bluuebunny/crossref_metadata_2025 huggingface dataset:
Coring project in Bighorn Basin: Drilling phase complete
Authors: Clyde William C. , Gingerich Philip D. , Wing Scott L.
Published: January 2012

Scientists have begun a new project to recover and study sediment cores in the Bighorn Basin of Wyoming to investigate the high‐frequency climatic and biotic variability of a continental depositional system during past episodes of greenhouse conditions. The project, called the Bighorn Basin Coring Project (BBCP), focuses on the early Paleogene (∼50–65 million years ago), the most recent interval of Earth history characterized by greenhouse climate conditions. During the early Paleogene, global mean annual temperatures were up to 10°C higher than today, and carbon dioxide (CO2) concentrations may have been more than 1000 parts per million [Zachos et al., 2008] compared with the roughly 390 parts per million today. Superimposed on this background greenhouse climate state were a series of short‐term extreme warming events called hyperthermals. The best known early Paleogene hyperthermal event is the Paleocene‐Eocene Thermal Maximum (PETM; ∼56 million years ago), which is characterized by a large global carbon isotope excursion and coincides with major changes to marine and continental ecosystems [McInerney and Wing, 2011]. At present, the causes of these hyperthermal events remain unknown, although several hypotheses exist.
Benoit B. Mandelbrot (1924–2010)
Authors: Barton Christopher C. , Lovejoy Shaun , Schertzer Daniel J. , Turcotte Donald L.
Published: January 2012

Benoit B. Mandelbrot, who advanced the concept of power law scaling as a fundamental property of a broad range of natural processes and patterns in geophysics, economics, mathematics, and virtually all of science, died on 14 October 2010 in Cambridge, Mass., at the age of 85. Mandelbrot, known as the “father of fractal geometry,” was a mathematician who developed the scaling concepts of self‐similarity and self‐affinity and found examples in spatial, temporal, and size patterns across a broad spectrum of disciplines. He coined the term “fractal” (from the Latin noun “fractus,” meaning fragmented) for shapes and patterns that exhibit self‐similarity, meaning that they are statistically scale independent. Such shapes are characterized by fractional power law exponents, between the integer (Euclidean) dimensions. He is best known through his books, including Les Objets Fractals: Forme, Hasard et Dimension; Fractals: Form, Chance and Dimension; The Fractal Geometry of Nature; andMultifractals and 1/f Noise: Wild Self‐Affinity in Physics [Mandelbrot, 1975, Mandelbrot 1977, Mandelbrot 1982, Mandelbrot 1999].
Effect of dust charging and trapped electrons on nonlinear solitary structures in an inhomogeneous magnetized plasma
Authors: Kumar Ravinder , Malik Hitendra K. , Singh Khushvant
Published: January 2012

Main concerns of the present article are to investigate the effects of dust charging and trapped electrons on the solitary structures evolved in an inhomogeneous magnetized plasma. Such a plasma is found to support two types of waves, namely, fast wave and slow wave. Slow wave propagates in the plasma only when the wave propagation angle θ satisfies the condition θ≥tan-1{(1+2σ)-[(ndlh(γ1-1))/(1+ndlhγ1)]-v0u0}, where v0(u0) is the z- (x-) component of ion drift velocity, σ = Ti/Teff, ndlh = nd0/(nel0 + neh0), and γ1=-1Φi0[1-Φi01+σ(1-Φi0)] together with Ti as ion temperature, nel0(neh0) as the density of trapped (isothermal) electrons, Φi0 as the dust grain (density nd0) surface potential relative to zero plasma potential, and Teff=(nelo+neho)TelTeh/(neloTeh+nehoTel), where Tel(Teh) is the temperature of trapped (isothermal) electrons. Both the waves evolve in the form of density hill type structures in the plasma, confirming that these solitary structures are compressive in nature. These structures are found to attain higher amplitude when the charge on the dust grains is fluctuated (in comparison with the case of fixed charge) and also when the dust grains and trapped electrons are more in number; the same is the case with higher temperature of ions and electrons. Slow solitary structures show weak dependence on the dust concentration. Both types of structures are found to become narrower under the application of stronger magnetic field. With regard to the charging of dust grains, it is observed that the charge gets reduced for the higher trapped electron density and temperature of ions and electrons, and dust charging shows weak dependence on the ion temperature.
Circular law and arc law for truncation of random unitary matrix
Authors: Dong Zhishan , Jiang Tiefeng , Li Danning
Published: January 2012

Let V be the m × m upper-left corner of an n × n Haar-invariant unitary matrix. Let λ1, …, λm be the eigenvalues of V. We prove that the empirical distribution of a normalization of λ1, …, λm goes to the circular law, that is, the uniform distribution on $\lbrace z\in \mathbb {C};\, |z|\le 1\rbrace${z∈C;|z|≤1} as m → ∞ with m/n → 0. We also prove that the empirical distribution of λ1, …, λm goes to the arc law, that is, the uniform distribution on $\lbrace z\in \mathbb {C};\, |z|=1\rbrace${z∈C;|z|=1} as m/n → 1. These explain two observations by Życzkowski and Sommers (2000).
Effects of static and temporally fluctuating tensions on semiflexible polymer looping
Authors: Shin Jaeoh , Sung Wokyung
Published: January 2012

