Soft-x-ray absorption spectroscopy of electron-doped (Nd,Sm${)}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ compounds

The oxygen 1s and copper 2p x-ray absorption spectra of the semiconductor, superconductor, and metallic compounds (Nd,Sm${)}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ have been measured. After cerium doping, the O 1s absorption edge does not give a change in the density of unoccupied O 2p–Cu 3d molecular-orbital states, but shows a decrease in the O 2p–Cu 4s molecular-orbital states with ${\mathit{a}}_1$ symmetry. The Cu 2p absorption spectra show a decrease in Cu 3${\mathit{d}}^9$, although a strong Cu 3${\mathit{d}}^{10}$ peak is not observed. As a result of the strong mixing between the O 2p and Cu 4s states of ${\mathit{a}}_1$ symmetry, a composite state of Cu 3d, Cu 4s, and O 2p orbitals can be formed in these compounds, as has been predicted by the cluster calculation of Tohyama, Ohta, and Maekawa [Physica C 158, 525 (1989)].