bluuebunny/crossref_metadata_2025 huggingface dataset:
Recent advances in the design of quasiaxisymmetric stellarator plasma configurations
Authors: Reiman A. , Ku L. , Monticello D. , Hirshman S. , Hudson S. , Kessel C. , Lazarus E. , Mikkelsen D. , Zarnstorff M. , Berry L. A. , Boozer A. , Brooks A. , Cooper W. A. , Drevlak M. , Fredrickson E. , Fu G. , Goldston R. , Hatcher R. , Isaev M. , Jun C. , Knowlton S. , Lewandowski J. , Lin Z. , Lyon J. F. , Merkel P. , Mikhailov M. , Miner W. , Mynick H. , Neilson G. , Nelson B. E. , Nührenberg C. , Pomphrey N. , Redi M. , Reiersen W. , Rutherford P. , Sanchez R. , Schmidt J. , Spong D. , Strickler D. , Subbotin A. , Valanju P. , White R.
Published: July 2002

Strategies for the improvement of quasiaxisymmetric stellarator configurations are explored. Calculations of equilibrium flux surfaces for candidate configurations are also presented. One optimization strategy is found to generate configurations with improved neoclassical confinement, simpler coils with lower current density, and improved flux surface quality relative to previous designs. The flux surface calculations find significant differences in the extent of islands and stochastic regions between candidate configurations. (These calculations do not incorporate the predicted beneficial effects of perturbed bootstrap currents.) A method is demonstrated for removing low order islands from candidate configurations by relatively small modifications of the configuration. One configuration is identified as having particularly desirable properties for a proposed experiment.
The interaction of two vortices on a beta-plane
Authors: McDonald N. Robb , Johnson E. R.
Published: July 2002

Two motion of two interacting vortices in the presence of a background gradient of potential vorticity due to the β-effect is considered. The vortices, and their interaction, are modeled using a two-layer, quasigeostrophic model. The paper extends that of McDonald [Philos. Trans. R. Soc. London, Ser. A 456, 1029 (2000)] who, using a similar model, considered only steady configurations of zonally aligned vortices, i.e., lying along the same parallel of latitude. Here Green’s function techniques are used to calculate the velocity of one vortex due to the presence of the Rossby wave wake of the other. This enables consideration of the more general case of vortices having arbitrary relative location, the only requirement being that the vortices are well-separated (i.e., several Rossby radii apart). An additional quasisteady approximation enables the time evolution of vortices to be studied. Of particular interest is the existence of equilibrium configurations in which the vortices do not move relative to each other and maintain their orientation with respect to the direction defined by the gradient of the background potential vorticity field. It is shown that necessary conditions for equilibrium are that the vortices are (a) of equal strength, or (b) aligned zonally. The stability of equilibria to small perturbations in the relative location of the vortices is investigated and the equilibria found to be weakly unstable in that the vortex trajectories drift slowly (but not exponentially fast) from their equilibrium paths. This is verified by numerical computation of the trajectories near equilibrium. For equal strength vortices not in equilibrium the distance between the vortices remains constant, but the line joining the two vortices rotates with constant angular velocity. For this case, examples of the trajectories of the individual vortices and the center of mass are given. For vortices of differing strength and arbitrary initial configurations, the motion is, in general, complicated. Some examples are presented.
Simulation of near-field photolithography using the finite-difference time-domain method
Authors: Tanaka Shuji , Nakao Masayuki , Umeda Mariko , Ito Kenchi , Nakamura Shigeru , Hatamura Yotaro
Published: July 2002

Illuminating a transparent mold under total internal reflection condition generates evanescent light. Near-field photolithography uses such light, and we simulated this exposure in two dimensions using the finite-difference time-domain (FDTD) method. Our simulation suggests the feasibility of resolving a 130 nm pitch grating pattern, which is finer than the diffraction limit of light. The simulation results showed fair agreement with our experimental results, confirming the strong influence of exposure light polarization to the distribution of optical near fields in photoresist films. This indicates that the FDTD simulation is promising to predict exposure results for designing molds. We further extended the simulation varying the thickness and refractive index of a photoresist film. Based on the simulation results, showing the good exposure contrast in the thin surface layer of the photoresist film, we suggested two methods to resolve a thick resist film: the multilayer resist method which allows us to use a sufficiently thin photoresist film, and the surface imaging technology which can completely dry develop a thick photoresist film even if its exposure area is confined in the surface layer.
Photon recycling and recombination processes in 0.53 eV p-type InGaAsSb
Authors: Borrego J. M. , Saroop S. , Gutmann R. J. , Charache G. W. , Donovan T. , Baldasaro P. F. , Wang C. A.
Published: July 2002

Transient radio frequency photoreflectance measurements were performed on 0.53 eV p-type InGaAsSb double heterostructures, grown by organometallic vapor phase epitaxy on lattice matched GaSb substrates, for determining excess carrier lifetime. Direct evidence of photon recycling was observed by changing the GaSb backsurface reflectivity and observing the change in excess carrier lifetime. Consistent with theory developed for this type of structure, effective lifetimes increased by 30%–40% when the backsurface was changed from an absorbing to a reflecting surface. The theory develops a closed-form expression for the total radiative recombination rate, starting with continuity equations for both excess minority carriers and the photon density. Lifetime measurements in these p-InGaAsSb structures with different doping concentrations and epitaxial layer thicknesses allow the extraction of minority carrier recombination parameters. Measurements indicate a value for the radiative recombination coefficient of B=5–6×10−11 cm3/s, for the Auger recombination coefficient of C=2–5×10−29 cm6/s, for the Shockley–Read–Hall lifetime of 100–150 ns and for the surface recombination velocity of 1–2×103 cm/s.
High-Reynolds-number flow in a narrow gap driven by solidification. I. Theory
Authors: Karcher Ch. , Steen P. H.
Published: July 2002

