Spin-polarized neutron matter: Critical unpairing and BCS-BEC precursor

TL;DR

This paper investigates how ultra-strong magnetic fields affect neutron pairing and superfluidity in neutron stars, revealing a critical field strength that destroys superfluidity and exploring the BCS-BEC crossover in spin-polarized neutron matter.

Contribution

It provides a systematic analysis of the critical magnetic field for neutron superfluidity destruction and studies the BCS-BEC precursor in spin-polarized neutron matter under extreme conditions.

Findings

Superfluidity is destroyed at magnetic fields B ≥ 10^{17} G.

Neutron matter exhibits a BCS-BEC precursor despite dineutron unbinding.

Phase diagram of spin-polarized neutron matter is constructed and analyzed.

Abstract

We obtain the critical magnetic field required for complete destruction of $S$-wave pairing in neutron matter, thereby setting limits on the pairing and superfluidity of neutrons in the crust and outer core of magnetars. We find that for fields $B \ge 10^{17}$ G the neutron fluid is non-superfluid -- if weaker spin-1 superfluidity does not intervene -- a result with profound consequences for the thermal, rotational, and oscillatory behavior of magnetars. Because the dineutron is not bound in vacuum, cold dilute neutron matter cannot exhibit a proper BCS-BEC crossover. Nevertheless, owing to the strongly resonant behavior of the $nn$ interaction at low densities, neutron matter shows a precursor of the BEC state, as manifested in Cooper-pair correlation lengths {being} comparable to the interparticle distance. We make a systematic quantitative study of this type of BCS-BEC crossover in the presence of neutron fluid spin-polarization induced by an ultra-strong magnetic field. We evaluate the Cooper pair wave-function, quasiparticle occupation numbers, and quasiparticle spectra for densities and temperatures spanning the BCS-BEC crossover region. The phase diagram of spin-polarized neutron matter is constructed and explored at different polarizations.