bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental tests of the eigenmode theory of auroral roar fine structure and its application to remote sensing
Authors: Ye Shengyi , LaBelle J. , Yoon P. H. , Weatherwax A. T.
Published: December 2007

Auroral roar is an electromagnetic emission near two and three times the electron gyrofrequency. The generation mechanism is believed to involve mode conversion of upper hybrid waves generated where the upper hybrid frequency matches a cyclotron harmonic and trapped in density enhancements. Theory predicts that if the density enhancement has dimensions comparable to the wavelength, the waves will be discretized, and indeed discrete features have been observed with the electric field detector on the HIBAR rocket experiment and with ground‐level instruments. We have generated a database of eigenfrequencies predicted by theory as a function of the dimension and amplitude of the trapping density enhancement, assuming a Lorentzian‐shaped cyclindrical trapping density enhancement embedded in the auroral F‐region ionosphere. A fitting method is developed to test these predictions against the rocket and ground‐based data. The fitting shows a significant discrepancy between theoretical predictions of density profiles and HIBAR observations. The predicted density enhancement scale size is of order ∼10 m, whereas rocket observations suggest scale size of order ∼100 m. A correction factor of 30 is used to reconcile the difference between the theoretical predictions and observation. A Versatile Electromagnetic Waveform (VIEW) receiver, operated at South Pole station during 2003, recorded many examples of auroral roar fine structures. Twenty‐one relatively stationary examples of fine structures were selected, and the fitting procedure was applied to them, resulting in statistics of the trapping density enhancements associated with them, assuming that the eigenmode theory applies. In this case, the eigenmode theory indicates that irregularities in the range 7–40 m, or 200–1200 m if the correction factor is applied, would be responsible for generating observed auroral roar fine structures.
Separation of spatial and temporal structure of auroral particle precipitation
Authors: Boudouridis A. , Spence H. E.
Published: December 2007

Knowledge of the dominant temporal and spatial scales of auroral features is instrumental in understanding the various mechanisms responsible for auroral particle precipitation. Single spacecraft data always suffer from temporal/spatial ambiguity. In an effort to separate the temporal and spatial variations of the aurora, we use electron and ion precipitation data from two co‐orbiting satellites, F6 and F8 of the Defense Meteorological Satellite Program (DMSP). The two spacecraft have almost identical polar orbits with a small difference in period. As a result the time difference between the two measurements varies with time. We use two statistical tools in order to determine the most probable lifetimes and spatial dimensions of the prevalent auroral features. The first tool is cross‐correlation analysis between the magnetic latitude series of electron and ion, number and energy fluxes measured by the two DMSP spacecraft. As one spacecraft overtakes the other, the variable time lag between the two measurements results in different cross‐correlation of the two series. We explore the dependence of this variation on the time lag between the satellites. We find that the electron precipitation exhibits a decreasing correlation between the two spacecraft with increasing time lag, whereas there is only a small similar effect for the ion precipitation data. The second statistical tool is cross‐spectral analysis, for which we compute the so‐called coherence function as a function of frequency (or inverse wavelength) and hence size of the auroral features. The coherence function is a measure of the stability of auroral features of different sizes. We investigate its variation as a function of the time separation between the two measurements. We show that the coherence function of both electrons and ions remains high for up to 1.5 min spacecraft separations for all features larger than about 100 km in width. For smaller features the coherence is lower even for time lags of a few seconds. The results are discussed in the context of characteristic temporal and spatial auroral scales deduced from complementary studies and expected from theory.
An adsorption model of insoluble particle activation: Application to black carbon
Authors: Henson B. F.
Published: December 2007

We present a model of insoluble particle activation based on a modification of the Köhler equation in which we introduce a term based on the activity of water adsorbed on the particle surface. We illustrate the model by application to activation data from black carbon (BC) particles. We parameterize the model using a free energy of adsorption that reflects the relative affinity for water vapor adsorbing on either the BC surface or the adsorbed water layer. This enables the parameterization of either chemically modified (hydrophilic) or graphitic (hydrophobic) BC. Several features of a suite of carbon activation data are captured by the model. In particular, upper and lower bounding curves are predicted for activation supersaturation as a function of diameter. We show a large body of recent activation data that all fall within these bounds. The model also predicts that activation of BC aerosol leads to activation diameters from 3 to 10 times smaller than activation of soluble particles of identical dry diameter. The activation of smaller particles may be expected to impact the size distribution of resulting cloud droplets and thus the aerosol first indirect effect on climate. Finally, we compare activation of BC aerosol as calculated with this model to activation of mixed particles of BC and ammonium sulfate. It is shown that for some range of the adsorption free energy a hydrophilic BC aerosol is predicted to activate at lower supersaturation than comparable mixed aerosol of low mass fraction in the soluble component, indicating the utility of a model of mixed particle activation based on the adsorption of water to form an interfacial solution.
Ozone columns obtained by ground‐based remote sensing in Kiev for Aura Ozone Measuring Instrument validation
Authors: Shavrina A. V. , Pavlenko Y. V. , Veles A. , Syniavskyi I. , Kroon M.
Published: December 2007

