bluuebunny/crossref_metadata_2025 huggingface dataset:
Thickness effects on imprint in chemical-solution-derived (Pb, La)(Zr, Ti)O3 thin films
Authors: Kim Seung-Hyun , Woo Hyun-Jung , Ha Jowoong , Hwang Cheol Seong , Kim Hae Ryoung , Kingon Angus I.
Published: July 2002

The film thickness-dependent imprinting behavior (voltage shift) of (Pb, La)(Zr, Ti)O3 capacitors was evaluated by a thermal stress process under a remanence bias. The remanent polarization (Pr) was found to be almost independent of the film thickness whereas in the 50–300 nm range the relative dielectric constant (εr) increased linearly with the square root of the film thickness. It was found that the voltage shift, which was attributed to the accumulation of charged defects near the electrode interface, also increased linearly with increasing film thickness. In addition, the charge accumulated thickness varied with the square root of the film thickness. This was established from a simple assumption that the level of charge accumulation is determined by the product of the total amount of charged defects (total film thickness×charged defect density) and the internal field that is generated by the Pr. Therefore, the imprint is much more a bulk-related degradation phenomenon compared to the fatigue.
Photoreflectance line shape symmetry and quantum-well ground-state exciton energy in vertical-cavity surface-emitting laser structures
Authors: Ghosh Sandip , Hosea Thomas J. C. , Constant Stephanie B.
Published: July 2002

We report photoreflectance studies on the coupling between the Fabry–Perot cavity mode (CM) and the quantum well (QW) ground-state excitonic feature in vertical-cavity surface-emitting laser structures. Changes in the symmetry of the CM-QW spectral feature occur when the angle of incidence of the probe beam is altered. Using detailed simulations, we explain how this is related to an unusual reversal of the roles of the Seraphin coefficients and QW dielectric function, in determining the line shape. Our study suggests a way to find the exciton energy, in situations where a distinct QW feature is not seen because of large broadening of the QW dielectric function combined with high reflectivity of Bragg mirrors and relatively narrow CM width.
A novel implicit Newton–Raphson geometry optimization method for density functional theory calculations
Authors: Filippone Francesco , Meloni Simone , Parrinello Michele
Published: July 2002

A novel Newton–Raphson geometry optimization method for density functional theory calculation is presented. Contrary to the standard Newton–Raphson procedure, our method does not require the calculation of the full Hessian matrix. We use instead density functional perturbation theory to evaluate the effect of the Hessian on a given atomic displacement and invert the Hessian using a conjugate gradient method. The method uses Cartesian coordinates and is very robust. In a variety of tested molecular systems, it has an efficiency comparable with that of more established methods. In difficult condensed matter systems, it appears to be far superior.
Generalization of the exponential variational ansatz in relative coordinates for bound state calculations in four-body systems
Authors: Frolov Alexei M. , Smith Vedene H.
Published: July 2002

A generalization of the exponential variational expansion in relative coordinates to four-body systems is presented. It is shown that all required matrix elements can be expressed as derivatives upon the nonlinear parameters of the so-called auxiliary or basic four-body (six-dimensional) integral. If such an integral can be computed analytically, then it is possible to use the proposed variational approach for highly accurate variational calculations for various four-body systems with arbitrary particle masses without adiabatic or similar approximations. For instance, this approach can be used for highly accurate, bound state, variational calculations of the positronium molecule Ps2, four-body muonic molecules, H2, D2, and DT molecules and many other systems. In fact, the present work opens a new avenue in the study of four-body systems.
Multiple pathway dynamics of the O(1D)+C2H6 reaction: A crossed beam study
Authors: Shu Jinian , Lin Jim J. , Lee Yuan T. , Yang Xueming
Published: July 2002

The O(1D)+C2H6 reaction has been reinvestigated using the improved universal crossed molecular beam technique. Four different reaction channels, OH+C2H5, CH3+H2COH/H3CO, H+H4C2OH/H5C2O and H2+H3C2OH/H4C2O, are observed. Among these reaction channels, the CH3 formation channel is the dominant pathway and the OH formation is also a significant process, while the H and H2 formation processes are minor. Different product angular distributions are also observed for these channels, indicating that the reaction product formation in these channels proceeds with distinctive reaction dynamics.
Space charge effect and the step voltages in metal/polyimide/rhodamine–dendorimer/polyimide/metal junctions
Authors: Noguchi Yutaka , Majima Yutaka , Iwamoto Mitsumasa
Published: July 2002

An electrostatic space charge layer of nanometer thickness is formed at organic film/metal interfaces due to the displacement of electrons from the metal to the film. This space charge layer directly affects the current–voltage (I–V) characteristics of artificially layered rhoadamine–dendorimer thin films, which show a step structure similar to a Coulomb staircase. The capacitance between one rhodamine–dendorimer molecule and a metal electrode is calculated using a sphere-plane model, assuming that excess electronic charges are displaced from the plane metal electrode into the film. This capacitance is found to increase as the excess charge increases which leads to a change in the voltage steps in the I–V characteristics as observed in metal/polyimide/ rhodamine–dendorimer/polyimide/metal junctions. Such a voltage step is derived theoretically, and will explain the difference between Au/polyimide/rhodamine–dendorimer/ polyimide/Al and Al/polyimide/rhodamine–dendorimer/polyimide/Al junctions.
Temperature effects on the electrical properties and structure of interfacial and bulk defects in Al/SiNx:H/Si devices
Authors: Martı́nez F. L. , San Andrés E. , del Prado A. , Mártil I. , Bravo D. , López F. J.
Published: July 2002

