bluuebunny/crossref_metadata_2025 huggingface dataset:
Bounce‐averaged advection and diffusion coefficients for monochromatic electromagnetic ion cyclotron wave: Comparison between test‐particle and quasi‐linear models
Authors: Su Zhenpeng , Zhu Hui , Xiao Fuliang , Zheng Huinan , Shen Chao , Wang Yuming , Wang Shui
Published: August 2012

The electromagnetic ion cyclotron (EMIC) wave has long been suggested to be responsible for the rapid loss of radiation belt relativistic electrons. The test‐particle simulations are performed to calculate the bounce‐averaged pitch angle advection and diffusion coefficients for parallel‐propagating monochromatic EMIC waves. The comparison between test‐particle (TP) and quasi‐linear (QL) transport coefficients is further made to quantify the influence of nonlinear processes. For typical EMIC waves, four nonlinear physical processes, i.e., the boundary reflection effect, finite perturbation effect, phase bunching and phase trapping, are found to occur sequentially from small to large equatorial pitch angles. The pitch angle averaged finite perturbation effect yields slight differences between the transport coefficients of TP and QL models. The boundary reflection effect and phase bunching produce an average reduction of >80% in the diffusion coefficients but a small change in the corresponding average advection coefficients, tending to lower the loss rate predicted by QL theory. In contrast, the phase trapping causes continuous negative advection toward the loss cone and a minor change in the corresponding diffusion coefficients, tending to increase the loss rate predicted by QL theory. For small amplitude EMIC waves, the transport coefficients grow linearly with the square of wave amplitude. As the amplitude increases, the boundary reflection effect, phase bunching and phase trapping start to occur. Consequently, the TP advection coefficients deviate from the linear growth with the square of wave amplitude, and the TP diffusion coefficients become saturated with the amplitude approaching 1 nT or above. The current results suggest that these nonlinear processes can cause significant deviation of transport coefficients from the prediction of QL theory, which should be taken into account in the future simulations of radiation belt dynamics driven by the EMIC waves.
The 4D imaging of the source of ground deformation at Campi Flegrei caldera (southern Italy)
Authors: D'Auria Luca , Giudicepietro Flora , Martini Marcello , Lanari Riccardo
Published: August 2012

We have applied a tomographic imaging technique to the inversion of a DInSAR data set at Campi Flegrei caldera. This technique allowed us to determine the temporal and spatial distribution of volumetric strain sources up to 5 km depth. Results have shown complex spatial and temporal patterns, identifying important features that were not noticed before. The first result is the observation of positive strain sources (expansion) migrating upward (in 2000 and 2006). We have interpreted them as hot fluid batches injected at the bottom of the geothermal reservoir, migrating upward and reaching the surface. Furthermore we have identified an injection episode (in 1997), which was not recognized before. This batch did not reach the surface and probably dissipated by diffusion and lateral advection without producing significant ground uplift. The injection of fluid batches does not occur at the center of the caldera, but along its borders. The three identified injection episodes (in 1997, 2000 and 2006) occur in different points. In 2000 and 2006, the injected fluids migrated, subsequently, toward the center of the caldera. Our findings agrees with results of other geophysical and geochemical studies. These results suggest a new framework for the modeling of Campi Flegrei geothermal system and for the interpretation of data recorded by the multiparametric monitoring networks on the caldera.
Observations of intermittent deep currents and eddies in the Gulf of Mexico
Authors: Kolodziejczyk Nicolas , Ochoa José , Candela Julio , Sheinbaum Julio
Published: August 2012

Intense currents having speeds between 10 and 40 cm.s−1from 1000 m below the surface to the bottom have been measured in the Loop Current (LC) region and in the Western Gulf of Mexico (WGM). The observations come from moored current meters sampling from the near‐surface to the bottom at 3300–3500 m and from August 2008 to August 2010. Ten intermittent intensified currents are documented. These events last 10 to 30 days and take place in the region just between the surface‐intensified anticyclonic‐cyclonic features, as deduced from altimetry and measured currents. In the WGM, they occur between the cores of anticyclone‐cyclone pairs. In the Eastern Gulf of Mexico (EGM), they occur in the Loop Current edge between its anticyclonic circulation and a neighboring cyclone. During each event, the analysis shows an increase in the barotropic contribution to the current. In the Loop Current region, these events are often associated with intensification of cyclonic vorticity in the deep layer.
Establishing a global climatology of marine phytoplankton phenological characteristics
Authors: Sapiano M. R. P. , Brown C. W. , Schollaert Uz S. , Vargas M.
Published: August 2012

