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Direct ab-initio molecular dynamic study of ultrafast phase change in Ag-alloyed Ge$_{2}$Sb$_{2}$Te$_{5}$

We employed ab-initio molecular dynamics to directly simulate the effects of Ag alloying ($\sim5%$ Ag concentration) on the phase change properties of Ge$_{2}$Sb$_{2}$Te$_{5}$. The short range order is preserved, whereas a slight improvement in the chemical order is observed. A slight decrease in the fraction of tetrahedral Ge (sp$^{3}$ bonding) is reflected in the reduction of the optical band gap and in the increased dielectric constant. Simulations of the amorphous to crystalline phase change cycle revealed the fact that the crystallization speed in Ag$_{0.5}$Ge$_{2}$Sb$_{2}$Te$_{5}$ is no less than that in Ge$_{2}$Sb$_{2}$Te$_{5}$. Moreover, the smaller density difference and the larger energy difference between the two phases of Ag$_{0.5}$Ge$_{2}$Sb$_{2}$Te$_{5}$ (compared to Ge$_{2}$Sb$_{2}$Te$_{5}$) suggest a smaller residual stress in devices due to phase transition and improved thermal stability for Ag$_{0.5}$Ge$_{2}$Sb$_{2}$Te$_{5}$. The potential viability of this material suggests the need for a wide exploration of alternative phase change memory materials.