Crossover effects in the temperature dependence of the critical current in YBa2Cu3O7- delta.

Abstract

Temperature dependence of the critical current ${I}_{c}$ was investigated in thin films and ceramics of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}(\mathrm{YBCO})$ over a temperature range 10-90 K and in magnetic fields up to 700 G. The measurements were performed using superconducting rings in a persistent mode. A scanning axial Hall probe was used to record the profile of the magnetic field across the ring, generated by the persistent current at the critical level. The magnitude of ${I}_{c}$ was inferred from the magnitude of the persistent current's self-field in the center of the ring. This technique eliminated the contribution of normal currents to the measured value of ${I}_{c}$ and allowed us to distinguish between depairing and depinning critical currents. The results revealed the crossover between an Ambegaokar-Baratoff-like temperature dependence of the critical current to a Ginzburg-Landau dependence. The crossover was observed for both depairing and depinning critical currents in $c$-axis-oriented YBCO thin films during reduction of ${T}_{c}$ from 91 down to 87 K and in a ceramic YBCO upon application of small magnetic fields. The experimental data imply the presence of superconducting microdomains of an effective diameter of about 30-40 \AA{}, coupled by Josephson tunnel junctions, inside the grains and in the intergrain microbridges of YBCO, in agreement with the Clem's model for ${I}_{c}(T)$ in strongly coupled granular superconductors [Clem et al., Phys. Rev. B 35, 6637 (1987)]. The size of these domains decreases with a decreasing oxygen content and with an increasing applied magnetic field.