bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion-irradiation induced hydrogen loss from polyethylene film
Authors: Taniike Akira , Kubota Naoyoshi , Takeuchi Masakatsu , Furuyama Yuichi , Kitamura Akira
Published: November 2002

Degradation of a polymer under ion irradiation is investigated quantitatively. The loss of hydrogen atoms in polyethylene films under bombardment of various kinds of ions has been measured by directly monitoring recoil hydrogen atoms. Fluence dependence of the decrease in the hydrogen density is analyzed using a modified model based on that of M. P. de Jong, A. J. H. Maas, L. J. van Ijzendoorn, S. S. Klein, and M. J. A. de Voigt, J. Appl. Phys. 82, 1058 (1997). It is shown that the dependence is well explained by the present model with a hydrogen trapping reaction constant having hydrogen density dependence. It is found that the product of a bond cleavage probability squared and an ion track area is proportional to z4/E3, where z and E are the atomic number and energy of the projectile, respectively.
Modeling the trap generation and relaxation effects in thin oxides under unipolar and bipolar high-field impulse stressing using stress-induced leakage current
Authors: Chim W. K. , Tan Y. N.
Published: November 2002

Stress-induced leakage current, or SILC, decreases when the time between pulses (Tbp) of a periodic ac-pulse stress wave form is increased. The SILC reduction with increasing Tbp becomes less significant when the stress voltage or the pulse duration increases. Both the SILC and positive trapped charges, generated as a result of the electrical stress, decrease with increasing Tbp. The SILC reduction with increasing Tbp was found to be a result of hole detrapping. As the SILC reduction with increasing Tbp is on the order of nanoseconds time scale, it suggests that short transit-time holes are responsible for the SILC in our work. A proposed model, describing the trap generation and relaxation processes in thin oxides during transient high-field stress using unipolar and bipolar pulse wave forms, is presented in this article. Additionally, the model was extended to calculate the amount of traps generated during write/erase endurance stress of a floating-gate tunnel oxide electrically erasable and programmable read-only memory. The measured and calculated (from the proposed model) trap concentrations were shown to be in good agreement.
Efficient energy transfer in organic thin films—implications for organic lasers
Authors: Sheridan A. K. , Buckley A. R. , Fox A. M. , Bacher A. , Bradley D. D. C. , Samuel I. D. W.
Published: November 2002

We show that efficient nonradiative (Förster) energy transfer between solid films of two highly luminescent perylene dyes blended in a solid film can be used to control the amplified spontaneous emission (ASE) emitted from the films under pulsed optical excitation. Perylene orange, which acts as the donor, and perylene red, which is the acceptor, are doped into a host matrix of poly(methylmethacrylate) (PMMA). We report the ASE behavior as a function of acceptor concentration, and observe a sudden change in the spectral position of the ASE at an acceptor:donor concentration of 1:9 by weight. Below this concentration, emission is at 590 nm, which is characteristic of ASE from undoped perylene orange:PMMA blends, whereas films with higher acceptor concentrations produced ASE spectra centered at 620 nm, which is characteristic of perylene red:PMMA blends. In order to understand this behavior, the rate constant for energy transfer between the dyes was measured and found to be 5.0±0.2×1011 s−1 (mol/dm3)−1. We used this to deduce an upper limit for the stimulated emission rate of 4.9±0.2×108 s−1.
Nonlinear evolution of thin free viscous films in the presence of soluble surfactant
Authors: Matar O. K.
Published: November 2002

The rupture of thin free viscous films is studied in the presence of soluble surfactant. In the limit of rapid surfactant bulk diffusion, higher-order long wavelength theory is used to derive a one-dimensional (1D) nonlinear model for the film thickness, tangential velocity, surfactant surface, and bulk concentrations, the latter being cross-sectionally averaged. For slow diffusion, an approximate (1D) model for the bulk concentration is derived; the predictions of this model in this limit are compared with those of the fully two-dimensional (2D) concentration model. Linear stability is investigated in detail for the 1D rapid diffusion model and numerical simulations of the 1D and 2D models for the symmetric (squeeze) mode are also conducted; this allows a parametric study of the nonlinear rupture time to be performed. Finally, self-similar scaling exponents for all flow variables as rupture is approached are extracted. Our results indicate that scaling exponents for rupture derived in the surfactant-free case are preserved even in the presence of soluble surfactant and absence of surface viscosity. Inclusion of a concentration-independent surface viscosity, however, alters the scalings giving rise to new exponents.
AlN growth kinetics during ion nitriding of aluminum
Authors: Fitz T. , Möller W.
Published: November 2002

To study the kinetics of aluminum ion nitriding, a series of experiments have been performed at an ion energy of 1.6 keV, an ion current density of 0.2 mA/cm2, and substrate temperatures varied from 250 to 400 °C. The nitride layers have been analyzed by nuclear reaction analysis and scanning electron microscopy. Binary collision computer simulations have been performed to calculate the sputtering yields of nitrogen and aluminum. Depending on the experimental conditions, the nitriding kinetics is either controlled by the supply of nitrogen atoms from the ion beam or by the diffusion of aluminum atoms through the growing nitride layer. Solutions of rate equations describing the growth of the nitride layer are fitted to the experimental data, from which diffusion and activation parameters for the aluminum transport are obtained.
Near-field spectroscopic characterization of a 10 μm aperture selectively oxidized vertical cavity surface emitting laser
Authors: Sharma Aditi , Yarrison-Rice Jan M. , Jackson Howard E. , Choquette K. D.
Published: November 2002

