bluuebunny/crossref_metadata_2025 huggingface dataset:
A new complete basis set model (CBS-QB3) study on the possible intermediates in chemiluminescence
Authors: Zhang Yong , Zeng Xi-Rui , You Xiao-Zeng
Published: July 2002

The new highly accurate complete basis set model, CBS-QB3, was employed here to elucidate the long experimentally discussed problem in a general class of chemiluminescent reactions involving peroxyoxalate systems. Both the stability comparison and the vibrational spectra favor that the intermediate is better to be recognized as the cyclic singlet 1,2-dioxetanedione with the C2v symmetry, which verifies the experimental suggestion yet provides more characterization information. Another two kinds of minimum species in its potential energy surface (PES) are two kinds of product: (1) two carbon dioxide and (2) two carbon monoxide and one oxygen, where the thermodynamic parameters correctly identify their relative yield in the experiment—the former is much more abundant than the latter. In a complete search of minimum states in its PES, the triplet C2v and D2h states were found, which is energetically unfavorable compared with the singlet C2v state. Their vibrational data also support some experimental conclusions of ruling out a radical intermediate. In contrast, the singlet D2h state was found to be a transition state for the “up” and “down” singlet C2v states. The complete active space self-consistent-field calculations with the second-order Möller–Plesset correlation energy correction also support that the most stable species is the singlet C2v state and the singlet D2h state is more energetically favorable than its triplet counterpart.
Effects of stiffness on the flow behavior of polymers
Authors: Dua Arti , Cherayil Binny J.
Published: July 2002

A recent model of the behavior of Gaussian chains in steady shear flow [J. Chem. Phys. 112, 8707 (2000)] is extended to include the effects of stiffness and finite extensibility. Calculations of the shear rate dependence of fractional elongation and of the time dependence of size fluctuations are found to be in good agreement with results from an experimental study of the behavior of single chains of DNA in steady shear flow. As in the earlier approach to the polymer-flow problem, we have ignored excluded volume and hydrodynamic interactions, but have instead added a bending energy contribution to the Hamiltonian of the chain, and have treated the usual connectivity term as a contribution to chain stretching that can be adjusted to ensure that the average size of the chain is fixed. The inclusion of stiffness and finite extensibility in the present treatment is found to produce significant improvements over the approach based purely on flexible chains.
Laser interferometric experiments for the TJ-II stellarator electron-density measurements
Authors: Lamela Horacio , Acedo Pablo ,   and the Optoelectronics and Laser Technology Group, Irby James
Published: July 2002

Laser two-color heterodyne interferometry is a proven method to measure electron density in fusion plasmas. Though only used in tokamaks with high electron densities, the idea of also using two-color laser interferometers for stellarators and small machines to replace far-infrared laser interferometers is being proposed. This will lead to low-cost, reliable, and easy to operate diagnostics for electron-density measurements. In this article, we present the interferometric experiments we have used for calibration of a two-color laser heterodyne system for electron-density measurements in the TJ-II stellarator. These experiments have been based on the use of a novel interferometric scheme: the heterodyne/homodyne interferometer. Finally, we describe the interferometer we have installed in the TJ-II stellarator and present the first results on mechanical-vibration subtraction and electron-density measurements with a two-color laser interferometer in a stellarator (TJ-II, Madrid, Spain).
Surface stress induced deflections of cantilever plates with applications to the atomic force microscope: Rectangular plates
Authors: Sader John Elie
Published: July 2002

Surface stress is a material property that underpins many physical processes, such as the formation of self-assembled monolayers and the deposition of metal coatings. Due to its extreme sensitivity, atomic force microscopy (AFM) has recently emerged as an important tool in the measurement of surface stress. Fundamental to this application is theoretical knowledge of the effects of surface stress on the deflections of AFM cantilever plates. In this article, a detailed theoretical study of the effects of surface stress on the deflections of rectangular AFM cantilever plates is given. This incorporates the presentation of rigorous finite element results and approximate analytical formulas, together with a discussion of their limitations and accuracies. In so doing, we assess the validity of Stoney’s equation, which is commonly used to predict the deflections of these cantilevers, and present new analytical formulas that greatly improve upon its accuracy.
Gas puff fueled H-mode discharges with good energy confinement above the Greenwald density limit on DIII-D
Authors: Osborne T. H. , Leonard A. W. , Mahdavi M. A. , Chu M. , Fenstermacher M. E. , La Haye R. , McKee G. , Petrie T. W. , Doyle E. , Staebler G. , Wade M. R. ,   DIII-D Team
Published: July 2002

