310 research outputs found
Preface: Oceanographic processes on the continental shelf: observations and modeling
No abstract available
Modified Gravity Away from a CDM Background
Within the effective field theory approach to cosmic acceleration, the
background expansion can be specified separately from the gravitational
modifications. We explore the impact of modified gravity in a background
different from a cosmological constant plus cold dark matter (CDM) on
the stability and cosmological observables, including covariance between
gravity and expansion parameters. In No Slip Gravity the more general
background allows more gravitational freedom, including both positive and
negative Planck mass running. We examine the effects on cosmic structure
growth, as well as showing that a viable positive integrated Sachs-Wolfe effect
crosscorrelation easily arises from this modified gravity theory. Using current
data we constrain parameters with a Monte Carlo analysis, finding a maximum
running . We provide the modified {\tt hi\_class} code
publicly on GitHub, now enabling computation and inclusion of the redshift
space distortion observable as well as the No Slip Gravity
modifications.Comment: 14 pages, 13 figures. Matches published version in JCAP, LCDM
discussion adde
Exploring Early and Late Cosmology with Next Generation Surveys
Perturbations from inflation evolve into large scale structure of the late
universe, and encode abundant cosmic structure formation physics. We allow
freedom in the primordial power spectrum, rather than assuming a power law
scale dependence, to study its impact on cosmological parameter determination.
Combining various generations of cosmic microwave background (CMB) data and
galaxy redshift survey data, we investigate the constraints on reconstruction
of the primordial curvature perturbation power spectrum and the late time
cosmology, especially the sum of neutrino masses. We quantify how each
successive generation, in CMB and galaxy surveys, provides significant
improvements, often by factors of several. By using CMB polarization
information over a broad range of angular scales, and galaxy redshift data in
many bins of redshift, one can allow inflationary freedom and still constrain
parameters comparably to assuming power law dependence. The primordial power
spectrum can be reconstructed at the subpercent level in a dozen wavenumber
bins, while simultaneously fitting the sum of neutrino masses to 14 meV.Comment: 19 pages, 12 figures, 2 tables, v2 accepted for publicatio
Data needs for hyperspectral detection of algal bloom diversity across the globe.
A group of 38 experts specializing in hyperspectral remote-sensing methods for aquatic ecosystems attended an interactive Euromarine Foresight Workshop at the Flanders Marine Institute (VLIZ) in Ostend, Belgium, June 4–6, 2019. The objective of this workshop was to develop recommendations for comprehensive, efficient, and effective laboratory and field programs to supply data for development of algorithms and validation of hyperspectral satellite imagery for micro-, macro- and endosymbiotic algal characterization across the globe. The international group of researchers from Europe, Asia, Australia, and North and South America (see online Supplementary Materials) tackled how to develop global databases that merge hyperspectral optics and phytoplankton group composition to support the next generation of hyperspectral satellites for assessing biodiversity in the ocean and in food webs and for detecting water quality issues such as harmful algal blooms. Through stimulating discussions in breakout groups, the team formulated a host of diverse programmatic recommendations on topics such as how to better integrate optics into phytoplankton monitoring programs; approaches to validating phytoplankton composition with ocean color measurements and satellite imagery; new database specifications that match optical data with phytoplankton composition data; requirements for new instrumentation that can be implemented on floats, moorings, drones, and other platforms; and the development of international task forces.
Because in situ observations of phytoplankton biogeography and abundance are scarce, and many vast oceanic regions are too remote to be routinely monitored, satellite observations are required to fully comprehend the diversity of micro-, macro-, and endosymbiotic algae and any variability due to climate change. Ocean color remote sensing that provides regular synoptic monitoring of aquatic ecosystems is an excellent tool for assessing biodiversity and abundance of phytoplankton and algae in aquatic ecosystems. However, neither the spatial, temporal, nor spectral resolution of the current ocean color missions are sufficient to characterize phytoplankton community composition adequately. The near-daily overpasses from ocean color satellites are useful for detecting the presence of blooms, but the spatial resolution is often too coarse to assess the patchy distribution of blooms, and the multiband spectral resolution is generally insufficient to identify different types of phytoplankton from each other, even if progress has undeniably been achieved during the last two decades (e.g., IOCGG, 2014). Moreover, the methods developed for multichannel sensor use are often highly tuned to a region but are inaccurate when applied broadly.
New orbital imaging spectrometers are being developed that cover the full visible and near-infrared spectrum with a large number of narrow bands dubbed “hyperspectral” (e.g., TROPOMI, PRISMA, EnMAP, PACE, CHIME, SBG). Hyper-spectral methods have been explored for many years to assess phytoplankton groups and map seafloor habitats. However, the utility of hyperspectral imaging still needs to be demonstrated across diverse aquatic regimes. Aquatic applications of hyperspectral imagery have been limited by both the technology and the ability to validate products. Some of the past hyperspectral space-based sensors have suffered from calibration artifacts, low sensitivity in aquatic ecosystems (e.g., CHRIS, HICO), and very low spatial resolution (e.g., SCIAMACHY), but the next generation of sensors are planned to have high signal-to-noise ratio and improved performance over aquatic targets. Providing data to develop and validate hyperspectral approaches to characterize phytoplankton groups across the globe poses new challenges. Several recent studies have documented gaps that need to be filled in order to assess algal diversity across the globe (IOCCG, 2014; Mouw et al., 2015; Bracher et al., 2017), which promoted/inspired the formation of this workshop
Quantum Tricritical Points in NbFe
Quantum critical points (QCPs) emerge when a 2nd order phase transition is
suppressed to zero temperature. In metals the quantum fluctuations at such a
QCP can give rise to new phases including unconventional superconductivity.
