bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetohydrodynamic simulations of transient transpolar potential responses to solar wind density changes
Authors: Ober D. M. , Wilson G. R. , Burke W. J. , Maynard N. C. , Siebert K. D.
Published: October 2007

Magnetohydrodynamic (MHD) simulations are used to examine the response of the transpolar potential (ΦTP) to changes in the solar wind density during periods of constant solar wind electric field. For increases (decreases) in the solar wind density ΦTP responds immediately by increasing (decreasing) from the steady state values. In both cases the response of ΦTP is transient, decaying to near initial steady state values even when the density change persists. The magnitude of the ΦTP response is proportional to both the rate of erosion of the dayside magnetopause and the ionospheric Pedersen conductance. In our MHD simulations ΦTP is driven entirely by the dayside merging rate and is insensitive to changes in the nightside reconnection rate. The observed relationship between the modeled dayside merging rate and ΦTP is well characterized by an L‐R circuit equation derived from integrating Faraday's Law around the Region 1 current loop. The inductive time constant for variations in the transpolar potential was found to be 6.5 (13) minutes for simulations using ionospheric Pedersen conductances of 6 (12) mhos. This corresponds in both cases to a magnetosphere‐ionosphere inductance of 65 Henries. Observations of the transpolar potential derived using the assimilative mapping of ionospheric electrodynamics (AMIE) model are presented and shown to be consistent with the simulation results.
Simulation of electrified storms with comparison of the charge structure and lightning efficiency
Authors: Barthe Christelle , Pinty Jean‐Pierre
Published: October 2007

An electrification and lightning flash scheme was run in a cloud‐resolving model to study the electrical structure of idealized convective storms with several charge separation parameterizations. The electrification of an intense multicellular storm was first simulated. The results confirm the sensitivity of the cloud polarity and lightning flash characteristics to three noninductive charging formulations. Furthermore, it is found that the inductive charging is an efficient mechanism to enhance the lower electric charge, which favors cloud‐to‐ground flashes. Then, microphysical and electrical budgets were calculated for the convective and stratiform regions of a two‐dimensional squall line. The simulation shows that the liquid water content is high enough to generate graupel by riming. Thus the noninductive separation process is efficient to charge the stratiform plume as well, and lightning flashes can be triggered. Finally, the application of various noninductive charging schemes to several convective storms showed the storm electrification variability, which heavily depends on the cloud dynamics and microphysics. The study reveals some remarkable features concerning the charge structure and the cloud‐to‐ground flash polarity.
Hybrid simulation of Titan's magnetic field signature during the Cassini T9 flyby
Authors: Simon S. , Kleindienst G. , Boesswetter A. , Bagdonat T. , Motschmann U. , Glassmeier K.‐H. , Schuele J. , Bertucci C. , Dougherty M. K.
Published: October 2007

Titan's induced magnetotail is analyzed in terms of a three‐dimensional hybrid model, treating the electrons as a massless, charge‐neutralizing fluid, whereas a completely kinetic approach is used to cover ion dynamics. Since during the T9 flyby of Cassini, the spacecraft passed through Titan's induced magnetotail in a distance of about 4 Titan radii downstream of the obstacle, this flyby offers a unique chance to study the magnetic lobe structure in Titan's wake region. The key features of the measured magnetic field signature have shown to be completely reproducible in the framework of the hybrid approximation. Besides, the influence of the ionospheric production rates as well as the upstream plasma direction is investigated. While changes of the ion production rate yield only a slight modification of the magnetotail structure, the magnetic field enhancement in the lobes is strongly modified by changes of the upstream flow direction.
Impact on modeled cloud characteristics due to simplified treatment of uniform cloud condensation nuclei during NEAQS 2004
Authors: Gustafson William I. , Chapman Elaine G. , Ghan Steven J. , Easter Richard C. , Fast Jerome D.
Published: October 2007

Subgrid‐scale cloud condensation nuclei (CCN) heterogeneity is not represented in global climate models (GCM) and potentially contributes systematic errors to simulated cloud effects. High‐resolution WRF‐Chem model simulations were performed to investigate the impact of assuming a uniform CCN distribution on cloud properties and surface radiation over a region the size of a GCM grid column. Results indicate that a prescribed CCN distribution allowing for vertical and temporal fluctuations does substantially better in simulating cloud properties and radiative effects than does a prescribed uniform and constant CCN distribution. Spatially and temporally averaged net effects on downwelling shortwave radiation are between −3 and −11 W m−2 for the fluctuating and uniform distributions, respectively, versus a control simulation with fully interactive aerosols. Both prescribed CCN distributions produce optically thicker clouds more often than the control, with the mean cloud optical depth increasing by over 25% when using the uniform and constant CCN distribution.
From Rankine‐Hugoniot relation fitting procedure: Tangential discontinuity or intermediate/slow shock?
Authors: Feng H. Q. , Lin C. C. , Chao J. K. , Wu D. J. , Lyu L. H. , Lee L.‐C.
Published: October 2007

