Heavy solitons in a fermionic superfluid

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Abstract

Solitons—solitary waves that maintain their shape as they propagate—occur as water waves in narrow canals, as light pulses in optical fibres and as quantum mechanical matter waves in superfluids and superconductors. Their highly nonlinear and localized nature makes them very sensitive probes of the medium in which they propagate. Here we create long-lived solitons in a strongly interacting superfluid of fermionic atoms and directly observe their motion. As the interactions are tuned from the regime of Bose–Einstein condensation of tightly bound molecules towards the Bardeen–Cooper–Schrieffer limit of long-range Cooper pairs, the solitons’ effective mass increases markedly, to more than 200 times their bare mass, signalling strong quantum fluctuations. This mass enhancement is more than 50 times larger than the theoretically predicted value. Our work provides a benchmark for theories of non-equilibrium dynamics of strongly interacting fermions.

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Figure 1: Creation and observation of solitons in a fermionic superfluid.
Figure 2: Soliton oscillations in the BEC–BCS crossover.
Figure 3: Soliton period and effective mass versus interaction strength in the BEC–BCS crossover.
Figure 4: Soliton motion in the unitary Fermi gas at various temperatures.
Figure 5: Effect of finite temperature on soliton motion.

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Acknowledgements

We thank L. Pitaevskii, S. Stringari, F. Dalfovo, W. Zwerger and D. Huse for discussions. This work was supported by the NSF, the ARO MURI on Atomtronics, AFOSR PECASE, ONR, a grant from the Army Research Office with funding from the DARPA OLE programme, and the David and Lucile Packard Foundation.

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T.Y. and A.T.S. contributed equally to the experimental work. All authors contributed extensively to the work presented in this paper.

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Correspondence to Martin W. Zwierlein.

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The authors declare no competing financial interests.

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Yefsah, T., Sommer, A., Ku, M. et al. Heavy solitons in a fermionic superfluid. Nature 499, 426–430 (2013). https://doi.org/10.1038/nature12338

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