1,178 research outputs found
Reply to Comment on ``Thermal Model for Adaptive Competition in a Market''
We reply to the Comment of Challet et al. [cond-mat/0004308] on our paper
[Phys. Rev. Lett. 83, 4429 (1999)]. We show that the claim of the Comment that
the effects of the temperature in the Thermal Minority Game ``can be eliminated
by time rescaling'' and consequently the behaviour is ``independent of T'' has
no general validity.Comment: 1 page, 1 figur
Theory of Self-organized Criticality for Problems with Extremal Dynamics
We introduce a general theoretical scheme for a class of phenomena
characterized by an extremal dynamics and quenched disorder. The approach is
based on a transformation of the quenched dynamics into a stochastic one with
cognitive memory and on other concepts which permit a mathematical
characterization of the self-organized nature of the avalanche type dynamics.
In addition it is possible to compute the relevant critical exponents directly
from the microscopic model. A specific application to Invasion Percolation is
presented but the approach can be easily extended to various other problems.Comment: 11 pages Latex (revtex), 3 postscript figures included. Submitted to
Europhys. Let
On the High-dimensional Bak-Sneppen model
We report on extensive numerical simulations on the Bak-Sneppen model in high
dimensions. We uncover a very rich behavior as a function of dimensionality.
For d>2 the avalanche cluster becomes fractal and for d \ge 4 the process
becomes transient. Finally the exponents reach their mean field values for
d=d_c=8, which is then the upper critical dimension of the Bak Sneppen model.Comment: 4 pages, 3 eps figure
Laplacian Fractal Growth in Media with Quenched Disorder
We analyze the combined effect of a Laplacian field and quenched disorder for
the generation of fractal structures with a study, both numerical and
theoretical, of the quenched dielectric breakdown model (QDBM). The growth
dynamics is shown to evolve from the avalanches of invasion percolation (IP) to
the smooth growth of Laplacian fractals, i. e. diffusion limited aggregation
(DLA) and the dielectric breakdown model (DBM). The fractal dimension is
strongly reduced with respect to both DBM and IP, due to the combined effect of
memory and field screening. This implies a specific relation between the
fractal dimension of the breakdown structures (dielectric or mechanical) and
the microscopic properties of disordered materials.Comment: 11 pages Latex (revtex), 3 postscript figures included. Submitted to
PR
Generalized Dielectric Breakdown Model
We propose a generalized version of the Dielectric Breakdown Model (DBM) for
generic breakdown processes. It interpolates between the standard DBM and its
analog with quenched disorder, as a temperature like parameter is varied. The
physics of other well known fractal growth phenomena as Invasion Percolation
and the Eden model are also recovered for some particular parameter values. The
competition between different growing mechanisms leads to new non-trivial
effects and allows us to better describe real growth phenomena.
Detailed numerical and theoretical analysis are performed to study the
interplay between the elementary mechanisms. In particular, we observe a
continuously changing fractal dimension as temperature is varied, and report an
evidence of a novel phase transition at zero temperature in absence of an
external driving field; the temperature acts as a relevant parameter for the
``self-organized'' invasion percolation fixed point. This permits us to obtain
new insight into the connections between self-organization and standard phase
transitions.Comment: Submitted to PR
TSA infrared measurements for stress distribution on car elements
Because of the continuous evolution of the market in terms of quality and
performance, the car production industry is being subjected to more and more
pressing technological challenges. In this framework the use of an advanced
measurement technique such as thermoelasticity allows the engineers to have a
fast and reliable tool for experimental investigation, optimization and
validation of the finite element method (FEM) of those critical parts, such
as parts of car-frame tables (Marsili and Garinei, 2013; Ju et al., 1997). In
this work it is shown how the thermoelastic measurement technique can be used
to optimize a Ferrari car frame, as a method of experimental investigation
and as a technique of validation of numerical models.The measurement technique developed for this purpose is described together
with the calibration method used in the test benches normally used for
fatigue testing and qualification of this car's components. The results
obtained show a very good agreement with FEM models and also the possibility
of experimentally identifying the concentration levels of stress in critical
parts with a very high spatial resolution and testing the effective geometry
and material structure
Nonequilibrium phase transition in a model for social influence
We present extensive numerical simulations of the Axelrod's model for social
influence, aimed at understanding the formation of cultural domains. This is a
nonequilibrium model with short range interactions and a remarkably rich
dynamical behavior. We study the phase diagram of the model and uncover a
nonequilibrium phase transition separating an ordered (culturally polarized)
phase from a disordered (culturally fragmented) one. The nature of the phase
transition can be continuous or discontinuous depending on the model
parameters. At the transition, the size of cultural regions is power-law
distributed.Comment: 5 pages, 4 figure
Financial instability from local market measures
We study the emergence of instabilities in a stylized model of a financial
market, when different market actors calculate prices according to different
(local) market measures. We derive typical properties for ensembles of large
random markets using techniques borrowed from statistical mechanics of
disordered systems. We show that, depending on the number of financial
instruments available and on the heterogeneity of local measures, the market
moves from an arbitrage-free phase to an unstable one, where the complexity of
the market - as measured by the diversity of financial instruments - increases,
and arbitrage opportunities arise. A sharp transition separates the two phases.
Focusing on two different classes of local measures inspired by real markets
strategies, we are able to analytically compute the critical lines,
corroborating our findings with numerical simulations.Comment: 17 pages, 4 figure
Physics and application of photon number resolving detectors based on superconducting parallel nanowires
The Parallel Nanowire Detector (PND) is a photon number resolving (PNR)
detector which uses spatial multiplexing on a subwavelength scale to provide a
single electrical output proportional to the photon number. The basic structure
of the PND is the parallel connection of several NbN superconducting nanowires
(100 nm-wide, few nm-thick), folded in a meander pattern. PNDs were fabricated
on 3-4 nm thick NbN films grown on MgO (TS=400C) substrates by reactive
magnetron sputtering in an Ar/N2 gas mixture. The device performance was
characterized in terms of speed and sensitivity. PNDs showed a counting rate of
80 MHz and a pulse duration as low as 660ps full width at half maximum (FWHM).
Building the histograms of the photoresponse peak, no multiplication noise
buildup is observable. Electrical and optical equivalent models of the device
were developed in order to study its working principle, define design
guidelines, and develop an algorithm to estimate the photon number statistics
of an unknown light. In particular, the modeling provides novel insight of the
physical limit to the detection efficiency and to the reset time of these
detectors. The PND significantly outperforms existing PNR detectors in terms of
simplicity, sensitivity, speed, and multiplication noise
Timing performance of 30-nm-wide superconducting nanowire avalanche photodetectors
We investigated the timing jitter of superconducting nanowire avalanche
photodetectors (SNAPs, also referred to as cascade switching superconducting
single photon detectors) based on 30-nm-wide nanowires. At bias currents (IB)
near the switching current, SNAPs showed sub 35 ps FWHM Gaussian jitter similar
to standard 100 nm wide superconducting nanowire single-photon detectors. At
lower values of IB, the instrument response function (IRF) of the detectors
became wider, more asymmetric, and shifted to longer time delays. We could
reproduce the experimentally observed IRF time-shift in simulations based on an
electrothermal model, and explain the effect with a simple physical picture
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