ELMing (edge-localized) H-mode discharges with densities as high as 40% above the Greenwald density and good energy confinement, HITER-89P=2, were obtained with D2 gas puffing on DIII-D [Chan et al., Proceedings of the 16th IAEA Conference, Montreal (International Atomic Energy Agency, Vienna, 1996), Vol. 1, p. 95]. These discharges have performance comparable to the best pellet fueled DIII-D discharges. Spontaneous peaking of the density profile was an important factor in obtaining high energy confinement. Without density profile peaking, the energy confinement at high density degraded with reduction in the H-mode pedestal pressure under the stiff temperature profile conditions observed at high density on DIII-D. Reduction in the pedestal pressure was associated with loss of access to the second stable regime for ideal ballooning modes at the edge, and change in the edge-localized mode (ELM) instability from a low to high toroidal mode number. Gyrokinetic stability calculations indicate that the core of the high-density discharges is dominated by ion temperature gradient mode turbulence. A turbulent transport simulation with the GLF23 [Waltz et al., Phys. Plasmas 4, 2482 (1997)] code produced stiff temperature profiles in agreement with the experiment and did not indicate the formation of an internal transport barrier. Helium transport studies showed an anomalous inward particle pinch at high density. The highest density discharges were terminated by onset of a magnetohydromagnetic instability, which is consistent with destabilization of neoclassical tearing modes through peaking of the pressure profile.
Stability of the Rankine vortex in a multipolar strain field
Authors: Eloy Christophe , Le Dizès Stéphane
Published: July 2002

In this paper, the linear stability of a Rankine vortex in an n-fold multipolar strain field is addressed. The flow geometry is characterized by two parameters: the degree of azimuthal symmetry n which is an integer and the strain strength ε which is assumed to be small. For n=2, 3 and 4 (dipolar, tripolar and quadrupolar strain fields, respectively), it is shown that the flow is subject to a three-dimensional instability which can be described by the resonance mechanism of Moore and Saffman [Proc. R. Soc. London, Ser. A 346, 413 (1975)]. In each case, two normal modes (Kelvin modes), with the azimuthal wave numbers separated by n, resonate and interact with the multipolar strain field when their axial wave numbers and frequencies are identical. The inviscid growth rate of each resonant Kelvin mode combination is computed and compared to the asymptotic values obtained in the large wave numbers limits. The instability is also interpreted as a vorticity stretching mechanism. It is shown that the inviscid growth rate is maximum when the perturbation vorticity is preferentially aligned with the direction of stretching. Viscous effects are also considered for the distinguished scalings: ν=O(ε) for n=2 and 3, ν=O(ε2) for n=4, where ν is the dimensionless viscosity. The instability diagram showing the most unstable mode combination and its growth rate as a function of viscosity is obtained and used to discuss the role of viscosity in the selection process. Interestingly, for n=2 in a high viscosity regime, a combination of Kelvin modes of azimuthal wave numbers m=0 and m=2 is found to be more unstable than the classical helical modes m=±1. For n=3 and 4, the azimuthal structure of the most unstable Kelvin mode combination is shown to be strongly dependent on viscous effects. The results are discussed in the context of turbulence and compared to recent observations of vortex filaments.
Modeling of the physics and chemistry of thermal plasma waste destruction
Authors: Murphy A. B. , McAllister T.
Published: July 2002

The application of thermochemical modeling, chemical kinetic modeling, and computational fluid dynamic modeling to waste destruction by thermal plasmas is considered. Destruction of liquid and gaseous wastes in the PLASCON™ waste destruction process is used as an example. It is demonstrated that thermochemical calculation of the mixing temperature is a useful tool to predict the level to which wastes are destroyed; however, chemical kinetic calculations are necessary to investigate the formation of byproducts in the process. Computational fluid dynamic modeling is required to obtain temperature and flow fields in two dimensions. When combined with chemical kinetics, composition fields can also be obtained. These points are illustrated using the examples of chlorobenzene and chlorofluorocarbon destruction.
Laser-driven in-tube accelerator
Authors: Sasoh Akihiro
Published: July 2002

A device of accelerating a projectile using laser power, “laser-driven in-tube accelerator,” has been developed. It is characterized by accelerating the projectile in a tube and by supplying the laser beam from the direction opposite to the projectile motion. The laser beam is incident on the nose of the projectile, reflected first on the nose and second on the inner wall of the projectile shroud, then focused behind. Operation performance is measured using a CO2 transversely excited atmospheric laser of a nominal output energy of 5 J/pulse. A coupling coefficient of 73 N/MW is obtained using the atmospheric air as the working fluid.
Writing and reading perpendicular magnetic recording media patterned by a focused ion beam
Authors: Lohau J. , Moser A. , Rettner C. T. , Best M. E. , Terris B. D.
Published: July 2002

We have written and read bit patterns on arrays of square islands cut with a focused ion beam into granular perpendicular magnetic recording media. Using a static write–read tester, we have written square-wave bit patterns on arrays of islands with sizes between 60 and 230 nm, matching the recording linear density to the pattern period. These measurements reveal the onset of single-domain behavior for islands smaller than 130 nm, in agreement with magnetic force microscope images. The recording performance of patterned regions is systematically compared to that of unpatterned regions.
An experimental testbed for the study of hydrodynamic issues in supernovae
Authors: Robey H. F. , Kane J. O. , Remington B. A. , Drake R. P. , Hurricane O. A. , Louis H. , Wallace R. J. , Knauer J. , Keiter P. , Arnett D. , Ryutov D. D.
Published: July 2002

