Propagation of fifth sound in superfluid helium films

Wave propagation in films of superfluid 4He and superfluid 3He−4He mixtures is investigated under the conditions of partial damping of the normal component of the liquid and without regard for effects associated with the condensation and evaporation of helium atoms from the film. It is shown that two types of surface waves are possible, the first associated with van der Waals interaction between the film and the substrate, and the second with the propagation of temperature waves (fifth sound). The propagation velocities and absorption coefficients are calculated for the latter waves. It is determined that the wave associated with van der Waals interaction is highly attenuated, whereas fifth sound is only slightly attenuated.