References of "Raty, Jean-Yves"
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See detailDynamical, dielectric, and elastic properties of GeTe investigated with first-principles density functional theory
Shaltaf, R.; Durgun, Engin ULg; Raty, Jean-Yves ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2008), 78

The dynamical, dielectric, and elastic properties of GeTe, a ferroelectric material in its low-temperature rhombohedral phase, have been investigated using first-principles density functional theory. We ... [more ▼]

The dynamical, dielectric, and elastic properties of GeTe, a ferroelectric material in its low-temperature rhombohedral phase, have been investigated using first-principles density functional theory. We report the electronic energy bands, phonon-dispersion curves, electronic and low-frequency dielectric tensors, infrared reflectivity, Born effective charges, and elastic and piezoelectric tensors and compare them with the existing theoretical and experimental results, as well as with similar quantities available for other ferroelectric materials, when appropriate. [less ▲]

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See detailTetrahedral clustering in molten lithium under pressure
Tamblyn, Isaac; Raty, Jean-Yves ULg; Bonev, Stanimir A.

in Physical Review Letters (2008), 101(7), 0757034

A series of electronic and structural transitions are predicted in molten lithium from first principles. A new phase with tetrahedral local order characteristic of sp 3 bonded materials and poor ... [more ▼]

A series of electronic and structural transitions are predicted in molten lithium from first principles. A new phase with tetrahedral local order characteristic of sp 3 bonded materials and poor electrical conductivity is found at pressures above 150 GPa and temperatures as high as 1000 K. Despite the lack of covalent bonding, weakly bound tetrahedral clusters with finite lifetimes are predicted to exist. The stabilization of this phase in lithium involves a unique mechanism of strong electron localization in interstitial regions and interactions among core electrons. The calculations provide evidence for anomalous melting above 20 GPa, with a melting temperature decreasing below 300 K, and point towards the existence of novel low-symmetry crystalline phases. [less ▲]

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See detailElectronic and structural transitions in dense liquid sodium
Raty, Jean-Yves ULg; Schwegler, E.; Bonev, S. A.

in Nature (2007), 449(7161), 448-451451

At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure 1-6. It ... [more ▼]

At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure 1-6. It was predicted from theory 1.2 - and later confirmed by experiment 3-5 - that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve 6 have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states - an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well. [less ▲]

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See detailOptical properties and structure of nanodiamonds
Raty, Jean-Yves ULg; Galli, G.

in Journal of Electroanalytical Chemistry (2005), 584(1 Sp. Iss. SI), 9-12

We present a theoretical study of the structure and optical properties of nanodiamonds. Using ab initio molecular dynamics simulation, we confirm that quantum confinement effects become negligible between ... [more ▼]

We present a theoretical study of the structure and optical properties of nanodiamonds. Using ab initio molecular dynamics simulation, we confirm that quantum confinement effects become negligible between 2 and 3 nm in size. In this size domain, specific surface reconstructions occur upon sample dehydrogenation, leading to fullerene-capped structures, or 'bucky diamonds' with absorption spectra similar to the experiment. We finally show that the HOMO and LUMO states are interface states in the reconstructed structures. (c) 2004 Elsevier B.V.. All rights reserved. [less ▲]

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See detailFirst principle study of nanodiamond optical and electronic properties
Raty, Jean-Yves ULg; Galli, G.

in Computer Physics Communications (2005), 169(1-3), 14-1919

Nanometer sized diamond has been found in meteorites, proto-planetary nebulae and interstellar dusts, as well as in residues of detonation and in diamond films. Remarkably, the size distribution of ... [more ▼]

Nanometer sized diamond has been found in meteorites, proto-planetary nebulae and interstellar dusts, as well as in residues of detonation and in diamond films. Remarkably, the size distribution of diamond nanoparticles appears to be peaked around 2-5 nm, and to be largely independent of preparation conditions. Using ab-initio calculations, we have shown that in this size range nanodiamond has a fullerene-like surface and, unlike silicon and germanium, exhibits very weak quantum confinement effects. We called these carbon nanoparticles bucky-diamonds: their atomic structure, predicted by simulations, is consistent with many experimental findings. In addition, we carried out calculations of the stability of nanodiamond, which provided a unifying explanation of its size distribution in extra-terrestrial samples, and in ultra-nano crystalline diamond (UNCD) films. [All rights reserved Elsevier] [less ▲]

