Phase Transitions in the Presence of Disorder

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9311918 Chan This project involves studies undertaken to further the understanding of the effect of frozen-in disorder (including the dilute limit) on the behavior of systems undergoing continuous phase transitions. This will be accomplished by placing 4He and 3He-4He mixtures inside porous gold and silica aerogel. These two substrates, owing to the different manufacturing processes, provide disorder that are respectively correlated and uncorrelated at large length scales -- thus allowing for a critical test of current theoretical ideas. Preliminary results indicate that when a 3He- 4He mixture is placed inside light aerogel, the coexistence region at low temperature detaches from the superfluid transition line, giving rise to a novel miscible superfluid mixture at high 3He concentration. This mixture may represent the realization of a dilute Bose gas system and suggests the possibility of observing an interpenetrating double superfluid in the limit of zero temperature. %%% In the last twenty years, physicists have made great advances in understanding the behavior of pure substances undergoing continuous phase transformations, for example from liquid to vapor or from superconducting or superfluid states to the normal state. This knowledge is not always applicable to realistic situations as most substances contain defects and impurities. This project will investigate the effects of impurities and disorder on the nature of the continuous phase transitions by placing nature's purest substance, liquid helium, inside a disordered environment -- porous glass or porouss gold. Since the open fraction of the porous glasses ranges from 0.98 to 0.3, this is akin to the situation where the impurities constitute from two to twenty percent of a sample. Preliminary results indicate that impurities have a profound and unexpected effect on both the liquid-vapor and the superfluid transitions, and suggest new mechanisms as to how fluid mixtures undergo mixing and demixi ng. ***