FLUX-PINNING AND THE IRREVERSIBILITY LINE IN BI2SR2CACU2O8 SINGLE-CRYSTALS

Measurements of the ac-susceptibility have been used to investigate th e irreversibility line (IL) for a Bi2Sr2CaCu2O8 Single crystal. The ac tivation energy, U0, determined from the frequency dependence of the a c-susceptibility, can be related to the IL if one employs the model of thermally activated flux creep. This correlation is clearly demonstra ted by the observation of a kink in the IL and the existence of two re gions in the U0(H) curve. These results show that for the magnetic fie ld strengths considered in this work, the IL is a depinning line marki ng a crossover from flux creep to flux flow regimes.