Temperature dependence of the critical current density in proton irradiated YBCO films by magneto-optical analysis.

In this paper we present a magneto-optical analysis of local current densities in YBCO films, before and after 3.5 MeV proton irradiation. The main issue consists into measuring and interpreting the temperature dependence of the critical current density (J_c) in samples with different, increasing defect density. Proton irradiation adds more point defects into the as-grown films. The new defect density as well as the related strain-induced modifications of the order parameter are pushed in our experiment up to temperature-modulated damage thresholds. First of all model-independent J_c data were analysed in the framework of different pinning models, all of them based on mechanisms related to the temperature induced change of the effective pinning centre distribution as well as to the shape of single pinning wells. It turns out that in such a framework the fit parameters are, generally speaking, not suitable to interpret the changes of the pinning landscape across the whole investigated temperature range. Then a model based on a vortex distribution across the whole sample, resulting in a current density that mirrors the current through a defect-modulated average short Josephson junction (JJ) row, is successfully tried. The J_c dependence in the whole temperature range and for all the considered defect densities is accounted for by means of a coherent set of fit parameters. It turns out that the chief quantity that allows applying the JJ formalism to a vortex distribution across the defected matrix is a suitably defined temperature-dependent magnetic thickness of the junctions, which substitutes the usual magnetic penetration in JJs. [ABSTRACT FROM AUTHOR]