bluuebunny/crossref_metadata_2025 huggingface dataset:
A quantitative comparison of lightning‐induced electron precipitation and VLF signal perturbations
Authors: Peter W. B. , Inan U. S.
Published: December 2007

VLF signal perturbations recorded on the Holographic Array for Ionospheric/Lightning Research (HAIL) are quantitatively related to a comprehensive model of lightning‐induced electron precipitation (LEP) events. The model consists of three major components: a test‐particle model of gyroresonant whistler‐induced electron precipitation, a Monte Carlo simulation of energy deposition into the ionosphere, and a model of VLF subionospheric signal propagation. For the two representative LEP events studied, the model calculates peak VLF amplitude perturbations within a factor of three of those observed, well within the expected variability of radiation belt flux levels. The phase response of the observed VLF signal to precipitation varied dramatically over the course of the two nights and this variability in phase response is not properly reproduced by the model. The model calculates a peak in the precipitation that is poleward displaced ∼6° from the causative lightning flash, consistent with observations. The modeled precipitated energy flux (E > 45 keV) peaks at ∼1 × 10−2 (ergs s−1 cm−2), resulting in a peak loss of ∼0.001% from a single flux tube at L ∼ 2.2, consistent with previous satellite measurements of LEP events. The precipitation calculated by the model is highly dependent on the near‐loss‐cone trapped radiation belt flux levels assumed, and hence our main objective is not to compare the model calculations and the VLF signal observations on an absolute basis but is rather to develop metrics with which we can characterize the VLF signal perturbations recorded on HAIL in terms of the associated precipitation flux. Metrics quantifying the ionospheric density enhancement (NILDE) and the electron precipitation (Γ) along a VLF signal path are strongly correlated with the VLF signal perturbations calculated by the model. A conversion ratio Ψ, relating VLF signal amplitude perturbations (ΔA) to the time‐integrated precipitation (100–300 keV) along the VLF path (Ψ = Γ/ΔA), of 1.2 ± 0.3 × 1010 (el m−1/dB) is suggested for precipitation events of similar location and characteristics to those examined. The total precipitation (100–300 keV) induced by one of the representative LEP events is estimated at ∼1.8 ± 0.4 × 1016 electrons, calculated directly from the conversion ratio Ψ and observations of VLF signal perturbations.
Potential phytoplankton responses to iron and stratification changes in the Southern Ocean based on a flexible‐composition phytoplankton model
Authors: Mongin Mathieu , Nelson David M. , Pondaven Philippe , Tréguer Paul
Published: December 2007

Under present climatic conditions, primary production in the Southern Ocean is limited by a combination of grazing pressure, the light/mixing regime, and iron. The response of the ecosystem to a permanent increase of iron supply and/or changes in the mixing regime is analyzed with a flexible‐composition phytoplankton model that includes C, N, Si, and Fe cycling. Limitation of phytoplankton growth by light and nutrients (Si, N, and Fe) is treated through their effects on cellular elemental composition. The model is applied to the KERFIX time series in the subantarctic region. Two physical scenarios are considered, normal and reduced mixed layer depth, with four different aeolian inputs of Fe in each case ranging from 1 to 1000 times the estimated present input. These simulations suggest that Fe supply via dust and rain must be increased by more than a factor of 10 to produce significant changes. Increased Fe supply alone causes the bloom to occur later in the season (summer rather than spring), and coupled with a decrease in the mixed layer depth it produces drastic changes in the bloom intensity while preserving its present temporal development. Seasonal interaction between light and Fe limitation plays a critical role in controlling the primary and export production. If aeolian Fe input is increased by a factor of 1000, and mixed layer depth is reduced at the same time, export of carbon increases by a factor of 3. Light limitation prevents complete drawdown of nitrate, even if Fe limitation is removed and mixed layer depth reduced. This sets an upper limit on the primary production that can be achieved under the present meteorological conditions in this sector of the Southern Ocean.
An examination of 1997–2007 surface layer temperature trends at two heights in Oklahoma
Authors: Lin X. , Pielke R. A. , Hubbard K. G. , Crawford K. C. , Shafer M. A. , Matsui T.
Published: December 2007

This study assesses near surface lapse rates and temperatures over the past decade at two heights from the Oklahoma Mesonet. A statistically significant change in lapse rate was detected of −0.21 ± 0.09°C (10 m)−1 per decade. The trend of nighttime lapse rate was about three times larger than the magnitude of trend of the daytime lapse rate. The lapse rate trends at the time of the daily maximum and minimum temperatures were larger during calm conditions. Significantly, changes of temperature trends at a single height were inconclusive when the data was not segmented by wind speed classes. For daily maximum and minimum station series at two heights, the temperature trends of these station series were the largest for daily minimum temperature at 1.5 m under calm conditions, and the second largest for daily minimum temperatures at 9.0 m under calm conditions. These observations document that monitoring long term near‐surface daily minimum temperature trends at a single level on light wind nights will not produce the same trends as for long term temperature trends at other heights near the surface.
Spectral properties of high‐density ducts
Authors: Streltsov A. V.
Published: December 2007

