bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Elicitation on Growth, Respiration, and Nutrient Uptake of Root and Cell Suspension Cultures of Hyoscyamusmuticus
Authors: Carvalho Edgard B. , Curtis Wayne R.
Published: July 2002

The elicitation of Hyoscyamus muticus root and cell suspension cultures by fungal elicitor from Rhizoctonia solani causes dramatic changes in respiration, nutrient yields, and growth. Cells and mature root tissues have similar specific oxygen uptake rates (SOUR) before and after the onset of the elicitation process. Cell suspension SOUR were 11 and 18 μmol O2/g FW·h for non‐elicited control and elicited cultures, respectively. Mature root SOUR were 11 and 24 μmol O2/g FW·h for control and elicited tissue, respectively. Tissue growth is significantly reduced upon the addition of elicitor to these cultures. Inorganic yield remains fairly constant, whereas yield on sugar is reduced from 0.532 to 0.352 g dry biomass per g sugar for roots and 0.614 to 0.440 g dry biomass per g sugar for cells. This reduction in yield results from increased energy requirements for the defense response. Growth reduction is reflected in a reduction in root meristem (tip) SOUR, which decreased from 189 to 70 μmol O2/g FW·h upon elicitation. Therefore, despite the increase in total respiration, the maximum local oxygen fluxes are reduced as a result of the reduction in metabolic activity at the meristem. This distribution of oxygen uptake throughout the mature tissue could reduce mass transfer requirements during elicited production. However, this was not found to be the case for sesquiterpene elicitation, where production of lubimin and solavetivone were found to increase linearly up to oxygen partial pressures of 40% O2 in air. SOUR is shown to similarly increase in both bubble column and tubular reactors despite severe mass transfer limitations, suggesting the possibility of metabolically induced increases in tissue convective transport during elicitation.
Conformational investigation of α,β‐dehydropeptides. X. Molecular and crystal structure of Ac‐ΔAla‐NMe2compared with those of Ac‐l‐Ala‐NMe2, Ac‐dl‐Ala‐NMe2and other dimethylamidesa
Authors: Rzeszotarska B. , Siodłak D. , Broda M.A. , Kozioł A.E. , Dybała I.
Published: July 2002

Abstract:A series of three homologous dimethyldiamides Ac‐ΔAla‐NMe2, Ac‐l‐Ala‐NMe2and Ac‐dl‐Ala‐NMe2has been synthesized and the structures of these amides determined from single‐crystal X‐ray diffraction data. To learn more about the conformational preferences of compounds studied, the fully relaxed (φ–ψ) conformational energy mapsin vacuo(AM1) of Ac‐ΔAla‐NMe2and Ac‐l‐Ala‐NMe2were obtained, and the calculated minima reoptimized with the DFT/B3LYP/6–31G** method. The crystal‐state results have been compared with the literature data. Ac‐ΔAla‐NMe2and other α,β‐dehydroamino acid dimethyldiamides, Ac‐ΔXaa‐NMe2adopt the conservative conformation of the torsion angles φ, ψ = ∼ −45°, ∼130°, which are located in the high‐energy region (region H) of Ramachandran diagram. Ac‐l‐Ala‐NMe2and Ac‐dl‐Ala‐NMe2, as well as other saturated amino acid dimethylamides Ac‐l/dl‐Xaa‐NMe2, present common peptide structures, and no conformational preferences are observed. Molecular packing of the amides analysed reveals two general hydrogen‐bonded motifs. Dehydro anddl‐species are paired into centrosymmetric dimers, andl‐compounds form catemers. However, Ac‐ΔAla‐NMe2and Ac‐DL‐Ala‐NMe2constitute exceptions: their molecules also link into catemers.
Skin‐sensitization structure‐activity relationships for aldehydes
Authors: Patlewicz Grace , Basketter David A. , Smith Camilla K. , Hotchkiss Sharon A. M. , Roberts David W.
Published: July 2002

A selection of 17 aldehydes (13 sensitizing and 4 non‐sensitizing), all of which possessed a benzene ring, were evaluated using structure‐activity relationships (SARs). The sensitizing compounds were classified as strong, moderate or weak skin sensitizers on the basis of in vivo data. The aldehydes were grouped into 4 distinct subcategories of functionally related aldehydes that were termed aryl‐substituted aliphatic, aryl, aryl with special features (that can undergo metabolism) and α,β‐unsaturated aldehydes. It was observed that a structure‐activity relationship could be derived for a subset of aldehydes that could react via the same chemical mechanism. This further supports the view that applying knowledge on reaction mechanisms to develop SAR models can provide a more accurate means of investigating and predicting the sensitization potential of structurally and functionally related chemicals.
The involvement of human ribonucleases H1 and H2 in the variation of response of cells to antisense phosphorothioate oligonucleotides
Authors: ten Asbroek Anneloor L. M. A. , van Groenigen Marjon , Nooij Marleen , Baas Frank
Published: July 2002

