Impurity Effects in Superfluid Helium Three

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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. ***