A New Scenario on the Metal-Insulator Transition in VO2

TL;DR

This paper proposes a new scenario for the metal-insulator transition in VO2, emphasizing the role of lattice distortion-induced orbital level splitting over traditional Coulomb interaction effects, supported by a three-band Hubbard model analysis.

Contribution

It introduces a novel mechanism where orbital level splitting triggers the transition, differing from the conventional Mott-Hubbard transition explanation.

Findings

Transition driven by orbital level splitting, not Coulomb strength increase.

Suppression of charge fluctuation due to spin state change.

Applicable to Ti2O3 as well.

Abstract

The metal-insulator transition in VO2 was investigated using the three-band Hubbard model, in which the degeneracy of the 3d orbitals, the on-site Coulomb and exchange interactions, and the effects of lattice distortion were considered. A new scenario on the phase transition is proposed, where the increase in energy level separation among the t_2g orbitals caused by the lattice distortion triggers an abrupt change in the electronic configuration in doubly occupied sites from an S=1 Hund's coupling state to a spin S=0 state with much larger energy, and this strongly suppresses the charge fluctuation. Although the material is expected to be a Mott-Hubbard insulator in the insulating phase, the metal-to-insulator transition is not caused by an increase in relative strength of the Coulomb interaction against the electron hopping as in the usual Mott transition, but by the level splitting among the t_2g orbitals against the on-site exchange interaction. The metal-insulator transition in Ti2O3 can also be explained by the same scenario. Such a large change in the 3d orbital occupation at the phase transition can be detected by linear dichroic V 2p x-ray absorption measurements.