DISLOCATION-MEDIATED FLUX CREEP IN BI2SR2CACU2O8+DELTA

We have investigated the possible role of two-dimensional vortex-lattice defects in thermally assisted flux flow for the very anisotropic high-temperature superconductor Bi2Sr2CaCu2O8+delta (Bi-Sr-Ca-Cu-O). At low current densities, this mechanism, which we shall refer to as plastic flux creep, is expected to prevail over creep of elastically correlated flux bundles, or elastic creep. It is assumed that vortices are pinned by oxygen vacancies, for which the elementary interaction is obtained. Measurements of the ac susceptibility in an applied dc field, B(c1) < mu-0H < 0.2B(c2), carried out using a sufficiently low driving-field amplitude, were performed on a Bi-Sr-Ca-Cu-O single-crystalline sample It is shown that under the above assumptions the experimental data may be consistently interpreted. The field dependence of the activation barrier is explained. Furthermore, the irreversibility line is reproduced, as well as the shape of the ac-susceptibility transition. Deviations from the plastic-flux-creep model can be qualitatively understood using a criterion describing the crossover to elastic flux creep.