  Photoexcitation is a powerful means in distinguishing different interactions
and manipulating the states of matter, especially in complex quantum systems.
As a well-known charge density wave (CDW) material, 1T-TaS2 has been widely
studied experimentally thanks to its intriguing photoexcited responses.
However, the microscopic atomic dynamics and underlying mechanism are still
under debate. Here, we demonstrate photoexcitation induced ultrafast dynamics
in 1T-TaS2 using time-dependent density functional theory molecular dynamics.
We discover a novel collective mode induced by photodoping, which is
significantly different from thermally-induced phonon mode in TaS2. In
addition, our finding validates nonthermal melting of CDW induced at low light
intensities, supporting that conventional hot electron model is inadequate to
explain photoinduced dynamics. Our results provide a deep insight on coherent
electron and lattice quantum dynamics during the formation and excitation of
CDW in 1T-TaS2.
