Flux Pinning and Weak Link Structure in Ca-Doped LaBa2Cu3O7-y

The effect of Ca doping on the flux pinning and the weak link structure of the LaBa₂Cu₃O_7-y system have been studied. The magnitude and the magnetic field dependence of the critical current density were improved by Ca doping. A small amount of impurity phases of Ca-related compounds such as Ca₂Cu₁O₃ and Ca₁Cu₂O₃ may work as a desirable flux pinning center. Moreover, it was found that the current-temperature characteristics for Ca-doped samples showed the evidence of two kinds of superconductor phases which have different transition temperatures T_c and T_c'. The experimental result agreed well with the Ambegaokar-Baratoff theory for asymmetric Josephson junctions (S-I-S') in the temperature range of T<T_c' and with the proximity junction theory (S-N-S) in the range of T_c'<T<T_c.