875 research outputs found
Moderne broederij en hygiene
Bij het ontwerp van een hedendaagse broederij dient men rekening te houden met strenge voorwaarden op het gebied van hygiëne, energieverbruik en arbeid. Om een zo gunstig mogelijke broedomgeving te creëren, moeten het gebouw, de machines, de automatiseringsapparatuur en de klimaatbeheersing aan verschillende eisen voldoen
Logarithmic Corrections in Dynamic Isotropic Percolation
Based on the field theoretic formulation of the general epidemic process we
study logarithmic corrections to scaling in dynamic isotropic percolation at
the upper critical dimension d=6. Employing renormalization group methods we
determine these corrections for some of the most interesting time dependent
observables in dynamic percolation at the critical point up to and including
the next to leading correction. For clusters emanating from a local seed at the
origin we calculate the number of active sites, the survival probability as
well as the radius of gyration.Comment: 9 pages, 3 figures, version to appear in Phys. Rev.
The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved
Two mutually exclusive mechanisms have been proposed to explain the improved electron injection by the insertion of a LiF layer between the metal cathode and the active organic layer of organic photoelectronic devices: the dipole and the doping mechanism. The possibility of the doping mechanism was studied by investigating the interface of poly[2-methoxy-5-(3',7'dimethyl-octyloxyl)-1,4-phenylenevinylene] (MDMO-PPV) or 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6,6]C-61 (PCBM) with Al, LiF, or Al/LiF. In this mechanism, Li dopes the organic layer, after liberation via the reaction Al+3LiF-->AlF3+3Li. If this reaction takes place, AlF3 should be detectable at the surface. However, SIMS measurements showed that AlF3 is not present at the Al/LiF/MDMO-PPV and Al/LiF/PCBM interfaces. This is evidence that the proposed reaction does not occur. Other evidence that the doping mechanism cannot be the general mechanism to explain the enhanced electron injection comes from the presence of LiF on both organic surfaces. XPS measurements indicate that there is a reaction of Al with the carboxylic oxygen of PCBM, and that a LiF layer between PCBM and Al prevents this reaction. (C) 2002 American Institute of Physic
Multifractality at the spin quantum Hall transition
Statistical properties of critical wave functions at the spin quantum Hall
transition are studied both numerically and analytically (via mapping onto the
classical percolation). It is shown that the index characterizing the
decay of wave function correlations is equal to 1/4, at variance with the
decay of the diffusion propagator. The multifractality spectra of
eigenfunctions and of two-point conductances are found to be
close-to-parabolic, and .Comment: 4 pages, 3 figure
Granular packings with moving side walls
The effects of movement of the side walls of a confined granular packing are
studied by discrete element, molecular dynamics simulations. The dynamical
evolution of the stress is studied as a function of wall movement both in the
direction of gravity as well as opposite to it. For all wall velocities
explored, the stress in the final state of the system after wall movement is
fundamentally different from the original state obtained by pouring particles
into the container and letting them settle under the influence of gravity. The
original packing possesses a hydrostatic-like region at the top of the
container which crosses over to a depth-independent stress. As the walls are
moved in the direction opposite to gravity, the saturation stress first reaches
a minimum value independent of the wall velocity, then increases to a
steady-state value dependent on the wall-velocity. After wall movement ceases
and the packing reaches equilibrium, the stress profile fits the classic
Janssen form for high wall velocities, while it has some deviations for low
wall velocities. The wall movement greatly increases the number of
particle-wall and particle-particle forces at the Coulomb criterion. Varying
the wall velocity has only small effects on the particle structure of the final
packing so long as the walls travel a similar distance.Comment: 11 pages, 10 figures, some figures in colo
Role of correlated two-pion exchange in scattering
A dynamical model for S-- and P--wave correlated (and )
exchange between a kaon and a nucleon is presented, starting from corresponding
amplitudes in the pseudophysical region, which
have been constructed from nucleon, --isobar and hyperon (,
) exchange Born terms and a realistic meson exchange model of the and amplitude. The
contribution in the s--channel is then obtained by performing a dispersion
relation over the unitarity cut. In the --channel, considerable
ambiguities exist, depending on how the dispersion integral is performed. Our
model, supplemented by short range interaction terms, is able to describe
empirical data below pion production threshold in a satisfactory way.Comment: 24 pages, REVTEX, figures available from the author
Composite vertices that lead to soft form factors
The momentum-space cut-off parameter of hadronic vertex functions
is studied in this paper. We use a composite model where we can measure the
contributions of intermediate particle propagations to . We show that
in many cases a composite vertex function has a much smaller cut-off than its
constituent vertices, particularly when light constituents such as pions are
present in the intermediate state. This suggests that composite
meson-baryon-baryon vertex functions are rather soft, i.e., they have \Lambda
considerably less than 1 GeV. We discuss the origin of this softening of form
factors as well as the implications of our findings on the modeling of nuclear
reactions.Comment: REVTex, 19 pages, 5 figs(to be provided on request
On-line monitoring van transpiratie en fotosynthese: de praktijk
WUR Glastuinbouw heeft monitoren ontwikkeld voor de on-line bepaling van de verdamping en fotosynthese. Deze monitoren zijn uitgetest bij diverse vruchtgroentetelers en slatelers. In het onderzoek is ook een nieuwe transpiratiemonitor ontwikkeld, op basis van een statische energie- en vochtbalan
Reaction-controlled diffusion: Monte Carlo simulations
We study the coupled two-species non-equilibrium reaction-controlled
diffusion model introduced by Trimper et al. [Phys. Rev. E 62, 6071 (2000)] by
means of detailed Monte Carlo simulations in one and two dimensions. Particles
of type A may independently hop to an adjacent lattice site provided it is
occupied by at least one B particle. The B particle species undergoes
diffusion-limited reactions. In an active state with nonzero, essentially
homogeneous B particle saturation density, the A species displays normal
diffusion. In an inactive, absorbing phase with exponentially decaying B
density, the A particles become localized. In situations with algebraic decay
rho_B(t) ~ t^{-alpha_B}, as occuring either at a non-equilibrium continuous
phase transition separating active and absorbing states, or in a power-law
inactive phase, the A particles propagate subdiffusively with mean-square
displacement ~ t^{1-alpha_A}. We find that within the accuracy of
our simulation data, \alpha_A = \alpha_B as predicted by a simple mean-field
approach. This remains true even in the presence of strong spatio-temporal
fluctuations of the B density. However, in contrast with the mean-field
results, our data yield a distinctly non-Gaussian A particle displacement
distribution n_A(x,t) that obeys dynamic scaling and looks remarkably similar
for the different processes investigated here. Fluctuations of effective
diffusion rates cause a marked enhancement of n_A(x,t) at low displacements
|x|, indicating a considerable fraction of practically localized A particles,
as well as at large traversed distances.Comment: Revtex, 19 pages, 27 eps figures include
A detailed study of giant pulses from PSR B1937-1-21 using the Large European Array for Pulsars
Contains fulltext :
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