13,774 research outputs found
Main-streaming participatory and cross-disciplinary approaches in animal science research in developing countries
Conventional research approaches have lost considerable momentum after their astonishing achieve-ments during the green revolution. The negative side of focusing rigorously on production improve-ment was eminent around 1980 and led to considerations of environmental, gender and equity aspects - making agricultural development much more complex than previously. In the search for new ways of addressing the persisting problems of food insecurity and malnutrition, new ways should be explored. Based on the experiences from three international, African research projects, the article argues the case of participatory action research and cross-disciplinarity as some of the key elements in future animal science research in developing countries. The benefits are outlined as well as the challenges for the researchers and the donor agencies
Shell structure and electron-electron interaction in self-assembled InAs quantum dots
Using far-infrared spectroscopy, we investigate the excitations of
self-organized InAs quantum dots as a function of the electron number per dot,
1<n<6, which is monitored in situ by capacitance spectroscopy. Whereas the
well-known two-mode spectrum is observed when the lowest s - states are filled,
we find a rich excitation spectrum for n=3, which reflects the importance of
electron-electron interaction in the present, strongly non-parabolic confining
potential. From capacitance spectroscopy we find that the electronic shell
structure in our dots gives rise to a distinct pattern in the charging energies
which strongly deviates from the monotonic behavior of the Coulomb blockade
found in mesoscopic or metallic structures.Comment: 4 pages, 3 PostScript figure
Far-infrared absorption in parallel quantum wires with weak tunneling
We study collective and single-particle intersubband excitations in a system
of quantum wires coupled via weak tunneling. For an isolated wire with
parabolic confinement, the Kohn's theorem guarantees that the absorption
spectrum represents a single sharp peak centered at the frequency given by the
bare confining potential. We show that the effect of weak tunneling between two
parabolic quantum wires is twofold: (i) additional peaks corresponding to
single-particle excitations appear in the absorption spectrum, and (ii) the
main absorption peak acquires a depolarization shift. We also show that the
interplay between tunneling and weak perpendicular magnetic field drastically
enhances the dispersion of single-particle excitations. The latter leads to a
strong damping of the intersubband plasmon for magnetic fields exceeding a
critical value.Comment: 18 pages + 6 postcript figure
Size effects in the magnetic behaviour of TbAl_2 milled alloys
The study of the magnetic properties depending upon mechanical milling of the
ferromagnetic polycrystalline TbAl_2 material is reported. The Rietveld
analysis of the X-ray diffraction data reveals a decrease of the grain size
down to 14 nm and -0.15 % of variation of the lattice parameter, after 300
hours of milling time. Irreversibility in the zero field cooled - field cooled
(ZFC-FC) DC-susceptibility and clear peaks in the AC susceptibility between 5
and 300 K show that the long-range ferromagnetic structure is inhibited in
favour of a disordered spin arrangement below 45 K. This glassy behaviour is
also deduced from the variation of the irreversibility transition with the
field (H^{2/3}) and frequency. The magnetization process of the bulk TbAl_2 is
governed by domain wall thermal activation processes. By contrast, in the
milled samples, cluster-glass properties arise as a result of cooperative
interactions due to the substitutional disorder. The interactions are also
influenced by the nanograin structure of the milled alloys, showing a variation
of coercivity with the grain size, below the crossover between the multi- and
single-domain behaviours.Comment: 23 pages, 11 figures, to appear in J. Phys.: Condens. Ma
Reduction of nickel oxide particles by hydrogen studied in an environmental TEM
In situ reduction of nickel oxide (NiO) particles is performed under 1.3mbar of hydrogen gas (H2) in an environmental transmission electron microscope (ETEM). Images, diffraction patterns and electron energy-loss spectra (EELS) are acquired to monitor the structural and chemical evolution of the system during reduction, whilst increasing the temperature. Ni nucleation on NiO is either observed to be epitaxial or to involve the formation of randomly oriented grains. The growth of Ni crystallites and the movement of interfaces result in the formation of pores within the NiO grains to accommodate the volume shrinkage associated with the reduction. Densification is then observed when the sample is nearly fully reduced. The reaction kinetics is obtained using EELS by monitoring changes in the shapes of the Ni L2,3 white lines. The activation energy for NiO reduction is calculated from the EELS data using both a physical model-fitting technique and a model-independent method. The results of the model-fitting procedure suggest that the reaction is described by Avrami models (whereby the growth and impingement of Ni domains control the reaction), in agreement with the ETEM observation
Time-Lapse Acoustic Imaging of Mesoscale and Fine-Scale Variability within the Faroe-Shetland Channel
