Ultrafast Collective Dynamics in the Charge-Density-Wave Conductor K${}_{0.3}$MoO${}_3$

Low-energy coherent charge-density wave excitations are investigated in blue bronze (K${}_{0.3}$MoO${}_3$) and red bronze (K${}_{0.33}$MoO${}_3$) by femtosecond pump-probe spectroscopy. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K${}_{0.33}$MoO${}_3$. The amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.Low-energy coherent charge-density wave excitations are investigated in blue bronze (K${}_{0.3}$MoO${}_3$) and red bronze (K${}_{0.33}$MoO${}_3$) by femtosecond pump-probe spectroscopy. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K${}_{0.33}$MoO${}_3$. The amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.