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 ($Q$ ≈ 0.2 Å$^{−1}$) 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 $Q$ ≈ 0.8 Å$^{−1}$ 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.