The list of interesting electrical properties exhibited by transition metal dichalcogenides has grown with the discovery of extremely large magnetoresistance (MR) and type-II Weyl semimetal behavior in ${\mathrm{WTe}}_2$ and ${\mathrm{MoTe}}_2$. The extremely large MR in ${\mathrm{WTe}}_2$ is still not adequately understood. Here, we systematically study the effect of Mo substitution on the quantum oscillations in the MR in ${\mathrm{WTe}}_2$. The MR decreases with Mo substitution, however, the carrier concentrations extracted from the quantum oscillations show that the charge compensation improves. We believe that earlier interpretations based on the two-band theory, which attribute the decrease in MR to charge imbalance, could be incorrect due to overparametrization. We attribute the decrease in MR in the presence of charge compensation to a fall in transport mobility, which is evident from the residual resistivity ratio data. The quantum scattering time and the effective masses do not change within experimental errors upon substitution.

Metastable states arise in a range of quantum systems and can be observed in various dynamical scenarios, including decay, bubble nucleation, and long-lived oscillations. The phenomenology of metastable states has been examined in quantum many-body systems, notably in one-dimensional (1D) ferromagnetic Ising spin systems and superfluids. In this paper, we study long-lived oscillations of metastable and ground states in 1D antiferromagnetic neutral atom chains with long-range Rydberg interactions. We use a staggered local detuning field to achieve confinement. Using theoretical and numerical models, we identify novel spectral signatures of quasiparticle oscillations distinct to antiferromagnetic neutral atom systems and interpret them using a classical energy model of short-range meson repulsion. Finally, we evaluate the experimental accessibility of our proposed setup on current neutral-atom platforms and discuss experimental feasibility and constraints.

We consider surface states in semiconductors with inverted-band structures, such as $\alpha$-Sn and HgTe. The main interest is the interplay of the effect of a strain of an arbitrary sign and that of the sample finite size. We consider, in particular, a model system composed of a gapless semiconductor (e.g., HgTe or $\alpha$-Sn) of finite-width sandwiched between layers of a regular-band semiconductor (e.g., CdTe or InSb). We clarify the origin of various transitions that happen at a given strain with the change of the sample thickness, in particular the transition between the Dirac semimetal and quasi-three-dimensional (quantized) topological insulator. Our conclusion opposes those reached recently by the majority of researchers. We show that near the transition point the surface state cannot be treated as a truly topological one since the parameters of the problem are such that an appreciable overlap of the surface states' wave functions located at opposite boundaries occur. As a result, a spin-conserving, elastic impurity scattering between the states located at opposite boundaries will induce substantial backscattering and destroy the robustness of the surface state. For the k-p Kane model we derive hard-wall boundary conditions in the case when the regular-band materials form high barriers for the carriers of the inner inverted-band semiconductor (for example, the CdTe/HgTe/CdTe and $\mathrm{CdTe}/\alpha \text{−}\mathrm{Sn/CdTe}$ cases). We show that in this case the boundary conditions have universal and simple form and allow investigation of the realistic case of finite mass of the heavy-hole band, and comparison of the results obtained within the Kane and Luttinger models. In particular, a new type of surface states (wing states) developes with application of strain in the Kane model and is absent in the Luttinger model.

We investigated the interlayer coupling effect in homobilayer structures of $M{\mathrm{Si}}_2{X}_4$ with $M=\mathrm{Mo},\mathrm{W}$ and $X=\mathrm{N},\mathrm{P},\mathrm{As}$. Through the combination of first-principles calculations and analytical formulations, the equilibrium interlayer distance, layer energy difference, and interlayer hopping strength are obtained for all six $M{\mathrm{Si}}_2{X}_4$ materials, which are found to be insensitive to the type of $M$ atom but differ significantly between $X=\mathrm{N}$ and $X=\mathrm{P},\mathrm{As}$. In homobilayers with twist angles close to 0, the interlayer charge redistribution introduces a stacking-dependent interlayer electrostatic potential with a magnitude reaching $0.1\mathrm{eV}$ in $M{\mathrm{Si}}_2{\mathrm{N}}_4$, suggesting that it can be an excellent candidate for studying the sliding ferroelectricity. The interlayer hopping strengths are found to be as large as several tens meV at valence band maxima positions $\mathbf{K}$ and $\mathbf{\Gamma }$, and $\sim 1\mathrm{meV}$ at the conduction band edge $\mathbf{K}$. The resultant layer hybridizations vary in a large range under different stacking registries, which can be used to simulate honeycomb lattice models with both trivial and nontrivial band topologies.