We have analyzed the response of a number of human cell lines to treatment with antisense oligodeoxynucleotides (ODNs) directed against RNA polymerase II, replication protein A, and Ha‐ras. ODN‐delivery to the cells was liposome‐mediated or via electroporation, which resulted in different intracellular locations of the ODNs. The ODN‐mediated target mRNA reduction varied considerably between the cell lines. In view of the essential role of RNase H activity in this response, RNase H was analyzed. The mRNA levels of RNase H1 and RNase H2 varied considerably in the cell lines examined in this study. The intracellular localization of the enzymes, assayed by green‐fluorescent protein fusions, showed that RNase H1 was present throughout the whole cell for all cell types analyzed, whereas RNase H2 was restricted to the nucleus in all cells except the prostate cancer line 15PC3 that expressed the protein throughout the cell. Whole cell extracts of the cell lines yielded similar RNase H cleavage activity in an in vitro liquid assay, in contrast to the efficacy of the ODNs in vivo. Overexpression of RNase H2 did not affect the response to ODNs in vivo. Our data imply that in vivo RNase H activity is not only due to the activity assayed in vitro, but also to an intrinsic property of the cells. RNase H1 is not likely to be a major player in the antisense ODN‐mediated degradation of target mRNAs. RNase H2 is involved in the activity assayed in␣vitro. The presence of cell‐type specific factors affecting the activity and localization of RNase H2 is strongly suggested.
Human cytokine responses induced by Gram-positive cell walls of normal intestinal microbiota
Authors: Chen T , Isomäki P , Rimpiläinen M , Toivanen P
Published: July 2002

The normal microbiota plays an important role in the health of the host, but little is known of how the human immune system recognizes and responds to Gram-positive indigenous bacteria. We have investigated cytokine responses of peripheral blood mononuclear cells (PBMC) to Gram-positive cell walls (CW) derived from four common intestinal indigenous bacteria, Eubacterium aerofaciens (Eu.a.), Eubacterium limosum (Eu.l.), Lactobacillus casei (L.c.), and Lactobacillus fermentum (L.f.). Our results indicate that Gram-positive CW of the normal intestinal microbiota can induce cytokine responses of the human PBMC. The profile, level and kinetics of these responses are similar to those induced by lipopolysaccharide (LPS) or CW derived from a pathogen, Streptococcus pyogenes (S.p.). Bacterial CW are capable of inducing production of a proinflammatory cytokine, tumour necrosis factor-alpha (TNF-α), and an anti-inflammatory cytokine, IL-10, but not that of IL-4 or interferon-gamma (IFN-γ). Monocytes are the main cell population in PBMC to produce TNF-α and IL-10. Induction of cytokine secretion is serum-dependent; both CD14-dependent and -independent pathways are involved. These findings suggest that the human cytokine responses induced by Gram-positive CW of the normal intestinal microbiota are similar to those induced by LPS or Gram-positive CW of the pathogens.
Elevated CTLA-4 expression on CD4 T cells from periodontitis patients stimulated withPorphyromonas gingivalisouter membrane antigen
Authors: Aoyagi T , Yamazaki K , Kabasawa-Katoh Y , Nakajima T , Yamashita N , Yoshie H , Hara K
Published: July 2002

To characterize the T cell response to Porphyromonas gingivalis, we examined the expression of costimulatory molecules on T cells derived from adult periodontitis patients with high serum antibody titre to P. gingivalis. The expression of CD28, CTLA-4, CD40 ligand (CD40L) on CD4+ T cells was analysed by flow cytometry. IL-10 and transforming growth factor-beta (TGF-β) mRNA expression were determined by reverse transcription-polymerase chain reaction (RT-PCR) and subsequent image analysis. Peripheral blood mononuclear cells (PBMC) derived from periodontitis patients showed higher proliferative responses to P. gingivalis outer membrane (OM) than those from healthy controls (P < 0.05). The percentage of CTLA-4+ cells within CD4+ T cells of patients was significantly higher than that of healthy controls after P. gingivalis OM stimulation (33.0% versus 11.9%, P < 0.01). There was no significant difference in the percentages of CD28+ cells and CD40L+ cells, and the percentage of CD40L+ cells was low in both groups even after stimulation. Stimulation of PBMC with P. gingivalis OM induced significantly higher IL-10 mRNA expression in periodontitis patients than in healthy controls (P < 0.05). The level of TGF-β mRNA expression of patients tended to be higher than that of healthy controls, but there was no significant difference. To elucidate the functional role of CTLA-4, we further investigated the secondary proliferative response to P. gingivalis OM. Interestingly, P. gingivalis OM stimulation did not enhance antigen-specific secondary response. Anti-CTLA-4 MoAb had no effect on proliferation in the presence of P. gingivalis OM. CTLA-4Ig suppressed the proliferative response significantly (P < 0.01). These results suggest that T cell responses to P. gingivalis OM may be regulated by CTLA-4 that is expressed at the late phase of T cell activation, and, in part, immunosuppressive cytokines. Taken together, CTLA-4 may play a crucial role in the pathogenesis of chronic inflammatory periodontal disease.
Generation and characterization of a recombinant adenovirus expressing vascular endothelial growth factor for studies of neovascularization in the eye
Authors: Yu Meaghan J T , Shen Wei‐Yong , Lai May C , Constable Ian J , Rakoczy Piroska E
Published: July 2002

There is accumulating evidence that an increased expression of vascular endothelial growth factor from retinal pigment epithelial cells may be important in choroidal neovascularization. In vivo studies have demonstrated that subretinal injection of recombinant adenovirus vectors produces long‐term transgene expression specifically within retinal pigment epithelial cells. A recombinant adenovirus encoding of vascular endothelial growth factor (Ad.RSV.VEGF) was therefore produced and characterized in order to determine whether an upregulation of vascular endothelial growth factor expression is sufficient to induce choroidal neovascularization. Ad.RSV.VEGF was produced by homologous recombination and its identity confirmed by restriction enzyme analysis. Ad.RSV.VEGF was characterized in vitro by the transduction of cultured retinal pigment epithelial cells. The in vitro characterization confirmed vascular endothelial growth factor mRNA and protein expression from Ad.RSV.VEGF and demonstrated the biological activity of the vascular endothelial growth factor protein. A preliminary in vivo study suggested that the subretinal injection of Ad.RSV.VEGF induced vascular leakage.
An axially vascularized meatal‐based flap for the repair of hypospadias
Authors: Elbakry  , Shamaa  , AL‐Atrash
Published: July 2002

Objectives

 To report the initial experience of hypospadias repair using an axially vascularized meatal‐based skin flap.