To identify an observed intermediate/slow shock, it is important to fit the measured magnetic fields and plasma on both sides using Rankine‐Hugoniot (R‐H) relations. It is not reliable to determine an intermediate/slow shock only by the shock properties and fitting procedure based on one spacecraft observation, though previous reported intermediate/slow shocks are confirmed in such a way. We investigated two shock‐like discontinuities, which satisfy the R‐H relations well. One meets the criterions of slow shocks and was reported as a slow shock, and another has all the characters of intermediate shock based on one spacecraft observation. However, both discontinuities also meet the requirements of tangential discontinuities and were confirmed as tangential discontinuities on large‐scale perspective by using multi‐spacecraft observations. We suggest that intermediate/slow shocks should be identified as carefully as possible and had better be determined by multi‐spacecraft.
Multiscale modeling of magnetospheric reconnection
Authors: Kuznetsova M. M. , Hesse M. , Rastätter L. , Taktakishvili A. , Toth G. , De Zeeuw D. L. , Ridley A. , Gombosi T. I.
Published: October 2007

In our efforts to bridge the gap between small‐scale kinetic modeling and global simulations, we introduced an approach that allows to quantify the interaction between large‐scale global magnetospheric dynamics and microphysical processes in diffusion regions near reconnection sites. We use the global MHD code BATS‐R‐US and replace an ad hoc anomalous resistivity often employed by global MHD models with a physically motivated dissipation model. The primary kinetic mechanism controlling the dissipation in the diffusion region in the vicinity of the reconnection site is incorporated into the MHD description in terms of nongyrotropic corrections to the induction equation. We developed an algorithm to search for reconnection sites in north‐south symmetric magnetotail. Spatial scales of the diffusion region and magnitude of the reconnection electric field are calculated consistently using local MHD plasma and field parameters. The locations of the reconnection sites are constantly updated during the simulations. To clarify the role of nongyrotropic effects in the diffusion region on the global magnetospheric dynamics, we perform simulations with steady southward interplanetary magnetic field driving of the magnetosphere. Ideal MHD simulations with magnetic reconnection supported by numerical resistivity often produce quasi‐steady configuration with almost stationary near‐Earth neutral line (NENL). Simulations with nongyrotropic corrections demonstrate dynamic quasi‐periodic response to the steady driving conditions. Fast magnetotail reconnection supported by nongyrotropic effects results in tailward retreat of the reconnection site with average speed of the order of 100 km/s followed by a formation of a new NENL in the near‐Earth thin current sheet. This approach allowed to model for the first time loading/unloading cycle frequently observed during extended periods of steady low‐mach‐number solar wind with southward interplanetary magnetic field.
Solar and interplanetary sources of major geomagnetic storms (Dst ≤ −100 nT) during 1996–2005
Authors: Zhang J. , Richardson I. G. , Webb D. F. , Gopalswamy N. , Huttunen E. , Kasper J. C. , Nitta N. V. , Poomvises W. , Thompson B. J. , Wu C.‐C. , Yashiro S. , Zhukov A. N.
Published: October 2007

We present the results of an investigation of the sequence of events from the Sun to the Earth that ultimately led to the 88 major geomagnetic storms (defined by minimum Dst ≤ −100 nT) that occurred during 1996–2005. The results are achieved through cooperative efforts that originated at the Living with a Star (LWS) Coordinated Data‐Analysis Workshop (CDAW) held at George Mason University in March 2005. On the basis of careful examination of the complete array of solar and in situ solar wind observations, we have identified and characterized, for each major geomagnetic storm, the overall solar‐interplanetary (solar‐IP) source type, the time, velocity, and angular width of the source coronal mass ejection (CME), the type and heliographic location of the solar source region, the structure of the transient solar wind flow with the storm‐driving component specified, the arrival time of shock/disturbance, and the start and ending times of the corresponding IP CME (ICME). The storm‐driving component, which possesses a prolonged and enhanced southward magnetic field (Bs), may be an ICME, the sheath of shocked plasma (SH) upstream of an ICME, a corotating interaction region (CIR), or a combination of these structures. We classify the Solar‐IP sources into three broad types: (1) S‐type, in which the storm is associated with a single ICME and a single CME at the Sun; (2) M‐type, in which the storm is associated with a complex solar wind flow produced by multiple interacting ICMEs arising from multiple halo CMEs launched from the Sun in a short period; (3) C‐type, in which the storm is associated with a CIR formed at the leading edge of a high‐speed stream originating from a solar coronal hole (CH). For the 88 major storms, the S‐type, M‐type, and C‐type events number 53 (60%), 24 (27%), and 11 (13%), respectively. For the 85 events for which the surface source regions could be investigated, 54 (63%) of the storms originated in solar active regions, 11 (13%) in quiet Sun regions associated with quiescent filaments or filament channels, and 11 (13%) were associated with coronal holes. Remarkably, nine (11%) CME‐driven events showed no sign of eruptive features on the surface or in the low corona (e.g., no flare, no coronal dimming, and no loop arcade, etc.), even though all the available solar observations in a suitable time period were carefully examined. Thus while it is generally true that a major geomagnetic storm is more likely to be driven by a frontside fast halo CME associated with a major flare, our study indicates a broad distribution of source properties. The implications of the results for space weather forecasting are briefly discussed.
Statistical analysis of the relationship between earthward flow bursts in the magnetotail and low‐latitude Pi2 pulsations
Authors: Kim K.‐H. , Takahashi K. , Ohtani S. , Sung S.‐K.
Published: October 2007

