Photoemission studies of the high T_Csuperconductor, YBa₂Cu₃O₇

Considerable changes are observed in the spectra of the valence band and O(ls) regions in YBa2Cu3O7, when the temperature is lowered from 300K to 80K. The results are interpreted in terms of a progressive formation of O2- 2-like species with decreasing temperature. This is accompanied by the transformation of Cu to Cu. It is suggested that the holes generated at the top of the valence band forming the O2- 2-like species may play a crucial role in the superconductivity of the oxide.