bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomic-scale chemical analyses of niobium oxide/niobium interfaces via atom-probe tomography
Authors: Yoon Kevin E. , Seidman David N. , Antoine Claire , Bauer Pierre
Published: October 2008

Niobium is the metal of choice for superconducting radio-frequency cavities for the future International Linear Collider. We present the results of atomic-scale characterization of the oxidation of niobium utilizing local-electrode atom-probe tomography employing picosecond laser pulsing. Laser pulsing is utilized to prevent a tip from fracturing as a buried niobium oxide/niobium interface is dissected on an atom-by-atom basis. The thickness of niobium oxide is about 15 nm, the root-mean-square chemical roughness is 0.4 nm, and the composition is close to Nb2O5, which is an insulator, with an interstitial oxygen concentration profile in Nb extending to a depth of 12 nm.
The metal-insulator transition in dimerized Hückel chains
Authors: Monari Antonio , Bendazzoli Gian Luigi , Evangelisti Stefano
Published: October 2008

The metal-insulator transition is investigated in the case of linear chains described by a one-electron Hückel Hamiltonian. In these systems, the transition is a consequence of a dimerization of the chain bond length, which induces a similar dimerization of the hopping integral. Three indicators of the chain character are considered: The highest occupied molecular orbital–lowest unoccupied molecular orbital gap, the polarizability, and the localization tensor. In the case of even open chains, the behavior of the large chains depends in a crucial way on the alternating structure of the hopping integrals. If the ending atoms of the chain are weakly bonded to their neighbors, the energy spectrum of the Hamiltonian shows two quasidegenerated eigenvalues, and all the indicators would predict a (spurious) metallic behavior. It is shown that if the corresponding eigenvectors are removed from the Hamiltonian, the ordinary insulating behavior of alternating chains is recovered.
The surface enhanced Raman spectroscopic study of the adsorption of C70 on the gold nanoparticles
Authors: Wang Peijie , Fang Yan
Published: October 2008

Surface enhanced Raman spectra (SERS) of C70 adsorbed on the gold nanoparticles coated on the filter paper were obtained by employing pyridine molecule as medium. As a new SERS substrate, dried gold coated filter paper has a high SERS activity, of which enhancement factor can be up to 8.2×106. When the C70 molecule is adsorbed on the gold nanoparticle surface, its symmetry is reduced. Different orientations on the surface of gold nanoparticles will lead to different symmetry lowering. Generally, symmetry lowering may lead to the splitting of the Raman bands due to the degenerate modes. It was demonstrated that the C70 molecule was adsorbed on its pentagonal face with its long axis perpendicular to the gold nanoparticles surface. Its surface enhanced Raman spectral behavior is consistent with the analysis based on the group theory.
Au
42
: A possible ground-state noble metallic nanotube
Authors: Wang Jing , Ning Hua , Ma Qing-Min , Liu Ying , Li You-Cheng
Published: October 2008

A large hollow tubelike Au42 is predicted as a new ground-state configuration based on the scalar relativistic density functional theory. The shape of this new Au42 cluster is similar to a (5,5) single-wall gold nanotube, the two ends of which are capped by half of a fullerenelike Au32. In the same way, a series of Aun (n=37,42,47,52,57,62,67,72,…, Δn=5) tubelike structures has been constructed. The highest occupied molecular orbital-lowest unoccupied molecular orbital gaps suggested a significant semiconductor-conductor alternation in n∊[32,47]. Similar to the predictions and speculation of Daedalus [D. E. H. Jones, New Sci. 32, 245 (1966); E. Osawa, Superaromaticity (Kagaku, Kyoto, 1970), Vol. 25, pp. 854–863; Z. Yoshida and E. Osawa, Aromaticity Chemical Monograph (Kagaku Dojin, Kyoto, Japan, 1971), Vol. 22, pp. 174–176; D. A. Bochvar and E. G. Gal’pern, Dokl. Akad. Nauk SSSR 209, 610 (1973)], here a large hollow ground-state gold nanotube was predicted theoretically.
Micron-scale measurements of the coefficient of thermal expansion by time-domain probe beam deflection
Authors: Zheng Xuan , Cahill David G. , Weaver Richard , Zhao Ji-Cheng
Published: October 2008

We describe a pump-probe optical technique, time-domain probe-beam deflection, that enables measurements of the coefficient of thermal expansion (CTE) with micron-scale spatial resolution. Our quantitative model for the beam deflection includes contributions from thermal expansion of thin films deposited on a substrate, thermal expansion of the substrate, elastic deformation of the substrate, heating of air above the surface of the sample, and the lateral temperature gradient at the surface of the sample. The usefulness of this technique for high-throughput CTE measurement is demonstrated by two experiments: we are able to find the Invar alloy composition directly from a simple Fe–Ni diffusion couple and we find abnormal CTE increase in an ≈100μm region of dentin adjacent to the dentin-enamel junction of a human tooth. Such a CTE abnormality would be difficult to find using other techniques.
Asymptotic performance of optimal state estimation in qubit system
Authors: Hayashi Masahito , Matsumoto Keiji
Published: October 2008

