bluuebunny/crossref_metadata_2025 huggingface dataset:
Detailed measurements of velocities and suspended sand concentrations over full‐scale ripples in regular oscillatory flow
Authors: van der Werf J. J. , Doucette J. S. , O'Donoghue T. , Ribberink J. S.
Published: May 2007

The knowledge and modeling of wave‐induced sand transport over rippled beds still has significant shortcomings, which is partly related to a lack of measurements of the detailed processes from controlled laboratory experiments. We have carried out new measurements of the detailed time‐dependent velocity and suspended sand concentration field around vortex ripples for regular oscillatory flow conditions. The fact that the ripples were mobile and the flow conditions were full‐scale makes these measurements unique. We made velocity measurements for 14 different flows and concentration measurements for three of these flows. The velocity and concentration field above ripples are dominated by the generation and ejection of vortices on the ripple flanks around the time of flow reversal. Vortex formation results in near‐ripple flow reversals ahead of free‐stream reversals and velocity maxima near the ripple crest that are much higher than the free‐stream maxima. Asymmetry in the free stream produces steady circulation cells with dominant offshore mean flow up the ripple lee slope, balanced by weaker onshore streaming up the ripple stoss slope as well as higher up in the flow. The time‐ and bed‐averaged horizontal velocity profile comprises an offshore streaming near the bed and an onshore drift higher up in the flow. The vortices are responsible for three main concentration peaks: one just after on‐offshore flow reversal associated with the passage of a sand‐laden vortex followed by two smaller peaks due to advected suspension clouds generated by vortex action at the neighboring onshore ripples. The sand flux field measured for one typical asymmetric flow condition is dominated by an offshore flux associated with the suspended sand cloud generated by vortex shedding from the ripple's lee slope around the time of on‐offshore flow reversal. The net (time‐averaged) current‐related and wave‐related horizontal sand fluxes are generally offshore directed and mostly contained within 1.5 ripple heights above the ripple crest. The wave‐related suspended transport component is larger, but the contribution of the current‐related suspended sand transport cannot be neglected. In addition to the measured offshore net transport of suspended sand, there is an onshore‐directed transport very close to the ripple surface. The total net transport is in the offshore direction for this specific asymmetric flow condition.
Conservation of soil organic matter through cryoturbation in arctic soils in Siberia
Authors: Kaiser Christina , Meyer Hildegard , Biasi Christina , Rusalimova Olga , Barsukov Pavel , Richter Andreas
Published: May 2007

Cryoturbation (mixing of soil layers due to repeated freeze‐thaw processes) is a major soil forming process in arctic regions, which may contribute to long‐term storage of C in soils of northern latitudes. Our goal was to determine the effect of subduction of organic matter by cryoturbation on microbial decomposition processes in tundra soils. Buried layers were situated at 30–60 cm depth, between Bg and B horizons, but exhibited a C and N content highly similar to present‐day A horizons. Radiocarbon dating revealed, however, that the mean age of C in the buried layer was three times higher (∼1300 years BP) than in the A horizon (∼400 years BP), suggesting that decomposition rates in the buried layer were delayed. The observed microbial processes support this result: gross C and N mineralization rates were substantially lower in the buried layers than in the respective A horizons. The amount of C stored in the buried layer still doubles the amount of C stored in topsoil horizons (O and A). Assuming that the buried layer originates from both O and A horizons, this indicates that O and A horizon at time of burying (800–1300 years BP) must have been significantly thicker and present‐day O and A horizon at this site may still have the capacity to accumulate additional C. Cryoturbation therefore may lead to additional long‐term storage of carbon in the system by (1) retarding decomposition processes of buried organic material and (2) enabling the soil to restart C accumulation in topsoil layers.
Dynamics of organic and inorganic carbon across contiguous mangrove and seagrass systems (Gazi Bay, Kenya)
Authors: Bouillon Steven , Dehairs Frank , Velimirov Branko , Abril Gwenaël , Borges Alberto Vieira
Published: May 2007

We report on the water column biogeochemistry in adjacent mangrove and seagrass systems in Gazi Bay (Kenya), with a focus on assessing the sources and cycling of organic and inorganic carbon. Mangrove and seagrass‐derived material was found to be the dominant organic carbon sources in the water column, and could be distinguished on the basis of their δ13C signatures and particulate organic carbon:total suspended matter (POC/TSM) ratios. Spatially, a distinct boundary existed whereby the dominance of mangrove‐derived material decreased sharply close to the interface between the mangrove forest and the dense seagrass beds. The latter is consistent with the reported export of mangrove‐derived material, which is efficiently trapped in the adjacent seagrass beds. There were significant net inputs of POC and dissolved organic carbon (DOC) along the Kidogoweni salinity gradient, for which the δ13CPOC signatures were consistent with those of mangroves. DOC was the dominant form of organic carbon in both mangrove and seagrass beds, with DOC/POC ratios typically between 3 and 15. Dynamics of dissolved inorganic carbon in the creeks were strongly influenced by diagenetic C degradation in the intertidal mangrove areas, resulting in significant CO2 emission from the water column to the atmosphere. Although highest partial pressure of CO2 (pCO2) values and areal CO2 flux rates were observed in the mangrove creeks, and the water column above the seagrass beds was in some locations a net sink of CO2, most of the ecosystems' emission of CO2 to the atmosphere occurred in the seagrass beds adjacent to the mangrove forest. The presence of dense seagrass beds thus had a strong effect on the aquatic biogeochemistry, and resulted in trapping and further mineralization of mangrove‐derived POC, intense O2 production and CO2 uptake. The adjacent seagrass beds provide a large area with conditions favorable to exchange of CO2 with the atmosphere, thereby limiting export of mangrove‐derived organic and inorganic carbon toward the coastal ocean.
Does the polar cap area saturate?
Authors: Merkin V. G. , Goodrich C. C.
Published: May 2007

