Neutral triplet Collective Mode as a new decay channel in Graphite

In an earlier work we predicted the existence of a neutral triplet collective mode in undoped graphene and graphite [Phys. Rev. Lett. {\bf 89} (2002) 16402]. In this work we study a phenomenological Hamiltonian describing the interaction of tight-binding electrons on honeycomb lattice with such a dispersive neutral triplet boson. Our Hamiltonian is a generalization of the Holstein polaron problem to the case of triplet bosons with non-trivial dispersion all over the Brillouin zone. This collective mode constitutes an important excitation branch which can contribute to the decay rate of the electronic excitations. The presence of such collective mode, modifies the spectral properties of electrons in graphite and undoped graphene. In particular such collective mode, as will be shown in this paper, can account for some part of the missing decay rate in a time-domain measurement done on graphite

Short range Coulomb correlations render massive Dirac fermions massless

Tight binding electrons on a honeycomb lattice are described by an effective Dirac theory at low energies. Lowering symmetry by an alternate ionic potential ($\Delta$) generates a single-particle gap in the spectrum. We employ the dynamical mean field theory (DMFT) technique, to study the effect of on-site electron correlation ($U$) on massive Dirac fermions. For a fixed mass parameter $\Delta$, we find that beyond a critical value $U_{c1}(\Delta)$ massive Dirac fermions become massless. Further increasing $U$ beyond $U_{c2}(\Delta)$, there will be another phase transition to the Mott insulating state. Therefore the competition between the single-particle gap parameter, $\Delta$, and the Hubbard $U$ restores the semi-metallic nature of the parent Hamiltonian. The width of the intermediate semi-metallic regime shrinks by increasing the ionic potential. However, at small values of $\Delta$, there is a wide interval of $U$ values for which the system remains semi-metal.Comment: 4 pages, 5 figure

Ultrafast electron-optical phonon scattering and quasiparticle lifetime in CVD-grown graphene

10.1021/nn200419zACS Nano543278-328