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

We investigate the stability of $π$-phase in a polarized superfluid Fermi gas ($N_\uparrow>N_\downarrow$, where $N_σ$ is the number of atoms in the hyperfine state described by pseudospin-$σ$). In our previous paper [T. Kashimura, S. Tsuchiya, and Y. Ohashi, Phys. Rev. A {\bf 82}, 033617 (2010)], we showed that excess atoms ($ΔN=N_\uparrow-N_\downarrow$) localized around a potential barrier embedded in the system induces the $π$-phase at T=0, where the phase of superfluid order parameter differ by $π$ across the junction. In this paper, we extend our previous work to include temperature effects within the mean-field theory. We show that the $π$-phase is stable even at finite temperatures, although transition from the $π$-phase to 0-phase eventually occurs at a certain temperature. Our results indicate that the $π$-phase is experimentally accessible in cold Fermi gases.