bluuebunny/crossref_metadata_2025 huggingface dataset:
Apparatus for high-temperature internal friction differential measurements
Authors: Gadaud P. , Guisolan B. , Kulik A. , Schaller R.
Published: July 2002

We present a new internal friction pendulum conceived and built to work at high temperature (up to 1800 K) with forced vibrations allowing measurements as a function of the frequency (from 10−4 to 10 Hz) as well as the temperature. The main difference with previous constructions is that data are obtained by comparison of the specimen deformation with the deformation of an elastic standard, instead of comparing with the excitation signal. At first we describe the mechanical part and the computed control of all the parameters during measurements. Then we present the first tests that show the great sensitivity of the measurements due to the quality of the mechanics and to the principle of differential acquisition. We also present the interest of internal friction tests to study the behavior of materials for which the plasticity appears at high temperatures (carbides, ceramics).
Counting statistics and loss corrections for the APS
Authors: Lee Wah Keat , Mills Dennis M.
Published: July 2002

It has been suggested that for timing experiments, it might be advantageous to arrange the bunches in the storage ring in an asymmetrical mode. In this paper, we determine the counting losses from pulsed x-ray sources from basic probabilistic arguments and from Poisson statistics. In particular the impact on single-photon counting losses of a variety of possible filling modes for the Advanced Photon Source (APS) is examined. For bunches of equal current, a loss of 10% occurs whenever the count rate exceeds 21% of the bunch repetition rate. This changes slightly when bunches containing unequal numbers of particles are considered. The results are applied to several common detector/electronics systems.
A high precision ultrasonic system for vibration measurements
Authors: Young M. S. , Li Y. C.
Published: July 2002

A microcomputer-aided ultrasonic system that can be used to measure the vibratory displacements of an object is presented. A pair of low cost 40-kHz ultrasonic transducers is used to transmit ultrasound toward an object and receive the ultrasound reflected from the object. The relative motion of the object modulates the phase angle difference between the transmitted and received ultrasound signals. A single-chip microcomputer-based phase detector was designed to record and analyze the phase shift information which is then sent to a PC-AT microcomputer for processing. We have developed an ingenious method to reconstruct the relative motion of an object from the acquired data of the phase difference changes. A digital plotter based experiment was also designed for testing the performance of the whole system. The measured accuracy of the system in the reported experiments is within ±0.4 mm and the theoretical maximal measurable speed of the object is 89.6 cm/s. The main advantages of this ultrasonic vibration measurement system are high resolution, low cost, noncontact measurement and it is easily installed.
Design of a constant tension thermocouple rake suitable for flame studies
Authors: Ahuja Sandeep , Miller David L.
Published: July 2002

An improved, spring-loaded thermocouple rake, suitable for studying flame structure, has been designed. This design keeps the thermocouple under tension thereby ensuring that the thermocouple does not droop due to the thermal expansion of the sensing wire when inserted in the flame. The present design allows the usage of thermocouple wire as small as 0.0508 mm and relative ease in changing thermocouple wire.
Fabrication and characterization of micromagnet arrays on top of GaAs/AlGaAs heterostructures
Authors: Ye P. D. , Weiss D. , von Klitzing K. , Eberl K. , Nickel H.
Published: July 2002

Using nanolithographic techniques we deposit an array of ferromagnetic dysprosium dots with period a=500 nm on top of GaAs–AlGaAs heterojunctions. The spatially periodic stray fields affect drastically the magnetotransport properties of the electrons in the underlying two-dimensional electron gas and give rise to the long predicted magnetic commensurability oscillations. We show that such a semiconductor-ferromagnet hybrid system can be used to study the magnetic properties of nanoscale particles.
Ion cyclotron emission on the Tokamak Fusion Test Reactor
Authors: Cauffman S. , Majeski R.
Published: July 2002

Preliminary observations of ion cyclotron emission (ICE) have been made in D-T plasmas on the Tokamak Fusion Test Reactor, using an array of seven rf probes located at the top and bottom of the torus. Fusion product line emission has been identified, and appears to originate at the low field equatorial plane, near the radius of the outboard limiter. While the peak intensity of D-T fusion product ICE exhibits a correlation with overall neutron production, the time evolution of the ICE signal does not follow that of the neutron rate. During high power neutral beam injection, beam-species ICE is also observed, with frequencies corresponding to the outer midplane edge of the plasma.
Fast, real-time multichannel scaler, construction and applications
Authors: Zieliński Marek , Karasek Krzysztof , Dygdala Roman S.
Published: July 2002

The construction of a fast, real-time multichannel scaler and its application to the analysis of signals composed of series of pulses are described. The device allows measurements in time intervals of up to 1.3 ms with a single time-channel width of 5 ns and accepts input pulses of 2 ns in width. The capacity of the single time channel and the repetition rate are up to 32 bits and 50 per second, respectively. Applications to laser deposition/ionization and three-photon-ionization experiments involving pulsed laser sources are briefly discussed.
A linearized Josephson–Fraunhofer magnetometer with high-temperature Josephson junction
Authors: Dolabdjian C. , Poupard P. , Martin V. , Gunther C. , Hamet J. F. , Robbes D.
Published: July 2002

