bluuebunny/crossref_metadata_2025 huggingface dataset:
The role of nuclear reactions and α-particle transport in the dynamics of inertial confinement fusion capsules
Authors: Garnier Josselin , Cherfils-Clérouin Catherine
Published: October 2008

This paper is devoted to the study of the deceleration phase of inertial confinement capsules. The purpose is to obtain a zero-dimensional model that has the form of a closed system of ordinary differential equations for the main hydrodynamic quantities. The model takes into account the energy released by nuclear reactions, a nonlocal model for the α-particle energy deposition process, and radiation loss by electron bremsstrahlung. The asymptotic analysis is performed in the case of a strong temperature dependence of the thermal conductivity. We finally study the beginning of the expansion phase after stagnation to derive an ignition criterion.
Heteroborane analogs of silicon clusters: Experimental and theoretical studies on Bi2Si5 and Bi2Si5−
Authors: Li Xiang , Wang Haopeng , Grubisic Andrej , Wang Di , Bowen Kit H. , Jackson Miley , Kiran Boggavarapu
Published: October 2008

We have investigated the electronic structure of anionic and neutral Bi2Si5 by means of anion photoelectron spectroscopy and density functional calculations. Both the experiments and calculations reveal that the Bi2Si5− anion prefers to adopt a distorted trigonal-bipyramidal structure with Bi2 bridges. Following the isolobal analogy between divalent Si and B–H group, we show that both neutral Bi2Si5 and neutral Bi2B5H5 adopt similar pentagonal-bipyrmidal geometries and have analogous orbital energy patterns.
Charged particle acceleration in dense plasma channels
Authors: Dodin I. Y. , Fisch N. J.
Published: October 2008

Reduced nonlinear equations are derived from the oscillation amplitude and the energy of a charged particle accelerated in a plasma channel. The maximum energy gain, as limited by dissipation, is described by three different scalings depending on the channel parameters.
Stability of ternary and binary mixtures in a vertical slot including the Soret effect
Authors: Zebib Abdelfattah
Published: October 2008

Convective instabilities in a vertical slot containing a single fluid are longitudinal if side heated and transverse in the presence of an adverse density gradient. Here we examine the influence of the concentration field generated by thermodiffusion in ternary and binary mixtures of otherwise uniform concentrations on these phenomena at parameter values of available experiments. In a binary mixture, a lighter component with positive separation ratio ε migrates to the hot wall, which augments thermal buoyancy enhancing instability. It is also convected upwards and the resulting stable stratification enhances stability. The preferred instability in this case is the longitudinal mode. When ε<0 thermal buoyancy is opposed, unstable stratification is created, and long unstable transverse modes prevail. This is also generally true for a ternary mixture when both the separation ratios of the two lighter components are negative or positive, and either scenario may hold when they assume opposite signs, depending on four diffusion and two concentration expansion coefficients.
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electronic spectrum of NCCN+ in the gas phase
Authors: Rudnev V. , Rice C. A. , Maier J. P.
Published: October 2008

The BΣu+2←XΠg2 absorption spectrum of NCCN+ in the gas-phase was observed using a two-color, two-photon photodissociation technique. This was measured at ∼20K in a 22-pole ion trap with laser bandwidths of less than a cm−1. The spectrum shows distinct vibrational structure, with the origin band near 11253cm−1, and the excitation of four normal modes in the excited state. The rotational structure of the 000 band in the gas phase could not be resolved, indicating that the BΣu+2 state has a lifetime of a few picoseconds because of a fast intramolecular process.
Analysis of (110) indium arsenide–gallium antimonide superlattices for infrared detection
Authors: Szmulowicz F. , Haugan H. J. , Brown G. J.
Published: October 2008

This paper presents an analysis of indium arsenide–gallium antimonide (InAs/GaSb) (110)-grown superlattices for use as infrared detectors. The analysis shows that InAs/GaSb superlattices grown on (110)-oriented substrates can be grown thinner, have higher mobilities, longer diffusion lengths, greater quantum efficiencies, and gains, thus higher responsivities, than those grown on (100)-oriented substrates for the same long wavelength threshold. Based on growth studies to date, (110) superlattices should also have higher minority carrier lifetimes, lower noise, and lower residual carrier densities, thus higher detectivities. The calculated electronic structure of a (110)-oriented superlattice based on the 8×8 envelope-function approximation shows the bands to be anisotropic and the oscillator strengths to be polarization dependent. Recommended layer widths for specific absorption thresholds are calculated.
Far-field sheaths due to fast waves incident on material boundaries
Authors: D’Ippolito D. A. , Myra J. R. , Jaeger E. F. , Berry L. A.
Published: October 2008

The problem of “far-field” sheath formation is studied with a new quantitative one-dimensional model. These radio-frequency (rf) sheaths occur when unabsorbed fast waves in the ion cyclotron range of frequencies are incident on a conducting surface not aligned with a flux surface. Use of a nonlinear sheath boundary condition gives self-consistent solutions for the wave fields and sheath characteristics, and it introduces a sheath-plasma-wave resonance which can enhance the sheath potential. The model is used to compute the parametric dependence of the far-field sheath potential. Its application to post-process the rf fields computed by a full-wave code for a typical D(H) minority heating scenario is also discussed. This work shows that two-dimensional effects (included heuristically) are essential in determining whether far-field sheath potentials are strong enough to cause significant edge interactions, such as impurity generation and reduced heating efficiency.
A time-resolved particle image velocimetry investigation of a cavity flow with a thick incoming turbulent boundary layer
Authors: Haigermoser C. , Vesely L. , Novara M. , Onorato M.
Published: October 2008

