bluuebunny/crossref_metadata_2025 huggingface dataset:
Timely detection of dynamical change in scalp EEG signals
Authors: Hively L. M. , Protopopescu V. A. , Gailey P. C.
Published: July 2002

We present a robust, model-independent technique for quantifying changes in the dynamics underlying nonlinear time-serial data. After constructing discrete density distributions of phase-space points on the attractor for time-windowed data sets, we measure the dissimilarity between density distributions via L1-distance and χ2 statistics. The discriminating power of the new measures is first tested on data generated by the Bondarenko “synthetic brain” model. We also compare traditional nonlinear measures and the new dissimilarity measures to detect dynamical change in scalp EEG data. The results demonstrate a clear superiority of the new measures in comparison to traditional nonlinear measures as robust and timely discriminators of changing dynamics.
Uniformity of semi-insulating InP wafers obtained by Fe diffusion
Authors: Fornari R. , Görög T. , Jimenez J. , De la Puente E. , Avella M. , Grant I. , Brozel M. , Nicholls M.
Published: July 2002

As-cut semiconducting InP wafers were submitted to Fe diffusion at high temperature. The subsequent electrical characterization showed that the wafers became semi-insulating, with resistivities well above 107 Ω cm and mobilities in the range of 3000–4000 cm2/V s. In this article a study on the uniformity of Fe-diffused semi-insulating InP wafers is presented. The photoconductivity, photoluminescence and resistivity mapping showed that the short range uniformity is improved with respect to typical as-grown Fe-doped InP. However, sometimes the wafers exhibit long range gradients of resistivity and luminescence intensity which are believed to be associated to temperature gradients inside the annealing furnace.
Oriented chlorine atoms as a probe of the nonadiabatic photodissociation dynamics of molecular chlorine
Authors: Alexander Andrew J. , Kim Zee Hwan , Kandel S. Alex , Zare Richard N. , Rakitzis T. Peter , Asano Yukako , Yabushita Satoshi
Published: July 2002

Molecular chlorine was photolyzed using circularly polarized radiation at 310 and 330 nm, and orientation moments of the chlorine-atom Cl(2Pj) photofragment distributions were measured by resonance enhanced multiphoton ionization using circularly polarized light with Doppler resolution. The product atoms were found to be strongly oriented in the laboratory as a result of both incoherent and coherent dissociation mechanisms, and the orientation moments contributed by each of these mechanisms have been separately measured. The experimental results can be explained by nonadiabatic transitions from the C 1Π1u state to higher states of Ω=1u symmetry, induced by radial derivative coupling. Ab initio calculations indicate strong Rosen–Zener–Demkov noncrossing-type radial derivative couplings between states of 1u symmetry. The observed angular distribution (β parameter) indicates that 88% of Cl*(2P1/2) fragments produced at 310 nm originate from a perpendicular transition to the C state. The orientation measurements suggest that 67±16% of Cl*35(2P1/2) atoms dissociate via the 1u(3Σ1u+) state, and 21±6% dissociate via the 1u(3Δ1u) state.
Oswald ripening and shape transitions of self-assembled PbSe quantum dots on PbTe (111) during annealing
Authors: Raab A. , Springholz G.
Published: July 2002

The thermal stability of faceted self-assembled PbSe quantum dots during annealing is investigated. With increasing annealing time, the dot density is found to decrease rapidly with a simultaneous increase of the average island volumes. In addition, a shape transition from pyramidal islands to truncated pyramids is observed for islands exceeding a critical height of 160 Å. The evolution of island volumes and densities is consistent with Oswald ripening by interface-reaction-limited mass transfer. This is a clear indication that the as-grown islands do not represent an equilibrium structure and that their narrow size dispersion is a purely kinetic effect.
High-mobility AlGaN/GaN heterostructures grown by molecular-beam epitaxy on GaN templates prepared by hydride vapor phase epitaxy
Authors: Manfra M. J. , Pfeiffer L. N. , West K. W. , Stormer H. L. , Baldwin K. W. , Hsu J. W. P. , Lang D. V. , Molnar R. J.
Published: July 2002

We report on the growth and transport properties of high-mobility two-dimensional electron gases (2DEGs) confined at the AlGaN/GaN interface grown by plasma-assisted molecular-beam epitaxy on GaN templates prepared by hydride vapor phase epitaxy. We have grown samples over a broad range of electron densities ranging from ns=6.9×1011 to 1.1×1013 cm−2, and at T=4.2 K, observe a peak mobility of 53 300 cm2/V s at a density of 2.8×1012 cm−2. Magnetotransport studies on these samples display exceptionally clean signatures of the quantum Hall effect. Our investigation of the dependence of 2DEG mobility on carrier concentration suggests that the low-temperature mobility in our AlGaN/GaN heterostructures is currently limited by the interplay between charged dislocation scattering and interface roughness.
Effects of growth conditions on the incorporation of oxygen in AlGaN layers grown by plasma assisted molecular beam epitaxy
Authors: Elsass C. R. , Mates T. , Heying B. , Poblenz C. , Fini P. , Petroff P. M. , DenBaars S. P. , Speck J. S.
Published: July 2002

