bluuebunny/crossref_metadata_2025 huggingface dataset:
Structures and vibrations of phenol(NH3)2−4 clusters
Authors: Schmitt M. , Jacoby Ch. , Gerhards M. , Unterberg C. , Roth W. , Kleinermanns K.
Published: July 2002

Vibronic spectra of PhOH(NH3)n clusters with n=2–4 have been obtained by resonant two-photon ionization, recorded at the mass channels of the fragment ions (NH3)nH+. The PhOH(NH3)2–4 spectra show long progressions of at least one low frequency vibration pointing to different S0 and S1 geometries along this coordinate. In addition, the vibronic bands of the n=2 cluster are split into two components. A tunneling motion is discussed, which may be responsible for these splittings. To get more information about the structure of PhOH(NH3)2 in the electronic ground state, IR–UV double resonance spectroscopy has been applied. Possible geometries for the n=2–4 clusters are considered based on a comparison between the experimental data and theoretical results from ab initio calculations, performed at the Hartree–Fock and second-order Møller–Plesset perturbation theory level.
Matrix effects and mechanisms of the spectral shifts of coumarin 440 doped in sol–gel-derived gel glass
Authors: Qian G. , Yang Z. , Yang Chunlei , Wang Minquan
Published: July 2002

The spectral and temporal changes of the emission from four types of gel glass doped with coumarin 440 prepared by using tetraethoxysilane (TEOS), 3-glycidoxypropyltrimethoxysilane (GPTMS), methyltriethoxysilane (MTES), and vinyltriethoxysilane (VTES) as sol–gel precursors, respectively, were studied. The observed significant spectral shifts of coumarin 440 in the TEOS– and TEOS/GPTMS–gel glass are due to two types of matrix effects on different time scales. The formation of hydrogen bonding between the carbonyl and amine groups of coumarin 440 and the residual Si–OH and H2O in the TEOS– and TEOS/GPTMS–gel glass shifts the emission spectra to longer wavelengths on the subnanosecond time scale with respect to that in the MTES– and VTES–gel glass. The temperature and time dependent spectral shifts on the nanosecond time scale are attributed to the reorientation of the surrounding matrix molecules during the lifetime of the excited state of coumarin 440. No residual Si–OH groups H2O molecules are found in the MTES– and VTES–gel glass prepared by acid-catalyzed hydrolysis and basic-catalyzed condensation, and the emission spectra of coumarin 440 in these matrices show no dependence on time and temperature.
Stress distribution in thin heteroepitaxial diamond films on Ir/SrTiO3 studied by x-ray diffraction, Raman spectroscopy, and finite element simulations
Authors: Schreck M. , Roll H. , Michler J. , Blank E. , Stritzker B.
Published: July 2002

The residual stress in thin diamond films with a strongly improved heteroepitaxial alignment has been studied by x-ray diffraction and micro-Raman spectroscopy. The measurements have been compared with the predictions from finite element simulations. The diamond films have been deposited by microwave plasma chemical vapor deposition at a temperature of 700°C on thin (200 nm) iridium buffer layers on SrTiO3(001). Three different regions have been found for a 600 nm thick diamond film: (I) a high quality epitaxial central area with >109 cm−2 oriented diamond grains showing a mosaic spread of only ≈1°; (II) a ringlike area of isolated epitaxial islands; and (III) a nontextured closed film at the edge of the sample. In area I the stress tensor was determined from the mean shift of the x-ray Bragg reflections. It can be interpreted in terms of a plane, biaxial stress state with σ=−4.9 GPa which is confirmed by micro-Raman measurements. Analyzing the diamond (004) and (311) peak profiles measured by x-ray diffraction (XRD) using monochromatic CuKα1 radiation allows us to distinguish a strongly shifted main component and a weaker, broader component with a minor shift. Finite element simulations predict a pronounced elastic relaxation of the thermal stress at rugged surfaces thus explaining this minor component. They also substantiate a stress reduction by more than 80% as observed by Raman measurements in area II. Combining all measurements taken in the different areas with the predictions of the simulation allows to separate four contributions, i.e., the thermal stress, elastic stress relaxation at a rugged surface, inhomogeneous stress contributions from the coalescence of the grains, and finally coherence stress due to lattice misfit.
Effect of Tb3+ co-doping on the electron population densities of Tm3+ in Ge–As–Ga–S glasses
Authors: Shin Yong Beom , Kim Ji Hyun , Kim You Song , Heo Jong
Published: July 2002

The 1.48-μm emission properties and electron population densities of the excited levels were investigated for Ge–As–Ga–S glasses doped with Tm3+ or Tm3+/Tb3+. Electron population inversion between the H43 and F43 levels in Tm3+ cannot be achieved without Tb3+ addition due to the strong cross-relaxation in Tm3+. Co-doping of Tb3+ resulted in the large decrease in the population density of the Tm3+:F43 level via the energy transfer of Tm3+:3F4→Tb3+:7F0,1,2. Calculated population densities of the two levels in Tm3+ using the rate equations showed that the population inversion became possible through the addition of Tb3+.
Electronic properties of silicon epitaxial layers deposited by ion-assisted deposition at low temperatures
Authors: Oberbeck Lars , Bergmann Ralf B.
Published: July 2002

