Chiral symmetry breaking (CSB) is promising to realize a series of exotic topological phenomena due to the coupling of Dirac fermions at inequivalent valleys. We propose a strategy to introduce CSB by applying a Kekulé distortion in a spinless honeycomb lattice, resulting in the emergence of a second-order topological phase as the period of the superlattice is $3m\times 3m$ $(m=1,2,\dots \mathrm{)}$. Following the strategy, $\mathrm{S}{\mathrm{i}}_x{\mathrm{C}}_y$ and circumcoronene-based two-dimensional (2D) honeycomb lattices are predicted to be second-order topological insulators (SOTIs) that are identified via the topological invariants and the presence of in-gap topological corner modes. In this paper, we provide an effective approach to realize CSB in a honeycomb lattice and greatly enrich 2D SOTIs families.

We performed a detailed study of the intrinsic electronic structure of ${\mathrm{YRu}}_2{\mathrm{Si}}_2$ employing angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT) based first-principles calculations. Electrical and magnetic measurements were conducted on well-oriented high-quality single crystals. Bulk physical measurements indicate that the compound exhibits slightly enhanced Pauli paramagnetic behavior, accompanied by electrical transport properties reminiscent of metals. Our ARPES data reveal fourfold symmetric Fermi surface with weakly dispersing bands around the $\overline{\mathrm{N}}$ point originating from Ru $d$ orbitals. We observed the anisotropic characteristics of the band near the $\overline{\mathrm{N}}$ point, showing weak dispersion in the $\overline{\mathrm{X}}\text{−}\overline{\mathrm{N}}\text{−}\overline{\mathrm{X}}$ direction and minimal dispersion along the $\overline{\mathrm{N}}\text{−}\overline{\mathrm{\Gamma }}\text{−}\overline{\mathrm{N}}$ direction. The electronic band structure near the Fermi level is primarily governed by the Ru $d$ orbital, with minor contributions from the Y $d$ and Si $p$ orbitals. Polarization-dependent ARPES results indicate the multiband and multiorbital band character of ${\mathrm{YRu}}_2{\mathrm{Si}}_2$. Due to the negligible correlation effect, the observed ARPES data are found to be in good agreement with the DFT results.

With the help of the numerical renormalization group method, we theoretically investigate the Josephson phase transition and the Yu-Shiba-Rusinov (YSR) states in parallel-coupled double quantum dot (QD) Josephson junctions. The interplay of competition and cooperation of multiple energy scales within the system results in distinct impacts on the 0-$\pi$ transition behavior and the evolution of YSR states when the QDs are in the half-filled case. This arises from the relative variation in the strengths of the interdot antiferromagnetic correlation and the Ruderman-Kittel-Kasuya-Yosida interaction. In addition, when the interdot coupling coefficient $t_{12}$ is fixed, by adjusting the phase difference between superconducting electrodes, we found that the magnetic order of two QDs and the YSR subgap states can be controlled in a delocalized manner. Our research explores the complexities of electron correlation effects in Josephson junctions, providing unique insights into the construction and regulation of superconducting nanodevices and quantum bits in the future. Furthermore, the system's spin and orbital degrees of freedom can be manipulated to satisfy the SU(4) symmetry by adjusting the QD levels and other parameters. An orbital YSR state is induced by a pseudospin-flip process involving pseudospins and quasiparticles outside the superconducting gap. We provide a detailed analysis of the various physical mechanisms underlying the emergence of spin and orbital YSR states through the application of an external magnetic field.

Recent angle-resolved photoemission spectroscopy (ARPES) for a newly discovered doped one-dimensional (1D) Mott insulator ${\mathrm{Ba}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_{3+\delta }$ proposed the presence of attractive charge interactions between neighboring sites if a 1D Hubbard model is employed to describe the material. On the other hand, long-range repulsive charge interactions were proposed to be crucial for understanding the optical excitations of undoped 1D Mott insulators. Motivated by these contrasting proposals, we perform density-matrix renormalization group calculations of the single-particle spectral function as well as dynamical spin and charge structure factors for two kinds of extended 1D Hubbard models. The spectral function for the long-range repulsive model less agrees with the ARPES data as compared with that for the attractive one. The dynamical spin and charge structure factors also exhibit contrasting behaviors between the two models: The attractive model shows less momentum dependence of the integrated weight in the dynamical spin structure factor and more accumulation of the weight on the upper edge of particle-hole-type excitation in the dynamical charge structure factor. These features give a good criterion of an appropriate model for doped 1D Mott insulators, which can be judged by resonant-inelastic x-ray scattering experiments.

