Using the recently developed time-dependent Landauer-Büttiker formalism and Jefimenko's retarded solutions to the Maxwell equations, we show how to compute the time-dependent electromagnetic field produced by the charge and current densities in nanojunctions out of equilibrium. We then apply this formalism to a benzene ring junction and show that geometry-dependent quantum interference effects can be used to control the magnetic field in the vicinity of the molecule. Then, treating the molecular junction as a quantum emitter, we demonstrate clear signatures of the local molecular geometry in the nonlocal radiated power.

A recent experiment reported two new noncentrosymmetric superconductors $\mathrm{Nb}{\mathrm{Ir}}_2{\mathrm{B}}_2$ and $\mathrm{Ta}{\mathrm{Ir}}_2{\mathrm{B}}_2$ with respective superconducting transition temperatures of 7.2 and 5.2 K and further suggested their superconductivity to be unconventional [K. Górnicka et al., Adv. Funct. Mater. 31, 2007960 (2021)]. Here, based on first-principles calculations and symmetry analysis, we propose that $T{\mathrm{Ir}}_2{\mathrm{B}}_2(T=\mathrm{Nb},\mathrm{Ta})$ are topological Weyl metals in the normal state. In the absence of spin-orbit coupling (SOC), we find that $\mathrm{Nb}{\mathrm{Ir}}_2{\mathrm{B}}_2$ has 12 Weyl points, and $\mathrm{Ta}{\mathrm{Ir}}_2{\mathrm{B}}_2$ has four Weyl points, i.e., the minimum number under time-reversal symmetry; meanwhile, both of them have a nodal net composed of three nodal lines. In the presence of SOC, a nodal loop on the mirror plane evolves into two hourglass Weyl rings, along with the Weyl points, which are dictated by the nonsymmorphic glide mirror symmetry. Besides the rings, $\mathrm{Nb}{\mathrm{Ir}}_2{\mathrm{B}}_2$ and $\mathrm{Ta}{\mathrm{Ir}}_2{\mathrm{B}}_2$ have 16 and 20 pairs of Weyl points, respectively. The surface Fermi arcs are explicitly demonstrated. On the (110) surface of $\mathrm{Ta}{\mathrm{Ir}}_2{\mathrm{B}}_2$, we find extremely long surface Fermi arcs ($\sim 0.6{\AA }^{-1}$) located 1.4 meV below the Fermi level, which should be readily probed in experiment. Combined with the intrinsic superconductivity and the nontrivial bulk Fermi surfaces, $T{\mathrm{Ir}}_2{\mathrm{B}}_2$ may, thus, provide a very promising platform to explore the three-dimensional topological superconductivity.

We have investigated plasma excitations in a disk-shaped two-dimensional electron system (2DES), the edge of which was covered by an overlaying metallic gate. We have found that the microwave response of the structure is dominated by the proximity plasma mode propagating along the disk edge. Significantly, we find that as the overlap between the 2DES and the gate tends to zero, the frequency of the proximity plasmon makes a transition to that of an ordinary 2D plasmon in the ungated area of the 2DES. We also observe that tuning the electron density under the gate results in a crossover from the proximity to the laterally screened 2D plasmon. The experimental findings are analyzed using the plasmonic lumped-element approach.

We discuss transport through an interferometer formed by the helical edge states of a quantum spin Hall insulator. Focusing on the effects induced by a strong magnetic impurity placed in one of the arms of an interferometer, we consider the experimentally relevant case of relatively high temperature as compared to the level spacing. We obtain the conductance and spin polarization in a closed form for an arbitrary tunneling amplitude of the contacts and arbitrary strength of the magnetic impurity. We demonstrate the existence of quantum effects which do not show up in previously studied cases of weak magnetic disorder. We find optimal conditions for spin filtering and demonstrate that the spin polarization of outgoing electrons can reach 100%.

