bluuebunny/crossref_metadata_2025 huggingface dataset:
Risk factors for stunting among under-fives in Libya
Authors: El Taguri Adel , Betilmal Ibrahim , Mahmud Salah Murad , Monem Ahmed Abdel , Goulet Olivier , Galan Pilar , Hercberg Serge
Published: September 2008

Objective

Stunting is a chronic condition reflecting poor nutrition and health. Our aim was to ascertain major predictors of stunting in children <5 years old in Libya.

Population and methods

A nationally representative, cross-sectional, two-stage stratified cluster sample survey enrolled 4549 under-fives from 6707 households. Logistic regression was used to determine individual risk factors in bivariate and multivariate analyses.

Results

Anthropometric measurements were available for 4498 children. Among the 929 stunted children (20·7 %), 495 were boys (53·3 %) and 434 were girls (46·5 %). In multivariate analysis, risk factors were young age (1–2 years: OR = 2·32, 95 % CI 1·67, 3·22; 2–3 years: OR = 1·64, 95 % CI 1·22, 2·21), resident of Al-Akhdar (OR = 1·67, 95 % CI 1·08, 2·58), being a boy (OR = 1·28, 95 % CI 1·05, 1·55), having a less educated father (illiterate: OR = 2·10, 95 % CI 1·17, 3·77; preparatory school: OR = 1·71, 95 % CI 1·11, 2·65), poor psychosocial stimulation (no family visits or trips: OR = 1·52, 95 % CI 1·07, 2·16; father rarely/never plays with child: OR = 2·24, 95 % CI 1·20, 4·16), filtered water (OR = 8·45, 95 % CI 2·31, 30·95), throwing garbage in the street (OR = 13·81, 95 % CI 2·33, 81·72), diarrhoea (OR = 1·58, 95 % CI 1·09, 2·29) and low birth weight (OR = 1·8, 95 % CI 1·17, 2·40). Protective factors were older age of father (OR = 0·53, 95 % CI 0·32, 0·90) and water storage (OR = 0·70, 95 % CI 0·54, 0·90). These variables only explained 20 % of cases of stunting.

Conclusion

Various multilevel actions are needed to improve nutritional status of under-fives in Libya. At risk-groups include those with young age (1–3 years), resident of Al-Akhdar region, boys, father’s low educational level, poor psychosocial stimulation, poor housing environment, diarrhoea and low birth weight.
An exploratory study of associations between social capital and self-assessed health in Norway
Authors: IVERSEN TOR
Published: September 2008

The objective of this study is to estimate associations between social capital and health when other factors are controlled for. Data from the standard-of-living survey by Statistics Norway are merged with data from several other sources. The merged files combine data at the individual level with data that describe indicators of community-level social capital related to each person’s county of residence. Both cross-sectional and panel data are used. We find that one indicator of community-level social capital – voting participation in local elections – is positively associated with self-assessed health in the cross-sectional study and in the panel data study. While we find that religious activity at the community-level has a positive effect in the cross-sectional survey and no effect in the panel survey, we find that sports organizations have a negative effect on health in the cross-sectional survey and no effect in the panel survey. The question is raised whether the welfare state diminishes the effect of structural community social capital, as represented by voluntary organizations, on health.
Access to psycho-social resources and health: exploratory findings from a survey of the French population
Authors: JUSOT FLORENCE , GRIGNON MICHEL , DOURGNON PAUL
Published: September 2008

We study the psycho-social determinants of self-assessed health in order to explain social inequalities in health in France. We use a unique general population survey to assess the respective impact on self-assessed health status of subjective perceptions of social capital, social support, and sense of control, controlling for standard socio-demographic factors (SES, income, education, age, and gender). The survey is unique in that it provides a variety of measures of self-perceived psycho-social resources (trust and civic engagement, social support, sense of control, and self-esteem). We find empirical support for the link between the subjective perception of psycho-social resources and health. Sense of control at work is the most important correlate of health status after income. Other important ones are civic engagement and social support. To a lesser extent, sense of being lower in the social hierarchy is associated with poorer health status. On the contrary, relative deprivation does not affect health in our survey. Since access to psycho-social resources is not equally distributed in the population, these findings suggest that psycho-social factors can partially explain of social inequalities in health in France.
Modeling the 20th century Arctic Ocean/Sea ice system: Reconstruction of surface forcing
Authors: Kauker Frank , Köberle Cornelia , Gerdes Rüdiger , Karcher Michael
Published: September 2008

The ability to simulate the past variability of the sea ice‐ocean system is of fundamental interest for the identification of key processes and the evaluation of scenarios of future developments. To achieve this goal atmospheric surface fields are reconstructed by statistical means for the period 1900 to 1997 and applied to a coupled sea ice‐ocean model of the North Atlantic/Arctic Ocean. We devised a statistical model using a redundancy analysis to reconstruct the atmospheric fields. Several sets of predictor and predictand fields are used for reconstructions on different time scales. The predictor fields are instrumental records available as gridded or station data sets of sea level pressure and surface air temperature. The predictands are surface fields from the NCAR/NCEP reanalysis. Spatial patterns are selected by maximizing predictand variance during a “learning” period. The reliability of these patterns is tested in a validation period. The ensemble of reconstructions is checked for robustness by mutual comparison and an “optimal” reconstruction is selected. Results of the simulations with the sea ice‐ocean model are compared with historical sea ice extent observations for the Arctic and Nordic Seas. The results obtained with the “optimal” reconstruction are shown to be highly consistent with these historical data. An analysis of simulated trends of the “early 20th century warming” and the recent warming in the Arctic complete the manuscript.
A statistical look at plasmaspheric drainage plumes
Authors: Borovsky Joseph E. , Denton Michael H.
Published: September 2008

