Studies of Solid 4-Helium

****非技术摘要****本研究项目集中于凝聚态物理中一个基本而重要的当前问题的研究：4He原子流过固体4He是否可能（如果可能的话，其机制是什么）。其他人先前的研究表明，这种奇怪的现象可能存在，并且可能存在“超固体”。固体氦在极低温度下可能存在的物质状态。超立体?将代表一种新的物质量子态，因此它的存在问题引起了凝聚态界的强烈兴趣，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。为了确定氦原子是否有可能流过固体氦，该项目将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，对液氦施加压力差，使其与固体在其侧面接触。该方法利用已知的4He行为在Vycor（多孔玻璃）中保持高压液体。对Vycor中的液氦施加压力，原子通过Vycor与固体的界面进入固体氦。参与这些研究的学生将获得基础物理和尖端技术的经验。他们将攻读博士学位，毕业时将为工业、国家实验室和学术环境中的科学研究和技术发展做出贡献。****技术摘要****本研究项目主要研究凝聚态物理中一个基本而重要的问题：氦原子是否有可能流过固体氦原子，如果有可能，其机制是什么？其他人先前的研究表明，这种流动是可能的，更具体地说，可能存在一种“超固体”。固体氦在极低温度下可能存在的物质状态。超立体?将代表一种新的物质量子态，因此，它的存在问题引起了凝聚态界的浓厚兴趣，激发了许多实验和理论辩论，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，固态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。为了研究通过固体4He的质量通量的可能性，该项目将通过一种独特的技术在固体上施加化学势梯度：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。这种方法利用氦的已知特性，在Vycor（多孔玻璃）的高压下保持液体状态，在这种压力下，氦块会变成固体。在Vycor两侧的固体氦上应用化学电位差将允许原子注入固体，而无需对晶格施加机械压力差。作为温度和压力函数的研究将为流动提供（或不提供）证据，并限制导致流动的可能机制。参与这些研究的学生将获得基础物理和尖端技术的经验。在完成博士论文后，他们将准备在工业，国家实验室和学术环境中为科学研究和发展做出贡献。

****NON-TECHNICAL ABSTRACT****This research project is centered on an investigation of a fundamental and important current question in Condensed Matter Physics: Is it possible (and if so, what is the mechanism) for 4He atoms to flow though solid 4He. Prior work by others has suggested that this strange phenomenon might be possible and that there may be a ?supersolid? state of matter that may exist in solid 4He at very low temperatures. A ?supersolid? would represent a new quantum state of matter, therefore the question of its existence 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. To determine whether it is possible for 4He atoms to flow through solid 4He, this project will impose 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 liquid helium that interfaces the solid on its sides. The approach employs the known behavior of 4He to remain a liquid at elevated pressure in Vycor (a porous glass). The pressure is applied to the liquid helium in the Vycor and atoms are fed into the solid helium through the interface where the Vycor meets the solid. The students involved in these studies will gain experience in fundamental physics and cutting-edge technology. They will work toward a Ph.D. degree and at graduation will be poised to contribute to scientific research and technological development in industrial, national laboratory, and academic settings.****TECHNICAL ABSTRACT****This research project is centered on an investigation of a fundamental and important current question in Condensed Matter Physics: Is it possible for 4He atoms to flow though solid 4He and if so, what is the mechanism. Prior work by others has suggested such flow might be possible and more specifically that there may be a ?supersolid? state of matter that may exist in solid 4He at very low temperatures. A ?supersolid? would represent a new quantum state of matter, therefore the question of its existence has aroused intense interest in the Condensed Matter community, stimulated a number of experiments and theoretical debate, 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. To investigate the possibility of mass flux through solid 4He, this project will impose a chemical potential gradient on the solid by a unique technique: application of a pressure difference to liquid helium that interfaces the solid instead of applying pressure directly to the 4He crystal lattice. This 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. Application of a chemical potential difference across solid helium flanked by Vycor will allow atoms to be injected into the solid without application of a mechanical pressure difference to the lattice. Studies as a function of temperature and pressure will provide evidence (or not) for flow and limit the set of possible mechanisms that are responsible for flow. The students involved in these studies will gain experience in fundamental physics and cutting-edge technology. Upon completion of Ph.D. dissertations they will be poised to contribute to scientific research and development in industrial, national laboratory, and academic settings.