We have characterized a 10 μm square aperture selectively oxidized 850 nm vertical cavity surface emitting laser (VCSEL) utilizing a near-field scanning optical microscope coupled to a spectrometer. We have obtained both spatially and spectrally resolved images of the VCSEL’s emission. The spatially resolved intensity distributions were measured for the transverse modes of the VCSELs in the near field and far field at three different excitation currents, at threshold as well as above threshold. We identify the modes as being Hermite–Gaussian. Two-dimensional maps of the total integrated intensity of the spectra reveal an inhomogeneous gain distribution. The relative intensities of the various individual modes as well as their spatial orientation reflect how the available gain of the cavity is used. The wavelengths of the transverse modes were observed to increase with increasing current; calculations show that a temperature-induced change in refractive index of the cavity is the factor contributing to this redshift.
Control of point defects and space charge in electroluminescent ZnS:Mn thin films
Authors: Lewis J. S. , Davidson M. R. , Holloway P. H.
Published: November 2002

The mechanisms leading to improved brightness, efficiency, and stability of alternating-current thin-film electroluminescent (ACTFEL) ZnS:Mn phosphors have been studied. Previously we have shown that ex situ codoping of the sputter deposited ZnS:Mn active layer with K and Cl results in a 53% improvement in brightness, a 62% improvement in efficiency, and better 100 h accelerated aging stability. In this work, we demonstrate that these improvements result from a 75% increase in excitation efficiency for conduction electrons, combined with a small decrease in both light outcoupling and nonradiative recombination. Electrical properties data were used to determine that there is a reduced amount of static space charge in the codoped films, resulting in a larger average field, increased excitation efficiency, and increased charge multiplication. The reduced space charge is attributed to the addition of charge compensating zinc vacancy–chlorine complexes and isolated chlorine point defects, which are acceptor and donor defects, respectively, and a reduction of zinc vacancy deep hole traps. It is postulated that higher average fields results in sufficient electron multiplication or donor ionization such that current for EL excitation is limited by the phosphor resistance rather than capacitance or density of interface states. The possibility of using these mechanisms to increase the efficiency of other ACTFEL phosphors is discussed.
Analysis of the g−(3P1)–B ′3 Π(0u−) system of I2 by perturbation-facilitated optical–optical double resonance
Authors: Motohiro Satoshi , Nakajima Shinsuke , Aoyama Kenji , Kagi Eriko , Fujiwara Hisashi , Fukushima Masaru , Ishiwata Takashi
Published: November 2002

We present the analysis of the 0g−(3P1)–B ′3Π(0u−) system of I2. Both of the states are optically forbidden to access from the X 1Σg+ ground state, and the perturbation-facilitated optical–optical double resonance technique is used to access the 0g−(3P1) ion-pair state in the stepwise excitation. The intermediate states we used are the B 3Π(0u+)∼(3)0u− states coupled by hyperfine interaction, which are identified in the high vibrational levels of the B 3Π(0u+) state near the dissociation limit. These coupled states have the mixed character of the 0u+ and 0u− states, and allow to combine the X 1Σg+ ground state with the 0g−(3P1) state in the (1+1) photon excitation following the optical selection rules for one photon transition: 0g−(3P1)←(3)0u−∼B 3Π(0u+)←X 1Σg+. We elucidate the B 3Π(0u+)∼(3)0u− coupling scheme in the intermediate states by analyzing the second step of double resonance. The 0g−(3P1) state is located on the absolute energy axis and its molecular constants are reported. The B ′3Π(0u−) valence state is analyzed by the dispersed fluorescence spectra from the 0g−(3P1) state in conjunction with the Franck-Condon factor calculations.
Negative ion photoelectron spectroscopy of (benzene)n− (n=53–124) and (toluene)n− (n=33–139): Solvation energetics of an excess electron in size-selected aromatic hydrocarbon nanoclusters
Authors: Mitsui Masaaki , Nakajima Atsushi , Kaya Koji
Published: November 2002

We present a negative ion photoelectron spectroscopic study on the solvation energetics of an excess electron in the size-selected aromatic hydrocarbon nanoclusters, (benzene)n− (n=53–124) and (toluene)n− (n=33–139). The formation and stability of these negatively charged clusters were investigated using mass spectrometry with two different sources: (1) low-energy (≈0.3 eV) electron attachment and (2) high-energy electron impact. The results reveal that very large coordination numbers (n∼25) are necessary for the formation of stable benzene and toluene cluster anions. This suggests that the second solvation layer (effect) is essential for stable binding of the excess electron in these clusters. The energetics of the cluster anions were also explored by negative ion photoelectron spectroscopy. The photoelectron spectra obtained were related to bulk parameters, e.g., solvent reorganization energy and conduction band minimum, via an analysis of vertical detachment energies versus n−1/3. The bulk solvent reorganization energies thus estimated have revealed the excess electron trapping levels in bulk benzene and toluene. The applicability of the generalized cluster size equation is examined in these nondipolar aromatic hydrocarbon systems for the first time.
Rotational spectrum of jet-cooled HfO2 and HfO
Authors: Lesarri Alberto , Suenram R. D. , Brugh Dale
Published: November 2002

