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Ni microparticles in nonmagnetic elastic matrices have a magnetic behavior which depends on particles percentage, temperature, intensity of magnetizing field and the induced strain. In particular, the elastic properties of the matrix give nonconventional effects of deformation on magnetization. When a compression is applied and the magnetizing field is high, i.e., near saturation, it becomes evident the importance of magnetic particles density increment. On the other hand, if the magnetizing field is little, below 1/4 of saturation field, the effect of elastic matrix deformation on particles' orientation and the consequent change of the magnetization intensity is prominent. We named the last peculiar behavior as ``elastomagnetic'' effect and a simple model to explain it is reported. A threshold field as a function of the particles concentration is experimentally determined which gives the prominence of one or the other of the above described mechanisms. The experimental results and the theoretical model explain that even when the intrinsic magnetoelastic effect is negligible one can find magnetoelastic effects due to coexistence of phases with different geometry, elastic and/or magnetic properties.

Interest in superconducting transistors is due to some potential benefits of these devices, such as the possibility to have large current gain and bandwidth at low temperatures. In this work we present experimental results concerning with a new superconducting 3-terminal device showing true transistor-like properties at T=4.2K. It is a stacked double tunnel junction structure where the intermediate film is a bilayer of superconducting Nb with an Al quasiparticle trap which can be either in the superconducting or in the normal metal state. Current amplification factors of up to 2.0 are observed at a temperature of 4.2 K when the Al is superconducting. When the Al is in the normal state, large current gains of more than 50 are observed. The operation is highly directional, i.e. practically no effect is observed when the roles of the injector and detector junctions are reversed. The results are explained according to the recently proposed QUAsiparticle TRApping TRANnsistor model.

Tunnel quasi-particle injection into a superconducting film provides useful information on the nonequilibrium state inside the perturbed superconductor as well as on the potential application to electronic devices. Three terminal injector-detector superconducting devices have a. long history in non-equilibrium superconductivity. In the recent past non-equilibrium phenomena have attracted again considerable attention because of many superconducting based detectors involve processes substantially non-equilibrium in nature. The possibility of using a stacked double tunnel junction to study the influence of Iron-equilibrium superconductivity on the Josephson critical current is now considered. All experimental study of the effect of quasi-particle injection on the Josephson current both in steady-state and pulsed experiments down to T=1.2 K is presented using 3 terminal Nb-based stacked double tunnel devices. The feasibility of a new class of particle detectors based on the direct measurement of the change in the Josephson current following the absorption Of 3 X-my quantum is also discussed in terms of non-equilibrium theories.

Josephson tunnel junctions are well known as low temperature electronics devices. However, they are also important as very powerful sensors. A novel class of sensors is constituted by radiation detectors based on superconducting tunnel junctions, which could lead in next future to an advanced instrumentation with an interesting applicative future. So far this area is restricted to low-temperature superconductors. Several superconducting materials have been considered. Among them, Niobium is very attractive because of its highest critical temperature T c We have measured detailed X-ray detection characteristics for Nb-based tunnel junction made with different fabrication processes. We will discuss the difference in the X-ray response in terms of the relevant junction's parameters, underlying the problems of the niobium as suitable material for this kind of device.

Eddy-current technique based on high temperature SQUID gradiometer is a promising tool for nondestructive evaluation of material defects which result from the machining process or ageing of electrically conductive structures. We present here the outline of a new project supported by the italian Istituto Nazionale per la Fisica della Materia (INFM), concerning the realization of such a prototype NDE instrumentation. We discuss both the NDE measurement set-up (in particular, the design of the cryostat and thermal shielding, and the x-y sample translation system) and the design of a YBCO SQUID gradiometer based on bi-crystal technology.

