bluuebunny/crossref_metadata_2025 huggingface dataset:
Variability of the Deep Western Boundary Current east of the Grand Banks
Authors: Schott Friedrich A. , Fischer Jürgen , Dengler Marcus , Zantopp Rainer
Published: October 2006

The Deep Western Boundary Current east of the Grand Banks has been observed during 1999–2005 by moored current‐meter stations and shipboard current profiling sections. These recent observations can be compared with those of a WOCE moored array deployed during 1993–95 at the same location. Overall, the observations of Deep Water currents east of the Grand Banks reported here do not support suggestions of a basin‐wide "slowdown" of the Atlantic Meridional Overturning Circulation.
Subsurface temperature maxima in the Labrador Sea and the subpolar North Atlantic
Authors: Karstensen Johannes , Avsic Tom , Fischer Jürgen , Send Uwe
Published: October 2006

Deep and shallow subsurface temperature maxima (Tmax) in the Labrador Sea are found to be the result of anomalous freshwater input during past decades in particular during the early 1990s. The deep Tmax is associated with a specific water mass imported into the Labrador Sea. The shallow Tmax is of local origin and created by anomalous heat input in 1999, not eroded by surface buoyancy forcing in recent years. Both are associated with stability maxima: the deep Tmax is a barrier for maximum convection depth, the shallow Tmax separates the water in a layer ventilated by overturning and a layer modified through lateral fluxes only. The shallow Tmax is exported into the subpolar gyre. A complementary shallow Tmax in the Greenland Sea suggest a concerted response of deep convection regions to anomalous freshwater input: diminished vertical mixing and dominance of lateral heat/salt fluxes underneath Tmax, shallow convection above it.
Infrared spectra of nitric acid trihydrate (β‐NAT): A comparison of available optical constants and implication for the detection of polar stratospheric clouds (PSCs)
Authors: Ortega I. K. , Maté B. , Moreno M. A. , Herrero V. J. , Escribano R.
Published: October 2006

The β phase of nitric acid trihydrate (β‐NAT) is assumed to be one of the essential components of polar stratospheric clouds (PSC) and may play an important role in ozone chemistry. Infrared spectra of vapour deposited films of β‐NAT, have been measured in transmission and in reflection‐absorption (RAIR). The spectra have been simulated with the different sets of optical constants available in the literature. The films are found to be optically isotropic. The refractive indices of Toon et al. (1994) give the best global fit to the data, but have low resolution and tend to smooth sharp features. The higher resolution indices of Höpfner et al. (2006) lead to worse overall spectral simulations, but can account for the comparatively narrow feature at 820 cm−1 recently identified in field measurements of β‐NAT particles. The best compromise is obtained with the “coa” indices of Höpfner et al.
Evidence of frictional melting from disk‐shaped black material, discovered within the Taiwan Chelungpu fault system
Authors: Hirono Tetsuro , Ikehara Minoru , Otsuki Kenshiro , Mishima Toshiaki , Sakaguchi Masumi , Soh Wonn , Omori Masahiro , Lin Weiren , Yeh En‐Chao , Tanikawa Wataru , Wang Chien‐Ying
Published: October 2006

The Taiwan Chelungpu‐fault Drilling Project penetrated three fault zones as the Chelungpu fault system, which slipped during the 1999 Chi‐Chi earthquake, discovering disk‐shaped black material (BM disk) within the middle and lower fault zones in Hole B. The microscopic features of the BM disks indicated that they were pseudotachylytes, and they showed high magnetic susceptibility, possibly the result of intense shearing or high temperature conditions. Inorganic carbon content of the BM disks was low, possibly because of thermal decomposition of carbonate minerals. The high temperatures might be related to frictional heating during the earthquake, implying that the BM disks were produced under intense shearing with frictional heating that reached melting temperature. Because the disks, which provide the only evidence of melting, pre‐date the 1999 earthquake, we concluded that frictional melting did not occur during the earthquake.
Interpreting multipoint observations of substructure at the quasi‐perpendicular bow shock: Simulations
Authors: Burgess D.
Published: October 2006

Two‐dimensional hybrid simulations of high Mach number quasi‐perpendicular collisionless shocks, as relevant to the Earth's bow shock, show strong fluctuations of density and magnetic field within the average shock profile. In particular, there are rapid variations moving across the shock surface, in terms of local changes in its position and the shock gradient normal direction on scales of order of the ion inertial length. Using virtual probes, we simulate the magnetic field time series that would be observed by a multipoint observation at a high Mach number, moderate plasma beta, quasi‐perpendicular shock as a function of shock crossing speed. For crossing speeds greater than twice the upstream Alfvén speed, the time series profiles are similar to the spatially averaged profile. As the crossing speed decreases, more structure becomes visible in the time series so that oscillations and major differences of amplitude and gradient are evident even between closely spaced probes. These signatures are directly linked to the spatiotemporal variability of the shock front. Even at slow crossing speeds sharp gradients can sometimes be seen in the time series owing to propagation of fluctuations across the shock surface. On the basis of the simulations, oscillatory substructure becomes increasingly significant when the crossing speed becomes less than the Alfvén speed. We suggest some signatures that may then be used observationally to infer the presence of the associated surface fluctuations.
A numerical study of atmospheric signals in the Earth‐ionosphere electromagnetic cavity with the Transmission Line Matrix method
Authors: Morente Juan A. , Portí Jorge A. , Besser Bruno P. , Salinas Alfonso , Lichtenegger Herbert I. M. , Navarro Enrique A. , Molina‐Cuberos Gregorio J.
Published: October 2006

