bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction of a wave packet with a thin electron beam spiraling in a magnetized plasma
Authors: Krafft C. , Volokitin A. S.
Published: July 2003

The nonlinear processes governing the resonant interaction of a packet of lower hybrid waves with a radially bounded electron beam spiraling in a magnetized plasma are investigated. In particular, the paper tries to answer the fundamental following questions: What are the causes of the beam bunching and the main agents of the beam self-organization occurring during the nonlinear wave–particle evolution? What is the influence of the wave packet on the stability of the formed bunches? This paper shows that, owing to the presence of wave energy dissipation out of the bounded beam volume, a competition takes place between the beam relaxation and the particle bunching processes, leading to the structuring of the beam over long distances from the injection point. In particular, two main mechanisms govern the behavior of the particles in resonance with the waves: First, the process of particle bunching which gives rise to dynamically stable and long living bunches of particles which keep resonance and strong correlations with several waves of the packet while continuously decelerated (Cherenkov resonance is considered) in the frame moving with the initial parallel beam velocity and, second, a process of particle diffusion in the velocity space concerning particles which do not experience trapping by waves or which leave a bunch through the action of large stochastic oscillations. This diffusion process cannot be well described in the frame of the weak turbulence theory, as it is noticeably perturbed by the presence of a large number of small unstable bunches which appear, merge together and disappear during the system’s evolution.
Parametric instabilities of Alfvén waves in a dusty plasma
Authors: Hertzberg M. P. , Cramer N. F. , Vladimirov S. V.
Published: July 2003

The excitation of low frequency modes of oscillations in a magnetized dusty plasma with parametric pumping of the magnetic field is analyzed. The dust is taken to be stationary, but acts as a sink for negative charge: The absence of a proportion of the electrons in the overall neutral plasma modifies the dispersion properties of the pump and excited waves. The pump is a magnetoacoustic wave modified by the dust, and the excited waves are modifications of Alfvén waves due to the dust and due to ion cyclotron effects. The equation of motion governing the perturbed plasma is derived and pump wave fields are found. The stability of the pump to modes propagating parallel to the magnetic field is investigated as a function of the plasma parameters. It is found that pairs of slow and fast parallel propagating modes are parametrically excited. The growth rates of the various interactions are calculated, and it is shown that the growth rates of slow–fast and fast–fast mode interactions can be maximized by varying the proportion of negative charge on the dust.
Destructive role of hot ions in the formation of electrostatic density humps and dips in dusty plasmas
Authors: Mahmood S. , Saleem H.
Published: July 2003

It is shown that the ion thermal energy is destructive for the ion acoustic solitons in the presence of dust, and it decreases the value of Mach number for the formation of solitary structures. The regions of ion density humps and dips are produced simultaneously, corresponding to positive and negative values of the electrostatic potential. The nonlinear electron density also behaves in a similar fashion as that of ions. However, the dust density increases in the regions where the ion and electron densities are depleted and vice versa.
Complete nondestructive diagnostic of nonneutral plasmas based on the detection of electrostatic modes
Authors: Amoretti M. , Bonomi G. , Bouchta A. , Bowe P. D. , Carraro C. , Cesar C. L. , Charlton M. , Doser M. , Fontana A. , Fujiwara M. C. , Funakoshi R. , Genova P. , Hangst J. S. , Hayano R. S. , Jørgensen L. V. , Lagomarsino V. , Landua R. , Rizzini E. Lodi , Macrı́ M. , Madsen N. , Manuzio G. , Testera G. , Variola A. , van der Werf D. P.
Published: July 2003

The detection of electrostatic nonneutral plasma modes in the ATHENA (ApparaTus for High precision Experiment on Neutral Antimatter) experiment [M. Amoretti, C. Amsler, G. Bonomi et al., Nature (London) 419, 456 (2002)] is described. A complete nondestructive diagnostic of the plasma based on a fit to the line shape of the function describing the power transmitted through the plasma around the frequency of the fundamental mode is developed and the experimental results are presented and discussed.
Direct and specific heat study of magnetocaloric effect in La0.845Sr0.155MnO3
Authors: Szewczyk A. , Gutowska M. , Piotrowski K. , Dąbrowski B.
Published: July 2003

Magnetocaloric effect and specific heat of a La0.845Sr0.155MnO3 sample were measured over a temperature range of 4.2–300 K, to verify whether large entropy changes induced by magnetic field make possible the use of manganites for magnetic refrigeration and whether a magnetostrictive contribution to magnetocaloric effect is significant. Isothermal entropy change and adiabatic temperature change induced by ΔB=7 T were determined as functions of temperature. Near to the Curie temperature, TC≈234 K, they reach, respectively, ∼−6.6 J/(kg K) and ∼3.4 K, i.e., the values ∼2 and ∼4 times smaller than for Gd. Despite the presence of large magnetostriction, its contribution to magnetocaloric effect was found negligible.
Rapid deformation of thin gold layers in polymer matrices studied by x-ray reflectivity
Authors: Shin Kwanwoo , Wang Howard , Satija Sushil K. , Han Charles C. , Josell Daniel , Bonevich John E.
Published: July 2003

