We observe a series of sharp resonant features in the differential conductance of graphene-hexagonal boron nitride-graphene tunnel transistors over a wide range of bias voltages between 10 and 200 mV. We attribute them to electron tunneling assisted by the emission of phonons of well-defined energy. The bias voltages at which they occur are insensitive to the applied gate voltage and hence independent of the carrier densities in the graphene electrodes, so plasmonic effects can be ruled out. The phonon energies corresponding to the resonances are compared with the lattice dispersion curves of graphene-boron nitride heterostructures and are close to peaks in the single phonon density of states.

We performed calculations of electronic, optical, and transport properties of graphene on hexagonal boron nitride with realistic moiré patterns. The latter are produced by structural relaxation using a fully atomistic model. This relaxation turns out to be crucially important for electronic properties. We describe experimentally observed features such as additional Dirac points and the "Hofstadter butterfly" structure of energy levels in a magnetic field. We find that the electronic structure is sensitive to many-body renormalization of the local energy gap.

Abstract. We study by molecular dynamics the structural properties of single layer h-BN in comparison to graphene. We show that the Tersoff bond order potential developed for BN by Albe, Möller and Heinig gives a thermally stable hexagonal single layer with a bending constant κ = 0.54 eV at T = 0. We find that the non-monotonic behaviour of the lattice parameter, the expansion of the interatomic distance and the growth of the bending rigidity with temperature are qualitatively similar to those of graphene. Conversely, the energetics of point defects is extremely different: instead of Stone Wales defects, the two lowest energy defects in h-BN involve either a broken bond or an out of plane displacement of a N atom to form a tetrahedron with three B atoms in the plane. We provide the formation energies and an estimate of the energy barriers.

First principle calculations of the phonons of graphene-h-BN heterostructures are presented and compared to those of the constituents. We show that AA and AB' stacking are not only energetically less favoured than AB but also dynamically unstable. We have identified low energy flat phonon branches of h-BN character with out of plane displacement and evaluated their coupling to electrons in graphene.

Abstract. We have studied the electronic excitation spectrum in periodically rippled graphene on Ru(0001) and flat, commensurate graphene on Ni(111) by means of high-resolution electron energy loss spectroscopy and a combination of density functional theory and tight-binding approaches.We show that the periodic moiré superlattice originated by the lattice mismatch in graphene/Ru(0001) induces the emergence of an extra mode, which is not present in graphene/Ni(111). Contrary to the ordinary intra-band plasmon of doped graphene, the extra mode is robust in charge-neutral graphene/metal contacts, having its origin in electron-hole inter-band transitions between van Hove singularities that emerge in the reconstructed band structure, due to the moiré pattern superlattice.