Magnetic field control of elastic scattering in a cold gas of fermionic lithium atoms.

We study elastic collisions in an optically trapped spin mixture of fermionic lithium atoms in the presence of magnetic fields up to 1.5 kG by measuring evaporative loss. Our experiments confirm the expected magnetic tunability of the scattering length by showing the main features of elastic scattering according to recent calculations. We measure the zero crossing of the scattering length at 530(3) G which is associated with a predicted Feshbach resonance at approximately 850 G. Beyond the resonance we observe the expected large cross section in the triplet scattering regime.