bluuebunny/crossref_metadata_2025 huggingface dataset:
Multipactor in a coaxial transmission line. II. Particle-in-cell simulations
Authors: Semenov V. E. , Zharova N. , Udiljak R. , Anderson D. , Lisak M. , Puech J.
Published: March 2007

The effects of nonuniform radiofrequency fields are analyzed for a cylindrical coaxial transmission line using a particle-in-cell code. The behavior predicted by the analytical analysis is confirmed by this study. In addition, by including an initial velocity spread and different maximum secondary electron yields, a more elaborate analysis is performed. It is found that in the case of high secondary emission, an increase in the spread of initial velocities results in an overlapping of the multipactor zones for both double and single sided multipactor. In the other case, when the secondary emission is low, a large enough initial velocity spread can lead to total suppression of the electron avalanche for the higher order modes. Comparison with available experimental data shows good agreement.
Application of the phenomenological Landau theory of improper ferroelectrics to perovskite rare-earth manganites
Authors: Dai L. , Jiang Q.
Published: March 2007

Based on the phenomenological Landau theory of improper ferroelectric phase transition, the magnetoelectric properties of TbMnO3 are investigated. We show that the appearance of the transverse magnetic moment of Mn as one of the primary order parameters is the key to the appearance of polarization, and its rotating is responsible for the magnetic-field-induced polarization flop. Our theoretical results are in good agreement with the experimental data.
Rheology of concentrated carbon nanotube suspensions
Authors: Hobbie E. K. , Fry D. J.
Published: March 2007

The rheological properties of non-Brownian carbon nanotube suspensions are measured over a range of nanotube volume fractions spanning the transition from semidilute to concentrated. The polymer-stabilized nanotubes are “sticky” and form a quiescent elastic network with a well-defined shear modulus and yield stress that both depend strongly on nanotube volume fraction with different but related critical exponents. We compare controlled-strain-rate and controlled-stress measurements of yielding in shear flow, and we study the effect of slow periodic stress reversal on yielding and the arrest of flow. Our measurements support a universal scaling of both the linear viscoelastic and steady-shear viscometric response. The former allows us to extract the elastic shear modulus of semidilute nanotube networks for values that are near or below the resolution limit of the rheometers used, while the latter provides a similar extrapolation of the yield stress. A simple scaling argument is used to model the dependence of yield stress and elastic modulus on concentration.
Composition dependence and the nature of endothermic freezing and exothermic melting
Authors: Ferrari C. , Tombari E. , Salvetti G. , Johari G. P.
Published: March 2007

The heat capacity, Cp, and enthalpy and entropy change of α-cyclodextrin, H2O, and 4-methylpyridine solutions have been studied during their freezing on heating, isothermal freezing, and the solid’s melting on cooling. Freezing occurs in several endothermic steps on heating to 383K and α-cyclodextrin rich solutions freeze in four steps. The melting rate becomes slower with decrease in temperature and its steps merge. Decreasing the amount of α-cyclodextrin decreases the Cp change on freezing. The endothermic freezing phenomenon differs from freezing of a pure liquid and is attributed to formation of a solid inclusion compound and its incongruent way of exothermic melting.
Optimized theory for simple and molecular fluids
Authors: Marucho M. , Montgomery Pettitt B.
Published: March 2007

An optimized closure approximation for both simple and molecular fluids is presented. A smooth interpolation between Perkus-Yevick and hypernetted chain closures is optimized by minimizing the free energy self-consistently with respect to the interpolation parameter(s). The molecular version is derived from a refinement of the method for simple fluids. In doing so, a method is proposed which appropriately couples an optimized closure with the variant of the diagrammatically proper integral equation recently introduced by this laboratory [K. M. Dyer et al., J. Chem. Phys. 123, 204512 (2005)]. The simplicity of the expressions involved in this proposed theory has allowed the authors to obtain an analytic expression for the approximate excess chemical potential. This is shown to be an efficient tool to estimate, from first principles, the numerical value of the interpolation parameters defining the aforementioned closure. As a preliminary test, representative models for simple fluids and homonuclear diatomic Lennard-Jones fluids were analyzed, obtaining site-site correlation functions in excellent agreement with simulation data.
Acoustic leakage in electromagnetic waveguides made from piezoelectric materials
Authors: Yang Jiashi , Soh A.-K. , Chen Xiaohong
Published: March 2007

We study the propagation of coupled acoustic and electromagnetic waves in a piezoelectric plate waveguide embedded in two half-spaces of another dielectric material. It is shown that certain waves are guided electromagnetically but not so acoustically and that these waves are effectively damped waves with acoustic leakage of energy. An estimate of the acoustic radiation damping is given.
Geometrical-confinement effects on two electrons in elliptical quantum dots
Authors: Liu Yong-Hui , Yang Fu-Hua , Feng Song-Lin
Published: March 2007

The effects of the geometrical shape on two electrons confined in a two-dimensional parabolic quantum dot and subjected to an external uniform magnetic field have been calculated using a variational-perturbation method based on a direct construction of trial wave functions. The calculations show that both the energy levels and the spin transition of two electrons in elliptical quantum dots are dramatically influenced by the shape of the dots. The ground states with total spin S=0 and S=1 are affected greatly by changing the magnetic field and the geometrical confinement. The quantum behavior of elliptical quantum dots show some relation to that of laterally coupled quantum dots. For a special geometric configuration of the confinement ωy∕ωx=2.0, we encounter a characteristic magnetic field at which spin singlet-triplet crossover occurs.
Comparative studies of transparent conductive Ti-, Zr-, and Sn-doped In2O3 using a combinatorial approach
Authors: Koida T. , Kondo M.
Published: March 2007

