bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical and dynamical analyses of generation mechanisms of solitary internal waves in the northern South China Sea
Authors: Zheng Quanan , Susanto R. Dwi , Ho Chung‐Ru , Song Y. Tony , Xu Qing
Published: March 2007

This study analyzes the effects of thermocline shoaling on the ocean internal wave (IW) generation in the north South China Sea (NSCS). Seven years of satellite synthetic aperture radar (SAR) images from 1995 to 2001 are used for the statistical analysis of IW occurrence, and field measurements of sea surface wind, sea state, and vertical temperature profiles are used for analyzing IW generation and SAR imaging conditions. Latitudinal distribution of IW packets shows that 22% of IW packets distributed in the east of 118°E obviously originate from the Luzon Strait, and 78% of IW packets west of 118°E may propagate from the east or evolve into the solitons originating from the east boundary owing to the fission effect of shoaling thermocline. The yearly distribution of IW occurrence frequencies reveals an interannual variability, implying that there are long‐term and large‐scale processes modifying the SAR‐observed IW occurrence. The monthly SAR‐observed IW occurrence frequencies show that the high frequencies are distributed from April to July and reach a peak in June with a maximum frequency of 20%. The low occurrence frequencies are distributed in winter from December to February of next year with a minimum frequency of 1.5% in February. This study proposes that the IW generation needs the necessary and sufficient conditions: initial disturbance formation and wave amplitude growth. Owing to the dissipation effect on the initial disturbance, only fully grown waves have a chance to radiate out of the source region. A physical model and PKdV equation are adopted for analyzing the sufficient conditions for solitary IW amplitude growth. The results indicate that the thermocline shoaling provides the forcing to soliton amplitude growth, so that the soliton amplitude growth ratio (SAGR) serves as a decisive factor for the IW occurrence frequency. Theoretical analysis predicts a linear relation between the two. Application of theoretical models to field measurements in the Luzon Strait gives a correlation coefficient as high as 0.845 with a confidence level of 99% for months from March to November. The linear regression gives a correlation coefficient (R2) of 0.6519 and a SAGR threshold (minimum) value of 0.90 for IW occurrence. According to the theoretical solutions, the eastward propagating disturbances have no chance to grow up, so that they hardly appear on the east side of the submarine ridges in the eastern Luzon Strait.
Anthropogenic heat island at Barrow, Alaska, during winter: 2001–2005
Authors: Hinkel Kenneth M. , Nelson Frederick E.
Published: March 2007

The village of Barrow (71°N latitude) is the largest native community in the Arctic, with a population of approximately 4500 people. Situated on the coast of the Arctic Ocean in northernmost Alaska, the area is entirely underlain by permafrost. Although most supplies must be imported, Barrow relies on local natural gas fields to meet all energy requirements for building heat and electrical power generation. This energy eventually dissipates into the atmosphere, and can be detected as a pronounced urban heat island (UHI) in winter. Since 2001, a 150 km2 area in and around Barrow has been monitored using ∼70 data loggers recording air temperature at hourly intervals. The mean daily temperature of the urban and rural areas is calculated using a representative sample of core sites, and the UHI magnitude (MUHI) is calculated as the difference in the group averages. The MUHI is most pronounced in winter months (December–March), with temperatures in the urban area averaging 2°C warmer than in the surrounding tundra and occasionally exceeding 6°C. The MUHI is maximized under cold and calm conditions, and decreases with wind speed and warmer temperatures. It is strongly and directly correlated to natural gas utilization on a monthly basis. Integrated over the home heating season, there is an 8% reduction in freezing degree days in the village. It is unlikely that anthropogenic heat contributes to the forward shift in the snow meltout date that has been observed near Barrow over the past 60 years.
Line splitting in the Schumann resonance oscillations
Authors: Nickolaenko A. P. , Sentman Davis D.
Published: March 2007

We discuss detection of line splitting in the global electromagnetic (Schumann) resonances. The lifting of resonance degeneracy is usually not visible in the ordinary power spectrum of either the electric or magnetic field components since splitting is small in comparison with the natural width of the resonance lines. Splitting may be detected by exploiting the spatial structure of the fields and/or the elliptical polarization of the magnetic field. The spatial properties were utilized in synchronous and coherent measurements of the vertical electric field at two longitudinally separated observatories. The results were attributed to line splitting. An alternative interpretation was also advanced that takes into account the source‐receiver separation. The lifting of degeneracy also appears as a frequency‐dependent elliptical polarization of the horizontal magnetic field vector, which has been found experimentally. We compare measurement and computational data, and their reciprocity proves the detection of Schumann resonance line splitting.
Reanalysis of plasma measurements at geosynchronous orbit
Authors: O'Brien T. P. , Lemon C. L.
Published: March 2007

