Probing The Valley Filtering Effect By Andreev Reflection In A Zigzag Graphene Nanoribbon

Due to find two-dimensional honeycomb lattice materials,valleytronics has a great advance in past decade.Graphene is a commonly used material in the study of valleytronics.Conduction and valence bands in graphene form conically shaped valleys,touching at a point called the Dirac point.The two inequivalent Dirac points are located at the Brillouin zone corners,so that they can be defined a new degree of freedom: energy valley.Similar to spintronics,the goals of valleytronics are using the valley degree of freedom to manipulate electrons to find applications in semiconductor electronics and quantum information.In valleytronics,valley filters play an important role,using it to generate and detect valley polarization currents.So many graphene-based valley filters have been proposed in recent years,but there is no experimental proposal to detect these valley filtering effect.The first valley filter scheme is a ballistic point contact with zigzag edges in graphene nanoribbon.In this paper,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 when valley is polarized.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.We studied Andreev reflection and ballistic point contact length dependence.As the length of the BPC increases,the conductance exhibits an oscillatory decay behavior,indicating a negative correlation between Andreev reflection and valley polarization.