Abrupt Interfaces and Andreev Conversion in Graphene-Superconductor Junctions

We theoretically study chiral transport between quantum Hall graphene and an s-wave superconductor in junctions with varied interface transparency and structure. We focus on the lowest Landau level with the minimum of two chiral edge or interface modes. We find that Andreev conversion is more robust to poor interfaces at strong magnetic field than in the zero field multimode regime. Since intervalley scattering is necessary in graphene for chiral transport around a crystallographic corner, a localized corner mode is essential in zigzag structures when the coupling to the superconductor is weak. At strong coupling, on the other hand, Andreev conversion is enabled by means of the valley content of the chiral Andreev edge state. By studying the regime between these two limits, we develop an understanding of the Andreev reflection probability in presence of imperfections. Moreover, we show how to influence the amount of Andreev versus normal scattering and, consequently, the current and the noise in these graphene devices.