Determination of the fermion pair size in a resonantly interacting superfluid

Fermionic superfluids: strong stuff Fermionic superfluidity requires the formation of particle pairs, the size of which varies depending on the system. Many properties of the superfluid depend on the pair size relative to the inter-particle spacing. For example, conventional superconductors comprise a superfluid of loosely bound, large Cooper pairs of electrons, while Bose-Einstein condensates contain tightly bound molecules. The microscopic properties of the fermion pairs can be probed with radio-frequency spectroscopy. However, previous results have been difficult to interpret due to strong final-state interactions that were not well understood. Schunck et al. realize a superfluid spin mixture in an ultracold gas of lithium atoms in which such interactions have negligible influence. They find that the spectroscopic pair size is smaller than the inter-particle spacing. These are the smallest pairs yet observed for fermionic superfluids. In a related experiment, Jin et al use a technique called photoemission spectroscopy to study the excitations in a strongly interacting gas of ultracold potassium atoms. Such studies are of interest because the physics is related to that of the high transition-temperature superconductors, which are not fully understood.