We theoretically study the power-law decay behavior of the local density of states (LDOS) oscillations near a line defect in a system with semi-Dirac points by using a low-energy $k·p$ Hamiltonian. We find that the LDOS oscillations are strongly anisotropic and sensitively depend on the orientation of the line defect. We analytically obtain the decay indexes of the LDOS oscillations near a line defect running along different directions by using the stationary phase approximation. Specifically, when the line defect is perpendicular to the linear dispersion direction, the decay index is $-5/4$ whereas it becomes $-1/4$ if the system is gapped, both of which are different from the decay index $-3/2$ in isotropic Dirac systems. In contrast, when the line defect is perpendicular to the parabolic dispersion direction, the decay index is always $-1/2$ regardless of whether the system is gapped or not, which is the same as that in a conventional semimetal. In general, when the defect runs along an arbitrary direction, the decay index sensitively depends on the incident energy for a certain orientation of the line defect. It varies from $-5/4$ to $-1/2$ due to the absence of strict stationary phase point. Our results indicate that the decay index $-5/4$ provides a fingerprint to identify semi-Dirac points in 2D electron systems.

In this Letter, we study two-dimensional Floquet conformal field theory, where the external periodic driving is described by iterated logistic or tent maps. These maps are known to be typical examples of dynamical systems exhibiting the order-chaos transition, and we show that, as a result of such driving, the entanglement entropy scaling develops fractal features when the corresponding dynamical system approaches the chaotic regime. For the driving set by the logistic map, the fractal contribution to the scaling dominates, making entanglement entropy a highly oscillating function of the subsystem size.

Proposals for realizing Majorana fermions in condensed-matter systems typically rely on magnetic fields, which degrade the proximitizing superconductor and plague the Majoranas' detection. We propose an alternative scheme to realize Majoranas based only on phase-biased superconductors. The phases (at least three of them) can be biased by a tiny magnetic field threading macroscopic superconducting loops, focusing and enhancing the effect of the magnetic field onto the junction, or by supercurrents. We show how a combination of the superconducting phase winding and the spin-orbit phase induced in closed loops (Aharonov-Casher effect) facilitates a topological superconducting state with Majorana end states. We demonstrate this scheme with an analytically tractable model as well as simulations of realistic setups comprising only conventional materials.

An important challenge in the field of many-body quantum dynamics is to identify nonergodic states of matter beyond many-body localization (MBL). Strongly disordered spin chains with non-Abelian symmetry and chains of non-Abelian anyons are natural candidates, as they are incompatible with standard MBL. In such chains, real space renormalization group methods predict a partially localized, nonergodic regime known as a quantum critical glass (a critical variant of MBL). This regime features a treelike hierarchy of integrals of motion and symmetric eigenstates with entanglement entropy that scales as a logarithmically enhanced area law. We argue that such tentative nonergodic states are perturbatively unstable using an analytic computation of the scaling of off-diagonal matrix elements and accessible level spacing of local perturbations. Our results indicate that strongly disordered chains with non-Abelian symmetry display either spontaneous symmetry breaking or ergodic thermal behavior at long times. We identify the relevant length and timescales for thermalization: Even if such chains eventually thermalize, they can exhibit nonergodic dynamics up to parametrically long timescales with a nonanalytic dependence on disorder strength.

We examine room temperature current-voltage (IV) characteristics of $\mathrm{CoFeB}\left|\mathrm{MgO}\right|\mathrm{CoFeB}$ type of perpendicularly magnetized tunnel junctions developed for memory applications. From their nonlinear bias voltage dependence, a conductance “cross-scaling” is seen that is consistent with the involvement of inelastic spin-flip scattering in electrodes. A phenomenological model is constructed that connects the parameters of spin-flip scatter-related inelastic events with both magnetoresistance and spin-transfer torque. The model provides measurable, electrode-specific properties such as interface exchange stiffness as it affects spin-torque performance.

