Numerical modeling of radiation effects in solids: Principal features, limitations and perspectives

International audienceThis paper focuses on atomistic computer modeling of radiation effects with special emphasis on the achievements of this approach as well as on its space and time scales inherent limitations. These last are the principal motivations for the development of a multiscale approach ideally consisting in a succession of models providing input information to the model acting at the immediate next time and space scales. Some methodologies used to bridge the gap between multiple space and time scales are considered together and reasons encountered preventing the approach from being predictive are examined. Moreover, we briefly comment methods improving modeling of radiation in solids

Stochastic description of a period-2 limit cycle

The effect of internal noise on the period-2 behavior is investigated in low-dimensional dynamical systems. To treat the morphological complexity of such attractors, a general stochastic formulation using a suitable moving frame in the Gaussian approximation is set up. Its applicability is illustrated on a specific model, the Willamowski-Rössler chemical scheme. Using this approach, we are able to interpret the direct simulations of the master equation which show significant changes with respect to the deterministic behavior. Specifically, a band merging in the multi-periodic regime which at first sight seems to disrupt the bifurcation cascade is observed. Marked inhomogeneities in the fluctuations distribution along the period-2 attractor explaining these features are brought out

Interfacial layering and capillary roughness in immiscible liquids

International audienceThe capillary roughness and the atomic density profiles of extended interfaces between immiscible liquids are determined as a function of the interface area by using molecular dynamics and Lennard-Jones (12-6) potentials. We found that with increasing area, the interface roughness diverges logarithmically, thus fitting the theoretical mean-field prediction. In systems small enough for the interfacial roughness not to blur the structural details, atomic density profiles across the fluid interface are layered with correlation length in the range of molecular correlations in liquids. On increasing the system size, the amplitude of the thermally excited position fluctuations of the interface increases, thus causing layering to rapidly vanish, if density profiles are computed without special care. In this work, we present and validate a simple method, operating in the direct space, for extracting from molecular dynamics trajectories the ``intrinsic'' structure of a fluid interface that is the local density profile of the interface cleaned from capillary wave effects. Estimated values of interfacial properties such as the tension, the intrinsic width, and the lower wavelength limit of position fluctuations are in agreement with results collected from the literature. (C) 2010 American Institute of Physics. [doi:10.1063/1.3471384

Étude par simulation numérique de l'annihilation en surface d'une dislocation coin

Nous avons étudié par simulation numérique à l'échelle atomique la stabilité d'une dislocation coin, de plan de glissement proche et parallèle à une surface libre. La même étude à été conduite pour le débris idéalisé d'annihilation d'une telle dislocation : une cavité de longueur infinie, de section rectangulaire et de profondeur variable, créée par élimination partielle d'atomes appartenant à deux plans {110} adjacents, perpendiculaires à une surface libre {111}. Le modèle de cohésion choisi est un potentiel semi-empirique, central, à n-corps, adapté à l'aluminium. Les résultats montrent que l'annihilation de la dislocation, éventuellement assistée par la contrainte et/ou la température, devient possible lorsque la distance entre son plan de glissement et la surface est inférieure à une distance critique d$_{\rm c}$. Ces résultats suggèrent que le débris d'annihilation peut engendrer une fissure en ouvrant ainsi la perspective d'une meilleure compréhension du rôle des surfaces en tant que sites privilégiés de germination de fissures

The structure of the solid-liquid interface: Atomic size effect

The atomic structure of the solid-liquid heterophase interface was investigated by using molecular dynamics. Two kinds of systems were studied; the first one was crystalline copper with (100) and (111) surface terminations in contact with liquid aluminium, while in the second one the interface was modelled by two systems in contact made of Lennard-Jones particles with different size ($\sigma$) and energy ($\epsilon$) parameters. We found that at the interface the liquid was layered whatever the crystallographic orientation of the surface. The layering of the liquid is still preserved when the ratio of particles sizes ($x=\sigma_1/\sigma_2$) changes while an epitaxial relationship is always found between the crystal and the first liquid layer. The average density of the latter is closely related to the $x$ value

Molecular dynamics study of the solid–liquid interface

International audienc

Combination of (100), (110) and (111) facets in MgO crystals shapes from dry to wet environment

International audienceAt the onset of dissolution in water, cubic MgO smoke crystals present (110) cuts of the edges of the cubes. Next, (111) facets progressively dominate the shape of the crystallites, which finally transform into truncated octahedra. The morphology of the crystallites that are derived from surface energies computed within the density functional theory (DFT), only involve (100) and (111) facets. We explain the unexpected (110) cuts via a ``constrained'' Wulff equilibrium shape that arises from a slower kinetics of formation of (111) facets than (110) ones. Experiment and theory fully agree on the hierarchy of hydroxylated surface energies: Gamma((111)) < Gamma((100)) < Gamma((110)), both supporting the partial dissociation of water on MgO(100). Finally, from low to high P(H2O) (high to low T), DFT-based calculations predict a switch from Wulff shapes involving dry (100) facets, in which the (100)/(111) area ratio decreases upon increasing P(H2O), to shapes involving hydroxylated (100) surfaces, in which the above ratio increases with P(H2O)