bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of the tilt angle on the bow shock shape and location
Authors: Jelínek K. , Němeček Z. , Šafránková J. , Merka J.
Published: May 2008

The bow shock is created in front of an obstacle immersed into a supersonic flow and its location depends on the size and shape of the obstacle. It was found that the obstacle (magnetopause) is scaled with the solar wind dynamic pressure and changes its dimensions and shape with the dipole tilt angle and interplanetary magnetic field orientation. Similar functional dependencies would be expected for the bow shock position, however, none of the bow shock models considers the parametrization of bow shock properties with the tilt angle. The present study employs a set of bow shock crossings registered during 1994–2002 by different spacecraft and demonstrates the tilt angle influence on the bow shock location. The study is based on a comparison of a recent bow shock model with observations and shows that the night–side bow shock moves in the direction of the positive ZGSM axis for positive tilt angles. The magnitude of the displacement can reach ≈3 RE. The analysis reveals that the high–latitude bow shock surface is significantly distorted near the dawn–dusk meridian. This effect was identified as a counterpart of the magnetopause indentation in the cusp region.
Crustal dynamics of Mount Vesuvius from 1998 to 2005: Effects on seismicity and fluid circulation
Authors: Madonia Paolo , Federico Cinzia , Cusano Paola , Petrosino Simona , Aiuppa Alessandro , Gurrieri Sergio
Published: May 2008

This paper presents the results of hydrogeochemical and seismological studies carried out at Mount Vesuvius during the period June 1998 to December 2005. Hydrogeochemical data show the occurrence of slowly varying long‐term variations in the total dissolved salts and bicarbonate contents of the groundwaters, accompanied by a general decline in water temperatures. The temporal distributions of air temperature and rainfall in the Vesuvius area suggest that these variations do not depend on changes in the hydrological regime. The changes in the geochemical parameters are accompanied by slight variations in both the seismicity rate and energy release. A further relationship between seismic activity and fluid discharge rate is highlighted by a particular episode that occurred in August 2005, when a soil thermal anomaly was observed a few weeks before the occurrence of a very shallow earthquake. Moment tensor analysis of this earthquake suggests that the most plausible source mechanism is a shear faulting combined with the opening of tensile crack. This feature is often observed in volcanic areas and it is usually related to fluid‐ and/or gas‐driven rock fracturing. The observed seismological, hydrological, and geochemical temporal changes are interpreted not as changes of the volcanic system but in terms of an external forcing as identified in the variation of the regional and local stress field acting on the volcano. This study has inferences onto the evaluation of the state of activity of volcanic systems and the eventual detection of unrest phenomena.
Spontaneous generation of pure ice streams via flow instability: Role of longitudinal shear stresses and subglacial till
Authors: Sayag Roiy , Tziperman Eli
Published: May 2008

A significant portion of the ice discharge in ice sheets is drained through ice streams, with subglacial sediment (till) acting as a lubricant. The known importance of horizontal friction in shear margins to ice stream dynamics suggests a critical role of longitudinal stresses. The effects of subglacial till and longitudinal stresses on the stability of an ice sheet flow are studied by linear stability analysis of an idealized ice‐till model in two horizontal dimensions. A power law‐viscous constitutive relation is used, explicitly including longitudinal shear stresses. The till, which has compressible viscous rheology, affects the ice flow through bottom friction. We examine the possibility that pure ice streams develop via a spontaneous instability of ice flow. We demonstrate that this model can be made intrinsically unstable for a seemingly relevant range of parameters and that the wavelengths and growth rates that correspond to the most unstable modes are in rough agreement with observed pure ice streams. Instabilities occur owing to basal friction and meltwater production at the ice‐till interface. The most unstable wavelength arise because of selective dissipation of both short and long perturbation scales. Longitudinal stress gradients stabilize short transverse wavelengths, while Nye diffusion stabilizes long transverse wavelengths. The selection of an intermediate unstable wavelength occurs, however, only for certain parameter and perturbation structure choices. These results do not change qualitatively for a Newtonian ice flow law, indicating no significant role to shear thinning, although this may very well be due to the restrictive assumptions of the model and analysis.
Distribution, transport, and deposition of mineral dust in the Southern Ocean and Antarctica: Contribution of major sources
Authors: Li Fuyu , Ginoux Paul , Ramaswamy V.
Published: May 2008

A model‐based investigation of the transport, distribution and deposition of mineral dust in the Southern Hemisphere (SH) is performed by using the GFDL Atmospheric Model (AM2). The study represents an attempt to quantify the contribution of the major sources by tagging dust based on its origin. We evaluate the contribution of each source to the emission, distribution, mass burden and deposition of dust in the Southern Ocean and Antarctica, and show that each source produces distinctive meridional transport, vertical distribution, and deposition patterns. The dust in SH originates primarily from Australia (120 Tg a−1), Patagonia (38 Tg a−1) and the inter‐hemispheric transport from Northern Hemisphere (31 Tg a−1). A small fraction of it (7 Tg a−1) is transported and deposited in the Southern Ocean and Antarctica, where dust from South America, Australia, and Northern Hemisphere are essentially located in the boundary layer, mid‐troposphere, and upper‐troposphere, respectively. These three sources contribute to nearly all the dust burden in the Southern Ocean and Antarctica. South America and Australia are the main sources of the dust deposition, but they differ zonally, with each one dominating half of a hemisphere along 120°E–60°W: the half comprising the Atlantic and Indian oceans in the case of the South American dust and the Pacific half in the case of the Australian dust. Our study also indicates a potentially important role of Northern Hemisphere dust, as it appears to be a significant part of the dust burden but contributing little to the dust deposition in Antarctica.
Bispectral analysis of the long‐term recording of surface pressure at Jakarta
Authors: Hocke Klemens , Kämpfer Niklaus
Published: May 2008

