We study the dynamic vortex Mott transition in two-dimensional superconducting arrays in a magnetic field with $f$ flux quantum per plaquette. The transition is induced by external driving current and thermal fluctuations near rational vortex densities set by the value of $f$ and has been observed experimentally from the scaling behavior of the differential resistivity. Recently, numerical simulations of interacting vortex models have demonstrated this behavior only near fractional $f$. A fine-grid vortex model is introduced, which allows us to consider both the cases of fractional and integer $f$. The critical behavior is determined from a scaling analysis of the current-voltage relation and voltage correlations near the transition and by Monte Carlo simulations. The critical exponents for the transition near $f=1/2$ are consistent with the experimental observations and previous numerical results from a standard vortex model. The same scaling behavior is obtained for $f=1$, in agreement with experiments. However, the estimated correlation-length exponent indicates that even at integer $f$, the critical behavior is not of mean-field type.

With dc magnetization and polarized neutron reflectometry we studied the ferromagnetic response of $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_7/\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{C}{\mathrm{a}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (YBCO/LCMO) multilayers that are grown with pulsed laser deposition. We found that whereas for certain growth conditions (denoted as A type) the ferromagnetic moment of the LCMO layer is strongly dependent on the structural details of the YBCO layer on which it is deposited, for others (B type) the ferromagnetism of LCMO is much more robust. Both kinds of multilayers are of similar structural quality, but electron energy-loss spectroscopy studies with a scanning transmission electron microscope reveal an enhanced average Mn oxidation state of +3.5 for the A-type as opposed to the B-type samples, for which it is close to the nominal value of +3.33. The related, additional hole doping of the A-type LCMO layers, which likely originates from La and/or Mn vacancies, can explain their fragile ferromagnetic order, since it places them close to the boundary of the ferromagnetic order at which even weak perturbations can induce an antiferromagnetic or glassy state. On the other hand, we show that the B-type samples allow one to obtain YBCO/LCMO heterostructures with very thick YBCO layers and, yet, strongly ferromagnetic LCMO layers.

Kramers-Kronig relations for propagating modes are a fundamental property of many but not all structures supporting wave propagation. While Kramers-Kronig relations for a transfer function of a causal system are always satisfied, it was discovered in the past that Kramers-Kronig relations for individual modes of a system may fail, e.g., in leaky structures. Our aim in this paper is to scrutinize whether magnetoinductive waves propagating in discrete metamaterial structures by virtue of interelement coupling satisfy Kramers-Kronig relations. Starting with the dispersion relations of magnetoinductive waves in the nearest-neighbor approximation we investigated the real and imaginary parts of two complex functions: the propagation coefficient and the transfer function. In order to show that the real and imaginary parts of these functions are related to each other by the Kramers-Kronig relations we had to modify the functions by deducting from them the values of the functions at infinite frequencies. It was shown that these modified functions and their Kramers-Kronig pairs perfectly coincided. The much more complicated case of magnetoinductive wave propagation with long-range coupling assumed was also investigated. The mode structure was derived for interactions up to the third neighbor. It was shown that the Kramers-Kronig relations are not applicable to the individual modes, but are valid for the transfer functions of the waveguide after solving the excitation problem and taking all the modes into account. Our method can be applied to practically relevant cases enabling rapid evaluation of transfer functions, e.g., in metamaterials used for wireless power transfer.

We theoretically show that the coupling of inhomogeneous strains to the Dirac fermions of three-dimensional topological insulators (3DTI) in thin film geometries results in the occurrence of phase transitions between topologically distinct insulating phases. By means of minimal $\mathbf{k}·\mathbf{p}$ models for strong 3DTI in the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ materials class, we find that in thin films of stoichiometric materials a strain-gradient induced structure inversion asymmetry drives a phase transition from a quantum spin-Hall phase to a topologically trivial insulating phase. Interestingly, in alloys with strongly reduced bulk band gaps strain gradients have an opposite effect and promote a topologically non-trivial phase from a parent normal band insulator. These strain-gradient assisted switchings between topologically distinct phases are expected to yield a strain gradient tunability of magnetism in magnetic topological insulator thin films.

