bluuebunny/crossref_metadata_2025 huggingface dataset:
Regulation by Per-Arnt-Sim (PAS) kinase of pancreatic duodenal homeobox-1 nuclear import in pancreatic β-cells
Authors: An R. , da Silva Xavier G. , Hao H.-X. , Semplici F. , Rutter J. , Rutter G.A.
Published: October 2006

The transcription factor PDX-1 (pancreatic duodenal homeobox-1) is required for normal pancreatic development and for the function of insulin-producing islet β-cells in mammals. We have shown previously that glucose regulates insulin gene expression in part through the activation and translocation of PDX-1 from the nuclear periphery to the nucleoplasm. We have also found that PASK [PAS (Per-Arnt-Sim) kinase], a member of the nutrient-regulated family of protein kinases, is activated in response to glucose challenge in β-cells and is involved in the regulation of expression of PDX-1. Purified PASK efficiently phosphorylated recombinant PDX-1 in vitro on a single site (Thr-152). To determine the impact of phosphorylation at this site, we generated wild-type and mutant (T152A, T152D and T152E) forms of PDX-1 and examined the distribution of each of these in clonal MIN6 β-cells by immunocytochemical analysis. Unexpectedly, only the T152D mutation significantly affected subcellular distribution, increasing the ratio of nuclear/cytosolic labelling at low and high glucose concentrations, suggesting that phosphorylation at Thr-152 inhibits nuclear uptake in response to glucose. Based on these results, experiments to examine the contribution of Thr-152 to the overall phosphorylation of PDX-1 in intact cells will be undertaken.
Nicotinamide nucleotide transhydrogenase: a link between insulin secretion, glucose metabolism and oxidative stress
Authors: Freeman H. , Shimomura K. , Cox R.D. , Ashcroft F.M.
Published: October 2006

This paper reviews recent studies on the role of Nnt (nicotinamide nucleotide transhydrogenase) in insulin secretion and detoxification of ROS (reactive oxygen species). Glucose-stimulated insulin release from pancreatic β-cells is mediated by increased metabolism. This elevates intracellular [ATP], thereby closing KATP channels (ATP-sensitive potassium channels) and producing membrane depolarization, activation of voltage-gated Ca2+ channels, Ca2+ influx and, consequently, insulin secretion. The C57BL/6J mouse displays glucose intolerance and reduced insulin secretion, which results from a naturally occurring deletion in the Nnt gene. Transgenic expression of the wild-type Nnt gene in C57BL/6J mice rescues the phenotype. Knockdown of Nnt in the insulin-secreting cell line MIN6 with small interfering RNA dramatically reduced Ca2+ influx and insulin secretion. Similarly, mice carrying ENU (N-ethyl-N-nitrosourea)-induced loss-of-function mutations in Nnt were glucose intolerant and secreted less insulin during a glucose tolerance test. Islets isolated from these mice showed impaired insulin secretion in response to glucose, but not to the KATP channel blocker tolbutamide. This is explained by the fact that glucose failed to elevate ATP in Nnt mutant islets. Nnt is a nuclear-encoded mitochondrial protein involved in detoxification of ROS. β-Cells isolated from Nnt mutant mice showed increased ROS production on glucose stimulation. We hypothesize that Nnt mutations enhance glucose-dependent ROS production and thereby impair β-cell mitochondrial metabolism, possibly via activation of uncoupling proteins. This reduces ATP production and lowers KATP channel activity. Consequently, glucose-dependent electrical activity and insulin secretion are impaired.
Mitochondrial diabetes and its lessons for common Type 2 diabetes
Authors: Maassen J.A. , 't Hart L.M. , Janssen G.M.C. , Reiling E. , Romijn J.A. , Lemkes H.H.
Published: October 2006

Multiple pathogenic pathways are able to deregulate glucose homoeostasis leading to diabetes. The 3243A>G mutation in the mtDNA (mitochondrial DNA)-encoded tRNALeu,UUR gene was found by us to be associated with a particular diabetic subtype, designated MIDD (maternally inherited diabetes and deafness). This mutation causes an imbalance in the mitochondrion between proteins encoded by the nuclear and mitochondrial genomes, resulting in a gradual deterioration of glucose homoeostasis during life. Remarkably, carriers of the 3243A>G mutation are generally not obese. The mutation also results in enhanced radical production by mitochondria. We propose that this mutation leads to the development of diabetes due to an inappropriate storage of triacylglycerols within adipocytes. The result is a fatty acid-induced deterioration of pancreatic β-cell function. In combination with an enhanced radical production in the β-cell due to the mutation, this leads to an age-dependent, accelerated decline in insulin production. In common Type 2 (non-insulin-dependent) diabetes, which is generally associated with obesity, a decline in mitochondrial function in adipose cells seems to result in an inappropriate scavenging of fatty acids by β-oxidation. As a consequence, a systemic overload with fatty acids occurs, leading to an enhanced decline in β-cell function due to lipotoxicity.
Mitochondrial damages and the regulation of insulin secretion
Authors: Maechler P. , de Andrade P.B.M.
Published: October 2006