We derive an asymptotic bound for the error of state estimation when we are allowed to use the quantum correlation in the measuring apparatus. It is also proven that this bound can be achieved in any statistical model in the qubit system. Moreover, we show that this bound cannot be attained by any quantum measurement with no quantum correlation in the measuring apparatus except for several specific statistical models. That is, in such a statistical model, the quantum correlation can improve the accuracy of the estimation in an asymptotic setting.
Band-gap engineering of Zn1−xGaxO nanopowders: Synthesis, structural and optical characterizations
Authors: Snure Michael , Tiwari Ashutosh
Published: October 2008

We report the preparation and detailed structural and optical characterizations of single phase gallium doped ZnO nanopowders. A low temperature solution-based technique was developed to synthesize high-purity Zn1−xGaxO (x:0–0.05) nanopowders. Structural and optical characterization experiments were performed using x-ray diffraction (XRD), energy dispersive x-ray spectroscopy, scanning electron microscopy, photoluminescence spectroscopy, Raman spectroscopy, and optical transmission spectroscopy. Analysis of XRD data showed a maximum in the lattice volume for x=0.03 as the Ga concentration increases in the Zn1−xGaxO series. Optical transmission spectroscopy results also showed an initial increase in the band gap (Eg) followed by a decrease for x>0.04. A maximum band gap was observed for the Zn0.96Ga0.04O sample, which has a value of 3.42 eV compared to the band-gap value of 3.31 eV for the undoped ZnO sample. All Zn1−xGaxO nanopowder samples showed a broad photoluminescence spectrum with significant shift in peak positions compared to bulk ZnO sample. The broadening and shifting in luminescence are understood to be caused by the nanocrystalline nature of the material and the presence of deep-level surface defects. These effects were further investigated by Raman microscopy where an increase in the intensity of E1(TO) mode was observed accompanied by the appearance of a few additional Raman active modes.
Charge carrier transport in thin films of colloidal CdSe quantum rods
Authors: Persano A. , Leo G. , Manna L. , Cola A.
Published: October 2008

Phototransport properties of organically capped colloidal CdSe quantum rod thin films deposited by spin coating are studied in air at room temperature in planar electrode configuration. Under optical excitation, the observed current-voltage characteristics and current transients are well described by a resonant tunneling model. A significant and irreversible current quenching of the photoresponse occurs with either the aging of the samples or the flowing of the current itself when above few picoamperes. The process, which is still interpreted in the frame of the model, can be attributed to the charge trapping by the defect states at the barrier between rods with a consequent increase in the barrier height.
Electronic and chemical properties of the TaN/a-SiOC:H stack studied by photoelectron spectroscopy for advanced interconnects
Authors: Martinez E. , Guedj C. , Mariolle D. , Licitra C. , Renault O. , Bertin F. , Chabli A. , Imbert G. , Delsol R.
Published: October 2008

Thin TaN metallic barriers are used to prevent copper diffusion into porous low-k dielectrics such as a-SiOC:H for advanced interconnects. We investigate the detailed electronic properties of the TaN/a-SiOC:H stack. Here we combine ultraviolet and x-ray photoelectron spectroscopy to measure the chemical composition and the whole band diagram of the TaN/a-SiOC:H stack. An original interpretation based on the image-force model used for internal photoemission is suggested to explain the electric field effect induced by negative bias of a-SiOC:H. This model is used to extrapolate the unbiased electron affinity of the dielectric. TaN work function, a-SiOCH band gap, valence band maximum and electron affinity of 4.6, 7.7, 4.0, and 3.8 eV are respectively obtained. Kelvin force microscopy and spectroscopic ellipsometry confirm TaN work function and a-SiOC:H band gap measurements, respectively. From the full band diagram of the TaN/a-SiOC:H stack, an interfacial barrier height of 0.8 eV is deduced.
Atomistic simulation of self-diffusion in Al and Al alloys under electromigration conditions
Authors: Sen F. G. , Aydinol M. K.
Published: October 2008

The effect of alloying elements on the self-diffusion behavior of Al under electromigration conditions was investigated using nonequilibrium molecular dynamics. The corresponding defect structures were also characterized energetically by Mott–Littleton approach. Pd, Cu, Mn, and Sn were found to be the most effective alloying elements that may retard the electromigration failure by increasing the activation energy for self-diffusion of Al. This increase in the activation energy is believed to be either because of the dragging effect that may be experienced in a coupled substitutional-vacancy defect structure or the energy penalty for the separation of this couple.
Influence of chromophore content on the steady-state space charge formation of poly[methyl-3-(9-carbazolyl) propylsiloxane]-based polymeric photorefractive composites
Authors: Oh Jin-Woo , Lee Choongkeun , Kim Nakjoong
Published: October 2008

We investigate the influence of chromophore content on the steady-state space charge formation. The chromophore is expected to act as a trap for hole transport in the matrix when the chromophore has a lower ionization potential as compared with the hole transporting material. We measured the photoconductivity and space charge field and then discussed their chromophore content dependence. At low chromophore concentrations, the photoconductivity decreased with increasing chromophore content due to the trap effect of chromophore. Above 3 wt % of chromophore concentration, however, the photoconductivity increased with the chromophore content through the increment of hole detrapping and the decrement of electron-hole recombination. The space charge field, which is dependent on the mobile hole and trap densities, increased with the chromophore content and saturated at about 15 wt % of chromophore. Numerical simulation work, the modified Schildkraut differential equation, was used to help us understand the experimental results. All experimental results are in excellent agreement with the simulation predictions.
Domain characterization of Pb(Zn1/3Nb2/3)O3−(6%–7%)PbTiO3 single crystals using scanning electron acoustic microscopy
Authors: Wong Meng Fei , Heng Xiangxin , Zeng Kaiyang
Published: October 2008

