bluuebunny/crossref_metadata_2025 huggingface dataset:
Explicit-filtering large-eddy simulation using the tensor-diffusivity model supplemented by a dynamic Smagorinsky term
Authors: Winckelmans Grégoire S. , Wray Alan A. , Vasilyev Oleg V. , Jeanmart Hervé
Published: July 2002

Large-eddy simulation (LES) with regular explicit filtering is investigated. The filtered-scale stress due to the explicit filtering is here partially reconstructed using the tensor-diffusivity model: It provides for backscatter along the stretching direction(s), and for global dissipation, both also attributes of the exact filtered-scale stress. The necessary LES truncations (grid and numerical method) are responsible for an additional subgrid-scale stress. A natural mixed model is then the tensor-diffusivity model supplemented by a dynamic Smagorinsky term. This model is reviewed, together with useful connections to other models, and is tested against direct numerical simulation (DNS) of turbulent isotropic decay starting with Reλ=90 (thus moderate Reynolds number): LES started from a 2563 DNS truncated to 643 and Gaussian filtered. The tensor-diffusivity part is first tested alone; the mixed model is tested next. Diagnostics include energy decay, enstrophy decay, and energy spectra. After an initial transient of the dynamic procedure (observed with all models), the mixed model is found to produce good results. However, despite expectations based on favorable a priori tests, the results are similar to those obtained when using the dynamic Smagorinsky model alone in LES without explicit filtering. Nevertheless, the dynamic mixed model appears as a good compromise between partial reconstruction of the filtered-scale stress and modeling of the truncations effects (incomplete reconstruction and subgrid-scale effects). More challenging 483 LES are also done: Again, the results of both approaches are found to be similar. The dynamic mixed model is also tested on the turbulent channel flow at Reτ=395. The tensor-diffusivity part must be damped close to the wall in order to avoid instabilities. Diagnostics are mean profiles of velocity, stress, dissipation, and reconstructed Reynolds stresses. The velocity profile obtained using the damped dynamic mixed model is slightly better than that obtained using the dynamic Smagorinsky model without explicit filtering. The damping used so far is however crude, and this calls for further work.
National Ignition Facility scale hohlraum asymmetry studies by thin shell radiography
Authors: Pollaine S. M. , Bradley D. K. , Landen O. L. , Wallace R. J. , Jones O. S. , Amendt P. A. , Suter L. J. , Turner R. E.
Published: July 2002

A necessary condition for igniting indirectly driven inertial confinement fusion (ICF) capsules on the National Ignition Facility (NIF) is controlling drive asymmetry to the 1% level [S. W. Haan, S. M. Pollaine, J. D. Lindl et al., Phys. Plasmas 2, 2480 (1995)]. Even flux-asymmetry modes (e.g., Legendre modes P2, P4, P6, and P8) must be reduced by hohlraum design and laser beam pointing. Odd flux-asymmetry modes (e.g., Legendre modes P1, P3, P5, etc.) are theoretically removed by reflection symmetry across the hohlraum midplane [S. M. Pollaine and D. Eimerl, Nucl. Fusion 38, 1523 (1998)], but will be produced by power imbalance, laser beam pointing errors, and target fabrication errors. An experimental campaign is now being conducted on the University of Rochester’s Omega laser to measure higher order (P4 and higher) flux asymmetry modes inside hohlraums that approximate the conditions of a NIF hohlraum during the 90 eV early drive phase [S. W. Haan, S. M. Pollaine, J. D. Lindl et al., Phys. Plasmas 2, 2480 (1995)]. These experiments use a new point-projection backlighting technique [O. L. Landen, D. R. Farley, S. G. Glendinning et al., Rev. Sci. Instrum. 72, 627 (2001)] to cast high quality 4.7 keV radiographs of thin 2 mm diameter Ge-doped CH shells designed to enhance sensitivity to drive asymmetries. Distortions in the position of the limb of the shells resulting primarily from drive asymmetries are measured to an accuracy of 2 μm. The linearity and sensitivity of thin imploding shells to flux asymmetry makes it possible to achieve this degree of accuracy, which is sufficient for NIF ignition symmetry tuning. The promising results to date permit the comparison of measured and predicted distortions and, by inference, drive asymmetries for the first eight asymmetry modes.
Nondestructive readout of photochromic optical memory using photocurrent detection
Authors: Tsujioka Tsuyoshi , Hamada Yuji , Shibata Kenichi , Taniguchi Akira , Fuyuki Takashi
Published: July 2002

We proposed and demonstrated a nondestructive readout method using photocurrent detection for photon-mode photochromic memory. The principle of this readout method, which utilized the ionization potential change according to photoisomerzation reaction, was confirmed by using a medium with a photochromic diarylethene layer and phthalocyanine photoabsorbing layer, and by using a near-infrared readout light. We demonstrated perfect nondestructive readout operations over 106 times.
Interactions of adsorbates with locally strained substrate lattices
Authors: Jakob P. , Gsell M. , Menzel D.
Published: July 2002

