Quantum Bose and Fermi gases with large negative scattering length in the 2-body S-matrix approximation

TL;DR

This paper investigates Bose and Fermi gases with negative scattering lengths using a 2-body S-matrix approximation, calculating virial coefficients and critical temperatures for Bose-Einstein condensation across different interaction regimes.

Contribution

It introduces a method that sums infinite many-body processes reducible to 2-body scatterings for arbitrary negative scattering lengths, extending virial coefficient calculations to bosons and the upper branch.

Findings

Second virial coefficient is exact.

Extended virial coefficient results to bosons and upper branch.

Mapped critical temperatures for Bose-Einstein condensation.

Abstract

We study both Bose and Fermi gases at finite temperature and density in an approximation that sums an infinite number of many body processes that are reducible to 2-body scatterings. This is done for arbitrary negative scattering length, which interpolates between the ideal and unitary gas limits. In the unitary limit, we compute the first four virial coefficients within our approximation. The second virial coefficient is exact, and we extend the previously known result for fermions to bosons, and also for both bosons and fermions for the upper branch on the other side of unitarity (infinitely large positive scattering length). Assuming bosons can exist in a meta-stable state before undergoing mechanical collapse, we map out the critical temperatures for strongly coupled Bose-Einstein condensation as a function of scattering length.