Measurement of the zero crossing in a Feshbach resonance of fermionic ⁶Li

We measure a zero crossing in the scattering length of a mixture of the two lowest hyperfine states of ⁶Li. To locate the zero crossing, we monitor the decrease in temperature and atom number arising from evaporation in a CO₂ laser trap as a function of magnetic field B. The temperature decrease and atom loss are minimized for B=52.8±0.4mT, consistent with no evaporation. We also present preliminary calculations using potentials that have been constrained by the measured zero crossing and locate a broad Feshbach resonance at ≈86mT, in agreement with previous theoretical predictions. In addition, our theoretical model predicts a second and much narrower Feshbach resonance near 55 mT.