We have fabricated a Josephson–Fraunhofer magnetometer based on the shrinking of the Fraunhofer pattern exhibited by high-Tc grain boundaries Josephson (GBJ) junctions. It is obtained using flux focuser and GBJ junction directly patterned on a single chip. The device is actively voltage biased and operates in a field locked loop as conventional superconducting quantum interference device systems. The amplitude of the auxiliary modulation field is adjusted to obtain the maximal sensitivity; the coupling between the feedback coil and the flux focuser has been studied to evaluate the magnetometer performances. The system bandwidth is 1 kHz and the measured noise level is 40 pT/√Hz above 10 Hz. The slew rate and the dynamic range of the magnetometer are 2 mT/s and 112 dB (1 Hz bandwidth), respectively.
New type of time-series sediment trap for the reliable collection of inorganic and organic trace chemical substances
Authors: Kremling K. , Lentz U. , Zeitzschel B. , Schulz-Bull D. E. , Duinker J. C.
Published: July 2002

The new sediment trap has a 0.5 m2 aperture, a funnel slope of 34° and is capable of collecting 21 samples at programmed intervals (1 min–1 year) during deployment in the deep ocean. The trap has been designed to allow reliable data on trace inorganic and organic components (such as trace elements, n-alkanes, PCBs, PAHs, amino/fatty acids) in addition to the standard biogeochemical variables in the collected particles. Due to the exclusive use of synthetic (such as fiberglass, PVC, PTFE, or POM-Delrin®) and highly resistant metallic materials contamination problems have been eliminated for these species. Blank values determined in several tests in the open ocean were as low as 1% or even less of the amounts present in trap material, even at low particulate loadings. Another major aim was the elimination of loss of dissolved components from the sample cups into seawater. Microbial and chemical processes modify the collected particles, thereby mobilizing originally particulate species into solution. It is thus essential to avoid greater losses of dissolved species through diffusion into the surrounding seawater after collection. This was achieved by means of an especially designed sealing mechanism. Tests with tracer substances during field studies proved that losses of dissolved components from supernatants during one year of deployment are as low as 10%. Additionally, the relationship between flow characteristics around the traps and their excursions from the vertical position in a bottom-tethered array was studied during a one-year deployment in the North Atlantic.
Compact floating ion energy analyzer for measuring energy distributions of ions bombarding radio-frequency biased electrode surfaces
Authors: Edelberg Erik A. , Perry Andrew , Benjamin Neil , Aydil Eray S.
Published: July 2002

A compact floating retarding-field ion energy analyzer and the accompanying electronics have been designed and built to measure the energy distribution of ions bombarding radio-frequency (rf) biased electrodes in high-density plasma reactors. The design consists of two main components, a compact retarding field vacuum probe and an integrated stack of floating electronics for providing output voltages, measuring currents and voltages and transmitting data to a computer. The operation and capabilities of the energy analyzer are demonstrated through ion energy distribution measurements conducted on a 4 MHz rf-biased electrostatic chuck in a 13.56 MHz high-density transformer coupled plasma (TCP) reactor. The analyzer is capable of operating while floating on several hundreds of volts of rf bias and at pressures up to 30 mTorr without differential pumping. The effects of pressure (2–30 mTorr), TCP power (500–1500 W), rf-bias power (0–800 W), gas composition, and ion mass on the ion energy distributions are demonstrated through Ar, Ne, and Ar/Ne discharges.
Spontaneous and stimulated emission from photopumped GaN grown on SiC
Authors: Zubrilov A. S. , Nikolaev V. I. , Tsvetkov D. V. , Dmitriev V. A. , Irvine K. G. , Edmond J. A. , Carter C. H.
Published: July 2002

Photoluminescence of GaN layers grown on 6H–SiC substrates was studied in the temperature range 77–900 K. GaN layers were grown by metalorganic chemical vapor deposition. The temperature dependence of the band gap of GaN was measured throughout the entire temperature range. Edge cavity stimulated emission from photopumped GaN layers was observed in the temperature range 77–450 K. The full width at half-maximum (FWHM) of the stimulated emission peak was ∼3 nm at 300 K and ∼7 nm at 450 K. Multipass stimulated emission with Fabry–Pérot modes was detected from GaN. The FWHM of Fabry–Pérot modes was ∼0.2 nm (300 K).
Single-electron transistor logic
Authors: Chen R. H. , Korotkov A. N. , Likharev K. K.
Published: July 2002

We present the results of numerical simulations of a functionally complete set of complementary logic circuits based on capacitively coupled single-electron transistors (CSETs). The family includes an inverter/buffer stage, as well as two-input NOR, NAND, and XOR gates, all using similar tunnel junctions, and the same dc bias voltage and logic levels. Maximum operation temperature, switching speed, power consumption, noise tolerances, error rate, and critical parameter margins of the basic gates have been estimated. When combined with the data from a preliminary geometrical analysis, the results indicate that implementation of the CSET logic family for operation at T∼20 K will require fabrication of structures with ∼2-nm-wide islands separated by ∼1-nm-wide tunnel gaps.
Formation and observation of 50 nm polarized domains in PbZr1−xTixO3 thin film using scanning probe microscope
Authors: Hidaka T. , Maruyama T. , Saitoh M. , Mikoshiba N. , Shimizu M. , Shiosaki T. , Wills L. A. , Hiskes R. , Dicarolis S. A. , Amano Jun
Published: July 2002

Nanometer size polarized domains were written in a PbZr1−xTixO3 (PZT) thin film using an atomic force microscope (AFM) and the relationship between the polarized domain and the grain of the film was investigated. The polarized domain was formed by applying a pulse voltage to the ferroelectric PZT thin film through a conductive AFM tip. The polarized domain structure was observed by imaging the piezoelectric-induced surface vibration by an AFM with an ac voltage applied between the tip and the bottom electrode of a sample. The polarized domains with a diameter of 50 nm were written within a single grain.
Pressure induced structural transitions in nanometer size particles of PbS
Authors: Qadri S. B. , Yang J. , Ratna B. R. , Skelton E. F. , Hu J. Z.
Published: July 2002

