Dynamics of phase-separated ³He-⁴He films

A Fröhlich-type Hamiltonian is derived for third sound and ³He quasi particles in phase-separated double layer of superfluid ⁴He and normal ³He liquid. It is stressed that our system is unique and valuable in that characteristic parameters can be varied in a wide range, simply by adjusting the film thickness. The effect of fermion-boson coupling on the velocity and damping of the third sound is examined. It is predicted that a rather drastic change in the third sound spectrum will occur when the Fermi velocity of ³He system and the third sound velocity are nearly the same. It is pointed out that the system under consideration may show a variety of interesting phenomena, in addition to the softening of the third sound.