bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of software changes: Transit dose and source position accuracy of the Eckert & Ziegler BEBIG GmbH MultiSource® high dose rate (HDR) brachytherapy treatment unit
Authors: Palmer A.
Published: August 2012

Purpose: Medical device performance checks are essential following changes to control system software. This work investigates the effects of new software on the performance of a high dose rate (HDR) brachytherapy treatment unit.

Methods and Materials: A performance assessment was undertaken of the Eckert & Ziegler BEBIG GmbH MultiSource® HDR treatment unit following software upgrade. Video recordings of source transits were used to calculate transit doses, and autoradiography used to measure source dwell positions. Results were compared to a previous study.1

Results: All results showed improved performance with the new compared to old control software. Optimal source movement profiles were observed with maximum transit speeds of 63 (+/−4) mm s−1 between dwells of 5.0 mm separation. The maximum error in transit dose correction with the new software was 2.5 % at 10.0 mm perpendicular from the source axis, compared to 5.6 % previously. The new software eliminated a causal relationship between curvature of the source transfer tubes and dwell position uncertainty.

Conclusions: This work demonstrates the need for comprehensive medical device system checks following software changes. Technical improvements in HDR device performance have been achieved with the new software; reducing transit doses, improving transit dose correction, and improving source positioning accuracy.
Comparative study of RF MEMS micro-contact materials
Authors: Broué Adrien , Dhennin Jérémie , Charvet Pierre-Louis , Pons Patrick , Ben Jemaa Nourredine , Heeb Peter , Coccetti Fabio , Plana Robert
Published: August 2012

A systematic comparison between several pairs of contact materials based on an innovative methodology early developed at NOVA MEMS is hereby presented. The technique exploits a commercial nanoindenter coupled with electrical measurements, and test vehicles specially designed to investigate the underlying physics driving the surface-related failure modes. The study provides a comprehensive understanding of micro-contact behavior with respect to the impact of low-to-medium levels of electrical current. The decrease of the contact resistance, when the contact force increases, is measured for contact pairs of soft material (Au/Au contact), harder materials (Ru/Ru and Rh/Rh contacts), and mixed configuration (Au/Ru and Au/Ni contacts). The contact temperatures have been calculated and compared with the theoretical values of softening temperature for each couple of contact materials. No softening behavior has been observed for mixed contact at the theoretical softening temperature of both materials. The enhanced resilience of the bimetallic contacts Au/Ru and Au/Ni is demonstrated.
Optimization of a 61.44 GHz BiCMOS HBT integrated PLL for ultra-fast settling time
Authors: Barghouthi Atheer , Hellfeld Marcu , Carta Corrado , Ellinger Frank
Published: August 2012

The design of a 61.44 GHz integrated Phase-locked loop (PLL) on a 180 GHz BiCMOS technology is presented. The PLL was optimized for a very fast settling time of 4 µs as required by the system specifications. Because the receiver is using a carrier recovery circuit that can follow the slow changes of the carrier such as phase noise, the sensitivity of the bit error rate to phase noise at the receiver end is very low. As a result, to achieve the required dynamic behavior, the phase noise performance could be sacrificed and the loop bandwidth was increased until the needed settling time was achieved, while taking the suppression of the reference spurs into consideration. Capacitor multiplication was used to enable the integration of the loop filter (LF) on chip and the effect of the capacitor multiplier on the total PLL phase noise performance was quantified and evaluated. In addition, a very close matching between the measured and simulated phase noise of the system was achieved. The PLL consumes a power of 200 mW from 2 and 3 V supply voltages, while delivering a differential output power of −7 dBm, sufficient to drive the following I/Q modulator without additional amplification.
Transient analysis of the EM field coupling to multi-conductor transmission lines using the NILT method
Authors: Rajhi Adnen , Ghnimi Said , Gharssallah Ali
Published: August 2012

A simple and efficient numerical inversion Laplace transform (NILT) algorithm is implemented in MATLAB environment based on the quotient difference method to solve the problem of electromagnetic (EM) field coupling to lossy or lossless multi-conductor transmission lines (MTL) illuminated by an EM incident field. Two major points are treated in this work for the lossy MTL system excited by an incident EM field; the first one is the optimum equivalent circuit taking into consideration the different physical concepts based on the transmission line theory and the second point deals with the choice and implementation of the numerical method for less computing time and for efficient results. In this paper, the effect of the EM coupling is treated and it is based on the superposition effect of each distributed voltage current sources using the NILT numerical method. Results of the near end and far end voltages and currents for an MTL system are presented and displayed for two types of microwave (MW) structures in the time domain for the case of a plane wave excitation. It has been shown that a non-homogeneous MW structure or multilayered system with a specific choice of the dielectric constant has an advantage for less transient EM coupling due to the external EM field.
An observational analysis of Southern Hemisphere tropical expansion
Authors: Lucas Christopher , Nguyen Hanh , Timbal Bertrand
Published: August 2012

