The experiments in quantum spin Hall insulator candidate materials, such as HgTe/CdTe and InAs/GaSb heterostructures, indicate that in addition to the topologically protected helical edge modes, these multilayer heterostructures may also support additional edge states, which can contribute to scattering and transport. We use first-principles calculations to derive an effective tight-binding model for HgTe/CdTe, HgS/CdTe, and InAs/GaSb heterostructures, and we show that all these materials support additional edge states which are sensitive to edge termination. We trace the microscopic origin of these states back to a minimal model supporting flat bands with a nontrivial quantum geometry that gives rise to polarization charges at the edges. We show that the polarization charges transform into additional edge states when the flat bands are coupled to each other and to the other states to form the Hamiltonian describing the full heterostructure. Interestingly, in HgTe/CdTe quantum wells the additional edge states are far away from the Fermi level so that they do not contribute to the transport, but in the HgS/CdTe and InAs/GaSb heterostructures they appear within the bulk energy gap, giving rise to the possibility of multimode edge transport. Finally, we demonstrate that because these additional edge modes are nontopological it is possible to remove them from the bulk energy gap by modifying the edge potential, for example, with the help of a side gate or chemical doping.

We study the intervalley scattering induced by the Coulomb repulsion as a purely electronic mechanism for the origin of superconductivity in a few layers of graphene. The pairing is strongly favored by the presence of van Hove singularities in the density of states. We consider three different heterostructures: twisted bilayer graphene, rhombohedral trilayer graphene, and Bernal bilayer graphene. We obtain trends and estimates of the superconducting critical temperature in agreement with the experimental findings, which might identify the intervalley Coulomb scattering as a universal pairing mechanism in a few layers of graphene.

Using quantum Monte Carlo simulations we have mapped out the zero-temperature phase diagram of a symmetric electron-hole bilayer with twofold valley degeneracy, as a function of the interlayer distance $d$ and in-layer density $n$. We find that the effect of the valley degeneracy is to shrink the region of stability of the excitonic condensate, in favor of quadriexcitons at small $d$ and of the four-component plasma at large $d$, with minor effects on the value of the excitonic condensate fraction. The enclosure of the condensate in a density window possibly explains why anomalous tunneling conductivity, interpreted as a signature of condensation, is observed only between two finite values of carrier density in graphene bilayers. Our phase diagram may provide directions to select device parameters for future experiments.

Heusler compounds represent a unique class of materials that exhibit a wide range of fascinating and tuneable properties such as exotic magnetic phases, superconductivity, band topology, or thermoelectricity. An exceptional, but for Heusler compounds common, feature is that they are prone to antisite defects and disorder. In this regard, the $\mathrm{Fe}{}_2\mathrm{VAl}$ Heusler compound has been a particularly interesting and disputed candidate. Even though various theoretical scenarios for the interplay of physical properties and disorder have been proposed, the metastable disordered A2 phase hitherto precluded experimental investigation in bulk samples. Here, we report experimental results on disorder-tuned $\mathrm{Fe}{}_2{\mathrm{VAl}}_{0.9}{\mathrm{Si}}_{0.1}$ alloys all the way toward the A2 phase, which we realized via rapidly quenching our samples from high temperatures. We measured the thermoelectric properties of these materials in a wide temperature range (4 to 700 K); they suggest a gradual semimetal/narrow-gap semiconductor $\to$ metal transition upon increasing the disorder. We also find a large anomalous Hall effect in the disordered A2 phase, arising from the side-jump scattering of charge carriers at the antisite magnetic moments. This is corroborated by measurements of the temperature- and field-dependent magnetization, which increases dramatically up to $\approx 2.5{\mu }_{\text{B}}/\mathrm{f}.\mathrm{u}.$ as compared to the ordered compound ($<0.1{\mu }_{\text{B}}/\mathrm{f}.\mathrm{u}.$). This study provides an experimental realization of the metastable A2 structure in bulk $\mathrm{Fe}{}_2\mathrm{VAl}$-based alloys and grants insight into the structure-property relationship of these materials. Our work confirms that temperature-induced antisite disorder, occurring during thermal heat treatment, can be a precisely tuneable parameter in the family of Heusler compounds.