We have used x-ray reflectivity to measure the morphological profiles of thin Au layers of three different average thicknesses sandwiched between two polystyrene layers with different molecular weights. The results showed that the equilibrium structures of the samples with the layers less than 4 nm thick, where discontinuous islands are expected, were very close to those of the nonannealed samples. However, the morphologies of samples with layers more than 4 nm thick, where a continuous layer structure was formed, were influenced by annealing. Comparison of transmission electron micrographs from unannealed and annealed samples showed that annealing promotes spherical shapes for the Au particles and breakup of an elongated, randomly connected structure visible prior to the annealing. The fractional area covered with Au obtained from transmission electron microscopy images is in good agreement with x-ray reflectivity results. These results were interpreted in terms of capillarity induced spheroidization of the ultrathin Au layers.
Intermixing and lateral composition modulation in GaAs/GaSb short period superlattices
Authors: Dorin C. , Mirecki Millunchick J. , Wauchope C.
Published: July 2003

Lateral composition modulation on the group V sublattice has been investigated in GaAs/GaSb short period superlattices. The effect of As species and growth temperature on the appearance of lateral composition modulation was studied. Cross-sectional transmission electron microscopy and x-ray diffraction reciprocal space maps reveal that structures grown using As tetramers are always disordered, defective, and phase separated. Also, in these structures the As-rich regions appear to be composed of stacked GaAs quantum dots embedded in a GaSb matrix. The structures grown with As dimers show improved crystalline quality. Short period superlattices grown at T<420 °C have flat interfaces and are laterally homogeneous, however, there is significant anion intermixing across the interfaces. Structures deposited at 420 °C<T<445 °C roughen during growth, and exhibit lateral composition modulation and anion intermixing. Growing at higher temperatures destroys both the superlattice structure and the lateral composition modulation. The As sticking coefficient was calculated and was found to range between 0.1⩽σ⩽0.17 depending on the growth temperature and As species.
Effect of particle size and shape on the order–disorder phase transition in diblock copolymers
Authors: Chervanyov Alexander I. , Balazs Anna C.
Published: July 2003

The effect of spherical and sphero-cylindrical fillers on the order–disorder transition (ODT) of a symmetric diblock copolymer melt is investigated. Self-consistent equations describing the copolymer density distribution in the presence of fillers are derived. Using these equations, we calculate the excess free energy due to the presence of the particles in the diblocks. The critical value of the segregation factor χN is recalculated with the effect of the fillers taken into account. We find that a relatively small volume fraction of fillers can cause a significant suppression of the ODT temperature. It is shown that smaller particles cause a greater suppression of the ODT temperature provided a constant particle volume fraction is maintained. The effect of the particle shape on the ODT is investigated. The ODT temperature shift is calculated for the sphero-cylindrical particles as a function of their aspect ratio at a given particle volume. It is found that sphero-cylinders with the smaller aspect ratio produce the bigger effect on the ODT. A scaling analysis of the presented results and a comparison with the experimental work are given.
A variational study of nuclear dynamics and structural flexibility of the CH2OH radical
Authors: Marenich Aleksandr V. , Boggs James E.
Published: July 2003

In order to describe correctly the complicated nature of the nuclear motion in CH2OH a dynamic model of CH2OH was developed. The strong structural flexibility due to the out-of-plane motions and the appreciable anharmonicity along some in-plane vibrational modes were taken into consideration. The dynamic problem was done separately for the out-of-plane and in-plane vibrational modes in a basis set of harmonic oscillator eigenfunctions. Potential energy surfaces dependent on displacements of all nine internal coordinates from their equilibrium values in the planar structure of Cs symmetry were built using the energies of 1793 geometries of CH2OH. Computations were done with correlation-consistent polarized valence basis sets of triple zeta quality and at the second (MP2) and fourth (MP4) orders of many body perturbation theory, and by the coupled cluster singles and doubles method with perturbative treatment of triple excitations [CCSD(T)]. Infrared intensities of the lowest excitations were estimated by calculation of matrix elements of the dipole moment for the corresponding transitions. The vibrationally averaged values of the geometric parameters of CH2OH were computed. The results obtained were found to be in good agreement with available experimental data.
Linear and nonlinear properties of dust-acoustic waves in collisional, magnetized dusty plasmas
Authors: Moslem Waleed M.
Published: July 2003

Effects of dust-neutral collision and densities of positive ions and electrons have been investigated for the propagation of dust-acoustic waves (DAWs) in magnetized dusty plasmas. It is found that, due to collisions, the DAWs damp waves and the damping rate of the waves depends mainly on the collision frequency (i.e., if there are no collisions the waves do not damp waves). The collisions are found to significantly change the basic properties (viz., the amplitude and the width) of the DAWs. The densities of positive ions and electrons have important roles in the behavior of the DAWs. The present analysis shows that only rarefactive solitary waves exist in the system. This investigation can be relevant to the DAWs in various space plasma environments, such as Jupiter’s ring, the F ring of Saturn, and interstellar dusty clouds.
Strong shock wave and areal mass oscillations associated with impulsive loading of planar laser targets
Authors: Velikovich A. L. , Schmitt A. J. , Metzler N. , Gardner J. H.
Published: July 2003

