1,720,981 research outputs found
Validity of Weyl fermion picture for transition metals monopnictides TaAs, TaP, NbAs, and NbP from ab initio studies
Full text linkWe investigate electronic and optical properties of the topological Weyl semimetals TaAs, TaP, NbAs and NbP crystallizing in bct geometry by means of the ab initio density functional theory with spin-orbit interaction within the independent-particle approximation. The small energetical overlap of Ta5d or Nb4d derived conduction and valence bands leads to electron and/or hole pockets near the Fermi energy at the 24 Weyl nodes. The nodes give rise to two-(three-)dimensional Dirac cones for the W1 (W2) Weyl type. The band dispersion and occupation near the Weyl nodes determine the infrared optical properties. They are dominated by interband transitions, which lead to a deviation from the expected constant values of the imaginary part of the dielectric function. The resulting polarization anisotropy is also visible in the real part of the optical conductivity, whose lineshape deviates from the expected linearity. The details of the Weyl nodes are discussed in relation to recent ARPES results for TaAs and NbP, and compared with measured optical spectra for TaAs. The spectral features of the anisotropic and tilted Weyl fermions are restricted to low excitation energies above absorption onsets due to the band occupation
Many-body study of the photoisomerization of the minimal model of the retinal protonated Schiff base
No full textWe investigate the optical properties of the tZt-penta-3,5-dieniminium cation, a simplified model for the protonated Schiff base of 11-cis retinal in rhodopsin, along the isomerization pathway by ab-initio calculations based on Many-Body Perturbation Theory using the GW method and the Bethe–Salpeter equation.
Our calculations are carried out on a few significant CASSCF geometrical configurations of the isomerization minimal energy path taken from the literature. Our excitation energies are qualitatively in agreement with previous Quantum Monte Carlo and post-Hartree–Fock calculations. We also employ TDDFT
based methods, and investigate the outcome of using different approximations and several exchange–
correlation functionals
Structural, electronic and optical properties of the two isomers of Si(111)2x1
No full textThe Si(111)2x1 is one of the most studied surfaces. Its reconstruction is described by the Pandey model with a buckling of the topmost atoms. With relation to the sign of the buckling, there are two slightly different geometric structures (isomers), conventionally named positive buckling and negative buckling. STM measurements suggest that the positive buckling isomer is the stable configuration at room temperature, but a recent work, involving STS measurements, has shown the coexistence of both the isomers, at very low temperature, for highly n-doped Si(111)2x1 specimens. There is hence the necessity to deepen the study of the negative buckling isomer, almost completely neglected in literature especially for what concerns its optical properties. In this work we have studied the structural, electronic and optical properties of both the isomers of Si(111)2x1 within the Density Functional Theory in the Local Density Approximation. Our results show that the response to light of the two isomers is sizearly different; Reflectance Anisotropy Spectroscopy would be the right experimental tool to investigate the coexistence of the two isomers
Si(111)2 x 1 surface isomers: DFT investigations on stability and doping effects
No full textThe relative energy stability between Si(111)2 x 1 positive and negative buckling is still a puzzle. Both isomers possess comparable energies in Density Functional Theory (DFT) calculations and, hence, should occur with the same probability. This is in contrast to experimental findings, which show that the positive buckling is the observed structure at low and room temperature. Here we investigate the role of the exchange-correlation (xc) approximation in DFT functionals. We find that the particular choice of the xc functional does not change the relative stability between the Si(111)2 x 1 isomers. Moreover, we investigate the effect of donor atoms on ground-state and optical properties of both isomers, highlighting the differences with respect to the undoped case for several possible positions of the phosphorus (P) donor atoms. Considering the most stable position of P, relative stability between the isomers is not strongly affected by doping whereas geometric parameters are different with respect to the no doping case. The optical response bears some resemblance with the undoped case but the intensity of reflectance anisotropy peak is decreased for both isomers. (C) 2013 Elsevier B.V. All rights reserved
Many-body study of the photoisomerization of the minimal model of the retinal protonated Schiff base
No full textWe investigate the optical properties of the tZt-penta-3,5-dieniminium cation, a simplified model for the protonated Schiff base of 11-cis retinal in rhodopsin, along the isomerization pathway by ab-initio calculations based on Many-Body Perturbation Theory using the GW method and the Bethe-Salpeter equation. Our calculations are carried out on a few significant CASSCF geometrical configurations of the isomerization minimal energy path taken from the literature. Our excitation energies are qualitatively in agreement with previous Quantum Monte Carlo and post-Hartree-Fock calculations. We also employ TDDFT based methods, and investigate the outcome of using different approximations and several exchange-correlation functionals. (C) 2011 Elsevier B.V. All rights reserved
Many-Body Perturbation Theory Extended to the Quantum Mechanics/Molecular Mechanics Approach: Application to Indole in Water Solution
No full textOptical properties of aromatic chromophores are used to probe complex biological processes, yet how the environment tunes their optical properties is far from being fully understood. Here we present a method to calculate such properties on large-scale systems, like biologically relevant molecules in aqueous solution. Our approach is based on many-body perturbation theory combined with a quantum mechanics/molecular mechanics (QM/MM) approach. We include quasiparticle and excitonic effects for the calculation of optical absorption spectra in a QM/MM scheme. We apply this scheme, together with the well-established TDDFT approach, to indole in water solution. Our calculations show that the solvent induces a red shift in the main spectral peak of indole, in quantitative agreement with the experiments, and they point to the relevance of both the electrostatic and geometrical origin of the shift
Non-destructive monitoring of molecular modifications in the restoration of works of art on paper ⋆ : Application of theoretical and experimental optical spectroscopy
No full textThe discoloration of paper, due to the development of oxidized groups acting as chromophores in its chief component, cellulose, is responsible for severe visual degradation of works of art on paper. By adopting a diagnostic method based on in situ non-invasive optical reflectance spectroscopy and time-dependent density functional theory ab initio calculations, it is possible to describe and quantify the chromophores in cellulose fibers in a non-destructive way. In order to recover the absorption coefficient of cellulose fibers from reflectance measurements a specific approach based on the Kubelka-Munk theory was applied. The concentrations of carbonyl groups acting as chromophores were obtained by fitting the experimental optical absorption spectra to those simulated by using ab initio calculations. This method was applied for monitoring the restoration interventions of two great format engravings Le Nozze di Psiche and Gesù Cristo e l’adultera by Diana Scultori (1547-1612), as well as a contemporary artwork by Renato Guttuso, Bozzetto per Crocifissione (dated 1940). All artefacts were affected by chromatic deterioration due to a strong oxidation of the paper. Results quantified the decreasing of chromophores concentration after washing and reducing treatments evidencing the different behavior of the carbonyl groups as a function of the specific protocol performed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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