SPONTANEOUS CONDUCTANCE FLUCTUATIONS AND PERCOLATION IN YBA2CU3O7 THIN-FILMS

Experimental results of the conductivity noise in the superconducting transition region of YBCO thin films prepared by co-evaporation and sp uttering are presented. In the case of ex-situ fabricated samples made by co-evaporation, Cooper-pair number fluctuations (induced by electr on trapping) have been identified in the high temperature part of the transition. Classical percolation noise was found in the low temperatu re part of the transition. In the case of in-situ fabricated samples m ade 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 fa bricated-samples, a new type of fluctuation has been discovered; the f luctuation of the volume concentration of the superconducting phase. T his implies new universal noise exponents very different from the expo nents of classical percolation models. This effect is a consequence of inter-grain critical current fluctuations and can be caused by for in stance magnetic flux motion, defect motion or trapping of electrons in the barriers between grains. Moreover, in the coevaporated samples, a dimensional crossover, 3D --> 2D, of the percolating network has been observed. From this effect, the length scale of the microscopic disor der can be estimated.