Picosecond Electron Diffraction Study of Superheating of Solids

研究了固体过热的可能性和限度。过热固体是一种在高于整体熔点Tm的温度下保持长程有序的固体。时间分辨电子衍射(TED)，时间分辨率高达1ps，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。虽然熔融状态的过冷是一种常见的现象，但观察到固体过热的情况却很少。表面和扩展的缺陷，如晶界，为Tm以下的无序提供了大量的形核位置，从而抑制了固体的过热。然而，一些单晶表面保持有序性，直到Tm，允许表面过热。本工作着重于透射式电子衍射的研究，以探测高达几百埃的薄膜，而不是以前工作中使用的反射式高能电子衍射(RHEED)中探测的最初几个原子层。时间分辨TED(1-ps或更小)将在单晶和多晶的金属和半导体薄膜以及由紧密堆积表面结合的微晶上进行。研究了脉冲宽度分别为80ps和100ps的激光脉冲加热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。虽然我们从热力学的角度对熔化的理解已经确立，但对这种相变的微观理解还很欠缺。熔融状态的过冷是一种常见的现象；然而，观察到固体过热的情况却很少。过冷液体是指在低于整体熔点的温度下保持液态一段时间的液体。过热固体是一种在高于整体熔点的温度下保持结构完整性的固体。熔化是在表面和缺陷处开始的，并传播到晶体的整体。这项研究的主要目的是调查过热固体的可能性和限度，从而为固体在其熔点之上存在多远和多长时间的问题提供一个答案。由于熔化的快速性，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头。我们使用超短持续时间的激光脉冲，即万亿分之一秒或更少，作为快速加热源。材料中随后的电子脉冲的衍射允许监测加热材料的结构。这些高时间分辨率的研究将在单晶和多晶的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，为他们在本世纪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.***