bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic process of domain switching in ferroelectric films
Authors: Wang Biao , Xia R. , Fan H. , Woo C. H.
Published: August 2003

In this article, a general variational formulation is established for the dynamics of domain switching in ferroelectric thin films. By incorporating the kinetic energy in the Landau free energy expansion, we obtain a second-order time-dependent Ginzburg–Landau evolution equation, which can be used to analyze the frequency-dependent dielectric and ferroelectric properties of the materials. A numerical model is developed to calculate the time-dependent polarization and electric-field distribution during the domain switching process. As an example of practical importance, a ferroelectric film with 180° domain boundaries perpendicular to the film surface is studied. Domain switching is found to proceed in two stages, the initial nucleation and growth of the ferroelectric domain, followed by the expansion of the domain through the sidewise motion of the 180° domain walls, until the spontaneous polarization in the entire electrified area is reversed.
Static and dynamic magnetic properties of spherical magnetite nanoparticles
Authors: Goya G. F. , Berquó T. S. , Fonseca F. C. , Morales M. P.
Published: August 2003

We present a detailed study of static and dynamic magnetic behavior of Fe3O4 nanoparticles with average particle sizes 〈d〉 ranging from 5 to 150 nm. Bulk-like properties such as saturation magnetization, hyperfine parameters, coercive field, and Verwey transition are observed in 150 nm particles. For decreasing particle size, the Verwey temperature, TV, shifts down to ∼20 K for 〈d〉=50 nm and is no longer observable for smaller particles. The smallest particles (〈d〉=5 nm) display superparamagnetic behavior at room temperature, with transition to a blocked state at TB∼45 K, which depends on the applied field. The existence of surface spin disorder can be inferred from the decrease of saturation magnetization MS at low temperatures, as the average particle size is reduced. This disordered surface did not show effects of exchange coupling to the particle core, as observed from hysteresis loops after field cooling in a 7 T magnetic field. For particles with 〈d〉=5 nm, dynamic ac susceptibility measurements show a thermally activated Arrhenius–Néel dependence of the blocking temperature with applied frequency. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy. From the calculus of the magnetic anisotropy, it is inferred that there is no structural transition from cubic to triclinic symmetry for 〈d〉=5 nm, in agreement with the absence of the Verwey transition. A value K1=4.68×105 erg/cm3 is obtained for the magnetocrystalline anisotropy constant of the cubic phase.
Luminescent properties and energy transfer in ZrO2:Sm3+ nanocrystals
Authors: De la Rosa-Cruz E. , Diaz-Torres L. A. , Salas P. , Rodrı́guez R. A. , Kumar G. A. , Meneses M. A. , Mosiño J. F. , Hernández J. M. , Barbosa-Garcı́a O.
Published: August 2003

The photoluminescence and crystalline structure characterization of undoped and several samarium doped ZrO2 samples are reported. Strong fluorescence emission produced by the transitions G5/24→6H5/2,7/2,9/2 of Sm3+ was obtained by the excitation of the host at 320 nm. The energy transfer process from the host to the samarium ion was confirmed by the analysis of the ZrO2 fluorescence decay curve. It is shown that the content of the active ions stabilizes the tetragonal structure of ZrO2 at 1000 °C, being 73% for 2 mol % Sm2O3 doped and 3% for undoped samples. The dependence between the fluorescence emission and the crystalline structure is discussed.
Periodic tunnel-current oscillations in metal droplets
Authors: Ruggiero S. T. , Ekkens T. B. , Arnold G. B.
Published: August 2003

We have observed strong current oscillations in a metal droplet system by point-contact tunneling with a scanning tunneling microscope at 77 K. The oscillations have a periodicity equal to the single-electron charging voltage, e/C=28 mV, where C is the metal droplet capacitance. The current oscillations are large in magnitude, robust, and insensitive to tip position. A simple model of the system based on a self-biased single-electron transistor suggests the contribution of multidroplet single-electron tunneling effects.
Two-dimensional electron gases induced by polarization charges in AlN/GaN heterostructure grown by plasma-assisted molecular-beam epitaxy
Authors: Jeganathan K. , Ide T. , Shimizu M. , Okumura H.
Published: August 2003

We discuss the growth and transport properties of two-dimensional electron gas confined at the AlN/GaN heterointerface grown by plasma-assisted molecular-beam epitaxy. The sheet carrier density was found to be highly dependent on the barrier thickness of AlN grown on a doped or undoped GaN layer. The carrier sheet density monotonously increased from 0.8×1012 to 1.1×1013 cm−2 as the AlN barrier thickness on a semi-insulating GaN layer increased from 15 to 25 Å due to spontaneous and piezoelectric polarization. An AlN barrier of 35 Å in thickness grown on n-GaN gave the highest sheet carrier density, which was 4.3×1013 cm−2. In thin AlN barrier layers, the sheet carrier density was low due to surface depletion.
Instability of surface dust acoustic waves in the presence of nonspherical and rotating charged dust grains
Authors: Shokri B. , Alavi S. Kh.
Published: August 2003

The dispersion relation of surface quasistatic waves for a semibounded dusty plasma containing elongated and rotating dust grains is obtained. The analytic modes in the frequency range of dust acoustic waves are derived and discussed with the aid of some graphical solutions. It will be shown that surface dust acoustic waves become unstable when the dust rotation frequency is smaller than the dust acoustic frequency.
Fabrication of 60-nm transistors on 4-in. wafer using nanoimprint at all lithography levels
Authors: Zhang Wei , Chou Stephen Y.
Published: August 2003

