CRITICAL AND GAUSSIAN CONDUCTIVITY FLUCTUATIONS IN YBA2CU3O7-DELTA

We report on systematic conductivity fluctuation measurements on three different samples of YBa2Cu3O7-delta. We show, using the temperature derivative of the resistivity and the logarithmic derivative of the conductivity with respect to temperature, that the transition is a two-step process. In the normal phase, contributions from Gaussian and critical fluctuations are clearly evidenced. Far from T(c), the Gaussian exponents indicate that a fractal topology might be adequate to describe the space dimensionality of the fluctuation spectrum. Closer to T(c) we observe a crossover to a three-dimensional (3D) homogeneous Gaussian regime. Still closer to T(c) we unambiguously identify the exponent lambda(cr) approximately 0.33, predicted by the simplest full dynamic scaling theory of critical superconducting fluctuations. The obtained exponent is consistent with a 3D, two-component, order parameter. Near the zero-resistance state, the.temperature dependence of our data is rather consistent with power-law behavior, suggesting the occurrence of a phase-transition phenomenon related to the percolation granular network.