ORIGIN OF THE DIFFERENCE IN THE TEMPERATURE DEPENDENCES OF DIFFUSION AND STRUCTURAL RELAXATION IN A SUPERCOOLED LIQUID

A supercooled binary mixture of soft disks in 2D, studied using molecu lar dynamics simulations, exhibits a breakdown of scaling between the self-diffusion constant and a structural relaxation time below a cross over temperature. Evidence is presented that this breakdown arises fro m two causes. First, the small displacements involved in structural re laxation correspond to the transition between ballistic and diffusive motion. Second, the different physical quantities averaged to obtain t ransport and relaxation time scales sample different parts of the incr easingly heterogeneous distribution of local relaxation times and, hen ce, exhibit different temperature variation.