Valence bands and Fermi-surface topology of untwinned single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.9}$

The cleaved surfaces of untwinned, single-crystal ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ have been probed with synchrotron-radiation photoemission, utilizing both high energy and angular resolution. Acute spectral structure was observed, both at the Fermi energy and at higher binding energies, particularly near the high-symmetry points of the two-dimensional Brillouin zone, Γ¯, X¯, Y¯, and S¯. Many band crossings of the Fermi energy were seen, with obvious and important differences between the bands near X¯ and those near Y¯. A large superconducting gap was not observed: The data are consistent with a gap of less than 10 meV. The assignment of bands and Fermi-level crossings to chain and plane states will be discussed, including comparisons to the predictions of theory, particularly local-density-approximation calculations.