FLUX PINNING AND CRITICAL CURRENTS IN SUPERCONDUCTING THALLIUM CUPRATES

The irreversibility lines H(irr)(T) for the series of superconducting compounds Tl(m)A2Ca(n-1)Cu(n)O2+m+2n, where A is Ba or Sr, are measured between 0 and 1 T for m = 1 and 2 and n = 1, 2, 3 and 4 and show a marked steepening with decreasing anisotropy, i.e. as m is lowered from 2 to 1. In addition, for m = 2 material H(irr)(T) is observed to steepen with decreasing number of CuO2 layers. The hole dependence of the irreversibility line is also investigated in Tl2Ba2CuO6+delta by changing the oxygen stoichiometry delta. H(irr)(T) progressively steepens with increasing hole concentration on the overdoped side of T(c)(max). These results are consistent with a model of flux motion arising from a cross-over from 3D vortex lines to 2D pancake vortices which are decoupled in the c-direction, and provide guidance for materials selection and optimum doping. The manufacture of superconducting, silver-clad tapes using Tl0.5Pb0.5Sr2Ca0.8Y0.2Cu2O7 is described. Zero field transport critical currents in excess of 10(4) A cm-2 are obtained but the material is untextured and weak-linked, as shown by the strong field dependence of J(c).