Phase diagrams of neutron-proton superfluid in asymmetric nuclear matter

TL;DR

This paper investigates the finite-temperature phase structures of neutron-proton superfluidity in asymmetric nuclear matter, revealing how angular dependence of the pairing gap influences phase stability and suppresses phase separation.

Contribution

It introduces the impact of angular dependence of the pairing gap on phase stability and identifies two stable orientations of the FFLO state in asymmetric nuclear matter.

Findings

Two stable orientations of the FFLO state identified

Angular dependence suppresses phase separation at low asymmetry

Cooper pair momentum suppresses phase separation at high asymmetry

Abstract

The finite-temperature phase structures for neutron-proton superfluidity in asymmetric nuclear matter are investigated, with a particular focus on the angular dependence of the pairing gap induced by the $^3SD_1$ $NN$ interaction. This angular dependence of the pairing gap results in the Cooper pair momentum in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state exhibiting exactly two distinct stable orientations: one orthogonal (FFLO-ADG-O) and the other parallel (FFLO-ADG-P) to the symmetry axis of the pairing gap. The FFLO-ADG-O state dominates at low asymmetries, while the FFLO-ADG-P state prevails at high asymmetries. Additionally, these analysis of normal-superfluid phase separation reveals that the angular dependence of the pairing gap eliminates phase separation in the low-asymmetry regime, whereas the Cooper pair momentum effectively suppresses phase separation at high asymmetries. These two mechanisms act in concert to significantly prevent the occurrence of normal-superfluid phase separation across the entire phase diagram, ensuring the stability of the homogeneous superfluid state over a broad range of asymmetries. These results provide new insights into the interplay between the angular dependence of the pairing gap and the stability of superfluidity in asymmetric nuclear matter.