bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigations of ultrafast nuclear response induced by resonant and nonresonant laser pulses
Authors: Kumar Anand T. N. , Rosca Florin , Widom Allan , Champion Paul M.
Published: July 2002

We analyze the nonstationary vibrational states prepared by ultrashort laser pulses interacting with a two electronic level molecular system. Fully quantum mechanical expressions are derived for all the moments of the coordinate and momentum operators for the vibrational density matrices associated with the ground and excited electronic states. The analysis presented here provides key information concerning the temperature and carrier frequency dependence of the moments, and relates the moments to equilibrium absorption and dispersion line shapes in a manner analogous to the “transform methods” previously used to describe resonance Raman scattering. Particular attention is focused on the first two moments, for which simple analytical expressions are obtained that are computationally easy to implement. The behavior of the first two moments with respect to various parameters such as the pulse carrier (center) frequency, pulse width, mode frequency, electron-nuclear coupling strength, and temperature is investigated in detail. Using rigorous analytical formulas, we also discuss the laser pulse induced squeezing of the nuclear distributions as well as the pulse induced vibrational heating/cooling in the ground and excited states. The moment analysis of the pump induced state presented here offers a convenient starting point for the analysis of signals measured in pump–probe spectroscopy. The moment analysis can also be used, in general, to better understand the material response following ultrashort laser pulse excitation.
Electrical activation of carbon in GaAs: Implantation temperature effects
Authors: Danilov I. , de Souza J. P. , Murel A. V. , Pudenzi M. A. A.
Published: July 2002

Carbon was implanted into GaAs at the energy of 1 MeV with doses between 1×1013 and 2×1015 cm−2 at temperatures of 80 K, nominal room temperature (RT), and 300 °C. A markedly higher electrical activation was obtained in the samples implanted at 80 K compared to those implanted at RT or 300 °C, attaining a maximum hole concentration of 2×1019 cm−3. The redistribution of the C profile during rapid thermal annealing at temperatures from 700 to 950 °C for 10 s was found negligible, independently of the implantation temperature. Similar improvements in the electrical properties were also verified in samples implanted at 80 K with a lower energy of 60 keV. We consider that despite the light mass of C ions, the reduced dynamic annealing at 80 K allows the accumulation of an abundance of As vacancies, which assist the C activation as a p-type dopant.
Asymptotic limits of SU(2) and SU(3) Wigner functions
Authors: Rowe D. J. , de Guise H. , Sanders B. C.
Published: July 2002

Asymptotic limits are given for the SU(2) Wigner Dmnj functions as j→∞ for three domains of m and n. Similar asymptotic limits are given for the SU(3) Wigner functions of an irrep with highest weight (λ,0) as λ→∞. The results are shown to be relevant to the analysis of experiments with quantum interferometers.
Magnetic recording medium with improved temporal stability
Authors: Jensen P. J.
Published: July 2002

The current effort to fabricate nonvolatile magnetic recording media with a high areal density is deteriorated by the increasing temporal instability of the stored information. If the stored energy per magnetic particle competes with the thermal energy, spontaneous magnetic reversal processes may occur. Deposition of the magnetic particles on top of an antiferromagnetic substrate will increase its energy barrier due to the exchange coupling between the two subsystems. For this, the magnetic moments of the antiferromagnet in the vicinity of the magnetic particle have to deviate from their undisturbed arrangement. This disturbance vanishes within a few lattice constants. In the framework of a classical spin model, we calculate the spin arrangements and the resulting energy barriers for typical systems.
π
*
–σ
*
hyperconjugation mechanism on the rotational barrier of the methyl group (III): Methyl-azabenzenes in the ground, excited, and anionic states
Authors: Kawamura Y. , Nagasawa T. , Nakai H.
Published: July 2002

We theoretically investigate the internal rotations of the methyl group in methyl-azabenzenes, such as o- and m-methylpyridines, 2-methylpyrazine, 4-methylpyrimidine, 4-methylpyridadine, and 4-methyl-1,2,3-triazine in the ground, excited, and anionic states. The calculated rotational barriers reproduce well the experimental data. Orbital pictures are given for the barrier changes by excitation and electron attachment. An idea of π*–σ* hyperconjugation is applied for a comprehensive interpretation of the barrier changes. A correlation is found between the rotational barriers and the splitting of the lowest and next-lowest unoccupied molecular orbitals.
Experimental study of the K-regime of breakdown in straight and swept wing boundary layers
Authors: Chernoray V. G. , Bakchinov A. A. , Kozlov V. V. , Löfdahl L.
Published: July 2002

In this Brief Communication, the nonlinear evolution of periodical disturbances generated by an external sound field in a swept wing boundary layer is presented. All experimental results are compared with corresponding data for a straight wing configuration. The Tollmien–Schlichting instability has been studied, and it was found that the disturbance flow field remained highly deterministic and periodic in both time and space until the latest stages of the transition. A frequency-wave number Fourier analysis shows that the disturbance spectra comprise only the fundamental wave and its higher harmonics. The K-type breakdown scenario was observed, and the nonsymmetry of flow patterns in the swept wing boundary layer was found to be due to the presence of the cross flow.
Acetylene on Si(100) from first principles: adsorption geometries, equilibrium coverages, and thermal decomposition
Authors: Silvestrelli Pier Luigi , Toigo Flavio , Ancilotto Francesco
Published: July 2002

