condensation into an unconventional $d_{x^2-y^2}+i{d}_{xy}$ phase, which is characterized by an integer topological invariant and gapless edge states. In contrast, the $s^{*}$-wave channel, which has no sign change in the pairing function, is favored by the electron doping. We further find the dominating $s^{*}$ wave pairing persists up to the Van Hove singularity. The results are closely related to the recent experimental observations in kagome compounds $A{\text{V}}_3{\text{Sb}}_5(A=\mathrm{K}, \mathrm{Rb}, \mathrm{Cs})$ and provide insight into the pairing mechanism of their superconducting states.

Approach-to-equilibrium molecular dynamics (AEMD) is a widely used molecular dynamics (MD) method to extract thermal transport properties in different material systems. Despite the success in many applications, the thermal transport mechanism in AEMD is not well understood. Although AEMD can simulate larger domain than other MD variants, it still suffers from simulation domain size effect. In addition, the size effect is quite different from that of the nonequilibrium molecular dynamics (NEMD) simulations. In this paper, we reveal the phonon transport mechanism in AEMD by comparing the size-dependent thermal conductivity values of AEMD and phonon Boltzmann transport equation. We show that the simulation size of AEMD should be defined as half of the size in the conventional AEMD simulations with periodic boundary conditions. Also, the size effect in AEMD originates from ballistic phonon transport. Different from NEMD, some phonons with long mean-free paths do not contribute to the thermal conductivity, resulting in a smaller thermal conductivity than NEMD with the same size. Based on the phonon transport mechanism in AEMD, we suggest an extrapolation method for AEMD to obtain bulk thermal conductivity.

Illumination is performed at low temperature on dopant-free two-dimensional electron gases (2DEGs) of varying depths, under unbiased (gates grounded) and biased (gates at a positive or negative voltage) conditions. Unbiased illuminations in 2DEGs located more than 70 nm away from the surface result in a gain in mobility at a given electron density, primarily driven by the reduction of background impurities. In 2DEGs closer to the surface, unbiased illuminations result in a mobility loss, driven by an increase in surface charge density. Biased illuminations performed with positive applied gate voltages result in a mobility gain, whereas those performed with negative applied voltages result in a mobility loss. The magnitude of the mobility gain (loss) weakens with 2DEG depth, and is likely driven by a reduction (increase) in surface charge density. Remarkably, this mobility gain/loss is fully reversible by performing another biased illumination with the appropriate gate voltage, provided both $n$-type and $p$-type Ohmic contacts are present. Experimental results are modeled with Boltzmann transport theory, and possible mechanisms are discussed.

We consider light scattering by two-dimensional arrays of high-index dielectric spheres arranged into a triangular and square lattices. We demonstrate the appearance of the double degenerate accidental super-BIC modes with extremely suppressed radiative losses in the vicinity of the $\mathrm{\Gamma }$ point of the leaky band of the triangular lattice. Two rings of BICs (circular lines of BICs in reciprocal space) with different polarization appear at the point of the super-BIC destruction. The radius of the ring BIC (RBIC) changes as a function of the sphere's radius. We propose a generic analytical expression to describe the behavior of the guided mode decay rate as a function of the sphere radii and the wave vector in the vicinity of the RBIC. The results are explained using a multipolar approach.

The van der Waals (vdW) heterostructures are a wonderful playground for condensed matter physics, and the recently discovered two-dimensional magnetism renovates this field. In this work, we construct vdW heterostructures composed of $p$- and $d$-magnetic layers, namely carbon-terminated $\mathrm{Si}\mathrm{C}\left(111\right)/\mathrm{Cr}{\mathrm{Br}}_3$ and hydrogenated graphene (HG)/$\mathrm{Cr}{\mathrm{Br}}_3$. By first-principles calculations, strong interlayer magnetic interaction is predicted, as the interlayer exchange energy is up to hundreds of meV, which may originate from the delocalization of magnetic $p$ orbitals in SiC(111) and HG. Moreover, upon transforming the interlayer magnetic order, the electronic structures of the heterostructures vary dramatically, such as the gap can change from 471 to 4 meV. This significant charge-spin coupling can be ascribed to spin-dependent $p-d$ orbital coupling across the vdW interface. The $p-d$ coupling is turned on/off by switching interlayer magnetic order, and shifts the energy level so that it alters the band gaps. Besides, the magnetic anisotropy of heterostructures also shows dependence on interlayer magnetic order. Our findings provide inspiration to design vdW heterostructures whose electronic structures can be effectively controlled by magnetism, which have potential applications in spintronics.

