Attenuation of superfluid two-phase sound

Non-universal Casimir effect in saturated superfluid4He films at Tλ

“…For a film with d = 300Å near T λ this is all frequencies below ω/2π = 6 MHz. Any possible modes at higher frequencies in the film would necessarily be waveguide modes reflecting from the free surface, and because second sound involves temperature oscillations these would excite sound waves in the helium vapor, leading to very strong attenuation from the dissipative Onsager reciprocity relations for the evaporation-condensation process [20,21]. Such waveguide modes coupled to vapor in bulk helium have been observed to be strongly attenuated (Q values of order 5) at only a few hundred Hz [21], so we believe there is almost no probability they could propagate at MHz frequencies in the film.If there is no second sound in the film, then K 2 in Eq.…”