Adsorption of acetylene on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is a “bridge” configuration, where the C2H2 molecule is bonded to two Si atoms. Instead, “pedestal” configurations, recently proposed as the lowest-energy structures, are found to be much higher in energy and, therefore, can represent only metastable adsorption sites. We have calculated the surface formation energies for two different saturation coverages, namely 0.5 and 1 monolayer, both observed in experiments. We find that although, in general, the full monolayer coverage is favored, a narrow range of temperatures exists in which the 0.5 monolayer coverage is the most stable one, where the acetylene molecules are adsorbed in a 2×2 structure. This result disagrees with the conclusions of a recent study and represents a possible explanation of apparently controversial experimental findings. The crucial role played by the use of a gradient-corrected density functional is discussed. Finally, we study thermal decomposition of acetylene adsorbed on Si(100) by means of finite-temperature molecular dynamics, and we observe an unexpected behavior of dehydrogenated acetylene molecules.
Local plasmon photonic transistor
Authors: Tominaga J. , Mihalcea C. , Büchel D. , Fukuda H. , Nakano T. , Atoda N. , Fuji H. , Kikukawa T.
Published: July 2002

A proposal for a photonic transistor is made and some basic proving experiments are described. These experiments show that by focusing two laser beams (405 and 635 nm) in one small spot on a high-speed rotating optical disk, a large signal enhancement is observed. It was found that a plasmon interaction generated between a silver light-scattering center and recorded small marks in the optical disk with a super-resolution near-field structure produced the large signal amplification in the spot (<1 μm). A modulated signal of the blue laser was enhanced by 60 times by controlling the red laser power from 1.5 to 3.5 mW. It has been shown that the system has the potential to realize all-thin-films photonic transistors by using local plasmon amplification.
New sample levitation initiation and imaging techniques for the processing of refractory metals with an electrostatic levitator furnace
Authors: Ishikawa Takehiko , Paradis Paul-François , Yoda Shinichi
Published: July 2002

Two new methods that substantially ease the processing and study of refractory metals, when an electrostatic levitation furnace is used, are reported. The first technique is concerned with preheating the sample on a pedestal, prior to launch, to a temperature (∼1500 K) at which thermionic emission dominates all other charging/discharging mechanisms that may be going on simultaneously. Launched into levitation at that temperature, the sample can be quickly heated to its molten state without encountering further charge loss problems. This procedure thus shortens substantially the time it takes to bring the samples to their final high temperature states at which their thermophysical properties can be measured. This technique can be applied to most materials whose melting temperatures are higher than their thermionic temperatures. The second technique described is an ultraviolet-based sample imaging configuration. Due to the excellent sample–background contrast it continuously provides during all phases of processing (from solid to overheated liquid sample), it allows the measurements of the density and the ratio of constant pressure heat capacity over hemispherical total emissivity of refractory metals. This method, compared with other imaging techniques, leads to more accurate density data at very high temperatures and to density temperature coefficients closer to those reported in the literature. This is exemplified in this article with zirconium and niobium liquid samples.
A balloon-carried electrometer for high-resolution atmospheric electric field measurements in clouds
Authors: Harrison R. Giles
Published: July 2002

A disposable sensor for electric field measurements within clouds is described, which operates in conjunction with a standard meteorological radiosonde carried beneath a helium balloon. The radiosonde supplies telemetry and meteorological data. The sensor is constructed to measure electric fields of order 100 V m−1 and has been calibrated to 400 V m−1, typical of weakly electrified nonthunderstorm clouds. It measures using a regular cycle of measurement, dynamic zeroing, and marker pulses, which permits the integrity of the received data to be assessed. The electric field sensor is optically connected to the radiosonde, allowing it to remain electrically floating from the balloon system. A low-leakage electrometer switch for dynamic zeroing is an integral part of the system, and successful electric field measurements have been obtained aloft in temperatures down to −60 °C.
Ab initio investigation of the vertical and adiabatic excitation spectrum of NO3
Authors: Eisfeld Wolfgang , Morokuma Keiji
Published: July 2002

The ground and excited electronic states up to approximately 100 000 cm−1 of the nitrate free radical (NO3) are investigated by complete active space self-consistent-field (CASSCF) and mulitreference–singles doubles configuration interaction calculations. Extended basis sets, containing polarization and diffuse functions, and an active space consisting of 13 orbitals and 17 electrons are used. A total of 28 electronic states is obtained within the D3h framework which split into 44 states in C2v. All calculated states are valence states and their character is discussed in detail. Oscillator strength and radiative lifetimes are determined from the CASSCF wave functions and give evidence for a strong transition that was not yet known experimentally. For the experimentally observed E″2 and E′2 states equilibrium geometries, harmonic frequencies, and adiabatic excitation energies are calculated in excellent agreement with experimental data. Important new insight is gained about the role of the low-lying electronic states in the photodissociation and the mechanism of this process is discussed.
Frequency upshifting of microwave radiation via resonant excitation of plasma oscillations in a thin layer of a time-varying plasma
Authors: Bakunov M. I. , Gildenburg V. B. , Nishida Y. , Yugami N.
Published: July 2002