This paper presents results from a study of propagation of very‐low‐frequency electromagnetic waves (whistlers) inside high‐density ducts (HDDs) in the Earth's magnetosphere. The main hypothesis investigated in this study is that whistlers can propagate without any attenuation inside HDD when the width of the duct channel is equal to the integer number of perpendicular wavelengths of the waves inside the duct. This condition provides a simple and efficient procedure to determine nonleaking eigenmodes of HDDs. Our analysis of this nonleaking condition reveals several new and important properties of HDDs which were never discussed before. In particular, this condition shows that any HDD has only a finite number of nonleaking eigenmodes. This number is proportional to the width of the duct and the magnitude of the plasma density inside the duct. Thus each value of the density inside the duct defines minimum width of the duct channel providing the nonleaking ducting, and each width of the duct channel defines a minimal density magnitude providing the nonleaking ducting. HDDs with the same value of the product of the duct width times the square root of the plasma density have the same number of nonleaking eigenmodes and frequencies of the corresponding eigenmodes are the same. A broader (or with higher density) HDD can guide without leakage whistlers with higher frequencies than a narrower (or with lower density) HDD.
Influence of sedimentation, local and regional hydrothermal circulation, and thermal rebound on measurements of seafloor heat flux
Authors: Hutnak M. , Fisher A. T.
Published: December 2007

We quantify the influence of environmental processes on measurements of seafloor heat flux with a new one‐dimensional thermal model that includes time‐varying sedimentation and boundary conditions, represents several scales of hydrothermal and transient conductive processes within basement, and allows fluid seepage through accumulating sediments. Variations in basement thermal conductivity, the extent of hydrothermal mixing in upper basement, and fluid seepage through sediments each influence seafloor heat flux by ≤2–8%. Conductive thermal rebound following the cessation of advective heat loss from the crust may lower seafloor heat flux values by ≥5–10%, even after several to several tens of million years. The new models indicate that thermal rebound takes much longer than suggested by earlier (analytical) calculations, mainly because earlier models did not account for the heat capacitance of the conductive lithosphere. Application of the new model to 3.5–3.6 Ma seafloor on the eastern flank of the Juan de Fuca Ridge suggests that anomalously low heat flux in this area is best explained by incomplete conductive thermal rebound in the last 100–200 ka, following the burial of numerous basement outcrops. Application of the new model to the eastern flank of the East Pacific Rise and extrapolation out to the age of some of the oldest remaining seafloor indicate that sedimentation corrections may be important even where accumulation rates are typical of global values. Model results also suggest that conductive thermal rebound through thick sediments may bias measurements made on moderate to old seafloor, even where there is little evidence at present for ridge‐flank hydrothermal circulation.
Tidal asymmetry and residual circulation over linear sandbanks and their implication on sediment transport: A process‐oriented numerical study
Authors: Sanay Rosario , Voulgaris George , Warner John C.
Published: December 2007

A series of process‐oriented numerical simulations is carried out in order to evaluate the relative role of locally generated residual flow and overtides on net sediment transport over linear sandbanks. The idealized bathymetry and forcing are similar to those present in the Norfolk Sandbanks, North Sea. The importance of bottom drag parameterization and bank orientation with respect to the ambient flow is examined in terms of residual flow and overtide generation, and subsequent sediment transport implications are discussed. The results show that although the magnitudes of residual flow and overtides are sensitive to bottom roughness parameterization and bank orientation, the magnitude of the generated residual flow is always larger than that of the locally generated overtides. Also, net sediment transport is always dominated by the nonlinear interaction of the residual flow and the semidiurnal tidal currents, although cross‐bank sediment transport can occur even in the absence of a cross‐shore residual flow. On the other hand, net sediment divergence/convergence increases as the bottom drag decreases and as bank orientation increases. The sediment erosion/deposition is not symmetric about the crest of the bank, suggesting that originally symmetric banks would have the tendency to become asymmetric.
Ice properties of single‐layer stratocumulus during the Mixed‐Phase Arctic Cloud Experiment: 2. Model results
Authors: Fridlind A. M. , Ackerman A. S. , McFarquhar G. , Zhang G. , Poellot M. R. , DeMott P. J. , Prenni A. J. , Heymsfield A. J.
Published: December 2007

Measurements from the US Department of Energy Atmospheric Radiation Measurement Program's 2004 Mixed‐Phase Arctic Cloud Experiment (M‐PACE) provide a unique opportunity to study poorly understood ice formation processes in mixed‐phase stratocumulus. Using meteorological, aerosol, and ice nucleus measurements to initialize large‐eddy simulations with size‐resolved microphysics, we compare predicted liquid and ice mass, number, and size distribution with observations from a typical flight. We find that ambient ice nuclei appear insufficient by a few orders of magnitude to explain observed ice, consistent with past literature. We also find that two processes previously hypothesized to explain the discrepancy, shatter of freezing drops and fragmentation during ice‐ice collisions, were not significant sources of ice based on parameterizations from existing studies. After surveying other mechanisms that have been hypothesized to explain ice formation in mixed‐phase clouds generally, we find two that may be strong enough: (1) formation of ice nuclei from drop evaporation residuals, a process suggested by sparse and limited measurements to date, and (2) drop freezing during evaporation, a process suggested only by inference at this time. The first mechanism can better explain the persistence of mixed‐phase conditions in simulations of less vigorous stratus observed during the Beaufort Arctic Storms Experiment (BASE). We consider conditions under which emission of nuclei from the ocean surface or activation through cloud‐phase chemistry could provide alternative explanations for M‐PACE observations. Additional process‐oriented measurements are suggested to distinguish among ice formation mechanisms in future field studies.
Role of atmospheric aerosol concentration on deep convective precipitation: Cloud‐resolving model simulations
Authors: Tao Wei‐Kuo , Li Xiaowen , Khain Alexander , Matsui Toshihisa , Lang Stephen , Simpson Joanne
Published: December 2007

