Valence electronic structure of ${\mathrm{Y}}_1$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

The occupied and unoccupied valence-band structure of ${\mathrm{Y}}_1$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ is probed with photoemission, inverse photoemission, and near-edge x-ray absorption. The Cu $3d$ and O $2p$ states nearly coincide, thereby maximizing their interaction. The occupied states are shifted down by 2 eV relative to ground-state band calculations, indicating localization and a Coulomb interaction $U>\mathrm{}2\mathrm{}$ eV. The bandwidth is 10 eV with the top of the band at $E_F+2.5\mathrm{}$ eV. Cu is found in the 2+ oxidation state. O has $2p$ holes which are assigned to specific oxygen atoms.