The rotational spectrum of jet-cooled hafnium dioxide obtained by laser ablation of a solid ceramic rod has been investigated by Fourier-transform microwave spectroscopy in the 8 to 28 GHz frequency range. Rotational transitions within the ground and several excited vibrational states of the lowest vibrational mode of the molecule have been assigned. The resulting spectra have been fit, yielding rotational parameters for the five most abundant isotopomers of HfO2. Centrifugal distortion effects are noticeable even for the lowest-J transitions. Very large quadrupole coupling effects for the isotopomers with nuclear quadrupole moments (179Hf (I=9/2) and Hf177 (I=7/2)) have been accounted for using the diagonal elements of the nuclear quadrupole coupling tensor. A ground-state effective C2v geometry has been obtained for HfO2, yielding ro(Hf–O)=1.7764(4) Å and ∠(O–Hf–O)=107.51(1)°. The electric dipole moment has been determined for HfO2180 from Stark-effect measurements, giving μ=26.42(3)×10-30C⋅m [7.92(1) D]. Ab initio calculations using density functional theory and relativistic core potentials are in satisfactory agreement with the experimental results. Finally, in the course of this investigation, the rotational spectrum of the diatomic molecule, HfO, has also been reexamined, and new results for vibrational satellites (up to v=18 in some cases) of the J=1←0 rotational transition are reported. A vibrational analysis using Le Roy’s form of the Dunham expansion has allowed the determination of atomic mass-dependent Born–Oppenheimer breakdown parameters for both atoms of HfO.
Metal nanoparticles as models of single crystal surfaces and supported catalysts: Density functional study of size effects for CO/Pd(111)
Authors: Yudanov Ilya V. , Sahnoun Riadh , Neyman Konstantin M. , Rösch Notker
Published: November 2002

Large octahedral and cuboctahedral palladium clusters, ranging from Pd55 to Pd146, have been investigated by means of all-electron relativistic density functional calculations. Adsorption of CO molecules on the (111) facets of these clusters was also studied. In particular, we focused on the interaction of CO (a single molecule per facet) with threefold hollow sites to inspect the variation of the calculated adsorption parameters with cluster size. We considered how observables calculated for that adsorption position on cluster facets relate to adsorption properties of the corresponding site at the single crystal surface Pd(111). We demonstrated for the first time that, with three-dimensional cluster models proposed here, one can reach cluster size convergence even for such a sensitive observable as the adsorption energy on a metal surface. We also addressed size effects on interatomic distances and the cohesive energy of bare Pd nanoclusters whose structure was fully optimized under the imposed Oh symmetry constraint. These quantities were found to correlate linearly with the average coordination number and the inverse of the cluster radius, respectively, allowing a rather accurate extrapolation to the corresponding values of Pd bulk. Finally, we considered the size convergence of adsorption properties of the optimized Pd clusters, as probed by CO adsorption. We also outlined implications of using these symmetric clusters for investigating adsorption and reactions on oxide-supported nanoparticles of model Pd catalysts.
Resonances in SN2 reactions: Two-mode quantum calculations for Cl−+CH3Br on a coupled-cluster potential energy surface
Authors: Schmatz Stefan , Botschwina Peter , Hauschildt Jan , Schinke Reinhard
Published: November 2002

An effective two-dimensional potential energy surface has been constructed for the SN2 reaction Cl−+CH3Br→ClCH3+Br− from coupled-cluster calculations with a large basis set. In the quantum dynamics calculations Radau coordinates were employed to describe the Cl–C and C–Br stretching modes. Making use of the filter diagonalization method and an optical potential, bound states as well as resonance states up to energies far above the dissociation threshold have been calculated. The resonance widths fluctuate over several orders of magnitude. In addition to a majority of Feshbach-type resonances there are also exceedingly long-lived shape resonances, which can only decay by tunneling. Owing to a smaller width of the potential barrier and a larger density of states, tunneling through the barrier is more important for Cl−+CH3Br than for Cl−+CH3Cl despite the larger total mass of this system. Excitation of the C–Br stretching vibration enhances the tunneling probability of the entrance channel complex.
Systematic construction of approximate one-matrix functionals for the electron-electron repulsion energy
Authors: Cioslowski Jerzy , Pernal Katarzyna , Ziesche Paul
Published: November 2002

The Legendre transform of an (approximate) expression for the ground-state energy E0(η,g) of an N-electron system yields the one-matrix functional Vee[Γ(x′,x)] for the electron-electron repulsion energy that is given by the function Vee(n;g) of the occupation numbers n pertaining to Γ(x′,x) and the two-electron repulsion integrals g computed in the basis of the corresponding natural spinorbitals. Extremization of the electronic energy functional, which is a sum of Vee[Γ(x′,x)] and the contraction of Γ(x′,x) with the core Hamiltonian, produces the (approximate) ground-state energy even if E0(η,g) itself is not variational. Thanks to this property, any electron correlation formalism can be reformulated in the language of the density matrix functional theory. Ten conditions that have to be satisfied by Vee(n;g) uncover several characteristics of Vee[Γ(x′,x)]. In particular, when applied in conjunction with the homogeneity property, the condition of volume extensivity imposes stringent constraints upon the possible dependence of Vee(n;g) on g.
Ring structure of the NO dimer radical cation: A possible new assignment of the mysterious IR absorption at 1424 cm−1
Authors: Xie Yaoming , Wang Wenning , Fan Kangnian , Schaefer Henry F.
Published: November 2002