Retrospective analyses of spacecraft anomalies, especially surface charging, require information about the local plasma environment. Such information is usually not available at geosynchronous orbit because most vehicles lack appropriate sensors. We have constructed a nearly continuous hourly database covering four plasma moments at geosynchronous orbit: density and temperature for hot (energies > 100 eV) protons and electrons. The four moments are provided for 24 hourly fiducial local time bins and cover the years 1990–2005. We apply an ensemble of simple models to “fill in” the local time gaps between plasma measurements at geosynchronous orbit made by Los Alamos sensors. Validation on artificial data gaps shows that the reconstruction has rank order correlations ranging from 0.65 to 0.97, depending on bulk moment and local time. As a demonstration, we include a comparison of our reanalysis electron temperature to on‐orbit surface charging measurements at a vehicle that lacks a plasma sensor. In addition to anomaly analyses, the reanalysis database we have produced can be used to provide geosynchronous boundary conditions for numerical simulations of the inner magnetosphere and temporal variations at a single location for statistical studies.
Oblique collision and accretion of the Netherlands Leeward Antilles to South America
Authors: Beardsley A. G. , Avé Lallemant H. G.
Published: March 2007

Accretion of the Netherlands Leeward Antilles volcanic island arc to northern South America (offshore Venezuela) was accompanied by deformation and clockwise block rotation. Structural analysis of Aruba, Curaçao, and Bonaire indicates that they were deformed during at least one ductile event (D1) and three brittle events (F1 − F3). The first deformation phase (D1 and F1) began in the Early Cretaceous, at approximately 110 Ma. Deformation features are consistent with displacement partitioning at the obliquely convergent Caribbean‐Atlantic (proto‐Caribbean) plate boundary. This deformation was accompanied by an approximate 90° clockwise rotation of the forearc terrane. The onset of the second phase of deformation (F2) occurred in the Eocene (∼55 Ma), also accompanied by a small amount of rotation, and structures are consistent with wrench tectonics in a pull‐apart regime. This coincided with the collision of the Caribbean plate with the Great Bahama Bank and the initiation of convergence between North and South America. The youngest deformation structures and rotation (F3) are also associated with wrench faulting. Likely, motion along the Oca fault and the Caribbean plate transform fault initiated a major restraining bend in the late Paleogene. Beginning ∼12 Ma, the northeastward escape of the Maracaibo block further increased the complexity of tectonism at the Caribbean–South American plate boundary. Deformation structures are consistent with a total of 135° clockwise rotation of the Leeward Antilles islands and regional diachronous deformation from west to east since the Early Cretaceous.
Experiments on dispersion of tracer stones under lower‐regime plane‐bed equilibrium bed load transport
Authors: Wong Miguel , Parker Gary , DeVries Paul , Brown Timothy M. , Burges Stephen J.
Published: March 2007

A common approach for estimating the bed load transport rate in gravel‐bed streams is to relate it to deterministic channel‐averaged driving parameters and corresponding resistance properties of the bed material. Notwithstanding the proven success of this approach in modeling various morphodynamic scenarios, it does not contain the mechanics necessary to relate the bulk sediment transport rate to the displacement patterns of individual particles. Experiments on entrainment, transport, and deposition of tracer stones in a flume described here were designed to address this issue. Predictors of statistics of bed elevation fluctuations at short timescales, total and elevation‐specific particle entrainment rates, particle step lengths, mean and associated probability density function for particle virtual velocity, and thickness of the active layer were developed. The working hypothesis tested in this paper is that the statistics of tracer displacements can be related to channel‐averaged hydraulic parameters, and thus linked to macroscopic aspects of bed load transport.
Hyperelastic behavior of single wall carbon nanotubes
Authors: Ling Xianwu , Atluri S. N.
Published: March 2007

Single wall carbon nanotubes (SWNTs) are shown to obey a hyperelastic constitutive model at moderate strains and temperatures. The finite temperature is considered via the local harmonic approach. The equilibrium configurations were obtained by minimizing the Helmholtz free energy of a representative atom in an atom-based cell model. While the concept of strain-dependent tangent modulus using linear elasticity was considered in prior literature, a constant μ for Ogden’s hyperelastic model [R. W. Ogden, Nonlinear Elastic Deformation (Horwood, England, 1984)] is found in the current work for large tubes subjected to moderately large strains up to 900 K.
Roughening transition in nanoporous hydrogenated amorphous germanium: Roughness correlation to film stress
Authors: Carroll M. S. , Verley J. C. , Sheng J. J. , Banks J.
Published: March 2007

Hydrogenated amorphous germanium (a-Ge:H) is a material of interest for optoelectronic applications such as solar cells and radiation detectors because of the material’s potential to extend the wavelength sensitivity of hydrogenated amorphous silicon (a-Si:H). An increase in porosity is observed in amorphous germanium compared to a-Si:H, and this increase in porosity has been correlated with a degradation of the electrical performance. Improved understanding of the mechanisms of porous formation in a-Ge:H films is therefore desirable in order to better control it. In this paper we describe a correlation between film stress and surface roughness, which evolves with increasing thickness of a-Ge:H. A roughening transition from planar two-dimensional growth to three-dimensional growth at a critical thickness less than 800Å results in a network of needlelike nanotrench cavities which stretch from the transition thickness to the surface in films up to 4000Å thick. Surface roughness measurements by atomic force microscope and transmission electron microscopy indicate that the transition is abrupt and that the roughness increases linearly after the transition thickness. The roughening transition thickness is, furthermore, found to correlate with the maxima of the integrated compressive stress. The compressive stress is reduced after this transition thickness due to the incorporation of nanovoids into the film that introduce tensile stress as the islands coalesce together. The roughening transition behavior is similar to that found in a general class of Volmer-Weber mode thin film deposition (e.g., Cu, Ag, and nonhydrogenated amorphous silicon), which offers additional insight into the underlying mechanisms of the stress and roughening in these a-Ge:H films. The suppression of the roughening transition by changing the kinetics of the deposition rates (e.g., slowing the deposition rate with a weak sputtering bias) is also observed and discussed.
Optically transparent and electrically conducting epitaxial Ta2O films
Authors: Gnanarajan S. , Lam S. K. H. , Foley C. P.
Published: March 2007

