34 research outputs found
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
Quasiparticle Transformation During a Metal-Insulator Transition in Graphene
Here we show, with simultaneous transport and photoemission measurements,
that the graphene terminated SiC(0001) surface undergoes a metal-insulator
transition (MIT) upon dosingwith small amounts of atomic hydrogen. We find the
room temperature resistance increases by about 4 orders of magnitude, a
transition accompanied by anomalies in the momentum-resolved spectral function
including a non-Fermi Liquid behaviour and a breakdown of the quasiparticle
picture. These effects are discussed in terms of a possible transition to a
strongly (Anderson) localized ground state.Comment: 11 pages, 4 figure
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
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
Scanning tunneling spectroscopy of inhomogeneous electronic structure in monolayer and bilayer graphene on SiC
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.Comment: Acknowledgment added. 11 pages, 3 figure
Controlling the balance between remote, pinhole, and van der Waals epitaxy of Heusler films on graphene/sapphire
Remote epitaxy on monolayer graphene is promising for synthesis of highly
lattice mismatched materials, exfoliation of free-standing membranes, and
re-use of expensive substrates. However, clear experimental evidence of a
remote mechanism remains elusive. In many cases, due to contaminants at the
transferred graphene/substrate interface, alternative mechanisms such as
pinhole-seeded lateral epitaxy or van der Waals epitaxy can explain the
resulting exfoliatable single-crystalline films. Here, we find that growth of
the Heusler compound GdPtSb on clean graphene on sapphire substrates produces a
30 degree rotated epitaxial superstructure that cannot be explained by pinhole
or van der Waals epitaxy. With decreasing growth temperature the volume
fraction of this 30 degree domain increases compared to the direct epitaxial 0
degree domain, which we attribute to slower surface diffusion at low
temperature that favors remote epitaxy, compared to faster surface diffusion at
high temperature that favors pinhole epitaxy. We further show that careful
graphene/substrate annealing () and consideration of the
film/substrate vs film/graphene lattice mismatch are required to obtain epitaxy
to the underlying substrate for a variety of other Heusler films, including
LaPtSb and GdAuGe. The 30 degree rotated superstructure provides a possible
experimental fingerprint of remote epitaxy since it is inconsistent with the
leading alternative mechanisms