A two‐dimensional cloud‐resolving model with detailed spectral bin microphysics is used to examine the effect of aerosols on three different deep convective cloud systems that developed in different geographic locations: south Florida, Oklahoma, and the central Pacific. A pair of model simulations, one with an idealized low cloud condensation nuclei (CCN) (clean) and one with an idealized high CCN (dirty environment), is conducted for each case. In all three cases, rain reaches the ground earlier for the low‐CCN case. Rain suppression is also evident in all three cases with high CCN. However, this suppression only occurs during the early stages of the simulations. During the mature stages of the simulations the effects of increasing aerosol concentration range from rain suppression in the Oklahoma case to almost no effect in the Florida case to rain enhancement in the Pacific case. The model results suggest that evaporative cooling in the lower troposphere is a key process in determining whether high CCN reduces or enhances precipitation. Stronger evaporative cooling can produce a stronger cold pool and thus stronger low‐level convergence through interactions with the low‐level wind shear. Consequently, precipitation processes can be more vigorous. For example, the evaporative cooling is more than two times stronger in the lower troposphere with high CCN for the Pacific case. Sensitivity tests also suggest that ice processes are crucial for suppressing precipitation in the Oklahoma case with high CCN. A comparison and review of other modeling studies are also presented.
Cloud water in windward and leeward mountain forests: The stable isotope signature of orographic cloud water
Authors: Scholl M. A. , Giambelluca T. W. , Gingerich S. B. , Nullet M. A. , Loope L. L.
Published: December 2007

Cloud water can be a significant hydrologic input to mountain forests. Because it is a precipitation source that is vulnerable to climate change, it is important to quantify amounts of cloud water input at watershed and regional scales. During this study, cloud water and rain samples were collected monthly for 2 years at sites on windward and leeward East Maui. The difference in isotopic composition between volume‐weighted average cloud water and rain samples was 1.4‰ δ18O and 12‰ δ2H for the windward site and 2.8‰ δ18O and 25‰ δ2H for the leeward site, with the cloud water samples enriched in 18O and 2H relative to the rain samples. A summary of previous literature shows that fog and/or cloud water is enriched in 18O and 2H compared to rain at many locations around the world; this study documents cloud water and rain isotopic composition resulting from weather patterns common to montane environments in the trade wind latitudes. An end‐member isotopic composition for cloud water was identified for each site and was used in an isotopic mixing model to estimate the proportion of precipitation input from orographic clouds. Orographic cloud water input was 37% of the total precipitation at the windward site and 46% at the leeward site. This represents an estimate of water input to the forest that could be altered by changes in cloud base altitude resulting from global climate change or deforestation.
Quantum phase transitions and thermodynamics of quantum antiferromagnets with competing interactions
Authors: Trippe C. , Klümper A.
Published: December 2007

We study the isotropic Heisenberg chain with nearest and next-nearest neighbor interactions. The ground state phase diagram is constructed in dependence on the additional interactions and an external magnetic field. The thermodynamics is studied by the use of finite sets of nonlinear integral equations resulting from integrability. The equations are solved numerically and analytically in suitable limiting cases. We find second- and first-order transition lines. The exponents of the low-temperature asymptotics at the phase transitions are determined.
Phase separation in nanosize samples of (LaSr)MnO3
Authors: Mazur A. S. , Krivoruchko V. N. , Danilenko I. A.
Published: December 2007

Comparative investigations of the nuclear magnetic resonance and nuclear spin-spin relaxation residents of Mn55 nuclei of nanopowders of the manganite La0.7Sr0.3MnO3 with different particle sizes 50–200nm (NP1) and 12nm (NP2) have been performed. It has been found that two local ferromagnetic phases (FgM and FbM) with different magnetic spin dynamics coexist in a wide temperature interval in the NP1 sample. The phase FgM phase is identified as a ferromagnetic metallic phase which is due to double exchange and the FbM phase is identified as a ferromagnetic phase with slower hole motion and weaker double exchange. It is established that only one ferromagnetic state, corresponding to the FbM phase, exists in the NP2 sample. The results of these investigations show that the stability of the nonuniform ferromagnetic state of lanthanum manganites can be limited below by a characteristic minimum scale.
Investigations of the magnetic properties of the granular system Co0.6(Al2On)0.4 possessing isotropic positive magnetoresistance
Authors: Timofeev A. A. , Ryabchenko S. M. , Lozenko A. F. , Trotsenko P. A. , Stogneĭ O. V. , Sitnikov A. V. , Avdeev S. F.
Published: December 2007

