Microscopic dynamics of collective acoustic excitations in simple liquids

Here, we systematically investigate the microscopic dynamics of collective vibrational excitations (CVEs) in simple liquids, of which the thermodynamic state is unified to the mean atomic free volume. Our results show that longitudinal acoustic CVEs are always observed in simple liquids, even when the liquids are viscous, in which the mean atomic free volume is larger than the cross point of the corresponding mean propagation length and the atomic diffusion limit. This is because some long-wavelength longitudinal acoustic CVEs can still propagate in viscous liquids. However, transverse acoustic CVEs in viscous liquids become localized since both short- and long-wavelength transverse acoustic CVEs have propagation lengths smaller than the atomic diffusion limit. Therefore, transverse acoustic CVEs may not be detected in simple liquids. The propagation length of macroscopic elastic and shear waves which are the mechanical response of long-wavelength longitudinal and transverse CVEs, respectively, is further calculated to quickly determine the propagation-to-localization crossover of CVEs in simple liquids. Our findings here advance the understanding of microscopic dynamics of CVEs in simple liquids.