Flow driven by solidification is studied in a limit when fluid dynamics and heat transfer decouple. The governing equations are derived by integrating the mass and momentum equations over a narrow gap of any shape within which rapid solidification occurs. These equations are then simplified by exploiting the high-Reynolds-number characteristics of the flow. The modeling leads to fluid dynamics with a suction-type boundary layer established at the solid/liquid interface due to solid pulling and phase change. In part II [Phys. Fluids 13, 834 (2001)], the method is applied to planar-flow melt-spinning where liquid metal, held by surface tension in the gap between the injection nozzle and spinning chill-wheel, forms a long thin puddle.
Different types of defects in silicon dioxide characterized by their transient behavior
Authors: Ielmini Daniele , Spinelli Alessandro S. , Beretta Marco , Lacaita Andrea L.
Published: July 2002

In this article we report results on the physical properties of defect levels generated by high-field stress in silicon dioxide. Carrier separation experiments were performed in p-channel metal–oxide–semiconductor transistors, measuring the transient and steady-state leakage currents. Different transient behaviors are observed in the channel and substrate currents, indicating that different kinds of traps are involved in the transient and steady-state leakage currents. This conclusion is also supported by time-relaxation experiments, where it is shown that the components of the current feature different relaxation behaviors. The impact of these results on the modeling of stress-induced leakage conduction is also addressed.
Growth and vertical correlation of CdSe/ZnSe quantum dots
Authors: Litvinov D. , Rosenauer A. , Gerthsen D. , Preis H. , Fuchs K. , Bauer S.
Published: July 2002

The growth and vertical organization of CdSe quantum dots in three-layer stacks consisting of CdSe with a nominal thickness of 2.5 monolayers (ML) and ZnSe spacers with thicknesses between 10 and 20 ML was investigated by reflection high energy electron diffraction during the growth and different transmission electron microscopy techniques. The samples were grown by molecular beam epitaxy at 400 °C. It was found that up to 10 ML spacer thickness all three CdSe layers and ZnSe spacers form one broad (Cd, Zn)Se alloy layer with a small Cd concentration containing Cd-rich islands with a size of ∼15 nm. For spacers with a larger thickness (12–20 ML) three separated ternary (Cd, Zn)Se layers are observed which contain Cd-rich inclusions (small islands) with a size of less than 10 nm. A preferential vertical correlation of the small islands occurs for the 12 ML spacer thickness. With increasing spacer thickness, the number of the correlated small islands is reduced displaying a tendency to uncorrelated growth.
Semiconductor impurity parameter determination from Schottky junction thermal admittance spectroscopy
Authors: Los Andrei V. , Mazzola Michael S.
Published: July 2002

The problem of interpretation of thermal admittance spectroscopy data for semiconductor impurity parameter extraction is considered. Traditional analysis predicts that the Arrhenius plot for conductance peak temperatures is a straight line with the slope proportional to impurity activation energy and the intercept determining its capture cross section. Using a general model of the Schottky junction admittance we show that conductance peak positions strongly depend on the impurity bulk occupation number and potential distribution in the space-charge region, and, as a result, the Arrhenius plot is nonlinear for some semiconductor parameters and experimental conditions, in particular for relatively shallow impurities. In this case, the traditional linear approximation of the Arrhenius plot yields inaccurate values of activation energy and capture cross section. We propose a more accurate procedure for admittance spectroscopy data analysis involving least-squares fitting using the general and the small-signal models of the junction admittance. Although much more computationally intensive, the general model is shown to provide a better fit to the data at low temperatures, where the small-signal approximation is invalid. This approach is applied for an example admittance data and yields a better fit of the theoretical curve to the data and an improved value of activation energy for the nitrogen donor in 6H-SiC.
Processing, microstructure, and electric properties of buried resistors in low-temperature co-fired ceramics
Authors: Yang Pin , Rodriguez Mark A. , Kotula Paul , Miera Brandon K. , Dimos Duane
Published: July 2002

The electrical properties of ruthenium oxide based devitrifiable resistors embedded within low-temperature co-fired ceramics were investigated from −100 °C to 100 °C. Special attention was given to the processing conditions and their effects on resistance and temperature coefficient of resistance (TCR). Results indicate that within this temperature range the conductance for these buried resistors is limited by tunneling of charge carriers through the thin glass layer between ruthenium oxide particles. A modified version of the tunneling barrier model is proposed to account for the microstructure ripening observed during thermal processing. The model parameters determined from curve fitting show that charging energy (i.e., the energy required for a charge carrier to tunnel through the glass barrier) is strongly dependent on particle size and particle–particle separation between ruthenium oxide grains. Initial coarsening of ruthenium oxide grains was found to reduce the charging energy and lower the resistance. However, when extended ripening occurs, the increase in particle–particle separation increases the charging energy, reduces the tunneling probability and gives rise to a higher resistance. The tradeoff between these two effects results in an optimum microstructure with a minimum resistance and TCR. Furthermore, the TCR of these buried resistors has been shown to be governed by the magnitude of the charging energy. Model parameters determined by our analysis appear to provide quantitative physical interpretations to the microstructural changes in the resistor, which in turn, are controlled by the processing conditions.
Interaction of water and dichlorine in the gas phase: An investigation of H2O⋯Cl2 by rotational spectroscopy and ab initio calculations
Authors: Davey J. B. , Legon A. C. , Thumwood J. M. A.
Published: July 2002