The properties of plasmaspheric drainage plumes are examined using cold‐plasma measurements in geosynchronous orbit. During high‐speed stream‐driven storms, 210 plume crossings are collected and statistically analyzed. Plumes that persist for 4 days are common, which was the duration of our search. Plumes weaken with age, becoming narrower in local time with plasma that becomes less dense. Cold‐plasma flow velocities are sunward in the plumes, with flow speeds decreasing as the storms progress. Plumes transfer typically 2 × 1026 ions/sec (1.2 ton/hr of protons) when they are young, and the rate of transport decreases with plume age. A total of approximately 2 × 1031 ions (34 tons of protons) are transported via plumes in the life of a storm. About half of the outer plasmasphere is drained in the first 20 hours of a storm. Large density fluctuations in the plumes indicate that the drainage plumes are lumpy, and large velocity fluctuations of the plasma flow indicate that the drainage plumes may be turbulent. Because of their persistence, drainage plumes are anticipated to be a regular feature of any ongoing geomagnetic storm.
Continuous monitoring of soil CO2 flux on Mt. Etna: The 2004–2005 eruption and the role of regional tectonics and volcano tectonics
Authors: Gurrieri Sergio , Liuzzo Marco , Giudice Gaetano
Published: September 2008

The soil CO2 flux on Mt. Etna as recorded by the ETNAGAS network (an automatic system for measuring soil CO2 flux and meteorological parameters) started to increase strongly about 5 months prior to the onset of the 2004–2005 eruption and decreased a few months before the end of the eruption. Time delays in the occurrences of anomalies in soil CO2 flux at different sites in the geochemical network constrain the relationship between soil CO2 flux distributions and the tectonic framework of Etna volcano. The anomalies observed before the 2004–2005 eruption support the intrusion of new undegassed magma into the upper feeding system of the volcano (<20 km below sea level). Magma subsequently rose slowly in the volcano conduits, thereby triggering the onset of the 2004–2005 eruption. The time delays in the occurrences of anomalies in combination with spectral analysis indicate the importance of tectonic and volcanotectonic structures in driving the ascent of deep gases within the crust. Moreover, greatest amplitude pulsations of the low‐frequency components of the CO2 flux signals were correlated with the paroxystic activities of the 2004–2005 eruption. This study confirms that CO2 flux variation is a useful indicator for volcanic activity in the surveillance of the Mt. Etna and similar basaltic volcanoes.
Modeling the mechanics of rate and state friction with linear viscoelasticity
Authors: Walsh Joseph B. , Goldsby David L.
Published: September 2008

In slide‐hold‐slide friction experiments, rock surfaces loaded by normal stress σ slide against one another at velocity V until steady‐state conditions prevail; the applied shear stress τ required for steady‐state sliding is fσ, where f is the friction coefficient. Sliding is then stopped abruptly for a specified hold time tH, and then resumed at velocity V. Shear stress τ and the separation Δw between the rough surfaces are measured throughout the test. During the hold period, both applied shear stress and separation are found to decay with time from their steady‐state values. We show that the constitutive behavior in shear during the hold period is in effect a relaxation test, and likewise, the closure measurements describe a creep test. Analyzing the data in detail, we find that they can be modeled as the response of a standard linear solid, with a characteristic time T1 for creep of 1900 s and a characteristic time T2 for relaxation of 1400 s. The shear stress required to initiate sliding at the end of the hold period is found to peak Δfσ above the steady‐state stress fσ, then decay over a distance Dc to the steady‐state value. In our experiments, measurements of separation and shear load were made for hold periods as long as 106 s, i.e., one hundred times longer than in similar previous studies. We find that our measured values of Δf increase gradually with increasing hold times—in agreement with previous measurements up to about 104 s—but then approach a limiting value asymptotically. In our analysis we show that Δf is equal to (1 + f2)ΔδH/fλ, where ΔδH is the total decrease in separation during the hold period and λ is a length parameter that describes the roughness of the surfaces. The parameter Dc is found to be given by ΔδH/Δf. Using measurements from our experiments, we find that both relationships are valid to a reasonable degree, considering the uncertainties in the data and the simplicity of the analysis.
In situ production of ash in pyroclastic flows
Authors: Dufek J. , Manga M.
Published: September 2008