An optically transparent and electrically conducting oxide Ta2O epitaxial thin film has been fabricated and characterized. In our experiments, we grew epitaxial Ta films on r-cut sapphire crystal substrates, which were subsequently oxidized into Ta2O films. The x-ray θ-2θ scan peaks match the American Society for Testing and Materials x-ray powder data card for Ta2O (18-1302) with a cubic structure. The x-ray photoelectron spectroscopic measurements indicated Ta 4f peak energy values between those for Ta and TaO whereas the O∕Ta atomic ratio is between 0.5 and 1. Resistivity-temperature measurements showed a metallic behavior with resistivity values in the range of (1.5–3.3)×10−4Ωcm at 293K. Optical measurements on 25–100nm thick films indicated that they are transparent in the range of 400–2000nm and have an energy band gap between 2.04 and 2.17eV. The Hall mobility and carrier density of the films were in the range of 62–110cm2V−1s−1 and (1.9–3.0)×1020cm−3, respectively.
Orbiting atoms and C60 fullerenes inside carbon nanotori
Authors: Hilder Tamsyn A. , Hill James M.
Published: March 2007

The discovery of carbon nanostructures, such as carbon nanotubes and C60 fullerenes, has generated considerable interest for potential nanoelectronic applications. One such device is the high frequency nanoscale gigahertz oscillator. Several studies investigating these oscillators demonstrate that sliding an inner-shell inside an outer-shell of a multiwalled carbon nanotube generates oscillatory frequencies in the gigahertz range. Research has shown that the oscillation is sensitive to the diameter and the helicity of the tube and that the inner tube length can be used to tune the frequency, such that the smaller the inner tube length the higher the frequency of oscillation, suggesting that a C60 fullerene might provide the ultimate core. Recently, researchers have observed single continuous toroidal nanotubes with no beginning or end, effectively a single-walled carbon nanotube closed around onto itself so that the two open ends fuse together, stabilized by van der Waals forces alone, to form a perfect “nanotorus.” The question arises as to whether it is possible to create a C60- nanotorus oscillator or orbiter, comprising a C60 fullerene orbiting around the inside of a nanotorus. The C60- nanotorus orbiter has yet to be constructed and the aim here is to assess its feasibility by examining the dominant mechanics of this potential nanoscale device. As in previous studies, the Lennard-Jones potential is used to calculate the interatomic forces acting on the fullerene due to the nonbonded interactions. Furthermore, other relevant forces are examined. Initially, we investigate the dynamics of an orbiting single atom followed by the corresponding analysis for an orbiting C60 fullerene. The equilibrium position depends on the radius of the nanotorus tube for both the atom and the C60 fullerene. Gravity is shown to be negligible, while the centrifugal forces are shown to move the orbiting body further from the center of the nanotorus. The theory also predicts that by changing the orbital position, the resulting frequencies, which are in the gigahertz range, may vary to as much as four times those obtained for the C60-nanotube oscillator.
Calculations of spin-disorder resistivity from first principles
Authors: Wysocki A. L. , Belashchenko K. D. , Velev J. P. , van Schilfgaarde M.
Published: March 2007

Spin-disorder resistivity of Fe and Ni is studied using the noncollinear density functional theory. The Landauer conductance is averaged over random disorder configurations and fitted to Ohm’s law. The distribution function is approximated by the mean-field theory. The dependence of spin-disorder resistivity on magnetization in Fe is found to be in excellent agreement with the results for the isotropic s-d model. In the fully disordered state, spin-disorder resistivity for Fe is close to experiment, while for fcc Ni it exceeds the experimental value by a factor of 2.3. This result indicates strong magnetic short-range order in Ni at the Curie temperature.
Effects of Co80Fe20 insertion layer on perpendicular exchange bias characteristics in [Pd∕Co]5∕FeMn bilayered thin films
Authors: Lin Lin , Kim Sunwook , Bae Seongtae , Joo Ho Wan , Hwang Do Guwn , Lee Sang Suk
Published: March 2007

