1,761 research outputs found
Nutrient Limitation of Periphyton in a Spring-Fed, Coastal Stream in Florida, USA.
There is strong evidence to suggest that ground-water
nitrate concentrations have increased in recent years and
further increases are expected along portions of the central
Gulf coast of Florida. Much of the nitrate enriched groundwater
is discharged into surface waters through numerous
freshwater springs that are characteristic of the area and the
potential for eutrophication of their receiving waters is a
legitimate concern. To test the potential effects of elevated
nutrient concentrations on the periphyton community an
in
situ
nutrient addition experiment was conducted in the
spring-fed Chassahowitzka River, FL, USA, during the summer
of 1999. Plastic tubes housing arrays of glass microscope
slides were suspended in the stream. Periphyton colonizing
the microscope slides was subjected to artificial increases in
nitrogen, phosphorus or a combination of both. Slides from
each tube were collected at 3- to 4- day intervals and the
periphyton communities were measured for chlorophyll concentration.
The addition of approximately 10 μg/L of phosphate
above ambient concentrations significantly increased
the amount of periphyton on artificial substrates relative to
controls; the addition of approximately 100 μg/L of nitrate
above ambient concentrations did not. The findings from
this experiment implicated phosphorus, rather than
nitrogen, as the nutrient that potentially limits periphyton
growth in this system.(PDF contains 4 pages.
Control of atomic currents using a quantum stirring device
We propose a BEC stirring device which can be regarded as the incorporation
of a quantum pump into a closed circuit: it produces a DC circulating current
in response to a cyclic adiabatic change of two control parameters of an
optical trap. We demonstrate the feasibility of this concept and point out that
such device can be utilized in order to probe the interatomic interactions.Comment: 5 pages, 4 figures, uses epl2.cls, revised versio
Effects of Uniaxial Stress on Antiferromagnetic Moment in the Heavy Electron Compound URu_2Si_2
We have performed the elastic neutron scattering experiments under uniaxial
stress \sigma along the tetragonal [100], [110] and [001] directions for
URu2Si2. For \sigma // [100] and [110], the antiferromagnetic moment \mu_o is
strongly enhanced from 0.02 \mu_B (\sigma=0) to 0.22 \mu_B (\sigma=2.5 kbar) at
1.5 K. The rate of increase d\mu_o/d\sigma is roughly estimated to be ~ 0.1
\mu_B/kbar, which is much larger than that for the hydrostatic pressure (~
0.025 \mu_B/kbar). Above 2.5 kbar, \mu_o shows a tendency to saturate similar
to the behavior in the hydrostatic pressure. For \sigma // [001], on the other
hand, \mu_o shows only a slight increase to 0.028 \mu_B (\sigma = 4.6 kbar)
with a rate of ~ 0.002 \mu_B/kbar. The observed anisotropy suggests that the
competition between the hidden order and the antiferromagnetic state in URu2Si2
is strongly coupled with the tetragonal four-fold symmetry and the c/a ratio,
or both.Comment: 3 pages, 3 eps figures, Proceedings of Int. Conf. on Strongly
Correlated Electrons with Orbital Degrees of Freedom (Sendai, Japan,
September 11-14, 2001
A Comprehensive Radio and Optical Study of Abell 2256: Activity from an Infalling Group
Abell 2256 is a nearby (z~0.06), rich cluster of galaxies with fascinating
observed properties across a range of wavelengths. Long believed to represent a
cluster merger, recent X-ray and optical results have suggested that in
addition to the primary cluster and subcluster there is evidence for a third,
poorer system. We present wide-field, high sensitivity 1.4 GHz VLA radio
observations of Abell 2256 in conjunction with optical imaging and additional
spectroscopy. Over 40 cluster radio galaxies are identified, with optical
spectroscopy indicating the emission source (star formation or AGN) for most of
them. While the overall fraction of galaxies exhibiting radio emission is
consistent with a large sample of other nearby clusters, we find an increase in
the activity level of galaxies belonging to the third system (hereafter, the
``Group''). Specifically, the Group has relatively more star formation than
both the primary cluster and main subcluster. The position of the Group is also
coincident with the observed cluster radio relic. We suggest that the Group
recently (~0.3 Gyr) merged with the primary cluster and that this merger, not
the ongoing merger of the primary and the main subcluster, might be responsible
for many of the unusual radio properties of Abell 2256. Furthermore, the
greater star formation activity of the Group suggests that the infall of groups
is an important driver of galaxy evolution in clusters.Comment: 21 pages plus 13 JPEG figures; to appear in the Astronomical Journa
Neutron Scattering Study on Competition between Hidden Order and Antiferromagnetism in U(Ru_{1-x}Rh_x)_2Si_2 (x <= 0.05)
We have performed elastic and inelastic neutron scattering experiments on the
solid solutions U(Ru_{1-x}Rh_x)_2Si_2 for the Ru rich concentrations: x=0,
0.01, 0.02, 0.025, 0.03, 0.04 and 0.05. Hidden order is suppressed with
increasing x, and correspondingly the onset temperature T_m (~ 17.5 K at x=0)
of weak antiferromagnetic (AF) Bragg reflection decreases. For x=0.04 and 0.05,
