bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of dipolarization at geosynchronous orbits and its response in the polar cap convection during extreme southward interplanetary magnetic field conditions
Authors: Jayachandran P. T. , MacDougall J. W. , Hamza A. M. , Henderson M. G.
Published: October 2007

Analysis of the polar cap convection and geosynchronous magnetic field measurements, in the context of dipolarization events, during extreme southward interplanetary magnetic field (IMF) conditions is presented in the paper. Clear dipolarization events at geosynchronous orbits are always followed by an increase in the polar cap convection with a time delay. The distribution of the time delay between the onset of dipolarization at geosynchronous orbit and polar cap convection response varied between 6 and 15 min with an average of 9 min. The polar cap convection response to the dipolarization events enabled the deduction of the dipolarization timescales at geosynchronous orbits and in the ionosphere. A comparison of these two timescales revealed that the timescales of dipolarization events deduced from geosynchronous magnetic field measurements were always shorter (average 11 min) than the timescales deduced from the polar cap convection measurements (average 33 min).
Relationship of electric field and charged particle density fluctuations to air turbulence in the mesosphere
Authors: Robertson Scott
Published: October 2007

A theoretical model is developed that is applicable to the electric field fluctuations that arise in the polar summer mesosphere as a result of the coupling of the charged species to the neutral air turbulence. The motions of electrons, ions, and charged aerosol particles are described as harmonic oscillators both driven and damped by the drag force exerted by the neutral air. The relative fluctuations in the ion density are found to be nearly the same as those in the neutral air as a consequence of the ions' high‐momentum transfer relaxation frequency. The aerosol density fluctuations follow those of the neutral air at frequencies below their relaxation frequency, which is in the acoustic range. The electrons move primarily in response to the electric force to partially cancel the net charge density of ions and aerosol particles except at wavelengths shorter than the Debye length. Electric field and charge density fluctuations are calculated for several sets of conditions. In “bite‐out” regions in which the electron density is reduced as a consequence of attachment to the aerosol particles the electric field fluctuations are found to be enhanced, which is consistent with observations.
Estimating the scaling of rain rate moments from radar and rain gauge
Authors: Paulson Kevin S. , Zhang Xaiobei
Published: October 2007

Rain rate is a physical parameter that is only defined over some spatial or spatial‐temporal integration volume. The moments of rain rate fields calculated from measurements derived from integration volumes of the same shape and a range of sizes are commonly used as a summarizing statistic of the rain rate process. Ranges of scales are known as stochastically scaling, either simple or multiscaling, when the moments are a power law of integration volume size. The identification of scaling ranges provides information about the dominant physical processes leading to rain rate variation and allow simulation models to be devised. The spatial moment scaling statistics of rain fields have been estimated from radar data many times. In this paper, three real and potential problems with the reported statistics are identified; that is, the existence of moments is not verified, the effects of interpolation have not been considered and the inhomogeneity introduced by variation in radar sample volume with range has been ignored. Radar data from the Chilbolton CAMRa radar in the UK are analyzed, and the existence of positive moments of all orders is demonstrated. An algorithm has been implemented for the calculation of moments from spatially averaged rain data on a regular polar grid. The resulting moments are consistent with cosited rain gauge data and show smooth variation across the scales considered, 300 m to 10 km and 10 s to 6 hours. The resulting moments are well approximated by two multiscaling ranges with a scale break around 3 km or 300 s.
Airborne validation of spatial properties measured by the CALIPSO lidar
Authors: McGill Matthew J. , Vaughan Mark A. , Trepte Charles R. , Hart William D. , Hlavka Dennis L. , Winker David M. , Kuehn Ralph
Published: October 2007

The Cloud‐Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) satellite provides a new and exciting opportunity to study clouds and aerosols in the Earth's atmosphere using range‐resolved laser remote sensing. Following the successful launch of the CALIPSO satellite, validation flights were conducted using the long‐established Cloud Physics Lidar (CPL) to verify CALIPSO's calibration and validate various CALIPSO data products. This paper presents results of the initial comparisons made between the spaceborne CALIPSO lidar and the airborne CPL. Results are presented to validate measurement sensitivity and the spatial properties reported in the CALIPSO data products. Cloud layer top determinations from CALIPSO are found to be in good agreement with those from CPL. Determinations of minimum detectable backscatter are in excellent agreement with theoretical values predicted prior to launch.
Rain attenuation prediction and modeling for line‐of‐sight links on terrestrial paths in South Africa
Authors: Fashuyi M. O. , Afullo T. J.
Published: October 2007

The different rain attenuation prediction models proposed by different authors on terrestrial paths are studied in this paper. Subsequently, for paths not exceeding 22 km, the rain attenuation exceeded for 0.01% of the time for these four geographical locations is estimated for South Africa using the ITU‐R Model, the Crane Global model, and the Moupfouma model, at different frequencies. Finally, the predicted attenuation values are compared on a monthly basis, as well with the measured upper and lower attenuation bounds for a 6.73‐km line‐of‐sight link operating at 19.5 GHz in Durban.
Analytical and numerical investigation of the radiation from concentric metamaterial spheres excited by an electric Hertzian dipole
Authors: Arslanagic Samel , Ziolkowski Richard W. , Breinbjerg Olav
Published: October 2007