The present paper aims for a better understanding of the flow in a rectangular cavity at very low Mach number and with a thick incoming turbulent boundary layer. In the present experiment the boundary layer is defined as thick because the ratio of the cavity length to the momentum thickness Θ is low (L/Θ=18) with respect to the claimed threshold (L/Θ>80) for cavity self-sustained oscillations to take place. Time-resolved particle image velocimetry (PIV) measurements were taken in planes normal and parallel to the wall over a cavity having aspect ratio L/H=4, at ReΘ=1041. The PIV data were analyzed to give the mean characteristics and the time behavior of the flow. The irregular vortex shedding observed in the shear layer was analyzed and interpreted with reference to the coherent structures in the incoming turbulent boundary layer. The unsteady load on the cavity walls and the consequent oscillating drag forces were studied by analyzing the instant momentum flux across the cavity mouth. A low frequency mode of about 2 Hz, corresponding to a Strouhal number based on the cavity length of StL=0.2, was revealed. This oscillation mode is due to the build up of the flow inside the cavity and not to the vortex shedding in the shear layer.
Magnetic design calculation and FRC formation modeling for the field reversed experiment liner
Authors: Dorf L. A. , Intrator T. P. , Awe T. , Renneke R. , Hsu S. C. , Wurden G. A. , Siemon R. , Semenov V. E.
Published: October 2008

Integrated magnetic modeling and design are important to meet the requirements for (1) formation, (2) translation, and (3) compression of a field reversed configuration (FRC) for magnetized target fusion. Off-the-shelf solutions do not exist for many generic design issues. A predictive capability for time-dependent magnetic diffusion in realistically complicated geometry is essential in designing the experiment. An eddy-current code was developed and used to compute the mutual inductances between driven magnetic coils and passive magnetic shields (flux excluder plates) to calculate the self-consistent axisymmetric magnetic fields during the first two stages. The plasma in the formation stage was modeled as an immobile solid cylinder with selectable constant resistivity and magnetic flux that was free to readjust itself. It was concluded that (1) use of experimentally obtained anomalously large plasma resistivity in magnetic diffusion simulations is sufficient to predict magnetic reconnection and FRC formation, (2) comparison of predicted and experimentally observed timescales for FRC Ohmic decay shows good agreement, and (3) for the typical range of resistivities, the magnetic null radius decay rate scales linearly with resistivity. The last result can be used to predict the rate of change in magnetic flux outside of the separatrix (equal to the back-emf loop voltage), and thus estimate a minimum θ-coil loop voltage required to form an FRC.
Effect of the sp2 carbon phase on n-type conduction in nanodiamond films
Authors: Ikeda Tomohiro , Teii Kungen , Casiraghi C. , Robertson J. , Ferrari A. C.
Published: October 2008

Structural and electrical conduction properties of nitrogen-doped nanocrystalline diamond films are studied as a function of deposition temperature (TD) in a microwave Ar-rich/CH4 plasma with 30%N2 addition. Hall- and Seebeck-effect measurements confirm n-type conduction for TD above 1100 K. For TD from 1100 and 1220 K, the electron concentration increases up to 1020 cm−3 and the electron mobility is in the range of 4–8 cm2 V−1 s−1. For TD above 1250 K, the mobility decreases to ∼1 cm2 V−1 s−1. Low conductivity films deposited at low TD exhibit semiconductorlike thermal activation in the Arrhenius plots, while high conductivity films deposited at high TD are almost temperature independent, indicative of quasimetallic conduction. The nitrogen concentration in the films is about 0.3 at. %, independent of TD. As TD is increased, the sp2 content and order increase. This is responsible for the appearance of midgap states, their delocalization, and the larger distance between diamond grains. The high conductivity at high TD is due to the amount and crystallinity of sp2 carbon, rather than the nitrogen concentration.
Evaluations of optical properties of Co based soft x-ray multilayer gratings
Authors: Ishino Masahiko , Koike Masato , Satou Futami , Terauchi Masami , Sano Kazuo , Sasai Hiroyuki
Published: October 2008

The multilayer gratings were fabricated by depositing the Co/Si and Co/SiO2 multilayers onto the surface of laminar-type holographic gratings having shallow grooves. The structures of these multilayer gratings observed by a transmission electron microscope were the well defined structures without serious defects. The structural property evaluated by comparing the measured and calculated diffraction conditions, i.e., incidence and diffraction angles, resulted that the diffraction conditions of multilayer gratings were affected by the refractive indices of multilayer coatings even at the photon energy of 8.05 keV. The measured low diffraction efficiencies of the Co/Si multilayer grating in the photon energy range of 1–2 keV can be attributed to the large interdiffusion in the Co/Si multilayer coating.
Heterostructure interface roughness characterization by chemical mapping: Application to InGaP/GaAs quantum wells
Authors: Tizei L. H. G. , Bettini J. , Carvalho M. M. G. , Ugarte D.
Published: October 2008

