Breakdown of the Stokes-Einstein Relation Above the Melting Temperature in a Liquid Phase-Change Material

The dynamic properties of liquid phase-change materials (PCMs), such as viscosity $\eta$ and atomic self-diffusion coefficients D, play an essential role in ultrafast phase switching behavior of novel non-volatile phase-change memory applications, as they are intimately related to crystallization kinetics and phase stabilities. To connect $\eta$ to D, the Stokes-Einstein relation (SER) is commonly assumed to be valid at high temperatures near or above the melting temperature $T_{m}$ and is frequently employed for assessing liquid fragility (or crystal growth velocity) of technologically important PCM compositions. However, using quasi-elastic neutron scattering (QENS), we give here experimental evidence for a breakdown of the SER even at temperatures above $T_{m}$ in the high-atomic-mobility state of a typical PCM, Ge$_{1}$Sb$_{2}$Te$_{4}$, where the decay of density correlation functions still remains exponential. The origin of the breakdown is thus unlikely the result of dynamical heterogeneities, as is usually postulated for viscous liquids. Rather, we discuss its possible connections to a metal-semiconductor and fragile-strong transition hidden below $T_{m}$.