bluuebunny/crossref_metadata_2025 huggingface dataset:
Parents’ attitudes to their HIV‐infected children being enrolled into a placebo‐controlled trial: the PENTA 1 trial
Authors:   Paediatric European Network for Treatment of AIDS (PENTA)
Published: July 2002

Objective The study aimed to explore the experience of parents/care‐givers to their child’s participation in a European randomized trial of immediate (zidovudine) with deferred (placebo) antiretroviral treatment in asymptomatic children with vertically acquired HIV infection (PENTA 1 trial).

Design One hundred and thirty‐three questionnaires were distributed to parents/care‐givers (68% of children in the trial) through their paediatrician prior to unblinding the individual child’s therapy (zidovudine/placebo) and 84 (63% response rate) were returned.

Method and results Thirty‐six (43%) parents described moderate (n = 30) or great (n = 6) interference with everyday life. This was more frequent among parents of children whose HIV disease progressed (P = 0.03, Fisher’s exact test) but was unrelated to ethnicity, country of origin, treatment allocated or adverse events. Invited comments suggested that concern about forgetting doses and the taste/volume of the trial medication contributed to interference with everyday life. Seventy‐six (90%) parents considered information received during the trial adequate. The eight expressing dissatisfaction were recruited in the same country and five of them were among the eight (10%) who stated that they would not want to enrol their child in another trial.

Conclusion There is a need for adequate ongoing feedback about trial progress to participating families. With increasing use of complex antiretroviral regimens, innovative ways of helping families with adherence issues require development and evaluation.
Informal Caregiving Time and Costs for Urinary Incontinence in Older Individuals in the United States
Authors: Langa Kenneth M. , Fultz Nancy H. , Saint Sanjay , Kabeto Mohammed U. , Herzog A. Regula
Published: July 2002

OBJECTIVES:To obtain nationally representative estimates of the additional time, and related cost, of informal caregiving associated with urinary incontinence in older individuals.

DESIGN:Multivariate regression models using data from the 1993 Asset and Health Dynamics Study, a nationally representative survey of people aged 70 and older (N = 7,443).

SETTING:Community‐dwelling older people.

PARTICIPANTS:National population‐based sample of community‐dwelling older people.

MEASUREMENTS:Weekly hours of informal caregiving, and imputed cost of caregiver time, for community‐dwelling older people who reported (1) no unintended urine loss, (2) incontinence that did not require the use of absorbent pads, and (3) incontinence that required the use of absorbent pads.

RESULTS:Thirteen percent of men and 24% of women reported incontinence. After adjusting for sociodemographics, living situation, and comorbidities, continent men received 7.4 hours per week of care, incontinent men who did not use pads received 11.3 hours, and incontinent men who used pads received 16.6 hours (P< .001). Women in these groups received 5.9, 7.6, and 10.7 hours (P< .001), respectively. Theadditionalyearly cost of informal care associated with incontinence was $1,700 and $4,000 for incontinent men who did not and did use pads, respectively, whereas, for women in these groups, theadditionalyearly cost was $700 and $2,000. Overall, this represents a national annual cost of more than $6 billion for incontinence‐related informal care.

CONCLUSIONS:The quantity of informal caregiving for older people with incontinence and its associated economic cost are substantial. Future analyses of the costs of incontinence, and the cost‐effectiveness of interventions to prevent or treat incontinence, should consider the significant informal caregiving costs associated with this condition.
22.8% efficient silicon solar cell
Authors: Blakers Andrew W. , Wang Aihua , Milne Adele M. , Zhao Jianhua , Green Martin A.
Published: July 2002

A new silicon solar cell structure, the passivated emitter and rear cell, is described. The cell structure has yielded independently confirmed efficiencies of up to 22.8%, the highest ever reported for a silicon cell.
State of bismuth in BaBiO3 and BaBi1−xPbxO3: Bi 4f photoemission and Bi L3 absorption spectroscopic studies
Authors: Kulkarni G. U. , Vijayakrishnan V. , Ranga Rao G. , Seshadri Ram , Rao C. N. R.
Published: July 2002

The 2p 6d feature in the Bi L3 spectra has different energies in the semiconducting (0.0≤x<0.7) and the superconducting (x=0.75) compositions of BaBi1−xPbxO3. The Bi 4f core level spectrum shows distinct features ascribable to Bi III and Bi V in BaBiO3 and in the semiconducting compositions; the width of the 4f peaks is also considerably larger in these compositions compared to that in BaBi0.25Pb0.75O3, which shows a single sharp Bi 4f feature.
Vertical-cavity, room-temperature stimulated emission from photopumped GaN films deposited over sapphire substrates using low-pressure metalorganic chemical vapor deposition
Authors: Khan M. Asif , Olson D. T. , Van Hove J. M. , Kuznia J. N.
Published: July 2002

We report the first observation of near-UV vertical-cavity stimulated emission from a photopumped GaN epilayer at room temperature. The epilayer was deposited over AIN-coated basal plane sapphire substrate using low-pressure metalorganic chemical vapor deposition. Epitaxy quality of a 1.5-μm-thick GaN layer was high enough to achieve stimulated emission at room temperature. The observed near-UV optical emission power was a nonlinear function of the pump power density. At threshold power density, we also observed line narrowing and a shift of the peak UV emission towards longer wavelengths. Data comparing the UV emission for the vertical-cavity and the edge emission geometry are also presented.
Ultrahigh spontaneous emission quantum efficiency, 99.7% internally and 72% externally, from AlGaAs/GaAs/AlGaAs double heterostructures
Authors: Schnitzer I. , Yablonovitch E. , Caneau C. , Gmitter T. J.
Published: July 2002

