**
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.
**

**
**