bluuebunny/crossref_metadata_2025 huggingface dataset:
FAD is a further essential cofactor of the NAD(P)H and O2‐dependent zeaxanthin‐epoxidase
Authors: Büch Karen , Stransky Harald , Hager Achim
Published: July 2002

In chloroplasts of plants the xanthophyll cycle is suggested to function as a protection mechanism against photodamage. Two enzymes catalyze this cycle. One of them, violaxanthin de‐epoxidase, transforms violaxanthin (Vio) to zeaxanthin (Zea) via antheraxanthin (Anth) and is bound to the lumenal surface of the thylakoid vesicles, when being in its active state. The other enzyme, Zea‐epoxidase, is responsible for the backward reaction (Zea → Anth → Vio) and is active at the stromal side of the thylakoid. For the epoxidation of Zea this enzyme requires NAD(P)H and O2 as cosubstrates. Using isolated thylakoid membranes we found that FAD enhances the epoxidase activity (decrease of apparent K
m for NAD(P)H and two‐fold increase of V
max). The flavin functions as a third cofactor which is partially lost during the isolation procedure of thylakoids. Other flavins, such as FMN or riboflavin are without effect. The involvement of FAD in the enzymatic reaction is also demonstrated by the inhibitory action of diphenyleneiodoniumchloride (DPI) (IC50 = 2.3 μM), a compound that blocks the reoxidation of reduced flavins within enzymes. The Zea‐epoxidase is a multicomponent enzyme system which can be classified as FAD‐containing, NAD(P)H‐ and O2‐dependent monooxygenase that is able to epoxidize 3‐hydroxy β‐ionone rings of xanthophylls in the 5,6 position.
Pregnancy and its outcome in quadriplegia due to Pott's spine
Authors: Vaidya M.K. , Shah G.V. , Bharucha K.E.
Published: July 2002

Pregnancy with quadriplegia is a problem sometimes encountered in obstetric practice. The etiology of quadriplegia in the developed world is mainly spinal cord tumor or accident, while in the developing countries the main cause is tuberculosis of the spine. We report the management of two pregnant patients with quadriplegia due to tuberculosis of the cervical spine. Worsening of the neurological condition necessitated early surgical intervention, and termination of pregnancy was advised in both patients. Literature on the subject makes frequent reference to the life‐threatening complication of autonomic hyperreflexia encountered during pregnancy and delivery. It is characterized by sweating, headache, severe hypertension leading to unconsciousness and convulsions. These complications, surprisingly, were absent in both of our patients.
On the mechanism of the inhibition of Na+, K+‐ATPase activity caused by homocysteine
Authors: Streck Emilio L. , Zugno Alexandra I. , Tagliari Bárbara , Sarkis João J.F. , Wajner Moacir , Wannmacher Clóvis M.D. , Wyse Angela T.S.
Published: July 2002

In the present work, we investigated the kinetics of the inhibition of Na+, K+‐ATPase activity caused by homocysteine (Hcy) in rat hippocampus. We also studied the interaction between Hcy and phenylalanine (Phe) and the kinetics of alanine (Ala) reversal of the inhibition of Na+, K+‐ATPase caused by Hcy. The apparent Km and Vmax of Na+, K+‐ATPase for ATP as substrate were 0.55 mM and 2.0 nmol Pi released per min per mg of protein, respectively. Ki value was approximately 0.1 mM, and the inhibition was of the non‐competitive type. The results also showed a competition between Hcy and Phe. Ala per se did not alter this enzyme, but prevented the inhibitory effect caused by Hcy, suggesting a common binding site for these substances. It is proposed that the inhibition of Na+, K+‐ATPase by Hcy may be one of the mechanisms related to the neuronal dysfunction observed in human homocystinuria.
Characterization of a Novel Biocatalyst System for Sulfide Oxidation
Authors: McComas Charles , Sublette Kerry L. , Jenneman Gary , Bala Greg
Published: July 2002

It has been demonstrated that an enrichment culture dominated by Thiomicrospira sp. CVO may be cultured on H2S(g) as an energy source under sulfide‐limiting conditions in suspended culture with nitrate as the electron acceptor. Hydrogen sulfide (10,000 ppmv) was completely removed from the feed gas and oxidized to sulfate in <3 s of gas‐liquid contacting time. Maximum loading of the biomass for sulfide oxidation was observed to be 5.8 mmol H2S/h‐g biomass protein, comparable to that reported previously for Thiobacillus denitrificans under similar conditions. However, the enrichment culture was shown to be more tolerant of extremes in pH and elevated temperature than T. denitrificans. Coupled with a reported tolerance of CVO for up to 10% NaCl, these observations suggest that a CVO‐based culture is potentially a more robust biocatalyst system for sulfide oxidation than cultures based on Thiobacilli.
Adsorption of Protein from Several Commercial Lipase Preparations onto a Hollow‐Fiber Membrane Module
Authors: Balcão Victor M. , Vieira Madalena C. , Malcata F. Xavier
Published: July 2002

Ten commercially available crude preparations of lipase from various microbial sources were adsorbed from aqueous buffers at several initial concentrations onto a bundle of hydrophobic hollow fibers made of poly(propylene) at pH 7.0 and 40 °C. The kinetics of adsorption were evaluated from measurements at various times of the protein content of the supernatant solution (using BSA as equivalent) in a well‐mixed reservoir placed in series with the hollow fiber module. Preliminary tracer experiments have indicated that the module and the tank can be simulated as a system consisting of a plug flow reactor in series with a continuous stirred tank reactor. A mechanistic model based on the hydrodynamic assumptions associated with this system coupled with the postulation of two reversible first‐order steps for the adsorption of protein was successfully fitted to the experimental data via nonlinear regression analysis. The statistical significance of the model was checked using tests for lack of fit. This work is useful in predicting the time period required to immobilize a (crude) lipase by adsorption onto a hydrophobic hollow fiber module, a configuration which has proved successful in the recent past for the performance of lipase‐catalyzed reactions.
Gaulin Homogenization: A Mechanistic Study
Authors: Lander R. , Manger W. , Scouloudis M. , Ku A. , Davis C. , Lee A.
Published: July 2002

