FLUCTUATION CONDUCTIVITY AND MICROSCOPIC GRANULARITY IN BI-BASED HIGH-TEMPERATURE SUPERCONDUCTORS

Systematic conductivity measurements near T-c in granular (Bi,Pb)-Sr-C a-Cu-O superconductors are presented, and focus is given on the interp lay between thermal fluctuations and disorder at micro-and mesoscopic levels. Experiments show that the resistive transition is a two-step-p rocess. In the normal phase, Gaussian and critical fluctuation conduct ivity regimes were identified. Both are affected by local disorder. Pa rticularly, the critical regime is characterized by a power law with e xponent lambda(cr)similar to 3, which we interpret as resulting from m icroscopic granularity. Below T-c and in the regime describing the app roach to the zero-resistance state, fluctuation conductivity diverges as expected in a paracoherent-coherent transition of a mesoscopic gran ular superconductor. The results show that, instead of trivially round ing the transition, disorder at micro-and mesoscopic levels preserves a true critical phenomenology in granular superconductors.