Abstract
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 sites undergo no observable spectroscopic 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.
Original language | English |
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Article number | 015002 |
Journal | Physical Review Materials |
Volume | 2 |
Issue number | 1 |
DOIs | |
State | Published - Jan 30 2018 |
Funding
P.C.R., A.H., and S.J.M. were supported by the Army Research Office, Grant No. W911NF-15-1-0133; A.X.G., R.U.C., and A.A. acknowledge support from the U.S. Army Research Office, under Grant No. W911NF-15-1-0181; J.M.R. was supported by the National Science Foundation through DMR-1729303. Film synthesis at Drexel utilized deposition instrumentation acquired through an Army Research Office DURIP grant (W911NF-14-1-0493). This work used resources at the Advanced Light Source, which is a DOE Office of Science User Facility under Contract No. DE-AC02-05CH11231, and at the Canadian Light Source, which is funded by the Canada Foundation for Innovation, NSERC, the National Research Council of Canada, the Canadian Institutes of Health Research, the Government of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. We thank Diamond Light Source for access to beamline I09 (Proposal No. SI17824) that contributed to the results presented here. X-ray diffraction performed at ORNL was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.