Search

With the ever-growing interest in quantum computing, understanding the behaviour of excitons in monolayer quantum dots has become a topic of great relevance. In this paper, we consider a Wannier exciton confined in a triangular quantum dot of hexagonal Boron Nitride. We begin by outlining the adequate basis functions to describe a particle in a triangular enclosure, analyzing their degeneracy and symmetries. Afterwards, we discuss the excitonic hamiltonian inside the quantum dot and study the influence of the quantum dot dimensions on the excitonic states., Comment: 5 figures

The use of graphene in surface plasmon resonance sensors, covering a metallic (plasmonic) film, has a number of demonstrated advantages, such as protecting the film against corrosion/oxidation and facilitating the introduction of functional groups for selective sensing. Recently, a number of works have claimed that few-layer graphene can also increase the sensitivity of the sensor. However, graphene was treated as an isotropic thin film, with an out-of-plane refractive index that is identical to the in-plane index. Here, we critically examine the role of single and few layers of graphene in the sensitivity enhancement of surface plasmon resonance sensors. Graphene is introduced over the metallic film via three different descriptions: as an atomic-thick two-dimensional sheet, as a thin effective isotropic material (same conductivity in the three coordinate directions), and as an non-isotropic layer (different conductivity in the perpendicular direction to the two-dimensional plane). We find that only the isotropic layer model, which is known to be incorrect for the optical modeling of graphene, provides sizable sensitivity increases, while the other, more accurate, models lead to a negligible contribution to the sensitivity.

Biased bilayer graphene, with its easily tunable band gap, presents itself as the ideal system to explore the excitonic effect in graphene based systems. In this paper we study the excitonic optical response of such a system by combining a tight binding model with the solution of the Bethe-Salpeter equation, the latter being solved in a semi-analytical manner, requiring a single numerical quadrature, thus allowing for a transparent calculation. With our approach we start by analytically obtaining the optical selection rules, followed by the computation of the absorption spectrum for the case of a biased bilayer encapsulated in hexagonal boron nitride, a system which has been the subject of a recent experimental study. An excellent agreement is seen when we compare our theoretical prediction with the experimental data., 5 fugues, one comparing experimental data and theory

In this paper we consider a honeycomb antiferromagnet subject to an external laser field. Obtaining a time-independent effective Hamiltonian, we find that the external laser renormalizes the exchange interaction between the in-plane components of the spin-operators, and induces a synthetic Dzyaloshinskii-Moria interaction (DMI) between second neighbors. The former allows the control of the magnon dispersion's bandwidth and the latter breaks time-reversal symmetry inducing non-reciprocity in momentum space. The eigen-excitations of the system correspond to squeezed magnons whose squeezing parameters depend on the properties of the laser. When studying how these spin excitations couple with cavity photons, we obtain a coupling strength which can be enhanced by an order of magnitude via careful tuning of the laser's intensity, when compared to the case where the laser is absent. The transmission plots through the cavity are presented, allowing the mapping of the magnons' dispersion relation., Comment: 9 pages, 5 figures, supplemental information available from the authors

Trilayer graphene is receiving an increasing level of attention due to its stacking--dependent magnetoelectric and optoelectric properties, and its more robust ferromagnetism relative to monolayer and bilayer variants. Additionally, rhombohedral stacked trilayer graphene presents the possibility of easily opening a gap via either an external electric field perpendicular to the layers, or via the application of external strain. In this paper, we consider an external electric field to open a bandgap in rhombohedral trilayer graphene and study the excitonic optical response of the system. This is done via the combination of a tight binding model with the Bethe--Salpeter equation, solved semi--analytically and requiring only a simple numerical quadrature. We then discuss the valley--dependent optical selection rules, followed by the computation of the excitonic linear optical conductivity for the case of a rhombohedral graphene trilayer encapsulated in hexagonal boron nitride. The tunability of the excitonic resonances via an external field is also discussed, together with the increasing localization of the excitonic states as the field increases., Comment: 17 pages, 6 figures

