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Density and quasiparticle excitations in liquid He 4

1992; American Physical Society; Volume: 45; Issue: 13 Linguagem: Inglês

10.1103/physrevb.45.7321

1095-3795

H. R. Glyde,

Cold Atom Physics and Bose-Einstein Condensates

The recent interpretation of the phonon-roton excitations in superfluid $^{4}\mathrm{He}$ 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, ${\mathit{n}}_{0}$(T), the quasiparticle response of liquid $^{4}\mathrm{He}$, becomes a component of the observed density dynamic structure factor, S(Q,\ensuremath{\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,\ensuremath{\omega}) is confined predominantly to a single peak. The peak broadens with temperature but remains well defined in normal $^{4}\mathrm{He}$, where ${\mathit{n}}_{0}$(T)=0. This peak is interpreted as a density mode. At the maxon and higher Q, S(Q,\ensuremath{\omega}) has a sharp peak plus a broad component. The sharp peak is interpreted as the quasiparticle component of S(Q,\ensuremath{\omega}). As T is increased and ${\mathit{n}}_{0}$(T) is decreased, the intensity in the sharp peak is reduced until it vanishes from S(Q,\ensuremath{\omega}) at ${\mathit{T}}_{\ensuremath{\lambda}}$. A simple model based on uncoupled quasiparticle and density excitations with coupling via ${\mathit{n}}_{0}$(T) is proposed. For simplicity, all model parameters are held independent of T. By allowing only ${\mathit{n}}_{0}$(T) to vary with T, the temperature dependence of S(Q,\ensuremath{\omega}) can be quite accurately reproduced.