Magnetostatic, magnetoresonance (FMR), and magnetotransport measurements are performed in the temperature range 100–360K in the granular ferromagnetic film Co0.59(Al2On)0.41 possessing isotropic (independent of the angle between the direction of the current and magnetization in the plane) positive magnetoresistance in magnetic fields ±500Oe are performed. It is established that this anomaly of the magnetoresistance is due to short-range order effects in the ordering imposed on the magnetic moments of granules in the absence of an external magnetic field by their dipole–dipole interaction. It is established that the blocking temperature of thermally activated re-orientations of the magnetic moments of the granules differs substantially with respect to the measurement time and the methods used for this investigation. It is about 200K for magnetization measurements and exceeds 470K for FMR and measurements of the magnetoresistance. It is shown that ferromagnetic resonance in the present system changes from the regime “independent resonance of individual clusters” to the regime “collective resonance of a system of magnetic granules in a film” when the direction of the magnetic field changes from in-plane to normal to the film.
Effect of electric field on quantized vortices in HeII
Authors: Natsik V. D.
Published: December 2007

A study is made of the electric polarization of quantized vortices in a superfluid Bose liquid and their interaction with electric field. Two types of vortex polarization are considered, both of which are due to the centrifugal forces exerted on the atoms of the liquid in their azimuthal motion around the vortex line. First, under the influence of these forces the atoms acquire dipole moments (inertial polarization in the absence of external field), and a nonuniform symmetric distribution of the polarization density arises around the vortex line. Here the vortex does not have an integral dipole moment, but each element of the vortex line possesses a quadrupole moment. Second, the centrifugal forces lead to a nonuniform distribution of the atomic density around the vortex line, and therefore the polarization density of the liquid in an external electric field is also nonuniform near the vortex line, and an individual element of the vortex line acquires an effective dipole moment proportional to the field (inductive polarization). Analytical expressions are obtained for the polarization density around rectilinear and ring-shaped vortex lines, and the effective dipole and quadrupole moments of such vortices are calculated. The distribution of ponderomotive forces acting on a superfluid liquid containing a quantized vortex in an electric field is analyzed, and the corrections to the vortex energy due to the field are found. Numerical estimates of these effects are made for the case of HeII.
Dynamic behavior of a superconducting flux qubit excited by a series of electromagnetic pulses
Authors: Kiyko A. S. , Omelyanchouk A. N. , Shevchenko S. N.
Published: December 2007

We study theoretically the behavior of a superconducting flux qubit subjected to a series of electromagnetic pulses. The possibility of controlling the system state via changing the parameters of the pulse is studied. We calculated the phase shift in the tank circuit weakly coupled to the qubit which can be measured by the impedance measurement technique. For the flux qubit we consider the possibility of estimating the relaxation rate from the impedance measurements by varying the delay time between the pulses.
Critical velocities in two-component superfluid Bose systems
Authors: Kravchenko L. Yu. , Fil D. V.
Published: December 2007

The question of the critical velocities of superfluid motion in a homogeneous, slightly nonideal two-component Bose gas with point interactions between particles is examined on the basis of the Landau criterion. It is shown that in the motion of the components with different velocities, the velocity of each component is not necessarily less than the minimal phase velocity of the elementary excitations in the nonmoving condensate. The Landau criterion leads to a joint condition on the values of the velocities of the components and the angle between them. It is found that the maximum value of the critical velocity of a given component can be achieved when the other component is at rest or when the components move in mutually perpendicular directions. The results are generalized to the case of a long-range interaction between particles and also for an inhomogeneous two-component Bose gas confined in a cylindrical harmonic potential. It is shown that in these cases the behavior of the critical velocities is qualitatively similar to that in a homogeneous two-component system with point interactions.
Magnetoresistive oscillations in a doubly connected SFS interferometer with a ferromagnetic segment longer than the thermal coherence length
Authors: Chiang Yu. N. , Shevchenko O. G.
Published: December 2007

The conductance of ferromagnetic Ni samples of macroscopic length between F/S interfaces closed by a superconductor (indium) — an SNS system in the “Andreev interferometer” (AI) geometry — is investigated. The macroscopic size of the system makes it possible to measure directly the conductance of each of the elements of the branched AI circuit and to arrange conditions under which the contribution of “side” effects, reminiscent of the proximity effect, are minimal. The hc∕2e oscillations of the resistance with respect to magnetic field (the Aharonov–Bohm effect in disordered conductors), with an amplitude corresponding to the positive interference contribution to the resistance of an F/S interfacial region with a thickness of the order of the coherence length of the subgap excitations upon Andreev reflection in the presence of an exchange field typical for ferromagnetic materials (∼1nm), are observed for the first time at lengths of the ferromagnetic segments exceeding the typical mesoscopic scale (∼1μm) by more than a factor of 103.
Fullerene C24 in the Hubbard model
Authors: Izergin E. D. , Mironov G. I.
Published: December 2007

