Nonequilibrium situations where selected currents are suppressed are of interest in fields like thermoelectrics and spintronics, raising the question of how the related noises behave. We study such zero-current charge, spin, and heat noises in a two-terminal mesoscopic conductor. In the presence of voltage, spin, and temperature biases, the nonequilibrium (shot) noises of charge, spin, and heat can be arbitrarily large, even if their average currents vanish. However, as soon as a temperature bias is present, additional equilibrium (thermal-like) noise necessarily occurs. We show that this equilibrium noise sets an upper bound on the zero-current charge and spin shot noises, even if additional voltage or spin biases are present. We demonstrate that these bounds can be overcome for heat transport by breaking the spin and electron-hole symmetries, respectively. By contrast, we show that the bound on the charge noise for strictly two-terminal conductors even extends into the finite-frequency regime.

Magnet-superconductor hybrid (MSH) systems are a key platform for custom-designed topological superconductors. Ideally, the ends of a one-dimensional MSH structure will host Majorana zero-modes (MZMs), the fundamental unit of topological quantum computing. However, some experiments with ferromagnetic (FM) chains show a more complicated picture. Due to tiny gap sizes and hence long coherence lengths, MZMs might hybridize and lose their topological protection. Recent experiments on a niobium surface have shown that both FM and antiferromagnetic (AFM) chains may be engineered, with the magnetic order depending on the crystallographic direction of the chain. While FM chains are well understood, AFM chains are less so. Here, we study two models inspired by the niobium surface: A minimal model to elucidate the general topological properties of AFM chains and an extended model to more closely simulate a real system by mimicking the proximity effect. We find that, in general, for AFM chains, the topological gap is larger than for FM ones, and thus, coherence lengths are shorter for AFM chains, yielding more pronounced localization of MZMs in these chains. While for some parameters AFM chains may be topologically trivial, we find in these cases that adding an adjacent chain can result in a nontrivial system, with a single MZM at each chain end.

Zero band gap in graphene and silicene is an important obstacle to their application in nanodevices. Combining carbon and silicon atoms to form a silicon carbide (${\mathrm{Si}}_x{\mathrm{C}}_y$) sheet is an applicable proposal to solve the band gap issue. Very recently, a large-scale monolayer $\mathrm{Si}{}_9\mathrm{C}{}_{15}$ was successfully synthesized and exhibited suitable band gap and environment stability [Gao et al., Adv. Mater. 34, 2204779 (2022)]. Motivated by this recent experiment, we explore the electronic, mechanical, optical, and thermoelectric properties of a $\mathrm{Si}{}_9\mathrm{C}{}_{15}$ monolayer using first-principles calculations. Our results reveal that the monolayer is an auxetic material with a negative Poisson ratio of 0.175. The monolayer is a semiconductor with a direct band gap of 2.54 eV calculated by the Heyd-Scuseria-Ernzerhof functional (HSE06) functional. The optical properties of the monolayer are investigated by solving the Bethe-Salpeter equation. It is revealed that the optical band gap of the monolayer is 2.73 eV, and the exciton binding energy is equal to 1.14 eV, indicating a strongly bound bright exciton. Thermoelectric properties of the monolayer are calculated by combining density functional theory and Green's function formalism in the linear response regime. The lattice thermal conductance of the monolayer is 0.85 nW/K at room temperature, and the maximum figure of merit is 1.2 at 800 K, higher than that of other silicon carbide allotropes. High flexibility, a suitable band gap, high mobility, high optical absorption in the UV-visible region, and high thermoelectric efficiency make the $\mathrm{Si}{}_9\mathrm{C}{}_{15}$ monolayer a promising candidate for applications in mechanics, optoelectronics, and energy converters.

