142 research outputs found
Hydrogenation of CO on a silica surface: an embedded cluster approach
The sequential addition of H atoms to CO adsorbed on a siliceous edingtonite surface is studied with an embedded cluster approach, using density functional theory for the quantum mechanical (QM) cluster and a molecular force field for the molecular mechanical (MM) cluster. With this setup, calculated QM/MM adsorption energies are in agreement with previous calculations employing periodic boundary conditions. The catalytic effect of the siliceous edingtonite (100) surface on CO hydrogenation is assessed because of its relevance to astrochemistry. While adsorption of CO on a silanol group on the hydroxylated surface did not reduce the activation energy for the reaction with a H atom, a negatively charged defect on the surface is found to reduce the gas phase barriers for the hydrogenation of both CO and H2C = O. The embedded cluster approach is shown to be a useful and flexible tool for studying reactions on (semi-)ionic surfaces and specific defects thereon. The methodology presented here could easily be applied to study reactions on silica surfaces that are of relevance to other scientific areas, such as biotoxicity of silica dust and geochemistry
Ab initio study of reflectance anisotropy spectra of a sub-monolayer oxidized Si(100) surface
The effects of oxygen adsorption on the reflectance anisotropy spectrum (RAS)
of reconstructed Si(100):O surfaces at sub-monolayer coverage (first stages of
oxidation) have been studied by an ab initio DFT-LDA scheme within a
plane-wave, norm-conserving pseudopotential approach. Dangling bonds and the
main features of the characteristic RAS of the clean Si(100) surface are mostly
preserved after oxidation of 50% of the surface dimers, with some visible
changes: a small red shift of the first peak, and the appearance of a distinct
spectral structure at about 1.5 eV. The electronic transitions involved in the
latter have been analyzed through state-by-state and layer-by-layer
decompositions of the RAS. We suggest that new interplay between present
theoretical results and reflectance anisotropy spectroscopy experiments could
lead to further clarification of structural and kinetic details of the Si(100)
oxidation process in the sub-monolayer range.Comment: 21 pages, 8 figures. To be published in Physical Rev.
Ab initio studies of phonon softening and high pressure phase transitions of alpha-quartz SiO2
Density functional perturbation theory calculations of alpha-quartz using
extended norm conserving pseudopotentials have been used to study the elastic
properties and phonon dispersion relations along various high symmetry
directions as a function of bulk, uniaxial and non-hydrostatic pressure. The
computed equation of state, elastic constants and phonon frequencies are found
to be in good agreement with available experimental data. A zone boundary (1/3,
1/3, 0) K-point phonon mode becomes soft for pressures above P=32 GPa. Around
the same pressure, studies of the Born stability criteria reveal that the
structure is mechanically unstable. The phonon and elastic softening are
related to the high pressure phase transitions and amorphization of quartz and
these studies suggest that the mean transition pressure is lowered under
non-hydrostatic conditions. Application of uniaxial pressure, results in a
post-quartz crystalline monoclinic C2 structural transition in the vicinity of
the K-point instability. This structure, intermediate between quartz and
stishovite has two-thirds of the silicon atoms in octahedral coordination while
the remaining silicon atoms remain tetrahedrally coordinated. This novel
monoclinic C2 polymorph of silica, which is found to be metastable under
ambient conditions, is possibly one of the several competing dense forms of
silica containing octahedrally coordinated silicon. The possible role of high
pressure ferroelastic phases in causing pressure induced amorphization in
silica are discussed.Comment: 17 pages, 8 figs., 8 Table
Kaplan-Narayanan-Neuberger lattice fermions pass a perturbative test
We test perturbatively a recent scheme for implementing chiral fermions on
the lattice, proposed by Kaplan and modified by Narayanan and Neuberger, using
as our testing ground the chiral Schwinger model. The scheme is found to
reproduce the desired form of the effective action, whose real part is gauge
invariant and whose imaginary part gives the correct anomaly in the continuum
limit, once technical problems relating to the necessary infinite extent of the
extra dimension are properly addressed. The indications from this study are
that the Kaplan--Narayanan--Neuberger (KNN) scheme has a good chance at being a
correct lattice regularization of chiral gauge theories.Comment: LaTeX 18 pages, 3 figure
Squeezing based on nondegenerate frequency doubling internal to a realistic laser
We investigate theoretically the quantum fluctuations of the fundamental
field in the output of a nondegenerate second harmonic generation process
occuring inside a laser cavity. Due to the nondegenerate character of the
nonlinear medium, a field orthogonal to the laser field is for some operating
conditions indepedent of the fluctuations produced by the laser medium. We show
that this fact may lead to perfect squeezing for a certain polarization mode of
the fundamental field. The experimental feasibility of the system is also
discussed.Comment: 6 pages, 5 figure
Sistemas de manejo do solo e de culturas em latossolo vermelho e sua relação com a produtividade de soja.