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See detailTemperature-induced density anomaly in Te-rich liquid Germanium tellurides: p versus sp 3 bonding?
Bichara, C.; Johnson, M.; Raty, Jean-Yves ULg

in Physical Review Letters (2005), 95(26), 2678011-26780142678014

The density anomaly of liquid Ge 0.15Te 0.85 measured between 633 and 733 K is investigated with ab initio molecular dynamics calculations at four temperatures and at the corresponding experimental ... [more ▼]

The density anomaly of liquid Ge 0.15Te 0.85 measured between 633 and 733 K is investigated with ab initio molecular dynamics calculations at four temperatures and at the corresponding experimental densities. For box sizes ranging from 56 to 112 atoms, an 8 k-points sampling of the Brillouin zone is necessary to obtain reliable results. Contrary to other Ge chalcogenides, no sp 3 hybridization of the Ge bonding is observed. As a consequence, the negative thermal expansion of the liquid is not related to a tetrahedral bonding as in the case of water or silica. We show that it results from the symmetry recovery of the local environment of Ge atoms that is distorted at low temperature by a Peierls-like mechanism acting in the liquid state in the same way as in the parent solid phases [less ▲]

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See detailGrowth of carbon nanotubes on metal nanoparticles: a microscopic mechanism from ab initio molecular dynamics simulations
Raty, Jean-Yves ULg; Gygi, F.; Galli, G.

in Physical Review Letters (2005), 95(9), 0961031-09610340961034

We report on ab initio molecular dynamics simulations of the early stages of single-walled carbon nanotube (SWCNT) growth on metal nanoparticles. Our results show that a sp2 bonded cap is formed on an ... [more ▼]

We report on ab initio molecular dynamics simulations of the early stages of single-walled carbon nanotube (SWCNT) growth on metal nanoparticles. Our results show that a sp2 bonded cap is formed on an iron catalyst, following the diffusion of C atoms from hydrocarbon precursors on the nanoparticle surface. The weak adhesion between the cap and iron enables the graphene sheet to "float" on the curved surface, as additional C atoms covalently bonded to the catalyst "hold" the tube walls. Hence the SWCNT grows capped. At the nanoscale, we did not observe any tendency of C atoms to penetrate inside the catalyst, consistent with total energy calculations showing that alloying of Fe and C is very unlikely for 1 nm particles. Root growth was observed on Fe but not on Au, consistent with experiment [less ▲]

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See detailA thermodynamic investigation of selenium confined in silicalite zeolite
Bichara, C.; Raty, Jean-Yves ULg; Pellenq, R.

in Molecular Simulation (2004), 30(9), 601-606

In this paper, we study the practical feasibility of selenium adsorption in silicalite-1 zeolite by performing Grand Canonical Monte Carlo (GCMC) simulations on a simulation box including the porous ... [more ▼]

In this paper, we study the practical feasibility of selenium adsorption in silicalite-1 zeolite by performing Grand Canonical Monte Carlo (GCMC) simulations on a simulation box including the porous matrix and its outer surface. This work aims at gaining insight on the stability of semi-conductor wires in microporous materials. The simulations at two different temperatures show two distinct behaviors: adsorption occurs inside the pores at 200degreesC while solely on the external surface at 650degreesC. This indicates that adsorption inside the pore network can only proceed below the pseudo-wetting transition temperature that lies between 200 and 650degreesC. The existence of such transition temperature is thus crucial if one aims to produce nanowires from microporous materials by adsorption from a gas phase. [less ▲]

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See detailStructure of liquid Te-based alloys used in rewritable DVDs
Delheusy, M.; Raty, Jean-Yves ULg; Detemple, R. et al

in Physica B: Condensed Matter (2004), 350(1-3 SUPPL. 1), 1055-1057

We analyze the structure of Te-based chalcogenide compounds that are used as materials for rewritable DVDs by using a combination of neutron diffraction and ab initio computer simulation. We show that in ... [more ▼]