At elevated pressure, PbS undergoes a first order phase transition from the NaCl or B1 structure to an orthorhombic structure. The effects of particle sizes in the nanometer range on this transition have been investigated using energy-dispersive x-ray diffraction of synchrotron produced wiggler radiation. Relative to the bulk crystals, the onset of transition pressure showed a significant increase with decreasing particle size. The results also show that compressibility increases with decreasing particle size: this increase is continuous through the phase transition.
Nanooxidation using a scanning probe microscope: An analytical model based on field induced oxidation
Authors: Stiévenard D. , Fontaine P. A. , Dubois E.
Published: July 2002

The formation of a nanometer-size oxide pattern on silicon using a scanning probe microscope (SPM) has been widely reported in the literature. No analytical model has been proposed, however, to explain the variation of the oxide height with both polarization and speed of the SPM tip. In this letter, we explain quantitatively the variation of the oxide height with the polarization and the speed of the tip with a model based on field induced oxidation. Data analysis also allows us to estimate the thermal activation energy of the oxidation process, (∼0.15 eV). This low value is compared with activation energies measured for thermal and plasma oxidation of silicon.
Estimating the effective magnetostriction of a composite: A simple model
Authors: Herbst J. F. , Capehart T. W. , Pinkerton F. E.
Published: July 2002

For a composite consisting of magnetostrictive particles dispersed in a nonmagnetostrictive matrix, a simple, approximate model for the effective magnetostriction, defined as the ratio of the magnetostriction of the composite to that of the magnetostrictive component, is described. The predictions of the model compare well with measurements on SmFe2-based composites having Al and Fe matrices.
Polarization switching in vertical-cavity surface emitting lasers observed at constant active region temperature
Authors: Martín-Regalado J. , Chilla J. L. A. , Rocca J. J. , Brusenbach P.
Published: July 2002

Polarization switching in gain-guided, vertical-cavity, surface-emitting lasers was studied as a function of the active region temperature. We show that polarization switching occurs even when the active region temperature is kept constant during fast pulse low duty cycle operation. This temperature independent polarization switching phenomenon is explained in terms of a recently developed model.
Anisotropic strains and magnetoresistance of La0.7Ca0.3MnO3
Authors: Koo T. Y. , Park S. H. , Lee K.-B. , Jeong Y. H.
Published: July 2002

Thin films of perovskite manganite La0.7Ca0.3MnO3 were grown epitaxially on SrTiO3(100), MgO(100) and LaAlO3(100) substrates by the pulsed laser deposition method. Microscopic structures of these thin film samples as well as a bulk sample were fully determined by x-ray diffraction measurements. The unit cells of the three films have different shapes, i.e., contracted tetragonal, cubic, and elongated tetragonal for SrTiO3, MgO, and LaAlO3, respectively, while the unit cell of the bulk is cubic. It is found that the samples with a cubic unit cell show smaller peak magnetoresistance at low fields (≲1 T) than the noncubic ones do. The present result demonstrates that the magnetoresistance of La0.7Ca0.3MnO3 at low fields can be controlled by lattice distortion via externally imposed strains.
Magnetic-field-enhanced rf argon plasma for ionized sputtering of copper
Authors: Wang W. , Foster J. , Wendt A. E. , Booske J. H. , Onuoha T. , Sandstrom P. W. , Liu H. , Gearhart S. S. , Hershkowitz N.
Published: July 2002

A multipole magnetic field was used to increase the ion density of an inductively coupled rf (13.5 MHZ) argon plasma for ionized magnetron sputtering of copper (Cu). Langmuir probe measurements showed an increase of plasma density over a factor of 2 with the application of the magnetic field. At an argon pressure of 15 mTorr and a rf power of 600 W, an ion density of 1.2×1012 ions/cm3 was achieved. When this plasma was applied to ionize the magnetron sputtered Cu vapor, a high emission intensity ratio from the Cu+ ion line to the Cu neutral line was observed from the optical emission spectroscopy, suggesting a high ionization fraction for the sputtered Cu vapor.
Efficiency of NH3 as nitrogen source for GaN molecular beam epitaxy
Authors: Mesrine M. , Grandjean N. , Massies J.
Published: July 2002

We show that optical reflectivity measurements can be used to evaluate the part of a NH3 flux which reacts with a Ga-terminated GaN surface or with a Ga-flux simultaneously impinging on the surface, as in standard molecular beam epitaxy situation. At least for temperatures not exceeding 700 °C, the ratio of the reacted part of the NH3 flux to the incident flux can be assimilated to the NH3 cracking efficiency. Being nearly zero below a threshold temperature of 450 °C, it increases with temperature but remains low (∼4%) explaining why an exceptionally high V/III flux ratio is necessary to grow GaN using NH3.
Model experiments on fatigue of Pb(Zr0.53Ti0.47)O3 ferroelectric thin films
Authors: Du Xiaofeng , Chen I-Wei
Published: July 2002

With an n-type or p-type semiconductor as a top electrode in contact with ferroelectric Pb(Zr0.53Ti0.47)O3 thin film, polarization fatigue has been studied to investigate the effect of charge carrier injection. Electron injection is shown to be correlated with fatigue while hole injection is not. Current blocking by an insulating SiO2 layer prevents fatigue as well. The enhanced mobility of oxygen vacancies, partially de-ionized by association with the injected electrons, is proposed to be the mechanism for the electron effect.
Effective magnetostriction of magnetostrictive composites
Authors: Nan Ce-Wen
Published: July 2002

