Recent experiments on the solid-liquid 4He interface: Onsager coefficient and roughening transitions

This article presents a general introduction to the history of the study of rough and smooth parts on the interface between superfluid and crystalline Helium. Two experiments performed recently, the measurement of the Onsager coefficient for rough surfaces and the discovery of a new roughening transition at 0.365 K which is related to the present controversy on the shape of He crystals at T = 0, are discussed. 24 references, 5 figures, 1 table