bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimating englacial radar attenuation using depth profiles of the returned power, central West Antarctica
Authors: Matsuoka K. , Morse D. , Raymond C. F.
Published: May 2010

We analyzed depth patterns of geometrically corrected returned power Pc from within the ice of central West Antarctica to develop a proxy for englacial radar attenuation. The depth patterns of [Pc]dB (Pc in the decibel scale) at individual sites were first approximated with least squares vertical gradients of the local mean [Pc]dB for five depth ranges. Variations of these gradients along radar tracks show identifiable trends but have local anomalous features over distances less than ∼5–10 km that are caused by smaller reflection from tilted internal layers above steep beds and other factors. Consequently, extraction of an attenuation proxy from the returned power requires mitigation of reflectivity variations. Next, returned power was synthesized only from bright layers assembled over distances much wider than the local anomalous features. Individual data ensembles show a clear upper (not lower) cutoff in [Pc]dB. The cutoff power decreases with depth linearly between ∼500 and ∼1600 m, which defines the upper envelope gradient. With the aid of attenuation and reflectivity modeling, we concluded that the upper envelope gradient can be an attenuation proxy in the isothermal ice that is expected in the upper half or more of central West Antarctica. The estimated attenuation rate in the upper ∼1600 m varies 5 dB km−1 (one way), equivalent to lateral temperature variations of about 2°C or chemistry variations of up to a factor of 2. This range indicates that the attenuation variations can significantly affect delineation of bed wetness on the basis of contrasts in power returned from the bed.
Microbial risk assessment in heterogeneous aquifers: 1. Pathogen transport
Authors: Molin S. , Cvetkovic V.
Published: May 2010

Pathogen transport in heterogeneous aquifers is investigated for microbial risk assessment. A point source with time‐dependent input of pathogens is assumed, exemplified as a simple on‐site sanitation installation, intermingled with water supply wells. Any pathogen transmission pathway (realization) to the receptor from a postulated infection hazard is viewed as a random event, with the hydraulic conductivity varying spatially. For aquifers where VAR[lnK] < 1 and the integral scale is finite, we provide relatively simple semianalytical expressions for pathogen transport that incorporate the colloid filtration theory. We test a wide range of Damkohler numbers in order to assess the significance of rate limitations on the aquifer barrier function. Even slow immobile inactivation may notably affect the retention of pathogens. Analytical estimators for microbial peak discharge are evaluated and are shown to be applicable using parameters representative of rotavirus and Hepatitis A with input of 10–20 days duration.
Severe Obesity Is Associated With Impaired Arterial Smooth Muscle Function in Young Adults
Authors: Ayer Julian G. , Harmer Jason A. , David Clementine , Steinbeck Katherine S. , Seale J. Paul , Celermajer David S.
Published: May 2010

The degree of arterial dilatation induced by exogenous nitrates (nitrate‐mediated dilatation, NMD) has been similar in obese and normal‐weight adults after single high‐dose glyceryl trinitrate (GTN). We examined whether NMD is impaired in obesity by performing a GTN dose‐response study, as this is a potentially more sensitive measure of arterial smooth muscle function. In this cross‐sectional study, subjects were 19 obese (age 31.0 ± 1.2 years, 10 male, BMI 44.1 ± 2.1) and 19 age‐ and sex‐matched normal‐weight (BMI 22.4 ± 0.4) young adults. Blood pressure (BP), triglycerides, high‐density lipoprotein (HDL), and low‐density lipoprotein (LDL)‐cholesterol, glucose, insulin, high‐sensitivity C‐reactive protein (hs‐CRP), carotid intima‐media thickness (CIMT), and flow‐mediated dilatation (FMD) were measured. After incremental doses of GTN, brachial artery maximal percent dilatation (maximal NMD) and the area under the dose‐response curve (NMD AUC) were calculated. Maximal NMD (13.4 ± 0.9% vs. 18.3 ± 1.1%, P = 0.002) and NMD AUC (54,316 ± 362 vs. 55,613 ± 375, P = 0.018) were lower in obese subjects. The obese had significantly higher hs‐CRP, insulin, and CIMT and lower HDL‐cholesterol. Significant bivariate associations existed between maximal NMD or NMD AUC and BMI‐group (r = −0.492, P = 0.001 or r = −0.383, P = 0.009), hs‐CRP (r = −0.419, P = 0.004 or r = −0.351, P = 0.015), and HDL‐cholesterol (r = 0.374, P = 0.01 or r = 0.270, P = 0.05). On multivariate analysis, higher BMI‐group remained as the only significant determinant of maximal NMD (r2 = 0.242, β = −0.492, P = 0.002) and NMD AUC (r2 = 0.147, β = −0.383, P = 0.023). In conclusion, arterial smooth muscle function is significantly impaired in the obese. This may be important in their increased cardiovascular risk.
Association of Maternal Short Sleep Duration With Adiposity and Cardiometabolic Status at 3 Years Postpartum
Authors: Taveras Elsie M. , Rifas‐Shiman Sheryl L. , Rich‐Edwards Janet W. , Gunderson Erica P. , Stuebe Alison M. , Mantzoros Christos S.
Published: May 2010