Using secondary ion mass spectroscopy we have shown that oxygen incorporation in AlGaN films is dependent upon the III/V growth conditions and the growth temperature of the films. AlGaN films grown under excess group III conditions (Ga-rich) exhibited step flow growth and at least a factor of 3 less oxygen incorporation than films grown under excess group V (N-rich conditions). We found that oxygen incorporation into AlGaN decreases as the growth temperature is increased. The lowest oxygen levels were achieved by growing at 750 °C under Ga-rich growth conditions. Possible sources of unwanted oxygen are discussed.
Dependence of polycrystalline silicon thin-film transistor characteristics on the grain-boundary location
Authors: Kimura Mutsumi , Inoue Satoshi , Shimoda Tatsuya , Eguchi Tsukasa
Published: July 2002

Dependence of transistor characteristics on the grain-boundary location in polycrystalline silicon (poly-Si) thin-film transistors (TFTs) has been analyzed using device simulation. In the linear region, degradation is similar wherever the grain boundary is located. On the other hand, in the saturation region, degradation is less when the grain boundary is in the pinch-off region near the drain edge and degradation is similar when the grain boundary is elsewhere. Although this dependence is similar to the dependence on the trap location in single-crystal silicon transistors, the mechanism is different. This dependence in poly-Si TFTs is because the coulombic potential barrier caused by the grain boundary is lowered in the high electric field in the pinch-off region. This is a kind of Poole–Frenkel effect.
Time-autocorrelation in scanning-tunneling-microscope-induced photon emission from metallic surface
Authors: Silly Fabien , Charra Fabrice
Published: July 2002

We analyze the statistical properties of the photons emitted at the junction of a scanning tunneling microscope by time-autocorrelated two-photon counting measurements, with time resolutions down to the nanosecond scale. In the case of a gold surface at air, we show that the tip-induced luminescence exhibits a clear bunching phenomenon at a 10 ns time scale. Moreover, this two-photon time correlation appears to be strongly bias dependent, with an onset of the bunching behavior near 2.5 V.
Variational principle for optimal accelerated neutralized flow
Authors: Fruchtman A. , Fisch N. J.
Published: July 2002

Minimizing the energy deposited in the electron current in neutralized flows, such as in the Hall thruster, is examined. Modifying the electron current along the channel by inserting emitting electrodes, can enhance the efficiency. By employing variational methods, an optimal electron current distribution is found. The efficiency enhancement due to this effect, however, is shown to be small.
Backward switching phenomenon from field forced ferroelectric to antiferroelectric phases in antiferroelectric PbZrO3 thin films
Authors: Bharadwaja S. S. N. , Krupanidhi S. B.
Published: July 2002

Antiferroelectric materials (example: lead zirconate and modified lead zirconate stannate), in which a field-induced ferroelectric phase transition is feasible due to a small free energy difference between the ferroelectric and the antiferroelectric phases, are proven to be very good candidates for applications involving actuation and high charge storage devices. The property of reverse switching from the field-induced ferroelectric to antiferroelectric phases is studied as a function of temperature, applied electric field, and sample thickness in antiferroelectric lead zirconate thin films deposited by pulsed excimer laser ablation. The maximum released charge density was 22 μC/cm2 from a stored charge density of 36 μC/cm2 in a 0.55 μ thick lead zirconate thin film. This indicated that more than 60% of the stored charge could be released in less than 7 ns at room temperature for a field of 200 kV/cm. The content of net released charge was found to increase with increasing field strength, whereas with increasing temperature the released charge was found to decrease. Thickness-dependent studies on lead zirconate thin films showed that size effects relating to extrinsic and intrinsic pinning mechanisms controlled the released and induced charges through the intrinsic switching time. These results proved that antiferroelectric PZ thin films could be utilized in high-speed charge decoupling capacitors in microelectronics applications.
Direct interband transitions in tris-(8-hydroxyquinoline) aluminum thin films
Authors: Xu X. L. , Xu Z. , Hou Y. B. , Chen X. H. , Wang Z. J. , Zhang X. Q. , Xu X. R. , Lau S. P. , Tay B. K.
Published: July 2002

The electroluminescent properties of three different device structures (A:ITO/SiO2/Alq3/SiO2/Al, B:ITO/Alq3/SiO2/Al, and C:ITO/SiO2/Alq3/Al) based on the tris-(8-hydroxyquinoline) aluminum (Alq3) were investigated. A blue electroluminescence at 457 nm was obtained from device (A) and (B), and the green emission at 518 nm was obtained from device (C). It is generally agreed that the green emission originates from the recombination of the singlet excitons. The blue emission, here, is attributed to the direct transitions between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital of Alq3. This is due to the electric field-induced excitons dissociation and the space charge accumulation at the interface. The high internal electric field enhances the dissociation of neutral singlet excitations into LUMO states and inhibits the formation of the singlet excitons, therefore enhances the probability for direct interband transitions of the relaxed carriers. The intensity of the blue emission is dependent on the operating frequency. This indicates that space charge accumulation time and effective internal electric field are responsible for the blue emission intensity.
Atom-scale optical determination of Si-oxide layer thickness during layer-by-layer oxidation: Theoretical study
Authors: Nakayama Takashi , Murayama Misao
Published: July 2002

