3,476 research outputs found
Grazing-angle scattering of electromagnetic waves in gratings with varying mean parameters: grating eigenmodes
A highly unusual pattern of strong multiple resonances for bulk
electromagnetic waves is predicted and analysed numerically in thick periodic
holographic gratings in a slab with the mean permittivity that is larger than
that of the surrounding media. This pattern is shown to exist in the geometry
of grazing-angle scattering (GAS), that is when the scattered wave (+1
diffracted order) in the slab propagates almost parallel to the slab (grating)
boundaries. The predicted resonances are demonstrated to be unrelated to
resonant generation of the conventional guided modes of the slab. Their
physical explanation is associated with resonant generation of a completely new
type of eigenmodes in a thick slab with a periodic grating. These new slab
eigenmodes are generically related to the grating; they do not exist if the
grating amplitude is zero. The field structure of these eigenmodes and their
dependence on structural and wave parameters is analysed. The results are
extended to the case of GAS of guided modes in a slab with a periodic groove
array of small corrugation amplitude and small variations in the mean thickness
of the slab at the array boundaries.Comment: 16 pages, 6 figure
Computational study of boron nitride nanotube synthesis: how catalyst morphology stabilizes the boron nitride bond
In an attempt to understand why catalytic methods for the growth of boron
nitride nanotubes work much worse than for their carbon counterparts, we use
first-principles calculations to study the energetics of elemental reactions
forming N2, B2 and BN molecules on an iron catalyst. We observe that in the
case of these small molecules, the catalytic activity is hindered by the
formation of B2 on the iron surface. We also observe that the local morphology
of a step edge present in our nanoparticle model stabilizes the boron nitride
molecule with respect to B2 due to the ability of the step edge to offer sites
with different coordination simultaneously for nitrogen and boron. Our results
emphasize the importance of atomic steps for a high yield chemical vapor
deposition growth of BN nanotubes and may outline new directions for improving
the efficiency of the method.Comment: submitted to physical review
Orientation of biological cells using plane-polarized Gaussian beam optical tweezers
Optical tweezers are widely used for the manipulation of cells and their
internal structures. However, the degree of manipulation possible is limited by
poor control over the orientation of trapped cells. We show that it is possible
to controllably align or rotate disc shaped cells - chloroplasts of Spinacia
oleracea - in a plane polarised Gaussian beam trap, using optical torques
resulting predominantly from circular polarisation induced in the transmitted
beam by the non-spherical shape of the cells.Comment: 9 pages, 6 figure
NIEL Dose Dependence for Solar Cells Irradiated with Electrons and Protons
The investigation of solar cells degradation and the prediction of its
end-of-life performance is of primary importance in the preparation of a space
mission. In the present work, we investigate the reduction of solar-cells'
maximum power resulting from irradiations with electrons and protons. Both GaAs
single junction and GaInP/GaAs/Ge triple junction solar cells were studied. The
results obtained indicate how i) the dominant radiation damaging mechanism is
due to atomic displacements, ii) the relative maximum power degradation is
almost independent of the type of incoming particle, i.e., iii) to a first
approximation, the fitted semi-empirical function expressing the decrease of
maximum power depends only on the absorbed NIEL dose, and iv) the actual
displacement threshold energy value (Ed=21 eV) accounts for annealing
treatments, mostly due to self-annealing induced effects. Thus, for a given
type of solar cell, a unique maximum power degradation curve can be determined
as a function of the absorbed NIEL dose. The latter expression allows one to
predict the performance of those solar cells in space radiation environment.Comment: To appear on the Proceedings of the 13th ICATPP Conference on
Astroparticle, Particle, Space Physics and Detectors for Physics
Applications, Villa Olmo (Como, Italy), 23--27 October, 2013, to be published
by World Scientific (Singapore
Forces from highly focused laser beams: modeling, measurement and application to refractive index measurements
The optical forces in optical tweezers can be robustly modeled over a broad
range of parameters using generalsed Lorenz-Mie theory. We describe the
procedure, and show how the combination of experimental measurement of
properties of the trap coupled with computational modeling, can allow unknown
parameters of the particle - in this case, the refractive index - to be
determined.Comment: 5 pages, 4 figures, presented at 17th AIP Congress, Brisbane, 200
Optical application and measurement of torque on microparticles of isotropic nonabsorbing material
We show how it is possible to controllably rotate or align microscopic
particles of isotropic nonabsorbing material in a TEM00 Gaussian beam trap,
with simultaneous measurement of the applied torque using purely optical means.