An analytical method is developed for the effective magnetostriction of magnetostrictive composites based on Green’s function technique. For an elastically isotropic composite of magnetostrictive and nonmagnetostrictive phases, the analytical results give explicit effective-medium approximate formulas for the effective magnetostriction that fit well with available experimental data for SmFe2-based composites.
Raman studies of nitrogen incorporation in GaAs1−xNx
Authors: Prokofyeva T. , Sauncy T. , Seon M. , Holtz M. , Qiu Y. , Nikishin S. , Temkin H.
Published: July 2002

We report direct-backscattering Raman studies of GaAs1−xNx alloys, for x⩽0.03, grown on (001) GaAs. The Raman spectra exhibit a two-mode behavior. The allowed GaAs-like longitudinal-optic phonon near 292 cm−1 is found to red shift at a rate of −136±10 cm−1/x. This is well described by the combined effects of strain and alloying. The GaN-like phonon near 470 cm−1 is observed to increase in intensity in direct proportion to x, and to systematically blue shift at a rate of 197±10 cm−1/x. This blue shift is likewise attributed to strain and alloying. The GaAs-like second-order features are also seen to broaden slightly and diminish in intensity with increasing nitrogen concentration. These results are attributed to a weak breakdown in the zincblende-crystal long-range order, possibly related to the presence of ordered domains within the random alloy.
Waveguide laser film in erbium-doped KTiOPO4 by pulsed laser deposition
Authors: Wang Ke-Ming , Shi Bo-Rong , Cue Nelson , Zhu Yong-Yuan , Xiao Rong-Fu , Lu Fei , Li Wei , Liu Yao-Gang
Published: July 2002

Erbium doping into potassium titanyl phosphate (KTiOPO4 or KTP) was performed by pulsed laser deposition. Waveguide laser film in Er-doped KTiOPO4 was formed using both KTiOPO4 and erbium as targets and optically polished KTiOPO4 as a substrate. Rutherford backscattering, photoluminescence spectrometry, x-ray diffraction, and prism coupling method are used to investigate the properties of Er-doped KTiOPO4. The results show that (1) Er-doped KTiOPO4 emits the characteristic photoluminescence of Er3+ around 1.54 μm corresponding to transition between the I13/24 and I15/24 manifolds of the Er3+ ions in KTiOPO4 host. (2) two bright modes in Er-doped KTiOPO4 were observed. This approach may be useful for direct fabrication of thin film waveguide laser for other optoelectrical materials.
Rapid imaging of calcite crystal growth using atomic force microscopy with small cantilevers
Authors: Paloczi George T. , Smith Bettye L. , Hansma Paul K. , Walters Deron A. , Wendman Mark A.
Published: July 2002

Using a 26 μm cantilever with a resonant frequency of 100 kHz in water, we were able to obtain sequential images of calcite crystal steps growing from a screw dislocation. The small cantilever permitted acquisition of 250 nm images at scan rates of 104 lines/s (1.2 s/image). From this sequence we directly measured critical step lengths (the length of the shortest step that can advance) of 6–21 nm. These values provided a rough estimate of (0.25±0.13 J/m2) for the step energy per unit length per unit step height on the (104) face of calcite.
Quantitative characterization of GaN quantum-dot structures in AlN by high-resolution transmission electron microscopy
Authors: Arlery M. , Rouvière J. L. , Widmann F. , Daudin B. , Feuillet G. , Mariette H.
Published: July 2002

GaN/AlN heterostructures grown by molecular beam epitaxy are studied by high-resolution transmission electron microscopy (HRTEM). The two-dimensional/three-dimensional Stranski–Krastanow growth mode transition of GaN allows the formation of GaN quantum-dot structures embedded in AlN. The nature of the wetting layer associated with these dots is determined by quantitative HRTEM analysis, based on comparison between interplanar distortion profiles of experimental and simulated images. This study demonstrates a low intermixing between GaN and AlN materials. Such result is also evidenced for the GaN dots.
Optical properties of molecular organic semiconductor thin films under intense electrical excitation
Authors: Kozlov V. G. , Burrows P. E. , Parthasarathy G. , Forrest S. R.
Published: July 2002

Using electro-optical pump and probe measurements, we studied optical properties of a thin film of tris-(8-hydroxyquinoline) aluminum (Alq3) doped with DCM2 laser dye under pulsed injection currents as high as 300 A/cm2. Pump-induced transparency and absorption were observed in the spectral ranges of 570–610 nm and 620–705 nm, respectively. We attribute these optical effects to molecular polarons formed under electrical excitation. Polaron absorption dominates the optical properties of electrically pumped Alq3:DCM2 films in the spectral range where optical gain is anticipated (i.e., from 630 to 700 nm). Polaronic effects may present a significant obstacle for realization of organic diode lasers based on this material system.
Laser-based method for fabricating monodisperse metallic nanoparticles
Authors: Bosbach J. , Martin D. , Stietz F. , Wenzel T. , Träger F.
Published: July 2002

