bluuebunny/crossref_metadata_2025 huggingface dataset:
No increase of the interplanetary electric field since 1926
Authors: Le Sager Philippe , Svalgaard Leif
Published: July 2004

The long‐term variation of the interplanetary electric field is inferred back to 1926 from a correlation analysis with the magnetograms recorded at Godhavn and Thule, two polar cap geomagnetic observatories. The method is reliable because of the large dependence of the magnetic perturbation on the cross‐polar cap electric field, i.e., the penetration and mapping of the interplanetary electric field into the magnetosphere‐ionosphere system. This dependence is isolated by minimizing Sq and the Svalgaard‐Mansurov effect. Both appear when an observatory moves closer to the polar cap boundary and are found to be a minimum in a direction almost perpendicular to the magnetic north. Strictly speaking, no secular trend in the solar wind‐magnetosphere large‐scale coupling is indicated for the past 77 years. This suggests that there is no secular trend in the interplanetary electric field and by inference in the Sun's open magnetic flux and in the solar wind speed. The method is independent of the aa geomagnetic index and the sunspot cycle characteristics.
Intercomparison of ionospheric electrodynamics from the Iridium constellation with global MHD simulations
Authors: Korth H. , Anderson B. J. , Wiltberger M. J. , Lyon J. G. , Anderson P. C.
Published: July 2004

Global magnetospheric simulation results of the field‐aligned current (FAC) density at low altitudes are compared with global two‐dimensional distributions of Birkeland currents at the topside ionosphere derived from magnetic field data of the Iridium satellite constellation. We present two events with opposite direction of the interplanetary magnetic field By, 23 November 1999 1400–1700 UT, By > 0, and 31 March 2000 1500–1800 UT, By < 0. In both observations and simulations the configuration of the FACs near noon displays the expected reversal of sense with latitude corresponding to the sign of By. The total Region‐1 current in the simulations (2–3 MA) is larger than that derived from Iridium (1–1.5 MA), consistent with known underestimates from Iridium, but there is a deficit of Region 2 in the simulations. The poleward limit of the simulated Region‐1 currents is significantly, ∼5°, poleward of the observed currents for standard resolution simulations (∼6° per grid point at 70° magnetic latitude). Doubling the simulation resolution leads to a marked improvement in the agreement with the observed spatial distribution of Region‐1 FACs. Moreover, the higher resolution leads to the appearance of some Region‐2 FACs in the 23 November event. The location of the inner simulation boundary, 2 RE or 3 RE, is found to have negligible effect on the distribution of Region‐1 currents, but positioning the inner boundary at 3 RE can be inadequate for capturing Region‐2 FACs. Nightside currents in the simulation underestimate the observed FACs. Separating FACs associated with ∇ · E, ∇ΣP, and ∇ΣH terms in the ionosphere, we find that the observed nightside currents agree well with the ∇ · E FACs and that the ∇ΣH term cancels some of the ∇ · E currents on the nightside, resulting in the discrepancy with the observations. The results imply that in addition to including a ring current module to properly account for the particle drift physics, a high simulation resolution and improved nighttime ionospheric conductivities are two factors that also need to be addressed to obtain reliable simulation results.
A convective vorticity vector associated with tropical convection: A two‐dimensional cloud‐resolving modeling study
Authors: Gao Shouting , Ping Fan , Li Xiaofan , Tao Wei‐Kuo
Published: July 2004

Although dry/moist potential vorticity (( · ∇θe)/ρ) is a useful physical quantity for meteorological analysis, it cannot be applied to the analysis of two‐dimensional (2‐D) simulations. A new vorticity vector ( × ∇θe)/ρ (convective vorticity vector (CVV)) is introduced in this study to analyze 2‐D cloud‐resolving simulation data associated with 2‐D tropical convection. The cloud model is forced by the vertical velocity, zonal wind, horizontal advection, and sea surface temperature obtained from the Tropical Ocean‐Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) and is integrated for a selected 10‐day period. The CVV has zonal and vertical components in the 2‐D x‐z frame. Analysis of zonally averaged and mass‐integrated quantities shows that the correlation coefficient between the vertical component of the CVV and the sum of the cloud hydrometeor mixing ratios is 0.81, whereas the correlation coefficient between the zonal component and the sum of the mixing ratios is only 0.18. This indicates that the vertical component of the CVV is closely associated with tropical convection. The tendency equation for the vertical component of the CVV is derived and the zonally averaged and mass‐integrated tendency budgets are analyzed. The tendency of the vertical component of the CVV is determined by the interaction between the vorticity and the zonal gradient of cloud heating. The results demonstrate that the vertical component of the CVV is a cloud‐linked parameter and can be used to study tropical convection.
Non‐Foster and connected planar arrays
Authors: Hansen R. C.
Published: July 2004

A planar array of dipoles, with contiguous collinear dipole ends connected by a fixed inductor or capacitor, is evaluated. A Moment Method simulation of a 20 × 20 array provides Scan Element Pattern (gain per element), Scan Impedance, and current distributions. Cases considered, over a 10:1 bandwidth (dipole length .05λ to .5λ), include zero impedance coupling, capacitor coupling, inductor coupling, and Non‐Foster negative inductor coupling. The last is outstanding, probably because the physics of the Non‐Foster coupling makes sense.
HMGB1 is an endogenous immune adjuvant released by necrotic cells
Authors: Rovere‐Querini Patrizia , Capobianco Annalisa , Scaffidi Paola , Valentinis Barbara , Catalanotti Federica , Giazzon Marta , Dumitriu Ingrid E , Müller Susanne , Iannacone Matteo , Traversari Catia , Bianchi Marco E , Manfredi Angelo A
Published: July 2004

