bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation into magma degassing at Nyiragongo volcano, Democratic Republic of the Congo
Authors: Sawyer G. M. , Carn S. A. , Tsanev V. I. , Oppenheimer C. , Burton M.
Published: February 2008

We report the first combined measurements of the composition and flux of gas emitted from Nyiragongo volcano by ground‐based remote‐sensing techniques. Ultraviolet spectroscopic measurements made in May/June 2005 and January 2006 indicate average SO2 emission rates of 38 kg s−1 and 23 kg s−1, respectively. Open‐path Fourier transform infrared spectroscopic measurements obtained in May/June 2005, January 2006, and June 2007 indicate average molar proportions of 70, 24, 4.6, 0.87, 0.26, 0.11, and 0.0016% for H2O, CO2, SO2, CO, HCl, HF, and OCS, respectively. The composition of the plume was remarkably similar in 2005, 2006, and 2007, with little temporal variation in proportions of CO2, SO2, and CO, in particular, on the scale of seconds or days or even between the three field campaigns that span a period of 24 months. This stability persisted despite a wide range of degassing behaviors on the surface of the summit crater's lava lake (including discrete strombolian bursts and lava fountains) and variations in the SO2 emission rate. We explain these observations by a regime of steady state degassing in which bubbles nucleate and ascend in chemical equilibrium with the convecting magma. Short‐term (seconds to minutes) temporal fluctuations in the SO2–HCl–HF composition were observed, and these are attributed to shallow degassing processes.
Teleseismic surface wave tomography in the western U.S. using the Transportable Array component of USArray
Authors: Yang Yingjie , Ritzwoller Michael H.
Published: February 2008

Fundamental mode Rayleigh waves recorded by the Transportable Array component of EarthScope/USArray from January 2006 through April 2007 are used to generate phase velocity maps at periods from 25 to 100 sec across the western U.S., including Washington, Oregon, California, Nevada and western Idaho. At short periods (25–33 s), low velocity anomalies are observed in western Washington, western and central Oregon, northern California, the southern Sierra Nevada and the Snake River Plain. At intermediate and long periods (50–100 s), high velocities are seen in the Cascades, the southern Central Valley of California, California's Transverse Range, and the Columbia River Flood Basalt Province. The phase velocity maps are consistent with those obtained from ambient noise tomography at comparable periods. Short period phase velocities from ambient noise tomography and the longer‐period phase velocities from teleseismic tomography, therefore, present natural data sets to invert jointly 3‐D structure across the western U.S.
Magma compressibility and the missing source for some dike intrusions
Authors: Rivalta Eleonora , Segall Paul
Published: February 2008

Dike intrusions are often accompanied by localized deflation, interpreted as depressurizing magma chambers feeding the dike. In some cases the inferred volume decrease is a factor of 4 or 5 less than the volume increase of the dike. Here we explore whether this discrepancy can be explained by compressibility of the magma combined with the fact that cracks are much more compliant than equidimensional magma chambers. If pressure changes are small, the magma compressibility βm is constant, and the dike ends up in hydrostatic equilibrium with an ellipsoidal magma chamber at the same depth, the ratio rV of the volume of the crack to the volume lost by the chamber is rV = 1 + 4μβm/3 > 1, where μ is the host rock rigidity. For gas poor magmas, βm = 0.6–2 · 10−10 Pa−1 and μ = 3–25 GPa, we find 1.2 < rV < 7.7. Large changes in magma compressibility due to gas exsolution increase rV.
Large‐scale structure of Saturn's E‐ring
Authors: Horányi Mihály , Juhász Antal , Morfill Gregor E.
Published: February 2008

Geysers on the recently discovered, geologically active south‐polar region of the moon Enceladus are now recognized as the dominant source of material in Saturn's E ring. The ring was traditionally thought to span the region between 3 to 8 RS, where RS is the radius of Saturn. However, new in situ dust measurements indicate that the density of small grains might continuously extend far beyond these boundaries, and the E ring could reach even beyond the orbit of Titan (20.3 RS). We report on the modeling results of the long‐term evolution of dust particles comprising the E ring to show that grains from Enceladus could indeed reach the outskirts of Saturn's magnetosphere.
Excess ejecta craters record episodic ice‐rich layers at middle latitudes on Mars
Authors: Black Benjamin A. , Stewart Sarah T.
Published: February 2008

We infer Amazonian climate change events from the presence of fresh craters with excess volumes of ejecta. Using the Mars Orbiter Laser Altimeter data, the geometrical properties of 572 fresh impact craters with rim diameters between 2.5 and 102 km were compiled in lowland and highland plains. The data reveal a class of fresh craters with anomalously high ejecta volumes preferentially located in Utopia Planitia. These fresh, “excess ejecta” craters have volumes of material above the preimpact surface larger than the crater cavity volumes by factors of 2.5 to 5.8. The excess volume corresponds to an excess thickness of ejecta of about 20 to 100 m averaged over the continuous ejecta blanket. The excess material cannot be accommodated by ejecta bulking alone and requires an external process to increase the apparent volume of the ejecta and/or the uplifted surface. On the basis of the geologic setting, ejecta morphology, and calculations of increased ice stability with burial, we conclude that the most likely origin of the excess ejecta volume is the presence of an ice‐rich layer tens of meters thick at the time of impact. The icy layer or a lag deposit is partially preserved beneath the ejecta blanket today. In this scenario, the icy layer has since been removed from areas unprotected by ejecta blankets, creating an apparent preimpact surface lower than the original elevation. The statistical occurrence of excess ejecta craters is consistent with climate model predictions of recent glacial periods on Mars.
Effects of low‐velocity fault zones on dynamic ruptures with nonelastic off‐fault response
Authors: Duan Benchun
Published: February 2008