We report on comparative studies of transparent conductive Ti-, Zr-, and Sn-doped In2O3 using a combinatorial approach. In2−2xMe2xO3 (Me:Ti, Zr, Sn) composition-spread epilayers (0≤x≤0.1) were fabricated on yttria-stabilized zirconia substrates using the combinatorial pulsed laser deposition technique, and structural, optical, and electrical properties for each composition were systematically investigated. In2−2xTi2xO3 (0.003≤x<0.01) and In2−2xZr2xO3 (0.003≤x<0.05) exhibited superior transparency in the near infrared wavelength region compared to In2−2xSn2xO3 without compromising the conductivity. The results are discussed in terms of scattering centers of electrons from temperature dependence of Hall mobility and the relationship between the values of the room temperature Hall mobility and carrier concentration.
Electronic structure of copper phthalocyanine: An experimental and theoretical study of occupied and unoccupied levels
Authors: Evangelista Fabrizio , Carravetta Vincenzo , Stefani Giovanni , Jansik Branislav , Alagia Michele , Stranges Stefano , Ruocco Alessandro
Published: March 2007

An experimental and theoretical study of the electronic structure of copper phthalocyanine (CuPc) molecule is presented. We performed x-ray photoemission spectroscopy (XPS) and photoabsorption [x-ray absorption near-edge structure (XANES)] gas phase experiments and we compared the results with self-consistent field, density functional theory (DFT), and static-exchange theoretical calculations. In addition, ultraviolet photoelectron spectra (UPS) allowed disentangling several outer molecular orbitals. A detailed study of the two highest occupied orbitals (having a1u and b1g symmetries) is presented: the high energy resolution available for UPS measurements allowed resolving an extra feature assigned to vibrational stretching in the pyrrole rings. This observation, together with the computed DFT electron density distributions of the outer valence orbitals, suggests that the a1u orbital (the highest occupied molecular orbital) is mainly localized on the carbon atoms of pyrrole rings and it is doubly occupied, while the b1g orbital, singly occupied, is mainly localized on the Cu atom. Ab initio calculations of XPS and XANES spectra at carbon K edge of CuPc are also presented. The comparison between experiment and theory revealed that, in spite of being formally not equivalent, carbon atoms of the benzene rings experience a similar electronic environment. Carbon K-edge absorption spectra were interpreted in terms of different contributions coming from chemically shifted C 1s orbitals of the nonequivalent carbon atoms on the inner ring of the molecule formed by the sequence of CN bonds and on the benzene rings, respectively, and also in terms of different electronic distributions of the excited lowest unoccupied molecular orbital (LUMO) and LUMO+1. In particular, the degenerate LUMO appears to be mostly localized on the inner pyrrole ring.
Dielectric properties of laser exploded clusters
Authors: Palastro John P. , Antonsen Thomas , Gupta Ayush
Published: March 2007

The optical properties of a gas of laser-pulse exploded clusters are determined by the time evolving polarizabilities of individual clusters. In turn, the polarizability of an individual cluster is determined by the time evolution of individual electrons within the cluster’s electrostatic potential. We calculate the linear cluster polarizability using the Vlasov equation. A quasistatic equilibrium is calculated from a bi-Maxwellian distribution that models both the hot and cold electrons, using inputs from a particle-in-cell simulation [T. Taguchi et al., Phys. Rev. Lett. 92, 205003 (2004)]. We then perturb the system to first order in the field and integrate the response of individual electrons to the self-consistent field following unperturbed orbits. The dipole spectrum depicts strong absorption at frequencies much smaller than ωp∕√2. This enhanced absorption results from a beating of the laser field with electron orbital motion.
Thermodynamic theory of intrinsic finite-size effects in PbTiO3 nanocrystals. I. Nanoparticle size-dependent tetragonal phase stability
Authors: Akdogan E. K. , Safari A.
Published: March 2007

We propose a phenomenological intrinsic finite-size effect model for single domain, mechanically free, and surface charge compensated PbTiO3 (PT) nanocrystals with no depolarization fields, undergoing a first-order tetragonal→cubic ferrodistortive phase transition. We then derive a Landau-Devonshire type free-energy functional, which is commensurate with the solution of the free-energy equation for the polar point group 4mm, wherein the nanoparticle size (ξ) is a variable. By using experimental particle size-dependent spontaneous polarization (P⃗s) data for PbTiO3, we compute the Landau coefficients up to the sixth order in the range <150 nm as a function of ξ. This thermodynamic potential takes into account the size dependence of the Landau coefficients in a consistent manner, and is able to predict the size-induced phase transition as well as the metastable tetragonal phase in the cubic phase field. We then construct a free-energy density surface in ΔG-P⃗s-ξ space, which describes the decrease in tetragonal phase stability with decreasing ξ rigorously.
Length characterization of DNA-wrapped carbon nanotubes using Raman spectroscopy
Authors: Chou S. G. , Son H. , Kong J. , Jorio A. , Saito R. , Zheng M. , Dresselhaus G. , Dresselhaus M. S.
Published: March 2007

