Abstract
Abstract Superconducting oxygen-vacancy ordered ortho-II phase samples of Ba 2 YCu 3 O 6.5 were prepared with a new method of low temperature annealing. Electron diffraction analyses confirmed the ordered structure with a doubled cell along the a -axis of the orthorhombic triple perovskite. Contrary to previously reported results for quenched samples of the same phase, the annealed samples exhibited a metallic temperature dependence of resistivity and an almost linearly increasing magnetic susceptibility with temperature above its T c (∼53 K). Significance of this temperature dependence of susceptibility without Curie-Weiss contribution is discussed in comparison with the recently proposed model for the La 2- x (Sr, Ba) x CuO 4 system. Resistivity measurements in magnetic fields up to 8 T revealed an anomaly around 35 K, implying existence of another superconducting phase. From low temperature specific heat measurements we found 4.0 mJ/mol-K 2 for the residual γ value in the superconducting state which is about the same found for good quality ortho-I samples.
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