bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of ice lenses and frost heave
Authors: Rempel A. W.
Published: June 2007

I examine the morphology of ice growth in porous media. Intermolecular forces cause premelted fluid to migrate and supply segregated ice growth (e.g., lenses) and frost heave. I account for the net effect of these microscopic interactions in a homogenized model formulated in terms of fundamental physical properties and characteristics of the porous medium that can be measured; no ad hoc parameterizations are required. Force equilibrium constraints yield the rate of fluid migration toward the ice lens boundary and predict the conditions under which new lenses are initiated. By combining this analysis with considerations of the heat flow problem in a step‐freezing (Stefan) configuration, I elucidate the boundaries between different regimes of freezing behavior. At higher overburden pressures and relatively warm surface temperatures, ice lenses cannot form, and freezing of the available liquid occurs within the pore space, with no accompanying deformation. When conditions allow a lens to form, water is drawn toward it. If the fluid supply is sufficiently rapid, the lens grows faster than the latent heat of fusion can be carried away, and its boundary temperature warms until it reaches a stable steady state configuration. At lower fluid supply rates, the lens boundary temperature cools until a new lens can form at a warmer temperature beneath. With subsequent freezing this lens grows until yet another lens forms and the process repeats. An approximate treatment leads to estimates of the evolving lens thickness and spacing, as well as the accumulated total heave.
Electrical resistivity tomography monitoring of permafrost in solid rock walls
Authors: Krautblatter Michael , Hauck Christian
Published: June 2007

This article describes the first attempt to conduct electrical resistivity tomography (ERT) in solid permafrost‐affected rock faces. Electrode design, instrument settings, and processing routines capable of measuring under relevant conditions were developed. Four transects, with NW, NE, east (E) and south (S) aspects, were installed in solid rock faces between Matter Valley and Turtmann Valley, Switzerland, at 3070–3150 m above sea level. DC resistivity in the transects was measured repeatedly during the summer and compared by applying a time‐lapse inversion routine. Resistivity values were calibrated using observed rock surface conditions of thawed, damp rocks (1–8 kΩ m), deeply frozen rocks (18–80 kΩ m), and the transition from damp thawed to frozen rocks (8–18 kΩ m). Mean surface layer resistivities of transects respond to air temperatures below 0°C with a rapid increase by a factor of 1.4 to 2.9 from values of 12–15 kΩ m to values of 22–31 kΩ m. Rock layers at depths of 2–6 m display a general trend of resistivity decrease in summer, corresponding to a persistent thawing process. Their response to anomalously cool August temperatures occurs with a time lag of 2 to 4 weeks. Only transects E, NE and NW display persistent, high‐resistivity permafrost bodies (>50 kΩ m) mostly at depths of 6–10 m. The maximum thaw depth of a continuous thawing front above permafrost is 6 m. However, the ERT results emphasize the role of heat transfer by deep‐reaching cleft water systems. Thus permafrost occurs in lenses rather than layers. ERT provides rapid detection of ice and water distribution in permafrost‐affected bedrock.
Carbon and water cycling in lake‐rich landscapes: Landscape connections, lake hydrology, and biogeochemistry
Authors: Cardille Jeffrey A. , Carpenter Stephen R. , Coe Michael T. , Foley Jonathan A. , Hanson Paul C. , Turner Monica G. , Vano Julie A.
Published: June 2007

Lakes are low‐lying connectors of uplands and wetlands, surface water and groundwater, and though they are often studied as independent ecosystems, they function within complex landscapes. One such highly connected region is the Northern Highland Lake District (NHLD), where more than 7000 lakes and their watersheds cycle water and carbon through mixed forests, wetlands, and groundwater systems. Using a new spatially explicit simulation framework representing these coupled cycles, the Lake, Uplands, Wetlands Integrator (LUWI) model, we address basic regional questions in a 72‐lake simulation: (1) How do simulated water and carbon budgets compare with observations, and what are the implications for carbon stocks and fluxes? (2) How do the strength and spatial pattern of landscape connections vary among watersheds? (3) What is the role of interwatershed connections in lake carbon processing? Results closely coincide with observations at seasonal and annual scales and indicate that the connections among components and watersheds are critical to understanding the region. Carbon and water budgets vary widely, even among nearby lakes, and are not easily predictable using heuristics of lake or watershed size. Connections within and among watersheds exert a complex, varied influence on these processes: Whereas inorganic carbon budgets are strongly related to the number and nature of upstream connections, most organic lake carbon originates within the watershed surrounding each lake. This explicit incorporation of terrestrial and aquatic processes in surface and subsurface connection networks will aid our understanding of the relative roles of on‐land, in‐lake, and between‐lake processes in this lake‐rich region.
Endolithic cyanobacteria in soil gypsum: Occurrences in Atacama (Chile), Mojave (United States), and Al‐Jafr Basin (Jordan) Deserts
Authors: Dong Hailiang , Rech Jason A. , Jiang Hongchen , Sun Henry , Buck Brenda J.
Published: June 2007

