The screening response of the kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ was obtained from high-resolution ac mutual inductance measurements. At zero applied magnetic field and low temperatures, we observe no evidence for gapless quasiparticles, while near $T_c$ we find evidence for enhanced fluctuations. A rich vortex state appears above $H_{c1}\approx 30$ Oe, exhibiting successive emergence of vortex phases. For a fixed magnetic field, lowering the temperature below $T_c\left(H\right)$ leads to a wide range of vortex liquid state in a landscape of weak pinning potential, which gives rise to an irreversibility line. Further lowering the temperature, we identify the vortex melting line followed by the peak effect manifested in enhanced vortex pinning strength and critical current. We suggest that such an unusual behavior, where the peak effect region is fully contained within the vortex lattice state below the irreversibility line, is a consequence of the strong anisotropy and weak bulk pinning in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.

In this paper, we propose a practical way to realize topological surface Dirac fermions with tunable attractive interaction between them. The approach involves coating the surface of a topological insulator with a thin-film metal and utilizing the strong-electron phonon coupling in the metal to induce interaction between the surface fermions. We found that for a given TI (topological insulator) and thin film, the attractive interaction between the surface fermions can be maximally enhanced when the Dirac point of the TI surface resonates with one of the quasi-2D quantum-well bands of the thin film. This effect can be considered to be an example of quantum-well resonance. We also demonstrate that the superconductivity of the resonating surface fermions can be further enhanced by choosing a strongly interacting thin-film metal or by tuning the spin-orbit coupling of the TI. This TI–thin-film hybrid configuration holds promise for applications in Majorana-based quantum computations and for the study of quantum critical physics of strongly attractively interacting surface topological matter with emergent supersymmetry.

Employing the projective formalism of determinant quantum Monte Carlo (DQMC) simulations, we meticulously explore the ground-state phase diagram and critical behavior of the half-filled Hubbard model on a square-hexagon-octagon (SHO) lattice. This lattice, a two-dimensional (2D) structure comprising squares, hexagons, and octagons, is representative of the biphenylene network (BPN). Our findings reveal an intriguing ground-state phase diagram, featuring an antiferromagnetic (AFM) Mott insulating phase enveloped by three valence-bond-solid-like (VBS-like) insulating phases. Analyzing the single-particle gap, spin gap, and single-particle spectral function, we observe that the metallic state in the noninteracting case becomes unstable under the influence of Hubbard $U$. This interaction drives the system into a hexagon insulating phase before transitioning into an AFM Mott insulating phase. To quantify the critical exponents, we use finite-size scaling techniques. The critical exponents of quantum critical points between the AFM Mott insulating phase and two insulating phases, plaquette insulator, and ethylene insulator, closely align with the 3D O(3) universality class. However, the critical exponents of quantum critical points between the hexagon insulating phase and the AFM Mott insulating phase deviate from the 3D O(3) universality class. This deviation can be attributed to the coupling effect between the fluctuations of magnetic order parameters and very low-energy fermionic excitations. Our comprehensive study not only advances the understanding of correlation effects on the SHO lattice but also sheds light on the less-explored critical exponents in a weakly insulating quantum critical point.

We consider the problem of magnetic charges in $(2+1)$ dimensions for a torus geometry in real space, subjected to an inverted Lorentz force due to an external electric field applied normal to the surface of the torus. We compute the Hall conductivity associated with transport of these charges for the case of finite energy gap between the ground state and excitations and global $\mathrm{U}\left(1\right)$ charge conservation symmetry, and find it is proportional to an integer-valued topological invariant $\mathcal{Q}$, corresponding to the magnetic quantum Hall effect (MQHE). We identify a lattice model realizing this physics in the absence of an external electric field. Based on this, we identify a generalization of the MQHE to be quantized transport of magnetic skyrmions, the quantum skyrmion Hall effect (QSkHE), with a $\mathrm{U}\left(1\right)$ easy-plane anisotropy/spin rotation symmetry of magnetic skyrmions and effective conservation of charge associated with magnetic skyrmions yielding incompressibility, provided a hierarchy of energy scales is respected. As the lattice model may be characterized both by a total Chern number and the topological invariant $\mathcal{Q}$, we furthermore outline a possible field theory for electric charges, magnetic charges, and correlations between magnetic and electric charges approximated as composite particles, on a two-torus, to handle the scenario of intermediate-strength correlations between electric and magnetic charges modeled as composite particles. We map this problem to a generalized $(4+1)\mathrm{D}$ theory of the quantum Hall effect for the composite particles.

