987 research outputs found
Electron-hole coexistence in disordered graphene probed by high-field magneto-transport
We report on magneto-transport measurement in disordered graphene under
pulsed magnetic field of up to 57T. For large electron or hole doping, the
system displays the expected anomalous Integer Quantum Hall Effect (IQHE)
specific to graphene up to filling factor . In the close vicinity of the
charge neutrality point, the system breaks up into co-existing puddles of holes
and electrons, leading to a vanishing Hall and finite longitudinal resistance
with no hint of divergence at very high magnetic field. Large resistance
fluctuations are observed near the Dirac point. They are interpreted as the the
natural consequence of the presence of electron and hole puddles. The magnetic
field at which the amplitude of the fluctuations are the largest is directly
linked to the mean size of the puddles
Search for neutrinos from transient sources with the ANTARES telescope and optical follow-up observations
The ANTARES telescope has the opportunity to detect transient neutrino
sources, such as gamma-ray bursts, core-collapse supernovae, flares of active
nuclei... To enhance the sensitivity to these sources, we have developed a new
detection method based on the optical follow-up of "golden" neutrino events
such as neutrino doublets coincident in time and space or single neutrinos of
very high energy. The ANTARES Collaboration has therefore implemented a very
fast on-line reconstruction with a good angular resolution. These
characteristics allow to trigger an optical telescope network; since February
2009. ANTARES is sending alert trigger one or two times per month to the two 25
cm robotic telescope of TAROT. This follow-up of such special events would not
only give access to the nature of the sources but also improves the sensitivity
for transient neutrino sources.Comment: 3 pages, 3 figures, Proceedings of the 31st ICRC, Lodz, Polan, July
200
Integer Quantum Hall Effect in Trilayer Graphene
The Integer Quantum Hall Effect (IQHE) is a distinctive phase of
two-dimensional electronic systems subjected to a perpendicular magnetic field.
Thus far, the IQHE has been observed in semiconductor heterostructures and in
mono- and bi-layer graphene. Here we report on the IQHE in a new system:
trilayer graphene. Experimental data are compared with self-consistent Hartree
calculations of the Landau levels for the gated trilayer. The plateau structure
in the Hall resistivity determines the stacking order (ABA versus ABC). We find
that the IQHE in ABC trilayer graphene is similar to that in the monolayer,
except for the absence of a plateau at filling factor v=2. At very low filling
factor, the Hall resistance vanishes due to the presence of mixed electron and
hole carriers induced by disorder.Comment: 5 pages, 4 figure
Microbiological aspects of methane emission in a ricefield soil from the Camargue (France) : 2. Methanotrophy and related microflora
Les principaux facteurs qui influencent l'activité méthanotrophe du sol (épaisseur du sol, humidité, addition d'engrais et dessiccation) ont été étudiés in vitro sur un sol de riziÚre de Camargue (France) afin de standardiser une méthode pour estimer l'activité méthanotrophe potentielle (PMA) d'un sol. La possibilité d'énumérer les méthanotrophes du sol a été étudiée et les souches dominantes du sol incubé pendant un mois sous un mélange de 80% air/20% CH4 ont été isolées. La méthanotrophie in vitro est un phénomÚne quantifiable par unité de surface plutÎt que par unité de poids de sol. L'activité maximale (PMA)(1.8 mole m-2 j-1) a été mesurée sur un échantillon à une humidité voisine de la capacité au champ et incubé pendant un mois sous air/CH4. Les consommations de CH4 et d'O2 ainsi que la production de CO2 sont fortement corrélées. La consommation d'O2 liée à l'activité hétérotrophe du sol est négligeable comparée à celle due à l'activité méthanotrophe. L'addition d'engrais à des concentrations utilisées en agriculture est sans effet sur la PMA qui est restée stable pendant deux ans dans le sol conservé à l'état sec. Une trÚs forte contamination rend les étalements sur boites de Pétri inexploitables pour dénombrer les bactéries méthanotrophes du sol. Les populations estimées par MPN sont faibles dans le sol sec (2.5 x 102 g-1) et atteignent 5 x 108 g-1 dans le sol enrichi. La morphologie des souches isolées indique leur appartenance aux bactéries méthanotrophes de type II. (Résumé d'auteur
ALMA Capabilities for Observations of Spectral Line Emission
The Atacama Large Millimeter/submillimeter Array (ALMA) combines large
collecting area and location on a high dry site to provide it with unparalleled
potential for sensitive millimeter/submillimeter spectral line observations.
