Ballistic displacements within string-like motions in a supercooled liquid

We perform a series of molecular dynamics simulations in a binary Lennard-Jones system at temperatures close to its mode-coupling temperature, T,. We demonstrate that the string-like movements carried out by a fraction of the particles are of ballistic nature. Depending on temperature these ballistic strings are found to become rare events related to free-volume effects on the sample. These facts point to a mechanistic description of the transport process and enables us to provide a microscopic reason for the occurrence of timescale separation. (C) 2004 Elsevier B.V. All rights reserved.