The long‐term series of surface pressure at Jakarta (6 S, 107 E) from 1842 to 2000 are analyzed for nonlinear three‐wave interaction. In the tropics, the variations of surface pressure are mainly caused by variations of the water vapor pressure. Jakarta is located in the strongest zone of water vapor convection of the Earth, and the pressure variations at Jakarta essentially influence or drive the circulation of the Earth's atmosphere. The bispectrum evaluates the phase relationships and the magnitudes of oscillation triads having the frequencies f1, f2, and f3 = f1 + f2. Analysis of the bispectrum from intraseasonal to decadal timescales quantifies the degree of nonlinearity in the troposphere and reveals the dominant oscillation triads. The bispectrum also informs about the energy and impulse transfer from low to high frequencies and vice versa. We find following main results for the Jakarta series: (1) a strong annual oscillation and strong harmonics up to the fourth order and higher, (2) strong interactions between the annual oscillation (AO) and the biennial oscillation (BO) and several components of the quasi‐biennial oscillation (QBO with periods at 2.79, 2.39, 1.72, 1.56‐year), (3) mixing processes between the annual harmonics and the interannual and decadal oscillations (10 to 40‐year) are obvious in the bispectrum, FFT‐spectrum, and in the temporal evolution of the amplitudes (derived from the bandpass‐filtered time series), and (4) a close relationship is found between the amplitudes of the semi‐annual oscillation and the quasi 4‐year oscillation (nonlinear interaction 2 cycles/a + 0.25 cycles/a = 2.25 cycles/a). The bispectral analysis of the Jakarta series gives us a detailed insight into the self‐organization of the atmospheric oscillations by nonlinear three‐wave interaction and permits the development of a strategy for improved detection, modeling, and forecasting of atmospheric variability.
Decadal‐scale changes in the effect of the QBO on the northern stratospheric polar vortex
Authors: Lu Hua , Baldwin Mark P. , Gray Lesley J. , Jarvis Martin J.
Published: May 2008

This study documents decadal‐scale changes in the Holton and Tan (HT) relationship, i.e., the influence of the lower stratospheric equatorial quasi‐biennial oscillation (QBO) on the northern hemisphere (NH) extratropical circulation. Using a combination of ECMWF ERA‐40 Reanalysis and Operational data from 1958–2006, we find that the Arctic stratosphere is indeed warmer under easterly QBO and colder under westerly QBO. During November to January, composite easterly minus westerly QBO signals in zonal wind extend from the lower stratosphere to the upper stratosphere and are centered at ∼5 hPa, 55–65°N with a magnitude of ∼10 m s−1. In temperature, the maximum signal is near ∼20–30 hPa at the pole with a magnitude of ∼4 K. During winter, the dominant feature is a poleward and downward transfer of wind and temperature anomalies from the midlatitude upper stratosphere to the high latitude lower stratosphere. For the first time, a statistically significant decadal scale change of the HT relationship during 1977–1997 is diagnosed. The main feature of the change is that the extratropical QBO signals reverse sign in late winter, resulting in fewer and delayed major stratospheric sudden warmings (SSWs), which occurred more often under westerly QBO. Consistent with earlier studies, it is found that the HT relationship is significantly stronger under solar minima overall, but the solar cycle does not appear to be the primary cause for the detected decadal‐scale change. Possible mechanisms related to changes in planetary wave forcing are discussed.
Formation and distribution of benzene on Titan
Authors: Vuitton V. , Yelle R. V. , Cui J.
Published: May 2008

We present a study of the formation and distribution of benzene (C6H6) on Titan. Analysis of the Cassini Mass Spectrometer (INMS) measurements of benzene densities on 12 Titan passes shows that the benzene signal exhibits an unusual time dependence, peaking ∼20 s after closest approach, rather than at closest approach. We show that this behavior can be explained by recombination of phenyl radicals (C6H5) with H atoms on the walls of the instrument and that the measured signal is a combination of (1) C6H6 from the atmosphere and (2) C6H6 formed within the instrument. In parallel, we investigate Titan benzene chemistry with a set of photochemical models. A model for the ionosphere predicts that the globally averaged production rate of benzene by ion‐molecule reactions is ∼107 cm−2 s−1, of the same order of magnitude as the production rate by neutral reactions of ∼4 × 106 cm−2 s−1. We show that benzene is quickly photolyzed in the thermosphere, and that C6H5 radicals, the main photodissociation products, are ∼3 times as abundant as benzene. This result is consistent with the phenyl/benzene ratio required to match the INMS observations. Loss of benzene occurs primarily through reaction of phenyl with other radicals, leading to the formation of complex aromatic species. These species, along with benzene, diffuse downward, eventually condensing near the tropopause. We find a total production rate of solid aromatics of ∼10−15 g cm−2 s−1, corresponding to an accumulated surface layer of ∼3 m.
A hybrid KA‐MoM algorithm for computation of scattering from a 3‐D PEC target above a dielectric rough surface
Authors: Ye Hongxia , Jin Ya‐Qiu
Published: May 2008