Effects of Co80Fe20 insertion layer on the perpendicular exchange bias characteristics in [Pd∕Co]5∕FeMn bilayered thin films have been investigated by changing the Co80Fe20 insertion layer thickness. Hysteresis loops for both in-plane and perpendicular to the film plane were measured by using a vibrating sample magnetometer and an extraordinary Hall effect. It was found that the thin Co80Fe20 insertion layer was very effective in improving the perpendicular exchange bias characteristics in [Pd∕Co]5∕FeMn bilayered thin films. By increasing Co80Fe20 insertion layer thickness from 0to1.24nm, the exchange bias field was significantly increased up to 332Oe and then decreased down to 60Oe, while the pinned coercivity was monotonically decreased. The effective magnetic anisotropy was kept at positive up to 0.6nm thick of Co80Fe20 insertion layer. It was experimentally confirmed that the perpendicular exchange bias field was promisingly larger than the pinned coercivity when the Co80Fe20 insertion layer had the thickness in the range between 0.37 and 0.56nm. The significant increase of the perpendicular exchange bias field and the reduction of pinned coercivity after inserting Co80Fe20 thin films are thought to be attributed to the modification of interfacial roughness between the FeMn and adjacent top Co layers and the stress-induced change of perpendicular magnetization direction of the top Co layer, which is adjacent to FeMn layer.
Exchange interactions between magnetic ions and free carriers in ZnMnO: An electron paramagnetic resonance analysis
Authors: Ben Mahmoud A. , von Bardeleben H. J. , Cantin J. L. , Chikoidze E. , Dumont Y. , Mauger A.
Published: March 2007

The magnetic interactions in n-type conductive Zn0.66Mn0.34O thin films with carrier concentrations of 2×1018cm−3 have been investigated by X-band electron paramagnetic resonance (EPR) technique. We observe exclusively a single exchange narrowed Mn related EPR spectrum characterized by antiferromagnetic exchange interactions. The presence of free carriers modifies the EPR spectra by shifting the resonance fields to lower values. The shift can be modeled by the carrier polarization induced local magnetic field.
Electron spin resonance and Raman studies of Mn-doped ZnO ceramics
Authors: Phan The-Long , Vincent Roger , Cherns David , Nghia Nguyen Xuan , Phan Manh-Huong , Yu Seong-Cho
Published: March 2007

In the present work, the influence of annealing on structure, electron spin resonance (ESR), and Raman scattering (RS) spectra of x at. % Mn-doped ZnO (x=4 and 8) ceramic compounds has been systematically investigated. The samples were annealed at temperatures (Tan) between 400 and 1000°C for 12h. The obtained results revealed the strong dependence of x-ray diffraction, ESR, and RS spectra on the annealing temperature Tan. Mn2+ ions did not substitute into Zn2+ sites in samples annealed in the range of 400–600°C but started to substitute into Zn2+ sites for annealing temperatures Tan>600°C. The results of this investigation provide further insights into the physical processes occurring in Mn-doped ZnO materials due to annealing.
Spin torque and heating effects in current-induced domain wall motion probed by transmission electron microscopy
Authors: Junginger F. , Kläui M. , Backes D. , Rüdiger U. , Kasama T. , Dunin-Borkowski R. E. , Heyderman L. J. , Vaz C. A. F. , Bland J. A. C.
Published: March 2007

Observations of domain wall motion and transformations due to injected current pulses in permalloy zigzag structures using off-axis electron holography and Lorentz microscopy are reported. Heating on membranes leads to thermally activated random behavior at low current densities and by backcoating the SiN membranes with Al, heating effects are significantly reduced. A set of indicators is devised to separate unambiguously spin torque effects from heating and it is shown that by using the Al layer the structures are sufficiently cooled to exhibit current-induced domain wall motion due to spin torque.
Theoretical modeling of half-metallic current-perpendicular-to-plane spin valves
Authors: Tan S. G. , Jalil M. B. A. , Bala Kumar S. , Li Kebin , Zheng Yuankai , Han Guchang
Published: March 2007

We conducted theoretical studies of the magnetoresistance (MR) behavior of the current-perpendicular-to-plane spin valves (SVs) with half-metallic (HM) layers replacing either the pinned layer or both the pinned and the free ferromagnetic (FM) layers. In the single HM layer SV, resistance matching between the HM and the FM layers is important in order to avoid the decrease in MR due to the dominance of the single FM layer with respect to the resistance. For the dual HM layer SV, the presence of the HM layers results in a plateau of high MR values over a wider range of HM resistivity. As expected, the MR ratio is suppressed in the low HM resistivity range because of low spin-dependent scattering. Unexpectedly, the MR also reduces to zero at extremely high HM resistivity due to the effect of spin flipping.
Asymmetric magnetic reversal of perpendicular exchange biased (Co∕Pt)5∕IrMn probed by magnetoresistance and magnetic force microscopy
Authors: Ji Xiaosong , Pakhomov Alexandre B. , Krishnan Kannan M.
Published: March 2007

Magnetic reversal of (Co∕Pt)5∕IrMn multilayers with perpendicular exchange bias has been studied by magnetoresistance and magnetic force microscopy (MFM). It has been found that as a function of perpendicular external field, the resistance decreases with field above saturation and has sharp maxima at the reversal fields due to the domain wall resistance effect. The latter contribution has been found to be asymmetric, suggesting a corresponding asymmetry in the domain state in the two branches of the hysteresis loop. This asymmetry correlates with the fractal dimension of the domain wall projection deduced from MFM images, which is larger on the descending branch of the hysteresis loop than along the ascending branch. This in turn can be explained as due to the different intensities of domain wall nucleation in the two branches.
Exact stochastic simulation of coupled chemical reactions with delays
Authors: Cai Xiaodong
Published: March 2007

