Comment to "Imaging the atomic orbitals of carbon atomic chains with field-emission electron microscopy"

The observation of a stable doublet pattern in the field-emission electron microscopy of a linear atomic chain requires a stable mechanism breaking the axial symmetry, which is not identified correctly by Mikhailovskij et al. [Phys. Rev. B 80, 165404 (2009)]. Using microscopic calculations, we attribute the observed pattern to the symmetry breaking produced by the ligand where the chain is attached, plus carbon pi-bonding alternation.Comment: 2 pages 1 fig, PRB commen

Surface versus crystal-termination effects in the optical properties of surfaces

We prove, by realistic microscopic calculations within the sp^3s^* Tight Binding method for GaAs (110) and (100), that the surface optical properties are not influenced by long-range crystal termination effects, and hence that they can be consistently studied considering slabs of limited thickness (20 - 30 \AA). The origin of derivative-like and bulk-like lineshapes in Reflection Anisotropy Spectra is also discussed, analyzing the effects arising from possible surface-induced reduction, broadening, and shifting of the bulk spectrum near the surface.Comment: 10 figures. In press in Physical Review B, scheduled 15 Aug. 199

Optical Properties of BN in the cubic and in the layered hexagonal phases

Linear optical functions of cubic and hexagonal BN have been studied within first principles DFT-LDA theory. Calculated energy-loss functions compare well with experiments and previous theoretical results both for h-BN and for c-BN. Discrepancies arise between theoretical results and experiments in the imaginary part of the dielectric function for c-BN. Possible explanation to this mismatch are proposed and evaluated; lattice constant variations, h-BN contamination in c-BN samples and self-energy effects.Comment: RevTex 42 pages, 16 postscript figures embedde

Ab initio calculation of excitonic effects in the optical spectra of semiconductors

An ab initio approach to the calculation of excitonic effects in the optical absorption spectra of semiconductors and insulators is formulated. It starts from a quasiparticle bandstructure calculation and is based on the relevant Bethe--Salpeter equation. An application to bulk silicon shows a substantial improvement with respect to previous calculations in the description of the experimental spectrum, for both peak positions and lineshape.Comment: 4 pages, 1 figur

Ab-initio calculation of the vibrational modes of SiH4, H2SiO, Si10H16, and Si10H14O

We have studied the normal modes of hydrogenated and oxidized silicon nanocrystals, namely SiH4 (silan), H2SiO (silanon), Si10H16 and Si10H14O. The small clusters (SiH4 and H2SiO) have been used for convergence tests and their bondlengths and frequencies have been compared with experimental and theoretical reference data. For the large clusters (Si10H16 and Si10H14O) we have investigated the vibrational density of states where we have identified the oxygen-related spectral features. The vibrational modes have been also analyzed with respect to the displacement patterns. The calculations have been carried out within the density-functional and density-functional perturbation theory using the local-density approximation.Comment: 9 pages, 7 figure

Rappresentanza o/e partecipazione: formazione della volontà «per» o/e «per mezzo di» altri: nei rapporti individuali e collettivi, di diritto privato e pubblico, romano e positivo

