Electric-field Effects On Shallow Impurity States In Gaas-(ga,al)as Quantum Wells

The influence of an applied electric field on shallow donor and acceptor states in GaAs-(Ga,Al)As quantum wells is studied. We work within the effective-mass approximation and adopt a trial envelope wave function for the impurity carrier, which leads to the exact results for vanishing applied electric fields and limiting values of the quantum-well thickness. Results for the binding energies and density of impurity states as functions of the impurity position, well thicknesses, and applied electric field are reported. Some results for the effects of electric field on the donor-related optical properties are also presented. As a general feature, the density of impurity states and impurity-related optical absorption for finite electric fields exhibit three van Hovelike singularities corresponding to the binding energies associated with impurities at the two edges of the quantum well and at the position at which the binding energy has a maximum. © 1990 The American Physical Society.42117069707

Las edades del Cenozoico mamalífero de la Argentina, con especial atención a aquellas del territorio bonaerense

La primera parte del trabajo ha sido dedicada a discutir los problemas de la Clasificación, Terminología y Nomenclatura estratigráfica en la Argentina. Sucintamente se analizan las dificultades que los estratígrafos argentinos actuales encuentran en estos aspectos por la falta de claridad que ha regido la distinción de las unidades y los nombres aplicados a las rocas, a las faunas y al tiempo. Como en los EE. UU. se han vivido situaciones semejantes, se analizan las proposiciones de su Código Estratigráfico, y su aplicabilidad a la Estratigrafía de Argentina. Especialmente se analizan las exigencias formales de la clasificación cronoestratigráfica y geocronológica, y la practicabilidad de sus equivalencias. Dado que en la distinción de las unidades cronoestratigráficas y geocronológicas no se han seguido en la mayoría de los casos las proposiciones del Congreso Geológico Internacional de Copenhague, de 1961, ni las del mencionado Código norteamericano, los autores eluden premeditadamente el uso de la clasificación cronoestratigráfica y sólo aplican la clasificación geocronológica. Pero al reconocer las Edades del Cenozoico mamalífero de Argentina lo hacen con el significado que le diera en los EE.UU. Savage, es decir, Edades-mamífero (“Land-Mammals Ages”), señalando que su correspondencia con las Edades reconocidas según las exigencias formales vigentes y su equivalencia con los Pisos respectivos serán determinadas por el avance de los estudios, y por el sometimiento más riguroso a los cánones estratigráficos. La segunda parte del trabajo se dedica al análisis de los criterios que se han seguido para reconocer las Edades-mamífero, y se analizan las bases de sus correlaciones continentales y extracontinentales, y por ellas se- las asigna a las Épocas universalmente reconocidas. Las conclusiones se sintetizan en dos cuadros: uno de todas las Edades-mamífero de Argentina y su correlación con las Edades de América del Norte y Europa, y otro de las Edades del Cenozoico mamalífero representadas en la provincia de Buenos Aires, indicando la asignación a ellas de las unidades litoestratigráficas fundamentales reconocidas hasta el momento. Finalmente se incluye una lista sistemática de los taxa de mamíferos de cada Edad, y se señalan para cada una de ellas los “fósiles guías” y los “fósiles característicos”, indicando además aquellos taxa que aparecen y desaparecen en cada una.The first part of the paper deals with the stratigraphical classification, terminology and nomenclature of units specially those mammal-bearing of Argentina. Difficulty is emphasized in applying stratigraphical codes in connexion with the stages and ages. Savage criteria is here adopted re- cognizing only Land-Mammal Ages. The second part analizes the criteria used for recognizing the Argentine Land-Mammals Ages specially those of the Province of Buenos Aires. An attempt is made to correlate them with those of Europe and North America. Finally a sistematic list ending with the genus with indication of “guide fossils” and “characteristic fossils”, etc., of each Land-Mammal Ages of the Province of Buenos Aires is displayed