Patients and methods

Twenty‐six children aged 2–5.5 years with subcoronal hypospadias underwent the procedure; three were circumcised and three had recurrent hypospadias. The neourethra was constructed using a meatal‐based flap extending obliquely from the hypospadiac orifice towards the prepuce or its remnants. The flap was supplied axially by the ventrolateral branch of the superficial external pudendal artery. The vascular pedicle was dissected and mobilized to allow tensionless reflection of the flap ventrally to be sutured to the urethral plate.

Results

Functional and cosmetic success was achieved in 25 of the patients (96%). Haematoma and disruption of the flap occurred in one patient, who underwent re‐operation, and slight and insignificant meatal retraction occurred in another child.

Conclusions

The procedure confirmed the technical feasibility of creating an axially vascularized meatal‐based flap. The oblique orientation of the flap permits it to be obtained from well‐developed skin. Continuity of the oblique flap with the urethral plate reduces the total length of suture lines and minimizes the possibility of fistula formation.
Prenatal ultrasound findings in a fetus with paternal uniparental disomy 14q12‐qter
Authors: Towner D. , Yang S. P. , Shaffer L. G.
Published: July 2002

We present the prenatal ultrasound findings in a case of postnatally identified paternal uniparental isodisomy 14q12‐qter. Increased nuchal translucency and a large omphalocele were identified at 14 weeks' gestation. Karyotyping revealed a normal male, 46,XY. As gestation advanced, polyhydramnios developed, skeletal abnormalities involving the long bones and chest became evident, hand contractures developed, and the presumed large omphalocele was in part found to be a large ventral hernia, as echogenic adipose tissue could be seen in the abdominal wall near to the cord insertion. Prenatal findings were confirmed after delivery and central nervous system imaging revealed lissencephaly. The combination of an abdominal wall defect with either increased nuchal translucency or skeletal abnormalities should prompt an investigation for uniparental disomy 14 even if the karyotype is normal. Copyright © 2001 International Society of Ultrasound in Obstetrics and Gynecology
4‐Hydroxynonenal Induces Oxidative Stress and Death of Cultured Spinal Cord Neurons
Authors: Malecki Andrzej , Garrido Rosario , Mattson Mark P. , Hennig Bernhard , Toborek Michal
Published: July 2002

Abstract: Primary spinal cord trauma can trigger a cascade of secondary processes leading to delayed and amplified injury to spinal cord neurons. Release of fatty acids, in particular arachidonic acid, from cell membranes is believed to contribute significantly to these events. Mechanisms of fatty acid‐induced injury to spinal cord neurons may include lipid peroxidation. One of the major biologically active products of arachidonic acid peroxidation is 4‐hydroxynonenal (HNE). The levels of HNE‐protein conjugates in cultured spinal cord neurons increased in a dose‐dependent manner after a 24‐h exposure to arachidonic acid. To study cellular effects of HNE, spinal cord neurons were treated with different doses of HNE, and cellular oxidative stress, intracellular calcium, and cell viability were determined. A 3‐h exposure to 10 μM HNE caused ∼80% increase in oxidative stress and 30% elevation of intracellular calcium. Exposure of spinal cord neurons to HNE caused a dramatic loss of cellular viability, indicated by a dose‐dependent decrease in MTS [3‐(4,5‐dimethylthiazol‐2‐yl)‐5‐(3‐carboxymethoxyphenyl)‐2‐(4‐sulfophenyl)‐2H‐tetrazolium, inner salt] conversion. The cytotoxic effect of HNE was diminished by pretreating neurons with ebselen or N‐acetylcysteine. These data support the hypothesis that formation of HNE may be responsible, at least in part, for the cytotoxic effects of membrane‐released arachidonic acid to spinal cord neurons.
Xylem Flow and its Driving Forces in a Tropical Liana: Concomitant Flow‐Sensitive NMR Imaging and Pressure Probe Measurements
Authors: Wistuba N. , Reich R. , Wagner H.‐J. , Zhu J. J. , Schneider H. , Bentrup F.‐W. , Haase A. , Zimmermann U.
Published: July 2002

Abstract: Flow‐sensitive NMR imaging and pressure probe techniques were used for measuring xylem water flow and its driving forces (i.e., xylem pressure as well as cell turgor and osmotic pressure gradients) in a tropical liana, Epipremnum aureum. Selection of tall specimens allowed continuous and simultaneous measurements of all parameters at various distances from the root under diurnally changing environmental conditions. Well hydrated plants exhibited exactly linearly correlated dynamic changes in xylem tension and flow velocity. Concomitant multiple‐probe insertions along the plant shoot revealed xylem and turgor pressure gradients with changing magnitudes due to environmental changes and plant orientation (upright, apex‐down, or horizontal). The data suggest that in upright and ‐ to a lesser extent ‐ in horizontal plants the transpirational water loss by the cells towards the apex during the day is not fully compensated by water uptake through the night. Thus, longitudinal cellular osmotic pressure gradients exist. Due to the tight hydraulic coupling of the xylem and the tissue cells these gradients represent (besides the transpiration‐induced tension in the xylem) an additional tension component for anti‐gravitational water movement from the roots through the vessels to the apex.
Distributed feedback lasers with multiple phase-shift regions
Authors: Agrawal G. P. , Geusic J. E. , Anthony P. J.
Published: July 2002

