Role of c-axis field components in making the transport E(J) characteristics of (Bi,Pb)2Sr2Ca2Cu3Ox tapes insensitive to the direction of field within the tape plane

Abstract

We report extended (over four decades) electric field versus current density characteristics of (Bi,Pb)2Sr2Ca2Cu3O10 tapes in magnetic fields applied along each of the three principal tape axes at 77 K, and along the two principal axes in the tape plane at 4.2 K. The characteristics are almost independent of field direction in the tape plane at both temperatures. Similar insensitivity of the critical current to magnetic field direction observed in other polycrystalline high temperature superconductors has been ascribed to the percolative nature of current flow, while the insensitivity in (Bi,Pb)2Sr2Ca2Cu3O10(2223) films and Bi2Sr2CaCu2O8 single crystals has been attributed to c-axis conduction across Josephson junctions. We show that the insensitivity of the critical current density to the field direction of these tapes follows naturally from the fact that the dissipation depends only on the field component lying along the c axis of individual 2223 grains. Lorenz-force driven motion of the pancake vortices that form within each 2223 grain provides a dissipation mechanism that involves only the c-axis components of the field. The angular dispersion of the grains in the tape ensures a c-axis field component in most grains, regardless of the applied field direction. This produces the remarkable insensitivity of E(J) over some four orders of magnitude of the electric field.