Measurement of the zero crossing in a Feshbach resonance of fermionic ${}^6\mathrm{Li}$

We measure a zero crossing in the scattering length of a mixture of the two lowest hyperfine states of ${}^6\mathrm{Li}.$ To locate the zero crossing, we monitor the decrease in temperature and atom number arising from evaporation in a ${\mathrm{CO}}_2$ laser trap as a function of magnetic field B. The temperature decrease and atom loss are minimized for $B=52.8\mathrm{}\pm 0.4\mathrm{mT},$ 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 $\approx 86\mathrm{mT},$ in agreement with previous theoretical predictions. In addition, our theoretical model predicts a second and much narrower Feshbach resonance near 55 mT.