Determining the origin of the insulating gap in the monoclinic VO2(M1) is a long-standing issue. The difficulty of this study arises from the simultaneous occurrence of structural and electronic transitions upon thermal cycling. Here, we compare the electronic structure of the M1 phase with that of single crystalline insulating VO2(A) and VO2(B) thin films to better understand the insulating phase of VO2. As these A and B phases do not undergo a structural transition upon thermal cycling, we comparatively study the origin of the gap opening in the insulating VO2 phases. By x-ray absorption and optical spectroscopy, we find that the shift of unoccupied t2g orbitals away from the Fermi level is a common feature, which plays an important role for the insulating behavior in VO2 polymorphs. The distinct splitting of the half-filled t2g orbital is observed only in the M1 phase, widening the bandgap up to ∼0.6 eV. Our approach of comparing all three insulating VO2 phases provides insight into a better understanding of the electronic structure and the origin of the insulating gap in VO2.
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This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Work for optical spectroscopy was supported by No. IBS-R009-D1. Computational work was supported by the National Institute of Supercomputing and Network/Korea Institute of Science and Technology Information with supercomputing resources including technical support No. KSC-2015-C3-034. Spectroscopic ellipsometry at the University of Kentucky was supported by the National Science Foundation grant DMR-1454200. Work at the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
Lee, Shinbuhm; Meyer, Tricia L.; Sohn, Changhee; Lee, Donghwa; Nichols, John; Lee, Dongkyu; Seo, Sung S. Ambrose; Freeland, John W.; Noh, Tae Won; and Lee, Ho Nyung, "Electronic Structure and Insulating Gap in Epitaxial VO2 Polymorphs" (2015). Physics and Astronomy Faculty Publications. 358.