Using a finite element method for elastoplastic dynamic analysis, we examine the effects of a low‐velocity fault zone (LVFZ) surrounding a fault on a spontaneous dynamic earthquake rupture. A Mohr‐Coulomb failure criterion is used to characterize nonelastic off‐fault response to earthquake ruptures. We find that the presence of a LVFZ alters the distribution of off‐fault plastic strain and results in larger final slip and larger peak slip velocities on the fault. These effects show a LVFZ‐width dependency. A supershear (relative to the shear wave velocity of country rocks) rupture transition is observed with a LVFZ width of 400 m or larger (up to 2 km). The presence of a LVFZ increases the amplitude of ground shaking at stations within the LVFZ and induces high‐frequency oscillations at stations within and near the LVFZ.
Open Science: Tools, approaches, and implications
Authors: Wu Shirley , Neylon Cameron
Published: February 2008

The Pacific Symposium on Biocomputing is an annual meeting whose topics are determined by proposals submitted by members of the community. This document is the proposal for a session on Open Science, submitted for consideration for the PSB meeting in 2009.
Mammalian Notum induces the release of glypicans and other GPI-anchored proteins from the cell surface
Authors: Traister Alexandra , Shi Wen , Filmus Jorge
Published: February 2008

Glypicans are heparan sulfate proteoglycans that are attached to the cell surface by a GPI (glycosylphosphatidylinositol) anchor. Glypicans regulate the activity of Wnts, Hedgehogs, bone morphogenetic proteins and fibroblast growth factors. In the particular case of Wnts, it has been proposed that GPI-anchored glypicans stimulate Wnt signalling by facilitating and/or stabilizing the interaction between Wnts and their cell surface receptors. On the other hand, when glypicans are secreted to the extracellular environment, they can act as competitive inhibitors of Wnt. Genetic screens in Drosophila have recently identified a novel inhibitor of Wnt signalling named Notum. The Wnt-inhibiting activity of Notum was associated with its ability to release Dlp [Dally (Division abnormally delayed)-like protein; a Drosophila glypican] from the cell surface by cleaving the GPI anchor. Because these studies showed that the other Drosophila glypican Dally was not released from the cell surface by Notum, it remains unclear whether this enzyme is able to cleave glypicans from mammalian cells. Furthermore, it is also not known whether Notum cleaves GPI-anchored proteins that are not members of the glypican family. Here, we show that mammalian Notum can cleave several mammalian glypicans. Moreover, we demonstrate that Notum is able to release GPI-anchored proteins other than glypicans. Another important finding of the present study is that, unlike GPI-phospholipase D, the other mammalian enzyme that cleaves GPI-anchored proteins, Notum is active in the extracellular environment. Finally, by using a cellular system in which GPC3 (glypican-3) stimulates Wnt signalling, we show that Notum can act as a negative regulator of this growth factor.
Acyl-CoA-binding protein (ACBP) localizes to the endoplasmic reticulum and Golgi in a ligand-dependent manner in mammalian cells
Authors: Hansen Jesper S. , Færgeman Nils J. , Kragelund Birthe B. , Knudsen Jens
Published: February 2008

In the present study, we microinjected fluorescently labelled liver bovine ACBP (acyl-CoA-binding protein) [FACI-50 (fluorescent acyl-CoA indicator-50)] into HeLa and BMGE (bovine mammary gland epithelial) cell lines to characterize the localization and dynamics of ACBP in living cells. Results showed that ACBP targeted to the ER (endoplasmic reticulum) and Golgi in a ligand-binding-dependent manner. A variant Y28F/K32A-FACI-50, which is unable to bind acyl-CoA, did no longer show association with the ER and became segregated from the Golgi, as analysed by intensity correlation calculations. Depletion of fatty acids from cells by addition of FAFBSA (fatty-acid-free BSA) significantly decreased FACI-50 association with the Golgi, whereas fatty acid overloading increased Golgi association, strongly supporting that ACBP associates with the Golgi in a ligand-dependent manner. FRAP (fluorescence recovery after photobleaching) showed that the fatty-acid-induced targeting of FACI-50 to the Golgi resulted in a 5-fold reduction in FACI-50 mobility. We suggest that ACBP is targeted to the ER and Golgi in a ligand-binding-dependent manner in living cells and propose that ACBP may be involved in vesicular trafficking.
Protein-tyrosine kinase CAKβ/PYK2 is activated by binding Ca2+/calmodulin to FERM F2 α2 helix and thus forming its dimer
Authors: Kohno Takayuki , Matsuda Eiko , Sasaki Hiroko , Sasaki Terukatsu
Published: February 2008

