bluuebunny/crossref_metadata_2025 huggingface dataset:
Convection suppression criteria applied to the MIT cumulus parameterization scheme for simulating the Asian summer monsoon
Authors: Chow K. C. , Chan Johnny C. L. , Pal Jeremy S. , Giorgi Filippo
Published: December 2006

It is shown in this study that applying some convection suppression criteria to the MIT or Emanuel cumulus parameterization scheme may significantly improve the performance of a regional climate model to simulate the Asian summer monsoon precipitation, particularly the precipitation over southeastern China and the Mei‐yu rainband over the East Asian region. With the original MIT cumulus scheme, the precipitation over the ocean particularly the South China Sea (SCS) region is generally over‐estimated, which may result in the under‐estimation of precipitation over China. It is found that the relative vorticity criterion (which shuts down the convection when the low‐level flow is anticyclonic and stronger than a certain threshold value) has the largest impact on suppressing the over‐estimated precipitation over the SCS.
Source of the July 2006 West Java tsunami estimated from tide gauge records
Authors: Fujii Yushiro , Satake Kenji
Published: December 2006

The source of the West Java tsunami of July 17, 2006, which was generated during a large earthquake near the Sunda trench, is constrained by tsunami waveforms that were recorded on six tide gauges around the Indian Ocean. The tsunami travel times poorly constrain the source area, probably because shallow bathymetry near these gauges is not well known. Inversion of tsunami waveforms, however, reveals that the tsunami source was about 200 km long. The largest slip, about 2.5 m for instantaneous rupture model, was located about 150 km east of the epicenter. Most of the slip occurred on shallow parts of the fault, indicating that this earthquake shares the same characteristics with “tsunami earthquakes” which generate abnormally large tsunamis compared with ground shaking. The slip distribution yields a total seismic moment of 7.0 × 1020 Nm (Mw = 7.8).
Seasonal reversal of the ENSO rainfall signal in the Philippines
Authors: Lyon Bradfield , Cristi Hannagrace , Verceles Ernesto R. , Hilario Flaviana D. , Abastillas Rusy
Published: December 2006

Seasonal rainfall in the Philippines is known to be modulated by the El Niño Southern Oscillation (ENSO) phenomenon, with ENSO warm (cold) events frequently contributing to drought (excessive rainfall) in many areas. Here, using rainfall data from 40 observing stations across the Philippines, it is shown that the seasonal rainfall response to ENSO reverses sign between boreal summer (July–September) and fall (October–December) for both El Niño and La Niña. Above‐median, boreal summer rainfall is observed at several stations in the north‐central Philippines during El Niño before anomalously dry conditions become manifest the subsequent fall. Conversely, during La Niña below‐median summer rainfall occurs at several stations before the onset of anomalously wet conditions in fall. The results are shown to be not simply a consequence of ENSO transitions during boreal spring, but are associated with changes in the large scale monsoon system during the life cycle of ENSO events.
Effect of flow depth and velocity on the scales of macroturbulent structures in gravel‐bed rivers
Authors: Marquis Geneviève A. , Roy André G.
Published: December 2006

Laboratory and field data show that the length of macroturbulent structures consistently scales with flow depth (Y). Scalings with average streamwise flow velocity (U) only exist for laboratory data. Our aim is to establish the relations between macroturbulent structures, low‐speed wedges and ejections, and U and Y in gravel‐bed rivers. Fifteen velocity profiles covering a wide range of U while controlling for Y were measured. The duration (time interval between the beginning and the end of an event) and length (distance between two successive similar events) of the macroturbulent events are analyzed. Strong scalings of the duration and length of the low‐speed wedges and ejections exist for Y. Ejection length is strongly related to U. Because the sampling design ensured the statistical independence between U and Y, these results should provide robust estimates of the scalings of macroturbulent structures in gravel‐bed rivers.
Surface eddy diffusivity for heat in a model of the northwest Atlantic Ocean
Authors: Zhai Xiaoming , Greatbatch Richard J.
Published: December 2006

Eddies influence the surface heat budget both by modifying the surface heat flux and by the lateral transfer of heat within the surface mixed layer. It is shown that the presence of eddies modifies the surface heat flux in a model of the northwest Atlantic Ocean by more than 100 W m−2 over the Gulf Stream system. The diffusive effect of eddies is then illustrated by comparing two model runs, in the second of which the surface heat flux acts only on large spatial scales and interaction with the mesoscale eddies is suppressed. This second run exhibits finer‐scale structure and tighter thermal fronts than in the fully interactive run. Finally, we estimate the surface eddy diffusivity associated with surface thermal damping from the fully interactive run. The estimated diffusivity takes large values (more than 103 m2 s−1) south of the Gulf Stream and smaller values elsewhere.
Estimating local plasma sheet PV5/3 from single‐spacecraft measurements
Authors: Wolf R. A. , Kumar V. , Toffoletto F. R. , Erickson G. M. , Savoie A. M. , Chen C. X. , Lemon C. L.
Published: December 2006

