605 research outputs found
A second eigenvalue bound for the Dirichlet Schroedinger operator
Let be the th eigenvalue of the Schr\"odinger
operator with Dirichlet boundary conditions on a bounded domain and with the positive potential . Following the spirit of the
Payne-P\'olya-Weinberger conjecture and under some convexity assumptions on the
spherically rearranged potential , we prove that . Here denotes the ball, centered at the
origin, that satisfies the condition .
Further we prove under the same convexity assumptions on a spherically
symmetric potential , that decreases
when the radius of the ball increases.
We conclude with several results about the first two eigenvalues of the
Laplace operator with respect to a measure of Gaussian or inverted Gaussian
density
Dislocation constriction and cross-slip in Al and Ag: an ab initio study
A novel model based on the Peierls framework of dislocations is developed.
The new theory can deal with a dislocation spreading at more than one slip
planes. As an example, we study dislocation cross-slip and constriction process
of two fcc metals, Al and Ag. The energetic parameters entering the model are
determined from ab initio calculations. We find that the screw dislocation in
Al can cross-slip spontaneously in contrast with that in Ag, which splits into
partials and cannot cross-slip without first being constricted. The dislocation
response to an external stress is examined in detail. We determine dislocation
constriction energy and critical stress for cross-slip, and from the latter, we
estimate the cross-slip energy barrier for the straight screw dislocations
An Isoperimetric Inequality for Fundamental Tones of Free Plates
We establish an isoperimetric inequality for the fundamental tone (first
nonzero eigenvalue) of the free plate of a given area, proving the ball is
maximal. Given , the free plate eigenvalues and eigenfunctions
are determined by the equation
together with certain natural boundary conditions. The boundary conditions are
complicated but arise naturally from the plate Rayleigh quotient, which
contains a Hessian squared term . We adapt Weinberger's method from
the corresponding free membrane problem, taking the fundamental modes of the
unit ball as trial functions. These solutions are a linear combination of
Bessel and modified Bessel functions.Comment: PhD thesis. Papers are in preparatio
Isolation and preliminary characterization of the jaagsiekte retrovirus (JSRV)
Jaagsiekte, or ovine pulmonary adenomatosis, is caused by a recently discovered retrovirus. The virus cannot be cultivated in vitro at present, but a procedure is described for the isolation and purification of small amounts in the form of immune complexes with IgA from affected lungs. The virion was shown to possess a 70S RNA genome which can be transcribed by an endogenous reverse transcriptase. Nine polypeptides, ranging in
size from 94 000 to 25 000 daltons, were found in purified preparations. Using neutralization of the viral reverse transcriptase and an enzyme immunoassay as criteria, no serological relationship could be demonstrated to representatives of type B, C and C oncoviruses, or to bovine leukemia virus, maedi-visna virus of sheep or caprine arthritis-encephalitis virus.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi.
Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format
Exo-hydrogenated Single Wall Carbon Nanotubes
An extensive first-principles study of fully exo-hydrogenated zigzag (n,0)
and armchair (n,n) single wall carbon nanotubes (CH), polyhedral
molecules including cubane, dodecahedrane, and CH points to
crucial differences in the electronic and atomic structures relevant to
hydrogen storage and device applications. CH's are estimated to be
stable up to the radius of a (8,8) nanotube, with binding energies proportional
to 1/R. Attaching a single hydrogen to any nanotube is always exothermic.
