Fermionic superfluidity with positive scattering length

TL;DR

This paper demonstrates that ultracold Fermi gases can exhibit superfluidity with positive scattering length through effective interactions near a narrow Feshbach resonance, challenging the traditional requirement of negative scattering length or bound states.

Contribution

It introduces a mechanism for superfluidity in Fermi gases with positive scattering length using narrow Feshbach resonances and energy-dependent effective interactions.

Findings

Superfluidity can occur with positive scattering length.

Effective interactions are attractive near the Fermi energy.

Superfluidity arises without two-body bound states.

Abstract

Superfluidity in an ultracold Fermi gas is usually associated with either a negative scattering length, or the presence of a two-body bound state. We show that none of these ingredients is necessary to achieve superfluidity. Using a narrow Feshbach resonance with strong repulsive background interactions, the effective interactions can be repulsive for small energies and attractive for energies around the Fermi energy, similar to the effective interactions between electrons in a metallic superconductor. This can result in BCS-type superfluidity while the scattering length is positive.