Observation of the density dependence of the closed-channel fraction of a 6Li superfluid

Abstract

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 ${Z_{{\rm{cc}}}}$ 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 ${Z_{{\rm{cc}}}}$ as a function of interaction strength and the Fermi temperature ${T_{\rm{F}}}$ in a trapped 6Li 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 ${{\rm{T}}_F}$. In particular, our data show ${Z_{{\rm{cc}}}} \propto T_{\rm{F}}^\alpha $ with $\alpha \ = {\rm{\ 1/2}}$ 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.