PHYSICAL REVIEW B
We present comprehensive broadband optical spectroscopy data on two insulating phases of vanadium dioxide (VO2): 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 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.
Huffman, T. J.; Hendriks, C.; Walter, E. J.; Yoon, Joonseok; Ju, Honglyoul; Smith, R.; Carr, G. L.; Krakauer, H. X.; and Qazilbash, M. M., Insulating phases of vanadium dioxide are Mott-Hubbard insulators (2018). PHYSICAL REVIEW B, 95(7).