Resonant excitation of an oxygen valence satellite in photoemission from high-Tcsuperconductors

A detailed analysis of the intensities of valence-band photoelectron features of superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ and semiconducting ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.05}$ ${\mathrm{CuO}}_4$ has revealed a resonance in the peak located at a binding energy of ∼9.5 eV for photon energies spanning the onset of O 2s excitations. This demonstrates conclusively that the feature is associated with oxygen excitations. The origin of the satellite is described and its disappearance on superconducting surfaces is explained.