Ultrafast control of the crystal structure in a topological charge-density-wave material

The optical control of crystal structures is a promising route to change physical properties including the topological nature of a targeting material. Time-resolved x-ray diffraction measurements using an x-ray free-electron laser are performed to study the ultrafast lattice dynamics of VTe₂, which is known to possess a unique charge-density-wave (CDW) ordering coupled to the topological surface states as a first-order phase transition. A significant oscillation of the CDW amplitude mode is observed at a superlattice reflection as well as Bragg reflections. The frequency of the oscillation is independent of the fluence of the pumping laser, which is prominent in the CDW ordering of the first-order phase transition. Furthermore, the timescale of the photoinduced 1T^'' to $1T$ phase transition is not equal to the half period of the CDW amplitude mode, which is typically the case for Peierls insulators.