Multiquantum well spin oscillator

A dc voltage biased II-VI semiconductor multiquantum well structure attached to normal contacts exhibits self-sustained spin-polarized current oscillations if one or more of its wells are doped with Mn. Without magnetic impurities, the only configurations appearing in these structures are stationary. Analysis and numerical solution of a nonlinear spin transport model yield the minimal number of wells (four) and the ranges of doping density and spin splitting needed to find oscillations.Comment: 11 pages, 2 figures, shortened and updated versio

Magnetoswitching of current oscillations in diluted magnetic semiconductor nanostructures

Strongly nonlinear transport through Diluted Magnetic Semiconductor multiquantum wells occurs due to the interplay between confinement, Coulomb and exchange interaction. Nonlinear effects include the appearance of spin polarized stationary states and self-sustained current oscillations as possible stable states of the nanostructure, depending on its configuration and control parameters such as voltage bias and level splitting due to an external magnetic field. Oscillatory regions grow in size with well number and level splitting. A systematic analysis of the charge and spin response to voltage and magnetic field switching of II-VI Diluted Magnetic Semiconductor multiquantum wells is carried out. The description of stationary and time-periodic spin polarized states, the transitions between them and the responses to voltage or magnetic field switching have great importance due to the potential implementation of spintronic devices based on these nanostructures.Comment: 14 pages, 4 figures, Revtex, to appear in PR

Productivity of a \u3cem\u3eLeucaena Leucocephala-Cynodon Nlemfuensis\u3c/em\u3e Silvopastoral System with Sheep in Yucatan, Mexico

Animal production in the tropics of Mexico is based on grazed grasslands of low productivity; this type of production system has reduced the areas of natural vegetation and damaged the ecology (erosion of flora, fauna and soil). Silvopastoral technologies may improve the welfare and economic conditions of the rural population and, consequently, preserve their natural resources. The current work was designed to assess the introduction of Leucaena leucocephala in a silvopastoral system with Cynodon nlemfuensis (star grass) grazed by sheep

Observational Characterization of the Downward Atmospheric Longwave Radiation at the Surface in the City of São Paulo

This work describes the seasonal and diurnal variations of downward longwave atmospheric irradiance (LW) at the surface in São Paulo, Brazil, using 5-min-averaged values of LW, air temperature, relative humidity, and solar radiation observed continuously and simultaneously from 1997 to 2006 on a micrometeorological platform, located at the top of a 4-story building. An objective procedure, including 2-step filtering and dome emission effect correction, was used to evaluate the quality of the 9-yr-long LW dataset. The comparison between LW values observed and yielded by the Surface Radiation Budget project shows spatial and temporal agreement, indicating that monthly and annual average values of LW observed in one point of São Paulo can be used as representative of the entire metropolitan region of São Paulo. The maximum monthly averaged value of the LW is observed during summer (389 ± 14 W m-2; January), and the minimum is observed during winter (332 ± 12 W m-2; July). The effective emissivity follows the LW and shows a maximum in summer (0.907 ± 0.032; January) and a minimum in winter (0.818 ± 0.029; June). The mean cloud effect, identified objectively by comparing the monthly averaged values of the LW during clear-sky days and all-sky conditions, intensified the monthly average LW by about 32.0 ± 3.5 W m-2 and the atmospheric effective emissivity by about 0.088 ± 0.024. In August, the driest month of the year in São Paulo, the diurnal evolution of the LW shows a minimum (325 ± 11 W m-2) at 0900 LT and a maximum (345 ± 12 W m-2) at 1800 LT, which lags behind (by 4 h) the maximum diurnal variation of the screen temperature. The diurnal evolution of effective emissivity shows a minimum (0.781 ± 0.027) during daytime and a maximum (0.842 ± 0.030) during nighttime. The diurnal evolution of all-sky condition and clear-sky day differences in the effective emissivity remain relatively constant (7% ± 1%), indicating that clouds do not change the emissivity diurnal pattern. The relationship between effective emissivity and screen air temperature and between effective emissivity and water vapor is complex. During the night, when the planetary boundary layer is shallower, the effective emissivity can be estimated by screen parameters. During the day, the relationship between effective emissivity and screen parameters varies from place to place and depends on the planetary boundary layer process. Because the empirical expressions do not contain enough information about the diurnal variation of the vertical stratification of air temperature and moisture in São Paulo, they are likely to fail in reproducing the diurnal variation of the surface emissivity. The most accurate way to estimate the LW for clear-sky conditions in São Paulo is to use an expression derived from a purely empirical approach

Self-similar chain conformations in polymer gels

We use molecular dynamics simulations to study the swelling of randomly end-cross-linked polymer networks in good solvent conditions. We find that the equilibrium degree of swelling saturates at Q_eq = N_e**(3/5) for mean strand lengths N_s exceeding the melt entanglement length N_e. The internal structure of the network strands in the swollen state is characterized by a new exponent nu=0.72. Our findings are in contradiction to de Gennes' c*-theorem, which predicts Q_eq proportional N_s**(4/5) and nu=0.588. We present a simple Flory argument for a self-similar structure of mutually interpenetrating network strands, which yields nu=7/10 and otherwise recovers the classical Flory-Rehner theory. In particular, Q_eq = N_e**(3/5), if N_e is used as effective strand length.Comment: 4 pages, RevTex, 3 Figure