Optically thin AlGaAs/GaAs/AlGaAs double heterostructures, (5000 Å), are floated off their substrates by the epitaxial liftoff technique and mounted on various high reflectivity surfaces. From the absolute photoluminescence intensity, we measure internal and external quantum efficiencies of 99.7% and 72%, respectively. High spontaneous emission quantum efficiency, is important for photon number squeezed light, diode lasers, single-mode light-emitting-diodes, optical interconnects, and solar cells.
New transparent vitroceramics codoped with Er3+ and Yb3+ for efficient frequency upconversion
Authors: Wang Yuhu , Ohwaki Junichi
Published: July 2002

Through thermal treatment of a fluoroaluminosilicate glass codoped with Er3+-Yb3+ at a suitable temperature, microcrystallites were successfully precipitated in the glass matrix but little change was observed in the optical transmission spectra even around the ultraviolet edge, meanwhile the green upconversion luminescence due to the 4S3/2-4I15/2 transition of Er3+ increased by a factor of ∼100 when a 0.97 μm laser diode (LD) (5 mW) was used for excitation. The infrared to green or to red upconversion efficiencies for the vitroceramics are evaluated to be nearly 2 or 10 times as high as fluoride glasses, respectively. Also higher mechanical and chemical stabilities and laser damage threshold over fluoride glasses or crystals are expected for the vitroceramics.
New type of magnetocaloric effect: Implications on low-temperature magnetic refrigeration using an Ericsson cycle
Authors: Takeya H. , Pecharsky V. K. , Gschneidner K. A. , Moorman J. O.
Published: July 2002

The low-temperature, high magnetic field heat capacity (1.5 to 70 K and 0 to 9.85 T), dc and ac magnetic behaviors of the compound (Gd0.54Er0.46)AlNi show that field-induced magnetic entropy change is significant and almost constant over the temperature region of ∼15 to ∼45 K. The resulting temperature dependence of the magnetocaloric effect, nearly constant over a 30+ K temperature range, is unprecedented (most magnetic materials have a caretlike shape temperature dependence). These data show that (Gd0.54Er0.46)AlNi can be used as an effective active magnetic regenerator material for an Ericsson-cycle magnetic refrigerator, and could substitute for complex composite layered materials suggested earlier.
Microwave performance of a 0.25 μm gate AlGaN/GaN heterostructure field effect transistor
Authors: Asif Khan M. , Kuznia J. N. , Olson D. T. , Schaff W. J. , Burm J. W. , Shur M. S.
Published: July 2002

We fabricated a 0.25 μm gate length AlGaN/GaN heterostructure field effect transistor (HFET) with a maximum extrinsic transconductance of 27 mS/mm (at room temperature) limited by the source series resistance. The device exhibited an excellent pinch-off and a low parasitic output conductance in the saturation regime. We measured the cutoff frequency fT and the maximum oscillation frequency fmax as 11 and 35 GHz, respectively. These values are superior to the highest reported values for field effect transistors based on other wide band-gap semiconductors such as SiC. These results demonstrate an excellent potential of AlGaN/GaN HFETs for microwave and millimeter wave applications.
Formation of ‘‘single crystalline films’’ of ferroelectric copolymers of vinylidene fluoride and trifluoroethylene
Authors: Ohigashi Hiroji , Omote Kenji , Gomyo Teruhisa
Published: July 2002

We find that a highly double-oriented film of practically unlimited size can be obtained from a uniaxially drawn film of P(VDF/TrFE) by crystallization in the paraelectric phase with its surfaces free from any constraint other than tensile stress along the chain axis. The c axis (the chain axis) and the [110] axis in the orthorhombic system of the poled crystalline film are parallel to the stretching axis and perpendicular to the film plane, respectively. Contrary to conventional crystalline polymer films, the present film has no amorphous region and no lamellar crystal. The film exhibits highly anisotropic characteristics in optical, mechanical, and piezoelectric properties.
Ultrasonic method for static pressure measurement
Authors: Diodati P.
Published: July 2002

A method to measure pressure in a fluid, using external gauges has been developed. A linear dependence has been found between the amplitude of an acoustic pulse propagating inside a part of the vessel wall and the pressure of the internal fluid, in the range from 1 to 650 bars. A simple interpretation of this dependence is given.
Two-dimensional imaging x-ray spectrometer using channel-plate collimator
Authors: Yamaguchi Naohiro , Aoki Sadao , Miyoshi Syoichi
Published: July 2002

A new method for two-dimensional imaging x-ray spectrometer is introduced. The optical system consists of a microchannel plate as a large-aperture collimator and a flat Bragg-type diffractor. This method can be applied to radiation covering from VUV to hard x ray. The feasibility has been demonstrated by using x rays of two different energies and a LiF crystal.
Principles and performance of a high-field time domain magnetic spectrometer
Authors: Strutz T. , Witowski A. M. , Wyder P.
Published: July 2002

