16,286 research outputs found
Free-Space Antenna Field/Pattern Retrieval in Reverberation Environments
Simple algorithms for retrieving free-space antenna field or directivity
patterns from complex (field) or real (intensity) measurements taken in ideal
reverberation environments are introduced and discussed.Comment: 6 pages, 2 figures, submitted to IEEE Antennas and Wireless
Propagation Letter
Parameterizing Quasiperiodicity: Generalized Poisson Summation and Its Application to Modified-Fibonacci Antenna Arrays
The fairly recent discovery of "quasicrystals", whose X-ray diffraction
patterns reveal certain peculiar features which do not conform with spatial
periodicity, has motivated studies of the wave-dynamical implications of
"aperiodic order". Within the context of the radiation properties of antenna
arrays, an instructive novel (canonical) example of wave interactions with
quasiperiodic order is illustrated here for one-dimensional (1-D) array
configurations based on the "modified-Fibonacci" sequence, with utilization of
a two-scale generalization of the standard Poisson summation formula for
periodic arrays. This allows for a "quasi-Floquet" analytic parameterization of
the radiated field, which provides instructive insights into some of the basic
wave mechanisms associated with quasiperiodic order, highlighting similarities
and differences with the periodic case. Examples are shown for quasiperiodic
infinite and spatially-truncated arrays, with brief discussion of computational
issues and potential applications.Comment: 29 pages, 10 figures. To be published in IEEE Trans. Antennas
Propagat., vol. 53, No. 6, June 200
Perspectives on Beam-Shaping Optimization for Thermal-Noise Reduction in Advanced Gravitational-Wave Interferometric Detectors: Bounds, Profiles, and Critical Parameters
Suitable shaping (in particular, flattening and broadening) of the laser beam
has recently been proposed as an effective device to reduce internal (mirror)
thermal noise in advanced gravitational wave interferometric detectors. Based
on some recently published analytic approximations (valid in the
infinite-test-mass limit) for the Brownian and thermoelastic mirror noises in
the presence of arbitrary-shaped beams, this paper addresses certain
preliminary issues related to the optimal beam-shaping problem. In particular,
with specific reference to the Laser Interferometer Gravitational-wave
Observatory (LIGO) experiment, absolute and realistic lower-bounds for the
various thermal noise constituents are obtained and compared with the current
status (Gaussian beams) and trends ("mesa" beams), indicating fairly ample
margins for further reduction. In this framework, the effective dimension of
the related optimization problem, and its relationship to the critical design
parameters are identified, physical-feasibility and model-consistency issues
are considered, and possible additional requirements and/or prior information
exploitable to drive the subsequent optimization process are highlighted.Comment: 12 pages, 9 figures, 2 table
Neutrino Quantum Kinetics
We present a formulation of the quantum kinetic equations (QKEs) which govern
the evolution of neutrino flavor at high density and temperature. Here, the
QKEs are derived from the ground up, using fundamental neutrino interactions
and quantum field theory. We show that the resulting QKEs describe coherent
flavor evolution with an effective mass when inelastic scattering is
negligible. The QKEs also contain a collision term. This term can reduce to the
collision term in the Boltzmann equation when scattering is dominant and the
neutrino effective masses and density matrices become diagonal in the
interaction basis. We also find that the QKE's include equations of motion for
a new dynamical quantity related to neutrino spin. This quantity decouples from
the equations of motion for the density matrices at low densities or in
isotropic conditions. However, the spin equations of motion allow for the
possibility of coherent transformation between neutrinos and antineutrinos at
high densities and in the presence of anisotropy. Although the requisite
conditions for this exist in the core collapse supernova and compact object
merger environments, it is likely that only a self consistent incorporation of
the QKEs in a sufficiently realistic model could establish whether or not
significant neutrino-antineutrino conversion occurs.Comment: Revised version, published in Physical Review
Entanglement spectrum of the Heisenberg XXZ chain near the ferromagnetic point
We study the entanglement spectrum (ES) of a finite XXZ spin 1/2 chain in the
limit \Delta -> -1^+ for both open and periodic boundary conditions. At
\Delta=-1 (ferromagnetic point) the model is equivalent to the Heisenberg
ferromagnet and its degenerate ground state manifold is the SU(2) multiplet
with maximal total spin. Any state in this so-called "symmetric sector" is an
equal weight superposition of all possible spin configurations. In the gapless
phase at \Delta>-1 this property is progressively lost as one moves away from
the \Delta=-1 point. We investigate how the ES obtained from the states in this
manifold reflects this change, using exact diagonalization and Bethe ansatz
calculations. We find that in the limit \Delta ->-1^+ most of the ES levels
show divergent behavior. Moreover, while at \Delta=-1 the ES contains no
information about the boundaries, for \Delta>-1 it depends dramatically on the
choice of boundary conditions. For both open and periodic boundary conditions
the ES exhibits an elegant multiplicity structure for which we conjecture a
combinatorial formula. We also study the entanglement eigenfunctions, i.e. the
eigenfunctions of the reduced density matrix. We find that the eigenfunctions
corresponding to the non diverging levels mimic the behavior of the state
wavefunction, whereas the others show intriguing polynomial structures. Finally
we analyze the distribution of the ES levels as the system is detuned away from
\Delta=-1.Comment: 21 pages, 8 figures. Minor corrections, references added. Published
versio
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