Substitution for copper in a high-T_c superconductor YBa ₂Cu₃O_7-δ

In spite of an overwhelming amount of research work¹ triggered by the discovery² of superconductivity above 90 K in an Y-Ba-Cu oxide, fundamental questions regarding the origin and mechanism of the high-T_c superconductivity in YBa₂Cu₃O _7-δ remain to be answered. Specifically, it has not been decided which of the two Cu sites mainly carries superconducting electrons. As an approach to the problem, we investigated superconductivity in a series of alloyed compounds, YBa₂ (Cu_1-xM_x) ₃O_7-δ with M = Fe, Co, Ni, Zn and Ga, in which metals M are partially substituted for Cu. The electrical resistivity measurements, as well as the X-ray analysis at low temperatures, show that T_c is not sensitive to the orthorhombic-tetragonal phase transition induced by the impurities, thus strongly suggesting that superconductivity along the one-dimensional Cu-O chains is unlikely.