Identifying valence band structure of transient phase in VO2 thin film by hard x-ray photoemission

We investigated valence band structures of a VO2 thin film around the metal-insulator transition (MIT) using bulk-sensitive hard x-ray photoemission spectroscopy and revealed that a second monoclinic M-2 phase was a Mott insulator. A spectrum of the M-2 phase partially existing around the MIT was identified from the spatially mixed valence band structures consisting of the rutile, monoclinic M-1 and M-2 phases. The identified spectrum of the M-2 phase has an energy gap at the Fermi level. The origin of the gap is thought to be due to the Coulomb repulsion energy of the 3d states in equally spaced vanadium atoms in the M-2 phase.