bluuebunny/crossref_metadata_2025 huggingface dataset:
p-type doping of GaAs nanowires using carbon
Authors: Salehzadeh O. , Zhang X. , Gates B. D. , Kavanagh K. L. , Watkins S. P.
Published: November 2012

We report on the electrical properties of Au-catalyzed C-doped GaAs nanowires (NWs) grown by metal organic vapor phase epitaxy. Transport measurements were carried out using a tungsten nanoprobe inside a scanning electron microscope by contacting to the Au catalyst particle of individual nanowires. The doping level could be varied from approximately (4 ± 1) × 1016 cm−3 to (1.0 ± 0.3) × 1019 cm−3 by varying the molar flow of the gas phase carbon precursor, as well as the group V to group III precursor ratio. It was found that the current transport mechanism switches from generation-recombination to tunnelling field emission by increasing the doping level to 1 × 1019 cm−3. Based on a diameter-dependent analysis of the apparent resistivity of the C-doped NWs, we propose that C incorporates into GaAs NWs through the triple boundary at the Au/NW interface. The p-type conductivity of the C-doped NWs was inferred by observing a rectification at negative bias (applied to the Au electrode) and confirmed by back-gating measurements performed on field effect transistor devices.
Lower bounds on blow up solutions of the three-dimensional Navier–Stokes equations in homogeneous Sobolev spaces
Authors: Robinson James C. , Sadowski Witold , Silva Ricardo P.
Published: November 2012

Suppose that u(t) is a solution of the three-dimensional Navier–Stokes equations, either on the whole space or with periodic boundary conditions, that has a singularity at time T. In this paper we show that the norm of u(T − t) in the homogeneous Sobolev space \documentclass[12pt]{minimal}\begin{document}$\dot{H}^s$\end{document}Ḣs must be bounded below by cst−(2s−1)/4 for 1/2 < s < 5/2 (s ≠ 3/2), where cs is an absolute constant depending only on s; and by \documentclass[12pt]{minimal}\begin{document}$c_s\Vert u_0\Vert _{L^2}^{(5-2s)/5}t^{-2s/5}$\end{document}cs‖u0‖L2(5−2s)/5t−2s/5 for s > 5/2. (The result for 1/2 < s < 3/2 follows from well-known lower bounds on blowup in Lp spaces.) We show in particular that the local existence time in \documentclass[12pt]{minimal}\begin{document}$\dot{H}^s({\mathbb R}^3)$\end{document}Ḣs(R3) depends only on the \documentclass[12pt]{minimal}\begin{document}$\dot{H}^s$\end{document}Ḣs-norm for 1/2 < s < 5/2, s ≠ 3/2.
Electronic excitation in bulk and nanocrystalline alkali halides
Authors: Bichoutskaia Elena , Pyper Nicholas C.
Published: November 2012

The lowest energy excitations in bulk alkali halides are investigated by considering five different excited state descriptions. It is concluded that excitation transfers one outermost halide electron in the fully ionic ground state to the lowest energy vacant s orbital of one closest cation neighbour to produce the excited state termed dipolar. The excitation energies of seven salts were computed using shell model description of the lattice polarization produced by the effective dipole moment of the excited state neutral halogen–neutral metal pair. Ab initio uncorrelated short-range inter-ionic interactions computed from anion wavefunctions adapted to the in-crystal environment were augmented by short-range electron correlation contributions derived from uniform electron-gas density functional theory. Dispersive attractions including wavefunction overlap damping were introduced using reliable semi-empirical dispersion coefficients. The good agreement between the predicted excitation energies and experiment provides strong evidence that the excited state is dipolar. In alkali halide nanocrystals in which each ionic plane contains only four ions, the Madelung energies are significantly reduced compared with the bulk. This predicts that the corresponding intra-crystal excitation energies in the nanocrystals, where there are two excited states depending on whether the halide electron is transferred to a cation in the same or in the neighbouring plane, will be reduced by almost 2 eV. For such an encapsulated KI crystal, it has been shown that the greater polarization in the excited state of the bulk crystal causes these reductions to be lowered to a 1.1 eV–1.5 eV range for the case of charge transfer to a neighbouring plane. For intra-plane charge transfer the magnitude of the polarization energy is further reduced thus causing the excitation in these encapsulated materials to be only 0.2 eV less than in the bulk crystal.
Green's function for reversible geminate reaction with volume reactivity
Authors: Khokhlova Svetlana S. , Agmon Noam
Published: November 2012

The kinetics of a diffusing particle near a reversible trap may be described by an extension of the Feynman-Kac equation to the case of reversible binding, which can occur within a finite reaction sphere. We obtain the Green's function solution for the Laplace transform of this equation when the particle is initially either bound or unbound. We study the solution in the time-domain by either inverting the Laplace transform numerically or propagating the partial differential equation in the time-domain. We show that integrals of this solution over the reaction sphere agree with previously obtained solutions.
Free unitary groups are (almost) simple
Authors: Chirvasitu Alexandru
Published: November 2012

We show that the quotients of Wang and Van Daele's universal quantum groups by their centers are simple in the sense that they have no normal quantum subgroups, thus providing the first examples of simple compact quantum groups with non-commutative fusion rings.
Detrimental influence of nanostructuring on the thermoelectric properties of magnesium silicide
Authors: Satyala Nikhil , Vashaee Daryoosh
Published: November 2012

