Motivated by recent experiments, we investigate the quantum spin Hall state in 2D topological insulator/ferromagnetic metal planar junctions by means of a tight-binding model and linear response theory. We demonstrate that whether the edge state Dirac cone is submerged into the ferromagnetic subbands and the direction of the magnetization dramatically affect (i) how the edge state percolates into the ferromagnet and (ii) the spin-momentum locking of the edge state. Laminar flows of room temperature persistent charge and spin currents near the interface are uncovered. In addition, the current-induced spin polarization at the edge of the 2D topological insulator is found to be dramatically enhanced near the impurities. The current-induced spin polarization in the ferromagnet is mainly polarized in the out-of-plane direction $\widehat{\mathbf{z}}$, rendering a current-induced spin torque that is predominantly fieldlike $\propto \mathbf{S}\times \widehat{\mathbf{z}}$.

The discovery of superconductivity in twisted bilayer graphene has triggered a resurgence of interest in flat-band superconductivity. Here, we investigate the square-octagon lattice, which also exhibits two perfectly flat bands when next-nearest-neighbor hopping or an external magnetic field are added to the system. We calculate the superconducting phase diagram in the presence of on-site attractive interactions and find two superconducting domes, as observed in several types of unconventional superconductors. The critical temperature shows a linear dependence on the coupling constant, suggesting that superconductivity might reach high temperatures in the square-octagon lattice. Our model could be experimentally realized using photonic or ultracold atoms' lattices.

The two classes of 3D, time-reversal-invariant insulators are known to subdivide into four classes in the presence of glide symmetry. Here, we extend this classification of insulators to include glide-symmetric Weyl metals and find a finer ${\mathbb{Z}}_4\oplus \mathbb{Z}$ classification. We further elucidate the smoking-gun experimental signature of each class in the photoemission spectroscopy of surface states. Measuring the ${\mathbb{Z}}_4$ topological invariant by photoemission relies on identifying the glide representation of the initial Bloch state before photoexcitation—we show how this is accomplished with relativistic selection rules, combined with standard spectroscopic techniques to resolve both momentum and spin. Our method relies on a spin-momentum locking that is characteristic of all glide-symmetric solids (inclusive of insulators and metals in trivial and topological categories). As an orthogonal application, given a glide-symmetric solid with an ideally symmetric surface, we may utilize this spin-momentum locking to generate a source of fully spin-polarized photoelectrons, which have diverse applications in solid-state spectroscopy. Our ab initio calculations predict ${\mathrm{Ba}}_2\mathrm{Pb}$, stressed ${\mathrm{Na}}_3\mathrm{Bi}$, and KHgSb to realize all three, nontrivial insulating phases in the ${\mathbb{Z}}_4$ classification.

Aharonov-Bohm cages correspond to an extreme confinement for two-dimensional tight-binding electrons in a transverse magnetic field. When the dimensionless magnetic flux per plaquette $f$ equals a critical value $f_c=1/2$, a destructive interference forbids the particle to diffuse away from a small cluster. The corresponding energy levels pinch into a set of highly degenerate discrete levels as $f\to f_c$. We show here that cages also occur for discrete-time quantum walks on either the diamond chain or the ${\mathcal{T}}_3$ tiling but require specific coin operators. The corresponding quasienergies versus $f$ result in a Floquet-Hofstadter butterfly displaying pinching near a critical flux $f_c$ and that may be tuned away from 1/2. The spatial extension of the associated cages can also be engineered.

We develop a hydrodynamic field theory of the three-dimensional fractional quantum Hall effect, which was recently proposed to exist in magnetic Weyl semimetals, when the Weyl nodes are gapped by strong repulsive interactions. This theory takes the form of a BF theory, which contains both one-form and two-form gauge fields, coupling to quasiparticle and loop excitations, respectively. It may be regarded as a generalization of the Chern-Simons theory of two-dimensional fractional quantum Hall liquids to three dimensions.

