Since the seminal work of Bardeen-Cooper and Schrieffer (BCS) that unveiled the microscopic mechanisms of superconductivity, it has been known that in fermionic ensembles superfluidity could arise through Cooper pairing of particles with opposite spins. The issue of the robustness of the BCS order against spin imbalance was first addressedby Clogston and Chandrasekar who suggested that the superfluid would oppose any attempt at mismatching the Fermi surface of the two species.This prediction received its confirmation by recent experiments on lithium atoms performed nearly simultaneously in Rice and MIT where it was observed that when the spin populations of a strongly interacting trapped superfluid are imbalanced, the cloud forms a shell structure, with at its center a perfectly balanced superfluid phase. Despite these converging results on the inner shell, a controversy arose on the number of different phases observed: While Rice's group observed a fully polarized superfluid at center surrounded by an ideal gas of majority atoms at the rim, MIT obtained an additional intermediate phase where both spin species coexist with unequal densities.Although the controversy is not fully settled, strong theoretical evidences based mainly on Monte Carlo simulations confirm the existence of the intermediate normal phase. We will show how the most salient features of the experiments can be recovered from a study of a single fermionic impurity immersed in the Fermi Sea of majority atoms. We will show in particular that we recover without any adjustable parameters some of the results obtained at MIT, such as the density profile or the critical polarization above which the superfluid core vanishes.
10 Minute Talk: Search for the electron's electric dipole moment with cold ThO molecules