bluuebunny/crossref_metadata_2025 huggingface dataset:
Shock wave response of ammonium perchlorate single crystals to 6 GPa
Authors: Yuan G. , Feng R. , Gupta Y. M. , Zimmerman K.
Published: July 2002

Plane shock wave experiments were carried out on ammonium perchlorate single crystals compressed along [210] and [001] orientations to peak stresses ranging from 1.2 to 6.2 GPa. Quartz gauge and velocity interferometer techniques were used to measure the elastic and plastic shock wave velocities, and stress and particle velocity histories in the shocked samples. The measured Hugoniot elastic limit (HEL) was 0.48±0.09 GPa. Above the HEL and up to about 6 GPa, the data show a clear two-wave structure, indicating an elastic-plastic response. Time-dependent elastic precursor decay and plastic wave ramping are discernable and orientation dependent in the low stress data. However, the orientation dependence of the peak state response is small. Hence, data for both orientations were summarized into a single isotropic, elastic-plastic-stress relaxation model. Reasonable agreement was obtained between the numerical simulations using this model and the measured wave profiles. At a shock stress of about 6 GPa and for the time duration and crystal orientations examined, we did not observe any features that may be identified as a sustained chemical reaction or a phase transformation.
Plasma response of electrons in GaAs/AlAs superlattices in the presence of strong localization
Authors: Pusep Yu. A. , Chiquito A. J.
Published: July 2002

Raman scattering and thermally stimulated current measurements were employed in investigation of the dynamic response of the localized interacting electrons in the doped GaAs/AlAs superlattices. The presence of the plasma resonances of the strongly localized electrons was unambiguously demonstrated.
Description and optimization of a wall-stabilized arc chamber
Authors: Brugeat S. , Coitout H. , Parizet M. J.
Published: July 2002

A wall-stabilized arc chamber has been built to make spectroscopic measurements in order to study thermal plasmas. An experimental design method has been carried out in order to control and to optimize all experimental setup parameters. Pure Ar, Ar–CO2, and Ar–N2 mixtures have been used to test time reliability and homogeneity of the plasma.
Retention and switching kinetics of protonated gate field-effect transistors
Authors: Devine R. A. B. , Herrera G. V.
Published: July 2002

The switching and memory retention time has been measured in 50 μm gate-length “pseudo” nonvolatile memory metal–oxide–semiconductor field-effect transistors containing protonated 40 nm gate oxides. Times of the order of 3.3 s are observed for fields of 3 MV cm−1. The retention time with protons placed either at the gate oxide/substrate or gate oxide/gate electrode interfaces is found to be better than 96% after 5000 s. Measurement of the time dependence of the source–drain current during switching provides clear evidence for the presence of dispersive proton transport through the gate oxide.
Turbulent steady-state configuration of an inductively driven, dissipative tokamak plasma
Authors: Chu T. K.
Published: July 2002

By taking Faraday’s equation into account and using a variational principle that optimizes dissipation, it is shown that the turbulent steady-state profile of current density of an inductively driven tokamak plasma is peaked at the axis and decreases monotonically to a finite value at the edge. It corresponds to a state of minimum rate of dissipation of poloidal magnetic field energy under the constraint that all tearing points on its rational surfaces are equally effective in reducing the slope of the current density across the surface. An alternative interpretation is that the configuration corresponds to a state of maximum poloidal magnetic field energy for the prescribed plasma current.
On the effect of synchrotron radiation and magnetic fluctuations on the avalanche runaway growth rate
Authors: Martı́n-Solı́s J. R. , Sánchez R. , Esposito B.
Published: July 2002

The avalanche runaway growth rate was recently determined by analysis of the trajectories of the secondary runaway electrons in momentum space [Parks et al., Phys. Plasmas 6, 2523 (1999)]. Here, such an approach is used to study the effect of the electron synchrotron radiation and the magnetic fluctuations on the secondary generation of runaway electrons. It is found that not only the threshold electric field for runaway generation is increased due to these effects, but also the creation rate of secondary runaway electrons may be noticeably reduced for electric fields close to the threshold field. It has been shown that avalanche runaway generation can be efficiently suppressed in large tokamaks, even during major disruptions, for stochastic magnetic fluctuation levels b̃>10−3.
Theoretical calculation of the acoustic force on a patterned silicon wafer during megasonic cleaning
Authors: Deymier P. A. , Khelif A. , Djafari-Rouhani B. , Vasseur J. O. , Raghavan S.
Published: July 2002

We have calculated, theoretically, the acoustic pressure field around a linear pattern on a silicon wafer immersed in water subjected to a megasonic beam. The method of calculation is based on a Green’s function formalism. The acoustic force applied on the pattern by the pressure field is determined as a function of frequency and the angle the incident megasonic beam makes with the wafer surface. The calculation is applied to two types of features that may be encountered in megasonic cleaning of integrated circuits prior to packaging, namely a micron-size silicon ridge and a metal wire (tens to hundreds of microns in diameter) bonded on a silicon substrate. The calculated acoustic shear stress is found to be orders of magnitude smaller than the shear strength of the features.
Photodissociation of the acetone cation at 355 nm using the velocity imaging technique
Authors: Jackson William M. , Xu Dadong
Published: July 2002