Hydrogenation of zigzag nanotubes is found to be more likely than armchair
nanotubes with similar radius. Our findings may have important implications for
selective functionalization and finding a way of separating similar radius
nanotubes from each other.Comment: 5 pages, 4 postscript figures, Revtex file, To be appear in Physical
Review
Ab initio and finite-temperature molecular dynamics studies of lattice resistance in tantalum
This manuscript explores the apparent discrepancy between experimental data
and theoretical calculations of the lattice resistance of bcc tantalum. We
present the first results for the temperature dependence of the Peierls stress
in this system and the first ab initio calculation of the zero-temperature
Peierls stress to employ periodic boundary conditions, which are those best
suited to the study of metallic systems at the electron-structure level. Our ab
initio value for the Peierls stress is over five times larger than current
extrapolations of experimental lattice resistance to zero-temperature. Although
we do find that the common techniques for such extrapolation indeed tend to
underestimate the zero-temperature limit, the amount of the underestimation
which we observe is only 10-20%, leaving open the possibility that mechanisms
other than the simple Peierls stress are important in controlling the process
of low temperature slip.Comment: 12 pages and 9 figure
A retrospective evaluation of the impact of a dedicated obstetric and neonatal transport service on transport times within an urban setting
OBJECTIVE:To determine whether the establishment of a dedicated obstetric and neonatal flying squad resulted in improved performance within the setting of a major metropolitan area.DESIGN AND SETTING:The Cape Town metropolitan service of the Emergency Medical Services was selected for a retrospective review of the transit times for the newly implemented Flying Squad programme. Data were imported from the Computer Aided Dispatch programme. Dispatch, Response, Mean Transit and Total Pre-hospital times relating to the obstetric and neonatal incidents was analysed for 2005 and 2008. RESULTS: There was a significant improvement between 2005 and 2008 in all incidents evaluated. Flying Squad dispatch performance improved from 11.7% to 46.6% of all incidents dispatched within 4 min (p < 0.0001). Response time performance at the 15-min threshold did not demonstrate a statistically significant improvement (p = 0.4), although the improvement in the 30-min performance category was statistically significant in both maternity and neonatal incidents. Maternity incidents displayed the greatest improvement with the 30-min performance increasing from 30.3% to 72.9%. The analysis of the mean transit times demonstrated that neonatal transfers displayed the longest status time in all but one of the categories. Even so, the introduction of the Flying Squad programme resulted in a reduction in a total pre-hospital time from 177 to 128 min. CONCLUSION: The introduction of the Flying Squad programme has resulted in significant improvement in the transit times of both neonatal and obstetric patients. In spite of the severe resource constraints facing developing nations, the model employed offers significant gains
Efficient Recursion Method for Inverting Overlap Matrix
A new O(N) algorithm based on a recursion method, in which the computational
effort is proportional to the number of atoms N, is presented for calculating
the inverse of an overlap matrix which is needed in electronic structure
calculations with the the non-orthogonal localized basis set. This efficient
inverting method can be incorporated in several O(N) methods for
diagonalization of a generalized secular equation. By studying convergence
properties of the 1-norm of an error matrix for diamond and fcc Al, this method
is compared to three other O(N) methods (the divide method, Taylor expansion
method, and Hotelling's method) with regard to computational accuracy and
efficiency within the density functional theory. The test calculations show
that the new method is about one-hundred times faster than the divide method in
computational time to achieve the same convergence for both diamond and fcc Al,
while the Taylor expansion method and Hotelling's method suffer from numerical
instabilities in most cases.Comment: 17 pages and 4 figure
Reversible Band Gap Engineering in Carbon Nanotubes by Radial Deformation
We present a systematic analysis of the effect of radial deformation on the
atomic and electronic structure of zigzag and armchair single wall carbon
nanotubes using the first principle plane wave method. The nanotubes were
deformed by applying a radial strain, which distorts the circular cross section
to an elliptical one. The atomic structure of the nanotubes under this strain
are fully optimized, and the electronic structure is calculated
self-consistently to determine the response of individual bands to the radial
deformation. The band gap of the insulating tube is closed and eventually an
insulator-metal transition sets in by the radial strain which is in the elastic
range. Using this property a multiple quantum well structure with tunable and
reversible electronic structure is formed on an individual nanotube and its
band-lineup is determined from first-principles. The elastic energy due to the
radial deformation and elastic constants are calculated and compared with
classical theories.Comment: To be appear in Phys. Rev. B, Apr 15, 200
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