The complete assignment of IR absorptions for the nitric oxide dimer radical cation has been a difficult task for some time. Although the 1619 cm−1 band was recently assigned to the antisymmetric N–O stretch mode for the trans and cis ONNO+ structures, the 1424 cm−1 band has remained a mystery. The ring or rectangular structure of the (NO)2+ cation was examined in this research with density functional theory (DFT) and high-level ab initio methods in the prospect that it might be an energetically low-lying isomer. In conjunction with the above methods, two sets of basis functions were utilized. One is double-ζ plus polarization, and another is triple-ζ plus double polarization with f functions. The ground state of the ring (NO)2+ cation is of Bu2 symmetry. The antisymmetric N–O stretch vibrational frequency is predicted to be nonphysically large with the self-consistent field method due to the symmetry breaking problem. This fundamental is predicted in the ∼1800 cm−1 region based upon DFT methods, but the result is also doubtful because (NO)2+ exhibits the inverse symmetry breaking problem. Since these problems may impair the theoretical vibrational frequencies, higher theoretical levels, namely Brueckner coupled-cluster methods, were ultimately applied, and the harmonic vibrational frequency of this challenging mode was eventually predicted to be about 1400 cm−1. This ring structure lies only ∼5 kcal/mol above the global minimum, so that it might be observable in the laboratory. Moreover, the ring structure is predicted to lie ∼10 kcal/mol below separated NO+NO+. Since no other low-lying isomers were found, it is plausible to assign the 1424 cm−1 band to the ring structure.
Time-dependent density functional calculations of fully π-conjugated zinc oligoporphyrins
Authors: Yamaguchi Yoichi
Published: November 2002

Time-dependent density functional theory (TDDFT) calculations were performed on the excitation energies and oscillator strengths of fully π-conjugated zinc oligoporphyrins. The TDDFT calculated results for the Q bands of the triply meso-meso-, β-β-, and β-β-linked zinc oligoporphyrins can well reproduce the significantly increasing redshifted and intensified absorption maximum for the increasing number of porphyrins in the experimental data, which originates from the greater stabilized lowest unoccupied molecular orbital (LUMO) and the increasing contribution of the highest occupied molecular orbital (HOMO)→LUMO excitation to the bands. In order to estimate the minimum value of the HOMO−LUMO energy gap (Eg) of the infinite size oligoporphyrins, the DFT solid-state calculations for the one-dimensional tapelike and two-dimensional sheetlike π-conjugated zinc porphyrins were carried out using periodic boundary conditions. The results show that there is a metallic energy gap, Eg≈0.0 eV for the tape, and a semiconducting one, Eg≈0.5 eV for the sheet, which is caused by the difference in the local periodic structures at the minimum energy gap between them, namely the directly and indirectly connected zinc porphyrins for the former and the latter, respectively. Similarly, the corresponding infinite fused free-base porphyrins were also examined for comparison, which showed that a metallic behavior is also expected for them. The present fused porphyrin arrays may be used as conducting molecular wires or films.
Separation of radiation and absorption losses in two-dimensional photonic crystal single defect cavities
Authors: Alvarado-Rodriguez I. , Yablonovitch E.
Published: November 2002

We have characterized the optical modes present in a two-dimensional photonic crystal single defect cavity fabricated in an InP/In0.53Ga0.47As/InP double heterostructure thin film on a glass slide. The cavity resonance was tuned to different frequencies in the 1.55 μm spectral region. Radiation losses and material absorption influence the measured value of cavity quality factor Q. We separated these two loss mechanisms by performing a curve fit of the loss rate 1/Q versus the wavelength-dependent absorption coefficient of In0.53Ga0.47As. By extrapolating this curve to zero absorption, the radiation loss rate 1/Qrad is obtained.
Modeling of space-charge effects in nanocrystalline ceramics: The influence of geometry
Authors: Lubomirsky Igor , Fleig Juergen , Maier Joachim
Published: November 2002

The distribution of mobile charge carriers in the space-charge regions at grain boundaries of ceramic materials was modeled. Delocalization effects are neglected, i.e., we consider ionic defects or polarons. The calculations were performed for cubic-shaped grains of equal size. When considering the size dependence, the standard free chemical potentials of the defects rather than the specific grain-boundary charge density or the defects’ boundary concentration were set to be constant in accordance with the core space-charge model. Although specific edge and corner effects are neglected in the present analysis and hence the structural potentials are invariant along grain boundaries, the accumulation or depletion of charge carriers turns out to be inhomogeneous along the grain boundary and to be particularly pronounced near grain edges and grain corners if the grain size was smaller than four Debye lengths. Especially the accumulation near grain edges can have a strong influence on the effective conductivity (though being a purely geometric effect). The modeling also predicts that a contact of two grains that differ only in size, leads to a redistribution of mobile ions between grains, provided that either one or both grain sizes are smaller than the double width of space-charge layers. Such a charge transfer between the grains can be viewed as a “heterosize charging.”
Modeling dielectric properties of composites by finite-element method
Authors: Krakovský Ivan , Myroshnychenko Viktor
Published: November 2002

