We investigated the metal-insulator transition for epitaxial thin films of the perovskite CaFeO3, a material with a significant oxygen ligand hole contribution to its electronic structure. We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard X-ray photoelectron spectroscopy, soft X-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe electron valence undergoes no observable change between the metallic and insulating states, whereas the O electronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for CaFeO3 and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by ~5-10% in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.
@article{arxiv.1801.05374,
title = {Electronic structure of negative charge transfer CaFeO3 across the metal-insulator transition},
author = {Paul C. Rogge and Ravini U. Chandrasena and Antonio Cammarata and Robert J. Green and Padraic Shafer and Benjamin M. Lefler and Amanda Huon and Arian Arab and Elke Arenholz and Ho Nyung Lee and Tien-Lin Lee and Slavomír Nemšák and James M. Rondinelli and Alexander X. Gray and Steven J. May},
journal= {arXiv preprint arXiv:1801.05374},
year = {2018}
}