Theory of Auger core-valence-valence processes in simple metals. II. Dynamical and surface effects on Auger line shapes

ABSTRACT: Auger CVV spectra of simple metals are generally believed to be well described by one-electronlike theories in the bulk which account for matrix elements and, in some cases, also static core-hole screening effects. We present here detailed calculations on Li, Be, Na, Mg, and Al using selfconsistent bulk wave functions and proper matrix elements. The resulting spectra differ markedly from experiment and peak at too low energies. To explain this discrepancy we investigate effects of the surface and dynamical effects of the sudden disappearance of the core hole in the final state. To study core-hole effects we solve Mahan —Nozieres —De Dominicis (MND) model numerically over the entire band. The core-hole potential and other parameters in the MND model are determined by self-consistent calculations of the core-hole impurity. The results are compared with simpler approximations based on the final-state rule due to von Barth and Grossmann. To study surface and mean-free-path effects we perform slab calculations for Al but use a simpler infinite-barrier model in the remaining cases. The model reproduces the slab spectra for Al with very good accuracy. .In all cases investigated either the effects of the surface or the effects of the core hole give important modifications and a much improved agreement with experiment

Theory of auger core-valence-valence processes in simple metals. I. Total yields and core-level lifetime widths

ABSTRACT: There is a considerable disagreement in the literature on the description of lifetime effects arising from core-valence transitions in solids. We calculate here Auger and radiative widths of shallow core levels in Li, Be, Na, Mg, and Al with use of principles consistent with dynamical theories of secondary-emission processes developed earlier. The lifetime has no simple relation to the usual self-energy but is instead directly related to emission yields. The problem of choosing reliable approximations for Auger rates and matrix elements is analyzed theoretically and computationally. We also comment on some earlier approaches. Much of our discussion pertains also to calculations of Auger line shapes from first principles. For long hole lifetimes the total and partial level widths obey an initial-state rule and follow from wave functions perturbed by a static core hole. To obtain these impurity wave .functions we perform self-consistent supercell calculations. The core-hole screening increases the Auger rates by factors of the order 2—4 compared with results from ground-state orbitals but has never been properly included before. The width of the 1s level in Li is rather accurately known because it monitors large effects of incomplete lattice relaxation. For Li we obtain here a width 17 meV in excellent agreement with the value 16 meV deduced earlier from measurements by Callcott et al

Caracterización de microcosmos naturales de bacterias magnetotácticas estuarinas

ABSTRACT: To date, no complete study of magnetotactic bacteria’s (MTB) natural microcosms in estuarine or tropical environments has been reported. Besides, almost all the studies around magnetotactic bacteria have been based on fresh waters away from the Equator. In this work, we focused the experimental region at the Equator and present a comprehensive mineralogical and physicochemical characterization of two estuarine bacterial microcosms. The results show that mineral lixiviation in the sediments may be an important factor in the solubilization of elements required by magnetotactic bacteria. Specifically, we show that clinochlore, phlogopite, nontronite, and halloysite could be among the main minerals that lixiviate iron to the estuarine microcosms. We conclude that nitrate concentration in the water should not be as low as those that have been reported for other authors to achieve optimal bacteria growth. It is confirmed that magnetotactic bacteria do not need large amounts of dissolved iron to grow or to synthesize magnetosomes.RESUMEN: No se ha reportado ningún estudio completo sobre microcosmos naturales de bacterias magnetotácticas (MTB) en estuarios o ambientes tropicales. Además, casi todos los estudios sobre las bacterias magnetotácticas se han desarrollado en aguas dulces alejadas del ecuador. Este trabajo se desarrolla sobre el ecuador y reporta una caracterización mineralógica y fisicoquímica detallada de dos microcosmos bacterianos estuarinos. Los resultados muestran que la lixiviación de minerales en los sedimentos puede ser un factor importante en la solubilización de elementos requeridos por las bacterias magnetotácticas. Específicamente, que el clinocloro, flogopita, nontronita y haloisita pueden estar entre los minerales más importantes en la lixiviación de hierro a los microcosmos estuarinos. Se concluye que la concentración de nitrato en el agua no debe ser tan baja como se ha reportado para lograr un crecimiento bacteriano óptimo. Las bacterias magnetotácticas no necesitan grandes cantidades de hierro disuelto para su crecimiento ni para la síntesis de magnetosomas

Vacancies and vacancy-oxygen complexes in silicon: Positron annihilation with core electrons

ABSTRACT: Various point defects in silicon are studied theoretically from the point view of positron annihilation spectroscopy. Properties of a positron trapped at a single vacancy, divacancy, vacancy-oxygen complexes (VOn), and divacancy-oxygen complex are investigated. In addition to the positron lifetime and positron binding energy to defects, we also calculate the momentum distribution of annihilation photons (MDAP) for high momenta, which has been recently shown to be a useful quantity for defect identification in semiconductors. The influence of atomic relaxations around defects on positron properties is also examined. Mutual differences among the high momentum parts of the MDAP for various defects studied are mostly considerable, which can be used for the experimental defect determination