More than a decade after the explosion of supernova 1987A, unresolved discrepancies still remain in attempts to numerically simulate the mixing processes initiated by the passage of a very strong shock through the layered structure of the progenitor star. Numerically computed velocities of the radioactive Ni56 and Co56, produced by shock-induced explosive burning within the silicon layer, for example, are still more than 50% too low as compared with the measured velocities. To resolve such discrepancies between observation and simulation, an experimental testbed has been designed on the Omega Laser for the study of hydrodynamic issues of importance to supernovae (SNe). In this paper, results are presented from a series of scaled laboratory experiments designed to isolate and explore several issues in the hydrodynamics of supernova explosions. The results of the experiments are compared with numerical simulations and are generally found to be in reasonable agreement.
Micromagnetic switching of patterned square magnetic nanostructures
Authors: Torres L. , Martinez E. , Lopez-Diaz L. , Iñiguez J.
Published: July 2002

Micromagnetic simulations of switching processes in planar square magnetic nanoelements are presented. Nanosquares of size “a” in the range 10 lex<a<110 lex (lex being the exchange length, lex=(A/2πMs2)1/2) and thickness “t”, 0.25 lex<t<2 lex have been studied. Two different switching modes have been observed. The first one is basically found for very small nanoelements and it is close to uniform rotation while the second one takes place in the large nanosquares and presents an intermediate state. Good agreement of the computations with recent experimental measurements in Supermalloy is achieved. The shape of the hysteresis loops, the values of the coercivity, and the maximum in coercivity dependence with size are reproduced by the simulations. Furthermore, the two reversal mechanisms are shown to be the cause of such maximum in coercivity such as the different shapes of hysteresis loops.
Dynamics of the water-catalyzed phototautomerization of 7-azaindole
Authors: Fernández-Ramos Antonio , Smedarchina Zorka , Siebrand Willem , Zgierski Marek Z.
Published: July 2002

Multidimensional ab initio proton tunneling rate constants are reported for the tautomerization of singlet-excited 7-azaindole complexed with water, represented by discrete water molecules with and without a dielectric continuum. The results are compared with experimental observations in cold beams and in room-temperature aqueous solutions. For complexes with one and two water molecules, potential-energy surfaces are calculated at the complete active space multiconfiguration self-consistent-field [CASSCF(8,8)] level. For comparison with solution data, the structures are reoptimized inside a spherical cavity according to the Onsager model. To compare the effect of the dielectric with that of a secondary solvent shell, the structure of 1:1 and 1:2 complexes solvated by four and three additional water molecules so as to form 1:5 complexes, are optimized at the CASSCF(8,8) level with single-point Onsager corrections. Based on these potential-energy surfaces, temperature-dependent multidimensional proton transfer rate constants are calculated with a recently developed version of the instanton approach. It is found that in gas-phase 1:1 and 1:2 complexes tautomerization occurs through concerted double and triple proton transfer, respectively. The calculated low-temperature rate constants agree with the observation that in these complexes no tautomerization occurs within the fluorescence lifetime of about 8 ns. Addition of a dielectric continuum within the Onsager model cannot explain the room-temperature rate constant of about 1010 s−1 observed as the fast tautomerization component of excited 7-azaindole in protic solutions. Addition of a secondary solvent shell of four water molecules to the 1:1 complex has only a minor effect on the proton transfer rate, but addition of a secondary shell of three water molecules to the cyclic 1:2 complex yields rate constants of the observed order of magnitude. This happens because the double bridge facilitates charge separation, which stabilizes an ion-pair structure for the transition state. As a result the barrier is lowered drastically and although the proton effective mass is also increased, the effect of the lower barrier dominates, leading to much faster proton transfer. It is concluded that the fast rate component observed in room-temperature tautomerization of excited 7-azaindole in water and alcohols corresponds to proton transfer through a bridge of two hydrogen-bonded water molecules, rather than through a single-molecule water bridge as previously assumed. The predicted mechanism involves a (meta)stable intermediate state.
The influence of the molecular charge on potential energy curves for the proton transfer in electronic ground and excited states
Authors: Kowal Marek , Roszak Szczepan , Leszczynski Jerzy
Published: July 2002

The potential energy curves were studied for the proton transfer in the electronic ground and excited states for the model systems H3O2−, H4O2, and H5O2+. The complete active space self-consistent-field calculations were performed for the ground state optimized structures. The potential energy curves for the proton transfer in the excited states undergo a dramatic change due to the different electronic density distribution as an effect of electronic excitations. In all cases of the studied excited states, the electron population on the transferred proton is higher compared to that in the ground state. The total charge of the system greatly influences the potential curves. Energy separation between ground and excited states is decreased due to the negative charge of the system and is increased when the studied species are positively charged. The vertical excitations of the complex are similar to those in the monomers, but the proton relaxation leads to significant energetical (energy barriers) and structural (H+ position) changes.
Direct evidence for implanted Fe on substitutional Ga sites in GaN
Authors: Wahl U. , Vantomme A. , Langouche G. , Correia J. G. , Peralta L. ,   ISOLDE Collaboration
Published: July 2002