Scattering of excitons by free carriers is a phenomena which is especially important when considering moderately to heavily doped semiconductors in low temperature experiments, where the interaction of excitons with acoustic and optical phonons is reduced. In this paper, we consider the scattering of excitons by free carriers in monolayer hexagonal Boron Nitride encapsulated by a dielectric medium. We describe the excitonic states by variational wave functions, modeling the electrostatic interaction via the Rytova--Keldysh potential. Making the distinction between elastic and inelastic scattering, the relevance of each transition between excitonic states is also considered. Finally, we discuss the contribution of free carrier scattering to the excitonic linewidth, analyzing both its temperature and carrier density dependence., 16 pages, 5 figures

N/A

This paper presents an improvement of the first-order Generalised Beam Theory (GBT) formulation proposed in [1] , which was developed for naturally curved thin-walled members with deformable cross-section and whose undeformed axis is a circular arc with no pre-twist. In this paper, the restrictions on the cross-section shape are removed (in the previous paper only rather simple cross-sections were dealt with) by proposing and discussing a novel and systematic procedure to obtain the cross-section deformation modes for arbitrary flat-walled cross-sections (open, closed or “mixed”). The proposed procedure retains the nomenclature of the deformation mode subsets defined in [2] , [3] , even though the kinematic constraints employed to subdivide the modes are much more complex than for prismatic members. A set of representative illustrative examples is presented, involving complex local-distortional-global deformation, to show the efficiency of the proposed procedure when used together with a standard displacement-based GBT finite element. It is demonstrated that extremely accurate results are obtained with rather few DOFs and that the GBT modal solution provides in-depth insight into the structural behaviour of naturally curved members.

This paper presents an extension of the linear Generalized Beam Theory (GBT) formulation for members with circular axis, previously developed by the authors Peres et al., (2016), Peres et al., (2018), to the linear dynamic case. The proposed formulation makes it possible to calculate natural frequencies (and associated vibration mode shapes) and time-history responses of members with circular axis and arbitrary flat-walled cross-sections, undergoing complex global–distortional–local deformation. The remarkable modal decomposition features of GBT, stemming from the fact that the kinematic description of the beam is based on a superposition of structurally meaningful cross-section deformation modes, render the finite element implementation of the proposed formulation extremely accurate and computationally efficient, as demonstrated in the illustrative numerical examples presented throughout the paper. Both standard (displacement-based) and mixed (strain–displacement-based) finite elements are implemented and it is concluded that the predictive capacity of the latter is superior, as it is insensitive to the various forms of membrane locking appearing in curved members, even though it does not involve additional DOFs (the strain DOFs are eliminated from the element stiffness matrix).

This paper presents a geometrically exact beam finite element for curved and twisted thin-walled bars undergoing large displacements and finite rotations, combined with cross-section in-plane and out-of-plane (warping) deformation. The underlying formulation is based on a previous work by the authors Goncalves et al. (2010), which is now improved and extended for the pre-curved/twisted case. Cross-section deformation is allowed for by adding kinematic DOFs, the so-called “cross-section deformation modes”, which are obtained using Generalised Beam Theory . All expressions required to implement the proposed finite element are derived. For validation purposes, several numerical examples are presented and discussed. In each case, the results obtained with the proposed element and refined shell finite element models are compared, and an excellent agreement is found. In particular, it is demonstrated that the proposed element predicts very accurately the spatial behaviour of curved beams undergoing complex coupling between large displacements, finite rotations and cross-section deformation.

This paper presents a first-order Generalised Beam Theory (GBT) formulation for naturally curved thin-walled members with deformable cross-section, whose undeformed axis is a circular arc with no pre-twist. First, the strain-displacement relations for naturally curved thin-walled members are derived and it is shown how the classic GBT assumptions concerning the strains can be incorporated, namely: (i) Kirchhoff's thin-plate assumption, (ii) Vlasov's null membrane shear strain assumption and (iii) the null membrane transverse extension assumption. The equilibrium equations are obtained in terms of GBT modal matrices and stress resultants. It is demonstrated that, for the so-called “rigid-body” deformation modes (extension, bending and torsion), the GBT equations coincide with those of the Winkler (in-plane case) and Vlasov (out-of-plane case) theories. A standard displacement-based GBT finite element is used to solve a set of representative illustrative examples involving complex local-global deformation. It is shown that the proposed GBT formulation leads to extremely accurate results with a reduced number of DOF and that the GBT modal solution provides an in-depth insight into the structural behaviour of naturally curved members.