The colossal magnetoresistive manganite ${\left({\mathrm{La}}_{0.6}{\mathrm{Pr}}_{0.4}\right)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ (LPCMO) undergoes a ferromagnetic (FM) metal to paramagnetic insulator phase transition at around 195 K, which develops via an intermediate polaronic state strongly susceptible to external magnetic fields. Transient reflectivity studies of LPCMO on timescales from sub-picoseconds to nanoseconds reveal that the overall system dynamics are strongly influenced by the phase transition with a sharp and strong decrease in the relaxation time while crossing from the ferromagnetic metallic phase into the paramagnetic insulating phase. We show that the long relaxation times of the reflectivity after nonequilibrium excitation within the FM phase are caused by the slow recovery of the ordered FM state after laser-induced demagnetization. Control of the nonequilibrium dynamics close to the phase transition is demonstrated by applying moderate magnetic fields below 1 T.

The bulk-edge correspondence (BEC) is the hallmark of topological systems. In continuous (nonlattice) Hermitian systems with an unbounded wave vector, it was recently shown that the BEC of Chern insulators is modified. How would it be further affected in non-Hermitian systems, experiencing loss and/or gain? In this work, we take the first step in this direction, by studying a bulk-Hermitian continuous system with non-Hermitian boundary conditions. We find in this case that edge modes emerge at the roots of the scattering matrix, as opposed to the Hermitian case, where they emerge at its poles (or, more accurately, coalescence of roots and poles). This entails a nontrivial modification to the relative Levinson's theorem. We then show that the topological structure remains the same as in the Hermitian case, and the generalized BEC holds, provided one employs appropriately modified contours in the wave-vector plane so that the scattering matrix phase winding counts the edge modes correctly. We exemplify all this using a paradigmatic model of waves in a shallow ocean or active systems in the presence of odd viscosity, as well as 2D electron gas with Hall viscosity. We use this opportunity to examine the case of large odd viscosity, where the scattering matrix becomes $2\times 2$, which has not been discussed in previous works on the Hermitian generalized BEC.

Recent angle-resolved $c$-axis resistivity measurements of the stripe-ordered ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ (LBCO) with $x=1/8$ revealed an unexpected dependence on the direction of the in-plane magnetic field. We argue that these and other available data for the $c$-axis transport point to the existence of superconducting pairs of two different types in the $x=1/8$ LBCO below the stripe ordering temperature. The pairs of one type carry finite momentum and are confined to the Cu-O planes; the pairs of other type (probably the conventional $d$ wave with zero momentum) propagate along narrow conducting channels traversing the sample in the $c$-axis direction. The evidence for this comes from the observed exponential temperature dependence of the $c$-axis resistivity ${\rho }_c\left(T\right)$ which we attribute to the thermally excited slips of the superconducting phase and flux flows. We present a simple theory to fit the observed $\pi /2$-periodic dependence of ${\rho }_c$ on the direction of the in-plane magnetic field and the other data.

We investigate laser emission at the interface of the topological and trivial phases in one dimension. The system is described by a generalized Su-Schrieffer-Heeger model with site-dependent hopping parameters involving the interface width parameter where gain (loss) is introduced only to the $A$ $\left(B\right)$ sites of the bipartite lattice. The topological interface state is described by the Jackiw-Rebbi state with a pure imaginary energy, reflecting the non-Hermiticity of the system. It feels only the gain effect since it is strictly localized at the $A$ sites. The Jackiw-Rebbi state exists for any value of the interface width. We, thus, obtain a large area single-mode laser by making the interface width wide enough. We also find a series of analytic solutions of excited states based on supersymmetry (SUSY) quantum mechanics where the $A$ and $B$ sites of the bipartite lattice form SUSY partners. Furthermore, we study the system containing loss and gain with saturation by extending the Jackiw-Rebbi mode to a nonlinear theory.

We present ${\mathrm{CoTe}}_2$ as a type-II Dirac semimetal supporting Lorentz-symmetry violating Dirac fermions in the vicinity of the Fermi energy. By combining first-principles ab initio calculations with experimental angle-resolved photoemission spectroscopy results, we show ${\mathrm{CoTe}}_2$ hosts a pair of type-II Dirac fermions around 90 meV above the Fermi energy. In addition to the bulk Dirac fermions, we find several topological band inversions in bulk ${\mathrm{CoTe}}_2$, which gives rise to a ladder of spin-polarized surface states over a wide range of energies. In contrast to the surface states which typically display Rashba-type in-plane spin splitting, we find that ${\mathrm{CoTe}}_2$ hosts interesting out-of-plane spin polarization as well. Our work establishes ${\mathrm{CoTe}}_2$ as a potential candidate for the exploration of Dirac fermiology and applications in spintronic devices, infrared plasmonics, and ultrafast optoelectronics.