Abrasion and comminution of pumice clasts during the propagation of pyroclastic flows have long been recognized as a potential source for the enhanced production of volcanic ash, however, their relative importance has eluded quantification. The amount of ash produced in situ can potentially affect runout distance, deposit sorting, the volume of ash introduced in the upper atmosphere, and internal pore pressure. We conduct a series of laboratory experiments on the collisional and frictional production of ash that may occur during different regimes of pyroclastic flow transport. Ash produced in these experiments is predominately 10–100 microns in size and has similar morphology to tephra fall ash from Plinian events. We find that collisional ash production rates are proportional to the square of impact velocity. Frictional ash production rates are a linear function of the velocity of the basal, particle‐enriched bed load region of these flows. Using these laboratory experiments we develop a subgrid model for ash production that can be included in analytical and multiphase numerical procedures to estimate the total volume of ash produced during transport. We find that for most flow conditions, 10–20% of the initial clasts comminute into ash with the percentage increasing as a function of initial flow energy. Most of the ash is produced in the high‐energy regions near the flow inlet, although flow acceleration on steep slopes can produce ash far from the vent. On level terrain, collisionally and frictionally produced ash generates gravity currents that detach from the main flow and can more than double the effective runout distance of these flows. Ash produced at the frictional base of the flow and in the collisional upper regions of the flow can be redistributed through the entirety of the flow, although frictionally produced ash accumulates preferentially near its source in the bed load. Flows that descend steep slopes produce the majority of their ash in the collisonally dominant flow head and flow snouts likely develop subangular to rounded pumice during this process.
Oceanic propagation of a potential tsunami from the La Palma Island
Authors: Løvholt F. , Pedersen G. , Gisler G.
Published: September 2008

The likelihood of a large scale tsunami from the La Palma Island is considered small by most. Nevertheless, the potential catastrophic consequences call for attention. Here we report on numerical simulations of a tsunami that might result from the extreme case of a flank collapse of the Cumbre Vieja volcano at the La Palma Island, done by combining a multimaterial model for the wave generation with Boussinesq models for the far‐field propagation. Our simulations show that the slide speed is close to critical, effectively generating an initial wave of several hundred meters height. Our main focus is the wave propagation which is genuinely dispersive. In the far‐field, propagation becomes increasingly complex due to the combined effects of dispersion, refraction, and interference in the direction of propagation. Constructive interference of the trailing waves are found to decrease the decay of the maximum amplitude with distance compared to classical asymptotic theory at transatlantic distances. Thus, the commonly used hydrostatic models fail to describe the propagation. Consequences of the La Palma scenario would be largest at the Canary Islands, but our findings also suggests that the whole central Atlantic would face grave consequences. However, the largest surface elevations are smaller than the most pessimistic reports found in literature. We also find undular bores towards the shorelines of America.
A stochastic convective approach to account for model uncertainty due to unresolved humidity variability
Authors: Tompkins A. M. , Berner J.
Published: September 2008

Forecast uncertainty can result from the neglect of humidity variability on spatial scales not resolved by forecast models. To account for this, a stochastic convective scheme for the European Centre for Medium‐Range Weather Forecasts (ECMWF) ensemble prediction system is presented that uses the subgrid humidity distributions provided by the cloud scheme. Each member of a forecast ensemble randomly samples this distribution to perturb the convective parcel's initial humidity and/or the humidity of the air entrained during ascent. Accounting for humidity variability with the new scheme has a smaller impact on tropical ensemble spread in the short range compared to the ECMWF operational scheme that represents uncertainty because of parameterization error. Combining the two schemes to account for both parameterization error and subgrid humidity variability simultaneously generally improves the skill of the operational system for most variables in the short to medium range in midlatitudes, while results in the tropics are mixed, with a notable deterioration in medium‐range probabilistic skill for temperature and zonal wind. This deterioration is a consequence of the methodology employed. Since the convective scheme is highly nonlinear, providing zero‐mean humidity perturbations to the scheme's input profiles does not lead to zero‐mean perturbations to the output tendencies. For some parameters, this results in an increased bias of the ensemble mean and a deterioration in probabilistic skill. As future convective schemes are optimized to maximize deterministic forecast skill, the methodology will require modification to ensure zero‐mean convective output perturbations.
Uniqueness and stability analysis of hydrogeophysical inversion for time‐lapse ground‐penetrating radar estimates of shallow soil hydraulic properties
Authors: Jadoon Khan Zaib , Slob Evert , Vanclooster Marnik , Vereecken Harry , Lambot Sébastien
Published: September 2008

Precise measurement of soil hydraulic properties at field scales is one of the prerequisites to simulate subsurface flow and transport processes, which is crucial in many research and engineering areas. In our study, we numerically analyze uniqueness and stability for integrated hydrogeophysical inversion of time‐lapse, off‐ground ground‐penetrating radar (GPR) data in estimating the unsaturated soil hydraulic properties. In the inversion, hydrodynamic modeling based on the one‐dimensional (1‐D) Richards equation is used to physically constrain a full‐waveform radar electromagnetic model. Synthetic GPR data, in terms of 3‐D multilayered media Green's functions, were generated for three different textured soils (coarse, medium, and fine) and assuming different infiltration events. Inversion was performed iteratively to estimate three key soil hydraulic parameters (α, n, and Ks) of the Mualem‐van Genuchten model using the global multilevel coordinate search optimization algorithm. For the coarse‐ and medium‐textured soils, inversions converged to the actual solution for all scenarios. For the fine soil, estimation errors occurred, mainly because of the higher attenuation of the electromagnetic waves in such a soil (high electric conductivity). The procedure appeared to be generally stable with respect to possible errors in the hydrodynamic and petrophysical model parameterization. However, we found that particular attention should be given to an accurate estimation of the saturated water content and infiltration flux for real field applications. The results from our numerical experiments suggest that, in theory, the proposed method is promising for the noninvasive identification of the shallow soil hydraulic properties at the field scale with a high spatial resolution.
Effects of stream‐aquifer disconnection on local flow patterns
Authors: Desilets Sharon L. E. , Ferré Ty P. A. , Troch Peter A.
Published: September 2008

