Third-sound wave in superfluid films as a probe of interface exchanges and film-entropy evaluation

We present simultaneous measurements of thickness and temperature oscillations in a third-sound wave propagating in various superfluid films. In pure 4He films the exchanges at liquid-gas interface drive the behavior. A good description is obtained for T<0.7 K. In dilute mixture films, the measurements also give, at T=0.6 K, a direct evaluation of the film entropy, which is essentially the entropy of a two-dimensional Fermi gas