Elementary excitations in normal liquid 3He

Abstract

The density and spin-density excitations in normal liquid 3He observed in recent neutron scattering experiments, are described here using a simple RPA model employing the Landau quasiparticle–quasihole interaction. The purpose is to test how well a simple extension of the Landau theory, known to describe excitations of vanishing wave vector Q and frequency ω, can describe the full dynamic form factor, S(Q,ω), at all Q and ω. The extension is made by replacing the Landau dynamic susceptibility, χ(Q,ω), by the RPA expression. For Q ≥ 10 nm−1 the spin-symmetric Landau interaction is also reduced to allow for multi-quasiparticle–quasihole contributions to the density excitations.At low temperature, T ≤ 50 mK, the model predicts a well defined 'zero-sound' mode in the density excitations and a 'paramagnon' peak at low frequency in the spin-density excitations. The good agreement of the calculated S(Q,ω) with that observed by Sköld, Pelizzari, Kleb, and Ostrowski at T = 15 mK for Q ≤ 15 nm−1 suggests that the extension works surprisingly well. The temperature dependence of S(Q,ω) is also investigated by allowing χ0(Q,ω) to vary with T and by including the temperature dependence of the Landau interaction, indicated by thermodynamic properties. This leads to a loss of structure in S(Q,ω) at higher temperature in agreement with the observations of Stirling, Scherm, Hilton, and Cowley at T = 0.63 K.