Microscopic dynamics of collective acoustic excitations in simple liquids

TL;DR

This study explores the microscopic dynamics of collective vibrational excitations in simple liquids, revealing persistent longitudinal excitations even in viscous states and the localization of transverse excitations, advancing understanding of liquid dynamics.

Contribution

It provides a unified thermodynamic framework and detailed analysis of how collective excitations propagate or localize in simple liquids, highlighting differences between longitudinal and transverse modes.

Findings

Longitudinal acoustic excitations persist in viscous liquids.

Transverse excitations tend to localize and are often undetectable.

Propagation lengths determine the crossover from propagation to localization.

Abstract

In this letter, we systematically investigate the microscopic dynamics of collective vibrational excitations in simple liquids. The thermodynamic states of simple liquids are unified to the mean atomic free volume. Our results show that longitudinal acoustic collective vibrational excitations are always observed in simple liquids even when the liquids are viscous, in which the 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 collective vibrational excitations can still propagate in viscous liquids. However, transverse acoustic collective vibrational excitations in viscous liquids become localized since both short- and long-wavelength transverse acoustic collective vibrational excitations have propagation lengths smaller than the atomic diffusion limit. Therefore, transverse acoustic collective vibrational excitations 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 collective vibrational excitations, respectively, is further calculated to quickly determine the propagation-to-localization crossover of collective vibrational excitations in simple liquids. Our findings here advance the understanding of the microscopic dynamics of collective vibrational excitations in simple liquids.