Several case studies have suggested that low‐latitude Pi2 pulsations are triggered or driven by earthward flow bursts in the magnetotail. Until now, no statistical study has investigated the causality between the flow bursts and Pi2 pulsations, so the overall significance of flow bursts to Pi2 excitation remains unclear. In this study we statistically examine the relationship between earthward flow bursts and low‐latitude Pi2 pulsations. We address the following two questions: (1) Does every flow burst trigger a low‐latitude Pi2 pulsation? (2) Does every low‐latitude Pi2 pulsation have a corresponding flow burst? We use magnetic field and ion plasma measurements made by Geotail and Pi2 measurements made at Kakioka from January 1995 to July 2001. Regarding the first question, we identified 849 Geotail flow burst events in the near‐Earth magnetotail and found that ∼90% of them occurred in the region characterized by ∣Bx∣ < 10 nT, and ∼75% occurred in the region characterized by ∣Bx∣ < 5 nT (i.e., at locations very close to the midplane of the plasma sheet). We found that 31 Pi2 events occurred within the 10‐min interval after the peak velocity of a flow burst; that is, only ∼4% of the flow burst events produced a low‐latitude Pi2. Regarding the second question, we identified 556 Pi2 events at Kakioka at times when Geotail was in the near‐Earth magnetotail, and of these, 31 were the subset of the 849 flow bursts identified above. This gives a seemingly low (∼6%) association of flow burst with Pi2. However, we find that the association rate jumped to ∼26% when Geotail was in the region of ∣Bx∣ < 5 nT, implying that there is a great chance of detecting a flow burst at the time of a ground Pi2 if plasma measurements are made very close to the midplane of the plasma sheet. We conclude that only a small fraction of plasma sheet flow bursts generate Pi2 pulsations, while each Pi2 likely has an associated flow burst somewhere in the near‐Earth magnetotail.
Hadriaca Patera: Insights into its volcanic history from Mars Express High Resolution Stereo Camera
Authors: Williams David A. , Greeley Ronald , Zuschneid Wilhelm , Werner Stephanie C. , Neukum Gerhard , Crown David A. , Gregg Tracy K. P. , Gwinner Klaus , Raitala Jouko
Published: October 2007

High Resolution Stereo Camera (HRSC) images of Hadriaca Patera, Mars, in combination with Mars Orbiter Camera (MOC), Mars Orbiter Laser Altimeter (MOLA), and Thermal Infrared Imaging System (THEMIS) data sets, reveal morphologic details about this volcano and enable determination of a chronology of the major geologic events through new cratering age assessments. New topographic measurements of the Hadriaca edifice were also made from a HRSC‐based high‐resolution (125 m) digital terrain model (DTM) and compared to the MOLA DTM. We find evidence for a complex formation and erosional history at Hadriaca Patera, in which volcanic, fluvial, and aeolian processes were all involved. Crater counts and associated model ages suggest that Hadriaca Patera formed from early shield‐building volcanic (likely explosive pyroclastic) eruptions at ∼3.7–3.9 Ga, with caldera formation no later than ∼3.5 Ga. A variety of geologic activity occurred in the caldera and on the northern flank and plains at ∼3.3–3.5 Ga, likely including pyroclastic flows (that partially filled a large crater NW of the caldera, and plains to the NE) and differential erosion/deposition by aeolian and/or fluvial activity. There were some resurfacing event(s) in the caldera and on the eastern flank at ∼2.4–2.6 Ga, in which the eastern flank's morphology is indicative of fluvial erosion. The most recent dateable geologic activity on Hadriaca Patera includes caldera resurfacing by some process (most likely differential aeolian erosion/deposition) in the Amazonian Period, as recent as ∼1.5 Ga. This is coincident with the resurfacing of the heavily channeled south flank by fluvial erosion. Unlike the Tharsis shields, major geologic activity ended at Hadriaca Patera over a billion years ago.
Differential interaction of Salmonella enterica serovars with lettuce cultivars and plant-microbe factors influencing the colonization efficiency
Authors: Klerks Michel M , Franz Eelco , van Gent-Pelzer Marga , Zijlstra Carolien , van Bruggen Ariena H C
Published: October 2007

The availability of knowledge of the route of infection and critical plant and microbe factors influencing the colonization efficiency of plants by human pathogenic bacteria is essential for the design of preventive strategies to maintain safe food. This research describes the differential interaction of human pathogenic Salmonella enterica with commercially available lettuce cultivars. The prevalence and degree of endophytic colonization of axenically grown lettuce by the S. enterica serovars revealed a significant serovar–cultivar interaction for the degree of colonization (S. enterica CFUs per g leaf), but not for the prevalence. The evaluated S. enterica serovars were each able to colonize soil-grown lettuce epiphytically, but only S. enterica serovar Dublin was able to colonize the plants also endophytically. The number of S. enterica CFU per g of lettuce was negatively correlated to the species richness of the surface sterilized lettuce cultivars. A negative trend was observed for cultivars Cancan and Nelly, but not for cultivar Tamburo. Chemotaxis experiments revealed that S. enterica serovars actively move toward root exudates of lettuce cultivar Tamburo. Subsequent micro-array analysis identified genes of S. enterica serovar Typhimurium that were activated by the root exudates of cultivar Tamburo. A sugar-like carbon source was correlated with chemotaxis, while also pathogenicity-related genes were induced in presence of the root exudates. The latter revealed that S. enterica is conditioned for host cell attachment during chemotaxis by these root exudates. Finally, a tentative route of infection is described that includes plant-microbe factors, herewith enabling further design of preventive strategies.
Dynamic polarizabilities of polyaromatic hydrocarbons using coupled-cluster linear response theory
Authors: Hammond Jeff R. , Kowalski Karol , deJong Wibe A.
Published: October 2007

