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

Xiang-Pei Liu; Xing-Can Yao; Hao-Ze Chen; Xiao-Qiong Wang; Yu-Xuan Wang; Yu-Ao Chen; Qijin Chen; Kathryn Levin; Jian-Wei Pan

doi:10.1093/nsr/nwab226

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

Natl Sci Rev. 2021 Dec 23;9(10):nwab226. doi: 10.1093/nsr/nwab226. eCollection 2022 Oct.

Authors

Xiang-Pei Liu^{1

2

3}, Xing-Can Yao^{1

2

3}, Hao-Ze Chen^{1

2

3}, Xiao-Qiong Wang^{1

2

3}, Yu-Xuan Wang^{1

2

3}, Yu-Ao Chen^{1

2

3}, Qijin Chen^{1

2

3}, Kathryn Levin⁴, Jian-Wei Pan^{1

2

3}

Affiliations

¹ Hefei National Laboratory for Physical Sciences at the Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China.
² Shanghai Branch, CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Shanghai 201315, China.
³ Shanghai Research Center for Quantum Sciences, Shanghai 201315, China.
⁴ James Franck Institute, University of Chicago, Chicago, IL 60637, USA.

Abstract

Atomic Fermi gases provide an ideal platform for studying 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 [Formula: see text] provides an effective probe for important many-body interacting effects, especially through its density dependence, which is absent from two-body theoretical predictions. Here we measure [Formula: see text] as a function of interaction strength and the Fermi temperature [Formula: see text] in a trapped ⁶Li superfluid throughout the entire Bardeen-Cooper-Schrieffer-Bose-Einstein-condensate 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 [Formula: see text] is sensitive to [Formula: see text]. In particular, our data show [Formula: see text] with [Formula: see text] at unitarity, in quantitative agreement with calculations of a two-channel pairing fluctuation theory, and [Formula: see text] increases rapidly into the BCS regime, reflecting many-body interaction effects as predicted.

Keywords: closed-channel fraction; density dependence; many-body effect; strongly interacting Fermi gas.