bluuebunny/crossref_metadata_2025 huggingface dataset:
Misdirections in Conservation Biology
Authors: Asquith Nigel M.
Published: July 2002

Abstract: The hypothesis that conservation biology is proceeding along two separate trajectories (Caughley 1994) has provoked extensive discussion. Caughley's dichotomy, a “small‐population” paradigm versus a “declining‐population” paradigm, has recently been exemplified in discussion of management strategies for conservation of the Javan gibbon (  Hylobates moloch). Recommendations from extensive fieldwork focused on reducing the major known threat to the species—habitat destruction—and proposed a strategy of forest management and protection. A population and habitat viability analysis focused on an entirely different issue—low genetic diversity—and proposed a program of single‐species genetic management. It is not surprising that geneticists see inbreeding as a major conservation problem, and it is not unusual that ecologists focus on how ecology relates to conservation. A problem results when managers assume that addressing only one or the other of these factors is the appropriate conservation action. Conservation biologists must learn to thoroughly analyze every conservation problem before trying to solve it. We must actively involve experts from all fields in forming and reviewing conservation strategies. If only captive breeding specialists or ecologists are invited to address a problem, then the techniques employed to solve it will be irrevocably biased. We do not all see the world in the same way, even within the relatively small field of conservation. To achieve effective conservation action, we must learn to balance and capitalize on our different perspectives.
Surgical Outcome of Corpus Callosotomy in Patients with Drop Attacks
Authors: Maehara Taketoshi , Shimizu Hiroyuki
Published: July 2002

Summary:  Purpose: We examined presurgical factors that independently influence surgical outcome after corpus callosotomy. For adequate measurement of the surgical outcome, we analyzed seizure outcome of drop attacks, postoperative overall daily function, and family satisfaction.

Methods: At least 2 years after callosotomy (mean, 40.0 months), we retrospectively investigated 52 patients with drop attacks. As presurgical factors, we analyzed the age at surgery, age at seizure onset, age at drop attack onset, sex, hemiparesis, severe mental retardation, EEG abnormality, MRI abnormality, and extent of callosal section (total or partial callosotomy). Stepwise logistic regression was used for analysis.

Results: Satisfactory outcome (seizure cessation or >90% seizure reduction) was achieved in 85% of patients with drop attacks, 32% of those with generalized tonic seizures, and 31% of those with generalized tonic‐clonic seizures. The families assessed the overall daily function as improved in 62% of patients, unchanged in 23%, and impaired in 15%. Family satisfaction with callosotomy was achieved in 83% of patients (definitely satisfied, 39%, somewhat satisfied, 44%). Total callosotomy is independently predictive of satisfactory reduction of drop attacks (p = 0.013). A younger age is independently predictive of improvement of overall daily function (impaired and improved: p = 0.004) and family satisfaction (unsatisfied and somewhat satisfied, p = 0.018; unsatisfied and definitely satisfied, p = 0.0006).

Conclusions: In the present study, we found that total callosotomy is more effective for treatment of drop attacks than partial callosotomy and that children receive more benefit than adults after callosotomy.
An Introduction to Geriatrics for First‐Year Medical Students
Authors: Alford Cynthia L. , Miles Toni , Palmer Ray , Espino David
Published: July 2002

OBJECTIVE: To prepare medical students to better serve their older patients while raising awareness of geriatrics as a career choice.

DESIGN: To determine the impact of a new educational program, attitudinal assessments were administered to the group before and after participation in the program and to a comparison group of nonintervention students.

SETTING: University of Texas Health Science Center, San Antonio.

PARTICIPANTS: Two hundred and three first‐year medical students.

MEASUREMENTS: Student response on a scale of one to six regarding four constructs: attitudes and beliefs about providing medical care for older people, knowledge and beliefs about aging, interest in pursuing clinical geriatrics, and interest in pursuing aging research.

RESULTS: Four factors were consistently formed in the analysis process: beliefs about physical decline; comfort with older people; beliefs about career opportunities; and interest in geriatric research. The intervention group made significant gains in two areas: comfort with older people and understanding of physical decline in aging. Two new factors emerged in post‐test data.

CONCLUSIONS: The impact of the program was mixed. Although awareness of geriatrics and comfort with older people was increased, there was little change in career aspirations. Students in the program increased their awareness of physical decline in old age, setting the stage for teaching them about the physician's role with regard to function, and learned that geriatrics is a low‐status specialty. J Am Geriatr Soc 49:782–787, 2001.
Picosecond GaAs-based photoconductive optoelectronic detectors
Authors: Smith F. W. , Le H. Q. , Diadiuk V. , Hollis M. A. , Calawa A. R. , Gupta S. , Frankel M. , Dykaar D. R. , Mourou G. A. , Hsiang T. Y.
Published: July 2002

A novel material deposited by molecular beam epitaxy at low substrate temperatures using Ga and As4 beam fluxes has been used as the active layer for a high-speed photoconductive optoelectronic switch. The high-speed photoconductive performance of the material was assessed by fabricating two devices: an Auston switch and a photoconductive-gap switch with a coplanar transmission line. In a coplanar transmission line configuration, the speed of response is 1.6 ps (full width at half maximum) and the response is 10 to 100 times greater than that of conventional photoconductive switches. Since the material is compatible with GaAs discrete device and integrated circuit technologies, this photoconductive switch may find extensive applications for high-speed device and circuit testing.
Low-temperature formation of multilayered Bi(Pb)-Sr-Ca-Cu-O thin films by successive deposition using laser ablation
Authors: Kanai Masaki , Kawai Tomoji , Kawai Shichio , Tabata Hitoshi
Published: July 2002

