Andreev reflection in graphene nanoribbons

We study Andreev reflection in graphene nanoribbon/superconductor hybrid junctions. By using a tightbinding approach and the scattering formalism we show that finite-size effects lead to notable differences with respect to the bulk-graphene case. At subgap voltages, conservation of pseudoparity, a quantum number characterizing the ribbon states, yields either a suppression of Andreev reflection when the ribbon has an even number of sites in the transverse direction or perfect Andreev reflection when the ribbon has an odd number of sites. In the former case the suppression of Andreev reflection induces an insulating behavior even when the junction is biased; electron conduction can however be restored by applying a gate voltage. Finally, we check that these findings remain valid also in the case of nonideal nanoribbons in which the number of transverse sites varies along the transport direction.