The purpose of this study was to examine the association of short sleep duration among women in the first year postpartum with adiposity and cardiometabolic status at 3 years postpartum. We studied 586 women in Project Viva, a prospective cohort. At 6 months and 1 year postpartum, women reported the number of hours they slept in a 24‐h period, from which we calculated a weighted average of daily sleep. We used multivariable regression analyses to predict the independent effects of short sleep duration (≤5 h/day vs. >5 h/day) on adiposity, glucose metabolism, lipid metabolism, and adipokines at 3 years postpartum. Women's mean (s.d.) hours of daily sleep in the first year postpartum was 6.7 (0.97) h. After adjusting for age, race/ethnicity, education, parity, prepregnancy BMI, and excessive gestational weight gain, we found that postpartum sleep ≤5 h/day was associated with higher postpartum weight retention (β 1.50 kg; 95% confidence interval (CI): 0.02, 2.86), higher subscapular + triceps skinfold thickness (β 3.94 mm; 95% CI: 1.27, 6.60) and higher waist circumference (β 3.10 cm; 95% CI: 1.25, 4.94) at 3 years postpartum. We did not observe associations of short sleep duration with measures of cardiometabolic status at 3 years postpartum. In conclusion, short sleep duration in the first year postpartum is associated with higher adiposity at 3 years postpartum.
A new sawtooth control mechanism relying on toroidally propagating ion cyclotron resonance frequency waves: Theory and Joint European Torus tokamak experimental evidence
Authors: Graves J. P. , Chapman I. T. , Coda S. , Johnson T. , Lennholm M. ,   JET-EFDA Contributors
Published: May 2010

The sawtooth control mechanism in plasmas employing toroidally propagating ion cyclotron resonance waves is extended. The asymmetrically distributed energetic passing ions are shown to modify the ideal internal kink mode when the position of the minority ion cyclotron resonance resides within a narrow region close to the q=1 surface. An analytical treatment of the internal kink mode in the presence of model distribution function with parallel velocity asymmetry is developed. The fast ion mechanism explains the strong sensitivity of sawteeth to resonance position, and moreover is consistent with dedicated Joint European Torus [F. Romanelli, Nucl. Fusion 49, 104006 (2009)] experiments which controlled sawteeth despite negligible current drive.
Synthesis, characterizations, and thermal induced structural transformation of silver-fullerene C60 nanocomposite thin films for applications in optical devices
Authors: Singhal R. , Agarwal D. C. , Mishra Y. K. , Kabiraj D. , Mattei G. , Pivin J. C. , Chandra R. , Avasthi D. K.
Published: May 2010

Nanocomposite thin films of fullerene C60 containing Ag nanoparticles (NPs) were synthesized by thermal codeposition. The surface plasmon resonance (SPR) band of the nanocomposite film was observed in the region 450–550 nm, showing a large redshift with increasing metal concentration. This is explained by the Maxwell–Garnett effective medium theory considering the absorbing nature of fullerene C60. The C60–Ag nanocomposite thin film with lowest Ag concentration was annealed at increasing temperatures in neutral atmosphere. The SPR band showed first a small redshift after annealing at small temperature then progressive blueshift at higher temperature. This behavior of SPR is explained by the increased particle–particle interaction due to the compaction of the fullerene C60 film upon annealing at low temperature and the transformation of fullerene C60 matrix into amorphous carbon at higher temperature. Rutherford backscattering spectrometry and transmission electron microscopy were used to quantify Ag metal content and the microstructural evolution of Ag NPs in the nanocomposite films, respectively. Thermal induced structural transformations of fullerene C60 molecules of host matrix were investigated by Raman spectroscopy.
Transient effects controlling the charge carrier population of organic field effect transistor channels
Authors: Chang Hsiu-Chuang , Ruden P. Paul , Liang Yan , Frisbie C. Daniel
Published: May 2010

We present device simulations exploring the effects of traps during transient processes in the conducting channel of organic field effect transistors (OFETs). The device structure explored resembles a typical organic thin-film transistor with one of the channel contacts removed. However, the channel length is much longer than in typical OFETs in order to increase the transit time. By measuring the displacement current in these long-channel capacitors, transient effects in the carrier transport in organic semiconductors may be studied. When carriers are injected into the device, a conducting channel is established while traps, which are initially empty, are being populated. The filling of the traps then modifies the transport characteristics of the injected charge carriers. In contrast, dc experiments as they are typically performed to characterize the transport properties of organic semiconductor channels investigate a steady state with traps partially filled. Numerical and approximate analytical models for the formation of the conducting channel and the resulting displacement current are discussed here. The temperature dependence of the effective mobility arising from the temperature dependence of the trap emission rate is explored, and calculated results are compared with experimental data. We show that displacement current measurements on OFET structures provide unique opportunities for the study of trap dynamics involving a wide range of time scales.
Cyclic operation of a MEMS-based resonant micro heat engine: Experiment and model
Authors: Bardaweel H. , Richards R. , Richards C. , Anderson M.
Published: May 2010

The operation of a MEMS-based micro heat engine at resonant and off-resonant conditions is presented. Both model and experiment are used to investigate resonant and off-resonant operation of the engine. In this work, we look at the pressure-volume (PV) diagrams of an engine operated at resonance and off-resonance. Model predictions of the PV diagram are in favorable agreement with measured data. The results show that resonant operation is beneficial. At resonance, the pressure and volume in the engine cavity are decoupled and more mechanical work is observed. The PV diagram describes an elliptical shape. However, for an off-resonant operation the pressure and volume become more coupled and less mechanical work is observed. The PV diagram is described by a sigmoidal shape.
Study of the characteristics of a piezoelectric lead zirconate titanate radiation detector using a pulsed xenon source
Authors: Miyachi Takashi , Fujii Masayuki , Hasebe Nobuyuki , Okudaira Osamu , Takechi Seiji , Kurozumi Atsuma , Morinaga Shinya , Uno Takefumi , Shibata Hiromi , Kobayashi Masanori , Murakami Takeshi , Uchihori Yukio , Okada Nagaya
Published: May 2010

