Vanadium dioxide (VO2) undergoes a metal-insulator transition at 340 K. The electronic phase transition is accompanied by a "simultaneous" structural change from a low-temperature monoclinic (M1) to a high-temperature rutile (R) structure. Despite decades of intensive research, the physics behind the transition remains a subject of unabated debate. Recently, it was found that the phase transition in VO2 can be also induced by photo-excitation, gating (using ionic liquid), or external stress. In particular, ultrafast experiments revealed a multi-timescale structural evolution ranging from sub-picoseconds to nanoseconds in VO2 upon photo-excitation.

In this talk, I will present a theory that is able to explain the insulator-metal transition in VO2 and the related transient and multi-time-scale structural dynamics upon photo-excitation. Holes created by photo-excitation (or thermal excitation) weaken the V-V bonds and eventually break V-V dimers in the M1 phase when the hole concentration 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 even the crystal lattice remains monoclinic. If time permits, I will also discuss a few related topics. For example, gating-induced phase transition in VO2 is likely of a very different origin, and the magnetic behavior of the metallic phase is still not well understood.