Critical and gaussian conductivity fluctuations in yba2cu3o7-delta

We report on systematic conductivity fluctuation measurements on three different samp1es of YBa2Cu30 7 - δ • We show, using the temperature derivative of the resistivity and the logarithmic derivative ofthe conductivity with respect to temperature, that the transition is a two-step process. In the normal phase, contributions from Gaussian and criticai ftuctuations are c1early evidenced. Far from Tc the Gaussian exponents indicate that a fractal topology might be adequate to describe the space dimensionality of the ftuctuation spectrum. Closer to Te we observe a crossover to a three-dimensional (3D) homogeneous Gaussian regime. Still closer to Tc we unambiguously identify the exponent λcr~O. 33, predicted by the simplest full dynamic scaling theory of criticai 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