Crystallized as-grown Bi(Pb)-Sr-Ca-Cu-O thin films are formed by the laser ablation method at a substrate temperature as low as 480 °C under N2O gas flow. Moreover, the as-grown thin film, which has a crystal structure consisting of four or five CuO2 layers between adjacent Bi2O2 layers, can be formed by the combination of N2O gas flow and the successive deposition method with excimer laser pulses.
Cavity-enhanced highly nondegenerate four-wave mixing in GaAlAs semiconductor lasers
Authors: Provost J. G. , Frey R.
Published: July 2002

Highly nondegenerate four-wave mixing (δω>100 GHz) has been observed in GaAlAs semiconductor lasers in the case where each of the pump, probe, and conjugate frequencies is close to that of a cavity mode. The high measured reflectivities (∼1000) have been demonstrated to depend on the relative position of the pump and probe cavity resonances. When the probe frequency was tuned, probe and conjugate spectra exhibited resonances on the cavity modes lying near the probe and conjugate frequencies. Such experiments may greatly help both in the modeling of laser action and in the measurement of laser parameters.
Biaxial elastic modulus of metallic films determined from vibrating circular membranes
Authors: Fartash A. , Schuller Ivan K. , Grimsditch M.
Published: July 2002

The biaxial moduli of free-standing metallic films have been determined from their behavior as elastically tensioned membranes. Errors due to thickness determination and warping of the films are eliminated, thereby opening up the possibility of solving a number of controversial issues. The method combines the determination of stress from the frequency of the membrane modes with optical measurements of strain. Tests on Ta and Ni films yield values of the biaxial modulus in good agreement with calculated literature data.
X-ray emission from plasmas generated by an XeCl laser picosecond pulse train
Authors: O’Neill F. , Turcu I. C. E. , Xenakis D. , Hutchinson M. H. R.
Published: July 2002

Tenfold enhancement of keV x-ray emission is observed from plasmas generated by an XeCl laser picosecond pulse train as compared to previous results obtained using a 25 ns KrF laser pulse. When a picosecond pulse train of only 165 mJ total energy is used to generate a plasma, the conversion efficiency to x rays at hν≊0.85 keV is 2.4%.
Concept, design, and use of the photoacoustic heat pipe cell
Authors: Jalink Henk , Bicanic Dane
Published: July 2002

A resonant photoacoustic cell suitable for studies of liquid samples having low vapor pressures has been developed and tested. The cell, the working of which is based on that of the heat pipe, is of a simple, compact design; its operational temperature range is limited only by the choice of working fluid and the material used to construct the cell. The feasibility of this novel-type cell has been demonstrated by obtaining the absorption spectrum of geraniol C10H18O at 403 K in the spectral region covered by the CO2 laser emission.
Enhanced coercivities in die-upset Nd-Fe-B magnets with diffusion-alloyed additives (Zn, Cu, and Ni)
Authors: Fuerst C. D. , Brewer E. G.
Published: July 2002

Rapidly solidified Nd-Fe-B ribbons were crushed and mixed with a fine powder (10–100μm) of a pure element before hot working into fully dense anisotropic magnets. At the high temperatures and pressures required for densification some additives diffused throughout the ribbon matrix and presumably into the neodymium-rich phase surrounding the Nd2Fe14B grains. In small concentrations (0.5–0.8 wt%), zinc, copper, and nickel additions enhanced the coercivities of die-upset magnets by 100%, 77%, and 53%, respectively. Additives which did not diffuse thoroughly, such as manganese, had no measurable effect on the coercivities.
Deposition of (100) oriented MgO thin films on sapphire by a spray pyrolysis method
Authors: DeSisto W. J. , Henry R. L.
Published: July 2002

Thin films of magnesia have been deposited by a novel spray pyrolysis method. An aqueous magnesium acetylacetonate solution was ultrasonically nebulized, transported in flowing oxygen, and thermally decomposed on silicon (100), sapphire, and fused silica at temperatures between 400 and 550 °C. The films were from 0.1 to 0.5 μm thick, optically transparent, and smooth. The MgO films were poorly crystalline as deposited. MgO films on sapphire crystallized with strong (100) orientation after annealing at 700 and 930 °C in flowing oxygen.
Delta-doped quantum well structures grown by molecular beam epitaxy
Authors: Liu D. G. , Lee C. P. , Chang K. H. , Wu J. S. , Liou D. C.
Published: July 2002

Delta doping in quantum well structures has been studied. The quantum wells consist of a strained InGaAs layer sandwiched between two GaAs layers. The layers were undoped except for a sheet of Si dopants deposited in the middle of the quantum well. Structures with various doses and quantum well thicknesses were studied and compared. Capacitance voltage measurements were carried out to determine the carrier distribution. A very narrow carrier profile with a full width at half maximum of only 12 Å has been achieved. This is the narrowest carrier profile ever reported for any growth technique.
Impulsive excitation of coherent phonons observed in reflection in bismuth and antimony
Authors: Cheng T. K. , Brorson S. D. , Kazeroonian A. S. , Moodera J. S. , Dresselhaus G. , Dresselhaus M. S. , Ippen E. P.
Published: July 2002