Nanoimprint lithography (NIL) is a paradigm-shift method that has shown sub-10-nm resolution, high throughput, and low cost. To make NIL a next-generation lithography tool to replace conventional lithography, one must demonstrate the needed overlay accuracy in multilayer NIL, large-area uniformity, and low defect density. Here, we present the fabrication of 60-nm channel metal–oxide–semiconductor field-effect transistors on whole 4-in. wafers using NIL at all lithography levels. The nanotransistors exhibit excellent operational characteristics across the wafer. The statistics from consecutive multiwafer processing show an average overlay accuracy of 500 nm over the entire 4-in. wafer. The accuracy is much better when the field size is reduced. The overlay accuracies are limited by the current alignment method and can be improved substantially. The work presents a significant advance in nanoimprint development and its applications in manufacturing of integrated electrical, optical, chemical, and biological nanocircuits.
Optical centers introduced in boron-doped synthetic diamond by near-threshold electron irradiation
Authors: Charles S. J. , Steeds J. W. , Butler J. E. , Evans D. J. F.
Published: August 2003

Near-threshold irradiation of B-doped synthetic diamonds has been performed using a transmission electron microscope operated at 200 kV. Both chemical vapor deposited and high-pressure high-temperature synthesized samples have been studied. The B levels were in the range 1017–1019 cm−3. After irradiation the samples were studied by low temperature (∼7 K) photoluminescence spectroscopy using various excitation wavelengths. A number of characteristic optical centers have been observed in the spectral range 500–800 nm and these centers are reviewed. Details of the properties of the optical centers have been investigated and the results are summarized. In particular, two zero-phonon lines (ZPLs) at 636 and 666 nm, formed in boron-doped diamond materials after near displacement-threshold electron radiation damage, were found to be related. The nature of this relationship is studied by laser power dependence (at different wavelengths) of their intensities over a wide temperature range. The results are interpreted in terms of a three-level model for a single optical center that involves a dipole-forbidden excited state of lower energy and a dipole-allowed state of 90 meV higher energy. Similar behavior of a further pair of ZPLs at 650 and 668 nm also formed in these materials is discussed. The spatial distribution of centers and their alteration by ultraviolet excitation was used to investigate the nature of the 636 and 666 nm centers.
Characteristics of deep levels associated with rhodium impurity in n-type GaAs
Authors: Majid A. , Iqbal M. Zafar , Khan S. Haidar , Ali Akbar , Zafar Nasim , Dadgar A. , Bimberg D.
Published: August 2003

Deep levels have been characterized in n-type GaAs crystalline films grown by metalorganic chemical vapor deposition, doped in situ with 4d transition metal, rhodium, using the deep level transient spectroscopy (DLTS) technique. Two prominent broad bands of deep levels are found to be associated with Rh impurity, one in the upper half and the other in the lower half of the band gap. The band of electron emitting deep levels is shown to extend from 0.57–0.67 eV below the conduction band edge while the hole emitting deep levels band peaks at Ev+0.42 eV. Detailed double-correlation DLTS measurements are performed to investigate the characteristics of the observed Rh-related deep levels. Both deep level bands are found to exhibit significant dependence of the thermal emission rates on the electric field from these measurements. The observed variation of the peak profile with the capture time for the electron emission band and theoretical analysis of the field dependence data support a band-like-extended-defect nature of this deep level band.
The Bohm criterion for the dusty plasma sheath
Authors: Liu Jin-Yuan , Wang Zheng-Xiong , Wang Xiaogang , Zhang Qiang , Zou Xiu , Zhang Yu
Published: August 2003

The Bohm criterion for the dusty plasma sheath is investigated with a fluid model. It is shown that the presence of dust particles near the interface between the bulk plasma and the sheath should have effects upon the ion Mach number. The dust particles charging and density variation are taken into account, and therefore the self-consistent relation between the dust surface potential and ion Mach number at the sheath edge is derived. The numerical results reveal that both the ion critical Mach number and the dust critical Mach number with variation of density ratio of dust-to-electron begin to decline after a maximum.
Iterative solution of the Grad–Shafranov equation in symmetric magnetic coordinates
Authors: Brambilla Marco
Published: August 2003

The inverse Grad–Shafranov equation for axisymmetric magnetohydrodynamic equilibria is reformulated in symmetric magnetic coordinates (in which magnetic field lines look “straight,” and the geometric toroidal angle is one of the coordinates). The poloidally averaged part of the equilibrium condition and Ampère law takes the form of two first-order ordinary differential equations, with the two arbitrary flux functions, pressure and force-free part of the current density, as sources. The condition for the coordinates to be flux coordinates, and the poloidally varying part of the equilibrium equation are similarly transformed into a set of first-order ordinary differential equations, with coefficients depending on the metric, and explicitly solved for the radial derivatives of the coefficients of the Fourier representation of the Cartesian coordinates in the poloidal angle. The derivation exploits the existence of Boozer–White coordinates, but does not require to find these coordinates explicitly; on the other hand, it offers a simple recipe to perform the transformation to Boozer–White coordinates, if required. Use of symmetric flux coordinates is advantageous for the formulation of many problems of equilibrium, stability, and wave propagation in tokamak plasmas, since these coordinates have the simplest metric of their class. It is also shown that in symmetric flux coordinates the Lagrangian equations of the drift motion of charged particles are automatically solved for the time derivatives, with right-hand sides closely related to the coefficients of the inverse Grad–Shafranov equation.
Effect of external mechanical constraints on the phase diagram of epitaxial PbZr1−xTixO3 thin films—thermodynamic calculations and phase-field simulations
Authors: Li Y. L. , Choudhury S. , Liu Z. K. , Chen L. Q.
Published: August 2003

