Self-sustained current oscillations in the kinetic theory of semiconductor superlattices

We present the first numerical solutions of a kinetic theory description of self-sustained current oscillations in n-doped semiconductor superlattices. The governing equation is a single-miniband Boltzmann-Poisson transport equation with a BGK (Bhatnagar-Gross-Krook) collision term. Appropriate boundary conditions for the distribution function describe electron injection in the contact regions. These conditions seamlessly become Ohm's law at the injecting contact and the zero charge boundary condition at the receiving contact when integrated over the wave vector. The time-dependent model is numerically solved for the distribution function by using the deterministic Weighted Particle Method. Numerical simulations are used to ascertain the convergence of the method. The numerical results confirm the validity of the Chapman-Enskog perturbation method used previously to derive generalized drift-diffusion equations for high electric fields because they agree very well with numerical solutions thereof.Comment: 26 pages, 16 figures, to appear in J. Comput. Phy

Algunas reflexiones para el debate

Ponencia presentada enPonencia presentada en el III Simposio Internacional de Inmigración “La contratación en origen: programas de contratación de inmigrantes”, Fundación Ciudadanía y Valores, 27 marzo 2009, MadridLa condición global del fenómeno de las migraciones internacionales requiere integrar perspectivas plurales de análisis, contextos diversos y conocimientos de distintos expertos en la materia. Es por esto, que la Fundación Ciudadanía y Valores ha organizado su III Simposio Internacional de Inmigración, proponiendo a debate un instrumento habilitado para la captación de mano de obra extranjera: los programas de contratación en origen de inmigrantes.el III Simposio Internacional de Inmigración “La contratación en origen: programas de contratación de inmigrantes”, Fundación Ciudadanía y Valores, 27 marzo 2009, MadridPeer reviewe

Ethnic businesses in Spain

[EN] Current migrations are very dynamic processes that are always readjusting to the changing conditions in both origin and destination areas. Among those readjusting strategies, ethnic businesses play an important role. The growing importance of ethnic business in Spain is reported using the data on foreign self-employed workers, published by the Ministry of Labor and Social Affaires. Current statistics about the establishment of small commercial enterprises skip the nationality of the owner/s, and therefore could not be used in this piece of research.Peer reviewe

Un modelo multirregional para la proyección de la población de las provincias españolas

La realización de proyecciones de población es un tema de estudio con cierta tradición pero que, a consecuencia de su complejidad y dificultad, no ha sido abordado adecuadamente en nuestro país, ya sea en número de trabajos como en validez de sus resultados - En especial se echan en falta proyecciones de la población española donde se tenga en cuenta explícitamente la dirección de los flujos migratorios interprovinciales. El objetivo de este trabajo es dar un paso adelante en este sentido. En la actualidad diversos grupos de investigación, tanto demógrafos como geógrafos, intentan desarrollar métodos y técnicas donde el espacio intervenga de modo explícito en los análisis demográficos.Peer reviewe

Aproximación al conocimiento de la estructura urbana de España y sus comunidades autónomas.

Sin resume

Seasonal variations of melatonin in ram seminal plasma are correlated to those of testosterone and antioxidant enzymes

<p>Abstract</p> <p>Background</p> <p>Some breeds of sheep are highly seasonal in terms of reproductive capability, and these changes are regulated by photoperiod and melatonin secretion. These changes affect the reproductive performance of rams, impairing semen quality and modifying hormonal profiles. Also, the antioxidant defence systems seem to be modulated by melatonin secretion, and shows seasonal variations. The aim of this study was to investigate the presence of melatonin and testosterone in ram seminal plasma and their variations between the breeding and non-breeding seasons. In addition, we analyzed the possible correlations between these hormones and the antioxidant enzyme defence system activity.</p> <p>Methods</p> <p>Seminal plasma from nine Rasa Aragonesa rams were collected for one year, and their levels of melatonin, testosterone, superoxide dismutase (SOD), glutathione reductase (GRD), glutathione peroxidase (GPX) and catalase (CAT) were measured.</p> <p>Results</p> <p>All samples presented measurable quantities of hormones and antioxidant enzymes. Both hormones showed monthly variations, with a decrease after the winter solstice and a rise after the summer solstice that reached the maximum levels in October-November, and a marked seasonal variation (P < 0.01) with higher levels in the breeding season. The yearly pattern of GRD and catalase was close to that of melatonin, and GRD showed a significant seasonal variation (P < 0.01) with a higher activity during the breeding season. Linear regression analysis between the studied hormones and antioxidant enzymes showed a significant correlation between melatonin and testosterone, GRD, SOD and catalase.</p> <p>Conclusions</p> <p>These results show the presence of melatonin and testosterone in ram seminal plasma, and that both hormones have seasonal variations, and support the idea that seasonal variations of fertility in the ram involve interplay between melatonin and the antioxidant defence system.</p