We report time domain observations of coherent lattice vibrations in bismuth and antimony. Phonons are impulsively generated, and detected through reflectivity modulation with 70 fs pulses of laser light at 1.98 eV. With this technique, we demonstrate that coherent lattice oscillations can be studied by reflection in opaque materials, but with selection rules which may differ from conventional impulsive stimulated Raman scattering.
Calibration of the Raman spectrum to the oxygen stoichiometry of nanophase TiO2
Authors: Parker J. C. , Siegel R. W.
Published: July 2002

Calibration curves of the Raman spectrum of nanophase TiO2 to the material’s oxygen stoichiometry are presented. The thermogravimetric analysis studies used to determine the oxygen-titanium ratio are also discussed.
Dose rate effects on damage accumulation in Si+-implanted gallium arsenide
Authors: Haynes T. E. , Holland O. W.
Published: July 2002

Ion-induced damage accumulation has been measured as a function of ion dose and dose rate following 100 keV Si+ room-temperature implants in GaAs. The dose rate has been found to have a strong effect on the total damage produced in GaAs over a range of dose between 1014 and 1015/cm2 and implantation current densities from 0.05 to 12 μA/cm2. Two distinct stages of damage formation have been identified. At low implantation doses, damage accumulates slowly and tends to saturate at a level of approximately 0.4×1017defects/cm2. However, beyond a threshold dose (≊1014 Si/cm2) which decreases with increasing dose rate, damage accumulates rapidly. In the second stage, the onset of which appears to be associated with the formation of more complex damage structures, the total damage and the damage accumulation rate were found to increase with dose rate for a fixed ion dose. For comparison, dose rate effects were also measured in Si and Ge under similar implant conditions and found to be weaker. The results for GaAs are correlated with recent observations of dose-rate-dependent electrical activation in Si-implanted GaAs.
Trivalent character of platinum in silicon
Authors: Zimmermann H. , Ryssel H.
Published: July 2002

A third dominant platinum level is reported. The concentrations of the new level and of the platinum donor level at 0.330 eV above the valence band are equal to each other within 20% over a range exceeding three orders of magnitude in concentration. The new level at approximately 0.067 eV above the valence band is characterized by the method of deep level transient spectroscopy. Its activation energy is shown to be dependent on the electric field. Platinum is suggested to be a double donor in p-type silicon. All three dominant platinum levels are believed to belong to the same platinum center in silicon.
Near resonance optical nonlinearities in nickel dithiolene complexes
Authors: Winter C. S. , Hill C. A. S. , Underhill A. E.
Published: July 2002

The third-order optical nonlinearity χ(3), linear absorption coefficient α, and the two-photon absorption coefficient β of two nickel dithiolene complexes are reported. The nonlinearities were measured at 1064 nm using 100 ps pulses. Molecular nonlinearities 103 times those of carbon disulphide were observed. Two figures of merit were calculated from the data Δnsat/αλ=5 and n2/2βλ≳2, where α and β are the linear and two-photon absorption coefficients and n2 the nonlinear refractive index. These figures are compatible with those required for simple all-optical switching devices.
Acceptor compensation mechanism by midgap defects in nitrogen-doped ZnSe films
Authors: Ohki A. , Kawaguchi Y. , Ando K. , Katsui A.
Published: July 2002

A new carrier compensation mechanism has been investigated in nitrogen-doped ZnSe films grown by low-pressure metalorganic vapor phase epitaxy. Photoconduction spectrum measurements have revealed the existence of deep defect levels located just above the center of the band gap in N-doped, highly compensated films. These results combined with photo-Hall measurements (under infrared illumination) have revealed that these midgap defect levels act as deep donors, and play an important role in carrier compensation in shallow acceptor doped ZnSe films.
Characterization of nanometer scale wear and oxidation of transition metal dichalcogenide lubricants by atomic force microscopy
Authors: Kim Yun , Huang Jin-Lin , Lieber Charles M.
Published: July 2002

Atomic force microscopy has been used to characterize wear and oxidation of transition metal dichalcogenide surfaces. Sequential images recorded on molybdenum disulfide (MoS2) and niobium diselenide (NbSe2) surfaces show that wear proceeds at defects, and that MoS2 wears at least five times more slowly than NbSe2. Images of thermally treated MoS2 and NbSe2 further demonstrate that oxidation creates surface defects on both materials. However, for similar oxidation conditions, NbSe2 surfaces show extensive degradation, while MoS2 surfaces only exhibit isolated defects. The implications of these results to understanding the tribological properties of the transition metal dichalcogenides are discussed.
Modeling of cross-well carrier transport in a multiple quantum well modulator
Authors: Hutchings D. C. , Park C. B. , Miller A.
Published: July 2002

A self-consistent time and spatially resolved model is developed to explain the nature of cross-well transport in multiple quantum well devices. It is this cross-well transport that sets fundamental limits on the speed of such devices. This model is solved numerically to obtain a fit to experimental excite-probe data. A good fit to rise times is obtained with resonant tunneling features predicted and observed. It is also predicted that as the transient features in the transmission data relax, many carriers are still bound in the quantum wells.
Electron-wave reflection by multi-quantum barrier in n-GaAs/i-AlGaAs/ n-GaAs tunneling diode
Authors: Takagi Takeshi , Koyama Fumio , Iga Kenichi
Published: July 2002