Van der Waals (vdW) heterostructures ---formed by stacking or growing two-dimensional (2D) crystals on top of each other--- have emerged as a new promising route to tailor and engineer the properties of 2D materials. Twisted bilayer graphene (tBLG), a simple vdW structure where the interference between two misaligned graphene lattices leads to the formation of a moir\'e pattern, is a test bed to study the effects of the interaction and misalignment between layers, key players for determining the electronic properties of these stackings. In this chapter, we present in a pedagogical way the general theory used to describe lattice mismatched and misaligned vdW structures. We apply it to the study of tBLG in the limit of small rotations and see how the coupling between the two layers leads both to an angle dependent renormalization of graphene's Fermi velocity and appearance of low-energy van Hove singularities. The optical response of this system is then addressed by computing the optical conductivity and the dispersion relation of tBLG surface plasmon-polaritons., H2020 -Horizon 2020 Framework Programme(706538)

In this paper a mixed finite element based on Generalised Beam Theory (GBT) is proposed to analyse the first-order behaviour of naturally curved beams with circular axis (without pre-twist) and having deformable thin-walled cross-sections. The element applies the assumed strain concept to the longitudinal strains terms that are not present in straight members and cause locking. The additional strain DOFs are condensed out and therefore the stiffness matrix of the mixed element shares the DOFs of the standard (displacement-based) GBT finite element. The superior predictive capacity of the proposed mixed element with respect to the standard element with reduced integration is demonstrated through several examples involving complex global-distortional-local cross-section deformation.

Este artigo aborda três questões inter-relacionadas. São os EUA uma potência unipolar? A resposta é um sim sem reservas; os EUA são a única grande potência da actualidade. Será a unipolaridade americana duradoura? Novamente a resposta é sim, pelo menos durante mais algumas décadas. É a unipolaridade (americana) pacífica? Ao contrário da sabedoria convencional, não há qualquer razão para esperar que o actual sistema unipolar seja pacífico. Qualquer das opções estratégicas à disposição dos EUA - domínio global, defesa do status quo ou desengajamento - resultará em conflitos frequentes, possibilitados pela ausência de um equilíbrio de poder no sistema internacional. This article will try to address three inter-connected issues. Is the US a unipolar power? The answer in an unambiguous yes: the United States is the sole international pole in the post- Cold War era and unipolarity will last for the next few decades. But will that situation guarantee a more peaceful world? We can’t be so sure about it. Contrary to received wisdom, there is no reason to suppose that American unipolarity will be peaceful. Whatever the strategy chosen by the United States - global primacy, defense of the status quo or disengagement - conflict will be the rule in an unbalanced international system.

Se expone el proceso de creación de una ludoteca en Pólo de Chaves (Portugal), como espacio común de formación de alumnos, padres, profesores y comunidad. Se describe el origen del proyecto y el contexto de desarrollo del mismo, los objetivos a alcanzar con su apertura y las actividades propuestas para la dinámica de formación, a realizar mediante actividades lúdicas, en dicho espacio. Por último, se incluye un balance de los resultados obtenidos tras su puesta en marcha. Galicia Biblioteca de Educación del Ministerio de Educación, Cultura y Deporte; Calle San Agustín, 5 - 3 Planta; 28014 Madrid; Tel. +34917748000; biblioteca@mecd.es ESP