We address the question of how the polar cap area (APC) and the open magnetic flux in the Earth's ionospheric polar caps depend on the strength of the interplanetary magnetic field (IMF) under conditions of steady driving. We use the Lyon‐Fedder‐Mobarry (LFM) global magnetohydrodynamic (MHD) model to analyze the relationship between these quantities and compare their behavior to that of the transpolar potential (ΦPC). In a series of idealized simulation runs we find that in the LFM model APC saturates faster than ΦPC as the IMF strength increases. The ionospheric conductance and the solar wind ram pressure have similar moderate effects on the saturated APC, while their influence on ΦPC is very different. It appears that APC saturates as a result of bulging of the geomagnetic lobes toward the sun, whereby the inner dipole magnetic field is shielded from further reconnection. The process is accompanied by dramatic changes in the global configuration of the magnetic field.
Agonist‐dependent consequences of proline to alanine substitution in the transmembrane helices of the calcitonin receptor
Authors: Bailey R J , Hay D L
Published: May 2007

Background and purpose:

Transmembrane proline (P) residues in family A G protein‐coupled receptors (GPCRs) form functionally important kinks in their helices. These residues are little studied in family B GPCRs but experiments with the VPAC1 receptor and calcitonin receptor‐like receptor (CL) show parallels with family A receptors. We sought to determine the function of these residues in the insert negative form of the human calcitonin receptor, a close relative of CL.

Experimental approach:

Proline residues within the transmembrane domains of the calcitonin receptor (P246, P249, P280, P326, P336) were individually mutated to alanine (A) using site‐directed mutagenesis. Receptors were transiently transfected into Cos‐7 cells using polyethylenimine and salmon and human calcitonin‐induced cAMP responses measured. Salmon and human calcitonin competition binding experiments were also performed and receptor cell‐surface expression assessed by whole cell ELISA.

Key results:

P246A, P249A and P280A were wild‐type in terms of human calcitonin‐induced cAMP activation. P326A and P336A had reduced function (165 and 12‐fold, respectively). In membranes, human calcitonin binding was not detectable for any mutant receptor but in whole cells, binding was detected for all mutants apart from P326A. Salmon calcitonin activated mutant and wild‐type receptors equally, although Bmax values were reduced for all mutants apart from P326A.

Conclusions and Implications:

P326 and P336 are important for the function of human calcitonin receptors and are likely to be involved in generating receptor conformations appropriate for agonist binding and receptor activation. However, agonist‐specific effects were observed, implying distinct conformations of the human calcitonin receptor.

British Journal of Pharmacology (2007) 151, 678–687; doi:10.1038/sj.bjp.0707246
Applications of Hydrogen Peroxide and Derivatives
Authors: Jones C W
Published: May 2007

Simple, but beautifully versatile. Perhaps not a description many would choose for hydrogen peroxide, but an accurate one none the less, and this unique book explains the reasons behind the description. Beginning with an historical overview, and guidelines for the safe handling of peroxygens, Applications of Hydrogen Peroxide and Derivatives goes on to cover key activation mechanisms, organic functional group oxidations and the use of hydrogen peroxide with heterogeneous catalysts. The clean-up of environmental pollutants; chemical purification; and extraction of metals from their ores are also discussed in detail, using actual examples from industry. The versatility of this reagent may well prove to be a key to integrated pollution control in the future. This book should therefore be read by academics and industrialists at all levels, to encourage wider applications of the use of hydrogen peroxide in laboratories.
Lasers and Current Optical Techniques in Biology
Authors: King Terry A , Luthy Willy , Weber Heinz , Unger Peter , Zellmer Holger , Tunnermann Andreas , Diffey Brian L , Diehl Roland , Nisoli Mauro , Bottiroli Giovanni , Croce Anna C , Gratton Enrico , Fantini Sergio , Taroni Paola , Valentini Gianluca , Marcu Laura , Schneckenburger Herbert , Fusi Franco , Monici Monica , Agati Giovanni , Ascoli Cesare , Gottardi Riccardo , Petracchi Donatella , Diaspro Alberto , Sampson David D , Jacques Steven L , Ramella-Roman Jessica C , Seeger Stefan , Hillman Timothy R ,   European Society Photobiology
Published: May 2007

The introduction of innovative light sources, fibre laser sources and light emitting diodes, is opening unexpected perspectives into optical techniques and is promising new exciting applications in the field of biomedicine. Lasers and Current Optical Techniques in Biology aims to provide an overview of light sources, together with an extensive and authoritative description of the optical techniques in bio-medicine. This book is designed to give biomedical researchers a strong feel for the capability of physical approaches, promote new interdisciplinary interests and persuade more practitioners to take advantage of optical techniques. Current developments in a variety of optical techniques, including Near-Infra Red Spectroscopy, and traditional and advanced fluorescence techniques are covered, ranging from those that are becoming common practice to those that need much more experimentation before they can be accepted as real breakthroughs. Further topics include optical coherence tomography and its variations, polarised light imaging and, principle laser and lamp sources- a usually fragmentary topic, often dispersed among specialist publications. The wide range of topics covered make Lasers and Current Optical Techniques in Biology of interest to a diverse range of scientific communities.
Packed Column SFC
Authors: Berger T A
Published: May 2007