Photodissociation of acetone cations, CH3COCH3+, at 355 nm has been studied by means of the ion velocity imaging technique. Acetone cations are produced via direct photoionization of a supersonic beam of acetone at 118 nm generated by frequency tripling the 355 nm laser. Only the acetyl cation, CH3CO+, could be detected as a dissociation product in the time-of-flight mass spectrometer. The acetyl ion signal depends upon the fifth power of the 355 nm laser energy, while the acetone ion signal depends upon the third power. This suggests that the fragment ion is produced via two-photon absorption of 355 nm photons by the acetone cation. The total translational energy distribution and angular distribution of acetyl cation were derived from the 2D images of CH3CO+ for the reaction CH3COCH3++2hν355 nm→CH3CO++CH3*. The translational energy distribution suggests that methyl radicals are produced in two electronically excited states, the Rydberg 3s 1 2A1′ and the valence 1 2A″ states. The anisotropy parameter β shows that the Rydberg state is formed via a perpendicular excitation and the valence state via a parallel transition.
Small polarons in La2/3TiO3−δ
Authors: Jung W. H. , Wakai H. , Nakatsugawa H. , Iguchi E.
Published: July 2002

The thermoelectric power and dc conductivity of La2/3TiO3−δ (δ=0.057, 0.07, and 0.16) were investigated. The thermoelectric power is negative between 80 and 350 K. The measured thermoelectric power of La2/3TiO3−δ increases linearly with temperature, in agreement with the model proposed by C. Wood and D. Emin [Phys. Rev. B 29, 4582 (1984)], and represented by A+BT. This temperature dependence indicates that the charge carrier in this material is a small polaron. There exists a linear relation between log(σT) and 1/T in the range of 200–300 K, the activation energies for small polaron hopping were 0.15, 0.21, and 0.24 eV for δ=0.16, 0.07, and 0.057, respectively. These properties are discussed in terms of a hopping process involving small polarons. This conclusion is confirmed theoretically. Based upon polaron energies obtained experimentally, several parameters relevant to small polaron transport in this material are estimated.
Photoluminescence, deep level transient spectroscopy and transmission electron microscopy measurements on MeV self-ion implanted and annealed n-type silicon
Authors: Schmidt D. C. , Svensson B. G. , Seibt M. , Jagadish C. , Davies G.
Published: July 2002

Deep-level transient spectroscopy (DLTS), photoluminescence (PL), and transmission electron microscopy (TEM) measurements have been made on n-type silicon after implanting with 5.6 MeV Si3+ ions using doses of 109–1014 cm−2 and anneals at 525 and 750 °C. In all the samples, there is only a small dependence of the widths and energies of the PL zero-phonon lines on implantation dose, allowing the high resolution of PL to be exploited. In samples annealed at 525 °C, the PL intensity can provide a measure of the concentration of defects over the implantation range, 109–1012 cm−2. Carbon-hydrogen complexes are identified as transient species with increasing dose, and the “T” center is related to a DLTS trap 0.20 eV below the conduction band energy Ec. At the highest doses in these samples, TEM imaging shows the presence of nanometer-sized clusters, and the PL spectra show that many previously unreported defects exist in the implanted zone, in addition to two broad bands centered on ∼885 and ∼930 MeV. The multiplicity of defects supports recent suggestions that a range of interstitial complexes is present in the annealed samples. Annealing at 750 °C produces complete recovery in both the DLTS and PL spectra for doses of less than 1013 cm−2. At higher doses, {113} self-interstitial aggregates are observed in TEM, along with the “903” PL signal associated with the {113} defects, and the Ec−0.33 eV “KA” DLTS trap. These data support the recent identification of that trap with the {113} defects. The well-resolved PL spectra show that many previously reported defects also exist in samples implanted with a dose of 1014 cm−2 and annealed at 750 °C, again implying the presence of a range of interstitial complexes.
Enhanced nucleation of oxide precipitates during Czochralski silicon crystal growth with nitrogen doping
Authors: Aihara K. , Takeno H. , Hayamizu Y. , Tamatsuka M. , Masui T.
Published: July 2002

Thermal stability of oxide precipitate nuclei has been investigated for Czochralski silicon crystals with nitrogen doping. The experimental result indicates that generation of the grown-in oxide precipitate nuclei stable over 800 °C is enhanced by nitrogen doping. On the other hand, even though we confirmed this existence, doped nitrogen shows no influence on further oxide precipitate nucleation during the isothermal annealing at 600 °C after an epitaxial silicon growth process. Thus, it is found that the nitrogen doping only enhances the oxide precipitate nucleation at higher temperature during crystal cooling. The enhanced precipitate nucleation during the cooling is considered to be through excess vacancies which are suppressed to agglomerate by nitrogen.
Nonlinear optical diagnosis of oxide traps formed during reactive ion etching
Authors: Fang J. , Heidbrink W. W. , Li G. P.
Published: July 2002

Oxide traps generated by reactive ion etching are studied using a pulsed femtosecond laser. The second harmonic generation (SHG) signal from the Si/SiO2 interface is sensitive to charged traps in the oxide. The time evolution of the SHG signal indicates that positive traps predominate. The angular dependence of the polarized signal shows that the electric field generated by the oxide traps alters the symmetry of the sample. The damage is greatest for an oxide thickness of 13 nm (for a plasma dc bias of 300 V). Thicker oxides have smaller SHG signals, presumably because the Fowler–Nordheim tunneling currents induced by plasma charging of the oxide surface are smaller. Very thin oxides also exhibit reduced damage. The time dependent SHG signals depend on the temperature of the samples; these data provide information on the trapping and detrapping of substrate electrons by oxide holes.
Comparison of three Monte Carlo conformational search strategies for a proteinlike homopolymer model: Folding thermodynamics and identification of low-energy structures
Authors: Gront Dominik , Kolinski Andrzej , Skolnick Jeffrey
Published: July 2002

