Correlation between Fast Relaxation, Shear Phonon Propagation, and Dynamic Heterogeneity in Glass-Forming Liquids

Fast relaxation is the precursor to α-relaxation, yet its microscopic origin and role in the emergence of glassy dynamics remain elusive. Using inelastic neutron scattering on the fragile glass-former Ca0.4K0.6(NO₃)1.4, we show that the fast relaxation dynamics are strongly modulated by local structure and exhibit a distinct temperature dependence that correlates with shear phonon propagation and the Arrhenius-to-super-Arrhenius crossover of α-relaxation. These results indicate that fast relaxation arises from the interplay of shear phonon softening and local structural arrangements, serving as the intermediate dynamical process that connects the elastic interactions to kinetic facilitation underlying the emergence of dynamic heterogeneity in supercooled liquids.