Experiments on Solid Helium

固体氦实验

• 批准号: 1602616
• 负责人: Robert Hallock
• 金额: $ 46.5万
• 依托单位: University of Massachusetts Amherst
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-06-01 至 2022-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1602616&HistoricalAwards=false
• 关键词: Experiments; Solid; Helium

****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.

* 非技术摘要 * 固体4 He是一种量子固体（即性质由量子效应主导），具有独特的性质。 例如，4 He原子显然能够流过固体4 He，就像它们流过液体一样。先前的工作表明，这种行为表明物质的“超固体”状态类似于超流性和超导性。 人们不再相信这种情况，但允许原子流过固体的机制仍然是一个谜。 在这个项目中，将使用一种不同的方法来研究原子的流动。通过一种独特的技术在固体上施加压差，这种技术不需要推动晶体的侧面。 使用这种技术，我们将研究这个迷人的系统作为流量和3 He浓度的函数的属性。 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。这些研究可能会导致材料科学的进步，从而产生重大的技术影响，例如在冶金学中。技术摘要 * 本项目主要研究凝聚态物理学中的一个基本而重要的问题：4 He原子显然能够流过充满固体4 He的样品池的机制是什么？其他人先前的工作最初认为，在非常低的温度下，固体4 He中可能存在大量的“超固态”物质。这是一个引起凝聚态社区强烈兴趣的问题，激发了许多实验和理论工作，并导致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，固态氦的完美晶体不可能是“超固体”，任何通过固体的质量通量都必须由缺陷携带。在这项研究中使用的实验方法是在固体上施加化学势梯度：例如，通过向固体界面的液氦施加压力差，而不是直接向4 He晶格施加机械压力;或者，应用温度差来利用超流体喷泉效应来驱动流动。该方法采用已知的行为4 He保持在多孔材料Vycor（多孔玻璃）中的高压液体，在压力下，散装4 He将是固体。作为温度和压力（和3 He杂质浓度）的函数的研究为液-固熔化曲线的流动提供了进一步的证据。将探讨这种流动的具体机制，包括探索的机制，其中3 He的存在显着抑制流量在浓度依赖性的温度。 一种方法是理解当固体受到使固体变形的校准的施加应力时，流动如何响应。 另一种方法是提高灵敏度，并探讨临界通量的可能性和可测量耗散的可能性。 另一种方法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时固体的生长。 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。离开该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。