Abstract
Various infrared divergencies characterizing the static particle Green's function and the momentum distribution of Bose superfluids are investigated through a direct analysis of the thermal and quantum fluctuations of the phase of the order parameter. We explicitly investigate the role of two-phonon effects which are responsible for logarithmic (at T=0) and 1/q (at T\ensuremath{\ne}0) divergencies in the longitudinal static Green's function of a three-dimensional Bose superfluid. The temperature dependence of the condensate fraction of superfluid $^{4}\mathrm{He}$ is finally discussed with special emphasis to the contribution arising from the thermal excitation of rotons.
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