A quantum system consisting of 24 atoms (fullerene C24) is investigated in the Hubbard model in the static-fluctuation approximation. The anticommutator Green functions, correlation functions, energy spectrum, and ground-state energy for C24 fullerene and for the C24+ fullerene ion are calculated in this model. An analysis of the results and a comparison of C24 with the individual structural components of the fullerene are presented. For comparison of the results with experimental data the energy band gap and ionization energy—fundamental characteristics of nanosystems—are calculated.
From optical spectra to phase diagrams—the binary mixture N2–CO
Authors: Vetter M. , Jodl H.-J. , Brodyanski A.
Published: December 2007

We investigate the T–c% phase diagram of the binary system N2–CO. From changes in IR spectra of all kinds of mode excitations (phonons, vibrons) we were able to determine the temperature of phase transitions (solid-solid, solid-liquid). The improvements in comparison to structural investigations by x-rays or electrons are the following: sample growing and handling with perfect optical and thermodynamic quality; determination of actual concentration (N2)x(CO)y from optical spectra; reduction of thermal hysteresis by careful cooling-heating cycles of the samples.
Effects of geometrical characteristics of surface roughness on droplet wetting
Authors: Sheng Yu-Jane , Jiang Shaoyi , Tsao Heng-Kwong
Published: December 2007

Surface roughness is known to alter the wettability on a solid substrate. In general, either Wenzel or Cassie-Baxter theory is adopted to describe the apparent contact angle. Following the minimum free energy pathway associated with the imbibition process, we have derived a generalized expression for the apparent contact angle on a textured surface and the liquid-gas contact area within the groove that plays a key role. Depending on the geometrical characteristics of the grooves, the surface wetting falls into three regimes: (i) single stable state which is either Wenzel (completely wetted roughness) or Cassie-Baxter (completely nonwetted roughness) state, (ii) two stable states (Wenzel and Cassie-Baxter) separated by an energy barrier, and (iii) single stable state with partially wetted roughness. The sufficient condition for each regime is derived and several groove geometries are given to show the free energy path. Alteration in the geometric parameters may lead to the wetting crossover. We also show that the Cassie-Baxter can occur at a hydrophilic surface for particular pore shapes.
Infrared absorption of gaseous CH3OO detected with a step-scan Fourier-transform spectrometer
Authors: Huang Deng-Ruei , Chu Li-Kang , Lee Yuan-Pern
Published: December 2007

C H 3 O O radicals were produced upon irradiation of a flowing mixture of CH3I and O2 with a KrF excimer laser at 248nm. A step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to record temporally resolved IR absorption spectra of reaction intermediates. Transient absorption bands with origins at 3033, 2954, 1453, 1408, 1183, 1117, 3020, and 1441cm−1 are assigned to ν1–ν6, ν9, and ν10 modes of CH3OO, respectively, close to wavenumbers reported for CH3OO isolated in solid Ar. Calculations with density-functional theory (B3LYP/aug-cc-pVTZ) predicted the geometry and the vibrational wavenumbers of CH3OO; the vibrational wavenumbers and relative IR intensities of CH3OO agree satisfactorily with these observed features. The rotational contours of IR spectra of CH3OO, simulated based on ratios of predicted rotational parameters for the upper and lower states and on experimental rotational parameters of the ground state, agree satisfactorily with experimental results; the mixing ratios of a-, b-, and c-types of rotational structures were evaluated based on the direction of dipole derivatives predicted quantum chemically. A feature at 995cm−1, ascribed to CH3OOI from a secondary reaction of CH3OO with I, was also observed.
Specific features of thermal expansion and polyamorphism in CH4–C60 solutions at low temperatures
Authors: Dolbin A. V. , Esel’son V. B. , Gavrilko V. G. , Manzhelii V. G. , Vinnikov N. A. , Gadd G. E. , Moricca S. , Cassidy D. , Sundqvist B.
Published: December 2007

The temperature dependence of the linear thermal expansion coefficient α(T) is investigated in the temperature range of 2.5 to 23K for two different CH4–C60 solutions in which CH4 molecules occupy 24 and 50% of the octahedral interstitial sites of the C60 lattice. In both cases, α(T) exhibits hysteresis, suggesting the existence of two types of orientational glass associated with these solutions. The temperature of the first-order phase transition between these two glasses is estimated, and the behavior of these two glasses is compared. The characteristic times of thermalization τ1, reorientation of the C60 molecules τ2, and of the phase transformation between the glasses τ′, are estimated for these solutions. Both the temperature dependence of α(T) and the characteristic thermalization time τ1 are found to have features near the phase transition temperature, and an explanation is put forward to explain these observed features.
Solvent-averaged potentials for alkali-, earth alkali-, and alkylammonium halide aqueous solutions
Authors: Hess Berk , van der Vegt Nico F. A.
Published: December 2007

We derive effective, solvent-free ion-ion potentials for alkali-, earth alkali-, and alkylammonium halide aqueous solutions. The implicit solvent potentials are parametrized to reproduce experimental osmotic coefficients. The modeling approach minimizes the amount of input required from atomistic (force field) models, which usually predict large variations in the effective ion-ion potentials at short distances. For the smaller ion species, the reported potentials are composed of a Coulomb and a Weeks-Chandler-Andersen term. For larger ions, we find that an additional, attractive potential is required at the contact minimum, which is related to solvent degrees of freedom that are usually not accounted for in standard electrostatics models. The reported potentials provide a simple and accurate force field for use in molecular dynamics and Monte Carlo simulations of (poly-)electrolyte systems.
Natural convection boundary layer in a 5:1 cavity
Authors: Barhaghi D. G. , Davidson L.
Published: December 2007

