SURFACE AND MIXED-VALENCE EFFECTS IN XPS SPECTRA FROM YBCO

We report X-ray photoemission data from YBa2Cu3O7 showing that the measured composition and the electronic structure depend significantly upon the photo-electron escape depth. For large take-off angles the valence band spectrum near the Fermi edge is well structured and shows a number of features, arising from Cu---O hybrid states. Their intensities are drastically reduced in the energy distribution curves measured at grazing angles. The results are interpreted assuming that the surface is mainly composed by Ba---O planes. This interpretation is shown to be consistent with electronic structure calculations for the bulk and the Ba---O surface