The canonical problem of an electric Hertzian dipole radiating in the presence of a pair of concentric double negative metamaterial spheres is investigated analytically and numerically. The spatial distribution of the near field as well as the total radiated power are examined. The results are compared to those for the corresponding structures made of conventional double positive materials. It is shown that electrically small concentric metamaterial spheres can be designed to be resonant and that these resonant designs lead to significant changes in the field radiated by the electric Hertzian dipole and, in particular, to significant enhancements of the total power radiated by it. The impact of the location of the electric Hertzian dipole in both radiating and scattering configurations is studied in detail. Furthermore, the influence of dispersion and loss is investigated. Finally, a few results for larger size metamaterial concentric spheres are given.
Type curve interpretation of late‐time pumping test data in randomly heterogeneous aquifers
Authors: Neuman Shlomo P. , Blattstein Ayelet , Riva Monica , Tartakovsky Daniel M. , Guadagnini Alberto , Ptak Thomas
Published: October 2007

The properties of heterogeneous media vary spatially in a manner that can seldom be described with certainty. It may, however, be possible to describe the spatial variability of these properties in terms of geostatistical parameters such as mean, integral (spatial correlation) scale, and variance. Neuman et al. (2004) proposed a graphical method to estimate the geostatistical parameters of (natural) log transmissivity on the basis of quasi–steady state head data when a randomly heterogeneous confined aquifer is pumped at a constant rate from a fully penetrating well. They conjectured that a quasi–steady state, during which heads vary in space‐time while gradients vary only in space, develops in a statistically homogeneous and horizontally isotropic aquifer as it does in a uniform aquifer. We confirm their conjecture numerically for Gaussian log transmissivities, show that time‐drawdown data from randomly heterogeneous aquifers are difficult to interpret graphically, and demonstrate that quasi–steady state distance‐drawdown data are amenable to such interpretation by the type curve method of Neuman et al. The method yields acceptable estimates of statistical log transmissivity parameters for fields having either an exponential or a Gaussian spatial correlation function. These estimates are more robust than those obtained using the graphical time‐drawdown method of Copty and Findikakis (2003, 2004a). We apply the method of Neuman et al. (2004) simultaneously to data from a sequence of pumping tests conducted in four wells in an aquifer near Tübingen, Germany, and compare our transmissivity estimate with estimates obtained from 312 flowmeter measurements of hydraulic conductivity in these and eight additional wells at the site. We find that (1) four wells are enough to provide reasonable estimates of lead log transmissivity statistics for the Tübingen site using this method, and (2) the time‐drawdown method of Cooper and Jacob (1946) underestimates the geometric mean transmissivity at the site by 30–40%.
EvolveAGene 3: A DNA coding sequence evolution simulation program
Authors: Hall Barry
Published: October 2007

EvolveAGene 3 is a realistic coding sequence simulation program that separates mutation from selection and allows the user to set selection conditions, including variable regions of selection intensity within the sequence and variation in intensity of selection over branches. Variation includes base substitutions, insertions and deletions. Output includes a log file, the true tree and both unaligned coding sequence and protein sequences and the true DNA and protein alignments.
The proteasome maturation protein POMP facilitates major steps of 20S proteasome formation at the endoplasmic reticulum
Authors: Fricke Benjamin , Heink Sylvia , Steffen Janos , Kloetzel Peter‐Michael , Krüger Elke
Published: October 2007

The quality control of proteins mediated by the plasticity of the proteasome system is regulated by the timely and flexible formation of this multisubunit proteolytic enzyme complex. Adaptable biogenesis of the 20S proteasome core complex is therefore of vital importance for adjusting to changing proteolytic requirements. However, the molecular mechanism and the cellular sites of mammalian proteasome formation are still unresolved. By using precursor complex‐specific antibodies, we now show that the main steps in 20S core complex formation take place at the endoplasmic reticulum (ER). Thereby, the proteasome maturation protein (POMP)—an essential factor of mammalian proteasome biogenesis—interacts with ER membranes, binds to α1–7 rings, recruits β‐subunits stepwise and mediates the association of mammalian precursor complexes with the ER. Thus, POMP facilitates the main steps in 20S core complex formation at the ER to coordinate the assembly process and to provide cells with freshly formed proteasomes at their site of function.
Axon growth and guidance genes identify T‐dependent germinal centre B cells
Authors: Yu Di , Cook Matthew C , Shin Dong‐Mi , Silva Diego G , Marshall Jennifer , Toellner Kai‐Michael , Havran Wendy L , Caroni Pico , Cooke Michael P , Morse Herbert C , MacLennan Ian CM , Goodnow Christopher C , Vinuesa Carola G
Published: October 2007