The first author gratefully acknowledges the financial support of FCT (Fundação para a Ciência e a Tecnologia, Portugal), through the doctoral scholarship SFRH/BD/120062/2016. This work assesses the first-order behavior of thin-walled bars with deformable cross-section and circular axis, without pre-twist, with the help of the Generalised Beam Theory (GBT) formulation previously developed by the authors (which dealt only with simple cross-sections without distortional deformation). Moreover, this paper presents a novel and systematic procedure to obtain the deformation modes for arbitrary flat-walled cross-sections (open, closed or “mixed”). The standard GBT kinematic assumptions, although much more complex than for the prismatic case, are employed to subdivide the modes in a meaningful way, maintaining the same nomenclature used for prismatic bars, and to reduce the number of DOFs necessary to achieve accurate results. It is also shown that the curvature of the bar has a significant influence on the deformation mode shapes. Finally, a standard displacement-based GBT finite element (FE) is employed to solve a set of representative examples, proving the efficiency of the proposed formulation and showing the peculiar behavior of curved bars. A comparison with shell FE models is also provided for validation purposes. publishersversion published

Mestrado em Biotecnologia - Biotecnologia Alimentar A qualidade do vinho espumante está intimamente relacionada com o seu aroma, volume e persistência da espuma. Os vinhos espumantes são obtidos após uma segunda fermentação em garrafa, seguido de um prolongado envelhecimento sur lies. Estes vinhos têm que conter no mínimo 3 bar de CO2 supersaturado. A autólise de leveduras ocorre durante o envelhecimento dos vinhos espumantes, causando a liberação de muitos compostos citoplasmáticos e da parede celular que têm uma grande influência sobre o vinho espumante de qualidade. A autólise é um processo demorado e por esse motivo tem-se vindo a procurar alternativas biotecnológicas para acelerar e melhorar esse processo. Este trabalho decorreu em ambiente empresarial, conjugando os interesses de uma empresa produtora de vinhos espumantes, a Aliança Vinhos de Portugal S.A., e uma empresa produtora de produtos enológicos, a Proenol Indústria Biotecnológica, Lda. Neste trabalho foram produzidos três vinhos base diferentes, provenientes do mesmo mosto, e nove modalidades de vinhos espumantes . Durante a primeira fermentação, para obtenção do vinho base, foi realizado um vinho base controlo, Affinity (Aff), um vinho base que fermentou com a aplicação do produto enológico Optimum White®, AffOW, e um terceiro, onde foi estudado a influência do uso de duas espécies de leveduras fermentativas diferentes (Sacaromyces cerevisiae e Torulaspora delbrueckii), AffBd, tendo sido avaliado a quantidade e estabilidade da espuma de cada um deles. Durante a segunda fermentação, cada um dos vinhos base obtidos foram divididos em três modalidades, sem adição de qualquer produto enológico, outro com a adição de Optimum White® e o terceiro com a adição de OptiRED®. Foram avaliadas as propriedades de espuma de todos os vinhos base e espumantes produzidos usando um método baseado no aparelho Mosalux. Foi também avaliada a composição de açúcares livres e polissacarideos dos vinhos base, vinhos espumantes e dos produtos enológicos utilizados para relacionar a composição destes com as propriedades de espuma. Aos vinhos espumantes obtidos foi realizada uma prova organolética descritiva e de preferência. No vinho base produzido, o AffBd revelou ter maior quantidade de manoproteinas e apresentava um maior volume de espuma. Os vinhos espumantes obtidos apresentam um intervalo de quantidade de manoproteínas de 1,3-7,5 g. L-1. Os vinhos espumantes que apresentam melhores propriedades de espuma, como a altura máxima da espuma (HM) após injeção de CO2, altura de estabilidade da espuma (HS) durante a injeção de CO2 e tempo de estabilidade da espuma (TS) após interrupção de CO2, apresentavam concentrações elevadas de