We describe the synthesis of bilayer graphene thin films deposited on insulating silicon carbide and report the characterization of their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic-scale electronic devices

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With the recent discovery of superconductivity in carbon nanotubes (CNTs), 1, 2 alkaline-metal-doped C 60 crystals, 3 and graphite intercalation compounds 4-6 (GICs) with relatively high transition temperatures, there is a strong interest in the influence of many-body interactions on the electron dynamics in these systems. Graphene is a sheet of carbon atoms distributed in a honeycomb lattice and is the building block for all of these materials;therefore it is a model system for this entire family. Recently, graphene has been isolated using exfoliation from graphite 7, 8 and graphitization of SiC, 9, 10 enabling, for the first time, the direct measurement of the manybody interactions fundamental to all of these carbon systems. These interactions could be especially interesting owing to the effectively massless, relativistic nature of the charge carriers, which follows from the linearity of the bands at the Dirac crossing energy E D = ħω D and the formal equivalence of the Schrödinger wave equation with the relativistic Dirac equation for graphene. 7,8,11

The unusual transport properties of graphene are the direct consequence of a peculiar band structure near the Dirac point. We determine the shape of the bands and their characteristic splitting, and find the transition from two-dimensional to bulk character for 1 to 4 layers of graphene by angle-resolved photoemission. By detailed measurements of the bands we derive the stacking order, layer-dependent electron potential, screening length, and strength of interlayer interaction by comparison with tight binding calculations, yielding a comprehensive description of multilayer graphene's electronic structure. DOI: 10.1103/PhysRevLett.98.206802 PACS numbers: 73.21.ÿb, 73.22.ÿf, 73.90.+f, 79.60.ÿi Much recent attention has been given to the electronic structure of multilayer films of graphene, the honeycomb carbon sheet which is the building block of graphite, carbon nanotubes, C 60 , and other mesoscopic forms of carbon [1]. Recent progress in synthesizing or isolating multilayer graphene films [2 -4] has provided access to their physical properties, and revealed many interesting transport phenomena, including an anomalous quantum Hall effect [5,6], ballistic electron transport at room temperature [7], micronscale coherence length [7,8], and novel many-body couplings [9].These effects originate from the effectively massless Dirac fermion character of the carriers derived from graphene's valence bands, which exhibit a linear dispersion degenerate near the so-called Dirac point energy E D [10].These unconventional properties of graphene offer a new route to room temperature, molecular-scale electronics capable of quantum computing [6,7]. For example, a possible switching function in bilayer graphene has been suggested by reversibly lifting the band degeneracy at the Fermi level (E F ) upon application of an electric field [11,12]. This effect is due to a unique sensitivity of the band structure to the charge distribution brought about by the interplay between strong interlayer hopping and weak interlayer screening, neither of which is currently well understood [13,14].In order to evaluate the interlayer screening, stacking order,and interlayer coupling, we have systematically studied the evolution of the band structure of one to four layers of graphene using angle-resolved photoemission spectroscopy (ARPES). We demonstrate experimentally that the interaction between layers and the stacking sequence affect the topology of the bands, the former inducing an electronic transition from 2D to 3D (bulk) character when going from one layer to multilayer graphene. The interlayer hopping integral and screening length are determined as a function of the number of graphene layers by exploiting the sensitivity of states to the Coulomb potential, and the layer-dependent carrier concentration is estimated.The films were synthesized on n-type (nitrogen, 1 10 18 cm ÿ3 ) 6H-SiC(0001) substrates (SiCrystal AG) that were etched in hydrogen at 1550 C. Annealing in a vacuum first removes the resulting silicate adlayer and then causes t...

We have investigated the effects of doping on a single layer of graphene using angle-resolved photoemission spectroscopy. We show that many-body interactions severely warp the Fermi surface, leading to an extended van Hove singularity (EVHS) at the graphene M point. The ground state properties of graphene with such an EVHS are calculated, analyzing the competition between a magnetic instability and the tendency towards superconductivity. We find that the latter plays the dominant role as it is enhanced by the strong modulation of the interaction along the Fermi line, leading to an energy scale for the onset of the pairing instability as large as 1 meV when the Fermi energy is sufficiently close to the EVHS. DOI: 10.1103/PhysRevLett.104.136803 PACS numbers: 73.22.Pr, 73.20.At, 74.70.Wz, 79.60.Ài The fundamental interactions between charge, spin, and lattice depend very much on the topology of electronic bands, particularly in two dimensions when the Fermi level E F is near a saddle point. Such a saddle point occurs where the curvature of the bands has opposite sign in two orthogonal directions, leading to a van Hove singularity (VHS), i.e., a divergence in the density of states (DOS).Much attention has been given to a ''VHS scenario'' in the cuprates, whereby the superconductivity is strongly influenced or even induced by a saddle point VHS [1,2]. Among the important effects of the VHS are (1) the presence of both electronlike and holelike carriers, leading to an attractive potential favoring pairing, (2) a high DOS, held to favor not only superconductivity but structural and magnetic instabilities, and (3) a perfect screening at wave vectors connecting VHSs, which acts to reduce repulsion. The topology of the VHS is important, especially when the curvature of one band vanishes; i.e., the electron or hole mass diverges. This increases the strength of the divergence in the DOS at such a so-called extended VHS (EVHS), which can increase the critical temperature for superconductivity [3].Graphene's band structure has saddle points at the M point of the Brillouin zone (BZ) occurring around AE2 eV from the charge neutrality point at the Dirac energy E D [4]. Since superconductivity occurs in graphene-related systems such as graphite-intercalation compounds and nanotubes, it is interesting to explore whether superconductivity or other instability can appear in graphene itself due to the influence of this VHS.In this Letter, we present the band structure of highly doped graphene determined by angle-resolved photoemission spectroscopy (ARPES). We show that by chemically doping graphene on both sides, a much higher level of doping can be achieved than previously obtained. By this method, we can induce an electronic topological transition in graphene for the first time, whereby the Fermi energy E F is brought to the position of the saddle point VHS in graphene. We find that the electronic structure is strongly renormalized by the resulting DOS divergence such that the VHS has an extended, not pointlike, character [5,6]...

We present a scanning tunneling spectroscopy (STS) study of the local electronic structure of single and bilayer graphene grown epitaxially on a SiC(0001) surface. Low voltage topographic images reveal fine, atomic-scale carbon networks, whereas higher bias images are dominated by emergent spatially inhomogeneous large-scale structure similar to a carbon-rich reconstruction of SiC(0001). STS spectroscopy shows a ~100meV gap-like feature around zero bias for both monolayer and bilayer graphene/SiC, as well as significant spatial inhomogeneity in electronic structure above the gap edge. Nanoscale structure at the SiC/graphene interface is seen to correlate with observed electronic spatial inhomogeneity. These results are important for potential devices involving electronic transport or tunneling in graphene/SiC.

Recently, it was demonstrated that the quasiparticle dynamics, the layer-dependent charge and potential, and the c-axis screening coefficient could be extracted from measurements of the spectral function of few layer graphene films grown epitaxially on SiC using angle-resolved photoemission spectroscopy (ARPES). In this article we review these findings, and present detailed methodology for extracting such parameters from ARPES. We also present detailed arguments against the possibility of an energy gap at the Dirac crossing E D . Symmetry Breaking in FLG Films

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