The transport properties of electromagnetic waves change at the transition of high-index dielectric photonic structures to the metamaterial regime. Here, we demonstrate the changes in the properties of the waves traveling through photonic quasicrystalline structures made of dielectric rods arranged in the nodes on a Penrose tiling lattice with $C_5$ rotation symmetry. We cannot use Bloch theorem in the study of aperiodic structures, so we consider full-scale structures to reveal Bragg- and Mie-type band gaps. A real-space metric allows us to define the period of the effective crystallographic planes in the quasicrystal and to relate the Bragg band gap to the lattice nodes in reciprocal space. We compared the quasicrystal structure with photonic crystals and found that transmission spectra in the band gap have similar profiles for both types of structures. The analysis of the magnetic field distribution in quasicrystal structures with high dielectric permittivity allowed us to recognize $\mu$ near-zero modes, which indicates that the structure acquires the metamaterial regime. The constructed phase diagram specified the metamaterial regime for the structure. Our results reveal the transport properties of photonic quasicrystalline systems in the metamaterial regime.

In delocalized systems, particle number fluctuations, also known as quantum surface roughness, and the mean-square displacement exhibit a temporal power law growth followed by a saturation to a system size–dependent value. We use simple scaling arguments to show that these quantities satisfy the Family-Vicsek scaling law and derive a dynamic scaling relation between the dynamical exponents, assuming that the saturation times of both quantities scale similarly with the system size. This relation clarifies the mechanism behind quantum surface roughness growth and suggests that diffusive quantum many-body systems belong to the Edwards-Wilkinson universality class. Moreover, it provides a convenient way to assess quantum transport in cold-atoms experiments. We numerically verify our results by studying two noninteracting models and one interacting model having regimes with distinct dynamical exponents.

Motivated by recent numerical studies reporting putative quantum paramagnetic behavior in spin-$1/2$ Heisenberg models on the maple-leaf lattice, we classify Abrikosov fermion mean-field Ansätze of fully symmetric U(1) and ${\mathbb{Z}}_2$ quantum spin liquids within the framework of projective symmetry groups. We obtain a total of $17\text{U}\left(1\right)$ and $12{\mathbb{Z}}_2$ algebraic PSGs, and, upon restricting their realization via mean-field Ansätze with nearest-neighbor amplitudes (relevant to the studied models), only 12 U(1) and 8 ${\mathbb{Z}}_2$ distinct phases are obtained. We present both singlet and triplet fields for all Ansätze up to third nearest-neighbor bonds and discuss their spinon dispersions as well as their dynamical spin structure factors. We further assess the effects of Gutzwiller projection on the equal-time spin structure factors.

Utilizing Migdal-Eliashberg theory of superconductivity within the first-principles calculations, we work out the role of electron-phonon coupling (EPC) and anisotropic superconducting properties of a recently discovered [Baidya et al., Phys. Rev. B 104, 174510 (2021)] 2D van der Waals heterostructure comprising a single layer of ${\mathrm{MoS}}_2$ and few layers of ${\mathrm{NbSe}}_2$. We find strong EPC and a softening of phonon modes in the lowest acoustic branch. While the single ${\mathrm{MoS}}_2$ layer does not actively contribute to the EPC, it significantly elevates the superconducting critical temperature ($T_c$) compared to monolayer ${\mathrm{NbSe}}_2$. This is attributed to the degradation of the charge-density wave by the ${\mathrm{MoS}}_2$ layer. Notably, we observe a two-gap superconductivity in ${\mathrm{NbSe}}_2/{\mathrm{MoS}}_2$ and extend our study to three layers of ${\mathrm{NbSe}}_2$. A reduction in $T_c$ with increasing thickness of ${\mathrm{NbSe}}_2$ is observed. Incorporation of spin-orbit coupling (SOC) suggests a possible mechanism for Ising superconductivity. We find that SOC reduces EPC while $T_c$ is suppressed concomitantly by about 5K, leading to a closer estimate of the experimental $T_c$.