Free radical‐based oxidation has been detected in the normal operating regime of the Gaulin homogenizer, demonstrating that cavitation occurs in this important industrial bioprocessing equipment. Free radical generation is suppressed by imposition of back pressure, proving that such cavitation occurs in the impingement section. The calculated value of the cavitation number is consistent with submerged jet cavitation, wherein a high‐speed jet exiting from the valve gap accelerates fluid in the impingement region, creating the vacuum conditions for cavitation. Using polysaccharides as a model shear‐sensitive compound, their breakage pattern in the homogenizer was characterized by molecular size and polydispersity and compared to those of fluid shear flows in capillary tubes and cavitating flow from a sonic horn. The results indicate that breakage occurs primarily by fluid shear, although a contribution by cavitation is also apparent when back pressure is applied. Because biological molecules can readily react with free radicals and the alterations caused thereby are subtle in nature, a thorough evaluation of the impact of free radicals in upstream homogenization is warranted.
Allergens from fish and egg
Authors: Poulsen Lars K. , Hansen Tine K. , Nørgaard Astrid , Vestergaard Helle , Stahl Skov Per , Bindslev‐Jensen Carsten
Published: July 2002

Allergens from fish and egg belong to some of the most frequent causes of food allergic reactions reported in the literature. Egg allergens have been described in both white and yolk, and the egg white proteins ovomucoid, ovalbumin, ovotransferrin and lysozyme have been adopted in the allergen nomenclature as Gal d1–d4. The most reported allergen from egg yolk seems to be alpha‐livitin. In fish, the dominating allergen is the homologues of Gad c1 from cod, formerly described as protein M. A close cross‐reactivity exists within different species of fish between this calcium‐binding protein family, denominated the parvalbumins. This cross‐reactivity has been indicated to be of clinical relevance for several species, since patients with a positive double‐blind, placebo‐controlled food challenge to cod will also react with other fish species, such as herring, plaice and mackerel. In spite of the importance of these two allergen systems, only a few studies have been performed, and the scarcity of cloned allergens from both of the systems is emphasized.
Mechanism of the antigen formation of carvone and related α, β‐unsaturated ketones
Authors: Nilsson Anna‐Malin , Gäfvert Elisabeth , Salvador Lourdes , Luthman Kristina , Bruze Magnus , Gruvberger Birgitta , Nilsson J. Lars G. , Karlberg Ann‐Therese
Published: July 2002

In the present study, the mechanism for the antigen formation of α, β‐unsaturated ketones was investigated. A series of analogues of carvone ((5R)‐5‐isopropenyl‐2‐methyl‐2‐cyclohexenone) with altered chemical reactivity and with retained overall structure or with retained reactivity and altered three‐dimensional structure were synthesized. These analogues were tested for cross‐reactivity in carvone‐sensitized animals. Cross‐reactivity was observed for analogue 3 ((5R)‐5‐isopropyl‐2‐methyl‐2‐cyclohexen‐1‐one). No cross‐reactions were observed for analogues 1 ((2R,5R)‐5‐isopropenyl‐2‐methyl cyclohexanone) and 4 ((5R)‐2,3‐dimethyl‐5‐isopropenyl‐2‐cyclohexene‐1‐one). Both those compounds also failed to induce sensitization. These findings demonstrate that α, β‐unsaturated ketones form antigens after a nucleophilic attack at the β‐carbon with soft nucleophiles such as thiol in cysteine and not with the formation of a Schiff’s base after a nucleophilic attack at the carbonyl carbon with nitrogen nucleophiles. Furthermore, no cross‐reactivity was observed between R‐ and S‐carvone indicating the importance of the 3‐dimensional structure of haptens (and antigens) in T‐cell recognition. The analogues were also tested for cross‐reactivity on patients allergic to carvone. The results from the animal study were confirmed.
Psychological stress in undergraduate dental students: baseline results from seven European dental schools
Authors: Humphris Gerry , Blinkhorn Andy , Freeman Ruth , Gorter Ronald , Hoad‐Reddick Gillian , Murtomaa Heikki , O'Sullivan Robin , Splieth Christian
Published: July 2002

Objectives: To determine the degree of psychological distress, the experience of emotional exhaustion, and the extent of stress associated with course work in dental students and to compare these measurements among seven European dental schools.

Design: Multi‐centred survey.

Setting: Dental Schools at Amsterdam, Belfast, Cork, Greifswald, Helsinki, Liverpool and Manchester.

Participants: 333 undergraduate first‐year dental students.

Measures: General Health Questionnaire (GHQ12), Maslach Burnout Inventory (MBI), Dental Environment Stress Questionnaire (DES), demographic variables.

Procedure: Questionnaire administered to all students attending first year course. Completed questionnaires sent to central office for processing.

Results: Seventy‐nine percent of the sampled students responded. Over a third of the students (36%) reported significant psychological distress (morbidity) at the recommended cut‐off point (>3 on GHQ). These scores were similar to those reported for medical undergraduates. Twenty‐two percent recorded comparatively high scores on emotional exhaustion. A wide variation in these 2 measurements was found across schools (p's<0.001). Stress levels indicated by the DES were less variable (p>0.5). Some evidence showed that contact with patients and the level of support afforded by living at home may be protective.

