751 research outputs found
Taming the magnetoresistance anomaly in graphite
At low temperatures, graphite presents a magnetoresistance anomaly which
manifests as a transition to a high-resistance state (HRS) above a certain
critical magnetic field . Such HRS is currently attributed
to a c-axis charge-density-wave taking place only when the lowest Landau level
is populated. By controlling the charge carrier concentration of a gated sample
through its charge neutrality level (CNL), we were able to experimentally
modulate the HRS in graphite for the first time. We demonstrate that the HRS is
triggered both when electrons and holes are the majority carriers but is
attenuated near the CNL. Taking screening into account, our results indicate
that the HRS possess a strong in-plane component and can occur below the
quantum limit, being at odds with the current understanding of the phenomenon.
We also report the effect of sample thickness on the HRS
Modelling the relative velocities of isolated pairs of galaxies
We study the comoving relative velocities, v12, of model isolated galaxy
pairs at z=0.5. For this purpose, we use the predictions from the GALFORM
semi-analytical model of galaxy formation and evolution based on a Lambda cold
dark matter cosmology consistent with the results from WMAP7. In real space, we
find that isolated pairs of galaxies are predicted to form an angle t with the
line-of-sight that is uniformily distributed as expected if the Universe is
homogeneous and isotropic. We also find that isolated pairs of galaxies
separated by a comoving distance between 1 and 3 Mpc/h are predicted to have
=0. For galaxies in this regime, the distribution of the angle t is
predicted to change minimally from real to redshift space, with a change
smaller than 5% in . However, the distances defining the comoving
regime strongly depends on the applied isolation criteria.Comment: 4 pages, 4 figures, SF2A 2013 Proceedin
Ballistic effects in a proximity induced superconducting diffusive metal
Using a Scanning Tunneling Microscope (STM), we investigate the Local Density
of States (LDOS) of artificially fabricated normal metal nano-structures in
contact with a superconductor. Very low temperature local spectroscopic
measurements (100 mK) reveal the presence of well defined subgap peaks at
energy |E|<Delta in the LDOS at various positions of the STM tip. Although no
clear correlations between the LDOS and the shape of the samples have emerged,
some of the peak features suggest they originate from quasi-particle bound
states within the normal metal structures (De Gennes St James states).
Refocusing of electronic trajectories induced by the granular srtucture of the
samples can explain the observation of spatially uncorrelated interference
effects in a non-ballistic medium.Comment: 4 pages, 4 figure
Jackknife resampling technique on mocks: an alternative method for covariance matrix estimation
We present a fast and robust alternative method to compute covariance matrix
in case of cosmology studies. Our method is based on the jackknife resampling
applied on simulation mock catalogues. Using a set of 600 BOSS DR11 mock
catalogues as a reference, we find that the jackknife technique gives a similar
galaxy clustering covariance matrix estimate by requiring a smaller number of
mocks. A comparison of convergence rates show that 7 times fewer
simulations are needed to get a similar accuracy on variance. We expect this
technique to be applied in any analysis where the number of available N-body
simulations is low.Comment: 11 pages, 11 figures, 2 table
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
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
Reoptimization of Some Maximum Weight Induced Hereditary Subgraph Problems
The reoptimization issue studied in this paper can be described as follows: given an instance I of some problem Î , an optimal solution OPT for Î in I and an instance IâČ resulting from a local perturbation of I that consists of insertions or removals of a small number of data, we wish to use OPT in order to solve Î in I', either optimally or by guaranteeing an approximation ratio better than that guaranteed by an ex nihilo computation and with running time better than that needed for such a computation. We use this setting in order to study weighted versions of several representatives of a broad class of problems known in the literature as maximum induced hereditary subgraph problems. The main problems studied are max independent set, max k-colorable subgraph and max split subgraph under vertex insertions and deletion
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
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