Domain structures of [001]T and [011]T-cut Pb(Zn1/3Nb2/3)O3−(6%−7%)PbTiO3 (PZN-PT) single crystals are studied using scanning electron acoustic microscope (SEAM) technique. The observation of the orientation of domain walls agree reasonably well with the trigonometric projection of rhombohedral and orthorhombic dipoles on the (001) and (011) surfaces, respectively. After mechanical loading with microindentation, domain switching is also observed to form a hyperbolic butterfly shape and extend preferentially along four diagonal directions, i.e., ⟨110⟩ on (001) surface and ⟨111¯⟩ on (011) surface. The critical shear stress to cause domain switching for PZN-PT crystal is estimated to be approximately 49 MPa for both {110} and {111¯} planes based on theoretical analysis. Generally, the SEAM technique has been successfully demonstrated to be a valid technique for observation of domain structures in single crystal PZN-PTs.
The effect of classical noise on a quantum two-level system
Authors: Aguilar Jean-Philippe , Berglund Nils
Published: October 2008

We consider a quantum two-level system perturbed by classical noise. The noise is implemented as a stationary diffusion process in the off-diagonal matrix elements of the Hamiltonian, representing a transverse magnetic field. We determine the invariant measure of the system and prove its uniqueness. In the case of Ornstein–Uhlenbeck noise, we determine the speed of convergence to the invariant measure. Finally, we determine an approximate one-dimensional diffusion equation for the transition probabilities. The proofs use both spectral-theoretic and probabilistic methods.
ac conductivity in Gd doped Pb(Zr0.53Ti0.47)O3 ceramics
Authors: Portelles J. , Almodovar N. S. , Fuentes J. , Raymond O. , Heiras J. , Siqueiros J. M.
Published: October 2008

This study is focused in the conduction processes taking place in 0.6 wt % Gd doped lead zirconate titanate samples PbZr0.53Ti0.47O3:Gd (PZT53/47:Gd) in the vicinity of the morphotropic phase boundary. Doped samples show very large dielectric permittivity with respect to that of undoped ones near the transition temperature. The frequency dependent ac conductivity of PZT53/47:Gd ceramics was studied in the 30–450 °C temperature range. X-ray diffraction analyses indicate the incorporation of Gd atoms to the structure. The changes in the dielectric properties as functions of temperature of the doped samples are taken as additional evidence of the incorporation of Gd into the crystal structure. Gd acts as donor center promoting extrinsic n-type conduction. The ac conductivity behavior obeys Jonscher universal relation in the 100 Hz–1 MHz frequency range for temperatures between 30 and 300 °C. The measured conductivity values for Gd doped PZT53/47 are higher than those of pure PZT53/47. According to the correlated barrier hopping model, the preponderant conduction mechanism in the frequency-temperature response was recognized as small polarons hopping mechanism.
Mechanical fatigue and dielectric relaxation of carbon black/polymer composites
Authors: Brosseau C. , Mdarhri A. , Vidal A.
Published: October 2008

Using a home-built experimental setup, we investigate the carbon black (CB) volume fraction-resolved, time-resolved, and frequency-resolved spectra of the room temperature (well above the glass transition temperature of the matrix) effective complex permittivity of well characterized CB filled ethylene butylacrylate copolymer samples that are submitted to a uniaxial tension. We focus here on three samples with CB volume fraction less, near, and well away the percolation threshold at about 8 vol %. Our primary observation is that the temporal evolution of the real and imaginary parts of the effective permittivity is distinctly different for samples containing a CB volume fraction below and above percolation threshold. For samples containing a CB volume fraction below and close to the percolation threshold, and at a given frequency, ε′ and ε″ remain constant over the time scale of our measurements. For the sample containing a CB volume fraction above the percolation threshold the evolution of ε′ and ε″ is different in the low and high elongation ratio regimes. At low strain, the temporal evolution of permittivity during aging under stress shows a logarithmic growth phase followed by a logarithmic decay phase, whereas for sufficiently large strain the permittivity behaviors coincide with those observed below the percolation threshold. To explain the issues involved, we argue that the phenomenology for physical aging in these materials is related to the change in the mesostructure, formed by the heterogeneous three-dimensional interconnected network of polymer and of aggregates (or agglomerates) of CB particles, as the composite is stretched. Indeed, it is possible to argue qualitatively that below the percolation threshold the time-independent permittivity behavior is reminiscent to the elasticity network properties of the polymeric matrix. The reorientation and breakdown of the CB aggregates are believed to be crucial for the physical aging understanding above the percolation threshold and low strain. An examination of the surface and volume morphological evolutions of these materials under the action of a mechanical stress at the microscale by scanning electron microscopy and atomic force microscopy indicates that aging during a few hundreds of hours, even at a moderate strain, generates voids and cracks that are aligned along the stretching direction. The overall behavior is compared to what occurs in another type of filled polymer system, i.e., plastoferrites, for which it was recently discovered that two characteristic time scales are required to describe physical aging.
Measurement and parametrization of proton elastic scattering cross sections for nitrogen
Authors: Radović I. Bogdanović , Siketić Z. , Jakšić M. , Gurbich A. F.
Published: October 2008

