Enhanced two-photon emission from a dressed biexciton

Radiative two-photon cascades from biexcitons in semiconductor quantum dots under resonant two-photon excitation are promising candidates for the generation of photon pairs. In this work, we propose a scheme to obtain two-photon emission that allows to operate under very intense driving fields. This approach relies on the Purcell enhancement of two-photon virtual transitions between states of the biexciton dressed by the laser. The richness provided by the biexcitonic level structure allows to reach a variety of regimes, from antibunched and bunched photon pairs with polarization orthogonal to the driving field, to polarization entangled two-photon emission. This evidences that the general paradigm of two-photon emission from a ladder of dressed states can find interesting, particular implementations in a variety of systems

Tuning the conductance of a molecular switch

The ability to control the conductance of single molecules will have a major impact in nanoscale electronics. Azobenzene, a molecule that changes conformation as a result of a trans/cis transition when exposed to radiation, could form the basis of a light-driven molecular switch. It is therefore crucial to clarify the electrical transport characteristics of this molecule. Here, we investigate theoretically charge transport in a system in which a single azobenzene molecule is attached to two carbon nanotubes. In clear contrast to gold electrodes, the nanotubes can act as true nanoscale electrodes and we show that the low-energy conduction properties of the junction may be dramatically modified by changing the topology of the contacts between the nanotubes and the molecules, and/or the chirality of the nanotubes (that is, zigzag or armchair). We propose experiments to demonstrate controlled electrical switching with nanotube electrodes

Towards convergence in the teaching of high school teachears: Bologna, a european instrument as a lever for the national reforms agendas

The Bologna Process aimed at creating a European Space of Higher Education i.e. at harmonizing European universities and fostering the mobility of students and workers across Europe. The Bologna process could be construed, as the European Union itself, as an example of the conceptions of globalization proposed by the Stanford Neo-institutional approach. This approach maintains the existence of an imperative world-level educational culture and ideology sustaining the diffusion and transfer of rationalized educational models, creating therefore a world-wide educational isomorphism. However, one of the arguably central objectives of the reform, the Secondary teacher training, maintains a high degree of heteromorphism throughout Europe almost two decades after the Bologna Accord was signed. As the comparison of the German and Spanish cases show, the national and regional academic cultures, the different power constellations and institutional inertia and path dependence re-created the national and (in Germany) regional differences. The secondary teacher training reforms implemented under Bologna´s aegis rather than a diffusion of a global or European model could be understand as idiosyncratic responses to the secondary education reforms that took place in both countries in previous decadesEl proceso de Bolonia, una de las políticas educativas que partiendo de algunos estados miembros elementos ha asumido la Unión Europea, se puso en marcha para crear un Espacio Europeo de Educación Superior (EEES), es decir, armonizar la educación superior y fomentar la movilidad de estudiantes y fuerza de trabajo en Europa. Bolonia podría considerarse un ejemplo perfecto de la globalización según la concepción neo-institucional de Stanford. Este paradigma mantiene la existencia de una cultura e ideología educativa mundial que sustentan la difusión-globalización de modelos o mitos racionalizados, creando un isomorfismo educativo global. Empero, elmantenimiento del heteromorfismo es evidente en uno de los objetivos centrales de la reforma, la formación de profesores de secundaria (FPrS). Como demuestra el análisis histórico-comparativo de los casos alemán y español, las diferentes constelaciones políticas, la cultura académica regional y nacional, y la inercia institucional reprodujeron las diferencias en la FPrS de ambos países y entre estados alemanes. En la práctica, las reformas del FPrS implementadas bajo la égida de Bolonia, más que la difusión de un modelo global o europeo, podrían entenderse como respuestas locales idiosincráticas a las reformas de la escuela secundaria que tuvieron lugar en ambos países durante las décadas anteriore

Urban Growth with Limited Prosperity: A History of Public Housing in Laredo, Texas -- 1938 to 2006

Public housing in the United States has been a controversial sociopolitical topic since the years of the Great Depression. The issue of appropriate and secure habitation for the country\u27s deserving poor continues to be of great importance as government subsidies become scarce in the early 21st century. This dearth of support for public housing is even more evident and prominent along the United States-Mexico border of South Texas, a territory described as having a third world environment. The dissertation is a narrative history of public housing in Laredo, Texas, a border community. Compiled from news media records and the archives of the Laredo Housing Authority, the study gives insight into methods used by this authority to achieve decent habitation for the underprivileged residents of one of the poorest cities in the United States. After a historical background of Laredo, the study follows a chronological development of federally funded housing through the six decades that began in 1938. The study accentuates the continuing need for such housing as its sponsoring federal agency; the Department of Housing and Urban Development fails to properly fund its subsidiary programs and projects. Principal governmental and nongovernmental sources substantiate the dearth of appropriate housing, with the author providing further insight to his native city\u27s plight. The conclusion outlines how funding, together with higher upkeep and energy costs, will continue in a downward spiral and will lead to an increase in the underserved poor population

Optical Transmittance Maximization in Superior Performance Tunnel Junctions for Very High Concentration Applications.

The light transmission through a tunnel junction in a multijunction solar cell depends on the optical properties and thickness of the whole solar cell layers stack, which configure the light absorption, reflection and interference processes taking place inside the semiconductor structure. In this paper the focus is put on the AlGaAs barrier layers of p++AlGaAs/n++GaAs and p++AlGaAs/n++GaInP tunnel junctions inserted into a GaInP/GaAs dualjunction solar cell. The aim is to analyze the effect of the thickness and Al-composition of these barrier layers on the light transmittance of the tunnel junction, using the bottom cell Jsc as the merit figure to appraise it. An intricate relation between this Jsc and the barrier layers parameters, caused by interferential reflectance, was observed. The importance of an appropriate optical design of the semiconductor structure was corroborated by a non-negligible gain in the bottom cell Jsc when choosing the appropriate barrier layers Al-compositions and thicknesses from a range of practical values for which the optical absorption is not the main contributor to the optical losses