Direct observation of elastic softening immediately after femtosecond-laser excitation in a phase-change material

Abstract

The generation and propagation of photoexcited elastic waves in crystalline $\mathrm{G}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{5}$ were analyzed by picosecond time-resolved x-ray diffraction using a femtosecond-laser pump and an x-ray free-electron laser probe technique. The crystalline lattice anisotropically expanded initially in approximately 20 ps after the excitation. This was followed by a periodic oscillation of the lattice strain. The elastic stiffness along the cubic $\ensuremath{\langle}111\ensuremath{\rangle}$ direction had significantly softened during the initial expansion, and the strain magnitude was the largest in the $\ensuremath{\langle}100\ensuremath{\rangle}$ and $\ensuremath{\langle}110\ensuremath{\rangle}$ directions. This indicates that femtosecond-laser excitation creates a shallower interlayer potential between the Te and Ge-Sb layers and eventually leads to softening of the elastic stiffness along the cubic $\ensuremath{\langle}111\ensuremath{\rangle}$ direction. Furthermore, this softened state increases the system's sensitivity to an external stress field. This residual internal stress in a thin film enhances the selective formation of a particular type of variant during the symmetry change from cubic to rhombohedra. This causes the subsequent anisotropic expansion. These phenomena are quite interesting and align with the ultrafast amorphization of this material.