499 research outputs found
Optoelectronic Properties and Excitons in Hybridized Boron Nitride and Graphene Hexagonal Monolayers
We explain the nature of the electronic band gap and optical absorption
spectrum of Carbon - Boron Nitride (CBN) hybridized monolayers using density
functional theory (DFT), GW and Bethe-Salpeter equation calculations. The CBN
optoelectronic properties result from the overall monolayer bandstructure,
whose quasiparticle states are controlled by the C domain size and lie at
separate energy for C and BN without significant mixing at the band edge, as
confirmed by the presence of strongly bound bright exciton states localized
within the C domains. The resulting absorption spectra show two marked peaks
whose energy and relative intensity vary with composition in agreement with the
experiment, with large compensating quasiparticle and excitonic corrections
compared to DFT calculations. The band gap and the optical absorption are not
regulated by the monolayer composition as customary for bulk semiconductor
alloys and cannot be understood as a superposition of the properties of
bulk-like C and BN domains as recent experiments suggested
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
How strong is the Second Harmonic Generation in single-layer monochalcogenides? A response from first-principles real-time simulations
Second Harmonic Generation (SHG) of single-layer monochalcogenides, such as
GaSe and InSe, has been recently reported [2D Mater. 5 (2018) 025019; J. Am.
Chem. Soc. 2015, 137, 79947997] to be extremely strong with respect to bulk and
multilayer forms. To clarify the origin of this strong SHG signal, we perform
first-principles real-time simulations of linear and non-linear optical
properties of these two-dimensional semiconducting materials. The simulations,
based on ab-initio many-body theory, accurately treat the electron-hole
correlation and capture excitonic effects that are deemed important to
correctly predict the optical properties of such systems. We find indeed that,
as observed for other 2D systems, the SHG intensity is redistributed at
excitonic resonances. The obtained theoretical SHG intensity is an order of
magnitude smaller than that reported at the experimental level. This result is
in substantial agreement with previously published simulations which neglected
the electron-hole correlation, demonstrating that many-body interactions are
not at the origin of the strong SHG measured. We then show that the
experimental data can be reconciled with the theoretical prediction when a
single layer model, rather than a bulk one, is used to extract the SHG
coefficient from the experimental data.Comment: 8 pages, 4 figure
FROM THE ROAD SIGN TO THE MAP: 3D MODELING IN SUPPORT OF THE URBAN AND RURAL ROAD CONDITIONS
Unmanned Aerial Vehicles (UAV), commonly known as a drone, and an Unmanned Aircraft Systems (UAS) have been spreading on a massive scale during the last few years, especially for civilian use. And this situation can have significant repercussions on the ways and purposes with which we make photogrammetry nowadays. In this brief article we take into account the italian road signs as a case study on which to apply the new potential of photogrammetry realized with the aid of drones. Our main purpose is to achieve a specific method which allows the calculation of centimeter precision measurements of solids reconstructed for a mapping of (public and private) road signs which require verification or replacement in urban, peri-urban and rural areas. Our hope is that this new approach to photogrammetry may arise opportunities for dialogue with policy makers especially where the usefulness of mapping could also appear predictive with respect to recurrent issues before they become consolidated
La competencia judicial en los procesos de amparo por acceso a información. Referida a polÃticas públicas de infancia y adolescencia
El presente trabajo aborda la temática de la competencia judicial en los procesos promovidos ante la negativa de la Administración de proporcionar información que es indispensable para comprobar el efectivo goce de derechos de la infancia y la adolescencia; verificar la forma en la cual se le da cumplimiento a principios recogidos en la normativa nacional o internacional que le reconoce los derechos humanos de la infancia y la adolescencia, o cuando dicha información es necesaria para realizar una tarea de contralor de las polÃticas públicas relativas a esos derechos
Ab initio energy loss spectra of Si and Ge nanowires
We report an ab initio investigation of fast electron energy-loss probability in silicon and germanium
nanowires. Computed energy loss spectra are characterized by a strong enhancement of the direct
interband transition peak at low energy, in good agreement with experimental data. Our calculations
predict an important diameter dependence of the bulk volume plasmon peak for very thin wires which is
consistent with the blue shift observed experimentally in thicker wires
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