Fermionic Superfluidity with Imbalanced Spin Populations, in January 27 issue of Science
2.18.2006

Whether it occurs in superconductors, helium-3 or inside a neutron
star, fermionic superfluidity requires pairing of fermions,
particles with half-integer spin. For an equal mixture of two
sorts of fermions ("spin up" and "spin down"), pairing can be
complete and the entire system will become superfluid. When the
two populations of fermions are unequal, not every particle can
find a partner. Will the system nevertheless stay superfluid?

This intriguing question was recently studied by a team at MIT led by Wolfgang Ketterle, in an unequal mixture of
strongly interacting ultracold fermionic atoms. As long as the number of unpaired atoms were not too large, they observed that the superfluid was stable - in a ballroom analogy, the superfluid "couples" did not mind the presence of some "loners" on the dancefloor. However, as the numbers of singles grew beyond a critical number, the superfluid state was observed to break down - the couples stopped dancing.
This final breakdown of superfluidity marks a quantum phase transition, known as the Pauli limit of superfluidity.

Ketterle's team consisted of graduate students Martin Zwierlein, Andre Schirotzek, and Christian Schunck, all of whom are members of the Center for Ultracold Atoms.

Reference:
Martin W. Zwierlein, Andre Schirotzek, Christian H. Schunck, and Wolfgang Ketterle
Fermionic Superfluidity with Imbalanced Spin Populations
Science 311, 492-496 (2006); published online on Science Express 22 December 2005