Neutron Spin Echo Observation of Spontaneous Phonon Decay in Superfluid 4He
Due to the peculiar shape of the phonon-roton dispersion curve in 4He, the spontaneous decay of one excitation into a number of others is forbidden for wavenumbers above qc ≃ 0.55 Å−1. Therefore, q > qc excitations can only decay by scattering on another, thermal excitation, and consequently, in the ground state T→ 0 their lifetime becomes infinite, as put to evidence by ballistic photon/roton experimentsl. In contrast the q ≤ qc phonons must have a finite lifetime at T→ 0 and we shall present the first quantitative observation of this phenomenon.
KeywordsThermal Excitation Spontaneous Decay Neutron Spin Finite Lifetime Quantitative Observation
- 1.A. F. G. Wyatt, in this volume and references therein.Google Scholar
- 2.F. Mezei, The Principle of Neutron Spin Echo, in: “Neutron Spin Echo”, F. Mezei, ed., Springer Verlag Heidelberg (1980).Google Scholar
- 3.F. Mezei and W. G. Stirling, Neutron Scattering study of the lifetime of phonons in superfluid 4He, 75th Jubilee Conf. on 4He, J.G.M. Armitage, ed., World Scientific, Singapore (1983).Google Scholar