3,490 research outputs found
First-Principle Description of Correlation Effects in Layered Materials
We present a first-principles description of anisotropic materials
characterized by having both weak (dispersion-like) and strong covalent bonds,
based on the Adiabatic--Connection Fluctuation--Dissipation Theorem within
Density Functional Theory. For hexagonal boron nitride the in-plane and out of
plane bonding as well as vibrational dynamics are well described both at
equilibrium and when the layers are pulled apart. Also bonding in covalent and
ionic solids is described. The formalism allows to ping-down the deficiencies
of common exchange-correlation functionals and provides insight towards the
inclusion of dispersion interactions into the correlation functional.Comment: Accepted for publication in Physical Review Letter
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
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
Stability of antiphase line defects in nanometer-sized boron-nitride cones
We investigate the stability of boron nitride conical sheets of nanometer
size, using first-principles calculations. Our results indicate that cones with
an antiphase boundary (a line defect that contains either B-B or N-N bonds) can
be more stable than those without one. We also find that doping the antiphase
boundaries with carbon can enhance their stability, leading also to the
appearance of localized states in the bandgap. Among the structures we
considered, the one with the smallest formation energy is a cone with a
carbon-modified antiphase boundary that presents a spin splitting of about 0.5
eV at the Fermi level.Comment: 5 two-column pages with 2 figures Accepted for publication in
Physical Review B (vol 70, 15 Nov.
Infrared Studies of Some Selenocyanato Complexes of Oxozirconium(IV) with 0 and N Donor Ligands
A series of new complexes of oxozirconium(IV) selenocyanate
with different ligands has been prepared. The IR spectra (4000-200
cm-1) of free ligands and complexes indicate coordination through
either oxygen or nitrogen, depending on the type of the ligand
Early Results from the Wisconsin H-Alpha Mapper Southern Sky Survey
After a successful eleven-year campaign at Kitt Peak, we moved the Wisconsin
H-Alpha Mapper (WHAM) to Cerro Tololo in early 2009. Here we present some of
the early data after a few months under southern skies. These maps begin to
complete the first all-sky, kinematic survey of the diffuse H-alpha emission
from the Milky Way. Much of this emission arises from the Warm Ionized Medium
(WIM), a significant component of the ISM that extends a few kiloparsecs above
the Galactic disk. While this first look at the data focuses on the H-alpha
survey, WHAM is also capable of observing many other optical emission lines,
revealing fascinating trends in the temperature and ionization state of the
WIM. Our ongoing studies of the physical conditions of diffuse ionized gas will
continue from the southern hemisphere following the H-alpha survey. In
addition, future observations will cover the full velocity range of the
Magellanic Stream, Bridge, and Clouds to trace the ionized gas associated with
these neighboring systems.Comment: 4 pages, 2 figures. To appear in "The Dynamic ISM: A celebration of
the Canadian Galactic Plane Survey," ASP Conference Serie
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
Study - Mucosal and peri-orificial involvement in Post-Kala-Azar dermal leishmaniasis
BACKGROUND AND AIMS: Lesions of post-kala-azar dermal leishmaniasis
(PKDL) usually affect the skin. Uncommonly, the involvement of oral and
genital mucosae has been reported. METHODS: Twenty five patients
clinically diagnosed as post-kala-azar dermal leishmaniasis were
studied for periorificial and mucosal lesions. Clinical examination,
skin smears and biopsy were done for the patients with periorificial or
mucosal lesions. RESULTS: Out of 25 patients of PKDL, seven patients
had lesions on the oral and/or genital mucosa. Three cases had oral
lesions; two had only genital lesions and three patients had both sites
involved. All the patients were having skin lesions elsewhere too
either as nodules and/or plaque or macules. Conclusion: While
examining a case of PKDL, mucosal involvement must also be examined
carefully
Therapeutic trial of sodium antimony gluconate alone and in combination with ketoconazole in post-kala-azar dermal leishmaniasis
BACKGROUND: Drugs used in PKDL include parenteral sodium antimony
gluconate (SAG), amphotericin-B, pentamidine, and ketoconazole (KTZ).
SAG is the most effective one. Given alone, SAG has to be given for a
long duration, leading to poor patient compliance and treatment
failure. This study was carried out to compare the effectiness of SAG
alone and a combination of SAG and KTZ for sixty days. METHODS: Ten
patients of PKDL were included in the study. Five patients (Group A)
were given SAG intravenously, in the dose of 20 mg/kg per day and five
(Group B) were given SAG (intravenously 20 mg/kg per day) and KTZ (200
mg twice daily orally). Both treatment regimens were given for sixty
days. RESULTS: In Group A, the nodules and/or plaques showed
approximate 80-85% clinical improvement, and macules showed 25-30%
improvement. In group B (SAG + KTZ), there was 90-95% clinical
improvement in the nodules and/or plaques and 25-30% in macules.
CONCLUSION: This study suggests the therapeutic superiority of the
combination treatment regimen in a shorter duration but is not
conclusive as the number of patients was low. Further trials are
recommended
Isomorphs in model molecular liquids
Isomorphs are curves in the phase diagram along which a number of static and
dynamic quantities are invariant in reduced units. A liquid has good isomorphs
if and only if it is strongly correlating, i.e., the equilibrium
virial/potential energy fluctuations are more than 90% correlated in the NVT
ensemble. This paper generalizes isomorphs to liquids composed of rigid
molecules and study the isomorphs of two systems of small rigid molecules, the
asymmetric dumbbell model and the Lewis-Wahnstrom OTP model. In particular, for
both systems we find that the isochoric heat capacity, the excess entropy, the
reduced molecular center-of-mass self part of the intermediate scattering
function, the reduced molecular center-of-mass radial distribution function to
a good approximation are invariant along an isomorph. In agreement with theory,
we also find that an instantaneous change of temperature and density from an
equilibrated state point to another isomorphic state point leads to no
relaxation. The isomorphs of the Lewis-Wahnstrom OTP model were found to be
more approximative than those of the asymmetric dumbbell model, which is
consistent with the OTP model being less strongly correlating. For both models
we find "master isomorphs", i.e., isomorphs have identical shape in the
virial/potential energy phase diagram.Comment: 20 page
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