  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.