A new hybrid analytical‐numerical iterative algorithm, which combines the Kirchhoff approximation (KA) and the Method of Moment (MoM), is developed for computing the electromagnetic scattering from a three‐dimensional (3‐D) perfect electric conducting (PEC) target above a 2‐D infinite randomly rough dielectric surface. The equations of difference scattering due to the target presence above the rough surface are derived. The induced difference fields on the rough surface due to the interactions between the target and the rough surface are calculated by using the KA method. The excitation term on the right‐hand‐side (RHS) of target's surface integral equation (SIE), which contains the difference scattering of the rough surface, is then updated for calculating new target currents with the Conjugate Gradient (CG) procedure. Then the target currents are used to compute the difference field induced on the rough surface with the KA method. Multiple iterations take account of the multiorder interactions between the target and the underlying rough surface. Numerical quadrature upon the rough surface is performed only once to compute the coupling scattering field from the rough surface to the target, and it takes N steps (N is the discretized mesh number of rough surface). By using this hybrid KA‐MoM algorithm, the requirements of memory and CPU time can be reduced significantly. Moreover, the validity conditions and the convergence performance of this hybrid algorithm are also discussed. With Monte‐Carlo generations of randomly rough surfaces, bistatic scattering from different‐shaped targets above a Gaussian rough surface is numerically simulated. Finally, dependence of bistatic scattering pattern on the surface dielectric property and the target geometry is discussed.
Evaluation of cloudless‐sky periods detected by shortwave and longwave algorithms using lidar measurements
Authors: Dupont Jean‐Charles , Haeffelin Martial , Long Charles N.
Published: May 2008

Identifying cloud‐free periods is important as they are used as common references in cloud and aerosol radiative forcing studies. Their identification requires precise methods to distinguish condensed water from other aerosols (e.g. mineral or moist hydrophilic aerosols). In this study we combine analyses of wide field of view shortwave (SW) and longwave (LW) irradiances and Lidar backscatter measurements to explore situations that are considered neither completely clear nor cloudy. We find that situations classified as cloud‐free by analysis of SW (LW) measurements are also classified as cloud free by the Lidar in more than 60% (50%) of situations. The remaining 40% (50%) situations are classified as cloudy by the Lidar, and are hence considered as hazy. These hazy situations are predominantly composed of high‐altitude cirrus clouds, partitioned equally between subvisible and semi‐transparent optical thickness classes. We find that, in hazy situations, the average cloud radiative forcing on surface SW irradiances ranges between −5 and −15 Wm−2.
Nonlinear saturation of mirror instability
Authors: Qu Hongpeng , Lin Zhihong , Chen Liu
Published: May 2008

Mechanism of the nonlinear saturation of compressible electromagnetic mirror instability is studied using the gyrokinetic particle simulation. Phase‐space particle trapping due to the mirror force is found to be the dominant saturation mechanism in the simulation of a single mirror mode with relatively weak drive. At the nonlinear saturation, the phase‐space island of the distribution function is formed. The oscillation frequency of the saturated perturbation amplitude is close to the bounce frequency of the trapped particles, which is comparable to the linear growth rate of the mirror mode. Scaling of the saturation amplitude is consistent with the onset of the particle trapping. With strong instability drive, relaxation toward marginal stability dominates the nonlinear saturation of the mirror instability. Phase‐space trapping, however, persists after the saturation and continues to regulate the nonlinear evolution of the mirror mode.
Function and evolution of the psbA gene family in marine Synechococcus: Synechococcus sp. WH7803 as a case study
Authors: Garczarek Laurence , Dufresne Alexis , Blot Nicolas , Cockshutt Amanda M , Peyrat Anne , Campbell Douglas A , Joubin Ludovic , Six Christophe
Published: May 2008

In cyanobacteria, the D1 protein of photosystem II (PSII) is encoded by the psbA multigene family. In most freshwater strains, a D1:1 isoform of this protein is exchanged for a D1:2 isoform in response to various stresses, thereby altering PSII photochemistry. To investigate PSII responses to stress in marine Synechococcus, we acclimated cultures of the WH7803 strain to different growth irradiances and then exposed them to high light (HL) or ultraviolet (UV) radiation. Measurement of PSII quantum yield and quantitation of the D1 protein pool showed that HL-acclimated cells were more resistant to UV light than were low light- (LL) or medium light- (ML) acclimated cells. Both UV and HL induced the expression of psbA genes encoding D1:2 and the repression of the psbA gene encoding D1:1. Although three psbA genes encode identical D1:2 isoforms in Synechococcus sp. WH7803, only one was strongly stress responsive in our treatment conditions. Examination of 11 marine Synechococcus genomic sequences identified up to six psbA copies per genome, with always a single gene encoding D1:1. In phylogenetic analyses, marine Synechococcus genes encoding D1:1 clustered together, while the genes encoding D1:2 grouped by genome into subclusters. Moreover, examination of the genomic environment of psbA genes suggests that the D1:2 genes are hotspots for DNA recombination. Collectively, our observations suggest that while all psbA genes follow a concerted evolution within each genome, D1:2 coding genes are subject to intragenome homogenization most probably mediated by gene conversion.
Mitochondrial Molecular Adaptations and Life History Strategies Coevolve in Plants
Authors: Krishnan Neeraja , Rao Basuthkar
Published: May 2008