Selection of B cells subjected to hypermutation in germinal centres (GC) during T cell‐dependent (TD) antibody responses yields memory cells and long‐lived plasma cells that produce high affinity antibodies biased to foreign antigens rather than self‐antigens. GC also form in T‐independent (TI) responses to polysaccharide antigens but failed selection results in GC involution and memory cells are not generated. To date there are no markers that allow phenotypic distinction of T‐dependent and TI germinal centre B cells. We compared the global gene expression of GC B cells purified from mice immunized with either TD or TI antigens and identified eighty genes that are differentially expressed in TD GC. Significantly, the largest cluster comprises genes involved in growth and guidance of neuron axons such as Plexin B2, Basp1, Nelf, Shh, Sc4mol and Sult4α. This is consistent with formation of long neurite (axon and dendrite)‐like structures by mouse and human GC B cells, which may facilitate T:B cell interactions within GC, affinity maturation and B cell memory formation. Expression of BASP1 and PLEXIN B2 protein is very low or undetectable in resting and TI GC B cells, but markedly upregulated in GC B cells induced in the presence of T cell help. Finally we show some of the axon growth genes upregulated in TD‐GC B cells including Basp1, Shh, Sult4α, Sc4mol are also preferentially expressed in post‐GC B cell neoplasms.
Human pathologies associated with defective RNA transport and localization in the nervous system
Authors: Dahm Ralf , Macchi Paolo
Published: October 2007

RNA localization is emerging as an important process to restrict certain proteins to specific subcellular domains and thus spatially control the expression of genes within cells. It is used, for instance, to compartmentalize the developing embryo during early embryogenesis. The localization of RNA also plays important roles later during development, such as in asymmetric cell divisions, cell migration and the outgrowth and pathfinding of axons and dendrites. In differentiated cells, it serves to subdivide the cell into functionally distinct compartments. For example, in mature neurons it is believed to contribute to the plastic changes of individual synapses underlying learning and memory. In this review, we highlight the importance of subcellular RNA localization for the function of the nervous system and neurological diseases associated with defective RNA localization and translation. These diseases include fragile X mental retardation syndrome, spinocerebellar ataxia and spinal muscular atrophy.
Infosys Technologies: Improving Organizational Knowledge Flows
Authors: Mehta Nikhil , Oswald Sharon , Mehta Anju
Published: October 2007

Knowledge is being discussed as one of the most important organizational resources. But these resources exist in specialized pockets dispersed across the organization, and dedicated knowledge management (KM) programs are required to improve their flow. However, high failure rates of such programs raise serious doubts about their ability to improve knowledge flows. This case traces the KM program of Infosys Technologies, Ltd – a Global Most Admired Knowledge Enterprise. The case describes how, in 1999, Infosys’ top management detected a severe lack of organizational knowledge flows while implementing a program aimed at continuously improving their core business processes. A more detailed examination exposed that the lack of knowledge flows stifled the effectiveness of their organizational structure and their business model. Alarmed by these critical findings, Infosys initiated their KM program. A five-stage knowledge maturity model (KMM) was conceptualized to aid KM implementation. With people, processes, and technology as the three pillars of Infosys’ KM program, KMM identified specific capabilities Infosys needed to develop in each of the five levels. Things worked fine till 2004 when Infosys began moving towards KMM Level 4, which required developing clear metrics to measure KM effectiveness, that is, improvements in knowledge flow. In the absence of such metrics, Infosys’ Board of Directors started questioning company's financial investment in the KM program. The CEO, who championed the KM program, knew that he faced two key challenges – to convince the Board of future revenue prospects of the KM program, and to identify metrics for assessing improvements in organizational knowledge flows.
Longitudinal spatial hole burning in terahertz quantum cascade lasers
Authors: Kröll Josef , Darmo Juraj , Unterrainer Karl , Dhillon Sukhdeep S. , Sirtori Carlo , Marcadet Xavier , Calligaro Michel
Published: October 2007

The propagation of a quasi-single-cycle terahertz pulse through an active terahertz quantum cascade laser is studied. The terahertz pulse is found to be diffracted on the optically induced longitudinal modulation of the refractive index of the laser active region. This modulation is a result of longitudinal spatial hole burning due to the formation of a standing wave in the laser cavity.
Unified view on multiconfigurational time propagation for systems consisting of identical particles
Authors: Alon Ofir E. , Streltsov Alexej I. , Cederbaum Lorenz S.
Published: October 2007

We show that the successful and formally exact multiconfigurational time-dependent Hartree method (MCTDH) takes on a unified and compact form when specified for systems of identical particles (MCTDHF for fermions MCTDHB for bosons). In particular the equations of motion for the orbitals depend explicitly and solely on the reduced one- and two-body density matrices of the system’s many-particle wave function. We point out that this appealing representation of the equations of motion opens up further possibilities for approximate propagation schemes.
Energies of ions in water and nanopores within density functional theory
Authors: Leung Kevin , Marsman Martijn
Published: October 2007

Accurate calculations of electrostatic potentials and treatment of substrate polarizability are critical for predicting the permeation of ions inside water-filled nanopores. The ab initio molecular dynamics method, based on density functional theory (DFT), accounts for the polarizability of materials, water, and solutes, and it should be the method of choice for predicting accurate electrostatic energies of ions. In practice, DFT coupled with the use of periodic boundary conditions in a charged system leads to large energy shifts. Results obtained using different DFT packages may vary because of the way pseudopotentials and long-range electrostatics are implemented. Using maximally localized Wannier functions, we apply robust corrections that yield relatively unambiguous ion energies in select molecular and aqueous systems and inside carbon nanotubes. Large binding energies are predicted for ions in metallic carbon nanotube arrays, while Na+ and Cl− energies are found to exhibit asymmetry in water that is smaller than but comparable with those computed using nonpolarizable water force fields.
Response to “Comment on ‘Glass transition in pure and doped amorphous solid: An ultrafast microcalorimetry study’ [J. Chem. Phys. 125, 094501 (2006)]”
Authors: Sadtchenko Vlad , Lu Haiping , McCartney S.
Published: October 2007

