Bulk superconductivity at 70 K in Tetragonal YBa2(Cu0.96Co0.04)3O7−y

Abstract In this paper we report on the superconducting and structural properties of YBa 2 (Cu 1−x Co x ) 3 O 7−y as a function of Co-doping x. We observe a structural phase transition from orthorhombic to tetragonal for x=0.028. The structural phase transition is accompanied by a drastic increase of the T c -suppression rate. As we find bulk superconductivity in the tetragonal samples, our results highly question the necessity of Cu-O linear chain formation for the high-T c superconductivity.