Soil sulfates are present in arid and hyperarid environments on Earth and have been found to be abundant in soils on Mars. Examination of soil gypsum from the Atacama Desert, Chile, the Mojave Desert, United States, and Al‐Jafr Basin, Jordan, revealed endolithic cyanobacteria communities just below the surface of soil gypsum samples. Optical and scanning electron microscope observations of the colonized layers indicated that the unicellularChroococcidiopsisis the dominant cyanobacterium in all studied communities. 16S rRNA gene analysis revealed that in addition toChroococcidiopsis, a few other cyanobacteria are present. Heterotrophic bacteria are also abundant in the colonized zones of the fine‐grained gypsum from the Atacama and Mojave Desert, but insignificant in the fibrous gypsum from the Jordan Desert. Endolithic life forms similar to these described here may exist or have existed on Mars and should be targeted by the Mars Science Laboratory and future in situ missions.
Tectonic evolution of the Mogok metamorphic belt, Burma (Myanmar) constrained by U‐Th‐Pb dating of metamorphic and magmatic rocks
Authors: Searle M. P. , Noble S. R. , Cottle J. M. , Waters D. J. , Mitchell A. H. G. , Hlaing Tin , Horstwood M. S. A.
Published: June 2007

The Mogok metamorphic belt (MMB) extends for over 1500 km along the western margin of the Shan‐Thai block, from the Andaman Sea north to the eastern Himalayan syntaxis. Previous geochronology has suggested that a long‐lasting Jurassic–early Cretaceous subduction‐related event resulted in emplacement of granodiorites and orthogneisses (171–120 Ma) and a poorly constrained Tertiary metamorphic event. On the basis of new U‐Pb isotope dilution thermal ionization mass spectrometry and U‐Th‐Pb laser ablation–multicollector–inductively coupled plasma mass spectrometer geochronology presented here, we propose two Tertiary metamorphic events affected the MMB in Burma. The first was a Paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at ∼59 Ma. A second metamorphic event spanned late Eocene to Oligocene (at least from 37, possibly 47, to 29 Ma). This resulted in the growth of metamorphic monazite at sillimanite grade, growth of zircon rims at 47–43 Ma, sillimanite + muscovite replacing older andalusite, and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45.5 ± 0.6 Ma and 24.5 ± 0.7 Ma. These data imply high‐temperature sillimanite + muscovite metamorphism peaking at 680°C and 4.9 kbar between 45 and 33 Ma, to around 606–656°C and 4.4–4.8 kbar at 29.3 ± 0.5 Ma. The later metamorphic event is older than 24.5 ± 0.3 Ma, the age of leucogranites that crosscut all earlier fabrics. Our structural and geochronological data suggest that the MMB links north to the unexposed middle or lower crust rocks of the Lhasa terrane, south Tibet, and east to high‐grade metamorphic core complexes in northwest Thailand.
A note on forced versus internal variability of the stratosphere
Authors: Hardiman Steven C. , Butchart Neal , Haynes Peter H. , Hare Sylvia H. E.
Published: June 2007

An ensemble of integrations of a troposphere–stratosphere general circulation model is used to distinguish between externally forced (principally by sea‐surface temperatures) and internally generated interannual variability in the zonal mean stratospheric circulation. Forced variability is identified as that evident in the ensemble mean. Internal variability is identified with differences between ensemble members. The spatial and seasonal variation in the partitioning between forced and internal variability is examined. It is argued that the significantly larger forced interannual variability seen in the tropical lower stratosphere is not simply an upward extension of the tropical tropospheric variability, but is associated with coherent interannual variability in subtropical stratospheric wave driving.
Subglacial topography and geothermal heat flux: Potential interactions with drainage of the Greenland ice sheet
Authors: van der Veen C. J. , Leftwich T. , von Frese R. , Csatho B. M. , Li J.
Published: June 2007

Many of the outlet glaciers in Greenland overlie deep and narrow trenches cut into the bedrock. It is well known that pronounced topography intensifies the geothermal heat flux in deep valleys and attenuates this flux on mountains. Here we investigate the magnitude of this effect for two subglacial trenches in Greenland. Heat flux variations are estimated for idealized geometries using solutions for plane slopes derived by Lachenbruch (1968). It is found that for channels such as the one under Jakobshavn Isbræ, topographic effects may increase the local geothermal heat flux by as much as 100%.
The CombiUgi Project and Closing the Open Science Loop
Authors: Bradley Jean-Claude , Parikh Rikesh , Zaharevitz Dan , Guha Rajarshi
Published: June 2007

This presentation combines 2 blog posts explaining the CombiUgi project and the predicted anti-tumor activity of a virtual library of 68,000 compounds.
Folding pathway of the B1 domain of protein G explored by a multiscale modeling
Authors: Kolinski Andrzej , Kmiecik Sebastian
Published: June 2007

The understanding of the folding mechanisms of single domain proteins is an essential step in the understanding of protein folding in general. Recently, we developed a mesoscopic CABS protein model which was successfully applied in protein structure prediction, studies of protein thermodynamics and modeling of protein complexes. In the present research this model is employed in a detailed characterization of the folding process of a simple globular protein, the B1 domain of IgG-binding protein G (GB1). There is a vast body of experimental facts and theoretical findings for this protein. Performing unbiased, ab initio simulations, we demonstrated that the GB1 folding proceeds via the successive formation of an extended folding nucleus, followed by slow structure fine-tuning. Remarkably, a subset of native interactions drives the folding from the very beginning. The emerging comprehensive picture of GB1 folding perfectly matches and extends the previous experimental and theoretical studies.
Supply and Demand in Cerebral Energy Metabolism: The Role of Nutrient Transporters
Authors: Simpson Ian A , Carruthers Anthony , Vannucci Susan J
Published: June 2007