We report results of ultrafast scanning calorimetry (USC) measurements of enthalpy relaxation time for propanol, toluene, pentanol, decalin, and 2-ethyl-1-hexanol in a temperature range from 120to180K. These new data show that the enthalpy relaxation times measured in USC experiments are within one order of magnitude of those derived from dielectric spectroscopy studies. Thus, we demonstrate that Johari’s critique of the USC study of pure and doped amorphous solid water is without merit, and that USC experiments do provide evidence against assignment of the glass transition temperature of amorphous solid water to 136K.
Interaction potentials and transport properties of coinage metal cations in rare gases
Authors: Yousef Ahlam , Shrestha Shraddha , Viehland Larry A. , Lee Edmond P. F. , Gray Benjamin R. , Ayles Victoria L. , Wright Timothy G. , Breckenridge W. H.
Published: October 2007

High-level ab initio calculations are performed on the coinage metal cations (Cu+, Ag+, and Au+) interacting with each of the rare gases [Rg (Rg=He to Rn)]. The RCCSD(T) procedure is employed, with basis sets being of approximately quintuple-ζ quality, but with the heavier species using relativistic effective core potentials. The interaction potentials are compared to experimental and theoretical data where they exist. In addition, transport coefficients for the mobility and diffusion of the cations in the rare gases are calculated. The latter have involved a rewriting of some of the programs used, and the required modifications are discussed. The mobility results indicate that, rather than being a rare occurrence, mobility minima may be common phenomena. Finally, a new estimate is put forward for the validity of zero-field mobilities in ion mobility spectrometry.
Properties of ices at 0 K: A test of water models
Authors: Aragones J. L. , Noya E. G. , Abascal J. L. F. , Vega C.
Published: October 2007

The properties of ices Ih, II, III, V, and VI at zero temperature and pressure are determined by computer simulation for several rigid water models (SPC/E, TIP5P, TIP4P/Ice, and TIP4P/2005). The energies of the different ices at zero temperature and pressure (relative to the ice II energy) are compared to the experimental results of Whalley [J. Chem. Phys. 81, 4087 (1984)]. TIP4P/Ice and TIP4P/2005 provide a qualitatively correct description of the relative energies of the ices at these conditions. In fact, only these two models provide the correct ordering in energies. For the SPC/E and TIP5P models, ice II is the most stable phase at zero temperature and pressure whereas for TIP4P/Ice and TIP4P/2005 ice Ih is the most stable polymorph. These results are in agreement with the relative stabilities found at higher temperatures. The solid-solid phase transitions at 0 K are determined. The predicted pressures are in good agreement with those obtained from free energy calculations.
A limit of stability in supercooled liquid clusters
Authors: Mendez-Villuendas Eduardo , Saika-Voivod Ivan , Bowles Richard K.
Published: October 2007

We examine the metastable liquid phase of a supercooled gold nanocluster by studying the free energy landscape using the largest solidlike embryo as an order parameter. Just below freezing, the free energy exhibits a local minimum at small embryo sizes and a maximum at a larger critical embryo size. At T=660K the free energy becomes a monotonically decreasing function of the order parameter as the liquid phase becomes unstable, indicating that we have reached a limit of stability. In contrast to the mean-field theory predictions for a spinodal, the size of the critical embryo remains finite as the limit of stability is approached. We also calculate the rate of nucleation, independently from our free energy calculations, and observe a rapid increase in its temperature dependence when the free energy barrier is on the order of kT. We suggest that this supports the idea that freezing becomes a barrierless process at low temperatures.
Umbrella sampling for nonequilibrium processes
Authors: Warmflash Aryeh , Bhimalapuram Prabhakar , Dinner Aaron R.
Published: October 2007

The authors introduce an algorithm for determining the steady-state probability distribution of an ergodic system arbitrarily far from equilibrium. By enforcing equal sampling of different regions of phase space, as in umbrella sampling simulations of systems at equilibrium, low probability regions are explored to a much greater extent than in physically weighted simulations. The algorithm can be used to accumulate joint statistics for an arbitrary number of order parameters for a system governed by any stochastic dynamics. They demonstrate the efficiency of the algorithm by applying it to a model of a genetic toggle switch which evolves irreversibly according to a continuous time Monte Carlo procedure.
Nucleation rate isotherms of argon from molecular dynamics simulations
Authors: Wedekind Jan , Wölk Judith , Reguera David , Strey Reinhard
Published: October 2007