Messenger RNA secondary structure prevents mutations at functionally important sites. Mutations at exposed sites would cause micro-adaptations, niche-specialization, and therefore, can be thought to promote K-strategists. Exposing, rather than protecting, conserved sites, is also potentially adaptive because they probably promote macro-adaptive changes. This presumably fits r-strategists: their population dynamics tolerate decreased survival. We found that helix-forming tendencies are greater at evolutionary conserved sites of plant mitochondrial mRNAs than at evolutionary variable sites in a majority (73%) of species–gene combinations. K-strategists preferentially protect conserved sites in short genes, r-strategists protect them most in larger genes. This adaptive scenario resembles our earlier findings in chloroplast genes. Protection levels at various codon positions also display disparity with respect to life history strategies of the plants. Conserved site protection increases overall mRNA folding stabilities for some genes, while decreases it for some others. This contrast exists between homologous genes of r- and K- strategists. Such compensating interactions between variability, mRNA size, codon position, and secondary structure factors within r- and K-strategists are most likely, molecular adaptations of plants belonging to the two extreme life history strategies. Our results suggest coevolution between molecular and ecological adaptive strategies.
Electrostatic Maxwell stress model of the shapes of condensed phase domains in monolayers at the air-water interface
Authors: Yamamoto Tetsuya , Taguchi Dai , Weis Martin , Manaka Takaaki , Iwamoto Mitsumasa
Published: May 2008

The formation mechanism of the shapes of condensed phase domains in monolayers at the air-water interface was investigated taking into account the surface pressure, line tension, and electrostatic energy due to the spontaneous polarization generated in normal and in-plane direction. By deriving the shape equation of monolayer domains as the mechanical balance at the domain boundary, we found that the electrostatic energy contributes to the shape equation as electrostatic Maxwell stress. Development of a cusp from condensed phase domains of fatty acid monolayers, which has been experimentally observed, was analyzed by the shape equation. It was found that the development of a cusp originated from the strong Maxwell stress, which was induced by the non-uniform orientational distribution in the fatty acid domain, and that cusped shapes gave a minimum of the free energy of the domain. It demonstrates that the shape equation with Maxwell stress, which is derived in the present study, is useful to study the formation mechanism of the shapes of condensed phase domains in monolayers.
A
-plane GaN epitaxial lateral overgrowth structures: Growth domains, morphological defects, and impurity incorporation directly imaged by cathodoluminescence microscopy
Authors: Bastek B. , Bertram F. , Christen J. , Wernicke T. , Weyers M. , Kneissl M.
Published: May 2008

The distinctly different growth domains of a-plane epitaxial lateral overgrown GaN on stripe masks oriented along [011̱0] direction were directly visualized by highly spatially and spectrally resolved cathodoluminescence microscopy. Clear cut microscopic regions dominated by differing individual peak wavelengths originating from either basal plane stacking faults, prismatic stacking faults, impurity related donor-acceptor pair or (D0,X) emission are explicitly correlated to the different growth domains. The luminescence in the domains grown in [0001] direction over the mask [epitaxial lateral overgrown wings] is dominated by the intense and sharp (D0,X) emission at 3.471eV. Here, no luminescence originating from morphological defects is found over several micrometers. This evidences the excellent material quality of the a-plane GaN, which is fully relaxed at the surface of the wings.
Power-law decaying oscillations of neutrally buoyant spheres in continuously stratified fluid
Authors: Biró István , Gábor Szabó K. , Gyüre Balázs , Jánosi Imre M. , Tél Tamás
Published: May 2008

The free sinking motion of small spheres is experimentally investigated in a laboratory tank filled up with continuously stratified salt solutions. Special attention is paid to the late stage oscillations around the gravitational equilibrium level. Such motion is generated by slowly decaying weak internal waves originating from the interaction of the solid sphere with the surrounding fluid. The temporal decay of oscillation amplitudes and velocities can be best described by a power law of exponent value close to −3∕2. Up to our best knowledge, none of the existing theories can predict a similar algebraic decay.
Direct numerical simulation of heated CO2 flows at supercritical pressure in a vertical annulus at Re=8900
Authors: Bae Joong Hun , Yoo Jung Yul , McEligot Donald M.
Published: May 2008

The present study is concerned with thermal physics of the fluid at supercritical pressure (SCP) where many singular phenomena are observed in turbulent heat transfer due to severe property variations of the fluid. Direct numerical simulation is conducted for upward annular flows of CO2 at a pressure of 8MPa with a constant-heat-flux boundary condition imposed on the inner wall. All simulations are made at the inlet bulk Reynolds number of 8900 with particular attention being paid to the structure of the heated boundary layers at SCP. It is shown that most singular phenomena at SCP occur when the pseudocritical temperature arises between the heated wall and bulk fluid temperatures. The mean velocity profile near the heated wall shows no logarithmic distribution in the inertial subrange because a large reduction in the Reynolds shear stress occurs in the viscous region. The computational flow visualization reveals that alternating low- and high-speed streaks in the viscous region are not clearly observed when the wall temperature peak is realized due to significant heat transfer deterioration. These results strongly suggest that the ejection and sweep motions of the fluid in the viscous region become so weakened that turbulent boundary layers at SCP cannot be self-sustained in the presence of strong stabilizing effects of the variable-property and buoyancy. It is also shown that streaky thermal pattern remains no longer similar to that of the velocity boundary layer when these coherent motions are reduced in the viscous region. In the meanwhile, the predicted normalized temperature profile at SCP shows a nearly flat distribution outside the viscous region although substantial amount of the radial turbulent heat flux is predicted there. This singular phenomenon at SCP can be considered as comparable to a phase change phenomenon at subcritical pressure, where the internal energy of the fluid is increased without changing the fluid temperature. At SCP, this transition occurs continuously across the pseudocritical point but the computational flow visualization shows very vigorous density fluctuations when it occurs.
Infrared absorption spectra of vinyl radicals isolated in solid Ne
Authors: Wu Yu-Jong , Lin Meng-Yeh , Cheng Bing-Ming , Chen Hui-Fen , Lee Yuan-Pern
Published: May 2008

