Spontaneous conductance fluctuations and percolation in YBa2 Cu3O7 thin films

Experimental results of the conductivity noise in the superconducting transition region of YBCO thin films prepared by co-evaporation and sputtering are presented. In the case of ex-situ fabricated samples made by co-evaporation Cooper-pair number fluctuations (induced by electron trapping) have been identified in the high temperature part of the transition. Classical percolation noise was found in the low temperature part of the transition. In the case of in-situ fabricated samples made by co-evaporation, electron mobility fluctuations, shunted by the conductance of Cooper-pairs, were identified in the upper temperature range. In the low temperature part of the transition of the in-situ fabricated samples, a new type of fluctuation has been discovered; the fluctuation of the volume concentration of the superconducting phase. This implies new universal noise exponents very different from the exponents of classical percolation models. This effect is a consequence of intergrain critical current fluctuations