A time domain magnetic spectrometer based on a pick-up coil is presented for the study of spin-lattice relaxation rates in paramagnetic samples at high magnetic fields and at liquid-helium temperatures. The equilibrium between the spin system and the lattice is disturbed either by heating the spin system under the electron spin resonance condition (using far-infrared laser pulses) or by heating the lattice (using nonresonant laser pulses). Resulting magnetization changes induce a voltage in a pick-up coil oriented parallel to the field. By measuring the temporal evolution of this voltage, one can observe the spin-lattice relaxation directly. The spin-lattice relaxation in the diluted magnetic semiconductor CdMnTe is measured in order to demonstrate the performance of the spectrometer. The two methods of disturbing the equilibrium between the spin system and the lattice are compared.
A nondestructive method for determining the spring constant of cantilevers for scanning force microscopy
Authors: Cleveland J. P. , Manne S. , Bocek D. , Hansma P. K.
Published: July 2002

The spring constant of microfabricated cantilevers used in scanning force microscopy (SFM) can be determined by measuring their resonant frequencies before and after adding small end masses. These masses adhere naturally and can be easily removed before using the cantilever for SFM, making the method nondestructive. The observed variability in spring constant—almost an order of magnitude for a single type of cantilever—necessitates calibration of individual cantilevers in work where precise knowledge of forces is required. Measurements also revealed that the spring constant scales with the cube of the unloaded resonant frequency, providing a simple way to estimate the spring constant for less precise work.
Perfect crystal and mosaic crystal quarter-wave plates for circular magnetic x-ray dichroism experiments
Authors: Giles C. , Malgrange C. , Goulon J. , de Bergevin F. , Vettier C. , Fontaine A. , Dartyge E. , Pizzini S. , Baudelet F. , Freund A.
Published: July 2002

Circular magnetic x-ray dichroism experiments require circularly polarized x rays usually provided by bending magnet synchrotron radiation below or above the orbit plane. It is shown here that a very promising alternative consists of using linearly polarized x rays converted to circularly polarized ones by quarter-wave plates made of nonabsorbing crystals like good diamond crystals and also mosaic beryllium crystals. Convincing experimental results obtained on the energy-dispersive absorption spectrometer at LURE (Orsay-France) are presented.
On the possibilities of x-ray phase contrast microimaging by coherent high-energy synchrotron radiation
Authors: Snigirev A. , Snigireva I. , Kohn V. , Kuznetsov S. , Schelokov I.
Published: July 2002

Coherent properties of the x-ray beam delivered at the ESRF allow the observation of very weak perturbations of the wave front, resulting in the phase contrast. A straightforward experimental setup for phase contrast imaging is proposed and used to record holographic images from organic samples of 10–100 μm at energy 10–50 keV with the contrast up to 50%–100%. The theory of phase contrast imaging is considered and some theoretical estimations are made to reveal the performance of the proposed technique in terms of resolution, sensitivity, geometrical requirements, and energy range applicability. It is found that for carbon-based fibers a detectable size with 2% contrast is 0.1 μm for 10 keV and −1 μm for 100 keV. It is demonstrated that the fine interference structure of the image is very sensitive to the shape, density variation, and internal structure of the sample. Some prospects for the practical use and future development of the new coherent techniques such as phase contrast microscopy, microtomography, holography, and interferometry at high energies are also discussed.
Optical fiber coupled multitelescope lidar system: Application for a Rayleigh lidar
Authors: Ishii Syoken , Shibata Takashi , Mizutani Kohei , Itabe Toshikazu
Published: July 2002

It is common knowledge that optical fibers may be used to couple several telescopes with smaller apertures into a system with an equivalent total receiving area. The combined system is much less expensive and often can be handled by one person, making it suitable for use in a transportable lidar system. In this article, we discuss the design considerations involved, and apply them to assemble a fiber-coupled lidar receiver. This receiving system has proven satisfactorily with expected performance, when used in a Rayleigh lidar to measure density and temperature in the middle atmosphere (30–80 km in altitude).
A photodiode for the measurement of soft x-ray radiation from plasma
Authors: Xiao Qingjun , Navratil Gerald
Published: July 2002

Photodiodes with three layer zirconium, titanium, and aluminum have been used as detectors on the high beta tokamak-extended pulse tokamak for soft x-ray radiation measurement. Signals from the photodiodes show sawteeth instabilities and magnetohydrodynamic oscillations.
A multioctave coaxially oriented beam-resonator arrangement Fourier-transform microwave spectrometer
Authors: Grabow Jens-Uwe , Stahl Wolfgang , Dreizler Helmut
Published: July 2002

The design, performance, and operation of a broadband (3–26.5 GHz) high resolution microwave spectrometer is described. In comparison to previously developed molecular beam Fabry–Perot resonator spectrometers the design presented here implements some significant improvements: a coaxially oriented beam resonator arrangement (COBRA) formed by a confocal pair of mirrors incorporating an electromechanical valve and employing two pairs of microwave antennas, and a multioctave Fourier-transform microwave (FTMW) instrument providing the pulsed excitation source with microwave pulse phase-inversion scheme and the low-noise receiving system employing image-rejection downconversion with superheterodyne as well as quadrature detection. The entire apparatus, fully automated for scanning operation, covers a frequency range of more than three octaves. The novel design of the FTMW instrument does not require any changes of the spectrometer hardware in order to reach all regions of its spectral range. While operated in high resolution mode the COBRA-FTMW spectrometer is achieving a linewidth (half-width at half-height, HWHH) relative to the molecular emission frequency of better than 10−7 HWHH. The sensitivity and resolution of the spectrometer is demonstrated by several examples.
Evidence for the presence of remnant strain in grey-tracked KTiOPO4
Authors: Satyanarayan M. N. , Bhat H. L. , Srinivasan M. R. , Ayyub P. , Multani M. S.
Published: July 2002