For the purpose of experimentally demonstrating the enhancement of electron-wave reflection by a multi-quantum barrier (MQB) consisting of multiple combinations of quarter de Broglie wavelength hetero-stacks, we have fabricated two types of n-GaAs/i-barrier/n-GaAs tunneling diodes with a bulk AlxGa1−xAs barrier and an AlxGa1−xAs/GaAs MQB. From the current-voltage characteristic at 77 K, it has been clarified that electrons are well reflected by the MQB. The potential barrier height, virtually increased by the MQB, is estimated to be about 80 meV, which is in good agreement with a theoretical expectation.
Beryllium foil transition-radiation source for x-ray lithography
Authors: Piestrup M. A. , Boyers D. G. , Pincus C. I. , Harris J. L. , Caplan H. S. , Silzer R. M. , Skopik D. M.
Published: July 2002

We have measured the total soft-x-ray power from a transition radiator composed of a stack of 25 beryllium foils each 1.0 μm thick which were penetrated by a relativistic electron beam whose maximum power was approximately 7 kW. The maximum total soft-x-ray power was measured to be 15.2 mW for a 245 MeV, 37 μA electron beam. The bandwith of the radiation at the full width half maximum points was calculated to be between 0.6 and 1.6 keV. In addition, we have exposed photoresist-coated silicon wafers at a distance of 3 m from the radiator. Exposure times of the bare resist were as short as 120 s for 5 cm2 of wafer are (resist sensitivity is 55.6 mJ/cm2). The shortest time for mask/wafer exposure was 180 s for 5 cm2.
Optical heterodyning of the phase-tuned femtosecond optical Kerr gate signal for the determination of complex third-order susceptibilities
Authors: Orczyk M. E. , Samoc M. , Swiatkiewicz J. , Manickam N. , Tomoaia-Cotisel M. , Prasad P. N.
Published: July 2002

We show that ultrafast optically stimulated birefringence and dichroism may be conveniently investigated by combining polarization sensitive optically heterodyned detection with phase tune-up between the optical Kerr gate signal and the local oscillator. The real and the imaginary parts of complex third-order optical nonlinearity can be effectively separated and their values and signs determined. 60 fs pulses at 620 nm were used in experiments carried on tetrahydrofuran solutions of canthaxanthin, a carotenoid important for photobiology. The values of both parts of the complex second hyperpolarizability γ as well as the sign of its real part determined by this method compare well with that obtained from the concentration dependence method employing the homodyne-detection optical Kerr gate technique.
Back-gated split-gate transistor: A one-dimensional ballistic channel with variable Fermi energy
Authors: Hamilton A. R. , Frost J. E. F. , Smith C. G. , Kelly M. J. , Linfield E. H. , Ford C. J. B. , Ritchie D. A. , Jones G. A. C. , Pepper M. , Hasko D. G. , Ahmed H.
Published: July 2002

We have studied the quantized conductance of a one-dimensional ballistic channel in the two-dimensional electron gas of a back-gated GaAs/AlGaAs heterostructure. A standard Schottky split-gate fabricated with electron-beam lithography techniques is used to define the one-dimensional channel, but we incorporate an epitaxially grown in situ back-gate, situated ∼1 μm below the electron gas, to provide additional control of the carrier density. Quantized conductance steps can be induced by changing the bias on either gate, highlighting the self-consistent nature of the electrostatics involved. We show that we can, in principle, achieve independent control of the one-dimensional carrier density and channel width.
Mechanisms of copper chemical vapor deposition
Authors: Cohen Susan L. , Liehr Michael , Kasi Srinandan
Published: July 2002

The mechanism of copper chemical vapor deposition from Cu(II)bis-hexafluoroacetylacetonate [Cu(hfac)2] and 1,5-cyclooctadiene-Cu(I)-hexafluoroacetylacetonate (COD-Cu-hfac) has been determined. The results explain the different processing conditions required for deposition from the precursors. Both molecules react at room temperature on Ag to form a similar Cu(I)-hfac surface intermediate. Subsequent reaction of the COD-Cu-hfac fragment can lead to loss of the organic ligands leaving clean Cu. In contrast, for Cu(hfac)2, the presence of one extra surface hfac requires the addition of an external reductant to produce a ligand-free Cu film.
Polarization reversal and high dielectric permittivity in lead magnesium niobate titanate thin films
Authors: Udayakumar K. R. , Chen J. , Schuele P. J. , Cross L. E. , Kumar V. , Krupanidhi S. B.
Published: July 2002

Ferroelectric thin films of the morphotropic phase boundary composition in the lead magnesium niobate-lead titanate solid solution system were fabricated through the sol-gel spin-on technique. The rapid thermally annealed films showed a very high dielectric constant of 2900, with a concomitant low dissipation factor of 0.02; the films were hysteretic with a saturation remanence of 11 μC/cm2 and a coercive voltage of 0.5 V. The storage charge density observed at 5 V was 210 fC/μm2. These films merit consideration for potential application in ferroelectric nonvolatile random access memories (NVRAMs), and in high bit density metal-oxide-semiconductor (MOS) dynamic random access memories (DRAMs).
Er-diffused Ti:LiNbO3 waveguide laser of 1563 and 1576 nm emission wavelengths
Authors: Becker P. , Brinkmann R. , Dinand M. , Sohler W. , Suche H.
Published: July 2002