The cross sections for the elastic scattering of protons from natural nitrogen at non-Rutherford scattering energies were measured at three laboratory scattering angles: 118°, 150°, and 165°. The experimental data were parametrized in the framework of nuclear physics models. A benchmark experiment was performed in order to prove that the excitation functions obtained in the present work can be used to adequately simulate the yield from a thick target containing nitrogen.
Inferring connectivity of interacting phase oscillators
Authors: Yu Dongchuan , Fortuna Luigi , Liu Fang
Published: October 2008

The question as to how network topology properties influence network dynamical behavior has been extensively investigated. Here we treat the inverse problem, i.e., how to infer network connection topology from the dynamic evolution, and suggest a control based topology identification method. This method includes two steps: (i) driving the network to a steady state and (ii) inferring all elements of the connectivity matrix by exploiting information obtained from the observed steady state response of each node. We adopt different strategies for model-dependent (i.e., each local phase dynamics and coupling functions are known) and model-free (i.e., each local phase dynamics and coupling functions are unknown) cases and give detailed conditions for both cases under which network topology can be identified correctly. The influence of noise on topology identification is discussed as well. All proposed approaches are motivated and illustrated with networks of phase oscillators. We argue that these topology identification methods can be extended to general dynamical networks and are not restricted to only networks of phase oscillators.
Plasma-liquid electrochemistry: Rapid synthesis of colloidal metal nanoparticles by microplasma reduction of aqueous cations
Authors: Richmonds Carolyn , Sankaran R. Mohan
Published: October 2008

We demonstrate operation of an aqueous electrochemical cell with an atmospheric-pressure microplasma cathode and a solid metal anode for the rapid production of colloidal metal nanoparticles. Microplasmas are miniaturized versions of low-pressure glow discharges that operate nonthermally at high pressures (∼1atm) and contain energetic electrons. Aqueous metal cations are directly reduced by electrons in the microplasma without the presence of a solid cathode or chemical reducing agents. Kinetic studies performed by UV-visible absorbance spectroscopy suggest that particle nucleation and growth are coupled to redox reactions initiated in the electrochemical cell through plasma-liquid interactions.
Planar Hall bead array counter microchip with NiFe/IrMn bilayers
Authors: Tu Bui Dinh , Hung Tran Quang , Thanh Nguyen Trung , Danh Tran Mau , Duc Nguyen Huu , Kim CheolGi
Published: October 2008

The planar Hall effect (PHE) magnetic bead array countermicrochip integrating 24 of single sensors based on a simple NiFe/IrMn bilayer structure with a patterned size of 3×3 μm2 has been fabricated and characterized. Single PHE sensors exhibit a sensitivity of about 2.5 mΩ/Oe. It was well applied for single Dynabeads® M-280 detection, where one bead can induce a signal change, ΔV∼2.2 mV, under the applied magnetic field of 20 Oe and a sensing current of 2 mA. This type of microchip is promising for quickly detecting and identifying multiple biological agents in the environment.
Method specific Cholesky decomposition: Coulomb and exchange energies
Authors: Boman Linus , Koch Henrik , Sánchez de Merás Alfredo
Published: October 2008

We present a novel approach to the calculation of the Coulomb and exchange contributions to the total electronic energy in self consistent field and density functional theory. The numerical procedure is based on the Cholesky decomposition and involves decomposition of specific Hadamard product matrices that enter the energy expression. In this way, we determine an auxiliary basis and obtain a dramatic reduction in size as compared to the resolution of identity (RI) method. Although the auxiliary basis is determined from the energy expression, we have complete control of the errors in the gradient or Fock matrix. Another important advantage of this method specific Cholesky decomposition is that the exchange energy and Fock matrix can be evaluated with a linear scaling effort contrary to the RI method or standard Cholesky decomposition of the two-electron integral matrix. The methods presented show the same scaling properties as the so-called local density fitting methods, but with full error control.
Excitation of parasitic modes in gyrotrons with fast voltage rise
Authors: Nusinovich G. S. , Vlasov A. N. , Antonsen T. M. , Lohr J. , Danly B. G. , Hogge J.-P.
Published: October 2008

Megawatt-class, long-pulse and continuous-wave gyrotrons operate in high-order modes in order to handle the thermal losses in the resonators. The spectral density of neighboring modes in these devices is high; consequently during the beam voltage rise parasitic modes can be excited. If such gyrotrons are used in plasma experiments, these parasites can be dangerous for the receiving channels of numerous diagnostic tools. It is shown that in gyrotrons utilizing diode-type magnetron injection guns the shortening of the voltage rise time to about one microsecond can practically eliminate excitation of parasitic modes even in devices with a 2MW power level. Some means for realizing such rise times in high-voltage modulators which can be used for megawatt-class gyrotrons are discussed. It is also shown that allowable beam voltage overshoot in such gyrotrons is about 2.5%.
Effect of electron temperature on dynamic characteristics of two-dimensional sheath in Hall thrusters
Authors: Yu Daren , Zhang Fengkui , Liu Hui , Li Hong , Yan Guojun , Liu Jinyuan
Published: October 2008