We report an optical spectroscopy study of a single-crystal sample of PrSb, one of the monoantimonide $R\mathrm{Sb}$ compounds, which show interesting properties, such as topological nontrivial surface states and extremely large magnetoresistance. The plasma frequency is revealed at about $4300{\mathrm{cm}}^{-1}$, suggesting a low carrier density. In addition, we found two quasilinear components with variable slopes in the real part of the optical conductivity ${\sigma }_1\left(\omega \right)$. In combination with theoretical calculations which reveal a band inversion, our results may provide optical spectroscopic evidence of a topological nontrivial property in PrSb.

We extend a previously proposed rotation and truncation scheme to optimize quantum Anderson impurity calculations with exact diagonalization [Y. Lu, M. Höppner, O. Gunnarsson, and M. W. Haverkort, Phys. Rev. B 90, 085102 (2014)] to density-matrix renormalization group (DMRG) calculations. The method reduces the solution of a full impurity problem with virtually unlimited bath sites to that of a small subsystem based on a natural impurity orbital basis set. The later is solved by DMRG in combination with a restricted-active-space truncation scheme. The method allows one to compute Green's functions directly on the real frequency or time axis. We critically test the convergence of the truncation scheme using a one-band Hubbard model solved in the dynamical mean-field theory. The projection is exact in the limit of both infinitely large and small Coulomb interactions. For all parameter ranges, the accuracy of the projected solution converges exponentially to the exact solution with increasing subsystem size.

To understand correlated insulating and unconventional superconducting states in twisted bilayer graphene, we perform a systematic study of spin and pairing correlations in an effective two-orbital Hubbard model, by using the ground-state constrained-path quantum Monte Carlo method. Our numerical simulations reveal that when only the on-site Hubbard $U$ is considered, the long-range magnetic order develops for $U\ge 3.2t$ at half filling. Upon doping away from half filling, an analysis based on the pairing correlations and corresponding vertex contributions identifies that pairing with the $d+id$ symmetry is the dominant pairing channel. As the on-site Coulomb interaction is increased, both spin correlation and effective pairing interaction are enhanced simultaneously, indicating the intimate relation between magnetism and superconductivity. An inclusion of nearest neighbor Coulomb interaction $V$ results in a suppression of the $d+id$ superconductivity, but it can survive in the small $V$ regime. Our findings are useful for clarifying the ongoing controversy on the superconducting pairing symmetry in twisted bilayer graphene.

We compute and compare the decay lengths of several correlation functions and effective coupling constants in the many-body localized (MBL) phase. To this end, we consider the distribution of the logarithms of these couplings and correlators: In each case, the log coupling follows a normal distribution with mean and variance that grow linearly with separation. Thus, a localization length is asymptotically sharply defined for each of these quantities. These localization lengths differ numerically from one another, but all of them remain short up to the numerically observed MBL transition, indicating stability of the MBL phase against isolated ergodic inclusions. We also show how these broad distributions may be extracted using interferometric probes such as double-electron-electron resonance and the statistics of local spin precession frequencies.

We theoretically study the role of the Berry curvature on neutral and charged excitons in two-dimensional transition metal dichalcogenides. The Berry curvature arises due to a strong coupling between the conduction and valence bands in these materials that can to great extent be described within the model of massive Dirac fermions. The Berry curvature lifts the degeneracy of exciton states with opposite angular momentum. Using an electronic interaction that accounts for nonlocal screening effects, we find a Berry-curvature induced splitting of $\sim 17\mathrm{meV}$ between the $2p_{-}$ and $2p_{+}$ exciton states in ${\mathrm{WS}}_2$. Furthermore, we calculate the trion binding energies in ${\mathrm{WS}}_2$ and ${\mathrm{WSe}}_2$ for a large variety of screening lengths and different dielectric constants for the environment. Our approach indicates the prominent role played by the Berry curvature along with nonlocal electronic interactions in the understanding of the energy spectra of neutral and charged excitons in transition metal dichalcogenides and in the interpretation of their optical properties.

We demonstrate that, at the onset of conduction, an electrostatically defined quantum wire in bilayer graphene with an interlayer asymmetry gap may act as a one-dimensional semimetal, due to the multiple-minivalley dispersion of its lowest subband. Formation of a nonmonotonic subband coincides with a near-degeneracy between the bottom edges of the lowest two subbands in the wire spectrum, suggesting an $8e^2/h$ step at the conduction threshold, and the semimetallic behavior of the lowest subband in the wire would be manifest as resonance transmission peaks on an $8e^2/h$ conductance plateau.

