Stable, Strongly Attractive, Two-State Mixture of Lithium Fermions in an Optical Trap

We use an all-optical trap to confine a strongly attractive two-state mixture of lithium fermions. By measuring the rate of evaporation from the trap, we determine the effective elastic scattering cross section $4\pi a^2$ to show that the magnitude of the scattering length $|a|$ is very large, in agreement with predictions. We show that the mixture is stable against inelastic decay provided that a small bias magnetic field is applied. For this system, the $s$-wave interaction is widely tunable at low magnetic field, and can be turned on and off rapidly via a Raman $\pi$ pulse. Hence, this mixture is well suited for fundamental studies of an interacting Fermi gas.