84 research outputs found
Empirical distributions of galactic spin parameters from the SDSS
Using simple dimensional arguments for both spiral and elliptical galaxies,
we present formulas to derive an estimate of the halo spin parameter
for any real galaxy, in terms of common observational parameters. This allows a
rough estimate of , which we apply to a large volume limited sample of
galaxies taken from the SDSS data base. The large numbers involved (11,597)
allow the derivation of reliable distributions, as signal adds up
significantly in spite of the errors in the inferences for particular galaxies.
We find that if the observed distribution of is modeled with a
log-normal function, as often done for this distribution in dark matter halos
that appear in cosmological simulations, we obtain parameters and , interestingly consistent with
values derived from simulations. For spirals, we find a good correlation
between empirical values of and visually assigned Hubble types,
highlighting the potential of this physical parameter as an objective
classification tool.Comment: 8 pages, 6 figures, expanded final version, MNRAS (in press
Edge-functionalized and substitutional doped graphene nanoribbons: electronic and spin properties
Graphene nanoribbons are the counterpart of carbon nanotubes in
graphene-based nanoelectronics. We investigate the electronic properties of
chemically modified ribbons by means of density functional theory. We observe
that chemical modifications of zigzag ribbons can break the spin degeneracy.
This promotes the onset of a semiconducting-metal transition, or of an
half-semiconducting state, with the two spin channels having a different
bandgap, or of a spin-polarized half-semiconducting state -where the spins in
the valence and conduction bands are oppositely polarized. Edge
functionalization of armchair ribbons gives electronic states a few eV away
from the Fermi level, and does not significantly affect their bandgap. N and B
produce different effects, depending on the position of the substitutional
site. In particular, edge substitutions at low density do not significantly
alter the bandgap, while bulk substitution promotes the onset of
semiconducting-metal transitions. Pyridine-like defects induce a
semiconducting-metal transition.Comment: 12 pages, 5 figure
Interplay between edge states and simple bulk defects in graphene nanoribbons
We study the interplay between the edge states and a single impurity in a
zigzag graphene nanoribbon. We use tight-binding exact diagonalization
techniques, as well as density functional theory calculations to obtain the
eigenvalue spectrum, the eigenfunctions, as well the dependence of the local
density of states (LDOS) on energy and position. We note that roughly half of
the unperturbed eigenstates in the spectrum of the finite-size ribbon hybridize
with the impurity state, and the corresponding eigenvalues are shifted with
respect to their unperturbed values. The maximum shift and hybridization occur
for a state whose energy is inverse proportional to the impurity potential;
this energy is that of the impurity peak in the DOS spectrum. We find that the
interference between the impurity and the edge gives rise to peculiar
modifications of the LDOS of the nanoribbon, in particular to oscillations of
the edge LDOS. These effects depend on the size of the system, and decay with
the distance between the edge and the impurity.Comment: 10 pages, 15 figures, revtex
DNA nucleotide-specific modulation of \mu A transverse edge currents through a metallic graphene nanoribbon with a nanopore
We propose two-terminal devices for DNA sequencing which consist of a
metallic graphene nanoribbon with zigzag edges (ZGNR) and a nanopore in its
interior through which the DNA molecule is translocated. Using the
nonequilibrium Green functions combined with density functional theory, we
demonstrate that each of the four DNA nucleotides inserted into the nanopore,
whose edge carbon atoms are passivated by either hydrogen or nitrogen, will
lead to a unique change in the device conductance. Unlike other recent
biosensors based on transverse electronic transport through DNA nucleotides,
which utilize small (of the order of pA) tunneling current across a nanogap or
a nanopore yielding a poor signal-to-noise ratio, our device concept relies on
the fact that in ZGNRs local current density is peaked around the edges so that
drilling a nanopore away from the edges will not diminish the conductance.
Inserting a DNA nucleotide into the nanopore affects the charge density in the
surrounding area, thereby modulating edge conduction currents whose magnitude
is of the order of \mu A at bias voltage ~ 0.1 V. The proposed biosensor is not
limited to ZGNRs and it could be realized with other nanowires supporting
transverse edge currents, such as chiral GNRs or wires made of two-dimensional
topological insulators.Comment: 6 pages, 6 figures, PDFLaTe
Electronic Structures of Porous Nanocarbons
We use large scale ab-initio calculations to describe electronic structures
of graphene, graphene nanoribbons, and carbon nanotubes periodically perforated
with nanopores. We disclose common features of these systems and develop a
unified picture that permits us to analytically predict and systematically
characterize metal-semiconductor transitions in nanocarbons with superlattices
of nanopores of different sizes and types. These novel materials with highly
tunable band structures have numerous potential applications in electronics,
light detection, and molecular sensing.Comment: 7 pages, 8 figure
Electronic and transport properties of boron and nitrogen doped graphene nanoribbons: an ab initio approach
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
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
Defect symmetry influence on electronic transport of zigzag nanoribbons
The electronic transport of zigzag-edged graphene nanoribbon (ZGNR) with local Stone-Wales (SW) defects is systematically investigated by first principles calculations. While both symmetric and asymmetric SW defects give rise to complete electron backscattering region, the well-defined parity of the wave functions in symmetric SW defects configuration is preserved. Its signs are changed for the highest-occupied electronic states, leading to the absence of the first conducting plateau. The wave function of asymmetric SW configuration is very similar to that of the pristine GNR, except for the defective regions. Unexpectedly, calculations predict that the asymmetric SW defects are more favorable to electronic transport than the symmetric defects configuration. These distinct transport behaviors are caused by the different couplings between the conducting subbands influenced by wave function alterations around the charge neutrality point
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