Fluctuation conductivity and microscopic granularity in Bi-based high-temperature superconductors

Abstract

Systematic conductivity measurements near ${T}_{c}$ in granular (Bi,Pb)-Sr-Ca-Cu-O superconductors are presented, and focus is given on the interplay between thermal fluctuations and disorder at micro- and mesoscopic levels. Experiments show that the resistive transition is a two-step process. In the normal phase, Gaussian and critical fluctuation conductivity regimes were identified. Both are affected by local disorder. Particularly, the critical regime is characterized by a power law with exponent ${\ensuremath{\lambda}}_{\mathrm{cr}}\ensuremath{\sim}3,$ which we interpret as resulting from microscopic granularity. Below ${T}_{c}$ and in the regime describing the approach to the zero-resistance state, fluctuation conductivity diverges as expected in a paracoherent-coherent transition of a mesoscopic granular superconductor. The results show that, instead of trivially rounding the transition, disorder at micro- and mesoscopic levels preserves a true critical phenomenology in granular superconductors.