Interface quality is an important factor for the functionality of semiconductor modern devices. Routinely, these characteristics are probed qualitatively by photoluminescence. However, quantitative microscopic structural information to corroborate models is not commonly available. Among different techniques, atomic resolution transmission electron microscopy images represent the basic experimental method to analyze the quality of buried interfaces. In this work we describe the analysis of chemical changes determined from the quantitative comparison of intensity distribution across an interface in high resolution transmission electronic microscopy (HRTEM) images. We have shown that a careful analysis of HRTEM images contrast can provide extremely useful quantitative information on interface roughness. We have characterized four different samples of InGaP/GaAs quantum wells grown with different interfacial schemes. Limits in the quantification from different sources, such as sample preparation, sampling, and statistics, have been thoroughly analyzed.
Gate dielectric scaling of top gate carbon nanoribbon on insulator transistors
Authors: Alam Khairul
Published: October 2008

The effects of gate dielectric constant and thickness on the performance of top gate carbon nanoribbon on insulator transistors are studied using a π-orbital quantum simulation model. The focus is on both the zero Schottky barrier (SB) source-drain contacts and the metal-oxide-semiconductor field effect transistor (MOSFET)-like doped source-drain contacts. The gate dielectric constant has little effect on the on/off current ratio, channel transconductance gm, and switching performance in SB contact devices. However, the on/off current ratio, the channel transconductance, and the switching performance significantly improve with high-K gate dielectric in doped contact devices. The physics is related to the modulation of the tunnel barrier. In SB contact devices, the on-state current is limited by the SB, and therefore, the improvement in on/off current ratio and transconductance is insignificant. However, the tunnel barrier modulation in the subthreshold regime is similar in both types of contact and the inverse subthreshold slope has similar improvement with high-K gate dielectrics. The unity current gain frequency (fT=gm/2πCg) degrades with high-K gate dielectric in SB contact devices and improves in doped contact devices. This is because the gate capacitance does not change much with dielectric constant and gm has a significant improvement in doped contact devices. The device performance improves with thinner gate oxide. The on/off current ratio and the inverse subthreshold slope scale as square root of oxide thickness.
Characterization of directly coupled dual-superconducting quantum interference device magnetometer
Authors: Wu C. H. , Chen J. H. , Jhan F. J. , Jeng J. T. , Yang H. C.
Published: October 2008

The effect on effective area and voltage modulation depth of a serial superconducting quantum interference device (SQUID) magnetometer was simulated and observed. The variation in the effective area with the number N of serial SQUIDs was simulated. To reduce the beat phenomenon and optimize the serial SQUID magnetometer, the distance x between two adjacent SQUIDs should exceed 40 μm or even 65 μm, or dummy SQUIDs should be added at the edge of the serial SQUIDs. The optimal layout of a high-Tc serial SQUID array is discussed.
An interatomic potential model for molecular dynamics simulation of silicon etching by Br+-containing plasmas
Authors: Ohta H. , Iwakawa A. , Eriguchi K. , Ono K.
Published: October 2008

An interatomic potential model for Si–Br systems has been developed for performing classical molecular dynamics (MD) simulations. This model enables us to simulate atomic-scale reaction dynamics during Si etching processes by Br+-containing plasmas such as HBr and Br2 plasmas, which are frequently utilized in state-of-the-art techniques for the fabrication of semiconductor devices. Our potential form is based on the well-known Stillinger–Weber potential function, and the model parameters were systematically determined from a database of potential energies obtained from ab initio quantum-chemical calculations using GAUSSIAN03. For parameter fitting, we propose an improved linear scheme that does not require any complicated nonlinear fitting as that in previous studies [H. Ohta and S. Hamaguchi, J. Chem. Phys. 115, 6679 (2001)]. In this paper, we present the potential derivation and simulation results of bombardment of a Si(100) surface using a monoenergetic Br+ beam.
High field dielectric properties of anisotropic polymer-ceramic composites
Authors: Tomer V. , Randall C. A.
Published: October 2008

Using dielectrophoretic assembly, we create anisotropic composites of BaTiO3 particles in a silicone elastomer thermoset polymer. We study a variety of electrical properties in these composites, i.e., permittivity, dielectric breakdown, and energy density as function of ceramic volume fraction and connectivity. The recoverable energy density of these electric-field-structured composites is found to be highly dependent on the anisotropy present in the system. Our results indicate that x-y-aligned composites exhibit higher breakdown strengths along with large recoverable energy densities when compared to 0-3 composites. This demonstrates that engineered anisotropy can be employed to control dielectric breakdown strengths and nonlinear conduction at high fields in heterogeneous systems. Consequently, manipulation of anisotropy in high-field dielectric properties can be exploited for the development of high energy density polymer-ceramic systems.
Electrodynamic enhancement of film cooling of turbine blades
Authors: Wang Chin-Cheng , Roy Subrata
Published: October 2008

Three concepts are numerically investigated to promote lateral mixing of the cold jets and to ensure their better attachment to the surface. First, we introduce electrodynamically enhanced interaction of cool air jets with hot crossflow for improved cooling of hot surfaces. We identify mechanisms to “push” or “pull” the essentially stagnant fluid just downstream of the hole by enforcing an active pressure pulsation in streamwise and crosswise directions. Such method utilizes electrodynamic force that induces attachment of cold jet to the work surface by actively altering the body force in the vicinity using a plasma actuator for different cooling hole geometries. Second, we employ a negative pressure region just downstream of the cooling hole. This may be generated by utilizing a suction vent or other mechanisms. Third, we propose three geometric modifications of the cooling hole exit for enhancing lateral tripping of the cold jet. Detailed computation of a single row of 35 degree round holes on a flat plate has been obtained for a select blowing ratio of 1.0 with a density ratio of 2.0. Results are compared with the published experimental results and other numerical predictions for the latest film cooling technology to identify effectiveness improvement. We have shown that a combination of plasma and geometric change can significantly improve the film cooling performance.
Unsteady contact melting of a rectangular cross-section material on a flat plate
Authors: Myers T. G. , Mitchell S. L. , Muchatibaya G.
Published: October 2008

