Link between the laws of geometrical optics and the radiative transfer equation in media with a spatially varying refractive index

We proposed in a previous paper [Opt. Commun. 228, 33 (2003)] a modified radiative transfer equation to describe radiative transfer in a medium with a spatially varying refractive index. The present paper is devoted to the demonstration that this equation perfectly works in the non-absorbing / non-scattering limit, what was contested by L. Mart\'i-L\'opez and coworkers [Opt. Commun. 266, 44 (2006)]. The assertion that this equation would imply a zero divergence of the rays is also commented.Comment: 14 pages, 3 figure

The Intermediate Band Solar Cell: Progress Toward the Realization of an Attractive Concept

The intermediate band (IB) solar cell has been proposed to increase the current of solar cells while at the same time preserving the output voltage in order to produce an efficiency that ideally is above the limit established by Shockley and Queisser in 1961. The concept is described and the present realizations and acquired understanding are explained. Quantum dots are used to make the cells but the efficiencies that have been achieved so far are not yet satisfactory. Possible ways to overcome the issues involved are depicted. Alternatively, and against early predictions, IB alloys have been prepared and cells that undoubtedly display the IB behavior have been fabricated, although their efficiency is still low. Full development of this concept is not trivial but it is expected that once the development of IB solar cells is fully mastered, IB solar cells should be able to operate in tandem in concentrators with very high efficiencies or as thin cells at low cost with efficiencies above the present ones

Can Impurities be Beneficial to Photovoltaics?

The state of the art of the intermediate band solar cells is presented with emphasis on the use of impurities or alloys to form bulk intermediate band materials. Quantum dot intermediate band solar cells start to present already attractive efficiencies but many difficulties jeopardize the immediate achievement of record efficiency cells. To complement this research it is worthwhile examining bulk materials presenting an IB. Four or perhaps more materials have already proven to have it and several paths for the research of more are today open but no solar cell has yet been published based on them. This topic has already attracted many researches and abundant funds for their development worldwide

"Comment on "Thermodynamics limits of quantum photovoltaic cell efficiency" [Appl. Phys. Lett. 91, 223507 (2007)]"

This article claims the photovoltaic cell efficiency calculated using the Luque and Marti model

Experimental advances in the next generation of solar cells

We consider next generation solar cells concepts those that have the potential to exceed the limiting efficiency calculated by Shockley and Queisser for single gap solar cells (40.7 %) and still have not been commercialized. Among these concepts, this paper deals with the multiple exciton generation (or impact ionization or multiple carrier generation) solar cell, the intermediate band solar cell and the hot carrier solar cell. These concepts were proposed theoretically more than ten years ago. In the last years, the number of experiments supporting the theories behind and paving the way towards their practical implementation has leaped forward. This work reviews these experimental advance

Long-term KS_S-band photometric monitoring of L dwarfs

(abridged) We perform photometric time-series analysis of a sample of ten early to mid-L dwarfs in the field over three years of $K_s$-band observations with the OMEGA 2000 infrared camera of the 3.5m telescope on Calar Alto Observatory between January 2010 and December 2012. We perform $K_s$-band differential photometry of our targets (with typical errors of $\pm$15-30~mmag at the 1$\sigma$ level) by subtracting a reference flux from each photometric measurement. This reference flux is computed using three nearby, probably constant stars in the target's field-of-view. We then construct and visually inspect the light curves to search for variability, and use four different periodogram algorithms to look for possible periods in our photometric data. Our targets do not display long-term variability over 1$\sigma$ compared to other nearby stars of similar brightness, nor do the periodograms unveil any possible periodicity for these objects, with two exceptions: 2MASS~J02411151-0326587 and G196-3B. In the case of 2MASS~J02411151-0326587 (L0), our data suggest a tentative period of 307$\pm$21~days, at 40% confidence level, which seems to be associated with peak-to-peak variability of 44$\pm$10~mmag. This object may also display variability in timescales of years, as suggested by the comparison of our Ks-band photometry with 2MASS. For G196-3B (L3), we find peak-to-peak variations of 42$\pm$10~mmag, with a possible photometric period of 442$\pm$7~days, at 95% confidence level. This is roughly the double of the astrometric period reported by Zapatero Osorio (2014). Given the significance of these results, further photometric data are required to confirm the long-term variability.These results suggest that early- to mid-L dwarfs are fairly stable in the $K_s$-band within $\pm$90 mmag at the 3 $\sigma$ level over months to years, which covers hundreds to tens of thousands of rotation cycles.Comment: Accepted for publication in Astronomy & Astrophysic

