bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of material mixing on planetary gamma ray spectroscopy
Authors: Squyres Steven W. , Evans Larry G.
Published: May 2009

In many types of compositional remote sensing, a key problem involves separating the signals of two or more compositionally distinct materials that are mixed on a planet's surface. We investigate mixing effects in planetary gamma ray spectroscopy. Two important sources of planetary gamma ray emissions are inelastic scatter reactions induced by energetic neutrons and prompt capture reactions induced by thermal neutrons. The nature of the neutron energy spectrum at a planet's surface therefore exerts an important influence on the nature of the gamma radiation emitted and on the interpretation of compositional information from that radiation. The most important factors controlling the neutron energy spectrum are the hydrogen content xH and the macroscopic thermal neutron absorption cross section ∑ of the surface material. If a planetary surface is composed of several mixed materials with differering values of xH and/or ∑, then the emitted gamma ray spectrum can depend sensitively on the physical nature of the mixing. An important example may be Mars, where one expects to find rocks with low xH intermixed with fine‐grained soil that may have a significantly higher xH due to inclusion of water of hydration. If rocky materials are mixed through the soil at a size scale that is finer than the characteristic length scale for neutron transport (tens of centimeters), then they will be immersed in the thermal neutrons produced by scattering in the H‐rich soil. Such rock may be expected to generate strong prompt capture lines. However, rock segregated in bodies larger than this length scale will be shielded from the thermal neutrons, so their emission will be dominated by inelastic scatter lines. We quantify these effects by modeling gamma ray emissions from the Martian surface. Our model includes production of neutrons by cosmic ray interactions, neutron scattering, gamma ray production by inelastic scatter, thermal neutron capture, and natural radionuclides, attenuation of the gamma ray signal by passage through surface materials and the Martian atmosphere, production of the gamma ray continuum background, and the physics and statistics of gamma ray detection from an orbiting spacecraft. In order to investigate the kinds of effects that can occur, we consider mixing of “Viking soil” with rocks of “mafic” and “felsic” composition, in three geometries: intimate mixing, in which the mixing length scale is small compared to neutron transport length scales; checkerboard mixing, in which it is large; and layered mixing, in which a thin layer of soil overlies a semi‐infinite layer of rock. We also consider mixing of soil and ice in these geometries. We show that mixing geometry can have a major impact on observed spectra and hence on geochemical interpretations. We also show that significant steps can be made toward characterizing the mixing geometry and disentangling the soil and rock signals if adequate independent knowledge of the composition and concentration of the soil component is available.
Geophysical models for the formation and evolution of coronae on Venus
Authors: Janes Daniel M. , Squyres Steven W. , Bindschadler Duane L. , Baer Gidon , Schubert Gerald , Sharpton Virgil L. , Stofan Ellen R.
Published: May 2009

Coronae are large circular features on Venus characterized by an annulus of concentric tectonic features, interior fracturing, volcanism, and generally upraised topography. They are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation. We examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the Magellan altimetry data and radar images. Our results show that mantle diapirism can produce the domical topography of novae, which may be coronae in the earliest stage of formation. The model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae. The dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick. Diapirs that have reached the top of the mantle are expected to spread and flatten, producing plateaulike rather than domical topography. We model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift. The volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae. We model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae.
Enhanced visualization for interpretation of Magellan radar data: Supplement to the Magellan Special Issue
Authors: Kirk Randolph L. , Soderblom Laurence A. , Lee Ella M.
Published: May 2009

The differences of radar data from more familiar photographic images, coupled with the unique geologic characteristics of Venus's surface, can make interpretation of the standard Magellan data products difficult for both planetary scientists and nonspecialists. We describe a set of digital processing techniques for transforming individual remote‐sensing datasets in order to make the information they contain more apparent, and for combining multiple datasets of diverse resolution and information content into synthetic images that allow the viewer to explore correlations among the data. The creation of derived data products by techniques like those we describe is becoming increasingly widespread and important, both for scientific research and for use in educational and media presentations. We apply our processing techniques to Magellan synthetic aperture radar images, altimetry, and microwave emissivity measurements of four areas that exemplify some of the most important styles of geologic activity on Venus.
Lunar magnetic fields: Implications for utilization and resource extraction
Authors: Hood Lon L.
Published: May 2009

Although the Moon possesses no large‐scale intrinsic magnetic field, locally strong crustal fields exist which must be considered in assessing lunar utilization scenarios. On the basis of available data, orbital anomalies most probably imply the presence of subsurface concentrations of metallic iron in the single‐domain size range. Such concentrations may indicate larger iron volume fractions than are characteristic of sampled materials, allowing more efficient extraction of metal than is now considered likely using nominal regolith compositions. Most other implications relate to the deflection of interplanetary and magnetospheric ions by crustal fields. Numerical simulations are employed to show that solar wind ion deflection by strong lunar magnetic anomalies can produce local increases, as well as decreases, in the implantation rate of solar wind hydrogen. The largest concentrations of implanted solar wind volatiles may therefore be found in local areas near strong anomalies. If water ice exists in permanently shadowed regions of the lunar poles, locally strong magnetic fields can shield these volatiles from sputter erosion losses by interplanetary and magnetospheric ion fluxes. Model simulations indicate that the ability of magnetic anomalies to shield the surface from incident ions increases with the angle of incidence and hence for most particle sources, with selenographic latitude. Finally, the possibility that relatively strong anomalies can provide significant protection of men and materials against major solar flare particle events is examined and found to be unlikely.
A spatial domain Stokes flow model for the gravity and topography of the middle latitudes of Venus
Authors: Bills Bruce G. , Fischer Mark A.
Published: May 2009

