Properties of Dense Fluid Hydrogen and Helium in Giant Gas Planets

Equilibrium properties of hydrogen-helium mixtures under thermodynamic conditions found in the interior of giant gas planets are studied by means of density functional theory molecular dynamics simulations. Special emphasis is placed on the molecular-to-atomic transition in the fluid phase of hydrogen in the presence of helium. Helium has a substantial influence on the stability of hydrogen molecules. The molecular bond is strengthened and its length is shortened as a result of the increased localization of the electron charge around the helium atoms, which leads to more stable hydrogen molecules compared to pure hydrogen for the same thermodynamic conditions. The {\it ab initio} treatment of the mixture enables us to investigate the structure of the liquid and to discuss hydrogen-hydrogen, helium-helium, and hydrogen-helium correlations on the basis of pair correlation functions.Comment: 6 pages, 3 figures, 1 table, proceedings PNP1

Structural models for the Si(553)-Au atomic chain reconstruction

Recent photoemission experiments on the Si(553)-Au reconstruction show a one-dimensional band with a peculiar ~1/4 filling. This band could provide an opportunity for observing large spin-charge separation if electron-electron interactions could be increased. To this end, it is necessary to understand in detail the origin of this surface band. A first step is the determination of the structure of the reconstruction. We present here a study of several structural models using first-principles density functional calculations. Our models are based on a plausible analogy with the similar and better known Si(557)-Au surface, and compared against the sole structure proposed to date for the Si(553)-Au system [Crain JN et al., 2004 Phys. Rev. B 69 125401 ]. Results for the energetics and the band structures are given. Lines for the future investigation are also sketched

Doping of Si nanoparticles: the effect of oxidation

The preferred location of boron and phosphorus in oxidized free-standing Si nanoparticles was investigated using a first-principles density functional approach. The calculated formation energies indicate that P should segregate to the silicon core, whereas B is equally stable in the Si and SiO_2 regions. Our models thus suggest that, in contrast with nanocrystals with H-terminated surfaces, the efficiency of phosphorus incorporation in oxidized Si nanoparticles can be improved by thermal annealing

Supported magnetic nanoclusters: Softlanding of Pd clusters on a MgO surface

Low-energy deposition of neutral Pd_N clusters (N=2-7 and 13) on a MgO(001) surface F-center (FC) was studied by spin-density-functional molecular dynamics simulations. The incident clusters are steered by an attractive "funnel" created by the FC, resulting in adsorption of the cluster, with one of its atoms bonded atop of the FC. The deposited Pd_2-Pd_6 clusters retain their gas-phase structures, while for N>6 surface-commensurate isomers are energetically more favorable. Adsorbed clusters with N > 3 are found to remain magnetic at the surface.Comment: 5 pages, 2 figs, Phys.Rev.Lett., accepte

High-Current Field Emission from an Atomic Quantum Wire

Linear chains of carbon atoms have been proposed as the electron emitting structures of open tip carbon nanotubes subject to an electric field. To better understand the implications of the results of Smalley and collaborators, the electromagnetic response of linear carbon chains to both static and dynamics fields have been studied, making use of ab-initio methods. It is found that the associated emission currents, plotted as a function of the bias potential, follow Fowler-Nordheim intensity-voltage curves typical of the field emission of metallic tips. Under standard bias conditions, linear carbon chains of one nanometer of length are expected to deliver currents of the order of one microampere. These systems behave, furthermore, as conducting needles in photoabsorption processes. Linear carbon chains are thus likely to constitute the ultimate atomic-scale realization of metallic wires.Comment: 10 pages, 4 figures, RevTe

Direct ab initio MD simulation of silver ion diffusion in chalcogenide glasses

In this paper, we present new models of germanium selenide chalcogenide glasses heavily doped with silver. The models were readily obtained with ab initio molecular dynamics and their structure agrees closely with diffraction measurements. Thermal molecular dynamics simulation reveals the dynamics of Ag+ ions and the existence of trapping centers as conjectured in other theory work. We show that first principles simulation is a powerful tool to reveal the motion of ions in glass.Comment: 3 pages, 3 figures, submitted to Phys. Stat. Sol. {b} Rapid Research Letter

Electron-Phonon Interaction in NbB_2 : A Comparison with MgB_2

We present a comparison of electron-phonon interaction in NbB_2 and MgB_2, calculated using full-potential, density-functional-based methods in P6/mmm crystal structure. Our results, described in terms of (i) electronic structure, (ii) phonon density of states F(\omega), (iii) Eliashberg function \alpha ^2F(\omega), and (iv) the solutions of the isotropic Eliashberg gap equation, clearly show significant differences in the electron-phonon interaction in NbB_2 and MgB_2. We find that the average electron-phonon coupling constant \lambda is equal to 0.59 for MgB_2 and 0.43 for NbB_2, leading to superconducting transition temperature T_c of around 22 K for MgB_2 and 3 K for NbB_2

Strain-Dependence of Surface Diffusion: Ag on Ag(111) and Pt(111)

Using density-functional theory with the local-density approximation and the generalized gradient approximation we compute the energy barriers for surface diffusion for Ag on Pt(111), Ag on one monolayer of Ag on Pt(111), and Ag on Ag(111). The diffusion barrier for Ag on Ag(111) is found to increase linearly with increasing lattice constant. We also discuss the reconstruction that has been found experimentally when two Ag layers are deposited on Pt(111). Our calculations explain why this strain driven reconstruction occurs only after two Ag layers have been deposited.Comment: 4 pages, 3 figures, Phys. Rev. B 55 (1997), in pres

Diffusion of Pt dimers on Pt(111)

We report the results of a density-functional study of the diffusion of Pt dimers on the (111) surface of Pt. The calculated activation energy of 0.37 eV is in {\em exact} agreement with the recent experiment of Kyuno {\em et al.} \protect{[}Surf. Sci. {\bf 397}, 191 (1998)\protect{]}. Our calculations establish that the dimers are mobile at temperatures of interest for adatom diffusion, and thus contribute to mass transport. They also indicate that the diffusion path for dimers consists of a sequence of one-atom and (concerted) two-atom jumps.Comment: Pour pages postscript formatted, including one figure; submitted to Physical Review B; other papers of interest can be found at url http://www.centrcn.umontreal.ca/~lewi

Localised spherical-wave basis set for O(N) total-energy pseudopotential calculations

Accepted versio