Síntesis de partículas magnéticas cubiertas con caseinato de sodio

The one-step coprecipitation method is used to obtain magnetic nanoparticles controlling the pH (10 and 12), and casein surfactant (CS) concentrations (1 % and 3 % (m/m)). CS has not been used so far for stabilizing magnetic iron oxide ferrofluids. The magnetic nanoparticles have a magnetite core with maghemite in surface, and a shell of polymer. The transmission electron images confirm the crystallinity, particle size distribution in the range of 5-10 nm, and the spinel structure of the nanoparticles. Mössbauer results at 80 K showed line shapes dominated by magnetic relaxation effects with sextets and combinations of sextets and doublets. The interactions of the surfactant with the nanoparticle surface are strong showing at least two surfactant layers. The magnetic behavior was evaluated by moment versus temperature and magnetic field measurements. The nanoparticles showed superparamagnetic behavior at room temperature and blocked (irreversible) behavior at 5 K. The saturation magnetization presented lower values than reported bulk systems due to the presence of a large layer of maghemite. The FC/ZFC magnetization vs. temperature curves confirmed the superparamagnetic nature of the iron oxide particles and the strong interactions for pH 12 samples and weak interactions for pH 10 samples. The particle growth was dominated by the surface properties of the nanoparticles.Se usa el método de coprecipitación para obtener nanopartículas magnéticas controlando el pH (10 y 12) y la concentración del caseinato de sodio (CS) (1 % y 3 %(m/m)). CS no se ha utilizado hasta el momento para estabilizar ferrofluidos magnéticos. Las partículas muestran un núcleo de magnetita, una capa de maghemita sobre el mismo, y otra capa exterior de la proteína. La microscopía electrónica de transmisión muestra partículas cristalinas, una distribución de tamaños entre 5-10 nm, y la estructura de espinela. Los resultados Mössbauer a 80 K muestran formas de línea dominadas por efectos de relajación magnética. La interacción de la proteína con la superficie de las nanopartículas es fuerte y muestra varias capas de proteína. El comportamiento magnético se evaluó mediante medidas termomagnéticas y de momento versus campo magnético. Estas revelaron un sistema superparamagnético a 300 K y bloqueado a 5 K. La magnetización de saturación mostró valores menores que en el volumen posiblemete debido a la presencia de la maghemita. Las medidas termomagnéticas confirmaron el superparamagnetismo y mostraron que las muestras obtenidas a pH 12 presentan interacciones fuertes mientras que las de pH 10 muestran interacciones débiles. El crecimiento de las partículas fue dominado por las propiedades superficiales de las partículas.Fil: Urquijo Morales, Jeaneth Patricia. Universidad de Antioquia; ColombiaFil: Casanova Yepes, Herley. Universidad de Antioquia; ColombiaFil: Morales Aramburo, Álvaro Luis. Universidad de Antioquia; ColombiaFil: Zysler, Roberto Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Universidad Nacional de Cuyo; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentin

Electron Raman scattering in pyramidal quantum dots

ABSTRACT: The Raman scattering related with electrons in pyramidal quantum dots is theoretically investigated. The electron Raman differential cross section is calculated making use of analytically determined quantum states within the effective mass approximation. The features of the Raman differential cross section are discussed in terms of their dependence on the changes of the quantum dot geometry.RESUMEN: En este artículo se investiga teóricamente la dispersión Raman de electrones en puntos cuánticos piramidales. Se reporta la sección transversal diferencial Raman electrónica usando los estados cuánticos determinados analíticamente dentro de la aproximación de masa efectiva. Las características de la sección transversal diferencial Raman se discuten en términos de su dependencia de los cambios de la geometría de punto cuántico

Stress effects on shallow-donor impurity states in symmetrical GaAs/AlxGa1_xAs double quantum wells

ABSTRACT: The effects of the compressive stress on the binding energy and the density of shallow-donor impurity states in symmetrical GaAs/AlxGa12xAs double quantum wells are calculated using a variational procedure within the effective-mass approximation. Results are for different well and barrier widths, shallow-donor impurity position, and compressive stress along the growth direction of the structure. We have found that independently of the well and barrier widths, for stress values up to 13.5 kbar ~in the direct-gap regime! the binding energy increases linearly with the stress. For stress values greater than 13.5 kbar ~indirect gap regime! and for impurities at the center of the wells, the binding energy increases up to a maximum and then decreases. For all impurity positions the binding energy shows a nonlinear behavior in the indirect gap regime due to the G-X crossing effect. The density of impurity states is calculated for a homogeneous distribution of donor impurities within the barriers and the wells of the low-dimensional heterostructures. We have found that there are three special structures in the density of impurity states: one associated with on-center-barrier-, the second one associated with on-center-well-, and the third one corresponding to on-external-edge-well-impurity positions. The three structures in the density of impurity states must be observed in valence–to–donor-related absorption and conduction–to–donor-related photoluminescence spectra, and consequently these peaks can be tuned at specific energies and convert the system in a stress detector

