Helium-three and Helium-four at Very Low Temperatures

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w:\awards\awards96\*.doc 9630930 Edwards The helium isotopes 4He and 3He form Bose and Fermion superfluids respectively. The solubility of 4He in 3He is very low at ultra- low temperatures but recent theoretical predictions have suggested a finite value at zero temperature. If so, a weakly interacting dilute Bose gas (the dissolved 4He quasiparticles) would coexist in a solution of Fermion superfluid and provide a unique opportunity to study interacting superfluids. A stringent test of Fermi liquid theory will result from the determination of the solubility. The free energy of the interface between the two coexisting phases in phase-separated liquid mixtures will be measured as a function of pressure and temperature. The results will indicate whether 3He is preferentially adsorbed near the interface and compared to fundamental many-body quantum theories. A measurement of the coalescence of two helium atom beams is planned to determine the nature of the scattering cross-section at very low velocities and investigate the dimerization of helium atoms. %%% A fundamental theory of matter, Fermi liquid theory, used to understand systems as disparate as neutron stars, nuclear matter, and electrons in ordinary and superconducting metals, is to be tested in a new way by studying 4He atoms dissolved in liquid 3He at extremely low temperatures. The experiment is to measure, at temperatures less than 0.1 degrees above absolute zero, the rate at which the 4He atoms diffuse through the liquid 3He and the maximum amount that may be dissolved as functions of the temperature and pressure. The theory predicts a relation between these effects. In a second low temperature experiment, two beams of colliding 4He atoms will be used to measure the very weak chemical bond between a pair of 4He atoms. ***