69 research outputs found
Interlayer interaction and electronic screening in multilayer graphene
The unusual transport properties of graphene are the direct consequence of a
peculiar bandstructure near the Dirac point. We determine the shape of the pi
bands and their characteristic splitting, and the transition from a pure 2D to
quasi-2D behavior for 1 to 4 layers of graphene by angle-resolved
photoemission. By exploiting the sensitivity of the pi bands to the electronic
potential, we derive the layer-dependent carrier concentration, screening
length and strength of interlayer interaction by comparison with tight binding
calculations, yielding a comprehensive description of multilayer graphene's
electronic structure
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
Experimental Determination of the Spectral Function of Graphene
A number of interesting properties of graphene and graphite are postulated to
derive from the peculiar bandstructure of graphene. This bandstructure consists
of conical electron and hole pockets that meet at a single point in momentum
(k) space--the Dirac crossing, at energy . Direct
investigations of the accuracy of this bandstructure, the validity of the
quasiparticle picture, and the influence of many-body interactions on the
electronic structure have not been addressed for pure graphene by experiment to
date. Using angle resolved photoelectron spectroscopy (ARPES), we find that the
expected conical bands are distorted by strong electron-electron,
electron-phonon, and electron-plasmon coupling effects. The band velocity at
and the Dirac crossing energy are both renormalized by these
many-body interactions, in analogy with mass renormalization by electron-boson
coupling in ordinary metals. These results are of importance not only for
graphene but also graphite and carbon nanotubes which have similar
bandstructures.Comment: pdf file, 10 pages, 4 figure
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
Van Hove Singularity and Apparent Anisotropy in the Electron-Phonon Interaction in Graphene
We show that the electron-phonon coupling strength obtained from the slopes
of the electronic energy vs. wavevector dispersion relations, as often done in
analyzing angle-resolved photoemission data, can differ substantially from the
actual electron-phonon coupling strength due to the curvature of the bare
electronic bands. This effect becomes particularly important when the Fermi
level is close to a van Hove singularity. By performing {\it ab initio}
calculations on doped graphene we demonstrate that, while the apparent strength
obtained from the slopes of experimental photoemission data is highly
anisotropic, the angular dependence of the actual electron-phonon coupling
strength in this material is negligible.Comment: 5 pages 4 figure
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