47,878 research outputs found
Transmission, reflection and localization in a random medium with absorption or gain
We study reflection and transmission of waves in a random tight-binding
system with absorption or gain for weak disorder, using a scattering matrix
formalism. Our aim is to discuss analytically the effects of absorption or gain
on the statistics of wave transport. Treating the effects of absorption or gain
exactly in the limit of no disorder, allows us to identify short- and long
lengths regimes relative to absorption- or gain lengths, where the effects of
absorption/gain on statistical properties are essentially different. In the
long-lengths regime we find that a weak absorption or a weak gain induce
identical statistical corrections in the inverse localization length, but lead
to different corrections in the mean reflection coefficient. In contrast, a
strong absorption or a strong gain strongly suppress the effect of disorder in
identical ways (to leading order), both in the localization length and in the
mean reflection coefficient.Comment: Important revisions and expansion caused by a crucial property of
$\hat Q
Reflection and transmission of waves in surface-disordered waveguides
The reflection and transmission amplitudes of waves in disordered multimode
waveguides are studied by means of numerical simulations based on the invariant
embedding equations. In particular, we analyze the influence of surface-type
disorder on the behavior of the ensemble average and fluctuations of the
reflection and transmission coefficients, reflectance, transmittance, and
conductance. Our results show anomalous effects stemming from the combination
of mode dispersion and rough surface scattering: For a given waveguide length,
the larger the mode transverse momentum is, the more strongly is the mode
scattered. These effects manifest themselves in the mode selectivity of the
transmission coefficients, anomalous backscattering enhancement, and speckle
pattern both in reflection and transmission, reflectance and transmittance, and
also in the conductance and its universal fluctuations. It is shown that, in
contrast to volume impurities, surface scattering in quasi-one-dimensional
structures (waveguides) gives rise to the coexistence of the ballistic,
diffusive, and localized regimes within the same sample.Comment: LaTeX (REVTeX), 12 pages with 14 EPS figures (epsf macro), minor
change
Classical limit of transport in quantum kicked maps
We investigate the behavior of weak localization, conductance fluctuations,
and shot noise of a chaotic scatterer in the semiclassical limit. Time resolved
numerical results, obtained by truncating the time-evolution of a kicked
quantum map after a certain number of iterations, are compared to semiclassical
theory. Considering how the appearance of quantum effects is delayed as a
function of the Ehrenfest time gives a new method to compare theory and
numerical simulations. We find that both weak localization and shot noise agree
with semiclassical theory, which predicts exponential suppression with
increasing Ehrenfest time. However, conductance fluctuations exhibit different
behavior, with only a slight dependence on the Ehrenfest time.Comment: 17 pages, 13 figures. Final versio
LANDSAT-D investigations in snow hydrology
Work undertaken during the contract and its results are described. Many of the results from this investigation are available in journal or conference proceedings literature - published, accepted for publication, or submitted for publication. For these the reference and the abstract are given. Those results that have not yet been submitted separately for publication are described in detail. Accomplishments during the contract period are summarized as follows: (1) analysis of the snow reflectance characteristics of the LANDSAT Thematic Mapper, including spectral suitability, dynamic range, and spectral resolution; (2) development of a variety of atmospheric models for use with LANDSAT Thematic Mapper data. These include a simple but fast two-stream approximation for inhomogeneous atmospheres over irregular surfaces, and a doubling model for calculation of the angular distribution of spectral radiance at any level in an plane-parallel atmosphere; (3) incorporation of digital elevation data into the atmospheric models and into the analysis of the satellite data; and (4) textural analysis of the spatial distribution of snow cover
Numerical simulation of moving rigid body in rarefied gases
In this paper we present a numerical scheme to simulate a moving rigid body
with arbitrary shape suspended in a rarefied gas. The rarefied gas is simulated
by solving the Boltzmann equation using a DSMC particle method. The motion of
the rigid body is governed by the Newton-Euler equations, where the force and
the torque on the rigid body is computed from the momentum transfer of the gas
molecules colliding with the body. On the other hand, the motion of the rigid
body influences the gas flow in its surroundings. We validate the numerical
results by testing the Einstein relation for Brownian motion of the suspended
particle. The translational as well as the rotational degrees of freedom are
taken into account. It is shown that the numerically computed translational and
rotational diffusion coefficients converge to the theoretical values.Comment: 16 pages, 8 figure
From Wires to Cosmology
We provide a statistical framework for characterizing stochastic particle
production in the early universe via a precise correspondence to current
conduction in wires with impurities. Our approach is particularly useful when
the microphysics is uncertain and the dynamics are complex, but only
coarse-grained information is of interest. We study scenarios with multiple
interacting fields and derive the evolution of the particle occupation numbers
from a Fokker-Planck equation. At late times, the typical occupation numbers
grow exponentially which is the analog of Anderson localization for disordered
wires. Some statistical features of the occupation numbers show hints of
universality in the limit of a large number of interactions and/or a large
number of fields. For test cases, excellent agreement is found between our
analytic results and numerical simulations.Comment: v3: minor changes and references added; matches published version in
JCA
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