11,401 research outputs found
Partial stratification of secant varieties of Veronese varieties via curvilinear subschemes
We give a partial "quasi-stratification" of the secant varieties of the order
Veronese variety of . It covers the set
of all points lying on the linear span of
curvilinear subschemes of , but two "quasi-strata" may overlap. For
low border rank two different "quasi-strata" are disjoint and we compute the
symmetric rank of their elements. Our tool is the Hilbert schemes of
curvilinear subschemes of Veronese varieties. To get a stratification we attach
to each the minimal label of a quasi-stratum
containing it.Comment: 16 page
B-Mode contamination by synchrotron emission from 3-years WMAP data
We study the contamination of the B-mode of the Cosmic Microwave Background
Polarization (CMBP) by Galactic synchrotron in the lowest emission regions of
the sky. The 22.8-GHz polarization map of the 3-years WMAP data release is used
to identify and analyse such regions. Two areas are selected with
signal-to-noise ratio S/N<2 and S/N<3, covering ~16% and ~26% fraction of the
sky, respectively. The polarization power spectra of these two areas are
dominated by the sky signal on large angular scales (multipoles l < 15), while
the noise prevails on degree scales. Angular extrapolations show that the
synchrotron emission competes with the CMBP B-mode signal for tensor-to-scalar
perturbation power ratio -- at 70-GHz in the 16%
lowest emission sky (S/N<2 area). These values worsen by a factor ~5 in the
S/N<3 region. The novelty is that our estimates regard the whole lowest
emission regions and outline a contamination better than that of the whole high
Galactic latitude sky found by the WMAP team (T/S>0.3). Such regions allow to be measured directly which approximately corresponds to the
limit imposed by using a sky coverage of 15%. This opens interesting
perspectives to investigate the inflationary model space in lowest emission
regions.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
The synchrotron foreground and CMB temperature-polarization cross correlation power spectrum from the first year WMAP data
We analyse the temperature-polarization cross-correlation in the Galactic
synchrotron template that we have recently developed, and between the template
and CMB temperature maps derived from WMAP data. Since the polarized
synchrotron template itself uses WMAP data, we can estimate residual
synchrotron contamination in the CMB angular spectrum. While
appears to be contamined by synchrotron, no evidence for
contamination is found in the multipole range which is most relevant for the
fit of the cosmological optical depth.Comment: Accepted for pubblication on MNRAS Lette
Kelvin probe characterization of buried graphitic microchannels in single-crystal diamond
In this work, we present an investigation by Kelvin Probe Microscopy (KPM) of
buried graphitic microchannels fabricated in single-crystal diamond by direct
MeV ion microbeam writing. Metal deposition of variable-thickness masks was
adopted to implant channels with emerging endpoints and high temperature
annealing was performed in order to induce the graphitization of the
highly-damaged buried region. When an electrical current was flowing through
the biased buried channel, the structure was clearly evidenced by KPM maps of
the electrical potential of the surface region overlying the channel at
increasing distances from the grounded electrode. The KPM profiling shows
regions of opposite contrast located at different distances from the endpoints
of the channel. This effect is attributed to the different electrical
conduction properties of the surface and of the buried graphitic layer. The
model adopted to interpret these KPM maps and profiles proved to be suitable
for the electronic characterization of buried conductive channels, providing a
non-invasive method to measure the local resistivity with a micrometer
resolution. The results demonstrate the potential of the technique as a
powerful diagnostic tool to monitor the functionality of all-carbon
graphite/diamond devices to be fabricated by MeV ion beam lithography.Comment: 21 pages, 5 figure
Forest-grassland transitions : How livestock and fire shape grassy biomes