Immune responses against pathogens require that microbial components promote the activation of antigen‐presenting cells (APCs). Autoimmune diseases and graft rejections occur in the absence of pathogens; in these conditions, endogenous molecules, the so‐called ‘innate adjuvants’, activate APCs. Necrotic cells contain and release innate adjuvants; necrotic cells also release high‐mobility group B1 protein (HMGB1), an abundant and conserved constituent of vertebrate nuclei. Here, we show that necrotic HMGB1−/− cells have a reduced ability to activate APCs, and HMGB1 blockade reduces the activation induced by necrotic wild‐type cell supernatants. In vivo, HMGB1 enhances the primary antibody responses to soluble antigens and transforms poorly immunogenic apoptotic lymphoma cells into efficient vaccines.
Helicase Motif III in SecA is essential for coupling preprotein binding to translocation ATPase
Authors: Papanikou Efrosyni , Karamanou Spyridoula , Baud Catherine , Sianidis Giorgos , Frank Miriam , Economou Anastassios
Published: July 2004

The SecA ATPase is a protein translocase motor and a superfamily 2 (SF2) RNA helicase. The ATPase catalytic core (‘DEAD motor’) contains the seven conserved SF2 motifs. Here, we demonstrate that Motif III is essential for SecA‐mediated protein translocation and viability. SecA Motif III mutants can bind ligands (nucleotide, the SecYEG translocase ‘channel’, signal and mature preprotein domains), can catalyse basal and SecYEG‐stimulated ATP hydrolysis and can be activated for catalysis. However, Motif III mutation specifically blocks the preprotein‐stimulated ‘translocation ATPase’ at a step of the reaction pathway that lies downstream of ligand binding. A functional Motif III is required for optimal ligand‐driven conformational changes and kinetic parameters that underlie optimal preprotein‐modulated nucleotide cycling at the SecA DEAD motor. We propose that helicase Motif III couples preprotein binding to the SecA translocation ATPase and that catalytic activation of SF2 enzymes through Motif‐III‐mediated action is essential for both polypeptide and nucleic‐acid substrates.
Dynamic movement of actin‐like proteins within bacterial cells
Authors: Defeu Soufo Hervé Joël , Graumann Peter L
Published: July 2004

Actin proteins are present in pro‐ and eukaryotes, and have been shown to perform motor‐like functions in eukaryotic cells in a variety of processes. Bacterial actin homologues are essential for cell viability and have been implicated in the formation of rod cell shape, as well as in segregation of plasmids and whole chromosomes. We have generated functional green fluorescent protein fusions of all three Bacillus subtilis actin‐like proteins (MreB, Mbl and MreBH), and show that all three proteins form helical filaments underneath the cell membrane, the pattern of which is distinct for each protein. Time‐lapse microscopy showed that the filaments are highly dynamic structures. A number of separate filaments of MreB and Mbl continuously move through the cell along helical tracks underneath the cell membrane. The speed of extension of the growing end of filaments is within the range of known actin polymerization (0.1 μm/s), generating a potential poleward or centreward pushing velocity at 0.24 μm/min for MreB or Mbl, respectively. During nutritional downshift and a block in topoisomerase IV activity, the filaments rapidly disintegrated, showing that movement occurs only in growing cells. Contrary to Mbl and MreBH filaments, MreB filaments were generally absent in cells lacking DNA, providing a further distinction between the three orthologues.
Complex oscillations in a simple model for the Briggs-Rauscher reaction
Authors: Kim Kyoung-Ran , Shin Kook Joe , Lee Dong J.
Published: July 2004

Complex oscillations in a simple model of the Briggs-Rauscher reaction mechanism in a continuously stirred tank reactor proposed by Kim et al. [J. Chem. Phys. 117, 2710 (2002)] are investigated numerically. The k0-[CH2(COOH)2]0 phase diagram is constructed first where k0 is the flow rate and […]0 is the input concentration. Within the region surrounded by the Hopf bifurcation curve, we find complex oscillation regions which are again separated from the regular oscillation region by the secondary Hopf bifurcation curves. Mixed mode oscillations with an incomplete Farey sequence, periodic-chaotic (or nonperiodic) sequence, and various types of burst oscillations are observed in complex oscillation regions. Also, chaotic burst oscillations, which are due to the transition from one kind of burst to another kind, are reported.
In situ high P-T Raman spectroscopy and laser heating of carbon dioxide
Authors: Santoro Mario , Lin Jung-fu , Mao Ho-kwang , Hemley Russell J.
Published: July 2004

