Intervalley conversion in bilayer-monolayer graphene junctions

We studied the valley dependent transport properties of bilayer-monolayer graphene junctions consisting of a device region of bilayer zigzag ribbon connected to monolayer zigzag ribbon leads. We determined numerically using the python package Kwant the valley dependent conductance through the junctions in an energy range in which each valley has at most two subbands in a single layer ribbon. There is a Fermi energy in this range in which the K to K conductance drops significantly to below $0.2e^2/h$ (spin degree of freedom is included), while the K to K′ conductance remains high. This means a high percentage of the incident K electrons are transferred to the K′ valley in the outgoing lead. A number of junctions can be designed with K to K′ conductance larger than $1.2e^2/h$, which have a conversion efficiency higher than 84% and there are also junctions with K to K′ conductance between $1.2e^2/h$ and $0.6e^2/h$ (the conversion efficiency > 66%). Changing the transverse dimension, the conductance features are shifted in energy without significant change to the qualitative behavior. When the longitudinal dimension of the junction is changed, the qualitative features are changed in a way, which resembles the relation between the band structure of armchair ribbons and the dimer numbers along the ribbon's transverse direction. For some junctions, this interesting intervalley scattering property can be explained by Fano resonance. Our results show that these bilayer-monolayer junctions can be used as efficient intervalley converters in valleytronic devices.