Packed Column SFC is the third title to be published in this series and has been produced as a result of the dramatic re-emergence, in the last three years, of packed column instrumentation. This has led to a redefinition of the technique and an urgent need for a practical guide that deals with its subtleties. This book fulfils that need and deals exclusively with packed column SFC. It places the emphasis on understanding the underlying chemistry in order to perform rapid, systematic optimizations and provides many practical tips to help the new user avoid problems unique to SFC. It also proposes a detailed scheme for method development and provides lists of prioritized guidelines. The book clears up some of the confusion that surrounds the analytical use of supercritical fluids and assists the user in understanding the power and utility of this technique. Detailed chapters cover the most promising new application areas for packed column SFC, which are often overlooked in the mainstream chromatography literature. Like the other books in this popular series, Packed Column SFC will prove an invaluable guide and is essential reading for graduates, postgraduates and researchers with interests in pharmaceuticals, agricultural chemistry, small polar drug molecules, chiral analysis, environmental chemistry, and chromatography/instrumentation.
The Chemistry of Medical and Dental Materials
Authors: Nicholson John W
Published: May 2007

Implants into the human body, such as hip joints, heart valves and dental crowns, have been increasingly used over the last 40 years or so, and many patients have benefited from their use. But how much is known about the metals, ceramics and polymers that are used in these repairs? This book provides a state-of-the-art account of the chemistry of the synthetic materials used in medicine and dentistry. It looks at the properties and interactions of these materials within the body at a molecular level, and includes discussion of bioengineering and cell biology. In addition, there is an account of the surgical procedures used, as well as extensive coverage of the possible biological reactions to the presence of foreign materials in the body. A brief look at the emerging field of tissue engineering completes the text. Fully referenced, with detailed reviews of the current literature, The Chemistry of Medical and Dental Materials will be an essential starting-point for all those in academia and industry who are involved in the development of new and improved repair materials.
The Maillard Reaction
Authors: Fayle Sian E , Gerrard Juliet A.
Published: May 2007

It is almost 100 years since the Maillard reaction was first described. Despite decades of research since then, the products of the reaction and the mechanistic pathways leading to their formation are only gradually being unravelled. Combining comprehensive information regarding the various methods that can be employed in the analysis of Maillard products with a discussion of the advantages and limitations of those methods, this unique book provides a 'one-stop' text from which methods of analysis may be obtained. It will be useful for both new and experienced researchers who are involved in solving the mysteries and complexities of Maillard chemistry.
Fine‐Scale Spatial Genetic Structure of the Distylous Primula veris in Fragmented Habitats
Authors: van Rossum F. , Triest L.
Published: May 2007

Abstract: In Flanders (northern Belgium), the distylous self‐incompatible perennial herb Primula veris is common, but mainly occurs in fragmented habitats. Distyly, which favours disassortative mating, is characterized in P. veris by two genetically determined floral morph types (pin or thrum). Using 18 polymorphic loci, we investigated fine‐scale spatial genetic structure (SGS) and spatial distribution of the morphs within four populations from two regions that differ in degree of habitat fragmentation. We studied the contributions made by sexual reproduction and clonal propagation and compared the SGS patterns between pin and thrum morph types. Clonal growth was very restricted to a few individuals and to short distances. One population showed a non‐random spatial distribution of the morphs. Pin and thrum individuals differed in SGS patterns at a small scale, suggesting intrapin biparental inbreeding, also related to high plant densities. This may be explained by partial self‐compatibility of the pin morph combined with restricted seed dispersal and pollinator behaviour. There is an indication of more pronounced SGS when populations occur in highly fragmented habitats. From our findings, we may hypothesize disruption of the gene flow processes if these large populations evolve into patchworks of small remnants, but also a possible risk for long‐term population survival if higher intrapin biparental inbreeding leads to inbreeding depression. Our study emphasizes the need for investigating the interactions between the heterostylous breeding system, population demographic and genetic structure for understanding population dynamics in fragmented habitats and for developing sustainable conservation strategies.
The Binding of Nuclear Factors to the as‐1 Element in the CaMV 35S Promoter is Affected by Cytosine Methylation in Vitro
Authors: Kanazawa A. , O'Dell M. , Hellens R. P.
Published: May 2007

Abstract: Transcriptional gene silencing (TGS) is often associated with an increased level of cytosine methylation in the affected promoters. The effect of methylation of the cauliflower mosaic virus (CaMV) 35S promoter sequence on its binding to factors present in the nuclei was analyzed by electrophoretic mobility shift assays using extracts of petunia flowers. Specific DNA‐protein interactions were detected in the region of the CaMV 35S promoter that contains the as‐1 element and the region between ‐ 345 and ‐ 208. The binding of protein factor(s) to the as‐1 element was influenced by cytosine methylation, whereas the binding to the region between ‐ 345 and ‐ 208 was unaffected. The results suggest that cytosine methylation of the as‐1 element potentially affects the activity of the CaMV 35S promoter.
Fine Structure and Phylogeny of Green Algal Photobionts in the Microfilamentous Genus Psoroglaena (Verrucariaceae, Lichen‐Forming Ascomycetes)
Authors: Nyati S. , Beck A. , Honegger R.
Published: May 2007