Pancreatic β-cells are able to respond to nutrients, principally glucose, as the primary stimulus for insulin exocytosis. This unique feature requires translation of metabolic substrates into intracellular messengers recognized by the exocytotic machinery. Central to this signal transduction mechanism, mitochondria integrate and generate metabolic signals, thereby coupling glucose recognition with insulin secretion. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic Ca2+, in the stimulation of insulin exocytosis. Mitochondrial defects, such as mutations and ROS (reactive oxygen species) production, might be associated with β-cell failure in the course of diabetes. mtDNA (mitochondrial DNA) mutation A3243G is associated with MIDD (mitochondrial inherited diabetes and deafness). A common hypothesis to explain the link between the genotype and the phenotype is that the mutation might impair mitochondrial metabolism expressly required for β-cell functions, although this assumption lacks direct demonstration. mtDNA-deficient cellular models are glucose-unresponsive and are defective in mitochondrial function. Recently, we used clonal cytosolic hybrid cells (namely cybrids) harbouring mitochondria derived from MIDD patients. Compared with control mtDNA from the same patient, the A3243G mutation markedly modified metabolic pathways. Moreover, cybrid cells carrying patient-derived mutant mtDNA exhibited deranged cell Ca2+ handling and elevated ROS under metabolic stress. In animal models, transgenic mice lacking expression of the mitochondrial genome specifically in β-cells are diabetic and their islets are incable of releasing insulin in response to glucose. These various models demonstrate the fragility of nutrient-stimulated insulin secretion, caused primarily by defective mitochondrial function.
The role of GABAA receptor biogenesis, structure and function in epilepsy
Authors: Mizielinska S. , Greenwood S. , Connolly C.N.
Published: October 2006

Maintaining the correct balance in neuronal activation is of paramount importance to normal brain function. Imbalances due to changes in excitation or inhibition can lead to a variety of disorders ranging from the clinically extreme (e.g. epilepsy) to the more subtle (e.g. anxiety). In the brain, the most common inhibitory synapses are regulated by GABAA (γ-aminobutyric acid type A) receptors, a role commensurate with their importance as therapeutic targets. Remarkably, we still know relatively little about GABAA receptor biogenesis. Receptors are constructed as pentameric ion channels, with α and β subunits being the minimal requirement, and the incorporation of a γ subunit being necessary for benzodiazepine modulation and synaptic targeting. Insights have been provided by the discovery of several specific assembly signals within different GABAA receptor subunits. Moreover, a number of recent studies on GABAA receptor mutations associated with epilepsy have further enhanced our understanding of GABAA receptor biogenesis, structure and function.
Do polymorphisms in the human 5-HT3 genes contribute to pathological phenotypes?
Authors: Krzywkowski K.
Published: October 2006

5-HT3 receptors are members of the Cys-loop superfamily of ligand-gated ion channels. In both the central and the peripheral nervous systems, 5-HT3 receptors excite postsynaptic cells and modulate the release of neurotransmitters from presynaptic neurons. 5-HT3 receptors are known to be involved in mediation of nausea/emesis caused by chemo/radio-therapy and anaesthesia, and more recently have also been found to be involved in irritable bowel syndrome. 5-HT3 receptors have also been suggested to play a role in a range of other indications, including various psychiatric disorders. This review summarizes the current evidence for the contribution of 5-HT3 subunit genes to disease phenotypes arising from association studies. Furthermore, it suggests how in vitro characterization of naturally occurring genetic variants can be used to obtain a better understanding of the causal relationship between gene and disease.
The structural biology of voltage-gated calcium channel function and regulation
Authors: Van Petegem F. , Minor D.L.
Published: October 2006

Voltage-gated calcium channels (CaVs) are large (∼0.5 MDa), multisubunit, macromolecular machines that control calcium entry into cells in response to membrane potential changes. These molecular switches play pivotal roles in cardiac action potentials, neurotransmitter release, muscle contraction, calcium-dependent gene transcription and synaptic transmission. CaVs possess self-regulatory mechanisms that permit them to change their behaviour in response to activity, including voltage-dependent inactivation, calcium-dependent inactivation and calcium-dependent facilitation. These processes arise from the concerted action of different channel domains with CaV β-subunits and the soluble calcium sensor calmodulin. Until recently, nothing was known about the CaV structure at high resolution. Recent crystallographic work has revealed the first glimpses at the CaV molecular framework and set a new direction towards a detailed mechanistic understanding of CaV function.
Do voltage-gated calcium channel α2δ subunits require proteolytic processing into α2 and δ to be functional?
Authors: Douglas L. , Davies A. , Wratten J. , Dolphin A.C.
Published: October 2006

The accessory α2δ subunits of voltage-gated calcium channels are type 1 transmembrane proteins that are highly glycosylated and possess multiple disulfide bonds. From studies of the topology and processing of skeletal-muscle α2δ-1, it has been shown to be post-translationally cleaved into an α2 and a δ subunit, which remain disulfide-bonded. In the present study, we have examined the processing of α2δ-2 subunits when stably or transiently expressed, in tsA (temperature-sensitive A)-201, Cos-7 and NG108-15 cells, and compared it with that observed in the cerebellum. Despite showing full functionality and being expressed on the plasma membrane, the vast majority of heterologously expressed α2δ-2 is not cleaved into α2-2 and δ-2, unlike endogenous α2δ-2 in the cerebellum. It remains an open question for future research whether α2δ-2 is functional in its calcium channel trafficking role in its proteolytically cleaved or non-cleaved state.
Redox regulation of the ryanodine receptor/calcium release channel
Authors: Zissimopoulos S. , Lai F.A.
Published: October 2006

