KOSTERLITZ-THOULESS TRANSITION AND FLUCTUATION EFFECTS IN YBA2CU3O7-XTHIN-FILMS

Fluctuation effects and temperature dependance of IV characteristics o f c-axis oriented YBCO thin films on STO(100) have been investigated w ithin the context of unbinding of vortex-antivortex pairs in the Koste rlitz-Thouless transition (KT) and Aslamazov-Larkin (AL) model. A crit ical jump in the nonlinearity parameter, alpha(V=I-alpha), is found at the universal value of alpha=3. The broadening of the jump in the tem perature dependance of a and associated exponential inverse square roo t temperature dependance of resistivity above T-c indicate quasi bi-di mensional behavior in the YBCO system. The correlation between AL fluc tuations and phase fluctuations (KT model) has also been studied.