Creation of bulk elementary excitations in superfluid helium-II by helium atomic beams at low temperatures

The experimental results of creating bulk elementary excitations ( BEEs) in isotopically pure liquid helium-II by helium atomic beams at low temperatures similar to 60 mK are presented. In the present experiment, BEE signals generated by He-4-atomic beams incident on the liquid free surface were detected by a bolometer positioned in the liquid helium-II. Some detected signals were very weak and depended on the heater power. Some examples of BEE detected signals are shown. Also, group velocities of the detected BEEs are evaluated and the threshold velocities of the helium atoms are discussed. The present experimental results demonstrate BEE creation, such as the third non-dispersive Zakharenko waves ( supra-thermal phonons), with energies similar to 17 K ( the Cooper pairing phenomenon doubles the supra-thermal phonon energy E-k similar to 2 x 17 K similar to 34 K in order to fulfil the energy conservation law) in the positive roton branch of the BEE energy spectra by helium atomic beams with suitable energies similar to 35 K, which perturb the liquid surface at incidence points similar to heaters.