An experimental method is presented for fabrication of almost monodisperse metal nanoparticles on substrate surfaces. It relies on substantial narrowing of broad size distributions through irradiation with short laser pulses by exploiting the size dependent optical absorption coefficient of the metal particles. Successive irradiation by applying two laser wavelengths completely removes the smallest clusters of the distribution and causes a size reduction of the largest particles. Finally, only clusters with diameters in a very narrow size interval remain on the surface. By using this method, Ag clusters with mean diameters of 〈d〉=10 nm and size distributions with standard deviations of Δd/〈d〉=0.13 have been prepared.
Ion-beam-induced surface damages on tris-(8-hydroxyquinoline) aluminum
Authors: Liao L. S. , Hung L. S. , Chan W. C. , Ding X. M. , Sham T. K. , Bello I. , Lee C. S. , Lee S. T.
Published: July 2002

Surface damage of tris-(8-hydroxyquinoline) aluminum (Alq3) film by 100 eV Ar+ irradiation has been studied by using both x-ray and ultraviolet photoelectron spectroscopies (XPS and UPS). XPS core level electron density curves revealed that some N–Al and C–O–Al bonds in Alq3 molecules were broken by the irradiation. Correspondingly, the valence band structure of the Alq3 molecule, as measured by UPS, was tremendously changed. The highest occupied state extended towards the Fermi level (EF), implying that a metal-like conducting surface was formed. This kind of damaged surface would cause nonradiative quenching in an electroluminescence device when electrons are injected from the cathode into the Alq3 layer, and possibly result in electrical shorts.
Sub-diffraction-limited patterning using evanescent near-field optical lithography
Authors: Alkaisi M. M. , Blaikie R. J. , McNab S. J. , Cheung R. , Cumming D. R. S.
Published: July 2002

Patterning at resolution below the diffraction limit for projection optical lithography has been demonstrated using evanescent near-field optical lithography with broadband illumination (365–600 nm). Linewidths of 50 nm and gratings with 140 nm period have been achieved. Ultrathin photoresist layers in conjunction with conformable photomasks are employed and a reactive ion etching process using SF6 has been developed to transfer the patterns to a depth of more than 100 nm into silicon. Full electromagnetic field simulations of the exposure process show that a high contrast image is present within the resist layer, and that the exposure is dominated by one polarization for the grating structures studied.
Current-induced realignment of magnetic domains in nanostructured Cu/Co multilayer pillars
Authors: Katine J. A. , Albert F. J. , Buhrman R. A.
Published: July 2002

We have developed a nanofabrication technique to facilitate current-perpendicular-to-plane transport measurements on magnetic multilayer pillar structures with diameters as narrow as 100 nm—a size scale at which the reversal of individual domains within the ferromagnetic layers may be detected. When large currents are passed through such pillars, the Oersted field produced by the current can affect the orientation of the magnetic moments of the layers. In pillars ranging from 250 to 500 nm, a stack of alternating hard and soft ferromagnetic layers can controllably be switched between high and low resistance states via this mechanism.
Mechanism of diffusional transport during ion nitriding of aluminum
Authors: Telbizova T. , Parascandola S. , Kreissig U. , Günzel R. , Möller W.
Published: July 2002

The mechanism of diffusional transport during low-energy ion nitriding of aluminum has been investigated using marker and isotope sequence techniques in connection with ion-beam analysis. For an ion energy of 1 keV and a temperature of 400 °C, it is shown that the nitride grows at the surface with aluminum being supplied by diffusion from the underlying bulk.
Generation of terahertz pulses by photoionization of electrically biased air
Authors: Löffler T. , Jacob F. , Roskos H. G.
Published: July 2002

We present an experimental demonstration of the generation of far-infrared (terahertz) pulses by photoionization of electrically biased air with amplified laser pulses. The current surge following photoionization of the air with an applied bias field of 10.6 kV/cm leads to the emission of THz pulses with an intensity which can be almost as high as that of THz pulses radiated from a large-area intrinsic-field GaAs emitter. The spectra peak at higher frequency than those of biased large-area GaAs emitters.
Recombination coefficients of O and N radicals on stainless steel
Authors: Singh Harmeet , Coburn J. W. , Graves David B.
Published: July 2002

Surface recombination coefficients of O and N radicals in pure O2 and N2 plasmas, respectively, have been estimated on the stainless steel walls of a low-pressure inductively coupled plasma reactor. The recombination coefficients are estimated using a steady state plasma model describing the balance between the volume generation of the radicals from electron-impact dissociation of the parent molecules, and the loss of the radicals due to surface recombination. The model uses radical and parent molecule number densities and the electron energy distribution function (EEDF) as input parameters. We have measured the radical number density using appearance potential mass spectrometry. The parent neutral number density is measured using mass spectrometry. The EEDF is measured using a Langmuir probe. The recombination coefficient of O radicals on stainless steel walls at approximately 330 K is estimated to be 0.17±0.02, and agrees well with previous measurements. The recombination coefficient of N radicals is estimated to be 0.07±0.02 on stainless steel at 330 K.
Quantum confinement in germanium nanocrystals
Authors: Niquet Y. M. , Allan G. , Delerue C. , Lannoo M.
Published: July 2002

The electronic structure of Ge nanocrystals is studied using a sp3 tight binding description. Analytical laws for the confinement energies, valid over the whole range of sizes, are derived. We validate our results with ab initio calculations in the local density approximation for smaller clusters. Comparing to experimental data, we conclude that, similar to the case of silicon: (a) the blue-green photoluminescence (PL) of Ge nanocrystals comes from defects in the oxide and (b) the size dependent PL in the near infrared probably involves a deep trap in the gap of the nanocrystals. We predict that the radiative lifetimes remain long in spite of the small difference (0.14 eV) between direct and indirect gaps of bulk Ge.
Thermal conductivity of fully and partially coalesced lateral epitaxial overgrown GaN/sapphire (0001) by scanning thermal microscopy
Authors: Florescu D. I. , Asnin V. M. , Pollak Fred H. , Jones A. M. , Ramer J. C. , Schurman M. J. , Ferguson I.
Published: July 2002