Grey tracks produced in KTiOPO4 (KTP) by applying a dc electric field have been studied through optical absorption, Raman scattering, and synchrotron x-ray topography. A study of the optical absorption and Raman scattering from the grey-tracked region suggests that their formation is accompanied by changes in the electronic levels of Ti4+. There is no evidence for a major structural change or disorder in the grey-tracked region. However, the x-ray topographs do indicate the presence of a remnant strain in the lattice, which might contribute to the observed changes in the Raman intensities.
Instrumental improvements in optical waveguide light mode spectroscopy for the study of biomolecule adsorption
Authors: Kurrat R. , Textor M. , Ramsden J. J. , Böni P. , Spencer N. D.
Published: July 2002

Optical waveguide light mode spectroscopy (OWLS) is a new technique that is particularly well suited to the in situ study of biomolecule adsorption kinetics on surfaces. Here we describe improvements to a commercial OWLS instrument in order to allow for easy combination with other ex situ surface-characterization methods, such as x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and atomic force microscopy. Further, the problem of contamination of the waveguide surface arising from the use of silicone in the flow-through cuvette with which biomolecules are brought into contact with the adsorbing surface had to be resolved, as it greatly altered the wetting and adsorption properties of the waveguide. Finally, through physical vapor deposition of thin, nanosized layers of titanium oxide onto the waveguide layer, it is possible to simulate the surface properties of oxide-covered titanium implant surfaces. However, scanning angle constraints set by the mechanics of the commercial instrument must be borne in mind.
Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen
Authors: Eppink André T. J. B. , Parker David H.
Published: July 2002

The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and support the appealing properties of this velocity mapping technique.
Levitation apparatus for structural studies of high temperature liquids using synchrotron radiation
Authors: Krishnan Shankar , Felten John J. , Rix James E. , Weber J. K. Richard , Nordine Paul C. , Beno Mark A. , Ansell S. , Price D. L.
Published: July 2002

A new levitation apparatus coupled to a synchrotron-derived x-ray source has been developed to study the structure of liquids at temperatures up to 3000 K. The levitation apparatus employs conical nozzle levitation using aerodynamic forces to stably position solid and liquid specimens at high temperatures. A 270 W CO2 laser was used to heat the specimens to desired temperatures. Two optical pyrometers were used to record the specimen temperature, heating curves, and cooling curves. Three video cameras and a video recorder were employed to obtain and record specimen views in all three dimensions. The levitation assembly was supported on a three-axis translation stage to facilitate precise positioning of the specimen in the synchrotron radiation beam. The levitation system was enclosed in a vacuum chamber with Be windows, connections for vacuum and gas flow, ports for pyrometry, video, and pressure measurements. The vacuum system included automatic pressure control and multi-channel gas flow control. A phosphor screen coupled to a high-resolution video microscope provided images of the x-ray beam and specimen shadow which were used to establish the specimen position. The levitation apparatus was integrated with x-ray diffractometers located at X-6B and X-25 beamlines at the National Synchrotron Light Source. X-ray structural measurements have been obtained on a number of materials including Al2O 3, Ni, Si, Ge, and other metallic and ceramic materials in the liquid state.
Destruction of hazardous air pollutants using a fast rise time pulsed corona reactor
Authors: Korzekwa R. A. , Grothaus M. G. , Hutcherson R. K. , Roush R. A. , Brown R.
Published: July 2002

Increasingly stringent environmental regulation imposed on both the military and civilian sectors has created a growing demand for alternative abatement methods for a variety of hazardous compounds. One alternative, the nonthermal plasma, shows promise of providing an efficient means for the destruction of dilute concentrations of hazardous air pollutants. The Dahlgren Laboratory of the Naval Surface Warfare Center has extensively investigated one type of nonthermal plasma discharge, the pulsed corona reactor, for the destruction of volatile organic compounds and chemical warfare agents. In this reactor, a fast rise time (∼10 ns), short duration (<100 ns), high-voltage pulse is repetitively delivered to a wire-cylinder electrode geometry, thereby producing a multitude of streamer discharges along its length. The resulting nonthermal plasma contains highly reactive chemical radicals which can interact with and destroy the hazardous molecules entrained in the ambient atmosphere flowing through the reactor volume. Increased electrical efficiency was obtained using a combination of high efficiency constant-current capacitor-charging, high repetition-rate spark gap switching, and resonant energy transfer to the reactor. Promising results have been obtained for toluene, methylene chloride, and dichlorodifluoromethane in air at concentrations of a few hundred parts per million. The device has been operated at voltages up to 30 kV, pulse repetition rates up to 1.4 kHz, and flow rates up to 60 ℓ/min. Detailed electrical measurements have been made to properly characterize the electrical properties of the pulsed corona reactor and to validate subsequent improvements in the reactor energy efficiency.
Wide-band, high-resolution soft x-ray spectrometer for the Electron Beam Ion Trap
Authors: Brown G. V. , Beiersdorfer P. , Widmann K.
Published: July 2002