Ground‐based observations with a Fourier transform spectrometer in the infrared region (FTIR) were performed in Kiev (Ukraine) during the time frames August–October 2005 and June–October 2006 within the Ozone Monitoring Instrument (OMI) validation project 2907 entitled “OMI validation by ground based remote sensing: ozone columns and profiles” in the frame of the international European Space Agency/Netherlands Agency for Aerospace Programmes/Royal Dutch Meteorological Institute OMI Announcement of Opportunity effort. Ozone column data for 2005 were obtained by modeling the ozone spectral band at 9.6 μm with the radiative transfer code MODTRAN3.5. Our total ozone column values were found to be lower than OMI Differential Optical Absorption Spectroscopy (DOAS) total ozone column data by 8–10 Dobson units (DU, 1 DU = 0.001 atm cm) on average, while our observations have a relatively small standard error of about 2 DU. Improved modeling of the ozone spectral band, now based on HITRAN‐2004 spectral data as calculated by us, moves our results toward better agreement with the OMI DOAS total ozone column data. The observations made during 2006 with a modernized FTIR spectrometer and higher signal‐to‐noise ratio were simulated by the MODTRAN4 model computations. For ozone column estimates the Aqua Atmospheric Infrared Sounder satellite water vapor and temperature profiles were combined with the Aura Microwave Limb Sounder stratospheric ozone profiles and Tropospheric Emission Monitoring Internet Service‐Koninklijk Nederlands Meteorologisch Instituut climatological profiles to create a priori input files for spectral modeling. The MODTRAN4 estimates of ozone columns from the 2006 observations compare rather well with the OMI total ozone column data: standard errors are of 1.11 DU and 0.68 DU, standard deviation are of 8.77 DU and 5.37 DU for OMI DOAS and OMI Total Ozone Mapping Spectrometer, respectively.
Aircraft observations of dust and pollutants over northeast China: Insight into the meteorological mechanisms of transport
Authors: Dickerson R. R. , Li C. , Li Z. , Marufu L. T. , Stehr J. W. , McClure B. , Krotkov N. , Chen H. , Wang P. , Xia X. , Ban X. , Gong F. , Yuan J. , Yang J.
Published: December 2007

The meteorological mechanisms for lofting trace gases and aerosols out of the planetary boundary layer (PBL) into the free troposphere are key to understanding local air pollution problems as well as regional and global atmospheric chemistry and climate issues. Over the North American continent, convective storms and lifting in warm conveyor belts transport pollutants into the free troposphere. Little is known about the vertical distribution of pollutants and dust over east Asia, and the processes leading to transport, transformation, and removal of these species remain uncertain. To provide insight into these mechanisms, we report on eight flights based out of Shenyang in NE China as part of the U.S./China EAST‐AIRE project conducted in April 2005. We evaluate profiles of trace species, along with back trajectories and satellite data, in the meteorological context of cyclonic systems. The warm‐sector PBL air ahead of a cold front was highly polluted, while in the free troposphere concentrations of trace gases and aerosols were lower, but well above background; we measured ∼300 ppb CO, ∼2 ppb SO2, ∼70 ppb O3, and ∼ 8 × 10−5 m−1 aerosol scattering between ∼1000 and 4000 m altitude. Satellite observations indicate that the entire plume contained almost 105 tons of SO2 and that the gas decayed with a lifetime of 3–5 d. Roughly the same mass of aerosol was transported into the free troposphere. Over the east Asian continent, dry convection appears to dominate with warm conveyor belts first coming into play as the cyclonic systems move off the coast.
Detection of tropospheric propagation effects from deep space links of the Cassini spacecraft
Authors: Morabito David D.
Published: December 2007

The bandwidth allocations for frequencies that lie below 10 GHz are limited, and thus they cannot accommodate high rate communications requirements for future deep space missions. The transition to higher‐frequency bands where there are higher bandwidth allocations is being considered for such missions. As communications frequencies increase, the degradation effects due to weather also increase. The statistics of such effects have been documented in a variety of experimental programs using geostationary satellites. Deep‐space missions operate at much weaker received signal levels than near‐Earth missions because of very large interplanetary distances. These weaker signal levels result in lower operating margins, and thus it becomes important to characterize propagation effects at these higher microwave frequencies. The Ka band frequency allocation (27–40 GHz) offers increased advantages over the X band allocation (8–12 GHz) for telecommunications. Such advantages include increased gain (using directional antennas) and increased bandwidth. Knowledge of weather degradation effects is essential for developing optimum telemetry return strategies at Ka band. This article will report on atmospheric induced effects that have been observed on a deep‐space Ka band carrier signal (32 GHz) emitted by the Cassini spacecraft between December 2002 and January 2003. The results of such work will be beneficial in the planning of operations on future spacecraft with Ka band telemetry links.
Long‐term occurrence probabilities of intense geomagnetic storm events
Authors: Tsubouchi K. , Omura Y.
Published: December 2007

