Tunable Low Temperature Phonon-induced Electronic Bi-stability in Vanadium Dioxide*

We use a joint theory and experimental approach to clarify the nature of the Metal-insulator transition in VO₂, by QSGW calculations in the frozen phonon approach, and pump-probe ultra-fast spectroscopy with THz radiation. We first show that the insulating state is induced by a Peierls instability, that is responsible for the orbital selection that drives the material to its insulating state, and is associated with a degenerate two-orbital electronic valence band. At the same time, Peierls excitations driven by the lattice dynamics. A 5.7 THz phonon modes split the degeneracy, in a particular, which in turn induces a rapid metallization. This mode is observed in THz pump measurements, far below the critical temperature in the M₁ phase. Our combined approach sheds light in the nature of the transition, which is in our view mediated by a combination of the lattice dynamics and electronic excitations. Finally, we report a possibility in the theory for a novel mechanism to induce an electronic metal-insulator hysteresis via the electron-phonon coupling to the 6.5 THz phonon mode.