C. Gandini

MgB2 thin films on silicon nitride substrates prepared by an in situ method

Large-area MgB2 thin films were deposited on silicon nitride and sapphire substrates by co-deposition of Mg and B. After a post-annealing in Ar atmosphere at temperatures between 773 and 1173?K depending on the substrate, the films showed a critical temperature higher than 35?K with a transition width less than 0.5?K. The x-ray diffraction pattern suggested a c-axis preferential orientation in films deposited on amorphous substrate. The smooth surface and the good structural properties of these MgB2 films allowed their reproducible patterning by a standard photolithographic process down to dimensions of the order of 10??m and without a considerable degradation of the superconducting properties.

Structural and electrical characterisation of Mo films for transition-edge sensors

Detectors based on transition-edge thermometers are under investigation for material analysis and space observations thanks to their high-energy resolution. However, fabrication of stable structures using superconducting/normal proximity bilayer is not trivial. For example, the characteristics of thermometers made of Al/Ag are quickly degraded by interdiffusion. In this work, we report details on fabrication and characterisation of Mo films and Mo/Ag bilayers that being immiscible and non-reactive with each other should have a long time stability. First results show that bilayers with transition widths of 1 mK are achievable, adequate to realise high sensitivity thermometers.

Properties of bilayers based on Mo films for transition-edge sensors

Transition-edge sensors (TES) are promising devices for materials science and space applications due to their spectroscopic capabilities and good count rates. In this work, we report details on fabrication and characterization of Mo films and Mo/Ag bilayers that, being immiscible and nonreactive with each other, should have a long time stability. Both single superconducting films and bilayers of normal-superconducting materials have been deposited in UHV by e-gun. Structural and morphological properties have been investigated by x-ray diffraction and AFM. Bilayers with transition widths of 1 mK are achievable, adequate to realize high sensitivity thermometers.

Current noise in stationary conditions in MgB 2 thin films

The present paper concerns the analysis of the current noise taken in stationary conditions at different stages of the transition process in MgB2 thin films. The specimen, during its transition, reaches an intermediate state characterized by resistive and superconductive regions. Except in the case where the specimen resistance is lower than about one tenth of its value at the end of the transition, both the amplitude and frequency behavior of the relative noise power spectra remain rigorously constant at different stages of the transition curve, until the transition is completed and the fully normal state is achieved [1]. A different picture appears at the beginning of the transition, where the specimen resistance increases slowly with the temperature, before reaching the steepest part of the transition curve. Contrarily to the previous case, now the specimen reaches a mixed state, and the noise is produced by depinning and motion of fluxoids. Actually the amplitude of the reduced noise power spectrum becomes very sensitive to both to the magnetic field and the specimen resistance, and is largely dominant when this resistance tends to zero.

Characterization of the Resistive Transition of

An original model for the interpretation of the noise produced during the resistive transition of disordered granular superconductive films, induced by a slow temperature change, has been recently developed and tested on MgB2 films. Both the amplitude and frequency behavior of the noise power spectrum, simulated on the basis of this model, are in very good agreement with the experimental data, practically without the introduction of adjustable parameters. The model is based on the onset of correlated transitions of large sets of grains, forming resistive layers through the film cross-section area during the transition process. The strong non-linear behavior and correlation of the grains produces abrupt resistance variations, giving rise to the large noise, of the 1/f3 type, observed in experiments. Presently this model is tested under more general conditions, producing the resistive transition under an external magnetic field. The results show that the field reduces the grain critical current density but does not change the mechanism of the transition process. An alternative transition model, based on fluxoids depinning and motion, which would produce a much lower, 1/f type transition noise, can thus be excluded by the present analysis.

{\rm MgB}_{2}

A brief review of the non stationary noise produced during the transition process of different types of superconducting materials is presented. Noise analysis evidence that very different mechanisms are at the base of the transition in monocrystalline, weak‐link dominated or metallic‐like granular high temperature superconductors. Models of the transition process which are in good agreement with the experimental results will be also given and discussed.

Nanogranular Films by Current Noise Analysis

We present a set of experimental results concerning the current noise produced during the resistive transition in a MgB2 polycrystalline thin film. Measurements of the power spectrum of the current noise, observed when the temperature is slowly changed across its critical value, are given and discussed. During the transition a large electrical noise component is generated, having a power spectrum of the 1/fn type (n ≈ 3) over a quite wide range of frequencies. This noise may be considered as generated by the abrupt creation of resistive strips across the specimen constituted by grains which have undergone the resistive transition. A computer model, based on this assumption, has been developed to simulate the resistive transition and to evaluate the noise power spectrum. The specimen has been represented as a two dimensional film characterized by randomly oriented grains having slightly different critical currents characterized by a gaussian distribution. When the temperature is incresed and reaches its critical value, resistive strips are formed according to a percolative process, giving rise to resistance steps which are at the origin of the noise. The theoretical results obtained by this model, concerning both the shape and intensity of the noise power spectrum, are compared with the experimental ones directly measured on the specimen.