A quantitative assessment of the occurrence probability of intense geomagnetic storms (peak Dst < −100 nT) has been investigated by analyzing the Dst index time series database from 1957 to 2001. The main purpose was to derive two parameters, the probable intensity ST and the occurrence frequency λt, that can act as proxies for long‐term space weather quantities. The intensity ST represents the expected maximum storm level with an occurrence rate of 1/T (a−1, where a is years) and has been derived from the probability density function (PDF) of extreme (∣Dst∣ > 280 nT) storms. The mathematical tool to determine this type of PDF is the extreme value modeling, which exhibits more accurate statistics for extreme behavior. Our results estimate S60 ≈ 589, indicating that the March 1989 storm (the event with the largest ∣Dst∣ in the database) corresponds to an event expected to occur only once every 60 a. The other parameter λt gives the average occurrence rate of storm events. We have tested the null hypothesis that the storm occurrence pattern can be modeled as a Poisson process represented by λt, where different λt exist for the active and quiet periods of the solar cycle. Ordinary χ2 tests of goodness of fit can not reject this hypothesis, except within the periods that include extremely frequent occurrences. The rate λt is approximately 2.3 (0.7) per 3 months in the active (quiet) period. A future practical application of this work is that the resultant Poisson probability will enable us to calculate the expected damage due to storms, which represent potential risks in space activities.
Simulating the dissolution of a complex dense nonaqueous phase liquid source zone: 2. Experimental validation of an interfacial area–based mass transfer model
Authors: Grant G. P. , Gerhard J. I.
Published: December 2007

A multiphase flow–aqueous phase transport numerical model (DNAPL3D‐MT) is developed to simulate the dissolution of complex source zones containing both pooled and residual dense nonaqueous phase liquids (DNAPLs). The multiphase flow model (DNAPL3D) is coupled to the aqueous species transport code (MT3D) via a flexible mass transfer function, which can employ the local equilibrium assumption or the single–boundary layer expression for rate‐limited dissolution either incorporating a lumped (correlation function) coefficient or explicitly accounting for the interfacial area (IFA) between the fluids. For the latter, this work employs the thermodynamically based Explicit IFA Submodel (Grant and Gerhard, 2007), which provides IFA as a function of saturation and saturation history. A bench‐scale experiment is presented involving the complete, natural dissolution of a DNAPL source zone emplaced by a point source release into heterogeneous porous media. DNAPL3D‐MT simulations of the experiment, involving no calibration to results, are compared with the observed evolution of both (1) measured downgradient dissolved phase concentrations and (2) DNAPL source zone configuration. The model, employing a mass transfer expression equipped with the Explicit IFA Submodel, simulates the experiment more accurately than when equipped with either a local equilibrium assumption or a published empirical correlation expression. Sensitivity simulations indicate that this model validation is sensitive to a number of the key assumptions in the Submodel derivation except one: the relationship between interfacial area and residual DNAPL saturations. The employed assumption of a single mass transfer coefficient value is supported by an analysis of the evolution of Peclet numbers throughout the DNAPL source zone, which reveals that the low hydraulic gradient employed resulted in diffusion‐dominated mass transfer conditions throughout the experiment. This study suggests that simulations of global mass flux from complex DNAPL source zones are sensitive to the interrelationship of rate‐limited mass transfer and groundwater velocity (and thus aqueous phase relative permeability and DNAPL saturation) at the local scale.
Non-Fermi-liquid behavior in the layered NaxCoO2
Authors: Zorkovská A. , Feher A. , Šebek J. , Šantavá E. , Bradaric I.
Published: December 2007

The specific heat in the temperature region from 100mKto360K and in magnetic fields of 0 and 9T and the susceptibility from 2to360K and in a magnetic field of 1T are measured in the layered NaxCoO2 (x=0.65, 0.70, and 0.75) oxides. Analysis of the data revealed non-Fermi-liquid behavior. The character of the observed anomalies seems to be connected to the mesoscopic phase separation, underlining the impact of disorder on the low-lying excitations in cobaltates.
Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine
Authors: Shubert V. Alvin , Baquero Esteban E. , Clarkson Jasper R. , James William H. , Turk Jeffrey A. , Hare Alissa A. , Worrel Kevin , Lipton Mark A. , Schofield Daniel P. , Jordan Kenneth D. , Zwier Timothy S.
Published: December 2007

