title: Signatures of Phonon and Defect-Assisted Tunneling in Planar Metal-Hexagonal Boron Nitride-Graphene Junctions.
abstract: Electron tunneling spectroscopy measurements on van der Waals heterostructures consisting of metal and graphene (or graphite) electrodes separated by atomically thin hexagonal boron nitride tunnel barriers are reported. The tunneling conductance, dI/dV, at low voltages is relatively weak, with a strong enhancement reproducibly observed to occur at around |V| ≈ 50 mV. While the weak tunneling at low energies is attributed to the absence of substantial overlap, in momentum space, of the metal and graphene Fermi surfaces, the enhancement at higher energies signals the onset of inelastic processes in which phonons in the heterostructure provide the momentum necessary to link the Fermi surfaces. Pronounced peaks in the second derivative of the tunnel current, d2I/dV2, are observed at voltages where known phonon modes in the tunnel junction have a high density of states. In addition, features in the tunneling conductance attributed to single electron charging of nanometer-scale defects in the boron nitride are also observed in these devices. The small electronic density of states of graphene allows the charging spectra of these defect states to be electrostatically tuned, leading to "Coulomb diamonds" in the tunneling conductance.
authors.affiliation: Institute for Quantum Information and Matter, Department of Physics, California Institute of Technology , 1200 East California Boulevard, Pasadena, California 91125, United States.
authors.forename: U
authors.initials: U
authors.lastname: Chandni
authors.affiliation: National Institute for Materials Science , 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Watanabe
authors.affiliation: National Institute for Materials Science , 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Taniguchi
authors.affiliation: Institute for Quantum Information and Matter, Department of Physics, California Institute of Technology , 1200 East California Boulevard, Pasadena, California 91125, United States.
authors.forename: J P
authors.initials: JP
authors.lastname: Eisenstein
keywords: Coulomb blockade; IETS; Tunneling; graphene; hBN; hBN defects; hexagonal boron nitride; phonon-assisted tunneling