bluuebunny/crossref_metadata_2025 huggingface dataset:
Atmospheric chemistry over southern Africa
Authors: Gatebe Charles K. , Levy Robert C. , Thompson Anne M.
Published: March 2012

Changing Chemistry in a Changing Climate: Human and Natural Impacts Over Southern Africa (C4‐SAR); Midrand, South Africa, 31 May to 3 June 2011 During the southern African dry season, regional haze from mixed industrial pollution, biomass burning aerosol and gases from domestic and grassland fires, and biogenic sources from plants and soils is worsened by a semipermanent atmospheric gyre over the subcontinent. These factors were a driver of several major international field campaigns in the 1990s and early 2000s and attracted many scientists to the region. Some researchers were interested in understanding fundamental processes governing chemistry of the atmosphere and interaction with climate change. Others found favorable conditions for evaluating satellite‐ derived measurements of atmospheric properties and a changing land surface. With that background in mind a workshop on atmospheric chemistry was held in South Africa. Sponsored by the International Commission on Atmospheric Chemistry and Global Pollution (ICACGP; http://www.icacgp.org/), the workshop received generous support from Eskom, the South African power utility; and the Climatology Research Group of the University of the Witwatersrand, Johannesburg, South Africa.
Fine aspherical ash source of Eyjafjallajökull's influence
Authors: Schultz Colin
Published: March 2012

The 2010 eruption of Eyjafjallajökull volcano was not a large event. Over months of volcanic activity the ash plume never pushed above 10 kilometers, and the mass flows peaked at 1 million kilograms per second, feeble amounts compared to some other volcanic eruptions. In total, the volcano spewed out only 270 million cubic meters of ash—a single day's activity for some eruptions. By any conventional measure, Eyjafjallajökull lacked power. Nevertheless, the eruption had a tremendous effect on society, leaving tens of thousands of people stranded as air traffic around western Europe was shut down.
A reassessment of lake and wetland feedbacks on the North African Holocene climate
Authors: Krinner G. , Lézine A.‐M. , Braconnot P. , Sepulchre P. , Ramstein G. , Grenier C. , Gouttevin I.
Published: March 2012

Large parts of the Sahara were vegetated during the early to mid Holocene. Several positive feedbacks, most notably related to vegetation, have been shown to have favored the northward migration of the desert boundary. During this period, numerous lakes and wetlands existed in the Sahara region and might have acted as a local moisture source. However, earlier model studies of the effects of open water surfaces on the mid‐Holocene North African climate suggested that these were weak and did not contribute significantly to this northward migration of the North African climate zones. Using a state‐of‐the‐art climate model, we suggest that the effect of open‐water surfaces on the mid‐Holocene North African climate might have been much stronger than previously estimated, regionally more than doubling the simulated precipitation rates. It is thus possible that this effect, combined to other known positive feedbacks, favored the appearance of the “Green Sahara”.
Sudden intensity increases and radial gradient changes of cosmic ray MeV electrons and protons observed at Voyager 1 beyond 111 AU in the heliosheath
Authors: Webber W. R. , McDonald F. B. , Cummings A. C. , Stone E. C. , Heikkila B. , Lal N.
Published: March 2012

Voyager 1 has entered regions of different propagation conditions for energetic cosmic rays in the outer heliosheath at a distance of about 111 AU from the Sun. The low energy 6–14 MeV galactic electron intensity increased by ~20% over a time period ≤10 days and the electron radial intensity gradient abruptly decreased from ∼19%/AU to ∼8%/AU at 2009.7 at a radial distance of 111.2 AU. At about 2011.2 at a distance of 116.6 AU a second abrupt intensity increase of ∼25% was observed for electrons. After the second sudden electron increase the radial intensity gradient increased to 18%/AU. This large positive gradient and the ∼13 day periodic variations of >200 MeV particles observed near the end of 2011 indicate that V1 is still within the overall heliospheric modulating region. The implications of these results regarding the proximity of the heliopause are discussed.
Nonstationarities of regional climate model biases in European seasonal mean temperature and precipitation sums
Authors: Maraun D.
Published: March 2012

Bias correcting climate models implicitly assumes stationarity of the correction function. This assumption is assessed for regional climate models in a pseudo reality for seasonal mean temperature and precipitation sums. An ensemble of regional climate models for Europe is used, all driven with the same transient boundary conditions. Although this model‐dependent approach does not assess all possible bias non‐stationarities, conclusions can be drawn for the real world. Generally, biases are relatively stable, and bias correction on average improves climate scenarios. For winter temperature, bias changes occur in the Alps and ice covered oceans caused by a biased forcing sensitivity of surface albedo; for summer temperature, bias changes occur due to a biased sensitivity of cloud cover and soil moisture. Precipitation correction is generally successful, but affected by internal variability in arid climates. As model sensitivities vary considerably in some regions, multi model ensembles are needed even after bias correction.
AMP-activated protein kinase, stress responses and cardiovascular diseases
Authors: Wang Shaobin , Song Ping , Zou Ming-Hui
Published: March 2012

