School of Physics and Astronomy, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK.
Sci Rep. 2013 Oct 16;3:2965. doi: 10.1038/srep02965.
The phonon spectrum of Ge₂Sb₂Te₅ is a signature of its crystallographic structure and underlies the phase transition process used in memory applications. Epitaxial materials allow coherent optical phonons to be studied in femtosecond anisotropic reflectance measurements. A dominant phonon mode with frequency of 3.4 THz has been observed in epitaxial Ge₂Sb₂Te₅ grown on GaSb(001). The dependence of signal strength upon pump and probe polarization is described by a theory of transient stimulated Raman scattering that accounts for the symmetry of the crystallographic structure through use of the Raman tensor. The 3.4 THz mode has the character of the 3 dimensional T₂ mode expected for the O(h) point group, confirming that the underlying crystallographic structure is cubic. New modes are observed in both Ge₂Sb₂Te₅ and GaSb after application of large pump fluences, and are interpreted as 1 and 2 dimensional modes associated with segregation of Sb.
锗锑碲的声子谱是其晶体结构的特征，也是用于内存应用的相变过程的基础。外延材料允许在飞秒各向异性反射测量中研究相干光学声子。在 GaSb(001) 上生长的外延 Ge₂Sb₂Te₅ 中已经观察到频率为 3.4 THz 的主导声子模式。通过瞬态受激拉曼散射理论来描述信号强度对泵浦和探测偏振的依赖关系，该理论通过使用拉曼张量来考虑晶体结构的对称性。3.4 THz 模式具有 O(h)点群预期的 3 维 T₂模式的特征，这证实了底层晶体结构是立方的。在施加大的泵浦强度后，在 Ge₂Sb₂Te₅ 和 GaSb 中都观察到了新的模式，并将其解释为与 Sb 分凝相关的 1 和 2 维模式。