Whereas antiferromagnetic QCPs have been studied in considerable detail
ferromagnetic (FM) QCPs are much harder to access. In almost all metals FM QCPs
are avoided through either a change to 1st order transitions or through an
intervening spin-density-wave (SDW) phase. Here, we study the prototype of the
second case, NbFe. We demonstrate that the phase diagram can be modelled
using a two-order-parameter theory in which the putative FM QCP is buried
within a SDW phase. We establish the presence of quantum tricritical points
(QTCPs) at which both the uniform and finite susceptibility diverge. The
universal nature of our model suggests that such QTCPs arise naturally from the
interplay between SDW and FM order and exist generally near a buried FM QCP of
this type. Our results promote NbFe as the first example of a QTCP, which
has been proposed as a key concept in a range of narrow-band metals, including
the prominent heavy-fermion compound YbRhSi.Comment: 21 pages including S
Evolution of magnetism in Yb(Rh_(1-x)Co_x)2Si2
We present a study of the evolution of magnetism from the quantum critical
system YbRh2Si2 to the stable trivalent Yb system YbCo2Si2. Single crystals of
Yb(Rh_(1-x)Co_x)2Si2 were grown for 0 < x < 1 and studied by means of magnetic
susceptibility, electrical resistivity, and specific heat measurements, as well
as photoemission spectroscopy. The results evidence a complex magnetic phase
diagram, with a non-monotonic evolution of T_N and two successive transitions
for some compositions resulting in two tricritical points. The strong
similarity with the phase diagram of YbRh2Si2 under pressure indicates that Co
substitution basically corresponds to the application of positive chemical
pressure. Analysis of the data proves a strong reduction of the Kondo
temperature T_K with increasing Co content, T_K becoming smaller than T_N for x
~ 0.5, implying a strong localization of the 4f electrons. Furthermore,
low-temperature susceptibility data confirm a competition between ferromagnetic
and antiferromagnetic exchange. The series Yb(Rh_(1-x)Co_x)2Si2 provides an
excellent experimental opportunity to gain a deeper understanding of the
magnetism at the quantum critical point in the vicinity of YbRh2Si2 where the
antiferromagnetic phase disappears (T_N=>0).Comment: 11 pages, 9 figure
Electro-nuclear transition into a spatially modulated magnetic state in YbRhSi
The nature of the antiferromagnetic order in the heavy fermion metal
YbRhSi, its quantum criticality, and superconductivity, which appears
at low mK temperatures, remain open questions. We report measurements of the
heat capacity over the wide temperature range 180 K - 80 mK, using current
sensing noise thermometry. In zero magnetic field we observe a remarkably sharp
heat capacity anomaly at 1.5 mK, which we identify as an electro-nuclear
transition into a state with spatially modulated electronic magnetic order of
maximum amplitude 0.1. We also report results of measurements in
magnetic fields in the range 0 to 70 mT, applied perpendicular to the c-axis,
which show eventual suppression of this order. These results demonstrate a
coexistence of a large moment antiferromagnet with putative superconductivity.Comment: 11 pages, 11 figures, including the supplementary informatio
Heavy-Fermions in LiV2O4: Kondo-Compensation vs. Spin-Liquid Behavior?
7Li NMR measurements were performed in the metallic spinel LiV2O4. The
temperature dependencies of the line width, the Knight shift and the
spin-lattice relaxation rate were investigated in the temperature range 30 mK <
T < 280 K. For temperatures T < 1 K we observe a spin-lattice relaxation rate
which slows down exponentially. The NMR results can be explained by a
spin-liquid behavior and the opening of a spin gap of the order 0.6 K
Surprisingly modest water quality impacts from expansion and intensification of large-scale commercial agriculture in the Brazilian Amazon-Cerrado region
© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Tropical Conservation Science 10 (2017): 1-5, doi:10.1177/1940082917720669.Large-scale commercial cropping of soybeans expanded in the tropical Amazon and Cerrado biomes of Brazil after 1990. More recently, cropping intensified from single-cropping of soybeans to double-cropping of soybeans with corn or cotton. Cropland expansion and intensification, and the accompanying use of mineral fertilizers, raise concerns about whether nutrient runoff and impacts to surface waters will be similar to those experienced in commercial cropland regions at temperate latitudes. We quantified water infiltration through soils, water yield, and streamwater chemistry in watersheds draining native tropical forest and single- and double-cropped areas on the level, deep, highly weathered soils where cropland expansion and intensification typically occurs. Although water yield increased four-fold from croplands, streamwater chemistry remained largely unchanged. Soil characteristics exerted important control over the movement of nitrogen (N) and phosphorus (P) into streams. High soil infiltration rates prevented surface erosion and movement of particulate P, while P fixation in surface soils restricted P movement to deeper soil layers. Nitrogen retention in deep soils, likely by anion exchange, also appeared to limit N leaching and export in streamwater from both single- and double-cropped watersheds that received nitrogen fertilizer. These mechanisms led to lower streamwater P and N concentrations and lower watershed N and P export than would be expected, based on studies from temperate croplands with similar cropping and fertilizer application practices.The work described here was supported by National Science Foundation grants EF 1655432, IOS 1457662 and ICER 1342953 and grants from the Fundação de Amparo à Pesquisa do Estado de São Paulo
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