The effect of the Bose broken symmetry on the dynamics of superfluid ⁴He: a review

A status report is given on recent theoretical work concerning the role the Bose condensate has on the dynamical properties of superfluid 4 He. A Bose broken symmetry is shown to be crucial in understanding why zero-sound-type density fluctuations play the role of quasi particles in the superfluid phase. A new unified treatment of long-range order in bulk Bose liquids and quasi-long-range order in Bose films is given. In addition, the strong evidence we now have concerning the existence and magnitude of the Bose condensate is critically reviewed. This includes (i) determination of the momentum distribution from inelastic neutron scattering and (ii) recent finite-temperature Monte Carlo calculations of the condensate fraction n 0 (T)and the normal-fluid density ρ N (T).

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