The effect of the Earth‐ionosphere electromagnetic cavity on the spectrum of an atmospheric signal generated by a broadband electrical current source is analyzed numerically by means of the Transmission Line Matrix (TLM) method. Two new TLM meshes are developed, one with transmission lines connected in parallel and the other with connections in series. The equations describing propagation through these parallel or series meshes are equivalent to the Maxwell equations for TEr or TMr modes in the spherical Earth‐ionosphere cavity, respectively. The numerical algorithm obtains Schumann resonance frequencies very close to the experimental ones, confirming that this methodology is a valid numerical tool for predicting these resonances on other planets and moons. Finally, the TEr and TMr modes with a higher order than the Schumann resonances are also analyzed, finding that the effect of atmospheric conductivity is to shift the peak frequencies toward higher values than the eigenfrequencies corresponding to the lossless system. For daytime conditions, these peak frequencies appear around 2, 4, 6, 8… kHz, connected to an effective aboveground ionosphere height of approximately 75 km. In the night region, the shift is slightly smaller and the effective ionosphere height is around 85 km in agreement with smaller values in the conductivity profile.
Reconnection of nonidentical flux tubes
Authors: Linton M. G.
Published: October 2006

An investigation of the three‐dimensional reconnection of flux tubes at low twist and of flux tubes with nonequal axial fluxes is presented. This is a generalization of the earlier investigation of highly twisted, identical flux tubes in the work of Linton et al. (2001). This generalization is important for determining how robust the three types of reconnection found in that study are and therefore how applicable they are to common space and laboratory magnetic field interactions. It is found that the merge reconnection of low twist tubes of nonequal flux occurs much as it does for the high twist, identical flux tubes. The slingshot of low twist, identical tubes is also much the same as for high twist, identical tubes. The slingshot of tubes of unequal flux is, however, different than that of identical tubes: the smaller tube reconnects with only the outer shell of the larger tube, and so it is left hanging off the unreconnected, core flux of the larger flux tube. Finally, it is found, surprisingly, that the tunnel occurs as easily for tubes of nonequal flux as it does for identical tubes.
A physically motivated index of subgrid‐scale pattern
Authors: Jupp Tim E. , Twiss Sean D.
Published: October 2006

Earth Observation data on land surface properties such as albedo are typically collected at a pixel resolution of 1 km or less. Global climate models, on the other hand, are constrained by current limits on computing power to run at a grid box resolution of 10 km or more. This mismatch in spatial scales means that large amounts of pixel‐scale information are condensed into a small number of grid‐box‐scale summary statistics before use in global climate models. Subgrid‐scale patterns in land surface properties may have a significant effect but the summary statistics currently in use, such as the grid box mean and grid box variance, are insensitive to the spatial arrangement of pixels. To address this gap in the information available to global climate models we define here a new grid‐box‐scale summary statistic, the Laplacian pattern index, that is sensitive to the spatial arrangement of pixels. This dimensionless index is based on the mean‐squared value of the Laplacian filter ∇2 within the grid box and is motivated by the physics of diffusive heat transport. We investigate the value of the index in some simple cases and show that it is a measure of the local correlation structure within a grid box. This allows us to generate random grid boxes in which the index takes (on average) a prescribed value. The Laplacian pattern index is designed to be a useful measure of subgrid‐scale pattern in numerical climate models, but can be used as a measure of pattern in any two‐dimensional array of real‐valued data.
How representative is a time series derived from a firn core? A study at a low‐accumulation site on the Antarctic plateau
Authors: Karlöf Lars , Winebrenner Dale P. , Percival Donald B.
Published: October 2006

The acquisition and interpretation of increasingly high‐resolution climate data from polar ice and firn cores motivates the question: What is the finest depth or timescale on which measurements on cores arrayed over a given area correlate? We analyze dated depth series of electrical and oxygen isotope measurements from a spatial array of firn cores with 3.5–7 km spacing in Dronning Maud Land, Antarctica, each with a temporal span of approximately 200 years. We use wavelet analysis to decompose the series into components associated with changes of averages on different scales, and thus deduce which scales are dominated by environmental noise, and which may contain a common signal. We find that common signals in electrical records have timescales of approximately 1–3 years. We identify only one electrical signal which rises significantly above the background in our 200‐year records, evidently corresponding to the Tambora eruption. Several smaller signals correlate in a few of pairs of cores, one of which may correspond to a known volcanic event, but the others appear to be spurious. We present a simulation‐based method for testing the significance of apparent electrical signal correlations, and highlight the importance of accurate relative dating between cores. In the case of oxygen‐isotope records, we find, surprisingly, no significant correlation on any scale in the records, for any of the pairs of cores. There is, however, a weak trend toward positive correlation at longer timescales (up to 16 years). Statistical theory for the relevant confidence intervals and the observed statistics of the records permit estimation of the length of a data series necessary to reliably detect a hypothetical correlation equal to that observed. For the highest correlation observed on 16‐year scales, core records of about 380 years (approximately 30 m at the Dronning Maud Land site) would be necessary to establish significance.
Transport ofCryptosporidium parvumin porous media: Long‐term elution experiments and continuous time random walk filtration modeling
Authors: Cortis Andrea , Harter Thomas , Hou Lingling , Atwill E. Robert , Packman Aaron I. , Green Peter G.
Published: October 2006