The natural convection boundary layer in a tall cavity with an aspect ratio of AR=5 is studied numerically. The Rayleigh number based on the width of the cavity is RaW=4.028×108. The large eddy simulation method together with different subgrid scale models are used to study the near-wall behavior of the boundary layer and the turbulence structure. It is found that the dynamic subgrid scale model is the most accurate model in terms of predicting the transition location. Results also indicate that the conventional grid resolutions expressed in viscous units that are used for forced convection flows are not appropriate in the case of the natural convection flows and higher grid resolutions are necessary. The turbulence statistics are studied in both the turbulent and the transition regions. Although the results of the fully turbulent region show no important grid dependency, it is found that the accuracy of the results in the transition region is highly grid dependent, suggesting that the subgrid scale fluctuations in the transition region are of different nature compared to the fully turbulent region. The budget of the momentum, temperature, and Reynolds stress transport equations are studied and the behavior of the important terms in the boundary layer is investigated.
On the conformal forms of the Robertson-Walker metric
Authors: Ibison M.
Published: December 2007

All possible transformations from the Robertson-Walker metric to those conformal to the Lorentz-Minkowski form are derived. It is demonstrated that the commonly known family of transformations and associated conformal factors are not exhaustive and that there exists another relatively less well known family of transformations with a different conformal factor in the particular case that K=−1. Simplified conformal factors are derived for the special case of maximally symmetric space-times. The full set of all possible cosmologically compatible conformal forms is presented as a comprehensive table. A product of the analysis is the determination of the set-theoretical relationships between the maximally symmetric space-times, the Robertson-Walker space-times, and functionally more general space-times. The analysis is preceded by a short historical review of the application of conformal metrics to cosmology.
Core and grain boundary sensitivity of tungsten-oxide sensor devices by molecular beam assisted particle deposition
Authors: Huelser T. P. , Lorke A. , Ifeacho P. , Wiggers H. , Schulz C.
Published: December 2007

In this study, we investigate the synthesis of WO3 and WOx (2.6≥x≤2.8) by adding different concentrations of tungsten hexafluoride (WF6) into a H2/O2/Ar premixed flame within a low-pressure reactor equipped with a particle-mass spectrometer (PMS). The PMS results show that mean particle diameters dp between 5 and 9 nm of the as-synthesized metal-oxides can be obtained by varying the residence time and precursor concentration in the reactor. This result is further validated by N2 adsorption measurements on the particle surface, which yielded a 91 m2/g surface area, corresponding to a spherical particle diameter of 9 nm (Brunauer-Emmett-Teller technique). H2/O2 ratios of 1.6 and 0.63 are selected to influence the stoichiometry of the powders, resulting in blue-colored WOx and white WO3 respectively. X-ray diffraction (XRD) analysis of the as-synthesized materials indicates that the powders are mostly amorphous, and the observed broad reflexes can be attributed to the orthorhombic structure of β-WO3. Thermal annealing at 973 K for 3 h in air resulted in crystalline WO3 comprised of both monoclinic and orthorhombic phases. The transmission electron microscope micrograph analysis shows that the particles exhibit spherical morphology with some degree of agglomeration. Impedance spectroscopy is used for the electrical characterization of tungsten-oxide thin films with a thickness of 50 nm. Furthermore, the temperature-dependent gas-sensing properties of the material deposited on interdigital capacitors are investigated. Sensitivity experiments reveal two contributions to the overall sensitivity, which result from the surface and the core of each particle.
Effect of sapphire-substrate thickness on the curvature of thick GaN films grown by hydride vapor phase epitaxy
Authors: Paskova T. , Becker L. , Böttcher T. , Hommel D. , Paskov P. P. , Monemar B.
Published: December 2007

The effect of sapphire-substrate thickness on the curvature and stress in thick hydride vapor phase epitaxial GaN films was studied by high-resolution x-ray diffraction at variable temperatures. The curvature was found to have the maximum value for comparable thicknesses of the film and the substrate, while the stress at the film surface decreases with increasing film thickness and increases with increasing substrate thickness, which is in very good agreement with the simulation results. The curvature at the growth temperature was found to be strongly influenced by the value of the intrinsic tensile strain, which is determined by the film/substrate thickness ratio.
Inelastic scattering and spin polarization in dilute magnetic semiconductor (Ga,Mn)Sb
Authors: Panguluri Raghava P. , Nadgorny B. , Wojtowicz T. , Liu X. , Furdyna J. K.
Published: December 2007

