1,381 research outputs found
Dynamic Many-Body Theory. II. Dynamics of Strongly Correlated Fermi Fluids
We develop a systematic theory of multi-particle excitations in strongly
interacting Fermi systems. Our work is the generalization of the time-honored
work by Jackson, Feenberg, and Campbell for bosons, that provides, in its most
advanced implementation, quantitative predictions for the dynamic structure
function in the whole experimentally accessible energy/momentum regime. Our
view is that the same physical effects -- namely fluctuations of the wave
function at an atomic length scale -- are responsible for the correct
energetics of the excitations in both Bose and Fermi fluids. Besides a
comprehensive derivation of the fermion version of the theory and discussion of
the approximations made, we present results for homogeneous He-3 and electrons
in three dimensions. We find indeed a significant lowering of the zero sound
mode in He-3 and a broadening of the collective mode due to the coupling to
particle-hole excitations in good agreement with experiments. The most visible
effect in electronic systems is the appearance of a ``double-plasmon''
excitation.Comment: submitted to Phys. Rev.
Informações geoespaciais para a gestão dos recursos naturais do Corredor de Nacala - Moçambique.
O projeto Embrapa-Moçambique foi baseado nas similaridades edafoclimáticas existentes entre as savanas moçambicanas e o Cerrado brasileiro e objetiva o levantamento e o mapeamento das potencialidades dos recursos naturais de Moçambique. Neste estudo, organizaram-se bases de dados geográficos existentes do Corredor de Nacala, levantaram-se aspectos da legislação ambiental moçambicana, e realizou-se o mapeamento de florestas em distritos específicos deste corredor para a visualização da gestão de recursos naturais. Como resultados já alcançados, destacam-se a geração da base de dados geográficos on-line, a análise preliminar da integração da legislação ambiental em ambiente de sistemas de informação geográfica e o levantamento do uso e da ocupação dos distritos de Mandimba, Cuamba, Malema e Ribaué
Uso da banda termal do satélite LANDSAT-5 e técnicas de geoestatística na estimativa da degradação da Atrazina no solo da Área de Proteção Ambiental de Tejupá.
Este estudo teve como objetivo avaliar a empregabilidade da banda termal do Landsat-5 e técnicas de geoestatística, para obtenção das estimativas da taxa de degradação da Atrazina na superfície do solo
C-Band All-Sky Survey: A First Look at the Galaxy
We present an analysis of the diffuse emission at 5 GHz in the first quadrant
of the Galactic plane using two months of preliminary intensity data taken with
the C-Band All Sky Survey (C-BASS) northern instrument at the Owens Valley
Radio Observatory, California. Combining C-BASS maps with ancillary data to
make temperature-temperature plots we find synchrotron spectral indices of
between 0.408 GHz and 5 GHz and between 1.420 GHz and 5 GHz for ,
. Through the subtraction of a radio recombination
line (RRL) free-free template we determine the synchrotron spectral index in
the Galactic plane () to be between
0.408 GHz and 5 GHz, with a contribution of per cent from free-free
emission at 5\,GHz. These results are consistent with previous low frequency
measurements in the Galactic plane. By including C-BASS data in spectral fits
we demonstrate the presence of anomalous microwave emission (AME) associated
with the HII complexes W43, W44 and W47 near 30 GHz, at 4.4 sigma, 3.1 sigma
and 2.5 sigma respectively. The CORNISH VLA 5 GHz source catalogue rules out
the possibility that the excess emission detected around 30\;GHz may be due to
ultra-compact HII regions. Diffuse AME was also identified at a 4 sigma level
within , between 5
GHz and 22.8 GHz.Comment: 16 pages, 9 figures, submitted to MNRAS, referee's corrections made,
awaiting for final approval for publicatio
Cadernos de geoprocessamento 9: métodos, técnicas e equipamentos para o processo de reambulação de mapas de uso e cobertura da terra.
