School of Physics Abstract

Physica B (1999) Submitted.

The creation of supra-thermal densities of high energy phonons in HeII

A.F.G. Wyatt, M.A.H. Tucker,

School of Physics, University of Exeter, Exeter, EX4 4QL (UK).

I.N. Adamenko, K.É. Nemchenko and A.V. Zhukov

Department of Physics and Technology, Kharkov State University, Kharkov 310077, Ukraine.

(Submitted 28 June 1999)

We report the new phenomenon that high energy phonons can be created from low energy phonons. This arises because the dynamics of phonons in propagating pulses are quite different to those in isotropic phonon distributions. A pulse of low energy phonons rapidly thermalises by three phonon processes. On a much longer time scale four phonon processes occur within this phonon cloud which create high energy (10K) phonons that cannot spontaneously decay. These phonons have a lower velocity and so are lost from the back of phonon cloud; their deficit is restored continuously by four phonon processes. These now isolated high energy phonons are very stable and propagate ballistically behind the low energy phonons, so giving the two pulses which are detected in experiments. For long pulses the high energy phonons may also decay within the cloud, however the available low energy phonons for scattering are confined to a narrow-angle cone, so the decay probability is very low because the four phonon process requires larger angle scattering. A supra-thermal density of these high energy phonons is predicted.

Keywords: liquid helium, phonons.