The fractional quantum Hall effect (FQHE) at the filling factor with an even denominator $\frac{5}{2}$ occurs despite the expectation, due to the electron statistics, that the denominator must be an odd number. It is believed that the Cooper pairing of underlying quasiparticles, composite fermions (CFs), leads to the explanation of this effect. Such a state should have a Pfaffian form of the BCS wave function (due to the fully polarized spin) and non-Abelian statistics of possible vortexlike excitations (due to the $p$-wave nature of the pairing). Here we expose the origin of pairing by using the effective dipole representation of the problem and show that pairing is encoded in a Hamiltonian that describes the interaction of the charge density with dipoles, i.e., the current of CFs. The necessary condition for the paired state to exist is the effective dipole physics at the Fermi level as a consequence of the nontrivial topology of the ideal band in which electrons live, a Landau level (LL); the paired state is a resolution of the unstable, critical behavior characterized by the distancing of correlation hole with respect to electron (and thus dipole) at the Fermi level due to the topology. We describe analytically this deconfined critical point, at which deconfinement of Majorana neutral fermions takes place. In the presence of large, short-range repulsive interaction inside a LL, the critical behavior may be stabilized into a regularized Fermi-liquid-like state, like the one that characterizes the physics in the lowest LL, but in general, for an interaction with slowly decaying pseudopotentials, the system is prone to pairing.

Charge order (CO) is ubiquitous in high-$T_c$ cuprate superconductors and is considered to compete with superconductivity in the underdoped region. However, how CO emerges from antiferromagnetic parent compounds is rarely studied and remains elusive. The existence of CO in extremely underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CuO}}_{6+\delta }$ (La-Bi2201) has only been observed by scanning tunneling microscopy, lacking comprehensive bulk evidence. Here, we use Cu $L_3$-edge resonant inelastic x-ray scattering to investigate the emergence of CO in insulating La-Bi2201 samples. Our findings reveal the absence of CO at doping $p=0.03$, while it becomes evident at $p=0.07$. The CO diminishes with increasing temperature, with an onset temperature around 270 K, similar to the behavior of charge order in underdoped and optimally doped Bi2201. These findings suggest that CO in La-Bi2201 forms a truncated dome-shaped phase diagram as doping decreases. This phase diagram originates from the inhomogeneity of the charge distribution, where localized regions of high charge density allow CO to exist at higher temperatures. The rapid reduction of these high-density regions accounts for the disappearance of CO at lower doping levels. Our findings provide bulk evidence of CO in extremely underdoped Bi2201 and reveal how CO emerges in inhomogeneous systems.

Kinetic Monte Carlo (KMC) can be used to simulate the behavior of a quantum system governed by incoherent transitions between its states. At each KMC step, the system evolves by executing a transition that is randomly chosen according to its rate, and a contribution to the average of a system observable is calculated. In traditional KMC, only the contribution of the transition that is actually chosen is accounted for when calculating system observables. We propose here an extension of KMC, which we call multiverse-weighted kinetic Monte Carlo (MKMC). In MKMC, also the contributions of all other transitions, that are not chosen but could have been chosen, are taken into account, weighted according to their rates. Because in MKMC the average of an observable is based on more information than in traditional KMC, one may expect that the observable can be calculated with higher accuracy. As a demonstration we apply MKMC to the simulation of several optoelectronic processes in organic light-emitting diodes. We demonstrate situations where the accuracy of a system observable calculated with MKMC is indeed much better than with traditional KMC and also identify situations where this is not the case.

We establish a universal theory to understand quasiparticle Hall effects and transverse charge-carrier transport in organic semiconductors. The simulations are applied to organic crystals inspired by rubrene and cover multiple transport regimes. This includes calculations of the intrinsic Hall conductivity in pristine crystals, which are connected with a simple description of semiclassical electron transport that involves the concept of closed electronic orbits in the band structure, which can be easily calculated in density functional theory. Furthermore, this framework is employed to simulate temperature-dependent longitudinal and transverse mobilities in rubrene. These simulations are compared to experimental findings, providing insights into these results by characterizing the nonideality of the Hall effect due to the influence of vibrational disorder. We finally investigate the conditions for the observation of Shubnikov–de Haas oscillations in the longitudinal resistivity and quantized Hall plateaus in the transverse resistivity. A clear picture why this is not observed in rubrene is developed. These insights into classical and quantum Hall effects and their intermediates in organic semiconductors establish a blueprint for future explorations in similar systems.