Ion-assisted deposition enables epitaxial growth of high-quality Si films with high deposition rates at low growth temperatures. The present study investigates structural and electronic properties of monocrystalline Si epitaxial layers deposited at temperatures between 470 and 800 °C and deposition rates between 0.07 and 0.18 μm/min. The density of extended defects decreases with increasing deposition rate and saturates at values around 2×104 cm−2 for deposition rates above 0.16 μm/min. We apply methods of design of experiment to systematically investigate the influence of deposition parameters on the electronic properties of epitaxial layers. The majority carrier mobility in p- and n-type layers reaches values comparable to those in Czochralski Si and does not depend on deposition rate, deposition temperature, and sample pretreatment prior to epitaxy in the investigated parameter range. The minority carrier diffusion length strongly increases with rising deposition temperature and reaches about 22 μm in a 7 μm thick epitaxial layer deposited at 710 °C. Extended structural defects limit the minority carrier diffusion length at deposition temperatures below 500 °C, while point defects limit the minority carrier diffusion length at deposition temperatures between 540 and 800 °C.
Multidimensional alignment structure for the liquid crystal director field
Authors: Hoke Charles D. , Bos Philip J.
Published: July 2002

An alignment technique for liquid crystal devices is presented that consists of polymer walls made from monomers that are oriented by an electric field while cured with a spatially patterned UV light source. The technique provides defined boundary conditions of the liquid crystal director field in three dimensions.
Heavy-particle hybrid modeling of transients in a direct-current argon discharge
Authors: Donkó Z.
Published: July 2002

A hybrid model incorporating heavy-particle processes was developed to describe the transient behavior of a low-pressure, cold-cathode, direct-current-excited argon discharge, recently studied experimentally by Jelenković and Phelps [J. Appl. Phys. 85, 7089 (1999)]. The transients were initiated by applying a high-voltage pulse to the discharge, which induced a transition from the Townsend state to the abnormal glow mode. The model made it possible to calculate the transient electrical characteristics, as well as the spatiotemporal changes of the light intensity distribution of the discharge. While in the initial state of the discharge most of the light emission occurs near the anode, with the development of the cathode sheath (taking place on the μs time scale), intensity peaks corresponding to the negative glow (created by electron-impact excitation) and the cathode glow (created by heavy-particle excitation) could be identified. The results of the simulations are in good agreement with the experimental observations, both in terms of electrical signals and light intensity distributions. It was also found that hybrid models not taking into account heavy-particle processes (and assuming that the secondary electron emission coefficient depends on the reduced electric field at the cathode surface) do not give satisfactory agreement with the experimental data.
Optimization of lamellar gratings for quantum-well infrared photodetectors
Authors: Dupont E.
Published: July 2002

This article presents the results of a self-consistent model of midinfrared intersubband absorption in n-doped quantum wells, which are optically coupled by one-dimensional reflective gratings. In the vicinity of the cutoff of the grating, multiple reflections at the well-barrier interfaces play an important role. With an optimized grating, a near-unity absorption quantum efficiency is predicted for the transverse-magnetic polarization. The influence of the top contact on the design of the grating is studied. Performances of standard detectors are compared to those using an additional aluminum-arsenide confinement layer below the bottom contact. Finally, the influence of the number of wells on the performance of the device is investigated.
Cavity formation energies for diatomic and spherical solutes in a diatomic hard body fluid
Authors: Ben-Amotz Dor , Omelyan Igor P.
Published: July 2002

The chemical potentials of spherical and diatomic dumbbell particles dissolved in a hard dumbbell fluid are determined using the Widom insertion Monte Carlo simulation method. Results obtained as a function of fluid density and solute–solvent size ratio are compared with previous simulation results and analytical hard body fluid expressions derived from bonded hard sphere (BHS), scaled particle theory (SPT), and corresponding hard sphere (CHS) equations of state. The BHS predictions best represent all the simulation results, while SPT predictions are comparably accurate except for small solute particles dissolved in high-density fluids, and CHS predictions are exact to first order in solute size and solvent density but somewhat less accurate for large particles at high densities. Simulations of the excess reaction free energy for model dissociation and isomerization processes illustrate the subtle effects of solute shape on cavity formation energy for particles with identical molecular volumes.
Hall constant in quantum-sized semimetal Bi films: Electric field effect influence
Authors: Butenko A. V. , Shvarts Dm. , Sandomirsky V. , Schlesinger Y. , Rosenbaum R.
Published: July 2002

We report on the influence of a gate voltage on the Hall constant [electric field effect Hall constant (EFE–HC)] in a quantum-sized Bi film. The dependence of EFE–HC on the applied electric field, film thickness, and temperature was measured. The electric field effect induces a change of several tens of percent in the Hall constant under an applied electric field of 108 V/m. The effect depends on the film thickness in an oscillatory manner similar to that observed in other quantum-size characteristics. We present an interpretation of the known temperature maximum of HC in quantum size Bi films in absence of EFE, by considering the temperature dependence of the electron and hole mobilities.
Origin of temperature-sensitive hole current at low gate voltage regime in ultrathin gate oxide
Authors: Ang C. H. , Ling C. H. , Cheng Z. Y. , Cho B. J.
Published: July 2002

