Superconductivity fluctuations in Bi(Pb) based granular ceramics superconductors: evidence for fractal behavior

We report precise measurements of the electrical resistivity in three different Bi(Pb) based granular ceramics superconductors. We show that a single critical exponent (5/6) describes the superconductivity fluctuations. Such a critical exponent indicates a fractal behavior of the superconducting path. Our results thus indicate a strict two dimensional fluctuation percolation set (below the superconductivity onset temperature 195 K down to T(c)), and provide some proof for Tarascon et al. "shell conductivity path" hypothesis. We estimate the shell thickness to be of the order of 10 angstrom