672 research outputs found
Lunar dust characterization by polarimetric signature. I. Negative polarization branch of sphere aggregates of various porosities
Context. In support of NASA's exploration program and the return to the Moon,
the polarimetric signature of dispersed individual Lunar regolith dust grains
is studied to enable the characterization of the dust exopsheric environment by
remote, in-situ, and standoff sensing. Aims. We explore the value of the
negative polarization branch (NPB) as a signature for characterizing individual
grains to determine if it can be used in the same way as for surfaces of
planets and atmosphereless bodies. Results. Calculations show that polarization
phase curves for spherical grains exhibit a sharp transition over a narrow
range of size parameter between two distinct regimes, one typical of Rayleigh
scattering and another dominated by a large NPB. The linear polarimetric
signature observed for aggregates is a composite of a) the polarization induced
by individual grains composing the aggregate and b) the polarization due to the
aggregate as a whole dust grain. The weight of each component varies depending
on the porosity of the aggregate. An NPB similar to the one observed for
atmosphereless astronomical bodies is present for different ranges of the size
parameter depending on the value of the porosity. It appears as a remnant of
the negative branch exhibited by the single spherical grains. The sharper,
narrow negative branch that is measured for some granular surfaces in the
laboratory or seen in astronomical observations is not observed here.Comment: 7 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Optical polarimetry and photometry of comet 17P/Holmes
Comet 17P/Holmes was observed for linear polarisation using the optical
polarimeter mounted on the 1.2m telescope atop Gurushikhar peak near Mt. Abu
during the period November-December 2007. Observations were conducted through
the IHW narrow band (continuum) filters. During the observing run the phase
angle was near at which the comet showed negative polarisation. On
the basis of the observed polarisation data we find comet 17P/Holmes to be a
typical comet with usual dust characteristics. We note that radial rate of
change of brightness in coma in red band is higher than that in blue band; it
has decreased by a factor of 3.6 and 2.5 respectively in red and blue bands
during the November - December run, indicating relative increase in the
abundance of smaller dust particles out ward. Radial brightness variation seen
near the nucleus on November 6 is indicative of the presence of a blob or
shocked region beyond 10" from the nucleus which has gradually smoothened by
December 13. The brightness distribution is found steeper during November 5-7
as compared to on December 13.Comment: 11pages, 7 figures, accepted for publication in MNRA
Effect of discrete impurities on electron transport in ultra-short MOSFET using 3D Monte Carlo simulation
This paper discusses the influence of the channel impurity distribution on
the transport and the drive current in short-gate MOSFET. In this purpose, a
careful description of electron-ion interaction suitable for the case of
discrete impurities has been implemented in a 3D particle Monte Carlo
simulator. This transport model is applied to the investigation of 50 nm MOSFET
operation. The results show that a small change in the number of doping
impurities or in the position of a single discrete impurity in the inversion
layer may significantly influence the drain current. This effect is not only
related to threshold voltage fluctuations but also to variations in transport
properties in the inversion layer, especially at high drain voltage. The
results are analyzed in terms of local fluctuations of electron velocity and
current density. In a set of fifteen simulated devices the drive current Ion,
determined at VGS = VDS = 0.6 V, is found to vary in a range of 23% according
to the position of channel impurities.Comment: 31 pages, 13 figures, revised version: discussions and references
added, to be published in IEEE Trans. Electron. Device
Monte Carlo study of coaxially gated CNTFETs: capacitive effects and dynamic performance
Carbon Nanotube (CNT) appears as a promising candidate to shrink field-effect
transistors (FET) to the nanometer scale. Extensive experimental works have
been performed recently to develop the appropriate technology and to explore DC
characteristics of carbon nanotube field effect transistor (CNTFET). In this
work, we present results of Monte Carlo simulation of a coaxially gated CNTFET
including electron-phonon scattering. Our purpose is to present the intrinsic
transport properties of such material through the evaluation of electron
mean-free-path. To highlight the potential of high performance level of CNTFET,
we then perform a study of DC characteristics and of the impact of capacitive
effects. Finally, we compare the performance of CNTFET with that of Si nanowire
MOSFET.Comment: 15 pages, 14 figures, final version to be published in C. R. Acad.