The first continuous wave (cw) and pulsed mode operation of an Er-diffused Ti:LiNbO3 monomode waveguide laser at 1563 nm (E⊥ c) and 1576 nm (E∥ c) wavelengths is reported. A tunable Tl:KCl color center laser was used as a pump source. With π-polarized (Ep∥ c) pump radiation of 1479 nm wavelength an oscillation threshold of 13 mW coupled pump power was achieved for the 1576 nm emission line. Above 25 mW pump power additional lasing at 1563 nm wavelength was observed. σ-polarized (Ep⊥ c) pumping led to a single line emission at 1563 nm throughout with the highest cw output power of 3 mW and a slope efficiency of 3%. Time averaged emission linewidths of about 0.6 nm were measured for both wavelengths at about two times the threshold pump power level. With pulsed excitation of about 1 W coupled peak power output pulses of up to 200 mW peak power were measured.
60 GHz sources using optically driven heterojunction bipolar transistors
Authors: Scott D. C. , Plant D. V. , Fetterman H. R.
Published: July 2002

Millimeter wave sources at 60 GHz have been demonstrated using optically driven heterojunction bipolar transistors configured as photodetectors. Two techniques were used to optically generate the millimeter waves; the mixing of two cw lasers and the mode locking of a semiconductor laser. The millimeter wave power generated from these two configurations was radiated into free space using integrated planar twin-dipole antennas and heterodyne detected with signal-to-noise ratios ≳40 dB. As part of these experiments, the dc optical gains and quantum efficiencies of the heterojunction bipolar transistor photodetectors were determined.
Ultrafast nanoscale metal-semiconductor-metal photodetectors on bulk and low-temperature grown GaAs
Authors: Chou S. Y. , Liu Y. , Khalil W. , Hsiang T. Y. , Alexandrou S.
Published: July 2002

Metal-semiconductor-metal photodetectors of finger spacing and width as small as 100 nm have been fabricated on bulk and low-temperature grown GaAs, and tested using a femtosecond pulse laser and high-speed electro-optic sampling. The fastest photodetectors have a measured full width at half maximum impulse response and a 3-dB bandwidth of 0.87 ps and 510 GHz, respectively, for low-temperature grown GaAs limited by carrier recombination time; and of 1.5 ps and 295 GHz for bulk GaAs, limited by the RC time constant. To our knowledge, they are the fastest detectors of their kinds reported to date.
High energy carbon ions implantation: An attempt to grow diamond inside copper
Authors: Zhang Z. H. , Chow L. , Paschke K. , Yu N. , Tao Y. K. , Matsuishi K. , Meng R. L. , Hor P. , Chu W. K.
Published: July 2002

1 MeV carbon ions were implanted into single-crystal copper which was then annealed in argon at temperatures ranging from 350 to 750 °C. Regrowth of the radiation-damaged copper was examined by RBS-channeling measurements. Carbon segregation occurred on annealing at 750 °C. Prolonged annealing at 750 °C caused blistering of the copper layer over the buried carbon. After removal of the blistered copper overlayer, the previously buried carbon layer was examined by Raman scattering, showing that graphite is the dominant phase.
Ambient scanning tunneling spectroscopy of n- and p-type gallium arsenide
Authors: Dagata J. A. , Tseng W.
Published: July 2002

Ambient scanning tunneling spectroscopy (STS) of n- and p-doped GaAs (110) and (100) surfaces, prepared with a stable, electrically transparent surface oxide, reveals that the current-voltage (I-V) characteristics of these surfaces are essentially identical to the I-V properties of the free (110) surface cleaved in ultrahigh vacuum. These results demonstrate for the first time that: (1) meaningful STS spectra of GaAs surfaces can be obtained in air, (2) the passivating layer, consisting of a stable, ultrathin oxide [J. A. Dagata, W. Tseng, J. Bennett, J. Schneir, and H. H. Harary, Appl. Phys. Lett. 59, 3288 (1991)], allows the scanning tunneling microscopy tip to probe the bulk electrical properties of the semiconductor, and (3) quantitative doping information, 1015<NA, ND<1019 cm−3, can be extracted from the STS data.
Observation of nearly degenerate and cavity-enhanced highly nondegenerate four-wave mixing in semiconductor lasers
Authors: Jiang Shijun , Dagenais M.
Published: July 2002

Both nearly degenerate four-wave mixing (FWM) and cavity-enhanced highly nondegenerate four-wave mixing have been studied in a GaAs/AlGaAs semiconductor laser operated above threshold. It is observed that the cavity-enhanced highly nondegenerate FWM signal is of the same order as the nearly degenerate FWM signal. A four-wave mixing signal enhancement of about 90 dB as compared to the background signal is observed at the pump-probe detuning corresponding to the round-trip frequency of the pump laser. The probe amplification is measured in the same frequency range and a large enhancement is also observed.
Minimum thickness MgO buffer layers for YBa2Cu3O7−x/GaAs structures: Assessment using photoluminescence of multiple-quantum-well structures
Authors: Tseng M. Z. , Hu S. Y. , Chang Y. L. , Jiang W. N. , Hu E. L.
Published: July 2002

A GaAs-AlGaAs multiple-quantum-well structure was used as a substrate for overgrowth by a MgO buffer layer and superconducting YBa2Cu3O7−X thin film. The multiple quantum well serves as an in-depth probe for possible damage incurred by the semiconductor substrate due to the relatively high temperature and active oxygen environment that characterizes the superconductor growth. The thickness of the MgO buffer layer was varied and correlated with both the superconducting properties of the overlying YBa2Cu3O7−X and the quality of the substrate as determined by photoluminescence measurements of the multiple quantum wells. Both high quality superconducting YBa2Cu3O7−X and excellent photoluminescence spectra of the substrate were obtained with a MgO thickness of 450 Å and quantum wells as close as 350 Å to the surface.
Diamond(001) single-domain 2×1 surface grown by chemical vapor deposition
Authors: Tsuno Takashi , Tomikawa Tadashi , Shikata Shin-ichi , Imai Takahiro , Fujimori Naoji
Published: July 2002

