Gardner transition

It is a long-standing and still unsolved question in the physics of condensed matter if the glass transition is a purely kinetic phenomenon or represents a kind of hidden thermodynamic phase transition. However, this ideal glass transition is not directly accessible in experiments because of the inevitable loss of thermodynamic equilibrium when cooling a liquid below its technical glass temperature Tg. Very recently, theoretical predictions have been made that, deep in the glassy state, a further phase transition termed the Gardner transition occurs, which is observable even under non-equilibrium conditions. At this transition, the existing basins of the free-energy landscape of the glassy state become fractionalized into sub-basins, explaining the universal low-temperature behavior of glasses (see Fig. 1). Indeed, it is believed that the Gardner transition is important for a large variety of disordered systems making it of broad interest for all kind of disordered matter.

gardner transition schema

Figure 1: Schematic energy-landscape plot explaining the α- and the Johari-Goldstein β relaxation. Inset: Suggested further roughening of the landscape caused by the Gardner transition occurring at TG

For the first time, we have provided systematic dielectric measurements of the Johari-Goldstein β relaxation at temperatures far below Tg (see Fig. 2). This universal relaxation process is commonly believed to arise from the local structure of the energy landscape and, thus, ideally suited to investigate the suggested variation of just this landscape at the Gardner transition. By a detailed analysis of our low-temperature data, we indeed find clear hints for the Gardner transition. This is only the second experimental work providing experimental evidence for this transition and the first performed for canonical structural glass formers.

xylitol dielectric-loss

Figure 2: Dielectric-loss spectra of xylitol for various temperatures very far below the glass-transition temperature (Tg = 248 K), extending down to 9 K. The peaks signify the Johari-Goldstein relaxation process; at low temperatures only the right flank of the peak is visible. The lines are fits by a combination of power laws and an empirical peak function. From the temperature dependence of the fit parameters, we conclude on an anomalous low-temperature broadening of the peaks, indicative of the Gardner transition.

To learn more, see:

Johari-Goldstein relaxation far below Tg: Experimental evidence for the Gardner transition in structural glasses?
K. Geirhos, P. Lunkenheimer, and A. Loidl, Phys. Rev. Lett. 120, 085705 (2018).