Coherent structural dynamics of a prototypical Charge-Density-Wave-to-Metal transition

In so called charge-density-wave compounds, the peculiar shape of the Fermi surface as well as electron-phonon coupling lead to a low-temperature broken symmetry ground state. This state is characterized by a modulation of the charge density (hence the name) and, via electron-phonon coupling, a distortion of the equilibrium lattice positions. Using femtosecond time resolved x-ray diffraction, we were able to follow the coherent structural dynamics during the nonthermal melting of a charge density wave in the textbook Peierls system K0.3MoO3. The rich measured dynamics after intense photoexcitation can be described by a simple phenomenological model. It is based on the idea that the structure evolves in an excitation dependent, electronically driven potential energy surface. The results allow us to naturally explain how the change of structural symmetry during an ultrafast phase transition can occur on a time scale given by an atomic vibration period.