Impurity Effects in Superfluid Helium Three

超流氦三中的杂质效应

• 批准号: 9703575
• 负责人: William Halperin
• 金额: $ 33万
• 依托单位: Northwestern University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1997
• 资助国家: 美国
• 起止时间: 1997-08-15 至 2000-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9703575&HistoricalAwards=false
• 关键词: Impurity; Effects; Superfluid; Helium; Three

9703575 Halperin High resolution nuclear resonance (NMR) experiments on helium 3 are proposed in order to study the normal and superfuid helium 3 states. Temperature dependent NMR frequency shifts in the normal state will be investigated as a function of magnetic field in order to identify their origin which appears to be of fundamental significance. High resolution NMR experiments and heat capacity measurements will be performed on superfluid helium 3 aerogels to understand the effects of impurities introduced by the aerogel into this important superfluid system. Close interaction with theoretical work should allow us to establish the extent to which anisotropic scattering models can account for our observations. The basic tools that we propose to use are low temperature nuclear demagnetization techniques to prepare samples of helium 3 imbibed into highly porous silica aerogel. The transition to superfluidity and the strength of the order parameter and energy gap will be determined mainly by nuclear magnetic resoncnce methods, complemented by acoustic excitation of order parameter collective modes, thermal conductivity, and heat capacity measurements. %%% The proposed work will further our understanding of a broad class of materials of considerable interest in condensed matter physics and of importance to a wide range of applications involving superconductivity. The study of impurity effects on the suppression of the transition temperature for superfluidity in helium 3 will give important insight into the unusual superfluid behavior in helium 3 as well as for other similar Superconducting systems. New phases and unusual properties concerning the impurity helium 3 problem have recently been discovered by several groups including our own at Northwestern. The impurities can be introduced in the helium 3 using very dilute aerogel. The aerogel consists of tenuous strands of silica polymer of very high porosity. The helium 3 impurity problem is a model for un derstanding the limitations and basic properties of new classes of superconducting materials including high temperature superconductors. The proposed work is also relevant to the behavior of neutron matter in the cores of neutron stars. These systems all have the common important characteristic of spontaneously broken symmetries beyond gauge symmetry which is itself responsible for the superfluid behavior. As such they are all extremely sensitive to impurity effects which must be investigated if the basic behavior of these systems is to be understood and if the potential for their application is to be realized. This situation is particularly relevant to the case of the high temperature superconductors for which the proposed work can be viewed as a paradigm. ***

为了研究氦3的正常态和超液态，提出了9703575个关于氦3的高分辨核磁共振实验。正常状态下的温度相关核磁共振频移将作为磁场的函数进行研究，以确定它们的来源，这似乎具有基本意义。高分辨率核磁共振实验和热容测量将在超流氦3气凝胶上进行，以了解气凝胶引入这一重要超流体系统的杂质的影响。与理论工作的密切互动应该使我们能够确定各向异性散射模型能在多大程度上解释我们的观测。我们建议使用的基本工具是低温核退磁技术，以制备吸入到高孔二氧化硅气凝胶中的氦3样品。向超流性的转变以及有序参数和能隙的强度将主要由核磁共振方法决定，辅之以有序参数集体模声激发、热导率和热容测量。拟议的工作将进一步加深我们对凝聚态物理中相当感兴趣的一大类材料的理解，并对涉及超导的广泛应用具有重要意义。研究杂质对抑制氦3中超流转变温度的影响，将对理解氦3中的异常超流行为以及其他类似的超导系统具有重要意义。最近，包括我们西北大学的几个小组已经发现了有关杂质氦3问题的新的相和不寻常的性质。可以使用非常稀薄的气凝胶将杂质引入氦3中。这种气凝胶由孔隙率非常高的细小的二氧化硅聚合物链组成。氦3杂质问题是理解包括高温超导体在内的新型超导材料的局限性和基本性质的一个模型。这项拟议的工作也与中子星核心中的中子物质的行为有关。这些系统都有一个共同的重要特征，即规范对称性以外的对称性自发破缺，这本身就是超流行为的原因。因此，它们都对杂质效应非常敏感，如果要了解这些系统的基本行为并实现它们的应用潜力，就必须研究杂质效应。这种情况与高温超导体的情况特别相关，拟议的工作可以被视为范例。***