8,627 research outputs found
Relativistic Tunneling Through Two Successive Barriers
We study the relativistic quantum mechanical problem of a Dirac particle
tunneling through two successive electrostatic barriers. Our aim is to study
the emergence of the so-called \emph{Generalized Hartman Effect}, an effect
observed in the context of nonrelativistic tunneling as well as in its
electromagnetic counterparts, and which is often associated with the
possibility of superluminal velocities in the tunneling process. We discuss the
behavior of both the phase (or group) tunneling time and the dwell time, and
show that in the limit of opaque barriers the relativistic theory also allows
the emergence of the Generalized Hartman Effect. We compare our results with
the nonrelativistic ones and discuss their interpretation.Comment: 7 pages, 3 figures. Revised version, with a new appendix added.
Slightly changes in the styles and captions of Figures 1 and 2. To appear in
Physical Review
Disorder and the effective Mn-Mn exchange interaction in GaMnAs diluted magnetic semiconductors
We perform a theoretical study, using {\it ab initio} total energy
density-functional calculations, of the effects of disorder on the
exchange interactions for diluted semiconductors. For a 128
atoms supercell, we consider a variety of configurations with 2, 3 and 4 Mn
atoms, which correspond to concentrations of 3.1%, 4.7%, and 6.3%,
respectively. In this way, the disorder is intrinsically considered in the
calculations. Using a Heisenberg Hamiltonian to map the magnetic excitations,
and {\it ab initio} total energy calculations, we obtain the effective \JMn,
from first () all the way up to sixth () neighbors. Calculated
results show a clear dependence in the magnitudes of the \JMn with the Mn
concentration . Also, configurational disorder and/or clustering effects
lead to large dispersions in the Mn-Mn exchange interactions, in the case of
fixed Mn concentration. Moreover, theoretical results for the ground-state
total energies for several configurations indicate the importance of a proper
consideration of disorder in treating temperature and annealing effects
A model for shock wave chaos
We propose the following model equation:
that predicts chaotic shock waves.
It is given on the half-line and the shock is located at for any
. Here is the shock state and the source term is assumed
to satisfy certain integrability constraints as explained in the main text. We
demonstrate that this simple equation reproduces many of the properties of
detonations in gaseous mixtures, which one finds by solving the reactive Euler
equations: existence of steady traveling-wave solutions and their instability,
a cascade of period-doubling bifurcations, onset of chaos, and shock formation
in the reaction zone.Comment: 4 pages, 4 figure
Hole concentration in a diluted ferromagnetic semiconductor
We consider a mean-field approach to the hole-mediated ferromagnetism in
III-V Mn-based semiconductor compounds to discuss the dependence of the hole
density on that of Mn sites in Ga_{1-x}Mn_xAs. The hole concentration, p, as a
function of the fraction of Mn sites, x, is parametrized in terms of the
product m*J_{pd}^2 (where m* is the hole effective mass and J_{pd} is the
Kondo-like hole/local-moment coupling), and the critical temperature Tc. By
using experimental data for these quantities, we have established the
dependence of the hole concentration with x, which can be associated with the
occurrence of a reentrant metal-insulator transition taking place in the hole
gas. We also calculate the dependence of the Mn magnetization with x, for
different temperatures (T), and found that as T increases, the width of the
composition-dependent magnetization decreases drammatically, and that the
magnetization maxima also decreases, indicating the need for quality-control of
Mn-doping composition in diluted magnetic semiconductor devices.Comment: 4 pages, 3 figures, RevTeX 3; Fig. 1 changed, new references adde
Gráfico de contorno preenchido: solução para a representação sintética de um conjunto de imagens de satélite.
