bluuebunny/crossref_metadata_2025 huggingface dataset:
Reconstruction of a 1436‐year soil moisture and vegetation water use history based on tree‐ring widths from Qilian junipers in northeastern Qaidam Basin, northwestern China
Authors: Yin Zhi‐Yong , Shao Xuemei , Qin Ningsheng , Liang Eryuan
Published: June 2007

Tree‐ring widths have been used widely in studies of environmental changes and reconstructions of past climate. Eleven tree‐ring chronologies of approximately 800–1500 years long were developed from Qilian junipers (Sabina przewalskii Kom.) for northeastern Qaidam Basin, along the margin of the Qinghai–Tibetan Plateau. Previous studies have revealed that water usage stress is the most limiting factor for tree growth in the study region. To evaluate the impact of the combined effects of temperature and precipitation changes over time, we performed water balance modelling using 1955–2002 meteorological data. We found that the tree‐ring widths were strongly correlated with variables representing soil moisture conditions obtained from the water balance model. Specifically we considered actual evapotranspiration (AE) to represent the combined effect of water use demand and moisture availability, deficit as the difference between potential evapotranspiration (PE) and AE to represent the severity of water use stress, and relative soil moisture as the measure of moisture availability. For certain individual monthly and seasonal combinations, the tree‐ring chronologies explained up to 80% of the variation in the soil moisture variables in regression analysis, indicating very good potential for reconstruction of regional soil moisture conditions in the past. These soil moisture variables outperformed precipitation and Palmer's drought severity index in most cases. We reconstructed the soil moisture conditions from 566 AD to 2001, which revealed major dry and wet periods and a general trend toward a wetter condition during the most recent 300 years. By comparing with other proxies in the region, we concluded that the moisture conditions reconstructed from tree‐ring widths very well reflected the climate variability at the interannual and interdecadal scales. Copyright © 2007 Royal Meteorological Society
PRG4 exchange between the articular cartilage surface and synovial fluid
Authors: Nugent‐Derfus G.E. , Chan A.H. , Schumacher B.L. , Sah R.L.
Published: June 2007

The boundary lubrication function of articular cartilage is mediated in part by proteoglycan 4 (PRG4) molecules, found both in synovial fluid (SF) and bound to the articular cartilage surface. Currently the mechanism by which PRG4 binds to the articular surface is not well understood. The objectives of this study were to determine (1) the effect of bathing fluid contents on PRG4 concentration at the articular surface ([PRG4]cart), and (2) whether native PRG4 can be removed from the surface and subsequently repleted with PRG4 from synovial fluid. In one experiment, cylindrical cartilage disks were stored in solutions of various PRG4 concentrations, either in phosphate‐buffered saline (PBS) or SF as the carrier fluid. In a separate experiment, cartilage disks were stored in solutions expected to remove native PRG4 from the articular surface and allow subsequent repletion with PRG4 from SF. [PRG4]cart was independent of PRG4 concentration of the bathing fluid, and was similar for both carrier fluids. PRG4 was removed from cartilage by treatment with hyaluronidase, reduction/alkylation, and sodium dodecyl sulphate, and was repleted fully by subsequent bathing in SF. These results suggest that the articular surface is normally saturated with tightly bound PRG4, but this PRG4 can exchange with the PRG4 in SF under certain conditions. This finding suggests that all tissues surrounding the joint cavity that secrete PRG4 into the SF may help to maintain lubrication function at the articular surface. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 25:1269–1276, 2007
Evidence for regionally wet conditions before the LGM in southeast Australia: OSL ages from a large palaeochannel in the Lachlan Valley
Authors: Kemp Justine , Spooner Nigel A.
Published: June 2007

Optically stimulated luminescence (OSL) techniques have been used to obtain the first ages on Late Pleistocene channels in the Lachlan Valley, southeastern Australia. Two OSL ages from channel sand and overlying source‐bordering dune sand indicate that large sinuous channels, with bankfull discharges six to eight times greater than the present river, were fully established by 34 ka BP. This conclusion is consistent with regional lake level and geomorphic evidence of cool, pluvial conditions that preceded the last glacial maximum (LGM), providing new information on a long‐standing palaeohydrological problem caused by the apparent synchroneity of large river systems and regional aridity. Copyright © 2007 John Wiley & Sons, Ltd.
A high‐resolution record of vegetation and climate through the last glacial cycle from Caledonia Fen, southeastern highlands of Australia
Authors: Kershaw A. P. , McKenzie G. M. , Porch N. , Roberts R. G. , Brown J. , Heijnis H. , Orr M. L. , Jacobsen G. , Newall P. R.
Published: June 2007

A blocked tributary has provided a rare site of long‐term sediment accumulation in montane southeastern Australia. This site has yielded a continuous, detailed pollen record through the last ca. 140 000 years and revealed marked vegetation and environmental changes at orbital to sub‐millennial scales. Radiocarbon and optically stimulated luminescence (OSL, or optical) ages provide some chronological control for the last ca. 70 000 years. Most of the sediment is inorganic but with well preserved pollen that accumulated under unproductive and probably largely ice‐covered lake conditions. The lake was surrounded by low‐growing plants with an alpine character. Exceptions include three discrete periods of high organic sedimentation in the basin and forest development in the surrounding catchment. The two major periods of forest expansion are related to the last interglacial and the Holocene, with the third, shorter period considered to represent an interstadial in the early part of Marine Isotope Stage (MIS) 3. The latter part of the last glacial period is characterised by abrupt sub‐millennial, amelioration events that may relate to documented global oscillations emanating from the North Atlantic. There are systematic changes through the record that can be partly attributed to basin infilling but the progressive reduction and regional extinction of some plant taxa is attributed to a long‐term trend towards climatic drying. Copyright © 2007 John Wiley & Sons, Ltd.
Excimer Lasers – Reliability and Higher Power Are Key Product Trends
Authors: Junger Johannes , Schmidt Kai
Published: June 2007