Dielectric spectra of binary composites consisting of equal-sized hard disks of one dielectric dispersed randomly in a matrix of another dielectric are modeled in two dimensions using the finite-element method. The effects of the volume fraction of the disks and difference in the component permittivities and ohmic conductivities on the effective dielectric behavior of the resulting composite are discussed. Numerical results are compared with the results predicted using a few mixture formulas chosen. For this type of composites, the comparison shows, that in the region of compositions and component properties explored, the Brugemann formula is in the best agreement with the numerical results [Ann. Phys. (Leipzig) 24, 636 (1935)].
Sensitive magneto-optical sensors for visualization of magnetic fields using garnet films of specific orientations
Authors: Klank M. , Hagedorn O. , Shamonin M. , Dötsch H.
Published: November 2002

Garnet films of high Faraday rotation are applied as magneto-optical sensors to visualize the spatial distribution of magnetic fields. Using specific orientations, the induced anisotropy can generate an easy plane of magnetization which is inclined with respect to the film plane. If the magnetization lies in this plane a very high sensitivity can be achieved. The dependence of the geometrical orientation of the easy plane on the growth direction is calculated and the sensitivity and dynamic range are derived. Experimental results of a [112] oriented garnet film are in good agreement with calculations.
Room temperature visible photoluminescence from undoped ZnS nanoparticles embedded in SiO2 matrices
Authors: Zhu Y. , Yuan C. L. , Ong P. P.
Published: November 2002

Thin films of ZnS nanoparticles embedded in SiO2 matrices were prepared by the pulsed laser deposition method using a rotating target. The as-prepared samples contained dispersed ZnS nanocrystallites mostly of less than 8 nm in diameter embedded in the SiO2 host. The nanoparticle size of the film increased with increasing sample annealing temperature and/or duration in the vacuum. Room temperature visible photoluminescence (PL) was found from the films without the need for any dopant. Contrary to the usual redshift following annealing, there was, instead, a blueshift of the PL which increased with increasing sample annealing. The mechanism of the PL and its blueshift is attributed to the S vacancies and the formation of ZnS(O2−) in the surface layers of the nanoparticles adjacent to the SiO2. The S vacancies arose partly from the laser ablation during the film preparation stage, and partly from the S-bond disruption at the surface of the ZnS particles during annealing. The ZnS(O2−) could be formed by the ZnS particles thermally chemisorbing oxygen from the SiO2 host in the interfacial regions between the nanoparticles and their host matrix. The effects on the PL due to S vacancies and ZnS(O2−) formation are similar to the effect due to impurities in the films which results in an effective size reduction of the ZnS nanocrystallites.
Fatigue in artificially layered Pb(Zr,Ti)O3 ferroelectric films
Authors: Jiang A. Q. , Scott J. F. , Dawber M. , Wang C.
Published: November 2002

We have performed fatigue tests on lead zirconate titanate (PZT) multilayers having stacks of Pb(Zr0.8Ti0.2)O3/Pb(Zr0.2Ti0.8)O3 with repeated distances of 12 formula groups. The results are compared with single-layer n-type (0.5 at. % Ta-doped) PZT films. We conclude that fatigue is dominated by space-charge layers in each case, but that in the multilayer such space charge accumulates at the layer interfaces, rather than at the electrode–dielectric interface. The model, which includes both drift and diffusion, is quantitative and yields a rate-limiting mobility of 6.9±0.9×10−12 cm2/V s, in excellent agreement with the oxygen vacancy mobility for perovskite oxides obtained from Zafar et al.
Effects of low-temperature Si buffer layer thickness on the growth of SiGe by molecular beam epitaxy
Authors: Lee S. W. , Chen H. C. , Chen L. J. , Peng Y. H. , Kuan C. H. , Cheng H. H.
Published: November 2002

The thickness of a low-temperature silicon (LT-Si) buffer layer has been found to affect the growth of a SiGe overlayer significantly. 300-nm-thick Si0.7Ge0.3 films were grown on 50- to 300-nm-thick LT-Si buffer layers at 450 °C by solid-source molecular beam epitaxy. The threading dislocation density was found to decrease with the thickness of the LT-Si buffer in the thickness range of 50–200 nm. The density remains at the same low level when the thickness was increased from 200 to 300 nm. A relatively dense misfit dislocation network was observed to form at the SiGe/Si interface in samples with the LT-Si buffer layer thickness exceeding 200 nm. It is suggested that the presence of more point defects in the thicker LT-Si buffer layer is more effective to block the propagation of threading dislocations.
Effect of low dose high energy O3+ implantation on refractive index and linear electro-optic properties in X-cut LiNbO3: Planar optical waveguide formation and characterization
Authors: Bentini G. G. , Bianconi M. , Chiarini M. , Correra L. , Sada C. , Mazzoldi P. , Argiolas N. , Bazzan M. , Guzzi R.
Published: November 2002

