bluuebunny/crossref_metadata_2025 huggingface dataset:
Cryogenically cooled multiple-substrate holder for high vacuum
Authors: Chaiken A. , Honea E. C. , Rupprecht W. S. , Torres S. , Michel R. P.
Published: July 2002

A novel design is presented for a cryogenically cooled multisubstrate sample holder for thin film deposition in high vacuum. The cryogen flows through tubing soldered onto a fixed copper plate, while the substrates are mounted on a separate tray that is supported by a linear/rotary-motion feedthrough. By lowering the substrate tray into firm contact with the copper plate, substrate temperatures as low as 120 K can be achieved.
High precision soft x-ray reflectometer
Authors: Fuchs D. , Krumrey M. , Müller P. , Scholze F. , Ulm G.
Published: July 2002

At the radiometry laboratory of the Physikalisch-Technische Bundesanstalt (PTB) a reflectometer for the soft x-ray spectral region has been operated for several years utilizing monochromatic radiation of a toroidal grating monochromator or a high-resolution plane grating monochromator. The monochromators cover the photon energy region from 35 to 1500 eV. New challenges due to the development of soft x-ray optical components led to the design of a second reflectometer with advanced capabilities. Samples with a diameter of up to 250 mm can be accommodated. The time required for sample exchange is reduced by using a lock chamber. The feasibility of sample positioning with high precision has been improved. Typical uncertainties of about 1%–2%, e.g., for the reflectance of multilayer mirrors, can be achieved. A detailed description of the reflectometer as well as some typical results showing the performance are presented.
Argonne National Laboratory X6B beamline at NSLS: A versatile facility
Authors: Huang K. G. , Ramanathan M. , Montano P. A.
Published: July 2002

With high-intensity, high-energy resolution, energy tunability, and flexibility of operation, the Argonne National Laboratory X6B beamline at the National Synchrotron Light Source (NSLS) has become a versatile facility for a variety of x-ray diffraction, scattering, and spectroscopy experiments. The beamline can be operated in either focused or unfocused beam mode, depending on the requirement of specific experiments. We describe the x-ray optics and beamline performance, and present selected experimental results to demonstrate the main features of the X6B beamline.
Integrated fiber optic light probe: Measurement of static deflections in rotating turbomachinery
Authors: Dhadwal Harbans S. , Mehmud Ali , Khan Romel , Kurkov Anatole
Published: July 2002

This paper describes the design, fabrication, and testing of an integrated fiber optic light probe system for monitoring blade tip deflections, vibrational modes, and changes in blade tip clearances in the compressor stage of rotating turbomachinery. The system comprises a set of integrated fiber optic light probes which are positioned to detect the passing blade tip at the leading and the trailing edges. In this configuration measurements of both blade vibrations and steady-state blade deflection can be obtained from the timing information provided by each light probe, which comprises an integrated fiber optic transmitting channel and a number of high numerical aperture receiving fibers, all mounted in the same cylindrical housing. A spatial resolution of 50 μm is possible with the integrated fiber optic technology, while keeping the outer diameter below 2.5 mm. Additionally, one fiber sensor provides a capability of monitoring changes in the blade tip clearance of the order of 10 μm. Measurements from a single stage compressor facility and an engine-fan rig in a 9 ft×15 ft subsonic wind tunnel are presented.
X-ray sensor based on a plastic optical fiber for monitoring high voltage interrupters
Authors: Pierre P. , Farenc J. , Destruel P. , Lescure M.
Published: July 2002

An x-ray sensor based on a fluorescent plastic optical fiber is described. The core is doped with an organic fluor while double cladding contains an inorganic product (ZnS) converting x-ray energy into visible light. The light emitted by the sensor is guided by a commercial PMMA fiber to a photodiode whose signal is amplified. The electronics have been optimized to increase sensitivity and reduce noise. The paper indicates the methods used to prepare fibers. It describes their characterization and gives the main characteristics of the amplifier. The sensor is used to visualize the x-rays emitted by vacuum interrupters during switching. Signal modification gives us information on the surface degradation of the electrodes. Among other advantages, the sensor can be used on site, without disconnecting the interrupter.
Conceptual design of a beam extraction electrode system for a 1 mN ion thruster
Authors: Sandonato G. M. , Barroso J. J.
Published: July 2002

Conceptual modeling of the multiaperture electrode system of a 5-cm-diam ring cusp ion thruster intended for producing a 1 mN thrust is described. A mathematical treatment of the ion extraction from the plasma has been developed to provide a method for calculating both the shape and potential of the plasma-emitting surface. The ion optics of the electrode system have been examined using a numerical code to evaluate the ion beam divergence. Close agreement is achieved between the predictions of the conceptual modeling presented here and published experimental results.
Plasma production via laser-induced photoemissiona)
Authors: Pickard D. S. , Kunkel W. B. , Leung K. N. , Young A. T.
Published: July 2002

The feasibility of laser-induced photoemission as a driving mechanism for short plasma pulse production is currently being investigated with a pulsed excimer laser at 248 nm. Low work function materials such as LaB6 and barium are used as cathode materials and the resulting plasma characteristics are being examined. Results from early measurements of the barium photocathode show a strong dependence of the photoemitted current on the source pressure and cathode voltage. Additionally, the temporal behavior of the emitted electron pulse is found to consist of two components: a short 50 ns burst corresponding to laser-induced photoemission, and a larger, longer 100 ns pulse increasing from the tail end of the photoemitted electron pulse.
The influence of the spectral emissivity on the signal phase at photothermal radiometry
Authors: Seidel U. , Walther H. G.
Published: July 2002