The carrier-separation characteristics of a p-channel metal–oxide–semiconductor field-effect transistor with 29 Å gate oxide has been measured at various temperatures from 90 to 375 K. It is found that the gate and source/drain currents at low gate voltage regime (below 0.5 V) were correlated and strongly dependent on temperature above 250 K. The earlier observation has been attributed to the existence of a temperature-sensitive hole direct-tunneling current due to the strong temperature sensitivity of surface hole’s concentration at low voltage regime.
Photodissociation dynamics of the CNN free radical
Authors: Bise Ryan T. , Hoops Alexandra A. , Choi Hyeon , Neumark Daniel M.
Published: July 2002

The spectroscopy and photodissociation dynamics of the Ã 3Π and B̃ 3Σ− states of the CNN radical have been investigated by fast beam photofragment translational spectroscopy. Vibronic transitions located more than 1000 cm−1 above the Ã 3Π←X̃ 3Σ− origin were found to predissociate. Photofragment yield spectra for the B̃ 3Σ−←X̃ 3Σ− band between 40 800 and 45 460 cm−1 display resolved vibrational progressions with peak spacing of ≈1000 cm−1 corresponding to symmetric stretch 10n and combination band 10n301 progressions. Ground state products C(3P)+N2 were found to be the major photodissociation channel for both the Ã 3Π and B̃ 3Σ− states. The translational energy distributions for the Ã 3Π state are bimodal with high and low translational energy components. The distributions for the B̃ 3Σ− state reveal partially resolved vibrational structure for the N2 photofragment and indicate extensive vibrational and rotational excitation of this fragment. These results suggest that bent geometries are involved in the dissociation mechanism and provide more accurate values: ΔfH0(CNN)=6.16±0.05 eV and ΔfH298(CNN)=6.15±0.05 eV. These values, coupled with recent D0(RH) and D298(RH) values from Clifford et al. [J. Phys. Chem. 102, 7100 (1998)], yield ΔfH0(HCNN)=5.02±0.18 eV, ΔfH298(HCNN)=4.98±0.18 eV, ΔfH0(H2CNN)=3.09±0.21 eV, and ΔfH0(H2CNN)=3.09±0.21 eV.
Ferroelectric Bi4Ti3O12 thin films on Pt-coated silicon by halide chemical vapor deposition
Authors: Schuisky Mikael , Hårsta Anders , Khartsev Sergey , Grishin Alex
Published: July 2002

A halide chemical vapor deposition technique has been developed to grow highly c-axis oriented submicron Bi4Ti3O12 films onto Pt(200 nm)/Ti(20 nm)/SiO2(1200 nm)/Si(100) substrates. BiI3, TiI4, and oxygen were used as precursors. The total gas pressure was found to be the critical processing parameter to grow films with good crystalline and dielectric properties. The 300 nm thick Bi4Ti3O12 films fabricated at 700 °C and total gas pressure of 3.8 Torr exhibit the best dielectric performance: dielectric constant around 200 and loss tan δ∼0.018 at 100 kHz, remnant polarization of 5.3 μC/cm2, induced polarization of 14.9 μC/cm2 at 560 kV/cm, coercive field of 150 kV/cm, electrical tunability of 51% at 350 kV/cm, resistivity of 2×109 Ω cm, and leakage current as low as 3×10−5 A/cm2 at 100 kV/cm. The effect of weak reduction of the remnant polarization and coercive field has been observed in the temperature range from 300 to 77 K and ascribed to the in-plane orientation of the polar a axis in fabricated Bi4Ti3O12 films.
Ligand-binding distributions in biopolymers
Authors: Poland Douglas
Published: July 2002

The probability distribution that a biopolymer has n ligands bound to it can be determined from the ligand-binding curve that gives the average number of ligands bound as a function of free-ligand concentration in solution. One fits the binding curve as a function of ligand concentration locally to an expansion in the ligand concentration. The expansion coefficients can be turned into moments of the ligand-binding distribution function which, using the maximum-entropy method, gives an accurate construction of the entire ligand-binding distribution function. A linear expansion gives two moments of the distribution while a cubic expansion gives four. In many cases two moments are sufficient to give a very accurate distribution function. The method is exactly analogous to the use of heat capacity data as a function of temperature to construct the enthalpy probability distribution. As with the case of the enthalpy distribution applied to proteins, knowledge of four moments of the distribution function is sufficient to resolve bimodal behavior in the distribution function. Several examples using model systems that involve independent units, cooperative units, and ligand-induced conformational changes (illustrating bimodal behavior) are given. We then examine literature data for the titration of ribonuclease and, using our method of moments, resolve all 30 average proton binding constants for the molecule.
A class of Liouville-integrable Hamiltonian systems with two degrees of freedom
Authors: McLenaghan Raymond G. , Smirnov Roman G.
Published: July 2002

A class of two-dimensional Liouville-integrable Hamiltonian systems is studied. Separability of the corresponding Hamilton–Jacobi equation for these systems is shown to be equivalent to other fundamental properties of Hamiltonian systems, such as the existence of the Lax and bi-Hamiltonian representations of certain fixed types. Applications to physical models, including the Calogero–Moser model, an integrable case of the Hénon-Heiles potential and the nonperiodic Toda lattice are presented.
Improvement of charge trapping by hydrogen post-oxidation annealing in gate oxide of 4H–SiC metal–oxide–semiconductor capacitors
Authors: Cho Won-ju , Kosugi Ryoji , Fukuda Kenji , Arai Kazuo , Suzuki Seiji
Published: July 2002

