8,822 research outputs found
Band-steaming reduces laborious hand-weeding in vegetables
Band-steaming is a new method that may reduce the need for hand-weeding in demanding row crops like carrot and drilled onion. Band-steaming only affects a soil volume equal to the intra-row area of the subsequent crop, and effectively kills the weed seeds in this soil volume. Side-effects on beneficial soil organisms are minimized as compared to current steaming technology, but still need to be assessed
Quantifying admissible undersampling for sparsity-exploiting iterative image reconstruction in X-ray CT
Iterative image reconstruction (IIR) with sparsity-exploiting methods, such
as total variation (TV) minimization, investigated in compressive sensing (CS)
claim potentially large reductions in sampling requirements. Quantifying this
claim for computed tomography (CT) is non-trivial, because both full sampling
in the discrete-to-discrete imaging model and the reduction in sampling
admitted by sparsity-exploiting methods are ill-defined. The present article
proposes definitions of full sampling by introducing four sufficient-sampling
conditions (SSCs). The SSCs are based on the condition number of the system
matrix of a linear imaging model and address invertibility and stability. In
the example application of breast CT, the SSCs are used as reference points of
full sampling for quantifying the undersampling admitted by reconstruction
through TV-minimization. In numerical simulations, factors affecting admissible
undersampling are studied. Differences between few-view and few-detector bin
reconstruction as well as a relation between object sparsity and admitted
undersampling are quantified.Comment: Revised version that was submitted to IEEE Transactions on Medical
Imaging on 8/16/201
The fundamental plane of elliptical galaxies with modified Newtonian dynamics
The modified Newtonian dynamics (MOND), suggested by Milgrom as an
alternative to dark matter, implies that isothermal spheres with a fixed
anisotropy parameter should exhibit a near perfect relation between the mass
and the fourth power of the velocity dispersion. This is consistent with the
observed Faber-Jackson relation for elliptical galaxies-- a luminosity-velocity
dispersion relation with large scatter. However, the observable global
properties of elliptical galaxies comprise a three parameter family; they lie
on a ``fundamental plane'' in a logarithmic space consisting of central
velocity dispersion, effective radius, and luminosity. The scatter
perpendicular to this plane is significantly less than that about the
Faber-Jackson relation. I show here that, in order to match the observed global
properties of elliptical galaxies with MOND, models must deviate from being
strictly isothermal and isotropic; such objects can be approximated by
high-order polytropic spheres with a radial orbit anisotropy in the outer
regions. MOND imposes boundary conditions on the inner Newtonian regions which
restrict these models to a dynamical fundamental plane which may differ from
that implied by the traditional virial theorem. Scatter about this plane is
relatively insensitive to the necessary deviations from homology.Comment: 9 pages, 6 eps figures, mn style. Shortened and revised version
includes more recent data on elliptical galaxies. Accepted for publication in
MNRA
An Iterative Receiver for OFDM With Sparsity-Based Parametric Channel Estimation
In this work we design a receiver that iteratively passes soft information
between the channel estimation and data decoding stages. The receiver
incorporates sparsity-based parametric channel estimation. State-of-the-art
sparsity-based iterative receivers simplify the channel estimation problem by
restricting the multipath delays to a grid. Our receiver does not impose such a
restriction. As a result it does not suffer from the leakage effect, which
destroys sparsity. Communication at near capacity rates in high SNR requires a
large modulation order. Due to the close proximity of modulation symbols in
such systems, the grid-based approximation is of insufficient accuracy. We show
numerically that a state-of-the-art iterative receiver with grid-based sparse
channel estimation exhibits a bit-error-rate floor in the high SNR regime. On
the contrary, our receiver performs very close to the perfect channel state
information bound for all SNR values. We also demonstrate both theoretically
and numerically that parametric channel estimation works well in dense
channels, i.e., when the number of multipath components is large and each
individual component cannot be resolved.Comment: Major revision, accepted for IEEE Transactions on Signal Processin
Single wall carbon nanotube double quantum dot
We report on two top-gate defined, coupled quantum dots in a semiconducting
single wall carbon nanotube, constituting a tunable double quantum dot system.