Entropy sampling Monte Carlo, the replica method, and the classical Metropolis scheme were applied in numerical studies of the collapse transition in a simple face-centered cubic lattice polymer. The force field of the model consists of pairwise, contact-type, long-range interactions and a short-range potential based on the β-sheet definition assumed in the model. The ability to find the lowest energy conformation by various Monte Carlo methods and the computational cost associated with each was examined. It is shown that all of the methods generally provide the same picture of the collapse transition. However, the most complete thermodynamic description of the transition derives from the results of entropy sampling Monte Carlo simulations, but this is the most time-consuming method. The replica method is shown to be the most effective and efficient in searching for the lowest energy conformation. The possible consequences of these findings for the development of simulation strategies for the folding of model proteins are discussed briefly.
The hydrogen fluoride dimer in liquid helium: A prototype system for studying solvent effects on hydrogen bonding
Authors: Nauta K. , Miller R. E.
Published: July 2002

High-resolution infrared spectra are reported for the “free” and “hydrogen bonded” H–F stretches of the hydrogen fluoride dimer solvated in helium nanodroplets. These rotationally resolved spectra provided detailed information concerning the effect of the helium solvent on the vibrational frequencies, rotational constants and tunneling dynamics of the dimer. The end-over-end rotation of the dimer is slowed by a factor of 2.2 by the helium, while the faster rotation about the a axis remains essentially unaffected. The interchange tunneling is reduced significantly (∼40%) in both the ground and vibrationally excited states. The effective tunneling barrier is higher than in the gas phase, making it easier to quench the tunneling motion with a large dc (direct current) electric field. Rapid rotational relaxation is observed from the Ka=1 state, resulting in significant broadening of the corresponding subband. Combination bands are observed for the intermolecular F–F stretch and trans-bend vibrations, providing further insights into the nature of the solvent effects.
C–H⋯O H-bonded complexes: How does basis set superposition error change their potential-energy surfaces?
Authors: Salvador P. , Simon S. , Duran M. , Dannenberg J. J.
Published: July 2002

Geometries, vibrational frequencies, and interaction energies of the CNH⋯O3 and HCCH⋯O3 complexes are calculated in a counterpoise-corrected (CP-corrected) potential-energy surface (PES) that corrects for the basis set superposition error (BSSE). Ab initio calculations are performed at the Hartree–Fock (HF) and second-order Mo/ller–Plesset (MP2) levels, using the 6-31G(d,p) and D95++(d,p) basis sets. Interaction energies are presented including corrections for zero-point vibrational energy (ZPVE) and thermal correction to enthalpy at 298 K. The CP-corrected and conventional PES are compared; the uncorrected PES obtained using the larger basis set including diffuse functions exhibits a double well shape, whereas use of the 6-31G(d,p) basis set leads to a flat single-well profile. The CP-corrected PES has always a multiple-well shape. In particular, it is shown that the CP-corrected PES using the smaller basis set is qualitatively analogous to that obtained with the larger basis sets, so the CP method becomes useful to correctly describe large systems, where the use of small basis sets may be necessary.
Spectral decay of a passive scalar in chaotic mixing
Authors: Toussaint Valérie , Carrière Philippe , Scott Julian , Gence Jean-Noël
Published: July 2002

In this paper we take a closer look at the decay phase of a passive, diffusing, scalar field undergoing steady, three-dimensional chaotic advection. The energy spectrum of the scalar is obtained by numerical simulation of the advection–diffusion equation at high Péclet number. At large times, the spectral decay is found to be exponential and self-similar. It is emphasized that the asymptotic decay-time is an important measure of mixing efficiency, alongside the time required for diffusion to first become effective. The large-wavenumber spectral form, representing the distribution of scalar energy over small scales, is analyzed. Power-law behavior is found at scales intermediate between the large ones, comparable in size with the entire flow volume, and the smallest ones, at which diffusion is effective and the spectrum falls off exponentially with increasing wavenumber. Fitting of the numerical results allows the exponent of the power-law to be estimated. It is observed to vary with the parameters of the flow, taking negative values which can be either less than or greater than −1. This implies that the dominant spectral energy at high P may be either at small, large or intermediate scales, depending on the flow. In consequence, the qualitative nature of the scalar field during the decay phase varies from flow to flow, resulting in differing behavior of the predicted decay times in the large P limit obtained by asymptotic analysis. The case in which the spectral exponent exceeds −1 is shown to produce more rapid mixing and the corresponding asymptotic expression for the decay time, independent of P and involving two spectral parameters, is suggested as a quantitative means for optimizing the flow.
Heat capacity of hydrogenated diamond-like carbon films
Authors: Hakovirta M. , Vuorinen J. E. , He X. M. , Nastasi M. , Schwarz R. B.
Published: July 2002

