118 research outputs found
Landau levels in deformed bilayer graphene at low magnetic fields
We review the effect of uniaxial strain on the low-energy electronic
dispersion and Landau level structure of bilayer graphene. Based on the
tight-binding approach, we derive a strain-induced term in the low-energy
Hamiltonian and show how strain affects the low-energy electronic band
structure. Depending on the magnitude and direction of applied strain, we
identify three regimes of qualitatively different electronic dispersions. We
also show that in a weak magnetic field, sufficient strain results in the
filling factor ff=+-4 being the most stable in the quantum Hall effect
measurement, instead of ff=+-8 in unperturbed bilayer at a weak magnetic field.
To mention, in one of the strain regimes, the activation gap at ff=+-4 is, down
to very low fields, weakly dependent on the strength of the magnetic field.Comment: 14 single-column pages, 5 figures, more details on material presented
in arXiv:1104.502
Strained bilayer graphene: Band structure topology and Landau level spectrum
We show that topology of the low-energy band structure in bilayer graphene
critically depends on mechanical deformations of the crystal which may easily
develop in suspended graphene flakes. We describe the Lifshitz transition that
takes place in strained bilayers upon splitting the parabollic bands at
intermediate energies into several Dirac cones at the energy scale of few meV.
Then, we show how this affects the electron Landau level spectra and the
quantum Hall effect.Comment: slightly over 4 pages, 3 figures, updated discussion and references;
almost identical to the published versio
Infrared absorption of closely-aligned heterostructures of monolayer and bilayer graphene with hexagonal boron nitride
We model optical absorption of monolayer and bilayer graphene on hexagonal
boron nitride for the case of closely-aligned crystal lattices. We show that
perturbations with different spatial symmetry can lead to similar absorption
spectra. We suggest that a study of the absorption spectra as a function of the
doping for almost completely full first miniband is necessary to extract
meaningful information about the moire characteristics from optical absorption
measurements and to distinguish between various theoretical proposals for the
physically realistic interaction. Also, for bilayer graphene, the ability to
compare spectra for the opposite signs of electric-field-induced interlayer
asymmetry might provide additional information about the moire parameters.Comment: 7 pages, 6 figures, minor changes, version accepted to PR
Band dispersion in the deep 1s core level of graphene
Chemical bonding in molecules and solids arises from the overlap of valence
electron wave functions, forming extended molecular orbitals and dispersing
Bloch states, respectively. Core electrons with high binding energies, on the
other hand, are localized to their respective atoms and their wave functions do
not overlap significantly. Here we report the observation of band formation and
considerable dispersion (up to 60 meV) in the core level of the carbon
atoms forming graphene, despite the high C binding energy of 284
eV. Due to a Young's double slit-like interference effect, a situation arises
in which only the bonding or only the anti-bonding states is observed for a
given photoemission geometry.Comment: 12 pages, 3 figures, including supplementary materia
Graphene-based modulation-doped superlattice structures
The electronic transport properties of graphene-based superlattice structures
are investigated. A graphene-based modulation-doped superlattice structure
geometry is proposed and consist of periodically arranged alternate layers:
InAs/graphene/GaAs/graphene/GaSb. Undoped graphene/GaAs/graphene structure
displays relatively high conductance and enhanced mobilities at elevated
temperatures unlike modulation-doped superlattice structure more steady and
less sensitive to temperature and robust electrical tunable control on the
screening length scale. Thermionic current density exhibits enhanced behaviour
due to presence of metallic (graphene) mono-layers in superlattice structure.
The proposed superlattice structure might become of great use for new types of
wide-band energy gap quantum devices.Comment: 5 figure
Transport Spectroscopy of Symmetry-Broken Insulating States in Bilayer Graphene
The flat bands in bilayer graphene(BLG) are sensitive to electric fields
E\bot directed between the layers, and magnify the electron-electron
interaction effects, thus making BLG an attractive platform for new
two-dimensional (2D) electron physics[1-5]. Theories[6-16] have suggested the
possibility of a variety of interesting broken symmetry states, some
characterized by spontaneous mass gaps, when the electron-density is at the
carrier neutrality point (CNP). The theoretically proposed gaps[6,7,10] in
bilayer graphene are analogous[17,18] to the masses generated by broken
symmetries in particle physics and give rise to large momentum-space Berry
curvatures[8,19] accompanied by spontaneous quantum Hall effects[7-9]. Though
recent experiments[20-23] have provided convincing evidence of strong
electronic correlations near the CNP in BLG, the presence of gaps is difficult
to establish because of the lack of direct spectroscopic measurements. Here we
present transport measurements in ultra-clean double-gated BLG, using
source-drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the
CNP. The gap can be closed by an electric field E\bot \sim13 mV/nm but
increases monotonically with a magnetic field B, with an apparent particle-hole
asymmetry above the gap, thus providing the first mapping of the ground states
in BLG.Comment: 4 figure
Anti-angiogenic effects of pterogynidine alkaloid isolated from Alchornea glandulosa
<p>Abstract</p> <p>Background</p> <p>Angiogenesis, a complex multistep process that comprehends proliferation, migration and anastomosis of endothelial cells (EC), has a major role in the development of pathologic conditions such as inflammatory diseases, tumor growth and metastasis. Brazilian flora, the most diverse in the world, is an interesting spot to prospect for new chemical leads, being an important source of new anticancer drugs. Plant-derived alkaloids have traditionally been of interest due to their pronounced physiological activities. We investigated the anti-angiogenic potential of the naturally occurring guanidine alkaloid pterogynidine (Pt) isolated from the Brazilian plant <it>Alchornea glandulosa</it>. The purpose of this study was to examine which features of the angiogenic process could be disturbed by Pt.</p> <p>Methods</p> <p>Human umbilical vein endothelial cells (HUVEC) were incubated with 8 μM Pt and cell viability, proliferation, apoptosis, invasion and capillary-like structures formation were addressed. Nuclear factor κB (NFκB), a transcription factor implicated in these processes, was also evaluated in HUVEC incubated with Pt. Quantifications were expressed as mean ± SD of five independent experiments and one-way analysis of variance (ANOVA) followed by the Dunnet test was used.</p> <p>Results</p> <p>A significant decrease in proliferation and invasion capacity and an effective increase in apoptosis as assessed by bromodeoxyuridine (BrdU), double-chamber and terminal transferase dUTP nick end labeling (TUNEL) assay, respectively, have been found. Pt also led to a drastic reduction in the number of capillary-like structures formation when HUVEC were cultured on growth factor reduced-Matrigel (GFR-Matrigel) coated plates. In addition, incubation of HUVEC with Pt resulted in reduced NFκB activity.</p> <p>Conclusion</p> <p>These findings emphasize the potential use of Pt against pathological situations where angiogenesis is stimulated as tumor development.</p
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