Real-time calculations in tensor networks are strongly limited in time by entanglement growth, restricting the achievable frequency resolution of Green's functions, spectral functions, self-energies, and other related quantities. By extending the time evolution to contours in the complex plane, entanglement growth is curtailed, enabling numerically efficient high-precision calculations of time-dependent correlators and Green's functions with detailed frequency resolution. Various approaches to time evolution in the complex plane and the required postprocessing for extracting the pure real-time and frequency information are compared. We benchmark our results on the examples of the single-impurity Anderson model using matrix product states and of the three-band Hubbard-Kanamori and Dworin-Narath models using a tree tensor network. Our findings indicate that the proposed methods are also applicable to challenging realistic calculations of materials.

We propose a simple phenomenological model for a hydrogen atom in a magnetic field which correctly reproduces the results of Makado and McGill's variational calculations for the $s$, $p$, $d$, and $f$ states for orbital quantum numbers $n\le 4$. The expression for the energy of the $1s$ state reveals that it never exhibits true free carrier behavior. Even at high magnetic fields the energy varies as ${\alpha }_n\hslash {\omega }_c/2$, where $\hslash {\omega }_c=\hslash eB/{\mu }_r$ is the cyclotron energy and ${\alpha }_n\simeq 0.84<1$. A Taylor expansion of the expression for the energy of the $1s$ state predicts a diamagnetic shift $\mathrm{\Delta }E=\sigma B^2$ where $\sigma ={\alpha }_n^2{e}^2{a}_B^2/4{\mu }_r$. This suggests that there is a missing factor of ${\alpha }_n^2\simeq 0.71$ in the generally accepted perturbation expression for the diamagnetic shift. This hypothesis is supported by a careful analysis of literature data for shallow donors and excitons in GaAs in the magnetic field.

We predict two topological phase transitions for cadmium arsenide $\left({\mathrm{Cd}}_3{\mathrm{As}}_2\right)$ thin films under in-plane magnetic field, taking advantage of a four-band $k·p$ model and effective $g$ factors calculated from first principles. Film thickness, growth direction, and in-plane Zeeman coupling strength can all serve as control parameters to drive these phase transitions. For (001) oriented ${\mathrm{Cd}}_3{\mathrm{As}}_2$ thin films, a two dimensional Weyl semimetal phase protected by $C_{2z}\mathcal{T}$ symmetry can be realized using an in-plane magnetic field, which has recently been reported in our companion paper [B. Guo, W. Miao, V. Huang, A. C. Lygo, X. Dai, and S. Stemmer, Phys. Rev. Lett. 131, 046601 (2023)]. We then put forth two pathways to achieve quantized in-plane anomalous Hall effects. By either introducing a trigonal warping term or altering the growth orientation, the emergent $C_{2z}\mathcal{T}$ symmetry can be broken. Consequently, in the clean limit and at low temperatures, quantized Hall plateaus induced by in-plane Zeeman fields become observable.

A perturbative framework is developed within the standard nonequilibrium field theory techniques to incorporate a temperature gradient across a thermoelectric device. The framework uses a temperature-dependent pseudo-Hamiltonian generated from the exact density matrix in the presence of nonuniform temperatures. We develop a perturbation theory for small temperature gradients in long wires and obtain the nonlinear thermal conductance as a function of temperature difference. The framework should be adaptable to more general cases of temperature inhomogeneities in either fermionic or bosonic systems.

In contrast to the chiral edge modes, the antichiral edge modes propagate in the same direction along two parallel boundaries, representing a new state that provides more possibility for the dissipationless transportation of information. In a recent work by Medina Dueñas et al. [Phys. Rev. Lett. 128, 066801 (2022)], a chiral phonon was shown to induce antichiral edge states on the zigzag edges of a honeycomb lattice through electron-phonon interaction (EPI). In this work we demonstrate that the EPI can promise antichiral edge modes even without the chirality of the phonon at $\mathit{K}/{\mathit{K}}^{\prime }$. To illustrate this, we consider a model of EPI with a nonchiral phonon and show that antichiral edge states appear independent of the phonon chirality. The antichiral edge modes carry quantized Zak phases if the model further respects the combination of time reversal ($\mathcal{T}$) and inversion ($\mathcal{I}$), i.e., $\mathcal{TI}$. Our work successfully extends the EPI-induced antichiral edge modes to the nonchiral phonon case, significantly increasing the experimental feasibility on realizing antichiral edge states in realistic materials.