Complex transport behavior other than advection‐dispersion, simple retardation, and first‐order removal has been observed in many biocolloid transport experiments in porous media. Such nonideal transport behavior is particularly evident in the late time elution of biocolloids at low concentrations. Here we present a series of saturated column experiments that were designed to measure the breakthrough and long‐term elution ofCryptosporidium parvumin medium sand for a few thousand pore volumes after the initial source of oocysts was removed. For a wide range of ionic strengths,I, we consistently observe slower‐than‐Fickian, power law tailing. The slope of the tail is flatter for higherI. At very high ionic strength the slope decays to a rate slower thant−1. To explain this behavior, we propose a new filtration model based on the continuous time random walk (CTRW) theory. Our theory upscales heterogeneities at both the pore‐scale geometry of the flow field and the grain surface physicochemical properties that affect biocolloid attachment and detachment. Pore‐scale heterogeneities in fluid flow are shown to control the breakthrough of a conservative tracer but are shown to have negligible effect on oocyst transport. In our experiments,C. parvumtransport is dominated by the effects of physicochemical heterogeneities. The CTRW model provides a parsimonious theory of nonreactive and reactive transport. The CTRW filtration process is controlled by three parameters, Λ,β, andc, which are related to the overall breakthrough retardation (R= 1 + Λ), the slope of the power law tail (β), and the transition to a slower thant−1decay (c).
Riparian vegetation distribution induced by river flow variability: A stochastic approach
Authors: Camporeale C. , Ridolfi L.
Published: October 2006

Riparian vegetation is part of one of the most diverse and fragile ecotones. The key role played by river discharge on the dynamics of riparian vegetation has been widely studied and documented. However, although randomness is a fundamental characteristic of river hydrology, very few quantitative vegetation studies take into account the random nature of river discharge. Here we propose a stochastic model of riparian vegetation ecosystem dynamics forced by random variations in river discharge. The model is solved, and the analytical expressions of the probability density function of the overall vegetation biomass and its first moments are obtained. These theoretical results are used to investigate the effect of river hydrology on the distribution of vegetation along the riparian transect transverse to the river. In particular, the influence of the type of riparian species and the statistical characteristics of discharge time series are discussed and compared with field observations.
Influence of simple fracture intersections with differing aperture on density‐driven immiscible flow: Wetting versus nonwetting flows
Authors: Ji Sung‐Hoon , Nicholl Michael J. , Glass Robert J. , Lee Kang‐Kun
Published: October 2006

We conducted laboratory experiments to evaluate the effects of simple fracture intersections with differing aperture on density‐driven immiscible wetting (water into air) and nonwetting (Trichloroethylene into water) flows, and analyzed them quantitatively. The experimental systems consisting of vertical and horizontal fractures were fabricated with glass for easy visualization. The aperture variation between intersecting fractures and the viscous force of the injected fluid were considered to be critical system parameters. Experimental results show the critical difference between the wetting and nonwetting flows by the intersection and viscous force, and subsequent mathematical analyses explain well our observations: The intersection acts as a capillary barrier (CB) for the wetting and capillary bridge for the nonwetting flows, and the viscous force of flowing fluids reduces the strength of CBs. The results of both laboratory experiments and mathematical analyses suggest that the fracture intersection with differing aperture can be a more significant factor controlling the network‐scale phase structure for the nonwetting than the wetting flows.
Optical design study for divertor observation at the stellarator W7-X
Authors: König R. , Hildebrandt D. , Hübner T. , Klinkhamer F. , Moddemeijer K. , Vliegenthart W.
Published: October 2006

The stellarator W7-X will be capable of running in a quasicontinuous operating mode with 10MW of electron cyclotron heating (ECRH) heating for 30min, the duration only being limited by the capacity of the available cooling reservoir. The integrated ten discrete water cooled divertor modules need to be closely monitored by ten high resolution infrared imaging (Δx∼10mm) real time control systems to prevent local overheating which could lead to a destruction of the tiles. Filter based (Hα, C II, C III, etc.) visible imaging systems will initially be used to study divertor symmetry. A first design study of a 2m long combined IR∕visible mirror based endoscope compatible with the boundary conditions of ∼107° viewing angle, thermal vessel movements, front end heat loads of ∼50–100kW∕m2, and maximum ECRH stray radiation levels of 50kW∕m2 has been performed. The system will be bakeable and contain an integrated shutter∕calibrator. A 45° mirror in front of the secondary mirror of the back-end Cassegrain optics gives extra flexibility to install further optical components, e.g., a vessel illumination system, a relative calibration light source, and spectroscopic systems such as a coherence imaging spectrometer or fiber optically coupled grating spectrometers, a scanning laser for an active IR diagnostic, or possibly even an erosion monitor.
Vertical localization of phase contrast imaging diagnostic in Alcator C-Mod
Authors: Lin L. , Edlund E. M. , Porkolab M. , Lin Y. , Wukitch S. J.
Published: October 2006

