5,440 research outputs found
Characterizing Quantum-Dot Blinking Using Noise Power Spectra
Fluctuations in the fluorescence from macroscopic ensembles of colloidal
semiconductor quantum dots have the spectral form of 1/f noise. The measured
power spectral density reflects the fluorescence intermittency of individual
dots with power-law distributions of "on" and "off" times, and can thus serve
as a simple method for characterizing such blinking behavior
Ultrafast resonant optical scattering from single gold nanorods: Large nonlinearities and plasmon saturation
We measure nonlinear optical scattering from individual Au nanorods excited
by ultrafast laser pulses on resonance with their longitudinal plasmon mode.
Isolating single rods removes inhomogeneous broadening and allows the
measurement of a large nonlinearity, much greater than that of nanorod
ensembles. Surprisingly, the ultrafast nonlinearity can be attributed entirely
to heating of conduction electrons and does not exhibit any response associated
with coherent plasmon oscillation. This indicates a previously unobserved
damping of strongly driven plasmons.Comment: Revised tex
From Dust To Planetesimal: The Snowball Phase ?
The standard model of planet formation considers an initial phase in which
planetesimals form from a dust disk, followed by a phase of mutual
planetesimal-planetesimal collisions, leading eventually to the formation of
planetary embryos. However, there is a potential transition phase (which we
call the "snowball phase"), between the formation of the first planetesimals
and the onset of mutual collisions amongst them, which has often been either
ignored or underestimated in previous studies. In this snowball phase, isolated
planetesimals move on Keplerian orbits and grow solely via the direct accretion
of sub-cm sized dust entrained with the gas in the protoplanetary disk. Using a
simplified model in which planetesimals are progressively produced from the
dust, we consider the expected sizes to which the planetesimals can grow before
mutual collisions commence and derive the dependence of this size on a number
of critical parameters, including the degree of disk turbulence, the
planetesimal size at birth and the rate of planetesimal creation. For systems
in which turbulence is weak and the planetesimals are created at a low rate and
with relatively small birth size, we show that the snowball growth phase can be
very important, allowing planetesimals to grow by a factor of 10^6 in mass
before mutual collisions take over. In such cases, the snowball growth phase
can be the dominant mode to transfer mass from the dust to planetesimals.
Moreover, such growth can take place within the typical lifetime of a
protoplanetary gas disk. A noteworthy result is that ... ...(see the paper).
For the specific case of close binaries such as Alpha Centauri ... ... (see the
paper). From a more general perspective, these preliminary results suggest that
an efficient snowball growth phase provides a large amount of "room at the
bottom" for theories of planet formation.Comment: Accepted for publication in the Astrophysical Journal. 15 pages, 4
figures, 1 tabl
Radio to Gamma-Ray Emission from Shell-type Supernova Remnants: Predictions from Non-linear Shock Acceleration Models
Supernova remnants (SNRs) are widely believed to be the principal source of
galactic cosmic rays. Such energetic particles can produce gamma-rays and lower
energy photons via interactions with the ambient plasma. In this paper, we
present results from a Monte Carlo simulation of non-linear shock structure and
acceleration coupled with photon emission in shell-like SNRs. These
non-linearities are a by-product of the dynamical influence of the accelerated
cosmic rays on the shocked plasma and result in distributions of cosmic rays
which deviate from pure power-laws. Such deviations are crucial to acceleration
efficiency and spectral considerations, producing GeV/TeV intensity ratios that
are quite different from test particle predictions. The Sedov scaling solution
for SNR expansions is used to estimate important shock parameters for input
into the Monte Carlo simulation. We calculate ion and electron distributions
that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron
emission, yielding complete photon spectra from radio frequencies to gamma-ray
energies. The cessation of acceleration caused by the spatial and temporal
limitations of the expanding SNR shell in moderately dense interstellar regions
can yield spectral cutoffs in the TeV energy range; these are consistent with
Whipple's TeV upper limits on unidentified EGRET sources. Supernova remnants in
lower density environments generate higher energy cosmic rays that produce
predominantly inverse Compton emission at super-TeV energies; such sources will
generally be gamma-ray dim at GeV energies.Comment: 62 pages, AASTeX format, including 1 table and 11 figures, accepted
for publication in The Astrophysical Journal (Vol 513, March 1, 1999
Population and Phase Coherence during the Growth of an Elongated Bose-Einstein Condensate
We study the growth of an elongated phase-fluctuating condensate from a
non-equilibrium thermal cloud obtained by shock-cooling. We compare the growth
of the condensate with numerical simulations, revealing a time delay and a
reduction in the growth rate which we attribute to phase fluctuations. We
measure the phase coherence using momentum Bragg spectroscopy, and thereby
observe the evolution of the phase coherence as a function of time. Combining
the phase coherence results with the numerical simulations, we suggest a simple
model for the reduction of the growth rate based on the reduction of bosonic
stimulation due to phase fluctuations and obtain improved agreement between
theory and experiment
Converse Lyapunov theorems for discrete-time linear switching systems with regular switching sequences
We present a stability analysis framework for the general class of
discrete-time linear switching systems for which the switching sequences belong
to a regular language. They admit arbitrary switching systems as special cases.
Using recent results of X. Dai on the asymptotic growth rate of such systems,
we introduce the concept of multinorm as an algebraic tool for stability
analysis.
We conjugate this tool with two families of multiple quadratic Lyapunov
functions, parameterized by an integer T >= 1, and obtain converse Lyapunov
Theorems for each.
Lyapunov functions of the first family associate one quadratic form per state
of the automaton defining the switching sequences. They are made to decrease
after every T successive time steps. The second family is made of the
path-dependent Lyapunov functions of Lee and Dullerud. They are parameterized
by an amount of memory (T-1) >= 0.
Our converse Lyapunov theorems are finite. More precisely, we give sufficient
conditions on the asymptotic growth rate of a stable system under which one can
compute an integer parameter T >= 1 for which both types of Lyapunov functions
exist.
As a corollary of our results, we formulate an arbitrary accurate
approximation scheme for estimating the asymptotic growth rate of switching
systems with constrained switching sequences
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