Surface state atoms and their contribution to the surface tension of quantum liquids

TL;DR

This paper introduces surfons, a new type of surface excitation in liquid helium, and demonstrates their significant contribution to surface tension, resolving long-standing discrepancies between theory and experiment.

Contribution

The study models surfons as a new surface excitation, estimating their properties and showing their crucial role in explaining surface tension behavior in helium.

Findings

Surfon activation energy and effective mass are estimated.

Surfon contribution explains the temperature dependence of surface tension.

High agreement between theoretical predictions and experimental data.

Abstract

We investigate the new type of excitations on the surface of liquid helium. These excitations, called surfons, appear because helium atoms have discrete energy level at the liquid surface, being attracted to the surface by the van der Waals force and repulsed at a hard-core interatomic distance. The concentration of the surfons increases with temperature. The surfons propagate along the surface and form a two-dimensional gas. Basing on the simple model of the surfon microscopic structure, we estimate the surfon activation energy and effective mass for both helium isotopes. We also calculate the contribution of the surfons to the temperature dependence of the surface tension. This contribution explains the great and long-standing discrepancy between theory and experiment on this temperature dependence in both helium isotopes. The achieved agreement between our theory and experiment is extremely high. The comparison with experiment allows to extract the surfon activation energy and effective mass. The values of these surfon microscopic parameters are in a reasonable agreement with the calculated from the proposed simple model of surfon structure.