The recent observations of the superconducting diode effect pose the challenge to fully understand the necessary ingredients for nonreciprocal phenomena in superconductors. In this theoretical work, we focus on the nonreciprocity of the critical current in a quasi-one-dimensional superconductor. We define the critical current as the value of the supercurrent at which the quasiparticle excitation gap closes (depairing). Once the critical current is exceeded, the quasiparticles can exchange energy with the superconducting condensate, giving rise to dissipation. Our minimal model can be microscopically derived as a low-energy limit of a Rashba spin-orbit coupled superconductor in a Zeeman field. Within the proposed model, we explore the nature of the nonreciprocal effects of the critical current both analytically and numerically. Our results quantify how system parameters such as spin-orbit coupling and quantum confinement affect the strength of the superconducting diode effect. Our theory provides a complementary description to Ginzburg-Landau theories of the effect.

Topological excitonic insulators combine topological edge states and spontaneous exciton condensation, with dual functionality of topological insulators and excitonic insulators. Yet, they are very rare and little is known about their formation. In this work, we find that a mechanism dubbed as parity frustration prevents excitonic instability in usual topological insulators, and those whose band inversion is independent of spin-orbit coupling are possible candidates. We verify this by first-principles calculations on monolayer transition-metal carbides, which show a robust thermal-equilibrium exciton condensation, being sufficient for topological applications at room temperature. Such a state can be identified by angle-resolved photoemission spectroscopy and transport measurement. Our work provides not only a guide for finding more topological excitonic insulators, but also a new platform for studying the interplay between nontrivial band topology and quantum many-body effects.

Extended reservoirs provide a framework for capturing macroscopic, continuum environments, such as metallic electrodes driving a current through a nanoscale contact, impurity, or material. We examine the application of this approach to periodically driven systems, specifically in the context of quantum transport. As with nonequilibrium steady states in time-independent scenarios, the current displays a Kramers' turnover including the formation of a plateau region that captures the physical, continuum limit response. We demonstrate that a simple stability criteria identifies an appropriate relaxation rate to target this physical plateau. Using this approach, we study quantum transport through a periodically driven tilted lattice coupled to two metallic reservoirs held at a finite bias and temperature. We use this model to benchmark the extended reservoir approach and assess the stability criteria. The approach recovers well-understood physical behavior in the limit of weak system-reservoir coupling. Extended reservoirs enable addressing strong coupling and nonlinear response as well, where we analyze how transport responds to the dynamics inside the driven lattice. These results set the foundations for the use of extended reservoir approach for periodically driven, quantum systems, such as many-body Floquet states.

Time reversal and inversion symmetric materials fail to yield linear and nonlinear responses since they possess net zero Berry curvature. However, higher-order Hall response can be generated in these systems upon constraining the crystalline symmetries. Motivated by the recently discovered third-order Hall (TOH) response mediated by Berry connection polarizability, namely, the variation of the Berry connection with respect to an applied electric field, here, we investigate the existence of such a Hall effect in the surface states of hexagonal warped topological insulators (e.g., ${\mathrm{Bi}}_2{\mathrm{Te}}_3$) under the application of only electric field. Using the semiclassical Boltzmann formalism, we investigate the effect of tilt and hexagonal warping on the Berry connection polarizability tensor and, consequently, the TOH effect, provided the Dirac cone remains gapless. We find that the magnitude of the response increases significantly with increasing tilt strength and warping, and therefore, they can provide the tunability of this effect. In addition, we also explore the effect of chemical doping on the TOH response in this system. Interestingly, we show, based on the symmetry analysis, that the TOH can be the leading-order response in this system, which can directly be verified in experiments.

Studying spin-$\frac{1}{2}$ fermions coupled to ${\mathbb{Z}}_2$ gauge fields on the square lattice, we show how a spatial modulation of the fermion hopping amplitude allows for the realization of various obstructed atomic insulators that host higher-order band topology. Including the effect of quantum dynamics of the gauge fields within a simplified model, we find a rich phase diagram of this system with a number of superfluid phases (which host distinct types of topological defects) arising from the attractive interactions meditated by the gauge fields. The evolution of the superfluid obtained by the destabilization of the obstructed atomic insulators from the Bardeen-Cooper-Schrieffer (BCS) type to a Bose-Einstein condensate (BEC) of tightly bound pairs occurs via the realization of these different superfluid phases separated by first-order transitions.