The detector characteristics of piezoelectric lead zirconate titanate (PZT) were studied by directly irradiating a multilayered PZT detector with 400 MeV/n xenon ions. An extracted beam was processed with a rotating slit. Thus, passed through ∼103 xenon ions were available for 50 to 250 μs. The effect of polarization on the output signal was discussed, and the optimal electrode configuration was determined. The output signal appeared as an isolated pulse whose amplitude was qualitatively understood by the Bethe–Bloch formula. However, the calculated and the observed values differed depending on the rotation speed of the slit. A process that can explain the differences is presented here. The output signal appearing beyond the range of 400 MeV/n xenon ion beam was discussed. The sensitivity was compared with that obtained with hypervelocity collision of dust.
Charge carrier density of organics with Gaussian density of states: Analytical approximation for the Gauss–Fermi integral
Authors: Paasch Gernot , Scheinert Susanne
Published: May 2010

Active layers in organic devices prepared in solution based preparation routes are usually disordered. Their highest occupied molecular orbitals and lowest unoccupied molecular orbitals, or the valence and conduction band states, show an energetic distribution which can be approximated by a Gaussian density of states (DOS). The resulting dependency of the (electron and hole) mobility on temperature, carrier density, and field can be easily implemented into advanced device simulation programs. However, in addition the charge carrier density is needed as the integral over the DOS multiplied with the Fermi–Dirac distribution. We denote this normalized quantity as the Gauss–Fermi integral. Since it cannot be evaluated analytically, similarly as in the case of the Fermi–Dirac integral F1/2, an analytical approximation is needed for efficient device simulation. In the present article, such an approximation is proposed with different expressions in the nondegenerate and degenerate regions with a continuous and differentiable transition between both regions. The approximation is also applicable to traps with a Gaussian DOS.
Strain relaxation of epitaxial (Ba0.6Sr0.4)(Zr0.3Ti0.7)O3 thin films grown on SrTiO3 substrates by pulsed laser deposition
Authors: Vayunandana Reddy Y. K. , Wolfman Jérôme , Autret-Lambert Cécile , Gervais Monique , Gervais François
Published: May 2010

High crystalline quality (Ba0.6Sr0.4)(Zr0.3Ti0.7)O3 (BSTZ) thin films were epitaxially grown by pulsed laser deposition on (001) SrTiO3 single crystal substrates. Their epitaxial nature was revealed by x-ray and electron diffraction. Thinnest film (∼9 nm) has largest out-of-plane lattice constant (4.135 Å) and tetragonality (1.06). Films are under compressive strain. Film thicknesses above ∼9 nm were started to relax as revealed from reciprocal space mapping. Thicknesses deduced from x-ray diffraction and transmission electron microscopy methods are in good agreement. Critical thickness to relieve strain of the BSTZ film is about 7.6±0.4 nm.
Nd
3
+
-sensitized upconversion white light emission of Tm3+/Ho3+ bridged by Yb3+ in β-YF3 nanocrystals embedded transparent glass ceramics
Authors: Lin Hang , Chen Daqin , Yu Yunlong , Shan Zhifa , Huang Ping , Wang Yuansheng , Yuan Junlin
Published: May 2010

Red (Ho3+:F55→I58), green (Ho3+:S52,F54→I58), and blue (Tm3+:D12→F34 and G14→H36) upconversion emissions were simultaneously generated in the transparent glass ceramics containing Nd3+/Yb3+/Tm3+/Ho3+:β-YF3 nanocrystals under 796 nm excitation. It was experimentally evidenced that Yb3+ ions acted as the bridging centers to prompt energy transfer from Nd3+ to Tm3+ and Ho3+. With appropriately optimizing the contents and relative ratios of the doped rare earth species, luminescence of various colors, including bright white light, was easily tuned in the glass ceramics. The related upconversion mechanisms were proposed.
A mathematical model of the modified Paschen’s curve for breakdown in microscale gaps
Authors: Go David B. , Pohlman Daniel A.
Published: May 2010

Traditionally, Paschen’s curve has been used to describe the breakdown voltage for gaseous ionization between two electrodes. However, experiments have shown that Paschen’s curve, which is based on Townsend effects, is not necessarily accurate in describing breakdown between electrodes spaced less than 15 μm apart. In this regime, electron field emission plays a significant role in the breakdown phenomenon, and recently an alternative mathematical description that accounts for ion-enhanced field emission was proposed to describe the breakdown voltage in small gaps. However, both Paschen’s curve and the small gap equation only work in certain regimes, and neither predicts the transition that occurs between Townsend and field emission effects—the so-called modified Paschen’s curve. In this work, a single, consistent mathematical description of the breakdown voltage is proposed that accounts for both Townsend ionization and ion-enhanced field emission mechanisms. Additionally, microscale breakdown experiments have been conducted in atmospheric air. The proposed formulation is compared to the present experiments and other atmospheric air experiments in the literature and describes the transition region in the breakdown curve. The proposed formulation represents a mathematical model for the modified Paschen’s curve.
Modification of edge mode dynamics by oxidation in Ni80Fe20 thin film edges
Authors: Zhu M. , McMichael R. D.
Published: May 2010