Bulk properties of SiNx:H thin film dielectrics and interface characteristics of SiNx:H/Si devices are studied by a combination of electrical measurements (capacitance–voltage and current–voltage characteristics) and defect spectroscopy (electron spin resonance). The SiNx:H films were deposited by an electron cyclotron resonance plasma method and subjected to rapid thermal annealing postdeposition treatments at temperatures between 300 and 1050 °C for 30 s. It is found that the response of the dielectric to the thermal treatments is strongly affected by its nitrogen to silicon ratio (N/Si=x) being above or below the percolation threshold of the Si–Si bonds in the SiNx:H lattice, and by the amount and distribution of the hydrogen content. The density of Si dangling bond defects decreases at moderate annealing temperatures (below 600 °C) in one order of magnitude for the compositions above the percolation threshold (nitrogen rich, x=1.55, and near stoichiometric, x=1.43). For the nitrogen rich films, a good correlation exists between the Si dangling bond density and the interface trap density, obtained from the capacitance measurements. This suggests that the observed behavior is mainly determined by the removal of states from the band tails associated to Si–Si weak bonds, because of the thermal relaxation of the bonding strain. At higher annealing temperatures the deterioration of the electrical properties and the increase of the Si dangling bonds seem to be associated with a release of trapped hydrogen from microvoids of the structure. For the silicon rich samples rigidity percolates in the network resulting in a rigid and strained structure for which the degradation phenomena starts at lower temperatures than for the other two types of samples.
Photovoltage imaging of a single As-vacancy at a GaAs(110) surface: Evidence for electron trapping by a charged defect?
Authors: Aloni S. , Nevo I. , Haase G.
Published: July 2002

We employed atomically-resolved surface photovoltage (SPV) imaging with a scanning tunneling microscope, using super- and sub-band gap energy photons, to map the potential distribution around a single As vacancy at a p-GaAs(110) surface. While the super-band gap (532 nm) SPV reflects the band bending (including the tip-induced effect) in the dark, the sub-band gap (1064 nm) SPV shows the change in band bending due to charge injection into the defect. We conclude that in the dark, the As vacancy introduces a surface state, ∼0.63 eV above the valence band maximum, that has to be filled with two electrons to maintain surface neutrality. However, under 1064 nm (sub-band gap) illumination at room temperature, we observed a large negative SPV feature (radius of 70–100 Å), and a zero or low positive SPV zone in its center, with a six-pointed star shape of ∼15 Å radius. We interpret our findings as either negative photo-charging of the defect, or positive charging that leads to a trapped electron state around the defect.
Al
x
Ga
1−x
N/GaN
band offsets determined by deep-level emission
Authors: Hang D. R. , Chen C. H. , Chen Y. F. , Jiang H. X. , Lin J. Y.
Published: July 2002

We present studies of the compositional dependence of the optical properties of AlxGa1−xN(0<x<0.22) alloys by modulation spectroscopy and photoluminescence. The yellow luminescence, which is well known in GaN and is generally assigned to shallow donor–deep acceptor pair recombination has also been observed in AlxGa1−xN. As aluminum concentration increases, the color of the band changes from yellow (2.2 eV) to blue (2.6 eV). The shift was less than that of the band gap. Together with previously published studies, it implies that the deep acceptor level is pinned to a common reference level to both materials, thus the deep level responsible for the yellow emission is used as a common reference level to determine the band alignment in AlxGa1−xN/GaN heterojunctions. Combining with the near-band-edge modulation spectra, the estimated ratio of conduction-to-valence band discontinuity is 65:35. Our results are close to the values obtained from PL measurements on Al0.14Ga0.86N/GaN quantum wells and those calculated by linear muffin-tin orbital method and linearized augmented plane wave method.
Density functional study of the first-row transition-metal complexes M–CH2, M–CHF, and M–CF2
Authors: Dalmázio Ilza , Anderson Duarte Hélio
Published: July 2002

Carbenes and fluorocarbenes are important intermediates in the hydrodechlorination of CFCs catalyzed by metal surfaces. However, the reaction mechanism at a molecular level is not completely understood. In this work, density functional calculations have been performed for the first-row transition-metal complexes M–CH2, M–CHF, and M–CF2 aiming to conbribute to the understanding of the metal/carbene interaction mechanism. Relative energies, geometries, and frequencies of the M–CXY complexes in different electronic states are reported. The binding mechanism is described through an analysis of the molecular orbitals. The binding energy of the M–CF2 is about 30% smaller than the respective M–CH2 binding energy. The electronic configuration of all complexes studied is presented in a diagram that allows one to predict qualitatively properties such as geometries, multiplicities, charge transfers, and relative bond lengths.
Strong self- and cross-phase modulation effects in chromium-doped KTiOPO4 crystals
Authors: Jurdik E. , Petukhov A. V. , Anema A. , van Etteger A. , Rasing Th.
Published: July 2002

