We predict that interfacing the known superconducting oxide $\mathrm{Ba}\left(\mathrm{Sr}\right){\mathrm{BiO}}_3$ with a polar terminated perovskite oxide leads to a two-dimensional (2D) electron and/or hole gasses with a density similar to superconducting K-doped bismuthates. These interfaces have the potential for high-$T_{\text{c}}$ superconductivity without the strong scattering present in randomly chemically substituted bismuthates. We present a detailed density functional theory (DFT) based study of the electronic structure of heterostructures involving polar 001 oriented ${\mathrm{LaLuO}}_3$ (LLO) and ${\mathrm{SrBiO}}_3$ (SBO) alternating layers. Beyond the thickness of four unit cells, 2D electron and hole gasses are formed at opposite interfaces of ${\mathrm{SrBiO}}_3$ with sheet charge densities on the order of ${10}^{14}{\mathrm{cm}}^{-2}$. The electron and hole gasses exist around the $Y$ ($X$) and $M$ points of the Brillouin zone, respectively. The separation of the gasses and therefore their interactions can be regulated by the SBO layer thickness. This provides a platform in which charge density waves, excitonic insulators or condensates, and superconductors compete for the ground state with the potential for new physical properties including high-$T_{\text{c}}$ superconductivity.

The electronic stopping power of a swift ion in matter can be obtained from ab initio calculations within time-dependent density functional theory. Most implementations rely today on a plane-wave plus pseudopotential approach, but at the expense of very cumbersome calculations. We show here that localized orbitals, especially with Gaussian-type orbitals, are a valuable alternative. These calculations can yield electronic stopping powers in quantitative agreement with the plane-wave results while maintaining a computational burden that is relatively low. These positive results are possible only when using Gaussian basis sets that were specially designed for the stopping power calculations. With this tool, we investigate the discrepancy between ab initio calculations and experiment at large velocity, the effect of the exchange-correlation functional, and the role of core excitations in the total stopping power. We rule out the widespread centroid path approximation as soon as the core electrons are involved in the process.

We experimentally study lateral electron transport between two $5-\mu \mathrm{m}$-spaced superconducting indium leads on a top of magnetic Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. For the disordered magnetic state of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ crystal, we observe only the Andreev reflection in the proximity of each of the leads, which is indicative of highly transparent In-${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ interfaces. If the sample is homogeneously magnetized, it demonstrates a well-developed anomalous Hall effect state. In this regime we find the Josephson current that takes place even for $5-\mu \mathrm{m}$-long junctions and show the unusual magnetic field and temperature dependencies. As a possible reason for the results obtained, we discuss the contribution to the proximity-induced spin-triplet Josephson current from the topologically protected Fermi-arc states on the surface of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$.

Recently, novel Coulomb drag mechanisms in capacitively coupled double quantum dots were uncovered by the $T$-matrix based master equation (TME). The TME is so far the primary approach to studying Coulomb drag in the weak-coupling regime; however, its accuracy and reliability remain unexplored. Here, we evaluate the performance of the TME for Coulomb drag via a comparison with numerically exact results obtained by the hierarchical equation-of-motion approach. We find that the TME can capture qualitative current evolutions versus dot levels, temperature, and effective coupling strengths but only partially succeeds at the quantitative level. Specifically, the TME gives highly inaccurate drag currents when large charge fluctuations on dots exist and the fourth-order tunneling processes make a leading-order contribution. This failure of the TME is attributed to the combined effect of the unique drag mechanisms and its overlook of the fourth-order single-electron tunnelings. We identify the reliable regions to facilitate further quantitative studies on Coulomb drag by the TME.

Topological nodal-line semimetals (TNLSMs) are a new state of quantum matter. It has one-dimensional energy band intersections where the conduction band and valence band cross to form one or more nodal rings. Here, based on the Landauer-Büttiker formula combined with the nonequilibrium Green's function method, we investigate the electron transport through TNLSM nanowires under a perpendicular magnetic field. We theoretically calculate the conductance of TNLSM nanowires and find that it is essentially different from other topological and trivial materials. The conductance of TNLSM nanowires presents the high plateaus with high steps in multiples of $e^2/h$. The height of the steps is determined by the size of the nanowire and for a TNLSM nanowire with the size of several tens of nanometers, its conductance plateaus can reach hundreds of $e^2/h$ with the step height being tens of $e^2/h$. The unique phenomenon of the high-stepped conductance plateaus originates from the peculiar energy spectrum of the TNLSMs, that is, there are a lot of subbands having the same energy minimum value. Furthermore, the high conductance plateaus and the high steps can well remain in the existence of magnetic fields, the moderate disorder, and the PT-symmetry-breaking mass term. This unique conductance characteristic can serve as a reliable signature of TNLSMs in experimental detections.