We analyze the structure of Te-based chalcogenide compounds that are used as materials for rewritable DVDs by using a combination of neutron diffraction and ab initio computer simulation. We show that in the liquid, the atoms have a low average coordination number, as the result of a Peierls distortion. The partial pair correlation functions are obtained from the computer simulation data. © 2004 Published by Elsevier B.V. [less ▲]

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See detailStructure of liquid semiconductors
Gaspard, Jean-Pierre ULg; Raty, Jean-Yves ULg

in Journal de Physique IV (2003), 111

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See detailProperties of molten Ge chalcogenides: an ab initio molecular dynamics study
Raty, Jean-Yves ULg; Gaspard, Jean-Pierre ULg; Bichara, C.

in Journal of Physics : Condensed Matter (2003), 15(1 Sp. Iss. SI), 167-173

In this study, we perform first-principles molecular dynamics simulations of the eutectic alloy Ge15Te85 at five different densities and temperatures. We obtain structures in agreement with the available ... [more ▼]

In this study, we perform first-principles molecular dynamics simulations of the eutectic alloy Ge15Te85 at five different densities and temperatures. We obtain structures in agreement with the available diffraction data and obtain a new view of the molten Ge chalcogenides. We show that the anomalous volume contraction observed in the liquid 30 K above the eutectic temperature corresponds to a significant change of the Ge-Te partial structure factor. The detailed structural analysis shows that volume variations observed upon melting in Ge15Te85, as in liquid, GeSe and GeTe, can be explained in terms of the competition between two types of local environment of the germanium atoms. A symmetrical coordination octahedron is entropically favoured at high temperature, while an asymmetrical octahedron resulting from the local manifestation of the Peierls distortion is electronically favoured at lower temperatures. [less ▲]

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See detailCalculation of optical absorption spectra of hydrogenated Si clusters: Bethe-Salpeter equation versus time-dependent local-density approximation
Benedict, L. X.; Puzder, A.; Williamson, A. J. et al

in Physical Review. B : Condensed Matter (2003), 68(8), 85310-1-85310-85310-885310-85310-8

We present calculations of the optical absorption spectra of clusters SiH 4, Si 10H 16, Si 17H 36, Si 29H 24, and Si 35H 36, as determined from two different methods: the Bethe-Salpeter equation (BSE ... [more ▼]

We present calculations of the optical absorption spectra of clusters SiH 4, Si 10H 16, Si 17H 36, Si 29H 24, and Si 35H 36, as determined from two different methods: the Bethe-Salpeter equation (BSE) with a model dielectric function, and the time-dependent density-functional theory within the adiabatic local-density approximation (TDLDA). Single-particle states are obtained from local-density approximation (LDA) calculations and, for the BSE calculation, a quasiparticle gap correction is provided by quantum Monte Carlo calculations. We find that the exchange-correlation kernel of the TDLDA has almost no effect on the calculated spectra, while the corresponding attractive part of the electron-hole interaction of the BSE produces enhanced absorptive features at low energies. For the smallest cluster SiH 4, the two methods produce markedly different results, with the TDLDA spectra appearing closer to the experimental result. The gross features of the TDLDA and BSE spectra for larger clusters are however similar, due to the strong repulsive Coulomb kernel present in both treatments [less ▲]

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See detailQuantum confinement and fullerenelike surface reconstructions in nanodiamonds
Raty, Jean-Yves ULg; Galli, G.; Bostedt, C. et al

in Physical Review Letters (2003), 90(3), 0374011-03740140374014

We present x-ray absorption and emission experiments and ab initio calculations showing that the size of carbon diamond must be reduced to at least 2 nm, in order to observe an increase of its optical gap ... [more ▼]

We present x-ray absorption and emission experiments and ab initio calculations showing that the size of carbon diamond must be reduced to at least 2 nm, in order to observe an increase of its optical gap, at variance with Si and Ge where quantum confinement effects persist up to 6-7 nm. In addition, our calculations show that the surface of nanodiamond particles larger than sime 1 nm reconstructs in a fullerenelike manner, giving rise to a new family of carbon clusters: bucky diamonds. Signatures of these surface reconstructions are compatible with pre-edge features observed in measured absorption spectra [less ▲]

