Cooperativity of short-time dynamics revisited

TL;DR

This study uses molecular dynamics simulations to investigate the size dependence of fast dynamics in glass-forming liquids, finding no temperature-dependent dynamic length scale for fast relaxation and clarifying the effects of acoustic modes and Johari-Goldstein relaxation.

Contribution

It revisits the cooperativity of short-time dynamics in glass formers, showing the absence of a temperature-dependent dynamic length scale for fast relaxation.

Findings

No temperature-dependent dynamic length scale for fast dynamics.

Size effects are due to acoustic mode cutoff, not cooperativity.

Johari-Goldstein relaxation shows size effects only in long-time relaxation.

Abstract

Using molecular dynamics simulations we examine the system size dependence of the fast dynamics in two model glass forming liquids, one of them a Lennard-Jones mixture for which cooperative fast relaxation has been reported. We find no indication of a temperature-dependent dynamic length scale characterizing these fast dynamics; the size effects in the short time range are temperature independent, and the consequence of cutting off of long wavelength acoustic modes. In a molecular liquid exhibiting a clear Johari-Goldstein (JG) relaxation, significant size effects are again present both for the vibrational motion and long-time {\alpha} relaxation (only the latter having a significant temperature dependence), but absent for the JG relaxation.