Probing the valley filtering effect by Andreev reflection in a zigzag graphene nanoribbon with a ballistic point contact

摘要

The ballistic point contact (BPC) with zigzag edges in graphene is a main candidate for a valley filter, in which the polarization of the valley degree of freedom can be selected by using a local gate voltage. Here, we propose to detect the valley filtering effect by Andreev reflection. By using both scattering matrix analysis and numerical simulation, we show that Andreev reflection is strongly suppressed as the incident electron and reflected hole propagate through different bands of BPC with opposite chirality. The conductance as a function of both the incident energy of electron and the local gate voltage at the BPC is obtained, with which the parametric region for a perfect valley filter and the direction of the valley polarization can be determined. As the length of the BPC increases, the conductance exhibits an oscillatory decay behavior, indicating a negative correlation between Andreev reflection and valley polarization.