Homogeneous Fermion Superfluid with Unequal Spin Populations

TL;DR

This paper demonstrates that quasiparticle interactions enable a homogeneous spin-polarized superfluid state in s-wave fermion systems, challenging the traditional view that such superfluids cannot support uniform spin polarization.

Contribution

It introduces the role of quasiparticle interactions in stabilizing a uniform spin-polarized superfluid, a novel insight contrasting with conventional theories.

Findings

Quasiparticle interactions induce triplet fluctuations and enable spin polarization.

The spin polarization at unitarity scales as the square root of the chemical potential.

Supports recent experimental evidence of homogeneous spin-polarized superfluidity.

Abstract

For decades, the conventional view is that an s-wave BCS superfluid can not support uniform spin polarization due to a gap $\Delta$ 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 $V_{qp}$, the spin polarization of at unitarity has the simple form $m\propto \mu^{1/2}$. Our study is motivated by the recent experiments at Rice which found evidence of a homogenous superfluid state with uniform spin polarization.