We report on systematic conductivity fluctuation measurements on three different samples of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\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 ${\mathit{T}}_{\mathit{c}}$, the Gaussian exponents indicate that a fractal topology might be adequate to describe the space dimensionality of the fluctuation spectrum. Closer to ${\mathit{T}}_{\mathit{c}}$ we observe a crossover to a three-dimensional (3D) homogeneous Gaussian regime. Still closer to ${\mathit{T}}_{\mathit{c}}$ we unambiguously identify the exponent ${\lambda }_{\mathrm{cr}}$∼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.

• Received 11 January 1993

DOI:https://doi.org/10.1103/PhysRevB.47.11420