Coupled-cluster theory with single and double excitations is applied to the calculation of optical properties of large polyaromatic hydrocarbons. Dipole polarizabilities are reported for benzene, pyrene, and the oligoacenes sequence n=2–6. Dynamic polarizabilities were calculated on polyacences as large as pentacene for a single frequency and for benzene and pyrene at many frequencies. The basis set effect was studied for benzene using a variety of basis sets in the Pople [Theor. Chim. Acta 28, 213 (1973)] and Dunning [J. Chem. Phys. 90, 1007 (1989)] families up to aug-cc-pVQZ and the Sadlej pVTZ basis [Collect. Czech. Chem. Commun. 53, 1995 (1998)], which was used exclusively for the largest molecules. Geometries were optimized using HF, B3LYP, PBE0, and MP2 and compared to experiment to measure method dependence and the possible role of bond-length alternation. Finally, the polarizability results were compared to four common density functionals (B3LYP, BLYP, PBE0, PBE).
Phase equilibrium of liquid mixtures: Experimental and modeled data using statistical associating fluid theory for potential of variable range approach
Authors: Giner Beatriz , Bandrés Isabel , Carmen López M. , Lafuente Carlos , Galindo Amparo
Published: October 2007

A study of the phase equilibrium (experimental and modeled) of mixtures formed by a cyclic ether and haloalkanes has been derived. Experimental data for the isothermal vapor liquid equilibrium of mixtures formed by tetrahydrofuran and tetrahydropyran and isomeric chlorobutanes at temperatures of 298.15, 313.15, and 328.15K are presented. Experimental results have been discussed in terms of both molecular characteristics of pure compounds and potential intermolecular interaction between them using thermodynamic information of the mixtures obtained earlier. The statistical associating fluid theory for potential of variable range (SAFT-VR) approach together with standard combining rules without adjustable parameters has been used to model the phase equilibrium. Good agreement between experiment and the prediction is found with such a model. Mean absolute deviations for pressures are of the order of 1kPa, while less than 0.013mole fraction for vapor phase compositions. In order to improve the results obtained, a new modeling has been carried out by introducing a unique transferable parameter kij, which modifies the strength of the dispersion interaction between unlike components in the mixtures, and is valid for all the studied mixtures being not temperature or pressure dependent. This parameter together with the SAFT-VR approach provides a description of the vapor-liquid equilibrium of the mixtures that is in excellent agreement with the experimental data for most cases. The absolute deviations are of the order of 0.005mole fraction for vapor phase compositions and less than 0.3kPa for pressure, excepting for mixtures containing 2-chloro-2-methylpropane which deviations for pressure are larger. Results obtained in this work in the modeling of the phase equilibrium with the SAFT-VR equation of state have been compared to the ones obtained in a previous study when the approach was used to model similar mixtures with clear differences in the thermodynamic behavior. We show how the SAFT-VR approach is capable of differentiating the molecular features of a relatively wide set of different mixtures.
Quasielastic neutron scattering of poly(methyl phenyl siloxane) in the bulk and under severe confinement
Authors: Chrissopoulou K. , Anastasiadis S. H. , Giannelis E. P. , Frick B.
Published: October 2007

Quasielastic neutron scattering was utilized to investigate the influence of confinement on polymer dynamics. Poly(methyl phenyl siloxane) chains were studied in the bulk as well as severely confined within the ∼1–2nm interlayer spacing of intercalated polymer/layered organosilicate nanohybrids. The temperature dependence of the energy resolved elastic scattering measurements for the homopolymer and the nanocomposites exhibit two distinct relaxation steps: one due to the methyl group rotation and one that corresponds to the phenyl ring flip and the segmental motion. Quasielastic incoherent measurements show that the very local process of methyl rotation is insensitive to the polymer glass transition temperature and exhibits a wavevector independent relaxation time and a low activation energy, whereas it is not affected at all by the confinement. At temperatures just above the calorimetric glass transition temperature, the observed motion is the phenyl ring motion, whereas the segmental motion is clearly identified for temperatures about 60K higher than the glass transition temperature. For the nanohybrid, the segmental motion is found to be strongly coupled to the motion of the surfactant chains for temperatures above the calorimetric glass transition temperature of the bulk polymer. However, the mean square displacement data show that the segmental motion in confinement is faster than that of the bulk polymer even after the contribution of the surfactant chains is taken into consideration.
Laterally pumped GaAs/AlGaAs quantum wells as sources of broadband terahertz radiation
Authors: Reeder Reeno , Ikonić Zoran , Harrison Paul , Udal Andres , Velmre Enn
Published: October 2007

