Acoustics of a "dirty" Fermi liquid: He-3 in aerogel

An acoustic cavity containing He-3 in 98% porous silica aerogel was used to investigate the effects of impurity scattering in a Fermi liquid. The pres sure and temperature dependence of the sound attenuation in the normal Ferm i liquid was extracted from the cavity response. The attenuation of sound d isplays behavior very different from the bulk owing to strong elastic scatt ering of quasi-particles by the silica strands. Using a visco-elastic model of the Fermi liquid, we find a mean free path restricted to similar to 340 nm. Information on the sound velocity is inferred from the pressure depend ence of the oscillation period of the cavity response. The data can be acco unted for by a Biot model of the He-3 liquid in the porous aerogel.