Nanostructuring techniques have steered the performance of many thermoelectric (TE) compounds towards significant improvement in performance in the last two decades. In this paper, we present a comprehensive study on the effect of bulk nanostructuring in magnesium silicide (Mg2Si) through simulation of thermoelectric properties using a multi-band semi-classical approach. It is shown that the magnitude of reduction in lattice thermal conductivity in nanostructured Mg2Si is comparable to that of reduction in charge carrier mobility for any chosen range of the grain sizes. The results are justified through a comparison with experimental data for both n-type and p-type Mg2Si characteristics versus temperature as well as doping concentration. In order to understand the underlying reasons for the detrimental effect of nanostructuring in Mg2Si, analogous calculations were performed on the well-known TE system of nanostructured Si0.8Ge0.2 and the results are compared. Model calculations show that in nanostructured Mg2Si a grain size of 20 nm results in approximately 40% reduction in lattice thermal conductivity, whereas the reduction in electrical conductivity is nearly 50% of its value in crystalline structures. For the case of nanostructured Si0.8Ge0.2, the loss in electrical conductivity was found to be a mere 20% of its magnitude in crystalline structures. The differential electrical and thermal conductivities versus charge carrier and phonon energies were calculated, respectively, and it was shown that the enhancement in Seebeck coefficient due to the energy filtering effect is also marginal. Therefore, it is conclusively shown that bulk nanostructuring in Mg2Si is not an efficient method to enhance ZT.
Theory of the dynamic Biot-Allard equations and their link to the quasi-static Biot system
Authors: Mikelić Andro , Wheeler Mary F.
Published: November 2012

We undertake establishing well-posedness of the dynamic Biot-Allard equations. It is obtained using the precise properties of the dynamic permeability matrix following the homogenization derivation of the model. By taking the singular limit of the contrast coefficient, the quasi-static Biot system can be obtained from the dynamic Biot equations. These results can be used to formulate an efficient computational algorithm for solving dynamic Biot-Allard equations for subsurface flows with the characteristic reservoir time scales larger than the intrinsical characteristic time. This result appears to be completely new in the literature on Biot's theory. We conclude by showing that in the case of periodic deformable porous media the dynamic permeability has the required properties.
Ab initio calculations on structural, elastic and dynamic stability of CdO at high pressures
Authors: Sahoo B. D. , Joshi K. D. , Gupta Satish C.
Published: November 2012

First principles calculations have been carried out to analyze structural, elastic, and dynamic stability, of CdO under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type (B2) structures determined as a function of compression suggests the B1 → B2 transition at ∼87 GPa, in good agreement with experimental value of 90.6 GPa [Liu et al. Phys. Rev. B 70, 0941141 (2004)]. Various physical quantities, such as zero pressure equilibrium volume, bulk modulus, pressure derivative of bulk modulus, Gruneisen parameter, and Debye temperature have been derived from the theoretically determined equation of state. All these physical quantities show a reasonably good agreement with the available experimental data. Additionally, employing the theoretically determined thermal equation of state in conjunction with Rankine Hugoniot relation, we have predicted the Hugoniot of B1 phase of this material. The single crystal elastic constants of B1 phase calculated up to the pressure of 166 GPa show that though, the shear elastic stability of B1 phase decreases monotonically with compression, it still remains stable elastically even at transition point. Our lattice dynamic calculations show that the B1 phase remains stable dynamically also for pressures <118 GPa. For B2 phase, our calculations demonstrate that this structure will be elastically stable beyond ∼38 GPa. However, to stabilize this phase dynamically a still higher pressure of ∼55 GPa will be required, which is less than B1 → B2 transition pressure.
Ultrafast time dynamics studies of periodic lattices with free electron laser radiation
Authors: Quevedo W. , Busse G. , Hallmann J. , Moré R. , Petri M. , Krasniqi F. , Rudenko A. , Tschentscher T. , Stojanovic N. , Düsterer S. , Treusch R. , Tolkiehn M. , Techert S. , Rajkovic I.
Published: November 2012

It has been proposed that radiation from free electron laser (FEL) at Hamburg (FLASH) can be used for ultrafast time-resolved x-ray diffraction experiments based on the near-infrared (NIR) pump/FEL probe scheme. Here, investigation probing the ultrafast structural dynamics of periodic nano-crystalline organic matter (silver behenate) with such a scheme is reported. Excitation with a femtosecond NIR laser leads to an ultrafast lattice modification which time evolution has been studied through the scattering of vacuum ultraviolet FEL pulses. The found effect last for 6 ps and underpins the possibility for studying nanoperiodic dynamics down to the FEL source time resolution. Furthermore, the possibility of extending the use of silver behenate (AgBh) as a wavelength and temporal calibration tool for experiments with soft x-ray/FEL sources is suggested.
Electron cyclotron maser based on the combination two-wave resonance
Authors: Savilov A. V.
Published: November 2012

A mechanism of a combination two-wave cyclotron interaction between an electron beam and the forward/backward components of a far-from-cutoff standing wave is analyzed. This regime can be promising for the realization of high-power continuous-wave electron cyclotron masers operating in the THz frequency range.
Transport properties of silver selenomolybdate glassy ionic conductors
Authors: Deb B. , Ghosh A.
Published: November 2012