In situ high P-T Raman spectra of solid CO2 up to 67 GPa and 1660 K have been measured, using a micro-optical spectroscopy system coupled with a Nd:YLF laser heating system in diamond anvil cells. A metallic foil was employed to efficiently absorb the incoming Nd:YLF laser and heat the sample. The average sample temperature was accurately determined by detailed balance from the anti-Stokes/Stokes ratio, and was compared to the temperature of the absorber determined by fitting the thermal radiation spectrum to the Planck radiation law. The transformation temperature threshold and the transformation dynamics from the molecular phases III and II to the polymeric phase V, previously investigated only by means of temperature quench experiments, was determined at different pressures. The P-T range of the transformation, between 640 and 1100 K in the 33–65 GPa pressure interval, was assessed to be a kinetic barrier rather than a phase boundary. These findings lead to a new interpretation of the high P-T phase diagram of carbon dioxide. Furthermore, our approach opens a new way to perform quantitative in situ Raman measurements under extremely high pressures and temperatures, providing unique information about phase relations and structural and thermodynamic properties of materials under these conditions.
Compressibility and specific heats of heavier condensed rare gases near the liquid-vapor critical point
Authors: March N. H.
Published: July 2004

This study concerns the way to describe some physical properties of simple liquids by using the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy. It constructively analyzes the recent contribution of Sarkisov [G. N. Sarkisov, J. Chem. Phys. 119, 373 (2003)] on the structural behavior of a simple fluid near the liquid-vapor critical point. His work, already compared with computer simulation studies, is here brought into direct contact for the heavier condensed rare gases Ar, Kr, and Xe with (a) experiment and (b) earlier theoretical investigations. Directions for future studies then emerge.
Decoherence in an anharmonic oscillator coupled to a thermal environment: A semiclassical forward-backward approach
Authors: Elran Yossi , Brumer Paul
Published: July 2004

The decoherence of an anharmonic oscillator in a thermal harmonic bath is examined via a semiclassical approach. A computational strategy is presented and exploited to calculate the time dependence of the purity and the decay of individual matrix elements in the energy representation for a variety of initial states. The time dependence of the decoherence is found to depend on the temperature of the bath, the coupling strength, the initial state of the oscillator, and the choice of quantity measuring the decoherence. Recurrences in the purity and in the off-diagonal matrix elements are observed, as well as the collapse of these matrix elements to the diagonal, providing evidence for the retention of quantum coherence for time scales longer than that indicated by the purity. The results are used to analyze the utility of the Caldeira-Leggett and Redfield models of decoherence and to assess the dependence of dephasing rates on the degree of structure in phase space. In several cases we find that the dephasing dynamics can be described as an initial Zeno-effect regime, followed by a Caldeira-Leggett region, followed by recurrences.
Density and wave function analysis of actinide complexes: What can fuzzy atom, atoms-in-molecules, Mulliken, Löwdin, and natural population analysis tell us?
Authors: Clark Aurora E. , Sonnenberg Jason L. , Hay P. Jeffrey , Martin Richard L.
Published: July 2004

Recent advances in computational methods have made it possible to calculate the wave functions for a wide variety of simple actinide complexes. Equally important is the ability to analyze the information contained therein and produce a chemically meaningful understanding of the electronic structure. Yet the performance of the most common wave function analyses for the calculation of atomic charge and bond order has not been thoroughly investigated for actinide systems. This is particularly relevant because the calculation of charge and bond order even in transition metal complexes is known to be fraught with difficulty. Here we use Mulliken, Löwdin, natural population analysis, atoms-in-molecules (AIM), and fuzzy atom techniques to determine the charges and bond orders of UO22+, PuO22+, UO2, UO2Cl42−, UO2(CO)52+, UO2(CO)42+, UO2(CN)53−, UO2(CN)42−, UO2(OH)53−, and UO2(OH)42−. This series exhibits a clear experimental and computational trend in bond lengths and vibrational frequencies. The results indicate that Mulliken and Löwdin populations and bond orders are unreliable for the actinyls. Natural population analysis performs well after modification of the partitioning of atomic orbitals to include the 6d in the valence space. The AIM topological partitioning is insensitive to the electron donating ability of the equatorial ligands and the relative atomic volume of the formally U(VI) center is counterintuitively larger than that of O2− in the UO22+ core. Lastly, the calibrated fuzzy atom method yields reasonable bond orders for the actinyls at significantly reduced computational cost relative to the AIM analysis.
The first observation of the rhodium monofluoride molecule: Jet-cooled laser spectroscopic studies
Authors: Li Runhua , Jensen Roy H. , Balfour Walter J. , Shepard Scott A. , Adam Allan G.
Published: July 2004

Rhodium monofluoride has been observed and spectroscopically characterized. RhF molecules were produced under jet-cooled conditions in a laser vaporization molecular beam source by the reaction of a laser-vaporized rhodium plasma with SF6 doped in helium, and studied with laser-induced fluorescence spectroscopy under both medium and high resolution. More than 25 bands have been observed in laser-induced fluorescence between 18 500 and 24 500 cm−1 and five of these have been recorded at 200 MHz resolution. All bands of appreciable intensity have been rotationally analyzed. The ground electronic levels has Ω=2, which is attributed to an inverted Π3 state from the 2δ46π312σ1 electron configuration. The ground level rotational constants are B=0.272 45 cm−1, D=1.035×10−7 cm−1. Very small ground level Λ doublings are evident in the spectrum. Excited states having Ω=1, 2, and 3 have been identified. Dispersed fluorescence spectroscopy from 11 excited levels has been used to locate a large number of low-lying vibronic states within the energy range up to 8000 cm−1. A ground state vibrational interval of ∼575 cm−1 is suggested.
The mechanism of N2O formation via the (NO)2 dimer: A density functional theory study
Authors: Burch R. , Daniells S. T. , Hu P.
Published: July 2004

