bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport in the static diffusion cloud chamber revisited
Authors: Anisimov M. P. , Shandakov S. D. , Polygalov Yu. I. , Heist R. H.
Published: July 2002

The static diffusion chamber (SDC) allows the measurement of critical supersaturation and of nucleation rates and it is a powerful instrument for the vapor nucleation study. Earlier, within the scope of the International Nucleation Workshop Group, nucleation rates of the n-pentanol–helium system have been measured using different experimental techniques. Disagreement of experimental data obtained using the static diffusion chamber and data obtained using other methods, particularly the laminar flow diffusion chamber, can be explained by re-examining the mass and energy transport analysis used to describe static diffusion chamber operation. In the present research we describe the mass and energy transport in the SDC modeled as an effectively open system with mass and energy transport in one direction with a nonzero diffusion flux at the system boundaries. Calculated values for vapor supersaturation are compared with the n-pentanol nucleation rate experimental results of the American–Czech group [M. Rudek, J. L. Katz, I. Y. Vidensky et al., J. Chem. Phys. 111, 3623 (1999)] and with a nucleation rate Reference Equation obtained from an earlier investigation involving the n-pentanol–helium system. From our results one can see that there is a significant difference in the calculated supersaturation for all of the data. The magnitude of this difference is quite large even for the relatively small vapor mass fractions at a nucleation temperature of 260 K. We also note that the calculated nucleation temperatures from our analysis are slightly larger than those reported in the work of Rudek et al.4 We performed our calculations with and without the thermal diffusion term. We observed that the effect of thermal diffusion on the transport process is relativelly small and is not particularly essential to include in this comparison that we are making the effects of the different flux boundary conditions.
A complementary metal–oxide–semiconductor anode array chip with two rows of 512 anodes and dual integrated analog read-out circuitry
Authors: Nevejans Dennis , Neefs Eddy , Kavadias Spyros , Merken Patrick , Van Hoof Chris , Gramegna Giuseppe , Bastiaens Johan , Dierickx Bart
Published: July 2002

A novel multianode electron detector chip (LEDA512) has been designed, fabricated and tested using a 0.7 μm complementary metal–oxide–semiconductor technology. This chip, mounted behind a stack of two microchannel plates, is used in a positive ion mass spectrometer selected by the European Space Agency for the ROSETTA cometary mission. Two functions are integrated on the chip: a dual row of 512 anodes, each having an area of 22 μm×8 mm for the collection of electrons, and two separate anode multiplexer and charge amplifier units. For redundancy reasons the chip has been designed such that the two anode rows with their associated electronics are identical and may be used independently with separate clocks and power supplies. Tests have confirmed that the LEDA512/microchannel plate combination is working properly as an ion detector and that excellent noise and response may be obtained in combination with the mass spectrometer.
Complete removal of threading dislocations from mismatched layers by patterned heteroepitaxial processing
Authors: Zhang X. G. , Rodriguez A. , Wang X. , Li P. , Jain F. C. , Ayers J. E.
Published: July 2002

We report the effectiveness of patterned heteroepitaxial processing (PHP) in removing threading dislocations (TDs) from ZnSe epitaxial layers grown on GaAs substrates by metalorganic vapor phase epitaxy. The PHP approach used here involves postgrowth patterning of continuous epitaxial layers followed by annealing. In this study, each as-grown ZnSe/GaAs sample was first cut into pieces forming four types of samples, namely: (1) as grown, (2) postgrowth annealed, (3) postgrowth patterned, and (4) PHP prepared (patterned and annealed). The epitaxial layers with thicknesses of 2000–6000 Å were patterned to create 500–6000-Å-high and 3–70-μm-wide square mesas that were separated by 20 μm trenches. TD densities were determined by the etch pit density (EPD) technique and comparisons were made between the four types of samples. The first three types of samples exhibited EPDs of approximately 107 cm−2, which indicate that neither patterning alone nor annealing alone was effective at reducing TDs. In contrast, PHP resulted in a complete removal of TDs from 70 μm×70 μm square layers with thicknesses of >3000 Å. This corresponds to an EPD less than 2.0×104 cm−2, and at least a 500-fold reduction compared to as-grown layers; in fact, this value is even lower than that of the GaAs substrate (EPD=105 cm−2). Thus TDs can be removed in PHP by glide to the sidewalls, as promoted by the presence of image forces.
Simple method for online correction of laser fluctuations in correlation measurements
Authors: Sghirlanzoni Andrea , Chirico Giuseppe
Published: July 2002

In this note we describe a method for the online correction of laser fluctuations that can be applied in the measurements of the fluorescence correlation measurements. The correction is performed by feeding a signal that varies with the laser output to the digital acquisition board as an external clock reference.
Cauchois–Johansson x-ray spectrograph for 1.5–400 keV energy range
Authors: Baronova E. O. , Stepanenko M. M. , Pereira N. R.
Published: July 2002

A Cauchois–Johansson optical scheme is proposed, based on a specially designed cylindrical quartz crystal. The corresponding x-ray spectrograph is manufactured, which provides high spectral resolution in transmission and reflection regimes and covers the wide energy range of 1.5–400 keV. The device is calibrated with an x-ray tube and is suggested to be used in high-temperature plasma diagnostics.
The Ã←X̃(1+1)REMPI spectrum and high-level ab initio calculations of the complex between NO and N2
Authors: Lozeille Jérôme , Daire Sophia E. , Gamblin Stuart D. , Wright Timothy G. , Lee Edmond P. F.
Published: July 2002