Disconnected stream‐aquifer systems are becoming increasingly common because of lowering groundwater tables. This work focuses on the pathways and rates of infiltration and seepage as streams transition from fully connected to disconnected conditions. HYDRUS‐2D simulations show for a connected stream, water infiltrates vertically then moves laterally below the top of the preflooding capillary fringe height, finally causing an upward displacement of antecedent water into the vadose zone. This contradicts the commonly held assumption that stream water moves laterally for some distance into the stream bank, forming a wedge above the antecedent water. Even for shallow disconnections (<1 m), there is an increase in infiltration losses from the stream, elimination of postflooding seepage, and increased lateral and vertical mixing of stream water and antecedent pore water.
How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006
Authors: Lean Judith L. , Rind David H.
Published: September 2008

To distinguish between simultaneous natural and anthropogenic impacts on surface temperature, regionally as well as globally, we perform a robust multivariate analysis using the best available estimates of each together with the observed surface temperature record from 1889 to 2006. The results enable us to compare, for the first time from observations, the geographical distributions of responses to individual influences consistent with their global impacts. We find a response to solar forcing quite different from that reported in several papers published recently in this journal, and zonally averaged responses to both natural and anthropogenic forcings that differ distinctly from those indicated by the Intergovernmental Panel on Climate Change, whose conclusions depended on model simulations. Anthropogenic warming estimated directly from the historical observations is more pronounced between 45°S and 50°N than at higher latitudes whereas the model‐simulated trends have minimum values in the tropics and increase steadily from 30 to 70°N.
Enhanced secondary organic aerosol formation due to water uptake by fine particles
Authors: Hennigan Christopher J. , Bergin Michael H. , Dibb Jack E. , Weber Rodney J.
Published: September 2008

This study characterizes the partitioning behavior of a significant fraction of the ambient organic aerosol through simultaneous measurements of gas and particle water‐soluble organic carbon (WSOC). During the summer in Atlanta, WSOC gas/particle partitioning showed a strong RH dependence that was attributed to particulate liquid water. At elevated RH levels (>∼70%), a significant increase in WSOC partitioning to the particle phase was observed and followed the predicted water uptake by fine particles. The enhancement in particle‐phase partitioning translated to increased median particle WSOC concentrations ranging from 0.3–0.9 μg C m−3. The results provide a detailed overview of the WSOC partitioning behavior in the summertime in an urban region dominated by biogenic emissions, and indicate that secondary organic aerosol formation involving partitioning to liquid water may be a significant aerosol formation route that is generally not considered.
Seasonal variations in nutrient concentrations and speciation in the Chena River, Alaska
Authors: Cai Yihua , Guo Laodong , Douglas Thomas A. , Whitledge Terry E.
Published: September 2008

To better understand the seasonal controls on nutrient abundances, speciation, and fluxes in a watershed underlain by discontinuous permafrost, we collected water samples biweekly from the Chena River during 2005–2006 to measure inorganic and organic N, P, and Si in dissolved and particulate phases. Nitrate concentrations were low (8–14 μM) during the winter and summer dry seasons but were elevated during the spring freshet (15–24 μM). Ammonium varied from 8 to 13 μM during the winter but dropped dramatically during the ice‐open season to 0.1–3 μM. Phosphate was very low throughout the year (ranging from 0.03 to 0.3 μM), reflecting the pristine condition of the watershed. Dissolved silica was high in the winter and reached its minimum during the spring freshet. DIN was the dominant species in the total N pool (60%), followed by DON (30%) and PN (10%). Most of the phosphorous was present in the particulate phase (74%), with phosphate and DOP only comprising 19% and 7%, respectively. Seasonal variations in nutrient concentrations and speciation were mostly controlled by the hydrological flow regime and biological activity in the river. Annual nutrient export fluxes from the Chena River during 2005–2006 were 51.1 × 106 mole‐N, 1.4 × 106 mole‐P, and 197 × 106 mole‐Si, corresponding to an annual yield of 9.8 × 103 mol‐N km−2, 0.28 × 103 mol‐P km−2, and 37.9 × 103 mol‐Si km−2, respectively. Within the annual export fluxes, the spring freshet contributed about 18% of TN, 27% of TP, and 10% of Si, while the winter season contributed 11% of TN, 12% of TP, and 20% of Si. Continued climatic warming in northern watersheds will likely increase the export of nutrient species from watersheds.
Effects of multiscale rainfall variability on flood frequency: Comparative multisite analysis of dominant runoff processes
Authors: Samuel Jos M. , Sivapalan Murugesu
Published: September 2008