Diamond homoepitaxial films were grown on an off-angle (001) substrate with a misorientation of 4.3° toward the [110] direction by microwave plasma-assisted chemical vapor deposition from a methane-hydrogen gas mixture. The single domain 2×1 surface was observed by low-energy electron diffraction (LEED) and scanning tunneling microscopy, after growth with methane concentration of 2%. With a methane concentration of 6%, the LEED superstructure spot intensity from another domain increased, suggesting a higher rate of two-dimensional nucleation.
Electrical characteristics of epitaxial CeO2 on Si(111)
Authors: Tye L. , El-Masry N. A. , Chikyow T. , McLarty P. , Bedair S. M.
Published: July 2002

Electrical properties of epitaxial CeO2 thin films on silicon (111) substrates grown in ultrahigh vacuum were studied, varying growth conditions and ex situ thermal treatments. Characterization using reflection high-energy electron diffraction and high resolution transmission electron microscopy reveal that while the ceramic layers have a good single-crystal structure, a dual amorphous layer of CeOx and SiO2 forms at the CeO2/Si interface. This structure has undesirable electrical properties, however, utilizing a post-anneal in dry oxygen, the α-CeOx layer was removed and the SiO2 amorphous layer was made thicker. This newly developed structure benefits from the SiO2/Si interface, having Dit=6×1011 cm−2, and Qf=5×1011 cm−2. The structure exhibits a high capacitance due to the large dielectric constant of CeO2, has electrical properties comparable with those of other reported gate insulators on Si, and has an epitaxial oxide lattice matched to Si.
Site-competition epitaxy for superior silicon carbide electronics
Authors: Larkin David J. , Neudeck Philip G. , Powell J. Anthony , Matus Lawrence G.
Published: July 2002

We present and discuss a novel dopant control technique for compound semiconductors, called site-competition epitaxy, which enables a much wider range of reproducible doping control and affords much higher and lower epilayer doping concentrations than was previously possible. Site-competition epitaxy is presented for the chemical vapor deposition of 6H-SiC epilayers on commercially available (0001)SiC silicon-face substrates. Results from utilizing site-competition epitaxy include the production of degenerately doped SiC epilayers for ohmic-as-deposited (i.e., unannealed) metal contacts as well as very low doped epilayers for electronic devices exhibiting SiC record-breaking reverse voltages of 300 and 2000 V for 3C- and 6H-SiC p-n junction diodes, respectively.
Gas filter correlation instrument for the remote sensing of gas leaks
Authors: Lee H. S. , Zwick H. H. , Till S. M.
Published: July 2002

An airborne remote sensor named GASPILS (gas pipeline leak sensor) has been developed and tested to detect leaks from natural gas pipelines by monitoring the amount of methane in the atmosphere above the leak. The sensor is a passive electro-optical system, with two independent channels, operating in a downward-looking profiling mode. It is designed to detect increases in the low levels of methane concentration by sensing the infrared spectral radiance signatures using a nondispersive gas filter correlation technique. This technique involves the use of a gas cell as a matched spectral filter, and it combines a high degree of sensitivity to the target gas with a high degree of specificity.
A technique for obtaining tunable, subnanosecond (≤300 ps) pulses from a commercial dye laser
Authors: Bourne O. L. , Alcock A. J. , Huo Y. S.
Published: July 2002

A simple technique to convert a commercial dye laser into a tunable source of subnanosecond (≤300 ps) pulses with a linewidth of ∼10 GHz (0.01 nm) is described. The technique used is a combination of stimulated Brillouin scattering and optical breakdown. The effectiveness of the system as a spectroscopic tool was demonstrated by using it to record a partial absorption spectrum of iodine.
Dual-beam laser deflection sensor
Authors: Pawliszyn Janusz , Weber Michael F. , Dignam Michael J.
Published: July 2002

A compact, dual-beam laser deflection system was built, mainly from commercially available parts. While the detector and electronics yielded noise levels at the theoretical level for the detector, pointing instability of the laser led to an increase in noise by a factor of about 103. The dual-beam arrangement gave only a factor of about 4 improvement over the single-beam mode due to alignment and drift problems. Further improvement requires a feedback system to minimize signal arising from common-mode beam displacements.
Nuclear techniques for determining the spatial and energy distribution of fast-confined alpha particles in ignited D–T plasma
Authors: Slaughter D. R.
Published: July 2002

There are several nuclear reactions between fast-confined α particles and low-z ions in ignited D–T plasma. Some of them produce unique and highly penetrating neutron or γ radiation. A study has begun to evaluate the feasibility of using these reactions to determine the spatial profile of fast-confined reaction product α particles, and to determine a crude envelope of their speed distribution as they thermalize. Particular attention is paid to the means of detecting these radiations, measuring their spectra, and most important of all, reducing and discriminating against the very much larger background of interfering radiation.
Method for using germanium thermometers in moderately high magnetic fields
Authors: Roy A. , Buchanan D. S. , Ginsberg D. M.
Published: July 2002