A systematic resonance Raman study has been carried out on DNA-wrapped single walled carbon nanotubes (SWCNTs) of three different average lengths ⟨Ltube⟩ using seven different values of laser excitation energy Elaser. The dependence of the intensity ratio of the D-band and G-band Raman features (ID∕IG) on ⟨Ltube⟩ indicates that nanotube length can be used as an important structural parameter for Raman characterization. By systematically varying Elaser, the ratio ID∕IG is found to be much stronger for metallic than for semiconducting SWCNTs but appears to have the same functional dependence on Elaser and ⟨Ltube⟩ or crystallite size as does nanographite.
Thermodynamic theory of intrinsic finite size effects in PbTiO3 nanocrystals. II. Dielectric and piezoelectric properties
Authors: Akdogan E. K. , Safari A.
Published: March 2007

We compute the intrinsic dielectric and piezoelectric properties of single domain, mechanically free, and surface charge compensated PbTiO3 nanocrystals (n-Pt) with no depolarization fields, undergoing a finite size induced first order tetragonal→cubic ferrodistortive phase transition. By using a Landau–Devonshire type free energy functional, in which Landau coefficients are a function of nanoparticle size, we demonstrate substantial deviations from bulk properties in the range <150 nm. We find a decrease in dielectric susceptibility at the transition temperature with decreasing particle size, which we verify to be in conformity with predictions of lattice dynamics considerations. We also find an anomalous increase in piezocharge coefficients near ∼15 nm , the critical size for n-Pt.
Symmetries in coherence theory of partially polarized light
Authors: Réfrégier Philippe
Published: March 2007

The symmetry properties of the second order statistical characteristics of partially polarized and partially coherent light that appear when one multiplies the electric fields by nonsingular Jones matrices at two different points in the space-time domain are analyzed. Since the application of such deterministic linear transformations does not introduce random fluctuations between the two fields, they do not modify their intrinsic coherence properties. For a better understanding of coherence properties, it is thus interesting to analyze the symmetry properties introduced by these transformations. It is first shown that the invariant subsets generated by the action of deterministic nonsingular Jones matrices applied to each electric field correspond to constant intrinsic degrees of coherence. Then, the subgroup of transformations that keep invariant the second order statistical properties is determined and it is demonstrated that four kinds of different symmetry groups can appear. It is shown that the recently introduced intrinsic degrees of coherence allow one to characterize the kind of symmetry group of the analyzed light, while such a characterization cannot be obtained with a unique scalar degree of coherence. A practical consequence of this result is that two degrees of coherence are necessary to characterize the kind of symmetry of the second order statistical properties of partially coherent and partially polarized light.
Thermal characterization of microscale conductive and nonconductive wires using transient electrothermal technique
Authors: Guo Jiaqi , Wang Xinwei , Wang Tao
Published: March 2007

In this paper, a transient technique is developed to characterize the thermophysical properties of one-dimensional conductive and nonconductive microscale wires. In this technique, the to-be-measured thin wire is suspended between two electrodes. When feeding a step dc to the sample, its temperature will increase and take a certain time to reach the steady state. This temperature evolution is probed by measuring the variation of voltage over the wire, which is directly related to resistance∕temperature change. The temperature evolution history of the sample can be used to determine its thermal diffusivity. A 25.4μm thick platinum wire is used as the reference sample to verify this technique. Sound agreement is obtained between the measured thermal diffusivity and the reference value. Applying this transient electrothermal technique, the thermal diffusivities of single-wall carbon nanotube bundles and polyester fibers are measured.
Characterization of the morphology and optical properties of InAs∕AlAs quantum dots with a GaAs insertion layer
Authors: Roh Cheong Hyun , Song Hong Joo , Kim Dong Ho , Park Joon Soo , Choi Yeon-Shik , Kim Hoon , Hahn Cheol-Koo
Published: March 2007

InAs self-assembled quantum dots by utilizing a thin GaAs insertion layer (IL) on a 1nm thick AlAs seed layer were grown on GaAs(100) substrates by using a molecular beam epitaxy technique. InAs quantum dots (QDs) were formed by varying the thickness of the GaAs IL from 1 to 9 ML (monolayer), and their morphological and optical properties were characterized by atomic force microscopy and photoluminescence (PL). As a result, when the GaAs IL was thicker than 5 ML, normal InAs QDs with an average diameter of 30nm and a density of 2×1010∕cm2 were formed, because the enhanced surface roughness due to the AlAs layer was leveled by the GaAs IL. However, when the thickness of the GaAs IL was decreased from 5 to 3 ML, the formed InAs QDs showed a bimodal size distribution, i.e., large dots with a lateral size of about 30nm and small dots with that of about 20nm. When the GaAs IL was below 1 ML, InAs QDs with an average diameter of less than 15nm and a high density of 1.5×1011∕cm2 were grown. Consequently, it was verified that the thickness parameter of the GaAs IL had an effect on the size distribution of InAs QDs. Furthermore, although the AlAs layer was used for the purpose of improving the density of the QDs, their PL intensity was comparable to that of the normal InAs QDs.
Thickness and temperature dependent electrical characteristics of crystalline BaxSr1−xTiO3 thin films
Authors: Panda B. , Roy A. , Dhar A. , Ray S. K.
Published: March 2007

