Superfluid Studies in Quantum Systems at Low Temperatures

这个低温物理项目涉及量子流体3He和4He的相行为，它们被限制在高度多孔的低密度‘固体’、Vycor和气凝胶中，或者被吸附在高表面积的固体上，如石墨。第一个主题将研究低密度氦系统中的玻色爱因斯坦跃迁(BEC)。在这项工作中，多孔Vycor玻璃被用来提供一种“捕获”势，将低密度的氦原子气体限制在三维空间中。在正常情况下，氦原子的气体在温度降低到足以发生BEC之前很久就会凝聚成液体，而对于吸附在多孔玻璃表面的可移动的氦原子，气相稳定到最低温度，实际上可以达到BEC温度。从稀玻色气体区域到几乎全液体密度连续变化粒子密度的能力，为研究在大范围相互作用参数范围内玻色凝聚系统的性质演化提供了独特的机会。第二个主题是研究3He-气凝胶体系中的超流相变。该系统因其独特的性能，目前正受到低温领域的广泛关注。该项目将为高分辨率热容和超流体密度测量做出贡献。第三个实验试图更清楚地了解在一定范围内吸附在六角基面石墨上的4He的第二层部分覆盖的重入超流相。试探性的解释是，超流体信号是通过第二层中二维液滴的渗流而产生的。超流体信号与温度的关系是非常不寻常的，几乎是对数的，目前还不清楚，因此仍然是一个有趣的悬而未决的问题。这项研究计划和之前的计划为倾向于工业或学术职业的本科生和研究生提供了一个极好的培训基础。这项培训开发了可转移到不同领域的实验性管理和决策技能，包括基因组学或管理咨询，以前的学生目前正在追求成功的职业生涯。相变，一种物质形式到另一种物质形式的变化，是自然界中普遍存在的现象。物理学的目标之一是实现对相变过程的更完整的理解。人们很自然地把低温的例子用来研究，因为它们是研究相变过程的几个最不复杂的系统。这笔赠款支持的研究将研究基本上相同的相变的三个截然不同的例子，其中一个从许多基本观点来看是重要的。它是从超流体状态到非超流体状态的转变。第一个例子是氦原子的低密度气体系统，它在很低的温度下经历超流体或玻色爱因斯坦凝聚(BEC)的转变。如果感兴趣的是，当粒子之间的相互作用强度随着原子密度以受控方式增加而增加时，这种转变的演变。第二个例子是携带在低密度气凝胶玻璃中的超流体3He。这是一个模型系统，用于探索淬火杂质这一复杂的气凝胶网络对3He液体超流转变性质的影响。最后一个例子探索了两维世界中的4He超流体转变，其中的情况与前面提到的3D例子有很大不同。多年来，人们已经知道4He-石墨系统显示出大量不同的二维相变。这些变化大多涉及被吸附的氦的晶体结构的变化，但在石墨中的不同覆盖层也观察到超流转变。这些转变中最不被理解的，也是将被研究的，是所谓的重入(一个性质作为实验变量的函数出现，然后消失)超流转变，观察到的是吸附的4He的第二层的部分覆盖。超流相随着第二层覆盖率的增加而出现，然后随着第二层的覆盖而消失。这一有趣的行为得到了尚未被理解的超流体信号的温度依赖性的补充。学生，包括本科生和研究生，都有机会在中等规模的项目中参与前沿研究。在这类研究中获得的科学管理和决策技能为几名毕业生带来了成功的职业生涯，他们目前在基因组研究和管理咨询等低温物理以外的领域工作。还有一些人在学术界和政府中追求着非常成功的物理职业生涯。

This low temperature physics project involves the phase behavior of quantum fluids, 3He and 4He, which are confined in highly porous low-density 'solids', vycor and aerogel, or adsorbed on high-surface-area solids such as graphite. The first subject will be a study of the Bose Einstein Transition (BEC) in low-density helium systems. In this work, porous Vycor glass is used to provide a " trapping" potential that confines a low-density gas of helium atoms in three-dimensional space. Under normal circumstances, a gas of helium atoms would condense into a liquid phase long before the temperature could be lowered enough for BEC to take place, however for mobile helium atoms adsorbed on the surface of the porous glass the gas phase is stable to the lowest temperatures and the BEC temperature can be reached in practice. The ability to continuously vary the particle density in this system, from the dilute Bose-gas regime to almost the full liquid density, provides a unique opportunity for the study of the evolution of the properties of a Bose-condensed system over a wide range of interaction parameter. The second topic is a study the superfluid transition in the 3He-aerogel system. This system is currently receiving much attention from the low temperature community because its unusual properties. This project will contribute a high-resolution heat capacity and superfluid density measurement. The third experiment seeks to provide a clearer understanding of the reentrant superfluid phase that is found for a certain range of partial coverage of the second layer for 4He adsorbed on hexagonal basal plane graphite. A tentative interpretation is that the superfluid signal arises through percolation of 2-D droplets in the second layer. The very unusual, almost logarithmic, temperature dependence of the superfluid signal is not understood at this time and so remains as fascinating open question. This research program and the previous program provide an excellent training ground for both undergraduate and graduate students inclined toward industrial or academic careers. This training develops experimental management and decision-making skills that are transferable to diverse areas, including genomics or management consulting where former students are currently pursuing successful careers.Phase transitions, the change of one matter form into another, are ubiquitous phenomena in nature. One of the goals of physics has been to achieve a more complete understanding of the phase transition process. It is natural to turn low temperature examples for study for these present several of the least complex systems in which to study the phase transition process. Research supported in this grant will study three contrasting examples of essentially the same phase transition, one that is important from a number of fundamental viewpoints. It is the transition from the superfluid to non-superfluid state. The first example is a system of low-density gas of helium atoms, which undergoes a superfluid or Bose Einstein Condensation (BEC) transition at very low temperatures. If interest is the evolution of this transition as the strength of the interaction between the particles by is increased as the density of the atoms is increased in controlled fashion. The second example is superfluid 3He entrained in low-density aerogel glass. This is a model system for the exploration of the influence of quenched impurities, the complex aerogel network, on the nature of the superfluid transition in liquid 3He. The final example explores the 4He superfluid transition in the world of two dimensions where things are quite different from the 3-D examples mentioned before. The 4He-graphite system has been known for many years to display a large number of different 2-D phase transitions. Most of these involve changes in crystallographic structure of the adsorbed helium, however the superfluid transition is also observed at various coverages in the graphite. One of the least understood of these transitions, and the one that will be investigated, is the so-called reentrant (a property appears and then disappears as a function of experimental variable) superfluid transition observed for partial coverage of the second layer of adsorbed 4He. The superfluid phase appears with increasing second layer coverage and then vanishes as the layer is completed. This interesting behavior is complemented by a yet to be understood temperature dependence for the superfluid signal. Students, both undergrads and graduate, are provided the opportunity to participate in leading-edge research, in projects of modest size. The scientific management and decision-making skills acquired in this type of research have launched successful careers for several graduates who are now working in areas outside of low temperature physics such as genome studies and management consulting. Still others have pursued notably successful physics careers in academe and government.