Optical anisotropy spectra of SiO2/Si(001) interfaces were theoretically investigated based on the sp3s* tight-binding calculation. In the spectra, we found three types of optical transitions originating from the E1 and E2 transitions of bulk Si, the interface Si–Si bonds, and the dangling-bond states at the interface. It was shown that the sign of these transitions oscillates during the layer-by-layer oxidation, which indicates that by counting the oscillation one can determine the layer thickness of oxidized Si layers in an atomic scale.
Laser emission in periodically modulated polymer films
Authors: Rocha Licinio , Dumarcher Vincent , Denis Christine , Raimond Paul , Fiorini Céline , Nunzi Jean-Michel
Published: July 2002

We report on the realization of a compact distributed feedback laser using luminescent polymer films where the optical feedback is provided by Bragg diffraction on an index grating. Permanent modulation of the polymer refractive index is achieved using an original technique for photoinduced patterning of surface relief grating, using laser-controlled mass-transport in azoaromatic polymers. We describe the fabrication of such surface gratings and show the laser emission properties resulting from a transversal one-photon pumping of the sinusoidally modulated polymer films upcovered with a luminescent-dye-doped film. Control of the laser wavelength by the grating pitch is evidenced.
Efficiency of different numerical methods for solving Redfield equations
Authors: Kondov Ivan , Kleinekathöfer Ulrich , Schreiber Michael
Published: July 2002

The numerical efficiency of different schemes for solving the Liouville–von Neumann equation within multilevel Redfield theory has been studied. Among the tested algorithms are the well-known Runge–Kutta scheme in two different implementations as well as methods especially developed for time propagation: the short iterative Arnoldi, Chebyshev, and Newtonian propagators. In addition, an implementation of a symplectic integrator has been studied. For a simple example of a two-center electron transfer system we discuss some aspects of the efficiency of these methods to integrate the equations of motion. Overall, for time-independent potentials the Newtonian method is recommended. For time-dependent potentials implementations of the Runge–Kutta algorithm are very efficient.
Staged Z pinch for controlled fusion
Authors: Ney P. , Rahman H. U. , Wessel F. J. , Rostoker N.
Published: July 2002

A staged Z pinch is considered in which an annular plasma shell made of a high Z material like Kr implodes onto a coaxial plasma target made of a low Z material like deuterium or a deuterium–tritium mixture. The target plasma could be made either by exploding a cryogenically extruded fiber or by filling the annular shell with a gas puff or a plasma puff. Modeling is performed with a two-dimensional (2D) radiation-MHD (magnetohydrodynamic) code. A parameter study is made to determine the sensitivity of this configuration to initial conditions of the shell and the target plasmas. An axial magnetic field is essential for a stable implosion and efficient energy coupling to the final load. Using a 50–50 mixture of deuterium–tritium as a target plasma, the fusion energy gain is optimized by adjusting the initial parameters. The calculations are based on the parameters of the University of California Irvine Z-pinch facility which has a maximum energy storage of 50 kJ.
Low temperature manipulation of big molecules in constant height mode
Authors: Moresco Francesca , Meyer Gerhard , Rieder Karl-Heinz , Tang Hao , Gourdon André , Joachim Christian
Published: July 2002

The possibility of extending the lateral manipulation process performed with the tip of a scanning tunneling microscope at low temperature to large molecules is demonstrated. Single Cu–TBPP molecules deposited on a Cu substrate were manipulated by means of the interaction between the molecule and the tip. Due to the complicated structure of the molecules and to their high conductance, we have explored the possibilities of performing controlled lateral manipulation at constant height. On Cu(111) this method makes possible translation combined with rotation of the molecules on the surface.
Reversible adsorption of calcium ions by imprinted temperature sensitive gels
Authors: Alvarez-Lorenzo Carmen , Guney Orhan , Oya Taro , Sakai Yasuzo , Kobayashi Masatoshi , Enoki Takashi , Takeoka Yukikazu , Ishibashi Toru , Kuroda Kenichi , Tanaka Kazunori , Wang Guoqiang , Grosberg Alexander Yu. , Masamune Satoru , Tanaka Toyoichi
Published: July 2002

With the aim of developing polymeric gels sensitive to external stimuli and able to reversibly adsorb and release divalent ions, copolymer gels of N-isopropylacrylamide (NIPA) and methacrylic (MAA) monomers were prepared. We chose calcium as a target divalent ion. Two MAAs form a complex with a calcium ion, and the NIPA component allows the polymers to swell and shrink reversibly in response to temperature. The adsorbing site develops an affinity to target ions when the adsorbing molecules come into proximity, but when they are separated, the affinity diminishes. To enhance the affinity to calcium, an imprinting technique was applied using Ca2+ and Pb2+ ions as templates in methylsulfoxide and dioxane media, respectively. The adsorption capacity of the imprinted gels was compared with that of the nonimprinted gels, and the effects of the templates, the solvents, and the amount of methacrylic monomers used in the synthesis and the medium temperature over the Ca2+ adsorption capacity of the gels from aqueous solutions were evaluated. The analysis of the adsorption revealed that (a) the adsorption can be described by the Langmuir isotherms; (b) there is an approximately linear relationship between saturation and methacrylic monomer concentration; (c) the affinity depends on the degree of gel swelling or shrinkage that can be switched on and off by temperature; (d) in the shrunken state, the affinity depends approximately linearly on the MAA concentration in the imprinted gels, whereas in the nonimprinted gels it is proportional to the square of MAA concentration; (e) the imprinted gels adsorb more than the nonimprinted gels when MAA concentration is less than that of permanent cross linkers. The success of imprinting of CaMAA2 and PbMAA2 complex is evidence for memory of such complex onto the weakly cross-linked gel.
Ion acoustic wave velocity measurement of the concentration of two ion species in a multi-dipole plasma
Authors: Hala A. M. , Hershkowitz N.
Published: July 2002