Conclusion: Higher than expected levels of emotional exhaustion were found in a large sample of first‐year undergraduate dental students in Europe.
Membrane depolarization-induced contraction of rat caudal arterial smooth muscle involves Rho-associated kinase
Authors: MITA Mitsuo , YANAGIHARA Hayato , HISHINUMA Shigeru , SAITO Masaki , WALSH Michael P.
Published: July 2002

Depolarization of the sarcolemma of smooth muscle cells activates voltage-gated Ca2+ channels, influx of Ca2+ and activation of cross-bridge cycling by phosphorylation of myosin catalysed by Ca2+/calmodulin-dependent myosin light-chain kinase (MLCK). Agonist stimulation of smooth muscle contraction often involves other kinases in addition to MLCK. In the present study, we address the hypothesis that membrane depolarization-induced contraction of rat caudal arterial smooth muscle may involve activation of Rho-associated kinase (ROK). Addition of 60mM K+ to de-endothelialized muscle strips in the presence of prazosin and propranolol induced a contraction that peaked rapidly and then declined to a steady level of force corresponding to approx. 30% of the peak contraction. This contractile response was abolished by the Ca2+-channel blocker nicardipine or the removal of extracellular Ca2+. An MLCK inhibitor (ML-9) inhibited both the phasic and tonic components of K+-induced contraction. On the other hand, the ROK inhibitors Y-27632 and HA-1077 abolished the tonic component of K+-induced contraction, and slightly reduced the phasic component. Phosphorylation levels of the 20-kDa light chain of myosin increased rapidly in response to 60mM K+ and subsequently declined to a steady-state level significantly greater than the resting level. Y-27632 abolished the sustained and reduced the phasic elevation of the phosphorylation of the 20-kDa light chain of myosin, without affecting the K+-induced elevation of cytosolic free Ca2+ concentration. These results indicate that ROK activation plays an important role in the sustained phase of K+-induced contraction of rat caudal arterial smooth muscle, but has little involvement in the phasic component of K+-induced contraction. Furthermore, these results are consistent with inhibition of myosin light-chain phosphatase by ROK, which would account for the sustained elevation of myosin phosphorylation and tension in response to membrane depolarization.
Flavonol 2,4‐dioxygenase from Aspergillus niger DSM 821, a type 2 CuII‐containing glycoprotein
Authors: Hund Hilda‐Katharina , Breuer Jörn , Lingens Franz , Hüttermann Jürgen , Kappl Reinhard , Fetzner Susanne
Published: July 2002

Flavonol 2,4‐dioxygenase, which catalyzes the cleavage of quercetin to carbon monoxide and 2‐protocatechuoyl‐phloroglucinol carboxylic acid, was purified from culture filtrate of Aspergillus niger DSM 821 grown on rutin. It is a glycoprotein (46–54% carbohydrate) with N‐linked oligo‐mannose type glycan chains. The enzyme was resolved in SDS polyacrylamide gels in a diffuse protein band that corresponded to a molecular mass of 130–170 kDa. When purified flavonol 2,4‐dioxygenase was heated, it dissociated into three peptides with apparent molecular masses of 63–67 kDa (L), 53–57 kDa (M), and 31–35 kDa (S), which occurred in a molar ratio of 1 : 1 : 1, suggesting a LMS structure. Crosslinking led to a 90–97 kDa species, concomitant with the decrease of staining intensity of the 63–67 kDa (L) and the 31–35 kDa (S) peptides. Analysis by matrix‐assisted laser desorption/ionization‐time of flight‐MS showed peaks at m/z≈ 69 600, m/z≈ 51 700, and m/z≈ 26 500 which are presumed to represent the three peptides of flavonol 2,4‐dioxygenase, and a broad peak at m/z≈ 96 300, which might correspond to the LS heterodimer as formed in the crosslinking reaction. Based on the estimated molecular mass of 148 kDa, 1 mol of enzyme contained 1.0–1.6 mol of copper. Ethylxanthate, which specifically reduces CuII to CuI ethylxanthate, is a potent inhibitor of flavonol 2,4‐dioxygenase. Metal chelating agents (such as diethyldithiocarbamate, diphenylthiocarbazone) strongly inhibited the enzymatic activity, but inactivation was not accompanied by loss of copper. The EPR spectrum of flavonol 2,4‐dioxygenase (as isolated) showed the characteristic parameters of a nonblue type 2 CuII protein. The Cu2+ is assumed to interact with four nitrogen ligands, and the CuII complex has a (distorted) square planar geometry.
The UBiT‐100 13CO2 Infrared Analyzer: Comparison Between Infrared Spectrometric Analysis and Mass Spectrometric Analysis
Authors: Ohara Shuichi , Kato Mototsugu , Asaka Masahiro , Toyota Takayoshi
Published: July 2002

Background.

Although the 13C‐urea breath test is the most accurate noninvasive method for detecting the presence of H. pylori infection, the requirement for an expensive mass spectrometer to analyze breath samples has prevented physicians from providing rapid results near the patient. Recently, a new type of infrared spectrometric analyzer, the UBiT‐100, was developed for analyzing 13CO2 in breath. The purpose of this study is to compare results analyzed by the UBiT‐100 to those analyzed by the mass spectrometric method.

Methods.

Four hundred and fifty‐three subjects participated in this study. Breath samples were collected before administration of 100~mg of 13C‐urea and at 10, 20, 30, 45 and 60~min after administration. Subjects were asked to hold their breath for 10~sec and then exhale in order to collect breath samples containing more than a 2% concentration of CO2. Samples were then analyzed by both methods.

