Motion of ³He quasiparticles in aerogel driven by fourth sound

While the shear viscous motion of the normal fluid component causes the energy loss of the fourth sound resonance in superfluid ³He confined in narrow pores, it is not yet clear what causes the energy loss of the fourth sound in the aerogel system. We prepared two aerogel samples of 99.0% and 97.5% porosities, which were directly grown in sintered silver sponges and performed the fourth sound resonance experiments. We found that the energy loss of the fourth sound resonance in aerogel is much smaller than that in pure liquid ³He, showing that the mean free path of ³He quasiparticles is strongly suppressed by aerogel strands. Such behavior is explained with the frictional flow. The temperature dependence of the frictional relaxation time will be discussed.