X-ray photoemission, bremsstrahlung isochromat, Auger-electron, and optical spectroscopy studies of Y-Ba-Cu-O thin films

We have investigated the electronic structure of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ thin films using a variety of spectroscopic methods. From a comparison of the copper and oxygen Auger spectra with the valence-band spectra we conclude that the on-site repulsion U between the valence electrons is fairly high both on copper and oxygen sites. A strong satellite is found in the copper 2p spectra, indicating a $d^9$ configuration of most copper atoms. The shape of the oxygen 1s line is found to depend strongly on the oxygen concentration. In both the occupied and unoccupied states as measured by x-ray photoemission spectroscopy and bremsstrahlung isochromat spectroscopy the density close to the Fermi level is very low and the consequences hereof are discussed. From the room-temperature optical spectra we find evidence for metallic behavior and a longitudinal mode at about 0.8 eV which we interpret as a plasmon.