Andreev reflection in layered structures: Implications for high-T-c grain-boundary Josephson junctions

Andreev reflection is investigated in layered anisotropic normal-metal-supe rconductor (N/S) systems in the case of an energy gap (Delta) in S not negl igible with respect to the Fermi energy (E-F), as it probably occurs with h igh critical temperature superconductors (HTSC's). We find that in these li mits retroreflectivity, which is a fundamental feature of Andreev reflectio n, is broken modifying sensitively transport across S/N interfaces. We disc uss the consequences for supercurrents in HTSC Josephson junctions and for the midgap states in S-N contacts.