Dissipation mechanisms in granular high T-c superconductors

We have studied the magnetoresistance and I-V characteristics of polycrystalline La2-xSrxCuO4-yCly samples as a function of temperature. Measurements were performed at temperatures near T-c and results were modeled taking into account the effect of thermal fluctuations as proposed by the Ambegaokar-Halperin model, the Anderson-Kim flux creep theory and the collective flux creep model (CFC). At low voltage, sub-ohmic results are better described by the CFC model. We found that the temperature dependence of the critical current density without thermal fluctuations indicates the presence of insulating barriers for non-chlorinated samples and metallic barriers for chlorinated samples. The obtained parameter mu similar or equal to 0.25 for both samples is, within the CFC model, in agreement with a picture where non-interacting vortices are pinned in a bidimensional defect array. (C) 1997 Elsevier Science B.V.