The lattice location of iron in thin-film, single-crystalline hexagonal GaN was studied by means of the emission channeling technique. Following 60 keV room temperature implantation of the precursor isotope Mn59 at a dose of 1.0×1013 cm−2 and annealing up to 900 °C, the angular distribution of β− particles emitted by the radioactive isotope Fe59 was measured by a position-sensitive electron detector. The β− emission patterns around the [0001], [1_102], [1_101], and [2_113] directions give direct evidence that the majority of Fe (80%) occupies substitutional Ga sites.
A new approach to the problem of noniterative corrections within the coupled-cluster framework
Authors: Meissner Leszek , Bartlett Rodney J.
Published: July 2002

Noniterative corrections to the coupled-cluster (CC) method with singles and doubles (CCSD) due to triple and higher excitations in the cluster operator are investigated. The derivation is based on the standard procedure for evaluating contributions coming from higher excitation rank cluster operators into the CC equations for singles and doubles. The noniterative nature of the approach leads to a direct modification of the CCSD energy through a posteriori corrections, however, unlike previous derivations, we take into account the coupling between the energy and cluster amplitudes in the CC equations. The coupling is not present in the fully iterative CC schemes due to the linked diagram theorem which makes the cluster amplitude equations energy independent. We show, however, that if the problem of unlinked contributions is re-examined in the context of noniterative approaches, then their complete cancellation does not occur. This leads to a partial restoration of the energy dependence. The energy dependence then gives the cluster amplitudes more flexibility in adjusting to the energy changes within the noniterative approach which is especially important in quasidegenerate situations when the standard energy corrections become large. The resulting modifications introduce disconnected contributions to the energy so size-extensivity is no longer preserved. This approach provides a new hierarchy of CC corrections in which the standard corrections, like CCSD[T] or CCSD(T), appear as a natural first step in the derivation. Some of the corrections can be easily identified as analogous to those recently proposed by Kowalski and Piecuch in the context of the method of moments of CC equations. We also suggest new approximations.
Modeling crown formation due to the splashing of a droplet
Authors: Trujillo M. F. , Lee C. F.
Published: July 2002

The typical crown formation created by the impact of a single drop on a slightly wetted target surface is treated as a series of two surfaces of discontinuity from which the jump momentum and mass equations are developed along with the governing equation for crown radius. The crown radius equation is solved in conjunction with the governing equations of the flow emanating from drop/wall impact. This flow is modeled initially as a cylindrical region of prescribed height and velocity. Both viscid and inviscid situations are treated. For the inviscid case of a crown moving through a motionless film, analytical solutions are found for the evolution of film height and velocity. For the viscid situation, a numerical scheme based on the discretization of the governing equations along the characteristic directions is employed. The results are validated by comparing with experimental and computational results from the literature. The effects of target surface film height, velocity, and wall friction on the crown dynamics are investigated.
Multifaceted asymmetric radiation from the edge-like asymmetric radiative collapse of density limited plasmas in the Large Helical Device
Authors: Peterson B. J. , Xu Yuhong , Sudo S. , Tokuzawa T. , Tanaka K. , Osakabe M. , Morita S. , Goto M. , Sakakibara S. , Miyazawa J. , Kawahata K. , Ohyabu N. , Yamada H. , Kaneko O. , Komori A. ,   LHD Experimental Group
Published: July 2002

Neutral beam injection heated discharges at the density limit in the Large Helical Device [O. Motojima, H. Yamada, A. Komori et al., Phys. Plasmas 6, 1843 (1999)] are terminated with asymmetric radiative collapse (ARC) exhibiting several properties in common with the MARFE (multifaceted asymmetric radiation from the edge) phenomenon: (1) A highly poloidally asymmetric radiation profile which is stronger on the inboard side. (2) This asymmetry is well correlated with the signal from the multichord interferometer. (3) Moreover, evidence from several diagnostics at different toroidal locations supports the possibility that ARC may be toroidally symmetric. However in contrast to MARFE, ARC is only observed in the period just prior to the quench of the plasma.
Physical and electrical properties of Ge-implanted SiO2 films
Authors: Fukuda H. , Sakuma S. , Yamada T. , Nomura S. , Nishino M. , Higuchi T. , Ohshima S.
Published: July 2002

Metal–oxide–semiconductor structures with a Ge nanocrystal embedded in SiO2 films were fabricated by Ge+ ion implantation and subsequent high-temperature annealing. The Raman spectra indicate the evidence of self-assembled Ge nanocrystals in the SiO2 films. The Ge size and its density were estimated to 3–5 nm and 1×1012/cm2, respectively. Photoluminescence spectra showed a strong blue–violet band around 400 nm and a weak near-infrared band around 750 nm, respectively. The several implantation-induced deficient centers are believed to be responsible for the blue-light luminescence. Capacitance–voltage characteristics exhibit the flatband voltage shifts of 1.02 V after the electron injection into the SiO2/Ge/SiO2 potential well. An anomalous leakage current was clearly observed in the current–voltage characteristics. The precise simulation of quantum electron transport in the SiO2 film indicates that the anomalous conduction is originated from resonant tunneling in the SiO2/Ge/SiO2 double-well band structure.
Time dependent analysis of the formation of a half-period surface relief grating on amorphous azopolymer films
Authors: Labarthet F. Lagugné , Buffeteau T. , Sourisseau C.
Published: July 2002