A new modeling strategy is developed and applied to the long‐wavelength gravity, topography, and internal density structure of Venus. Geodynamic modeling of planetary scale phenomena is usually carried out in the wavenumber transform domain (spherical harmonics or Fourier transforms). However, because of limitations associated with the gravity and topography data available for Venus, a spatial domain approach is preferable. Smoothed representations of the gravity and topography of Venus are developed in terms of quadratic B splines on 10° centers in latitude and longitude, over the latitude interval from 45°S to 45°N. These models reproduce 83.6% and 98.3% of their respective data variances. A similarly constructed model is then presented for the dynamic support of gravity and topography on Venus. Internal mass anomalies are specified at two discrete depths (50 and 500 km), with lateral variations determined by two constraints: (1) that the slow viscous flow induced by the anomalies will distort the free surface into a form matching the observed topography and (2) that the sum of the gravitational effects of the internal anomalies plus that due to the surface distortion they cause will match the observed gravity. There are four major features evident in the gravity and topography data sets: Atla, Beta, Ovda, and Thetis regiones. All four have compensating mass patterns consistent with a mantle plume origin. The surficial flow pattern associated with this model does not resemble terrestrial plate tectonic patterns.
Volatile history of Mangala Valles, Mars
Authors: Zimbelman James R. , Craddock Robert A. , Greeley Ronald , Kuzmin Ruslan O.
Published: May 2009

Mangala Valles are unique among Martian outflow channels because they emanate from a point source determined by the interaction of tectonism, volcanism, and volatile migration. Present topography and morphology place constraints on the magnitude of the discharge into the channels, supporting a release of water which spanned from tens to thousands of days. The sequence of events associated with the release of volatiles in the southern (proximal) reaches of Mangala Valles indicates that structural features around the source area played a crucial role in determining the location of the apparent source of volatile release onto the Martian surface. Postulated locations for storage of the water released into the Mangala Valles system include aquifer systems in the highlands south of the release point, within the layered flows of the Tharsis plains, and a surface lake in the Daedalia Planum area. Hydraulic conductivity within the aquifer could have inhibited a sudden release of all the stored water, lengthening the total discharge duration toward the longer time frame indicated above. Topographic and structural constraints favor groundwater flow from the Tharsis region, which may have manifested itself at the Mangala source area as an artesian flow. Fluid release into the Mangala Valles system likely included sheet flows and episodic surges of water resulting from temporary ice dams within the channel system and may have included releases from more than a single location.
A Mars magnetic field: Constraints from molecular ion escape
Authors: Lammer H. , Bauer S. J.
Published: May 2009

A model has been developed to investigate the transport of the upward flowing ionospheric ions from closed field lines of the Martian day and nightside “polar” magnetosphere into the solar wind interaction region. Guiding center ion trajectories in the polar regions, for a range of injection conditions of the convection electric field, gravitation, and magnetic gradient force using a simplified three‐dimensional magnetospheric model were calculated. The dependence of the heavy ion escape on the Martian magnetopause positions and the solar wind ram pressure was also investigated. The calculated parallel escape fluxes of molecular ions are in good agreement with the Phobos 2 measurements and other theoretical calculations. Our investigation of molecular ion escape from high latitudes strongly suggests the presence of a weak intrinsic magnetic moment, (estimated to be about 1022 G cm3), since only then are molecular ions capable of reaching the solar wind interaction region. The detailed properties of a Martian magnetic field, however, should become known from the magnetometer measurements on the forthcoming Mars Observer mission.
Diatom PtCPF1 is a new cryptochrome/photolyase family member with DNA repair and transcription regulation activity
Authors: Coesel Sacha , Mangogna Manuela , Ishikawa Tomoko , Heijde Marc , Rogato Alessandra , Finazzi Giovanni , Todo Takeshi , Bowler Chris , Falciatore Angela
Published: May 2009

Members of the cryptochrome/photolyase family (CPF) are widely distributed throughout all kingdoms, and encode photosensitive proteins that typically show either photoreceptor or DNA repair activity. Animal and plant cryptochromes have lost DNA repair activity and now perform specialized photoperceptory functions, for example, plant cryptochromes regulate growth and circadian rhythms, whereas mammalian and insect cryptochromes act as transcriptional repressors that control the circadian clock. However, the functional differentiation between photolyases and cryptochromes is now being questioned. Here, we show that the PtCPF1 protein from the marine diatom Phaeodactylum tricornutum shows 6‐4 photoproduct repair activity and can act as a transcriptional repressor of the circadian clock in a heterologous mammalian cell system. Conversely, it seems to have a wide role in blue‐light‐regulated gene expression in diatoms. The protein might therefore represent a missing link in the evolution of CPFs, and act as a novel ultraviolet/blue light sensor in marine environments.
The non‐apoptotic role of p53 in neuronal biology: enlightening the dark side of the moon
Authors: Tedeschi Andrea , Di Giovanni Simone
Published: May 2009