The work in this paper concerns a mathematical model of the contact melting process of a rectangular material in contact with a hot plate. The problem is described by a coupled system of heat equations in the solid and melt layer, fluid flow in the melt, a Stefan condition at the melt interface, and a force balance between the weight of the solid and the fluid pressure. Since the melt layer remains thin throughout the process, we use the lubrication approximation to the fluid equations and assume that the heat flow in the fluid is dominated by conduction across the thin film. In the solid we employ a heat balance integral method. Results show that the film height has initial and final rapid increases, whereas for intermediate times the height slowly increases. The quasisteady state of previous models is never attained: This is shown to be an effect of neglecting the change in mass and conduction in the solid. The previously observed initial infinite velocity of the melt is shown to be a result of the perfect thermal contact assumption. For a water-ice system the melting rate is shown to be approximately linear, this allows us to reduce the problem to solving a single first order differential equation for the liquid layer thickness. The main analysis is carried out in two dimensions, but we briefly highlight the extension to three dimensions. The method is verified by comparison with previously published experimental results on the melting of n-octadecane.
Boundary-layer theory for Taconis oscillations in a helium-filled tube
Authors: Sugimoto N. , Shimizu D.
Published: October 2008

This paper develops a one-dimensional formulation to simulate Taconis oscillations in a helium-filled, quarter-wavelength tube in cryogenics within a framework of the boundary-layer theory. Dividing an acoustic field in the tube into a boundary layer on the wall and a main-flow region outside of it, the fluid-dynamical equations are averaged over the whole cross section of the tube, from which the equations averaged over the main-flow region are derived by using the boundary-layer solutions. Nonlinear theory is employed for the main-flow region, whereas the boundary layer is assumed to be described by the linear theory. Resultant equations for the main-flow region are posed in the form of integrodifferential equations due to memory effects by the boundary layer. An initial- and boundary-value problem is solved numerically for the evolution of a small disturbance. It is demonstrated that for temperature distribution of a smooth, step function, a transient behavior leading to emergence of self-excited Taconis oscillations can be simulated numerically. Although the first-order theory in the boundary-layer thickness has been regarded as being incapable of describing Taconis oscillations, it turns out to be applicable to a case with plausible temperature gradient.
Effects of charge carrier trapping on polycrystalline PbO x-ray imaging detectors
Authors: Kabir M. Z.
Published: October 2008

The effects of charge carrier trapping on the x-ray sensitivity, resolution, and detective quantum efficiency (DQE) in polycrystalline lead oxide (PbO) x-ray imaging detectors are theoretically analyzed. The theoretical models for calculating carrier trapping-limited x-ray sensitivity and modulation transfer function (MTF), and DQE incorporating polyenergetic x-ray spectrum are described. A cascaded linear system model is developed for calculating the spatial frequency f dependent DQE of PbO detectors by incorporating the effects of bulk charge carrier trapping on the MTF and the x-ray interaction depth dependent charge collection efficiency and noise. The theoretical calculations are compared with the published experimental data and show a very good agreement. From the fittings of the sensitivity and MTF curves, the electron and hole ranges in polycrystalline PbO are found to be 3.5×10−7 and ∼10−8 cm2/V, respectively. The f dependent noise power spectrum NPS(f) and DQE(f) performances are analyzed as a function of applied electric field and carrier lifetimes. The calculation shows that the carrier trapping is responsible for a 21% reduction in the MTF at the applied field of 1.0 V/μm at the Nyquist frequency (fN) and the aliased NPS at fN is reduced to 63% of that at zero spatial frequency. The sensitivity, MTF, NPS(f), and DQE(f) strongly depend on the applied electric field and carrier ranges. It is found that the DQE(f) for fluoroscopic applications can be improved dramatically by simply enhancing the carrier lifetimes through improving material properties of the PbO sample.
Quantification of the bond-angle dispersion by Raman spectroscopy and the strain energy of amorphous silicon
Authors: Roura P. , Farjas J. , Roca i Cabarrocas P.
Published: October 2008

A thorough critical analysis of the theoretical relationships between the bond-angle dispersion in a-Si,Δθ, and the width of the transverse optical Raman peak, Γ, is presented. It is shown that the discrepancies between them are drastically reduced when unified definitions for Δθ and Γ are used. This reduced dispersion in the predicted values of Δθ together with the broad agreement with the scarce direct determinations of Δθ is then used to analyze the strain energy in partially relaxed pure a-Si. It is concluded that defect annihilation does not contribute appreciably to the reduction of the a-Si energy during structural relaxation. In contrast, it can account for half of the crystallization energy, which can be as low as 7 kJ/mol in defect-free a-Si.
The sintering temperature effects on the electrical and dielectric properties of Li0.05Ti0.02Ni0.93O ceramics prepared by a direct thermal decomposition method
Authors: Thongbai Prasit , Yamwong Teerapon , Maensiri Santi
Published: October 2008

