Sound propagation in superfluid 3He in aerogel

Sound propagation in superfluid 3He confined in aerogel is studied on the basis of the Keldysh Green's function theory, taking into account the dragged aerogel motion due to collisions with 3He-quasiparticles. The collision drag effect is important when the sound frequency ω is smaller than the quasiparticle-aerogel collision rate 1/τ, i.e. when the system is in the hydrodynamic regime ωτ<1. The condition ωτ<1 is fully satisfied in experiments so far reported on the acoustics of superfluid 3He in aerogel. We calculate key quantities for describing hydrodynamic sound propagation in aerogel, including the frictional relaxation time (which characterizes the collision drag effect) and the shear viscosity coefficient. The numerical results are in reasonable agreement with the acoustic experiment by the Northwestern University group.