3 years ago

The electronic structure of negative charge transfer CaFeO$_3$ across the metal-insulator transition.

Padraic Shafer, Arian Arab, Tien-Lin Lee, Elke Arenholz, Amanda Huon, Paul C. Rogge, Robert J. Green, Benjamin M. Lefler, Steven J. May, James M. Rondinelli, Antonio Cammarata, Alexander X. Gray, Ravini U. Chandrasena, Slavomír Nemšák, Ho Nyung Lee

We investigated the metal-insulator transition for epitaxial thin films of the perovskite CaFeO$_3$, 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 CaFeO$_3$ 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 ${\sim}$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.

Publisher URL: http://arxiv.org/abs/1801.05374

DOI: arXiv:1801.05374v1

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