VALENCE BANDS AND FERMI-SURFACE TOPOLOGY OF UNTWINNED SINGLE-CRYSTAL YBA2CU3O6.9

The cleaved surfaces of untwinned, single-crystal YBa2Cu3O6.9 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, GAMMABAR, XBAR, YBAR, and SBAR. Many band crossings of the Fermi energy were seen, with obvious and important differences between the bands near XBAR and those near YBAR. 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.