AMPK (AMP-activated protein kinase) is one of the key players in maintaining intracellular homoeostasis. AMPK is well known as an energy sensor and can be activated by increased intracellular AMP levels. Generally, the activation of AMPK turns on catabolic pathways that generate ATP, while inhibiting cell proliferation and biosynthetic processes that consume ATP. In recent years, intensive investigations on the regulation and the function of AMPK indicates that AMPK not only functions as an intracellular energy sensor and regulator, but is also a general stress sensor that is important in maintaining intracellular homoeostasis during many kinds of stress challenges. In the present paper, we will review recent literature showing that AMPK functions far beyond its proposed energy sensor and regulator function. AMPK regulates ROS (reactive oxygen species)/redox balance, autophagy, cell proliferation, cell apoptosis, cellular polarity, mitochondrial function and genotoxic response, either directly or indirectly via numerous downstream pathways under physiological and pathological conditions.
Magnetic NiFe/Au barcode nanowires with self-powered motion
Authors: Jeon In Tak , Yoon Seung Jae , Kim Bong Gun , Lee Ji Sung , An Boo Hyun , Ju Jae-Seon , Wu Jun Hua , Kim Young Keun
Published: March 2012

NiFe/Au barcode nanowires were synthesized by pulsed electrodeposition using anodic aluminum oxide nanotemplate, comprising magnetic, catalytic, and optical segments, respectively. The self-powered motion of the BNWs due to the catalytic reaction was observed in aqueous H2O2. The approach demonstrates how sophistication in barcode nanoarchitecture can be used to synthesize a wide range of hybrid materials.
Enhanced magnetoimpedance effect in Co-based amorphous ribbons coated with carbon nanotubes
Authors: Chaturvedi Anurag , Stojak Kristen , Laurita Nicholas , Mukherjee Pritish , Srikanth Hariharan , Phan Manh-Huong
Published: March 2012

We report upon the enhancement of the giant magnetoimpedance (GMI) effect in Co-based amorphous ribbons coated with non-magnetic carbon nanotubes (CNTs). In our study, the CNTs were drop-casted onto the surface of a Metglas® 2714 A ribbon with three different concentrations (5, 10, and 15 μL of CNTs). Relative to the plain ribbon, a 15% enhancement of the GMI effect was observed in the ribbon coated with 10 μL of CNTs. The GMI effect first increased with the CNT concentration, to a maximum of 10 μL of CNTs, and then decreased at higher concentrations. Noticeably, at a measured frequency of 10 MHz, the magnetic field-induced ac resistance change was about 35% larger for the ribbon coated with 10 μL of CNTs than for the plain ribbon. These observations may reveal a new perspective for developing CNT-based gas sensors that operate using the principle of the GMI effect.
Field dependence of the interfacial Cu in Cu-coated γ-Fe2O3 nanoparticles
Authors: Desautels R. D. , Chen Y.-Y. , Ouyang H. , Lo S.-C. , Freeland J. W. , van Lierop J.
Published: March 2012

Evidence of elemental copper magnetism in Cu-coated γ-Fe2O3 nanoparticles was revealed by element specific x-ray absorption spectroscopy and magnetic circular dichroism (XMCD). An interfacial layer of magnetic CuO was discovered that interacted with the Fe3+ surface magnetic moments. This unexpected exchange interaction canted the moments of the CuO, so that a non-zero net magnetic moment was measured. XMCD measurements as a function of field on the copper coated γ-Fe2O3 nanoparticles indicated that the CuO magnetic moment was aligned with the octahedral Fe3+ and its magnetization was modified by the changing applied field. With increasing copper thickness, a stronger field dependence of the Fe3+ and Cu2+ magnetization was observed.
Transmission electron microscopy studies of sintered Nd-Fe-B magnets prepared by cyclic sintering
Authors: Woo Kim Jin , Hoon Kim Se , Yong Song Sun , Do Kim Young
Published: March 2012

A cyclic sintering process was demonstrated to achieve higher magnetic and mechanical properties due to homogeneous distribution of the Nd-rich phase during sintering. Densification greater than 98% of the theoretical density was achieved by the cyclic sintering of the Nd-Fe-B powder. In this study, transmission electron microscopy was used to evaluate the microstructural changes of the cyclically sintered Nd-Fe-B magnets. The Nd-rich triple junction phase in the cyclic sintered sample was h-Nd2O3 and the Nd-rich grain boundary phase was a mixture of h-Nd2O3 and h-Nd amorphous thin layers. The coercivity and bending strength increased from 1666.8 to 1828.5 kA/m and from 301.8 to 520 MPa, respectively.
Large change of perpendicular magnetic anisotropy in Cobalt ultrathin film induced by varying capping layers
Authors: Zhang Xianmin , Mizukami Shigemi , Kubota Takahide , Ma Qinli , Naganuma Hiroshi , Oogane Mikihiko , Ando Yasuo , Miyazaki Terunobu
Published: March 2012