Biopolymer looping is a dynamic process that occurs ubiquitously in cells for gene regulation, protein folding, etc. In cellular environments, biopolymers are often subject to tensions which are either static or temporally fluctuating far away from equilibrium. We study the dynamics of semiflexible polymer looping in the presence of such tensions by using Brownian dynamics simulation combined with an analytical theory. We show a minute tension dramatically changes the looping time, especially for long chains. Considering a dichotomically flipping noise as a simple example of the nonequilibrium tension, we find the phenomenon of resonant activation, where the looping time can be the minimum at an optimal flipping time. We discuss our results in connection with recent experiments.
The effect of q-distributed electrons on the head-on collision of ion acoustic solitary waves
Authors: Ghosh Uday Narayan , Chatterjee Prasanta , Roychoudhury Rajkumar
Published: January 2012

The head-on collision of ion acoustic solitary waves (IASWs) in two component plasma comprising nonextensive distributed electrons is investigated. Two opposite directional Kortewg-de-vries (KdV) equations are derived and the phase shift due to collision is obtained using the extended version of Poincaré-Lighthill-Kuo method. Different ranges of nonextensive parameter q are considered and their effects on phase shifts are observed. It is found that the presence of nonextensive distributed electrons plays a significant role on the nature of collision of ion acoustic solitary waves.
Efficiency enhancement in free-electron laser amplifier with one dimensional helical wiggler and ion-channel guiding
Authors: Jafari Bahman F. , Maraghechi B.
Published: January 2012

A method for efficiency enhancement in free-electron laser is studied which uses both tapered wiggler magnetic field and ion-channel density. Derivation of a set of nonlinear and coupled differential equations leads to the self-consistent description of the evolution of both an ensemble of electrons and the electromagnetic radiation. Numerical solution of these equations reveals considerable enhancements of the interaction efficiency. In order to obtain a better insight into physical basis of the problem, a modified pendulum equation for the interaction is derived and a small signal theory of the efficiency enhancement is developed.
Fluid flow due to collective non-reciprocal motion of symmetrically-beating artificial cilia
Authors: Khaderi S. N. , den Toonder J. M. J. , Onck P. R.
Published: January 2012

Using a magneto-mechanical solid-fluid numerical model for permanently magnetic artificial cilia, we show that the metachronal motion of symmetrically beating cilia establishes a net pressure gradient in the direction of the metachronal wave, which creates a unidirectional flow. The flow generated is characterised as a function of the cilia spacing, the length of the metachronal wave, and a dimensionless parameter that characterises the relative importance of the viscous forces over the elastic forces in the cilia.
The effect of a compliant substrate on the electromigration-driven surface morphological stabilization of an epitaxial thin film
Authors: Sfyris Georgios I. , Gungor M. Rauf , Maroudas Dimitrios
Published: January 2012

We analyze the surface morphological stability of a coherently strained thin film that has been grown epitaxially on a compliant substrate of finite thickness and is subjected simultaneously to an external electric field, which drives surface electromigration. The compliant substrate has the ability to accommodate elastically some of the misfit strain that is developed in the epitaxial film due to the lattice mismatch between the film and substrate materials. We develop a three-dimensional model for the surface morphological evolution of the thin film and conduct a linear stability analysis for the morphological stability of the heteroepitaxial film’s planar state; of particular importance for the analysis is the elastostatic boundary-value problem for the heteroepitaxial film/substrate system. The analysis shows that surface electromigration due to a properly applied and sufficiently strong electric field can inhibit Stranski-Krastanow-type instabilities. Furthermore, we determine the critical electric-field strength as a function of material properties and heteroepitaxial system parameters, as well as the optimal direction of the electric field for the most efficient stabilization of the surface morphology. We find that using a compliant substrate reduces the critical strength of the externally applied electric field required for planar film surface stabilization by approximately two orders of magnitude compared to that needed to stabilize the planar surface of the same thin film when grown epitaxially on a practically infinite substrate. This critical electric-field strength also is found to be substantially lower than that required for planar film surface stabilization for the same film grown on an elastic substrate that is clamped to a holder and has thickness equal to the compliant substrate thickness. This critical strength requirement can be reduced further by decreasing the ratio of the film’s shear modulus with that of the substrate, and it can be minimized for an optimal value of the compliant substrate thickness. We conclude that surface electromigration can be used to control the onset of island formation on epitaxial film surfaces and that the required electric-field strength for such control can be optimized by efficient use of substrate engineering techniques.
Effect of guide field on lower-hybrid drift instabilities in current sheet containing energetic particles
Authors: Huang Feng , Chen Yinhua , Li Yibao , Yu M. Y.
Published: January 2012

The effect of a guide field on the linear lower-hybrid drift instability (LHDI) in a thin current sheet containing energetic particles is investigated using kinetic theory. It is found that the symmetry properties of the LHD modes are destroyed by the guide field. The LHDI growth rate decreases with the strength of the latter, and the perturbed magnetic field is much higher than that of the guide-field free case.
Spatial redistribution of turbulent and mean kinetic energy
Authors: Manz P. , Xu M. , Fedorczak N. , Thakur S. C. , Tynan G. R.
Published: January 2012

