Zero-energy bound states in superconductor/ferromagnet superlattices

Andreev bound states in monoatomic superconductor-ferromagnet (S/F) superla ttices are studied theoretically, assuming tunneling between S and F layers in perpendicular direction. Andreev reflection at S/F interfaces is strong ly affected by the exchange interaction h in F layers. In the ground state, only for h not equal 0 zero-energy states (ZES) are formed on S and F laye rs. For h = 0, corresponding to superconductor-normal metal (S/N) superlatt ices, ZES may appear in the nonequilibrium phase, phi = pi. This is found b oth for s-wave and d-wave symmetry of the order parameter in S. The conditi ons for ZES are obtained as a function of h, of the transfer integral t for movement of quasiparticles (QPs) between S and F layers, and of the corres ponding ground state phase difference phi(eq) between two neighboring S lay ers. (C) 1999 Elsevier Science B.V. All rights reserved.