The single wall carbon nanotubes are contacted by titanium electrodes, and
gated by three narrow top-gate electrodes as well as a back-gate. We show that
a bias spectroscopy plot on just one of the two quantum dots can be used to
extract the addition energy of both quantum dots. Furthermore, honeycomb charge
stability diagrams are analyzed by an electrostatic capacitor model that
includes cross capacitances, and we extract the coupling energy of the double
quantum dot.Comment: Published in Applied Physics Letters 4 December 2006.
http://link.aip.org/link/?APL/89/23211
Can on-farm bioenergy production make organic farming more sustainable? - A model for energy balance, nitrogen losses, and green house gas emissions in a 1000 ha energy catchment with organic dairy farming and integrated bioenergy production
Can biogas and bioethanol production make organic farming more sustainable?
- Results from a model for the fossil energy balance, Nitrogen losses, and greenhouse gas emissions in a 1000 ha energy catchment with organic dairy farming and integrated biogas and bioethanol production.
Dalgaard T1, Pugesgaard S1, Jørgensen U1, Olesen JE1, Møller HB1 and Jensen ES2
1) Dept. Agroecology and Environment. Faculty of Agricultural Sciences (DJF), University of Aarhus. DK-8830 Tjele. Denmark. Contact: [email protected]
2) Biosystems Department, Risø DTU, The National Laboratory for Sustainable Energy, The Technical University of Denmark DK-4000 Roskilde, Denmark
The vision of organic farming systems, independent of fossil energy resources, with significantly lower nutrient losses, and no net contribution to the greenhouse gas emissions might be fulfilled via the integration of biogas production. This is an important hypothesis investigated in the www.bioconcens.elr.dk/uk/ research project.
This poster illustrates preliminary results from a model for the fossil energy balance, Nitrogen losses, and greenhouse gas emissions in a 1000 ha energy catchment with organic dairy farming and integrated biogas production in Denmark. The model will draw on results from previous models (e.g the farmGHG model), and includes a number of organic dairy farm type components, including information on livestock production, housing, manure storage, manure and fodder import/export, crop rotations, yield levels, and soil types. In addition, a biogas plant model component evaluates effects of the inclusion of variable amounts of manures and crop residues from the specified farm types, into the biogas energy production.
The model is intended to result in an overall catchment balance for the following three types of indicators: 1) the fossil energy use – i.e. the net fossil energy use minus the bioenergy production, 2) losses of Nitrogen in the form of nitrates, ammonia and nitrous oxide, and 3) the emission of the three main greenhouse gasses from agriculture: carbon dioxide, nitrous oxide and methane, measured in carbon dioxide equivalents. Moreover, these indicator values are specified for each of the farm types included in the model, and for the biogas plant component. Finally, selected model results are discussed in relation to the overall hypothesis of the research project, and it is discussed how the integration of biogas production in organic farming, can help to improve the self-sufficiency in Nitrogen, and thereby reduce the import of nutrients to the organic farming systems
MOND and the lensing Fundamental Plane: No need for dark matter on galaxy scales
Bolton et al. (2007) have derived a mass-based fundamental plane using
photometric and spectroscopic observations of 36 strong gravitational lenses.
The lensing allows a direct determination of the mass-surface density and so
avoids the usual dependence on mass-to-light ratio. We consider this same
sample in the context of modified Newtonian dynamics (MOND) and demonstrate
that the observed mass-based fundamental plane coincides with the MOND
fundamental plane determined previously for a set of high-order polytropic
spheres chosen to match the observed range of effective radii and velocity
dispersions in elliptical galaxies. Moreover, the observed projected mass
within one-half an effective radius is consistent with the mass in visible
stars plus a small additional component of ``phantom dark matter'' resulting
from the MOND contribution to photon deflection.Comment: Minor revisions in response to referee. Revised title. Accepted in
MNRA
Critical Current 0- Transition in Designed Josephson Quantum Dot Junctions
We report on quantum dot based Josephson junctions designed specifically for
measuring the supercurrent. From high-accuracy fitting of the current-voltage
characteristics we determine the full magnitude of the supercurrent (critical
current). Strong gate modulation of the critical current is observed through
several consecutive Coulomb blockade oscillations. The critical current crosses
zero close to, but not at, resonance due to the so-called 0- transition in
agreement with a simple theoretical model.Comment: 5 pages, 4 figures, (Supplementary information available at
http://www.fys.ku.dk/~hij/public/nl_supp.pdf
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