When a rippled surface of a planar target is irradiated with a short (subnanosecond) laser pulse, the shock wave launched into the target and the mass distribution of the shocked plasma will oscillate. These oscillations are found to be surprisingly strong compared, for example, to the case when the laser radiation is not turned off but rather keeps pushing the shock wave into the target. Being stronger than the areal mass oscillations due to ablative Richtmyer–Meshkov instability and feedout in planar targets, which have recently been observed at the Naval Research Laboratory (NRL) [Aglitskiy et al., Phys. Plasmas 9, 2264 (2002)], these oscillations should therefore be directly observable with the same diagnostic technique. Irradiation of a target with a short laser pulse represents a particular case of an impulsive loading, a fast release of finite energy in a thin layer near the surface of a target. Renewed interest to the impulsive loading in the area of direct-drive laser fusion is due to the recent proposals of using a short pulse prior to the drive pulse to make the target more resistant to laser imprint and Rayleigh–Taylor growth. Impulsive loading produces a shock wave that propagates into the target and is immediately followed by an expansion wave, which gradually reduces the shock strength. If the irradiated surface is rippled, then, while the shock wave propagates through the target, its modulation amplitude grows, exceeding the initial ripple amplitude by a factor of 2 or more. The oscillating areal mass reaches the peak values that exceed the initial mass modulation amplitude (density times ripple height) by a factor of 5–7 or more, and reverses its phase several times after the laser pulse is over. The oscillatory growth is more pronounced in fluids with higher shock compressibility and is probably related to the Vishniac’s instability of a blast wave. Frequency of the oscillations is determined by the speed of sound in the shocked material, and could be used as a tuning fork to probe its equation of state. The analytical theory and numerical simulations describing such oscillations are reported, and the opportunities available for their experimental observation are discussed.
Comprehensive study of ZnO films prepared by filtered cathodic vacuum arc at room temperature
Authors: Wang Y. G. , Lau S. P. , Lee H. W. , Yu S. F. , Tay B. K. , Zhang X. H. , Tse K. Y. , Hng H. H.
Published: July 2003

Room temperature deposition of high crystal quality zinc oxide (ZnO) films was realized by the filtered cathodic vacuum arc (FCVA) technique. Detrimental macroparticles in the plasma as byproducts of arcing process are removed with an off-plane double bend magnetic filter. The influence of oxygen pressure on the structural, electrical and optical properties of ZnO films were investigated in detail. The crystal structure of ZnO is hexagonal with highly c-axis orientation. Intrinsic stress decreases with an increase of chamber pressure, and near stress-free film was obtained at 1×10−3 Torr. Films with optical transmittance above 90% in the visible range and resistivity as low as 4.1×10−3 Ω cm were prepared at pressure of 5×10−4 Torr. Energetic zinc particles in the cathodic plasma and low substrate temperature enhance the probability of formation of zinc interstitials in the ZnO films. The observation of strong ultraviolet photoluminescence and weak deep level emission at room temperature manifest the high crystal quality of the ZnO films prepared by FCVA. Enlargement of the band gap is observed in the absorption and photoluminescence spectra, the band gap shifts towards lower energy with an increase of oxygen pressure. This phenomenon is attributed to the Burstein–Moss effect.
Characteristics and polarization-enhanced model of wurtzite aluminum nitride thin films synthesized on Si(100) substrates by pulsed laser deposition
Authors: An Zhenghua , Men Chuanling , Yu Jian , Chu Paul K. , Lin Chenglu
Published: July 2003

AlN thin films were fabricated on silicon (100) substrates by pulsed laser deposition and their properties are investigated. Our results indicate that the AlN films have a wurtzite crystalline structure with (002) preferential orientation over a large range of temperatures from room temperature to 800 °C. With an increase in substrate temperature, the films undergo a transition from nanocrystalline to large polycrystalline morphology, and at the same time the surface roughness increases due to larger columnar grain size. Electrical measurements show that there is a high dynamic charge density in the AlN films, and a polarization-enhanced mechanism is proposed to interpret the voltage–charge hysteresis loops observed in the samples.
Photoyield and x-ray-photoelectron spectroscopic studies of O2-annealing effects on SrBi2Ta2O9 thin films prepared by pulsed laser deposition
Authors: Takahashi M. , Noda M. , Okuyama M.
Published: July 2003

Electronic properties of various SrBi2Ta2O9 (SBT) thin films were studied by ultraviolet (UV)-ray photoyield spectroscopy (PYS) and x-ray photoelectron spectroscopy (XPS). The UV-PYS studies have shown that O2 annealing increases the Fermi level in the SBT thin film surface which was prepared by pulsed laser deposition (PLD) by about 0.34 eV, while the other SBT thin films deposited by metalorganic decomposition (MOD) have shown almost the same Fermi levels as the O2-annealed PLD-SBT thin film. The XPS studies of those SBT thin films have shown that the PLD-SBT thin film is more susceptible to deoxidization by Ar+ bombardment than MOD-SBT thin films. This implies that the PLD-SBT thin film includes more defective (Bi2O2)2+ layers than MOD-SBT thin films. The O2-annealing effects on the PLD-SBT thin films are thought to oxidize defective (Bi2O2)2+ layers on the PLD-SBT surface and to shift the surface Fermi level towards the center of the band gap.
Proton spectra from ultraintense laser–plasma interaction with thin foils: Experiments, theory, and simulation
Authors: Allen M. , Sentoku Y. , Audebert P. , Blazevic A. , Cowan T. , Fuchs J. , Gauthier J. C. , Geissel M. , Hegelich M. , Karsch S. , Morse E. , Patel P. K. , Roth M.
Published: July 2003