Using scanning tunneling microscopy the effect of local strain at a Ru(001) surface on the adsorption of various adsorbates has been studied. Local strain fields have been produced by Ar-ion implantation and annealing. Thereby the accompanying surface sputter damage is fully healed out with the exception of subsurface cavities filled with argon atoms which have aggregated by bulk diffusion. The resulting nanometer-sized structures contain surface areas of expanded lattice at the tops of the protrusions while around their rim the lattice is compressed relative to the flat surface. Various adsorbates are found to react sensitively to these local lattice distortions. Oxygen atoms adsorb preferentially in the regions of expanded lattice. This preference prevails for all coverages up to the full monolayer with the successive formation of the well-known (2×2)-O, (2×1)-O, (2×2)-3O, and (1×1)-O ordered overlayers on the various parts of the surface. CO at coverages in excess of 0.33 monolayers is found to behave similarly. The experimental results are complemented by investigations of the mixed (O+CO) coadsorbate layer. The reported influence of surface strain on the adsorption energy can be considered as the reverse of strain induction by adsorption, and their direct local demonstration can be used to test theoretical predictions. We also find direct evidence for a compressed lattice zone close to step edges, which extends about 10–20 Å into the terraces.
Simple chemical routes to diamond-cubic germanium–tin alloys
Authors: Taraci Jennifer , Tolle John , Kouvetakis J. , McCartney M. R. , Smith David J. , Menendez J. , Santana M. A.
Published: July 2002

We report the development of a simple chemical route to growing Ge1−xSnx semiconductors using ultrahigh-vacuum chemical vapor deposition and the molecular precursor (Ph)SnD3 as the source of Sn atoms. Thin films were deposited on oxidized and oxide-free Si by reactions of (Ph)SnD3 with Ge2H6 at 350 °C. The composition, microstructure, and bonding properties of the films were characterized by Rutherford backscattering, high-resolution analytical electron microscopy, and Raman spectroscopy. As-deposited Ge1−xSnx on oxidized Si displayed good crystallinity which improved significantly by annealing at 400 °C. High-resolution electron microscopy and diffraction indicated a diamond-cubic structure with lattice constants intermediate to those of Ge and α-Sn. As-deposited Ge1−xSnx on pure Si was monocrystalline and epitaxial. Nanoprobe analysis in plan view and cross section revealed that the as-deposited and annealed materials were homogeneous with good chemical purity. The Raman spectra showed bands corresponding to Ge–Ge and Sn–Ge vibrations with frequencies consistent with a random tetrahedral alloy.
Activated escape of periodically driven systems
Authors: Dykman M. I. , Golding B. , McCann L. I. , Smelyanskiy V. N. , Luchinsky D. G. , Mannella R. , McClintock P. V. E.
Published: July 2002

We discuss activated escape from a metastable state of a system driven by a time-periodic force. We show that the escape probabilities can be changed very strongly even by a comparatively weak force. In a broad parameter range, the activation energy of escape depends linearly on the force amplitude. This dependence is described by the logarithmic susceptibility, which is analyzed theoretically and through analog and digital simulations. A closed-form explicit expression for the escape rate of an overdamped Brownian particle is presented and shown to be in quantitative agreement with the simulations. We also describe experiments on a Brownian particle optically trapped in a double-well potential. A suitable periodic modulation of the optical intensity breaks the spatio-temporal symmetry of an otherwise spatially symmetric system. This has allowed us to localize a particle in one of the symmetric wells.
Multipactor experiment on a dielectric surface
Authors: Anderson R. B. , Getty W. D. , Brake M. L. , Lau Y. Y. , Gilgenbach R. M. , Valfells A.
Published: July 2002

A novel experiment to investigate single-surface multipactor on a dielectric surface was developed and tested. The compact apparatus consists of a small brass microwave cavity in a high vacuum system. The cavity is ∼15 cm in length with an outer diameter of ∼10 cm. A pulsed variable frequency microwave source at ∼2.4 GHz, 2 kW peak excites the TE111 mode with a strong electric field parallel to a dielectric plate (∼0.2 cm thickness) that is inserted at midlength of the cavity. The microwave pulses are monitored by calibrated microwave diodes. An electron probe measures electron current and provides temporal measurements of the multipactor electron current with respect to the microwave pulses. Phosphor on the dielectric surface is used to detect multipactor electrons by photoemission. The motivation of this experiment is to test recent theoretical calculations of single-surface multipactor on a dielectric.
One- and two-center energy components in the atoms in molecules theory
Authors: Salvador P. , Duran M. , Mayer I.
Published: July 2002

The energy decomposition scheme proposed in a recent paper has been realized by performing numerical integrations. The sample calculations carried out for some simple molecules show excellent agreement with the chemical picture of molecules, indicating that such an energy decomposition analysis can be useful from the point of view of connecting quantum mechanics with the genuine chemical concepts.
Dry laser cleaning of particles from solid substrates: Experiments and theory
Authors: Zheng Y. W. , Luk’yanchuk B. S. , Lu Y. F. , Song W. D. , Mai Z. H.
Published: July 2002

