682 research outputs found
Broad distribution effects in sums of lognormal random variables
The lognormal distribution describing, e.g., exponentials of Gaussian random
variables is one of the most common statistical distributions in physics. It
can exhibit features of broad distributions that imply qualitative departure
from the usual statistical scaling associated to narrow distributions.
Approximate formulae are derived for the typical sums of lognormal random
variables. The validity of these formulae is numerically checked and the
physical consequences, e.g., for the current flowing through small tunnel
junctions, are pointed out.Comment: 14 pages, 9 figures. Minor changes + Gini coefficient and 4 refs.
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Fractional dynamics in the L\'evy quantum kicked rotor
We investigate the quantum kicked rotor in resonance subjected to momentum
measurements with a L\'evy waiting time distribution. We find that the system
has a sub-ballistic behavior. We obtain an analytical expression for the
exponent of the power law of the variance as a function of the characteristic
parameter of the L\'evy distribution and connect this anomalous diffusion with
a fractional dynamics
Crossover Time in Relative Fluctuations Characterizes the Longest Relaxation Time of Entangled Polymers
In entangled polymer systems, there are several characteristic time scales,
such as the entanglement time and the disengagement time. In molecular
simulations, the longest relaxation time (the disengagement time) can be
determined by the mean square displacement (MSD) of a segment or by the shear
relaxation modulus. Here, we propose the relative fluctuation analysis method,
which is originally developed for characterizing large fluctuations, to
determine the longest relaxation time from the center of mass trajectories of
polymer chains (the time-averaged MSDs). Applying the method to simulation data
of entangled polymers (by the slip-spring model and the simple reptation
model), we provide a clear evidence that the longest relaxation time is
estimated as the crossover time in the relative fluctuations.Comment: 17 pages, 9 figures, to appear in J. Chem. Phy
Fractal time random walk and subrecoil laser cooling considered as renewal processes with infinite mean waiting times
There exist important stochastic physical processes involving infinite mean
waiting times. The mean divergence has dramatic consequences on the process
dynamics. Fractal time random walks, a diffusion process, and subrecoil laser
cooling, a concentration process, are two such processes that look
qualitatively dissimilar. Yet, a unifying treatment of these two processes,
which is the topic of this pedagogic paper, can be developed by combining
renewal theory with the generalized central limit theorem. This approach
enables to derive without technical difficulties the key physical properties
and it emphasizes the role of the behaviour of sums with infinite means.Comment: 9 pages, 7 figures, to appear in the Proceedings of Cargese Summer
School on "Chaotic dynamics and transport in classical and quantum systems
Dephasing by a nonstationary classical intermittent noise
We consider a new phenomenological model for a classical
intermittent noise and study its effects on the dephasing of a two-level
system. Within this model, the evolution of the relative phase between the
states is described as a continuous time random walk (CTRW). Using
renewal theory, we find exact expressions for the dephasing factor and identify
the physically relevant various regimes in terms of the coupling to the noise.
In particular, we point out the consequences of the non-stationarity and
pronounced non-Gaussian features of this noise, including some new anomalous
and aging dephasing scenarii.Comment: Submitted to Phys. Rev.
Phase transitions driven by L\'evy stable noise: exact solutions and stability analysis of nonlinear fractional Fokker-Planck equations
Phase transitions and effects of external noise on many body systems are one
of the main topics in physics. In mean field coupled nonlinear dynamical
stochastic systems driven by Brownian noise, various types of phase transitions
including nonequilibrium ones may appear. A Brownian motion is a special case
of L\'evy motion and the stochastic process based on the latter is an
alternative choice for studying cooperative phenomena in various fields.
Recently, fractional Fokker-Planck equations associated with L\'evy noise have
attracted much attention and behaviors of systems with double-well potential
subjected to L\'evy noise have been studied intensively. However, most of such
studies have resorted to numerical computation. We construct an {\it
analytically solvable model} to study the occurrence of phase transitions
driven by L\'evy stable noise.Comment: submitted to EP
Power-law tail distributions and nonergodicity
We establish an explicit correspondence between ergodicity breaking in a
system described by power-law tail distributions and the divergence of the
moments of these distributions.Comment: 4 pages, 1 figure, corrected typo
Optical extinction in a single layer of nanorods
We demonstrate that almost 100 % of incident photons can interact with a
monolayer of scatterers in a symmetrical environment. Nearly-perfect optical
extinction through free-standing transparent nanorod arrays has been measured.
The sharp spectral opacity window, in the form of a characteristic Fano
resonance, arises from the coherent multiple scattering in the array. In
addition, we show that nanorods made of absorbing material exhibit a 25-fold
absorption enhancement per unit volume compared to unstructured thin film.
These results open new perspectives for light management in high-Q, low volume
dielectric nanostructures, with potential applications in optical systems,
spectroscopy, and optomechanics
Bardeen-Petterson effect and the disk structure of the Seyfert galaxy NGC 1068
VLBA high spatial resolution observations of the disk structure of the active
galactic nucleus NGC 1068 has recently revealed that the kinematics and
geometry of this AGN is well characterized by an outer disk of H2O maser
emission having a compact milliarcsecond (parsec) scale structure, which is
encircling a thin rotating inner disk surrounding a ~10^7 M_\sun compact
mass, likely a black hole. A curious feature in this source is the occurrence
of a misalignment between the inner and outer parts of the disk, with the
galaxy's radio jet being orthogonal to the inner disk. We interpret this
peculiar configuration as due to the Bardeen-Petterson effect, a general
relativistic effect that warps an initially inclined (to the black hole
equator) viscous disk, and drives the angular momentum vector of its inner part
into alignment with the rotating black hole spin. We estimate the time-scale
for both angular momenta to get aligned as a function the spin parameter of the
Kerr black hole. We also reproduce the shape of the parsec and kiloparsec scale
jets, assuming a model in which the jet is precessing with a period and
aperture angle that decrease exponentially with time, as expected from the
Bardeen-Petterson effect.Comment: 12 pages, 3 figures, accepted for publication in The Astrophysical
Journa
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