A beam of high energy ions and protons is observed from targets irradiated with intensities up to 5×1019 W/cm2. Maximum proton energy is shown to strongly correlate with laser-irradiance on target. Energy spectra from a magnetic spectrometer show a plateau region near the maximum energy cutoff and modulations in the spectrum at approximately 65% of the cutoff energy. Presented two-dimensional particle-in-cell simulations suggest that modulations in the proton spectrum are caused by the presence of multiple heavy-ion species in the expanding plasma.
Self-consistent equilibrium model of low aspect-ratio toroidal plasma with energetic beam ions
Authors: Belova E. V. , Gorelenkov N. N. , Cheng C. Z.
Published: July 2003

A theoretical model is developed which allows the self-consistent inclusion of the effects of energetic beam ions in equilibrium calculations of low-aspect-ratio toroidal devices. A two-component plasma is considered, where the energetic ions are treated using a kinetic Vlasov description, while a one-fluid magnetohydrodynamic description is used to represent the thermal plasma. The model allows for an anisotropic distribution function and a large Larmor radius of the beam ions. Numerical results are obtained for neutral-beam-heated plasmas in the National Spherical Torus Experiment (NSTX) [M. Ono, S. M. Kaye, Y.-K. M. Peng et al., Nucl. Fusion 40, 557 (2000)]. Self-consistent equilibria with an anisotropic fast ion distribution have been calculated for NSTX. It is shown for typical experimental parameters that the contribution of the energetic neutral beam ions to the total current can be comparable to that of the background plasma, and that the kinetic modifications of the equilibrium can be significant. The range of validity of the finite-Larmor-radius expansion and of the reduced kinetic descriptions for the beam ions in NSTX is discussed. The calculated kinetic equilibria can be used for self-consistent numerical studies of beam-ion-driven instabilities in NSTX.
Stress and its effect on optical properties of GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire
Authors: Zhao D. G. , Xu S. J. , Xie M. H. , Tong S. Y. , Yang Hui
Published: July 2003

The stress states in unintentionally doped GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire, and their effects on optical properties of GaN films were investigated by means of room-temperature confocal micro-Raman scattering and photoluminescence techniques. Relatively large tensile stress exists in GaN epilayers grown on Si and 6H-SiC while a small compressive stress appears in the film grown on sapphire. The latter indicates effective strain relaxation in the GaN buffer layer inserted in the GaN/sapphire sample, while the 50-nm-thick AlN buffer adopted in the GaN/Si sample remains highly strained. The analysis shows that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual strain in the films. Finally, a linear coefficient of 21.1±3.2 meV/GPa characterizing the relationship between the luminescent bandgap and the biaxial stress of the GaN films is obtained.
Atomic resolution composition analysis by scanning transmission electron microscopy high-angle annular dark-field imaging
Authors: Carlino E. , Modesti S. , Furlanetto D. , Piccin M. , Rubini S. , Franciosi A.
Published: July 2003

The silicon concentration profile in Si–GaAs (001) superlattices grown by molecular beam epitaxy was investigated using scanning transmission electron microscopy high-angle annular dark-field (HAADF) imaging. Comparison with atomic resolution results obtained through cross-sectional scanning tunneling microscopy indicates that, by choosing appropriate experimental conditions, HAADF imaging can be used to gauge the Si distribution in GaAs on the atomic scale even without any image simulation.
Simulations of diocotron instability using a special-purpose computer, MDGRAPE-2
Authors: Yatsuyanagi Yuichi , Kiwamoto Yasuhito , Ebisuzaki Toshikazu , Hatori Tadatsugu , Kato Tomokazu
Published: July 2003

The diocotron instability in a low-density non-neutral electron plasma is examined via numerical simulations. For the simulations, a current-vortex filament model and a special-purpose computer, MDGRAPE-2 are used. In the previous work, a simulation method based on the current-vortex filament model, which is called “current-vortex method,” is developed. It is assumed that electric current and vorticity have discontinuous filamentary distributions, and both point electric current and point vortex are confined in a filament, which is called “current-vortex filament.” In this paper, the current-vortex method with no electric current is applied to simulations of the non-neutral electron plasma. This is equivalent to the traditional point-vortex method. MDGRAPE-2 was originally designed for molecular dynamics simulations. It accelerates calculations of the Coulomb interactions, the van der Waals interactions and so on. It can also be used to accelerate calculations of the Biot–Savart integral. The diocotron modes reproduced by the simulations agree with the result predicted by linear theory. This indicates that the current-vortex method is applicable to problems of the non-neutral plasma. The linear growth rates of the diocotron instability in the simulations also agree with the theoretical ones. This implies that MDGRAPE-2 gives the sufficiently accurate results for the calculations of the current-vortex method. A mechanism of merging of electron clumps is demonstrated by the simulations. It is concluded that the electric field induced by the conducting wall makes the nonlinear stage unstable and causes the clumps to merge.
Raman spectroscopy of GaN nucleation and free-standing layers grown by hydride vapor phase epitaxy on oxidized silicon
Authors: Konenkova E. V. , Zhilyaev Yu. V. , Fedirko V. A. , Zahn D. R. T.
Published: July 2003

