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Labini F.S.,Enrico Fermi Center | Labini F.S.,CNR Institute for Complex Systems | Baryshev Y.V.,Saint Petersburg State University
Journal of Cosmology and Astroparticle Physics | Year: 2010

Cosmological density fields are assumed to be translational and rotational invariant, avoiding any special point or direction, thus satisfying the Copernican Principle. A spatially inhomogeneous matter distribution can be compatible with the Copernican Principle but not with the stronger version of it, the Cosmological Principle which requires the additional hypothesis of spatial homogeneity. We establish criteria for testing that a given density field, in a finite sample at low redshifts, is statistically and/or spatially homogeneous. The basic question to be considered is whether a distribution is, at different spatial scales, self-averaging. This can be achieved by studying the probability density function of conditional fluctuations. We find that galaxy structures in the SDSS samples, the largest currently available, are spatially inhomogeneous but statistically homogeneous and isotropic up to ∼ 100 Mpc/h. Evidences for the breaking of self-averaging are found up to the largest scales probed by the SDSS data. The comparison between the results obtained in volumes of different size allows us to unambiguously conclude that the lack of self-averaging is induced by finite-size effects due to long-range correlated fluctuations. We finally discuss the relevance of these results from the point of view of cosmological modeling. © 2010 IOP Publishing Ltd and SISSA.

Ristic D.,Ruder Boskovic Institute | Mazzola M.,CNR Institute for Photonics and Nanotechnologies | Chiappini A.,CNR Institute for Photonics and Nanotechnologies | Rasoloniaina A.,CNRS Foton Laboratory | And 7 more authors.
Optics Letters | Year: 2014

The modal dispersion of a whispering gallery mode (WGM) resonator is a very important parameter for use in all nonlinear optics applications. In order to tailor the WGM modal dispersion of a microsphere, we have coated a silica microsphere with a high-refractive-index coating in order to study its effect on the WGM modal dispersion. We used Er3+ ions as a probe for a modal dispersion assessment. We found that, by varying the coating thickness, the geometrical cavity dispersion can be used to shift overall modal dispersion in a very wide range in both the normal and anomalous dispersion regime. © 2014 Optical Society of America.

Every A.G.,University of Witwatersrand | Warmbier R.,University of Witwatersrand | Quandt A.,University of Witwatersrand | Quandt A.,Enrico Fermi Center
Optical and Quantum Electronics | Year: 2016

We derive and analyze a 2D model for plasmons, in order to understand the general preconditions for the appearance of THz plasmons in low-dimensional nanosystems like graphene. Using experimental data and back of the envelope type calculations, we discuss the typical frequency ranges of plasmon resonances in such systems. Next we compare our results to recent ab initio calculations for ideal graphene, and show that these are consistent with the predictions of a 3D plasmon model, rather than a 2D model. The validity of the ab initio calculation does not extend to long-wavelength regime where our 2D model holds. © 2016, Springer Science+Business Media New York.

Joyce M.,University Pierre and Marie Curie | Labini F.S.,Enrico Fermi Center | Labini F.S.,National Research Council Italy
Monthly Notices of the Royal Astronomical Society | Year: 2013

Beyond convergence studies and comparison of different codes, there are essentially no controls on the accuracy in the non-linear regime of cosmological N-body simulations, even in the dissipationless limit. We propose and explore here a simple test which has not been previously employed: when cosmological codes are used to simulate an isolated overdensity, they should reproduce, in physical coordinates, those obtained in open boundary conditions without expansion. In particular, the desired collisionless nature of the simulations can be probed by testing for stability in physical coordinates of virialized equilibria. We investigate and illustrate the test using a suite of simulations in an Einstein-de Sitter cosmology from initial conditions which rapidly settle to virial equilibrium. We find that the criterion of stable clustering allows one to determine, for given particle number N in the 'halo' and force smoothing ε, a maximum redshift range over which the collisionless limit may be represented with desired accuracy. We also compare our results to the so-called layzer-Irvine test, showing that it provides a weaker, but very useful, tool to constrain the choice of numerical parameters. Finally, we outline in some detail how these methods could be employed to test the choice of the numerical parameters used in a cosmological simulation. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Sylos Labini F.,Enrico Fermi Center | Sylos Labini F.,CNR Institute for Complex Systems
Astronomy and Astrophysics | Year: 2010

Aims: We study the statistical properties of the gravitational field generated by galaxy distribution observed by the Sloan Digital Sky Survey (DR7). We characterize the probability density function (PDF) of gravitational force fluctuations and relate its limiting behaviors to the correlation properties of the underlying density field. In addition, we study whether the PDF converges to an asymptotic shape within sample volumes. Methods: We consider several volume-limited samples of the Sloan Digital Sky Survey and we compute the gravitational force probability density function (PDF). The gravitational force is computed in spheres of varying radius as is its PDF. Results: We find that (i) the PDF of the force displays features that can be understood in terms of galaxy two-point correlations and (ii) density fluctuations on the largest scales probed, i.e. r ≈ 100 Mpch-1, still contribute significantly to the amplitude of the gravitational force. Conclusions: Our main conclusion is that fluctuations in the gravitational force field generated by galaxy structures are also relevant on scales ∼100 Mpch-1. By assuming that the gravitational fluctuations in the galaxy distribution reflect those in the whole matter distribution, and that peculiar velocities and accelerations are simply correlated, we may conclude that large-scale fluctuations in the galaxy density field may be the source of the large-scale flows recently observed. © ESO 2010.

