  Revealing the bonding and time-evolving atomic dynamics in functional
materials with complex lattice structures can update the fundamental knowledge
on rich physics therein, and also help to manipulate the material properties as
desired. As the most prototypical chalcogenide phase change material, Ge2Sb2Te5
has been widely used in optical data storage and non-volatile electric memory
due to the fast switching speed and the low energy consumption. However, the
basic understanding of the structural dynamics on the atomic scale is still not
clear. Using femtosecond electron diffraction, structure factor calculation and
TDDFT-MD simulation, we reveal the photoinduced ultrafast transition of the
local correlated structure in the averaged rock-salt phase of Ge2Sb2Te5. The
randomly oriented Peierls distortion among unit cells in the averaged rock-salt
phase of Ge2Sb2Te5 is termed as local correlated structures. The ultrafast
suppression of the local Peierls distortions in individual unit cell gives rise
to a local structure change from the rhombohedral to the cubic geometry within
~ 0.3 ps. In addition, the impact of the carrier relaxation and the large
amount of vacancies to the ultrafast structural response is quantified and
discussed. Our work provides new microscopic insights into contributions of the
local correlated structure to the transient structural and optical responses in
phase change materials. Moreover, we stress the significance of femtosecond
electron diffraction in revealing the local correlated structure in the subunit
cell and the link between the local correlated structure and physical
properties in functional materials with complex microstructures.
