3,397 research outputs found
Spectroscopic characterization of graphene films grown on Pt (111) surface by chemical vapor deposition of ethylene
This work reports the peculiar properties of a graphene film prepared by the
chemical vapor deposition (CVD) of ethylene in high vacuum on a well oriented
and carefully cleaned Pt(111) crystal surface maintained at high temperature.
In-situ and ex-situ characterization techniques (low energy electron
diffraction, high resolution electron energy loss spectroscopy, scanning
electron microscopy and Raman micro-spectroscopy) used here indicate the
prevalence of single layer regions and the presence of two different
orientations of the graphene sheets with respect to the Pt(111) substrate. In
most of the deposited area evidence is found of a compressive stress for the
graphene lattice, as a net result of the growth process on a metal substrate.
This graphene film grown on Pt(111) exhibits a lower degree of order and of
homogeneity with respect to the exfoliated graphene on Si/SiO2, as it is found
generally for graphene on metals, but several characterization techniques
indicates a better quality than in previous deposition experiments on the same
metal substrate.Comment: 18 pages, 5 figures, Journal of Raman spectroscopy 201
Anisotropic fluxes and nonlocal interactions in MHD turbulence
We investigate the locality or nonlocality of the energy transfer and of the
spectral interactions involved in the cascade for decaying magnetohydrodynamic
(MHD) flows in the presence of a uniform magnetic field at various
intensities. The results are based on a detailed analysis of three-dimensional
numerical flows at moderate Reynold numbers. The energy transfer functions, as
well as the global and partial fluxes, are examined by means of different
geometrical wavenumber shells. On the one hand, the transfer functions of the
two conserved Els\"asser energies and are found local in both the
directions parallel (-direction) and perpendicular (-direction)
to the magnetic guide-field, whatever the -strength. On the other
hand, from the flux analysis, the interactions between the two
counterpropagating Els\"asser waves become nonlocal. Indeed, as the -intensity is increased, local interactions are strongly decreased and the
interactions with small modes dominate the cascade. Most of the energy
flux in the -direction is due to modes in the plane at , while
the weaker cascade in the -direction is due to the modes with .
The stronger magnetized flows tends thus to get closer to the weak turbulence
limit where the three-wave resonant interactions are dominating. Hence, the
transition from the strong to the weak turbulence regime occurs by reducing the
number of effective modes in the energy cascade.Comment: Submitted to PR
Scaling properties of three-dimensional magnetohydrodynamic turbulence
The scaling properties of three-dimensional magnetohydrodynamic turbulence
are obtained from direct numerical simulations of decaying turbulence using
modes. The results indicate that the turbulence does not follow the
Iroshnikov-Kraichnan phenomenology.In the case of hyperresistivity, the
structure functions exhibit a clear scaling range yielding absolute values of
the scaling exponents . The scaling exponents agree with a modified
She-Leveque model , corresponding to Kolmogorov
scaling but sheet-like geometry of the dissipative structures
A cDNA Microarray Gene Expression Data Classifier for Clinical Diagnostics Based on Graph Theory
Despite great advances in discovering cancer molecular profiles, the proper application of microarray technology to routine clinical diagnostics is still a challenge. Current practices in the classification of microarrays' data show two main limitations: the reliability of the training data sets used to build the classifiers, and the classifiers' performances, especially when the sample to be classified does not belong to any of the available classes. In this case, state-of-the-art algorithms usually produce a high rate of false positives that, in real diagnostic applications, are unacceptable. To address this problem, this paper presents a new cDNA microarray data classification algorithm based on graph theory and is able to overcome most of the limitations of known classification methodologies. The classifier works by analyzing gene expression data organized in an innovative data structure based on graphs, where vertices correspond to genes and edges to gene expression relationships. To demonstrate the novelty of the proposed approach, the authors present an experimental performance comparison between the proposed classifier and several state-of-the-art classification algorithm
How to Make a Singleton sdB Star via Accelerated Stellar Evolution
Many hot subdwarf B stars (sdBs) are in close binaries, and the favored
formation channels for subdwarfs rely on mass transfer in a binary system to
strip a core He burning star of its envelope. However, these channels cannot
account for sdBs that have been observed in long period binaries nor the narrow
mass distribution of isolated (or "singleton") sdBs. We propose a new formation
channel involving the merger of a helium white dwarf and a low mass, hydrogen
burning star, which addresses these issues. Hierarchical triples whose inner
binaries merge and form sdBs by this process could explain the observed long
period subdwarf+main sequence binaries. This process would also naturally
explain the observed slow rotational speeds of singleton sdBs. We also briefly
discuss the implications of this formation channel for extreme horizontal
branch morphology in globular clusters and the UV upturn in elliptical
galaxies.Comment: 5 pages, 3 figures, accepted for publication ApJ
Masses and envelope binding energies of primary stars at the onset of a common envelope
We present basic properties of primary stars that initiate a common envelope
(CE) in a binary, while on the giant branch. We use the population-synthesis
code described in Politano et al. (2010) and follow the evolution of a
population of binary stars up to the point where the primary fills its Roche
lobe and initiates a CE. We then collect the properties of each system, in
particular the donor mass and the binding energy of the donor's envelope, which
are important for the treatment of a CE. We find that for most CEs, the donor
mass is sufficiently low to define the core-envelope boundary reasonably well.
We compute the envelope-structure parameter {\lambda_\mathrm{env}} from the
binding energy and compare its distribution to typical assumptions that are
made in population-synthesis codes. We conclude that {\lambda_\mathrm{env}}
varies appreciably and that the assumption of a constant value for this
parameter results in typical errors of 20--50%. In addition, such an assumption
may well result in the implicit assumption of unintended and/or unphysical
values for the CE parameter {\alpha_\mathrm{CE}}. Finally, we discuss accurate
existing analytic fits for the envelope binding energy, which make these
oversimplified assumptions for {\lambda_\mathrm{env}}, and the use of
{\lambda_\mathrm{env}} in general, unnecessary.Comment: 6 pages, 3 figures, 1 table; to be published in the proceedings of
the conference "Binary Star Evolution", in Mykonos, Greece, held in June
22-25, 201
Non-gaussian probability distribution functions in two dimensional Magnetohydrodynamic turbulence
Intermittency in MHD turbulence has been analyzed using high resolution 2D
numerical simulations. We show that the Probability Distribution Functions
(PDFs) of the fluctuations of the Elsasser fields, magnetic field and velocity
field depend on the scale at hand, that is they are self-affine. The departure
of the PDFs from a Gaussian function can be described through the scaling
behavior of a single parameter lambda_r^2 obtained by fitting the PDFs with a
given curve stemming from the analysis of a multiplicative model by Castaing et
al. (1990). The scaling behavior of the parameter lambda_r^2 can be used to
extract informations about the intermittency. A comparison of intermittency
properties in different MHD turbulent flows is also performed.Comment: 7 pages, with 5 figure
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