We have studied the atomic structure and phase transition of a dense metallic monolayer of In on Si(111). Although the monolayer phase was previously considered to have a $(\sqrt{7}\times \sqrt{3})$ periodicity, low-energy electron diffraction and scanning tunneling microscopy observations have revealed that the phase has an incommensurate structure with the In overlayer uniaxially contracted by 2% from $(\sqrt{7}\times \sqrt{3})$. The uniaxially contracted structure was found to be more stable than the commensurate $(\sqrt{7}\times \sqrt{3})$ structure by first-principles calculations. We also observed a phase transition to a $(\sqrt{7}\times \sqrt{7})$ phase at 250–210 K upon cooling. Angle-resolved photoelectron spectroscopy and macroscopic four-point-probe conductivity measurements demonstrated that the transition induces the disappearance of metallic surface states and a sharp drop in sheet conductivity, respectively. These results indicate an electronic metal-insulator transition.

Surfaces of multilayer semiconductors typically have regions of atomically flat terraces separated by atom-high steps. Here we investigate the properties of the low-energy states appearing at the surface atomic steps in ${\mathrm{Sn}}_{1-x}{\mathrm{Pb}}_x{\mathrm{Te}}_{1-y}{\mathrm{Se}}_y$. We identify the important approximate symmetries and use them to construct relevant topological invariants. We calculate the dependence of mirror- and spin-resolved Chern numbers on the number of layers and show that the step states appear when these invariants are different on the two sides of the step. Moreover, we find that a particle-hole symmetry can protect one-dimensional Weyl points at the steps. Since the local density of states is large at the step the system is susceptible to different types of instabilities, and we consider an easy-axis magnetization as one realistic possibility. We show that magnetic domain walls support low-energy bound states because the regions with opposite magnetization are topologically distinct in the presence of nonsymmorphic chiral and mirror symmetries, providing a possible explanation for the zero-bias conductance peak observed in the recent experiment [Mazur et al., Phys. Rev. B 100, 041408(R) (2019)].

We apply linked cluster expansion techniques to study the polarized high-field phase of a spin-$1/2$ antiferromagnet on the kagome lattice with Heisenberg and Dzyaloshinskii-Moriya interactions (DMI). We find that the Dirac points of the single-magnon spectrum without DMI are robust against arbitrary DMI when the magnetic field lies in the kagome plane. Unlike the typical case where DMI gaps the spectrum, here we find that varying the DMI merely shifts the location of the Dirac points. In contrast, a magnetic field with a component out of the kagome plane gaps the spectrum, leading to topological magnon bands. We map out a topological phase diagram as the couplings are varied by computing the band Chern numbers. A pair of phase transitions are observed and we find an enhanced thermal Hall conductivity near the phase boundary.

A two-dimensional Falicov-Kimball model, equivalent to the Hubbard model in an unconstrained slave-spin representation, is studied by quantum Monte Carlo simulations. The focus is on a fractionalized metallic phase that is characterized in terms of spectral, thermodynamic, and transport properties, including a comparison to the half-filled Hubbard model. The properties of this phase, most notably a single-particle gap but gapless spin and charge excitations, can in principle be understood in the framework of orthogonal metals. However, important and interesting differences arise in the present setting compared to single-particle mean-field theories and other models. We also discuss the role of the local constraints from the slave-spin representation within an extended phase diagram that includes the spatial dimension as a parameter, thereby making contact with previous work in infinite dimensions. Finally, we highlight the absence of $\pi$-flux configurations in the slave-spin formulation, in particular in the context of topologically ordered fractional phases predicted at the mean-field level.