Results.

The correlation analysis using values at 20~min after the administration of the study drug (433 points) was excellent with the regression equation of Y~=~1.034x − 0.203;r~=~.996. The results of the UBiT‐100 were available in 6~min, making the entire testing procedure less than 30~min.

Conclusions.

The UBiT‐100 infrared analyzer provides a simple and accurate method of performing the urea breath test while the patient is still in the doctor's office.
Measurement of nonlinear properties in Ag-ion exchange waveguides using degenerate four-wave mixing
Authors: Ashley Paul R. , Bloemer Mark J. , Davis Jack H.
Published: July 2002

Degenerate four-wave mixing was used to evaluate third-order nonlinearities in Ag-Na exchange waveguides. Guided wave linear absorption analysis was used to correlate metallic Ag content with nonlinear properties. Measured conjugate reflectivities approached 4%.
Surface-stress-induced structure and elastic behavior of thin films
Authors: Wolf D.
Published: July 2002

Computer simulations of unsupported (111), (001), and (011) thin films of gold, using an embedded-atom-method potential, demonstrate a direct correlation between the bulk-surface stress and the film dimensions. The considerably more complex elastic behavior, by contrast, appears to be dominated by the atomic structure of the film surfaces, and not by the stress-induced anisotropic lattice parameter changes.
Large photoinduced birefringence in an optically nonlinear polyester polymer
Authors: Shi Yongqiang , Steier William H. , Yu Luping , Chen Mai , Dalton Larry R.
Published: July 2002

A nonlinear optical polyester with disperse red side groups exhibited a stable photoinduced birefringence, from 0.14 at 850 nm to 0.21 at 633 nm, after exposure to linearly polarized short-wavelength visible or ultraviolet light. Thin-film wave plates, birefringent diffraction gratings and waveguides were written in this polymer using photoexposure. Thin- film wave plates of <1 μm thickness showed ≳π/4 phase delay and good temporal stability. A 9:1 diffraction efficiency ratio of the two polarization eigenmodes was measured from thin birefringent gratings. This ratio is in good agreement with the theoretical prediction obtained using a simple three-level model.
Electron holographic study of ferroelectric domain walls
Authors: Zhang X. , Hashimoto T. , Joy D. C.
Published: July 2002

Electron holography performed in a transmission electron microscope, has been used to measure the width of 90° domain walls in tetragonal BaTiO3, and to determine the magnitude and distribution of the polarization across these walls. The walls are found to be between 15 and 25 Å in width. The polarization and its measured variation are in good agreement with other estimates. Fringe splitting has been observed in some holograms and it is suggested that this is due to the presence of charged oxygen vacancies on the walls.
High-resolution recoil spectrometry for separate characterization of Ga and As in AlxGa(1−x)As structures
Authors: Hult Mikael , Whitlow Harry J. , Östling Mikael
Published: July 2002

Mass and energy-dispersive recoil spectrometry, where the recoil energy is derived from the recoil time of flight, has been used to characterize the depth distribution of Al, Ga, and As in an AlxGa(1−x)As quantum-well structure. Signals characterizing the Al, Ga, and As distribution with good separation between Ga and As (average crosstalk <2%) could be obtained from depths less than 560 nm from the surface. The depth resolution of the As signal at the surface was 16 nm FWHM, which is considerably better than achieved using a silicon particle detector (34 nm).
Magnetic switching of THz beams
Authors: Zhang X.-C. , Jin Y. , Hewitt T. D. , Sangsiri T. , Kingsley L. E. , Weiner M.
Published: July 2002

We demonstrate the use of a magnetic field to switch and to control the direction and polarization of a THz beam radiated from a semiconductor emitter.
Effects of crystalline quality and electrode material on fatigue in Pb(Zr,Ti)O3 thin film capacitors
Authors: Lee J. , Johnson L. , Safari A. , Ramesh R. , Sands T. , Gilchrist H. , Keramidas V. G.
Published: July 2002

Pb(Zr0.52Ti0.48)O3(PZT)/Y1Ba2Cu3Ox(YBCO) heterostructures have been grown by pulsed laser deposition, in which PZT films were epitaxial, highly oriented, or polycrystalline. These PZT films were obtained by varying the deposition temperature from 550 to 760 °C or by using various substrates such as SrTiO3(100), MgO(100), and r-plane sapphire. PZT films with Pt top electrodes exhibited large fatigue with 35%–50% loss of the remanent polarization after 109 cycles, depending on the crystalline quality. Polycrystalline films showed better fatigue resistance than epitaxial or highly oriented films. However, PZT films with both top and bottom YBCO electrodes had significantly improved fatigue resistance for both epitaxial and polycrystalline films. Electrode material seems to be a more important parameter in fatigue than the crystalline quality of the PZT films.
Microstructure and pseudomorphism in molecular beam epitaxially grown ZnCdS on GaAs(001)
Authors: Guha S. , Wu B. J. , Cheng H. , DePuydt J. M.
Published: July 2002

We have carried out an electron-microscopy-based microstructural study of molecular beam epitaxially grown ZnxCd1−xS epilayers on GaAs(001) substrates. We find that sulfur attacks bare GaAs substrates, resulting in pits that act as nucleating centers for defects that propagate into the epilayer. Such pitting may be avoided by capping the GaAs substrate with a ZnSe buffer in a sulfur-free separate growth chamber. We also investigate the pseudomorphic zinc blende to wurtzite crystal structure change in epitaxial ZnxCd1−xS and propose a microstructural model for this structural transition.
Interface roughness scattering in semiconducting and semimetallic InAs-Ga1−xInxSb superlattices
Authors: Hoffman C. A. , Meyer J. R. , Youngdale E. R. , Bartoli F. J. , Miles R. H.
Published: July 2002