Polycrystalline Ba1−xSrxTiO3 (BST) thin films with three different compositions have been deposited by radio-frequency magnetron sputtering technique on platinum coated silicon substrates. Samples with buffer and barrier layers for different film thicknesses and processing temperatures have been studied. Crystallite size of BST films has been found to increase with increasing substrate temperature. Thickness dependent dielectric constant has been studied and discussed in the light of an interfacial dead layer and the finite screening length of the electrode. Ferroelectric properties of the films have also been studied for various deposition conditions. The electrical resistivity of the films measured at different temperatures shows a positive temperature coefficient of resistance under a constant bias voltage.
Effect of intertube interaction on the transport properties of a carbon double-nanotube device
Authors: Li Xiao-Fei , Chen Ke-Qiu , Wang Ling-Ling , Long Meng-Qiu , Zou B. S. , Shuai Z.
Published: March 2007

By applying nonequilibrium Green’s functions and first-principles calculations, we investigate the transport behaviors of the bitube device with two single-walled nanotubes attached to metal electrodes. The results show that the intertube interactions play an important role in the conducting behavior of these systems. By adjusting the intertube distance and the orientational order, namely changing the magnitude of the intertube interactions, a different transport behavior can be observed in the system.
Simulation on the time dependence of breakdown strength in insulating polymers
Authors: Wu Kai , Cheng Yonghong
Published: March 2007

On the basis of the percolation model for electrical breakdown in insulating polymers, a simulation approach was put forward for the process of electrical aging. The deteriorated regions formed by bond scission are expressed as the sites with lower trap barriers, and the local deterioration rate is related to both the electrical field and the size of existing deteriorated regions. Then the reduction of breakdown strength (or the percolation threshold) with the increase of deteriorated sites can be simulated. Assuming a parameter β to reflect the effect of the size of existing deteriorated region, simulation results show two types of breakdown: percolationlike breakdown for β⩽2 (in which the percolation threshold is reduced due to the gradual global increase of deteriorated sites) and avalanchelike breakdown for β>2 (in which breakdown occurs by the rapid growth of a filamentary structure). Moreover, it is indicated that the lifetime might not be in inverse proportion to the local deterioration rate in the case of percolationlike breakdown.
Anomalous diffusion of Au in mega-electron-volt Au implanted SiO2∕Si(100)
Authors: Mohapatra S. , Ghatak J. , Joseph B. , Lenka H. P. , Kuiri P. K. , Mahapatra D. P.
Published: March 2007

Thermal annealing induced redistribution behavior of Au (3 MeV, 6.0×1015 ions cm−2), implanted into SiO2∕Si(100) substrates, has been investigated using Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy (XTEM). Sequential annealing in the temperature range of 550−750 °C has been found to result in rejection of Au atoms from Si toward the SiO2∕Si interface. Above 750 °C a significant fraction of the implanted Au atoms has been found to back diffuse into deeper regions, well below the projected range Rp of Au. Direct annealing of a sample at 850 °C also shows a similar anomalous diffusion of Au into deeper regions, well below Rp. However, direct annealing at 1050 °C has been found to result in an enhanced accumulation of Au at a deeper layer. XTEM micrographs for the same sample show the presence of Au-rich nanoparticles and dislocations, decorated with Au-rich nanoparticles in this region. Trails of Au-Si liquid nanodroplets along with dislocations extending into this region have also been observed. The observed enhanced accumulation of Au in the deeper layer has been explained as due to efficient gettering of diffusing Au atoms at dislocations in this region, together with the migration of Au-Si liquid nanodroplets into it.
Structural and magnetic properties of chemically synthesized Sn1−xMnxO2 nanocrystalline powders
Authors: Duan L. B. , Rao G. H. , Yu J. , Wang Y. C. , Liu G. Y. , Liang J. K.
Published: March 2007

Sn 1 − x Mn x O 2 nanocrystalline powders were synthesized by a wet chemical method (x⩽0.08). X-ray diffraction reveals that single-phase rutile Sn1−xMnxO2 was obtained for x up to about 0.06. Evolution of cell parameters, x-ray photoelectron spectroscopy measurements, thermogravimetric analysis, and quantitative evaluation of the magnetization data all indicate that Mn element acts as Mn4+ for x=0.01 and as Mn3+ for x=0.02 and 0.04 at room temperature, but as Mn3+ for all the samples at 5K. Magnetization data show that the average magnetic moment per Mn site decreases with increasing x at 5K, while the effective fraction of the Mn ions contributing to the magnetization decreases. The magnetic properties of the Sn1−xMnxO2 are discussed based on the competition between the free carrier mediated Ruderman-Kittel-Kasuya-Yosida interaction and antiferromagnetic superexchange interaction in the system.
Internal-energy measurements of angle-resolved product CO2 in catalytic CO oxidation by means of infrared chemiluminescence
Authors: Yamanaka Toshiro , Matsushima Tatsuo
Published: March 2007

Measurements of both vibrational and rotational energies of product CO2 in CO oxidation on palladium surfaces have been successfully performed as a function of the desorption angle by means of infrared chemiluminescence. The remarkable angle dependences of both energies indicate facile energy partitioning in repulsive desorption and provide new dimensions in the study of surface reaction dynamics as well as additional insights into the product formation site. Details of the apparatus for energy analysis of angle-resolved products are described, especially on how to pick up extremely weak infrared emission signals.
Self-limiting growth of tantalum oxide thin films by pulsed plasma-enhanced chemical vapor deposition
Authors: Seman Michael , Robbins Joshua J. , Agarwal Sumit , Wolden Colin A.
Published: March 2007