GaN nucleation layers (NL-GaN) and GaN free-standing (FS-GaN) layers are studied using Raman spectroscopy and atomic force microscopy. The layers are deposited onto oxidized silicon substrates by hydride vapor phase epitaxy at 520 °C (NL layers) and 970 °C (FS layers). The effect of high-temperature annealing (1010 °C) on the properties of FS-GaN layers is investigated. The average height of the islands in the NL-GaN layers is found to increase from 15 to 400 nm when the growth time is increased from 10 to 200 min. The average growth rate of NLs is found to be very low, namely, ≈1×10−2 nm/s. E2 (566 cm−1) and A1 (longitudinal optical) (730 cm−1) peaks are observed on NL-GaN layers when the average size of the islands increases to 400 nm, scattering by E2 (567.3 cm−1) and E1 [transverse optical (TO)] (558.3 cm−1) modes is detected on FS-GaN layers. High-temperature annealing of the FS-GaN layers results in an increase of the intensity of E2 and E1(TO) peaks detected from the front side whereas no effect is observed for detection from the side exposed by removal of the substrate.
Experimental investigation of isotope scaling of anomalous ion thermal transport
Authors: Sokolov V. , Sen A. K.
Published: July 2003

There is a significant divergence between tokamak experimental results and most theoretical predictions in isotopic mass scaling of transport. It indicates a fundamental open physics issue of great importance, as well as a serious question for magnetic fusion. This divergence has motivated a series of basic physics experiments in the Columbia Linear Machine (CLM) [R. Scarmozzino et al., Phys. Rev. Lett. 57, 1729 (1986)], which are reported here. The experimental focus is on the anomalous ion thermal conduction due to ion temperature gradient driven modes in two different gases: hydrogen and deuterium. Unlike in tokamaks, all plasma parameters are kept nearly identical in the CLM for both gases. The results indicate inverse dependence of ion thermal conductivity on the isotopic mass close to K⊥∼Ai−0.5 to −0.8, where Ai is the mass number of the isotope of hydrogen. This is close to the tokamak results, but in contradiction to most theoretical models, especially of the Bohm/gyro-Bohm-types.
Neutral desorption from intense electron beam impact on solid surfaces
Authors: Davis H. A. , Olson R. T. , Moir D. C.
Published: July 2003

Neutrals desorbed from solid surfaces by electron beam impact can be ionized by the beam and trapped in the beam potential causing beam disruption. This can increase the beam diameter on radiographic bremsstrahlung targets degrading resolution. Measurements of the number and species of neutrals desorbed by impact of a 19.8 MeV, 1.7 kA, 60 ns electron beam on thin-foil targets are made in open and closed geometries. The study focuses on aluminum and graphite foil surfaces. A fast Bayard–Alpert ionization gauge measures the number of neutral molecules released, and a quadrupole mass spectrometer measures the species mix. At high beam current density, where thermal desorption due to target heating is expected, the dominant neutral species is H2O, and approximately one monolayer of desorbed gas is released. Other prominent species such as H2, CO, and CO2 are not thermally desorbed until current densities near foil destruction are approached. The observations are in agreement with a previous hypothesis based on the comparison of computer calculations with beam-dynamics observations.
Design of improved magnetic channel for superconducting cyclotrons by the random search method
Authors: Sarma P. R.
Published: July 2003

Magnetic channels are used in cyclotrons and accelerators for extraction of ion beams. Passive magnetic channels use iron bars to lower the magnetic field locally and set up a field gradient. In order that the beam quality does not deteriorate, this field gradient should be highly uniform. We have used the random search method for improving the field quality of a magnetic channel by modifying the geometric parameters of a conventional channel.
Enhancement of the weak scattered signal in apertureless near-field scanning infrared microscopy
Authors: Stebounova Larissa , Akhremitchev Boris B. , Walker Gilbert C.
Published: July 2003

An interferometric method is used to enhance the weak scattered signal in apertureless near-field scanning infrared microscopy. The method involves introducing a homodyning reference field, and amplifies the desired signal field by the magnitude of the reference field. This method markedly improves the signal-to-noise ratio of the detected signal, over the nonhomodyned experiment. A model for the dependence of the near-field signal, as a function of the normal distance of the tip from the surface, is discussed. Application of a model in which the tip is represented by two spherical scatterers, one large and one small, indicates the electromagnetic field enhancement is 90-fold greater at the sharp apex of the metallic probe tip.
Controlling a telescope chopping secondary mirror assembly using a signal deconvolution technique
Authors: Houde Martin , Holt Lynn C. , Yoshida Hiroshige , Nelson Patrick M.
Published: July 2003

We describe a technique for improving the response of a telescope chopping secondary mirror assembly by using a signal processing method based on the Lucy deconvolution technique. This technique is general and could be used for any systems, linear or nonlinear, where the transfer function(s) can be measured with sufficient precision. We demonstrate how the method was implemented and show results obtained at the Caltech Submillimeter Observatory using different chop throw amplitudes and frequencies. No intervention from the telescope user is needed besides the selection of the chop throw amplitude and frequency. All the calculations are done automatically once the appropriate command is issued from the user interface of the observatory’s main computer.
High-temperature furnace for dynamic neutron radiography
Authors: Kahle Andreas , Winkler Björn , Hennion Bernard , Boutrouille Philippe
Published: July 2003

