Pairing effects in ultra-cold fermionic lithium

In fermionic systems, superfluidity arises through particle pairing: a pair of fermions is indeed a boson that can enter a superfluid state at sufficiently low temperature. Depending on the strength of the interactions, two simple limiting cases can he considered. In the strongly interacting regime, particles form tightly bound molecules. The many-body ground state is therefore very well described by a molecular Bose-Einstein condensate (BEC). By contrast, in the weakly interacting regime particles form delocalized Cooper pairs and the well known BCS (Bardeen-Cooper-Schrieffer) theory is applicable. One of the- major challenges of contemporary theoretical physics is to understand the transition between these two regimes, the so-called BEC-BCS crossover. Scattering properties can be strongly modifiai when a bound state becomes resonant with the energy of the incoming atoms. In Feshbach resonances, this condition can be achieved by applying a variable magnetic field and offers the unique possibility of controlling very precisely both the magnitude and the sign of atomic interactions. This phenomenon led to the recent observations of superfluidity in ferrnionic ultra-cold gases and otters a unique opportunity of studying experimentally the BEC-BCS crossover. We present cxperimcuts on ultra-cold fermionic ⁶Li confined in a crossed dipole trap. ⁶Li possesses Feshbach resonances both in the s-wave and p-wave channels. This features enables the exploration of both s-wave and p-wave pairing mechanisms. We first discuss our observation of Bose-Einstein condensation of ⁶Li₂ molecules in the low-magnetic field wing the s-wave Feshbach located at ∼ 834 G [1]. The molecular BEC offers a good starting point for the exploration of the BEC-BCS crossover. We have also located p-wave Feshbach resonances in the two lowest hyperfine states of ⁶Li. The experimental positions of these resonances are in good agreement with those predicted by coupled channel calculations. We have studied the atom losses close to these resonances and have developed a semi-analytical model capturing our experimental findings [2].