Picosecond Electron Diffraction Study of Superheating of Solids

研究了固体过热的可能性和极限。过热固体是指在高于体熔点Tm的温度下，长程序仍然存在的固体。时间分辨透射电子衍射（TED），具有高达1ps的时间分辨率，将用于探测激光加热薄膜的结构。激光提供超快加热脉冲，而超快电子衍射探测晶体的结构完整性，以检测任何亚稳态过热状态。虽然熔融状态的过冷是一种常见现象，但观察到固体的过热是罕见的。表面和延伸的缺陷，如晶界，为Tm以下的无序提供了大量的形核位点，从而抑制了固体的过热。然而，一些单晶表面保持有序到Tm，允许表面过热。目前的工作重点是透射电子衍射研究，以探测多达数百个A薄膜，而不是像以前的工作那样在反射高能电子衍射（RHEED）中探测最初的几个原子层。时间分辨TED （1-ps或更低）将在单晶和多晶薄金属和半导体薄膜上进行，并在紧密排列表面的微晶上进行。研究了在80-fs和100-ps脉冲脉冲下加热和熔化薄膜的结构完整性、原子平均振动幅度和晶格间距。研究生和本科生将参与本研究。熔融是自然界中最常见的相变之一。虽然我们从热力学角度对熔融的理解是很好的，但对这种相变的微观理解是缺乏的。熔融态过冷是一种常见现象；然而，观察到固体的过热是罕见的。过冷液体是在低于体积熔点的温度下保持液体状态一段时间的液体。过热固体是指其结构完整性在高于体积熔点的温度下仍然存在的固体。熔化开始于表面和缺陷，并扩展到晶体的整体。本研究的主要目的是研究过热固体的可能性和局限性，从而为固体在熔点以上存在多远和多长时间的问题提供答案。由于熔化的快速性质，本研究需要使用快速加热源和能够监测熔化过程中原子排列的探针。我们使用持续时间超短的激光脉冲，只有一万亿分之一秒或更短，作为快速热源。材料中随后的电子脉冲的衍射允许监测被加热材料的结构。这些高时间分辨率的研究将在单晶和多晶薄金属和半导体薄膜以及微晶上进行。研究生和本科生将参与本研究。他们将接受培训，为在本世纪未来几十年进入工业、政府或学术就业市场做准备

The possibility and limits of superheating solids are studied. A superheated solid is one in which long-range order remains present at temperatures higher than the bulk melting point, Tm. Time-resolved transmission electron diffraction (TED), with up to 1 ps temporal resolution, will be used to probe the structure of laser-heated thin films. The laser provides an ultrafast heating pulse, while ultrafast electron diffraction probes the structural integrity of the crystal to detect any metastable superheated state. While supercooling of the molten state is a common phenomenon, observations of superheating of solids are rare. Surfaces and extended defects, such as grain boundaries, provide vast nucleation sites for disorder below Tm and, thus, inhibit superheating of solids. Some single-crystal surfaces, however, remain ordered up to Tm allowing a surface to be superheated. The present work focuses on transmission electron diffraction studies to probe up to several hundred A thin films, rather than the first few atomic layers probed in reflection high-energy electron diffraction (RHEED) used in previous work. Time-resolved TED (1-ps or less) will be conducted on single-crystal and polycrystalline thin metal and semiconductor films and on microcrystallites bounded by close-packed surfaces. The structural integrity, atomic mean vibrational amplitude, and lattice spacing of thin films subjected to heating and melting with laser pulses of 80-fs and 100-ps pulse durations will be probed.. Graduate and undergraduate students will participate in this research.%%% Melting is one of the most common phase transformation in nature. While our understanding of melting from a thermodynamic point of view is well established, microscopic understanding of this phase transformation is lacking. Supercooling of the molten state is a common phenomenon; however, observations of superheating of solids are rare. A supercooled liquid is one that remains in a liquid state for some time at temperature below the bulk melting point. A superheated solid is one in which the structural integrity remains present at temperatures higher than the bulk melting point. Melting is initiated at surfaces and defects and propagates into the bulk of the crystal. The main objective of this study is to investigate the possibility and limits of superheating solids, thus, providing an answer to the question of how far and for how long can a solid exists above its melting point. Because of the fast nature of melting, this study requires the use of a fast heating source and a probe capable of monitoring the atomic arrangement during melting. We use laser pulses of ultra-short duration, one trillionth of one second or less, as a fast heating source. Diffraction of a subsequent electron pulse in the material allows for monitoring the structure of the heated material. These high time resolution studies will be conducted on single-crystal and polycrystalline thin metal and semiconductor films and on microcrystallites. Graduate and undergraduate students will participate in this research. They will receive training that prepares them for entry into the industrial, government, or academic job market during the coming decades of the century.***