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and in Toward the controllable quantum state ( H. Takayanagi and J. Nitta Eds) Word Scientific, pagg. 320-327 (2002).

Three-terminal superconducting tunnel devices offer a powerful tool to investigate nonequilibrium phenomena in superconductors. We study the role of the proximity effect in a Nb/Al bilayer forming the intermediate electrode of a stacked structure configuration. The spatial profile of the pair potential along the common electrode and the effective density of states near the detector tunnel barrier are determined by combining the deconvolution of the current-voltage (I-V) injector characteristics and the numerical solution of the microscopic proximity model equations. The Josephson current variation following a steady-state injection was measured down to 1.5 K. Results are discussed in the framework of a simple nonequilibrium model, which leads also to an estimate of the effective relaxation time limiting quasiparticle scattering processes in the Nb/Al bilayer.

Nowadays, tailoring the material properties is essential for advanced product design in engineering systems. The need to provide key information about micro- and macro-structural behaviour of materials, without destructively sectioning the sample, has spurred the development of nondestructive evaluation methodologies. These techniques are required during material production, quality testing of components during manufacturing, and in-service inspection of structural integrity. To ensure the highest possible operational safety along with an economic efficiency, it is necessary to carry out inspections with a high sensitivity and a proven reliability. Due to its unparalleled magnetic field sensitivity over a wide frequency range and large dynamic range, SQUID-based nondestructive evaluation has unique advantages for materials and structures characterization. We will present an overview of eddy-current nondestructive analysis utilizing high T c SQUIDs with an emphasis on examples relevant to the aeronautical industry. These include the detection of deep-lying defects in multi-layer structures of Al–Ti alloys and damage of extremely lightweight graphite/epoxy composites. Both of these can be successfully treated by this approach where conventional electromagnetic probes often fail. In addition some results based on the volume integral formulation, successfully developed to simulate the response of the system to different type of flaws in Al-alloy planar structures and to solve the inverse electromagnetic problem, will be shown.

Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe 0.5Te0.5 and optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides both in the superconducting state are now presented and discussed. Modelling the T-dependence of relaxation times an estimation of both electron-phonon constant and superconducting energy gap in the excitation spectrum of both Fe(Se,Te) and Ba-122 compounds is obtained.

We present characterization of Ba(Fe1-x Cox) 2As2 thin film using a magneto-optical Kerr effect setup based on photoelastic modulation. The magneto optical technique has been used to analyse the thickness and the temperature dependence of the magnetic properties of epitaxial Ba(Fe1-x Cox)2As2/Fe thin films grown on MgO substrate. The first harmonics, of the reflected laser beam intensity from pnictide thin films is compared with a reference Fe thin film. The hysteretic loops of such samples at room temperature and at 5K are presented.

A simple and high-reproducible method for the synthesis of polymer-protected silt-er cluster of controlled size is described. UV-visible spectroscopy has been used for investigating the influence of the aging of the protective poly (vinylpyrrolidone) layer on the cluster growth rate at different reaction temperatures and poly (vinylpyrrolidone)/ethylene glycol weight ratios. The obtained results show that the aging tune of the polymeric stabilizer solution plays a fundamental role in the reproducibility of the cluster growth process. A model for the metal cluster formation-grow process is also proposed.

The potential of superconducting devices as radiation detectors spans over a large variety of applications such as particle detections, X-my astronomy, dark matter search, mass spectrometry. In particular, Superconductive Tunnel Junctions (STJ) offer great possibilities for high energy resolution and fast response combined with an excellent radiation hardness. In the frame of our research activity all refractory Nb junctions with low leakage current have been realized by different preparation processes. New experiments on injection-detection experiments with a discussion of the underlying nonequilibrium superconductivity involved are reported. Interesting results have been obtained also on the delicate issue of junction geometry which plays a fundamental role in determining the detector performances. The required high transparency of the tunneling barrier for a higher charge collection implies the occurrence of Josephson effect. This, in turn, represents a drawback for the bias requirements of the detector which has to operate in the quasiparticle tunneling regime when devoted to energy spectroscopy. Suppression of zero-voltage current and current steps at a finite voltage can be realized by a proper junction geometry. Experimental results are reported which show a great potential for further improvements of junction detectors.

A theoretical model to describe the spectral response of SIS tunnel junctions under irradiation in the presence of diffusion and edge losses of non-equilibrium quasiparticles has been developed. The model takes into account the back-tunnelling effect, in the general case of asymmetrical rates of tunnelling, gamma(12) not equal gamma(21). It provides a quantitative estimate of parameters like the edge reflectivity, the diffusion length and the diffusion coefficient, which are important parameters for a diagnostic of STJ-detectors. The predictions of the theory are compared with an experimental spectrum obtained from a Nb-based STJ. (C) 2000 Elsevier Science B.V. All rights reserved.

International audience; We present here recent results on detection of surface and subsurface artificial features in Al-Ti planar structures, to show current performance of our eddy-current nondestructive evaluation system based on HTc SQUIDs. The anomalous magnetic fields generated by flaws with known electromagnetic characteristics have been modeled by three-dimensional codes based on finite element method and volume integral formulation and developed for the investigated problem. Both numerical solutions have correctly predicted the shape of the complicated magnetic field response which is mainly the result of the shape of the defect, the geometry of the inducing coil and the characteristics of the SQUID gradiometer.