The presence of chromium impurities in the crystal matrix of chromium-doped KTiOPO4 is shown to be at the origin of a strong cubic optical nonlinearity of this material. As a result, a pronounced self-phase modulation of a Gaussian laser beam at 532 nm is observed in the far field as a diffraction pattern consisting of alternating dark and bright rings. The induced nonparabolic and polarization sensitive refractive index change is also used to modulate a weak probe laser beam at 633 nm. A simple theoretical model incorporating a laser heating process and a thermo-optical effect accounts for the main features observed in our experiments and elucidates the mechanisms leading to the diffraction pattern formation.
Renner–Teller induced predissociation of HNO(Ã 1A″): Rotational-state dependent linewidths of quasibound states
Authors: Weiß Jan , Schinke Reinhard
Published: July 2002

The predissociation dynamics in the Ã 1A″ electronic state of HNO is investigated. The quantum mechanical dynamics calculations take into account the Renner–Teller (or electronic Coriolis) coupling with the electronic ground state X̃ 1A′, which appears to be the dominant decay mechanism for the quasi-bound vibrational states in the Ã 1A″ upper state. All three internal vibrational degrees of freedom are incorporated and two ab initio potential energy surfaces are used. The linewidths, Γ, are directly calculated by the filter diagonalization method and an absorbing optical potential in the exit channel. They are generally small (∼1 cm−1) and increase with excitation of the bending mode (bent-to-bent transition). On average, Γ increases with K, the a-axis rotational quantum number. However, for some vibrational states the linewidth shows a non-monotonic behavior with K, which is the result of mixing with highly excited vibrational levels in the continuum of the ground electronic state. This effect is even more striking, when the total angular momentum quantum number J is varied: In some cases, the linewidth exhibits a pronounced resonancelike behavior as function of J. The agreement with recent experimental spectroscopic data is satisfactory. The calculated linewidths are of the same order (within a factor of 2 or so) as the experimental ones. However, because the two ab initio potential energy surfaces do not reproduce sufficiently well the X̃–Ã excitation energies, the resonancelike effects are not quantitatively described. Potential energy surfaces with considerably higher accuracy are required.
Dynamic space charge mechanism in SrS:Ce ac thin-film electroluminescent devices
Authors: Peter Manuela , Murayama Masatoshi , Nishimura Satoru , Ohmi Koutoku , Tanaka Shosaku , Kobayashi Hiroshi
Published: July 2002

Dynamic space charge in SrS:Ce ac thin-film electroluminescent (ACTFEL) devices is not a SrS host property. In SrS:Ce ACTFEL devices, a lack of charge injection from interface states leads to space charge formation and to dynamic space charge formation. The dynamic space charge is formed by the impact ionization of Ce and of Ce induced deep level traps. The crystal field symmetry of the Ce3+ site determines its ionization probability. Ce3+ sites of cubic crystal field symmetry have a lower ionization probability than Ce3+ sites with axial crystal field symmetry. A prerequisite for dynamic space charge, i.e., short lived space charge, is the annihilation of space charge. The charge responsible for space charge annihilation originates from trap levels with an activation energy of about 9 meV. The temperature dependence of the annihilation of the space charge is responsible for the observed temperature dependence of the dynamic space charge formation, the conduction charge, and the quantum efficiency.
Urbach’s tail in III-nitrides under an electric field
Authors: Rodrigues Clóves G. , Vasconcellos Áurea R. , Luzzi Roberto , Freire V. N.
Published: July 2002

We consider electron-hole recombination in wide-gap strong-polar semiconductors of the III-nitride family under high electric fields. The calculated low-energy side of the luminescense spectrum displays the so-called Urbach’s tail, which is characterized as resulting from the presence of sidebands in the form of replicas of the main band, corresponding to recombination with accompanying emission of one, two, etc., LO phonons. The influence of the nonequilibrium macroscopic state of hot carriers and phonons on the luminescence spectrum is evidenced. Our results for a 45 kV/cm electric field intensity point to 50, 120, and 220 meV Urbach tail widths in, respectively, wurtzite InN, GaN, and AlN.
Two-photon spectroscopy of autoionizing states of ammonia
Authors: Raptis C. A. , Pratt S. T.
Published: July 2002

Two-photon ionization spectra of jet-cooled NH3 have been obtained in the region between the adiabatic ionization potential and the NH3+ X̃ 2A2″(0200) ionization threshold. Fairly intense np and nf Rydberg series are observed converging to selected rotational levels of the X̃ 2A2″(0100) and (0200) states. Rydberg series converging to higher vibrational thresholds and requiring vibrational autoionization by Δvi⩽−2 are also observed. Photoelectron spectroscopy reveals that many of the molecules in the latter series do not autoionize, but instead are ionized by the absorption of an additional photon. The new two-photon spectra are compared with previously reported double resonance spectra in the same energy region, providing insight into questions of the spectroscopic assignments.
An improved selective excitation double Mössbauer spectrometer
Authors: van Lierop J. , Ryan D. H.
Published: July 2002

