The optical and infrared properties of films of vanadium dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$ and vanadium sesquioxide $\left({\mathrm{V}}_2{\mathrm{O}}_3\right)$ 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) $6\mathrm{eV}$. 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 $\mathrm{V}{\mathrm{O}}_2$.

• Received 19 September 2007

DOI:https://doi.org/10.1103/PhysRevB.77.115121