339 research outputs found
Origins of anomalous electronic structures of epitaxial graphene on silicon carbide
On the basis of first-principles calculations, we report that a novel
interfacial atomic structure occurs between graphene and the surface of silicon
carbide, destroying the Dirac point of graphene and opening a substantial
energy gap there. In the calculated atomic structures, a quasi-periodic
domain pattern emerges out of a larger commensurate
periodic interfacial reconstruction,
resolving a long standing experimental controversy on the periodicity of the
interfacial superstructures. Our theoretical energy spectrum shows a gap and
midgap states at the Dirac point of graphene, which are in excellent agreement
with the recently-observed anomalous angle-resolved photoemission spectra.
Beyond solving unexplained issues of epitaxial graphene, our atomistic study
may provide a way to engineer the energy gaps of graphene on substrates.Comment: Additional references added; published version; 4 pages, 4 figure
Massive enhancement of electron-phonon coupling in doped graphene by an electronic singularity
The nature of the coupling leading to superconductivity in layered materials
such as high-Tc superconductors and graphite intercalation compounds (GICs) is
still unresolved. In both systems, interactions of electrons with either
phonons or other electrons or both have been proposed to explain
superconductivity. In the high-Tc cuprates, the presence of a Van Hove
singularity (VHS) in the density of states near the Fermi level was long ago
proposed to enhance the many-body couplings and therefore may play a role in
superconductivity. Such a singularity can cause an anisotropic variation in the
coupling strength, which may partially explain the so-called nodal-antinodal
dichotomy in the cuprates. Here we show that the topology of the graphene band
structure at dopings comparable to the GICs is quite similar to that of the
cuprates and that the quasiparticle dynamics in graphene have a similar
dichotomy. Namely, the electron-phonon coupling is highly anisotropic,
diverging near a saddle point in the graphene electronic band structure. These
results support the important role of the VHS in layered materials and the
possible optimization of Tc by tuning the VHS with respect to the Fermi level.Comment: 8 page
Morphology of graphene thin film growth on SiC(0001)
Epitaxial films of graphene on SiC(0001) are interesting from a basic physics
as well as applications-oriented point of view. Here we study the emerging
morphology of in-vacuo prepared graphene films using low energy electron
microscopy (LEEM) and angle-resolved photoemission (ARPES). We obtain an
identification of single and bilayer of graphene film by comparing the
characteristic features in electron reflectivity spectra in LEEM to the PI-band
structure as revealed by ARPES. We demonstrate that LEEM serves as a tool to
accurately determine the local extent of graphene layers as well as the layer
thickness
Electron states of mono- and bilayer graphene on SiC probed by STM
We present a scanning tunneling microscopy (STM) study of a
gently-graphitized 6H-SiC(0001) surface in ultra high vacuum. From an analysis
of atomic scale images, we identify two different kinds of terraces, which we
unambiguously attribute to mono- and bilayer graphene capping a C-rich
interface. At low temperature, both terraces show
quantum interferences generated by static impurities. Such interferences are a
fingerprint of -like states close to the Fermi level. We conclude that the
metallic states of the first graphene layer are almost unperturbed by the
underlying interface, in agreement with recent photoemission experiments (A.
Bostwick et al., Nature Physics 3, 36 (2007))Comment: 4 pages, 3 figures submitte
Electron-Phonon Coupling in Highly-Screened Graphene
Photoemission studies of graphene have resulted in a long-standing
controversy concerning the strength of the experimental electron-phonon
interaction in comparison with theoretical calculations. Using high-resolution
angle-resolved photoemission spectroscopy we study graphene grown on a copper
substrate, where the metallic screening of the substrate substantially reduces
the electron-electron interaction, simplifying the comparison of the
electron-phonon interaction between theory and experiment. By taking the
nonlinear bare bandstructure into account, we are able to show that the
strength of the electron-phonon interaction does indeed agree with theoretical
calculations. In addition, we observe a significant bandgap at the Dirac point
of graphene.Comment: Submitted to Phys. Rev. Lett. on July 20, 201
Large area quasi-free standing monolayer graphene on 3C-SiC(111)
Large scale, homogeneous quasi-free standing monolayer graphene is obtained
on cubic silicon carbide, i.e. the 3C-SiC(111) surface, which represents an
appealing and cost effective platform for graphene growth. The quasi-free
monolayer is produced by intercalation of hydrogen under the interfacial,
(6root3x6root3)R30-reconstructed carbon layer. After intercalation, angle
resolved photoemission spectroscopy (ARPES) reveals sharp linear pi-bands. The
decoupling of graphene from the substrate is identified by X-ray photoemission
spectroscopy (XPS) and low energy electron diffraction (LEED). Atomic force
microscopy (AFM) and low energy electron microscopy (LEEM) demonstrate that
homogeneous monolayer domains extend over areas of hundreds of
square-micrometers.Comment: 4 pages, 3 figures, Copyright (2011) American Institute of Physics.
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Revealing the atomic structure of the buffer layer between SiC(0001) and epitaxial graphene
On the SiC(0001) surface (the silicon face of SiC), epitaxial graphene is
obtained by sublimation of Si from the substrate. The graphene film is
separated from the bulk by a carbon-rich interface layer (hereafter called the
buffer layer) which in part covalently binds to the substrate. Its structural
and electronic properties are currently under debate. In the present work we
report scanning tunneling microscopy (STM) studies of the buffer layer and of
quasi-free-standing monolayer graphene (QFMLG) that is obtained by decoupling
the buffer layer from the SiC(0001) substrate by means of hydrogen
intercalation. Atomic resolution STM images of the buffer layer reveal that,
within the periodic structural corrugation of this interfacial layer, the
arrangement of atoms is topologically identical to that of graphene. After
hydrogen intercalation, we show that the resulting QFMLG is relieved from the
periodic corrugation and presents no detectable defect sites
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