The phase diagram of a PbZr1−xTixO3 (PZT) film constrained by a much thicker substrate was studied using both thermodynamic calculations and phase-field approach. It was found that the ferroelectric transition temperature is increased with substrate constraint regardless of the nature of the constraint, i.e., tensile or compressive. The maximum increase in the transition temperature occurs near x=0.5, and the morphotropic phase boundary is shifted considerably by the substrate constraint. It is shown that the orthorhombic phase (|P1|=|P2|≠0, P3=0) that does not exist in the bulk becomes stable under a tensile constraint, and the rhombohedral phase (|P1|=|P2|=|P3|≠0) in the bulk is distorted in the constrained film, i.e., |P1|=|P2|≠0, |P3|≠0. The phase diagrams obtained by the phase-field approach indicated that the stability region for the tetragonal phase is much wider than that obtained from the thermodynamic calculations assuming a single-domain, especially under tensile substrate constraint. The discrepancy between the two methods becomes larger as substrate constraint changes from compressive to tensile, implying that thermodynamic calculations are unreliable for constructing stability diagram of PZT, particularly under a tensile constraint.
Proton implantation effects on electrical and luminescent properties of p-GaN
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Pearton S. J. , Zavada J. M.
Published: August 2003

The electrical properties, deep level spectra, and microcathodoluminescence (MCL) spectra of p-GaN films implanted with 100 keV protons are reported. Measurable decreases of the MCL intensity began for doses as low as 1012 cm−2, while measurable decreases of the hole concentration started for doses of 1013 cm−2. The main deep traps introduced by protons had activation energies of 0.3, 0.6, and 0.9 eV. The 0.3 and 0.9 eV traps are located in the lower half of the band gap and determine the temperature dependence of the forward current in heavily proton implanted Schottky diodes.
High density current operation in nanographite fiber synthesized by chemical vapor deposition
Authors: Tsukagoshi K. , Suzuki At. , Yagi I. , Watanabe E. , Aoyagi Y. , Ago H. , Ohshima S. , Yumura M.
Published: August 2003

We report an electrical characterization of nanographite fibers (NGFs) by measuring current–voltage characteristics of the individual NGFs in air at room temperature in order to optimize the NGF mass production process. The NGFs were synthesized by thermal chemical vapor deposition with thermal annealing. The NGF contains many defects in comparison with a multiwalled carbon nanotube (MWNT) synthesized by arc discharge, which can be clearly confirmed in the current–voltage characteristic. The maximum current density, however, is typically 108 A/cm2, which is comparable to that in the MWNT. The high-density current operation mechanism is inferred from the measured diameter dependence of the current-voltage characteristic in various diameter NGFs.
Cross-phase modulation effects in surface-wave-sustained plasmas
Authors: Marinov K. , Schlüter H. , Shivarova A. , Stoev L.
Published: August 2003

The study extends models of the cw-regime of operation of diffusion-controlled discharges by incorporating the actual shape of the high-frequency signal producing the discharge as a narrow-band signal. The slow variations of the wave envelope determining slowly varying Joule heating of the electrons in the wave field leads to a low-frequency plasma-density response which influences the propagation properties of the high-frequency signal. Depending on the frequency shift of the spectral components from the carrier-wave frequency of the signal, the low-frequency plasma response appears as stationary or nonstationary. The derived nonlinear evolution equation for the wave envelope shows cross-phase modulation acting simultaneously through self-action and mutual action of coupled spectral components. A strong impact of the mutual action of the coupled spectral components is found. It removes the effect of self-action: The decay of the spectral components associated with a stationary low-frequency response transforms into an instability, and the monotonic variations of the amplitudes of the spectral components associated with a nonstationary low-frequency plasma response are replaced by space modulation. For verification of the origin of the effects, also the case of axially homogeneous plasma columns is treated in which obtaining analytical solutions of the evolution equations is possible.
Correspondence of the plasticity of rocksalt structure ceramics and tetrahedrally coordinated semiconductors
Authors: Siethoff Hans
Published: August 2003

Glide and creep in three crystal structures (rocksalt ceramics, and cubic and wurtzite semiconductors) are quantitatively related. Two deformation regimes with different temperature dependences are observed. The underlying activation energies obey the same laws in each regime, and are interpreted in terms of diffusion-controlled mechanisms. The physical origin of this correspondence may be sought in the interatomic binding that is predominantly covalent for the different crystal structures. These results eventually corroborate a former suggestion, that the kink mechanism of dislocation glide of tetrahedrally coordinated semiconductors may be basically governed by an elementary diffusion step. A bond-breaking mechanism, however, may not be totally ruled out.
Photoluminescence characterization of GaInNAs/GaAs quantum well carrier dynamics
Authors: Ng T. K. , Yoon S. F. , Wang S. Z. , Lin L.-H. , Ochiai Y. , Matsusue T.
Published: August 2003