The Debye sheath has a significant effect on the performance of Hall thrusters. The dynamic characteristics of the two-dimensional sheath is investigated using the 2D-3V particle-in-cell method in this paper. The numerical results show that while the sheath exhibits the one-dimensional stability when the electron temperature is relatively low, it behaves as a two-dimensional (both in time and space) oscillating characteristic when the electron temperature is high. Moreover, it is found that the oscillating frequency is the same order as the electron plasma frequency and the spatial wavelength is equal to the length of the electrostatic wave.
Extreme ultraviolet emission from dense plasmas generated with sub-10-fs laser pulses
Authors: Osterholz J. , Brandl F. , Cerchez M. , Fischer T. , Hemmers D. , Hidding B. , Pipahl A. , Pretzler G. , Rose S. J. , Willi O.
Published: October 2008

The extreme ultraviolet (XUV) emission from dense plasmas generated with sub-10-fs laser pulses with varying peak intensities up to 3×1016W∕cm2 is investigated for different target materials. K shell spectra are obtained from low Z targets (carbon and boron nitride). In the spectra, a series limit for the hydrogen- and helium-like resonance lines is observed, indicating that the plasma is at high density and that pressure ionization has removed the higher levels. In addition, L shell spectra from titanium targets were obtained. Basic features of the K and L shell spectra are reproduced with computer simulations. The calculations include hydrodynamic simulation of the plasma expansion and collisional radiative calculations of the XUV emission.
Propagation regimes for an electromagnetic beam in magnetized plasma
Authors: Sharma Ashutosh , Kourakis Ioannis , Sodha M. S.
Published: October 2008

The propagation of a Gaussian electromagnetic beam along the direction of magnetic field in a plasma is investigated. The extraordinary (Ex+iEy) mode is explicitly considered in the analysis, although the results for the ordinary mode can be obtained upon replacing the electron cyclotron frequency ωc by −ωc. The propagating beam electric field is coupled to the surrounding plasma via the dielectric tensor, taking into account the existence of a stationary magnetic field. Both collisionless and collisional cases are considered, separately. Adopting an established methodological framework for beam propagation in unmagnetized plasmas, we extend to magnetized plasmas by considering the beam profile for points below the critical curve in the beam-power versus beam-width plane, and by employing a relationship among electron concentration and electron temperature, provided by kinetic theory (rather than phenomenology). It is shown that, for points lying above the critical curve in the beam-power versus beam-width plane, the beam experiences oscillatory convergence (self-focusing), while for points between the critical curve and divider curve, the beam undergoes oscillatory divergence and for points on and below the divider curve the beam suffers a steady divergence. For typical values of parameters, numerical results are presented and discussed.
Growth of carbon nanocone arrays on a metal catalyst: The effect of carbon flux ionization
Authors: Levchenko I. , Ostrikov K. , Khachan J. , Vladimirov S. V.
Published: October 2008

The growth of carbon nanocone arrays on metal catalyst particles by deposition from a low-temperature plasma is studied by multiscale Monte Carlo/surface diffusion numerical simulation. It is demonstrated that the variation in the degree of ionization of the carbon flux provides an effective control of the growth kinetics of the carbon nanocones, and leads to the formation of more uniform arrays of nanostructures. In the case of zero degree of ionization (neutral gas process), a width of the distribution of nanocone heights reaches 360nm with the nanocone mean height of 150nm. When the carbon flux of 75% ionization is used, the width of the distribution of nanocone heights decreases to 100nm, i.e., by a factor of 3.6. A higher degree of ionization leads to a better uniformity of the metal catalyst saturation and the nanocone growth, thus contributing to the formation of more height-uniform arrays of carbon nanostructures.
Wafer-scale silicon nanopillars and nanocones by Langmuir–Blodgett assembly and etching
Authors: Hsu Ching-Mei , Connor Stephen T. , Tang Mary X. , Cui Yi
Published: October 2008

We have developed a method combining Langmuir–Blodgett assembly and reactive ion etching to fabricate nanopillars with uniform coverage over an entire 4 inch wafer. We demonstrated precise control over the diameter and separation between the nanopillars ranging from 60 to 600 nm. We can also change the shape of the pillars from having vertical to tapered sidewalls with sharp tips exhibiting a radius of curvature of 5 nm. This method opens up many possible opportunities in nanoimprinting, solar cells, batteries, and scanning probes.
Influence of uniaxial tension on the microwave absorption properties of filled polymers
Authors: Brosseau C. , NDong W. , Mdarhri A.
Published: October 2008

The evolution of the frequency-dependent absorption spectra for plastoferrites and carbon black-filled ethylene butylacrylate copolymer composites subjected to a uniaxial tension is evaluated using a vector network analyzer as a function of the elongation ratio over the frequency range of 0.3–5 GHz and at room temperature. The absorption spectrum has a double-peaked structure with two broad and symmetrical components. The positions of the lower-frequency peak (1.5 GHz) and the higher-frequency (≅4±0.3 GHz) do not change with extension over the considered range of strain, however, their relative absorption energy changes versus tensile stress can be explained in terms of a Gaussian molecular network model (affine behavior) provided that the elasticity network in the material occurs in a manner that is topologically similar to the elasticity network of a conventional rubber. The sensitivity of the microwave absorption to elongation ratio can be exploited to provide artificially structured materials with tunable electromagnetic properties.
Terahertz emission from a laser pulse with tilted front: Phase-matching versus Cherenkov effect
Authors: Bakunov M. I. , Bodrov S. B. , Tsarev M. V.
Published: October 2008