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See detailUltradispersity of diamond at the nanoscale
Raty, Jean-Yves ULg; Galli, G.

in Nature Materials (2003), 2(12), 792-795795

Nanometre-sized diamond has been found in meteorites, protoplanetary nebulae, and interstellar dusts, as well as in residues of detonation and in diamond films. Remarkably, the size distribution of ... [more ▼]

Nanometre-sized diamond has been found in meteorites, protoplanetary nebulae, and interstellar dusts, as well as in residues of detonation and in diamond films. Remarkably, the size distribution of diamond nanoparticles seems to be peaked around 2-5 nm, and to be largely independent of preparation conditions. We have carried out ab initio calculations of the stability of nanodiamond as a function of surface hydrogen coverage and of size. We have found that at about 3 nm, and for a broad range of pressures and temperatures, particles with bare, reconstructed surfaces become thermodynamically more stable than those with hydrogenated surfaces, thus preventing the formation of larger grains. Our findings provide an explanation of the size distribution of extraterrestrial and of terrestrial nanodiamond found in ultradispersed and ultracrystalline diamond films. They also provide an atomistic structural model of these films, based on the topology and structure of 2-3 nm diamond clusters consisting of a diamond core surrounded by a fullerene-like carbon network [less ▲]

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See detailLocal structure of liquid GeTe via neutron scattering and ab initio simulations
Raty, Jean-Yves ULg; Godlevsky, V. V.; Gaspard, Jean-Pierre ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2002), 65

We examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe. We employ hot-neutron two-axis diffraction at three temperatures above the melting point and compare ... [more ▼]

We examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe. We employ hot-neutron two-axis diffraction at three temperatures above the melting point and compare these results with ab initio molecular dynamics simulations. The simulations were based on interatomic forces derived from pseudopotentials constructed within density functional theory. At the melting temperature, the Peierls distortion responsible for the lower-temperature crystal phase is shown to manifest itself within the liquid structure. At higher temperatures in the liquid, increasing disorder in the Ge environment determines the eventual semiconductor-metal transition. The calculated kinematic viscosity of the liquid is found to agree with the experimental value and is shown to arise from the small diffusion coefficient of the Te atoms. [less ▲]

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See detailFirst-principles computation of material properties: the ABINIT software project
Gonze, X.; Beuken, J. M.; Caracas, R. et al

in Computational Materials Science (2002), 25(3), 478-492

The density functional theory (DFT) computation of electronic structure, total energy and other properties of materials, is a field in constant progress. In order to stay at the forefront of knowledge, a ... [more ▼]

The density functional theory (DFT) computation of electronic structure, total energy and other properties of materials, is a field in constant progress. In order to stay at the forefront of knowledge, a DFT software project can benefit enormously from widespread collaboration, if handled properly. Also, modern software engineering concepts can considerably ease its development. The ABINIT project relies upon these ideas: freedom of sources, reliability, portability, and self-documentation are emphasised, in the development of a sophisticated plane-wave pseudopotential code. We describe ABINITv3.0, distributed under the GNU General Public License. The list of ABINITv3.0 capabilities is presented, as well as the different software techniques that have been used until now: PERL scripts and CPP directives treat a unique set of FORTRAN90 source files to generate sequential (or parallel) object code for many different platforms; more than 200 automated tests secure existing capabilities; strict coding rules are followed; the documentation is extensive, including online help files, tutorials, and HTML-formatted sources. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailAdsorption of selenium wires in silicalite-1 zeolite: a first order transition in a microporous system
Bichara, C.; Raty, Jean-Yves ULg; Pellenq, RJ-M

in Physical Review Letters (2002), 89(1), 016101-1-016101-016101-4016101-016101-4

A tight binding grand canonical Monte Carlo simulation of the adsorption of selenium in silicalite-1 zeolite is presented. The calculated adsorption-desorption isotherms exhibit characteristic features of ... [more ▼]