Phase-shifted distributed-feedback (PSDFB) lasers with multiple phase-shift regions are proposed to reduce the effect of spatial hole burning in conventional PSDFB lasers where a single phase shift leads to a highly nonuniform axial distribution of the mode intensity. We analyze multiple phase-shift PSDFB lasers by solving numerically the coupled-wave equations. Our results show that the use of even two phase-shift regions can substantially reduce the axial inhomogeneity of the mode intensity. Although the gain margin is generally reduced by the use of multiple phase shifts, it is still large enough for most practical purposes if the phase shifts are suitably optimized.
Electrical properties of ytterbium-doped InP grown by metalorganic chemical vapor deposition
Authors: Whitney Peter S. , Uwai Kunihiko , Nakagome Hiroshi , Takahei Ke’ichiro
Published: July 2002

A study has been carried out on the electrical properties of ytterbium-doped InP grown by metalorganic chemical vapor deposition for the purpose of clarifying the electrical behavior of ytterbium impurities in InP. The epilayers were characterized by Hall effect measurements and by deep level transient spectroscopy. The results strongly suggest that ytterbium introduces an acceptor-like level 0.03±0.01 eV below the conduction band which is related to isolated ytterbium atoms or single ytterbium atoms complexed with native defects. No signs of precipitation or clustering of ytterbium are observed for concentrations below 1018 cm−3 . Contrary to previous hypotheses, no evidence has been found for the existence of an ytterbium-related acceptor level in the lower half of the band gap.
Optimization of quantum well materials and structures for excitonic electroabsorption effects
Authors: Nojima S. , Wakita K.
Published: July 2002

A method to enhance excitonic electroabsorption effects in semiconductor quantum well structures is presented, in which the well thickness and the band gap of the quantum well materials are optimized so that the figure of merit for electroabsorption is maximized. Application of this method to InGaAsP/InP and InGaAlAs/InAlAs quantum well structures enhances this figure of merit by 60 and 16 times, respectively, as compared with that of conventional InGaAs/InP and InGaAs/InAlAs quantum well structures.
Thermal wave imaging by photothermally modulated ferromagnetic resonance
Authors: Orth T. , Netzelmann U. , Pelzl J.
Published: July 2002

Thermal wave modulated ferromagnetic resonance has been used to investigate local magnetic properties of ferromagnetic materials. The sample inside the microwave cavity is exposed to a modulated laser beam and the reflected microwave power is measured as a function of beam position and modulation frequency. First results are obtained from magnetic recording tapes.
Epitaxial barium hexaferrite on sapphire by sputter deposition
Authors: Yuan M. S. , Glass H. L. , Adkins L. R.
Published: July 2002

Epitaxial films of barium hexaferrite (BaFe12 O19 ) have been prepared on basal-oriented sapphire substrates by rf sputter deposition of an amorphous ferrite film and crystallization of the film by annealing. Liquid phase epitaxy has been used to deposit an additional barium hexaferrite layer on top of the sputter deposited and annealed film. In this way films having symmetric ferromagnetic resonance peaks as narrow as 41 Oe at 60 GHz have been obtained.
Scanning tunneling microscope study of microcrystalline silicon surfaces in air
Authors: Tanaka Ichiro , Osaka Fukunobu , Kato Takashi , Katayama Yoshifumi , Muramatsu Shin-ichi , Shimada Toshikazu
Published: July 2002

Surfaces of microcrystalline silicon films prepared by the glow discharge method have been investigated by a scanning tunneling microscope (STM) in air. Grain-like structures of 30–80 nm size which correspond to transmission electron microscope data have been observed. The film surface was found to be geometrically rather flat but the structure was observed electrically, that is, the resistivity seemed to be inhomogeneous due to preferential oxidation. Also, degradation of STM images of a HF-etched microcrystalline silicon surface has been observed for the first time.
Catalytic effect of SiO on thermomigration of impurities in SiO2
Authors: Celler G. K. , Trimble L. E.
Published: July 2002

We have found that encapsulated thin Si films, adjacent to SiO2, are dissolved at elevated temperatures by a reaction: Si+SiO2→2SiO, with SiO diffusing through the silica network. The presence of SiO in SiO2 in a concentration ≤3 at. % has a profound effect on thermomigration of dopant impurities in the oxide. We propose a model for transport of As-, Sb-, P-, and Ge-rich precipitates, 50–1000 Å in diameter, based on a reaction between SiO2 and the precipitates that is mediated by a catalytic influence of SiO molecules. Only in the presence of SiO are the bonds in the SiO2 network broken ahead of the drifting clusters and regrown behind them. The model predicts that the migration of precipitates is controlled by diffusivity of SiO in silica, consistent with the narrow range of drift velocities for clusters of different chemical compositions. The data also suggest that SiO2 dissociation at the Si/SiO2 interface is diffusion limited, which explains why decomposition of thin SiO2 in vacuum is spatially inhomogeneous, while thick films used in our experiments react uniformly.
Properties of Ga vacancies in AlGaAs materials
Authors: Kahen K. B. , Peterson D. L. , Rajeswaran G. , Lawrence D. J.
Published: July 2002

Intermixing of AlGaAs-based interfaces is known to be enhanced by capping wafers with a layer of SiO2. Assuming that this enhancement results from the introduction of additional Ga vacancies into the sample, it is possible to obtain the temperature-dependent equilibrium Ga vacancy diffusivity. Experiments are performed whereby SiO2-capped quantum well samples are annealed at temperatures ranging from 800 to 1025 °C. Calculated photoluminescence shifts are compared with the measured spectra, and a relation for the Ga vacancy diffusivity of the form 0.962 exp(−2.72/kBT) cm2/s is obtained. Using this relation, the equilibrium Ga vacancy concentration can be computed via an ensemble Monte Carlo simulation. The resulting expression is 1.25×1031 exp(−3.28/kBT) cm−3.
Reduction of interface-state density by F2 treatment in a metal-oxide-semiconductor diode prepared from a photochemical vapor deposited SiO2 film
Authors: Inoue Kohji , Nakamura Masakazu , Okuyama Masanori , Hamakawa Yoshihiro
Published: July 2002