The design, operation and performance of a selective excitation double Mössbauer (SEDM) spectrometer are described. An innovative drive-mounted conversion electron detector for scattered γ-ray energy analysis provides a three orders of magnitude improvement in counting efficiency over previous SEDM equipment. Simple digital circuitry together with Wissel Mössbauer velocity transducers and electronics supplies energy synchronization for indefinite collection times. Magnetic materials containing resonant Mössbauer nuclei (e.g., Fe57) can now be conveniently studied using SEDM, offering a unique insight into dynamic processes.
Characterizing conformers and torsional potentials of nitrosoformaldehyde and N-nitrosomethanimine
Authors: Sancho-Garcı́a J. C. , Pérez-Jiménez A. J. , Pérez-Jordá J. M. , Moscardó F.
Published: July 2002

A detailed ab initio study on the torsional potential of two heterobutadienes (nitrosoformaldehyde and N-nitrosomethanimine) it is performed by using state-of-the-art coupled-cluster methodologies. Special emphasis is given to basis set incompleteness with a profuse comparison of several cc-pVnZ basis sets and to the performance of common extrapolation formulas for estimating the complete basis set limit. We give high quality torsional functions from a fit to our best calculations for its use in current force field methods. We analyze similarities and differences between the present heterobutadienes and similar compounds such as 1,3-butadiene and glyoxal. Finally, we provide accurate estimates to the proton affinities of nitrosoformaldehyde and N-nitrosomethanimine.
SUSY transformations for quasinormal modes of open systems
Authors: Leung P. T. , Maassen van den Brink Alec , Suen W. M. , Wong C. W. , Young K.
Published: July 2002

Supersymmetry (SUSY) in quantum mechanics is extended from square-integrable states to those satisfying the outgoing-wave boundary condition, in a Klein–Gordon formulation. This boundary condition allows both the usual normal modes and quasinormal modes with complex eigenvalues ω. The simple generalization leads to three features: The counting of eigenstates under SUSY becomes more systematic; the linear-space structure of outgoing waves (nontrivially different from the usual Hilbert space of square-integrable states) is preserved by SUSY; and multiple states at the same frequency (not allowed for normal modes) are also preserved. The existence or otherwise of SUSY partners is furthermore relevant to the question of inversion: Are open systems uniquely determined by their complex outgoing-wave spectra?
The Lax pairs for elliptic Cn and BCn Ruijsenaars–Schneider models and their spectral curves
Authors: Chen Kai , Hou Bo-yu , Yang Wen-li
Published: July 2002

We study the elliptic Cn and BCn Ruijsenaars–Schneider models which are elliptic generalization of systems given in previous paper by the present authors [Chen et al., J. Math. Phys. 41, 8132 (2000)]. The Lax pairs for these models are constructed by Hamiltonian reduction technology. We show that the spectral curves can be parametrized by the involutive integrals of motion for these models. Taking nonrelativistic limit and scaling limit, we verify that they lead to the systems corresponding to Calogero–Moser and Toda types.
Nonplanar adsorption and orientational ordering of porphyrin molecules on Au(111)
Authors: Yokoyama Takashi , Yokoyama Shiyoshi , Kamikado Toshiya , Mashiko Shinro
Published: July 2002

Saddle-shaped deformation of planar porphyrin molecules is accomplished by rotations of four phenyl-based substituents, which results from optimum adsorption onto Au(111) surface. The nonplanar macrocyclic conformation is clearly visualized by using low-temperature scanning tunneling microscopy and confirmed by molecular orbital calculations. Inside of the supramolecular molecular islands, we find that two different orientations of the nonplanar porphyrins are randomly distributed. An orientational ordering is obtained after short thermal excitations, which should be associated with steric intermolecular interactions between substituents.
On the correlation energy features in planar heteroatomic molecular systems
Authors: Smith David M. , Barić Danijela , Maksić Zvonimir B.
Published: July 2002

The correlation energy in planar heteroatomic open chain polyene systems involving N, O, and F atoms is considered by the CASSCF and CASPT2 methods employing a number of the cc-pVmZ (VDZ, VTZ, etc.) correlation consistent basis sets. A thorough study of the smallest molecules shows that the nondynamical correlation energy is virtually independent of the quality of the basis set. In contrast, the dynamical correlation energy is very sensitive to the basis set and, in estimating reliable dynamical correlation effects for larger systems, one has to rely on adequate extrapolation formulas to obtain the infinite basis set limit. We find that a method recently proposed by Truhlar offers economical yet reasonable estimates of the complete basis set results. Investigation of the sensitivity of the results to the choice of active space and the comparison to single reference MP2 calculations indicate that such extrapolations offer a good general method for saturating the basis set in multireference calculations. Thus a simple refinement of the conventional multireference coupled cluster method is proposed. It is also shown that both nondynamical and dynamical correlation energies follow very simple additivity rules in linear and branched planar chain heteroatomic polyenes, making possible their prediction in very large systems without calculation.
Monte Carlo simulation on thermodynamic properties of a heteropolymer chain
Authors: Wang Yuanyuan , Chen Hanning , Liang Haojun
Published: July 2002

