Surface excitations in liquid helium nanofilms

The structural dynamic factor S(Q, omega) of liquid He-4 has been measured by inelastic neutron scattering in films of different thickness at temperatures from 0.6 to 0.05 K. The measurements were performed on an IN6 spectrometer (Institut Laue-Langevin, France). Analysis of the obtained experimental data on S(Q, omega) made it possible to establish the main parameters of surface excitations arising in a helium film and their dependences on the film temperature and thickness. The measurements showed that surface excitations arise in a helium film at a thickness of about two to three atomic layers, and the film thickness plays a decisive role in the formation of surface excitations.