Ultrafast Electron Scattering to Understand and Control Material Properties

点击翻译按钮获取中文摘要

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.