Interface steps in field effect devices

The charge doped into a semiconductor in a field effect transistor (FET) is generally confined to the interface of the semiconductor. A planar step at the interface causes a potential drop due to the strong electric field of the FET, which in turn is screened by the doped carriers. We analyze the dipolar electronic structure of a single step in the Thomas-Fermi approximation and find that the transmission coefficient through the step is exponentially suppressed by the electric field and the induced carrier density as well as by the step height. In addition, the field enhancement at the step edge can facilitate the electric breakthrough of the insulating layer. We suggest that these two effects may lead to severe problems when engineering FET devices with very high doping. On the other hand steps can give rise to interesting physics in superconducting FETs by forming weak links and potentially creating atomic size Josephson junctions.Comment: 6 pages, 4 figures, submitted to J. Appl. Phy

From Molecules to Ecosystems: Topics, Challenges, and Players in Environmental Chemistry

This article is meant to be appetizer and introduction to this special issue of CHIMIA on 'Chemistry at EAWAG'. Two major topics presently challenge environmental chemistry, not only at EAWAG: The human impact on global and regional biogeochemical cycles and the pollution of the environment by anthropogenic chemicals. In order to tackle the various tasks associated with these problems, environmental chemists have to build bridges between the molecular scope of laboratory experiments and the systems approach of comprehensive field and modeling studies. This requires the ability to collaborate with other disciplines and to combine their knowledge with a profound understanding of chemistry. Some consequences of these requirements for research and education in environmental chemistry are also addressed in this article

Jahn-Teller effect versus Hund's rule coupling in C60N-

We propose variational states for the ground state and the low-energy collective rotator excitations in negatively charged C60N- ions (N=1...5). The approach includes the linear electron-phonon coupling and the Coulomb interaction on the same level. The electron-phonon coupling is treated within the effective mode approximation (EMA) which yields the linear t_{1u} x H_g Jahn-Teller problem whereas the Coulomb interaction gives rise to Hund's rule coupling for N=2,3,4. The Hamiltonian has accidental SO(3) symmetry which allows an elegant formulation in terms of angular momenta. Trial states are constructed from coherent states and using projection operators onto angular momentum subspaces which results in good variational states for the complete parameter range. The evaluation of the corresponding energies is to a large extent analytical. We use the approach for a detailed analysis of the competition between Jahn-Teller effect and Hund's rule coupling, which determines the spin state for N=2,3,4. We calculate the low-spin/high-spin gap for N=2,3,4 as a function of the Hund's rule coupling constant J. We find that the experimentally measured gaps suggest a coupling constant in the range J=60-80meV. Using a finite value for J, we recalculate the ground state energies of the C60N- ions and find that the Jahn-Teller energy gain is partly counterbalanced by the Hund's rule coupling. In particular, the ground state energies for N=2,3,4 are almost equal

Improving the establishment submodel of a forest patch model to assess the long-term protective effect of mountain forests

Simulation models such as forest patch models can be used to forecast the development of forest structural attributes over time. However, predictions of such models with respect to the impact of forest dynamics on the long-term protective effect of mountain forests may be of limited accuracy where tree regeneration is simulated with little detail. For this reason, we improved the establishment submodel of the ForClim forest patch model by implementing a more detailed representation of tree regeneration. Our refined submodel included canopy shading and ungulate browsing, two important constraints to sapling growth in mountain forests. To compare the old and the new establishment submodel of ForClim, we simulated the successional dynamics of the Stotzigwald protection forest in the Swiss Alps over a 60-year period. This forest provides protection for an important traffic route, but currently contains an alarmingly low density of tree regeneration. The comparison yielded a significantly longer regeneration period for the new model version, bringing the simulations into closer agreement with the known slow stand dynamics of mountain forests. In addition, the new model version was applied to forecast the future ability of the Stotzigwald forest to buffer the valley below from rockfall disturbance. Two scenarios were simulated: (1) canopy shading but no browsing impact, and (2) canopy shading and high browsing impact. The simulated stand structures were then compared to stand structure targets for rockfall protection, in order to assess their long-term protective effects. Under both scenarios, the initial sparse level of tree regeneration affected the long-term protective effect of the forest, which considerably declined during the first 40years. In the complete absence of browsing, the density of small trees increased slightly after 60years, raising hope for an eventual recovery of the protective effect. In the scenario that included browsing, however, the density of small trees remained at very low levels. With our improved establishment submodel, we provide an enhanced tool for studying the impacts of structural dynamics on the long-term protective effect of mountain forests. For certain purposes, it is important that predictive models of forest dynamics adequately represent critical processes for tree regeneration, such as sapling responses to low light levels and high browsing pressur

