463 research outputs found
Hybridations entre Globodera rostochiensis (Wollenweber), G. pallida (Stone), G. virginiae (Miller and Gray), G. solanacearum (Miller and Gray) et G. "mexicana" (Campos-Vela) : description et devenir des hybrides
On the energy growth of some periodically driven quantum systems with shrinking gaps in the spectrum
We consider quantum Hamiltonians of the form H(t)=H+V(t) where the spectrum
of H is semibounded and discrete, and the eigenvalues behave as E_n~n^\alpha,
with 0<\alpha<1. In particular, the gaps between successive eigenvalues decay
as n^{\alpha-1}. V(t) is supposed to be periodic, bounded, continuously
differentiable in the strong sense and such that the matrix entries with
respect to the spectral decomposition of H obey the estimate
|V(t)_{m,n}|0,
p>=1 and \gamma=(1-\alpha)/2. We show that the energy diffusion exponent can be
arbitrarily small provided p is sufficiently large and \epsilon is small
enough. More precisely, for any initial condition \Psi\in Dom(H^{1/2}), the
diffusion of energy is bounded from above as _\Psi(t)=O(t^\sigma) where
\sigma=\alpha/(2\ceil{p-1}\gamma-1/2). As an application we consider the
Hamiltonian H(t)=|p|^\alpha+\epsilon*v(\theta,t) on L^2(S^1,d\theta) which was
discussed earlier in the literature by Howland
IceCube expectations for two high-energy neutrino production models at active galactic nuclei
We have determined the currently allowed regions of the parameter spaces of
two representative models of diffuse neutrino flux from active galactic nuclei
(AGN): one by Koers & Tinyakov (KT) and another by Becker & Biermann (BB). Our
observable has been the number of upgoing muon-neutrinos expected in the
86-string IceCube detector, after 5 years of exposure, in the range 10^5 <
E/GeV < 10^8. We have used the latest estimated discovery potential of the
IceCube-86 array at the 5-sigma level to determine the lower boundary of the
regions, while for the upper boundary we have used either the AMANDA upper
bound on the neutrino flux or the more recent preliminary upper bound given by
the half-completed IceCube-40 array (IC40). We have varied the spectral index
of the proposed power-law fluxes, alpha, and two parameters of the BB model:
the ratio between the boost factors of neutrinos and cosmic rays,
Gamma_nu/Gamma_{CR}, and the maximum redshift of the sources that contribute to
the cosmic-ray flux, zCRmax. For the KT model, we have considered two
scenarios: one in which the number density of AGN does not evolve with redshift
and another in which it evolves strongly, following the star formation rate.
Using the IC40 upper bound, we have found that the models are visible in
IceCube-86 only inside very thin strips of parameter space and that both of
them are discarded at the preferred value of alpha = 2.7 obtained from fits to
cosmic-ray data. Lower values of alpha, notably the values 2.0 and 2.3 proposed
in the literature, fare better. In addition, we have analysed the capacity of
IceCube-86 to discriminate between the models within the small regions of
parameter space where both of them give testable predictions. Within these
regions, discrimination at the 5-sigma level or more is guaranteed.Comment: 24 pages, 6 figures, v2: new IceCube-40 astrophysical neutrino upper
bound and IceCube-86 discovery potential used, explanation of AGN flux models
improved, only upgoing neutrinos used, conclusions strengthened. Accepted for
publication in JCA
Adhesion mechanics of graphene membranes
The interaction of graphene with neighboring materials and structures plays
an important role in its behavior, both scientifically and technologically. The
interactions are complicated due to the interplay between surface forces and
possibly nonlinear elastic behavior. Here we review recent experimental and
theoretical advances in the understanding of graphene adhesion. We organize our
discussion into experimental and theoretical efforts directed toward: graphene
conformation to a substrate, determination of adhesion energy, and applications
where graphene adhesion plays an important role. We conclude with a brief
prospectus outlining open issues.Comment: Review article to appear in special issue on graphene in Solid State
Communication
Electron transport and energy relaxation in dilute magnetic alloys
We consider the effect of the RKKY interaction between magnetic impurities on
the electron relaxation rates in a normal metal. The interplay between the RKKY
interaction and the Kondo effect may result in a non-monotonic temperature
dependence of the electron momentum relaxation rate, which determines the Drude
conductivity. The electron phase relaxation rate, which determines the
magnitude of the weak localization correction to the resistivity, is also a
non-monotonic function of temperature. For this function, we find the
dependence of the position of its maximum on the concentration of magnetic
impurities. We also relate the electron energy relaxation rate to the
excitation spectrum of the system of magnetic impurities. The energy relaxation
determines the distribution function for the out-of-equilibrium electrons.
Measurement of the electron distribution function thus may provide information
about the excitations in the spin glass phase.Comment: 15 pages, 5 figure
Sub-terahertz, microwaves and high energy emissions during the December 6, 2006 flare, at 18:40 UT
The presence of a solar burst spectral component with flux density increasing
with frequency in the sub-terahertz range, spectrally separated from the
well-known microwave spectral component, bring new possibilities to explore the
flaring physical processes, both observational and theoretical. The solar event
of 6 December 2006, starting at about 18:30 UT, exhibited a particularly
well-defined double spectral structure, with the sub-THz spectral component
detected at 212 and 405 GHz by SST and microwaves (1-18 GHz) observed by the
Owens Valley Solar Array (OVSA). Emissions obtained by instruments in
satellites are discussed with emphasis to ultra-violet (UV) obtained by the
Transition Region And Coronal Explorer (TRACE), soft X-rays from the
Geostationary Operational Environmental Satellites (GOES) and X- and gamma-rays
from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The sub-THz
impulsive component had its closer temporal counterpart only in the higher
energy X- and gamma-rays ranges. The spatial positions of the centers of
emission at 212 GHz for the first flux enhancement were clearly displaced by
more than one arc-minute from positions at the following phases. The observed
sub-THz fluxes and burst source plasma parameters were found difficult to be
reconciled to a purely thermal emission component. We discuss possible
mechanisms to explain the double spectral components at microwaves and in the
THz ranges.Comment: Accepted version for publication in Solar Physic
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