We have used differential scanning calorimetry to measure the heat capacity of diamond-like carbon (DLC) film prepared by a plasma immersion ion processing method. The same calorimeter was used to measure the heat capacity of single crystal natural diamond and of high purity graphite. The amount of atomic hydrogen trapped in the DLC films was determined by elastic-recoil-detection spectrometry. The present data and literature values were used to deduce an expression for the specific heat that factors out the contribution from the sp3/sp2 bonding and from the atomic hydrogen trapped in the carbon. The data shows that the hydrogen contribution to the specific heat of carbon is independent of the sp3/sp2 bonding and amounts to about 0.63kB per hydrogen atom. We propose a simple method to determine the sp3/sp2 bonding ratio in hydrogenated DLC films based on measuring the specific heat and the hydrogen content of the sample.
Determination of velocity distribution, angular distribution, and vector correlation of photofragments using sub-Doppler fluorescence-imaging method
Authors: Chang Jia-lin , Chen Kuo-mei , Lin Wei-yu , Lee Kuo-huei , Chen Yit-Tsong
Published: July 2002

With sub-Doppler resolution, the fluorescence-imaging techniques can be modified to determine velocity distribution, angular distribution, and vector correlation of state-selected photofragments, even in an uncollimated molecular beam. This new method is proposed as “sub-Doppler fluorescence-imaging” in which two experimental schemes are developed. The dependence of fluorescence intensities, at any selected velocity and recoil angle in the scattering plane, with respect to the variation of polarization vectors of the probe laser and emitted fluorescence is derived using density matrix formalism. The intensity patterns of photofragments with v–J and μ–v–J correlations are simulated. The laser ablation of B atoms at 248 nm demonstrates the feasibility of this method. Two-dimensional velocity distribution of the laser-ablated B(2P1/2,3/20) atoms is measured and the ablation mechanism is discussed.
Intrinsic and extrinsic pressure effects in La0.7Ca0.3MnO3 thin films
Authors: Moshnyaga V. , Klimm S. , Gommert E. , Tidecks R. , Horn S. , Samwer K.
Published: July 2002

The influence of quasihydrostatic pressure on the electrical transport in epitaxial La0.7Ca0.3MnO3/MgO(100) thin films prepared by pulsed laser deposition (PLD) and metalorganic aerosol deposition (MAD) was studied for temperatures T=4.2–300 K and magnetic fields B=0–12 T. Usual or “intrinsic” pressure effect, characterized by a decrease of the resistance in the region of the metal–insulator transition TMI was observed in PLD film for p=0–16 kbar and also in MAD films for p⩽8 kbar. An anomalous, or “extrinsic” pressure effect, characterized by a 2–3 order of magnitude increase of the resistance at low temperatures, persisting after the pressure release, was found for p>8 kbar in films with extraordinarily high values of the pressure coefficient d(TMI)/dp=2.70–3.85 K/kbar. The lattice strain originated from the substrate due to epitaxy was found to control the TMI values and intrinsic pressure behavior in the films. The extrinsic pressure effect in MAD films is caused by a plastic deformation of the film, yielding irreversible changes of the microstructure accompanied by a magnetoresistance, ΔR(B)/R(B), more than 103% at 4.2 K<T<TMI.
Three-dimensional strain field calculations in coupled InAs/GaAs quantum dots
Authors: Jogai B.
Published: July 2002

A detailed calculation of the three-dimensional strain field in and around InAs/GaAs quantum dots is presented. The strain field is calculated by minimizing the elastic strain energy on a cubic grid. Surface boundary conditions are incorporated to enable the strain field at the surface of the cap layer to be examined in detail. This has important implications for the vertical and lateral ordering of subsequent layers of dots. Results are presented for a single dot as well as two and four coupled dots. It is found that the elastic energy density at the surface of the epilayer above a layer of seed dots exhibits strong minima directly above the seed dots, facilitating vertical ordering. An intriguing result is that, under certain conditions, satellite minima also occur at interstitial points, raising the possibility that a second layer of dots above the seed layer can have significantly more dots than the seed layer because of lateral ordering.
The role of thermal instabilities in limiting the density in DIII-D
Authors: Stacey W. M. , Petrie T. W.
Published: July 2002

The mechanisms which apparently govern the maximum achievable density in several DIII-D [Luxon, Anderson, Batty et al., Plasma Physics on Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 159] shots in which different operating procedures were used in building up the density were investigated and compared with the predictions of thermal instability theory. Core MARFEs (multifaceted asymmetric radiation from the edge) followed by a H–L (high-to-low confinement mode) transition limit the density well below the Greenwald limit in continuous gas puffed lower single-null discharges with low triangularity. Similar continuous gas puff fueled discharges with higher triangularity or with pumping in the lower divertor achieve densities at or above the Greenwald value, apparently limited by confinement degradation, without the formation of core MARFEs. Pellet fueled discharges achieve densities up to twice the Greenwald value, limited by global radiative collapse. Thermal instability theory predictions of the limiting core MARFEs, confinement degradation or global radiative collapse are in good agreement with the experimental observations for the shots examined. Evidence for an important role of neutral particles in the plasma edge in core MARFE onset and in confinement degradation was identified.
Combined kinetic and transport modeling of radiofrequency current drive
Authors: Dumont R. , Giruzzi G. , Barbato E.
Published: July 2002

