Showing papers by "Roberto Car published in 1998"

Identification of Raman defect lines as signatures of ring structures in vitreous silica

Abstract: By analyzing the vibrational eigenmodes of a model structure obtained previously by quantum molecular dynamics, the two sharp features appearing in the Raman spectrum of vitreous silica could conclusively be assigned to three- and four-membered ring structures embedded in the network. The projections of the eigenmodes onto oxygen breathing motions in these rings yield well defined peaks at frequencies in striking correspondence to the positions of the Raman lines. The good agreement between calculated and measured shifts upon isotopic substitution further confirms this assignment.

Fully Unconstrained Approach to Noncollinear Magnetism: Application to Small Fe Clusters

Abstract: We develop a plane-wave pseudopotential scheme for noncollinear magnetic structures, based on a generalized local spin-density theory in which the direction of the magnetization is a continuous variable of position. We allow the atomic and magnetic structures to relax simultaneously and self-consistently. Application to small Fe clusters yields noncollinear magnetic structures for Fe-3 and Fe-5. The components of the magnetization density vary smoothly with position. The spin direction undergoes sizable changes only in the regions of small charge and spin density between the atoms and is generally uniform in the magnetic regions of the atoms.

Interface structure between silicon and its oxide by first-principles molecular dynamics

Abstract: The requirement for increasingly thin (<50 A) insulating oxide layers in silicon-based electronic devices highlights the importance of characterizing the Si–SiO2 interface structure at the atomic scale. Such a characterization relies to a large extent on an understanding of the atomic-scale mechanisms that govern the oxidation process. The widely used Deal–Grove model invokes a two-step process in which oxygen first diffuses through the amorphous oxide network before attacking the silicon substrate, resulting in the formation of new oxide at the buried interface1. But it remains unclear how such a process can yield the observed near-perfect interface2,3,4,5,6,7,8,9,10,11,12. Here we use first-principles molecular dynamics13,14,15 to generate a model interface structure by simulating the oxidation of three silicon layers. The resulting structure reveals an unexpected excess of silicon atoms at the interface, yet shows no bonding defects. Changes in the bonding network near the interface occur during the simulation via transient exchange events wherein oxygen atoms are momentarily bonded to three silicon atoms — this mechanism enables the interface to evolve without leaving dangling bonds.

Frustration effects and microscopic growth mechanisms for BN nanotubes

Abstract: Using ab initio molecular dynamics simulations, we have studied the growth mechanisms of boron-nitride (BN) nanotubes. In this experimental conditions of temperature, the behavior of single-wall BN nanotubes strongly depends on their helicity. Open-ended "zigzag" tubes close into an amorphouslike tip, preventing further growth. In the "armchair" case, the formation of squares traps the tip into a flat cap able to revert to a growing hexagonal network by incorporating incoming atoms. These findings are related to the greater stability of B-N bonds as compared to B-B or N-N bonds.

First principles study of adsorbed cun (n=1-4) microclusters on mgo(100): structural and electronic properties

Abstract: We present a density functional study of the structural and electronic properties of small Cun (n=1,4) aggregates on defect-free MgO(100). The calculations employ a slab geometry with periodic boundary conditions, supercells with up to 76 atoms, and include full relaxation of the surface layer and of all adsorbed atoms. The preferred adsorption site for a single Cu adatom is on top of an oxygen atom. The adsorption energy and Cu–O distance are ES−A=0.99 eV and dS−A=2.04 A using the Perdew–Wang gradient corrected exchange correlation functional. The saddle point for surface diffusion is at the “hollow” site, with a diffusion barrier of around 0.45 eV. For the adsorbed copper dimer, two geometries, one parallel and one perpendicular to the surface, are very close in energy. For the adsorbed Cu3, a linear configuration is preferred to the triangular geometry. As for the tetramer, the most stable adsorbed geometry for Cu4 is a rhombus. The adsorption energy per Cu atom decreases with increasing the size of th...

Dynamic structure factor of vitreous silica from first principles: Comparison to neutron-inelastic-scattering experiments

Abstract: Using a first-principles approach, we study the vibrational properties of vitreous SiO2 which are measured in neutron-scattering experiments. We adopt a model structure consisting of corner-sharing tetrahedra, which was previously generated using first-principles molecular dynamics. We calculate the dynamic, structure function S(q,E) as a function of wave vector q and energy E by taking explicitly into account the correlations between different atoms as given by the normal modes. The effects of temperature and finite displacements are also considered. Overall, the agreement with experiment is very good, as illustrated by the comparison for the density of states. However, the calculated and measured S(q,E) differ in some cases up to a factor of 2 in absolute intensity. Nevertheless, the oscillations in S(q,E) describing the correlations between the motions of the atoms are accurately reproduced. The neutron effective density of states obtained directly from S(q,E) yields a good representation of the actual density of states. By introducing a comprehensive scheme, we clarify the relation between neutron and infrared spectre. In particular, we show that the neutron density of states does not distinguish between longitudinal and transverse excitations. Other properties such as the mean-square displacements and the elastic structure factor are also evaluated and found to be in good agreement with experiment.

Microscopic structure of liquid GeSe2: The problem of concentration fluctuations over intermediate range distances

Abstract: A first-principle molecular dynamics study on liquid GeSe2 yields structural properties in good agreement with detailed neutron scattering data. The short range order is accurately described by a variety of bonding configurations, in which regular tetrahedra coexist with an important fraction of homopolar bonds and threefold centers. The first sharp diffraction peak in the total structure factor, which characterizes order over intermediate range distances, is also well reproduced. However, this level of theory does not yield concentration fluctuations over such distances as observed in experiment.

Structure of liquid GexSe1-x at the stiffness threshold composition

Abstract: We investigate by first-principles molecular dynamics the structural properties of liquid GeSe4, i.e., GexSe1-x at x = 0.2. This composition is very close to the so-called stiffness threshold composition, at which dramatic changes in a series of experimental properties occur. The calculated total neutron structure factor is in very good agreement with experiment. The results show that liquid GeSe4 is a good prototype of a chemically ordered network. It consists of GeSe4 tetrahedra that are connected by either shared Se atoms or Se chains.

Cu++ and Li+ interaction with polyethylene oxide by ab initio molecular dynamics

Abstract: Equilibrium positions on the Li+–PEO and on the Cu++–PEO ground state potential energy surfaces have been determined by ab initio molecular dynamics. Our results confirm the previously proposed jump mechanism for ion diffusion in polymer electrolytes. The energy barriers for Li+ and Cu++ ionic diffusion along the PEO chain have been estimated.

Core-level shifts in Si(001)-SiO2 systems: The value of first-principle investigations

Abstract: A first-principle approach allows the study of relaxed structural models for surfaces and interfaces This is a powerful tool for the study of the local bonding The utility of the first-principle theory is substantially extended through the calculation of core-level shifts Such results can be used in conjunction with measured photoemission spectra to make progress in understanding the local atomic structure at interfaces We review the quantitative comparison of the calculated core level shifts with experiment for a series of molecules We then describe results for the Si(001)-SiO2 interface