Historical radiosonde data are analyzed using the tropopause height frequency method to investigate the variation of the Southern Hemisphere tropical edge from 1979/80–2010/11, independently of reanalysis‐derived data. Averaged across the hemisphere we identify a tropical expansion trend of 0.41 ± 0.37 deg dec−1, significant at the 90% level. A comparison with four reanalyses shows generally consistent results between radiosondes and reanalyses. Estimated rates of tropical expansion in the SH are broadly similar, as is the interannual variability. However, notable differences remain. Some of these differences are related to the methodology used to identify the height of the tropopause in the reanalyses, which produces inconsistent results in the subtropics. Differences between radiosondes and reanalyses are also more manifest in data‐poor regions. In these regions, the reanalyses are not fully constrained, allowing the internal model dynamics to drive the variability. The performance of the reanalyses varies temporally compared to the radiosonde data. These differences are particularly apparent from 1979 to 1985 and from 2001 to 2010. In the latter period, we hypothesize that the increased availability and quality of satellite‐based data improves the results from the ERA Interim reanalysis, creating an inconsistency with earlier data. This apparent inhomogeneity results in a tropical expansion trend in that product that is inconsistent with the radiosonde‐based observations. These results confirm the need for careful evaluation of reanalysis‐based data for use in studies of long‐term climate variability.
Subduction of oceanic asthenosphere: Evidence from sub‐slab seismic anisotropy
Authors: Song Teh‐Ru Alex , Kawakatsu Hitoshi
Published: August 2012

The oceanic asthenosphere is characterized as a low viscosity channel down to 200–300 km depth separating the cold lithosphere from above, and it is intimately linked to a layer of low seismic velocity and prominent seismic anisotropy observed globally beneath ocean basins. While subduction of tectonic plates in convergent margins is well recognized, the fate of oceanic asthenosphere remains enigmatic. We demonstrate that subduction of the oceanic asthenosphere characterized by weak azimuthal anisotropy and strong radial anisotropy explains the essence of sub‐slab shear‐wave splitting patterns, where the fast splitting direction changes from predominantly trench‐parallel (or sub‐parallel) under relatively steep subduction zones to frequently trench‐normal under shallow subduction zones. To explain the observed splitting time, the thickness of the subducted asthenosphere is estimated to be 100 ± 50 km.
Multispacecraft observation of magnetic cloud erosion by magnetic reconnection during propagation
Authors: Ruffenach A. , Lavraud B. , Owens M. J. , Sauvaud J.‐A. , Savani N. P. , Rouillard A. P. , Démoulin P. , Foullon C. , Opitz A. , Fedorov A. , Jacquey C. J. , Génot V. , Louarn P. , Luhmann J. G. , Russell C. T. , Farrugia C. J. , Galvin A. B.
Published: August 2012

During propagation, Magnetic Clouds (MC) interact with their environment and, in particular, may reconnect with the solar wind around it, eroding away part of its initial magnetic flux. Here we quantitatively analyze such an interaction using combined, multipoint observations of the same MC flux rope by STEREO A, B, ACE, WIND and THEMIS on November 19–20, 2007. Observation of azimuthal magnetic flux imbalance inside a MC flux rope has been argued to stem from erosion due to magnetic reconnection at its front boundary. The present study adds to such analysis a large set of signatures expected from this erosion process. (1) Comparison of azimuthal flux imbalance for the same MC at widely separated points precludes the crossing of the MC leg as a source of bias in flux imbalance estimates. (2) The use of different methods, associated errors and parametric analyses show that only an unexpectedly large error in MC axis orientation could explain the azimuthal flux imbalance. (3) Reconnection signatures are observed at the MC front at all spacecraft, consistent with an ongoing erosion process. (4) Signatures in suprathermal electrons suggest that the trailing part of the MC has a different large‐scale magnetic topology, as expected. The azimuthal magnetic flux erosion estimated at ACE and STEREO A corresponds respectively to 44% and 49% of the inferred initial azimuthal magnetic flux before MC erosion upon propagation. The corresponding average reconnection rate during transit is estimated to be in the range 0.12–0.22 mV/m, suggesting most of the erosion occurs in the inner parts of the heliosphere. Future studies ought to quantify the influence of such an erosion process on geo‐effectiveness.
The SuperMAG data processing technique
Authors: Gjerloev J. W.
Published: August 2012

In this paper I outline the data processing technique which is used in the SuperMAG initiative. SuperMAG is a worldwide collaboration of organizations and national agencies that currently operate more than 300 ground based magnetometers. SuperMAG provides easy access to validated ground magnetic field perturbations in the same coordinate system, identical time resolution and with a common baseline removal approach. The purpose of SuperMAG is to provide scientists, teachers, students and the general public easy access to measurements of the magnetic field at the surface of the Earth. Easy access to data, plots and derived products maximizes the utilization of this unique data set. It is outlined how SuperMAG processes the observations obtained by the individual data provider. Data are rotated into a local magnetic coordinate system by determining a time dependent declination angle. This angle displays a slow gradual change and a yearly periodic variation attributed to changes in the Earth main field and season temperature variations. The baseline is determined from the data itself in a three step process: (1) a daily baseline, (2) a yearly trend, and (3) a residual offset. This technique does not require so‐called quiet days and thus it avoids all the well‐known problems associated with their identification. The residual offset for the N‐ and Z‐components shows a distinct latitudinal dependence while the E‐component is independent of the latitude. This result is interpreted as being due to a weak ring current (likely asymmetric) which is present even during official quiet days. For the purpose of M‐I research using 1‐min data I find no difference between observatories and variometers. I finally argue that there is no correct baseline determination technique since we do not have a set of ground‐truth observations required to make an objective evaluation. Instead, the user must keep in mind the assumptions on which the baseline was determined and draw conclusions accordingly.
Spatial distribution and spectral characteristics of energetic electrons in Mercury's magnetosphere
Authors: Ho George C. , Krimigis Stamatios M. , Gold Robert E. , Baker Daniel N. , Anderson Brian J. , Korth Haje , Slavin James A. , McNutt Ralph L. , Winslow Reka M. , Solomon Sean C.
Published: August 2012

