Abstract

Submitted to Phys. Rev. B (2003) .

High-Energy Phonon Pulses in Liquid ⁴He

Ruslan V. Vovk, C.D.H. Williams and A.F.G. Wyatt

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

(Submitted 5 December 2003 )

We have discovered a method for selectively detecting high energy phonons which has enabled us to make detailed measurements of the shapes of high-energy phonon pulses in liquid ⁴He at ca 50mK. These phonons are created from pulses of low-energy phonons, injected into the helium by a heater. We suppress the detection of the low energy phonons by orienting a thin film superconducting Zn bolometer at an angle to the phonon beam. We find that the high-energy phonons are created continuously along the propagation path. As there is also velocity dispersion, the signal at any time is due to a range of phonon energies. Computer simulations of the signals from short pulses, reproduce the main features of the measured results. The behaviour of the high-energy phonon signals as functions of heater power and pulse length are analysed and this leads us to suggest how the low-energy phonon pulse develops. In another paper (Adamenko et al. Submitted to PRB), the pulse shape for short pulses is derived analytically.

Keywords: liquid helium, phonons.