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

通过超快 X 射线衍射研究结晶固体的可逆结构变化

• 批准号: 5372774
• 负责人: Privatdozent Dr. Michael Wörner
• 金额: --
• 依托单位: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI)
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2002
• 资助国家: 德国
• 起止时间: 2001-12-31 至 2008-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/5372774?language=en
• 关键词: Reversible; structural; changes; crystalline; solids

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.

该项目旨在理解基本的微观机制，这些机制是晶体固体的超快可逆结构变化的基础。通过使用新型的等离子体源对硬X射线脉冲的重复速率使用1 kHz的硬X射线脉冲，将改善超快时间域中X射线衍射实验的灵敏度。这样，实验将可以访问固定衍射信号的微小变化。计划了以下实验：（i）将在GAAS/ALAS半导体超晶格中研究声音子的相干动力学。超晶格周期性通过限制性k-vector范围内冲动飞秒的连贯的晶格激发型，声子分散的变化将允许冲动飞秒的一致晶格激发。单个激发脉冲和脉冲序列都将用于激发。晶格中的原子运动将通过延迟的硬X射线脉冲的衍射实时监测。将分析声音声子波袋的传播特性和冲动性声子生成的机制。此外，还将探索对声子激发的连贯控制的场景。 （ii）具有相关电子系统的固体中的相变，特别是高过渡 - 温度超导体，代表了该项目的第二个主题。 LA2-XBAXCUO4将研究从原晶到四方对称性的结构相过渡到四方对称的动力和机制。中心问题是，这种相变的纯粹性质，即由晶格温度确定，和/或相关电子系统的非平衡激发有助于结构变化。时间分辨的衍射实验应允许通过其明显不同的动态来分离此类贡献。在项目的后期，我们计划研究旋转结构和磁性特性可逆变化的系统。