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. We create long-lived solitary waves in a strongly interacting superfluid of fermionic atoms and directly observe their motion [1]. 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 waves’ effective mass increases dramatically, to more than 200 times their bare mass. This mass enhancement is more than 50 times larger than the theoretically predicted value for planar solitons. I will present new experiments that reveal the microscopic nature of the observed solitary waves. Our work provides a benchmark for theories of non-equilibrium dynamics of strongly interacting fermions.

[1] Tarik Yefsah, Ariel T. Sommer, Mark J.H. Ku, Lawrence W. Cheuk, Wenjie Ji, Waseem S. Bakr, Martin W. Zwierlein, Heavy Solitons in a Fermionic Superfluid, Nature 499, 426-430 (2013)