6,210 research outputs found
Sub-milliarcsec-scale structure of the gravitational lens B1600+434
In the gravitational lens system B1600+434 the brighter image, A, is known to
show rapid variability which is not detected in the weaker image, B (Koopmans &
de Bruyn 2000). Since correlated variability is one of the fundamental
properties of gravitational lensing, it has been proposed that image A is
microlensed by stars in the halo of the lensing galaxy (Koopmans & de Bruyn
2000). We present VLBA observations of B1600+434 at 15 GHz with a resolution of
0.5 milliarcsec to determine the source structure at high spatial resolution.
The surface brightness of the images are significantly different, with image A
being more compact. This is in apparent contradiction with the required
property of gravitational lensing that surface brightness be preserved. Our
results suggest that both the lensed images may show two-sided elongation at
this resolution, a morphology which does not necessarily favour superluminal
motion. Instead these data may suggest that image B is scatter-broadened at the
lens so that its size is larger than that of A, and hence scintillates less
than image A.Comment: 4 pages, 2 figures, accepted in AA Letter
NDICEA as a user friendly model tool for crop rotation planning in organic farming
For organic farming systems, the challenge is to become more specific in practices to maintain high standards in sustainability. Soil processes need to be clearly understood if rotations and manure applications are to become more precise. Simulation models like the NDICEA model help in the design and maintenance of these farming systems. These models play a key-role in the design of organic precision farming.
The NDICEA model has been calibrated for a number of long-term crop rotation experiments. Recently, the model was validated using research data from more than 35 organic farms all over the country. The model is used to calculate soil-specific mineralization rates in precision applications. In a new easy-to-use application, it was developed to design crop rotations and evaluate performance of crop rotations. This application is used to evaluate the sustainability of farming systems
Significance of interface anisotropy in laser induced magnetization precession in ferromagnetic metal films
Laser induced ultrafast demagnetization in ferromagnetic metals was
discovered almost 20 years ago, but currently there is still lack of consensus
on the microscopic mechanism responsible for the corresponding transfer of
angular momentum and energy between electron, lattice and spin subsystems. A
distinct, but intrinsically correlated phenomenon occurring on a longer
timescale is the magnetization precession after the ultrafast demagnetization
process, if a magnetic field is applied to tilt the magnetization vector away
from its easy direction, which can be attributed to the change of anisotropy
after laser heating. In an in-plane magnetized Pt/Co/Pt thin film with
perpendicular interface anisotropy, we found excellent agreement between
theoretical prediction with plausible parameters and experimental data measured
using time resolved magneto-optical Kerr effect. This agreement confirms that
the time evolution of the anisotropy field, which is driven by the interaction
between electrons and phonons, determines the magnetization precession
completely. A detailed analysis shows that, even though the whole sample is
magnetized in-plane, the dynamic interface anisotropy field dictates the
initial phase of the magnetization precession, highlighting the significance of
the interface anisotropy field in laser induced magnetization precession.Comment: 11 pages, 2 figure
Increasing weaning age of piglets from 4 to 7 weeks reduces stress, increases post-weaning feed intake but does not improve intestinal functionality
This study tested the hypothesis that late weaning and the availability of creep feed during the suckling period compared with early weaning, improves feed intake, decreases stress and improves the integrity of the intestinal tract. In this study with 160 piglets of 16 litters, late weaning at 7 weeks of age was compared with early weaning at 4 weeks, with or without creep feeding during the suckling period, on post-weaning feed intake, plasma cortisol (as an indicator of stress) and plasma intestinal fatty acid binding protein (I-FABP; a marker for mild intestinal injury) concentrations, intestinal morphology, intestinal (macro)molecular permeability and intestinal fluid absorption as indicators of small intestinal integrity. Post-weaning feed intake was similar in piglets weaned at 4 weeks and offered creep feed or not, but higher (P <0.001) in piglets weaned at 7 weeks with a higher (P <0.05) intake for piglets offered creep feed compared with piglets from whom creep feed was witheld. Plasma cortisol response at the day of weaning was lower in piglets weaned at 7 weeks compared with piglets weaned at 4 weeks, and creep feed did not affect cortisol concentration. Plasma I-FABP concentration was not affected by the age of weaning and creep feeding. Intestinal (macro)molecular permeability was not affected by the age of weaning and creep feeding. Both in uninfected and enterotoxigenic Escherichia coli-infected small intestinal segments net fluid absorption was not affected by the age of weaning or creep feeding. Creep feeding, but not the age of weaning, resulted in higher villi and increased crypt depth. In conclusion, weaning at 7 weeks of age in combination with creep feeding improves post-weaning feed intake and reduces weaning stress but does not improve functional characteristics of the small intestinal mucos
Absorption and generation of femtosecond laser-pulse excited spin currents in non-collinear magnetic bilayers
Spin currents can be generated on an ultrafast timescale by excitation of a