The equilibrium thermodynamic properties of a heteropolymer chain of 4 hydrophobic and 26 hydrophilic segments with Lennard-Jones interaction among segments were studied by multicanonical Monte Carlo methods in a three-dimensional off-lattice model. It is approved that the multicanonical algorithms provide us a practical way to accurately calculate thermodynamic properties of the chain at lower temperature. The collapse transition of a heteropolymer chain cooled from higher temperature is able to be separated in three steps: First, the hydrophobic segments associate together to form a hydrophobic core; then, the hydrophilic segments collapse on the surface of the core; finally, the heteropolymer chain reorganizes into its lowest energy state.
Effect of magnetic coupling on the magnetoresistive properties in La0.67Sr0.33MnO3/BaFe11.3(ZnSn)0.7O19 composites
Authors: Huang Q. , Li J. , Huang X. J. , Ong C. K. , Gao X. S.
Published: July 2002

The La0.67Sr0.33MnO3/BaFe11.3(ZnSn)0.7O19 (LSMO/BaM) composites have been prepared by cosintering the LSMO and the BaM powders. The microstructural, magnetic, and magnetoresistive (MR) properties of those systems were systematically studied. Due to the magnetic coupling between the LSMO (a soft magnet metal) and BaM (a hard magnet insulator) grains, the low-field MR sensitivity was reduced and the high-field MR slope was enhanced for the composites. In addition, an abnormal MR hysteresis (resistance reaches its maximum before field reversal) was identified for the composite when the field is applied vertical to the sample plane. Its origin was attributed to a coplay of the LSMO/BaM spin coupling and the demagnetization effect. Finally, an anisotropic magnetoresistance (AMR) effect was observed in the composites. The AMR value decreased with increasing the BaM content, which can be explained by the changes of the local effective field on the grain boundaries.
Bias stress in organic thin-film transistors and logic gates
Authors: Zilker S. J. , Detcheverry C. , Cantatore E. , de Leeuw D. M.
Published: July 2002

Threshold voltage instabilities of all-organic thin-film transistors are investigated as a function of stress time and stress bias. The dominant effect is a positive threshold shift for negative gate bias stress which is explained by mobile ions drifting in the insulator when a gate field is applied. Trapping of charge carriers at the semiconductor–insulator interface plays only a minor role. Furthermore, we investigate the stress behavior of a basic logic element, an inverter. In comparison to a single transistor, we observe improved stability which arises from partial compensation of the parametric shifts during operation.
Hexagonally ordered 100 nm period nickel nanowire arrays
Authors: Nielsch K. , Wehrspohn R. B. , Barthel J. , Kirschner J. , Gösele U. , Fischer S. F. , Kronmüller H.
Published: July 2002

The magnetic behavior of 100 nm period arrays of Ni nanowires embedded in a highly ordered alumina pore matrix were characterized by magnetometry and magnetic force microscopy. Reducing the diameter of the nanowires from 55 to 30 nm while keeping the interwire distance constant leads to increasing coercive fields from 600 to 1200 Oe and to increasing remanence from 30% to 100%. The domain structure of the arrays exhibits in the demagnetized state a labyrith-like pattern. These results show that stray field interactions of single domain nanowires are entirely dependent on the nanowire diameter.
Nonlinear optical response of molecule in inhomogeneous solvation environment: A response theory formalism
Authors: Jørgensen Solvejg , Ratner Mark A. , Mikkelsen Kurt V.
Published: July 2002

We present a method for determining quadratic response molecular properties of heterogeneously solvated molecules. The method enables the calculation of frequency-dependent hyperpolarizabilities and two-photon transition matrix elements of molecules that are (i) located at the surface of a metal or (ii) solvated alongside the surface of a metal. The metal is represented as a perfect conductor and the solvent is described by a dielectric medium. The method is able to describe the electronic structure of the molecular systems both at the uncorrelated and correlated electronic structure levels. The latter is given by the multiconfigurational self-consistent field method. We have completed sample calculations of the frequency-dependent first-order hyperpolarizability tensor and two-photon matrix elements for heterogeneously solvated CO. We find that the molecular properties depend strongly on the heterogeneous solvent configuration.
Nonlinear stability of field-reversed configurations with self-generated toroidal field
Authors: Omelchenko Y. A. , Schaffer M. J. , Parks P. B.
Published: July 2002

The field-reversed configuration (FRC) is a high-beta compact toroidal plasma confinement scheme in which the external poloidal field is reversed on the geometric axis by azimuthal (toroidal) plasma current. A quasineutral, hybrid, particle-in-cell (PIC) approach [Y. A. Omelchenko and R. N. Sudan, Phys. Plasmas 2, 2773 (1995)] is applied to study long-term nonlinear stability of computational FRC equilibria to a number of toroidal modes, including the most disruptive tilt mode. In particular, a self-generated toroidal magnetic field is found to be an important factor in mitigating the instability and preventing the confinement disruption. This is shown to be a unique FRC property resulting from the Hall effect in the regions of vanishing poloidal magnetic field. The instability-driven toroidal field stabilizes kink formation by increasing the magnetic field energy without destabilizing curvature-driven plasma motion. Finally, the tilt instability saturates due to nonlinear, finite Larmor radius (FLR) effects and plasma relaxation to a quasisteady kinetic state. During this transition the FRC is shown to dissipate a substantial amount of initially trapped flux and plasma energy. These effects are demonstrated for kinetic and fluid-like, spherical and prolate FRCs.
New solutions of the Jacobi equations for three-dimensional Poisson structures
Authors: Hernández-Bermejo Benito
Published: July 2002

