308 research outputs found
Automatically Discovering Hidden Transformation Chaining Constraints
Model transformations operate on models conforming to precisely defined
metamodels. Consequently, it often seems relatively easy to chain them: the
output of a transformation may be given as input to a second one if metamodels
match. However, this simple rule has some obvious limitations. For instance, a
transformation may only use a subset of a metamodel. Therefore, chaining
transformations appropriately requires more information. We present here an
approach that automatically discovers more detailed information about actual
chaining constraints by statically analyzing transformations. The objective is
to provide developers who decide to chain transformations with more data on
which to base their choices. This approach has been successfully applied to the
case of a library of endogenous transformations. They all have the same source
and target metamodel but have some hidden chaining constraints. In such a case,
the simple metamodel matching rule given above does not provide any useful
information
Cumulant ratios in fully developed turbulence
In the context of random multiplicative cascade processes, we derive
analytical solutions for one- and two-point cumulants with restored
translational invariance. On taking ratios of cumulants in ln epsilon,
geometrical effects due to spatial averaging cancel out. These ratios can
successfully distinguish between splitting functions while multifractal scaling
exponents and multiplier distributions cannot.Comment: 9th Workshop on Multiparticle Production (Torino), 9 pages latex,
incl 9 figs and espcrc2.st
Stochastic energy-cascade model for 1+1 dimensional fully developed turbulence
Geometrical random multiplicative cascade processes are often used to model
positive-valued multifractal fields such as the energy dissipation in fully
developed turbulence. We propose a dynamical generalization describing the
energy dissipation in terms of a continuous and homogeneous stochastic field in
one space and one time dimension. In the model, correlations originate in the
overlap of the respective spacetime histories of field amplitudes. The
theoretical two- and three-point correlation functions are found to be in good
agreement with their equal-time counterparts extracted from wind tunnel
turbulent shear flow data
Interplay between interferences and electron-electron interactions in epitaxial graphene
We separate localization and interaction effects in epitaxial graphene
devices grown on the C-face of a 4H-SiC substrate by analyzing the low
temperature conductivities. Weak localization and antilocalization are
extracted at low magnetic fields, after elimination of a geometric
magnetoresistance and subtraction of the magnetic field dependent Drude
conductivity. The electron electron interaction correction is extracted at
higher magnetic fields, where localization effects disappear. Both phenomena
are weak but sizable and of the same order of magnitude. If compared to
graphene on silicon dioxide, electron electron interaction on epitaxial
graphene are not significantly reduced by the larger dielectric constant of the
SiC substrate
Translationally invariant cumulants in energy cascade models of turbulence
In the context of random multiplicative energy cascade processes, we derive
analytical expressions for translationally invariant one- and two-point
cumulants in logarithmic field amplitudes. Such cumulants make it possible to
distinguish between hitherto equally successful cascade generator models and
hence supplement lowest-order multifractal scaling exponents and multiplier
distributions.Comment: 11 pages, 3 figs, elsart.cls include
Ballistic spin transport in exciton gases
Traditional spintronics relies on spin transport by charge carriers, such as
electrons in semiconductor crystals. This brings several complications: the
Pauli principle prevents the carriers from moving with the same speed; Coulomb
repulsion leads to rapid dephasing of electron flows. Spin-optronics is a
valuable alternative to traditional spintronics. In spin-optronic devices the
spin currents are carried by electrically neutral bosonic quasi-particles:
excitons or exciton-polaritons. They can form highly coherent quantum liquids
and carry spins over macroscopic distances. The price to pay is a finite
life-time of the bosonic spin carriers. We present the theory of exciton
ballistic spin transport which may be applied to a range of systems where
bosonic spin transport has been reported, in particular, to indirect excitons
in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit
interaction of electrons and holes on the exciton spin, account for the Zeeman
effect induced by external magnetic fields, long range and short range exchange
splittings of the exciton resonances. We also consider exciton transport in the
non-linear regime and discuss the definitions of exciton spin current,
polarization current and spin conductivity.Comment: 16 pages, 10 figures to be published in Phys. Rev.
Growth of monolayer graphene on 8deg off-axis 4H-SiC (000-1) substrates with application to quantum transport devices
Using high temperature annealing conditions with a graphite cap covering the
C-face of an 8deg off-axis 4H-SiC sample, large and homogeneous single
epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of
the almost free-standing character of these monolayer graphene sheets, which
was confirmed by magneto-transport measurements. We find a moderate p-type
doping, high carrier mobility and half integer Quantum Hall effect typical of
high quality graphene samples. This opens the way to a fully compatible
integration of graphene with SiC devices on the wafers that constitute the
standard in today's SiC industry.Comment: 11 pages, 4 figures , Submitted in AP
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