A supercooled binary mixture of soft disks in 2 dimensions was studied by using molecular dynamics simulations. It was seen to exhibit a breakdown in scaling, between the self-diffusion coefficient and structural relaxation time, below a certain cross-over temperature. The results suggested that this breakdown arose from 2 causes. Firstly, the small displacements which were involved in structural relaxation corresponded to a transition between ballistic and diffusive motion. Secondly, the various physical quantities which were averaged in order to obtain transport and relaxation time scales tended to sample different parts of an increasingly heterogeneous distribution of local relaxation times, and therefore exhibited differing temperature variations.

Origin of the Difference in the Temperature Dependences of Diffusion and Structural Relaxation in a Supercooled Liquid D.N.Perera, P.Harrowell: Physical Review Letters, 1998, 81[1], 120-3