Conductance noise and percolation in YBa2Cu3O7 thin films

Abstract

Experimental results of the conductivity noise in the superconducting transition region of YBCO thin films prepared by co-evaporation are presented. In the case of ex situ fabricated samples, 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 samples made by the in situ method, the noise is smaller by several orders of magnitude in the upper part of the transition. This indicates a much more ordered microstructure in these samples. Electron mobility fluctuations, shunted by the conductance of Cooper-pairs, were identified in this temperature range. In the low-temperature part of the transition, a new type of fluctuation has been discovered; the fluctuation of the volume fraction of the superconducting phase. This implies new scaling exponents very different from the exponents of classical percolation models. This effect is a consequence of intergrain critical current fluctuations and can be caused by, for instance, magnetic flux motion, defect motion or trapping of electrons in the barriers between grains. Moreover, in the in situ fabricated samples, a dimensional crossover, 3D→2D, of the percolating network has been observed. From this effect, the length scale of the microscopic disorder can be estimated.