The experimental analysis of dry laser cleaning efficiency is done for certified spherical particle (SiO2, 5.0, 2.5, 1.0, and 0.5 μm) from different substrates (Si, Ge, and NiP). The influence of different options (laser wavelength, incident angle, substrate properties, i.e., type of material, surface roughness, etc.) on the cleaning efficiency is presented in addition to commonly analyzed options (cleaning efficiency versus laser fluence and particle size). Found laser cleaning efficiency demonstrates a great sensitivity to some of these options (e.g., laser wavelength, angle of incidence, etc.). Partially these effects can be explained within the frame of the microelectronics engineering (MIE) theory of scattering. Other effects (e.g., influence of roughness) can be explained along the more complex line, related to examination of the problem “particle on the surface” beyond the MIE theory. The theory of dry laser cleaning, based on one-dimensional thermal expansion of the substrate, demonstrates a great sensitivity of the cleaning efficiency on laser pulse shape. For the reasonable pulse shape this theory yields the threshold fluence by the order of magnitude larger than the experimental one. At the same time the theory, which takes into account the near-field optical enhancement and three-dimensional thermal expansion effects, yields the correct values for threshold.
Aluminum-doped n-type ZnTe layers grown by molecular-beam epitaxy
Authors: Chang J. H. , Takai T. , Koo B. H. , Song J. S. , Handa T. , Yao T.
Published: July 2002

N-type ZnTe layers with high electron concentration are grown by molecular-beam epitaxy using aluminum as the donor species. The ZnTe:Al layers show a high structural quality with a narrow x-ray diffraction linewidth (24 arcsec) and a high carrier concentration up to n=4×1018 cm−3 with low resistivity (ρ=0.017 Ω cm). The dependence of the electron mobility on the carrier concentration suggests that the dominant scattering mechanisms in the ZnTe:Al layers are ionized impurity scattering and polar optical phonon scattering. The photoluminescence spectrum of moderately doped ZnTe layers shows strong Al–donor-related bound exciton lines: I2 (2.378 eV) and donor–acceptor pair emission (zero phonon energy=2.324 eV) with a weak deep-level emission (2.19 eV). Highly Al-doped layers show an increase in the deep-level emission intensity and a decrease in carrier mobility, which are interpreted in terms of the increase in the carrier compensation.
The approximate deconvolution model for large-eddy simulations of compressible flows and its application to shock-turbulent-boundary-layer interaction
Authors: Stolz S. , Adams N. A. , Kleiser L.
Published: July 2002

A formulation of the approximate deconvolution model (ADM) for the large-eddy simulation (LES) of compressible flows in complex geometries is detailed. The model is applied to supersonic compression ramp flow where shock-turbulence interaction occurs. With the ADM approach an approximation to the unfiltered solution is obtained from the filtered solution by a series expansion involving repeated filtering. Given a sufficiently good approximation of the unfiltered solution at a time instant, the flux terms of the underlying filtered transport equations can be computed directly, avoiding the need to explicitly compute subgrid-scale closures. The effect of nonrepresented scales is modeled by a relaxation regularization involving a secondary filter operation and a dynamically estimated relaxation parameter. Results of the large-eddy simulation of the turbulent supersonic boundary layer along a compression ramp compare well with filtered DNS data. The filtered shock solution is correctly predicted by the ADM procedure, demonstrating that turbulent and nonturbulent subgrid-scales are properly modeled. We found that a computationally expensive shock-capturing technique was not necessary for stable integration. As a consequence, the computational effort for simulations with ADM is approximately as large as for a coarse-grid DNS with a hybrid compact-upwind-ENO scheme, since the additional computational cost for the subgrid-scale model is more than compensated due to the fact that in the LES flux-derivatives can be computed by linear central finite differences on the entire domain.
Numerical simulations of molecular orientation using strong, nonresonant, two-color laser fields
Authors: Kanai Tsuneto , Sakai Hirofumi
Published: July 2002

We have investigated the possibility of molecular orientation using an asymmetric potential created by the superposition of strong, nonresonant, two-color (ω+2ω) laser fields. The time-independent Schrödinger equations are solved numerically for FCN molecules as an example in an adiabatic regime where the orientation proceeds slowly compared to the rotational period of molecules. We show the laser intensity dependence of the orientation parameter 《cos θ》 and the alignment parameter 《cos2 θ》 where θ is the angle between the polarization axis and the molecular axis, and their time evolution during the laser fields, suggesting that our approach can be used to orient polar molecules if they are rotationally cold.
Dimers of phenol in argon and neon matrices
Authors: Plokhotnichenko A. M. , Radchenko E. D. , Blagoı̆ Yu. P. , Karachevtsev V. A.
Published: July 2002

The IR absorption spectra of phenol molecules in solid rare gas matrices of argon (10–12 K) and neon (4.5–5 K) are investigated at molar ratios of phenol:matrix of 1:1000 to 1:30 in the frequency range 400–4000 cm−1. Bands of dimers and larger complexes of phenol molecules are observed in the absorption spectrum of both matrices as the matrix ratio decreases. The first additional bands to appear in the spectral region of the stretching vibrations of the O–H group as the phenol concentration increases are two bands attributed to dimers with one and two hydrogen bonds. The absorption coefficients are determined for the bands of stretching vibrations of the O–H and C–O groups, O–H planar bending vibrations of monomers, and the stretching vibrations of the hydrogen-bonded O–H groups of the phenol molecules. The features of the formation of H-bonded complexes in low-temperature matrices are discussed. A model is proposed which permits calculation of the number of monomers, dimers, and larger complexes in argon and neon matrices for molecules which are close in size to the phenol molecule.
Large, continuously controllable nematic pretilt from vertical orientation
Authors: Sinha Ghanshyam P. , Wen Bing , Rosenblatt Charles
Published: July 2002