A new technique for the reduction of the interface-state density Nss of a metal-oxide-semiconductor (MOS) diode has been developed. The SiO2 film was deposited on Si from Si2 H6 and O2 by direct photochemical vapor deposition (CVD) using vacuum ultraviolet (VUV) light of a D2 lamp. The new technique is a F2 treatment of the Si surface prior to the deposition of SiO2 film. Typically, 5% F2 gas diluted in He was introduced into the CVD chamber at 20 Pa for 5 min under UV light irradiation. The minimum value of the Nss was ∼5×109 cm−2 eV−1 at the Si midgap for the film deposited at 180 °C.
Growth of single-crystal metastable (GaAs)1−x(Si2)x alloys on GaAs and (GaAs)1−x(Si2)x/GaAs strained-layer superlattices
Authors: Mei D. H. , Kim Y.-W. , Lubben D. , Robertson I. M. , Greene J. E.
Published: July 2002

Epitaxial zinc blende structure metastable (GaAs)1−x(Si2)x alloys have been grown with 0<x<0.3 on As-stabilized GaAs(100) substrates by a hybrid sputter deposition/evaporation technique. The films, typically 2–3 μm thick, were deposited at 570 °C with growth rates between 0.7 and 1 μm h−1. Alloys with 0<x<0.12 were defect-free as judged by plan-view and cross-sectional transmission electron microscopy (TEM and XTEM) with x-ray diffraction peak widths approximately the same as that of the substrate, 30 arcsec 2θ. XTEM lattice images showed smooth abrupt interfaces. (GaAs)1−x(Si2)x alloys with x>0.12 exhibited increasing evidence of interfacial defects associated with lattice strain when grown on GaAs. However, defect-free alloys with x up to 0.3 were obtained using (GaAs)1−x(Si2)x/GaAs strained-layer superlattice buffer layers to provide a better lattice match.
Observations of ferroelectric polarization reversal in sol-gel processed very thin lead-zirconate-titanate films
Authors: Sanchez Lloyd E. , Wu Shu-Yau , Naik Ishver K.
Published: July 2002

Very thin lead-zirconate-titanate films, 500 Å or less in thickness, have been prepared on Pt-Ti metallized silicon wafers by a sol-gel processing technique. Excellent ferroelectric, dielectric, and structural properties have been demonstrated. The maximum polarization and the remanent polarization at a switching voltage of 3 V on a film with a Zr/Ti ratio of about 50/50 are 33 and 12 μC/cm2, respectively. The capability of reaching a saturation polarization at a low voltage (∼3 V) on these films suggests that they are very attractive for use in radiation-hard low-voltage nonvolatile programmable random access memories.
Photoemission studies of Si surface oxidation using synchrotron radiation
Authors: Nakazawa M. , Sekiyama H.
Published: July 2002

The electronic states of Si(111), (110), and (100) surfaces in the early oxidation stages are examined using high-resolution photoelectron spectroscopy (ΔE<0.3 eV) with synchrotron radiation. The experimental results reveal stronger intensities of Si3+ and Si4+ oxide components for the Si(111) surface oxidation than for the Si(110) and (100) surface oxidations. Additionally, the depth distribution of intermediary components (SiOx: 0<x≤2) obtained from the tunability of synchrotron radiation shows that the Si3+ and Si4+ oxidation states form at the first and second outermost layers of the Si(111) surface. Features of the oxidation process are also discussed.
Interpretation of the fragmentation phenomenon in single-filament composite experiments
Authors: Wagner H. D. , Eitan A.
Published: July 2002

The fiber fragmentation phenomenon in the single-filament composite test is currently among the most intensively researched aspects of composite micromechanics. By this method the interfacial shear strength, a physical parameter which reflects the quality of the fiber/matrix bonding, is calculated. In most studies various statistical models for the distribution of fragment lengths have been assumed without any justification other than a good fit to experimental data. Here we argue that if the flaws are assumed to be distributed along the fiber length according to a spatial Poisson process, then far from the saturation point the resulting fragment lengths must exactly follow a shifted exponential distribution. At the saturation limit, the distribution of fragment lengths is still approximately exponential. This is confirmed by single-filament composite experiments with high strength carbon fibers embedded in epoxy. Cumulative distributions of interdefect spacings at various levels of stress (or strain) are made available by means of an experimental technique described here. It is shown that the resulting data may be used to unfold the average strength versus length dependence in the single fiber, and to calculate the relevant Weibull shape parameter.
Correlation between midgap interface state density and thickness-averaged oxide stress and strain at Si/SiO2 interfaces formed by thermal oxidation of Si
Authors: Bjorkman C. H. , Fitch J. T. , Lucovsky G.
Published: July 2002

Correlations between midgap interface state density (Dit) and thickness-averaged stress in thermally grown SiO2 thin films have been investigated by infrared spectroscopy, an optical beam deflection technique, and capacitance-voltage measurements. We find no correlations between Dit and either (i) the maximum stress in the Si or SiO2 at the Si/SiO2 interface or (ii) the stress gradient in the SiO2 film. By direct measurements of the strain-induced bending of the Si wafer, and by calculating the microscopic strain from the SiOSi bond-stretching vibrational frequency, we have established linear relationships between Dit and the thickness-averaged stress and strain in the oxide.
Photofluxonic detection: A new mechanism for infrared detection in superconducting thin films
Authors: Kadin A. M. , Leung M. , Smith A. D. , Murduck J. M.
Published: July 2002