The concentration of two species in a multi-dipole plasma was determined by measuring the ion acoustic wave group velocity and the electron temperature. The wave was launched from a grid immersed in the plasma and was detected by a Langmuir probe. Electron temperature was found separately from an I–V characteristic trace. The measurements were performed in helium/xenon and argon/xenon plasmas. Typical parameters of the plasma were Te∼0.5–3 eV, density 1010 cm−3, plasma potential of 3–5 V, and pressure range from 1 to 20 mTorr. The accuracy of the measurement was from 2% to 4% depending on the mass difference between the two species and how accurately the group velocity and electron temperature are measured.
Three-dimensional simulation of carrier transport effects in the base of rear point contact silicon solar cells
Authors: Kotsovos K. , Misiakos K.
Published: July 2002

A method is proposed for a thorough analysis of rear point contact solar cell structures. The method is based on the numerical solution through Fast fourier transform of the minority and majority carrier transport equations in three dimensions in the base of the cell. Their illuminated current–voltage characteristics are simulated and the series resistance of the cell is calculated. Results are presented which show the influence of the rear contact size and the majority transport in the base to the solar cell performance.
Control of the electric-field profile in the Hall thruster
Authors: Fruchtman A. , Fisch N. J. , Raitses Y.
Published: July 2002

Control of the electric-field profile in the Hall thruster through the positioning of an additional electrode along the channel is shown theoretically to enhance the efficiency. The reduction of the potential drop near the anode by use of the additional electrode increases the plasma density there, through the increase of the electron and ion transit times, causing the ionization in the vicinity of the anode to increase. The resulting separation of the ionization and acceleration regions increases the propellant and energy utilizations. An abrupt sonic transition is forced to occur at the axial location of the additional electrode, accompanied by the generation of a large (theoretically infinite) electric field. This ability to generate a large electric field at a specific location along the channel, in addition to the ability to specify the electric potential there, allows us further control of the electric-field profile in the thruster. In particular, when the electron temperature is high, a large abrupt voltage drop is induced at the vicinity of the additional electrode, a voltage drop that can comprise a significant part of the applied voltage.
Material properties of indium oxide films prepared by oxygen ion assisted deposition
Authors: Cho Jun-Sik , Yoon Ki Hyun , Koh Seok-Keun
Published: July 2002

Transparent conducting In2O3 films were deposited on glass substrates by an oxygen ion assisted deposition. Microstructure, chemical composition, electrical, and optical properties of the deposited films were investigated as a function of arrival ratio and ion energy. At 60 eV oxygen ion bombardment, the microstructure was changed from grain structure to domain structure with increasing the arrival ratio. At 500 eV oxygen ion bombardment, the only grain structure was observed, irrespective of the arrival ratio. The ion assisted films had higher oxygen content than the evaporated film without oxygen ion bombardment because of the enhanced reactivity of oxygen due to ionization. Electrical and optical properties of the ion assisted films were significantly dependent on the change of microstructure and chemical composition with varying the arrival ratio and the ion energy. Compared to the films having the grain structure, the films with the domain structure had larger carrier concentration and mobility due to a low grain boundary density acting as a trap and a reduced grain boundary scattering. The high transmittance (≈89%) in the visible spectral range was obtained in the ion assisted films with the large oxygen content and the small surface roughness.
Budgets of Reynolds stress, kinetic energy and streamwise enstrophy in viscoelastic turbulent channel flow
Authors: Dimitropoulos Costas D. , Sureshkumar R. , Beris Antony N. , Handler Robert A.
Published: July 2002

The budgets of the Reynolds stress, turbulent kinetic energy and streamwise enstrophy are evaluated through direct numerical simulations for the turbulent channel flow of a viscoelastic polymer solution modeled with the Finitely Extensible Nonlinear Elastic with the Peterlin approximation (FENE-P) constitutive equation. The influence of viscoelasticity on the budgets is examined through a comparison of the Newtonian and the viscoelastic budgets obtained for the same constant pressure drop across the channel. It is observed that as the extensional viscosity of the polymer solution increases there is a consistent decrease in the production of Reynolds stress in all components, as well as in the other terms in the budgets. In particular, the effect of the flow elasticity, which is associated with the reduction in the intensity of the velocity-pressure gradient correlations, potentially leads to a redistribution of the turbulent kinetic energy among the streamwise, the wall-normal and the spanwise directions. In this work, we also show that in the presence of viscoelasticity there is a significant reduction in all components of the production of streamwise enstrophy. This is consistent with a proposed mechanism for polymer-induced drag reduction through the inhibition of vortex stretching by the high extensional viscosity of the polymer solution.
Granular segregation in the double-cone blender: Transitions and mechanisms
Authors: Alexander Albert W. , Shinbrot Troy , Muzzio Fernando J.
Published: July 2002

