26 research outputs found
Non Linear Current Response of a Many-Level Tunneling System: Higher Harmonics Generation
The fully nonlinear response of a many-level tunneling system to a strong
alternating field of high frequency is studied in terms of the
Schwinger-Keldysh nonequilibrium Green functions. The nonlinear time dependent
tunneling current is calculated exactly and its resonance structure is
elucidated. In particular, it is shown that under certain reasonable conditions
on the physical parameters, the Fourier component is sharply peaked at
, where is the spacing between
two levels. This frequency multiplication results from the highly nonlinear
process of photon absorption (or emission) by the tunneling system. It is
also conjectured that this effect (which so far is studied mainly in the
context of nonlinear optics) might be experimentally feasible.Comment: 28 pages, LaTex, 7 figures are available upon request from
[email protected], submitted to Phys.Rev.
Resonant tunneling through ultrasmall quantum dots: zero-bias anomalies, magnetic field dependence, and boson-assisted transport
We study resonant tunneling through a single-level quantum dot in the
presence of strong Coulomb repulsion beyond the perturbative regime. The level
is either spin-degenerate or can be split by a magnetic field. We, furthermore,
discuss the influence of a bosonic environment. Using a real-time diagrammatic
formulation we calculate transition rates, the spectral density and the
nonlinear characteristic. The spectral density shows a multiplet of Kondo
peaks split by the transport voltage and the boson frequencies, and shifted by
the magnetic field. This leads to zero-bias anomalies in the differential
conductance, which agree well with recent experimental results for the electron
transport through single-charge traps. Furthermore, we predict that the sign of
the zero-bias anomaly depends on the level position relative to the Fermi level
of the leads.Comment: 27 pages, latex, 21 figures, submitted to Phys. Rev.
A importùncia da associação obesidade e gravidez
Characteristics of the evolution of pregnancy in obese women were studied for their effect on newborn infants. Two control groups were observed - one of normal weight pregnant women, one of obese. The variables selected were: the socio-economic status of the family and the mother's age, height, arm circunference, prepregancy weight, total number of pregnancies, parity, weight gain during pregnancy, obstetric complications, birth weight, and fetal vitality. Results showed that pregnancy in obese women differs from that in normal weight women and that they show a larger incidence of obstetric complications. Children of obese mothers had a higher mortality rate principally in the perinatal period; moreover, there was also a higher rate of prematurity and a higher proportion of overweight babies among obese mothers. As a result, the distribution of the curve of the birth weight of infants of obese morthers was higher than that of infants of normal weight mothers. The conclusion reached was that whenever a pregnant obese woman reduced foot intake, with resultant insufficient weight gain, intrauterine growth was affected. Thus, it follows that pregnancy is not the best time for the obese mother to lose weight; for this reason, it is important that she receive adequate guidance in regard to diet. Obesity, therefore, is a factor contributing to high-risk pregnancy which can affect both mother and child.Foram estudados dois grupos de gestantes, sendo um de grĂĄvidas normais e outro de obesas, com a finalidade de reconhecer algumas caracterĂsticas da evolução da gravidez, em mulheres obesas, e suas repercussĂ”es sobre o concepto. Foram relacionadas as seguintes variĂĄveis: status sĂłcio-econĂŽmico familiar, idade, altura, perĂmetro braquial, peso habitual, nĂșmero de gestaçÔes anteriores, paridade materna, ganho de peso durante a gestação, idade gestacional, intercorrĂȘncias durante a gestação, peso ao nascer e vitalidade do recĂ©m-nascido. Pelos resultados concluiu-se que as gestantes obesas sĂŁo diferentes das normais e apresentam maior incidĂȘncia de complicaçÔes obstĂ©tricas. Os recĂ©m-nascidos, filhos de obesas, registraram Ăndice maior de mortalidade, principalmente no perĂodo perinatal. Houve maior incidĂȘncia de prematuridade e de fetos macrossĂŽmicos, sendo a curva de distribuição de peso ao nascer diferente da dos recĂ©m-nascidos das gestantes normais. A mĂ©dia de peso ao nascer das crianças das gestantes obesas Ă© maior que o das normais. Concluiu-se ainda que toda vez que a gestante obesa sofre restrição alimentar, com ganho de peso inadequado, o crescimento intra-uterino Ă© afetado; nĂŁo sendo, portanto, a Ă©poca da gravidez a melhor para a obesa perder peso, mas, ao contrĂĄrio, ela deveria receber uma orientação alimentar adequada. A obesidade Ă© pois um fator de aumento do risco gravĂdico, que pode afetar tanto a mĂŁe como o concepto
Influence de la Puissance Maritime dans l'histoire : 1660-1783
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