The formation of a surface relief grating (SRG) on amorphous copolymer films containing azobenzene chromophores in side chain positions is conducted using two linearly polarized interfering beams with orthogonal polarization directions (p+s). Time dependent analyses of the diffraction efficiency were carried out by measuring simultaneously the transmitted zero order (I(0)) as well as the first (I(±1)) and second (I(±2)) diffracted orders. Two main contributions to the diffraction efficiency are evidenced: an anisotropic (birefringence) part coming from the normal period grating plus another anisotropic and surface relief part (SRG) due to the half-period grating. The origin of the weak amplitude SRG stems from the interference of particular I(±1) diffracted orders, whose resulting polarization and angular Bragg’s conditions are in accordance with the existence of a double frequency modulation. Using the matricial products of Jones, theoretical expressions of transmitted and diffracted orders are thus derived. Extracted values of the phase shifts and, therefore, of the birefringence (Δn,Δn′) and surface relief amplitude (2Δd) are then compared to data from optical and topographic atomic force microscopy measurements. Time dependent calculations of these relevant parameters as well as analyses of the polarization states of transmitted and diffracted orders are in good agreement with the proposed model.
Dipole-allowed excited states of N2: Potential energy curves, vibrational analysis, and absorption intensities
Authors: Spelsberg Dirk , Meyer Wilfried
Published: July 2002

The three lowest adiabatic potential energy curves for each of the two dipole-allowed symmetries, Σu+1 and Πu1, are calculated in the multireference configuration–interaction framework. Diabatic potentials and corresponding coupling elements are obtained by diagonalizing the electronic operator r2 which serves to discriminate Rydberg and valence type states. A large basis set and judiciously chosen active orbital and configuration spaces furnish smooth and reliable potential curves. However, a vibrational analysis of the coupled systems in diabatic representation still shows some disappointing deviations from the experimental interference patterns of overlapping absorption bands that are highly sensitive to potential energy differences. Starting from the calculated curves, a fitting procedure accounting also for empirical information yields potential energy curves and diabatic coupling elements that reproduce all details of the experiment very well. These recommended results also serve to identify residual defects in the ab initio curves mainly as vertical shifts. The performance of other commonly used ab initio methods for the calculation of excited states is briefly discussed.
Effects of piezoelectricity and spontaneous polarization on electronic and optical properties of wurtzite III–V nitride quantum wells
Authors: Wan Shou-pu , Xia Jian-bai , Chang Kai
Published: July 2002

A theoretical model accounting for the macropolarization effects in wurtzite III–V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction–valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times.
Dependence of the magnetization and remanence of single-domain particles on the second cubic anisotropy constant
Authors: Geshev J. , Pereira L. G. , Schmidt J. E. , Mikhov M.
Published: July 2002

Magnetization and remanent magnetization curves for noninteracting single-domain particles with cubic magnetocrystalline anisotropy have been calculated, taking into account the first two anisotropy constants. The dependencies of the saturation remanence, coercivity, remanence coercivity, and δM plots on the anisotropy constants ratio have been discussed. It has been found that the calculated saturation remanence shows maxima in the vicinity of the spin-reorientation transition points for both negative and positive first cubic anisotropy constants. The remanent magnetization for the case when the face diagonals 〈110〉 are the easiest magnetization orientations has been analytically determined as well. It has been shown that when more than one type of easiest directions coexist, the remanence of the system can only be obtained numerically because of the history dependence of the remanent magnetization for some particles’ configurations. The predicted remanence behavior for systems representing spin-reorientation has been compared with that of a Tb0.6Ho0.4Fe2 powder sample, and an excellent agreement between numerical and experimental data has been found. © 2001 American Institute of Physics.
Visible photoluminescence from ruthenium-doped multiwall carbon nanotubes
Authors: Dickey Elizabeth C. , Grimes Craig A. , Jain Mahaveer K. , Ong Keat G. , Qian Dali , Kichambare P. D. , Andrews Rodney , Jacques David
Published: July 2002

Visible photoluminescence at 515 nm of ruthenium-doped multiwall carbon nanotubes, fabricated on quartz substrates using a chemical vapor deposition technique, is reported. The well-aligned nanotubes serve as templates for the luminescent, residual ruthenium–iron catalyst particles contained within the nanotubes, restricting the particle size to about 10 nm. The synthesis technique can be readily extended to other luminescent dopants; moreover, since nanotube arrays can be readily grown from patterned substrates, nanotube-based optoelectronic devices may be achieved.
On the measurement of toroidal rotation for the impurity and the main ion species on the Joint European Torus
Authors: Testa D. , Giroud C. , Fasoli A. , Zastrow K.-D. ,   EFDA–JET Team
Published: July 2002

The neoclassical moment approach of Hirshman and Sigmar [Nucl. Fusion 21, 1079 (1981)] has been used for the first time on the Joint European Torus [J. Wesson, Tokamaks, 2nd ed. (Oxford Science Publication, Oxford, 1997), p. 581] to deduce the toroidal rotation frequency of the main ion species from that measured for the impurity ions. A significant differential rotation between these two species is found where the second radial derivative of the ion temperature is large. The position of radially localized magnetohydrodynamic instabilities is then deduced from the Doppler shift in the frequency and verified using the cross-correlation between the magnetic and the electron cyclotron emission measurements. Accurate agreement between these estimates of the mode location is only found when using the toroidal rotation frequency of the main ion species. This result fully verifies the neoclassical approach and can be used directly as a diagnostic tool to localize magnetohydrodynamic instabilities.
n-
GaN
surface treatments for metal contacts studied via x-ray photoemission spectroscopy
Authors: Rickert K. A. , Ellis A. B. , Himpsel F. J. , Sun Jingxi , Kuech T. F.
Published: July 2002

