Band theory of insulating transition-metal monoxides: Band-structure calculations

The electronic structure of the insulating antiferromagnetic transition-metal compounds MnO, FeO, CoO, and NiO, which have been regarded as the prototypes of the concept of a Mott insulator, is discussed with use of energy-band theory based on the local-spin-density treatment of exchange and correlation. It is shown that the band structure is very sensitive to the magnetic ordering and that the ground-state magnetic ordering is special in the sense that it makes the e₆ (x^2-y^2, 3z^2-r^2) band particularly narrow, which is crucial to the insulating nature of NiO. A detailed analysis is made of this particular aspect of the ground-state magnetic ordering. As for FeO and CoO, it is suggested that the population imbalance among the t₂₆ (xy, yz, zx) orbitals induced by the intra-atomic exchange interaction may cause a gap to open at the Fermi level.