Fast sound in two-component liquids

A new high-frequency and short-wavelength collective mode specific to binary liquid mixtures with large mass difference is observed in a computer simulation of ${\mathrm{Li}}_{0.8}$ ${\mathrm{Pb}}_{0.2}$ and discussed within the framework of the Mori-Zwanzig formalism. The mode shows linear dispersion in a wave-number regime 0.1 A${\mathrm{̊}}^{\mathrm{-}1}$≤q≤0.6 A${\mathrm{̊}}^{\mathrm{-}1}$, but its propagation velocity is higher than the ordinary sound velocity by more than a factor of 3. Its attenuation is only weakly q-dependent in contrast to the damping of ordinary sound. In ${\mathrm{Li}}_{0.8}$ ${\mathrm{Pb}}_{0.2}$ ‘‘fast sound’’ entails motion of the lighter atoms only.