Andreev reflections in layered structures : Implications ifor high-Tc grain-boundary junctions

Andreev reflection is investigated in layered anisotropic normal-metal–superconductor (N/S) systems in the case of an energy gap (Δ) 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.