We present results of a comparative modeling analysis of the effects of multiscale rainfall variability (within‐event, between‐event, seasonal, interannual, and interdecadal) on estimated flood frequency curves for three catchments located in Perth, Newcastle, and Darwin, Australia. The analysis is performed using the derived distribution approach by combining long‐term rainfall time series generated by a stochastic rainfall model with a continuous rainfall‐runoff flood model that is able to generate runoff variability over a multiplicity of timescales. Similarities and differences of the flood frequency curves (FFCs) in these rather diverse catchments are then interpreted on the basis of differences in the dominant runoff generation processes. In Newcastle, annual maximum flood peaks are caused by saturation excess overland flow over the entire range of annual exceedance probabilities (AEPs) or return periods. On the other hand, in Darwin, the shape of the FFC is determined strongly by seasonal climatic variability, which, in combination with deep soils, leads to a switch of dominant runoff mechanisms contributing to annual maximum flood peaks, from subsurface stormflow at high AEPs (low return periods) to saturation excess overland flow at low AEPs (high return periods). This leads to FFCs exhibiting a consistent break in slope in the Darwin catchment but not so in Newcastle. On the other hand, the FFCs in Perth are affected by both seasonality and long‐term climate variability and produce a variety of shapes depending on the relative strengths of these climatic controls. Because of the fact that in Perth and Darwin the shapes of the flood frequency curves depend on a possible switch of the dominant runoff generation mechanisms with increasing return period, uncertainty in hydrological model parameters relating to landscape properties contributes significantly to the uncertainty in the flood frequency curves. This uncertainty is much less pronounced in Newcastle because of the absence of such a switch of runoff generation mechanisms.
Analysis of terahertz pulses from large-aperture biased semi-insulating and arsenic-ion-implanted GaAs antennas
Authors: Chou Rone-Hwa , Liu Tze-An , Pan Ci-Ling
Published: September 2008

We investigate the characteristics of terahertz radiation pulses generated using biased semi-insulating and arsenic-ion-implanted GaAs photoconductive antennas with 1.5 cm aperture size under various pump fluences and bias fields. Compared with semi-insulating GaAs antenna, our arsenic-ion-implanted GaAs antenna exhibits larger bandwidth and better emission efficiency. Our simulation verifies that the superior characteristics for the latter can be partly attributed to larger optical absorption in the ion-implanted layer. For both types of antennas, we observe that the radiated peak terahertz amplitude displays an anomalous dependence on pump fluence, which deviates from the prediction given by the scaling rule. Analyzing the theoretical and simulation results, we infer that this behavior arises from band filling and two-photon absorption effects. At specific pump fluence, we find that the dependence of peak terahertz amplitude on bias field is distinct from the usual linear relationship predicted by the scaling rule.
Exact solution of the phase problem in in situ x-ray reflectometry of a growing layered film
Authors: Kozhevnikov Igor , Peverini Luca , Ziegler Eric
Published: September 2008

The recent development of an exact solution of the phase retrieval problem is applied to the case of in situ x-ray reflectometry measurements performed during the growth of a tungsten film. The measurement of the phase variation during deposition provides information about the depth distribution of the material density, even though the grazing angle of the probe beam is fixed. A comparison of two different approaches for reconstructing the material density profile is presented. The effect of roughness on the phase retrieval is analyzed.
Ballistic transport properties in spin field-effect transistors
Authors: Jiang K. M. , Yang Jun , Zhang R. , Wang Hongyan
Published: September 2008

We investigate conductance properties in ballistic spin field-effect transistors (SFETs) by taking into account the Rashba spin-orbit coupling (SOC), interface scattering, the presence of an in-plane magnetic field, band mismatch, and spin polarization in the ferromagnetic electrodes. It is shown that the conductance of the SFET has high peaks for high potential barriers at the contact/channel interfaces. Therefore, switching on or off can be easily realized by tuning either the band mismatch, the Rashba SOC strength, or the magnitude or direction of the magnetic field. Moreover, in the SFET with Ohmic-contact interfaces, the conductance modulation results from a mixing between Fabry–Pérot-type spin channels interference and spin precession and becomes more and more pronounced as the spin polarization in the contacts increases.
Deoxidation of graphene oxide nanosheets to extended graphenites by “unzipping” elimination
Authors: Chua Lay-Lay , Wang Shuai , Chia Perq-Jon , Chen Lan , Zhao Li-Hong , Chen Wei , Wee Andrew T.-S. , Ho Peter K.-H.
Published: September 2008

Low-temperature scanning tunneling microscopy on alkyl-surface-functionalized graphene oxide nanosheets reveals the formation of low-dimensional graphenite nanostructures with extended π-conjugation at deoxidation temperatures above 150°C. The elimination of these alkyl chains from the surface of the nanosheets does not occur uniformly, but in distinctive patterns that correspond to the formation of an underlying network of graphenite one-dimensional “tracks” and “dots.” Atomic-resolution imaging of these graphenite regions reveals a defective honeycomb lattice characteristic of single-layer graphenes. These extended graphenite structures percolate the nanosheet even for moderate levels of deoxidation and regraphenization of the basal plane. The formation of extended conjugation indicates a regioselective rather than random elimination of the oxygen atoms and alkyl chains. The resultant network morphology allows bandlike transport of charge carriers across the sheets despite defects and disorder. The sub-meV apparent activation energies for the field-effect mobilities at low temperatures (70–30K) for both electrons and holes rule out significant electron-phonon coupling. This suggests a remarkable potential for electronic applications of these solution-processable functionalized graphene oxide nanosheets.
Enhancement of film-forming reactions for microcrystalline Si growth in atmospheric-pressure plasma using porous carbon electrode
Authors: Kakiuchi Hiroaki , Ohmi Hiromasa , Inudzuka Ryohei , Ouchi Kentaro , Yasutake Kiyoshi
Published: September 2008