An analysis of magnetotransport results for InAs-Ga1−xInxSb superlattices with a range of layer thicknesses demonstrates that interface roughness scattering dominates the electron mobility under most conditions of interest for infrared detector applications. However, the dependence on well thickness is much weaker than the d16 relation observed in other systems with thicker barriers, which is consistent with predictions based on the sensitivity of the energy levels to roughness fluctuations. Theory also correctly predicts an abrupt mobility decrease at the semiconductor-to-semimetal transition point, as well as the coexistence of two electron species in semimetallic samples.
Structure determination of very thin epitaxial layers of metastable body centered cubic Co with ion channeling
Authors: Dekoster J. , Bemelmans H. , De Wachter J. , Moons R. , Langouche G.
Published: July 2002

The structure of a 20 Å thin Co layer embedded between Fe layers grown with molecular beam epitaxy on MgO is determined with ion beam channeling. From the position and the ratio of the widths of the angular yield profiles for different crystallographic directions, we show that Co is forced in the metastable body centered cubic structure. The presence of tetragonal distortion in the Co layer is observed.
A laser heating system that stabilizes and controls the temperature: Diamond anvil cell applications
Authors: Heinz Dion L. , Sweeney Jeffrey S. , Miller Peter
Published: July 2002

A laser heating system is described for use with diamond anvil high pressure cells that directly senses and stabilizes visible thermal radiation emitted by hot samples. This technique stabilizes sample temperatures better than other methods and allows superior temperature control. Calibration of the system was checked by measuring the melting temperatures of five metals at ambient pressure. Assuming literature values for spectral emissivity, the calibration was found to be accurate to 3.3% (based upon one standard deviation of the percentage error from published melting temperatures). Performance of the laser heating system was verified by heating iron foil at 13 GPa. With the sample intensity unstabilized, mean temperature was 3003 K with a standard deviation of 144 K, while with it stabilized, mean temperature was 3051 K with a standard deviation of 8 K. For a given wavelength-dependent emissivity, the difference between the actual temperature and the greybody temperature increases as the temperature increases. Therefore, to accurately determine temperatures at the high temperatures applicable to solid Earth geophysics (1500–6000 K), wavelength-dependent emissivity cannot be ignored.
The inverted pendulum as a probe of anelasticity
Authors: Saulson Peter R. , Stebbins Robin T. , Dumont Frank D. , Mock Scott E.
Published: July 2002

We discuss the rich dynamics of an inverted pendulum kept from falling over by an anelastic flexure. The anelastic character of the flexure is dramatically emphasized by the pendulum. We report measurements showing internal friction in one flexure to be independent of frequency over a band of three to four decades. This anelastic behavior in flexures can limit the sensitivity in gravitational wave detectors and in other precision measurements.
The upgrade to the OMEGA laser system
Authors: Boehly T. R. , Craxton R. S. , Hinterman T. H. , Kelly J. H. , Kessler T. J. , Kumpan S. A. , Letzring S. A. , McCrory R. L. , Morse S. F. B. , Seka W. , Skupsky S. , Soures J. M. , Verdon C. P.
Published: July 2002

The upgrade to the OMEGA laser system, a 30-kJ, 351-nm, 60-beam direct-drive laser-fusion system is reported here. The system configuration is presented along with the status of its construction and the plans for system activation. The system utilizes rod and disk amplifiers and frequency tripling to produce UV pulses which are smoothed using phase conversion and smoothing by spectral dispersion. Dual driver lines will feed the propagation of two co-axial beams, which have different pulse widths and occupy different portions of the laser aperture. The laser system will be completed in November 1994 and the target area in March 1995.
Polycrystalline diamond photoconductive device with high UV-visible discrimination
Authors: McKeag Robert D. , Chan Simon S. M. , Jackman Richard B.
Published: July 2002

Planar metal–diamond–metal photoconductive devices have been fabricated from free standing large grain (20–30 μm) polycrystalline thin film diamond. An interdigitated electrode design with spacings of 20 μm was used to produce effective UV photodetecting devices at bias values in the range 0.1–10 V. A methane-air treatment has been used to modify the structures such that unprecedented performance characteristics have been recorded (106 higher response to 200 nm than visible wavelengths, <0.1 nA dark currents); spectral features similar to those observed in natural diamond crystals have been observed indicating that the treatment used led to near ideal electronic characteristics from polycrystalline material.
Large second-order optical nonlinearities in pulsed laser ablated silicon carbide thin films
Authors: Lundquist P. M. , Ong H. C. , Lin W. P. , Chang R. P. H. , Ketterson J. B. , Wong G. K.
Published: July 2002

Large second-order nonlinear optical response has been observed in silicon carbide thin films deposited by pulsed laser ablation on sapphire and fused silica substrates; films on both substrates were uniform and optically transparent but exhibited distinct orientations. The d33 values of the sapphire-substrate samples were determined to be 10 pm/V.
Photodetachment photoelectron spectroscopy of multiply charged anions using electrospray ionization
Authors: Wang Lai-Sheng , Ding Chuan-Fan , Wang Xue-Bin , Barlow S. E.
Published: July 2002