We have devised a simple method for extending the zero-field calibration of a germanium resistance thermometer to include the effects of magnetic fields up to 5 T. We describe the application of this method to the use of a germanium thermometer at liquid-helium temperatures. We outline a similar procedure to take into account the temperature variation of the calibration of a Hall probe.
High sensitivity or streak mode interferometer for pulsed plasma diagnostics
Authors: Smith Richard S. , Doggett Wesley O.
Published: July 2002

A Michelson interferometer using a 5-mW He–Ne laser can either be used in a photodiode detector mode to resolve transient fringe shifts ≲10−4 fringe or in a streak mode to obtain entire nanosecond resolved density profiles in a single shot. Extreme sensitivity is achieved by avoiding most noise sources. The effects of plasma light and of ray deflection due to density gradients are compensated for by the use of a lens, stop, and filter combination. Vibration induced signals are nearly eliminated by the use of feedback stabilization. A differential light detector system which subtracts signals at opposite sides of a fringe protects against laser noise and other spurious signals while doubling the sensitivity. In the streak mode a cylindrical lens beam expander allows utilization of nearly all of the laser light.
Data-acquisition system for measurement of thermal diffusivity and propagation properties of thermal waves by a non-steady-state method
Authors: Santucci A. , Verdini L. , Verdini P. G.
Published: July 2002

We describe an implementation of the experimental technique used for measuring thermal diffusivity and propagation properties of thermal waves by means of a non-steady-state method. The values of the phase shift and amplitude ratio of the temperature oscillation at two fixed points of the specimen are obtained directly by a data-acquisition and processing system which uses a properly interfaced microcomputer. Moreover, in order to reduce substantially the constant temperature gradient along the specimen, the heat oscillating source was replaced by a Peltier device supplied with an alternating current.
Pulsed NMR preamplifier protection at ultrahigh frequency: An improved scheme
Authors: Gonord Patrick , Kan Siew
Published: July 2002

The preamplifier of a pulsed NMR spectrometer using a single-coil probe is usually protected by a pair of crossed diodes to prevent voltage breakdown of the input transistor during transmitter-on periods and also to reduce recovery time. It has been observed that in uhf operations, the peak voltage across the input terminals of the preamplifier can reach a destructive level for a spin excitation power of the order of 20 W. This is due to the lead inductance of the commerically available glass encapsulated diodes. A scheme is proposed which incorporates a quasi-λ/2 line associated with the conventional λ/4 line to tune out this parasitic inductance, thus reducing the transmitter-on input voltage to that of the intrinsic voltage drop across the p-n junction of the diode.
Real-time imaging of analyzed areas in surface analysis
Authors: Grazulis L. , Grant J. T.
Published: July 2002

A system for real-time imaging of the analyzed area of specimens used in surface analysis is described. Images can be displayed on either an inexpensive TV or video monitor. Circuits are provided for the construction of such an imaging system. Its application to correct an improperly calibrated slit arrangement on a hemispherical analyzer used for x-ray photoelectron spectroscopy is described.
Simple model of pinhole closure in high-energy-density experiments
Authors: Harrach R. J.
Published: July 2002

The time-dependent transmission through an aperture of intense signals present in high-energy density experiments is controlled by the opacity of the ablation plasma produced by the signal itself. An analytical model for predicting or correcting for the closure of diagnostic collimators and windows in these experiments is presented. The model provides insight into the physics of the closure process and shows how it scales with properties of the pinhole material and radiation source. Results of the model are shown to be in good agreement with a recent experiment measuring transmissivity of a pinhole in copper subjected to an intense broadband flux of x rays.
Observations of centrally peaked impurity profiles following pellet injection in the Alcator-C tokamak
Authors: Petrasso R. D. , Hopf J. E. , Wenzel K. W. , Sigmar D. J. , Terry J. L. , Greenwald M.
Published: July 2002

Differently filtered x-ray arrays have been used to instantaneously measure central carbon and molybdenum profiles, Alcator’s dominant light and heavy impurities, following pellet injection. Approximately 40 ms after the pellet, the width of the carbon profile was close to the neoclassical prediction of a source-free equilibrium state. (Carbon is a plateau impurity.) The experimental molybdenum profile, which is more uncertain, was between a factor of 1.3 and 3.0 that of the asymptotic prediction of neoclassical theory. Carbon, the dominant nonhydrogenic contributor to Zeff, was found to dramatically affect sawtooth dynamics by altering the central resistivity. Specifically, following injection of the pellet the carbon profile became progressively more peaked, the effect of which was to lengthen the sawtooth period. Approximately 40 ms after the pellet, an internal disruption occurred which reduced on-axis carbon and molybdenum by a factor of 3. (In comparison, the corresponding temperature drop was only about 10%.) After this internal disruption, the sawtooth period shortened as a consequence of carbon not dramatically repeaking. The instrumentation, as well as the plasma conditions needed to make such measurements, will be described. Details of this work are contained in the report PFC/JA-85-41. This work was supported in part by the U.S. DOE Contract No. DE-AC02-78ET51013.
Asymmetry of Stark-broadened Lyman lines from laser-produced plasmas
Authors: Joyce R. F. , Woltz L. A. , Hooper C. F.
Published: July 2002

Three significant causes of spectral line asymmetry: the ion-quadrupole interaction, the quadratic Stark effect, and fine-structure splitting, are included in the calculation of Lyman line profiles emitted by highly ionized hydrogenic radiators in a dense, hot plasma. The line asymmetries are shown to be strongly dependent on the plasma density, indicating that the asymmetry may be of use as a density diagnostic. This work was supported by Lawrence Livermore National Laboratory under sub-contract ♯5962705.
Theoretical dielectric satellite lines of the neon X resonance line
Authors: Bhalla C. P. , Karim K. R.
Published: July 2002