The magnetic films of Co with Si/SiO2/Pt/Co/molecule structure were fabricated and their structural properties and magnetic anisotropy were investigated by varying both Co (0.5–1.8 nm) thickness and molecular capping layers of 5,6,11,12-tetraphenylnaphthacene (rubrene) and copper phthalocyanine (CuPc), respectively. The crystal structures were characterized using x-ray diffraction (XRD) and the magnetization curves were measured using vibrating sample magnetometer with an applied field both in parallel and perpendicular to a film plane. It was found that the thickness of Co for the maximum perpendicular magnetic anisotropy (PMA) is around 0.7 nm for both group films. However, the estimated effective magnetic anisotropy energy for Co was 2.9 ± 0.3 × 106 erg/cc for rubrene-capped sample, which was smaller than the value of 4.9 ± 0.4 × 106 erg/cc for CuPc-capped sample. The XRD patterns showed the crystal structure of rubrene layer was of amorphous structure and CuPc layer was polycrystalline. The different interface effects of Co/CuPc and Co/rubrene were discussed to analyze the change of PMA.
Anomalous lattice softening of Ni2MnGa austenite due to magnetoelastic coupling
Authors: Heczko Oleg , Seiner Hanuš , Sedlák Petr , Kopeček Jaromír , Landa Michal
Published: March 2012

Elastic constants of the cubic Ni2MnGa austenite phase and corresponding mechanical damping were determined in the temperature range from 220 K to 400 K and magnetic field up to 2 T using ultrasound pulse-echo method and resonant ultrasound spectroscopy. The shear coefficient c′ increases from 3.6 GPa in the demagnetized state to 5.9 GPa at magnetic saturation, whereas the damping decreased nearly six times. The changes of other elastic constants, c11 and c44 with an applied field were less than 1%. In the ferromagnetic state, the c′ was proportional to the square of magnetization. Above the Curie point, the coefficient c′ and damping were field-independent. The anomalous shear softening is attributed to strong magnetoelastic coupling enhanced by low magnetic anisotropy.
The role of dipole−dipole interaction in modulating the step-like magnetization of Ca3Co2O6
Authors: Xie Y. L. , Lin L. , Yan Z. B. , Wang K. F. , Liu J.-M.
Published: March 2012

While the magnetic ground state associated with the step-like magnetization in Ca3Co2O6 remains to be an issue under debate, we investigate the effect of dipole−dipole (D−D) interaction on the magnetic ground state using the Wang-Landau algorithm. It is revealed that the D−D interaction is equivalent to an effective short-range interaction term. An inclusion of this term allows a columnar-state platform and a 1/2 platform in addition to the two-step feature in the magnetization in response to magnetic field, suggesting that the role of the D−D interaction in modulating the multistep magnetization in Ca3Co2O6 is non-negligible although our model may not reproduce all the experimentally observed effects.
Flow and noise predictions for the tandem cylinder aeroacoustic benchmark
Authors: Brès Guillaume A. , Freed David , Wessels Michael , Noelting Swen , Pérot Franck
Published: March 2012

Flow and noise predictions for the tandem cylinder benchmark are performed using lattice Boltzmann and Ffowcs Williams–Hawkings methods. The numerical results are compared to experimental measurements from the Basic Aerodynamic Research Tunnel and Quiet Flow Facility (QFF) at NASA Langley Research Center. The present study focuses on two configurations: the first configuration corresponds to the typical setup with uniform inflow and spanwise periodic boundary condition. To investigate installation effects, the second configuration matches the QFF setup and geometry, including the rectangular open jet nozzle, and the two vertical side plates mounted in the span to support the test models. For both simulations, the full span of 16 cylinder diameters is simulated, matching the experimental dimensions. Overall, good agreement is obtained with the experimental surface data, flow field, and radiated noise measurements. In particular, the presence of the side plates significantly reduces the excessive spanwise coherence observed with periodic boundary conditions and improves the predictions of the tonal peak amplitude in the far-field noise spectra. Inclusion of the contributions from the side plates in the calculation of the radiated noise shows an overall increase in the predicted spectra and directivity, leading to a better match with the experimental measurements. The measured increase is about 1 to 2 dB at the main shedding frequency and harmonics, and is likely caused by reflections on the spanwise side plates. The broadband levels are also slightly higher by about 2 to 3 dB, likely due to the shear layers from the nozzle exit impacting the side plates.
Pressure-induced dehydration and the structure of ammonia hemihydrate-II
Authors: Wilson C. W. , Bull C. L. , Stinton G. , Loveday J. S.
Published: March 2012

The structure of the crystalline ammonia-bearing phase formed when ammonia monohydrate liquid is compressed to 3.5(1) GPa at ambient temperature has been solved from a combination of synchrotron x-ray single-crystal and neutron powder-diffraction studies. The solution reveals that rather than having the ammonia monohydrate (AMH) composition as had been previously thought, the structure has an ammonia hemihydrate composition. The structure is monoclinic with spacegroup P21/c and lattice parameters a = 3.3584(5) Å, b = 9.215(1) Å, c = 8.933(1) Å and β = 94.331(8)° at 3.5(1) GPa. The atomic arrangement has a crowned hexagonal arrangement and is a layered structure with long N–D⋯N hydrogen bonds linking the layers. The existence of pressure-induced dehydration of AMH may have important consequences for the behaviour and differentiation of icy planets and satellites.
Response calculations based on an independent particle system with the exact one-particle density matrix: Excitation energies
Authors: Giesbertz K. J. H. , Gritsenko O. V. , Baerends E. J.
Published: March 2012

