Resonant-photoemission study of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$

We have studied the photoemission spectra of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ (0.05≤x≤0.2) using synchrotron radiation in the Cu 3p→3d, Nd 4d→4f, and Ce 4d→4f core absorption regions. The valence-band satellite corresponding to ${\mathrm{Cu}}^{2+}$ shows a resonance behavior. The Nd 4f–derived photoemission spectrum shows a double-peak structure, which is characteristic of the localized 4${\mathit{f}}^3$ ground state and strong 4f–O 2p hybridization. The Ce 4f–derived photoemission is distributed predominantly near the top of the O 2p band. Comparing the results with those of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$, we find that the Fermi level is located only ∼0.7 eV above the top of the valence band in spite of the large (1.5–eV) band gap of ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$, indicating that the Fermi level of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ lies within the band gap of ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$.