Transport properties of silver selenomolybdate glassy ionic conductors have been reported for wide composition and temperature ranges. It has been observed that the transport properties of these glasses depend strongly on the modifier content as well as on the glass formers ratio. A direct correlation between the ion transport and the modification of the glass structure has been predicted. Transport properties of these glasses are also strongly influenced by the existence of dual character of SeO2 as a glass former and a glass modifier. Structural models for different compositions have also been proposed.
Structure and interactions in fluids of prolate colloidal ellipsoids: Comparison between experiment, theory, and simulation
Authors: Cohen A. P. , Janai E. , Rapaport D. C. , Schofield A. B. , Sloutskin E.
Published: November 2012

The microscopic structure of fluids of simple spheres is well known. However, the constituents of most real-life fluids are non-spherical, leading to a coupling between the rotational and translational degrees of freedom. The structure of simple dense fluids of spheroids – ellipsoids of revolution – was only recently determined by direct experimental techniques [A. P. Cohen, E. Janai, E. Mogilko, A. B. Schofield, and E. Sloutskin, Phys. Rev. Lett. 107, 238301 (2011)10.1103/PhysRevLett.107.238301]. Using confocal microscopy, it was demonstrated that the structure of these simple fluids cannot be described by hard particle models based on the widely used Percus-Yevick approximation. In this paper, we describe a new protocol for determining the shape of the experimental spheroids, which allows us to expand our previous microscopy measurements of these fluids. To avoid the approximations in the theoretical approach, we have also used molecular dynamics simulations to reproduce the experimental radial distribution functions g(r) and estimate the contribution of charge effects to the interactions. Accounting for these charge effects within the Percus-Yevick framework leads to similar agreement with the experiment.
Dissociation mechanisms of excited CH3X (X = Cl, Br, and I) formed via high-energy electron transfer using alkali metal targets
Authors: Hayakawa Shigeo , Tsujinaka Taiga , Fujihara Akimasa
Published: November 2012

High-energy electron transfer dissociation (HE-ETD) on collisions with alkali metal targets (Cs, K, and Na) was investigated for CH3X+ (X = Cl, Br, and I) ions by a charge inversion mass spectrometry. Relative peak intensities of the negative ions formed via HE-ETD strongly depend on the precursor ions and the target alkali metals. The dependency is explained by the exothermicities of the respective dissociation processes. Peak shapes of the negative ions, especially of the X– ions, which comprise a triangle and a trapezoid, also strongly depend on the precursor ions and the target alkali metals. The trapezoidal part of the I– peak observed with the Na target is more dominant and much broader than that with the Cs target. This dependence on the targets shows an inverse relation between the peak width and the available energy, which corresponds to the exothermicity assuming formation of fragment pair in their ground internal states. From a comparison of the kinetic energy release value calculated from the trapezoidal shape of I– with the available energy of the near-resonant level on the CH3I potential energy curve reported by ab initio calculations, the trapezoidal part is attributed to the dissociation to CH3 + I(2P3/2) via the repulsive 3Q1 state of CH3I, which is not dominant in the photo-dissociation of CH3I. The observation of trapezoid shape of the CH2I– peak with the Cs target indicates spontaneous dissociation via repulsive potential from the 3R2 Rydberg state, although the correlation between the 3R2 Rydberg state and relevant repulsive states has not been reported by any theoretical calculation.
Spin-up and spin-down in a half cone: A pathological situation or not?
Authors: Li L. , Patterson M. D. , Zhang K. , Kerswell R. R.
Published: November 2012

The spin-up and spin-down of a fluid in a rapidly rotating, fluid-filled, and closed half cone are studied both numerically and experimentally. This unusual set up is of interest because it represents a pathological case for the classical linear theory of Greenspan and Howard [J. Fluid Mech. 17, 385–404 (1963)10.1017/S0022112063001415] since there are no closed geostrophic contours nor a denumerable set of inertial waves (even a modified theory incorporating Rossby waves by Pedlosky and Greenspan [J. Fluid Mech. 27, 291–304 (1967)10.1017/S0022112067000321]—relies on geostrophy to leading order). The linearised spin-up/spin-down dynamics in a half cone is found to be dominated by topographical effects which force an ageostrophic leading balance and cause the large-scale starting vorticity to coherently move into the “westward” corner of the half cone for both spin-up and spin-down. Once there, viscous boundary layer effects take over as the dominant process ensuring that the spin-up/spin-down time scales conventionally with E−1/2, where E is the Ekman number. The numerical coefficient in this time scale is approximately a quarter of that for a full cone when the semi-angle is 30°. Nonlinear spin up from rest is also studied as well as an impulsive 50% reduction in the rotation rate which shows boundary layer separation and small scales. We conclude that spin-up in a rapidly rotating half cone is not pathological because the fluid dynamics is fundamentally the same as that in a container with small topography: in both topography-forced vortex stretching is to the fore.
Homogeneous pinhole free 1 nm Al2O3 tunnel barriers on graphene
Authors: Dlubak B. , Martin M.-B. , Deranlot C. , Bouzehouane K. , Fusil S. , Mattana R. , Petroff F. , Anane A. , Seneor P. , Fert A.
Published: November 2012