We present a method to solve the dynamical Bethe-Salpeter equation numerically. The method allows one to investigate the effects of dynamical dielectric screening on the spectral position of excitons in transition-metal dichalcogenide monolayers. The dynamics accounts for the response of optical phonons in the materials below and on top of the monolayer to the electric field lines between the electron and hole of the exciton. The inclusion of this effect unravels the origin of a counterintuitive energy blueshift of the exciton resonance, observed recently in monolayer semiconductors that are supported on ionic crystals with large dielectric constants. A surprising result is that while energy renormalization of a free electron in the conduction band or a free hole in the valence band is controlled by the low-frequency dielectric constant, the band-gap energy introduces a phase between the photoexcited electron and hole, rendering contributions from the high-frequency dielectric constant also important when evaluating self-energies of the exciton components. As a result, band-gap renormalization of the exciton is not the sum of independent contributions from energy shifts of the conduction and valence bands. The theory correctly predicts the energy shifts of exciton resonances in various dielectric environments that embed two-dimensional semiconductors.

Electric field-induced modulation of the optical properties is crucial for amplitude and phase modulators used in photonic devices. Here, we present a comprehensive study of the band geometry induced electro-optic effect, specifically focusing on the Fermi surface and disorder-induced contributions. These contributions are crucial for metallic and semimetallic systems and for optical frequencies comparable to or smaller than the scattering rates. We highlight the importance of the quantum metric and metric connection in generating the phenomenon in parity-time reversal ($\mathcal{PT}$) symmetric systems such as CuMnAs. Our findings establish the electro-optic effect as a tool to probe band geometric effects and open different avenues to design electrically controlled efficient amplitude and phase modulators for photonic applications.

Based on first-principles calculations, we report that the easy magnetization axis in two-dimensional (2D) van der Waals (vdW) magnets can be controlled by the sliding direction, a phenomenon we termed sliding ferromagnetism, directly linked to the second-order perturbation effects of spin-orbit coupling. Furthermore, we construct a ${\mathrm{MnSiSe}}_3$/graphene-bilayer-BN-graphene/${\mathrm{MnSiSe}}_3$ vdW multiferroic tunnel junction (MFTJ) where the direction of sliding, rather than electric or magnetic fields, drives the device. Here, the sliding ferroelectric bilayer $h$-BN acts as the insulating area, and the sliding ferromagnetic mirrored-bilayer ${\mathrm{MnSiSe}}_3$ serves as the electrodes. Our nonequilibrium Green's function calculations demonstrate the existence of multilevel resistance states in this MFTJ. These findings present an approach to manipulating the spin orientation in two-dimensional magnets without magnetic fields, and thus a colorful platform for designing tunable spintronic devices.

Employing the low-energy effective theory alongside a combination of analytical and numerical techniques, we explore the Landau level collapse phenomenon, uncovering previously undisclosed features. We consider both finite-width graphene ribbons and semi-infinite geometries subjected to a perpendicular magnetic field and an in-plane electric field, applied perpendicular to both zigzag and armchair edges. In the semi-infinite geometry the hole (electron)-like Landau levels collapse as the ratio of electric and magnetic fields reaches the critical value $+(-)1$. On the other hand, the energies of the electron (hole)-like levels remain distinct near the edge and deeply within the bulk, approaching each other asymptotically for the same critical value. In the finite geometry, we show that the electron (hole)-like levels become denser and merge, forming a band.

We study the Kondo effect of spin-$1/2$ magnetic impurities in gapped $Z_2$ spin liquids on two-dimensional lattices. We find that if the impurity is placed at a high-symmetry location, a nontrivial spinon fractionalization class of the impurity site symmetry group will necessarily lead to a non-Kramers doublet in the Kondo screening regime, protected by associated crystalline symmetries. This is in sharp contrast to a featureless screening phase in the usual Kondo effect. We demonstrate this symmetry-protected topological degeneracy by an exactly solvable model and by the large-$N$ theory. Based on this effect, we discuss how thermodynamic measurements in the limit of dilute magnetic impurities can be used to detect symmetry fractionalization in gapped $Z_2$ spin liquids.