We have constructed two wide-band, high-resolution vacuum flat crystal spectrometers and implemented them on the Electron Beam Ion Trap located at the Lawrence Livermore National Laboratory. Working in unison, these spectrometers can measure an x-ray bandwidth ⩽9 Å in the soft x-ray region below 21 Å. In order to achieve this large bandwidth each spectrometer houses either two 125 mm × 13 mm × 2 mm RAP (rubidium acid phthalate, 2d=26.121 Å), two 114 mm × 13 mm × 2 mm TlAP (thallium acid phthalate, 2d=25.75 Å) crystals, or some combination thereof, for dispersion and two position sensitive proportional counters for detection of x rays. The spectrometers are used to measure wavelengths and relative intensities of the L-shell line emission from Fe XVII–XXIV for comparison with spectra obtained from astrophysical and laboratory plasmas. The wide wavelength coverage attainable by these spectrometers makes it possible to measure all the L-shell emission from a given iron ion species simultaneously.
Real-time, interferometrically measuring atomic force microscope for direct calibration of standards
Authors: Gonda S. , Doi T. , Kurosawa T. , Tanimura Y. , Hisata N. , Yamagishi T. , Fujimoto H. , Yukawa H.
Published: July 2002

An atomic force microscope with a high-resolution three-axis laser interferometer for real-time correction of distorted topographic images has been constructed and investigated. With this apparatus, standard samples for a scanning probe microscope can be directly calibrated on the basis of stabilized wavelength of He–Ne lasers. The scanner includes a three-sided mirror block as a mobile target mirror for the interferometer, which realizes a rectangular coordinate system. The position coordinates of the sample is independently and simultaneously acquired with high-resolution (0.04 nm) X/Y/Z interferometer units and fed back for XY servo scanning and height image construction. The probe is placed on the sample surface at the intersection of the three optical axes of the interferometer with good reproducibility, so that the Abbe error caused by the rotation of the scanner is minimized. Image distortion in the XY plane and vertical overshoot/undershoot due to nonlinear motion of piezo devices, hysteresis, and creep are eliminated. The optical properties of the interferometers, mechanical characteristics of the scanner, and system performances in dimensional measurements for calibration standards are demonstrated.
A four-point constant-current/temperature controlled circuit for anemometric applications
Authors: Lige̢za P.
Published: July 2002

A four-point constant-current/temperature controlled circuit is an electronic system for supplying a resistance measuring sensor in two modes of operation: either in constant-current or in constant-temperature mode. This circuit is destined for anemometric applications. It is new nonbridge, four-point constant-current/temperature anemometer circuit. Separation of current and voltage leads of the sensor makes it possible to eliminate the effects of leads’ resistance on the preset value of sensor supply parameters. In such a circuit no cable adjustment is required. The value of sensor current or its resistance is set with digital signals. This article presents the design of the original measuring circuit and its principle of operation. Design of measuring circuit based on those principles is also presented. Attention is also given to its applications. The most important advantages of the new circuit are: both constant-current or constant-temperature modes of operation are available, precise digital control of sensor current or resistance can be performed, and four-point sensor operation in both modes is possible. It enables high-precision laboratory anemometric measurements, especially under the conditions of low sensor resistance, low overheat ratio range, and long sensor supply cables.
Velocity imaging photoionization coincidence apparatus for the study of angular correlations between electrons and fragment ions
Authors: Takahashi M. , Cave J. P. , Eland J. H. D.
Published: July 2002

A new apparatus has been developed to detect and measure angular correlations between energy-selected photoelectrons and coincident mass-analyzed fragment ions from photoionization at selected wavelengths. It achieves velocity imaging for electrons and ions simultaneously and has high collection efficiency for both particles, with moderate mass and energy resolution. Angular and energy correlations between the two particles are measured, as are the angular distributions of each particle independently relative to the light polarization direction. Fixed-molecule electron angular distributions are deduced in cases of pure axial recoil. Examples of angular distributions from photoionization of diatomic molecules are reported.
Fundamental limits to force detection using quartz tuning forks
Authors: Grober Robert D. , Acimovic Jason , Schuck Jim , Hessman Dan , Kindlemann Peter J. , Hespanha Joao , Morse A. Stephen , Karrai Khaled , Tiemann Ingo , Manus Stephan
Published: July 2002

This paper explores the fundamental limits of the use of quartz tuning forks as force detectors in scanned probe microscopy. It is demonstrated that at room temperature, pressure, and atmosphere these force sensors have a noise floor of 0.62 pN/Hz and exhibit a root mean square Brownian motion of only 0.32 pm. When operated as a shear force sensor both dissipative and reactive forces are detected on approach to the sample. These forces are sufficient to reduce the amplitude of motion of the probe nearly to zero without physically contacting the surface. It is also demonstrated that conventional proportional-integral feedback control yields closed loop responses at least 40 times faster than their open loop response.
Edge-emitting quantum well heterostructure laser diodes with auxiliary native-oxide vertical cavity confinement
Authors: Evans P. W. , Holonyak N. , Maranowski S. A. , Ries M. J. , Chen E. I.
Published: July 2002