A numerical model for predictive simulations of radiofrequency current drive in magnetically confined plasmas is developed. It includes the minimum requirements for a self-consistent description of such regimes, i.e., a three-dimensional kinetic equation for the electron distribution function, one-dimensional heat and current transport equations, and resonant coupling between velocity space and configuration space dynamics, through suitable wave propagation equations. The model finds its full application in predictive studies of complex current profile control scenarios in tokamaks, aiming at the establishment of internal transport barriers by the simultaneous use of various radiofrequency current drive methods. The basic properties of this nonlinear numerical system are investigated and illustrated by simulations applied to reversed magnetic shear regimes obtained by Lower Hybrid and Electron Cyclotron current drive for parameters typical of the Tore Supra tokamak [Equipe Tore Supra, in Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, France, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 1, p. 9].
Comment on “The multilayer-modified Stoney’s formula for laminated polymer composites on a silicon substrate” [J. Appl. Phys. 86, 5474 (1999)]
Authors: Klein Claude A.
Published: July 2002

There is a vast amount of literature dealing with the strains, the stresses, and the curvature induced by thermal cycling of multilayered laminates, but much of the published work fails to provide proper derivations of the applicable formulas. For example, Kim et al. recently proposed [J. Appl. Phys. 86, 5474 (1999)] a formula — the multilayer-modified Stoney’s formula — for estimating the bow of structures consisting of thin films mounted on a thick substrate, which they validate on the basis of experimental data. In this communication, it is shown that the formula can be derived from available closed-form solutions for the curvature of elastically isotropic multilayer laminates and has no conceptual connection with Stoney’s formula.
Capacitance–voltage and admittance spectroscopy of self-assembled Ge islands in Si
Authors: Miesner C. , Asperger T. , Brunner K. , Abstreiter G.
Published: July 2002

We investigate the electrical properties of self-assembled Ge islands embedded in Si Schottky diode structures by means of capacitance–voltage measurements and admittance spectroscopy. The Ge islands form at T=550 °C by self-assembly in the Stranski–Krastanow growth mode with an area density of 4.5×109 cm−2. Their diameter and height are 70 and 6.5 nm, respectively. A linear increase of the thermal activation energy observed in voltage-dependent admittance spectroscopy shows that the ensemble of Ge islands has a low, continuous, averaged density of states.
Response to “Comment on ‘An ab initio cluster study of the structure of the Si(001) surface’ ” [J. Chem. Phys. 113, 9353 (2000)]
Authors: Gordon Mark S. , Shoemaker James R. , Burggraf Larry W.
Published: July 2002

Given the agreement among TCSCF, MP2, and MRMP methods, it is concluded that the structure of Si9H12 is symmetric, that dynamic correlation does not play a major role in determining the structure, and that DFT incorrectly predicts Si9H12 to be buckled.
Electron-pair momentum densities of heavy atoms Cs through Lr
Authors: Koga Toshikatsu , Matsuyama Hisashi
Published: July 2002

Spherically averaged electron-pair intracule (relative motion) h̄(v) and extracule (center-of-mass motion) d̄(P) densities in momentum space are constructed for the 49 heavy atoms Cs (atomic number Z=55) through Lr(Z=103), by using the numerical Hartree–Fock method. As known for the 53 lighter atoms from He(Z=2) to Xe(Z=54), the electron-pair densities h̄(v) and d̄(P) of the heavy atoms are found to be classified into three and two types, respectively, based on their modalities. No new types are observed even when f subshells are occupied. Subshell-pair analysis clarifies that different modalities reflect different valence electron configurations. The intracule 〈vn〉 and extracule 〈Pn〉 moments and the electron–electron coalescence h̄(0) and counterbalance d̄(0) densities are also obtained. The present results for Z=55–103, together with the previous ones [J. Chem. Phys. 107, 8510 (1997); 108, 3424 (1998)] for Z=2–54, provide a compilation of accurate Hartree–Fock values of the momentum-space electron-pair properties for the 102 neutral atoms in the periodic table.
Tokamak, reversed field pinch and intermediate structures as minimum-dissipative relaxed states
Authors: Bhattacharyya R. , Janaki M. S. , Dasgupta B.
Published: July 2002

The principle of minimum energy dissipation rate is utilized to develop a unified model for relaxation in toroidal discharges. The Euler–Lagrange equation for such relaxed states is solved in toroidal coordinates for an axisymmetric torus by expressing the solutions in terms of Chandrasekhar–Kendall (C–K) eigenfunctions analytically continued in the complex domain. The C–K eigenfunctions are hypergeometric functions that are solutions of the scalar Helmholtz equation in toroidal coordinates in the large-aspect-ratio approximation. Equilibria are constructed by assuming the total current J=0 at the edge. This yields the eigenvalues for a given aspect-ratio. The most novel feature of the present model is that solutions allow for tokamak, low-q as well as reversed field pinch-like behavior with a change in the eigenvalue characterizing the relaxed state.
Homeotropic polar anchoring energy of a nematic liquid crystal using the fully leaky waveguide technique
Authors: Yang Fuzi , Ruan Lizhen , Sambles J. R.
Published: July 2002

Optical excitation of a series of fully leaky guided modes has been used to explore the director distortion of a hybrid-aligned nematic liquid crystal with a negative dielectric anisotropy (Merck-BDH, MLC 6608) under application of an ac electric field. Hybrid alignment is realized by the use of a lecithin film on the top glass plate and rubbed polyimide on the bottom plate, both plates having indium–tin–oxide coatings. Continuum theory is used to model the director profile through the cell. Fitting the fully leaky guided mode data both in transmission and reflection using optical multilayer theory together with continuum modeling gives the director profile both with and without applied fields. From this fitting the changes in the surface tilt angle and its gradient are obtained. Using this information the polar anchoring coefficient of the homeotropic surface Wθ is deduced. At 23.1 °C we find Wθ=(4.6±0.2)×10−4 J m−2.
Formation of nanomagnetic thin films by dispersed fullerenes
Authors: Zheng Lingyi A. , Lairson Bruce M. , Barrera Enrique V. , Shull Robert D.
Published: July 2002

