Elastic scattering of intermediate-energy electrons from C_(60) molecules

Experimental and calculated differential cross sections for elastic scattering of electrons by C_(60) molecules at collision energies of 100 to 500 eV are reported. The elastic differential cross sections were measured in a standard crossed-beam apparatus, while the calculations were performed employing the Schwinger multichannel technique at the static-exchange level. Diffraction effects, some due to the overall spherical-cage structure and some to scattering by individual C atoms, are observed in both the measured and calculated cross sections

The Impact of BeamCal Performance at Different ILC Beam Parameters and Crossing Angles on \tilde{tau} searches

The ILC accelerator parameters and detector concepts are still under discussion in the world-wide community. As will be shown, the performance of the BeamCal, the calorimeter in the very forward area of the ILC detector, is very sensitive to the beam parameter and crossing angle choices. We propose here BeamCal designs for a small (0 or 2 mrad) and large (20 mrad) crossing angles and report about the veto performance study done. As illustration, the influence of several proposed beam parameter sets and crossing-angles on the signal to background ratio in the stau search is estimated for a particular realization of the super-symmetric model.Comment: Talk given by V. Drugakov at the Linear Collider Workshop "LCWS06'', 9-13 March 2006, I.I.Sc Bangalore, Indi

Electrical Transport in High Quality Graphene pnp Junctions

We fabricate and investigate high quality graphene devices with contactless, suspended top gates, and demonstrate formation of graphene pnp junctions with tunable polarity and doping levels. The device resistance displays distinct oscillations in the npn regime, arising from the Fabry-Perot interference of holes between the two pn interfaces. At high magnetic fields, we observe well-defined quantum Hall plateaus, which can be satisfactorily fit to theoretical calculations based on the aspect ratio of the device.Comment: to appear in a special focus issue in New Journal of Physic

3114 Optical zone centration of excimer laser photo-refractive keratectomy for myopia relative to the pupil with and without the use of an active eye tracking system