Catalytic formation of N2O via a (NO)2 intermediate was studied employing density functional theory with generalized gradient approximations. Dimer formation was not favored on Pt(111), in agreement with previous reports. On Pt(211) a variety of dimer structures were studied, including trans-(NO)2 and cis-(NO)2 configurations. A possible pathway involving (NO)2 formation at the terrace near to a Pt step is identified as the possible mechanism for low-temperature N2O formation. The dimer is stabilized by bond formation between one O atom of the dimer and two Pt step atoms. The overall mechanism has a low barrier of approximately 0.32 eV. The mechanism is also put into the context of the overall NO+H2 reaction. A consideration of the step-wise hydrogenation of O(ads) from the step is also presented. Removal of O(ads) from the step is significantly different from O(ads) hydrogenation on Pt(111). The energetically favored structure at the transition state for OH(ads) formation has an activation energy of 0.63 eV. Further hydrogenation of OH(ads) has an activation energy of 0.80 eV.
UV spectra of benzene isotopomers and dimers in helium nanodroplets
Authors: Schmied Roman , Çarçabal Pierre , Dokter Adriaan M. , Lonij Vincent P. A. , Lehmann Kevin K. , Scoles Giacinto
Published: July 2004

We report spectra of various benzene isotopomers and their dimers in helium nanodroplets in the region of the first Herzberg-Teller allowed vibronic transition 601 1B2u←1A1g (the A00 transition) at ∼260 nm. Excitation spectra have been recorded using both beam depletion detection and laser-induced fluorescence. Unlike for many larger aromatic molecules, the monomer spectra consist of a single “zero-phonon” line, blueshifted by ∼30 cm−1 from the gas phase position. Rotational band simulations show that the moments of inertia of C6H6 in the nanodroplets are at least six-times larger than in the gas phase. The dimer spectra present the same vibronic fine structure (though modestly compressed) as previously observed in the gas phase. The fluorescence lifetime and quantum yield of the dimer are found to be equal to those of the monomer, implying substantial inhibition of excimer formation in the dimer in helium.
High-level ab initio calculations for the four low-lying families of minima of (H2O)20. I. Estimates of MP2/CBS binding energies and comparison with empirical potentials
Authors: Fanourgakis George S. , Aprà Edoardo , Xantheas Sotiris S.
Published: July 2004

We report estimates of complete basis set (CBS) limits at the second-order Møller-Plesset perturbation level of theory (MP2) for the binding energies of the lowest-lying isomers within each of the four major families of minima of (H2O)20. These were obtained by performing MP2 calculations with the family of correlation-consistent basis sets up to quadruple ζ quality, augmented with additional diffuse functions (aug-cc-pVnZ, n=D, T, Q). The MP2/CPS estimates are −200.1 (dodecahedron, 30 hydrogen bonds), −212.6 (fused cubes, 36 hydrogen bonds), −215.0 (face-sharing pentagonal prisms, 35 hydrogen bonds), and −217.9 kcal/mol (edge-sharing pentagonal prisms, 34 hydrogen bonds). The energetic ordering of the various (H2O)20 isomers does not follow monotonically the number of hydrogen bonds as in the case of smaller clusters such as the different isomers of the water hexamer. The dodecahedron lies ca. 18 kcal/mol higher in energy than the most stable edge-sharing pentagonal prism isomer. The TIP4P, ASP-W4, TTM2-R, AMOEBA, and TTM2-F empirical potentials also predict the energetic stabilization of the edge-sharing pentagonal prisms with respect to the dodecahedron, albeit they universally underestimate the cluster binding energies with respect to the MP2/CBS result. Among them, the TTM2-F potential was found to predict the absolute cluster binding energies to within <1% from the corresponding MP2/CBS values, whereas the error for the rest of the potentials considered in this study ranges from 3% to 5%.
Feedback controlled nanocantilever device
Authors: Ke Changhong , Espinosa Horacio D.
Published: July 2004

A switchable carbon-nanotube-based nanoelectromechanical systems (NEMS) with close-loop feedback is examined. The device consists of a cantilever carbon nanotube clamped to a top electrode and actuated by a bottom electrode. The actuation circuit includes a source and a feedback resistor. The pull-in/pull-out and tunneling characteristics of the system are investigated by means of an electromechanical analysis. The model includes the concentration of electrical charge, at the end of the nanocantilever, and the van der Waals force. The analysis shows that the device has two well-defined stable equilibrium positions as a result of tunneling and the incorporation of a feedback resistor to the circuit. Potential applications of the device include NEMS switches, random-access memory elements, logic devices, electron counters, and gap sensing devices.
Formation and structure of the microemulsion phase in two-dimensional ternary AB+A+B polymeric emulsions
Authors: Düchs Dominik , Schmid Friederike
Published: July 2004

We present an analysis of the structure of the fluctuation-induced microemulsion phase in a ternary blend of balanced AB diblock copolymers with equal amounts of A and B homopolymers. To this end, graphical analysis methods are employed to characterize two-dimensional configuration snapshots obtained with the recently introduced field-theoretic Monte Carlo method. We find that a microemulsion forms when the mean curvature diameter of the lamellar phase coincides roughly with the periodicity of the lamellar phase. Further, we provide evidence to the effect of a subclassification of the microemulsion into a genuine and a defect-driven region.
Study of the hydrostatic pressure dependence of the Raman spectrum of single-walled carbon nanotubes and nanospheres
Authors: Amer Maher S. , El-Ashry Mostafa M. , Maguire John F.
Published: July 2004