Photothermal radiometric (PTR) measurements from paint coated samples are compared with calculations taking into account the real ir spectrum of the coating. The photothermal signal phase was found to depend sensitively on the detected wavelength range. This fact has to be taken into consideration for the interpretation of PTR measurements.
Frequency response of metal-clad inductive magnetic field probes
Authors: Strait E. J.
Published: July 2002

The frequency response of an inductive magnetic field probe enclosed in a metal shell is calculated, including the important effect of inductive coupling between the probe coil and the shell. With an appropriate passive external circuit, the bandwidth of the system can be extended significantly beyond the cutoff frequency of the shell alone. A simple, noninvasive test shows that the model accurately predicts the behavior of magnetic probes in the DIII-D tokamak.
High-resolution surface sensing device using Bragg diffraction from multiplexed holograms in photorefractive crystal
Authors: Shiratori Akira , Obara Minoru
Published: July 2002

A novel high-resolution surface sensing device is developed and demonstrated, which utilizes Bragg diffraction from the volume holograms in the photorefractive BaTiO3 crystal. In this device, the angular information of a surface is obtained as a diffraction pattern from the multiplexed holograms stored in the crystal. The resolution of this device is as high as submilliradian due to the Bragg selectivity. Additionally, a new method of surface structure comparison between two samples is also demonstrated using this device, which is applicable to the nondestructive surface inspection of optical components.
Improving the SOPRA DMDP2000 spectrometer by a Michelson interferometer
Authors: Crescentini Luca , Amoruso Antonella
Published: July 2002

A new class of multiple dispersion grating spectrometer was commercialized a few years ago (SOPRA DMDP2000). In practice, mechanical tolerances do not let it operate at its very best characteristics. The instrument can be considerably improved by using a simple Michelson interferometer.
New developments in photoconductive detectors (invited)
Authors: Han S.
Published: July 2002

Nearly ideal for detecting ionizing radiation, wide band-gap semiconductors present a possibility of having outstanding radiation hardness, fast charge collection, and low leakage current that will allow them to be used in high radiation, high temperature, and chemically aggressive environments. Over the past few years, the improvements in the electrical quality of wide band-gap semiconductors have progressed enormously. One particular wide band-gap semiconductor, diamond, has properties that may be ideal for radiation detection. Since the discovery of low pressure and low temperature deposition of diamond, the possibility of large area diamond films has become a reality. Over the past few years, great progress has been made in advancing the electrical quality of chemical-vapor-deposited (CVD) diamond. Presently, unprecedented diamond wafer size of 7 in. diam is possible. Due to both the present electrical quality and the available size, the utilization of diamond in radiation detection applications is not just a dream but a reality. The progression of CVD diamond’s electrical properties in the last few years will be presented along with what is currently possible. Applications of CVD diamond for the National Ignition Facility (NIF) diagnostics will be reviewed. In addition, a brief review concerning other possible wide band-gap semiconductors for ICF diagnostics will be presented.
Development of a small-probe pulsed nuclear magnetic resonance system for measuring magnetic field uniformity
Authors: Kato Shuichi , Maehata Keisuke , Ishibashi Kenji , Wakuta Yoshihisa , Shintomi Takakazu
Published: July 2002

The pulsed nuclear magnetic resonance (NMR) field measurement system is suitable for high-accuracy field homogeneity measurement for accelerator dipole magnets. The measurement has to use NMR probes as small as a few mm in diameter. The coaxial cable connecting the NMR probe and preamplifier has a length of 10 m or more. The coaxial cable has to be treated as a distributed-constant circuit for high-field measurement. We studied the optimum condition of the radio-frequency signal propagation in the coaxial cable, and found that a cable length of nλNMR/2 (n=1,2,…) gives optimum propagation characteristics. In accordance with this result, we designed and fabricated a prototype field measurement system using pulsed NMR. The use of a phase-locked loop circuit facilitated the measurement of the frequency of the free-induction-decay signal. To confirm the performance of this system, the magnetic field in the 2 T region of the superconducting solenoid magnet was measured with the NMR probe of which the shape was a cylinder of 4 mm in diameter.
A sample translatory type insert for automated magnetocaloric effect measurements
Authors: Gopal B. R. , Chahine R. , Bose T. K.
Published: July 2002

This paper describes the development of an automated sample translatory type apparatus capable of measuring the magnetocaloric effect of potential magnetic refrigerants over the temperature range 10 to 325 K in applied fields of up to 9 T. A Quantum Design® Physical Property Measurement System has been used as the host platform. The emphasis of the design is on rapid and accurate automated measurements with field slew rates of 9 T per second. Measurement repeatabilities of ±30 mK have been achieved. Results are presented for the rare earth metals gadolinium, dysprosium and holmium, and a gadolinium-yttrium alloy.
Description of a pulsed capacitor discharge ionization source for the generation of intense cluster ion beams
Authors: Brock Ansgar , Cedeño David L. , Manzanares I. Carlos
Published: July 2002

