Excitations in thin 3He-4He superfluid films

We explore the behavior of He-3 single-particle states in thin He-4 superfluid films by analyzing both third sound and magnetization data. The third-sound speed is sensitive to the transverse, quantum-mechanical extent of motion of the adsorbed He-3. We are thus able to extract from experiment the size of the He-3 surface ground state. As this motion is state-dependent, we can interpret a high coverage feature as the signal that the first excited state has begun to be populated. For a 13.2 A He-4 film on Nuclepore, we find that the surface state is approximate to 3.9 Angstrom thick and that the onset coverage for first excited-state occupation is similar or equal to 0.6 (i.e., similar or equal to 0.039 Angstrom(-2)). This result combined with an analysis of magnetization step data show that the onset coverage decreases with increasing film thickness in the thin-film limit. [S0163-1829(99)01105-4].