We have investigated the behavior of single-walled carbon nanotubes and nanospheres (C60) under high hydrostatic pressure using Raman spectroscopy over the pressure range 0.2–10 GPa using a diamond anvil cell. Different liquid mixtures were used as pressure transmission fluids (PTF). Comparing the pressure dependence of the Raman peak positions for the nanotubes and the nanospheres in different PTF leads to the observation of a number of new phenomena. The observed shift in Raman peak position of both radial and tangential modes as a function of applied pressure and their dependence on the PTF chemical composition can be rationalized in terms of adsorption of molecular species from the of PTF on to the surface of the carbon nanotubes and/or nanospheres. The peak shifts are fully reversible and take place at a comparatively modest pressure (2–3 GPa) that is far below pressures that might be required to collapse the nanoparticles. Surface adsorption of molecular species on the nanotube or nanospheres provides a far more plausible rational for the observed phenomena than ideas based on the notion of tube collapse that have been put forward in the recent literature.
An ab initio study of the hyperfine structure in the X 2Π electronic state of CCCH
Authors: Perić Miljenko , Mladenović Milena , Engels Bernd
Published: July 2004

The results of an ab initio study of the magnetic hyperfine structure in the X 2Π electronic state of CCCH are reported. The potential surfaces for two components of the X 2Π electronic state were computed by means of an extensive configuration interaction approach. The electronically averaged hyperfine coupling constants of H and 13C for 12C 12C 12CH, 13C 12C 12CH, 12C 13C 12CH, and 12C 12C 13CH are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. The model Hamiltonian is expressed in terms of the normal bending coordinates. It is found that, due to the generally strong geometry dependence of the hyperfine coupling constants, it is necessary to carry out the vibronic averaging of the corresponding functions in order to obtain the values which can be compared to the results of the measurements. The results of the present study help to reliably interpret the experimental data previously published. They also predict the yet unobserved hyperfine structure in excited vibronic states.
A finite temperature linear tetrahedron method for electronic structure calculations of periodic systems
Authors: Yazyev Oleg V. , Brothers Edward N. , Kudin Konstantin N. , Scuseria Gustavo E.
Published: July 2004

A finite-temperature linear tetrahedron method for electronic structure calculations of periodic systems is developed. When compared to widely used simple temperature broadening, the number of k points necessary for accurate integration at finite temperatures is reduced. The utility of the method is demonstrated with benchmark calculations on 1D, 2D, and 3D systems.
Comparison of random-walk density functional theory to simulation for bead-spring homopolymer melts
Authors: Frischknecht Amalie L. , Curro John G.
Published: July 2004

Density profiles for a homopolymer melt near a surface are calculated using a random-walk polymeric density functional theory, and compared to results from molecular dynamics simulations. All interactions are of a Lennard-Jones form, for both monomer-monomer interactions and surface-monomer interactions, rather than the hard core interactions which have been most investigated in the literature. For repulsive systems, the theory somewhat overpredicts the density oscillations near a surface. Nevertheless, near quantitative agreement with simulation can be obtained with an empirical scaling of the direct correlation function. Use of the random phase approximation to treat attractive interactions between polymer chains gives reasonable agreement with simulation of dense liquids near neutral and attractive surfaces.
Dynamical heterogeneities close to a colloidal gel
Authors: Puertas Antonio M. , Fuchs Matthias , Cates Michael E.
Published: July 2004

Dynamical heterogeneities in a colloidal fluid close to gelation are studied by means of computer simulations. A clear distinction between some fast particles and the rest, slow ones, is observed yielding a picture of the gel composed of two populations with different mobilities. Analyzing the statics and dynamics of both sets of particles, it is shown that the slow particles form a network of stuck particles, whereas the fast ones are able to move over long distances. Correlation functions show that the environment of the fast particles relaxes much faster than that of the slow ones, but at short times the bonds between fast particles are longer lived due to the flexibility of their structure. No stringlike motion is observed for the fast particles, but they occupy preferential sites in the surface of the structure formed by the slow ones.
Near infrared study of water-benzene mixtures at high temperatures and pressures
Authors: Jin Yusuke , Ikawa Shun-ichi
Published: July 2004

Near-infrared absorption of water-benzene mixtures has been measured at temperatures and pressures in the ranges of 473–673 K and 100–400 bar, respectively. Concentrations of water and benzene in the water-rich phase of the mixtures were obtained from the integrated absorption intensities of the OH stretching overtone transition of water and the CH stretching overtone transition of benzene, respectively. Using these concentrations, the densities of the water-rich phase were estimated and compared with the average densities before mixing, which were calculated from literature densities of neat water and neat benzene. It is found that anomalously large volume expansion on the mixing occurs in the region enclosed by an extended line of the three-phase equilibrium curve and the one-phase critical curve of the mixtures, and the gas-liquid equilibrium curve of water. Furthermore, magnitude of the relative volume change increases with decreasing molar fraction of benzene in the present experimental range. It is suggested that dissolving a small amount of benzene in water induces a change in the fluid density from a liquidlike condition to a gaslike condition in the vicinity of the critical region.
Theoretical study of nitric oxide adsorption on Au clusters
Authors: Ding Xunlei , Li Zhenyu , Yang Jinlong , Hou J. G. , Zhu Qingshi
Published: July 2004