The effect of hydrogen postoxidation annealing (POA) on the reliability of gate oxide formed in 4H–SiC metal–oxide–semiconductor (MOS) capacitors has been investigated. Argon POA at 1200 °C and hydrogen POA were carried out over a temperature range of 400–1000 °C to improve the properties of 4H–SiC/SiO2 interface and thermal gate oxide. Interface state density Dit decreases as the temperature of hydrogen POA increases and saturates at 800 °C. Additionally, the characteristics of charge trapping in gate oxide against the electron injection was greatly improved by the hydrogen POA above 800 °C. Hence, we can conclude that the hydrogen POA at high temperature is effective for improving the Dit and reliability of gate oxide formed in 4H–SiC MOS devices.
Experimental observation of a “convective cell” in electron phase space in an inductively coupled radio-frequency plasma
Authors: Kortshagen U. , Heil B.
Published: July 2002

Spatially resolved two-dimensional Langmuir-probe measurements of energy-resolved electron fluxes have been performed in an inductively coupled radio-frequency plasma. A flux pattern reminiscent of a “convection cell” in energy-configuration space has been observed. The measurements are interpreted in terms of a total-energy picture of the plasma electrons.
Eley–Rideal reaction of O+ with oxidized Si(100)
Authors: Quinteros C. L. , Tzvetkov T. , Jacobs D. C.
Published: July 2002

The reaction of 10–60 eV O+ ions with a silicon oxide thin film produces scattered O2−. Isotopic labeling experiments demonstrate that the O2− product is formed by an abstraction reaction and not by physical sputtering. Energy and angle resolved detection reveals a correlation between the scattered and incident particle momenta, indicative of a direct process in which the incoming oxygen atom reacts with an adsorbed oxygen atom through an Eley–Rideal mechanism.
Thermochemistry of solvation: A self-consistent three-dimensional reference interaction site model approach
Authors: Kovalenko Andriy , Truong Thanh N.
Published: July 2002

We developed a self-consistent three-dimensional reference interaction site model integral equation theory with the molecular hypernetted chain closure (SC-3D-RISM/HNC) for studying thermochemistry of solvation of ionic solutes in a polar molecular solvent. It is free from the inconsistency in the positions of the ion–solvent site distribution peaks, peculiar to the conventional RISM/HNC approach and improves the predictions for the solvation thermodynamics. The SC-3D-RISM treatment can be readily generalized to the case of finite ionic concentrations, including the consistent dielectric corrections to provide a consistent description of the dielectric properties of ion–molecular solution. The proposed theory is tested for hydration of the Na+ and Cl− ions in ambient water at infinite dilution. An improved agreement of the ion hydration structure and thermodynamics with molecular simulation results is found as compared to the conventional RISM/HNC treatment.
Implementation and refinement of the modified-conductorlike screening quantum mechanical solvation model at the MP2 level
Authors: Baldridge Kim K. , Jonas Volker
Published: July 2002

A modified conductorlike screening continuum solvation model, implemented in the quantum chemistry program GAMESS, has been extended to second order perturbation theory (MP2). Two possible schemes have been considered: (a) the calculation of the MP2 energy using the solvated Hartree–Fock (HF) orbitals, and (b) the implementation of a double-iterative procedure where the HF density is updated with respect to the MP2 surface charges. The influence of the self-consistency of the surface charge distribution with respect to the MP2 density has been analyzed for a small dataset of 21 neutral molecules and 13 ions. In addition, the details of the distribution of surface charge density (σ profiles) and the effects of electron correlation on the accuracy of such distributions is analyzed in terms of the overall concept of deviation of continuum models from dielectric theory, leading to insights into higher order models.
Fatigue anisotropy in single crystal Pb(Zn1/3Nb2/3)O3–PbTiO3
Authors: Takemura Koichi , Ozgul Metin , Bornand Veronique , Trolier-McKinstry Susan , Randall Clive A.
Published: July 2002

Fatigue rates in the Pb(Zn1/3Nb2/3)O3–PbTiO3 perovskite solid solution were investigated for different compositions in the rhombohedral and tetragonal phases in a variety of directions, at various field levels. It was found that the fatigue rates depend on both the ferroelectric phase and crystallographic orientation. In the rhombohedral phase when the field is applied along the [001]C direction (where the direction is given in terms of the cubic prototype), excellent fatigue resistance was obtained.
Spectroscopic analysis of photoinduced degradation of polyimide for liquid crystal alignment: Linear and nonlinear effects
Authors: Newsome C. J. , O’Neill M.
Published: July 2002

Photoinduced liquid crystal alignment is achieved by irradiation of polyimide at 248 nm with polarized excimer laser light below the threshold fluence for significant material removal. The liquid crystal-surface anchoring conditions are found to depend nonlinearly on the incident pulse energy: saturated alignment is achieved with a total incident fluence of only 92 mJ cm−2 when a pulse fluence of 18.4 mJ cm−2 is used, but requires an exposure of 1350 mJ cm−2 for pulses fluences of 2.8 mJ cm−2. Spectroscopic measurements show that the photodegradation mechanism also depends on the pulse energy. When pulses of fluence 0.58 mJ cm−2 are incident, photo-oxidation and deimidization occur and many photoproducts remain on the surface. At higher fluences there is more ablation with material removal from the surface. Saturated alignment is achieved with less photodegradation as well as shorter exposure times when higher pulse energies are used.
Drift mode calculations for the Large Helical Device
Authors: Rewoldt G. , Ku L.-P. , Tang W. M. , Sugama H. , Nakajima N. , Watanabe K. Y. , Murakami S. , Yamada H. , Cooper W. A.
Published: July 2002

