95 research outputs found
Statistics of Lyapunov exponent in one-dimensional layered systems
Localization of acoustic waves in a one dimensional water duct containing
many randomly distributed air filled blocks is studied. Both the Lyapunov
exponent and its variance are computed. Their statistical properties are also
explored extensively. The results reveal that in this system the single
parameter scaling is generally inadequate no matter whether the frequency we
consider is located in a pass band or in a band gap. This contradicts the
earlier observations in an optical case. We compare the results with two
optical cases and give a possible explanation of the origin of the different
behaviors.Comment: 6 pages revtex file, 6 eps figure
Localization of electromagnetic waves in a two dimensional random medium
Motivated by previous investigations on the radiative effects of the electric
dipoles embedded in structured cavities, localization of electromagnetic waves
in two dimensions is studied {\it ab initio} for a system consisting of many
randomly distributed two dimensional dipoles. A set of self-consistent
equations, incorporating all orders of multiple scattering of the
electromagnetic waves, is derived from first principles and then solved
numerically for the total electromagnetic field. The results show that
spatially localized electromagnetic waves are possible in such a simple but
realistic disordered system. When localization occurs, a coherent behavior
appears and is revealed as a unique property differentiating localization from
either the residual absorption or the attenuation effects
Diffusive and localization behavior of electromagnetic waves in a two-dimensional random medium
In this paper, we discuss the transport phenomena of electromagnetic waves in
a two-dimensional random system which is composed of arrays of electrical
dipoles, following the model presented earlier by Erdogan, et al. (J. Opt. Soc.
Am. B {\bf 10}, 391 (1993)). A set of self-consistent equations is presented,
accounting for the multiple scattering in the system, and is then solved
numerically. A strong localization regime is discovered in the frequency
domain. The transport properties within, near the edge of and nearly outside
the localization regime are investigated for different parameters such as
filling factor and system size. The results show that within the localization
regime, waves are trapped near the transmitting source. Meanwhile, the
diffusive waves follow an intuitive but expected picture. That is, they
increase with travelling path as more and more random scattering incurs,
followed by a saturation, then start to decay exponentially when the travelling
path is large enough, signifying the localization effect. For the cases that
the frequencies are near the boundary of or outside the localization regime,
the results of diffusive waves are compared with the diffusion approximation,
showing less encouraging agreement as in other systems (Asatryan, et al., Phys.
Rev. E {\bf 67}, 036605 (2003).)Comment: 8 pages 9 figure
Development of a tight-binding potential for bcc-Zr. Application to the study of vibrational properties
We present a tight-binding potential based on the moment expansion of the
density of states, which includes up to the fifth moment. The potential is
fitted to bcc and hcp Zr and it is applied to the computation of vibrational
properties of bcc-Zr. In particular, we compute the isothermal elastic
constants in the temperature range 1200K < T < 2000K by means of standard Monte
Carlo simulation techniques. The agreement with experimental results is
satisfactory, especially in the case of the stability of the lattice with
respect to the shear associated with C'. However, the temperature decrease of
the Cauchy pressure is not reproduced. The T=0K phonon frequencies of bcc-Zr
are also computed. The potential predicts several instabilities of the bcc
structure, and a crossing of the longitudinal and transverse modes in the (001)
direction. This is in agreement with recent ab initio calculations in Sc, Ti,
Hf, and La.Comment: 14 pages, 6 tables, 4 figures, revtex; the kinetic term of the
isothermal elastic constants has been corrected (Eq. (4.1), Table VI and
Figure 4
Many-body aspects of positron annihilation in the electron gas
We investigate positron annihilation in electron liquid as a case study for
many-body theory, in particular the optimized Fermi Hypernetted Chain (FHNC-EL)
method. We examine several approximation schemes and show that one has to go up
to the most sophisticated implementation of the theory available at the moment
in order to get annihilation rates that agree reasonably well with experimental
data. Even though there is basically just one number to look at, the
electron-positron pair distribution function at zero distance, it is exactly
this number that dictates how the full pair distribution behaves: In most
cases, it falls off monotonously towards unity as the distance increases. Cases
where the electron-positron pair distribution exhibits a dip are precursors to
the formation of bound electron--positron pairs. The formation of
electron-positron pairs is indicated by a divergence of the FHNC-EL equations,
from this we can estimate the density regime where positrons must be localized.