We report on the topographical and electrical characterisations of 1 nm thick Al2O3 dielectric films on graphene. The Al2O3 is grown by sputtering a 0.6 nm Al layer on graphene and subsequentially oxidizing it in an O2 atmosphere. The Al2O3 layer presents no pinholes and is homogeneous enough to act as a tunnel barrier. A resistance-area product in the mega-ohm micrometer-square range is found. Comparatively, the growth of Al2O3 by evaporation does not lead to well-wetted films on graphene. Application of this high quality sputtered tunnel barrier to efficient spin injection in graphene is discussed.
The selective removal of metallic carbon nanotubes from As-grown arrays on insulating substrates
Authors: Tunnell Andrew , Ballarotto Vincent , Cumings John
Published: November 2012

We present a method of selectively removing metallic single-walled carbon nanotubes (SWCNTs) from as-grown arrays on quartz substrates. The process utilizes an external silicon piece as a temporary global top gate to increase the resistance of the semiconducting SWCNTs while current is passed through the metallic SWCNTs, causing electrical breakdown through joule heating. The resulting SWCNT field-effect transistors (FETs) consistently produce on/off current ratios greater than 1000. Additionally, we find that the high frequency parasitic losses between 1 GHz and 6 GHz on the completed SWCNT FETs are significantly lower than on comparable SWCNT FETs fabricated on silicon substrates.
Evaluation of the nonlinear optical properties for an expanded porphyrin Hückel-Möbius aromaticity switch
Authors: Torrent-Sucarrat Miquel , Anglada Josep M. , Luis Josep M.
Published: November 2012

The conformational flexibility of the expanded porphyrins allows them to achieve different topologies with distinct aromaticities and nonlinear optical properties (NLOP). For instance, it is possible to switch between Möbius and Hückel topologies applying only small changes in the external conditions or in the structure of the ring. In this work, we evaluate the electronic and vibrational contributions to static and dynamic NLOP of the Hückel and Möbius conformers of A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1) synthesized by Latos-Grażyński and co-workers [Angew. Chem., Int. Ed. 46, 7869 (2007)10.1002/anie.200700555]. Calculations are performed at the HF, M052X, and CAM-B3LYP levels using the 6-31G, 6-311G(d), and 6-31+G(d) basis sets. Our results conclude that M052X/6-31G and CAM-B3LYP/6-31G methods provide a correct qualitative description of the electronic and vibrational contributions for the NLOP of expanded porphyrins. The studied systems show high NLOP with large differences between the Möbius and Hückel conformations (around 1 × 106 a.u. for $\bar \gamma$γ¯). The obtained results indicate that the expanded porphyrins are promising systems to manufacture Hückel-to-Möbius topological switches.
Numerical study on aerodynamic performances of the wind turbine rotor with leading-edge rotation
Authors: Zhuang Yueqing , Sun Xiaojing , Huang Diangui , Wu Guoqing
Published: November 2012

The flow control using leading-edge rotating cylinder on both two-dimensional NACA 63418 airfoil section and three-dimensional two-bladed wind turbine rotor has been studied by means of numerical simulations. For the two-dimensional NACA 63418 airfoil with a fasting spinning cylinder at the leading-edge, flow separation occurs on the pressure side of the airfoil near the clearance between rotating cylinder and stationary airfoil at small angles of attack; however, flow separation on the suction side of the airfoil can be suppressed at large angles of attack. The aerodynamic performances of the leading-edge rotation (LER) wind turbine rotors with two twisted/non-tapered blades and with two twisted/tapered blades have been studied, respectively. The result shows that the power efficiency of the LER wind turbine with twisted/non-tapered blades is superior to the uncontrolled wind turbine rotor remarkably at high tip speed ratios. The aerodynamic control effects of the leading-edge rotating cylinder have also been studied under various wind speeds. On the basis of the aerodynamic characteristics of non-tapered LER wind turbine rotor, the power output of the rotor can be optimized through adjusting the cylinder rotational speed in various wind speeds.
Cluster glasses of ultrasoft particles
Authors: Coslovich Daniele , Bernabei Marco , Moreno Angel J.
Published: November 2012

We present molecular dynamics (MD) simulations results for dense fluids of ultrasoft, fully penetrable particles. These are a binary mixture and a polydisperse system of particles interacting via the generalized exponential model, which is known to yield cluster crystal phases for the corresponding monodisperse systems. Because of the dispersity in the particle size, the systems investigated in this work do not crystallize and form disordered cluster phases. The clustering transition appears as a smooth crossover to a regime in which particles are mostly located in clusters, isolated particles being infrequent. The analysis of the internal cluster structure reveals microsegregation of the big and small particles, with a strong homo-coordination in the binary mixture. Upon further lowering the temperature below the clustering transition, the motion of the clusters’ centers-of-mass slows down dramatically, giving way to a cluster glass transition. In the cluster glass, the diffusivities remain finite and display an activated temperature dependence, indicating that relaxation in the cluster glass occurs via particle hopping in a nearly arrested matrix of clusters. Finally we discuss the influence of the microscopic dynamics on the transport properties by comparing the MD results with Monte Carlo simulations.
Optical properties of Mg-doped VO2: Absorption measurements and hybrid functional calculations
Authors: Hu Shuanglin , Li S.-Y. , Ahuja R. , Granqvist C. G. , Hermansson K. , Niklasson G. A. , Scheicher R. H.
Published: November 2012