We have investigated the structural and electrical properties of microcrystalline silicon (μc-Si:H) films deposited with high rates (≥5 nm/s) at 220 °C in atmospheric-pressure He/H2/SiH4 plasma excited by a 150 MHz, very high-frequency (VHF) power. For this purpose, Si films are prepared varying the deposition parameters, such as H2 and SiH4 flow rates (H2 and SiH4 concentrations) and VHF power density, using two types of electrode (porous carbon and cylindrical rotary electrodes). In the case of using the porous carbon electrode, a μc-Si:H film having a crystalline volume fraction of 71.9% is obtained even when hydrogen is not added to the process gas mixture (H2/SiH4=0). In addition, the films exhibit considerably low defect densities of (3–5)×1016 cm–3 despite the high deposition rates. Such high-rate depositions of good-quality films are realized primarily due to the chemical and physical excitations of the film-growing surface by the atmospheric-pressure plasma while suppressing ion damage and excessive heating of the surface. On the other hand, when using the cylindrical rotary electrode, the phase transition from amorphous to microcrystalline occurs at around H2/SiH4=70. The enhancement of the film-forming reactions by the porous carbon electrode are discussed from the viewpoint of the gas residence time in the plasma.
Magnetohydrodynamic turbulence in a channel with spanwise magnetic field
Authors: Krasnov Dmitry , Zikanov Oleg , Schumacher Jörg , Boeck Thomas
Published: September 2008

The effect of a uniform spanwise magnetic field on a turbulent channel flow is investigated for the case of a low magnetic Reynolds number. Direct numerical simulation (DNS) and large eddy simulation (LES) computations are performed for two values of the hydrodynamic Reynolds number (104 and 2×104) and with the Hartmann number varying in a wide range. It is shown that the main effect of the magnetic field is the suppression of turbulent velocity fluctuations and momentum transfer in the wall-normal direction. This leads to drag reduction and transformation of the mean flow profile. The centerline velocity grows, the mean velocity gradients near the wall decrease, and the typical horizontal dimensions of the coherent structures enlarge upon increasing the Hartmann number. Comparison between LES and DNS results shows that the dynamic Smagorinsky model accurately reproduces the flow transformation.
Schottky barriers on silicon nanowires influenced by charge configuration
Authors: Piscator J. , Engström O.
Published: September 2008

Due to the geometry offered by nanowires, it is possible to introduce electric fields directed from the wire wall toward a Schottky contact positioned on the end surface of a wire. In the present work a simple model demonstrating the effect of charge on the wire walls close to the metal semiconductor interface is presented. This is also compared to measurements on fabricated nanowire devices, showing that additional positive charge close to the interface will lower the effective Schottky barrier height.
Excitonic spin lifetimes in InGaN quantum wells and epilayers
Authors: Brown J. , Wells J.-P. R. , Kundys D. O. , Fox A. M. , Wang T. , Parbrook P. J. , Mowbray D. J. , Skolnick M. S.
Published: September 2008

We have studied the exciton spin relaxation times in InxGa1−xN/GaN multiquantum wells as a function of well width and indium concentration for temperatures from 10 to 180 K. Well widths from 2 to 8 nm and indium concentrations from x=0.02 to 0.15 have been investigated. In contrast to 1 nm wide quantum wells where spin beats were observed [J. Brown et al., Phys. Status Solidi B 243, 1643 (2006)], no spin beats were observed in any of our samples due to the fast spin relaxation times and a reduction in the exchange energy. In all samples for which a net spin polarization could be generated, the measured spin relaxation time was 1 ps or faster. The fast exciton spin decay time is caused by the influence of the holes via the exchange interaction, while the temperature dependence can be largely attributed to exciton-phonon scattering. In the widest wells (8 nm thick), the quantum confined Stark effect precluded the possibility of observing the spin dynamics. Similar measurements on an In0.1Ga0.9N epilayer yielded a spin relaxation time of 0.45 ps.
Effect of in-plane shear strain on phase states and dielectric properties of epitaxial ferroelectric thin films
Authors: Zembilgotov A. G. , Böttger U. , Waser R.
Published: September 2008

A modified thermodynamic potential based on the eight-order Landau–Devonshire polynomial is derived for ferroelectric thin films grown on dissimilar substrates that induce anisotropic tensile/compressive strains and the shear strain in the film plane. The effect of the shear strain on the ferroelectric phase transition occurring in considered films is analyzed theoretically. It is shown that the application of the shear strain suppresses the formation of ferroelectric c phase and raises the temperature of aa∗ phase formation. The directional dependence of in-plane dielectric permittivity of a ferroelectric film is also calculated and compared with the dependence observed in (Ba0.60Sr0.40)TiO3 films deposited on NdGaO3.
Dielectric properties of Sn2P2(Se0.05S0.95)6 single crystal
Authors: Tyagur Iryna
Published: September 2008