Gillespie’s exact stochastic simulation algorithm (SSA) [J. Phys. Chem. 81, 2350 (1977)] has been widely used to simulate the stochastic dynamics of chemically reacting systems. In this algorithm, it is assumed that all reactions occur instantly. While this is true in many cases, it is also possible that some chemical reactions, such as gene transcription and translation in living cells, take certain time to finish after they are initiated. Thus, the product of such reactions will emerge after certain delays. Apparently, Gillespie’s SSA is not an exact algorithm for chemical reaction systems with delays. In this paper, the author develops an exact SSA for chemical reaction systems with delays, based upon the same fundamental premise of stochastic kinetics used by Gillespie in the development of his SSA. He then shows that an algorithm modified from Gillespie’s SSA by Barrio et al. [PLOS Comput. Biol. 2, 1017 (2006)] is also an exact SSA for chemical reaction systems with delays, but it needs to generate more random variables than the author’s algorithm.
Molecular dynamics study of the stability of methane structure H clathrate hydrates
Authors: Alavi Saman , Ripmeester J. A. , Klug D. D.
Published: March 2007

Molecular dynamics simulations are used to study the stability of structure H (sH) methane clathrate hydrates in a 3×3×3 sH unit cell replica. Simulations are performed at experimental conditions of 300K and 2GPa for three methane intermolecular potentials. The five small cages of the sH unit cell are assigned methane guest occupancies of one and large cage guest occupancies of one to five are considered. Radial distribution functions, unit cell volumes, and configurational energies are studied as a function of large cage CH4 occupancy. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies. Large cage occupancy of five is the most stable configuration for a Lennard-Jones united-atom potential and the Tse-Klein-McDonald potential parametrized for condensed methane phases and two for the most stable configuation for the Murad and Gubbins potential.
Wetting layer states of InAs∕GaAs self-assembled quantum dot structures: Effect of intermixing and capping layer
Authors: Sęk G. , Ryczko K. , Motyka M. , Andrzejewski J. , Wysocka K. , Misiewicz J. , Li L. H. , Fiore A. , Patriarche G.
Published: March 2007

The authors present a modulated reflectivity study of the wetting layer (WL) states in molecular beam epitaxy grown InAs∕GaAs quantum dot (QD) structures designed to emit light in the 1.3–1.5μm range. A high sensitivity of the technique has allowed the observation of all optical transitions in the QD system, including low oscillator strength transitions related to QD ground and excited states, and the ones connected with the WL quantum well (QW). The support of WL content profiles, determined by transmission electron microscopy, has made it possible to analyze in detail the real WL QW confinement potential which was then used for calculating the optical transition energies. We could conclude that in spite of a very effective WL QW intermixing, mainly due to the Ga–In exchange process (causing the reduction of the maximum indium content in the WL layer to about 35% from nominally deposited InAs), the transition energies remain almost unaffected. The latter effect could be explained in effective mass envelope function calculations taking into account the intermixing of the QW interfaces described within the diffusion model. We have followed the WL-related transitions of two closely spaced QD layers grown at different temperatures, as a function of the In content in the capping layer. We have shown that changing the capping layer from pure GaAs to In0.236Ga0.764As has no significant influence on the composition profile of the WL itself and the WL QW transitions can be usually interpreted properly when based on the cap-induced modification of the confinement potential within a squarelike QW shape approximation. However, some of the observed features could be explained only after taking into consideration the effects of intermixing and InGaAs cap layer decomposition.
First principles molecular dynamics of molten NaCl
Authors: Galamba N. , Costa Cabral B. J.
Published: March 2007

First principles Hellmann-Feynman molecular dynamics (HFMD) results for molten NaCl at a single state point are reported. The effect of induction forces on the structure and dynamics of the system is studied by comparison of the partial radial distribution functions and the velocity and force autocorrelation functions with those calculated from classical MD based on rigid-ion and shell-model potentials. The first principles results reproduce the main structural features of the molten salt observed experimentally, whereas they are incorrectly described by both rigid-ion and shell-model potentials. Moreover, HFMD Green-Kubo self-diffusion coefficients are in closer agreement with experimental data than those predicted by classical MD. A comprehensive discussion of MD results for molten NaCl based on different ab initio parametrized polarizable interionic potentials is also given.
Ground-state path integral Monte Carlo simulations of positive ions in He4 clusters: Bubbles or snowballs?
Authors: Paolini Stefano , Ancilotto Francesco , Toigo Flavio
Published: March 2007

The local order around alkali (Li+ and Na+) and alkaline-earth (Be+, Mg+, and Ca+) ions in He4 clusters has been studied using ground-state path integral Monte Carlo calculations. The authors apply a criterion based on multipole dynamical correlations to discriminate between solidlike and liquidlike behaviors of the He4 shells coating the ions. As it was earlier suggested by experimental measurements in bulk He4, their findings indicate that Be+ produces a solidlike (“snowball”) structure, similar to alkali ions and in contrast to the more liquidlike He4 structure embedding heavier alkaline-earth ions.
Euler-Poincaré reduction for discrete field theories
Authors: Vankerschaver Joris
Published: March 2007

