Ballistic transport of graphene pnp junctions with embedded local gates

We fabricated graphene pnp devices, by embedding pre-defined local gates in an oxidized surface layer of a silicon substrate. With neither dielectric-material deposition nor electron-beam irradiation on the graphene, we obtained high-quality graphene pnp devices without degradation of the carrier mobility even in the local-gate region. The corresponding increased mean free path leads to the observation of ballistic and phase-coherent transport across a 130-nm-wide local gate, which is about an order of magnitude wider than reported previously. Furthermore, in our scheme, we demonstrated independent control of the carrier density in the local-gate region, with a conductance map very distinctive from top-gated devices. It was caused as the electric field arising from the global back gate is strongly screened by the embedded local gate. Our scheme allows the realization of ideal multipolar graphene junctions with ballistic carrier transport.