Pulsed capacitor discharge ionization in supersonic expansions was investigated for the production of intense beams of molecular cluster ions from seeded and ablated compounds. A low cost high voltage high current pulser based on a triggered spark gap switch is described as a mean for ionization and ablation. In order to optimize the ion beam intensity and its composition, several nozzle geometries and electrode arrangements are presented. The individual components of the molecular beam setup and the timing sequence for discharge ionization are given. The performance of the instrument is illustrated by showing the styrene ion signal distribution at different delay times of detection. Examples with benzene clusters and Cu+−(acetone)n clusters illustrate other capabilities of the instrument.
Role of C and B clusters in transient diffusion of B in silicon
Authors: Cowern N. E. B. , Cacciato A. , Custer J. S. , Saris F. W. , Vandervorst W.
Published: July 2002

Transient diffusion of ion-implanted B is inhibited in the presence of high C or B concentrations, due to the formation of interstitial clusters stabilized by impurity atoms. Comparison between experiments and simulations suggests that the number of self-interstitials trapped per clustered impurity atoms is ≊1.15 for C and ∼1 for B, consistent with a volume compensation mechanism.
Nucleation layer evolution in metal-organic chemical vapor deposition grown GaN
Authors: Wu X. H. , Kapolnek D. , Tarsa E. J. , Heying B. , Keller S. , Keller B. P. , Mishra U. K. , DenBaars S. P. , Speck J. S.
Published: July 2002

The structure and morphology of low growth temperature GaN nucleation layers have been studied using atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED), and transmission electron microscopy (TEM). The nucleation layers were grown at 600 °C by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire. The layers consist of predominantly cubic GaN (c-GaN) with a high density of stacking faults and twins parallel to the film/substrate interface. The average grain size increases with increasing layer thickness and during the transition from low temperature (600 °C) to the high temperatures (1080 °C) necessary for the growth of device quality GaN. Upon heating to 1080 °C the nucleation layer partially converts to hexagonal GaN (h-GaN) while retaining a high stacking fault density. The mixed cubic-hexagonal character of the nucleation layer region is sustained after subsequent high-temperature GaN growth.
A near-field scanning optical microscopy study of the photoluminescence from GaN films
Authors: Liu Jutong , Perkins N. R. , Horton M. N. , Redwing J. M. , Tischler M. A. , Kuech T. F.
Published: July 2002

We have achieved spatially resolved photoluminescence from GaN films using a near-field scanning optical microscope (NSOM). GaN films grown by hydride vapor phase epitaxy (HVPE) and metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates have been studied. We have performed spatial scans of topography, band edge, and yellow luminescence signals. Atomic force microscopy measurements were also made and compared with the NSOM topography. We have found spatial variations in photoluminescence characteristics at the submicron scale for both HVPE and MOVPE GaN. The observed enhancement of yellow luminescence at multiatomic step edges on the HVPE GaN surface suggests that the yellow luminescence is associated with chemical impurities incorporated during the growth of GaN films.
Photon-assisted tunneling in GaAs/AlGaAs superlattices up to room temperature
Authors: Zeuner S. , Allen S. J. , Maranowski K. D. , Gossard A. C.
Published: July 2002

Photon-assisted transport is shown to be a remarkably robust phenomenon in sequential resonant superlattices with a large energy separation between the ground state and the first excited state of the quantum wells. Photon-assisted tunneling involving up to seven photons per tunneling event, stands out very clearly. The one-photon channel is observed up to 300 K. This implies that these superlattices are a gain medium at THz frequencies even at room temperature and potentially important for the future design of THz sources.
Study of nonlinear optical effects in GaN:Mg epitaxial film
Authors: Zhang H. Y. , He X. H. , Shih Y. H. , Schurman M. , Feng Z. C. , Stall R. A.
Published: July 2002

The second-order nonlinear optical coefficients d33, d31, and d15 of GaN:Mg epitaxial film were studied by the standard Maker fringe of an anisotropic medium. The measured d33=−(16.5±1.3) pm/V which is 55 times of the d11 of quartz. The measured ratios of d33/d31 and d31/d15 showed that the crystalline film is close to an ideal wurzite structure. The refractive indices and the dispersive curves of ne, n0 were also determined by TM and TE waveguide mode measurements.
Computer-assisted optical emission tomography in a radiofrequency capacitively coupled plasma
Authors: Kitajima T. , Izawa M. , Hashido R. , Nakano N. , Makabe T.
Published: July 2002

Temporal characteristics of two-dimensional (2D) spatial images of a net production rate in a capacitively coupled plasma (CCP) reactor driven at high frequency (13.56 MHz) are investigated by newly developed computer-assisted optical emission tomography. Typical examples are presented for the absolute net formation rate of Ar(3p5) in Ar for 2 Torr and 13.56 MHz. The tomographic imaging enables us to understand a complex radial discharge structure in a CCP reactor, as well as in the central axial region.
Mobility enhancements in AlGaN/GaN/SiC with stair-step and graded heterostructures
Authors: Lin C. F. , Cheng H. C. , Huang J. A. , Feng M. S. , Guo J. D. , Chi G. C.
Published: July 2002

