  The application of amorphous chalcogenide alloys as data-storage media relies
on their ability to undergo an extremely fast (10-100 ns) crystallisation once
heated at sufficiently high temperature. However, the peculiar features that
make these materials so attractive for memory devices still lack a
comprehensive microscopic understanding. By means of large scale molecular
dynamics simulations, we demonstrate that the supercooled liquid of the
prototypical compound GeTe shows a very high atomic mobility (D \sim 10^(-6)
cm2/s) down to temperatures close to the glass transition temperatures. This
behaviour leads to a breakdown of the Stokes-Einstein relation between the
self- diffusion coefficient and the viscosity in the supercooled liquid. The
results suggest that the fragility of the supercooled liquid is the key to
understand the fast crystallisation process in this class of materials.