Phase contrast imaging (PCI) diagnostic has been used to study mode conversion physics of ion cyclotron range of frequency waves [E. Nelson-Melby et al., Phys. Rev. Lett. 90, 155004 (2003)], plasma turbulence [A. Mazurenko et al., Phys. Rev. Lett. 89, 225004 (2002); N. Basse et al., Phys. Plasmas 12, 052512 (2005)], and Alfvén Cascades [M. Porkolab et al., IEEE Trans. Plasma Sci. 34, 229 (2006)] in Alcator C-Mod. The C-Mod PCI system measures line integrated electron density fluctuations along 32 vertical chords with a sampling frequency of 10MHz and wavenumber resolution up to 30cm−1. Although PCI normally lacks localization along the chords, the vertical variation of the magnetic field pitch angle allows for localized measurements for large k⊥ fluctuations. A system consisting of a partially masked phase plate on a rotatable stage has been installed and quasicoherent modes with wave number ∼5cm−1 associated with the enhanced DαH mode at the top and bottom of the plasma have been differentiated. In future experiments, for k∼30cm−1, a spatial resolution of r∕a∼0.3 can be achieved under ideal conditions.
Soft x-ray power diagnostic improvements at the Omega Laser Facility
Authors: Sorce C. , Schein J. , Weber F. , Widmann K. , Campbell K. , Dewald E. , Turner R. , Landen O. , Jacoby K. , Torres P. , Pellinen D.
Published: October 2006

Soft x-ray power diagnostics are essential for evaluating high temperature laser plasma experiments. The Dante soft x-ray spectrometer, a core diagnostic for radiation flux and temperature measurements of Hohlraums, installed on the Omega Laser Facility at the Laboratory for Laser Energetics has recently undergone a series of upgrades. Work performed at Brookhaven National Laboratory for the development of the National Ignition Facility (NIF) Dante spectrometer enables the Omega Dante to offer a total of 18 absolutely calibrated channels in the energy range from 50eVto20keV. This feature provides Dante with the capability to measure higher, NIF relevant, radiation temperatures with increased accuracy including a differentiation of higher energy radiation such as the Au M and L bands. Diagnostic monitoring using experimental data from directly driven Au spherical shots is discussed.
Imaging system for hypervelocity dust injection diagnostic on NSTX
Authors: Dorf L. A. , Roquemore A. L. , Wurden G. A. , Ticos C. M. , Wang Zhehui
Published: October 2006

The novel hypervelocity dust injection diagnostic will facilitate our understanding of basic aspects of dust-plasma interaction and magnetic field topology in fusion plasma devices, by observing “comet tails” associated with the injected micron-size dust particles. A single projection of the tail onto an image plane will not provide sufficient information; therefore, we plan to use two views, with intensified DiCam-Pro cameras on two NSTX ports. Each camera can furnish up to five overlaying sequential images with gate times greater than 3ns and 1280×1024pixel resolution. A coherent fiber bundle with 1500×1200 fibers will relay the image from an imaging lens installed directly on the port to the camera optics. The lens receives light from the outer portion of the NSTX cross section and focuses a 1cm tail onto at least 60 fibers for adequate resolution. The estimated number of photons received by the camera indicates signal-to-noise ratios of 102–104, with the use of a 10nm bandwidth filter. The imaging system with one camera was successfully tested on NSTX in 2005. Photographing lithium pellets yielded bright and distinctive pictures of the tails nearly aligned with B lines. We also observed that the bright “filaments”—plasma cords with high density and temperature—are present in both top and bottom portions of the machine.
Competition between ultrafast relaxation and photoionization in excited prefluorescent states of tryptophan and indole
Authors: Sherin P. S. , Snytnikova O. A. , Tsentalovich Yu. P. , Sagdeev R. Z.
Published: October 2006

The quantum yield of photoionization of TrpH and IndH from the nonrelaxed prefluorescent state S* increases with the temperature decrease. This effect is attributed to the competition between temperature independent ionization and ultrafast thermal relaxation S*→S1. The rate constant of the relaxation does not depend on the solvent and on the presence of the amino acid side chain: the temperature dependences of photoionization quantum yield, obtained for TrpH and IndH in different solvents, practically coincide. The activation energy for the relaxation rate constant Er≈4.5kJ∕mol probably corresponds to intramolecular process or to the formation of the vibronically excited transient complex between photoexcited molecule and solvent molecules.
Electron hydration dynamics in water clusters: A direct ab initio molecular dynamics approach
Authors: Tachikawa Hiroto
Published: October 2006

Electron attachment dynamics of excess electron in water cluster (H2O)n (n=2 and 3) have been investigated by means of full-dimensional direct ab initio molecular dynamics (MD) method at the MP2∕6-311++G(d,p) level. It was found that the hydrogen bond breaking due to the excess electron is an important process in the first stage of electron capture in water trimer. Time scale of electron localization and hydrogen bond breaking were determined by the direct ab initio MD simulation. The initial process of hydration in water cluster is clearly visualized in the present study. In n=3, an excess electron is first trapped around the cyclic water trimer with a triangular form, where the excess electron is equivalently distributed on the three water molecules at time zero. After 50fs, the excess electron is concentrated into two water molecules, while the potential energy of the system decreases by −1.5kcal∕mol from the vertical point. After 100fs, the excess electron is localized in one of the water molecules and the potential energy decreases by −5.3kcal∕mol, but the triangular form still remained. After that, one of the hydrogen bonds in the triangular form is gradually broken by the excess electron, while the structure becomes linear at 100–300fs after electron capture. The time scale of hydrogen bond breaking due to the excess electron is calculated to be about 300fs. Finally, a dipole bound state is formed by the linear form of three water molecules. In the case of n=2, the dipole bound anion is formed directly. The mechanism of electron hydration dynamics was discussed on the basis of theoretical results.
Relationships between three-body and two-body interactions in fluids and solids
Authors: Wang Liping , Sadus Richard J.
Published: October 2006

