THERMALLY ACTIVATED DISSIPATION IN EPITAXIAL BI2SR2CACU2O8- EVIDENCE FOR THE OCCURRENCE OF VORTEX STRINGS WITH 3-DIMENSIONAL FLUCTUATIONS IN LOW MAGNETIC-FIELDS(DELTA FILMS )

The thermally activated dissipation in highly anisotropic Bi2Sr2CaCu2O 8+delta superconducting thin films was investigated by conventional re sistive measurements, with the magnetic field B=mu0H applied perpendic ular to the (a, b) plane and ranging between 5 mT and 3 T. We observed a significant change in the form of the ''irreversibility line'' (det ermined with a constant-resistivity criterion) and of the magnetic-fie ld dependence of the activation energy at an applied field value B(cr) almost-equal-to 30 mT. This change is discussed in connection to the predicted ''dimensional crossover'' in the vortex system, in the case of highly anisotropic superconductors. For the investigated specimen, we found that the model of thermally activated plastic flux motion wit h double vortex kink formation describes the irreversibility line at i ntermediate field values. This indicates that the ''pancake'' vortices in the (CuO2)2 blocks are not completely decoupled, at least up to 3 T. In the low field region, due to the predicted increase of the tilt modulus in the vortex system, the above model becomes inappropriate, a nd the form of the irreversibility line can be explained in terms of t hermally assisted flux flow in a three-dimensional vortex ensemble, wi th a coherence length along the flux line of the order of 50 nm.