  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.
