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

We analyze the experimental data [1] on inelastic neutron scattering by a thin 5-atomic-layer film of liquid helium at three different temperatures: T = 0.4, 0.98, and 1.3 K. 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 ~4.5 K 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. 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. We also analyze previous numerical results on excitations in liquid helium and argue that surface excitations similar to surfons have been previously obtained by numerical calculations and called resonance interface states [21].