Photon emission in a constant magnetic field in 2+1 dimensional space-time

We calculate by the proper-time method the amplitude of the two-photon emission by a charged fermion in a constant magnetic field in (2+1)-dimensional space-time. The relevant dynamics reduces to that of a supesymmetric quantum-mechanical system with one bosonic and one fermionic degrees of freedom.Comment: 18 pages. v2: references added, some significant changes in the introductio

Muridae e Erinacidae da ilha de Santa Maria com uma nota sobre um ectoparasita de Erinaceus europaeus

XIV Expedição Científica do Departamento de Biologia - Santa Maria 2009.Realizámos capturas de Muridae e Erinacidae utilizando armadilhas Sherman e Tomahawk. Capturámos quatro indivíduos da espécie Mus musculus, um da espécie Rattus rattus e quatro da espécie Erinaceus europaeus, dos quais apresentamos dados morfométricos externos. Registamos o parasitismo de E. europaeus pela espécie de carraça Rhipicephalus sanguineus

Ab-initio study of the relation between electric polarization and electric field gradients in ferroelectrics

The hyperfine interaction between the quadrupole moment of atomic nuclei and the electric field gradient (EFG) provides information on the electronic charge distribution close to a given atomic site. In ferroelectric materials, the loss of inversion symmetry of the electronic charge distribution is necessary for the appearance of the electric polarization. We present first-principles density functional theory calculations of ferroelectrics such as BaTiO3, KNbO3, PbTiO3 and other oxides with perovskite structures, by focusing on both EFG tensors and polarization. We analyze the EFG tensor properties such as orientation and correlation between components and their link with electric polarization. This work supports previous studies of ferroelectric materials where a relation between EFG tensors and polarization was observed, which may be exploited to study ferroelectric order when standard techniques to measure polarization are not easily applied.Comment: 9 pages, 6 figures, 5 tables, corrected typos, as published in Phys. Rev.

Monitorização da adulteração de suplementos alimentares à base de plantas comercializados em Portugal com inibidores de fosfodiesterase tipo 5 (PDE-5)

Na ultima década, assistiu-se a um consumo crescente de suplementos alimentares pela população mundial, especialmente, no que diz respeito ao consumo de suplementos alimentares a base de plantas 1. Estes são utilizados com diversas finalidades, tais como equilibrar a dieta, compensar uma falta nutricional, manter a saúde, prevenir doenças crónicas, melhorar a aparência e o bem-estar incluindo a condição mental, sexual e desportiva, entre outras 1. São produtos frequentemente rotulados como "naturais" o que transmite uma falsa sensação de segurança aos consumidores. Contudo, recentemente foi referida a ocorrência de adulterações nestes produtos, o que compromete a sua segurança. Entre os casos de fraude detetados destaca-se a adição de substâncias ilegais. Atualmente, os suplementos para melhorar o desempenho sexual estão entre os suplementos alimentares mais populares entre os consumidores. A disfunção eréctil e uma doença que afeta 150 milhões de homens mundialmente sendo o tratamento recomendado a administração de fármacos inibidores da enzima fosfodiesterase tipo 5 (PDE-5) 2. Os inibidores PDE-5 legalmente comercializados a nível europeu são: citrato de sildenafil, tadalafil, hidrocloreto de vardenafil e avanafil 3. Estas são substâncias com efeitos secundários graves quando administrado concomitantemente com medicamentos como nitratos ou a-bloqueadores 4. 0 objetivo deste trabalho foi, por isso, a monitorização da presença de adulterantes do tipo inibidores PDE-5 em suplementos alimentares a base de plantas vendidos com o objetivo de melhorar a performance sexual. Foram analisados doze suplementos alimentares, adquiridos no mercado português. Estes suplementos foram pesquisados quanto a presença de sildenafil, acetildenafil, tiosildenafil, tadalafil, vardenafil e yombina, atraves de metodo de crornatografia líquida de elevada performance acoplada a detetor de díodos e detetor de fluorescência em série (HPLC-DAD-FL). Dos doze suplementos analisados, seis revelaram a presença de um ou mais dos inibidores PDE-5 referidos. Adicionalmente, dois outros suplementos revelaram a presença de dois compostos, cujo espectro UV-Vis leva a suspeição de uma possível adulteração com compostos análogos às substâncias pesquisadas.info:eu-repo/semantics/publishedVersio

Accuracy of Siri and Brozek equations in the percent body fat estimation in older adults

To identify which equation, Siri or Brozek, based on the two compartment model, provides a more accurate conversion of body density (BD) in percent body fat (%BF) in a group of older adults. Cross-sectional study. Research Centre in Physical Activity, Health and Leisure, Faculty of Sport, University of Porto. 60 older adults, aged 60-92 years. Skinfold thickness was used to estimate BD through Visser et al. prediction equation. The conversion of BD to %BF was done with Siri (%BF-Siri) and Brozek (%BF-Brozek) formulas and these determined values were both compared to Dual-Energy X-ray Absorptiometry (%BF-DXA) evaluations. A strong correlation between the %BF-DXA value and %BF-Siri (r=0.91, p < 0.001) and %BF-Brozek (r=0.91, p < 0.001) was found, although %BF-Siri and %BF-Brozek overestimated %BF-DXA (p < 0.001). The comparison of the %BF-Siri and %BF-Brozek mean values also revealed significant differences (p < 0.001). The %BF-Brozek reflects a better agreement than the %BF-Siri with %BF-DXA with respectively a mean difference of -4.0%BF (limits of agreement = -10.9 to 2.9%) and -5.7%BF (-12.6 to 1.2). The Bland and Altman plots confirmed that %BF-Brozek reflects a better agreement with %BF-DXA. The results of the present study show that the use of Brozek equation may correspond to a more accurate alternative than Siri equation for the conversion of BD in %BF in older adults

Analytical solution of a model for complex food webs

We investigate numerically and analytically a recently proposed model for food webs [Nature {\bf 404}, 180 (2000)] in the limit of large web sizes and sparse interaction matrices. We obtain analytical expressions for several quantities with ecological interest, in particular the probability distributions for the number of prey and the number of predators. We find that these distributions have fast-decaying exponential and Gaussian tails, respectively. We also find that our analytical expressions are robust to changes in the details of the model.Comment: 4 pages (RevTeX). Final versio

Universality classes for rice-pile models

We investigate sandpile models where the updating of unstable columns is done according to a stochastic rule. We examine the effect of introducing nonlocal relaxation mechanisms. We find that the models self-organize into critical states that belong to three different universality classes. The models with local relaxation rules belong to a known universality class that is characterized by an avalanche exponent $\tau \approx 1.55$, whereas the models with nonlocal relaxation rules belong to new universality classes characterized by exponents $\tau \approx 1.35$ and $\tau \approx 1.63$. We discuss the values of the exponents in terms of scaling relations and a mapping of the sandpile models to interface models.Comment: 4 pages, including 3 figure