Probing the many-body physics via measurement of the closed-channel fraction in a 6 Li superfluid

Atomic Fermi gases provide an ideal platform for studying the pairing and superfluid physics, using a Feshbach resonance between closed channel molecular states and open channel scattering states. The closed-channel fraction $Z$ contains important many-body interacting effects. Here we probe the many-body physics of interacting Fermi gases of $^6$Li by measuring $Z$ in the entire system. We have observed a significant departure from two-body physics. Away from the deep BEC regime, the fraction $Z$ is sensitive to the Fermi temperature $T_F$. In particular, our data show $Z \propto \sqrt{T_F}$ at unitarity, in good agreement with theory. Further more, we have found a progressive departure of our measured $Z$ value from theory as the system enters the BCS regime, calling for further investigation in both theory and experiment.