We investigate granular segregation in one of the most common industrial devices used in granular processing: the double-cone blender. We report several new and spontaneously occurring segregation patterns, including stripes, bands, and a symmetry-breaking state in which one species vacates half the tumbler. By varying the tumbling speed along with particle size and size ratio, we find that the transitions between segregated patterns are extremely sharp: Changes in fill level or speed of under one percent are sufficient to produce a reproducible qualitative change in the observed pattern. We show that the several distinct segregation patterns observed experimentally can be reproduced from a simplified model in which outward rolling on the convective granular cascade competes against inertial motion of rapidly moving large particles. Finally, we identify a cutoff particle size ratio above which large particles become buried in the cascading flow, and segregation appears to cease.
Surface-wave-produced plasmas in a diffusion-controlled regime
Authors: Makasheva K. , Shivarova A.
Published: July 2002

A numerical model of surface-wave-produced plasmas in a diffusion-controlled regime is presented in this study. With the wide gas-pressure range (p=0.05–5 Torr) considered in a combination with a frequency (f=144 MHz) in the rf-range, the model covers discharge maintenance under different conditions with respect to mechanisms of electron heating and discharge self-consistency. The precise description of the wave dispersion properties and field distribution in inhomogeneous collisional plasmas included in the electrodynamical part of the model and the specification of the discharge kinetics in its gas-discharge part provide the quantitative description of the discharge production. The model results in the self-consistent structure of the discharge composed by interrelated variations, along the discharge, of wave number, damping rate, field distribution, power absorbed in average by an electron, plasma density, electron temperature, and Joule heating. The total absorbed power finally obtained in its relation to the discharge length is the power which should be externally applied for the discharge maintenance.
Symmetry specificity in the unimolecular decay of the Cl−⋯CH3Cl complex: Two-mode quantum calculations on a coupled-cluster [CCSD(T)] potential energy surface
Authors: Schmatz S. , Botschwina P. , Hauschildt J. , Schinke R.
Published: July 2002

The decay of resonance states in the complex-forming nucleophilic substitution reaction Cl−+CH3Cl is investigated by means of two-dimensional quantum mechanical calculations on a coupled-cluster [CCSD(T)] potential energy surface. The dynamics calculations employ Radau coordinates to describe the two C–Cl stretching degrees of freedom, filter diagonalization, and an absorbing (optical) potential. The resonance widths and the corresponding decay rates vary by several orders-of-magnitude, reflecting the large degree of separability of the intramolecular and the intermolecular mode. The decay is found to be strongly symmetry specific: For energies above the reaction barrier, the smallest rates of the ungerade states are about two orders-of-magnitude smaller than the smallest rates of the gerade states. An explanation is given in terms of an adiabatic model formulated in hyperspherical coordinates. The nonadiabatic coupling elements, which control the energy transfer between the two modes and therefore determine the decay rate, are substantially larger for the gerade states. Ultimately, the differences are caused by the different structures of the gerade and the ungerade wave functions at the barrier.
Gyrokinetic turbulence simulations with kinetic electrons
Authors: Chen Yang , Parker Scott
Published: July 2002

Gyrokinetic turbulence simulations are presented with full drift-kinetic electron dynamics including both trapped and passing particle effects. This is made possible by using a generalization of the split-weight scheme [I. Manuilskiy and W. W. Lee, Phys. Plasmas 7, 1381 (2000)] that allows for a variable adiabatic part, as well as use of the parallel canonical momentum formulation. Linear simulations in shearless slab geometry and nonlinear simulations using representative tokamak parameters demonstrate the applicability of this generalized split-weight scheme to the turbulence transport problem in the low β regime [β(mi/me)⩽1]. The issues relating to difficulties at higher β, and initial three-dimensional toroidal simulations results will be discussed.
C
4
N
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The first CnN radical with stable cyclic isomers
Authors: Ding Yi-hong , Liu Jin-long , Huang Xu-ri , Li Ze-sheng , Sun Chia-chung
Published: July 2002

The potential-energy surface of the interstellar molecule C4N is explored at the B3LYP/6-311G(d) level of theory. Thirteen isomers including the linear, three-membered ring, four-membered ring, A-like, Y-like, and cage-like structures are located as minima connected by 23 interconversion transition states. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by single-point energy calculations at the MP4SDTQ, CCSD(T), and QCISD(T) levels with the 6-311G(2df) basis set. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d) level, the lowest-lying isomer is a linear structure CCCCN 1 followed by a CCC three-membered ring structure 4 with exocyclic CCN bonding that lies only 2.8 kcal/mol higher. The third and fourth low-lying isomers possess a CCC three-membered ring structure 5 with exocyclic CNC bonding at 21.4 kcal/mol and a linear structure CCCNC 2 at 23.4 kcal/mol, respectively. All the four isomers 1, 2, 4, and 5 and another high-lying isomer 3 with a linear CCNCC structure at 62.5 kcal/mol are shown to have considerable kinetic stability towards isomerization and dissociation. Thus, all the five isomers may be experimentally observable. Possible formation of these five stable C4N isomers in interstellar space is discussed. Finally, our calculations show that the Møller–Plesset methods fail to predict even qualitatively the energetic properties between the four low-lying isomers 1, 2, 4, and 5, in comparison with the QCISD and CCSD results. This paper indicates that C4N may be the first interstellar molecule with stable low-lying cyclic isomers among the CnN radical series to be detectable in near future. The results presented in this paper may provide useful information for future laboratory and interstellar identification of various C4N isomers.
Electronic coupling between Watson–Crick pairs for hole transfer and transport in desoxyribonucleic acid
Authors: Voityuk Alexander A. , Jortner Joshua , Bixon M. , Rösch Notker
Published: July 2002