Ground-state rotational spectra of the seven isotopomers H2O⋯35Cl2, H2O⋯37Cl35Cl, H2O⋯35Cl37Cl, D2O⋯35Cl2, D2O⋯37Cl35Cl, HDO⋯35Cl2, and HDO⋯37Cl35Cl of the complex formed between water and dichlorine were observed by pulsed-jet, Fourier-transform microwave spectroscopy. Rotational constants B0 and C0, centrifugal distortion constants ΔJ and ΔJK, and Cl nuclear quadrupole coupling constants χaa(Clx) and {χbb(Clx)−χcc(Clx)}, where x=i (inner) or o (outer), were obtained via spectral analyses. The spectroscopic constants were interpreted, on the basis of models for the complex, to give the geometry, the binding strength, and the extent of inter- and intramolecular electron transfer on complex formation. The zero-point geometry is defined in terms of the effective values r(O⋯Cli)=2.8479(3) Å and φ=43.4(3)°, where φ is the angle made by the bisector of the HÔH angle with the O⋯Cl internuclear line. The intermolecular stretching force constant kσ=8.0(1) N m−1. Fractions δi=0.005(5) and δp=0.034(3) of an electron were estimated to be transferred from O to Cl and from Cli to Clo, respectively. The geometrical conclusions are supported by ab initio calculations at the aug-cc-pVDZ/MP2 level of theory, with good agreement for φ and r(O⋯Cli). A comparison of the properties of H2O⋯Cl2 with those of H2O⋯HCl provides evidence in support of a recent proposal of a halogen bond B⋯XY that is the analog of the hydrogen bond B⋯HX, where X and Y are halogen atoms.
High-efficiency red electrophosphorescence devices
Authors: Adachi Chihaya , Baldo Marc A. , Forrest Stephen R. , Lamansky Sergey , Thompson Mark E. , Kwong Raymond C.
Published: July 2002

We demonstrate high-efficiency red electrophosphorescent organic light-emitting devices employing bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetylacetonate) [Btp2Ir(acac)] as a red phosphor. A maximum external quantum efficiency of ηext=(7.0±0.5)% and power efficiency of ηp=(4.6±0.5) lm/W are achieved at a current density of J=0.01 mA/cm2. At a higher current density of J=100 mA/cm2, ηext=(2.5±0.3)% and ηp=(0.56±0.05) lm/W are obtained. The electroluminescent spectrum has a maximum at a wavelength of λmax=616 nm with additional intensity peaks at λsub=670 and 745 nm. The Commission Internationale de L’Eclairage coordinates of (x=0.68, y=0.32) are close to meeting video display standards. The short phosphorescence lifetime (∼4 μs) of Btp2Ir(acac) leads to a significant improvement in ηext at high currents as compared to the previously reported red phosphor, 2,3,7,8,12,13,17,18-octaethyl-12H, 23H-prophine platinum (II) PtOEP with a lifetime of ∼50 μs.
Theoretical and experimental studies of the dissociation dynamics of methaniminium cation, CH2NH2+→CHNH++H2: Reaction path bifurcation
Authors: Hoon Choi Tae , Tae Park Sang , Soo Kim Myung
Published: July 2002

The unimolecular dissociation of CH2NH2+ has been investigated experimentally and theoretically. Kinetic energy release distribution was obtained by analyzing the mass-analyzed ion kinetic energy profile. Critical configurations along the reaction path were investigated by electronic structure calculations at the HF, MP2, QCISD, and B3LYP levels using the 6-31G** and 6-311+G** basis sets. Reaction path bifurcation was observed at all the levels. The bifurcation point was in the entrance region (before the transition state) at the HF level. This point moved to the exit region when the electron correlation effect was included at the MP2, QCISD, and B3LYP levels. A global potential energy surface incorporating this bifurcation feature was constructed by interpolation at the MP2/6-311+G** level. Classical trajectories were calculated on this surface and product mode-specific energies were evaluated. Based on these data, various experimental observations, lack of hydrogen scrambling in particular, could be adequately explained.
Weak mixing and anomalous kinetics along filamented surfaces
Authors: Zaslavsky G. M. , Edelman M.
Published: July 2002

We consider chaotic properties of a particle in a square billiard with a horizontal bar in the middle. Such a system can model field-line windings of the merged surfaces. The system has weak-mixing properties with zero Lyapunov exponent and entropy, and it can be also interesting as an example of a system with intermediate chaotic properties, between the integrability and strong mixing. We show that the transport is anomalous and that its properties can be linked to the ergodic properties of continued fractions. The distribution of Poincaré recurrences, distribution of the displacements, and the moments of the truncated distribution of the displacements are obtained. Connections between different exponents are found. It is shown that the distribution function of displacements and its truncated moments as a function of time exhibit log-periodic oscillations (modulations) with a universal period Tlog=π2/12 ln 2. We note that similar results are valid for a family of billiard, particularly for billiards with square-in-square geometry.
Three-dimensional HYDRA simulations of National Ignition Facility targets
Authors: Marinak M. M. , Kerbel G. D. , Gentile N. A. , Jones O. , Munro D. , Pollaine S. , Dittrich T. R. , Haan S. W.
Published: July 2002