The RyR (ryanodine receptor)/calcium release channel contains a number of highly reactive thiol groups that endow it with redox sensitivity. In general, oxidizing conditions favour channel opening, while reducing conditions have the opposite effect. Thiol modification affects the channel sensitivity to its principal effectors, Ca2+, Mg2+ and ATP, and alters RyR protein interactions. Here, we give a brief account of the major findings and prevailing views in the field.
Pro-survival signalling from the NMDA receptor
Authors: Hardingham G.E.
Published: October 2006

Ca2+ influx through the NMDA (N-methyl-D-aspartate) subtype of ionotropic glutamate receptors plays a Jekyll and Hyde role in the mammalian central nervous system. While it mediates excitotoxic death triggered by stroke and other acute trauma, there is growing evidence that physiological levels of NMDA receptor activity promote survival. Understanding the mechanisms that underlie these opposing effects may lead to strategies to selectively block pro-death signalling, which could have considerable clinical benefits.
Linking proximal and downstream signalling events in hepatic ischaemia/reperfusion injury
Authors: Jeyabalan G. , Tsung A. , Billiar T.R.
Published: October 2006

Hepatic I/R (ischaemia/reperfusion) injury occurs in a variety of clinical settings including transplantation, elective liver resections and trauma. One of the challenges in studying the pathophysiology of I/R injury is the fact that the liver plays a central role in a variety of metabolic pathways in addition to governing aspects of immune surveillance and tolerance. The pathways activated in response to insults as varied as toxins, microbial and endogenous ligands and I/R may share common elements. The multiple intracellular signalling cascades involved in this process and the initiating events are still under investigation. Recent work on the role of TLRs (Toll-like receptors) in I/R injury has elucidated some of the more proximal signalling events in the pathway. In addition to the well-established role of signalling molecules such as NO (nitric oxide) in mediating damage or protection following hepatic I/R, more recent studies have focused on the participation of endogenous danger signals or DAMPs (damage-associated molecular patterns) such as HMGB1 (high-mobility group box 1). The complex interplay between HMGB1, TLRs and the many intracellular signalling molecules and pathways is illustrative of how our understanding of hepatic I/R injury is continually evolving.
Superoxide, peroxynitrite and oxidative/nitrative stress in inflammation
Authors: Salvemini D. , Doyle T.M. , Cuzzocrea S.
Published: October 2006

A considerable body of evidence suggests that formation of potent reactive oxygen species and resulting oxidative/nitrative stress play a major role in acute and chronic inflammation and pain. Much of the knowledge in this field has been gathered by the use of pharmacological and genetic approaches. In this mini review, we will evaluate recent advances made towards understanding the roles of reactive oxygen species in inflammation, focusing in particular on superoxide and peroxynitrite. Given the limited space to cover this broad topic, here we will refer the reader to comprehensive review articles whenever possible.
Folding and unfolding for binding: large-scale protein dynamics in protein–protein interactions
Authors: Roberts G.C.K.
Published: October 2006

The role of dynamics in the function of proteins, from enzymes to signalling proteins, is widely recognized. In many cases, the dynamic process is a relatively localized one, involving motion of a limited number of key residues, while in others large-scale domain movements may be involved. These motions all take place within the context of a folded protein; however, there is increasing evidence for the existence of some proteins where a transition between folded and unfolded structures is required for function.
Ligand migration and escape pathways in haem proteins
Authors: Lavalette D. , Tétreau C. , Mouawad L.
Published: October 2006

Biophysical techniques developed during the last three decades have provided an increasingly detailed description of the internal processes associated with ligand capture and release by haem proteins. Myoglobin has long been the paradigm for these studies. More recently, cytochrome P450cam (the camphor-metabolizing cytochrome P450 from Pseudomonas putida) has also received considerable interest. In spite of sharing the same prosthetic group, the Fe(II)-haem, these proteins are structurally unrelated and they perform different functions. Recent works show that both proteins exhibit a common feature: a series of permanent or fluctuating, mostly hydrophobic, cavities of the protein matrix are providing transient docking sites as well as migration, escape and possibly entry pathways for the ligand. Remarkably, these systems of cavities connect the distal and the proximal regions of the haem, a disposition that may contribute to ligand capture enhancement.
Cell biochemistry studied by single-molecule imaging
Authors: Mashanov G.I. , Nenasheva T.A. , Peckham M. , Molloy J.E.
Published: October 2006

Over the last decade, there have been remarkable developments in live-cell imaging. We can now readily observe individual protein molecules within living cells and this should contribute to a systems level understanding of biological pathways. Direct observation of single fluorophores enables several types of molecular information to be gathered. Temporal and spatial trajectories enable diffusion constants and binding kinetics to be deduced, while analyses of fluorescence lifetime, intensity, polarization or spectra give chemical and conformational information about molecules in their cellular context. By recording the spatial trajectories of pairs of interacting molecules, formation of larger molecular complexes can be studied. In the future, multicolour and multiparameter imaging of single molecules in live cells will be a powerful analytical tool for systems biology. Here, we discuss measurements of single-molecule mobility and residency at the plasma membrane of live cells. Analysis of diffusional paths at the plasma membrane gives information about its physical properties and measurement of temporal trajectories enables rates of binding and dissociation to be derived. Meanwhile, close scrutiny of individual fluorophore trajectories enables ideas about molecular dimerization and oligomerization related to function to be tested directly.
An accurate analytic potential function for ground-state N2 from a direct-potential-fit analysis of spectroscopic data
Authors: Le Roy Robert J. , Huang Yiye , Jary Calvin
Published: October 2006

