Ultrafast Structural Dynamics in Solids and Nanoparticles

在原子和分子尺度上，原子的超快运动驱动着所有的结构变化，例如固体的熔化。对原子运动在时间和长度尺度上的动力学行为的透彻理解为控制这些结构演化提供了指导原则，这将在固体物理、化学、生物和材料科学中有重要的科学应用。这个项目的目标是通过实时直接观察相关的原子和分子运动来了解调节固体材料结构变化的机制。该项目将使用最近开发的一种技术来研究几个具有基本重要性和技术意义的凝聚态物理问题。其中包括激光诱导金属和金属纳米颗粒的超快结构转变，以及测量磁性材料通过加热其传导电子而产生的热膨胀特性。参与该项目的研究生和博士后学者将获得当代凝聚态物理和超快科学前沿的培训和知识，这将为学术、工业和国家实验室的职业生涯做好准备。这个个人研究人员奖支持一个项目，该项目将在原子时间和长度尺度上研究金属和金属纳米颗粒的结构动力学。动力学过程将由飞秒脉冲激光的脉冲激发来启动。它们的时间演化将通过使用新的飞秒电子衍射直接监测单个原子振动周期的时间尺度上的原子运动，以及相关自由度的飞秒时间分辨光学光谱测量来探索。这个项目的最终目标是超越目前的时间平均热力学观点，获得对结构动力学的微观理解。本项目要解决的问题包括：(1)固体中激光诱导超快熔化的机制，特别是相干晶格振动在有序-无序转变中的作用；(2)金属纳米粒子中电子-声子耦合和熔化的尺寸相关动力学，以及再结晶过程；(3)利用新的瞬时加热方法测量低居里温度磁性材料中的电子格吕内森常数，避免了传统低温方法的局限性。该项目将研究和教育相结合，培养研究生和博士后研究人员，掌握当代凝聚态物理和超快科学前沿领域的尖端技术和现代方法。

******NON-TECHNICAL ABSTRACT******At the atomic and molecular scale, the ultrafast motions of atoms drive all structural changes, such as the melting of a solid. A thorough understanding of such dynamic behavior on the time and length scales of atomic motions provides the guiding principles to control these structural evolutions, which will have important scientific applications in solid-state physics, chemistry, biology and materials science. The goal of this project is to understand the mechanisms that regulate structural changes in solid materials by directly observing the associated atomic and molecular motion in real time. The project will use a recently developed technique to study several condensed matter physics problems with both fundamental importance and technological implications. They include laser-induced ultrafast structural transformations in metals and metal nanoparticles, and the measurement of the thermal expansion properties of magnetic materials induced by heating their conduction electrons. Graduate students and postdoctoral scholars involved in the project will acquire training and knowledge in the forefronts of contemporary condensed matter physics and ultrafast science, which will prepare trainees for careers in academe, industry, and national laboratories.****** TECHNICAL ABSTRACT******This individual investigator award supports a project that will investigate the structural dynamics in metals and metal nanoparticles on the atomic time and length scales. The dynamical processes will be initiated by impulsive excitation using a femtosecond pulse laser. Their temporal evolutions will be probed by directly monitoring atomic motions on the timescale of a single atomic vibrational period using the novel femtosecond electron diffraction, together with femtosecond time-resolved optical spectroscopy measurements of the relevant degrees of freedom. The ultimate goal of this project is to gain a microscopic understanding of structural dynamics beyond the current time-averaged thermodynamic point of view. Issues and questions to be addressed in this project include: (1) the mechanism of laser-induced ultrafast melting in solids, in particular the role of coherent lattice vibration in the order-disorder transformations; (2) size-dependent dynamics of electron-phonon coupling and melting in metal nanoparticles, and the re-crystallization processes; (3) measurement of electronic Gruneisen constants in magnetic materials with low Curie temperature, using the novel approach of transient heating which circumvents the limitation of traditional low-temperature approaches. This project integrates research and education to train graduate students and postdoctoral researchers for cutting-edge techniques and modern methods in the forefront areas of contemporary condensed matter physics and ultrafast science.