A. Barone

A new fabrication process of superconducting Nb tunnel junctions with ultralow leakage current for X-ray detection

A new fabrication process is presented to make Nb/Al/AlOx/Nb superconducting tunnel junctions with ultralow subgap leakage current at low temperatures below 4.2 K for X-ray detection. A Nb anodization process has been adopted to make electric insulation at the periphery of an ultrathin AlOx tunnel barrier through use in combination with the conventional SiO self-aligned insulation process. The temperature dependence of the subgap current of the junctions fabricated by the present process is in good agreement with that predicted by the Bardeen-Cooper-Schrieffer (BCS) theory, while junctions made by the conventional process show a weak temperature dependence of the subgap current. In addition, the Vm value of the junction fabricated by the present process was found to be more than 3000 mV at 1.2 K. It has been clarified that the anodization process introduces improvements to make the subgap leakage current extremely small in the temperature range from 4.2 K down to 1.2 K.

Thermal hopping and retrapping of a Brownian particle in the tilted periodic potential of a NbN/MgO/NbN Josephson junction

We report on the occurrence of multiple hopping and retrapping of a Brownian particle in a tilted washboard potential. The escape dynamic has been studied experimentally by measuring the switching current distributions as a function of temperature in a moderately damped NbN/MgO/NbN Josephson junction. At low temperatures the second moment of the distribution increases in agreement with calculations based on Kramers thermal activation regime. After a turn-over temperature T*, the shape of the distributions starts changing and width decreases with temperature. We analyze the data through fit of the switching probability and Monte Carlo simulations and we find a good agreement with a model based on a multiple retrapping process.

Superconducting device with transistor-like properties including large current amplification

We have fabricated and studied a stacked superconducting double tunnel junction device with transistor-like properties. The intermediate electrode is a bilayer consisting of a Nb film together with an Al film that acts as a quasiparticle trap. Large current gains of more than 50 are observed at 4.2 K when the Al layer is normal. The operation is highly directional. Results are explained on the basis of trapping of quasiparticles from a superconductor into a normal metal, together with a conversion of relaxation energy into electronic excitations. Similar devices should have wide applications in low-temperature measurement and detection systems.

Quantum crossover in moderately damped epitaxial NbN/MgO/NbN junctions with low critical current density

High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d=1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm

Superconducting NbNbxOyPb tunnel junctions as high resolution detectors for nuclear spectroscopy: Preliminary results

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Nb-based Josephson junction devices for nuclear radiation detection: Design and preliminary experimental results

have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN.

Vortices in layered superconductors

Abstract Preliminary experimental results are reported on the development of an energy sensitive detector based on a superconducting tunnel junction. High resolution is expected due to the low value of the energy ϵ0 required per collected elementary charge and to the uniform response which can be obtained with suitable junction geometry. Experimental data are consistent with a value of ϵ0 ⩽ 20 meV.

Non Destructive Evaluation of Advanced Composite Materials for Aerospace Application Using HTS SQUIDs

In this work we apply the Electrotechnical Laboratory Josephson junction fabrication technology to realize an integrated device for nuclear radiation detection. We have integrated in a single 5×5 mm2 chip 240 Josephson tunnel junctions (JTJs) having various geometrical configurations and interconnections, in order to investigate both the effects of various detection geometrical configurations and the fabrication process uniformity and reproducibility. The junction quality was extensively tested by measurements of the current‐voltage characteristics, down to T=1.2 K, showing an unexpected saturation of the Vm quality factor. Preliminary results concerning the response to α‐particle radiation of these samples have been obtained. Pulse height and rise time spectra of both ‘‘island’’ type and series connected JTJs are reported. Signals generated by nuclear radiation have been observed at temperatures up to 2 K, where a reduction of the collected charge occurs because of the enhancement of the recombination ra...

Magnetic properties of annular Josephson junctions for radiation detection: Experimental results

The vortex structure in high-Tc superconductors is investigated on the basis of the anisotropic structures of these materials. An analysis is developed in order to determine the GinzburgLandau expression of the free energy for the layered structure. The two cases ofξ(T)≫d andɛ(T)≪d corresponding to the existence of Abrikosov and Josephson vortex types are discussed.

Escape dynamics in moderately damped Josephson junctions (Review Article)

The main advantage of a HTS SQUID magnetometer in NDE applications is represented by its unrivalled magnetic flux sensitivity down to very low frequencies, which allows the detection of weak magnetic field variations due to defects also in materials characterized by a very low electrical conductivity. The imaging obtained by means of the magnetic flux variations can be an useful technique for an easier interpretation of SQUID magnetic responses getting along without post-processing algorithms and independent of the operator. Therefore, the SQUIDs NDE system output is compatible with the other conventional non destructive testing equipment for data fusion of aircraft inspections.