3,594 research outputs found
Detection of QED vacuum nonlinearities in Maxwell's equations by the use of waveguides
We present a novel method for detecting nonlinearities, due to quantum
electrodynamics through photon-photon scattering, in Maxwell's equation. The
photon-photon scattering gives rise to self-interaction terms, which are
similar to the nonlinearities due to the polarisation in nonlinear optics.
These self-interaction terms vanish in the limit of parallel propagating waves,
but if instead of parallel propagating waves the modes generated in wavesguides
are used, there will be a non-zero total effect. Based on this idea, we
calculate the nonlinear excitation of new modes and estimate the strength of
this effect. Furthermore, we suggest a principal experimental setup.Comment: 4 pages, REVTeX3. To appear in Phys. Rev. Let
Application Of Generalized Snoek\u27s Law Over A Finite Frequency Range: A Case Study
Generalized Snoek\u27s law proposed in an integral form by Acher and coauthors is a useful tool for investigation of high-frequency properties of magnetic materials. This integral law referred to as Acher\u27s law allows for evaluating the ultimate performance of RF and microwave devices which employ magnetic materials. It may also be helpful in obtaining useful information on the structure and morphology of the materials. The key factor in practical application of Acher\u27s law is an opportunity to employ either measured or calculated data available over a finite frequency range. The paper uses simple calculations to check the applicability of Acher\u27s law in cases when the frequency range is limited, and the magnetic loss peak is comparatively wide and has a distorted shape. The cases of large magnetic damping pronounced skin effect, and inhomogeneity of the material are considered. It is shown that in most cases calculation of the integral through fitting of actual magnetic frequency dispersion by the Lorentzian dispersion law results in accurate estimations of the ultimate high-frequency performance of magnetic materials
Influence Of Higher-order Modes In Coaxial Waveguide On Measurements Of Material Parameters
The use of a coaxial air-filled line as a test fixture for measuring complex permittivity and permeability often shows odd resonance-like behavior of material parameters as functions of frequency. This effect is typically either ascribed to the half-wavelength resonance at the sample length, or erroneously misinterpreted as intrinsic resonance behavior of the material. However, as is shown in this paper, such behavior can be attributed to excitation of the higher-order modes on the surface of the sample resulting in resonance absorption of electromagnetic energy in the test fixture. Herein, analytical, numerical, and experimental results show that there can actually be a significant impact of higher-order modes in a coaxial line on the extracted constitutive material parameters of samples
Box and Gaussian plume models of the exhaust composition evolution of subsonic transport aircraft in- and out of the flight corridor
Evidence of Microfossils in Carbonaceous Chondrites
Investigations have been carried out on freshly broken, internal surfaces of the Murchison, Efremovka and Orgueil carbonaceous chondrites using Scanning Electron Microscopes (SEM) in Russia and the Environmental Scanning Electron Microscope (ESEM) in the United States. These independent studies on different samples of the meteorites have resulted in the detection of numerous spherical and ellipsoidal bodies (some with spikes) similar to the forms of uncertain biogenicity that were designated "organized elements" by prior researchers. We have also encountered numerous complex biomorphic microstructures in these carbonaceous chondrites. Many of these complex bodies exhibit diverse characteristics reminiscent of microfossils of cyanobacteria such as we have investigated in ancient phosphorites and high carbon rocks (e.g. oil shales). Energy Dispersive Spectroscopy (EDS) analysis and 2D elemental maps shows enhanced carbon content in the bodies superimposed upon the elemental distributions characteristic of the chondritic matrix. The size, distribution, composition, and indications of cell walls, reproductive and life cycle developmental stages of these bodies are strongly suggestive of biology' These bodies appear to be mineralized and embedded within the meteorite matrix, and can not be attributed to recent surface contamination effects. Consequently, we have interpreted these in-situ microstructures to represent the lithified remains of prokaryotes and filamentous cyanobacteria. We also detected in Orgueil microstructures morphologically similar to fibrous kerite crystals. We present images of many biomorphic microstructures and possible microfossils found in the Murchison, Efremovka, and Orgueil chondrites and compare these forms with known microfossils from the Cambrian phosphate-rich rocks (phosphorites) of Khubsugul, Northern Mongolia
Multivariate Granger Causality and Generalized Variance
Granger causality analysis is a popular method for inference on directed
interactions in complex systems of many variables. A shortcoming of the
standard framework for Granger causality is that it only allows for examination
of interactions between single (univariate) variables within a system, perhaps
conditioned on other variables. However, interactions do not necessarily take
place between single variables, but may occur among groups, or "ensembles", of
variables. In this study we establish a principled framework for Granger
causality in the context of causal interactions among two or more multivariate
sets of variables. Building on Geweke's seminal 1982 work, we offer new
justifications for one particular form of multivariate Granger causality based
on the generalized variances of residual errors. Taken together, our results
support a comprehensive and theoretically consistent extension of Granger
causality to the multivariate case. Treated individually, they highlight
several specific advantages of the generalized variance measure, which we
illustrate using applications in neuroscience as an example. We further show
how the measure can be used to define "partial" Granger causality in the
multivariate context and we also motivate reformulations of "causal density"
and "Granger autonomy". Our results are directly applicable to experimental
data and promise to reveal new types of functional relations in complex
systems, neural and otherwise.Comment: added 1 reference, minor change to discussion, typos corrected; 28
pages, 3 figures, 1 table, LaTe
Adaptive density estimation for stationary processes
We propose an algorithm to estimate the common density of a stationary
process . We suppose that the process is either or
-mixing. We provide a model selection procedure based on a generalization
of Mallows' and we prove oracle inequalities for the selected estimator
under a few prior assumptions on the collection of models and on the mixing
coefficients. We prove that our estimator is adaptive over a class of Besov
spaces, namely, we prove that it achieves the same rates of convergence as in
the i.i.d framework
Spectral Analysis of Multi-dimensional Self-similar Markov Processes
In this paper we consider a discrete scale invariant (DSI) process with scale . We consider to have some fix number of
observations in every scale, say , and to get our samples at discrete points
where is obtained by the equality
and . So we provide a discrete time scale
invariant (DT-SI) process with parameter space . We find the spectral representation of the covariance function of
such DT-SI process. By providing harmonic like representation of
multi-dimensional self-similar processes, spectral density function of them are
presented. We assume that the process is also Markov
in the wide sense and provide a discrete time scale invariant Markov (DT-SIM)
process with the above scheme of sampling. We present an example of DT-SIM
process, simple Brownian motion, by the above sampling scheme and verify our
results. Finally we find the spectral density matrix of such DT-SIM process and
show that its associated -dimensional self-similar Markov process is fully
specified by where is
the covariance function of th and th observations of the process.Comment: 16 page
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