New noise exponents in random conductor-superconductor and conductor-insulator mixtures

Time dependent fluctuations of the fraction of normal-conducting part in random resistor-superconductor (RS) and resistor-insulator (RI) networks lead to a novel effect close to the percolation threshold. The normalized noise scales as a function of the resistance with a characteristic exponent λ. The value of λ is different from the value found in classical percolation models but can be related to the resistivity exponent s (t) of the RS (RI) transition by a simple scaling relation: λ=2/s (2/t). Results of recent experiments on high-T_c superconducting thin films are interpreted in terms of this new effect and a crossover from three to two dimensional percolation behavior is found.