5,816 research outputs found
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
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
The Levi-Civita spacetime
We consider two exact solutions of Einstein's field equations corresponding
to a cylinder of dust with net zero angular momentum. In one of the cases, the
dust distribution is homogeneous, whereas in the other, the angular velocity of
dust particles is constant [1]. For both solutions we studied the junction
conditions to the exterior static vacuum Levi-Civita spacetime. From this study
we find an upper limit for the energy density per unit length of the
source equal for the first case and for the second
one. Thus the homogeneous cluster provides another example [2] where the range
of is extended beyond the limit value previously found in
the literature [3,4]. Using the Cartan Scalars technics we show that the
Levi-Civita spacetime gets an extra symmetry for or
. We also find that the cluster of homogeneous dust has a superior
limit for its radius, depending on the constant volumetric energy density
Melanoma em Idade Pediátrica: Epidemiologia, Patogénese, Diagnóstico e Tratamento
Pediatric melanoma is the most common skin cancer in children. However, it is extremely rare this population, being even rarer in younger than 10 years of age. Its diagnosis is often difficult, due to its rarity and atypical presentations. There are three main subtypes of pediatric melanoma: Spitzoid melanoma, melanoma arising in a congenital melanocytic nevus and conventional melanoma. Congenital melanomas exist and are exceptionally rare, although they do not constitute a different subtype of melanoma. Spitzoid melanoma is the most common subtype affecting children younger than 11 years. Despite presenting with local aggressive features and frequent nodal involvement, it encompasses an excellent prognosis. The risk of malignant transformation of congenital melanocytic nevi varies widely accordingly to the projected adult size, number, and concomitant abnormalities found in the central nervous system. The surveillance and treatment of melanoma arising in a congenital melanocytic nevus is challenging, enclosing poor outcomes. In adolescents, the most common subtype is the conventional (adult-type). Contrary to the adult population, the majority of conventional pediatric melanoma arises from previous nevi but follows the general adult epidemiology and risk factors. Specific guidelines for management of pediatric melanoma do not exist and it is treated similarly to melanoma in the adult.O melanoma é o cancro cutâneo mais comum da idade pediátrica. No entanto, é extremamente raro nesta população, sendo ainda mais raro nas crianças com menos de 11 anos. O diagnóstico é frequentemente difícil, devido à sua raridade e apresentações atípicas. Existem três subtipos de melanoma em idade pediátrica: melanoma Spitzoide; melanoma que emerge de nevo melanocítico congénito e o melanoma convencional. O melanoma congénito pode ocorrer, embora seja excepcionalmente raro, não constituindo um subtipo distinto de melanoma. O melanoma Spitzoide é o subtipo mais comum que afecta crianças com idade inferior a 11 anos. Embora apresente características locais agressivas, com envolvimento ganglionar frequente, acarreta um prognóstico excelente. A ocorrência de transformação maligna de nevos melanocíticos congénitos apresenta uma grande variabilidade, de acordo com a sua dimensão, número e afeção concomitante do sistema nervoso central. A vigilância e tratamento desta transformação maligna é problemática, sendo associada a pobres resultados. Nos adolescentes, o subtipo de melanoma mais comum é o convencional (tipo adulto). Contrariamente à população adulta, a maioria dos melanomas pediátricos convencionais tem origem em nevos pré-existentes, seguindo a epidemiologia e factores de risco associados ao do adulto. Não existem orientações específicas para a gestão e tratamento do melanoma pediátrico, pelo que esta doença é tratada de forma semelhante ao melanoma do adulto
The Influence of the Distributor Plate on the Bottom Zone of a Fluidized Bed Approaching the Transition from Bubbling to Turbulent Fluidization
The dynamics of the bottom zone of a narrow fluidized bed, from bubbling to turbulent regimes, was studied in a cold model of 0.1 m i.d. and 1.3 m high. Tested distributor types were perforated perspex plates, with six different perforation grids, metallic mesh and porous ceramic, with pressures drops ranging from 0.05 to 350 kPa, corresponding to superficial air velocities from 0.1 to 2.3m s-1. Group B silica ballotini, within the range 0.355-0.425 mm, were used as bed material. The experimental data consisted of pressure drop and absolute pressure fluctuating signals, together with visual observations. The bottom zone presented a dynamic condition that produced higher pressure drop values than those expected with the incipient fluidization condition, particularly for the distributor plates with a higher open area. A simple model is used to describe the axial solids distribution and the dynamics characteristics of the voids created in that zone, and an attempt is made to differentiate the bubble voidage from that of the dense phase, with a value of through flow estimated in a systematic way.http://www.sciencedirect.com/science/article/B8JGF-4RT04WJ-5/1/35ae43b23db71e43caf98b5be93bab0