Glucose is the obligate energetic fuel for the mammalian brain, and most studies of cerebral energy metabolism assume that the majority of cerebral glucose utilization fuels neuronal activity via oxidative metabolism, both in the basal and activated state. Glucose transporter (GLUT) proteins deliver glucose from the circulation to the brain: GLUT1 in the microvascular endothelial cells of the blood—brain barrier (BBB) and glia; GLUT3 in neurons. Lactate, the glycolytic product of glucose metabolism, is transported into and out of neural cells by the monocarboxylate transporters (MCT): MCT1 in the BBB and astrocytes and MCT2 in neurons. The proposal of the astrocyte—neuron lactate shuttle hypothesis suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energetics, especially during activation. Since the identification of the GLUTs and MCTs in brain, much has been learned about their transport properties, that is capacity and affinity for substrate, which must be considered in any model of cerebral glucose uptake and utilization. Using concentrations and kinetic parameters of GLUT1 and −3 in BBB endothelial cells, astrocytes, and neurons, along with the corresponding kinetic properties of the MCTs, we have successfully modeled brain glucose and lactate levels as well as lactate transients in response to neuronal stimulation. Simulations based on these parameters suggest that glucose readily diffuses through the basal lamina and interstitium to neurons, which are primarily responsible for glucose uptake, metabolism, and the generation of the lactate transients observed on neuronal activation.
Dielectric and thermal evidence of phase transitions in the system of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 crystals
Authors: Wang Haixia , Pan Xiaomin , Lin Di , Luo Haosu , Yin Zhiwen , Elouadi Brahim
Published: June 2007

Dielectric permittivity, differential scanning calorimetry nonisothermal measurements, and hysteresis loops were performed for oriented (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 unpoled crystals. Two distinguished branches were obtained where spontaneous ferroelectric to relaxor or ferroelectric phase transition was observed within the peculiar relaxor to normal ferroelectric region. The percolating polar region induced by the addition of PbTiO3 tends to gradually develop comparable ferroelectric order changing slightly with temperature from the nonpolar matrix, associated with a weak first or first order phase transition. Small frequency dispersion due to relaxor polarization at or below transition temperature TFE-RorFE indicated the changes of ferroelectric multidomain and polar nanoregion in this disordered system.
Knudsen diffusion through thin fibrous films
Authors: Li Xiangning , Strieder William C.
Published: June 2007

Variational upper and multiple scattering lower bounds on the Knudsen transport rates are applied to several slab geometries of finite thickness made up of parallel, randomly placed, overlapping long cylindrical fibers, one with fibers aligned perpendicular and a second with fibers parallel to the film edges. Estimates and rigorous error bounds are generated for various film thickness. Insight into collision paths useful for thin film vapor deposition, reactive matrix formation of composites, and gas membrane transport are obtained.
Hard-surface effects in polymer self-consistent field calculations
Authors: Meng Dong , Wang Qiang
Published: June 2007

We have investigated several effects due to the confinement of polymer melts by impenetrable (hard) surfaces in the self-consistent field calculations. To adequately represent such confinement, the total (normalized) polymer segmental density (volume fraction) is usually constrained to an imposed profile that continuously decreases from 1 in the interior of confined melts to 0 at the surfaces over a short distance. The choice of this profile strongly influences the numerical performance of the self-consistent field calculations. In addition, for diblock copolymers A-B the hard-surface confinement has both energetic and entropic effects: On one hand, the decrease of polymer density from 1 reduces A-B repulsion and favors morphologies with more A-B interfaces near the surfaces. On the other hand, the enrichment of chain ends and depletion of middle segments near the surfaces favor parallel morphologies where chains orient mainly perpendicular to the surfaces. These two effects are comparable in magnitude, and for asymmetric diblock copolymers result in an entropic preference of a neutral surface for the shorter block as proposed previously [Q. Wang et al., Macromolecules 34, 3458 (2001)]. The hard-surface effects are weak in practice and thus manifested only when the surfaces are nearly neutral.
Blast-wave diagnosis of self-focusing of an intense laser pulse in a cluster medium
Authors: Symes Daniel R. , Moore Alastair S. , Comley Andrew J. , Lazarus James , Hohenberger Matthias , Tisch John W. G. , Smith Roland A.
Published: June 2007

Self-focusing of intense laser pulses in a gas of atomic clusters is diagnosed in both long (>700fs) and short (<100fs) pulse regimes. This investigation uses blast-wave analysis techniques, which are sensitive to deposited energy, as a tool to identify locations of self-focusing. The detection of highly energetic x rays from the interaction of the short pulse with the clusters suggests the activation of electron acceleration in the self-focused high-intensity channels produced. The self-focusing is attributed to the optical properties of the clusters since it occurs at moderate laser powers and the cluster parameters are critical to the extent of the channel that forms.
One- and two-center physical space partitioning of the energy in the density functional theory
Authors: Salvador P. , Mayer I.
Published: June 2007

A conceptually new approach is introduced for the decomposition of the molecular energy calculated at the density functional theory level of theory into sum of one- and two-atomic energy components, and is realized in the “fuzzy atoms” framework. (Fuzzy atoms mean that the three-dimensional physical space is divided into atomic regions having no sharp boundaries but exhibiting a continuous transition from one to another.) The new scheme uses the new concept of “bond order density” to calculate the diatomic exchange energy components and gives them unexpectedly close to the values calculated by the exact (Hartree-Fock) exchange for the same Kohn-Sham orbitals.
Adsorbed states and scanning tunneling microscopy induced migration of acetylene on Cu(110)
Authors: Kumagai T. , Hatta S. , Okuyama H. , Aruga T.
Published: June 2007

