1,963 research outputs found
Non-Gaussian Fluctuations in Biased Resistor Networks: Size Effects versus Universal Behavior
We study the distribution of the resistance fluctuations of biased resistor
networks in nonequilibrium steady states. The stationary conditions arise from
the competition between two stochastic and biased processes of breaking and
recovery of the elementary resistors. The fluctuations of the network
resistance are calculated by Monte Carlo simulations which are performed for
different values of the applied current, for networks of different size and
shape and by considering different levels of intrinsic disorder. The
distribution of the resistance fluctuations generally exhibits relevant
deviations from Gaussianity, in particular when the current approaches the
threshold of electrical breakdown. For two-dimensional systems we have shown
that this non-Gaussianity is in general related to finite size effects, thus it
vanishes in the thermodynamic limit, with the remarkable exception of highly
disordered networks. For these systems, close to the critical point of the
conductor-insulator transition, non-Gaussianity persists in the large size
limit and it is well described by the universal Bramwell-Holdsworth-Pinton
distribution. In particular, here we analyze the role of the shape of the
network on the distribution of the resistance fluctuations. Precisely, we
consider quasi-one-dimensional networks elongated along the direction of the
applied current or trasversal to it. A significant anisotropy is found for the
properties of the distribution. These results apply to conducting thin films or
wires with granular structure stressed by high current densities.Comment: 8 pages, 4 figures. Invited talk at the 18-th International
Conference on Noise and Fluctuations, 19-23 September 2005, Salamanc
Non-Gaussianity of resistance fluctuations near electrical breakdown
We study the resistance fluctuation distribution of a thin film near
electrical breakdown. The film is modeled as a stationary resistor networkunder
biased percolation. Depending on the value of the external current,on the
system sizes and on the level of internal disorder, the fluctuation
distribution can exhibit a non-Gaussian behavior. We analyze this
non-Gaussianity in terms of the generalized Gumbel distribution recently
introduced in the context of highly correlated systems near criticality. We
find that when the average fraction of defects approaches the random
percolation threshold, the resistance fluctuation distribution is well
described by the universal behavior of the Bramwell-Holdsworth-Pinton
distribution.Comment: 3 figures, accepted for publication on Semicond Sci Tec
Freight transport in Europe : policy issues and future scenarios on trans-border alpine connections
This paper presents an overview of European policy on interconnected cross-border transport networks as well as of severe problems in estimatingempirically the avalanche of goods movements in the European Union (EU). In particular, it deals with the Transalpine freight transport case, whichrepresents one of the most challenging operational and policy issues of the present and future - both international (EU) and national (the Alpinecountries) - freight transport developments. The paper is organised to briefly describe the main objectives of EU transport policy, to generallyintroduce the concept of intermodal transport with particular emphasis on intermodal freight transport and to describe past, present and futuredevelopments of the Trans.Alpine inter-modal transport. Various scenarios on the future development of Trans.Alpine intermodal transport are analysed as well
A Review on suboptimal power allocation schemes for WSN localization
This paper considers a review of two proposed power allocation algorithms for increasing accuracy in localization scenarios, a deeper theoretical analysis and a detailed performance comparison. Appropriate power allocation (PA) among beacons is an effective tool to implement localization with improved precision. At first, a brief review on existing optimal PA strategies is presented. Subsequently, the first PA algorithm is discussed: a function called uncertainty area is defined according to the interaction of beacons in a pair-wise selection procedure.
A general selection strategy among allocated transmission powers for each beacon completes the algorithm structure. In the literature, on one hand the commonly made assumption about ranging measures is that their ideal values are equal to their corresponding Cramer-Rao bounds but, on the other hand, at high signal-to-noise ratios, real ranging estimators are characterized by different lower limits on their performance, mainly as a result of maximum sampling rates and computational load available in the sensors. The second PA algorithm develops a type of adaptive PA (APA) directly based on measured SNRs and, consequently, much simpler than other techniques
The index of symmetry of compact naturally reductive spaces
We introduce a geometric invariant that we call the index of symmetry, which measures how far is a Riemannian manifold from being a symmetric space. We compute, in a geometric way, the index of symmetry of compact naturally reductive spaces. In this case, the so-called leaf of symmetry turns out to be of the group type. We also study several examples where the leaf of symmetry is not of the group type. Interesting examples arise from the unit tangent bundle of the sphere of curvature 2, and two metrics in an Aloff-Wallach 7-manifold and the Wallach 24-manifold.submittedVersionFil: Olmos, Carlos Enrique. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Reggiani, Silvio Nicolás. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Tamuru, Hiroshi. Universidad de Hiroshima. Escuela de Ciencias. Departamento de Matemática; Japón.Matemática Pur
European freight transport and the environment. Empirical applications and scenarios
Environmental issues have received a prominent place in transport policies of most European countries. The coordination of such policies however, is fraught with many difficulties. The increasing freight flows after the European integration are a source of concern, but have not yet led to straightforward and effective environmental strategies. The paper focuses on the Trans Alpine freight transport systems in the light of the future integration of single national transport systems into the European transport network. The environmental, social and institutional peculiarities of this ‘region’ have favoured—in the past—the development of strong nationally-oriented policies, partly in contrast with the goals promoted by the European Union. The present analysis aims to highlight opportunities, and limits inherent, in the implementation of various infrastructure projects oriented towards a drastic change of the Alpine transport systems structure. The Alpine countries, viz. Austria and Switzerland, play a central role in the promotion of environmental benign modes of transport of goods, with a clear focus on rail. The route choice and modal split of freight flows in Europe are taking place simultaneously. In this paper the results of European freight flow models (based on logit analysis and neural networks) will be presented. An important exercise is then to assess the consequences of various types of eco-taxes on road transport in Europe. In this context, several policy scenarios will be dealt with
Tuning the Correlation Decay in the Resistance Fluctuations of Multi-Species Networks
A new network model is proposed to describe the resistance noise
in disordered materials for a wide range of values ().
