Carrier and Light Trapping in Graded Quantum Well Laser Structures

We investigated the carrier and light trapping in GaInAs/AlGaAs single quantum well laser structures by means of time resolved photoluminescence and Raman spectroscopy. The influence of the shape and depth of the confinement potential and of the cavity geometry was studied by using different AlGaAs/GaAs short-period superlattices as barriers. Our results show that grading the optical cavity improves considerably both carrier and light trapping in the quantum well, and that the trapping efficiency is enhanced by increasing the graded confining potential.Comment: PDF-format, 15 pages (including 4 figures), Applied Physics Letters (June 2000

Materias primas para la fabricación de clinker de cemento portland en el área de Madrid.

En Madrid y su area de influencia se ha incrementado la demanda de materiales de construcción, como consecuencia del espectacular desarrollo de las obras de infraestructura y de la edificación. Los problemas derivados del impacto ambiental y la futura expansión y/o renovación de las plantas de fabricación de cemento portland, aconsejan una investigación en el area, de las materias primas para la fabricación de clinker. Los materiales aflorantes en la cuenca Terciaria sobre la que se asienta Madrid, han sido divididos en tres unidades estratigráficas, con importantes diferencias litológicas. La Unidad Superior, presenta un volumen de mas de 1500 Mt de calizas de buena calidad para ser usadas como componente principal . La Unidad Inferior o Salina, esta formada localmente por sedimentos lutítico-arcillosos, que pueden ser usados en función de su naturaleza mineralógica y química, como un buen componente secundario. Estos materiales constituyen la materia prima para la fabricación de cerámica basta, por lo que en algunas zonas ambos tipos de uso deben de coexistir. Los yesos detríticos y primarios de la Unidad Intermedia, pueden ser usados como componente regulador de fraguado. Los modelos de dosificación elaborados muestran que no es necesaria la adición de un tercer componente al crudo del portland, para obtener una composición química adecuada

Electron tomography of (In,Ga)N insertions in GaN nanocolumns grown on semi-polar (112̄ 2) GaN templates

We present results of scanning transmission electron tomography on GaN/(In,Ga)N/GaN nanocolumns (NCs) that grew uniformly inclined towards the patterned, semi-polar GaN( 11 2 ̄ 2 ) substrate surface by molecular beam epitaxy. For the practical realization of the tomographic experiment, the nanocolumn axis has been aligned parallel to the rotation axis of the electron microscope goniometer. The tomographic reconstruction allows for the determination of the three-dimensional indium distribution inside the nanocolumns. This distribution is strongly interrelated with the nanocolumn morphology and faceting. The (In,Ga)N layer thickness and the indium concentration differ between crystallographically equivalent and non-equivalent facets. The largest thickness and the highest indium concentration are found at the nanocolumn apex parallel to the basal planes

Segmental relaxation in semicrystalline polymers: a mean field model for the distribution of relaxation times in confined regimes

The effect of confinement in the segmental relaxation of polymers is considered. On the basis of a thermodynamic model we discuss the emerging relevance of the fast degrees of freedom in stimulating the much slower segmental relaxation, as an effect of the constraints at the walls of the amorphous regions. In the case that confinement is due to the presence of crystalline domains, a quasi-poissonian distribution of local constraining conditions is derived as a result of thermodynamic equilibrium. This implies that the average free energy barrier $\Delta F$ for conformational rearrangement is of the same order of the dispersion of the barrier heights, $\delta (\Delta F)$, around $\Delta F$. As an example, we apply the results to the analysis of the $\alpha$-relaxation as observed by dielectric broad band spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized from either an isotropic or an oriented glass. It is found that in the latter case the regions of cooperative rearrangement are significantly larger than in the former.Comment: 10 pages, 4 figures .ep

Potential reactivity of the andesitic rocks from Cabo de Gata (SE Spain)

8 páginas, 4 figuras, 3 tablas.[ES] Las rocas andesíticas producen una reacción álcalis-sílice con los componentes de la fase intersticial del hormigón. Esta reacción puede encuadrarse dentro de las de cinética lenta, ya que la disolución incongruente de los minerales constituyentes de las rocas, da lugar a la formación tardía de geles silicocalco- alcalinos. En consecuencia, la utilización convencional del método de ensayo de barras de mortero no detecta la reacción hasta transcurridos al menos 90 días.[EN] Andesitic rocks produce an alkali-silica reaction with the components of the interstitial phase of concrete. This reaction can be considered as belonging to the slow-kinetic type, since the incongruent dissolution of the rock-forming minerals leads to the late formation of siliceous calc-alkaline gel. As a conquence, the conventional mortar-bar method does not detect such reaction until 90 days later.Peer reviewe

Cuantificación mediante estudio estereológico del sistema microfisural del granito de El Berrocal (Sistema Central, Toledo, España)

This paper describes an exarnple of the use of stereological techniques for the quantification of some characteristics of the microcrack network of rocks, in this case the El Berrocal granite (Sistema Central, Toledo, Spain). From data measured in sections, therefore 2-D, 3-D characteristics of the microcrack network have been obtained. The quantified characteristics are the microcrack density (in terms of the frequency distribution of the specific surface of microcracks in relation to their apertures) and the relative frequency of the microcrack aperture. These procedures have been applied to images under scanning electron microscopy, so microcracks with apparent apertures over 0,l pm. have been measured. The so called "vertical sections" have been prepared to assure a correct sampling. From the measurement of the apparent apertures of microcracks and applying stereological procedures, the real apertures and the specific surface of each class of microcracks are obtained. Besides of the characteristics of the microcrack network of the rock as a whole, data of the microcracks associated to each of the main minerals (quartz, feldspars and micas) are presented separately, allowing the interpretation of the contribution of each mineral to the microfractography of the rock. In El Berrocal granite, the microcracks with a real aperture less than 1 m are the most abundant. The microcracks are more important in quartz and, above all, in feldspars (with more than 60% of the total network of the rock), than in micas. When considering the microcrack density of each mineral, and taking into account the mineral composition of the rock too, the feldspar is also the more cracked mineral in the rock. Nevertheless, the frequency distribution of the specific surface in relation to the microcrack aperture is quite similar in any of the three mirierals. It is intended with this paper to show the possibilities of the stereology that, using simple, fast and economic procedures, provides quantitative information about the characteristics of the microcrack network of rocks that can be very useful in many geological studies