A theory that describes the terahertz generation via optical rectification of a femtosecond laser pulse with tilted front in an electro-optic crystal is developed. The theory accounts for the transverse size of the laser pulse and allows us to explore the dependence of the terahertz yield on laser focusing. Two typical experimental situations—LiNbO3 excited with Ti:sapphire laser at room and cryogenic temperatures—are considered. It is shown that depending on temperature the dominant generation mechanism can be either phase-matching or Cherenkov effect. The parameters of the laser pulse (transverse size, tilt angle, and pulse duration) and crystal size maximizing the terahertz yield for LiNbO3 are calculated.
Undoped ZnO phosphor with high luminescence efficiency grown by thermal oxidation
Authors: Xiao Zhiyan , Okada Morihro , Han Gui , Ichimiya Masayoshi , Michibayashi Katsuyoshi , Itoh Tadashi , Neo Yoichiro , Aoki Toru , Mimura Hidenori
Published: October 2008

Various ZnO morphologies with hexagonal cross sections were fabricated by a thermal oxidation method. Depending on the growth temperature, the lateral lengths of the hexagonal cross section varied from several hundred nanometers to the micrometer order. Comparing the cathodoluminescence (CL) of the synthesized ZnO to that of commercial ZnO:Zn phosphor revealed that a strong luminescence can be realized in undoped ZnO by fabricating one-dimensional structures. Specifically, in the micropyramid and towerlike structures, the CL intensity is locally concentrated near the hexagonal boundary. The enhanced green emission in these structures can be qualitatively explained by whispering gallery resonance modes produced in the hexagonal microcavity.
Effects of Ru underlayer on microstructures and magnetic properties of Co72Pt28 thin films
Authors: Pandey K. K. M. , Chen J. S. , Lim B. C. , Chow G. M.
Published: October 2008

The effects of Ru underlayer on crystallographic structure, microstructures, and magnetic properties of Co72Pt28 films were investigated. It was observed that Ru deposition pressure played a more important role in determining the crystallographic texture compared to Ru thickness. Ru had a preferred (0002) texture when deposited at low Ar pressure and induced perpendicular magnetic anisotropy for Co72Pt28. However, at high Ar pressure, Ru (101¯0) and Ru (101¯1) peaks appeared that changed the easy axes of magnetization of Co72Pt28 film from out-of-plane to in-plane directions. With increasing thickness of Ru top layer in the dual-layer structure of Ru underlayer, coercivities increased due to increased magnetic anisotropy and reduced intergranular magnetic interaction.
Origin of trapping in multicrystalline silicon
Authors: Gundel Paul , Schubert Martin C. , Warta Wilhelm
Published: October 2008

Defect sites in silicon, which temporarily capture excess charge carriers (traps), are a promising source of information on defect structures relevant for photovoltaic application of the material. In this work the correlation between traps in p-type silicon, structural crystal defects, and impurities is explored in order to find the origin of these traps in multicrystalline silicon. The trap density is compared to the density of different impurities and structural crystal defects. These comparisons reveal that the trap density is positively correlated with the oxygen density and negatively correlated with the density of the metallic impurities analyzed. In addition we show that structural crystal defects are necessary but not sufficient for the existence of high trap densities. In summary, structural crystal defects that are decorated by oxygen precipitates arise as likely origin of trap centers.
Ex situ variable angle spectroscopic ellipsometry studies on chemical vapor deposited boron-doped diamond films: Layered structure and modeling aspects
Authors: Gupta S. , Dudipala A. , Williams O. A. , Haenen K. , Bohannan E.
Published: October 2008

We report the optical property measurements on boron-doped diamond (BDD) films which were synthesized by microwave plasma-assisted chemical vapor deposition technique on Si (100) using methane in high hydrogen dilution and trimethylboron as precursors with varying boron concentration such that [B]∕[C]gas=100, 500, 1000, 2000, 4000, and 6467ppm. These BDD films were investigated using a rotating analyzer variable angle spectroscopic ellipsometry (SE) from the near IR to UV range (830–193nm). By applying the conventional Bruggeman effective medium approximation and linear regression analyses to the raw SE data that is, [ψ(λi),Δ(λi)] and pseudodielectric function (⟨εr(λi)⟩,⟨εi(λi)⟩), we determined the most appropriate model fit. The SE modeling was performed through the normal and point-by-point fit methods combined with the coupled and uncoupled bulk and surface layer approaches providing the details about the thin films’ microstructure in terms of the (a) multilayer (component and surface) structure and component layer thickness of the films, (b)volume fraction of constituents [fsp3C, fsp2C and void (fv) in the component layer], (c) inhomogeneity of the structure along the growth axis and its variation with boron concentration, and (iv) surface roughness layer thickness (ds) with dimensions less than the optical wavelength that is not otherwise available. A simplified three-layer structural model consisting of an interfacial layer, an intermediate (or bulk) layer, and a top surface roughness layer has been proposed, which simulates the ellipsometry data reasonably well with coupled point-by-point method. An estimator, i.e., mean squared error (χ2), is used to assess the accuracy of the model fit. The results (surface roughness and constituents’ fraction) obtained through SE modeling are also compared with those from atomic force microscopy (AFM) and Raman spectroscopy to validate the layered model employed. Typically, high surface roughness values around 6nm were found for films grown under different boron concentrations which is almost five times smaller than determined from AFM. In this context, we determined an approximate linear relationship between these two variables. The relatively smaller surface roughness for BDD films indicates the combined role of boron-hydrogen (B, H) in diamond (C) while keeping the substrate temperature constant. We also discussed the variation of (fv and fnd) for the bulk and surface layers with increasing boron concentration.
Off-axis beaming from subwavelength apertures
Authors: Caglayan Humeyra , Bulu Irfan , Ozbay Ekmel
Published: October 2008