Resonant two-photon ionization (R2PI), resonant ion-dip infrared (RIDIR), and UV-UV hole-burning spectroscopies have been employed to obtain conformation-specific infrared and ultraviolet spectra under supersonic expansion conditions for O-(2-acetamidoethyl)-N-acetyltyramine (OANAT), a doubly substituted aromatic in which amide-containing alkyl and alkoxy side chains are located in para positions on a phenyl ring. For comparison, three single-chain analogs were also studied: (i) N-phenethyl-acetamide (NPEA), (ii) N-(p-methoxyphenethyl-acetamide) (NMPEA), and (iii) N-(2-phenoxyethyl)-acetamide (NPOEA). Six conformations of OANAT have been resolved, with S0-S1 origins ranging from 34536to35711cm−1, denoted A–F, respectively. RIDIR spectra show that conformers A–C each possess an intense, broadened amide NH stretch fundamental shifted below 3400cm−1, indicative of the presence of an interchain H bond, while conformers D–F have both amide NH stretch fundamentals in the 3480–3495cm−1 region, consistent with independent-chain structures with two free NH groups. NPEA has a single conformer with S0-S1 origin at 37618cm−1. NMPEA has three conformers, two that dominate the R2P1 spectrum, with origin transitions between 35 580 and 35632cm−1. Four conformations, one dominate and three minor, of NPOEA have been resolved with origins between 35 654 and 36423cm−1. To aid the making of conformational assignments, the geometries of low-lying structures of all four molecules have been optimized and the associated harmonic vibrational frequencies calculated using density functional theory (DFT) and RIMP2 methods. The S0-S1 adiabatic excitation energies have been calculated using the RICC2 method and vertical excitation energies using single-point time-dependent DFT. The sensitivity of the S0-S1 energy separation in OANAT and NPOEA primarily arises from different orientations of the chain attached to the phenoxy group. Using the results of the single-chain analogs, tentative assignments have been made for the observed conformers of OANAT. The RIMP2 calculations predict that interchain H-bonded conformers of OANAT are 25–30kJ∕mol more stable than the extended-chain structures. However, the free energies of the interchain H-bonded and extended structures calculated at the preexpansion temperature (450K) differ by less than 10kJ∕mol, and the number of extended structures far outweighs the number of H-bonded conformers. This entropy-driven effect explains the presence of the independent-chain conformers in the expansion, and cautions future studies that rely solely on relative energies of conformers in considering possible assignments.
Real-time propagation of the reduced one-electron density matrix in atom-centered Gaussian orbitals: Application to absorption spectra of silicon clusters
Authors: Sun Jin , Song Jian , Zhao Yi , Liang Wan-Zhen
Published: December 2007

We solve the time-dependent density functional theory equation by propagating the reduced one-electron density matrix in real-time domain. The efficiency of several standard solvers such as the short-iterative Krylov-subspace propagator, the low-order Magnus integration method with the matrix polynomial (MP) or Chebyshev matrix polynomial (CMP) expansion of the evolution operator, and Runge-Kutta algorithm are assessed. Fast methods for summing MP and CMP are implemented to speed the calculation of the matrix exponential. It is found that the exponential propagators can tolerate large time step size and retain the computational accuracy whereas the Krylov-subspace algorithm is a little inferior for a larger time step size compared with the second-order Magnus integration method with the MP/CMP expansion of the evolution operator in both weak and intense fields. As an application, we calculate the absorption spectra of hydrogen-passivated silicon nanoparticles Si29Hx. The popular hybrid and generalized gradient approximation exchange-correlation functionals are applied. We find that the experimental spectra can be reproduced by using B3LYP and that the silicon particles with sizes of 1nm and the optical excitations at 3.7, 4.0, and 4.6eV may consist of 29 Si atoms surrounded by 24 hydrogen atoms.
Tuning intramolecular anharmonic vibrational coupling in 4-nitroaniline by solvent-solute interaction
Authors: Dreyer J. , Kozich V. , Werncke W.
Published: December 2007

Applying a combined experimental and theoretical approach we demonstrate that doublets of the νs(NO2) band of 4-nitroaniline which have been observed in several environments originate from Fermi resonances. Changes of the line shapes typical for Fermi resonances are reported also for other isotopomers of 4-nitroaniline, however, for each of them in different solvents and solvent mixtures. Simulations of the infrared spectra based on the solvatochromic frequency shifts of the νs(NO2) vibration determined experimentally together with calculated cubic couplings with overtones and combination bands account for the experimental findings.
Reflection of Alfvén waves from boundaries with different conductivities
Authors: Leneman D.
Published: December 2007

The reflection of Alfvén waves from the ionosphere plays a crucial role because the reflected wave can reduce or enhance the electric field pattern of the incident wave. The ionosphere is typically treated as a conducting surface, which has a height integrated Pederson conductivity. This approximation is appropriate in considering the reflection of Alfvén waves because the wavelengths along the magnetic field are large compared to the height of the ionosphere. Shear Alfvén wave reflection experiments have been performed in the large plasma device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. of Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles. A single frequency wave is launched from an antenna and reflects from a large plate inserted into the plasma column. By alternatively using a conducting and an insulating plate, the two extremes of conductivity relative to the Alfvén conductivity, 1∕(μovA) are tested. The data are compared with the expected theoretical behavior of the interference pattern of incident and reflected waves. Perhaps due to experimental effects, the conducting reflector is found to behave in much the same fashion as the insulator.
Depletion induced isotropic-isotropic phase separation in suspensions of rod-like colloids
Authors: Jungblut S. , Tuinier R. , Binder K. , Schilling T.
Published: January 2008

When non-adsorbing polymers are added to an isotropic suspension of rod-like colloids, the colloids effectively attract each other via depletion forces. We performed Monte Carlo simulations to study the phase diagram of such rod-polymer mixture. The colloidal rods were modeled as hard spherocylinders; the polymers were described as spheres of the same diameter as the rods. The polymers may overlap with no energy cost, while the overlap of polymers and rods is forbidden. Large amounts of depletant cause phase separation of the mixture. We estimated the phase boundaries of isotropic-isotropic coexistence both in the bulk and in confinement. To determine the phase boundaries we applied the grand canonical ensemble using successive umbrella sampling [J. Chem. Phys. 120, 10925 (2004)], and we performed a finite size scaling analysis to estimate the location of the critical point. The results are compared with predictions of the free volume theory developed by Lekkerkerker and Stroobants [Nuovo Cimento D 16, 949 (1994)]. We also give estimates for the interfacial tension between the coexisting isotropic phases and analyze its power-law behavior on the approach of the critical point.
Correlated transport and high resolution transmission electron microscopy investigations on inorganic-filled single-walled carbon nanotubes showing negative differential resistance
Authors: Ilie Adelina , Egger Stefan , Friedrichs Steffi , Kang Dae-Joon , Green Malcolm L. H.
Published: December 2007