The transcription factor p53 protects neurons from transformation and DNA damage through the induction of cell‐cycle arrest, DNA repair and apoptosis in a range of in vitro and in vivo conditions. Indeed, p53 has a crucial role in eliciting neuronal cell death during development and in adult organisms after exposure to a range of stressors and/or DNA damage. Nevertheless, accumulating evidence challenges this one‐sided view of the role of p53 in the nervous system. Here, we discuss how—unexpectedly—p53 can regulate the proliferation and differentiation of neural progenitor cells independently of its role in apoptosis, and p53 post‐translational modifications might promote neuronal maturation, as well as axon outgrowth and regeneration, following neuronal injury. We hope to encourage a more comprehensive view of the non‐apoptotic functions of p53 during neural development, and to warn against oversimplifications regarding its role in neurons. In addition, we discuss how further insight into the p53‐dependent modulation of these mechanisms is necessary to elucidate the decision‐making processes between neuronal cell death and differentiation during development, and between neuronal degeneration and axonal regeneration after injury.
A finite element method for transient analysis of concurrent large deformation and mass transport in gels
Authors: Zhang Jiaping , Zhao Xuanhe , Suo Zhigang , Jiang Hanqing
Published: May 2009

A gel is an aggregate of polymers and solvent molecules. The polymers crosslink into a three-dimensional network by strong chemical bonds and enable the gel to retain its shape after a large deformation. The solvent molecules, however, interact among themselves and with the network by weak physical bonds and enable the gel to be a conduit of mass transport. The time-dependent concurrent process of large deformation and mass transport is studied by developing a finite element method. We combine the kinematics of large deformation, the conservation of the solvent molecules, the conditions of local equilibrium, and the kinetics of migration to evolve simultaneously two fields: the displacement of the network and the chemical potential of the solvent. The finite element method is demonstrated by analyzing several phenomena, such as swelling, draining and buckling. This work builds a platform to study diverse phenomena in gels with spatial and temporal complexity.
Optical dielectric constant inhomogeneity along the growth axis in ZnO-based transparent electrodes deposited on glass substrates
Authors: Ikegami Keiichi , Yoshiyama Takashi , Maejima Keigou , Shibata Hajime , Tampo Hitoshi , Niki Shigeru
Published: May 2009

Simple optical measurements using a conventional spectrometer have revealed that in heavily doped ZnO films deposited on glass, the optical dielectric constant is inhomogeneous along the growth axis. Analyses based on Drude’s model have suggested that the origin of this inhomogeneity is the shorter carrier scattering time τc in the portion in contact with the glass substrate, while τc in the major portion of the films has been estimated to be as long as 1×10−14 s at infrared frequencies. This may imply that if better crystallinity is achieved in the initial stage of film deposition, higher conductivity will be attainable without sacrificing the high transparency of the electrode-on-glass system. In addition, the analyses have disclosed the correlation between the high transparency of the major portion of the films and the frequency dependence of τc that is caused by the dopants (Ga or Al) and makes τc even longer at visible-light frequencies.
Nanoscale capacitance spectroscopy characterization of AlGaN/GaN heterostructure by current-sensing atomic force microscopy
Authors: Zeng Huizhong , Sun Haoming , Luo Wenbo , Huang Wen , Wang Zhihong , Li Yanrong
Published: May 2009

A current-sensing atomic force microscopy was set up to measure the nanoscale capacitance spectroscopy of the two-dimensional electron gas (2DEG) of AlGaN/GaN heterostructures. The variation in the contact radius of the probe, which is important for the quantitative characterization of 2DEG, was experimentally determined by monitoring the resistive variation of the probe-sample contact and theoretically studied by numerical calculation. The current-sensing method is able to detect the change in Schottky barriers of the probe-sample contact, which affects the measurement of the threshold voltage of 2DEG. By fitting the nanoscale capacitance spectroscopy with numerical calculations, the concentration of 2DEG was quantitatively characterized. The result was comparable with that of macroscopic measurements.
Intrinsic ferroelectric properties of the nonstoichiometric perovskite oxide Ba1−xTi1−yO3−x−2y
Authors: Lee Soonil , Rossetti George A. , Liu Zi-Kui , Randall Clive A.
Published: May 2009

