Excitations of inhomogeneous liquid He4: A density functional study

Excitations of the free surface, films and bounded channels of liquid⁴He are investigated with a non-local density functional theory at zero temperature in the Feynman approximation. The nature of the various branches is discussed. At the free surface, for large momentum (k2 Å^–1), a hybridization occurs between ripplons and bulk rotons. For low momentum, the ripplon dispersion relation in thick films and the third sound mode in thin films are recovered, including the expected oscillations of the third sound velocity as a function of film thickness due to the layering of the film close to the substrate. Low energy 2-dimensional (2D) rotons confined at the liquid-wall interface are found on most substrates, except on the weakest binding surfaces such as Cs, for which we consider a channel geometry. In thin films, non trivial coverage dependence of the heat capacity may result from the interplay between the contribution of the low momentum part of the spectrum, characterized by the third sound velocity oscillations, and that of the large momentum part, dominated by the 2D rotons.Unité de Recherche des Universités Paris XI et Paris VI associée au CNRS.