Structural anomalies, oxygen ordering and superconductivity in oxygen deficient Ba 2YCu 3O x

Abstract

We report the characterization of series of oxygen deficient Ba 2YCu 3O x samples for 7 ≥ x ≥ 6 prepared by Zr gettered annealing at 440°C. Measurements include complete crystal structure analysis at 5 K by powder neutron diffraction, electron microscopy study of the oxygen ordering, and magnetic measurements of the superconducting transitions, with particular attention to the transition widths. The results show for the first time that the 90 K and 60 K plateaus in T c as a function of oxygen stoichiometry are associated with plateaus in the effective valence of the plane coppers. We also correlate the disappearance of superconductivity for x < 6.5 with an abrupt transfer of negative charge into the CuO 2 planes. We propose that different ordering schemes of oxygen have different characteristic T c's between 90 and 60 K.