O sistema plantio direto (SPD) é o sistema de manejo mais importante para a sustentabilidade dos agroecossitemas. Porém persistem dúvidas relacionadas a formação de camadas compactadas em SPD e os efeitos residuais da escarificação do solo. Objetivou-se avaliar o efeito da rotação de culturas, da escarificação periódica e do tempo de adoção do SPD em Latossolo Vermelho Distroférrico na produtividade de grãos de soja. O delineamento experimental foi de blocos ao acaso, em esquema fatorial 5x2 (manejos do solo x modelos de produção), com quatro repetições. Os sistemas de manejo do solo foram: (i) sistema de preparo convencional (SPC); (ii) SPD escarificado a cada ano (SPDE1); (iii) SPD escarificado a cada três anos (SPDE3); (iv) SPD contínuo por 11 anos (SPDC11); (v) SPD contínuo por 24 anos (SPDC24). Os modelos de produção foram: (i) rotação e; (ii) sucessão de culturas. A produtividade de grãos da soja não foi influenciada pelos modelos de produção. A produtividade de grãos de soja no SPDC24 foi maior do que no SPC e não se observou diferenças produtivas de soja entre os tempos de adoção do SPD. O SPDE1 e SPDE3 não favoreceram aumentos de produtividade de grãos de soja em relação ao SPD. A escarificação do solo foi ineficiente para melhorias de produção, mostrando-se uma prática desnecessária. O aumento do tempo de adoção do SPD favorece aumentos de produtividade de grãos da soja em comparação com o SPC, demonstrando ser uma importante prática de manejo a ser adotada em sistemas agrícolas de produção
Photoelasticity of crystalline and amorphous silica from first principles
Based on density-functional perturbation theory we have computed from first
principles the photoelastic tensor of few crystalline phases of silica at
normal conditions and high pressure (quartz, -cristobalite,
-cristobalite) and of models of amorphous silica (containig up to 162
atoms), obtained by quenching from the melt in combined classical and
Car-Parrinello molecular dynamics simulations. The computational framework has
also been checked on the photoelastic tensor of crystalline silicon and MgO as
prototypes of covalent and ionic systems. The agreement with available
experimental data is good.
A phenomenological model suitable to describe the photoelastic properties of
different silica polymorphs is devised by fitting on the ab-initio data.Comment: ten figure
Numerical atomic orbitals for linear scaling
The performance of basis sets made of numerical atomic orbitals is explored
in density-functional calculations of solids and molecules. With the aim of
optimizing basis quality while maintaining strict localization of the orbitals,
as needed for linear-scaling calculations, several schemes have been tried. The
best performance is obtained for the basis sets generated according to a new
scheme presented here, a flexibilization of previous proposals. The basis sets
are tested versus converged plane-wave calculations on a significant variety of
systems, including covalent, ionic and metallic. Satisfactory convergence
(deviations significantly smaller than the accuracy of the underlying theory)
is obtained for reasonably small basis sizes, with a clear improvement over
previous schemes. The transferability of the obtained basis sets is tested in
several cases and it is found to be satisfactory as well.Comment: 9 pages with 2 encapsulated postscript figures, submitted to Phys.
Rev.
Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4
An extensive theoretical study is performed for wide bandgap crystalline
oxides and nitrides, namely, SiO_{2}, GeO_{2}, Al_{2}O_{3}, Si_{3}N_{4}, and
Ge_{3}N_{4}. Their important polymorphs are considered which are for SiO_{2}:
-quartz, - and -cristobalite and stishovite, for
GeO_{2}: -quartz, and rutile, for Al_{2}O_{3}: -phase, for
Si_{3}N_{4} and Ge_{3}N_{4}: - and -phases. This work
constitutes a comprehensive account of both electronic structure and the
elastic properties of these important insulating oxides and nitrides obtained
with high accuracy based on density functional theory within the local density
approximation. Two different norm-conserving \textit{ab initio}
pseudopotentials have been tested which agree in all respects with the only
exception arising for the elastic properties of rutile GeO_{2}. The agreement
with experimental values, when available, are seen to be highly satisfactory.
The uniformity and the well convergence of this approach enables an unbiased
assessment of important physical parameters within each material and among
different insulating oxide and nitrides. The computed static electric
susceptibilities are observed to display a strong correlation with their mass
densities. There is a marked discrepancy between the considered oxides and
nitrides with the latter having sudden increase of density of states away from
the respective band edges. This is expected to give rise to excessive carrier
scattering which can practically preclude bulk impact ionization process in
Si_{3}N_{4} and Ge_{3}N_{4}.Comment: Published version, 10 pages, 8 figure
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