The Superfluid State of a Bose Liquid as a Superposition of a Suppressed Bose-Eistein Condensate and an Intensive Pair Coherent Condensate

TL;DR

This paper presents a self-consistent model of superfluid Bose liquids incorporating both a weak Bose-Einstein condensate and a strong pair coherent condensate, explaining experimental excitation spectra in superfluid helium-4.

Contribution

It introduces a novel model combining BEC and PCC in superfluid Bose liquids, accounting for interaction renormalization and matching experimental spectra.

Findings

The model reproduces the experimental quasiparticle spectrum of superfluid helium-4.

It demonstrates the emergence of a pair coherent condensate due to effective attraction.

The ratio of BEC density to total density is a key small parameter in the model.

Abstract

A self-consistent model of the superfluid (SF) state of a Bose liquid with strong interaction between bosons is considered, in which at T=0, along with a weak single-particle Bose-Einstein condensate (BEC), there exists an intensive pair coherent condensate (PCC) of bosons, analogous to the Cooper pair condensate of fermions. Such a PCC emerges due to an effective attraction between bosons in some regions of momentum space, which results from an oscillating sign-changing momentum dependence of the Fourier component V(p) of the interaction potential. The collective many-body effects of renormalization ("screening") of the initial interaction, which are described by the bosonic polarization operator \Pi(p,\omega), lead to a suppression of the repulsion [V(p)>0] and an enhancement of the effective attraction [V(p)<0] in the respective domains of nonzero momentum transfer. The ratio of the BEC density n_0 to the total particle density n of the Bose liquid is used as a small parameter of the model. A closed system of nonlinear integral equations for the normal and anomalous self-energy parts is obtained with account for the terms of first order in the BEC density. In the framework of the ``soft spheres'' model with the single fitting parameter--the value of the repulsion potential at r=0, a theoretical quasiparticle spectrum E(p) is obtained, which is in good accordance with the experimental spectrum E_{exp}(p) of elementary excitations in superfluid $^4$He.