On the metal-insulator transitions in VO2 and Ti2O3 from a unified viewpoint

The metal-insulator (M-I) transitions in VO2 and Ti2O3 were investigated using the three-band Hubbard model in connection with the on-site exchange interaction and lattice distortions. Although these two compounds have different crystal structures, the mechanism of the phase transitions can be understood from a unified viewpoint; an increase in energy level separation among the t(2g) orbitals caused by the lattice distortion triggers an abrupt change in the electron configuration in doubly occupied sites from an S = 1 Hund's coupling state to a low-spin S = 0 state with much larger energy and this strongly suppresses the charge fluctuation, resulting in localization of electrons. These M-I transitions are not induced by an increase in relative strength of the Coulomb interaction against the electron hopping as in the conventional scenario of the Mott-Hubbard transition but by the level splitting among the t(2g) orbitals against the on-site exchange interaction. Switching of the nearest-neighbor spin and orbital correlations and large change in the orbital occupation occur at the M-I transitions. Observation of the change in the orbital occupation at the M-I transitions by the linear dichroic V or Ti 2p x-ray absorption spectroscopy can be a crucial check to this theory.