Irradiation of samples of solid Ne near 3.0K containing ethene (C2H4) with vacuum ultraviolet radiation at 120nm from synchrotron yielded new spectral lines at 3141.0, 2953.6, 2911.5, 1357.4, 677.1, 895.3, and 857.0cm−1. These features are assigned to α-CH stretching (ν1), CH2 antisymmetric stretching (ν2), CH2 symmetric stretching (ν3), CH2-bending (ν5), HCCH cis bending (ν7), CH2 out-of-plane bending (ν8), and α-CH out-of-plane bending (ν9) modes of C2H3, respectively, based on results of C13- and D-isotopic experiments and quantum-chemical calculations. These calculations using density-functional theory (B3LYP and PW91PW91/aug-cc-pVTZ) predict vibrational wavenumbers, IR intensities, and isotopic ratios of vinyl radical that agree satisfactorily with our experimental results.
Depth profile photoemission study of thermally diffused Mn/GaAs (001) interfaces
Authors: Osafune Y. , Song G. S. , Hwang J. I. , Ishida Y. , Kobayashi M. , Ebata K. , Ooki Y. , Fujimori A. , Okabayashi J. , Kanai K. , Kubo K. , Oshima M.
Published: May 2008

We have performed a depth profile study of thermally diffused Mn/GaAs (001) interfaces using photoemission spectroscopy combined with Ar+-ion sputtering. We found that Mn ion was thermally diffused into the deep region of the GaAs substrate and completely reacted with GaAs. In the deep region, the Mn 2p core-level and Mn 3d valence-band spectra of the Mn/GaAs (001) sample heated to 600 °C were similar to those of Ga1−xMnxAs, zinc-blend-type MnAs dots, and/or interstitial Mn in tetrahedrally coordinated by As atoms, suggesting that the Mn atoms do not form any metallic compounds but are tetrahedrally coordinated by ligand atoms, and Mn 3d states are hybridized with ligand orbitals lized but were hybridized with the electronic states of the host GaAs. Ferromagnetism was observed in the dilute Mn phase.
Phononic band gap crystals with periodic fractal inclusions: Theoretical study using numerical analysis
Authors: Norris Ryan C. , Hamel John S. , Nadeau Phillip
Published: May 2008

A numerical study is conducted using finite difference time-domain analysis to determine the impact of periodic fractal-shaped inclusions on the frequency response of two-dimensional phononic or acoustic band gap crystals. Both solid host with solid inclusions and solid host with fluid inclusions were studied with increasing fractal order iteration or increasing fractal dimensionality for different types of fractals generated from square inclusions in a square lattice formation. Solid-fluid systems showed marked changes in frequency response including increasing multiplicity of bands compared to regular square inclusion shapes. Fractal inclusions in the solid host/fluid inclusion systems studied exhibited the ability to provide band gaps for much longer wavelengths than those of the solid host/solid inclusion systems studied. This behavior was attributed to the possibility of resonant localization effects that a fractal fluid inclusion can exhibit compared to fractal solid inclusions.
Linear field amplification for magnetoresistive sensors
Authors: Trindade I. G. , Fermento R. , Sousa J. B. , Chaves R. C. , Cardoso S. , Freitas P. P.
Published: May 2008

In this article, we describe the fabrication and characterization of a spin valve (SV) magnetoresistive (MR) sensor, located in the gap of two magnetically soft flux guides (FGs) that enhance the sensor magnetic field sensitivity, while keeping in the sensor a quasilinear Barkhausen-noise-free response. Top pinned SV sensors were fabricated into stripes, having lengths of 100μm and widths of 2 and 3μm, by optical lithography and ion-milling etching. The FGs consisted of poles and yokes of an amorphous alloy of Co88.4Zr3.3Nb8.3, prepared by physical vapor deposition and were lithographically defined by a lift-off process. The SV sensor MR responses to applied uniform magnetic fields Ha, when either isolated or located in the gap of two types of FGs, were characterized in terms of the saturation field, coercive force, and sensitivity. The impact of the FG geometry in the magnetic field amplification and sensor response characteristics were studied. Magnetic force microscopy analysis was performed to identify the presence of multidomain states in the FGs and of a remanent field in their gap. SV sensors in the gap of FGs using long poles and having a sensing area of 1000μm2 exhibit a linear sensitivity of 50mV∕Oe in the field range of a couple of oersteds. The SV sensor in the gap of magnetically soft FGs exhibits enhanced hysteresis, characterized by a coercive force of approximately 1Oe. Two schemes are proposed to reduce the hysteresis in the sensor response.
Morphology, structure, chemical composition, and light emitting properties of very thin anodic silicon films fabricated using short single pulses of current
Authors: Gardelis S. , Nassiopoulou A. G. , Petraki F. , Kennou S. , Tsiaoussis I. , Frangis N.
Published: May 2008

