Home page Journal Club ISC

• Giovedi' 31 Maggio 2007 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Massimo Cencini

Titolo: Inertial Particles in Turbulent Flows

Abstract: Small impurities like dust or droplets suspended in turbulent flows are finite-size particles whose density may differ from that of the fluid and thus cannot be modeled as fluid tracers. The description of their motion, which is much more complex than that of fluid tracers, must account for their finite response time, i.e. their inertia. Remarkably such particles, which are relevant to both natural phenomena and industrial applications, display strong spatial inhomogeneities even if the carrier flow is incompressible, whose statistical description is an open issue with many industrial and environmental applications. Results of high resolution direct numerical simulations and simple random flows are here reviewed. The statistics of their distribution is investigated at very small scales where they form (multi)fractal objects; such clustering is very efficient for particles lighter than the fluid which transport them. At larger scales, particles do not cluster into fractal objects and different approaches are needed for understanding their statistics.

• Mercoledi' 4 Maggio 2007 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Andrea Perali

Titolo: BCS-BEC crossover e fenomeni di pseudogap in atomi fermionici intrappolati

Abstract: Nella prima parte del seminario, dopo un'introduzione generale ai gas di fermioni in trappole magneto-ottiche e alle risonanze di Fano-Feshbach, verrà presentato un approccio teorico microscopico al crossover tra una condensazione di gas di fermioni di tipo BCS e una di tipo Bose-Einstein elaborato dal gruppo di Camerino e recentemente esteso ai gas di fermioni intrappolati.

Le previsioni di questa teoria, quali diagramma di fase, profili di densità, densità di condensato, potenziale chimicoverranno confrontat e con i più recenti esperimenti.Inoltre, nel caso di lunghezza di scattering "infinita" (limite unitario), i nostri risultati sono in accordo con le simulazioni Quantum Monte Carlo. Tali confronti mostrano i punti di forza della nostra teoria e possibili direzioni per miglioramenti futuri.

Nella seconda parte del seminario verranno analizzati gli aspetti teorici dei fenomeni di pseudogap nei gas di atomi fermionici intrappolati. La grandezza fisica che verrà studiata è lo spettro di assorbimento nella regione delle radio frequenze (RF) e i recenti esperimenti su gas di atomi fermionici di Litio verranno confrontati con le previsioni ottenute numericamente dalla nostra teoria. Verrà proposto un metodo per estrarre dagli spettri RF la gap o la pseudogap di eccitazione, confrontando i risultati ottenuti con le informazioni fornite in modo diretto dalla funzione spettrale per le eccitazioni di singola particella.

• Giovedi' 29 Marzo 2007 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Andrea Gabrielli

Titolo: Diffusion, super-diffusion, clusterization and cohalescence from single steps

Abstract: From the exact single step equation of the two-point correlation function of a particle distribution subjected to a stochastic displacement field, we derive different dynamical regimes when the displacement field is iterated to build a stochastic velocity field. These regimes range from simple diffusion to super-diffusion and from particle clustering to coalescence. The first two regimes are found in absence of displacement-displacement correlations, while clusterization and coalescence are observed in presence of such correlations. A transition between simple clusterization and coalescence is found depending on the nature of the small scale correlations in the velocity field. This corresponds exactly to the behavior of a well known family of models, called the Kraichnan ensemble, introduced to describe the advection of passive scalars from fully turbulent flows. For this ensemble we give also a new method of solution.

• Giovedi' 15 Marzo 2007 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Claudio Castellano

Titolo: Scaling and universality in proportional elections

Abstract: A most debated topic of the last years is whether simple statistical physics models can explain collective features of social dynamics. A necessary step in this line of endeavour is to find regularities in data referring to large scale social phenomena, such as scaling and universality. We show that, in proportional elections, the distribution of the number of votes received by candidates is a universal scaling function, identical in different countries and years. This finding reveals the existence in the voting process of a general microscopic dynamics that does not depend on the historical, political and/or economical context where voters operate. A simple dynamical model for the behaviour of voters, based on a branching process, reproduces the universal distribution.

