SIZE LIMITATION OF VORTEX FLUCTUATIONS IN TRANSPORT-CURRENT-CARRYING BI2SR2CA2CU3O10 HIGH-TEMPERATURE SUPERCONDUCTORS WITH PREFERENTIAL GRAIN-ORIENTATION

The dissipation process in transport-current-carrying Bi2Sr2Ca2CU3O10 samples with preferential grain orientation in zero applied magnetic f ield and close to the mean-field critical temperature T-c0 was investi gated. The T-c0 value was obtained from an analysis of the fluctuation -induced excess de conductivity in the framework of the Aslamazov-Lark in theory of paraconductibility. The measured transport critical-curre nt density, which, at least at high temperatures, is essentially given by the contribution of good superconducting intergrain contacts in th e sample, has a relatively large component decreasing linearly with te mperature and vanishing clearly below T-c0. The temperature dependence of the transport critical current as well as the form of the current- voltage characteristics at low voltages can be understood by consideri ng a current-dependent vortex-antivortex unbinding, size limited at th e intergranular contacts, and by taking into account the contribution of entropy to the unbinding process.