Binding Energies And Density Of Impurity States Of Shallow Hydrogenic Impurities In Cylindrical Quantum-well Wires

The binding energies of hydrogenic impurities in both infinite and finite GaAs-(Ga,Al)As cylindrical quantum-well wires are calculated as functions of the wire radius and of the impurity location in the well for different radii of the wires using a variational procedure within the effective-mass approximation. Assuming there is no intentional doping, we treat the impurity position as a random variable and define a density of impurity states that we have calculated as a function of the impurity binding energy. As a general feature, the density of impurity states presents two structures associated with impurities at the center and at the edge of the quantum-well wire that may be important in the understanding of absorption and photoluminescence experiments of doped GaAs-(Ga,Al)As quantum-well wires. © 1991 The American Physical Society.4321824182

High temperatures behaviour of mortars with incorporation of phase change materials

In a society increasingly concerned with sustainability and good construction practices, it becomes urgent to develop and study the behaviour of the materials when submitted to the unfavourable conditions. Mortars are extensively used in building. The mortars with incorporation of phase change materials (PCM) have the ability to regulate the temperature inside buildings, contributing to the thermal comfort and reduction the use of heating and cooling equipment, using only the energy supplied by the sun. Currently, it emerges the need to develop functional mortars that can contribute to the energy efficiency. However, it is important that they present a good behaviour when subjected to aggressive conditions, such as high temperatures. The main purpose of this study was the behaviour at high temperatures of mortars with PCM, based in different binders. The binders studied were aerial lime, hydraulic lime, gypsum and cement. For each type of binder, different mortars were developed with different content of PCM. The proportion of PCM studied was 0% and 40% of the mass of the sand. It was possible to observe that the exposure to high temperatures, generally leads to a decrease in the mechanical properties of the studied mortars

Fascículo IV. Vertebrata

El presente trabajo, como su nombre lo indica, responde al propósito de los que promovieron su realización desde la Comisión Científica de la Provincia de Buenos Aires, es decir, ofrecer fundamentalmente a los geólogos que trabajan en el campo un elemento bibliográfico con buena iconografía, que les hiciera accesible una primera determinación de los vertebrados fósiles para los fines estratigráficos que ellos manejan. No obstante, los autores han querido aprovechar la oportunidad de un trabajo de esta envergadura para ampliar un poco las diagnosis y llegar en casi todos los casos a categorías sistemáticas superiores al género, con el fin de transformar el trabajo en algo más que una simple Paleontografía ; la intención es facilitar el desarrollo de alguna vocación, o simplemente proveer un mayor número de elementos para aquéllos que no sólo se contentan con saber cuán antiguas son las rocas que contienen determinados fósiles, sino que desean adentrarse en el conocimiento y problemática de la paleozoología de los Vertebrados. Por otro lado, pocos —por no decir ninguno— son los libros en castellano donde pueden encontrarse condensados los datos aquí consignados, y bien valió esta ocasión para hacerlo. Aun los trabajos generales en otros idiomas difícilmente incluyen una documentación tan amplia como la que se ha conden- sado en éste, y bien podemos decir que mucha es absolutamente original, producto de la investigación personal de los autores. De acuerdo al convenio oportunamente tratado, esta parte dedicada a los Vertebrados sólo ha llegado hasta las diagnosis de los géneros. Intentar profundizar más hubiera sido tarea de muy largo aliento, difícilmente concretable en unos pocos años, puesto que prácticamente imponía la revisión crítica de todo lo hecho hasta el momento en el campo de los Vertebrados fósiles.  Digitalizado en SEDICI-CIC Digita

Acoustic Phonon-Assisted Resonant Tunneling via Single Impurities

We perform the investigations of the resonant tunneling via impurities embedded in the AlAs barrier of a single GaAs/AlGaAs heterostructure. In the $I(V)$ characteristics measured at 30mK, the contribution of individual donors is resolved and the fingerprints of phonon assistance in the tunneling process are seen. The latter is confirmed by detailed analysis of the tunneling rates and the modeling of the resonant tunneling contribution to the current. Moreover, fluctuations of the local structure of the DOS (LDOS) and Fermi edge singularities are observed.Comment: accepted in Phys. Rev.

