Hole-lattice coupling and photoinduced insulator-metal transition in VO2

Photoinduced insulator-metal transition in VO${}_{2}$ and the related transient and multi-time-scale structural dynamics upon photoexcitation are explained within a unified framework. Holes created by photoexcitation weaken the V-V bonds and eventually break V-V dimers in the ${M}_{1}$ phase of VO${}_{2}$ when the laser fluence reaches a critical value. The breaking of the V-V bonds in turn leads to an immediate electronic phase transition from an insulating to a metallic state while the crystal lattice remains monoclinic in shape. The coupling between excited electrons and the 6.0-THz phonon mode is found to be responsible for the observed zigzag motion of V atoms upon photoexcitation and is consistent with coherent phonon experiments.