The authors have studied adsorption of acetylene on Cu(110) by means of low-temperature scanning tunneling microscopy. Adsorbed molecules preferentially aggregate at 40K to yield dimer, trimer, and larger islands on the surface. Isolated species (monomer) adsorbs on the fourfold hollow site with ∼sp3 rehybridization as characterized by inelastic electron tunneling spectroscopy. Tunneling electron induces an acetylene molecule to migrate along the trough of Cu(110). The migration proceeds in two steps: the molecule first hops to the adjacent long-bridge site and then to the next fourfold site. The voltage and current dependencies of the hopping probability show that the migration is induced by inelastic electron tunneling that causes vibrational excitation of mainly C–H stretch mode.
Excess electron relaxation dynamics at water/air interfaces
Authors: Madarász Ádám , Rossky Peter J. , Turi László
Published: June 2007

We have performed mixed quantum-classical molecular dynamics simulations of the relaxation of a ground state excess electron at interfaces of different phases of water with air. The investigated systems included ambient water/air, supercooled water/air, Ih ice/air, and amorphous solid water/air interfaces. The present work explores the possible connections of the examined interfacial systems to finite size cluster anions and the three-dimensional infinite, fully hydrated electron. Localization site analyses indicate that in the absence of nuclear relaxation the electron localizes in a shallow potential trap on the interface in all examined systems in a diffuse, surface-bound (SB) state. With relaxation, the weakly bound electron undergoes an ultrafast localization and stabilization on the surface with the concomitant collapse of its radius. In the case of the ambient liquid interface the electron slowly (on the 10ps time scale) diffuses into the bulk to form an interior-bound state. In each other case, the excess electron persists on the interface in SB states. The relaxation dynamics occur through distinct SB structures which are easily distinguishable by their energetics, geometries, and interactions with the surrounding water bath. The systems exhibiting the most stable SB excess electron states (supercooled water/air and Ih ice/air interfaces) are identified by their characteristic hydrogen-bonding motifs which are found to contain double acceptor-type water molecules in the close vicinity of the electron. These surface states correlate reasonably with those extrapolated to infinite size from simulated water cluster anions.
Transport properties of liquid nickel near the melting point: An ab initio molecular dynamics study
Authors: Jakse N. , Wax J. F. , Pasturel A.
Published: June 2007

The authors have investigated the dynamic properties of liquid nickel near the melting point by means of first principles molecular dynamics simulations in the framework of the density functional theory. Single-atom as well as collective dynamic properties are determined and transport coefficients are deduced. The calculation of the shear viscosity from the transverse current-current correlation function is examined in detail, and finite size effects are analyzed through a reference model to show the feasibility of this approach from first principles. The role played by the anharmonic character of the interactions is discussed.
Dynamic effects of nonequilibrium solvation: Potential and free energy surfaces for Z/E isomerization in solvent-solute coordinates
Authors: Dhaliwal Manjinder , Basilevsky Michael V. , Weinberg Noham
Published: June 2007

A novel definition of a solvent coordinate associated with a given reaction is formulated in terms of molecular-dynamic trajectories of the solvent and is applied to discuss the topography of potential energy and free energy surfaces of model liquid phase Z/E isomerization reactions in solvent-solute coordinates. It is shown that the arrangement of the reactant and product valleys on these surfaces can vary from consecutive to parallel, depending on the strength of the solvent-solute interactions.
Experiments with planar inductive ion source meant for creation of H+ beams
Authors: Vainionpaa J. H. , Kalvas T. , Hahto S. K. , Reijonen J.
Published: June 2007

In this article the effects of different engineering parameters of rf-driven ion sources with an external spiral antenna and a quartz rf window are studied. This article consists of three main topics: the effect of source geometry on the operation gas pressure, the effect of source materials and magnetic confinement on extracted current density and ion species, and the effect of different antenna geometries on the extracted current density. The effect of source geometry was studied using three cylindrical plasma chambers with different inner diameters. The chamber materials were studied using two materials, aluminum (Al) and alumina (Al2O3). The removable 14 magnet multicusp confinement arrangement enabled us to compare the effects of the two wall materials with and without the magnetic confinement. The highest measured proton fractions were measured using Al2O3 plasma chamber and no multicusp confinement. For the compared ion sources the source with multicusp confinement and Al2O3 plasma chamber yields the highest current densities. Multicusp confinement increased the maximum extracted current by up to a factor of 2. Plasma production with different antenna geometries were also studied. The highest current density was achieved using 4.5 loop solenoid antenna with 6.0cm diameter. A slightly lower current density with lower pressure was achieved using a tightly wound 3 loop spiral antenna with 3.3cm inner diameter and 6cm outer diameter.
The infinitesimal operator for the semigroup of the Frobenius-Perron operator from image sequence data: Vector fields and transport barriers from movies
Authors: Santitissadeekorn N. , Bollt E. M.
Published: June 2007

