Reversible structural changes of crystalline solids studied by ultrafast x-ray diffraction

本项目旨在了解晶体固体超快速可逆结构变化的基本微观机制。采用一种新的等离子体源，以1khz的重复频率输出硬x射线脉冲，可以提高x射线衍射实验在超快时域的灵敏度。这样，静止衍射信号的微小变化就可以用于实验。计划进行以下实验：(1)在GaAs/AlAs半导体超晶格中研究声子的相干动力学。由超晶格周期性引起的声子色散的变化将允许在有限的k向量范围内产生脉冲飞二代相干晶格激发。将采用单激励脉冲和脉冲序列进行激励。晶格中的原子运动将通过延迟硬x射线脉冲的衍射实时监测。分析了声子波包的传播特性和脉冲声子产生的机理。此外，将探讨声子激发的相干控制方案。（ii）具有相关电子系统的固体中的相变，特别是高转变温度超导体，是本项目的第二个主题。本文将研究La2-xBaxCuO4从正交对称向四方对称转变的动力学和机制。中心问题是这种相变是否具有纯粹的热性质，即由晶格温度决定，以及/或相关电子系统的非平衡激发是否有助于结构变化。时间分辨衍射实验应该允许通过它们明显不同的动力学来分离这些贡献。在项目的后期阶段，我们计划研究自旋结构和磁性发生可逆变化的系统。

This project aims at understanding fundamental microscopic mechanisms which underlie ultrafast reversible structural changes of crystalline solids. The sensitivity of x-ray diffraction experiments in the ultrafast time domain will be improved by using a novel plasma source for hard x-ray pulses which works at a repetition rate of 1 kHz. In this way, small changes of stationary diffraction signals will become accessible for experiments. The following experiments are planned: (i) Coherent dynamics of acoustic phonons will be studied in GaAs/AlAs semiconductor superlattices. The changes of the phonon dispersion by the superlattice periodicity will allow for the impulsive femtosecond generation of coherent lattice excitations in a restricted range of k-vectors. Both single excitation pulses and pulse sequences will be applied for excitation. Atomic motions in the lattice will be monitored in real-time by diffraction of delayed hard x-ray pulses. The propagation properties of acoustic phonon wavepackets and the mechanisms of impulsive phonon generation will be analyzed. In addition, scenarios of coherent control of phonon excitations will be explored. (ii) Phase transitions in solids with a correlated electronic system, in particular high-transition-temperature superconductors, represent the second topic of this project. The dynamics and the mechanisms of the structural phase transition from an orthorhombic to a tetragonal symmetry will be investigated in La2-xBaxCuO4. Central problems are if this phase transition is of purely thermal nature, i.e. determined by the lattice temperature, and/or if nonequilibrium excitations of the correlated electronic system contribute to the structural change. Time-resolved diffraction experiments should allow for a separation of such contributions through their markedly different dynamics. At a later stage of the project, we plan to investigate systems in which reversible changes of the spin structure and the magnetic properties occur.