Homogeneous Fermion Superfluid with Unequal Spin Populations

For decades, the conventional view is that an s-wave BCS superfluid can not support uniform spin polarization due to a gap Δ in the quasiparticle excitation spectrum. We show that this is an artifact of the dismissal of quasiparticle interactions Vqp in the conventional approach at the outset. Such interactions can cause triplet fluctuations in the ground state and hence non-zero spin polarization at “magnetic field” h<Δ. The resulting ground state is a pairing state of quasiparticles on the “BCS vacuum”. For sufficiently large Vqp, the spin polarization of at unitarity has the simple form m∝μ1/2. Our study is motivated by the recent experiments on Fermi gases with spin imbalance in the strongly interacting regime.