Ultrasound in Superfluid Helium at Microkelvin Temperatures

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The energy gap in superfluid 3He-B is to be measured over a wide range of pressure and at low T/Tc. Ultrasound will be used to measure the pair-breaking edge, 2 (P,T), in the T-- O limit, for the first systematic test of the quasiclassical weak-coupling + WCP theory of Serene and Raianer. In addition, the proposed work will provide a stringent self-consistency check on the WCP model and the heat-capacity and phase-diagram results of Greywall. Performing the experiments in the microkelvin temperature regime will make the results less sensitive to problems in theomometry compaired with work at higher temperatures where very high accuracy is required. Furthermore, the very-low-temperature work will permit a study of the possible defects in the theoretical picture, as suggested by experiments at high temperature and high pressure. Finally, the reearch requires the development of broadband acoustic techniques and their extension to miocrokelvin temperatures. This research examaines the role of quantum mechanics in determining the fundamental properties of matter through high- frequency acoustic probing of liquid 3He at temperatures less than lnK above absolute zero.