93 research outputs found
Electrons scattering in the monolayer graphene with the short-range impurities
Scattering problem for electrons in monolayer graphene with short-range
perturbations of the types "local chemical potential" and "local gap" has been
solved. Zero gap and non-zero gap kinds of graphene are considered. The
determined S-matrix can be used for calculation of such observables as
conductance and optical absorption
Bound electron states in the monolayer graphene with short-range impurities
Bound electron states in impure graphene are considered. Short-range
perturbations for defect and impurities of the types "local chemical potential"
and "local gap" are taken into account.Comment: 3 figure
Strain Modulated Superlattices in Graphene
Strain engineering of graphene takes advantage of one of the most dramatic
responses of Dirac electrons enabling their manipulation via strain-induced
pseudo-magnetic fields. Numerous theoretically proposed devices, such as
resonant cavities and valley filters, as well as novel phenomena, such as snake
states, could potentially be enabled via this effect. These proposals, however,
require strong, spatially oscillating magnetic fields while to date only the
generation and effects of pseudo-gauge fields which vary at a length scale much
larger than the magnetic length have been reported. Here we create a periodic
pseudo-gauge field profile using periodic strain that varies at the length
scale comparable to the magnetic length and study its effects on Dirac
electrons. A periodic strain profile is achieved by pulling on graphene with
extreme (>10%) strain and forming nanoscale ripples, akin to a plastic wrap
pulled taut at its edges. Combining scanning tunneling microscopy and atomistic
calculations, we find that spatially oscillating strain results in a new
quantization different from the familiar Landau quantization observed in
previous studies. We also find that graphene ripples are characterized by large
variations in carbon-carbon bond length, directly impacting the electronic
coupling between atoms, which within a single ripple can be as different as in
two different materials. The result is a single graphene sheet that effectively
acts as an electronic superlattice. Our results thus also establish a novel
approach to synthesize an effective 2D lateral heterostructure - by periodic
modulation of lattice strain.Comment: 18 pages, 5 figures and supplementary informatio
Quantum Transport in Chemically-modified Two-Dimensional Graphene: From Minimal Conductivity to Anderson Localization
An efficient computational methodology is used to explore charge transport
properties in chemically-modified (and randomly disordered) graphene-based
materials. The Hamiltonians of various complex forms of graphene are
constructed using tight-binding models enriched by first-principles
calculations. These atomistic models are further implemented into a real-space
order-N Kubo-Greenwood approach, giving access to the main transport length
scales (mean free paths, localization lengths) as a function of defect density
and charge carrier energy. An extensive investigation is performed for epoxide
impurities with specific discussions on both the existence of a minimum
semi-classical conductivity and a crossover between weak to strong localization
regime. The 2D generalization of the Thouless relationship linking transport
length scales is here illustrated based on a realistic disorder model.Comment: 14 pages, 18 figures, submitte
Exact eigenstate analysis of finite-frequency conductivity in graphene
We employ the exact eigenstate basis formalism to study electrical
conductivity in graphene, in the presence of short-range diagonal disorder and
inter-valley scattering. We find that for disorder strength, 5, the
density of states is flat. We, then, make connection, using the MRG approach,
with the work of Abrahams \textit{et al.} and find a very good agreement for
disorder strength, = 5. For low disorder strength, = 2, we plot the
energy-resolved current matrix elements squared for different locations of the
Fermi energy from the band centre. We find that the states close to the band
centre are more extended and falls of nearly as as we move away
from the band centre. Further studies of current matrix elements versus
disorder strength suggests a cross-over from weakly localized to a very weakly
localized system. We calculate conductivity using Kubo Greenwood formula and
show that, for low disorder strength, conductivity is in a good qualitative
agreement with the experiments, even for the on-site disorder. The intensity
plots of the eigenstates also reveal clear signatures of puddle formation for
very small carrier concentration. We also make comparison with square lattice
and find that graphene is more easily localized when subject to disorder.Comment: 11 pages,15 figure
Anderson Localization of Thermal Phonons Leads to a Thermal Conductivity Maximum
Our elastic model of ErAs disordered GaAs/AlAs superlattices exhibits a local thermal conductivity maximum as a function of length due to exponentially suppressed Anderson-localized phonons. By analyzing the sample-to-sample fluctuations in the dimensionless conductance, g, the transition from diffusive to localized transport is identified as the crossover from the multichannel to single-channel transport regime g ≈ 1. Single parameter scaling is shown to hold in this crossover regime through the universality of the probability distribution of g that is independent of system size and disorder strength.Solid-State Solar-Thermal Energy Conversion Center (Award DE-FG02-09ER46577
Visualizing Individual Nitrogen Dopants in Monolayer Graphene
In monolayer graphene, substitutional doping during growth can be used to
alter its electronic properties. We used scanning tunneling microscopy (STM),
Raman spectroscopy, x-ray spectroscopy, and first principles calculations to
characterize individual nitrogen dopants in monolayer graphene grown on a
copper substrate. Individual nitrogen atoms were incorporated as graphitic
dopants, and a fraction of the extra electron on each nitrogen atom was
delocalized into the graphene lattice. The electronic structure of
nitrogen-doped graphene was strongly modified only within a few lattice
spacings of the site of the nitrogen dopant. These findings show that chemical
doping is a promising route to achieving high-quality graphene films with a
large carrier concentration.Comment: Science 201
Chemically-induced Mobility Gaps in Graphene Nanoribbons: A Route for Upscaling Device Performances
We report a first-principles based study of mesoscopic quantum transport in
chemically doped graphene nanoribbons with a width up to 10 nm. The occurrence
of quasibound states related to boron impurities results in mobility gaps as
large as 1 eV, driven by strong electron-hole asymmetrical backscattering
phenomena. This phenomenon opens new ways to overcome current limitations of
graphene-based devices through the fabrication of chemically-doped graphene
nanoribbons with sizes within the reach of conventional lithography.Comment: Nano Letters (in press
Transport length scales in disordered graphene-based materials: Strong localization regimes and dimensionality effects
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