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Determination of the fermion pair size in a resonantly interacting superfluid

Abstract

Fermionic superfluidity requires the formation of particle pairs, the size of which varies from the femtometre scale in neutron stars and nuclei to the micrometre scale in conventional superconductors. Many properties of the superfluid depend on the pair size relative to the interparticle spacing. This is expressed in ‘BCS–BEC crossover’ theories1,2,3, describing the crossover from a Bardeen–Cooper–Schrieffer (BCS)-type superfluid of loosely bound, large Cooper pairs to Bose–Einstein condensates (BECs) of tightly bound molecules. Such a crossover superfluid has been realized in ultracold atomic gases where high-temperature superfluidity has been observed4,5. The microscopic properties of the fermion pairs can be probed using radio-frequency spectroscopy. However, previous work6,7,8 was difficult to interpret owing to strong final-state interactions that were not well understood. Here we realize a superfluid spin mixture in which such interactions have negligible influence and present fermion pair dissociation spectra that reveal the underlying pairing correlations. This allows us to determine that the spectroscopic pair size in the resonantly interacting gas is 20 per cent smaller than the interparticle spacing. These are the smallest pairs so far observed in fermionic superfluids, highlighting the importance of small fermion pairs for superfluidity at high critical temperatures9. We have also identified transitions from fermion pairs to bound molecular states and to many-body bound states in the case of strong final-state interactions.

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Figure 1: Line shape of the pair dissociation spectrum in the BEC and BCS limits and the evolution of the fermion pair size in the BCS–BEC crossover5,11,20.
Figure 2: Radio-frequency dissociation spectra in the BCS–BEC crossover.
Figure 3: Comparison of line shapes and density effects.
Figure 4: Effect of final-state interactions on radio-frequency spectroscopy: bound–bound and bound–free spectra in the BCS–BEC crossover of the (1, 3) mixture.

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Acknowledgements

We thank M. Zwierlein, W. Zwerger, E. Mueller and S. Basu for discussions and A. Keshet for the experiment control software. This work was supported by the NSF and ONR, through a MURI program, and under ARO Award W911NF-07-1-0493 with funds from the DARPA OLE programme.

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Correspondence to Christian H. Schunck.

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The file contains Supplementary Figures 1-3 and Legends; Supplementary Discussion and additional references. (PDF 192 kb)

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Schunck, C., Shin, Yi., Schirotzek, A. et al. Determination of the fermion pair size in a resonantly interacting superfluid. Nature 454, 739–743 (2008). https://doi.org/10.1038/nature07176

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