12,881 research outputs found
Remark on the (Non)convergence of Ensemble Densities in Dynamical Systems
We consider a dynamical system with state space , a smooth, compact subset
of some , and evolution given by , , ;
is invertible and the time may be discrete, , , or continuous, . Here we show that starting with a
continuous positive initial probability density , with respect
to , the smooth volume measure induced on by Lebesgue measure on , the expectation value of , with respect to any
stationary (i.e. time invariant) measure , is linear in , . depends only on and vanishes
when is absolutely continuous wrt .Comment: 7 pages, plain TeX; [email protected],
[email protected], [email protected], to appear in Chaos: An
Interdisciplinary Journal of Nonlinear Science, Volume 8, Issue
Scaling of the electron dissipation range of solar wind turbulence
Electron scale solar wind turbulence has attracted great interest in recent
years. Clear evidences have been given from the Cluster data that turbulence is
not fully dissipated near the proton scale but continues cascading down to the
electron scales. However, the scaling of the energy spectra as well as the
nature of the plasma modes involved at those small scales are still not fully
determined. Here we survey 10 years of the Cluster search-coil magnetometer
(SCM) waveforms measured in the solar wind and perform a statistical study of
the magnetic energy spectra in the frequency range []Hz. We show that a
large fraction of the spectra exhibit clear breakpoints near the electon
gyroscale , followed by steeper power-law like spectra. We show that
the scaling below the electron breakpoint cannot be determined unambiguously
due to instrumental limitations that will be discussed in detail. We compare
our results to recent ones reported in other studies and discuss their
implication on the physical mechanisms and the theoretical modeling of energy
dissipation in the SW.Comment: 10 pages, submitte
Spin-orbit induced interference in polygon-structures
We investigate the spin-orbit induced spin-interference pattern of ballistic
electrons travelling along any regular polygon. It is found that the
spin-interference depends strongly on the Rashba and Dresselhaus spin-orbit
constants as well as on the sidelength and alignment of the polygon. We derive
the analytical formulae for the limiting cases of either zero Dresselhaus or
zero Rashba spin-orbit coupling, including the result obtained for a circle. We
calculate the nonzero Dresselhaus and Rashba case numerically for the square,
triangle, hexagon, and circle and discuss the observability of the
spin-interference which can potentially be used to measure the Rashba and
Dresselhaus coefficients.Comment: 17 pages, 4 figure
Axiomatic geometrical optics, Abraham-Minkowski controversy, and photon properties derived classically
By restating geometrical optics within the field-theoretical approach, the
classical concept of a photon (and, more generally, any elementary excitation)
in arbitrary dispersive medium is introduced, and photon properties are
calculated unambiguously. In particular, the canonical and kinetic momenta
carried by a photon, as well as the two corresponding energy-momentum tensors
of a wave, are derived from first principles of Lagrangian mechanics. As an
example application of this formalism, the Abraham-Minkowski controversy
pertaining to the definitions of these quantities is resolved for linear waves
of arbitrary nature, and corrections to the traditional formulas for the photon
kinetic energy-momentum are found. Several other applications of axiomatic
geometrical optics to electromagnetic waves are also presented
Nonclassical correlations of phase noise and photon number in quantum nondemolition measurements
The continuous transition from a low resolution quantum nondemolition
measurement of light field intensity to a precise measurement of photon number
is described using a generalized measurement postulate. In the intermediate
regime, quantization appears as a weak modulation of measurement probability.
In this regime, the measurement result is strongly correlated with the amount
of phase decoherence introduced by the measurement interaction. In particular,
the accidental observation of half integer photon numbers preserves phase
coherence in the light field, while the accidental observation of quantized
values increases decoherence. The quantum mechanical nature of this correlation
is discussed and the implications for the general interpretation of
quantization are considered.Comment: 16 pages, 5 figures, final version to be published in Phys. Rev. A,
Clarifications of the nature of the measurement result and the noise added in
section I
Challenges in modelling the random structure correctly in growth mixture models and the impact this has on model mixtures
Lifecourse trajectories of clinical or anthropological attributes are useful for identifying how our early-life experiences influence later-life morbidity and mortality. Researchers often use growth mixture models (GMMs) to estimate such phenomena. It is common to place constrains on the random part of the GMM to improve parsimony or to aid convergence, but this can lead to an autoregressive structure that distorts the nature of the mixtures and subsequent model interpretation. This is especially true if changes in the outcome within individuals are gradual compared with the magnitude of differences between individuals. This is not widely appreciated, nor is its impact well understood. Using repeat measures of body mass index (BMI) for 1528 US adolescents, we estimated GMMs that required variance-covariance constraints to attain convergence. We contrasted constrained models with and without an autocorrelation structure to assess the impact this had on the ideal number of latent classes, their size and composition. We also contrasted model options using simulations. When the GMM variance-covariance structure was constrained, a within-class autocorrelation structure emerged. When not modelled explicitly, this led to poorer model fit and models that differed substantially in the ideal number of latent classes, as well as class size and composition. Failure to carefully consider the random structure of data within a GMM framework may lead to erroneous model inferences, especially for outcomes with greater within-person than between-person homogeneity, such as BMI. It is crucial to reflect on the underlying data generation processes when building such models
Theory of four-wave mixing of matter waves from a Bose-Einstein condensate
A recent experiment [Deng et al., Nature 398, 218(1999)] demonstrated
four-wave mixing of matter wavepackets created from a Bose-Einstein condensate.
The experiment utilized light pulses to create two high-momentum wavepackets
via Bragg diffraction from a stationary Bose-Einstein condensate. The
high-momentum components and the initial low momentum condensate interact to
form a new momentum component due to the nonlinear self-interaction of the
bosonic atoms. We develop a three-dimensional quantum mechanical description,
based on the slowly-varying-envelope approximation, for four-wave mixing in
Bose-Einstein condensates using the time-dependent Gross-Pitaevskii equation.
We apply this description to describe the experimental observations and to make
predictions. We examine the role of phase-modulation, momentum and energy
conservation (i.e., phase-matching), and particle number conservation in
four-wave mixing of matter waves, and develop simple models for understanding
our numerical results.Comment: 18 pages Revtex preprint form, 13 eps figure
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