Explosive volcanic eruptions can eject large quantities of particulate matter that, along with other aerosol droplets and trace gases, are carried upwards into the atmosphere by the buoyant eruption column and then dispersed by winds aloft. The presence in the atmosphere of volcanic ash is a sporadic yet important factor that can threaten human health, affect the urban built environment, disrupt aerial navigation and, for very large eruptions, alter both atmospheric composition and chemistry. Once volcanic ash is injected into the atmosphere, it can be transported over great distances, even circumnavigating the entire planet. Volcanic ash modeling systems are used to simulate the atmospheric dispersion of volcanic ash and to generate operational short-term forecasts to support civil aviation and emergency management. The efficiency of response and mitigation actions is directly associated to the accuracy of the volcanic ash cloud detection. Volcanic ash modeling systems normally require an emission or source term model to characterize the eruption column; a dispersal model to simulate the atmospheric transport, dispersion and ground deposition of ash particles; and a meteorological model for the description of the atmospheric conditions. Traditional forecasts for volcanic ash build on off-line coupled modeling systems, where meteorological variables are only updated at the specified coupling intervals. Although this approach is computationally advantageous is some cases, there is a concern that it can lead to a number of accuracy issues and limitations that can be corrected by on-line modeling strategies. Despite these concerns, to date, no on-line coupled model is available for operational forecast of volcanic ash. In addition, the quantification of the limitations associated to the off-line systems has received no attention. This Ph.D. thesis describes and evaluates NMMB-MONARCH-ASH, a novel fully coupled on-line multiscale meteorological and atmospheric transport model designed to predict ash cloud trajectories, concentration of ash at relevant flight levels, and the expected deposit thickness for both regional and global domains in research and operational set-ups. The first activity targeted a model validation against several well-characterized events including, the Mt. Etna 2001, Eyjafjallajökull 2010, and Cordón Caulle 2011 eruptions. The model has shown to be robust, scalable, and capable to reproduce the spatial and temporal dispersal variability of the ash cloud and tephra deposits, showing promising results and improving the performance from well-known off-line operational models. The second activity quantified the model shortcomings and systematic errors associated to traditional off-line forecasts employed in operational set-ups. NMMB-MONARCH-ASH demonstrated that off-line forecasts could fail to reproduce up to 45-70% of the ash cloud of an on-line forecast, considered to be the best estimate of the true outcome. The uncertainty associated to off-line systems was found to be as relevant (same order of magnitude) as those uncertainties attributed to the source term. The third activity focused on a global application of NMMB-MONARCH-ASH to analyze the potential impacts of ash dispersal from Antarctic volcanoes. Numerical simulations suggested that volcanic ash emitted from Antarctic volcanoes could potentially encircle the globe, leading to significant consequences for global aviation safety. The last activity included a novel computational inversion method to account, for the first time, for the Plinian and co-ignimbrite phases of the 39 ka Campanian Ignimbrite super-eruption. This particular application employed the off-line coupled FALL3D model, found to be more suitable from a computational point of view. The outcome of this Ph.D. thesis encourages operational groups responsible for real-time advisories for aviation to consider using computationally efficient on-line coupled ash dispersal models.Las erupciones volcánicas explosivas pueden emitir una gran cantidad de material que, junto con otros aerosoles y gases traza, son inyectados en la atmósfera por la columna eruptiva para luego ser dispersados por los vientos en altura. La presencia en la atmósfera de cenizas volcánicas es un factor esporádico aunque importante, que puede llegar a amenazar la salud humana, afectar las infraestructuras urbanas, interrumpir la navegación aérea y, en el caso de grandes erupciones, alterar la composición atmosférica y química. Una vez en la atmósfera, la ceniza puede ser transportada a grandes distancias, llegando incluso a circunnavegar todo el planeta. Los sistemas de modelado de cenizas volcánicas se utilizan para simular la dispersión atmosférica de estas partículas, y para generar pronósticos operacionales a corto plazo empleados para dar soporte a la aviación civil y a la gestión de emergencias. La eficacia para responder a estos eventos está directamente asociada a la precisión de los modelos de transporte de cenizas volcánicas. Los sistemas de modelado de cenizas volcánicas requieren de un modelo de emisión de partículas para la caracterización de la columna eruptiva; un modelo de dispersión para la simulación del transporte atmosférico y la deposición de cenizas; y de un modelo meteorológico para la descripción de las condiciones atmosféricas. Los pronósticos tradicionales se basan en sistemas de modelado desacoplados (off-line), donde las variables meteorológicas sólo se actualizan a intervalos de tiempo especificados. Aunque este enfoque presenta ventajas desde el punto de vista computacional, existe la preocupación de que puede estar asociado a limitaciones y problemas de precisión que, por el contrario, pueden ser corregidos mediante estrategias de modelado acoplado (on-line). A pesar de estas preocupaciones, hasta la fecha no hay un modelo acoplado on-line disponible para el pronóstico operativo de la cenizas volcánicas. Además, tampoco existe una cuantificación de las limitaciones asociadas a los sistemas off-line. Este doctorado describe y evalúa NMMB-MONARCH-ASH, un modelo de transporte meteorológico y atmosférico multiescalar (regional/global) completamente acoplado on-line, para su uso en investigación y predicción operacional. El modelo está diseñado para predecir trayectorias de cenizas volcánicas, concentración de ceniza en niveles de vuelo (flight levels), y el correspondiente espesor de depósito. La primera actividad de esta tesis se centra en la validación de modelo mediante erupciones bien caracterizadas (Mt. Etna 2001, Eyjafjallajökull 2010, y del Cordón Caulle 2011). El modelo ha demostrado ser robusto, escalable y capaz de reproducir la variabilidad de la dispersión espacial y temporal de los depósitos y de las nubes de ceniza, ostrando resultados prometedores y mejorando el rendimiento de modelos operacionales. La segunda actividad cuantifica los errores sistemáticos asociados a los pronósticos off-line. NMMB-MONARCH-ASH demuestra que estps pronósticos podrían no reproducir hasta un 45-70% de la nube de cenizas de un pronóstico on-line, considerado éste último como la mejor estimación de la realidad. Esta actividad concluye que la incertidumbre asociada a los sistemas off-line puede llegar a ser tan relevante como aquellas incertidumbres atribuidas al término fuente. La tercera actividad se centra en una aplicación global de NMMB-MONARCH-ASH para analizar los posibles impactos asociados a la dispersión de ceniza de volcanes antárticos. Los resultados alertan de las posibles consequencias de estas erupciones en la aviación a nivel mundial . La última actividad incluye un nuevo método de inversión computacional para identificar, por primera vez, las fases Pliniana y coignimbrita de la super-erupción de la Ignimbrita Campaniana (39 ka) con FALL3D. Los resultados de este Ph.D. alientan a considerar el uso de modelos acoplados on-line para generar pronósticos operacionales de ceniza volcánica

Four-terminal magneto-transport in graphene p-n junctions created by spatially selective doping

In this paper we describe a graphene p-n junction created by chemical doping. We find that chemical doping does not reduce mobility in contrast to top-gating. The preparation technique has been developed from systematic studies about influences on the initial doping of freshly prepared graphene. We investigated the removal of adsorbates by vacuum treatment, annealing and compensation doping using NH3. Hysteretic behavior is observed in the electric field effect due to dipolar adsorbates like water and NH3. Finally we demonstrate spatially selective doping of graphene using patterned PMMA. 4-terminal transport measurements of the p-n devices reveal edge channel mixing in the quantum hall regime. Quantized resistances of h/e^2, h/3e^2 and h/15e^2 can be observed as expected from theory.Comment: 18 pages, 5 figure

Effect of vegetation on the Late Miocene ocean circulation

International audienceA weak and shallow thermohaline circulation in the North Atlantic Ocean is related to an open Central American gateway and exchange with fresh Pacific waters. We estimate the effect of vegetation on the ocean general circulation using the atmospheric circulation model simulations for the Late Miocene climate. Caused by an increase in net evaporation in the Miocene North Atlantic, the North Atlantic water becomes more saline which enhances the overturning circulation and thus the northward heat transport. This effect reveals a potentially important feedback between the ocean circulation, the hydrological cycle and the land surface cover for Cenozoic climate evolution

