Surface states on NiO (100) and the origin of the contrast reversal in atomically resolved scanning tunneling microscope images

The local spin-density approximation+$U$ method has been applied to identify the origin of surface electronic states on the (100) surface of nickel monoxide. The results of ab initio calculations show a substantial reduction of the forbidden gap at the surface and point to the presence of two types of electronic states associated with the termination of the crystal lattice. The energy of one of the surface states corresponds to the top of the valence band and the wave function of this state is predominantly localized on the oxygen surface sites. The other state is situated at the bottom of the conduction band and its wave function is localized on the nickel sites. The presence of these surface states explains the origin of the contrast reversal of scanning tunneling microscope images of the (100) NiO surface observed experimentally by altering the sign of the bias applied to the sample.