Valence charge fluctuations in YBa2Cu3O7-delta from core level spectroscopies

From x-ray photoemission and Auger measurements of the Cu 2p and O1s core levels of YBCO as a function of the oxygen concentration delta, the average copper charge is determined. Evidence is found of dynamic charge fluctuations on the oxygen sublattice giving rise to a greater concentration of trivalent copper at the Cu(1) sites with respect to that determined by the analysis of neutron-diffraction data. On the basis of our experimental results, we introduce a molecular cluster description for the Cu states. The lowest final states configurations of Cu2+ and Cu3+ are c3d10L and c3d10L2, respectively, where c and L denote core holes on copper and oxygen atoms. Oxygen holes have high mobility and a Hubbard correlation energy less than 2 eV, a signature of their delocalization.The effect of temperature on the spectra is minor.Surface degradation modifies the relative intensity of the structures,particularly those of the O spectrum.