A magnetic-bottle time-of-flight (TOF) photoelectron spectrometer, coupled with an electrospray ionization source, has been developed for the investigation of multiply charged anions in the gas phase. Anions formed in the electrospray source are guided by a radio-frequency quadrupole ion guide into a quadrupole ion trap, where the ions are accumulated. A unique feature of this apparatus involves the coupling of a TOF mass spectrometer to the ion trap with perpendicular ion extraction. The ion trap significantly improves the duty cycle of the experiments and allows photodetachment experiments to be performed with low repetition-rate lasers (10–20 Hz). This novel combination makes the photodetachment photoelectron spectroscopy studies of multiply charged anions possible for the first time. Furthermore, the perpendicular extraction of ions, pulsed out of the ion trap, to the TOF mass spectrometer allows the ion energies to be conveniently referenced to ground, simplifying the configuration of the TOF mass spectrometer and the subsequent magnetic-bottle TOF photoelectron spectrometer. The mass resolution (M/ΔM) achieved is about 800 for smaller ions. The magnetic-bottle photoelectron spectrometer resolution is about 11 meV full width at half maximum for 0.5 eV photoelectrons with an overall resolution of ΔE/E∼2%. The detailed design, construction, and operation of the new apparatus are presented.
Use of a random phase plate as a KrF laser beam homogenizer for thin film deposition applications
Authors: Lewis C. L. S. , Weaver I. , Doyle L. A. , Martin G. W. , Morrow T. , Pepler D. A. , Danson C. N. , Ross I. N.
Published: July 2002

Fabrication of devices based on thin film structures deposited using the pulsed laser deposition technique relies on reproducibility and control of deposition rates over substrate areas as large as possible. Here we present an application of the random phase plate technique to smooth and homogenize the intensity distribution of a KrF laser footprint on the surface of a target which is to be ablated. It is demonstrated that intensity distributions over millimeter-sized spots on the target can be made insensitive to the typical changes that occur in the near-field intensity distribution of the ultraviolet output from a KrF laser.
Thermal characterization of film-on-substrate systems with modulated thermoreflectance microscopy
Authors: Li Bincheng , Pottier L. , Roger J. P. , Fournier D. , Welsch E.
Published: July 2002

Computer simulations are performed in this article to show the feasibility of simultaneous determination of the film diffusivity, the substrate diffusivity, and the thermal boundary resistance of film-on-substrate systems by modulated thermoreflectance microscopy and multiparameter fitting. The dependences of the phase on the probe-to-pump beam separation, measured at four different modulation frequencies, are simultaneously fitted to an appropriate thermal diffusion model to extract the three thermal parameters. The selection of the optimal frequency combination is analyzed. Three samples: an 80 nm gold film on LaAlO3 and diamond substrates, and a 300 nm YBaCuO film on LaAlO3 substrate, are simulated. Experimental results are also presented to discuss the influence of the shape and irregularity of the beam on the fitting. The simulation results show that the statistical mean values of the three thermal parameters are very close to the input values, and the statistical errors of the film diffusivity and substrate diffusivity are comparable to the overall experimental error. However, the error of the thermal boundary resistance depends largely on the absolute thermal resistance value and the diffusivity difference between the film and the substrate. Under typical experiment conditions, the measurement errors of the two diffusivities are ∼5%, and error of the thermal boundary resistance is 10%–20%.
Lock-in common-mode rejection demodulation: Measurement technique and applications to thermal-wave detection: Theoretical
Authors: Mandelis Andreas , Paoloni Stefano , Nicolaides Lena
Published: July 2002

The ability of conventional single-ended photothermal techniques to detect weak inhomogeneities in a given material is mainly limited by two instrumental factors: the signal-to-noise ratio (SNR) and the amplitude dynamic range. The amplitude level is limited by the output signal baseline, and may be too high to monitor relatively small variations introduced by the presence of weak inhomogeneities. The purpose of this work is to introduce a novel photothermal signal generation methodology, the principle of which can be broadly applied to any technique utilizing a lock-in analyzer demodulation scheme of periodic signal wave forms. Unlike the conventional single-ended periodic excitation wave form, which uses a 50% duty-cycle square wave or sinusoidal modulation of the pump laser heating beam, a more complicated periodic modulation wave form is employed, resulting in the equivalent of differential-signal demodulation. The new wave form takes advantage of the real-time differential action performed by the lock-in amplifier weighing function over the two half periods of the modulated signal. This results in enhanced signal dynamic range due to the efficient suppression of the baseline and a substantial improvement in the SNR. The main features of this technique are investigated with a theoretical model for an arbitrary repetitive signal wave form and, in particular, for a photothermal signal. The dependence of the signal on the wave form parameters is also discussed.
Investigations on exponential lifetime measurements for fluorescence thermometry
Authors: Fernicola V. C. , Rosso L. , Galleano R. , Sun T. , Zhang Z. Y. , Grattan K. T. V.
Published: July 2002

Lifetime-based methods have been, on the whole, one of the most successful schemes for fiber optic temperature sensing, using fluorescent materials whose response is intensity independent. Several approaches for determining the fluorescence lifetime, and with that the measurand, have been investigated. An experimental comparison of direct and indirect measurement methods, i.e., involving actual signals from representative optical media instead of simply using Monte Carlo simulations, has been carried out. Direct fitting methods, including Marquardt, log-fit and Prony, were used to estimate the fluorescence lifetime of a Cr3+:YAG-based sensor system and the results were compared. An agreement to better than 0.5% between Marquardt and log-fit algorithms and an agreement of about 1.5% between Marquardt and Prony approaches was found. Thus, a temperature reproducibility, of 0.5 and 1.2 °C, respectively, can be obtained with the Cr3+:YAG sensor system. An indirect measurement approach based on a phase-locked (analog-to-digital signal processor) (A-DSP) was also tested. It was found that when the A-DSP output is used to estimate the lifetime, it performs only slightly better than using direct fitting methods. On the contrary, when the whole A-DSP sensor system was directly calibrated against temperature, the measurement accuracy improves by at least a factor of 10.
Growth and characterization of bulk InGaN films and quantum wells
Authors: Keller S. , Keller B. P. , Kapolnek D. , Abare A. C. , Masui H. , Coldren L. A. , Mishra U. K. , Den Baars S. P.
Published: July 2002