ferromagnetic (FM) thin film with a femtosecond laser-pulse. Recently, it has
been demonstrated that these ultrafast spin currents can transport angular
momentum to neighbouring FM layers, being able to change both the magnitude and
orientation of the magnetization in the adjacent layer. In this work, both the
generation and absorption of these optically excited spin currents are
investigated. This is done using non-collinear magnetic bilayers, i.e. two FM
layers separated by a conductive spacer. Spin currents are generated in a Co/Ni
multilayer with out-of-plane (OOP) anisotropy, and absorbed by a Co layer with
an in-plane (IP) anisotropy. This behaviour is confirmed by careful analysis of
the laser-pulse induced magnetization dynamics, whereafter it is demonstrated
that the transverse spin current is absorbed very locally near the injection
interface of the IP layer (90% within the first approx. 2 nm). Moreover, it
will also be shown that this local absorption results in the excitation of THz
standing spin waves within the IP layer. The dispersion measured for these high
frequency spin waves shows a discrepancy with respect to the theoretical
predictions, for which a first explanation involving intermixed interface
regions is proposed. Lastly, the spin current generation is investigated using
different number of repeats for the Co/Ni multilayer, which proves to be of
great relevance for identifying the optical spin current generation mechanism
Controlling skyrmion bubble confinement by dipolar interactions
Large skyrmion bubbles in confined geometries of various sizes and shapes are
investigated, typically in the range of several micrometers. Two fundamentally
different cases are studied to address the role of dipole-dipole interactions:
(I) when there is no magnetic material present outside the small geometries and
(II) when the geometries are embedded in films with a uniform magnetization. It
is found that the preferential position of the skyrmion bubbles can be
controlled by the geometrical shape, which turns out to be a stronger influence
than local variations in material parameters. In addition, independent
switching of the direction of the magnetization outside the small geometries
can be used to further manipulate these preferential positions, in particular
with respect to the edges. We show by numerical calculations that the observed
interactions between the skyrmion bubbles and structure edge including the
overall positioning of the bubbles are fully controlled by dipole-dipole
interactions
Tunable chiral spin texture in magnetic domain-walls
Magnetic domain-walls (DWs) with a preferred chirality exhibit very efficient
current-driven motion. Since structural inversion asymmetry (SIA) is required
for their stability, the observation of chiral domain walls in highly symmetric
Pt/Co/Pt is intriguing. Here, we tune the layer asymmetry in this system and
observe, by current-assisted DW depinning experiments, a small chiral field
which sensitively changes. Moreover, we convincingly link the observed
efficiency of DW motion to the DW texture, using DW resistance as a direct
probe for the internal orientation of the DW under the influence of in-plane
fields. The very delicate effect of capping layer thickness on the chiral field
allows for its accurate control, which is important in designing novel
materials for optimal spin-orbit-torque-driven DW motion.Comment: 12 pages, 5 figure
Femtosecond Demagnetization and Hot Hole Relaxation in Ferromagnetic GaMnAs
We have studied ultrafast photoinduced demagnetization in GaMnAs via
two-color time-resolved magneto-optical Kerr spectroscopy. Below-bandgap
midinfrared pump pulses strongly excite the valence band, while near-infrared
probe pulses reveal sub-picosecond demagnetization that is followed by an
ultrafast (1 ps) partial recovery of the Kerr signal. Through comparison
with InMnAs, we attribute the signal recovery to an ultrafast energy relaxation
of holes. We propose that the dynamical polarization of holes through -
scattering is the source of the observed probe signal. These results support
the physical picture of femtosecond demagnetization proposed earlier for
InMnAs, identifying the critical roles of both energy and spin relaxation of
hot holes.Comment: 7 pages, 6 figure
The initial mass function of early-type galaxies
We determine an absolute calibration of the initial mass function (IMF) of
early-type galaxies, by studying a sample of 56 gravitational lenses identified
by the SLACS Survey. Under the assumption of standard Navarro, Frenk & White
dark matter halos, a combination of lensing, dynamical, and stellar population
synthesis models is used to disentangle the stellar and dark matter
contribution for each lens. We define an "IMF mismatch" parameter
\alpha=M*(L+D)/M*(SPS) as the ratio of stellar mass inferred by a joint lensing
and dynamical models (M*(L+D)) to the current stellar mass inferred from
stellar populations synthesis models (M*(SPS)). We find that a Salpeter IMF
provides stellar masses in agreement with those inferred by lensing and
dynamical models (=0.00+-0.03+-0.02), while a Chabrier IMF
underestimates them (=0.25+-0.03+-0.02). A tentative trend is
found, in the sense that \alpha appears to increase with galaxy velocity
dispersion. Taken at face value, this result would imply a non universal IMF,
perhaps dependent on metallicity, age, or abundance ratios of the stellar
populations. Alternatively, the observed trend may imply non-universal dark
matter halos with inner density slope increasing with velocity dispersion.
While the degeneracy between the two interpretations cannot be broken without
additional information, the data imply that massive early-type galaxies cannot
have both a universal IMF and universal dark matter halos.Comment: 10 pages 4 figures. Resubmitted to ApJ taking into account referee's
comment
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