Why only half of the fermionic atoms were converted to molecules by a Feshbach resonance?

TL;DR

The paper explains why only about half of fermionic atoms are converted into molecules near a Feshbach resonance, attributing this to atom-molecule coupling effects and suggesting the formation of a molecular condensate.

Contribution

It introduces a microscopic model that accounts for the 50% conversion efficiency and links this to the atom-molecule coupling term near the resonance.

Findings

Atom-molecule coupling explains the 50% efficiency.

Experiments likely produced a molecular condensate.

Model matches observed transfer efficiency.

Abstract

In some recent experiments an ultracold gas of $^{40}$K$_2$ (or $^{6}$Li$_2$) molecules has been produced from a degenerate two-component Fermi gas of $^{40}$K (or $^{6}$Li) atoms by adiabatic passage through a Feshbach resonance. The maximum atom-molecule transfer efficiency is reported to be about 50%. We propose a simple microscopic model to characterize the ground state of the gas in the vicinity of the resonance, and show that the term describing the atom-molecule coupling is responsible for the observed efficiency 50%. Our result also suggests that the experiments have produced a molecular condensate.