Identification of the phase responsible for high-temperature superconductivity in Y–Ba–Cu oxides

Abstract

The discovery of Lá–Ba(Sr)–Cu oxides of K₂NiF₄ structure exhibiting superconductivity in the 30–40 K region was reported early this year¹. The recent announcement of superconductivity above the temperature of liquid nitrogen in Y–Ba–Cu oxides² has belied all expectations. Unfortunately, all the Y–Ba–Cu oxide compositions exhibiting this remarkable feature were biphasic or multiphasic. We have studied various compositions of the Y_3–xBa_3+xCu₆O₁₄ and now report the isolation of the pure oxide phase responsible for high T_c superconductivity. This oxide has the composition Y_0.33Ba_0.67CuO_2.33±δ, with an oxygen defect perovskite structure. The pure oxide shows the onset of superconductivity at 120 K, attaining zero resistance at 87 K. It also exhibits the highest Meissner effect of all the high-T_c oxide superconductors.