By applying a hydrodynamic representation of nonrelativistic scalar electrodynamics to the superconducting order parameter, we predict a negative (attractive) pressure between planar superconducting bodies. For conventional superconductors with London penetration depth ${\lambda }_{\text{L}}\approx 100\text{nm}$, the pressure reaches tens of ${\text{N/mm}}^2$ at angstrom separations. The resulting surface energies are in better agreement with experimental values than those predicted by the Hartree-Fock theory, and the emergent electric-field screening length is comparable to that of the Thomas-Fermi theory. The model circumvents the bulk limitations of the Bardeen-Cooper-Schrieffer and Ginzburg-Landau theories to the analysis of superconducting quantum devices.

Mielke and Tasaki's theoretical proposal of flatband ferromagnetism with the Hubbard model has attracted much attention due to the promising possibility of ferromagnetic order in electronic materials. Using first-principles density-functional theory calculations and tight-binding analysis, we present hole-doping-induced flatband ferromagnetism in the van der Waals layered insulating electride material ${\mathrm{Y}}_2{\mathrm{Cl}}_3$, whose structural framework consists of an array of weakly coupled one-dimensional (1D) Y wires. It is revealed that ${\mathrm{Y}}_2{\mathrm{Cl}}_3$, featuring a 1D paired, puckered diamond lattice of Y atoms, possesses three occupied valence states: the first- and second-highest ($S_1$ and $S_2$) states give rise to flatbands due to the destructive interference of Bloch wavefunctions, whereas the third-highest ($S_3$) state exhibits a dispersive band along the interstitial space within the paired diamond lattice. Upon partial hole doping of the $S_1$ band with a density larger than 0.3 holes per unit cell, we predict the emergence of ferromagnetism by satisfying the Stoner criterion, enabled by a high density of states at the Fermi level. Interestingly, the spin polarization of the $S_1$ band induces the nearly equal spin splitting of the $S_2$ and $S_3$ bands via the facilitated exchange interactions with the presence of interstitial anionic excess electrons. Our findings offer theoretical insights into an intricate flatband ferromagnetism in the experimentally synthesized 1D electride ${\mathrm{Y}}_2{\mathrm{Cl}}_3$ by hole doping, thereby enriching the family of 1D electride materials for spintronic applications.

Employing transformations of planar Ising models (decoration-decimation followed by star-triangle), the frustrated antiferromagnetic kagomé Ising model is mapped to the known disordered region of the ferromagnetic triangular Ising model. Thereafter, with supplemental aid of mapping theorems and linear algebraic correlation identities, correlations in the frustrated kagomé Ising model are determined at all temperatures, enabling exact solutions to be obtained in the neutron scattering problem. More particularly, the scattering function $S^{xx}(\vec{q},\omega )$ is uniform in momentum $\vec{q}$ and exhibits Dirac $\delta$ functions at select scattering frequencies, where each select frequency is associated with a temperature-dependent scattering amplitude. The scattering solutions are compared with those of the ferromagnetic kagomé Ising model.

The coexistence of ferrovalley and band topology has garnered significant attention, due to its valley-polarized quantum anomalous Hall effect in condensed matter physics for two-dimensional (2D) ferromagnetic materials, in which the spontaneous out-of-plane magnetization and strong spin-orbit coupling effect are both satisfied. Here, using first-principles calculations and a tight-binding model, we reported the valley-polarized band topological phase transition and perpendicular magnetocrystalline anisotropy in a series of 2D Janus magnetic transition metal dihalides $H-M\mathrm{ClBr}$ ($M$ = Ru, Os) via strain manipulation. Due to both the inversion and time-reversal asymmetry, valley polarization could be spontaneously achieved in hexagonal RuClBr and hexagonal OsClBr monolayers, with considerable values of 247 and 150 meV, respectively, which exceed those of most reported 2D ferrovalley materials. By applying an external biaxial strain, it is noteworthy that the easy magnetization axis of $H$-RuClBr can be switched to the out-of-plane direction due to distinct spin channel interactions between the Ru $d$ orbitals. Moreover, transitions from the anomalous valley Hall effect to the nontrivial topological state in both $H$-RuClBr and $H$-OsClBr are observed. Due to the spontaneous valley polarization, band topology phase transition, and perpendicular magnetocrystalline anisotropy, it is expected that 2D Janus magnetic transition metal dihalides expand the application for the interplay among valleytronics, spintronics, and band topology.