The timing or phenology of the annual cycle of phytoplankton biomass can be monitored to better understand the underpinnings of the marine ecosystem and assess its response to environmental change. Ten‐year, global maps of the mean date of bloom onset, peak concentration and termination of bloom duration were constructed by extracting these phenological metrics from Generalized Linear Models (GLM) fit to time series of 1° × 1° daily estimates of SeaWiFS chlorophyll concentrations dating from September 1997 to December 2007 as well as to MODIS chlorophyll concentrations from July 2002 to July 2010. The fitted models quantitatively define the annual cycle of phytoplankton throughout the global ocean and from which a baseline of phenological characteristics was extracted. The analysis revealed regionally consistent patterns in the shape and timing of the annual cycle of chlorophyll concentration that are broadly consistent with other published studies. The results showed that a single bloom predominates over the global ocean with secondary, autumn blooms being limited in both location and spatial extent. Bloom duration tended to be zonally consistent, but meridionally complex and did not become progressively shorter with increasing latitude as is sometimes depicted. Both the shape of the annual cycle and the phenological climatologies can be used in future studies to detect significant departures over time.
Effect of permeability anisotropy on buoyancy‐driven flow for CO2 sequestration in saline aquifers
Authors: Myint Philip C. , Bestehorn Michael , Firoozabadi Abbas
Published: August 2012

Solubility trapping of carbon dioxide (CO2) in deep saline aquifers is considered one of the most effective methods for carbon sequestration. Dissolution of CO2 into the brine may create gravitational instabilities that lead to the onset of convection, which greatly enhances the storage efficiency and reduces the possibilities of leakage. Convection appears in the form of downward traveling fingers of relatively dense, CO2‐dissolved fluid. Many natural aquifer formations display considerable permeability anisotropy, where the horizontal permeability kh may be several times greater than the vertical permeability kz. It has been previously found that increasing kh for a fixed kz reduces the critical time tc at which onset occurs and the critical wavelength λc with which the fingers initially form. We extend earlier work by showing how and why this occurs. Our results reveal new insights about λc. We have studied the behavior for times greater than tc using high‐resolution numerical simulations. We show that the enhanced dissolution from convection can become significant much earlier in anisotropic media. Furthermore, the effects of anisotropy may be sustained for a long period of time. Our results suggest that permeability anisotropy can allow a wider range of aquifer formations to be considered for effective sequestration.
Activation of B cells by non‐canonical helper signals
Authors: Cerutti Andrea , Cols Montserrat , Puga Irene
Published: August 2012

Cognate interaction between T and B lymphocytes of the adaptive immune system is essential for the production of high‐affinity antibodies against microbes, and for the establishment of long‐term immunological memory. Growing evidence shows that—in addition to presenting antigens to T and B cells—macrophages, dendritic cells and other cells of the innate immune system provide activating signals to B cells, as well as survival signals to antibody‐secreting plasma cells. Here, we discuss how these innate immune cells contribute to the induction of highly diversified and temporally sustained antibody responses, both systemically and at mucosal sites of antigen entry.
Cell‐nonautonomous control of the UPR
Authors: Martínez Gabriela , Hetz Claudio
Published: August 2012

A study published in this issue of EMBO reports shows that the UPR at the intestine is under the control of the nervous system, to facilitate response to stress. This new way of understanding how the UPR is controlled at an organismal level is discussed here.
The pig as a model for investigating the role of neutrophil serine proteases in human inflammatory lung diseases
Authors: Bréa Déborah , Meurens François , Dubois Alice V. , Gaillard Julien , Chevaleyre Claire , Jourdan Marie-Lise , Winter Nathalie , Arbeille Brigitte , Si-Tahar Mustapha , Gauthier Francis , Attucci Sylvie
Published: August 2012

The serine proteases released by activated polymorphonuclear neutrophils [NSPs (neutrophil serine proteases)] contribute to a variety of inflammatory lung diseases, including CF (cystic fibrosis). They are therefore key targets for the development of efficient inhibitors. Although rodent models have contributed to our understanding of several diseases, we have previously shown that they are not appropriate for testing anti-NSP therapeutic strategies [Kalupov, Brillard-Bourdet, Dade, Serrano, Wartelle, Guyot, Juliano, Moreau, Belaaouaj and Gauthier (2009) J. Biol. Chem. 284, 34084–34091). Thus NSPs must be characterized in an animal model that is much more likely to predict how therapies will act in humans in order to develop protease inhibitors as drugs. The recently developed CFTR−/− (CFTR is CF transmembrane conductance regulator) pig model is a promising alternative to the mouse model of CF [Rogers, Stoltz, Meyerholz, Ostedgaard, Rokhlina, Taft, Rogan, Pezzulo, Karp, Itani et al. (2008) Science 321, 1837–1841]. We have isolated blood neutrophils from healthy pigs and determined their responses to the bacterial pathogens Pseudomonas aeruginosa and Staphylococcus aureus, and the biochemical properties of their NSPs. We used confocal microscopy and antibodies directed against their human homologues to show that the three NSPs (elastase, protease 3 and cathepsin G) are enzymatically active and present on the surface of triggered neutrophils and NETs (neutrophil extracellular traps). All of the porcine NSPs are effectively inhibited by human NSP inhibitors. We conclude that there is a close functional resemblance between porcine and human NSPs. The pig is therefore a suitable animal model for testing new NSP inhibitors as anti-inflammatory agents in neutrophil-associated diseases such as CF.
Logarithmically regularized inviscid models in borderline sobolev spaces
Authors: Chae Dongho , Wu Jiahong
Published: August 2012

