13,829 research outputs found
Making electromagnetic wavelets
Electromagnetic wavelets are constructed using scalar wavelets as
superpotentials, together with an appropriate polarization. It is shown that
oblate spheroidal antennas, which are ideal for their production and reception,
can be made by deforming and merging two branch cuts. This determines a unique
field on the interior of the spheroid which gives the boundary conditions for
the surface charge-current density necessary to radiate the wavelets. These
sources are computed, including the impulse response of the antenna.Comment: 29 pages, 4 figures; minor corrections and addition
A Weak Gravitational Lensing Analysis of Abell 2390
We report on the detection of dark matter in the cluster Abell 2390 using the
weak gravitational distortion of background galaxies. We find that the cluster
light and total mass distributions are quite similar over an angular scale of
\simeq 7^\prime \;(1 \Mpc). The cluster galaxy and mass distributions are
centered on the cluster cD galaxy and exhibit elliptical isocontours in the
central \simeq 2^\prime \; (280 \kpc). The major axis of the ellipticity is
aligned with the direction defined by the cluster cD and a ``straight arc''
located to the northwest. We determined the radial
mass-to-light profile for this cluster and found a constant value of , which is consistent with other published
determinations. We also compared our weak lensing azimuthally averaged radial
mass profile with a spherical mass model proposed by the CNOC group on the
basis of their detailed dynamical study of the cluster. We find good agreement
between the two profiles, although there are weak indications that the CNOC
density profile may be falling more steeply for
(420\kpc).Comment: 14 pages, latex file. Postscript file and one additional figure are
available at
ftp://magicbean.berkeley.edu/pub/squires/a2390/massandlight.ps.g
Global stability analysis of birhythmicity in a self-sustained oscillator
We analyze global stability properties of birhythmicity in a self-sustained
system with random excitations. The model is a multi-limit cycles variation of
the van der Pol oscillatorintroduced to analyze enzymatic substrate reactions
in brain waves. We show that the two frequencies are strongly influenced by the
nonlinear coefficients and . With a random excitation, such as
a Gaussian white noise, the attractor's global stability is measured by the
mean escape time from one limit-cycle. An effective activation energy
barrier is obtained by the slope of the linear part of the variation of the
escape time versus the inverse noise-intensity 1/D. We find that the
trapping barriers of the two frequencies can be very different, thus leaving
the system on the same attractor for an overwhelming time. However, we also
find that the system is nearly symmetric in a narrow range of the parameters.Comment: 17 pages, 8 figures, to appear on Choas, 201
Symmetrization and enhancement of the continuous Morlet transform
The forward and inverse wavelet transform using the continuous Morlet basis
may be symmetrized by using an appropriate normalization factor. The loss of
response due to wavelet truncation is addressed through a renormalization of
the wavelet based on power. The spectral density has physical units which may
be related to the squared amplitude of the signal, as do its margins the mean
wavelet power and the integrated instant power, giving a quantitative estimate
of the power density with temporal resolution. Deconvolution with the wavelet
response matrix reduces the spectral leakage and produces an enhanced wavelet
spectrum providing maximum resolution of the harmonic content of a signal.
Applications to data analysis are discussed.Comment: 12 pages, 8 figures, 2 tables, minor revision, final versio
Helicity, polarization, and Riemann-Silberstein vortices
Riemann-Silberstein (RS) vortices have been defined as surfaces in spacetime
where the complex form of a free electromagnetic field given by F=E+iB is null
(F.F=0), and they can indeed be interpreted as the collective history swept out
by moving vortex lines of the field. Formally, the nullity condition is similar
to the definition of "C-lines" associated with a monochromatic electric or
magnetic field, which are curves in space where the polarization ellipses
degenerate to circles. However, it was noted that RS vortices of monochromatic
fields generally oscillate at optical frequencies and are therefore
unobservable while electric and magnetic C-lines are steady. Here I show that
under the additional assumption of having definite helicity, RS vortices are
not only steady but they coincide with both sets of C-lines, electric and
magnetic. The two concepts therefore become one for waves of definite frequency
and helicity. Since the definition of RS vortices is relativistically invariant
while that of C-lines is not, it may be useful to regard the vortices as a
wideband generalization of C-lines for waves of definite helicity.Comment: 5 pages, no figures. Submitted to J of Optics A, special issue on
Singular Optics; minor changes from v.