Ta 2 O 5 thin films were fabricated by pulsed plasma-enhanced chemical vapor deposition (PECVD) with simultaneous delivery of O2 and the metal precursor. By appropriately controlling the gas-phase environment self-limiting deposition at controllable rates (∼1Å/pulse) was obtained. The process was insensitive to substrate temperature, with a constant deposition rate observed from 90to350°C. As-deposited Ta2O5 films under these conditions displayed good dielectric properties. Performance improvements correlate strongly with film density and composition as measured by spectroscopic ellipsometry and Fourier transform infrared spectroscopy. Pulsed PECVD eliminates the need for gas actuation and inert purge steps required by atomic layer deposition.
On the formation of voids in internal tin Nb3Sn superconductors
Authors: Scheuerlein C. , Di Michiel M. , Haibel A.
Published: March 2007

The formation of voids during the reaction heat treatment of internal tin Nb3Sn strands degrades the physical superconductor properties. The authors describe three void growth mechanisms on the basis of combined synchrotron microtomography and x-ray diffraction results obtained during in situ heating cycles. Initially, void growth is driven by a reduction of void surface area. The main void volume increase is caused by density changes during the formation of Cu3Sn in the strand. Long duration temperature ramps and isothermal holding steps neither reduce the void volume nor improve the chemical strand homogeneity prior to the superconducting A15 phase nucleation and growth.
Full-band study of current across silicon nanowire transistors
Authors: Nehari K. , Cavassilas N. , Michelini F. , Bescond M. , Autran J. L. , Lannoo M.
Published: March 2007

The authors report an atomistic study of the ballistic current through silicon nanowire metal-oxide-semiconductor transistors. A self-consistent quantum ballistic transport model is used to calculate the current in gate-all-around nanowire transistors, taking into account the full-band structure of the quantum wire with a sp3 tight-binding approach. The authors demonstrate the occurence of an optimal wire cross section for which the on-state/off-state current ratio is maximum, a result which cannot be obtained in a standard bulk effective mass description.
Plasma-assisted molecular-beam epitaxy of AlN(112¯2) on m sapphire
Authors: Lahourcade L. , Bellet-Amalric E. , Monroy E. , Abouzaid M. , Ruterana P.
Published: March 2007

The authors report on the plasma-assisted molecular-beam epitaxy of semipolar AlN(112¯2) films on (11¯00) m-plane sapphire. AlN deposited on m sapphire settles into two main crystalline orientation domains, AlN(112¯2) and AlN(101¯0), whose ratio depends on the III/V ratio. The in-plane epitaxial relationships of AlN(112¯2) on m-plane sapphire are [112¯3¯]AlN‖[0001]sapphire and [11¯00]AlN‖[112¯0]sapphire. In the case of AlN(101¯0), the in-plane epitaxial relationships were [12¯10]AlN‖[0001]sapphire and [0001]AlN‖[112¯0]sapphire. Growth under moderate nitrogen-rich conditions enables them to isolate the (112¯2) orientation and to improve the surface morphology of the layers.
Analytic models for the kinetics of generating a voltage contrast signal from contact plugs used in integrated circuits
Authors: Shadman K. , De I.
Published: March 2007

Electron beams are commonly used to inspect wafers for defective contact plugs during the manufacturing of semiconductor integrated circuits. The plugs form a part of the overall electrical connection to the transistors. These plugs can fail to make adequate electrical contact to the underlying circuitry. They may also be connected to faulty circuits. As a result, the voltage from such defective plugs evolves differently upon irradiation by an electron beam. The paths of the secondary electrons emitted from a defective plug respond to this voltage, thereby modifying the fraction of the emitted current that reaches a detector as compared to the fraction obtained from a healthy contact plug. This paper analyzes the fundamental kinetics that ultimately produces this contrast in a scanning electron microscope designed for wafer inspection. In particular, the paper investigates the kinetics of secondary electron emission from an isolated, biased plug embedded in a charge-neutral dielectric. It presents analytic models for the dependence of the electron collection efficiency with the plug voltage and an applied vertical field at the wafer. The analytic results are compared with those from numerical simulations to test the assumptions that enter the models. The mathematical derivations may ultimately be used to estimate the signal that can be extracted from plugs at dissimilar potentials.
Generalized theory of annularly bounded helicon waves
Authors: Yano Masayuki , Walker Mitchell L. R.
Published: March 2007

The generalized dispersion relationship for annularly bounded helicon plasma is derived. The theory considers the effect of finite electron mass, which results in the plasma field described by superposition of classical helicon solution and the Trivelpiece-Gould (TG) wave. The solution is obtained for an insulating boundary. The solution shows that the wave structure is heavily affected by the presence of the TG wave, even when a strong axial magnetic field is applied. However, the electric field profile shows that a pair of external rf antennas can still be coupled to the strong radial electric field in the plasma to excite the wave. Moreover, the general solution permits an almost continuous spectrum of parallel wave numbers. The study of k-f and n0-B0 diagrams shows that the linear relation obtained using classical theory holds everywhere except at low applied magnetic field strength.
Laser-induced Coulomb mirror effect: Applications for proton acceleration
Authors: Velchev I. , Fourkal E. , Ma C.-M.
Published: March 2007

Using two-dimensional particle-in-cell simulations, the temporal evolution of the uncompensated charge on an ultra-thin (400nm) foil target ionized by a relativistically intense laser pulse is studied in detail. The analysis reveals a new dynamic regime of acceleration of light ions/protons that allows particles to experience the maximum acceleration potential created by the laser. As an alternative to the conventional double-layer target, a new target geometry in which the proton energy is enhanced by ≈30% is proposed.
Gravitational capillary viscometer for low-temperature liquids
Authors: Hilton David K. , Van Sciver Steven W.
Published: March 2007