Interaction of a p-polarized microwave radiation with a thin (compared to the wavelength) plasma layer, whose density slowly (compared to the wave period) grows in time due to ionization by an external source, e.g., laser pulse, is considered. Unlike previous papers on the frequency upshifting of electromagnetic radiation in a time-varying plasma, which treated mainly the cases of unbounded plasma or thick plasma slab, a novel mechanism of frequency upshifting is presented. It is connected with the excitation of free oscillations in the plasma layer at the moment when the plasma density passes the critical value and, thus, plasma resonance in the layer occurs. The frequency of the excited plasma oscillations increases in time following adiabatically the growing plasma frequency, whereas the oscillations themselves gradually radiate. Due to this mechanism, the upshifted electromagnetic radiation with the frequency equal to the maximum value of the plasma frequency in the layer can be effectively generated. Concrete estimations of the effect for a real experimental setup are given.
Isotropic tensile strength of molecular glasses
Authors: Utz Marcel , Debenedetti Pablo G. , Stillinger Frank H.
Published: July 2002

The relationship between the bulk density and pressure of configurations corresponding to local minima on the potential energy surface of molecular models of ethane, n-pentane, and cyclopentane (the equation of state of their energy landscape) has been explored. Like simpler, atomic fluids, these systems exhibit a limiting bulk density below which minimum energy configurations are no longer spatially homogeneous, but consist instead of a locally dense fraction and large, system-spanning voids. In the case of n-pentane, the sampling of the minima on the energy landscape was found to depend strongly on temperature, due to changing Boltzmann factors associated with the different conformers in the liquid. The pressures of the minimum energy configurations, in contrast, were found to be essentially independent of the liquid temperature in all cases. The highest amount of isotropic tension (negative pressure) that minimum energy configurations can sustain is reached at the limiting densities, and is of similar magnitude (approximately 250 MPa) for all three model substances. Crystalline configurations of ethane and n-pentane, in contrast, were found to exhibit higher isotropic tensile strength than their amorphous counterparts. A pronounced segregation of end groups on the boundary of large voids was observed in the minimum energy configurations of low bulk density pentane.
An optimum feedback coil position for active stabilization of resistive wall modes
Authors: Jhang Hogun , Ku S. H. , Kim Jin-Yong
Published: July 2002

A study on the feedback stabilization of resistive wall modes in a cylindrical geometry is presented. The effect of radial separation of feedback coils from sensor loops is investigated. It is shown that there is an optimum radial position for feedback coils where sideband mode coupling parameter is minimized, hence enhancing the efficacy of a system of coils for feedback stabilization of resistive wall modes.
Growth of high-quality GaN on Si(111) substrate by ultrahigh vacuum chemical vapor deposition
Authors: Kim Min-Ho , Bang Young-Churl , Park Nae-Man , Choi Chel-Jong , Seong Tae-Yeon , Park Seong-Ju
Published: July 2002

An ultrahigh vacuum chemical vapor deposition (UHVCVD) system was employed to grow high-quality GaN on a Si (111) substrate using a thin AlN buffer layer. X-ray diffraction, high-resolution electron microscopy (HREM), and photoluminescence (PL) data indicate that a single crystalline GaN layer with a wurtzite structure was epitaxially grown on a silicon substrate. The full width at half maximum (FWHM) of the x-ray rocking curve for the GaN (0002) diffraction was 16.7 arc min. A cross-sectional HREM image showed an amorphous SiNx layer at the Si/AlN interface, as well as stacking faults and inversion domain boundaries in the GaN epilayer. An intense PL emission line, which is associated with the recombination of the donor bound exciton, was observed at 10 K PL spectra (FWHM=6.8 meV) and a strong band edge emission was obtained (FWHM=33 meV) as well, even at room temperature. These results indicate that high-quality GaN can be grown on Si (111) substrates using a UHVCVD growth method.
Resonantly amplified vibronic symmetry breaking
Authors: Rathbone G. J. , Poliakoff E. D. , Bozek John D. , Lucchese R. R.
Published: July 2002

The energy dependence of the vibrational branching ratio for exciting one quantum of bending is determined for CO2 4σg−1 photoionization. This nominally forbidden transition becomes allowed for a photoionization transition as a result of instantaneous symmetry breaking due to zero point motion, and is strongly enhanced by a continuum shape resonance.
Nonrelativistic closed string theory
Authors: Gomis Jaume , Ooguri Hirosi
Published: July 2002

We construct a Galilean invariant nongravitational closed string theory whose excitations satisfy a nonrelativistic dispersion relation. This theory can be obtained by taking a consistent low energy limit of any of the conventional string theories, including the heterotic string. We give a finite first order worldsheet Hamiltonian for this theory and show that this string theory has a sensible perturbative expansion, interesting high energy behavior of scattering amplitudes and a Hagedorn transition of the thermal ensemble. The strong coupling duals of the Galilean superstring theories are considered and are shown to be described by an eleven-dimensional Galilean invariant theory of light membrane fluctuations. A new class of Galilean invariant nongravitational theories of light-brane excitations are obtained. We exhibit dual formulations of the strong coupling limits of these Galilean invariant theories and show that they exhibit many of the conventional dualities of M theory in a nonrelativistic setting.
Late-time hohlraum pressure dynamics in supernova remnant experiments
Authors: Hurricane O. A. , Glendinning S. G. , Remington B. A. , Drake R. P. , Dannenberg K. K.
Published: July 2002