We report six nucleation rate isotherms of vapor-liquid nucleation of Lennard-Jones argon from molecular dynamics simulations. The isotherms span three orders of magnitude in nucleation rates, 1023<J∕cm−3s−1<1026, in a temperature range of 45–70K below the triple point. The rates are very accurately determined using the concept of mean first-passage times, which also allows a determination of the critical cluster size directly from the kinetics. The results deviate from classical nucleation theory (CNT) by two to seven orders of magnitude, which nevertheless is much smaller than the more than 20 orders of magnitude encountered in recent experiments in a similar temperature range. The extended modified liquid drop-dynamical nucleation theory (EMLD-DNT) shows excellent agreement with the simulation results with deviations of less than one order of magnitude over the entire studied temperature range. Both simulation and experiment confirm the same incorrect temperature trend of CNT, which seems to be corrected in the EMLD-DNT model. However, the predictions of CNT for the critical cluster sizes agree well with the results obtained from the simulations using the nucleation theorem, supporting the notion that CNT successfully estimates the location of the transition state but severely fails to predict its height.
Coupled cluster investigation on the low-lying electronic states of CuCN and CuNC and the ground state barrier to isomerization
Authors: Paul Ankan , Yamaguchi Yukio , Schaefer Henry F.
Published: October 2007

The observation of several metal cyanides and isocyanides in interstellar space has raised much interest these molecules. Optimum molecular structures, harmonic vibrational frequencies, and dipole moments of the ground electronic states (X̃Σ+1), triplet excited states, and open shell singlet excited states of CuCN and CuNC were determined using different levels of nonrelativistic and scalar relativistic (Douglas-Kroll) [Ann. Phys. 82, 89 (1979)] coupled cluster theory in conjunction with atomic natural orbital basis sets and correlation consistent basis sets. For the relativistic computations the specially contracted correlation consistent Douglas-Kroll (DK) basis sets were used. Moreover, barriers to isomerization from CuCN to CuNC were computed. The predicted structures of the X̃Σ+1 state for CuCN are re(Cu–C)=1.826Å and re(C–N)=1.167Å, at the most sophisticated level of theory, the scalar relativistic DK-CCSD(T)/cc-pVQZ(DK) method. These results are in excellent agreement with the experimentally determined Cu–C bond length of 1.829Å and C–N bond distance of 1.162Å. At the same level of theory, the zero-point corrected barrier to isomerization from CuCN to CuNC is estimated to be 14.7kcalmol−1, and the cyanide is more stable than the isocyanide by 11.5kcalmol−1. For both CuCN and CuNC the Σ+3 state is the lowest lying excited electronic state. At the DK-CCSD/cc-pVQZ(DK) level of theory, the energetic ordering of excited states of CuCN and CuNC is X̃Σ+1<ãΣ+3<b̃Π3<2Σ+1≈Δ3<Π1<Δ1. The variations of CN bond lengths in the optimized structures for the different electronic states and the CN stretching frequencies of the ground state and the excited states suggest that metal dπ to ligand π charge transfer is insignificant, in contrast to previous results for isoelectronic NiCO.
Nonmonotonic energy harvesting efficiency in biased exciton chains
Authors: Vlaming S. M. , Malyshev V. A. , Knoester J.
Published: October 2007

We theoretically study the efficiency of energy harvesting in linear exciton chains with an energy bias, where the initial excitation is taking place at the high-energy end of the chain and the energy is harvested (trapped) at the other end. The efficiency is characterized by means of the average time for the exciton to be trapped after the initial excitation. The exciton transport is treated as the intraband energy relaxation over the states obtained by numerically diagonalizing the Frenkel Hamiltonian that corresponds to the biased chain. The relevant intraband scattering rates are obtained from a linear exciton-phonon interaction. Numerical solution of the Pauli master equation that describes the relaxation and trapping processes reveals a complicated interplay of factors that determine the overall harvesting efficiency. Specifically, if the trapping step is slower than or comparable to the intraband relaxation, this efficiency shows a nonmonotonic dependence on the bias: it first increases when introducing a bias, reaches a maximum at an optimal bias value, and then decreases again because of dynamic (Bloch) localization of the exciton states. Effects of on-site (diagonal) disorder, leading to Anderson localization, are addressed as well.
Wire core and coronal plasma expansion in wire-array Z pinches with small numbers of wires
Authors: Shelkovenko T. A. , Pikuz S. A. , Douglass J. D. , Blesener I. C. , Greenly J. B. , McBride R. D. , Hammer D. A. , Kusse B. R.
Published: October 2007

Wire core and coronal plasma formation and expansion in wire-array Z pinches with small numbers of wires have been studied on a 1MA, 100ns rise time pulsed power generator and a 500kA, 50ns generator. Two-frame point-projection x-ray imaging and three-frame laser optical imaging and interferometry were the principal diagnostic methods used for these studies. The x-ray images show that dense coronal plasma forms and is maintained close to each dense wire core in the array. A less dense, rapidly expanding (∼10μm∕ns) coronal plasma, best seen in the laser images, surrounds the ∼100μm radius dense corona. These results are in agreement with computer simulations and modeling carried out by Yu et al. [Phys. Plasmas 14, 022705 (2007)]. Results are also presented for the dependence of the wire core and coronal plasma expansion rates on the wire diameter, number of wires and current through individual wires and the overall configuration for Al, Cu, and W wire arrays. For example, the W wire dense core expansion rate increases with increasing initial wire diameter from 5.1μm (0.1μm∕ns)to12.7μm diameter (0.3μm∕ns).
Fundamental emission via wave advection from a collapsing wave packet in electromagnetic strong plasma turbulence
Authors: Jenet F. A. , Melatos A. , Robinson P. A.
Published: October 2007