We report on the anharmonic signatures in dissipative polaritons' stationary energy distribution and thermodynamics under external periodic driving. First, we introduce a dynamic model for the dissipative anharmonic Jaynes-Cummings polariton with a generic time-periodic interaction representing modulations of the polariton's energy due to an external force or field. We characterize the stationary state in terms of the exciton, phonon, and interaction energy dependence on the phonon anharmonicity, exciton-phonon coupling strength, and intensity and form of the external field-polariton coupling. Our model also captures the quantum thermodynamics of the driven polariton, which we analyze in connection with the irreversible heat, maximum power, and efficiency of the process. We find considerable differences in energy distribution and thermodynamics between harmonic, moderate, and strongly anharmonic polaritons. Moreover, comparing the external modulations to the phonon and exciton energy, we conclude that the former enhances the polariton's energy storage capacity and is occasionally limited by interference effects and energy saturation at the exciton.

We study the influence of quantum geometry on the magnetic responses of quadratic band crossing semimetals. More explicitly, we examine the Landau levels, quantum Hall effect, and magnetic susceptibility of a general two-band Hamiltonian that has fixed isotropic quadratic band dispersion but with tunable quantum geometry, in which the interband coupling is fully characterized by the maximum quantum distance $d_{\max }$. By continuously tuning $d_{\max }$ in the range of $0\le d_{\max }\le 1$, we investigate how the magnetic properties of the free-electron model with $d_{\max }=0$ evolve into those of the bilayer graphene with $d_{\max }=1$. We demonstrate that despite sharing the same energy dispersion $\epsilon \left(\mathit{p}\right)=\pm \frac{p^2}{2m}$, the charge carriers in the free-electron model and bilayer graphene exhibit entirely distinct Landau levels and quantum Hall responses due to the nontrivial quantum geometry of the wave functions.

The interplay between Floquet driving and non-Hermitian gain/loss could give rise to intriguing phenomena including topological funneling of light, edge-state delocalization, anomalous topological transitions and Floquet non-Hermitian skin effects. In this work, we uncover two unique phenomena in Floquet systems caused by gain and loss. First, multiple topological transitions from anomalous Floquet second-order topological insulators to anomalous Floquet first-order topological insulators and then to normal insulators can be induced by gain and loss. Interestingly, the resulting anomalous Floquet insulators further carry hybrid skin-topological boundary modes, which could either be fully localized or localized to different edges at different time slices and traversing along all edges in a single driving period. The topological phase transitions are also shown to be detectable through studies of transmission properties in the setting of coupled ring resonators. Second, gain and loss are found to induce singularities in the Floquet spectral, around which anomalous transmissions at flat quasienergy bands are predicted. These discoveries not only enhanced our understanding of topological matter and phase transitions in driven non-Hermitian systems, but also promoted their experimental realizations in optical and acoustic settings.

A theory of detection of microwave photons with a flux qubit based detector is presented. We consider a semiclassical approximation with the electromagnetic field being in a coherent state. The flux qubit is considered as a multilevel quantum system ($\mathrm{qu}d\mathrm{it}$). By solving the Lindblad equation, we describe the time evolution of occupations of the qudit's levels for readout and reset stages of detection. When considering the reset stage, the time evolution is described by multiple avoided-level crossings, thus providing a multilevel Landau-Zener-Stückelberg-Majorana (LZSM) problem. In addition to numerical calculations, we present an approximate solution for the description of the reset stage dynamics based on the adiabatic-impulse approximation and rate equation approach. Our theory may be useful for the theoretical description of driven-dissipative dynamics of $\mathrm{qu}d\mathrm{its}$, including applications such as single-photon detection.

Topological photonics has been an emerging research field in optics and a promising area within the broader realm of topological physics. One recent focus has been on the exploration of large-area topological edge states with tunable widths. At the same time, there has been considerable interest in higher-order topological corner states that can manipulate wave localization. Challenges remain in achieving multiband large-area topological edge states and higher-order corner states to flexibly manipulate the localization and transmission of electromagnetic waves. In this paper, we propose a valley Hall photonic crystal (VPC) with two types of interfaces capable of achieving both large-area topological edge states and large-area higher-order corner states in the dual-band frequency ranges. We demonstrate numerically and experimentally that large-area edge states enable width-tunable waveguide transmission and topological cavities in two frequency ranges. In addition, we also construct corner structures based on the two interfaces and achieve large-area higher-order corner states at dual-band frequency ranges. Therefore, our proposed VPC possesses both large-area topological edge states and large-area corner states at dual-band frequency ranges. It is expected that this VPC can serve as a multifunctional device for manipulating electromagnetic waves.