• Giovedi' 8 Marzo 2007 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Keith S Cover (VU University Medical Center)

Titolo: Is there a chance that the anisotropies in the cosmic microwave background (CMB) detected by COBE are an artefact of COBE's image reconstruction?

Abstract: In the early 90's its was reported that the COBE satellite had detected anisotropies in the cosmic microwave background at 10ppm. The WMAP satellite launched early in this decade reported similar results. Rather than measuring the CMB directly, both satellites measured the difference between points in the sky and reconstructed images from the differential measurements. The Planck satellite, to be launched in 2008, will be able to detect the CMB directly, without the use of image reconstruction. Thus, it will provide a check on the image reconstruction used in both the COBE and WMAP analysis. Will Planck confirm the detection of the anisotropies reported by COBE and WMAP?

A key assumption that appears to have been made in the official COBE analysis is that all sources of noise are ideal and thus average out as well as possible. The original COBE time ordered data (TOD) was reanalysed using a novel statistical technique that extends the concept of rejecting the null hypothesis to image reconstruction. When ideal noise was assumed, the detection of the anisotropies was confirmed to 28 sigma. However, when COBE's calibration error of 0.7% was taken into account, the COBE TOD did not reject the null hypothesis and thus there is no statistically significant detection. With a modified scanning strategy, the WMAP satellite may also be able to detect the reported anisotropies without image reconstruction.

The author's reanalysis of the COBE image reconstruction is presented in http://arxiv.org/abs/astro-ph/0607484.

• Giovedi' 1 Marzo 2007 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Federico Becca

Titolo: Variational description of Mott insulators

Abstract: We give a review of recent developpements on the possibility to describe strongly correlated systems by improved variational wave functions. We describe how it is possible to describe the metal-insulator transition by using the Gutzwiller wave function supplemented with a long-range Jastrow factor. We give examples for the fermionic Hubbard model in one and two dimensions. An appealing interpretation in terms of the binding-unbinding Kosterlitz-Thouless transition is obtained through the mapping onto a classical model. The role of dimensionality is also discussed on the bosonic Hubbard model, where the numerically exact solution is knwon by Monte Carlo simulations.

• Giovedi' 15 Febbraio 2007 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Ciro Cattuto

Titolo: Stochastic models for collaborative tagging

Abstract: The collaborative character underlying many "Web 2.0" applications puts them in the spotlight of complex systems science. Modern web applications for organizing and sharing data are open-ended, dynamically evolving, collaborative and therefore intrinsically social.

The new paradigm of "collaborative tagging", in particular, has been massively adopted because of its ability to recruit the activity of web users into effectively organizing and sharing vast amounts of information. In web-based applications like del.icio.us (http://del.icio.us/), Flickr (http://flickr.com/), BibSonomy (http://www.bibsonomy.org) users enrich resources - ranging from photographs to scientific references and web pages - with semantically meaningful metadata in the form of text labels, or "tags".

We collected data from a popular system and investigated the statistical properties of tag co-occurrence. We introduced a stochastic model of user behavior embodying two main aspects of collaborative tagging: (i) a frequency-bias mechanism related to the idea that users are exposed to each other's tagging activity; (ii) a notion of memory - or aging of resources - in the form of a heavy-tailed access to the past state of the system. Remarkably, this simple modeling is able to account quantitatively for the observed experimental features. This points in the direction of a universal behavior of users, who - despite the complexity of their own cognitive processes and the uncoordinated and selfish nature of their tagging activity - appear to follow simple activity patterns.

• Giovedi' 1 Febbraio 2007 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Carmine Ortix

Titolo: Mesoscopic phase separation

Abstract: Phase separation is generally expected in systems with a density-driven first order phase transition. In electronic systems the ordinary macroscopic phase coexistence phenomenon is hampered by the long-range Coulomb interaction. Thus the system breaks in mesoscopic domains to guarantee large-scale neutrality. Recently, mesoscopic inhomogeneities have been observed in a variety of strongly correlated electronic systems (cuprates-manganites-ruthanates-2d electron gas) where phases of different nature compete with each other. In this talk I will review the relevant experiments and show how one can perform a general analysis of the inhomogeneous states independently of the specific competing phases. I will discuss the main properties of the phase separated states and the different role of the long-range Coulomb interaction and screening in two and three dimensional systems.