The results of two separate studies of the complex between NO and N2 are reported. The (1+1) REMPI spectrum of the Ã←X̃ transition of the complex between NO and N2 is presented of improved quality over that reported previously, and the appearance of the spectrum is discussed. The results of high-level ab initio calculations [RCCSD(T)/aug-cc-pVQZ//QCISD/6-311+G(2d)] on the X̃ 2Π state are also reported. The indications are that the NO moiety is more freely rotating in the complex than is N2, and that a wide angular space is sampled in the zero-point energy level. The appearance of the REMPI spectrum suggests that the Ã 2Σ+ state is (close to) linear, and RCCSD(T)//QCISD calculations on the Ã state, using Rydberg-function-augmented basis sets, suggest that the lowest energy linear isomer is the ON⋅N2 linear orientation. It is clear, however, that the understanding of this complex, and its spectroscopy, is far from complete, and will be challenging.
The effects of librations on the C13 chemical shift and H2 electric field gradient tensors in β-calcium formate
Authors: Hallock Kevin J. , Lee Dong Kuk , Ramamoorthy A.
Published: July 2002

The magnitudes and orientations of the principal elements of the C13 chemical shift anisotropy (CSA) tensor in the molecular frame of the formate ion in β-calcium formate is determined using one-dimensional dipolar-shift spectroscopy. The magnitudes of the principal elements of the C13 CSA tensor are σ11C=104 ppm, σ22C=179 ppm, and σ33C=233 ppm. The least shielding element of the C13 CSA tensor, σ33C, is found to be collinear with the C–H bond. The temperature dependence of the C13 CSA and the H2 quadrupole coupling tensors in β-calcium formate are analyzed for a wide range of temperature (173–373 K). It was found that the span of the C13 CSA and the magnitude of the H2 quadrupole coupling interactions are averaged with the increasing temperature. The experimental results also show that the H2 quadrupole coupling tensor becomes more asymmetric with increasing temperature. A librational motion about the σ22C axis of the C13 CSA tensor is used to model the temperature dependence of the C13 CSA tensor. The temperature dependence of the mean-square amplitude of the librational motion is found to be 〈α2〉=2.6×10−4(T) rad2 K−1. The same librational motion also accounts for the temperature-dependence of the H2 quadrupole coupling tensor after the relative orientation of the C13 CSA and H2 electric field gradient tensors are taken into account. Reconsideration of the results of a previous study found that the librational motion, not the vibrational motion, accounts for an asymmetry in the H1–C13 dipolar coupling tensor of α-calcium formate at room temperature.
Bright red organic light-emitting diodes doped with a fluorescent dye
Authors: Mitsuya Masayuki , Suzuki Takayuki , Koyama Toshiki , Shirai Hirofusa , Taniguchi Yoshio , Satsuki Makoto , Suga Sadaharu
Published: July 2002

We have evaluated a synthetic red fluorescent dye, 6-methyl-3-[3-(1,1,6,6-tetramethyl-10-oxo2,3,5,6-tetrahydro-1H,4H,10H-11 -oxa-3a-aza-benzo[de]anthracen-9-yl)-acryloyl]-pyran-2,4-dione (AAAP), as a dopant for an organic light-emitting diode (LED). Bright emission of a good red (maximum luminance: 5600 cd/m2, chromaticity coordinates: x=0.63, y=0.36) was obtained. The device consisted of ITO/TPD(50 nm)/Alq3 doped with AAAP(1.5 mol %,15 nm)/bOXDF(20 nm)/Alq3(25 nm)/Mg:Ag (ITO: indium tin oxide, TPD: N, N′-diphenyl- N,N′-di(3-methylphenyl)-1, 1′biphenyl-4,4′-diamine, Alq3: tris (8-hydroxyquinolinato)-aluminum (III), bOXDF:2,2-bis[5-(4-biphenyl)-1,3,4-oxadiazole-2-yl-4,1 -phenylene]-hexafluoropropane). The bands of fluorescence of the Alq3 host and of absorption of the AAAP dopant overlap in an advantageous way, and insertion of the bOXDF layer between the emission layer, doped with AAAP, and Alq3 layer, made for a device with good properties.
Memory function for dielectric relaxation
Authors: Dı́az-Calleja R. , Garcı́a-Bernabé A. , Sanchis M. J. , del Castillo L. F.
Published: July 2002

The second-order memory function (SOMF) for the dicyclohexylmetyl-2metyl succinate is obtained by using simple numerical manipulation of the experimental dielectric data. According to the prescription given in a previous paper [J. Chem. Phys. 109, 9057 (1998)], the frequency behavior of the real and imaginary parts of the SOMF is discussed in terms of the Havriliak-Negami equation of the dielectric function, and together with the three-variable model describing the evolution of the torque-autocorrelation function. Furthermore, in this paper we present the temperature dependence of the parameters, which characterize the SOMF behavior for two ester substances.
Equilibrium geometries of cyclic SiC3 isomers
Authors: Stanton John F. , Gauss Jürgen , Christiansen Ove
Published: July 2002

Equilibrium geometries of two isomers of cyclic SiC3 are determined by both large scale ab initio calculation and a procedure involving the use of experimental data from microwave spectroscopy and calculated harmonic and cubic force fields. Internuclear distances corresponding to structures obtained by both procedures agree to better than 0.003 Å in all cases, allowing for precise recommendations of equilibrium structures. Rather large residual inertial defects obtained from moments of inertia based on rotational constants adjusted for effects of vibration–rotation interaction were found to be reduced significantly by inclusion of electronic contributions as estimated from calculations of rotational g-tensors.
A microemulsion model on sc, bcc and fcc lattices: Ground state properties
Authors: Buzano C. , Pretti M.
Published: July 2002

