Observation of the density dependence of the closed-channel fraction of a ${}^6$Li superfluid

Abstract

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. Of particular interest is the strongly interacting regime. We show that the closed-channel fraction $Z_{cc}$ provides an effective probe for the important many-body interacting effects, especially through its density dependence, which is absent from two-body theoretical predictions. Here we measure $Z_{cc}$ as a function of interaction strength and the Fermi temperature $T_{\text{F}}$ in a trapped ${}^6$Li superfluid throughout the entire BCS--BEC crossover, in quantitative agreement with theory when important thermal contributions outside the superfluid core are taken into account. Away from the deep BEC regime, the fraction $Z_{cc}$ is sensitive to $T_{\text{F}}$. In particular, our data show $Z_{cc}\propto T_{\text{F}}^{\alpha }$ with $\alpha =1/2$ at unitarity, in quantitative agreement with calculations of a two-channel pairing fluctuation theory, and $\alpha$ increases rapidly into the BCS regime, reflecting many-body interaction effects as predicted.

Updated with the final version. This submission contains supplementary information as an ancillary file