The Energetic Particle Spectrometer (EPS) on the MESSENGER spacecraft, in orbit about Mercury since March 2011, has detected bursts of low‐ and moderate‐energy (tens to hundreds of keV) electrons during portions of most orbits. There have been periods when such bursts were observed regularly on every orbit over a span of several weeks, and other periods when electrons were not observed for several days at a time. We have systematically characterized these energetic events on the basis of particle intensity over the 12‐month period since MESSENGER began orbital operations. Now that MESSENGER has sampled most Mercury longitudes and local times, it is evident that the largest burst events were either at high northern latitudes or near local midnight. Lower‐energy events were also seen near the equator but were mostly absent in both the dawn and dusk local time sectors. The high‐latitude and nightside events are similar in particle intensity, spectra, and pitch angle and are interpreted to be the result of acceleration by the same mechanism. Another group of events occurred upstream of Mercury's bow shock. For two examples of this group of upstream events with good pitch angle coverage, the particles were field‐aligned and traveling away from the bow shock. This group of events is interpreted to be similar to upstream events found at Earth during which particles are accelerated at the bow shock and subsequently travel upstream into the solar wind.
The influence of microstructure on deformation of olivine in the grain‐boundary sliding regime
Authors: Hansen L. N. , Zimmerman M. E. , Kohlstedt D. L.
Published: August 2012

Although microstructural evolution is critical to strain‐dependent processes in Earth's mantle, flow laws for dunite have only been calibrated with low‐strain experiments. Therefore, we conducted a series of high‐strain torsion experiments on thin‐walled cylinders of iron‐rich olivine aggregates. Experiments were performed in a gas‐medium apparatus at 1200°C and constant strain rate. In our experiments, each at a different strain rate, a peak stress was observed followed by significant strain weakening. We first deformed samples to high enough strain that a steady state microstructure was achieved and then conducted strain rate stepping tests to characterize the creep behavior of each sample with constant microstructure. A global fit to the data yields a stress exponent of 4.1 and a grain‐size exponent of 0.73, values which agree well with those from previous small‐strain experiments conducted on olivine in the dislocation‐accommodated grain‐boundary sliding (GBS) regime. Strong crystallographic preferred orientations provide support for GBS accommodated by movement of (010)[100] dislocations. The observed strain weakening is not entirely explained by grain‐size reduction; thus, we propose that the remaining 30% reduction in stress is related to CPO development. To incorporate microstructural evolution in a constitutive description of GBS in olivine, we (1) derive a flow law for high‐strain deformation with steady state microstructure, which results in an apparent stress exponent of 5.0, and (2) present a system of evolution equations that recreate the observed strain weakening. Our results corroborate flow‐law parameters and microstructural observations from low‐strain experiments and provide a means for incorporating strain weakening into geodynamic simulations.
Observed variation in the decay time of oceanic whitecap foam
Authors: Callaghan Adrian H. , Deane Grant B. , Stokes M. Dale , Ward Brian
Published: August 2012

Whitecap foam decay times for 552 individual breaking waves determined from digital images of the sea surface are reported. The images had sub‐centimeter pixel resolution and were acquired at frame rates between 3 and 6 frames per second at the Martha's Vineyard Coastal Observatory over a 10‐day period in 2008, subdivided into 4 observation periods. Whitecap foam decay times for individual events varied between 0.2 s to 10.4 s across the entire data set. A systematic positive correlation between whitecap foam decay time and maximum whitecap foam patch area was found for each observation period. For a given whitecap size within each observation period, the decay times varied between a factor of 2 and 5, with the largest variation occurring during unsteady environmental forcing conditions. Within observation periods, bin‐averaged decay times varied by up to a factor of 4 across the range of foam patch areas. Between observation periods, the effective whitecap foam decay time, which we define as the area‐weighted mean decay time, varied by a factor of 3.4 between 1.4 s and 4.8 s. We found a weak correlation between decay times and individual event‐averaged breaking wave speeds. The variation in the active breaking area across all 4 observation periods was small, indicating relatively uniform surface whitecap area generating potential. We speculate that the variation in the foam decay times may be due to (i) the effect of surfactants on bubble and foam stability, and (ii) differences between bubble plume characteristics caused by a variation in breaking wave type.
Using the Murchison Widefield Array to observe midlatitude space weather
Authors: Coster Anthea , Herne David , Erickson Philip , Oberoi Divya
Published: August 2012