Hoje inúmeros sensores satelitários e aerotransportados, bem como múltiplas fontes de dados auxiliares, encontram-se disponíveis para mapeamentos de uso e cobertura da terra, em diferentes escalas e com classes estabelecidas de acordo com distintos objetivos. Uma componente fundamental da atividade, entretanto, é a verificação da concordância entre as classes mapeadas durante a execução do processo cartográfico e as classes observadas no terreno (?verdade terrestre?). Já em 1990, Dicks e Lo (1990) comentavam o fato de que, por dificuldades orçamentárias, falta de tempo ou de treinamento adequado, muitos mapas de uso e cobertura são produzidos sem que se quantifique e documente sua acuracidade temática. Ainda, segundo os autores, tal prática se tornou mais evidente com o advento de sistemas de processamento de imagens bastante intuitivos, que permitem a usuários relativamente inexperientes gerar mapas aparentemente precisos em curto espaço de tempo e com mínimo esforço. Os levantamentos de campo ou verificação in situ na área objeto de mapeamento podem servir a três propósitos: adquirir pontos de controle que serão usados como amostras de treinamento no processo de classificação de imagens digitais; adquirir pontos amostrais independentes, destinados à avaliação da acuracidade temática do mapa produzido ou; coletar informações sobre o uso e cobertura das feições já mapeadas, porém sobre as quais se tem dúvidas que podem comprometer a qualidade do mapeamento. Tratando-se desse último caso, o processo de verificação em campo é conhecido, genericamente, pelo nome de reambulação.bitstream/item/166829/1/CT-409-1520.pd
A blind detection of a large, complex, Sunyaev--Zel'dovich structure
We present an interesting Sunyaev-Zel'dovich (SZ) detection in the first of
the Arcminute Microkelvin Imager (AMI) 'blind', degree-square fields to have
been observed down to our target sensitivity of 100{\mu}Jy/beam. In follow-up
deep pointed observations the SZ effect is detected with a maximum peak
decrement greater than 8 \times the thermal noise. No corresponding emission is
visible in the ROSAT all-sky X-ray survey and no cluster is evident in the
Palomar all-sky optical survey. Compared with existing SZ images of distant
clusters, the extent is large (\approx 10') and complex; our analysis favours a
model containing two clusters rather than a single cluster. Our Bayesian
analysis is currently limited to modelling each cluster with an ellipsoidal or
spherical beta-model, which do not do justice to this decrement. Fitting an
ellipsoid to the deeper candidate we find the following. (a) Assuming that the
Evrard et al. (2002) approximation to Press & Schechter (1974) correctly gives
the number density of clusters as a function of mass and redshift, then, in the
search area, the formal Bayesian probability ratio of the AMI detection of this
cluster is 7.9 \times 10^4:1; alternatively assuming Jenkins et al. (2001) as
the true prior, the formal Bayesian probability ratio of detection is 2.1
\times 10^5:1. (b) The cluster mass is MT,200 = 5.5+1.2\times 10^14h-1M\odot.
(c) Abandoning a physical model with num- -1.3 70 ber density prior and instead
simply modelling the SZ decrement using a phenomenological {\beta}-model of
temperature decrement as a function of angular distance, we find a central SZ
temperature decrement of -295+36 {\mu}K - this allows for CMB primary
anisotropies, receiver -15 noise and radio sources. We are unsure if the
cluster system we observe is a merging system or two separate clusters.Comment: accepted MNRAS. 12 pages, 9 figure
Am empirical comparison of the performance of classical power indices
Power indices are general measures of the relative voting power of individual members of a voting body. They are useful in helping understand and design voting bodies particularly those which employ weighted voting, in which different members having different numbers of votes. It is well known that in such bodies a member's voting power, in the sense of their capacity to affect the outcomes of votes called, rarely corresponds to the actual number of votes allocated to him. Many voting bodies for which this is an important consideration exist: examples include international organisations (notably the World Bank, the IMF, the European Union), the US presidential Electoral College and corporations in which votes are proportionate to stockholdings.
