Superfluid density reduction and spin-imbalanced pairing in a fermionic superfluid due to dynamical boson exchange

TL;DR

This paper investigates how dynamical boson exchange affects fermionic superfluids, leading to superfluid density reduction, spin susceptibility at zero temperature, and implications for ultracold mixtures and neutron stars.

Contribution

It introduces the effects of dynamical boson exchange on superfluid properties, highlighting superfluid density reduction and spin susceptibility in fermionic superfluids.

Findings

Dynamical boson exchange causes superfluid density reduction at strong pairing.

It results in nonzero spin susceptibility at zero temperature.

The effects are significant in ultracold Fermi-Bose mixtures and neutron star matter.

Abstract

We explore novel features of a nonrelativistic fermionic superfluid in which the pairing interaction includes a contribution from the exchange of a dynamical bosonic mode. We show that the dynamical boson exchange (DBE), which causes a retarded pairing interaction and thus violates the Galilean invariance of the fermion sector, generically leads to a quantum reduction of the superfluid density and hence a nonzero normal fraction even at zero temperature. For spin-singlet pairing, the DBE also leads to a nonvanishing spin susceptibility at zero temperature, providing a mechanism for the coexistence of pairing and magnetization. While these effects are negligible for weak pairing, they become sizable at strong pairing. For the double superfluidity in ultracold Fermi-Bose mixtures, the superfluid density reduction for the fermion sector induced by the DBE just gives rise to the Andreev-Bashkin drag effect, indicating a strong entrainment between the two superfluid components. The DBE may also provide a new source for the superfluid fraction reduction of neutron matter, which is crucial for models of neutron star glitches based on neutron superfluidity.