3He impurities on the bulk surface of liquid4He: Possible existence of excited states

We study, using a density-functional approach, the properties of the two-dimensional system formed by³He atoms on the surface of liquid⁴He, as a function of³He coverageN_s, at zero temperature. We find several types of surface states accessible to the³He atoms. For small values ofN_s, the surface tension is, as expected, linear inN_s². For a coverage of about half a monolayer, a new type of surface state starts being occupied, and this produces a change in the slope of as a function ofN_s² and, more clearly, a step in the surface specific heat, which increases by a factor of almost two. Another step is predicted to occur for a coverage of 1.3 monolayer. Existing data are compatible with this structure of surface states but are not numerous enough to prove or disprove the existence of steps.Unité de Recherche des Universités Paris 11 et Paris 6 Associée au CNRS.