Adiabatic response time-dependent density functional theory (TDDFT) suffers from the restriction to basically an occupied → virtual single excitation formulation. Adiabatic time-dependent density matrix functional theory allows to break away from this restriction. Problematic excitations for TDDFT, viz. bonding-antibonding, double, charge transfer, and higher excitations, are calculated along the bond-dissociation coordinate of the prototype molecules H2 and HeH+ using the recently developed adiabatic linear response phase-including (PI) natural orbital theory (PINO). The possibility to systematically increase the scope of the calculation from excitations out of (strongly) occupied into weakly occupied (“virtual”) natural orbitals to larger ranges of excitations is explored. The quality of the PINO response calculations is already much improved over TDDFT even when the severest restriction is made, to virtually the size of the TDDFT diagonalization problem (only single excitation out of occupied orbitals plus all diagonal doubles). Further marked improvement is obtained with moderate extension to allow for excitation out of the lumo and lumo+1, which become fractionally occupied in particular at longer distances due to left-right correlation effects. In the second place the interpretation of density matrix response calculations is elucidated. The one-particle reduced density matrix response for an excitation is related to the transition density matrix to the corresponding excited state. The interpretation of the transition density matrix in terms of the familiar excitation character (single excitations, double excitations of various types, etc.) is detailed. The adiabatic PINO theory is shown to successfully resolve the problematic cases of adiabatic TDDFT when it uses a proper PI orbital functional such as the PILS functional.
Experimental observation and analytical model of the stress gradient inversion in 3C-SiC layers on silicon
Authors: Zielinski M. , Michaud J. F. , Jiao S. , Chassagne T. , Bazin A. E. , Michon A. , Portail M. , Alquier D.
Published: March 2012

A detailed study of the static bending of micro-cantilevers has been performed for structures created from thin 3C-SiC films grown on (100) and (111) oriented silicon substrates. The biaxial stress distribution in the direction of the film normal has been evaluated based on analysis of the deformation profiles of clamped-free 3C-SiC beams of various thicknesses. Surprisingly, the obtained results clearly indicate that for as-grown samples of both studied orientations, the absolute value of the intrinsic stress increases from the interface to the surface of the film. We propose a simple analytical model of a relaxation process that explains in a quantitative way this unexpected phenomenon of stress gradient inversion.
Engineering enhanced thermoelectric properties in zigzag graphene nanoribbons
Authors: Karamitaheri Hossein , Neophytou Neophytos , Pourfath Mahdi , Faez Rahim , Kosina Hans
Published: March 2012

We theoretically investigate the thermoelectric properties of zigzag graphene nanoribbons in the presence of extended line defects, substrate impurities, and edge roughness along the nanoribbon’s length. A nearest-neighbor tight-binding model for the electronic structure and a fourth nearest-neighbor force constant model for the phonon bandstructure are used. For transport, we employ quantum mechanical non-equilibrium Green’s function simulations. Starting from the pristine zigzag nanoribbon structure that exhibits very poor thermoelectric performance, we demonstrate how after a series of engineering design steps the performance can be largely enhanced. Our results could be useful in the design of highly efficient nanostructured graphene nanoribbon–based thermoelectric devices.
The speed of sound and derived thermodynamic properties of pure water at temperatures between (253 and 473) K and at pressures up to 400 MPa
Authors: Lin C.-W. , Trusler J. P. M.
Published: March 2012

The speed of sound in high-purity water has been measured in the temperature range (253 to 473) K at pressures up to 400 MPa. The experimental technique used was based on a double-path pulse-echo method with a single 5-MHz ultrasound transducer placed between two unequally spaced reflectors. The cell was calibrated in water at T = 298.15 K and p = 1 MPa against the speed of sound given by the 1995 equation-of-state formulation of the International Association for the Properties of Water and Steam (IAPWS-95) which, for that state point, has an uncertainty of 0.005%. Corrections for the effects of temperature and pressure on the path length difference are considered in detail. The estimated expanded relative uncertainty of the speed of sound determined in this work is shown to be between 0.03% and 0.04% at a confidence level of 95%. The density and isobaric specific heat capacity of water have been obtained in the temperature range (253.15 to 473.15) K at pressure up to 400 MPa by thermodynamic integration of the sound-speed data subject to initial values computed from IAPWS-95 on the isobar at p = 0.1 MPa. The speed of sound, density, and isobaric specific heat capacity were compared with IAPWS-95 with corresponding absolute relative deviations within 0.3%, 0.03%, and 1%, respectively at T ≥ 273.15 K and p ≤ 400 MPa; larger deviations, especially for heat capacity, were found at lower temperatures. The results imply that the uncertainties of properties computed from IAPWS-95 may be significantly reduced over the major part of the region investigated in this work.
Quantification of errors in large-eddy simulations of a spatially evolving mixing layer using polynomial chaos
Authors: Meldi M. , Salvetti M. V. , Sagaut P.
Published: March 2012