CAKβ (cell adhesion kinase β)/PYK2 (proline-rich tyrosine kinase 2) is the second protein-tyrosine kinase of the FAK (focal adhesion kinase) subfamily. It is different from FAK in that it is activated following an increase in cytoplasmic free Ca2+. In the present study we have investigated how Ca2+ activates CAKβ/PYK2. Calmodulin-agarose bound CAKβ/PYK2, but not FAK, in the presence of CaCl2. An α-helix (F2-α2) present in the FERM (band four-point-one, ezrin, radixin, moesin homology) F2 subdomain of CAKβ/PYK2 was the binding site of Ca2+/calmodulin; a mutant of this region, L176A/Q177A (LQ/AA) CAKβ/PYK2, bound to Ca2+/calmodulin much less than the wild-type. CAKβ/PYK2 is known to be prominently tyrosine phosphorylated when overexpressed from cDNA. The enhanced tyrosine phosphorylation was inhibited by W7, an inhibitor of calmodulin, and by a cell-permeable Ca2+ chelator and was almost defective in the LQ/AA-mutant CAKβ/PYK2. CAKβ/PYK2 formed a homodimer on binding of Ca2+/calmodulin, which might then induce a conformational change of the kinase, resulting in transphosphorylation within the dimer. The dimer was formed at a free-Ca2+ concentration of 8–12 μM and was stable at 500 nM Ca2+, but dissociated to a monomer in a Ca2+-free buffer. The dimer formation of CAKβ/PYK2 FERM domain was partially defective in the LQ/AA-mutant FERM domain and was blocked by W7 and by a synthetic peptide with amino acids 168–188 of CAKβ/PYK2, but not by a peptide with its LQ/AA-mutant sequence. It is known that the F2-α2 helix is found immediately adjacent to a hydrophobic pocket in the FERM F2 lobe, which locks, in the autoinhibited FAK, the C-lobe of the kinase domain. Our results indicate that Ca2+/calmodulin binding to the FERM F2-α2 helix of CAKβ/PYK2 releases its kinase domain from autoinhibition by forming a dimer.
Role of histidine residues in EcoP15I DNA methyltransferase activity as probed by chemical modification and site-directed mutagenesis
Authors: Jois Prashanth S. , Madhu Nagaraj , Rao Desirazu N.
Published: February 2008

Towards understanding the catalytic mechanism of M.EcoP15I [EcoP15I MTase (DNA methyltransferase); an adenine methyltransferase], we investigated the role of histidine residues in catalysis. M.EcoP15I, when incubated with DEPC (diethyl pyrocarbonate), a histidine-specific reagent, shows a time- and concentration-dependent inactivation of methylation of DNA containing its recognition sequence of 5′-CAGCAG-3′. The loss of enzyme activity was accompanied by an increase in absorbance at 240 nm. A difference spectrum of modified versus native enzyme shows the formation of N-carbethoxyhistidine that is diminished by hydroxylamine. This, along with other experiments, strongly suggests that the inactivation of the enzyme by DEPC was specific for histidine residues. Substrate protection experiments show that pre-incubating the methylase with DNA was able to protect the enzyme from DEPC inactivation. Site-directed mutagenesis experiments in which the 15 histidine residues in the enzyme were replaced individually with alanine corroborated the chemical modification studies and established the importance of His-335 in the methylase activity. No gross structural differences were detected between the native and H335A mutant MTases, as evident from CD spectra, native PAGE pattern or on gel filtration chromatography. Replacement of histidine with alanine residue at position 335 results in a mutant enzyme that is catalytically inactive and binds to DNA more tightly than the wild-type enzyme. Thus we have shown in the present study, through a combination of chemical modification and site-directed mutagenesis experiments, that His-335 plays an essential role in DNA methylation catalysed by M.EcoP15I.
ODCp, a brain- and testis-specific ornithine decarboxylase paralogue, functions as an antizyme inhibitor, although less efficiently than AzI1
Authors: Snapir Zohar , Keren-Paz Alona , Bercovich Zippi , Kahana Chaim
Published: February 2008

ODC (ornithine decarboxylase), the first enzyme in the polyamine biosynthesis pathway in mammalian cells, is a labile protein. ODC degradation is stimulated by Az (antizyme), a polyamine-induced protein, which in turn is regulated by an ODC-related protein termed AzI (Az inhibitor). Recently, another ODCp (ODC paralogue) was suggested to function as AzI, on the basis of its ability to increase ODC activity and inhibit Az-stimulated ODC degradation in vitro. We show in the present study that ODCp is indeed capable of negating Az functions, as reflected by its ability to increase ODC activity and polyamine uptake and by its ability to provide growth advantage in stably transfected cells. However, ODCp is less potent than AzI1 in stimulating ODC activity, polyamine uptake and growth rate. The superiority of AzI1 to ODCp in inhibiting the Az-stimulated ODC degradation is also demonstrated using an in vitro degradation assay. We show that the basis for the inferiority of ODCp as an AzI is its lower affinity towards Az (Az1 and Az3). Further, we show here that ODCp, like AzI, is degraded in a ubiquitin-dependent manner, in a reaction that does not require either interaction with Az or the integrity of its C-terminus. Interaction with Az actually stabilizes ODCp by interfering with its ubiquitination. This results in sequestration of Az into a stable complex with ODCp, which is the central feature contributing to the ability of ODCp to function as AzI.
Negative regulation of cell-cycle progression by RINGO/Speedy E
Authors: Dinarina Ana , Ruiz E. Josué , O'loghlen Ana , Mouron Silvana , Perez Laurent , Nebreda Angel R.
Published: February 2008