The performance of a targets designed for the National Ignition Facility (NIF) are simulated in three dimensions using the HYDRA multiphysics radiation hydrodynamics code. [M. Marinak et al., Phys. Plasmas 5, 1125 (1998)] In simulations of a cylindrical NIF hohlraum that include an imploding capsule, all relevant hohlraum features and the detailed laser illumination pattern, the motion of the wall material inside the hohlraum shows a high degree of axisymmetry. Laser light is able to propagate through the entrance hole for the required duration of the pulse. Gross hohlraum energetics mirror the results from an axisymmetric simulation. A NIF capsule simulation resolved the full spectrum of the most dangerous modes that grow from surface roughness. Hydrodynamic instabilities evolve into the weakly nonlinear regime. There is no evidence of anomalous low mode growth driven by nonlinear mode coupling.
Investigation of the rise time and damping of spin excitations in Ni81Fe19 thin films
Authors: Wu J. , Hughes N. D. , Moore J. R. , Hicken R. J.
Published: July 2002

The rise and damping of spin excitations in three Ni81Fe19 films of thickness 50, 500, and 5000 Å have been studied with an optical pump–probe technique in which the sample is pumped with an optically triggered magnetic field pulse. The motion of the magnetization was described by the uniform mode solution of the Landau–Lifshitz–Gilbert equation. The rise time of the pulsed field within the film was smallest in the 50 Å sample and was generally greater when the pulsed field was perpendicular to the film plane. The damping constant was smallest in the 500 Å sample. The variations in the rise time and damping are attributed to the presence of eddy currents and structural disorder in the films. Under certain excitation conditions a second mode was observed in the 5000 Å sample which we believe to be a magnetostatic surface mode.
Dependence of dielectric properties on internal stresses in epitaxial barium strontium titanate thin films
Authors: Li Hao , Roytburd A. L. , Alpay S. P. , Tran T. D. , Salamanca-Riba L. , Ramesh R.
Published: July 2002

A series of heteroepitaxial BaxSr1−xTiO3 thin films with composition x=0.50 were deposited on (001) MgO substrates by pulsed-laser deposition. The thickness of the films was varied from 14 to 500 nm to produce a systematically decreasing level of in-plane tensile stresses. The microstructural and crystallographic features of the films were determined via transmission electron microscopy and x-ray diffraction. A theoretical treatment of the in-plane misfit strain as a function of film thickness is in agreement with the measured out-of-plane lattice parameters. Electrical measurements indicate a drop in the dielectric constant from 2350 for highly stressed thin films to 1700 for relaxed thicker films. The variation in the dielectric constant with the misfit strain is in accordance with a thermodynamic model developed. The relationship between the dielectric constant and electric field is also described by extending the thermodynamic model and taking the effect of electric field into account. A new definition of tunability is adopted to study the effect of strain on tunability.
Athermal annealing of low-energy boron implants in silicon
Authors: Donnelly D. W. , Covington B. C. , Grun J. , Fischer R. P. , Peckerar M. , Felix C. L.
Published: July 2002

Silicon samples that have been ion implanted with boron at energies below 3 keV have been athermally annealed. The annealing process has been characterized using secondary ion mass spectrometry and infrared absorption spectroscopy. The athermally annealed samples show activation comparable to that for thermally annealed samples, but with much less boron diffusion. The activation in the athermally annealed samples is shown to be much higher than would be achieved by recrystallization of the amorphous layer.
Quantum state distributions of HCl from the ultraviolet photodissociation of HCl dimer
Authors: Picconatto Carl A. , Ni Hong , Srivastava Abneesh , Valentini James J.
Published: July 2002

The rovibrational state distributions for the HCl product of the 193 nm photodissociation of (HCl)2, (HCl)2+hν→H+Cl+HCl, have been measured. The HCl dimer is prepared in a supersonic expansion of HCl in Ar, and its photoproduct detected by resonant multiphoton ionization under collisionless conditions. The state distributions are extremely “cold,” with very little of the available energy deposited in either rotation or vibration of the surviving HCl molecule. Only v′=0 product is observed, and linear rotational surprisal analysis yields an extremely large surprisal parameter, θr′ of 95(8). The results are in excellent agreement with theoretical predictions.
Chemical vapor deposition and deep level analyses of 4H-SiC(112̄0)
Authors: Kimoto Tsunenobu , Yamamoto Toshiyuki , Chen Zhi Ying , Yano Hiroshi , Matsunami Hiroyuki
Published: July 2002

