Ph. D. Thesis - The University of Texas at Austin

Angle-resolved photoemission spectroscopy (ARPES) is a powerful probeof A(k, ω) in 2D crystals because it achieves momentum k resolution [61].Two recent experiments [62,63] have reported ARPES spectra for singlelayergraphene samples prepared by graphitizing the surface of Silicon Carbide(SiC) [64]. Although the data in Refs. [62,63] are similar, the physical interpretationsof the experimental findings are very different. Ref. [62] discussesthe ARPES spectra in terms of electron-phonon [65] and electron-plasmon interactions,while Ref. [63] focuses mainly on the apparent band-gap opening atthe Dirac point. A gap at the Dirac point can be explained without electronelectroninteractions by assuming strong inversion symmetry breaking in thegraphene layer due to coupling with the SiC substrate. Our theoretical resultsappear to allow an intrinsic interpretation for this feature, although it isclear that present experimental data is still partially obscured by incompletelycontrolled interactions with the substrate and by sample inhomogeneity whichproduces momentum space broadening.The self-energy in a system of fermions can be separated into an exchangecontribution due to interactions with occupied states in the static Fermisea, and a correlation contribution due to the sea’s quantum fluctuations [40].Graphene differs [21] from the widely studied 2D systems in semiconductorquantum wells because its quasiparticles are chiral and because it is gaplessand therefore has interband quantum fluctuations on the Fermi energy scale.In graphene, band eigenstate chirality endows exchange interactions with anew source of momentum dependence which renormalizes the quasiparticle63