A new model is proposed in which a photon of energy E=hf is absorbed by a superconducting film to create a pair of equal and opposite fluxons (or vortices), each with quantized flux Φ0=h/2e. An applied current sweeps these fluxons to opposite edges of the film, causing a voltage pulse with time-integrated magnitude Φ0, and leading to a time-averaged voltage responsivity Rv = Φ0/E = 1/(2ef). This is directly analogous to photoconductive detection in a semiconductor via creation of electron-hole pairs. Data on an ultrathin granular NbN film are presented which indicate a responsivity of 6000 V/W in red light, in agreement with the model. This is promising for the development of a sensitive, high-speed infrared detector using thin films of either low or high Tc superconductors.
Hydrogen in carbon-doped GaAs grown by metalorganic molecular beam epitaxy
Authors: Kozuch D. M. , Stavola Michael , Pearton S. J. , Abernathy C. R. , Lopata J.
Published: July 2002

Atomic profiles show that hydrogen is incorporated in GaAs:C that has been grown by metalorganic molecular beam epitaxy. The hydrogen concentration has been found to be about 5% of the carbon concentration for our growth conditions. An infrared absorption study shows that this hydrogen is involved in stable C-H complexes. At the lower C concentrations (<1019 cm−3) the CAs-H complex is the dominant species involving C and H. At higher C concentrations new complexes involving C and H appear.
Room-temperature blue lasing action in (Zn,Cd)Se/ZnSe optically pumped multiple quantum well structures on lattice-matched (Ga,In)As substrates
Authors: Jeon H. , Ding J. , Nurmikko A. V. , Luo H. , Samarth N. , Furdyna J. K. , Bonner W. A. , Nahory R. E.
Published: July 2002

We report on studies of optically pumped laser action in (Zn,Cd)Se/ZnSe multiple quantum well structures prepared by molecular beam epitaxy on lattice-matched bulk (Ga,In)As substrates. Room-temperature lasing under pulsed excitation with threshold pump intensity at I≊500 kW/cm2 has been achieved, together with high repetition ‘‘quasi-continuous’’ mode operation at temperatures so far up to 100 K.
Ion irradiation enhanced crystal nucleation in amorphous Si thin films
Authors: Im J. S. , Atwater Harry A.
Published: July 2002

The nucleation kinetics of the amorphous-to-crystal transition of Si films under 1.5 MeV Xe+ irradiation have been investigated by means of in situ transmission electron microscopy in the temperature range T=500–580 °C. After an incubation period during which negligible nucleation occurs, a constant nucleation rate was observed in steady state, suggesting that homogeneous nucleation occurred. Compared to thermal crystallization, a significant enhancement in the nucleation rate during high-energy ion irradiation (five to seven orders of magnitude) was observed with an apparent activation energy of 3.9±0.75 eV.
Ballistic current vortex excitations in electron waveguide structures
Authors: Lent Craig S.
Published: July 2002

Coherent ballistic transport through a cavity in a quantum electron waveguide is examined. Electrons passing through the cavity excite vortices in the current density in the cavity. The circulating currents may be larger than the current in the waveguide leads. Vortex excitations are possible even when the incoming current is totally reflected and no net current passes through the cavity.
Correlation of light-induced changes with hydrogen content in hydrogenated amorphous silicon films
Authors: Nevin W. A. , Yamagishi H. , Yamaguchi M. , Tawada Y.
Published: July 2002

By annealing H-rich intrinsic a-Si:H films at 340 °C in a hydrogen atmosphere, we have prepared a series of samples with varying hydrogen content (CH), but essentially constant photoconductivity, Urbach energy, and microstructure. In this way, we have been able to eliminate some of the interfering factors contributing to the light-induced effect, thus enabling the influence of CH to be determined. A strong correlation is found between the stability and CH, supporting the involvement of hydrogen in the light-induced mechanism.
Cross-linked stable second-order nonlinear optical polymer by photochemical reaction
Authors: Mandal Braja K. , Chen Yong M. , Lee Jun Y. , Kumar Jayant , Tripathy Sukant
Published: July 2002

A new method for obtaining cross-linked second-order nonlinear optical polymer by photochemical reaction has been described. An active dye was processed and poled in a photoreactive polymer in a manner similar to doped polymer system, and photo-cross-linked in the poled phase by ultraviolet irradiation. The dye doped (20% by weight) cross-linked polymer has a nonlinear optical coefficient d33 of 5.1 pm/V at 1.540 μm and shows no relaxation after being subjected to thermal treatment at 60 °C for 0.5 h.
Anomalous electric field and temperature dependence of collector multiplication in InP/Ga0.47In0.53As heterojunction bipolar transistors
Authors: Ritter D. , Hamm R. A. , Feygenson A. , Panish M. B.
Published: July 2002

The collector multiplication in InP/Ga0.47In0.53As heterojunction bipolar transistors was found to increase with temperature, and to have a weak electric field dependence. This anomalous behavior has a profound impact on device characteristics.
S-passivated InP (100)-(1×1) surface prepared by a wet chemical process
Authors: Tao Y. , Yelon A. , Sacher E. , Lu Z. H. , Graham M. J.
Published: July 2002

A highly stable crystalline S-passivated InP(100) surface has been obtained by a simple wet chemical process, using illumination and heated (NH4)2S solution. Low-energy electron diffraction studies show a (1×1) diffraction pattern, which persists even after 3 days of exposure to the atmosphere. High-resolution photoemission studies show that the surface is terminated with a monolayer of sulfur, which forms bridge bonds only to indium. The P 2p core level is identical to that of a vacuum-cleaved InP surface. A possible structural model is presented.
Coimplantation and electrical activity of C in GaAs: Stoichiometry and damage effects
Authors: Moll A. J. , Yu Kin Man , Walukiewicz W. , Hansen W. L. , Haller E. E.
Published: July 2002

