Relationship between vibrations and dynamical heterogeneity in a model glass former: extended soft modes but local relaxation

TL;DR

This study explores how vibrational modes relate to relaxation dynamics in a model glass, revealing that localized excitations influence vibrational properties and challenge the idea that extended soft modes drive relaxation.

Contribution

It demonstrates that localised excitations, not extended soft modes, are key to relaxation, providing a new perspective on vibrational-dynamical heterogeneity in glass formers.

Findings

Localized excitations propagate relaxation.

Correlation between low-frequency modes and high mobility regions.

Extended soft modes are not the primary cause of relaxation.

Abstract

We study the relation between short-time vibrational modes and long-time relaxational dynamics in a kinetically constrained lattice gas with harmonic interactions between neighbouring particles. We find a correlation between the location of the low (high) frequency vibrational modes and regions of high (low) propensity for motion. This is similar to what was observed in continuous force systems, but our interpretation is different: in our case relaxation is due to localised excitations which propagate through the system; these localised excitations act as background disorder for the elastic network, giving rise to anomalous vibrational modes. Our results show that a correlation between spatially extended low frequency modes and high propensity regions does not imply that relaxational dynamics originates in extended soft modes. We consider other measures of elastic heterogeneity, such as non-affine displacement fields and mode localisation lengths, and discuss implications of our results to interpretations of dynamic heterogeneity more generally.