Tunable attractive interaction via Feshbach resonance Feshbach resonance involves a bound state near a continuum level Unitary limit BEC Contact potential intermediate nteraction is described BCS by scattering length 60 100 magnetic field (mT) ← ncreasing attractionTunable attractive interaction via Feshbach resonance Increasing attraction k -k Feshbach resonance involves a bound state near a continuum level. Contact potential. Interaction is described by scattering length. R Unitary limit