The role of pair correlations in the formation of the ground state and theelementary excitation spectrum in a superfluid Bose liquid (A review)

The paradoxes and disparities in the contemporary microscopic theory of sup erfluid helium (He-II) are discussed along with possible ways of resolving them by taking pair correlations of He-4 atoms into consideration. It is sh own that most paradoxes are associated with the commonly accepted initial a ssumption concerning the dominating role of single-particle Bose condensate (SPBC) in the quantum microstructure of the superfluid component rho(s). T he existence of intensive SPBC leads to a strong hybridization of the eleme ntary excitation branches and to a common dispersion law for all boson bran ches, which is identified with the quasiparticle spectrum E( p) observed ex perimentally from slow neutron scattering in liquid helium. However, the st ability of this spectrum during a transition through the lambda-point and t he large value of the gap in the vicinity of the "rotonic'' minimum contrad ict both the Landau theoretical criterion of superfluidity and the small va lue of experimentally measured critical velocity. At the same time, a stron g interaction between particles in the Bose liquid He-4 strongly suppresses the SPBC which amounts to less than 1% of all He-4 atoms and hence cannot be the main constituent of the superfluid component, unlike the case of a w eakly nonideal Bose gas. Moreover, for a quite strong attraction between pa rticles in a certain region of the momentum space, bound pairs of bosons ca n be formed in the superfluid Bose liquid, and a coherent pair condensate ( CPC) analogous to the Cooper pair condensate in superconductors may appear. Such a strong CPC may completely suppress the weak SPBC. In this case, the one-particle spectrum epsilon(p) of elementary excitations does not hybrid ize with the collective (two-particle) spectrum and does not appear in the structure of the dynamic form factor S( p, epsilon), i.e., does not coincid e with the spectrum measured from neutron scattering. The dispersion of one -particle spectrum is defined by the momentum dependence of the pair order parameter <(Psi)over tilde>(p) and may have a minimum or a point of inflect ion at p not equal 0. This peculiarity in the one-particle spectrum of a Bo se liquid with CPC but without SPBC vanishes together with <(Psi)over tilde >(p) at the temperature T-c=T-lambda of the phase transition from the super fluid to the normal state (unlike the rotonic minimum in the collective spe ctrum!, while the corresponding critical velocity v(c)=min[epsilon(p)/p] va nishes at the lambda-point in accordance with the Landau criterion and the experimental data. The assumption that the strong "Cooper-like'' CPC is res ponsible for the quantum structure of the superfluid component rs is confir med indirectly by the successful application of the Justrow approximation ( based on strong pair correlations) for describing the properties of liquid He-4 and quantum liquid mixtures He-3-He-4 on one hand, and by an anomalous ly large effective mass of He-3 impurity atoms in He-4, which is approximat ely equal to total mass of He-3 and He-4 atoms, thus pointing to the existe nce of helium atoms in superfluid liquid He-II. The value of the superfluid velocity circulation quantum in the Onsager-Feynman vortices in a Bose liq uid with CPC but without SPBC is discussed as well as the critical velociti es of superfluid He-4 in ultrathin films and channels in which the creation and motion of quantum vortices are ruled out, and the quasiparticle spectr um undergoes dimensional quantization. (C) 1999 American Institute of Physi cs. [S1063-777X(99)00102-4].