We reported the effects of grain size on high dielectric and related electrical properties of Li0.05Ti0.02Ni0.93O (LTNO) ceramics, which were prepared by a direct thermal decomposition method. The analysis of complex impedance indicated that these LTNO ceramics were electrically heterogeneous consisting of conducting grains and insulating grain boundaries (GBs). Interestingly, our results revealed that the dielectric permittivity (ε′) increases with the increase in grain size, which can be well described by Maxwell–Wagner relaxation model. Furthermore, we also found that the activation energy required for relaxation process (Ea) and related activation energy of the conductivity in the grain interior (Eg) decreased with the increase in grain size. These results suggested that the different microstructures resulted in chemical change (e.g., oxygen vacancies) inside the grains, leading to the changes in electrical properties of the LTNO ceramics.
Thermoelectric properties of polygrained icosahedral Al71−xGaxPd20Mn9 (x=,2,3,4) quasicrystals
Authors: Takagiwa Y. , Kamimura T. , Hosoi S. , Okada J. T. , Kimura K.
Published: October 2008

The electrical and thermal transport properties of quaternary icosahedral Al71−xGaxPd20Mn9 (x=0,2,3,4) quasicrystals, obtained by replacing Al in icosahedral Al–Pd–Mn quasicrystal with Ga, have been measured in accordance with the guiding principle of “weakly bonded rigid heavy clusters” (WBRHCs). While the electrical conductivity and Seebeck coefficient were not dramatically changed, the thermal conductivity effectively decreased with increasing Ga concentration except for the sample with x=4. Although the thermoelectric properties do not obey the WBRHCs, the dimensionless figure of merit increased by a factor of 1.4 from 0.18 for Al71Pd20Mn9 to 0.26 for Al68Ga3Pd20Mn9 quasicrystal.
Comment on “Nonlinear properties of small amplitude dust ion acoustic solitary waves” [Phys. Plasmas 7, 3594 (2000)]
Authors: Duha S. S. , Mamun A. A.
Published: October 2008

The aim of this comment is to show how the model equations used by Ghosh et al. [Phys. Plasmas 7, 3594 (2000)] are completely inconsistent, and to provide a guideline for a consistent dusty plasma model which is appropriate for the study of the nonlinear properties of the dust ion acoustic solitary waves.
Effect of the transverse nonuniformity of the radiofrequency field on the start current and efficiency of gyrodevices with confocal mirrors
Authors: Nusinovich Gregory S. , Chainani Samir , Granatstein Victor L.
Published: October 2008

The theory is developed for analyzing the effect of transverse nonuniformity of the radiofrequency (rf) field on the starting conditions and efficiency of such gyrotron oscillators as gyromonotrons and gyro-backward-wave oscillators (gyro-BWO). The formalism allows one to study this effect in oscillators operating in the regimes of soft and hard self-excitation. Results obtained for a device with a confocal waveguide (or resonator) are compared with the results for conventional gyrodevices where the rf field acting on electrons with different guiding centers is the same. It is shown how to use results of the classical small-signal theory of backward-wave oscillators driven by linear electron beams for calculating the start currents in gyro-BWOs. The effect of the wave attenuation in waveguide walls on the start current is analyzed, which is important for the design of frequency-tunable gyro-backward-wave oscillators in the THz (and sub THz) frequency range.
10-fold detection range increase in quadrant-photodiode position sensing for photonic force microscope
Authors: Perrone Sandro , Volpe Giovanni , Petrov Dmitri
Published: October 2008

We propose a technique that permits one to increase by one order of magnitude the detection range of position sensing for the photonic force microscope with quadrant photodetectors (QPDs). This technique takes advantage of the unavoidable cross-talk between output signals of the QPD and does not assume that the output signals are linear in the probe displacement. We demonstrate the increase in the detection range from 150 to 1400 nm for a trapped polystyrene sphere with radius of 300 nm as probe.
Enhanced high-gain harmonic generation for x-ray free-electron laser
Authors: Jia Qika
Published: October 2008

The harmonic generation free-electron laser (HG-FEL) is capable of producing temporally and spatially coherent pulses in the vacuum ultraviolet and shorter wavelength regions. In this letter an easy-to-implement scheme that significantly enhances the performance of HG-FEL and extends its short-wavelength range is proposed. By adding a short phase-reversed modulator after the dispersive section, the electron energy spread is reduced and simultaneously the electron microbunching is enhanced. Three-dimensional simulations demonstrate the energy spread reduction and the bunching enhancement and show that more powerful higher harmonic radiation can be obtained.
Melting transition in a two-dimensional complex plasma heated by driven acoustic instability
Authors: Sheridan T. E.
Published: October 2008

The melting transition in a two-dimensional complex (dusty) plasma is studied experimentally. A system consisting of ≈3900 microspheres is heated by amplitude modulating the rf discharge power with a square wave at the vertical resonance frequency. The vertical motion couples to an in-plane dust-acoustic instability at one-half the modulation frequency, thereby increasing the complex plasma’s effective temperature. The “thermodynamic” phase of the system is characterized for increasing levels of amplitude modulation at constant neutral pressure (35mTorr Ar) and average rf power using the Lindemann ratio, defect density, bond-orientational correlation function, and pair correlation function. A melting transition showing evidence for an intermediate hexatic phase is observed.
Local control of spin polarization in a semiconductor by microscale current loops
Authors: Chen Y. S. , Halm S. , Neshataeva E. , Kümmell T. , Bacher G. , Wiater M. , Wojtowicz T. , Karczewski G.
Published: October 2008