In this letter, we report the observation of the enhanced electron mobility in three different Al0.08Ga0.92N/GaN heterostructures. These structures were deposited on (0001) 6H-SiC substrates by using the low-pressure metalorganic chemical vapor deposition method. The structure was composed of 500 Å AlGaN compositional stair-step layer deposited onto 1.3 μm GaN epitaxial layer. There is a 100 Å GaN Cap layer on top of the AlGaN layer to prevent the oxidation of the AlGaN layer. By using the van der Pauw method of Hall measurement, the sheet carrier density and mobility for Al0.08Ga0.92N/GaN heterostructure are 6.6×1012 cm-2 and 5413 cm2/Vs at 77 K, respectively. This high value of mobility is, to the best of our knowledge, the first observed two-dimensional electron gas (2DEG) at the AlGaN/GaN stair-step type of heterostructure. Other interface structures such as graded composition and bulk composition structures were also prepared and 2DEG properties were observed.
Generation of 11 fs pulses tunable across the visible by optical parametric amplification
Authors: Cerullo Giulio , Nisoli Mauro , De Silvestri Sandro
Published: July 2002

We have generated pulses with 11 fs duration, tunable between 500 and 700 nm, using a noncollinear β-barium borate optical parametric amplifier pumped by the second harmonic of a Ti:sapphire laser.
Monte Carlo analysis of ultrafast electron transport in quantum well infrared photodetectors
Authors: Ryzhii M. , Ryzhii V.
Published: July 2002

A self-consistent ensemble Monte Carlo simulation is used to study ultrafast electron transport in quantum well infrared photodetectors (QWIPs). It is shown that transient photocurrent triggered by a short infrared radiation pulse reveals a sharp peak followed by a relatively slow decay. The photocurrent peak is associated with the electron velocity overshoot effect while the slow component of the photocurrent is determined by the electron transit time and capture effects. The velocity overshoot effect results in the existence of a plateau in the QWIP frequency-dependent responsivity in the range of terahertz frequencies.
Temperature fiber sensor based on semiconductor nanocrystallite-doped phosphate glasses
Authors: Barmenkov Yu. O. , Starodumov A. N. , Lipovskii A. A.
Published: July 2002

We propose a temperature fiber sensor based on a CdSe semiconductor nanocrystallite-doped phosphate glass. For 6 nm semiconductor particles the reversible temperature-induced absorption edge shift is demonstrated in the range of 0 °C–150 °C. The prototype of the fiber sensor with sensitivity of 0.12 nm/K and a simple direct intensity detection scheme are reported.
Resistive transition of niobium superconducting hot-electron bolometer mixers
Authors: Floet D. Wilms , Baselmans J. J. A. , Klapwijk T. M. , Gao J. R.
Published: July 2002

We present a model for the description of the resistive transition in hot-electron bolometer mixers. We show that the transition is a property of a superconducting microbridge connected to normal conducting cooling pads. Using the concepts of the superconducting proximity effect, charge-imbalance generation, and Andreev reflection, we have calculated the resistance versus temperature of the device and demonstrate its dependence on the length of the microbridge, both theoretically and experimentally. The analysis reopens the question of the relationship between the resistive transition and the situation in which the device is optimally operated as a heterodyne mixer.
Saturation of intraband absorption and electron relaxation time in n-doped InAs/GaAs self-assembled quantum dots
Authors: Sauvage S. , Boucaud P. , Glotin F. , Prazeres R. , Ortega J.-M. , Lemaı̂tre A. , Gérard J.-M. , Thierry-Flieg V.
Published: July 2002

We have observed the saturation of intraband absorption in InAs/GaAs self-assembled quantum dots. The investigated n-doped self-assembled quantum dots exhibit an intraband absorption within the conduction band, which is peaked at an 8 μm wavelength. The saturation of the intraband absorption is achieved with an infrared pump delivered by a pulsed free-electron laser. The saturation of the transition is observed for an intensity around ≈0.6 MW cm−2. The electron relaxation time under intraband excitation is measured by time-resolved pump–probe experiments. An electron relaxation time T1≈3 ps is reported.
Thermionic emission cooling in single barrier heterostructures
Authors: Shakouri Ali , LaBounty Chris , Piprek Joachim , Abraham Patrick , Bowers John E.
Published: July 2002

Nonisothermal transport in InGaAsP-based heterostructure integrated thermionic coolers is investigated experimentally. Cooling on the order of a degree over 1 μm thick barriers has been observed. This method can be used to enhance thermoelectric properties of semiconductors beyond what can be achieved with the conventional Peltier effect.
Photoreflectance investigations of the bowing parameter in AlGaN alloys lattice-matched to GaN
Authors: Ochalski Tomasz J. , Gil Bernard , Lefebvre Pierre , Grandjean Nicolas , Leroux Mathieu , Massies Jean , Nakamura Shuji , Morkoç Hadis
Published: July 2002

Room temperature photoreflectance investigations have been performed on a series of AlGaN layers grown both by metalorganic vapor phase epitaxy and molecular beam epitaxy on c-plane sapphire substrates. The aluminum composition was ranging between 0% and 20%, and was determined independently in the different growth laboratories, by various methods. It is found that within the experimental uncertainty, there is no detectable bowing parameter in these alloys. This contradicts some previous experimental investigations and confirms other ones.
Multiple scattering attenuation and anisotropy of ultrasonic surface waves
Authors: Scales John A. , Van Wijk Kasper
Published: July 2002