Over the last decade, excimer lasers have transitioned from scientific instruments into industrial‐grade workhorses capable of supporting 24/7 manufacturing. This is because excimer laser manufacturers have made significant improvements in product lifetime, performance, reliability and stability. This article reviews the recent technological advances that have enabled these improvements, and looks forward at ongoing product development trends.
CO2 Lasers: Modern Workhorses in the World of Industrial Manufacturing
Authors: Rath Wolfram
Published: June 2007

For more than 30 years lasers have been successfully used for industrial production. Modern efficient manufacturing would be not imaginable without lasers and laser processes. Since the beginning of laser applications, CO2 lasers dominated the market of continuous cutting and welding applications (referred here as “macro” applications) because of their high power and electrical efficiency, reliability and cost efficiency.
Disk Lasers Enable Industrial Manufacturing – What Was Achieved and What Are the Limits?
Authors: Brockmann Rüdiger
Published: June 2007

In the past few years, CO2 lasers as well as lamp‐pumped solid‐state lasers have become established solutions for industrial cutting and welding applications. Due to its high beam quality and output powers up to 20 kW the RF excited fast axial flow CO2 laser has enabled the largest single laser application in the world: Cutting of steel up to one inch thickness. So far, these lasers are also the preferred industrial choice for deep penetration welding of thick plate and narrow seam geometries.
Fiber Laser – New Challenges for the Materials Processing
Authors: Thieme J.
Published: June 2007

Continuous wave (CW) and pulsed Ytterbium fiber lasers in the power range from several Watts up to the high Multikilowatt range are commercially available standard products. The systems have been tested in numerous applications and are in stable operation for several years already. The demand for fiberlasers is rising fast and the future is promising.
Effect of cathodic polarization and sulfate reducing bacteria on mechanical properties of different steels in synthetic sea water
Authors: Domżalicki P. , Lunarska E. , Birn J.
Published: June 2007

At the slow strain rate tensile tests done using the specially designed facility, the decrease in the elongation to fracture, reduction of area, fracture energy and no effect on the strength have been stated for the low alloy ferrite‐pearlite and sorbite steels, polarized in synthetic sea water at potentials corresponding to the cathodic protection (− 800 to − 1400 mVSCE). Presence of SRB promotes the plasticity loss, being especially pronounced at potentials − 1100 to − 1200 mVSCE. At higher cathodic polarization, the plasticity estimated in inoculated and in sterile water equalizes. The effects have been correlated with the contents of absorbed and of permeable hydrogen. The promotion of hydrogen charging and the plasticity loss by SRB at the low and medium applied cathodic polarization has been accounted for the observed production of S−2 ions and inhibition of deposit formation. The negligible effect of SRB at the high cathodic polarization has been suggested to be a result of the suppression the SRB growth due to the high alkalization of the near surface solution. The same amount of hydrogen produces the less detrimental effect on the sorbite than on the ferrite‐pearlite steel. However, at the similar cathodic polarization, the sorbite steel absorbs the highest amount of hydrogen and reveals the most pronounced degradation. Cathodic protection of constructions subjected to the action of SRB in the sea water should provide the conditions, under which no fragment of marine construction could be polarized by potential corresponding to the maximum degradation of the plastic properties of steels (− 1100 to − 1200 mVSCE).
Kinetics of high temperature corrosion of a low Cr‐Mo steel in aqueous NaCl Solution
Authors: Bayoumi F.M.
Published: June 2007

The kinetics of corrosion of a low Cr‐Mo steel alloy were studied over the temperature range of 75–250°C in 1 m NaCl in absence and in presence of various levels of contamination with CuCl2. Corrosion rates, weights of corrosion product (magnetite) film and total (integral) weight loss of the alloy over exposure times from 1 up to 480 h were measured. The corrosion rate decreases rapidly with time, before it levels off at longer time, indicating the formation of a protective corrosion product film. The ability of the alloy to retain an adherent corrosion product (magnetite) film was expressed in terms of a retention coefficient. This was found to increase with temperature and exposure time and to decrease with the level of contamination with CuCl2. This effect of temperature was attributed to the improvement of the crystallinity of the corrosion product. On the other hand, the effect of CuCl2 was attributed to the electrodeposition of Cu and its impregnation within the corrosion product, which becomes less adherent. The free corrosion potential was found to be affected by the presence of the CuCl2, in a fashion compatible with the Wagner‐Traud theory of mixed potential.
Corrosion behavior of laser melted AZ91HP magnesium alloy
Authors: Gao Y. , Wang C. , Yao M. , Liu H.
Published: June 2007

AZ91HP magnesium alloy was melted by CO2 laser. Compared with as‐received Mg alloy, the grain of the melted layer was refined significantly and the content of Al was increased. The corrosion resistance of the melted layer was improved because of the grain refinement, the redistribution of β‐Mg17Al12 and the increasing of the Al content. As compared to the non‐overlapping zone, the overlapping zone of the melted layer was liable to be corroded.
The cyclooxygenase‐2‐mediated prostaglandin signaling is causally related to epithelial carcinogenesis
Authors: Müller‐Decker Karin , Fürstenberger Gerhard
Published: June 2007