Two types of combined-isotopologue analysis have been performed on an extensive spectroscopic data set for ground-state N2 involving levels up to v=19, which is bound by half the well depth. Both a conventional Dunham-type analysis and a direct-potential-fit (DPF) analysis represent the data within (on average) the estimated experimental uncertainties. However, the Dunham-type parameters do not yield realistic predictions outside the range of the data used in the analysis, while the potential function obtained from the DPF treatment yields quantum mechanical accuracy over the data region and realistic predictions of the energies and properties of unobserved higher vibrational levels. Our DPF analysis also introduces a compact new analytic potential function form which incorporates the two leading inverse-power terms in the long-range potential.
Large chiral diffeomorphisms on Riemann surfaces and W-algebras
Authors: Bandelloni G. , Lazzarini S.
Published: October 2006

The diffeomorphism action lifted on truncated (chiral) Taylor expansion of a complex scalar field over a Riemann surface is presented in the paper under the name of large diffeomorphisms. After an heuristic approach, we show how a linear truncation in the Taylor expansion can generate an algebra of symmetry characterized by some structure functions. Such a linear truncation is explicitly realized by introducing the notion of Forsyth frame over the Riemann surface with the help of a conformally covariant algebraic differential equation. The large chiral diffeomorphism action is then implemented through a Becchi-Rouet-Stora (BRS) formulation (for a given order of truncation) leading to a more algebraic setup. In this context the ghost fields behave as holomorphically covariant jets. Subsequently, the link with the so-called W-algebras is made explicit once the ghost parameters are turned from jets into tensorial ghost ones. We give a general solution with the help of the structure functions pertaining to all the possible truncations lower or equal to the given order. This provides another contribution to the relationship between Korteweg-de Vries (KdV) flows and W-diffeomorphims.
Cell adhesion measurement by laser-induced stress waves
Authors: Hu Lili , Zhang Xuan , Miller Phillip , Ozkan Mihrimah , Ozkan Cengiz , Wang Junlan
Published: October 2006

Cell adhesion is a fundamental property of living cells and influences cell morphology, proliferation, and differentiation. The affinity of cells to relevant substrates plays an important role in tissue response to implanted devices and tissue regeneration. Directivity and precisely quantifying cell adhesion are paramount to the successful development of biomedical and hybrid devices. In this work, a laser-induced stress wave technique previously developed for thin solid film adhesion measurement is modified to investigate the cell-substrate adhesion. High-amplitude short-duration stress wave pulses induced by laser pulse absorption were used to detach cells from the substrate. The results obtained in this work proved the laser-induced stress wave technique to be an effective means for investigating the adhesion between biological cells and inorganic substrates.
Ion transport and electrochemical tuning of Fermi level in single-wall carbon nanotube probed by in situ Raman scattering
Authors: Gupta S. , Robertson J.
Published: October 2006

In situ Raman spectroscopy technique was employed to investigate the ion transport process and to determine the concomitant electrochemical tuning of Fermi level in single-wall carbon nanotube. The variation of structural bonding in single-wall carbon nanotube bundle dipped in aqueous alkaline earth halide electrolyte such as CaCl2 with electrochemical biasing was monitored. It is because Raman can detect changes in C–C bond length through radial breathing mode at ∼184cm−1 that varies inversely with the nanotube diameter and the G band at ∼1590cm−1 that varies with the axial bond length. Consistent reversible and substantial variations in Raman intensity of both the modes induced by electrode potential point at the fine and continuous tuning (alternatively, emptying/depleting or filling) of the specific bonding and antibonding states. Qualitatively, the results were explained in terms of changes in the energy gaps between the one-dimensional van Hove singularities present in the electron density of states arising possibly due to the alterations in the overlap integral of π bonds between the p orbitals of the adjacent carbon atoms. We estimated the extent of variation of the absolute potential of the Fermi level and overlap integral (γ0) between the nearest-neighbor carbon atoms from modeling the electrochemical potential dependence of Raman intensity. Observations also suggest that the work function of the tube is larger for the metallic nanotubes in contrast to the simultaneously present semiconducting nanotubes.
Grating based electro-optic switch with azo nonlinear optical polymers
Authors: Sarailou Edris , Gharavi Alireza , Javadpour Sirus , Shkunov Vladimir
Published: October 2006

Depth grating was inscribed into nonlinear optical (NLO) polymers functionalized with azo chromophores by a simple interferometric apparatus using a continuous wave laser within the absorption bands of NLO polymer. An attenuation of more than 99.5% with a full width at half maximum of ∼1.4nm was obtained for a single grating with a length of ∼1mm. By making use of the Pockels effect, the refractive index of the poled polymer was changed, and consequently the Bragg wavelength and filter transmission shifted. An ∼0.17nm shift in an applied field of about 116V∕μm was obtained in a weakly poled material.
Folding kinetics of a lattice protein via a forward flux sampling approach
Authors: Borrero Ernesto E. , Escobedo Fernando A.
Published: October 2006