La noción jurídica de “representación” nace y tiene sentido come parte de una específica forma del binomio constituido por la concepción y el régimen unitarios de la pluralidad de hombres. Esta ‘forma específica’ es “persona jurídica y representación”. La “abstracción” de la pluralidad de hombres, conseguida con la noción de “persona jurídica”, es instrumental para su “substitición” volitiva (es decir, substitución en el ejercicio de su poder) por parte del “uno” o de los “pocos”, obtenida con la noción de “representación”. El ‘binomio’ recibe esta ‘forma específica’ en aplicación de la lógica organizativa feudal: inicialmente durante la edad media avanzada y luego durante la época moderna y la contemporánea, en una progresión científica, que continúa. La Pandectística ha reforzado el binomio feudal a nivel de verdadero postulado de razón, sobrescribiendolo en la lógica jurídica romana, que se “olvida”. Al refuerzo contribuye la aplicación de la noción de representación también a la categoría de los actos de volición individual con intermediarios (mandatum – procura). El resultado se resume con la fórmula “actuar por otros”, la cual expresa el protagonismo del “representante” (sujeto de la acción) y la eclipse del “representado” (“otros”). A pesar de las voces críticas (en particular contra la “representación política”, más expuesta a ellas como “hierro de lanza” de la parte operativa del binomio feudal), este sigue apareciendo sin alternativas, justo en virtud de su superposición al Derecho romano durante el siglo XIX; superposición que, en el curso del siglo XX no se niega sino que se historiza. Por lo tanto, parece necesaria y se propone una relectura de las fuentes romanas, la que, contra la lógica del “actuar por otros”, pueda recuperar la lógica del “actuar por medio de otros”, propia del “dominus negotii”; y, contra el binomio feudal de la “desmaterialización” de la pluralidad de hombres en la “persona jurídica” y de su consiguiente “substitución” volitiva con el “representante”, pueda recuperar el binomio republicano del “cuerpo societario concreto” y de su consecuente “participación” en la volición, a la que “colabora” un “encargado subalterno”. La nozione giuridica di “rappresentanza” nasce e ha senso come parte di una specifica forma del binomio costituito dalla concezione e dal regime unitari della pluralità di uomini. Tale ‘specifica forma’ è “persona giuridica e rappresentanza”. La “astrazione” della pluralità di uomini, ottenuta con la nozione di “persona giuridica”, è strumentale alla loro “sostituzione” volitiva (alla sostituzione cioè nell’esercizio del loro potere) da parte dell’ “uno” o dei “pochi”, ottenuta con la nozione di “rappresentanza”. Tale ‘specifica forma’ è data al ‘binomio’ in applicazione della logica organizzativa feudale; inizialmente durante la epoca medievale avanzata e quindi durante le epoche moderna e contemporanea, con una progressione scientifica tuttora in corso. La Pandettistica ha potenziato il ‘binomio feudale’ a postulato di ragione, sopra-scrivendolo sulla logica giuridica romana, la quale è “dimenticata”. Al ‘potenziamento’ concorre la applicazione della nozione di “rappresentanza” anche alla categoria degli atti di volizione individuale per mezzo di intermediari (mandatum – procura). Il risultato è sintetizzato con la formula “agire per altri”, la quale esprime il protagonismo del “rappresentante” (soggetto della azione) e la eclisse del “rappresentato” (“altri”). Nonostante le voci critiche (in particolare contro la “rappresentanza politica”, più esposta ad esse in quanto ‘ferro di lancia’ della parte operativa del binomio feudale) questo continua ad apparire privo di alternative, proprio in forza della sua sovrapposizione ’800esca sul Diritto romano, la quale durante il ’900 è stata storicizzata, non negata. Pertanto, appare necessaria e si propone una rilettura delle fonti romane, la quale, contro la logica dell’ “agire per altri”, recuperi la logica dell’ “agire per mezzo di altri”, propria del “dominus negotii”, e, contro il binomio feudale della “smaterializzazione” della pluralità di uomini nella “persona giuridica” e della loro conseguente “sostituzione” volitiva con il “rappresentante”, recuperi il binomio repubblicano del concreto corpo societario e della sua conseguente “partecipazione” alla volizione, cui “coopera” un preposto in posizione subordinata

Anisotropic Effects of Oxygen Vacancies on Electrochromic Properties and Conductivity of γ\gamma-Monoclinic WO3_3

Tungsten trioxide (WO$_3$) is a paradigmatic electrochromic material, whose peculiar optical properties in the presence of oxygen vacancies or intercalated alkali atoms have been observed and investigated for a long time. In this paper we propose a rationalization of experiments based on first-principles calculations of optical and electrical properties of oxygen deficient (reduced) WO$_3$. Our approach is based on a parameter-free dielectric-dependent hybrid density functional methodology, used in combination with the charge transition levels formalism, for studying excitation mechanisms in the presence of defects. Our results indicate that oxygen vacancies lead to a different physics in $\gamma$-monoclinic WO$_3$, depending on the orientation of the W-O-W chain where the vacancy is created, thus evidencing strong anisotropic effects rooted in the peculiar structural properties of the original nondefective monoclinic cell. Different types of oxygen vacancies can hence be classified on the basis of the calculated ground state properties, electronic structure, and excitation/emission energies, giving a satisfactory explanation to a range of experimental observations made on oxygen deficient WO$_3$.Comment: Accepted for publication in J. Phys. Chem.

Defect calculations in semiconductors through a dielectric-dependent hybrid DFT functional: the case of oxygen vacancies in metal oxides

We investigate the behavior of oxygen vacancies in three different metal-oxide semiconductors (rutile and anatase TiO2, monoclinic WO3, and tetragonal ZrO2) using a recently proposed hybrid density-functional method in which the fraction of exact exchange is material-dependent but obtained ab initio in a self-consistent scheme. In particular, we calculate charge-transition levels relative to the oxygen-vacancy defect and compare computed optical and thermal excitation/emission energies with the available experimental results, shedding light on the underlying excitation mechanisms and related materials properties. We find that this novel approach is able to reproduce not only ground-state properties and band structures of perfect bulk oxide materials, but also provides results consistent with the optical and electrical behavior observed in the corresponding substoichiometric defective systems.Comment: Accepted for publication in J. Chem. Phy

Ab-initio self-energy corrections in systems with metallic screening

The calculation of self-energy corrections to the electron bands of a metal requires the evaluation of the intraband contribution to the polarizability in the small-q limit. When neglected, as in standard GW codes for semiconductors and insulators, a spurious gap opens at the Fermi energy. Systematic methods to include intraband contributions to the polarizability exist, but require a computationally intensive Fermi-surface integration. We propose a numerically cheap and stable method, based on a fit of the power expansion of the polarizability in the small-q region. We test it on the homogeneous electron gas and on real metals such as sodium and aluminum.Comment: revtex, 14 pages including 5 eps figures v2: few fixe