3,177 research outputs found
Limits of the energy-momentum tensor in general relativity
A limiting diagram for the Segre classification of the energy-momentum tensor
is obtained and discussed in connection with a Penrose specialization diagram
for the Segre types. A generalization of the coordinate-free approach to limits
of Paiva et al. to include non-vacuum space-times is made. Geroch's work on
limits of space-times is also extended. The same argument also justifies part
of the procedure for classification of a given spacetime using Cartan scalars.Comment: LaTeX, 21 page
Limits of space-times in five dimensions and their relation to the Segre Types
A limiting diagram for the Segre classification in 5-dimensional space-times
is obtained, extending a recent work on limits of the energy-momentum tensor in
general relativity. Some of Geroch's results on limits of space-times in
general relativity are also extended to the context of five-dimensional
Kaluza-Klein space-times.Comment: Late
Charge-density waves in one-dimensional Hubbard superlattices
We study the formation of charge density waves (CDW's) in one-dimensional
Hubbard superlattices, modeled by a repeated pattern of repulsive (U>0) and
free (U=0) sites. By means of Lanczos diagonalizations for the ground state, we
calculate the charge structure factor. Our results show that while the
superlattice structure affects the modulation of the charge density waves, the
periodicity can still be predicted through an effective density. We also show
that, for a fixed repulsive layer thickness, the periodicity of the CDW is an
oscillatory function of the free layer thickness.Comment: 4 pages, 4 figure
Modulation of charge-density waves by superlattice structures
We discuss the interplay between electronic correlations and an underlying
superlattice structure in determining the period of charge density waves
(CDW's), by considering a one-dimensional Hubbard model with a repeated
(non-random) pattern of repulsive (U>0) and free (U=0) sites. Density matrix
renormalization group diagonalization of finite systems (up to 120 sites) is
used to calculate the charge-density correlation function and structure factor
in the ground state. The modulation period can still be predicted through
effective Fermi wavevectors, k_F*, and densities, and we have found that it is
much more sensitive to electron (or hole) doping, both because of the narrow
range of densities needed to go from q*=0 to \pi, but also due to sharp
2k_F*-4k_F* transitions; these features render CDW's more versatile for actual
applications in heterostructures than in homogeneous systems.Comment: 4 pages, 5 figures, to appear in Phys Rev
Comparação entre variedades de mangueira em relação a características do fruto nas condições do Submédio do São Francisco.
Este trabalho objetivou avaliar atributos do fruto de 14 variedades nas condições do Submédio do Vale do São Francisc
Variabilidade para teor de tanino em sorgo (Sorghum bicolor L.) e sua associação com a resistência a pássaros.
O objetivo do presente trabalho foi verificar a variabilidade existente entre diversos materiais geneticos quanto ao teor de tanino e resistencia a passaros nas fases de grao leitoso, grao pastoso, maturacao fisiologicas e de colheita. O experimento foi conduzido no delineamento blocos casualizados em tres repeticoes e semeadura em outubro/89 no Centro Nacional de Pesquisa de Milho e Sorgo (CNPMS/EMBRAPA) em Sete Lagoas, Estado de Minas Gerais. Os resultados mostraram maior teor de tanino na materia seca na fase de grao leitoso. No entanto, quando se considerou o conteudo de tanino na materia verde do grao, os resultados mostraram maior teor nas fases de maturacao fisiologicas ou de colheita para sorgo de alto tanino (valores maiores que 1%). O dano causado por passaro iniciou-se na fase de grao pastoso, sendo que todos os materiais geneticos resistentes apresentaram alto teor de tanino no grao na fase de colheita
Valoração da erosão do solo e a sustentabilidade ambiental no espaço rural do município de Araras, SP.
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