Interface between Two Thin ³He-⁴He Liquid Mixtures Films

Abstract

The reduction of one dimension size in liquids is of course a quite natural way to enhance the influencp of surface effects with respect to bulk contribution. The very first experiments on thin ⁴He films started in 1949. It was quickly understood that the study of such films, which thicknesses being as low as one or a few atomic layers, opened a new and fascinating research field: the two dimensional (2D) world and also the transition from this 2D to a three dimensional (3D) behaviour. Among the most spectacular results or discoveries on these films study, there is obviously the description of the superfluid transition in films as belonging to an universal class of transitions characteristic of the 2D world. This Kosterlitz Thouless transition describes the order disappearance by the thermal activation of a free vortex in the fluid¹. Another quite interesting concept can be analysed in its limits with the help of helium mixtures films. It is the notion and existence of interface when it should appear between very thin films. Obviously the spatial extension and the localisation of an interface is directly related to the density profile in fluid systems. The region where the profile density sharply changes from one constant value corresponding to the bulk value of the first phase to another constant value associated to the bulk value of the second phase is the interface region. The spatial extension of the interface can reach a few atomic layers. So what is the meaning of an interface, when the sizes of the two phases become also of the order of a few atomic layers ? Do we expect to observe a one-phase system with a rather continuous density profile or is it possible that the two-phases system will still exist and that the spatial extension of the interface cold b_y reduced to one or less than one atomic layer ? To answer this question we can use³He-⁴He mixtures films as a very appropriate experimental system.