In this paper, we present an approach to approximate the Frobenius-Perron transfer operator from a sequence of time-ordered images, that is, a movie dataset. Unlike time-series data, successive images do not provide a direct access to a trajectory of a point in a phase space; more precisely, a pixel in an image plane. Therefore, we reconstruct the velocity field from image sequences based on the infinitesimal generator of the Frobenius-Perron operator. Moreover, we relate this problem to the well-known optical flow problem from the computer vision community and we validate the continuity equation derived from the infinitesimal operator as a constraint equation for the optical flow problem. Once the vector field and then a discrete transfer operator are found, then, in addition, we present a graph modularity method as a tool to discover basin structure in the phase space. Together with a tool to reconstruct a velocity field, this graph-based partition method provides us with a way to study transport behavior and other ergodic properties of measurable dynamical systems captured only through image sequences.
Correlated atomic motions in liquid deuterium fluoride studied by coherent quasielastic neutron scattering
Authors: Fernandez-Alonso F. , McLain S. E. , Taylor J. W. , Bermejo F. J. , Bustinduy I. , Ruiz-Martín M. D. , Turner J. F. C.
Published: June 2007

The collective dynamics of liquid deuterium fluoride are studied by means of high-resolution quasielastic and inelastic neutron scattering over a range of four decades in energy transfer. The spectra show a low-energy coherent quasielastic component which arises from correlated stochastic motions as well as a broad inelastic feature originating from overdamped density oscillations. While these results are at variance with previous works which report on the presence of propagating collective modes, they are fully consistent with neutron diffraction, nuclear magnetic resonance, and infrared/Raman experiments on this prototypical hydrogen-bonded fluid.
Molecular dynamics simulation of the recrystallization of amorphous Si layers: Comprehensive study of the dependence of the recrystallization velocity on the interatomic potential
Authors: Krzeminski C. , Brulin Q. , Cuny V. , Lecat E. , Lampin E. , Cleri F.
Published: June 2007

The molecular dynamics method is applied to simulate the recrystallization of an amorphous/crystalline silicon interface. The atomic structure of the amorphous material is constructed with the method of Wooten, Winer, and Weaire. The amorphous on crystalline stack is annealed afterward on a wide range of temperature and time using five different interatomic potentials: Stillinger-Weber, Tersoff, EDIP, SW115, and Lenosky. The simulations are exploited to systematically extract the recrystallization velocity. A strong dependency of the results on the interatomic potential is evidenced and explained by the capability of some potentials (Tersoff and SW115) to correctly handle the amorphous structure, while other potentials (Stillinger-Weber, EDIP, and Lenosky) lead to the melting of the amorphous. Consequently, the interatomic potentials are classified according to their ability to simulate the solid or the liquid phase epitaxy.
Dual layered surface crystallization of 30BaO–15TiO2–55GeO2 glass by stepwise heat treatment
Authors: Masai Hirokazu , Fujiwara Takumi , Mori Hiroshi , Benino Yasuhiko , Komatsu Takayuki
Published: June 2007

In the crystallization process of 30BaO–15TiO2–55GeO2 (BTG55), we demonstrated by an x-ray diffraction analysis that a stepwise heat treatment enables a selective crystallization of fresnoite and benitoite, two crystalline phases obtained from the BTG55 mother glass and that these two phases form a dual surface layer. The result indicates that the stepwise heat treatment generates a compositional heterogeneity inside the glass matrix, which facilitates the nucleation of the fresnoite phase. The mechanism for the generation of the dual surface layer is discussed. Due to a marked difference in optical nonlinearity of the two phases, the selective crystallization may open the possibility of an optimized heat-treatment condition to achieve a large optical nonlinearity in crystallized glass. Also, two types of fresnoite, tetragonal and orthorhombic, were observed in the crystallized phase after a stepwise heat treatment of 690∕720°C.
New calibration technique for multiple-component stress wave force balances
Authors: Abdel-jawad Madhat M. , Mee David J. , Morgan Richard G.
Published: June 2007

The measurement of forces in hypervelocity expansion tubes is not possible using conventional techniques. The stress wave force balance technique can be applied in expansion tubes to measure forces despite the short test times involved. This article presents a new calibration technique for multiple-component stress wave force balances where an impulse response created using a load distribution is required and no orthogonal surfaces on the model exist. This new technique relies on the tensorial superposition of single-component impulse responses analogous to the vectorial superposition of the calibration loads. The example presented here is that of a scale model of the Mars Pathfinder, but the technique is applicable to any geometry and may be useful for cases where orthogonal loads cannot be applied.
Raman investigation of A2CoBO6 (A=Sr and Ca, B=Te and W) double perovskites
Authors: Ayala A. P. , Guedes I. , Silva E. N. , Augsburger M. S. , del C. Viola M. , Pedregosa J. C.
Published: June 2007

Raman scattering is used to investigate the room temperature phonons of tetragonal (I4∕m) Sr2CoWO6 and monoclinic (P21∕n) Ca2CoWO6, Sr2CoTeO6, and Ca2CoTeO6 double perovskites. Both tetragonal and monoclinic structures result from small distortions of the Fm3¯m cubic lattice. Therefore, phonons were assigned according to correlations with those observed in the prototype A2B′B″O6 Fm3¯m cubic structure and the internal vibrational modes of the oxygen octahedra. The observed shifts of some vibrations in the A2CoBO6 compounds upon changing either A or B elements are tentatively explained.
Long-life vibration-free 4.5K sorption cooler for space applications
Authors: Burger J. F. , ter Brake H. J. M. , Holland H. J. , Meijer R. J. , Veenstra T. T. , Venhorst G. C. F. , Lozano-Castelló D. , Coesel M. , Sirbi A.
Published: June 2007