In this note, we develop a theory of Euler-Poincaré reduction for discrete Lagrangian field theories. We introduce the concept of Euler-Poincaré equations for discrete field theories, as well as a natural extension of the Moser-Veselov scheme, and show that both are equivalent. The resulting discrete field equations are interpreted in terms of discrete differential geometry. An application to the theory of discrete harmonic mappings is also briefly discussed.
Idealized gas turbine combustor for performance research and validation of large eddy simulations
Authors: Williams Timothy C. , Schefer Robert W. , Oefelein Joseph C. , Shaddix Christopher R.
Published: March 2007

This paper details the design of a premixed, swirl-stabilized combustor that was designed and built for the express purpose of obtaining validation-quality data for the development of large eddy simulations (LES) of gas turbine combustors. The combustor features nonambiguous boundary conditions, a geometrically simple design that retains the essential fluid dynamics and thermochemical processes that occur in actual gas turbine combustors, and unrestrictive access for laser and optical diagnostic measurements. After discussing the design detail, a preliminary investigation of the performance and operating envelope of the combustor is presented. With the combustor operating on premixed methane/air, both the equivalence ratio and the inlet velocity were systematically varied and the flame structure was recorded via digital photography. Interesting lean flame blowout and resonance characteristics were observed. In addition, the combustor exhibited a large region of stable, acoustically clean combustion that is suitable for preliminary validation of LES models.
Motion of a droplet near an evaporating liquid-gas interface
Authors: Fisher L. S. , Golovin A. A.
Published: March 2007

The motion of a droplet placed in a gas phase near an evaporating, planar liquid-gas interface is considered. Due to latent heat of evaporation, the gradients of temperature and vapor concentration are sustained in the system. The temperature gradient results in the thermocapillary force acting on the droplet and on the planar liquid-gas interface causing thermocapillary flows in the three phases. The corresponding hydrodynamic and diffusion problems are solved using bispherical coordinates, and the velocity of the droplet as a function of the drop separation distance from the interface is found. It is shown that, in the absence of gravity, there exists a critical separation distance above which the drop escapes from the interface and below which the drop is captured by the interface. In the presence of gravity, it is shown that there exists a steady-state separation distance at which the drop can levitate above the interface.
Spin transfer oscillators emitting microwave in zero applied magnetic field
Authors: Devolder T. , Meftah A. , Ito K. , Katine J. A. , Crozat P. , Chappert C.
Published: March 2007

Using pillar-shaped spin valves with the magnetization of the reference layer being pinned perpendicularly to the easy axis of the free layer, we show that spin-transfer-induced microwave emission can be obtained at exactly zero applied magnetic field and in its vicinity. The frequency tunability (typically 150 MHz/mA), the spectral purity (quality factor up to 280), and the power (up to 5 nV/Hz1/2) of the emission compares well with other spin-transfer oscillators based on spin-valve nanopillars. This ability to get satisfactory microwave emission without needing bulky magnetic field sources may arise from a small nonvanishing field-like term in the current-induced torque. It may be of interest for the design of submicron microwave sources.
Criterion for bulk behavior of a Yukawa disk
Authors: Sheridan T. E.
Published: March 2007

A Yukawa disk is a two-dimensional system of n particles interacting through a Yukawa potential (i.e., a screened Coulomb or Debye potential) with Debye length λ and confined in an isotropic parabolic well where the single-particle oscillation frequency is ω0. One example of a Yukawa disk is a two-dimensional complex (dusty) plasma. The emergence of bulk (macroscopic) behavior in a strongly coupled Yukawa disk is studied by considering the dependence of the normalized, squared breathing frequency ωbr2∕ω02 (i.e., the bulk modulus) on n, λ, the disk radius R0, and the nearest-neighbor distance a. An analytical expression for ωbr2∕ω02 is derived for the bulk limit, R0≫λ, with a∕λ finite. In the plasma regime a≲λ, so that each particle interacts with many other particles, ωbr2∕ω02≈4 independent of a∕λ. In the nearest-neighbor regime a≳λ, short-range interactions dominate and ωbr2∕ω02∼a∕λ. Computational solutions of the model for n=100−3200 particles approach the theoretical bulk limit as n increases. Solutions with n=3200 and a∕λ between 0.25 and 0.5 are found to give the best approximation to an unbounded plasma.
Laser-induced fluorescence within subwavelength metallic arrays of nanoholes indicating minimal dependence on hole periodicity
Authors: Kim Je Hong , Moyer Patrick J.
Published: March 2007

Fluorescence measurements are obtained in transmission from a thin film of rhodamine 6G molecules immersed into a 40nm thick polymethylmethacrylate film, which has been spin coated onto an array of nanoholes in a thin gold film. The fluorescence is shown to be minimally correlated to the white light spectra of the same films and not highly dependent upon film periodicity, but rather highly dependent upon hole size and shape. The authors suggest that the spectroscopic signature originates from a determination of the allowed modes inside of the hole, which are recognized by the excited state molecular fluorophore before the radiative field is generated.
Anisotropic strain relaxation in a-plane GaN quantum dots
Authors: Founta S. , Coraux J. , Jalabert D. , Bougerol C. , Rol F. , Mariette H. , Renevier H. , Daudin B. , Oliver R. A. , Humphreys C. J. , Noakes T. C. Q. , Bailey P.
Published: March 2007