We describe and analyze the results of a threeâdimensional seismic (i.e. acoustic) reflection survey from the FaroeâShetland Channel that is calibrated with nearâcoincident hydrographic and satellite observations. 54 vertical seismic transects were acquired over a period of 25 days. On each transect, a 250ââ400 m band of reflections is observed within the water column. Hydrographic measurements demonstrate that this reflective band is caused by temperature variations within the pycnocline that separates warm, nearâsurface waters of Atlantic origin from cold, deep waters which flow southward from the Nordic Seas. Tilting of reflective surfaces records geostrophic shear between these nearâsurface and deep waters. Measurements of temporal changes of pycnoclinic depth and of reflection tilt are used to infer the existence of an anticyclonic vortex that advects northeastward. Comparison with satellite measurements of seaâsurface temperature and height suggests that this vortex is caused by meandering of the Continental Slope Current. A model of a Gaussian vortex is used to match seismic and satellite observations. This putative vortex has a core radius of 20â30 km and a maximum azimuthal velocity of 0.3ââ0.4 m sâ1. It translates at 0.01ââ0.1 m sâ1. Within the pycnocline, diapycnal diffusivity, K , is estimaed by analyzing the turbulent spectral subrange of tracked reflections. K varies between 10â5.7 and 10â5.0 m 2 sâ1 in a pattern that is broadly consistent with translation of the vortex. Our integrated study demonstrates the ability of timeâlapse seismic reflection surveying to dynamically resolve the effects that mesoscale activity has upon deep thermohaline structure on scales from meters to hundreds of kilometers.Natural Environment Research Council (NERC)
Engineering and Physical Science Research Council
794 Program Grant EP/K034529/
Absorption Cross Section of Scalar Field in Supergravity Background
It has recently been shown that the equation of motion of a massless scalar
field in the background of some specific p branes can be reduced to a modified
Mathieu equation. In the following the absorption rate of the scalar by a D3
brane in ten dimensions is calculated in terms of modified Mathieu functions of
the first kind, using standard Mathieu coefficients. The relation of the latter
to Dougall coefficients (used by others) is investigated. The S-matrix obtained
in terms of modified Mathieu functions of the first kind is easily evaluated if
known rapidly convergent low energy expansions of these in terms of products of
Bessel functions are used. Leading order terms, including the interesting
logarithmic contributions, can be obtained analytically.Comment: latex, 42 page
Absence of a Finite-Temperature Melting Transition in the Classical Two-Dimensional One-Component Plasma
Vortices in thin-film superconductors are often modelled as a system of
particles interacting via a repulsive logarithmic potential. Arguments are
presented to show that the hypothetical (Abrikosov) crystalline state for such
particles is unstable at any finite temperature against proliferation of
screened disclinations. The correlation length of crystalline order is
predicted to grow as as the temperature is reduced to zero, in
excellent agreement with our simulations of this two-dimensional system.Comment: 3 figure
Optimal Investment in the Development of Oil and Gas Field
Let an oil and gas field consists of clusters in each of which an investor
can launch at most one project. During the implementation of a particular
project, all characteristics are known, including annual production volumes,
necessary investment volumes, and profit. The total amount of investments that
the investor spends on developing the field during the entire planning period
we know. It is required to determine which projects to implement in each
cluster so that, within the total amount of investments, the profit for the
entire planning period is maximum.
The problem under consideration is NP-hard. However, it is solved by dynamic
programming with pseudopolynomial time complexity. Nevertheless, in practice,
there are additional constraints that do not allow solving the problem with
acceptable accuracy at a reasonable time. Such restrictions, in particular, are
annual production volumes. In this paper, we considered only the upper
constraints that are dictated by the pipeline capacity. For the investment
optimization problem with such additional restrictions, we obtain qualitative
results, propose an approximate algorithm, and investigate its properties.
Based on the results of a numerical experiment, we conclude that the developed
algorithm builds a solution close (in terms of the objective function) to the
optimal one
From Spinor Geometry to Complex General Relativity
An attempt is made of giving a self-contained (although incomplete)
introduction to holomorphic ideas in general relativity, following work over
the last thirty years by several authors. The main topics are complex
manifolds, two-component spinor calculus, conformal gravity, alpha-planes in
Minkowski space-time, alpha-surfaces and twistor geometry, anti-self-dual
space-times and Penrose transform, spin-3/2 potentials, heaven spaces and
heavenly equations.Comment: With kind permission from Springer Science and Business Media to use
material in the first 5 sections taken from the 1995 Kluwer book "Complex
General Relativity" by G. Esposito. In the revised version, 11 References
have been adde
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