Kinetic Monte Carlo simulation of the nitridation of the GaAs (100) surfaces

We present, in this work, our preliminary results of a systematic theoretical study of the adsorption of N over As-terminated GaAs (100) (2$\times$1) surfaces. We analyzed the changes in the bond-lenghts, bond-angles and the energetics involved before and after deposition. Our results show that the N-atoms will prefer the unoccupied sites of the surface, close to the As dimer. The presence of the N pushes the As dimer out of the surface, leading to the anion exchange between the N and As atoms. Based on our results, we discussed about the kinetics of the N islands formation during epitaxial growth of the III-Nitrides.Comment: 4 pages, 7 figures, accepted for publication in Braz. J. Phys., special number, Proceedings of BWSP-12, 12th Brazilian Workshop on Semiconductor Physic

Induced Lorentz-violating terms at finite temperature

We study the radiatively induced Lorentz-violating terms at finite temperature, namely, the higher-derivative term and the Chern-Simons term. These terms are induced by integrating out the fermions coupled to the coefficient $g^{\kappa\mu\nu}$. The calculation of the resulting expressions is performed by using the derivative expansion and the Matsubara formalism. The Chern-Simons terms is nonzero only at finite temperature, whereas the higher-derivative term is finite at zero temperature, however, it goes to zero as the temperature grows to infinity. We also obtain a higher-derivative Chern-Simons term, nevertheless, it vanishes asymptotically.Comment: 12 pages, 4 figures, revtex4; v2: EPL versio

Increased cell efficiency in InGaAs thin film solar cells with dielectric and metal back reflectors

Compound single junction and multijunction solar cells enable very high photovoltaic efficiencies by virtue of employing different band gap materials in seriesconnected tandem cells to access the full solar spectrum. Researchers focused on improving the electrical properties of solar cells by optimizing the material growth conditions, however relatively little work to date has been devoted to light trapping and enhanced absorption in III-V compound solar cells using back reflectors. We studied absorption enhancement in InGaAs and InGaAsP thin film solar cells by means of numerical modeling. Flat dielectric and metal back reflectors that might be introduced into the solar cell via wafer-bonding, epitaxial lift-off or deposition techniques have been shown to increase the short circuit current and the photovoltaic efficiency of solar cells

On Advanced Mobility Concepts for Intelligent Planetary Surface Exploration

Surface exploration by wheeled rovers on Earth's Moon (the two Lunokhods) and Mars (Nasa's Sojourner and the two MERs) have been followed since many years already very suc-cessfully, specifically concerning operations over long time. However, despite of this success, the explored surface area was very small, having in mind a total driving distance of about 8 km (Spirit) and 21 km (Opportunity) over 6 years of operation. Moreover, ESA will send its ExoMars rover in 2018 to Mars, and NASA its MSL rover probably this year. However, all these rovers are lacking sufficient on-board intelligence in order to overcome longer dis-tances, driving much faster and deciding autonomously on path planning for the best trajec-tory to follow. In order to increase the scientific output of a rover mission it seems very nec-essary to explore much larger surface areas reliably in much less time. This is the main driver for a robotics institute to combine mechatronics functionalities to develop an intelligent mo-bile wheeled rover with four or six wheels, and having specific kinematics and locomotion suspension depending on the operational terrain of the rover to operate. DLR's Robotics and Mechatronics Center has a long tradition in developing advanced components in the field of light-weight motion actuation, intelligent and soft manipulation and skilled hands and tools, perception and cognition, and in increasing the autonomy of any kind of mechatronic systems. The whole design is supported and is based upon detailed modeling, optimization, and simula-tion tasks. We have developed efficient software tools to simulate the rover driveability per-formance on various terrain characteristics such as soft sandy and hard rocky terrains as well as on inclined planes, where wheel and grouser geometry plays a dominant role. Moreover, rover optimization is performed to support the best engineering intuitions, that will optimize structural and geometric parameters, compare various kinematics suspension concepts, and make use of realistic cost functions like mass and consumed energy minimization, static sta-bility, and more. For self-localization and safe navigation through unknown terrain we make use of fast 3D stereo algorithms that were successfully used e.g. in unmanned air vehicle ap-plications and on terrestrial mobile systems. The advanced rover design approach is applica-ble for lunar as well as Martian surface exploration purposes. A first mobility concept ap-proach for a lunar vehicle will be presented

Insights into channel function via channel dysfunction

The nicotinic synapse has been a touchstone for advances in neuroscience ever since Jean Nicot, the French ambassador to Portugal, sent some tobacco seeds home to Paris in 1550 with a note that the New World plant had interesting effects when smoked. Now the muscle nicotinic acetylcholine receptor (nAChR) is a well-studied example of ligand-gated ion channels. After a motor neuron is stimulated, the nerve impulse reaches the presynaptic terminal, where it evokes release of acetylcholine (ACh) into the synapse. The nAChR depolarizes the postsynaptic muscle and triggers muscle action potentials; muscle contraction follows. To date, several nAChR subtypes have been successfully isolated, purified, imaged, and expressed, and unitary currents have been recorded from these channels (1). Researchers continue to unravel the molecular mechanisms of these macromolecules that are embedded in membranes at vertebrate nerve-muscle synapses, at invertebrate nicotinic synapses (which explains why nicotine-producing tobacco plants have a select advantage against invertebrate pests), and in the vertebrate central system (which explains Jean Nicot’s fascination with those leaves). However, the precise structural events that trigger channel opening or "gating" remain mostly unknown

Should Cash Transfers Be Confined to the Poor? Implications for Poverty and Inequality in Latin America

This paper compares for 13 Latin American countries the poverty and inequality impacts of cash transfer programs that are given to all children and the elderly (that is, "categorical" transfers), to programs of equal budget that are confined to the poor within each population group (that is, "poverty targeted" transfers). The analysis finds that both the incidence of poverty and the depth of the poverty gap are important factors affecting the relative effectiveness of categorical versus poverty targeted transfers. The comparison of transfers to children and the elderly also supports the view that choosing carefully categories of beneficiaries is almost as important as targeting the poor for achieving a high poverty and inequality impact. Overall, the findings suggest that although in the Latin American context poverty targeting tends to deliver higher poverty impacts, there are circumstances under which categorical targeting confined to geographical regions (sometimes called "geographic targeting") may be a valid option to consider. This is particularly the case in low-income countries with widespread pockets of poverty.cash transfers, targeting, social assistance, poverty

Phonon-assisted and phonon-free radiative transitions in sulfur-doped GaAs diffused junctions

In the course of electroluminescence studies of GaAs junctions we investigated the emission spectra of sulfur-doped zinc-diffused diodes. The peaks in the spectra correspond to three main transitions and their associated phonon emission satellites