Incidence of community-acquired lower respiratory tract infections and pneumonia among older adults in the United Kingdom: a population-based study.

Community-acquired lower respiratory tract infections (LRTI) and pneumonia (CAP) are common causes of morbidity and mortality among those aged ≥65 years; a growing population in many countries. Detailed incidence estimates for these infections among older adults in the United Kingdom (UK) are lacking. We used electronic general practice records from the Clinical Practice Research Data link, linked to Hospital Episode Statistics inpatient data, to estimate incidence of community-acquired LRTI and CAP among UK older adults between April 1997-March 2011, by age, sex, region and deprivation quintile. Levels of antibiotic prescribing were also assessed. LRTI incidence increased with fluctuations over time, was higher in men than women aged ≥70 and increased with age from 92.21 episodes/1000 person-years (65-69 years) to 187.91/1000 (85-89 years). CAP incidence increased more markedly with age, from 2.81 to 21.81 episodes/1000 person-years respectively, and was higher among men. For both infection groups, increases over time were attenuated after age-standardisation, indicating that these rises were largely due to population aging. Rates among those in the most deprived quintile were around 70% higher than the least deprived and were generally higher in the North of England. GP antibiotic prescribing rates were high for LRTI but lower for CAP (mostly due to immediate hospitalisation). This is the first study to provide long-term detailed incidence estimates of community-acquired LRTI and CAP in UK older individuals, taking person-time at risk into account. The summary incidence commonly presented for the ≥65 age group considerably underestimates LRTI/CAP rates, particularly among older individuals within this group. Our methodology and findings are likely to be highly relevant to health planners and researchers in other countries with aging populations

Sex Difference and Rupture Rate of Intracranial Aneurysms : An Individual Patient Data Meta-Analysis

Background and Purpose: In previous studies, women had a higher risk of rupture of intracranial aneurysms than men, but female sex was not an independent risk factor. This may be explained by a higher prevalence of patient- or aneurysm-related risk factors for rupture in women than in men or by insufficient power of previous studies. We assessed sex differences in rupture rate taking into account other patient- and aneurysm-related risk factors for aneurysmal rupture. Methods: We searched Embase and Pubmed for articles published until December 1, 2020. Cohorts with available individual patient data were included in our meta-analysis. We compared rupture rates of women versus men using a Cox proportional hazard regression model adjusted for the PHASES score (Population, Hypertension, Age, Size of Aneurysm, Earlier Subarachnoid Hemorrhage From Another Aneurysm, Site of Aneurysm), smoking, and a positive family history of aneurysmal subarachnoid hemorrhage. Results: We pooled individual patient data from 9 cohorts totaling 9940 patients (6555 women, 66%) with 12 193 unruptured intracranial aneurysms, and 24 357 person-years follow-up. Rupture occurred in 163 women (rupture rate 1.04%/person-years [95% CI, 0.89-1.21]) and 63 men (rupture rate 0.74%/person-years [95% CI, 0.58-0.94]). Women were older (61.9 versus 59.5 years), were less often smokers (20% versus 44%), more often had internal carotid artery aneurysms (24% versus 17%), and larger sized aneurysms (>= 7 mm, 24% versus 23%) than men. The unadjusted women-to-men hazard ratio was 1.43 (95% CI, 1.07-1.93) and the adjusted women/men ratio was 1.39 (95% CI, 1.02-1.90). Conclusions: Women have a higher risk of aneurysmal rupture than men and this sex difference is not explained by differences in patient- and aneurysm-related risk factors for aneurysmal rupture. Future studies should focus on the factors explaining the higher risk of aneurysmal rupture in women.Peer reviewe