The properties of many electronic materials are governed by defect type, distribution, and concentration. In this paper, we demonstrate the influence of equilibrated nonstoichiometry on the ferroelectric properties of the perovskite BaTiO3. It is shown from measurements of the transition temperature, thermal hysteresis, latent heat, and transformation strain that the concentration of partial and/or full Schottky defects significantly alters the nature of the weak first-order transition between paraelectric and ferroelectric phases. The coefficients of a conventional 2-4-6 Ginzburg–Landau polynomial, determined from the measured transition parameters, were also found to vary as functions of the defect type and concentration. These results illustrate for the first time the strong sensitivity of the Landau coeffecients characterizing the properties of the monodomain ferroelectric state to nonstoichiometry involving partial Schottky disorder reactions in the perovskite-structured oxides. The strong dependence of the Landau coefficients on the defect concentration points to the fact that most classical descriptions of the ferroelectric state for these materials, as found in the literature, are based on materials that are nonstoichiometric. The results underscore the importance of attaining an equilibrium state through controlled high-temperature processing in furthering a fundamental understanding of ferroelectricity in perovskite materials.
A simple device for long-term radar cross section recordings
Authors: Eskelinen Pekka , Ruoskanen Jukka , Peltonen Jouni
Published: May 2009

A sample and hold circuit with settable delay can be used for recording of radar echo amplitude variations having time scales up to 100 s at the selected range bin in systems utilizing short rf pulses. The design is based on two integrated circuits and gives 1% uncertainty for 70 ns pulses. The key benefit is a real-time display of lengthy amplitude variations because the sample rate is defined by the radar pulse repetition frequency. Additionally we get a reduction in file size at least by the inverse of the radar's duty cycle. Examples of 10 and 100 s recordings with a Ka-band short pulse radar are described.
Droplet formation in matrix-assisted pulsed-laser evaporation direct writing of glycerol-water solution
Authors: Lin Yafu , Huang Yong , Chrisey Douglas B.
Published: May 2009

Matrix-assisted pulsed-laser evaporation direct-write (MAPLE DW) is emerging as a promising technique for printing microelectronics as well as fabricating biological constructs. For disparate MAPLE DW-based microfabrication applications, the droplet formation during MAPLE DW should be first carefully understood. Toward this goal, this study aims to study the effects of laser fluence and material properties of material to be transferred on the formed droplet in direct writing glycerol-water droplets using MAPLE DW. It was found that (1) at a given glycerol concentration ratio, the droplet diameter was linearly dependent on the laser fluence, and the slope of this relationship was dependent on the glycerol concentration, and (2) the droplet diameter had no systematic relationship with the glycerol concentration ratio. This study reveals important phenomena for droplet formation in MAPLE DW; further theoretical modeling is expected to further explain these observations.
Negative pressures in CaWO4 nanocrystals
Authors: Manjón F. J. , Errandonea D. , López-Solano J. , Rodríguez-Hernández P. , Muñoz A.
Published: May 2009

Tetragonal scheelite-type CaWO4 nanocrystals recently prepared by a hydrothermal method show an enhancement of its structural symmetry with the decrease in nanocrystal size. The analysis of the volume dependence of the structural parameters in CaWO4 nanocrystals with the help of ab initio total-energy calculations shows that the enhancement of the symmetry in the scheelite-type nanocrystals is a consequence of the negative pressure exerted on the nanocrystals; i.e., the nanocrystals are under tension. Besides, the behavior of the structural parameters in CaWO4 nanocrystals for sizes below 10 nm suggests an onset of a scheelite-to-zircon phase transformation in good agreement with the predictions from our ab initio calculations. CaWO4 nanocrystals exhibit a reconstructive-type mechanism for the scheelite-to-zircon phase transition that seems to follow the tetragonal path that links both structures. This result is in contrast with the mechanism recently proposed for this transition in bulk ZrSiO4 where the transition goes through an intermediate monoclinic phase.
Heat transfer enhancement by using pulsating flows
Authors: Benavides Efrén Moreno
Published: May 2009

The paper presents a theoretical model of convective heat exchangers working with internal pulsating flows. It aims for a better physical understanding of the processes leading to a heat transfer enhancement inside these devices. When the frequency of the pulsation is increased, some geometries exhibit a maximum response, measured by its temperature rise, similar to those obtained in some dynamical resonant systems. The work explains the nature of this characteristic behavior and produces a simplified theoretical model that isolates the main physical features of the fluid dynamics involved. Two characteristic frequencies, measured by its Strouhal numbers, are theoretically found. The first one is associated with the spatial-averaged thermal response of the fluid near the wall and the second with the response of the velocity field. It is found, for a general device, that both Strouhal numbers and the maximum enhancement are mainly defined by the geometry of the device. Finally, the heat transfer enhancement of a straight channel, a backward facing step channel, and a two heated blocks inside an adiabatic channel are used to validate the model. Enhancements calculated with the present model are compared with the results reported in the scientific literature showing a good agreement for the tested cases.
Photoinduced current transient spectroscopy technique applied to the study of point defects in polycrystalline CdS thin films
Authors: El Akkad Fikry , Ashour Habib
Published: May 2009

