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

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 $N_σ$ is the number of atoms with pseudospin $σ=\uparrow,\downarrow$), 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 $π$-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.