RESISTIVE TRANSITION AND PARACONDUCTIVITY IN YBACUO-AG COMPOSITES

We report on detailed measurements of the resistive transition of nonrandom superconducting composites formed by YBa2Cu3O7 mixed with silver. We study samples with 6, 11 and 23 wt% Ag. These systems are representative of intrinsically granular superconductors. The pure YBaCuO system is also measured. Our results show that the transition proceeds in two steps. First, superconductivity stabilizes in a subsystem (the grains) in a temperature T(c) given approximately by the maximum of the temperature derivative of the resistivity. Critical current density measurements near T(c) reveal effects of pair-tunneling. Above T(c), the analysis of the paraconductivity data indicates that the superconducting thermal fluctuations occur in a space having fractal topology. Close to the zero resistance temperature, T(co), a power law regime is found, which suggests the occurrence of a percolation process leading to long-range superconducting order through the metallic weak-links.