An easy-to-build and comparatively inexpensive high-temperature graphite tube furnace has been designed, fabricated, and tested for measuring physical properties of silicate melts with dynamic neutron radiography up to temperatures of 2300 K. This furnace allows in situ and real-time observations of large melt volumes at these high temperatures. The furnace has been used for the visualization of the interaction of two chemically distinct melts and for a determination of melt viscosities of silicates with the falling sphere method.
Cellophane as a half-wave plate and its use for converting a laptop computer screen into a three-dimensional display
Authors: Iizuka Keigo
Published: July 2003

It was experimentally verified that an ordinary 25-μm-thick cellophane sheet possesses properties of a wide wavelength spectrum half-wave plate. Moreover, cellophane displayed superior performance when used for rotating the direction of polarization of white light than a commercially available half-wave plate with a specified wavelength. The retardance of the cellophane was measured to be 170°. The availability of a half-wave plate of an extra large size with low cost opens up usages for large size displays. As an example of its application, an ordinary screen of a laptop personal computer was converted into that of a three-dimensional display by the cellophane half-wave plate. It may be added that the price per square cm2 of the cellophane half-wave plate is about 1/3500 of that of a commercially available half-wave plate.
Efficiency enhancement of ideal photovoltaic solar cells by photonic excitations in multi-intermediate band structures
Authors: Peng R. W. , Mazzer M. , Barnham K. W. J.
Published: July 2003

We present an efficiency analysis of ideal photovoltaic solar cells based on multi-intermediate band structures. It is shown that the difference between the thermodynamic limit of photovoltaic conversion and the limit of efficiency of traditional bulk semiconductor solar cells can be gradually bridged if an optimum energy band structure is achieved. Efficiency enhancement originates from photonic excitations among multiple energy bands. Several possible ways to design the optimum energy band structures are proposed.
Comparison of intraband absorption and photocurrent in InAs/GaAs quantum dots
Authors: Adawi A. M. , Zibik E. A. , Wilson L. R. , Lemaı̂tre A. , Cockburn J. W. , Skolnick M. S. , Hopkinson M. , Hill G.
Published: July 2003

We present a comparative study of mid-infrared absorption and photocurrent measurements of self assembled InAs/GaAs quantum dots. A thermally activated bound/state–bound-state transition, as well as bound–wetting-layer and bound–continuum transitions are identified. By analyzing the temperature dependence of these transitions using absorption and photocurrent spectroscopies we are able to explain the previously reported discrepancies between the two measurement techniques. The activation energy (≅100 meV) for the bound–bound transition indicates that thermal escape of electrons occurs directly to continuum states. Evidence for preferential escape within the inhomogeneous distribution of dots is presented.
Effect of a ferromagnetic wall on low β tearing modes in the Japan Atomic Energy Research Institute Fusion Torus-2 Modified
Authors: Bakhtiari M. , Azumi M. , Tsuzuki K. , Kamiya K. , Kawashima H. , Kusama Y. , Sato M. , Hoshino K. ,   JFT-2M Group
Published: July 2003

A ferritic steel wall, which might be employed in fusion demonstration reactors, has been installed in Japan Atomic Energy Research Institute Fusion Torus-2 Modified [Nucl. Fusion 41, 257 (2001)] to study the applicability of ferromagnetic walls to plasmas. The tearing mode equation is solved in presence of a ferromagnetic/resistive wall to study the effect of the ferromagnetism on the tearing mode stability in tokamaks. It is shown that a ferromagnetic wall has a weak effect on the surface tearing modes depending on the current profile. The destabilization effect due to the attraction of the perturbed magnetic field by permeability is shown to be very small compared to the stabilization effect of the conducting wall.
Low angular-dispersion microwave absorption of a dual-pitch nondiffracting metal bigrating
Authors: Lockyear Matthew J. , Hibbins Alastair P. , Sambles J. Roy , Lawrence Christopher R.
Published: July 2003

The surface plasmon modes supported by a nondiffracting 90° bigrating consisting of three grooves per repeat period with one slightly shallower than the other two are characterized by studying the reflectivity from the structure as a function of the angle of incidence and the incident wavelength (11.3<λ0<16.7 mm). This structure supports two remarkably angle-independent modes plus a further, lower-energy mode which is more dispersive. Experimental reflectivity is compared with that calculated using a finite element model. In addition, to understand the character of each of the modes, the spatial form of the electromagnetic fields at the resonant frequencies are explored.
Bright red-emitting electrophosphorescent device using osmium complex as a triplet emitter
Authors: Kim Joo Hyun , Liu Michelle S. , Jen Alex K.-Y. , Carlson Brenden , Dalton Larry R. , Shu Ching-Fong , Dodda Rajasekhar
Published: July 2003

A series of efficient and bright double-layer light-emitting devices have been fabricated using the osmium (Os) complex as the triplet emissive dopant in both a blue-emitting polyfluorene derivative (PF–TPA–OXD) containing hole-transporting triphenylamine (TPA) and electron-transporting oxadiazole (OXD) as side chains and a blend of 2-(4-t-butylphenyl)-5(4-biphenylyl)-1,3,4-oxadizole (PBD) in poly(N-vinylcarbazole) (PVK). Due to a balanced charge injection and transport in PF–TPA–OXD and very efficient energy transfer from this polymer to the Os complex, the resulting device (indium tin oxide/HTL/OsCF3:PF–TPA–OXD/Ca/Ag) reaches a maximum external quantum efficiency of 2.1% with a peak brightness of 2920 cd/m2. These results are significantly higher than those obtained from the commonly used host, PVK:PBD (0.49% and 1270 cd/m2).
VIRTIS-M flight lamps
Authors: Melchiorri R. , Piccioni G. , Mazzoni A.
Published: July 2003

