Motivated by recent experimental realizations of topological edge states in Su-Schrieffer-Heeger (SSH) chains, we theoretically study a ladder system whose legs are comprised of two such chains. We show that the ladder hosts a rich phase diagram and related edge-mode structure dictated by choice of interchain and intrachain couplings. Namely, we exhibit three distinct physical regimes: a topological hosting localized zero-energy edge modes, a topologically trivial phase having no edge-mode structure, and a regime reminiscent of a weak topological insulator having unprotected edge modes resembling a twin-SSH construction. In the topological phase, the SSH ladder system acts as an analog of the Kitaev chain, which is known to support localized Majorana fermion end modes, with the difference that bound states of the SSH ladder having the same spatial wave-function profiles correspond to Dirac fermion modes. Further, inhomogeneity in the couplings can have a drastic effect on the topological phase diagram of the ladder system. In particular for quasiperiodic variations of the interchain coupling, the phase diagram reproduces Hofstadter's butterfly pattern. We thus identify the SSH ladder system as a potential candidate for experimental observation of such fractal structure.

Topological superconductors are among the most actively studied materials these days. They are a promising candidate for hosting Majorana fermions either on their boundaries or in vortex cores. Detecting one-dimensional (1D) edge current around the periphery of a 2D $p_x+i{p}_y$ superconductor would be a hallmark signature of topological superconductivity, but Majorana fermions are not amenable to electronic current measurements due to their charge-neutral nature. Thermal conductivity measurements, such as the thermal Hall effect, are alternatively proposed, but material synthesis must come first. Superfluid ${}^3\mathrm{He}-A$, on the other hand, is a known $p_x+i{p}_y$ superfluid whose angular momentum generated by the edge current can be measured with a gyroscopic technique. Here, we propose a microelectromechanical system based gyroscope that will not only have enough signal sensitivity to measure the angular momentum but also be used to observe dimensionality induced phase transitions between different topological superfluids.

We propose to realize a one-dimensional chiral topological superconducting state at the magnetic domain walls stripe of a magnetic topological insulator coupled with a conventional $s$-wave superconductor. The localized Majorana zero modes can be constructed in a reconfigurable manner through magnetic domain writing by magnetic force microscopy. This proposal could be further extended to the Majorana zero modes at domain walls on superconducting spin-helical Dirac surface states and may be applicable to the two-dimensional time-reversal invariant topological superconductor on the ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ surface.

Superconductivity was recently observed in CrAs as the helimagnetic order is suppressed by applying pressure, suggesting possible unconventional superconductivity. To reveal the nature of the superconducting order parameter of CrAs, here we report the angular dependence of the upper critical field under pressure. Upon rotating the field by ${360}^{\circ }$ in the $bc$ plane, six maxima are observed in the upper critical field, where the oscillations have both sixfold- and twofold-symmetric components. Our analysis suggests the presence of an unconventional odd-parity spin-triplet state.

Motivated by recent studies on ferroelectriclike order coexisting with metallicity, we investigate ferroelectric (FE) superconductivity in which a FE-like structural phase transition occurs in the superconducting state. We consider a two-dimensional $s$-wave superconductor with Rashba-type antisymmetric spin-orbit coupling (ASOC). Assuming a linear relationship between the polar lattice displacement and the strength of the ASOC, we treat the Rashba-type ASOC as a molecular field of FE-like order. It is shown that the FE-like order is induced by the magnetic field when the system is superconducting. Furthermore, we clarify the FE superconductivity in a low carrier density regime, which was recently discovered in doped ${\mathrm{SrTiO}}_3$. It is demonstrated that the FE superconducting state can be stable in this regime in the absence of the magnetic field. Our results open a way to control the electric polarization by superconductivity, that is, superconducting multiferroics.

Motivated by the recent observation of the time-reversal symmetry broken state in K-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ superconducting alloys, we theoretically study the collective modes and the short time dynamics of the superconducting state with $s+is$-wave order parameter using an effective four-band model with two hole and two electron pockets. The superconducting $s+is$ state emerges for incipient electron bands as a result of hole doping and appears as an intermediate state between $s^{\pm }$ (high number of holes) and $s^{++}$ (low number of holes). The amplitude and phase modes are coupled giving rise to a variety of collective modes. In the $s^{\pm }$ state, we find that the collective excitations are the Higgs (amplitude) modes, while the Leggett mode is absent due to strong interband interaction. In the $s+is$ and $s^{++}$ state, we uncover a new coupled collective soft mode. Finally we compare our results with the $s+id$ solution and find similar behavior of the collective modes as in the $s+is$ state.