We revisit the computation of particle number fluctuations and the Rényi entanglement entropy of a two-dimensional Fermi gas using multidimensional bosonization. In particular, we compute these quantities for a circular Fermi surface and a circular entangling surface. Both quantities display a logarithmic violation of the area law, and the Rényi entropy agrees with the Widom conjecture. Lastly, we compute the symmetry-resolved entanglement entropy for the two-dimensional circular Fermi surface and find that, while the total entanglement entropy scales as $RlnR$, the symmetry-resolved entanglement scales as $\sqrt{RlnR}$, where $R$ is the radius of the subregion of our interest.

Spin-flip excitations in a quantum Hall electron system at fixed filling factor $\nu =2$ are modeled and studied under conditions of a strong Coulomb interaction when the “Landau level mixing” is a dominant factor determining the excitation energy. The “one-exciton” approach used for the purely electronic excitations in question allows us to describe the Stoner transition from the unpolarized/paramagnet state to the polarized/ferromagnet one. The theoretical results are compared with the available experimental data.

Motivated by recent scanning tunneling microscopy experiments on single magnetic impurities on superconducting surfaces, we present here a comprehensive theoretical study of the interplay between Yu-Shiba-Rusinov bound states and (multiple) Andreev reflections. Our theory is based on a combination of an Anderson model with broken spin degeneracy and nonequilibrium Green's function techniques that allows us to describe the electronic transport through a magnetic impurity coupled to superconducting leads for arbitrary junction transparency. Using this combination, we are able to elucidate the different tunneling processes that give a significant contribution to the subgap transport. In particular, we predict the occurrence of a large variety of Andreev reflections mediated by Yu-Shiba-Rusinov bound states that clearly differ from the standard Andreev processes in nonmagnetic systems. Moreover, we provide concrete guidelines on how to experimentally identify the subgap features originating from these tunneling events. Overall, our work provides new insight into the role of the spin degree of freedom in Andreev transport physics.

We study the Casimir effect in a system composed of two Weyl semimetals (WSMs) separated by a gap filled with a chiral medium. We calculate the optical response of the material to chiral photons in order to calculate the Casimir force. We find that if the medium between the two WSMs is a Faraday material, a repulsive Casimir force can be obtained. The magnitude of this repulsive force is found to be greatly enhanced at distances of a few microns, when experimentally accessible parameters are used. Moreover, in the system under consideration various parameters can be modified. Some of them are intrinsic to the materials employed (the absolute value of the Hall conductivity of the WSM, the Verdet constant of the Faraday material), while some of them can be manipulated externally even for a fixed sample, such as the external magnetic field and the orientation of the plates which determines the sign of their conductivity. Suitable combinations of these parameters can be used to switch from attraction to repulsion, and to place the trapping distance, in which no force acts on the plates, at any desired distance between them.

In this work, we have developed an ab initio photoemission model that accurately describes the photoemission process for the most diverse range of photocathode materials to date. Compared to previous photoemission models, this is accomplished by considerably reducing the number of approximations and assumptions used in representing the photoemission process and the photoemitting material itself. Notably, our model directly includes the full electronic structure of the material, photoexcitation probabilities for all direct optical transitions, and an improved surface-vacuum barrier transmission probability. To test the performance of our model, we perform validations with experimental measurements for all photocathode materials studied in this work. Whereas previous models have often qualitatively disagreed with the measured photoemission properties of some materials, our model is found to provide quantitative agreement with experimental measurements for all tested materials. As an example, our method predicts the root-mean-square transverse momentum of electrons emitted from PbTe up to an excess energy of 1.0 eV with a mean absolute error that is ∼5× less than from previously derived expressions. Perhaps more importantly, our model is able to match experimentally observed decreases in intrinsic emittance with increasing photon energy—a feat that current analytical models are unable to achieve. We expect that the broad applicability of our model will greatly accelerate the rate of discovery, characterization, and scientific understanding of photocathodes and other photonic devices.