We report Monte Carlo studies of lattices of up to ${10}^5$ skyrmions treated as particles with negative core energy and repulsive interaction obtained from a microscopic spin model. Temperature dependence of translational and orientational correlations has been investigated for different experimental protocols and initial conditions. Cooling the skyrmion liquid from a fully disordered high-temperature state results in the formation of a skyrmion polycrystal. A perfect skyrmion lattice prepared at $T=0$, on raising temperature undergoes a first-order melting transition into a polyhexatic state that consists of large orientationally ordered domains of fluctuating shape. On the further increasing temperature, these domains decrease in size, leading to a fully disordered liquid of skyrmions.

We report a combined polarized and unpolarized neutron diffraction study on a multiferroic ${\mathrm{Sr}}_2{\mathrm{CoSi}}_2{\mathrm{O}}_7$ (SCSO) single crystal below and above the antiferromagnetic ordering temperature ${\mathrm{T}}_N$ = 6.5 K. Unpolarized neutron diffraction measurements at 15 K confirm the melilite-type tetragonal $P\overline{4}{2}_{1}m$ space group as the parent structure of SCSO. The low temperature study at 2.3 K, in contrast, reveals symmetry lowering with the orthorhombic $Cm{m}^{\prime }{2}^{\prime }$ and $P{2}_1{2}_1^{\prime }{2}^{\prime }$ magnetic space groups being equally possible. In these $Cm{m}^{\prime }{2}^{\prime }$ and $P{2}_1{2}_1^{\prime }{2}^{\prime }$ magnetic space groups we obtain a very similar ordered magnetic moment about 2.86 and $2.94{\mu }_B/{\mathrm{Co}}^{2+}$, respectively, which lies in the $\mathit{ab}$ plane. Our spin polarized flipping ratio measurements under an applied magnetic field of 6 T in the paramagnetic state support the results of our bulk magnetization data, indicating strong easy-plane spin anisotropy, responsible for the in-plane order below ${\mathrm{T}}_N$.

The magnetic order in a series of nonsuperconducting $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ single crystals is investigated in detail by polarized neutron diffraction. The Fe and Eu magnetic sublattices are found to be almost magnetically decoupled in this system. With the Ni doping, the Eu sublattice shows a dramatic change in its magnetic ground state, from the A-type antiferromagnetic order for $\mathit{x}\le 0.02$, in which the spins lie in the $\mathit{ab}$ plane, to a pure canted ferromagnetic order for $0.04\le \mathit{x}\le 0.20$, in which the spins are canted with a small angle off the $\mathit{c}$ axis. By comparison with the $\mathit{c}$-axis perfectly aligned ferromagnetic structure widely observed in other superconducting compounds, the lack of superconductivity in $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ might be associated with the formation of in-plane ferromagnetism. The doping-induced change of the lattice constants and accordingly the variation of the strength of indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the ${\mathrm{Eu}}^{2+}$ moments is speculated to be responsible for the dramatic change of the Eu spin structure. In addition, a spin reorientation of the ${\mathrm{Eu}}^{2+}$ moments in an intermediate temperature range is observed for $\mathit{x}=0.20$, in which some in-plane spin fluctuations persists above the long-range ordering temperature $T_{\mathrm{Eu}}$.

We investigate the sampling efficiency for the simulations of quantum many-body systems at finite temperatures when initial sampling states are generated by applying Trotter gates to random phase product states (RPPSs). We restrict the number of applications of Trotter gates to be proportional to the system size, and thus the preparation would be easily accomplished in fault-tolerant quantum computers. When the Trotter gates are made from a nonintegrable Hamiltonian, we observe that the sampling efficiency increases with system size. This trend means that almost ideal sampling of initial states can be achieved in sufficiently large systems. We also find that the sampling efficiency is almost equal to that obtained by a thermal pure quantum (TPQ) state method utilizing Haar random sampling in some cases. These findings suggest that chaotic Hamiltonian dynamics can transform RPPSs into an alternative to TPQ states for evaluating thermal expectation values.

