2,640 research outputs found
The Ultramassive White Dwarf EUVE J1746-706
We have obtained new optical and extreme ultraviolet (EUV) spectroscopy of
the ultramassive white dwarf EUVE J1746-706. We revise Vennes et al.'s (1996a,
ApJ, 467, 784) original estimates of the atmospheric parameters and we measure
an effective temperature of 46,500 +/- 700 K and a surface gravity log g = 9.05
+/- 0.15 (~1.2 M_o), in agreement with Balmer line profiles and the EUV
continuum. We derive an upper limit on the atmospheric abundance of helium of
He/H = 1.3 x 10^{-4} and a neutral hydrogen column density in the local
interstellar medium N_HI = 1.8 +/- 0.4 x 10^{19} cm^{-2} from the EUV spectrum.
Our upper limit corresponds to half the helium abundance observed in the
atmosphere of the ultramassive white dwarf GD 50. We discuss the possibility
that EUVE J1746-706 represents an earlier phase of evolution relative to GD 50
and may, therefore, help us understand the origin and evolution of massive
white dwarfs.Comment: 6 pages, 4 postscript figures, uses aastex, to be published in ApJ
Letter
Influence of Coulomb interaction on the Aharonov-Bohm effect in an electronic Fabry-Perot interferometer
We study the role of Coulomb interaction in an electronic Fabry-Perot
interferometer (FPI) realized with chiral edge states in the integer quantum
Hall regime in the limit of weak backscattering. Assuming that a compressible
Coulomb island in a bulk region of the FPI is formed, we develop a capacitance
model which explains the plethora of experimental data on the flux and gate
periodicity of conductance oscillations. It is also shown that a suppression of
finite-bias visibility stems from a combination of weak Coulomb blockade and a
nonequilibrium dephasing by the quantum shot noise
Tunnel current in self-assembled monolayers of 3-mercaptopropyltrimethoxysilane
The current density-voltage (J-V) characteristics of self assembled
monolayers of 3-mercaptopropyltrimethoxysilane (MPTMS) chemisorbed on the
native oxide surface of p+-doped Si demonstrate the excellent tunnel dielectric
behavior of organic monolayers down to 3 carbon atoms. The J-V characteristics
of MPTMS SAMs on Si are found to be asymmetric, and the direction of
rectification has been found to depend upon the applied voltage range. At
voltages < 2.45V, the reverse bias current was found to be higher than forward
bias current; while at higher voltages this trend was reversed. This result is
in agreement with Simmons theory. The tunnel barrier heights for this short
chain (2.56 and 2.14 eV respectively at Au and Si interfaces) are in good
agreement with the ones for longer chains (>10 carbon atoms) if the chain is
chemisorbed at the electrodes. These results extend all previous experiments on
such molecular tunnel dielectrics down to 3 carbon atoms. This suggests that
these molecular monolayers, having good tunnel behavior (up to 2.5 eV) over a
large bias range, can be used as gate dielectric well below the limits of
Si-based dielectrics.Comment: Small, in pres
The max-plus finite element method for solving deterministic optimal control problems: basic properties and convergence analysis
We introduce a max-plus analogue of the Petrov-Galerkin finite element method
to solve finite horizon deterministic optimal control problems. The method
relies on a max-plus variational formulation. We show that the error in the sup
norm can be bounded from the difference between the value function and its
projections on max-plus and min-plus semimodules, when the max-plus analogue of
the stiffness matrix is exactly known. In general, the stiffness matrix must be
approximated: this requires approximating the operation of the Lax-Oleinik
semigroup on finite elements. We consider two approximations relying on the
Hamiltonian. We derive a convergence result, in arbitrary dimension, showing
that for a class of problems, the error estimate is of order or , depending on the
choice of the approximation, where and are respectively the
time and space discretization steps. We compare our method with another
max-plus based discretization method previously introduced by Fleming and
McEneaney. We give numerical examples in dimension 1 and 2.Comment: 31 pages, 11 figure
Kernels for Feedback Arc Set In Tournaments
A tournament T=(V,A) is a directed graph in which there is exactly one arc
between every pair of distinct vertices. Given a digraph on n vertices and an
integer parameter k, the Feedback Arc Set problem asks whether the given
digraph has a set of k arcs whose removal results in an acyclic digraph. The
Feedback Arc Set problem restricted to tournaments is known as the k-Feedback
Arc Set in Tournaments (k-FAST) problem. In this paper we obtain a linear
vertex kernel for k-FAST. That is, we give a polynomial time algorithm which
given an input instance T to k-FAST obtains an equivalent instance T' on O(k)
vertices. In fact, given any fixed e>0, the kernelized instance has at most
(2+e)k vertices. Our result improves the previous known bound of O(k^2) on the
kernel size for k-FAST. Our kernelization algorithm solves the problem on a
subclass of tournaments in polynomial time and uses a known polynomial time
approximation scheme for k-FAST
Experimental evidence of shock mitigation in a Hertzian tapered chain
We present an experimental study of the mechanical impulse propagation
through a horizontal alignment of elastic spheres of progressively decreasing
diameter , namely a tapered chain. Experimentally, the diameters of
spheres which interact via the Hertz potential are selected to keep as close as
possible to an exponential decrease, , where the
experimental tapering factor is either ~% or ~%.