We investigate the normal state of the superconducting compound ${\mathrm{PuCoGa}}_5$ using the combination of density functional theory (DFT) and dynamical mean-field theory (DMFT), with the continuous time quantum Monte Carlo (CTQMC) and the vertex-corrected one-crossing approximation (OCA) as the impurity solvers. Our DFT+DMFT (CTQMC) calculations suggest a strong tendency of Pu-$5f$ orbitals to differentiate at low temperatures. The renormalized $5f_{5/2}$ states exhibit a Fermi-liquid behavior whereas one electron in the $5f_{7/2}$ states is at the edge of a Mott localization. We find that the orbital differentiation is manifested as the removing of $5f_{7/2}$ spectral weight from the Fermi level relative to DFT. We corroborate these conclusions with DFT+DMFT (OCA) calculations which demonstrate that $5f_{5/2}$ electrons have a much larger Kondo scale than the $5f_{7/2}$.

Based on first-principles calculations and symmetry analysis, we report that the three-dimensional nodal-line (NL) semimetal phases can be realized in the lead dioxide family ( $\alpha$ -${\mathrm{PbO}}_2$ and $\beta$ -${\mathrm{PbO}}_2$) and its derivatives. The $\beta$ -${\mathrm{PbO}}_2$ features two orthogonal nodal rings around the Fermi level, protected by the mirror reflection symmetry. The effective model is developed and the related parameters are given by fitting with the HSE06 band structures. The NLs mainly come from the $p$ orbitals of the light element O and are rather robust against such tiny spin-orbit coupling. The NL phase of the $\alpha$ -${\mathrm{PbO}}_2$ can be effectively tailored by strain, making a topological phase transition between a semiconductor phase and a NL phase. In addition, the exploration of $\beta$ -${\mathrm{PbO}}_2$ derivatives (i.e., $\beta$ -${\mathrm{PbS}}_2$ and $\beta$ -${\mathrm{PbSe}}_2$) and confirmation of their topological semimetallicity greatly enrich the NL semimetal family. These findings pave a route for designing topological NL semimetals and spintronic devices based on the realistic ${\mathrm{PbO}}_2$ family.

We investigate the quantum mechanical origin of resistive phase transitions in solids driven by a constant electric field in the vicinity of a metal-insulator transition. We perform a nonequilibrium mean-field analysis of a driven-dissipative symmetry-broken insulator, which we solve analytically for the most part. We find that the insulator-to-metal transition (IMT) and the metal-to-insulator transition (MIT) proceed by two distinct electronic mechanisms: Landau-Zener processes and the destabilization of the metallic state by Joule heating, respectively. However, we show that both regimes can be unified in a common effective thermal description, where the effective temperature $T_{\mathrm{eff}}$ depends on the state of the system. This explains recent experimental measurements in which the hot-electron temperature at the IMT was found to match the equilibrium transition temperature. Our analytic approach enables us to formulate testable predictions on the nonanalytic behavior of $I-V$ relation near the insulator-to-metal transition. Building on these successes, we propose an effective Ginzburg-Landau theory which paves the way to incorporating spatial fluctuations and to bringing the theory closer to a realistic description of the resistive switchings in correlated materials.

We address the steady-state behavior of a system consisting of several correlated monoatomic layers sandwiched between two metallic leads under the influence of a bias voltage. In particular, we investigate the interplay of the local Hubbard and the long-range Coulomb interaction on the charge redistribution at the interface, in the paramagnetic regime of the system. We provide a detailed study of the importance of the various system parameters, like Hubbard $U$, lead-correlated region coupling strength, and the applied voltage on the charge distribution in the correlated region and in the adjacent parts of the leads. In addition, we also present results for the steady-state current density and double occupancies. Our results indicate that, in a certain range of parameters, the charge on the two layers at the interface between the leads and the correlated region display opposite signs, producing a dipolelike layer at the interface. Our results are obtained within nonequilibrium (steady-state) real-space dynamical mean-field theory, with a self-consistent treatment of the long-range part of the Coulomb interaction by means of the Poisson equation. The latter is solved by the Newton-Raphson method and we find that this significantly reduces the computational cost compared to existing treatment. As the impurity solver for real-space dynamical mean-field theory, we use the auxiliary master equation approach, which addresses the impurity problem within a finite auxiliary system coupled to Markovian environments.

