Ultrafast Dynamics in Ferromagnetic Metals and Nanoparticles

*非技术摘要*这一个人研究人员奖支持一个项目，目的是通过实时直接观察相关的原子运动和重排，了解调节固体材料结构变化和转变的机制。在之前国家科学基金会的支持下，开发了一种工具，该工具具有在原子运动的时间和长度尺度上探测结构动力学的能力。利用这项新技术，该项目将研究超快激光加热产生的热电子在磁性材料的磁性和结构行为中的作用。此外，还将研究纳米颗粒的结构动力学。这一结果可能会影响我们对技术上重要材料的行为的理解。这项技术的持续发展可能会对包括物理、化学和生物在内的许多领域的研究产生影响。该项目将通过研究和教育相结合的方式进行。参与该项目的本科生和研究生将获得当代凝聚态物理和超快科学前沿的培训和知识，这将为他们在学术、工业和国家实验室的职业生涯做好准备。*技术摘要*本项目旨在了解铁磁性金属和纳米颗粒在原子时间和长度尺度上支配结构动力学和结构-功能关联的物理机制。将使用一种最新开发的技术，该技术涉及飞秒电子衍射的直接和实时结构探针。动力学过程将由飞秒光脉冲的脉冲激发来启动，并将通过拍摄激发脉冲后不同时间点的衍射图快照来探索其时间演化。在这项研究中，飞秒时间分辨光学光谱测量也将被用来通过探测相关自由度的相关动力学来获得完整的物理图像。本项目要解决的问题包括：(1)测量过渡金属和稀土金属中的电子Gruneisen常数及其与磁有序的关系；(2)通过探索相关的结构动力学来了解退磁动力学，并获得关于光沉积能量如何在亚ps时间尺度上从电荷转移到自旋子系统的物理见解；(3)研究金属纳米粒子中的电子-声子耦合和熔化的动力学及其尺寸相关性。该项目将把研究和教育结合起来，培养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的研究生。

****NON-TECHNICAL ABSTRACT****This individual investigator award supports a project with a goal of understanding the mechanisms that regulate structural changes and transformations in solid materials by directly observing the associated atomic motions and rearrangements in real time. With previous support from the National Science Foundation, a tool was developed that has the capability of probing structural dynamics at the time and length scales of atomic motion. Using this novel technique, the project will investigate the role of hot electrons generated by ultrafast laser heating in the magnetic and structural behavior of magnetic materials. In addition the structural dynamics of nanoparticles will be investigated. The results may have an impact on our understanding of the behavior of technologically important materials. The continued development of the technique may have an impact on studies in many fields including physics, chemistry, and biology. This project will be carried out by integrating research and education. The undergraduate and graduate students involved in the project will acquire training and knowledge in the forefronts of contemporary condensed matter physics and ultrafast science, which will prepare them for careers in academe, industry, and national laboratories. ****TECHNICAL ABSTRACT****This project seeks to understand the physics that governs structural dynamics and the structure-function correlation at the atomic time and length scales in ferromagnetic metals and nanoparticles. A recently developed technique involving a direct and real-time structural probe of femtosecond electron diffraction will be utilized. The dynamical processes will be initiated by impulsive excitation using femtosecond optical pulses, and their temporal evolution will be probed by taking snapshots of diffraction patterns at different time points after the excitation pulses. In the study, femtosecond time-resolved optical spectroscopy measurements will also be used to gain a complete physical picture by probing associated dynamics in the relevant degrees of freedom. Issues and questions to be addressed in this project include (1) measurements of electronic Gruneisen constants in transition and rare earth metals, and their correlation to the magnetic ordering; (2) understanding the demagnetization dynamics by probing the correlated structural dynamics, and gaining physical insights about how the photo-deposited energy is transferred from charge to spin subsystem in sub-ps timescale; (3) investigating the dynamics of electron-phonon coupling and melting in metal nanoparticles, and their size dependence. This project will integrate research and education to train graduate students for cutting-edge techniques and modern methods in the forefront areas of contemporary condensed matter physics and ultrafast science.