A systematic investigation of the skew-symmetric solutions of the three-dimensional Jacobi equations is presented. As a result, three disjoint and complementary new families of solutions are characterized. Such families are very general, thus unifying many different and well-known Poisson structures seemingly unrelated which now appear embraced as particular cases of a more general solution. This unification is not only conceptual but allows the development of algorithms for the explicit determination of important properties such as the symplectic structure, the Casimir invariants and the Darboux canonical form, which are known only for a limited sample of Poisson structures. These common procedures are thus simultaneously valid for all the particular cases which can now be analyzed in a unified and more economic framework, instead of using a case-by-case approach. In addition, the methods developed are valid globally in phase space, thus ameliorating the usual scope of Darboux’ reduction which is only of local nature. Finally, the families of solutions found present some new nonlinear superposition principles which are characterized.
Phase diagram for Abrikosov lattice
Authors: Dutour Mathieu
Published: July 2002

Working in the framework of Abrikosov’s theory, we propose a mathematical definition of the phase diagram and prove mathematically the validity of what is usually drawn in the physical literature.
Slice imaging: A new approach to ion imaging and velocity mapping
Authors: Gebhardt Christoph R. , Rakitzis T. Peter , Samartzis Peter C. , Ladopoulos Vlassis , Kitsopoulos Theofanis N.
Published: July 2002

In this article we present a new approach to the already popular methods of ion imaging and velocity mapping. The novelty of this approach is that the speed and angular distributions are measured directly from the images without the need of inverse Abel transformation as in the conventional approaches. This is achieved by using delayed pulsed extraction of the ions following photodissociation and positioning of the nascent products. Delayed pulsed extraction causes a sufficient velocity spread in the ion cloud such that the time width of the ion packet at the detector is on the order of 500 ns. By using a narrow detector time gate (<40 ns) we are able to image only the center slice of the ion packet. The result is equivalent to that obtained by conventional methods using the inverse Abel transform, however, the artificial noise introduced by this transform is eliminated. The energy resolution of the new approach is at least comparable to that achieved with the velocity mapping technique.
Onset of thermohaline convection in a rectangular porous cavity in the presence of vertical vibration
Authors: Jounet Arnaud , Bardan Gérald
Published: July 2002

Double diffusive convection in a two-dimensional rectangular cavity with imposed vertical differences in temperature and concentration is considered in the presence of high-frequency vertical vibrations. Linear stability analysis permits the domains of oscillatory and stationary convection thresholds to be delimited. Depending on the governing parameters, vibrations are found to delay or speed up the onset of convection. The effect of vibration on the flow structure near the bifurcation is also analyzed, showing different possible behaviors. The stationary bifurcation is then studied by means of a weakly nonlinear analysis. In agreement with the symmetries present in the problem, this bifurcation is found to be pitchfork. The parametric ranges where it can be subcritical or supercritical are precisely delimited. Finally, direct numerical simulations have been performed to confirm and illustrate these findings.
Interaction of a shock wave with two counter-rotating vortices: Shock dynamics and sound production
Authors: Pirozzoli S. , Grasso F. , D’Andrea A.
Published: July 2002

In the present paper the interaction of a planar shock wave with a pair of counter-rotating vortices is studied by means of extensive numerical simulations. The main objective of the study is to characterize the shock pattern and the nature of the acoustic field. For the purpose of shedding some light into the mechanism of sound generation, the acoustic analogy developed by the present authors for the interaction of a shock wave with an isolated vortex has been extended to the case of a vortex pair, so as to obtain a closed form solution for the acoustic far field pressure in the limit of weak vortices. The effect of various parameters, such as the shock and vortex strengths and the initial vortex separation distance has been investigated, and their influence on the dynamics has been assessed. In addition, we have also considered both the case of passing and colliding pairs. In the case of a colliding pair five types of interactions are found to occur, depending upon the value of the interaction parameter, defined as the ratio of the vortex pair intensity over the shock strength. With regard to the acoustic field, the simulations have shown that the latter is the result of two mechanisms: the one directly associated to the interaction itself, and the one related to the coupling process of the vortex pair. The former mechanism is controlling if the interaction is weak, while for strong interactions the acoustic field is also controlled by the sound generated through the coupling process. For all cases the acoustic pressure field exhibits a multipolar directivity pattern with at least four sound peaks along the radial direction; however, if the vortex pair intensity is large the sound waves trailing the precursor are not clearly distinguishable. Shock wave–passing vortex pair interactions exhibit only three types of shock patterns, as in the case of shock wave–isolated vortex interaction; however, the acoustic field exhibits the same features as in the colliding case.
Scroll waves in spherical shell geometries
Authors: Chávez Francisco , Kapral Raymond , Rousseau Guillaume , Glass Leon
Published: July 2002

