Superfluid/ferromagnet/superfluid junction and $\pi$-phase in a superfluid Fermi gas

TL;DR

This paper explores the formation of a superfluid/ferromagnet/superfluid junction in a superfluid Fermi gas, demonstrating the emergence of a $\pi$-phase where the order parameter changes sign, using a Hubbard model at zero temperature.

Contribution

It introduces a simple model showing how population imbalance induces a ferromagnetic barrier, creating a $\pi$-phase in a superfluid Fermi gas, analogous to superconductor junctions.

Findings

Magnetized barrier localizes excess spins in the Fermi gas.

A $\pi$-phase with sign change of the order parameter is realized.

The model provides insights into magnetic effects on fermion superfluidity.

Abstract

We investigate the possibility of superfluid/ferromagnet/superfluid (SFS)-junction in a superfluid Fermi gas. To examine this possibility in a simple manner, we consider an attractive Hubbard model at $T=0$ within the mean-field theory. When a potential barrier is embedded in a superfluid Fermi gas with population imbalance ($N_\uparrow>N_\downarrow$, where $N_\sigma$ is the number of atoms with pseudospin $\sigma=\uparrow,\downarrow$), this barrier is shown to be {\it magnetized} in the sense that excess $\uparrow$-spin atoms are localized around it. The resulting superfluid Fermi gas is spatially divided into two by this {\it ferromagnet}, so that one obtains a junction similar to the superconductor/ferromagnet/superconductor-junction discussed in superconductivity. Indeed, we show that the so-called $\pi$-phase, which is a typical phenomenon in the SFS-junction, is realized, where the superfluid order parameter changes its sign across the junction. Our results would be useful for the study of magnetic effects on fermion superfluidity using an ultracold Fermi gas.