OCCUPIED ELECTRONIC-STRUCTURE AND FERMI-SURFACE OF YBA2CU3O6.8

We have determined the occupied electronic structure and Fermi surface of twinned YBa2Cu3O6.8 single crystals near the Fermi level along the high-symmetry directions of the Brillouin zone at temperatures below 20 K by angle-resolved photoemission spectroscopy of high angular and energy resolution. We observe pronounced structures dispersing from about 0.25 eV at the GAMMA-point upward to the Fermi level, which they cross along GAMMA-X, Y and GAMMA-S. Comparison to a recent high-precision local-density-functional band-structure calculation yields only partial agreement. In particular, for several of the theoretically predicted bands we find no experimental evidence, which may in part be explained by the finite oxygen deficiency of our samples. Although the measurements are performed at temperatures well below T(c), we find no indication for the superconductivity energy gap, but are able to identify Fermi-Dirac cutoffs in the spectra within an accuracy of 10 meV. This observation is interpreted as the suppression of superconductivity at the surface and is discussed in relation to the recent discovery of an electronic surface state on YBa2Cu3O6.8.