ISI Foundation

Sant'Ambrogio di Torino, Italy

ISI Foundation

Sant'Ambrogio di Torino, Italy
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Petri G.,ISI Foundation | Expert P.,King's College London
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2014

We present a method to find the best temporal partition at any time scale and rank the relevance of partitions found at different time scales. This method is based on random walkers coevolving with the network and as such constitutes a generalization of partition stability to the case of temporal networks. We show that, when applied to a toy model and real data sets, temporal stability uncovers structures that are persistent over meaningful time scales as well as important isolated events, making it an effective tool to study both abrupt changes and gradual evolution of a network mesoscopic structures. © 2014 American Physical Society.

Vanossi A.,CNR Institute of Materials | Vanossi A.,International School for Advanced Studies | Manini N.,International School for Advanced Studies | Manini N.,University of Milan | And 6 more authors.
Reviews of Modern Physics | Year: 2013

The physics of sliding friction is gaining impulse from nanoscale and mesoscale experiments, simulations, and theoretical modeling. This Colloquium reviews some recent developments in modeling and in atomistic simulation of friction, covering open-ended directions, unconventional nanofrictional systems, and unsolved problems. © 2013 American Physical Society.

Allegra M.,University of Turin | Giorda P.,ISI Foundation | Paris M.G.A.,University of Milan
Physical Review Letters | Year: 2010

We address the degradation of continuous variable (CV) entanglement in a noisy channel focusing on the set of photon-number entangled states. We exploit several separability criteria and compare the resulting separation times with the value of non-Gaussianity at any time, thus showing that in the low-temperature regime: (i) non-Gaussianity is a bound for the relative entropy of entanglement and (ii) Simon's criterion provides a reliable estimate of the separation time also for non-Gaussian states. We provide several evidences supporting the conjecture that Gaussian entanglement is the most robust against noise, i.e., it survives longer than a non-Gaussian one, and that this may be a general feature for CV systems in Markovian channels. © 2010 The American Physical Society.

Shekhawat A.,Cornell University | Zapperi S.,National Research Council Italy | Zapperi S.,ISI Foundation | Sethna J.P.,Cornell University
Physical Review Letters | Year: 2013

We present a unified theory of fracture in disordered brittle media that reconciles apparently conflicting results reported in the literature. Our renormalization group based approach yields a phase diagram in which the percolation fixed point, expected for infinite disorder, is unstable for finite disorder and flows to a zero-disorder nucleation-type fixed point, thus showing that fracture has a mixed first order and continuous character. In a region of intermediate disorder and finite system sizes, we predict a crossover with mean-field avalanche scaling. We discuss intriguing connections to other phenomena where critical scaling is only observed in finite size systems and disappears in the thermodynamic limit. © 2013 American Physical Society.

Giorda P.,ISI Foundation | Paris M.G.A.,University of Milan
Physical Review Letters | Year: 2010

We extend the quantum discord to continuous variable systems and evaluate Gaussian quantum discord C(at) for bipartite Gaussian states. In particular, for squeezed-thermal states, we explicitly maximize the extractable information over Gaussian measurements: C(at) is minimized by a generalized measurement rather than a projective one. Almost all squeezed-thermal states have nonzero Gaussian discord: They may be either separable or entangled if the discord is below the threshold C(at)=1, whereas they are all entangled above the threshold. We elucidate the general role of state parameters in determining the discord and discuss its evolution in noisy channels. © 2010 The American Physical Society.

Laurson L.,ISI Foundation | Miguel M.-C.,University of Barcelona | Alava M.J.,Aalto University
Physical Review Letters | Year: 2010

Dislocation assemblies exhibit a jamming or yielding transition at a critical external shear stress value σ=σc. Here we study the heterogeneous and collective nature of dislocation dynamics within a crystal plasticity model close to σc, by considering the first-passage properties of the dislocation dynamics. As the transition is approached in the moving phase, the first-passage time distribution exhibits scaling, and a related peak dynamical susceptibility χ4* diverges as χ4*∼(σ- σc)-α, with α ≈ 1.1. We relate this scaling to an avalanche description of the dynamics. While the static structural correlations are found to be independent of the external stress, we identify a diverging dynamical correlation length ξy in the direction perpendicular to the dislocation glide motion. © 2010 The American Physical Society.