The theory of plasma transport in Earth's plasma sheet depends critically on the entropy parameter PV5/3, where P is particle pressure and V is the volume of a closed flux tube containing one unit of magnetic flux. Theory suggests that earthward moving flow bursts that inject plasma into the inner magnetosphere consist of flux tubes that have PV5/3 values that are lower than those of neighboring slow‐moving flux tubes. However, there is no way to measure flux tube volume from one spacecraft or a small number of spacecraft. We propose a formula for estimating local PV5/3 from a single spacecraft in the plasma sheet based on a simple two‐dimensional analytic model of plasma in force equilibrium, with some parameters set from local measurements at a spacecraft and other parameters set to fit a series of equilibrated Tsyganenko models. To gain an idea of the expected error, the resulting formula is then tested against various relaxed Tsyganenko models, an equilibrium magnetic field/plasma model with a depleted channel and also a thin‐filament MHD calculation. The formula is used to estimate the entropy parameter of flux tubes injected in two substorms, using spacecraft measurements near X = −10 RE in the central plasma sheet.
Interannual variations in precipitation: The effect of the North Atlantic and Southern oscillations as seen in a satellite precipitation data set and in models
Authors: Kyte Elizabeth A. , Quartly Graham D. , Srokosz Meric A. , Tsimplis Michael N.
Published: December 2006

Precipitation is a parameter that varies on many different spatial and temporal scales. Here we look at interannual variations associated with the North Atlantic Oscillation (NAO) and the Southern Oscillation (SO), comparing the spatial and temporal changes as shown by three data sets. The Global Precipitation Climatology Project (GPCP) product is based upon satellite data, whereas both the National Centers for Environmental Prediction (NCEP) and European Centre for Medium‐Range Weather Forecasts (ECMWF) climatologies are produced through reanalysis of atmospheric circulation models. All three products show a consistent response to the NAO in the North Atlantic region, with negative states of the NAO corresponding to increases in precipitation over Greenland and southern Europe, but to a decrease over northern Europe. None of the climatologies display any net change in total rainfall as a result of the NAO, but rather a redistribution of precipitation patterns. However, this redistribution of rain is important because of its potential effect on oceanic overturning circulation. Similarly, all three data sets concur that the SO has a major effect on precipitation in certain tropical regions; however, there is some disagreement amongst the data sets as to the regional sensitivity, with NCEP showing a much weaker response than GPCP and ECMWF over Indonesia. The GPCP and NCEP climatologies show that the various phases of El Niño and La Niña act to redistribute, rather than enhance, the freshwater cycle. Given that the models incorporate no actual observations of rain, and are known to be imperfect, it is surprising how well they represent these interannual phenomena.
Long‐period seismology on Europa: 1. Physically consistent interior models
Authors: Cammarano F. , Lekic V. , Manga M. , Panning M. , Romanowicz B.
Published: December 2006

In order to examine the potential of seismology to determine the interior structure and properties of Europa, it is essential to calculate seismic velocities and attenuation for the range of plausible interiors. We calculate a range of models for the physical structure of Europa, as constrained by the satellite's composition, mass, and moment of inertia. We assume a water‐ice shell, a pyrolitic or a chondritic mantle, and a core composed of pure iron or iron plus 20 weight percent of sulfur. We consider two extreme mantle thermal states: hot and cold. Given a temperature and composition, we determine density, seismic velocities, and attenuation using thermodynamical models. While anelastic effects will be negligible in a cold mantle and the brittle part of the ice shell, strong dispersion and dissipation are expected in a hot convective mantle and the bulk of the ice shell. There is a strong relationship between different thermal structures and compositions. The “hot” mantle may maintain temperatures consistent with a liquid core made of iron plus light elements. For the “cold scenarios,” the possibility of a solid iron core cannot be excluded, and it may even be favored. The depths of the ocean and core‐mantle boundary are determined with high precision, 10 km and 40 km, respectively, once we assume a composition and thermal structure. Furthermore, the depth of the ocean is relatively insensitive (4 km) to the core composition used.
Long‐period seismology on Europa: 2. Predicted seismic response
Authors: Panning M. , Lekic V. , Manga M. , Cammarano F. , Romanowicz B.
Published: December 2006

Previous studies have documented the potential for using relatively short‐period body waves and intermediate‐period surface waves to explore the structure and tectonics of Europa. We show that long‐period measurements (0.001 to 0.1 Hz) may have large amplitudes of displacement (millimeters to centimeters) and are potentially measurable from orbit without requiring a lander. To accurately model the long‐period response of Europa, we use normal modes calculated from physically self‐consistent models of Europa's structure developed in part 1 (Cammarano et al., 2007). On the basis of the geometry of observed faults, we estimate that faulting events of magnitude 5 or larger may occur regularly. Synthetic seismograms show that long‐period displacement measurements with millimeter accuracy could detect current tectonic activity and determine the thickness of Europa's ice shell, and confirm the presence of a subsurface ocean. Determination of deeper structure with seismic measurements, however, is more challenging in the presence of a global liquid ocean, which acts to decouple deeper seismic energy from the surface.
Crustal magnetization equivalent source model of Mars constructed from a hierarchical multiresolution inversion of the Mars Global Surveyor data
Authors: Chiao Ling‐Yun , Lin Ja‐Ren , Gung Yuan‐Cheng
Published: December 2006

