Role of calcium in suppressing the superconductivity of Y_1-xCaxBa₂Cu₃O_7-δ

Abstract

Partial substitution of Ca²⁺ at the Y³⁺ site in the YBa₂Cu₃O_7-δ system shows several interesting features, namely (1) creation of oxygen vacancies without any change in the lattice parameters, (2) no change in the effective copper valence, and (3) decrease in the critical temperature T_c with substitution, accompanied by a decrease in the normal-state conductivity. A host of measurements such as low-temperature resistivity, magnetic susceptibility, X-ray diffraction, oxygen stoichiometry, and the effective copper valence, reported here, suggest that T_c depression with Ca substitution is unlikely to be due to an overdoping effect. An alternate explanation, which takes stock of the above observations, and involves the creation of disorder in CuO₂ planes, is invoked.