Specular 4H-SiC layers have been homoepitaxially grown on 4H-SiC(112̄0), parallel to the c axis (〈0001〉), by chemical vapor deposition at 1500 °C. An x-ray diffraction analysis has revealed that a lattice-mismatch strain between n− epilayers and n+ substrates could be minimized by introducing n-type buffer layers. The donor concentration of unintentionally doped epilayers could be reduced down to 1×1014 cm−3 under a C-rich growth condition. Through isothermal capacitance transient spectroscopy measurements, three acceptor-like traps with activation energies of 0.27, 0.32, and 0.66 eV have been detected with a total trap concentration as low as 3.8×1012 cm−3. The capture cross section of the deepest trap, the Z1 center, at high temperatures has been determined.
Electrodeposition of highly uniform magnetic nanoparticle arrays in ordered alumite
Authors: Sun Ming , Zangari Giovanni , Shamsuzzoha Mohammad , Metzger Robert M.
Published: July 2002

We report the fabrication of nanometer scale ordered arrays of magnetic cylindrical nanoparticles with low aspect ratio (height/radius a=0.2–7) and ultrahigh uniformity. Anodization and electrochemical deposition are employed for template synthesis and metal particle growth, respectively. Particle uniformity is achieved by an electrodeposition scheme, utilizing pulse reverse voltage wave forms to control nucleation and growth of the particles. The resulting nanoparticles are polycrystalline and grains are randomly oriented. The magnetic properties of the array are dominated by particle shape and by interparticle magnetostatic interactions. A very clear transition of the anisotropy from perpendicular to in plane is observed at an aspect ratio a of about two. The arrays exhibit good thermal stability, demonstrating a great potential of these structures as future recording media in a patterned scheme. The pulse reverse electrodeposition technique shows great promise for the synthesis of nanostructures of various nature.
Raman spectra and structural phase transition in nanocrystalline lead lanthanum titanate
Authors: Zhou Q. F. , Chan H. L. W. , Zhang Q. Q. , Choy C. L.
Published: July 2002

The effects of particle size on the structure and Raman spectra of lead lanthanum titanate nanocrystalline particles are investigated. Samples with different particle sizes were prepared by the sol–gel process using different calcination temperatures. The sizes of the crystallites of particles were determined by x-ray diffraction with the aid of Scherrer’s equation. Raman vibration modes of samples with various particle sizes were also studied by Raman scattering at room temperature. The results show that the tetragonal distortion of the unit cell apparently decreases with decreasing particle size: the soft A1(1TO) phonon mode which is related to the spontaneous polarization of ferroelectric materials shifts toward lower frequency, and other peaks exhibit broadening and intensity changes with decreasing particle size. These are attributed to a phase transformation from the ferroelectric tetragonal to the paraelectric cubic phase. The above phenomena are related to a grain size-induced structural phase transition.
Model of ferromagnetic clusters in amorphous rare earth and transition metal alloys
Authors: Wang L. , Ding J. , Li Y. , Feng Y. P. , Phuc N. X. , Dan N. H.
Published: July 2002

Experimental results on amorphous rare earth and transition metal alloys suggest the presence of Fe-rich clusters. A model is proposed in which the magnetic units are magnetic clusters. The magnetization of the clusters decreases with the increase of temperature. In this model, there are two critical temperatures, Tcsystem and Tccluster. Tccluster is the Curie temperature of the magnetic clusters, which is also the Curie temperature of the sample. Tcsystem is the measurement of the strength of interactions between clusters. Between Tccluster and Tcsystem, the system exhibits superparamagnetism with strong cluster interactions. The strong cluster interactions result in the ferromagnetic state below the critical temperature (Tcsystem), which is called a cluster ferromagnetism. Our experimental data (magnetization curves and susceptibility values of amorphous Y60Fe30Al10 and Nd60Fe30Al10 ribbons) support the cluster ferromagnetic model. The zero temperature coercivity and the relationship between Tblock and Tcsystem are also discussed in this article.
Control of strain in GaN by a combination of H2 and N2 carrier gases
Authors: Yamaguchi Shigeo , Kariya Michihiko , Kosaki Masayoshi , Yukawa Yohei , Nitta Shugo , Amano Hiroshi , Akasaki Isamu
Published: July 2002

We study the effect of a combination of N2 and H2 carrier gases on the residual strain and crystalline properties of GaN, and we propose its application to the improvement of crystalline quality of GaN/Al0.17Ga0.83N multiple quantum well (MQW) structures. GaN was grown with H2 or N2 carrier gas (H2– or N2–GaN) on an AlN low-temperature-deposited buffer layer. A (0001) sapphire substrate was used. N2–GaN was grown on H2–GaN. The total thickness was set to be 1.5 μm, and the ratio of N2–GaN thickness to the total thickness, x, ranged from 0 to 1. With increasing x, the tensile stress in GaN increased. Photoluminescence intensity at room temperature was much enhanced. Moreover, the crystalline quality of GaN/Al0.17Ga0.83N MQW was much higher when the MQW was grown with N2 on H2–GaN than when it was grown with H2 on H2–GaN. These results were due to the achievement of control of strain in GaN using a combination of N2–GaN and H2–GaN.
Low resistive p-type GaN using two-step rapid thermal annealing processes
Authors: Scherer M. , Schwegler V. , Seyboth M. , Kirchner C. , Kamp M. , Pelzmann A. , Drechsler M.
Published: July 2002