Abstract: According to the literature the microfilamentous thalli of lichen‐forming ascomycetes of the genus Psoroglaena are assumed to harbour vivid green “prochlorophyte” cyanobacterial photobionts. As this would be the first report of terrestrial “prochlorophytes” we investigated the fine structure and two molecular markers (SSU rDNA and rbcL) of the photobionts of P. stigonemoides (Orange) Henssen and P. epiphylla Lücking. Both Psoroglaena spp. had unicellular green algal photobionts, representatives of the Trebouxiophyceae. The photobiont of P. stigonemoides is closely related to the non‐symbiotic Auxenochlorella protothecoides and to a Chlorella endosymbiont of the freshwater polyp Hydra viridis. The putative photobiont of P. epiphylla may be related to Chlorella luteoviridis, C. saccharophila, and a Pseudochlorella isolate. In contrast to other microfilamentous lichens, which derive their shape from filamentous green algae or cyanobacterial colonies overgrown and ensheathed by the fungal partner, Psoroglaena mycobionts position their unicellular photobiont in uni‐ or multiseriate rows which strongly resemble the situation in filamentous cyanobacterial colonies.
Investigating Cytoskeletal Function in Chloroplast Protrusion Formation in the Arctic‐Alpine Plant Oxyria digyna
Authors: Holzinger A. , Wasteneys G. O. , Lütz C.
Published: May 2007

Abstract: Arctic and alpine plants like Oxyria digyna have to face enhanced environmental stress. This study compared leaves from Oxyria digyna collected in the Arctic at Svalbard (78°N) and in the Austrian Alps (47°N) at cellular, subcellular, and ultrastructural levels. Oxyria digyna plants collected in Svalbard had significantly thicker leaves than the samples collected in the Austrian Alps. This difference was generated by increased thickness of the palisade and spongy mesophyll layers in the arctic plants, while epidermal cells had no significant size differences between the two habitats. A characteristic feature of arctic, alpine, and cultivated samples was the occurrence of broad stroma‐filled chloroplast protrusions, 2 ‐ 5 μm broad and up to 5 μm long. Chloroplast protrusions were in close spatial contact with other organelles including mitochondria and microbodies. Mitochondria were also present in invaginations of the chloroplasts. A dense network of cortical microtubules found in the mesophyll cells suggested a potential role for microtubules in the formation and function of chloroplast protrusions. No direct interactions between microtubules and chloroplasts, however, were observed and disruption of the microtubule arrays with the anti‐microtubule agent oryzalin at 5 ‐ 10 μM did not alter the appearance or dynamics of chloroplast protrusions. These observations suggest that, in contrast to studies on stromule formation in Nicotiana, microtubules are not involved in the formation and morphology of chloroplast protrusions in Oxyria digyna. The actin microfilament‐disrupting drug latrunculin B (5 ‐ 10 μM for 2 h) arrested cytoplasmic streaming and altered the cytoplasmic integrity of mesophyll cells. However, at the ultrastructural level, stroma‐containing, thylakoid‐free areas were still visible, mostly at the concave sides of the chloroplasts. As chloroplast protrusions were frequently found to be mitochondria‐associated in Oxyria digyna, a role in metabolite exchange is possible, which may contribute to an adaptation to alpine and arctic conditions.
Excessive Variation in Y Chromosomal DNA in Rumex acetosa (Polygonaceae)
Authors: Korpelainen H. , Kostamo K.
Published: May 2007

Abstract: Rumex acetosa is one of the few angiosperms that possesses sex chromosomes. The same types of abundant repetitive sequences cover both heterochromatic Y chromosomes present in males. The aim of this study was to investigate genetic variation in paternally inherited Y chromosomal DNA and in maternally inherited cpDNA, and to find out whether the examined genomic regions are suited to a phylogeographic study in R. acetosa. DNA sequence polymorphisms present in the 850‐bp heterochromatic segment on the Y chromosomes were compared to variation in the 409‐bp long chloroplast section (trnL‐trnF spacer) in R. acetosa originating from several European locations and from the Altai mountains in Russia. A great amount of genetic variation was detected within the Y chromosomal region while only four chloroplast genotypes were detected. Although the chloroplast haplotypes possessed some geographic pattern, no clear phylogeographic pattern was detected based on the variable Y chromosomes. The mean Y chromosomal nucleotide diversity among all samples equaled 6.6 %, and the mean proportion of polymorphic sites per individual equaled 8.2 % among SNP sites and 1.7 % among all sites investigated. The high number of substitutions detected in the Y chromosomal DNA shows that this heterochromatic sequence has a high mutation rate. The diversity pattern indicates that gene flow via pollen is extensive and it blurs any geographical pattern in the Y chromosomal variation. The high number of repeats and uncertainty concerning the extent of recombination between the two Y chromosomes impair the usability of the Y chromosomal segment for phylogeographic or population genetic studies.
Involvement of the Phosphoinositide Signaling Pathway in the Anti‐Senescence Effect of Low‐Concentration Stressors on Detached Barley Leaves
Authors: Nyitrai P. , Mayer M. , Óvári M. , Keresztes Á.
Published: May 2007