The turbulent spatial redistribution of turbulent and mean kinetic energy and its exchange is investigated in the framework of an K-ε model in a magnetized plasma column. The kinetic energy is transferred from the turbulence into the zonal flow around the shear layer. The Reynolds stress profile adjusts to the shear layer, which results in a transport of mean kinetic energy from the region around the shear layer into the shear layer.
Vibrational energy relaxation of benzene dimer and trimer in the CH stretching region studied by picosecond time-resolved IR-UV pump-probe spectroscopy
Authors: Kusaka Ryoji , Inokuchi Yoshiya , Ebata Takayuki
Published: January 2012

Vibrational energy relaxation (VER) of the Fermi polyads in the CH stretching vibration of the benzene dimer (Bz2) and trimer (Bz3) has been investigated by picosecond (ps) time-resolved IR-UV pump-probe spectroscopy in a supersonic beam. The vibrational bands in the 3000–3100 cm−1 region were excited by a ps IR pulse and the time evolutions at the pumped and redistributed (bath) levels were probed by resonance enhanced multiphoton ionization with a ps UV pulse. For Bz2, a site-selective excitation in the T-shaped structure was achieved by using the isotope-substituted heterodimer hd, where h = C6H6 and d = C6D6, and its result was compared with that of hh homodimer. In the hd heterodimer, the two isomers, h(stem)d(top) and h(top)d(stem), show remarkable site-dependence of the lifetime of intracluster vibrational energy redistribution (IVR); the lifetime of the Stem site [h(stem)d(top), 140–170 ps] is ∼2.5 times shorter than that of the Top site [h(top)d(stem), 370–400 ps]. In the transient UV spectra, a broad electronic transition due to the bath modes emerges and gradually decays with a nanosecond time scale. The broad transition shows different time profile depending on UV frequency monitored. These time profiles are described by a three-step VER model involving IVR and vibrational predissociation: initial → bath1(intramolecular) → bath2(intermolecular) → fragments. This model also describes well the observed time profile of the Bz fragment. The hh homodimer shows the stepwise VER process with time constants similar to those of the hd dimer, suggesting that the excitation-exchange coupling of the vibrations between the two sites is very weak. Bz3 also exhibited the stepwise VER process, though each step is faster than Bz2.
A pathwise derivative approach to the computation of parameter sensitivities in discrete stochastic chemical systems
Authors: Sheppard Patrick W. , Rathinam Muruhan , Khammash Mustafa
Published: January 2012

Characterizing the sensitivity to infinitesimally small perturbations in parameters is a powerful tool for the analysis, modeling, and design of chemical reaction networks. Sensitivity analysis of networks modeled using stochastic chemical kinetics, in which a probabilistic description is used to characterize the inherent randomness of the system, is commonly performed using Monte Carlo methods. Monte Carlo methods require large numbers of stochastic simulations in order to generate accurate statistics, which is usually computationally demanding or in some cases altogether impractical due to the overwhelming computational cost. In this work, we address this problem by presenting the regularized pathwise derivative method for efficient sensitivity analysis. By considering a regularized sensitivity problem and using the random time change description for Markov processes, we are able to construct a sensitivity estimator based on pathwise differentiation (also known as infinitesimal perturbation analysis) that is valid for many problems in stochastic chemical kinetics. The theoretical justification for the method is discussed, and a numerical algorithm is provided to permit straightforward implementation of the method. We show using numerical examples that the new regularized pathwise derivative method (1) is able to accurately estimate the sensitivities for many realistic problems and path functionals, and (2) in many cases outperforms alternative sensitivity methods, including the Girsanov likelihood ratio estimator and common reaction path finite difference method. In fact, we observe that the variance reduction using the regularized pathwise derivative method can be as large as ten orders of magnitude in certain cases, permitting much more efficient sensitivity analysis than is possible using other methods.
Collisionless distribution function for the relativistic force-free Harris sheet
Authors: Stark C. R. , Neukirch T.
Published: January 2012

A self-consistent collisionless distribution function for the relativistic analogue of the force-free Harris sheet is presented. This distribution function is the relativistic generalization of the distribution function for the non-relativistic collisionless force-free Harris sheet recently found by Harrison and Neukirch [Phys. Rev. Lett. 102, 135003 (2009)], as it has the same dependence on the particle energy and canonical momenta. We present a detailed calculation which shows that the proposed distribution function generates the required current density profile (and thus magnetic field profile) in a frame of reference in which the electric potential vanishes identically. The connection between the parameters of the distribution function and the macroscopic parameters such as the current sheet thickness is discussed.
Laser heating of finite two-dimensional dust clusters: A. Experiments
Authors: Schablinski Jan , Block Dietmar , Piel Alexander , Melzer André , Thomsen Hauke , Kählert Hanno , Bonitz Michael
Published: January 2012

Laser manipulation allows to control the kinetic particle temperature in dusty plasmas. Different methods of laser heating for plasma crystals are benchmarked experimentally. The methods are analyzed with respect to homogeneity and isotropy in a spatial, temporal, and statistical sense. It is shown that it is possible to achieve particle dynamics very close to thermal equilibrium and that laser heating methods allow for a detailed study of phase transitions in finite size systems.
Evidence cross-validation and Bayesian inference of MAST plasma equilibria
Authors: von Nessi G. T. , Hole M. J. , Svensson J. , Appel L.
Published: January 2012

