Valence-electron states of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ characterized by combined x-ray and photoemission studies

X-ray-emission (XES), x-ray-excited photoelectron (XPS), and ultraviolet-excited photoelectron spectra of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ have been measured and arranged on a common binding-energy scale by XPS core lines. Three main valence-band (VB) structures at 2.7, 3.7, and 4.8 eV relative to the Fermi level (${\mathit{E}}_{\mathit{F}}$=0) could be identified representing hybridized Cu 3d/4p–O 2p states. The VB XES and photoelectron spectra are strongly influenced by correlation and charge-transfer effects and hence reproduce the density of states (DOS) of excited multiple-hole states but not the one-hole ground-state DOS. These processes are responsible for the observed differences between high-energy spectroscopic results and one-electron theory. From the XES compared with calculations we estimate a ‘‘local partial correlation shift’’ of the theoretical Fermi level (Δ${\mathit{E}}_{\mathit{F}}$s/${\mathrm{r}}^{\mathit{c}\mathit{o}\mathit{r}\mathit{r}}$) in the energy range 0.6–1.6 eV. Charge transfer and screening processes contribute to the x-ray transitions of the selective-type Cu 2${\mathit{pd}}^{10}$L→${\mathit{d}}^9$L and O 1${\mathit{sp}}^6$Cu 3${\mathit{d}}^9$→${\mathit{d}}^9$L owing to Cu Lα and O Kα, respectively. With the mechanism of charge transfer and band shift acting in highly correlated electron systems, such as high-temperature superconducting compounds, a possible explanation for the unusually low DOS at ${\mathit{E}}_{\mathit{F}}$ in excitation spectra is given.