Electronic matrix elements for hole transfer between Watson–Crick pairs in desoxyribonucleic acid (DNA) of regular structure, calculated at the Hartree–Fock level, are compared with the corresponding intrastrand and interstrand matrix elements estimated for models comprised of just two nucleobases. The hole transfer matrix element of the GAG trimer duplex is calculated to be larger than that of the GTG duplex. “Through-space” interaction between two guanines in the trimer duplexes is comparable with the coupling through an intervening Watson–Crick pair. The gross features of bridge specificity and directional asymmetry of the electronic matrix elements for hole transfer between purine nucleobases in superstructures of dimer and trimer duplexes have been discussed on the basis of the quantum chemical calculations. These results have also been analyzed with a semiempirical superexchange model for the electronic coupling in DNA duplexes of donor (nuclobases)–acceptor, which incorporates adjacent base–base electronic couplings and empirical energy gaps corrected for solvation effects; this perturbation-theory-based model interpretation allows a theoretical evaluation of experimental observables, i.e., the absolute values of donor–acceptor electronic couplings, their distance dependence, and the reduction factors for the intrastrand hole hopping or trapping rates upon increasing the size of the nucleobases bridge. The quantum chemical results point towards some limitations of the perturbation-theory-based modeling.
Hybrid approach for including electronic and nuclear quantum effects in molecular dynamics simulations of hydrogen transfer reactions in enzymes
Authors: Billeter Salomon R. , Webb Simon P. , Iordanov Tzvetelin , Agarwal Pratul K. , Hammes-Schiffer Sharon
Published: July 2002

A hybrid approach for simulating proton and hydride transfer reactions in enzymes is presented. The electronic quantum effects are incorporated with an empirical valence bond approach. The nuclear quantum effects of the transferring hydrogen are included with a mixed quantum/classical molecular dynamics method in which the hydrogen nucleus is described as a multidimensional vibrational wave function. The free energy profiles are obtained as functions of a collective reaction coordinate. A perturbation formula is derived to incorporate the vibrationally adiabatic nuclear quantum effects into the free energy profiles. The dynamical effects are studied with the molecular dynamics with quantum transitions (MDQT) surface hopping method, which incorporates nonadiabatic transitions among the adiabatic hydrogen vibrational states. The MDQT method is combined with a reactive flux approach to calculate the transmission coefficient and to investigate the real-time dynamics of reactive trajectories. This hybrid approach includes nuclear quantum effects such as zero point energy, hydrogen tunneling, and excited vibrational states, as well as the dynamics of the complete enzyme and solvent. The nuclear quantum effects are incorporated during the generation of the free energy profiles and dynamical trajectories rather than subsequently added as corrections. Moreover, this methodology provides detailed mechanistic information at the molecular level and allows the calculation of rates and kinetic isotope effects. An initial application of this approach to the enzyme liver alcohol dehydrogenase is also presented.
Discovery of magnetite in the exhausted material from a diesel engine
Authors: Abdul-Razzaq W. , Gautam M.
Published: July 2002

Magnetite was detected in the particulate matter collected from diesel engine exhaust using a total exhaust dilution tunnel. This discovery is very important in determining the health effects of exposure to magnetite or its interaction with static magnetic fields or low-frequency electromagnetic fields. Magnetite is the best absorber of microwave radiation of any biological material in the 0.5–10 GHz frequency range through the process of electromagnetic resonance. This includes the frequencies that are normally used in the cellular telephone industry.
Gallium arsenide crystalline nanorods grown by molecular-beam epitaxy
Authors: Lee Hae Gwon , Jeon Hee Chang , Kang Tae Won , Kim Tae Whan
Published: July 2002

Gallium arsenide (GaAs) crystalline nanorods were grown by molecular-beam epitaxy (MBE). Scanning electron microscopy, transmission electron microscopy, and energy dispersive x-ray fluorescence measurements showed that the grown GaAs nanorods were straight single crystals with diameters between 70 and 80 nm, lengths of up to 5 μm, and were doped with Si impurity. The formation mechanism of the Si-doped GaAs crystalline nanorods is described. These results indicate that Si-doped GaAs crystalline nanorods can be grown by using the MBE technique and that the nanorods hold promise for potential applications in next-generation electronic and optoelectronic devices.
Stability of zirconium silicate films on Si under vacuum and O2 annealing
Authors: Morais J. , da Rosa E. B. O. , Miotti L. , Pezzi R. P. , Baumvol I. J. R. , Rotondaro A. L. P. , Bevan M. J. , Colombo L.
Published: July 2002

The effect of postdeposition annealing in vacuum and in dry O2 on the atomic transport and chemical stability of chemical vapor deposited ZrSixOy films on Si is investigated. Rutherford backscattering spectrometry, narrow nuclear resonance profiling, and low energy ion scattering spectroscopy were used to obtain depth distributions of Si, O, and Zr in the films. The chemical environment of these elements in near-surface and near-interface regions was identified by angle-resolved x-ray photoelectron spectroscopy. It is shown that although the interface region is rather stable, the surface region presents an accumulation of Si after thermal annealing.
Free energies based on generalized entropies and H-theorems for nonlinear Fokker–Planck equations
Authors: Shiino Masatoshi
Published: July 2002

