Parallel valence- and core-level photoemission studies of the metal-to-nonmetal transition in YBa2Cu3O7-x.

Valence-level and Cu 2p core-level photoemission spectra for YBa2Cu3O7-x in both metallic (x0.5) phases are presented together for the first time. The core spectra show evidence for holes in the hybrid Cu3d-O 2p valence levels in the metallic state, but nonetheless the density of states at the Fermi energy seen in valence-level photoemission is at most 0.05 states/eV cell. © 1989 The American Physical Society