The surface chemistry and electronic properties of n-GaN surfaces were studied via x-ray photoemission spectroscopy before and after wet chemical treatments. Shifts of the surface Fermi level were measured with the change in position of the Ga 3d core level peak. HCl treatment of n-GaN led to a 0.9 eV shift of the surface Fermi level toward the conduction band minimum, while KOH treatment led to a 0.3 eV shift of the surface Fermi level toward the valance band maximum. These shifts lead to a reduction in the surface barrier for HCl-treated n-GaN and for KOH-treated p-GaN, potentially improving contact resistance. The changes in surface chemistry indicate that a N (or Ga) deficiency with HCl(KOH) treatment alters the surface state density through the formation of donor (acceptor)-like states.
Electronic properties of antidot lattices fabricated by atomic force lithography
Authors: Dorn A. , Sigrist M. , Fuhrer A. , Ihn T. , Heinzel T. , Ensslin K. , Wegscheider W. , Bichler M.
Published: July 2002

Antidot lattices were fabricated by atomic force lithography using local oxidation. High quality finite 20×20 lattices are demonstrated with periods of 300 nm. The low-temperature magnetoresistance shows well developed commensurability oscillations as well as a quenching of the Hall effect around zero magnetic field. In addition, we find B-periodic oscillations superimposed on the classical commensurability peaks at temperatures as high as 1.7 K. These observations indicate the high electronic quality of our samples.
Hooge parameter in buried-channel metal-oxide-semiconductor field-effect transistors
Authors: Omura Yasuhisa
Published: July 2002

This article describes the low-frequency noise characteristics of various buried-channel metal-oxide-semiconductor field-effect transistors. Experimental results show that their are peculiar behaviors in Hooge parameter. However, the original mobility-based model for the Hooge parameter is shown to provide partial understanding of the experimental results on the basis of a phenomenological consideration. It is also observed that, in contrast to previous theories, the interpretation of some aspects of the Hooge parameter strongly depends on how the two fluctuation modes, the carrier-density fluctuation and the mobility fluctuation, correlate. The proposed model gives a fundamental physical basis that allows aspects of the Hooge parameter to be interpreted. The gate voltage dependence of the Hooge parameter can be explained reasonably well by introducing the correlation of the mobility fluctuation and the carrier density fluctuation. Regarding the drain voltage dependence of the Hooge parameter, however, the present model for the Hooge parameter needs further modification.
Area scaling and voltage dependence of time-to-breakdown in magnetic tunnel junctions
Authors: Das J. , Degraeve R. , Roussel P. , Groeseneken G. , Borghs G. , Boeck J. De
Published: July 2002

The development of magnetic random access memory requires very reliable magnetic tunnel junctions (MTJs). However these devices are prone to dielectric breakdown. To investigate the reliability of the MTJs, we performed constant voltage stress tests. This study shows the area scaling and the voltage dependence of the time-to-breakdown tBD of exchange biased, naturally oxidized MTJs. The fraction of broken devices F is plotted on a Weibull scale. The Weibull fit of our data shows clearly that tBD scales with the area, meaning that the breakdown sites are randomly distributed. Moreover, the Weibull distributions have a constant slope β (=0.35) and show no tails. This suggests that only one breakdown mode is observed. It is not clear yet whether an intrinsic or extrinsic failure mechanism is causing the breakdown, but considering the small Weibull slope, extrinsic breakdown caused by randomly distributed process-induced defects in the oxide is most plausible. Second, the voltage dependence of tBD is studied. To fit the data, a linear dependence of log(tBD) on stress voltage is assumed. It is however not yet proven whether this model is applicable for ultrathin Al2O3 barriers.
Nonvolatile field programmable spin-logic for reconfigurable computing
Authors: Richter R. , Bär L. , Wecker J. , Reiss G.
Published: July 2002

We have fabricated field programmable spin-logic gates based on spin-dependent tunneling (SDT) elements. Here we show their feasibility down to a width of 0.6 μm of the SDT elements that form spin-logic gates. We further demonstrate the clocked operation of a hybrid spin-logic gate consisting of SDT elements and a semiconductor-based sense amplifier. Apart from the nonvolatility of the inputs, the output and the programming information, the experimentally demonstrated concept seems to be suitable for reconfigurable computing operations.
Role of atomic arrangements at interfaces on the phase control of epitaxial TiO2 films
Authors: Park B. H. , Huang J. Y. , Li L. S. , Jia Q. X.
Published: July 2002