Several inviscid models in hydrodynamics and geophysics such as the incompressible Euler vorticity equations, the surface quasi-geostrophic equation, and the Boussinesq equations are not known to have even local well-posedness in the corresponding borderline Sobolev spaces. Here Hs is referred to as a borderline Sobolev space if the L∞-norm of the gradient of the velocity is not bounded by the Hs-norm of the solution but by the \documentclass[12pt]{minimal}\begin{document}$H^{\widetilde{s}}$\end{document}Hs̃-norm for any \documentclass[12pt]{minimal}\begin{document}$\widetilde{s}>s$\end{document}s̃>s. This paper establishes the local well-posedness of the logarithmically regularized counterparts of these inviscid models in the borderline Sobolev spaces.
Progress in obtaining an absolute calibration of a total deuterium–tritium neutron yield diagnostic based on copper activation
Authors: Ruiz C. L. , Chandler G. A. , Cooper G. W. , Fehl D. L. , Hahn K. D. , Leeper R. J. , McWatters B. R. , Nelson A. J. , Smelser R. M. , Snow C. S. , Torres J. A.
Published: August 2012

The 350-keV Cockroft-Walton accelerator at Sandia National laboratory's Ion Beam facility is being used to calibrate absolutely a total DT neutron yield diagnostic based on the 63Cu(n,2n)62Cu(β+) reaction. These investigations have led to first-order uncertainties approaching 5% or better. The experiments employ the associated-particle technique. Deuterons at 175 keV impinge a 2.6 μm thick erbium tritide target producing 14.1 MeV neutrons from the T(d,n)4He reaction. The alpha particles emitted are measured at two angles relative to the beam direction and used to infer the neutron flux on a copper sample. The induced 62Cu activity is then measured and related to the neutron flux. This method is known as the F-factor technique. Description of the associated-particle method, copper sample geometries employed, and the present estimates of the uncertainties to the F-factor obtained are given.
The phase diagram of ammonium nitrate
Authors: Chellappa Raja S. , Dattelbaum Dana M. , Velisavljevic Nenad , Sheffield Stephen
Published: August 2012

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH4NO3] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3–11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.
Coalescence of surfactant covered drops in extensional flows: Effects of the interfacial diffusivity
Authors: Vannozzi Carolina
Published: August 2012

Boundary integral simulations and scaling theory were employed to study the effects of insoluble surfactant surface diffusivity Ds and concentration Γ on the coalescence process of two equal-sized viscous drops. The drops underwent head-on collisions in a biaxial extensional flow, in the Stokes flow limit and low capillary numbers. The simulations were compared with the drainage time experiments of Yoon et al. [Phys. Fluids 19, 023102 (2007)10.1063/1.2409735] concerned with a polymeric system, polybutadiene (PBd) drops in a polydimethylsyloxane (PDMS) matrix, stabilized by block-copolymers acting as insoluble surfactants to explain the mechanism underneath their findings. An ad hoc equation of state, derived by mean field theory, specific for the block-copolymers in the experiments of Yoon et al., able to match the experimental surface tension data without fitting parameters, was used. We were able to reproduce the experimental drainage time data, although an additional attractive force, besides the usual van der Waals interactions, had to be introduced for high block-copolymer concentrations, probably as a result of the entropic attraction between the two facing dry brushes formed in the thin film between the two drops. According to simulations, the puzzling experimental drainage time transition for low surfactant concentrations, from high drainage time to low drainage time as Ca increases, was a consequence of the oscillating behavior of the minimum film thickness, which takes place for Marangoni numbers Ma < 5 and surface Peclet number Pes > 60. In this regard, a master curve was obtained for the scaled relative minimum film thickness attained during the oscillation as a function of Ma. This enabled to determine both the minimum value of the dimensionless attractive forces to avoid coalescence for each concentration studied and the range of Ma that favors early coalescence. The coalescence process was found very sensitive to Pes and for Pes O(100–1000) even trace amounts of surfactants can be as effective stabilizers as high surfactant concentrations. Moreover, for the polymeric system of interest, the range of Ds in which the drainage time changes from the saturation value to the clean interface value was computed as a function of the surfactant concentration. In the specific, for the PDMS/PBd system of interest the Ds range studied was O(10−12–10−5 cm2 s−1). Additionally, our scaling analysis further validates our simulations, also highlighting the effect on the drainage process of the different parameters, in particular, of the external pushing force, which is increased compared to a clean interface system, as Pes is increased or as the surfactant concentration is increased, because of the reduction in the interfacial mobility of the drop. Finally, our study suggests that matching simulations with four-roll mill drainage time experiments can be an effective method to determine block-copolymer surface diffusivity.
What are the most efficient basis set strategies for correlated wave function calculations of reaction energies and barrier heights?
Authors: Papajak Ewa , Truhlar Donald G.
Published: August 2012