Cell-cycle transitions are controlled by CDKs (cyclin-dependent kinases), whose activation is usually associated with the binding of cyclins. RINGO/Speedy proteins can also bind to and activate CDKs, although they do not have amino acid sequence homology with cyclins. The RINGO/Speedy family members studied so far positively regulate cell-cycle progression. In the present paper, we report the biochemical and functional characterization of RINGO/Speedy E. We show that RINGO/Speedy E is a functionally distant member of this protein family that negatively affects cell-cycle progression. RINGO/Speedy E overexpression inhibits the meiotic progression in Xenopus oocytes as well as the proliferation of mammalian cells. RINGO/Speedy E can bind to endogenous CDK1 and CDK2 in both cellular systems. However, the RINGO/Speedy E-activated CDKs have different substrate specificity than the CDKs activated by other RINGO/Speedy proteins, which may account for their different effects on the cell cycle. Our results indicate that, although all RINGO/Speedy family members can activate CDKs, they may differently regulate cell-cycle progression.
EGF and amphiregulin differentially regulate Cbl recruitment to endosomes and EGF receptor fate
Authors: Stern Kathryn A. , Place Trenton L. , Lill Nancy L.
Published: February 2008

EGF-R [EGF (epidermal growth factor) receptor] ligands can promote or inhibit cell growth. The biological outcome of receptor activation is dictated, at least in part, by ligand-specified patterns of endocytic trafficking. EGF-R trafficking downstream of the ligands EGF and TGF-α (transforming growth factor-α) has been investigated extensively. However, less is known about EGF-R fates induced by the ligands BTC (betacellulin) and AR (amphiregulin). We undertook comparative analyses to identify ligand-specific molecular events that regulate EGF-R trafficking and degradation. EGF (17 nM) and BTC (8.5 nM) induced significant EGF-R degradation, with or without ectopic expression of the ubiquitin ligase Cbl. Human recombinant AR (17 nM) failed to affect receptor degradation in either case. Notably, levels of ligand-induced EGF-R ubiquitination did not correlate strictly with receptor degradation. Dose–response experiments revealed that AR at a saturating concentration was a partial agonist at the EGF-R, with approx. 40% efficacy (relative to EGF) at inducing receptor tyrosine phosphorylation, ubiquitination and association with Cbl. EGF-R down-regulation and degradation also were compromised upon cell stimulation with AR (136 nM). These outcomes correlated with decreased degradation of the Cbl substrate and internalization inhibitor hSprouty2. Downstream of the hSprouty2 checkpoint in AR-stimulated cells, Cbl-free EGF-R was incorporated into endosomes from which Cbl–EGF-R complexes were excluded. Our results suggest that the AR-specific EGF-R fate results from decreased hSprouty2 degradation and reduced Cbl recruitment to underphosphorylated EGF-R, two effects that impair EGF-R trafficking to lysosomes.
The Hsp90 molecular chaperone: an open and shut case for treatment
Authors: Pearl Laurence H. , Prodromou Chrisostomos , Workman Paul
Published: February 2008

The molecular chaperone Hsp90 (90 kDa heat-shock protein) is a remarkably versatile protein involved in the stress response and in normal homoeostatic control mechanisms. It interacts with ‘client proteins’, including protein kinases, transcription factors and others, and either facilitates their stabilization and activation or directs them for proteasomal degradation. By this means, Hsp90 displays a multifaceted ability to influence signal transduction, chromatin remodelling and epigenetic regulation, development and morphological evolution. Hsp90 operates as a dimer in a conformational cycle driven by ATP binding and hydrolysis at the N-terminus. The cycle is also regulated by a group of co-chaperones and accessory proteins. Here we review the biology of the Hsp90 molecular chaperone, emphasizing recent progress in our understanding of structure–function relationships and the identification of new client proteins. In addition we describe the exciting progress that has been made in the development of Hsp90 inhibitors, which are now showing promise in the clinic for cancer treatment. We also identify the gaps in our current understanding and highlight important topics for future research.
Two-dimensional/three-dimensional simulations for the optimization of an electron-beam-generated-plasma-based-type ion source
Authors: Mhamed M. Cheikh , Lau C. , Essabaa S.
Published: February 2008

To fulfill the need of a plasma ion source for SPIRAL-2 and EURISOL, capable of producing radioactive ion beams under strong radiation, the first prototype of the IRENA (Ionization by Radial Electrons Neat Adaptation) ion source has been designed. For designing an optimized prototype based on the first one, the influence of the geometrical parameters on the electron trajectories is investigated by means of two-dimensional/three-dimensional (3D) simulations. Due to the strong space charge effect in this kind of ion source, 3D simulations help particularly to better estimate effects on ion confinement and extraction. Simulation constructions will be presented and results discussed.
H
−
ion source developments at the SNS
Authors: Welton R. F. , Stockli M. P. , Murray S. N. , Pennisi T. R. , Han B. , Kang Y. , Goulding R. H. , Crisp D. W. , Sparks D. O. , Luciano N. P. , Carmichael J. R. , Carr J.
Published: February 2008

The U.S. Spallation Neutron Source (SNS) will require substantially higher average and pulse H− beam currents than can be produced from conventional ion sources such as the base line SNS source. H− currents of 40–50mA (SNS operations) and 70–100mA (power upgrade project) with a rms emittance of 0.20–0.35πmmmrad and a ∼7% duty factor will be needed. We are therefore investigating several advanced ion source concepts based on rf plasma excitation. First, the performance characteristics of an external antenna source based on an Al2O3 plasma chamber combined with an external multicusp magnetic configuration, an elemental Cs system, and plasma gun will be discussed. Second, the first plasma measurements of a helicon-driven H− ion source will also be presented.
Charge redistribution and photoacidity: Neutral versus cationic photoacids
Authors: Spry D. B. , Fayer M. D.
Published: February 2008