The microemulsion model proposed by Ciach, Høye, and Stell [J. Phys. A 21, L777 (1988)] is a lattice gas model for ternary mixtures of water, oil, and amphiphile, which explicitly takes into account orientational degrees of freedom of amphiphilic molecules. Usually only a discrete set of orientations (towards nearest-neighbor sites) is permitted, and the model is lattice dependent. An investigation about the ground state phase diagram on the body-centered cubic (bcc) and face-centered cubic (fcc) lattices, compared with previously known results on the simple cubic (sc) lattice, is performed. Different structured phases are found, resembling experimentally observed phases and not always considered in the literature. The role of the lattice structure is discussed.
Differentiating 180° and 90° switching of ferroelectric domains with three-dimensional piezoresponse force microscopy
Authors: Roelofs A. , Böttger U. , Waser R. , Schlaphof F. , Trogisch S. , Eng L. M.
Published: July 2002

Three-dimensional (3D) piezoresponse force microscopy is applied in order to differentiate 90° and 180° domain switching in PbTiO3 (PTO) thin films. The 3D domain configuration is recorded both statically, revealing the surface crystallographic orientation of PTO films on the nanometer scale, and dynamically by simultaneously mapping the in-plane and out-of-plane hysteresis loops. We show that exclusively 180° switching occurs, also switching only half of the grain volume.
Model for the aggregation state of living anionic polymers
Authors: Frischknecht Amalie L. , Milner Scott T.
Published: July 2002

To better understand the aggregation of living polymers formed in anionic polymerization, we develop a theoretical model for the aggregation state of living polymers with polar headgroups in hydrocarbon solvents. In particular, we have examined polymers with butadienyllithium headgroups in benzene. The system is modeled as a population of star-like micelles coexisting with a broadly polydisperse population of long cylindrical micelles. We use numerical solutions of the self-consistent mean-field equations to quantitatively calculate the stretching energy of the polymer chains in the micelles, and semiempirical and ab initio quantum chemistry methods to calculate headgroup geometries and binding energies. We find that the general aggregation state of the model consists of nearly Gaussian-distributed spherical micelles for long polymer chain lengths, which cross over to exponentially-distributed cylindrical micelles at short chain lengths, with a region of coexistence of the two types of micelles in a narrow intermediate range of chain length. For physically reasonable values of the model parameters, the predicted aggregation state is consistent with experimental features observed for this system by Stellbrink et al., and in particular with the formation of star-like micelles with mean aggregation numbers of 10 or larger.
Flow enhanced diffusion-limited polymerization of rodlike molecules
Authors: Jain Sumeet , Agge Ameya , Khakhar D. V.
Published: July 2002

Polymerization of rigid rodlike molecules with reactive end groups requires near parallel orientation of the molecules. The reaction is diffusion limited because of the low mobility of the molecules in the later stages of the reaction. Experimental studies have shown that flow-induced molecular orientation enhances the rate of polymerization [Agarwal and Khakhar, Nature 360, 53 (1992)]. Here a theoretical study of the polymerization under axisymmetric extensional flow is carried out to obtain the effective reaction rate constant (keff) for the reaction. Computations show that an increase in the intrinsic rate constant (kh) results in a decrease in the relative rate constant krel=keff/kh. Reduction in the rotational diffusivity (Dr) results in a significant reduction in krel; however, the variation of the translational diffusion coefficient perpendicular to the rod axis D⊥ has only a small effect for D⊥/D‖≪1, where D‖ is the diffusivity parallel to the rod axis. The imposition of flow increases the effective rate constant, however, the variation of krel with other parameters remains qualitatively similar at different Peclet numbers (Pe=ε/Dr, where ε is the extensional rate). To simulate the variation of the rate constant during polymerization, computations are carried out for different rod lengths using correlations to estimate rod diffusivities. Results indicate that krel initially decreases and then increases after a certain critical degree of polymerization, which reduces with increase in Peclet number. For sufficiently high extensional rates (ε∼200 s−1) the rate constant becomes higher than the intrinsic value (krel>1).
Time-reversal of the evolution of a dipole-coupled, many-spin system under continuous resonant irradiation
Authors: Michal Carl A. , Tycko Robert
Published: July 2002

Time-reversal of the evolution of a dipole-coupled, many-spin system under continuous resonant excitation with a radio-frequency (rf) field of arbitrary amplitude is demonstrated in solid-state H1 nuclear magnetic resonance (NMR) experiments on polycrystalline adamantane. Time-reversed evolution is accomplished with an rf pulse sequence that generates an effective nuclear spin Hamiltonian that includes both dipole–dipole coupling and rf interaction terms, with signs opposite to those in forward evolution. The amplitude of the effective continuous rf field is varied by varying the phases of rf pulses in the sequence. Experiments show echo-like NMR signals under time-reversed evolution after forward evolution to an apparent quasiequilibrium state under continuous rf excitation. Such echolike signals are inconsistent with the hypothesis of spin temperature in the rotating frame, according to which the approach to quasiequilibrium under continuous rf excitation is an irreversible process. The use of this time-reversed evolution in multiple quantum (MQ) NMR spectroscopy is also demonstrated. MQ NMR spectra obtained with increasing excitation times exhibit a partial confinement of nuclear spin order to zero- and one-quantum operators. This novel behavior is shown to be a consequence of energy conservation.
Two mechanisms of the negative-effective-mass instability in p-type quantum well-based ballistic p+pp+-diodes: Simulations with a load
Authors: Bashirov R. R. , Gribnikov Z. S. , Vagidov N. Z. , Mitin V. V.
Published: July 2002

