Insulating phases of vanadium dioxide are Mott-Hubbard insulators

We present comprehensive broadband optical spectroscopy data on two insulating phases of vanadium dioxide (VO₂): monoclinic M₂ and triclinic. The main result of our work is that the energy gap and the electronic structure are essentially unaltered by the first-order structural phase transition between the M₂ and triclinic phases. Moreover, the optical interband features in the M₂ and triclinic phases are remarkably similar to those observed in the well-studied monoclinic M₁ insulating phase of VO₂. As the energy gap is insensitive to the different lattice structures of the three insulating phases, we rule out vanadium-vanadium pairing (the Peierls component) as the dominant contributor to the opening of the gap. Rather, the energy gap arises primarily from intra-atomic Coulomb correlations.