7,201 research outputs found
Role of Self-Interaction Effects in the Geometry Optimization of Small Metal Clusters
By combining the Self-Interaction Correction (SIC) with pseudopotential
perturbation theory, the role of self-interaction errors inherent to the Local
Density Approximation (LDA) to Density Functional Theory is estimated in the
determination of ground state and low energy isomeric structures of small
metallic clusters. Its application to neutral sodium clusters with 8 and 20
atoms shows that the SIC provides sizeable effects in Na_8, leading to a
different ordering of the low lying isomeric states compared with ab-initio LDA
predictions, whereas for Na_20, the SIC effects are less pronounced, such that
a quantitative agreement is achieved between the present method and ab-initio
LDA calculations.Comment: RevTeX, 4 pages, 1 figure available from [email protected]
Equilibrium configurations for quark-diquark stars and the problem of Her X-1 mass
We report new calculations of the physical properties of a quark-diquark
plasma. A vacuum contribution is taken into account and is responsible for the
appearance of a stable state at zero pressure and at a baryon density of about
2.2 times the nuclear matter density in this model. The resulting equation of
state was used to integrate numerically the Tolman-Oppenheimer-Volkoff
equations. The mass-radius relationship has been derived from a series of
equilibrium configurations constituted by a mixture of quarks and diquarks.
These stellar models, which are representative of a whole class, may be helpful
to understand the possible compactness of the X-ray source Her X-1 and related
objects.Comment: 15 pp., PlainTex file + 3 figures available upon request at
[email protected]. Submitted to Int. Jour. Mod. Phys.
Utilização da estatística Pi na predição de ganhos por seleção em famílias de meios-irmãos de milho-pipoca.
Neste trabalho utiliza-se da estatistica Pi, medida de estabilidade, adaptabilidade e adaptacao, como criterio alternativo a selecao com base nas medias da analise de variancia conjunta. Foram utilizadas as avaliacoes de 166 familias de meios-irmaos de milho-pipoca CMS 43, em Sete Lagoas, MG, e Coimbra, MG, no delineamento em blocos casualizados, com duas repeticoes. Utilizou-se o estimador baseado no diferencial de selecao para estimar os ganhos preditos. Nos dois locais esses ganhos, com base na estatistica Pi, foram semelhantes aos ganhos preditos com base nos dados da analise de variancia conjunta. Considera-se que a selecao com base na estatistica Pi e um novo procedimento valido e de interesse na fase de predicao de ganhos de um programa de melhoramento
Número mínimo de famílias de meios-irmãos para representar uma população de milho-pipoca.
O numero minimo de familias de meios-irmaos, no melhoramento genetico, e aquele que representa uma populacao, permitindo obter estimativas consistentes e estaveis dos parametros geneticos. A media e a variancia genetica foram escolhidas para caracterizar a populacao de 166 familias de meios-irmaos de milho-pipoca CMS 43, avaliada em Sete Lagoas e em Coimbra, MG, no deliamento em blocos casualizados, com duas repeticoes, 1997/1998. A tecnica de simulacao bootstrap foi utilizada para se obterem as informacoes necessarias. A determinacao do numero minimo e feita para analise visual dos graficos da estabilizacao da media e da variancia de subamostras. Na caracteristica prolificidade foram necessarias 20 familias na caracteristica producao de graos/ha, 161; e na capacidade de expansao, 141. Este e o numero minimo de familias considerado adequado para representar esta populacao. O metodo de simulacao de subamostras proposto contribui para reduzir os trabalhos e os custos da pesquisa agronomica, mantendo a precisao e a exatidao desejada
Predição de ganhos em famílias de meios irmãos do milho-pipoca CMS 43.
Foram avaliadas 166 famílias de meios irmãos de milho-pipoca (Zea mays L), da população CMS 43, em Sete Lagoas, Minas Gerais, em 1997-1998. O objetivo foi predizer ganhos simultâneos nas características capacidade de expansão e produtividade e identificar características positivamente correlacionadas com esses dois caracteres. Usou-se o delineamento em blocos casualizados, com duas repetições e com testemunhas. Os ganhos diretos preditos foram superiores aos ganhos correlacionados. Os descritores produtividade e capacidade de expansão foram negativamente correlacionados, inferindo-se que a seleção para um deles implicará em perdas prováveis na expressão do outro. O descritor altura de planta poderá ser usado na seleção indireta para aumento simultâneo de produtividade e capacidade de expansão
Using the best linear predictor (BLP) in the selection between and among half-sib progenies of the CMS-39 maize population.
