Insulator-metal transitions in the T phase Cr-doped and M1 phase undoped VO2 thin films

VO2 exhibits several insulating phases (monoclinic M1, M2, and triclinic T), and the study of these phases is important for understanding the true nature of the insulator-to-metal transition (IMT) in VO2. These insulating phases have small but discernible crystallographic differences in the vanadium chains forming the dimers. Peculiarities of the electron localizations in the dimerized chains for many of the probes such as NMR make it difficult to characterize the true character of these phases [T. J. Huffman et al., Phys. Rev. B 95, 075125 (2017); J. Pouget et al., Phys. Rev. B 10, 1801 (1974)]. Here we present structural, electrical, ultrafast-reflectivity, and electronic structure studies of the T phase Cr-doped VO2 and the M1 phase pure VO2 thin films, both grown by pulsed laser deposition under identical conditions. An intermediate M2 structure is observed in the Cr-doped VO2, while the pure VO2 directly goes from the insulating monoclinic M1 structure to a metallic rutile R structure, manifested by temperature-dependent Raman spectroscopy. Temperature-dependent electronic structure studies utilizing x-ray near-edge absorption spectroscopy reveal that all these insulating phases (monoclinic M1 and M2 and triclinic T) have similar electronic structures which place these insulating phases into the list of Mott-Hubbard insulators. This is a first combined experimental report on the electronic structure of all the three insulating phases, monoclinic M1, M2, and triclinic T.