We describe a strategy for fabricating devices on perforated silicon nitride membranes that is ideal for systems with modulated structure, in particular for nanotubes with endohedral inorganic compounds or molecules, or nanotubes externally functionalized with soft materials. It uses dynamic nanostenciling and focussed ion beam while avoiding nanotubes’ exposure to chemicals or beams. Physical properties and high resolution transmission electron microscopy are then correlated. Devices with individual single-walled carbon nanotubes partially filled with potassium iodide (KI) showed negative differential resistance (NDR). We attribute the NDR to electrostatic potential modulation on the nanotube due to permanent dipoles in the KI chain.
Magnetization easy axis in martensitic Heusler alloys estimated by strain measurements under magnetic field
Authors: Aksoy Seda , Krenke Thorsten , Acet Mehmet , Wassermann Eberhard F. , Moya Xavier , Mañosa Lluís , Planes Antoni
Published: December 2007

We study the temperature dependence of strain under constant magnetic fields in Ni–Mn based ferromagnetic Heusler alloys in the form Ni–Mn–X (X: Ga, In, Sn, and Sb) which undergo a martensitic transformation. We discuss the influence of the applied magnetic field on the nucleation of ferromagnetic martensite and extract information on the easy axis of magnetization in the martensitic state.
Mitigation of fast ions generated from laser-produced Sn plasma for extreme ultraviolet light source by H2 gas
Authors: Nakamura Daisuke , Tamaru Koji , Hashimoto Yuki , Okada Tatsuo , Tanaka Hiroki , Takahashi Akihiko
Published: December 2007

One of the serious problems in the laser-produced plasma for an extreme ultraviolet (EUV) light source used for the next generation lithography is the generation of fast ions that damage the EUV collector optics. In this study, the mitigation of fast ions from a laser-produced Sn plasma by a H2 background gas was investigated. It has been confirmed that H2 buffer gas at a pressure of 13.3Pa has little influence on the transmission of 13.5nm light with an optical path length of 200mm. The sputtering of a dummy mirror by the fast ions generated from the laser-produced Sn plasma and their mitigation were investigated by visualizing the spatial distributions of sputtered atoms using the laser-induced fluorescence (LIF) imaging method. It was evaluated that the sputtering rate by the fast ions was reduced to less than approximately 5% by H2 gas with a column density of 1.2×1021l∕m2. The dynamics and the chemical reaction of the plasma plume containing the high energy and high density ions in a H2 background gas were also investigated by a time-resolved emission spectroscopy as well as by LIF.
Pairs and heptamers of C70 molecules ordered via PTCDI-melamine supramolecular networks
Authors: Silly Fabien , Shaw Adam Q. , Porfyrakis Kyriakos , Briggs G. A. D. , Castell Martin R.
Published: December 2007

In this paper, we report on the use of two PTCDI-melamine supramolecular networks on Au(111) to trap C70 molecules. The different supramolecular networks were formed by changing the postannealing temperature after molecular deposition. We observed, using scanning tunneling microscopy, that the deposition of C70 onto a PTCDI melamine network with parallelogram cavities results in the long-range ordering of paired C70, whereas the deposition of C70 molecules onto a PTCDI-melamine honeycomb network results in the trapping of C70 heptamers.
Gravitational effects on the deformation of a droplet adhering to a horizontal solid surface in shear flow
Authors: Dimitrakopoulos P.
Published: December 2007

In this paper we investigate the gravitational effects on the deformation of a three-dimensional droplet adhering to a horizontal rough solid surface in steady shear Stokes flows. Our study considers both positive and negative Bond numbers for viscous and inviscid droplets. When the interfacial system is initially at hydrostatic equilibrium, our study shows that the Bond number affects the deformation of viscous droplets with moderate and large initial contact angles in a different way than those for small angles owing to the interplay between the viscous and surface tension forces. Inviscid droplets with different initial contact angles show similar behavior as the Bond number increases, i.e., their deformation is monotonically decreased owing to the monotonic decrease of the droplets’ height and thus the exerted pressure force. Our study identifies the gravitational effects of the onset of interfacial sliding, i.e., on the portions of the contact line which slide first due to violation of the hysteresis condition. When the interfacial system is not at hydrostatic equilibrium at the flow initiation, its dynamic evolution is more complicated owing to the combined action of the shear flow with the gravitational forcing due to the difference between the initial shape with the hydrostatic one. Our computational results are accompanied with an analysis of the forces on the droplet which provides physical insight and identifies the three-dimensional nature of the interfacial deformation.
Stress relaxation in GaN by transfer bonding on Si substrates
Authors: Hsu S. C. , Pong B. J. , Li W. H. , Beechem Thomas E. , Graham Samuel , Liu C. Y.
Published: December 2007