A polyimide that is designed to produce homeotropic liquid-crystal orientation is baked at high temperature and rubbed with a cotton cloth. It is found that the resulting pretilt angle of the liquid-crystal pentylcyanobiphenyl is continuously controllable from 0° to nearly 40°.
Atomistic Monte Carlo simulation of cis-1,4 polyisoprene melts. II. Parallel tempering end-bridging Monte Carlo simulations
Authors: Doxastakis M. , Mavrantzas V. G. , Theodorou D. N.
Published: July 2002

Results are presented for the temperature dependence of the thermodynamic and conformational properties of cis-1,4 polyisoprene (PI) melts from detailed atomistic parallel-tempering end-bridging Monte Carlo (ptEBMC) simulations. The simulations have been performed with a C80 cis-1,4 PI melt system which was simultaneously equilibrated at 10 different temperatures, ranging from T=328 K up to T=513 K, in the semigrand NnPTμ* statistical ensemble. This strategy allowed system equilibration at temperatures as low as T=328 K (where most available experimental data have been obtained), for which the performance of the single temperature end-bridging Monte Carlo (EBMC) algorithm was seen to deteriorate. Results for the variation of the specific volume of the cis-1,4 PI melt with temperature at constant mean chain length are found to be always within 1% of experimentally reported values and analytical fits to these values. Additional results for the equilibrium mean-square chain end-to-end distance 〈R2〉0, which can be fully equilibrated with the algorithm employed here, show that the model predictions for 〈R2〉0 are rather insensitive to temperature variations, at least over the range of temperatures studied here. This behavior is explained by investigating the temperature variation of the distributions of the three types of torsion angles in a PI molecule. The conformational predictions of the ptEBMC simulations are closest to experiment at the lower temperatures, where they almost match the measured value of 〈R2〉0. Detailed results are also presented for the performance of the ptEBMC algorithm, demonstrating its unique capability to equilibrate the atomistically detailed cis-1,4 PI melt.
X-ray diffraction and Raman scattering study of SrBi2Ta2O9 ceramics and thin films with Bi3TiNbO9 addition
Authors: Zhu J. S. , Qin H. X. , Bao Z. H. , Wang Y. N. , Cai W. Y. , Chen P. P. , Lu W. , Chan H. L. W. , Choy C. L.
Published: July 2002

Good ferroelectric properties have previously been reported for both the (1−x)SrBi2Ti2O9–xBi3TiNbO9 bulk ceramics and thin films. In this work, x-ray diffraction and Raman scattering were used to investigate the effect of the incorporation of Bi3TiNbO9 into SrBi2Ta2O9 bulk ceramics and thin films. A better crystallization, larger grain size and larger displacement of the Ta–O(4) or Ta–O(5) ions are the origin for the good ferroelectric properties of (1−x)SrBi2Ta2O9–xBi3TiNbO9 with x=0.3–0.4.
Refractive index of Ag nanocrystals composite films in the neighborhood of the surface plasmon resonance
Authors: de Sande J. C. G. , Serna R. , Gonzalo J. , Afonso C. N. , Hole D. E. , Naudon A.
Published: July 2002

Nanocomposite thin films formed by Ag nanocrystals embedded in an amorphous aluminum oxide (Al2O3) host were prepared by alternating-target pulsed laser deposition. Spectroscopic ellipsometry was used to determine the effective refractive index (n=n+ik). When the Ag volume fraction is over 2%, the linear optical properties of the nanocomposite films differ from those of the pure dielectric host. The extinction coefficient shows a maximum around 435 nm that is related to the surface plasmon resonance. Near this wavelength, the real part of the refractive index undergoes anomalous dispersion, leading to a significant increase of the n value of the composite compared to that of the matrix.
Electrostrictive response of an ideal polar rubber
Authors: Yamwong T. , Voice A. M. , Davies G. R.
Published: July 2002

This article considers the electrostrictive response of an ideal polar rubber in which each random link carries a dipole which is constrained to remain perpendicular to the link but is free to rotate about it. It thus mimics typical polar polymers with dipoles essentially perpendicular to the chain backbone and which rotate preferentially about the local chain axis (Stockmayer type B). In such materials, there is a genuine interplay between mechanical deformation and the electrical permittivity since an applied strain can produce preferential segmental orientation, align dipole rotation axes, and thus cause a change in permittivity. Though this is a plausible mechanism of electrostriction in polar rubbers, the quantitative analysis presented in this article suggests that the effect makes only a small contribution to the observed deformation of actuators based on typical polar rubber films with compliant electrodes.
Compensation of temperature effects in quartz crystal microbalance measurements
Authors: Rahtu Antti , Ritala Mikko
Published: July 2002