The adsorption properties of NO molecule on anionic, cationic, and neutral Aun clusters (n=1–6) are studied using the density functional theory with the generalized gradient approximation, and with the hybrid functional. For anionic and cationic clusters, the charge transfer between the Au clusters and NO molecule and the corresponding weakening and elongation of the N–O bond are essential factors of the adsorption. The neutral Au clusters have also remarkable adsorption ability to NO molecule. The adsorption energies of NO on the cationic clusters are generally greater than those on the neutral and anionic clusters.
H atom transfer along an ammonia chain: Tunneling and mode selectivity in 7-hydroxyquinoline⋅(NH3)3
Authors: Manca Carine , Tanner Christian , Coussan Stephane , Bach Andreas , Leutwyler Samuel
Published: July 2004

Excitation of the 7-hydroxyquinoline⋅(NH3)3 [7HQ⋅(NH3)3] cluster to the S1 1ππ* state results in an O-H→NH3 hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S1↔S0 spectra of the 7HQ⋅(NH3)3 and the d2-7DQ⋅(ND3)3 clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S1←S0 spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200–250 cm−1 for 7HQ⋅(NH3)3 accelerates the HAT reaction by an order of magnitude or more. The d2-7DQ⋅(ND3)3 cluster exhibits a more gradual threshold from 300 to 650 cm−1. For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H→NH3 coordinate. The barrier results from a conical intersection of the optically excited ππ*1 state with an optically dark πσ*1 state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H⋯NH3 distance and the energetic separation between the ππ*1 and πσ*1 states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the ππ*1↔1πσ* interaction to a much smaller extent.
Nitrogen self-diffusion in silicon nitride thin films probed with isotope heterostructures
Authors: Schmidt H. , Borchardt G. , Rudolphi M. , Baumann H. , Bruns M.
Published: July 2004

The self-diffusion of nitrogen is measured with secondary ion mass spectrometry in isotopically enriched polycrystalline and amorphous Si3N414∕Si3N415∕Si3N414 isotope heterostructures which were produced by reactive magnetron sputtering. The N diffusivities of polycrystalline films in the temperature range between 1130 and 1700°C follow an Arrhenius law over four orders of magnitude with a single activation enthalpy of ΔH=4.9eV and a pre-exponential factor of D0=1×10−6m2∕s. The calculated entropy of diffusion of ΔS≈0kB indicates a diffusion mechanism with localized point defects, in contrast to extended point defects usually found in semiconductors, like Si, Ge, and GaAs. The diffusivities in the amorphous state between 1130 and 1180°C do not differ significantly from those in the polycrystalline state.
Molecular dynamics study of the catalyst particle size dependence on carbon nanotube growth
Authors: Ding Feng , Rosén Arne , Bolton Kim
Published: July 2004

The molecular dynamics method, based on an empirical potential energy surface, was used to study the effect of catalyst particle size on the growth mechanism and structure of single-walled carbon nanotubes (SWNTs). The temperature for nanotube nucleation (800–1100 K), which occurs on the surface of the cluster, is similar to that used in catalyst chemical vapor deposition experiments, and the growth mechanism, which is described within the vapor-liquid-solid model, is the same for all cluster sizes studied here (iron clusters containing between 10 and 200 atoms were simulated). Large catalyst particles, which contain at least 20 iron atoms, nucleate SWNTs that have a far better tubular structure than SWNTs nucleated from smaller clusters. In addition, the SWNTs that grow from the larger clusters have diameters that are similar to the cluster diameter, whereas the smaller clusters, which have diameters less than 0.5 nm, nucleate nanotubes that are ≈0.6–0.7 nm in diameter. This is in agreement with the experimental observations that SWNT diameters are similar to the catalyst particle diameter, and that the narrowest free-standing SWNT is 0.6–0.7 nm.
Interpolation of diabatic potential energy surfaces
Authors: Evenhuis Christian R. , Collins Michael A.
Published: July 2004

A method is presented for constructing diabatic potential energy matrices from ab initio quantum chemistry data. The method is similar to that reported previously for single adiabatic potential energy surfaces, but correctly accounts for the nuclear permutation symmetry of diabatic potential energy matrices and other complications that arise from the derivative coupling of electronic states. The method is tested by comparison with an analytic model for the two lowest energy states of H3.
High-performance photorefractive polymers sensitized by cadmium selenide nanoparticles
Authors: Fuentes-Hernandez Canek , Suh Duck Jong , Kippelen Bernard , Marder Seth R.
Published: July 2004

We report on efficient and fast hybrid photorefractive polymer sensitized with cadmium selenide (CdSe) quantum dots. The surface of the quantum dots was treated with 4-methylbenzenethiol. This surfactant is responsible for efficient photoinduced charge generation in the composite, leading to fast grating build-up times of 100ms and below. Overmodulation of the diffraction efficiency was observed at an applied field of 60V∕μm.
Instrumentation for trace detection of high explosives
Authors: Moore D. S.
Published: July 2004

There is at present an urgent need for trace detection of high explosives, with applications to screening of people, packages, luggage, and vehicles. A great concern, because of recent terrorist activities, is for the development of methods that might allow detection and identification of explosives at a stand off distance. Nearly every analytical chemical method has been or is being applied to this problem. This review outlines the properties of explosives that might be utilized in detection schemes, discusses sampling issues, presents recent method developments with particular attention to detection limits, speed of analysis, and portability, and looks towards future developments.
Depth sensing and dissipation in tapping mode atomic force microscopy
Authors: Bodiguel Hugues , Montes Hélène , Fretigny Christian
Published: July 2004