The stress state of GaN epilayers transferred onto Si substrates through a Au–Si bonding process was studied by micro-Raman scattering and photoluminescence techniques. By increasing the Au bonding thickness from 1to40μm, the high compressive stress state in GaN layer was relieved. A 10μm Au bonding layer thickness is shown to possess the maximum compressive stress relief and also the deformation potential of the quantum well was found to be ∼85meV. A nonlinear parabolic relation between luminescent bandgap and the biaxial stress of the transferred GaN epilayer in the compressive region was observed.
Chemical vapor synthesis of nanocrystalline perovskites using laser flash evaporation of low volatility solid precursors
Authors: Winterer Markus , Srdic Vladimir V. , Djenadic Ruzica , Kompch Alexander , Weirich Thomas E.
Published: December 2007

One key requirement for the production of multinary oxide films by chemical vapor deposition (CVD) or nanocrystalline multinary oxides particles by chemical vapor synthesis (CVS) is the availability of precursors with high vapor pressure. This is especially the case for CVS where much higher production rates are required compared to thin films prepared by CVD. However, elements, which form low valent cations such as alkaline earth metals, are typically only available as solid precursors of low volatility, e.g., in form of β-diketonates. This study describes laser flash evaporation as precursor delivery method for CVS of nanocrystalline perovskites. Laser flash evaporation exploits the nonequilibrium evaporation of solid metal organic precursors of low vapor pressure by absorption of the infrared radiation of a CO2 laser. It is shown that stoichiometric, nanocrystalline particles consisting of SrZrO3 and SrTiO3 can be formed from corresponding mixtures of β-diketonates which are evaporated nonselectively and with high rates by laser flash evaporation.
Sliding orientation effects on the tribological properties of polytetrafluoroethylene
Authors: Jang Inkook , Burris David L. , Dickrell Pamela L. , Barry Peter R. , Santos Catherine , Perry Scott S. , Phillpot Simon R. , Sinnott Susan B. , Sawyer W. Gregory
Published: December 2007

The chemical inertness, high melting point, and intrinsic lubricity of polytetrafluoroethylene (PTFE) have been used to develop solid lubricating parts for operation in extreme environments, from frying pans to satellites. The atomic-level mechanisms associated with friction and wear at PTFE surfaces are elucidated here by systematic investigations of the frictional anisotropy measured with respect to chain orientation. In particular, a combination of atomic-scale simulations, nanometer-scale atomic force microscopy experiments, micrometer-scale microtribometers experiments, and macroscale pin-on-disk experiments are used. Data across these length scales, from both the computational and experimental approaches, provide a consistent view of the mechanisms by which the structural orientation of PTFE contributes to its unique tribological properties.
Ignition of thermally sensitive explosives between a contact surface and a shock
Authors: Sharpe Gary J. , Short Mark
Published: December 2007

The dynamics of ignition between a contact surface and a shock wave is investigated using a one-step reaction model with Arrhenius kinetics. Both large activation energy asymptotics and high-resolution finite activation energy numerical simulations are employed. Emphasis is on comparing and contrasting the solutions with those of the ignition process between a piston and a shock, considered previously. The large activation energy asymptotic solutions are found to be qualitatively different from the piston driven shock case, in that thermal runaway first occurs ahead of the contact surface, and both forward and backward moving reaction waves emerge. These waves take the form of quasisteady weak detonations that may later transition into strong detonation waves. For the finite activation energies considered in the numerical simulations, the results are qualitatively different to the asymptotic predictions in that no backward weak detonation wave forms, and there is only a weak dependence of the evolutionary events on the acoustic impedance of the contact surface. The above conclusions are relevant to gas phase equation of state models. However, when a large polytropic index more representative of condensed phase explosives is used, the large activation energy asymptotic and finite activation energy numerical results are found to be in quantitative agreement.
Excitation of ion-acoustic perturbations by incoherent kinetic Alfvén waves in plasmas
Authors: Mendonça J. T. , Shukla P. K.
Published: December 2007

The dispersion relation for ion-acoustic perturbations (IAPs) in the presence of incoherent kinetic Alfvén waves (KAWs) in plasmas is derived. The wave-kinetic-approach is used to study the nonlinear interactions between an ensemble of random phase KAWs and IAPs. It is found that incoherent KAW spectrum is unstable against IAPs. The instability growth rates for particular cases are obtained. The present instability offers the possibility of heating ions in a turbulent magnetoplasma composed of incoherent KAWs.
Effective hard x-ray spectrum of a tabletop Mather-type plasma focus optimized for flash radiography of metallic objects
Authors: Raspa V. , Moreno C. , Sigaut L. , Clausse A.
Published: December 2007

The effective spectrum of the hard x-ray output of a Mather-type tabletop plasma focus device was determined from attenuation data on metallic samples using commercial radiographic film coupled to a Gd2O2S:Tb phosphor intensifier screen. It was found that the radiation has relevant spectral components in the 40–150 keV range, with a single maximum around 60−80 keV. The radiation output allows for 50 ns resolution, good contrast, and introspective imaging of metallic objects even through metallic walls. A numerical estimation of the induced voltage on the focus during the compressional stage is briefly discussed.
Influence of spot size on propagation dynamics of laser-produced tin plasma
Authors: Harilal S. S.
Published: December 2007