Data are presented demonstrating edge-emitting laser diode operation of AlyGa1−yAs– GaAs–InxGa1−xAs quantum well heterostructures modified by the formation of a buried native-oxide distributed Bragg reflecting (DBR) mirror adding vertical confinement to the longitudinal laser cavity. The bottom DBR mirror, combined with the highly reflective top p-contact metallization (Ag), forms a thin broadband vertical cavity. The auxiliary vertical mirrors are tuned to improve the coupling of the spontaneous emission to the longitudinal lasing mode, resulting in reduced threshold currents and modified emission characteristics below threshold.
The vacuum ultraviolet phase-matching characteristics of nonlinear optical KBe2BO3F2 crystal
Authors: Chen Chuangtian , Xu Zuyan , Deng Daoqun , Zhang J. , Wong G. K. L. , Wu Baichang , Ye Ning , Tang Dingyan
Published: July 2002

The nonlinear optical crystal KBe2BO3F2 has been used to produce vacuum ultraviolet second-harmonic generation with an output range of 200–184.7 nm. The Sellmeier equation further indicates that this new crystal can generate the sixth harmonic from Nd-based laser systems.
Activation of acceptors in Mg-doped GaN grown by metalorganic chemical vapor deposition
Authors: Götz W. , Johnson N. M. , Walker J. , Bour D. P. , Street R. A.
Published: July 2002

The activation kinetics of acceptors was investigated for heteroepitaxial layers of GaN, doped with Mg. After growth, the samples were exposed to isochronal rapid thermal anneals in the temperature range from 500 to 775 °C. The samples were studied by variable temperature Hall effect measurements and photoluminescence (PL) spectroscopy in the as-grown condition and after each temperature step. The thermal treatment reduced the resistivity by six orders of magnitude and the p-type conductivity was found to be dominated by an acceptor with an activation energy of ∼170 meV. This acceptor is attributed to Mg atoms substituting for Ga in the GaN lattice and the activation process is consistent with dissociation of electrically inactive Mg–H complexes. It is shown that the appearance of a blue emission band in the PL spectrum of Mg-doped GaN does not directly correlate with the increase in p-type conductivity.
Enhanced electron injection in organic electroluminescence devices using an Al/LiF electrode
Authors: Hung L. S. , Tang C. W. , Mason M. G.
Published: July 2002

A bilayer is used as an electrode for organic electroluminescent devices. The bilayer consists of an ultrathin LiF layer adjacent to an electron-transporting layer and an aluminum outerlayer. Devices with the bilayer electrode showed enhanced electron injection and high electroluminescence efficiency as compared with a Mg0.9Ag0.1 cathode. Similar effects were observed when replacing MgO for LiF. The improvements are attributed to band bending of the organic layer in contact with the dielectrics.
Epitaxial Pb–Fe–O film with large planar magnetic anisotropy on (0001) sapphire
Authors: Dorsey P. C. , Qadri S. B. , Grabowski K. S. , Knies D. L. , Lubitz P. , Chrisey D. B. , Horwitz J. S.
Published: July 2002

Epitaxial films (1.7 μm thick) with the composition PbFe12.9O22.9 and hexagonal lattice parameters a=5.12 Å and c=23.67 Å have been pulsed laser deposited at 600 °C in 50 mTorr of O2 onto single-crystal (0001) sapphire substrates. Epitaxy was determined using standard powder x-ray diffraction (XRD) and grazing incidence XRD. The films were deposited using a single-phase polycrystalline PbFe12O19 target. The composition of the films is PbFe12.9O22.9, which was measured using Rutherford backscattering spectrometry. Static magnetic measurements were performed using a vibrating sample magnetometer and SQUID magnetometer in order to measure magnetic anisotropy, magnetic remanence (Mr), coercive field (Hc), and saturation magnetization (4πMs) of the films. The PbFe12.9O22.9 films exhibit magnetically isotropic behavior in the film plane with remanence ratio (Mr/Ms) and Hc values of 88±2.9% and 2500±97 Oe, respectively. However, the films are anisotropic with respect to the film normal such that the c axis is a magnetically hard direction and all directions normal to the c axis are magnetically easy (i.e., a planar anisotropy field, HA, with an estimated magnitude of 77.5 kOe). The 4πMs value for the films is 630 Gauss at room temperature.
Dynamic flow and switching bistability in twisted nematic liquid crystal cells
Authors: Qian Tie-Zheng , Xie Zhi-Liang , Kwok Hoi-Sing , Sheng Ping
Published: July 2002

We investigate the switching bistability based on the interaction between dynamic flow and director rotation in twisted nematic liquid crystal cells. Numerical calculation shows that there exists a general type of bistable twisted director configuration. Two specific cases are verified experimentally.
Measurement of piezoelectrically induced charge in GaN/AlGaN heterostructure field-effect transistors
Authors: Yu E. T. , Sullivan G. J. , Asbeck P. M. , Wang C. D. , Qiao D. , Lau S. S.
Published: July 2002