Molecular dynamics data are reported for two-body and three-body interactions in noble gases at densities covering the gas, liquid, and solid phases. The data indicate that simple relationships exist between three- and two-body interactions in both fluid and solid phases. The relationship for liquids has a simple density dependence with only one external parameter. In contrast, the solid phase relationship depends both on density and on the square of density and requires the evaluation of two parameters. The relationships are tested for both system-size and temperature dependences. The values of the relationship parameters are only sensitive to system size when a small number of atoms are involved. For 500 or more atoms, they remain nearly constant. The relationships are valid for both subcritical and slightly supercritical temperatures. A practical benefit of the relationships is that they enable the use of two-body intermolecular potentials for the prediction of the properties of real systems without the computational expense of three-body calculations.
Theoretical investigation on H1 and C13 NMR chemical shifts of small alkanes and chloroalkanes
Authors: d’Antuono Philippe , Botek Edith , Champagne Benoît , Spassova Milena , Denkova Pavletta
Published: October 2006

Using density functional theory (DFT) with the B3LYP, PBE, and PBE0 exchange-correlation functionals as well as the Møller-Plesset second-order perturbation theory (MP2) combined with a series of rather extended basis sets, H1 and C13 chemical shifts of small alkanes and chloroalkanes (with different numbers of chlorine atoms on specific positions) have been simulated and compared to experimental data. For the H1 chemical shifts, theory tends to reproduce experiment within the limits of the experimental errors. In the case of C13 chemical shift, the differences between theory and experiment increase monotonically with the number of chlorine atoms and exhibit a deviation from additivity. This behavior is related to the saturation of the experimental C13 chemical shifts with the number of chlorine atoms, whereas the evolution is mostly linear at both DFT and MP2 levels of approximation. This difference has been traced back to the relativistic spin-orbit coupling effects, which are exalted as a result of the enhancement of the s character of the C atom when increasing the number of linked Cl atoms. Thus, it was demonstrated that not only electron correlation but also relativistic effects have to be considered for estimating the C13 chemical shifts when several Cl atoms are directly attached to the C atom. Linear (theory/experiment) regressions have then been performed for the different types of C atoms, i.e., bearing one, two, and three Cl atoms, with excellent correlation coefficients. The linear correlation relationships so obtained can then serve to predict and facilitate the interpretation of the nuclear magnetic resonance spectra of more complex compounds. Furthermore, by investigating the basis set effects, the correlation between the chemical shifts calculated using the 6-311+G(2d,p) basis set and the more extended 6-311+G(2df,p) and aug-cc-pvtz basis sets is excellent, demonstrating that the choice of the 6-311+G(2d,p) basis set for calculating the H1 and C13 chemical shifts is relevant.
A “partitioned leaping” approach for multiscale modeling of chemical reaction dynamics
Authors: Harris Leonard A. , Clancy Paulette
Published: October 2006

We present a novel multiscale simulation approach for modeling stochasticity in chemical reaction networks. The approach seamlessly integrates exact-stochastic and “leaping” methodologies into a single partitioned leaping algorithmic framework. The technique correctly accounts for stochastic noise at significantly reduced computational cost, requires the definition of only three model-independent parameters, and is particularly well suited for simulating systems containing widely disparate species populations. We present the theoretical foundations of partitioned leaping, discuss various options for its practical implementation, and demonstrate the utility of the method via illustrative examples.
Dynamical motions of lipids and a finite size effect in simulations of bilayers
Authors: Klauda Jeffery B. , Brooks Bernard R. , Pastor Richard W.
Published: October 2006

Molecular dynamics (MD) simulations of dipalmitoylphosphatidylcholine bilayers composed of 72 and 288 lipids are used to examine system size dependence on dynamical properties associated with the particle mesh Ewald (PME) treatment of electrostatic interactions. The lateral diffusion constant Dℓ is 2.92×10−7 and 0.95×10−7cm2∕s for 72 and 288 lipids, respectively. This dramatic finite size effect originates from the correlation length of lipid diffusion, which extends to next-nearest neighbors in the 288 lipid system. Consequently, diffusional events in smaller systems can propagate across the boundaries of the periodic box. The internal dynamics of lipids calculated from the PME simulations are independent of the system size. Specifically, reorientational correlation functions for the slowly relaxing phosphorus-glycerol hydrogen, phosphorus-nitrogen vectors, and more rapidly relaxing CH vectors in the aliphatic chains are equivalent for the 72 and 288 lipid simulations. A third MD simulation of a bilayer with 72 lipids using spherical force-shift electrostatic cutoffs resulted in interdigitated chains, thereby rendering this cutoff method inappropriate.
Superconducting quantum interference device instruments and applications
Authors: Fagaly R. L.
Published: October 2006

