Orbital-Assisted Metal-Insulator Transition in VO₂

We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO₂. We have used soft-x-ray absorption spectroscopy at the V L_2,3 edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.