Abstract: The effect of low concentration of some stress‐inducing compounds of different toxicity and chemical nature like Pb and Ti salts or DCMU on the senescence of chloroplasts was investigated in detached primary leaves of barley (Hordeum vulgare cv. Omega). These agents stimulated chlorophyll accumulation, photosynthetic activity (14CO2 fixation), and decreased the number of plastoglobuli in chloroplasts compared to the control, thus delaying senescence. Low‐concentration stressors did not increase the level of active cytokinins of leaves during the treatment. Lithium and stearoylcarnitine chloride inhibited the stimulating effect of stressors. This points to the involvement of the PIP2‐IP3/DAG signal transduction pathway in generation of the specific responses.
Determination of stacking fault probability in fcc Fe–Mn–Si–Al alloy by electron diffraction
Authors: Wang X. D. , Huang B. X. , Rong Y. H. , Wang L.
Published: May 2007

The stacking disorder in the fcc structure of Fe–23Mn–2Si–2Al alloy after tensile testing at −75°C was investigated by electron diffraction, using a relationship between the stacking fault probability and the shift of diffraction spots deduced in the present work (and based on Kakinoki’s theory [Acta Crystallogr. 23, 875 (1967)] and Kajiwara’s previous work [Jpn. J. Appl. Phys. 9, 385 (1970); J. Phys. Soc. Jpn. 22, 795 (1967)]). Shifts of diffraction spots along the [111] direction in the fcc structure were observed, and the stacking fault probabilities in two selected areas with different densities of stacking faults were determined as α=0.15 and α=0.35. The stacking fault probabilities measured by electron diffraction are much larger than average values determined by x-ray diffraction, indicating that the distribution of stacking faults is localized. A mechanism for the γfcc→εhcp transformation is suggested, whereby during deformation the following sequences take place: dissociation of perfect dislocation→localization of stacking faults→evolution from stacking disorder to stacking order→the formation of the perfect hcp martensite (α=1) or the hcp martensite with stacking faults (α→1). This proposed mechanism for strain-induced hcp martensite formation should be also applicable to the thermally induced hcp martensite.
Spin polarization of Ni2MnIn and Ni80Fe20 determined by point-contact Andreev spectroscopy
Authors: Bocklage Lars , Scholtyssek Jan M. , Merkt Ulrich , Meier Guido
Published: May 2007

We present point-contact Andreev reflection (PCAR) spectroscopy on Permalloy (Ni80Fe20) and on the half-metallic Heusler alloy Ni2MnIn. The thin Permalloy films are deposited on Si, the Ni2MnIn films on Si as well as on in situ cleaved (110) surfaces of InAs. A highly conductive layer under the ferromagnetic film almost eliminates a series resistance and thus facilitates the determination of the spin polarization from the differential conductance curves. We obtain a spin polarization of Permalloy of 35%. The spin polarization of Ni2MnIn depends on the substrate, presumably due to the growth of different crystal structures. It is shown that the surface sensitive PCAR spectroscopy cannot determine the spin polarization of the bulk material of half-metals where the degree of spin polarization strongly depends on the crystal structure.
Ferromagnetic nanotubes by atomic layer deposition in anodic alumina membranes
Authors: Daub M. , Knez M. , Goesele U. , Nielsch K.
Published: May 2007

In this paper, two methods for the synthesis of magnetic nanotubes inside the pores of anodic alumina membranes by atomic layer deposition (ALD) are compared. The precursors were nickelocene or cobaltocene, and H2O or O3. The first method consists of a three-step ALD cycle: First, the sample is exposed to the metal-organic precursor, subsequently to water, and finally, to hydrogen. In the second method, metal oxide is deposited by a conventional two-step ALD cycle. After the ALD process, the sample is reduced under hydrogen atmosphere. The magnetic nanotubes obtained by the second method have a smaller grain size and improved magnetic properties. The magnetic nanotubes with diameters ranging from 35to60nm exhibit a preferential magnetization direction along the nanowire axis. The Ni or Co nanotubes with larger diameters (around 160nm) show a nearly isotropic magnetic behavior, with the magnetic moments arranged in a vortex state at zero field.
Microwave absorption of patterned arrays of nanosized magnetic stripes with different aspect ratios
Authors: Malkinski Leszek M. , Yu Minghui , Vovk Andriy Y. , Scherer Donald J. , Spinu Leonard , Zhou Weilie , Whittenburg Scott , Davis Zachary , Jung Jin-Seung
Published: May 2007