De Santis S.,University of Rome La Sapienza | De Santis S.,Santa Lucia Foundation | Gabrielli A.,University of Rome La Sapienza | Gabrielli A.,CNR Institute for Complex Systems | And 5 more authors.
Magnetic Resonance in Medicine | Year: 2011

A new method to investigate anomalous diffusion in human brain, inspired by the stretched-exponential model proposed by Hall and Barrick, is proposed here, together with a discussion about its potential application to cerebral white matter characterization. Aim of the work was to show the ability of anomalous diffusion indices to characterize white matter structures, whose complexity is only partially accounted by diffusion tensor imaging indices. MR signal was expressed as a stretched-exponential only along the principal axes of diffusion; whereas, in a generic direction, it was modeled as a combination of three stretched-exponentials. Indices to quantify the tissue anomalous diffusion and its anisotropy, independently of the experiment reference frame, were derived. Experimental results, obtained on 10 healthy subjects at 3T, show that the new parameters are highly correlated to intrinsic local geometry when compared with Hall and Barrick indices. Moreover, they offer a different contrast in white matter regions when compared with diffusion tensor imaging. Specifically, the new indices show a higher capability to discriminate among areas of the corpus callosum associated to different distribution in axonal densities, thus offering a new potential tool to detect more specific patterns of brain abnormalities than diffusion tensor imaging in the presence of neurological and psychiatric disorders. Copyright © 2010 Wiley-Liss, Inc.

Di Pietro G.,Italian Institute of Technology | Di Pietro G.,University of Rome La Sapienza | Palombo M.,University of Rome La Sapienza | Capuani S.,University of Rome La Sapienza | Capuani S.,Enrico Fermi Center
Applied Magnetic Resonance | Year: 2014

Two techniques used for evaluating internal magnetic field gradient (G i), spin-echo (SE) and diffusion decay internal field (DDIF), were investigated at 9.4 T and compared in porous systems characterized by different pores size ranging from 4 to 96 μm with magnetic susceptibility difference between solid and liquid phase, Δχ ≈ 1.6 ppm. Since diffusion of a fluid in a solid porous matrix plays a role in both SE and DDIF methods, we investigated these two different methods by highlighting their dependence on characteristic parameters and length scales used to describe diffusion behavior of fluids in porous systems. Therefore, G i behavior as a function of the dephasing length (l g), diffusion length (l d) and pores size (l s) was obtained. Moreover G i was evaluated by using both free diffusion and measured apparent diffusion coefficient of water, to quantify diffusion effect in different porous samples. This study gives more insight into the physical dynamics process to explain contrast mechanisms recently exploited by DDIF and SE applications for cancellous bone quality measurements. © 2014 Springer-Verlag Wien.

Gili T.,Enrico Fermi Center | Gili T.,Santa Lucia Foundation | Cercignani M.,Santa Lucia Foundation | Serra L.,Santa Lucia Foundation | And 6 more authors.
Journal of Neurology, Neurosurgery and Psychiatry | Year: 2011

Objective: To assess the contribution of regional grey matter (GM) atrophy and functional disconnection in determining the level of cognitive decline in patients with Alzheimer's disease (AD) at different clinical stages. Methods: Ten patients with amnesic mild cognitive impairment (a-MCI), 11 patients with probable AD and 10 healthy controls were recruited. T1 volumes were obtained from each subject and postprocessed according to an optimised voxel based morphometry protocol. Resting state functional MRI data were also collected from the same individuals and analysed to produce connectivity maps after identification of the default mode network (DMN) by independent component analysis. Results: Compared with healthy controls, both AD and a- MCI patients showed a similar regional pattern of brain disconnection between the posterior cingulate cortex (PCC) and the medial prefrontal cortex and the rest of the brain. Conversely, the distribution of GM atrophy was significantly more restricted in a-MCI than in AD patients. Interestingly, the PCC showed reduced connectivity in a- MCI patients in the absence of GM atrophy, which was, in contrast, detectable at the stage of fully developed AD. Conclusions: This study indicates that disconnection precedes GM atrophy in the PCC, which is a critical area of the DMN, and supports the hypothesis that GM atrophy in specific regions of AD brains likely reflects a long term effect of brain disconnection. In this context, our study indicates that GM atrophy in PCC accompanies the conversion from MCI to AD.

Righini G.C.,Enrico Fermi Center | Righini G.C.,CNR Institute of Neuroscience | Chiappini A.,CNR Institute of Neuroscience
Optical Engineering | Year: 2014

Glasses, either pure or suitably doped, constitute an excellent material for the development of integrated optical circuits. A brief review is presented of the most widely used processes for the fabrication of passive and active glass waveguides. Brilliant prospects of glass-based platforms for the development of photonic integrated circuits are outlined. © The Authors.

Parisi G.,University of Rome La Sapienza | Rizzo T.,Enrico Fermi Center
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We compute the probability of positive large deviations of the free energy per spin in mean-field spin-glass models. The probability vanishes in the thermodynamic limit as P (Δf) ∞exp [- N2 L2 (Δf)]. For the Sherrington-Kirkpatrick model we find L2 (Δf) =O ( Δf ) 12/5 in good agreement with numerical data and with the assumption that typical small deviations of the free energy scale as N1 /6. For the spherical model we find L2 (Δf) =O ( Δf ) 3 in agreement with recent findings on the fluctuations of the largest eigenvalue of random Gaussian matrices. The computation is based on a loop expansion in replica space and the non-Gaussian behavior follows in both cases from the fact that the expansion is divergent at all orders. The factors of the leading order terms are obtained resumming appropriately the loop expansion and display universality, pointing to the existence of a single universal distribution describing the small deviations of any model in the full-replica-symmetry-breaking class. © 2010 The American Physical Society.

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