THE CRITICAL CURRENT-DENSITY IN HIGH FIELDS IN EPITAXIAL THIN-FILMS OF Y1BA2CU3O7-DELTA - FLUX PINNING AND PAIR BREAKING

The critical current density (J(c)) of two epitaxial thin films of Y1Ba2Cu3O7-delta has been measured as a function of temperature in magnetic fields up to 27 T. A detailed study of one of the films shows that when the transport current is in the a-b plane and the magnetic field is orthogonal to the c axis, the functional form of J(c) is consistent with both a polynomial (flux pinning) description and an exponential (pair breaking) description. In contrast, when the current is in the a-b plane and the applied field is parallel to the c axis of the film, J(c) is unambiguously exponential of the form J(c) = alpha(T)exp[- mu0H/beta(T)]. These results explain why there are conflicting suggestions in the literature as to the mechanism that determines J(c) in high magnetic fields in thin films for J perpendicular-to H perpendicular-to c axis and provide evidence that pair breaking operates for J perpendicular-to H parallel-to c axis. However, the standard phenomenological superconductor-normal-superconductor junction model derived in the clean limit for the normal metal cannot describe this pair-breaking mechanism.