VIRTIS-M is a visible-infrared (VIS-IR) image spectrometer designed for the Rosetta mission; it intends to provide detailed informations on the physical, chemical, and mineralogical nature of comets and asteroids. The in-flight performances of VIRTIS-M are expected to be influenced by various disturbances, like the initial strong vibrations of the rocket, the long duration of the experiment (from 2003 to 2010), as well as other possible environmental changes; therefore, an in-flight recalibration procedure is mandatory. Quite often in such kinds of missions, a light emission diode (LED) is employed to calibrate the on-board spectrometers by taking advantage of the relative small dimensions, stability, and hardness of these sources. VIRTIS-M is the first image spectrometer that will use a new generation of lamps for internal calibrations. These new lamps are characterized by a wide spectral range with a blackbody-like emission with an effective temperature of about (2400–2600 K), thereby covering the whole VIRTIS-M’s spectral range (0.2–5 μm); i.e., they offer the possibility of a wider spectral calibration in comparison with the quasimonochromatic LED emission. A precise spectral calibration is achieved by adding special filters for visible and infrared ranges in front of the window source, containing many narrow absorption lines. In the present article, we describe the calibration and tests of some flight prototypes of these lamps (VIS and IR), realized by the Officine Galileo and calibrated by the Consiglio Nazionale delle Ricerche–Istituto di Astrofisica Spaziale e Fisica Cosmica.
Apertureless near-field scanning optical microscope based on a quartz tuning fork
Authors: De Wilde Y. , Formanek F. , Aigouy L.
Published: July 2003

The design of an apertureless near-field scanning optical microscope (ANSOM) is presented. The system operates in tapping mode with a tungsten tip mounted on a quartz tuning fork, which periodically scatters the near field at the sample surface. The tip-tuning fork structure is made short so that it can accommodate a standard high numerical aperture microscope objective. We describe the mounting of the tip on the tuning fork. Topographical and optical images, in the visible (λ=655 nm) and in the infrared (λ=10.6 μm), of subwavelength holes in a chromium film demonstrate that the ANSOM routinely achieves a resolution of a few tens of nanometers.
High-sensitivity noncontact atomic force microscope/scanning tunneling microscope (nc AFM/STM) operating at subangstrom oscillation amplitudes for atomic resolution imaging and force spectroscopy
Authors: Oral A. , Grimble R. A. , Özer H. Ö. , Pethica J. B.
Published: July 2003

We describe a new, highly sensitive noncontact atomic force microscope/scanning tunneling microscope (STM) operating in ultrahigh vacuum (UHV) with subangstrom oscillation amplitudes for atomic resolution imaging and force–distance spectroscopy. A novel fiber interferometer with ∼4×10−4 Å/Hz noise level is employed to detect cantilever displacements. Subangstrom oscillation amplitude is applied to the lever at a frequency well below the resonance and changes in the oscillation amplitude due to tip–sample force interactions are measured with a lock-in amplifier. Quantitative force gradient images can be obtained simultaneously with the STM topography. Employment of subangstrom oscillation amplitudes lets us perform force–distance measurements, which reveal very short-range force interactions, consistent with the theory. Performance of the microscope is demonstrated with quantitative atomic resolution images of Si(111)(7×7) and force–distance curves showing short interaction range, all obtained with <0.25 Å lever oscillation amplitude. Our technique is not limited to UHV only and operation under liquids and air is feasible.
Real time femtosecond optical pulse measurement using a video-rate frequency-resolved optical gating system
Authors: Garduño-Mejı́a J. , Ramsay E. , Greenaway A. , Reid D. T.
Published: July 2003

We describe an improved instrument for measuring at video rate (30 frames per second) the second-harmonic frequency-resolved optical gating trace of femtosecond pulses from a mode-locked laser oscillator. The system comprises separate scanning acquisition and pulse retrieval elements which together enable the exact pulse profile to be viewed in real time with a typical refresh rate of 1 Hz. Details are given of the optical system used, the electronic synchronization circuits and the acquisition and retrieval software employed.
Suppression of hot electrons in threshold photoelectron photoion coincidence spectroscopy using velocity focusing optics
Authors: Sztáray Bálint , Baer Tomas
Published: July 2003

Velocity focusing of electrons is combined with photoelectron photoion coincidence (PEPICO) spectroscopy to achieve a true threshold PEPICO signal without contributions from energetic electrons. Ions are generated by a continuous vacuum ultraviolet light source. Electrons, extracted by a field of 20 V/cm, pass through a 13 cm drift region and are dispersed in space on a multichannel plate detector by velocity focusing optics. The ions are extracted in the opposite direction by the same electric field, further accelerated by a second field, and collected after passing through a 30 cm drift region. Ions are measured in coincidence with electrons collected from the central 3.2 mm electrode as well as a ring electrode (inner and outer diameters of 5.6 and 8.1 mm). The central ring electrode contains mostly true threshold electrons along with a background of “hot” electrons, whereas the outer ring electrode collects only hot electrons. By subtracting the latter from the former, true threshold photoelectron photoion coincidence spectra are obtained. The major advantages of this approach are the high electron energy resolution with the use of high direct current extraction fields, and the complete suppression of energetic electrons.
Space applications of microelectromechanical systems: Southwest Research Institute® vacuum microprobe facility and initial vacuum test results
Authors: McComas D. J. , Miller G. P. , Mitchell J. N. , Pope S. E. , Valek P. W.
Published: July 2003

