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Showing papers by "Roberto Car published in 2001"


Journal ArticleDOI
02 Feb 2001-Science
TL;DR: The structure of the hydrated Cu(II) complex is determined by both neutron diffraction and first-principles molecular dynamics and it is argued that this picture is also consistent with experimental data obtained previously by visible near-infrared absorption, x-ray absorption near-edge structure, and nuclear magnetic resonance methods.
Abstract: We determined the structure of the hydrated Cu(II) complex by both neutron diffraction and first-principles molecular dynamics. In contrast with the generally accepted picture, which assumes an octahedrally solvated Cu(II) ion, our experimental and theoretical results favor fivefold coordination. The simulation reveals that the solvated complex undergoes frequent transformations between square pyramidal and trigonal bipyramidal configurations. We argue that this picture is also consistent with experimental data obtained previously by visible near-infrared absorption, x-ray absorption near-edge structure, and nuclear magnetic resonance methods. The preference of the Cu(II) ion for fivefold instead of sixfold coordination, which occurs for other cations of comparable charge and size, results from a Jahn-Teller destabilization of the octahedral complex.

356 citations


Journal ArticleDOI
TL;DR: In this paper, a generalized gradient approximation for the exchange and correlation energy of liquid GeSe2 was used to describe both the short-range and intermediate-range structure of the liquid, and a very good agreement with experiment was obtained for the total neutron structure factor over the entire range of momentum transfer.
Abstract: First-principles molecular dynamics simulations are carried out to study the structural properties of liquid GeSe2. We use a generalized gradient approximation for the exchange and correlation energy, which we find to improve significantly upon the local density approximation in describing both the short- and the intermediate-range structure. A very good agreement with experiment is obtained for the total neutron structure factor over the entire range of momentum transfer. In particular, the first sharp diffraction peak (FSDP) is well reproduced. We carry out a detailed comparison between partial structure factors and partial pair correlations in theory and experiment to assess the quality of our simulation model. The short-range and intermediate-range structure are well described overall. However, residual differences between theory and experiment, such as the absence of a FSDP in the concentration-concentration structure factor, appear and are traced back to the Ge-Ge correlations. An analysis of the bonding configurations indicates that liquid GeSe2 is a defective network consisting of predominant Ge-centered tetrahedral units, but Ge- and Se-centered triads and homopolar bonds occur in non-negligible amounts. The number of Ge-Ge homopolar bonds and of ordered fourfold rings compare favorably with experimental estimates. Chemical disorder manifests through an important percentage of Se-rich odd-membered rings. We characterized the intermediate-range order by studying the relation between real-space distances and the FSDP. We found that this feature appears when correlations beyond 5 Angstrom are accounted for. The evaluation of bond lifetimes reflect the higher stability of Ge-Se bonds with respect to homopolar bonds, consistent with the predominance of tetrahedral units.

73 citations


Journal ArticleDOI
TL;DR: In this article, a model for the carbon (C)-induced structure of a Si-C-Heterodimers was proposed, in which surface carbon atoms are stabilized in SiC heterodimmers, with a surface density of about 0.25 ML.
Abstract: A combination of low-energy electron diffraction, x-ray and ultraviolet photoelectron spectroscopy, and scanning-tunneling microscopy studies, in conjunction with ab initio calculations leads us to suggest a model for the carbon (C)-induced $\mathrm{Si}(001)c(4\ifmmode\times\else\texttimes\fi{}4)$ atomic structure. This surface superstructure is obtained in a defined range of ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ exposures at 600 \ifmmode^\circ\else\textdegree\fi{}C. Experimental probes show that the $c(4\ifmmode\times\else\texttimes\fi{}4)$ superstructure involves C atoms in both surface and subsurface sites. This is reflected in well-marked features in photoemission valence- and core-level spectra. Surface carbon atoms are stabilized in Si-C heterodimers, with a surface density of about 0.25 monolayer (ML) [i.e., two C atoms per $c(4\ifmmode\times\else\texttimes\fi{}4)$ unit cell of eight atoms]. In the subsurface region, carbon atoms substitute for Si atoms in well-defined sites of the third or fourth layers of the Si substrate. The subsurface C density increases with ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ exposure time up to a limit value of about 0.5 ML, within the $c(4\ifmmode\times\else\texttimes\fi{}4)$ surface structure. Further exposure disrupts the $c(4\ifmmode\times\else\texttimes\fi{}4)$ reconstruction and leads to a $(2\ifmmode\times\else\texttimes\fi{}1)$ low-energy electron diffraction pattern. Interaction with atomic hydrogen shows that the surface contains a mixture of heterodimers (Si-C) and homodimers (Si-Si), with an 1:1 proportion. These assignments are supported by first-principle calculations, which yield valence band and core level states in fairly good agreement with the experiment. Furthermore, total energy calculations strongly favor C incorporation in surface Si-C dimers and in third and fourth layer sites, and rule out C incorporation in sites of the second Si layer. The most stable $c(4\ifmmode\times\else\texttimes\fi{}4)$ surface configuration, suggested by our calculations, consists of alternate Si-C and Si-Si dimer lines. In such a configuration, surface carbon atoms in Si-C dimers induce a surface stress that leads to charge redistribution and atomic relaxation of the adjacent Si-Si dimers, consistent with scanning-tunneling microscopy images. Additional C atoms (in excess of those accommodated in surface sites) are forced in selected compressive (\ensuremath{\alpha}) sites of the third and fourth layers. This model is discussed with respect to the previous models published in the literature.