Multiple scattering of waves induces bulk effects such as attenuation and anisotropy that are important in seismology, optics, medical imaging, and other fields involving propagation in disordered media. We report here measurements of ultrasonic surface wave propagation in a strong-scattering, quasiperiodic medium consisting of a grooved surface of aluminum. Using noncontacting optical methods we have tracked the evolution of surface wave pulses within the scattering medium. Waves propagating parallel to the grooves propagate nearly attenuation and dispersion free, whereas waves propagating normal to the grooves are dispersed and exponentially attenuated with distance as energy is transferred from the direct pulse into the multiple-scattering coda. We measure this attenuation length and show that there is, in addition, a scattering induced anisotropy in the phase velocity.
A plasma process for ultrafast deposition of SiGe graded buffer layers
Authors: Rosenblad C. , Känel H. von , Kummer M. , Dommann A. , Müller E.
Published: July 2002

Low energy plasma enhanced chemical vapor deposition (LEPECVD) has been applied to the synthesis of Si-modulation doped field effect transistor structures, comprising a SiGe relaxed buffer layer and a modulation doped strained Si channel. A growth rate of at least 5 nm/s for the relaxed SiGe buffer layer is well above that obtainable by any other technique. Due to the low ion energies involved in LEPECVD, ion damage is absent, despite a huge plasma density. The structural quality of the LEPECVD grown SiGe buffer layers is comparable to that of state-of-the-art material. The electronic properties of the material were evaluated by growing modulation doped Si quantum wells on the buffer layers. We obtain a low temperature (2 K) Hall mobility of μH=2.5×104 cm2/Vs for the electrons in the Si channel at an electron sheet density of ns=8.6×1011 cm−2.
An optimal quasisuperlattice design to further improve thermal stability of tantalum nitride diffusion barriers
Authors: Chen G. S. , Huang S. C. , Chen S. T. , Yang T. J. , Lee P. Y. , Jou J. H. , Lin T. C.
Published: July 2002

X-ray diffraction and transmission electron microscopy, along with electrical and film stress measurements, were used to evaluate the effectiveness of 40-nm-thick amorphous Ta2N and microcrystalline TaN diffusion barriers, both single and multilayered, against Cu penetration. Failure of the single-layered Ta2N diffusion barrier upon annealing is initialized by crystallization/grain growth, mainly helped by frozen-in compressive stress (3–4 GPa) to transform itself into a columnar structure with a comparable grain size to the thickness of the barrier. However, when subjected to annealing, the Ta2N/TaN alternately layered diffusion barrier with an optimum bilayer thickness (10 nm) remains almost stress-free (0–0.7 GPa) and transforms itself into an equiaxed structure with grain sizes of only ⩽3 nm. Such quasisuperlattice films can present lengthening and complex grained structures to effectively block Cu diffusion, thus acting as much more effective barriers than Ta2N (and TaN) single-layered films.
Electro-optic polymer modulators with 0.8 V half-wave voltage
Authors: Shi Yongqiang , Lin Weiping , Olson David J. , Bechtel James H. , Zhang Hua , Steier William H. , Zhang Cheng , Dalton Larry R.
Published: July 2002

We report the fabrication and test results for polymeric electro-optic modulators with a half-wave voltage of 0.8 V and a half-wave voltage-interaction length product of 2.2 V cm. These modulators employ an optical push–pull architecture and are made from poly(methylmethacrylate) with a high molecular hyperpolarizability polyene bridge-type chromophore. An electro-optic coefficient of 58 pm/V was obtained at a 1318 nm wavelength. The guest–host polymer system exhibited a thermal stability to 75 °C and a relatively stable nonlinearity at ambient conditions. The experimental results have demonstrated not only the sub-1 V half-wave voltage electro-optic polymer modulator but also the potential of polymeric electro-optic materials for photonic applications.
Transport process of high-speed impurity pellets through curved guide tubes
Authors: Nihei Hitoshi , Ogawa Yuichi , Nakajima Toru , Morita Shigeru
Published: July 2002

Curved guide tubes to transport high-speed pellets are used for pellet injection into plasma devices. This article describes the transport process of the high-speed pellets through curved guide tubes. The pellets pass through the curved tubes with repeating reflections. Using the equation of friction at each reflection point, the velocity and the direction of the output pellet are calculated by following the pellet trajectory. The analytic expressions, with respect to the velocity and the direction of the output pellet, are also derived. It has been confirmed that there is good correspondence between the results obtained by following the pellet trajectories, those by the analytic expressions and those by the experiments. The analytic expressions are particularly useful for the design and making of the curved guide tubes used in the pellet injection.
A modified dual-slope method for heat capacity measurements of condensable gases
Authors: Pilla S. , Hamida J. A. , Sullivan N. S.
Published: July 2002

A high resolution nonadiabatic method for measuring the heat capacity (CP) of bulk samples of condensable gases in the range of 7.5–70 K is described. In this method CP is evaluated by directly comparing the heating and cooling rates of the sample temperature for two algebraically independent heat pulse sequences without explicit use of the thermal conductance between sample and thermal bath. A fully automated calorimeter for rapid measurement of CP of molecular solids utilizing this technique is presented. The technique along with the automated calorimeter with a provision to apply external electric and magnetic fields is particularly useful for the study of continuous phase transitions in molecular solids as well as field induced changes in the heat capacity.
Transverse magnetic field propellant isolator
Authors: Foster John E.
Published: July 2002