In this paper, current profiles for plasma discharges on the mega-ampere spherical tokamak are directly calculated from pickup coil, flux loop, and motional-Stark effect observations via methods based in the statistical theory of Bayesian analysis. By representing toroidal plasma current as a series of axisymmetric current beams with rectangular cross-section and inferring the current for each one of these beams, flux-surface geometry and q-profiles are subsequently calculated by elementary application of Biot-Savart’s law. The use of this plasma model in the context of Bayesian analysis was pioneered by Svensson and Werner on the joint-European tokamak [Svensson and Werner,Plasma Phys. Controlled Fusion 50(8), 085002 (2008)]. In this framework, linear forward models are used to generate diagnostic predictions, and the probability distribution for the currents in the collection of plasma beams was subsequently calculated directly via application of Bayes’ formula. In this work, we introduce a new diagnostic technique to identify and remove outlier observations associated with diagnostics falling out of calibration or suffering from an unidentified malfunction. These modifications enable a good agreement between Bayesian inference of the last-closed flux-surface with other corroborating data, such as that from force balance considerations using EFIT++ [Appel et al., "A unified approach to equilibrium reconstruction" Proceedings of the 33rd EPS Conference on Plasma Physics (Rome, Italy, 2006)]. In addition, this analysis also yields errors on the plasma current profile and flux-surface geometry as well as directly predicting the Shafranov shift of the plasma core.
Synchronous imaging for rapid visualization of complex vibration profiles in electromechanical microresonators
Authors: Linzon Yoav , Joe Daniel J. , Ilic Bojan , Topolancik Juraj , Parpia Jeevak M. , Craighead Harold G. , Krylov Slava
Published: January 2012

Synchronous imaging is used for the dynamic space-domain studies of vibration profiles in capacitively driven, thin n + doped polysilicon microbridges oscillating at rf frequencies. Fast and high-resolution actuation profile measurements of micromachined resonators are useful when significant device nonlinearities are present. For example, bridges under compressive stress near the critical Euler value often reveal complex dynamics stemming from a state close to the onset of buckling. This leads to enhanced sensitivity of the vibration modes to external conditions, such as pressure, temperatures, and chemical composition, the global behavior of which can be conveniently evaluated using synchronous imaging combined with spectral measurements. We performed an experimental study of high drive amplitude and ambient pressure effect on the resonant vibration profiles in electrically driven microbridges near critical buckling. Numerical analysis of electrostatically driven post-buckled microbridges supports the richness of complex vibration dynamics that are possible in such microelectromechanical devices.
A new experimental setup designed for the investigation of irradiation of nanosystems in the gas phase: A high intensity mass-and-energy selected cluster beam
Authors: Bruny G. , Eden S. , Feil S. , Fillol R. , El Farkh K. , Harb M. M. , Teyssier C. , Ouaskit S. , Abdoul-Carime H. , Farizon B. , Farizon M. , Märk T. D.
Published: January 2012

DIAM (Dispositif d’Irradiation d’Agrégats Moléculaires) is a new experimental setup devoted to investigate processes induced by irradiation at the nanoscale. The DIAM apparatus is based on a combination of techniques including a particle beam from high-energy physics, a cluster source from molecular and cluster physics, and mass spectrometry form analytical sciences. In this paper, we will describe the first part of the DIAM apparatus that consists of an ExB double spectrometer connected to a cluster ion source based on a continuous supersonic expansion in the presence of ionizing electrons. This setup produces high intensities of energy-and-mass selected molecular cluster ion beams (1000 s of counts s−1). The performance of the instrument will be shown through measurements of 6–8 keV beams of protonated water clusters, (H2O)nH+ (n = 0–21) and mixed protonated (or deprotonated) water-pyridine cluster ions: PyrH+(H2O)n (n = 0–15), Pyr2H+ (H2O)n (n = 0–9), and (Pyr-H)+ (H2O).
An optically pumped XeF(C-A) laser with repetitive rate of 10 Hz
Authors: Li Yu , Lian-ying Ma , Ai-ping Yi , Chao Huang , Feng Zhu , Xiao-xia An , Yan-long Shen , Hang Qian , Xin Huang , Ke Huang , Ying Tang , Xi-sheng Ye , Jing-ru Liu
Published: January 2012

A novel XeF(C-A) laser which can be operated in repetition mode has been developed based on surface discharge optical pumping technique. Its maximum repetitive rate is up to 10 Hz. The influence of repetitive rate and gas flow rate on the stability of output energy is studied and the main factor which influences the stability of output energy is analyzed. The experimental results show that increasing the gas flow rate into laser chamber can improve the stability of the output energy. The ideal output energy results of 20 laser pulses under different repetitive rates and their optimal experimental conditions are presented. Output energies of more than 4 J and better stability can be obtained when the laser device operates at 1, 2, and 5 Hz, respectively. When the gas feed rate is larger than 53 l/s, the stability of output energy is improved obviously at the repetitive rate of 10 Hz, and the average energy of 20 laser pulses is up to 3.2 J.
Spark discharge formation in an inhomogeneous electric field under conditions of runaway electron generation
Authors: Shao Tao , Tarasenko Victor F. , Zhang Cheng , Lomaev Mikhail I. , Sorokin Dmitrii A. , Yan Ping , Kozyrev Andrei V. , Baksht Evgeni Kh.
Published: January 2012

