Broader perspective on the high-temperature superconducting YBa2Cu3Oy system: The real role of the oxygen content

A series of new YBa2Cu3Oy-like materials are reported in which the oxygen content and the average [Cu-O] charge (or Cu valence) are separately varied over a wide range. We find a distinct boundary between samples showing anomalous insulating behavior and those with high Tc. These data are interpreted to demonstrate that the [Cu-O] charge in the sheets largely determines Tc and that the variable oxygen content and the chains merely provide an insulating reservoir of charge. © 1988 The American Physical Society.