Decoupled ultrafast electronic and structural phase transitions in photoexcited monoclinic VO 2 .

Photoexcitation has emerged as an efficient way to trigger phase transitions in strongly correlated materials. There are great controversies about the atomistic mechanisms of structural phase transitions (SPTs) from monoclinic ( M 1 -) to rutile ( R -) VO 2 and its association with electronic insulator-metal transitions (IMTs). Here, we illustrate the underlying atomistic processes and decoupling nature of photoinduced SPT and IMT in nonequilibrium states. The photoinduced SPT proceeds in the order of dilation of V-V pairs and increase of twisting angles after a small delay of ~40 fs. Dynamic simulations with hybrid functionals confirm the existence of isostructural IMT. The photoinduced SPT and IMT exhibit the same thresholds of electronic excitations, indicating similar fluence thresholds in experiments. The IMT is quasi-instantaneously (<10 fs) generated, while the SPT takes place with time a constant of 100 to 300 fs. These findings clarify some key controversies in the literature and provide insights into nonequilibrium phase transitions in correlated materials.