Electronic structure near ${\mathit{E}}_{\mathit{F}}$ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ for 6.35≤x≤6.9: A photoemission study

High-resolution angle-resolved photoemission measurements are reported for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ 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 Γ¯-S¯, two bands dispersing through ${\mathit{E}}_{\mathit{F}}$ were observed in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$. 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 ${\mathit{E}}_{\mathit{F}}$ 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 Γ¯-Y¯(X¯), one band dispersing through ${\mathit{E}}_{\mathit{F}}$ was observed in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$. 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 ${\mathit{E}}_{\mathit{F}}$ in the oxygen stoichiometry range x≥6.4, where the material shows metallic behavior.