In this work, we investigate the thermodynamic behavior of exchange graded ferromagnetic films using Monte Carlo simulations. The systems are modeled by using a classical Heisenberg Hamiltonian, considering only nearest neighbor exchange interactions and a linear depth-dependent effective magnetic exchange coupling strength profile. Our quantitative assessment of the local physical quantities shows that each layer exhibits rather isolated thermodynamic behavior, since both layerwise magnetization and susceptibility data indicate layer-specific “local” Curie temperatures as a consequence of the depth-dependent change in the exchange coupling strength. We also propose and evaluate a predictive formulation for such profiles of “local” Curie temperatures, whose only input is the pre-selected exchange coupling profile. Having this very precise predictive tool, we show how it can be used to obtain the temperature-dependent ferromagnetic state including its depth-dependent magnetization profile at any given temperature. Thus, it is possible to predict which exchange profile will produce a desired thermodynamic behavior and associated functionality, without the need to perform complex experiments or time-consuming computations. With this study, we furthermore demonstrate that nonlocal aspects of the thermodynamic state formation in graded magnetic materials are relevant only over very short length scales, which is in outstanding qualitative agreement with prior numerical and experimental work.

Some features of the phase diagram of the two-dimensional axial next-nearest-neighbor Ising model have long been debated. The extended structural correlations and long relaxation times associated with its Kosterlitz-Thouless phase indeed result in analytical and numerical treatments making contradictory predictions. Here, we introduce a numerical transfer matrix approach that bypasses these problems and thus clears up various ambiguities. In particular, we confirm the transition temperatures and the order of the transition to the floating incommensurate phase. Our approach motivates considering transfer matrices for solving long-standing problems in related models.

We study electron correlation driven superconductivity on a decorated honeycomb lattice (DHL), which has a low-energy flat band. On doping, we find singlet superconductivity with extended-$s$, extended-$d$, and $f$-wave symmetry mediated by magnetic exchange. $f$-wave singlet pairing is enabled by the lattice decoration. The critical temperature is predicted to be significantly higher than on similar lattices lacking flat bands. We discuss how high-temperature superconductivity could be realized in the DHL materials such as ${\mathrm{Rb}}_3\mathrm{TT}·2{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{Mo}}_3{\mathrm{S}}_7{\left(\mathrm{dmit}\right)}_3$.

In this paper, we consider an extension of the Rosenzweig-Porter model, the Lévy-RP (L-RP) model, in which the off-diagonal matrix elements are broadly distributed, providing a more realistic benchmark to develop an effective description of nonergodic extended (NEE) states in interacting many-body disordered systems. We put forward a simple, general, and intuitive argument that allows one to unveil the multifractal structure of the minibands in the local spectrum when hybridization is due to anomalously large transition amplitudes in the tails of the distribution. The idea is that the energy spreading of the minibands can be determined self-consistently by requiring that the maximal hybridization rate ${\mathcal{H}}_{ij}$ between a site $i$ and the other $N^{D_1}$ sites of the support set is of the same order of the Thouless energy itself $N^{D_1-1}$. This argument yields the fractal dimensions that characterize the statistics of the multifractal wave functions in the NEE phase, as well as the whole phase diagram of the L-RP ensemble. Its predictions are confirmed both analytically, by a thorough investigation of the self-consistent equation for the local density of states obtained using the cavity approach, and numerically, via extensive exact diagonalizations.

The magnetic properties of ${\mathrm{Cs}}_3{\mathrm{Fe}}_2{\mathrm{Cl}}_9$ single crystals were investigated by magnetic and thermal measurements. The crystal structure of ${\mathrm{Cs}}_3{\mathrm{Fe}}_2{\mathrm{Cl}}_9$ consists of a bilayer triangular lattice of ${\mathrm{Fe}}^{3+}$ ions. The analysis of magnetic susceptibility clarified that the intra- and interdimer interactions are comparable with each other, and ${\mathrm{Cs}}_3{\mathrm{Fe}}_2{\mathrm{Cl}}_9$ is regarded as a spin-5/2 $J_1\text{−}{J}_2$ buckled honeycomb antiferromagnet with the ferromagnetic interlayer interaction $J_3$. The susceptibility for the magnetic field $H\left|\right|c$ axis exhibits a sudden drop at the Néel temperature $T_{\mathrm{N}}=5.4\mathrm{K}$, indicating a first-order magnetic phase transition. One of the characteristic features is the linear temperature dependence below $T_{\mathrm{N}}$. Furthermore, ${\mathrm{Cs}}_3{\mathrm{Fe}}_2{\mathrm{Cl}}_9$ undergoes successive magnetic phase transitions in high magnetic fields along the $c$ axis. We obtained a rich H-T phase diagram in which the $M_{\mathrm{s}}/2$ magnetization-plateau phase is included. These unique magnetic behaviors probably originate from the competition of magnetic interactions and the easy-axis anisotropy in the antiferromagnetic $J_1\text{−}{J}_2$ buckled honeycomb network.