A breadboard 4.5K helium sorption cooler for use in vibration-sensitive space missions was developed and successfully tested. This type of cooler has no moving parts and is, therefore, essentially vibration-free. The absence of moving parts also simplifies scaling down of the cooler to small sizes, and it contributes to achieving a very long lifetime. In addition, the cooler operates with limited dc’s so that hardly any electromagnetic interference is generated. This cooler is a favorite option for future missions such as ESA’s Darwin mission, a space interferometer in which the sensitive optics and detectors can hardly accept any vibration. The system design consists of a hydrogen stage cooling from 80to14.5K and a helium stage establishing 5mW at 4.5K. Both stages use microporous activated carbon as the adsorption material. The two cooler stages need about 3.5W of total input power and are heat sunk at two passive radiators at temperatures of about 50 and 80K—radiators which are constructed at the cold side of the spacecraft. We developed, built, and tested a demonstrator of the helium cooler. This demonstrator has four sorption compressor cells in two compressor stages. Test experiments on this cooler showed that it performs within all specifications imposed by ESA. The cooler delivered 4.5mW at 4.5K with a long-term temperature stability of 1mK and an input power of 1.96W. So far, the cooler has operated continuously for a period of 2.5months and has not shown any sign of performance degradation.
Soft chemical synthesis and characterization of Ni0.65Zn0.35Fe2O4 nanoparticles
Authors: Parvatheeswara Rao B. , Rao G. S. N. , Mahesh Kumar A. , Rao K. H. , Murthy Y. L. N. , Hong S. M. , Kim Chong-Oh , Kim CheolGi
Published: June 2007

Mixed ferrite nanoparticles of Ni0.65Zn0.35Fe2O4 using metal nitrates were prepared by three different low temperature methods of sol gel in polyvinyl alcohol matrix, coprecipitation with NaOH as coprecipitating base, and sol gel in citric acid matrix, separately to estimate the influence of synthesis on the structural and magnetic parameters of this high saturation magnetization NiZn ferrite composition. In the final stage of the synthesis, the coprecipitated sample was washed and filtered several times before drying and finally heat treated at 350°C for 1h to get the ultrafine powder. Whereas the other two samples, which were transformed into dried gel after mixing and drying, were heat treated at 350°C for 1h, respectively, to find out the dried gel burnt out in a self-propagating combustion manner to form a fluffy powder. X-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer measurements are performed on all three samples. Though the composition is the same for all three samples, the results of these characterizations are found to be different for different synthesis methods and also much different from their corresponding bulk counterpart. However, the magnetic performance of these nanoparticles is primarily determined by their respective particle size in all three samples. The results are analyzed in terms of the microstructure and magnetic performance.
Magnetization process of a single magnetic ring detected by nonlocal spin valve measurement
Authors: Kimura T. , Otani Y.
Published: June 2007

We investigate the magnetization process of a 200-nm-wide Permalloy ring using a nonlocal spin-valve measurement technique in a lateral geometry. The nonlocal spin signal is found to reveal the chirality of the flux closure state in the magnetic ring. The angular dependence of the magnetization process of the Permalloy ring are also studied.
Quantum dynamics study of H+NH3→H2+NH2 reaction
Authors: Zhang Xu Qiang , Cui Qian , Zhang John Z. H. , Han Ke Li
Published: June 2007

We report in this paper a quantum dynamics study for the reaction H+NH3→NH2+H2 on the potential energy surface of Corchado and Espinosa-Garcia [J. Chem. Phys. 106, 4013 (1997)]. The quantum dynamics calculation employs the semirigid vibrating rotor target model [J. Z. H. Zhang, J. Chem. Phys. 111, 3929 (1999)] and time-dependent wave packet method to propagate the wave function. Initial state-specific reaction probabilities are obtained, and an energy correction scheme is employed to account for zero point energy changes for the neglected degrees of freedom in the dynamics treatment. Tunneling effect is observed in the energy dependency of reaction probability, similar to those found in H+CH4 reaction. The influence of rovibrational excitation on reaction probability and stereodynamical effect are investigated. Reaction rate constants from the initial ground state are calculated and are compared to those from the transition state theory and experimental measurement.
Diffusion coefficients for crystal nucleation and growth in deeply undercooled glass-forming liquids
Authors: Fokin Vladimir M. , Schmelzer Jürn W. P. , Nascimento Marcio L. F. , Zanotto Edgar D.
Published: June 2007

We calculate, employing the classical theory of nucleation and growth, the effective diffusion coefficients controlling crystal nucleation of nanosize clusters and the subsequent growth of micron-size crystals at very deep undercoolings, below and above Tg, using experimental nucleation and growth data obtained for stoichiometric Li2O∙2SiO2 and Na2O∙2CaO∙3SiO2 glasses. The results show significant differences in the magnitude and temperature dependence of these kinetic coefficients. We explain this difference showing that the composition and/or structure of the nucleating critical clusters deviate from those of the stable crystalline phase. These results for diffusion coefficients corroborate our previous conclusion for the same glasses, based on different experiments, and support the view that, even for the so-called case of stoichiometric (polymorphic) crystallization, the nucleating phase may have a different composition and/or structure as compared to the parent glass and the evolving macroscopic crystalline phase. This finding gives a key to explain the discrepancies between calculated (by classical nucleation theory) and experimentally observed nucleation rates in these systems, in particular, and in deeply undercooled glass-forming liquids, in general.
Experimental studies of fluctuation field and temperature coefficient of coercivity in advanced data backup tapes
Authors: Nishio Hiroaki , Yamamoto Hiroshi
Published: June 2007