Medium energy ion scattering (MEIS) has been used to measure at the monolayer scale the strain profile of self-organized GaN quantum dots grown on (11–20) or a-plane AlN by molecular-beam epitaxy. By confronting the MEIS results with a structural analysis carried out by atomic force microscopy, it is established that the strain profile is anisotropic, i.e., fully elastic along [1–100] and a combination of plastic and elastic along [0001]. High resolution transmission electron microscopy measurements reveal the presence of misfit dislocations with 1/2 [0001] Burgers vector, consistent with MEIS data.
Pore scale mixing and macroscopic solute dispersion regimes in polymer flows inside two-dimensional model networks
Authors: D’Angelo Maria Veronica , Auradou Harold , Allain Catherine , Hulin Jean-Pierre
Published: March 2007

A change of solute dispersion regime with the flow velocity has been studied both at the macroscopic and pore scales in a transparent array of capillary channels, using an optical technique allowing for simultaneous local and global concentration mappings. Two solutions of different polymer concentrations (500 and 1000ppm) have been used at different Péclet numbers. At the macroscopic scale, the displacement front displays a diffusive spreading: for Pe≤10, the dispersivity ld is constant with Pe and increases with the polymer concentration; for Pe>10, ld increases as Pe1.35 and is similar for the two concentrations. At the local scale, a time lag between the saturations of channels parallel and perpendicular to the mean flow has been observed and studied as a function of the flow rate. These local measurements suggest that the change of dispersion regime is related to variations of the degree of mixing at the junctions. For Pe≤10, complete mixing leads to pure geometrical dispersion enhanced for shear thinning fluids; for Pe>10, weaker mixing results in higher correlation lengths along flow paths parallel to the mean flow and in a combination of geometrical and Taylor dispersion.
Enhanced light output from aligned micropit InGaN-based light emitting diodes using wet-etch sapphire patterning
Authors: Cuong T. V. , Cheong H. S. , Kim H. G. , Kim H. Y. , Hong C.-H. , Suh E. K. , Cho H. K. , Kong B. H.
Published: March 2007

The authors have demonstrated an effective method to obtain high light output power of GaN-based light-emitting diodes (LEDs) by simultaneous enhancement of internal quantum efficiency and light extraction efficiency. Micropit InGaN∕GaN LEDs were fabricated on hexagonal-shaped GaN template through wet-etched substrate patterning. The result indicated that under optimized growth condition of high temperature GaN template, micropits could be formed and distributed in an aligned manner by growing on wet-etch patterned sapphire substrate. The LED structures showed superior optical output power, which directly resulted from not only effective elimination of threading dislocation of the epitaxial layers but also significant increase in light extraction efficiency via the inclined facets of aligned micropits.
Effect of self-gravitation and dust-charge fluctuations on the shielding and energy loss of N×M projectiles in a collisional dusty plasma
Authors: Sarwar M. Adnan , Mirza Arshad M.
Published: March 2007

A simple derivation of the electrostatic potential and energy loss of N×M test charge projectiles traveling through dusty plasma has been presented. The effect of dust-charge fluctuations, dust neutral collisions, and self-gravitation on the shielded potential and energy loss of charge projectiles has been investigated both analytically as well as numerically. An interference contribution of these projectiles to the shielded potential and energy loss has been observed, which depends upon their relative orientation and separation distance. A comparison has been made for correlated and uncorrelated motion of the two projectiles. The amplitude of the shielded potential is enhanced with the increase of dust Jeans frequency for separation less than the effective Debye length. The dust-charge fluctuations produce a potential well for a slow charge relaxation rate and energy is gained, not lost, by the test charge projectiles. However, a fast charge relaxation rate with a fixed value of Jeans frequency enhances the energy loss. The dust neutral collisions are also found to enhance the energy loss for the test charge velocities greater than the dust acoustic speeds. The present investigation might be useful to explain the coagulation of dust particles such as those in molecular clouds, the interstellar medium, comet tails, planetary rings, etc.
Axisymmetric gravity currents at high Reynolds number: On the quality of shallow-water modeling of experimental observations
Authors: Ungarish Marius
Published: March 2007

The motion of an axisymmetric gravity current that is released from a lock and then propagates over a horizontal boundary is considered. Attention is focused on the interpretation of the experimentally observed propagation (for Boussinesq systems at high Reynolds number) by means of the shallow-water inviscid model. Comparisons of the theoretical one-layer model results with previously published measurements of M. A. Hallworth, H. E. Huppert, and M. Ungarish [J. Fluid Mech. 447, 1 (2001)] and M. D. Patterson, J. E. Simpson, S. B. Dalziel, and G. J. F. van Heijst [Phys. Fluids 18, 046601 (2006)] prove that the model provides reliable insights and fairly accurate predictions (sometimes even better than obtained by full Navier-Stokes simulations) of distance of propagation and shape of interface, except for a very short initial phase. The limit of validity of the inviscid approximation is calculated, and it is shown that the above-mentioned experiments are within it. Some apparent discrepancies and scatter of various results are briefly discussed, and some reconciliations are suggested.
Three-photon absorption in ZnSe and ZnSe∕ZnS quantum dots
Authors: Lad Amit D. , Prem Kiran P. , Ravindra Kumar G. , Mahamuni Shailaja
Published: March 2007