43 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigate the feasibility of reconstructing frozen-core all-electron molecular orbitals from corresponding pseudo-orbitals, and apply a transformation due to Blochl [Phys. Rev. B 50, 17953 (1994) to each calculated pseudo-orbital to obtain a corresponding frozen core all-electric molecular orbital.
Abstract: We investigate the numerical feasibility of reconstructing frozen-core all-electron molecular orbitals from corresponding pseudo-orbitals. We perform density-functional calculations on simple atomic and molecular model systems using ultrasoft pseudopotentials to represent the atomic cores. We apply a transformation due to Blochl [Phys. Rev. B 50, 17953 (1994)] to each calculated pseudo-orbital to obtain a corresponding frozen-core all-electron molecular orbital. Our model systems include the reconstruction of the 5d orbital of a gold atom, and the occupied valence states of the TiO2 molecule. Comparison of the resulting all-electron orbitals to corresponding ones that were obtained from calculations in which the core electrons were explicitly included indicates that all-electron molecular orbital reconstruction is a feasible and useful operation in reproducing the correct behavior of molecular orbitals in the nuclear core regions.

33 citations


Book ChapterDOI
01 Jan 2001
TL;DR: In this paper, a characterization of the Si(001)-SiO2 interface at the atomic scale is presented, which is intimately related to the understanding of the atomistic mechanisms that govern the oxidation process.
Abstract: The rapid rate of development of the electronics industry is predicated on aggressive scaling rules for robust transistor design. In particular, each successive reduction in the size of the silicon based field effect transistor requires further thinning of the oxide layer which separates the polysilicon gate electrode from the channel. For integrated circuits in production now, this gate oxide has a thickness well below 50 A. In this thickness regime, which is only an order of magnitude away from typical bond distances, a characterization of the Si(001)-SiO2 interface at the atomic scale is of critical importance. Knowledge of the atomic structure at this interface is intimately related to the understanding of the atomistic mechanisms that govern the oxidation process. In particular, the oxidation process used to fabricate integrated circuits yields a nearly perfect interface between amorphous silica and the silicon substrate for reasons that are still not well understood.

6 citations


Book ChapterDOI
01 Jan 2001
TL;DR: In this article, the results of first-principles dynamical simulations of both single and double-walled carbon nanotube edges were reported. And they showed that the open end of carbon single-wall nanotubes (SWNTs) spontaneously closes by forming a graphitic dome in the 2500-3000 K temperature range of synthesis experiments.
Abstract: The growth of carbon (C) and boron nitride (BN) nanotubes cannot be directly observed and the underlying microscopic mechanism is a controversial subject. Here we report on the results of first-principles dynamical simulations of both single-and double-walled carbon nanotube edges. We find that the open end of carbon single-walled nanotubes (SWNTs) spontaneously closes by forming a graphitic dome in the 2500-3000 K temperature range of synthesis experiments. On the other hand, “lip-lip” interactions, consisting of chemical bonding between the edges of adjacent coaxial tubes, trap the end of the double-walled carbon nanotube into a metastable energy minimum, preventing dome closure. The resulting end geometry is highly chemically active, and can easily accommodate incoming carbon fragments, thus allowing for growth by chemisorption from the vapour phase.

1 citations