A fully kinetic assessment of the stability properties of toroidal drift modes has been obtained for a case for the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. This calculation retains the important effects in the linearized gyrokinetic equation, using the lowest-order “ballooning representation” for high toroidal mode number instabilities in the electrostatic limit. Results for toroidal drift waves destabilized by trapped particle dynamics and ion temperature gradients are presented, using three-dimensional magnetohydrodynamic equilibria reconstructed from experimental measurements. The effects of helically trapped particles and helical curvature are investigated.
Effect of poloidal electric field on electron cyclotron current drive in a tokamak geometry
Authors: Lee Sanghyuk , Chang C. S.
Published: July 2002

Effect of a poloidal electric field on the electron cyclotron current drive is studied in a tokamak geometry. A general discussion of its influence on the electron phase-space dynamics and current drive efficiency is presented. It is shown that the modification to the current drive efficiency increases as the heating location is moved out in the major radius. It is concluded that the modification is only moderate to insignificant (<10% for low magnetic field side heating and <3% for high field side heating) for a poloidal electrostatic potential variation of order inverse aspect ratio under an efficient current drive.
Confinement criterion for a highly bunched beam
Authors: Hess Mark , Chen Chiping
Published: July 2002

The nonrelativistic motion is analyzed for a highly bunched beam propagating through a perfectly conducting cylindrical pipe confined radially by a constant magnetic field parallel to the conductor axis. In the present analysis, the beam is treated as either a thin rod distribution representing a continuous (unbunched) beam or periodic collinear point charges representing a highly bunched beam. Use is made of a Green’s function to compute the electrostatic force on the beam due to the induced surface charge in the conductor wall. By analyzing the Hamiltonian dynamics, a criterion is derived for the confinement of unbunched and bunched beams. It is shown that for the confinement of beams with the same charge per unit length, the maximum value of the effective self-field parameter is 2ωp2/ωc2≅2a/L for a highly bunched beam with a≪L. This value is significantly lower than the Brillouin density limit for an unbunched beam 2ωp2/ωc2=1. Here, a is the radius of the conducting cylinder, and L is the periodic spacing of the bunches.
Drift resonance of impurity ions in a toroidal magnetic trap with rotational transform
Authors: Shishkin A. A. , Zolotukhin A. V.
Published: July 2002

Impurity ion motion in a helical field under time-dependent magnetic field perturbations is considered using guiding center equations. One main aspect studied here is the role of the drift resonance in the motion of passing impurity ions. The ultimate goal is to find out ways to control impurity ion motion in stellarator/torsatron type traps, and especially, to remove impurities from the confinement volume or to protect the confinement volume from impurity ion penetration.
Modeling hydrodynamic instabilities in inertial confinement fusion targets
Authors: Goncharov V. N. , McKenty P. , Skupsky S. , Betti R. , McCrory R. L. , Cherfils-Clérouin C.
Published: July 2002

In inertial confinement fusion experiments, a cold target material is accelerated by a hot, low-density plasma. The interface between the heavy and light materials is Rayleigh–Taylor (RT) unstable. To estimate the perturbation growth in accelerated targets, a postprocessor to the results of one-dimensional codes is developed. The postprocessor is based on the sharp-boundary model that takes into account time variation in the unperturbed state, mode interaction of neighboring interfaces in the target, effects of spherical convergence, and the mass ablation. The model reveals a new stabilizing effect of ablation for modes with wavelengths longer than the shell thickness. For such modes with γcl>Va/d, the perturbation growth is reduced to η∼m(t)/m(0)e∫dt′γcl2−kVblVa/(2d), where γcl=kg is the classical RT growth rate of interface perturbations in the semi-infinite slab subject to gravitational field g, k is the wave number, d and m(t) are the slab thickness and mass, and Va and Vbl are the ablation and blowoff velocities, respectively. The perturbation evolution calculated by using the developed postprocessor is shown to be in good agreement with the results of two-dimensional hydrodynamic simulations.
Strong narrow-band electron cyclotron emission from a mirror plasma heated by electron cyclotron waves
Authors: Ito K. , Kiwamoto Y. , Saito T. , Tatematsu Y.
Published: July 2002

Strong electromagnetic radiation is observed in a narrow band slightly above the frequency of a heating wave that is absorbed by electrons near the electron cyclotron resonance layer in a magnetic beach. The frequency spectrum consists of a sharp component and a broad background, which is enhanced by more than 30 dB and 10 dB, respectively, above the radiation of thermal electrons. This observation is explained in terms of cyclotron radiation emitted by electrons, localized in a magnetic mirror, that are resonantly heated and bunched by a strong monochromatic wave.
The virial coefficients of the pearl-necklace model
Authors: Vega C. , Labaig J. M. , MacDowell L. G. , Sanz E.
Published: July 2002