Global model simulations of the impact of ocean-going ships on aerosols, clouds, and the radiation budget

International shipping contributes significantly to the fuel consumption of all transport related activities. Specific emissions of pollutants such as sulfur dioxide (SO<sub>2</sub>) per kg of fuel emitted are higher than for road transport or aviation. Besides gaseous pollutants, ships also emit various types of particulate matter. The aerosol impacts the Earth's radiation budget directly by scattering and absorbing the solar and thermal radiation and indirectly by changing cloud properties. Here we use ECHAM5/MESSy1-MADE, a global climate model with detailed aerosol and cloud microphysics to study the climate impacts of international shipping. The simulations show that emissions from ships significantly increase the cloud droplet number concentration of low marine water clouds by up to 5% to 30% depending on the ship emission inventory and the geographic region. Whereas the cloud liquid water content remains nearly unchanged in these simulations, effective radii of cloud droplets decrease, leading to cloud optical thickness increase of up to 5–10%. The sensitivity of the results is estimated by using three different emission inventories for present-day conditions. The sensitivity analysis reveals that shipping contributes to 2.3% to 3.6% of the total sulfate burden and 0.4% to 1.4% to the total black carbon burden in the year 2000 on the global mean. In addition to changes in aerosol chemical composition, shipping increases the aerosol number concentration, e.g. up to 25% in the size range of the accumulation mode (typically >0.1 μm) over the Atlantic. The total aerosol optical thickness over the Indian Ocean, the Gulf of Mexico and the Northeastern Pacific increases by up to 8–10% depending on the emission inventory. Changes in aerosol optical thickness caused by shipping induced modification of aerosol particle number concentration and chemical composition lead to a change in the shortwave radiation budget at the top of the atmosphere (ToA) under clear-sky condition of about −0.014 W/m² to −0.038 W/m² for a global annual average. The corresponding all-sky direct aerosol forcing ranges between −0.011 W/m² and −0.013 W/m². The indirect aerosol effect of ships on climate is found to be far larger than previously estimated. An indirect radiative effect of −0.19 W/m² to −0.60 W/m² (a change in the atmospheric shortwave radiative flux at ToA) is calculated here, contributing 17% to 39% of the total indirect effect of anthropogenic aerosols. This contribution is high because ship emissions are released in regions with frequent low marine clouds in an otherwise clean environment. In addition, the potential impact of particulate matter on the radiation budget is larger over the dark ocean surface than over polluted regions over land

Envelhecimento, trabalho e tempo livre: elaborando projetos de vida.

bitstream/item/71542/1/ID-30980.pd

Evolution of Global Ocean Tide Levels Since the Last Glacial Maximum

This study addresses the evolution of global tidal dynamics since the Last Glacial Maximum focusing on the extraction of tidal levels that are vital for the interpretation of geologic sea-level markers. For this purpose, we employ a truly-global barotropic ocean tide model which considers the non-local effect of Self-Attraction and Loading. A comparison to a global tide gauge data set for modern conditions yields agreement levels of 65%–70%. As the chosen model is data-unconstrained, and the considered dissipation mechanisms are well understood, it does not have to be re-tuned for altered paleoceanographic conditions. In agreement with prior studies, we find that changes in bathymetry during glaciation and deglaciation do exert critical control over the modeling results with minor impact by ocean stratification and sea ice friction. Simulations of 4 major partial tides are repeated in time steps of 0.5–1 ka and augmented by 4 additional partial tides estimated via linear admittance. These are then used to derive time series from which the tidal levels are determined and provided as a global data set conforming to the HOLSEA format. The modeling results indicate a strengthened tidal resonance by M2, but also by O1, under glacial conditions, in accordance with prior studies. Especially, a number of prominent changes in local resonance conditions are identified, that impact the tidal levels up to several meters difference. Among other regions, resonant features are predicted for the North Atlantic, the South China Sea, and the Arctic Ocean

Visual orbit for the low-mass binary Gliese 22 AC from speckle interferometry

Based on 14 data points obtained with near-infrared speckle interferometry and covering an almost entire revolution, we present a first visual orbit for the low-mass binary system Gliese 22 AC. The quality of the orbit is largely improved with respect to previous astrometric solutions. The dynamical system mass is 0.592 +- 0.065 solar masses, where the largest part of the error is due to the Hipparcos parallax. A comparison of this dynamical mass with mass-luminosity relations on the lower main sequence and theoretical evolutionary models for low-mass objects shows that both probably underestimate the masses of M dwarfs. A mass estimate for the companion Gliese 22 C indicates that this object is a very low-mass star with a mass close to the hydrogen burning mass limit.Comment: Accepted by Astronomy and Astrophysics, 6 pages, 2 figure