We use “edge mode” ferromagnetic resonance to probe the magnetic properties of oxidized Ni80Fe20 (Py) nanostripe edges. The oxidation is carried out using either oxygen plasma or thermal annealing in an oxygen ambient. We find that for both treatments the edge saturation field decreases systematically with increasing oxidation due to reduced magnetization near the edges. However, the change of effective out-of-plane anisotropy field shows opposite trends for these two oxidation methods. Micromagnetic simulations suggest that thermally annealed samples may have an additional reduction in the bulk magnetization, possibly due to a compositional change in Py stripes. The two distinct oxidation profiles also result in different changes in resonance linewidth; we find little damping change for plasma treatment but an enhanced effective damping for thermal annealing.
Current-induced domain wall motion in Co/Pt nanowires: Separating spin torque and Oersted-field effects
Authors: Heinen J. , Boulle O. , Rousseau K. , Malinowski G. , Kläui M. , Swagten H. J. M. , Koopmans B. , Ulysse C. , Faini G.
Published: May 2010

We report on low temperature current induced domain wall depinning experiments on (Co/Pt) multilayer nanowires with perpendicular magnetization. Using a special experimental scheme, we are able to extract the different contributions of the Oersted field and spin torque from the dependence of the depinning field on the injected current for selected magnetization configurations. The spin torque contribution is found to be dominant with a small contribution of the Oersted field leading to a nonadiabaticity factor β in line with previous measurements.
Kinetic processes in supercooled monosaccharides upon melting: Application of dielectric spectroscopy in the mutarotation studies of D-ribose
Authors: Wlodarczyk P. , Kaminski K. , Haracz S. , Dulski M. , Paluch M. , Ziolo J. , Wygledowska-Kania M.
Published: May 2010

Dielectric spectroscopy has been recently used to monitor mutarotation in undercooled D-fructose. This method can be viewed as a universal method to study mutarotation phenomenon in the whole family of monosaccharides. In this paper, we studied kinetics of mutaration of anhydrous D-ribose at ambient pressure as well as pressure effect on the rate constant of this process. Ribose mutarotation behavior is compared to the one obtained for D-fructose. In addition, we attempted to determine the “direction” of mutarotation in undercooled monosaccharides after quenching the melted sample. To this end, analysis of dipole moments of different tautomers of D-fructose and D-ribose have been performed. Conformational analysis of studied carbohydrates was done with use of density functional theory. Geometry optimizations as well as calculations of dipole moments were done on the 6-311++G(d,p)/B3LYP level. Finally, it turned out that data obtained from the mutarotation experiment might be helpful in understanding the origin of γ-process occurring in the whole family of carbohydrates.
Growth kinetics and electronic properties of unintentionally doped semi-insulating GaN on SiC and high-resistivity GaN on sapphire grown by ammonia molecular-beam epitaxy
Authors: Tang H. , Fang Z. Q. , Rolfe S. , Bardwell J. A. , Raymond S.
Published: May 2010

Growth of unintentionally doped (UID) semi-insulating GaN on SiC and highly resistive GaN on sapphire using the ammonia molecular-beam epitaxy technique is reported. The semi-insulating UID GaN on SiC shows room temperature (RT) resistivity of 1011 Ω cm and well defined activation energy of 1.0 eV. The balance of compensation of unintentional donors and acceptors is such that the Fermi level is lowered to midgap, and controlled by a 1.0 eV deep level defect, which is thought to be related to the nitrogen antisite NGa, similar to the “EL2” center (arsenic antisite) in unintentionally doped semi-insulating GaAs. The highly resistive GaN on sapphire shows RT resistivity in range of 106–109 Ω cm and activation energy varying from 0.25 to 0.9 eV. In this case, the compensation of shallow donors is incomplete, and the Fermi level is controlled by levels shallower than the 1.0 eV deep centers. The growth mechanisms for the resistive UID GaN materials were investigated by experimental studies of the surface kinetics during growth. The required growth regime involves a moderate growth temperature range of 740–780 °C, and a high ammonia flux (beam equivalent pressure of 1×10−4 Torr), which ensures supersaturated coverage of surface adsorption sites with NHx radicals. Such highly nitrogen rich growth conditions lead to two-dimensional layer by layer growth and reduced oxygen incorporation.
Localized vibrational modes in bars and plates
Authors: Ma Jing , Maris Humphrey J.
Published: May 2010

We consider the localized vibrational modes that can exist at the edge of a semi-infinite plate and at the end of a semi-infinite bar of small thickness. It is known that for certain special values of Poisson’s ratio σ these modes are perfectly localized, are uncoupled to bulk modes, and thus do not lose energy by acoustic radiation. We show that for other values of σ it is possible to modify the shape of the end of the plate or bar in a way such that a perfectly localized edge mode is formed. Finally, we discuss the effect of this localization phenomenon on the vibrational modes of plates and bars of finite length.
A precise high-resolution near infrared continuous wave cavity ringdown spectrometer using a Fourier transform based wavelength calibration
Authors: Fehling Carsten , Friedrichs Gernot
Published: May 2010

A wavelength calibration technique is described, which is based on a combination of a Fourier transform wavelength meter and a distributed feedback laser locked to a molecular transition as a frequency marker in the spectrum. The technique provides a reliable wavelength scale to be used in high resolution continuous wave cavity ringdown spectroscopy without need for stabilization of the probe laser and accurately known molecular transitions in the scanned wavelength range. Due to a continuous reference measurement, ambient influences on the laser sources are effectively suppressed. As an example, we measured highly resolved cavity ringdown spectra of N2O isotopomers and determined the line strength of several absorption lines at a wavelength around 1687 nm. A near infrared wavelength precision of 6×10−8 and an absolute accuracy on the order of 1×10−7 was readily achieved. The general concept is easy to implement and can be further refined by using additional reference lasers, thus holding the potential of even higher wavelength accuracy.
Mechanisms controlling the phase and dislocation density in epitaxial silicon films grown from silane below 800 °C
Authors: Teplin Charles W. , Alberi Kirstin , Shub Maxim , Beall Carolyn , Martin Ina T. , Romero Manuel J. , Young David L. , Reedy Robert C. , Stradins Paul , Branz Howard M.
Published: May 2010

