Experiments on Solid Helium

****非技术摘要****固体4它是一种具有独特性质的量子固体（即性质由量子效应支配）。例如，氦原子显然能够像流过液体一样流过固体氦。先前的研究表明，这种行为表明物质的“超固体”状态类似于超流动性和超导性。人们不再相信这是事实，但原子在固体中流动的机制仍然是个谜。在这个项目中，将使用一种不同的方法来研究原子的流动。压力差将通过一种独特的技术施加在整个固体上，这种技术不需要在晶体两侧施加压力。利用这种技术，我们将研究这个令人着迷的体系的性质，作为流速和3He浓度的函数。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尖端技术方面的经验，并为未来的研究、教学或工业环境工作做好准备。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。****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：4He原子显然能够流过充满固体4He的样品细胞的机制是什么？其他人先前的工作最初表明，在非常低的温度下，固体4He中可能存在大块的“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。本研究中使用的实验方法是在固体上施加化学势梯度：例如，通过对固体界面的液氦施加压力差，而不是直接对4He晶格施加机械压力；或者，应用温差来利用超流体喷泉效应来驱动流动。该方法利用氦的已知特性，在多孔材料Vycor（一种多孔玻璃）的高压下保持液体状态，在这种压力下，氦体将成为固体。研究温度和压力（以及3He杂质浓度）的函数为液固熔融曲线的流动提供了进一步的证据。我们将探索这种流动的具体机制，包括探索3He的存在在浓度相关温度下显著抑制流动的机制。一种方法是了解当固体受到使固体变形的校准施加应力时，流动是如何响应的。另一种方法将是提高灵敏度，并探讨临界通量的可能性和可测量耗散的可能开始的问题。然而，另一种方法将是探索压力在多大程度上抑制质量注入和在各种条件下存在流动的固体的生长。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。离开小组的人员将准备在工业、国家实验室或学术环境中为科学研究和技术发展做出贡献。

****Non-Technical Abstract****Solid 4He is a quantum solid (i.e. the properties are dominated by quantum effects) with unique properties. For example, 4He atoms are apparently able to flow though solid 4He as they do through a liquid. Prior work suggested that this behavior indicated a "supersolid" state of matter analogous to superfluidity and superconductivity. This is no longer believed to be the case but the mechanism allowing atoms to flow through the solid remains a mystery. In this project a different approach to study the flow of atoms will be used. A pressure difference will be applied across the solid by a unique technique that does not employ pushing on the crystal sides. Using this technique we will study the properties of this fascinating system as a function of flow rate and 3He concentration. Participating undergraduate, graduate and post-graduate (postdocs) students will gain experience in fundamental physics and cutting-edge technology and be prepared for future work in research, teaching or industrial settings. These investigations may lead to advances in materials science that could have significant technological implications, for example in metallurgy.****Technical Abstract****This project is centered on an investigation of a fundamental and important question in Condensed Matter Physics: What is the mechanism by which 4He atoms are apparently able to flow through a sample cell that is filled with solid 4He? Prior work by others initially suggested that there may be a bulk "supersolid" state of matter that may exist in solid 4He at very low temperatures. This is an issue that 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. The experimental approach used in this research is to impose a chemical potential gradient on the solid: for example, by application of a pressure difference to liquid helium that interfaces the solid instead of applying mechanical pressure directly to the 4He crystal lattice; or, the application of a temperature difference to utilize the superfluid Fountain Effect to drive the flow. The approach employs the known behavior of 4He to remain a liquid at elevated pressure in the porous material 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) provided further evidence for flow of the liquid-solid melting curve. The specific mechanism for this flow will be explored including an exploration of the mechanism by which the presence of 3He dramatically suppresses the flow at a concentration-dependent temperature. One approach will be to understand how the flow responds when the solid is subjected to calibrated applied stress that deforms the solid. Another approach will be to increase the sensitivity and explore questions about the possibility of a critical flux and a possible onset of measurable dissipation. Yet another approach will be to explore the extent to which pressure suppresses mass injection and the growth of the solid in the presence of flow under various conditions. The students (undergraduate, graduate) and post-graduate (postdocs) involved in these studies will gain experience in fundamental physics and cutting-edge technology. Personnel who leave the group will be poised to contribute to scientific research and technological development in industrial, national laboratory, or academic settings.