The evolution of scroll waves in excitable media with spherical shell geometries is studied as a function of shell thickness and outer radius. The motion of scroll wave filaments that are the locii of phaseless points in the medium and organize the wave pattern is investigated. When the inner radius is sufficiently large the filaments remain attached to both the inner and outer surfaces. The minimum size of the sphere that supports spiral waves and the maximum number of spiral waves that can be sustained on a sphere of given size are determined for both regular and random initial distributions. When the inner radius is too small to support spiral waves the filaments detach from the inner surface and form a curved filament connecting the two spiral tips in the surface. In certain parameter domains the filament is an arc of a circle that shrinks with constant shape. For parameter values close to the meandering border, the filament grows and collisions with the sphere walls lead to turbulent filament dynamics.
Three-dimensional swirling flow with a precessing vortex breakdown in a rotor-stator cylinder
Authors: Serre E. , Bontoux P.
Published: July 2002

Time-dependent swirling flows are studied inside an enclosed cylindrical rotor-stator cavity with aspect ratio H/R=4, larger than those usually considered in the literature. The study is carried out by a numerical resolution of the 3D Navier–Stokes equations, based on high-order spectral approximations. The bifurcation to the oscillatory regime is axisymmetric at Re=3700. The transition to a three-dimensional flow associated with a vortex breakdown is investigated and is shown to be brought about the centrifugal instability originated from the vertical sidewall layer. At Re=6500, the time-dependent flow is three dimensional. A modulated rotating wave with azimuthal wave number k=8 arises first in the vertical layer. The vortex breakdown evolves from an axisymmetric bubble to an S-shape type and gyrates around the axis (Ωt=1500) inducing a new time-dependent regime. This precession motion is associated to an antisymmetric k=1 mode of an instability.
State-resolved rotationally inelastic collisions of highly rotationally excited CN(A 2Π) with helium: Influence of the interaction potential
Authors: Nizamov Boris , Dagdigian Paul J. , Alexander Millard H.
Published: July 2002

A collaborative experimental and theoretical study of rotationally inelastic collisions of CN (A 2Π, v=3, N=60–62) fine-structure Λ-doublet levels with helium is presented. Experimental state-to-state rate constants were determined with an optical–optical double resonance technique. The CN radical was prepared by 193 nm photolysis of BrCN diluted in slowly flowing helium at a total pressure of ∼0.3 Torr. Specific levels of CN(A 2Π,v=3,N=60–62) were prepared by excitation with a pulsed dye laser on various rotational lines in the A 2Π–X 2Σ+ (3,0) band, and collisionally populated levels were probed after a short delay by laser fluorescence excitation in the B 2Σ+–A 2Π (3,3) band. Final state distributions (relative state-to-state rate constants) are reported. To calibrate their magnitude, absolute total removal rate constants were determined. The measured rate constants were compared with theoretical rate constants computed in a quantum scattering treatment of the dynamics based on ab initio CN(A 2Π)–He potential energy surfaces. The agreement of measured and computed rate constants is very good. The state-to-state rate constants display propensities for conservation of the Λ-doublet and fine structure labels for the range of initial levels studied. The significant differences in the final state distributions in CN(A 2Π)–He collisions and the previously studied CN(A 2Π)–Ar system are shown to arise from differences in the interaction potentials for these two systems.
A direct dynamics study of the H2 elimination from 2,5-dihydrofuran
Authors: Martínez-Núñez Emilio , Marques Jorge M. C. , Vázquez Saulo A.
Published: July 2002

The H2 elimination from 2,5-dihydrofuran (DHF) was investigated by classical trajectories propagated with the energy and forces taken directly from PM3 semiempirical calculations. The dehydrogenation of DHF was first studied by MP2, B3LYP, and CBS-QB3 calculations and the results were found to be in good agreement with experiment. The CBS-QB3 outcomes were subsequently employed to obtained specific reaction parameters for the PM3 Hamiltonian. Product energy distributions were computed for several ensembles of trajectories initiated at the transition state. Three ensembles based on the quasiclassical barrier sampling (QCBS) technique were used to investigate the problem of zero-point energy (ZPE) leakage in the classical trajectories during the formation of the H2 product molecule. The calculations predict that both the translational energy and vibrational energy distributions may be substantially influenced by nonrandom initial excitations.
Inter-basin dynamics on multidimensional potential surfaces. I. Escape rates on complex basin surfaces
Authors: Despa Florin , Berry R. Stephen
Published: July 2002

In this report, we present a general prescription for computing the escape rate of the system from a basin with full consideration of the topographical fingerprint of that basin. The method is based on a solution of the reduced Fokker–Planck equation and built up to allow the separation of the inter-basin dynamics from that of the intra-basin motion. The main result is that when local well populations thermalize within a basin, local minima, especially those of higher energy, enhance the escape rate from the basin. Also, numerical analyses lead to the inference that kinetic traps of “wrong” structures are distinctive topographical patterns which may produce kinetic properties similar to those of the primary basin, i.e., that containing the global minimum, but lie in other basins.
Defect engineering in annealed n-type GaAs epilayers using SiO2/Si3N4 stacking layers
Authors: Deenapanray Prakash N. K. , Martin A. , Jagadish C.
Published: July 2002

