Structure of Valence and Conduction Levels in NiO

Abstract

Photoemission measurements on cleaved NiO(100) identify both the O $2p$ $\pi$ valence orbitals and the previously undetected O $2p$ $\sigma$ orbitals. An upper limit of 0.8±0.1 eV is set on the width of the Ni $3d$ $t_{2g}\downarrow$ emission. A large maximum in the empty density of states is detected in the secondary-electron spectrum, explaining previously debated photoabsorption data. We conclude from the available data that NiO is not a conventional band-gap insulator, as has been suggested recently, but is in fact a Mott insulator.