Dielectric mismatch and shallow donor impurities in GaN/HfO2 quantum wells

In this work we investigate electron-impurity binding energy in GaN/HfO$_2$ quantum wells. The calculation considers simultaneously all energy contributions caused by the dielectric mismatch: (i) image self-energy (i.e., interaction between electron and its image charge), (ii) the direct Coulomb interaction between the electron-impurity and (iii) the interactions among electron and impurity image charges. The theoretical model account for the solution of the time-dependent Schr\"odinger equation and the results shows how the magnitude of the electron-impurity binding energy depends on the position of impurity in the well-barrier system. The role of the large dielectric constant in the barrier region is exposed with the comparison of the results for GaN/HfO$_2$ with those of a more typical GaN/AlN system, for two different confinement regimes: narrow and wide quantum wells.Comment: 6 Pages, 7 figure

Assessing the Harvest Maturity of Brazilian Mangoes

No clear criterion exists to determine the optimum time to harvest mango. Some empirical relations are used to assess maturity, such as shoulder development. Moreover, as a result of the typical growing conditions in tropical climates, a huge variation in maturity and ripeness exists, seriously hampering the export of fruit in the global chain. The consequence for consumers in western countries is that sometimes mangoes are overripe at the retailer, or have to be kept for several days, even weeks, to reach the edible state, provided they do not rot in the meantime. To ensure an edible quality, the chlorophyll content in the fruit flesh, measured at harvest by Time-resolved Reflectance Spectroscopy (TRS), could be used as a maturity criterion for mango fruit. Commercially grown fruit were harvested in Brazil and transported to Italy by plane. Fruits were measured using TRS at 630 nm for absorption coefficient (µa) and skin colour. The development of µa was followed on 60 fruits during 15 days of storage at 20°C. The remainders of fruit were used to measure firmness destructively. Absorption coefficient decreased during shelf life according to a logistic pattern, as expected for colour development. Taking the variation between the individual fruit into account, 72% of the variation was accounted for. Nevertheless, µa assessed at harvest could be converted into a biological shift factor (BSF), as an expression of the maturity at harvest of each individual fruit. This biological shift factor explained about 70% of the variation in firmness development in individual fruit. These preliminary results indicate that TRS methodology coupled with BSF theory could be useful in assessing maturity at harvest and assuring acceptable eating quality of mango

Chemical Potential and the Nature of the Dark Energy: The case of phantom

The influence of a possible non zero chemical potential $\mu$ on the nature of dark energy is investigated by assuming that the dark energy is a relativistic perfect simple fluid obeying the equation of state (EoS), $p=\omega \rho$ ($\omega <0, constant$). The entropy condition, $S \geq 0$, implies that the possible values of $\omega$ are heavily dependent on the magnitude, as well as on the sign of the chemical potential. For $\mu >0$, the $\omega$-parameter must be greater than -1 (vacuum is forbidden) while for $\mu < 0$ not only the vacuum but even a phantomlike behavior ($\omega <-1$) is allowed. In any case, the ratio between the chemical potential and temperature remains constant, that is, $\mu/T=\mu_0/T_0$. Assuming that the dark energy constituents have either a bosonic or fermionic nature, the general form of the spectrum is also proposed. For bosons $\mu$ is always negative and the extended Wien's law allows only a dark component with $\omega < -1/2$ which includes vacuum and the phantomlike cases. The same happens in the fermionic branch for $\mu 0$ are permmited only if $-1 < \omega < -1/2$. The thermodynamics and statistical arguments constrain the EoS parameter to be $\omega < -1/2$, a result surprisingly close to the maximal value required to accelerate a FRW type universe dominated by matter and dark energy ($\omega \lesssim -10/21$).Comment: 7 pages, 5 figure

Vortices in the presence of a nonmagnetic atom impurity in 2D XY ferromagnets

Using a model of nonmagnetic impurity potential, we have examined the behavior of planar vortex solutions in the classical two-dimensional XY ferromagnets in the presence of a spin vacancy localized out of the vortex core. Our results show that a spinless atom impurity gives rise to an effective potential that repels the vortex structure.Comment: 6 pages, 2 figures, RevTex

Structural behaviour assessment and material characterization of traditional adobe constructions

