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

Abstract

The experimental results of creating bulk elementary excitations (BEEs) in isotopically pure liquid helium-II by helium atomic beams at low temperatures ∼ 60 mK are presented. In the present experiment, BEE signals generated by ⁴He-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 ∼17 K (the Cooper pairing phenomenon doubles the supra-thermal phonon energy E _k ∼ 2 × 17 K∼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 ∼ 35 K, which perturb the liquid surface at incidence points similar to heaters.

Acknowledgements

It is a pleasure to acknowledge support for this work from Engineering and Physical Sciences Research Council (EPSRC) with which it was possible for the author during 2000–2001 academic year to study low temperature Physics at the Exeter University, England, UK. Also, it is a pleasure to thank Dr S. Roshko and Dr M. A. H. Tucker for their help in preparing the experiment.

(This paper is written in memory of Academician P. L. Kapitza)