Two-step thermal annealing processes were investigated for electrical activation of magnesium- doped galliumnitride layers. The samples were studied by room-temperature Hall measurements and photoluminescence spectroscopy at 16 K. After an annealing process consisting of a short-term step at 960 °C followed by a 600 °C dwell step for 5 min a resistivity as low as 0.84 Ω cm is achieved for the activated sample, which improves the results achieved by standard annealing (800 °C for 10 min) by 25% in resistivity and 100% in free hole concentration. Photoluminescence shows a peak centered at 3.0 eV, which is typical for Mg-doped samples with high free hole concentrations.
High resolution measurement of the carbon localized vibrational mode in gallium arsenide
Authors: Nagai Naoto
Published: July 2002

The localized vibrational mode of carbon substituted at arsenic sites in gallium arsenide (GaAs) was measured with infrared absorption spectroscopy at 0.005 cm−1 resolution. Well-resolved fine structures were observed, yielding quantitative information on the line half widths and intensities. The relative intensities of the isotope lines are well fitted to a probability factor calculation using a natural abundance of 55.3% Ga69. One explanation considered is that the dipole moment due to the relative displacement of carbon with respect to the gallium atoms has a tendency to be larger when the neighboring gallium atoms are heavier (71Ga).
Photoinduced in-plane switching of a photochromic nematic liquid crystal
Authors: Komitov L. , Yamamoto J. , Yokoyama H.
Published: July 2002

Photoinduced fast in-plane switching of the optic axis of a photochromic nematic liquid crystal is found in a sandwich cell with substrates promoting a twofold degenerate anchoring. The switching process is governed by the modification of the anchoring conditions associated with the photoisomerization of the photochromic nematic liquid crystal. Photoinduced in-plane reorientation of the sample optic axis of about 80° has been found. Together with the model of photoinduced in-plane switching, some implementations of this effect are briefly discussed.
Self-assembly of the symmetric diblock copolymer in a confined state: Monte Carlo simulation
Authors: He Xuehao , Song Mo , Liang Haojun , Pan Caiyuan
Published: July 2002

Self-assembly of symmetric diblock copolymers in confined state has been investigated by means of Monte Carlo simulation method. The symmetric diblock copolymers were confined in two- (parallel walls or circle) or in three-dimensional (spherical or cylindrical) space. There are interactions between these boundaries and the symmetric diblock polymers. These interactions and boundary shape resulted in the formation of novel self-assemble structures, e.g., strip, circle, core-multishell, and multibarrel-layer structures. Simulation results predicated that it is possible to design different phase structures for block copolymers by adjusting boundary shape and boundary-block copolymer interactions.
Composition dependence of the optical phonon energies in hexagonal AlxGa1−xN
Authors: Holtz M. , Prokofyeva T. , Seon M. , Copeland K. , Vanbuskirk J. , Williams S. , Nikishin S. A. , Tretyakov V. , Temkin H.
Published: July 2002

We report optical studies on AlxGa1−xN alloy layers across the full composition range. The series of alloy layers was grown on (111)-oriented silicon substrates using gas-source molecular beam epitaxy. From reflectance measurements, we determine the composition dependence of the energy gap to be Eg=3.42+1.18x+1.56x2, in good agreement with previous work. By combining Fourier transform infrared and Raman spectroscopy studies, we determine the composition dependence of phonons having A1(TO), E1(TO), E22, A1(LO), and E1(LO) symmetry. The longitudinal optic phonons exhibit one-mode alloy behavior. Two-mode alloy behavior is observed for all transverse optic and the E22 phonons (i.e., each mode has AlN-like and GaN-like branches). All phonons are seen to blueshift with increasing x. The influence of stress on the phonon energies is discussed.
Continuous monitoring of the zinc-phosphate acid-base cement setting reaction by proton nuclear magnetic relaxation
Authors: Apih T. , Lebar A. , Pawlig O. , Trettin R.
Published: July 2002

Proton nuclear magnetic relaxation is a well-established technique for continuous and non destructive monitoring of hydration of conventional Portland building cements. Here, we demonstrate the feasibility of nuclear magnetic resonance (NMR) monitoring of the setting reaction of zinc-phosphate acid-base dental cements, which harden in minutes as compared to days, as in the case of Portland cements. We compare the setting of cement powder (mainly, zinc oxide) prepared with clinically used aluminum-modified orthophosphoric acid solution with the setting of a model system where cement powder is mixed with pure orthophosphoric acid solution. In contrast to previously published NMR studies of setting Portland cements, where a decrease of spin-lattice relaxation time is attributed to enhanced relaxation at the growing internal surface, spin-lattice relaxation time T1 increases during the set of clinically used zinc-phosphate cement. Comparison of these results with a detailed study of diffusion, viscosity, and magnetic-field dispersion of T1 in pure and aluminum-modified orthophosphoric acid demonstrates that the increase of T1 in the setting cement is connected with the increase of molecular mobility in the residual phosphoric acid solution. Although not taken into account so far, such effects may also significantly influence the relaxation times in setting Portland cements, particularly when admixtures with an effect on water viscosity are used.
Dynamics of laser ablation of microparticles prior to nanoparticle generation
Authors: Lee Jaemyoung , Becker Michael F. , Keto John W.
Published: July 2002