no magnetic order is detected in the investigated temperature range down to 1.4
K. In the middle range, 0.02 <= x <= 0.03, we found that the AF Bragg
reflection is strongly enhanced. At x=0.02, this takes place at ~ 7.7 K (=T_M),
which is significantly lower than T_m (~ 13.7 K). T_M increases with increasing
x, and seems to merge with T_m at x=0.03. If the AF state is assumed to be
homogeneous, the staggered moment \mu_o estimated at 1.4 K increases from
0.02(2) \mu_B/U (x=0) to 0.24(1) \mu_B/U (x=0.02). The behavior is similar to
that observed under hydrostatic pressure (\mu_o increases to ~ 0.25 \mu_B/U at
1.0 GPa), suggesting that the AF evolution induced by Rh doping is due to an
increase in the AF volume fraction. We also found that the magnetic excitation
observed at Q=(1,0,0) below T_m disappears as T is lowered below T_M.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
Forest structure, diversity, and primary production in relation to disturbance severity
Differential disturbance severity effects on forest vegetation structure, species diversity, and net primary production (NPP) have been long theorized and observed. Here, we examined these factors concurrently to explore the potential for a mechanistic pathway linking disturbance severity, changes in light environment, leaf functional response, and wood NPP in a temperate hardwood forest.Using a suite of measurements spanning an experimental gradient of tree mortality, we evaluated the direction and magnitude of change in vegetation structural and diversity indexes in relation to wood NPP. Informed by prior observations, we hypothesized that forest structural and species diversity changes and wood NPP would exhibit either a linear, unimodal, or threshold response in relation to disturbance severity. We expected increasing disturbance severity would progressively shift subcanopy light availability and leaf traits, thereby coupling structural and species diversity changes with primary production.Linear or unimodal changes in three of four vegetation structural indexes were observed across the gradient in disturbance severity. However, disturbance‐related changes in vegetation structure were not consistently correlated with shifts in light environment, leaf traits, and wood NPP. Species diversity indexes did not change in response to rising disturbance severity.We conclude that, in our study system, the sensitivity of wood NPP to rising disturbance severity is generally tied to changing vegetation structure but not species diversity. Changes in vegetation structure are inconsistently coupled with light environment and leaf traits, resulting in mixed support for our hypothesized cascade linking disturbance severity to wood NPP.We examined multiple metrics of vegetation structural and biological diversity concurrently to explore the potential for a mechanistic pathway linking disturbance severity, changes in light environment, leaf functional response, and wood NPP in a temperate hardwood forest. Significant linear or unimodal changes in three of four vegetation structural indexes were observed across the gradient in disturbance severity, although disturbance‐related changes in vegetation structure were not consistently correlated with shifts in light environment, leaf traits, and wood NPP. We conclude that, in our study system, the sensitivity of wood NPP to rising disturbance severity is generally tied to changing vegetation structure but not species diversity.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155474/1/ece36209.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155474/2/ece36209_am.pd
Dijet resonances, widths and all that
The search for heavy resonances in the dijet channel is part of the on-going
physics programme, both at the Tevatron and at the LHC. Lower limits have been
placed on the masses of dijet resonances predicted in a wide variety of models.
However, across experiments, the search strategy assumes that the effect of the
new particles is well-approximated by on-shell production and subsequent decay
into a pair of jets. We examine the impact of off-shell effects on such
searches, particularly for strongly interacting resonances.Comment: Version published in JHE
Roy-Steiner equations for pion-nucleon scattering
Starting from hyperbolic dispersion relations, we derive a closed system of
Roy-Steiner equations for pion-nucleon scattering that respects analyticity,
unitarity, and crossing symmetry. We work out analytically all kernel functions
and unitarity relations required for the lowest partial waves. In order to
suppress the dependence on the high-energy regime we also consider once- and
twice-subtracted versions of the equations, where we identify the subtraction
constants with subthreshold parameters. Assuming Mandelstam analyticity we
determine the maximal range of validity of these equations. As a first step
towards the solution of the full system we cast the equations for the
partial waves into the form of a Muskhelishvili-Omn\`es
problem with finite matching point, which we solve numerically in the
single-channel approximation. We investigate in detail the role of individual
contributions to our solutions and discuss some consequences for the spectral
functions of the nucleon electromagnetic form factors.Comment: 106 pages, 18 figures; version published in JHE
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