Instantaneous band gap collapse in photoexcited monoclinic VO$_2$ due to   photocarrier doping
  Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO$_2$ quasi-instantaneously into a metal. Thereby, we exclude an 80 femtosecond structural bottleneck for the photoinduced electronic phase transition of VO$_2$. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO$_2$ bandgap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructral insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening \emph{before} significant hot-carrier relaxation or ionic motion has occurred. 
Daniel Wegkamp, Marc Herzog, Lede Xian, Matteo Gatti, Pierluigi   Cudazzo, Christina L. McGahan, Robert E. Marvel, Richard F...
cond-mat.str-el cond-mat.mtrl-sci