Tapping mode atomic force microscopy is frequently used to image the surface of soft materials; it is also a powerful technique for nanomechanical analysis of surfaces. We report here an investigation of the depth sensing of the method on soft polymers. The chosen approach is based on the analysis of phase images of a model filled elastomer material. It leads to the determination of the depths of the hard particles lying under the surface. We found that tapping mode can probe interfaces buried under up to 80 nm of polymer. Under given tapping conditions, the penetration depth of the tip into the polymer is observed to depend on the layer thickness. However we show that, for a given penetration depth, the dissipated energy is independent of the thickness of the polymer layer under the tip. This suggests that the phase signal does not originate in the bulk viscoelasticity of the elastomer. Our observations support the hypothesis that, in tapping mode experiments on elastomers, the phase signal has an adhesive origin. Then, on surfaces with uniform interfacial properties, the phase images may reflect the local elastic properties of the sample, since they modify the tip-surface adhesive interactions.
Ultra-stable oven designed for x-ray reflectometry and ellipsometry studies of liquid surfaces
Authors: Brown Matt , Uran Serif , Law Bruce , Marschand Lyle , Lurio Larry , Kuzmenko I. , Gog T.
Published: July 2004

Stable temperature control is highly desirable for reflectivity studies of binary liquid mixtures. In this article we report on the construction of an oven that possesses good temperature stability (∼1 mK/day) and small transverse temperature gradients (<1 mK/cm). The oven has a horizontal geometry and can be used for either x-ray reflectometry or ellipsometry measurements from the liquid/vapor surfaces of such systems. Details of the oven design together with test results are provided.
Sensitivity improvement in surface infrared spectroscopy: Design, characteristics, and application of a high-temperature graphite source
Authors: Herceg Eldad , Celio Hugo , Trenary Michael
Published: July 2004

An infrared source designed to increase the sensitivity of the technique of reflection absorption infrared spectroscopy (RAIRS) for the detection of molecular adsorbates at submonolayer coverages on metal surfaces is described. The source is based on a graphite element with a lifetime of 500 h when operated at a temperature of ∼2300 K in a static pressure of 800 Torr of argon. The design allows for rapid and easy replacement of the low cost graphite element. The signal-to-noise ratio (SNR) achieved with this source for spectra obtained with both mercury cadmium telluride and indium antimonide detectors is a factor of 3-higher than obtained with a standard silicon carbide (SiC) source operated at ∼1500 K. With the higher SNR available with the graphite source it was possible to detect two vibrational features, δs(CH3) of methyl at 1247 cm–1, and ν(CH) of methylidyne at 2956 cm–1, that were not detected in previous RAIRS studies.
Effects of carbon nanosolids on the electro-optical properties of a twisted nematic liquid-crystal host
Authors: Lee Wei , Wang Chun-Yu , Shih Yu-Cheng
Published: July 2004

We present results of the electro-optical effect in 90° twisted nematic cells of pristine and doped liquid crystals under an applied dc voltage. The doped cells were fabricated with a minute addition of either buckminsterfullerene C60 or multiwalled carbon nanotubes (CNTs). Investigated were the switching behaviors as well as the hystereses and time evolutions of both the optical transmittance and electrical capacitance of the display samples. It is shown that doping with nanotubes can effectively reduce the dc driving voltage and improve the switching behavior.
Band offsets at the interfaces of GaAs(100) with GdxGa0.4-xO0.6 insulators
Authors: Afanas’ev V. V. , Stesmans A. , Passlack M. , Medendorp N.
Published: July 2004

Electron energy band alignment in GaAs(100)∕Ga2O3∕GdxGa0.4−xO0.6 structures was determined using internal photoemission and photoconductivity measurements. Two band gap values associated with Ga2O3(4.8eV) and Gd2O3(5.8eV) subnetworks were revealed. They yield potential barriers between the GaAs valence band and the bottom of the Ga2O3 and Gd2O3 derived conduction bands of 2.2 and 2.9eV, respectively. The corresponding conduction band offsets at the GaAs∕oxide interface, 0.8 and 1.5eV, indicate the possibility of significant reduction of electron injection in Gd-rich oxides.
Vortex arrays for sinh-Poisson equation of two-dimensional fluids: Equilibria and stability
Authors: Gurarie D. , Chow K. W.
Published: July 2004

The sinh-Poisson equation describes a stream function configuration of a stationary two-dimensional (2D) Euler flow. We study two classes of its exact solutions for doubly periodic domains (or doubly periodic vortex arrays in the plane). Both types contain vortex dipoles of different configurations, an elongated “cat-eye” pattern, and a “diagonal” (symmetric) configuration. We derive two new solutions, one for each class. The first one is a generalization of the Mallier–Maslowe vortices, while the second one consists of two corotating vortices in a square cell. Next, we examine the dynamic stability of such vortex dipoles to initial perturbations, by numerical simulations of the 2D Euler flows on periodic domains. One typical member from each class is chosen for analysis. The diagonally symmetric equilibrium maintains stability for all (even strong) perturbations, whereas the cat-eye pattern relaxes to a more stable dipole of the diagonal type.
Deoxyribonucleic acid sensing device with 40-nm-gap-electrodes fabricated by low-cost conventional techniques
Authors: Hashioka Shingi , Saito Masato , Tamiya Eiichi , Matsumura Hideki
Published: July 2004