The body-centered-tetragonal antiferromagnet ${\mathrm{EuGa}}_4$ was recently identified as a Weyl nodal-line semimetal that exhibits the topological Hall effect below its reported antiferromagnetic (AFM) ordering temperature $T_{\mathrm{N}}=15–16.5$ K which we find to be $T_{\mathrm{N}}=16.4\left(2\right)$ K. The ${\mathrm{Eu}}^{+2}$ ions are located at the corners and body centers of the unit cells. ${\mathrm{EuGa}}_4$ exhibits A-type antiferromagnetic order below $T_{\mathrm{N}}$, where the ${\mathrm{Eu}}^{2+}$ spin-7/2 moments are ferromagnetically aligned in the $ab$ plane with the Eu moments in adjacent Eu planes along the $c$ axis aligned antiferromagnetically. Low-field magnetization versus field $M\left(H_{ab}\right)$ data at $T=2$ K with the field aligned in the $ab$ plane are reported that exhibit anomalous positive curvature up to a critical field $H_{\mathrm{c}1}$ at which a second-order transition occurs with $H_{c1}\approx 0.85$ kOe for $\mathbf{H}\parallel [1,1,0]$ and $\approx 4.8$ kOe for $\mathbf{H}\parallel [1,0,0]$. For larger fields, the linear behavior $M_{ab}=\chi \left(T_{\mathrm{N}}\right)H_{ab}$ is followed until the critical field $H_{\mathrm{c}}$ is reached at which all moments become aligned with the applied field. A theory is formulated for $T=0$ K that fits the observed $M\left(H_{ab}\right)$ behavior at $T=2$ K well, where domains of $\mathrm{A}$-type AFM order with fourfold rotational symmetry occur in the AFM state in zero field. The moments in the four domains reorient to become almost perpendicular to ${\mathbf{H}}_{ab}$ at $H_{c1}$, followed by increasing canting of all moments toward the field with increasing field up to $H_{\mathrm{c}}$ which is reported to be 71 kOe. A first-order transition in $M\left(H_{ab}\right)$ at $H_{ab}=H_{\mathrm{c}1}$ is predicted by the theory for $T=0$ K when ${\mathbf{H}}_{ab}$ is at a small angle from the [1,0,0] direction.

Domain-wall skyrmions are skyrmions trapped inside a domain wall. We investigate domain-wall skyrmions in chiral magnets using a fully analytic approach. Treating the Dzyaloshinskii-Moriya (DM) interaction perturbatively, we construct the low-energy effective theory of a magnetic domain wall in an $O\left(3\right)$ sigma model with the DM interaction and an easy-axis potential term, yielding a sine-Gordon model. We then construct domain-wall skyrmions as sine-Gordon solitons along the domain wall. We also construct domain-wall skyrmions on top of a pair of a domain wall and an anti-domain wall. One characteristic feature of domain-wall skyrmions is that both skyrmions and antiskyrmions are equally stable inside the domain wall, unlike the bulk in which only one of them is stable.

A chiral anomaly Landau level emerges when a Weyl semimetal is subjected to an external magnetic field. Recently, it was demonstrated that similar chiral anomaly bulk states exist within two-dimensional Dirac semimetals if proper boundary conditions are applied. The resulting chiral bulk states disperse linearly with the slope determined by a combination of the boundary condition and chirality of the Dirac cone, i.e., whether it is at $K$ or $K$′ point. In this paper, we show that the slope (the sign of group velocity) near the $K$ and $K$′ points can be reversed under a periodically staggered potential. We first analyze the parameter dependence of this phenomenon with a nearest-neighbor tight-binding model. Then, we give a semianalytical solution for the dispersion of the chiral anomaly bulk states. In the end, we provide a photonic crystal system and prove with full-wave simulations that such a phenomenon can indeed be observed.

