CRITICAL AND GAUSSIAN FLUCTUATIONS IN SUPERCONDUCTING OXIDES

We discuss the transport properties of high-Tc cuprates as resulting from a two-step transition: a superconducting intragranular transition at T„ and an intergrandular coherence transition at Tc < Tc s. The first one can be described by mean-field theory, while the second one displays scaling properties characteristic of critical phenomena. A variety of behaviours are predicted and found for the maximum supercurrent at T <Tc Critical phase fluctuations typical of granular systems account for the excess conductivity at T > Tc. The rounding of the resistive transition at T ≥ Tc s can be explained by small (gaussian) fluctuations of the order parameter amplitude, once the role of granularity is taken into account. We also propose an explanation for the apparently intrinsic granularity of superconducting oxides. © 1991, Taylor & Francis Group, LLC. All rights reserved.