A method of forming magnetic materials using dispersed fullerenes in ferromagnetic materials has been studied. Fullerenes (C60) have been integrated into the matrix of Co, Fe, CoFe thin films by thermal vapor codeposition. The size effects and interaction of the C60 molecules to the metallic atoms promote a self-assembly grain growth mode to produce thin films with unique evoluted microstructures characterized by nanosize columnar grains with uniformly dispersed C60 on the grain boundaries. These nanocrystalline films have displayed a series of promising magnetic properties, such as high out of plane remanence, high coercivity, fast magnetic switching, and unusual hysteresis behavior.
B-site vacancy as the origin of spontaneous normal-to-relaxor ferroelectric transitions in La-modified PbTiO3
Authors: Kim Tae-Yong , Jang Hyun M.
Published: July 2002

The site vacancy responsible for the manifestation of a spontaneous normal-to-relaxor transition was investigated using La-modified PbTiO3 as a model perovskite system. The two cation-site defects relevant to the present study are the A-site (Pb site) and B-site (Ti site) vacancies. To clearly elucidate the type of vacancy involved in the spontaneous transition, we have developed a sintering route that suitably adjusts the relative concentration of these two distinct cation-site vacancies. Using this processing scheme, we have shown that the microcompositional inhomogeneity caused by the B-site vacancy is primarily responsible for the manifestation of relaxor behavior from normal ferroelectricity in La-modified PbTiO3.
Substrate and chain length dependencies of the thermal behavior of [CF3(CF2)m(CH2)nCOO]2Cd single monolayers investigated by infrared reflection absorption spectroscopy
Authors: Ren Yanzhi , Asanuma Morito , Iimura Ken-ichi , Kato Teiji
Published: July 2002

Temperature-variable grazing incidence reflection absorption (GIR) spectra were recorded for the single monolayer of [CF3(CF2)m(CH2)nCOO)]2Cd [(m,n)=(7,10), (7,16), (7,22), (5,22), and (3,22)], transferred from aqueous Cd2+ subphase to gold- and aluminum-evaporated glass substrates. The spectra reveal that these monolayers have better thermal stability on Al substrates than on Au. An “interaction band” is identified at 1484∼1480 cm−1, due to the νs(COO−) mode of carboxylate headgroups in ionic bonding with the Al surface. It is found that both the van der Waals interaction between the trans zig-zag hydrocarbon chains and the overlapping interaction between the fluorocarbon helixes are responsible for the systematic variation of the monolayer thermal behavior with (m,n). The thermal behavior of a single monolayer of cadmium stearate, serving as a model system, has been investigated to further confirm the spectral interpretation about the partially fluorinated monolayer. In addition, temperature-dependent friction measurements show that the single monolayers of (m,n)=(7,16), (7,22), (5,22), and (3,22) are potential molecular lubricants that can be used in the range of 25∼140 °C.
Chemical sensing in Fourier space
Authors: Thundat T. , Finot E. , Hu Z. , Ritchie R. H. , Wu G. , Majumdar A.
Published: July 2002

Chemical sensing using optical diffraction from an array of microcantilevers is demonstrated. Properly fashioned arrays of micromachined silicon-nitride cantilevers containing embedded deformable diffraction gratings are functionalized with chemically selective coatings. Adsorption of specific molecules on the cantilever leads to bending, which changes the diffraction pattern of a laser beam reflecting off the array. Quantitative chemical information can be obtained by monitoring the displacement of diffraction peaks as a function of analyte exposure.
Further development of a novel pulling device for crystal growth
Authors: Lang P. L. , He Y. Z. , Zhou F. , Zheng D. N. , Zhao Z. X.
Published: July 2002

We have proposed a new method that could increase the pulling stroke length in a thermal expansion pulling device used for crystal growth. In this method, the thermal expansion part consists of more than one expansion rod linked by materials with low thermal expansion coefficients. We have constructed an experimental setup to demonstrate the advantage of the new design over the previous one. The measured data showed that the stroke length can be increased by 40% with two expansion rods. Also, a linear temperature dependence of the length was observed, and the reason for this observation was discussed.
High room-temperature hole mobility in Ge0.7Si0.3/Ge/Ge0.7Si0.3 modulation-doped heterostructures
Authors: Madhavi S. , Venkataraman V. , Xie Y. H.
Published: July 2002

Modulation-doped two-dimensional hole gas structures consisting of a strained germanium channel on relaxed Ge0.7Si0.3 buffer layers were grown by molecular-beam epitaxy. Sample processing was optimized to substantially reduce the contribution from the parasitic conducting layers. Very high hall mobilities of 1700 cm2/V s for holes were observed at 295 K which are the highest reported to date for any kind of p-type silicon-based heterostructures. Hall measurements were carried out from 13 to 300 K to determine the temperature dependence of the mobility and carrier concentration. The carrier concentration at room temperature was 7.9×1011 cm−2 and decreased by only 26% at 13 K, indicating very little parallel conduction. The high-temperature mobility obeys a T−α behavior with α∼2, which can be attributed to intraband optical phonon scattering.
A semiclassical approach to the dynamics of many-body Bose/Fermi systems by the path integral centroid molecular dynamics
Authors: Kinugawa Kenichi , Nagao Hidemi , Ohta Koji
Published: July 2002