Mg-doped VO2 thin films with thermochromic properties were made by reactive DC magnetron co-sputtering onto heated substrates, and spectral absorption was recorded at room temperature in the 0.5 < ħω < 3.5 eV energy range. Clear evidence was found for a widening of the main band gap from 1.67 to 2.32 eV as the Mg/(V + Mg) atomic ratio went from zero to 0.19, thereby significantly lowering the luminous absorption. This technologically important effect could be reconciled with spin-polarized density functional theory calculations using the Heyd-Scuseria-Ernzerhof [Heyd et al., J. Chem. Phys. 118, 8207 (2003); ibid. 124, 219906 (2006)] hybrid functional. Specifically, the calculated luminous absorptance decreased when the Mg/(V + Mg) ratio was increased from 0.125 to 0.250.
A perfect plasmonic quarter-wave plate
Authors: Gorodetski Yuri , Lombard Emmanuel , Drezet Aurélien , Genet Cyriaque , Ebbesen Thomas W.
Published: November 2012

The excitation of surface plasmons on an elliptical grating followed by the transmission through a subwavelength aperture can modify the polarization state of the incoming light. The combined effect of the elliptical grooves and a slightly elliptical central hole allows to fully control the birefringence and retardation of the structure, providing a simple approach for polarization state design by the geometry of the structure. From this combination, a perfect plasmonic quarter-wave plate is obtained.
Sustainability of superhydrophobicity under pressure
Authors: Samaha Mohamed A. , Vahedi Tafreshi Hooman , Gad-el-Hak Mohamed
Published: November 2012

Prior studies have demonstrated that superhydrophobicity of submerged surfaces is influenced by hydrostatic pressure and other environmental effects. Sustainability of a superhydrophobic surface could be characterized by both how long it maintains the trapped air in its surface pores, so-called “longevity,” and the pressure beyond which it undergoes a global wetting transition, so-called “terminal pressure.” In this work, we investigate the effects of pressure on the performance of electrospun polystyrene fibrous coatings. The time-dependent hydrophobicity of the submerged coating in a pressure vessel is optically measured under elevated pressures. Rheological studies are also performed to determine the effects of pressure on drag reduction and slip length. The measurements indicate that surface longevity exponentially decays with increasing pressure in perfect agreement with the studies reported in the literature at lower pressures. It is found, however, that fibrous coatings could resist hydrostatic pressures significantly higher than those of previously reported surfaces. Our observations indicate that superhydrophobic fibrous coatings could potentially be used for underwater applications.
The role of upstream distal electrodes in mitigating electrochemical degradation of ionic liquid ion sources
Authors: Brikner Natalya , Lozano Paulo C.
Published: November 2012

Ionic liquid ion sources produce molecular ions from micro-tip emitters wetted with room-temperature molten salts. When a single ion polarity is extracted, counterions accumulate and generate electrochemical reactions that limit the source lifetime. The dynamics of double layer formation are reviewed and distal electrode contacts are introduced to resolve detrimental electrochemical decomposition effects at the micro-tip apex. By having the emitter follow the ionic liquid potential, operation can be achieved for an extended period of time with no apparent degradation of the material, indicating that electrochemistry can be curtailed and isolated to the upstream distal electrode.
The fixed-bias Langmuir probe on the Communication/Navigation Outage Forecast System satellite: Calibration and validation
Authors: Klenzing J. , Rowland D.
Published: November 2012

A fixed-bias spherical Langmuir probe is included as part of the Vector Electric Field Instrument (VEFI) suite on the Communication/Navigation Outage Forecast System (C/NOFS) satellite. C/NOFS gathers data in the equatorial ionosphere between 400 and 860 km, where the primary constituent ions are H+ and O+. The ion current collected by the probe surface per unit plasma density is found to be a strong function of ion composition. The calibration of the collected current to an absolute density is discussed, and the performance of the spherical probe is compared to other in situ instruments on board the C/NOFS satellite. The application of the calibration is discussed with respect to future fixed-bias probes; in particular, it is demonstrated that some density fluctuations will be suppressed in the collected current if the plasma composition rapidly changes along with density. This is illustrated in the observation of plasma density enhancements on C/NOFS.
Note: Electronic circuit for two-way time transfer via a single coaxial cable with picosecond accuracy and precision
Authors: Prochazka Ivan , Kodet Jan , Panek Petr
Published: November 2012

We have designed, constructed, and tested the overall performance of the electronic circuit for the two-way time transfer between two timing devices over modest distances with sub-picosecond precision and a systematic error of a few picoseconds. The concept of the electronic circuit enables to carry out time tagging of pulses of interest in parallel to the comparison of the time scales of these timing devices. The key timing parameters of the circuit are: temperature change of the delay is below 100 fs/K, timing stability time deviation better than 8 fs for averaging time from minutes to hours, sub-picosecond time transfer precision, and a few picoseconds time transfer accuracy.
Controlled formation and manipulation of colloidal lattices by dynamically reconfigurable three dimensional interferometric optical traps
Authors: Xavier Jolly , Dasgupta Raktim , Ahlawat Sunita , Joseph Joby , Gupta Pradeep Kumar
Published: November 2012