Superconducting quantum interference devices (SQUIDs) have been a key factor in the development and commercialization of ultrasensitive electric and magnetic measurement systems. In many cases, SQUID instrumentation offers the ability to make measurements where no other methodology is possible. We review the main aspects of designing, fabricating, and operating a number of SQUID measurement systems. While this article is not intended to be an exhaustive review on the principles of SQUID sensors and the underlying concepts behind the Josephson effect, a qualitative description of the operating principles of SQUID sensors and the properties of materials used to fabricate SQUID sensors is presented. The difference between low and high temperature SQUIDs and their suitability for specific applications is discussed. Although SQUID electronics have the capability to operate well above 1MHz, most applications tend to be at lower frequencies. Specific examples of input circuits and detection coil configuration for different applications and environments, along with expected performance, are described. In particular, anticipated signal strength, magnetic field environment (applied field and external noise), and cryogenic requirements are discussed. Finally, a variety of applications with specific examples in the areas of electromagnetic, material property, nondestructive test and evaluation, and geophysical and biomedical measurements are reviewed.
Low sticking probability in the nonactivated dissociation of N2 molecules on W(110)
Authors: Alducin M. , Díez Muiño R. , Busnengo H. F. , Salin A.
Published: October 2006

The six-dimensional potential energy surface for the dissociation of N2 molecules on the W(110) surface has been determined by density functional calculations and interpolated using the corrugation reducing procedure. Examination of the resulting six-dimensional potential energy surface shows that nonactivated paths are available for dissociation. In spite of this, the dissociation probability goes to a very small value when the impact energy goes to zero and increases with increasing energy, a behavior usually associated with activated systems. Statistics on the dynamics indicate that this unconventional result is a consequence of the characteristics of the potential energy surface at long distances. Furthermore, two distinct channels are identified in the dissociation process, namely, a direct one and an indirect one. The former is responsible for dissociation at high energies. The latter, which includes long-lasting dynamic trapping in the vicinity of a potential well above the W top position, is the leading mechanism at low and intermediate energies.
Two-dimensional electron-electron two-stream instability of an inertial electrostatic confinement device
Authors: Marocchino A. , Lapenta G. , Evstatiev E. G. , Nebel R. A. , Park J.
Published: October 2006

Theoretical works by Barnes and Nebel [D. C. Barnes and R. A. Nebel, Phys. Plasmas 5, 2498 (1998); R. A. Nebel and D. C. Barnes, Fusion Technol. 38, 28 (1998)] have suggested that a tiny oscillating ion cloud (referred to as the periodically oscillating plasma sphere or POPS) may undergo a self-similar collapse in a harmonic oscillator potential formed by a uniform electron background. A major uncertainty in this oscillating plasma scheme is the stability of the virtual cathode that forms the harmonic oscillator potential. The electron-electron two-stream stability of the virtual cathode has previously been studied with a fluid model, a slab kinetic model, a spherically symmetric kinetic model, and experimentally [R. A. Nebel and J. M. Finn, Phys. Plasmas 8, 1505 (2001); R. A. Nebel et al., Phys. Plasmas 12, 040501 (2005)]. Here the mode is studied with a two-dimensional particle-in-cell code. Results indicate stability limits near those of the previously spherically symmetric case.
Neutron powder diffraction studies of sulfuric acid hydrates. I. The structure of sulfuric acid hemitriskaidekahydrate D2SO4∙612D2O
Authors: Fortes A. D. , Wood I. G. , Knight K. S.
Published: October 2006

We report the first neutron diffraction data from D2SO4∙612D2O. The crystal is monoclinic, space group Cm, with four formula units per unit cell. At 4.2K the unit cell dimensions are a=6.25326(4)Å, b=26.81362(10)Å, c=5.90845(2)Å, and β=112.1939(3)° [V=917.286(6)Å3 and ρ(deuterated)=1664.14(2)kgm−3]. The deuteron positions refined from the neutron data are in agreement with those established by single crystal x-ray analysis [D. Mootz and A. Merschenz-Quack, Z. Naturforsch. B 42, 1231 (1987)], but not with those found from the ab initio simulation of Hirsch and Ojamäe [Acta Crystallogr, Sect. B: Struct. Sci. 60, 179 (2004)]. The crystal consists of SO42−, D3O+ ions, and D2O molecules hydrogen bonded to form a layered structure in which sheets of “icelike” D3O+ and D2O are separated by layers of opposing SO42− tetrahedra.
Cumulative effect of the filamentation and Weibel instabilities in counterstreaming thermal plasmas
Authors: Lazar M. , Schlickeiser R. , Shukla P. K.
Published: October 2006