An alternative high voltage propellant isolator for electric propulsion and ground-based ion source applications has been designed and tested. This design employs a transverse magnetic field that increases the breakdown voltage. The design can greatly enhance the operating range of laboratory propellant isolators used for high voltage applications.
Microstructure and magnetic properties of strained La0.7Sr0.3MnO3 thin films
Authors: Haghiri-Gosnet A. M. , Wolfman J. , Mercey B. , Simon Ch. , Lecoeur P. , Korzenski M. , Hervieu M. , Desfeux R. , Baldinozzi G.
Published: July 2002

The lattice deformation of dense strained La0.7Sr0.3MnO3 (LSMO) films is shown to control the easy direction of the magnetization. Optimized pulsed laser deposited conditions allow the fabrication of dense LSMO thin films which present an exceptional flatness with a peak–valley roughness (Rp–v) of 1 Å, associated to epitaxial grains as large as 1 μm. Electron microscopy coupled with x-ray diffraction have been used to study the unit cell distortion of both tensile and compressive dense LSMO films as a function of the thickness. No relaxation of the lattice distortion imposed by substrate has been observed in the thickness range 10–60 nm. The Curie temperature is not significantly affected by the nature of the substrate: a TC of 350 K is observed for both SrTiO3 (STO) and LaAlO3 (LAO) substrates, i.e., close to the bulk material (369 K). In contrast, the easy direction of magnetization depends on the substrate. For tensile films deposited on the STO substrate, the unit cell is elongated along the film’s plane (ain-plane=3.905 Å) with a reduced perpendicular parameter (cperp=3.85 Å): an easy direction of magnetization M in the plane of the film is observed. For compressive films deposited on LAO substrate, the situation is reversed with a unit cell elongated along the direction of growth (cperp=4.00 Å and ain-plane=3.79 Å) and an easy axis for M along this perpendicular out-plane direction. It is thus demonstrated that the larger cell parameter, ain-plane for films deposited on STO and cperp for films deposited on LAO, is fully correlated to the direction of the easy magnetization.
Fast relaxation, boson peak, and anharmonicity in Li2O–B2O3 glasses
Authors: Kojima S. , Novikov V. N. , Kodama M.
Published: July 2002

Glasses with a composition xLi2O⋅(1−x)B2O3 were investigated by low-frequency Raman scattering in the composition range x=0–0.28. The evolution of the quasielastic line, the boson peak, the Debye frequency, and some other glass parameters with the composition was analyzed. The frequency of the boson peak ωb shifts with changing x by a factor of 3 and the width of the quasielastic spectrum at room temperature is always equal to ∼0.24ωb. The Grüneisen parameter of the glasses is estimated on the basis of the light scattering data for the boson peak frequency within the frames of the anharmonic model of the fast relaxation and using the sound velocity data—for the Debye frequency. The anharmonic properties are compared with the fragility of these glassformers; it is shown that the fragility increases with anharmonicity. It is shown also that the width of the glass transition region correlates with the anharmonic properties.
Polarized reflectometer for the investigation of surface magnetism, the new polarized neutron reflectometer with polarization analysis at the Laboratoire Léon Brillouin
Authors: Fermon C. , Ott F. , Legoff G. , Glättli H. , Wintenberger V.
Published: July 2002

We present the new polarized reflectometer for the investigation of surface magnetism, which has been built at the Laboratoire Léon Brillouin. This polarized neutron reflectometer with polarization analysis has been optimized for the study of small magnetic samples (1 cm2 range). We describe the new multilayer monochromator which has permitted us to obtain a wider wavelength distribution (∼4%) and the focusing neutron guide which has allowed the focalization of a 100 mm high beam onto a region 15 mm high. This new optical setup has allowed a large neutron flux increase, which is now of 2×105 n/cm2 s.
Development and evaluation of a surface acoustic intensity probe
Authors: Driesch Patricia L. , Iwata Hisao , Koopmann Gary H. , Dosch Jeff
Published: July 2002

Commonly, sound power radiated from vibrating machinery is measured either with a two-microphone intensity probe or in a reverberant chamber using a reference source. However, for sound source identification, it is difficult to isolate the sound power radiated from specific areas from the total sound power using these conventional methods. To facilitate this type of measurement, a type of intensity probe has been developed that measures sound intensity directly on the surface of a vibrating structure. It is comprised of an accelerometer and a microphone and requires a straightforward calibration which measures the phase shift in the microphone relative to the accelerometer. This is accomplished with a device that supports the probe on the top of a piezoelectric actuator and encloses both within a small cavity. This method provides a frequency-dependent phase correction for the microphone referenced to the accelerometer. For evaluation at low frequencies where the probe is most applicable, its performance compares favorably (to within 1 dB) with the two-microphone intensity probe measurement for the case of sound radiating from a square, clamped plate.
Relative energies of the C2H2S2 isomers 1,2-dithiete and dithioglyoxal: Peculiar basis set dependencies of density functional theory and ab initio methods
Authors: Timoshkin Alexey , Frenking Gernot
Published: July 2002