Two classical power indices dominate the literature: the Shapley-Shubik index and the Banzhaf index (also known by other names). Both are based on the idea that a member's power depends on the relative number of times they can change a coalition from losing to winning by joining it and adding their vote. They may be defined in probabilistic terms as the probability of being able to swing the result of a vote, where all possible outcomes are taken as equiprobable. The indices differ however in the way they count voting coalitions. In probabilistic terms they use different coalition models and therefore differ in precisely what is meant by equiprobable outcomes.
The indices have been used in a number of empirical applications but their relative performance has remained an open question for many years, a factor, which has hindered the wider acceptance of the approach.
Where both the indices have been used for the same case, they have often given different results, sometimes substantially so, and theoretical studies of their properties have not been conclusive. There is therefore a need for comparative testing of their relative performance in practical contexts. Very little work of this type has been done however for a number of reasons: lack of independent indicators of power in actual voting bodies with which to compare them, difficulties in obtaining consistent data on a voting body over time with sufficient variation in the disposition of votes among members of actual legislatures and the lack of independent criteria against which the results of the indices may be judged. It has also been hampered to some extent by lack of easily available algorithms for computing the indices in large games.
This paper assesses the indices against a set of reasonable criteria in terms of shareholder voting power and the control of the corporation in a large cross section of British companies. Each company is a separate voting body and there is much variation in the distribution of voting shares among them. Moreover reasonable criteria exist against which to judge the indices. New algorithms for the Shapley-Shubik and Banzhaf indices are applied to detailed data on beneficial ownership of 444 large UK companies without majority control. Because some of the data is missing, both finite and oceanic games of shareholder voting are studied to overcome this problem.
The results, judged against these criteria, are unfavorable to the Shapley-Shubik index and suggest that the Banzhaf index much better reflects the variations in the power of shareholders between companies as the weights of shareholder blocks vary
The structure of the Shiga toxin 2a A-subunit dictates the interactions of the toxin with blood components
Hemolytic uremic syndrome (eHUS) is a severe complication of human infections with Shiga toxins (Stxs)-producing Escherichia coli. A key step in the pathogenesis of eHUS is the interaction of Stxs with blood components before the targeting of renal endothelial cells. Here, we show that a single proteolytic cleavage in the Stx2a A-subunit, resulting into two fragments (A1 and A2) linked by a disulfide bridge (cleaved Stx2a), dictates different binding abilities. Uncleaved Stx2a was confirmed to bind to human neutrophils and to trigger leukocyte/platelet aggregate formation, whereas cleaved Stx2a was ineffective. Conversely, binding of complement factor H was confirmed for cleaved Stx2a and not for uncleaved Stx2a. It is worth noting that uncleaved and cleaved Stx2a showed no differences in cytotoxicity for Vero cells or Raji cells, structural conformation, and contaminating endotoxin. These results have been obtained by comparing two Stx2a batches, purified in different laboratories by using different protocols, termed Stx2a(cl; cleaved toxin, Innsbruck) and Stx2a(uncl; uncleaved toxin, Bologna). Stx2a(uncl) behaved as Stx2a(cl) after mild trypsin treatment. In this light, previous controversial results obtained with purified Stx2a has to be critically re-evaluated; furthermore, characterisation of the structure of circulating Stx2a is mandatory to understand eHUS-pathogenesis and to develop therapeutic approaches
Beam-helicity asymmetries for single-hadron production in semi-inclusive deep-inelastic scattering from unpolarized hydrogen and deuterium targets
A measurement of beam-helicity asymmetries for single-hadron production in
deep-inelastic scattering is presented. Data from the scattering of 27.6 GeV
electrons and positrons off gaseous hydrogen and deuterium targets were
collected by the HERMES experiment. The asymmetries are presented separately as
a function of the Bjorken scaling variable, the hadron transverse momentum, and
the fractional energy for charged pions and kaons as well as for protons and
anti-protons. These asymmetries are also presented as a function of the three
aforementioned kinematic variables simultaneously
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