The relationship between H-theorems and free energies is studied on the basis of generalized entropies. Two kinds of nonlinear Fokker–Planck equations with different nonlinear diffusion terms that exhibit the power-law-type equilibrium distributions of Tsallis thermostatistics are investigated from the viewpoint of nonequilibrium free energies and stability analysis of their solutions. Using the generalized entropies Liapunov functions are constructed to show H-theorems, which ensure uniqueness of and convergence to the equilibrium distributions of the nonlinear Fokker–Planck equations.
Influence of sensitizer on organic electroluminescence
Authors: Chen Zhijian , Yu Junsheng , Sakuratani Yuuki , Li Minrun , Sone Masato , Miyata Seizo , Watanabe Toshiyuki , Wang Xiaoqing , Sato Hisaya
Published: July 2002

We synthesized a polymer, poly(N,N′-diphenyl-N,N′-bis(4-methylphenyl)-1,4-phenylenediamine1,3-diisopropenylbenzene) (PDAIPB), of which the energy of the highest occupied molecular orbital was measured to be 5.6 eV below vacuum level. In this article, PDAIPB was used as a hole transporter in a light emitting diode, with tri(8-hydroxyquinoline)-aluminum (Alq3) as an electron transporter and emitting layer to ameliorate the stability of the device, and fullerene doped in the hole transporting layer as a sensitizer to study the influence of sensitizer on organic electroluminescence (EL). Internal EL quantum efficiency as high as 7.8% was obtained under an applied electric field of 1.2×106 V/cm and doping fullerene concentration of 0.9 wt %. A function for EL quantum efficiency in terms of the dopant concentration is derived. The theoretical analysis is consistent with the experimental results.
Harmonic distortion and intermodulation products in the microstrip amplifier based on a superconducting quantum interference device
Authors: Mück Michael , Clarke John
Published: July 2002

The harmonic distortion and intermodulation products have been measured for a microstrip amplifier based on a dc superconducting quantum interference device. When the flux bias is adjusted to maximize the gain, in general both second harmonic and a series of odd harmonics are observed. The magnitude of the third harmonic is in good agreement with a simple model. The magnitude of the second harmonic, however, depends critically on the flux bias and is generally a minimum when the gain is slightly less than the maximum value. Similar results are found for the two lowest order intermodulation products.
The Cl-35−–H2 and Cl-35−–D2 anion complexes: Infrared spectra and radial intermolecular potentials
Authors: Wild D. A. , Weiser P. S. , Bieske E. J. , Zehnacker A.
Published: July 2002

Rotationally resolved mid-infrared spectra of the Cl-35–H2 and Cl-35–D2 anion complexes are measured in the regions associated with the H2 and D2 stretch vibrations. The Cl-35–H2 spectrum contains a single Σ–Σ transition assigned to the more abundant ortho H2 containing species. The corresponding Cl-35–D2 spectrum consists of two overlapping Σ–Σ transitions whose origins are separated by 0.24 cm−1, and which are due to absorptions by complexes containing para and ortho D2. The spectra are consistent with linear equilibrium structures for Cl−–H2 and Cl−–D2, although zero-point bending vibrational excursions are expected to be substantial. Ground state vibrationally averaged intermolecular separations between Cl− and the diatomic center-of-mass are deduced to be 3.195±0.003 Å (35Cl−–H2) and 3.159±0.002 Å (35Cl−–D2). Vibrational excitation of the diatomic core profoundly affects the intermolecular interaction and leads to contractions of 0.118 Å (35Cl−–H2) and 0.078 Å (35Cl−–D2) in the vibrationally averaged intermolecular separations. Effective one-dimensional radial potential energy curves are developed. Their form near the equilibrium separation is determined by Rydberg–Klein–Rees inversion of the spectroscopic data, and at longer ranges by averaging the dominant long range electrostatic and induction potentials over the angular motion of the atom–diatomic system. On the basis of these potentials the dissociation energies for Cl-35–H2(o), Cl-35–D2(p), and Cl-35–D2(o) are estimated as 488, 499, and 559 cm−1.
High-order laser harmonics detection in the EUV and soft x-ray spectral regions
Authors: Poletto Luca , Tondello Giuseppe , Villoresi Paolo
Published: July 2002

The design and performance of a spectroscopic instrument able to simultaneously measure spectral features, angular divergence, and conversion efficiency of high-order harmonics generated by an ultrashort pulsed laser focused onto a gas jet are presented. It combines an advanced optical setup with a EUV-sensitive bidimensional detector. The grazing-incidence spectrometer consists of two optical elements: a toroidal mirror and a spherical variable-line-spaced grating with flat-field properties. The mirror focuses the radiation tangentially on the grating entrance plane, in order to increase the flux collected by the grating. The grating has a variable groove spacing with parameters optimized to produce an almost flat focal surface in the 5–40 nm region. A 40 mm diameter microchannel plate intensifier is used to acquire the spectrum. The mirror does not compensate for the grating astigmatism, so the length of the spectral lines on the spectrum is proportional to the sagittal angular divergence of the high-order harmonic emission. The absolute response of the system was measured in order to obtain the conversion efficiency of the harmonic generation process. Some experimental examples reported here show very high sensitivity, which makes the system able to work in the single-shot operation, thus allowing us to follow the dynamic evolution of the harmonic generation process.
Anti-cooperativity in hydrophobic interactions: A simulation study of spatial dependence of three-body effects and beyond
Authors: Shimizu Seishi , Chan Hue Sun
Published: July 2002