It is shown that laser driven hohlraums obtain significant internal pressures which affect the hydrodynamics of high-energy density shock-tube experiments. By incorporating this previously neglected hohlraum pressure effect (in addition to the usual x-ray drive) into computer simulations which model the NOVA laser driven supernova remnant experiment [R. P. Drake, S. G. Glendinning, K. Estabrook, B. A. Remington, R. McCray, R. J. Williams, L. J. Suter, T. B. Smith, J. J. Carroll III, R. A. London, and E. Liang, Phys. Rev. Lett. 81, 2068 (1998)], calculations are able to reproduce the observed structure of hydrodynamic features.
Nonlocality and intermittency in three-dimensional turbulence
Authors: Laval J-P. , Dubrulle B. , Nazarenko S.
Published: July 2002

Numerical simulations are used to determine the influence of the nonlocal and local interactions on the intermittency corrections in the scaling properties of three-dimensional turbulence. We show that neglect of local interactions leads to an enhanced small-scale energy spectrum and to a significantly larger number of very intense vortices (“tornadoes”) and stronger intermittency (e.g., wider tails in the probability distribution functions of velocity increments and greater anomalous corrections). On the other hand, neglect of the nonlocal interactions results in even stronger small-scale spectrum but significantly weaker intermittency. Thus, the amount of intermittency is not determined just by the mean intensity of the small scales, but it is nontrivially shaped by the nature of the scale interactions. Namely, the role of the nonlocal interactions is to generate intense vortices responsible for intermittency and the role of the local interactions is to dissipate them. Based on these observations, a new model of turbulence is proposed, in which nonlocal (rapid distortion theory-like) interactions couple large and small scale via a multiplicative process with additive noise and a turbulent viscosity models the local interactions. This model is used to derive a simple version of the Langevin equations for small-scale velocity increments. A Gaussian approximation for the large scale fields yields the Fokker–Planck equation for the probability distribution function of the velocity increments. Steady state solutions of this equation allows one to qualitatively explain the anomalous corrections and the skewness generation along scale. A crucial role is played by the correlation between the additive and the multiplicative (large-scale) process, featuring the correlation between the stretching and the vorticity.
Mapping of local stress distributions in SiGe/Si optical channel waveguide
Authors: Rho H. , Jackson Howard E. , Weiss B. L.
Published: July 2002

Micro-Raman scattering experiments have been carried out to study a SiGe/Si photoelastic optical channel waveguide structure. A Si3N4 stressor stripe was fabricated on a SiGe/Si planar waveguide to create local stress variation beneath the stripe. To characterize the local stress distributions which create the channel waveguide structure, we obtained Si–Si Raman peak shifts in the SiGe layer by utilizing spatially resolved Raman spectra both from the top surface across the width of the Si3N4 stripe and from an edge (i.e., a waveguiding region) beneath the stripe. Longitudinal optical phonon shifts observed from the top surface show a significant increase in compressive stress within the stripe width and in tensile stress just outside of stripe edges for a sample annealed at 600 °C. To map local changes of stresses in the waveguiding region beneath the stripe, spectra from two transverse optical phonons were measured from the edge and interpreted using the Raman polarization selection rules. The Si–Si Raman peak shifts obtained were used to find strain components and, in turn, to model resultant local refractive index variations which give rise to optical confinement via the photoelastic effect. Model calculations show good agreement with the experimental data and provide mode profiles for both transverse magnetic and transverse electric propagation. These experiments contribute to a more detailed understanding of the complex stress and refractive index distributions present in photoelastic semiconductor optical channel waveguide structures, structures which are potentially useful for optical device technology.
Rotational spectra of the H2–HCN cluster observed by millimeter-wave spectroscopy combined with a pulsed supersonic jet technique
Authors: Ishiguro Masazumi , Tanaka Takehiko , Harada Kensuke , Whitham Christopher J. , Tanaka Keiichi
Published: July 2002

Millimeter-wave spectroscopy combined with a pulsed supersonic jet technique was used to observe rotational lines of the H2–HCN cluster containing the ortho as well as para hydrogen molecule. The Σ symmetry in the ground state was confirmed for both species. From the observed rotational constants, the average distance between the center of mass of H2 and that of HCN was derived to be 3.9613 (35) Å for the ortho species and 4.229 (11) Å for the para species. The results suggest totally different configurations in the ortho and para species: H2 is attached to the hydrogen end of HCN in the para species, while to the nitrogen end in the ortho species. The nuclear quadrupole interaction constants show that the HCN part executes a floppy motion with a large mean amplitude of about 30° in the ortho, as well as para, species.
Local trap spectroscopy in superconducting tunnel junctions
Authors: Kozorezov A. G. , Wigmore J. K. , Peacock A. , Poelaert A. , Verhoeve P. , den Hartog R. , Brammertz G.
Published: July 2002

We show that thermal activation of quasiparticles from local traps is responsible for the temperature variation of responsivity observed for some superconducting tunneling junction photon detectors. With this model, the depth of the local traps in two different proximized Ta structures was found to be the same, 0.20±0.02 meV.
Microscopic imaging with high energy x-rays by Fourier transform holography
Authors: Leitenberger Wolfram , Snigirev Anatoly
Published: July 2002

