Phase separation in low-density neutron matter

TL;DR

This paper investigates phase separation phenomena in low-density neutron matter, revealing how polarization influences the coexistence of superfluid and normal phases, with implications for neutron star physics and ultracold atomic systems.

Contribution

It provides the first ab initio microscopic simulations of polarized low-density neutron matter, exploring phase coexistence and critical polarization dependence.

Findings

Normal phase dominates at low to intermediate polarization levels.

Phase coexistence occurs beyond a critical polarization.

Critical polarization varies with density.

Abstract

Low-density neutron matter has been studied extensively for many decades, with a view to better understanding the properties of neutron-star crusts and neutron-rich nuclei. Neutron matter is beyond experimental control, but in the past decade it has become possible to create systems of fermionic ultracold atomic gases in a regime close to low-density neutron matter. In both these contexts pairing is significant, making simple perturbative approaches impossible to apply and necessitating ab initio microscopic simulations. Atomic experiments have also probed polarized matter. In this work, we study population-imbalanced neutron matter (possibly relevant to magnetars and to density functionals of nuclei) arriving at the lowest-energy configuration to date. For small to intermediate relative fractions the system turns out to be fully normal, while beyond a critical polarization we find phase coexistence between a partially polarized normal neutron gas and a balanced superfluid gas. As in cold atoms, a homogeneous polarized superfluid is close to stability but not stable with respect to phase separation. We also study the dependence of the critical polarization on the density.