4,220 research outputs found

    Group theory for structural analysis and lattice vibrations in phosphorene systems

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    Group theory analysis for two-dimensional elemental systems related to phosphorene is presented, including (i) graphene, silicene, germanene and stanene, (ii) dependence on the number of layers and (iii) two stacking arrangements. Departing from the most symmetric D6h1D_{6h}^{1} graphene space group, the structures are found to have a group-subgroup relation, and analysis of the irreducible representations of their lattice vibrations makes it possible to distinguish between the different allotropes. The analysis can be used to study the effect of strain, to understand structural phase transitions, to characterize the number of layers, crystallographic orientation and nonlinear phenomena.Comment: 24 pages, 3 figure

    Modelling and analysis of time dependent processes in a chemically reactive mixture

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    In this paper, we study the propagation of sound waves and the dynamics of local wave disturbances induced by spontaneous internal fluctuations in a reactive mixture. We consider a non-diffusive, non-heat conducting and non-viscous mixture described by an Eulerian set of evolution equations. The model is derived from the kinetic theory in a hydrodynamic regime of a fast chemical reaction. The reactive source terms are explicitly computed from the kinetic theory and are built in themodel in a proper way. For both time-dependent problems, we first derive the appropriate dispersion relation, which retains the main effects of the chemical process, and then investigate the influence of the chemical reaction on the properties of interest in the problems studied here. We complete our study by developing a rather detailed analysis using the Hydrogen–Chlorine system as reference. Several numerical computations are included illustrating the behavior of the phase velocity and attenuation coefficient in a low-frequency regime and describing the spectrum of the eigenmodes in the small wavenumber limit.The paper is partially supported by the Research Centre of Mathematics of the University of Minho, with the Portuguese Funds from the Foundation for Science and Technology (FCT) through the Project UID/MAT/00013/2013. We wish to thank the anonymous Referees for their valuable comments and suggestions that helped us to improve the paper.info:eu-repo/semantics/publishedVersio

    Group Theory analysis of phonons in two-dimensional Transition Metal Dichalcogenides

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    Transition metal dichalcogenides (TMDCs) have emerged as a new two dimensional materials field since the monolayer and few-layer limits show different properties when compared to each other and to their respective bulk materials. For example, in some cases when the bulk material is exfoliated down to a monolayer, an indirect-to-direct band gap in the visible range is observed. The number of layers NN (NN even or odd) drives changes in space group symmetry that are reflected in the optical properties. The understanding of the space group symmetry as a function of the number of layers is therefore important for the correct interpretation of the experimental data. Here we present a thorough group theory study of the symmetry aspects relevant to optical and spectroscopic analysis, for the most common polytypes of TMDCs, i.e. 2Ha2Ha, 2Hc2Hc and 1T1T, as a function of the number of layers. Real space symmetries, the group of the wave vectors, the relevance of inversion symmetry, irreducible representations of the vibrational modes, optical selection rules and Raman tensors are discussed.Comment: 32 pages, 4 figure

    Procedure for Sr and Nd isotope analysis in the terrigenous component of marine sediments

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    Neste trabalho, apresenta-se um protocolo analítico para a obtenção das razões isotópicas 87Sr/86Sr e 143Nd/144Nd na fracção detrítica de sedimentos marinhos. Dado ser comum estes materiais apresentarem uma fracção biogénica carbonatada significativa, foram testadas diversas metodologias com o objectivo de extrair selectivamente os carbonatos sem comprometer a composição da componente terrígena. O ataque com ácido acético 5N durante 12h, à temperatura ambiente, revelou ser eficaz na remoção dos carbonatos e, simultaneamente, preservar fases detríticas que incorporam concentrações significativas de lantanídeos. A aplicabilidade do protocolo desenvolvido é ilustrado com a análise isotópica de amostras de um core colhido no talude continental da Galiza. Os resultados obtidos, em ambos os sistemas de decaimento radioactivo, para além de serem consistentes com a assinatura isotópica de materiais resultantes da erosão de crosta continental, também permitem detectar, através do decréscimo dos valores de εNd em determinados níveis do core, alguns episódios (eventos de Heinrich) em que a fonte crustal terá sido bastante diferente da habitual.In this paper, an experimental procedure for measuring 87Sr/86Sr and 143Nd/144Nd ratios in the detrital fraction of marine sediments is presented. Since these sediments commonly include a significant biogenic carbonate fraction, several methodologies were tested in order to leach the carbonates without compromising the composition of the terrigenous component. The extraction with 5N acetic acid for 12 hours, at room temperature, reveals (a) to be effective in the removal of carbonates, and (b) to preserve detrital phases that incorporate important amounts of REE. The applicability of this chemical procedure is illustrated with analyses done on samples from a core collected in the Galician continental slope. The results for both radiogenic decay systems, on one hand, are consistent with the isotope signature of material resulting from the erosion of continental crust and, one the other hand, allow the identification of several episodes of modification of the source areas involved

    Informações meteorológicas para pesquisa e planejamento agrícola, referentes ao município de Santo Antonio de Goiás, GO, 2007.

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    Os dados meteorológicos aqui apresentados, permitirão aos usuários do setor agrícola, obter informações sobre as variações dos elementos climáticos do município de Santo Antônio de Goiás, GO, a partir de observações contínuas das variáveis climáticas.bitstream/CNPAF-2009-09/27607/1/doc_221.pd
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