On the Metal–Insulator Transitions in VO2and Ti2O3from a Unified Viewpoint

Abstract

The metal–insulator (M–I) transitions in VO 2 and Ti 2 O 3 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 2 g 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 2 g orbitals against the on-site exchange interaction. Switching of the nearest-neighbor spin and...