In this article we report on work where the formation of a spark in nanosecond high-voltage discharges was studied in nitrogen, nitrogen-methane mixtures, and air at increased pressures under the conditions of runaway electron generation. Voltage pulses of amplitude ∼90 and ∼250 kV were applied to a point-to-plane gap with a planar anode and a cathode of small curvature radius. Cathode spots appeared early in the discharge, within ∼200 ps of a corona discharge at high rate of rise of the voltage (∼5 × 1014 V/s) across centimeter point-to-plane gap spacing. The spark leader that bridged the point-to-plane gap propagated from the planar anode with cathode spots and a voltage pulse rise time of less than 1 ns. The discharge from diffuse clouds took the form of diffuse jets with increasing pulse repetition rate, thus achieving the accumulation effect in a repetitively pulsed discharge. Characteristic emission spectra are presented for spark diffuse and corona discharges.
Propagating waves in quasi-one dimensional MoO3 ribbons investigated by Raman spectroscopy and scanning near-field optical microscope
Authors: Li Zhipeng , Liu Ning , Yang Zhilin , Dong Bin
Published: January 2012

MoO3 ribbons have proven to be a good quasi-one dimensional waveguide. Here, we show that propagating waves can be launched in very thin MoO3 ribbons with the thickness of only 150 nm. By using Raman image method, it was found that the 1/e light damping length at 632.8 nm is 5 ∼ 6 μm. Scanning near-field optical measurement was carried to provide distribution of propagating waves near the surface of the waveguide, where Fabry-Pérot interference with the period ∼ 0.33 μm was observed. Furthermore, numerical calculations reveal the guiding waves in thin ribbons are in leak mode with a large portion of energy guided over the MoO3 surface. Applications of these kinds of thin MoO3 ribbons can be found in combined nano-photonic devices and remote surface enhanced Raman sensing.
Efficiency improvement in fullerene-layer-inserted organic bulk-heterojunction solar cells
Authors: Liu Zhihai , Lee Eun-Cheol
Published: January 2012

We have improved power conversion efficiencies of organic bulk-heterojunction solar cells with the active layer composed of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) by inserting a PCBM layer between the active layer and adjacent Al cathode. This improvement is mainly due to the increase in short-circuit current, which prevails over the changes in other parameters. The largest improvement of 11.7% is obtained at the PCBM thickness of 10 nm, while further increasing of the thickness degrades the fill factor and eventually induces S-kinks in the current-voltage characteristics. We discuss possible models explaining the efficiency variations with the PCBM thickness and the occurrence of the S-kinks.
Nucleation of lateral compositional modulation in InGaP epitaxial films grown on (001) GaAs
Authors: Schmidbauer M. , Ugur A. , Wollstein C. , Hatami F. , Katmis F. , Caha O. , Masselink W. T.
Published: January 2012

The nucleation of the one dimensional periodic surface corrugations that form during epitaxy along the [-110] direction on the In0.48Ga0.52 P lattice matched to (001) GaAs is investigated using x-ray diffuse scattering in the grazing incidence geometry. Comparing samples with differing In0.48Ga0.52 P thickness shows that the corrugations are not detectable for a 10-nm film, but develop gradually for thicker films. Very pronounced corrugations are well developed for 200-nm thick films and are associated with a lateral strain field and enhanced In composition. The data support a lateral compositional modulation as the cause of the corrugations.
Optoelectronic surface-related properties in boron-doped and irradiated diamond thin films
Authors: Nemashkalo A. , Chapagain P. R. , Peters R. M. , Farmer J. , Gupta S. , Strzhemechny Y. M.
Published: January 2012

Elucidation of microscopic properties of synthetic diamond films, such as formation and evolution of bulk and surface defects, chemistry of dopants, is necessary for a reliable quality control and reproducibility in applications. Surface photovoltage (SPV) spectroscopy and photoluminescence (PL) spectroscopy were employed to study diamond thin films grown on silicon by microwave plasma-assisted chemical vapor deposition and hot-filament chemical vapor deposition with different levels of boron doping in conjunction with gamma irradiation. SPV experiments showed that while the increase of boron concentration leads to a semiconductor-metal transition, subsequent gamma irradiation reverts quasi-metallic samples back to a semiconducting state by compensating electrical activity of boron possibly via hydrogen. One of the most pronounced common transitions observed at ∼3.1–3.2 eV in the SPV spectra was also present in all of the PL spectra. It is likely that this is a signature of the sp2-hybridized carbon clusters in or in the vicinity of grain boundaries.
The coupled cluster singles, doubles, and a hybrid treatment of connected triples based on the split virtual orbitals
Authors: Shen Jun , Kou Zhuangfei , Xu Enhua , Li Shuhua
Published: January 2012