X-cut LiNbO3 crystals were implanted at room temperature by 5.0 MeV O3+ ions with doses ranging from 1.0×1014 to 6.0×1014 O/cm2. Secondary ion mass spectrometry profiles of atomic species migration as well as damage profiles by the Rutherford backscattering channeling technique and refractive index variation were investigated as a function of dose and subsequent annealing conditions. Two different kinds of damage produced by oxygen implantation were seen: near-surface damage correlated to electronic stopping, which causes an increase of the extraordinary refractive index, and end-of-ion range damage generated by collision cascades, which decreases the extraordinary refractive index values. The different nature of the two kinds of damage is also seen by the different temperature conditions needed for recovery. Low loss planar optical waveguides were obtained and characterized by the prism coupling technique.
Self-consistent three-dimensional model of dust particle transport and formation of Coulomb crystals in plasma processing reactors
Authors: Vyas Vivek , Hebner Gregory A. , Kushner Mark J.
Published: November 2002

Dust particle transport in low-temperature plasmas has received considerable attention due to the desire to minimize contamination of wafers during plasma processing of microelectronic devices and for their use to study nonideal plasmas. Dust particles in radio frequency discharges form Coulomb crystals and display collective behavior under select conditions. In this article, we discuss results from a self-consistent three-dimensional model for dust particle transport in plasma processing reactors. The consequences of varying the bias voltage of the capacitively coupled discharge, plasma density, particle diameter, and the number of particles on the propensity for Coulomb crystal formation in argon plasmas will be discussed. We found that a single one-layer lattice spontaneously breaks up into separate lattices as the substrate bias is increased due to a redistribution of plasma forces. At high substrate biases, a void occurs in the plasma crystal which tends to close upon addition of electronegative gases such as O2 and Cl2 to argon. The interparticle spacing in the lattice depends on the number of particles in the lattice due to gravitational compressive forces; and on the plasma density due to the change in shielding length.
Formation of the charged exciton complexes in self-assembled InAs single quantum dots
Authors: Moskalenko E. S. , Karlsson K. F. , Holtz P. O. , Monemar B. , Schoenfeld W. V. , Garcia J. M. , Petroff P. M.
Published: November 2002

We have studied the low-temperature photoluminescence (PL) of the self-assembled InAs single quantum dots (QDs) using conventional micro-PL setup to detect PL from an individual QD. It is demonstrated, that at certain experimental conditions, what concerns the laser excitation energy, the laser power and the crystal temperature, several additional lines, redshifted relative to the ground state transition, appear in the PL spectra. These are interpreted in terms of charged exciton complexes which form due to the population of quantum dots with a nonequal amount of electrons and holes. The latter phenomenon is determined by the excess energies of photogenerated carriers and is proposed as an effective optical method to create and study charged exciton complexes in QDs.
Photoluminescence study of InAs quantum dots embedded in GaNAs strain compensating layer grown by metalorganic-molecular-beam epitaxy
Authors: Zhang X. Q. , Ganapathy Sasikala , Kumano Hidekazu , Uesugi Kasturi , Suemune Ikuo
Published: November 2002

Self-assembled InAs quantum dots (QDs) embedded in GaN0.007As0.993 strain compensating layers have been grown by metalorganic-molecular-beam epitaxy on a GaAs (001) substrate with a high density of 1×1011 cm−2. The photoluminescence properties have been studied for two periods of InAs quantum dots layers embedded in GaN0.007As0.993 strain compensating layers. Four well-resolved excited-state peaks in the photoluminescence spectra have been observed from these highly packed InAs QDs embedded in the GaN0.007As0.993 strain compensating layers. This indicates that the InAs QDs are uniformly formed and that the excited states in QDs due to the quantum confinement effect are well defined. This is explained by tensile strain in GaNAs layers instead of the usual GaAs layers to relieve the compressive strain formed in InAs QDs to keep the total strain of the system at a minimum.
Relaxation of a tethered polymer using dynamic mean field calculation
Authors: Furuichi Kenji , Nonomura Chisato , Kawakatsu Toshihiro , Doi Masao
Published: November 2002

Relaxation process of a stretched tethered polymer chain was investigated using the dynamic mean field technique. We confirmed that the relaxation of the end-to-end distance of the chain is consistent with that of Rouse dynamics.
Evaluating shear viscosity: Power dissipated versus entropy produced
Authors: Holian Brad Lee
Published: November 2002

For nonequilibrium molecular-dynamics simulations of shear flow in a fluid, we show that evaluation of the shear viscosity by means of an expression derived from the dissipated mechanical power is an order of magnitude more efficient than that for entropy production (heat flow into the thermostat).
The gradual transition from mass-controlled to diffusion-controlled kinetics during melt polymerization
Authors: Wang J. , Johari G. P.
Published: November 2002

