Electronic structure near EF in YBa2Cu3Ox for 6.35≤x≤6.9: A photoemission study

High-resolution angle-resolved photoemission measurements are reported for YBa2Cu3Ox when oxygen stoichiometries are in the range 6.35≤x≤6.9. The stoichiometry dependence of spectral features at energies close to the Fermi level was monitored along major symmetry lines in the two-dimensional Brillouin zone. Along bar Γ-bar S, two bands dispersing through EF were observed in YBa2Cu3O6.9. Band calculations indicate that these are ''plane''-related features. These two bands and the Fermi surfaces that they define are nearly independent of oxygen stoichiometry. The spectral weight near EF is relatively constant for 6.4≤x≤6.9. However, the spectral weight falls off significantly when x=6.35 (insulating), although the dispersive behavior that is characteristic of the higher oxygen stoichiometries persists. This abrupt intensity change might signal the onset of an electronic structure governed by strong correlation. Along bar Γ-bar Y(bar X), one band dispersing through EF was observed in YBa2Cu3O6.9. As oxygen was removed, the intensity of this dispersive feature decreased systematically. Predictions of band theory indicate that this band is a ''chain''-related feature. In general, predictions of band theory appear to be quite reliable near EF in the oxygen stoichiometry range x≥6.4, where the material shows metallic behavior