We have proposed a simple strategy for splitting the virtual orbitals with a large basis set into two subgroups (active and inactive) by taking a smaller basis set as an auxiliary basis set. With the split virtual orbitals (SVOs), triple or higher excitations can be partitioned into active and inactive subgroups (according to the number of active virtual orbitals involved), which can be treated with different electron correlation methods. In this work, the coupled cluster (CC) singles, doubles, and a hybrid treatment of connected triples based on the SVO [denoted as SVO-CCSD(T)-h], has been implemented. The present approach has been applied to study the bond breaking potential energy surfaces in three molecules (HF, F2, and N2), and the equilibrium properties in a number of open-shell diatomic molecules. For all systems under study, the SVO-CCSD(T)-h method based on the unrestricted Hartree–Fock (UHF) reference is an excellent approximation to the corresponding CCSDT (CC singles, doubles, and triples), and much better than the UHF-based CCSD(T) (CC singles, doubles, and perturbative triples). On the other hand, the SVO-CCSD(T)-h method based on the restricted HF (RHF) reference can also provide considerable improvement over the RHF-based CCSD(T).
Time-dependent degradation of AlGaN/GaN high electron mobility transistors under reverse bias
Authors: Meneghini Matteo , Stocco Antonio , Bertin Marco , Marcon Denis , Chini Alessandro , Meneghesso Gaudenzio , Zanoni Enrico
Published: January 2012

This paper describes a detailed analysis of the time-dependent degradation kinetics of GaN-based high electron mobility transistors submitted to reverse-bias stress. We show that: (1) exposure to reverse-bias may induce recoverable changes in gate leakage and threshold voltage, due to the accumulation of negative charge within the AlGaN layer, and of positive charge at the AlGaN/GaN interface. (2) Permanent degradation consists in the generation of parasitic leakage paths. Several findings support the hypothesis that permanent degradation is due to a defect percolation process: (2(a)) for sufficiently long stress times, degradation occurs even below the “critical voltage” estimated by step stress experiments; (2(b)) before permanent degradation, gate current becomes noisy, indicating an increase in defect concentration; and (2(c)) time to breakdown strongly depends on the initial defectiveness of the samples.
Gallium-droplet behaviors of self-catalyzed GaAs nanowires: A transmission electron microscopy study
Authors: Heon Kim Young , Woo Park Dong , Jun Lee Sang
Published: January 2012

When the arsenic (As) precursor was supplied during cool down, the gallium (Ga) droplets on top of GaAs nanowires (NWs) were extinct on the final products. The Ga droplets were conserved, and their behaviors depended on the size of the NWs when the As supply was cut off during the cooling: The contact angles of the droplets on the NWs with diameters larger than 60 nm were fixed to approximately 113°. However, they were decreased and fluctuated on the NWs less than 60 nm in diameter. Finally, the NWs less than 46 nm were free from the Ga droplet.
The relationship between structural evolution and electrical percolation of the initial stages of tungsten chemical vapor deposition on polycrystalline TiN
Authors: Rozenblat A. , Haimson S. , Shacham-Diamand Y. , Horvitz D.
Published: January 2012

This paper presents experimental results and a geometric model of the evolution of sheet resistance and surface morphology during the transition from nucleation to percolation of tungsten chemical vapor deposition over ultrathin polycrystalline titanium nitride (TiN). We observed two mechanisms of reduction in sheet resistance. At deposition temperatures higher than 310 °C, percolation effect is formed at ∼35% of surface coverage, θ, and characterized with a sharp drop in resistance. At temperature below 310 °C, a reduction in resistance occurs in two steps. The first step occurs when θ = 35% and the second step at θ = 85%. We suggest a geometric model in which the electrical percolation pass is modulated by the thickness threshold of the islands at the instant of collision.
Below band-gap optical absorption and photoluminescence excitation spectroscopy at room temperature in low-defect-density bulk GaN:Fe
Authors: Gladkov P. , Hulicius E. , Paskova T. , Preble E. , Evans K. R.
Published: January 2012

We present a detailed study of the below band-gap optical absorption at room temperature in bulk semi-insulating GaN:Fe versus the Fe-doping. It was established that the 1.24 eV photoluminescence band at 300 K consists of only vibrational replicas of the Fe3+ 4T1(G)→6A1(S) internal transition. We also studied the below band-gap photoluminescence excitation of the 1.24 eV band. The identical exponential rise of the photoluminescence excitation and the optical absorption coefficient identify the Fe3+/2+ charge-transfer as the main contributor to the 300 K optical absorption in the range 400-500 nm. Practical implications of these results for Fe-doping determination are discussed.
Communication: Improving the density functional theory+U description of CeO2 by including the contribution of the O 2p electrons
Authors: Plata José J. , Márquez Antonio M. , Sanz Javier Fdez.
Published: January 2012

Density functional theory (DFT) based approaches within the local-density approximation or generalized gradient approximation frameworks fail to predict the correct electron localization in strongly correlated systems due to the lack of cancellation of the Coulomb self-interaction. This problem might be circumvented either by using hybrid functionals or by introducing a Hubbard-like term to account for the on site interactions. This latter DFT+U approach is less expensive and therefore more practical for extensive calculations in solid-state computational simulations. By and large, the U term only affects the metal electrons, in our case the Ce 4f ones. In the present work, we report a systematic analysis of the effect of adding such a U term also to the oxygen 2p electrons. We find that using a set of Uf = 5 eV and Up = 5eV effective terms leads to improved description of the lattice parameters, band gaps, and formation and reduction energies of CeO2.
Two beam surface fluctuation specular reflection spectroscopy
Authors: Raudsepp Allan , Fretigny Christian , Lequeux François , Talini Laurence
Published: January 2012

