SPONTANEOUS CONDUCTIVITY FLUCTUATIONS IN Y-BA-CU-O THIN-FILMS - SCALING OF FLUCTUATIONS, EXPERIMENTAL-EVIDENCE OF PERCOLATION AT THE SUPERCONDUCTING TRANSITION

Spontaneous conductivity fluctuations in high-Tc superconducting films on sapphire and SrTiO3 substrates have been measured through the superconducting transition. It is found that the normalized spectrum for the sapphire substrate film is two orders of magnitude larger than for the SrTiO3 substrate film, due to a stronger disorder. At sufficiently low temperatures, in the superconducting transition region, the normalized spectrum scales with the conductivity. This is experimental evidence of a temperature controlled percolating superconducting network in high-Tc superconductors. © 1990.