Are supercooled liquids Fickian yet non Gaussian?

TL;DR

This paper critiques a recent study claiming that glass-forming liquids exhibit Fickian yet non-Gaussian diffusion, arguing that the reported scales are not well-defined and that the conclusions are not supported.

Contribution

The authors clarify misconceptions about the scales in glass-formers and dispute the claim that these systems show Fickian non-Gaussian behavior.

Findings

The reported length and time scales are either previously characterized or ill-defined.

The paper disputes the existence of Fickian non-Gaussian diffusion in glass-formers.

It questions the interpretation of dynamic heterogeneity in the context of the criticized study.

Abstract

Comment on `Fickian Non-Gaussian Diffusion in Glass-Forming Liquids', by Rusciano et al., Phys. Rev. Lett. 128, 168001 (2022). In a recent Letter, Rusciano et al. examined the statistics of individual particles displacements in two-dimensional glass-formers and concluded that the corresponding probability distribution is non-Gaussian in a time regime where the mean-squared displacement is Fickian. Here, we clarify that the multiple length scales and time scales reported in this work have either been characterized before, or are not well-defined. This leads us to dispute the conclusions that glass-formers display Fickian non-Gaussian behaviour and that this analogy fruitfully addresses the central questions regarding the nature of dynamic heterogeneity in these systems.