In surface fluctuation specular reflection spectroscopy (SFSRS) deflections of a specularly reflected laser beam are used to characterize thermally excited surface waves. Here we report on a new two beam version of SFSRS in which the deflections of two reflected laser beams from separate locations on a surface are correlated. We demonstrate that this new two beam SFSRS technique can be used to determine directly the power spectrum of height fluctuation of thermally excited surface waves over a large range of both frequencies and wavevectors. In addition, we show that the technique is well suited for materials ranging from simple liquids to complex liquids and soft solids, including turbid materials.
Positive/negative ion velocity mapping apparatus for electron-molecule reactions
Authors: Wu Bin , Xia Lei , Li Hong-Kai , Zeng Xian-Jin , Xi Tian Shan
Published: January 2012

In molecular dissociative ionization by electron collisions and dissociative electron attachment to molecule, the respective positively and negatively charged fragments are the important products. A compact ion velocity mapping apparatus is developed for the angular distribution measurements of the positive or negative fragments produced in the electron-molecule reactions. This apparatus consists of a pulsed electron gun, a set of ion velocity mapping optic lenses, a two-dimensional position detector including two pieces of micro-channel plates, and a phosphor screen, and a charge-coupled-device camera for data acquisition. The positive and negative ion detections can be simply realized by changing the voltage polarity of ion optics and detector. Velocity sliced images can be directly recorded using a narrow voltage pulse applied on the rear micro-channel plate. The efficient performance of this system is evaluated by measuring the angular distribution of O− from the electron attachments to NO at 7.3 and 8.3 eV and O+ from the electron collision with CO at 40.0 eV.
Enhancing interwall load transfer by vacancy defects in carbon nanotubes
Authors: Santo Pietro David , Tang Chun , Chen Changfeng
Published: January 2012

Structural defects usually reduce the load carrying capacity of materials. Here, we show by molecular dynamics simulations an anomalous vacancy-defect-induced enhancement of interwall load transfer in annealed carbon nanotubes (CNTs). The large increase of load transfer mainly stems from the hybrid sp2-sp3 interwall bonds at the vacancy edges, which are stronger than the sp3 interwall bonds in pristine CNTs. This finding offers insights for better understanding and designing CNT-based high-performance nanocomposites.
Morphological control of hybrid polymer-quantum dot solar cells with electron acceptor ligands
Authors: Boivin Mathieu , Lamarre Sébastien , Tessier Jonathan , Lecavalier Marie-Ève , Najari Ahmed , Dufour-Beauséjour Sophie , Brown Dussault Evelyne , Collin Pierre , Allen Claudine Nì.
Published: January 2012

We integrate the electro-attractive conjugated molecule tetrafluoro-tetracyano-quinodimethane (F4TCNQ) in the active layer of polymer-CdSe colloidal quantum dot (cQD) solar cells. The addition of this molecule enhances cQD dispersion inside the polymer. In tuning its concentration, we can optimize the active layer morphology for charge separation and transport. A smoother morphology is likely the result of polymer chain adsorption on cQDs via F4TCNQ which increases the steric barrier between cQDs. Our most optimized device has a F4TCNQ:cQDs weight ratio of 0.5% improving the power conversion efficiency by a factor ∼2.3.
Could implementation of Australia's National Gay Men's Syphilis Action Plan have an indirect effect on the HIV epidemic?
Authors: Hoare Alexander , Gray Richard T. , Wilson David P.
Published: January 2012

Objectives
The number of incident infections of syphilis and HIV have increased over the past decade across Australia, particularly among gay men. In other industrialised settings, syphilis epidemics have also resurged coincidentally with increases in HIV diagnoses. Sexually transmissible infections (STI) are a biologically plausible cofactor for increasing HIV transmission. We pose the question: could strategies purely targeting syphilis also have an indirect impact on HIV incidence? Methods: We developed an agent-based computer model that simulates the transmission and disease progression of HIV and syphilis among a population of sexually active gay men, calibrated to reflect the epidemics in Victoria, Australia. The model was informed by detailed behavioural data from a variety of sources and was used to investigate the potential epidemiological impact of different public health interventions. Results: Assuming that syphilis could act as a biological cofactor for HIV transmission, from no effect to increasing risk by five-fold, our model indicates that if Australia’s syphilis action plan is effectively implemented then the number of HIV infections could decrease by up to 48% over the next decade in the absence of any specific HIV interventions. Conclusion: It is plausible that effective implementation of interventions targeting syphilis epidemics can have an indirect effect of mitigating the spread of HIV. The possible effects of STI should be considered in the design, implementation and evaluation of public health strategies and programs.
Ribosome clearance by FusB-type proteins mediates resistance to the antibiotic fusidic acid
Authors: Cox Georgina , Thompson Gary S. , Jenkins Huw T. , Peske Frank , Savelsbergh Andreas , Rodnina Marina V. , Wintermeyer Wolfgang , Homans Steve W. , Edwards Thomas A. , O'Neill Alexander J.
Published: January 2012

