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. in the quasiparticle excitation spectrum. We show that this is an artifact of the dismissal of quasiparticle interactions V-qp 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 < Delta.. 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 alpha mu(1/ 2). Our study is motivated by the recent experiments on Fermi gases with spin imbalance in the strongly interacting regime.