CdS thin films of variable thickness (between 160 and 1200 nm) were prepared using rf magnetron sputtering. X-ray diffraction measurements showed that the films have hexagonal structure and that the crystallites are preferentially oriented with the ⟨002⟩ axis perpendicular to the substrate surface. The results of electrical conductivity measurements as a function of film thickness and of temperature provide evidence that the conductivity is controlled by a thermally activated mobility in the presence of an intergrain barrier. The room temperature barrier height ϕ decreases with the increase in film thickness. Values of ϕ between 0 and 0.25 eV were determined. Photoinduced current transient spectroscopy performed on five samples having different thicknesses showed the presence of 11 traps with activation energies in the range 0.08–1.06 eV; deeper traps being observed on thinner films. By comparison with literature results, seven traps are attributed to native defects and foreign impurities (mainly Cu, Au, and Ag). Four other traps, not previously observed, are attributed to residual defects. The observation that deeper traps are detected in samples with larger barrier heights has been discussed and interpreted in terms of the energy band profile near the grain boundary.
A tetragonal phase of superhard BC2N
Authors: Zhou Xiang-Feng , Sun Jian , Qian Quang-Rui , Guo Xiaoju , Liu Zhongyuan , Tian Yongjun , Wang Hui-Tian
Published: May 2009

Based on first-principles calculation, we predict a tetragonal phase of BC2N (t-BC2N), which matches the experiment well because the estimated Vickers hardness and the simulated X-ray diffraction (XRD) and Raman patterns are in excellent agreement with the experimental results besides the low total energy and formation energy. In particular, we acquire the underlying physics behind the apparent lack of the transverse optical mode in the measured Raman spectrum and find the interesting electronic structure transition among the polymorph of superhard BC2N compounds.
Ion energy distribution near a plasma meniscus for multielement focused ion beams
Authors: Mathew Jose V. , Bhattacharjee Sudeep
Published: May 2009

The axial ion energy spread near a plasma meniscus for multielement focused ion beams is investigated experimentally in atomic and molecular gaseous plasmas of krypton, argon, and hydrogen by tailoring the magnetic field in the region. In the case of magnetic end plugging, the ion energy spread reduces by ∼50% near the meniscus as compared to the bulk plasma, thereby facilitating beam focusing. A quadrupole filter can be used to control the mean energy of the ions. Comparison with standard Maxwellian and Druyvesteyn distributions with the same mean energy indicates that the ion energy distribution in the meniscus is deficient in the population of low and high energy tail ions, resulting in a Gaussian-like profile with a spread of ∼4 and ∼5 eV for krypton and argon ions, respectively. By carefully tuning the wave power, plasma collisionality, and the magnetic field in the meniscus, the spread can be made lower than that of liquid metal ion sources, for extracting focused ion beams of other elements with adequate current density, for research and applications in nanosystems
How silylene defects at (100) Si surfaces can account for the anomalous features observed via x-ray photoelectron spectroscopy
Authors: Cerofolini G. F. , Giorgi G. , Sgamellotti A. , Belanzoni P.
Published: May 2009

A theoretical analysis of the hydrogen-terminated (100) surface of silicon leads to the identification of a new configuration, formed by a silylene center interacting with vicinal silicon dihydrides. This structure may be viewed as a metastable configuration of 2×1 (100) (SiH)2. Silylene can however be stabilized via interaction with water. The paper proposes that some of the anomalous features observed at the hydrogen-terminated or oxidized (100) Si can be attributed to silylene centers datively stabilized by oxo groups or to structures resulting from their decomposition.
Surface tension of room temperature ionic liquids measured by dynamic light scattering
Authors: Osada R. , Hoshino T. , Okada K. , Ohmasa Y. , Yao M.
Published: May 2009

Using dynamic light scattering techniques, we obtained the surface tension σ, surface excess entropy S(σ), surface excess enthalpy H(σ), and viscosity η for the following seven room temperature ionic liquids in a wide temperature range from 30 to around 140 °C: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide, N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-hexyl-3-methylimidazolium hexafluorophosphate, and 1-octyl-3-methyl imidazolium hexafluorophosphate. We have found that σ increases systematically with decreasing the anion size and the alkyl side chain length. On the other hand, S(σ) and η increase with decreasing the anion size but decrease with decreasing the alkyl chain length. H(σ) seems to decrease with increasing the anion size, but it has no clear dependence on the alkyl chain length. We discuss the bulk and surface properties, referring to the Coulomb interactions and van der Waals interactions.
On the algebra of quantum observables for a certain gauge model
Authors: Rudolph G. , Schmidt M.
Published: May 2009

We prove that the algebra of observables of a certain gauge model is generated by unbounded elements in the sense of Woronowicz. The generators are constructed from the classical generators of invariant polynomials by means of geometric quantization.
Two-plasmon-decay instability in direct-drive inertial confinement fusion experiments
Authors: Seka W. , Edgell D. H. , Myatt J. F. , Maximov A. V. , Short R. W. , Goncharov V. N. , Baldis H. A.
Published: May 2009