A tight binding grand canonical Monte Carlo simulation of the adsorption of selenium in silicalite-1 zeolite is presented. The calculated adsorption-desorption isotherms exhibit characteristic features of a first order transition, unexpected for adsorption in a microporous system with pore size of the order of 0.5 to 0.6 nm. We analyze this behavior as a result of the favored twofold coordinated chain structure of selenium that grows inside the complex three-dimensional microchannel network of silicalite. This analysis is confirmed by simpler calculations of a lattice gas-type model [less ▲]

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See detailComputer simulation of liquid semiconductors
Bichara, C.; Gaspard, Jean-Pierre ULg; Raty, Jean-Yves ULg

in Journal of Non-Crystalline Solids (2002), 312

We discuss two examples of computer simulation of liquid semiconductors by two different techniques. Both examples are concerned with the relationship between thermodynamic properties and the atomic ... [more ▼]

We discuss two examples of computer simulation of liquid semiconductors by two different techniques. Both examples are concerned with the relationship between thermodynamic properties and the atomic structure in the liquid state. By means of ab initio molecular dynamics we analyze the atomic structure of liquid Ge15Te85 eutectic alloys. We show that the changes observed in the experimental total structure factor S(q) are located in the GeTe partial structure factor, the TeTe partial structure factor remaining essentially unaltered. Using a semi-empirical tight binding approach, coupled with Gibbs ensemble and constant pressure Monte Carlo calculations, we can calculate the liquid vapor equilibrium of selenium. We obtain a liquid-vapor equilibrium curve and a critical point in the correct range of magnitude and an atomic structure of the liquid phase in good agreement with the experimental data. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailDistance correlations and dynamics of liquid GeSe: An ab initio molecular dynamics study
Raty, Jean-Yves ULg; Godlevsky, V. V.; Gaspard, Jean-Pierre ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2001), 64

We analyze the structure and dynamics of semiconducting liquid GeSe using ab initio molecular-dynamics simulations. We show the local order of the liquid to be close to that of the low-temperature ... [more ▼]

We analyze the structure and dynamics of semiconducting liquid GeSe using ab initio molecular-dynamics simulations. We show the local order of the liquid to be close to that of the low-temperature crystalline phase. alpha -GeSe. In particular, we show that the Peierls distortion, which defines the a phase and vanishes in the high-temperature beta crystalline phase, reenters GeSe in the melt. Examining the distance histograms allows one to analyze the Ge environment as consisting of a Gese(3) unit and having one Ge-Ge defective bond. Evidence is presented that Peierls distortion is directly responsible for the semiconducting behavior of the melt. The calculated viscosity and electrical conductivity are in agreement with the experiment. An additional neutron-diffraction experiment indicates that this liquid structure is unmodified 200 K above the melting point. [less ▲]

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See detailAb initio simulations of liquid semiconductors using the pseudopotential-density functional method
Chelikowsky, J. R.; Derby, J. J.; Godlevsky, V. V. et al

in Journal of Physics : Condensed Matter (2001), 13(41), 817-854854

One of the most difficult problems in condensed matter physics is describing the microscopic nature of the liquid state. Owing to the dynamical nature of the liquid state, it is not possible to discuss a ... [more ▼]

One of the most difficult problems in condensed matter physics is describing the microscopic nature of the liquid state. Owing to the dynamical nature of the liquid state, it is not possible to discuss a particular microscopic structure; only ensemble averages can be specified. Such averages can be performed via well crafted molecular dynamics simulations: the length of the simulation, the size of the ensemble and the nature of the interatomic forces must all be carefully analysed. Historically, a problematic issue in doing such simulations is that of how to describe the interatomic forces in the liquid state. This matter is especially challenging for the melt of semiconductors, such as silicon or gallium arsenide, where the chemical bond contains a strong covalent component. It is difficult to use pairwise interatomic potentials in such cases. Although many-body potentials can be utilized for simulations of these materials, one must map quantum phenomena such as hybridization onto classical interatomic potentials. This mapping is complex and difficult. In this review, we illustrate how one can avoid this problem by utilizing quantum forces to simulate liquids. Our focus is on the pseudopotential-density functional method. Within the pseudopotential method, only the valence electrons are explicitly treated and within the density functional theory, exchange and correlation terms are mapped onto an effective one-electron potential [less ▲]

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