We demonstrate a method to electrically manipulate the spin polarization in a semiconductor on a micrometer length scale and a submicrosecond time scale. A variable magnetic field induced by a microscale current loop magnetizes the Mn2+ ions in a CdMnTe/CdMgTe diluted magnetic semiconductor quantum well, and via sp-d exchange interaction polarizes photogenerated electron-hole pairs. A maximum spin polarization degree of ±8.5% is obtained at 4.2 K without external magnetic field. The induced carrier spin polarization and the thermal heating of Mn2+ spins are resolved spatially and temporally by microphotoluminescence measurements.
Approaches for achieving highly efficient exciplex-based organic light-emitting devices
Authors: Lai S. L. , Chan M. Y. , Tong Q. X. , Fung M. K. , Wang P. F. , Lee C. S. , Lee S. T.
Published: October 2008

We studied the performance of exciplex-based organic light-emitting devices (OLEDs) made of different electron transporting materials (ETMs) with similar electron affinities to minimize the effect of the lowest unoccupied molecular orbital levels. A strong correlation was observed between the intensity of exciplex emission and the choice of ETMs. The intensity of exciplex emission relied on interfacial charge accumulation densities at organic/organic contacts, which in turn determined device color and efficiency. Contrary to common belief, highly efficient exciplex-based OLEDs can be achieved, provided that the involved organic materials have high carrier mobility, high photoluminescence quantum yield, and suitable electron energy levels.
Spatial distribution of manganese and room temperature ferromagnetism in manganese-doped ZnO nanorods
Authors: Yan H. L. , Wang J. B. , Zhong X. L. , Zhou Y. C.
Published: October 2008

Monochromatic cathodoluminescence image of Mn-doped ZnO nanorods shows that Mn elements are mainly distributed on the surface of nanorods. The intensity of ultraviolet luminescence decreases with the increment in Mn-doped concentration. Mn-doped ZnO nanorods exhibit a room temperature ferromagnetic characteristic with a saturation magnetization (MS) of 0.39 μB/Mn and a coercive field of 50 Oe.
Measurements and calculations of two-dimensional band gap structure composed of narrowly slit tubes
Authors: Cui Zhan You , Chen Tian Ning , Wu Jiu Hui , Chen Hua Ling , Zhang Bo
Published: October 2008

In this letter, large band gap properties of a two-dimensional stop band gap system composed of steel tubes are reported, which are embedded with periodic narrow slits in each half of the annular tube. The band gap with finite thickness samples is experimentally investigated and theoretically calculated. The results show that without enlarging the lattice constant, the starting frequency of the band gap is reduced, and using the periodic narrow slits with multiwidth can enlarge the stop band of this system due to the multicavity resonance.
On the appearance of Moffatt eddies in viscous cavity flow as the aspect ratio varies
Authors: Heaton C. J.
Published: October 2008

Two-dimensional Stokes flow in a rectangular driven cavity is studied. Flow in very tall or shallow cavities has many counter-rotating eddies lying along the cavity centerline. This structure is investigated by constructing asymptotic approximations to the flow based on the assumptions A⪡1 and A⪢1, where A is the cavity’s aspect ratio. We show that the number of eddies increases as A tends to infinity or zero and derive asymptotic formulas for the values of the aspect ratio at which the streamline topology bifurcates and a new eddy appears. We concentrate particularly on flow driven by translation of the top and bottom cavity walls with equal and opposite velocities. For this benchmark problem our asymptotics are able to connect existing computations (performed in the approximate range of 10−1≤A≤101) with Moffatt’s 1964 theory for an infinite channel (i.e., A=0 or ∞). We show that tall cavities have sequences of eddies which match the infinite channel flow, as observed in the previous computations. In contrast, shallow cavities have only half the number of eddies and a more complicated streamline topology. In both cases our asymptotic approximations give analytic formulas for the number of eddies and the shape of the streamlines. Other flows, corresponding to different driving at the boundaries, are also discussed and can be treated by the asymptotic methods we derive.
High speed scanning of terahertz pulse by a rotary optical delay line
Authors: Kim Geun-Ju , Jeon Seok-Gy , Kim Jung-Il , Jin Yun-Sik
Published: October 2008

We demonstrate high speed scanning of a time-domain terahertz pulse by a rotary optical delay line (RODL) incorporated into a photoconductive antenna based terahertz system. The delay line of RODL consists of six rotating convex reflective blades with a rotating speed controlled for a wide range of scan repetition rates. It can perform path length scans of 2.1 cm at a scanning speed of 8.4 m/s while maintaining linearity. The comparison of the terahertz temporal waveform and the frequency spectrum measured by using the RODL and a linear stage optical delay line confirms the linearity of the RODL up to 140 ps at high scan repetition rates of 400 scan/s. A high signal-to-noise ratio, up to a few thousand, can be obtained by averaging multiple terahertz pulses at a high scan speed.
Ionic liquid as additive to increase sensitivity, resolution, and diffraction efficiency of photopolymerizable hologram material
Authors: Lin Hechun , Oliveira Peter W. , Veith Michael
Published: October 2008