A semiempirical theory for the excitation and subsequent relaxation of nonthermal electrons is described. The theory, which is applicable to ultrafast-laser excited metals, is based on the Boltzmann transport equation for the carrier distribution function $f(\epsilon ,t)$ and includes electron-phonon, electron-electron, and electron-photon scattering integrals in forms that explicitly depend on the electronic density of states. Electron-phonon coupling is treated by extending the theory of Allen [Phys. Rev. Lett. 59, 1460 (1987)] to include highly-excited nonthermal electron distributions, and is used to determine the energy transfer rate between a nonthermal electron subsystem and a thermal phonon subsystem. Electron-electron scattering is treated with a simple energy-conserving electron-electron scattering integral. The electron-photon integral assumes photon absorption is phonon assisted. We apply the theory to analyze prior ultrafast thermionic emission, two-color photoemission, and electronic inelastic light (Raman) scattering experiments on Au. These analyses show that getting the details of $f(\epsilon ,t)$ is necessary for proper interpretation of each experiment. Together, the photoemission and Raman-scattering analyses indicate an electron excited 1 eV above the Fermi level has an electron-electron scattering time in the range of 25 to 55 fs.

By constraining the directions of magnetic moments in first-principles calculations, one can investigate the physical properties of desirable magnetic structures and extract parameters of magnetic interactions. However, the current methods for constraining the directions of magnetism in first-principles calculations may lead to quenched magnetic moments mathematically and practically. In this paper, we propose a method to constrain the spin directions in both collinear and noncollinear cases by imposing a local parallel magnetic field. The effectiveness of this method is confirmed by performing calculations on ${\mathrm{GdMn}}_2{\mathrm{O}}_5$ and MnSi by using a modified vasp package. Through these examples, we find that by adding an external parallel local magnetic field, we are able to constrain the spin directions more efficiently than existing methods.

Heavy metal (HM) multilayers with opposite spin-Hall angle have attracted extensive attentions due to their rich physical properties and controversial reported results. Here, we systematically investigated the spin-orbit torque (SOT) efficiency in $\mathrm{Ta}\left(1\right)/{\left[\mathrm{Pt}\left(t_{\mathrm{Pt}}\right)/\mathrm{Ta}\left(1-t_{\mathrm{Pt}}\right)\right]}_5/\mathrm{Pt}\left(1.3\right)/\mathrm{Co}$ multilayers using harmonic Hall voltage response methods. We observed that the SOT efficiency of Pt/Ta multilayer can be continuously tuned with varying Pt or Ta thickness, and the strong thermoelectric effect in Pt/Ta multilayers causes a large mismatch of low-field and high-field harmonic results. Combined with finite element analysis, we demonstrated the enhanced thermoelectric effect in Pt/Ta multilayers originates from the increased resistivity of HM layers and shunt effect, as well as the change of anomalous Nernst coefficient. Furthermore, the accurate SOTs strength can be obtained by ruling out the thermoelectric effect. Our work not only confirms the importance of thermoelectric effect in electrical transport measurements, but also holds significant implications for the precise determination of SOT strength and the application of SOT devices.

Giant anomalous Hall effect (AHE) and magneto-optical activity can emerge in magnets with topologically nontrivial degeneracies. However, identifying the specific band-structure features such as Weyl points, nodal lines, or planes which generate the anomalous response is a challenging issue. Since the low-energy interband transitions can govern the static AHE, we addressed this question in the prototypical magnetic Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ also hosting nodal lines by broadband polarized reflectivity and magneto-optical Kerr effect spectroscopy with a focus on the far-infrared range. In the linear dichroism spectrum we observe a strong resonance at 40 meV, which also appears in the optical Hall conductivity and primarily determines the static AHE, and thus confirms its intrinsic origin. Our material-specific theory reproduces the experimental data remarkably well and shows that strongly tilted nodal-line segments around the Fermi energy generate the resonance. While the Weyl points only give vanishing contributions, these segments of the nodal lines gapped by the spin-orbit coupling dominate the low-energy optical response and generate the giant AHE.

We investigate the tunneling magnetoresistance (TMR) effect using the lattice models which describe the magnetic tunnel junctions (MTJ). First, taking a conventional ferromagnetic MTJ as an example, we show that the product of the local density of states (LDOS) at the center of the barrier traces the TMR effect qualitatively. The LDOS inside the barrier has the information on the electrodes and the electron tunneling through the barrier, which enables us to easily evaluate the tunneling conductance more precisely than the conventional Julliere's picture. We then apply this method to the MTJs with collinear ferrimagnets including antiferromagnets. The TMR effect in the ferrimagnetic MTJs changes depending on the interfacial magnetic structures originating from the sublattice structure, which can also be captured by the LDOS. Our findings will reduce the computational cost for the qualitative evaluation of the TMR effect and be useful for a broader search for the materials which work as the TMR devices showing high performance.

The inclusion of long-range couplings in the Kitaev chain is shown to modify the universal scaling of topological states close to the critical point. By means of the scattering approach, we prove that the Majorana states soften, becoming increasingly delocalized at a universal rate which is only determined by the interaction range. This edge mechanism can be related to a change in the value of the bulk topological index at criticality, upon careful redefinition of the latter. The critical point turns out to be topologically akin to the trivial phase rather than interpolating between the two phases. Our treatment moreover showcases how various topological aspects of quantum models can be investigated analytically.