Variable temperature photoluminescence (PL) measurement of a thermal-annealed 6 nm GaInNAs/GaAs quantum well (QW) is carried out to understand its low-temperature carrier dynamic characteristics. It is found that the effect of carrier localization, which remained after a thermal anneal, is due possibly to a center characterized by a transition with activation energy of 13.7 meV below the e1 state. This result is deduced from fitting the integrated PL intensity versus the temperature data with a single-activation-energy model. A comparable value of 11 meV was also obtained between the low-energy (main localized state) and high-energy (e1 state) Gaussian functions used to fit the low-temperature PL spectrum. The localization effect in the thermal-annealed GaInNAs/GaAs QW is further confirmed by time-resolved PL measurements at 17 K, which showed emission-energy-dependent PL decay time characteristic for the low-energy regime (below 1.045 eV) and nearly constant decay time of about 0.14 ns at the high-energy regime, which corresponds to the e1−hh1 transition lifetime. The main localization center could form carrier traps that reduce the e1−hh1 PL transition efficiency.
Dynamical structure of irregular constrained systems
Authors: Mišković Olivera , Zanelli Jorge
Published: August 2003

Hamiltonian systems with functionally dependent constraints (irregular systems), for which the standard Dirac procedure is not directly applicable, are discussed. They are classified according to their behavior in the vicinity of the constraint surface into two fundamental types. If the irregular constraints are multilinear (type I), then it is possible to regularize the system so that the Hamiltonian and Lagrangian descriptions are equivalent. When the constraints are power of a linear function (type II), regularization is not always possible and the Hamiltonian and Lagrangian descriptions may be dynamically inequivalent. It is shown that the inequivalence between the two formalisms can occur if the kinetic energy is an indefinite quadratic form in the velocities. It is also shown that a system of type I can evolve in time from a regular configuration into an irregular one, without any catastrophic changes. Irregularities have important consequences in the linearized approximation to nonlinear theories, as well as for the quantization of such systems. The relevance of these problems to Chern–Simons theories in higher dimensions is discussed.
Tri-Hamiltonian formulation for certain integrable lattice equations
Authors: Sahadevan R. , Khousalya S.
Published: August 2003

A systematic investigation of integrable differential–difference equations with two independent variables admitting multi-Hamiltonian structure is presented. Considering the Volterra (VL), Toda (TL), Relativistic Toda (RT), Belov–Chaltikian (BC) and Blaszak–Marciniak both three (BM3) and four (BM4) coupled lattice equations it is shown that they admit a sequence of operators out of which only three are Hamiltonian ones and so they are tri-Hamiltonian systems only. It is observed that the constructed third operator for VL and BC lattice equations is Hamiltonian only if the field variable is periodic with even period.
Tunneling microscopy and spectroscopy of multiwalled boron nitride nanotubes
Authors: Czerw R. , Webster S. , Carroll D. L. , Vieira S. M. C. , Birkett P. R. , Rego C. A. , Roth S.
Published: August 2003

Atomic resolution images of multiwalled boron nitride nanotubes have been obtained using scanning tunneling microscopy operating at tunneling currents below 20 pA and biases of approximately −2.5 V. Lattice images acquired with negative sample biases exhibit trigonal symmetry that is interpreted as resulting from nitrogen states. Tunneling spectroscopy confirms band gaps between 4.5 eV and 4.8 eV for tube diameters above 5 nm. Tunneling barrier height measurements made using standard current–distance analysis yields Φ∼6.3±0.7 eV for the boron nitride nanotubes.
Investigation of electrostatic self-assembly as a means to fabricate and interfacially modify polymer-based photovoltaic devices
Authors: Durstock Michael F. , Spry Robert J. , Baur Jeffery W. , Taylor Barney E. , Chiang Long Y.
Published: August 2003

This work focuses on studying a water-based processing method for fabricating and modifying polymer-based photovoltaic devices based on donor–acceptor type complexes. Electrostatic self-assembly is a simple technique that involves immersion of a substrate into dilute aqueous solutions of positively and negatively charged polymers. Extremely thin layers of these polymers are adsorbed onto the surface and their structure can be tailored by manipulating deposition conditions such as the concentration, pH, and salt content. Poly(p-phenylene vinylene) (PPV) containing bilayers were examined as the donor block and water soluble, functionalized C60 molecules were investigated for the acceptor block. By varying the number of bilayers deposited in each individual block (i.e., the block thickness), we have been able to demonstrate a peak in device performance. By controlling the thickness of both the donor and acceptor blocks, we have determined the optimal device architecture for this system. Additionally, we have demonstrated that the interface between the donor and acceptor layers can be modified by inserting thin interfacial layers in between the blocks. The insertion of only two interfacial bilayers apparently combines the donor and acceptor functionalities such that the efficiency can be improved by a factor of 3. From this, it is apparent that one strength of electrostatic self-assembly lies in the modification of surfaces and interfaces, which is a key capability for further development of polymeric photovoltaics.
Dynamics of a two-dimensional electron-hole system in a coupled quantum well
Authors: Cruz H. , Luis D.
Published: August 2003