We present an approach for simulating x-ray nanobeam Bragg coherent diffraction patterns based on the Takagi-Taupin equations. Compared to conventional methods, the current approach can be universally applied to any weakly strained system including semi-infinite crystals that diffract dynamically. It addresses issues such as the curved wave front and redivergence of the focused incident beam. We show excellent agreement with experimental data for a strained ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ thin film on a ${\mathrm{SrTiO}}_3$ substrate and a path to extracting physical information using automatic differentiation.

Altermagnets are a class of antiferromagnetic materials which have electron bands with lifted spin degeneracy in momentum space but vanishing net magnetization and no stray magnetic fields. Because of these properties, altermagnets have attracted much attention for potential use in spintronics. We here show that despite the absence of bulk magnetization, the itinerant electrons in altermagnets can generate a magnetization close to edges and vacuum interfaces. We find that surface-induced magnetization can also occur for conventional antiferromagnets with spin-degenerate bands, where the magnetization from the itinerant electrons originates from a subtle yet nonvanishing redistribution of the probability density on the unit-cell level. An intuitive explanation of this effect in a phenomenological model is provided. In the altermagnetic case, the induced magnetization has a different spatial dependence than in the antiferromagnetic case due to the anisotropy of the spin-polarized Fermi surfaces, causing the edge-induced Friedel oscillations of the spin-up and -down electron densities to have different periods. We employ both a low-energy effective continuum model and lattice tight-binding calculations. Our results have implications for the usage of altermagnets and antiferromagnets in nanoscale spintronic applications.

Magnetometry, neutron diffraction experiments, and high-resolution transmission electron microscopy (HRTEM) were performed to study the magnetic behavior of ${\mathrm{CuCr}}_{2-x}{\mathrm{Sn}}_x{\mathrm{S}}_2{\mathrm{Se}}_2$ ($0.2\le x\le 1.0$) solid solutions and experimentally determine the appropriate magnetic structure for these systems. For all samples, the main phase with normal spinel-type structure ($Fd\overline{3}m$) was refined. For low Sn concentration a minority monoclinic phase appears also corroborated with HRTEM analyses. Together with the results from magnetization experiments, neutron diffraction measurements allowed to establish that for samples with $x\le 0.4$ there is a ferromagnetic long-range order at high temperatures labeled with the $3d$ irrep $\mathrm{m}{\mathrm{\Gamma }}_4^{+}$ of $Fd\overline{3}m.1^{\prime }$, while for $x>0.4$ no magnetic signal is observed, indicating that the ferromagnetic behavior is suppressed and replaced with a spin-glass-like state.

Understanding the stability of skyrmions and skyrmioniums is crucial for potential applications, as thermal fluctuations can cause them to transform into other magnetic structures. In this study, we used micromagnetic simulations to examine the regions of stable existence for skyrmion and skyrmionium structures in Co/Pt ferromagnetic nanodisks at 300 K and beyond. We identified stability regions for different values of anisotropy energy ($K_z$) and interfacial Dzyaloshinskii-Moriya interaction (DMI). Interestingly, these structures share a mutual region of stability. Depending on the thermal energy system, transitions can occur from skyrmionium to skyrmion. We further compute the corresponding thermal activation energy barrier for this transition as $\mathrm{\Delta }E/k_B=(15\pm 2)\times {10}^2 \mathrm{K}$, with an average lifetime of ${\tau }_0$ = 0.9 $\pm$ 0.4 ns. This transition can also be triggered by a magnetic field applied along the $z$-axis.

It is known that the eigenspectra and eigenfields of wave systems are sensitive to boundary conditions. We show a counterintuitive result that two hard-boundary sound cavities of different boundary shapes and of different sizes have a common set of eigenfrequencies. Moreover, their eigenfields at shared frequencies are the same in some of their common subregions. The reason is that both cavities can be created by cutting along locally mirror-symmetric lines in a honeycomb lattice, with their boundaries compatible with the hexagonal cavity's eigenfields at those common eigenfrequencies. Based on this observation, we have discovered a phenomenon where cavity eigenfields remain unaffected by obstacles of certain shapes within the cavity. The key lies in aligning the obstacles’ boundaries with the antinodal lines of the cavity eigenfields, which are easy to find when the cavity contains some local mirror symmetries. All the results can be generalized to electromagnetic waves and three-dimensional cavities.