Arrays consisting of nanosized stripes of Permalloy with different length-to-width ratios have been fabricated using electron beam nanolithography, magnetron sputtering, and lift-off process. These stripes have a thickness of 100nm, a width of 300nm, and different lengths ranging from 300nmto100μm. The stripes are separated by a distance of 1μm. Magnetization hysteresis loops were measured using a superconducting quantum interference device susceptometer. Microwave absorption at 9.8GHz was determined by means of ferromagnetic resonance technique. The dependence of the resonant field on the angle between the nanostructure and the in-plane dc magnetic field indicates the presence of uniaxial magnetic anisotropy associated with the aspect ratio of the stripes. A maximum change of the resonant field of 1600Oe was observed in the longest stripes, yet it was only 200Oe for square shaped stripes. The linewidth of the resonant curve varied with the angle, in the range from 120to300Oe. Most of the ferromagnetic resonance spectra exhibited multiple resonant peaks due to dimensional confinement of spin waves in the nanosized stripes. The maximum squareness of the magnetization hysteresis loop was for the field applied along the stripes, but the coercivity did not have a monotonic angular dependence as expected from the Stoner-Wohlfarth model for coherent magnetization rotation of the systems with uniaxial anisotropy.
Pattern formation in type-I superconducting films
Authors: Jeudy V. , Gourdon C. , Cēbers A. , Okada T.
Published: May 2007

We report on extensive experimental and theoretical studies of domain patterns in type-I superconductors. Both superconducting (SC) and normal state (NS) domains are analyzed using magneto-optical imaging. We measure the average width (diameter) of lamellar (circular) domains. The experimental values and predicted equilibrium values are shown to present a significantly better agreement when the screening of the magnetic flux is taken into account. On this basis, we analyze the contribution of the elongation instability of circular (NS and SC) domains on the formation of the intermediate state. The most surprising and specific aspects of the formation of domains are shown to originate from the perfect diamagnetism of the superconducting phase.
Perpendicular interlayer coupling through oscillatory Ruderman-Kittel-Kasuya-Yosida interaction between Co∕Pt multilayers and Co∕TbCo bilayers
Authors: Lin Meng-Shian , Lai Chih-Huang
Published: May 2007

The oscillatory perpendicular interlayer-coupling field and strength as a function of Ru spacer thickness were studied for samples of the [Co∕Pt]5∕Co∕Ru∕Co∕TbCo structure. For Ru thickness in the range of 0.5–1nm, a strong antiferromagnetic (AF) coupling (J∼0.38erg∕cm2) was observed. When AF coupling existed, the coercivity of Co∕TbCo bilayers was enhanced because the Co∕Pt multilayers stabilized the magnetization direction of Co∕TbCo. Consequently, oscillatory coercivity of Co∕TbCo with Ru thickness was also observed. Since the magnetization direction of the Co sublattice in TbCo can be tuned by adjusting composition or measuring temperature, the direction of the shifted minor loop of Co∕Pt multilayers can be varied accordingly while keeping the same Ru thickness.
Magnetic frustration behavior of ferroelectric ferromagnet YbMnO3 epitaxial films
Authors: Fujimura N. , Takahashi T. , Yoshimura T. , Ashida A.
Published: May 2007

Yb Mn O 3 has an antiferromagnetic (AFM) and field-induced ferromagnetic transitions at 80 and 3K, respectively. It also exhibits a ferroelectric transition at much higher temperature (∼1000K), making it a rare example of systems having both ferroelectric and ferromagnetic transitions. In the present study, magnetic field induced ferromagnetism of an YbMnO3 epitaxial film was recognized at 80K, which is much higher temperature than that of single crystal. The magnetic frustration behaviors were also observed below 87K. Regarding the cross-correlation phenomena, a coupling between the ferroelectric domain switching and the AFM spin ordering is demonstrated using a real multiferroic material, YbMnO3 epitaxial film.
A model of the magnetic properties of coupled ferromagnetic∕antiferromagnetic bilayers
Authors: Choo D , Chantrell R. W. , Lamberton R , Johnston A , O’Grady K
Published: May 2007

A granular level model which is capable of predicting the bulk magnetic properties of coupled ferromagnetic and antiferromagnetic layers is described. The model is used in an extensive investigation of the effect of the thermal instability of the antiferromagnetic layer as a function of the layer thickness, grain diameter, temperature, and the grain size distribution σ. The calculations give good qualitative agreement with experiment and provide an understanding of the role of the antiferromagnetic layer in determining the exchange bias field and the coercivity.
Role of thermodynamic fluctuations in magnetic recording (invited)
Authors: Skomski Ralph
Published: May 2007

The thermal stability of the information stored in magnetic recording media is determined by a complex hierarchy. The leading consideration is the static or zero-temperature magnetization reversal complemented by the intrinsic temperature dependence of the micromagnetic parameters. Thermally activated Arrhenius (or Néel-Brown) processes modify the reversal by realizing paths close to static reversal, whereas “giant fluctuations” corresponding to reversal fields much higher than the nucleation field can safely be excluded. Thermally activated reversal in very thin elongated nanoparticles limits the thermal stability of magnetic recording media but degenerates into coherent rotation as the temperature is lowered, thereby reconciling micromagnetism and thermodynamics. A particularly complicated situation is encountered in alloys, where sublattices containing heavy transition-metal atoms act like earthquakes that modify the energy landscape.
Targets for direct-drive fast ignition at total laser energy of 200–400kJ
Authors: Atzeni Stefano , Schiavi Angelo , Bellei Claudio
Published: May 2007