The reverse field dependence of the normalized magnetization decay and the fluctuation field (Hf)was examined for advanced data backup tapes prepared from ultrafine acicular metal particulate (MP) composite and hexagonal platelet Ba-ferrite (BF) particles. Also, the effect of the ratio of Hf to coercivity (Hc) on the temperature coefficient of coercivity [α(Hc)] was studied at 300 K. In comparison to MP and BF tapes with approximately the same mean volume of particles (Vphy), the normalized magnetization decay of BF tapes was much larger than that of MP tapes. The smaller the Vphy was, the larger the normalized magnetization decay became. The values of Hf were not constant, but were largely dependent on the reverse field. In the low reverse fields, Vphy was smaller than the activation volume for both tapes with very small Vphy. The value of α(Hc), which is very important for the thermal stability of high-density recording media, increased as the ratio Hf∕Hc increased for both tapes.
Bond length variation in Ga1−xInxAs crystals from the Tersoff potential
Authors: Titantah J. T. , Lamoen D. , Schowalter M. , Rosenauer A.
Published: June 2007

In this work we show that a reparametrized Tersoff potential accurately reproduces the bond length variations observed in ternary Ga1−xInxAs mixed crystals. The reparametrization is based on accurate first-principles electronic structure calculations. Previous parametrizations of the Tersoff potential for GaAs and InAs structures, although they accurately reproduce the properties of the zinc-blende GaAs and InAs crystals, are shown to be unable to reproduce the bond length variations in these mixed crystals. In addition to correcting the bond length inconsistencies, the new set of parameters is also shown to yield the elastic constants of GaAs and InAs that agree fairly well with measurements and to reproduce accurately their respective melting temperature.
Raman scattering from epitaxial TaNx(0.94≤x≤1.37) layers grown on MgO(001)
Authors: Stoehr M. , Shin C.-S. , Petrov I. , Greene J. E.
Published: June 2007

Single-crystal group-V δ-TaNx transition-metal (TM) nitride layers, with x ranging from 0.94 to 1.37, are grown on MgO(001) and analyzed by Raman spectroscopy. In perfect NaCl-structure crystals, the first-order Raman modes are not allowed by the Oh crystal symmetry. Since first-order acoustic and optical bands are observed for all δ-TaNx samples, this indicates the presence of point defects, even for stoichiometric TaN. The large mass difference between Ta and N leads to a correspondingly large separation between the first-order acoustic and optical bands centered at 150 and 550 cm−1, respectively. A Raman peak at 440 cm−1, overlapping the first-order optical band, is identified as the difference between the first-order optical and acoustic modes. Contrary to the case for most TM nitrides, there is no peak visible in the second-order high-frequency range (700–1500 cm−1). In addition, the first-order transverse (120 cm−1) and longitudinal acoustic peaks (175 cm−1) do not exhibit the frequency shift with N/Ta ratio x (i.e., with the valence electron density) that has been reported for group-IV TM nitrides. This is consistent with the resistivity of TaNx remaining essentially constant, as well as with the very weak dependence of the lattice parameter as a function of the N/Ta ratio, and suggests that the wide single-phase region in this system is due to the presence of isoelectronic antisite substitutions rather than, as has been proposed previously, cation and anion vacancies.
Cathode sputtering and the resulting formation of carbon nanometer-size dust
Authors: Dominique C. , Arnas C.
Published: June 2007

Nanometer-size particles are produced in parallel electrode glow discharges. Supersaturated carbon vapor produced by the sputtering of a graphite cathode is at the origin of their formation. In this context, the energy distribution and the flux of the sputtering particles are estimated. The energy distribution of the emitted carbon atoms is also evaluated as a function of the distance to the cathode, taking into account the collisions with the discharge gas atoms. These collisions induce the carbon vapor cooling and then, a nucleation-condensation phase giving rise to dust precursor clusters. A linear scaling law of growth is established experimentally as a function of time when the dust size is higher than 20 nm. Within the considered time range, this scaling law shows that the growth comes from neutral deposition on the dust surface.
Making waves: Kinetic processes controlling surface evolution during low energy ion sputtering
Authors: Chan Wai Lun , Chason Eric
Published: June 2007

When collimated beams of low energy ions are used to bombard materials, the surface often develops a periodic pattern or “ripple” structure. Different types of patterns are observed to develop under different conditions, with characteristic features that depend on the substrate material, the ion beam parameters, and the processing conditions. Because the patterns develop spontaneously, without applying any external mask or template, their formation is the expression of a dynamic balance among fundamental surface kinetic processes, e.g., erosion of material from the surface, ion-induced defect creation, and defect-mediated evolution of the surface morphology. In recent years, a comprehensive picture of the different kinetic mechanisms that control the different types of patterns that form has begun to emerge. In this article, we provide a review of different mechanisms that have been proposed and how they fit together in terms of the kinetic regimes in which they dominate. These are grouped into regions of behavior dominated by the directionality of the ion beam, the crystallinity of the surface, the barriers to surface roughening, and nonlinear effects. In sections devoted to each type of behavior, we relate experimental observations of patterning in these regimes to predictions of continuum models and to computer simulations. A comparison between theory and experiment is used to highlight strengths and weaknesses in our understanding. We also discuss the patterning behavior that falls outside the scope of the current understanding and opportunities for advancement.
GaAs metal-oxide-semiconductor device with HfO2∕TaN gate stack and thermal nitridation surface passivation
Authors: Gao Fei , Lee S. J. , Chi D. Z. , Balakumar S. , Kwong D.-L.
Published: June 2007

