Fermionic Superfluidity with Imbalanced Spin Populations
Dr. Martin Zwierlein (Massachusetts Institute of Technology)

Fermionic superfluidity, whether it occurs in superconductors, in helium-3 or inside a neutron star, requires pairing of fermions. In an equal mixture of "spin up" and "spin down" fermions, pairing can be complete and the entire system will become superfluid. When the two populations of fermions are unequal, however, not every particle can find a partner. Can superfluidity persist in response to such a population imbalance? We studied this intriguing question in a two-state mixture of trapped, ultracold fermionic atoms. The observation of vortices established superfluidity for imbalanced spin populations. When the fraction of unpaired atoms increased beyond a critical value, we observed a quantum phase transition to the normal state, known as the Clogston limit of superfluidity.