Crossover of quasi-localized dynamics and diffusion in supercooled liquids

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.