Superfluid Studies in Quantum Systems at Low Temperatures

低温量子系统中的超流体研究

• 批准号: 0203244
• 负责人: John Reppy
• 金额: --
• 依托单位: Cornell University
• 依托单位国家: 美国
• 项目类别: Continuing grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-05-15 至 2006-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0203244&HistoricalAwards=false
• 关键词: Superfluid; Studies; Quantum; Systems; Low

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.

这个低温物理项目涉及量子流体的相行为，3 He和4 He，它们被限制在高度多孔的低密度“固体”，vycor和气凝胶中，或吸附在高表面积的固体上，如石墨。 第一个主题是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玻璃是用来提供一个“捕获”的潜力，限制在三维空间中的低密度气体的氦原子。 在正常情况下，氦原子的气体在温度降低到足以发生BEC之前很久就会冷凝成液相，然而对于吸附在多孔玻璃表面上的移动的氦原子，气相在最低温度下是稳定的，并且实际上可以达到BEC温度。 在这个系统中的粒子密度连续变化的能力，从稀玻色气体制度几乎全液体密度，提供了一个独特的机会，在广泛的相互作用参数的玻色凝聚系统的性质的演变的研究。 第二个课题是研究~ 3 He-气凝胶体系的超流转变。 该系统目前正受到低温界的广泛关注，因为它具有不寻常的性质。 该项目将有助于高分辨率的热容和超流体密度测量。 第三个实验旨在提供一个更清晰的理解的可重入超流相，发现在一定范围内的第二层的部分覆盖的4 He吸附在六方基底平面石墨。一个试探性的解释是，超流信号产生通过渗透的2-D液滴在第二层。 超流体信号的温度依赖性是非常不寻常的，几乎是对数的，目前还不清楚，因此仍然是一个令人着迷的开放性问题。 这个研究计划和以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很好的培训基地。 该培训培养实验管理和决策技能，可转移到不同领域，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，一种物质形式转变为另一种物质形式，是自然界中普遍存在的现象。 物理学的目标之一是更全面地理解相变过程。 这是自然的，把低温的例子进行研究，因为这些目前的几个最不复杂的系统中，研究相变过程。 这项资助支持的研究将研究本质上相同的相变的三个对比例子，其中一个从一些基本观点来看很重要。 它是从超流状态到非超流状态的过渡。 第一个例子是氦原子的低密度气体系统，它在非常低的温度下经历超流体或玻色爱因斯坦凝聚（BEC）转变。 如果感兴趣的是这种转变的演变，因为粒子之间的相互作用的强度随着原子密度的增加而增加。 第二个例子是夹带在低密度气凝胶玻璃中的超流3 He。 这是一个模型系统的淬火杂质的影响，复杂的气凝胶网络，在液体3 He的超流转变的性质的探索。 最后一个例子探讨了二维世界中的4 He超流转变，其中的事物与前面提到的3D例子有很大的不同。 多年来，4 He-石墨系统已经被认为显示出大量不同的2-D相变。 其中大部分涉及吸附的氦的晶体结构的变化，然而，超流转变也观察到在不同的覆盖率的石墨。 这些转变中最不了解的一个，也是将要研究的一个，是所谓的再入（一种属性出现，然后作为实验变量的函数消失）超流转变，观察到部分覆盖的第二层吸附的4 He。随着第二层覆盖率的增加，超流相出现，然后随着层的完成而消失。 这种有趣的行为是由一个尚未被理解的温度依赖性的超流体信号补充。 学生，无论是本科生和研究生，都有机会参与前沿研究，在适度规模的项目。 在这种类型的研究中获得的科学管理和决策技能已经为一些毕业生推出了成功的职业生涯，他们现在正在低温物理学以外的领域工作，如基因组研究和管理咨询。 还有一些人在议会和政府中从事着非常成功的物理学事业。