We construct a phase diagram for silicon layer growth on (001) Si by hot-wire chemical vapor deposition (HWCVD), for rates from 10 to 150 nm/min and for substrate temperatures from 500 to 800 °C. Our results show that a mixed mono and dihydride surface termination during growth causes polycrystalline growth; some H-free sites are needed for epitaxy. For epitaxial films (T>620 °C), the dislocation density decreases with increasing growth temperature because of reduced O contamination of the surface. The best HWCVD epitaxial layers have dislocation densities of 105 cm−2.
Stripe/kink microstructures formed in mechanical peeling of highly orientated pyrolytic graphite
Authors: Liu Ze , Zheng Quan-shui , Liu Jefferson Z.
Published: May 2010

Mechanical exfoliation is nowadays the primary method to produce isolated graphenes. A stripe/kink microstructure is observed in our graphite flakes produced by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG). It composes a series of parallel stripes with width of about 100 microns separated by kinking microstructures (∼2 microns) in the graphite flake plane. The formation of such structure is attributed to the sliding between adjacent layers of the HOPG under the mechanical peeling. A theoretical model is presented to understand the persistence of such kinking structures in terms of the interlayer shear force locking effect.
XYG3s: Speedup of the XYG3 fifth-rung density functional with scaling-all-correlation method
Authors: Zhang Igor Ying , Luo Yi , Xu Xin
Published: May 2010

Recently, we proposed a new version of doubly hybrid functional, XYG3 [Y. Zhang, X. Xu, and W. A. Goddard III, Proc. Natl. Acad. Sci. U.S.A. 106, 4963 (2009)], which not only has a nonlocal orbital-dependent component in the exchange term (Hartree–Fock-like exchange), but also contains information about the unoccupied Kohn–Sham orbitals in the correlation part (PT2 double excitation). In the present work, we examine the frozen-core approximation and Truhlar’s scaling-all-correlation (SAC) method in order to accelerate the PT2 evaluations. We adopt the SAC idea and extend XYG3 to propose XYG3s that uses a uniform scaling parameter for the PT2 correlation. We conclude that XYG3s is both fast and accurate for thermochemistry, bond dissociation enthalpies, reaction barrier heights, and nonbonded interactions of main group molecules. In addition, the accuracy remains nearly constant with system size.
Potential energy and dipole moment surfaces of H3− molecule
Authors: Ayouz M. , Dulieu O. , Guérout R. , Robert J. , Kokoouline V.
Published: May 2010

A new potential energy surface for the electronic ground state of the simplest triatomic anion H3− is determined for a large number of geometries. Its accuracy is improved at short and large distances compared to previous studies. The permanent dipole moment surface of the state is also computed for the first time. Nine vibrational levels of H3− and 14 levels of D3− are obtained, bound by at most ∼70 and ∼126 cm−1, respectively. These results should guide the spectroscopic search of the H3− ion in cold gases (below 100K) of molecular hydrogen in the presence of H− ions.
Direct evaluation of the temperature dependence of the rate constant based on the quantum instanton approximation
Authors: Buchowiecki Marcin , Vaníček Jiří
Published: May 2010

A general method for the direct evaluation of the temperature dependence of the quantum-mechanical reaction rate constant in many-dimensional systems is described. The method is based on the quantum instanton approximation for the rate constant, thermodynamic integration with respect to the inverse temperature, and the path integral Monte Carlo evaluation. It can describe deviations from the Arrhenius law due to the coupling of rotations and vibrations, zero-point energy, tunneling, corner-cutting, and other nuclear quantum effects. The method is tested on the Eckart barrier and the full-dimensional H+H2→H2+H reaction. In the temperature range from 300 to 1500 K, the error of the present method remains within 13% despite the very large deviations from the Arrhenius law. The direct approach makes the calculations much more efficient, and the efficiency is increased even further (by up to two orders of magnitude in the studied reactions) by using optimal estimators for reactant and transition state thermal energies. Which of the estimators is optimal, however, depends on the system and the strength of constraint in a constrained simulation.
Large-eddy simulation of turbulent collision of heavy particles in isotropic turbulence
Authors: Jin Guodong , He Guo-Wei , Wang Lian-Ping
Published: May 2010

The small-scale motions relevant to the collision of heavy particles represent a general challenge to the conventional large-eddy simulation (LES) of turbulent particle-laden flows. As a first step toward addressing this challenge, we examine the capability of the LES method with an eddy viscosity subgrid scale (SGS) model to predict the collision-related statistics such as the particle radial distribution function at contact, the radial relative velocity at contact, and the collision rate for a wide range of particle Stokes numbers. Data from direct numerical simulation (DNS) are used as a benchmark to evaluate the LES using both a priori and a posteriori tests. It is shown that, without the SGS motions, LES cannot accurately predict the particle-pair statistics for heavy particles with small and intermediate Stokes numbers, and a large relative error in collision rate (up to 60%) may arise when the particle Stokes number is near StK=0.5. The errors from the filtering operation and the SGS model are evaluated separately using the filtered-DNS (FDNS) and LES flow fields. The errors increase with the filter width and have nonmonotonic variations with the particle Stokes numbers. It is concluded that the error due to filtering dominates the overall error in LES for most particle Stokes numbers. It is found that the overall collision rate can be reasonably predicted by both FDNS and LES for StK>3. Our analysis suggests that, for StK<3, a particle SGS model must include the effects of SGS motions on the turbulent collision of heavy particles. The spectral analysis of the concentration fields of the particles with different Stokes numbers further demonstrates the important effects of the small-scale motions on the preferential concentration of the particles with small Stokes numbers.
Magnetotransport properties of Mn–Si–C based nanostructures
Authors: Kang Sungmu , Brewer Greg A. , Jugdersuren Battogtokh , DiPietro Robert , Heiman Don , Buechele Andrew C. , McKeown David A. , Pegg Ian L. , Philip John
Published: May 2010