We have designed and built the first fully functional vacuum microprobe test facility specifically intended to optimize the development of microelectromechanical systems (MEMS) devices for space applications. This facility includes an ion-pumped, ultraclean vacuum system outfitted with four three-axis precision microprobe stages. The testing is monitored with a long focal length microscope through a thin sapphire window. Testing of several initial MEMS designs shows extremely promising results for using such devices in space applications. In particular, we show that significantly reduced voltages are adequate to resonantly drive some MEMS devices in vacuum owing to significantly reduced damping and the consequent much higher Q of the systems (∼1000×) in the absence of air. We also show the results of a many cycle (>1010) test of a comb-driven, force–distance multiplied sliding aperture door and demonstrate that potential show-stopper issues such as stiction and vacuum welding can be overcome in MEMS devices properly designed for the vacuum environment.
Temperature determination for nanosecond pulsed laser heating
Authors: Rekhi Sandeep , Tempere Jacques , Silvera Isaac F.
Published: July 2003

Materials can be heated to thousands of degrees Kelvin using a high power focused laser beam. For continuous-wave laser heating the temperature can be remotely measured to a high precision by fitting the spectral irradiance to a blackbody curve. Heating with a pulsed laser has a number of advantages, but the temperature rises and falls in times measured in nanoseconds, which would require fast electronics and tedious procedures to determine the temperature by measuring the blackbody curve. We present a method of temperature determination using time-averaged measurements with a charge-coupled-device detector. The method is tested on fixed points (the melting of stainless steel and platinum), and provides accurate easy temperature determinations.
High-pressure ultrasonic setup using the Paris–Edinburgh press: Elastic properties of single crystalline germanium up to 6 GPa
Authors: Gauthier Michel , Lheureux Davy , Decremps Frédéric , Fischer Myriam , Itié Jean Paul , Syfosse Gérard , Polian Alain
Published: July 2003

A setup for the measurement of ultrasonic properties of single crystals at high pressure in the 10 GPa range is presented. In order to validate this new device, based on the “Paris–Edinburgh” press, the elastic properties of germanium have been measured and compared with the published data. The transit times of the ultrasonic waves are directly measured. The three elastic moduli and their pressure derivatives are obtained up to 6 GPa through three different methods of calculation, which are presented and compared. These results are in good agreement with previous results and hence validate the device. The Paris–Edinburgh press enables in situ measurement of ultrasonic and x-ray diffraction on the studied sample.
Compact high pressure unit for ultraviolet-visible-near-infrared spectroscopic measurements at pressures up to 400 MPa
Authors: Zahl Achim , Igel Peter , Weller Manfred , Khoshtariya Dimitri E. , Hamza Mohamed S. A. , van Eldik Rudi
Published: July 2003

The construction of a compact high pressure unit for UV-VIS-NIR spectroscopic measurements at pressures up to 400 MPa is described. The pressure generating system can be operated with different pressure liquids, depending on the spectral characteristics required for specific applications. The system can be used for thermodynamic as well as a variety of kinetic measurements.
Electrostatic hexapole state selector: Honeycomb field for integrating intensity of oriented molecular beams
Authors: Shimizu Y. , Che D.-C. , Hashinokuchi M. , Fukuyama T. , Suzui M. , Watanabe M. , Kasai T.
Published: July 2003

A type of electrostatic state selector that consists of seven sets of adjacent hexapole fields forming a honeycomb field has been developed in order to integrate intensities of seven oriented molecular beam lines. The cross section of the electric field is gradually diminished from the inlet to the outlet in order to sum up the seven molecular beam lines and focus on a focal point located 1000 mm downstream from the honeycomb field outlet. We first used nonfocusable noble gases: Ar and Kr to check the assembly alignment, and then we used the polar molecule of acetonitrile (CH3CN, μ=3.92 D) seeded in the noble gases to characterize the quantum state selectivity of the honeycomb field. We obtained the dependence of the focused CH3CN beam intensity on the honeycomb electrode voltages. We could reproduce the experimental focusing curve by use of a modified trajectory simulation applicable to the honeycomb electric field. The present beam character (i.e., rotational temperature and stream velocity) was confirmed to be maintained.
Influence of surface oxides on hydrogen-sensitive Pd:GaN Schottky diodes
Authors: Weidemann O. , Hermann M. , Steinhoff G. , Wingbrant H. , Lloyd Spetz A. , Stutzmann M. , Eickhoff M.
Published: July 2003

The hydrogen response of Pd:GaN Schottky diodes, prepared by in situ and ex situ deposition of catalytic Pd Schottky contacts on Si-doped GaN layers is compared. Ex situ fabricated devices show a sensitivity towards molecular hydrogen, which is about 50 times higher than for in situ deposited diodes. From the analysis of these results, we conclude that adsorption sites for atomic hydrogen in Pd:GaN sensors are provided by an oxidic intermediate layer. In addition, in situ deposited Pd Schottky contacts reveal lower barrier heights and drastically higher reverse currents. We suggest that the passivation of the GaN surface before ex situ deposition of Pd also results in quenching of leakage paths caused by structural defects.