Thermally Activated Flux Motion in 2223 Phase High Temperature Superconductors

Isothermal V-I characteristics of Bi_1.8Pb_0.2Sr₂Ca₂Cu₃O_y high temperature superconductors (HTSCs) have been systematically studied. Below the irre-versibility temperature T_r(H), two important features were observed: 1) Both positive and negative curvatures can coexist in the same isothermal V-I curve, so it is different from the scaling behavior V∝exp(-(J₀/J)^μ). 2) The activa-ion volume of flux motion is found temperature dependent as (1=T/T_r(H))^-2v with v about 0.5. This behavior supports the existence of a two-dimensional secondary phase transition below H_c2(T) in this Bi-based HTSCs.