We present optical properties of nanowires consisting of both proximitized superconductor/ferromagnet bilayers of NbN/NiCu and pure superconducting NbN layers. Thin NbN/NiCu hetero-structures were deposited on MgO(100) substrates by in situ dc magnetron sputtering, while nanowires were patterned using standard electron beam lithography and reactive ion etching. Photo-response signals were recorded from 100-nm-wide, three-parallel and six-series NbN(8 nm)/NiCu nanowires by 850-nm pulsed laser irradiation (time width 400 ps, repetition rate ~ 100 MHz). For the latter configuration the fall time of voltage responses was found to be close to 1.7 ns, and it gradually increased with increasing the NiCu film thickness. Moreover, three discrete families of photo-response signals recorded for the three-parallel and six-series nanowires were also observed.

We have designed and tested 5 × 5 mm 2 chips containing island type STJs with different geometries, fabricated on Si substrates in the configuration Nb/AlAlO x /Nb and using a NbN overlayer acting both as wiring and radiation absorber. The aim is to investigate the advantages of the trapping effect due to the Nb layer trap. In fact, for the NbN Nb bilayer the scattering time is much faster than the values calculated for other Nb based bilayers, leading to more efficient trapping processes. The junctions have been characterized by tunneling and Josephson current measurements down to 0.5 K. Experimental results concerning the response of the devices under X-ray irradiation down to 60 mK are presented and discussed.

A theroretical analysis of the behaviour of an STJ detector in the non equilibrium regime induced by some external radiation has been carried, based on the kinetic equations for both junction electrodes. The analytical expression of the tunnel times and of the non equilibrium current has been obtained. The effect of the backtunneling process on the amount of collected charge is shown.

X-ray STJ detector response is analyzed in the framework of the kinetic transport equation for the non-equilibrium quasi-particles. Numerical solutions of the resulting equation which describes the diffusion of excitations in the plane of the junction are presented and discussed. A particular attention is paid to the limits of “small” and “large” junctions, with respect to the effective diffusion length of quasi-particles. An almost linear behaviour is expected, under suitable conditions, for “large” Nb-based devices, in agreement with experiments reported in the current literature. Deviations from linearity are however possible in “small” devices or in case of short electronic mean free path.

The behavior of a superconducting tunnel junction detector (STJ) in the regime of low excitation is described, and the characteristic times of the dynamical response are discussed. The effect of a proximity layer in the STJ is analyzed in terms of the McMillan model. A set of phenomenological equations for the proximized STJ driven out of equilibrium is obtained, showing the importance of the role of the proximity effect in the dynamics of the device.

Superconducting Tunnel Junctions are considered for their use as new nuclear radiation detectors with high energy resolution, for radioastronomy. In fact they have potentiality for 0.05% resolution, more than one order of magnitude better than conventional detectors. The reasons and the advantages of using this tunnel junctions are briefly introduced. A description of the working principle and the basic experimental set up are also outlined. The attention is focused on Nb/AlOx/Nb junction which is presently the state of art of superconductive tunnel junction technology with respect to stability and cyclability. Junctions with high dynamical resistance in the biasing region are in fact necessary to optimize the signal to noise ratio and then improve the high energy resolution. The characterization at low temperatures (down to 1.2 K) of samples fabricated with a special <> geometry is presented to demonstrate the feasibility of their use because they satisfy the rather stringent quality requirements. Finally, preliminary results concerning the junction responsivity to nuclear radiation at relatively high temperature (T=1.5 K) are reported. Although the obtained resolution of 700 eV with 6 keV X-rays is far from the theoretical limit, better results can be expected working with the devices at lower temperatures and improving the structure configuration.

We have designed and tested a 5x5 mm2 chip which includes 24 ''island'' Josephson tunnel junctions (STJs) with square, circular and diamond geometries, two arrays of 100 ''island'' junctions and other detection devices. The junctions were based on Nb-AlOx-Nb trilayers, obtained on Si substrate by a process consisting of underlayer Nb deposition, self-aligned insulation and anodization steps. The junction quality was extensively tested by measurements of the I-V characteristics down to T=1.2 K. I(c)R(n) values up to 2.16 mV have been obtained Preliminary results concerning the response of these samples to 6 keV X-ray radiation are discussed. Effects of the substrate on the detector response have also been observed and reported A rough estimation of the energy resolution gives about 700 eV. The quality of the junctions is such that the electronic noise is the predominant limitation to the ultimate resolution.