We studied the band structure of AB-stacked multilayer graphene with odd numbers of layers by conducting experiments to measure resistance ridge structures which were recently found to appear in the plot of the resistance with respect to a carrier density and a perpendicular electric flux density. The resistance ridges were found to exhibit qualitatively different structure depending on the parity of the number of layers, which determines the presence or absence of a monolayerlike band. In the perpendicular electric field, pairs of nearly flat bands (or heavy mass band) are formed at the bottoms of bilayerlike band because of the formation of the energy gap, and result in the split resistance ridge structures in the even numbers of layers. However, a monolayerlike band, which is present in AB-stacked graphene with odd numbers of layers, hybridizes with the bilayerlike bands; number of nearly flat bands, and thus, the number of resistance ridges, reduced as compared with the case of AB-stacked graphene with even numbers of layers. The mixing also opened an energy gap at the bottom of the monolayerlike band. The resistance ridge provides detailed information on the dispersion relation in multilayer graphene.

We present a method of extracting information about the topological order from the ground state of a strongly correlated two-dimensional system computed with the infinite projected entangled pair state (iPEPS). For topologically ordered systems, the iPEPS wrapped on a torus becomes a superposition of degenerate, locally indistinguishable ground states. Projectors in the form of infinite matrix product operators onto states with well-defined anyon flux are used to compute topological $S$ and $T$ matrices (encoding mutual and self-statistics of emergent anyons). The algorithm is shown to be robust against a perturbation driving string-net toric code across a phase transition to a ferromagnetic phase. Our approach provides accurate results near quantum phase transition, where the correlation length is prohibitively large for other numerical methods. Moreover, we used numerically optimized iPEPS describing the ground state of the Kitaev honeycomb model in the toric code phase and obtained topological data in excellent agreement with theoretical prediction.

Quadrupole topological insulator is a symmetry-protected higher-order topological phase with the intriguing topology of Wannier bands, which, however, has not yet been realized in plasmonic metamaterials. Here, we propose a lattice of plasmon-polaritonic nanocavities which can realize quadrupole topological insulators by exploiting the geometry-dependent sign reversal of the couplings between the daisylike nanocavities. The designed system exhibits various topological and trivial phases as characterized by the nested Wannier bands and the topological quadrupole moment which can be controlled by the distances between the nanocavities. Our study opens a pathway toward plasmonic topological metamaterials with quadrupole topology.

Recently, a novel planar Hall effect (PHE), which results from the resistivity anisotropy induced by an in-plane magnetic field, was discovered on the surface of topological insulators (TIs). While the PHE phenomenon in Weyl/Dirac semimetals is understood as a consequence of the chiral anomaly, the origin of the PHE in TIs, however, remains unclear theoretically. Several theories and experiments have ascribed the appearance of the PHE to the anisotropic backscattering induced by magnetic disorders, where the magnetization of the scatterers is indispensable. Instead, we here show that the anisotropic backscattering can arise from the tilt of the Dirac cone by an in-plane magnetic field, which emerges if nonlinear momentum terms are included, irrelevant to the magnetic nature of the scatterers. We further find that a relatively strong scalar potential can further enhance the PHE magnitude significantly, and the resulting impurity resonant state together with the tilted cone can produce the double-peak structure of the PHE and the sign change of the anisotropic magnetoresistivity. Our theory provides another perspective to understand the nontopological origin of the experimentally observed PHE in topological materials.

We present a rigorous theoretical investigation on the collective magnetic resonance of a circular array of subwavelength gyromagnetic rods. We derive the closed-form solutions to the eigenmodes within the framework of the dynamic dipole approximation using an eigenresponse theory. Owing to the lack of time-reversal symmetry and Lorentz reciprocity, the eigenpolarizability spectra exhibit significant nonreciprocal characteristics: only a single branch of rotating magnetic dipoles is supported, whereas another one corresponding to inversely rotating magnetic dipoles is totally suppressed. We found that the collective coupled magnetic dipole and coupled electric dipole (CED) modes of the circular arrays display remarkable distinction in both the resonant frequency and the radiative width in comparison with the isolated gyromagnetic rod. We further carry out full-wave numerical simulation to test the accuracy of the analytical results on the CED mode.