The plume dynamics in the presence of an ambient gas is very intriguing physics. The expansion of a laser-produced plasma in the presence of an ambient gas leads to internal plume structures, plume splitting, sharpening, confinement, etc. We investigated propagation dynamics of an expanding tin plume for various spot sizes using a fast visible plume imaging and Faraday cup diagnostic tools. Our results indicate that the sharpening of the plume depends strongly on the spot size. With a smaller spot size, the lateral expansion is found to be higher and the plume expansion is spherical while with a larger spot size the plume expansion is more cylindrical. Analysis of time resolved imaging also showed internal structures inside the plume.
Effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster
Authors: Yamamoto Naoji , Kondo Shinya , Chikaoka Takayuki , Nakashima Hideki , Masui Hirokazu
Published: December 2007

The effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster were investigated in order to improve thrust performance. First, the extracted ion beam current was measured for various levels of strength of the magnetic field. It was found that there is an optimum magnitude of the magnetic field. That this is due to the tradeoff between magnetic mirror confinement and microwave-plasma coupling was confirmed by measurement of the ion saturation current into the antenna of the ion thruster. The ion saturation current was found to decrease with an increase in magnetic field strength, due to the improvement in magnetic mirror confinement. The estimated electron temperature also decreases with an increase in magnetic field strength. This result shows that the increase in magnetic field strength leads to a decrease in microwave-plasma coupling. Next, the ion beam current for three magnetic field shapes was measured by changing the length of the central yoke. The results show that the optimum magnetic field shape depends on the mass flow rate because of the tradeoff between magnetic confinement and ionization probability. For the configurations tested, the 3 mm length central yoke is optimal for low mass flow, whereas 7 mm is the best for high mass flow. Overall, the extracted ion beam current is 21.4 mA, at a xenon mass flow rate of 0.036 mg/s, beam voltage of 1500 V, and incident microwave power of 16 W.
Dynamics of virtual cathode oscillation analyzed by impedance changes in high-power diodes
Authors: Li Limin , Men Tao , Liu Lie , Wen Jianchun
Published: December 2007

Investigations on the dynamics of virtual cathode oscillation are performed with a reflex triode virtual cathode oscillator powered by a 450 kV, 400 ns voltage pulse. Based on the diode impedance changes, the dynamics of virtual cathode oscillation are discussed. Four ensuing processes, corresponding to different stages of microwave emission, exist in the temporal behavior of diode impedance changes. When the turning point of diode impedance forms, the beam current develops into the parapotential flow. The process of microwave emission can be described by diode impedance changes. To confirm this mechanism, self-pinching of the beam current and current-voltage characteristics are introduced. This mechanism is independent of the cathode material and structure. When the experiments were carried out with a carbon fiber cathode, the 300 MW, 180 ns microwave emission was obtained, the beam current density reached kA/cm2 level, and the plasma expansion rate was 1.3 cm/μs. These results show that the carbon fiber cathode is a potential candidate for high-power microwave sources.
Space-charge-limited, two-dimensional unmagnetized flow in a wedge geometry
Authors: Rokhlenko A. , Lebowitz J. L.
Published: December 2007

This paper studies the space-charge-limited current in an infinite wedge geometry in two dimensions. This geometry permits a reduction of the problem to a set of easily solved ordinary differential equations. The system, though very simplified, exhibits features similar to those expected to occur in many realistic systems with inhomogeneous electric fields. We obtain, in particular, a universal form for the particle trajectories and a nonmonotone charge distribution with accumulation at both the cathode and the anode. The explicit solution of the model can be useful for testing numerical schemes. The case of a very low density current is also considered. Relaxation of the geometrical limitations of the model are studied using conformal mapping techniques. Possible applications to realistic systems, which can be tested by simple experiments, are presented.
Flow over a stretchable disk
Authors: Fang Tiegang
Published: December 2007

In this work, an exact solution for the steady state Navier–Stokes equation in cylindrical coordinates is presented. The solution is an extension of the well-known von Kármán viscous pump problem to the configuration with a stretchable disk with or without rotation. The results also offer new examples for three-dimensional computation code validation.
Connection between quantum mechanical and classical time evolution of certain dissipative systems via a dynamical invariant
Authors: Schuch Dieter
Published: December 2007

In a former publication [D. Schuch and M. Moshinsky, Phys. Rev. A 73, 062111 (2006)] we have shown that the time evolution of a quantum system with at most quadratic Hamiltonian that can be described by different methods, namely, the time-dependent Schrödinger equation, the time propagator or Feynman kernel, and the Wigner function is connected via a dynamical invariant, the so-called Ermakov invariant. Since exact invariants of this type also exist for certain effective descriptions of dissipative quantum systems, we will show the relations between these descriptions and the corresponding invariants and will discuss how the results obtained for the conservative systems must be modified in the presence of a linear velocity dependent frictional force.
Mechanism for zirconium oxide atomic layer deposition using bis(methylcyclopentadienyl)methoxymethyl zirconium
Authors: Elam J. W. , Pellin M. J. , Elliott S. D. , Zydor A. , Faia M. C. , Hupp J. T.
Published: December 2007

