Bogolyubov quasiparticle analysis of the phonon-roton spectrum.

The phonon-roton spectrum of the superfluid helium-4 presents a foundational behavior of a quntum bose fluid. While this spectrum has been measured in the past with an ever increasing precision [1], no simple theoretical description or a physically intuitive picture has been developed since the foundational works of Feynman and Feynman and Cohen [2]. Here, we present an analysis of the phonon-roton excitation spectrum measured in superfluid helium-4 at several pressures in terms of an effective Bogolyubov quasiparticle Hamiltonian. The obtained effective mass of the Bogolyubov quasiparticle is in good agreement with the previous calculations [3]; the parameters of the interaction in the effective Hamiltonian are consistent with the physical picture of weakly interacting helium atoms "dressed" by the Bose condensate. The spectrum is strongly modified by the interaction with the two-particle continuum [4]; this effect is more pronounced at higher pressures. The analysis of the spectrum end point according to Ref. 3 indicates q_c = 3.3(1) Å^-1, roughly independent of pressure.