arXiv subjects

Normand Mousseau

Publications and source records attributed to Normand Mousseau.

50 records · Page 3Linked to original sources

arXiv2002

Stretched exponential dynamics in a chain of coupled chaotic oscillators

We measure stretched exponential behavior, exp(- (t/t_0)**beta), over many decades in a one-dimensional array of coupled chaotic electronic elements just above a crisis-induced intermittency transition. There is strong spatial heterogeneity and individual sites display a dynamics ranging from near power law ($β=0$) to near exponential ($β=1$) while the global dynamics, given by a spatial average, remains stretched exponential. These results can be reproduced quantitatively with a one-dimensional coupled-map lattice and thus appear to be system independent. In this model, local stretched exponential dynamics is achieved without frozen disorder and is a fundamental property of the coupled system. The heterogeneity of the experimental system can be reproduced by introducing quenched disorder in the model. This suggests that the stretched exponential dynamics can arise as a purely chaotic phenomenon.

cond-mat↗

arXiv2000

Efficient tight-binding Monte Carlo structural sampling of complex materials

While recent work towards the development of tight-binding and ab-initio algorithms has focused on molecular dynamics, Monte Carlo methods can often lead to better results with relatively little effort. We present here a multi-step Monte Carlo algorithm that makes use of the possibility of quickly evaluating local energies. For the thermalization of a 1000-atom configuration of {\it a}-Si, this algorithm gains about an order of magnitude in speed over standard molecular dynamics. The algorithm can easily be ported to a wide range of materials and can be dynamically optimized for a maximum efficiency.

cond-mat.stat-mech↗

arXiv2000

Cooperative motion in Lennard-Jones binary mixtures below the glass transition

Using the activation-relaxation technique (ART), we study the nature of relaxation events in a binary Lennard-Jones system above and below the glass transition temperature (T_g). ART generates trajectories with almost identical efficiency at all temperature, thus avoiding the exponential slowing down below T_g and providing extensive sampling everywhere. Comparing these runs, we find that the number of atoms involved in an event decreases strongly with temperature. In particular, while in the supercooled liquid activated events are collective, involving on average thirty atoms or more, events below T_g involve mostly single atoms and produce minimal disturbance of the local environment. These results confirm the interpretation and the generality of recent NMR results by Tang et al (Nature 402, 160 (1999)).

cond-mat.dis-nn↗

arXiv1999

Activated mechanisms in amorphous silicon: an activation-relaxation-technique study

At low temperatures, dynamics in amorphous silicon occurs through a sequence of discrete activated events that locally reorganize the topological network. Using the activation-relaxation technique, a data base containing over 8000 such events is generated, and the events are analyzed with respect to their energy barrier and asymmetry, displacement and volume expansion/contraction. Special attention is paid to those events corresponding to diffusing coordination defects. The energetics is not clearly correlated with the displacement, nor with the defect density in well relaxed configurations. We find however some correlation with the local volume expansion: it tends to increase by about 4 eV/Å$^3$. The topological properties of these events are also studied; they show an unexpectedly rich diversity.

cond-mat.dis-nn↗

arXiv1999

Elementary mechanisms governing the dynamics of silica

A full understanding of glasses requires an accurate atomistic picture of the complex activated processes that constitute the low-temperature dynamics of these materials. To this end, we generate over five thousand activated events in silica glass, using the activation-relaxation technique; these atomistic mechanisms are analysed and classified according to their activation energies, their topological properties and their spatial extend. We find that these are collective processes, involving ten to hundreds of atoms with a continuous range of activation energies; that diffusion and relaxation occurs through the creation, annihilation and motion of single dangling bonds; and that silicon and oxygen have essentially the same diffusivity.

cond-mat↗

arXiv1998

Exploring structural mechanisms in disordered materials using the activation-relaxation technique

Structural mechanisms in disordered materials like amorphous semi-conductors and glasses can be explored with the activation-relaxation technique (ART). The application of a sequence of such mechanisms allows for the generation of well-relaxed structures. The method and its application in the study of the microscopic changes in amorphous silicon and silica glass are reviewed, and two recent improvements are presented.

cond-mat↗

arXiv1998

Identification of relaxation and diffusion mechanisms in amorphous silicon

The dynamics of amorphous silicon at low temperatures can be characterized by a sequence of discrete activated events, through which the topological network is locally reorganized. Using the activation-relaxation technique, we create more than 8000 events, providing an extensive database of relaxation and diffusion mechanisms. The generic properties of these events - size, number of atoms involved, activation energy, etc. - are discussed and found to be compatible with experimental data. We introduce a complete and unique classification of defects based on their topological properties and apply it to study of events involving only four-fold coordinated atoms. For these events, we identify and present in detail three dominant mechanisms.

