Third-sound study of a layered superfluid film

Isothermal scans of third-sound velocity as a function of He4 coverage on graphite exhibit pronounced dips integral layer periodicity. The minimum He4 coverage on graphite needed to support third sound is nearly three layers. The heat-capacity isotherm exhibits minima at layer completion but there is no noticeable feature related to the onset of superfluidity. High-resolution adsorption isotherms below 1 K show that He4 film grows layer by layer through at least seven layers.