Sci. Pari
The Circumstellar Disk of the Butterfly Star in Taurus
We present a model of the circumstellar environment of the so-called
``Butterfly Star'' in Taurus (IRAS 04302+2247). The appearance of this young
stellar object is dominated by a large circumstellar disk seen edge-on and the
light scattering lobes above the disk. The model is based on multi-wavelength
continuum observations: Millimeter maps and high-resolution near-infrared
images obtained with HST/NICMOS.
It was found that the disk and envelope parameters are comparable with those
of the circumstellar environment of other young stellar objects. A main result
is that the dust properties must be different in the circumstellar disk and in
the envelope: While a grain size distribution with grain radii up to 100 micron
is required to reproduce the millimeter observations of the disk, the envelope
is dominated by smaller grains similar to those of the interstellar medium.
Preprint with high figure quality available at:
http://spider.ipac.caltech.edu/staff/swolf/homepage/public/preprints/i04302.psComment: 32 pages, 9 figure
Polarization Studies of Comet C/2000 WM1 (LINEAR)
Linear polarization observations were carried out on comet C/2000 WM1 with
the 1.2m telescope at Mt. Abu Observatory during November 2001 and March 2002.
The observations in November were at low phase angle (<~22\degr) when the
polarization is negative and where the data for most of the comets are rather
meager. The observations during March were made when the phase angle was
47\degr. Observations were conducted through the IHW narrow band and BVR broad
band filters. Based on these polarization observations we infer that the comet
C/2000 WM1 belongs to high polarization class i.e. the dusty comet family.Comment: 7 pages, 3 figures, accepted for publication in Astron. & Astrop
Electron effective mobility in strained Si/Si1-xGex MOS devices using Monte Carlo simulation
Based on Monte Carlo simulation, we report the study of the inversion layer
mobility in n-channel strained Si/ Si1-xGex MOS structures. The influence of
the strain in the Si layer and of the doping level is studied. Universal
mobility curves mueff as a function of the effective vertical field Eeff are
obtained for various state of strain, as well as a fall-off of the mobility in
weak inversion regime, which reproduces correctly the experimental trends. We
also observe a mobility enhancement up to 120 % for strained Si/ Si0.70Ge0.30,
in accordance with best experimental data. The effect of the strained Si
channel thickness is also investigated: when decreasing the thickness, a
mobility degradation is observed under low effective field only. The role of
the different scattering mechanisms involved in the strained Si/ Si1-xGex MOS
structures is explained. In addition, comparison with experimental results is
discussed in terms of SiO2/ Si interface roughness, as well as surface
roughness of the SiGe substrate on which strained Si is grown.Comment: 25 pages, 8 figures, 1 table, revised version, discussions and
references adde
Multiple scattering of polarized radiation by non-spherical grains: first results
We present the first numerical radiative transfer simulation of multiple
light scattering in dust configurations containing aligned non-spherical
(spheroidal) dust grains. Such models are especially important if one wants to
explain the circular polarization of light, observed in a variety of
astronomical objects. The radiative transfer problem is solved on the basis of
the Monte Carlo method. Test simulations, confirming the correct numerical
implementation of the scattering mechanism, are presented. As a first
application, we investigate the linear and circular polarization of light
coming from a spherical circumstellar shell. This shell contains perfectly
aligned prolate or oblate spheroidal grains. The most remarkable features of
the simulated linear polarization maps are so-called polarization null points
where the reversal of polarization occurs. They appear in the case when the
grain alignment axis is perpendicular to the line of sight. The maps of
circular polarization have a sector-like structure with maxima at the ends of
lines inclined to the grain alignment axis by \pm 45\degr.Comment: 13 pages, 14 figures, accepted by A&
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