We study the effect of Haldane flux in the bilayer $\alpha \text{−}{\mathcal{T}}_3$ lattice system, considering possible nonequivalent, commensurate stacking configurations with a tight-binding formalism. The bilayer $\alpha \text{−}{\mathcal{T}}_3$ lattice comprises six sublattices in a unit cell, and its spectrum consists of six bands. In the absence of Haldane flux, threefold band crossings occur at the two Dirac points for both valence and conduction bands. The introduction of Haldane flux in a cyclically stacked bilayer $\alpha \text{−}{\mathcal{T}}_3$ lattice system separates all six bands, including two low-energy, corrugated nearly flat bands, and assigns nonzero Chern numbers to each band, rendering the system topological. We demonstrate that the topological evolution can be induced by modifying the hopping strength between sublattices with the scaling parameter $\alpha$ in each layer. In the dice lattice limit $(\alpha =1)$ of the Chern-insulating phase, the Chern numbers of the three pairs of bands, from low energy to higher energies, are $\pm 2$, $\pm 3$, and $\pm 1$. Interestingly, a continuous change in the parameter $\alpha$ triggers a topological phase transition through band crossings between the two lower energy valance and conduction band. These crossings occur at different values for two different BZ-points and depend further on the next-nearest-neighbor hopping strength. Moreover, there are multiple topological phase transitions are found for $0<\alpha <1$.

Self-assembled InAs quantum dots (QDs), which have long hole spin coherence times and are amenable to optical control schemes, have long been explored as building blocks for qubit architectures. One such design consists of vertically stacking two QDs to create a quantum dot molecule. The two dots can be resonantly tuned to form “moleculelike” coupled hole states from the hybridization of hole states otherwise localized in each respective dot. Furthermore, spin mixing of the hybridized states in dots offset along their stacking direction enables qubit rotation to be driven optically, allowing for an all-optical qubit control scheme. Increasing the magnitude of this spin mixing is important for optical quantum control protocols. To enhance the tunnel coupling and spin mixing across the dots, we introduce Bi in the GaAs interdot barrier. Previously, we showed how to model InAs/${\mathrm{GaBi}}_x{\mathrm{As}}_{1-x}$ in an atomistic tight-binding formalism, and how the dot energy levels are affected by the alloy. In this paper, we discuss the lowering of the tunnel barrier, which results in a threefold increase of hole tunnel coupling strength in the presence of a 7% alloy. Additionally, we show how an asymmetric strain between the two dots caused by the alloy shifts the resonance. Finally, we discuss device geometries for which the introduction of Bi is most advantageous.

Electro-optic (EO) effects relate the change of optical constants by low-frequency electric fields. Thanks to the advent of density functional perturbation theory, the EO properties of bulk three-dimensional materials can now be calculated in an ab initio way. However, the use of periodic boundary conditions in most density functional theory codes requires us to simulate two-dimensional (2D) materials using slabs surrounded by a large layer of vacuum. The EO coefficients predicted from such calculations, if not rescaled properly, can severely deviate from the real EO properties of 2D materials. The present work discusses the issue and introduces the rescaling relationships that enable us to recover the true EO properties.

Due to their extremely long propagation lengths compared to the wavelengths, surface plasmon polaritons (SPPs) have been considered as a key in enhancing thermal conductivity in thin metal films. This study explores the conditions at which the plasmon thermal conductivity is maximized, considering the thickness-dependent metal permittivity. We derived the analytical solutions for the plasmon thermal conductivity in both the thin-film and thick-film limits to analyze the effect of the permittivities of metals and substrates. From the analytical solutions of plasmon thermal conductivity, we deduced that the plasmon thermal conductivity is proportional to the electron thermal conductivity based on the Wiedemann-Franz law. Additionally, we analyzed the conditions where the enhancement ratio of the thermal conductivity via SPPs is maximized. Metals with high plasma frequency and low damping coefficient are desirable for achieving the maximum plasmon thermal conductivity, as well as the maximum enhancement ratio of thermal conductivity among metals. Significantly, 10-cm-long and 14-nm-thick Al film demonstrates the most superior in-plane heat transfer via SPPs, showing a 53.5% enhancement in thermal conductivity compared to its electron thermal counterpart on a lossless glass substrate.

We model the electronic properties of thin films of binary compounds with stacked layers where each layer is a two-dimensional honeycomb lattice with two atoms per unit cell. The two atoms per cell are assigned different on-site energies in order to consider six different stacking orders: ABC, ABA, AA, ${\mathrm{ABC}}^{\prime }, {\mathrm{ABA}}^{\prime }$, and ${\mathrm{AA}}^{\prime }$. Using a minimal tight-binding model with nearest-neighbor hopping, we consider whether a fault in the texture of on-site energies in the vertical, stacking direction supports localized states, and we find localized states within the bulk band gap for ABC, ABA, and ${\mathrm{AA}}^{\prime }$ stacking. Depending on the stacking type, parameter values, and whether the soliton is atomically sharp or a smooth texture, there are a range of different band structures including soliton bands that are either isolated or that hybridize with other states, such as surface states, and soliton bands that are either dispersive or flat, the latter yielding narrow features in the density of states. We discuss the relevance of our results to specific materials including graphene, hexagonal boron nitride, and other binary compounds.

