Crossover of quasi-localized dynamics and diffusion in supercooled liquids

Abstract Secondary relaxation, also known as β JG relaxation, is a dynamical process observed in supercooled liquids and glasses. It is distinct from the slower, so-called α relaxation, which is associated with particles escaping the cages formed by their surrounding neighbours. The secondary relaxation affects several properties of the glass, including the crystallization propensity and mechanical response. Traditionally studied via dielectric and mechanical spectroscopy, the β JG relaxation is often linked to quasi-localized particle motion. Here we use a wavevector-resolved X-ray technique to access microscopic, real-space information on the β JG relaxation in a hydrogen-bonded glass former and show that the α and β JG relaxations cannot be described as two independent processes. Moreover, we provide evidence that the β JG relaxation is associated with a sublinear growth of the mean-squared molecular displacement preceding the onset of diffusion. Our findings thus clarify that the β JG relaxation can be described as the critical rattling of molecules within the cage formed by their neighbours just before escaping from it.