Graphene-based spin-pumping transistor

We demonstrate with a fully quantum-mechanical approach that graphene can function as gate-controllable transistors for pumped spin currents, i.e., a stream of angular momentum induced by the precession of adjacent magnetizations, which exists in the absence of net charge currents. Furthermore, we propose as a proof of concept how these spin currents can be modulated by an electrostatic gate. Because our proposal involves nano-sized systems that function with very high speeds and in the absence of any applied bias, it is potentially useful for the development of transistors capable of combining large processing speeds, enhanced integration and extremely low power consumption

Graphene as a non-magnetic spin-current lens

In spintronics, the ability to transport magnetic information often depends on the existence of a spin current traveling between two different magnetic objects acting as source and probe. A large fraction of this information never reaches the probe and is lost because the spin current tends to travel omni-directionally. We propose that a curved boundary between a gated and a non-gated region within graphene acts as an ideal lens for spin currents despite being entirely of non-magnetic nature. We show as a proof of concept that such lenses can be utilized to redirect the spin current that travels away from a source onto a focus region where a magnetic probe is located, saving a considerable fraction of the magnetic information that would be otherwise lost.Comment: 9 pages, 3 figure

Dynamic RKKY interaction between magnetic moments in graphene nanoribbons

Graphene has been identified as a promising material with numerous applications, particularly in spintronics. In this paper we investigate the peculiar features of spin excitations of magnetic units deposited on graphene nanoribbons and how they can couple through a dynamical interaction mediated by spin currents. We examine in detail the spin lifetimes and identify a pattern caused by vanishing density of states sites in pristine ribbons with armchair borders. Impurities located on these sites become practically invisible to the interaction, but can be made accessible by a gate voltage or doping. We also demonstrate that the coupling between impurities can be turned on or off using this characteristic, which may be used to control the transfer of information in transistor-like devices.Comment: 10 pages, 10 figure

Cosmic String Wakes in Scalar-Tensor Gravities

The formation and evolution of cosmic string wakes in the framework of a scalar-tensor gravity are investigated in this work. We consider a simple model in which cold dark matter flows past an ordinary string and we treat this motion in the Zel'dovich approximation. We make a comaprison between our results and previous results obtained in the context of General Relativity. We propose a mechanism in which the contribution of the scalar field to the evolution of the wakes may lead to a cosmological observation.Comment: Replaced version to be published in the Classical and Quantum Gravit

Experimental observation of quantum entanglement in low dimensional spin systems

We report macroscopic magnetic measurements carried out in order to detect and characterize field-induced quantum entanglement in low dimensional spin systems. We analyze the pyroborate MgMnB_2O_5 and the and the warwickite MgTiOBO_3, systems with spin 5/2 and 1/2 respectively. By using the magnetic susceptibility as an entanglement witness we are able to quantify entanglement as a function of temperature and magnetic field. In addition, we experimentally distinguish for the first time a random singlet phase from a Griffiths phase. This analysis opens the possibility of a more detailed characterization of low dimensional materials

Neonatal Morbi-Mortality in Very Low Birth Weight in Europe: the Portuguese Experience

The aim of this study was to access evolution in care of very low birth weight (VLBW) infants after the implementation of a regionalization policy in Portugal. The data of the National Portuguese Network of VLBW infants are analyzed concerning mortality, morbidity, and quality of regionalization. A total of 12,826 VLBW infants born from 1 January 1994 to 31 December 2008 were enrolled, with a prevalence of 0.66%-0.99% of all live born. The global mortality was 11%. The major improvement in survival is in the babies more than 1000 g. Since 2004, the threshold of viability is 25 weeks, but the intact survival is around 28 weeks. In the last 10 years with more efficient regionalization more VLBW babies are born in the right place. The improvement in neonatal mortality rate was determinant in the good evolution of perinatal and infant mortalities. After reinforcement of regionalization policies, we found improvements in mortality for VLBW infants. The aims of regionalization were achieved. The reform of perinatal care in Portugal is an example of how a good diagnosis and adequate proposals combined with a strong political will is crucial for changing.info:eu-repo/semantics/publishedVersio

Aproveitamento sustentável de recursos geotérmicos no campo geotérmico da Ribeira Grande, São Miguel, Açores

ABSTRACT: The use of geothermal resources for power production on São Miguel Island has almost 40 years of accumulated experience. In the last decade, the stable power output from Pico Vermelho and Ribeira Grande geothermal plants have been providing up to 44% of the electricity needs of São Miguel Island. Throughout the history of the project, EDA RENOVÁVEIS has been carefully monitoring the Ribeira Grande reservoir conditions and well discharges to assess the resource response to the production load. The results from the last 10 years of monitoring data indicate that production did not cause any significant impact on the reservoir pressure or temperature and it can be maintained in the long-term without the risk of over-exploiting the geothermal resource.RESUMO: O uso de recursos geotérmicos para produção de eletricidade na ilha de São Miguel tem quase 40 anos de experiência acumulada. Na última década, a estabilidade da produção das centrais geotérmicas do Pico Vermelho e da Ribeira Grande tem garantido até 44% das necessidades de energia elétrica da ilha de São Miguel. Ao longo da história do projeto, a EDA RENOVÁVEIS tem promovido a monitorização do reservatório geotérmico da Ribeira Grande, bem como dos parâmetros de produção dos poços geotérmicos, com vista a avaliar a resposta do recurso ao esforço de exploração. Os resultados dos últimos 10 anos de monitorização indicam que a exploração do recurso geotérmico não causou impacto significativo quer na pressão, quer na temperatura do reservatório, prevendo-se que esta poderá manter-se a longo prazo sem o risco de sobre-exploração do recurso geotérmico.info:eu-repo/semantics/publishedVersio