Scientists study whirlpools in fermion superfluids.

The structure of vortices and vortex rings in Bose-Einstein condensates (BECs) has been a subject of intense interest during the past few years and has been studied experimentally at JILA and theoretically in Gaithersburg. Vortices in BECs are similar to the familiar whirlpool motion that forms around a bathtub drain when water flows out, yet they are capable of circulating indefinitely due to the superfluid properties of BECs.

Vortices exist also in superfluids consisting of fermion particles, such as electrons in a superconductor, liquid 3He, or neutron stars. Despite extensive investigations over the past 30 years, the energy and density profile of a vortex in a fermion superfluid had not yet been calculated from first principles. There is even controversy over the applicable characteristic length scale: whether it is the so-called Bardeen-Cooper-Schrieffer (BCS) coherence length, or a much smaller length that is inversely proportional to the Fermi energy of the system.

Scientists from NIST, in collaboration with colleagues at Niels Bohr Institute (Denmark), and the University of Calgary (Canada), have now performed the first such calculation for a superfluid fermion gas. Their results, published in Physical Review Letters (Vol. 90, p. 210402, 2003), establish that the characteristic size of the vortex core is indeed proportional to the BCS coherence length. Furthermore, they make the first quantitative prediction of the critical rotational frequency at which a vortex will appear in the ground state of a rotating fermion superfluid. Discovery of such a state would be a definitive signature of super-fluidity in a fermion gas, which has been long sought but not yet observed.

CONTACT: Nicolai Nygaard, [phone]; nicolai. [e-mail].