Epidemiologic, pharmacologic, clinical, and experimental studies document the importance of prostaglandin (PG) signaling in cancer development, including non‐melanoma skin cancer lesions in humans and mice. First of all, enzymes involved in PG biosynthesis, such as cyclooxygenase (COX)‐2 and/or membrane prostaglandin E synthase (mPGES)‐1, were found to be overexpressed in a wide range of premalignant and malignant epithelial tumors, including those of the skin, breast, esophagus, stomach, colorectum, pancreas, and bladder. On the other hand, 15‐hydroxy‐prostaglandin dehydrogenase (15‐PGDH), which is involved in the degradation pathway of PG including PGE2, thus counteracting the activities of COX‐2 and PGES, was found to be downregulated in human epithelial tumors, indicating a tumor suppressor activity of this enzyme. Most remarkably, genetic studies showed that mice, which are deficient in COX‐2 and/or PGES are resistant to the development of cancer of skin, colon, and stomach. In contrast, the forced overexpression of COX‐2 in proliferative compartments of simple or stratified epithelia such as skin epidermis, urinary bladder, mammary gland, and pancreas results in spontaneous hyperplasia and dysplasia in transgenic mice. In skin, the pathological changes are found to be due to an abnormal process of terminal differentiation, while in other tissues, hyperproliferation seems to be the main contributer to the pre‐invasive neoplasms. Moreover, the COX‐2 transgenic mouse lines are sensitized for cancer development. © 2007 Wiley‐Liss, Inc.
Cardiomyopathy in Friedreich's ataxia‐assessment by cardiac MRI
Authors: Meyer Chris , Schmid Gebhard , Görlitz Sabine , Ernst Monika , Wilkens Christian , Wilhelms Inga , Kraus Peter H. , Bauer Peter , Tomiuk Jürgen , Przuntek Horst , Mügge Andreas , Schöls Ludger
Published: June 2007

Cardiomyopathy is an important and frequently life limiting manifestation of Friedreich's ataxia (FA), the most prevalent form of autosomal recessive ataxia. Left ventricular mass is used as primary outcome measure in recent intervention studies but systematic analyses of FA cardiomyopathy are sparse. To assess cardiac hypertrophy by cardiac magnetic resonance imaging (MRI) in vivo, we assessed 41 adult patients with genetically confirmed FA and 33 age‐ and sex‐matched healthy controls by cardiac MRI and echocardiogarphy. Septal hypertrophy and left ventricular mass index were determined by two independent raters. MRI revealed hypertrophy of the interventricular septum in 40% and increased left ventricular mass index in 29% of patients. Interobserver variability was less than 5% for both measures. GAA repeat length had only minor influence on interventricular septum thickness. Left ventricular mass index decreased with age. Severity of ataxia did not correlate with cardiac disease. In echocardiography wall diameter was assessable only in 31 of 41 FA patients with 32% of patients presenting septal hypertrophy and 6% increased left ventricular mass index. We conclude that cardiac hypertrophy is present only in a minority of adult FA patients. If despite this limitation intervention studies use left ventricular mass as outcome measure, MRI is recommended as the most accurate assessment of cardiac anatomy in vivo. © 2007 Movement Disorder Society
Impaired cytotrophoblast cell–cell fusion is associated with reduced Syncytin and increased apoptosis in patients with placental dysfunction
Authors: Langbein Manuela , Strick Reiner , Strissel Pamela L. , Vogt Nicole , Parsch Hans , Beckmann Matthias W. , Schild Ralf L.
Published: June 2007

Preeclampsia (PE), Hemolysis Elevated Liver Enzymes and Low Platelets (HELLP)‐syndrome, and intrauterine growth restriction (IUGR) are associated with abnormal placentation. In early pregnancy, placental cytotrophoblasts fuse and form multinuclear syncytiotrophoblasts. The envelope gene of the human endogenous retrovirus‐W, Syncytin, is a key factor for mediating cell–cell fusion of cytotrophoblasts. This study investigated clinical parameters of PE and HELLP‐associated IUGR and analyzed the cell–cell fusion index and β‐human chorionic gonadotropin (β‐hCG) secretion of cytotrophoblasts isolated and cultured from placentas of these patients. In addition, we performed absolute quantitation of Syncytin and determined the apoptosis rate in both cultured cytotrophoblasts and placental tissues. Cultured cytotrophoblasts from PE and HELLP‐associated IUGR correlated with a pronounced lower cell–cell fusion index, 1.8‐ and 3.6‐fold; less nuclei per syncytiotrophoblast, 1.4‐ and 2.0‐fold; a significantly decreased β‐hCG secretion, 4.3‐ and 17.2‐fold and a reduction of Syncytin gene expression, 8.1 (P = 0.019) and 222.7‐fold (P = 0.011) compared with controls, respectively. In contrast, a significantly 2.3‐fold higher apoptosis rate was observed in cultured PE/IUGR cytotrophoblasts (P = 0.043). Importantly, Syncytin gene expression in primary placental tissues of PE/IUGR was 5.4‐fold lower (P = 0.047) and in HELLP/IUGR 10.6‐fold lower (P = 0.019) along with a 1.8‐ and 1.9‐fold significant increase in the apoptosis rate compared with controls, respectively. Low Syncytin expression in both cultured cytotrophoblasts and primary tissues from pathological placentas supports an intrinsic placenta‐specific deregulation of cell–cell fusion in the formation of syncytiotrophoblasts leading to increased apoptosis. These processes could contribute to the development and severity of PE and HELLP‐associated IUGR. Mol. Reprod. Dev. 75: 175–183, 2008. © 2007 Wiley‐Liss, Inc.
On the successive supersymmetric rank‐1 decomposition of higher‐order supersymmetric tensors
Authors: Wang Yiju , Qi Liqun
Published: June 2007

