Layer-polarized PN and point junctions in double bilayer graphene

We fabricate twisted double bilayer graphene devices with zero twist angle and a set of local top and bottom gates aligned perpendicularly to each other. A 1D PN junction can be electrostatically defined when the gate voltages applied to the top gates are the same but different on the bottom gates. Resistance peaks are observed at finite doping instead of at the charge neutrality points, exhibiting an unconventional broken-cross shape that arises from layer polarization of the P and N region, which can be further enhanced with finite magnetic fields. A 0D point junction can be electrostatically defined by applying different gate voltages to the top and bottom gates such that the P and N sides of the device are connected at a single point in the center of the device. As the finite magnetic field B increases, the quantum Hall states are selectively brought into contact or away from each other depending on their layer polarization, leading to unconventional quantum oscillations that characterize the layer-polarized band-crossing. Our work provides new insights into understanding band-structure evolution and layer polarization in twisted bilayers and paves the way for new device functionality based on manipulating layer-polarized electronic states.