Several magnetic field models of Mars have been constructed since the Mars Global Surveyor data became available. Three distinct schemes formulated through spherical harmonic functions, discrete equivalent dipoles, and the continuous magnetic field kernels have yielded results that are grossly compatible but with very different details. Models of internal potential function in terms of spherical harmonics tend to yield divergent high‐degree Mauersberger‐Lowes spectra, whereas crustal magnetization models exhibit flat but still significant spectra up to high degrees. To have a better fitting to the observed data seems to have dominated previous efforts that have yielded fine details with wavelengths shorter than the lateral track spacing. The variance‐reduction versus model‐variance tradeoff analysis is invoked in this study for the determination of the appropriate regularization. Taking advantage of the recently developed multiscale inversion, we are able to conservatively retain only the model components that are robustly constrained by the data rather than unilaterally pushing for a higher degree of fitting. With the variance reduction around 82%, we find that to reach a reasonably fair data fitting without high model variance, the high‐degree power spectra of our preferred model exhibit an obvious decaying trend, implying that a lot of the short‐wavelength energy embedded within established models is either not robustly resolvable or is of external origin or is simply reflecting the nonuniform distribution of sampling at short scales. The reason that models based on spherical harmonics have greater high‐degree power is attributed to the spectral leakage due to the truncated representation.
Mössbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity's journey across sulfate‐rich outcrop, basaltic sand and dust, and hematite lag deposits
Authors: Morris R. V. , Klingelhöfer G. , Schröder C. , Rodionov D. S. , Yen A. , Ming D. W. , de Souza P. A. , Wdowiak T. , Fleischer I. , Gellert R. , Bernhardt B. , Bonnes U. , Cohen B. A. , Evlanov E. N. , Foh J. , Gütlich P. , Kankeleit E. , McCoy T. , Mittlefehldt D. W. , Renz F. , Schmidt M. E. , Zubkov B. , Squyres S. W. , Arvidson R. E.
Published: December 2006

The Mössbauer (MB) spectrometer on Opportunity measured the Fe oxidation state, identified Fe‐bearing phases, and measured relative abundances of Fe among those phases at Meridiani Planum, Mars. Eight Fe‐bearing phases were identified: jarosite (K,Na,H3O)(Fe,Al)(OH)6(SO4)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and metallic Fe (kamacite). Burns Formation outcrop rocks consist of hematite‐rich spherules dispersed throughout S‐rich rock that has nearly constant proportions of Fe3+ from jarosite, hematite, and npOx (29%, 36%, and 20% of total Fe). The high oxidation state of the S‐rich rock (Fe3+/FeT ∼ 0.9) implies that S is present as the sulfate anion. Jarosite is mineralogical evidence for aqueous processes under acid‐sulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite‐rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite‐rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe3+/FeT ∼ 0.2–0.4), with the fine‐grained and bright aeolian deposits being the most oxidized. Average proportions of total Fe from olivine, pyroxene, npOx, magnetite, and hematite are ∼33%, 38%, 18%, 6%, and 4%, respectively. The MB parameters of outcrop npOx and basaltic‐soil npOx are different, but it is not possible to infer mineralogical information beyond octahedrally coordinated Fe3+. Basaltic soils at Meridiani Planum and Gusev crater have similar Fe‐mineralogical compositions.
Mixing relationships and the effects of secondary alteration in the Wishstone and Watchtower Classes of Husband Hill, Gusev Crater, Mars
Authors: Hurowitz Joel A. , McLennan Scott M. , McSween Harry Y. , DeSouza Paulo A. , Klingelhöfer Göstar
Published: December 2006

The Wishstone and Watchtower Class rocks on Husband Hill preserve evidence for a geochemical relationship consistent with two‐component mixing between a high Al2O3, TiO2, CaO, Na2O, P2O5 end‐member and a second end‐member enriched in the elements MgO, Zn, S, Br, and Cl. The first end‐member appears to be reasonably well represented by rocks of the Wishstone Class, while the second end‐member is consistent with a chemical component, not represented by any lithology encountered by Spirit. The Watchtower Class appears to be an intermediate in the mixture. The concentration of the redox sensitive elements Fe and Mn display no systematic variation between rock classes, and the Fe‐redox state of the Watchtower Class is elevated relative to the Wishstone Class. Combined with evidence for two‐component mixing, these observations suggest that the mixture was generated by interaction between an initially Wishstone‐like deposit and a fluid phase enriched in the elements MgO, Zn, S, Br, and Cl. The fluid phase may have been weakly acidic (pH > 4) to basic (pH ≥ 9), resulting in oxidation and immobilization of Fe and Mn. A number of secondary rock surface alteration processes which obscure the mixing relationship are also identified. These include (1) removal of CaO and P2O5 from rock surfaces by dissolution of Ca‐phosphate minerals under acidic, low water‐to‐rock ratio conditions, (2) addition of SO3 to rock surfaces via aeolian deposition of high SO3 soil and/or precipitation of sulfate salts from solution, and (3) homogenization of rock surface chemistry by soil contamination.
Remote sensing of groundwater storage changes in Illinois using the Gravity Recovery and Climate Experiment (GRACE)
Authors: Yeh Pat J.‐F. , Swenson S. C. , Famiglietti J. S. , Rodell M.
Published: December 2006

