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We propose graphite intercalation compounds ͑GICs͒ as a material system with precisely the same electronic properties as doped few layer graphene. Despite the fact that GICs have been around for the last four decades, this fact has gone unnoticed so far. Especially, we focus on the electronic energy bands of KC 8 which correspond to a doped graphene monolayer. We provide extensive theoretical and experimental evidence for this claim employing a combined angle-resolved photoemission and theory approach using tight-binding, standard density-functional theory and including electron-electron correlation on a GW level. We observe a strong momentum-dependent kink in the quasiparticle dispersion at 166 meV highlighting electron-phonon coupling to an in-plane transversal optical phonon. These results are key for understanding both the unique electronic properties of doped graphene layers and superconductivity in KC 8 .

Site-specific and doping-dependent electronic structure of YBa2Cu~O"probed by 0 1s and Cu 2p I-ray-absorption spectroscopy

The two-dimensional mapping of the phonon dispersions around the K point of graphite by inelastic x-ray scattering is provided. The present work resolves the longstanding issue related to the correct assignment of transverse and longitudinal phonon branches at K. We observe an almost degeneracy of the three TO-, LA-, and LO-derived phonon branches and a strong phonon trigonal warping. Correlation effects renormalize the Kohn anomaly of the TO mode, which exhibits a trigonal warping effect opposite to that of the electronic band structure. We determined the electron-phonon coupling constant to be 166 ͑eV/ Å͒ 2 in excellent agreement to GW calculations. These results are fundamental for understanding angle-resolved photoemission, doubleresonance Raman and transport measurements of graphene-based systems.

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Electrons in isolated graphene layers are a two-dimensional gas of massless Dirac Fermions. In realistic devices, however, the electronic properties are modified by elastic deformations, interlayer coupling and substrate interaction. Here, we unravel the electronic structure of noninteracting, doped graphene layers by revisiting the stage one graphite intercalation compound KC 8 . To this end we apply angle-resolved photoemission spectroscopy and ab initio calculations. The full experimental dispersion is in excellent agreement with calculations of doped graphene once electron correlations are included at the GW level ͑Greens function G of the Coulomb interaction W͒. This highlights that KC 8 has negligible interlayer coupling allowing us to access the full experimental Dirac cone. The recent discovery of two-dimensional metastable graphene sheets has sparked enormous interest in their lowenergy electronic structure.1,2 Angle-resolved photoemission spectroscopy ͑ARPES͒ has been proven to be a key tool to determine the electronic structure of one-and few-layer graphene 3 and graphite. 4-6 A major problem for the investigation of graphene on SiC and metal substrates is that there is a significant modification of the electronic structure due to interaction with the substrates yielding charge transfer and hybridization. 7,8 Upon intercalation of Au into the graphene/ substrate interface, the Fermi level could be brought back to the Dirac point. 9 However, the graphene electronic bands are still strongly renormalized by a kink at 0.95 eV due to interaction with the Au film.9 Thus, neither graphene on SiC nor graphene on metal substrates provides access to the full Dirac cone of noninteracting graphene. One way to overcome this problem is to measure single crystalline graphite, which has no substrate interaction. In this case however, a k z dispersion of two valence bands 6 and a small gap 10 were observed because of the AB stacking of graphene layers ͑AA stacking does not show the gap opening͒.11 Another important issue is the renormalization of the bare electronic band structure due to doping-dependent electron-electron correlation, 6 electron-phonon coupling ͑EPC͒ 5,12 or electronplasmon coupling. 13In the present work, we take a different perspective to circumvent the problems of substrate interaction, strong bilayer splitting and electron-electron correlation by revisiting stage one graphite intercalation compound ͑GIC͒ KC 8 using a combination of ARPES and ab initio calculations. We show that the electronic properties of intercalated graphite and graphene are equivalent, therefore, we can use the former to understand the physics of the latter. In particular, we provide the full experimental Dirac cone of doped graphene layers in KC 8 using ARPES. We compare the ARPES intensities to ab initio calculations and find good agreement with calculations at the GW level. In contrast to previous works on undoped graphite 6 and graphene monolayers on metal substrates 7,14 the electronic energy band structure of graphene sh...

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