Oxides induced Fermi level pinning at the interface between the GaAs and high-k gate dielectric is a major obstacle for developing high performance GaAs metal-oxide-semiconductor (MOS) devices. In this letter, thermal nitridation treatment on GaAs surface prior to the high-k deposition is proposed to solve the issue of interface pinning. It is found that an optimized nitride layer formed during the thermal nitridation surface treatment can effectively suppress the oxides formation and minimize the Fermi level pinning at the interface between the GaAs and HfO2. By using thermal nitridation treatment and in situ metal-organic chemical vapor deposition HfO2 as high-k gate dielectric, GaAs MOS capacitor with improved capacitance-voltage characteristics and reduced gate leakage current is achieved.
ZnO-based nonvolatile memory thin-film transistors with polymer dielectric/ferroelectric double gate insulators
Authors: Noh Seok Hwan , Choi Wonjun , Oh Min Suk , Hwang D. K. , Lee Kimoon , Im Seongil , Jang Sungjin , Kim Eugene
Published: June 2007

The authors report on the fabrication of a top-gate ZnO thin-film transistor (TFT) with a polymer dielectric/ferroelectric double-layer gate insulator that was formed on patterned ZnO through a sequential spin-casting process of 450-nm-thick poly-4-vinylphenol (PVP) and 200-nm-thick poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)]. Compared to the single P(VDF/TrFE) layer, double layer shows remarkably reduced leakage current with the aid of the PVP buffer. TFT with the PVP/P(VDF/TrFE) double layer exhibits a field effect mobility of 0.36cm2∕V and a large memory hysteresis in the transfer characteristics due to the ferroelectric P(VDF/TrFE). The retention of the device lasted over 2h.
Liquid crystal pretilt angle control using adjustable wetting properties of alignment layers
Authors: Ahn Han Jin , Kim Jong Bok , Kim Kyung Chan , Hwang Byoung Har , Kim Jong Tae , Baik Hong Koo , Park Jin Seol , Kang Daeseung
Published: June 2007

The authors demonstrate the production of amorphous fluorinated carbon (a-C:F) thin film with adjustable wetting properties, inducing variable liquid crystal (LC) pretilt angles. To control the surface wetting properties, they apply a dual radio frequency magnetron system with a controlled power ratio of targets. In this manner we obtain various compositional surfaces with fluorine and carbon components and adjust the surface energy with regard to the various compositions. Whereas the fluorine-rich a-C:F layer shows a preference for homeotropic (vertical) LC alignment, the carbon-rich a-C:F layer shows a planar LC alignment. To achieve uniform LC alignment with a proper pretilt angle, an accelerated Ar+ ion beam irradiates the films after the deposition process. The ion beam selectively destroys the surface bonding of the a-C:F films, yielding an intermediate pretilt angle.
Strain-free Ge∕GeSiSn quantum cascade lasers based on L-valley intersubband transitions
Authors: Sun G. , Cheng H. H. , Menéndez J. , Khurgin J. B. , Soref R. A.
Published: June 2007

The authors propose a Ge∕Ge0.76Si0.19Sn0.05 quantum cascade laser using intersubband transitions at L valleys of the conduction band which has a “clean” offset of 150meV situated below other energy valleys (Γ,X). The entire structure is strain-free because the lattice-matched Ge and Ge0.76Si0.19Sn0.05 layers are to be grown on a relaxed Ge buffer layer on a Si substrate. Longer lifetimes due to the weaker scattering of nonpolar optical phonons reduce the threshold current and potentially lead to room temperature operation.
Charge storage characteristics of atomic layer deposited RuOx nanocrystals
Authors: Maikap S. , Wang T. Y. , Tzeng P. J. , Lin C. H. , Lee L. S. , Yang J. R. , Tsai M. J.
Published: June 2007

The charge storage characteristics of atomic layer deposited RuOx nanocrystals embedded in high-k HfO2∕Al2O3 films in a metal/Al2O3∕RuOx∕HfO2∕SiO2∕n-Si structure have been investigated. The size and density of RuOx nanocrystals have been measured using transmission electron microscopy. The RuOx nanocrystals show a density of ∼1×1012∕cm2 and a diameter of 5–8nm. A large hysteresis memory window of ∼13.3V at a gate voltage of 9V has been observed for RuOx nanocrystal memory capacitors. A hysteresis memory window of 0.7V has also been observed under a small sweeping gate voltage of 1V. A promising memory window of RuOx nanocrystals has been observed as compared with those of pure HfO2 and Al2O3 charge trapping layers, due to charge storage in the RuOx metal nanocrystals. The RuOx nanocrystal memory capacitor has similar leakage current with the pure HfO2 and Al2O3 charge trapping layers. The RuOx memory capacitor has a large breakdown voltage of ∼13.8V.