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Abstract

Phys. Rev. B Submittted (2004)

Pressure dependence of interacting phonons 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)

We have measured the pressure dependence of a single phonon sheet and the interaction of two such sheets, in liquid helium, from 0 to 21 bar and show it is dominated by three phonon processes (3pp). Pressure affects the 3pp scattering by changing the shape of the dispersion curve. The scattering varies from very strong at P = 0 to zero at P = 19 bar. The 3pp is small angle scattering at P = 0 and, as pressure is increased, the angles decrease further and eventually become zero. We find that the signal from a single phonon sheet increases considerably with pressure to a maximum at P = 7.5 bar and then decreases to a minimum at ~15 bar, and becomes independent of pressure at > 19 bar. We discuss this behaviour in terms of the effect of 3pp on the cone of occupied states in momentum space, the expansion of the phonon sheet and the creation of high energy phonons. The signal from the collision of two sheets, at 8.8° to each other, largely follows the pressure dependence of the energy density in the individual sheets but there is also direct evidence that the interaction between two phonon sheets is caused by 3pp scattering. In another paper, the pressure dependence of colliding sheets is considered theoretically.

Keywords: superfluidity, phonons.
 

                                                                                                                                                                                                                                                                       

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