A pressurized gravitational capillary viscometer was developed for subcooled liquefied gases such as oxygen, nitrogen, hydrogen, and methane. It acquires accurate absolute viscosity measurements with an uncertainty of 1% at a 95.5% confidence level, necessary for the demands of aerospace engineering. The viscometer includes a coiled capillary of electroformed nickel that hydraulically connects two reservoirs located at equal heights. Using helium gas to temporarily drive liquid from a third supply reservoir, a level difference is created between the first two reservoirs without the need to tilt or invert the viscometer. Helium gas is then used to pressurize the first two reservoirs equally. Each reservoir holds a capacitive liquid-level sensor that both measure the flow induced by gravity through the capillary more precisely than by visual observations through windows. Viscosity data for liquid oxygen were acquired in the pressure domain from 0.15to1.0MPa and in the temperature domain from the normal boiling point to near the freezing point. Published viscosity data and correlations for subcooled liquid oxygen are not precise nor complete in this density regime. Accordingly, the viscosity data delivered by the viscometer of the present work are superior.
Mechanism for the ablation of Si⟨111⟩ with pairs of ultrashort laser pulses
Authors: Hu Zhan , Singha Sima , Liu Yaoming , Gordon Robert J.
Published: March 2007

Pairs of ultrafast laser pulses are used to ablate Si⟨111⟩. The fluorescence from Si atoms and ions was observed to increase by an order of magnitude as the delay between the pulses was increased. From the dependence of the fluorescence enhancement on the laser fluence and the pulse delay, it is deduced that the first pulse melts the surface and that the second pulse interacts more strongly with the liquid phase.
Synthetic antiferromagnet for hard layer of exchange coupled composite media
Authors: Hernandez Stephanie , Kapoor Manish , Victora R. H
Published: March 2007

It has been shown that exchange coupled composite (ECC) media exhibit a superior thermal stability to switching field ratio compared to conventional perpendicular media. However, optimal media designs have employed a low magnetization hard layer. A more physically achievable design may consist of replacing the hard layer with a synthetic antiferromagnet consisting of two ferromagnetic hard layers coupled by an antiferromagnetic exchange interaction. This approach is shown to decrease the switching field and increases the thermal stability ratio when compared to ECC media with a single hard layer.
Rectified elongational streaming due to asymmetric electro-osmosis induced by ac polarization
Authors: Wu Jie-Tang , Du Jiong-Rong , Juang Yi-Je , Wei Hsien-Hung
Published: March 2007

We demonstrate a microelongational streaming generated by nonlinear electro-osmosis with high-frequency ac polarization. The phenomenon is attributed to the unique rectification mechanism that coordinates three-dimensional flow interactions between adjacent microvortices in an asymmetric quadrupole electric field. This streaming exhibits a stagnation-point structure with velocity of 300μm∕s at 100Hz due to Faradaic polarization, but is reversed with slower velocity at 1kHz by Ohmic charging. The measured extensional rate shows a quadratic dependence on the field in line with nonlinear Smoluchowski scale. The flow can be ready to control fluid transport and manipulate particles in microdevices.
Sound generation by a street of vortices in a nonuniform flow
Authors: Zheng Ting-hui , Vatistas Georgios H. , Povitsky Alex
Published: March 2007

Propagation of acoustic waves originating from periodic vortices deforming in a nonuniform flow about a rigid body is examined numerically using a high-order compact finite-difference approximation. The governing equations are approximated by the linearized Euler equations in terms of disturbances. The aim of the study is to determine the sound directivity and strength as a result of the vortex street interaction with a solid body under subsonic base flow conditions. Both the vortex core diameter and vortex street spacing have a minor influence on the amplitude of the produced sound wave. When low-frequency vortex streets interact with a cylinder, the produced sound waves are very different from those that originated from high-frequency vortex streets. The interaction mechanism, sound generation, and propagation in a nonuniform flow are quite different for Taylor and Vatistas’ vortex streets. In the case of a low-frequency Vatistas vortex street, the root-mean-square (RMS) value of the acoustic pressure has a well-defined sound directivity and amplitude. The former is greatly affected by the Mach number of the mean flow. For a high-frequency Vatistas vortex street, the RMS of acoustic pressure becomes highly nonmonotonic in the angular direction, while the mean flow Mach number has a moderate effect on the RMS angular profile. The striking differences in the sound amplitude and directivity for Taylor and Vatistas vortices are discussed in terms of their vorticity distribution.
Dissipative particle dynamics simulation of multiphase fluid flow in microchannels and microchannel networks
Authors: Liu Moubin , Meakin Paul , Huang Hai
Published: March 2007

Multiphase fluid motion in microchannels and microchannel networks involves complicated fluid dynamics and is fundamentally important to diverse practical engineering applications such as ink-jet printing, DNA and protein micro-/nano-arraying, and fabrication of particles and capsules for controlled release of medicines. This paper presented the simulations of multiphase fluid motion in microchannels and microchannel networks using a modified dissipative particle dynamics method that employs a new conservative particle-particle interaction combining short-range repulsive and long-range attractive interactions to simulate multiphase systems. This new conservative particle-particle interaction allows the behavior of multiphase systems consisting of gases, liquids, and solids to be simulated. Three numerical examples that are closely related to engineering applications were simulated. These examples involve multiple fluid motions in (i) a simple microchannel within two parallel plates; (ii) an inverted Y-shaped microchannel junction consisting of a vertical channel that divides into two branch channels with the same aperture; and (iii) a microchannel network. The numerical results obtained by using DPD agreed well with those from other sources, and clearly demonstrated the potential value of this DPD method for modeling and analyzing multiphase flow in microchannels and microchannel networks.
Self-similarity in decaying two-dimensional stably stratified adjustment
Authors: Sukhatme Jai , Smith Leslie M.
Published: March 2007