There exist two regimes of the negative-effective-mass (NEM) instability in ballistic p+pp+-diodes with two-dimensional hole gas in the p-base: the instability of homogeneous NEM-hole distribution in a quasineutral plasma region, and the instability of a thin accumulation layer, which forms inside a depletion region and contains NEM holes. Both instabilities lead to the development of terahertz oscillatory regimes. The regimes’ simulation in the inductance-loaded diodes with base lengths 0.05–0.15 μm demonstrates that such loads substantially enlarge the voltage range of the second regime and give rise to oscillatory regimes, which do not appear in unloaded diodes at all. Efficiencies of different oscillatory regimes are estimated.
Tunable subnanosecond pulse generation in a dye laser using overlapped pump pulses
Authors: Delev Andrey , Deneva Margarita , Nenchev Marin , Stoykova Elena , Slavov Dimitar
Published: July 2002

We report a simple technique for generation of tunable subnanosecond pulses (0.2–0.4 ns) based on single spike selection from the relaxation oscillations in a dye laser using preformed and overlapping in time excitation pulses. The pulses were obtained by temporal division of a 5–50 ns initial pulse using a Pockels cell with a 1–2 ns switching time and an optical delay time. The proposed technique widens the choice of suitable dye lasers in which the single spike selection technique may be applied, to include dye lasers excited by a Q-switched solid-state laser or a Cu-vapor laser. Other advantages to this technique are the shortened length of the generated subnanosecond pulse, the increased energetic output, higher reproducibility, and the possibility for synchronization of spike generation to an external signal accurate to within 2–3 ns.
A mathematical proof of a result derived from the application of the Thompson–Lampard theorem of electrostatics
Authors: Morales Daniel A.
Published: July 2002

We give an entirely mathematical demonstration and a generalization of a recent result obtained from the application of the Thompson–Lampard theorem of electrostatics, i.e., ∑n even[8 sin2(nπa/2)sinh2(nπa/2)]/[n sinh(nπ)]+∑n odd[8 cos2(nπa/2)cosh2(nπa/2)]/[n sinh(nπ)]=ln 2. It is shown that, as it should be, the result of the left-hand side, ln 2, is independent of the value of a.
Quantum-mechanical study of vibrational relaxation of HF in collisions with Ar atoms
Authors: Krems Roman V. , Marković Nikola , Buchachenko Alexei A. , Nordholm Sture
Published: July 2002

Vibrational relaxation cross sections and rate constants of HF(v=1) by Ar are calculated on a recent semiempirical potential energy surface (PES) [J. Chem. Phys. 111, 2470 (1999)] using the quantum-mechanical coupled states approach. Accurate theoretical estimations of rate coefficients for vibrational relaxation of HF(v=1) at temperatures between 100 and 350 K are obtained. The vibrational relaxation is shown to be of a quasiresonant character and occur mostly to two nearest rotational levels of the ground vibrational state. The weak isotope effect after substitution of HF by DF is investigated and explained. The cross sections for vibrational relaxation of HF(v,j=0), where v=1,2,3,4, are calculated and shown to increase significantly as v increases. In the same calculations we observe a dramatic increase of multiple quantum vibrational transitions as the difference between the initial and final states falls in close resonance with the collision energy. A comparison of the cross sections obtained from the coupled states calculations with those performed with rotational infinite-order-sudden approximation proves a crucial role of molecular rotations for vibrational relaxation. Finally, we describe the close coupling coupled states calculations for relaxation and rotational excitation of HF(v=1, j=0) with a reduced number of open channels in the basis set and show that it is possible to obtain converged results for rotationally inelastic transitions between the various levels of v=1 neglecting all states below v=1, j=0.
Electron distribution function in laser heated plasmas
Authors: Fourkal E. , Bychenkov V. Yu. , Rozmus W. , Sydora R. , Kirkby C. , Capjack C. E. , Glenzer S. H. , Baldis H. A.
Published: July 2002

A new electron distribution function has been found in laser heated homogeneous plasmas by an analytical solution to the kinetic equation and by particle simulations. The basic kinetic model describes inverse bremsstrahlung absorption and electron–electron collisions. The non-Maxwellian distribution function is comprised of a super-Gaussian bulk of slow electrons and a Maxwellian tail of energetic particles. The tails are heated due to electron–electron collisions and energy redistribution between superthermal particles and light absorbing slow electrons from the bulk of the distribution function. A practical fit is proposed to the new electron distribution function. Changes to the linear Landau damping of electron plasma waves are discussed. The first evidence for the existence of non-Maxwellian distribution functions has been found in the interpretation, which includes the new distribution function, of the Thomson scattering spectra in gold plasmas [Glenzer et al., Phys. Rev. Lett. 82, 97 (1999)].
Structural and bonding properties of carbon nitride films synthesized by low energy nitrogen-ion-beam-assisted pulsed laser deposition with different laser fluences
Authors: Zhao J. P. , Chen Z. Y. , Yano T. , Ooie T. , Yoneda M. , Sakakibara J.
Published: July 2002