The electronic structures of two-dimensional hexagonal materials for a given spin direction can be generally written by the Haldane model, which shows circular dichroism or valley polarization due to broken time-reversal and inversion symmetries, respectively. By using the Kubo formula, we calculate longitudinal and transversal (Hall) optical conductivities to obtain the absorption spectra. The absorption spectra for circularly polarized lights are calculated by solving the Maxwell equations with a boundary conditions at the two-dimensional material. We found that circular dichroism and valley polarization depend on the properties of the imaginary part of the Hall conductivity, while the real part of the Hall conductivity generates large Faraday and Kerr rotations up to a few degrees.

The melting curve of gold has been measured up to 110 GPa using laser-heated diamond anvil cells and synchrotron x-ray diffraction techniques. Accurate pyrometry temperature measurements and a homogeneous heating of the gold sample were achieved by implementing a sample assembly consisting of two boron-doped diamond cupped disks sandwiching the gold sample. In the investigated pressure range, the fcc solid gold remains stable up to melting. A clear structural signature of bulk melting is observed. Ab initio molecular dynamics simulations within the two-phase approach give a melting curve in good agreement with the experimental one. We discuss the validity of calculations based on the Lindemann criteria of melting which have been up to now used to obtain the melting line of Au in the 100 GPa range.

Annealing temperature dependency of the optical and structural properties of Cu films is carefully studied using multiple means such as spectroscopic ellipsometry, x-ray diffraction, white-light interferometry, atomic force microscopy, scanning electron microscope (SEM), and the four-probe method. A wide annealing temperature range from 300 to 1200 K is examined experimentally. We find that the optical and structural properties of Cu films annealed at above 600 K cannot be naturally extended from the data commonly achieved on the samples annealed at the temperature below 600 K. When the annealing temperature is increasing, two transition points at around 600 and 900 K have been found by the Drude-Tauc-Lorentz analysis of the dielectric functions covering the energy range from 0.73 to 6.42 eV. The transition feature at 600 K can be attributed to the improvement of the crystalline state caused by annealing, which can be interpreted by the sharper diffraction peak shown in x-ray diffraction (XRD). The transition feature at 900 K was caused by the emergence of island structure due to the high-temperature annealing, which was supported by the SEM images as well. In fact, both the conspicuous scattering characteristics in the pseudodielectric function and the spurious peaks in the XRD pattern indicate the dramatically morphologic changes induced by the migration and reaggregation of Cu nanoparticles. The downward trend of dc resistivity captured in dielectric function analysis was supported by that of the sheet resistivity measured by the four-probe method.

An insulator to metal (IMT) transition $(T_t\sim 341\mathrm{K})$ in the ${\mathrm{VO}}_2$ accompanies a transition from an infrared (IR) transparent to IR opaque phase. Tailoring of the IMT and associated IR switching behavior can offer potential thermochromic applications. Here we report on effects of the W and the Tb doping on the IMT and associated structural, electronic structure, and optical properties of the ${\mathrm{VO}}_2$ thin film. Our results show that the W doping significantly lowers IMT temperature ($\sim 292\mathrm{K}$ to $\sim 247\mathrm{K}$ for 1.3% W to 3.7% W) by stabilizing the metallic rutile, $R$, phase while Tb doping does not alter the IMT temperature much and retains the insulating monoclinic, $\mathit{M}\mathit{1}$, phase at room temperature. It is observed that the W doping albeit significantly reduces the IR switching temperature but is detrimental to the solar modulation ability, contrary to the Tb doping effects where higher IR switching temperature and solar modulation ability is observed. The IMT behavior, electrical conductivity, and IR switching behavior in the W and the Tb-doped thin films are found to be directly associated with the spectral changes in the V $3d_{\parallel }$ states.

We introduce and study a toy model for anomalous transport and Griffiths effects in one-dimensional (1D) quantum disordered isolated systems near the many-body localization (MBL) transition. The model is constituted by a collection of 1D tight-binding chains with on-site random energies, locally coupled to a weak perturbation, corresponding to a random matrix which belongs to the Gaussian orthogonal ensemble (GOE) and mimics the effect of thermal inclusions due to delocalizing interactions by providing a local broadening of the Poisson spectrum. While in the absence of such a coupling the model is localized as expected for the one-dimensional Anderson model, increasing the coupling with the GOE perturbation we find a transition to a conducting phase driven by the proliferation of quantum avalanches, which does not fit the standard paradigm of the Anderson transition. In particular an intermediate Griffiths region emerges, where exponentially distributed insulating segments coexist with a few rare resonances. Typical correlations decay exponentially fast, while average correlations decay as stretched exponentials and diverge with the length of the chain, indicating that the conducting inclusions have a fractal structure and that the localization length is broadly distributed at the critical point. This behavior is consistent with a Kosterlitz-Thouless-like criticality of the MBL transition. Transport and relaxation are dominated by rare resonances and rare strong insulating regions, and show anomalous behaviors strikingly similar to those observed in recent simulations and experiments in the bad metal delocalized phase preceding MBL. In particular, we find subdiffusive transport and power-law decay of the return probability at large times, with exponents that gradually change as one moves across the intermediate region. Concomitantly, the ac conductivity vanishes near zero frequency with an anomalous power law.