This is a simple and general method of rotation, requiring only that the
particle is elongated along one direction. Thus, this method can be used to
rotate or align a wide range of naturally occurring particles. The ability to
measure the applied torque enables the use of this method as a quantitative
tool--the rotational equivalent of optical tweezers based force measurement. As
well as being of particular value for the rotation of biological specimens,
this method is also suitable for the development of optically-driven
micromachines.Comment: 8 pages, 6 figure
Optical measurement of torque exerted on an elongated object by a non-circular laser beam
We have developed a scheme to measure the optical torque, exerted by a laser
beam on a phase object, by measuring the orbital angular momentum of the
transmitted beam. The experiment is a macroscopic simulation of a situation in
optical tweezers, as orbital angular momentum has been widely used to apply
torque to microscopic objects. A hologram designed to generate LG02 modes and a
CCD camera are used to detect the orbital component of the beam. Experimental
results agree with theoretical numerical calculations, and the strength of the
orbital component suggest its usefulness in optical tweezers for
micromanipulation.Comment: 6 pages, 7 figures, v2: minor typographical correction
Repeated or intermittent levosimendan treatment in advanced heart failure: An updated meta-analysis.
Abstract Introduction Advanced heart failure is a malignant disease characterized by a debilitating late course, with increasingly frequent hospitalisations and high rate of mortality. Levosimendan, an inodilator developed for the treatment of acutely decompensated chronic heart failure, has been recently proposed also as a repetitive treatment of advanced heart failure. Several studies on the use of levosimendan in this settings report mortality data. Independent meta-analyses on the effect on mortality of repetitive or intermittent levosimendan administration in advanced heart failure has been published but were criticized in regard to the selection of the studies. Meanwhile new data became available. We therefore updated the selection of studies and re-analyzed all the available data. Methods & results Data from seven randomized trial and a total of 438 adult patients using intermittent levosimendan in a cardiological setting were included in the present analysis. The average follow-up period was 8±3.8months. The use of levosimendan was associated with a significant reduction in mortality at the longest follow-up available [41 of 257 (16%) in the levosimendan group vs. 39 of 181 (21.5%) in the control arm, OR=0.54 (95% CI 0.32–0.91), p for effect=0.02, p for heterogeneity=0.64, I2=0%]. Conclusions The updated results suggest that repetitive or intermittent levosimendan administration in advanced heart failure is associated with a significant reduction in mortality at the longest follow-up available. There is therefore a strong rationale for a randomized clinical trial with adequate power on mortality
Induced superconductivity in noncuprate layers of the BiSrCaCuO high-temperature superconductor: Modeling of scanning tunneling spectra
We analyze how the coherence peaks observed in Scanning Tunneling
Spectroscopy (STS) of cuprate high temperature superconductors are transferred
from the cuprate layer to the oxide layers adjacent to the STS microscope tip.
For this purpose, we have carried out a realistic multiband calculation for the
superconducting state of BiSrCaCuO (Bi2212) assuming a
short range d-wave pairing interaction confined to the nearest-neighbor Cu
orbitals. The resulting anomalous matrix elements of the Green's
function allow us to monitor how pairing is then induced not only within the
cuprate bilayer but also within and across other layers and sites. The symmetry
properties of the various anomalous matrix elements and the related selection
rules are delineated.Comment: 9 pages, 2 figures. Accepted for publication in Phys. Rev.
Origin of electron-hole asymmetry in the scanning tunneling spectrum of
We have developed a material specific theoretical framework for modelling
scanning tunneling spectroscopy (STS) of high temperature superconducting
materials in the normal as well as the superconducting state. Results for
(Bi2212) show clearly that the tunneling process
strongly modifies the STS spectrum from the local density of states (LDOS) of
the orbital of Cu. The dominant tunneling channel to the surface
Bi involves the orbitals of the four neighbouring Cu atoms. In
accord with experimental observations, the computed spectrum displays a
remarkable asymmetry between the processes of electron injection and
extraction, which arises from contributions of Cu and other orbitals
to the tunneling current.Comment: 5 pages, 4 figures, published in PR
- …