Electronic structure and charge fluctuations in YBa2Cu3O7 from EELS, XPS and Auger spectroscopies

Electron energy loss, x-ray photoemission and Auger spectroscopies have been used to study the valence states and the Cu 2p and O 1s core levels of YBa_2Cu_3O_7-delta as a function of the oxygen concentration delta. 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. The lowest final states configurations of Cu2+ and Cu3+ are found to be 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