Evidence of growing spatial correlations at the glass transition from nonlinear response experiments

TL;DR

This study measures the nonlinear dielectric response of glycerol near its glass transition, revealing increasing spatial correlations and collective dynamics as the liquid approaches the glassy state.

Contribution

It provides the first direct experimental estimate of the temperature dependence of the number of dynamically correlated molecules in a glass-forming liquid.

Findings

Peak in $ ext{chi}_3( ext{omega}, T)$ as a function of frequency

Scaling of $ ext{chi}_3$ with $ ext{omega} au(T)$

Increase of $N_{corr}(T)$ as the system approaches the glass transition

Abstract

The ac nonlinear dielectric response $\chi_3(\omega,T)$ of glycerol was measured close to its glass transition temperature $T_g$ to investigate the prediction that supercooled liquids respond in an increasingly non-linear way as the dynamics slows down (as spin-glasses do). We find that $\chi_3(\omega,T)$ indeed displays several non trivial features. It is peaked as a function of the frequency $\omega$ and obeys scaling as a function of $\omega \tau(T)$, with $\tau(T)$ the relaxation time of the liquid. The height of the peak, proportional to the number of dynamically correlated molecules $N_{corr}(T)$, increases as the system becomes glassy, and $\chi_3$ decays as a power-law of $\omega$ over several decades beyond the peak. These findings confirm the collective nature of the glassy dynamics and provide the first direct estimate of the $T$ dependence of $N_{corr}$.