Critical and gaussian fluctuations in superconducting oxides

Abstract

We discuss the transport properties of high-T_c cuprates as resulting from a two-step transition: a superconducting intragranular transition at T _cs and an intergrandular coherence transition at T_c < T _cs. 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 < T_c . Critical phase fluctuations typical of granular systems account for the excess conductivity at T ≳ T_c . The rounding of the resistive transition at T ≳ T_cs 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.