Localization and band-gap pinning in semiconductor superlattices with layer-thickness fluctuations

We consider (AlAs)_n/(GaAs)_n superlattices with random thickness fluctuations Delta-n around the nominal period n. Using three-dimensional pseudopotential plane-wave band theory, we show that (i) any amount Delta-n/n of thickness fluctuations leads to band-edge wavefunction localization, (ii) for small Delta-n/n the SL band gap is pinned at the gap level produced by a single layer with ``wrong'' thickness n + Delta-n, (iii) the bound states due to monolayer thickness fluctuations lead to significant band-gap reductions, (iv) AlAs/GaAs SL's with monolayer thickness fluctuations have a direct band gap, while the ideal SL's are indirect for n<4.Comment: 10 pages, Revtex. 3 figures available at http://www.cecam.fr/~mader/elstruc.html . Published in Europhys. Lett. 31, 107 (95

Electronic structure of intentionally disordered AlAs/GaAs superlattices

We use realistic pseudopotentials and a plane-wave basis to study the electronic structure of non-periodic, three-dimensional, 2000-atom (AlAs)_n/(GaAs)_m (001) superlattices, where the individual layer thicknesses n,m = {1,2,3} are randomly selected. We find that while the band gap of the equivalent (n = m = 2) ordered superlattice is indirect, random fluctuations in layer thicknesses lead to a direct gap in the planar Brillouin zone, strong wavefunction localization along the growth direction, short radiative lifetimes, and a significant band-gap reduction, in agreement with experiments on such intentionally grown disordered superlattices.Comment: 10 pages, REVTeX and EPSF macros, 4 figures in postscript. e-mail to [email protected]

Flowers, leaves or both? How to obtain suitable images for automated plant identification

Background: Deep learning algorithms for automated plant identification need large quantities of precisely labelled images in order to produce reliable classification results. Here, we explore what kind of perspectives and their combinations contain more characteristic information and therefore allow for higher identification accuracy. Results: We developed an image-capturing scheme to create observations of flowering plants. Each observation comprises five in-situ images of the same individual from predefined perspectives (entire plant, flower frontal- and lateral view, leaf top- and back side view). We collected a completely balanced dataset comprising 100 observations for each of 101 species with an emphasis on groups of conspecific and visually similar species including twelve Poaceae species. We used this dataset to train convolutional neural networks and determine the prediction accuracy for each single perspective and their combinations via score level fusion. Top-1 accuracies ranged between 77% (entire plant) and 97% (fusion of all perspectives) when averaged across species. Flower frontal view achieved the highest accuracy (88%). Fusing flower frontal, flower lateral and leaf top views yields the most reasonable compromise with respect to acquisition effort and accuracy (96%). The perspective achieving the highest accuracy was species dependent. Conclusions: We argue that image databases of herbaceous plants would benefit from multi organ observations, comprising at least the front and lateral perspective of flowers and the leaf top view

Biological soil quality as a factor of efficient resource utilization in organic farming systems

In 1978 the DOK long-term field experiment was installed at Therwil close to Basel comparing the farming systems „bio-Dynamic“, „bio-Organic“ and „(K)conventional“. In the first years of the trial, crop yield and feasibility of organic farming were investigated. Soils were analysed with respect to long-term effects on fertility and were evaluated in the view of farming effects on the environment. Today the interrelation of diversity and efficiency and the quality of organic products is the research focus. Long-term trials like the DOK-trial offer unique opportunities for this kind of research. The DOK-trial compares the three systems mentioned above on the basis of the same intensity of organic fertilization (i.e. the same number of animals per area), the same crop rotation and the same soil tillage. Fertilization and plant protection are different and done according to the farming system. A minerally fertilized conventional treatment is mimicking stockless farming and unfertilized plots serve as controls. Crop yields of the organic systems averaged over 21 experimental years at 80% of the conventional ones. The fertilizer input, however, was 34 – 51% lower, indicating an efficient production. The organic farming systems used 20 – 56% less energy to produce a crop unit and per land area this difference was 36 – 53%. Maintenance of soil fertility is important for a sustainable land use. In DOK field plots the organically treated soils were biologically more active than conventional, whereas chemical and physical soil parameters differed less significantly. Soil organic matter after 21 years shows substantially higher values in farming systems with manure fertilization and especially in biodynamic field plots fertilized with composted farmyard manure. Organic farming systems are suggested to utilize non-renewable resources more efficiently due to a higher inherent soil fertility

Ecological study of aquatic midges and some related insects with special reference to feeding habits

Die Schweiz ist ein reiches Land. Sie verfügt über viele Millionäre. Der große Reichtum konzentriert sich auf wenige Familien und Personen. In der Schweiz leben aber auch eine halbe Million der Bevölkerung (7,5 Mio.) in Haushalten von Erwerbstätigen, die weniger als das Existenzminimum verdienen. Über 200‘000 Personen sind auf Sozialhilfe angewiesen. Bei den Vermögen und den verfügbaren Einkommen hat sich in den letzten Jahren die Kluft zwischen den obersten und untersten zehn Prozent verschärft. Die Zunahme der sozialen Ungleichheit erhöht die soziale Brisanz, was mehr zu ergründen ist. Die soziale Differenzierung dokumentiert Prozesse der Globalisierung. Sie reproduziert und spezifiziert alte soziale Ungleichheiten. Wichtig ist, dass die Soziale Arbeit das thematisiert und weiter theoretisiert