Plant associations are determined by complex interactions with their environment depending on resource availability, landscape features, and periodic disturbances that shape the structure and functions of these communities. Forests, savannas and grasslands extend across the global land surface, contribute to planetary processes and provide ecosystems services sustaining local production. However, the factors that explain the distribution of trees and determine these biomes are still not well understood. In this thesis, long-standing questions about the origins and distribution of these ecosystems are discussed in light of new evidence suggesting that a feedback of fire and grasses may maintain forests, savannas and grasslands as alternative tree cover states. I also address how anthropogenic land use, including the introduction of livestock, may be affecting these dynamics, particularly in the neotropics, with consequences in terms of potential transitions in tree cover regimes. I analyze the distribution of trees in the grasslands of subtropical South America, looking at what may determine current tree cover and change dynamics (Chapters 2 & 3). The results suggest that, in non-cultivated areas, the expansion of trees into grasslands is likely limited by fire, livestock and precipitation, and that livestock likely reduces fire frequency (Chapter 2). The analyses also suggest that in the Uruguayan Campos of southeastern South America, where fire frequency is low and livestock densities are high, a release in livestock density may cause a moderate expansion of forests into grasslands (Chapter 3). To understand the consequences of a potential transition to higher tree cover by increasing precipitation, I looked at the effects of tree cover in subtropical rangelands (Chapter 5). The results indicated that isolated trees can improve the forage quality and abundance of these rangelands, with potential benefits in terms of biodiversity and ecosystem services. Lastly, I analyzed correlational patterns relating livestock density to vegetation structure across the global tropics and subtropics (Chapter 4), in an attempt to generalize the findings of Chapter 2. The results indicate that extensive livestock systems reduce fire frequency and impact vegetation structure, maintaining savannas and grasslands with low tree cover, low fire frequency and a higher presence of shrubs and dwarf trees.</p
Microelectrode arrays of diamond-insulated graphitic channels for real time detection of exocytotic events from cultured chromaffin cells and slices of adrenal glands
A microstructured graphitic 4x4 multielectrode array was embedded in a single
crystal diamond substrate (4x4 {uG-SCD MEA) for real-time monitoring of
exocytotic events from cultured chromaffin cells and adrenal slices. The
current approach relies on the development of a parallel ion beam lithographic
technique, which assures the time effective fabrication of extended arrays with
reproducible electrode dimensions. The reported device is suitable for
performing amperometric and voltammetric recordings with high sensitivity and
temporal resolution, by simultaneously acquiring data from 16 rectangularly
shaped microelectrodes (20x3.5 um^2) separated by 200 um gaps. Taking advantage
of the array geometry we addressed the following specific issues: i) detect
both the spontaneous and KCl-evoked secretion simultaneously from several
chromaffin cells directly cultured on the device surface, ii) resolve the
waveform of different subsets of exocytotic events, iii) monitoring quantal
secretory events from thin slices of the adrenal gland. The frequency of
spontaneous release was low (0.12 Hz and 0.3 Hz respectively for adrenal slices
and cultured cells) and increased up to 0.9 Hz after stimulation with 30 mM KCl
in cultured cells. The spike amplitude as well as rise and decay time were
comparable with those measured by carbon fiber microelectrodes and allowed to
identify three different subsets of secretory events associated to "full
fusion" events, "kiss and-run" and "kiss-and-stay" exocytosis, confirming that
the device has adequate sensitivity and time resolution for real-time
recordings. The device offers the significant advantage of shortening the time
to collect data by allowing simultaneous recordings from cell populations
either in primary cell cultures or in intact tissues
STOCHASTIC PERTURBATION OF A CUBIC ANHARMONIC OSCILLATOR
We perturb with an additive noise the Hamiltonian system associated to a cubic anharmonic oscillator. This gives rise to a system of stochastic differential equations with quadratic drift and degenerate diffusion matrix. Firstly, we show that such systems possess explosive solutions for certain initial conditions. Then, we carry a small noise expansion's analysis of the stochastic system which is assumed to start from initial conditions that guarantee the existence of a periodic solution for the unperturbed equation. We then investigate the probabilistic properties of the sequence of coefficients which turn out to be the unique strong solutions of stochastic perturbations of the well-known Lamé's equation. We also obtain explicit expressions of these in terms of Jacobi elliptic functions. Furthermore, we prove, in the case of Brownian noise, a lower bound for the probability that the truncated expansion stays close to the solution of the deterministic problem. Lastly, when the noise is bounded, we provide conditions for the almost sure convergence of the global expansion
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