As electronic structure methods are being used to obtain quantitatively accurate reaction energies and barrier heights for increasingly larger systems, the choice of an efficient basis set is becoming more critical. The optimum strategy for achieving basis set convergence can depend on the way that electron correlation is treated and can take advantage of flexibility in the order in which basis functions are added. Here we study several approaches for estimating accurate reaction energies and barrier heights from post-Hartree–Fock electronic structure calculations. First and second, we evaluate methods of estimating the basis set limit of second order Møller-Plesset perturbation theory and of coupled cluster theory with single and double excitations and a quasiperturbative treatment of connected triple excitations by using explicitly correlated basis functions (in the F12a implementation) along with valence, polarization, and diffuse one-electron basis functions. Third, we test the scheme of adding a higher-order correction to MP2 results (sometimes called MP2/CBS + ΔCCSD(T)). Finally, we evaluate the basis set requirements of these methods in light of comparisons to Weizmann-3.2, Weizmann-4, and CCSDT(2)Q/CBS+CV+R results.
Ion irradiation of electronic-type-separated single wall carbon nanotubes: A model for radiation effects in nanostructured carbon
Authors: Rossi Jamie E. , Cress Cory D. , Helenic Alysha R. , Schauerman Chris M. , DiLeo Roberta A. , Cox Nathanael D. , Messenger Scott R. , Weaver Brad D. , Hubbard Seth M. , Landi Brian J.
Published: August 2012

The structural and electrical properties of electronic-type-separated (metallic and semiconducting) single wall carbon nanotube (SWCNT) thin-films have been investigated after irradiation with 150 keV 11B+ and 150 keV 31P+ with fluences ranging from 1012 to 1015 ions/cm2. Raman spectroscopy results indicate that the ratio of the Raman D to G′ band peak intensities (D/G′) is a more sensitive indicator of SWCNT structural modification induced by ion irradiation by one order of magnitude compared to the ratio of the Raman D to G band peak intensities (D/G). The increase in sheet resistance (Rs) of the thin-films follows a similar trend as the D/G′ ratio, suggesting that the radiation induced variation in bulk electrical transport for both electronic-types is equal and related to localized defect generation. The characterization results for the various samples are compared based on the displacement damage dose (DDD) imparted to the sample, which is material and damage source independent. Therefore, it is possible to extend the analysis to include data from irradiation of transferred CVD-graphene films on SiO2/Si substrates using 35 keV C+ ions, and compare the observed changes at equivalent levels of ion irradiation-induced damage to that observed in the SWCNT thin-film samples. Ultimately, a model is developed for the prediction of the radiation response of nanostructured carbon materials based on the DDD for any incident ion with low-energy recoil spectra. The model is also related to the defect concentration, and subsequently the effective defect-to-defect length, and yields a maximum defect concentration (minimum defect-to-defect length) above which the bulk electrical transport properties in SWCNT thin-films and large graphene-based electronic devices rapidly degrade when exposed to harsh environments.
Molecular simulation of the phase behavior of fluids and fluid mixtures using the synthetic method
Authors: Sadus Richard J.
Published: August 2012

A new molecular simulation procedure is reported for determining the phase behavior of fluids and fluid mixtures, which closely follows the experimental synthetic method. The simulation procedure can be implemented using Monte Calro or molecular dynamics in either the microcanonical or canonical statistical ensembles. Microcanonical molecular dynamics simulations are reported for the phase behavior of both the pure Lennard-Jones fluid and a Lennard-Jones mixture. The vapor pressures for the pure fluid are in good agreement with Monte Carlo Gibbs ensemble and Gibbs-Duhem calculations. The Lennard-Jones mixture is composed of equal size particles, with dissimilar energy parameters (ɛ2/ɛ1 = 1/2, ɛ12/ɛ1 = 1/$\sqrt 2$2). The binary Lennard-Jones mixture exhibits liquid-liquid equilibria at high pressures and the simulation procedure allows us to estimate the coordinates of the high-pressure branch of the critical curve.
Vertical impedance measurements on concrete bridge decks for assessing susceptibility of reinforcing steel to corrosion
Authors: Bartholomew Paul D. , Guthrie W. Spencer , Mazzeo Brian A.
Published: August 2012

Corrosion is a pressing problem for aging concrete infrastructure, especially bridge decks. Because of its sensitivity to factors that affect corrosion of reinforcing steel in concrete, resistivity is an important structural health indicator for reinforced concrete structures. In this research, an instrument was developed to measure vertical impedance on concrete bridge decks. Measurements of vertical impedance on slabs prepared in the laboratory, on slabs removed from decommissioned bridge decks, and on an in-service bridge deck in the field demonstrate the utility of the new apparatus.
A near-field scanning microwave microscope for characterization of inhomogeneous photovoltaics
Authors: Weber J. C. , Schlager J. B. , Sanford N. A. , Imtiaz A. , Wallis T. M. , Mansfield L. M. , Coakley K. J. , Bertness K. A. , Kabos P. , Bright V. M.
Published: August 2012

