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.