In reviewing the large variety of possibilities offered by superconductivity in the small scale applications, it is often neglected the important role which can be played by superconducting tunnel junctions as radiation detectors in physics and astrophysics. The growing interest of such devices in this context lies in the perspective of outstanding performances, partially already demonstrated, in the high energy spectrometry, in the fast discrimination, in the spatial resolution. In this article we shall confine our attention to the first topic. As far as the drawbacks of cryogenic requirements are concerned, it should be point out that conventional semiconductor junction detectors require liquid nitrogen not only during operation but also for storage since those devices are not thermally cyclable. In the other hand helium cryostat technology is dramatically improving and, if confined to a temperature of the order of a few degrees (1–4 K), cryocoolers extremely compact within today can provide the necessary cryogenic environment for space based equipment.

Time-resolved transient pump-probe spectroscopy measurements on proximized ferromagnet/superconductor (F/S) hybrid structures are presented. We focused our attention on both low and high critical temperature superconductors such as Nb and YBCO, while for F the weak-ferromagnetic alloy Ni0.48Cu0.52 has been used. Dynamics of the electron-phonon relaxation process has been investigated as a function of both the temperature and the F-film thickness. In the case of NiCu/Nb bilayers the presence of a thin F overlayer reduces the slow bolometric component of the photoresponse, while in YBCO-based structures faster relaxation times were measured. F/S nanobilayers are very attractive toward the development of novel hybrid superconducting photodetectors.

Some of the main anomalies in conductance characteristics of high-T C tunnel junctions are considered. Mainly, the extreme, depression of gap structures, the presence of conductance peaks in corrispondence to the counterelectrode gap, and «zero-bias anomalies» will be examined. The possibility of application of the proximity McMillan model to describe the behaviour in conductance of high-T C junctions is considered. Discussion and preliminary comparison with experimental data on YBCO junctions are also reported.

Photoluminescence spectra of Au/Ag nanoparticles have been investigated experimentally. The clusters were obtained inside an amorphous polymeric matrix by thermal simultaneous decomposition of both gold and silver mercaptides. The optical characterization in terms of photoluminescence showed a red shift of position of the emission peak as a function of the Ag mercaptide concentration. The red shift is qualitatively explained in terms of the Jellium model along with the quantum size effects.

An extension of the Mie [Ann. Phys. 25, 377 (1908) ] theory for the calculation of optical absorption spectra in bimetallic nanoparticles has been developed. The nanoparticle dielectric function is assumed to be a weighted linear combination of dielectric functions for single particles. Accordingly, analytical expressions for the resonance light absorption frequency, the spectrum maximum value, and the full width at half maximum have been derived, taking into account the interband transitions in the dielectric functions. Experiments have been performed on polymer-embedded Ag/Au nanoparticles prepared by reducing the presence of poly(vinyl pyrrolidone) at room temperature. Experimental absorption spectra have been compared to numerical curves derived by the model in the case of Ag/Au systems at different relative compositions, and they are in good agreement. The time dependence of both Ag/Au nanoparticle size and chemical composition during the growth process has also been investigated.

Nb−Al−AlOx−Al−Nb−Al−AlOx−Nb Josephson Tunnel structures have been fabricated and investigated. The configuration of such structures allows to measure separately top and bottom junctions, i.e. injection and detection junctions respectively. Current-voltage characteristics have been measured at temperatures down to 1.2 K for different values of the injection current. The detection efficiency of the device has been also investigated in order to get information about the non-equilibrium relaxation processes which take pace into a steady-state non-equilibrium Nb layer.

Superconducting Tunnel Junctions are promising devices in next generation nuclear and optical detectors. Unfortunately, a number of relaxation processes in the number of initially produced excitations degrades the performances of such devices. In order to investigate the role of charge loss mechanisms in Nb and/or at the Nb/Si interface we have fabricated three terminal in-plane double tunnel junctions, in which quasi-particles are injected into a Nb common strip and detected at different distances by tunnel junctions. The detection efficiency η decreases of about one order of magnitude from T=4.2K to T=1.2K for any injector-detector distance. The presence of charge trapping centers, activated at low temperatures, can qualitatively explain such a result. The estimation of the quasi-particle diffusion constant D=28±5 cm2/s at T=1.2 K is in agreement with other similar values reported in literature.