For most chemical reactions, the rate coefficient, k, is independent of the extent of reaction, α, and varies with the temperature, T, according to the Arrhenius equation, but, for melt polymerization at a fixed T, the diffusion coefficient, D, of the reactant pairs decreases as α increases, and for viscous liquids in general D varies with T according to the Vogel–Fulcher–Tammann equation. We propose that a change in the mass-controlled to diffusion-controlled reaction kinetics during a melt’s polymerization would be seen first as k begins to decrease with increase in α, and second as the temperature dependence of k for a fixed α deviates from the Arrhenius to the Vogel–Fulcher–Tammann type. The range of α and T over which this change occurs may be determined by calorimetry or related experiments. Polymerization of a liquid mixture to a random network structure has been studied by calorimetry. It is shown that, (i) the ln(k) against α plot at a fixed T bends downwards progressively more as α increases, and (ii) over a given range of T, the ln(k) against 1/T plot at a fixed α is a straight line when α is low, and bends downwards when α is high. The onset temperature of this bend increases as α is increased. Thus the gradual onset of diffusion control varies with both α and T. The simulated dα/dt for mass-controlled kinetics is higher than that for diffusion-controlled kinetics up to a certain time and lower thereafter. The ratio of dα/dt for the two kinetics shows a local maximum at a certain time. The procedure developed here would be useful for studying diffusion control in biological processes.
Real-space mean-field approach to polymeric ternary systems
Authors: Komura Shigeyuki , Kodama Hiroya , Tamura Keizo
Published: November 2002

Phase separated structure of ternary blends of A and B homopolymers and symmetric AB diblock copolymer is investigated using a lattice (real-space) self-consistent field theory. This paper includes the detailed description of our published results [Kodama, Komura, and Tamura, Europhys. Lett. 53, 46 (2001)] as well as more extended calculations. We consider the symmetric case, namely, (i) both A and B homopolymers have the same degree of polymerization NA=NB; (ii) AB diblock copolymer of length NAB is symmetric; (iii) average volume fractions of A and B homopolymers are equal. We looked into the influence of relative chain lengths α=NA/NAB on the phase separated structure. Our numerical simulations are performed in the real space without assuming the symmetry of the structure a priori. For the fixed copolymer length and α<1, the typical length scale of the microphase separated structure become smaller for relatively shorter homopolymer chains (small α). In other words, the homopolymers becomes more efficient to swell the microphase separated structure for longer homopolymer chains (large α). Detailed free-energy analysis revealed that the stability of the lamellar phase is marginal for small block copolymer volume fraction. For α>1, on the other hand, three-phase coexistence either between the disorder, A-rich and B-rich phases or between the lamellar, A-rich and B-rich phases is observed.
Car–Parrinello molecular dynamics simulation of the hydrated calcium ion
Authors: Bakó I. , Hutter J. , Pálinkás G.
Published: November 2002

First principles molecular dynamics has been used to investigate the structural, vibrational, and energetic properties of [Ca(H2O)n]2+ clusters with n=1–9, and the hydration shell of a calcium ion in a periodically repeated box with 54 water molecules. We find that, while stable highly symmetric Ca–water clusters can be formed with up to eight water molecules, the n=9 cluster dissociates into the last stable [Ca(H2O8]2+ complex. In solution the first hydration shell around the Ca2+ ion contains six water molecules in an octahedral arrangement. The electronic structure of nearest neighbor hydration shell water molecules has been examined with a localized orbital analysis. The average dipole moments of hydration water molecules was found to be increased by about 0.4 Debye relative to that of pure water.
Derivation and reflection properties of a transmission-free absorbing potential
Authors: Manolopoulos David E.
Published: November 2002

The criterion for the validity of the JWKB approximation is used to derive a first order differential equation for a negative imaginary absorbing potential, −iε(r). The resulting absorbing potential switches on as rapidly as possible with increasing r without causing too much reflection, and it has a second order pole at the end of the absorbing region which eliminates all transmission. It is completely specified by a single physical parameter: the minimum energy Emin at which absorption is required; the width of the absorbing region can be chosen to correspond to the de Broglie wavelength at this minimum energy. With this choice, the exact quantum mechanical reflection probability of the potential is <1% at E=Emin, <0.1% at E=2Emin, and <0.01% for all E⩾3Emin. Because of its lack of parameters and its convenient reflection properties, we anticipate that the new potential (and other potentials of a similar form) will find useful application in both time-dependent and time-independent quantum scattering calculations.
Determination of bound-free dissociative couplings via classical Fourier coefficients
Authors: Tannenbaum Emmanuel , Heller Eric J.
Published: November 2002

This paper presents an approach to compute couplings between bound and unbound wave functions using only classical Fourier coefficients of the Hamiltonian. This approach is an extension of the well-known technique of using Fourier coefficients in the action-angle representation to compute bound-state to bound-state couplings. We develop the analogous bound-free approach for one-dimensional Hamiltonians and demonstrate it for several coupling potentials. The generalization to higher dimensions is also discussed.
Semiclassical initial value representation for the Boltzmann operator in thermal rate constants
Authors: Zhao Yi , Miller William H.
Published: November 2002

The thermal rate constant for a chemical reaction, k(T), can be expressed as the long time limit of the flux-side correlation Cfs(t)=tr[e−βĤ/2F̂e−βĤ/2eiĤt/ℏĥe−iĤt/ℏ]. Previous work has focused on semiclassical (SC) approximations [implemented via an initial value representation (IVR)] for the time evolution operators exp(±iĤt/ℏ) in the correlation function, and this paper shows how an SC-IVR can also be used to approximate the Boltzmann operators exp(−βĤ/2). Test calculations show that over a wide temperature range little error is introduced in the rate constant by this SC approximation for the Boltzmann operator.
Wave packet dynamics on adaptive moving grids
Authors: Wyatt Robert E.
Published: November 2002