Photonic crystal waveguides and metallic subwavelength apertures are promising tools for light manipulation. It is possible to obtain enhanced directional beams by using these structures via coupling to surface waves. In addition, these apertures can be designed to steer such directional beams by introducing asymmetrical gratings on the output surface. In the present paper, we report directional yet off-axis beaming from subwavelength apertures at microwave frequencies. The full width at half maximum of the beam is 10° while the beaming angle is 15°. Our results show that it is possible to steer the beam by the appropriate modification of the output surface.
Selective domain wall depinning by localized Oersted fields and Joule heating
Authors: Ilgaz Dennis , Kläui Mathias , Heyne Lutz , Boulle Olivier , Zinser Fabian , Krzyk Stephen , Fonin Mikhail , Rüdiger Ulrich , Backes Dirk , Heyderman Laura J.
Published: October 2008

Using low temperature magnetoresistance measurements, the possibility to selectively move a domain wall locally by applying current pulses through a Au nanowire adjacent to a permalloy element is studied. We find that the domain wall depinning field is drastically modified with increasing current density due to the Joule heating and the Oersted field of the current, and controlled motion due to the Oersted field without any externally applied fields is achieved. By placing the domain wall at various distances from the Au wire, we determine the range of the Joule heating and the Oersted field and both effects can be separated.
InSbN alloys prepared by two-step ion implantation for infrared photodetection
Authors: Zhang D. H. , Liu W. , Wang Y. , Chen X. Z. , Li J. H. , Huang Z. M. , Zhang Sam S. Y.
Published: October 2008

InSbN alloys are fabricated by two-step nitrogen ion implantation into InSb (111) wafers. X-ray photoelectron spectroscopy indicates that most of the implanted nitrogen ions substitute Sb to form In–N bonds. The percentage of the In–N bonds is found to decrease with the increase in the implanted nitrogen. Such alloys can effectively detect long wavelength infrared radiation and the absorption peak energies can be controlled by monitoring the implanted nitrogen dose. The measured peak wavelengths are consistent with the band gaps of the alloys calculated using a ten-band k⋅p model.
Self-focusing of electromagnetic pulsed beams in collisional plasmas
Authors: Faisal Mohammad , Verma M. P. , Sodha Mahendra Singh
Published: October 2008

In this paper, the self-focusing of an electromagnetic pulsed beam in a collisional plasma has been investigated in the paraxial approximation, following the formalism developed by Akhmanov. The energy balance equation for electrons, the equation expressing the equality of pressure gradient (of electrons and ions) to the force due to space charge field, and the equation for the beam width parameter f (obtained by following Akhmanov’s approach) have been simultaneously solved for given initial (z=0) time profile of the pulse to obtain f as a function of ξ (cz∕ωr02) and t′=t−z∕Vg, where Vg is the group velocity. Both Gaussian and sine time profiles of the pulse have been investigated.
Evolutions of non-steady-state magnetic reconnection
Authors: Wan Weigang , Lapenta Giovanni
Published: October 2008

The full evolutions of collisionless non-steady-state magnetic reconnection are studied with full kinetic particle-in-cell simulations. There are different stages of reconnection: the onset or early growing stage when the out-of-plane electric field (Ey) structure is a monopole at the X-point, the bipolar stage when the Ey structure is bipolar and the outer electron diffusion region (EDR) is being elongated over time, and the possible final steady-state stage when Ey is uniform in the reconnection plane. We find the change of reconnection rate is not empowered or dependent on the length of the EDR. During the early growing stage, the EDR is elongated while the reconnection rate is growing. During the later stage, the reconnection rate may significantly decrease but the length of the inner EDR is largely stable. The results indicate that reconnection is not controlled by the downstream physics, but rather by the availability of plasma inflows from upstream. The physical mechanism of the EDR elongation is studied. The Hall current induced by the quadrupole magnetic field (By) is discovered to play an important role in this process. The condition of forming an extended electron super-Alfvénic outflow jet structure in nature is discussed. The jet structure could be elongated during the bipolar stage, and remains stable during steady state. The sufficiency of the electron inflow is crucial for the elongation. Open boundary conditions are applied in the outflow direction.
Resonant and phonon-assisted tunneling transport through silicon quantum dots embedded in SiO2
Authors: Berghoff B. , Suckow S. , Rölver R. , Spangenberg B. , Kurz H. , Dimyati A. , Mayer J.
Published: October 2008

Charge transport through SiO2∕Si∕SiO2 double-barrier structures (DBSs) and SiO2 single-barrier structures is investigated by low temperature I-V measurements. Resonant tunneling signatures accompanied by a negative differential conductance are observed if silicon quantum dots (Si QDs) are embedded in the amorphous SiO2 matrix. The I-V characteristics are correlated with the morphology of Si QDs extracted from transmission electron microscopy and photoluminescence. Evidence for phonon-assisted tunneling at low voltages has been found in the DBSs. These results show the potential but also the limitation for charge extraction from Si QDs embedded in SiO2.
Amplified dispersive Fourier-transform imaging for ultrafast displacement sensing and barcode reading
Authors: Goda Keisuke , Tsia Kevin K. , Jalali Bahram
Published: October 2008

