Insulating phases of vanadium dioxide are Mott-Hubbard insulators

Abstract

We present comprehensive broadband optical spectroscopy data on two insulating phases of vanadium dioxide $(\mathrm{V}{\mathrm{O}}_{2})$: monoclinic ${M}_{2}$ 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 ${M}_{2}$ and triclinic phases. Moreover, the optical interband features in the ${M}_{2}$ and triclinic phases are remarkably similar to those observed in the well-studied monoclinic ${M}_{1}$ insulating phase of $\mathrm{V}{\mathrm{O}}_{2}$. 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.