Nonadiabatic quantum effects, often experimentally observed in semiconductor nanodevices such as single-electron pumps operating at high frequencies, can result in undesirable and uncontrollable behavior. However, when combined with the valley degree of freedom inherent to silicon, these unfavourable effects may be leveraged for quantum information processing schemes. By using an explicit time evolution of the Schrödinger equation, we study numerically nonadiabatic transitions between the two lowest valley states of an electron in a quantum dot formed in a SiGe/Si heterostructure. The presence of a single-atomic layer step at the top SiGe/Si interface opens an anticrossing in the electronic spectrum as the center of the quantum dot is varied. We show that an electric field applied perpendicularly to the interface allows tuning of the anticrossing energy gap. As a result, by moving the electron through this anticrossing, and by electrically varying the energy gap, it is possible to electrically control the probabilities of the two lowest valley states.

We theoretically study emergent edge and corner states in monolayer blue phosphorus (blue phosphorene) using the first-principles calculation and tight-binding model. We show that the existence of the Wannier orbitals at every bond center yields edge states both in zigzag and armchair nanoribbons. The properties of the edge states can be well described by a simple effective Hamiltonian for uncoupled edge orbitals, where the structural relaxation near the boundary significantly affects the edge band structure. For corner states, we examine two types of corner structures consisting of zigzag and armchair edges, where we find that multiple corner states emerge in the bulk gap as a consequence of hybridization of edge and corner uncoupled orbitals. In the armchair corner, in particular, we demonstrate that corner states appear right at the Fermi energy, which leads to the emergence of fractional corner charge due to filling anomaly. Finally, we discuss the relationship between blue phosphorene and black phosphorene, and show that two systems share the equivalent Wannier orbital positions and similar edge/corner state properties even though their atomic structures are totally different.

Two-dimensional (2D) ferroelectricity has attracted extensive attention since its discovery in the monolayers of van der Waals materials. Here we show that 2D ferroelectricity induced by octahedral rotation distortion is widely present in the perovskite bilayer system through first-principles calculations. The perovskite tolerance factor plays a crucial role in the lattice dynamics and ground-state structure of the perovskite monolayers and bilayers, thus providing an important indicator for screening this hybrid improper ferroelectricity. Generally, the ferroelectric switching via an orthorhombic twin state has the lowest energy barrier. Epitaxial strain can effectively tune the ferroelectric polarization and ferroelectric switching by changing the amplitude of octahedral rotation and tilt distortion. The increasing compressive strain causes a polar to nonpolar phase transition by suppressing the tilt distortion. The cooperative effect of octahedral distortion at the interface with the substrate can reduce the energy barrier of the reversing rotation mode and can even change the lowest-energy ferroelectric switching path.

We propose indicators of the discriminant for systems with generalized inversion symmetry which are computed from data only at high-symmetry points in the Brillouin zone. Our approach captures the exceptional points and their symmetry-protected variants without ambiguity arising from the reference energy, which is an advantage over the previously known indicators for non-Hermitian systems. As demonstrations, we systematically analyze $3\times 3$ Hamiltonians where the proper choice of the reference energy is not obvious.

All previous cuprate superconductors display a set of common features: (i) vicinity to a Cu $3d^9$ configuration; (ii) separated ${\mathrm{CuO}}_2$ planes; and (iii) superconductivity for doping $\delta \sim 0.1–0.3$. Recently, Li et al. [Proc. Natl. Acad. Sci. USA 116, 12156 (2019)] challenged this picture by discovering “highly overdoped” superconducting ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+y}$. Using density-functional theory plus dynamical mean-field theory, we reveal a bilayer structure of ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3.2}$ of alternating quasi-two-dimensional (2D) and quasi-one-dimensional (1D) character. Correlations tune an interlayer self-doping leading to an almost half-filled, strongly nested, quasi-1D $d_{b^2-c^2}$ band, which is prone to strong antiferromagnetic fluctuations, possibly at the origin of superconductivity in ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+y}$.

