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

TL;DR

This study uses molecular dynamics simulations to reveal that GeTe's supercooled liquid state exhibits high atomic mobility and a breakdown of the Stokes-Einstein relation, linking fragility to rapid crystallization in phase change materials.

Contribution

It provides a microscopic understanding of the high mobility and Stokes-Einstein breakdown in GeTe's supercooled liquid, highlighting fragility as key to fast crystallization.

Findings

High atomic mobility near glass transition temperatures

Breakdown of Stokes-Einstein relation in supercooled liquid

Fragility correlates with rapid crystallization

Abstract

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.