Introducing a thermal particle distribution is important for a realistic investigation of counterstreaming plasmas with finite temperatures. Such counterstreaming thermal plasmas are described by the particle distributions, which include the counterstreams and thermal distribution as well. Two nonrelativistic counterstreams are considered here, with a bi-Maxwellian thermal anisotropy for each of them. This type of distribution is often expected to be found in both laboratory or cosmic plasmas, and it is able to cumulate the effects of the filamentation and Weibel instability. Comparing with the growth rates of each of these instabilities, the cumulative effect provides larger values, if they are emitted in the same direction. If the thermal anisotropy is negative, which means that Weibel instability develops on a perpendicular direction with respect to the filamentation instability, then their cumulative effect will suppress the instability. In both of these cases, the cumulative effect of the filamentation and Weibel instabilities can modify significantly the effective growth rate of the electromagnetic unstable modes.
Saturable absorption of highly stacked InAs quantum dot layer in 1.5μm band
Authors: Inoue Jun , Isu Toshiro , Akahane Kouichi , Tsuchiya Masahiro
Published: October 2006

The authors examined 1.5μm band absorption saturation properties of 150 stacked InAs quantum dot layers on InP substrate. The transmission change at the vertical incidence based on the saturable absorption of the quantum dots was as much as 1%. Theoretical prediction shows that the transmission change could be greatly increased by removing the InP substrate. After grinding the substrate and antireflection coating on both surfaces, transmittance change was build up to 3%. It is thus concluded that highly stacked quantum dot layers are potentially applicable to a planar-type mode locker for short optical pulse generation in the 1.5μm band.
Origins of linewidth in H1 magic-angle spinning NMR
Authors: Zorin Vadim E. , Brown Steven P. , Hodgkinson Paul
Published: October 2006

A detailed study of the factors determining the linewidth (and hence resolution) in H1 solid-state magic-angle spinning NMR is described. Although it has been known from the early days of magic-angle spinning (MAS) that resolution of spectra from abundant nuclear spins, such as H1, increases approximately linearly with increasing sample rotation rate, the difficulty of describing the dynamics of extended networks of coupled spins has made it difficult to predict a priori the resolution expected for a given sample. Using recently developed, highly efficient methods of numerical simulation, together with experimental measurements on a variety of test systems, we propose a comprehensive picture of H1 resolution under MAS. The “homogeneous” component of the linewidth is shown to depend primarily on the ratio between an effective local coupling strength and the spin rate, modified by geometrical factors which loosely correspond to the “dimensionality” of the coupling network. The remaining “inhomogeneous” component of the natural linewidth is confirmed to have the same properties as in dilute-spin NMR. Variations in the NMR frequency due to chemical shift effects are shown to have minimal impact on H1 resolution. The implications of these results for solid-state NMR experiments involving H1 and other abundant-spin nuclei are discussed.
Self-focusing of intense laser beam in magnetized plasma
Authors: Jha Pallavi , Mishra Rohit K. , Upadhyaya Ajay K. , Raj Gaurav
Published: October 2006

In this paper, evolution of spot size of an intense laser beam in cold, underdense, magnetized plasma has been studied. The plasma is embedded in a uniform magnetic field perpendicular to both, the direction of propagation and electric vector of the radiation field. Nonlinear current density is set up and the source dependent expansion method is used to determine the evolution of the spot size of a laser beam having a Gaussian profile. It is shown that transverse magnetization of plasma enhances the self-focusing property of the laser beam leading to reduction in critical power required to self-focus the beam.
Propagation of a dense relativistic electron beam through a gas
Authors: Frolov B. K. , Krasheninnikov S. I.
Published: October 2006

The ionization front induced by a relativistic high-density electron beam in a gas was studied in a one-dimensional (1D) approximation. The expression for the beam density corresponding to a steady front propagation was found in the limit of large (∼c) and small (⪡c) front velocities. The corresponding expression for front velocity was also found and the validity of the approximations used was verified. Finally, the values of front velocity and beam density were calculated for a wide range of beam energies and gas densities. The calculated values of the front velocity are in good agreement with the experimental data [D. Batani, S. D. Baton, M. Manclossi et al., Phys. Rev. Lett. 94, 055004 (2005).].
New technique for studying nanoscale friction at sliding velocities up to 200mm∕s using atomic force microscope
Authors: Tao Zhenhua , Bhushan Bharat
Published: October 2006

Micro-/nanoelectromechanical devices and components operate at high sliding velocities. Studies of tribological properties of materials by atomic force microscope (AFM) are limited to low velocity regime (<100–250μm∕s) due to inherent instrument limitation of AFM. To overcome the limitation of AFM working velocity, a custom-calibrated ultrahigh velocity stage has been incorporated for tribological investigation up to 200mm∕s. The stage is driven by a piezoactuator which is controlled using proportional-integral-derivative algorithm. Friction data are obtained by processing the AFM laser photodiode signals using a high sampling rate data acquisition system. The friction data obtained from the modified setup at high sliding velocities are compared with the results from conventional AFM friction measurement. The effects of scan size, rest time, and velocity on the friction force and adhesive force for single crystal silicon (100) with native oxide have been studied.
Efficient electrochemical etching method to fabricate sharp metallic tips for scanning probe microscopes
Authors: Kim Pilkyu , Kim Jun Ho , Jeong Mun Seok , Ko Do-Kyeong , Lee Jongmin , Jeong Sungho
Published: October 2006

A new technique based on electrochemical etching for the fabrication of sharp metallic tips for scanning probe microscopes is introduced. In the proposed method, a small Teflon mass is attached to the end of an immersed tungsten wire using an aluminum tape, which leads to a significant enhancement of yield rate of sharp tungsten tips with an apex size below 100nm to over 60%. The functionality of the tungsten tips fabricated by the proposed method is verified by measuring the topography of a standard sample using a shear-force scanning probe microscope.
Elongation of InAs∕GaAs quantum dots from magnetophotoluminescence measurements
Authors: Křápek V. , Kuldová K. , Oswald J. , Hospodková A. , Hulicius E. , Humlíček J.
Published: October 2006