In this work, we have numerically integrated in space and time the effective-mass Schrödinger equation for a two-dimensional electron-hole system in a coupled quantum well system. Considering a time-dependent Hartree potential and an external electric field, we derive the nonlinear dynamical evolution of the carrier wave function. It is found that charge dynamically trapped in both wells produces a reaction field that modifies the system resonant condition. At different electron-hole sheet densities we show the possibility of multiple resonant tunneling peaks in a bilayer system.
Sodium stearate, an effective amphiphilic molecule buffer material between organic and metal layers in organic light-emitting devices
Authors: Zhan Y. Q. , Xiong Z. H. , Shi H. Z. , Zhang S. T. , Xu Z. , Zhong G. Y. , He J. , Zhao J. M. , Wang Z. J. , Obbard E. , Ding H. J. , Wang X. J. , Ding X. M. , Huang W. , Hou X. Y.
Published: August 2003

Tris (8-hydroxyquinoline) aluminum (Alq3)-based organic light-emitting devices using an amphiphilic molecule sodium stearate (NaSt) layer between aluminum (Al) cathode and Alq3 have been fabricated. By comparing the devices with those containing a LiF buffer layer, the results demonstrate that both have almost the same high electroluminescent (EL) brightness but the former is more stable. The amphiphilic property of NaSt is considered as the main reason for this enhancement.
Turbulent edge structure formation in complex configurations
Authors: Kendl A. , Scott B. D. , Ball R. , Dewar R. L.
Published: August 2003

Formation of nonlinear structures in drift-Alfvén turbulence is investigated in the often complex edge geometries of stellarator and tokamak configurations, by analysis of drift wave turbulence simulations using a model in which three-dimensional magnetic geometries are approximated. The structures of parallel mode extension, radially sheared zonal flows and perpendicular mode spectra are highlighted in particular for three-dimensional stellarator magnetic fields and shaped tokamaks. Specific characteristics of advanced stellarators in comparison to (lower aspect ratio) circular tokamaks are a less pronounced ballooning structure of the modes, a strong influence of local magnetic shear on amplitude structure and average, and stronger level of zonal flows due to lower geodesic curvature.
Vavilov–Cherenkov and transition radiations on the dielectric and metallic spheres
Authors: Afanasiev G. N. , Kartavenko V. G. , Stepanovsky Yu. P.
Published: August 2003

Closed expressions are obtained for angular and frequency radiation intensities produced by a charge moving inside the dielectric sphere S, with observations made outside S (in fact, this is a typical experimental situation when a charge moves in one medium while measurements are made in the other one). It is shown that the difference in media properties inside and outside S drastically affects angular and frequency distributions. Also, a charge motion is considered which begins and terminates in medium 2 and which passes either through the dielectric sphere filled with medium 1 or through the metallic one. The energy flux in medium 2 involves the Vavilov–Cherenkov, transition radiation and the one arising from the charge instantaneous beginning and termination of motion. The evaluated angular and frequency distributions for various charge velocities and medium properties inside and outside S show that the standard identification of the charge velocity by its radiation on the part of the charge trajectory where βn>1 is not always valid. We analyze also the frequently used interpretation of the transition radiation in terms of instantaneous charge deceleration in one medium and its sudden acceleration in another one, and find them as to be insufficient. On the other hand, attempts to interpret the transition radiation in terms of semi-infinite motions terminating in one medium and beginning in the other one turn out to be correct if one takes into account the terms corresponding to the Vavilov–Cherenkov radiation.
The propagation of ion Bernstein waves in heliotron/torsatron magnetic configurations
Authors: Torii Y. , Kumazawa R. , Saito K. , Watari T. , Watanabe T. , Idei H. , Kubo S. , Shoji T. , Mutoh T. , Seki T. , Yamamoto T. , Takeuchi N. , Xie J. , Wan B.
Published: August 2003

The propagation and absorption of ion Bernstein waves were studied by use of the ray tracing method to predict the result of an experiment planned in the Large Helical Device (LHD) [M. Fujiwara et al., Nucl. Fusion 41, 1355 (2001)]. Three-dimensional data of the magnetic field (obtained from Biot–Savart’s law) and the associated magnetic flux given in an analytical expression were used in this calculation. Scans of various parameters were made: magnetic field strength, profiles of electron and ion temperatures and plasma density at the edge region, initial value of wave number parallel to the magnetic lines of force, starting position and direction of wave number. In many practical cases, however, electron Landau damping was so strong that the wave was absorbed in the peripheral region. For comparison, the same code was applied to a tokamak and a straight helical device. The difference of the wave damping processes of LHD from others may be attributed to the fact that the magnetic configuration of the LHD has mixed toroidal and helical ripples.
Distribution of built-in electrical potential in GaInP2/GaAs tandem-junction solar cells
Authors: Jiang C.-S. , Friedman D. J. , Geisz J. F. , Moutinho H. R. , Romero M. J. , Al-Jassim M. M.
Published: August 2003

Distributions of built-in potential in GaInP2/GaAs tandem-junction solar cells were investigated by scanning Kelvin probe microscopy. Two states of potential distribution resulting from flattening of band bending and charge accumulation on either the top or bottom p–n junction were observed under short circuit, depending on the illumination spectra. With an external bias voltage, the voltage change always happened on the junction with the charge accumulation, and the potential distribution between the two states became less sensitive to illumination spectra.
Stability of oblique modulation of dust-acoustic waves in a warm dusty plasma with dust charge variation
Authors: Tang Rong-An , Xue Ju-Kui
Published: August 2003

