13,367 research outputs found
Classical Analog of Electromagnetically Induced Transparency
We present a classical analog for Electromagnetically Induced Transparency
(EIT). In a system of just two coupled harmonic oscillators subject to a
harmonic driving force we can reproduce the phenomenology observed in EIT. We
describe a simple experiment performed with two linearly coupled RLC circuits
which can be taught in an undergraduate laboratory class.Comment: 6 pages, two-column, 6 figures, submitted to the Am. J. Phy
Digital Image Compression Using Artificial Neural Networks
The problem of storing, transmitting, and manipulating digital images is considered. Because of the file sizes involved, large amounts of digitized image information are becoming common in modern projects. Our goal is to described an image compression transform coder based on artificial neural networks techniques (NNCTC). A comparison of the compression results obtained from digital astronomical images by the NNCTC and the method used in the compression of the digitized sky survey from the Space Telescope Science Institute based on the H-transform is performed in order to assess the reliability of the NNCTC
Interferometric observations of SiO thermal emission in the inner wind of M-type AGB stars IK Tauri and IRC+10011
Context. AGB stars go through a process of strong mass-loss that involves
pulsations of the atmosphere, which extends to a region where the conditions
are adequate for dust grains to form. Radiation pressure acts on these grains
which, coupled to the gas, drive a massive outflow. The details of this process
are not clear, including which molecules are involved in the condensation of
dust grains.
Aims. To study the role of the SiO molecule in the process of dust formation
and mass-loss in M-type AGB stars.
Methods. Using the IRAM NOEMA interferometer we observed the SiO and
SiO , emission from the inner circumstellar envelope of the
evolved stars IK Tau and IRC+10011. We computed azimuthally averaged emission
profiles to compare the observations to models using a molecular excitation and
ray-tracing code for SiO thermal emission.
Results. We observed circular symmetry in the emission distribution. We also
found that the source diameter varies only marginally with radial velocity,
which is not the expected behavior for envelopes expanding at an almost
constant velocity. The adopted density, velocity, and abundance laws, together
with the mass-loss rate, which best fit the observations, give us information
on the chemical behavior of the SiO molecule and its role in the dust formation
process.
Conclusions. The results indicate that there is a strong coupling between the
depletion of gas phase SiO and gas acceleration in the inner envelope. This
could be explained by the condensation of SiO into dust grains
Geiger-Mode Avalanche Photodiodes in Particle Detection
It is well known that avalanche photodiodes operated in the Geiger mode above
the breakdown voltage offer a virtually infinite sensitivity and time accuracy
in the picosecond range that can be used for single photon detection. However,
their performance in particle detection remains still unexplored. In this
contribution, we are going to expose the different steps that we have taken in
order to prove the efficiency of Geiger mode avalanche photodiodes in the
aforementioned field. In particular, we will present an array of pixels of
1mmx1mm fabricated with a standard CMOS technology for characterization in a
test beam.Comment: 7 pages, 2 figures, Proceedings of LCWS1
Evaporative cooling in a radio-frequency trap
A theoretical investigation for implementing a scheme of forced evaporative
cooling in radio-frequency (rf) adiabatic potentials is presented. Supposing
the atoms to be trapped by a rf field RF1, the cooling procedure is facilitated
using a second rf source RF2. This second rf field produces a controlled
coupling between the spin states dressed by RF1. The evaporation is then
possible in a pulsed or continuous mode. In the pulsed case, atoms with a given
energy are transferred into untrapped dressed states by abruptly switching off
the interaction. In the continuous case, it is possible for energetic atoms to
adiabatically follow the doubly-dressed states and escape out of the trap. Our
results also show that when the frequencies of the fields RF1 and RF2 are
separated by at least the Rabi frequency associated with RF1, additional
evaporation zones appear which can make this process more efficient.Comment: 12 pages, 11 figure
Bulk dynamics for interfacial growth models
We study the influence of the bulk dynamics of a growing cluster of particles
on the properties of its interface. First, we define a {\it general bulk growth
model} by means of a continuum Master equation for the evolution of the bulk
density field. This general model just considers arbitrary addition of
particles (though it can be easily generalized to consider substraction) with
no other physical restriction. The corresponding Langevin equation for this
bulk density field is derived where the influence of the bulk dynamics is
explicitly shown. Finally, when it is assumed a well-defined interface for the
growing cluster, the Langevin equation for the height field of this interface
for some particular bulk dynamics is written. In particular, we obtain the
celebrated Kardar-Parisi-Zhang (KPZ) equation. A Monte Carlo simulation
illustrates the theoretical results.Comment: 6 pages, 2 figure
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