A series of pyrene photoacids is used to investigate excited-state proton transfer with time-dependent pump-probe spectroscopy. The deprotonation dynamics of a cationic photoacid, 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS), shows single exponential dynamics(∼30ps) in water. This is in contrast to what is observed for the neutral photoacids 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and 8-hydroxy-N,N,N′,N′,N″,N″-hexamethylpyrene-1,3,6-trisulfonamide, which display biexponential dynamics. For the cationic photoacid, the vast majority of the intramolecular charge redistribution does not occur in the protonated state. Instead, the charge redistribution, which is responsible for the photoacidity and the observed spectroscopic changes, occurs primarily following the excited-state proton transfer. The lack of charge redistribution prior to proton transfer causes APTS to display single exponential kinetics. In contrast, the dynamics for the neutral photoacids are multiexponential because major charge redistribution precedes proton transfer followed by additional charge redistribution that accompanies proton transfer. Previous studies of HPTS in water are discussed in terms of the results presented here.
Vibrational action spectroscopy of the C–H and C–D stretches in partially deuterated formic acid dimer
Authors: Yoon Y. Heidi , Hause Michael L. , Case Amanda S. , Crim F. Fleming
Published: February 2008

Vibrational action spectroscopy of jet-cooled formic acid dimer measures the frequency of the C–H(D) stretching vibration and its coupling to nearby states. The action spectrum of (DCOOH)2 reveals a specific Fermi resonance between the C–D stretch and two antisymmetric combination states formed from the C–O stretch and DCO bend. A three-state deperturbation analysis shows that there is a relatively strong coupling between the fundamental vibration and each of the combination vibrations (∣13cm−1∣) as well as between the combination states themselves (∣7cm−1∣). This situation contrasts with that for the action spectrum of (HCOOD)2, where the C–H oscillator is isolated and not strongly coupled to other states.
Many-body decomposition of the binding energies for OH⋅(H2O)2 and OH⋅(H2O)3 complexes
Authors: Du Shiyu , Francisco Joseph S. , Schenter Gregory K. , Garrett Bruce C.
Published: February 2008

We use ab initio electronic structure methods to calculate the many-body decomposition of the binding energies of the OH⋅(H2O)n (n=2,3) complexes. We employ MP2 and CCSD(T) levels of theory with aug-cc-pVDZ and aug-cc-pVTZ basis sets and analyze the significance of the nonpairwise interactions between OH radical and the surrounding water molecules. We also evaluate the accuracy of our newly developed potential function, the modified Thole-type model, for predicting the many-body terms in these complexes. Our analysis of the many-body contributions to the OH⋅(H2O)n binding energies clearly shows that they are just as important in the OH interactions with water as they are for interactions in pure water systems.
Shear viscosity of liquid copper at experimentally accessible shear rates: Application of the transient-time correlation function formalism
Authors: Desgranges Caroline , Delhommelle Jerome
Published: February 2008

We report on nonequilibrium molecular dynamics (NEMD) simulations results on the shear viscosity of liquid copper, modeled by a many-body embedded-atoms model potential. Because conventional NEMD methods are restricted to very high shear rates (at least of the order of 1010 s−1, that is several orders of magnitude larger than those accessible by experiment), previous work only provided access to the response of the fluid in the shear-thinning regime. Using the transient-time correlation function formalism, we show how NEMD simulations can be extended to study the rheological properties of liquid copper subjected to low, experimentally accessible, shear rates. Our results provide a full picture of the rheology of the system, in the Newtonian regime as well as in the shear-thinning regime.
Intrinsic optical bistability of thin films of linear molecular aggregates: The two-exciton approximation
Authors: Klugkist Joost A. , Malyshev Victor A. , Knoester Jasper
Published: February 2008

We generalize our recent work on the optical bistability of thin films of molecular aggregates [J. A. Klugkist et al., J. Chem. Phys. 127, 164705 (2007)] by accounting for the optical transitions from the one-exciton manifold to the two-exciton manifold as well as the exciton-exciton annihilation of the two-exciton states via a high-lying molecular vibronic term. We also include the relaxation from the vibronic level back to both the one-exciton manifold and the ground state. By selecting the dominant optical transitions between the ground state, the one-exciton manifold, and the two-exciton manifold, we reduce the problem to four levels, enabling us to describe the nonlinear optical response of the film. The one- and two-exciton states are obtained by diagonalizing a Frenkel Hamiltonian with an uncorrelated on-site (diagonal) disorder. The optical dynamics is described by means of the density matrix equations coupled to the electromagnetic field in the film. We show that the one- to two-exciton transitions followed by a fast exciton-exciton annihilation promote the occurrence of bistability and reduce the switching intensity. We provide estimates of pertinent parameters for actual materials and conclude that the effect can be realized.
Spin-dependent bandgap structure and resonant transmission of electrons in ferromagnetic metal/semiconductor cascade junctions
Authors: Zhang R. L. , Zhang Z. J. , Peng R. W. , Wu X. , Li De , Li Jia , Cao L. S.
Published: February 2008

