Helium3 and Mixtures of Helium3/Helium4 in Aerogel, and High-Q Mechanical Detectors at Low Temperatures

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9424131 Parpia This project explores the effect of impurities that are one dimensional (strands of aerogel) or zero dimensional (4He atoms) on the superfluidity of 3He. The presence of impurities modifies the phase diagram of pure 3He in ways that are incompletely understood and are often surprising. The research will employ both pure 3He and mixtures of 3He and 4He. Of considerable interest is the distribution of 3He and the 4He in mixtures: Phase separation may occur, one of the isotopes being preferentially adsorbed near the aerogel colloid, the superfluid properties of the system may be changed, etc. There is the possibility that the work could lead to discovery of conditions in which two different superfluids coexist in a mixture of 3He and 4He. The work will use torsional oscillators and sound resonators to map the transition temperature, superfluid fraction, sound velocity and heat capacity of these quantum fluids in aerogels. The research is of a fundamental nature and will provide important information about the physics of superfluid behavior. %%% This is an experimental low temperature physics project. The research deals with superfluid liquids consisting of the isotopes of helium: helium-3, helium-4, or mixtures thereof. Separately, helium-3 or helium-4 can be cooled to the superfluid state (vanishing viscosity). Very recently, it was discovered that mixtures of helium-3 and helium-4 can have properties surprisingly different from the parent superfluids. Further, it has been found that the presence of a filamentary aerogel (a silica-gel) "impurity" colloid, which comprises only 1% of the sample volume, can profoundly affect the superfluid properties of the isotope mixtures. The effect is related to the very large surface area of the aerogel. A range of low temperature techniques are being employed to investig ate these fundamental problems, which bear of the physics of superfluids. The results may also be relevant to other quantum phenomena such as superconductivity. ***