The two-plasmon-decay (TPD) instability in direct-drive irradiation OMEGA [J. M. Soures, R. L. McCrory, C. P. Verdon, et al., Phys. Plasmas 3, 2108 (1996)] experiments is seen in the half-integer harmonic emission. Experimental time-resolved ω/2 and 3ω/2 spectra indicate that the linear theory for the absolute TPD instability reasonably predicts TPD thresholds. The plasma wave spectra do not, however, agree at all with the predictions of the linear theory. This is most likely a consequence of the nonlinear evolution of this instability once it is above threshold. This is demonstrated with spectral data obtained from spherical implosion experiments as well as planar target experiments. In the latter, Thomson scattering shows the importance of the Landau cutoff. For the TPD instability, the Landau cutoff is found to be respected in all spherical and planar target experiments. In addition, the maximum plasma wave amplitudes appear to occur near the Landau cutoff.
The line shape problem in the near-infrared spectrum of self-colliding CO2 molecules: Experimental investigation and test of semiclassical models
Authors: Casa Giovanni , Wehr Richard , Castrillo Antonio , Fasci Eugenio , Gianfrani Livio
Published: May 2009

An intensity-stabilized diode laser absorption spectrometer was developed and used to perform a highly accurate study of the line shape of CO2 absorption lines, in the spectral region around 5000 cm−1, belonging to the ν1+2ν20+ν3 combination band, at a temperature of 296.00 K. Standard and complex semiclassical models, including Dicke narrowing and speed-dependent broadening effects, were applied, tested, and compared in the pressure range between 0.7 and 4 kPa, in order to single out the model best reproducing the absorption profile and, hence, the physical situation of self-colliding CO2 molecules. Line intensity factors and self-broadening coefficients were determined. The 1-σ overall accuracy of our determinations is at a level of 0.1%, which is, to our knowledge, the highest ever reached.
Effect of local E×B flow shear on the stability of magnetic islands in tokamak plasmas
Authors: Fitzpatrick R. , Waelbroeck F. L.
Published: May 2009

The influence of local E×B flow shear on a relatively wide, constant-ψ, magnetic island embedded in a large-aspect-ratio, low-β, circular cross-section tokamak plasma is examined, using a slab approximation to model the magnetic geometry. It is found that there are three separate solution branches characterized by low, intermediate, and high values of the shear. Flow shear is found to have a stabilizing effect on island solutions lying on the low and high shear branches, via a nonlinear modification of the ion polarization term in the Rutherford island width evolution equation, but to have a destabilizing effect on solutions lying on the intermediate shear branch. Moreover, the effect is independent of the sign of the shear. The modification of island stability by local E×B flow shear is found to peak when the magnitude of the shear is approximately vi/Ls, where vi is the ion thermal velocity, and Ls the magnetic shear length.
Highly multireferenced arynes studied with large active spaces using two-electron reduced density matrices
Authors: Greenman Loren , Mazziotti David A.
Published: May 2009

Using the active-space two-electron reduced density matrix (2-RDM) method, which scales polynomially with the size of the active space [G. Gidofalvi and D. A. Mazziotti, J. Chem. Phys. 129, 134108 (2008)], we were able to use active spaces as large as 24 electrons in 24 orbitals in computing the ground-state energies and properties of highly multireferenced arynes. Because the conventional complete-active-space self-consistent-field (CASSCF) method scales exponentially with the size of the active space, its application to arynes was mainly limited to active spaces of 12 electrons in 12 orbitals. For these smaller active spaces the active-space 2-RDM method accurately reproduces the results of CASSCF. However, we show that the larger active spaces are necessary for describing changes in energies and properties with aryne chain length such as the emergence of polyradical character. Furthermore, the addition of further electron correlation by multireference perturbation theory is demonstrated to be inadequate for removing the limitations of the smaller active spaces.
Brownian dynamics mean first passage time of two hard disks diffusing in a channel
Authors: Mon K. K.
Published: May 2009

We use Brownian dynamics simulations of two hard disks in a channel to study the mean first passage time to pass each other. The disks have a diameter σ and are confined in a channel with hard reflective walls. The mean first passage time diverges with an exponent η as the channel width (2Rp) approaches that of the nonpassing limit (2σ). There are two different theoretical predictions for the exponent η of the two disk hopping time divergences. Transition state theory and a Fick–Jacobs type of dimensional reduction approach predict exponents of 2 and 32, respectively. Previous Brownian dynamics simulations results have a range of effective exponents and are inconclusive. Here, we present extensive Brownian dynamics simulations results which are consistent with the predictions of transition state theory. The new data show that one must be close to the nonpassing limit to observe the asymptotic scaling exponent. The scaling dependence crosses over from the bulk limit to the nonpassing limit as the width of the channel narrows, corresponding to a range of effective exponents between 0 and 2. This crossover provides an explanation of the inconclusive results reported in previous Brownian dynamics simulations.
Cumulative reaction probabilities and transition state properties: A study of the H++H2 and H++D2 proton exchange reactions
Authors: Jambrina P. G. , Aoiz F. J. , Eyles C. J. , Herrero V. J. , Sáez Rábanos V.
Published: May 2009