The thickness dependence of spin-orbit torque and magnetoresistance in ferromagnet/heavy-metal bilayers is studied using the first-principles nonequilibrium Green's function formalism combined with the Anderson disorder model. A systematic expansion in orthogonal vector spherical harmonics is used for the angular dependence of the torque. The dampinglike torque in Co/Pt and Co/Au bilayers can be described as a sum of the spin-Hall contribution, which increases with thickness in agreement with the spin-diffusion model, and a comparable interfacial contribution. The magnetoconductance in the plane perpendicular to the current in Co/Pt bilayers is of the order of a conductance quantum per interfacial atom, exceeding the prediction of the spin-Hall model by more than an order of magnitude. This suggests that the “spin-Hall magnetoresistance,” similarly to the dampinglike torque, has a large interfacial contribution unrelated to the spin-Hall effect.

High field magnetization measurements in pulsed fields up to 65 T have been performed on ${\mathrm{FePS}}_3$, which is nominally a good example of a two-dimensional Ising-like antiferromagnet on a honeycomb lattice. Measurements with the field parallel to the moment direction confirm the presence of two first-order transitions above 35 T, to $M/M_{\mathrm{sat}}=1/2$ and $M/M_{\mathrm{sat}}=1$, respectively, at 4 K. The measurements are in contradiction with published estimates for the magnetic exchange parameters, but the contradiction can be resolved by allowing for anisotropic exchange parameters in the Hamiltonian. The magnetization with the field perpendicular to the moment direction is anisotropic, with no transitions observed for fields along the a axis while a cascade of first-order transitions is observed for fields above 50 T along the b axis, the latter case also showing a strong degradation of the sample after repeated pulses. The results indicate a strong magnetolattice coupling in ${\mathrm{FePS}}_3$. Temperature-dependent measurements hint at a possible tricritical point.

Elucidating the nature of spin excitations is important to understanding the mechanism of superconductivity in the iron pnictides. Motivated by recent inelastic neutron scattering measurements in the nearly 100% detwinned ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, we study the spin dynamics of an $S=1$ frustrated bilinear-biquadratic Heisenberg model in the antiferromagnetic phase with wave vector $(\pi ,0)$. The biquadratic interactions are treated in a dynamical way using a flavor-wave theory in an $SU\left(3\right)$ representation. Besides the dipolar spin wave (magnon) excitations, the biquadratic interactions give rise to quadrupolar excitations at high energies. We find that the quadrupolar wave significantly influences, in an energy dependent way, the anisotropy between the spin excitation spectra along the $(\pi ,0)$ and $(0,\pi )$ directions in the wave vector space. Our theoretical results capture the essential behavior of the spin dynamics measured in the antiferromagnetic phase of the detwinned ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. More generally, our results underscore the importance of electron correlation effects for the microscopic physics of the iron pnictides.

We theoretically analyze two setups of low-energy single-photon counters based on Josephson junctions (JJs). For this, we propose two simple and general models based on the macroscopic quantum tunneling formalism (MQT). The first setup is similar to the photon counter based on the “cold-electron bolometer” (CEB), where the JJ replaces the CEB in the center of the superconducting antenna. In the second setup, the JJ is capacitively coupled to the antenna. We derive the Hamiltonians for the two setups, and we write the Schrödinger equations, taking into account both the antenna and the JJ. The quantum particles of the MQT models move in two-dimensional potential landscapes, which are parabolic along one direction and may have the form of a washboard potential along another direction. Such a potential landscape has a series of local minima, separated by saddle points. If the particle is prepared in the initial state in the metastable “ground state” of a local minimum, then the photon absorption causes it to jump into an excited state. If the excitation energy is bigger than the potential barrier seen by the quantum particle (the difference between the ground state and the saddle point), the photon is detected. The models are simple and allow us to do mostly analytical calculations. We show that the two setups are equivalent from the MQT point of view since one Hamiltonian can be transformed into the other by changes in variables. For typical values of the JJ and antenna parameters, the setups may work as counters of photons of wavelengths up to at least 1 cm. Dark count rates due to the phase particle tunneling directly from the ground state into the running state have also been evaluated.