We have studied the normal and superconductive state characteristics (resistivity, Hall coefficient, heat capacity, and magnetization) of model strongly correlated electronic systems ${\mathrm{Lu}}_x{\mathrm{Zr}}_{1-x}{\mathrm{B}}_{12}$ with cooperative Jahn-Teller instability of the boron rigid cage and with dynamic charge stripes. It was found that these metals are s-wave dirty limit superconductors with a small mean free path of charge carriers $l=5–140\AA$ and with a Cooper pair size changing nonmonotonously in the range 450–4000 Å. The parent ${\mathrm{ZrB}}_{12}$ and ${\mathrm{LuB}}_{12}$ borides are type-I superconductors, and Zr to Lu substitution induces a type-I to type-II phase transition providing a variation of the Ginzburg-Landau-Maki parameter in the limits $0.65\le {\kappa }_{1,2}\le 6$. We argue in favor of the two-band scenario of superconductivity in ${\mathrm{Lu}}_x{\mathrm{Zr}}_{1-x}{\mathrm{B}}_{12}$ with gap values ${\mathrm{\Delta }}_1\sim 14\mathrm{K}$ and ${\mathrm{\Delta }}_2\sim 6–8\mathrm{K}$, with pairing corresponding to strong coupling limit $({\lambda }_{\text{e-ph}}\sim 1)$ in the upper band, and to weak coupling $({\lambda }_{\text{e-ph}}\sim 0.1–0.4)$ in the lower one. A pseudogap ${\mathrm{\Delta }}_{\text{ps-gap}}\sim 60–110\mathrm{K}$ is observed in ${\mathrm{Lu}}_x{\mathrm{Zr}}_{1-x}{\mathrm{B}}_{12}$ above $T_c$. We discuss also the possibility of anisotropic single-band superconductivity with stripe-induced both pair breaking and anisotropy, and analyze the origin of a unique enhanced surface superconductivity detected in these model compounds.

Quantum spin Hall (QSH) effect with great promise for the potential application in spintronics and quantum computing has attracted extensive research interest from both theoretical and experimental researchers. Here, we predict monolayer ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ can be a QSH insulator based on first-principles calculations. The interlayer binding energy in the layered van der Waals compound ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ is $19.6\mathrm{meV}/{\AA }^2$; thus, its monolayer/thin-film structures could be readily obtained by exfoliation. The band inversion near the Fermi level ($E_F$) is an intrinsic characteristic, which happens between $\mathrm{Ta}\text{−}5d$ and $\mathrm{Pd}\text{−}4d$ orbitals without spin-orbit coupling (SOC). The SOC effect opens a global gap and makes the system a QSH insulator. With the $d\text{−}d$ band-inverted feature, the nontrivial topology in monolayer ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ is characterized by the time-reversal topological invariant ${\mathbb{Z}}_2=1$, which is computed by the one-dimensional (1D) Wilson loop method as implemented in our first-principles calculations. The helical edge modes are also obtained using surface Green's function method. Our calculations show that the QSH state in ${\mathrm{Ta}}_2{M}_3{\mathrm{Te}}_5$ ($M=\mathrm{Pd}$, Ni) can be tuned by external strain. These monolayers and thin films provide feasible platforms for realizing QSH effect as well as related devices.

