Superconducting and electrical properties of Ca-substituted YBa 2Cu 3O 6

Abstract

Superconducting properties of sintered Y 1− x Ca x Ba 2 Cu 3 O 6 (0 < x ≦ 0.31)have been investigated. Measurements of resistivity, Hall effect and X-ray diffraction as a function of Ca content x were carried out. It was found that Y 1- x Ca xBa 2Cu 3O 6 becomes superconducting above x ≦ ∼ 0.2, as had been reported previously, and T c shows a maximum at x ∼ 0.26 and declines at larger x. By increasing the Ca content, the carrier number in the CuO 2 plane increased suddenly in the range x ∼ 0.2, where samples were superconducting. A part of the carrier introduced by Ca substitution, however, was localized at Cu sites forming CuO chains. About 40% of the doped carrierr was conductive in x < ∼ 0.2 and ∼ 80% was conductive in x ≧ ∼ 0.2 at room temperature and these results agree fairly well with the analysis for the results of the iodometric titration measurement. Finally, we pointed out that slight oxidation for these samples brought about the release of carrier localization at low temperatures and a remarkable rise of T c, indicating the crucial importance of the CuO chain or orthorhombicity for high T c in YBa 2Cu 3O 6+ z .