We present real-time controlled manipulation of microparticles optically trapped in three dimensional (3D)-trap array lattices generated by dynamically reconfigurable n + 1 non-coplanar multiple plane wave interference in an umbrella-like configuration. Through a programmable spatial light modulator-assisted approach, reconfigurable stable 3D interferometric optical traps belonging to diverse transverse rotational symmetry are realized and used to trap micro beads in multi-layers. Dynamically controlled translation as well as rotation of trapped array of particles are also demonstrated using presented approach. Further, the optical stacking of microparticles in an array of 3D single-stranded chiral traps portrays the versatility in tailoring axially tunable trap arrays.
The role of Anderson-Gruneisen parameter in the estimation of self-diffusion coefficients in alkaline earth oxides
Authors: Dologlou Elizabeth
Published: November 2012

In a previous publication [J. Appl. Phys. 110, 036103 (2011)], we have shown that the bulk expansivity and elastic data can reproduce the self-diffusion coefficients in MgO over a wide range of values, i.e., 20 orders of magnitude. This publication was crossed with recent studies supporting the view that the Anderson-Gruneisen parameter δ is independent of the temperature in alkaline earth oxides. Here, we take this view and using the resulting elastic and expansivity parameters, we repeat the calculation for the diffusion coefficient of O in MgO. The results obtained agree with the experimental data.
Free volume change of elongated polyethylene films studied using a positron probe microanalyzer
Authors: Oka Toshitaka , Oshima Nagayasu , Suzuki Ryoichi , Uedono Akira , Fujinami Masanori , Kobayashi Yoshinori
Published: November 2012

Free volume change of low density polyethylene (LDPE) and high density polyethylene (HDPE) films upon mechanical deformation was microscopically investigated by positron probe microanalysis (PPMA). The ortho-positronium (o-Ps) lifetimes were gradually shortened by uniaxial deformation, indicative of shrinkage of the free volume. The o-Ps intensity for HDPE increased by deformation, whereas that for LDPE varied little. It suggests that destruction of crystallites plays an important role in the deformation of HDPE. PPMA is demonstrated to be a promising, powerful probe investigating free volume changes, at different local points subjected to different degrees of deformation, in elongated polymers.
Unipolar resistive switching behavior of Pt/LixZn1−xO/Pt resistive random access memory devices controlled by various defect types
Authors: Lin Chun-Cheng , Tseng Zong-Liang , Lo Kuang-Yao , Huang Chih-Yu , Hong Cheng-Shong , Chu Sheng-Yuan , Chang Chia-Chiang , Wu Chin-Jyi
Published: November 2012

The unipolar resistive switching behavior of Pt/LixZn1−xO/Pt structures fabricated via radio-frequency magnetron sputtering is investigated. Various Li doping concentrations influence the defect types (i.e., VO, Lii, and LiZn) in LixZn1−xO films for adjustable resistance ratio. The resistance ratio reaches 108 due to the minimized leakage current in the high-resistance state (HRS) at 6 at. % Li dopants. The dominant conduction mechanisms are explained in terms of Ohmic behavior and Poole-Frenkel (PF) emission. The coefficient r of the PF emission in the HRS is evaluated to confirm that the total number of defects in LixZn1−xO films decreases with increasing Li content.
Study of nonlinear ohmic heating and ponderomotive force effects on the self-focusing and defocusing of Gaussian laser beams in collisional underdense plasmas
Authors: Etehadi Abari M. , Shokri B.
Published: November 2012

In the present paper, the propagation characteristics of a linearly polarized gaussian laser beam in a non-isothermal underdense collisional plasma is studied. By considering the effects of the ponderomotive force and ohmic heating of plasma electrons as the nonlinear mechanisms, the second order differential equation of the dimensionless beam width parameter has been obtained and solved at several initial ion temperatures. Furthermore, by using the nonlinear dielectric permittivity of the mentioned plasma medium in the paraxial approximation and its dependence on the propagation characteristics of the gaussian laser pulse, the perturbed electron density ne/n0e is obtained and its variation in terms of the dimensionless plasma length is analyzed at different initial ion temperatures. Our results show that the dimensionless beam width parameter is strongly influenced by the initial plasma ion temperature. It is found that, for the self-focusing regime, the plasma electron density perturbation continuously oscillates between the initial density distribution and a minimum, and for the defocusing regime, the plasma electron density perturbation continuously oscillates between the initial density distribution and a maximum.
Assessment of hydrokinetic energy near Rose Dhu Island, Georgia
Authors: Bomminayuni Sandeep , Bruder Brittany , Stoesser Thorsten , Haas Kevin
Published: November 2012

The presented study reports on numerical simulations of flows in tidal channels near Rose Dhu Island, GA, which is used to identify hotspots of hydrokinetic energy and to assess the tidal stream energy potential at this site. The numerical simulations are complemented with field measurements of local current velocities and water surface heights, which are used to validate the simulations. Both velocity distributions and water surface heights as predicted by the numerical model are in good agreement with observed data. The simulations reveal a tidal asymmetry in the encompassing Ogeechee estuary with the ebb tide currents dominating over the flood tide ones. The model is able to successfully predict the distribution of discharge into the smaller creeks around Rose Dhu Island and thereby capturing the location of local hotspots of hydrokinetic energy. It is found that local hotspots do exist near the island, and the analysis suggests the maximum available annual power of 4.75 MW, with a peak estimated extraction surpassing 4 KW during Spring tides.
Point of common coupling voltage recovery capability improvement of offshore wind farm connected to a weak grid through high voltage direct current link
Authors: Chung Phan Dinh , Ro Kyoung-Soo
Published: November 2012