To better understand the process of nanoparticle formation when microspheres are ablated by a high-energy laser pulse, we investigated the Nd:YAG laser-induced breakdown of 20 μm glass microspheres using time-resolved optical shadow images and Schlieren images. Time-resolved imaging showed the location of the initial breakdown and the shockwave motion over its first 300 μm of expansion. From these measurements, we determined the shockwave velocity dependence on laser fluence. Measured shockwave velocities were in the range of 1–10 km/s. We also developed a numerical model that simulated breakdown in the glass microsphere and the propagation of this disturbance through the edge of the sphere where it could launch an air shock. Our objective was to simulate the shockwave velocity dependence on laser fluence and to generate glass density, temperature, and mass velocity profiles after breakdown. The simulation and experimental data compared favorably.
Measurement of femtosecond resonant nonlinear refraction in Nd:YVO4 by degenerate pump-probe spectroscopy
Authors: Luo L. , Chen L. , Qiu Z. R. , Yu X. Y. , Dai D. C. , Zhou J. Y. , Kuhl J.
Published: July 2002

The resonant nonlinear refraction in rare-earth-doped solids is studied with femtosecond degenerate pump-probe two-beam coupling spectroscopy. The phase modulation induced by the pump pulses is shown to result in a significant contribution to the differential change of the probe transmission. An ultrafast resonant nonlinear refractive index of the order of 1×10−18 m2/W is measured in Nd:YVO4 crystals.
Phosphorous ion implantation in C60 for the photovoltaic applications
Authors: Narayanan K. L. , Yamaguchi M.
Published: July 2002

Thin films of C60 deposited on p-type Si(100) wafer are implanted with low energy phosphorous ions for the photovoltaic applications. An attempt has been made on the device fabrication with phosphorous ion implanted C60 films grown on the p-type Si wafer. The photovoltaic properties of the solar cell structure are discussed with the dark and illuminated J–V characteristics. The efficiency of the structure in the multiple energy phosphorous ion implanted C60 film/p-Si heterojunction is found to be 0.01% under air mass 1.5 conditions. The low efficiency is attributed to the ion implantation induced damage effects and subsequent larger series resistance values.
Electron vortices in magnetized plasmas
Authors: Kuvshinov B. N. , Rem J. , Schep T. J. , Westerhof E.
Published: July 2002

This article is devoted to a systematic development of the theory of distributed electron vortices in magnetized plasmas. Such vortices are nonlinear stationary propagating solutions of the model of electron magnetohydrodynamics. Two types of vortices are investigated: two-dimensional dipole and spherical vortices. In both cases dispersion relations are derived and vortex structures are analyzed. The dynamics and stability properties of dipole vortices are investigated numerically. It is found numerically that dipole vortices propagating strictly perpendicular to the magnetic field are always unstable. Obliquely propagating vortices with a single separatrix are found to be stable, while vortices with a double separatrix turn out to be unstable.
Intrinsic bond strengths of multiple gallium–gallium bonds: A compliance matrix study using multiconfiguration self-consistent field wave functions and hybrid density functionals
Authors: Grunenberg Jörg
Published: July 2002

Inverted matrices of force constants (compliance matrices) are used to reassign the bond order of the Ga–Ga bond in a recently synthesized compound Na2{GaC6H3–2,6-Trip2}2 (Trip=C6H2–2,4,6–Pr3). The results of multiconfiguration wave functions using the complete active space self-consistent field method are compared to hybrid density functionals and Hartree–Fock wave functions.
Anomalous phosphorus diffusion in Si during post-implantation annealing
Authors: Kim Ryangsu , Furuta Yoshikazu , Hayashi Syunsuke , Hirose Tetsuya , Shano Toshihumi , Tsuji Hiroshi , Taniguchi Kenji
Published: July 2002

The transient behavior of P diffusion in Si implanted with As or Ge above the amorphizing threshold has been investigated. Annealing at 720 °C after Ge implantation induces extensive P segregation into the extended defect layer formed by implantation damage. This segregation is attributed to P trapping to end-of-range {311} defects and dislocation loops. For As implantation, P segregation was also observed only after 1 min annealing. However, in contrast to the Ge implantation, in the As-implanted samples, significant P depletion occurs in the As-tail region after further annealing. Nonequilibrium simulation that takes into account both Fermi-level and electric field effects shows the P depletion during transient enhanced diffusion. Furthermore, simulation results based on the coexistence of neutral and positively charged phosphorus-interstitial pairs agree well with the obtained experimental results.
Hartree approximation III: Symmetry breaking
Authors: Prodan E. , Nordlander P.
Published: July 2002

We consider a one dimensional fermionic gas confined on a circle of finite length. At finite temperatures, in the absence of any background potential, one can show that a constant density of particles is a solution to the Hartree equation, regardless of the type of interparticle interaction. Moreover, at finite temperatures and small coupling constants, our previous analysis shows that this is the only solution of the Hartree equations. We show in this paper that, at zero temperature, there is another solution, which has an asymptotic expansion: n(x)=n0(1+ct.cos 2kFx)+o(λ), where λ is the coupling constant. Moreover, this solution is stable while the trivial solution become unstable at zero temperature.
Damage production in a-Si under low-energy self-atom bombardment
Authors: Koster Monika , Urbassek Herbert M.
Published: July 2002