Bound states at sharp corners have been widely viewed as the hallmark of two-dimensional second-order topological insulators and superconductors. In this paper, we show that the existence of sublattice degrees of freedom can enrich the tunability of bound states on the boundary and hence lift the constraint on their locations. We take the Kane-Mele model with honeycomb-lattice structure to illustrate the underlying physics. With the introduction of an in-plane exchange field to the model, we find that the boundary Dirac mass induced by the exchange field has a sensitive dependence on the boundary sublattice termination. We find that the sensitive sublattice dependence can lead bound states to emerge at a specific type of boundary defects named as sublattice domain walls if the exchange field is of ferromagnetic nature, even in the absence of any sharp corner on the boundary. Remarkably, this sensitive dependence of the boundary Dirac mass on the boundary sublattice termination allows the positions of bound states to be manipulated to any place on the boundary for an appropriately-designed sample. With a further introduction of conventional $s$-wave superconductivity to the model, we find that, no matter whether the exchange field is ferromagnetic, antiferromagnetic, or ferrimagnetic, highly controllable Majorana zero modes can be achieved at the sublattice domain walls. Our paper reshapes the understanding of boundary physics in second-order topological phases, and meanwhile opens potential avenues to realize highly controllable bound states for potential applications.

Correlated insulators are frequently observed in magic-angle twisted bilayer graphene at even fillings of electrons or holes per moiré unit cell. Whereas theory predicts these insulators to be intervalley coherent excitonic phases, the measured gaps are routinely much smaller than theoretical estimates. We explore the effects of random strain variations on the intervalley coherent phase, which have a pair-breaking effect analogous to magnetic disorder in superconductors. We find that the spectral gap may be strongly suppressed by strain disorder, or vanish altogether, even as intervalley coherence is maintained. We discuss predicted features of the tunneling density of states, show that the activation gap measured in transport experiments corresponds to the diminished gap, and thus offer a solution for the apparent discrepancy between the theoretical and experimental gaps.

The exposure to intense electromagnetic radiation can induce distortions and symmetry breaking in the crystal structure of solids, providing a route for the all-optical control of their properties. In this paper, we formulate a unified theoretical approach to describe the coherent lattice dynamics in presence of external driving fields, electron-phonon and phonon-phonon interactions, and quantum nuclear effects. The main mechanisms for the excitation of coherent phonons—including infrared absorption, displacive excitation, impulsive stimulated Raman scattering, and ionic Raman scattering—can be seamlessly accounted for. We apply this formalism to a model consisting of two coupled phonon modes, where we illustrate the influence of quantum nuclei on structural distortions induced by ionic Raman scattering. Besides validating the widely employed classical models for the coherent lattice dynamics, our paper provides a versatile approach to methodically explore the emergence of quantum nuclear effects in the structural response of crystal lattices to strong fields.

The valley Hall effect provides a popular route to engineer robust waveguides for bosonic excitations such as photons and phonons. The almost complete absence of backscattering in many experiments has its theoretical underpinning in a smooth-envelope approximation that neglects large momentum transfer and is accurate only for small bulk band gaps and/or smooth domain walls. For larger bulk band gaps and hard domain walls, backscattering is expected to become significant. Here, we show that in this experimentally relevant regime, the reflection of a wave at a sharp corner becomes highly sensitive to the orientation of the outgoing waveguide relative to the underlying lattice. Enhanced backscattering can be understood as being triggered by resonant tunneling transitions in quasimomentum space. Tracking the resonant tunneling energies as a function of the waveguide orientation reveals a self-repeating fractal pattern that is also imprinted in the density of states and the backscattering rate at a sharp corner.

