Coexisting Bose & Fermi Superfluids and Fermi Liquid Transport in the Presence of Disorder

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This low temperature physics project will investigate the behavior of 3He, and phase-separated mixtures of 3He and 4He, in Aerogel. Aerogel, a dilute glass that occupies between 0.1% and 3% of the volume is the only means by which a structural "disorder" can be introduced to the 3He. One project will examine flow and the onset of dissipation (the breakdown of superfluidity) in this disordered superfluid. Magnetic resonance will be used to characterize the superfluidity of isolated "bubbles" of superfluid 3He trapped in the isotope mixtures. This will establish dimensional constraints that may limit superfluidity. These demanding experiments, which involve cutting edge instrumentation and development of new techniques, provide a challenging environment where graduate students and post-doctoral researchers acquire skills and analytic tools to prepare them for careers in the Nation's scientific and technological infrastructure.Liquid 3He, available only near "absolute zero" is one of the purest materials that can be prepared by any means. Impurities simply freeze out during the arduous cooling and liquefaction procedures required to obtain the liquid. Like its more abundant sister isotope 4He, 3He can be obtained in a "superfluid" state. But the superfluid is complicated by the magnetism of the 3He atoms, each of which acts like a tiny compass needle. The atoms themselves undergo mutual orbital motion, and their compass needles pair up in various ways depending on the orbital motion of the atoms. Because of its complex magnetic character, the superfluid properties of 3He are of fundamental interest, and in some circumstances can be related to more common phenomena like the flow of electrons in a wire. One project under this award will study how the ultra pure superfluid 3He changes when it is diluted and 'disordered' by the addition of Aerogel, a dilute glass that occupies less than 2% of the total volume of the container. The disorder introduced into the helium changes the orbital motions of 'paired' atoms, and in some conditions of temperature and pressure can destroy superfluidity altogether. The research will also look for changes in the pairing when the helium is confined to small "bubbles", which approximate a so-called zero-dimensional superfluid "dot". Finally the effects of fluid flow on the disordered superfluid will be examined with the objective of detecting energy dissipation, analogous to heating of a wire by current flow. This is signaled by loss of superfluidity. Besides adding to the understanding of quantum systems and superfluid flow, this research provides a demanding experimental environment that educates and trains graduate students and post-doctoral researchers for successful careers in the Nation's scientific and technological infrastructure.