Conversion efficiency of ultracold fermionic atoms to bosonic molecules via Feshbach resonance sweep experiments

We explain why the experimental efficiency observed in the conversion of ultracold Fermi gases of K-40 and Li-6 atoms into diatomic Bose gases is limited to 0.5 when the Feshbach resonance sweep rate is sufficiently slow to pass adiabatically through the Landau-Zener transition but faster than the collision rate in the gas, and increases beyond 0.5 when it is slower. The 0.5 efficiency limit is due to the preparation of a statistical mixture of two spin states, required to enable s-wave scattering. By constructing the many-body state of the system we show that this preparation yields a mixture of even and odd parity pair states, where only even parity can produce molecules. The odd parity spin-symmetric states must decorrelate before the constituent atoms can further Feshbach scatter, thereby increasing the conversion efficiency; the collision rate is the pair decorrelation rate.