Midlatitude space weather events are being monitored at a location in Western Australia (−26.7° latitude, 116.7° longitude, and −38° magnetic latitude). This location is that of the Murchison Widefield Array (MWA), a new low radio frequency array (80 to 300 MHz) being built with the objective of developing powerful new capabilities for radio astronomy, solar, heliospheric, and ionospheric science. The MWA is a radio interferometer that images large fields of view with unprecedented fidelity and dynamic range at these frequencies. With respect to the ionosphere, it will be able to make important new measurements of ionospheric scintillation and improve our understanding of midlatitude space weather events in the Southern Hemisphere. For example, by studying midlatitude space weather events in the Australian sector, the seasonal and geomagnetic dependencies of storm enhanced density (SED) can be characterized and separated from other forces that may impact its formation elsewhere, such as in the longitudinal sector of the South Atlantic Anomaly (SAA). After full commissioning, the MWA will consist of 2048 dipole antennas arranged as 128 “tiles,” each being a 4 × 4 array of dual‐polarization dipoles. Currently, an engineering prototype comprising 32 tiles has been installed and has been used to observe the Faraday rotation from a linearly polarized satellite beacon on the DMSP‐15 satellite. We will report on these initial observations of ionospheric Faraday rotation and on results from GPS receivers located at the site and elsewhere in the Australian continent. We will also discuss future space weather products enabled by the MWA.
Impact of sound attenuation by suspended sediment on ADCP backscatter calibrations
Authors: Sassi M. G. , Hoitink A. J. F. , Vermeulen B.
Published: August 2012

Although designed for velocity measurements, acoustic Doppler current profilers (ADCPs) are being widely used to monitor suspended particulate matter in rivers and in marine environments. To quantify mass concentrations of suspended matter, ADCP backscatter is generally calibrated with in situ measurements. ADCP backscatter calibrations are often highly site specific and season dependent, which is typically attributed to the sensitivity of the acoustic response to the number of scatterers and their size. Besides being a joint function of the concentration and the size of the scatterers, the acoustic backscatter can be heavily affected by the attenuation due to suspended matter along the two‐way path to the target volume. Our aim is to show that accounting for sound attenuation in ADCP backscatter calibrations may broaden the range of application of ADCPs in natural environments. The trade‐off between the applicability and the accuracy of a certain calibration depends on the variation in size distribution and concentration along the sound path. We propose a simple approach to derive the attenuation constant per unit concentration or specific attenuation, based on two water samples collected along the sound path of the ADCP. A single calibration was successfully applied at five locations along the River Mahakam, located up to 200 km apart. ADCP‐derived estimates of suspended mass concentration were shown to be unbiased, even far away from the transducer.
Biogeography of Rhizobium radiobacter and distribution of associated temperate phages in deep subseafloor sediments
Authors: Engelhardt Tim , Sahlberg Monika , Cypionka Heribert , Engelen Bert
Published: August 2012

Bacteriophages might be the main ‘predators’ in the marine deep subsurface and probably have a major impact on indigenous microbial communities. To identify their function within this habitat, we have determined their abundance and distribution along the sediment columns of two continental margin and two open ocean sites that were recovered during Leg 201 of the Ocean Drilling Program. For all investigated sites, viral abundance followed the total cell numbers with a virus-to-cell ratio between 1 and 10 in the upper 100 mbsf (meters below seafloor). An increasing ratio of about 20 in deeper layers indicated an ongoing viral production in up to 11 Ma old sediments. We have used Rhizobium radiobacter as the most frequently isolated organism from the deep subsurface with a high in situ abundance to identify the frequency of associated rhizobiophages. In this study, 16S rRNA gene copies of R. radiobacter accounted for up to 5.6% of total bacterial 16S rRNA genes (average: 0.75%) as detected by quantitative PCR. A distinctive distribution was identified for R. radiobacter as indicated by a site-specific arrangement of genetically similar populations. Whole genome information of rhizobiophage RR1-A was used to generate a primer system for quantitative PCR specifically targeting the prophage antirepressor gene, indicative for temperate phages. The quantification of this gene within various sediment horizons showed a contribution of temperate phages of up to 14.3% to the total viral abundance. Thus, the high amount of temperate phages within the sediments and among all investigated isolates indicates that lysogeny is the main viral proliferation mode in deep subsurface populations.
Coarse-grain model for lipid bilayer self-assembly and dynamics: Multiparticle collision description of the solvent
Authors: Huang Mu-Jie , Kapral Raymond , Mikhailov Alexander S. , Chen Hsuan-Yi
Published: August 2012

A mesoscopic coarse-grain model for computationally efficient simulations of biomembranes is presented. It combines molecular dynamics simulations for the lipids, modeled as elastic chains of beads, with multiparticle collision dynamics for the solvent. Self-assembly of a membrane from a uniform mixture of lipids is observed. Simulations at different temperatures demonstrate that it reproduces the gel and liquid phases of lipid bilayers. Investigations of lipid diffusion in different phases reveals a crossover from subdiffusion to normal diffusion at long times. Macroscopic membrane properties, such as stretching and bending elastic moduli, are determined directly from the mesoscopic simulations. Velocity correlation functions for membrane flows are determined and analyzed.
N(4S /2D)+N2: Accurate ab initio-based DMBE potential energy surfaces and surface-hopping dynamics
Authors: Galvão B. R. L. , Caridade P. J. S. B. , Varandas A. J. C.
Published: August 2012