Temperature investigations of the dielectric permittivity ε|, dielectric loss ε∥, dielectric dissipation factor tan δ, and conductivity σ of Sn2P2(Se0.05S0.95)6 single crystal at several frequencies are made. Measurements were realized by heating the sample in the oil medium from ∼293 K over the phase transition temperature to ∼353 K and slow cooling to room temperature. The phase transition is observed at temperature T0,c=329.10 K. The following parameters for Sn2P2(Se0.05S0.95)6 are obtained: extrapolated Curie temperature T0=331.56 K, Curie–Weiss temperature T01=327.88 K, and T02=332.00 K, the difference ΔT=(T0.c−T01)=1.22 K, what is obvious for the second-order phase transitions. Also, the Curie–Weiss temperature constants for the ferroelectric Cw,fe and paraelectric Cw,pa phases are calculated as Cw,fe=−30 708 and Cw,pa=96 154, and a ratio of |Cw,pa/Cw,fe|=3.10.
Glass forming ability and short-range order in a binary bulk metallic glass by ab initio molecular dynamics
Authors: Jakse N. , Pasturel A.
Published: September 2008

We have performed a series of ab initio molecular dynamics simulations of Cu64Zr36 alloy, in order to study the glass forming ability in connection to the local order. The results show that Cu64Zr36 is characterized by a pronounced icosahedral short-range order (ISRO) in the liquid, undercooled, and amorphous states associated with the existence of Cu7Zr6 and Zr6Cu10 clusters centered by Cu and Zr atoms, respectively. The latter increase the structural incompatibility of liquid and amorphous states with competing crystalline phases and give support to the view that good glass formers have high degree of ISRO already present in the liquid.
An experimentally guided umbrella sampling protocol for biomolecules
Authors: Mills Maria , Andricioaei Ioan
Published: September 2008

We present a simple method for utilizing experimental data to improve the efficiency of numerical calculations of free energy profiles from molecular dynamics simulations. The method involves umbrella sampling simulations with restraining potentials based on a known approximate estimate of the free energy profile derived solely from experimental data. The use of the experimental data results in optimal restraining potentials, guides the simulation along relevant pathways, and decreases overall computational time. In demonstration of the method, two systems are showcased. First, guided, unguided (regular) umbrella sampling simulations and exhaustive sampling simulations are compared to each other in the calculation of the free energy profile for the distance between the ends of a pentapeptide. The guided simulation use restraints based on a simulated “experimental” potential of mean force of the end-to-end distance that would be measured by fluorescence resonance energy transfer (obtained from exhaustive sampling). Statistical analysis shows a dramatic improvement in efficiency for a 5 window guided umbrella sampling over 5 and 17 window unguided umbrella sampling simulations. Moreover, the form of the potential of mean force for the guided simulations evolves, as one approaches convergence, along the same milestones as the extensive simulations, but exponentially faster. Second, the method is further validated by replicating the forced unfolding pathway of the titin I27 domain using guiding umbrella sampling potentials determined from actual single molecule pulling data. Comparison with unguided umbrella sampling reveals that the use of guided sampling encourages unfolding simulations to converge faster to a forced unfolding pathway that agrees with previous results and produces a more accurate potential of mean force.
Observation of relativistic E⊗e vibronic coupling in Rb3 and K3 quartet states on helium droplets
Authors: Auböck G. , Nagl J. , Callegari C. , Ernst W. E.
Published: September 2008

Spectroscopy of alkali-metal complexes on helium droplets (HeN, N=1000–10000) provides the unique opportunity to observe high-spin species, which exhibit prominent spin-orbit (SO) effects. In this work we present laser-induced-fluorescence and magnetic-circular-dichroism (MCD) spectra of the (2)E′4←(1)A2′4 band of Rb3 and K3 on HeN. For Rb3 we find a progression of four SO split bands, weakly perturbed by linear vibronic [Jahn–Teller (JT)] coupling. The K3 transition was previously observed and interpreted in terms of linear and quadratic JT effects in the (2)E′4 state [J. H. Reho et al., J. Chem. Phys. 115, 10265 (2001)]. The structure of the MCD spectrum, however, clearly reveals the importance of SO coupling also for the K3 transition and suggests a different assignment with weak linear JT and nonsignificant quadratic JT coupling. The observed strong C-type MCD spectra arise from different populations of the ground-state Zeeman sublevels. Thus a quantitative analysis allows a determination of the spin temperature, and since alkali-metal complexes are situated on the surface of the helium droplet this corresponds to a determination of the surface temperature. Our results from the trimer spectra are consistent with the value of ≈0.4K, which is found in the interior of a droplet and which we already confirmed for the surface from the spectra of triplet-state alkali-metal dimers in a previous work.
Energy and charge transfer in blends of dendronized perylenes with polyfluorene
Authors: Jaiser Frank , Neher Dieter , Meisel Andreas , Nothofer Heinz-Georg , Miteva Tzenka , Herrmann Andreas , Müllen Klaus , Scherf Ullrich
Published: September 2008