Genetic variation for nutrient use efficiency in maize under different tillage and fertilization regimes with special emphasis to plant microbe interaction

Conservation tillage (no-till and reduced tillage) brings many benefits with respect to soil fertility and energy use, but it also has drawbacks regarding the need for synthetic fertilizers and herbicides. To promote conversation tillage in organic farming systems, crop rotation, fertilization and weed control have to be optimized. In addition, crop varieties are needed with improved nutrient use efficiency (NUE) and high weed competitiveness or tolerance

Predicion of charge separation in GaAs/AlAs cylindrical Russian Doll nanostructures

We have contrasted the quantum confinement of (i) multiple quantum wells of flat GaAs and AlAs layers, i.e. (\GaAs)_{m}/(\AlAs)_n/(\GaAs)_p/(\AlAs)_q, with (ii) ``cylindrical Russian Dolls'' -- an equivalent sequence of wells and barriers arranged as concentric wires. Using a pseudopotential plane-wave calculation, we identified theoretically a set of numbers ($m,n,p$ and $q$) such that charge separation can exist in ``cylindrical Russian Dolls'': the CBM is localized in the inner GaAs layer, while the VBM is localized in the outer GaAs layer.Comment: latex, 8 page

Nitrogen Budgets and Soil Nitrogen Stocks of Organic and Conventional Cropping Systems: Trade-Off between Efficiency and Sustainability of Nitrogen Use

Organic and conventional cropping systems differ in the nature and amounts of nitrogen (N) inputs, which may affect efficiency and sustainability of N use. In the DOK (bio-Dynamic, bio-Organic, Konventionell) field experiment, organic and conventional cropping systems have been compared since 1978 at two fertilization levels. Nitrogen inputs via manure and/or mineral fertilizers, and N exports from plots with harvested products have throughout been recorded. For all treatments, N outputs with harvests have exceeded the inputs with fertilizers. Over the past years, symbiotic N2 fixation by soybean and clover grown in the trial has additionally been assessed, indicating average annual inputs of about 100 kg ha-1 yr-1 of N fixed from the atmosphere. Soil surface budgets opposing N inputs via fertilization, symbiotic fixation, seeds and deposition to N outputs via harvested products have been computed at the plot level for the duration from 1985 to 2012. The resulting balances range from negative values of about -20 kg N ha-1 yr-1 (where outputs exceed the sum of said N inputs) to surpluses of about +50 kg N ha-1 yr-1. The budget based N use efficiency (NUE; N output via harvested products divided by sum of N inputs) in the case of negative balances suggests irrationally high NUE (>100%), while positive balances are related to lower NUE for treatments with inputs exceeding outputs. Negative balances, however, indicate soil N mining, while surpluses point to a risk of N losses, and/or N accumulation in the soil. Estimation of soil N stock changes based on yearly total N concentration measurements in the topsoil layer is currently ongoing. Preliminary results suggest that soil N stocks in the topsoil decreased under all treatments more than expected from the N balance, and that positive N balances are needed to maintain topsoil N stocks. An increase in soil N concentration was observed in none of the treatments. In conclusion, the results indicate an efficiency-sustainability trade-off. Treatments with a higher NUE lose more soil stock N than those with a lower NUE. Treatments with lower NUE indicate higher N losses from the studied crop-topsoil system. Sustainable soil N management in addition to organic fertilizer inputs might at this site require reduced soil tillage. The significance of N contained in deeper soil layers, and deep rooting crops in recovering leached N should as well be investigated

Carbon sequestration and stabilization in a 40-year agronomic long-term experiment

Soils contain more carbon (C) in the form of organic matter (soil organic matter = SOM) than the entire atmosphere and global vegetation put together. They are thus a central component of the global C cycle and its largest dynamic reservoir. On the one hand, intelligent agricultural practices are discussed as a way of mitigating climate change because they can increase the amount of SOM and thus actively remove C from the atmosphere. On the other hand, all intensively used soils lose C in the long term. Central questions in this context revolve around the extent and dynamics of storage, the stabilisation mechanisms involved and the impact of agricultural use on the C budget

New insights in below ground nitrogen of clover-grass mixtures

Estimates of symbiotic nitrogen fixation (SNF) of clover in mixtures usually consider only aboveground clover nitrogen (N). However, belowground inputs of clover N derived from SNF via roots and rhizodeposition and its transfer to associated grass may contribute significantly to the amount of symbiotically fixed clover N. A microplot study with a red clover ( L.)-perennial ryegrass ( L.) model mixture was conducted within zero fertilised, bio-organic and conventional field plots of the DOK (bio-Dynamic, bio-Organic, Konventionell) long-term experiment during two consecutive years