Calculations of the atomic parameters for the dielectric satellite lines of the Ne x resonance line are presented. These satellite lines correspond to transition from doubly excited states of heliumlike neon with electron configurations nl n′l′. The values of n and n′ considered in this paper are 2, 3, and 4 with all allowed values of l and l′. The atomic model used in the present calculation is the self-consistent Hartree–Fock–Slater model with the inclusion of effects arising from the spin-orbit coupling and the electron configuration interaction.
Optical and UV x-ray imaging diagnostics for imploding plasma experiments
Authors: Lee P. H. Y. , Price R. H. , Reay J. , Pecos J. , Seagrave J. , McGurn J. , Cochrane B. , Anderson B.
Published: July 2002

The Trailmaster/Pioneer 1 series of imploding plasma experiments are aimed at using an inductive storage driver to implode an ultrathin aluminum foil with a multimegampere, submicrosecond electrical pulse. The power pulse is produced by an explosive flux compression generator and a fast plasma compression opening switch. The goal is to obtain an intense source of soft x rays from the thermalization of the plasma kinetic energy when pinch occurs on axis. An important target diagnostic is a fast camera which measures the dynamics of foil run-in and implosion symmetry. These measurements are made in the visible, UV, and x-ray portions of the electromagnetic spectrum. UV/x-ray images are first converted to visible light, then transmitted by visible light optics to a framing camera, which is located at a safe distance. For UV/soft x-ray imaging, we mount a disposable pinhole camera with a p-terphenyl-coated converter screen on the target chamber. For soft/hard x-ray imaging, a microchannel plate is used in front of the p-terphenyl-coated screen to boost quantum detection efficiency and signal gain. For faster temporal response the p-terphenyl can be replaced by NE-111 or other fast fluors. Image transmission is accomplished by means of two large mirrors (150–250 mm diam) and a large catadioptric telescope. The framing camera consists of four gated microchannel plates with adjustable gate and interframe times. The framing camera is a versatile and rugged instrument, it has performed satisfactorily for each and every Pioneer shot. Experimental data indicate that this camera, which uses individual gated microchannel plates, is far superior to commercial streak/framing cameras, because it has a dynamic range which is orders of magnitude larger than commercial cameras, each channel can be attenuated separately, further increasing the dynamic range of the imaging system. This makes our framing camera especially suitable for recording sequences of events where the brightness changes by many orders of magnitude. This work was performed under the auspices of U.S. DOE.
Toroidal variation of power radiated from TFTR
Authors: Bush C. E. , Schivell J. , Medley S. S. , Ulrickson M.
Published: July 2002

Bolometers located at a number of toroidal and poloidal positions are used to determine the toroidal distribution of impurity radiation losses from the Tokamak Fusion Test Reactor (TFTR) plasmas. Distributions for both OH and beam-heated D+ and He++ plasmas have been determined. The distributions are highly peaked at the limiter sector for plasmas run on the outer limiter but more symmetric toroidally when run on the inner bumper limiter. Plasmas detached from both inner and outer limiters have distributions which are basically symmetric toroidally and poloidally. The widths of toroidally peaked distributions are ≤1 sector of the 20 sector torus. Scalings of the enhanced radiative losses at the limiter with input power are indicated.
X-ray spectroscopic methods for the diagnosis of laser-imploded targets (invited)
Authors: Yaakobi B. , Barnouin O. , Richardson M. C. , Soures J. M. , Hauer A. , Post Benjamin
Published: July 2002

Several methods involving x-ray spectroscopic methods for diagnosing laser-imploded targets are discussed. The first method involves the recording of absorption lines formed in the target tamper, out of the continuum emitted by a hotter compressed core. This method is applied to ablatively imploded targets having a thin KCl signature layer. The tamper ρΔR is deduced from the area within the absorption lines, whereas the tamper temperature is deduced from the intensity distribution among absorption lines of adjacent charge states. In a second method, doubly diffracting crystals can give two-dimensional monochromatic images of thin signature layers in spherical targets. Such information is useful in studying stability and mixing. Experimental results relevant to these methods will be shown and the limitations on their application to laser-target experiments will be discussed.
Deconvolution of H-α profiles measured by Thomson scattering collecting optics
Authors: LeBlanc B. , Grek B.
Published: July 2002

A method using a multipoint Thomson scattering system to obtain spatially resolved local emissivity is described. The method involves Fourier decomposition with auxiliary constraints. Optimum solution is determined from an analysis of the entropy and the error of the inversion. The technique is applied to H-α emission profiles from the PDX tokamak.
Magnetic-fusion data acquisition at Los Alamos
Authors: Wilkins R. W. , Klare K. A.
Published: July 2002

The Controlled Thermonuclear Research (CTR) division uses a single software package to acquire data from hundreds of CAMAC recorders. For the past five years, this package has been used by all experiments, both manual and computer controlled. A variety of signals are stored in transient digitizers and other memories. The computer retrieves the data and applies an efficient compression algorithm before storing it on disk. The physicist can plan his next shot from the acquired data even as the rest is read. The readout is list directed with device, amplifier, and time-base types and their settings selected by the physicist. A new experiment may be set up in minutes.