This occurs in our calculations in the range 9.4 <= r_s <=10, where r_s is the
dimensionless density parameter of the electron liquid.Comment: To appear in Phys. Rev. B (2003
Energetics, forces, and quantized conductance in jellium modeled metallic nanowires
Energetics and quantized conductance in jellium modeled nanowires are
investigated using the local density functional based shell correction method,
extending our previous study of uniform in shape wires [C. Yannouleas and U.
Landman, J. Phys. Chem. B 101, 5780 (1997)] to wires containing a variable
shaped constricted region. The energetics of the wire (sodium) as a function of
the length of the volume conserving, adiabatically shaped constriction leads to
formation of self selecting magic wire configurations. The variations in the
energy result in oscillations in the force required to elongate the wire and
are directly correlated with the stepwise variations of the conductance of the
nanowire in units of 2e^2/h. The oscillatory patterns in the energetics and
forces, and the correlated stepwise variation in the conductance are shown,
numerically and through a semiclassical analysis, to be dominated by the
quantized spectrum of the transverse states at the narrowmost part of the
constriction in the wire.Comment: Latex/Revtex, 11 pages with 5 Postscript figure
On the selection of AGN neutrino source candidates for a source stacking analysis with neutrino telescopes
The sensitivity of a search for sources of TeV neutrinos can be improved by
grouping potential sources together into generic classes in a procedure that is
known as source stacking. In this paper, we define catalogs of Active Galactic
Nuclei (AGN) and use them to perform a source stacking analysis. The grouping
of AGN into classes is done in two steps: first, AGN classes are defined, then,
sources to be stacked are selected assuming that a potential neutrino flux is
linearly correlated with the photon luminosity in a certain energy band (radio,
IR, optical, keV, GeV, TeV). Lacking any secure detailed knowledge on neutrino
production in AGN, this correlation is motivated by hadronic AGN models, as
briefly reviewed in this paper.
The source stacking search for neutrinos from generic AGN classes is
illustrated using the data collected by the AMANDA-II high energy neutrino
detector during the year 2000. No significant excess for any of the suggested
groups was found.Comment: 43 pages, 12 figures, accepted by Astroparticle Physic
Search for Neutrino-Induced Cascades with AMANDA
We report on a search for electro-magnetic and/or hadronic showers (cascades)
induced by high energy neutrinos in the data collected with the AMANDA II
detector during the year 2000. The observed event rates are consistent with the
expectations for atmospheric neutrinos and muons. We place upper limits on a
diffuse flux of extraterrestrial electron, tau and muon neutrinos. A flux of
neutrinos with a spectrum which consists of an equal mix
of all flavors, is limited to at
a 90% confidence level for a neutrino energy range 50 TeV to 5 PeV. We present
bounds for specific extraterrestrial neutrino flux predictions. Several of
these models are ruled out.Comment: 18 pages, 12 figure
New results from the Antarctic Muon And Neutrino Detector Array
We present recent results from the Antarctic Muon And Neutrino Detector Array
(AMANDA) on searches for high-energy neutrinos of extraterrestrial origin. We
have searched for a diffuse flux of neutrinos, neutrino point sources and
neutrinos from GRBs and from WIMP annihilations in the Sun or the center of the
Earth. We also present a preliminary result on the first energy spectrum above
a few TeV for atmospheric neutrinos.Comment: 8 pages, 8 figures, to be published in Nuclear Physics B (Proceedings
Supplement): Proceedings of the XXIst International Conference on Neutrino
Physics and Astrophysics, Paris, June 14-19, 200
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