In this work we consider lateral current pumped GaAs/AlGaAs quantum wells as sources of incoherent terahertz radiation. The lateral field heats the electrons in a two-dimensional quantum layer and increases the population of higher subbands, hence also increasing the radiation power generated in spontaneous intersubband emission processes. Digitally graded quasi-parabolic and simple square quantum wells are considered, and the advantages of both types are discussed. Calculations at lattice temperatures of 77 K and 300 K, for electric fields up to 10 kV/cm, show that the optical output power of ∼100−200 W/m2 may be achieved for the 7 THz source. The main peak of the spectrum, at 7 THz, of the quasi-parabolic quantum well exceeds the black body radiation at 300 K by approximately a factor of two and by two orders of magnitude at 77 K.
Spectroscopic and energy transfer studies of Eu3+ centers in GaN
Authors: Peng Hongying , Lee Chang-Won , Everitt Henry O. , Munasinghe Chanaka , Lee D. S. , Steckl Andrew J.
Published: October 2007

Photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved PL spectroscopies have been carried out at room temperature and 86K on transitions from D25, D15, and D05 excited states to numerous FJ7 ground states of Eu-doped GaN films grown by conventional solid-source molecular beam epitaxy (MBE) and interrupted growth epitaxy MBE. Within the visible spectral range of 1.8–2.7eV, 42 spectral features were observed and assignments were attempted for each transition. PL and PLE indicate that four Eu3+ centers exist in the GaN lattice whose relative concentration can be controlled by the duration of growth interruption. The energy levels for these four sites are self-consistently obtained, and time-resolved photoluminescence measurements reveal details about the radiative and nonradiative relaxations of excitation among these levels. The data indicate a near-resonant cross relaxation among these sites. The D25 and D15 states are observed to decay nonradiatively by filling the D05 state with characteristic times of 2.4 and 2.8μs, respectively. The D05 state is found to relax in a manner that depends slightly on the final state and dopant site.
Molecular dynamics simulation study of the ejection and transport of polymer molecules in matrix-assisted pulsed laser evaporation
Authors: Leveugle Elodie , Zhigilei Leonid V.
Published: October 2007

The physical mechanisms and molecular-level picture of laser-induced material ejection from frozen solutions of polymer molecules in a volatile matrix are investigated in a series of coarse-grained molecular dynamics simulations. The simulations are performed for polymer concentrations up to 6wt% and laser fluences covering the range from the regime where molecular ejection is limited to matrix evaporation from the surface up to more than twice the threshold fluence for the onset of the collective molecular ejection or ablation. The results of the simulations are related to experimental observations obtained in matrix-assisted pulsed laser evaporation (MAPLE) thin film depositions and are used to address unresolved research questions that are of direct relevance to MAPLE performance. Contrary to the original picture of the ejection and transport of individual polymer molecules in MAPLE, the simulations indicate that polymer molecules are only ejected in the ablation regime and are always incorporated into polymer-matrix clusters/droplets generated in the process of the explosive disintegration of the overheated matrix. The entanglement of the polymer molecules facilitates the formation of intricate elongated viscous droplets that can be related to the complex morphologies observed in polymer films deposited by MAPLE. Analysis of the state of the irradiated target reveals a substantial increase of the polymer concentration and complex surface morphology generated in the new surface region by the ablation process. The ramifications of the computational predictions for interpretation of experimental data and the directions for future experimental exploration are discussed based on the physical picture of molecular ejection and transport in MAPLE emerging from the simulations.
Intense 1540nm emission from Er doped Ce:YAG phosphor
Authors: Meng Jian Xin , Cheah Kok Wai , Shi Zhao Pu , Li Jin Qing
Published: October 2007

A very efficient energy transfer from Ce3+ to Er3+ in yttrium aluminum garnet (YAG) matrix is observed. The transfer gives about 1000 times enhancement to the Er3+ 1540nm emission. This is due to efficient absorption by the allowed 4f-5d transition of Ce3+ ions then transfer to Er3+ ions via a matched energy level F9∕24. Thus the 1540nm emission was strongly sensitized by exciting an allowed transition in an inorganic matrix. In fact, the 1540nm emission was further enhanced via the cross-relaxation process F5∕22→2F7∕2(Ce3+), I11∕24→4I13∕2(Er3+). The result means that a new class of material, Ce Er:YAG, was produced and it potentially can be an efficient near-IR emitter.
Dielectric properties of hydrogen-incorporated chemical vapor deposited diamond thin films
Authors: Liu Chao , Xiao Xingcheng , Wang Jian , Shi Bing , Adiga Vivekananda P. , Carpick Robert W. , Carlisle John A. , Auciello Orlando
Published: October 2007

Diamond thin films with a broad range of microstructures from a ultrananocrystalline diamond (UNCD) form developed at Argonne National Laboratory to a microcrystalline diamond (MCD) form have been grown with different hydrogen percentages in the Ar∕CH4 gas mixture used in the microwave plasma enhanced chemical vapor deposition (CVD) process. The dielectric properties of the CVD diamond thin films have been studied using impedance and dc measurements on metal-diamond-metal test structures. Close correlations have been observed between the hydrogen content in the bulk of the diamond films, measured by elastic recoil detection (ERD), and their electrical conductivity and capacitance-frequency (C-f) behaviors. Addition of hydrogen gas in the Ar∕CH4 gas mixture used to grow the diamond films appears to have two main effects depending on the film microstructure, namely, (a) in the UNCD films, hydrogen incorporates into the atomically abrupt grain boundaries satisfying sp2 carbon dangling bonds, resulting in increased resistivity, and (b) in MCD, atomic hydrogen produced in the plasma etches preferentially the graphitic phase codepositing with the diamond phase, resulting in the statistical survival and growth of large diamond grains and dominance of the diamond phase, and thus having significant impact on the dielectric properties of these films.
Influence of As4 flux on the growth kinetics, structure, and optical properties of InAs∕GaAs quantum dots
Authors: Garcia A. , Mateo C. M. , Defensor M. , Salvador A. , Hui H. K. , Boothroyd C. B. , Philpott E.
Published: October 2007

