Evidence of growing spatial correlations at the glass transition from nonlinear susceptibility measurements

We have measured the ac nonlinear dielectric response χ 3 ( ω , T ) of an archetypical glassformer (glycerol) just above its glass transition temperature T g ∼190 K. Our measurements were performed at temperatures between T g + 4 K and T g + 35 K, which corresponds to 5 decades of variation of the relaxation time τ ( T ) of the liquid. We find that χ 3 ( ω , T ) is peaked for ωτ ∼ 0.2 and that the height of the peak grows as one approaches T g . A powerlaw in frequency beyond the peak of χ 3 is observed and χ 3 ( ω , T ) displays scaling as a function of ωτ . These features correspond to the main predictions of some recent theoretical works relating the nonlinear susceptibility of supercooled liquids to the average number N corr of dynamically correlated molecules. We thus interpret the experimental increase of the peak of nonlinear susceptibility when T decreases, as an increase of N corr when T → T g . This strongly reinforces the collective picture of the dynamics of supercooled liquids close to the glass transition. Finally, we discuss some heating effects and show that they do not affect significantly our conclusions. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)