Epitaxial rutile-TiO2 and anatase-TiO2 films were grown at 800 °C on Al2O3(1̄102̄) and LaAlO3(001), respectively, using pulsed laser deposition. Both films showed high crystalline quality, evidenced by x-ray diffraction and high-resolution electron microscopy. The formation of different phases on different substrates could be qualitatively explained by the atomic arrangements at the interfaces. We also deposited epitaxial rutile-TiO2 and anatase-TiO2 films on conductive RuO2 and La0.5Sr0.5CoO3 electrodes, respectively. Using a Kelvin probe, we measured the photovoltaic properties of these multilayer structures. A rutile-TiO2 film grown on RuO2 showed a very broad peak in the visible light region. An epitaxial anatase-TiO2 film grown on La0.5Sr0.5CoO3 showed a strong peak with a threshold energy of 3.05 eV.
Parallel correlation measurements in the scrape-off layer of the Joint European Torus
Authors: Thomsen H. , Endler M. , Bleuel J. , Chankin A. V. , Erents S. K. , Matthews G. F. ,   Contributors to the EFDA-JET workprogramme
Published: July 2002

In the scrape-off layer of the Joint European Torus [P.-H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)], Langmuir probes were used to measure the correlation of ion saturation current fluctuations parallel to the magnetic-field B. It was possible to realize three different probe tip separations (connection lengths) ∥B under almost identical plasma conditions and magnetic configuration. Among these, a probe tip separation of 66 m ∥B represents the largest value ever realized for this type of experiment. It was found that the correlation is reduced from ⩾80% at 0.75 m probe tip separation ∥B in the divertor to ≈40% at 23 m, but remains at ≈40% even for the 66 m separation. An upper limit for the ratio between the average wave numbers parallel and perpendicular to the magnetic field |k̄∥/k̄⊥|<0.0022 is found, but the results are in good agreement with a parallel wave number k̄∥=0.
Transverse demagnetizing factors of long rectangular bars: I. Analytical expressions for extreme values of susceptibility
Authors: Chen D.-X. , Prados C. , Pardo E. , Sanchez A. , Hernando A.
Published: July 2002

The demagnetizing problem should be studied numerically in nonellipsoids with nonzero susceptibility χ, except for a few limiting cases where analytical treatment turns out to be practical. In this work, for an infinitely long bar with rectangular cross section 2a×2b and χ=∞ and −1 in a uniform transverse applied field Ha along dimension a or b, analytical expressions for the surface pole or current distributions and the fluxmetric and magnetometric demagnetizing factors Nf,m are derived using a technique of conformal transformation. From the new as well as the existing formulas, Nf,m(χ=∞,0,−1) are evaluated, plotted, and tabulated as functions of a/b.
Excited singlet (S1) state interactions of 2,2′- and 4,4′-biphenyldiols with chloroalkanes: Photoinduced dissociative electron transfer
Authors: Mohanty J. , Pal H. , Sapre A. V.
Published: July 2002

Interactions of the excited singlet (S1) state of 2,2′- and 4,4′-biphenyldiols with a number of chloroalkanes (CA) have been investigated in acetonitrile solutions using fluorescence quenching measurements. For any particular diol–CA pair, the bimolecular quenching constants kq obtained from steady-state and time-resolved measurements are found to be the same, indicating the dynamic nature of the interaction. The kq values for different diol–CA pairs are seen to increase as the oxidation potential of the diol becomes less positive or the reduction potential of the CA becomes less negative, indicating the electron transfer (ET) type of interaction for the observed quenching. Following Marcus’ outer-sphere ET theory, the correlation of the observed kq values with the free-energy changes for such reactions (ΔG0) results in the recovery of an unusually higher intramolecular reorganization energy (λin), indicating that the ET in the systems studied might not be of outer sphere in nature. Since the CAs are prone to undergo C–Cl bond cleavage following their reduction, a dissociative ET (DET) mechanism has been proposed for the observed fluorescence quenching. The evidence for the DET mechanism has been obtained by characterizing and estimating the Cl− ions in the photolyzed diol–CA solutions. Following a suitable theory for concerted DET reactions, it is seen that the observed kq values correlate well with the free-energy changes (ΔGDET0) for such reactions. It is seen that the reorganization energy recovered from such correlation accounts well for the C–Cl bond dissociation energy of the CAs, supporting a concerted DET mechanism in these systems.
Symmetries of supersymmetric integrable hierarchies of KP type
Authors: Nissimov E. , Pacheva S.
Published: July 2002

This article is devoted to the systematic study of additional (non-isospectral) symmetries of constrained (reduced) supersymmetric integrable hierarchies of KP type—the so-called SKP(R;MB,MF) models. The latter are supersymmetric extensions of ordinary constrained KP hierarchies which contain as special cases basic integrable systems such as (m)KdV, AKNS, Fordy–Kulish, Yajima–Oikawa, etc. As a first main result it is shown that any SKP(R;MB,MF) hierarchy possesses two different mutually (anti-)commuting types of superloop superalgebra additional symmetries corresponding to the positive- and negative-grade parts of certain superloop superalgebras. The second main result is the systematic construction of the full algebra of additional Virasoro symmetries of SKP(R;MB,MF) hierarchies, which requires nontrivial modifications of the Virasoro flows known from the general case of unconstrained Manin–Radul super-KP hierarchies (the latter flows do not define symmetries for constrained SKP(R;MB,MF) hierarchies). As a third main result we provide systematic construction of the supersymmetric analogs of multi-component (matrix) KP hierarchies and show that the latter contain, among others, the supersymmetric version of the Davey–Stewartson system. Finally, we present an explicit derivation of the general Darboux–Bäcklund solutions for the SKP(R;MB,MF) super-tau functions (supersymmetric “soliton”-like solutions) which preserve the additional (non-isospectral) symmetries.
On the comparison of turbulence intensities from large-eddy simulation with those from experiment or direct numerical simulation
Authors: Winckelmans G. S. , Jeanmart H. , Carati D.
Published: July 2002

