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arXiv 1504.05053cond-mat.mtrl-sciphysics.comp-phphysics.plasm-ph
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飞秒X射线辐照下硅的热熔化和非热熔化

Thermal and nonthermal melting of silicon under femtosecond x-ray irradiation

Nikita Medvedev, Zheng Li, Beata Ziaja

摘要:

从可见光实验可知,如果从价带到导带激发的电子密度超过某个临界值,那么硅在飞秒脉冲辐照下可以经历所谓的“非热熔化”。这种超快跃迁是由原子势能面的强烈变化引起的,这种变化触发了原子的重新定位。然而,由于电子-声子耦合引起的材料加热也可以导致相变,称为“热熔融”。即使激发电子密度太低而不能引起非热效应,这种热熔化也会发生。为了研究相变,特别是,在硅的热效应和非热效应的相互作用下的飞秒X射线照射,我们提出了他们的统一处理超越Born-Oppenheimer近似在我们的混合模型的基础上紧束缚分子动力学。与我们的扩展模型,我们确定在辐照硅的各种相变的损伤阈值。我们发现,电子-声子耦合触发相变的固体硅到一个低密度的液相,如果沉积到样品中的能量是每个原子的$\sim0.65$ eV以上。对于沉积的剂量超过$\sim0.9$ eV每原子,固体硅经历相变到高密度液相之间的相互作用,电子-声子加热和非热效应触发。这些阈值远低于Born-Oppenheimer近似($\sim2.1$ eV/atom)的预测值,表明电子-声子耦合对激光激发硅弛豫的重要贡献。我们期望这些结果将刺激专门的实验研究,详细揭示激光辐照硅内结构弛豫的各种路径。

英文摘要:

As it is known from visible light experiments, silicon under femtosecond pulse irradiation can undergo the so-called 'nonthermal melting' if the density of electrons excited from the valence to the conduction band overcomes a certain critical value. Such ultrafast transition is induced by strong changes in the atomic potential energy surface, which trigger atomic relocation. However, heating of a material due to the electron-phonon coupling can also lead to a phase transition, called 'thermal melting'. This thermal melting can occur even if the excited-electron density is much too low to induce non-thermal effects. To study phase transitions, and in particular, the interplay of the thermal and nonthermal effects in silicon under a femtosecond x-ray irradiation, we propose their unified treatment by going beyond the Born-Oppenheimer approximation within our hybrid model based on tight binding molecular dynamics. With our extended model we identify damage thresholds for various phase transitions in irradiated silicon. We show that electron-phonon coupling triggers the phase transition of solid silicon into a low-density liquid phase if the energy deposited into the sample is above $\sim0.65$ eV per atom. For the deposited doses of over $\sim0.9$ eV per atom, solid silicon undergoes a phase transition into high-density liquid phase triggered by an interplay between electron-phonon heating and nonthermal effects. These thresholds are much lower than those predicted with the Born-Oppenheimer approximation ($\sim2.1$ eV/atom), and indicate a significant contribution of electron-phonon coupling to the relaxation of the laser-excited silicon. We expect that these results will stimulate dedicated experimental studies, unveiling in detail various paths of structural relaxation within laser-irradiated silicon.

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