Experimental results obtained with an x-ray microscope based on Fourier transform holography are discussed. The experiments were done with monochromatic x rays at an energy of 14 keV. The x-ray source was a high brilliance undulator with a small vertical source size providing a spatial coherent beam. The necessity of the increase of the spatial coherence by spatial filtering is discussed together with the generation of a smallest possible reference point source. A Fresnel zone plate was used as a beam splitter. It formed the reference point source by the first order diffracted beam. The sample to be imaged is placed close to the first order focal spot and in parallel the sample is illuminated by the zero order transmitted beam. The hologram is generated by the interference of the reference wave and the object wave at a large distance from the object. A characteristic of these holograms is that a small distance in the object plane gives rise to large fringe spacing in the hologram plane and vice versa. In principle the minimum lateral resolution in a reconstruction of such holograms is not limited by the lateral resolution of the detector but other factors which will be discussed. The holograms were recorded with a charge coupled device based x-ray microscope. The properties of holograms of test structures are investigated by the evaluation of the fringe spacing in the hologram. Fourier transform holograms were successfully numerically reconstructed and structures of 3.5 μm size are visible in the reconstruction.
Interference induced oscillations in the tunneling current through ultrathin gate insulators
Authors: Mohaidat Jihad M.
Published: July 2002

Through the numerical solution of the time-dependent Schrödinger equation, the tunneling current through ultrathin gate oxides in metal-oxide-semiconductor (MOS) structures was computed. The tunneling current exhibited oscillatory behavior, which was attributed to wave interference. It is found that the oscillation amplitude decreases as the oxide thickness is increased. This amplitude decrease used to be attributed to different scattering mechanisms. However, we will show that the oscillation amplitude decrease as oxide thickness is increased is an intrinsic quantum-tunneling feature. Furthermore, we will also show that the oscillation amplitude decreases as well when the semiconductor effective mass in the MOS structures is increased, vanishing when the semiconductor effective mass reaches that of a metal, thus forming a metal-insulator-metal (MIM) structure. This result pinpointed why tunneling current oscillation has never been observed experimentally in MIM-like structures.
Fabrication and integration possibilities of ultrasmall quantum dots in silicon-on-insulator material
Authors: Tilke A. , Blick R. H. , Lorenz H.
Published: July 2002

Single-electron transistors utilizing Coulomb blockade effects are promising candidates for future silicon based nanoelectronics. We present the fabrication of such transistors and measurements that reveal Coulomb blockade behavior. Various silicon quantum dots are investigated up to room temperature. We employ a dual gate configuration with which we are able to control our devices by both a metallic top gate as well as by an in-plane gate. This design principle enhances the integration density.
Inward turbulent particle transport in a helical plasma heated by neutral beam injection
Authors: Ohkuni K. , Toi K. , Akiyama R. , Goto M. , Isobe M. , Matsunaga G. , Matsuoka K. , Minami T. , Morita S. , Nishimura S. , Okamura S. , Osakabe M. , Shimizu A. , Takagi S. , Takahashi C. , Takechi M. , Tanaka K. , Yoshimura Y.
Published: July 2002

In neutral-beam-injection (NBI) heated plasmas of the Compact Helical System (CHS) heliotron/torsatron [K. Matsuoka et al., Plasma Physics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 441], inward turbulent particle flux is observed at a particular radial location near the plasma edge. The observed reversal of the particle flux is connected to the change in the cross-phase between electron density and poloidal electric field fluctuations. This phenomenon is observed in the parameter regime of low collisionality at the electron temperature Te>15 eV, electron density ne<3×1018 m−3, which approximately corresponds to the effective collision frequency normalized by the transit frequency of a circulating particle ν*≲1. The large inward particle flux (≳1×1020 m−2 s−1) correlates with the positive shear of the radial electric field Er′ (≳1×106 V m−2), but does not have clear correlation with the radial electric field Er and the curvature Er″.
A family of completely integrable multi-Hamiltonian systems explicitly related to some celebrated equations
Authors: Fan Engui
Published: July 2002

By introducing a spectral problem with an arbitrary parameter, we derive a Kaup–Newell-type hierarchy of nonlinear evolution equations, which is explicitly related to many important equations such as the Kundu equation, the Kaup–Newell (KN) equation, the Chen–Lee–Liu (CLL) equation, the Gerdjikov–Ivanov (GI) equation, the Burgers equation, the modified Korteweg-deVries (MKdV) equation and the Sharma–Tasso–Olver equation. It is shown that the hierarchy is integrable in Liouville’s sense and possesses multi-Hamiltonian structure. Under the Bargann constraint between the potentials and the eigenfunctions, the spectral problem is nonlinearized as a finite-dimensional completely integrable Hamiltonian system. The involutive representation of the solutions for the Kaup–Newell-type hierarchy is also presented. In addition, an N-fold Darboux transformation of the Kundu equation is constructed with the help of its Lax pairs and a reduction technique. According to the Darboux transformation, the solutions of the Kundu equation is reduced to solving a linear algebraic system and two first-order ordinary differential equations. It is found that the KN, CLL, and GI equations can be described by a Kundu-type derivative nonlinear Schrödinger equation involving a parameter. And then, we can construct the Hamiltonian formulations, Lax pairs and N-fold Darboux transformations for the Kundu, KN, CLL, and GI equations in explicit and unified ways.
Characteristics of metalorganic remote plasma chemical vapor deposited Al2O3 gate stacks on SiC metal–oxide–semiconductor devices
Authors: Lazar H. R. , Misra V. , Johnson R. S. , Lucovsky G.
Published: July 2002