The relation between the Reynolds stresses from experiment or direct numerical simulation (DNS) and large-eddy simulation (LES) is reviewed. As is well known, the Reynolds stresses can only be reconstructed from a LES when the average contribution from the subgrid-scale model is taken into account. However, in the case of LES using traceless models (e.g., effective viscosity models: Smagorinsky model, dynamic Smagorinsky model, etc.), or even partially traceless models (e.g., mixed models), only the deviatoric Reynolds stresses can be reconstructed. This obvious point is often overlooked in the literature. It has important consequences in all flows with at least one inhomogeneous direction (channels, boundary layers, wakes, shear layers, jets, etc.): as far as the rms turbulence intensities are concerned, one can only properly reconstruct, and thus directly compare with experimental or DNS data, their deviation from isotropy.
Structure-function equations for scalars
Authors: Hill Reginald J.
Published: July 2002

An exact equation is obtained which relates the products of two-point differences of a scalar quantity and those differences with the difference of velocity and other quantities. The averages of such products are structure functions. Equations that follow from the scalar equation but with no other approximations are called “exact” here. Exact equations for structure functions are obtained. Ensemble, temporal, and spatial averages are all considered because each one produces different statistical equations from the others and because they differ in application to theoretical purposes, experiment, and numerical simulation of turbulence. The midpoint and the difference of the two points at which the hydrodynamic quantities are obtained are X and r, respectively. Use of X and r as independent variables combined with Reynolds decomposition reveals quantifiable effects of inhomogeneity and large-scale structures. Exact equations obtained by averaging over a sphere in r space have a particularly simple form. The case of a numerical simulation that has periodic boundary conditions leads to particularly simple equations. For the case of grid-generated wind-tunnel turbulence, approximate equations are derived relating structure functions of fluctuations. Applications of the equations to experiments and direct numerical simulation are discussed.
Hierarchy of Dirac, Pauli, and Klein–Gordon conserved operators in Taub-NUT background
Authors: Cotăescu Ion I. , Visinescu Mihai
Published: July 2002

The algebra of conserved observables of the SO(4,1) gauge-invariant theory of the Dirac fermions in the external field of the Kaluza–Klein monopole is investigated. It is shown that the Dirac conserved operators have physical parts associated with Pauli operators that are also conserved in the sense of the Klein–Gordon theory. In this way one gets simpler methods of analyzing the properties of the conserved Dirac operators and their main algebraic structures including the representations of dynamical algebras governing the Dirac quantum modes.
On the conditional variance and covariance equations for second-order conditional moment closure
Authors: Kim Seung Hyun
Published: July 2002

The consistency of the conditional variance and covariance equations derived by the decomposition method and by the probability density function method are addressed in this paper. The complete and consistent equations are derived and the limitations of the previous closure hypotheses are discussed. It is shown from a numerical experiment that conditional turbulent fluxes in conserved scalar space, which have been neglected in the previous closure, can have significant contribution to the transport of conditional variances and covariances in conserved scalar space. A gradient transport model for the turbulent fluxes is proposed.
GaN–GaN junctions with ultrathin AlN interlayers: Expanding heterojunction design
Authors: Keller S. , Heikman S. , Shen L. , Smorchkova I. P. , DenBaars S. P. , Mishra U. K.
Published: July 2002

The formation of a two-dimensional electron gas (2DEG) was observed at GaN–GaN junctions, when an AlN layer of a thickness greater than or equal to 0.5 nm was inserted, and the GaN cap layer was modulation-doped with silicon. No 2DEG was found for undoped samples. When the AlN interlayer thickness was increased from 0.5 to 1 nm, the electron mobility increased from 720 to 1250 cm2/Vs at 300 K and from 6400 to 12 000 cm2/Vs at 77 K. The electron mobility was also affected by the Si-spike doping concentration with the highest electron mobility of 13 800 cm2/Vs at 77 K measured for a doping of 5×1012 cm−2. The formation of the 2DEG was attributed to the polarization-induced discontinuity in the conduction band caused by the extremely strong polarization fields in the pseudomorphically strained AlN interlayers.
A general formulation of focus wave modes
Authors: Sezginer A.
Published: July 2002

A general formulation is given for electromagnetic pulses which remain localized in a multidimensional space, and which propagate at the speed of light without dispersing (focus wave modes). It is shown that such modes necessarily have infinite electromagnetic energy in the source-free, three-dimensional space. Finite-energy focus wave modes cannot exist without sources. A set of complete focus wave modes with Hermite–Gaussian transverse variation is derived. The relation of focus wave modes to the solutions of the paraxial wave equation is established.