Disorder effects at low temperatures in La_{0.7-x}Y_{x}Ca_{0.3}MnO_{3} manganites

With the aim of probing the effect of magnetic disorder in the low-temperature excitations of manganites, specific-heat measurements were performed in zero field, and in magnetic fields up to 9 T in polycrystalline samples of La_{0.7-x}Y_{x}Ca_{0.3}MnO_{3}, with Y concentrations x=0, 0.10, and 0.15. Yttrium doping yielded the appearance of a cluster-glass state, giving rise to unusual low-temperature behavior of the specific-heat. The main feature observed in the results is a strong enhancement of the specific-heat linear term, which is interpreted as a direct consequence of magnetic disorder. The analysis was further corroborated by resistivity measurements in the same compounds.Comment: 9 pages, 2 figure

Parallel structurally-symmetric sparse matrix-vector products on multi-core processors

We consider the problem of developing an efficient multi-threaded implementation of the matrix-vector multiplication algorithm for sparse matrices with structural symmetry. Matrices are stored using the compressed sparse row-column format (CSRC), designed for profiting from the symmetric non-zero pattern observed in global finite element matrices. Unlike classical compressed storage formats, performing the sparse matrix-vector product using the CSRC requires thread-safe access to the destination vector. To avoid race conditions, we have implemented two partitioning strategies. In the first one, each thread allocates an array for storing its contributions, which are later combined in an accumulation step. We analyze how to perform this accumulation in four different ways. The second strategy employs a coloring algorithm for grouping rows that can be concurrently processed by threads. Our results indicate that, although incurring an increase in the working set size, the former approach leads to the best performance improvements for most matrices.Comment: 17 pages, 17 figures, reviewed related work section, fixed typo

Application of non-parametric geostatistical methods to the identification of aggregate deposits on the continental shelf

More than 1 400 samples were used in the assessment of the sand and gravel potential of the continental shelf north of Espinho (northern Portugal). This study deals primarily with the determination of the probabilities of the occurrence of some textural (gravel, sand, silt and clay contents) and chemical (carbonate content) parameters and of some textural groups, according to Nickless' (1973) classification. Sediment samples were collected between 1986 and 1989, during several cruises sponsored by the Portuguese Hydrographic Institute. For the purpose of the present study, a non-parametric geostatistical methodology was used on the whole set of samples, encompassing the following steps: a) a preliminary variography analysis based on the raw data, to determine structural features, such as trends, anisotropies and nested models; b) binary codification of the initial data, in ‘less than' or ‘equal to' specific cut-offs determined with economic criteria; c) calculation and modelling of variograms of this indicator; and d) application of indicator kriging to obtain probability (risk) maps. The results obtained by the application of the methodology to the two data groups (sediment texture and Nickless classification) were compared in order to characterise the aggregate deposits which could be economically exploited in the near future. This study should help decision-makers to define an exploitation strategy, to be implemented in the near future. However, this preliminary approach requires further studies, namely by using cross-variography procedures and conditional probabilities.Más de 1 400 muestras sedimentarias fueron utilizadas en la evaluación del potencial en gravas y arenas que se pueden explotar para áridos naturales y de trituración para la construcción y obras públicas. El presente estudio se atiene a la determinación de la probabilidad de ocurrencia de algunos parámetros texturales (contenido en gravas, en arena y en fango) y químicos (contenido en carbonatos) y de determinados grupos texturales, según la clasificación de Nickless (1973). Las muestras fueron recogidas entre 1986 y 1989 en el transcurso de diversas campañas oceanográficas realizadas por el Instituto Hidrográfico de la Marina Portuguesa. Se ha realizado un estudio de geoestadística no paramétrica incluyendo las siguientes etapas: a) variografía preliminar de los datos de base para identificar características estructurales (por ejemplo, tendencias, anisotropías y estructuras imbricadas); b) se codificaron los datos iniciales en formato binario, según valores inferiores a o iguales a un patamar específico determinado por un criterio económico; c) se calcularon y modelaron variogramas de esta indicatriz; y d) la indicatriz fue krijeada para obtener mapas de probabilidad o riesgo. La referida metodología fue aplicada sobre los dos tipos de datos (granulometría sedimentaria y clasificación de Nickless). Se compararon los resultados obtenidos para caracterizar los depósitos de gravas y arenas que pueden ser explotados en un futuro próximo. La presente aplicación pretende ayudar a los organismos decisorios a definir una futura estrategia de explotación. Sin embargo, éste es solamente un estudio preliminar. Se necesitan estudios más profundos, utilizando, por ejemplo, procedimientos de variografía cruzada y probabilidades condicionales.Instituto Español de Oceanografí

Energy efficiency of mmWave massive MIMO precoding with low-resolution DACs

With the congestion of the sub-6 GHz spectrum, the interest in massive multiple-input multiple-output (MIMO) systems operating on millimeter wave spectrum grows. In order to reduce the power consumption of such massive MIMO systems, hybrid analog/digital transceivers and application of low-resolution digital-to-analog/analog-to-digital converters have been recently proposed. In this work, we investigate the energy efficiency of quantized hybrid transmitters equipped with a fully/partially-connected phase-shifting network composed of active/passive phase-shifters and compare it to that of quantized digital precoders. We introduce a quantized single-user MIMO system model based on an additive quantization noise approximation considering realistic power consumption and loss models to evaluate the spectral and energy efficiencies of the transmit precoding methods. Simulation results show that partially-connected hybrid precoders can be more energy-efficient compared to digital precoders, while fully-connected hybrid precoders exhibit poor energy efficiency in general. Also, the topology of phase-shifting components offers an energy-spectral efficiency trade-off: active phase-shifters provide higher data rates, while passive phase-shifters maintain better energy efficiency.Comment: Published in IEEE Journal of Selected Topics in Signal Processin

Cuidados com a ovelha prenhe e seu cordeiro.

Alimentação; Diagnóstico de gestação - ultra-som; Limpeza pré-parto; Vacinações; Dosificações; Potreiros abrigados; Revisando o rebanho; Ovelha caída; Hipotermia; Vigilância contra predadores.bitstream/item/110569/1/CUIDADOS-COM-A-OVELHA-DE.pd

Symmetry, incommensurate magnetism and ferroelectricity: the case of the rare-earth manganites RMnO3

The complete irreducible co-representations of the paramagnetic space group provide a simple and direct path to explore the symmetry restrictions of magnetically driven ferroelectricity. The method consists of a straightforward generalization of the method commonly used in the case of displacive modulated systems and allows us to determine, in a simple manner, the full magnetic symmetry of a given phase originated from a given magnetic order parameter. The potential ferroic and magneto-electric properties of that phase can then be established and the exact Landau free energy expansions can be derived from general symmetry considerations. In this work, this method is applied to the case of the orthorhombic rare-earth manganites RMnO3. This example will allow us to stress some specific points, such as the differences between commensurate or incommensurate magnetic phases regarding the ferroic and magnetoelectric properties, the possible stabilization of ferroelectricity by a single irreducible order parameter or the possible onset of a polarization oriented parallel to the magnetic modulation. The specific example of TbMnO3 will be considered in more detail, in order to characterize the role played by the magneto-electric effect in the mechanism for the polarization rotation induced by an external magnetic field.Comment: Conference: Aperiodic`0