Boron-incorporated Mn5SiC nanowires were grown using chemical vapor deposition method. The nanowire cluster exhibits magnetic hysteresis loops at room temperature and the strength of the magnetic behavior depends on the concentration of the boron incorporation. Mn5SiC nanowire-based devices exhibit spin dependent transport properties which shows significant changes with boron content. Large magnetoresistance is observed in lightly boron-incorporated nanowire devices and it decreases with increase in boron content.
The mechanical and the electrical properties of conducting polypyrrole fibers
Authors: Foroughi J. , Ghorbani S. R. , Peleckis G. , Spinks G. M. , Wallace G. G. , Wang X. L. , Dou S. X.
Published: May 2010

The mechanical and the electrical properties of polypyrrole (PPy) fibers and electrochemically deposited PPy films were studied. It was found that the PPy fibers showed a significantly higher strength than the PPy films due to better orientation of the molecular structure. The electrochemically prepared PPy films had a higher electrical conductivity than that of the fibers at high temperature. At low temperature, the PPy fibers showed the higher conductivity. The conductivity results were analyzed in the frame of the three-dimensional variable range hopping model. The results showed that at room temperature the average hopping distance for the fibers was about 4 Å while for the films it increases to about 5.7 Å. This corresponds to about 1 and 2 monomer units in length for the fiber and film samples, respectively.
On the nonlinearity of the Langmuir turbulence excited by a weak electron beam-plasma interaction
Authors: Nariyuki Y. , Umeda T.
Published: May 2010

In the present study, we analyze the data sets produced by a one-dimensional Vlasov–Poisson simulation of the weak electron beam-plasma instability to clarify the nonlinearity of the Langmuir turbulence excited by the weak-beam interaction. The growth of wave number modes is analyzed by using the momentum equation of the whole electrons. The analysis shows that the primary Langmuir wave mode is almost linear, while the nonlinear terms play important roles in the growth of the lower harmonic mode and the secondary higher harmonic mode. After the linear growth saturates, while the wave power of the primary mode is much larger than the other modes, linear and nonlinear interactions occurring in both lower harmonic and secondary higher harmonic modes are more active than those in the primary mode. Nonlinearity in the system comes from the advection rather than the ponderomotive forces.
Aufbau derived from a unified treatment of occupation numbers in Hartree–Fock, Kohn–Sham, and natural orbital theories with the Karush–Kuhn–Tucker conditions for the inequality constraints ni≤1 and ni≥
Authors: Giesbertz K. J. H. , Baerends E. J.
Published: May 2010

In the major independent particle models of electronic structure theory—Hartree–Fock, Kohn–Sham (KS), and natural orbital (NO) theories—occupations are constrained to 0 and 1 or to the interval [0,1]. We carry out a constrained optimization of the orbitals and occupation numbers with application of the usual equality constraints ∑i∞ ni=N and ⟨ϕi|ϕj⟩=δij. The occupation number optimization is carried out, allowing for fractional occupations, with the inequality constraints ni≥0 and ni≤1 with the Karush–Kuhn–Tucker method. This leads in all cases to an orbital energy spectrum with (only for NO and KS) possibly fractionally occupied degenerate levels at energy equal to the Lagrange multiplier ϵ for the first equality constraint, completely occupied levels at lower energies and completely unoccupied levels at higher energies. Aufbau thus follows in all cases directly from this general derivation.
C
-axis critical current of a PrFeAsO0.7 single crystal
Authors: Kashiwaya H. , Shirai K. , Matsumoto T. , Shibata H. , Kambara H. , Ishikado M. , Eisaki H. , Iyo A. , Shamoto S. , Kurosawa I. , Kashiwaya S.
Published: May 2010

The c-axis transport properties of a high-pressure synthesized PrFeAsO0.7 single crystal are studied using s-shaped junctions. Resistivity anisotropy of about 120 detected at 50 K shows the presence of strong anisotropy in the electronic states. The obtained critical current density for the c-axis of 2.9×105 A/cm2 is two orders of magnitude larger than that in Bi2Sr1.6La0.4CuO6+δ. The appearance of a hysteresis in the current-voltage curve below Tc is the manifestation of the intrinsic Josephson effect similar to that in cuprate superconductors. The suppression of the critical current-normal resistance (IcRn) product is explained by an inspecular transport in s±-wave pair potential.
Performance improvement of phase change memory cell by using a cerium dioxide buffer layer
Authors: Shang Fei , Zhai Jiwei , Song Sannian , Song Zhitang , Wang Changzhou
Published: May 2010

The performance of phase change memory (PCM) cell, based on Ge2Sb2Te5, was significantly improved by using a CeO2 buffer layer. The presence of a buffer layer remarkably reduced the reset voltage of the PCM cell. Even at voltage pulse width of 10 ns, the buffered PCM cell could accomplish reset operation. Theoretical thermal simulation of reset process was conducted. It is concluded that the improved performance of the PCM cell with a CeO2 buffer layer can be attributed to the fact that the buffer layer not only acted as heating layer but also reduced efficiently the cell dissipated power.
Communications: A rigorous transition state based approach to state-specific reaction dynamics: Full-dimensional calculations for H+CH4→H2+CH3
Authors: Schiffel Gerd , Manthe Uwe
Published: May 2010