InGaN bulk layers and single quantum wells were grown by atmospheric pressure metalorganic chemical vapor deposition on c-plane sapphire. We have found that the incorporation efficiency of indium into InGaN epitaxial layers is strongly dependent on the growth rate of the films. Narrow and bright band edge related luminescence was observed for InGaN films up to an indium content of 20% grown at 700 °C. In0.16Ga0.84N single quantum wells with graded InxGa1−xN barriers showed intense luminescence, with an energy shift towards shorter wavelength with decreasing quantum well thickness. The photoluminescence full width at half-maximum of the 50 Å thick well was as low as 7.9 nm (59 meV) at 300 K.
Role of hydrogen in doping of GaN
Authors: Neugebauer Jörg , Van de Walle Chris G.
Published: July 2002

We investigate the interactions between hydrogen and dopant impurities in GaN, using state-of-the-art first-principles calculations. Our results for energetics and migration reveal a fundamental difference in the behavior of hydrogen between p-type and n-type material; in particular, we explain why hydrogen concentrations in n-type GaN are low, and why hydrogen has a beneficial effect on acceptor incorporation in p-type GaN. Our results identify the conditions under which hydrogen can be used to control doping in semiconductors in general.
Experimental determination of the absorption strength of C–H vibrations for infrared analysis of hydrogenated carbon films
Authors: Jacob W. , Unger M.
Published: July 2002

Plasma-deposited, hydrogenated carbon thin films were investigated by infrared analysis and high-energy ion analysis. The absolute quantities of hydrogen in the films, as measured by ion beam analysis, and the strength of the C–H vibrational bands around 3000 cm−1 were compared to determine the average absorption strength for these bands. The absorption strength was found to vary by a factor of 4 depending on the structure of the hydrocarbon films.
Electroluminescence from triplet excited states of benzophenone
Authors: Hoshino Satoshi , Suzuki Hiroyuki
Published: July 2002

We report the electroluminescent characteristics of organic multilayer light-emitting diodes (LEDs) with a benzophenone (BP) dispersed poly(methylmethacrylate) (PMMA) film as an emitting layer. The electroluminescence (EL) intensity of these LEDs increases with decreasing temperature from 273 to 100 K when they are operated at the same voltages or the same current densities. The EL spectrum of the LEDs, which peaks at around 450 nm, is identical to the phosphorescence spectrum of BP in PMMA. In addition, the EL decay time was determined as 46.8 μs at 100 K by applying a rectangular voltage pulse. These results indicate that the EL of the LED originates from the triplet excited states of BP.
Hot electron tunable supercurrent
Authors: Morpurgo A. F. , Klapwijk T. M. , van Wees B. J.
Published: July 2002

A Josephson junction has been realized in which the supercurrent flow is regulated by a “normal” control current traversing the normal metal in between the superconducting electrodes. The principle of operation of the devices is based on the existing relation between the magnitude of the supercurrent and the electronic distribution function in the junction. This method for controlling the supercurrent has clear advantages over other known methods and is relevant for superconducting electronics applications.
Potentiometry and repair of electrically stressed nanowires using atomic force microscopy
Authors: Hersam M. C. , Hoole A. C. F. , O’Shea S. J. , Welland M. E.
Published: July 2002

Using an atomic force microscope equipped with a conducting diamond tip, the surface potential on a current carrying gold nanowire was measured with microvolt potential sensitivity and nanometer spatial resolution. Potentiometry images illustrate the stages of failure of nanowires subjected to current stressing. During this failure process, a discontinuity in the potential gradient and an enhanced resistance region were observed at the failure site until a complete fracture was formed. By increasing the repulsive force and accurately positioning the tip, gold could be manipulated into the nanoscale fracture so that the electrical conductivity of the nanowire was regained.
Evidence for surfactant mediated nucleation and growth of diamond
Authors: Lannon J. M. , Gold J. S. , Stinespring C. D.
Published: July 2002

Auger electron spectroscopy has been used as an in situ diagnostic in ultrahigh vacuum studies of diamond nucleation and growth on silicon. It is demonstrated that sp3-bonded carbon can be formed under ultrahigh vacuum conditions in the absence of excess hydrogen using a flux of C2H4 molecules. For the conditions reported here, a silicon adlayer is always present on the surface of the growing thin film. The presence of this adlayer suggests that the formation of sp3-bonded carbon occurs by a surfactant mediated process. Specifically, silicon is thought to maintain the template for sp3-carbon growth by minimizing the surface free energy for this structure. Further, the silicon adlayer is thought to enhance the growth kinetics of sp3 carbon by providing a more reactive growth surface than a hydrogen-terminated carbon surface.
Ultrafast generation of blue light by efficient second-harmonic generation in periodically-poled bulk and waveguide potassium titanyl phosphate
Authors: Wang Y. , Petrov V. , Ding Y. J. , Zheng Y. , Khurgin J. B. , Risk W. P.
Published: July 2002