In this paper, a successive supersymmetric rank‐1 decomposition of a real higher‐order supersymmetric tensor is considered. To obtain such a decomposition, we design a greedy method based on iteratively computing the best supersymmetric rank‐1 approximation of the residual tensors. We further show that a supersymmetric canonical decomposition could be obtained when the method is applied to an orthogonally diagonalizable supersymmetric tensor, and in particular, when the order is 2, this method generates the eigenvalue decomposition for symmetric matrices. Details of the algorithm designed and the numerical results are reported in this paper. Copyright © 2007 John Wiley & Sons, Ltd.
Local boundary value problems for the error in FE approximation of non‐linear diffusion systems
Authors: Carnes B. , Carey G. F.
Published: June 2007

In this work we investigate the a posteriori error estimation for a class of non‐linear, multicomponent diffusion operators, which includes the Stefan–Maxwell equations. The local error indicators for the global error are based on local boundary value problems, which are chosen to approximate either the global residual of the finite element approximation or the global linearized error equation. Using representative numerical examples, it is shown that the error indicators based on the latter approach are more accurate for estimating the global error for this problem class as the problem becomes more non‐linear, and can even produce better adaptive mesh refinement (AMR). In addition, we propose a new local error indicator for the error in output functionals that is accurate, inexpensive to compute, and is suitable for AMR, as demonstrated by numerical examples. Copyright © 2007 John Wiley & Sons, Ltd.
Preparation of hemispherical hollow silica microcapsules with different affinity surface by using spherical vaterite calcium carbonate as template
Authors: Fuchigami Kiyomi , Taguchi Yoshinari , Tanaka Masato
Published: June 2007

We developed a method to prepare hemispherical hollow silica microcapsules (HHSM) with different affinity surfaces using spherical vaterite calcium carbonate (SVCC) as a template. The preparation process composed of the adhesion of calcium carbonate onto the surface of methyl methacrylate (MMA) droplets followed by suspension polymerization, the partial etching of calcium carbonate on the polymethyl methacrylate (PMMA) mother particle, the formation of silicon dioxide powder by sol–gel reaction and their deposition onto the etched flat surface of calcium carbonate, the surface modification of deposited silicon dioxide with silane coupling agent, the removal of the mother particle with acetone, the formation of silicon dioxide powder by sol–gel reaction and deposition onto the exposed hemispherical surface of calcium carbonate, and the surface modification of deposited silicon dioxide with silane coupling agent. The synthesized microcapsules had a complete hemispherical structure and both hydrophilic and hydrophobic surfaces. Copyright © 2007 John Wiley & Sons, Ltd.
Preparation, characterization, and sterilization of hydrogel sponges for iontophoretic drug‐delivery use
Authors: Eljarrat‐Binstock Esther , Bentolila Alfonso , Kumar Neeraj , Harel Hannah , Domb Abraham J.
Published: June 2007

Hydrogel sponges of hydroxyethyl methacrylate (HEMA) crosslinked with ethylene glycol dimethacrylate (EGDMA) were prepared and characterized for iontophoretic drug delivery. Various parameters were examined, including: morphological structure, thermal behavior, mechanical strength, absorption capacity, swelling profile, and hydrogel stability. Three sterilization methods, gamma irradiation, ethylene oxide, and autoclave sterilizations were assessed for revealing the optimal sterilization process avoiding alteration in hydrogel properties. Hydrogels prepared with 75% water and polymerized for 24 hr seem to preserve their physical properties, swelling capacity, and absorption kinetics at least over 5 years from preparation. Gamma irradiation was found to be the most suitable as sterilization method for the dried hydrogels causing minimal alteration of hydrogel properties. This work confirmed the feasibility of preparing stable hydrogel sponges of different shapes with desired mechanical strength and swelling capacities at a rapid rate profile suitable for iontophoretic application and drug delivery. Copyright © 2007 John Wiley & Sons, Ltd.
Role of architecture and molecular weight in the formation of tailor‐made ultrathin multilayers using dendritic macromolecules and click chemistry
Authors: Vestberg Robert , Malkoch Michael , Kade Matthew , Wu Peng , Fokin Valery V. , Barry Sharpless K. , Drockenmuller Eric , Hawker Craig J.
Published: June 2007

The high efficiency and mild reaction conditions associated with the Cu(I) catalyzed cycloaddition of azides and alkynes were exploited for the covalent layer‐by‐layer synthesis of dendritic thin films on silicon wafers. The preparation of azide and alkyne‐terminated dendrimers based on bisMPA was accomplished by a divergent strategy; combinations of these monodisperse building blocks from the 2nd to the 5th generation were used for construction of the thin films. The layer‐by‐layer self assembly process proceeds under ambient conditions and was monitored by ellipsometry, XPS, and ATR‐IR, which showed extremely regular growth of the dendritic thin films. Film thickness could be accurately controlled by both the size/generation number of the dendrimers as well as the number of layers. In comparison with linear analogues, the growth of the dendritic films was significantly more controlled and defect‐free with each layer being thinner than the corresponding films prepared from the isomeric linear polymers, demonstrating the well‐defined, three‐dimensional nature of the dendritic architecture. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2835–2846, 2007
Redox properties and evolution of human glutaredoxins
Authors: Sagemark Johan , Elgán Tobias H. , Bürglin Thomas R. , Johansson Catrine , Holmgren Arne , Berndt Kurt D.
Published: June 2007