A stochastic approach based on generalized polynomial chaos (gPC) is used to quantify the error in large-eddy simulation (LES) of a spatially evolving mixing layer flow and its sensitivity to different simulation parameters, viz., the grid stretching in the streamwise and lateral directions and the subgrid-scale (SGS) Smagorinsky model constant (CS). The error is evaluated with respect to the results of a highly resolved LES and for different quantities of interest, namely, the mean streamwise velocity, the momentum thickness, and the shear stress. A typical feature of the considered spatially evolving flow is the progressive transition from a laminar regime, highly dependent on the inlet conditions, to a fully developed turbulent one. Therefore, the computational domain is divided in two different zones (inlet dependent and fully turbulent) and the gPC error analysis is carried out for these two zones separately. An optimization of the parameters is also carried out for both these zones. For all the considered quantities, the results point out that the error is mainly governed by the value of the CS constant. At the end of the inlet-dependent zone, a strong coupling between the normal stretching ratio and the CS value is observed. The error sensitivity to the parameter values is significantly larger in the inlet-dependent upstream region; however, low-error values can be obtained in this region for all the considered physical quantities by an ad hoc tuning of the parameters. Conversely, in the turbulent regime the error is globally lower and less sensitive to the parameter variations, but it is more difficult to find a set of parameter values leading to optimal results for all the analyzed physical quantities. A similar analysis is also carried out for the dynamic Smagorinsky model, by varying the grid stretching ratios. Comparing the databases generated with the different subgrid-scale models, it is possible to observe that the error cost function computed for the streamwise velocity and for the momentum thickness is not significantly sensitive to the used SGS closure. Conversely, the prediction of the shear stress is much more accurate when using a dynamic subgrid-scale model and the variance of the error is lower in magnitude.
Temperature-dependent ballistic transport in a channel with length below the scattering-limited mean free path
Authors: Arora Vijay K. , Zainal Abidin Mastura Shafinaz , Tan Michael L. P. , Riyadi Munawar A.
Published: March 2012

The temperature-dependent ballistic transport, using nonequilibrium Arora distribution function (NEADF), is shown to result in mobility degradation with reduction in channel length, in direct contrast to expectation of a collision-free transport. The ballistic mean free path (mfp) is much higher than the scattering-limited long-channel mfp, yet the mobility is amazingly lower. High-field effects, converting stochastic velocity vectors to streamlined ones, are found to be negligible when the applied voltage is less than the critical voltage appropriate for a ballistic mfp, especially at cryogenic temperatures. Excellent agreement with the experimental data on a metal-oxide-semiconductor field-effect transistor is obtained. The applications of NEADF are shown to cover a wide spectrum, covering regimes from the scattering-limited to ballistic, from nondegenerate to degenerate, from nanowire to bulk, from low- to high-temperature, and from a low electric field to an extremely high electric field.
Energy localization in HMX-Estane polymer-bonded explosives during impact loading
Authors: Barua A. , Horie Y. , Zhou M.
Published: March 2012

We report the results of a mechanistic study of energy localization in aHMX (High Melting point eXplosive octahydro-1,3,5,7-tetranitro-1,2,3,5-tetrazocine)/Estane PBX system during dynamic loading. The focus is on the thermal-mechanical response over the strain rate range of 104 – 105 s−1 under different confinement conditions. A recently developed cohesive finite element method is used to track and analyze the contributions to heating from different constituents, interfaces, deformation and fracture mechanisms, and internal friction. In particular, energy dissipations due to viscoelastic deformation, grain fracture, interfacial debonding, and friction along crack faces are quantified as functions of time and overall deformation. The materials analyzed have HMX volume fractions between 0.69 and 0.82. Calculations show that variation in strain rate can significantly affect the spatial distribution but not the overall number of hot spots. Higher confining stresses lead to more intense heating in the binder and more uniform distribution of hot spots. The evolution of hot spots is quantified as a function of loading condition, deformation and microstructural attributes. The microstructure-response relations obtained can be used to assess the initiation sensitivity of energetic composites.
On the accuracy and reliability of different fluid models of the direct current glow discharge
Authors: Rafatov I. , Bogdanov E. A. , Kudryavtsev A. A.
Published: March 2012

We developed and tested 2D “extended fluid model” of a dc glow discharge using COMSOL MULTIPHYSICS software and implemented two different approaches. First, assembling the model from COMSOL’s general form pde’s and, second, using COMSOL’s built-in Plasma Module. The discharge models are based on the fluid description of ions and excited neutral species and use drift-diffusion approximation for the particle fluxes. The electron transport as well as the rates of electron-induced plasma-chemical reactions are calculated using the Boltzmann equation for the EEDF and corresponding collision cross-sections. The self-consistent electric field is calculated from the Poisson equation. Basic discharge plasma properties such as current-voltage characteristics and electron and ion spatial density distributions as well as electron temperature and electric field profiles were studied. While the solutions obtained by two different COMSOL models are essentially identical, the discrepancy between COMSOL and CFD-ACE+ model solutions is about several percents and caused by the difference in the models due to undocumented details in the software packages. We also studied spatial distributions of particle fluxes in discharge plasma and identified the existence of vortex component of the discharge current.
Rethinking the application of the first nucleation theorem to particle formation
Authors: Vehkamäki Hanna , McGrath Matthew J. , Kurtén Theo , Julin Jan , Lehtinen Kari E. J. , Kulmala Markku
Published: March 2012