Quartz crystal microbalance (QCM) is a very powerful method for in situ monitoring of thin film growth processes. However, especially at high temperatures, QCM is very sensitive to already minor temperature changes during the measurement. Here, a method for compensating the temperature effects on the QCM is introduced. In this method, the baseline drift during the growth is calculated using a function obtained by fitting the signal measured before and after the growth. The present method is compared with the earlier published reference crystal method where two closely spaced quartz crystals are used, one being protected against film growth and thus serving as a reference crystal for compensating the temperature effects on the measurement crystal. Both methods were tested with TiO2 and Al2O3 atomic layer deposition processes and the temperature effect was successfully compensated. The differences between these methods are discussed.
Dose-energy match for the formation of high-integrity buried oxide layers in low-dose separation-by-implantation-of-oxygen materials
Authors: Chen Meng , Wang Xiang , Chen Jing , Liu Xianghua , Dong Yeming , Yu Yuehui , Wang Xi
Published: July 2002

High-quality low-dose separation-by-implantation-of-oxygen (SIMOX) silicon-on-insulator (SOI) wafers have been fabricated from a series of good matches of dose-energy combinations. The results reveal that a wafer fabricated at an optimum dose-energy match has a superior SOI layer with a low threading dislocation density, a high-integrity buried oxide (BOX) layer with a minimal detectable silicon island density and a low pinhole density. This work introduces an approach to flexibly control the thickness of both SOI and BOX layers, allowing the fabrication of ultrathin SIMOX wafers with ultrathin SOI and BOX layers, and improving the throughput capacity by selecting good dose-energy matches. A possible mechanism is discussed.
Magnetic sublevel specific stimulated Raman pumping of molecular H2
Authors: Cureton-Chinn Sarah , Kelly Peter B. , Augustine Matthew P.
Published: July 2002

The magnetic sublevel specific preparation of the |X 1Σg+,v=1,J=1〉 state of molecular H2 using stimulated Raman pumping with circularly polarized light to saturate the Q01(1) transition is reported. The intensities of S branch rotational Raman scattering signals for the v=0 and v=1 states combined with their depolarization ratios measured as a function of pumping light polarization provide direct evidence of the mJ selective nature of the pumping process. This Raman method of non-Boltzmann population preparation does not require a permanent dipole moment and is generally applicable to a wide range of systems.
Free electron distribution in AlGaN/GaN heterojunction field-effect transistors
Authors: Jogai B.
Published: July 2002

A detailed calculation of the free electron concentration and conduction and valence band edges of AlGaN/GaN heterojunction field-effect transistors is presented. The model is based on a self-consistent solution of the Schrödinger, Poisson, and charge balance equations and includes the effect of exchange correlation on the Coulomb interaction. It also includes surface acceptor and donor states populated according to Fermi–Dirac statistics. The piezoelectric and spontaneous polarization discontinuities across the material interfaces are rigorously taken into account. The influence of the polarization discontinuity on the magnitude of the charge in the two-dimensional electron gas is investigated. From charge conservation, it is shown that the polarization discontinuity does not behave as dopants in the same manner as substitutional impurities. Any free electrons within the structure must originate from some other source, either from the surface through surface donors, or from the bulk through unintentional n-type dopants. The present model allows for δ doping and multiple AlGaN/GaN interfaces.
Dynamics of the C+C2H2 reaction from differential and integral cross-section measurements in crossed-beam experiments
Authors: Cartechini L. , Bergeat A. , Capozza G. , Casavecchia P. , Volpi G. G. , Geppert W. D. , Naulin C. , Costes M.
Published: July 2002

The reaction between atomic carbon and acetylene has been investigated using complementary crossed molecular beam techniques. Differential cross sections have been obtained for the reactions of both ground and excited carbon atoms, C(3PJ, 1D2)+C2H2(X 1Σg+), in experiments conducted with continuous supersonic beams, mass spectrometric detection, and time-of-flight analysis at a relative translational energy of 29.3 kJ mol−1. The reaction C(3PJ)+C2H2(X 1Σg+) has been found to lead to C3H+H and C3+H2 products in comparable amounts. Both H and H2 elimination pathways are found to proceed through the formation of a C3H2 long-lived intermediate complex whose lifetime may be comparable to its rotational period. The spin-forbidden H2 elimination channel is attributed to the occurrence of intersystem-crossing between the triplet and singlet manifolds of the C3H2 potential-energy surfaces. The reaction C(1D2)+C2H2(X 1Σg+) has been found to lead to formation of C3H+H, with a C3H center-of-mass angular distribution strongly forward peaked, indicating a short-lived intermediate complex. Integral cross sections have been obtained for the C(3PJ)+C2H2(X 1Σg+)→C3H+H(2S1/2) reaction in experiments conducted with pulsed, supersonic molecular beams in the range of relative translational energies ET=0.38−25.5 kJ mol−1, the H(2S1/2) product being detected by laser-induced fluorescence. The reaction has been found to be without a barrier, relative integral cross sections being proportional to (ET)−0.80±0.03 below ET=10 kJ mol−1. These findings provide direct evidence that the C(3PJ)+C2H2 reaction can occur under the physical conditions prevailing in dense interstellar clouds and, in particular, that it may be the source of both C3H and C3 species in these extreme environments.
Scaling effects on vertical-cavity surface-emitting lasers static and dynamic behavior
Authors: Jungo Marc , di Sopra Fabrice Monti , Erni Daniel , Baechtold Werner
Published: July 2002

