In the framework of the model of multiple Andreev reflections of quasiparticles (Averin-Bardas model) the current-voltage characteristics and the dI/dV-spectra of dynamic conductivity of ballistic ...contacts "superconductor - normal metal - superconductor" (SNS) in high transparency mode (for D = 0,9÷0,98) were calculated. In this mode the energy gap of superconductors comprising SNS-contact is oscillatory in the energy range 60-100 meV due to the strong electron-phonon interaction. The features of subharmonic gap structures are revealed in the considered contacts at voltages V4 = 2Δ/4e, VS=2Δ/5e, V6=2Δ/6e and V7=2Δ/7e as transforming minimums into maximums at the increase of transparency coefficient from 0,9 to 0,98 (or reduction of the barrier height from 0,33 to 0,144). The results satisfactorily correlate with experimental data.
We report new measurements of microwave-induced perturbations of the sub-harmonic energy gap structures in the current–voltage characteristics of superconductor–semiconductor-superconductor ...junctions. Around the sub-gap bias voltages associated with the enhanced quasi-particle transfer mediated by multiple Andreev reflection processes we observe microwave induced satellites, shifted in voltage by multiples of
hf/
en, where
hf is the photon energy and
n is the number of quasi-particle traversals as determined by the Andreev processes. The observed behavior is the analogue of the so-called photon-assisted tunneling but here associated with the multiple Andreev reflections.
After the pioneering work of Giaever Phys. Rev. Lett. 5 (1960) 147. 1 and Shapiro in the 1960s, tunneling spectroscopy has become an important method for studying different properties of ...superconductors, such as the superconducting gap and the density of states. At the interface with a superconductor there is a variety of tunneling processes depending on barrier properties and the type of materials in both sides of the junction. Among other phenomena, we have single electron (Giaever) tunneling, cooper pairs (Josephson) tunneling B.D. Josephson, Phys. Rev. Lett. 1 (1962) 251. 17 and mixed (Andreev-Saint-James reflections A.F. Andreev, Zh. Eksp. Teor. Fiz. 46 (1964) 1823, Saint-James D., J. Phys 25 (1964) 899) tunneling. In high-Tc superconductors these phenomena are enriched by the anisotropy of the energy gap, which adds new features, like the so-called ‘Andreev-Saint-James bound states’, which originate the zero bias conductance peak. These peculiarities add to this technique the capability of studying the symmetry of the superconducting gap and the normal state pseudogap. In this report we present a brief review of tunneling spectroscopy applied to high-Tc superconductors and some results about the symmetry of the superconducting gap, supporting the hypothesis of a quantum critical point, which may be at the origin of the superconductive transition.
We explore local Andreev reflection and crossed Andreev reflection in a normal metal (N)–superconductor (S)–normal metal (N) hybrid junction based on the α−T3 lattice. Initially, both the left and ...right sections feature dice lattice structures (α=1). We compute probabilities for electron reflection, local Andreev reflection, electron transmission, and crossed Andreev reflection, as well as local conductance, electron transmission conductance, and crossed Andreev reflection conductance. Subsequently, we analyze structures with the left side featuring dice lattice and the right side graphene (α=0), as well as the reverse configuration. Our findings reveal that crossed Andreev reflections are more prevalent in the graphene–superconductor–dice lattice (GSD) junction, particularly with moderate superconducting region lengths, larger electron incidence angles, and lower electron incidence energy.
•Investigate local Andreev reflection and crossed Andreev reflection within a normal metal-superconductor-normal metal hybrid junction based on the α−T3 lattice.•Explore scattering processes in NSN hybridized junctions, with both the left and right sections featuring a dice lattice structure (α=1).•Subsequently, we evaluate two configurations: one with dice lattice on the left side and graphene (α=0) on the right side, and the reverse configuration.•The transport properties of these three junctions are systematically compared.
In recent years, the study of heat to work conversion has been re-invigorated by nanotechnology. Steady-state devices do this conversion without any macroscopic moving parts, through steady-state ...flows of microscopic particles such as electrons, photons, phonons, etc. This review aims to introduce some of the theories used to describe these steady-state flows in a variety of mesoscopic or nanoscale systems. These theories are introduced in the context of idealized machines which convert heat into electrical power (heat-engines) or convert electrical power into a heat flow (refrigerators). In this sense, the machines could be categorized as thermoelectrics, although this should be understood to include photovoltaics when the heat source is the sun. As quantum mechanics is important for most such machines, they fall into the field of quantum thermodynamics. In many cases, the machines we consider have few degrees of freedom, however the reservoirs of heat and work that they interact with are assumed to be macroscopic. This review discusses different theories which can take into account different aspects of mesoscopic and nanoscale physics, such as coherent quantum transport, magnetic-field induced effects (including topological ones such as the quantum Hall effect), and single electron charging effects. It discusses the efficiency of thermoelectric conversion, and the thermoelectric figure of merit. More specifically, the theories presented are (i) linear response theory with or without magnetic fields, (ii) Landauer scattering theory in the linear response regime and far from equilibrium, (iii) Green–Kubo formula for strongly interacting systems within the linear response regime, (iv) rate equation analysis for small quantum machines with or without interaction effects, (v) stochastic thermodynamic for fluctuating small systems. In all cases, we place particular emphasis on the fundamental questions about the bounds on ideal machines. Can magnetic-fields change the bounds on power or efficiency? What is the relationship between quantum theories of transport and the laws of thermodynamics? Does quantum mechanics place fundamental bounds on heat to work conversion which are absent in the thermodynamics of classical systems?
Direct detection of superconductivity has long been a key strength of point-contact Andreev reflection. However, its applicability to atomic-scale imaging is limited by the mechanical contact of the ...Andreev probe. To this end, we present a new method to probe Andreev reflection in a tunnel junction, leveraging tunneling spectroscopy and junction tunability to achieve quantitative detection of Andreev scattering. This method enables unambiguous assignment of superconducting origins of current-carrying excitations, as well as detection of higher order Andreev processes in atomic-scale junctions. We furthermore revealed distinct sensitivity of Andreev reflection to natural defects, such as step edges, even in classical superconductors. The methodology opens a new path to nano- and atomic-scale imaging of superconducting properties, including disordered superconductors and proximity to phase transitions.
In this work, we used the double exchange model and studied the transport properties of the electronic states across the superconducting skyrmion crystal (SkX)–normal SkX interface. By calculating ...the energy dispersion and the real-space wave function of the electronic states we found topologically protected edge zero modes in the superconducting SkX nanoribbon. By consideration of the symmetry of the Hamiltonian of the bulk superconducting SkX and its nanoribbon, we found that they belong to the symmetry class D and support Majorana edge modes. To demonstrate the transport properties of these zero edge modes, we used the Green's function method and calculated the Andreev reflection coefficients of the electronic states across the superconducting SkX–normal SkX interface. Zero-mode-induced Andreev reflection peaks are demonstrated and also reflected in the tunneling current. Single-channel conductance is quantized at 2e2/h and 4e2/h for two-fold and four-fold degenerate Majorana bound states, respectively.
•The non-equilibrium Green's function method is extended to the double-exchange model of the superconducting skyrmion crystal.•Majorana states at the edges of the infinitely long superconducting skyrmion crystal nanoribbon are found.•Transport properties of the superconducting skyrmion crystal—normal skyrmion crystal junction are investigated.
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