5,609 research outputs found
On the two-dimensional state in driven magnetohydrodynamic turbulence
The dynamics of the two-dimensional (2D) state in driven tridimensional (3D)
incompressible magnetohydrodynamic turbulence is investigated through
high-resolution direct numerical simulations and in the presence of an external
magnetic field at various intensities. For such a flow the 2D state (or slow
mode) and the 3D modes correspond respectively to spectral fluctuations in the
plan and in the area . It is shown that if
initially the 2D state is set to zero it becomes non negligible in few turnover
times particularly when the external magnetic field is strong. The maintenance
of a large scale driving leads to a break for the energy spectra of 3D modes;
when the driving is stopped the previous break is removed and a decay phase
emerges with alfv\'enic fluctuations. For a strong external magnetic field the
energy at large perpendicular scales lies mainly in the 2D state and in all
situations a pinning effect is observed at small scales.Comment: 11 pages, 11 figure
Using Chemistry to Unveil the Kinematics of Starless Cores: Complex Radial Motions in Barnard 68
We present observations of 13CO, C18O, HCO+, H13CO+, DCO+ and N2H+ line
emission towards the Barnard 68 starless core. The line profiles are
interpreted using a chemical network coupled with a radiative transfer code in
order to reconstruct the radial velocity profile of the core. Our observations
and modeling indicate the presence of complex radial motions, with the inward
motions in the outer layers of the core but outward motions in the inner part,
suggesting radial oscillations. The presence of such oscillation would imply
that B68 is relatively old, typically one order of magnitude older than the age
inferred from its chemical evolution and statistical core lifetimes. Our study
demonstrates that chemistry can be used as a tool to constrain the radial
velocity profiles of starless cores.Comment: 12 pages, 3 figures, to appear in the Astrophysical Journal Letter
Activated aging dynamics and negative fluctuation-dissipation ratios
In glassy materials aging proceeds at large times via thermal activation. We
show that this can lead to negative dynamical response functions and novel and
well-defined violations of the fluctuation-dissipation theorem, in particular,
negative fluctuation-dissipation ratios. Our analysis is based on detailed
theoretical and numerical results for the activated aging regime of simple
kinetically constrained models. The results are relevant to a variety of
physical situations such as aging in glass-formers, thermally activated domain
growth and granular compaction.Comment: 4 pages, 4 figs; v2 final version (minor modifs) published in Phys.
Rev. Let
Complex-network analysis of combinatorial spaces: The NK landscape case
We propose a network characterization of combinatorial fitness landscapes by
adapting the notion of inherent networks proposed for energy surfaces. We use
the well-known family of NK landscapes as an example. In our case the inherent
network is the graph whose vertices represent the local maxima in the
landscape, and the edges account for the transition probabilities between their
corresponding basins of attraction. We exhaustively extracted such networks on
representative NK landscape instances, and performed a statistical
characterization of their properties. We found that most of these network
properties are related to the search difficulty on the underlying NK landscapes
with varying values of K.Comment: arXiv admin note: substantial text overlap with arXiv:0810.3492,
arXiv:0810.348
Electronic structure of amorphous germanium disulfide via density functional molecular dynamics simulations
Using density functional molecular dynamics simulations we study the
electronic properties of glassy g-GeS. We compute the electronic density of
states, which compares very well with XPS measurements, as well as the partial
EDOS and the inverse participation ratio. We show the electronic contour plots
corresponding to different structural environments, in order to determine the
nature of the covalent bonds between the atoms. We finally study the local
atomic charges, and analyze the impact of the local environment on the charge
transfers between the atoms. The broken chemical order inherent to amorphous
systems leads to locally charged zones when integrating the atomic charges up
to nearest-neighbor distances.Comment: 13 pages, 9 figures; to appear in Phys. Rev.
CCA: An R Package to Extend Canonical Correlation Analysis
Canonical correlations analysis (CCA) is an exploratory statistical method to highlight correlations between two data sets acquired on the same experimental units. The cancor() function in R (R Development Core Team 2007) performs the core of computations but further work was required to provide the user with additional tools to facilitate the interpretation of the results. We implemented an R package, CCA, freely available from the Comprehensive R Archive Network (CRAN, http://CRAN.R-project.org/), to develop numerical and graphical outputs and to enable the user to handle missing values. The CCA package also includes a regularized version of CCA to deal with data sets with more variables than units. Illustrations are given through the analysis of a data set coming from a nutrigenomic study in the mouse.
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