Glutaredoxins (Grxs) are glutathione‐dependent oxidoreductases that belong to the thioredoxin superfamily catalyzing thiol‐disulfide exchange reactions via active site cysteine residues. Focusing on the human dithiol glutaredoxins having a C‐X‐Y‐C active site sequence motif, the redox potentials of hGrx1 and hGrx2 were determined to be −232 and −221 mV, respectively, using a combination of redox buffers, protein–protein equilibrium and thermodynamic linkage. In addition, a nonactive site disulfide was identified between Cys28 and Cys113 in hGrx2 using redox buffers and chemical digestion. This disulfide confers nearly five kcal mol−1additional stability by linking the C‐terminal helix to the bulk of the protein. The redox potential of this nonactive site disulfide was determined to be −317 mV and is thus expected to be present in all but the most reducing conditionsin vivo. As all human glutaredoxins contain additional nonactive site cysteine residues, a full phylogenetic analysis was performed to help elucidate their structural and functional roles. Three distinct groups were found: Grx1, Grx2, and Grx5, the latter representing a highly conserved group of monothiol glutaredoxins having a C‐G‐F‐S active site sequence, with clear homologs from bacteria to human. Grx1 and Grx2 diverged from a common ancestor before the origin of vertebrates, possibly even earlier in animal evolution. The highly stabilizing nonactive site disulfide observed in hGrx2 is found to be a conserved feature within the deuterostomes and appears to be the only additional conserved intramolecular disulfide within the glutaredoxins. Proteins 2007. © 2007 Wiley‐Liss, Inc.
Diapause disruption with tebufenozide for early‐instar control of the spruce budworm, Choristoneura fumiferana
Authors: Doucet Daniel , Frisco Caroline , Cusson Michel , Bauce Éric , Palli Subba Reddy , Tomkins Bill , Arif Basil , Retnakaran Arthur
Published: June 2007

In North America, the eastern spruce budworm, Choristoneura fumiferana Clem., is an important coniferous pest against which tebufenozide has proven effective as a control product. By acting as an ecdysone agonist, tebufenozide can induce precocious moulting in late (fifth–sixth) instars but can also be carried over to the next generation owing to its persistence on foliage. The authors conducted laboratory experiments on first‐instar larvae treated with tebufenozide dissolved in acetone. Larvae exposed to doses equal to or above 0.1 µg cm−2 displayed precocious moulting in the second instar after hibernaculum spinning, which effectively disrupted diapause. Larger doses induced moulting in first instars. Evidence is provided that this dose–response difference is related to whether or not an effective dose of tebufenozide is ingested by the first instar prior to the peak of moulting hormone (20‐hydroxyecdysone) in first instars. Doses ineffective to kill first instars are carried over to the second instar, where they induce a precocious moult. This type of response to tebufenozide is dependent on the presence of a moulting machinery (the EcR–USP receptor complex) that is ready for ecdysone transduction. Interestingly, ecdysone levels are low in second instars, as measured by a radioimmunoassay, which suggests that diapause in spruce budworm is maintained by a suppression of ecdysone production. Thus, diapause disruption by tebufenozide may well provide an alternative control strategy for this important pest. Copyright © 2007 Society of Chemical Industry
Crystal field parameters of praseodymium in oxides
Authors: Novák P. , Diviš M.
Published: June 2007

The crystal field parameters of the Pr4+ ion in PrO2 and PrBaO3 are calculated using the density of states projected on the 4f states of the Pr4+ ion. The orbital‐dependent potential is used and the procedure involves an adjustable parameter Δf that determines the relative position of the center of the 4f levels. Δf can be estimated using the LDA + U method. For reasonable values of Δf the calculated crystal field parameters give a splitting close to the one observed. It is shown that the hybridization of the 4f states is important. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Tricritical behavior of Ising metamagnet in both external longitudinal and transverse fields
Authors: Liu Jing , Wei Guozhu , Miao Hailing , Du An
Published: June 2007

A two‐sublattice Ising metamagnet in both external longitudinal and transverse fields is studied within the mean‐field approach based on Bogoliubov's inequality for the Gibbs free energy. The phase diagrams in the longitudinal field‐temperature plane and in the transverse field‐temperature plane are determined, respectively. The results show that the tricritical points can occur in a certain region of the external longitudinal and transverse fields, the temperature of the tricritical point monotonically increases with decreasing the transverse magnetic field Ω and increasing the longitudinal magnetic field h. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electronic and optical properties of Ir3Si5
Authors: Migas D. B. , Shaposhnikov V. L. , Rodin V. N. , Dorozhkin N. N. , Borisenko V. E.
Published: June 2007

First‐principles calculations of the band structure and the dielectric function of Ir3Si5are presented. We find this compound to be characterized by an indirect band gap of 0.97 eV. A number of direct transitions of about 1 eV have been also determined. Features in the dependence of the imaginary and real parts of the dielectric function on photon energy are discussed in comparison with data for some other semiconducting silicides. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Spin‐polarized transport through two coupled quantum dots
Authors: Trocha P. , Barnaś J.
Published: June 2007

