Third sound: a study of the superfluid‐helium film

Liquid‐helium films of only a few atomic layers thickness exhibit a remarkable mobility as a result of the condensed superfluid state. The atoms are tightly bound to the substrate surface by the van der Waals attraction leaving almost complete freedom of motion in the lateral directions. Disturbances in the film thickness then propagate as waves called third sound. A wealth of information concerning the static and dynamic properties of the film have come from third‐sound group‐velocity measurements primarily through detecting small temperature oscillations accompanying the wave. We have learned that these waves can propagate in essentially monolayer films thus exhibiting a two‐dimensional superfluid. A very important quantum‐mechanical length describing the coherence of the condensate atoms is now known from static third‐sound velocity measurements. Experiments on the Doppler shift of flowing films have delineated the superflow phase region. Recently experiments have demonstrated the potential flow nature of the film and measured its ability to trap circulation. Subsequent measurements of the decays of these persistent currents are yielding new insights into the general stability of the superflow state.