We have used SiO2/Si3N4 stacking layers to control the creation of defects in rapid thermally annealed epitaxial GaAs layers. Annealing at 900 °C introduces three electron traps S1 (Ec−0.23 eV), S2 (Ec−0.53 eV), and S4 (Ec−0.74 eV) in SiO2/n-GaAs. The concentrations of S1 and S4 decreased by factors of ∼28 and ∼19, respectively, in Si3N4/SiO2/n-GaAs. The overlap of a hole trap with the S2 peak in Si3N4/SiO2/n-GaAs results in an apparent decrease in the concentration of S2 by over two orders of magnitude. The lower concentration of defects in the region probed by deep level transient spectroscopy is explained by the tensile stress which the Si3N4 layer imposes on the structure during annealing. In addition to S1 and S4, hole traps H1 (Ev+0.28 eV) and H2 (Ev+0.42 eV) are observed in Si3N4/n-GaAs and SiO2/Si3N4/n-GaAs, respectively. The concentration of defects is larger by ∼1.5 times in the latter structure. SiO2/Si3N4 stacking layers can, therefore, be used to achieve spatially selective modification of GaAs-based structures using defect engineering.
Theoretical study of photochromism of N-salicylidene-α-methylbenzylamine
Authors: Zgierski Marek Z.
Published: July 2002

It is shown that the direct precursor for the formation of a photochrome in Schiff bases is a “twisted” geometry S1 state of a keto tautomer in which the O⋯N–H hydrogen bond is broken. In photochromic Schiff bases this state is close in energy to the ππ* state of the cis-ketone obtained from the initially optically excited enol species by an ultrafast excited-state proton transfer, in particular, in N-salicylidene-α-methylbenzylamine (SMB), it lies within 1 kcal/mol from the ππ* state of cis-ketone and ∼16 kcal/mol below the initial ππ* state of enol. The excited-state proton transfer in SMB is predicted to take ∼15 fs and to be nearly barrierless. It is also shown that the initial ππ* state of enol can decay via competing route to a twisted geometry nNπ* state of enol with the energy ∼20 kcal/mol below that state.
“Single ring theorem” and the disk-annulus phase transition
Authors: Feinberg Joshua , Scalettar R. , Zee A.
Published: July 2002

Recently, an analytic method was developed to study in the large N limit non-Hermitian random matrices that are drawn from a large class of circularly symmetric non-Gaussian probability distributions, thus extending the existing Gaussian non-Hermitian literature. One obtains an explicit algebraic equation for the integrated density of eigenvalues from which the Green’s function and averaged density of eigenvalues could be calculated in a simple manner. Thus, that formalism may be thought of as the non-Hermitian analog of the method due to Brézin, Itzykson, Parisi, and Zuber for analyzing Hermitian non-Gaussian random matrices. A somewhat surprising result is the so called “single ring” theorem, namely, that the domain of the eigenvalue distribution in the complex plane is either a disk or an annulus. In this article we extend previous results and provide simple new explicit expressions for the radii of the eigenvalue distribution and for the value of the eigenvalue density at the edges of the eigenvalue distribution of the non-Hermitian matrix in terms of moments of the eigenvalue distribution of the associated Hermitian matrix. We then present several numerical verifications of the previously obtained analytic results for the quartic ensemble and its phase transition from a disk shaped eigenvalue distribution to an annular distribution. Finally, we demonstrate numerically the “single ring” theorem for the sextic potential, namely, the potential of lowest degree for which the “single ring” theorem has nontrivial consequences.
Surface-emitting channel drop filters using single defects in two-dimensional photonic crystal slabs
Authors: Chutinan Alongkarn , Mochizuki Masamitsu , Imada Masahiro , Noda Susumu
Published: July 2002

We report a theoretical analysis of trapping and emission of photons by a single defect in a two-dimensional photonic crystal slab. We show that the tuning of emission wavelength is possible by selecting appropriate geometry of structure while keeping the emission efficiency maximal. The results suggest the possibility of applying it to ultra-small channel add/drop device in wavelength division multiplexed optical communications.
Planar trajectories in a monopole field
Authors: Azizi Azizollah
Published: July 2002

Motion of a Yang–Mills particle in a monopole field is proposed and planar orbits are observed. The planar orbits are studied further with some numerical analysis of the equations of motion.
Quasi-direct current plasma immersion ion implantation
Authors: Zeng Xuchu , Fu Ricky K. Y. , Kwok Dixon T. K. , Chu Paul K.
Published: July 2002

Quasi-dc (direct current) plasma immersion ion implantation (PIII) is demonstrated in the long-pulse mode. To prevent plasma extinction as a result of the sheath reaching the vacuum chamber wall in long-pulse experiments, a grounded grid is used to partition the chamber into two halves. The pulse width can be readily increased to 500 μs that is more than 10 times longer than that in typical low-pressure PIII experiments for monoenergetic implantation (ion mean free path≫sheath thickness). The electron saturation current measured by the Langmuir probe indicates that the grounded grid indeed stops the propagation of the plasma sheath. After the plasma sheath reaches the grounded grid, the pulse current drops to a smaller value indicative of the quasi-dc PIII mode. The plasma recovery time is found to be 800 μs thereby limiting the maximum pulsing frequency to below 1 kHz, and the preferred pulse duration window is between 100 and 500 μs. The secondary ion mass spectrometry profiles show that low energy ions are reduced using long pulses. This operation mode thus offers the unique advantage of a smaller low-energy ion component, that is, more monoenergetic ion distribution, and less surface damage compared to conventional short-pulse PIII. When compared to dc-PIII, this mode retains the discharge characteristics and works well for insulators.