Its wide frequency coverage, superb receivers and flexible spectrometer will
ensure that its potential is met. Since the 1999 meeting on ALMA
Science\cite{RefA}, the ALMA team has substantially enhanced its capability for
line observations. ALMA's sensitivity increased when Japan joined the project,
bringing the 16 antennas of the Atacama Compcat Array (ACA), equivalent to
eight additional 12m telescopes. The first four receiver cartridges for the
baseline ALMA (Japan's entry has brought two additional bands to ALMA's
receiver retinue) have been accepted, with performance above the
already-challenging specifications. ALMA's flexibility has increased with the
enhancement of the baseline correlator with additional channels and
flexibility, and with the addition of a separate correlator for the ACA. As an
example of the increased flexibility, ALMA is now capable of
multi-spectral-region and multi-resolution modes. With the former, one might
observe e.g. four separate transitions anywhere within a 2 GHz band with a high
resolution bandwidth. With the latter, one might simultaneously observe with
low spectral resolution over a wide bandwidth and with high spectral resolution
over a narrow bandwidth; this mode could be useful for observations of
pressure-broadened lines with narrow cores, for example. Several science
examples illustrate ALMA's potential for transforming millimeter and
submillimeter astronomy.Comment: 6 pages, for the Proceedings of Science with ALMA: a new era for
Astrophysics: International Conference, 2006 held 13 - 17 November 2006 in
Madrid, Spai
Approximability of Connected Factors
Finding a d-regular spanning subgraph (or d-factor) of a graph is easy by
Tutte's reduction to the matching problem. By the same reduction, it is easy to
find a minimal or maximal d-factor of a graph. However, if we require that the
d-factor is connected, these problems become NP-hard - finding a minimal
connected 2-factor is just the traveling salesman problem (TSP).
Given a complete graph with edge weights that satisfy the triangle
inequality, we consider the problem of finding a minimal connected -factor.
We give a 3-approximation for all and improve this to an
(r+1)-approximation for even d, where r is the approximation ratio of the TSP.
This yields a 2.5-approximation for even d. The same algorithm yields an
(r+1)-approximation for the directed version of the problem, where r is the
approximation ratio of the asymmetric TSP. We also show that none of these
minimization problems can be approximated better than the corresponding TSP.
Finally, for the decision problem of deciding whether a given graph contains
a connected d-factor, we extend known hardness results.Comment: To appear in the proceedings of WAOA 201
Cosmological inference including massive neutrinos from the matter power spectrum: biases induced by uncertainties in the covariance matrix
Data analysis from upcoming large galaxy redshift surveys, such as Euclid and
DESI will significantly improve constraints on cosmological parameters. To
optimally extract the information from these galaxy surveys, it is important to
control with a high level of confidence the uncertainty and bias arising from
the estimation of the covariance that affects the inference of cosmological
parameters. In this work, we are addressing two different but closely related
issues: (i) the sampling noise present in a covariance matrix estimated from a
finite set of simulations and (ii) the impact on cosmological constraints of
the non-Gaussian contribution to the covariance matrix of the power spectrum.
We focus on the parameter estimation obtained from fitting the matter power
spectrum in real space, using the DEMNUni N-body simulations. Regarding the
first issue, we adopt two different approaches to reduce the sampling noise in
the precision matrix that propagates in the parameter space: on the one hand
using an alternative estimator of the covariance matrix based on a non-linear
shrinkage, NERCOME; and on the other hand employing a method of fast generation
of approximate mock catalogs, COVMOS. We find that NERCOME can significantly
reduce the noise induced on the posterior distribution of parameters, but at
the cost of a systematic overestimation of the error bars on the cosmological
parameters. We show that using a COVMOS covariance matrix estimated from a
large number of realisations (10~000) results in unbiased cosmological
constraints. Regarding the second issue, we quantify the impact on cosmological
constraints of the non-Gaussian part of the power spectrum covariance purely
coming from non-linear clustering. We find that when this term is neglected,
both the errors and central values of the estimated parameters are affected for
a scale cut \kmax > 0.2\ \invMpc.Comment: 21 pages, 2 appendices, 20 figures. Submitted to A&
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