Two inherent crossovers of the diffusion process in glass-forming liquids

Abstract

We report on incoherent quasielastic neutron scattering measurements examining a self-diffusion process in two types of glass-forming liquids, namely a molecular liquid (3-methylpentane) and an ionic liquid [1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide]. We have experimentally demonstrated that both liquids exhibit two crossovers in the momentum transfer (Q) dependence of the self-correlation function, which is basically described by the stretched exponential function, exp[-(t/τ)β]. The first crossover point (Q≈0.2Å-1) corresponds to a crossover from Fickian (β=1) to non-Fickian (β≠1) diffusion attributed to dynamical correlation. On the other hand, the second one at Q≈0.8Å-1 is associated with the crossover from Gaussian to non-Gaussian behavior. It is remarkable that the stretching exponent β gradually changes in between the two crossover points. We consider that the two crossovers are the universal feature for glass-forming liquids.