The point contact Andreev reflection technique has already been used to measure the spin polarization of some of the dilute magnetic semiconductors, such as narrow-band (In,Mn)Sb, as well as wider gap (Ga,Mn)As. While conventional Andreev reflection has been demonstrated in (In,Mn)Sb, quasiparticle density of states broadening has been observed in (Ga,Mn)As, possibly due to inelastic scattering effects. Here, we investigate the spin polarization, magnetic, and transport properties of epitaxially grown (Ga,Mn)Sb films with the Curie temperature of ∼10K. The spin polarization of 57±5% was measured. Spectrum broadening in (Ga,Mn)Sb has also been observed.
Reducing exciton–longitudinal optical phonon coupling with increasing Mg incorporation in MgZnO powders
Authors: Pan Ching-Ju , Lin Kuo-Feng , Hsu Wei-Tse , Hsieh Wen-Feng
Published: December 2007

The coupling between exciton and longitudinal optical (LO) phonon was investigated in the use of temperature-dependent photoluminescence from MgxZn1−xO powders prepared by a sol-gel method in the range of 0≤x≤0.05. The exciton binding energy increases to 73 meV for 5 at. % Mg incorporated powders. The strength of exciton–LO phonon coupling was deduced from the energy shift of exciton emission with temperature variation. The increase of the exciton binding energy results from a decrease of the exciton Bohr radius that is responsible for reducing the coupling strength of exciton–LO phonon as increasing Mg content.
The hyperfine interaction in the AΠ1∕22 and XΣ+2 states of ytterbium monofluoride
Authors: Steimle Timothy C. , Ma Tongmei , Linton Colan
Published: December 2007

The fine and hyperfine interaction parameters in the AΠ1∕22(v=0) and XΣ+2(v=0) states of the odd metal nuclear spin isotopologues of ytterbium monofluoride, Yb171F and Yb173F, have been determined from an analysis of high-resolution laser induced fluorescence spectra of the AΠ1∕22←XΣ+2(0,0) band. The observed ground XΣ+2(v=0) state Yb171(I=1∕2) Fermi contact parameter is significantly smaller than that determined from the matrix isolation electron spin resonance measurement [Van Zee et al., J. Phys. Chem. 82, 1192 (1978)]. An interpretation of the Yb173,171 magnetic hyperfine and nuclear electric quadrupole coupling parameters is given.
Correlation of microstructure and magnetic properties for exchange-biased Co ferromagnets grown above and below the diluted antiferromagnet Co(Mg)O
Authors: Leo Titus , Hong Jung-Il , Berkowitz Ami E. , Smith David J.
Published: December 2007

The microstructure of ferromagnetic/antiferromagnetic (FM/AFM) coupled bilayers consisting of 6 nm Co films, grown above and below 20 nm CoO layers diluted with nonmagnetic Mg, is correlated with measurements of exchange-bias field and coercivity as a function of Mg content. Increased Mg dilution is found to slightly decrease the grain width of the columnar oxide layers but no suggestion of Mg segregation at grain boundaries is detected using small-probe microanalysis. The order of growth and the amount of Mg dilution both play an important role in determining the size of the exchange bias field. The large increase in exchange coupling that is measured for Co-below bilayers is attributed to partial oxidation of the upper Co surface during initiation of the AFM deposition. The large decrease in coercivity measured for Co-above bilayers with high Mg dilution is closely correlated with the falloff in exchange-bias field. Overall, these results provide strong support for the uncompensated spin model of exchange bias.
Concentration and temperature effects on the spectroscopic properties of Tb3+ doped borosilicate glasses
Authors: Yamashita Tatsuya , Ohishi Yasutake
Published: December 2007

The spectroscopic properties of Tb3+ doped borosilicate glasses were investigated to assess their potential as gain media in the 0.54μm band. Analysis of the concentration and temperature dependences of the optical transitions from the D45 level of Tb3+ was carried out. The emission lifetime of the D45 level at 300K was found to be 2.61±0.03ms and almost invariant for Tb3+ concentrations less than 15wt% (1.68×1021ions∕cm3). Judd-Ofelt analysis was used to calculate the emission transition probabilities from the D45 level, and the emission cross section at 300K for the D45→F57 transition was 8.1×10−22cm2 at 541nm. Temperature dependent nonradiative relaxation of the D45 level was observed in samples with 5 and 20wt% Tb3+. The quenching process of the D45 level in the temperature range of 50–373K might have been caused by a two-photon one-site resonant process, whereas it was dominated by a two-site nonresonant process at temperatures below 40K. The results obtained indicate that Tb3+ doped borosilicate glass is a promising candidate for optical gain media in the green (0.54μm) band.
Nonlinear trapping and self-guiding of magnetized Langmuir waves due to thermal plasma filamentation
Authors: Nazarov Vladimir V. , Starodubtsev Mikhail V. , Kostrov Alexander V.
Published: December 2007

Nonlinear interaction of Langmuir waves with a laboratory magnetoplasma has been studied under the conditions relevant to the ionospheric heating experiments. Self-guiding of magnetized Langmuir waves is observed at critical plasma density (ω=ωp): Langmuir waves are trapped inside a narrow, magnetic-field-aligned plasma density depletion region, which is formed by trapped waves due to thermal plasma nonlinearity, i.e., due to local plasma heating and consequent thermodiffusion. Magnetized Langmuir waves are trapped inside the depletion region through their specific dispersion properties; this fact has been shown using the kinetically modified dispersion relation. The threshold of the nonlinear wave trapping exhibits significant growth in the vicinity of harmonics of the electron gyrofrequency.
Acoustical resonant absorption of pulsed laser radiation by a spherical absorber
Authors: Faraggi Eshel , Gerstman Bernard S.
Published: December 2007

