Tunneling in graphene–topological insulator hybrid devices

Abstract

Hybrid graphene--topological insulator (TI) devices were fabricated using a mechanical transfer method and studied via electronic transport. Devices consisting of bilayer graphene (BLG) under the TI ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ exhibit differential conductance characteristics which appear to be dominated by tunneling, roughly reproducing the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ density of states. Similar results were obtained for BLG on top of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, with tenfold greater conductance consistent with a larger contact area due to better surface conformity. The devices further show evidence of inelastic phonon-assisted tunneling processes involving both ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and graphene phonons. These processes favor phonons which compensate for momentum mismatch between the TI $\mathrm{\ensuremath{\Gamma}}$ and graphene $K,{K}^{\ensuremath{'}}$ points. Finally, the utility of these tunnel junctions is demonstrated on a density-tunable BLG device, where the charge neutrality point is traced along the energy-density trajectory. This trajectory is used as a measure of the ground-state density of states.