We have loaded an ultracold gas of fermionic atoms into a far-off resonance optical dipole trap and precisely controlled the spin composition of the trapped gas. We have measured a magnetic-field Feshbach resonance between atoms in the two lowest energy spin states, $|9/2,-9/2〉$ and $|9/2,-7/2〉$. The resonance peaks at a magnetic field of $201.5\pm 1.4\mathrm{G}$ and has a width of $8.0\pm 1.1\mathrm{G}$. Using this resonance, we have changed the elastic collision cross section in the gas by nearly 3 orders of magnitude.

• Received 29 November 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.173201