Active metasurfaces have attracted a great deal of attention due to the unlimited ways of controlling electromagnetic radiation. In this work, we theoretically study the THz electromagnetic properties of photogenerated metasurfaces in an InAs slab. We use a spatially modulated optical pump to modify the permittivity. Those modifications are calculated by solving the ambipolar rate equation for the photocarriers. This work reveals the crucial impact of the photocarrier diffusion for realizing optimal metasurfaces for the control of THz radiation. We demonstrate that InAs is a promising semiconductor that could be used to manufacture fast and efficient on-chip THz components. We also demonstrate that low pump fluence in the continuous regime of only tens of $\text{W}{\text{cm}}^{-2}$ is sufficient to modulate the absorption of the THz waves up to $67\%$ over a broad frequency range from 1 to 3 THz.

Silicon-vacancy (SiV) center in diamond is a photoluminescence (PL) center with a characteristic zero-phonon line energy at 1.681 eV that acts as a solid-state single-photon source and, potentially, as a quantum bit. The majority of the luminescence intensity appears in the zero-phonon line; nevertheless, about 30% of the intensity manifests in the phonon sideband. Since phonons play an essential role in the operation of this system, it is of importance to understand the vibrational properties of the SiV center in detail. To this end, we carry out density functional theory calculations of dilute SiV centers by embedding the defect in supercells of a size of a few thousand atoms. We find that there exist two well-pronounced quasilocal vibrational modes (resonances) with $A_{2u}$ and $E_u$ symmetries, corresponding to the vibration of the Si atom along and perpendicular to the defect symmetry axis, respectively. Isotopic shifts of these modes explain the isotopic shifts of prominent vibronic features in the experimental SiV PL spectrum. Moreover, calculations show that the vibrational frequency of the $A_{2u}$ mode increases by about 30% in the excited state with respect to the ground state, while the frequency of the $E_u$ mode increases by about 5%. These changes explain experimentally observed isotopic shifts of the zero-phonon-line energy. We also emphasize possible dangers of extracting isotopic shifts of vibrational resonances from finite-size supercell calculations, and instead propose a method to do this correctly.

We study quantum noise in a nonequilibrium, periodically driven, open system attached to static leads. Using a Floquet Green's function formalism we show, both analytically and numerically, that local voltage noise spectra can detect the rich structure of Floquet topological phases unambiguously. Remarkably, both regular and anomalous Floquet topological bound states can be detected, and distinguished, via peak structures of noise spectra at the edge around zero-, half-, and full-drive frequency. We also show that the topological features of local noise are robust against moderate disorder. Thus, local noise measurements are sensitive detectors of Floquet topological phases.

We report the observation of an extreme magnetoresistance (XMR) in HoBi with a large magnetic moment from Ho $f$ electrons. Neutron scattering is used to determine the magnetic wave vectors across several metamagnetic transitions on the phase diagram of HoBi. Unlike other magnetic rare-earth monopnictides, the field dependence of resistivity in HoBi is nonmonotonic and reveals clear signatures of every metamagnetic transition in the low-temperature and low-field regime, at $T<2$ K and $H<2.3$ T. The XMR appears at $H>2.3$ T after all the metamagnetic transitions are complete and the system is spin polarized by the external magnetic field. The existence of an onset field for XMR and the intimate connection between magnetism and transport in HoBi are unprecedented among the magnetic rare-earth monopnictides. Therefore, HoBi provides a unique opportunity to understand the electrical transport in magnetic XMR semimetals.

We report the magnetotransport properties of HoSb, a semimetal with antiferromagnetic ground state. HoSb shows extremely large magnetoresistance (XMR) and Shubnikov-de Haas (SdH) oscillation at low temperature and high magnetic field. Different from previous reports in other rare earth monopnictides, kinks in $\rho \left(B\right)$ and ${\rho }_{xy}\left(B\right)$ curves and the field dependent resistivity plateau are observed in HoSb, which result from the magnetic phase transitions. The fast Fourier transform analysis of the SdH oscillation reveals the split of Fermi surfaces induced by the nonsymmetric spin-orbit interaction. The Berry phase extracted from SdH oscillation indicates the possible nontrivial electronic structure of HoSb in the presence of magnetic field. The Hall measurements suggest that the XMR originates from the electron-hole compensation and high mobility.

