135,725 research outputs found
Superradiance in spin- particles: Effects of multiple levels
We study the superradiance dynamics in a dense system of atoms each of which
can be generally a spin- particle with an arbitrary half-integer. We
generalize Dicke's superradiance point of view to multiple-level systems, and
compare the results based on a novel approach we have developed in {[}Yelin
\textit{et al.}, arXiv:quant-ph/0509184{]}. Using this formalism we derive an
effective two-body description that shows cooperative and collective effects
for spin- particles, taking into account the coherence of transitions
between different atomic levels. We find that the superradiance, which is
well-known as a many-body phenomenon, can also be modified by multiple level
effects. We also discuss the feasibility and propose that our approach can be
applied to polar molecules, for their vibrational states have multi-level
structure which is partially harmonic.Comment: 11 pages, 7 figure
Thermal instabilities in protogalactic clouds
The means by which a protogalaxy can fragment to form the first generation of stars and globular clusters remains an important problem in astrophysics. Gravitational instabilities grow on timescales too long to drive fragmentation before the background density grows by many orders of magnitude (see Murray and Lin 1989a, and references therein). Thermal instability provides a much more likely mechanism. After its initial collapse, a protogalactic cloud is expected to be shock heated to its virial temperature approx. 10(exp 6) K. Cooling by H and He+ below 10(exp 6) K has a negative slope, so that the cloud is subject to strong thermal instabilities. Density enhancements may then grow rapidly, fragmenting the protogalaxy as it cools to lower temperatures. The role of dynamical effects upon the growth of perturbations is considered here. The method used is similar to that used in Murray and Lin (1989a; see also the Erratum to appear September 15), which examined the growth of thermal instabilities with a one-dimensional Lagrangian hydrodynamics code, written for spherical symmetry. Perturbed regions therefore take the form of shells. The dynamical variables are integrated explicitly, while the temperature, ionization fraction, and molecular fraction are integrated implicitly, and account is taken for non-equilibrium values of these quantities
Thermalization and temperature distribution in a driven ion chain
We study thermalization and non-equilibrium dynamics in a dissipative quantum
many-body system -- a chain of ions with two points of the chain driven by
thermal bath under different temperature. Instead of a simple linear
temperature gradient as one expects from the classical heat diffusion process,
the temperature distribution in the ion chain shows surprisingly rich patterns,
which depend on the ion coupling rate to the bath, the location of the driven
ions, and the dissipation rates of the other ions in the chain. Through
simulation of the temperature evolution, we show that these unusual temperature
distribution patterns in the ion chain can be quantitatively tested in
experiments within a realistic time scale.Comment: 5 pages, 5 figure
Green's function for the Relativistic Coulomb System via Sum Over Perturbation Series
We evaluate the Green's function of the D-dimensional relativistic Coulomb
system via sum over perturbation series which is obtained by expanding the
exponential containing the potential term in the path integral
into a power series. The energy spectra and wave functions are extracted from
the resulting amplitude.Comment: 13 pages, ReVTeX, no figure
Toward a Deterministic Model of Planetary Formation VII: Eccentricity Distribution of Gas Giants
The ubiquity of planets and diversity of planetary systems reveal planet
formation encompass many complex and competing processes. In this series of
papers, we develop and upgrade a population synthesis model as a tool to
identify the dominant physical effects and to calibrate the range of physical
conditions. Recent planet searches leads to the discovery of many
multiple-planet systems. Any theoretical models of their origins must take into
account dynamical interaction between emerging protoplanets. Here, we introduce
a prescription to approximate the close encounters between multiple planets. We
apply this method to simulate the growth, migration, and dynamical interaction
of planetary systems. Our models show that in relatively massive disks, several
gas giants and rocky/icy planets emerge, migrate, and undergo dynamical
instability. Secular perturbation between planets leads to orbital crossings,
eccentricity excitation, and planetary ejection. In disks with modest masses,
two or less gas giants form with multiple super-Earths. Orbital stability in
these systems is generally maintained and they retain the kinematic structure
after gas in their natal disks is depleted. These results reproduce the
observed planetary mass-eccentricity and semimajor axis-eccentricity
correlations. They also suggest that emerging gas giants can scatter residual
cores to the outer disk regions. Subsequent in situ gas accretion onto these
cores can lead to the formation of distant (> 30AU) gas giants with nearly
circular orbits.Comment: 54 pages, 14 Figures; accepted for publication in Astrophysical
Journa
Probability of undetected error after decoding for a concatenated coding scheme
A concatenated coding scheme for error control in data communications is analyzed. In this scheme, the inner code is used for both error correction and detection, however the outer code is used only for error detection. A retransmission is requested if the outer code detects the presence of errors after the inner code decoding. Probability of undetected error is derived and bounded. A particular example, proposed for NASA telecommand system is analyzed
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