Giant Phonon-induced Conductance in Scanning Tunneling Spectroscopy of Gate-tunable Graphene

TL;DR

This study reveals a giant phonon-induced conductance in graphene, where phonons control electron tunneling, leading to a gap-like feature in the spectrum that is independent of charge density.

Contribution

It uncovers a phonon-mediated inelastic tunneling channel in graphene, demonstrating phonons as a 'floodgate' for electron flow in gate-tunable 2D materials.

Findings

Observation of a gap-like feature pinned to the Fermi level

Giant enhancement of tunneling at higher energies due to phonons

Phonons act as a 'floodgate' controlling tunneling flow

Abstract

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