4,678 research outputs found
The Effects of Tree Density on Pasture Production Under Acacia Melanoxylon
Silvi-pastoral systems could contribute to greater total productivity and more sustainable land management of grazed pastures in New Zealand. Despite this potential, little is known of the interactions of tree species and environment on pasture growth. In this paper we report the effects of tree density on pasture yield in an Acacia melanoxylon silvi-pastoral system. The results show decreasing pasture yield with increasing tree density, although the extent of the reduction is less than would be expected under Pinus radiata. Results suggest that shade is the dominant factor causing yield decline in this study, while reduced N availability is clearly implicated in previous studies with pine trees. This study will provide more data on which a mechanistic model of tree-pasture interactions can be based
Vortex stability in nearly two-dimensional Bose-Einstein condensates with attraction
We perform accurate investigation of stability of localized vortices in an
effectively two-dimensional ("pancake-shaped") trapped BEC with negative
scattering length. The analysis combines computation of the stability
eigenvalues and direct simulations. The states with vorticity S=1 are stable in
a third of their existence region, , where is
the number of atoms, and is the corresponding collapse
threshold. Stable vortices easily self-trap from arbitrary initial
configurations with embedded vorticity. In an adjacent interval, , the unstable vortex
periodically splits in two fragments and recombines. At , the fragments do not recombine, as each one collapses by
itself. The results are compared with those in the full 3D Gross-Pitaevskii
equation. In a moderately anisotropic 3D configuration, with the aspect ratio
, the stability interval of the S=1 vortices occupies
of their existence region, hence the 2D limit provides for a reasonable
approximation in this case. For the isotropic 3D configuration, the stability
interval expands to 65% of the existence domain. Overall, the vorticity
heightens the actual collapse threshold by a factor of up to 2. All vortices
with are unstable.Comment: 21 pages, 8 figures, to appear in Physical Review
Slow Quenches Produce Fuzzy, Transient Vortices
We examine the Zurek scenario for the production of vortices in quenches of
liquid in the light of recent experiments. Extending our previous
results to later times, we argue that short wavelength thermal fluctuations
make vortices poorly defined until after the transition has occurred. Further,
if and when vortices appear, it is plausible that that they will decay faster
than anticipated from turbulence experiments, irrespective of quench rates.Comment: 4 pages, Revtex file, no figures Apart from a more appropriate title,
this paper differs from its predecessor by including temperature, as well as
pressure, quenche
New Experiments for Spontaneous Vortex Formation in Josephson Tunnel Junctions
It has been argued by Zurek and Kibble that the likelihood of producing
defects in a continuous phase transition depends in a characteristic way on the
quench rate. In this paper we discuss an improved experiment for measuring the
Zurek-Kibble scaling exponent for the production of fluxons in
annular symmetric Josephson Tunnel Junctions. We find .
Further, we report accurate measurements of the junction gap voltage
temperature dependence which allow for precise monitoring of the fast
temperature variations during the quench.Comment: 12 pages, 5 figures, submitted to Phys. Rev.
Size-selective concentration of chondrules and other small particles in protoplanetary nebula turbulence
Size-selective concentration of particles in a weakly turbulent
protoplanetary nebula may be responsible for the initial collection of
chondrules and other constituents into primitive body precursors. This paper
presents the main elements of this process of turbulent concentration. In the
terrestrial planet region, both the characteristic size and size distribution
of chondrules are explained. "Fluffier" particles would be concentrated in
nebula regions which were at a lower gas density and/or more intensely
turbulent. The spatial distribution of concentrated particle density obeys
multifractal scaling}, suggesting a close tie to the turbulent cascade process.
This scaling behavior allows predictions of the probability distributions for
concentration in the protoplanetary nebula to be made. Large concentration
factors (>10^5) are readily obtained, implying that numerous zones of particle
density significantly exceeding the gas density could exist. If most of the
available solids were actually in chondrule sized particles, the ensuing
particle mass density would become so large that the feedback effects on gas
turbulence due to mass loading could no longer be neglected. This paper
describes the process, presenting its basic elements and some implications,
without including the effects of mass loading.Comment: 34 pages, 7 figures; in press for Astrophys. J; expected Jan 01 2001
issu
Tzitz\'eica transformation is a dressing action
We classify the simplest rational elements in a twisted loop group, and prove
that dressing actions of them on proper indefinite affine spheres give the
classical Tzitz\'eica transformation and its dual. We also give the group point
of view of the Permutability Theorem, construct complex Tzitz\'eica
transformations, and discuss the group structure for these transformations
Scalar second order evolution equations possessing an irreducible sl-valued zero curvature representation
We find all scalar second order evolution equations possessing an
sl-valued zero curvature representation that is not reducible to a proper
subalgebra of sl. None of these zero-curvature representations admits a
parameter.Comment: 10 pages, requires nath.st
The formation of vortex loops (strings) in continuous phase transitions
The formation of vortex loops (global cosmic strings) in an O(2) linear sigma
model in three spatial dimensions is analyzed numerically. For over-damped
Langevin dynamics we find that defect production is suppressed by an
interaction between correlated domains that reduces the effective spatial
variation of the phase of the order field. The degree of suppression is
sensitive to the quench rate. A detailed description of the numerical methods
used to analyze the model is also reported.Comment: LaTeX, 17 pages, 6 eps figures 2 references and a footnote adde
Testing the Kibble-Zurek Scenario with Annular Josephson Tunnel Junctions
In parallel with Kibble's description of the onset of phase transitions in
the early universe, Zurek has provided a simple picture for the onset of phase
transitions in condensed matter systems, strongly supported by agreement with
experiments in He3. In this letter we show how experiments with annular
Josephson tunnel Junctions can and do provide further support for this
scenario.Comment: Revised version with correct formula for the Swihart velocity. The
results are qualitatively the same as with the previous version but differ
quantitatively. 4 pages, RevTe
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