Structure of Valence and Conduction Levels in NiO

Photoemission measurements on cleaved NiO(100) identify both the O $2p$ π valence orbitals and the previously undetected O $2p$ σ orbitals. An upper limit of 0.8±{}0.1 eV is set on the width of the Ni $3d$ t2g↓ 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.