This work gives a full account of the N(4S/2D)+\documentclass[12pt]{minimal}\begin{document}${\rm N}_2(\sideset{^1}{_{g}^{+}}{\Sigma})$\end{document}N2(Σg+1) interactions via accurate electronic structure calculations and study of the involved exchange reactions. A 2 × 2 diabatic representation of the potential energy surface is suggested for N3(2A′), which, combined with the two previously reported adiabatic forms for 2A″ and another for 4A″, completes the set of five global potentials required to study the title collisional processes. The trajectory results provide the first N(2D)+N2 rate constants, and allow a comparison with the ones for N(4S)+N2. Nonadiabatic effects are estimated by surface hopping, and the geometrical phase effect assessed by following the trajectories that encircle the crossing seam.
Quasigraded bases in loop algebras and classical rational r-matrices
Authors: Skrypnyk T.
Published: August 2012

In the present paper we construct quasigraded bases in the loop algebras associated with each classical rational r-matrix. We show that they are orthogonal and agreed with the decomposition of the loop algebra into sum of subalgebras that corresponds to this rational r-matrix. Using the quasigraded character of the constructed algebras we define the infinite sequence of the embedded ideals in the each subalgebra of the decomposition and apply this result to the theory of finite-dimensional integrable systems.
Appearance potential spectroscopy with a photon counting detector and multiple scattering spectral interpretation
Authors: Amidani L. , Pasquini L. , Boscherini F.
Published: August 2012

We describe a soft x-ray appearance potential spectroscopy apparatus, which uses a windowless hyperpure Ge detector operated in the photon counting mode. Direct comparisons of recorded spectra with the self-convolution of x-ray absorption spectra and with ab initio simulations in the multiple scattering framework are reported and discussed.
Application of the quasi-variational coupled cluster method to the nonlinear optical properties of model hydrogen systems
Authors: Robinson James B. , Knowles Peter J.
Published: August 2012

We present a pilot application of the recently proposed quasi-variational coupled cluster method to the energies, polarizabilities, and second hyperpolarizabilities of model hydrogen chains. Relative to other single-reference methods of equivalent computational complexity, we demonstrate this method to be highly robust and especially useful when traditional coupled cluster theory fails to perform adequately. In particular, our results indicate it to be a suitable method for the black-box treatment of multiradicals, making it of widespread general interest and applicability.
Filamentary streamer discharges in argon at atmospheric pressure excited by surface plasmon polaritons
Authors: Chen Zhaoquan , Xia Guangqing , Zhou Qiyan , Hu Yelin , Zheng Xiaoliang , Zheng Zhen , Hong Lingli , Li Ping , Huang Yourui , Liu Minghai
Published: August 2012

This paper describes a microwave plasma jet in an argon atmosphere capable of generating filamentary streamer discharges within the entire quartz tube excited by surface waves of surface plasmon polaritons (SPPs) located in the tube. Several discharge streamers are immediately produced at the end of the copper wire when incident power reaches 20 W. From simulations, the wavelength of the surface wave was found to be approximately 5.7 cm. Although the developing streamers induce E-field enhancements favoring discharging, more streamer bifurcations requiring additional energy to maintain discharging diminish the resonant enhanced E-field. The underlying mechanism of the proposed plasma jet is resonant excitation of SPPs and its interaction with plasmas.
Evaluation of the importance of spin-orbit couplings in the nonadiabatic quantum dynamics with quantum fidelity and with its efficient “on-the-fly” ab initio semiclassical approximation
Authors: Zimmermann Tomáš , Vaníček Jiří
Published: August 2012

We propose to measure the importance of spin-orbit couplings (SOCs) in the nonadiabatic molecular quantum dynamics rigorously with quantum fidelity. To make the criterion practical, quantum fidelity is estimated efficiently with the multiple-surface dephasing representation (MSDR). The MSDR is a semiclassical method that includes nuclear quantum effects through interference of mixed quantum-classical trajectories without the need for the Hessian of potential energy surfaces. Two variants of the MSDR are studied, in which the nuclei are propagated either with the fewest-switches surface hopping or with the locally mean field dynamics. The fidelity criterion and MSDR are first tested on one-dimensional model systems amenable to numerically exact quantum dynamics. Then, the MSDR is combined with “on-the-fly” computed electronic structure to measure the importance of SOCs and nonadiabatic couplings in the photoisomerization dynamics of \documentclass[12pt]{minimal}\begin{document}${\rm CH}_2 {\rm NH}_2^+$\end{document} CH 2 NH 2+ considering 20 electronic states and in the collision of F + H2 considering six electronic states.
Modified reactive empirical bond-order potential for heterogeneous bonding environments
Authors: Hur Jaewoong , Stuart Steven J.
Published: August 2012

