ROLE OF CALCIUM IN SUPPRESSING THE SUPERCONDUCTIVITY OF Y1-XCAXBA2CU3O7-DELTA

Partial substitution of Ca2+ at the Y3+ site in the YBa2Cu3O7-delta 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.