We report the effects of variations in As4 growth flux on the evolution of molecular beam epitaxy grown InAs quantum dots (QDs) and their structures and optical properties. For InAs QDs grown under As-stable conditions, evaluated through photoluminescence and atomic force microscopy (AFM) measurements, it is evident that QD size increases with As4 pressure along with improvement in size uniformity. Furthermore, transmission electron microscopy measurements for InAs layers of critical thicknesses (∼1.7 ML) showed decreasing QD density with increasing As4 pressure accompanied by a strong reduction in photoluminescence (PL) integral intensity. These show that high As4 fluxes suppress InAs QD formation while the decreasing PL intensity seems to indicate cluster formation that features nonradiative recombination. AFM measurements show larger and denser QDs for samples grown at higher As4 pressures. These are explained on the basis of adatom condensation during surface cooling and the influence of As4 pressure on indium incorporation.
Nonlinear hole transport and nonequilibrium thermodynamics in group III-nitrides under the influence of electric fields
Authors: Rodrigues Clóves G. , Vasconcellos Áurea R. , Luzzi Roberto
Published: October 2007

A theoretical study on the nonlinear transport of holes and of the nonequilibrium thermodynamic characteristics of p-doped wurtzite gallium nitride (GaN), aluminium nitride (AlN), and indium nitride (InN), under the influence of moderate to high electric fields, is presented. It is based on a nonlinear quantum kinetic theory which provides a description of the dissipative phenomena developing in the system. The ultrafast time evolution of the hole drift velocity and of the quasitemperatures of holes and longitudinal optical phonons are obtained. The steady state is analyzed by determining the dependence on the electric field of the nonequilibrium thermodynamic state and of the non-Ohmic mobility. A velocity overshoot is evidenced.
Physical models of magnetoimpedance
Authors: Ciureanu P. , Melo L. G. C. , Seddaoui D. , Ménard D. , Yelon A.
Published: October 2007

We recall the methods for the rigorous calculation of the electromagnetic behavior of magnetic metallic samples and their application to the modeling of ferromagnetic resonance and of giant magnetoimpedance experiments. We explain the effect of various approximations and simplifications, particularly of the neglect of the exchange-conductivity effect, which has been the subject of confusion and of misconceptions in the literature, as have questions of domain wall motion and of nonlinear behavior. We show that the rigorous treatment provides a satisfactory description of experimental results, while the simplifications can only do so under limited circumstances.
Structural modifications in InP nanostructures prepared by Ar+-ion irradiation
Authors: Mohanta S. K. , Soni R. K. , Gosvami N. N. , Vajpeyi A. P. , Tripathy S.
Published: October 2007

The evolution of nanopatterned InP surfaces by low-energy Ar+-ion irradiation and their dependence on incidence angle were investigated by field emission scanning electron microscopy, atomic force microscopy, and Raman scattering. Ordered surface nanodots of high aspect ratio were created. At large ion incidence angle the dot density decreases and the size varies in the range of 65–130nm with height of around 25–30nm. Rapid thermal annealing of the patterned surface shows cluster formation at annealing temperatures of 400°C and above, with some micro-cracks at ion incidence angle of 45°. With increasing ion incidence angle, the optical phonon Raman modes display systematic downward shift and large asymmetric broadening associated with increased contribution of disorder activated LO and TO modes from the patterned surface. The lowering of phonon frequencies, induced by the phonon wave function confinement, signifies the presence of embedded nanocrystallites in the large sized nanodot patterned surface. The surface damage recovery is achieved by rapid thermal annealing of the samples as reflected in the increased optical phonon frequencies and reduced line shape broadening with annealing temperature. For large ion incident angle, the strain relaxation in the irradiated surface region leads to micro-crack formation in the patterned surface and further hardening of the phonon frequencies.
Estimation of surface density of molecules adsorbed on a whispering gallery mode resonator: Utility of isotropic polarizability
Authors: Teraoka Iwao , Arnold Stephen
Published: October 2007

A protocol is presented to estimate the surface density and anisotropic polarizability of molecules adsorbed on the surface of a dielectric resonator of uniform refractive index. Measurement of resonance wavelength shift of transverse electric and transverse magnetic whispering gallery modes in the resonator gives the product of the surface density and the polarizability components normal and tangential to the resonator surface. The isotropic mean of the two products is independent of the molecular orientation, allowing us to estimate the surface density with the knowledge of the isotropic polarizability. If the adsorbate consists of anisotropic molecules, one can obtain information on the orientation of the molecules from the polarizability components. We also show that the effect of dipoles induced in nearby adsorbed molecules is in practice negligible up to the highest density in random sequential adsorption.
Fe-doped La0.5Sr0.5TiO3−δ nanoparticles: A diluted magnetic oxide system
Authors: Maensiri Santi , Wongsaprom Kwanruthai , Swatsitang Ekaphan , Seraphin Supapan
Published: October 2007