Donor impurity-related linear and nonlinear intraband optical absorption coefficients in quantum ring : effects of applied electric field and hydrostatic pressure

ABSTRACT: The linear and nonlinear intraband optical absorption coefficients in GaAs three-dimensional single quantum rings are investigated. Taking into account the combined effects of hydrostatic pressure and electric field, applied along the growth direction of the heterostructure, the energies of the ground and first excited states of a donor impurity have been found using the effective mass approximation and a variational method. The energies of these states are examined as functions of the dimensions of the structure, electric field, and hydrostatic pressure. We have also investigated the dependencies of the linear, nonlinear, and total optical absorption coefficients as a function of incident photon energy for several configurations of the system. It is found that the variation of distinct sizes of the structure leads to either a redshift and/or a blueshift of the resonant peaks of the intraband optical spectrum. In Maddition, we have found that the application of an electric field leads to a redshift, whereas the influence of hydrostatic pressure leads to a blueshift (in the case of on-ring-center donor impurity position) of the resonant peaks of the intraband optical spectrum

Adaptation of a strain Acidithiobacillus ferrooxidans compatible on concentrates of chalcopyrite (CuFeS2), sphalerite (ZnS) and galena (PbS)

ABSTRACT: In this study the adaptation of Acidithiobacillus ferrooxidans-like to high concentrations of chalcopyrite, sphalerite and galena were evaluated with two mineral-particle sizes: 200 and 325 Tyler mesh. The strain was adapted using two simultaneous processes. The first one consisted in a gradual decreasing of the main energy source, ferrous sulphate. The second one consisted in a gradual increasing of the mineral content. Finally, a test was made without ferrous sulphate. The serial subculturing was found to be an efficient strategy to adapt Acidithiobacillus ferrooxidans-like to higher concentrations of chalcopyrite, sphalerite and galena. This indicates that a suitable protocol was employed. The results showed that Acidithiobacillus ferrooxidans-like is more resistant to high concentration of sphalerite, chalcopyrite and galena in descendant order. The particle size played an important role in the adaption of microorganism to the mineral.RESUMEN: En este estudio se evaluó la adaptación de una cepa compatible con Acidithiobacillus ferrooxidans a altas densidades de pulpa de calcopirita, esfalerita y galena, con dos distribuciones de tamaño de partícula, -200 y -325 serie Tyler de tamices. Los microorganismos fueron adaptados por la disminución gradual de la fuente principal de energía, sulfato ferroso, y el aumento en el contenido de mineral, para finalmente realizar un subcultivo sin la adición de fuente de energía externa. La realización de subcultivos en serie resultó ser una estrategia eficaz para la adaptación a altas densidades de pulpa de esfalerita, calcopirita y galena indicando que el protocolo empleado es adecuado. Los resultados muestran que la cepa compatible con Acidithiobacillus ferrooxidans es más resistente a altas concentraciones de esfalerita, seguido por calcopirita y finalmente por galena. El tamaño de partícula juega un papel fundamental en la adaptación de los microorganismos al mineral

Would you give a quantum ring to your fiancee?

ABSTRACT: A quantum ring is a physical object similar to a real life ring that we find in our daily life, the only difference is in its very small nanometer dimensions (0.000000001 m). Furthermore, the ring is made of a semiconductor material called gallium arsenide (GaAs), as opposed to ordinary rings generally made of gold or silver. Another difference with the real ring is that it only has two dimensions, this means that it looks like a hole coin with negligible thickness. To the naked eye, this system could appear too ideal and not useful; but, these structures can be produced experimentally with very small thicknesses close to the system studied here and they have large enough important applications for the actual world.RESUMEN: Un anillo cuántico es un sistema físico similar a un anillo cualquiera que encontramos en nuestra vida cotidiana, la única diferencia es que sus dimensiones son muy pequeñas, en el rango de nanómetros (0.000000001 m). Además, en este caso el anillo es de un material semiconductor llamado arseniuro de galio (GaAs), a diferencia de los anillos que usamos en la vida cotidiana que son de oro y plata. Otra diferencia es que el anillo que se estudió en este trabajo solo tiene dos dimensiones, esto quiere decir que se parece a una moneda hueca con un espesor despreciable. A simple vista, este sistema puede parecer muy ideal y poco útil; pero, estas estructuras se pueden producir experimentalmente, con un espesor muy delgado de unas cuantas capas atómicas, muy cercanas al sistema aquí considerado y tienen muchas aplicaciones importantes en el mundo actual