Zakharov simulations of nonlinear wave collapse in continuously driven two-dimensional, electromagnetic strong plasma turbulence with electron thermal speeds v⩾0.01c show that for v≲0.1c, dipole radiation occurs near the plasma frequency, mainly near arrest, but for v≳0.1c, a new mechanism applies in which energy oscillates between trapped Langmuir and transverse modes until collapse is arrested, after which trapped transverse waves are advected into incoherent interpacket turbulence by an expanding annular density well, where they detrap. The multipole structure, Poynting flux, source current, and radiation angular momentum are computed.
Influence of substrate temperature on the stability of glasses prepared by vapor deposition
Authors: Kearns Kenneth L. , Swallen Stephen F. , Ediger M. D. , Wu Tian , Yu Lian
Published: October 2007

Physical vapor deposition of indomethacin (IMC) was used to prepare glasses with unusual thermodynamic and kinetic stability. By varying the substrate temperature during the deposition from 190K to the glass transition temperature (Tg=315K), it was determined that depositions near 0.85Tg (265K) resulted in the most stable IMC glasses regardless of substrate. Differential scanning calorimetry of samples deposited at 265K indicated that the enthalpy was 8J∕g less than the ordinary glass prepared by cooling the liquid, corresponding to a 20K reduction in the fictive temperature. Deposition at 265K also resulted in the greatest kinetic stability, as indicated by the highest onset temperature. The most stable vapor-deposited IMC glasses had thermodynamic stabilities equivalent to ordinary glasses aged at 295K for 7months. We attribute the creation of stable IMC glasses via vapor deposition to enhanced surface mobility. At substrate temperatures near 0.6Tg, this mobility is diminished or absent, resulting in low stability, vapor-deposited glasses.
Excitation of the l=2 azimuthal mode in a pure electron plasma
Authors: Bettega G. , Cavaliere F. , Paroli B. , Cavenago M. , Pozzoli R. , Romé M.
Published: October 2007

The effect of an externally imposed time-varying quadrupole electrostatic perturbation on a pure electron plasma trapped in a Malmberg-Penning trap is investigated experimentally. A resonance occurs when the frequency of the drive matches the frequency of the azimuthal l=2, kz=0 mode. Very good agreement is found between the experimentally determined resonance frequency and the real part of the frequency calculated from a two-dimensional linear model for a top-hat density profile, while nonlinear deviations for the mode amplitude are found especially close to the resonance.
Influence of oxygen contents of carbohydrate dopants on connectivity and critical current density in MgB2 tapes
Authors: Gao Zhaoshun , Ma Yanwei , Zhang Xianping , Wang Dongliang , Yang Huan , Wen Haihu , Watanabe K.
Published: October 2007

More recently, it has been reported that carbohydrate doping resulted in an effective substitution of carbon for boron in MgB2, and hence achieved significant flux pinning enhancement. However, the authors found that the oxygen amount in carbohydrate additives played an important role in improving Jc-B performance in MgB2. In this work, the effect of maleic anhydride (C4H2O3) and maleic acid (C4H4O4) dopants on the superconducting properties of in situ MgB2 tapes has been studied. Compared to the undoped counterpart, both doped tapes showed an enhancement of Jc values by more than an order of magnitude in high-field region; however, the transport Jc of C4H4O4 doped tapes was apparently lower than that of C4H2O3 added samples. The poor grain connectivity as a result of more MgO and voids formed during the reaction in C4H4O4 doped samples can be responsible for the reduced Jc values.
Relationship between the bubble temperature and main oxidant created inside an air bubble under ultrasound
Authors: Yasui Kyuichi , Tuziuti Toru , Kozuka Teruyuki , Towata Atsuya , Iida Yasuo
Published: October 2007

Numerical simulations of nonequilibrium chemical reactions in a pulsating air bubble have been performed for various ultrasonic frequencies (20kHz, 100kHz, 300kHz, and 1MHz) and pressure amplitudes (up to 10bars). The results of the numerical simulations have indicated that the main oxidant is OH radical inside a nearly vaporous or vaporous bubble which is defined as a bubble with higher molar fraction of water vapor than 0.5 at the end of the bubble collapse. Inside a gaseous bubble which is defined as a bubble with much lower vapor fraction than 0.5, the main oxidant is H2O2 when the bubble temperature at the end of the bubble collapse is in the range of 4000–6500K and O atom when it is above 6500K. From the interior of a gaseous bubble, an appreciable amount of OH radical also dissolves into the liquid. When the bubble temperature at the end of the bubble collapse is higher than 7000K, oxidants are strongly consumed inside a bubble by oxidizing nitrogen and the main chemical products inside a bubble are HNO2, NO, and HNO3.
Adsorption∕desorption hysteresis of simple fluids confined in realistic heterogeneous silica mesopores of micrometric length: A new analysis exploiting a multiscale Monte Carlo approach
Authors: Puibasset Joël
Published: October 2007