In this work, the morphology, structure, surface chemical composition, and optical properties of very thin (10–70 nm) anodic silicon films grown on a silicon substrate by electrochemical dissolution of bulk crystalline silicon in the transition regime between the porous formation and electropolishing were investigated in detail. Anodization was performed by using short single pulses of anodization current in low and high hydrofluoric acid (HF) concentration electrolytes. A systematic comparison was made between films grown at low and high HF concentration electrolytes. The morphology and structure of the films were investigated by combining atomic force microscopy and transmission electron microscopy (TEM), while x-ray and ultraviolet photoelectron spectroscopies were used to investigate the chemical composition of the films. Photoluminescence was used to investigate the optical properties. It was found that films that formed at low HF concentrations were much thinner than films that formed at high HF concentrations due to surface dissolution of the films during anodization. High resolution TEM images revealed an amorphouslike structure (porous) in all of the films in which discrete Si nanocrystals (NCs) were identified. NC size was, on the average, larger in films fabricated in low HF concentration electrolytes and these films were not luminescent. On the other hand, films fabricated in high HF concentration electrolytes were thicker and contained smaller NCs. A silicon oxide layer covered the internal surface of all films, this oxide being much thinner in films grown at high HF concentrations. This last effect was attributed to self-limiting oxidation of the very small NCs constituting these films.
Plane wave methodology for single quantum dot electronic structure calculations
Authors: Vukmirovć Nenad , Tomić Stanko
Published: May 2008

The development of the plane wave methodology for the calculation of the electronic structure of single quantum dots within the framework of multiband envelope function theory was presented. The methodology developed enables one to use a small embedding box, sufficient to eliminate electronic coupling, without introducing the artificial interaction with periodically replicated array of quantum dots caused by periodic boundary conditions. The appropriate formulas for Fourier transforms of strain tensor components on the embedding box that eliminate the strain field of the neighboring dots were derived. The expressions that enable the evaluation of Coulomb integrals in inverse space without the introduction of artificial electrostatic interactions with surrounding dots were presented. It was also shown how symmetry can be exploited to further reduce the computational effort in the case of quantum dots of symmetric shape. Numerical results illustrating the application of the methods to the calculation of single-particle states, as well as the configuration interaction calculation of exciton, biexciton, and negative trion states in InAs∕GaAs quantum dots were given.
Negative differential resistance and tunable peak-to-valley ratios in a silicon nanochain
Authors: Sun Z. Z. , Wang X. R. , Zhang R. Q. , Lee S. T.
Published: May 2008

The current-voltage characteristics of a silicon nanochain is investigated. The nanochain is viewed as a superlattice structure of quantum dots (QDs), where silicon cores in a chain act as QDs while silicon dioxides covering the cores act as potential barriers. It is found that the whole nanochain structure can display the negative differential conductance (NDC) feature as the tunneling current through each barrier has the NDC property individually. Importantly, large peak-to-valley ratios of the current are observed and tunable by the number of QDs involved. This feature will be useful in device design.
Strongly enhanced current-carrying performance in MgB2 tape conductors by C60 doping
Authors: Zhang Xianping , Ma Yanwei , Gao Zhaoshun , Wang Dongliang , Wang Lei , Liu Wei , Wang Chunru
Published: May 2008

By utilizing C60 as a viable alternative dopant, we demonstrate a simple and industrially scalable route that yields a 10∼15-fold improvement in the in-high-field current densities of MgB2 tape conductors. For example, a Jc value higher than 4×104 A/cm2 (4.2 K, 10 T), which exceeds that for NbTi superconductor, can be realized on the C60 doped MgB2 tapes. It is worth noting that this value is even higher than that fabricated using strict high energy ball milling technique under Ar atmosphere. At 20 K, Hirr was ∼10 T for C60 doped MgB2 tapes. A large amount of nanometer-sized precipitates and grain boundaries were found in MgB2 matrix. The special physical and chemical characteristics of C60, in addition to its C containing intrinsic essence, are a key point in enhancing the superconducting performance of MgB2 tapes.
Investigating minority-carrier lifetime in small spherical Si using microwave photoconductance decay
Authors: Liu Zhengxin , Masuda Atsushi , Kondo Michio
Published: May 2008

We have studied minority-carrier recombination lifetime in small spherical Si (<1mm in diameter) using microwave photoconductance decay (μPCD). It was tested that 0.01molar quinhydrone in methanol solution has the best passivation effect, which results in perfect exponential photoconductance decay curve and the highest average lifetime. However, the average lifetime of single crystalline spheres that were formed by polishing single crystalline Czochralski bulks was only 4.8μs, and that of tear-drop-like spheres that were formed by a dropping method was 2.6μs. These values are significantly lower compared with crystalline Si wafers. It has been found that the low lifetime is induced by the small size, and the spherical geometry does not have influence on the lifetime value. The surface recombination velocity becomes more dominant compared to bulk recombination as the size of sphere becomes smaller. Therefore, the principal of surface equivalent lifetime in the sphere is essentially different from the case of the planar wafer.
Highly polarized luminescence from aligned conjugated polymer electrospun nanofibers
Authors: Campoy-Quiles Mariano , Ishii Yuya , Sakai Heisuke , Murata Hideyuki
Published: May 2008

