Density and quasiparticle excitations in liquid 4He.

Abstract

The recent interpretation of the phonon-roton excitations in superfluid {sup 4}He as a density mode at low wave vector and a quasiparticle excitation at higher {ital Q} proposed by Glyde and Griffin is developed. When there is a condensate, {ital n}{sub 0}({ital T}), the quasiparticle response of liquid {sup 4}He, becomes a component of the observed density dynamic structure factor, {ital S}({ital Q},{omega}). The quasiparticle and density response functions also share a common denominator, due to a coupling via the condensate. At low {ital Q}, the observed {ital S}({ital Q},{omega}) is confined predominantly to a single peak. The peak broadens with temperature but remains well defined in normal {sup 4}He, where {ital n}{sub 0}({ital T})=0. This peak is interpreted as a density mode. At the maxon and higher {ital Q}, {ital S}({ital Q},{omega}) has a sharp peak plus a broad component. The sharp peak is interpreted as the quasiparticle component of {ital S}({ital Q},{omega}). As {ital T} is increased and {ital n}{sub 0}({ital T}) is decreased, the intensity in the sharp peak is reduced until it vanishes from {ital S}({ital Q},{omega}) at {ital T}{sub {lambda}}. A simple model based on uncoupled quasiparticle and density excitations with coupling via {ital n}{submore » 0}({ital T}) is proposed. For simplicity, all model parameters are held independent of {ital T}. By allowing only {ital n}{sub 0}({ital T}) to vary with {ital T}, the temperature dependence of {ital S}({ital Q},{omega}) can be quite accurately reproduced.« less