A sensing device for the detection of a very small amount of the deoxyribonucleic acid (DNA) with the order of femtoliter is described. Such a sensing device has metal electrodes with a 40nm gap, and is fabricated by low-cost technology with the conventional photolithography and anodic oxidation. Gold (Au) electrodes are used to fix DNA, and the electric currents between two electrodes are measured to detect the existence of DNA. The results indicate that the present device is promising as future very low-cost DNA analyzing chips.
Interface structure and non-stoichiometry in HfO2 dielectrics
Authors: Baik Hion Suck , Kim Miyoung , Park Gyeong-Su , Song Se Ahn , Varela Maria , Franceschetti A. , Pantelides S. T. , Pennycook S. J.
Published: July 2004

High-resolution electron microscopy, electron energy-loss spectroscopy, and first-principles theory are used to investigate the composition and electronic structure of HfO2 dielectric layers deposited directly onto Si. A thin, nonstoichiometric, but Hf-free SiO2 layer forms between the HfO2 dielectric and the substrate, consistent with one-dimensional spinodal decomposition. Rapid thermal annealing crystallizes the HfO2, and the resulting grain boundaries within the HfO2 are found to be O-depleted, with localized states within the bandgap. These localized states are thought to act as significant leakage pathways, and may be responsible for Fermi-level pinning at the dielectric/contact interface.
Interdiffusion in a polydisperse polymer blend
Authors: Yashin Victor V. , Balazs Anna C.
Published: July 2004

We present a theoretical description of interdiffusion in a binary blend of polymers that exhibit polydispersity in length. The diffusion equations are formulated in terms of the volume fractions and the chain concentrations of the components. This choice of variables is equivalent to the assumption that the local molecular weight distributions of the components are described by the Flory distribution. The Onsager kinetic coefficients are obtained based on the Green-Kubo equation and correspond to the fast-mode interdiffusion theory. As demonstrated by numerical simulations, the resulting equations describe the simultaneous processes of the evolution of blend composition and the relaxation of the local molecular weight distributions of the components. The developed approach can be used to study polymer systems in which the degree of polymerization changes due to interfacial or bulk chemical reactions.
Atomic-layer deposition of Lu2O3
Authors: Scarel G. , Bonera E. , Wiemer C. , Tallarida G. , Spiga S. , Fanciulli M. , Fedushkin I. L. , Schumann H. , Lebedinskii Yu. , Zenkevich A.
Published: July 2004

Rare earth oxides could represent a valuable alternative to SiO2 in complementary metal–oxide–semiconductor devices. Lu2O3 is proposed because of its predicted thermodynamical stability on silicon and large conduction band offset. We report on the growth by atomic-layer deposition of lutetium oxide films using the dimeric {[C5H4(SiMe3)]2LuCl}2 complex, which has been synthesized for this purpose, and H2O. The films were found to be stoichiometric, with Lu2O3 composition, and amorphous. Annealing in nitrogen at 950°C leads to crystallization in the cubic bixbyite structure. The dielectric constant of the as-grown Lu2O3 layers is 12±1.
Magnetooptic properties of Fe∕Pd and Co∕Pd bilayers under hydrogen absorption
Authors: Lederman D. , Wang Y. , Morales E. H. , Matelon R. J. , Cabrera G. B. , Volkmann U. G. , Cabrera A. L.
Published: July 2004

The magnetooptical (MO) properties of hydrogenated Fe∕Pd and Co∕Pd bilayers were studied as a function of the H2 pressure. For samples with a Pd overlayer thickness 5.3nm and Fe film thickness in the range of 4.0–11.0nm, the fractional change in MO response under hydrogen loading, ΔθK∕θK0, increases approximately 12%, independent of the Fe film thickness. For H2 pressures less than PH2=25Torr, the enhancement obeys Sievert’s law (ΔθK∕θK0∝PH21∕2). Thicker Pd overlayers increase the MO enhancement, with remarkably large enhancement of 50% for a Fe(4.1nm)∕Pd(10.0nm) sample. Because ΔθK∕θK0 is independent of the Fe thickness, this effect results from a change in the optical properties of the Pd overlayer. This was confirmed by vibrating sample magnetometry. In contrast, no effect is observed in the Co∕Pd bilayers, presumably due to a small amount Co interdiffusion that prevents the Pd from absorbing H2.
Short-period intrinsic Stark GaN∕AlGaN superlattice as a Bloch oscillator
Authors: Litvinov V. I. , Manasson A. , Pavlidis D.
Published: July 2004

We discuss the properties of AlGaN∕GaN superlattice (SL) related to the feasibility of a terahertz-range oscillator. The distortion of the conduction-band profile by the polarization fields has been taken into account. We have calculated the conduction-band offset between the pseudomorphic AlGaN barrier and the GaN quantum well, the first miniband width and energy dispersion, as functions of Al content in the barrier. As the short-period SL miniband energy dispersion contains contributions from next to nearest neighbors, it causes anharmonic electron oscillations at the multiples of the fundamental Bloch frequency. The Al content and SL period that favor high-frequency oscillations have been determined.