Metalorganic remote plasma chemical vapor deposited SiO2/Al2O3 stacks were deposited on 6H p-type silicon SiC to fabricate a high-k gate stack SiC metal–oxide–semiconductor capacitors. Capacitance–voltage (C–V) and current–voltage (I–V) measurements were performed. C–V characteristics showed excellent properties at room and higher temperatures. Samples exhibited a slight negative flatband shift from which the net oxide charge (Qox) was calculated. Low leakage currents were observed even at high temperatures. I–V characteristics of Al2O3 were superior to those observed on AlN and SiO2 dielectrics on SiC.
Computational study of shock interaction with a vortex ring
Authors: Ding Z. , Hussaini M. Y. , Erlebacher G. , Krothapalli A.
Published: July 2002

The problem of shock interaction with a vortex ring is investigated within the framework of axisymmetric Euler equations solved numerically by a shock-fitted sixth-order compact difference scheme. The vortex ring, which is based on Lamb’s formula, has an upstream circulation Γ=0.01 and its aspect ratio R lies in the range 8⩽R⩽100. The shock Mach number varies in the range 1.1⩽M1⩽1.8. The vortex ring/shock interaction results in the streamwise compression of the vortex core by a factor proportional to the ratio of the upstream and downstream mean velocity U1/U2, and the generation of a toroidal acoustic wave and entropy disturbances. The toroidal acoustic wave propagates and interacts with itself on the symmetry axis of the vortex ring. This self-interaction engenders high amplitude rarefaction/compression pressure peaks upstream/downstream of the transmitted vortex core. This results in a significant increase in centerline sound pressure levels, especially near the shock (due to the upstream movement of the rarefaction peak) and in the far downstream (due to the downstream movement of the compression peak). The magnitude of the compression peak increases nonlinearly with M1. For a given M1, vortex rings with smaller aspect ratios (R<20) generate pressure disturbances whose amplitudes scale inversely with R, while vortex rings with larger aspect ratios (R>40) generate pressure disturbances whose amplitudes are roughly independent of R.
Electromigration of eutectic SnPb and SnAg3.8Cu0.7 flip chip solder bumps and under-bump metallization
Authors: Lee T. Y. , Tu K. N. , Frear D. R.
Published: July 2002

The electromigration damage in flip chip solder bumps of eutectic SnPb and SnAg3.8Cu0.7 was studied after current stressing at 120 °C with 1.5 A. The diameter of the bumps was about 200 μm. The under-bump metallization (UBM) on the chip side was electroless Ni and on the board side was electroplated Cu. Surface marker motion was used to measure the atomic flux driven by electromigration and to calculate the effective charge number, Z*, of the solder. For eutectic SnPb, Z* is about 36–100 after 39.5 h of electromigration, but for SnAg3.8Cu0.7 the marker movement was too small to measure Z* even after 200 h of current stressing. While the effect of electromigration in SnAg3.8Cu0.7 is much smaller than that in eutectic SnPb, hillocks of intermetallic compounds at the anode have been observed in the former. An extensive growth of Ni–Cu–Sn intermetallic compounds in the matrix of both solder bumps was found. A localized dissolution of electroless Ni UBM was observed under current stressing, yet no dissolution of the Cu UBM by electromigration was found.
Vacancies and deep levels in electron-irradiated 6H SiC epilayers studied by positron annihilation and deep level transient spectroscopy
Authors: Kawasuso A. , Redmann F. , Krause-Rehberg R. , Frank T. , Weidner M. , Pensl G. , Sperr P. , Itoh H.
Published: July 2002

The annealing behavior of defects in n-type 6H SiC epilayers irradiated with 2 MeV electrons have been studied using positron annihilation and deep level transient spectroscopy. Vacancy-type defects are annealed at 500–700 °C and 1200–1400 °C. From the analysis of Doppler broadening spectra (core electron momentum distribution), the latter annealing process is attributed to the disappearance of complexes related to silicon vacancies and not to nearest neighbor divacancies. Among the observed deep levels, the E1/E2 levels show similar annealing behavior to that of positron annihilation centers above 1000 °C. It is thus proposed that the E1/E2 levels originate from complexes containing silicon vacancies.
Solubility limit and precipitate formation in Al-doped 4H-SiC epitaxial material
Authors: Linnarsson M. K. , Janson M. S. , Zimmermann U. , Svensson B. G. , Persson P. O. Å. , Hultman L. , Wong-Leung J. , Karlsson S. , Schöner A. , Bleichner H. , Olsson E.
Published: July 2002

Heavily Al-doped 4H–SiC structures have been prepared by vapor phase epitaxy. Subsequent anneals have been carried out in an Ar atmosphere in a rf-heated furnace between 1500 °C and 2000 °C for 0.5 to 3 h. Secondary ion mass spectrometry has been utilized to obtain Al concentration versus depth as well as lateral distributions (ion images). Transmission electron microscopy (TEM) has been employed to study the crystallinity and determine phase composition after heat treatment. A solubility limit of ∼2×1020 Al/cm3 (1900 °C) is extracted. Three-dimensional ion images show that the Al distribution does not remain homogeneous in layers heat treated at 1700 °C or above when the Al concentration exceeds 2×1020 cm−3. Al-containing precipitates are identified by energy-filtered TEM.
Spatiotemporal reaction-diffusion patterns emerging on cylindrical surfaces due to global coupling
Authors: Savin I. , Nekhamkina O. , Sheintuch M.
Published: July 2002