We investigate spin-dependent transport in ferromagnetic metal (FM)/semiconductor (SC) cascade junctions, which can be denoted as (FM∕SC)n∕FM. Here, n is the repeated number of FM/SC junction. In the Landauer framework of ballistic transport, we have calculated the spin-dependent transmission and the spin polarization in these cascade junctions. It is shown that spin-up and spin-down electrons possess different bandgap structures against the Rashba spin-orbit wave vector. As a result, high spin polarization can be achieved. Besides, resonant transmission for spin-up or spin-down electrons can be observed within the bandgap when we intentionally change the magnetization of FM in the center of the cascade junctions. Around resonant wave vector, spin polarization will be reversed. Our investigations may have potential applications in spin filters and spin switches.
Stability and magnetic properties of transition metal atoms endohedral BnNn (n=12–28) cages
Authors: Wang Jianguang , Ma Li , Zhao Jijun , Wang Baolin , Wang Guanghou
Published: February 2008

First-principles calculations have been conducted to systemically investigate the stability and magnetic properties of 3d and 4d transitional-metal (TM) atoms doped in the BnNn (n=12,16,20,24,28) cages. Among those cages, the B24N24 is the optimal one for encapsulating 3d and 4d TM atoms according to the computed heat of formation. Inside B24N24 cage, 3d and 4d TM dopants belonging to the same group in the Periodic Table exhibit similar magnetic behaviors. Most of the 3d and 4d TM atoms remain magnetic after doped in the B24N24 cage except for Ni, Zr, and Pd. The magnitudes of the remaining moments for 3d (except for Sc, Ti, and V) and 4d dopants are reduced from those of free atoms. The energy gaps are localized at the doped transition metal atoms. Encapsulations of two TM atoms inside the B24N24 cage were also considered.
Dynamic evolution of light-induced orientation of dye-doped liquid crystals in liquid phase studied by time-resolved optically heterodyned optical Kerr effect technique
Authors: Yang Pei , Liu Liying , Xu Lei
Published: February 2008

Transient evolution of light-induced molecular reorientation both in 1-amino-anthraquinone (1AAQ) dye and azobenzene doped isotropic liquid crystals (LCs) were studied by time-resolved optically heterodyned optical Kerr effect method. The results give clear direct experimental proof that under short pulse (30ps) excitation, LC molecules orientate toward the excitation light polarization direction in the 1AAQ/LC system. However, LC molecular orientation becomes orthogonal to the light polarization in azobenzene/LC system. Time-resolved excited-state absorption of 1AAQ and wavelength dependent excited-state absorption of azobenzene were also observed and their contributions to the early dynamics of the third order optical responses of the two systems were confirmed. A simplified two-level mean-field theory was derived to reveal the intensity dependence of orientation enhancement factor in azobenzene/LC system considering the photoisomerization process.
Nickel-silica nanocomposite: Variation of the blocking temperature with magnetic field and measuring frequency
Authors: Singh V. , Seehra M. S. , Bonevich J.
Published: February 2008

The variations of the blocking temperature TB with measuring frequency fm and applied field H are reported for Ni nanoparticles (NPs) embedded in SiO2 matrix with the nominal composition Ni∕SiO2 (15∕85). Transmission electron microscopy of the sol-gel prepared NPs yielded a log-normal distribution with diameter D=3.8(0.2)nm whereas x-ray diffraction data yielded D≃3.3nm. The TB versus fm variation for fm=0.1–1000Hz fits the equation TB=Ta∕ln(f0∕fm) with f0=2.6×109Hz and Ta=270K whereas TB versus H data follows the equation TB(H)=33(1−H∕H0)m with H0=3200Oe and m=2(3∕2) for lower (higher) H. These results are compared with the predictions of the Néel-Brown model of magnetic relaxation in noninteracting NPs.
Thermal effects on Farley–Buneman waves at nonzero aspect and flow angles. I. Dispersion relation
Authors: Kissack R. S. , Kagan L. M. , St.-Maurice J.-P.
Published: February 2008

With the increasingly accepted notion that E-region electrostatic plasma waves move at their instability threshold speed when they attain their maximum amplitude, it has become more important to determine just what that threshold speed is supposed to be. An accurate determination requires a calculation of the electron temperature fluctuation level, which can sometimes dramatically influence the results. In this paper, the Grad 8-moment transport equations have been employed with the addition of a generalized electron cooling rate to account for cooling via inelastic collisions, and a general electron volume heating rate that includes frictional heating as a special case. The derived generalized dispersion relation is presented in such a way that a variety of other earlier treatments, including the isothermal and adiabatic limits and an instability associated with electron friction, all fall out transparently, allowing us to easily compare our results with earlier ones while tracking down the physical processes involved. The presentation format is also geared to facilitate the computation of the dispersion relation for the purpose of comparing the theoretical results with observations.
Thermal effects on Farley–Buneman waves at nonzero aspect and flow angles. II. Behavior near threshold
Authors: Kissack R. S. , Kagan L. M. , St.-Maurice J.-P.
Published: February 2008