The path integral formalism is the building block of many powerful numerical methods for quantum impurity problems. However, existing fermionic path integral-based numerical calculations have only been performed in either the imaginary-time or the real-time axis, while the most generic scenario formulated on the L-shaped Kadanoff-Baym contour is left unexplored. In this work, we extended the recently developed Grassmann time-evolving matrix product operator (GTEMPO) method to solve quantum impurity problems directly on the Kadanoff-Baym contour. The resulting method is numerically exact, with only two sources of numerical errors, e.g., the time discretization error and the matrix product state bond truncation error. The accuracy of this method is numerically demonstrated against exact solutions in the noninteracting case and against existing calculations on the real- and imaginary-time axes for the single-orbital Anderson impurity model. We also show that the numerical errors of the method can be well suppressed as we refine the hyperparameters. Our method is a perfect benchmarking baseline for its alternatives which often employ less-controlled approximations, and can also be used as a real-time impurity solver in dynamical mean field theory.

We consider a linearly dispersing quantum impurity interacting through a contact density-density term with a one-dimensional (1D) superfluid described by the Tomonaga-Luttinger liquid theory. Using a linked-cluster expansion we characterize the impurity dynamics by calculating approximate expressions for the single-particle Green's function and for the time evolution of the density profile. We show the existence of two different dynamical regimes: (i) a quasiparticle regime, dominated by the presence of a finite lifetime for the host impurity, and (ii) an infrared-dominated regime, where the impurity causes a Anderson's orthogonality catastrophe of the $1\text{D}$ bath. We discuss the possible experimental consequences of these findings for cold-atom experiments.

We investigate magic-angle twisted bilayer graphene (MATBG) with proximity-induced Ising and Rashba spin-orbit couplings (SOC) in the top layer, as recently achieved experimentally. Utilizing the Bistritzer-MacDonald model with SOCs, we reveal a rich single-particle topological phase diagram featuring topological flat bands across different twist angles and interlayer hopping energies. The evolution of Dirac cones and Chern numbers is examined to understand the topological phase transitions. We find that all phases can be achieved with an experimentally accessible SOC strength ($\sim 1$ meV) in systems with angles very close to the magic angle. Furthermore, the van Hove singularity for each topological flat band splits in the presence of SOC, significantly altering the electronic properties. Additionally, we investigate the spin textures of each band in momentum space, discovering a skyrmion-like spin texture in the center of the moiré Brillouin zone, which is correlated with the topological phase transitions and can be tuned via the SOCs and an out-of-plane electric field. Our findings provide a comprehensive understanding of the topological flat bands, establishing a foundation for grasping the intrinsic and rich roles of SOCs in MATBG.

Stanene, a two-dimensional counterpart to graphene, has the potential to exhibit novel quantum phenomena when grown on a magnetic topological insulator (MTI). This work demonstrates the formation of up to bilayer stanene on 30% Sb-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBST), a well-known MTI, albeit with a buffer layer (BL) in between. Angle-resolved photoemission spectroscopy (ARPES), when combined with density functional theory (DFT), reveals stanene-related bands such as two holelike bands and an inverted parabolic band around the $\overline{\mathrm{\Gamma }}$ point. An outer holelike band traverses the Fermi level ($E_F$) and gives rise to a hexagonal Fermi surface, showing that stanene on MBST is metallic. In contrast, a band gap of 0.8 eV is observed at the $\overline{K}$ point. We find that DFT shows good agreement with ARPES only when the BL and hydrogen passivation of the top Sn layer are considered in the calculation. Scanning tunneling microscopy (STM) establishes the honeycomb buckled structure of stanene. A stanene-related component is also detected in the Sn $d$ core level spectra, in addition to a BL-related component. The BL, which forms because of the chemical bonding between Sn and the top two layers of MBST, has an ordered crystal lattice with random antisite defects. The composition of the BL is estimated to be Sn:Te:Bi/Sb $\approx$ 2:1:1 from x-ray photoelectron spectroscopy. Low-energy electron diffraction shows that the lattice constant of stanene is marginally larger than that of MBST, and the STM result aligns with this. The BL bridges this disparity and provides a platform for stanene growth.