Regional groundwater storage changes in Illinois are estimated from monthly GRACE total water storage change (TWSC) data and in situ measurements of soil moisture for the period 2002–2005. Groundwater storage change estimates are compared to those derived from the soil moisture and available well level data. The seasonal pattern and amplitude of GRACE‐estimated groundwater storage changes track those of the in situ measurements reasonably well, although substantial differences exist in month‐to‐month variations. The seasonal cycle of GRACE TWSC agrees well with observations (correlation coefficient = 0.83), while the seasonal cycle of GRACE‐based estimates of groundwater storage changes beneath 2 m depth agrees with observations with a correlation coefficient of 0.63. We conclude that the GRACE‐based method of estimating monthly to seasonal groundwater storage changes performs reasonably well at the 200,000 km2 scale of Illinois.
Expanding Lie (super)algebras through Abelian semigroups
Authors: Izaurieta Fernando , Rodríguez Eduardo , Salgado Patricio
Published: December 2006

We propose an outgrowth of the expansion method introduced by de Azcárraga et al. [Nucl. Phys. B 662, 185 (2003)]. The basic idea consists in considering the direct product between an Abelian semigroup S and a Lie algebra g. General conditions under which relevant subalgebras can systematically be extracted from S×g are given. We show how, for a particular choice of semigroup S, the known cases of expanded algebras can be reobtained, while new ones arise from different choices. Concrete examples, including the M algebra and a D’Auria-Fré-like superalgebra, are considered. Finally, we find explicit, nontrace invariant tensors for these S-expanded algebras, which are essential ingredients in, e.g., the formulation of supergravity theories in arbitrary space-time dimensions.
Sign reversal and light controlled tuning of circular polarization in semimagnetic CdMnSe quantum dots
Authors: Schmidt T. , Scheibner M. , Worschech L. , Forchel A. , Slobodskyy T. , Molenkamp L. W.
Published: December 2006

Circularly polarized luminescence of CdMnSe quantum dots in magnetic fields up to 5 T is studied for nominal Mn concentrations of 0%, 1%, and 2% by using a photoelastic modulator technique. The exciton g factors as well as spin relaxation times were determined from the polarized luminescence taking into account the exciton lifetimes, which were also extracted by means of time-resolved photoluminescence spectroscopy. For quantum dots without Mn and with 2% Mn exciton g factors of −1.62 and +1.32, respectively, were found. The quantum dots with 1% Mn show a vanishing small value of g for small excitation powers. For this structure the polarization properties are dominated by the optical orientation. Interestingly, for the 1% Mn quantum dots with increasing excitation power considerable changes of the polarization and the exciton g factor were observed which are interpreted in terms of heating effects. From the power dependence indirect heating via phonons and above a critical value direct heating due to photocarriers were identified to result in drastic changes of the circular polarized quantum dot emission.
Growth of short-period InAs∕GaSb superlattices
Authors: Haugan H. J. , Mahalingam K. , Brown G. J. , Mitchel W. C. , Ullrich B. , Grazulis L. , Elhamri S. , Wickett J. C. , Stokes D. W.
Published: December 2006

The purpose of this work is to explore materials for midinfrared detectors that can operate at room temperature. Shorter-period InAs∕GaSb superlattices (SLs) have larger intervalance band separations, which are beneficial for reducing Auger recombination and tunneling current, thus making room temperature operation possible. To test these possibilities, several short-period SLs ranging from 50to11Å were grown and their morphological properties were carefully monitored by transmission electron microscopy. The effect of structural degradation caused by the period reduction on the optical properties was studied using low-temperature photoluminescence (PL). The samples with larger periods (50–32Å) showed excellent structural qualities and produced narrow full width at half maximum (FWHM) of the PL peak (5meV). As the period approached 24Å, slight layer thickness undulations within the SLs were observed and these undulations intensified as the period further reduced to 17Å. These structural degradations strongly influence optical properties causing significant broadening in the FWHM and higher residual carrier concentration in the shorter-period SLs. With slower growth rates, samples with periods as thin as 19Å were grown without significant layer undulations.
Visualization of the curved liquid surface by means of the optical method
Authors: Dong Jun , Miao Runcai , Qi Jianxia
Published: December 2006

A simple technique for visualizing the curved liquid surface was developed, which is based on analyzing the reflection patterns from the curved liquid surface. Due to the wetting effect, a symmetric meniscus appeared around the needle which was inserted into the liquid. A round dark area reflection pattern was observed, when a collimated, expanded laser beam illuminates upon the curved liquid surface in a vertical direction. Particularly, the size of the dark area will change with the diameter of the laser beam. The relationships between them were theoretically analyzed, and the corresponding analytic expression of the curved liquid surface profile was derived. Based on the analytic expression, the surface tension of the liquid was obtained. Compared with the results of other methods, such as the Wilhelmy plate method and the rising method, they are in good agreement. It shows that an effective and practical technique for visualizing the curved liquid surface was found.
Contact effects on electronic transport in donor-bridge-acceptor complexes interacting with a thermal bath
Authors: Volkovich Roie , Peskin Uri
Published: December 2006

