OPTICAL SPECTROSCOPY OF THE M$_{\mathrm{2}}$ AND T PHASES OF VANADIUM DIOXIDE

The salient feature of the familiar structural transition that accompanies the metal-insulator transition in bulk VO$_{\mathrm{2}}$ is a pairing of all of the vanadium ions in the M$_{\mathrm{1}}$ insulating phase. This pairing has long been thought critical to the emergence of insulating behavior. However, there exist two less familiar insulating states, M$_{\mathrm{2}}$ and T. These phases notably exhibit distinctly different V-V pairing. In the M$_{\mathrm{2}}$ phase, only half of the vanadium ions exhibit pairing while the other half carry local spin 1/2 magnetic moments and are equally spaced in quasi-one dimensional chains. The T phase has two types of inequivalent vanadium chains, each consisting of V-V pairs but with different spacing between V ions in the pairs. The M$_{\mathrm{1}}$ phase has been studied extensively with optical spectroscopy. By studying the two less familiar insulating phases, M$_{\mathrm{2}}$ and T, one can investigate how changes in V-V pairing affect the properties of the VO$_{\mathrm{2}}$ insulating state. We performed infrared and optical spectroscopy on the M$_{\mathrm{2}}$ and T phases in the same sample. Despite a clear change in the lattice structure, the inter-band transitions are insensitive to changes in the V-V pairing. This result conclusively establishes that intra-atomic Coulomb repulsion between electrons provides the dominant contribution to the energy gap in all insulating phases of VO$_{\mathrm{2}}$. Our work highlights the necessity of considering the M$_{\mathrm{2}}$ and T phases of VO$_{\mathrm{2}}$ in future experimental and theoretical research.