We have coimplanted carbon and a series of elements (B, N, Al, P, Ar, Ga, As, and Kr) in GaAs to study the effect of both implant damage and stoichiometry on activation. Electrical activity of C was found to increase due to the additional damage caused by coimplantation of a heavy element regardless of the chemical nature of the coimplant. Maintaining stoichiometry by coimplanting a group III element further increased activation in substrates heavily damaged during implantation. Activation of 65±3%, corresponding to a sheet free-carrier concentration of 3.5×1014 cm−2, was achieved by coimplanting Ga and annealing at 950 °C for 10 s.
Large two-dimensional arrays of phase-locked vertical cavity surface emitting lasers
Authors: Orenstein M. , Kapon E. , Harbison J. P. , Florez L. T. , Stoffel N. G.
Published: July 2002

The phase-locking of two-dimensional (2D) arrays incorporating a large number of electrically pumped, vertical cavity surface emitting lasers (VCSELs) is described. The InGaAs/GaAs quantum well VCSELs are phase locked by patterning the reflectivity of the laser back mirrors. Structures with both weak and strong modulation of the mirror reflectivity have been studied. The strongly modulated (weakly coupled) structures exhibit superior coherence and beam patterns, with up to 27×27 lasers oscillating in virtually a single (highest order) supermode. The weakly modulated (strongly coupled) arrays also emit highly coherent beams, but with lower threshold currents and higher differential efficiencies. Weakly modulated arrays of 20×20 elements (120×120 μm2) exhibited threshold currents of <1 mA per laser and ≳300 mW pulsed output powers. These coherent 2D arrays should be useful for optical signal processing and high optical power applications.
Novel organic molecular second harmonic generation crystal: 3-methoxy-4-hydroxy-benzaldehyde
Authors: Tao X. T. , Yuan D. R. , Zhang N. , Jiang M. H. , Shao Z. S.
Published: July 2002

In considering the trade-off between second-order nonlinearity and transparency, a new organic molecular crystal 3-methoxy-4-hydroxy-benzaldehyde (MHBA) has been grown from solution by the temperature lowering method for the first time. MHBA belongs to the monoclinic system, and its space group is P21. The unit cell dimensions are: a=14.057, b=7.875, c=15.037 Å with β=115.45°. The powder second harmonic generation of MHBA is 30 times higher than that of urea and the cutoff wavelength is 370 nm.
Amorphization of M-Si (M=Ni, Co, Mo, Mn, and Cr) powders by ball milling using revolution-step-like-decreasing mode
Authors: Omuro K. , Miura H.
Published: July 2002

Amorphous alloy powders of the silicide type M-Si (M=Ni, Co, Mo, Mn, and Cr) are produced from crystalline elemental powders by high-energy ball milling using revolution-step-like-decreasing (RSD) mode. In the ball milling using the RSD mode, after promoting true chemical alloying in milled powder samples under high-vial rotation of the ball mill in the earlier stage, the medium- or low-vial rotations in the successive milling stages lead to amorphization of powder products due to making recovery processes, like nucleation of more stable phases, less prevailing. Amorphicity of the RSD milled powder samples is examined by x-ray diffraction and/or differential scanning calorimetry.
Theory of the strain-induced magnetic anisotropy of transition metal-noble metal multilayers
Authors: Yamaguchi Akira , Ogu Shinzo , Soe We-Hyo , Yamamoto Ryoichi
Published: July 2002

It is shown that the magnetoelastic anisotropy energies of the transition metal-noble metal multilayers which have fcc [111] perfectly flat interfaces can be expressed with biaxial moduli, magnetostrictions, and lattice mismatch. The calculated magnetoelastic anisotropy energies are larger than the shape anisotropy energy of Ni and large enough for the effective uniaxial magnetic anisotropy energies to be positive. For films with a large interfacial roughness, we have found the strain-induced magnetic anisotropy reduced considerably.
Ga0.47In0.53As multiquantum well heterostructures, confined by pseudoquaternary (InP)n/(Ga0.47In0.53As)m short period superlattices lattice-matched to InP
Authors: Dotor M. L. , Huertas P. , Golmayo D. , Briones F.
Published: July 2002

(InP)n/(Ga0.47In0.53As)m lattice-matched short-period superlattices have been used as pseudoquaternary material to confine Ga0.47In0.53As multiquantum wells in a GaxIn1−xAs/GaxIn1−xAsyP1−y/InP heterostructure. The samples have been grown by low-temperature atomic layer molecular beam epitaxy, using fast operating valved solid sources to generate P2 and As4 beams. X-ray diffraction was used to assess the structural quality of the samples. The effect of the superlattice period on the pseudoquaternary band gap is reported. Room-temperature photoluminescence wavelength of the multiquantum well structure is close to 1.55 μm. Growing short-period superlattices results in a much easier method to control band-gap energy than growing alternative quaternary material.
Al-Ga interdiffusion through group III-vacancy second nearest-neighbor hopping
Authors: Olmsted B. L. , Houde-Walter S. N.
Published: July 2002

We report on a systematic study of impurity-free Al-Ga interdiffusion in AlGaAs/GaAs superlattices in sealed ampoules. Three ambients were explored: along the Ga-rich solidus, with no excess Ga or As in the evacuated ampoule, and with excess As less than that required to reach the As-rich solidus limit. In each of the ambients the Arrhenius dependence of the Al-Ga interdiffusion coefficient is represented by a single activation energy throughout the temperature range investigated (700–1050 °C). These results were obtained using four structures with superlattice periods ranging from 90 to 520 Å. Excellent agreement was obtained for the Al-Ga interdiffusion coefficients measured using superlattices on Si-doped and undoped GaAs substrates. With proper normalization to a constant As overpressure, pAs4 = 1 atm, the Ga- and As-rich activation energies are 3.26±0.12 and 4.91±0.23 eV, respectively. These activation energies are in the range predicted for Al-Ga interdiffusion mediated by group III-vacancy second nearest-neighbor hopping. The increase in energy when going from Ga- to As-rich conditions is attributed to a shift in the Fermi-level position toward the valence band with an increase in the ionized group III-vacancy concentration.
Time-resolved scanning tunneling microscopy through tunnel distance modulation
Authors: Freeman M. R. , Nunes G.
Published: July 2002

