Observation of a new thermal wave in a planar superfluid helium layer

The existence of a new thermal wave in He II has been experimentally confirmed in a channel formed between two plane-parallel glass plates. The thermal wave is the limiting case of second sound in narrow channels under the condition that the channel width is small compared to the penetration depth of a viscous wave. It is a strongly attenuated temperature or entropy wave, which is coupled to pressure oscillations, and it shows dispersion. The phase velocity of this wave mode was determined in dependence on the frequency, the channel width, and the temperature. At temperatures between 1.1 K and the $\lambda$ point, the results are in very good agreement with a thermohydrodynamic theory. Below 1.1 K the phase velocity is affected by mean-free-path effects of the elementary excitations, and deviations from this theory occur. They can be described by a simple kinetic theory.