An improvement to the AIREBO potential for hydrocarbons is presented in which contributions to the bond order are determined by the local bonding environment around the bond, rather than the average of the environments around the two constituent atoms. This bond-centric approach decreases the errors by ∼80% in the fullerene-type systems for which the original approach leads to the most severe errors. With the newly developed and parameterized method, energy errors are less than 0.7 eV for a collection of hydrocarbon molecules not used in the fitting. This modified AIREBO potential is expected to be more useful not only for the molecular hydrocarbons and fullerene isomers studied here, but also for the full range of carbon and hydrocarbon systems to which the AIREBO potential has been applied.
Static surface temperature effects on the dissociation of H2 and D2 on Cu(111)
Authors: Wijzenbroek M. , Somers M. F.
Published: August 2012

A model for taking into account surface temperature effects in molecule-surface reactions is reported and applied to the dissociation of H2 and D2 on Cu(111). In contrast to many models developed before, the model constructed here takes into account the effects of static corrugation of the potential energy surface rather than energy exchange between the impinging hydrogen molecule and the surface. Such an approximation is a vibrational sudden approximation. The quality of the model is assessed by comparison to a recent density functional theory study. It is shown that the model gives a reasonable agreement with recently performed ab initio molecular dynamics calculations, in which the surface atoms were allowed to move. The observed broadening of the reaction probability curve with increasing surface temperature is attributed to the displacement of surface atoms, whereas the effect of thermal expansion is found to be primarily a shift of the curve to lower energies. It is also found that the rotational quadrupole alignment parameter is generally lowered at low energies, whereas it remains approximately constant at high energies. Finally, it is shown that the approximation of an ideal static surface works well for low surface temperatures, in particular for the molecular beams for this system (Ts = 120 K). Nonetheless, for the state-resolved reaction probability at this surface temperature, some broadening is found.
Electronic spectroscopy and electronic structure of diatomic TiFe
Authors: Krechkivska Olha , Morse Michael D. , Kalemos Apostolos , Mavridis Aristides
Published: August 2012

Diatomic TiFe, a 12 valence electron molecule that is isoelectronic with Cr2, has been spectroscopically investigated for the first time. In addition, the first computational study that includes the ground and excited electronic states is reported. Like Cr2, TiFe has a 1Σ+ ground state that is dominated by the 1σ2 2σ2 1π4 1δ4 configuration. Rotationally resolved spectroscopy has established a ground state bond length of 1.7024(3) Å, quite similar to that found for Cr2 (r0 = 1.6858 Å). Evidently, TiFe exhibits a high degree of multiple bonding. The vibronic spectrum is highly congested and intense to the blue of 20 000 cm−1, while two extremely weak band systems, the [15.9]3Π1 ← X 1Σ+ and [16.2]3Π0+ ← X 1Σ+ systems, are found in the 16 000–18 500 cm−1 region. The bond lengths, obtained by inversion of the Be′ values, and vibrational frequencies of the two upper states are nearly identical: 1.886 Å and 344 cm−1 for [15.9]3Π1 and 1.884 Å and 349 cm−1 for [16.2]3Π0+. The measured spin-orbit splitting of the 3Π state is consistent with its assignment to the 1σ2 2σ2 1π4 1δ3 2π1 configuration, as is also found in the ab initio calculations.
Two-phase simulation of the crystalline silicon melting line at pressures from –1 to 3 GPa
Authors: Dozhdikov V. S. , Basharin A. Yu. , Levashov P. R.
Published: August 2012

Results of a numerical investigation of crystalline silicon melting line within the range of pressures from –1 to 3 GPa are presented. A two-phase molecular dynamics method is applied to obtain temperature, pressure, and densities of solid and liquid phases on the melting line. Using a special procedure we ensure the strict control of the two-phase equilibrium in the simulation cell. To describe the interaction between the atoms four classic potentials have been chosen: the Stillinger–Weber one and three modified variants of the Tersoff potential. For the Stillinger–Weber and Tersoff potentials in the modification by Kumagai–Izumi–Hara–Sakai a good coincidence with experimental data on crystalline Si melting temperature is obtained within the range of pressure from 0 to 3 GPa. Calculations of the solid and liquid phase densities on the silicon melting line for the Stillinger–Weber potential are also in close agreement with experiments.
Hanle effect as candidate for measuring magnetic fields in laboratory plasmas
Authors: Presura R.
Published: August 2012

Weak or turbulent magnetic fields are generally difficult to measure in laboratory plasmas. A new technique to address this problem may be based on the Hanle effect, used for magnetic field measurements in solar and stellar atmospheres. The effect consists in the modification of the polarization state of the resonance-line scattered radiation. It applies for magnetic field strengths for which the shift of magnetic sublevels remains comparable to the natural width of the sublevels. Experimental configurations and parameter ranges of applicability of a Hanle effect-based diagnostics technique are discussed.
Active spectroscopic measurements of the bulk deuterium properties in the DIII-D tokamak (invited)
Authors: Grierson B. A. , Burrell K. H. , Chrystal C. , Groebner R. J. , Kaplan D. H. , Heidbrink W. W. , Muñoz Burgos J. M. , Pablant N. A. , Solomon W. M. , Van Zeeland M. A.
Published: August 2012