Full-dimensional quantum dynamics calculations studying all initial state-selected reaction probabilities of the H+CH4→H2+CH3 reaction relevant at total energies below 0.58 eV are presented. The calculations employ a flux correlation function based approach to obtain the initial state-selected reaction probabilities: A complete set of wavepackets is generated at the top of the reaction barrier and propagated into the reactant asymptotic region. The results obtained show that rotational excitation decreases the reaction probability even when comparing reaction probabilities at equivalent collision energies. The efficiency of different types of reactant vibrational energy in supporting the reaction processes can nicely be explained by a transition state view which considers the vibrational states of the activated complex.
Liquid cell with plasmon lenses for surface enhanced Raman spectroscopy
Authors: Vlasko-Vlasov V. , Joshi-Imre A. , Bahns J. T. , Chen L. , Ocola L. , Welp U.
Published: May 2010

High-fidelity surface enhanced Raman spectra (SERS) of Rhodamine 6G and 2-mercaptopyrimidine liquid solutions are measured using a microfluidic delivery system constructed on a flat silver substrate. Microscopic plasmon lenses patterned in the silver film focus surface plasmons into a subwavelength spot which yields the light amplification required for SERS. The system provides an efficiency similar to traditional colloidal substrates, and allows multiple sample loading. We find that the main contribution to the spectra comes from the molecules directly attached to the silver surface, which gives strong evidence for the chemical enhancement of SERS.
Terahertz optical sideband emission in self-assembled quantum dots
Authors: Sandall I. C. , Porter N. E. , Wagner M. , Schneider H. , Winnerl S. , Helm M. , Wilson L.
Published: May 2010

A multilayer quantum dot sample has been excited with a strong terahertz (THz) electric field and probed with a near-infrared (NIR) laser. First- and second-order THz optical sidebands are generated on the NIR probe beam by driving quantum dot intersublevel resonances with the THz fields. A conversion efficiency of 3×10−6 was obtained for the conversion of NIR power into sideband emission at 4 K, decreasing by a factor of 20 up to room temperature. The sideband emission wavelength can be tuned over ∼20 nm by selection of appropriate NIR and THz frequencies, due to the inhomogeneous broadening of the dot ensemble.
Supercooling of aqueous dimethylsulfoxide solution at normal and high pressures: Evidence for the coexistence of phase-separated aqueous dimethylsulfoxide solutions of different water structures
Authors: Kanno H. , Kajiwara K. , Miyata K.
Published: May 2010

Supercooling behavior of aqueous dimethylsulfoxide (DMSO) solution was investigated as a function of DMSO concentration and at high pressures. A linear relationship was observed for TH (homogeneous ice nucleation temperature) and Tm (melting temperature) for the supercooling of aqueous DMSO solution at normal pressure. Analysis of the DTA (differential thermal analysis) traces for homogeneous ice crystallization in the bottom region of the TH curve for a DMSO solution of R=20 (R: moles of water/moles of DMSO) at high pressures supported the contention that the second critical point (SCP) of liquid water should exist at Pc2=∼200 MPa and at Tc2<−100 °C (Pc2: pressure of SCP, Tc2: temperature of SCP). The presence of two TH peaks for DMSO solutions (R=15, 12, and 10) suggests that phase separation occurs in aqueous DMSO solution (R≤15) at high pressures and low temperatures (<−90 °C). The pressure dependence of the two TH curves for DMSO solutions of R=10 and 12 indicates that the two phase-separated components in the DMSO solution of R=10 have different liquid water structures [LDL-like and HDL-like structures (LDL: low-density liquid water, HDL: high-density liquid water)] in the pressure range of 120–230 MPa.
Anisotropy of electrical conductivity in a pentacene crystal grain on SiO2 evaluated by atomic-force-microscope potentiometry and electrostatic simulation
Authors: Ohashi Noboru , Tomii Hiroshi , Sakai Masatoshi , Kudo Kazuhiro , Nakamura Masakazu
Published: May 2010

Conductivity anisotropy in a crystal grain of thin-film-phase pentacene has been estimated by a combination of atomic-force-microscope potentiometry (AFMP) and electrostatic simulation. The surface potential distribution and topography of a grain in a working pentacene thin-film transistor are simultaneously measured by AFMP. Then, the nonlinear potential profile due to the thickness variation is simulated by changing the anisotropic ratio of conductivity. The anisotropic ratio (corresponding to the anisotropy of carrier drift mobility) is estimated to be σx:σz=45:1, where x is the horizontal (harmonic mean of those in a- and b-axes) direction and z is the vertical (c-axis) direction.
Dual rf-optical slot waveguide for ultrabroadband modulation with a subvolt Vπ
Authors: Shi Shouyuan , Prather Dennis W.
Published: May 2010

In this letter, we propose a high speed and ultralow driving voltage traveling wave electro-optic (EO) modulator based on a dual horizontal slot waveguide. The proposed design harnesses the unique properties of ferroelectric materials such as LiNBO3 and exotic, organic EO polymers to tightly confine both the optical and rf modes in a nanoscale slot maximizing the nonlinear interaction with the electric field. Larger electrode separation allows for significant reduction in rf propagation loss. Simulations of the half-wavelength voltage-length product and electro-optic response of the proposed device reveal ultrabroadband operation, up to 250 GHz, and subvolt driving voltage for a 1 cm long modulator.
High-temperature thermoelectric properties of thallium-filled skutterudites
Authors: Harnwunggmoung Adul , Kurosaki Ken , Muta Hiroaki , Yamanaka Shinsuke
Published: May 2010

