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.

Laser micromachining techniques using femtosecond laser pulses are employed for the fabrication of a wide range of devices, from optical waveguides and fiber Bragg gratings to microfluidic systems. In this perspective, it is important to characterize how transparent materials respond to the high-intensity laser field in order to fully understand and control the strong-field ionization process at the core of such techniques. Here, we characterize the laser polarization dependence of $y$-cut and $x$-cut ferroelectric lithium niobate (${\mathrm{LiNbO}}_3$) upon ionization. Using linearly polarized 1800-nm femtosecond pulses, we perform single-shot laser-induced ablation measurements to obtain a macroscopic observable with a large contrast compared to all-optical techniques. In the tunnel ionization regime, the crystal orientation can be correlated with the structural symmetry/asymmetry of the material. This is revealed through a variation of the ablated area when the laser polarization is rotated with respect to the $c$ axis of the crystal. It is further found that ablation is more pronounced when the laser polarization is oriented towards angular regions containing Nb-O bonds of the unit cell, identifying the main chemical bonds contributing to the ionization of the material. The experimental results are supported by numerical simulations based on a two-band model for ${\mathrm{LiNbO}}_3$.

We investigate the mapping between magnetic susceptibility and entanglement in the metallic, insulating, conventional, and exotic polarized superfluid phases of one-dimensional fermionic lattice systems as described by the Hubbard model. Motivated by recent proposals for determining and quantifying entanglement via magnetic susceptibility measurements, we numerically study the intrinsic relationship between the two quantities at zero temperature. We find signatures of the metal-insulator transition and of the BCS-BEC crossover, but the most relevant result is that for conventional and exotic superfluids the mapping between magnetic susceptibility and entanglement is surprisingly simple: Directly proportional. This linear behavior is found to be universal for conventional superfluids and therefore could be exploited to quantify entanglement in current cold-atom and condensed-matter experiments.

Bi-based cuprate superconductors are important materials for both fundamental research and applications. As in other cuprates, the superconducting phase in the Bi compounds lies close to an antiferromagnetic phase. Our density functional theory calculations based on the strongly-constrained-and-appropriately-normed exchange correlation functional in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ reveal the persistence of magnetic moments on the copper ions for oxygen concentrations ranging from the pristine phase to the optimally hole-doped compound. We also find the existence of ferrimagnetic solutions in the heavily doped compounds, which are expected to suppress superconductivity.

Near the transverse-field-induced quantum critical point of the Ising chain, an exotic dynamic spectrum consisting of exactly eight particles was predicted, which is uniquely described by an emergent quantum integrable field theory with the symmetry of the $E_8$ Lie algebra, but rarely explored experimentally. Here we use high-resolution terahertz spectroscopy to resolve quantum spin dynamics of the quasi-one-dimensional Ising antiferromagnet $\mathrm{BaC}{\mathrm{o}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ in an applied transverse field. By comparing to an analytical calculation of the dynamical spin correlations, we identify $E_8$ particles as well as their two-particle excitations.

In this work, we construct an exact projector Hamiltonian with interactions, which is given by a sum of mutually commuting operators called stabilizers. The model is based on the recently studied Creutz ladder of fermions, in which flat-band structure and strong localization are realized. These stabilizers are local integrals of motion from which many-body localization (MBL) is realized. All energy eigenstates are explicitly obtained even in the presence of local disorders. All states are MBL states, that is, this system is a full many-body localized (FMBL) system. We show that this system has a topological order and stable gapless edge modes exist under the open boundary condition. By the numerical study, we investigate stability of the FMBL and topological order.

We carried out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy and investigated cation valence states and spin and orbital magnetic moments in inverse-spinel ferrimagnet $\mathrm{N}{\mathrm{i}}_{1-x}\mathrm{C}{\mathrm{o}}_{2+y}{\mathrm{O}}_{4-z}$ (NCO) epitaxial films with perpendicular magnetic anisotropy. We show that the oxygen pressure $P_{\mathrm{O}2}$ during film growth by pulsed laser deposition influences not only the cation stoichiometry (site occupation) but also the cation valence state. Our XAS results show that the Ni in the $O_h$-site is in the intermediate valence state between +2 and +3, $\mathrm{N}{\mathrm{i}}^{\left(2+\delta \right)+}$ ($0<\delta <1$), whose nominal valence state (the δ value) varies depending on $P_{\mathrm{O}2}$. On the other hand, the Co in the octahedral ($O_h$) and tetrahedral ($T_d$) sites, respectively, have a valence state close to +3 and +2. We also find that the XMCD signals originate mainly from the $T_d$-site $\mathrm{C}{\mathrm{o}}^{2+}$ ($\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$) and $O_h$-site $\mathrm{N}{\mathrm{i}}^{\left(2+\delta \right)+}$ ($\mathrm{N}{\mathrm{i}}_{\mathrm{Oh}}$), indicating that these cation valence states are the key to determining the magnetic and transport properties of NCO films. Interestingly, the valence state of $\mathrm{N}{\mathrm{i}}^{\left(2+\delta \right)+}$ that gives rise to the XMCD signal remains unchanged independent of $P_{\mathrm{O}2}$. The electronic structure of $\mathrm{N}{\mathrm{i}}^{\left(2+\delta \right)+}$ that is responsible for the magnetic moment and electrical conduction differs from those of $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{N}{\mathrm{i}}^{3+}$. In addition, the orbital magnetic moment originating from $\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$ is as large as $0.14{\mu }_{\mathrm{B}}/\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$ and parallel to the magnetization, while the $\mathrm{N}{\mathrm{i}}_{\mathrm{Oh}}$ orbital moment is as small as $0.07{\mu }_{\mathrm{B}}/\mathrm{N}{\mathrm{i}}_{\mathrm{Oh}}$ and is rather isotropic. $\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$, therefore, plays the key role in the perpendicular magnetic anisotropy of the films. Our results demonstrate the significance of the site-dependent cation valence states for the magnetic and transport properties of $\mathrm{NiC}{\mathrm{o}}_2{\mathrm{O}}_4$ films.

