CORRELATION EFFECTS IN NiO: COMPARISON OF NEAR THRESHOLD EXCITATION SPECTROSCOPIES

Abstract

At UHV-cleaved single-crystal NiO(100) surfaces core electron energy loss spectra (CEELS) at low primary energy and soft x-ray appearance potential spectra (SXAPS) have been measured at the oxygen K-threshold: the near-edge fine-structures have been investigated up to 40 eV above the threshold. Both spectroscopic methods probe in different ways the structure of the lowest empty states. CEELS is closely related to x-ray absorption spectroscopy (XAS), in both cases the excited final configuration consists of one core hole and one additional electron near the Fermi level E F. In APS we can study correlation effects very directly because the excited system consists of one core hole and two additional interacting electrons in localized resp. delocalized states near E F. The interpretation of the CEELS and SXAPS spectra at the oxygen K-edge is in agreement with a description of NiO as an intermediate valence system, where the charge-transfer gap is smaller than the Hubbard correlation energy. The comparison of our measured spectra with the experimental XAS of NiO shows the dominance of optically allowed channels in both spectroscopies.