A model for electron transfer in donor-bridge-acceptor complexes with electronic coupling to nuclear bridge modes is studied using the Redfield formulation. We demonstrate that the transport mechanism through the molecular bridge is controlled by the location of the electronic-nuclear coupling term along the bridge. As the electronic-nuclear coupling term is shifted from the donor/acceptor-bridge contact sites into the bridge, the mechanism changes from kinetic transport (incoherent, thermally activated, and bridge-length independent) to coherent tunneling oscillations. This study joins earlier works aiming to explore the factors which control the mechanism of electronic transport through molecular bridges and molecular wires.
Fractional supersymmetry and hierarchy of shape invariant potentials
Authors: Daoud M. , Kibler M. R.
Published: December 2006

Fractional supersymmetric quantum mechanics is developed from a generalized Weyl-Heisenberg algebra. The Hamiltonian and the supercharges of fractional supersymmetric dynamical systems are built in terms of the generators of this algebra. The Hamiltonian gives rise to a hierarchy of isospectral Hamiltonians. Special cases of the algebra lead to dynamical systems for which the isospectral supersymmetric partner Hamiltonians are connected by a (translational or cyclic) shape invariance condition.
Band gap gratings using quantum well intermixing for quasi-phase-matching
Authors: Helmy A. S. , Bryce A. C. , Hutchings D. C. , Aitchison J. S. , Marsh J. H.
Published: December 2006

In this work, the spatial resolution of two quantum well intermixing processes has been obtained using spatially resolved photoluminescence. The processes investigated are impurity-free vacancy disordering using SiO2∕SiO2:P caps and sputtered silica induced intermixing. These studies aimed to choose a suitable intermixing technology to realize the band gap gratings for domain disordering quasi-phase-matching in GaAs∕AlGaAs heterostructures. From the photoluminescence studies it was established that the process of impurity-free vacancy disordering using SiO2∕SiO2:P caps has a spatial resolution on the order of 7μm, while the process of sputtered silica induced intermixing has a spatial resolution on the order of 3μm. From these measurements it was demonstrated that the sputtered silica induced intermixing process is more suitable for the fabrication of the gratings needed for quasi-phase-matching in the samples studied here. Successful quasi-phase-matching demonstrated through second harmonic generation at 775nm has been produced in GaAs∕AlAs short superlattice waveguides using sputtered silica induced intermixing through domain disordering quasi-phase-matching. The gratings have shown a duty cycle far from the targeted 1:1 design, which has implications on the conversion efficiency.
Automatic integration of the reaction path using diagonally implicit Runge-Kutta methods
Authors: Burger Steven K. , Yang Weitao
Published: December 2006

The diagonally implicit Runge-Kutta framework is shown to be a general form for constructing stable, efficient steepest descent reaction path integrators, of any order. With this framework tolerance driven, adaptive step-size methods can be constructed by embedding methods to obtain error estimates of each step without additional computational cost. There are many embedded and nonembedded, diagonally implicit Runge-Kutta methods available from the numerical analysis literature and these are reviewed for orders two, three, and four. New embedded methods are also developed which are tailored to the application of reaction path following. All integrators are summarized and compared for three systems: the Müller-Brown [Theor. Chem. Acta 53, 75 (1979)] potential and two gas phase chemical reactions. The results show that many of the methods are capable of integrating efficiently while reliably keeping the error bound within the desired tolerance. This allows the reaction path to be determined through automatic integration by only specifying the desired accuracy and transition state.
Influence of carrier conductivity and injection on efficiency and chromaticity in small-molecule white organic light-emitting diodes based on 4,4′-bis(2,2′-diphenylvinyl)-1,1′-spirobiphenyl and rubrene
Authors: van Gemmern Philipp , van Elsbergen Volker , Grabowski Stefan Peter , Boerner Herbert , Löbl Hans-Peter , Becker Heinrich , Kalisch Holger , Heuken Michael , Jansen Rolf H.
Published: December 2006

Organic light-emitting devices (OLEDs) employing yellow-emitting 5,6,11,12-tetraphenylnaphthacene (rubrene) and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-spirobiphenyl are optimized using a vacuum thermal evaporator. The influence of various hole injection/hole transport stacks and electron transport materials on the device performance and the electroluminescence spectra are discussed. Device characteristics are explained by the charge carrier distribution among the organic layers. OLEDs with warm-white emission with color coordinates of x=0.43 and y=0.42 were produced with power and current efficiencies of 5lm∕W and 10.9cd∕A, respectively, at a luminance of 1000cd∕m2. The maximum external quantum efficiency at a current density of 20mA∕cm2 was 4.6%.
Effect of oxidation on the chemical bonding structure of PECVD SiNx thin films
Authors: Jehanathan Neerushana , Liu Yinong , Walmsley Byron , Dell John , Saunders Martin
Published: December 2006