We investigate the thermomechanical response of a spherical absorber to pulsed laser radiation and the potential for causing damage to the absorber and the surrounding material due to shockwave and bubble formation. We calculate the expected response of a spherical absorber to a series of laser pulses as a function of the gap duration between the pulses. We model two common absorbers that have different characteristics: a 1 μm melanosome found in the retina, and a 100 nm gold particle. We find that the thermomechanical response strongly depends on the duration between pulses and displays resonant effects with a characteristic period that depends on the absorber properties. This allows tuning the duration between pulses to channel a greater or lesser fraction of the absorbed energy into shockfront and bubble production, presenting various possibilities such as delivering large amounts of laser energy to produce strong thermal effects while suppressing unwanted pressure effects in the surrounding material. Resonance can also be used to target absorbers of a specific size, allowing generation of shockfronts in localized target regions to destroy specific cells. This specificity can also be used to sort particles by size.
Charge generation and trapping in bisphenol-A-polycarbonate/N-isopropylcarbazole mixture: A study by electron bombardment-induced conductivity
Authors: Santos S. , Caraballo D.
Published: December 2007

Electron bombardment-induced conductivity measurements were carried out on cast films of N-isopropylcarbazole (NIPC) dispersed into an amorphous matrix of bisphenol-A-polycarbonate. The charge generation was studied by estimating the hole yield (g), the fraction of charge escaping recombination, as a function of electric field and concentration of NIPC at room temperature. The hole yield, besides increasing by increasing the content of NIPC, was observed to increase with the electric field in the manner predicted by the Onsager theory of geminate recombination. Deep trapping levels were studied by filling under electron bombardment and observing transients. The deep traps were neutral in nature with a concentration on the order of 8.0×1014 cm−3, which was low enough not to degrade transport under normal conditions.
Electronic structures and doping of SnO2, CuAlO2, and CuInO2
Authors: Falabretti Barbara , Robertson John
Published: December 2007

The electronic structure and density of states of SnO2, CuAlO2, and CuInO2 have been calculated using the plane wave pseudopotential method, in order to study the limits to doping in n, p, and bipolar semiconductor systems. For each material, we have calculated the charge neutrality level and we have related it to the system’s electronic structure. We have found that the charge neutrality level energy increases in the band gap, going from p-type to n-type materials, giving an insight into the type of doping achievable in these oxides.
Measurement of gas temperature and convection velocity profiles in a dc atmospheric glow discharge
Authors: Stepaniuk Vadim P. , Ioppolo Tindaro , Ötügen M. Volkan , Sheverev Valery A.
Published: December 2007

Gas temperature and convective velocity distributions are presented for an unconfined glow discharge in air at atmospheric pressure, with electric currents ranging between 30 and 92 mA. The vertically oriented discharge was formed between a pin anode (top) and an extended cathode. The temperature and velocity profiles were measured using laser-induced Rayleigh scattering and laser Doppler anemometry techniques, respectively. The temperature field exhibited a conical shape with the radius of hot temperature zone increasing toward the anode. A maximum temperature of 2470 K was observed on the discharge axis with the discharge current of 92 mA. Air velocity measurements around the discharge demonstrated that the shape and magnitude of the temperature field are strongly affected by natural convection. Estimates indicate that convective losses may account for more than 50% of the power input into the positive column of the discharge. The measured temperature fields and convective velocity profiles provide a set of data that is important for the evaluation of dc atmospheric glow discharges in various applications such as sound manipulation and acoustic noise mitigation.
Band structure of FeBO3: Implications for tailoring the band gap of nanoparticles
Authors: Shang Shunli , Wang Yi , Liu Zi-Kui , Yang Chia-En , Yin Shizhuo
Published: December 2007

The energy band structure of a transparent, green, room-temperature ferromagnetic FeBO3 has been calculated using density functional theory within the generalized gradient approximation (GGA) and the GGA+U approach. The electronic structure of FeBO3 is predicted to be antiferromagnetic and insulating, both in perfect agreement with available experiments. The predicted band structure, in particular, the shapes of the valence and conduction band edges, provides great insights into tailoring the optical band gap of FeBO3 nanoparticles: for the particle size close to 3nm, the visible spectral region extends from green up to blue due to quantum confinement.
Density measurement of particles in rf silane plasmas by the multipass laser extinction method
Authors: Seon C. R. , Choe W. , Park H. Y. , Kim Junghee , Park S. , Seong D. J. , Shin Y. H.
Published: December 2007

Measurement of the time evolution of the particle number density was investigated in rf silane plasmas by using the multipass laser extinction method. A He–Ne laser beam underwent multiple reflections on one horizontal plane of the plasma. The extinction signal increased in proportion to the beam pass numbers. A 1011cm−3 density of 8nm radius particles was measured at 10s in a 32mTorr and 50W discharge using nine passes. The primary particle density was obtained by comparing the measured particle sizes with the calculated sizes from the light extinction signals and the Brownian free molecule coagulation model.