The insulating phases of vanadium dioxide are Mott-Hubbard insulators

TL;DR

This study shows that different insulating phases of VO2 have similar electronic structures and energy gaps, which are primarily due to Coulomb correlations rather than structural Peierls effects.

Contribution

First broadband optical spectroscopy comparison of M2 and triclinic VO2 phases, revealing the gap's insensitivity to structural changes and emphasizing Coulomb correlations.

Findings

Energy gap remains unchanged across M2 and triclinic phases.

Optical features are similar to the M1 phase.

Peierls effects are not the main cause of the gap.

Abstract

We present the first 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 Peierls effects as the dominant contributor to the opening of the gap. Rather, the energy gap arises from intra-atomic Coulomb correlations.