manoproteínas na ordem dos 2,7-4,1 g. L-1. Apesar de existir uma relação entre a quantidade de manoproteínas e os valores de HM e HS, a partir de valores de concentração superiores a 4,1 g. L-1 não se verificou um incremento significativo nos valores de HM e HS. No entanto, para o mesmo valor de concentração de manoproteínas, o valor de TS varia de 22 s para 78 s, não sendo a concentração de manoproteínas o único parâmetro diretamente relacionado com o aumento da estabilidade da espuma. A análise sensorial revelou uma preferência para o vinho com adição de Optimum White® nas duas fermentações e uma preferência generalizada para os vinhos que tiveram em contato com este produto enológico durante a primeira fermentação. A preferência dos provadores incide sobre os vinhos que obtiveram melhores propriedades de espuma, nomeadamente na quantidade e estabilidade da mesma. The quality of the sparkling wine is closely related to its aroma, volume and persistence of foam. Sparkling wines are obtained after a second fermentation in the bottle, followed by a prolonged aging sur lies. These wines must contain at least 3 bar of supersaturated CO2. The autolysis of yeast occurs during aging sparkling wines, enhancing the release of cytoplasmatic and cell wall compounds that have influence on the quality of sparkling wine. The autolysis is a slow process, therefore has been efforts for biotechnological alternatives in order to accelerate and enhance this process. This work was carried on industry, combining the interests of a winery company, the Aliança Vinhos de Portugal S.A.and a producer of oenological products, Proenol Indústria Biotecnológica, Lda. In this work three different base wines were produced from the same must, and nine different sparkling wines. Upon the first fermentation were produced three base wines, a control base wine, Affinity (Aff), a base wine fermented with the application of oenological product Optimum White®, AffOW and a third base wine, fermented with two different yeasts species (Sacaromyces cerevisiae and Torulaspora delbrueckii), AffBd, the foamability and foam stability were evaluated in the three base wines. In order to perform the second fermentation, each base wine were divided into three different lots, one lot was the control wine, without adding any oenological product, one lot with the addition of Optimum White®, and the third lot with the addition of OptiRED®. The foam properties were evaluated for all base and sparkling wines produced using the methodology based on Mosalux device. In order to relate the sugar composition with foam properties it was also evaluated the composition of free sugars and polysaccharides of the base wines, sparkling wines and oenological products. Sparkling wines obtained were also evaluated by a descriptive organoleptic and preference test. Concerning the base wine produced, the AffBd showed higher amount of mannoproteins and exhibited the highest volume of foam. The sparkling wines showed an amount of mannoproteins ranging from 1.3 to 7.5 g. L-1. The sparkling wines that have better foam properties such as the maximum height of foam (HM) after injection of CO2, height of the foam on stability (HS) during the injection of CO2 and foam stability time (TS) after cessation of CO2 presented high concentrations of mannoproteins in the range of 2.7 to 4.1 g. L-1. Although there was a relationship between the amount of mannoproteins and HM and HS values, for concentrations higher than 4.1 g. L-1 no significant increase was observed. However, for the same value of mannoprotein concentration, the range of TS was from 22 s to 78 s, and the concentration of mannoproteins are not the only parameter related to the increase in foam stability. Sensory analysis showed a preference for wine with the addition of Optimum White® in the two fermentations and a general preference for wines that had contact with this oenological product during the first fermentation. The consumer preferred the wines with better foam properties, namely in the amount and stability of the foam.

