Breakdown of the Stokes-Einstein relation in supercooled liquids: A cage-jump perspective

R Pastore and T Kikutsuji and F Rusciano and N Matubayasi and K Kim and F Greco, JOURNAL OF CHEMICAL PHYSICS, 155, 114503 (2021).

DOI: 10.1063/5.0059622

The breakdown of the Stokes-Einstein relation in supercooled liquids, which is the increase in the ratio tau alpha/tau D between the two macroscopic times for structural relaxation and diffusion on decreasing the temperature, is commonly ascribed to dynamic heterogeneities, but a clear-cut microscopic interpretation is still lacking. Here, we tackle this issue exploiting the single-particle cage-jump framework to analyze molecular dynamics simulations of soft disk assemblies and supercooled water. We find that tau(alpha)/tau(D) alpha < t(p)>/< t(c)>, where < t(p)> and < t(c)> are the cage-jump times characterizing slow and fast particles, respectively. We further clarify that this scaling does not arise from a simple term-by-term proportionality; rather, the relations tau(alpha) alpha < t(p)>/ and T-D alpha < t(p)>/ effectively connect the macroscopic and microscopic timescales, with the mean square jump length shrinking on cooling. Our work provides a microscopic perspective on the Stokes-Einstein breakdown and generalizes previous results on lattice models to the case of more realistic glass-formers.

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