Carbon nitride films were deposited by pulsed Nd:yttrium–aluminum–garnet laser ablation of graphite with assistance of low energy nitrogen-ion-beam bombardment. The nitrogen to carbon (N/C) atomic ratio, surface morphology, bonding state, and microstructure of the deposited carbon nitride films were characterized by x-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, micro-Raman spectroscopy, atomic force microscopy (AFM), and x-ray diffraction. The influence of laser fluence on the synthesis of carbon nitride films was investigated. The N/C atomic ratio of the carbon nitride films can reach the maximum at the highest laser fluence. XPS and FTIR analyses indicated that the bonding state between the carbon and nitrogen in the deposited films was significantly influenced by the laser fluence during deposition. The carbon–nitrogen bonding of C–N and C=N were observed in the films. In addition, α and β C3N4 phases were found to coexist in the carbon nitride films with relative low degree of ordering in the crystal lattice. AFM results indicated that the laser fluence also had a critical effect on the surface structure of the carbon nitride films.
Generalization of the Darboux transform
Authors: Morales J. , Peña J. J. , López-Bonilla J. L.
Published: July 2002

This article presents a generalization of the standard Darboux transform applied to Sturm–Liouville differential equations. This is achieved with the aid of an ansatz as a particular solution for the Riccati relationship involved, which in turn led us to obtain its generalized Darboux solution that contains, as a particular case, the standard Darboux transform. The proposed generalized Darboux transform (GDT), applied to the quantum mechanical field, gives the opportunity to prove the existence of standard and generalized Darboux potentials that match with the so-called isospectral potentials. This is exemplified by obtaining, through the GDT, a set of standard and generalized Darboux potentials that form the partner of the one-dimensional harmonic oscillator model for any quantum principal number. The worked example indicates how the GDT can be used to obtain the isospectral potentials associated to any known specific potential. We consider also the application of our method as proposed to the theory of solitons in order to show why the GDT will be important in other fields of application where the standard Darboux transform is usually concerned.
Simple versus cooperative relaxations in complex correlated systems
Authors: Mano J. F. , Lanceros-Méndez S.
Published: July 2002

A method for investigating the nature of thermally activated relaxations in terms of their cooperative character is tested in both polymer and low molecular weight crystal systems. This approach is based on analysis of the activation entropy in order to describe thermally activated relaxations. The betaine arsenate/phosphate mixed system of low molecular weight crystals was selected for investigation because pure compounds of this system show ferro-/antiferroelectric phase transitions and the mixed crystals undergo different kinds of relaxation processes involving both dipole–dipole and dipole–lattice interactions. The polymer chosen was a side chain liquid-crystalline polysiloxane, which shows the β-relaxation characteristic of disordered systems and amorphous materials. The cooperative versus local character of the relaxations is described in terms of “complex” and “simple” relaxations based on calculations of the activation entropies. The initial assumptions of the theory, as well as the resulting equations, were found to be applicable to the systems studied.
Lattice misfit versus performance of thin film electroluminescent structures
Authors: Mastio E. A. , Cranton W. M. , Thomas C. B.
Published: July 2002

Thin polycrystalline electroluminescent thin films (TFEL) of ZnS:Mn (phosphor) and Y2O3 (insulator) were deposited individually or as multilayers onto Si (100) substrates. Their crystallinity and the luminescent efficiency of the phosphor films were investigated at varying thermal annealing temperatures. It is shown that the luminescent quality of the phosphor layer increases up to 700 °C, whereas the electroluminescence operating intensity of TFEL devices saturates at 500 °C. The structural analysis of the insulating and phosphor layers shows that they recrystallize at annealing temperatures of, respectively, 500 and 600 °C, and that their lattice misfit doubles at processing temperatures>=500 °C. Since TFEL devices should benefit from enhanced luminescence efficiency and crystallinity at high annealing temperatures, we suggest that the lack of improvement in device performance beyond 500 °C is due to interface alterations. According to previous works, we propose that the lattice misfit increase between the phosphor and dielectric thin films modifies the morphology of the phosphor–insulator boundary inducing a modification of the interface states density, and hence, modifying high field electron transport properties of TFEL devices.
Effects of small amounts of Al in GaN grown on sapphire (0001) by molecular beam epitaxy
Authors: Zsebök O. , Thordson J. V. , Zhao Q. X. , Andersson T. G.
Published: July 2002

A series of 0.7 μm thick GaN layers have been grown by solid-source rf-plasma assisted molecular beam epitaxy on sapphire (0001) substrates with the addition of 0.10%–3.30% Al. The Al concentration was determined by secondary ion-mass spectrometry and Auger-electron spectroscopy, while the layer quality was assessed by photoluminescence, Hall effect measurement, and high-resolution scanning electron microscopy. Microscopy revealed a surface roughness varying with Al content. The smallest surface roughness was obtained at 0.10% and 3.30% Al. Low-temperature photoluminescence revealed dominating peaks attributed to the neutral donor-bound exciton. Its energy increased slightly with Al concentration, which established a correlation between the Al concentration and the band gap. The surface morphology, the corresponding optical and electrical properties, showed a clear improvement of the GaN layer quality for the range of 0.10%–0.17% Al content.
Growth of carbon nanotubes by gas source molecular beam epitaxy
Authors: Wan J. , Luo Y. H. , Choi Sung D. , Li R. G. , Jin G. , Liu J. L. , Wang K. L.
Published: July 2002