The efficiency of the charge collecting mechanism of a radiation detector based on superconducting tunnel junctions depends strongly on the competition between tunneling and losses of quasiparticles in the electrodes of the junction. In this article we discuss the loss of quasiparticles, both in the bulk and at the edges of the electrodes. The quasiparticle loss taking place during their diffusion in the junction limits the amount of charge carriers tunneling and thereby the energy resolution of the detector. A charge amplification process caused by multiple tunneling of quasiparticles is beneficial to higher signal to noise ratios. Such multiple tunneling is always present in the case of junctions based on symmetrical electrodes. In this article we present a theory for the detector response which takes into account both multiple tunneling and the diffusion of quasiparticles. A model is presented for the spectral response of the detector based on four parameters one of which is the degree of back-tunneling present in the device. Analytical solutions of the model for rectangular and circular junctions are discussed. The predictions of the theory are compared with an experimental pulse height spectrum obtained on a Nb/Al2O3/Nb junction. © 1999 American Institute of Physics.

The attainment of high-quality Josephson junctions on electro-optical substrates constitutes a key step toward the fabrication of functional superconducting opto-electronics. We report on the fabrication of high-quality all-refractory niobium superconducting tunnel junctions deposited on X-cut LiNbO3 single crystals. A high quality factor Vm = 0.7 ?IgR2 mV (where ?Ig is the current jump at the sum gap voltage and R2 mV the static resistance at V = 2 mV) up to 61 mV has been measured at T = 4.2 K. The very low contribution of subgap leakage currents is demonstrated by the BCS-like quasiparticle current measured at V = 0.1 mV down to T = 1.9 K.

superconducting thin films by Mo75Re25 single-target magnetron sputtering at various temperatures on sapphire substrates were prepared. Sharp superconductive transitions andT c values above 10 K were achieved, and the deposited films showed a good metallic behavior. According to structural analyses (EDS and X-rays), all the samples have the same stoichiometry as the target and, even at low deposition temperatures, they exhibit theA15 phase. From measured residual resistivities and superconducting critical temperatures the theoretical BCS surface resistances were estimated and compared to those o Mo60Re40 films, presented in a previous work. Both these alloys, in fact, being characterized by low surface resistances and high critical fields, are good candidates as coating films in r.f. cavities.

Current-voltage characteristics and dV/dI curves of a YBCO/Nb bulk junction at 4.2 K are reported. The presence of the Josephosn effect is clearly shown. The dV/dI spectrum shows sharp structures which can be related to the Niobium and the YBCO energy gaps Δnb and ΔNb and ΔYBCO. A value ΔYBCO = 26.2 ± 1.0 meV can be estimated, confirming the strong coupling character of such a compound.

Coherent passage of Cooper pairs in a Josephson junction (JJ) above the liquid nitrogen temperature has been the first impressive revolutionary effect induced by high critical temperature superconductors (HTS) in the domain of the study of Josephson effect (JE). But this has been only the start. A d-wave order parameter has lead to significant novel insights in the physics of the JE turning into a device the notion of a ?-junction. Spontaneous currents in a frustrated geometry, Andreev bound states, long-range proximity effect have rapidly become standard terms in the study of the JE, standing as a reference bench for conventional systems based on low critical temperature superconductors (LTS) and inspiring analogies for junctions based on novel superconductors discovered in the meantime. The extreme richness of the physics of HTS JJs has not been adequately supported by the expected impact in the applications, the main reason lying in the complexity of these materials and in the consequent unsatisfactory yield and reproducibility of the performances of the JJs within the required limits. The continuous progress in material science, and specifically in the realization of oxide multi-layers, and in nanotechnologies applied to superconductors, accompanied by the advances in a better understanding of the properties of HTS and of HTS devices, has as a matter of fact opened possible novel scenarios and interest in the field. We intend to give a brief overview on interesting new problems concerning HTS JJs of inspiration also for other systems. We also review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation, which characterize a large majority of low critical current Josephson devices and are therefore an unavoidable consequence of nanotechnology applied more and more to Josephson devices.

The moderately damped regime (MDR) in a Josephson junction (JJ) is quite distinct from the well-known case of underdamped systems and quite common in devices characterized by low critical currents. Measurements of switching current distribution (SCD) are a direct way of discriminating the phase dynamics and the transport also in nontrivial cases of moderate damping. MDR is going to be more and more common with advances in nanopatterning superconductors and in materials science finalized to build hybrid devices. We report on measurements of SCDs, both in the thermal and quantum regime, on moderately damped NbN/MgO/NbN JJs. A transition from the Thermal Activation (TA) regime to Macroscopic Quantum Tunnelling (MQT) occurs at about 90 mK, while above 1.6 K a phase diffusive dynamics is clearly observed. The combined analysis of these regimes through numerical simulations allowed us to extract the fundamental parameters, which completely characterize the phase dynamics of such moderately damped JJs.

Weak ferromagnet/superconductor (F/S) proximity bilayers have been fabricated and characterized for basic physics studies concerning the ultrafast carrier dynamics in layered materials. The normalized reflectivity change (Δ R / R ) as a function of the time delay between the pump and the probe laser beams has been measured in F/S heterostructures formed by a low critical temperature superconductor (Nb) with a NiCu overlayer, and a high T c superconductor (YBCO) covered by Au/NiCu and manganite (LSMO) overlayers. The attention is mainly focused to the investigation of nonequilibrium excitation dynamics inside different bilayers in the low temperature region. The presence of the weak ferromagnetic overlayer produces faster optical relaxation times with respect to sole superconducting films. The results are promising in view of potential applications as ultrafast kinetic inductance superconducting photodetectors as confirmed by preliminary time-resolved photoimpedance experiments on both Nb and NiCu(21 nm)/Nb(70 nm) samples.