Floating bonds and gap states in a-Si and a-Si:H from first principles calculations

We study in detail by means of ab-initio pseudopotential calculations the electronic structure of five-fold coordinated (T_5) defects in a-Si and a-Si:H, also during their formation and their evolution upon hydrogenation. The atom-projected densities of states (DOS) and an accurate analysis of the valence charge distribution clearly indicate the fundamental contribution of T_5 defects in originating gap states through their nearest neighbors. The interaction with hydrogen can reduce the DOS in the gap annihilating T_5 defects.Comment: To appear in Europhysics Let

El pilotaje obligatorio en los estrechos utilizados para la navegación internacional

La introducción del pilotaje obligatorio para los buques que transitan por los estrechos utilizados para la navegación internacional plantea una delicada cuestión jurídica en la medida en que a nivel internacional falta una normativa clara que reconozca a los Estados costeros de los estrechos el derecho a declarar obligatorio este servicio como medida para proteger el medio ambiente marino. Este artículo se centra en la adopticón por parte de algunos Estados ribereños de una legislación nacional que prevé un servicio de pilotaje obligatorio en los estrechos de su jurisdicción

Coordination defects in a-Si and a-Si:H : a characterization from first principles calculations

We study by means of first-principles pseudopotential method the coordination defects in a-Si and a-Si:H, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the ``electron localization function'' (ELF) allows to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly three-fold (T_3) and five-fold (T_5) coordinated defects. We found that electronic states in the gap can be associated to both kind of defects, and that in both cases the interaction with hydrogen can reduce the density of states in the gap.Comment: To appear in Philos. Ma

AFLOW-QHA3P: Robust and automated method to compute thermodynamic properties of solids

Accelerating the calculations of finite-temperature thermodynamic properties is a major challenge for rational materials design. Reliable methods can be quite expensive, limiting their applicability in autonomous high-throughput workflows. Here, the three-phonon quasiharmonic approximation (QHA) method is introduced, requiring only three phonon calculations to obtain a thorough characterization of the material. Leveraging a Taylor expansion of the phonon frequencies around the equilibrium volume, the method efficiently resolves the volumetric thermal expansion coefficient, specific heat at constant pressure, the enthalpy, and bulk modulus. Results from the standard QHA and experiments corroborate the procedure, and additional comparisons are made with the recently developed self-consistent QHA. The three approaches—three-phonon, standard, and self-consistent QHAs—are all included within the open-source ab initio framework aflow, allowing the automated determination of properties with various implementations within the same framework

Geomorphological control of gold distribution and gold particle evolution in glacial and fluvioglacial placers of the Ancocala-Ananea Basin : southeastern Andes of Peru

Etude sédimentologique et minéralogique des placer d'or d'origine glaciaire ou fluvio-glaciaire du bassin d'Ancocala-Ananea (Andes, Sud du Pérou). L'analyse morphologique et morphométrique des particules d'or permet d'en définir les différents milieux de sédimentation