First Principles theory of carrier doping of prototype Mott insulators MnO and NiO—polaron trapping vs. band conductivity

We investigate theoretically the microscopic mechanisms of carrier doping in the paradigmatic antiferromagnetic (AFM) and paramagnetic (PM) phases of MnO and NiO by first principles supercell calculation allowing symmetry breaking. To correctly describe strong correlation effects, we extend density-functional theory to comply with Koopmans’ linearity. Transitions are described by relaxed total energy differences, not orbital energies. For NiO, we find delocalized perturbed host band states for both electron and hole doping, in both AFM and PM phases, including a split-off small polaron like state just 0.1 eV above the ground state, in agreement with observations.  For MnO, we find a clear asymmetry between electron and hole doping: holes get trapped in split-off deep gap states, whereas electrons are delocalized in conduction band states. For the split-off small polaron states in both MnO and NiO, the charge trapping changes the on-site screening, which via a charge self-regulation mechanism not only induces new gap states akin to Zhang-Rice states, but also induces ultra-deep states that lie below the valence band.