In this contribution we show highly polarized photoluminescence (PL) from aligned polyethyleneoxide: polyphenylenevinylene derivative composite nanofibers. We demonstrate PL polarization ratios (parallel to perpendicular) greater than 13. This ratio is further increased (up to ∼25) by stretching the nanofibers. Stretching also results in an increase in conjugation length, fiber density, and PL lifetime. We argue that the effect of stretching is equivalent to applying a permanent and strong pressure. Our results open up the possibility for new optoelectronic devices and fundamental science studies based on polymer nanofibers.
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heterojunction: Sol-gel fabrication, photoresponse properties, and transport mechanism
Authors: Dutta M. , Basak D.
Published: May 2008

p - Zn O ∕ n - Si heterojunction is achieved by depositing Al–N codoped p-type ZnO film on n-Si by low-cost sol-gel technique. The junction shows good diode characteristics with rectification ratio (IF∕IR)∼10 at 4V in the dark. The photoresponse of the heterojunction is investigated by studying the current-voltage characteristics under the ultraviolet (370nm) and visible light (450nm) illuminations. By fitting the experimental data, we have proposed the current transport mechanism to be dominated by the recombination tunneling at lower and by the space-charge limited current at higher forward voltages, which are further supported by the photocapacitance and photocurrent spectra.
Impedance measurements on a DNA junction
Authors: Hong Sungmin , Jauregui Luis A. , Rangel Norma L. , Cao Huan , Day B. Scott , Norton Michael L. , Sinitskii Alexander S. , Seminario Jorge M.
Published: May 2008

1 μ m double-stranded DNA molecules are immobilized between pairs of gold and pairs of platinum microelectrodes with gaps of 0.4 and 1μm, respectively, and their electrical characteristics are determined under the application of constant and sinusoidal bias voltages. Due to their extremely high impedance for constant voltage bias, the samples of DNA are excellent insulators; however, their impedances show strong frequency dependence in the range of 10Hz–7.5MHz. Favorable response in the gold electrodes is attributed to the higher ability of DNA molecules to bridge the narrower gold electrode gaps in contrast to that in the wider platinum junctions.
The vacancy in silicon: Identical diffusion properties at cryogenic and elevated temperatures
Authors: Watkins George D.
Published: May 2008

The vacancy contribution to self-diffusion in silicon, recently revealed in the study by Shimitzu et al. [Phys. Rev. Lett. 98, 095901 (2007)], is analyzed to demonstrate that the pre-exponential term, as well as the enthalpy term, matches accurately to that of the isolated neutral vacancy, as determined many years earlier at cryogenic temperatures by electron paramagnetic resonance and deep level transient spectroscopy. The persistent suggestion of an “extended” vacancy with very different migrational properties at elevated temperatures is therefore incorrect. There is only one vacancy.
Cesium carbonate as a functional interlayer for polymer photovoltaic devices
Authors: Chen Fang-Chung , Wu Jyh-Lih , Yang Sidney S. , Hsieh Kuo-Huang , Chen Wen-Chang
Published: May 2008

The device characteristics of polymer solar cells with cesium carbonate (Cs2CO3) as an electron-injection interlayer have been investigated. It is found that the insertion of Cs2CO3 at the cathode interface improves the device power conversion efficiency from 2.3% to 3.1%. In order to further understand the mechanism, the interfacial interaction between the active organic layer and the cathode was studied by x-ray photoemission spectroscopy (XPS). The results of XPS measurement indicate the fact that a portion of electrons transfer from the interlayer into the organic layer, resulting in n-type doping. The n-doping effect enhances the efficiency of electron injection and collection. Further, the maximum open-circuit voltage (Voc) was determined from its temperature dependence. For the device with Cs2CO3, the maximum Voc is extremely close to the corresponding value of the energy difference between the highest occupied molecular orbital of the electron donor and the lowest unoccupied molecular orbital of the electron acceptor, suggesting a better Ohmic contact. All evidences indicated that Cs2CO3 is a promising candidate as an interlayer to improve the device performance.
Intersubband absorption of quantum cascade laser structures and its application to laser modulation
Authors: Lyakh A. , Maulini R. , Tsekoun A. , Go R. , Patel C. K. N.
Published: May 2008

We have analyzed light absorption in a quantum cascade laser (QCL) structure under forward and reverse bias. Strong laser frequency absorption modulation is predicted and observed for the voltage variation within the high differential resistance voltage range. We propose to use this mechanism for monolithically integrated intracavity modulation of QCLs with suppressed thermal chirp. In addition, the described method allows for extraction of the intersubband absorption component of the total waveguide losses.
Phase transition in nanomagnetite
Authors: Dézsi I. , Fetzer Cs. , Gombkötő Á. , Szűcs I. , Gubicza J. , Ungár T.
Published: May 2008