Electron concentration profiles have been obtained for AlxGa1−xN/GaN heterostructure field-effect transistor structures. Analysis of the measured electron distributions demonstrates the influence of piezoelectric effects in coherently strained layers on III-V nitride heterostructure device characteristics. Characterization of a nominally undoped Al0.15Ga0.85N/GaN transistor structure reveals the presence of a high sheet carrier density in the GaN channel which may be explained as a consequence of piezoelectrically induced charges present at the Al0.15Ga0.85N/GaN interface. Measurements performed on an Al0.15Ga0.85N/GaN transistor structure with a buried Al0.15Ga0.85N isolation layer indicate a reduction in electron sheet concentration in the transistor channel and accumulation of carriers below the Al0.15Ga0.85N isolation layer, both of which are attributable to piezoelectric effects.
Laser-induced breakdown spectroscopy for sizing and elemental analysis of discrete aerosol particles
Authors: Hahn D. W.
Published: July 2002

The laser-induced breakdown spectroscopy technique has been extended to provide quantitative analysis of the mass and elemental composition of individual, submicrometer to micrometer-sized aerosol particles. A two-part approach was used for calibration of the overall mass concentration response, and of the characteristic plasma volume, equal to 2.5×10−4 cm3. Laboratory results are presented for submicrometer-sized particles containing a known concentration of magnetite. Additional data are presented for fine particulate matter measured in ambient air, including magnesium-containing particles with an average size of 313 nm.
Au–Cu alloy nanoclusters in silica formed by ion implantation and annealing in reducing or oxidizing atmosphere
Authors: Gonella F. , Mattei G. , Mazzoldi P. , Sada C. , Battaglin G. , Cattaruzza E.
Published: July 2002

The formation of binary alloy clusters in sequentially ion-implanted Au–Cu silica glass has been studied as a function of the annealing atmosphere. Alloy formation has been unambiguously evidenced in the as-implanted samples. The selective influence on Au precipitation of either oxygen or hydrogen annealing atmosphere leads to separation of gold and copper or to Au–Cu alloy cluster formation, respectively.
Fabrication of metallic electrodes with nanometer separation by electromigration
Authors: Park Hongkun , Lim Andrew K. L. , Alivisatos A. Paul , Park Jiwoong , McEuen Paul L.
Published: July 2002

A simple yet highly reproducible method to fabricate metallic electrodes with nanometer separation is presented. The fabrication is achieved by passing a large electrical current through a gold nanowire defined by electron-beam lithography and shadow evaporation. The current flow causes the electromigration of gold atoms and the eventual breakage of the nanowire. The breaking process yields two stable metallic electrodes separated by ∼1 nm with high efficiency. These electrodes are ideally suited for electron-transport studies of chemically synthesized nanostructures, and their utility is demonstrated here by fabricating single-electron transistors based on colloidal cadmium selenide nanocrystals.
Blocking temperature, energy barrier, and reversal field variation of fine magnetic particles
Authors: Huang Huei Li , Lu Jing Ju
Published: July 2002

Blocking temperature, energy barrier, and reversal field variation of interacting fine magnetic particles display similar characteristics as a function of the bonding angle and interparticle distance in the context of the dipole interaction. The energy barrier and reversal field exhibit local maxima at the bonding angles β=0° and π/2 and a global minimum at β≃60°. Thus, the sample average of these quantities is found to reduce with decreasing interparticle distance, in agreement with the latest experimental data. For a system with easy-axis misorientation, magnetization reversal will traverse along the direction in which the misorientation angle and bonding angle have the same sign.
The elastic constants of single crystal β-Si3N4
Authors: Vogelgesang R. , Grimsditch M. , Wallace J. S.
Published: July 2002

Using Brillouin scattering, we have determined the complete set of elastic constants of the technologically important β-Si3N4. Availability of single crystals has allowed us to measure all five independent cij, which (in units of GPa) are: c11=433(3), c33=574(3), c13=127(5), c44=108(2), c66=119(4), and c12=c11−2c66=195(8), giving a bulk modulus of 259. Our values are compared with existing calculations and cij determined from nano-indentation results combined with empirical relations and trends in related materials.
Relaxation in glassforming liquids and amorphous solids
Authors: Angell C. A. , Ngai K. L. , McKenna G. B. , McMillan P. F. , Martin S. W.
Published: July 2002

The field of viscous liquid and glassy solid dynamics is reviewed by a process of posing the key questions that need to be answered, and then providing the best answers available to the authors and their advisors at this time. The subject is divided into four parts, three of them dealing with behavior in different domains of temperature with respect to the glass transition temperature, Tg, and a fourth dealing with “short time processes.” The first part tackles the high temperature regime T>Tg, in which the system is ergodic and the evolution of the viscous liquid toward the condition at Tg is in focus. The second part deals with the regime T∼Tg, where the system is nonergodic except for very long annealing times, hence has time-dependent properties (aging and annealing). The third part discusses behavior when the system is completely frozen with respect to the primary relaxation process but in which secondary processes, particularly those responsible for “superionic” conductivity, and dopart mobility in amorphous silicon, remain active. In the fourth part we focus on the behavior of the system at the crossover between the low frequency vibrational components of the molecular motion and its high frequency relaxational components, paying particular attention to very recent developments in the short time dielectric response and the high Q mechanical response.
An electron impact cross section set for CHF3
Authors: Kushner Mark J. , Zhang Da
Published: July 2002

