Quantum Monte Carlo studies of superfluid Fermi gases

We report results of quantum Monte Carlo calculations of the ground state of dilute Fermi gases with attractive short-range two-body interactions. The strength of the interaction is varied to study different pairing regimes which are characterized by the product of the s-wave scattering length and the Fermi wave vector, ak(F), We report results for the ground-state energy, the pairing gap Delta, and the quasiparticle spectrum. In the weak-coupling regime, 1/ak(F)<-1, we obtain Bardeen-Cooper-Schrieffer (BCS) superfluid and the energy gap Delta is much smaller than the Fermi gas energy E-FG. When a> 0, the interaction is strong enough to form bound molecules with energy E-mol. For 1/ak(F)greater than or similar to0.5, we find that weakly interacting composite bosons are formed in the superfluid gas with Delta and gas energy per particle approaching E-mol//2. In this region, we seem to have Bose-Einstein condensation (BEC) of molecules. The behavior of the energy and the gap in the BCS-to-BEC transition region, -0.5<1/ak(F)<0.5, is discussed.