Rosti J.,Aalto University | Koivisto J.,Aalto University | Laurson L.,ISI Foundation | Alava M.J.,Aalto University
Physical Review Letters | Year: 2010

The spatial fluctuations of deformation are studied in the creep in Andrade's power law and the logarithmic phases, using paper samples. Measurements by the digital image correlation technique show that the relative strength of the strain rate fluctuations increases with time, in both creep regimes. In the Andrade creep phase characterized by a power-law decay of the strain rate t∼t-θ, with θ 0.7, the fluctuations obey Δ t∼t-γ, with γ ≤ 0.5. The local deformation follows a data collapse appropriate for a phase transition. Similar behavior is found in a crystal plasticity model, with a jamming or yielding transition. © 2010 The American Physical Society.

Gauvin L.,ISI Foundation | Panisson A.,ISI Foundation | Cattuto C.,ISI Foundation
PLoS ONE | Year: 2014

The increasing availability of temporal network data is calling for more research on extracting and characterizing mesoscopic structures in temporal networks and on relating such structure to specific functions or properties of the system. An outstanding challenge is the extension of the results achieved for static networks to time-varying networks, where the topological structure of the system and the temporal activity patterns of its components are intertwined. Here we investigate the use of a latent factor decomposition technique, non-negative tensor factorization, to extract the community-ctivity structure of temporal networks. The method is intrinsically temporal and allows to simultaneously identify communities and to track their activity over time. We represent the time-varying adjacency matrix of a temporal network as a three-way tensor and approximate this tensor as a sum of terms that can be interpreted as communities of nodes with an associated activity time series. We summarize known computational techniques for tensor decomposition and discuss some quality metrics that can be used to tune the complexity of the factorized representation. We subsequently apply tensor factorization to a temporal network for which a ground truth is available for both the community structure and the temporal activity patterns. The data we use describe the social interactions of students in a school, the associations between students and school classes, and the spatio-temporal trajectories of students over time. We show that non-negative tensor factorization is capable of recovering the class structure with high accuracy. In particular, the extracted tensor components can be validated either as known school classes, or in terms of correlated activity patterns, i.e., of spatial and temporal coincidences that are determined by the known school activity schedule. © 2014 Gauvin et al.

Rasetti M.,ISI Foundation
International Journal of Quantum Information | Year: 2014

The possibility of approaching quantum simulation of "topological turbulence" is discussed. In the latter, the topological features of turbulent flow are ascribed to a purely group-theoretic and topological view through the group SDiff of volume-preserving diffeomorphisms and the Mapping Class Group for the 2D surfaces that foliate the 3D flow domain. A class of representations of the group of mapping classes for compact boundaryless surfaces is explicitly constructed in terms of the Witt-Virasoro Hopf-coalgebra and implemented in terms of su(1,1). This leads to a complex, nonlinear Hamiltonian naturally associated with the fluid diffeomorphisms that could lend itself to the quantum simulation of turbulence in Fock space. © 2014 World Scientific Publishing Company.

Budrikis Z.,ISI Foundation | Zapperi S.,ISI Foundation | Zapperi S.,CNR Institute for Energetics and Interphases
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2013

We perform large-scale simulations of a two-dimensional lattice model for amorphous plasticity with random local yield stresses and long-range quadrupolar elastic interactions. We show that as the external stress increases towards the yielding phase transition, the scaling behavior of the avalanches crosses over from mean-field theory to a different universality class. This behavior is associated with strain localization, which significantly depends on the short-range properties of the interaction kernel. © 2013 American Physical Society.

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