Ab initio calculations at the levels of Hartree–Fock (HF), second order Møller–Plesset perturbation theory (MP2), coupled-cluster theory with singles, doubles, and estimated triple excitations [CCSD(T)], and density functional theory (DFT) using the functionals B3LYP and B3PW91 of the relative energies of the C2H2S2 isomers 1,2-dithiete (2a), and dithioglyoxal (2b) show a peculiar dependence of the results on the f-type polarization functions. The ab initio calculations with 6-31G(nd) basis sets with n=1–3 incorrectly predict that 2a is higher in energy than 2b. The relative energies at the MP2 and CCSD(T) levels change by more than 6 kcal/mol in favor of 2a if the basis set is augmented by one set of f functions. The DFT calculations also give a higher stability of 2a relative to 2b if f functions are included in the basis sets, but the change in the relative energy is only ∼2 kcal/mol. The large change in the relative energies which are calculated at MP2 and CCSD(T) are mainly due to the functions at sulfur, while the effect of the f functions in the DFT calculations is mainly due to the f functions at carbon.
Camera length and field of view in Makyoh-topography instruments
Authors: Riesz Ferenc
Published: July 2002

Makyoh topography is an optical method for the study of the large-scale morphology of mirror-like surfaces. Formulas are derived for the two basic parameters, the camera length and the field of view of Makyoh instruments. The relation between these parameters and the sensitivity are established. The formulas allow the proper design and setting of Makyoh instruments. Different types of Makyoh instruments are compared.
Volume phase transitions of nematic gels under an external field
Authors: Matsuyama Akihiko , Kato Tadaya
Published: July 2002

The effect of external (magnetic or electric) fields on the swelling of nematic gels dissolved in an isotropic solvent are discussed. We calculate the swelling ratio of the gel, orientational order parameter, and phase diagrams on a temperature–swelling ratio plane under an external field. For a weak external field, we find a first-order volume phase transition between two nematic gels with different orientational ordering. Upon increasing the strength of an external field, a critical point appears on the phase diagram. We also discuss the swelling of nematic gels as a function of the strength of an external field for various temperatures.
On the problem of algebraic completeness for the invariants of the Riemann tensor: I
Authors: Zakhary E. , Carminati J.
Published: July 2002

We present a new determining set, CZ, of Riemann invariants which possesses the minimum degree property. From an analysis on the possible independence of CZ, we are led to the division of all space–times into two distinct, invariantly characterized, classes: a general class MG+, and a special, singular class MS. For each class, we provide an independent set of invariants (IG+⊂CZ and IS⊂CZ, respectively) which, with the results of a sequel paper, will be shown to be algebraically complete.
Numerical modeling of magnetohydrodynamic activity in the Swarthmore Spheromak Experiment
Authors: Lukin V. S. , Qin G. , Matthaeus W. H. , Brown M. R.
Published: July 2002

Results from a three-dimensional axisymmetric resistive magnetohydrodynamic (MHD) simulation are compared to experimental data from the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)]. The MHD simulation is run under conditions and with dimensionless parameters similar to the experiment (Lundquist number S=1000, plasma beta β=0.1). The simulation is shown to reproduce global equilibrium magnetic field profiles of the spheromaks as well as much of the detailed reconnection dynamics measured when two spheromaks are merged. It is concluded that SSX merger dynamics may be characterized as MHD reconnection, with the likelihood that extensions are needed to account for kinetic effects in the associated current sheet. High spatial and temporal resolution MHD simulation data will be used as input for a particle orbit and energization code.
Collisional, magnetic, and nonlinear skin effect in radio-frequency plasmas
Authors: Chen Francis F.
Published: July 2002

The penetration of radio-frequency energy into cylindrical devices is relevant to the production of uniform plasmas for etching and deposition processes in the production of semiconductor circuits. The so-called “anomalous skin effect” has been invoked to explain irregularities not predicted by classical electromagnetic theory. These expectations are summarized for the collisionality regimes of interest, and new results are given for nonkinetic effects caused by small direct current magnetic fields and the ponderomotive force.
Growth, structure, and transport properties of thin (>10 nm) n-type microcrystalline silicon prepared on silicon oxide and its application to single-electron transistor
Authors: Kamiya T. , Nakahata K. , Tan Y. T. , Durrani Z. A. K. , Shimizu I.
Published: July 2002

Microcrystalline silicon (μc-Si:H) thin films were prepared at 300 °C on glass. Their structure and transport properties were studied in a wide range of film thickness ranging from 10 nm to 1 μm. The crystal fraction increases monotonously from ∼64% to ∼100% as film thickness increases. Electron mobility first increases with increasing film thickness at thicknesses smaller than 50 nm but saturates at larger thickness. This mobility behavior is explained by percolation transport through crystalline grains. These results are different from those obtained with preferentially oriented polycrystalline silicon films. It is related to the difference in the microstructure evolution in which subsequent film growth is influenced by the growth surface structure. A single-electron transistor fabricated in 30-nm-thick μc-Si:H exhibits Coulomb blockade effects at 4.2 K. This result indicates that amorphous phases which exist between crystalline grains behave as tunnel barrier for electrons.
Long time scale evolution of collisionless driven reconnection in a two-dimensional open system
Authors: Pei Wenbing , Horiuchi Ritoku , Sato Tetsuya
Published: July 2002