In non-Hermitian crystal systems under open boundary conditions (OBCs), it is generally believed that the OBC modes with frequencies containing positive imaginary parts, when excited by external driving, will experience exponential growth in population, thereby leading to instability. However, our work challenges this conventional understanding. In such a system, we find an anomalous response that grows exponentially with the frequencies aligned with those of saddle points. The frequencies of these saddle points on the complex plane are below the maximum imaginary part of the OBC spectrum, but they can lie within or beyond the OBC spectrum. We derive general formulas of excitation–response relationships and find that this anomalous response can occur because the excitation of OBC modes eventually evolves toward these saddle points at long times. Only when the frequencies of all these saddle points are below the real axis do the non-Hermitian crystal systems remain stable under periodic excitation. Thus our results also provide insights on the stability criterion of non-Hermitian crystal systems.

This study is focused on the dispersion properties of a wire medium formed by a rectangular lattice of parallel wires at frequencies close to its plasma frequency. While the effective medium theory predicts isotropic behavior of transverse magnetic (TM) waves in the structure, numerical simulations reveal noticeable anisotropic properties. This anisotropy is dependent on the lattice rectangularity and reaches over 6% and over 75% along and across the wires respectively for thick wires with radii about 20 times smaller than the smallest period. This conclusion is confirmed by line-of-current approximation theory. The revealed anisotropy effect is observed when the wavelength at the plasma frequency is comparable to the period of the structure. The effect vanishes in the case of extremely thin wires. A dispersion relation for TM waves in the vicinity of the $\mathrm{\Gamma }$ point is obtained in a closed form. This provides an analytical description of the anisotropy effect.

It is found that subtle changes in the external magnetic field and temperature result in dramatic changes in the ultrafast response of spins to a femtosecond laser excitation in a ferrimagnetic Gd/FeCo multilayer. A total of six distinct types of spin dynamics were observed and explained by considering the spin-flop transition to the noncollinear phase and the concept of a tricritical point in the $H\text{−}T$ phase diagram. A particularly interesting type of dynamics is the exchange-driven reversal. These exchange-driven dynamics provide insights into the tricritical point, which is shown to separate two thermodynamically distinct noncollinear phases with the transition-metal magnetization pointing on adjacent sides of the anisotropy plane.

We study the effect of ferromagnetic metals (FM) on the circularly polarized modes of an electromagnetic cavity and show that broken time-reversal symmetry leads to a dichroic response of the cavity modes. With one spin-split band, the Zeeman coupling between the FM electrons and cavity modes leads to an anticrossing for mode frequencies comparable to the spin splitting. However, this is only the case for one of the circularly polarized modes, while the other is unaffected by the FM, allowing for the determination of the spin-splitting of the FM using polarization-dependent transmission experiments. Moreover, we show that for two spin-split bands, also the lifetimes of the cavity modes display a polarization-dependent response. The change in photon lifetimes can be understood as a suppression due to level attraction with a continuum of Stoner modes with the same wave vector. The reduced lifetime of modes of only one polarization could potentially be used to engineer and control circularly polarized cavities.

Chiral superconductors are one of the predominant quantum electronic states of matter where topology, symmetry, and Fermiology intertwine. This is pushed to a new limit by further invoking the coupling between spin and charge degrees of freedom, which fundamentally affects the principal nature of the Cooper pair wave function. We investigate the onset of superconductivity in the Rashba-Hubbard model on the triangular lattice, which is symmetry classified by the associated irreducible representations (irrep) of the hexagonal point group. From an instability analysis by means of the truncated-unity functional renormalization group (TUFRG) we find the $E_2$ irrep to dominate a large fraction of phase space and to lead up to an energetically preferred gapped, chiral superconducting state. The topological phase space classification associated with the superconducting order parameter obtained from TUFRG reveals a fragmentation of the $E_2$ domain into different topological sectors with vastly differing Chern numbers. It hints at a potentially applicable high sensitivity and tunability of chiral superconductors with respect to topological edge modes and phase transitions.

The moiré superlattice in twisted bilayer graphene has been proven to be a versatile platform for exploring exotic quantum phases. Extensive investigations have been invoked focusing on the zero-magnetic-field phase diagram at the magic twist angle around $\theta =1.1^{\circ }$, which has been indicated to be an exclusive regime for exhibiting a flat band with the interplay of strong electronic correlation and untrivial topology in the experiment so far. In contrast, electronic bands in non-magic-angle twisted bilayer graphene host dominant electronic kinetic energy compared to Coulomb interaction. By quenching the kinetic energy and enhancing Coulomb exchange interactions by means of an applied perpendicular magnetic field, here we unveil gapped flat Hofstadter subbands at large magnetic flux that yield correlated insulating states in minimally twisted bilayer graphene at $\theta =0.{41}^{\circ }$. These states appear with isospin symmetry breaking due to strong Coulomb interactions. Our work provides a platform to study the phase transition of the strongly correlated Hofstadter spectrum.