We investigate the influence of oxide aperture size and number of top distributed Bragg reflector pairs on the performance of oxide confined vertical-cavity surface emitting lasers. Several counteracting mechanisms are shown to result in nonmonotonic behavior, which limits the performance of very small cavities. Static, dynamic, and noise behavior are considered. We examine static operation by means of steady-state measurements, whereas dynamic behavior and noise performance are described by the intrinsic relaxation oscillation frequency, damping coefficient, and Schawlow–Townes linewidth. These parameters are extracted from relative intensity noise measurements. Reducing the oxide aperture up to a given optimal diameter is shown to improve the device’s characteristics. We attribute the performance degradation below this value to increased diffraction losses, reduced confinement factor, and enhanced spontaneous emission. Similarly, increasing the number of Bragg reflector pairs first yields better overall performance as a consequence of reduced mirror losses. However, beyond an optimal value, significant reduction of the differential gain is observed that is attributed to gain compression and possibly thermal effects, degrading both the steady-state and high-frequency performance of the device.
Precise control of multiwall carbon nanotube diameters using thermal chemical vapor deposition
Authors: Siegal M. P. , Overmyer D. L. , Provencio P. P.
Published: July 2002

We grow multiwall carbon nanotube (CNT) films using thermal chemical vapor deposition at atmospheric pressure using a mixture of acetylene and nitrogen from a 4-nm-thick Ni film catalyst. CNTs are characterized using electron microscopy and Rutherford backscattering spectrometry. CNTs grown with this method are extremely uniform in diameter, both throughout the sample and within the lengths of individual tubes. Nanotube outer diameters, ranging from 5–350 nm, and the total deposition of carbon material, increase exponentially with growth temperature from 630 °C–790 °C.
Surface photovoltage spectroscopy and normal-incidence reflectivity characterization of a 1.3 μm InGaAlAs/InP vertical-cavity surface-emitting laser structure
Authors: Huang Y. S. , Malikova L. , Pollak Fred H. , Debray J.-P. , Hoffman R. , Amtout A. , Stall R. A.
Published: July 2002

We have investigated a 1.3 μm InGaAlAs/InP (001) vertical-cavity surface-emitting laser (VCSEL) structure using surface photovoltage spectroscopy (SPS) and normal-incidence reflectivity. The SPS measurements were performed as a function of angle of incidence relative to the normal (0°–55°) and temperature (300 K<T<420 K). The SPS spectra exhibit both the fundamental conduction to heavy-hole excitonic transition and cavity mode plus a rich interference pattern related to the mirror stack. The advantages of SPS in relation to other methods to study VCSELs, such as photoreflectance and photocurrent spectroscopy, are discussed. This experiment demonstrates the considerable potential of SPS for the characterization of these device structures.
Phase diagrams for a multistep potential model of n-alkanes by discontinuous molecular dynamics and thermodynamic perturbation theory
Authors: Cui Jingyu , Elliott J. Richard
Published: July 2002

Discontinuous molecular dynamics simulations and thermodynamic perturbation theory have been used to study thermodynamic properties for chain molecules. A multistep potential model is considered in this work through characterization of the vapor pressure and saturated liquid density for the n-alkanes from ethane to octane. The multistep potential model provides ∼4% accuracy for the entire molecular weight range. By contrast, previous studies of the square-well potential showed that the range of the square-well needed to vary with molecular weight to provide comparable accuracy. Therefore, having multiple steps with different depths for each functional group appears to be an essential part of making the potential model transferable. The depths of the wells vary such that the CH2 potential appears flatter than the CH3 potential, in an apparent attempt to moderate the overlaps implicit in the interaction site model.
Regular obstructed categories and topological quantum field theory
Authors: Duplij Steven , Marcinek Wladyslaw
Published: July 2002

A proposal of the concept of n-regular obstructed categories is given. The corresponding regularity conditions for mappings, morphisms, and related structures are considered. An n-regular topological quantum field theory is introduced. The connection of time nonivertibility with the regularity is shown.
On the required shape corrections to the local density and generalized gradient approximations to the Kohn–Sham potentials for molecular response calculations of (hyper)polarizabilities and excitation energies
Authors: Grüning Myrta , Gritsenko Oleg V. , van Gisbergen Stan J. A. , Jan Baerends Evert
Published: July 2002

