Paraconductivity of YBCO thin films with different anisotropy factors

We present paraconductivity measurements on YBCO thin films obtained by means of pulsed laser ablation. The films, grown under different oxygen pressures, exhibit critical temperatures ranging from 81 to 92 K. The excess conductivity has been analyzed in order to study the critical phenomena. We observe the Lawrence-Doniach crossover and, at higher temperatures, the crossover related to the breakdown of the Ginzburg-Landau (GL) approximation due to the increasing role of short wave fluctuations; the first crossover depends on the sample anisotropy and shifts to lower temperatures as the critical temperature of the film decreases, while for all the samples the second occurs roughly at the same reduced temperature. This proves the universality of the 2D paraconductivity behavior even out of the GL region. From the Lawrence-Doniach crossover values, we calculate the anisotropy factor which comes out to be nearly five when the critical temperature varies between 92-89 K, and increases up to 9.6 when the critical temperature is 81.5 K. (C) 1999 Published by Elsevier Science B.V. All rights reserved.