Ressenyes

Index de les obres ressenyades: Héctor LUCENA, Lo laboral en tiempos de transició

Crisis económica, austeridad y sus efectos en las transiciones laborales en mercado de trabajos flexibles : los ejemplos de España y el Reino Unido

La gran recesión de 2007 ha afectado de forma muy intensa los mercados de trabajo europeos. Al mismo tiempo, diferentes países han implementado desde 2010 paquetes de austeridad que han afectado la regulación del empleo, las políticas sociales, y el sector público. El presente artículo analiza los casos de dos países que desde diferentes modelos de empleo y variedades institucionales se han visto fuertemente afectados por la crisis económica y que se encuentran al frente de las políticas de austeridad: España y el Reino Unido. A través de los datos del EU-SILC se analiza como los cambios en el contexto económico e institucional afectan a las transiciones entre diferentes situaciones en el mercado de trabajo: empleo, desempleo, educación y tareas domésticas/otras formas de inactividad. Se concluye que el contexto ha afectado fuertemente la frecuencia y perfiles de las transiciones y que durante el periodo de austeridad se han aumentado las dinámicas de inseguridad y precariedad en ambos países, y que igualmente se han reducido las opciones de los grupos con menos recursos.La gran recessió de 2007 ha afectat de forma molt intensa els mercats de treball europeus. Al mateix temps, diferents països han implementat des de 2010 paquets d'austeritat que han afectat la regulació de l'ocupació, les polítiques socials, i el sector públic. El present article analitza els casos de dos països que, des de diferents models d'ocupació i varietats institucionals, s'han vist fortament afectats per la crisi econòmica i que es troben al capdavant de les polítiques d'austeritat: Espanya i el Regne Unit. A través de les dades de l'EU-SILC s'analitza com els canvis en el context econòmic i institucional afecten a les transicions entre diferents situacions al mercat de treball: ocupació, desocupació, educació i tasques domèstiques/altres formes d'inactivitat. Es conclou que el context ha afectat fortament la freqüència i perfils de les transicions i que durant el període d'austeritat s'han augmentat les dinàmiques d'inseguretat i precarietat en tots dos països, i que igualment s'han reduït les opcions dels grups amb menys recursos.The big recession of 2007 has had a strong effect in European labour markets. Moreover, the austerity policies implemented since 2010 in many European countries lead to important changes in employment regulation, social policies and the public sector. This paper explores the effects of recession and austerity in two countries with different employment models and institutional settings but strongly affected by the economic downturn and at the forefront of the austerity policies: Spain and the United Kingdom. The panel data provided by EU-SILC is used to analyse how the changes in the economic and institutional context affect labour market transitions: employment, unemployment, education and domestic tasks, care, and other inactivity situations. The findings show a strong influence of the changing context in the frequency and patterns of the transitions and identify the increasing insecurity and precariousness in both countries, especially during the so called austerity turn. Furthermore, they suggest that during this period the options for those with less resources have been further diminished

Les reformes de la prestació d'atur a l'Estat espanyol en el marc de l'Estratègia Europea d'Ocupació

La reforma de la prestació d'atur constitueix un element significatiu per analitzar les transformacions i els canvis entorn de la manera com s'entenen el treball i els treballadors des de les polítiques públiques. Des d'aquest punt de vista, en les transformacions de la prestació d'atur, hi podem assenyalar un seguit de canvis, que es troben en bona mesura enunciats per les definicions de la política europea d'ocupació, destinats a augmentar la responsabilització individual i l'activació dels treballadors en el mercat laboral. Igualment, l'evolució de la prestació d'atur a l'Estat espanyol ens mostra també aquesta tendència i ens enuncia un nou model o estatut a l'hora de considerar tant els aturats com el conjunt dels treballadors.The reform of the unemployment benefit is a significant element in order to analyze the transformations about how public policies conceptualise labour and workers. From this point of view, in the reforms we can indicate some changes, largely developed by the definitions of the European employment policy, destined to increase the individual responsibility and workers' activation in the labour market. In this way, the evolution of the unemployment benefit in Spain also shows this tendency and enunciates us a new model or statute for the unemployed and the workers