One of the main advantages of reciprocal bianisotropic metasurfaces is their capability to produce asymmetric scattering depending from which side they are illuminated and on the handedness of circularly polarized illuminations. For most applications, these metasurfaces are designed for illumination by a single source at a time. The resulting bianisotropic metasurface has a specific and usually complex geometrical structure that ensures the expected scattering produced under various illuminations. Here we show that geometrical asymmetry of metasurfaces can be emulated by using nonbianisotropic layers in presence of coherent illumination, which allows us to replicate and optically control the desired asymmetric scattering and chirality effects. In particular, the concept is developed on an example of emulating asymmetric scattering needed to create a ${180}^{\circ }$ hybrid junction for plane waves. We show that this device can be realized either using a bianisotropic metasurface or a set of simple sheets with electric response under simultaneous illumination by two coherent waves.

Recently, graphene triangulene chains have been synthesized, and their magnetic response has been analyzed by scanning tunneling microscopy methods by Mishra et al. [Nature (London) 598, 287 (2021)]. Motivated by this study, we determine the exchange bilinear and biquadratic constants of the triangulene chains by calculating two-spin rotations in the spirit of the magnetic force theorem. We then analyze open-ended, odd-numbered chains, whose edge states pair up forming a triplet ground state. We propose three experimental approaches that enable us to trigger and control a singlet-triplet spin transition. Two of these methods are based on applying a mechanical distortion to the chain. We finally show that the transition can be controlled efficiently by the application of an electric field.

Plasmon excitations in graphene, silicene, and germanene are studied using linear-response time-dependent density functional theory within the random phase approximation (RPA). In this work, we examine both the plasmon dispersion behavior and lifetime of extrinsic and intrinsic plasmons for these three materials. For extrinsic plasmons, we found that their properties are closely related to Landau damping. In the region without single-particle excitation (SPE), the plasmon dispersion shows a $\sqrt{q}$ behavior and the lifetime is infinite at the RPA level, while in the SPE region, the plasmon dispersion shows a quasilinear behavior and the lifetime is finite. Moreover, for intrinsic plasmons, unlike graphene, the plasmon dispersion behavior of silicene and germanene exhibits a two-peak structure, which can be attributed to the complex and hybridized band structure of these two materials.

We study a random circuit model of constrained fracton dynamics, in which particles on a one-dimensional lattice undergo random local motion subject to both charge and dipole moment conservation. The configuration space of this system exhibits a continuous phase transition between a weakly fragmented (“thermalizing”) phase and a strongly fragmented (“nonthermalizing”) phase as a function of the number density of particles. Here, by mapping to two different problems in combinatorics, we identify an exact solution for the critical density $n_c$. Specifically, when evolution proceeds by operators that act on $\ell$ contiguous sites, the critical density is given by $n_c=1/(\ell -2)$. We identify the critical scaling near the transition, and we show that there is a universal value of the correlation length exponent $\nu =2$. We confirm our theoretical results with numeric simulations. In the thermalizing phase the dynamical exponent is subdiffusive, $z=4$, while at the critical point it increases to $z_c\gtrsim 6$.

We calculate the theoretical contribution to the doping and temperature ($T$) dependence of electrical resistivity due to scattering by acoustic phonons in Bernal bilayer graphene (BBG) and rhombohedral trilayer graphene (RTG). We focus on the role of nontrivial geometric features of the detailed, anisotropic $\mathrm{k}·\mathrm{p}$ band structures of these systems—e.g., Van Hove singularities, Lifshitz transitions, Fermi surface anisotropy, and band curvature near the gap—whose effects on transport have not yet been systematically studied. We find that these geometric features strongly influence the temperature and doping dependencies of the resistivity. In particular, the band geometry leads to a nonlinear $T$ dependence in the high-$T$ equipartition regime, complicating the usual $T^4$ to $T$ Bloch-Grüneisen crossover. Our focus on BBG and RTG is motivated by recent experiments in these systems that have discovered several exotic low-$T$ superconductivity proximate to complicated hierarchies of isospin-polarized phases. These interaction-driven phases are intimately related to the geometric features of the band structures, highlighting the importance of understanding the influence of band geometry on transport. While resolving the effects of the anisotropic band geometry on the scattering times requires nontrivial numerical solution, our approach is rooted in intuitive Boltzmann theory. We compare our results with recent experiments and discuss how our predictions can be used to elucidate the relative importance of various scattering mechanisms in these systems.