We consider the virial coefficients of an idealized model polymer under good solvent conditions, the so-called pearl-necklace model, consisting of a fully flexible chain of m tangent hard spheres. We employ an efficient algorithm recently proposed to determine the second, third, and fourth virial coefficients of chains of up to 100 monomers. We also include some preliminary results for chains of up to 200 monomers. These results, which include the first off-lattice calculations of third and fourth virial coefficients of polymer models, are compared with predictions obtained from Wertheim’s equation of state. It is shown that, despite the good agreement of Wertheim’s equation of state for the compressibility factor, the predictions for the individual virial coefficients are far from satisfactory. It it shown that for the values of m considered in this work, the truncated virial expansion correctly describes the equation of state up to packing fractions of 0.25. A new equation of state which describes the low and high density regimes of the pearl-necklace model is proposed.
Tensile strength of single-walled carbon nanotubes directly measured from their macroscopic ropes
Authors: Li F. , Cheng H. M. , Bai S. , Su G. , Dresselhaus M. S.
Published: July 2002

20 mm long ropes consisting of soundly aligned single-walled carbon nanotube (SWNT) ropes, synthesized by the catalytic decomposition of hydrocarbons, were employed for direct tensile strength measurements. The average tensile strength of SWNT rope composites is as high as 3.6±0.4 GPa, similar to that of carbon fibers. The tensile strength of SWNT bundles was extrapolated from the strength of the composites to be 2.3±0.2 to 14.2±1.4 GPa after simply taking into account the volume fraction of SWNT bundles in the minicomposite, and the tensile strength of single SWNTs was estimated to be as high as 22.2±2.2 GPa. The excellent mechanical properties of SWNTs will make them an ideal reinforcement agent for high performance composite materials.
Rotational dynamics of nondipolar probes in alkane–alkanol mixtures: Microscopic friction on hydrogen bonding and nonhydrogen bonding solute molecules
Authors: Dutt G. B.
Published: July 2002

Rotational dynamics of two structurally similar nondipolar probes; 2,5-dimethyl-1,4-dioxo-3,6-di- phenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) has been studied in mixtures of squalane–1-butanol with the idea of finding out the role of size, chemical composition, and viscosity of the solvent on the friction experienced by hydrogen bonding (DPP) and nonhydrogen bonding (DMDPP) solute molecules. Although the reorientation times of both the probes followed a power law dependence on the solvent viscosity, DPP is found to rotate two to three times slower than DMDPP due to solute–solvent hydrogen bonding. The observed size effects of DMDPP have been modeled using the quasihydrodynamic theory of Gierer–Wirtz (GW). The rotational dynamics of DPP, however, follows stick hydrodynamics in the butanol rich region due to solute–solvent hydrogen bonding. But at higher concentrations of squalane, DPP gets preferentially located in a cagelike structure formed by butanol molecules and even this DPP–1-butanol complex experiences microscopic friction.
Nonequilibrium formulation of infrared frequencies and intensities in solution: Analytical evaluation within the polarizable continuum model
Authors: Cappelli Chiara , Corni Stefano , Cammi Roberto , Mennucci Benedetta , Tomasi Jacopo
Published: July 2002

We present a nonequilibrium approach to the analytical evaluation of infrared frequencies and intensities for molecules in solution within the polarizable continuum model framework. Vibrational frequencies and intensities are obtained in the harmonic approximation as the second derivatives of the suitable nonequilibrium free energy. A numerical application to the calculation of carbonyl stretching frequencies and intensities of a series of ketones at the density functional level is shown. In order to effectively compare theoretical and experimental data the coupling between the solvent and the probing field is also taken into account. The dependence of the results on the choice of the functional and of the basis set, as well as on the cavity geometry, is considered.
Three-dimensionally periodic dielectric layered structure with omnidirectional photonic band gap
Authors: Johnson Steven G. , Joannopoulos J. D.
Published: July 2002

A three-dimensionally periodic dielectric structure with a large complete photonic band gap (PBG) is presented. The structure is distinguished by a sequence of planar layers, identical except for a horizontal offset, and repeating every three layers to form an fcc lattice. The layers can be thought of as an alternating stack of the two basic two-dimensional (2D) PBG slab geometries: rods in air and air cylinders in dielectric. These high-symmetry planar cross-sections should simplify the integration of optical devices and components by allowing modification of only a single layer, using simple defects of the same form as in the corresponding 2D systems. A process for fabricating the structure with conventional planar microfabrication technology is described. Gaps of over 21% are obtained for Si/air substrates. Reasonable gaps, over 8%, can be achieved even for the moderate index ratio of 2.45 (Si/SiO2).
A numerical and experimental investigation of “inverse” triple flames
Authors: Aggarwal Suresh K. , Puri Ishwar K. , Qin Xiao
Published: July 2002