Based on the linear dispersion relation of Kissack et al., Phys. Plasmas 15, 022901 (2008), the physical processes that define altitude behavior of marginally stable Farley–Buneman waves in the equatorial electrojet are investigated. The expressions derived for the angular frequency and growth rate are presented in such a way as to make it easy to track the dominant physical processes and to see the relation with earlier work. Two dimensionless parameters are identified that are helpful in showing the transition between altitude and wavelength domains where different thermal processes dominate. The difference in phase velocity between vertical and off-vertical transmissions (zero versus nonzero flow angles) is found to be due to Dimant–Sudan effects, which are preferentially less important at higher altitudes and shorter wavelengths.
Development of a vacuum ultraviolet laser-based angle-resolved photoemission system with a superhigh energy resolution better than 1meV
Authors: Liu Guodong , Wang Guiling , Zhu Yong , Zhang Hongbo , Zhang Guochun , Wang Xiaoyang , Zhou Yong , Zhang Wentao , Liu Haiyun , Zhao Lin , Meng Jianqiao , Dong Xiaoli , Chen Chuangtian , Xu Zuyan , Zhou X. J.
Published: February 2008

The design and performance of the first vacuum ultraviolet (VUV) laser-based angle-resolved photoemission (ARPES) system are described. The VUV laser with a photon energy of 6.994eV and bandwidth of 0.26meV is achieved from the second harmonic generation using a novel nonlinear optical crystal KBe2BO3F2. The new VUV laser-based ARPES system exhibits superior performance, including superhigh energy resolution better than 1meV, high momentum resolution, superhigh photon flux, and much enhanced bulk sensitivity, which are demonstrated from measurements on a typical Bi2Sr2CaCu2O8 high temperature superconductor. Issues and further development related to the VUV laser-based photoemission technique are discussed.
New ultrasonic Bleustein-Gulyaev wave method for measuring the viscosity of liquids at high pressure
Authors: Kiełczyński P. , Szalewski M. , Siegoczyński R. M. , Rostocki A. J.
Published: February 2008

In this paper, a new method for measuring the viscosity of liquids at high pressure is presented. To this end the authors have applied an ultrasonic method using the Bleustein-Gulyaev (BG) surface acoustic wave. By applying the perturbation method, we can prove that the change in the complex propagation constant of the BG wave produced by the layer of liquid loading the waveguide surface is proportional to the shear mechanical impedance of the liquid. In the article, a measuring setup employing the BG wave for the purpose of measuring the viscosity of liquids at high pressure (up to 1GPa) is presented. The results of high-pressure viscosity measurements of triolein and castor oil are also presented. In this paper the model of a Newtonian liquid was applied. Using this new method it is also possible to measure the viscosity of liquids during the phase transition and during the decompression process (hysteresis of the dependence of viscosity on pressure).
Magnetotransport properties of polycrystalline and epitaxial chromium dioxide nanowires
Authors: Zou Xiaojing , Xiao Gang , Huang Sunxiang , Chen Tingyong , Chien Chia-Ling
Published: February 2008

Temperature dependent magnetotransport measurements were performed on polycrystalline and epitaxial chromium dioxide (CrO2) nanowires fabricated using the selective-area growth technique. Polycrystalline nanowires showed a negative temperature coefficient of resistivity at low temperatures because of strong grain boundary scattering. The magnetoresistance (MR) value exhibited a width dependence, reaching a maximum of 20% for a 150nm wide wire. In contrast, the MR response of single crystal CrO2 wires was mainly determined by magnetocrystalline and shape anisotropy.
On the high contraction ratio anomaly of axisymmetric contraction of grid-generated turbulence
Authors: Ertunç Özgür , Durst Franz
Published: February 2008

The present paper deals with the anomalous increase in second-order moments of longitudinal velocity fluctuations reported in experiments carried out to measure properties of axisymmetric strained turbulence when the strain is provided by axisymmetric contractions (nozzles). Experimental evidence is provided that the increase is due to inaccuracies in the measurements. It is shown that the imperfect spatial resolution of X-wire probes, the mass flow rate fluctuations in the flow facility, and the electronic noise contaminate the hot-wire measurements of not only the longitudinal but also the transverse velocity fluctuation components. These kinds of contaminations start to dominate the measurements, especially when the contraction ratio of the nozzle is high and/or the turbulence level is low and turbulence length scales are small. A series of measurement and correction methods is proposed employing two single normal wires and one inclined wire to detect and eliminate all three types of contaminations of longitudinal and transverse velocity fluctuation measurements. These methods are subsequently employed to carry out turbulence measurements of grid turbulence strained in a converging nozzle with a high contraction ratio. The resulting Reynolds stress developments along the nozzle do not involve an erroneous increase in longitudinal turbulent velocity fluctuations and are consistent with theoretical expectations such as the direct numerical simulations and the rapid distortion theory. The corrected results revealed that turbulence along the nozzle reaches the two-component axisymmetric state when looked at through the anisotropy of Reynolds stresses. Furthermore, the anisotropy of Reynolds stresses exhibits zero gradient at that state. For the first time in the literature, experimental confirmation of rapid distortion theory is supplied for the axisymmetric contraction of turbulence within nozzles having a contraction ratio higher than 9. Finally, a set of experimental data is provided for the validation of turbulence models.
A generalized Bhatnagar–Gross–Krook model for nonequilibrium flows
Authors: Xu Kun
Published: February 2008