In agreement with recent numerical results, an impulse initiated in a
monodisperse chain (a chain of identical beads) propagates without shape
changes, and progressively transfer its energy and momentum to a propagating
tail when it further travels in a tapered chain. As a result, the front pulse
of this wave decreases in amplitude and accelerates. Both effects are
satisfactorily described by the hard spheres approximation, and basically, the
shock mitigation is due to partial transmissions, from one bead to the next, of
momentum and energy of the front pulse. In addition when small dissipation is
included, a better agreement with experiments is found. A close analysis of the
loading part of the experimental pulses demonstrates that the front wave adopts
itself a self similar solution as it propagates in the tapered chain. Finally,
our results corroborate the capability of these chains to thermalize
propagating impulses and thereby act as shock absorbing devices.Comment: ReVTeX, 7 pages with 6 eps, accepted for Phys. Rev. E (Related papers
on http://www.supmeca.fr/perso/jobs/
How Hertzian solitary waves interact with boundaries in a 1-D granular medium
We perform measurements, numerical simulations, and quantitative comparisons
with available theory on solitary wave propagation in a linear chain of beads
without static preconstrain. By designing a nonintrusive force sensor to
measure the impulse as it propagates along the chain, we study the solitary
wave reflection at a wall. We show that the main features of solitary wave
reflection depend on wall mechanical properties. Since previous studies on
solitary waves have been performed at walls without these considerations, our
experiment provides a more reliable tool to characterize solitary wave
propagation. We find, for the first time, precise quantitative agreements.Comment: Proof corrections, ReVTeX, 11 pages, 3 eps (Focus and related papers
on http://www.supmeca.fr/perso/jobs/
Typical Borel measures on satisfy a multifractal formalism
In this article, we prove that in the Baire category sense, measures
supported by the unit cube of typically satisfy a multifractal
formalism. To achieve this, we compute explicitly the multifractal spectrum of
such typical measures . This spectrum appears to be linear with slope 1,
starting from 0 at exponent 0, ending at dimension at exponent , and it
indeed coincides with the Legendre transform of the -spectrum associated
with typical measures .Comment: 17 pages. To appear in Nonlinearit
Multiscale fractal dimension analysis of a reduced order model of coupled ocean–atmosphere dynamics
Atmosphere and ocean dynamics display many complex features and are characterized by a wide variety of processes and couplings across different timescales. Here we demonstrate the application of multivariate empirical mode decomposition (MEMD) to investigate the multivariate and multiscale properties of a reduced order model of the ocean–atmosphere coupled dynamics. MEMD provides a decomposition of the original multivariate time series into a series of oscillating patterns with time-dependent amplitude and phase by exploiting the local features of the data and without any a priori assumptions on the decomposition basis. Moreover, each oscillating pattern, usually named multivariate intrinsic mode function (MIMF), represents a local source of information that can be used to explore the behavior of fractal features at different scales by defining a sort of multiscale and multivariate generalized fractal dimensions. With these two complementary approaches, we show that the ocean–atmosphere dynamics presents a rich variety of features, with different multifractal properties for the ocean and the atmosphere at different timescales. For weak ocean–atmosphere coupling, the resulting dimensions of the two model components are very different, while for strong coupling for which coupled modes develop, the scaling properties are more similar especially at longer timescales. The latter result reflects the presence of a coherent coupled dynamics. Finally, we also compare our model results with those obtained from reanalysis data demonstrating that the latter exhibit a similar qualitative behavior in terms of multiscale dimensions and the existence of a scale dependency of the statistics of the phase-space density of points for different regions, which is related to the different drivers and processes occurring at different timescales in the coupled atmosphere–ocean system. Our approach can therefore be used to diagnose the strength of coupling in real applications
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