We have measured high spatial/depth resolution (∼2–3 μm) thermal conductivity (κ) at 300 K of both fully and partially coalesced GaN/sapphire (0001) samples fabricated by lateral epitaxial overgrowth. On the fully coalesced sample we found 1.86W/cm K<κ<2.05 W/cm K over a distance of approximately 50 μm. One of the partially coalesced samples had 2.00 W/cm K<κ<2.10 W/cm K on the overgrown regions, as identified by atomic force microscopy imaging. These latter results are the highest thermal conductivity values reported on GaN material. A correlation between low threading dislocation density and high thermal conductivity values was established.
Analysis of solvation structure and thermodynamics of methane in water by reference interaction site model theory using an all-atom model
Authors: Cui Qizhi , Smith Vedene H.
Published: July 2002

An all-atom molecular model and optimized site–site pair potential parameters are employed for methane solute. By use of the reference interaction site model theory, the methane hydration structure is analyzed at the atomic level and its hydration free energies, energies, enthalpies, and entropies for four different thermodynamic states: temperature T=283.15, 298.15, 313.15, 328.15 K and the corresponding bulk water density ρ=0.9997, 0.9970, 0.9922, 0.9875 g cm−3 are calculated. Our calculated thermodynamic properties are in rather good agreement with experiments.
Knudsen cell mass spectrometric investigation of the B2N molecule
Authors: Meloni G. , Baba M. Sai , Gingerich K. A.
Published: July 2002

High-temperature Knudsen cell mass spectrometry has been used to study the equilibria involving the B2N molecule over the Si–BN system. Thermal functions needed in the evaluation of the mass spectrometric equilibrium data have been calculated from available experimental and theoretical molecular parameters. The enthalpy changes for the reactions 2B(g)+Si2N(g)=B2N(g)+2Si(g), and BN(s)+B(g)=B2N(g) have been measured. The following atomization enthalpy, ΔaH0o, and enthalpy of formation, ΔfH298.15o, in kJ mol−1, of 1045.5±18 and 551.3±18 for the B2N molecule have been determined from these reaction enthalpies. Atomization energies of similar molecules have been compared and discussed.
Effect of gate leakage current on noise properties of AlGaN/GaN field effect transistors
Authors: Rumyantsev S. L. , Pala N. , Shur M. S. , Gaska R. , Levinshtein M. E. , Khan M. Asif , Simin G. , Hu X. , Yang J.
Published: July 2002

The effect of the gate leakage current fluctuations on noise properties of AlGaN/GaN heterostructure field effect transistors (HFETs) has been studied in conventional HFET structures and in AlGaN/GaN metal-oxide-semiconductor heterostructure field effect transistors (MOS-HFETs). The comparison of the noise properties of conventional AlGaN/GaN HFETs and AlGaN/GaN MOS-HFETs fabricated on the same wafer, allowed us to estimate the contribution of the gate current noise to the HFET’s output noise. The effect of the gate current fluctuations on output noise properties of HFETs depends on the level of noise in the AlGaN/GaN HFETs. For the transistors with a relatively high magnitude of the Hooge parameter α∼10−3, even a relatively large leakage current Ig (Ig/Id∼10−3–10−2, where Id is the drain current) does not contribute much to the output noise. In HFETs with a relatively small values of α (α∼10−5–10−4), the contribution of the leakage current to output noise can be significant even at Ig/Id∼10−4–10−3. For such transistors, a very rapid increase of the 1/f noise with gate bias was observed. The differences in the noise behavior can be linked to the material quality of the AlGaN and GaN layers in different types of HFETs.
Thermoelectric detection of spherical tin inclusions in copper by magnetic sensing
Authors: Carreon Hector , Nagy Peter B. , Nayfeh Adnan H.
Published: July 2002

Inclusions and other types of imperfections in metals can be nondestructively detected by noncontacting magnetic measurements that sense the thermoelectric currents around such flaws when the specimen is subjected to directional heating and cooling. This article presents experimental data for the magnetic field produced by thermoelectric currents around surface-breaking spherical tin inclusions in copper under external thermal excitation for different lift-off distances between the sensor and the surface of the specimen. The diameter of the inclusions and the lift-off distance varied from 2.4 to 12.7 mm and from 12 to 20 mm, respectively. A fairly modest 0.7 °C/cm temperature gradient in the specimen produced peak magnetic flux densities ranging from 1 to 250 nT. These results were found to be in good agreement with recently published theoretical predictions [P. B. Nagy and A. H. Nayfeh, J. Appl. Phys. 87, 7481 (2000)].
Young–Laplace equation for liquid crystal interfaces
Authors: Rey Alejandro D.
Published: July 2002

This letter uses the classical theories of liquid crystal physics to derive the Young–Laplace equation of capillary hydrostatics for interfaces between viscous isotropic (I) fluids and nematic liquid crystals (NLC’s), and establishes the existence of four energy contributions to pressure jumps across these unusual anisotropic interfaces. It is shown that in addition to the usual curvature contribution, bulk and surface gradient elasticity, elastic stress, and anchoring energy contribute to pressure differentials across the interface. The magnitude of the effect is proportional to the elastic moduli of the NLC, and to the bulk and surface orientation gradients that may be present in the nematic phase. In contrast to the planar interface between isotropic fluids, flat liquid crystal interfaces support pressure jumps if elastic stresses, bulk and surface gradient energy, and/or anchoring energies are finite.
Orbital angular momentum (Renner–Teller) effects in the Πi2 ground state of silicon methylidyne (SiCH)
Authors: Smith Tony C. , Li Haiyang , Hostutler David A. , Clouthier Dennis J. , Merer Anthony J.
Published: July 2002

