Enhancement of the superconducting transition temperature by the Sr-substitution for Ba in (Yb,Ca)Ba₂Cu₃O₆

We have investigated the effect of the Sr-substitution for Ba on the superconductivity in Yb_0.6Ca_0.4Ba_{2- y}Sr_yCu₃O₆ without O_chain forming the so-called Cu(1)O chain in the blocking layer. It has been found that T_c increases with increasing y even though no holes are doped through the Sr²⁺-substitution for Ba²⁺. The increase in T_c is because holes doped by the Ca-substitution for Yb are more likely supplied to the superconducting Cu(2)O₂ plane than the blocking layer of Cu(1) owing to the contraction of the Cu(2)O₂ plane through the substitution of smaller Sr²⁺ for larger Ba²⁺. The highest T_c ^onset = 73 K observed for y(Sr) = 0.6 is greatly higher than the previous record of T_c ^onset = 50 K in (RE,Ca)Ba₂Cu₃O₆ (RE: rare-earth element). The reason why it is lower than ∼90 K for REBa₂Cu₃O₇ is the low hole-concentration in the Cu(2)O₂ plane.