The electronic band structures of two-dimensional materials are significantly different from those of their bulk counterparts due to quantum confinement and strong modifications of electronic screening. An accurate determination of electronic states is a prerequisite to design electronic or optoelectronic applications of two-dimensional materials; however, most of the theoretical methods we have available to compute band gaps are either inaccurate, computationally expensive, or only applicable to bulk systems. Here we show that reliable band structures of nanostructured systems can now be efficiently calculated using density-functional theory with the local modified Becke-Johnson exchange-correlation functional that we recently proposed. After reoptimizing the parameters of this functional specifically for two-dimensional materials, we show, for a test set of almost 300 systems, that the obtained band gaps are of comparable quality as those obtained using the best hybrid functionals but at a very reduced computational cost. These results open the way for accurate, high-throughput studies of band structures of two-dimensional materials and for the study of van der Waals heterostructures with large unit cells.

The spectrum of electromagnetic collective excitations in Dirac semimetals placed in a quantizing magnetic field is considered. We have found the Landau damping regions using the energy and momentum conservation law for allowed transitions between one-particle states of electron excitations. Analysis of dispersion equations for longitudinal and transverse waves near the window boundaries in the Landau damping regions reveals different types of collective excitations. We also indicate the features of universal broadening of cyclotron absorption for a magnetic field variation in systems with linear dispersion of the electron spectrum. The use of the obtained spectrum also allows us to predict a number of oscillation and resonance effects in the field of magneto-optical phenomena.

We present calculations of the equilibrium current density and Chern numbers for a two-dimensional electron gas in a sinusoidal periodic potential with infinite strip geometry and a perpendicular magnetic field. We consider a triangular lattice of antidots with large ($a=120$ nm) lattice spacing. Such a system is known as artificial graphene (AG). To compute the current density we numerically diagonalize the AG Hamiltonian over a set of Landau level basis states; this takes into account coupling between different Landau levels. Our calculations show that, at magnetic fields typical for quantum Hall measurements, extended streams of current are present in the bulk of the sample when the chemical potential lies within a bulk band gap. We investigate the scaling of these streams with potential strength. Knowledge of the AG energy levels allows us to compute the Chern number associated with each energy gap. We demonstrate that in tuning the height of the potential modulation the Chern number can undergo a transition between two different values.

We present a Schwinger-Keldysh scheme theory for full counting statistics applicable to the solid-state entangler, which consists of two coupled superconducting/normal-conducting/normal-conducting capacitively coupled single-electron transistors (double S/N/N C-SET). We focus on the case when the superconducting gap energy is larger than the charging energy since flexible control of entanglement information can be expected by the charging effect for various bias conditions, and we study the double S/N/N C-SET with and without dissipative environment. We derive the cumulant generation function for the double S/N/N C-SET under current continuity conditions, from which arbitrary order of current noise cumulants are obtained. We explicitly show that the synchronized Coulomb oscillation, a celebrated experimental finding by Hofstetter et al. [Nature (London) 461, 960 (2009)], originate from the crossed Andreev current conveying quantum entanglement. We also investigate current in the double S/N/N C-SET, its current components, and the resulting cross-correlation of current noise $S_{\mathrm{LR}}(\omega =0)$ in the superconducting subgap region. It is shown that the bunching-antibunching nature strongly depends on the relative location of the Coulomb gap regions for the relevant current components since the bunching-antibunching nature is determined by the competition between the contributions of relevant current components. Depending on the bias conditions for each of the two C-SETs, $S_{\mathrm{LR}}(\omega =0)$ exhibits a sign crossover from positive (bunching) to negative (antibunching), which is followed by restoration to bunching correlations. The effect of the dissipative environment tends to reduce $S_{\mathrm{LR}}(\omega =0)$ because of a reduction in relevant currents. Although crossover and restoration become less conspicuous, the way the bunching-antibunching nature appears is essentially the same as in the case of the inductive environment.

