Impurity Effects in Superfluid Helium Three

为了研究氦3的正常态和超液态，提出了氦3的高分辨率核磁共振实验。在正常状态下，温度相关的核磁共振频移将作为磁场的函数进行研究，以确定其起源，这似乎具有根本意义。将对超流氦3气凝胶进行高分辨率核磁共振实验和热容测量，以了解气凝胶引入的杂质对这一重要超流系统的影响。与理论工作的密切相互作用应使我们能够确定各向异性散射模型能够解释我们的观测结果的程度。我们建议使用的基本工具是低温核退磁技术来制备氦3吸入到高多孔硅气凝胶中的样品。向超流体的过渡以及有序参数和能隙的强度将主要由核磁共振方法确定，并辅以有序参数集体模式的声激励、导热系数和热容测量。所提出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。研究杂质对氦3中超流体转变温度抑制的影响，将对氦3中不寻常的超流体行为以及其他类似超导体系提供重要的认识。最近，包括我们西北大学在内的几个研究小组发现了有关杂质氦3问题的新相和不寻常的性质。杂质可以用非常稀的气凝胶引入氦3中。气凝胶由非常高孔隙率的硅聚合物细丝组成。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。所提出的工作也与中子星核心中中子物质的行为有关。这些系统都具有超越规范对称的自发破缺对称性的共同重要特征，这本身就是超流体行为的原因。因此，它们都对杂质效应非常敏感，如果要了解这些体系的基本行为，并且要实现它们的应用潜力，就必须对杂质效应进行研究。这种情况与高温超导体的情况特别相关，所提出的工作可以被视为一个范例。* * *

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. ***