MIT Physics News Spotlight
A new wave in the quantum world: Solitons in fermionic superfluids
July 18, 2013
Solitons - solitary waves that do not spread as they propagate – moving in a
strongly interacting fermionic superfluid of ultracold lithium-6 atoms created at MIT.
The pictures show four absorption images, about 0.3 mm in height, in a time
sequence with 0.1 seconds per frame. Image courtesy of Martin Zwierlein
A type of wave first observed in a Scottish canal has been used as a sensitive probe to investigate the physics of a strongly interacting, ultracold gas of lithium atoms. As reported online in Nature this week,
a team led by professor of physics Martin Zwierlein has created long-lived ‘solitons’, or solitary waves, in superfluid lithium-6, and reports a significant departure of their behavior from theoretical expectations.
Solitons — wave packets that maintain their shape as they propagate — occur in nonlinear systems ranging from shallow waterways, optical fibers to DNA. In a superfluid, solitons take the form of matter-waves where the particle density is locally reduced (a ‘dark’ soliton) or increased (a ‘bright’ soliton). Solitons have been created and studied in the weakly interacting ultracold quantum gases known as Bose–Einstein condensates. But until now, they have not been seen in a strongly interacting system of particles with half-integer spins (‘fermions’), which – just like electrons in a superconductor - have to form pairs to condense into the superfluid state.
Zwierlein’s team members were MIT graduate students Ariel Sommer, Mark Ku, Lawrence Cheuk, Wenjie Ji, and postdoctoral fellows Waseem Bakr and Tarik Yefsah. The team creates dark solitons in ultracold lithium-6, and then observes their motion as the interaction strength in the system was varied from weak (the Bose–Einstein condensate regime) to strong (towards the Bardeen–Cooper–Schrieffer regime, which describes superconductivity of electron pairs). As the interaction strength increases, the effective mass of the solitons increases by a factor of at least 200 — more than 50 times larger than the theoretically predicted value. The authors argue that this behavior can be explained by strong quantum fluctuations, which are not described by current theory. The new observations thus provide an important benchmark for evolving theories of the nonlinear dynamics of strongly interacting Fermi gases, relevant for our understanding of high-temperature superconductors and neutron stars.