Abstract
Sign reversal of Berry curvature across two oppositely gated regions in bilayer graphene can give rise to counter-propagating 1D channels with opposite valley indices. Considering spin and sub-lattice degeneracy, there are four quantized conduction channels in each direction. Previous experimental work on gate-controlled valley polarizer achieved good contrast only in the presence of an external magnetic field. Yet, with increasing magnetic field the ungated regions of bilayer graphene will transit into the quantum Hall regime, limiting the applications of valley-polarized electrons. Here we present improved performance of a gate-controlled valley polarizer through optimized device geometry and stacking method. Electrical measurements show up to two orders of magnitude difference in conductance between the valley-polarized state and gapped states. The valley-polarized state displays conductance of nearly 4e2/h and produces contrast in a subsequent valley analyzer configuration. These results pave the way to further experiments on valley-polarized electrons in zero magnetic field.
Highlights
Sign reversal of Berry curvature across two oppositely gated regions in bilayer graphene can give rise to counter-propagating 1D channels with opposite valley indices
We start with a discussion on optimizing device geometry
By carefully adjusting the spacing, the induced charge carriers can be reduced by a factor of 5, which is a significant amount and bring the level of gate-induced disorder into a range that is comparable to the expected charge-carrier fluctuations already present in the device. This demonstrates that careful adjustments to the device design, based on specific measurements of the device geometry during the fabrication process, can improve device fabrication, the results show that gate geometry must be precisely defined in the nanometer range
Summary
Sign reversal of Berry curvature across two oppositely gated regions in bilayer graphene can give rise to counter-propagating 1D channels with opposite valley indices. Along the split junction there exist valley-polarized 1D channels with quantized conductance 4e2/h in the ballistic limit (Fig. 1b). We find that asymmetric gating can exceed this, but gateinduced charge disorder can be minimized by adjusting the topgate geometry according to measured device parameters.
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