When it Pays to Rush: Interpreting Morphogen Gradients Prior to Steady-State

During development, morphogen gradients precisely determine the position of gene expression boundaries despite the inevitable presence of fluctuations. Recent experiments suggest that some morphogen gradients may be interpreted prior to reaching steady-state. Theoretical work has predicted that such systems will be more robust to embryo-to-embryo fluctuations. By analysing two experimentally motivated models of morphogen gradient formation, we investigate the positional precision of gene expression boundaries determined by pre-steady-state morphogen gradients in the presence of embryo-to-embryo fluctuations, internal biochemical noise and variations in the timing of morphogen measurement. Morphogens that are direct transcription factors are found to be particularly sensitive to internal noise when interpreted prior to steady-state, disadvantaging early measurement, even in the presence of large embryo-to-embryo fluctuations. Morphogens interpreted by cell-surface receptors can be measured prior to steady-state without significant decrease in positional precision provided fluctuations in the timing of measurement are small. Applying our results to experiment, we predict that Bicoid, a transcription factor morphogen in Drosophila, is unlikely to be interpreted prior to reaching steady-state. We also predict that Activin in Xenopus and Nodal in zebrafish, morphogens interpreted by cell-surface receptors, can be decoded in pre-steady-state.Comment: 18 pages, 3 figure

Amino acid and chlorin based degradation indicators in freshwater systems

Lakes cover a global area that is about 35 times smaller than the oceans, but carbon burial in lakes and oceans are on the same order of magnitude. Hence, understanding the processes for such high organic carbon burial in lacustrine systems is essential. We applied proxies typically used for marine environments including amino acid (AA) content and their nitrogen stable isotope composition to the water columns and sediments of three lakes that differ in their trophic states and depositions rates of sedimentary terrestrial organic matter. Additionally, we used carbon isotope fingerprinting of AAs to characterise their sources and fates. We show that this set of proxies tracks sources and degradation processes in eutrophic lakes with high sedimentary total organic carbon and nitrogen content. Those lakes also have a high total hydrolysable amino acid (THAA) content as well as higher pigment concentrations. While the Chlorin degradation Index (CI) showed increasing values with depth, the patterns were less consistent for the Degradation Index (DI). In general, all parameters failed to describe degradation in the most oligotrophic lake due to the very low organic carbon and nitrogen content in the sediment. We show that AAs are mostly of autochthonous origin and that AA contribute 5–45% to the organic material in plankton, POM, and sediment. Proxies based on AA for bacterial reworking (ΣV) or trophic level (TL) showed increasing values in the water column but relatively stable values in the sediments. Furthermore, we show that methanotrophic bacteria led to increased values for the bacterial reworking proxy (ΣV) and extraordinarily low δ13C AA values (−30 to −60‰).1. Introduction 2. Experimental 2.1. Sample locations and collection 2.2. Elemental and isotopic composition 2.3. Chlorin Index 2.4. Hydrolysation, derivatisation and quantification of amino acids 2.5. Compound specific stable isotope analysis of amino acids 2.6. Degradation index 2.7. Trophic level calculation 2.8. Statistical methods 3. Results 3.1. Bulk properties 3.2. Composition of total hydrolysable amino acids (THAA) 3.3. Degradation indicators 3.4. Compound specific stable nitrogen isotope analysis 3.5. Compound specific stable carbon isotope analysis 4. Discussion 4.1. Degradation indicators 4.2. Amino acids as source markers 4.3. Bacterial reworking 4.4. Trophic level 4.5. Carbon isotopes of amino acids 4.6. Implications 5. Conclusion

Influence of human impact and bedrock differences on the vegetational history of the Insubrian Southern Alps

