Dynamic heterogeneities above and below the mode-coupling temperature: Evidence of a dynamic crossover

We examine dynamic heterogeneities in a model glass-forming fluid, a binary harmonic sphere mixture, above and below the mode-coupling temperature T-c. We calculate the ensemble independent susceptibility chi(4)(tau(alpha)) and the dynamic correlation length. 4(tau(alpha)) at the a-relaxation time tau(alpha) a. We also examine in detail the temperature dependence of ta and the diffusion coefficient D. For higher temperatures, we find that the standard Stokes-Einstein relationship, D similar to tau(-1)(alpha) a, holds, but at lower temperatures a fractional Stokes-Einstein relationship, D similar to tau(-sigma)(alpha) a with sigma = 0.65, emerges. By examining the relationships between tau(alpha), D, chi(4)(tau(alpha)), and xi(4)(tau(alpha)) we determine that the emergence of the fractional Stokes-Einstein relationship is accompanied by a dynamic crossover from tau(alpha) similar to e(k2 xi 4) at higher temperatures to tau(alpha) similar to e(k1 xi 3/2) 4 at lower temperatures. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773321]