The high temperature thermoelectric (TE) properties of polycrystalline Tl-filled skutterudites TlxCo4Sb12 (x=0, 0.05, 0.10, 0.15, 0.20, and 0.25) were examined from room temperature to 750 K. All samples exhibited negative Seebeck coefficients. The electrical resistivity, absolute values of the Seebeck coefficient, and the lattice thermal conductivity were found to decrease with increasing Tl content. Tl0.25Co4Sb12 exhibited the best TE performance; the maximum value for the dimensionless figure of merit ZT was 0.90 at 600 K.
Reduced low frequency noise in electron beam evaporated MgO magnetic tunnel junctions
Authors: Diao Z. , Feng J. F. , Kurt H. , Feng G. , Coey J. M. D.
Published: May 2010

We compare low frequency noise in magnetic tunnel junctions with MgO barriers prepared by electron-beam evaporation with those prepared by radiofrequency sputtering, both showing a high tunneling magnetoresistance. The normalized noise parameter in the parallel state of junctions with evaporated barriers is at least one order of magnitude lower than that in junctions with sputtered barriers, and exhibits a weaker bias dependence. The lowest normalized noise is in the 10−11 μm2 range. A lower density of oxygen vacancies acting as charge trap states in the evaporated MgO is responsible for the lower noise.
Terahertz surface plasmon propagation in nanoporous silicon layers
Authors: Lo Shu-Zee A. , Murphy Thomas E.
Published: May 2010

We describe the fabrication and measurement of a terahertz surface plasmon waveguide in which the optical mode is localized within a nanoporous silicon slab. We compare the propagation characteristics among waveguides with different porous layer thickness, and present an analytical model that accurately describes the dispersion and loss in the waveguides.
Spectroscopic signatures of proton transfer dynamics in the water dimer cation
Authors: Kamarchik Eugene , Kostko Oleg , Bowman Joel M. , Ahmed Musahid , Krylov Anna I.
Published: May 2010

Using full-dimensional EOM-IP-CCSD/aug-cc-pVTZ potential energy surfaces, the photoelectron spectrum, vibrational structure, and ionization dynamics of the water dimer radical cation, (H2O)2+, were computed. We also report an experimental photoelectron spectrum which is derived from photoionization efficiency measurements and compares favorably with the theoretical spectrum. The vibrational structure is also compared to the recent experimental work of Gardenier et al. [J. Phys. Chem. A 113, 4772 (2009)] and the recent theoretical calculations by Cheng et al. [J. Phys. Chem. A 113, 13779 (2009)]. A reduced-dimensionality nuclear Hamiltonian was used to compute the ionization dynamics for both the ground state and first excited state of the cation. The dynamics show markedly different behavior and spectroscopic signatures depending on which state of the cation is accessed by the ionization. Ionization to the ground state cation surface induces a hydrogen transfer which is complete within 50 fs, whereas ionization to the first excited state results in a much slower process.
Effect of oxygen partial pressure on the Fermi level of ZnO1−x films fabricated by pulsed laser deposition
Authors: Min Chul-Hee , Cho Suyeon , Lee Seung-Hyuk , Cho Deok-Yong , Park Won Goo , Chung Jae Gwan , Lee Eunha , Lee Jae Cheol , Anass Benayad , Lee Jae Hak , Hwang Cheol Seong , Oh Se-Jung
Published: May 2010

We investigated the influence of oxygen deficiency on the Fermi level (EF) of ZnO thin film prepared by pulsed laser deposition (PLD). For this purpose, we adopted in situ x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The oxygen deficiency was effectively controlled by varying the oxygen partial pressure [P(O2)] during the PLD. The EF shifted by +0.6 eV as the P(O2) decreased from 10 to 3.3 Pa. This shift indicates a significant change in the energy balance in the oxygen-deficient ZnO films. This fact suggests that the very large change in the resistivity of ZnO thin films resulting from the oxygen deficiency could be attributed to the EF shift rather than grain boundary formation in the ZnO film.
GaN avalanche photodiodes grown on m-plane freestanding GaN substrate
Authors: Vashaei Z. , Cicek E. , Bayram C. , McClintock R. , Razeghi M.
Published: May 2010

M-plane GaN avalanche p-i-n photodiodes on low dislocation density freestanding m-plane GaN substrates were realized using metal-organic chemical vapor deposition. High quality homoepitaxial m-plane GaN layers were developed; the root-mean-square surface roughness was less than 1 Å and the full-width-at-half-maximum value of the x-ray rocking curve for (101¯0) diffraction of m-plane GaN epilayer was 32 arcsec. High quality material led to a low reverse-bias dark current of 8.11 pA for 225 μm2 mesa photodetectors prior to avalanche breakdown, with the maximum multiplication gain reaching about 8000.
Organic floating-gate transistor memory based on the structure of pentacene/nanoparticle-Al/Al2O3
Authors: Wang Wei , Ma Dongge
Published: May 2010

An organic thin-film transistor memory, based on the pentacene semiconductor/nanoparticle-Al floating-gate/Al2O3 tunneling layer, is demonstrated by a simple fabrication process. The floating-gate transistor exhibits significant hysteresis behaviors in current–voltage characteristics and these hysteresis loops size depends on the gate voltage sweeping range. The memory windows of 32.5, 50, and 67.5 V and the memory ratio of 13, 32, and 70 can be obtained by the writing/erasing pulse of ±40 V, ±50 V, and ±60 V, respectively. The charge storage mechanism is discussed well via holes inject or eject the floated gate by F–N tunneling.