We present a formalism of the path integral centroid molecular dynamics (CMD) extended to Bose and Fermi statistics as a semiclassical approach to explore the dynamics of quantum many-body systems. The validity of the method is examined in relation to the time correlation functions. The presently proposed scheme, refined from our previous derivation [Chem. Phys. Lett. 307, 187 (1999)], is aimed at the calculations of not the exact quantum-mechanical dynamics but the semiclassical dynamics under certain approximations. The formalism is based on the projection operator with which the Bose/Fermi system is mapped onto a particular type of pseudo-Boltzmann system. In the pseudo-Boltzmann system the correlation due to the Bose/Fermi statistics is introduced via an extra pseudopotential called the permutation potential and its relevant operator. Using the present semiclassical formalism, the time correlation function of centroid position, which is evaluated from the CMD trajectories in the pseudo-Boltzmann system, is an approximation to the Kubo canonical correlation function of position operator of the exact quantum-statistical system composed of bosons or fermions. There is no such apparent relation between the momentum operator and the corresponding momentum centroid.
Strain profiles in pyramidal quantum dots by means of atomistic simulation
Authors: Kikuchi Y. , Sugii H. , Shintani K.
Published: July 2002

The minimum energy configurations of the atomic structure of a Ge island on a Si(001) substrate are calculated by using the conjugate gradient minimization of the potential energy of the system. The island is assumed to be covered or uncovered by a Si layer and assumed to be of pyramidal shape with the sidewalls of {110} or {105} facets; the base length of the island ranges from 5.43 to 10.9 nm. Two empirical potentials, the Keating and Stillinger–Weber potentials, are used. At the interfaces between the regions occupied by the atoms of different species, the potential parameters for such bondings are properly adopted. The strain profiles along the three selected paths within the structure and along the cap surface are calculated. While the profiles of the normal strain component εxx obtained by the two potentials are in good agreement with each other except within the substrate and at the edges of the island in the uncovered structures, the two profiles of the normal strain component εzz show a considerable difference in their magnitude, and the use of the Stillinger–Weber potential is recommended for the islands of the small sizes below 10 nm. The validity of the valence force field model with the Keating potential for such small islands is questionable although this model is widely recognized to be applicable to the calculation of strains in the quantum dot structures. The strain relaxation in the uncovered island is discussed through the comparison with that in the covered island. The strain profile along the cap surface explains vertical self-organization of stacked dots.
Simulation of wet oxidation of silicon based on the interfacial silicon emission model and comparison with dry oxidation
Authors: Uematsu Masashi , Kageshima Hiroyuki , Shiraishi Kenji
Published: July 2002

Silicon oxidation in wet ambients is simulated based on the interfacial silicon emission model and is compared with dry oxidation in terms of the silicon-atom emission. The silicon emission model enables the simulation of wet oxidation to be done using the oxidant self-diffusivity in the oxide with a single activation energy. The amount of silicon emission from the interface during wet oxidation is smaller than that during dry oxidation. The small emission rate for wet oxidation is responsible for the insignificant initial oxidation enhancement and the linear pressure dependence of the oxidation rate observed in wet oxidation. Using a unified set of parameters, the whole range of oxide thickness is fitted for both (100) and (111) substrates in a wide range of oxidation temperatures (800 °C–1200 °C) and pressures (1–20 atm).
Theoretical study of space focusing in linear time-of-flight mass spectrometers
Authors: Seccombe D. P. , Reddish T. J.
Published: July 2002

In response to continued improvements in the production of “cold” atoms, molecular beams, and in electronic timing resolution, the issue of space focusing in linear time-of-flight (TOF) mass spectrometers is reevaluated. Starting with the Wiley–McLaren [W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum. 26, 1150 (1955)] condition for first-order space focusing in the conventional two-field system, we extend the approach to higher orders in more complicated situations. A general, solvable, set of equations for satisfying n-order space focusing in an m-field regime is derived. We demonstrate quantitatively that if higher orders of space focus are employed, then provided the initial velocity distribution of the ions is sufficiently narrow, a significant improvement in the mass resolution can be achieved. The conclusions drawn have important implications for the design of the next generation of TOF instruments.
Simultaneous measurement of free-surface velocity and surfactant concentration via a common laser probe
Authors: Vogel Michael J. , Hirsa Amir H. , Kelley Joseph S. , Korenowski Gerald M.
Published: July 2002

A new technique was developed to simultaneously measure the velocity and surfactant surface concentration at an air/water interface via a common laser probe. The surfactant concentration is measured using the nonlinear optical technique of reflected second-harmonic generation (SHG), utilizing an Nd:YAG laser. The transmitted portion of the same laser beam provides illumination for velocity measurements at the interface using the technique of boundary-fitted digital particle image velocimetry (DPIV). The combined SHG and DPIV system was utilized to scan along the free surface of a uniform water flow with a surface barrier trapping hemicyanine, an insoluble monolayer. Measurements were made downstream of the Reynolds ridge, which marks the contamination front and the leading edge of a free-surface boundary layer.
Distributed Bragg reflectors based on (Zn, Cd, Mg)Se for use in the visible spectral range
Authors: Maksimov O. , Guo S. P. , Zeng L. , Tamargo M. C. , Peiris F. C. , Furdyna J. K.
Published: July 2002