We report room-temperature ferromagnetism in ∼11.4–14.6nm nanoparticles of La0.5Sr0.5Ti0.985Fe0.015O3−δ. Nanoparticles of undoped and Fe-doped samples were synthesized by a polymerized complex method and were characterized using x-ray diffraction, transmission electron microscopy, and vibrating-sample magnetometer. The undoped samples exhibit a diamagnetic behavior, whereas the Fe-doped samples are ferromagnetic having the specific magnetizations of 0.041–0.101emu∕g at 10kOe. Our results indicate that the ferromagnetic property of La0.5Sr0.5Ti0.985Fe0.015O3−δ system is intrinsic and is not a result of any secondary magnetic phase or cluster formation. The finding of room-temperature ferromagnetism in this system would stimulate further interest in the area of diluted magnetic oxides.
Nanobuckling and x-ray photoelectron spectra of carbyne-rich tetrahedral carbon films deposited by femtosecond laser ablation at cryogenic temperatures
Authors: Hu A. , Griesing S. , Rybachuk M. , Lu Q.-B. , Duley W. W.
Published: October 2007

The growth, surface morphology, and electronic binding states of diamondlike films deposited by femtosecond laser ablation on Si wafers at 77 K have been studied in order to elucidate the mechanical properties of this material. Nanoscale buckling has been observed and is found to have a morphology that exhibits a strong dependence on film thickness. Nanobuckling takes the form of quasiperiodic discrete pointlike excursions extending over widths of 50–100 nm. This morphology converts to a regular structure of grooves/ripples with a modulation period of 30–50 nm as the film thickness increases to 300–600 nm. We find that microhardness is not changed in regions where nanobuckling is present. Analysis of Raman and x-ray photoelectron spectra (XPS) demonstrate that nanobuckling can be attributed to the relaxation of internal stress and to the formation of strong C-Si covalent bonds at the C-Si interface. XPS spectra show that the C 1s peak is broadened compared to that found in spectra of films deposited using nanosecond laser ablation. This is found to be consistent with a composition that includes sp, sp2, and sp3-bonded carbon. The unique composition of these films suggests that these materials may find application in electromechanical devices.
Cerium-doped barium halide scintillators for x-ray and γ-ray detections
Authors: Selling J. , Schweizer S. , Birowosuto M. D. , Dorenbos P.
Published: October 2007

Single crystals of Ce-activated BaCl2, BaBr2, and BaI2 were investigated under x-ray and γ-ray excitation. The Ce3+-related x-ray excited luminescence in BaBr2 shifts significantly to longer wavelengths upon increasing the doping level from 0.1% to 1%. In Ce-activated BaCl2 only a slight shift can be observed. BaI2 does not show any Ce3+-related emission. Scintillation decay time measurements show that the decay in BaBr2 is delayed due to migration processes. Additional K doping has a large influence on the scintillation decay time properties but not on the overall efficiency.
The effect of ion-surface and ion-bulk interactions during hydrogenated amorphous silicon deposition
Authors: Smets A. H. M. , Kessels W. M. M. , van de Sanden M. C. M.
Published: October 2007

The ion-bombardment induced surface and bulk processes during hydrogenated amorphous silicon (a-Si:H) deposition have been studied by employing an external rf substrate bias (ERFSB) in a remote Ar–H2–SiH4 expanding thermal plasma (ETP). The comparison of the ETP chemical vapor deposition without and with ERFSB enables us to identify some important ion-surface and ion-bulk interactions responsible for film property modifications. Employing ERFSB creates an additional growth flux and the low energetic ions deliver an extra 5–10eV per Si atom deposited at typical deposition rates of 10–42Å∕s which is a sufficient ion dose to modify the film growth. It is demonstrated that the extra surface and bulk process during a-Si:H growth, induced by the additional ion bombardment, provide an extra degree of freedom to manipulate the a-Si:H microstructure. An ion-film interaction diagram is introduced, which is used to discriminate ion-surface interactions from ion-bulk interactions. According to this ion-film interaction diagram, the a-Si:H grown with ERFSB can be roughly classified in three phases. In phase I the only ion-surface process activated is Si surface atom displacement. In phase II also ion-induced Si bulk atom displacement is sufficiently activated, whereas in phase III ion-induced Si atom sputtering is significant. Phase I is characterized by a reduction in the nanosized void density, a reduction in defect density, and an improvement of the photoresponse. We find that the Si surface displacement is the process responsible for various improvements of the material properties via the enhanced surface migration. Phase II is characterized by an enhancement of vacancy incorporation. In accordance with the introduced ion-film interaction diagram, the Si atom bulk displacement process is responsible for the incorporation of additional vacancies. Phase III is characterized by the decrease in growth flux and the increase in void density. The significant contribution of ion-sputtering processes is responsible for the effects observed in phase III.
Decoupling of silicon carbide optical sensor response for temperature and pressure measurements
Authors: Chakravarty A. , Quick N. R. , Kar A.
Published: October 2007

