Role of single-particle and pair condensates in Bose systems with arbitrary intensity of interaction

TL;DR

This paper investigates a superfluid Bose system considering both single-particle and pair condensates without assuming weak interactions, revealing how condensate fractions depend on density and aligning with experimental data.

Contribution

It introduces a half-phenomenological theory for Bose liquids that accounts for arbitrary interaction strength and analyzes condensate behavior at various densities.

Findings

Single-particle condensate fraction is less than 10% at liquid helium density.

Pair condensate fraction increases with density and can become dominant.

Theoretical predictions agree with experimental and numerical data.

Abstract

We study a superfluid Bose system with single-particle and pair condensates on the basis of a half-phenomenological theory of a Bose liquid not involving the weakness of interparticle interaction. The coupled equations describing the equilibrium state of such system are derived from the variational principle for entropy. These equations are analyzed at zero temperature both analytically and numerically. It is shown that the fraction of particles in the single-particle and pair condensates essentially depends on the total density of the system. At densities attainable in condensates of alkali-metal atoms, almost all particles are in the single-particle condensate. The pair condensate fraction grows with an increasing total density and becomes dominant. It is shown that at density of liquid helium, the single-particle condensate fraction is less than 10%, which agrees with experimental data on inelastic neutron scattering, Monte Carlo calculations and other theoretical predictions. The ground state energy, pressure, and compressibility are found for the system under consideration. The spectrum of single-particle excitations is also analyzed.