Distributed Bragg reflectors (DBRs) based on ZnCdSe and ZnCdMgSe quarter wave layers have been grown on InP (001) substrates by molecular beam epitaxy. Their reflectivity maxima were controlled by the individual thicknesses of constituent layers, and were adjusted in the range of 615–500 nm, covering the red, green, and blue-green regions of the visible spectrum. The crystal quality of the structures was assessed by double crystal x-ray diffraction measurements. Reflectivity as high as 95% was achieved for the DBR structures that consisted of 16 periods. Theoretical calculations, based on the transfer matrix method, predict that a reflectivity above 99% can be achieved by increasing the number of periods to between 20 and 26.
Characterization of SnTe-doped InP grown by solid-source atomic layer molecular beam epitaxy
Authors: Postigo P. A. , Dotor M. L. , Garcı́a F. , Golmayo D. , Briones F.
Published: July 2002

SnTe-doped InP layers were grown at low temperature by solid-source atomic layer molecular beam epitaxy. The samples were characterized by Hall measurements versus temperature, low temperature photoluminescence, x-ray diffraction, and secondary ion mass spectroscopy. The temperature of the SnTe effusion cell was varied from 320 to 440 °C, and the free electron concentration measured at room temperature ranged between 2.0×1016 cm−3 and 5.6×1018 cm−3 with the corresponding Hall mobility varying from 2320 to 1042 cm2/V s.
Versatile cavity for optically pumped molecular lasers
Authors: Nilaya J. Padma , Biswas Dhruba J.
Published: July 2002

We describe here an optically pumped molecular laser cavity configuration which allows near total transmission of pump radiation into the cavity maintaining, at the same time, a high Q value at the lasing wavelength. This also has built into it the capability of elimination of unabsorbed pump radiation, isolation of pump cavity, and suppression of self-focusing effect. The efficient performance of this versatile cavity has been demonstrated in the pulsed operation of a NH3 laser pumped by a transversely excited atmospheric CO2 laser. The low lasing threshold observed here suggests that this cavity can also be employed for the continuous operation of such lasers.
Residual lattice strain in thin silicon-on-insulator bonded wafers: Effects on electrical properties and Raman shifts
Authors: Iida Tsutomu , Itoh Takamasa , Noguchi Daisuke , Takanashi Yoshifumi , Takano Yukio , Kanda Yozo
Published: July 2002

The effects of the residual strain in thin bonded silicon-on-insulator (SOI) wafers on the Hall mobilities and the Raman shifts were investigated as a function of the annealing temperature and duration. For samples with an n-type Si layer of 5 μm thickness and a p-type layer of 2.2 μm thickness, mobilities identical to the theoretical values calculated from the piezoresistive effect were found. However, a sample with an n-type SOI layer of 1.5 μm thickness showed anomalous variation in the electron mobility. While samples with little strain had mobilities close to the corresponding value of the bulk Si, the presence of residual strain brought about a significant decrease to one tenth of the corresponding value of the bulk Si with increasing residual strain. The observed drop in the mobility in the sample N1.5 cannot be explained by the piezoresistive effect only. A Raman microprobe was used to examine the change in the residual strain of SOI layers. A single Raman peak observed for the SOI specimens was nearly identical to that of bulk Si in intensity and half width. However, a shift associated with the residual strain was observed. From a combination of the observed Raman shifts and the precisely evaluated built-in strain in the SOI layers, the results were compared with relationships that are commonly used for quantitative determination of the existing built-in strain and associated stress from the results of Raman characterization.
Photoreflectance characterization of an AlInAs/GaInAs heterojunction bipolar transistor structure with a chirped superlattice
Authors: Mourokh Lev G. , Malikova L. , Pollak Fred H. , Shi B. Q. , Nguyen C.
Published: July 2002

We have measured the photoreflectance spectrum at 300 K from an AlInAs/GaInAs (lattice matched to InP) heterojunction bipolar transistor structure with a chirped superlattice (ChSl) grown by molecular-beam epitaxy. From the observed Franz–Keldysh oscillations we have evaluated the built-in dc electric fields and associated doping levels in the n-GaInAs collector and n-AlInAs emitter regions. The oscillatory signal originating from the ChSl is caused both by the uniform quasielectric and nonuniform space-charge fields in this region.
Localization of helium at an aromatic molecule in superfluid helium clusters
Authors: Kwon Yongkyung , Whaley K. Birgitta
Published: July 2002

Analysis of the helium distribution around a benzene molecule in a 4HeN cluster with the path integral method shows evidence of near complete localization of two 4He atoms at positions above and below the molecular plane. These two atoms are only very weakly coupled to the remainder of the first solvation shell by permutation exchanges, implying that these atoms are effectively removed from the superfluid solvation. The implications of such localization for molecular spectra in helium clusters are discussed.
Langmuir probe measurement of plasma splitting during pulsed laser deposition
Authors: Wild J. , Kudrna P. , Gronych T. , Brož J. , Zelinger Z. , Kubát P. , Civiš S.
Published: July 2002

The properties of plasma formed during pulsed laser deposition from a Bi–Sr–Ca–Cu–O target were measured using a Langmuir probe in the electron accelerating region “Plasma splitting” was observed at pressures in the range of 1–5 Pa and distances of the probe from the target of 3–14 mm.