Optical investigations on electronic structure changes related to the   metal-insulator transition in VO2 film
  We investigated optical absorption coefficient spectra of an epitaxial VO2 film in wide photon energy (0.5 - 5.0 eV) and temperature (100 - 380 K) regions. In its insulating phase, we observed two d-d transition peaks around 1.3 eV and 2.7 eV and a charge transfer peak around 4.0 eV. As temperature goes above the metal-insulator transition temperature near 340 K, a large portion of the spectral weight of the peak around 4.0 eV becomes redistributed and a Drude-like peak appears. We initially applied the band picture to explain the details of the spectral weight changes, especially the temperature-dependent shift at 2.7 eV, but failed. To check whether the spectral changes are optical signatures of the electron-electron correlation effects, we applied the Hubbard model which takes into account orbital degeneracy. This orbitally degenerate Hubbard model could explain the details of the temperature-dependent peak shifts quite well. In addition, from the peak assignments based on the orbitally degenerate Hubbard model, we could obtain the values of U + delta(~ 3.4 eV) and JH (~ 0.7 eV), where U, delta, and JH are the on-site Coulomb repulsion energy, the crystal field splitting between the t2g bands, and the Hund's rule exchange energy, respectively. Our spectroscopic studies indicate that the electron-electron correlation could play an important role in the metal-insulator transition of VO2. 
C. H. Koo, J. S. Lee, M. W. Kim, Y. J. Chang, T. W. Noh, J. H. Jung B.   G. Chae, and H. -T. Kim