  We study Andreev reflection in normal metal-superconductor junctions by using
an extended Blonder-Tinkham-Klapwijk model combined with transport calculations
based on density functional theory. Starting from a parameter-free description
of the underlying electronic structure, we perform a detailed investigation of
normal metal-superconductor junctions, as the separation between the
superconductor and the normal metal is varied. The results are interpreted by
means of transverse momentum resolved calculations, which allow us to examine
the contributions arising from different regions of the Brillouin zone.
Furthermore we investigate the effect of a voltage bias on the normal
metal-superconductor conductance spectra. Finally, we consider Andreev
reflection in carbon nanotubes sandwiched between normal and superconducting
electrodes.
