High Temperature Superconductivity in Y-Ba-Cu-O: Identification of a Copper-rich Superconducting Phase

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For the past 15 years, advances in superconductivity have come about only slowly and even a 0.5 K increase in transition temperature was noteworthy. Until April 1986, the highest transition temperatures were near 23 K. At that time, Bednorz and Muller reported superconductivity in La-Ba-Cu-0 compounds above 30K. Following this breakthrough, there were further studies of these phases and superconductivity was found also in similar solids containing Sr and Ca instead of Ba. The superconducting phase was identified as a layered perovskite A2-xBxCu04 with tetragonal symmetry, where A = La3+ and B = Ca2+, Sr2+, or Ba2+. The Sr compounds with x = 0.15 show the highest transition temperatures with Tc near 36 K.5 These studies lead to the investigation of other ternary copper oxides and soon thereafter Chu and co-worker found that compounds with a nominal composition of Y1.2Ba0.8Cu04could be prepared with superconducting transitions near 100 K. In contrast to the La2-x,SrXCuO4 phases,the analogous Y2-xBa,Cu04 materials contain more than one phase. We report here the composition and properties of a superconducting phase containing Y, Ba, Cu, and 0 which is copper-rich compared with Y1.2Ba0.8Cu04.