302 research outputs found
Dispersion Properties, Nonlinear Waves and Birefringence in Classical Nonlinear Electrodynamics
Using the very basic physics principles, we have studied the implications of
quantum corrections to classical electrodynamics and the propagation of
electromagnetic waves and pulses.
The initial nonlinear wave equation for the electromagnetic vector potential
is solved perturbatively about the known exact plane wave solution in both the
free vacuum case, as well as when a constant magnetic field is applied. A
nonlinear wave equation with nonzero convective part for the (relatively)
slowly varying amplitude of the first-order perturbation has been derived. This
equation governs the propagation of electromagnetic waves with a reduced speed
of light, where the reduction is roughly proportional to the intensity of the
initial pumping plane wave. A system of coupled nonlinear wave equations for
the two slowly varying amplitudes of the first-order perturbation, which
describe the two polarization states, has been obtained for the case of
constant magnetic field background.
Further, the slowly varying wave amplitude behavior is shown to be similar to
that of a cnoidal wave, known to describe surface gravity waves in shallow
water. It has been demonstrated that the two wave modes describing the two
polarization states are independent, and they propagate at different wave
frequencies. This effect is usually called nonlinear birefringence.Comment: 11 pages, 3 figure
In memory of the âTwo Helmutsâ: the lives, legacies, and historical impact of Helmut Schmidt and Helmut Kohl: a forum with Clayton Clemens, Ronald Granieri, Mathias Haeussler, Mary Elise Sarotte, Kristina Spohr, and Christian Wicke
Between them, the chancellorships of the âtwo Helmutsâ span nearly a quarter-century of German history. Helmut Schmidt led the country from 1974 to 1982; his successor, Helmut Kohl, served until 1998. But the verdict on their respective tenures has been very different. Kohl was seen as a bumbling provincial when he came to office in 1982 but, by the end of his second term, he had won a place in the history books as the âChancellor of Unityâ (Einheitskanzler). By the time he lost the election for what would have been his fifth term, he was hailed as the âmaster-builderâ (Baumeister) of Europe for his decisive role in furthering the European Community's political and economic integration through the Maastricht Treaty and the introduction of the Euro. Schmidt, by contrast, came to office with a reputation for high administrative competence and intellectual prowess, but left the chancellery under a cloud. Der Spiegel spoke for many commentators when it dismissed him as a âgood chancellor with a bad recordâ; few features of his period in office stood out as âproof of success.â Schmidt, it was said, had been a mere crisis manager and âproblem-solverâ (Macher) who lacked broader vision, so that âlittle endured of historical significance.â This has also been the verdict of many historians
Supercooled confined water and the Mode Coupling crossover temperature
We present a Molecular Dynamics study of the single particle dynamics of
supercooled water confined in a silica pore. Two dynamical regimes are found:
close to the hydrophilic substrate molecules are below the Mode Coupling
crossover temperature, , already at ambient temperature. The water closer
to the center of the pore (free water) approaches upon supercooling as
predicted by Mode Coupling Theories. For free water the crossover temperature
and crossover exponent are extracted from power-law fits to both the
diffusion coefficient and the relaxation time of the late region.Comment: To be published, Phys. Rev. Lett., 4 pages, 3 figures, revTeX, minor
changes in the figures, references added, changes in the tex
Magneto-sensitive nickel nanowires fabricated by electrodeposition into multi- and single-ion track templates
Polycarbonate templates of (30±1) ÎŒm thickness containing cylindrical etched-track nanochannels of (500±50) nm diameter were used for electrodeposition of Ni nanowires. Using 104 channels per cm2, the most favourable deposition potential of âââ1.0 V was determined in a potentiostatic mode by varying the deposition potential with respect to an Ag/AgCl reference electrode over a range between âââ0.1 V and âââ1.5 V. The deposition efficiency at âââ1.0 V was estimated around 10%. The resulting single wires had a resistance around 200 Ω and showed an anisotropic magnetoresistance (AMR) effect of 1%, applicable to directionally sensitive magnetic field sensors
Notes towards a history of Khoi literature
