4,440 research outputs found
Packing Fractions and Maximum Angles of Stability of Granular Materials
In two-dimensional rotating drum experiments, we find two separate influences
of the packing fraction of a granular heap on its stability. For a fixed grain
shape, the stability increases with packing fraction. However, in determining
the relative stability of different grain shapes, those with the lowest average
packing fractions tend to form the most stable heaps. We also show that only
the configuration close to the surface of the pile figures prominently.Comment: 4 pages, 4 figure
Ballistic magnetoresistance in nickel single-atom conductors
Large ballistic magnetoresistance (BMR) has been measured in Ni single-atom
conductors electrodeposited between microfabricated thin films. These
measurements irrefutably eliminate any magnetostriction related artifacts in
the BMR effect.Comment: 12 pages, 3 Figure
Broad boron sheets and boron nanotubes: An ab initio study of structural, electronic, and mechanical properties
Based on a numerical ab initio study, we discuss a structure model for a
broad boron sheet, which is the analog of a single graphite sheet, and the
precursor of boron nanotubes. The sheet has linear chains of sp hybridized
sigma bonds lying only along its armchair direction, a high stiffness, and
anisotropic bonds properties. The puckering of the sheet is explained as a
mechanism to stabilize the sp sigma bonds. The anisotropic bond properties of
the boron sheet lead to a two-dimensional reference lattice structure, which is
rectangular rather than triangular. As a consequence the chiral angles of
related boron nanotubes range from 0 to 90 degrees. Given the electronic
properties of the boron sheets, we demonstrate that all of the related boron
nanotubes are metallic, irrespective of their radius and chiral angle, and we
also postulate the existence of helical currents in ideal chiral nanotubes.
Furthermore, we show that the strain energy of boron nanotubes will depend on
their radii, as well as on their chiral angles. This is a rather unique
property among nanotubular systems, and it could be the basis of a different
type of structure control within nanotechnology.Comment: 16 pages, 17 figures, 2 tables, Versions: v1=preview, v2=first final,
v3=minor corrections, v4=document slightly reworke
Excitons and Many-Electron Effects in the Optical Response of Single-Walled Boron Nitride Nanotubes
We report first-principles calculations of the effects of quasiparticle
self-energy and electron-hole interaction on the optical properties of
single-walled BN nanotubes. Excitonic effects are shown to be even more
important in BN nanotubes than in carbon nanotubes. Electron-hole interactions
give rise to complexes of bright (and dark) excitons, which qualitatively alter
the optical response. Excitons with binding energy larger than 2 eV are found
in the (8,0) BN nanotubes. Moreover, unlike the carbon nanotubes, theory
predicts that these exciton states are comprised of coherent supposition of
transitions from several different subband pairs, giving rise to novel
behaviors.Comment: 4 pages, 4 figure
Noninvasive Embedding of Single Co Atoms in Ge(111)2x1 Surfaces
We report on a combined scanning tunneling microscopy (STM) and density
functional theory (DFT) based investigation of Co atoms on Ge(111)2x1 surfaces.
When deposited on cold surfaces, individual Co atoms have a limited diffusivity
on the atomically flat areas and apparently reside on top of the upper
pi-bonded chain rows exclusively. Voltage-dependent STM imaging reveals a
highly anisotropic electronic perturbation of the Ge surface surrounding these
Co atoms and pronounced one-dimensional confinement along the pi-bonded chains.
DFT calculations reveal that the individual Co atoms are in fact embedded in
the Ge surface, where they occupy a quasi-stationary position within the big
7-member Ge ring in between the 3rd and 4th atomic Ge layer. The energy needed
for the Co atoms to overcome the potential barrier for penetration in the Ge
surface is provided by the kinetic energy resulting from the deposition
process. DFT calculations further demonstrate that the embedded Co atoms form
four covalent Co-Ge bonds, resulting in a Co4+ valence state and a 3d5
electronic configuration. Calculated STM images are in perfect agreement with
the experimental atomic resolution STM images for the broad range of applied
tunneling voltages.Comment: 19 pages, 15 figures, 3 table
Graphitic-BN Based Metal-free Molecular Magnets From A First Principle Study
We perform a first principle calculation on the electronic properties of
carbon doped graphitic boron nitride graphitic BN. It was found that carbon
substitution for either boron or nitrogen atom in graphitic BN can induce
spontaneous magnetization. Calculations based on density functional theory with
the local spin density approximation on the electronic band structure revealed
a spin polarized, dispersionless band near the Fermi energy. Spin density
contours showed that the magnetization density originates from the carbon atom.
The magnetization can be attributed to the carbon 2p electron. Charge density
distribution shows that the carbon atom forms covalent bonds with its three
nearest neighbourhood. The spontaneous magnetization survives the curvature
effect in BN nanotubes, suggesting the possibility of molecular magnets made
from BN. Compared to other theoretical models of light-element or metal-free
magnetic materials, the carbon-doped BN are more experimentally accessible and
can be potentially useful.Comment: 8 pages, 4 figure
Resynthesized Brassica juncea lines with novel organellar genome constitution obtained through protoplast fusion
This article does not have an abstract
Development of insect-resistant transgenic cabbage plants expressing a synthetic cryIA(b) gene from Bacillus thuringiensis
A synthetic cryIA(b) gene coding for an insecticidal crystal protein of Bacillus thuringiensis (Bt) was transferred to cabbage cultivar 'Golden Acre' by co-cultivating hypocotyl explants with Agrobacterium tumefaciens. Transformed plants resistant to kanamycin were regenerated. Hybridization experiments demonstrated gene integration and mRNA expression. Immunoblot analysis revealed high-level expression of Bt toxin protein in the transgenic plants. The expression resulted in a significant insecticidal activity of transgenic cabbage plants against the larvae of diamondback moth (Plutella xylostella). The results also demonstrated that a synthetic gene based on monocot codon usage can be expressed in dicotyledonous plants for insect control
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