Dispersive Fourier transformation is a powerful technique in which the spectrum of an optical pulse is mapped into a time-domain waveform using chromatic dispersion. It replaces a diffraction grating and detector array with a dispersive fiber and single photodetector. This simplifies the system and, more importantly, enables fast real-time measurements. Here we describe a novel ultrafast barcode reader and displacement sensor that employs internally amplified dispersive Fourier transformation. This technique amplifies and simultaneously maps the spectrally encoded barcode into a temporal waveform. It achieves a record acquisition speed of 25MHz—four orders of magnitude faster than the current state of the art.
A new mark shearing technique for strain measurement using digital image correlation method
Authors: Hua Tao , Xie Huimin , Pan Bing , Wang Qinghua , Dai Fulong
Published: October 2008

The digital image correlation (DIC) method is a noncontact, full-field surface deformation measuring method, which is widely used in experimental mechanics. Although the DIC method has a high precision in displacement measurement, its precision in strain determination is a bit low. In this paper, a new mark shearing technique is proposed to improve the accuracy of the strain measurement of the DIC method. A wedge mirror is used to introduce a shearing distance of the marks, which are made on the specimen to calculate the strain. The measurement principle of the mark shearing technique is described in detail. From the analysis, it can be concluded that this method is suitable for both large-scaled and small-scaled specimens, and thus has a wider adaptability than the common DIC method. Using this method, the maximum gauge length is 80 mm, and the accuracy of strain measurement can reach 4 μm strains. A tensile experiment with aluminum sample was conducted, and the successful results demonstrated the feasibility of this method.
Analysis of peer-to-peer locking of magnetrons
Authors: Pengvanich P. , Lau Y. Y. , Cruz E. , Gilgenbach R. M. , Hoff B. , Luginsland J. W.
Published: October 2008

The condition for mutual, or peer-to-peer, locking of two magnetrons is derived. This condition reduces to Adler’s classical phase-locking condition in the limit where one magnetron becomes the “master” and the other becomes the “slave.” The formulation is extended to the peer-to-peer locking of N magnetrons, under the assumption that the electromagnetic coupling among the N magnetrons is modeled by an N-port network.
Dominant ultraviolet light emissions in packed ZnO columnar homojunction diodes
Authors: Kong Jieying , Chu Sheng , Olmedo Mario , Li Lin , Yang Zheng , Liu Jianlin
Published: October 2008

The growth of Sb-doped p-type ZnO∕Ga-doped n-type ZnO homojunction on Si (100) substrate by molecular beam epitaxy led to closely packed vertical ZnO columns with lateral diameters ranging from 100to400nm. Mesa structures were defined and Ohmic contact of both n-type ZnO and p-type ZnO was realized with Au∕Ti and Au∕NiO, respectively. I-V and C-V curves present typical electrical properties of a diode, indicating that reliable p-type ZnO was formed. Electroluminescence shows dominant ultraviolet emissions with insignificant deep-level related yellow/green band emissions at different drive currents from 60to100mA at room temperature.
Transport properties of three-terminal ballistic junctions realized by focused ion beam enhanced etching in InGaAs/InP
Authors: Frimmer Martin , Sun Jie , Maximov Ivan , Xu H. Q.
Published: October 2008

Three-terminal junction devices are realized in an InGaAs/InP quantum well by focused ion beam (FIB) implantation and selective wet etching. Room temperature electrical measurements show that the fabricated devices exhibit strong nonlinear electrical properties. The results are discussed in terms of ballistic electron transport. It is demonstrated that FIB-enhanced etching processing can be exploited as a maskless, resist-free technique for fabrication of high-quality and functional nanoelectronic devices.
Nitrogen-polar GaN growth evolution on c-plane sapphire
Authors: Sun Q. , Cho Y. S. , Lee I.-H. , Han J. , Kong B. H. , Cho H. K.
Published: October 2008

This letter presents a study on N-polar GaN growth evolution on sapphire using a low-temperature GaN buffer, which is distinctly different from the two-step growth of Ga-polar GaN according to both in situ reflectance and ex situ microscopy. Annealed N-polar GaN buffer exhibits densely packed tiny grains, serving as a template for the subsequent high-temperature GaN growth, which starts in a quasi-two-dimensional mode without any roughening-recovery process. Atomically smooth N-polar GaN has been achieved with no stacking fault or inversion domain observed. The mosaic microstructure, electrical, and optical properties of N-polar GaN are compared with those of Ga-polar GaN.
Mechanism of flatband voltage roll-off studied with Al2O3 film deposited on terraced oxide
Authors: Choi Kisik , Wen Huang-Chun , Bersuker Gennadi , Harris Rusty , Lee Byoung Hun
Published: October 2008

The mechanism of flatband voltage roll-off (FVRO) has been investigated with capacitors fabricated on terraced oxide with various thicknesses of Al2O3 film deposited by atomic layer deposition. It is found that the FVRO is mainly controlled by the thickness of the bottom interfacial layer and is independent of Al2O3 thickness. This indicates that the dipole at the SiO2/Al2O3 interface is mainly responsible for the FVRO phenomenon rather than the charges. Electrostatic analysis suggests that the disruption of the interface dipole by oxygen vacancy generated in thinner bottom interfacial layer is a potential cause of the FVRO.