Investigations of Solid Helium

****技术摘要****本项目主要研究凝聚态物理中一个基本而重要的问题：氦原子明显能够流过固体氦的机制是什么？其他人先前有争议的工作表明，在非常低的温度下，固体4He中可能存在物质的“超固体”状态。这个问题引起了凝聚态物质界的强烈兴趣，激发了许多实验和理论工作，并产生了许多可能的解释和一些实质性的悖论。理论上的争论坚持认为，固体氦的完美晶体不可能是“超固体”，任何通过固体的质量流都必须由缺陷携带。人们提出了各种缺陷机制。一些人认为，一种以前未知的可塑性机制可能在起作用。本研究中使用的实验方法是通过一种独特的技术在固体上施加化学势梯度：例如，在固体界面的液氦上施加压力差，而不是直接向4He晶格施加压力；或者，应用温差来利用超流体喷泉效应。这种方法利用了氦的已知特性，在Vycor（一种多孔玻璃）的高压下保持液体状态，在这种压力下，氦会变成固体。研究温度和压力（以及3He杂质浓度）的函数将为流动提供进一步的证据，并限制导致流动的可能机制。参与这些研究的学生（本科生、研究生）和研究生（博士后）将获得基础物理和尖端技术的经验。毕业的学生将在工业、国家实验室和学术环境中为科学研究和技术发展做出贡献。****非技术摘要****本研究集中在凝聚态物理中一个基本和重要的当前问题的调查：什么是机制，使4He原子显然能够流过固体4He？其他人先前有争议的工作表明，在非常低的温度下，固体4He中可能存在物质的“超固体”状态。这个问题引起了凝聚态物质界的强烈兴趣，激发了许多实验和理论工作，并产生了许多可能的解释和一些实质性的悖论。一些人认为，一种以前未知的可塑性机制可能在起作用。大多数人认为，固体中的无序是一个关键因素。例如，本研究中使用的方法是通过一种独特的技术在固体上施加压力差，这种技术不需要在固体的晶体侧面施加推力；相反，利用超流氦在含有超流体的多孔材料中的独特性质，对超流液氦施加压力差，使其与固体在其侧面接触。参与的本科生、研究生和研究生（博士后）学生将获得基础物理和前沿技术方面的经验。这些研究可能导致材料科学的进步，这些进步可能具有重大的技术影响，例如在冶金学方面。

****Technical Abstract****This project is centered on an investigation of a fundamental and important current question in Condensed Matter Physics: What is the mechanism by which 4He atoms are apparently able to flow though solid 4He? Prior controversial work by others has suggested that there may be a "supersolid" state of matter that may exist in solid 4He at very low temperatures. This is an issue that has aroused intense interest in the Condensed Matter community, stimulated a number of experiments and theoretical works, and resulted in a number of possible explanations and some substantial paradoxes. The theoretical debate insists that perfect crystals of solid helium cannot be a "supersolid", and that any mass flux through the solid must be carried by defects. Various defect mechanisms have been proposed. Some believe that a previously unknown plasticity mechanism may be at work. The experimental approach used in this research is to impose a chemical potential gradient on the solid by a unique technique: for example, application of a pressure difference to liquid helium that interfaces the solid instead of applying pressure directly to the 4He crystal lattice; or, the application of a temperature difference to utilize the superfluid Fountain Effect. The approach employs the known behavior of 4He to remain a liquid at elevated pressure in Vycor (a porous glass), at pressures at which bulk 4He would be a solid. Studies as a function of temperature and pressure (and 3He impurity concentration) will provide further evidence for flow and limit the set of possible mechanisms that are responsible for flow. The students (undergraduate, graduate) and post-graduate (postdocs) involved in these studies will gain experience in fundamental physics and cutting-edge technology. Graduating students will be poised to contribute to scientific research and technological development in industrial, national laboratory, and academic settings.****Non-Technical Abstract****This research is centered on an investigation of a fundamental and important current question in Condensed Matter Physics: What is the mechanism by which 4He atoms are apparently able to flow though solid 4He? Prior controversial work by others has suggested that there may be a "supersolid" state of matter that may exist in solid 4He at very low temperatures. This is an issue that has aroused intense interest in the Condensed Matter community, stimulated a number of experiments and theoretical works, and resulted in a number of possible explanations and some substantial paradoxes. Some believe that a previously unknown plasticity mechanism may be at work. Most agree that disorder in the solid is a crucial ingredient. The approach used in this research is to impose, for example, a pressure difference across the solid by a unique technique that does not employ pushing on the crystal sides of the solid; instead, application of a pressure difference is made to superfluid liquid helium that interfaces the solid on its sides using the unique properties of superfluid helium in a porous material that contains the superfluid. Participating undergraduate, graduate and post-graduate (postdocs) students will gain experience in fundamental physics and cutting-edge technology. These investigations may lead to advances in materials science that could have significant technological implications, for example in metallurgy.