DIRECT EVIDENCE FOR A CHANGE AT 50 K IN THE CRITICAL-CURRENT PROPERTIES OF A BI2-XPBXSR2CA2CU3O-DELTA TAPE IN HIGH MAGNETIC-FIELDS

The transport critical current density (J(C)) of a monocore Bi2-xPbxSr2Ca2Cu3Odelta silver-sheathed tape has been measured for three sample orientations: (i) B \\ c-axis, (ii) J perpendicular to B, B perpendicular to c-axis, (iii) J \\ B, B perpendicular to c-axis, from 4.2-110 K in fields up to 15 T. For all three orientations in high fields, J(C)(B,T) = alpha(T) exp(-B/beta(T)) where there is a marked change in the temperature dependence of alpha(T) and beta(T) at 50 K, such that below 50 K alpha(T) is much less temperature dependent whereas beta(T) is much more temperature dependent. We attribute this change to a quasi-2D dislocation mediated Kosterlitz-Thouless transition when the lattice markedly softens. We discuss the evidence that J(C) is determined by the average field component parallel to the c-axis (as has been found in epitaxial Bi2Sr2Ca2Cu3Odelta thin films) and the intergrain connectivity.