Features of the temperature dependence and magnetic-field dependence of the critical current density close to the critical temperature in YBa2Cu3O7-δ thin films

The temperature dependence of the critical density of superconducting current j(c)(tau) [tau=(1-T/T-c), and T-c is the critical temperature] of two epitaxial c-oriented YBa2Cu3O7-delta thin films grown on LaAlO3 and SrTiO3 substrates has been investigated by the method of dynamic magnetic susceptibility. In both samples, the j(c)(tau) dependence has two sections with power dependences j(c)similar to tau(3/2) and j(c)similar to tau(2) in the temperature region from 77 K to T-c, with crossover between them. Measurements were made of the temperature dependences of the dynamic relaxation rate Q(tau) (magnetic flux creep), which turned out to be unconnected with the observed crossovers. It is shown that the observed features of j(c)(tau) are not caused, as was assumed earlier, by the limitation of the critical depairing current density in the interdislocation gaps of the interblock boundaries of YBa2Cu3O7-delta epitaxial thin films. Based on numerical calculations, it is shown that the existence of crossovers and the breakdown of H/tau scaling detected in one of the samples (H is the dc magnetic field) in the j(c)(tau,H) magnetic-field dependences close to T-c can be explained by the manifestation of macroscopic inhomogeneity of the samples in the local values of the critical temperature T-c (T-c inhomogeneity). The absence of observable breakdown of the H/tau scaling in the second sample is explained by the smaller width of the distribution over T-c in it by comparison with the first sample.