Pairing of a trapped Fermi gas with unequal spin populations

We investigate pairing in a two-component degenerate gas of trapped fermionic 6Li atoms at a broad Feshbach resonance by in-situ imaging of real-space density distributions. From even mixtures of the two spin components, we measured the β factor, describing the universal energy of strongly interacting paired fermions. In uneven spin mixtures, pairing and corresponding phases show a temperature dependence that is consistent with a phase diagram having a tricritical point. At the lowest temperatures, an unpolarized core separates from the excess unpaired atoms by a sharp boundary, which is consistent with a phase separation driven by a first-order phase transition. Moreover, the unpolarized core deforms with increasing polarization, in violation of local density approximation (LDA). In contrast, at higher but still degenerate temperatures, an unpolarized central core remains up to a critical polarization, but does not deform. In this case, the boundaries are not sharp, indicating a partially-polarized shell between the core and the unpaired atoms, consistent with a second-order phase boundary.