Using a molecular-dynamics simulation, we study the buildup of damage in an a-Si specimen bombarded by Si atoms with energies between 10 and 150 eV for fluences up to 1.4×1015 cm−2, i.e., an equivalent of 2 monolayer growth. The production rate of overcoordinated atoms increases with the bombarding energy; we analyze its fluence and bombarding-energy dependence in detail. The number of undercoordinated atoms decreases for low-energy bombardment due to the saturation of dangling bonds at the surface; for higher bombarding energies, it increases slightly, but shows only little dependence on bombarding energy. The depth distribution of the damage, of the induced stress, and of the atom relocation in the target demonstrate that bombardment modifies the target at considerably greater depths than the ion range.
Experimental evidence of monomer contribution to the static magnetic birefringence in magnetic fluids
Authors: Bakuzis A. F. , Skeff Neto K. , Silva L. P. , Azevedo R. B. , Morais P. C.
Published: July 2002

Manganese–ferrite and nickel–ferrite ionic magnetic fluids (MFs) were investigated using static magnetic birefringence (SMB). Transmission electron microscopy (TEM) was used to obtain the particle diameter polydispersity profile of the MF samples. The model used in the present study to fit the SMB data includes the field dependence of the magnetic permeability associated to the magnetic structures (incoherent monomer and dimer) and allowed estimation of the magnetic surface anisotropy. The combined analysis of the SMB and TEM data support the contribution of incoherent monomer to the SMB signal in MFs and allows the estimation of its characteristic diameter. It was found that the incoherent monomer diameter is sensitive to the nanoparticle nature. Finally, the surface anisotropy depends upon the nanoparticle diameter and falls within the range of 0.1–1.5 erg/cm2.
Planar Hall effect in NiFe/NiMn bilayers
Authors: Lu Z. Q. , Pan G. , Lai W. Y.
Published: July 2002

The exchange anisotropy in NiFe/NiMn bilayers was studied by using the planar Hall effect. The sputtered NiFe/NiMn films were patterned into strips of 1 mm in length and 200 μm in width and with six terminals for anisotropy magnetoresistance and planar Hall voltage measurements by a photolithographic process. It is shown that the planar Hall effect is an effective method to characterize the exchange anisotropy in ferromagnetic/antiferromagnetic (AF) systems. It can be used to accurately determine the exchange field and describe the magnetization reversal processes. The effective uniaxial anisotropy field HK eff, the effective unidirectional anisotropy field Hud, and AF domain wall energy Hw can be obtained by fitting the experimental results. We found that in the NiFe/NiMn bilayer system, the parameters HK eff, Hud, and Hw have the same values in reversible and irreversible measurements, and the domain wall energy in AF layer is larger than interfacial unidirectional anisotropy.
Continuous wave ultraviolet laser source based on self-frequency-sum-mixing in Nd3+:YAl3(BO3)4 nonlinear laser crystal
Authors: Jaque D. , Romero J. J.
Published: July 2002

In this work, we experimentally demonstrate continuous wave laser generation in the 330–380 nm range by intracavity self-frequency-sum-mixing processes in a Nd3+:YAl3(BO3)4 crystal. Ultraviolet laser radiation has been obtained from a simple hemispherical linear cavity by nonlinear interaction between the pump radiation from an argon laser and the infrared laser radiations at 1.06 or 1.33 μm of Nd3+ ions. In nonoptimal preliminary experiments, up to 0.2 mW of ultraviolet laser power has been obtained at 250 mW of absorbed pump power.
A new scintillation-type time-of-flight anion detector
Authors: Pramann Axel , Rademann Klaus
Published: July 2002

A new scintillation-type anion time-of-flight detector of high transmittance is reported. The precursor type of this detector, which is known as the Daly detector, is often used in molecular beam experiments in combination with time-of-flight mass spectrometry. We present an improved and simple design which has several advantages compared to most common anion detectors.
Increased dielectric permittivity of SiO2 thin films
Authors: Holten Stephan , Kliem Herbert
Published: July 2002

The dielectric permittivity of amorphous SiO2 thin films could be increased by the evaporation of palladium electrodes. The evaporation of gold electrodes does not show a comparable effect. Gold electrodes reproduce the literature value for the dielectric permittivity of SiO2. Because the preparation processes of both types of electrodes have been carried out in the same manner, the detected increase of the dielectric permittivity must be caused by the physical properties of the used evaporated material. A significant difference between gold and palladium is the ability of palladium to absorb large amounts of hydrogen. Therefore we assume in the case of evaporated Pd electrodes a diffusion process of hydrogen atoms into the insulator, which cause additional relaxation centers. The contribution of these relaxation centers to the polarization is measured as an enhanced dielectric permittivity of the SiO2 thin film. Measurements of the small signal response have been carried out in the frequency domain, the time domain, where a Kohlrausch law is found, and in dependence of the temperature. The large signal response was studied using isochronal time domain measurements. A double well potential model is discussed to interpret the experimental results quantitatively. It is shown that a distribution of relaxation times is caused by a distribution of distances between neighbored atoms in the amorphous state.
A pulsed mixing valve for generating reactive complexes in a supersonic expansion
Authors: DeBoer Gary , Patel Piyush , Prince Amy Preszler , Young Mark A.
Published: July 2002

A pulsed source for producing clusters of reactive species in a supersonic expansion has been successfully implemented. The design utilizes two commercial pulsed valves to mix cluster components directly in the expansion, forming cold complexes and minimizing or eliminating the complications due to prereaction of the sample. The duty cycle of the source is well matched to pulsed laser probe techniques and minimizes gas loading on the vacuum system. The source has been tested with an unreactive system and also employed to produce strongly interacting aliphatic–I2 species.