140 research outputs found
Universal properties of branching random walks in confined geometries
Characterizing the occupation statistics of a radiation flow through confined
geometries is key to such technological issues as nuclear reactor design and
medical diagnosis. This amounts to assessing the distribution of the travelled
length and the number of collisions performed by the underlying
stochastic transport process, for which remarkably simple Cauchy-like formulas
were established in the case of branching Pearson random walks with
exponentially distributed jumps. In this Letter, we show that such formulas
strikingly carry over to the much broader class of branching processes with
arbitrary jumps, provided that scattering is isotropic and the average jump
size is finite.Comment: 5 pages, 3 figure
The critical catastrophe revisited
The neutron population in a prototype model of nuclear reactor can be
described in terms of a collection of particles confined in a box and
undergoing three key random mechanisms: diffusion, reproduction due to
fissions, and death due to absorption events. When the reactor is operated at
the critical point, and fissions are exactly compensated by absorptions, the
whole neutron population might in principle go to extinction because of the
wild fluctuations induced by births and deaths. This phenomenon, which has been
named critical catastrophe, is nonetheless never observed in practice: feedback
mechanisms acting on the total population, such as human intervention, have a
stabilizing effect. In this work, we revisit the critical catastrophe by
investigating the spatial behaviour of the fluctuations in a confined geometry.
When the system is free to evolve, the neutrons may display a wild patchiness
(clustering). On the contrary, imposing a population control on the total
population acts also against the local fluctuations, and may thus inhibit the
spatial clustering. The effectiveness of population control in quenching
spatial fluctuations will be shown to depend on the competition between the
mixing time of the neutrons (i.e., the average time taken for a particle to
explore the finite viable space) and the extinction time.Comment: 16 pages, 6 figure
The Stochastic complexity of spin models: Are pairwise models really simple?
Models can be simple for different reasons: because they yield a simple and
computationally efficient interpretation of a generic dataset (e.g. in terms of
pairwise dependences) - as in statistical learning - or because they capture
the essential ingredients of a specific phenomenon - as e.g. in physics -
leading to non-trivial falsifiable predictions. In information theory and
Bayesian inference, the simplicity of a model is precisely quantified in the
stochastic complexity, which measures the number of bits needed to encode its
parameters. In order to understand how simple models look like, we study the
stochastic complexity of spin models with interactions of arbitrary order. We
highlight the existence of invariances with respect to bijections within the
space of operators, which allow us to partition the space of all models into
equivalence classes, in which models share the same complexity. We thus found
that the complexity (or simplicity) of a model is not determined by the order
of the interactions, but rather by their mutual arrangements. Models where
statistical dependencies are localized on non-overlapping groups of few
variables (and that afford predictions on independencies that are easy to
falsify) are simple. On the contrary, fully connected pairwise models, which
are often used in statistical learning, appear to be highly complex, because of
their extended set of interactions
Cauchy's formulas for random walks in bounded domains
Cauchy's formula was originally established for random straight paths
crossing a body and basically relates the average
chord length through to the ratio between the volume and the surface of the
body itself. The original statement was later extended in the context of
transport theory so as to cover the stochastic paths of Pearson random walks
with exponentially distributed flight lengths traversing a bounded domain. Some
heuristic arguments suggest that Cauchy's formula may also hold true for
Pearson random walks with arbitrarily distributed flight lengths. For such a
broad class of stochastic processes, we rigorously derive a generalized
Cauchy's formula for the average length travelled by the walkers in the body,
and show that this quantity depends indeed only on the ratio between the volume
and the surface, provided that some constraints are imposed on the entrance
step of the walker in . Similar results are obtained also for the average
number of collisions performed by the walker in , and an extension to
absorbing media is discussed.Comment: 12 pages, 6 figure
Acid/base-triggered photophysical and chiroptical switching in a series of helicenoid compounds
International audienceA series of molecules that possess two quinolines, benzoquinolines, or phenanthrolines connected in a chiral fashion by a biaryl junction along with their water-soluble derivatives was developed and characterized. The influence of the structure on the basicity of the nitrogen atoms in two heterocycles was examined and the photophysical and chiroptical switching activity of the compounds upon protonation was studied both experimentally and computationally. The results demonstrated that changes in the electronic structure of the protonated vs. neutral species, promoting a bathochromic shift of dominant electronic transitions and alternation of their character from π-to-π* to charge-transfer-type, when additionally accompanied by the high structural flexibility of a system, leading to changes in conformational preferences upon proton binding, produce particularly pronounced modifications of the spectral properties in acidic medium. The latter combined with reversibility of the read-out make some of the molecules in this series very promising multifunctional pH probes
Intrinsically Stretchable Biphasic (Solid–Liquid) Thin Metal Films
Stretchable biphasic conductors are formed by physical vapor deposition of gallium onto an alloying metal film. The properties of the photolithography-compatible thin metal films are highlighted by low sheet resistance (0.5 Ω sq−1) and large stretchability (400%). This novel approach to deposit and pattern liquid metals enables extremely robust, multilayer and soft circuits, sensors, and actuators
Broadband optical limiting optimisation by combination of carbon nanotubes and two-photon absorbing chromophores in liquids
International audienceWe report here on the optical limiting studies performed with nanosecond laser pulses on several families of multiphoton absorbers in chloroform, with carbon nanotubes suspended in solutions. Performances of these samples are compared with those of simple multiphoton absorber solutions and carbon nanotube suspensions, and the differences observed are interpreted in terms of cumulative NLO effects and adverse aggregation phenomenon
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