We present a near-field scanning microwave microscope (NSMM) that has been configured for imaging photovoltaic samples. Our system incorporates a Pt-Ir tip inserted into an open-ended coaxial cable to form a weakly coupled resonator, allowing the microwave reflection S11 signal to be measured across a sample over a frequency range of 1 GHz – 5 GHz. A phase-tuning circuit increased impedance-measurement sensitivity by allowing for tuning of the S11 minimum down to −78 dBm. A bias-T and preamplifier enabled simultaneous, non-contact measurement of the DC tip-sample current, and a tuning fork feedback system provided simultaneous topographic data. Light-free tuning fork feedback provided characterization of photovoltaic samples both in the dark and under illumination at 405 nm. NSMM measurements were obtained on an inhomogeneous, third-generation Cu(In,Ga)Se2 (CIGS) sample. The S11 and DC current features were found to spatially broaden around grain boundaries with the sample under illumination. The broadening is attributed to optically generated charge that becomes trapped and changes the local depletion of the grain boundaries, thereby modifying the local capacitance. Imaging provided by the NSMM offers a new RF methodology to resolve and characterize nanoscale electrical features in photovoltaic materials and devices.
Nanoparticle collisions in the gas phase in the presence of singular contact potentials
Authors: Ouyang Hui , Gopalakrishnan Ranganathan , Hogan Christopher J.
Published: August 2012

Collisional growth and ionization is commonplace for gas phase nanoparticles (i.e., in aerosols). Nanoparticle collisions in atmospheric pressure environments occur in the mass transfer transition regime, and further attractive singular contact potentials (which arise when modeling nanoparticles as condensed matter and for which the potential energy approaches −∞ when two entities contact) often have a non-negligible influence on collision processes. For these reasons collision rate calculations for nanoparticles in the gas phase are not straightforward. We use mean first passage time calculations to develop a simple relationship to determine the collision rate in the gas phase, accounting for the influences of both the transition regime and singular contact potentials (specifically the non-retarded van der Waals and image potentials). In the presented analysis, methods to determine the degree of enhancement in collision rate due to attractive singular potentials in the continuum (diffusive) regime, ηC, and the degree of enhancement in the free molecular (ballistic) regime, ηFM, are first reviewed. Accounting for these enhancement factors, with mean first passage time calculations it is found that the collision rate for gas phase nanoparticles with other gas phase entities can be determined from a relationship between the dimensionless collision rate coefficient, H, and the diffusive Knudsen number, KnD, i.e., the ratio of the mean collision persistence distance to the collision length scale. This coincides with the H(KnD) relationship found to appropriately describe collisions between entities interacting via a hard-sphere potential, but with ηC and ηFM incorporated into the definitions of both H and KnD, respectively. The H(KnD) relationship is compared to the predictions of flux matching theory, used prevalently in prior work for collision rate calculation, and through this comparison it is found that at high potential energy to thermal energy ratios, flux matching theory predictions underestimate the true collision rate. Finally, a series of experimental measurements of nanoparticle-nanoparticle collision rates are compared to the determined H(KnD) expression, considering that nanoparticles interact via non-retarded van der Waals potentials. Very good agreement is found with collision rates inferred from experiments, with almost all measured values from four separate studies within 25% of model predictions.
Level density distribution for one-dimensional vertex models related to Haldane-Shastry like spin chains
Authors: Banerjee Pratyay , Basu-Mallick B.
Published: August 2012

The energy level density distributions of some Haldane-Shastry like spin chains associated with the AN − 1 root system have been computed recently by Enciso et al., exploiting the connection of these spin systems with inhomogeneous one-dimensional vertex models whose energy functions depend on the vertices through specific polynomials of first or second degree. Here we consider a much broader class of one-dimensional vertex models whose energy functions depend on the vertices through arbitrary polynomials of any possible degree. We estimate the order of mean and variance for such energy functions and show that the level density distribution of all vertex models belonging to this class asymptotically follow the Gaussian pattern for large number of vertices. We also present some numerical evidence in support of this analytical result.
Relativistically corrected electric field gradients calculated with the normalized elimination of the small component formalism
Authors: Filatov Michael , Zou Wenli , Cremer Dieter
Published: August 2012

Based on the analytic derivatives formalism for the spin-free normalized elimination of the small component method, a new computational scheme for the calculation of the electric field gradient at the atomic nuclei was developed and presented. The new computational scheme was tested by the calculation of the electric field gradient at the mercury nucleus in a series of Hg-containing inorganic and organometallic compounds. The benchmark calculations demonstrate that the new formalism is capable of reproducing experimental and theoretical reference data with high accuracy. The method developed can be routinely applied to the calculation of large and very large molecules and holds considerable promise for the interpretation of the experimental data of biologically relevant compounds containing heavy elements.
Magnetism and structure of graphene nanodots with interiors modified by boron, nitrogen, and charge
Authors: Philpott Michael R. , Kawazoe Yoshiyuki
Published: August 2012

