The evolution of the electronic structures of strongly correlated insulators with doping has long been a central fundamental question in condensed matter physics; it is also of great practical relevance for applications. We have studied the evolution of NiO under hole {\em and} electron doping using high-quality thin film and a wide range of experimental and theoretical methods. The evolution is in both cases very smooth with dopant concentration. The band gap is asymmetric under electron and hole doping, consistent with a charge-transfer insulator picture, and is reduced faster under hole than electron doping. For both electron and hole doping, occupied states are introduced at the top of the valence band. The formation of deep donor levels under electron doping and the inability to pin otherwise empty states near the conduction band edge is indicative that local electron addition and removal energies are dominated by a Mott-like Hubbard U-interaction even though the global bandgap is predominantly a charge-transfer type gap.
@article{arxiv.2001.11436,
title = {Doped NiO: the Mottness of a charge transfer insulator},
author = {Friederike Wrobel and Hyowon Park and Changhee Sohn and Haw-Wen Hsia and Jian-Min Zuo and Hyeondeok Shin and Ho Nyung Lee and P. Ganesh and Anouar Benali and Paul R. C. Kent and Olle Heinonen and Anand Bhattacharya},
journal= {arXiv preprint arXiv:2001.11436},
year = {2020}
}