Andreev reflection in layered structures: implications for high TC grain boundary Josephson junctions

Andreev reflection is investigated in layered anisotropic normal-metal–superconductor ~N/S! systems in the case of an energy gap (D) in S not negligible with respect to the Fermi energy (EF), as it probably occurs with high critical temperature superconductors ~HTSC’s!. We find that in these limits retroreflectivity, which is a fundamental feature of Andreev reflection, is broken modifying sensitively transport across S/N interfaces. We discuss the consequences for supercurrents in HTSC Josephson junctions and for the midgap states in S-N contacts.