Large offshore wind farms using high voltage direct current transmission system have been considered and exploited in a few countries in the world. Maintaining stable operation of the wind farm is an important issue. Previous studies only proposed some methods to improve the fault ride-through capability of the wind farm as it is connected to a quite strong system, resulting in recovery of the voltage at the point of common coupling (PCC) after clearance of onshore grid-fault. This paper will evaluate the PCC voltage recovery ability of the wind farm when it is connected to a weak grid and suggest a new method to overcome unrecoverable voltage problem. The effectiveness of the proposed control strategies is proven by simulation results.
On stability of collisional coupling between relativistic electrons and ions in hot plasmas
Authors: Marushchenko I. , Azarenkov N. A. , Marushchenko N. B.
Published: November 2012

The collisional coupling of relativistic electrons and non-relativistic ions in hot plasmas has been analysed. It is found that relativistic effects produce a new feature: while the condition Te<3Ti guarantees a stable collisional coupling between electrons and ions in low-temperature plasmas, relativistic effects shift the upper Te/Ti boundary of stability to higher values. Moreover, for sufficiently high temperatures, Te,i>75 keV, collisional decoupling between relativistic electrons and ions becomes impossible.
Synthesis and magnetostrictive properties of Pr(Fe1.95B0.05)1.93 bulk nanocrystalline alloy
Authors: Shi Y. G. , Hu C. C. , Fan J. Y. , Shi D. N. , Lv L. Y. , Tang S. L.
Published: November 2012

The structure, magnetic properties, and magnetostriction of Pr(Fe1.95B0.05)1.93 alloys prepared by annealing its precursor amorphous ribbons under high pressure were investigated. It was found that Pr(Fe1.95B0.05)1.93 single cubic Laves phase could be obtained only when the pressure is up to 3 GPa. The average grain size about 20 nm is found in the sample synthesized under 6 GPa. A large linear magnetostriction of 541 ppm at 3 kOe is observed in the Pr(Fe1.95B0.05)1.93 compound synthesized under 6 GPa, which is 25% larger than that under 3 GPa. The present work offers an effective method to obtain bulk nanocrystalline magnetostrictive compounds.
Imaging of a patterned and buried molecular layer by coherent acoustic phonon spectroscopy
Authors: Hettich Mike , Jacob Karl , Ristow Oliver , He Chuan , Mayer Jan , Schubert Martin , Gusev Vitalyi , Bruchhausen Axel , Dekorsy Thomas
Published: November 2012

A molecular layer of aminopropyltriethoxysilane is patterned with a focused ion beam and subsequently covered by a gold film. The gold-polymer-substrate structures are afterwards imaged by ultrafast coherent acoustic phonon spectroscopy in reflection geometry. We demonstrate that the lateral structure of the covered polymer layer can be detected via the damping time of the vibrational mode of the gold film. Furthermore, we utilize Brillouin oscillations originating from the silicon substrate to map the structures and to estimate the molecular layer thickness.
Indium incorporation efficiency and critical layer thickness of (202¯1) InGaN layers on GaN
Authors: Ploch Simon , Wernicke Tim , Frentrup Martin , Pristovsek Markus , Weyers Markus , Kneissl Michael
Published: November 2012

In this study, the indium incorporation efficiency and critical layer thickness for misfit dislocation formation of thick (202¯1) oriented InGaN layers were investigated. InGaN layers with an indium content between 1.7% and 16% were grown by metalorganic vapor phase epitaxy. A reduced In-incorporation efficiency was found in comparison to (0001) oriented layers at 725°C. At lower growth temperature, the difference in incorporation efficiency between the two orientations is reduced. All (202¯1) InGaN layers, strained and relaxed with an In-content up to 12%, exhibit smooth surface morphologies with a rms roughness below 1 nm. In contrast to (0001) InGaN, hardly any strain is reduced by 3D-growth. The critical layer thickness for misfit dislocation formation of InGaN (202¯1) exhibits a behavior as predicted by the Matthews and Blakeslee model [J. W. Matthews and A. E. Blakeslee, J. Cryst. Growth 27, 118 (1974)]. Deviations, however, indicate that modifications of the formula are needed.
Formation of native defects in the γ-ray detector material Cs2Hg6S7
Authors: Im Jino , Jin Hosub , Li Hao , Peters John A. , Liu Zhifu , Wessels Bruce W. , Kanatzidis Mercouri G. , Freeman Arthur J.
Published: November 2012

Semiconductor γ-ray detectors have broad applications, yet finding superior detector materials is a challenge because of its contradictory requirements. Here, we investigated a large set of native defects in Cs2Hg6S7 that has been suggested as a promising candidate for detector materials. Using first-principles calculations, we showed that S-vacancy and HgCs-antisite defect provide life-time limiting deep levels, and Cs-vacancy forms a shallow acceptor level, resulting in low resistivity. To decrease such detrimental effects, concentrations of defects and carriers were examined in various chemical environments, which reveal that carrier densities can be extremely reduced by adjusting Cs partial pressure.
Tomographic effects of near-field microwave microscopy in the investigation of muscle cells interacting with multi-walled carbon nanotubes
Authors: Farina Marco , Di Donato Andrea , Monti Tamara , Pietrangelo Tiziana , Da Ros Tatiana , Turco Antonio , Venanzoni Giuseppe , Morini Antonio
Published: November 2012

