Self-diffusion of adatoms, dimers, and vacancies on Cu(100)

We use ab initio static relaxation methods and semi-empirical molecular-dynamics simulations to investigate the energetics and dynamics of the diffusion of adatoms, dimers, and vacancies on Cu(100). It is found that the dynamical energy barriers for diffusion are well approximated by the static, 0 K barriers and that prefactors do not depend sensitively on the species undergoing diffusion. The ab initio barriers are observed to be significantly lower when calculated within the generalized-gradient approximation (GGA) rather than in the local-density approximation (LDA). Our calculations predict that surface diffusion should proceed primarily via the diffusion of vacancies. Adatoms are found to migrate most easily via a jump mechanism. This is the case, also, of dimers, even though the corresponding barrier is slightly larger than it is for adatoms. We observe, further, that dimers diffuse more readily than they can dissociate. Our results are discussed in the context of recent submonolayer growth experiments of Cu(100).Comment: Submitted to the Physical Review B; 15 pages including postscript figures; see also http://www.centrcn.umontreal.ca/~lewi

Reconnaissance de parentèle chez Aleochara bilineata (Coleoptera: Staphylinidae) (mécanismes proximaux et optimalité évolutive)

La reconnaissance de parentèle, correspondant à l identification et au traitement différentiel des individus apparentés aux dépens des individus non-apparentés, est l'une des étapes préalables à la mise en place de la socialité. Cette capacité existe dans tout le règne animal, des organismes unicellulaires aux êtres humains. Pourtant, chez les insectes elle n a été étudiée que chez les espèces sociales ou clonales. Chez le staphylin non-social Aleochara bilineata (Coleoptera; Staphylinidae), les larves de premier stade, qui se développent en parasitoïdes des pupes de la mouche du chou Delia radicum (Diptera; Anthomyiidae) et les adultes sont capables de reconnaissance de parentèle. Cette dernière est étudiée chez cette espèce selon ses contextes d expression, ses mécanismes proximaux et son optimalité évolutive. Les facteurs pouvant entraîner la mise en place de la socialité dans les populations animales sont ainsi abordés.Kin recognition, defined as the identification and differential treatments of kin and non-kin, allows to identify the factors influencing the sociality establishment. This ability exists in all the animal kingdom, from unicellular organisms to human being. However, in insects, kin recognition has only been studied in social or clonal species. In the non-social staphylinid beetle Aleochara bilineata (Coleoptera; Staphylinidae), the first instar larvae, which develop as pupal parasitoid of the cabbage root fly Delia radicum (Diptera; Anthomyiidae), and the adults are able of kin recognition. In this species, kin recognition is studied according to its expression contexts, proximal mechanisms and its evolutionary optimality. The factors leading to sociality establishment in animal populations are reached.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

9 SCIENTIFIC HIGHLIGHT OF THE MONTH Partial dislocations in wurtzite GaN

The atomic structures and energies of 1/6 < 20¯23> and 1/3 < 10¯10> partial dislocations in wurtzite GaN are modelled using an empirical interatomic potential in combination with topological theory and anisotropic elasticity calculations. Twelve stable configurations of the 1/6 < 20¯23> edge and mixed partial dislocations that bound I1 intrinsic basal stacking faults are obtained for each polarity and their core radii, energies, and atomic configurations are given. Moreover, the 1/3 < 10¯10> edge and mixed partial dislocations along the junction lines between inversion domain boundaries (IDBs) and I1 stacking faults are studied. Twenty eight stable junction line configurations have been identified, sixteen of them resulting in stable transformations of the energetically favourable IDB * defect to the electrically active Holt type IDB. The majority of the dislocation core structures possessed either dangling bonds or highly strained bonds. Since reduced coordination or strained bonds can introduce gap states, such dislocations could be electrically active. 1

Analytical determination of the Landau-Ginzburg parameters of (100) metal homoepitaxial systems

In the initial stages of homoepitaxial growth on the (100) surface of metals such as Ag, Fe, Cu, Ni, and Pd, where the clean surface does not reconstruct, two-dimensional islands with compact, near-square shapes are formed. In order to determine the phenomenological material parameters of the nonlinear and nonlocal Landau-Ginzburg theory, which describes the metal homoepitaxial systems mentioned above, an atomistic model for these systems is developed. Based on this model, we derive analytical relationships between the Landau-Ginzburg parameters A, B, C, and D, and the parameters of the homoepitaxial system (such as coverage, first-neighbour interaction energy, etc.). We find that the Landau-Ginzburg parameters of the system depend on the specific material as well as on the coverage of the surface. We then apply the method to the Ag/Ag(100) system. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200668.47.De Metallic surfaces, 61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling, 64.60.Qb Nucleation,