Enhanced irreversibility field and current stabilization by Pb substitution in aligned Tl-1223 high-T_c superconductor

Using magnetic methods, we have studied the effects of Pb substitution on the irreversibility line and other features of c-axis aligned Tl based cuprate superconductors with the "1223" structure. Specifically, the properties of (Tl_0.5Pb_0.5)Sr₂Ca₂Cu₃O₉ are compared with those of grain-aligned TlBa₂Ca₂Cu₃O₉ and Tl(BaSr)₂Ca₂Cu₃O₉ materials. The irreversibility line H_irr(T) is notably higher for the Pb substituted material ( 1.7 T at 77 K with Hc) and markedly anisotropic for all of the samples. Also, results for the persistent current density are reported. Flux-creep studies of the current-decay rate show that the current is the most stable in the aligned (Tl_0.5Pb_0.5)Sr₂Ca₂Cu₃O₉, and that the decay rate of all three materials increases monotonically as the temperature increases.