Imagens de satélite podem ser obtidas sobre extensas áreas da superfície terrestre e têm grande utilidade no monitoramento do uso do solo. Uma região, definida por contornos geopolíticos, pode ser imageada diversas vezes por um mesmo sensor ao longo de um intervalo de tempo, como o ano civil, produzindo um enorme e rico volume de dados, mas cuja interpretação conjunta é complexa. O objetivo do presente trabalho foi o de apresentar uma forma de visualização de dados de sensoriamento remoto, relativamente simples, que permite reunir em um único gráfico de contorno preenchido (filled contour plot), informações de diversas imagens obtidas ao longo de um período de tempo sobre uma região. Para ilustrar a técnica proposta, dados do índice de vegetação EVI2 de quatro municípios do Estado de São Paulo, obtidos a partir de 23 imagens MODIS/Terra datadas de 2008, foram analisados. Os resultados permitem afirmar que é possível associar, de maneira lógica, o aspecto do gráfico e o uso predominante do solo, ao se considerar a fenologia e o manejo das principais atividades agrícolas dos municípios estudados. A principal vantagem do método gráfico apresentado é que ele permite resumir, em uma única figura, o comportamento espectral (nesse caso, o EVI2), em uma região geopolítica (município), durante um período de tempo(ano), representado por várias imagens (23). Assim, torna-se possível comparar conjuntos de imagens obtidas durante períodos de duração variada, entre regiões de tamanhos e formas diferentes, como os municípios, utilizando sempre um mesmo padrão gráfico de fácil visualização e interpretação
Regional and Fishery-specific Patterns of Age and Growth of Yellowtail Snapper, Ocyurus chrysurus
We sampled yellowtail snapper, Ocyurus chrysurus, from commercial and recreational fisheries and fishery-independent surveys in the Atlantic Ocean off south Florida from 1980 through 2002. Specimens were collected primarily from two areas: Palm Beach and Monroe counties; collections were divided at 26° latitude into northern and southern populations. We collected sagittal otoliths and corresponding morphometric data from each population. Fork lengths (FL) ranged from 115 to 605 mm with a mean length of 312 mm. Yellowtail snapper were aged using sagittal otoliths with a high degree of precision [average percent error (APE) \u3c1%]. Ages ranged from 1 to 17 years, with mean ages of 3.96 years for the commercial fishery, 3.33 years for the recreational fishery, and 3.00 years for fishery-independent surveys. Yellowtail snapper entered the commercial and recreational fisheries by age 2; both fisheries were dominated by 2 and 3 year olds. The commercial fishery indicated the influence of a strong 1994 year class; this was not apparent in the recreational and fishery-independent surveys possibly due to small sample size. The von Bertalanffy growth curve parameters for all years and fishing modes combined [Lt = 410(1 - e-0.27(t+2.03))] were similar to previously published estimates for yellowtail snapper. The instantaneous total mortality rate of yellowtail snapper for all years and fishing modes combined (Z = 0.49) was also similar to previously published estimates. The total mortality rate for the northern population, Z = 0.67, was greater than for the southern population, Z = 0.45. Weight-length relationships were significantly different between northern and southern populations (P \u3c 0.001), and yellowtail snapper from the southern population were significantly larger and older than those from the northern population (P \u3c 0.001). Size-at-age was significantly larger for the most common ages (1--4 years) in the northern population compared to the southern population (age 1, P = 0.002; age 2--4, P \u3c 0.001 ). This may be due in part to differential fishing pressure; additional site-specific sampling is needed to elucidate the demographic differences between populations
Micofenolato mofetil pode proteger contra a pneumonia por Pneumocystis carinii em transplantados renais
Pneumocystis carinii pneumonia (PCP) is usually prevented in transplanted patients by prophylactic trimethoprim-sulfamethoxazol (TMS). Mycophenolate mofetil (MMF) has been shown to have a strong protective effect against PCP in rats. This effect is also suggested in humans by the absence of PCP in patients receiving MMF. After January 1998 MMF has been used with no TMS prophylaxis in renal transplanted patients. In azathioprine (AZA) treated patients TMS prophylaxis was maintained. The incidence of PCP was analyzed in both groups. Data were collected in order to have a minimum 6-month follow-up. Two hundred and seventy-two patients were eligible for analysis. No PCP occurred either in patients under MMF without TMS prophylaxis nor in patients under AZA. MMF may have an effective protective role against PCP as no patient under MMF, despite not receiving TMS coverage, developed PCP. A larger, controlled, trial is warranted to consolidate this information.A pneumonia por Pneumocystis carinii (PPC) em transplantados renais é, habitualmente, prevenida pelo uso profilático de trimetoprim-sulfametoxazol (TMS). Foi demonstrado que o micofenolato mofetil (MMF) exerce um poderoso efeito protetor sobre a PPC experimental em ratos. Este efeito também foi sugerido em humanos pela ausência de PPC em pacientes recebendo MMF. A partir de janeiro de 1998 passamos a usar o MMF em transplantados renais sem profilaxia por TMS. Nos pacientes recebendo azatioprina (AZA) a profilaxia com TMS continuou a ser empregada. A incidência de PPC foi analisada em ambos os grupos. Os dados foram coletados após um mínimo de seis meses de seguimento. Foram analisados 272 pacientes. Não ocorreu nenhum caso de PPC tanto nos pacientes recebendo MMF como naqueles recebendo AZA. O MMF pode ter exercido um efeito protetor contra a PPC, já que nenhum paciente sob MMF e sem receber profilaxia por TMS desenvolveu PPC. Estudos maiores e controlados se fazem necessários para confirmar estas informações
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