The protected surface conduction of topological insulators is in high demand for the next generation of electronic devices. What is needed to move forward are robust settings where topological surface currents can be controlled by simple means, ideally by the application of external stimuli. Surprisingly, this direction is only a little explored. In this work we demonstrate that we can boost the surface conduction of a topological insulator by both light and electric field. This happens in a fully controlled way and the additional Dirac carriers exhibit ultralong lifetimes. We provide a comprehensive understanding, namely that carriers are injected from the bulk to the surface states across an intrinsic Schottky barrier. We expect this mechanism to be at play in a broad range of materials and experimental settings.

Exotic properties emerge from the electronic structure of few-layer transition-metal dichalcogenides (TMDs), such as direct band gaps in monolayers and moiré excitons in twisted bilayers, which are exploited in modern optoelectronic devices and twistronics. Here, Compton scattering in a transmission electron microscope (TEM) is used to probe the nature of the interlayer electronic coupling in the TMD material $\mathrm{W}{\mathrm{S}}_2$. The high spatial resolution and strong scattering in the TEM enables a complete analysis of individual $\mathrm{W}{\mathrm{S}}_2$ domains, including their crystal structure. Compton measurements show that the electrons in an ${18}^{\circ }$ twisted bilayer are more localized than in a monolayer. Density functional theory simulations reveal this is caused by a twist-induced charge buildup in the interlayer region, directly shielding the energetically unfavorable overlapping tungsten atoms. This unexpected result uncovers the precise role of twist angle on interlayer coupling, and therefore the physical properties that depend on it.

We introduce trilayer and multilayer moiré heterostructures that cannot be viewed from the “moiré-of-moiré” perspective of helically twisted trilayer graphene. These “intrinsically trilayer” moiré systems feature periodic modulation of a local quasicrystalline structure. They open the door to realizing moiré heterostructures with vastly more material constituents because they do not constrain the lattice constants of the layers. In this manuscript, we define intrinsically multilayer patterns, provide a recipe for their construction, derive their local configuration space, and connect the visual patterns to physical observables in material systems.

Phonon polariton modes in layered anisotropic heterostructures are a key building block for modern nanophotonic technologies. The light-matter interaction for evanescent excitation of such a multilayer system can be theoretically described by a transfer-matrix formalism. This method allows us to compute the imaginary part of the $p$-polarized reflection coefficient $Im\left(r_{pp}\right)$, whose resonant features are commonly used to evaluate the polariton dispersion of the multilayer structure. This reflection coefficient, however, does not reveal how the different layers contribute to these resonances. We present an approach to compute layer-resolved polariton resonance intensity in arbitrarily anisotropic layered heterostructures, based on calculating the Poynting vector extracted from the transfer-matrix formalism under evanescent light excitation. Our approach is independent of the experimental excitation conditions, and it fulfills a strictly proved conservation law for the energy flux. As a testing ground, we study two state-of-the-art nanophotonic multilayer systems, covering strong coupling and tunable hyperbolic surface phonon polaritons in twisted ${\text{MoO}}_{\text{3}}$ double layers. Providing a new level of insight into the polaritonic response, our method holds great potential for understanding, optimizing, and predicting new forms of polariton heterostructures in the future.

Nonlocal correlations play an essential role in correlated electron systems, especially in the vicinity of phase transitions and crossovers, where two-particle correlation functions display a distinct momentum dependence. In nonequilibrium settings, the effect of nonlocal correlations on dynamical phase transitions, prethermalization phenomena, and trapping in metastable states is not well understood. In this paper, we introduce a dynamical mean field theory (DMFT) extension to the nonequilibrium two-particle self-consistent (TPSC) approach, which allows to perform nonequilibrium simulations capturing short- and long-ranged nonlocal correlations in the weak-, intermediate-, and strong-correlation regimes. The method self-consistently computes local spin and charge vertices, from which a momentum-dependent self-energy is constructed. Replacing the local part of the self-energy by the DMFT result within this self-consistent scheme provides an improved description of local correlation effects. We explain the details of the formalism and the implementation, and demonstrate the versatility of DMFT+TPSC with interaction quenches and dimensional crossovers in the Hubbard model.

Topological materials provide an interesting platform in the study of thermoelectric effect due to their novel electronic properties. Here we report the magneto-Seebeck (MS) effect, large Nernst effect, and a possible temperature-induced Lifshitz transition in topological semimetal ${\mathrm{YbMnSb}}_2$. It exhibits evident quantum oscillations in the MS effect from which six frequencies are extracted and a light effective mass of $F_4$ is obtained. The large Nernst effect with the Nernst thermopower of about 40 $\mu {\mathrm{VK}}^{-1}$ is discovered around 120 K and 14 T. Combining the analysis of the first-principles calculations and the Hall effect, we suggest that the large Nernst signal could mainly stem from the Dirac bands-induced high mobility and small Fermi energy (${\varepsilon }_F$). Besides, it could be further enhanced by the modified carrier properties resulting from the temperature-induced Lifshitz transition through the so-called ambipolar effect.