Fickian crossover and lenght scales from two point functions in supercooled liquids

D. STARIOLO; G. FABRICIUS

JOURNAL OF CHEMICAL PHYSICS

American Institute of Physics

Año: 2006 vol. 125 p. 64505 - 6450510

0021-9606

Particle motion of a Lennard-Jones supercooled liquid near the glass transition is studied bymolecular dynamics simulations. We analyze the wave vector dependence of relaxation times in theincoherent self-scattering function and show that at least three different regimes can be identifiedand its scaling properties determined. The transition from one regime to another happens atcharacteristic length scales. The length scale associated with the onset of Fickian diffusioncorresponds to the maximum size of heterogeneities in the system, and the characteristic time scaleis several times larger than the alpha relaxation time. A second crossover length scale is observed,which corresponds to the typical time and length of heterogeneities, in agreement with results fromfour point functions. The different regimes can be traced back to the behavior of the van Hovedistribution of displacements, which shows a characteristic exponential regime in the heterogeneousregion before the crossover to Gaussian diffusion and should be observable in experiments. Ourresults show that it is possible to obtain characteristic length scales of heterogeneities through thecomputation of two point functions at different times.