The neutral-beam induced Dα emission spectrum contains a wealth of information such as deuterium ion temperature, toroidal rotation, density, beam emission intensity, beam neutral density, and local magnetic field strength magnitude |B| from the Stark-split beam emission spectrum, and fast-ion Dα emission (FIDA) proportional to the beam-injected fast ion density. A comprehensive spectral fitting routine which accounts for all photoemission processes is employed for the spectral analysis. Interpretation of the measurements to determine physically relevant plasma parameters is assisted by the use of an optimized viewing geometry and forward modeling of the emission spectra using a Monte-Carlo 3D simulation code.
The effects of electronic polarization on water adsorption in metal-organic frameworks: H2O in MIL-53(Cr)
Authors: Cirera Jordi , Sung Jeffrey C. , Howland Porter B. , Paesani Francesco
Published: August 2012

The effects of electronic polarization on the adsorption of water in the MIL-53(Cr) metal-organic framework are investigated using molecular dynamics simulations. For this purpose a fully polarizable force field for MIL-53(Cr) was developed which is compatible with the ab initio-based TTM3-F water model. The analysis of the spatial distributions of the water molecules within the MIL-53(Cr) nanopores calculated as a function of loading indicates that polarization effects play an important role in the formation of hydrogen bonds between the water molecules and the hydroxyl groups of the framework. As a result, large qualitative differences are found between the radial distribution functions calculated with non-polarizable and polarizable force fields. The present analysis suggests that polarization effects can significantly impact molecular adsorption in metal-organic frameworks under hydrated conditions.
A self healing model based on polymer-mediated chromophore correlations
Authors: Ramini Shiva K. , Kuzyk Mark G.
Published: August 2012

Here we present a model of self healing in which correlations between chromophores, as mediated by the polymer, are key to the recovery process. Our model determines the size distribution of the correlation volume using a grand canonical ensemble through a free energy advantage parameter. Choosing a healing rate that is proportional to the number of undamaged molecules in a correlated region, and a decay rate proportional to the intensity normalized to the correlation volume, the ensemble average is shown to correctly predict decay and recovery of the population of disperse orange 11-DO11 (1-amino-2-methylanthraquinone) molecules doped in PMMA polymer as a function of time and concentration as measured with amplified spontaneous emission and linear absorption spectroscopy using only three parameters that apply to the full set of data. Our model also predicts the temperature dependence of the process. One set of parameters should be characteristic of a particular polymer and dopant chromophore combination. Thus, the use of the model in determining these parameters for various materials systems should provide the data needed to test fundamental models of the underlying mechanism responsible for self healing.
Molecular dynamics simulation of quasi-two-dimensional water clusters on ice nucleation protein
Authors: Murakami Daisuke , Yasuoka Kenji
Published: August 2012

We performed molecular dynamics simulations of systems that consisted of the ice nucleation protein and the quasi-two-dimensional water cluster on it. The angle distributions, percolation probabilities, mean cluster sizes, cluster size distributions, and hydrogen bond relaxation times were analyzed. We concluded that the behavior of the water clusters on the ice nucleation protein was elaborately intertwined by the interaction between the ice nucleation protein and water, the interaction between the water molecules and the effect of temperature. The percolation probability and mean cluster size depended on the interactions and temperatures.
Focused ion beam induced structural modifications in thin magnetic films
Authors: Roshchupkina O. D. , Grenzer J. , Strache T. , McCord J. , Fritzsche M. , Muecklich A. , Baehtz C. , Fassbender J.
Published: August 2012

Focused ion beam techniques are one way to modify locally the properties of magnetic thin films. We report on structural investigations of 50 nm thick non-ordered nano-crystalline Permalloy (Ni81Fe19) films modified by 30 keV Ga+ focused ion beam (FIB) irradiation. From the x-ray diffraction (XRD) measurements a considerable crystallite growth and a material texturing towards (111)-direction with a linearly increasing lattice constant was observed. In addition, cross-sectional transmission electron microscope (XTEM) images show that crystallites are growing through the entire film at high irradiation fluences. Extended x-ray absorption fine structure (EXAFS) analysis shows a perfect near-order coordination corresponding to a face-centered (fcc) unit cell for both Fe, Ni and Ga atom surrounding. The structural changes are accompanied by a decrease of saturation polarization with increasing ion fluence. Such a behavior is attributed to the incorporation of non-magnetic Ga atoms in the Permalloy film.
Restricted dynamics of water around a protein–carbohydrate complex: Computer simulation studies
Authors: Jana Madhurima , Bandyopadhyay Sanjoy
Published: August 2012

Water-mediated protein–carbohydrate interaction is a complex phenomenon responsible for different biological processes in cellular environment. One of the unexplored but important issues in this area is the role played by water during the recognition process and also in controlling the microscopic properties of the complex. In this study, we have carried out atomistic molecular dynamics simulations of a protein–carbohydrate complex formed between the hyaluronan binding domain of the murine Cd44 protein and the octasaccharide hyaluronan in explicit water. Efforts have been made to explore the heterogeneous influence of the complex on the dynamic properties of water present in different regions around it. It is revealed from our analyses that the heterogeneous dynamics of water around the complex are coupled with differential time scales of formation and breaking of hydrogen bonds at the interface. Presence of a highly rigid thin layer of motionally restricted water molecules bridging the protein and the carbohydrate in the common region of the complex has been identified. Such water molecules are expected to play a crucial role in controlling properties of the complex. Importantly, it is demonstrated that the formation of the protein–carbohydrate complex affects the transverse and longitudinal degrees of freedom of the interfacial water molecules in a heterogeneous manner.
Effect of tensor interaction in the Dirac-attractive radial problem under pseudospin symmetry limit
Authors: Hamzavi M. , Eshghi M. , Ikhdair S. M.
Published: August 2012