Mestrado em Bioquímica - Bioquímica Alimentar A produção de um vinho espumante, segundo o método Champanhês, consiste na produção de um vinho base, a partir do qual se realiza uma segunda fermentação alcoólica em garrafa pela adição de leveduras e açúcar com produção de CO2. A eletrodiálise é um processo usado para a estabilização tartárica do vinho base, que consiste na remoção de iões, tais como potássio e hidrogeno tartarato, de forma a evitar a formação de precipitados, durante a vinificação e armazenamento do vinho. Porém, este processo pode levar à remoção de outros iões que são essenciais para o crescimento da levedura S. cerevisiae durante a segunda fermentação alcoólica, para a produção de vinho espumante. O primeiro objetivo deste trabalho consistiu em avaliar o impacto da eletrodiálise nos parâmetros enológicos nos vinhos base para espumante e na sua composição em catiões, tais como potássio, magnésio, manganês, cálcio, cobre, ferro, zinco, molibdénio e cobalto. O segundo objetivo do trabalho foi avaliara influência nas características físico-químicas e sensoriais do vinho da adição de produtos enológicos, constituídos por extratos de levedura inativa, como suplementos durante a primeira e segunda fermentação. Dois vinhos espumantes brancos, um fermentado com adição de produto enológico e outro sem adição de qualquer produto, foram analisados ao longo do processo de vinificação, em parâmetros como teor alcoólico, pH, acidez volátil, acidez total, massa volúmica, conteúdo de extrato seco, dióxido de enxofre livre e total, atividade antioxidante e compostos fenólicos totais e turbidez. O seu conteúdo em catiões antes e após o tratamento por eletrodiálise foi também determinado. Após a obtenção destes dois vinhos base e antes da segunda fermentação, a cada vinho foram novamente adicionados, dois produtos enológicos diferentes. Vinhos espumantes sem adição destes produtos foram também produzidos para serem usados como controlo. Os níveis de catiões foram novamente determinados após a adição destes produtos. Todas as variedades de vinho espumante produzidos foram sujeitos a uma prova organolética. Pelos resultados obtidos, verificou-se que a eletrodiálise não afeta os vinhos nos diferentes parâmetros enológicos analisados. No entanto, a eletrodiálise diminuiu significativamente as concentrações de alguns catiões, magnésio, manganês, zinco e cobalto. A adição do produto enológico na primeira fermentação não teve qualquer influência nos níveis minerais do vinho base . A adição dos produtos enológicos ao vinho base como suplementos de catiões antes da segunda fermentação aumentou os níveis de cobre e cobalto num vinho, e de potássio, magnésio, cobre e cobalto no outro vinho. A análise sensorial aos vinhos mostrou que a adição dos produtos enológicos diminuiu a tonalidade amarela aumentando a verde, aumentou o carácter frutado dos vinhos. Em relação à espuma, a adição de qualquer um dos produtos enológicos aumentou o potencial de espuma dos vinhos. The production of sparkling wine, according to the champagne method consists in the production of a base wine, from which is carried a second fermentation in bottle by the addition of yeast and sugar. The electrodialysis is a process used for the tartaric stabilization of wine, which consists in removing ions, such as potassium and hydrogen tartrate, in order to prevent the occurrence of precipitates during vinification and wine storage. However, this process may lead to the removal of other ions that are essential for S. cerevisiae growth during the second fermentation for sparkling wine production The first objective of this study was to assess the impact of electrodialysis in in the base wine oenological parameters and their composition on cations like potassium, magnesium, manganese, calcium, copper, iron, zinc, molybdenum, and cobalt. The second objective was to evaluate the addition of two oenological products, composed by inactive yeast extracts, in the physico-chemical and sensorial properties of the sparkling wine. Two white sparkling wines, one fermented without the addition of oenological product, and another with the oenological product addition before the first alcoholic fermentation, were analyzed throughout the winemaking process, on parameters such as alcohol content, pH, volatile acidity, total acidity, density, dry matter content, free and total sulfur dioxide, antioxidant activity and total phenolic compounds and turbidity. The content of cations before and after electrodialysis treatment was also determined. After obtaining these two base wines, each wine was used to produce two different sparkling wines by the addition of two different oenological products. Sparkling wines without addition of these products have also been produced to be use as control. The cations content were determined after the addition of these products. All varieties of sparkling wine produced were subjected to sensorial analysis. The results obtained allow to concluded that the electrodialysis does not affect the oenological parameters of the wines. However, the treatment significantly reduced the concentrations of some elements such as magnesium, manganese, zinc, and cobalt. The addition of oenological product in the first fermentation had no influence on the mineral levels of the base wine The addition of oenological products to the base wine for sparkling wine production, as ions supplements before the second fermentation, increased copper and cobalt levels in one wine, and potassium, magnesium, copper and cobalt in the other wine. A sensorial analysis of the wines showed that the addition of oenological products decreased the yellow tone increasing the green one, increased the fruity character of the wines. Regarding the foam, the addition of any of the oenological product increased foam potential of the wines.