Section E. Phase Transitions and Diffusion Processes in Condensed Matter and Gases NONPERTURBATIVE APPROACH TO SUPERFLUID BOSE SYSTEMS

In the present paper, we study a superfluid Bose system with single-particle and pair condensates without any restriction on intensity of interparticle interaction. To this end, we employ the formalism [1, 2] developed by us earlier to describe a Bose superfluid. From the principle of maximum entropy, we present a brief derivation of the coupled equations describing the system at finite temperature. Their numerical analysis at zero temperature shows that for dilute systems (gases of alkali-metal atoms), the pair condensate fraction is negligibly small in comparison with single-particle condensate fraction. Such systems, as expected, are well described by the Gross-Pitaevskii equation [3, 4]. The role of pair condensate becomes determinative for dense systems. In particular, it is found that the single-particle condensate fraction in a superfluid He is less than 10%. This result is in a good agreement with experiments [5–7] on inelastic neutron scattering in superfluid He.