CRITICAL-CURRENT MAGNETIC-FIELD DEPENDENCE OF YBA2CU3O7

J(c)(B, T), the critical current density as a function of applied magnetic field and temperature of a superconductor is analysed in terms of flux-line-lattice (FLL)-shear and Josephson junction grain-boundary weak-link mechanisms. For low-T(c) superconductors, J(c)(B) Of FLL-shear decreases monotonically from its peak at zero field, J(c)(0) = J(c0), to zero at the upper critical field Jc(B(c2)) = 0. For high-T(c) superconductors J(c)(B) decreases to zero at an irreversible field B*, a field considerably smaller than B(c2). The J(c)(B) profile between the two boundaries, J(c)(0) and J(c)(B*), is influenced primarily by material defect density. J(c)(B) of the Josephson junction weak-link model is the Fraunhofer diffraction function. In this formalism, J(c) decreases to zero at a field H-0, and at larger fields J(c)(B) exhibits sidelobes. Most J(c)(B) data of two proton irradiated YBa2Cu3O7 crystals analysed in this investigation fit the FLL-shear model well. For the more heavily irradiated crystal, systematic deviations from the FLL-Shear model occur for applied magnetic fields larger than 2-3 T and temperatures above 70 K. J(c)(B) calculated from the Josephson junction model is inconsistent with the data. J(c)(B) fitted to a two-component model, the linear sum Of FLL-shear and Josephson junction mechanisms, improves fits at all fields and temperatures. This may imply that J(c)(B) mechanisms change with increasing defect density from FLL-shear to Josephson junction weak-links.