Two generations of polyphenylene dendrimers with a perylene diimide core are compared with a nondendronized tetraphenoxyperylene diimide model compound regarding their application in organic light-emitting diodes (OLEDs). Single layer devices with blends of the first and second generation dendrimers in polyfluorene are investigated as active layers in OLEDs, and the effect of dendronization on the emission color and electroluminescence intensity is studied. In photoluminescence, higher degrees of dendronization lead to a reduction in Förster transfer from the polyfluorene host to the perylene, resulting in a larger contribution of the blue host emission in the photoluminescence spectra. In electroluminescence, the dopants appear to act as active traps for electrons, resulting in a predominant generation of excitons on the dye. This gives rise to a remarkably stronger contribution of red emission in electroluminescence than in photoluminescence where energy is exchanged exclusively via Förster transfer. The pronounced color change from red to blue with higher degrees of dendronization and larger driving voltages is explained by the competition of the recombination of free electrons with holes and trapping of electrons by the emitting guest.
Tunnel junctions with a ferroelectric-ferromagnetic composite barrier
Authors: Wang Jian , Li Z. Y.
Published: September 2008

A theoretical model for a tunnel junction with a ferroelectric-ferromagnetic (insulator) composite barrier separating two metallic electrodes is proposed. By using free electron direct quantum tunneling method and transfer matrix formalism, taking into account screening of polarization charges in metallic electrodes and dielectric response in ferromagnetic barrier, we investigate the spin filtering (SF) effect, tunneling magnetoresistance (TMR), and electroresistance (TER) effects in the junction. It is shown that the large SF effect, hence TMR and TER effects, can be achieved. Eight resistive states in the junction can also be realized by the reversal of electric polarization in the ferroelectric layer and magnetization either in the ferromagnetic layer or in the electrodes.
Enhanced optical transmission: Role of the localized surface plasmon
Authors: Wu Shan , Wang Qian-jin , Yin Xiao-gang , Li Jia-qi , Zhu Ding , Liu Shi-qiang , Zhu Yong-yuan
Published: September 2008

We report the observation of enhanced optical transmission through the metal subwavelength hole arrays with nanoparticle inside the hole, and demonstrate the transmission enhancement and the suppression due to the excitation of the localized surface plasmon (LSP). We interpret this phenomenon with the Fano theory considering the real and imaginary parts of the polarizability of the LSP.
Mapping local defects of extended media using localized structures
Authors: Pedaci F. , Tissoni G. , Barland S. , Giudici M. , Tredicce J.
Published: September 2008

The stable positions of localized structures depend on spatial gradients in the system parameters and on the local defects of the hosting medium. We propose a general method to disclose and visualize the local defects of the medium structure, otherwise not detected. The method is based on the observation of the spatiotemporal behavior of localized structures in the presence of controlled gradients in the experimental parameters. We experimentally show an application of this method in a broad-area semiconductor vertical cavity surface emitting laser with optical injection. The comparison of the experimental results with numerical simulations shows a very good agreement.
Quality factor due to roughness scattering of shear horizontal surface acoustic waves in nanoresonators
Authors: Palasantzas G.
Published: September 2008

In this work we study the quality factor associated with dissipation due to scattering of shear horizontal surface acoustic waves by random self-affine roughness. It is shown that the quality factor is strongly influenced by both the surface roughness exponent H and the roughness amplitude w to lateral correlation length ξ ratio. Indeed, quality factors for roughness exponents H≥0.5 are comparable to quality factors due to intrinsic dissipation mechanisms (e.g., thermoelastic losses and phonon-phonon scattering) especially for wave vectors <1/ξ. Our results indicate that this dissipation mechanism should be carefully considered in the design micro/nanoelectromechanical systems.
Rashba spin-orbit interaction induced spin-polarized Andreev-reflection current through a double Aharonov–Bohm interferometer
Authors: Ye Cheng-Zhi , Li Z.-J. , Nie Y.-H. , Liang J.-Q.
Published: September 2008

Using the standard nonequilibrium Green’s function techniques, we investigate the spin-polarization dependent Andreev-reflection currents through a double Aharonov–Bohm interferometer with Rashba spin-orbit interaction. It is demonstrated that the spin polarization is induced by the Rashba spin-orbit interaction, which results in a phase difference between two quantum dots of the interferometer. The spin polarization of currents can be adjusted by tuning the Rashba spin-orbit interaction strength, the magnetic flux, and the interdot coupling as well, which lead to the transport current of complete spin polarization (either spin up or down) in both cases, with or without the interdot coupling. However, the current of complete spin polarization can approach its maximum value only in the absence of the interdot coupling.
Electron trajectory and growth rate in a two-stream electromagnetically pumped free electron laser and axial guide field
Authors: Mehdian H. , Saviz S.
Published: September 2008

The effects of two-stream on electromagnetic wiggler free electron lasers (TSEMWFEL) with an axial guiding magnetic field are studied. An analysis of the two-stream steady-state electron trajectories is given by solving the equation of motion in the axial guiding magnetic field and the electromagnetic wiggler. Numerical calculations are made to illustrate the effects of the dual electron beam on the trajectories. The dispersion relation is derived employing linear fluid theory. The characteristics of the dispersion relation are analyzed numerically. The result shows that the growth rate is considerably enhanced in comparison with single-stream. The maximum growth rate is studied numerically as a function of axial guiding magnetic field for multiple electron trajectories. It is shown that the maximum growth rate of TSEMWFEL increases and decreases with respect to the axial guiding field for different trajectories.