Theory of the pi 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 ph ase difference k in the ground state can be zero or pi. Three basic paramet ers - the transfer integral t between S and F layers, the exchange field h in F, and the pairing constant Lambda in S - characterize the system. We fi nd that the critical Josephson current has a nonmonotonic dependence on h, becoming zero at the critical value h = h(crit)(T), corresponding to the tr ansition between k = 0 and k = pi in the ground state. We calculate the den sities of states on S and F layers and show that the quasiparticle spectra are strongly influenced by the values of h and t. [S0163-1829(98)01546-X].