We present first-principles calculations of Schottky barrier heights at interfaces relevant for silicon-based merged-element transmon qubit devices. Focusing on Al(111)/Si(111) and ${\mathrm{CoSi}}_2$(111)/Si(111), we consider various possible interfacial structures, for which we study the relaxations of the atoms near the interface, calculate the formation energies and Schottky barrier heights, and provide estimates of the Josephson critical currents based on the Wentzel-Kramers-Brillouin (WKB) tunneling formalism as implemented in the Simmons/Tsu-Esaki model. We find that the formation energies and SBHs are very similar for all Al(111)/Si(111) structures, yet vary significantly for the ${\mathrm{CoSi}}_2$(111)/Si(111) structures. We attribute this to the more covalent character of bonding at ${\mathrm{CoSi}}_2$/Si, which leads to configurations with distinct atomic and electronic structure. Our estimated Josephson critical currents, which govern the behavior of merged-element transmons, provide insight into the trends as a function of Schottky-barrier height. We show that desirable qubit frequencies of 4–5 GHz can be obtained with a Si barrier thickness of about 5–10 nm, and demonstrate that the critical current density as a function of Schottky barrier height can be modeled based on the tunneling probability for a rectangular barrier.

We investigate gradual development of the quasiparticle states in two quantum dots attached to opposite sides of the topological superconducting nanowire, hosting the boundary modes. Specifically, we explore the nonequilibrium cross-correlations transmitted between these quantum dots via the zero-energy Majorana modes. Our analytical and numerical results reveal the nonlocal features observable in the transient behavior of electron pairing, which subsequently cease while the hybrid structure evolves towards its asymptotic steady-state configuration. We estimate duration of these temporary phenomena. Using the nonperturbative scheme of the time-dependent numerical renormalization group technique we also analyze nonequilibrium signatures of the correlation effects competing with the proximity induced electron pairing. These dynamical processes could manifest themselves in braiding protocols imposed on the topological and/or conventional superconducting quantum bits, using superconducting hybrid nanostructures.

Novel fermions with relativistic linear dispersion can emerge as low-energy excitations in topological semimetal materials. Here, we show that the orbital moment contribution in the circular photomagnetic effect for these topological semimetals exhibit an unconventional ${\omega }^{-1}$ frequency scaling, leading to significantly enhanced response in the infrared window, which can be orders of magnitude larger than previous observations on conventional materials. Furthermore, the response tensor is directly connected to the Chern numbers of the emergent fermions, manifesting their topological character. Our work reveals a new signature of topological semimetals and suggests them as promising platforms for optoelectronics and spintronics applications.

Significant efforts have been dedicated to achieving excitonic insulators. In this paper, we explore a problem of doping excitons into a Mott insulator instead of a band insulator. Specifically, we start with a Mott insulator on a triangular moiré superlattice in a transition metal dichalcogenides layer and inject excitons by either transferring particles to a different layer or optically pumping electrons from the valence to the conduction band. In both cases, the excitons move in the presence of local spin moments inherited from the Mott insulator. When the Heisenberg spin coupling $J$ is small, the kinetic energy of the excitons decides the magnetism, akin to Nagaoka ferromagnetism in hole-doped Mott insulators. Through density-matrix renormalization-group calculations, we demonstrate that the spin moments originating from the Mott insulator form ${120}^{\circ }$ antiferromagnetic or ferromagnetic order for the two signs of the exciton hoppings over a broad range of exciton densities. Notably, the optical pump case may result in an antiferromagnetic to ferromagnetic transition with increasing exciton density, indicating a potential mechanism for light-induced ferromagnetism. A similar exciton-induced ferromagnetism could be achieved in a moiré-monolayer system where the monolayer is electron-doped while the moiré Mott insulator is hole-doped. Our works demonstrates a possibility to engineering magnetism through doping neutral excitons.

