Magneto-optical analysis of the critical current density dependence on temperature in proton irradiated YBCO films

A quantitative magneto-optical analysis of YBCO thin films, as grown and irradiated with 3.5 MeV protons, is presented. Induction field maps are drawn in the temperature range 5–70 K and, by means of the Biot–Savart law inversion, the local current density distributions are evaluated. The paper is focused on the temperature dependence of the critical current densities (Jcs) before and after irradiation at increasing fluence. Temperature-modulated damage thresholds are reached. To account for the Jc dependence on temperature for all the considered defect densities and in the whole investigated temperature range, a model based on a Josephson junction formalism is proposed. Namely, the vortex distribution across the whole sample results in a current density that mirrors the current through an average short Josephson junction (JJ) row. The JJ length is modulated by the defect distribution. By applying this model, a satisfying agreement between the experimental data and the model is found in the whole temperature range, both before and after irradiation. A suitably defined temperature-dependent magnetic thickness of the junctions is the chief quantity that allows applying this JJ formalism to the vortex distribution across the defected matrix.