Superfluidity enhanced by spin-flip tunnelling in the presence of a magnetic field

TL;DR

This paper shows that spin-flip tunnelling in a bilayer ultracold Fermi gas system can enhance superfluidity under magnetic fields by opening a spin-triplet gap, changing the phase transition nature, and is experimentally feasible.

Contribution

It introduces a mechanism where spin-flip tunnelling increases the critical magnetic field for superfluidity, a novel approach in ultracold Fermi gases.

Findings

Spin-flip tunnelling opens a spin-triplet gap near the Fermi surface.

The critical magnetic field for superfluidity is significantly increased.

The phase transition changes from first to second order with sufficient tunnelling.

Abstract

It is well-known that when the magnetic field is stronger than a critical value, the spin imbalance can break the Cooper pairs of electrons and hence hinder the superconductivity in a spin-singlet channel. In a bilayer system of ultra-cold Fermi gases, however, we demonstrate that the critical value of the magnetic field at zero temperature can be significantly increased by including a spin-flip tunnelling, which opens a gap in the spin-triplet channel near the Fermi surface and hence reduces the influence of the effective magnetic field on the superfluidity. The phase transition also changes from first order to second order when the tunnelling exceeds a critical value. Considering a realistic experiment, this mechanism can be implemented by applying an intralayer Raman coupling between the spin states with a phase difference between the two layers.