Electrodynamics of the vanadium oxidesVO2andV2O3

Abstract

The optical and infrared properties of films of vanadium dioxide $(\mathrm{V}{\mathrm{O}}_{2})$ and vanadium sesquioxide $({\mathrm{V}}_{2}{\mathrm{O}}_{3})$ have been investigated via ellipsometry and near-normal incidence reflectance measurements from far infrared to ultraviolet frequencies. Significant changes occur in the optical conductivity of both $\mathrm{V}{\mathrm{O}}_{2}$ and ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ across the metal-insulator transitions at least up to (and possibly beyond) $6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We argue that such changes in optical conductivity and electronic spectral weight over a broad frequency range are evidence of the important role of electronic correlations to the metal-insulator transitions in both of these vanadium oxides. We observe a sharp optical transition with possible final state (exciton) effects in the insulating phase of $\mathrm{V}{\mathrm{O}}_{2}$. This sharp optical transition occurs between narrow ${a}_{1g}$ bands that arise from the quasi-one-dimensional chains of vanadium dimers. Electronic correlations in the metallic phases of both $\mathrm{V}{\mathrm{O}}_{2}$ and ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ lead to reduction of the kinetic energy of the charge carriers compared to band theory values, with paramagnetic metallic ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ showing evidence of stronger correlations compared to rutile metallic $\mathrm{V}{\mathrm{O}}_{2}$.