The critical cluster is the threshold size above which a cluster will be more likely to grow than to evaporate. In field and laboratory measurements of new particle formation, the number of molecules of a given species in the critical cluster is commonly taken to be the slope of the log-log plot of the formation rate versus the concentration of the species. This analysis is based on an approximate form of the first nucleation theorem, which is derived with the assumption that there are no minima in the free energy surface prior to the maximum at the critical size. However, many atmospherically relevant systems are likely to exhibit such minima, for example, ions surrounded by condensable vapour molecules or certain combinations of acids and bases. We have solved numerically the birth-death equations for both an electrically neutral one-component model system with a local minimum at pre-critical sizes and an ion-induced case. For the ion-induced case, it is verified that the log-log slope of the nucleation rate versus particle concentration plot gives accurately the difference between the cluster sizes at the free energy maximum and minimum, as is expected from the classical form of the ion-induced nucleation rate. However, the results show that applying the nucleation theorem to neutral systems with stable pre-nucleation clusters may lead to erroneous interpretations about the nature of the critical cluster.
The fluoroformyloxyl radical geometry and ground-state rotational spectra of the free FC18O2· radical
Authors: Koucký Jan , Kolesniková Lucie , Uhlíková Tereza , Varga Juraj , Kania Patrik , Beckers Helmut , Willner Helge , Urban Štěpán
Published: March 2012

The rotational spectra of the isotopically substituted free fluoroformyloxyl radical FC18O2· were measured using the Prague millimeter-wave high-resolution spectrometer. More than 110 rotational-fine-hyperfine transition lines were observed and assigned to appropriate quantum numbers in the frequency range of 235–270 GHz. The obtained transition frequencies were analyzed with standard effective Hamiltonians to acquire a set of precise rotational, centrifugal-distortion, fine, and hyperfine structure molecular constants. Merging the new FC18O2· isotopologue molecular parameters with those previously obtained for the ordinary FC16O2· radical, the substitution molecular geometry in the ground vibronic state was evaluated. The molecular parameters for both radical isotopologues were also calculated by several quantum chemistry methods and their calculated mutual ratios are in remarkable agreement with the experimental FC16O2·/FC18O2· parameter ratios. The measurements, assignments of the 18-oxygen isotopologue FC18O2· radical millimeter-wave transitions, as well as the derivation of the fluoroformyloxyl radical ground-state geometry have been carried out for the first time.
Nontrivial augmentations in mixing performance through integrated active and passive mixing in serpentine microchannels
Authors: Biswas Sujay K. , Das Tamal , Chakraborty Suman
Published: March 2012

Achievement of efficient mixing in microfluidic systems appears to be a highly challenging proposition, as attributable to typical low Reynolds number hydrodynamics over small scales. To circumvent these constraints, numerous strategies, either relying upon a modulation in the microchannel geometry or involving active flow perturbations have been proposed in the literature. However, while the geometric or passive means suffer from a lack of dynamic control on the mixing process, the active methods can be unfavorably energy expensive. Here we show that the problem of controllability and energy efficiency can be optimized to a large extent by combining the active and passive strategies within an integrated microfluidic platform, in the form of serpentine microchannel geometry with embedded electrodes. We demonstrate, both theoretically and experimentally, that in specific operating regimes, the mixing effectiveness (expressed in terms of a quantifiable index) of the designed system can be nontrivially higher than the algebraic sum of effectivenesses realized from pure active and passive mixing configurations, leading to a nonlinear amplification in the separation efficiency. Results of our experiments may be used a generic design principle for optimized mixing performance of lab-on-a-chip microdevices, with a judicious combination of the active and passive mixing paradigms.
Edge-stress-induced spontaneous twisting of graphene nanoribbons
Authors: Ramasubramaniam Ashwin , Koskinen Pekka , Kit Oleg O. , Shenoy Vivek B.
Published: March 2012

We present a continuum model for spontaneous twisting of graphene nanoribbons driven by compressive edge stresses. Based on a geometrically nonlinear theory of plates, we identify scaling laws for the dependence of twist angles on ribbon width. Strikingly, we find the existence of a critical width below which a ribbon will not undergo spontaneous twisting, preferring an in-plane stretching mode instead. The model predictions are shown to be in excellent qualitative and quantitative agreement with density-functional tight-binding simulations. More generally, our model provides a unifying picture of twisting in graphene nanoribbons with different edge orientations and chemical functionalizations that have been reported recently in the literature.
A numerical solution of the linear Boltzmann equation using cubic B-splines
Authors: Khurana Saheba , Thachuk Mark
Published: March 2012

A numerical method using cubic B-splines is presented for solving the linear Boltzmann equation. The collision kernel for the system is chosen as the Wigner-Wilkins kernel. A total of three different representations for the distribution function are presented. Eigenvalues and eigenfunctions of the collision matrix are obtained for various mass ratios and compared with known values. Distribution functions, along with first and second moments, are evaluated for different mass and temperature ratios. Overall it is shown that the method is accurate and well behaved. In particular, moments can be predicted with very few points if the representation is chosen well. This method produces sparse matrices, can be easily generalized to higher dimensions, and can be cast into efficient parallel algorithms.
The effect of step geometry in copper oxidation by hyperthermal O2 molecular beam: Cu(511) vs Cu(410)
Authors: Okada Michio , Vattuone Luca , Rocca Mario , Teraoka Yuden
Published: March 2012