We study the momentum and temperature dependencies of magnetic susceptibilities and magnetic exchange in paramagnetic fcc iron by a combination of density functional theory and supercell dynamical mean-field theory (DFT+DMFT). We find that in agreement with experimental results the antiferromagnetic correlations with the wave vector close to $(0,0,2\pi )$ dominate at low temperatures (as was also obtained previously theoretically), while the antiferromagnetic and ferromagnetic correlations closely compete at the temperatures $T\sim 1000$ K, where $\gamma$-iron exists in nature. Inverse staggered susceptibility has linear temperature dependence at low temperatures, with negative Weiss temperature ${\theta }_{\mathrm{stagg}}\approx -340$ K; the inverse local susceptibility is also linear at not too low temperatures, showing well formed local moments. Analysis of magnetic exchange shows that the dominant contribution comes from first two coordination spheres. In agreement with the analysis of the susceptibility, the nearest-neighbor exchange is found to be antiferromagnetic at low temperatures, while at temperature of the $\alpha -\gamma$ structural phase transition its absolute value becomes small, and the system appears on the boundary between the regimes with strongest antiferro- and ferromagnetic correlations.

We study the hybrid excitations due to the coupling between surface optical phonons of a polar insulator substrate and plasmons in the valley-spin-polarized metal phase of silicene under an exchange field. We perform the calculations within the generalized random-phase approximation where the plasmon-phonon coupling is taken into account by the long-range Fröhlich interaction. Our investigation on two hybridized plasmon branches in different spin and valley subbands shows distinct behavior compared to the uncoupled case. Interestingly, in one valley, it is found that while the high-energy hybrid branch is totally damped in the spin-up state, it can be well defined in the spin-down state. Moreover, we show that the electron-phonon coupling is stronger in both spin-down subbands, regardless of valley index, due to their higher electron densities. In addition, we study the effects of electron-phonon coupling on the quasiparticle scattering rate of four distinct spin-valley locked subbands. The results of our calculations predict a general enhancement in the scattering rate for all subbands and a jump in the case of spin-down states. This sharp increase associated with the damping of hybrid plasmon modes is almost absent in the uncoupled case. The results suggest an effective way for manipulating collective modes of valley-spin-polarized silicene which may become useful in future valleytronic and spintronic applications.

We examine excitons formed in the bulk of a topological insulator as the system is tuned via a parameter between topological and trivial insulating phases, arguing that nontrivial topology has fingerprints in the spectrum of these excitons. The closely related hydrogen atom problem is well known to have a degeneracy due to a hidden symmetry, and the changes to the excitonic spectrum that we find can be understood as a result of breaking of this underlying symmetry due to the Berry phase. Furthermore, this phase is found to affect the spectrum in the topological parameter regime much more strongly than in the trivial regime. We first construct a semiclassical model of the system to develop qualitative intuition for the effects at play, then we move to a more robust numerical simulation of the full quantum system, working with the Bernevig-Hughes-Zhang model of a two-dimensional topological insulator.

Using first-principles density functional theory calculations, we investigate a family of stable two-dimensional crystals with chemical formula $A_2{B}_2$, where $A$ and $B$ belong to groups IV and V, respectively ($A=\mathrm{C}$, Si, Ge, Sn, Pb; $B=\mathrm{N}$, P, As, Sb, Bi). Two structural symmetries of hexagonal lattices $P\overline{6}m2$ and $P\overline{3}m1$ are shown to be dynamically stable, named as $\alpha$- and $\beta$ -phases correspondingly. Both phases have similar cohesive energies, and the $\alpha$ phase is found to be energetically favorable for structures except CP, CAs, CSb, and CBi, for which the $\beta$ phase is favored. The effects of spin-orbit coupling and Hartree-Fock corrections to exchange correlation are included to elucidate the electronic structures. All structures are semiconductors except CBi and PbN, which have metallic character. SiBi, GeBi, and SnBi have direct band gaps, whereas the remaining semiconductor structures have indirect band gaps. All structures have quartic dispersion in their valence bands, some of which make the valence band maximum and resemble a mexican-hat shape. SnAs and PbAs have purely quartic valence band edges, i.e., $E-\alpha k^4$, a property reported for the first time. The predicted materials are candidates for a variety of applications. Owing to their wide band gaps, CP, SiN, SiP, SiAs, GeN, GeP can find their applications in optoelectronics. The relative band positions qualify a number of the structures as suitable for water splitting, where CN and SiAs are favorable at all pH values. Structures with quartic band edges are expected to be efficient for thermoelectric applications.