The effects of the electron-phonon interaction on optical excitations can be understood in terms of exciton-phonon coupling and require a careful treatment in low-dimensional materials with strongly bound excitons or strong electron-hole interaction in general. Through phonon absorption and emission processes, the optically accessible excitons are scattered into otherwise optically dark finite-momentum exciton states. We derive a practical expression for the phonon-induced term of the exciton self-energy (denoted as the exciton-phonon self-energy) that gives the temperature dependence of the optical transition energies and their lifetime broadening resulting from the exciton's interaction with the phonons. We illustrate this theory on a two-dimensional model and show that our expression for the exciton-phonon self-energy differs qualitatively from previous expressions found in the literature that neglect the exciton binding or electron-hole correlations.

We investigate under which conditions the three-dimensional (3D) multicomponent Abelian-Higgs (AH) field theory (scalar electrodynamics) is the continuum limit of statistical lattice gauge models, i.e., when it characterizes the universal behavior at critical transitions occurring in these models. We perform Monte Carlo simulations of the lattice AH model with compact gauge fields and $N$-component scalar fields with charge $q\ge 2$ for $N=15$ and 25. Finite-size scaling analyses of the Monte Carlo data show that the transitions along the line separating the confined and deconfined phases are continuous and that they belong to the same universality class for any $q\ge 2$. Moreover, they are in the same universality class as the transitions in the lattice AH model with noncompact gauge fields along the Coulomb-to-Higgs transition line. We finally argue that these critical behaviors are described by the stable charged fixed point of the renormalization-group flow of the 3D AH field theory.

Recent advances in topological artificial systems open the door to realizing topological states in dimensions higher than the usual three-dimensional space. Here we present a “tensor product” theory, which offers a method to construct Chern insulators with arbitrarily high dimensions and Chern numbers. Particularly, we show that the tensor product of a $d_A\mathrm{D}$ Chern insulator $\langle {\mathcal{H}}_A^{\left({\kappa }_A\right)},C_A\rangle$ with a $d_B\mathrm{D}$ Chern insulator $\langle {\mathcal{H}}_B^{\left({\kappa }_B\right)},C_B\rangle$ leads to a $(d_A+d_B)\mathrm{D}$ Chern insulator $\langle {\mathcal{H}}_{AB}^{\left({\kappa }_A★{\kappa }_B\right)},-2C_A{C}_B\rangle$, where in the brackets, ${\mathcal{H}}^{\left(\kappa \right)}$ is the $d\mathrm{D}$ Hamiltonian with $d$ even, $C$ is the corresponding $(d/2)\mathrm{th}$ Chern number, and $\kappa$ labels the five nonchiral Altland-Zirnbauer symmetry classes A, AI, D, AII, and C. The four real classes AI, D, AII, and C form a Klein four-group under the multiplication “$★,$” with class AI the identity and class A is the zero element. Our theory leads to novel higher-dimensional topological physics. (i) The construction can generate large higher-order Chern numbers, e.g., for some cases the resultant classification is $8\mathbb{Z}$. (ii) Fascinatingly, the boundary states feature flat nodal surfaces with nontrivial Chern charges. For the constructed $(d_A+d_B)\mathrm{D}$ Chern insulator, a boundary perpendicular to a direction of ${\mathcal{H}}_A$ generically hosts $|C_A|d_B\mathrm{D}$ nodal surfaces, each of which has topological charge $\pm 2C_B$. Under perturbations, each nodal surface bursts into stable unit nodal points with the total Chern charge conserved. Examples are given to demonstrate our theory, which can be experimentally realized in artificial systems such as acoustic crystals, electric-circuit arrays, ultracold atoms, or mechanical networks.

We present photoluminescence measurements in monolayer ${\mathrm{WSe}}_2$, which point to the importance of the interaction between charged particles and excitonic complexes. The theoretical analysis highlights the key role played by exchange scattering, referring to cases wherein the particle composition of the complex changes after the interaction. For example, exchange scattering renders bright excitonic complexes dark in monolayer ${\mathrm{WSe}}_2$ on accounts of the unique valley-spin configuration in this material. In addition to the ultrafast energy relaxation of hot excitonic complexes following their interaction with electrons or holes, our analysis sheds light on several key features that are commonly seen in the photoluminescence of this monolayer semiconductor. In particular, we can understand why the photoluminescence intensity of the neutral bright exciton is strongest when the monolayer is hole doped rather than charge neutral or electron doped. Similarly, we can understand the reason for the dramatic increase of the photoluminescence intensity of negatively charged excitons (trions) as soon as electrons are added to the monolayer. To self-consistently explain the findings, we further study the photoluminescence spectra at different excitation energies and analyze the behavior of the elusive indirect exciton.