It is well known that shape corrections have to be applied to the local-density (LDA) and generalized gradient (GGA) approximations to the Kohn–Sham exchange–correlation potential in order to obtain reliable response properties in time dependent density functional theory calculations. Here we demonstrate that it is an oversimplified view that these shape corrections concern primarily the asymptotic part of the potential, and that they affect only Rydberg type transitions. The performance is assessed of two shape-corrected Kohn–Sham potentials, the gradient-regulated asymptotic connection procedure applied to the Becke–Perdew potential (BP–GRAC) and the statistical averaging of (model) orbital potentials (SAOP), versus LDA and GGA potentials, in molecular response calculations of the static average polarizability α, the Cauchy coefficient S−4, and the static average hyperpolarizability β. The nature of the distortions of the LDA/GGA potentials is highlighted and it is shown that they introduce many spurious excited states at too low energy which may mix with valence excited states, resulting in wrong excited state compositions. They also lead to wrong oscillator strengths and thus to a wrong spectral structure of properties like the polarizability. LDA, Becke–Lee–Yang–Parr (BLYP), and Becke–Perdew (BP) characteristically underestimate contributions to α and S−4 from bound Rydberg-type states and overestimate those from the continuum. Cancellation of the errors in these contributions occasionally produces fortuitously good results. The distortions of the LDA, BLYP, and BP spectra are related to the deficiencies of the LDA/GGA potentials in both the bulk and outer molecular regions. In contrast, both SAOP and BP-GRAC potentials produce high quality polarizabilities for 21 molecules and also reliable Cauchy moments and hyperpolarizabilities for the selected molecules. The analysis for the N2 molecule shows, that both SAOP and BP–GRAC yield reliable energies ωi and oscillator strengths fi of individual excitations, so that they reproduce well the spectral structure of α and S−4.
Application of HfSiON as a gate dielectric material
Authors: Visokay M. R. , Chambers J. J. , Rotondaro A. L. P. , Shanware A. , Colombo L.
Published: July 2002

Physical and electrical properties of HfSiON that make this material desirable as the gate dielectric in a standard metal–oxide–semiconductor flow are reported. Sputtering was used to deposit films with minimal low dielectric constant interface layers, equivalent oxide thicknesses below 13 Å, and leakage current density at least two orders of magnitude lower than SiO2. The presence of nitrogen in the film enhances the thermal stability relative to HfSiO, and no crystallization was observed for anneals up to 1100 °C.
Magnetic coupling between liquid 3He and solid insulators (Review)
Authors: Naletov V. V. , Tagirov M. S. , Tayurskiĭ D. A.
Published: July 2002

Theoretical and experimental results are presented on the magnetic coupling between liquid 3He (in the normal phase) and single crystals and fine powders of insulating Van Vleck paramagnets and their diamagnetic analogs.
Reaction network reduction for distributed systems by model training in lumped reactors: Application to bifurcations in combustion
Authors: Raimondeau S. , Gummalla M. , Park Y. K. , Vlachos D. G.
Published: July 2002

A new methodology is presented to derive reduced reaction mechanisms for distributed reacting flows by model training in a lumped parameter system (a continuous-stirred tank reactor). The method identifies the relevant transport time scales in the reaction zone of a distributed system along with the local composition vector, over a range of operation conditions. A training box in the parameter space of pressure-transport time scale-composition is then identified. Sensitivity and principal component analyses are subsequently performed at bifurcation points in a lumped parameter system at representative conditions of the training box. The most inclusive chemistry derived in the lumped system captures the proper transport–chemistry coupling and is suitable for the distributed reactor. Application to ignition of hydrogen/air and methane/air mixtures is presented and validated for premixed and diffusion flames in a stagnation flow geometry.
The flow of heat and the motion of the weld pool in penetration welding with a laser
Authors: Dowden John , Davis Michael , Kapadia Phiroze
Published: July 2002

Lasers are well suited to penetration welding as they can supply large amounts of power to a very small volume far from the surface of the workpiece. This paper reports the results of calculations performed with the aid of a mathematical model for the flow of heat and the motion of the molten metal surrounding the keyhole formed by the laser. The equations were solved with the aid of an approximate analytical solution for low welding speeds, and a computer program for higher speeds. A number of different materials are considered and graphs of the temperature distribution and the stream function describing the fluid motion are shown for a typical case. Relationships between the weld width, the power absorbed per unit thickness of the workpiece, and the speed of welding are investigated in detail. Over the range of welding speeds considered, the relationships given by the approximate analytical solution were found to be close to those obtained by numerical integration of the exact equations, even though the fine details of the solution, such as the ratio of the weld width to the total length of the weld pool at a given instant, or the details of the fluid motion, are substantially different.
Ordering phenomena of particles dispersed in magnetic fluids (invited)
Authors: Skjeltorp A. T.
Published: July 2002

Various order-disorder phenomena have been studied from microscopic observations of μm-sized polystyrene particles in magnetic fluids (ferrofluids). The basis for this is that the holes created by the particles in a magnetized fluid appear to possess magnetic moments antiparallel to an external field. The effective dipolar interactions between the particles (holes) may be made attractive or repulsive and varied relative to the thermal energy due to Brownian motion. Various many-body effects are visualized including formation of triangular, chain, and lamellar structures of spheres. Nonspherical and magnetic particles are shown to form structures with competing interactions. The results are interpreted qualitatively and compared with existing model calculations.
Magnetostatic energy of magnetic thin-film edge having volume and surface charges
Authors: Nonaka K. , Hirono S. , Hatakeyama I.
Published: July 2002