We implement a forward flux sampling approach [R. J. Allen et al., J. Chem. Phys. 124, 194111 (2006)] for calculating transition rate constants and for sampling paths of protein folding events. The algorithm generates trajectories for the transition between the unfolded and folded states as chains of partially connected paths, which can be used to obtain the transition-state ensemble and the properties that characterize these intermediates. We apply this approach to Monte Carlo simulations of a model lattice protein in open space and in confined spaces of varying dimensions. We study the effect of confinement on both protein thermodynamic stability and folding kinetics; the former by mapping free-energy landscapes and the latter by the determination of rate constants and mechanistic details of the folding pathway. Our results show that, for the range of temperatures where the native state is stable, confinement of a protein destabilizes the unfolded state by reducing its entropy, resulting in increased thermodynamic stability of the folded state. Relative to the folding in open space, we find that the kinetics can be accelerated at temperatures above the temperature at which the unconfined protein folds fastest and that the rate constant increases with the number of constrained dimensions. By examining the statistical properties of the transition-state ensemble, we detect signs of a classical nucleation folding mechanism for a core of native contacts formed at an early stage of the process. This nucleus acts as folding foci and is composed of those residues that have higher probability to form native contacts in the transition-state intermediates, which can vary depending on the confinement conditions of the system.
Fast and accurate nonlinear spectral method for image recognition and registration
Authors: da Fontoura Costa Luciano , Bollt Erik
Published: October 2006

This letter addresses the problem of two- and higher-dimensional pattern matching, i.e., the identification of instances of a template within a larger signal space, which is a form of registration. Unlike traditional correlation, the authors aim at obtaining more selective matchings by considering more strict comparisons of gray-level intensity. In order to achieve fast matching, a nonlinear thresholded version of the fast Fourier transform is applied to a gray-level decomposition of the original two-dimensional image. The potential of the method is substantiated with respect to real data involving the selective identification of neuronal cell bodies in gray-level images.
Stretching of proteins in a uniform flow
Authors: Szymczak P. , Cieplak Marek
Published: October 2006

Stretching of a protein by a fluid flow is compared to that in a force-clamp apparatus. The comparison is made within a simple topology-based dynamical model of a protein in which the effects of the flow are implemented using Langevin dynamics. We demonstrate that unfolding induced by a uniform flow shows a richer behavior than that in the force clamp. The dynamics of unfolding is found to depend strongly on the selection of the amino acid, usually one of the termini, which is anchored. These features offer potentially wider diagnostic tools to investigate structure of proteins compared to experiments based on the atomic force microscopy.
Spectral analysis of radial Dirac operators in the Kerr-Newman metric and its applications to time-periodic solutions
Authors: Winklmeier Monika , Yamada Osanobu
Published: October 2006

We investigate the existence of time-periodic solutions of the Dirac equation in the Kerr-Newman background metric. To this end, the solutions are expanded in a Fourier series with respect to the time variable t, and the Chandrasekhar separation ansatz is applied so that the question of existence of a time-periodic solution is reduced to the solvability of a certain coupled system of ordinary differential equations. First, we prove the already known result that there are no time-periodic solutions in the nonextreme case. Then, it is shown that in the extreme case for fixed black hole data there is a sequence of particle masses (mN)N∊N for which a time-periodic solution of the Dirac equation does exist. The period of the solution depends only on the data of the black hole described by the Kerr-Newman metric.
Room temperature thermal conductance of alkanedithiol self-assembled monolayers
Authors: Wang Robert Y. , Segalman Rachel A. , Majumdar Arun
Published: October 2006

Solid-solid junctions with an interfacial self-assembled monolayer (SAM) are a class of interfaces with very low thermal conductance. Au–SAM–GaAs junctions were made using alkanedithiol SAMs and fabricated by nanotransfer printing. Measurements of thermal conductance using the 3ω technique were very robust and no thermal conductance dependence on alkane chain length was observed. The thermal conductances using octanedithiol, nonanedithiol, and decanedithiol SAMs at room temperature are 27.6±2.9, 28.2±1.8, and 25.6±2.4MWm−2K−1, respectively.
Formation and distribution of neutral vanadium, niobium, and tantalum oxide clusters: Single photon ionization at 26.5eV
Authors: Dong F. , Heinbuch S. , He S. G. , Xie Y. , Rocca J. J. , Bernstein E. R.
Published: October 2006