Multiwall carbon nanotubes have been grown by gas source molecular beam epitaxy in the presence of Ni catalyst. Some nanotubes show thinner bases compared with their heads. First- and second-order Raman scattering spectra are used to study the structure of samples with different initial thicknesses of Ni layers. The second-order 2D Raman mode of carbon nanotubes shows a downshift compared with the graphite-like structure. The growth of carbon nanotubes is found to depend on the size of the metal droplets. When the initial Ni layer is either too thick or too thin, few carbon nanotubes are observed. The Raman spectra show graphite and glassy carbon structures for too thick and too thin initial Ni layer films, respectively. Only when a proper range of Ni catalyst film is used, carbon nanotubes could be found.
B
11
and B10 magic-angle spinning nuclear magnetic resonance studies of cubic boron nitride films prepared by plasma chemical vapor deposition
Authors: Zhang W. J. , Tansho M. , Matsumoto S. , Mori T.
Published: July 2002

B 11 and B10 magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was conducted to characterize cubic boron nitride films prepared by plasma chemical vapor deposition. Similarities and differences between cubic boron nitride films and polycrystals synthesized at high pressure and high temperature were clarified by chemical shift and linewidth. The same local structure and tetrahedral symmetry of boron atoms in both forms was demonstrated, and a higher defective density appeared to exist in the film form, agreeing well with results from Raman measurements. Noncubic-phase impurities, i.e., amorphous, turbostratic, and hexagonal phases, in films were also detected in B11 MAS NMR spectra, and the possibility of removing these impurities by chemical etching was demonstrated.
On the existence of stationary states for quantum dynamical semigroups
Authors: Fagnola Franco , Rebolledo Rolando
Published: July 2002

We provide two criteria on the existence of stationary states for quantum dynamical semigroups. The first one is based on the semigroup itself, while the second criterion is based on the generator which is in general unbounded and interpreted as a sesquilinear form. These results are illustrated by physical examples drawn from quantum optics.
On 30 MHz TE11 mode piston attenuator
Authors: Swarup R. , Anand J. R. , Negi P. S.
Published: July 2002

A precision 30 MHz TE11 mode waveguide below cut-off piston attenuator has been designed for 0–100 dB attenuation range to serve as transfer standard of attenuation. The attenuator has been fabricated using a precision cylindrical copper waveguide and resonant inductive antennas with type-N connector at its input and output ports. The 30 MHz matched antennas have been incorporated in the cylindrical waveguide in such a manner that one serves as fixed transmitter and the other as moveable receiver for changing the attenuation. A Faraday screen type mode filter has also been fabricated to attenuate the amplitude of unwanted TM01 mode to an attenuation level of 60 dB. The piston attenuator has approximately 20 dB insertion loss and has been calibrated in steps of 10 dB over the 0–60 dB range. The results were found to lie within ±0.01 dB/10 dB.
Solid-phase crystallization of Si1−xGex alloy layers
Authors: Yamaguchi S. , Sugii N. , Park S. K. , Nakagawa K. , Miyao M.
Published: July 2002

Solid-phase crystallization of Si1−xGex (x=0–1.0) alloy layers deposited on a Si (100) substrate was investigated by ellipsometric spectroscopy. From a dispersion analysis of dielectric spectra, we deduced a crystallinity corresponding to the degree of average lattice alignment of the composed polycrystalline Si1−xGex layers and investigated the dynamical change in crystallinity during crystallization. We found that the crystallinity and crystallization temperature (TC) rapidly decreased with increasing Ge concentration (x). When x was small (=0–0.3), the highest crystallinity was ∼0.8 of that for single crystals while the lowest one was considerably below 0.6 when x>0.8. Moreover, the crystallinity decreased with increasing temperature above TC. We investigated the nucleation rate during crystallization and found that the decrease in crystallinity at both large Ge concentration and high temperature can be explained by a trade-off between the nucleation and crystallization rates; nucleation was dominant under these conditions. An overview of the crystallinity of solid-phase crystallized Si1−xGex alloy layers is provided.
X-ray transmission through a plasma window
Authors: Pinkoski B. T. , Zacharia I. , Hershcovitch A. , Johnson E. D. , Siddons D. P.
Published: July 2002

Traditional solid window materials used for x-ray synchrotron beamlines may introduce undesirably high attenuation, or are subject to failure under high heat loads. A plasma window can in principle obviate these problems over a wide range of energies. Experiments were performed at the Brookhaven National Laboratory National Synchrotron Light Source on beamline X6A to study the transmission characteristics of a plasma window using argon as the arc gas. Measurements were made around the Ar K edge and far from resonance. The “white-line” absorption at the K edge was actually suppressed during arc operation as compared to room temperature gas at the same pressure. This is attributed to the high degree of ionization in the plasma. The relative strength of the white line to the edge jump does not seem to be a strong function of arc current at the argon K edge. Away from resonance (∼3 times the edge energy) x-ray attenuation was negligible.
Domain structures and planar defects in SrBi2Ta2O9 single crystals observed by transmission electron microscopy
Authors: Zhu Xinhua , Zhu Jianmin , Zhou Shunhua , Li Qi , Liu Zhiguo , Ming Naiben
Published: July 2002