Using the truncated Wigner approximation (TWA) we study quench dynamics of two-dimensional lattice systems consisting of interacting spinless fermions with potential disorder. First, we demonstrate that the semiclassical dynamics generally relaxes faster than the full quantum dynamics. We obtain this result by comparing the semiclassical dynamics with exact diagonalization and Lanczos propagation of one-dimensional chains. Next, exploiting the TWA capabilities of simulating large lattices, we investigate how the relaxation rates depend on the dimensionality of the studied system. We show that strongly disordered one-dimensional and two-dimensional systems exhibit a transient, logarithmic-in-time relaxation, which was recently established for one-dimensional chains. Such relaxation corresponds to the infamous $1/f$ noise at strong disorder.

We perform a symmetry analysis of the ABC-stacked kagome planes of Mn atoms common to the $L{1}_2 {\mathrm{Mn}}_{3}X$ and antiperovskite ${\mathrm{Mn}}_{3}AB$ alloys. In addition to the single-ion-anisotropy and Kitaev-type anisotropic exchange known to stabilize ${120}^{\circ }$ spin structures in these materials, our analysis results in a staggered Dzyaloshinskii-Moriya interaction and further gamma-type anisotropic exchange between nearest-neighbor spins. The presence of these new terms is shown not to affect the energetics of the ${120}^{\circ }$ ground states, which explains their absence in prior minimal magnetic models. We go on to show, however, that their influence becomes apparent when spin-wave excitations are considered. We highlight these effects by calculating inelastic neutron-scattering cross sections to illustrate experimental means of probing the existence and relative strengths of these cloaked interactions.

The magnetic-field-dependent critical current $I_{\text{c}}\left(B\right)$ of a Josephson junction is determined by the screening currents in its electrodes. In macroscopic junctions, a local vector potential drives the currents; however, in thin-film planar junctions, with electrodes of finite size and various shapes, they are governed by nonlocal electrodynamics. This complicates the extraction of parameters such as the geometry of the effective junction area, the effective junction length, and the critical current density distribution from the $I_{\text{c}}\left(B\right)$ interference patterns. Here, we provide a method to tackle this problem by simulating the phase differences that drive the shielding currents and use those to find $I_{\text{c}}\left(B\right)$. To this end, we extend the technique proposed by Clem [Phys. Rev. B 81, 144515 (2010)] to find $I_{\text{c}}\left(B\right)$ for Josephson junctions separating a superconducting strip of length $L$ and width $W$ with rectangular, ellipsoid, and rhomboid geometries. We find the periodicity of the interference pattern ($\mathrm{\Delta }B$) to have geometry-independent limits for $L\gg W$ and $L\ll W$. By fabricating elliptically shaped superconductor–normal-metal–superconductor junctions with various aspect ratios, we experimentally verify the $L/W$ dependence of $\mathrm{\Delta }B$. Finally, we incorporate these results to correctly extract the distribution of critical currents in the junction by the Fourier analysis of $I_{\text{c}}\left(B\right)$, which makes these results essential for the correct analysis of topological channels in thin-film planar Josephson junctions.

We report the first comprehensive microscopic description of the effect of strong correlations and thermal fluctuations on the properties of noncentrosymmetric superconductors in presence of an in-plane Zeeman field. Away from the weak coupling regime the Bardeen-Cooper-Schrieffer theory breaks down and the superconducting transitions are dictated by the pairing field phase fluctuations. Using a nonperturbative numerical technique viz. static path approximation, we demonstrate that short-range fluctuating superconducting pair correlations give rise to Fermi-surface segmentation with direction-dependent pair breaking and hot spots for quasiparticle scattering. A fluctuation-driven finite-temperature topological transition of the Fermi surface is realized, characterized by a shift of the corresponding Dirac point from $\mathbf{k}=0$ to $\mathbf{k}\ne 0$. Our results provide key benchmarks for the thermal scales and regimes of thermal stability of the properties of these systems, which are important for device applications. Our numerical estimates are in fairly good qualitative agreement with the recent differential conductance and quasiparticle interference measurements on ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$ hybrid. A generic theoretical framework for the finite momentum scattering of quasiparticles and the associated spectroscopic features is proposed, which is expected to be applicable to a wide class of superconducting materials.