Models for third sound in thin 3He-4He films

Third sound speeds in He-3-He-4 thin films are sensitive to the transverse single-particle state occupied by the He-3. The third sound speed in the low coverage region with the He-3 occupying the transverse ground-state can be understood quantitatively. The onset of occupation of the first excited transverse state is signaled by a high coverage feature in the third sound speed. Three third sound models for the high coverage region, differing in their assumptions about the spatial distribution of rite excited states, are introduced. Using the experimental third sound data, these models can also be used to infer the fractional population of the He-3 in the first excited state as a function of coverage. It is found that the third sound analyses each predict a larger fractional population in the first excited stare at monolayer completion than a recent analysis of magnetization step data taken on the same system.