Two theorems on electron states in helimagnets are proved. They reveal a Kramers-like degeneracy in a helical magnetic field. Since a commensurate helical magnetic system is transitionally invariant with two multiple periods (ordinary translations and generalized ones with rotations), the band structure turns out to be topologically nontrivial. Together with the degeneracy, this gives an unusual spin structure of electron bands. A two-dimensional model of nearly free electrons is proposed to describe conductive hexagonal palladium layers under an effective field of magnetically ordered ${\mathrm{CrO}}_2$ spacers in ${\mathrm{PdCrO}}_2$. The spin texture of the Fermi surface leads to abnormal conductivity.

Nontrivial topological superconductivity has received enormous attention due to its potential applications in topological quantum computing. The intrinsic issue concerning the correlation between a topological insulator and a superconductor is, however, still widely open. Here, we systemically report an emergent superconductivity in a cross junction composed of a magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and a conventional superconductor ${\mathrm{NbSe}}_2$. Remarkably, the interface indicates the existence of a reduced superconductivity at the surface of ${\mathrm{NbSe}}_2$ and a proximity-effect-induced superconductivity at the surface of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Furthermore, the in-plane angular-dependent magnetoresistance measurements unveil distinctive features indicative of unconventional pairing symmetry in these superconducting gaps. Our findings extend our views and ideas of topological superconductivity in the superconducting heterostructures with time-reversal symmetry breaking, offering an exciting opportunity to elucidate the cooperative effects on the surface state of a topological insulator aligning a superconductor.

We investigated the optical properties of single crystals of one-dimensional van der Waals crystal ${\mathrm{Ta}}_4{\mathrm{SiTe}}_4$, which exhibited high thermoelectric performance below room temperature. Optical conductivity estimated from reflectivity spectra indicates the presence of a small energy gap of 0.1–0.15 eV at the Fermi energy. At the lowest energy, optical conductivity along the ${\mathrm{Ta}}_4{\mathrm{SiTe}}_4$ chain is an order of magnitude higher than that perpendicular to this direction, reflecting anisotropic electron conduction in ${\mathrm{Ta}}_4{\mathrm{SiTe}}_4$. These results suggest that the coexistence of a small band gap and moderate anisotropy in electron conduction is a promising strategy for developing high-performance thermoelectric materials for low-temperature applications.

Parafermion zero modes can arise in hybrid structures composed of $\nu =1/m$ fractional quantum Hall edges' proximity coupled by an $s$-wave superconductor. Here we consider a Josephson junction formed in such hybrid structures in addition to parafermion tunneling, Cooper pair tunneling, and backscattering. We find that the $4\pi m$ periodicity due to parafermion-only tunneling reduces, in the presence of backscattering, to $4\pi$-periodic at zero temperature and $2\pi$-periodic at finite temperature unless the fermion parity is fixed. Nevertheless, a clear signature of parafermion tunneling remains in the shape of the current-phase relation.

The wear-free sliding of layers or flakes of graphene-like 2D materials, important in many experimental systems, may occur either smoothly or through stick-slip, depending on driving conditions, corrugation, twist angles, as well as edges and defects. No single parameter has been so far identified to discriminate a priori between the two sliding regimes. Such a parameter, $\eta$, does exist in the ideal (Prandtl-Tomlinson) problem of a point particle sliding across a 1D periodic lattice potential. In that case $\eta >1$ implies mechanical instability, generally leading to stick-slip, with $\eta =\frac{2{\pi }^2{U}_0}{K_{\mathrm{p}}{a}^2}$, where $U_0$ is the potential magnitude, $a$ the lattice spacing, and $K_{\mathrm{p}}$ the pulling spring constant. Here we show, supported by a repertoire of graphene flake/graphene sliding simulations, that a similar stick-slip predictor ${\eta }_{\mathrm{eff}}$ can be defined with the same form but suitably defined $U_{\mathrm{eff}}$, $a_{\mathrm{eff}}$, and $K_{\mathrm{eff}}$. Remarkably, simulations show that $a_{\mathrm{eff}}=a$ of the substrate remains an excellent approximation, while $K_{\mathrm{eff}}$ is an effective stiffness parameter, combining equipment and internal elasticity. Only the effective energy barrier $U_{\mathrm{eff}}$ needs to be estimated in order to predict whether stick-slip sliding of a 2D island or extended layer is expected or not. In a misaligned defect-free circular graphene sliding island of contact area $A$, we show that $U_{\mathrm{eff}}$, whose magnitude for a micrometer size diameter is of order 1 eV, scales as $A^{1/4}$, thus increasing very gently with size. The PT-like parameter ${\eta }_{\mathrm{eff}}$ is therefore proposed as a valuable tool in 2D layer sliding.