Trifluoromethane, CHF3, is used for plasma etching of silicon compounds for microelectronics fabrication, and so there is interest in developing computer models for plasmas sustained in CHF3. Recent measurements of electron swarm parameters, and electron impact dissociation and ionization cross sections, have provided a sufficient basis to develop a working electron impact cross section set for CHF3. Such a cross section set is reported here. We found that increased energy losses from dissociative electronic excitation processes were required to reproduce experimental ionization coefficients. The cross sections for attachment are small with there being some uncertainty in their magnitude at low energies. The cross sections were used in a plasma equipment model for an inductively coupled plasma reactor and compared to discharges sustained in C2F6. For otherwise identical operating conditions, plasmas sustained in CHF3 had higher electron and lower negative ion densities.
Manifestation of Hubbard and covalent correlations in the absorption spectra of YBa2Cu3O6+x films
Authors: Eremenko V. V. , Samovarov V. N. , Svishchev V. N. , Vakula V. L. , Libin M. Yu. , Uyutnov S. A.
Published: July 2002

The absorption spectra of single-crystal YBa2Cu3O6+x films with various doping levels in the range from x≈0.35 to x≈0.9 are measured in the energy region 0.3–3 eV. An analysis of the spectral composition of the absorption is made with allowance for intraband and interband transitions and the local dd transitions in the Cu2+ ion. It is concluded that the dd band (the transition dxy→dx2−y2 at 1.5 eV) reflects the enhancement of the covalent bonding (pd hybridization) upon metallization and that the spectral feature at ≈1.8 eV carries information about the contribution of electronic correlations, since it is sensitive to the opening of a spin gap in the insulator and to antiferromagnetic fluctuations in the metal. Although the covalent (≈1.5 eV) and correlation (≈1.8 eV) absorption peaks compete with each other, the coexistence of these bands in the metal supports the validity of a model based on the correlation polaron — a charge carrier which creates a region of covalent bonding in a Hubbard matrix of antiferromagnetic fluctuations.
Quick deposition of a fluid on the wall of a tube
Authors: Aussillous Pascale , Quéré David
Published: July 2002

We are interested in the amount of fluid left behind a drop moved inside a capillary tube. Long ago, Taylor showed that for very viscous liquids moved at small velocities, the film thickness is a monotonic increasing function of the capillary number. New data obtained with liquids of low viscosity are reported here and compared with Taylor’s law. Two successive effects are observed: above a threshold in capillary number, the film is thicker than a Taylor film; at a very high speed, the deposition law becomes a decreasing function of the drop velocity. Both behaviors are analyzed thanks to scaling arguments and shown to be consequences of inertia.
A nonlocal electron conduction model for multidimensional radiation hydrodynamics codes
Authors: Schurtz G. P. , Nicolaï Ph. D. , Busquet M.
Published: July 2002

Numerical simulation of laser driven Inertial Confinement Fusion (ICF) related experiments require the use of large multidimensional hydro codes. Though these codes include detailed physics for numerous phenomena, they deal poorly with electron conduction, which is the leading energy transport mechanism of these systems. Electron heat flow is known, since the work of Luciani, Mora, and Virmont (LMV) [Phys. Rev. Lett. 51, 1664 (1983)], to be a nonlocal process, which the local Spitzer–Harm theory, even flux limited, is unable to account for. The present work aims at extending the original formula of LMV to two or three dimensions of space. This multidimensional extension leads to an equivalent transport equation suitable for easy implementation in a two-dimensional radiation-hydrodynamic code. Simulations are presented and compared to Fokker–Planck simulations in one and two dimensions of space.
Kinetics of the graphitization of dispersed diamonds at “low” temperatures
Authors: Butenko Yu. V. , Kuznetsov V. L. , Chuvilin A. L. , Kolomiichuk V. N. , Stankus S. V. , Khairulin R. A. , Segall B.
Published: July 2002

The bulk density of graphitized ultradisperse diamond (UDD) was measured by a gamma-ray attenuation method at 1370–1870 K. These data combined with small angle x-ray scattering and true density measurements of the samples heated at various fixed temperatures were used to study the graphitization kinetics of the UDD. The reaction rate was modeled as a migration rate of the interface between the developing graphite-like carbon and the remaining diamond phase. A “reducing sphere” model was used to obtain the rates from the changes in densities. The estimated kinetic parameters in an Arrhenius expression, namely the activation energy, E=45±4 kcal/mol, and the pre-exponential factor, A=74±5 nm/s, allow quantitative calculations of the diamond graphitization rates in and around the indicated temperature range. The calculated graphitization rates agree well with the graphitization rates of diamonds with different dispersity estimated from high-resolution transmission electron microscopy data. The large difference between the rates and the kinetic parameters obtained in this study and those estimated by G. Davies and T. Evans [Proc. R. Soc. London 328, 413 (1972)] for the temperature range 2150–2300 K indicates that there are different graphitization mechanisms operating in the “low” and “high” temperatures regions.
The geometry of photon surfaces
Authors: Claudel Clarissa-Marie , Virbhadra K. S. , Ellis G. F. R.
Published: July 2002

The photon sphere concept in Schwarzschild space–time is generalized to a definition of a photon surface in an arbitrary space–time. A photon sphere is then defined as an SO(3)×R-invariant photon surface in a static spherically symmetric space–time. It is proved, subject to an energy condition, that a black hole in any such space–time must be surrounded by a photon sphere. Conversely, subject to an energy condition, any photon sphere must surround a black hole, a naked singularity or more than a certain amount of matter. A second order evolution equation is obtained for the area of an SO(3)-invariant photon surface in a general nonstatic spherically symmetric space–time. Many examples are provided.