A diluted oxide interface of $\mathrm{LaA}{\mathrm{l}}_{1-x}\mathrm{M}{\mathrm{n}}_x\mathrm{O}/\mathrm{SrTi}{\mathrm{O}}_3$ (LAMO/STO) provides a new way of tuning the ground states of the interface between the two band insulators of LAO and STO from metallic/superconducting to highly insulating. Increasing the Mn doping level $\left(x\right)$ leads to a delicate control of the carrier density as well as a raise in the electron mobility and spin polarization. Herein, we demonstrate a tunable Rashba spin-orbit coupling (SOC) and spin polarization of LAMO/STO $\left(0.2\le x\le 0.3\right)$ by applying a back gate. The presence of SOC causes splitting of the energy band into two branches by a spin splitting energy. The maximum spin splitting energy depends on the Mn doping and decreases with the increasing Mn content and then vanishes at $x=0.3$. The carrier density dependence of the spin splitting energy for different compositions shows a dome-shaped behavior with a maximum at different normalized carrier densities. These findings have not yet been observed in LAO/STO interfaces. A fully back-gate-tunable spin-polarized two-dimensional electron liquid is observed at the interface with $x=0.3$ where only $d_{xy}$ orbits are populated $(5.3\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-2}\le n_s\le 1.0\times {10}^{13}\mathrm{c}{\mathrm{m}}^{-2})$. The present results shed light on unexplored territory in SOC at STO-based oxide heterostructures and make LAMO/STO an intriguing platform for spin-related phenomena in $3d$ electron systems.

In spin-density-functional theory for noncollinear magnetic materials, the Kohn-Sham system features exchange-correlation (xc) scalar potentials and magnetic fields. The significance of the xc magnetic fields is not very well explored; in particular, they can give rise to local torques on the magnetization, which are absent in standard local and semilocal approximations. We obtain exact benchmark solutions for two electrons on four-site extended Hubbard lattices over a wide range of interaction strengths, and compare exact xc potentials and magnetic fields with approximations obtained from orbital-dependent xc functionals. The xc magnetic fields turn out to play an increasingly important role as systems becomes more and more correlated and the electrons begin to localize; the effects of the xc torques, however, remain relatively minor. The approximate xc functionals perform overall quite well, but tend to favor symmetry-broken solutions for strong interactions.

The optical properties of ${\mathrm{YbMnSb}}_2$ have been measured in a broad frequency range from room temperature down to 7 K. With decreasing temperature, a flat region develops in the optical conductivity spectra at about $300{\mathrm{cm}}^{-1}$, which cannot be described by the well-known Drude-Lorentz model. A frequency-independent component has to be introduced to model the measured optical conductivity. Our first-principles calculations show that ${\mathrm{YbMnSb}}_2$ possesses a Dirac nodal line near the Fermi level. A comparison between the measured optical properties and calculated electronic band structures suggests that the frequency-independent optical conductivity component arises from interband transitions near the Dirac nodal line, thus demonstrating that ${\mathrm{YbMnSb}}_2$ is a Dirac nodal-line semimetal.

We investigate the electron states and optical absorption in square- and hexagonal-shaped two-dimensional (2D) HgTe quantum dots and quantum rings in the presence of a perpendicular magnetic field. The electronic structure is modeled by means of the $s{p}^3{d}^5{s}^{*}$ tight-binding method within the nearest-neighbor approximation. Both bulklike and edge states appear in the energy spectrum. The bulklike states in quantum rings exhibit Aharonov-Bohm oscillations in magnetic field, whereas no such oscillations are found in quantum dots, which is ascribed to the different topology of the two systems. When magnetic field varies, all the edge states in square quantum dots appear as quasibands composed of almost fully flat levels, whereas some edge states in quantum rings are found to oscillate with magnetic field. However, the edge states in hexagonal quantum dots are localized like in rings. The absorption spectra of all the structures consist of numerous absorption lines, which substantially overlap even for small line broadening. The absorption lines in the infrared are found to originate from transitions between edge states. It is shown that the magnetic field can be used to efficiently tune the optical absorption of HgTe 2D quantum dot and quantum ring systems.

Low temperature photoluminescence spectra of the phonon sidebands of a color center in hexagonal boron nitride are compared to an independent boson model. We infer that the LA-phonon sideband is described by deformation coupling proportional to in-plane strain, resulting in a phonon bath that is effectively two-dimensional. For optical phonons, sharp resonances close to turning points in the phonon dispersion are observed. We infer that the TO-electron coupling is described by deformation coupling proportional to in-plane lattice displacement, resulting in a TO bath that is also effectively two-dimensional. By contrast, for the LO branches we infer that the Fröhlich coupling depends on the interactions between adjacent layers, and propose that the 200-meV peak is a signature of a few-layer sample. This work highlights features of electron-phonon coupling that arise from the layered structure of h-BN.