To better understand the energetics of hydrophobic core formation in protein folding under ambient conditions, the potential of mean force (PMF) for different three-methane configurations in an aqueous environment is computed by constant-pressure Monte Carlo sampling using the TIP4P model of water at 25 °C under atmospheric pressure. Whether the hydrophobic interaction is additive, cooperative or anti-cooperative is determined by whether the directly simulated three-methane PMF is equal to, more favorable, or less favorable than the sum of two-methane PMFs. To ensure that comparisons between PMFs are physically meaningful, a test-particle insertion technique is employed to provide unequivocal correspondence between zero PMF value and the nonexistent inter-methane interaction (zero reference-state free energy) experienced by a pair of methanes infinitely far apart. Substantial deviations from pairwise additivity are observed. Significantly, a majority of the three-methane configurations investigated exhibit anti-cooperativity. Previously simulated three-methane PMFs were defined along only one single coordinate. In contrast, our technique enables efficient computation of a three-methane PMF that depends on two independent position variables. The new results show that the magnitude and sign of nonadditivity exhibit a prominent angular dependence, highlighting the complexity of multiple-body hydrophobic interactions. Packing consideration of crystal-like constructs of an infinite number of methanes and analysis of methane sublimation and hydration data suggest that anti-cooperativity may be a prevalent feature in hydrophobic interactions. Ramifications for protein folding are discussed.
On the mechanism of laser-induced desorption–ionization of organic compounds from etched silicon and carbon surfaces
Authors: Alimpiev Sergey , Nikiforov Sergey , Karavanskii Vladimir , Minton Tim , Sunner Jan
Published: July 2002

The laser-induced desorption/ionization of organic compounds from etched carbon and silicon substrate surfaces was investigated. Two different etching procedures were used. Silicon surfaces were etched either by galvanostatic anodization to produce porous silicon or by a hyperthermal (∼5 eV) F-atom beam to produce nonporous silicon. Atomic force microscopy (AFM) images showed that both etching procedures yielded surfaces with sub-micrometer structures. Highly oriented pyrolytic graphite was etched with hyperthermal O atoms. A 337 nm ultraviolet (UV) laser and a 3.28 μm infrared (IR) laser were used for desorption. Analytes were deposited on the substrates either from the liquid or the gas phase. Mass spectra were obtained provided that three conditions were fulfilled. First, sufficient laser light had to be absorbed. When the IR laser was employed, a thin physisorbed solvent layer was required for sufficient laser light absorption to occur. Though the required fluence of IR and UV light differed by a factor of about 20, the calculated maximum surface temperatures were similar, about 1000 K. The second requirement was that the substrate had a “rough” surface. The third requirement, for the observation of protonated analytes, was that the aqueous pKa-value of the analyte be larger than about 4. These observations support the conclusion that the desorption–ionization mechanisms of analytes from porous and nonporous surfaces are very similar or essentially the same.
An octopole electrodynamic balance for three-dimensional microparticle control
Authors: Zheng F. , Qu X. , Davis E. J.
Published: July 2002

A new octopole electrode design improves the particle trapping stability of an electrodynamic balance (EDB) and allows one to make three-dimensional force measurements on a trapped particle. A conventional double-ring EDB was modified by splitting each ring into four equal segments that are electrically independent. Three dc sources were combined such that eight potentials were applied to the eight segments of the electrodes. An additional ac voltage was superimposed on each ring segment as in a conventional double-ring EDB. The resulting electric field has dc components in the x, y, and z directions, which can be controlled independently by the three dc supplies. The z component is used to balance and measure vertical forces such as gravity, radiation pressure, or phoretic forces. The x and y fields can be used to suppress lateral oscillations of the trapped particles, and lateral forces on the particle can be measured in terms of the x and y dc voltages. The apparatus and the electric fields are described herein, and the operation of the device is demonstrated for spheres and aggregated particles.
Electrical properties of the TiSi2–Si transition region in contacts: The influence of an interposed layer of Nb
Authors: Åberg J. , Persson S. , Hellberg P.-E. , Zhang S.-L. , Smith U. , Ericson F. , Engström M. , Kaplan W.
Published: July 2002

The influence of an interposed ultrathin Nb layer between Ti and Si on the silicide formation and the electrical contact between the silicide formed and the Si substrate is investigated. The presence of the Nb interlayer results in the formation of ternary alloy (Nb,Ti)Si2 in the C40 crystallographic structure adjacent to the Si substrate. Depending on the nature of the Si substrates and/or the amount of the initial Nb, the interfacial C40 (Nb,Ti)Si2 leads, in turn, to either epitaxial growth of a highly faulted metastable C40 TiSi2 or formation of the desired C54 TiSi2 at a lower temperature than needed for it to form in reference samples with Ti deposited directly on Si. On p-type substrates doped to various concentrations, the Nb also leads to a considerably lower specific contact resistivity than that obtained in the reference samples: a twofold to fourfold reduction in the contact resistivity is found using cross-bridge Kelvin structures in combination with two-dimensional numerical simulation. As C40 (Nb,Ti)Si2 forms at the interface when an interfacial Nb is present, the interface characterized is likely to represent the one between (Nb,Ti)Si2 and Si. For the reference samples, the interface studied is between TiSi2 and Si.