Basic issues for the design of moderate-gain fast ignition targets at total laser energy of 200–400kJ (with less than 100kJ for the igniting beams) are discussed by means of a simple integrated gain model. Gain curves are generated and their sensitivity to several parameters is analyzed. A family of scaled target is designed, based on 1D hydrodynamic simulations of the implosion stage and 2D model simulations of ignition and burn. It is found that ignition and propagating burn can be achieved by targets compressed by 100–150kJ, properly shaped laser pulses (with wavelength λc=0.35μm), and ignited by 80–100kJ pulses. This requires adiabat shaped implosions to limit Rayleigh-Taylor instability, at the same time keeping the fuel entropy at a very low level. In addition, the igniting beam should be coupled to the fuel with an efficiency of about 25%, and the hot-electron average penetration depth should be at most 1.2–1.5g∕cm2. According to the present understanding of ultraintense laser-matter interaction, this limits the wavelength of the ignition beam to λig≤0.5μm. With the same assumptions, energy gain G=100 can be achieved by targets driven by a 250kJ compression laser pulse and an 80–100kJ ignition pulse.
Stark effect and oscillator strength in a Si1−xGex∕Si quantum disk
Authors: Liu Jin-Long , Zhu Jing
Published: May 2007

In the framework of the effective mass approximation, we develop a method to study the quantum-confined Stark effect in a Si1−xGex∕Si quantum disk. The hole energy levels are calculated presence of a vertically applied electric field adiabatic approximation using an exact diagonalization technique. The effects of strain, finite offset, and the difference between effective masses of holes in different materials are taken into account. The oscillator strength of the Si1−xGex∕Si quantum disk system under a vertically applied electric field is also studied. Our calculated results are useful for the application of Si1−xGex∕Si quantum disks in photoelectric devices.
Optical nonlinearities in delta-doped AlGaN/GaN quantum well heterostructures
Authors: Saidi I. , Bouzaïene L. , Maaref H. , Mejri H.
Published: May 2007

The linear and nonlinear optical properties such as second-harmonic generation susceptibility, optical absorption and refractive index change associated with intersubband transitions in AlGaN/GaN quantum well heterostructures have been investigated theoretically. The electronic states of these structures are calculated using both the envelope wave function approximation and the compact-density matrix formalism. The confining potential has been assumed to be semiparabolic. The results obtained for typical AlGaN/GaN single-heterostructure field effect transistors reveal that these properties strongly depend on the Al composition and the doping concentration as well. Also calculated is the electric field effect on the susceptibility, the refractive index, and the absorption coefficient. It was thus found that the applied electric field can improve considerably the nonlinear optical behavior of the AlGaN/GaN heterostructures studied.
Analysis of hysteresis behavior of pentacene field effect transistor characteristics with capacitance-voltage and optical second harmonic generation measurements
Authors: Lim Eunju , Manaka Takaaki , Tamura Ryosuke , Iwamoto Mitsumasa
Published: May 2007

Analyzing pentacene field effect transistors (FETs) with Au source and drain electrodes as Maxwell-Wagner effect elements, electron and hole injection from the Au electrodes into the FET channel were examined using capacitance-voltage (C−V) and optical second harmonic generation (SHG) measurements. The C−V characteristics show a hysteresis behavior that depends on gate-source (drain) stress biasing, Vgs(Vgd). Charge carriers forming the conducting channel of pentacene FET are mainly holes injected from Au electrodes. Results suggest that this hysteresis behavior is attributable to carriers trapped in the FET channel injected from the Au electrodes, and that hole injection is suppressed after Vgs<0 stress biasing, whereas it is assisted after Vgs>0. To further clarify the carrier injection mechanism for a different stress biasing condition, the modulation of the electric field along the FET channel by injected carriers was examined using SHG measurements. At the on state, the SHG signal was well diminished due to the hole injection from the Au electrodes, whereas the SHG was enhanced at the off state. However, interestingly, the enhanced SHG at the off state decayed gradually with a relaxation time of 103 s during the Vgs(=Vgd)=+100 V stress biasing. Results suggest that electron injection occurs from the Au source and drain electrodes into the pentacene and that they are subsequently trapped around Au electrodes. Based on these results, a mechanism of the hole injection assisted by trapped electrons in the pentacene is proposed for the C−V hysteresis behavior after stress-biasing Vgs>0.
Spontaneous and induced dynamic correlations in glass formers. II. Model calculations and comparison to numerical simulations
Authors: Berthier L. , Biroli G. , Bouchaud J.-P. , Kob W. , Miyazaki K. , Reichman D. R.
Published: May 2007

We study in detail the predictions of various theoretical approaches, in particular, mode-coupling theory (MCT) and kinetically constrained models (KCMs), concerning the time, temperature, and wave vector dependence of multipoint correlation functions that quantify the strength of both induced and spontaneous dynamical fluctuations. We also discuss the precise predictions of MCT concerning the statistical ensemble and microscopic dynamics dependence of these multipoint correlation functions. These predictions are compared to simulations of model fragile and strong glass-forming liquids. Overall, MCT fares quite well in the fragile case, in particular, explaining the observed crucial role of the statistical ensemble and microscopic dynamics, while MCT predictions do not seem to hold in the strong case. KCMs provide a simplified framework for understanding how these multipoint correlation functions may encode dynamic correlations in glassy materials. However, our analysis highlights important unresolved questions concerning the application of KCMs to supercooled liquids.
Two-electron reduced density matrices from the anti-Hermitian contracted Schrödinger equation: Enhanced energies and properties with larger basis sets
Authors: Mazziotti David A.
Published: May 2007

