We have performed angle-integrated photoemission measurements at 20 K on well-oxygenated ($T_c$=92 K) single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.9}$ cleaved n situ, and find a relatively large, resolution-limited Fermi edge which shows large amplitude variations with photon energy, indicative of band-structure final-state effects. Some dispersion is seen even in our angle-integrated measurements. Our best estimate of N($E_F$) per Cu atom is that it is about 20% that of Cu metal with about a 20-80 mix of Cu 3d and O 2p orbitals. Dispersive and final-state effects are seen throughout the valence bands. The line shapes of the spectra as a function of photon energy are very well reproduced by band-structure predictions, indicating a correct mix of 2p and 3d orbitals in the calculations, while the energy positions of the peak agree with calculated bands to within ≊0.5 eV. We conclude that a Fermi-liquid approach to conductivity is appropriate.

• Received 20 March 1989

DOI:https://doi.org/10.1103/PhysRevB.40.2268