  The time-resolved reflectivity measurements were carried out on the
interfacial phase change memory (iPCM) materials
([(GeTe)$_{2}$(Sb$_{2}$Te$_{3}$)$_{4}$]$_{8}$ and
[(GeTe)$_{2}$(Sb$_{2}$Te$_{3}$)$_{1}$]$_{20}$) as well as conventional
Ge$_{2}$Sb$_{2}$Te$_{5}$ alloy at room temperature and above the RESET-SET
phase transition temperature. In the high-temperature phase, coherent phonons
were clearly observed in the iPCM samples while drastic attenuation of coherent
phonons was induced in the alloy. This difference strongly suggests the atomic
rearrangement during the phase transition in iPCMs is much smaller than that in
the alloy. These results are consistent with the unique phase transition model
in which a quasi-one-dimensional displacement of Ge atoms occurs for iPCMs and
a conventional amorphous-crystalline phase transition takes place for the
alloy.