This study investigated the effect of oxidation on the chemical bonding structures of silicon nitride thin films synthesized by a low-temperature plasma-enhanced chemical vapor deposition (PECVD) method. These films were heat treated to different temperatures up to 1373 K. The bonding structures were studied by means of x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy. It was found that the amorphous PECVD SiNx films were subjected to oxidation in air at elevated temperatures. The oxidation caused the formation of crystalline silicon dioxide within the matrix of amorphous silicon nitride, conforming to the “random mixing” model. The crystalline silicon dioxide formed is believed to be stoichiometric SiO2, whereas the remaining matrix is believed to be a nonstoichiometric silicon oxynitride with a structure conforming to the “random bonding” model.
Density functional theory investigation of N interstitial migration in GaN
Authors: Wixom R. R. , Wright A. F.
Published: December 2006

Using density-functional total energy calculations, we investigated N interstitial migration in GaN. Two migration paths were considered. The first path confines motion to a single c-plane of the lattice, while the second path involves movement both perpendicular and parallel to the c-axis. The latter path has a lower barrier for the positive charge states and will be the dominant mechanism for migration of the N interstitial in p-type GaN. The calculated barriers are 1.79, 2.12, and 1.98 eV for the +1, +2, and +3 charge states. These barriers are consistent with recent experimental results and indicate that interstitials will be mobile at typical processing temperatures.
Discrimination of dipicolinic acid and its interferents by femtosecond coherent Raman spectroscopy
Authors: Huang Yu , Dogariu Arthur , Avitzour Yoav , Murawski Robert K. , Pestov Dmitry , Zhi Miaochan , Sokolov Alexei V. , Scully Marlan O.
Published: December 2006

Measurements of the beat frequencies between vibrational modes of dipicolinic acid (DPA) and a series of other molecules (interferents) are presented. The results were obtained from femtosecond time-resolved coherent Raman scattering, and the vibrational level spacings were determined from a Fourier transform of the signal versus probe pulse delay. The entire spectrum of the generated signal is recorded in order to demonstrate multimode excitation and to explain the variety of qualitatively different traces that can be obtained for the same molecule. Since the spectral signature of DPA is unique enough to be used for identification purposes, this technique has the potential to detect hazardous bacterial species, such as anthrax spores.
Fitting a free-form scattering length density profile to reflectivity data using temperature-proportional quenching
Authors: Laub Charles F. , Kuhl Tonya L.
Published: December 2006

A technique for fitting a free-form scattering length density profile to reflectivity data via least-squares minimization is presented. The approach combines aspects of simulated annealing with a parametrized representation of the scattering length density profile. The ability of the algorithm to accurately recover the scattering length density profile from arbitrary initial parameter values is demonstrated for simulated and experimental data.
Characterization of the single crystalline iron sulfide nanowire array synthesis by pulsed electrodeposition
Authors: Yue G. H. , Yan P. X. , Fan X. Y. , Wang M. X. , Qu D. M. , Yan D. , Liu J. Z.
Published: December 2006

Magnetic material Fe7S8 nanowire arrays have been successfully synthesized by pulsed electrodeposition in porous anodized aluminum oxide template. X-ray diffraction results show that the as-synthesized nanowires are single crystalline and have a highly preferential orientation. Scanning electron microscopy and transmission electron microscopy observations indicate that the ordered Fe7S8 nanowire arrays are uniform with a diameter of 50nm and length up to several tens of micrometers. The temperature variation of the magnetic properties has also been studied.
Changes in thermodynamic quantities upon contact of two solutes in solvent under isochoric and isobaric conditions
Authors: Kinoshita Masahiro , Harano Yuichi , Akiyama Ryo
Published: December 2006

The changes in excess thermodynamic quantities upon the contact of two solutes immersed in a solvent are analyzed using the radial-symmetric and three-dimensional versions of the integral equation theory. A simple model mimicking a solute in water is employed. The solute-solute interaction energy is not included in the calculations. Under the isochoric condition, the solute contact always leads to a positive entropy change irrespective of the solute solvophobicity or solvophilicity. The energy change is negative for solvophobic solutes while it is positive for solvophilic ones. Under the isobaric condition, the contact of solvophobic solutes results in system-volume compression but that of solvophilic ones gives rise to expansion. Effects of the compression and expansion on the changes in enthalpy and entropy are enlarged with rising temperature. When the solute solvophobicity is sufficiently high, the entropy change (multiplied by the absolute temperature) can become negative due to the compression, except at low temperatures with the result of an even larger, negative enthalpy change. The expansion in the case of solvophilic solutes leads to a large, positive entropy change accompanied by an even larger, positive enthalpy change. The changes in enthalpy and entropy are strongly dependent on the temperature. However, the changes in enthalpy and entropy are largely cancelled out and the temperature dependency of the free-energy change is much weaker. The authors also discuss possible relevance to the enthalpy-entropy compensation experimentally known for a variety of physicochemical processes in aqueous solution such as protein folding.
Application of fractals to the contact of carbon-carbon surfaces
Authors: Marx David T. , Khor Eng-Keng , Policandriotes Tod
Published: December 2006

Fractal contact mechanics has been applied to the study of carbon-carbon aircraft brake materials. Prepared aluminum surfaces were also studied. The approach originally taken by Yan and Komvopoulos [J. Appl. Phys. 84, 3617 (1998)] has been corrected and modified to determine the real contact area as a function of applied load for any surfaces in contact. The methods are then applied to determine the real area of contact for the carbon brake materials in their application. A key finding of this approach is that the measured friction coefficient is shown to strongly depend on the contact area.
Estimation of higher-order contribution to viscosity of hydrogen plasmas including electronically excited states
Authors: Singh Gurpreet , Singh Kuldip
Published: December 2006