We have investigated second-harmonic generation in short-period periodically-poled bulk and waveguide potassium titanyl phosphate crystals to generate blue light using subpicosecond laser pulses. For the bulk, the highest conversion efficiency is ≈5.5%, which is about two orders of magnitude larger than that achieved previously. For the waveguide, the highest conversion efficiency is ≈32%, which is about a factor of 4 higher than that obtained before. These measured values are in good agreement with our theoretical results. We have observed saturation of conversion efficiency, which sets a limit to the maximum efficiency that can be achieved.
Highly sensitive photopolymerizable dry film for use in real time holography
Authors: Blaya S. , Mallavia R. , Carretero L. , Fimia A. , Madrigal R. F.
Published: July 2002

A photopolymerizable holographic material has been obtained and characterized using agent N,N′-dihydroxiethylenbisacrylamide as crosslinking. A sensitivity of 5 mJ/cm2 and a diffraction efficiency around of 70% have been reached for holograms recorded at 633 nm with a spatial frequency of 1000 lines/mm. High sensitivities can be obtained with low intensities, and this is important in applications such as holographic interferometry and the fabrication of holographic optical elements.
Single-walled carbon nanotube probes for high-resolution nanostructure imaging
Authors: Wong Stanislaus S. , Woolley Adam T. , Odom Teri Wang , Huang Jin-Lin , Kim Philip , Vezenov Dimitri V. , Lieber Charles M.
Published: July 2002

Single-walled carbon nanotube (SWNT) tips have been used to image nanostructures with high resolution. Studies of gold nanocrystal standards showed that SWNT tips provide a significant improvement in lateral resolution with respect to multi-walled nanotube tips and microfabricated Si tips. The nanotube tips were also used to resolve substructure within SWNTs deposited on surfaces. These results suggest that observed 1.5 nm high structures can correspond to several SWNTs aligned in parallel. In addition, SWNT tips exhibited superior resolution compared to conventional tips when imaging biological nanostructures, such as double-stranded DNA. The potential and future challenges of SWNT tips are discussed.
Bonding configurations of nitrogen absorption peak at 960 cm−1 in silicon oxynitride films
Authors: Ono Haruhiko , Ikarashi Taeko , Miura Yoshinao , Hasegawa Eiji , Ando Koichi , Kitano Tomohisa
Published: July 2002

We investigated bonding configurations of nitrogen atoms in silicon oxynitride films, resulting in a 960 cm−1 absorption peak, which is a higher frequency than that for Si3N4 (840 cm−1). The 960 cm−1 peak was observed in the films for which an N 1s x-ray photoemission peak was observed with a binding energy of about 398.6 eV, which has been reported as a binding energy associated with the ≡Si–N–Si≡ structure. However, the 960 cm−1 peak was absent in the films for which the N 1s peak was observed at about 397.8 eV, being close to the binding energy associated with the Si3≡N structure. We conclude that the absorption peak at 960 cm−1 arises from the ≡Si–N–Si≡ structure of doubly bonded N atoms with two Si atoms, not affected by any oxygen atoms.
In situ boron incorporation and activation in silicon carbide using excimer laser recrystallization
Authors: Russell S. D. , Ramirez A. D.
Published: July 2002

In situ incorporation and activation of boron into silicon carbide is demonstrated using excimer laser recrystallization in a boron trifluoride ambient. Rutherford backscattering spectroscopy and x-ray analysis demonstrate that there is no crystalline damage during recrystallization at laser fluences below ∼1.4 J/cm2. Point-contact current–voltage measurements confirm dopant activation, and the formation of shallow (∼90 nm) pn  junctions in silicon carbide. This technique may be applied to the fabrication of shallow junctions and low resistance contacts in silicon carbide power devices without ion implantation and furnace annealing.
Atomic geometry and energetics of vacancies and antisites in cubic boron nitride
Authors: Orellana W. , Chacham H.
Published: July 2002

We use first-principles calculations to investigate the atomic geometries and formation energies of vacancies (VN,VB) and antisites (BN,NB) in cubic boron nitride. We find that VN and VB are the most stable defects in p-type and n-type conditions, respectively. They also exhibit intrinsic donor (VN) and acceptor (VB) characters, which makes them good candidates for compensation. The equilibrium geometries show large outward breathing relaxations for both vacancies and for BN, with a slight Jahn–Teller distortion from Td symmetry. For NB in neutral and negatives charge states, we find an off-center distortion, inducing a negative-U behavior.
Thermoelectric properties of Tl2SnTe5 and Tl2GeTe5
Authors: Sharp Jeff W. , Sales Brian C. , Mandrus David G. , Chakoumakos Bryan C.
Published: July 2002

We report on the thermoelectric properties of two ternary tellurides with known crystal structures. Both compounds have a very low lattice thermal conductivity. Tl2SnTe5 appears to have a p-type figure of merit about the same as that of Bi2Te3, the best thermoelectric material among binary compounds. We have synthesized mainly polycrystalline samples, but a few small crystals have been grown and used for electrical measurements. Prospects for further improvement of these materials are discussed.
Oxygen-induced amorphous structure of tungsten thin films
Authors: Shen Y. G. , Mai Y. W. , McBride W. E. , McKenzie D. R. , Zhang Q. C.
Published: July 2002

A combination of energy-filtered electron diffraction, electron energy-loss spectroscopy, transmission electron microscopy, and x-ray diffraction are used to establish that oxygen impurities incorporated in tungsten films prepared by magnetron sputtering in the early stage of the deposition play a dominant role in the formation of an amorphous phase. Energy-filtered electron diffraction data collected from a range of amorphous films were Fourier transformed to a reduced density function (RDF) and matched with an amorphous model. The results show that better agreement with the experimental RDF is achieved if the amorphous model consists of a random continuous matrix of clusters with W3O-like symmetry.