Neutral vanadium, niobium, and tantalum oxide clusters are studied by single photon ionization employing a 26.5eV/photon soft x-ray laser. During the ionization process the metal oxide clusters are almost free of fragmentation. The most stable neutral clusters of vanadium, niobium, and tantalum oxides are of the general form (MO2)0,1(M2O5)y. M2O5 is identified as a basic building unit for these three neutral metal oxide species. Each cluster family (Mm, m=1,…,9) displays at least one oxygen deficient and/or oxygen rich cluster stoichiometry in addition to the above most stable species. For tantalum and niobium families with even m, oxygen deficient clusters have the general formula (MO2)2(M2O5)y. For vanadium oxide clusters, oxygen deficient clusters are detected for all cluster families Vm (m=1,…,9), with stable structures (VO2)x(V2O5)y. Oxygen rich metal oxide clusters with high ionization energies (IE>10.5eV, 118nm photon) are detected with general formulas expressed as (MO2)2 (M2O5)y O1,2,3. Oxygen rich clusters, in general, have up to three attached hydrogen atoms, such as VO3H1,2, V2O5H1,2, Nb2O5H1,2, etc.
Atomistic simulations of shock-induced alloying reactions in Ni∕Al nanolaminates
Authors: Zhao Shijin , Germann Timothy C. , Strachan Alejandro
Published: October 2006

We employ molecular dynamics simulations with a first principles-based many body potential to characterize the exothermic alloying reactions of nanostructured Ni∕Al multilayers induced by shock loading. We introduce a novel technique that captures both the initial shock transit as well as the subsequent longer-time-scale Ni3Al alloy formation. Initially, the softer Al layers are shock heated to a higher temperature than the harder Ni layers as a result of a series of shock reflections from the impedance-mismatched interfaces. Once initiated, the highly exothermic alloying reactions can propagate in a self-sustained manner by mass and thermal diffusion. We also characterize the role of voids on the initiation of alloying. The interaction of the shock wave with the voids leads not only to significant local heating (hot spots) but also directly aids the intermixing between Al and Ni; both of these phenomena contribute to a significant acceleration of the alloying reactions.
Turbulent E×B advection of charged test particles with large gyroradii
Authors: Hauff T. , Jenko F.
Published: October 2006

The turbulent E×B advection of charged test particles with large gyroradii is investigated. To this aim, a recently developed theory—the so-called decorrelation trajectory method—is used together with direct numerical simulations and analytical calculations. It is found that for Kubo numbers larger than about unity, the particle diffusivity is almost independent of the gyroradius as long as the latter does not exceed the correlation length of the electrostatic potential. The underlying physical mechanisms leading to this surprising and initially counterintuitive behavior are identified.
Reaction of niobium and tantalum neutral clusters with low pressure, unsaturated hydrocarbons in a pickup cell: From dehydrogenation to Met-Car formation
Authors: He S.-G. , Xie Y. , Dong F. , Bernstein E. R.
Published: October 2006

Neutral niobium and tantalum clusters (Nbn and Tan) are generated by laser ablation and supersonic expansion into a vacuum and are reacted in a pickup cell with various low pressure (∼1mTorr) unsaturated hydrocarbons (acetylene, ethylene, propylene, 1-butene, 1,3-butadiene, benzene, and toluene) under nearly single collision conditions. The bare metal clusters and their reaction products are ionized by a 193nm laser and detected by a time of flight mass spectrometer. Partially and fully dehydrogenated products are observed for small (n⩽m) and large (n⩾m) neutral metal clusters, respectively, with m ranging from 2 to 5 depending on the particular hydrocarbon. In addition to primary, single collision products, sequential addition products that are usually fully dehydrogenated are also observed. With toluene used as the reactant gas, carbon loss products are observed, among which Nb8C12 and Ta8C12 are particularly abundant, indicating that the Met-Car molecule M8C12 can be formed from the neutral metal cluster upon two collisions with toluene molecules. The dehydrogenation results for low pressure reactions are compared with those available from previous studies employing flow tube (high pressure) reactors. Low pressure and high pressure cluster ion reactions are also compared with the present neutral metal cluster reactions. Reactions of unsaturated hydrocarbons and metal surfaces are discussed in terms of the present neutral cluster results.
Photodissociation dynamics of allyl bromide at 234, 265, and 267nm
Authors: Ji Lei , Tang Ying , Zhu Rongshu , Wei Zhengrong , Zhang Bing
Published: October 2006

The photodissociation dynamics of allyl bromide was investigated at 234, 265, and 267nm. A two-dimensional photofragment ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent Br* (P1∕22) and Br (P3∕22) atoms. The Br fragments show a bimodal translational energy distribution, while the Br* fragments reveal one translational energy distribution. The vertical excited energies and the mixed electronic character of excited states were calculated at ab initio configuration interaction method. It is presumed that the high kinetic energy bromine atoms are attributed to the predissociation from (ππ*)1 or (πσ*)1 state to the repulsive (nσ*)1 state, and to the direct dissociation from (nσ*)3 and (πσ*)3 states, while the low kinetic energy bromine atoms stem from internal conversion from the lowest (ππ*)3 state to (πσ*)3 state.
Simultaneous generation of shift and injection currents in (110)-grown GaAs∕AlGaAs quantum wells
Authors: Bieler M. , Pierz K. , Siegner U.
Published: October 2006

We have generated shift and injection currents in unstrained, undoped (110)-grown GaAs∕Al0.3Ga0.7As quantum wells with a single optical pulse and detected them via free-space terahertz experiments. By properly choosing the polarization state of the excitation pulse, it is possible to generate both currents alone along certain crystal directions or to simultaneously generate them along the same crystal direction. A comparison of injection and shift currents allows us to estimate the strength of the injection current. At an excitation energy of 1.53eV the injection current tensor element is ≈i2×107A∕(V2s). This corresponds to an injection of electrons with an average velocity of ≈10km∕s. Moreover, a comparison of the intensity dependence of shift and injection currents under identical experimental conditions demonstrates a stronger saturation of the shift current.
Classification of hexagonal adlayer arrangements by means of collective geometrical properties
Authors: Tkatchenko Alexandre , Batina Nikola
Published: October 2006

