DENSITY AND QUASI-PARTICLE EXCITATIONS IN LIQUID-HE-4

The recent interpretation of the phonon-roton excitations in superfluid He-4 as a density mode at low wave vector and a quasiparticle excitation at higher Q proposed by Glyde and Griffin is developed. When there is a condensate, n0(T), the quasiparticle response of liquid He-4, becomes a component of the observed density dynamic structure factor, S(Q, omega). The quasiparticle and density response functions also share a common denominator, due to a coupling via the condensate. At low Q, the observed S(Q, omega) is confined predominantly to a single peak. The peak broadens with temperature but remains well defined in normal He-4, where n0(T) = 0. This peak is interpreted as a density mode. At the maxon and higher Q, S(Q, omega) has a sharp peak plus a broad component. The sharp peak is interpreted as the quasiparticle component of S(Q, omega). As T is increased and n0(T) is decreased, the intensity in the sharp peak is reduced until it vanishes from S(Q, omega) at T(lambda). A simple model based on uncoupled quasiparticle and density excitations with coupling via n0(T) is proposed. For simplicity, all model parameters are held independent of T. By allowing only n0(T) to vary with T, the temperature dependence of S(Q, omega) can be quite accurately reproduced.