ZnSe and ZnSe∕ZnS core/shell quantum dots (QDs) of two different sizes (4.5 and 3.5nm) have been synthesized. The nonlinear absorption is measured at 1064nm using a 35ps laser with an open aperture Z-scan setup. Three-photon absorption (3PA) has been observed in ZnSe and ZnSe∕ZnS QDs. 3PA cross section is found to be about four orders of magnitude larger than bulk ZnSe, and three orders of magnitude higher than ZnS QDs. 3PA cross section is found to be increased in ZnSe and in ZnSe∕ZnS QDs with decreasing size from 4.5to3.5nm, due to strong confinement effect.
Control of reverse twist domain near a pixel edge using strong vertical electric field in the fringe-field switching liquid crystal device
Authors: Kim Mi Sook , Seen Seung Min , Lee Seung Hee
Published: March 2007

In the fringe-field switching (FFS) liquid crystal (LC) device, competition between two opposite forces to twist LCs exists near the pixel edge. The authors proposed an advanced FFS structure with the strong vertical field near the pixel edge which was induced between Cr black matrix (BM) and pixel electrode due to electrical connection between the Cr BM and the common electrode. The vertical field induces a tilt angle of LCs which increases with increasing field intensity. As a result, the degree of twist in the reverse direction decreases with increasing the pixel voltage, and thus voltage-dependent LC dynamics becomes very stable even at high applied voltages.
Oxygen-dependent phosphorus networking in ZnO thin films grown by low temperature rf sputtering
Authors: Pugel D. Elizabeth , Vispute R. D. , Hullavarad S. S. , Venkatesan T. , Varughese B.
Published: March 2007

Radio frequency (rf) sputtered films of 10at.% P2O5-doped zinc oxide (ZnO) were deposited at temperatures (Td) below the sublimation point of P2O5 (Td<350°C) and at a range of oxygen pressures p(O2). Ultraviolet-visible optical transmission measurements, x-ray photoelectron spectroscopy (XPS), and x-ray diffraction were used to examine the effects of p(O2) during deposition on the band gap and on the bonding behavior of phosphorus. At both deposition temperatures studied (room temperature with unintentional heating and 125°C), an increase in phosphorus concentration with increasing p(O2) was observed. However, the dependence of the band gap behavior on p(O2) was observed to be dramatically different for the two deposition temperatures: room-temperature-deposited films show a redshift while films deposited at 125°C show a blueshift. Analysis of the oxygen 1s XPS peak shows a progressive formation of nonbridging (Zn–O–P) bond networks for room temperature films, whereas films grown at 125°C show increased (P–O–P) bond networks with increasing p(O2). This indicates that a small degree of thermal activation considerably modifies the bonding behavior of phosphorus in ZnO. Implications of these results for the use of phosphorus as a p-type dopant for ZnO are discussed.
The influence of target irradiation conditions on the parameters of laser-produced plasma jets
Authors: Kasperczuk A. , Pisarczyk T. , Borodziuk S. , Ullschmied J. , Krousky E. , Masek K. , Pfeifer M. , Rohlena K. , Skala J. , Pisarczyk P.
Published: March 2007

Recent experimental results demonstrate that the forming of plasma jets is a fundamental process accompanying the laser-produced plasma expansion, if a massive planar target with relatively high atomic number is irradiated by a defocused laser beam. In this paper some new results on the influence of target irradiation conditions on plasma jet parameters are presented. The experiment was carried out at the Prague Asterix Laser System (PALS) iodine laser [K. Jungwirth, A. Cejnarova, L. Juha, B. Kralikova, J. Krasa, E. Krousky, P. Krupickova, L. Laska, K. Masek, A. Prag, O. Renner, K. Rohlena, B. Rus, J. Skala, P. Straka, and J. Ullschmied, Phys. Plasmas 8, 2495 (2001)]. with the third harmonic beam of the pulse duration of 250ps. The beam energies varied in the range of 13–160J. The planar massive targets used in the experiment were made of copper. For measurements of the electron density evolution a three frame interferometric system was employed. The jets were produced in the whole range of the laser energy used. Calculations of the efficiency of the plasma jet production show that it decreases with increasing the laser energy.
The determinants of hydrogen concentrations in hydrogenated amorphous silicon films prepared using a triode deposition system
Authors: Shimizu Satoshi , Matsuda Akihisa , Kondo Michio
Published: March 2007

It is important to reduce the hydrogen concentration, in particular, the Si–H2 bond concentration, in a hydrogenated amorphous silicon film to improve its light-soaking stability. In a previous study, we found that a triode configuration plasma enhanced chemical vapor deposition method provides high quality and a very low hydrogen concentration film; however, the origin of the hydrogen reduction has been unknown. In this article, we investigate the essential factor causing the very low hydrogen concentrations observed in the triode system. In several experiments, we observed strong influences of deposition precursors on the resulting hydrogen concentrations. We propose that due to a steric hindrance, the hydrogen elimination process during film growth is disturbed when higher silane radicals stick to a growth surface. In a triode system, corresponding with the separation of the film growth surface from the precursor generation region, the contribution of higher silane radicals to film growth is suppressed due to their short diffusion length and frequent collisions with silane molecules.