There are many recent studies on the linear magnetotransport of Weyl semimetals in the presence of a weak electric field. In this paper, we theoretically investigate the nonlinear current response of type-I Weyl semimetals to strong electric fields $\mathbf{E}$ in the ultraquantum regime (with strong magnetic field $\mathbf{B}\parallel \mathbf{E}$). Our nonperturbative approach based on the stochastic Liouville equation reveals the nonmonotonic dependence of the current on the electric field. In the weak-field regime, the time-average current $〈j〉\propto v_F\left|\mathbf{B}\right|\left|\mathbf{E}\right|\tau$ (with $\tau$ being the relaxation time) as a consequence of the chiral anomaly. In the strong-field regime, $〈j〉\propto v_F\left|\mathbf{B}\right|/\left(\right|\mathbf{E}\left|\tau \right)$. The current achieves the maximal value when the corresponding Bloch oscillation frequency matches the relaxation rate. Moreover, the nonlinear magnetotransport properties are independent of the tilting of the Weyl node. We also point out the difference between the lattice model and low-energy continuum model. Our study makes an important step toward the understanding of nonlinear magnetotransport in Weyl semimetals.

Current fluctuations generated by tunnel junctions are known to be non-Gaussian. However, this property is lost when fluctuations are measured at high frequency and limited bandwidth. We show that the fluctuations of the electric field generated by a tunnel junction at frequency $f$ displays third order correlations, i.e., skewness, when the junction is electrically driven at $3f$, revealing the Poisonnian statistic of charge transfer by the barrier even at short timescales. In addition to this intrinsic contribution from the junction, we observe extra correlations induced by the environmental noise at frequency $f$ as well as a feedback effect coming from the environmental impedance not only at frequency $f$ but also at some multiples of $f$.

We present a detailed theoretical study of chiral topological superconductor phases in proximity-superconducting graphene systems based on an effective model inspired by density functional theory simulations. Inducing $s$-wave superconductivity in quantum anomalous Hall effect systems leads to chiral topological superconductors. For out-of-plane magnetization we find topological superconducting phases with even numbers of chiral Majorana fermions per edge, which is correlated with the opening of a nontrivial gap in the bulk system in the $K$ points and their connection under particle-hole symmetry. We show that in a quantum anomalous Hall insulator with in-plane magnetization and a nontrivial gap opening at $M$, the corresponding topological superconductor can be tuned to host only single chiral Majorana states at its edge, which is promising for proposals exploiting such states for braiding operations.

Ultrafast observation of multiple asymmetric Fano line shapes in energetically overlapped single continuum and multiple discrete states is of great theoretical interest in different domains of physics, though their real-time experimental demonstrations are yet to be achieved. Here, we present an experimental observation on time-domain response of the double Fano asymmetries in a metal–two-dimensional semiconductor hybrid prototype at room temperature. The ultrafast interactions in two discrete spin-resolved excitons of ${\mathrm{MoS}}_2$ and a single metal plasmon-continuum allow us to observe the generation and evolution of the double Fano line profiles in real time. Apart from the subpicosecond development of double Fano line shapes, we calculate all the time-dependent nonlinear double Fano parameters. These results suggest a better understanding of ultrafast Fano physics in condensed-matter systems and have potential prospects in ultrafast photonic devices using double Fano resonances even at room temperature.

We study an insulator-metal transition (IMT) in vanadium dioxide films. We argue that the main features of the voltage-induced IMT in these systems can be understood within a theory of the blow-up overheating instability. In the blow-up regime, the IMT occurs locally even in uniform films: a narrow “hot” metallic channel along the current direction arises in the insulating “cool” environment. We derive an instability criterion and analytical formulas for a characteristic instability time and the hot-channel width. We apply our results to the experiments on the epitaxial films available in the literature and our measurements performed on the specially prepared ${\mathrm{VO}}_2$ granular films. The main features of the voltage-induced IMT are the same for all considered samples and can be described in the framework of the proposed theory. Interestingly, the mathematical description of the voltage-induced IMT in the vanadium oxide is similar to that proposed for the superconductor-normal state transition in high-$T_c$ tapes. However, the hot channel in high $T_c$ is directed across the current flow, while in the case of the IMT it is directed along the current.