Spin‐dependent transport through two coupled single‐level quantum dots attached to ferromagnetic leads with collinear (parallel and antiparallel) magnetizations is analyzed theoretically. Generally, the intra‐ and inter‐dot Coulomb correlations are taken into account. Transport characteristics, including conductance and tunnel magnetoresistance associated with the magnetization rotation from the parallel to antiparallel configurations, are calculated by the noneqiulibrium Green function technique. The relevant Green functions are derived by the equation of motion method. To close the set of equations we employ the Hartree–Fock decoupling scheme. The dot occupation numbers and the Green functions are calculated self‐consistently. We focus on the interplay of interference (Fano resonance) and Coulomb interaction effects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Construction of superconducting pairs in the two‐dimensional electron gas for the model of itinerant electrons
Authors: Inglot Michał , Wal Andrzej , Lulek Tadeusz
Published: June 2007

We consider a 3 × 3 finite plane lattice with Born–von Karman boundary conditions, threaded by magnetic field perpendicular to it, such that two quanta of flux cover exactly three Bravais cells. The corresponding magnetic translation group is determined, and the single‐electron energy levels of the system are labelled by three‐dimensional irreps of this group. We perform also the Clebsch–Gordan decomposition, and interpret selected irreps as the relevant superconducting pairs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the CTMA for the infinite U Anderson impurity model
Authors: Keiter Hellmut , Baumgartner Klaus , Duffe Sebastian
Published: June 2007

If one wants to deal with strongly correlated electron lattice systems within dynamical mean field theory, one needs an impurity solver, i.e. a solution for the dynamics of the Anderson magnetic impurity problem. The most ambitious approach to that problem has been the consistent t‐matrix approximation (CTMA). The basic integral equations could be solved with a slow variable approximation, which compared well with a direct numerical solution. Fermi‐liquid behavior was not obtained, presumably the normalscattering potential was treated approximately only. We show, how the normal scattering potential can be included exactly. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A study of the optical and EPR spectra for Ni2+ ion at the Cd2+(II) site of CsCdCl3 crystal
Authors: Feng Wen‐Lin , Wu Xiao‐Xuan , Zheng Wen‐Chen
Published: June 2007

The optical absorption bands and spin‐Hamiltonian parameters (g ‐factors g‖, g⊥ and zero‐field splitting D measured by EPR spectra) of Ni2+ ion at the Cd2+ (II) site of CsCdCl3 crystals are calculated by using the method of diagonalizing the complete energy matrix of 3d8 ions in trigonal symmetry. The calculated results are in reasonable agreement with the observed values. The defect structure of this Ni2+ impurity center is also estimated from the calculations. The results are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Annihilation of threading dislocations in strain relaxed nano‐porous GaN template for high quality GaN growth
Authors: Hartono H. , Soh C. B. , Chua S. J. , Fitzgerald E. A.
Published: June 2007

Porous GaN subjected to heat treatment under in‐situ MOCVD chamber annealing at 850 °C for 3 minutes has shown significant annihilation of threading dislocations and improvement of its optical quality. Similar phenomenon is not observed for the porous GaN samples annealed at lower temperatures. The annealing was done in a mixed of N2 and NH3 ambient, which resulted in air‐gap formation. Encounter of threading dislocations with the air‐gap might cause them to bend horizontally and annihilate each other. Dark‐field cross‐section TEM image confirms the annihilation of threading dislocations within the porous/air‐gap region. This dislocation density reduction causes a significant enhancement of the optical quality as shown by doubled Raman intensity of E2 phonon peak after the annealing as compared to the as‐fabricated porous GaN. Because the fabricated porous GaN samples are already relaxed, no further strain relaxation is observed after annealing. Such a template is suitable for use in high quality GaN growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Subband structure and transport properties of two‐dimensional electron gas in AlxGa1–xN/GaN heterostructures
Authors: Han Xiuxun , Honda Yoshio , Narita Tetsuo , Yamaguchi Masahito , Sawaki Nobuhiko
Published: June 2007

We report on Hall mobility and Shubnikov‐de Haas (SdH) measurements of the two dimensional electron gas (2DEG) formed in a series of AlxGa1–xN/GaN heterostructures with different Al composition grown on sapphire substrates. In samples of x = 0.15 and 0.20, we found single periodical oscillations. In a sample of x = 0.30 a double periodical one was clearly resolved, which indicates that more than one subband has been occupied by electrons. Fast Fourier transform (FFT) analysis yielded information about the electron density and subband structure of the 2DEGs. The oscillatory part of the SdH signal was analyzed to extract the quantum scattering time at 4.2 K. It was found that as the increase of the Al content, quantum scattering time of the ground subband decreases gradually. We attributed it to the enhanced interface roughness scattering and the onset of the inter‐subband scattering in the sample with higher Al content. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Mega‐cone blue LEDs based on ZnO/GaN direct wafer bonding
Authors: Murai A. , Thompson D. B. , Masui H. , Fellows N. , Mishra U. K. , Nakamura S. , DenBaars S. P.
Published: June 2007

We demonstrate the fabrication of hexagonal pyramid shaped light‐emitting diodes (LEDs), “mega‐cone” LEDs based on ZnO/GaN wafer bonding. Compared with a conventional type LEDs with thin Ni/Au p‐type electrode, external quantum efficiency was 3.1 times higher at 20 mA of current injection condition. We also report on the structural, optical, and electrical characteristics of the mega‐cone LED. The results here indicate the high potential of these mega‐cone LEDs for light sources. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Observations on surface morphologies and dislocations of a‐plane GaN grown by metal organic chemical vapor deposition
Authors: Ko T. S. , Wang T. C. , Chen H. G. , Gao R. C. , Huang G. S. , Lu T. C. , Kuo H. C. , Wang S. C.
Published: June 2007