The ground state vibrational energy levels of jet-cooled SiCH and SiCD have been studied by a combination of laser-induced fluorescence and wavelength-resolved fluorescence techniques. The radicals were produced by a pulsed electric discharge at the exit of a supersonic expansion using tetramethylsilane or methyltrichlorosilane as the precursor. Emission spectra have been obtained by pumping both perpendicular and parallel (vibronically induced) bands, providing complementary information on the Si–C stretching and Si–C–H bending modes. Ground state energy levels up to 4000 cm−1 have been assigned and fitted using a vibrational Hamiltonian that incorporates Renner–Teller, spin–orbit, vibrational anharmonicity, and Fermi resonance interactions. The validity of the derived parameters has been tested using the isotope relations.
Point vortex dynamics within a background vorticity patch
Authors: Jin Dezhe Z. , Dubin Daniel H. E.
Published: July 2002

Recent experiments and simulations have observed that the interaction of strong vortices with a low vorticity background can strongly affect the dynamics of both vortices and background. This paper considers an idealized model of this interaction. The background is treated as a patch of uniform vorticity with a nearly circular shape. The strong vortices are treated as point vortices, and their total circulation λ is assumed to be small compared to that of the background. It is found that Kelvin waves on the boundary of the background patch can be driven to large amplitude by the strong vortices, eventually resulting in wave breaking and filamentation. A multiscale analysis finds that the wave-breaking time scales as λ−1 ln λ−1, in agreement with contour dynamics simulations.
Random telegraph signal noise mechanisms in reverse base current of hot carrier-degraded submicron bipolar transistors: Effect of carrier trapping during stress on noise characteristics
Authors: Pogany D. , Chroboczek J. A. , Ghibaudo G.
Published: July 2002

We investigate physical mechanisms of random telegraph signal (RTS) noise in reverse base current of hot carrier-degraded polysilicon emitter bipolar junction transistors. RTS noise, analyzed in the time domain, is studied as a function of reverse base-emitter bias, temperature, and additional reverse-bias stress. Two-level RTS with a relative amplitude as high as 100% is observed at room temperature. The RTS amplitude varies exponentially with the applied reverse base-emitter voltage and depends weakly on temperature. The additional hot carrier stress is observed to induce changes in RTS amplitude and mean pulse widths (independent or correlated), and a disappearance/reappearance of the RTS fluctuations. The results are interpreted by a model where the RTS noise is caused by fluctuations of generation-recombination (g-r) parameters (i.e., capture cross sections and energy position in the gap) of a stress-induced complex bistable defect (CBD) at the Si/SiO2 interface. The complex defect is assumed to be either a two-state fast interface state or an interacting pair of a fast interface state with a slow neighboring border trap. The RTS amplitude is well explained by fluctuations in a single-defect electric-field-enhanced g-r rate between a finite value and naught. The RTS amplitude-bias characteristics and their temperature dependence are satisfactorily accounted for by an expression for a phonon-assisted tunneling current via a single deep-level state. The model parameters are the g-r parameters of the defect and its spatial position in the base-emitter p–n junction. The stress-induced changes in the RTS noise are attributed to the influence of log-time trapping of hot carriers on border states laying in the vicinity of a CBD center. The charged border traps interact with a CBD, changing both its g-r parameters and the RTS switching behavior. The variations in RTS parameters are related to the microscopic nature of the interaction and are discussed for the two types of the CBDs.
Influence of molecular structure on anisotropic photoinduced decomposition of polyimide molecules
Authors: Usami Kiyoaki , Sakamoto Kenji , Ushioda Sukekatsu
Published: July 2002

We have investigated the anisotropic decomposition of polyimide molecules induced by irradiation of linearly polarized ultraviolet light (LPUVL). Three polyimides were examined: poly [4,4′-oxydiphenylene-pyromellitimide], poly [4,4′-oxydiphenylene-1,2,3,4-butanetetracarboximide] and poly [4,4′-oxydiphenylene-1,2,3,4-cyclobutanetetracarboximide] (CBDA-ODA). Anisotropic decomposition was monitored by measuring the polarized infrared absorption spectra of ∼10 nm thick films as a function of LPUVL exposure. Among the three polyimides, CBDA-ODA showed the largest anisotropy of decomposition in the initial stage of LPUVL exposure. This result suggests that CBDA-ODA is a suitable polyimide material for photoinduced liquid crystal alignment based on photodecomposition reactions. The details of the LPUVL exposure dependence of the film anisotropy are discussed.
On dense granular flows down flat frictional inclines
Authors: Louge Michel Y. , Keast Stephen C.
Published: July 2002

We consider dense, relatively shallow flows of 3 mm glass spheres moving down a chute with a flat, frictional base of 3.6 m length. Sustained flows are observed at inclinations corresponding to an effective friction between the static and dynamic friction of individual grains. A capacitance instrument records the formation of waves with a dominant component traveling upstream. Simultaneous measurements of granular temperature at the base using a load cell reveal that the waves are accompanied by substantial reduction in granular agitation. A theory incorporating contributions from impulsive and enduring interactions with the base produces quantitative predictions for the range of sustained flows observed in the experiments. Closure of the theory is achieved using a balance between the production and dissipation of angular momentum in a narrow basal shear layer. A linear stability analysis of the corresponding hydraulic equations further suggests the origin of the waves.
Electron paramagnetic resonance investigation of the photochromic effect in near-stoichiometric LiNbO3 with applications to holographic storage
Authors: Lee Myeongkyu , Gyoo Kim In , Takekawa Shunji , Furukawa Yasunori , Uchida Yoshishige , Kitamura Kenji , Hatano Hideki
Published: July 2002