Unequal-sphere packing model is applied for the simulation of large number of hexagonal adlayer structures with surface coverage between θ=13 and θ=1 on the hexagonal substrate, with atomic radius of the adsorbate and substrate atoms as the only input. Each structure is characterized with respect to collective adlayer properties: the average adlayer height and the adlayer roughness. The distribution of hexagonal arrangements is presented in a special plot, which can be used for identification and characterization of hexagonal adlayers of different surface coverages and atomic registries. The most likely structures are related to the extreme values of our model parameters. The usefulness of this methodology is successfully demonstrated by comparison with some real adsorbate-substrate systems, i.e., halogens and rare gases adsorbed on (111) surface. Besides the agreement with experimental results, our model offers new insight into the formation of atomic adlayers and detailed analysis of the atomic registry. We believe that our approach will be of use for identification of probable structures among the large number of combinatorial possibilities in theoretical studies and for better interpretation of experimental results (i.e., scanning-tunneling microscopy images of atomic adlayers).
Robust response-compensation scheme for estimating the thermal time-constants of fine-wire temperature sensors
Authors: Kato K. , Tagawa M.
Published: October 2006

A robust response-compensation scheme has been developed to estimate in situ the thermal time constants of fine-wire temperature sensors in the frequency domain. The time-constant values can be obtained by maximizing a coherence function of two “compensated” temperature signals measured simultaneously by two sensors having different time constants. The scheme was validated experimentally and showed the highest tolerance to instrumentation noise compared with our previous methods.
Virtual instrument for the diagnosis of surface wave discharges
Authors: Carabaño O. , Ballesteros J. , Gamero A. , Sola A.
Published: October 2006

This article presents an experimental system that controls the automatic measuring procedure for the electromagnetic and optical diagnosis of a surface wave discharge by using a LABVIEW virtual instrument. The system also processes the measured data to determine some parameters that characterize the plasma, based on previously developed techniques. The structure of the system is implemented in such a way that permits the diagnosis of the surface wave discharges in both continuous and pulsed modes. The system has been tested and validated in a cylindrical discharge but it can be used to carry out measurement in a coaxial configuration too.
Dynamic magnetization processes in magnetostrictive amorphous wires
Authors: Chen A. P. , Zhukov A. , González J. , Domínguez L. , Blanco J. M.
Published: October 2006

We have performed the theoretical studies on the longitudinal dynamic magnetization process of magnetostrictive amorphous wires characterized by a large single Barkhausen jump (magnetic bistability) based on our previous experimental measurements on these wires. The domain structures of these wire samples consist of a single domain inner core with axial magnetization surrounded by the outer domain shell with the magnetization oriented perpendicular (λs>0) or circular (λs<0) to the wire axis. In the present work we use the resultant magnetization vector M⃗ tilting θ angle to z axis to describe the sample’s domain structures. In terms of solving the Landau-Lifshitz-Gilbert equation followed by M⃗ the analytical solution of the dimensionless axial component of the magnetization mz=MZ∕Ms has been obtained, and mz[t(H0,fe),τ,γ] is a function of the field amplitude H0, field frequency fe, and the samples’ material parameters such as the damping constant τ and the gyromagnetic ratio γ. The function mz[t(H0,fe),τ,γ] allows us to study the dynamic properties of the magnetization process of a wire sample. It has been found that the switching time ts, the switching field Hsw, and the dynamic coercive field Hdc depend on a magnetic field and material parameters. We found that the parameter α=γτ∕(1+τ2) related to the rate of M⃗, rotating the direction of the effective field, plays an important role in the magnetization process. By fitting the experimental data to the theoretical magnetization curve the value of the damping constant τ of the magnetostrictive amorphous wires can be estimated.
Ink-jet printing of organic metal electrodes using charge-transfer compounds
Authors: Hiraoka M. , Hasegawa T. , Abe Y. , Yamada T. , Tokura Y. , Yamochi H. , Saito G. , Akutagawa T. , Nakamura T.
Published: October 2006

In this work the authors fabricated patterned thin films made of highly conductive organic charge-transfer complexes using the ink-jet printing technique. The overprinted films of BO9(C14-TCNQ)4 exhibit low sheet resistivity and are available for carrier injection and interconnection of organic field-effect transistors (FETs). The performance of pentacene thin film FETs, where the top contact and bottom contact are ink-jet-printed organic electrodes, demonstrates the potential of organic/organic heterointerfaces.
Experimental and theoretical investigations of the He⋯I2 rovibronic spectra in the I2 B–X, 20–0 region
Authors: Ray Sara E. , McCoy Anne B. , Glennon John J. , Darr Joshua P. , Fesser Elizabeth J. , Lancaster Jeffrey R. , Loomis Richard A.
Published: October 2006

