Impact of graphene quantum capacitance on transport spectroscopy

We demonstrate experimentally that graphene quantum capacitance C-q can have a strong impact on transport spectroscopy through the interplay with nearby charge reservoirs. The effect is elucidated in a field-effect-gated epitaxial graphene device, in which interface states serve as charge reservoirs. The Fermi-level dependence of C-q is manifested as an unusual parabolic gate voltage (V-g) dependence of the carrier density, centered on the Dirac point. Consequently, in high magnetic fields B, the spectroscopy of longitudinal resistance (R-xx) vs V-g represents the structure of the unequally spaced relativistic graphene Landau levels (LLs). R-xx mapping vs V-g and B thus reveals the vital role of the zero-energy LL on the development of the anomalously wide nu = 2 quantum Hall state.