In this study, we grew non‐polar a‐plane GaN thin films on r‐plane sapphire using a series of growth conditions by metal‐organic chemical vapor deposition. The results showed that high temperature and low‐pressure conditions benefited two‐dimension growth could lead to a fully coalesced a‐plane GaN layer with a very smooth surface. The best surface morphology with an excellent mean roughness of 10.5 Å was obtained. The different thickness AlN as a nucleation layer and the different δ/β ratio were also considered. The results revealed that the surface morphology would get worse when the thickness of nucleation layer and δ/β ratio were away from the values of optimal condition. The observation of transmission electronic microscopy shown the lowest density of threading dislocations was 1.85×1010/cm2. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Thermal and electrical properties of high‐quality freestanding GaN wafers with high carrier concentration
Authors: Oshima Yuichi , Yoshida Takehiro , Eri Takeshi , Shibata Masatomo , Mishima Tomoyoshi
Published: June 2007

The thermal and electrical properties of GaN were investigated using high‐quality freestanding crystals in the high carrier concentration region between 1.0x1018 and 1.24x1019 cm‐3. The carrier concentration n increased almost linearly with increasing Si doping concentration. A very low electrical resistivity of 2.5 mΩcm was obtained at n = 1.24x1019 cm‐3, combined with its high electron mobility. Thermal expansion coefficients along 〈11$ \bar 2 $0〉 and 〈10$ \bar 1 $0〉 were confirmed to be approximately constant at 5.1x10‐6 K‐1 (298‐573 K) in the measured doping concentration range. Thermal conductivity remained high in the measured carrier concentration range at approximately 2.0 Wcm‐1K‐1 for n = 1.0x1018 cm‐3, and 1.87 Wcm‐1K‐1 for n = 1.24x1019 cm‐3. The compatibility of low electrical resistivity and high thermal conductivity has potential to contribute significantly to the realization of high‐performance high‐power nitride devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigations of the growth conditions for GaN‐bulk crystals grown by the sublimation technique
Authors: Rost H.‐J. , Siche D. , Müller R. , Gogova D. , Schulz T. , Albrecht M. , Fornari R.
Published: June 2007

Spontaneous nucleation of GaN and the growth of thick polycrystalline layers by the sublimation technique in nitrogen atmosphere have been studied as a function of growth environment and conditions. Average growth rates higher than 10 microns per hour were realised by reaction of gallium, evaporated from an elemental source, with nitrogen from ammonia gas. The growth temperature was in the range of 1000 – 1300 °C; the pressure was varied between 200 ‐ 800 hPa and the ammonia flow between 50‐500 sccm min–1. It was found that the growth rate only moderately decreases during a long‐term experiment when using an optimised set of growth parameters. Furthermore, a strong influence of the growth environment on the growth rate and impurity content was found. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Practical issues in carrier‐contrast imaging of GaN structures
Authors: Sumner J. , Oliver R. A. , Kappers M. J. , Humphreys C. J.
Published: June 2007

Scanning capacitance microscopy (SCM), scanning spreading resistance microscopy (SSRM) and secondary electron emission within a scanning electron microscope (SEM) were used to image two magnesium‐doped GaN samples, grown under identical conditions. In only one of these samples were the dopants activated by annealing. Under normal imaging conditions for SCM and SSRM, no discernable change in contrast was observed in the unactivated sample, whilst for SEM imaging contrast increased over ca. 40 s of imaging time, indicating some activation of the dopants by the electron beam. In addition, various sample preparation techniques were attempted in order to minimise the roughness of the GaN cross‐section used for SCM imaging. Some techniques which reduce the surface roughness lead to a poor SCM signal‐to‐noise ratio. However, a cleaving technique is described which produces suffi‐ciently large and flat areas within multi‐layered GaN specimens for the extraction of useful SCM data. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Realization of AlGaN/GaN HEMTs on Si‐on‐polySiC substrates
Authors: Cordier Yvon , Chenot Sébastien , Laügt Marguerite , Tottereau Olivier , Joblot Sylvain , Semond Fabrice , Massies Jean , Di Cioccio Léa , Moriceau Hubert
Published: June 2007

AlGaN/GaN High Electron Mobility Transistors have been realized on resistive Si(111)/SiO2/polySiC substrates. The epitaxial structures were grown by Molecular Beam Epitaxy. The structural, optical, and electrical properties of these films are studied and compared with those of reference heterostructures grown on thick Si(111) substrates. Channel mobility of 1470 cm2/Vs at room temperature and sheet carrier density of 6.65x1012 cm–2 have been achieved. Field effect transistors with 3.6 µm x 100 µm gates have been fabricated on these layers and exhibit a drain current superior to 450 mA/mm and a transconductance of 123 mS/mm. Furthermore, to evaluate the influence of heat transfer onto electrical properties, the resistance change with the power dissipated inside Transmission Line Measurement devices has been compared with those in devices realized with layers grown on thick Si(111) and 4H‐SiC substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
GaN/air gap based micro‐opto‐electro‐mechanical (MOEM) Fabry‐Pérot filters
Authors: Cho E. , Pavlidis D. , Sillero E.
Published: June 2007