The evolution of large-scale density perturbations is studied in a stably stratified, two-dimensional flow governed by the Boussinesq equations. As is known, initially smooth density (or temperature) profiles develop into fronts in the very early stages of evolution. This results in a frontally dominated k−1 potential energy spectrum. The fronts, initially characterized by a relatively simple geometry, spontaneously develop into severely distorted sheets that possess structure at very fine scales, and thus there is a transfer of energy from large to small scales. It is shown here that this process culminates in the establishment of a k−5∕3 kinetic energy spectrum, although its scaling extends over a shorter range as compared to the k−1 scaling of the potential energy spectrum. The establishment of the kinetic energy scaling signals the onset of enstrophy decay, which proceeds in a mildly modulated exponential manner and possesses a novel self-similarity. Specifically, the self-similarity is seen in the time invariant nature of the probability density function (PDF) associated with the normalized vorticity field. Given the rapid decay of energy at this stage, the spectral scaling is transient and fades with the emergence of a smooth, large-scale, very slowly decaying, (almost) vertically sheared horizontal mode with most of its energy in the potential component, i.e., the Pearson-Linden regime.
Perpendicular magnetization of L1-ordered FePt films in the thinnest limit
Authors: Imada S. , Yamasaki A. , Suga S. , Shima T. , Takanashi K.
Published: March 2007

L 1 0 -ordered FePt ultrathin films with thickness down to subnanometer range were fabricated by alternate monatomic layer (ML) deposition of Fe and Pt on Pt (001) substrate and were capped by Pt. Perpendicular Fe magnetic moment under magnetic field, together with that under remanence, was detected by soft x-ray magnetic circular dichroism. Pt∕(Fe1ML∕Pt1ML)3∕Pt (001), whose FePt layer is ∼1nm thick, showed perpendicular remanent magnetization at room temperature. At low temperatures, perpendicular remanent magnetization was found even at the thinnest end, i.e., 1 ML Fe sandwiched by Pt (001).
( 1 − x ) Ba Ti O 3 − x ( Na 0.5 K 0.5 ) Nb O 3 ceramics for multilayer ceramic capacitors
Authors: Choi Chang-Hak , Ahn Cheol-Woo , Nahm Sahn , Hong Jung-Oh , Lee Jung-Soo
Published: March 2007

( 1 − x ) Ba Ti O 3 − x ( Na 0.5 K 0.5 ) Nb O 3 ceramics with 0.0⩽x⩽0.08 formed a homogeneous solid solution. As (Na0.5K0.5)NbO3 (NKN) was added, the Curie temperature of the BaTiO3 (BT) ceramics decreased and specimens became a relaxor as x exceeded 0.04. The grain size of the BT ceramics decreased with increasing NKN. The average grain size was approximately 0.4μm at x=0.06. The dielectric constant (εr) of the specimen increased with increasing NKN and high εr of 7402 with low dielectric loss of <1.0% was observed for the 0.94BT-0.06NKN ceramic. Therefore, the 0.94BT-0.06NKN ceramic is proposed as a promising candidate material for thin multilayer ceramic capacitors with high capacitance.
Noise characteristics of 100nm scale GaAs∕AlxGa1−xAs scanning Hall probes
Authors: Hicks C. W. , Luan L. , Moler K. A. , Zeldov E. , Shtrikman H.
Published: March 2007

The authors have fabricated and characterized GaAs∕AlxGa1−xAs two-dimensional electron gas scanning Hall probes for imaging perpendicular magnetic fields at surfaces. The Hall crosses range from 85×85to1000×1000nm2. They study low-frequency noise in these probes, especially random telegraph noise, and show that low-frequency noise can be significantly reduced by optimizing the voltage on a gate over the Hall cross. The authors demonstrate a 100nm Hall probe with a sensitivity of 0.5G∕√Hz (flux sensitivity of 0.25mΦ0∕√Hz; spin sensitivity of 1.2×104μB∕√Hz) at 3Hz and 9K.
Magnetic alignment of ZnO nanowires for optoelectronic device applications
Authors: Lee Sang-Won , Jeong Min-Chang , Myoung Jae-Min , Chae Gee-Sung , Chung In-Jae
Published: March 2007

Ni caps on ZnO nanowires were synthesized to control the direction and position of these nanowires using magnetism. Vertically ordered ZnO nanowires were grown 15μm in length by metal organic chemical vapor deposition and then ferromagnetic Ni was capped on top of the nanowires by sputtering. The synthesized nanowires were aligned at the edge of the magnetized Ni patterns by applying a magnetic field. The density of the aligned nanowires depended on the magnetic force of the Ni patterns. Nanowire-based ultraviolet sensors were demonstrated using the magnetic alignment method.
High-efficiency low operation voltage organic light-emitting diodes
Authors: Di Chong-an , Yu Gui , Liu Yunqi , Xu Xinjun , Song Yabin , Zhu Daoben
Published: March 2007

