Towards a gate-controlled valley-analyzer in bilayer graphene.

The inequivalent valleys K and K' in k-space of 2D-hexagonal materials offer a new valley-degree of freedom similar to spin and controllable via electrostatic gating. In bilayer graphene, the sign of Berry curvature in each valley is determined by the direction of the external electric field whereas the sign of Berry curvature in each valley has opposite sign. Theorists have predicted that at 1D-boundaries at which the sign of Berry curvature changes, counter-propagating valley-polarized 1D-channels emerge. Including spin and sublattice degeneracy, there are 4 quantized conduction channels in each direction. This approach provides a gate-controlled platform for valley-polarizers and analyzers, yet it is technically challenging to build such a nanoscale system. Here, we fabricate such system with four pairs of dual-split gates with optimized geometry and stacking methods. Low temperature measurements show large conductance contrast with a factor of 100 between different gate configurations. In our devices, we achieve conductivity of nearly 4 conduction channels in each valley polarized state, much closer to the ballistic limit than previous work. Ultimately, one pair can be valley polarizer and the subsequent pair could detect the valley polarization and serve as valley analyzer.