Experimental Evidence of a Fluctuating Charge State in Cupric Oxide

This letter reports on a X-ray photoelectron spectroscopy study of cupric oxide which demonstrates that the split observed in the O-1 s core line arises from the fluctuating valence character of the Cu-O bond. This state is also confirmed by the valence band spectrum where the lowest binding-energy transition in the 3 d 9 → 3 d 10 L manifold is at the Fermi level allowing valence charge fluctuations at zero energy cost. Since valence fluctuation phenomena lead to a strong electron-phonon interaction, the observation of this effect also in HTSCs (high-temperature superconductors) would be a noteworthy point for an understanding of the mechanisms that originate high- T c in cupric-oxide-based compounds.