Large-scale atomistic simulations of laser-excited bismuth with density functional theory accuracy: unified description of nonthermal and thermal ultrafast relaxation effects

当固体被强烈的飞秒激光脉冲照射时，电子突然被加热到极高的温度。这导致原子键的显著变化和具有非热特性的结构变化。由于电子和离子之间的碰撞，热电子和冷离子的瞬变状态在皮秒时间尺度上衰变。这些碰撞导致电子冷却，并向离子晶格释放能量。结果，形成了热状态，其中可能发生结构变化(热结构变化)。我们计划第一次在实验的长度和时间尺度上，以相同的精度水平(密度泛函理论的精度)模拟铋在飞秒激光脉冲激发后的热和非热结构变化。铋在正常条件下以Peierls扭曲的A7结构结晶。由于这种特殊的晶体结构，许多飞秒激光诱导的现象可以并且已经在实验上观察到：键软化、固体到固体的转变、相干声子的激发、超快熔化和烧蚀。此外，铋具有很大的散射截面，这使得它对泵浦探测实验很有吸引力，比如时间分辨结晶学。在这些实验中，用飞秒激光脉冲激发薄铋膜，然后用超短X射线或电子脉冲探测，延迟几百飞秒。从时间分辨的衍射图可以得出飞秒激光激发后原子运动的结论。然而，单个原子的精确运动是无法重建的。我们计划缩小这一差距，并提供飞秒激光激发后铋的结构动力学的“分子电影”：为此，我们将进行考虑数百万个原子的分子动力学模拟，以一对一地与现有实验进行比较，并预测激光诱导的材料转变。然而，在铋中，相对论效应扮演着重要的角色，因此对大规模原子模拟提出了挑战。在本项目中，我们将模拟激光诱导的铋的热和非热结构变化，包括自旋-轨道耦合。为了实现这些模拟，我们将基于我们将要产生的大量密度泛函理论数据来构造第一个与电子温度相关的铋原子间相互作用势。然后，这种原子间相互作用势将在大规模双温模型分子动力学(TTM-MD)模拟中实现。

When a solid is irradiated by an intense femtosecond laser pulse, the electrons are abruptly heated to an extremely high temperature. This leads to significant changes in the atomic bonds and to structural changes that have a non-thermal character. The transient state with hot electrons and cold ions decays on a picosecond time scale due to collisions between electrons and ions. These collisions cause the electrons to cool down and release energy to the ion lattice. As a result, a thermal state is formed, in which structural changes can occur (thermal structural changes). We plan, for the first time, to simulate thermal and non-thermal structural changes following a femtosecond laser pulse excitation of bismuth at the same level of accuracy (density functional theory accuracy) at length- and time scales of experimental relevance. Bismuth crystallizes under normal conditions in a Peierls-distorted A7 structure. Due to this particular crystal structure, many femtosecond laser-induced phenomena can be and have been experimentally observed: bond softening, a solid-to-solid transition, excitation of coherent phonons, ultrafast melting, and ablation. Moreover, bismuth exhibits a large scattering cross section, which makes it attractive for pump probe experiments, like time-resolved crystallography. In those experiments, thin bismuth films are excited with a femtosecond laser pulse and then probed, with a delay of a few hundred femtoseconds, with an ultrashort X-ray or electron pulse. From the time-resolved diffraction patterns, conclusions can be drawn about the atomic motions following a femtosecond laser excitation. However, the precise motions of individual atoms cannot be reconstructed. We plan to close this gap and provide a "molecular movie" of the structural dynamics in bismuth after femtosecond laser excitation: To do so, we will perform molecular dynamics simulations considering several millions of atoms to compare one-to-one with existing experiments and to make predictions regarding laser-induced material transformations. In bismuth, however, relativistic effects play a significant role and therefore pose a challenge for large scale atomistic simulations. In the present project we will simulate laser induced thermal and non-thermal structural changes in bismuth including spin-orbit coupling. In order to realize these simulations, we will construct the first electron temperature-dependent interatomic potential for bismuth based on extensive density functional theory data, which we will produce. This interatomic potential will then be implemented in large-scale two temperature model molecular dynamics (TTM-MD) simulations.