Phase diagrams of BCS-BEC crossover in asymmetric nuclear matter

Abstract

The phase structure of the BCS-BEC crossover for neutron-proton superfluid in asymmetric nuclear matter is systematically investigated, with particular focus on the roles of the angle-dependent gap (ADG) and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states. Phase diagrams in the T-alpha, alpha-rho, and T-rho planes are constructed using both angle-averaged and angle-dependent gap treatments, enabling a unified analysis of the interplay between the FFLO pairing, ADG, and normal-superfluid phase separation (PS). The results confirm that the crossover is primarily density-driven. In the weak-coupling BCS regime, isospin asymmetry suppresses the stability of the homogeneous superfluid phase and drives the system toward PS, while the FFLO and ADG mechanisms partially alleviate this suppression. Although the ADG itself does not extend the asymmetry window for superfluidity, in combination with the FFLO state it enlarges the asymmetry range over which superfluidity survives and significantly reduces PS. At high density, these combined effects can nearly eliminate PS. However, as density decreases, ADG-induced suppression of PS is progressively weakened, due to both the reduced destructive effect of isospin asymmetry and the decreasing D-wave fraction in the 3SD1 channel. In general, the system evolves smoothly from a D-wave-dominated superfluid at high density to an S-wave superfluid at low density, with a corresponding weakening of ADG effects. Furthermore, the ADG lifts the orientational degeneracy of the FFLO state, resulting in two distinct FFLO-ADG phases separated by a first-order transition. In contrast, in the BEC regime, the FFLO and ADG states vanish, while the PS persists, leading to an inhomogeneous mixed phase at low temperatures and large asymmetries, where the superfluid component forms a BEC of deuterons.