With the consideration of the dust charge variation, the oblique modulational instability of the dust-acoustic waves in an unmagnetized warm dust plasma is studied. A nonlinear Schrödinger equation governing the slow modulation of the wave amplitude is derived. The effect of the dust temperature, the dust charge variation, and the constituents of the dust grains, electrons, and ions on the oblique modulational instability of the dust-acoustic wave is presented. It is found that the dust temperature, the dust charge variation, and the constituents of the dust grains, electrons, and ions would modify the oblique modulational instability domain in the k–θ plane significantly.
Energy spectrum of turbulent fluctuations in boundary driven reduced magnetohydrodynamics
Authors: Dmitruk Pablo , Gómez Daniel O. , Matthaeus William H.
Published: August 2003

The nonlinear dynamics of a bundle of magnetic flux ropes driven by stationary fluid motions at their endpoints is studied, by performing numerical simulations of the magnetohydrodynamic (MHD) equations. The development of MHD turbulence is shown, where the system reaches a state that is characterized by the ratio between the Alfvén time (the time for incompressible MHD waves to travel along the field lines) and the convective time scale of the driving motions. This ratio of time scales determines the energy spectra and the relaxation toward different regimes ranging from weak to strong turbulence. A connection is made with phenomenological theories for the energy spectra in MHD turbulence.
Low-cost gated system for monitoring phosphorescence lifetimes
Authors: Kostov Yordan , Rao Govind
Published: August 2003

A low-cost, gated system for measurements of phosphorescence lifetimes is presented. An extensive description of the system operating principles and metrological characteristics is given. Remarkably, the system operates without optical filtering of the light emitting diode excitation source. A description of a practical system is also given and its performance is discussed. Because the device effectively suppresses high-level background fluorescence and scattered light, it is expected to find widespread application in bioprocess, environmental, and biomedical fields.
Patterning of particulate films using Faraday waves
Authors: Wright P. H. , Saylor J. R.
Published: August 2003

Faraday waves were used to create a patterned particulate film on the surface of a glass substrate. The process involves deposition of a thin liquid film on the substrate, where the liquid is comprised of water and a concentrated suspension of particles. Faraday waves were created on the surface of the liquid film by subjecting the substrate and film to a vertical oscillation. Because the films are thin, the standing waves create velocity fluctuations that are not negligible near the substrate surface. As the particles in the liquid settled to the bottom, the flow field caused by the waves caused preferential particle deposition organized in a pattern determined by the Faraday wave field. Subsequent evaporation of the liquid film left a particulate film on the solid substrate that retained the pattern of the original wave field. We refer to these particulate films as Faraday films. The length scale of the patterns in these films was on the order of millimeters in the experiments described here. Use of higher frequency oscillations could permit the formation of Faraday films having smaller spatial scales.
High-resolution grazing-incidence scattering using a combination of analyzer crystal and linear detector
Authors: Smilgies Detlef M.
Published: August 2003

Mesoscopic structures on length scales of 10–1000 nm are of high interest for the study of nanostrucured surfaces and thin films. Such structures can be analyzed with the help of high-resolution x-ray scattering using crystal optics. In order to improve the speed of data collection, it is convenient to combine the analyzer crystal with a linear detector. A simple analytical theory in angular space is presented, how to correct distortions in intensity maps obtained this way, and how to optimize the accessible angular range. The anisotropic resolution that can be achieved with this combination—high-resolution scattering in the sample plane, large scattering range normal to the surface—is useful for grazing-incidence small-angle scattering and high-resolution grazing-incidence diffraction from surfaces and thin films, on both solid substrates as well as at the air–water interface.
Effect of graphite and molybdenum wall tile reflections on visible light diagnostics in tokamak experiments
Authors: Hollmann E. M. , Pigarov A. Yu. , Doerner R. P.
Published: August 2003

Visible light reflectivities are measured for graphite and molybdenum tokamak wall tiles as a function of incident angle and wavelength. Using the measured reflectivities together with a ray-tracing routine, estimates of the effect of wall reflections on Dα light measurements in the DIII-D and Alcator C-Mod tokamaks are made. It is found that the contribution of reflections is small (typically <10%) for divertor view channels, but can be significant (>50%) for other view chords. Of the nondivertor channels, near-midplane view chords are found to be least susceptible to reflections from the divertor and are therefore best suited for main chamber recycling measurements.
Continuous wave optical spectroscopic system for use in magnetic resonance imaging scanners for the measurement of changes in hemoglobin oxygenation states in humans
Authors: Hulvershorn Justin , Bloy Luke , Leigh John S. , Elliott Mark A.
Published: August 2003

A continuous wave near infrared three-wavelength laser diode spectroscopic (NIRS) system designed for use in magnetic resonance imaging (MRI) scanners is described. This system measures in vivo changes in the concentrations of oxyhemoglobin (HbO) and deoxyhemoglobin (Hb) in humans. An algorithm is implemented to map changes in light intensity to changes in the concentrations of Hb and HbO. The system’s signal to noise ratio is 3.4×103 per wavelength on an intralipid phantom with 10 Hz resolution. To demonstrate the system’s performance in vivo, data taken on the human forearm during arterial occlusion, as well as data taken on the forehead during extended breath holds, are presented. The results show that the instrument is an extremely sensitive detector of hemodynamic changes in human tissue at high temporal resolution. NIRS directly measures changes in the concentrations of hemoglobin species. For this reason, NIRS will be useful in determining the sources of MRI signal changes in the body due to hemodynamic causes, while the precise anatomic information provided by MRI will aid in localizing NIRS contrast and improving the accuracy of models of light transport through tissue.
Design of a composite by-pass versus bearing testing apparatus
Authors: Zarco-González J. C. , Fellows N. A. , Durodola J. F.
Published: August 2003