Resistance to the antibiotic fusidic acid (FA) in the human pathogen
Staphylococcus aureus
usually results from expression of FusB-type proteins (FusB or FusC). These proteins bind to elongation factor G (EF-G), the target of FA, and rescue translation from FA-mediated inhibition by an unknown mechanism. Here we show that the FusB family are two-domain metalloproteins, the C-terminal domain of which contains a four-cysteine zinc finger with a unique structural fold. This domain mediates a high-affinity interaction with the C-terminal domains of EF-G. By binding to EF-G on the ribosome, FusB-type proteins promote the dissociation of stalled ribosome⋅EF-G⋅GDP complexes that form in the presence of FA, thereby allowing the ribosomes to resume translation. Ribosome clearance by these proteins represents a highly unusual antibiotic resistance mechanism, which appears to be fine-tuned by the relative abundance of FusB-type protein, ribosomes, and EF-G.
Tobias Walther: Floating ideas on lipids
Authors: Sedwick Caitlin
Published: January 2012

Walther studies how cells use and store lipids to maintain membrane composition.
The Ras protein superfamily: Evolutionary tree and role of conserved amino acids
Authors: Rojas Ana Maria , Fuentes Gloria , Rausell Antonio , Valencia Alfonso
Published: January 2012

The Ras superfamily is a fascinating example of functional diversification in the context of a preserved structural framework and a prototypic GTP binding site. Thanks to the availability of complete genome sequences of species representing important evolutionary branch points, we have analyzed the composition and organization of this superfamily at a greater level than was previously possible. Phylogenetic analysis of gene families at the organism and sequence level revealed complex relationships between the evolution of this protein superfamily sequence and the acquisition of distinct cellular functions. Together with advances in computational methods and structural studies, the sequence information has helped to identify features important for the recognition of molecular partners and the functional specialization of different members of the Ras superfamily.
The synaptic vesicle SNARE neuronal Synaptobrevin promotes endolysosomal degradation and prevents neurodegeneration
Authors: Haberman Adam , Williamson W. Ryan , Epstein Daniel , Wang Dong , Rina Srisha , Meinertzhagen Ian A. , Hiesinger P. Robin
Published: January 2012

Soluble NSF attachment protein receptors (SNAREs) are the core proteins in membrane fusion. The neuron-specific synaptic v-SNARE n-syb (neuronal Synaptobrevin) plays a key role during synaptic vesicle exocytosis. In this paper, we report that loss of n-syb caused slow neurodegeneration independent of its role in neurotransmitter release in adult Drosophila melanogaster photoreceptor neurons. In addition to synaptic vesicles, n-Syb localized to endosomal vesicles. Loss of n-syb lead to endosomal accumulations, transmembrane protein degradation defects, and a secondary increase in autophagy. Our evidence suggests a primary defect of impaired delivery of vesicles that contain degradation proteins, including the acidification-activated Cathepsin proteases and the neuron-specific proton pump and V0 adenosine triphosphatase component V100. Overexpressing V100 partially rescued n-syb–dependent degeneration through an acidification-independent endosomal sorting mechanism. Collectively, these findings reveal a role for n-Syb in a neuron-specific sort-and-degrade mechanism that protects neurons from degeneration. Our findings further shed light on which intraneuronal compartments exhibit increased or decreased neurotoxicity.
Novel plant SUN–KASH bridges are involved in RanGAP anchoring and nuclear shape determination
Authors: Zhou Xiao , Graumann Katja , Evans David E. , Meier Iris
Published: January 2012

Inner nuclear membrane Sad1/UNC-84 (SUN) proteins interact with outer nuclear membrane (ONM) Klarsicht/ANC-1/Syne homology (KASH) proteins, forming linkers of nucleoskeleton to cytoskeleton conserved from yeast to human and involved in positioning of nuclei and chromosomes. Defects in SUN–KASH bridges are linked to muscular dystrophy, progeria, and cancer. SUN proteins were recently identified in plants, but their ONM KASH partners are unknown. Arabidopsis WPP domain–interacting proteins (AtWIPs) are plant-specific ONM proteins that redundantly anchor Arabidopsis RanGTPase–activating protein 1 (AtRanGAP1) to the nuclear envelope (NE). In this paper, we report that AtWIPs are plant-specific KASH proteins interacting with Arabidopsis SUN proteins (AtSUNs). The interaction is required for both AtWIP1 and AtRanGAP1 NE localization. AtWIPs and AtSUNs are necessary for maintaining the elongated nuclear shape of Arabidopsis epidermal cells. Together, our data identify the first KASH members in the plant kingdom and provide a novel function of SUN–KASH complexes, suggesting that a functionally diverged SUN–KASH bridge is conserved beyond the opisthokonts.
Continental Divide: The Canadian Banking and Currency Laws of 1871 in the Mirror of the United States
Authors: Smith Andrew
Published: January 2012

In the wake of the 2008 financial crisis, international observers praised the stability of Canada's banks. When financial institutions in the United States and the United Kingdom collapsed, Canada did not experience any bank failures. The World Economic Forum'sGlobal Competitiveness Reportrated Canada's banking system as the most sound in the world. Historically, bank failures have been quite rare in Canada. Some authors argue that the stabilizing features of Canada's financial system were established in the first five years after the creation of the Canadian nation-state in 1867. This paper will examine the making of the Canadian banking law in 1871, an event widely regarded as a crucial turning point in Canadian financial history.