In Aveiro district, adobe can be found with abundance in rural and urban buildings, many of which are of cultural, historical and architectonic recognized value. A great number of the existing adobe constructions present pronounced structural damage, which results, in great part, from the lack of maintenance and the limited available knowledge concerning the mechanical behaviour of these structures. It is thus recognised the urgency for the development of research work in the structural analysis and strengthening of this important legacy. This work is of great value to the reduction of the seismic risk associated to these adobe constructions. The University of Aveiro has been developing studies to help filling the technical information gap concerning the structural behaviour of existing adobe constructions. Cylindrical adobe specimens were subjected to compressive and “splitting” tests, and prismatic mortar specimens were subjected to compressive tests. Small wallets, constructed with materials representative of those found in existing adobe constructions, were subjected to compression tests, perpendicularly and diagonally to the bed joints. The structural non-linear response of adobe walls has also been investigated in a series of full-scale tests, in the laboratory and in situ, with imposed horizontal cyclic displacements. This article describes the studies carried out and discusses the principal results. These studies aim to establish a basis of knowledge that can support the interpretation of structural pathologies, calibration of numerical models, structural safety assessment, and design of strengthening solutions for the existing adobe constructions, and even support the design and execution of new edifications

Vacuum fluctuations and the conditional homodyne detection of squeezed light

Conditional homodyne detection of quadrature squeezing is compared with standard nonconditional detection. Whereas the latter identifies nonclassicality in a quantitative way, as a reduction of the noise power below the shot noise level, conditional detection makes a qualitative distinction between vacuum state squeezing and squeezed classical noise. Implications of this comparison for the realistic interpretation of vacuum fluctuations (stochastic electrodynamics) are discussed.Comment: 14 pages, 7 figures, to appear in J. Opt. B: Quantum Semiclass. Op

Experimental characterization of the mechanical behaviour of adobe bricks of existing constructions in Aveiro district

A significant number of existing adobe constructions in Aveiro district presents pronounced structural degradation, which results, in great part, from the lack of sensibility for the preservation of this patrimony, and also from the shortage of knowledge concerning the behaviour of the applied materials. An important difficulty for the technicians working in the rehabilitation of this type of buildings lays indeed in the scarcity of knowledge on the mechanical behaviour of adobe units and adobe walls. The mechanical properties of adobes taken from houses and land dividing walls, representative of the construction in Aveiro district, were investigated. Cylindrical specimens extracted from the collected adobe samples were subjected to compression and splitting tests. This article describes the tests that were carried out and presents the obtained results, discussing them. The achieved results will support the behaviour characterization of existing structures, and help in the calibration of numerical models for the simulation of the adobe constructions behaviour

Caracterização e reabilitação de construções existentes em terra

O adobe é um material de construção que apresenta muitas características atractivas: é barato, disponível localmente, biodegradável, reciclável, adaptado a uma vasta gama de solos; apresenta excelentes propriedades térmicas e acústicas; e encontra-se associado a métodos construtivos simples e expeditos que apenas requerem um pequeno consumo de energia. É, assim, um material economicamente viável, cujo uso respeita o equilíbrio ambiental. Em alguns países a construção em adobe está mesmo a tornar-se economicamente competitiva em relação aos comuns métodos construtivos. A reabilitação e o reforço das construções tradicionais em adobe, tendo em consideração exigências e necessidades modernas, contribuem para a melhoria da qualidade de vida daqueles que as utilizam, e para um aumento dos níveis de segurança associados, particularmente se for assegurado o reforço sísmico. A preservação destas construções também contribui para o reconhecimento das vantagens que o adobe, enquanto material construtivo, apresenta, encorajando o seu uso em construções novas, e permitindo a redução dos esforços e recursos afectos às novas edificações que geralmente substituem as existentes em adobe. Na Beira Litoral, e em particular no distrito de Aveiro, o adobe foi um material de construção largamente utilizado até ao final da segunda metade do século XX. Actualmente, o adobe pode ainda ser encontrado com abundância em edifícios rurais e urbanos, muitos dos quais possuem valor arquitectónico reconhecido. Uma parte significativa destas construções apresenta, contudo, degradação estrutural e não estrutural pronunciada. Um grupo de investigação, pertencente ao Departamento de Engenharia Civil da Universidade de Aveiro, tem vindo a desenvolver trabalho para caracterização do vasto parque tradicional construído em adobe existente no distrito de Aveiro. Foram conduzidos ensaios para avaliação da composição e comportamento mecânico de unidades de adobe e de argamassas, e ainda ensaios para análise do comportamento estrutural de paredes de alvenaria e arcos em adobe. Estes estudos foram estimulados pelo reconhecimento da importância da preservação desta herança cultural e arquitectónica e da promoção do adobe como material de construção. Têm como objectivo a criação de uma base de conhecimento que possa suportar a interpretação de patologias estruturais, calibração de modelos numéricos, avaliação da segurança estrutural, e desenvolvimento de soluções de reforço caracterizadas por simplicidade, eficiência e baixo custo