508 research outputs found
Subdiffusion via dynamical localization induced by thermal equilibrium fluctuations
We reveal the mechanism of subdiffusion which emerges in a straightforward,
one dimensional classical nonequilibrium dynamics of a Brownian ratchet driven
by both a time-periodic force and Gaussian white noise. In a tailored parameter
set for which the deterministic counterpart is in a non-chaotic regime,
subdiffusion is a long-living transient whose lifetime can be many, many orders
of magnitude larger than characteristic time scales of the setup thus being
amenable to experimental observations. As a reason for this subdiffusive
behaviour in the coordinate space we identify thermal noise induced dynamical
localization in the velocity (momentum) space. This novel idea is distinct from
existing knowledge and has never been reported for any classical or quantum
systems. It suggests reconsideration of generally accepted opinion that
subdiffusion is due to road distributions or strong correlations which reflect
disorder, trapping, viscoelasticity of the medium or geometrical constraints.Comment: in press in Scientific Reports (2017
Direct observation of Levy flight of holes in bulk n-InP
We study the photoluminescence spectra excited at an edge side of n-InP slabs
and observed from the broadside. In a moderately doped sample the intensity
drops off as a power-law function of the distance from the excitation - up to
several millimeters - with no change in the spectral shape.The hole
distribution is described by a stationary Levy-flight process over more than
two orders of magnitude in both the distance and hole concentration. For
heavily-doped samples, the power law is truncated by free-carrier absorption.
Our experiments are near-perfectly described by the Biberman-Holstein transport
equation with parameters found from independent optical experiments.Comment: 4 pages, 3 figure
Front Propagation in Random Media
This PhD thesis deals with the problem of the propagation of fronts under random circumstances. A
statistical model to represent the motion of fronts when are evolving in a media characterized by
microscopical randomness is discussed and expanded, in order to cope with three distinct
applications: wild-land fire simulation, turbulent premixed combustion, biofilm modeling. In the
studied formalism, the position of the average front is computed by making use of a sharp-front
evolution method, such as the level set method. The microscopical spread of particles which takes
place around the average front is given by the probability density function linked to the underlying
diffusive process, that is supposedly known in advance. The adopted statistical front propagation
framework allowed a deeper understanding of any studied field of application. The application of
this model introduced eventually parameters whose impact on the physical observables of the front
spread have been studied with Uncertainty Quantification and Sensitivity Analysis tools. In
particular, metamodels for the front propagation system have been constructed in a non intrusive
way, by making use of generalized Polynomial Chaos expansions and Gaussian Processes.The Thesis received funding from Basque Government through the BERC 2014-2017 program.
It was also funded by the Spanish Ministry of Economy and Competitiveness MINECO via the BCAM Severo Ochoa SEV-2013-0323 accreditation.
The PhD is fundend by La Caixa Foundation through the PhD grant “La Caixa 2014”.
Funding from “Programma Operativo Nazionale Ricerca e Innovazione” (PONRI 2014-2020) , “Innotavive PhDs with Industrial Characterization” is kindly acknowledged for a research visit at the department of Mathematics and Applications “Renato Caccioppoli” of University “Federico II” of Naples
L\'evy walks
Random walk is a fundamental concept with applications ranging from quantum
physics to econometrics. Remarkably, one specific model of random walks appears
to be ubiquitous across many fields as a tool to analyze transport phenomena in
which the dispersal process is faster than dictated by Brownian diffusion. The
L\'{e}vy walk model combines two key features, the ability to generate
anomalously fast diffusion and a finite velocity of a random walker. Recent
results in optics, Hamiltonian chaos, cold atom dynamics, bio-physics, and
behavioral science demonstrate that this particular type of random walks
provides significant insight into complex transport phenomena. This review
provides a self-consistent introduction to L\'{e}vy walks, surveys their
existing applications, including latest advances, and outlines further
perspectives.Comment: 50 page
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