Adsorption∕desorption isotherms in porous materials are commonly used for characterization. In order to analyze the data, accurate calculations of fluid adsorption in various complex pore models are required. The reversible, low adsorption portion of the isotherm is generally well described by molecular simulation, since the relevant fluid∕substrate interactions are described at molecular level. This molecular approach is, however, ineffective in the hysteresis region because the large scale spatial distribution of heterogeneities in the pore network is beyond the computer capabilities. On the other hand, coarse grained approaches are more suited to take into account this porous network complexity at large scale and discuss the hysteresis nature, but the molecular description is lost. In this paper, a multiscale approach is introduced which allows both a molecular description of fluid∕substrate interactions, and taking into account the connectivity between the various domains in a porous material. The case of argon confined in heterogeneous tubular silica mesopores (MCM-41 or oxidized porous silicon) is considered. Comparison with the simple independent domain theory shows the strong influence of quenched disorder. It is also shown that the independent pore model significantly overestimates the hysteresis width. The effect of pore ends open at only one or at both ends is addressed.
Scanning tunneling microscopy image simulation of the rutile (110) TiO2 surface with hybrid functionals and the localized basis set approach
Authors: Di Valentin Cristiana
Published: October 2007

In this work we present a simplified procedure to use hybrid functionals and localized atomic basis sets to simulate scanning tunneling microscopy (STM) images of stoichiometric, reduced and hydroxylated rutile (110) TiO2 surface. For the two defective systems it is necessary to introduce some exact Hartree-Fock exchange in the exchange functional in order to correctly describe the details of the electronic structure. Results are compared to the standard density functional theory and planewave basis set approach. Both methods have advantages and drawbacks that are analyzed in detail. In particular, for the localized basis set approach, it is necessary to introduce a number of Gaussian function in the vacuum region above the surface in order to correctly describe the exponential decay of the integrated local density of states from the surface. In the planewave periodic approach, a thick vacuum region is required to achieve correct results. Simulated STM images are obtained for both the reduced and hydroxylated surface which nicely compare with experimental findings. A direct comparison of the two defects as displayed in the simulated STM images indicates that the OH groups should appear brighter than oxygen vacancies in perfect agreement with the experimental STM data.
An efficient transition path sampling algorithm for nanoparticles under pressure
Authors: Grünwald Michael , Dellago Christoph , Geissler Phillip L.
Published: October 2007

We apply transition path sampling to the simulation of nanoparticles under pressure. As a barostat we use a bath of ideal gas particles that form a stochastically updated atmosphere around the nanoparticle. We justify this algorithm by showing that it preserves the distribution of an ideal gas at constant temperature and pressure by satisfying detailed balance. Based on this result, we present a simple and efficient transition path sampling scheme for the study of activated processes in nanoparticles under pressure. As a first application, we investigate the h-MgO to rocksalt transformation in faceted CdSe nanocrystals. Starting from an artificial mechanism involving a uniform motion of all atoms, trajectories quickly converge towards the dominant mechanism of nucleation and growth along parallel (100) planes.
Theoretical study of the multiplet branching of the SD product in the S(D1)+D2→SD(Π2)+D reaction
Authors: Kłos Jacek A. , Dagdigian Paul J. , Alexander Millard H.
Published: October 2007

The statistical model of atom-diatom insertion reactions is combined with coupled-states capture theory to calculate integral cross sections for formation of specific rotational/fine-structure states of the SD product of the title reaction. The four electronic potential energy surfaces that correlate with the products (A′1,3 and A″1,3) and an accurate description of the electronic and spin-orbit couplings between them have been determined from ab initio calculations. The dependence of the cross sections upon the product rotational quantum number shows a statistical behavior similar to that computed with the simple prior statistical model. We predict a significant preference for formation of the lower (F1) versus the upper (F2) spin-orbit manifold but essentially equal A′ and A″ Λ-doublet populations. The computed SD v=0 rotational/fine-structure state distribution is in good agreement with the distribution measured experimentally for this reaction by Khachatrian and Dagdigian [J. Chem. Phys. 122, 024303 (2005)]. The calculations predict the F1: F2 spin-orbit population ratio to be slightly larger than experimentally observed.
Forward and backward diodelike rectifying properties of the heterojunctions composed of La1−xSrxCoO3−δ and 0.7wt% Nb-doped SrTiO3
Authors: Li G. , Zhou T. F. , Hu D. D. , Yao Y. P. , Hou Y. , Li X. G.
Published: October 2007

Heterojunctions composed of La1−xSrxCoO3−δ (x=0.4 and 0.5) and 0.7wt% Nb-doped SrTiO3 were fabricated and exhibited good rectifying properties. The asymmetric current-voltage relations for the x=0.4 and x=0.5 junctions display opposite shapes: the former has a forward shape, while the latter shows a backward one. The x=0.4 junction can be viewed as a Schottky diode. For the x=0.5 junction, the forward bias currents show much less temperature dependence than the reverse ones and the reverse voltage Vchar, at which the bias current is 10μA, displays a distinct change at the ferromagnetic transition temperature of the cobalt oxide of the junction. These observations in the x=0.5 junction can be understood by a nearly degenerate model together with its temperature-dependent magnetism.
Anomalous lineshapes and aging effects in two-dimensional correlation spectroscopy
Authors: Šanda František , Mukamel Shaul
Published: October 2007