Steps are known to be often the active sites for the dissociation of O2 molecules and the nucleation sites of oxide films since they provide paths for subsurface migration and oxygen incorporation. In order to unravel the effect of their morphology on the oxidation of Cu surfaces, we present here a detailed investigation of the O2 interaction with Cu(511) and compare it with previous results for Cu(410), a surface exhibiting terraces of similar size and geometry but different step morphology. As for Cu(410) we find, by x-ray photoemission spectroscopy performed with synchrotron radiation, that Cu2O formation gradually starts above half a monolayer oxygen coverage and that the ignition of oxidation can be lowered to room temperature by dosing O2 via a supersonic molecular beam at hyperthermal energy. The oxidation rate for Cu(511) comes out to be lower than for Cu(410) at normal incidence, about the same when the O2 molecules impinge towards the ascending step rise, but higher when they hit the surface along trajectories even slightly inclined towards the descending step rise. These findings can be rationalized by a collision induced absorption mechanism.
On the efficiency of Hamiltonian-based quantum computation for low-rank matrices
Authors: Cao Zhenwei , Elgart Alexander
Published: March 2012

We present an extension of adiabatic quantum computing (AQC) algorithm for the unstructured search to the case when the number of marked items is unknown. The algorithm maintains the optimal Grover speedup and includes a small counting subroutine. Our other results include a lower bound on the amount of time needed to perform a general Hamiltonian-based quantum search, a lower bound on the evolution time needed to perform a search that is valid in the presence of control error and a generic upper bound on the minimum eigenvalue gap for evolutions. In particular, we demonstrate that quantum speedup for the unstructured search using AQC type algorithms may only be achieved under very rigid control precision requirements.
CO2 laser pulse shortening by laser ablation of a metal target
Authors: Donnelly T. , Mazoyer M. , Lynch A. , O’Sullivan G. , O’Reilly F. , Dunne P. , Cummins T.
Published: March 2012

A repeatable and flexible technique for pulse shortening of laser pulses has been applied to transversely excited atmospheric (TEA) CO2 laser pulses. The technique involves focusing the laser output onto a highly reflective metal target so that plasma is formed, which then operates as a shutter due to strong laser absorption and scattering. Precise control of the focused laser intensity allows for timing of the shutter so that different temporal portions of the pulse can be reflected from the target surface before plasma formation occurs. This type of shutter enables one to reduce the pulse duration down to ∼2 ns and to remove the low power, long duration tails that are present in TEA CO2 pulses. The transmitted energy is reduced as the pulse duration is decreased but the reflected power is ∼10 MW for all pulse durations. A simple laser heating model verifies that the pulse shortening depends directly on the plasma formation time, which in turn is dependent on the applied laser intensity. It is envisaged that this plasma shutter will be used as a tool for pulse shaping in the search for laser pulse conditions to optimize conversion efficiency from laser energy to useable extreme ultraviolet (EUV) radiation for EUV source development.
Electron density and plasma dynamics of a spherical theta pinch
Authors: Teske C. , Liu Y. , Blaes S. , Jacoby J.
Published: March 2012

A spherical theta pinch for plasma stripper applications has been developed and investigated regarding the electron density and the plasma confinement during the pinching sequence. The setup consists of a 6 μH induction coil surrounding a 4000 ml spherical discharge vessel and a capacitor bank with interchangeable capacitors leading to an overall capacitance of 34 μF and 50 μF, respectively. A thyristor switch is used for driving the resonant circuit. Pulsed coil currents reached values of up to 26 kA with maximum induction of 500 mT. Typical gas pressures were 0.7 Pa up to 120 Pa with ArH2 (2.8% H2)-gas as a discharge medium. Stark broadening measurements of the Hβ emission line were carried out in order to evaluate the electron density of the discharge. In accordance with the density measurements, the transfer efficiency was estimated and a scaling law between electron density and discharge energy was established for the current setup. The densities reached values of up to 8 × 1022 m−3 for an energy of 1.6 kJ transferred into the plasma. Further, the pinching of the discharge plasma was documented and the different stages of the pinching process were analyzed. The experimental evidence suggests that concerning the recent setup of the spherical theta pinch, a linear scaling law between the transferred energy and the achievable plasma density can be applied for various applications like plasma strippers and pulsed ion sources.
Thermocapillary flows and interface deformations produced by localized laser heating in confined environment
Authors: Chraïbi Hamza , Delville Jean-Pierre
Published: March 2012

The deformation of a fluid-fluid interface due to the thermocapillary stress induced by a continuous Gaussian laser wave is investigated analytically. We show that the direction of deformation of the liquid interface strongly depends on the viscosities and the thicknesses of the involved liquid layers. We first investigate the case of an interface separating two different liquid layers while a second part is dedicated to a thin film squeezed by two external layers of same liquid. These results are predictive for applications fields where localized thermocapillary stresses are used to produce flows or to deform interfaces in presence of confinement, such as optofluidics.
Accurate and low jitter time-interval generators based on phase shifting method
Authors: Kwiatkowski P. , Jachna Z. , Różyc K. , Kalisz J.
Published: March 2012

