1,183 research outputs found
Is transport in time-dependent random potentials universal ?
The growth of the average kinetic energy of classical particles is studied
for potentials that are random both in space and time. Such potentials are
relevant for recent experiments in optics and in atom optics. It is found that
for small velocities uniform acceleration takes place, and at a later stage
fluctuations of the potential are encountered, resulting in a regime of
anomalous diffusion. This regime was studied in the framework of the
Fokker-Planck approximation. The diffusion coefficient in velocity was
expressed in terms of the average power spectral density, which is the Fourier
transform of the potential correlation function. This enabled to establish a
scaling form for the Fokker-Planck equation and to compute the large and small
velocity limits of the diffusion coefficient. A classification of the random
potentials into universality classes, characterized by the form of the
diffusion coefficient in the limit of large and small velocity, was performed.
It was shown that one dimensional systems exhibit a large variety of novel
universality classes, contrary to systems in higher dimensions, where only one
universality class is possible. The relation to Chirikov resonances, that are
central in the theory of Chaos, was demonstrated. The general theory was
applied and numerically tested for specific physically relevant examples.Comment: 5 pages, 3 figure
On the attractors of two-dimensional Rayleigh oscillators including noise
We study sustained oscillations in two-dimensional oscillator systems driven
by Rayleigh-type negative friction. In particular we investigate the influence
of mismatch of the two frequencies. Further we study the influence of external
noise and nonlinearity of the conservative forces. Our consideration is
restricted to the case that the driving is rather weak and that the forces show
only weak deviations from radial symmetry. For this case we provide results for
the attractors and the bifurcations of the system. We show that for rational
relations of the frequencies the system develops several rotational excitations
with right/left symmetry, corresponding to limit cycles in the four-dimensional
phase space. The corresponding noisy distributions have the form of hoops or
tires in the four-dimensional space. For irrational frequency relations, as
well as for increasing strength of driving or noise the periodic excitations
are replaced by chaotic oscillations.Comment: 9 pages, 5 figure
Polaron Recombination in Pristine and Annealed Bulk Heterojunction Solar Cells
The major loss mechanism of photogenerated polarons was investigated in
P3HT:PCBM solar cells by the photo-CELIV technique. For pristine and annealed
devices, we find that the experimental data can be explained by a bimolecular
recombination rate reduced by a factor of about ten (pristine) and 25
(annealed) as compared to Langevin theory. Aided by a macroscopic device model,
we discuss the implications of the lowered loss rate on the characteristics of
polymer:fullerene solar cells.Comment: 3 pages, 4 figure
Direct optical observations of surface thermal motions at sub-shot noise levels
We measure spectral properties of surface thermal fluctuations of liquids,
solids, complex fluids and biological matter using light scattering methods.
The random thermal fluctuations are delineated from random noise at sub-shot
noise levels. The principle behind this extraction, which is quite general and
is not limited to surface measurements, is explained. An optical lever is used
to measure the spectrum of fluctuations in the inclinations of surfaces down to
at W optical intensity, corresponding
to in the vertical displacement, in the
frequency range . The dynamical evolution of the
surface properties is also investigated. The measurement requires only a short
amount of time and is essentially passive, so that it can be applied to a wide
variety of surfaces.Comment: 5pp, 5 figure
Anomalous temperature dependence of surface tension and capillary waves at liquid gallium
The temperature dependence of surface tension \gamma(T) at liquid gallium is
studied theoretically and experimentally using light scattering from capillary
waves. The theoretical model based on the Gibbs thermodynamics relates the
temperature derivative of \gamma to the surface excess entropy -\Delta S.
Although capillary waves contribute to the surface entropy with a positive sign
the effect of dipole layer on \Delta S is negative. Experimental data collected
at a free Ga surface in the temperature range from 30 to 160 C show that the
temperature derivative of the tension changes sign near 100 C.Comment: 11 pages, 1 Postscript figure, submitted to J. Phys.
Spectral properties of thermal fluctuations on simple liquid surfaces below shot noise levels
We study the spectral properties of thermal fluctuations on simple liquid
surfaces, sometimes called ripplons. Analytical properties of the spectral
function are investigated and are shown to be composed of regions with simple
analytic behavior with respect to the frequency or the wave number. The derived
expressions are compared to spectral measurements performed orders of magnitude
below shot noise levels, which is achieved using a novel noise reduction
method. The agreement between the theory of thermal surface fluctuations and
the experiment is found to be excellent.Comment: 9 pages, 5 figure
Fluctuating and dissipative dynamics of dark solitons in quasi-condensates
The fluctuating and dissipative dynamics of matter-wave dark solitons within
harmonically trapped, partially condensed Bose gases is studied both
numerically and analytically. A study of the stochastic Gross-Pitaevskii
equation, which correctly accounts for density and phase fluctuations at finite
temperatures, reveals dark soliton decay times to be lognormally distributed at
each temperature, thereby characterizing the previously predicted long lived
soliton trajectories within each ensemble of numerical realizations (S.P.
Cockburn {\it et al.}, Phys. Rev. Lett. 104, 174101 (2010)). Expectation values
for the average soliton lifetimes extracted from these distributions are found
to agree well with both numerical and analytic predictions based upon the
dissipative Gross-Pitaevskii model (with the same {\it ab initio} damping).
Probing the regime for which , we find average
soliton lifetimes to scale with temperature as , in agreement
with predictions previously made for the low-temperature regime .
The model is also shown to capture the experimentally-relevant decrease in the
visibility of an oscillating soliton due to the presence of background
fluctuations.Comment: 17 pages, 14 figure
A simple measure of memory for dynamical processes described by the generalized Langevin equation
Memory effects are a key feature in the description of the dynamical systems
governed by the generalized Langevin equation, which presents an exact
reformulation of the equation of motion. A simple measure for the estimation of
memory effects is introduced within the framework of this description.
Numerical calculations of the suggested measure and the analysis of memory
effects are also applied for various model physical systems as well as for the
phenomena of ``long time tails'' and anomalous diffusion
Observation of enhanced rate coefficients in the H + H H + H reaction at low collision energies
The energy dependence of the rate coefficient of the H reaction has been measured in the range of
collision energies between K and
mK. A clear deviation of the rate coefficient from the value expected on the
basis of the classical Langevin-capture behavior has been observed at collision
energies below K, which is attributed to the joint
effects of the ion-quadrupole and Coriolis interactions in collisions involving
ortho-H molecules in the rotational level, which make up 75% of the
population of the neutral H molecules in the experiments. The experimental
results are compared to very recent predictions by Dashevskaya, Litvin, Nikitin
and Troe (J. Chem. Phys., in press), with which they are in agreement.Comment: 14 pages, 3 figure
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