Origin of band and localized electron states in photoemission of NiO

In a variety of model studies it has been shown that the problem of a single hole in a Mott insulator can be quite well addressed by assuming that all that matters is the interaction between the propagating hole and the spin waves of the insulator. NiO has been often taken as the archetypical example of a Mott insulator and recent angular resolved photoemission studies have revealed that holes in this material share both itinerant and localized aspects that are very hard to understand either in conventional band-structure theory or from purely localized approaches. Starting from a strongly coupled electronic multiband Hubbard model, we derive a generalized strong-coupling spin-fermion model. The model includes the multiplet structure of the electronic excitations and describes the interaction of the O(2p) holes moving in oxygen bands with the spins localized on Ni ions. In linear spin-wave order we find an effective Hamiltonian describing the scattering of the bandlike holes on the spin waves. This problem is solved in rainbow order, and we find that the outcomes resemble well the experimental findings. In contrast to earlier impurity interpretations stressing spatial locality, we find that momentum dependencies are dominating the hole dynamics.