A condensed polynomial model, that captures the main features of high- or low-pressure catalytic oscillations, is used to simulate spatiotemporal patterns in a cylindrical catalytic surface. This model includes a single autocatalytic variable (activator) and a slow changing and localized inhibitor subject to a global interaction mechanism which maintains the spatial average of the activator at the set point. While for very short (small length L) or very narrow (small perimeter P) cylinders the pattern preserves the structures of the corresponding one-dimensional problems (a ring or a wire), two-dimensional patterns emerge for comparable L and P showing a large multiplicity of spatiotemporal behavior because of a very high sensivity to initial conditions. The effect of kinetic parameters and system size is studied. Approximate solutions for the bifurcation from one- to two-dimension patterns are derived.
Cathodoluminescence study of diluted magnetic semiconductor quantum well/micromagnet hybrid structures
Authors: Kossut J. , Yamakawa I. , Nakamura A. , Cywiński G. , Fronc K. , Czeczott M. , Wróbel J. , Kyrychenko F. , Wojtowicz T. , Takeyama S.
Published: July 2002

Cathodoluminescence (CL) was studied in hybrid structures consisting of a diluted magnetic semiconductor (DMS) Cd1−xMnxTe (x=0.06 and 0.09) quantum well buried 300 Å below the surface on which Fe islands with micrometric dimensions were deposited. The CL at T=10 K collected from areas far away from the Fe island was consistent with the photoluminescence spectra obtained prior to Fe deposition as were the raster scans and spot excited CL spectra taken in nonmagnetized structures close to the Fe islands. After a magnetization at a magnetic field of 3 T, the CL peak related to DMS quantum well (QW) shifts by up to 4 meV to lower energy only when the exciting beam is focused close to edges of an island. The observed shifts are interpreted as due to a fringe field, affecting the DMS QW, of magnetic domains formed in the Fe islands. The experiments prove a feasibility of the concept of usage of the fringe fields to achieve further confinement of excitons in submicron DMS/ferromagnet hybrid structures.
Electronic states in modulation doped p-AlGaN/GaN superlattices
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Osinsky A. V. , Norris P. E. , Pearton S. J. , Van Hove J. , Wowchack A. M. , Chow P. P.
Published: July 2002

The properties of p-AlGaN/GaN modulation doped superlattices (SLs) prepared by molecular beam epitaxy were studied by means of conductivity versus temperature, admittance spectroscopy, photoinduced current spectroscopy, microcathodoluminescence (MCL) spectra measurements, and measurements of effective diffusion lengths. It is shown that in SLs grown on GaN underlayers the sheet resistivity is about two orders of magnitude lower than for reference p-GaN films and the resistivity of SLs remains lower up to temperatures of about 350 °C. For SLs grown on AlGaN underlayers the gain in resistivity is much more moderate and certain advantages in using such SLs are envisaged only for temperatures below room temperature. The reason for this lower gain is a considerable decrease in hole mobility compared to p-GaN. The effect is somewhat tentatively attributed to worse crystalline perfection of these SLs. It is also shown that such SLs are characterized by a strongly broadened MCL peak and the presence of additional hole traps with activation energy of about 0.4 eV. Despite that, the photosensitivity and MCL intensity of these SLs are much better than for reference p-GaN samples.
Ion imaging measurement of collision-induced rotational alignment in Ar-NO scattering
Authors: Cline Joseph I. , Lorenz K. Thomas , Wade Elisabeth A. , Barr James W. , Chandler David W.
Published: July 2002

Collision-induced rotational alignment of NO X 2Π1/2(v=0,j=8.5) is measured for rotationally inelastic scattering of NO X 2Π1/2(v=0,j=0.5) with Ar at 65 meV collision energy. The experiments are performed by velocity-mapped ion imaging with polarized 1+1′ REMPI probing of the scattered NO products. It is shown that the azimuthal information intrinsic to imaging detection allows the measurement of additional alignment moments not previously reported. The measured alignment shows only qualitative agreement with the predictions of the kinematic apse conservation model.
Towards multireference equivalents of the G2 and G3 methods
Authors: So/lling Theis I. , Smith David M. , Radom Leo , Freitag Mark A. , Gordon Mark S.
Published: July 2002

The effect of replacing the standard single-determinant reference wave functions in variants of G2 and G3 theory by multireference (MR) wave functions based on a full-valence complete active space has been investigated. Twelve methods of this type have been introduced and comparisons, based on a slightly reduced G2-1 test set, are made both internally and with the equivalent single-reference methods. We use CASPT2 as the standard MR-MP2 method and MRCl+Q as the higher correlation procedure in these calculations. We find that MR-G2(MP2,SVP), MR-G2(MP2), and MR-G3(MP2) perform comparably with their single-reference analogs, G2(MP2,SVP), G2(MP2), and G3(MP2), with mean absolute deviations (MADs) from the experimental data of 1.41, 1.54, and 1.23 kcal mol−1, compared with 1.60, 1.59, and 1.19 kcal mol−1, respectively. The additivity assumptions in the MR-Gn methods have been tested by carrying out MR-G2/MRCI+Q and MR-G3/MRCI+Q calculations, which correspond to large-basis-set MRCI+Q+ZPVE+HLC calculations. These give MADs of 1.84 and 1.58 kcal mol−1, respectively, i.e., the agreement with experiment is somewhat worse than that obtained with the MR-G2(MP2) and MR-G3(MP2) methods. In a third series of calculations, we have examined pure MP2 and MR-MP2 analogs of the G2 and G3 procedures by carrying out large-basis-set MP2 and CASPT2(+ZPVE+HLC) calculations. The resultant methods, which we denote G2/MP2, G3/MP2, MR-G2/MP2, and MR-G3/MP2, give MADs of 4.19, 3.36, 2.01, and 1.66 kcal mol−1, respectively. Finally, we have examined the effect of using MCQDPT2 in place of CASPT2 in five of our MR-Gn procedures, and find that there is a small but consistent deterioration in performance. Our calculations suggest that the MR-G3(MP2) and MR-G3/MP2 procedures may be useful in situations where a multireference approach is desirable.
Real time ultrafast spectroscopy of shock front pore collapse
Authors: Hambir Selezion A. , Kim Hackjin , Dlott Dana D. , Frey Robert B.
Published: July 2002