The authors have used magnetophotoluminescence for the determination of the lateral anisotropy of buried quantum dots. While the calculated shifts of the energies of higher radiative transitions in magnetic field are found to be sensitive to the lateral elongation, the shift of the lowest transition is determined mainly by the exciton effective mass. This behavior can be used for a fairly reliable determination both the effective mass and the elongation from spectra containing at least two resolved bands.
Growth and characteristics of ultralow threshold 1.45μm metamorphic InAs tunnel injection quantum dot lasers on GaAs
Authors: Mi Z. , Bhattacharya P. , Yang J.
Published: October 2006

The molecular beam epitaxial growth and characteristics of 1.45μm metamorphic InAs quantum dot tunnel injection lasers on GaAs have been studied. Under optimized growth conditions, the quantum dots exhibit photoluminescence linewidths ∼30meV and high intensity at room temperature. The lasers are characterized by ultralow threshold current (63A∕cm2), large frequency response (f−3dB=8GHz), and near-zero α parameter and chirp.
Potential profiling of the nanometer-scale charge-depletion layer in n-ZnO∕p-NiO junction using photoemission spectroscopy
Authors: Ishida Yukiaki , Fujimori Atsushi , Ohta Hiromichi , Hirano Masahiro , Hosono Hideo
Published: October 2006

The authors have performed a depth-profile analysis of an all-oxide p-n junction diode n-ZnO∕p-NiO using photoemission spectroscopy combined with Ar-ion sputtering. Systematic core-level shifts were observed during the gradual removal of the ZnO overlayer, and were interpreted using a model based on charge conservation. Spatial profile of the potential around the interface was deduced, including the charge-depletion width of 2.3nm extending on the ZnO side and the built-in potential of 0.54eV.
Arbitrary surface structuring of amorphous silicon films based on femtosecond-laser-induced crystallization
Authors: Lee Geon Joon , Song Seok Ho , Lee YoungPak , Cheong Hyeonsik , Yoon Chong Seung , Son Yong Duck , Jang Jin
Published: October 2006

The arbitrary surface structuring of amorphous silicon (a-Si) films was performed by applying the Fourier-transform (FT) method to the femtosecond-laser-induced crystallization. In order to realize the arbitrary structuring, the logo q-Psi was produced in the a-Si film by the FT of a computer-generated hologram. The crystallization of a-Si was performed using the near-infrared femtosecond-laser pulses. By micro-Raman spectroscopy, scanning-electron microscopy, and transmission-electron microscopy, it was found that the femtosecond-laser pulses induced a localized phase transformation from the amorphous to the crystalline phase, and the spatially selected crystallization of the a-Si was responsible for the formation of the two-dimensional pattern.
Near-infrared wavelength intersubband transitions in GaN∕AlN short period superlattices
Authors: DeCuir E. A. , Fred Emil , Passmore B. S. , Muddasani A. , Manasreh M. O. , Xie Jinqiao , Morkoç Hadis , Ware M. E. , Salamo G. J.
Published: October 2006

Intersubband transitions in GaN∕AlN short period superlattices prepared by molecular beam epitaxy were investigated using the optical absorption technique. The peak position wavelengths of these transitions are found to span the spectral range of 1.35–2.90μm for samples cut into 45° waveguides with GaN quantum well thicknesses ranging between 1.70 and 2.41nm. The Fermi energy levels are estimated from the carrier concentrations, which were measured using an electrochemical capacitance-voltage profiler. The well widths were inferred from comparing the measured peak position energy of the intersubband transitions and the bound state energy levels calculated using the transfer matrix method.
Improved properties of epitaxial YNixMn1−xO3 films by annealing under high magnetic fields
Authors: Ma Yanwei , Xu Aixia , Li Xiaohang , Zhang Xianping , Guilloux-Viry M. , Peña O. , Awaji S. , Watanabe K.
Published: October 2006

The effect of annealing under a magnetic field on the microstructure and properties of YNixMn1−xO3 (x=0.33 and 0.5) films has been investigated. It is found that the ferromagnetic transition temperature is significantly enhanced after postannealing in the presence of an 8T magnetic field. Characterization study shows that the microstructure is affected, obtaining larger grains of uniform size when films are annealed under a magnetic field. The improvement in the ordering temperature of all films is interpreted in terms of the grain growth caused by the magnetic field driving force for boundary motion where the exchange coupling is high.
Method to assess the grain crystallographic orientation with a submicronic spatial resolution using Kelvin probe force microscope
Authors: Gaillard Nicolas , Gros-Jean Mickael , Mariolle Denis , Bertin François , Bsiesy Ahmad
Published: October 2006

In thin polycrystalline copper film, a direct correlation between the grain crystallographic orientation and the work function has been evidenced by comparing Kelvin probe force microscope (KFM) mapping and electron backscattered diffraction analysis performed over the same region. As a result, work function mapping provided by KFM technique can be used to assess the crystallographic properties of thin layers with a spatial resolution better than 100nm.