Surface Levels and Their Contribution to the Surface Tension of Quantum Liquids

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.

Inelastic Neutron Scattering As an Indication of a New Type of Gapped Surface Excitations in Liquid He

“…These data were partially published previously,[2-4] but here we present them in a better quality and at various temperatures. The neutron scattering intensity plots, in addition to the previously know dispersion of phonons and ripplons, suggest a branch of gapped surface excitations with activation energy ∼ 4.5K and the dispersion similar to that expected for surfons -the bound quantum states of helium atoms above liquid helium surface, proposed and investigated theoretically [5,6]. These data, probably, provide the first direct experimental confirmation of surfons.…”

“…[2][3][4] Recently, an additional new type of surface excitations has been proposed semi-phenomenologically to explain too strong temperature dependence of surface tension coefficient α (T ). [5,6] These excitations, called surfons, can be considered as the quantum states of He atoms localized above liquid surface. Surfons resemble the Andreev states of 3 He atoms in the 3 He-4 He mixture [27], or the states of 3 He and 4 He atoms on the surface of liquid hydrogen.…”

“…These data, probably, provide the first direct experimental confirmation of surfons. Before these surface excitations received only indirect experimental substantiation, based on the temperature dependence of surface tension coefficient [5,6] and on their interaction with surface electrons [7,8]. The existence of surfons as an additional type of surface excitations, although being debated yet, is very important for various physical properties of He surface.…”

Neutrons on a surface of liquid helium

“…These excitations, called surfons, are He atoms in a quasistationary discrete quantum energy level above the liquid helium surface [48][49][50]. The state is formed by the combination of the van-der-Waals attractive potential of the bulk helium and the hard-core repulsion between He atoms.…”

“…The activation energy of the surfon has been obtained from fitting the temperature dependence of the surface tension coefficient of liquid 4 He to the experimental data [49]. Its value, ∆ s0 ≈ k B × 2.67 K = 3.7 × 10 −16 erg, is significantly smaller than the evaporation energy −µ = k B × 7.17 K of a 4 He atom.…”