Transport properties of YBa2Cu3O7−δ thin films near the critical state with no applied magnetic field

Transport measurements carried out on twinned YBa2Cu3O7−δ films are compared to the predictions of a previously proposed model suggesting that the vortices move along the films twin boundaries that behave as rows of Josephson weak links [P. Bernstein, J.F. Hamet, J. Appl. Phys. 95 (2004) 2569]. The obtained results suggest that, except if the films are very thin, the twin boundaries consist of superimposed rows of weak links with mean height, d¯s, whose mean length along the TBs is an universal function of T/Tc, the reduced temperature. This conclusion yields a general expression for the critical surface current density of the films as a function of T/Tc and of the number of superimposed weak links rows, while the critical current density depends on d¯s. A comparison of the measurements reported by various authors shows that the nature of the substrate and the growth technique have both a strong effect on d¯s. The existence of superimposed weak links rows is attributed to extended defects generated by Y2O3 inclusions.