The laser-induced fluorescence and action spectra of I2 in a helium supersonic expansion have been recorded in the I2 B–X, 20–0 region. Two features are identified within the spectra. The lower-energy feature arises from transitions between states that are localized in a T-shaped conformation on both the X- and B-state potentials. The higher-energy feature reflects transitions from states that are localized in a linear conformation on the X state to states that have energies that are larger than the barrier for free rotation of the rare gas atom about the I2 molecule on the B-state potential. Ground-state binding energies of 16.6(6) and 16.3(6)cm−1 were determined for the T-shaped and linear conformers, respectively. These spectra are compared to those calculated using the experimentally determined rotational temperatures. Based on the agreement between the experimental and calculated spectra, the binding energies of the J′=0 states with 0 and 2–6 quanta of excitation in the He⋯I2 bending mode on the B state were determined. Several models for the B-state potential were used to investigate the origins of the shape of the contour of the higher-energy feature in the spectra for He⋯I2 and He⋯Br2. The shape of the contours was found to be relatively insensitive to the choice of potential. This leads us to believe that the spectra of these systems are relatively insensitive to the parameters of the B-state potential energy surface and are more sensitive to properties of the halogen molecule.
Oxygen vacancy motion in Er-doped barium strontium titanate thin films
Authors: Wang Junling , Trolier-McKinstry Susan
Published: October 2006

Amphoteric dopants are widely used in BaTiO3-based dielectrics to improve capacitor reliability. In this work, an analogous approach was explored for barium strontium titanate thin films. Ba0.7Sr0.3TiO3 thin films were prepared by chemical solution deposition. Er was used as a dopant to decrease the leakage current and improve the film lifetime under dc electric field. The (Ba+Sr)∕Ti ratio in the precursor solution was modified to facilitate doping on the A site, the B site, or both the A and B sites. It was observed that when (Ba+Sr)∕Ti=1, the dopant has little effect on the dielectric constant, but decreases both the loss tangent and the leakage current. A current transient (peak) was observed prior to resistance degradation in both the undoped and Er-doped samples and was related to oxygen migration under dc bias. It is shown that Er doping effectively decreases the oxygen vacancy mobility, which may lead to longer lifetime under dc field in thin film Ba0.7Sr0.3TiO3 capacitors.
Directional assembly of biomolecules on Si
Authors: Aurongzeb Deeder
Published: October 2006

Formation of oriented biomolecules of Si from large-scale filamental fungal growth is reported. The temperature induced process resulted in formation of ∼50nm width nanowires and nanoislands. By using scaling theory, it is shown that such self-organization is consistent with diffusive model. When 5nm Fe is deposited on Si substrate, exchange mechanism is observed, however, the effect is reduced on biomolecules. Films on self-organized molecules also exhibit high saturation magnetization.
Simulation of micro gas bubble generation of uniform diameter in an ultrasonic field by a boundary element method
Authors: Makuta Toshinori , Takemura Fumio
Published: October 2006

Micro gas bubbles of uniform diameter are generated periodically when liquid under pressure near the tip of a cylindrical needle is oscillated by an ultrasonic wave. Here, using a boundary element method, we simulated this gas-liquid interface behavior previously reported by Makuta et al. [J. Fluid Mech. 548, 113 (2006)]. Although the simulation model is simple because the flow field around the needle is assumed axisymmetrical and irrotational, the simulation results of the periodic interface behavior and of the effects of liquid viscosity and gas pressure inside the needle qualitatively agreed with experimental results reported in the above-mentioned reference. Simulation and experimental results revealed that a micro gas bubble is generated by detachment of the top of a projection produced by propagation of a surface wave traveling from the outside to the inside of the needle. The surface wave is produced by the difference in velocity near the needle wall, and is then propagated by surface tension. The size of the generated micro gas bubbles becomes uniform because the gas-liquid interface converges on periodic behavior due to the influence of liquid viscosity.
Photoemission study of the valence band offset between low temperature GaAs and (GaMn)As
Authors: Adell M. , Adell J. , Ilver L. , Kanski J. , Sadowski J.
Published: October 2006

Using synchrotron based photoelectron spectroscopy (GaMn)As∕GaAs interfaces prepared in situ by low temperature molecular beam epitaxy have been studied. No band offset between the two systems is observed. The continuous transition is explained as an effect of dilution of the (GaMn)As by GaAs adlayers.
Fabrication and photoluminescent characteristics of ZnO∕Mg0.2Zn0.8O coaxial nanorod single quantum well structures
Authors: Bae Jun Young , Yoo Jinkyoung , Yi Gyu-Chul
Published: October 2006

The authors report on fabrication and photoluminescent (PL) properties of ZnO∕Mg0.2Zn0.8O coaxial nanorod quantum structures with various quantum well and barrier layer thicknesses. Employing catalyst-free metal-organic vapor-phase epitaxy, coaxial nanorod single quantum well structures were fabricated by the alternate heteroepitaxial growth of ZnO and Mg0.2Zn0.8O layers over the entire surfaces of the ZnO nanorods with fine thickness controls of the layers. The quantum confinement effect of carriers in coaxial nanorod quantum structures depends on the Mg0.2Zn0.8O quantum barrier layer thickness as well as the thickness of the ZnO quantum well layer. The temperature-dependent PL characteristics of the coaxial nanorod quantum structures are also discussed.