In this work, the domain structures and structural planar defects in SrBi2Ta2O9 (SBT) single crystals with (001) orientation were investigated by transmission electron microscopy. The 90° domain walls are identified by the 90° rotation relationship of the electron diffraction pattern about the [001] zone axis, and which exhibit irregular configurations. Antiphase boundaries (APBs) in SBT single crystals are also observed, which exhibit ribbon-like morphologies. Fourfold vertices formed by four APBs meeting together are observed as predominant singularities, and are explained by a four-state clock model, in which the four states are considered as the TaO6 octahedra tilting left, right, forward, or backward along the crystallographic directions. Some threefold vertices are also observed since both threefold and fourfold vertices are energetically allowed in the present model.
Exact solution of the Dirac equation for a spin-12 charged particle in two-dimensional and three-dimensional Euclidean spaces with shape invariance symmetry
Authors: Fakhri H. , Abbasi N.
Published: July 2002

It is shown that two types of shape invariance with respect to quantum numbers n and m lead us to solve Dirac equation for a spin-12 charged particle in two-dimensional and three-dimensional Euclidean spaces in the presence of magnetic fields. Also, we introduce supersymmetry algebra and shape invariance symmetry represented by two bunches of spinors obtained from the above solutions.
A simple avalanche model for astroplasma and laboratory confinement systems
Authors: Chapman S. C. , Dendy R. O. , Hnat B.
Published: July 2002

An avalanche or “sandpile” model is discussed that generalizes the original self-organized criticality avalanche model of Bak, Tang, and Wiesenfeld [Phys. Rev. Lett. 59, 381 (1987)] to include spatially extended local redistribution. A single control parameter specifies the spatial extent of local redistribution when the critical gradient is exceeded: this has profound consequences for nonlocal avalanching transport and for the dynamical behavior of the system, which are insensitive to other details such as the initial conditions and fluctuations in fueling or the critical gradient. The model possesses essentially two regimes of behavior. If the scale of nonlocal transport is of the order of the system size, the system is in the vicinity of a fixed point; in consequence the statistics of energy dissipation and length of avalanches are power law, and the time evolution is irregular (“intermittent”). If this scale is significantly smaller than the system size, the time evolution is quasiregular and follows a limit cycle. The first of these regimes appears relevant to the earth’s magnetosphere, where bursty transport and large scale reconfiguration (substorms) are observed. In this case the avalanche statistics have been inferred from observations of patches of intensity in the aurora, which may map to energy dissipation events in the magnetotail. The second regime displays significant links to the observed confinement phenomenology of magnetic fusion plasmas, corresponding to a broader range of model parameter space. For example, there is correlation between sandpile profiles, stored energy, and edge steepening on the one hand, and the control parameter on the other.
Low-frequency method for magnetothermopower and Nernst effect measurements on single crystal samples at low temperatures and high magnetic fields
Authors: Choi E. S. , Brooks J. S. , Qualls J. S. , Song Y. S.
Published: July 2002

We describe an alternating current method for the measurement of the longitudinal (Sxx) and transverse (Sxy, i.e., Nernst) thermopower of millimeter-size crystal samples at low temperatures (T<1 K) and high magnetic fields (B∼30 T). A low-frequency (33 mHz) heating method is used to increase the resolution and to determine the temperature gradient reliably in high magnetic fields. Samples are mounted between two thermal blocks which are heated by a sinusoidal frequency f0 with a π/2 phase difference. The phase difference between two heater currents gives a temperature gradient at 2f0. The corresponding thermopower and Nernst effect signals are extracted by using a digital signal processing method due to the low frequency of the measurement. An important component of the method involves a superconducting link, YBa2Cu3O7+δ, which is mounted in parallel with sample to remove the background magnetothermopower of the lead wires. The method is demonstrated for the quasi-two-dimensional organic conductor α-(BEDT–TTF)2KHg(SCN)4, which exhibits a complex, magnetic field dependent ground state above 22.5 T at low temperatures.
Strength of plasma sprayed turbine-blade coatings using an advanced spallation technique
Authors: Baumung K. , Müller G. , Singer J. , Kanel G. I. , Razorenov S. V.
Published: July 2002

We present an application of the spall technique for studying the strength, homogeneity, and adhesion of plasma sprayed coatings on a metal substrate. We used a flyer plate impact and a pulsed high-power proton beam to generate short, intense pressure pulses. To study adhesion it is necessary to provide a spall fracture at the interface. This was realized due to the bell-shaped power profile in the ion beam cross section. As a result, the spall fracture inside the samples occurred in tests with the ion beam, at different distances from the surface, including the interface between the coating and the substrate. Using a line-imaging laser-Doppler velocimeter, we were able to measure the free surface velocity histories for a range of load parameters in each experiment. The results of the measurements demonstrate the great influence of annealing on the homogeneity and strength of the coating and a lesser influence of the substrate temperature at coating on the adhesion.
Single-polarization single-mode intraband guidance in supersquare photonic crystals fibers
Authors: Ferrando Albert , Miret Juan José
Published: July 2002

We present a two-dimensional (2D) photonic crystal structure exhibiting appealing guiding properties as an optical fiber. The structure can be regarded as a dislocation of a variant from a 2D square photonic crystal and shows a large photonic band gap for on-axis illumination. The introduction of an off-lattice hole in the structure acts as a defect and generates a highly anisotropic intraband guidance. The combination of very strong anisotropy and intraband guidance gives rise to a mechanism of polarization discrimination in optical fiber propagation that enables this guiding structure to operate as a single-polarization single-mode fiber over a wide wavelength window.
Uniaxial anisotropy induced by field annealing in (001) NiMn/Co films
Authors: Wang Yung-Hung , Lai Chih-Huang , Chang Ching-Ray , Yang Jyh-Shinn , Lo C. K.
Published: July 2002