We approximately investigated pseudospin symmetric solutions of the Dirac equation for attractive radial potential, including a Coulomb-like tensor interaction under pseudospin symmetry limit for any spin-orbit quantum number κ. By using the parametric generalization of the Nikiforov-Uvarov method, the energy eigenvalues equation and the corresponding wave functions have been obtained in closed forms. Some numerical results are also given. We presented tensor interaction removes degeneracy between two states in pseudospin doublets.
New infrared bands of nonpolar OCS dimer and experimental frequencies for two intermolecular modes
Authors: Afshari M. , Dehghany M. , McKellar A. R. W , Moazzen-Ahmadi N.
Published: August 2012

Spectra of the nonpolar carbonyl sulfide dimer in the region of the OCS ν1 fundamental band were observed in a slit-jet supersonic expansion. The jet was probed using radiation from a tunable diode laser employed in a rapid-scan signal averaging mode. Six new bands were observed and analyzed, all of which originate from the dimer ground vibrational state. Three were vibrational fundamentals involving the (18OCS)2 and 16OCS-18OCS isotopologues. They enabled an estimate to be made of the frequency of the infrared-forbidden mode corresponding to in-phase vibration of the OCS monomers in the dimer, a value needed to obtain an intermolecular vibrational frequency from one of the observed combination bands. A relatively weak b-type dimer band centered at 2103.105 cm−1 was assigned to the OCS 4ν2 (l = 0) bending overtone. Combination bands were observed involving the geared bend and van der Waals stretch intermolecular modes. The resulting experimental frequencies of 37.5(20) cm−1 for the bend and 42.9727(1) cm−1 for the stretch are in good agreement with a recent high level theoretical calculation.
Recent results regarding affine quantum gravity
Authors: Klauder John R.
Published: August 2012

Recent progress in the quantization of nonrenormalizable scalar fields has found that a suitable non-classical modification of the ground state wave function leads to a result that eliminates term-by-term divergences that arise in a conventional perturbation analysis. After a brief review of both the scalar field story and the affine quantum gravity program, examination of the procedures used in the latter surprisingly shows an analogous formulation which already implies that affine quantum gravity is not plagued by divergences that arise in a standard perturbation study. Additionally, guided by the projection operator method to deal with quantum constraints, trial reproducing kernels are introduced that satisfy the diffeomorphism constraints. Furthermore, it is argued that the trial reproducing kernels for the diffeomorphism constraints may also satisfy the Hamiltonian constraint as well.
Reversible-strain criteria of ferromagnetic shape memory alloys under cyclic 3D magneto-mechanical loadings
Authors: He Y. J. , Chen X. , Moumni Z.
Published: August 2012

Recent researches revealed that ferromagnetic shape memory alloys (FSMA) in 2D/3D configurations (with multi-axial stresses) had much more advantages (e.g., higher working stress and more application flexibility) than that in 1D configuration (with uniaxial stress). In literature, however, there is no simple criterion to judge whether a cyclic 3D magneto-mechanical loading can induce a large reversible strain (via martensite reorientation in FSMA). In this paper, a 3D magneto-mechanical energy analysis is proposed and incorporated into a phase diagram in terms of deviatoric stresses (including mechanical and magneto-stresses) to study the path-dependent (hysteretic) martensite reorientation in FSMA under 3D cyclic loadings. Based on the phase diagram (a plane graph), general criteria for obtaining reversible strain under cyclic magneto-mechanical loadings are derived, which provide basic guidelines for FSMA’s applications under multi-axial loadings. Particularly for FSMA actuators driven by cyclic magnetic fields, the criteria of setting the 3D mechanical stresses to allow field-induced reversible strain are formulated. The 3D criteria can be reduced to 1D and 2D criteria which agree with the existing 1D/2D theoretical and experimental studies.
Crystalline structure of GeTe layer in GeTe/Sb2Te3 superlattice for phase change memory
Authors: Soeya Susumu , Shintani Toshimichi
Published: August 2012

Low electric-power switching between set and reset has been confirmed to occur in a GeTe/Sb2Te3 superlattice (SL) film. This sort of operation will be essential for future phase-change random access memories. However, the crystalline structure of the SL, especially the key layer of GeTe, has not been studied sufficiently. The GeTe crystalline structure in the SL was thus investigated in XRD experiments, and the order parameter S was estimated. With the bulk crystalline structure of GeTe, S was about 1.76–1.77, which is quite different from the desirable value of around 1. To solve this unreasonable S, GeTe crystalline structures with conceivable real vacancies were modeled, and their Ss’ were estimated. Consequently, a GeTe crystalline structure with 0.7 ≦ S ≦ 1 was discovered which differed from bulk. It was also verified that the previously thought to be imaginary vacancy layer of GeTe in the SL estimated from a first-principle computer simulation is very likely to be a real vacancy layer.