Giant phonon-induced conductance in scanning tunnelling spectroscopy of gate-tunable graphene

The honeycomb lattice of graphene is a unique two-dimensional system where the quantum mechanics of electrons is equivalent to that of relativistic Dirac fermions(1,2). Novel nanometre-scale behaviour in this material, including electronic scattering(3,4), spin-based phenomena(5) and collective excitations(6), is predicted to be sensitive to charge-carrier density. To probe local, carrier-density-dependent properties in graphene, we have carried out atomically resolved scanning tunnelling spectroscopy measurements on mechanically cleaved graphene flake devices equipped with tunable back-gate electrodes. We observe an unexpected gap-like feature in the graphene tunnelling spectrum that remains pinned to the Fermi level (E-F) regardless of graphene electron density. This gap is found to arise from a suppression of electronic tunnelling to graphene states near E F and a simultaneous giant enhancement of electronic tunnelling at higher energies due to a phonon-mediated inelastic channel. Phonons thus act as a 'floodgate' that controls the flow of tunnelling electrons in graphene. This work reveals important new tunnelling processes in gate-tunable graphitic layers.