Ensembles of gold nanoparticles present a magnetic behavior which is at odds with the weakly diamagnetic response of bulk gold. In particular, an unusual ferromagnetic order has been unveiled by several experiments. Here we investigate if the combined effect of orbital magnetism of conduction electrons and interparticle dipolar interaction can lead to magnetic ordering. Using different model systems of interacting mesoscopic magnetic dipoles, together with a microscopic description of the electron dynamics within the nanoparticles, we find that a spontaneous magnetic moment may arise in dimers of metallic nanoparticles when the latter are characterized by a large orbital paramagnetic susceptibility.

Thermoelectric materials have attracted significant interest owing to their wide range of applications, such as power generators and refrigerators. Along with the evaluation of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and its alloys, the search for materials that show good thermoelectric performance has risen considerably. The efficiency of thermoelectric conversion is quantified by the figure of merit, $ZT$, and $ZT$ greater than unity has been considered as a target. For a large $ZT$, acquiring a large Seebeck coefficient and a reduction of phonon thermal conductivity are significant given its definition. In this work, we propose that a thin film of topological nodal line semimetals can be a new and promising thermoelectric material. We also discuss the correlation effect and spin current induced by the temperature gradient. The obtained results give various insights into the quest for the best thermoelectric materials.

We theoretically investigate optical injection processes, including one- and two-photon carrier injection and two-color coherent current injection, in twisted bilayer graphene with moderate angles. The electronic states are described by a continuum model, and the spectra of injection coefficients are numerically calculated for different chemical potentials and twist angles, where the transitions between different bands are understood by the electron energy resolved injection coefficients. The comparison with the injection in monolayer graphene shows the significance of the interlayer coupling in the injection processes. For undoped twisted bilayer graphene, all spectra of injection coefficients can be divided into three energy regimes, which vary with the twist angle. For very low photon energies in the linear dispersion regime, the injection is similar to graphene with a renormalized Fermi velocity determined by the twist angle; for very high photon energies where the interlayer coupling is negligible, the injection is the same as that of graphene; and in the middle regime around the transition energy of the Van Hove singularity, the injection shows fruitful fine structures. Furthermore, the two-photon carrier injection diverges for the photon energy in the middle regime due to the existence of double resonant transitions.

The recent discovery of higher-order topology has largely enriched the classification of topological materials. Theoretical and experimental studies have unveiled various higher-order topological insulators that exhibit topologically protected corner or hinge states. More recently, higher-order topology has been introduced to topological semimetals. Thus far, realistic models and experimental verifications on higher-order topological semimetals are still very limited. Here we design and demonstrate a three-dimensional photonic crystal that realizes a higher-order Dirac semimetal phase. Numerical results on the band structure show that the designed three-dimensional photonic crystal is able to host two fourfold Dirac points, which are connected in the momentum-space projections via higher-order hinge states localized at the hinge. The higher-order topology can be characterized by the topological invariant ${\chi }^{\left(6\right)}$ at different values of $k_z$. An experiment at microwave frequencies is also presented to measure the hinge state dispersion. Our work demonstrates the physical realization of a higher-order Dirac semimetal phase and paves the way to explore higher-order topological semimetal phases in three-dimensional photonic systems.

Quantum many-body scars (QMBS) constitute a new quantum dynamical regime in which rare “scarred” eigenstates mediate weak ergodicity breaking. One open question is to understand the most general setting in which these states arise. In this work, we develop a generic construction that embeds a new class of QMBS, rainbow scars, into the spectrum of an arbitrary Hamiltonian. Unlike other examples of QMBS, rainbow scars display extensive bipartite entanglement entropy while retaining a simple entanglement structure. Specifically, the entanglement scaling is volume-law for a random bipartition, while scaling for a fine-tuned bipartition is subextensive. When internal symmetries are present, the construction leads to multiple, and even towers, of rainbow scars revealed through distinctive non-thermal dynamics. Remarkably, certain symmetries can lead rainbow scars to arise in translation-invariant models. To this end, we provide an experimental road map for realizing rainbow scar states in a Rydberg-atom quantum simulator, leading to coherent oscillations distinct from the strictly sub-volume-law QMBS previously realized in the same system.