Within the framework of the Chapman-Enskog method, the influence of electronically excited states on viscosity and its higher-order contributions has been investigated for a partially ionized thermal hydrogen plasma. A strong dependence of viscosity and its higher-order contributions on the presence of electronically excited states (especially at high pressure) has been observed. In the present work, a simplified relationship has been suggested for estimating higher-order contribution to viscosity, which avoids the cumbersome computational procedure involved in its exact evaluation. The results thus obtained agree with the exact contributions satisfactorily.
Electron acceleration by a chirped Gaussian laser pulse in vacuum
Authors: Sohbatzadeh F. , Mirzanejhad S. , Ghasemi M.
Published: December 2006

Electron acceleration by a chirped Gaussian laser pulse is investigated numerically. A linear and negative chirp is employed in this study. At first, a simple analytical description for the chirp effect on the electron acceleration in vacuum is provided in one-dimensional model. The chirp mechanism is then extended to the interaction of a femtosecond laser pulse and electron. The electron final energy is obtained as a function of laser beam waist, laser intensity, chirp parameter, and initial phase of the laser pulse. It is shown that the electron final energy depends strongly on the chirp parameter and the initial phase of the laser pulse. There is an optimal value for the chirp parameter in which the electron acceleration takes place effectively. The energy gain increases with laser beam waist and intensity. It is also shown that the electron is accelerated within a few degrees to the axial direction. Emphasis is on the important aspect of the chirp effect on the energy gained by an electron from the electromagnetic wave.
Mechanical scratching induced phase transitions and reactions of boron carbide
Authors: Chen Mingwei , McCauley James W.
Published: December 2006

The structural and chemical stabilities of single-crystal boron carbide (B4C) under severe mechanical scratching in air and water were investigated by transmission electron microscopy (TEM). Amorphous and nanocrystalline B4C as well as nanostructured boron nitride (BN) were observed in the scratched fragments. Energy-filtered TEM analysis and thermodynamic calculations suggested that the BN nanophase results from the reaction of very small B4C fragments with nitrogen in the ambience.
Structural properties of epitaxial SrTiO3 thin films grown by molecular beam epitaxy on Si(001)
Authors: Delhaye G. , Merckling C. , El-Kazzi M. , Saint-Girons G. , Gendry M. , Robach Y. , Hollinger G. , Largeau L. , Patriarche G.
Published: December 2006

This work reports on the structural properties of an epitaxial SrTiO3 (STO) layer grown by molecular beam epitaxy on a Si(001) substrate with a two step process. The study, which includes a complete characterization of large scale plane-view images of the STO layer, is based on a careful analysis of x-ray spectra and transmission electron microscopy images. The STO layer presents a good crystalline quality and a slight texturation related to the presence of extended defects. A thin Ti-rich amorphous silicate layer (thickness ≈1.3nm) is formed at the interface between the STO and the Si substrate, evidencing the thermodynamic instability of the STO/Si interface. The difference between the thermal expansion coefficients of Si and STO is shown to be at the origin of an increased in-plane lattice parameter (3.927Å) of the STO layer as compared to its bulk value (3.905Å). This effect of differential thermal expansion is expected to be responsible for the formation of at least part of the extended defects of the STO layer.
Vibrational energy relaxation of azide in water
Authors: Li Shuzhou , Schmidt J. R. , Skinner J. L.
Published: December 2006

Vibrational lifetimes of the asymmetric stretch fundamental of azide anion in normal and heavy water have been measured experimentally, with results in the range of a few picoseconds. This is an interesting problem for theoretical study because of the competition between intramolecular (relaxation to the other excited vibrational states of azide) and purely intermolecular (relaxation to azide’s ground vibrational state) pathways. In addition it is important to understand the origin of the solvent isotope effect. Building on the seminal work of Morita and Kato [J. Chem. Phys. 109, 5511 (1998)], the authors develop a simple model based on a two-dimensional description of the azide stretching vibrations. A novel aspect of their theory is the use of an “on-the-fly” optimized quantum mechanical/molecular mechanical approach to calculate the system-bath coupling. Their theoretical lifetimes are in good agreement with experiment for azide in both normal and heavy water. They find that the predominant relaxation pathway is intramolecular. The solvent isotope effect arises from the different librational frequencies in normal and heavy water.
Subsonic choked flow in the microchannel
Authors: Chong Xie
Published: December 2006

Choked gas flows in the microchannel were studied. With use of the direct simulation Monte Carlo (DSMC) method, a serial of microchannels (length L=10,100,and200μm) was simulated and gained a flow-field evolutional trend with pressure ratio. Also, the effects of the boundary condition setting on the mass flux and static pressure distribution were analyzed during the nonequilibrium flows. It is shown that for the microchannel, the sonic point (Ma=1) only appears at the center point of the outlet section when it is choked. Finally, this paper compares the mass flux between the isothermal limiting Navier-Stokes (NS) and DSMC results, and it is concluded that the limiting NS result can be used to estimate the mass flux when the length of the channel is very long (the ratio of length to height is larger than 1000).
Experimental study of the suppression of Rayleigh-Bénard instability in a cylinder by combined rotating and static magnetic fields
Authors: Grants I. , Pedchenko A. , Gerbeth G.
Published: December 2006