This article presents the work carried out to design and build an apparatus to enable the bearing versus by-pass failure envelope to be determined for a range of composites. The apparatus incorporates a removable rig (consisting of a loading frame, actuators and additional load cell) with a servo-hydraulic testing machine to enable the application of both by-pass and bearing load. The advantages of this system are that independence between the loads can be achieved with minimum cost while enabling the servo-hydraulic machine to be utilized for other test types. Additionally, a variety of specimen geometries, bolt sizes, clamp-up areas, clamp-up pressures, by-pass loads and bearing loads can be applied. Initial tests have shown that the apparatus performs well.
The infrared synchrotron radiation beamline at the third generation light source ELETTRA
Authors: Nucara Alessandro , Lupi Stefano , Calvani Paolo
Published: August 2003

The synchrotron radiation emitted in the infrared range from a bending magnet of the third generation storage ring ELETTRA is studied in detail by simulation techniques. Edge effects are taken into account. Preliminary experimental checks on an existing port for beam diagnostics are reported. The design of the infrared beamline under construction, including an evaluation of the tolerance on mirror parameters and of thermal distortions, is presented. Finally, the gain in flux and brightness with respect to a conventional source and the polarization degree of the infrared radiation are calculated and discussed.
Experimental study of the optical gain and loss in InAs/GaInSb interband cascade lasers
Authors: Suchalkin S. , Bruno J. , Tober R. , Westerfeld D. , Kisin M. , Belenky G.
Published: August 2003

Optical gain and loss have been measured in type II InAs/GaInSb interband cascade lasers operating in the 3.4–3.6 μm wavelength range. The maximum temperature of cw operation was found to be limited by strong gain saturation due to active region overheating, while the temperature increase of the total optical loss was relatively small. In devices with a longer lasing wavelength and a thinner substrate-side cladding layer, a strong periodic modulation of the optical gain spectra was observed. This effect is consistent with resonant optical leakage into the substrate.
Magnetovibrational coupling in small cantilevers
Authors: Kovalev Alexey A. , Bauer Gerrit E. W. , Brataas Arne
Published: August 2003

A nanomagnetomechanical system consisting of a cantilever and a thin magnetic film is predicted to display magnetovibrational modes, which should enable applications for sensors and actuators. The “polaritonic” modes can be detected by line splittings in ferromagnetic resonance spectra.
Control of profiles and transport by ion Bernstein waves in the Hefei Tokamak-7
Authors: Wan Baonian , Zhao Yanping , Li Jiangang , Zhao Junyu , Xu Guosheng , Song Mei , Shi Yuejiang , Jie Yinxian , Zhu Yubao , Liu Haiqing , Xu Yuchun , Hu Liqun , Ling Bili , Gentle K. , Rowan B. , Phillipe P. , Huang H. , Watari T. , Seki T.
Published: August 2003

Electron pressure profile and transport behaviors in ion Bernstein waves (IBW) heated plasmas were investigated at three frequencies (24, 27, and 30 MHz) in the Hefei Tokamak-7. Both on-axis and off-axis electron heating with global peaked and locally steepened electron pressure profiles were realized, if the ion cyclotron resonant layer was properly selected. Improvement of the IBW-heated plasma confinement was observed except in the 24 MHz case. Reduction of the local electron heat transport was evident from sawtooth heat pulse propagation. High performance plasma volume was extended in the IBW off-axis heating mode, which resulted in improvement of the global plasma confinement. These studies demonstrate that IBW is a potentially powerful tool for active control of plasma profiles and transport.
Positron acceleration to ultrarelativistic energies by a shock wave in a magnetized electron–positron–ion plasma
Authors: Hasegawa Hiroki , Usami Shunsuke , Ohsawa Yukiharu
Published: August 2003

Positron acceleration in oblique shock waves is studied with relativistic, electromagnetic, particle simulations with full particle dynamics. In the simulations, some positrons have been accelerated to energies γ∼600, where γ is the Lorentz factor. The electric field parallel to the magnetic field plays an essential role in the acceleration; it prevents some positrons from passing through the shock wave. For certain shock propagation velocities and angles, these positrons stay around the shock front for long periods of time, moving roughly parallel to the external magnetic field, and gain great energies. These properties are in good agreement with a proposed acceleration model.
Ordered plasma structures in the interspace of two independently working discharges
Authors: Strat M. , Strat Georgeta , Gurlui S.
Published: August 2003

The plasma formations that appear in the contact region between two negative glow discharges have been studied by means of electrical and spectral methods. The experimental and theoretical research of the plasma formations show that the double layer is the final stage of a more complex plasma structure which is formed first.
Electrical characteristics of Au and Ag Schottky contacts on n-ZnO
Authors: Polyakov A. Y. , Smirnov N. B. , Kozhukhova E. A. , Vdovin V. I. , Ip K. , Heo Y. W. , Norton D. P. , Pearton S. J.
Published: August 2003