Multitime correlation functions provide useful probes for the ensembles of trajectories underlying the stochastic dynamics of complex systems. These can be obtained by measuring their optical response to sequences of ultrashort optical pulse. Using the continuous time random walk model for spectral diffusion, we analyze the signatures of anomalous relaxation in two-dimensional four wave mixing signals. Different models which share the same two point joint probability distribution show markedly different lineshapes and may be distinguished. Aging random walks corresponding to waiting time distributions with diverging first moment show dependence of 2D lineshapes on initial observation time, which persist for long times.
Applications of Wang-Landau sampling to determine phase equilibria in complex fluids
Authors: Ganzenmüller Georg , Camp Philip J.
Published: October 2007

Applications of the Wang-Landau algorithm for simulating phase coexistence at fixed temperature are presented. The number density is sampled using either volume scaling or particle insertion/deletion. The resulting algorithms, while being conceptually easy, are of comparable efficiency to existing multicanonical methods but with the advantage that neither the chemical potential nor the pressure at phase coexistence has to be estimated in advance of the simulation. First, we benchmark the algorithm against literature results for the vapor-liquid transition in the Lennard-Jones fluid. We then demonstrate the general applicability of the algorithm by studying vapor-liquid coexistence in two examples of complex fluids: charged soft spheres, which exhibit a transition similar to that in the restricted primitive model of ionic fluids, being characterized by strong ion pairing in the vapor phase; and Stockmayer fluids with high dipole strengths, in which the constituent particles aggregate to form chains, and for which the very existence of a transition has been widely debated. Finally, we show that the algorithm can be used to locate a weak isotropic-nematic transition in a fluid of Gay-Berne mesogens.
Single molecule probing of the glass transition phenomenon: Simulations of several types of probes
Authors: Vallée R. A. L. , Paul W. , Binder K.
Published: October 2007

Molecular dynamics simulations of a system of short bead-spring chains containing an additional dumbbell are presented and analyzed. This system represents a coarse-grained model for a melt of short, flexible polymers containing fluorescent probe molecules at very dilute concentration. It is shown that such a system is very well suited to study aspects of the glass transition of the undercooled polymer melt via single molecule spectroscopy, which are not easily accessed by other methods. Such aspects include data which can be extracted from a study of fluctuations along a trajectory of the single molecule, probing the rugged energy landscape of the glass-forming liquid and transitions from one metabasin of this energy landscape to the next one. Such an information can be inferred from “distance maps” constructed from trajectories characterizing the translational and orientational motion of the probe. At the same time, determining autocorrelation functions along such trajectories, it is shown for several types of probes (differing in their size and/or mass within reasonable limits) that this time-averaged information of the probe is fully compatible with ensemble averaged information on the relaxation of the glass-forming matrix, accessible from bulk measurements. The analyzed quantities include the fluorescence lifetime, linear dichroism, and also various orientational correlation functions of the probe, in order to provide guidance to experimental work. Similar to earlier findings from simulations of bulk molecular fluids, deviations from the Stokes-Einstein and Stokes-Einstein-Debye relations are observed.
Electronic memory effects in diodes of zinc oxide nanoparticles in a matrix of polystyrene or poly(3-hexylthiophene)
Authors: Verbakel Frank , Meskers Stefan C. J. , Janssen René A. J.
Published: October 2007

Electronic memory effects in metal-insulator-metal devices with aluminum and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) as electrodes and a solution processed active layer consisting of zinc oxide (ZnO) nanoparticles embedded in a matrix of poly(3-hexylthiophene) or polystyrene are investigated. After an initial forming process, the devices show a reversible change in conductivity. The forming process itself is interpreted in terms of desorption of molecular oxygen from the ZnO nanoparticle surface, induced by injection of holes via the PEDOT:PSS contact, leading to a higher n-type conductivity via interparticle ZnO contacts. The forming can also be induced with ultraviolet light and the process is studied with electron paramagnetic resonance, photoinduced absorption spectroscopy, and field effect measurements. Also, the composition of the active layer is varied and the memory effects can by influenced by changing the ZnO content and the polymer, allowing for data storage with lifetime >14h.
Low temperature transport and structural properties of misch-metal-filled skutterudites
Authors: Yang J. , Meisner G. P. , Rawn C. J. , Wang H. , Chakoumakos B. C. , Martin J. , Nolas G. S. , Pedersen B. L. , Stalick J. K.
Published: October 2007

Skutterudites, such as CoSb3, are a promising class of thermoelectric materials, particularly when the voids in the crystal structure are filled with guest atoms. We report a comprehensive study of the effects of filling skutterudites with misch-metal (Mm), a rare-earth alloy having the naturally occurring La, Ce, Pr, and Nd composition. Our power diffraction experiments show that Mm filling causes a larger expansion and an unusual distortion of the CoSb3 lattice compared with single-element-filled skutterudites. We probed the response of crystal lattice, electronic structure, and carrier and phonon scattering mechanisms to Mm filling using neutron powder diffraction, Hall effect, electrical resistivity, thermopower, and thermal conductivity measurements between 2 and 300 K on a series of MmyFe4−xCoxSb12 samples. The thermoelectric properties of these Mm-filled skutterudites in this low temperature range are comparable to those of pure Ce-filled skutterudites despite the anomalous lattice expansion and distortion. We expect that these materials will have high thermoelectric figures of merit at elevated temperatures.