Pair fraction in a finite temperature Fermi gas on the BEC side of the BCS-BEC crossover

TL;DR

This paper studies pairing phenomena in a strongly interacting Fermi gas on the BEC side, revealing coexistence of unbound fermions and pairs at finite temperature, and compares experimental data with theoretical models.

Contribution

It provides the first systematic experimental investigation of pairing in a finite temperature Fermi gas on the BEC side, testing classical atom-molecule equilibrium models against data.

Findings

Classical atom-molecule equilibrium model fits experimental data well

Pairing occurs above the superfluid critical temperature

Coexistence of unbound fermions and pairs observed

Abstract

We investigate pairing in a strongly interacting two-component Fermi gas with positive scattering length. In this regime, pairing occurs at temperatures above the superfluid critical temperature; unbound fermions and pairs coexist in thermal equilibrium. Measuring the total number of these fermion pairs in the gas we systematically investigate the phases in the sectors of pseudogap and preformed-pair. Our measurements quantitatively test predictions from two theoretical models. Interestingly, we find that already a model based on classical atom-molecule equilibrium describes our data quite well.