Slides (1.1 MB)

• Giovedi' 18 Gennaio 2007 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Francesco Sylos Labini

Titolo: Gravitational dynamics of an infinite self-gravitating particle distribution

Abstract: I present the main elements of the dynamics of an infinite self-gravitating particle distribution which starts from a uniform initial condition represented by a slightly perturbed perfect cubic lattice, by considering several N-body simulations in a periodic box. I discuss the various phases of the evolution: the formation of non-linear correlations at small scales and the amplification of small fluctuations at large scales which proceed in agreement with the prediction of the linearized fluid theory. Non-linear two-point correlations are characterized by a simple spatio-temporal scaling whose time dependence is in agreement with the prediction of fluid linear theory; however in a first transient phase they are well-accounted solely by two-body correlations, i.e. by a particle-like dynamics. To study the role of fluctuations at different scales in the system, I then focus on the comparison between the evolution of such a system and that of ``daughter'' coarse-grained particle distributions. This study allows to understand the origin of the spatio-temporal scaling, the role of discreteness in the formation of non linear structures and the continuum limit of these simulations.

Slides (8.9 MB)

• Giovedi' 14 Dicembre 2006 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Sergio Gaudio

Titolo: Fermi liquid behavior close to a Feshbach resonance

Abstract: I will show that the exchange fluctuations close to a Feshbach resonance in a two component Fermi gas can result in an effective p-wave attractive interaction. On the BCS side of the resonance, the magnitude of this effective interaction is comparable to the s-wave interaction, therefore leading to a spin-triplet superfluid in the range of temperatures of actual experiments. I will compare the results for our effective scattering length to the mean field result and to the observed binding energy. I will show that by including particle-hole exchange in the fluctuations, the divergence of the effective scattering length disappearswe derive the thermodynamic properties of a Fermi gas, deep into the quantum degenerate regime. Then I will prove that a first order phase transition occurs in the normal phase as a function of the interaction strength, U, as a consequence of a jump occurring in the compressibility, spin susceptibility and specific heat. The signature of the transition is given by the presence of a non-zero latent heat. Furthemore, A volume change occurs at finite temperatures from the BEC to the BCS side of the Feshbach resonance, in the normal phase. The transition has an end point close to the BCS critical temperature. Thus, observation of these properties will require suppression of the superfluid phase. Also we show that a paramagnetic system in equilibrium, close to a diverging scattering length, expels any applied magnetic field and as a consequence, there is no Clogston limit in the superfluid phase.

Slides (1.3 MB)

Speaker: Francesco Rao

Titolo: Complex network analysis of free-energy landscapes

Abstract: The kinetics of biomolecular isomerization processes, such as protein folding, is governed by a free-energy surface of high dimensionality and complexity. As an alternative to projections into one or two dimensions, the free-energy surface can be mapped into a weighted network where nodes and links are configurations and direct transitions among them, respectively. In this work, the free energy basins and barriers of the alanine dipeptide are determined quantitatively using an algorithm to partition the network into clusters (i.e. states) according to the equilibrium transitions sampled by molecular dynamics. The network-based approach allows for the analysis of the thermodynamics and kinetics of biomolecule isomerization without reliance on arbitrarily chosen order parameters.

Slides (6.9 MB)

• Mercoledi' 15 Novembre 2006 - ore 11.30, ISC-CNR, via dei Taurini 19 (IV piano)

Speaker: Benedetta Cerruti

Titolo: Dipolar interactions in ferromagnetic thin films

Abstract: One of the still open questions in the physics of magnetic systems is the comprehension of the hysteretic behavior of ferromagnetic thin films. Experiments on two dimensional specimens highlight different features from bulk systems, due to the greater complexity of domain structure in two dimensions. In this contribution we analyse the effects of dipolar interactions on the domain walls dynamics. We present two models for the study of zigzag and parallel domain walls motion, with particular interest to hysteretic properties and Barkhausen noise.

Sildes (pdf): parte I (728 KB), parte II (478 KB), parte III (773 KB)