Transport Properties of ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}$ Thin Films Near the Critical State

Transport measurements carried out on YBa 2 Cu 3 O 7-delta films are compared to a model suggesting that the films boundary planes behave as rows of Josephson weak links. The obtained results suggest that, except if the films are very thin, the boundary planes do not consist of a single row but of superimposed rows across the film thickness, whose existence is attributed to extended defects generated by Y 2 O 3 inclusions. An expression for the critical surface current density valid in a large temperature range above a threshold value and depending on T, T c and the number of superimposed weak links rows is derived from this conclusion and found in good agreement with the measurements. The critical current densities measured below the threshold are accounted for supposing that the oxygen content is not uniform in the samples.