Valence charge fluctuations inYBa2Cu3O7−δfrom core-level spectroscopies

From x-ray photoemission and Auger measurements of the $\mathrm{Cu} 2p$ and $\mathrm{O} 1s$ core levels of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ 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. One the basis of our experimental results, we introduce a molecular cluster description for the Cu states. The lowest final-states configurations of ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cu}}^{3+}$ are $\mathit{c}{3d}^{10}\mathit{L}$ and $\mathit{c}{3d}^{10}{\mathit{L}}^2$, respectively, where $\mathit{c}$ and $\mathit{L}$ 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.