Two-electron reduced density matrices (2-RDMs) have recently been directly determined from the solution of the anti-Hermitian contracted Schrödinger equation (ACSE) to obtain 95%–100% of the ground-state correlation energy of atoms and molecules, which significantly improves upon the accuracy of the contracted Schrödinger equation (CSE) [D. A. Mazziotti, Phys. Rev. Lett. 97, 143002 (2006)]. Two subsets of the CSE, the ACSE and the contraction of the CSE onto the one-particle space, known as the 1,3-CSE, have two important properties: (i) dependence upon only the 3-RDM and (ii) inclusion of all second-order terms when the 3-RDM is reconstructed as only a first-order functional of the 2-RDM. The error in the 1,3-CSE has an important role as a stopping criterion in solving the ACSE for the 2-RDM. Using a computationally more efficient implementation of the ACSE, the author treats a variety of molecules, including H2O, NH3, HCN, and HO3−, in larger basis sets such as correlation-consistent polarized double- and triple-zeta. The ground-state energy of neon is also calculated in a polarized quadruple-zeta basis set with extrapolation to the complete basis-set limit, and the equilibrium bond length and harmonic frequency of N2 are computed with comparison to experimental values. The author observes that increasing the basis set enhances the ability of the ACSE to capture correlation effects in ground-state energies and properties. In the triple-zeta basis set, for example, the ACSE yields energies and properties that are closer in accuracy to coupled cluster with single, double, and triple excitations than to coupled cluster with single and double excitations. In all basis sets, the computed 2-RDMs very closely satisfy known N-representability conditions.
Lie algebraic noncommuting structures from reparametrization symmetry
Authors: Gangopadhyay Sunandan
Published: May 2007

We extend our earlier work of revealing both space-space and space-time noncommuting structures in various models in particle mechanics exhibiting reparametrization symmetry. We show explicitly (in contrast to the earlier results in our paper [R. Banerjee et al. J. Phys. A 38, 957 (2005), e-print hep-th∕0405178]) that for some special choices of the reparametrization parameter ϵ, one can obtain space-space noncommuting structures which are Lie algebraic in form even in the case of the relativistic free particle. The connection of these structures with the existing models in the literature is also briefly discussed. Further, there exists some values of ϵ for which the noncommutativity in the space-space sector can be made to vanish. As a matter of internal consistency of our approach, we also study the angular momentum algebra in details.
Effect of elastic strain redistribution on electronic band structures of compressively strained GaInAsP/InP membrane quantum wires
Authors: Ferdous F. , Haque A.
Published: May 2007

The effect of redistribution of elastic strain relaxation on the energy band structures of GaInAsP/InP compressively strained membrane quantum wires fabricated by electron-beam lithography, reactive-ion etching and two-step epitaxial growth is theoretically studied using an 8-band k⋅p method. Anisotropic strain analysis by the finite element method shows that due to etching away the top and the bottom InP clad layers in membrane structures, redistribution of strain occurs. It is found that strain redistribution increases the effective bandgap of membrane quantum wire structures causing a blueshift of the emission frequency. Comparison with effective bandgap calculations neglecting confinement and band mixing demonstrates that neglect of these effects leads to an overestimation of the change in the bandgap. We have also investigated the effect of variation of wire width, barrier strain compensation, number of stacked quantum wire layers, and thickness of the top and the bottom residual InP layers in membrane structures on the change in the effective bandgap of membrane structures.
A systematic enumeration of local topological relaxation mechanisms in amorphous networks and their efficiency in network relaxation
Authors: Hudson T. S. , Harrowell P.
Published: May 2007

We consider the infinite hierarchy of local collective rearrangements on bond networks that preserves the valency of each atom and explicitly enumerate those involving 4, 5, and 6 particles. The only 4-particle rearrangement is identical to the Wooten-Winer-Weaire (WWW) mechanism. Each rearrangement mechanism is applied in a Monte Carlo (MC) algorithm in order to determine the rate at which it equilibrate a network and relax the structure at equilibrium. At low temperature the 4-particle mechanism provides the fastest relaxation rate but we find that there is a crossover with increasing temperature to 5-particle mechanisms.
Frequency dependence of effective permittivity of carbon nanotube composites
Authors: Liu L. , Matitsine S. , Gan Y. B. , Chen L. F. , Kong L. B. , Rozanov K. N.
Published: May 2007

Dependence of the permittivity of single-walled and multiwalled carbon nanotube composites on frequency and concentration was investigated experimentally using the coaxial air-line method over 0.1–10 GHz. The results are in good agreement with that obtained using the impedance method. It is found that scaling law based on the percolation theory provides a good description of the frequency dependence of measured permittivity in carbon nanotube composites. Parameters of the scaling law and deviations from the percolation theory are also discussed.
Nanographite domains in carbon nanowalls
Authors: Kobayashi K. , Tanimura M. , Nakai H. , Yoshimura A. , Yoshimura H. , Kojima K. , Tachibana M.
Published: May 2007

The nanostructure of carbon nanowalls (CNWs), composed of a stack of several graphite sheets, was investigated by transmission electron microscopy. Our detailed analysis revealed that numerous graphite regions with an average size of about 20 nm were formed in the CNWs. The formation of these regions originates from the introduction of lattice defects such as dislocations and the slight rotation of the graphite sheets. On this basis, it is concluded that the graphite regions, so-called “nanographite domains,” are the constitutional units of the CNWs.