We show that antiparallel triplet pairing correlations are generated in superfluids with purely $s$-wave interactions whenever population imbalance enforces anisotropic Fulde-Ferrell (FF) or inhomogeneous Larkin-Ovchinikov (LO) states. These triplet correlations appear in the Cooper pair wave function, while the triplet part of the gap remains zero. The same set of quasiparticle states contributes to the triplet component and to the polarization, thus spatially correlating them. In the LO case, this set forms a narrow band of Andreev states centered on the nodes of the $s$-wave order parameter. This picture naturally provides a unifying explanation of previous findings that attractive $p$-wave interaction stabilizes FFLO states. We also study a similar triplet mixing which occurs when a balanced two-component system displays FFLO-type oscillations due to a spin-dependent optical lattice. We discuss how this triplet component can be measured in systems of ultracold atoms using a rapid ramp across a $p$-wave Feshbach resonance. This should provide a smoking gun signature of FFLO states.

• Received 8 June 2011

DOI:https://doi.org/10.1103/PhysRevLett.109.155304