We provide a quantitative characterization of generic weakly first-order thermal phase transitions out of planar spin-nematic states in three-dimensional spin-one quantum magnets, based on calculations using Poisson-Dirichlet distributions (PDs) within a universal loop model formulation, combined with large-scale quantum Monte Carlo calculations. In contrast to earlier claims, the thermal melting of the nematic state is not continuous, instead a weakly first-order transition is identified from both thermal properties and the distribution of the nematic order parameter. Furthermore, based on PD calculations, we obtain exact results for the order parameter distribution and Binder cumulants at the discontinuous melting transition. Our findings establish the thermal melting of planar spin-nematic states as a generic platform for quantitative approaches to weakly first-order phase transitions in quantum systems with a continuous SU(2) internal symmetry.

The centrosymmetric tetragonal itinerant magnet ${\mathrm{EuAl}}_4$ exhibits an intricate magnetic phase diagram including rhombic and square skyrmion-lattice (SkL) phases in the external magnetic field. Here, we report a multi-axis dilatometric investigation of ${\mathrm{EuAl}}_4$ by means of a newly designed fiber-Bragg-grating technique complemented by a resonant x-ray scattering experiment, revealing anisotropic magnetostriction and magnetovolume effect associated with successive phase transitions. The rhombic and square SkL phases are found to possess $\sim 0.10$% and $\sim 0.03$% orthorhombic structural distortion within the $ab$ plane, respectively. We propose that the coupling between the spin system and the lattice deformation should be essential for the structural instability in ${\mathrm{EuAl}}_4$, yielding a rich variety of topological spin textures with spontaneous rotational-symmetry breaking as well as a potential controllability of the SkL phases by uniaxial stress or pressure.

The breaking of chiral and time-reversal symmetries provides a pathway to exotic quantum phenomena and topological phases. Recent work has extensively explored the resulting emergence of chiral charge orders and chiral spin liquids (CSLs) on the kagome lattice. Such CSLs are closely tied to bosonic fractional quantum Hall states with anyonic quasiparticles; however, their connection to nearby ordered states has remained a mystery. Here, we use spin-wave theory, parton Gutzwiller wave functions, and exact diagonalization to show that two distinct magnetic orders with uniform scalar chirality—the XYZ umbrella state and the octahedral spin crystal–emerge as competing orders in close proximity to the CSL. In this letter, we highlight the intimate link between a topologically ordered liquid and broken symmetry states with nontrivial real-space topology.

To determine how electron correlations open a gap in two-dimensional massless Dirac electrons in the organic compounds $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ [$\alpha \text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{I}}_3$] and $\alpha \text{−}{\left(\text{BEDT-}\mathrm{TSeF}\right)}_2{\mathrm{I}}_3$ [$\alpha -{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$], we derive and analyze ab initio low-energy effective Hamiltonians for these two compounds. We find that the horizontal stripe charge ordering opens a gap in the massless Dirac electrons in $\alpha \text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{I}}_3$, while an insulating phase without explicit symmetry breaking appears in $\alpha -{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$. We clarify that a combination of anisotropic transfer integrals and electron correlations induces a dimensional reduction in the spin correlations, i.e., one-dimensional spin correlations develop in $\alpha -{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$. We show that one-dimensional spin correlations open a gap in the massless Dirac electrons. Our finding paves the way for opening gaps for massless Dirac electrons using strong electron correlations.