Institute of Physics of the National Academy of Sciences of Ukraine, 46 Nauki Ave., Kyiv 03028, Ukraine
Received Jule 21, 2009
The temperature dependences of critical superconducting current density jc(t) (t) = (1−T/Tc), Tc being the critical temperature) of two thin epitaxial c-oriented YBa2Cu3O7–δ films grown on LaAlO3 and SrTiO3 substrates are investigated by the method of dynamic magnetic susceptibility. At temperatures ranged from 77 K to Tc the dependence of jc(t) for both samples has two regions with power laws jc ~ t3/2 and jc ~ t2 divided by a crossover. The temperature dependences of dynamic relaxation rate Q(t) (magnetic flow creep) are measured. The results of the above measurements turn out to be not related to observed crossovers. It is shown, that the features in jc(t) are not related to the restriction of critical density of depairing current in the interdislocation
intervals of interblock borders of the YBa2Cu3O7–δ thin epitaxial films as supposed earlier. Using the numerical calculations, it is suggested that the existence of crossovers and the
infringement of H / t -scaling (H being the applied magnetic field) observed only in one of the samples in the magnetic field dependences jc (t,H) near Tc may be explained by the macroscopic heterogeneity of the samples in local
values of critical temperature Tc (Tc -heterogeneity).
The absence of this effect in the second
sample is explained by a smaller width of Tc distribution in it as compared to the first sample.
PACS: 74.72.–h Cuprate superconductors; PACS: 74.78.–w Superconducting films and lowdimensional structures; PACS: 74.25.fc Electric and thermal conductivity.
Key words: YBCO, flux creep, crossover, Pearl vortex.