Cumulative reaction probabilities (CRPs) have been calculated by accurate (converged, close coupling) quantum mechanical (QM), quasiclassical trajectory (QCT), and statistical QCT (SQCT) methods for the H++H2 and H++D2 reactions at collision energies up to 1.2eV and total angular momentum J=0–4. A marked resonance structure is found in the QM CRP, most especially for the H3+ system and J=0. When the CRPs are resolved in their ortho and para contributions, a clear steplike structure is found associated with the opening of internal states of reactants and products. The comparison of the QCT results with those of the other methods evinces the occurrence of two transition states, one at the entrance and one at the exit. At low J values, except for the quantal resonance structure and the lack of quantization in the product channel, the agreement between QM and QCT is very good. The SQCT model, that reflects the steplike structure associated with the opening of initial and final states accurately, clearly tends to overestimate the value of the CRP as the collision energy increases. This effect seems more marked for the H++D2 isotopic variant. For sufficiently high J values, the growth of the centrifugal barrier leads to an increase in the threshold of the CRP. At these high J values the discrepancy between SQCT and QCT becomes larger and is magnified with growing collision energy. The total CRPs calculated with the QCT and SQCT methods allowed the determination of the rate constant for the H++D2 reaction. It was found that the rate, in agreement with experiment, decreases with temperature as expected for an endothermic reaction. In the range of temperatures between 200 and 500K the differences between SQCT and QCT rate results are relatively minor. Although exact QM calculations are formidable for an exact determination of the k(T), it can be reliably expected that their value will lie between those given by the dynamical and statistical trajectory methods.
Resistive ferromagnetic wall modes in theory and experiment
Authors: Pustovitov V. D.
Published: May 2009

Effects of the ferromagnetic resistive wall on the plasma stability are analyzed. The analysis is based on the equations describing the perturbation dynamics outside the plasma, assuming a linear plasma response. A single-mode cylindrical model is used with two features that differ from the standard case: the wall magnetic permeability is incorporated and the thin-wall approximation is waived. The derivations are performed so that the results can be applied to both tokamaks and line-tied pinches. This is done to allow conclusions for tokamaks from comparison of the developed theory with the experimental data on the resistive and ferromagnetic wall modes in the Wisconsin rotating wall machine with and without a ferritic wall [W. F. Bergerson, D. A. Hannum, C. C. Hegna, R. D. Kendrick, J. S. Sarff, and C. B. Forest, Phys. Rev. Lett. 101, 235005 (2008)]. The model shows that the ferromagnetic wall effect is always destabilizing. However, it must be small under standard conditions in tokamaks. The effect can be much stronger in the pinch with lower magnetic field and larger wall permeability. The dispersion relation obtained here makes possible an explanation of the experimental results available so far, including those from the Wisconsin machine reported recently as strongly contradictory to expectations based on earlier models. Also, an easy practical solution for compensating the destabilizing ferromagnetic effect in tokamaks is proposed.
Study of the effect of dielectric porosity on the stress in advanced Cu/low-k interconnects using x-ray diffraction
Authors: Wilson C. J. , Zhao C. , Zhao L. , Metzger T. H. , Tőkei Zs. , Croes K. , Pantouvaki M. , Beyer G. P. , Horsfall A. B. , O’Neill A. G.
Published: May 2009

High intensity x-rays at an advanced light facility were used to probe the strained atomic spacing of Cu interconnects embedded in ultralow-k dielectrics of different porosities. The assumption that the strain-free atomic spacing of a thin film can be extrapolated from a relationship based on the material stiffness coefficients used in literature is tested and demonstrated experimentally using a series of blanket wafers. This is used to calculate the stress of 100 nm interconnects. High porosity materials show in-plane relaxation reducing the stress. The air gap architecture is shown to provide little constraint with the lowest Cu stress.
Fatigue rates of monocrystalline silicon thin films in harsh environments: Influence of stress amplitude, relative humidity, and temperature
Authors: Theillet P.-O. , Pierron O. N.
Published: May 2009

This study investigates the separate influence of stress, temperature, and relative humidity (RH) on the fatigue behavior of 10-μm-thick, monocrystalline silicon (Si) films at 40 kHz, under fully reversed loading. The fatigue rates are most sensitive to stress, with four orders of magnitude decrease from 3.2 to 1.5–2 GPa, confirming a size effect associated with the fatigue behavior of Si under bending load. The fatigue rates are also much more sensitive to RH than temperature or partial pressure of water, indicating that the effective environmental parameter is the adsorbed water layer. The implications on the relevant fatigue process(es) are discussed.
Twisted-nematic liquid-crystal displays with small grayscale inversion and wide viewing angle
Authors: Lin Chi-Huang
Published: May 2009

This work proposes a twisted-nematic liquid-crystal display (TN-LCD) with a small grayscale inversion and wide viewing angle. The electro-optical properties of a tilted TN cell are studied to develop the proposed TN-LCD. Calculations reveal that when the applied voltage exceeds a critical voltage, the effective optical property of the tilted TN cell is optically uniaxial with the effective optical axis parallel to the bisector of the twist angle. This bisector effect is used to improve the grayscale inversion of TN-LCDs. A viewing angle of over the entire 80° viewing cone of the proposed TN-LCD is also demonstrated theoretically and experimentally.
Origin of the abnormal behavior of contact resistance in Ohmic contacts to laser-irradiated n-type GaN
Authors: Jang Ho Won , Lee Jong-Lam
Published: May 2009

