G. Costabile

Superradiant emission from an array of long Josephson junctions

Abstract We report measurements on lumped arrays of eight long Josephson junctions, series dc biased and coupled to a microwave transmission line. Fluxon oscillations in different junctions can be mutually locked and, concurrently, the power of the emitted radiation increases according to the conjecture of superradiance.

Microwave oscillator using arrays of long Josephson junctions

The authors report on measurements performed on integrated superconducting devices based on arrays of long Josephson tunnel junctions operating in the resonant fluxon oscillation regime (i.e. biased on the zero field steps). The electromagnetic coupling among the junctions causes a mutual phase-locking of the fluxon oscillations with a corresponding increase of the emitted power and a decrease of the signal linewidth. This phase-locked state can be controlled by means of an external DC bias current and magnetic field. The effect of the generated microwave signal has been observed on a small Josephson tunnel junction coupled to the array via a microstrip transmission line. The feasibility of the reported devices as local oscillators in an integrated microwave Josephson receiver is discussed. >

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.

Influence of the idle region on the dynamic properties of window Josephson tunnel junctions

The processes developed in the last years for the fabrication of high quality, reliable all‐refractory Josephson tunnel junctions open a window in an insulating region, called the idle region, which provides the passivation of the base electrode all around the junction itself. We report experimental data showing that the phase velocity of electromagnetic waves in the junction barrier can be considerably modified by the presence of the insulating region surrounding the window Josephson tunnel junctions. We have investigated this phenomenon in unidimensional junctions, measuring the asymptotic voltage of the Fiske resonances in the presence of an externally applied magnetic field. We have considered the effect of an idle region: (i) along the long junction dimension and (ii) at the extremities of the junction. When comparing the behavior to that of a bare junction, in the first case the phase velocity has been found to increase and to be affected by dispersion, while in the latter case it decreased and was ...

Experimental study of the interaction between fluxon arrays in stacked Josephson junctions

Abstract Stacks of two Nb/Al/AlOx/Nb long Josephson junctions have been investigated experimentally. The fabrication process of the samples provides a connection to their common electrode. We report the first experimental data which show the interaction between fluxon arrays in the two junctions.

Series array of highly hysteretic Josephson junctions coupled to a microstrip resonator

We have tested a new device based on a 12 junction array coupled to a resonator. We have explored the feasibility of the phase lock for all the junctions at the same biasing current, which yields voltage quantization across each junction, eliminating the need to individually bias the junctions. The whole rf structure has been realized by stripline technology. The resonator is fed by a 50‐Ω line and is decoupled from the dc circuit by elliptical low‐pass filters inserted in the bias leads.

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.

Static properties and current steps in one‐dimensional parallel arrays of Josephson tunnel junctions in the presence of a magnetic field

Static and dynamical states in one‐dimensional parallel arrays of small, all refractory Josephson tunnel junctions, are experimentally investigated at 4.2 K. The dependence of the maximum Josephson current on the applied magnetic field in parallel arrays shows asymmetric behavior around secondary maxima which are discussed in terms of a nonuniform applied magnetic field. Current steps and linear branches induced by the applied magnetic field are observed together in the current‐voltage characteristics for the first time in the strongly underdamped regime. The resonant structures appearing in the current‐voltage characteristics of the arrays show features similar to high‐order Fiske modes in a long Josephson junction.