Theory of the π junctions formed in atomic-scale superconductor/ferromagnet superlattices

Using the Green's-functions method we investigate theoretically a model of atomic-scale superconductor/ferromagnet $(S/F)$ superlattices. In this model the phase of the order parameter changes periodically, and the intrinsic phase difference k in the ground state can be zero or {π}. Three basic parameters --- the transfer integral t between S and F layers, the exchange field h in $F,$ and the pairing constant Λ in S --- characterize the system. We find that the critical Josephson current has a nonmonotonic dependence on h, becoming zero at the critical value h=hcrit(T), corresponding to the transition between $k=0$ and k=π in the ground state. We calculate the densities of states on S and F layers and show that the quasiparticle spectra are strongly influenced by the values of h and t.