The authors report a simple approach to reduce power consumption of organic light-emitting diodes. Introducing a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline layer into the emissive layer can dramatically improve the carrier recombination efficiency. The presence of an interlayer of tris(8-quinolinolato)aluminum (Alq3) within the hole-transport layer also gives rise to the similar phenomenon. Furthermore, they modified the anode with the ultrathin hexadecafluorocopper phthalocyanine layer in order to further enhance the electroluminescent properties. They obtained the high performance light-emitting diodes using Alq3 as the emissive layer with a high current efficiency of 7.63cd∕A and a low turn-on voltage of 2.89V.
High-performance InAs quantum-dot infrared photodetectors grown on InP substrate operating at room temperature
Authors: Lim H. , Tsao S. , Zhang W. , Razeghi M.
Published: March 2007

The authors report a room temperature operating InAs quantum-dot infrared photodetector grown on InP substrate. The self-assembled InAs quantum dots and the device structure were grown by low-pressure metal-organic chemical vapor deposition. The detectivity was 2.8×1011cmHz1∕2∕W at 120K and a bias of −5V with a peak detection wavelength around 4.1μm and a quantum efficiency of 35%. Due to the low dark current and high responsivity, a clear photoresponse has been observed at room temperature, which gives a detectivity of 6.7×107cmHz1∕2∕W.
Chemical inhibition of the TFIIH-associated kinase Cdk7/Kin28 does not impair global mRNA synthesis
Authors: Kanin Elenita I. , Kipp Ryan T. , Kung Charles , Slattery Matthew , Viale Agnes , Hahn Steven , Shokat Kevan M. , Ansari Aseem Z.
Published: March 2007

The process of gene transcription requires the recruitment of a hypophosphorylated form of RNA polymerase II (Pol II) to a gene promoter. The TFIIH-associated kinase Cdk7/Kin28 hyperphosphorylates the promoter-bound polymerase; this event is thought to play a crucial role in transcription initiation and promoter clearance. Studies using temperature-sensitive mutants of Kin28 have provided the most compelling evidence for an essential role of its kinase activity in global mRNA synthesis. In contrast, using a small molecule inhibitor that specifically inhibits Kin28
in vivo
, we find that the kinase activity is not essential for global transcription. Unlike the temperature-sensitive alleles, the small-molecule inhibitor does not perturb protein–protein interactions nor does it provoke the disassociation of TFIIH from gene promoters. These results lead us to conclude that other functions of TFIIH, rather than the kinase activity, are critical for global gene transcription.
Melanomas require HEDGEHOG-GLI signaling regulated by interactions between GLI1 and the RAS-MEK/AKT pathways
Authors: Stecca Barbara , Mas Christophe , Clement Virginie , Zbinden Marie , Correa Rafael , Piguet Vincent , Beermann Friedrich , Ruiz i Altaba Ariel
Published: March 2007

Melanoma is one of the most aggressive cancers, and its incidence is increasing. These tumors derive from the melanocyte lineage and remain incurable after metastasis. Here we report that SONIC HEDGEHOG (SHH)-GLI signaling is active in the matrix of human hair follicles, and that it is required for the normal proliferation of human melanocytes in culture. SHH-GLI signaling also regulates the proliferation and survival of human melanomas: the growth, recurrence, and metastasis of melanoma xenografts in mice are prevented by local or systemic interference of HH-GLI function. Moreover, we show that oncogenic RAS-induced melanomas in transgenic mice express Gli1 and require Hh-Gli signaling
in vitro
and
in vivo
. Finally, we provide evidence that endogenous RAS-MEK and AKT signaling regulate the nuclear localization and transcriptional activity of GLI1 in melanoma and other cancer cells. Our data uncover an unsuspected role of HH-GLI signaling in melanocytes and melanomas, demonstrate a role for this pathway in RAS-induced tumors, suggest a general integration of the RAS/AKT and HH-GLI pathways, and open a therapeutic approach for human melanomas.
Viral activation and recruitment of metacaspases in the unicellular coccolithophore,
Emiliania huxleyi
Authors: Bidle Kay D. , Haramaty Liti , Barcelos e Ramos Joana , Falkowski Paul
Published: March 2007

Lytic viral infection and programmed cell death (PCD) are thought to represent two distinct death mechanisms in phytoplankton, unicellular photoautotrophs that drift with ocean currents. Here, we demonstrate an interaction between autocatalytic PCD and lytic viral infection in the cosmopolitan coccolithophorid,
Emiliania huxleyi
. Successful infection of
E. huxleyi
strain 374 with a lytic virus, EhV1, resulted in rapid internal degradation of cellular components, a dramatic reduction in the photosynthetic efficiency (F
v
/F
m
), and an up-regulation of metacaspase protein expression, concomitant with induction of caspase-like activity. Caspase activation was confirmed through
in vitro
cleavage in cell extracts of the fluorogenic peptide substrate, IETD-AFC, and direct,
in vivo
staining of cells with the fluorescently labeled irreversible caspase inhibitor, FITC-VAD-FMK. Direct addition of z-VAD-FMK to infected cultures abolished cellular caspase activity and protein expression and severely impaired viral production. The absence of metacaspase protein expression in resistant
E. huxleyi
strain 373 during EhV1 infection further demonstrated the critical role of these proteases in facilitating viral lysis. Together with the presence of caspase cleavage recognition sequences within virally encoded proteins, we provide experimental evidence that coccolithoviruses induce and actively recruit host metacaspases as part of their replication strategy. These findings reveal a critical role for metacaspases in the turnover of phytoplankton biomass upon infection with viruses and point to coevolution of host–virus interactions in the activation and maintenance of these enzymes in planktonic, unicellular protists.