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

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

Greenhouse gas emissions (CO2, CH4, and N2O) from several perialpine and alpine hydropower reservoirs by diffusion and loss in turbines

We investigated greenhouse gas emissions (CO2, CH4, and N2O) from reservoirs located across an altitude gradient in Switzerland. These are the first results of greenhouse gas emissions from reservoirs at high elevations in the Alps. Depth profiles were taken in 11 reservoirs located at different altitudes between the years 2003 and 2006. Diffusive trace gas emissions were calculated using surface gas concentrations, wind speeds and transfer velocities. Additionally, methane entering with the inflowing water and methane loss at the turbine was assessed for a subset of the reservoirs. All reservoirs were emitters of carbon dioxide and methane with an average of 970±340mgm−2day−1 (results only from four lowland and one subalpine reservoir) and 0.20±0.15mgm−2day−1, respectively. One reservoir (Lake Wohlen) emitted methane at a much higher rate (1.8±0.9mgm−2day−1) than the other investigated reservoirs. There was no significant difference in methane emissions across the altitude gradient, but average dissolved methane concentrations decreased with increasing elevation. Only lowland reservoirs were sources for N2O (72±22μgm−2day−1), while the subalpine and alpine reservoirs were in equilibrium with atmospheric concentrations. These results indicate reservoirs from subalpine/alpine regions to be only minor contributors of greenhouse gases to the atmosphere compared to other reservoir

Organic matter governs N and P balance in Danube Delta lakes

The transformation of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorous (SRP), and the release of dissolved organic and particulate N and P, were analyzed in two lake complexes (Uzlina-Isac and Puiu-Rosu-Rosulet) of the Danube Delta wetland during flood conditions in May and at low water level in September 2006. The Uzlina-Isac complex was hydrologically tightly-connected with the Danube River and was flushed with river-borne nutrients and organic matter. These lakes acted as effective transformers for nutrients and produced large amounts of fresh biomass, that promoted the excretion of dissolved organic N and P during active growth. Biomass breakdown created particulate matter (<0.45μm), which was widely liberated during low flow in the fall. The Puiu-Rosu-Rosulet complex was characterized by a more distant position to the Danube and proximity to the Black Sea, and received dominantly transformed organic compounds from the flow-through water and vast vegetation cover. Due to reduced nutrient input, the internal production of organic biomass also was reduced in these more remote lakes. Total N and P export from the lake nearest to the shelf was governed by dominantly dissolved organic and particulate compounds (mean 58 and 82%, respectively). Overall, this survey found that these highly productive wetlands efficiently transform nutrients into a large pool of dissolved organic and particulate N and P. Hence, wetland lakes may behave widely as net sources of organic N and P to downstream waters and coastal marine system

Wg/Wnt Signal Can Be Transmitted through Arrow/LRP5,6 and Axin Independently of Zw3/Gsk3β Activity

AbstractActivation of the Wnt signaling cascade provides key signals during development and in disease. Here we provide evidence, by designing a Wnt receptor with ligand-independent signaling activity, that physical proximity of Arrow (LRP) to the Wnt receptor Frizzled-2 triggers the intracellular signaling cascade. We have uncovered a branch of the Wnt pathway in which Armadillo activity is regulated concomitantly with the levels of Axin protein. The intracellular pathway bypasses Gsk3β/Zw3, the kinase normally required for controlling β-catenin/Armadillo levels, suggesting that modulated degradation of Armadillo is not required for Wnt signaling. We propose that Arrow (LRP) recruits Axin to the membrane, and that this interaction leads to Axin degradation. As a consequence, Armadillo is no longer bound by Axin, resulting in nuclear signaling by Armadillo

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

Role of multiple subband renormalization in the electronic transport of correlated oxide superlattices

Metallic behavior of band-insulator/ Mott-insulator interfaces was observed in artificial perovskite superlattices such as in nanoscale SrTiO3/LaTiO3 multilayers. Applying a semiclassical perspective to the parallel electronic transport we identify two major ingredients relevant for such systems: i) the quantum confinement of the conduction electrons (superlattice modulation) leads to a complex, quasi-two dimensional subband structure with both hole- and electron-like Fermi surfaces. ii) strong electron-electron interaction requires a substantial renormalization of the quasi-particle dispersion. We characterize this renormalization by two sets of parameters, namely, the quasi-particle weight and the induced particle-hole asymmetry of each partially filled subband. In our study, the quasi-particle dispersion is calculated self-consistently as function of microscopic parameters using the slave-boson mean-field approximation introduced by Kotliar and Ruckenstein. We discuss the consequences of strong local correlations on the normal-state free-carrier response in the optical conductivity and on the thermoelectric effects.Comment: 11 pages, 4 figure

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

Optical interconnect with densely integrated plasmonic modulator and germanium photodetector arrays

We demonstrate the first chip-to-chip interconnect utilizing a densely integrated plasmonic Mach-Zehnder modulator array operating at 3 x 10 Gbit/s. A multicore fiber provides a compact optical interface, while the receiver consists of germanium photodetectors

Optical interconnect solution with plasmonic modulator and Ge photodetector array

We report on an optical chip-to-chip interconnect solution, thereby demonstrating plasmonics as a solution for ultra-dense, high-speed short-reach communications. The interconnect comprises a densely integrated plasmonic Mach-Zehnder modulator array that is packaged with standard driving electronics. On the receiver side, a germanium photodetector array is integrated with trans-impedance amplifiers. A multicore fiber provides a compact optical interface to the array. We demonstrate 4 × 20 Gb/s on-off keying signaling with direct detection.ISSN:1041-1135ISSN:1941-017