Excitations in thin He-3-He-4 superfluid films

We explore the behavior of He-3 single-particle states in thin He-4 superfl uid films by analyzing both third sound and magnetization data. The third-s ound speed is sensitive to the transverse, quantum-mechanical extent of mot ion of the adsorbed He-3. We are thus able to extract from experiment the s ize 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 f ind that the surface state is approximate to 3.9 Angstrom thick and that th e 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 wi th an analysis of magnetization step data show that the onset coverage decr eases with increasing film thickness in the thin-film limit. [S0163-1829(99 )01105-4].