Heat diffusion in normal4He and3He thin films

We report measurements of heat diffusion at the surface of a glass substrate coated by normal⁴He and³He thin films. As the normal Helium film is grown, we observe for both isotopes that the diffusion of heat is strongly altered. Clearly a new diffusive channel is associated to the presence of the normal film itself, and for an³He film thickness of 7a.l. two distinct characteristic diffusion times are measured. The shorter time is characteristic of the diffusion of heat in the bare substrate while the longer one is associated to the diffusive process taking place in the normal adsorbed film. Considering the two diffusive channels as independent, we are able to obtain the diffusive contribution of the normal He film. The analysis of these data leads us to propose the picture of phonons traveling in the film with a wavevector parallel to the substrate plane. The existence of this independent diffusive mode, with a characteristic time larger than that of the substrate, is supported by the very slow decrease at long times of the measured temperature deviation for the coated substrate.