Irreversibility field and coherence length of Ca-substituted YBCO single crystals

The irreversibility fields of Ca-YBCO single crystals are estimated from torque measurement and dc-resistivity. The small anisotropy about 3 is related to the heavy overdoping with help of Ca-substitution. For the optimally doped Ca-YBCO, the irreversibility field is lower than that for the Ca-free YBCO, when compared at a fixed T/Tc. Since the former has more oxygen deficiencies, this result contradicts with the general understanding that CuO-chain deficiency is considered to enhance vortex pinning. The resistivity in magnetic fields shows that fluctuation effect is largely suppressed for heavily overdoped Ca-YBCO.