Role of the lattice in the light-induced insulator-to-metal transition in vanadium dioxide

Vanadium dioxide is a model material system which exhibits a metal-to-insulator transition at 67 degrees C. This holds potential for future ultrafast switching in memory devices but typically requires a purely electronic process to avoid the slow lattice response. The role of lattice vibrations is thus important, but it is not fully understood and it has been a long-standing source of controversy. We use ultrafast spectroscopy and ab initio quantum calculations to study the mechanism responsible for the transition. We identify an atypical Peierls vibrational mode associated with the transition and study its associated properties. Our findings led to theoretical prediction of a new electron-phonon pathway for a purely reversible electronic transition in a true bistable fashion under specific conditions. This transition is very atypical, as it involves purely chargelike excitations and requires only small nuclear displacement and might prompt future experimental investigations.