Fermionic Superfluidity with Imbalanced Spin Populations

We have observed fermionic superfluidity with population imbalance and the Clogston limit of superfluidity in an ultracold Fermi mixture [Zwierlein et al,2006]. Importantly, this demonstrated that in the case of population imbalance, there is no longer a smooth crossover from a Bose-Einstein condensate of molecules to a BCS type superfluid. Instead, as the pairing strength is varied, phase transitions occur between the superfluid and the normal state.