G. Carapella

Josephson effect in Nb/Al2O3/Al/MgB2 large-area thin-film heterostructures

We report the demonstration of dc and ac Josephson effects in Nb/Al2O3/Al/MgB2 thin-film heterostructure. The heterostructure exhibits moderately hysteretic current–voltage characteristic with a dc Josephson current branch and regular microwave-induced Shapiro steps. From conductance spectrum, a gap of about 2 meV at 7.7 K is estimated for the proximized surface of MgB2 electrode.

Experimental investigation of flux motion in exponentially shaped Josephson junctions

We report an experimental and numerical analysis of exponentially shaped long Josephson junctions with lateral current injection. Quasilinear flux flow branches are observed in the current-voltage characteristic of the junctions in the absence of a magnetic field. A strongly asymmetric response to an applied magnetic field is also exhibited by the junctions. Experimental data are found in agreement with numerical predictions and demonstrate the existence of a geometry-induced potential experienced by the flux quanta in nonuniform width junctions.

Nonlinear current-voltage characteristics due to quantum tunneling of phase slips in superconducting Nb nanowire networks

We report on the transport properties of an array of N∼30 interconnected Nb nanowires, grown by sputtering on robust porous Si substrates. The analyzed system exhibits a broad resistive transition in zero magnetic field, H, and highly nonlinear V(I) characteristics as a function of H, which can be both consistently described by quantum tunneling of phase slips.

Nonequilibrium fluctuations as a distinctive feature of weak localization

Two-dimensional materials, such as graphene, topological insulators, and two-dimensional electron gases, represent a technological playground to develop coherent electronics. In these systems, quantum interference effects, and in particular weak localization, are likely to occur. These coherence effects are usually characterized by well-defined features in dc electrical transport, such as a resistivity increase and negative magnetoresistance below a crossover temperature. Recently, it has been shown that in magnetic and superconducting compounds, undergoing a weak-localization transition, a specific low-frequency 1/f noise occurs. An interpretation in terms of nonequilibrium universal conductance fluctuations has been given. The universality of this unusual electric noise mechanism has been here verified by detailed voltage-spectral density investigations on ultrathin copper films. The reported experimental results validate the proposed theoretical framework, and also provide an alternative methodology to detect weak-localization effects by using electric noise spectroscopy.

Bistable Abrikosov vortex diode made of a Py–Nb ferromagnet-superconductor bilayer structure

We report magnetotransport measurements on Py/Nb bilayers patterned in a Hall strip geometry, with Permalloy allowing a weak stripe domain regime. After application of a weak magnetic field in the plane of the bilayer perpendicular to the transport current, the strip behaves as a bistable superconducting diode and a nonvolatile superconducting valve as well. The observed behavior can be accounted for by the stray fields at the edges of the strip that modulate the distribution of the stray fields from the stripe domains in the ferromagnetic layer, generating an asymmetric and bistable magnetic forces background for the Abrikosov vortices moving in the superconducting layer.

Experimental realization of a relativistic fluxon ratchet

We report the observation of the ratchet effect for a relativistic flux quantum trapped in an annular Josephson junction embedded in an inhomogeneous magnetic field. In such a solid state system mechanical quantities are proportional to electrical quantities, so that the ratchet effect represents the realization of a relativistic-flux-quantum-based diode. Mean static voltage response, equivalent to directed fluxon motion, is experimentally demonstrated in such a diode for deterministic current forcing both in the overdamped and in the underdamped dynamical regime. In the underdamped regime, the recently predicted phenomenon of current reversal is also recovered in our fluxon ratchet.

Magneto-transport properties of curved mesoscopic superconducting strips

Transport properties of curved mesoscopic superconducting strips are investigated in the framework of the time-dependent Ginzburg?Landau formalism. The geometries of the superconducting strips considered here are either a section of a cylindrical shell or a full cylindrical shell in which the magnetic field is applied perpendicular to the axis. The cylindrical section can exhibit considerably asymmetric transport properties, making it potentially interesting as a sub-micrometer scale superconducting current rectifier. The full cylindrical surface exhibits well developed dissipative branches in the voltage?current curves, that can be accounted for by kinematic vortex?antivortex phase slip lines. Such kinds of phase slip lines cause voltage oscillations in a frequency range higher than the one associated with the familiar flux flow regime.

Experimental investigation of double-barrier SINIS (superconductor-insulator-normal-insulator-superconductor) structures

Double-barrier Nb/AlOx /Al/AlOx /Nb devices were fabricated and investigated in direct-current and alternating-current (ac) electromagnetic fields. The occurrence of current singularities and of the ac Josephson effect at T = 4.2 K are demonstrated. In a three-terminal configuration, a transistor-like behaviour of the structure is also observed.

Current driven transition from Abrikosov-Josephson to Josephson-like vortex in mesoscopic lateral S/S’/S superconducting weak links

Vortices are topological defects accounting for many important effects in superconductivity, superfluidity, and magnetism. Here we address the stability of a small number of such excitations driven by strong external forces. We focus on Abrikosov-Josephson vortex that appears in lateral superconducting S/S’/S weak links with suppressed superconductivity in S’. In such a system the vortex is nucleated and confined in the narrow S’ region by means of a small magnetic field and moves under the effect of a force proportional to an applied electrical current with a velocity proportional to the measured voltage. Our numerical simulations show that when a slow moving Abrikosov-Josephson vortex is driven by a strong constant current it becomes unstable with respect to a faster moving excitation: the Josephon-like vortex. Such a current-driven transition explains the structured dissipative branches that we observe in the voltage-current curve of the weak link. When vortex matter is strongly confined phenomena as magnetoresistance oscillations and reentrance of superconductivity can possibly occur. We experimentally observe these phenomena in our weak links.

A single Abrikosov vortex trapped in a mesoscopic superconducting cylindrical surface

We investigate the behaviour of a single Abrikosov vortex trapped in a mesoscopic superconducting cylindrical surface with a magnetic field applied transverse to its axis. In the framework of the time-dependent Ginzburg-Landau formalism we show that, provided the transport current and the magnetic field are not large, the vortex behaves as an overdamped quasi-particle in a tilted washboard potential. The cylindrical thin strip with the trapped vortex exhibits E(J) curves and time-dependent electric fields very similar to the ones exhibited by a resistively shunted Josephson weak link.