50 research outputs found
Rashba splitting of 100 meV in Au-intercalated graphene on SiC
Intercalation of Au can produce giant Rashba-type spin-orbit splittings in
graphene but this has not yet been achieved on a semiconductor substrate. For
graphene/SiC(0001), Au intercalation yields two phases with different doping.
Here, we report the preparation of an almost pure p-type graphene phase after
Au intercalation. We observe a 100 meV Rashba-type spin-orbit splitting at 0.9
eV binding energy. We show that this giant splitting is due to hybridization
and much more limited in energy and momentum space than for Au-intercalated
graphene on Ni
The Origin of Doping in Quasi-Free Standing Graphene on Silicon Carbide
We explain the robust p-type doping observed for quasi-free standing graphene
on hexagonal silicon carbide by the spontaneous polarization of the substrate.
This mechanism is based on a bulk property of SiC, unavoidable for any
hexagonal polytype of the material and independent of any details of the
interface formation. We show that sign and magnitude of the polarization are in
perfect agreement with the doping level observed in the graphene layer. With
this mechanism, models based on hypothetical acceptor-type defects as they are
discussed so far are obsolete. The n-type doping of epitaxial graphene is
explained conventionally by donor-like states associated with the buffer layer
and its interface to the substrate which overcompensate the polarization
doping.Comment: 10 pages, 1 figur
The quasi-free-standing nature of graphene on H-saturated SiC(0001)
We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001)
which was prepared by intercalation of hydrogen under the buffer layer. Using
infrared absorption spectroscopy we prove that the SiC(0001) surface is
saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer
layer into graphene which exhibits a slight tensile strain and short range
defects. The layers are hole doped (p = 5.0-6.5 x 10^12 cm^(-2)) with a carrier
mobility of 3,100 cm^2/Vs at room temperature. Compared to graphene on the
buffer layer a strongly reduced temperature dependence of the mobility is
observed for graphene on H-terminated SiC(0001)which justifies the term
"quasi-free-standing".Comment: 3 pages, 3 figures, accepted for publication in Applied Physics
Letter
Visualizing Atomic-Scale Negative Differential Resistance in Bilayer Graphene
We investigate the atomic-scale tunneling characteristics of bilayer graphene on silicon carbide using the scanning tunneling microscopy. The high-resolution tunneling spectroscopy reveals an unexpected negative differential resistance (NDR) at the Dirac energy, which spatially varies within the single unit cell of bilayer graphene. The origin of NDR is explained by two near-gap van Hove singularities emerging in the electronic spectrum of bilayer graphene under a transverse electric field, which are strongly localized on two sublattices in different layers. Furthermore, defects near the tunneling contact are found to strongly impact on NDR through the electron interference. Our result provides an atomic-level understanding of quantum tunneling in bilayer graphene, and constitutes a useful step towards graphene-based tunneling devices. DOI: 10.1103/PhysRevLett.110.036804X11109sciescopu
Multi-component magneto-optical conductivity of multilayer graphene on SiC
Far-infrared diagonal and Hall conductivities of multilayer epitaxial
graphene on the C-face of SiC were measured using magneto-optical absorption
and Faraday rotation in magnetic fields up to 7 T and temperatures between 5
and 300 K. Multiple components are identified in the spectra, which include:
(i) a quasi-classical cyclotron resonance (CR), originating from the highly
doped graphene layer closest to SiC, (ii) transitions between low-index Landau
levels (LLs), which stem from weakly doped layers and (iii) a broad optical
absorption background. Electron and hole type LL transitions are optically
distinguished and shown to coexist. An electron-hole asymmetry of the Fermi
velocity of about 2% was found within one graphene layer, while the Fermi
velocity varies by about 10% across the layers. The optical intensity of the LL
transitions is several times smaller than what is theoretically expected for
isolated graphene monolayers without electron-electron and electron-phonon
interactions.Comment: 9 pages, 6 figure
Extremely flat band in bilayer graphene
We propose a novel mechanism of flat band formation based on the relative biasing of only one sublattice against other sublattices in a honeycomb lattice bilayer. The mechanism allows modification of the band dispersion from parabolic to "Mexican hat"-like through the formation of a flattened band. The mechanism is well applicable for bilayer graphene-both doped and undoped. By angle-resolved photoemission from bilayer graphene on SiC, we demonstrate the possibility of realizing this extremely flattened band (< 2-meV dispersion), which extends two-dimensionally in a k-space area around the K point and results in a disk-like constant energy cut. We argue that our two-dimensional flat band model and the experimental results have the potential to contribute to achieving superconductivity of graphene- or graphite-based systems at elevated temperatures
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