Tribrachial or triple flames represent a class of partially premixed flames that generally contain three spatially distinct but synergistically coupled reaction zones, namely a rich premixed, a lean premixed, and a nonpremixed reaction zone. The generally considered flow arrangement for burner-stabilized triple flames involves a rich mixture issuing from a central port and a lean mixture from two outer ports, which we call a reference configuration (RC). Herein, we examine an inverse configuration (IC) in which a fuel-lean stream is flanked by two fuel-rich streams. The reaction zone topology in this configuration is richer and more complex compared to that in a RC flame. A numerical-experimental investigation is conducted to characterize the fundamental differences and similitude between the RC and IC methane–air triple flames in both spatial and mixture fraction based coordinates. The detailed structure of IC triple flames and their response to variations in the rich and lean equivalence ratios are examined. Finally, the transient behavior of the flames in both configurations is described. The IC and RC flames have markedly different spatial structures. In the inverse configuration, the global flame contains five reaction zones. The predicted and measured topologies of the various reaction zones are in excellent agreement. The modified mixture fraction (ξ) is found to be effective in characterizing the structure of both RC and IC flames. The scalar profiles in terms of ξ clearly illustrate the similitude between the two flames. The three reaction zones in the RC flame have a structure that is similar to that of the corresponding five reaction zones of the IC flame. The two nonpremixed (or the two rich premixed reaction zones) in the IC flames are not differentiated in terms of ξ. Both the flames are subjected to a buoyancy-induced instability at normal gravity that generates large vortex structures, which cause the reaction zones to flicker. The flame–vortex interaction is initiated in the nonpremixed reaction zone for the IC flame and in the lean premixed reaction zone for the RC flame. Consequently, the IC flame flickers with higher amplitude but lower frequency as compared to the RC flame. There is good agreement between the measured and predicted flickering frequencies for the two flames. As φrich is increased, the height of the two rich premixed reaction zones in the IC flames increases, their tips open, and the chemical activity in these zones decreases. While the oscillation frequency is essentially constant, the oscillation amplitude increases as φrich is increased. The effect of increasing φlean is to enhance chemical activity in the lean premixed zone. However, the two rich premixed zones and the outer nonpremixed zone are relatively unaffected by variations in φlean. The oscillation amplitude and frequency also remain unaffected by variations in φlean.
Mean first passage times across a potential barrier in the lumped state approximation
Authors: Mapes Eric J. , Schumaker Mark F.
Published: July 2002

The lumped state approximation (LSA) is a method for handling boundary conditions for diffusion on an interval which simplifies the description of transitions into and out of the interval. It was originally motivated by the problem of proton conduction through the ion channel gramicidin. This paper discusses the mean first passage time of a diffuser crossing a potential barrier in the lumped state approximation. The LSA mean first passage time is shown to be identical to a different quantity, the interior mean first passage time, clarifying the nature of the approximation. We also discuss a variant of the LSA in which dependence on an applied electrical potential is made explicit; an optimal value for an effective electrical distance is found. A detailed comparison is made of the LSA mean first passage time with several other formulations of the mean time to cross a barrier.
Stability of ultrathin amorphous carbon films deposited on smooth silicon substrates by radio frequency sputtering
Authors: Lu W. , Komvopoulos K. , Yeh S. W.
Published: July 2002

The mechanical stability of amorphous carbon (a-C) films deposited on ultrasmooth Si(100) substrates by radio-frequency sputtering under different energetic ion bombardment conditions was investigated in light of results obtained from aging and annealing experiments. The a-C films were annealed at 495 °C in the high-vacuum chamber of an x-ray photoelectron spectroscopy (XPS) system with a base pressure of 10−8 Torr. The annealing process consisted of three sequential heating cycles of temperature 495 °C and duration 5, 10, and 70 min, respectively. Atomic force microscopy and XPS studies were conducted to reveal possible changes in the surface topography, microstructure, and composition of the a-C films. To investigate the effect of annealing on the nanomechanical properties of the a-C films, nanoindentation experiments were performed with a surface force microscope. Only subtle changes in the surface topography, microstructure, composition, and nanomechanical properties of the a-C films were observed after aging for about two years. Film agglomeration during annealing due to residual stress relaxation was found to strongly depend on the kinetics of film deposition. It is shown that the stability of the a-C films is affected by residual stresses produced from the energetic ion bombardment during film growth. The magnitude of the residual stress and the film thickness exhibits a pronounced effect on the thermodynamics and kinetics of film agglomeration. The experimental results demonstrate that increasing the residual stress and/or film thickness decreases the mechanical stability of the a-C films.
Molecular composition of films and solid particles polymerized in fluorocarbon plasmas
Authors: Takahashi Kazuo , Tachibana Kunihide
Published: July 2002

In fluorocarbon (C4F8) plasmas, formation mechanisms of polymers were investigated by the characterization with x-ray photoelectron spectroscopy (XPS) and gel permeation chromatography (GPC). The molecular compositions of the polymers in the films deposited on the substrate and in the particles formed in the gas phase were elucidated by these chemical analyses. The XPS results showed that the particles were carbon-rich and composed of highly branched molecules in contrast to the film composition. From the GPC measurements, the particles were found to contain ultrahigh mass polymers, whose molecular weights were around 100 000. On the contrary, the deposited film contained polymers with molecular weights distributed below 2000, in which oligomers, monomers, and fragmented products were included. Present study suggests that these polymers are involved in the formation of crosslinked networks of the films and the particles via surface reactions, where the crosslinking is enhanced by the ion bombardment.
Nearside–farside analysis of differential cross sections using Jacobi functions of the first and second kinds: Application to Ar+N2 rotationally inelastic scattering
Authors: McCabe P. , Connor J. N. L. , Sokolovski D.
Published: July 2002

