Electrodynamics of the vanadium oxides VO₂ and V₂O₃

The optical and infrared properties of films of vanadium dioxide (VO₂) and vanadium sesquioxide (V₂O₃) 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 VO₂ and V₂O₃ across the metal-insulator transitions at least up to (and possibly beyond) 6 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 VO₂. 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 VO₂ and V₂O₃ lead to reduction of the kinetic energy of the charge carriers compared to band theory values, with paramagnetic metallic V₂O₃ showing evidence of stronger correlations compared to rutile metallic VO₂.