592 research outputs found
Coherent Destruction of Photon Emission from a Single Molecule Source
The behavior of a single molecule driven simultaneously by a laser and by an
electric radio frequency field is investigated using a non-Hermitian
Hamiltonian approach. Employing the renormalization group method for
differential equations we calculate the average waiting time for the first
photon emission event to occur, and determine the conditions for the
suppression and enhancement of photon emission. An abrupt transition from
localization-like behavior to delocalization behavior is found.Comment: 5 pages, 4 figure
Theory of Single File Diffusion in a Force Field
The dynamics of hard-core interacting Brownian particles in an external
potential field is studied in one dimension. Using the Jepsen line we find a
very general and simple formula relating the motion of the tagged center
particle, with the classical, time dependent single particle reflection and transmission coefficients. Our formula describes rich
physical behaviors both in equilibrium and the approach to equilibrium of this
many body problem.Comment: 4 Phys. Rev. page
Distribution of Time-Averaged Observables for Weak Ergodicity Breaking
We find a general formula for the distribution of time-averaged observables
for systems modeled according to the sub-diffusive continuous time random walk.
For Gaussian random walks coupled to a thermal bath we recover ergodicity and
Boltzmann's statistics, while for the anomalous subdiffusive case a weakly
non-ergodic statistical mechanical framework is constructed, which is based on
L\'evy's generalized central limit theorem. As an example we calculate the
distribution of : the time average of the position of the particle,
for unbiased and uniformly biased particles, and show that exhibits
large fluctuations compared with the ensemble average .Comment: 5 pages, 2 figure
Analytical and Numerical Study of Internal Representations in Multilayer Neural Networks with Binary Weights
We study the weight space structure of the parity machine with binary weights
by deriving the distribution of volumes associated to the internal
representations of the learning examples. The learning behaviour and the
symmetry breaking transition are analyzed and the results are found to be in
very good agreement with extended numerical simulations.Comment: revtex, 20 pages + 9 figures, to appear in Phys. Rev.
Fluctuations of noise and the low frequency cutoff paradox
Recent experiments on blinking quantum dots and weak turbulence in liquid
crystals reveal the fundamental connection between noise and power law
intermittency. The non-stationarity of the process implies that the power
spectrum is random -- a manifestation of weak ergodicity breaking. Here we
obtain the universal distribution of the power spectrum, which can be used to
identify intermittency as the source of the noise. We solve an outstanding
paradox on the non integrability of noise and the violation of Parseval's
theorem. We explain why there is no physical low frequency cutoff and therefore
cannot be found in experiments.Comment: 5 pages, 2 figures, supplementary material (4 pages
Linear and non linear response in the aging regime of the 1D trap model
We investigate the behaviour of the response function in the one dimensional
trap model using scaling arguments that we confirm by numerical simulations. We
study the average position of the random walk at time tw+t given that a small
bias h is applied at time tw. Several scaling regimes are found, depending on
the relative values of t, tw and h. Comparison with the diffusive motion in the
absence of bias allows us to show that the fluctuation dissipation relation is,
in this case, valid even in the aging regime.Comment: 5 pages, 3 figures, 3 references adde
Residence Time Statistics for Normal and Fractional Diffusion in a Force Field
We investigate statistics of occupation times for an over-damped Brownian
particle in an external force field. A backward Fokker-Planck equation
introduced by
Majumdar and Comtet describing the distribution of occupation times is
solved. The solution gives a general relation between occupation time
statistics and probability currents which are found from solutions of the
corresponding problem of first passage time. This general relationship between
occupation times and first passage times, is valid for normal Markovian
diffusion and for non-Markovian sub-diffusion, the latter modeled using the
fractional Fokker-Planck equation. For binding potential fields we find in the
long time limit ergodic behavior for normal diffusion, while for the fractional
framework weak ergodicity breaking is found, in agreement with previous results
of Bel and Barkai on the continuous time random walk on a lattice. For
non-binding potential rich physical behaviors are obtained, and classification
of occupation time statistics is made possible according to whether or not the
underlying random walk is recurrent and the averaged first return time to the
origin is finite. Our work establishes a link between fractional calculus and
ergodicity breaking.Comment: 12 page
Diffusion of Tagged Particle in an Exclusion Process
We study the diffusion of tagged hard core interacting particles under the
influence of an external force field. Using the Jepsen line we map this many
particle problem onto a single particle one. We obtain general equations for
the distribution and the mean square displacement of the tagged
center particle valid for rather general external force fields and initial
conditions. A wide range of physical behaviors emerge which are very different
than the classical single file sub-diffusion $ \sim t^{1/2}$ found
for uniformly distributed particles in an infinite space and in the absence of
force fields. For symmetric initial conditions and potential fields we find
$ = {{\cal R} (1 - {\cal R})\over 2 N {\it r} ^2} $ where $2 N$ is
the (large) number of particles in the system, ${\cal R}$ is a single particle
reflection coefficient obtained from the single particle Green function and
initial conditions, and $r$ its derivative. We show that this equation is
related to the mathematical theory of order statistics and it can be used to
find even when the motion between collision events is not Brownian
(e.g. it might be ballistic, or anomalous diffusion). As an example we derive
the Percus relation for non Gaussian diffusion
Correlations between hidden units in multilayer neural networks and replica symmetry breaking
We consider feed-forward neural networks with one hidden layer, tree
architecture and a fixed hidden-to-output Boolean function. Focusing on the
saturation limit of the storage problem the influence of replica symmetry
breaking on the distribution of local fields at the hidden units is
investigated. These field distributions determine the probability for finding a
specific activation pattern of the hidden units as well as the corresponding
correlation coefficients and therefore quantify the division of labor among the
hidden units. We find that although modifying the storage capacity and the
distribution of local fields markedly replica symmetry breaking has only a
minor effect on the correlation coefficients. Detailed numerical results are
provided for the PARITY, COMMITTEE and AND machines with K=3 hidden units and
nonoverlapping receptive fields.Comment: 9 pages, 3 figures, RevTex, accepted for publication in Phys. Rev.
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