Valence charge fluctuations in YBa2Cu3O7-δ from core-level spectroscopies

From x-ray photoemission and Auger measurements of the Cu 2p and O 1s core levels of YBa2Cu3O7-δ as a function of the oxygen concentration δ, 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. One 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.