The properties (geometry, spin, and charge distribution) of a series of flat hexagonal zigzag edged graphene nanodots (GNDs), with interiors modified by centrally located substituent atoms boron and nitrogen and by positive and negative charge, have been calculated using ab initio density functional theory. The doped series X-GND has the stoichiometry \documentclass[12pt]{minimal}\begin{document}${\rm C}_{6m^{2}-1}$\end{document}C6m2−1XH6m, zigzag size index m = 2, 4, 6, 8, 10 and substituent X = B or N. The undoped parents \documentclass[12pt]{minimal}\begin{document}${\rm C}_{6m^{2}}$\end{document}C6m2H6m with m ⩽ 8 have spin paired ground states and the parent m = 10 has a spin polarized singlet ground state with edges that alternate α- and β-spin. The spin on the substituent atom decreases to zero with size index m and magnetization builds on the edges of all the X-GND. This demonstrates translocation of substituent spin and a proximity or directional effect for small m as the edges show different degrees of magnetization. For the largest X-GND (m = 10) the magnetization on edges resembles the calculated triplet S = 1(a) configuration of the parent (four edge spins up and two down) and has a higher apparent symmetry than the C2v point group of X-GND. For charged (m = 10) GNDs the edge magnetization has strength comparable to the parent on two parallel edges and weak on the other four in a perimeter pattern that resembles the triplet S = 1(b) configuration of the undoped parent and not the ground configuration of the isoelectronic X-GND molecule. Many of the results can be interpreted by simple Kekul\documentclass[12pt]{minimal}\begin{document}$\acute{e}$\end{document}é valence bond structures for an unpaired spin on a network where the substituent site group symmetry is not compatible with the perimeter. A deeper understanding is provided by the properties of the Kohn-Sham orbitals. The calculations of the X-doped GNDs reveal limitations in the use of the hex-radical hypothesis of the parent ground state to systems where foreign atoms lower symmetry and perturb the π- and σ-bond manifolds.
High-spin organic molecules with dominant spin-orbit contribution and unprecedentedly large magnetic anisotropy
Authors: Misochko Eugenii Ya. , Akimov Alexander V. , Masitov Artem A. , Korchagin Denis V. , Yakushchenko Igor K. , Chapyshev Sergei V.
Published: August 2012

High-spin organic molecules with dominant spin-orbit contribution to magnetic anisotropy are reported. Quintet 4-azido-3,5-dibromopyridyl-2,6-dinitrene (Q-1), quintet 2-azido-3,5-dibromopyridyl-4,6-dinitrene (Q-2), and septet 3,5-dibromopyridyl-2,4,6-trinitrene (S-1) were generated in solid argon matrices by ultraviolet irradiation of 2,4,6-triazido-3,5-dibromopyridine. The zero-field splitting (ZFS) parameters, derived from electron spin resonance spectra, show unprecedentedly large magnitudes of the parameters D: |DQ1| = 0.289, |DQ2| = 0.373, and |DS1| = 0.297 cm−1. The experimental ZFS parameters were successfully reproduced by density functional theory calculations, confirming that magnetic anisotropy of high-spin organic molecules can considerably be enhanced by the “heavy atom effect.” In bromine-containing high-spin nitrenes, the spin-orbit term is dominant and governs both the magnitude and the sign of magnetic anisotropy. The largest negative value of D among septet trinitrenes is predicted for 1,3,5-trinitrenobenzene bearing three heavy atoms (Br) in positions 2, 4, and 6 of the benzene ring.
Density-functional theory for polymer-carbon dioxide mixtures: A perturbed-chain SAFT approach
Authors: Xu Xiaofei , Cristancho Diego E. , Costeux Stéphane , Wang Zhen-Gang
Published: August 2012

We propose a density-functional theory (DFT) describing inhomogeneous polymer-carbon dioxide mixtures based on a perturbed-chain statistical associating fluid theory equation of state (PC-SAFT EOS). The weight density functions from fundamental measure theory are used to extend the bulk excess Helmholtz free energy to the inhomogeneous case. The additional long-range dispersion contributions are included using a mean-field approach. We apply our DFT to the interfacial properties of polystyrene-CO2 and poly(methyl methacrylate) CO2 systems. Calculated values for both solubility and interfacial tension are in good agreement with experimental data. In comparison with our earlier DFT based on the Peng-Robinson-SAFT EOS, the current DFT produces quantitatively superior agreement with experimental data and is free of the unphysical behavior at high pressures (>35 MPa) in the earlier theory.
Asymmetric orientation of toluene molecules at oil-silica interfaces
Authors: Ledyastuti Mia , Liang Yunfeng , Kunieda Makoto , Matsuoka Toshifumi
Published: August 2012

The interfacial structure of heptane and toluene at oil-silica interfaces has previously been studied by sum frequency generation [Z. Yang et al., J. Phys. Chem. C. 113, 20355 (2009)]10.1021/jp9043122. It was found that the toluene molecule is almost perpendicular to the silica surface with a tilt angle of about 25°. Here, we have investigated the structural properties of toluene and heptane at oil-silica interfaces using molecular dynamics simulations for two different surfaces: the oxygen-bridging (hydrophobic) and hydroxyl-terminated (hydrophilic) surfaces of quartz (silica). Based on the density profile, it was found that both heptane and toluene oscillate on silica surfaces, with heptane showing more oscillation peaks. Furthermore, the toluene molecules of the first layer were found to have an asymmetric distribution of orientations, with more CH3 groups pointed away from the silica surface than towards the silica surface. These findings are generally consistent with previous experiments, and reveal enhanced molecular structures of liquids at oil-silica interfaces.
Formation of metastable structures by phase separation triggered by initial composition gradients in thin films
Authors: Jaiswal Prabhat K. , Binder Kurt , Puri Sanjay
Published: August 2012