Long time scale evolution of collisionless driven reconnection in an open system is investigated by means of two-dimensional full particle simulation based on an open boundary model. Collisionless reconnection is externally driven by the plasma inflow, which is mainly controlled by two key parameters of an external driving electric field, i.e., the strength E0 and the early nonuniformity scale xd. The strength E0 controls reconnection rate, while the scale xd controls the current layer shape and thus the magnetic-field configuration. It is found that the dynamical behavior of collisionless reconnection is sensitive to xd and less to E0 in our simulation parameter range. In the small xd case, the system evolves toward a steady state in which the reconnection rate is balanced with the external driving field E0. As xd increases, the reconnection evolution exhibits an intermittent phenomenon because of the frequent excitation of magnetic islands near the original X point.
Comparison of full multiple spawning, trajectory surface hopping, and converged quantum mechanics for electronically nonadiabatic dynamics
Authors: Hack Michael D. , Wensmann Amanda M. , Truhlar Donald G. , Ben-Nun M. , Martı́nez Todd J.
Published: July 2002

We present calculations employing the simplest version of the full multiple spawning method, FMS-M or minimal FMS, for electronically nonadiabatic quantum dynamics using three model potential energy matrices with different strengths and ranges for the diabatic coupling. We first demonstrate stability of the branching probabilities and final energy distributions with respect to the parameters in the FMS-M method. We then compare the method to a variety of other semiclassical methods, as well as to accurate quantum mechanical results for three-dimensional atom–diatom reactions and quenching processes; the deviations of the semiclassical results from the accurate quantum mechanical ones are averaged over nine cases. In the adiabatic electronic representation, the FMS-M method provides some improvement over Tully’s fewest switches trajectory surface hopping method. However, both methods, irrespective of electronic representation, systematically overpredict the extent of reaction in comparison to the exact quantum mechanical results. The present work provides a baseline for understanding the simplest member of the hierarchy of FMS methods and its relationship to established surface hopping methods.
Nonlinear spinor in a Kerr–Schild background
Authors: Cadavid A. C. , Finkelstein R. J.
Published: July 2002

We study the nonlinear spinor field in a Kerr–Schild background by first looking for solitonic solutions in the absence of rotation, given that in the special relativistic limit there are solitonic solutions in this approximation. Since for the scalar field problem at least two independent radial functions are needed for the solitonic solutions to exist, we introduce a dilation field by deforming the Kerr–Schild metric by a Weyl factor. We find that for the parameter space studied there are no solitonic solutions in the spherically symmetric approximation. While in the present work we have studied the spinor field, we also had in mind a corresponding study of a scalar field in the same background where it may be of interest for describing a rotating stellar soliton. The proposed strategy in both cases begins by looking for solitonic solutions in the absence of rotation and then continues by perturbatively correcting for the rotation required by the angular dependence of the Kerr background. The strategy appears to be unsuccessful.
Performance of Co/Al2O3/NiFe magnetic tunnel junctions prepared by a two-step rf plasma oxidation method
Authors: Yoon K. S. , Park J. H. , Choi J. H. , Yang J. Y. , Lee C. H. , Kim C. O. , Hong J. P. , Kang T. W.
Published: July 2002

A two-step rf plasma oxidation technique of an insulating layer has been performed to enhance electrical and structural properties of magnetic tunnel junction (MTJ) devices. Comparison was made by analyzing properties of the MTJ oxidized by conventional rf and two-step rf plasma oxidation methods. Experimentally observed results give improved surface imaging and sufficient oxygen contents of the insulating layer under the two-step oxidation method. In addition, electrical breakdown voltage and magnetoresistance of the MTJ were increased from 0.7 to 1.8 V and from 4.5% to 6.8%, respectively, correlated with improved structural information.
Detection of stacking faults in carbon dioxide clusters
Authors: Kovalenko S. I. , Solnyshkin D. D. , Verkhovtseva É. T. , Eremenko V. V.
Published: July 2002

An electron-diffraction study of clusters formed in supersonic jets of carbon dioxide is reported. The mean size of the clusters varies from 4×102 to 2×104 molecules/cluster. From an analysis of the diffraction patterns obtained it is established that clusters of these sizes have the structure of the bulk crystal. Stacking faults are observed for the first time in clusters with an average size of less than 1.7×103 molecules/cluster; the density of these stacking faults increases with decreasing size of the aggregations. It is found that the density of stacking faults in (CO2)N clusters is considerably lower than rare-gas clusters.
Low-resistance and thermally stable ohmic contact on p-type GaN using Pd/Ni metallization
Authors: Jang Ho Won , Kim Ki Hong , Kim Jong Kyu , Hwang Soon-Won , Yang Jung Ja , Lee Kang Jae , Son Sung-Jin , Lee Jong-Lam
Published: July 2002

We report a low-resistance thermally stable ohmic contact on p-type GaN using a promising contact scheme of Pd/Ni. Specific contact resistance as low as 5.7×10−5 Ω cm2 was obtained from the Pd (30 Å)/Ni (70 Å) contact annealed at 500 °C under an oxidizing ambient. NiO that formed at the surface prevented Pd atoms from outdiffusing, promoting the formation of Pd gallides, Ga2Pd5 and Ga5Pd. This reaction produces Ga vacancies below the contact, leading to enhancement of the thermal stability as well as reduction of the contact resistivity.