The paper describes two time-interval generators based on the phase shifting method. The first one utilizes the digital clock manager units integrated in a field programmable gate array (FPGA) device and has jitter below 65 ps (rms) over the range of 4 ns–50 ms, while the second one utilizes a separate direct digital synthesizer and has jitter below 15 ps (rms) over the range of 10.2 ns–50 ms. The phase shifting method can be used to design new low-cost and high-precision time-interval generators using the popular FPGA technology.
Stark spectroscopy of a probe lithium beam excited with two dye lasers as a technique to study a high-power ion-beam diode
Authors: Knyazev B. A. , An W. , Bluhm H.
Published: March 2012

A non-disturbing measurement of electric field distributions is a subject of special interest in plasma physics and high-voltage devices. In this paper we describe a diagnostic technique for remote sensing of electric fields via injection of a probe beam of lithium atoms and cascade excitation of resonance fluorescence with two broadband dye lasers. The fluorescence spectrum was recorded using a monochromator equipped with an optical multi-channel analyser. The magnitude of the local electric field was retrieved from the Stark-shifted components of the 3d-2plithium spectral line. The technique was applied to measurements of the electric field in the applied-B-field high-voltage diode of the 1 TW KALIF ion-beam accelerator.
Low energy (e,2e) measurements of CH4 and neon in the perpendicular plane
Authors: Nixon Kate L. , Murray Andrew James , Chaluvadi Hari , Amami Sadek , Madison Don H. , Ning Chuangang
Published: March 2012

Low energy experimental and theoretical triple differential cross sections for the highest occupied molecular orbital of methane (1t2) and for the 2p atomic orbital of neon are presented and compared. These targets are iso-electronic, each containing 10 electrons and the chosen orbital within each target has p-electron character. Observation of the differences and similarities of the cross sections for these two species hence gives insight into the different scattering mechanisms occurring for atomic and molecular targets. The experiments used perpendicular, symmetric kinematics with outgoing electron energies between 1.5 eV and 30 eV for CH4 and 2.5 eV and 25 eV for neon. The experimental data from these targets are compared with theoretical predictions using a distorted-wave Born approximation. Reasonably good agreement is seen between the experiment and theory for neon while mixed results are observed for CH4. This is most likely due to approximations of the target orientation made within the model.
Micro-Raman spectroscopy of mechanically exfoliated few-quintuple layers of Bi2Te3, Bi2Se3, and Sb2Te3 materials
Authors: Shahil K. M. F. , Hossain M. Z. , Goyal V. , Balandin A. A.
Published: March 2012

Bismuth telluride (Bi2Te3) and related compounds have recently attracted strong interest, owing to the discovery of the topological insulator properties in many members of this family of materials. The few-quintuple films of these materials are particularly interesting from the physics point of view. We report results of the micro-Raman spectroscopy study of the “graphene-like” exfoliated few-quintuple layers of Bi2Te3, Bi2Se3, and Sb2Te3. It is found that crystal symmetry breaking in few-quintuple films results in appearance of A1u-symmetry Raman peaks, which are not active in the bulk crystals. The scattering spectra measured under the 633-nm wavelength excitation reveals a number of resonant features, which could be used for analysis of the electronic and phonon processes in these materials. In order to elucidate the influence of substrates on the few-quintuple-thick topological insulators, we examined the Raman spectra of these films placed on mica, sapphire, and hafnium-oxide substrates. The obtained results help to understand the physical mechanisms of Raman scattering in the few-quintuple-thick films and can be used for nanometrology of topological insulator films on various substrates.
The role of C-H and C-C stretching modes in the intrinsic non-radiative decay of triplet states in a Pt-containing conjugated phenylene ethynylene
Authors: Köhler Anna , Khan Amena L. T. , Wilson Joanne S. , Dosche Carsten , Al-Suti Mohammed K. , Shah Hakikulla H. , Khan Muhammad S.
Published: March 2012

The intrinsic non-radiative decay (internal conversion) from the triplet excited state in phosphorescent dyes can be described by a multi-phonon emission process. Since non-radiative decay of triplet excitons can be a significant process in organic light-emitting diodes, a detailed understanding of this decay mechanism is important if the overall device efficiency is to be controlled. We compare a deuterated Pt(II)-containing phenylene ethynylene with its non-deuterated counterpart in order to investigate which phonon modes control to the non-radiative decay path. We observe that deuteration does not decrease the non-radiative decay rate. A Franck-Condon analysis of the phosphorescence spectra shows that the electronic excitation is coupled strongly to the breathing mode of the phenyl ring and the C≡C carbon stretching modes, while high-energy C-H or C-D stretching modes play an insignificant role. We, therefore, associate the internal conversion process with the carbon-carbon stretching vibrations.
Ultrafast laser inscribed waveguide lattice in glass for direct observation of transverse localization of light
Authors: Ghosh Somnath , Psaila Nicholas D. , Thomson R. R. , Pal Bishnu P. , Varshney R. K. , Kar Ajoy K.
Published: March 2012

We present initial results of the direct observation of the signature of localized light in an ultrafast laser-inscribed (ULI) disordered lattice that contains an array of evanescently coupled, one-dimensional optical waveguides in glass in which certain amount of disorder in refractive index was introduced. Numerical simulations were carried out to test the feasibility of the initial experimental design. Such configurable ULI disordered waveguide lattices should open up a platform for investigating the phenomenon of transverse localization of light and its statistical nature.