Low-energy collective excitations in a charge-density wave conductor K0.3MoO3

We report on time-resolved ultrafast optical measurements on a blue bronze (K0.3MoO3) single crystal grown by the electro-crystallization of high-temperature solution method. The differential reflectivity shows coherent THz and GHz oscillations due to the excitation of amplitudon and phason of charge-density waves in the long-wavelength limit. The wavelength dependence of the period of such oscillations allows us to determine the dispersion relationship of these modes. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy. 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.