We consider experimentally transitions in a liquid metal cylinder heated from below and subject to superimposed rotating and static magnetic fields. The applied static magnetic fields are too weak to influence the characteristic velocity of the rotating field driven basic flow. Being itself turbulent, a strong enough magnetically driven flow suppresses considerably the temperature fluctuations due to the thermogravitational convection. The remaining background fluctuations are caused by unsteady Taylor vortices generated near the sidewall. Our experiment shows that the superimposed static “cusp” magnetic field reduces the amplitude of these remaining temperature fluctuations by a factor of 4, compared to the case with a superimposed uniform axial field. The observed behavior agrees well with the static field effect on the amplitude of the additional unstable Taylor vortex-type solutions. These solutions bifurcate subcritically and represent the governing structures in the background turbulence. Thus, the observations are consistent with the description of the background turbulence as an irregular phase trajectory around the skeleton of the subcritical flow states. If this “skeleton” is compressed by an external influence (the cusp static field in our case), then also the amplitude of turbulent fluctuations decreases by the same factor. Another effect of the cusp field is to sharpen the transition between buoyancy and magnetic forcing dominated regimes. This allowed us to obtain an empirical expression for the conditions of this transition. We conclude that the rotating magnetic-field-driven flow suppresses the buoyant flow at a much lower angular velocity than a rigid-body mechanical rotation.
Insight into the periodicity of Schallamach waves in soft material friction
Authors: Rand Charles J. , Crosby Alfred J.
Published: December 2006

A dominant mechanism in friction of soft material interfaces is the onset and propagation of Schallamach waves. Schallamach waves are “tunnels” of air that provide relative displacement between the slider and the substrate rather than the instantaneous interfacial failure involved with stick-slip. Here, through model experiments and analysis, the authors present a fundamental relationship between the periodicity of Schallamach waves (λ) and the ratio of interfacial adhesion (Gc) over complex elastic modulus (E*). This deconvolution of bulk and interfacial contributions to the friction of soft materials leads to interesting predictions that will impact material design for a wide range of applications.
Stability of a dragged viscous thread: Onset of “stitching” in a fluid-mechanical “sewing machine”
Authors: Ribe Neil M. , Lister John R. , Chiu-Webster Sunny
Published: December 2006

A thin thread of viscous fluid that falls on a moving belt acts like a fluid-mechanical “sewing machine,” exhibiting a rich variety of “stitch” patterns including meanders, translated coiling, slanted loops, braiding, figures-of-eight, W-patterns, side kicks, and period-doubled patterns. Using a numerical linear stability analysis, we determine the critical belt speed and oscillation frequency of the first bifurcation, at which a steady dragged viscous thread becomes unstable to transverse oscillations or “meandering.” The predictions of the stability analysis agree closely with the experimental measurements of Chiu-Webster and Lister [J. Fluid Mech. 569, 89 (2006)]. Moreover, the critical belt speed and onset frequency for meandering are nearly identical to the contact-point migration speed and angular frequency, respectively, of steady coiling of a viscous thread on a stationary surface, implying a remarkable degree of dynamical similarity between the two phenomena.
Diffusion-limited binding to a site on the wall of a membrane channel
Authors: Dagdug Leonardo , Berezhkovskii Alexander M.
Published: December 2006

The authors develop a theory of diffusion-controlled reactions with a site located on the wall of a cylindrical membrane channel that connects two reservoirs containing diffusing particles which are trapped by the site at the first contact. An expression for the Laplace transform of the rate coefficient, k(t), is derived assuming that the size of the site is small compared to the channel radius. The expression is used to find the stationary value of the rate coefficient, k(∞), as a function of the length and radius of the channel, the radius of the site, and its position inside the channel (distances from the two ends of the channel) as well as the particle diffusion constants in the bulk and in the channel. Their derivation is based on the one-dimensional description of the particle motion in the channel, which is generalized to include binding to the site into consideration. The validity of the approximate one-dimensional description of diffusion and binding was checked by three-dimensional Brownian dynamics simulations. They found that the one-dimensional description works reasonably well when the size of the site does not exceed 0.2 of the channel radius.
Using the histogram test to quantify reaction coordinate error
Authors: Peters Baron
Published: December 2006

Many schemes for calculating reaction rates and free energy barriers require an accurate reaction coordinate, but it is difficult to quantify reaction coordinate accuracy for complex processes like protein folding and nucleation. The histogram test, based on estimated committor probabilities, is often used as a qualitative indicator for good reaction coordinates. This paper derives the mean and variance of the intrinsic committor distribution in terms of the mean and variance of the histogram of committor estimates. These convenient formulas enable the first quantitative calculations of reaction coordinate error for complex systems. An example shows that the approximate transition state surface from Peters’ and Trout’s reaction coordinate for nucleation in the Ising model gives a mean committor probability of 0.495 and a standard deviation of 0.042.