Recently, the application of nanosized magnetite particles became an area of growing interest for their potential practical applications. Nanosized magnetite samples of 36 and 9nm sizes were synthesized. Special care was taken on the right stoichiometry of the magnetite particles. Mössbauer spectroscopy measurements were made in 4.2–300K temperature range. The temperature dependence of the intensities of the spectral components indicated size dependent transition taking place in a broad temperature range. For nanosized samples, the hyperfine interaction values and their relative intensities changed above the Verwey transition temperature value of bulk megnetite. The continuous transition indicated the formation of dendritelike granular assemblies formed during the preparation of the samples.
Dual electrode mode electrospinning of biodegradable polymers
Authors: Li Mei , He Yadong , Xin Chunling , Wei Xiaopeng , Li Qingchun , Lu Chunmeng , Juang Yi-Je
Published: May 2008

A high speed dual electrode mode electrospinning was developed in this study. With coexistence of both positively and negatively charged nozzles, the electrospun fibers interact and bridge grafting between nozzles, interconnect with each other, and are stretched on a movable collecting roll. The results demonstrate that a highly intertwined, three-dimensional isotropic network structure can be obtained by using the dual electrode mode. Moreover, the productivity was increased more than 170 times compared to that using the conventional single electrode mode process, which is essential for commercialization of the electrospinning process.
Quenching of local magnetic moment in oxygen adsorbed graphene nanoribbons
Authors: Veiga R. G. A. , Miwa R. H. , Srivastava G. P.
Published: May 2008

The electronic and magnetic properties of oxidized zigzag and armchair graphene nanoribbons, with hydrogen passivated edges, have been investigated from ab initio pseudopotential calculations within the density functional scheme. The oxygen molecule in its triplet state is adsorbed most stably at the edge of a zigzag nanoribbon. The Stoner metallic behavior of the ferromagnetic nanoribbons and the Slater insulating (ground state) behavior of the antiferromagnetic nanoribbons remain intact upon oxygen adsorption. The formation of a spin-paired C–O bond drastically reduces the local atomic magnetic moment of carbon at the edge of the ferromagnetic zigzag ribbon.
Photoluminescence and x-ray diffraction measurements of InN epifilms grown with varying In∕N ratio by plasma-assisted molecular-beam epitaxy
Authors: Yao Yong-zhao , Sekiguchi Takashi , Ohashi Naoki , Adachi Yutaka , Ohgaki Takeshi
Published: May 2008

Photoluminescence (PL) and x-ray diffraction (XRD) have been performed to examine the In∕N ratio effect on the InN films. The PL of N-rich InN are more asymmetric, broadened, and have higher energy compared with In-rich ones. An important finding is that the N-rich and In-rich InN show a sharp difference in their PL peak energies. XRD shows that lattice constant a is affected more by the In∕N ratio rather than lattice constant c. The PL behavior of N-rich and In-rich InN films are explained in terms of two different types of native point defects that dominate the two growth regimes.
Photoelectron spectroscopy of anions at 118.2nm: Observation of high electron binding energies in superhalogens MCl4− (M=Sc, Y, La)
Authors: Yang Jie , Wang Xue-Bin , Xing Xiao-Peng , Wang Lai-Sheng
Published: May 2008

High energy photon is needed for photoelectron spectroscopy (PES) of anions with high electron binding energies, such as superhalogens and O-rich metal oxide clusters. The highest energy photon used for anion PES in the laboratory has been 157nm (7.866eV) from F2 eximer lasers. Here, we report an anion PES experiment using coherent vacuum ultraviolet radiation at 118.2nm (10.488eV) by tripling the third harmonic output (355nm) of a Nd:YAG laser in a Xe∕Ar cell. Our study focuses on a set of superhalogen species, MCl4− (M=Sc, Y, La), which were expected to possess very high electron binding energies. While the 157nm photon can only access the ground state detachment features for these species, more transitions to the excited states at binding energies higher than 8eV are observed at 118.2nm. The adiabatic detachment energies are shown to be, 6.84, 7.02, and 7.03eV for ScCl4−, YCl4−, and LaCl4−eV, respectively, whereas their corresponding vertical detachment energies are measured to be 7.14, 7.31, and 7.38eV.
Synchronisation of canine germinal vesicle stage oocytes prior to in vitro maturation alters the kinetics of nuclear progression during subsequent resumption of meiosis
Authors: Hanna Carol , Menges Suzanne , Kraemer Duane , Long Charles R.
Published: May 2008

Inhibition of meiosis before in vitro maturation (IVM) can improve meiotic competence in immature mammalian oocytes. Therefore, meiosis-inhibiting agents were evaluated singularly for the ability to arrest and synchronise germinal vesicle (GV) stage canine oocytes, and the most effective treatments were combined to improve meiotic resumption rates. Oocytes cultured in 2 ng mL–1 oestradiol (E2), 10 IU mL–1 eCG, or both (EG) for 72 h resulted in significantly fewer oocytes resuming meiosis in EG than the control, E2, or with eCG. Oocytes cultured in 50 or 100 μmol L–1 of butyrolactone 1 or roscovitine (ROS) for up to 48 h did not resume meiosis nor increase subsequent meiotic resumption rates following IVM. A combination of 50 μmol L–1 ROS and EG treatment for 48 h significantly increased the proportion of canine oocytes in meiotic arrest. More importantly, following 48 h of IVM, ROS+EG-treated oocytes demonstrated a dramatic increase in the ability to resume meiosis compared with the non-treated controls (51.3 ± 8.2% and 10.8 ± 4.5%, respectively; P < 0.05). These data indicate that chemical and biological meiotic inhibitors are effective at inducing GV arrest in canine oocytes. Furthermore, these inhibitors are reversible and beneficial to subsequent meiotic resumption in vitro.