Effective Fokker-Planck Equation for Birhythmic Modified van der Pol Oscillator
We present an explicit solution based on the phase-amplitude approximation of
the Fokker-Planck equation associated with the Langevin equation of the
birhythmic modified van der Pol system. The solution enables us to derive
probability distributions analytically as well as the activation energies
associated to switching between the coexisting different attractors that
characterize the birhythmic system. Comparing analytical and numerical results
we find good agreement when the frequencies of both attractors are equal, while
the predictions of the analytic estimates deteriorate when the two frequencies
depart. Under the effect of noise the two states that characterize the
birhythmic system can merge, inasmuch as the parameter plane of the birhythmic
solutions is found to shrink when the noise intensity increases. The solution
of the Fokker-Planck equation shows that in the birhythmic region, the two
attractors are characterized by very different probabilities of finding the
system in such a state. The probability becomes comparable only for a narrow
range of the control parameters, thus the two limit cycles have properties in
close analogy with the thermodynamic phases
Coherent light transport in a cold Strontium cloud
We study light coherent transport in the weak localization regime using
magneto-optically cooled strontium atoms. The coherent backscattering cone is
measured in the four polarization channels using light resonant with a J=0 to
J=1 transition of the Strontium atom. We find an enhancement factor close to 2
in the helicity preserving channel, in agreement with theoretical predictions.
This observation confirms the effect of internal structure as the key mechanism
for the contrast reduction observed with an Rubidium cold cloud (see: Labeyrie
et al., PRL 83, 5266 (1999)). Experimental results are in good agreement with
Monte-Carlo simulations taking into account geometry effects.Comment: 4 pages, 2 figure
Thermoelectric power in one-dimensional Hubbard model
The thermoelectric power S is studied within the one-dimensional Hubbard
model using the linear response theory and the numerical exact-diagonalization
method for small systems. While both the diagonal and off-diagonal dynamical
correlation functions of particle and energy current are singular within the
model even at temperature T>0, S behaves regularly as a function of frequency
and T. Dependence on the electron density n below the half-filling
reveals a change of sign of S at n_0=0.73+/-0.07 due to strong correlations, in
the whole T range considered. Approaching half-filling S is hole-like and can
become large for U>>t although decreasing with T.Comment: 6 pages, 4 figure
Beam Effects on the Cryogenic System of LEP2
The LEP collider was operated during 1996 for the first time with superconducting cavities at the four interaction points. During operation for physics it was observed that the dissipated heat in the cavities is not only a function of the acceleration gradient, but depends also on beam characteristics such as intensity, bunch length and beam current. These beam effects had not been foreseen in the original heat budget of the LEP cryogenic system. The observations indicating the beam effect and its origin are presented. The available capacity of the refrigerators demonstrates that cryogenics might become a limiting factor for the performance of the LEP collider
Evolution of Lyman Break Galaxies Beyond Redshift Four
The formation rate of luminous galaxies seems to be roughly constant from z~2
to z~4 from the recent observations of Lyman break galaxies (LBGs) (Steidel et
al 1999). The abundance of luminous quasars, on the other hand, appears to drop
off by a factor of more than twenty from z~2 to z~5 (Warren, Hewett, & Osmer
1994; Schmidt, Schneider, & Gunn 1995). The difference in evolution between
these two classes of objects in the overlapping, observed redshift range,
z=2-4, can be explained naturally, if we assume that quasar activity is
triggered by mergers of luminous LBGs and one quasar lifetime is ~10^{7-8} yrs.
If this merger scenario holds at higher redshift, for the evolutions of these
two classes of objects to be consistent at z>4, the formation rate of luminous
LBGs is expected to drop off at least as rapidly as exp(-(z-4)^{6/5}) at z>4.Comment: in press, ApJ Letters, 15 latex pages plus 1 fi
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