In this work, we introduce a hybrid atomic-force/near-field scanning microwave microscope, exploiting the tomographic capabilities of the microwave microscopy to explore structures of relevant interest, namely, samples involving both biological and non-biological materials at the same time. In particular, we show imaging of C2C12 muscle cells grown in the presence of bundles of multi-walled carbon nanotubes: here, the microwave microscopy, by virtue of its tomographic potentiality, highlights how cells incorporate some nanotubes in their fibers.
Loop-mirror-based slot waveguide refractive index sensor
Authors: Kou Jun-long , Xu Fei , Lu Yan-qing
Published: November 2012

Loop mirror has been widely used in fiber optical devices and systems for it provides a smart way to make use of the fiber birefringence properties and can enhance the sensitivity greatly. On the other hand, slot waveguide is very promising for optical sensing applications because of their peculiar spatial mode profile. In this paper, we propose and analyze a loop-mirror-based slot waveguide (LMSW) sensor which can be routinely fabricated in modern high-volume complementary metal-oxide–semiconductor (CMOS) process. The finite element method (FEM) simulation results show that the birefringence can be as high as 0.8 which is orders of magnitude than that in conventional birefringent fiber loop mirror. High sensitivity up to 6 × 103 nm/RIU (refractive index unit) is achieved by this scheme.
Linking ligand perception by PEPR pattern recognition receptors to cytosolic Ca
2+
elevation and downstream immune signaling in plants
Authors: Ma Yi , Walker Robin K. , Zhao Yichen , Berkowitz Gerald A.
Published: November 2012

Little is known about molecular steps linking perception of pathogen invasion by cell surface sentry proteins acting as pattern recognition receptors (PRRs) to downstream cytosolic Ca
2+
elevation, a critical step in plant immune signaling cascades. Some PRRs recognize molecules (such as flagellin) associated with microbial pathogens (pathogen-associated molecular patterns, PAMPs), whereas others bind endogenous plant compounds (damage-associated molecular patterns, DAMPs) such as peptides released from cells upon attack. This work focuses on the
Arabidopsis
DAMPs plant elicitor peptides (Peps) and their receptors, PEPR1 and PEPR2. Pep application causes in vivo cGMP generation and downstream signaling that is lost when the predicted PEPR receptor guanylyl cyclase (GC) active site is mutated. Pep-induced Ca
2+
elevation is attributable to cGMP activation of a Ca
2+
channel. Some differences were identified between Pep/PEPR signaling and the Ca
2+
-dependent immune signaling initiated by the flagellin peptide flg22 and its cognate receptor Flagellin-sensing 2 (FLS2). FLS2 signaling may have a greater requirement for intracellular Ca
2+
stores and inositol phosphate signaling, whereas Pep/PEPR signaling requires extracellular Ca
2+
. Maximal FLS2 signaling requires a functional Pep/PEPR system. This dependence was evidenced as a requirement for functional PEPR receptors for maximal flg22-dependent Ca
2+
elevation, H
2
O
2
generation, defense gene [
WRKY33
and
Plant Defensin 1.2
(
PDF1.2
)] expression, and flg22/FLS2-dependent impairment of pathogen growth. In a corresponding fashion, FLS2 loss of function impaired Pep signaling. In addition, a role for PAMP and DAMP perception in bolstering effector-triggered immunity (ETI) is reported; loss of function of either FLS2 or PEPR receptors impaired the hypersensitive response (HR) to an avirulent pathogen.
Averaging principle for second-order approximation of heterogeneous models with homogeneous models
Authors: Fibich Gadi , Gavious Arieh , Solan Eilon
Published: November 2012

Typically, models with a heterogeneous property are considerably harder to analyze than the corresponding homogeneous models, in which the heterogeneous property is replaced by its average value. In this study we show that any outcome of a heterogeneous model that satisfies the two properties of differentiability and symmetry is
O
(ɛ
2
) equivalent to the outcome of the corresponding homogeneous model, where ɛ is the level of heterogeneity. We then use this averaging principle to obtain new results in queuing theory, game theory (auctions), and social networks (marketing).
Hierarchical mesoporous perovskite La
0
.5
Sr
0.5
CoO
2.91
nanowires with ultrahigh capacity for Li-air batteries
Authors: Zhao Yunlong , Xu Lin , Mai Liqiang , Han Chunhua , An Qinyou , Xu Xu , Liu Xue , Zhang Qingjie
Published: November 2012

Lithium-air batteries have captured worldwide attention due to their highest energy density among the chemical batteries. To provide continuous oxygen channels, here, we synthesized hierarchical mesoporous perovskite La
0.5
Sr
0.5
CoO
2.91
(LSCO) nanowires. We tested the intrinsic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity in both aqueous electrolytes and nonaqueous electrolytes via rotating disk electrode (RDE) measurements and demonstrated that the hierarchical mesoporous LSCO nanowires are high-performance catalysts for the ORR with low peak-up potential and high limiting diffusion current. Furthermore, we fabricated Li-air batteries on the basis of hierarchical mesoporous LSCO nanowires and nonaqueous electrolytes, which exhibited ultrahigh capacity, ca. over 11,000 mAh⋅g
–1
, one order of magnitude higher than that of LSCO nanoparticles. Besides, the possible reaction mechanism is proposed to explain the catalytic activity of the LSCO mesoporous nanowire.