Shock-wave induced nanopore collapse (average diameter 100 nm) at 4.2 GPa in a 3-μm-thick poly-methyl methacrylate (PMMA) layer is measured in real time using coherent anti-Stokes Raman spectroscopy (CARS). Pore collapse is monitored via CARS transitions of a dye probe embedded in the porous medium. A pore collapse time constant of 3 ns in PMMA is in poor agreement with hydrodynamic pore collapse models but in excellent agreement with a viscoplastic model that uses the “shock viscosity” determined from the PMMA viscoelastic response to shock. The shock viscosity is more than 12 orders of magnitude smaller than the ordinary viscosity. A downstream gauge of polycrystalline anthracene monitors changes in the steeply rising shock front (<25 ps rise time) after passing through the porous medium or a scattering medium with 100-nm-diam scatterers. The anthracene is a two-dimensional (2D) shock gauge that provides a time sequence of CARS spectra S(t,λ). The 2D gauge is shown to be capable of discriminating between a shock front that gradually rises with time constant tr or a bunch of steeply rising shocklets with an arrival time spread equal to tr. The transmitted shock front is shown to consist of a bunch of steep shocklets with an arrival time spread of 550 ps.
Real and imaginary components of the alternating current magnetic susceptibility of RAl2 (R=Gd, Dy, and Er) in the ferromagnetic region
Authors: Levin E. M. , Pecharsky V. K. , Gschneidner K. A.
Published: July 2002

The real (χac′) and imaginary (χac″) components of the ac magnetic susceptibility of the polycrystalline GdAl2, DyAl2, and ErAl2 have been measured as functions of temperature, ac and bias dc magnetic field amplitude, and ac magnetic field frequency. Both χac′ and χac″ of the ferromagnetic DyAl2 and ErAl2 are strongly dependent on the ac magnetic field parameters, especially when compared with those of the ferromagnetic GdAl2. The observed behavior of the ac magnetic susceptibility is determined mainly by the domain dynamics, rather than by a change of the exchange interactions below Curie temperature.
Oxidation states in scanning-probe-induced Si3N4 to SiOx conversion studied by scanning photoemission microscopy
Authors: Klauser R. , Hong I.-H. , Su H.-J. , Chen T. T. , Gwo S. , Wang S.-C. , Chuang T. J. , Gritsenko V. A.
Published: July 2002

The biased conductive probe of an atomic force microscope can induce local oxidation in ambience for converting silicon nitride films to silicon oxides with high reaction rate. Spatially resolved photoemission analysis with submicron resolution has been utilized to study the oxidation states of converted silicon oxide patterns in comparison with the surrounding Si3N4 layer. The core level shift of the Si 2p photoelectron peak and the spectral features in the valence band reveal a complete conversion of silicon nitride to silicon oxide at a bias voltage of 10 V, with no remaining nitrogen left. The major oxide is SiO2. The observed oxidation states of Si4+, Si3+, and Si2+ show a gradient depth distribution indicating excess silicon in the layer.
Temperature-induced change from p to n conduction in metallofullerene nanotube peapods
Authors: Chiu P. W. , Gu G. , Kim G. T. , Philipp G. , Roth S. , Yang S. F. , Yang S.
Published: July 2002

Metallofullerene nanotube peapods were prepared by introducing Dy@C82 into the interior space of single-walled carbon nanotubes. Transport measurements show that the Dy@C82 molecules function as electron donors and transfer charge to the carbon nanotube host. The amount of charge transferred varies with the temperature. At room temperature, the doped nanotube shows p-type behavior as seen from the response to a back gate. As the temperature decreases, the conductance becomes n type and at T<215 K metallic behavior is observed, indicating the degenerate state by doping. Below about 75 K, single-electron charging phenomena dominate the transport and show irregular Coulomb blockade oscillation, implying that the insertion of Dy@C82 splits the tube into a series of several quantum dots.
Copper wetting of a tantalum silicate surface: Implications for interconnect technology
Authors: Zhao X. , Leavy M. , Magtoto N. P. , Kelber J. A.
Published: July 2002

X-ray photoelectron spectroscopy data show that sputter-deposited Cu (300 K) displays conformal growth on oxidized TaSi films (TaSiO6). The TaSiO6 films, 6 Å thick, were formed by sputter deposition of Ta onto ultrathin SiO2 substrates at 300 K, followed by annealing to 600 K in 2 Torr O2. The photoelectron spectra of the films are characterized by a Si(2p) binding energy at 102.1 eV, indicative of silicate formation. Annealing the film to >900 K resulted in silicate decomposition and SiO2 formation. Cu(I) formation and conformal growth were not observed for the annealed films. The results are similar to those previously reported for oxidized TaSiN, and indicate that Si-modified Ta barriers should maintain Cu wettability under oxidizing conditions for Cu interconnect applications.