Fermionic condensation in ultracold atoms, nuclear matter and neutron stars

TL;DR

This paper explores fermionic pair condensation across ultracold atomic gases, neutron matter, and neutron stars, revealing how pairing behavior varies with interaction strength and density in these diverse superfluid systems.

Contribution

It provides a comparative analysis of fermionic condensation phenomena in three different superfluid systems, highlighting the BCS-BEC crossover and spatial variation of condensate fractions.

Findings

Fermionic pairing transitions from BCS to BEC regimes in ultracold gases.

Neutron matter exhibits a BCS-quasiunitary crossover with increasing density.

Maximum condensate fraction occurs in the crust of neutron stars.

Abstract

We investigate the Bose-Einstein condensation of fermionic pairs in three different superfluid systems: ultracold and dilute atomic gases, bulk neutron matter, and neutron stars. In the case of dilute gases made of fermionic atoms the average distance between atoms is much larger than the effective radius of the inter-atomic potential. Here the condensation of fermionic pairs is analyzed as a function of the s-wave scattering length, which can be tuned in experiments by using the technique of Feshbach resonances from a small and negative value (corresponding to the Bardeen-Cooper-Schrieffer (BCS) regime of Cooper Fermi pairs) to a small and positive value (corresponding to the regime of the Bose-Einstein condensate (BEC) of molecular dimers), crossing the unitarity regime where the scattering length diverges. In the case of bulk neutron matter the s-wave scattering length of neutron-neutron potential is negative but fixed, and the condensate fraction of neutron-neutron pairs is studied as a function of the total neutron density. Our results clearly show a BCS-quasiunitary-BCS crossover by increasing the neutron density. Finally, in the case of neutron stars, where again the neutron-neutron scattering length is negative and fixed, we determine the condensate fraction as a function of the distance from the center of the neutron star, finding that the maximum condensate fraction appears in the crust of the neutron star.