We report the first nearside–farside (NF) analysis of angular scattering for an inelastic molecular collision in which the partial wave series for the scattering amplitude is expanded in a basis set of reduced rotation matrix elements dmf,miJ(θ), where θ is the scattering angle, J is the total angular momentum quantum number, and mi,mf are initial and final helicity quantum numbers, respectively. The practical implementation of the NF theory is described in detail; it exploits in an essential way the properties of a function that we denote emf,miJ(θ) and call a reduced rotation matrix element of the second kind. The caustic structure of dmf,miJ(θ) and emf,miJ(θ) is taken into account via a restricted nearside–farside (resNF) decomposition of the scattering amplitude. The NFres theory is used to analyze polarization and degeneracy averaged differential cross sections for the Ar+N2(ji=2,mi=0,±1,±2)→Ar+N2(jf=2,mf=0,±1,±2) collision system, treated as an atom+rigid-rotor. The NFres analysis always provides a clear physical interpretation of the scattering (except sometimes for θ≈0°,180°) for phenomena such as diffraction oscillations and potential rainbows, as well as for more complicated (unnamed) interference effects. We also report results for some approximations to the NFres theory. Mathematical properties of the emf,miJ(θ) required for the NFres analysis are derived.
Electrical properties of Ta-doped SnO2 thin films prepared by the metal–organic chemical-vapor deposition method
Authors: Lee Sang woo , Kim Young-Woon , Chen Haydn
Published: July 2002

Undoped and Ta-doped SnO2 (Sn1−xTaxO2) thin films were prepared on Corning 7059 glass substrates by the metal–organic chemical-vapor deposition method. The relative amount of Ta, CTa=XTa/(XTa+XSn), varied from 0 to 7.13 at. %. For the five compositions studied, the lowest resistivity at room temperature was 2.01×10−4 Ω cm at CTa=3.75% with charge carrier density and mobility of 1.27×1021 cm−3 and 24.5 cm2/V s, respectively. In microstructural investigation, 3.75% Ta-doped film maintains a growth pattern of initial stage growth while 7.13% Ta-doped film has a high population of small grains at the interface, which results in large grains through competitive growth. The resistivity of the undoped film was 0.17 Ω cm with charge carrier density and mobility of 1.31×1018 cm−3 and 28.1 cm2/V s obtained from Hall measurement. This study suggests that Ta is an excellent n-type dopant in SnO2.
Existence of a CuAu–I-type ordered structure in lattice-mismatched InxGa1−xAs/InyAl1−yAs multiple quantum wells
Authors: Kim T. W. , Lee D. U. , Lee H. S. , Lee J. Y. , Kim M. D.
Published: July 2002

Selected area electron diffraction pattern (SADP) and transmission electron microscopy (TEM) measurements were carried out to investigate the ordered structures in lattice-mismatched InxGa1−xAs/InyAl1−yAs multiple quantum wells (MQWs). The SADP showed two sets of extra spots with asymmetrical intensity, and the high-resolution TEM image showed doublet periodicity in the contrast of the (001) lattice planes. The results of the SADP and the TEM measurements showed that a CuAu–I-type ordered structure was observed near the lattice-mismatched InxGa1−xAs/InyAl1−yAs heterointerfaces. This CuAu–I-type ordered structure had an antiphase boundary in the periodically regular InxGa1−xAs/InyAl1−yAs lattice-mismatched region. The existence of a CuAu–I-type ordered structure in InxGa1−xAs/InyAl1−yAs MQWs might originate from the lattice mismatch between the InxGa1−xAs and the InyAl1−yAs layers. These results provide important information on the microstructural properties for improving operating efficiencies in long-wavelength optoelectronic devices, such as strain compensated electroabsorption modulators utilizing lattice-mismatched InxGa1−xAs/InyAl1−yAs MQWs.
Statistical thermodynamic foundation for photovoltaic and photothermal conversion. IV. Solar cells with larger-than-unity quantum efficiency revisited
Authors: Badescu Viorel , Landsberg Peter T. , De Vos Alexis , Desoete Bart
Published: July 2002

A detailed balance solar energy conversion model offering a single treatment of both photovoltaic and photothermal conversion is expounded. It includes a heat rejection mechanism. The effect of multiple impact ionizations on the solar cell efficiency is reconsidered by including the constraints dictated by the first law of thermodynamics (which already exist in the model) and it improves of course the solar cell efficiency. However the upper bound efficiencies previously derived are too optimistic as they do not take into consideration the necessary increase in solar cell temperature. The cell efficiency operating under unconcentrated radiation is a few percent lower than in the ideal case (i.e., with perfect cooling). Wider band gap materials are recommended for those applications where the cell cooling is not effective. The best operation of naturally ventilated cells is under unconcentrated or slightly concentrated solar radiation. Increasing the (forced) ventilation rate allows an increase of the optimum concentration ratio. Additional effects such as the radiation reflectance and radiative pair recombination efficiency are also considered. A sort of threshold minimum band gap depending on the last effect is emphasized: materials with band gaps narrower than this threshold are characterized by very low cell efficiency.
Comparitive behavior of radiation and thermally generated protons in amorphous SiO2
Authors: Devine R. A. B.
Published: July 2002

Measurements of the transport of thermally and radiation generated protons in amorphous SiO2 gate dielectrics of metal–oxide–semiconductor field effect transistors are compared. I conclude that there is no significant behavioral difference between the two types of protons other than the fact that radiation generated protons transport more dispersively—this may be related to the proton densities involved which are typically more than 50 times smaller. From the inversion channel carrier mobility and subthreshold slope measurements on devices with thermally generated protons I conclude that the interfaces are essentially unpassivated. This observation explains why no proton induced interface state generation is observed in the case of devices containing thermally generated protons.