Size limitation of vortex fluctuations in transport-current-carrying Bi2Sr2Ca2Cu3O10 high-temperature superconductors with preferential grain orientation.

Abstract

The dissipation process in transport-current-carrying ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ samples with preferential grain orientation in zero applied magnetic field and close to the mean-field critical temperature ${\mathit{T}}_{\mathit{c}0}$ was investigated. The ${\mathit{T}}_{\mathit{c}0}$ value was obtained from an analysis of the fluctuation-induced excess dc conductivity in the framework of the Aslamazov-Larkin theory of paraconductibility. The measured transport critical-current density, which, at least at high temperatures, is essentially given by the contribution of good superconducting intergrain contacts in the sample, has a relatively large component decreasing linearly with temperature and vanishing clearly below ${\mathit{T}}_{\mathit{c}0}$. 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 considering a current-dependent vortex-antivortex unbinding, size limited at the intergranular contacts, and by taking into account the contribution of entropy to the unbinding process.