Effect of inhomogeneity on s-wave superconductivity in the attractive Hubbard model
Inhomogeneous s-wave superconductivity is studied in the two-dimensional,
square lattice attractive Hubbard Hamiltonian using the Bogoliubov-de Gennes
(BdG) mean field approximation. We find that at weak coupling, and for
densities mainly below half-filling, an inhomogeneous interaction in which the
on-site interaction takes on two values, results in a larger
zero temperature pairing amplitude, and that the superconducting can also
be significantly increased, relative to a uniform system with on all
sites. These effects are observed for stripe, checkerboard, and even random
patterns of the attractive centers, suggesting that the pattern of
inhomogeneity is unimportant. Monte Carlo calculations which reintroduce some
of the fluctuations neglected within the BdG approach see the same effect, both
for the attractive Hubbard model and a Hamiltonian with d-wave pairing
symmetry.Comment: 5 pages, 4 figure
s-wave Superconductivity Phase Diagram in the Inhomogeneous Two-Dimensional Attractive Hubbard Model
We study s-wave superconductivity in the two-dimensional square lattice
attractive Hubbard Hamiltonian for various inhomogeneous patterns of
interacting sites. Using the Bogoliubov-de Gennes (BdG) mean field
approximation, we obtain the phase diagram for inhomogeneous patterns in which
the on-site attractive interaction U_i between the electrons takes on two
values, U_i=0 and -U/(1-f) (with f the concentration of non-interacting sites)
as a function of average electron occupation per site n, and study the
evolution of the phase diagram as f varies. In certain regions of the phase
diagram, inhomogeneity results in a larger zero temperature average pairing
amplitude (order parameter) and also a higher superconducting (SC) critical
temperature T_c, relative to a uniform system with the same mean interaction
strength (U_i=-U on all sites). These effects are observed for stripes,
checkerboard, and even random patterns of the attractive centers, suggesting
that the pattern of inhomogeneity is unimportant. The phase diagrams also
include regions where superconductivity is obliterated due to the formation of
various charge ordered phases. The enhancement of T_{c} due to inhomogeneity is
robust as long as the electron doping per site n is less than twice the
fraction of interacting sites [2(1-f)] regardless of the pattern. We also show
that for certain inhomogeneous patterns, when n = 2(1-f), increasing
temperature can work against the stability of existing charge ordered phases
for large f and as a result, enhance T_{c}.Comment: 16 pages, 11 figure
Dispersion of carbon nanotubes in poly(lactic acid)
Carbon nanotubes (CNT) present excellent mechanical, electrical and thermal properties, and are expected to impart these properties into their composites. However, the CNT are grown as entangled bundles that are difficult to disperse in polymer matrices, or even in solvents. Several approaches have been tried for the efficient dispersion of CNT in polymer matrices, ranging from the CNT chemical modification to the use of different mixing methods. Previous studies have shown that the resulting filler dispersion is strongly dependent on the characteristics of the melt mixing equipment [1]. It is also known that the dispersion level of nanofillers strongly affects the final nanocomposite properties [2].
The present work reports the optimization of the CNT dispersion in poly (lactic acid) (PLA) using a small-scale twin-screw extruder. The CNT were chemically modified for improved interaction with PLA. The effect of varying the mixing parameters on the dispersion level of the CNT and functionalized CNT was evaluated using optical and electron microscopy. The electrical resistivity and mechanical properties of the composites were measured. It was observed that the incorporation of 1% (weight) of CNT reduced the electrical resistivity of the composite to 400 Ohm.m, and that 3% CNT rendered the composite conductive, with an electrical resistivity of 0.6 Ohm.m.Fundação para a Ciência e a Tecnologia (FCT) - POCI/QUI/59835/2004, bolsa de doutoramento SFRH/BD/32189/2006
A comparative study of the dispersion of carbon nanofibres in polymer melts
The dispersion of carbon nanofibres (CNF) in a polymer matrix using two melt mixing methods is studied. Distributive and dispersive mixing were evaluated by optical and electron microscopy. The CNF were chemically modified to improve the interface with the matrix. The results showed that the two methods produced good distribution of the filler, but extensional stresses induced higher dispersion. The latter correlated well with a decrease in electrical resistivity. Also, the chemical modification largely improved the CNF/polymer interfaceFundação para a Ciência e a Tecnologia (FCT
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