Surface Band Gaps and Superexchange Interaction in Transition Metal Oxides

In this paper, we discuss the change in the band gap of charge transfer insulators for different surface terminations. We have calculated the Madelung potential of the unreconstructed (100) surfaces of the rocksalt structured TM-O compounds (TM = Mn, Fe, Co and Ni). We also considered possible step defects on a (100) surface. The presented results are calculated in both the purely ionic case (TM2+O2−) and in the strong ligand p - cation 4s, phybridization (TM1+O1−) case. In both cases the charge transfer gap, Δ, for the surface is reduced compared to the bulk value. As a consequence of this reduction there is a large increase of the surface superexchange interaction, Jsur, and a decrease of the band gap.