The mechanism for zirconium oxide atomic layer deposition using bis(methylcyclopentadienyl)methoxymethyl zirconium and H2O was examined using ab initio calculations of hydrolysis energies to predict the order of ligand loss. These predictions were tested using in situ mass spectrometric measurements which revealed that the methyl ligand, and 65% of the methylcyclopentadienyl ligands are lost during the zirconium precursor adsorption. The remaining 35% of the methylcyclopentadienyl ligands and the methoxy ligand are lost during the subsequent H2O exposure. These measurements agree very well with the predictions, demonstrating that thermodynamic calculations are a simple and accurate predictor for the reactivities of these compounds.
Optical polarization properties of M-plane GaN films investigated by transmittance anisotropy spectroscopy
Authors: Bhattacharyya Jayeeta , Ghosh Sandip , Arora B. M. , Brandt O. , Grahn H. T.
Published: December 2007

The authors investigate the in-plane optical polarization properties of [11¯00]-oriented (M-plane) GaN films on γ-LiAlO2(100) substrates by transmittance anisotropy spectroscopy (TAS). This technique is sensitive to the difference in the transmittance between light polarized parallel and perpendicular to the c axis of GaN, which for M-plane GaN lies in the film plane. The TAS spectrum exhibits a clear resonance in the vicinity of the fundamental bandgap. Simulations demonstrate that this resonance directly reflects the polarization-dependent shift of the bandgap. The zero crossings of the differential TAS spectrum are shown to be a measure for the polarization-dependent transition energies.
Low temperature conduction and scattering behavior of Ga-doped ZnO
Authors: Ahn Byung Du , Oh Sang Hoon , Kim Hyun Jae , Jung Myung Hwa , Ko Young Gun
Published: December 2007

This paper dealt with the electrical properties of Ga:ZnO film deposited via pulsed laser deposition and the mechanisms responsible for conduction and scattering behavior in terms of temperature. Such film having a degenerate band exhibited the metallic characteristics except those observed below 100 and 60K for films fabricated at 298 and 773K, respectively. This metal semiconductor transition (MST) was explained based on weakly localized electrons found in disorder conductor. It was found that the predominant scattering mechanism was ionized-impurity scattering below MST region, while lattice vibration and grain boundary scattering were important in the regime above MST temperature.
Laser beam operation mode dependent grain morphology of alumina
Authors: Samant Anoop N. , Harimkar Sandip P. , Dahotre Narendra B.
Published: December 2007

The effect of laser beam operation mode on transition of grain morphology was studied during surface modification of alumina ceramic. A 4 kW HL 4006D continuous wave Nd:yttrium-aluminum-garnet (YAG) laser and a JK 701 pulsed Nd:YAG laser were employed to conduct surface modification of alumina. The laser beam operated in continuous wave mode generated faceted grain structure on the surface of the ceramic while the beam operated in pulsed mode produced nonfaceted grain structure. Such transition in grain morphology is mainly attributed to different levels of undercooling which is a function of cooling rates encountered in two different modes of laser operation.
Light scattering study of acoustic phonon modes and central peaks in Pb[(Mg1∕3Nb2∕3)0.45Ti0.55]O3 single crystals
Authors: Ko Jae-Hyeon , Kojima Seiji , Bokov Alexei A. , Ye Zuo-Guang
Published: December 2007

The dynamic polar nanoregions (PNRs) which form below the Burns temperature and determine the unique properties of relaxor ferroelectrics were predicted [Tinte et al., Phys. Rev. Lett. 97, 137601 (2006)] to be pinned to the quenched chemically ordered regions (CORs) usually observed in lead-containing relaxors. In Pb[(Mg1∕3Nb2∕3)0.45Ti0.55]O3 crystal where CORs are known to be absent, we have found the phenomena typically related to the relaxation of dynamic PNRs, namely, the significant Brillouin quasielastic scattering, the softening of the longitudinal acoustic mode, and the deviation from the Curie-Weiss law above the Curie point. This implies that PNRs may appear in crystals without CORs.
Electric-field-driven nuclear spin control using diluted magnetic semiconductors
Authors: Ogura M. , Akai H.
Published: December 2007

We propose a technique for electric-field-controlled nuclear magnetic resonance (NMR) using a heterostructure constructed from diluted magnetic semiconductors. The effect on NMR of a bias voltage on an (In,Mn,As)As∕AlSb system is calculated using first-principles electronic structure methods. The calculations show that the hyperfine field at the antisite As is very sensitive to an external electric field. This technique can be used to control nuclear spins in quantum computer that uses nuclear spins as its quantum bits.
Chemical disorder-induced magnetism in FeSi2 nanoparticles
Authors: Chen Y. Y. , Lee P. C. , Tsai C. B. , Neeleshwar S. , Wang C. R. , Ho J. C. , Hamdeh H. H.
Published: December 2007

Iron disilicide in a bulk form is practically nonmagnetic. In contrast, nanoparticles of FeSi2 exhibit superparamagnetism with blocking temperatures ranging from 8K(15nm)to34K(55nm). Their relatively low saturation magnetization suggests that the magnetic behavior is associated with only a small fraction of Fe ions, which have a sufficient number of other Fe as nearest neighbors. The chemical disorder is presumably induced in the formation of nanoparticles. A spin glass-type anomaly below 10K observed in specific heat data gives a further evidence for the compositional heterogeneity.