Single crystal silicon carbide is a chemically inert transparent material with superior oxidation-resistant properties at elevated temperatures compared to black polycrystalline silicon carbide substrates. These improved properties make crystalline silicon carbide a good optical sensor material for harsh environments such as combustion chambers and turbine systems. Interferometric optical sensors are orders of magnitude more sensitive than electrical sensors and are proposed for these applications. Silicon carbide itself behaves as a Fabry-Pérot etalon eliminating the need for an external interferometer for any measurement using this silicon carbide as a sensor. The principle of the optical sensor in this study is the temperature- and pressure-dependent refractive index of silicon carbide, which can be used to determine the temperatures and pressures of gases that are in contact with silicon carbide. Interference patterns produced by a silicon carbide (4H-SiC) wafer due to multiple reflections of a helium-neon laser beam of wavelength of 632.8nm have been obtained at temperatures up to 500°C and pressures up to 600psi. The pattern changes for the same gas at different temperatures and pressures and for different gases at the same temperature and pressure. The refractive index at the wafer-gas interface is calculated from the interference pattern and the refractive index gradients with respect to temperature and pressure, respectively, are also determined. Decoupling temperature and pressure using these gradients and the measured reflectivity data are discussed in this paper.
Computation of the dynamic thermal dissipation properties of porous media by Brownian motion simulation: Application to an open-cell aluminum foam
Authors: Perrot Camille , Panneton Raymond , Olny Xavier
Published: October 2007

This paper reports simulation results of frequency dependent heat conduction through three-dimensional reconstructed unit cells of an open-cell aluminum foam under acoustic excitations. First, a three-dimensional random walk based algorithm is proposed to calculate the dynamic thermal permeability or dynamic bulk modulus of periodic complex porous geometries. Second, the error and convergence of the implemented calculation algorithm are quantified in terms of the random walk population, normalized trapping distance, and type of geometry. Finally, the algorithm is applied to the calculation of the dynamic bulk modulus of an aluminum foam and compared to laboratory measurements. Good agreement is obtained between simulations and measurements.
X-ray diffraction studies of the growth of vanadium dioxide nanoparticles
Authors: Pauli S. A. , Herger R. , Willmott P. R. , Donev E. U. , Suh J. Y. , Haglund R. F.
Published: October 2007

We have characterized for the first time in situ the growth of vanadium dioxide nanoparticulate films prepared by pulsed-laser deposition, using a five-circle x-ray diffractometer, in order to provide structural information as the films are grown. A vanadium metal target was ablated in the presence of a pulsed N2O reactive gas source, and films were grown on Si(001) and Al2O3(0001) substrates. Optical measurements confirmed that the films deposited in this way exhibit the well-known VO2 metal–insulator transition at approximately 70 °C. The VO2 films grown at room temperature on silicon substrates are amorphous and extremely smooth. These become considerably rougher upon thermal annealing, as the VO2 phase crystallizes out in the form of hemispherical islands. These films also contain traces of a V2O5 nanoparticle phase in the first few monolayers, although the degree of VO2 crystallinity in the nanoparticles is quite high. In contrast, the VO2 nanoparticles grown on the sapphire substrates are in the form of preferentially oriented rodlike islands, with a maximum rod width of about 100 nm. This in-plane texture is in keeping with the relatively good lattice match between film and substrate, indicating good potential for the growth of VO2 epitaxial films and single-crystal nanoparticles on such substrates.
Density functional theory study of β-hydride elimination of ethyl on flat and stepped Cu surfaces
Authors: Li Xin , Gellman Andrew J. , Sholl David S.
Published: October 2007

Plane wave density functional theory calculations have been used to characterize the transition states for β-hydride elimination of ethyl on Cu(100), Cu(110), Cu(111), and Cu(221). The reaction rates predicted by these calculations have been compared to experiments by including tunneling corrections within harmonic transition state theory. Tunneling corrections are found to be important in describing the peak temperatures observed using temperature programed desorption experiments on Cu(110), Cu(111), and Cu(221). Once these corrections are included, the effective activation energies obtained from our calculations are in good agreement with previous experimental studies of this reaction on these four Cu surfaces. The transition states determined in our calculations are used to examine two general hypotheses that have been suggested to describe structure sensitivity in metal-catalyzed surface reactions.
Time-resolved dynamic compaction and tensile fracture of low-porosity aluminum under impact loading
Authors: Wang Yonggang , He Hongliang , Qi Meilan , Shen Liang , Bai Bin
Published: October 2007

Unlike the solid dense medium, the low-porosity metals exhibit many unique dynamic behaviors. These properties have been investigated in this work for aluminum with a porosity of 3.3% under high velocity and planar plate impact. A push-pull-type velocity interferometer system for any reflector was used to measure the rear free surface velocity profiles of the samples. Time-resolved dynamic compaction and tensile fracture properties are presented. Results demonstrated that there are three distinct shock compressive waves in succession, including elastic wave, compaction wave, and stable shock wave. The dynamic tensile fracture occurs after the porosity being fully compacted and the stable shock wave being formed. With the increase of impact velocity, the dynamic tensile strength becomes higher and the spalled layer becomes thicker. Nevertheless, the dynamic tensile strength of the shock compacted aluminum is apparently lower than that of the solid dense aluminum, and is approaching that of the aluminum alloy 6061-T6. Physical explanations are illustrated and discussed.