Near-stoichiometric LiNbO3 (LiNbO3, LiNbO3:Tb, and LiNbO3:Tb,Fe) crystals have exhibited a reversible photochromic effect at room temperature. Irradiation by UV light induced a visible absorption band extending from λ≈650 nm to the absorption edge, which could be bleached by subsequent illumination with visible light. Electron paramagnetic resonance (EPR) analysis at 15 K revealed an EPR signal of g≈2.0 appearing after UV irradiation with a simultaneous decrease in the Fe3+ signal intensity. This implies that the Fe3+ ions (natural impurities or dopants) act as electron traps and the induced absorption band is caused by the charge transfer from UV-sensitive absorption centers to Fe3+ ions via the conduction band. Using the induced absorption band, a high-sensitive holographic recording has been achieved with a continuous-wave green laser at λ≈532 nm. In addition, the fast response to UV light made it possible to quickly erase the stored information by UV illumination.
Characterization of the keyhole formed during pulsed Nd–YAG laser interaction with a Ti–6Al–4V metallic target
Authors: Perret O. , Bizouard M. , Naudy Ph. , Pascal G. , Noré D. , Hordé Y. , Delaisse Y.
Published: July 2002

We observed, with an x-ray apparatus, the keyhole formed during the interaction of a pulsed Nd–YAG laser beam with a metallic target of a Ti–6Al–4V titanium alloy. We noted that the keyhole formation is not instantaneous and that it grows linearly during the duration of the interaction. We highlighted that its depth, at the end of the laser pulse, is practically equal to the melting-zone depth in the material. Finally, we showed from the results of these tests that a quick estimation of the melting depth can be made simply from the ratio of the power densities used with a reference test.
Exponentially correlated Gaussian functions in variational calculations. Momentum space properties of the ground state helium dimer
Authors: Komasa Jacek
Published: July 2002

Microhartree accuracy wave functions composed of exponentially correlated Gaussians were transformed in closed form to momentum space representation and applied to compute various isotropic momentum space properties of helium dimer in the ground state. The set of properties includes electron momentum density distribution, expectation values of powers of the electronic momentum operator, and the Compton profile. Calculations were performed at many internuclear separations R including the united atom (beryllium) and the separated atoms (helium) limits.
On two-dimensional magnetohydrodynamic turbulence
Authors: Biskamp D. , Schwarz E.
Published: July 2002

Numerical studies of decaying two-dimensional (2D) magnetohydrodynamic (MHD) turbulence are presented. Results concern mainly the decay laws of integral quantities and the scaling properties of spectra and structure functions. The self-similarity of the decay is characterized by the invariance of the ratios of resistive and viscous dissipation rates, kinetic and magnetic energies, and mean parallel and perpendicular velocities. The energy decay rate is primarily determined by the conservation of the mean-square magnetic potential, but the process is somewhat retarded at intermediate times by current density condensation in the magnetic eddies. In contrast to the 3D case, the energy spectrum agrees with the Iroshnikov–Kraichnan (IK) law Ek∼k−3/2, but is modified by a strong anomalous bottleneck effect. Structure functions of the Elsässer fields satisfy the property of extended self-similarity, which provides accurate values of the relative scaling exponents. Also the structure functions themselves show clear scaling properties, particularly at the highest Reynolds numbers studied. The scaling exponent ζ2 is consistent with the IK spectrum, but ζ3 and ζ4 remain distinctly below unity, which is surprising in view of an exact relation suggesting ζ3=1. Finally, the role of the tearing mode in 2D MHD turbulence is revisited.
Trefoil symmetries I. Clover extensions beyond Coleman–Mandula theorem
Authors: Wills-Toro L. A.
Published: July 2002

A graded minimal Lie algebraic extension of the space–time symmetry is constructed involving only spin-1 multiplets as novel generators. The extension involves Z4×Z4 graded parameters and generators. It provides a bosonic analog to supersymmetry since the composition of three symmetric vector charges produces a space–time translation. There arise three noncommutative four-dimensional manifolds with pseudometric.
Visual analysis of two-dimensional magnetohydrodynamics
Authors: Frank M. , Barleon L. , Müller U.
Published: July 2002

Magnetohydrodynamics (MHD) offers a unique opportunity to study the behavior of two-dimensional turbulent flows. A strong external magnetic field B perpendicular to the flow direction of an electrically conducting fluid will suppress velocity gradients in the direction of B. The resulting approximation is known as quasi-two-dimensional MHD. An experimental configuration is presented which meets this requirement, along with a spatially extended probe used to visualize the two-dimensional flow kinematics inside the opaque liquid metal flow. As a prototypical example, the wake behind a circular cylinder is investigated for Reynolds numbers up to R=10 000. New and unexpected vortex patterns are observed that deviate significantly from usual hydrodynamic flows. Also, stability limits for the transition from stationary to nonstationary flow patterns are experimentally determined for the cylinder wake and another type of shear flow profile. These results confirm existing theoretical predictions and thus validate the quasi-two-dimensional approach.