Andreev reflection in a Y-shaped graphene-superconductor device

Abstract

Using the nonequilibrium Green function method, we study the Andreev reflection in a Y-shaped graphene-superconductor device by tight-binding model. Considering both the zigzag and armchair terminals, we confirm that the zigzag terminals are the better choice for detecting the Andreev reflection in the absence of external field. Due to scattering from the boundaries of the finite-size center region, the difference between Andreev retroreflection and specular reflection is hard to distinguish. Although adjusting the size of the device makes the difference visible, to distinguish them quantitatively is still impossible through the transport conductance of a three-terminal device. The problem is circumvented when applying a perpendicular magnetic field on the center region, which makes the incident electrons and the reflected holes propagate along the edge or the interface. In this case, the retroreflected and specular reflected holes from the different bands have opposite effective masses, therefore the moving direction of one is opposite to the other. Which external terminal the reflected holes flow into depends entirely on the kind of the Andreev reflection. Therefore, the specular Andreev reflection can be clearly distinguished from the retroreflected one in the presence of strong magnetic field, even for the device with finite size.

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