Johari-Goldstein relaxation far below Tg: Experimental evidence for the Gardner transition in structural glasses?

TL;DR

This study presents experimental dielectric measurements of glass formers at very low temperatures, providing evidence that supports the existence of the Gardner transition deep within the glassy state.

Contribution

It offers the first experimental hints of the Gardner transition in canonical structural glasses through dielectric response analysis.

Findings

Unusual broadening of Johari-Goldstein beta relaxation below Tg

Evidence consistent with the Gardner transition in sorbitol and xylitol

Broadening exceeds expectations from energy barrier distributions

Abstract

Experimental evidence for the Gardner transition, theoretically predicted to arise deep in the glassy state of matter, is scarce. At this transition, the energy landscape sensed by the particles forming the glass is expected to become more complex. In the present work, we report the dielectric response of two typical glass formers with well-pronounced Johari-Goldstein beta relaxation following this response down to unprecedented low temperatures, far below the glass transition. As the Johari-Goldstein process is believed to arise from the local structure of the energy landscape, its investigation seems an ideal tool to seek evidence for the Gardner transition. Indeed, we find an unusual broadening of the beta relaxation below TG ~ 110 K for sorbitol and TG ~ 100 K for xylitol, in excess of the expected broadening arising from a distribution of energy barriers. Thus, these results provide hints at the presence of the Gardner transition in canonical structural glass formers.