Evidence that pair breaking at the grain boundaries of Bi₂Sr₂Ca₂Cu₃O_x tapes determines the critical current density above 10 K in high magnetic fields

The critical current density of an aligned Ag doped Bi₂Sr₂Ca₂Cu₃O_x tape has been measured from 10 K up to T_c (105 K) in magnetic fields up to 15 T. Measurements of the critical current density (J_c,(B, T)) have been completed for three different configurations of magnetic field, transport current, and superconducting tape: (i) J perpendicular to c axis, J perpendicular to B, and B perpendicular to c axis; (ii) J perpendicular to c axis, J perpendicular to B, and B//c axis; (iii) J perpendicular to c axis, J//B, and B perpendicular to c axis. In all three configurations at high magnetic fields up to 15 T, we find the critical current density can be described by J_c(B, T)= alpha ( phi , T) exp(- mu ₀H/ beta ( phi , T)) where ( beta phi , T) is of a separable variable form beta ( phi , T)= beta /sup /(T)/f( phi ) where beta /sup /(T) is a function of temperature alone and f( phi ) is a constant dependent only on the orientation of B, J, and the c axis. A similar but not identical exponential magnetic field dependence has been found in Bi₂Sr₂Ca₂Cu₃O_x, Bi₂Sr₂CaCu₂O_x and YBa₂Cu₃O_x, thin films. We discuss the evidence that the exponential magnetic field dependence suggests that a pair breaking mechanism operates at barriers in these materials. We conclude that the grain boundaries determine J_c(B, T) in the Bi₂Sr₂Ca₂Cu₃O_x textured tape.