Ultrafast electronic charge redistribution processes in ionic crystals by femtosecond x-ray diffraction

这个项目旨在了解晶体物质中超快原子运动和电子电荷重新分布过程的基本微观机制。主要的科学目标是对位移铁电体中所谓的软模声子进行详细的空间表征(具有原子分辨率)。软模是离子运动和离子间电子电荷转移过程的复杂相互作用。它是铁电材料相干软模振荡所固有的电子密度图对称性破缺变化的相变前兆。计划中的实验旨在通过旋转方法使用飞秒X射线衍射来研究这种变化。为此，我们将为飞秒X射线旋转法建立一个新的探测器终端站，该方法将允许同时测量多达12个反射。我们计划的实验是基于用于硬X射线脉冲的激光等离子体源和用于向晶体施加强电场瞬变的强太赫兹源相结合。该方法对单胞内原子位置的场驱动各向异性变化(即结构变化)和单胞内各种离子之间的电子转移过程都是敏感的。软模的空间分辨特性将阐明这种混合模式中电子和离子运动的相互作用，这有望反过来更好地理解铁电相变。第二系列实验旨在研究电场诱导的轻元素离子晶体(如LiBH4，LiH，LiOH，NaBH4等)中电子密度图的准瞬时形变。飞秒X射线旋转法在大量等效反射上的应用将为研究材料的基态电子关联提供新的实验见解。此外，对晶体施加强THz场而不是光场，将使我们对空间离子位移与电子电荷转移对宏观极化的相对贡献有新的认识。

This project aims at understanding fundamental microscopic mechanisms which underlie ultrafast atomic motions and electronic charge redistribution processes in crystalline matter. The main scientific goal is a detailed spatial characterization (with atomic resolution) of the so called soft mode phonons in displacive ferroelectrics. The soft-mode is a complex interplay of ionic motions and inter-ionic electronic charge transfer processes. It is a sort of precursor of the phase transition with symmetry breaking changes of the electron density map which are inherent to coherent soft mode oscillations of ferroelectrics. The planned experiments aim at studying such changes using femtosecond x-ray diffraction by means of the rotation method. To this end, we will set up a new detector end station for the femtosecond x-ray rotation method which will allow for measuring up to 12 reflections simultaneously. Our planned experiments are based on a laser plasma source for hard x-ray pulses in combination with an intense terahertz source for applying strong electric field transients to the crystal. The proposed method will be sensitive to both field-driven anisotropic changes of the atomic positions within the unit cell, i.e. structural changes, and to electron transfer processes between various ions within the unit cell. A spatially resolved characterization of the soft-mode will clarify the interplay of electronic and ionic motions of this hybrid mode which is expected to lead in turn a better understanding of the ferro- to para-electric phase transition.A second series of experiments aims at electric field-induced quasi-instantaneous deformations of the electron density map in ionic crystals made of light elements (e.g. LiBH4, LiH, LiOH, NaBH4 etc.). The femtosecond x-ray rotation method on a multitude of equivalent reflections will give new experimental insights into ground state electron correlations of the materials. Furthermore, applying strong THz fields instead of optical fields to the crystals will give new insights in the relative contributions of spatial ionic shifts vs. electronic charge transfer to the macroscopic polarization.