We study the influence of ferromagnetic nonlocal exchange on correlated electrons in terms of an SU(2)-Hubbard-Heisenberg model and address the interplay of on-site interaction induced local moment formation and the competition of ferromagnetic direct and antiferromagnetic kinetic exchange interactions. In order to simulate thermodynamic properties of the system in a way that largely accounts for the on-site interaction driven correlations in the system, we advance the correlated variational scheme introduced in [M. Schüler et al., Phys. Rev. Lett. 111, 036601 (2013)] to account for explicitly symmetry broken electronic phases by introducing an auxiliary magnetic field. After benchmarking the method against exact solutions of a finite system, we study the SU(2) Hubbard-Heisenberg model on a square lattice. We obtain the $U-J$ finite temperature phase diagram of a SU(2)-Hubbard-Heisenberg model within the correlated variational approach and compare it to static mean-field theory. While the generalized variational principle and static mean-field theory yield transitions from dominant ferromagnetic to antiferromagnetic correlations in similar regions of the phase diagram, we find that the nature of the associated phase transitions differs between the two approaches. The fluctuations accounted for in the generalized variational approach render the transitions continuous, while static mean-field theory predicts discontinuous transitions between ferro- and antiferromagnetically ordered states.

We experimentally investigate charge transport through the interface between a gold electrode and a black phosphorus single crystal. The experimental $dI/dV\left(V\right)$ curves are characterized by well developed zero-bias conductance peak and two strongly different branches. We find that two branches of asymmetric $dI/dV\left(V\right)$ curves correspond to different band gap limits, which is consistent with the theoretically predicted band gap reconstruction at the surface of black phosphorus under electric field. This conclusion is confirmed by experimental comparison with the symmetric curves for narrow-gap (${\mathrm{WTe}}_2$) and wide-gap (GaSe) metal-semiconductor structures. In addition, we demonstrate p-type dopants redistribution at high bias voltages of different sign, which opens a way to use the interface structures with black phosphorus in resistive memory applications.

We report a two-dimensional (2D) Dirac semimetal in an orthorhombic $\mathit{Pmma}$ structure, an NP monolayer, based on first-principles calculations. The NP monolayer is found to be stable according to phonon dispersion and molecular dynamical simulations. It exhibits a pair of anisotropic Dirac cones exactly at the Fermi level, located along high symmetry lines in the Brillouin zone, and shows highly anisotropic in-plane stiffness. First-principles calculations show that Dirac cones are locally stable under strains along the $x$ or $y$ direction and electric fields perpendicular to the monolayer. The Dirac nodes are protected from gap opening by the mirror-reflection symmetry in the $x$ direction. We construct an effective two-band tight-binding model to characterize the Dirac states. Our result provides a promising 2D material for special electronics applications.

We report on the formation and comprehensive study of atomic and electronic structures of a new Tl-Mg two-dimensional compound reconstruction with $\sqrt{3}\times \sqrt{3}$ periodicity on a Si(111) substrate. Owing to properties of its constituent elements and crystal structure with a breathing kagome lattice of Mg and a dense Tl layer (4/3 monolayer), this double-layer system displays promising features, including spin-split metallic bands, nearly flat bands, and a saddle point with corresponding divergences in the local density of states. These features indicate the possibility that strongly correlated electronic states exist in this structure after tuning the Fermi level position. In accordance with the $C_3$ symmetry, we observe spin-orbit-coupling-driven Rashba-type spin splitting with spin degeneracy at the $\overline{M}$ point and Zeeman-like splitting at the $\overline{K}$ point. In contrast to the previously reported cases, splitting at the $\overline{K}$ point is significant to produce fully out-of-plane spin-polarized cones. Due to opposite Berry curvatures at the $\overline{K}$ and ${\overline{K}}^{\prime }$ points of the hexagonal surface Brillouin zone, spins at these valleys have opposite signs, which ensure effective spin-pseudospin coupling.

The electronic properties of two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention during the last decade. We show how a diagrammatic ab initio coupled cluster singles and doubles (CCSD) treatment paired with a careful thermodynamic limit extrapolation in two dimensions can be used to obtain converged band gaps for monolayer materials in the ${\mathrm{MoS}}_2$ family. We find CCSD gaps to lie in the upper range of the spread of $\mathit{GW}$ approximation based on density functional theory (DFT) simulations, and also find slightly higher effective hole masses compared to previous reports. We also investigate the ability of CCSD to describe trion states, finding a reasonable qualitative structure, but poor excitation energies due to the lack of screening of three-particle excitations in the effective Hamiltonian. Our study provides an independent high-level benchmark of the role of many-body effects in 2D TMDs and showcases the potential strengths and weaknesses of diagrammatic coupled cluster approaches for realistic materials.