Preparation procedure and sample characterization of 112 BiSCCO pellets exhibiting high-Tc superconductivity are discussed. TypicalI–V and dV/dI curves of Nb/BiSCCO point conctact junctions at various temperatures are presented.

Results concerning Nb/BiSCCO point contact junctions (PCJs) are presented. I-V (current voltage) and (dV/dI)-V measurements were performed at various temperatures and contact pressures. Differential resistance measurements performed by a standard modulation technique exhibits structures which can be related to the energy gap of BiSCCO. Features of the derivative plots are discussed in connection with models proposed previously for similar junctions. Nb-Nb PCJs based on sintered pellets of granular Nb have been investigated for comparison. >

We report measurements of the switching induced by a rapidly varying magnetic field in a single, small, fully hysteretic Josephson junction. We demonstrate that the variation of the magnetic field, generated by a current step in a control line deposited on the top of the junction, can control the transitions from Josephson branch to the resistive branch and vice versa. The transition from resistive branch to Josephson branch is not trivial for an hysteretic junction. With a suitable choice of the magnetic pulse shape, a "deterministic flip-flop regime" can be achieved in this system.

Ultrathin microstrips based on polycrystalline NbRe films were investigated in order to preliminarily test the suitability of this material for the realization of superconducting single-photon detectors. The voltage-current characteristics measured on these samples show clear hysteresis. This is a fundamental ingredient for investigating single-photon detection as well as single vortex fluctuation phenomena in 2D NbRe-based devices.

Eddy currents non-destructive evaluation (NDE) consists in the use of electromagnetic techniques through which the inspection is carried out by detecting magnetic anomalies in the material under examination. The exploitation of the very high sensitivity of SQUID superconducting magnetometers to magnetic flux variations makes it an interesting and promising candidate for magnetic field detection. SQUID (superconducting quantum interference device) magnetometry can allow to overcome the traditional limits due to the lack of sensitivity in conventional electromagnetic NDE techniques. In this paper we present a new project concerning the design of a prototype of a high-temperature SQUID-based NDE instrumentation and its mains applications.

The gap voltage shift of Nb Josephson junctions fabricated on different substrates is measured under optical illumination at several conditions of light intensity and chopping frequency. The signal dependence on the chopping frequency for junctions realized on glass substrates is bolometer-like with a strong thermal coupling between film and substrate. The responsivity of a junction of 50 μ m × 20 μ m is 50 V/W in vacuum and at liquid helium temperature, and its time constant is lower than 10 μ s . The linearity of the response on the optical power extends over five order of magnitude. The response of junctions fabricated on silicon substrates is flat versus the chopping frequency and about 100 times lower than a junction on glass with the same incident optical power. The shift of the gap at temperatures between 1.2 K and 4.2 K has been measured to separate the nonequilibrium effects from the heating effects.

We have experimentally investigated the effects of intense proton beam irradiation (up to 10/sup 15/ p/cm/sup 2/) on Josephson junctions and junction arrays. The devices we have studied were realized using state of the art full-Nb technology, employing same materials and thicknesses of common Josephson digital circuit designs. We have analysed in detail the magnetic field dependence of the junction critical current, and the quasiparticle tunneling current, in order to observe possible occurrence of permanent changes produced by the ionizing particles. No evidence of radiation induced damage on the properties of the junctions has been found.

Several approximations are usually introduced in the modelling of current distribution and ac losses in multistrand cables. In particular the mutual induction coupling between different strands is often limited to a local coupling that neglects the mutual induction between strand sectors distant along the cable length. Moreover, the longitudinal variations of the mutual induction coefficients along the cable length are usually neglected. This is usually due to the large memory requirements in the computer implementation. In this paper the effects of these approximations are analysed and discussed in the case of a CIC conductor with straight axis.