Speaker
Description
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. 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 often employed for assessing liquid fragility (or crystal growth velocity) of technologically important PCMs. However, using quasi-elastic neutron scattering (QENS), we provide experimental evidence for a breakdown of the SER even at temperatures above T$_m$ in the high-atomic-mobility state of well-known PCMs Ge$_1$Sb$_2$Te$_4$, Ge$_2$Sb$_2$Te$_5$, Ag$_4$In$_3$Sb$_{67}$Te$_{26}$(AIST), and GeTe. This implies that although the viscosity may have strongly increased during cooling, the diffusivity can remain high due to the early decoupling, being a favorable for fast phase switching behavior of the high-fluidity PCM. We discuss the origin of the observation and propose the possible connection to a metal-semiconductor and fragile-strong transition hidden below T$_m$. In addition, the infinite-frequency shear modulus is experimentally determined ranging from 2 to 3GPa for liquid PCMs, which permits extracting viscosity from microscopic structural relaxations usually accessible to simulations and scattering techniques.