A new hybrid moving grid approach to wave packet dynamics is described. Exterior points within a nonrigid frame follow Lagrangian trajectories obtained by solving the hydrodynamic equations of motion. Internal grid points within one or more open windows follow non-Lagrangian adaptive paths. Within these windows, problems encountered with quantum trajectories near wave function nodes are circumvented by directly solving the moving path transform of the Schrödinger equation. Excellent results are obtained for evolution of the density in a double well potential even though multiple ripples develop in the density.
Transitions in rotations of a nonspherical particle in a three-dimensional moderate Reynolds number Couette flow
Authors: Qi Dewei , Luo Lishi
Published: November 2002

The rotational states of three-dimensional nonspherical particles including cylindrical and disk-shaped, as well as prolate and oblate, ellipsoidal in a Couette flow are studied by using a lattice Boltzmann simulation. We discover that rotation of a nonspherical particle exhibits several different states, depending on the ranges of the particle Reynolds numbers and the geometric shape of the particle. As the Reynolds number increases, the rotation transits from one state to another state.
Transport properties of heavy particles in high Reynolds number turbulence
Authors: Olla Piero
Published: November 2002

The statistical properties of heavy particle trajectories in high Reynolds numbers turbulent flows are analyzed. Dimensional analysis assuming Kolmogorov scaling is compared with the result of numerical simulation using a synthetic turbulence advecting field. The non-Markovian nature of the fluid velocity statistics along the solid particle trajectories is put into evidence, and its relevance in the derivation of Lagrangian transport models is discussed.
Vibrational relaxation rates in the direct simulation Monte Carlo method
Authors: Gimelshein N. E. , Gimelshein S. F. , Levin D. A.
Published: November 2002

Exact relationship is developed that connects the vibrational relaxation number, ZvDSMC, used in the direct simulation Monte Carlo method and that employed in continuum simulations. An approximate expression for ZvDSMC is also derived that is cost-effective and applicable when translational temperature is larger than vibrational temperature.
Study of diluted magnetic semiconductor: Co-doped ZnO
Authors: Lee Hyeon-Jun , Jeong Se-Young , Cho Chae Ryong , Park Chul Hong
Published: November 2002

We report on the high-temperature ferromagnetism in Co-doped ZnO films fabricated by the sol–gel method above 350 K. The lattice constant of c axis of wurtzite Zn1−xCoxO follows Vergard’s law for 0<x<0.25. For Zn1−xCoxO with x⩾0.25, a secondary phase is detected. The Zn1−xCoxO exhibits ferromagnetic behavior with a Curie temperature higher than 350 K. By the results of x-ray photoelectron spectroscopy measurement, we assume that Co occupied the Zn site without changing the wurtzite structure. In the case of x=0.2, the coercive field measured by a magnetization–magnetic field hysteresis curve at 350 K was nearly 80 Oe. Additionally, we investigated the electric structure through first-principles pseudopotential plane-wave calculation.
Some dynamical properties of a differential model for the bursting cycle in the near-wall turbulence
Authors: Porporato A. , Ridolfi L.
Published: November 2002

In the last years several investigations have been devoted to model the bursting cycle in the near-wall turbulence by means of low-dimensional systems, with the aim of having simple mathematical models for the dynamics of coherent structures. The present paper deals with a low-dimension differential model, recently proposed by the authors. It is directly deduced from the velocity time series measured in a turbulent flow and well mimics the velocity oscillations typical of the bursting events. After studying the linear stability of the model, its behavior when an external forcing is added, both deterministic and stochastic, is analyzed. It is found that the essential characteristic of the dynamics described by the model is a Hopf bifurcation that, when excited by a stochastic forcing, produces time series with fluctuations that have similarities with the real turbulence signals.
Variable sphere molecular model for inverse power law and Lennard-Jones potentials in Monte Carlo simulations
Authors: Matsumoto Hiroaki
Published: November 2002

The variable sphere (VS) molecular model for the Monte Carlo simulation of rarefied gas flow is introduced to provide consistency for diffusion and viscosity cross-sections with those of any realistic intermolecular potential. It is then applied to the inverse power law (IPL) and Lennard-Jones (LJ) potentials. The VS model has a much simpler scattering law than either the variable hard sphere (VHS) or variable soft sphere (VSS) models; also, it has almost the same computational efficiency as the VHS and VSS models. A simulation of velocity relaxation in a homogeneous space and two comparative simulations of molecular diffusion in a homogeneous heat-bath gas and normal shock wave structure in a monatomic gas are made to examine VS model validity. The relaxation to a Maxwellian distribution function and equipartition between all degrees of freedom are well established; good agreement is shown in the molecular diffusion and shock wave structure between the VS model and the IPL and LJ potentials. The VS model is combined with the statistical inelastic cross-section (SICS) model and applied to simulation of translational and rotational energy relaxation in a homogeneous space. The VS model shows the relaxation of Maxwellian and Boltzmann distribution functions and equipartition between all degrees of freedom. Comparative calculation between the VS model with the SICS (VS-SICS) model and the VSS model with the SICS (VSS-SICS) model is made for rotational relaxation in a nitrogen normal shock wave. Good agreement is shown in the shock wave structure and rotational energy distribution function between the VS-SICS model and the VSS-SICS model. This study demonstrates that diffusion and viscosity cross-sections, rather than the scattering law of each molecular collision, affect macroscopic transport phenomena.