Superfluid Phase Transitions and Effects of Thermal Pairing Fluctuations in Asymmetric Nuclear Matter

We investigate superfluid phase transitions of asymmetric nuclear matter at finite temperature (T) and density (ρ) with a low proton fraction (Y_p ≤ 0.2), which is relevant to the inner crust and outer core of neutron stars. A strong-coupling theory developed for two-component atomic Fermi gases is generalized to the four-component case, and is applied to the system of spin-1/2 neutrons and protons. The phase shifts of neutron-neutron (nn), proton-proton (pp) and neutron-proton (np) interactions up to k = 2 fm⁻¹ are described by multi-rank separable potentials. We show that the critical temperature Tcnn of the neutron superfluidity at Y_p = 0 agrees well with Monte Carlo data at low densities and takes a maximum value Tcnn= 1.68 MeV at ρ/ρ=0.14 with ρ₀ = 0.17 fm⁻³. Also, the critical temperature Tcnn of the proton superconductivity for Y_p ≤ 0.2 is substantially suppressed at low densities due to np-pairing fluctuations, and starts to dominate over Tcnn only above ρ/ρ=0.70(0.77) for Y_p = 0.1(0.2), and (iii) the deuteron condensation temperature Tcd is suppressed at Y_p ≤ 0.2 due to a large mismatch of the two Fermi surfaces.