More precisely, we have considered the resistance fluctuations of a thin
resistor with granular structure in different stationary states: from nearly
equilibrium up to far from equilibrium conditions. This system has been
modelled as a network made by different species of resistors, distinguished by
their resistances, temperature coefficients and by the energies associated with
thermally activated processes of breaking and recovery. The correlation
behavior of the resistance fluctuations is analyzed as a function of the
temperature and applied current, in both the frequency and time domains. For
the noise frequency exponent, the model provides at low
currents, in the Ohmic regime, with decreasing inversely with the
temperature, and at high currents, in the non-Ohmic regime.
Since the threshold current associated with the onset of nonlinearity also
depends on the temperature, the proposed model qualitatively accounts for the
complicate behavior of versus temperature and current observed in many
experiments. Correspondingly, in the time domain, the auto-correlation function
of the resistance fluctuations displays a variety of behaviors which are tuned
by the external conditions.Comment: 26 pages, 16 figures, Submitted to JSTAT - Special issue SigmaPhi200
Electron effective masses of Scx Al1- x N and Alx Ga1- x N from first-principles calculations of unfolded band structure
The electron effective masses of Sc x Al 1 - xN and Al x Ga 1 - xN, two of the most promising wide bandgap materials for power and RF electronic applications, have been calculated using the predictions of the density functional theory (DFT). More specifically, the unfolding technique has been adopted to extract the effective band structure of the two alloys under investigation. It has been found that the AlGaN effective masses m - approximately follow the Vegard law. On the contrary, due to the larger amount of disorder inside the crystal, the ScAlN shows a non-monotonic change of m - as a function of the Sc concentration, which requires the DFT calculations to be consistently performed for an accurate prediction. The ScAlN effective masses as a function of Sc content have been reported in the range 0 ≤ x ≤ 0.25 for the first time
Searching for gas giant planets on Solar System scales - A NACO/APP L'-band survey of A- and F-type Main Sequence stars
We report the results of a direct imaging survey of A- and F-type main
sequence stars searching for giant planets. A/F stars are often the targets of
surveys, as they are thought to have more massive giant planets relative to
solar-type stars. However, most imaging is only sensitive to orbital
separations 30 AU, where it has been demonstrated that giant planets are
rare. In this survey, we take advantage of the high-contrast capabilities of
the Apodizing Phase Plate coronagraph on NACO at the Very Large Telescope.
Combined with optimized principal component analysis post-processing, we are
sensitive to planetary-mass companions (2 to 12 ) at Solar System
scales (30 AU). We obtained data on 13 stars in L'-band and detected one
new companion as part of this survey: an M dwarf companion around HD
984. We re-detect low-mass companions around HD 12894 and HD 20385, both
reported shortly after the completion of this survey. We use Monte Carlo
simulations to determine new constraints on the low-mass (80 )
companion frequency, as a function of mass and separation. Assuming solar-type
planet mass and separation distributions, normalized to the planet frequency
appropriate for A-stars, and the observed companion mass-ratio distribution for
stellar companions extrapolated to planetary masses, we derive a truncation
radius for the planetary mass companion surface density of 135 AU at 95%
confidence.Comment: Accepted for publication in MNRAS, 8 pages, 4 figure
Model for heterogeneous catalysis on metal surfaces with application to hypersonic flows
A model for heterogeneous catalysis for Copper, Nickel, and Platinum has been devised. The model simulates the heterogeneous chemical kinetics of dissociated air flow impinging metal surfaces. Elementary phenomena such as, atomic and molecular adsorption, Eley-Rideal and Langmuir-Hinshelwood recombinations, and thermal desorptions have been accounted for. Comparisons with experimental results for Nitrogen and Oxygen recombination show good agreement. In the second part of this work, the finite rate catalysis model has been used to analyzed numerically the problems of the heterogeneous catalysis similarity between hypersonic ground testing and reentry flight. Therefore the flow around a blunt cone under these conditions has been calculated and results for heat fluxes and for a suggested similarity parameter have been compared and discussed
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