3,099 research outputs found
Optimal Regularizing Effect for Scalar Conservation Laws
We investigate the regularity of bounded weak solutions of scalar
conservation laws with uniformly convex flux in space dimension one, satisfying
an entropy condition with entropy production term that is a signed Radon
measure. The proof is based on the kinetic formulation of scalar conservation
laws and on an interaction estimate in physical space.Comment: 24 pages, assumption (11) in Theorem 3.1 modified together with the
example on p. 7, one remark added after the proof of Lemma 4.3, some typos
correcte
Negative reflection of elastic guided waves in chaotic and random scattering media
The propagation of waves in complex media can be harnessed either by taming
the incident wave-field impinging on the medium or by forcing waves along
desired paths through its careful design. These two alternative strategies have
given rise to fascinating concepts such as time reversal or negative
refraction. Here, we show how these two processes are intimately linked through
the negative reflection phenomenon. A negative reflecting mirror converts a
wave of positive phase velocity into its negative counterpart and vice versa.
In this article, we experimentally demonstrate this phenomenon with elastic
waves in a 2D billiard and in a disordered plate by means of laser
interferometry. Despite the complexity of such configurations, the negatively
reflected wave field focuses back towards the initial source location, thereby
mimicking a phase conjugation operation while being a fully passive process.
The super-focusing capability of negative reflection is also highlighted in a
monochromatic regime. The negative reflection phenomenon is not restricted to
guided elastic waves since it can occur in zero-gap systems such as photonic
crystals, chiral metamaterials or graphene. Negative reflection can thus become
a tool of choice for the control of waves in all fields of wave physics.Comment: 9 pages, 6 figure
Synthetic observations of first hydrostatic cores in collapsing low-mass dense cores II. Simulated ALMA dust emission maps
First hydrostatic cores are predicted by theories of star formation, but
their existence has never been demonstrated convincingly by (sub)millimeter
observations. Furthermore, the multiplicity at the early phases of the star
formation process is poorly constrained. The purpose of this paper is twofold.
First, we seek to provide predictions of ALMA dust continuum emission maps from
early Class 0 objects. Second, we show to what extent ALMA will be able to
probe the fragmentation scale in these objects. Following our previous paper
(Commer\c{c}on et al. 2012, hereafter paper I), we post-process three
state-of-the-art radiation-magneto-hydrodynamic 3D adaptive mesh refinement
calculations to compute the emanating dust emission maps. We then produce
synthetic ALMA observations of the dust thermal continuum from first
hydrostatic cores. We present the first synthetic ALMA observations of dust
continuum emission from first hydrostatic cores. We analyze the results given
by the different bands and configurations and we discuss for which combinations
of the two the first hydrostatic cores would most likely be observed. We also
show that observing dust continuum emission with ALMA will help in identifying
the physical processes occurring within collapsing dense cores. If the magnetic
field is playing a role, the emission pattern will show evidence of a
pseudo-disk and even of a magnetically driven outflow, which pure
hydrodynamical calculations cannot reproduce. The capabilities of ALMA will
enable us to make significant progress towards understanding fragmentation at
the early Class 0 stage and discovering first hydrostatic cores.Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
The influence of residual oxidizing impurities on the synthesis of graphene by atmospheric pressure chemical vapor deposition
The growth of graphene on copper by atmospheric pressure chemical vapor
deposition in a system free of pumping equipment is investigated. The emphasis
is put on the necessity of hydrogen presence during graphene synthesis and
cooling. In the absence of hydrogen during the growth step or cooling at slow
rate, weak carbon coverage, consisting mostly of oxidized and amorphous carbon,
is obtained on the copper catalyst. The oxidation originates from the
inevitable occurrence of residual oxidizing impurities in the reactor's
atmosphere. Graphene with appreciable coverage can be grown within the
vacuum-free furnace only upon admitting hydrogen during the growth step. After
formation, it is preserved from the destructive effect of residual oxidizing
contaminants once exposure at high temperature is minimized by fast cooling or
hydrogen flow. Under these conditions, micrometer-sized hexagon-shaped graphene
domains of high structural quality are achieved.Comment: Accepted in Carbo
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