In-plane magnetic anisotropy was studied in Co films epitaxially grown on NiMn (001) films. The hysteresis loops measured in-plane perpendicular to the exchange field show double shifted loops, and the magnetization of Co to rotate coherently. Magnetic anisotropy constants can be determined from these double shifted loops. Exchange coupling in NiMn/Co not only induces a unidirectional anisotropy but also a uniaxial anisotropy. This induced uniaxial anisotropy may contribute to the enhancement of Co coercivity.
Magnetic anisotropy of permalloy films grown on an Mo(001) stepped surface
Authors: Yu C. C. , Huang J. C. A. , Hu Y. M.
Published: July 2002

The Mo(001) stepped surface was self-assembled on Al2O3(1-102) substrate by molecular beam epitaxy. Ni80Fe20 films were subsequently deposited on the fresh Mo(001) stepped surface at growth temperature (Tg) between 30 and 400 °C. Interestingly, in the low Tg regime (<100 °C) the Ni80Fe20 films were grown as (001) single crystal while high Tg (>300 °C) results in the growth of Ni80Fe20 (110) quadcrystal. For low Tg samples, the easy axis of the Ni80Fe20 film is directed parallel to the underlying Mo step edge direction and the M-H loop along the hard axis reveals a two-step magnetic moment rotation process. In contrast, the high Tg samples display isotropic-like magnetic behavior with a much enhanced coercivity due likely to the surface roughness effect. We conclude that the growth temperature of Ni80Fe20 film on the Mo stepped surface plays an important role on the crystal structure and magnetic anisotropy of Ni80Fe20 film.
Magnetic anisotropy and interlayer coupling in Fe0.5Co0.5(100) films on GaAs(100)
Authors: Pechan Michael J. , Compton Robert L. , Bennett Douglas , Chen L. C. , Palmstro/m C. J. , Allen S. James
Published: July 2002

Epitaxial Fe0.5Co0.5(100) films have been grown via MBE on GaAs (100) at reduced temperatures in order to minimize interfacial reactions. In-plane magnetization measurements reveal an overall twofold anisotropy superimposed with a fourfold component. Ferromagnetic resonance (FMR) measurements as a function of in-plane angle confirmed the mixed anisotropy state and provided energy densities of −2.4×105 erg/cm3 and −2.2×105 erg/cm3 for the twofold and fourfold terms, respectively. Two FeCo/InAs/FeCo trilayer samples were prepared to examine interlayer coupling effects. The samples differed in growth temperature (95 °C and 150 °C) of the InAs layer. FMR shows evidence for interlayer coupling in the 95 °C, but not the 150 °C sample.
Surface-step-induced magnetic anisotropy of p(1×1) Fe on W(100)
Authors: Mireles Hector C. , Erskine J. L.
Published: July 2002

Magneto-optic Kerr effect measurements of ultrathin p(1×1) Fe films on graded-step-density W(100) are used to study step-induced magnetic anisotropy. Spot-profile-analysis low-energy-electron diffraction is used to characterize the stepped W(100) surface prior to film growth and the epitaxial Fe layer after vapor deposition. The experimental results are qualitatively compatible with prior experiments and with theoretical predictions based on the Néel model and on a one-dimensional micromagnetic model proposed by Hyman, Zangwell, and Stiles (HZS). The observed evolution of hysteresis loop shape as a function of step density and anisotropy strength (which was varied by chemisorption of oxygen) is observed to be consistent with a hysteresis loop phase diagram based on the HZS model. However, the measured variation of switching field versus vicinal angle α for 2 monolayer thick Fe films differs significantly from the quadratic dependence predicted by the Néel model and from the dependence predicted by HZS. The surface-step-induced anisotropy vanishes at high vicinality (α∼10°) and novel two-state switching is observed at specific vicinal angles.
Interaction between supply circuits and hysteretic cores under rotational flux excitations
Authors: Bottauscio O. , Chiampi M. , Ragusa C. , Rege L. , Repetto M.
Published: July 2002

An advanced model, coupling the electric supply circuits with the electromagnetic field and including scalar and vector Preisach model of hysteresis, is presented in this article. The model is based on the finite element solution of the two-dimensional field problem, through the development of an iterative procedure, under periodic supply conditions. An experimental setup, constituted by a three limbed ferromagnetic core, has been especially designed and built to be used as a test bench for validation of the proposed model. Both unidirectional and rotational flux patterns can be generated by modifying the supply conditions. The reliability of the model is shown by comparing computed and measured results under different operating conditions. Finally, a voltage sinusoidal supply at the primary winding with a given load at the secondary is considered and discussed.
Dynamic generalization of Stoner–Wohlfarth model
Authors: Magni A. , Bertotti G. , Serpico C. , Mayergoyz I. D.
Published: July 2002

A dynamic generalization of the Stoner–Wohlfarth (SW) model is presented, in which the applied field component haz parallel to the particle anisotropy axis is slowly varied, whereas the one in the perpendicular plane, ha⊥, is rotated at the angular frequency ω. The Landau–Lifshitz–Gilbert equation is solved to calculate the system response under the constraint of spatially uniform magnetization at all times. Switching under slowly varying haz is discussed. Various switching modes can occur in correspondence of different types of bifurcation present in the dynamics. In addition, it is shown that the system response can become quasiperiodic, with spontaneous magnetization oscillations at a frequency definitely lower than ω.