Abnormal behavior of contact resistance with annealing in Ohmic contacts to laser-irradiated n-GaN is investigated. Ti/Al contacts on as-grown n-GaN shows no change in contact resistivity with annealing at the temperature range of 100–400 °C. However, the annealing results in the significant increase in contact resistivity in the contacts on laser-irradiated n-GaN. Synchrotron radiation photoemission study reveals the reduction of the concentration of donor-like N vacancies near the surface by the annealing. These results suggest that preventing the annihilation of N vacancies in the laser-irradiated n-GaN is important in improving the performance of vertical-structure GaN-based light-emitting diodes fabricated by laser lift-off.
Geoffrey Malcolm Badger 1916 - 2002
Authors: Rae Ian D.
Published: May 2009

Geoffrey Malcolm Badger was Professor of Organic Chemistry at the University of Adelaide from 1955 to 1964 and, after serving briefly as a member of the CSIRO Executive, Vice-Chancellor from 1967 to 1977. Elected to Fellowship of the Australian Academy of Science in 1960, he served on the Council and was President of the Academy from 1974 to 1978. He was President of the Royal Australian Chemical Institute in 1965 and Chairman of the Australian Science and Technology Council (ASTEC) from 1977 to 1982. During the Second World War, while working as a Lieutenant Instructor for the British Navy, he developed an interest in maritime navigation, and especially in Captain James Cook. Later, he edited the book Captain Cook: Navigator and Scientist and, in retirement, he wrote two books, Explorers of the Pacific (1988) and The Explorers of Australia (2001). He was admitted to the order of Australia (AO) in 1975 and knighted in 1979.
Geoffrey Donald Thorburn 1930 - 1996
Authors: Jenkin G. , Challis J. R. G. , Robinson J. S. , Young I. R.
Published: May 2009

Professor Geoffrey Donald Thorburn AO passed away on 28 October 1996. He graduated in Medicine from the University of Sydney in 1956 and began his scientific career as a cardiovascular-renal physiologist. His career, in which he undertook pioneering research and teaching in many fields of physiology, spanned 35 years and three continents. He is recognised internationally as one of the founders of contemporary fetal physiology research. He was Head of the Department of Physiology at Monash University from 1981 until his retirement in 1995, but continued active research right up until his death. He was made an Officer of the Order of Australia in 1995 'for service to medicine and research in the field of fetal physiology'. Geoff was a wonderful mentor and friend to all who were touched by him. He was an outstanding scientist and academic.
Abortive activation of CD4 T cell responses during competitive priming in vivo
Authors: Weaver Jason M. , Chaves Francisco A. , Sant Andrea J.
Published: May 2009

Immunodominance refers to the highly selective peptide reactivity of T cells during an immune response. In this study, we tested the hypothesis that persistence of peptide:class II complexes is one key parameter that selects the final specificity of CD4 T cells. We found that low-stability peptide:class II complexes support the initial priming and expansion of CD4 T cells, but the expansion becomes strikingly aborted in the presence of competitive T cell responses to unrelated peptides. Our experiments revealed that for inhibition to occur, the competitive responses must be initiated by the same antigen presenting cell, and it is not because of competition for MHC binding. These studies not only provide an insight into the events that regulate competitive CD4 T cell priming in vivo, but also provide a previously undescribed conceptual framework to understand the parameters that select the final specificity of the T cell repertoire during pathogen or vaccine-induced immune responses.
The tail binds to the head–neck domain, inhibiting ATPase activity of myosin VIIA
Authors: Umeki Nobuhisa , Jung Hyun Suk , Watanabe Shinya , Sakai Tsuyoshi , Li Xiang-dong , Ikebe Reiko , Craig Roger , Ikebe Mitsuo
Published: May 2009

Myosin VIIA is an unconventional myosin, responsible for human Usher syndrome type 1B, which causes hearing and visual loss. Here, we studied the molecular mechanism of regulation of myosin VIIA, which is currently unknown. Although it was originally thought that myosin VIIA is a dimeric myosin, our electron microscopic (EM) observations revealed that full-length
Drosophila
myosin VIIA (DM7A) is a monomer. Interestingly, the tail domain markedly inhibits the actin-activated ATPase activity of tailless DM7A at low Ca
2+
but not high Ca
2+
. By examining various deletion constructs, we found that deletion of the distal IQ domain, the C-terminal region of the tail, and the N-terminal region of the tail abolishes the tail-induced inhibition of ATPase activity. Single-particle EM analysis of full-length DM7A at low Ca
2+
suggests that the tail folds back on to the head, where it contacts both the motor core domain and the neck domain, forming an inhibited conformation. We concluded that unconventional myosin that may be present a monomer in the cell can be regulated by intramolecular interaction of the tail with the head.