1,459 research outputs found
Magnetism of Covalently Functionalized Carbon Nanotubes
We investigate the electronic structure of carbon nanotubes functionalized by
adsorbates anchored with single C-C covalent bonds. We find that, despite the
particular adsorbate, a spin moment with a universal value of 1.0 per
molecule is induced at low coverage. Therefore, we propose a mechanism of
bonding-induced magnetism at the carbon surface. The adsorption of a single
molecule creates a dispersionless defect state at the Fermi energy, which is
mainly localized in the carbon wall and presents a small contribution from the
adsorbate. This universal spin moment is fairly independent of the coverage as
long as all the molecules occupy the same graphenic sublattice. The magnetic
coupling between adsorbates is also studied and reveals a key dependence on the
graphenic sublattice adsorption site.Comment: final version, improved discussion about calculations and defect
concentratio
Magnetism of Substitutional Co Impurities in Graphene: Realization of Single -Vacancies
We report {\it ab initio} calculations of the structural, electronic and
magnetic properties of a graphene monolayer substitutionally doped with Co
(Co) atoms. We focus in Co because among traditional ferromagnetic
elements (Fe, Co and Ni), only Co atoms induce spin-polarization in
graphene. Our results show the complex magnetism of Co substitutional impurites
in graphene, which is mapped into simple models such as the -vacancy and
Heisenberg model. The links established in our work can be used to bring into
contact the engineering of nanostructures with the results of -models in
defective graphene. In principle, the structures considered here can be
fabricated using electron irradiation or Ar ion bombardment to create
defects and depositing Co at the same time
First-principles study of the atomic and electronic structure of the Si(111)-(5x2-Au surface reconstruction
We present a systematic study of the atomic and electronic structure of the
Si(111)-(5x2)-Au reconstruction using first-principles electronic structure
calculations based on the density functional theory. We analyze the structural
models proposed by Marks and Plass [Phys. Rev. Lett.75, 2172 (1995)], those
proposed recently by Erwin [Phys. Rev. Lett.91, 206101 (2003)], and a
completely new structure that was found during our structural optimizations. We
study in detail the energetics and the structural and electronic properties of
the different models. For the two most stable models, we also calculate the
change in the surface energy as a function of the content of silicon adatoms
for a realistic range of concentrations. Our new model is the energetically
most favorable in the range of low adatom concentrations, while Erwin's "5x2"
model becomes favorable for larger adatom concentrations. The crossing between
the surface energies of both structures is found close to 1/2 adatoms per 5x2
unit cell, i.e. near the maximum adatom coverage observed in the experiments.
Both models, the new structure and Erwin's "5x2" model, seem to provide a good
description of many of the available experimental data, particularly of the
angle-resolved photoemission measurements
Effect of electron and hole doping on the structure of C, Si, and S nanowires
We use ab initio density functional calculations to study the effect of
electron and hole doping on the equilibrium geometry and electronic structure
of C, Si, and S monatomic wires. Independent of doping, all these nanowires are
found to be metallic. In absence of doping, C wires are straight, whereas Si
and S wires display a zigzag structure. Besides two preferred bond angles of 60
deg and 120 deg in Si wires, we find an additional metastable bond angle of 90
deg in S wires. The equilibrium geometry and electronic structure of these
nanowires is shown to change drastically upon electron and hole doping.Comment: 5 pages including 5 figure
Magnetic field induced transition in a wide parabolic well superimposed with superlattice
We study a parabolic quantum wells (PQW) with
square superlattice. The magnetotransport in PQW with
intentionally disordered short-period superlattice reveals a surprising
transition from electrons distribution over whole parabolic well to
independent-layer states with unequal density. The transition occurs in the
perpendicular magnetic field at Landau filling factor and is
signaled by the appearance of the strong and developing fractional quantum Hall
(FQH) states and by the enhanced slope of the Hall resistance. We attribute the
transition to the possible electron localization in the x-y plane inside the
lateral wells, and formation of the FQH states in the central well of the
