Picosecond Electron Diffraction Study of Superheating of Solids

研究了固体过热的可能性和限度。过热固体是在高于本体熔点Tm的温度下仍存在长程有序的固体。时间分辨透射电子衍射（TED），与高达1 ps的时间分辨率，将被用来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。虽然熔融态的过冷是一种常见的现象，但固体过热的观察很少。表面和扩展的缺陷，如晶界，提供了巨大的成核位点的无序低于Tm，因此，抑制过热的固体。然而，一些单晶表面保持有序直到Tm，允许表面过热。目前的工作重点是透射电子衍射研究，探测到几百个薄膜，而不是前几个原子层探测反射高能电子衍射（RHEED）在以前的工作中使用。时间分辨TED（1-ps或更少）将进行单晶和多晶薄金属和半导体薄膜和微晶由紧密堆积的表面。结构完整性，原子平均振动幅度，和晶格间距的薄膜进行加热和熔化与80 fs和100 ps脉冲宽度的激光脉冲将被探测。研究生和本科生将参与这项研究。%熔化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化的理解已经很好地建立起来，但对这种相变的微观理解仍然缺乏。熔融态的过冷是一种常见的现象;然而，固体过热的观察是罕见的。过冷液体是在低于本体熔点的温度下保持液态一段时间的液体。过热固体是在高于本体熔点的温度下结构完整性仍然存在的固体。熔化在表面和缺陷处开始，并传播到晶体的本体中。这项研究的主要目的是调查过热固体的可能性和限制，从而提供一个答案的问题，多远和多久可以存在一个固体高于其熔点。由于熔化的快速性质，本研究需要使用快速加热源和能够在熔化期间监测原子排列的探针。我们使用超短持续时间的激光脉冲，万亿分之一秒或更短，作为快速加热源。随后的电子脉冲在材料中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这项研究。他们将接受培训，为他们在世纪的未来几十年进入工业、政府或学术就业市场做好准备。

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.***