Experiments on Solid Helium

*非技术摘要*固体4He是一种具有独特性质的量子固体(即其性质由量子效应主导)。例如，4He原子显然能够流过固体4He，就像它们流过液体一样。先前的工作表明，这一行为表明物质处于类似于超流体和超导电性的“超固态”状态。人们不再认为情况是这样，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流。压差将通过一种独特的技术在固体上施加，这种技术不会对晶体侧面施加推力。利用这项技术，我们将研究这个令人着迷的系统的性质作为流量和~3He浓度的函数。参与的本科生、研究生和研究生(博士后)将获得基础物理和尖端技术方面的经验，并为未来在研究、教学或工业环境中的工作做好准备。这些研究可能导致材料科学的进步，这些科学可能具有重大的技术意义，例如在冶金方面。*技术摘要*这个项目的中心是对凝聚态物理中一个基本和重要的问题的研究：4He原子显然能够通过什么机制流过装满固体4He的样品池？其他人之前的工作最初表明，在非常低的温度下，固体4He中可能存在一种大块的“超固态”物质。这个问题引起了凝聚态的强烈兴趣，激发了许多实验和理论工作，并导致了一些可能的解释和一些实质性的悖论。理论上的争论坚持认为，完美的固体氦晶体不可能是“超固体”的，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对固体施加化学势梯度：例如，通过对与固体界面的液氦施加压差，而不是直接对4He晶格施加机械压力；或者，通过应用温差来利用超流体喷泉效应来驱动流动。这种方法利用了4He的已知行为，在多孔性材料Vycor(一种多孔性玻璃)中，在高压下保持液体，在压力下4He是固体。对温度和压力(以及3He杂质浓度)的研究为液-固熔化曲线的流动提供了进一步的证据。将探索这种流动的具体机制，包括探索3He的存在在浓度依赖的温度下显著抑制这种流动的机制。一种方法是了解当固体受到校准的外加应力使固体变形时，流体如何响应。另一种方法将是提高敏感性，并探索关于临界通量的可能性和可能开始可测量的耗散的问题。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方面的经验。离开集团的人员将为工业、国家实验室或学术机构的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.