Breakdown of Stokes-Einstein relation in the supercooled liquid state of phase change materials

The application of amorphous chalcogenide alloys as data-storage media relies on their ability to undergo an extremely fast (10100?ns) crystallization 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 similar to 10-6?cm2?s-1) down to temperatures close to the glass transition temperatures. This behavior leads to a breakdown of the StokesEinstein 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 crystallization process in this class of materials.