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

Temperature dependence of the critical current I-c was investigated in thin films and ceramics of YBa2Cu3O7-delta(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 Angstrom, 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.