Intermediate Diffusive-Ballistic Electron Conduction Around Mesoscopic Defects in Graphene

Nondiffusive effects in charge transport become relevant as device sizes and features become comparable to the electronic mean free path. As a model system, we investigated the electric transport around mesoscopic defects in graphene with scanning tunneling potentiometry. Diffusive and ballistic contributions to the scattering dipole are probed by simultaneously resolving the nanoscale topography of pits in the graphene layer and measuring the local electrochemical potential in the surrounding area. We find evidence of transport in the intermediate regime between the diffusive and ballistic limits such that the magnitude of the electrochemical potential around the defects is substantially underestimated by diffusive models. Our experiments and modeling are supported by lattice-Boltzmann simulations, which highlight the importance of the ratio between defect size and mean free path in the intermediate transport regime. The magnitude of the scattering dipole depends on the shape of the pits in both the ballistic and diffusive transport modes. Remarkably, ballistic contributions to the electron transport are found at feature sizes larger than the mean free path and rapidly increase at lower sizes, having a noticeable impact already on mesoscopic length scales.