arXiv subjects

Z. Neda

Publications and source records attributed to Z. Neda.

30 records · Page 2Linked to original sources

arXiv2001

Collective dynamics of two-mode stochastic oscillators

We study a system of two-mode stochastic oscillators coupled through their collective output. As a function of a relevant parameter four qualitatively distinct regimes of collective behavior are observed. In an extended region of the parameter space the periodicity of the collective output is enhanced by the considered coupling. This system can be used as a new model to describe synchronization-like phenomena in systems of units with two or more oscillation modes. The model can also explain how periodic dynamics can be generated by coupling largely stochastic units. Similar systems could be responsible for the emergence of rhythmic behavior in complex biological or sociological systems.

cond-mat.stat-mech↗

arXiv2001

Evolution of the social network of scientific collaborations

The co-authorship network of scientists represents a prototype of complex evolving networks. By mapping the electronic database containing all relevant journals in mathematics and neuro-science for an eight-year period (1991-98), we infer the dynamic and the structural mechanisms that govern the evolution and topology of this complex system. First, empirical measurements allow us to uncover the topological measures that characterize the network at a given moment, as well as the time evolution of these quantities. The results indicate that the network is scale-free, and that the network evolution is governed by preferential attachment, affecting both internal and external links. However, in contrast with most model predictions the average degree increases in time, and the node separation decreases. Second, we propose a simple model that captures the network's time evolution. Third, numerical simulations are used to uncover the behavior of quantities that could not be predicted analytically.

cond-mat.soft↗

arXiv2001

Measuring preferential attachment for evolving networks

A key ingredient of current models proposed to capture the topological evolution of complex networks is the hypothesis that highly connected nodes increase their connectivity faster than their less connected peers, a phenomenon called preferential attachment. Measurements on four networks, namely the science citation network, Internet, actor collaboration and science coauthorship network indicate that the rate at which nodes acquire links depends on the node's degree, offering direct quantitative support for the presence of preferential attachment. We find that for the first two systems the attachment rate depends linearly on the node degree, while for the latter two the dependence follows a sublinear power law.

cond-mat.stat-mech↗

arXiv2001

Spiral cracks without twisting

A fascinating class of patterns, often encountered in nature as meandering cracks on rocks, dried-out fields and tectonic plates is produced by the fracture of solids. Here we report the observation and modeling of an unusual type of patterns consisting of spiral cracks within fragments of a thin layer of drying precipitate. We find that this symmetry-breaking cracking mode arises naturally not from twisting forces, but from a propagating stress front induced by the fold-up of the fragments.

cond-mat.mtrl-sci↗

arXiv2000

Physics of the rhythmic applause

We discuss in detail a human scale example of the synchronization phenomenon, namely the dynamics of the rhythmic applause. After a detailed experimental investigation, we describe the phenomenon with an approach based on the classical Kuramoto model. Computer simulations based on the theoretical assumptions, reproduce perfectly the observed dynamics. We argue that a frustration present in the system is responsible for the interesting interplay between synchronized and unsynchronized regimes

cond-mat.stat-mech↗

arXiv2000

Computer simulation of the microstructure and rheology of semi-solid alloys under shear

The rheological behavior of metallic alloys containing both solid and liquid phases is investigated in the low solid fraction range (<50%). This behavior depends on both the solid fraction and the shear rate. The concept of Effective Volume Fraction (EVF) is used to decorrelate the influence of these two parameters. At high shear rate the slurry behaves like a suspension of hard spheres, whereas at lower shear rate, particles tend to aggregate in clusters, entrapping liquid and thus, increasing the EVF and the viscosity. A lattice model is introduced to simulate the aggregation / break-up processes within a slurry under shear. When the steady state is reached, the entrapped liquid fraction is calculated, leading to a viscosity estimation. Simulation results for the viscosity and 3D cluster structure are in good agreement with experimental results.

cond-mat.mtrl-sci↗

arXiv2000

The sound of many hands clapping

We report on a series of measurements aimed to characterize the development and the dynamics of the rhythmic applause in concert halls. Our results demonstrate that while this process shares many characteristics of other systems that are known to synchronize, it also has features that are unexpected and unaccounted for in many other systems. In particular, we find that the mechanism lying at the heart of the synchronization process is the period doubling of the clapping rhythm. The characteristic interplay between synchronized and unsynchronized regimes during the applause is the result of a frustration in the systems. All results are understandable in the framework of the Kuramoto model.

cond-mat.stat-mech↗

arXiv1999

Limits of the Quantum Monte Carlo method

We consider the one-dimensional quantum-statistical problem of interacting spin-less particles in an infinite deep potential valley and on a ring. Several limits for the applicability of the Quantum Monte Carlo (QMC) methods were revealed and discussed. We show the inapplicability of the QMC method for ring-like geometries, realize an unphysical frustration for interacting fermions and a minus-sign problem for interacting bosons.

cond-mat.stat-mech↗

arXiv1999

Spatial stochastic resonance in 1D Ising systems

The 1D Ising model is analytically studied in a spatially periodic and oscillatory external magnetic field using the transfer-matrix method. For low enough magnetic field intensities the correlation between the external magnetic field and the response in magnetization presents a maximum for a given temperature. The phenomenon can be interpreted as a resonance phenomenon induced by the stochastic heatbath. This novel "spatial stochastic resonance" has a different origin from the classical stochastic resonance phenomenon.

cond-mat.stat-mech↗

arXiv1998

Comment on "Percolation Thresholds in the Three-Dimensional Stick System"

We discuss a classical mistake, made in earlier publications on the stick percolation problem in 3D, for generating the right isotropic configuration of sticks. We explain the observed systematic deviations from the excluded volume rule. New MC simulations considered by us confirm nicely the applicability of the excluded volume theory.

cond-mat.stat-mech↗

arXiv1998

Response in kinetic Ising model to oscillating magnetic fields

Ising models obeying Glauber dynamics in a temporally oscillating magnetic field are analyzed. In the context of stochastic resonance, the response in the magnetization is calculated by means of both a mean-field theory with linear-response approximation, and the time-dependent Ginzburg-Landau equation. Analytic results for the temperature and frequency dependent response, including the resonance temperature, compare favorably with simulation data.

cond-mat.stat-mech↗

arXiv1993

Instantaneous Configurations of the Bloch Walls in a Two-Dimensional and s=1/2 Model

We show that instantaneous configurations of 180 degree domain walls constructed on a square lattice in a two-dimensional and S=1/2 Ising-type model exhibit fractal structure. The fractal dimension depends on the coupling parameters and it is a continious function of the temperature. The wall thickness in the neighbourhood of Tc presents scaling properties in good agreement with the classical theory by Landau.

cond-mat↗