Ionic liquid was used as additive in photosensitive photopolymerizable materials. Ionic liquid as additive in photopolymerizable hologram material increases the sensitivity, the resolution, and the refractive index modulation of the material efficiently, which present strong diffusion after stopping the exposure due to the diffusion-controlled polymerization in ionic liquid. The diffraction efficiency was raised up to the theoretical maximum of 34% in the thin hologram.
Toward conductive traces: Dip Pen Nanolithography® of silver nanoparticle-based inks
Authors: Wang Hung-Ta , Nafday Omkar A. , Haaheim Jason R. , Tevaarwerk Emma , Amro Nabil A. , Sanedrin Raymond G. , Chang Chih-Yang , Ren Fan , Pearton Stephen J.
Published: October 2008

Low cost, direct writing of conductive traces is highly desired for applications in nanoelectronics, photonics, circuit repair, flexible electronics, and nanoparticle-based gas detection. The unique ability of Dip Pen Nanolithography (DPN®) to direct write a variety of materials onto suitable surfaces with nanoscale resolution and area-specific patterning is leveraged in this work. We present a direct-write approach toward creating traces with commercially available silver nanoparticle (AgNP)-based inks using DPN. In this work we demonstrate submicron AgNP feature creation together with a discussion on the ink transport mechanism.
Investigation of Eu–Mn energy transfer in A3MgSi2O8:Eu2+, Mn2+ (A=Ca,Sr,Ba) for light-emitting diodes for plant cultivation
Authors: Ma Liang , Wang Da-jian , Mao Zhi-yong , Lu Qi-fei , Yuan Zhi-hao
Published: October 2008

We report a plant growth lamp utilizing near-ultraviolet light-emitting diode and A3MgSi2O8:Eu2+, Mn2+ (A=Ca,Sr,Ba) phosphors with emission spectra consisting of blue and red bands, which conform well to the action spectra of plants and absorption spectra of chlorophylls. Either peak positions of the two bands or intensity ratio of red to blue band can be tailored by tuning the compositions of the cations. Eu–Mn energy transfer in the phosphors was investigated on the bases of photoluminescence, life decay time, and microstructure. This functional solid-state lighting provides a simple, compact, and efficient solution to illuminator for plant cultivation.
Programmable manipulation of motile cells in optoelectronic tweezers using a grayscale image
Authors: Choi Wonjae , Nam Seong-Won , Hwang Hyundoo , Park Sungsu , Park Je-Kyun
Published: October 2008

This paper describes a grayscale optoelectronic tweezers (OET) which allows adjustment of the electric field strength at each position of OET. A grayscale light image was used to pattern vertical electric field strength on an OET. As an electric field depends on the brightness at each point, the brighter light patterns generate the stronger electric field in the OET. Its feasibility for application to cell manipulation was demonstrated by aligning highly motile protozoan cells in vertical direction. Depending on the brightness of each pixel, the behaviors of aligned cells varied due to the different electric field strength to each cell.
Frequency modulation effect on microwave assisted magnetization switching
Authors: Okamoto S. , Kikuchi N. , Kitakami O.
Published: October 2008

Frequency modulation (FM) effect on the microwave assisted switching (MAS) of magnetization has been studied by calculating the Landau–Lifshitz–Gilbert equation. Application of FM microwave can significantly reduce the switching field compared with constant frequency wave, mainly due to more effective microwave energy absorption by the spin system. The FM effect on the MAS is remarkable especially for smaller Gilbert damping of α⩽0.02. For sufficient switching probability, the FM-MAS requires somewhat longer time of a few nanoseconds for the dc magnetic field parallel to the easy axis. The tilt dc field can effectively reduce the switching time.
Electronically patterning through one-dimensional nanostripes with high density of states on single-crystalline Al2O3 domain
Authors: Hsu Pin-Jui , Wu Chii-Bin , Yen Hong-Yu , Wong Sheng-Syun , Lin Wen-Chin , Lin Minn-Tsong
Published: October 2008

Self-assembled one-dimensional nanostripes on the single-crystalline Al2O3 domains are found to be the nucleation sites of nanoparticles through an enhanced density of states observed by the scanning tunneling microscopy and spectroscopy. Bias-dependent topographic images and the conductivity spectra indicate that these nanostripes have both enhanced occupied and unoccupied states within the oxide bandgap. These more metallic nanostripes have stronger electronically trapping ability than the oxide domain, which can be used as a one-dimensional electronically self-patterned template for the guided growth of nanostructures.
Beryllium acceptor binding energy in AlN
Authors: Sedhain A. , Al Tahtamouni T. M. , Li J. , Lin J. Y. , Jiang H. X.
Published: October 2008

The acceptor binding energy of an alternative dopant, Be, in AlN epilayers has been probed by time-resolved photoluminescence (PL) spectroscopy. The binding energy of excitons bound to Be acceptors in AlN is determined to be about 33meV, which implies that the Be acceptor binding energy in AlN is about 0.33eV in accordance with Haynes’ rule. The measured PL decay lifetimes of the acceptor-bound exciton transitions in Be- and Mg-doped AlN (93 and 119ps, respectively) also indicate that the binding energy of Be acceptor is smaller than that of the most common acceptor dopant in AlN, namely, Mg. The smaller activation energy of Be in AlN has the potential to partly address the critical p-type doping issue in AlN- and Al-rich AlGaN by increasing the room temperature free hole concentration by ∼103 compared to the case of Mg doping.
Direct observation of the birth of a nanocrystalline nucleus in an amorphous matrix
Authors: Rauf Ijaz A.
Published: October 2008

