Pairs and loners in a polarized fermi gas

In recent years, low-temperature fermion gases have demonstrated some remarkable properties. For example, under certain circumstances a trapped sample can be magnetically tuned through a so-called Feshbach resonance (see Physics Today, August 2004, page 12). On one side of the resonance, the spin-up and spin-down fermions pair strongly into molecules and form a superfluid Bose–Einstein condensate (BEC); on the other side, they pair weakly into a superconducting-type superfluid. Two independent groups—one at Rice University, the other at MIT—have now explored what happens when the spin-up and spin-down populations of lithium-6 atoms are unequal. Both groups found signatures in which leftover fermions are expelled into a halo surrounding the superfluid core. The MIT group found that when the polarization exceeded a critical value, the superfluid state collapsed under the pressure of the unpaired fermions. The Rice group observed that the phase separation occurred only above a critical polarization; for...