This article puts forward a revisionist history of Khoi literature, and also presents a number of translated Khoi narratives that have not been available in English before. Compared to the large volume of Bushman literature and scholarship, there has been very little Khoi literature and engagement with it, and an argument is presented to account for this gap in South African cultural history. Until now, the major source of Khoi literature was Wilhelm Bleekâs Reynard the Fox in South Africa (1864), and this text is critically interrogated as a limiting version of Khoi orature. An alternative corpus of Khoi narratives is presented that was originally published in Leonard Schultzeâs Aus Namaland und Kalahari (1907).Web of Scienc
Single-neutron transfer from 11Be gs via the (p,d) reaction with a radioactive beam
The 11Be(p,d)10Be reaction has been performed in inverse kinematics with a
radioactive 11Be beam of E/A = 35.3 MeV. Angular distributions for the 0+
ground state, the 2+, 3.37 MeV state and the multiplet of states around 6 MeV
in 10Be were measured at angles up to 16 deg CM by detecting the 10Be in a
dispersion-matched spectrometer and the coincident deuterons in a silicon
array. Distorted wave and coupled-channels calculations have been performed to
investigate the amount of 2+ core excitation in 11Be gs. The use of "realistic"
11Be wave functions is emphasised and bound state form factors have been
obtained by solving the particle-vibration coupling equations. This calculation
gives a dominant 2s component in the 11Be gs wave function with a 16% [2+ x 1d]
core excitation admixture. Cross sections calculated with these form factors
are in good agreement with the present data. The Separation Energy prescription
for the bound state wave function also gives satisfactory fits to the data, but
leads to a significantly larger [2 x 1d] component in 11Be gs.Comment: 39 pages, 12 figures. Accepted for publication in Nuclear Physics A.
Added minor corrections made in proof to pages 26 and 3
Morphology of supported polymer electrolyte ultra-thin films: a numerical study
Morphology of polymer electrolytes membranes (PEM), e.g., Nafion, inside PEM
fuel cell catalyst layers has significant impact on the electrochemical
activity and transport phenomena that determine cell performance. In those
regions, Nafion can be found as an ultra-thin film, coating the catalyst and
the catalyst support surfaces. The impact of the hydrophilic/hydrophobic
character of these surfaces on the structural formation of the films has not
been sufficiently explored yet. Here, we report about Molecular Dynamics
simulation investigation of the substrate effects on the ionomer ultra-thin
film morphology at different hydration levels. We use a mean-field-like model
we introduced in previous publications for the interaction of the hydrated
Nafion ionomer with a substrate, characterized by a tunable degree of
hydrophilicity. We show that the affinity of the substrate with water plays a
crucial role in the molecular rearrangement of the ionomer film, resulting in
completely different morphologies. Detailed structural description in different
regions of the film shows evidences of strongly heterogeneous behavior. A
qualitative discussion of the implications of our observations on the PEMFC
catalyst layer performance is finally proposed
World distribution, diversity and endemism of aquatic macrophytes
To test the hitherto generally-accepted hypothesis that most aquatic macrophytes have broad world distributions, we investigated the global distribution, diversity and endemism patterns of 3457 macrophyte species that occur in permanent, temporary or ephemeral inland freshwater and brackish waterbodies worldwide. At a resolution of 10âŻĂâŻ10° latitude x longitude, most macrophyte species were found to have narrow global distributions: 78% have ranges (measured using an approach broadly following the IUCN-defined concept âextent of occurrenceâ) that individually occupy <10% of the world area present within the six global ecozones which primarily provide habitat for macrophytes. We found evidence of non-linear relationships between latitude and macrophyte α- and Îł-diversity, with diversity highest in sub-tropical to low tropical latitudes, declining slightly towards the Equator, and also declining strongly towards higher latitudes. Landscape aridity and, to a lesser extent, altitude and land area present per gridcell also influence macrophyte diversity and species assemblage worldwide. The Neotropics and Orient have the richest ecozone species-pools for macrophytes, depending on Îł-diversity metric used. The region around Brasilia/GoiĂĄs (Brazil: gridcell 10â20âŻÂ°S; 40â50âŻÂ°W) is the richest global hotspot for macrophyte α-diversity (total species α-diversity, ST: 625 species/gridcell, 350 of them Neotropical endemics). In contrast, the Sahara/Arabian Deserts, and some Arctic areas, have the lowest macrophyte α-diversity (ST <20 species/gridcell). At ecozone scale, macrophyte species endemism is pronounced, though with a>5-fold difference between the most species-rich (Neotropics) and species-poor (Palaearctic) ecozones. Our findings strongly support the assertion that small-ranged species constitute most of Earthâs species diversity
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