Phase Transitions and Phase Stability in Superfluid Helium Three

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*****NON-TECHNICAL ABSTRACT*****:At very low temperatures helium becomes a liquid, at still lower temperatures it becomes a superfluid. A superfluid will flow with no friction. In that way it is similar to a superconductor, which carried an electric current with no resistance. Therefore understanding superfluid helium can provide insight into superconductors. This research project will study how impurities modify the class of ordered states that include the high temperature superconductors, using superfluid 3He (one type of helium) as a model system. Prior studies of superfluidity in liquid 3He contained within very low density silica aerogels (a very low density, highly porous material), which act as the impurities, found that the ordered state of the superfluid near its transition temperature was changed by the impurities, in a manner that was independent of the density of the impurities. The present research project will extend the range of density of the aerogel (impurities) to see how low the density can be and still modify the 3He ordered state; and to see if other ordered states can be stabilized over different ranges of impurity densities. Longitudinal magnetic resonance studies of the ordered state will be carried out to identify the microscopic nature of the state stabilized by the aerogel. The studies will also be carried out with different degrees of strain applied to the aerogel, to see how the affect of the aerogel 'impurities' depends upon anisotropy. This project will involve the training of undergraduate and graduate students, and the PI gives numerous technical and public lectures as a Nobel Laureate.*****TECHNICAL ABSTRACT*****:This award supports a study of how impurity scattering in unconventional BCS states can affect the identity of the ordered state, using superfluidity in 3He as a model system, and low density silica aerogels as model impurities. Prior work has shown that aerogels with densities (fractional volume occupied by the aerogel) as low as 0.5% can stabilize a state near Tc that is not stable in the bulk, and that this new state is unchanged over a factor of two in variation in the aerogel density. The present research will extend the range of aerogel densities to 0.2%, and will include longitudinal resonance studies in the new state to help determine its microscopic identity. The application of variable longitudinal strain will help determine if the affects of the aerogel depend upon its anisotropy. Separately, the 3He A to B transition is a valuable model for studies of first order phase nucleation. A study of the A to B transition will be carried out to better understand the role of surface roughness in nucleation and to differentiate between the two proposed nucleation mechanisms involving nucleation by ionizing radiation. This work will involve the training of both undergraduate and graduate research students.