Excitations, Emergent Facilitation and Glassy Dynamics in Supercooled Liquids

In supercooled liquids, dynamical facilitation is the phenomenon where motion begets motion nearby [1], leading to spatially heterogeneous dynamics. Such a phenomenon is central to the glassy dynamics of these liquids, where relaxation timescales exhibit super-Arrhenius growth with decreasing temperature. Despite the ubiquity of supercooled liquids, our mechanistic understanding of dynamical facilitation remains incomplete. Here, we present a theory that unveils the microscopic origins of dynamical facilitation. We show that dynamics progress through localized bond-exchange events (excitations), accumulating elastic stresses with which new excitations can interact [2,3]. At low temperatures, these elastic interactions lead to emergent facilitation where prior excitations lead to the creation of new excitations nearby. Employing principles from linear elasticity and Markov processes, we created a theory and model replicating multiple facets of glassy dynamics, including the stretched exponential decay of relaxation functions, super-Arrhenius timescales, 2D finite-size effects, and subdiffusion at intermediate timescales. We also investigate phonon contributions to diffusion and relaxation, demonstrating how their combination with excitation contributions leads to two-step relaxation processes and the ballistic-subdiffusive-diffusive crossover commonly observed in supercooled liquids. These results show that localized excitations and elastic interactions between them play a role in emergent facilitation and glassy dynamics.

[1] Keys at al. Phys. Rev. X 1 021013 (2011)

[2] Hasyim, M.R. and Mandadapu, K.K. 155 (4), 44504 (2021)

[3] Hasyim, M.R. and Mandadapu, K.K. arXiv:2310.06584 (2023)