Au and Ag Schottky contacts on the epiready (0001)Zn surface of bulk n-ZnO crystals show Schottky barrier heights of 0.65–0.70 eV from capacitance–voltage measurements, activation energies for reverse saturation currents of 0.3–0.4 eV and saturation current densities ranging from 10−5 A cm−2 on surfaces etched in HCl to 8×10−7 A cm−2 on solvent cleaned samples. The diode ideality factors were in the range 1.6–1.8 under all conditions. The properties of both the Au and the Ag Schottky diodes were degraded by heating in vacuum to temperatures even as low as 365 K. The degradation mechanisms during annealing were different in each case, with the Au showing reaction with the ZnO surface and the Ag contacts showing localized delamination. Mechanical polishing of the ZnO surface prior to contact deposition produced a high-resistivity damaged layer with prominent deep level defects present with activation energies of 0.55 and 0.65 eV.
Investigation of high-Q channel drop filters using donor-type defects in two-dimensional photonic crystal slabs
Authors: Akahane Yoshihiro , Asano Takashi , Song Bong-Shik , Noda Susumu
Published: August 2003

This letter describes experimental investigations of surface-emitting channel drop filters using donor-type point defect cavities and line-defect waveguides in two-dimensional photonic crystal slabs. By using donor-type defect cavities with three and four linearly aligned missing air holes, filter quality factors of around 2600 and 6400, respectively, are achieved experimentally, compared to the quality factor of 400 of previous acceptor-type defect cavities. Radiation patterns and polarization properties of light emitted from the defects are also discussed. The results indicate that these donor-type defects are very useful for the development of ultrasmall high-performance channel add/drop filters.
Temperature- and field-dependent leakage current of Pt/(Ba0.7Sr0.3)TiO3 interface
Authors: Yang Hao , Chen Bin , Tao Kun , Qiu Xianggang , Xu Bo , Zhao Bairu
Published: August 2003

Pt/(Ba 0.7 Sr 0.3 ) TiO 3 ( BST)/YBa 2 Cu 3 O 7− x (YBCO ) capacitors on SrTiO3 substrates were fabricated. The temperature and field dependence of the leakage current of Pt/BST interface were studied in the temperature range from 100 K to 320 K and in an electric field up to 3 MV/cm. For a middle electric field (<1 MV/cm), the leakage current shows space-charge-limited-current behavior in the temperature range from 100 K to 200 K. In the case of a high electric field (>1.8 MV/cm), the leakage mechanism is governed by the Fowler–Nordheim tunneling in the whole measured temperature range.
Dielectric and piezoelectric properties of relaxor Pb(Sc1/2Nb1/2)O3 thin films
Authors: Kuh B. J. , Choo W. K. , Brinkman K. , Damjanovic D. , Setter N.
Published: August 2003

Pure perovskite Pb(Sc1/2Nb1/2)O3 thin films without pyrochlore phase were prepared by the sol–gel method on TiO2/Pt/TiO2/SiO2/Si substrates. Films exhibited (111) preferred orientation and columnar microstructure. Diffuse phase transitions with permittivity maximum decreasing in value and shifting toward higher temperature with increasing frequency and slim polarization-electric field hysteresis loops typical for relaxors were observed. The maximum field-induced piezoelectric d33 coefficient measured with ac electric field of 14 kV/cm was 58 pm/V. The d33–dc electric field relation is virtually hysteresis free.
Current rectification in a single GaN nanowire with a well-defined p–n junction
Authors: Cheng Guosheng , Kolmakov Andrei , Zhang Youxiang , Moskovits Martin , Munden Ryan , Reed Mark A. , Wang Guangming , Moses Daniel , Zhang Jinping
Published: August 2003

This letter discusses Mg incorporation in GaN nanowires with diameters ∼35 nm, fabricated by vapor–liquid–solid synthesis in p-type nanowires. Turning on the Mg doping halfway through the synthesis produced nanowires with p–n junctions that showed excellent rectification properties down to 2.6 K. The nanowires are shown to possess good-quality, crystalline, hexagonal GaN inner cores surrounded by an amorphous GaN outer layer. Most wires grow such that the crystalline c axis is normal to the long axis of the nanowire. The temperature dependence of the current–voltage characteristics is consistent with electron tunneling through a voltage-dependent barrier.
Universal description of channel conductivity for nanotube and nanowire transistors
Authors: Rotkin S. V. , Ruda H. E. , Shik A.
Published: August 2003

A theory of drift-diffusion transport in a low-dimensional field-effect transistor is developed. Two cases of a semiconductor nanowire and a single-wall nanotube are considered using self-consistent electrostatics to obtain a general expression for the transconductance. This quantum-wire channel device description is shown to differ from a classical device theory because of the specific nanowire charge density distribution.
Strain tunable light transmission through a 90° bend waveguide in a two-dimensional photonic crystal
Authors: Malkova Natalia , Kim Sungwon , Gopalan Venkatraman
Published: August 2003

We report a device based on strain-tunable light propagation through a 90° bend waveguide based on a two-dimensional photonic crystal. This is achieved by the splitting of a doubly degenerate defect state, by means of a symmetrical distortion of the lattice, locally near the bend. The resonant coupling of the photon modes between the two waveguide arms across the bend can be tuned by the symmetry and the magnitude of the local distortion of the lattice.