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Submitted to Phys. Rev. B (2004) .
Energy from Colliding Phonon Sheets in Liquid 4He
D.H.S. Smith, R.V. Vovk, C.D.H. Williams and A.F.G. Wyatt
School of Physics, University of Exeter, Exeter, EX4 4QL (UK).
(Submitted April 2004 )
When two phonon-sheets collide they can produce a hot line in liquid 4He. We have measured the dependence of the energy in such a hot line on the angle between the two sheets and their energy density. We find that it is linearly proportional to both at small angles and falls to zero at high angles. We have modelled the dynamic equilibrium of the hot line by equating the energy gain from the consumption of the sheets with the energy loss due to the creation of high-energy phonons. We find agreement with the experimental results using the theoretical values for the creation rate of high-energy phonons from low energy phonons. We show that the key factor is the occupied solid-angle in momentum space of the low-energy phonons, and conjecture that this solid angle is proportional to the initial energy density in a phonon system.
Keywords: liquid helium, phonons.