Christian Schunck, Yong-il Shin, Andre Schirotzek, Wolfgang Ketterle (MIT)

Fermionic superfluidity with imbalanced spin populations has been an intriguing problem for many decades. Cooper pairing is the underlying mechanism for the conventional Bardeen-Cooper-Schrieffer (BCS) superfluid state of equal mixture of two fermionic components. However, population imbalance between the two components distresses the fully-paired state, eventually leading to depairing and the breakdown of superfluidity. This is known as the Chandrasekhar-Clogston (CC) limit. We have experimentally investigated the superfluid and normal phases of a population-imbalanced Fermi gas in a strongly interacting regime. We have observed that an imbalanced Fermi mixture maintains superfluidity up to a critical imbalance, showing the behavior of the CC limit. The superfluid region in an imbalanced mixture was found to have equal densities of the two components and be spatially separated from a normal region with unequal densities. Rf-spectroscopy was used to study the pairing gap energy in the strongly-interacting normal phase. Surprisingly, we have observed that minority components are fully paired even above the critical imbalance and the measured pairing gap was a large fraction of Fermi energy. This observation clearly demonstrated that the strongly-correlated normal phase has non-vanishing pairing gap energy without superfluidity.