Vegetation history for the study region is reconstructed on the basis of pollen, charcoal and AMS14C investigations of lake sediments from Lago del Segrino (calcareous bedrock) and Lago di Muzzano (siliceous bedrock). Late-glacial forests were characterised byBetula andPinus sylvestris. At the beginning of the Holocene they were replaced by temperate continental forest and shrub communities. A special type of temperate lowland forest, withAbies alba as the most important tree, was present in the period 8300 to 4500 B.P. Subsequently,Fagus, Quercus andAlnus glutinosa were the main forest components andA. alba ceased to be of importance.Castanea sativa andJuglans regia were probably introduced after forest clearance by fire during the first century A.D. On soils derived from siliceous bedrock,C. sativa was already dominant at ca. A.D. 200 (A.D. dates are in calendar years). In limestone areas, however,C. sativa failed to achieve a dominant role. After the introduction ofC. sativa, the main trees were initially oak (Quercus spp.) and later the walnut (Juglans regia). Ostrya carpinifolia became the dominant tree around Lago del Segrino only in the last 100-200 years though it had spread into the area at ca. 5000 cal. B.C. This recent expansion ofOstrya is confirmed at other sites and appears to be controlled by human disturbances involving especially clearance. It is argued that these forests should not be regarded as climax communities. It is suggested that under undisturbed succession they would develop into mixed deciduous forests consisting ofFraxinus excelsior, Tilia, Ulmus, Quercus andAce

Three-Dimensional Dirac Electrons at the Fermi Energy in Cubic Inverse Perovskites: Ca_3PbO and its Family

The band structure of cubic inverse perovskites, Ca_3PbO and its family, are investigated with the first-principles method. A close observation of the band structure reveals that six equivalent Dirac electrons with a very small mass exist on the line connecting the Gamma- and X-points, and at the symmetrically equivalent points in the Brillouin zone. The discovered Dirac electrons are three-dimensional and remarkably located exactly at the Fermi energy. A tight-binding model describing the low-energy band structure is also constructed and used to discuss the origin of the Dirac electrons in this material. Materials related to Ca_3PbO are also studied, and some design principles for the Dirac electrons in this series of materials are proposed.Comment: 4.2 pages, refined versio

Chemical Imaging of Evolving Amyloid Plaque Pathology and Associated Aβ Peptide Aggregation in a Transgenic Mouse Model of Alzheimer's Disease

One of the major hallmarks of Alzheimer's disease (AD) pathology is the formation of extracellular amyloid β (Aβ) plaques. While Aβ has been suggested to be critical in inducing and, potentially, driving the disease, the molecular basis of AD pathogenesis is still under debate. Extracellular Aβ plaque pathology manifests itself upon aggregation of distinct Aβ peptides, resulting in morphologically different plaque morphotypes, including mainly diffuse and cored senile plaques. As plaque pathology precipitates long before any clinical symptoms occur, targeting the Aβ aggregation processes provides a promising target for early interventions. However, the chain of events of when, where and what Aβ species aggregate and form plaques remains unclear. The aim of the current study was to investigate the potential of MALDI-IMS as a tool to study the evolving pathology in transgenic mouse models for AD. To that end, we used an emerging, chemical imaging modality - MALDI imaging mass spectrometry - that allows for delineating Aβ aggregation with specificity at the single plaque level. We identified that plaque formation occurs first in cortical regions and that these younger plaques contain higher levels of 42 amino acid-long Aβ (Aβ1-42). Plaque maturation was found to be characterized by a relative increase in deposition of Aβ1-40, which was associated with the appearance of a cored morphology of the plaques. Finally, other C-terminally truncated Aβ species (Aβ1-38 and Aβ1-39) exhibited a similar aggregation pattern as Aβ1-40, suggesting that these species have similar aggregation characteristics. These results suggest that initial plaque formation is seeded by Aβ1-42; a process that is followed by plaque maturation upon deposition of Aβ1-40 as well as deposition by other C-terminally modified Aβ species

Enhancing Tc in field-doped Fullerenes by applying uniaxial stress

Capitalizing on the two-dimensional nature of superconductivity in field-effect doped C60, we show that it should be possible to increase the transition temperature Tc by applying uniaxial stress perpendicular to the gate electrode. This method not only holds the promise of substantially enhancing Tc (by about 30 K per GPa), but also provides a sensitive check of the current understanding of superconductivity in the doped Fullerenes.Comment: 3 pages RevTe