Phase separation kinetics of a binary (A,B) mixture contained in a thin film of thickness D induced by a quench from the one-phase region into the miscibility gap is studied by simulations using a Cahn–Hilliard–Cook model. The initial randomly mixed state (50% A, 50% B) contains a concentration gradient perpendicular to the film, while the surfaces of the film are “neutral” (no preference for either A or B). In thermal equilibrium, a pattern of large A-rich and B-rich domains must result, separated by domain walls oriented perpendicularly to the external surfaces of the thin film. However, it is shown that for many choices of D and the strength of the initial gradient Ψg, instead a very long-lived metastable layered structure forms, with two domains separated by a single interface parallel to the external walls. The transient time evolution that leads to this structure is interpreted in terms of a competition between domain growth in the bulk and surface-directed spinodal decomposition caused by the gradient during the initial stages. A surprising and potentially useful finding is that a moderate concentration gradient perpendicular to the film does not favor the layered structure but facilitates the approach toward the true equilibrium with just two domain walls perpendicular to the film. This mechanism may have useful applications in producing layered materials.
A new instrument for dynamic helical squeeze flow which superposes oscillatory shear and oscillatory squeeze flow
Authors: Kim Jae Hee , Ahn Kyung Hyun
Published: August 2012

Simple shear flow has been widely used as a model flow system to investigate the rheological properties of complex fluids. Although it is useful and easy to apply, the shear flow is too simple compared with the complicated flows encountered in real processes. For this reason, it is necessary to study the rheological properties of viscoelastic fluids subjected to more complicated flow field. In this paper, we report a new instrument in which dynamic helical squeeze flow is generated. It superimposes the oscillatory squeeze flow on the oscillatory shear flow, and was designed to get more information on the microstructural change of complex fluids in the flow field which is more complex than conventional simple shear but is still well-defined. This instrument has advantages: the shear and normal stresses can be obtained simultaneously under the coupled flow field; the present design can be used to investigate the structural anisotropy in the direction of both rotational and vertical motion. This study provides a new design and methodology to investigate the rheological responses of complex fluids under dynamic helical squeeze flow.
Geometric integration of the Vlasov-Maxwell system with a variational particle-in-cell scheme
Authors: Squire J. , Qin H. , Tang W. M.
Published: August 2012

A fully variational, unstructured, electromagnetic particle-in-cell integrator is developed for integration of the Vlasov-Maxwell equations. Using the formalism of discrete exterior calculus [Desbrun et al., e-print arXiv:math/0508341 (2005)], the field solver, interpolation scheme, and particle advance algorithm are derived through minimization of a single discrete field theory action. As a consequence of ensuring that the action is invariant under discrete electromagnetic gauge transformations, the integrator exactly conserves Gauss’s law.
Correlation between in-plane strain and optical polarization of Si-doped AlGaN epitaxial layers as a function of Al content and Si concentration
Authors: Kurai Satoshi , Shimomura Kazuhide , Murotani Hideaki , Yamada Yoichi , Miyake Hideto , Hiramatsu Kazumasa
Published: August 2012

We have investigated the relationship between in-plane strain and optical polarization in AlGaN epitaxial layers as a function of Al content and of Si concentration. Al content and in-plane strain were measured by reciprocal space mapping using a simple relationship from the tensor of elastic moduli. The change of valence band energy separation based on the measured in-plane strain was in good agreement with the change of polarization degree as a function of Al content. The dependence of polarization degree on Si concentration was also explained by the change of in-plane strain, and polarization switching occurred around the Al content and strain as calculated by the k · p approach with the cubic approximation.
Phenomenological theory of electric-field-induced phase transition behavior of antiferroelectric ceramic (Pb,Ba,La)(Zr,Sn,Ti)O3 under uniaxial compressive pre-stress
Authors: Xu Yonghao , Feng Yujun , Zhang Na , Yan Yangxi , Liao Liming
Published: August 2012

The phase transition behavior of antiferroelectric ceramic (Pb,Ba,La)(Zr,Sn,Ti)O3 under uniaxial compressive pre-stress is observed at stress up to 100 MPa using a homebuilt experimental setup in conjunction with a modified Sawyer-Tower circuit. The results reveal that the saturation polarization (Psat) decrease with increasing compressive pre-stress, while the phase transition fields increase as the compressive pre-stress increases. Meanwhile the applied pre-stress virtually does not affect the transition field hysteresis (ΔE). The longitudinal strain (S) increases with increasing compressive pre-stress and reaches to maximum at 20 MPa, and then gradually decreases. A phenomenological thermodynamic theory has been developed to model the first-order phase transition behavior of the material.
Stationary electron velocity distribution function in crossed electric and magnetic fields with collisions
Authors: Shagayda Andrey
Published: August 2012