The ability to perform time-resolved measurements of fast transient signals with a scanning tunneling microscope has been achieved through direct control of the tunneling distance on short time scales. Modulation of the tip-sample separation is provided by the use of a magnetostrictive tip driven by a local magnetic field coil. The technique offers a broadly applicable means of obtaining fast dynamical information in tunneling microscopy, and is demonstrated here on nanosecond time scales.
Magnetic easy axis randomly in-plane oriented barium hexaferrite thin film media
Authors: Sui Xiaoyu , Kryder Mark H.
Published: July 2002

High coercivity, c-axis randomly in-plane oriented barium hexaferrite thin films have been successfully fabricated by using conventional rf diode sputtering and post-deposition annealing. Coercivities as high as 4100 Oe have been achieved. Ms, Sq, S*, SFD, and SFDr reach the values of about 270 emu/cc, 0.63, 0.90, 0.12, and 0.084, respectively. CoTi doping not only reduces the coercivity, but also reduces the grain size and improves the c-axis in-plane orientation. The composition of doped films can be controlled through different diffusion processes. In this way coercivities in a range between several hundred oersteds and 4000 Oe can be easily obtained.
Electron paramagnetic resonance forbidden transitions from hydrogen in polycrystalline diamond films
Authors: Holder S. L. , Rowan L. G. , Krebs J. J.
Published: July 2002

Investigation of polycrystalline diamond films by electron paramagnetic resonance at 9.5 and 35 GHZ has revealed the presence of forbidden transitions resulting from a simultaneous microwave induced flipping of unpaired electron spins and environmental nuclear spins. The spacing of the resonance lines identifies hydrogen as the atom neighboring the paramagnetic active site.
Defect generation in 3.5 nm silicon dioxide films
Authors: Buchanan D. A. , DiMaria D. J. , Chang C-A. , Taur Y.
Published: July 2002

The generation of defects in thin 3.5 nm SiO2 films has been measured as a function of the average electron energy and total injected fluence. It is found that the generation of defects during electron injection for both positive and negative bias manifests itself as positive charge as measured from the increase in the current for a given bias. Positive charge generation is seen for electrons injected into the silicon dioxide conduction band, with the generation rate increasing with increasing electron energy. Electrons that traverse the oxide film via direct quantum-mechanical tunneling do not generate measurable defects. These results are consistent with previously published results on thicker films where the defects were attributed to positive charge found near the anode.
Growth and band gap of strained 〈110〉 Si1−xGex layers on silicon substrates by chemical vapor deposition
Authors: Liu C. W. , Sturm J. C. , Lacroix Y. R. J. , Thewalt M. L. W. , Perovic D. D.
Published: July 2002

We report chemical vapor deposition growth of strained Si1−xGex alloy layers on 〈110〉 Si substrates. Compared to the same growth conditions on 〈100〉 substrates, a slightly lower Ge composition and a much lower growth rate was observed. From photoluminescence measurements, the band gap of these films for 0.16≤x≤0.43 is evaluated and compared to theory. Finally, a surprisingly large ‘‘no-phonon’’ replica line strength ratio was observed as compared with that observed in 〈100〉 layers.
Microwave interferometer for shock wave, detonation, and material motion measurements
Authors: McCall Gene H. , Bongianni Wayne L. , Miranda Gilbert A.
Published: July 2002

A microwave interferometer system which provides a continuous measurement of the position of interfaces, such as shock fronts, detonation fronts, or material surfaces, has been developed. The use of low-mass microcoaxial cable, some of which is commercially available, and stripline for conducting the microwave signal into the interior of an experiment makes the perturbation of the measuring system on the experiment small. Other microwave systems have measured the time of peak amplitude of a reflected signal, but the present system with proper matching produces a continuous record of position as a function of time. It is believed that this technique can be used to replace most uses of electrical contact or fiber-optic pins in high-explosive experiments. The amount of data far exceeds that which it is possible to obtain from pins or optical fibers. The matching of the microcoax to the microwave generator has an important effect on the accuracy of the technique. The effect of matching on accuracy will be described in detail, and matching methods will be discussed. A position accuracy of 0.2 mm has been attained. Several methods for extracting position information as a function of time are possible. These techniques are discussed, and their application to the analysis of an experiment is demonstrated. The data from the interferometer system is compared to a streak camera record in a phased, shock-breakout experiment. Agreement is quite good, and the superiority of this method over the pin method is demonstrated by comparing details of the streak and interferometer records.
On the magnetic field of rectangular coils
Authors: Briguet A. , Chankji J.
Published: July 2002

A derivation of the magnetic field, created by rectangular single-turn coils, is achieved with the aid of magnetic scalar potential. The transformation properties of spherical harmonics under rotation permit one to analyze the magnetic field of a system of several coils. Application to a four, rectangular coil system, generating a magnetic field gradient, leads to the optimal dimensions of each element of the system: h=1.732, b=2.486, when a=1 and β=45°, where a represents the half-width of the rectangle, b its half-length, h the distance between the origin of the reference frame and the center of each coil, and β is the angle between the directions of the normals to the planes of two neighboring coils.