cond-mat↗

arXiv1998

Tight-binding molecular-dynamics studies of defects and disorder in covalently-bonded materials

Tight-binding (TB) molecular dynamics (MD) has emerged as a powerful method for investigating the atomic-scale structure of materials --- in particular the interplay between structural and electronic properties --- bridging the gap between empirical methods which, while fast and efficient, lack transferability, and ab initio approaches which, because of excessive computational workload, suffer from limitations in size and run times. In this short review article, we examine several recent applications of TBMD in the area of defects in covalently-bonded semiconductors and the amorphous phases of these materials.

cond-mat.mtrl-sci↗

arXiv1998

Surface diffusion coefficients by thermodynamic integration: Cu on Cu(100)

The rate of diffusion of a Cu adatom on the Cu(100) surface is calculated using thermodynamic integration within the transition state theory. The results are found to be in excellent agreement with the essentially exact values from molecular-dynamics simulations. The activation energy and related entropy are shown to be effectively independent of temperature, thus establishing the validity of the Arrhenius law over a wide range of temperatures. Our study demonstrates the equivalence of diffusion rates calculated using thermodynamic integration within the transition state theory and direct molecular-dynamics simulations.

cond-mat.mtrl-sci↗

arXiv1997

Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique

A detailed description of the activation-relaxation technique (ART) is presented. This method defines events in the configurational energy landscape of disordered materials, such as a-Si, glasses and polymers, in a two-step process: first, a configuration is activated from a local minimum to a nearby saddle-point; next, the configuration is relaxed to a new minimum; this allows for jumps over energy barriers much higher than what can be reached with standard techniques. Such events can serve as basic steps in equilibrium and kinetic Monte Carlo schemes.

cond-mat↗

arXiv1997

Structural, electronic, and dynamical properties of amorphous gallium arsenide: a comparison between two topological models

We present a detailed study of the effect of local chemical ordering on the structural, electronic, and dynamical properties of amorphous gallium arsenide. Using the recently-proposed ``activation-relaxation technique'' and empirical potentials, we have constructed two 216-atom tetrahedral continuous random networks with different topological properties, which were further relaxed using tight-binding molecular dynamics. The first network corresponds to the traditional, amorphous, Polk-type, network, randomly decorated with Ga and As atoms. The second is an amorphous structure with a minimum of wrong (homopolar) bonds, and therefore a minimum of odd-membered atomic rings, and thus corresponds to the Connell-Temkin model. By comparing the structural, electronic, and dynamical properties of these two models, we show that the Connell-Temkin network is energetically favored over Polk, but that most properties are little affected by the differences in topology. We conclude that most indirect experimental evidence for the presence (or absence) of wrong bonds is much weaker than previously believed and that only direct structural measurements, i.e., of such quantities as partial radial distribution functions, can provide quantitative information on these defects in a-GaAs.

cond-mat.mtrl-sci↗

arXiv1996

Topology of amorphous tetrahedral semiconductors on intermediate lengthscales

Using the recently-proposed ``activation-relaxation technique'' for optimizing complex structures, we develop a structural model appropriate to a-GaAs which is almost free of odd-membered rings, i.e., wrong bonds, and possesses an almost perfect coordination of four. The model is found to be superior to structures obtained from much more computer-intensive tight-binding or quantum molecular-dynamics simulations. For the elemental system a-Si, where wrong bonds do not exist, the cost in elastic energy for removing odd-membered rings is such that the traditional continuous-random network is appropriate. Our study thus provides, for the first time, direct information on the nature of intermediate-range topology in amorphous tetrahedral semiconductors.

cond-mat.mtrl-sci↗

arXiv1996

Event-based relaxation of continuous disordered systems

A computational approach is presented to obtain energy-minimized structures in glassy materials. This approach, the activation-relaxation technique (ART), achieves its efficiency by focusing on significant changes in the microscopic structure (events). The application of ART is illustrated with two examples: the structure of amorphous silicon, and the structure of Ni80P20, a metallic glass.

cond-mat↗

arXiv1996

Randomly connected cellular automata: A search for critical connectivities

I study the Chate-Manneville cellular automata rules on randomly connected lattices. The periodic and quasi-periodic macroscopic behaviours associated with these rules on finite-dimensional lattices persist on an infinite-dimensional lattice with finite connectivity and symmetric bonds. The lower critical connectivity for these models is at C=4 and the mean-field connectivity, if finite, is not smaller than C=100. Autocorrelations are found to decay as a power-law with a connectivity independent exponent approx. equal to -2.5. A new intermitten chaotic phase is also discussed.

cond-mat↗