Analytical studies and numerical simulations show that the electron velocity distribution function in a Hall thruster discharge with crossed electric and magnetic fields is not Maxwellian. This is due to the fact that the mean free path between collisions is greater than both the Larmor radius and the characteristic dimensions of the discharge channel. However in numerical models of Hall thrusters, a hydrodynamic approach is often used to describe the electron dynamics, because discharge simulation in a fully kinetic approach requires large computing resources and is time consuming. A more accurate modeling of the electron flow in the hydrodynamic approximation requires taking into account the non-Maxwellian character of the distribution function and finding its moments, an approach that reflects the properties of electrons drifting in crossed electric and magnetic fields better than the commonly used Euler or Navier-Stokes approximations. In the present paper, an expression for the electron velocity distribution function in rarefied spatially homogeneous stationary plasma with crossed electric and magnetic fields and predominance of collisions with heavy particles is derived in the relaxation approximation. The main moments of the distribution function including longitudinal and transversal temperatures, the components of the viscous stress tensor, and of the heat flux vector are calculated. Distinctive features of the hydrodynamic description of electrons with a strongly non-equilibrium distribution function and the prospects for further development of the proposed approach for calculating the distribution function in spatially inhomogeneous plasma are discussed.
Conduction band caused by oxygen vacancies in aluminum oxide for resistance random access memory
Authors: Nigo Seisuke , Kubota Masato , Harada Yoshitomo , Hirayama Taisei , Kato Seiichi , Kitazawa Hideaki , Kido Giyuu
Published: August 2012

As a next-generation memory, we have developed a rare-metal-free memory using Al oxide with a high-density of oxygen vacancies (Vos). The electronic structure has been simulated using first-principles calculations. In this paper, we report the electronic structure of the band gap, analyzed using thermally stimulated current measurements, to evaluate the simulated results. We observed electronic states corresponding to resistance changes for the first time. These results show that Vo+2 (electron empty Vo) changes to Vo+1 by electron injection; the overlapped Vo+1 electron changes into a “Vo conduction band” (VoCB), and the changed structure is stabilized by structural relaxation of Al ions around Vo. VoCB is considered as a kind of mid-gap impurity band. The origin of the on/off switching is considered to be generation/degeneration of the VoCB caused by increasing/decreasing numbers of Vo electrons. Based on knowledge of the electronic mechanism, we have changed metal/insulator/metal structure to a metal/insulator/semiconductor structure and decreased the reset-current to 7 μA. The Vos of Al oxide are considered to be useful for electronic memory storage.
Properties of coupled surface plasmon-polaritons in metal-dielectric-metal structures
Authors: Chen N. C. , Lu C. Y. , Huang Y. L. , Liao C. C. , Ke W. C. , Huang B. R.
Published: August 2012

The properties of surface plasmon-polaritons (SPPs) in symmetrical metal-dielectric-metal (MDM) structures were theoretically investigated and experimentally demonstrated. These SPPs exhibit several intriguing properties that are not observed in single-interface SPPs: they can directly couple to radiation outside the MDM structure; their frequency may fall within the plasmon gap; their group velocities can be positive, negative, or even zero, and their density of states can be tuned. All of these properties are favorable in devices that exploit the Purcell effect of SPPs to improve performance. Analytical descriptions of the dispersion curves in some important regions are presented.
Three dimensional particle-in-cell simulation of particle acceleration by circularly polarised inertial Alfven waves in a transversely inhomogeneous plasma
Authors: Tsiklauri D.
Published: August 2012

The process of particle acceleration by left-hand, circularly polarised inertial Alfven waves (IAW) in a transversely inhomogeneous plasma is studied using 3D particle-in-cell simulation. A cylindrical tube with, transverse to the background magnetic field, inhomogeneity scale of the order of ion inertial length is considered on which IAWs with frequency 0.3ωci are launched that are allowed to develop three wavelength. As a result time-varying parallel electric fields are generated in the density gradient regions which accelerate electrons in the parallel to magnetic field direction. Driven perpendicular electric field of IAWs also heats ions in the transverse direction. Such numerical setup is relevant for solar flaring loops and earth auroral zone. This first, 3D, fully kinetic simulation demonstrates electron acceleration efficiency in the density inhomogeneity regions, along the magnetic field, of the order of 45% and ion heating, in the transverse to the magnetic field direction, of 75%. The latter is a factor of two times higher than the previous 2.5D analogous study and is in accordance with solar flare particle acceleration observations. We find that the generated parallel electric field is localised in the density inhomogeneity region and rotates in the same direction and with the same angular frequency as the initially launched IAW. Our numerical simulations seem also to suggest that the “knee” often found in the solar flare electron spectra can alternatively be interpreted as the Landau damping (Cerenkov resonance effect) of IAWs due to the wave-particle interactions.