The Bhatnagar–Gross–Krook (BGK) model approximates the collision term of the Boltzmann equation by a single relaxation process from a nonequilibrium state to an equilibrium state. From the BGK model, the Navier–Stokes, Burnett, and Super-Burnett equations can be derived. However, the success of these derived macroscopic equations in the description of highly nonequilibrium flow is limited. In order to extend the validity of the BGK model to cope with complicated physical relaxation processes, we propose a generalized BGK (GBGK) model, where the gas distribution function settles to an equilibrium state through multiple particle collisional processes. The GBGK model can be used to capture the transitional, rotational, and vibrational nonequilibrium flow phenomena. More importantly, it may provide a theoretical framework to derive new macroscopic governing equations which are valid in the near continuum flow regime.
Whistler waves in plasmas with magnetic field irregularities: Experiment and theory
Authors: Gushchin M. E. , Zaboronkova T. M. , Koldanov V. A. , Korobkov S. V. , Kostrov A. V. , Krafft C. , Strikovsky A. V.
Published: February 2008

The properties of whistler waves propagating in a large laboratory magnetoplasma with magnetic field irregularities have been studied. Two types of ambient magnetic field inhomogeneities have been considered: (i) a localized “lenslike” perturbation and (ii) an elongated “ductlike” irregularity. The magnetic field was perturbed by immersing into the plasma, without creating any significant plasma density disturbances, additional current-carrying coils. It has been found that the presence of magnetic field irregularities causes the whistler wave’s diffraction and affects their patterns substantially. Plasma regions with locally enhanced magnetic field strength focus oblique whistlers; oppositely, local magnetic field minima debunch the whistler waves. In case of prolonged magnetic field irregularity formation—encompassing several whistler wavelengths along its size—the diffraction effects are distinctly pronounced; even the comparatively weak magnetic field disturbances at the level of 10% lead to strong modifications of the whistler waves’ pattern. Theoretical calculations are presented which confirm the related experimental measurements. The obtained results are of great importance for laboratory plasmas as well as magnetospheric physics, and represent a new look at the problem of whistler waves’ scattering and ducting, caused not by the plasma density ducts and gradients, but by magnetic field irregularities.
Plasma physics and related challenges of millimeter-wave-to-terahertz and high power microwave generation
Authors: Booske John H.
Published: February 2008

Homeland security and military defense technology considerations have stimulated intense interest in mobile, high power sources of millimeter-wave (mmw) to terahertz (THz) regime electromagnetic radiation, from 0.1 to 10THz. While vacuum electronic sources are a natural choice for high power, the challenges have yet to be completely met for applications including noninvasive sensing of concealed weapons and dangerous agents, high-data-rate communications, high resolution radar, next generation acceleration drivers, and analysis of fluids and condensed matter. The compact size requirements for many of these high frequency sources require miniscule, microfabricated slow wave circuits. This necessitates electron beams with tiny transverse dimensions and potentially very high current densities for adequate gain. Thus, an emerging family of microfabricated, vacuum electronic devices share many of the same plasma physics challenges that are currently confronting “classic” high power microwave (HPM) generators including long-life bright electron beam sources, intense beam transport, parasitic mode excitation, energetic electron interaction with surfaces, and rf air breakdown at output windows. The contemporary plasma physics and other related issues of compact, high power mmw-to-THz sources are compared and contrasted to those of HPM generation, and future research challenges and opportunities are discussed.
Lithographically patterned servo position error signal patterns in perpendicular disks
Authors: Moneck Matthew T. , Zhu Jian-Gang , Tang Yawshing , Moon Ki-Seok , Lee Hyung Jai , Zhang Shuyu , Che Xiaodong , Takahashi Nobuyuki
Published: February 2008

For discrete track media to become a viable alternative, it is essential to produce accurate servo patterns in a cost effective manner. This study presents a spin stand analysis and comparison of position error signals generated from various lithographically defined servo patterns and servo bursts written on continuous regions of the same perpendicular recording media. It is demonstrated that the edge variation on the patterned servo elements is much less than that found on conventional servo bursts. In addition, evidence shows that despite some amplitude loss resulting from the removal of magnetic material, patterned servo bursts produce good quality position error signals when compared to servo bursts written on continuous media.
Oscillator-Morse–Coulomb mappings and algebras for constant or position-dependent mass
Authors: Quesne C.
Published: February 2008

The bound-state solutions and the su(1,1) description of the d-dimensional radial harmonic oscillator, the Morse, and the D-dimensional radial Coulomb Schrödinger equations are reviewed in a unified way using the point canonical transformation method. It is established that the spectrum generating su(1,1) algebra for the first problem is converted into a potential algebra for the remaining two. This analysis is then extended to Schrödinger equations containing some position-dependent mass. The deformed su(1,1) construction recently achieved for a d-dimensional radial harmonic oscillator is easily extended to the Morse and Coulomb potentials. In the last two cases, the equivalence between the resulting deformed su(1,1) potential algebra approach and a previous deformed shape invariance one generalizes to a position-dependent mass background a well-known relationship in the context of constant mass.
Transverse anisotropy in the mixed-valent Mn2IIMn4IIIMn3IV single-molecule magnet
Authors: Datta Saiti , Milios Constantinos J. , Brechin Euan , Hill Stephen
Published: February 2008

High-frequency electron paramagnetic resonance measurements have been performed on a single-crystal sample of a recently discovered mixed valent Mn2IIMn4IIIMn3IV single-molecule magnet, with a spin S=17∕2 ground state. Frequency, temperature and field-orientation dependent studies confirm previously reported axial magnetic anisotropy parameters and also provide clear evidence for higher order (fourth and sixth) transverse terms that are responsible for the magnetic quantum tunneling observed in this system.