Superfluidity in asymmetric nuclear matter

The onset of superfluidity in isospin-asymmetric nuclear matter is investigated within the BCS theory. A neutron-proton superfluid state in the channel ³S₁-³D₁ comes about from the interplay between thermal excitations and separation δμ of the two Fermi surfaces. The superfluid state disappears above the threshold value of the density-asymmetry parameter α=(nₙ-nₚ)/n0.35. For large enough shift between the two Fermi surfaces δμ=1/2(μₙ-μₚ) the transition to the normal state becomes a first-order transition and a second gap solution develops. This solution, however, corresponds to a metastable superfluid state which is unstable with respect to the transition to the normal state.