superlattice, driven by electron-electron interaction.Comment: 5 pages, 4 figure
Density waves and star formation in grand design spirals
HII regions in the arms of spiral galaxies are indicators of recent
star-forming processes. They may have been caused by the passage of the density
wave or simply created by other means near the arms. The study of these regions
may give us clues to clarifying the controversy over the existence of a
triggering scenario, as proposed in the density wave theory. Using H
direct imaging, we characterize the HII regions from a sample of three grand
design galaxies: NGC5457, NGC628 and NGC6946. Broad band images in R and I were
used to determine the position of the arms. The HII regions found to be
associated with arms were selected for the study. The age and the star
formation rate of these HII regions was obtained using measures on the
H line. The distance between the current position of the selected HII
regions and the position they would have if they had been created in the centre
of the arm is calculated. A parameter, T, which measures whether a region was
created in the arm or in the disc, is defined. With the help of the T parameter
we determine that the majority of regions were formed some time after the
passage of the density wave, with the regions located `behind the arm' (in the
direction of the rotation of the galaxy) the zone they should have occupied had
they been formed in the centre of the arm. The presence of the large number of
regions created after the passage of the arm may be explained by the effect of
the density wave, which helps to create the star-forming regions after its
passage. There is clear evidence of triggering for NGC5457 and a co-rotation
radius is proposed. A more modest triggering seems to exist for NGC628 and non
significant evidence of triggering are found for NGC6946.Comment: 10 pages, 20 figures, accepted for publication in A&
Crossover between distinct mechanisms of microwave photoresistance in bilayer systems
We report on temperature-dependent magnetoresistance measurements in balanced
double quantum wells exposed to microwave irradiation for various frequencies.
We have found that the resistance oscillations are described by the
microwave-induced modification of electron distribution function limited by
inelastic scattering (inelastic mechanism), up to a temperature of T*~4 K. With
increasing temperature, a strong deviation of the oscillation amplitudes from
the behavior predicted by this mechanism is observed, presumably indicating a
crossover to another mechanism of microwave photoresistance, with similar
frequency dependence. Our analysis shows that this deviation cannot be fully
understood in terms of contribution from the mechanisms discussed in theory.Comment: 7 pages, 4 figure
Role of metal-dependent regulation of ESX-3 secretion in intracellular survival of Mycobacterium tuberculosis
More people die every year from Mycobacterium tuberculosis infection than from infection by any other bacterial pathogen. Type VII secretion systems (T7SS) are used by both environmental and pathogenic mycobacteria to secrete proteins across their complex cell envelope. In the nonpathogen Mycobacterium smegmatis, the ESX-1 T7SS plays a role in conjugation, and the ESX-3 T7SS is involved in metal homeostasis. In M. tuberculosis, these secretion systems have taken on roles in virulence, and they also are targets of the host immune response. ESX-3 secretes a heterodimer composed of EsxG (TB9.8) and EsxH (TB10.4), which impairs phagosome maturation in macrophages and is essential for virulence in mice. Given the importance of EsxG and EsxH during infection, we examined their regulation. With M. tuberculosis, the secretion of EsxG and EsxH was regulated in response to iron and zinc, in accordance with the previously described transcriptional response of the esx-3 locus to these metals. While iron regulated the esx-3 expression in both M. tuberculosis and M. smegmatis, there is a significant difference in the dynamics of this regulation. In M. smegmatis, the esx-3 locus behaved like other iron-regulated genes such as mbtB. In M. tuberculosis, both iron and zinc modestly repressed esx-3 expression. Diminished secretion of EsxG and EsxH in response to these metals altered the interaction of M. tuberculosis with macrophages, leading to impaired intracellular M. tuberculosis survival. Our findings detail the regulatory differences of esx-3 in M. tuberculosis and M. smegmatis and demonstrate the importance of metal-dependent regulation of ESX-3 for virulence in M. tuberculosis
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