Data of corn ear production (kg/ha) of 196 half-sib progenies (HSP) of the maize population CMS-39 obtained from experiments carried out in four environments were used to adapt and assess the BLP method (best linear predictor) in comparison with to the selection among and within half-sib progenies (SAWHSP). The 196 HSP of the CMS-39 population developed by the National Center for Maize and Sorghum Research (CNPMS-EMBRAPA) were related through their pedigree with the recombined progenies of the previous selection cycle. The two methodologies used for the selection of the twenty best half-sib progenies. BLP and SAWHSP, led to similar expected genetic grains. There was a tendency in the BLP methodology to select a greater number of related progenies because of the previous generation (pedigree) than the other method. This implies that greater care with the effictive size of the population must be taken with this method. The SAWHSP methodology was efficient in isolating tha aditive genetic variance component from the phenotypic component. The pedigree system, although unnecessary for the routine use of the SAWHSP methodology, allowed the prediction of an increase in the inbreeding of the population in the long term SAWHSP selection when recombination is simultaneous to creation of new progenies
A análise de fatores na predição de ganhos por seleção em milho (Zea mays L.).
Analysis of prediction factors in yields by maize selection (Zea mays L.). The history and use of the Factor Analysis in agronomic studies, especially for the prediction of selection yields, is presented. The Factor Analysis was applied on data provided by the evaluation of 166 half-sib families of popcorn at the National Research Center in Maize and Sorghum, Sete Lagoas MG Brazil, in 1997-1998. Selection yield prediction based on scores from the Fator Analysis favored yields in the most important traits. Scores were used for yield prediction by direct and indirect selection and by several selection indices. The Factor Analysis is a useful multivariate method for prediction of selection yields and may either substitute the traditional method or supplement selection by selection indices
A bright nanowire single photon source based on SiV centers in diamond
The practical implementation of many quantum technologies relies on the
development of robust and bright single photon sources that operate at room
temperature. The negatively charged silicon-vacancy (SiV-) color center in
diamond is a possible candidate for such a single photon source. However, due
to the high refraction index mismatch to air, color centers in diamond
typically exhibit low photon out-coupling. An additional shortcoming is due to
the random localization of native defects in the diamond sample. Here we
demonstrate deterministic implantation of Si ions with high conversion
efficiency to single SiV- centers, targeted to fabricated nanowires. The
co-localization of single SiV- centers with the nanostructures yields a ten
times higher light coupling efficiency than for single SiV- centers in bulk
diamond. This enhanced photon out-coupling, together with the intrinsic
scalability of the SiV- creation method, enables a new class of devices for
integrated photonics and quantum science.Comment: 15 pages, 5 figure
Stacking Gravitational Wave Signals from Soft Gamma Repeater Bursts
Soft gamma repeaters (SGRs) have unique properties that make them intriguing
targets for gravitational wave (GW) searches. They are nearby, their burst
emission mechanism may involve neutron star crust fractures and excitation of
quasi-normal modes, and they burst repeatedly and sometimes spectacularly. A
recent LIGO search for transient GW from these sources placed upper limits on a
set of almost 200 individual SGR bursts. These limits were within the
theoretically predicted range of some models. We present a new search strategy
which builds upon the method used there by "stacking" potential GW signals from
multiple SGR bursts. We assume that variation in the time difference between
burst electromagnetic emission and burst GW emission is small relative to the
GW signal duration, and we time-align GW excess power time-frequency tilings
containing individual burst triggers to their corresponding electromagnetic
emissions. Using Monte Carlo simulations, we confirm that gains in GW energy
sensitivity of N^{1/2} are possible, where N is the number of stacked SGR
bursts. Estimated sensitivities for a mock search for gravitational waves from
the 2006 March 29 storm from SGR 1900+14 are also presented, for two GW
emission models, "fluence-weighted" and "flat" (unweighted).Comment: 17 pages, 16 figures, submitted to PR
A comparative analysis of the observed white dwarf cooling sequence from globular clusters
We report our study of features at the observed red end of the white dwarf
cooling sequences for three Galactic globular clusters: NGC\,6397, 47\,Tucanae
and M\,4. We use deep colour-magnitude diagrams constructed from archival
Hubble Space Telescope (ACS) to systematically investigate the blue turn at
faint magnitudes and the age determinations for each cluster. We find that the
age difference between NGC\,6397 and 47\,Tuc is 1.98\,Gyr,
consistent with the picture that metal-rich halo clusters were formed later
than metal-poor halo clusters. We self-consistently include the effect of
metallicity on the progenitor age and the initial-to-final mass relation. In
contrast with previous investigations that invoked a single white dwarf mass
for each cluster, the data shows a spread of white dwarf masses that better
reproduce the shape and location of the blue turn. This effect alone, however,
does not completely reproduce the observational data - the blue turn retains
some mystery. In this context, we discuss several other potential problems in
the models. These include possible partial mixing of H and He in the atmosphere
of white dwarf stars, the lack of a good physical description of the
collision-induced absorption process and uncertainties in the opacities at low
temperatures. The latter are already known to be significant in the description
of the cool main sequence. Additionally, we find that the present day local
mass function of NGC\,6397 is consistent with a top-heavy type, while 47\,Tuc
presents a bottom-heavy profile.Comment: Accepted for publication in MNRAS (16 pages, 19 figures
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