Session A27: Invited Session: Excitonic and Correlation Effects in Single-Layer Graphene

Dynamic screening and the effective fine structure constant of graphene

Electrons in graphene behave, in the low energy sector, like massless Dirac fermions. The degree to which Coulomb correlations influence these fermions is still subject to debate. In this talk I will describe inelastic x-ray scattering experiments on crystals of graphite, to which we have applied newly developed reconstruction algorithms to image the dynamical screening of charge in (effectively) a freestanding graphene sheet. We found that the polarizability of the Dirac fermions in graphene is amplified by excitonic effects in the particle-hole spectrum, which enhances screening of interactions among low energy quasiparticles. I will argue that interactions should be characterized by an effective, screened fine-structure constant, $\alpha_g^*({\bf k},\omega)$, which our measurements suggest converges to the value $\alpha_g^*=0.14 \pm 0.092$ in the small wave vector limit. This value is smaller than the bare $\alpha_g=2.2$, and suggests that the strength of interactions in graphene is similar to that in band semiconductors like Si or GaAs. I will discuss the implications of this result for other Dirac systems, such as nodal quasiparticles in cuprates and topological insulator surface states.   

Observation of strong excitonic effects in the optical spectrum of graphene

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Electron-electron interaction and excitonic effects in graphene systems

Owing to the unique electronic structure of graphene and enhanced electron-electron and electron-hole interactions in lower dimensions, graphene structures exhibit interesting and novel electronic and optical properties. We discuss in this talk results from recent theoretical studies on several graphene systems. First-principles calculations, based on the GW-Bethe-Salpeter Equation approach, have predicted very strong features in the optical absorption spectra of single-layer graphene and multi-layer graphene, arising from resonant excitons. Many of these features have since been observed experimentally. We also explore the effects of charge carrier doping and of having an external electric field on the absorbance of graphene and bilayer graphene. Graphene nanoribbons are semiconductors that, owing to electron-electron interactions, also exhibit extraordinarily magnetic and excitonic effects. Another intriguing phenomenon is one in the magneto-optical response of graphene, which involves resonant transitions of absorption of a photon together with the simultaneous creation of an intervalley, intra-Landau level exciton and a K-phonon. Finally, first-principles results on the plasmon satellite structures in the spectral function of graphene computed with the GW method and beyond are presented. We discuss the origin of these phenomena and make comparison with experiments.   

Graphene Update

I will overview the latest experimental progress made by our group in Manchester. This will cover several subjects with particular attention being paid to interaction effects that have been observed in graphene and its double layer heterostructures.   

Electron-electron interactions in doped graphene sheets

In this talk I will review some of the most important electronic properties of graphene. I will first discuss the appearance of plasmaron satellite bands in both angle-resolved photoemission [1] and STM spectra [2,3], emphasizing the important role of the sublattice pseudospin degree of freedom. I will then illustrate some unusual features, which appear only beyond the widely used Random Phase Approximation, characterizing plasmons and Drude weight of the electron gas in this material~[4].\\[4pt] [1] A. Bostwick et al., Science 328, 999 (2010).\\[0pt] [2] V.W. Brar et al., Phys. Rev. Lett. 104, 036805 (2010).\\[0pt] [3] A. Principi, M. Polini, and A.H. MacDonald, to be submitted\\[0pt] [4] S.H. Abedinpour et al., Phys. Rev. B 84, 045429 (2011).