The Study of Ultrafast Structural Dynamics in Solid Materials with Femtosecond Electron Diffraction

本项目将研究固体中的结构相变动力学和锰矿石中的动态Jahn-Teller畸变。动力学过程将由飞秒脉冲激光的脉冲激发能量引发。它们的时间演变将通过新开发的飞秒电子衍射技术来探测，该技术直接监测单个原子振动周期的时间尺度上的原子运动。这些测量伴随着互补的飞秒时间分辨光谱，对相关自由度也很敏感。其目的是获得超越当前时间平均热力学观点的这些动态过程的微观理解；例如，过渡金属氧化物中局部结构变形对整体性能的相互作用。需要解决的问题包括：(1)在微观的原子尺度上，固体有序-无序结构转变中的熔融和再结晶如何随时间演变；(2)原子在固-固结构转化过程中如何自我重排和演化、时间尺度和可能途径；(3)动力学Jahn-Teller畸变在巨磁阻效应中的作用，特别是动力学Jahn-Teller畸变与锰的其他物理性质（如电荷/轨道有序和电导率）之间的关系。参与该项目的研究生在当代凝聚态物理学的一个前沿领域获得培训和知识，为在学术界，工业界或政府的高级研究或就业做准备。本项目旨在研究固体材料中涉及超快结构变化的物理过程，特别是激光诱导的结构相变和巨磁阻材料中动态Jahn-Teller畸变的作用。对这些动态行为的透彻理解将对激光相关材料的加工和制造以及磁数据记录和存储行业新材料的开发具有重要的科学意义。参与该项目的研究生在当代凝聚态物理学的一个前沿领域获得培训和知识，为在学术界，工业界或政府的高级研究或就业做准备。

This project will investigate structural phase transition dynamics in solids and the dynamical Jahn-Teller distortion in manganites. The dynamical processes will be initiated by impulsive excitation energy from a femtosecond pulse laser. Their temporal evolutions will then be probed via the newly developed technique of femtosecond electron diffraction, which directly monitors atomic motions on the timescale of a single atomic vibrational period. These measurements are accompanied by complementary femtosecond time-resolved optical spectra that are also sensitive to the relevant degrees of freedom. The aim is to gain a microscopic understanding of these dynamic processes beyond the current time-averaged thermodynamic point of view; for example, the interplay of local structural deformations on the global properties in transition-metal oxides. Issues and questions to be addressed include: (1) How the melting and recrystallization in order-disorder structural transformations in solids evolve with time, microscopically on the atomic scale; (2) How atoms rearrange themselves and evolve, time scales and possible pathways, from one distinct structure to another in solid-solid structural transformations; and (3) The role of the dynamical Jahn-Teller distortion in the colossal magnetoresistance effect, in particular, the correlations between the dynamical Jahn-Teller distortion and the other physical properties in manganites, such as the charge/orbital ordering and conductivity. Graduate students involved in the project acquire training and knowledge in one the forefront areas of contemporary condensed matter physics in preparation for advanced studies or employment in academe, industry or government.This project is directed at the study of physical processes involving ultrafast structural changes in solid materials, in particular, laser-induced structural phase transition and the role of dynamical Jahn-Teller distortion in colossal magnetoresistance materials. A thorough understanding of these dynamic behaviors will have important scientific implications in laser-related materials processing and manufacturing, and in developing new materials in the magnetic data recording and storage industry. Graduate students involved in the project acquire training and knowledge in one the forefront areas of contemporary condensed matter physics in preparation for advanced studies or employment in academe, industry or government.