The experimental verification of chiral anomaly in Weyl semimetals is an active area of investigation in modern condensed matter physics, which typically relies on the combined signatures of longitudinal magnetoconductance (LMC) along with the planar Hall effect (PHE). It has recently been shown that for weak nonquantizing magnetic fields, a sufficiently strong finite intervalley scattering drives the system to switch the sign of LMC from positive to negative. Here we unravel another independent source that produces the same effect. Specifically, a smooth lattice cutoff to the linear dispersion, which is ubiquitous in real Weyl materials, introduces nonlinearity in the problem and also drives the system to exhibit negative LMC for noncollinear electric and magnetic fields even in the limit of vanishing intervalley scattering. We examine longitudinal magnetoconductivity and the planar Hall effect semianalytically for a lattice model of tilted Weyl fermions within the Boltzmann approximation. We independently study the effects of a finite lattice cutoff and tilt parameters and construct phase diagrams in relevant parameter spaces that are relevant for diagnosing chiral anomaly in real Weyl materials.

Anomalous metals are observed in numerous experiments on disordered two-dimensional systems proximate to superconductivity. A characteristic feature of an anomalous metal is that its low-temperature conductivity has a weakly temperature dependent value, significantly higher than that of a disordered Fermi liquid. We propose a dynamical mean-field model of an anomalous metal: interacting electrons similar in structure to that of the well-studied universal Hamiltonian of mesoscopic metallic grains, but with independent random interactions between pairs of sites, involving Cooper pair hopping and spin exchange. We find evidence for critical anomalous phases or points between a superconducting phase and a disordered Fermi liquid phase in this model. Our results are obtained by a renormalization group analysis in a weak coupling limit, and a complementary solution at large $M$ when the spin symmetry is generalized to $\mathrm{USp}\left(M\right)$. The large $M$ limit describes the anomalous metal by fractionalization of the electron into spinons, holons, and doublons, with these partons forming critical non-Fermi liquids in the Sachdev-Ye-Kitaev class. We compute the low-temperature conductivity in the large $M$ limit and find temperature-independent values moderately enhanced from that in the disordered metal.

We study the temperature dependence of energy diffusion in two chaotic gapped quantum spin chains, a tilted-field Ising model and an XZ model, using an open-system approach. We introduce an energy imbalance by coupling the chain to thermal baths at its boundary and study the nonequilibrium steady states of the resulting Lindblad dynamics using a matrix product operator ansatz for the density matrix. We define an effective local temperature profile by comparing local reduced density matrices in the steady state with those of a uniform thermal state. We then measure the energy current for a variety of driving temperatures and extract the temperature dependence of the energy diffusion constant. For the Ising model, we are able to study temperatures well below the energy gap and find a regime of dilute excitations where rare three-body collisions control energy diffusion. A kinetic model correctly predicts the observed exponential increase of the energy diffusion constant at low temperatures. For the XZ model, we are only able to access intermediate to high temperatures relative to the energy gap, and we show that the data are well described by an expansion around the infinite temperature limit. We also discuss the limitations of the particular driving scheme and suggest that lower temperatures can be accessed using larger baths.

Recent studies have predicted that magnetic ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ can host several different topological states depending on its magnetic order, including a single pair of Weyl points. Here we report on the bulk properties and band inversion induced by pressure in the nonmagnetic analogs $AE{\mathrm{Cd}}_2{\mathrm{As}}_2$ $(AE=\mathrm{Ca}, \mathrm{Sr}, \mathrm{Ba})$ as studied with density functional theory calculations. Under ambient pressure we find that these compounds are narrow band gap semiconductors, in agreement with experiment. The size of the band gap is dictated by both the increasing ionicity across the AE series which tends to increase the band gap, as well as the larger nearest neighbor Cd-As distance from increasing atomic size which can decrease the band gap because the conduction band edge is an antibonding state derived mostly from Cd $5s$ orbitals. The combination of these two competing effects results in a nonmonotonic change of the band gap size across the AE series with ${\mathrm{SrCd}}_2{\mathrm{As}}_2$ having the smallest band gap among the three compounds. The application of negative pressure reduces this band gap and causes the band inversion between the Cd $5s$ and As $4p$ orbitals along the $\mathrm{\Gamma }\text{−}A$ direction to induce a pair of Dirac points. The topological nature of the Dirac points is then confirmed by finding the closed Fermi arcs on the $\left(10\overline{1}0\right)$ surface.