Nucleation of crystals within an amorphous phase can be induced using electron beam irradiation in an electron microscope. In contrast to generally believed two-step phase transformation involving nucleation and growth, we observe a three-step transformation: a two-step nucleation stage followed by the growth process. The two steps in the nucleation stage are: the formation of a basic crystalline skeleton followed by the diffusion of excess defects to the periphery of the crystalline skeleton.
Pulsed laser triggered high speed microfluidic switch
Authors: Wu Ting-Hsiang , Gao Lanyu , Chen Yue , Wei Kenneth , Chiou Pei-Yu
Published: October 2008

We report a high-speed microfluidic switch capable of achieving a switching time of 10 μs. The switching mechanism is realized by exciting dynamic vapor bubbles with focused laser pulses in a microfluidic polydimethylsiloxane (PDMS) channel. The bubble expansion deforms the elastic PDMS channel wall and squeezes the adjacent sample channel to control its fluid and particle flows as captured by the time-resolved imaging system. A switching of polystyrene microspheres in a Y-shaped channel has also been demonstrated. This ultrafast laser triggered switching mechanism has the potential to advance the sorting speed of state-of-the-art microscale fluorescence activated cell sorting devices.
Decomposition of dicyandiamide (DCD) in three contrasting soils and its effect on nitrous oxide emission, soil respiratory activity, and microbial biomass—an incubation study
Authors: Singh Jagrati , Saggar S. , Giltrap D. L. , Bolan Nanthi S.
Published: October 2008

The objective of this work was to study the degradation kinetics of a nitrification inhibitor (NI), dicyandiamide (DCD), and evaluate its effectiveness in reducing nitrous oxide (N2O) emissions in different types of soils. Three soils contrasting in texture, mineralogy, and organic carbon (C) content were incubated alone (control) or with urine at 600 mg N/kg soil with 3 levels of DCD (0, 10, and 20 mg/kg). Emissions of N2O and carbon dioxide (CO2) were measured during the 58-day incubation. Simultaneously, subsamples were collected periodically from the incubating soils (40-day incubation) and the amounts of DCD, NH4+, and NO3− were determined. Our results showed that the half-life of DCD in these laboratory incubating soils at 25°C was 6–15 days and was longer at the higher rate of DCD application. Of the 3 soils studied, DCD degradation was fastest in the brown loam allophanic soil (Typic orthic allophanic) and slowest in the silt loam non-allophanic soil (Argillic-fragic Perch-gley Pallic). The differences in DCD degradation among these soils can be attributed to the differences in the adsorption of DCD and in the microbial activities of the soils. Among the 3 soils the highest reduction in N2O emissions with DCD from the urine application was measured in the non-allophanic silt loam soil followed by non-allophanic sandy loam soil and allophanic brown loam soil. There was no adverse impact of DCD application on soil respiratory activity or microbial biomass.
Efficient simulation of wildfire spread on an irregular grid
Authors: Johnston Paul , Kelso Joel , Milne George J.
Published: October 2008

A cell-based wildfire simulator that uses an irregular grid is presented. Cell-based methods are simpler to implement than fire front propagation methods but have traditionally been plagued by fire shape distortion caused by the fire only being able to travel in certain directions. Using an irregular grid randomises the error introduced by the grid, so that the shape of simulated fire spread is independent of the direction of the wind with respect to the underlying grid. The cell-based fire spread simulator is implemented using discrete event simulation, which is a much more efficient computational method than conventional wildfire simulation techniques because computing resources are not used in repeatedly computing small updates to parts of the fire whose dynamics change infrequently, namely those areas of a fire that move slowly. The resulting simulator is comparable in accuracy with traditional fire front propagation schemes but is much faster and can therefore be used as an engine for fire simulation applications that require large numbers of simulations, such as in the role of a risk analysis engine.
The relationship between the monsoonal summer rain and dry-season fire activity of northern Australia
Authors: Harris S. , Tapper N. , Packham D. , Orlove B. , Nicholls N.
Published: October 2008

Fire is an essential element of the northern Australian ecosystems with extensive areas burnt each year. The basic climate condition of high rainfall during the summer monsoon, followed by an extended warm dry winter, along with highly combustible vegetation (much of which grows rapidly during summer and senesces during winter), creates a highly flammable environment. These vegetation conditions change under various naturally occurring climate oscillations such as El Niño–Southern Oscillation (ENSO). The present paper investigates the link between burnt areas of northern Australia, rainfall, the Southern Oscillation Index (SOI) and sea surface temperatures (SST) for a 9-year period (1997–2005). The burnt area distribution is compared with the strength and timing of the monthly averaged rainfall, SOI and SST. Results indicate a strong relationship between antecedent rainfall and ENSO indices with area burnt. This is especially strong between the burnt areas of June–October and the preceding rainfall of November–March (r = 0.90), the SOI of November–February (r = 0.78) and the SST of June–August (r = –0.64). The results from the present study reveal the ability to forecast annual burnt areas and present some of the dynamics of the climate–fire interactions and their value for management systems.