Ultrafast Electron Scattering to Understand and Control Material Properties

目前，全世界都在努力开发和应用新的实验方法，以便能够直接“观察”物质随时间演变的结构。这些方法结合了联合收割机最先进的飞秒激光器（2018年诺贝尔物理学奖）和超短X射线或电子脉冲源，以获得时间分辨的衍射/散射图案和图像。在这些仪器可达到的最高时间分辨率（<100 fs）下，原子运动在观察过程中基本上是冻结的，人们可以完全遵循微观动力学来产生许多基本过程的“分子电影”。Siwick小组长期以来一直是实验室规模的超快电子散射方法及其在材料物理学中广泛应用的先驱。2016-2017年取得了一项非常重大的突破，RF电子脉冲压缩方法的完善首次将时间分辨率推到100 fs以下，并导致麦吉尔设计，建造和运行的仪器成为世界上同类仪器中最强大的。这为拟议的研究计划奠定了基础，专注于凝聚态物理学的中心问题;理解电荷，自旋，轨道和晶格结构自由度之间的复杂相互作用，从而产生材料的新兴宏观特性。应对这一广泛的挑战需要进一步增强仪器，包括电子光谱学（超快q-EELS），提高时间，空间和动量分辨率以及电子束相干性，这将通过当前的提案来实现。这些都是通过我们最近开发的RF腔与飞秒激光器的鲁棒同步实现的。利用这些工具，我们将追求材料物理学中的一系列紧迫问题：I）使用超快电子漫散射和计算材料方法提高我们对热电材料中热和电子输运的基本基础的理解，以促进新的热电材料的发现。研究如何利用光激发来控制强关联材料的性质。III）进一步了解强各向异性（1D和2D）材料中晶格和电荷自由度内部和之间的动量相关耦合，这些材料决定了半导体和金属中的超导性，电荷密度波，热电性，光致发光性和载流子迁移率等多种现象。IV）揭示光伏材料中光吸收之后的许多过程。这项拟议的研究计划直接针对美国能源部一份开创性报告中概述的基础能源科学面临的5大挑战中的3个，预计将产生巨大影响。

There is currently an enormous, world-wide effort directed at the development and application of new experimental methods that make it possible to directly `watch' the time-evolving structure of matter. These approaches combine state-of-the-art femtosecond lasers (Nobel Prize in Physics, 2018) and sources of either ultrashort Xray or electron pulses to acquire time-resolved diffraction/scattering patterns and images. At the highest time-resolution achievable in these instruments (<100 fs), atomic motion is essentially frozen during an observation and one can completely follow the microscopic dynamics to produce a "molecular movie" of many fundamental processes. The Siwick group has long been a pioneer in the development of lab-scale ultrafast electron scattering methods and their application to a wide range of fundamental problems in materials physics. A very significant breakthrough was made in 2016-2017 with the perfection of RF electron pulse compression methods which pushed time resolution below 100 fs for the first time and resulted in the instrumentation designed, built and operating at McGill being the most powerful of its kind anywhere in the World. This has set the stage for the proposed program of study, focused squarely on the central question of condensed matter physics; understanding the complex interplay between charge, spin, orbital and lattice-structural degrees of freedom that gives rise to the emergent macroscopic properties of materials. Addressing this broad challenge requires further enhancements in instrumentation, including electron spectroscopy (ultrafast q-EELS), improved time, space and momentum resolution and electron beam coherence that will be pursued through the current proposal. Each of these have been enabled by the robust synchronization of RF cavities to femtosecond lasers that we recently developed. Leveraging these tools, we will pursue a series of pressing problems in materials physics: I) To improve our understanding of the fundamental basis of heat and electronic transport in thermoelectric materials using ultrafast electron diffuse scattering and computational materials approaches in order to facilitate new thermoelectric materials discovery. II) To investigate how optical excitation can be used to control the properties of strongly correlated materials. III) To further our understanding of the momentum-dependent couplings within and between lattice and charge degrees of freedom in strongly anisotropic (1D and 2D) materials that determine phenomena as diverse as superconductivity, charge density waves, thermoelectricity, photovoltaicity and carrier mobility in semiconductors and metals. IV) To shed new light on the many processes that follow the absorption of light in photovoltaic materials. This proposed research program directly targets 3 of the 5 the Grand Challenges for the Basic Energy Sciences outlined in a seminal DOE report and is expected to have enormous impact.