53,055 research outputs found
In field N transfer, build-up, and leaching in ryegrass-clover mixtures
Two field experiments investigating dynamics in grass-clover mixtures were conducted, using 15N- and 14C-labelling to trace carbon (C) and nitrogen (N) from grass (Lolium perenne L.) and clover (Trifolium repens L. and Trifolium pratense L.). The leaching of dissolved inorganic nitrogen (DIN), as measured in pore water sampled by suction cups, increased during the autumn and winter, whereas the leaching of dissolved organic nitrogen (DON) was fairly constant during this period. Leaching of 15N from the sward indicated that ryegrass was the direct source of less than 1-2 percent of the total N leaching measured, whereas N dynamics pointed to clover as an important contributor to N leaching. Sampling of roots indicates that the dynamics in smaller roots were responsible for N and C build-up in the sward, and that N became available for transfer among species and leaching from the root zone. The bi-directional transfer of N between ryegrass and clover could however not be explained only by root turnover. Other processes like direct uptake of organic N compounds, may have contributed
A solvable non-conservative model of Self-Organized Criticality
We present the first solvable non-conservative sandpile-like critical model
of Self-Organized Criticality (SOC), and thereby substantiate the suggestion by
Vespignani and Zapperi [A. Vespignani and S. Zapperi, Phys. Rev. E 57, 6345
(1998)] that a lack of conservation in the microscopic dynamics of an SOC-model
can be compensated by introducing an external drive and thereby re-establishing
criticality. The model shown is critical for all values of the conservation
parameter. The analytical derivation follows the lines of Broeker and
Grassberger [H.-M. Broeker and P. Grassberger, Phys. Rev. E 56, 3944 (1997)]
and is supported by numerical simulation. In the limit of vanishing
conservation the Random Neighbor Forest Fire Model (R-FFM) is recovered.Comment: 4 pages in RevTeX format (2 Figures) submitted to PR
Quantum spin configurations in Tb2Ti2O7
Low energy collective angular momentum states of the Tb3+ ions in Tb2Ti2O7
are classified according to the irreducible representations of the octahedral
point group. Degeneracy lifting due to the exchange interaction is discussed.
Diffuse neutron scattering intensity patterns are calculated for each
collective angular momentum state and the ground state is inferred by comparing
to experiment.Comment: 5 pages, 1 colour figure. Slight corrections and additions to text
and figur
The Infati Data
The ability to perform meaningful empirical studies is of essence in research
in spatio-temporal query processing. Such studies are often necessary to gain
detailed insight into the functional and performance characteristics of
proposals for new query processing techniques.
We present a collection of spatio-temporal data, collected during an
intelligent speed adaptation project, termed INFATI, in which some two dozen
cars equipped with GPS receivers and logging equipment took part. We describe
how the data was collected and how it was "modified" to afford the drivers some
degree of anonymity.
We also present the road network in which the cars were moving during data
collection.
The GPS data is publicly available for non-commercial purposes. It is our
hope that this resource will help the spatio-temporal research community in its
efforts to develop new and better query processing techniques
Correlated Gaussian method for dilute bosonic systems
The weakly interacting trapped Bose gases have been customarily described
using the mean-field approximation in the form of the Gross-Pitaevskii
equation. The mean-field approximation, however, has certain limitations, in
particular it can not describe correlations between particles. We introduce
here an alternative variational approach, based on the correlated Gaussian
method, which in its simplest form is as fast and simple as the mean-field
approximation, but which allows successive improvements of the trial
wave-function by including correlations between particles.Comment: 9 pages, Workshop on Nuclei and Mesoscopic Physics, NSCL MSU, 200
Phase coherence length and quantum interference patterns at step edges
The accepted approximation used to describe quantum interference patterns at
steps is shown to be incorrect. As a result, electron lifetimes determined
using it are in error by a factor 2.Comment: 3 pages, 1 figur
Structural distortion and the spin liquid state in Tb2Ti2O7
It is shown that a k=0, A_{2u} distortion of the terbium tetrahedral network
in Tb2Ti2O7 accounts for the apparent isolation of single tetrahedra as seen in
neutron scattering studies. Single tetrahedron collective spin states, rather
than individual spins, account for the main features of the spin liquid state,
namely, fluctuating local moments and the absence of long range order. Singlet
and doublet collective spin ground states are considered. An effective
interaction between tetrahedra on the fcc lattice is derived and found to be
weak and anisotropic.Comment: 5 page
Oscillations and temporal signalling in cells
The development of new techniques to quantitatively measure gene expression
in cells has shed light on a number of systems that display oscillations in
protein concentration. Here we review the different mechanisms which can
produce oscillations in gene expression or protein concentration, using a
framework of simple mathematical models. We focus on three eukaryotic genetic
regulatory networks which show "ultradian" oscillations, with time period of
the order of hours, and involve, respectively, proteins important for
development (Hes1), apoptosis (p53) and immune response (NFkB). We argue that
underlying all three is a common design consisting of a negative feedback loop
with time delay which is responsible for the oscillatory behaviour
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