Showing papers by "Roberto Car published in 1997"

Electronic structure and localized states at carbon nanotube tips

Abstract: Topology related changes in the local density of states near the ends of closed carbon nanotubes are investigated using spatially resolved scanning tunneling spectroscopy and tight binding calculations. Sharp resonant valence band states are observed in the experiment at the tube ends, dominating the valence band edge and filling the band gap. Calculations show that the strength and position of these states with respect to the Fermi level depend sensitively on the relative positions of pentagons and their degree of confinement at the tube ends.

Microscopic growth mechanisms for carbon nanotubes

Abstract: The uncatalyzed edge growth of carbon nanotubes was investigated by first-principles molecular dynamics simulations. At experimental temperatures the open end of single-walled nanotubes closed spontaneously into a graphitic dome, which may explain why these nanotubes do not grow in the absence of transition metal catalysts, On the other hand, chemical bonding between the edges of adjacent coaxial tubes (''lip-lip'' interactions) trapped the end oi a double-walled nanotube in a metastable energy minimum, thus preventing dome closure. These calculations show that this end geometry exhibits a high degree of chemical activity and easily accommodates incoming carbon fragments, supporting a model of growth by chemisorption from the vapor phase.

Theory of composite BxCyNz nanotube heterojunctions

Abstract: Examines the stability and electronic properties of composite B[sub x]C[sub y]N[sub z] nanotube heterojunctions. Use of ab initio density functional calculations and semi-empirical approaches; Advantage of nanotubes; Independence of junction characteristics from nanotube factors.

Structure and Hyperfine Parameters of E 1 ′ Centers in α -Quartz and in Vitreous SiO 2

Abstract: We report a first-principle study of the E(1) defect in alpha-quartz and of the analogous E'(gamma) defect in amorphous SiO2. Our calculation supports the attribution of both these defects to a positively charged oxygen vacancy. The ground-state configuration of these defects is characterized by a large local relaxation of the atomic network, which leads to a localization of the unpaired electron on a Si dangling bond. Using the calculated electronic spin densities, we fully characterize the hyperfine interactions with nearby Si-29. Our results explain well both the strong and the weak features that are observed in the experimental spectra.

Origin of the High-Frequency Doublet in the Vibrational Spectrum of Vitreous SiO2

Abstract: The vibrational properties of amorphous SiO2 were studied within first-principles density functional theory The calculated spectrum is in good agreement with neutron data, showing, in particular, a double peak in the high-frequency region This doublet results from different local modes of the tetrahedral subunits and cannot be ascribed to a longitudinal-optic-transverse-optic (LO-TO) effect This solves a long-standing controversy about the origin of the doublet in neutron spectra A LO-TO splitting is recovered only when the long-wavelength limit is probed, as in optical experiments These findings should be a general feature of tetrahedral AX2 amorphous networks

Nitrogen incorporation at Si(001)-SiO2 interfaces: Relation between N 1s core-level shifts and microscopic structure

Abstract: Using a first-principles approach, we study the incorporation of nitrogen at the Si(001)-SiO2 interface by calculating N Is core-level shifts for several relaxed interface models. The unusually large shift with oxide thickness of the principal peak in photoemission spectra is explained in terms of a single first-neighbor configuration in which the N atom Is always bonded to three Si atoms, both in the interfacial region and further in the oxide. Core-hole relaxation and second nearest neighbor effects concur in yielding larger binding energies in the oxide than at the interface. The calculations do not support the occurrence of N-O bonds at nitrided Si(001)-SiO2 interfaces.

Dynamical charge tensors and infrared spectrum of amorphous sio2

Abstract: Using a model structure consisting of a network of corner sharing tetrahedra, we calculate the infrared spectrum of amorphous SiO2 within a first-principles approach and find good agreement with experiment both for the positions and the intensities of the main peaks. In addition to the vibrational properties, this required the dynamical charge tensors, which were obtained applying the recent quantum polarization theory. The relative intensities in the spectrum depend sensitively on the charge tensors and their anisotropic components are crucial to obtain good agreement with experiment. We find that the Born charges can be correlated to the local structural properties.

First principles study of adsorbed Cu_n (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 Cu_n (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 E_S-A = 0.99 eV and d_S-A = 2.04 Angstroems 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 Cu_3, a linear configuration is preferred to the triangular geometry. As for the tetramer, the most stable adsorbed geometry for Cu_4 is a rhombus. The adsorption energy per Cu atom decreases with increasing the size of the cluster, while the Cu-Cu cohesive energy increases, rapidly becoming more important than the adsorption energy.

A Novel Scheme for Accurate Md Simulations of Large Systems

Abstract: We present a simple and informationally efficient approach to electronic-structure-based simulations of large material science systems. The algorithm is based on a flexible embedding scheme, in which the parameters of a model potential are fitted at run time to some precise information relevant to localised portions of the system. Such information is computed separately on small subsystems by electronic-structure “black box” subprograms, e.g. based on tight-binding and/or ab initio models. The scheme allows to enforce electronic structure precision only when and where needed, and to minimise the computed information within a desired accuracy, which can be systematically controlled. Moreover, it is inherently linear scaling, and highly suitable for modern parallel platforms, including those based on non-uniform processing. The method is demonstrated by performing computations of tight-binding accuracy on solid state systems in the ten thousand atoms size scale.

Microscopic Growth Mechanisms for Carbon Nanotubes

Abstract: The uncatalyzed edge growth of carbon nanotubes was investigated by first-principles molecular dynamics simulations. At experimental temperatures the open end of single-walled nanotubes closed spontaneously into a graphitic dome, which may explain why these nanotubes do not grow in the absence of transition metal catalysts. On the other hand, chemical bonding between the edges of adjacent coaxial tubes (“lip-lip” interactions) trapped the end of a double-walled nanotube in a metastable energy minimum, thus preventing dome closure. These calculations show that this end geometry exhibits a high degree of chemical activity and easily accommodates incoming carbon fragments, supporting a model of growth by chemisorption from the vapor phase.

Amorphous indium phosphide from first principles

Abstract: We report detailed and extensive first-principles molecular-dynamics (MD) simulations of the structure and electronic properties of amorphous InP produced by rapid quenching from the liquid. The structure of the material is found to be strongly ordered chemically, even though there is a significant number of coordination defects and despite the presence of odd-membered rings. We find, as a consequence, that there exists ``wrong bonds'' in the system, in an amount of about 8%; these result from the presence of coordination defects, not of local composition fluctuations, as has been conjectured. The system, in fact, is found to be over-coordinated, which might be the reason for the observed higher density of a-InP compared to c-InP. We have also investigated the possibility of pressure-amorphizing InP. Our calculations indicate that the cost of a transformation of the compressed zinc-blende crystal into an amorphous phase is so large that it is very unlikely that it would take place.