Structural and optical properties of Fabry‐Pérot filters (FPFs) with GaN/air gap based distributed Bragg reflectors (DBRs) were studied. Reflectance of GaN/air gap DBRs on sapphire substrate was calculated from the standard transmission matrix method and results showed that 98% reflectance is achievable with only 3.5 pairs at a center wavelength of 450 nm. The thickness of the GaN layer and the first AlN layer was determined according to the deformation induced by the residual stress. In‐plane strain corresponding to growth conditions and the thickness of the GaN epilayer was considered for this analysis. Optical tuning efficiency and spectral range were found to be 0.27 and 25 nm respectively for FPFs with GaN/air gap (322 nm/113 nm) based DBRs and a λ0/2 air resonant cavity. The calculated pull‐in voltage was 1.5 V. Crack free AlN grown on GaN by in‐house MOCVD showed an etching rate of 0.2 nm/min. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Self‐separated freestanding GaN grown on patterned substrate by Hydride Vapor Phase Epitaxy
Authors: Lai Chih‐Ming , Liu Wen Yu , Tsay Jenq‐Dar , Liu Po Chun , Guo Yih‐Der , Huang Hsin‐Hsiung , Chang Yu Hsiang , Yeh Jul Chin
Published: June 2007

A freestanding Gallium Nitride bulk is obtained without using laser lift‐off technology. Two techniques such as mask‐less ELOG and air‐bridge structure are applied on patterned substrate. A self‐separated result is successfully demonstrated by using mask‐less ELOG and air‐bridge structure. Due to the thermal coefficient of expansion (TCE) mismatch between the GaN layer and the non‐native (foreign) substrate, separation happens during cooling down after HVPE growth. Both two structures successfully help to lift GaN off the substrate. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Normally‐off operation in AlGaN/GaN/AlGaN double heterojunction field effect transistors
Authors: Shimizu Mitsuaki , Inada Masaki , Yagi Shuichi , Piao Guanxi , Okumura Hajime , Arai Kazuo , Yano Yoshiki , Akutsu Nakao
Published: June 2007

Nearly normally‐off operation of the GaN‐based heterojunction field effect transistors was obtained using AlGaN/GaN/AlGaN double heterojunction channel. The aluminum compositions of the AlGaN heterobarriers are designed so as to deplete the electron carrier in the AlGaN/GaN/AlGaN channel, which gives the threshold gate voltage of about 0 V. The maximum drain current density was over 170 mA/mm. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
An analysis of the increase in sheet resistance with the lapse of time of Al0.3Ga0.7N/GaN HEMT structure wafers
Authors: Kikawa Junjiroh , Yamada Tomoyuki , Tsuchiya Tadayoshi , Kamiya Shinichi , Kosaka Kenichi , Hinoki Akihiro , Araki Tsutomu , Suzuki Akira , Nanishi Yasushi
Published: June 2007

We have observed the increase in sheet resistance with the lapse of time for some Al0.3Ga0.7N/GaN HEMT structure wafers, and this was due to the decrease in electron mobility rather than the decrease in carrier concentration. In the wafers, a lot of triangular micro‐cracks extending to 〈11$ \bar 2 $0〉 directions were observed by AFM, which were a few tiny lines initially. On the other hand, a wafer that was kept in the nitrogen atmosphere was little increased in sheet resistance. It is plausible that oxygen decreases surface energy of cleaved planes, and micro‐cracks progress. The reciprocal lattice map of XRD showed no lattice relaxation of the AlGaN/GaN system. Monochromatic CL image of 359 nm wavelength showed those micro‐cracks as dark lines, which means that the micro‐cracks work as non‐radiative centers, and the charge of trapped electrons might deplete or scatter 2DEG beneath the micro‐cracks and cause the increase in sheet resistance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Off‐state drain current and breakdown voltage of AlGaN/GaN MIS‐HEMT with multilayered gate insulator
Authors: Yagi S. , Shimizu M. , Inada M. , Okumura H. , Ohashi H. , Yano Y. , Akutsu N.
Published: June 2007

We report the fabrication of AlGaN/GaN high electron mobility transistors (HEMT) with high‐k /SiN and high‐k /SiO2/SiN gate structures. HfO2, ZrO2, TiO2 and Ta2O5 were used as high‐k materials. Both high‐k /SiN and high‐k /SiO2/SiN metal‐insulator‐semiconductor (MIS) HEMTs showed good operating characteristics. However, in the case of high‐k /SiN MIS‐HEMT, the off‐state drain current was large. On the other hand, in the case of the high‐k /SiO2/SiN MIS‐HEMT, the off state current was significantly reduced due to the presence of the SiO2 layer, and the breakdown voltage characteristics were improved. The breakdown voltages of HfO2/SiO2/SiN MIS‐HEMT and ZrO2/SiO2/SiN at gate‐drain distance Lgd = 28 μm were 1.8 kV and 1.7 kV, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Coherent manipulation of A and B excitons in GaN
Authors: Ishiguro T. , Toda Y. , Adachi S. , Hazu K. , Sota T. , Chichibu S. F.
Published: June 2007

An optical coherent manipulation of excitonic polarizations in hexagonal bulk GaN is achieved by means of an interferometric technique combined with spectrally‐resolved degenerate four‐wave‐mixing (FWM). Initial phase of the FWM quantum beats between A and B excitonic polarizations are arbitrarily controlled, in which simultaneous increase (decrease) of the beats in both polarizations are observed. In addition, 80% selective amplification and 40% suppression of one of the excitonic polarizations are realized. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)