Broader perspective on the high-temperature superconduct- ing YBa2Cu3O/sub y/ system: The real role of the oxygen content

A series of new YBa2Cu3O/sub y/-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 T/sub c/. These data are interpreted to demonstrate that the [Cu-O] charge in the sheets largely determines T/sub c/ and that the varying oxygen content and the chains merely provide an insulating reservoir of charge