Scaling spontaneous fluctuations in high-Tc superconductor films

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 film is two orders of magnitude larger than for the SrTiO3 film, due to a stronger disorder. At sufficiently low temperatures, the normalized spectrum scales with the conductivity, which proves the existence of a percolation superconducting network.