BAND-LIKE ELECTRONIC STRUCTURE AND FERMI EDGE OF $YBa2Cu3O6.9$ SINGLE CRYSTALS AS DETERMINED BY PHOTOEMISSION SPECTROSCOPY

Work supported by the U.S. Dept. of EnergyAuthor Institution: Los Alamos National Laboratory; Ames National Laboratory, USDOE Iowa State University; Materials Science Division, Argonne National Laboratory; $^{\ast\ast}$ Sandia National Laboratories, AlbuquerqueWe have re-examined the electronic structure of $YBa _{2}Cu_{3}O_{6.9}$ single crystals via photoemission spectroscopy, making the measurements at 20 K in order to maintain surface oxygen stoichiometry. We find that typical spectra display a large sharp (resolution limited) Fermi edge consistent with metallic behavior. $N(E_{F})$ per Cu atom is very roughly estimated to be as large as 20\% of that of Cu metal and contains about 20-80 mix of Cu-3d and O-2p orbitals. Dispersion and final state enhancements, phenomena associated with well defined bands, are observed at $EF$ and throughout the valence bands. We find excellent agreement with a calculated DOS if the DOS is shifted to higher binding energies by $\sim 0. 5$ eV (except for the feature at $E_{F}$). Not only peak positions, but also line shapes of the spectra appear to agree with previous calculations, which would indicate that the calculations contain the correct mix of 3d and 2p orbitals throughout the valence bands. Thus much of the O-2p intensity is peaked in a narrow, 2eV wide band near $E_{F}$ with a long tail extending to -7 eV, while the Cu-3d band is centered at -4 eV and has a width of about 4 eV. Our results complement well the angle - resolved measurements of Takahashi, which likewise point to a hybridized band-like behavior of the superconducting oxides. The very large resonance of the -12 eV satellite, which decreases in intensity relative to the valence bands as the surface deteriorates, is probably a result of a SCK autoionization process and points to the existence of empty Cu-3d states