We develop a theory of a Coulomb interaction-mediated contribution to the valley Hall effect (VHE) in two-dimensional noncentrosymmetric gapped Dirac materials. We assume that the bare valley Hall current occurs in the system due to the presence of disorder caused by impurities and is determined by the valley-selective anisotropic skew scattering. Applying the Boltzmann transport equation to describe the electron and hole transport in the material, we calculate the renormalized VHE conductivity due to electron-electron and electron-hole scattering processes, considering two regimes: (i) an $n$-doped monolayer hosting a degenerate electron gas, and (ii) an intrinsic semiconductor with the Boltzmann statistics of electron and hole gases. In both regimes, the dominant mechanism of interparticle scattering is due to particles residing in different valleys. Moreover, in case (ii), in addition to direct scattering, electron-hole annihilation starts to play a role with the increase in temperature. It might even become the dominant mechanism of the Coulomb interaction-mediated VHE.

In the realm of thermal expansion determination, the quasiharmonic approximation (QHA) stands as a widely embraced method for discerning minima of free energies across diverse temperatures such that the temperature dependence of lattice parameters as well as internal atomic positions can be determined. However, this methodology often imposes substantial computational demand, necessitating numerous costly calculations of full phonon spectra in a possibly many-dimensional geometry parameter space. Focusing on the volumetric thermal expansion only, the volume-constrained zero static internal stress approximation (v-ZSISA) within QHA allows one to limit significantly the number of phonon spectra determinations to typically less than 10. The linear Grüneisen approach goes even further with only two phonon spectra determinations to find the volumetric thermal expansion, but a deterioration of the accuracy of the computed thermal expansion is observed, except at low temperatures. We streamline this process by introducing further intermediate approximations between the linear Grüneisen and the v-ZSISA-QHA, corresponding to different orders of the Taylor expansion. The minimal number of phonon spectra calculations that is needed to maintain precise outcomes is investigated. The different approximations are tested on a representative set of 12 materials. For the majority of materials, three full phonon spectra, corresponding to quadratic order, is enough to determine the thermal expansion in reasonable agreement with the v-ZSISA-QHA method up to 800 K. Near perfect agreement is obtained with five phonon spectra. This study paves the way to multidimensional generalizations, beyond the volumetric case, with the expectation of much bigger benefits.

The circularly polarized photogalvanic effect (CPGE) is studied in chiral Weyl semimetals with short-range quenched disorder. Without disorder, the topological properties of chiral Weyl semimetals lead to quantization of the CPGE, which is a second-order optical response. Using a combination of diagrammatic perturbation theory in the continuum and exact numerical calculations via the kernel polynomial method on a lattice model, we show that disorder perturbatively destabilizes the quantization of the CPGE.

Bilayers made of the high-$T_c$ cuprate superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ assembled with a twist angle close to ${45}^{\circ }$ have been recently shown to spontaneously break time-reversal symmetry $\mathcal{T}$, consistent with theoretical predictions for emergent chiral topological $d_{x^2-y^2}+i{d}_{xy}$ phase in such twisted $d$-wave superconductors. Here we use a minimal microscopic model to estimate the size of spontaneous chiral edge currents expected to occur in the $\mathcal{T}$-broken phase. In accord with previous theoretical studies of chiral $d$-wave superconductors we find small but nonvanishing edge currents whose magnitude and spatial distribution are sensitive to the type of the edge or domain wall. We nevertheless predict these to be above the detection threshold of the state-of-the-art magnetic scanning probe microscopy. In addition, by deriving a simple relation between the edge current and the electron spectral function we help elucidate the longstanding disparity between the size of edge currents in chiral $d$-wave and $p$-wave superconductors.