Matsubara E, Okada S, Ichitsubo T, Kawaguchi T, Hirata A, Guan P F, Tokuda K, Tanimura K, Matsunaga T, Chen M W, Yamada N
Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan.
Office of Society-Academia Collaboration for Innovation, Kyoto University, Kyoto 611-0011, Japan.
Phys Rev Lett. 2016 Sep 23;117(13):135501. doi: 10.1103/PhysRevLett.117.135501. Epub 2016 Sep 21.
Despite the fact that phase-change materials are widely used for data storage, no consensus exists on the unique mechanism of their ultrafast phase change and its accompanied large and rapid optical change. By using the pump-probe observation method combining a femtosecond optical laser and an x-ray free-electron laser, we substantiate experimentally that, in both GeTe and Ge_{2}Sb_{2}Te_{5} crystals, rattling motion of mainly Ge atoms takes place with keeping the off-center position just after femtosecond-optical-laser irradiation, which eventually leads to a higher symmetry or disordered state. This very initial rattling motion in the undistorted lattice can be related to instantaneous optical change due to the loss of resonant bonding that characterizes GeTe-based phase change materials. Based on the amorphous structure derived by first-principles molecular dynamics simulation, we infer a plausible ultrafast amorphization mechanism via nonmelting.
尽管相变材料被广泛用于数据存储，但对于其超快相变的独特机制及其伴随的巨大且快速的光学变化，目前尚无共识。通过使用结合飞秒光学激光和X射线自由电子激光的泵浦-探测观测方法，我们通过实验证实，在GeTe和Ge₂Sb₂Te₅晶体中，飞秒光学激光照射后，主要是Ge原子的晃动运动在保持偏心位置的情况下发生，最终导致更高的对称性或无序状态。这种在未变形晶格中的初始晃动运动可能与基于GeTe的相变材料所特有的共振键合丧失导致的瞬时光学变化有关。基于第一性原理分子动力学模拟得出的非晶结构，我们推断出一种可能的非熔化超快非晶化机制。
