Effects of Disorder on Phase Transitions of Quantum Fluids

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w:\awards\awards96\*.doc 9630736 Chan This research will explore the effects of disorder on phase transitions, particularly the superfluid transition in Helium-4 and phase separation in Helium-3/Helium-4 mixtures. This is accomplished by introducing liquid Helium-4 and Helium-3/Helium-4 mixtures into porous media, such as silica aerogel or porous gold, two substrates made by the investigator. In this random environment of the porous media, the superfluid transition of confined pure Helium-4 was found to be completely altered from the bulk liquid although the transition remains remarkably sharp and exhibits well defined simple power law behavior. The power law exponents are significantly modified even though the silica based aerogel constitutes only a few percent of the total volume. When Helium-3/Helium-4 mixtures are added to the aerogel the phase diagram of Helium-3/Helium-4 mixture is completely altered giving rise to a new Helium-3 rich superfluid phase in addition to the usual Helium-3 rich superfluid. The phase diagram and behavior of mixtures when the volume fraction of the aerogel is further reduced will be studied. %%% In the last two decades, a great deal has been learned about how one phase of pure matter transforms into another, e.g., liquid into gas and superconductor into normal metal. In the real world, disorder and impurities are very often present in these systems. This project will provide an understanding of the effect of such impurities on the nature of a specific system, namely the transformation of liquid Helium-4 from the superfluid to the normal state. This system can be carefully controlled so that conclusions can be applied to technologically important materials. Impurities are introduced by adding liquid helium into porous aerogel or porous gold of well characterized porosity and topology. Experiments to date reveal unexpected results including an extraordinary sharp transition inspite of the random environment and the creation of a new 3He rich superfluid phase. Study is planned of this unusual behavior and the variation of these properties with temperature, pressure and concentration of the two helium isotopes. ***