DISSIPATION MECHANISMS IN GRANULAR HIGH T-C SUPERCONDUCTORS

We have studied the magnetoresistance and I-V characteristics of polyc rystalline La2-xSrxCuO4-yCly samples as a function of temperature. Mea surements were performed at temperatures near T-c and results were mod eled taking into account the effect of thermal fluctuations as propose d 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 tempe rature dependence of the critical current density without thermal fluc tuations indicates the presence of insulating barriers for non-chlorin ated samples and metallic barriers for chlorinated samples. The obtain ed parameter mu similar or equal to 0.25 for both samples is, within t he CFC model, in agreement with a picture where non-interacting vortic es are pinned in a bidimensional defect array. (C) 1997 Elsevier Scien ce B.V.