Kosterlitz-Thouless transition and fluctuation effects in YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB> thin films

Fluctuation effects and temperature dependance of IV characteristics of c-axis oriented YBCO thin films on STO(100) have been investigated within the context of unbinding of vortex-antivortex pairs in the Kosterlitz-Thouless transition (KT) and Aslamazov-Larkin (AL) model. A critical jump in the nonlinearity parameter, &#945; (V=I&#945;), is found at the universal value of &#945; = 3. The broadening of the jump in the temperature dependance of &#945; and associated exponential inverse square root temperature dependance of resistivity above Tc indicate quasi bi-dimensional behavior in the YBCO system. The correlation between AL fluctuations and phase fluctuations (KT model) has also been studied