This paper gives formulas for analyzing the curling state. Magnetostatic energy of magnetic thin films having volume and surface charges can be calculated from the formulas, expressed in terms of film thickness T, film width W, saturation magnetization M, length of curling region D, and angle θ0 between the magnetic moment and the film edge surface. Using the formulas, we have quantified the magnetization distribution in the film edge layer and the decrease in energy caused by conversion from surface charges into volume charges.
High-purity GaAs grown by molecular-beam epitaxy
Authors: Wang W. I. , Marks R. F. , Vina L.
Published: July 2002

We have grown high-purity GaAs on (100), (311)A, and (311)B orientations by molecular-beam epitaxy (MBE). While undoped GaAs grown on (100) and (311)A are typically p type, growth on (311)B orientation has yielded n type with a liquid-nitrogen electron mobility of 1.3×105 cm2 V−1 s−1, which is among the highest mobilities reported for MBE-grown materials. Low-temperature photoluminescence showed well-resolved impurity bound exciton peaks consistent with electrical results. The possible incorporations of impurities, especially carbon, are discussed. Our work demonstrates that the previously reported undoped p-type GaAs(100) are compensated.
Surface properties of metal-nitride and metal-carbide films deposited on Nb for radio-frequency superconductivity
Authors: Garwin E. L. , King F. K. , Kirby R. E. , Aita O.
Published: July 2002

Various effects occur which can prevent attainment of the high Q’s and/or the high gradient fields necessary for the operation of radio-frequency (rf) superconducting cavities. One of these effects, multipactor, both causes the cavity to detune during filling due to resonant secondary electron emission at the cavity walls, and lowers the quality factor (Q) by dissipative processes. TiN deposited onto the high-field regions of room-temperature Al cavities has been used at the Stanford Linear Accelerator Center to successfully reduce multipactor in the past. We have therefore studied TiN and its companion materials, NbN, NbC, and TiC, all on Nb substrates under several realistic conditions: (1) as-deposited, (2) exposed to air, and (3) electron bombarded. The studied films (up to 14-nm thickness) were sputter deposited onto sputter-cleaned Nb substrates. Results indicate that all the materials tested gave substantially the same results. The maximum secondary electron yields for as-deposited films were reduced to nearly the preoxidized values after electron bombardment (2–3×1017 electrons cm−2 in the case of NbN and NbC). X-ray photoelectron spectroscopy analysis showed that the oxides (e.g., TiO2 in the case of TiN films) formed during air exposure were slightly reduced (converted to lower oxides) by the electron-beam exposure. Auger electron spectroscopy (AES) showed a slight reduction in the surface O concentration following beam exposure. These results suggest that the chemical nature of top surface layers is responsible for the substantial changes in secondary electron yield observed upon electron-beam exposures and that AES does not reflect this change strongly because of the difficulty in extracting chemical (versus elemental) information from AES. The results indicate that any of these films would be poor choices if simply deposited and exposed to air, but, in fact, the in situ electron bombardment which occurs during cavity operation serves to reduce the secondary electron yield and thereby causes a substantial reduction in multipacting.
Silicon double-injection electro-optic modulator with junction gate control
Authors: Friedman Lionel , Soref Richard A. , Lorenzo Joseph P.
Published: July 2002

Novel structures for carrier-induced electro-optical phase modulation in crystalline silicon are examined. A new dual-injection field-effect transistor structure for guided-wave light modulation at 1.3 μm is proposed and analyzed. It consists of an elongated cathode-anode-gate structure integrated in a rib waveguide. Dual-gate and single-gate control are considered. The overlap between the plasma charge density and the optical guided mode is computed. For a cathode-anode voltage of 0.32 V, the effective refractive index of the waveguide mode changes by ΔN=1×10−3 when the gate voltage is altered by 12 V. Numerical estimates of the bias current, pinchoff voltage, interaction length, and modulator speed are given.
Ferromagnetic interactions in organic solids: An overview of theory and experiment (invited)
Authors: Torrance J. B. , Bagus P. S. , Johannsen I. , Nazzal A. I. , Parkin S. S. P. , Batail P.
Published: July 2002

Recent progress in preparing organic solids with ferromagnetic interactions between spins is reviewed. The three principal models are discussed together with relevant materials that have been prepared in Japan, at Columbia University, in Moscow, and at IBM, Almaden. These are compared with the molecular ferromagnets recently developed by Miller and Epstein, which are not strictly organic.
Glass-forming range in mechanically alloyed Ni-Zr and the influence of the milling intensity
Authors: Eckert J. , Schultz L. , Hellstern E. , Urban K.
Published: July 2002

Amorphous Ni-Zr powders have been prepared by mechanical alloying from crystalline elemental powders. The glass-forming range has been determined by x-ray diffraction, differential scanning calorimetry and saturation magnetization measurements. From 27 to 83 at. % Ni the powders become amorphous. This shows that deep eutectics do not play any role, contrary to amorphization by melt spinning. Crystallization temperatures, crystallization enthalpies, and wave numbers Qp, obtained from x-ray diffraction investigations, are compared with the data received for rapidly quenched samples. In addition, the effect of the milling intensity on the glass formation has been studied for the first time. If the intensity is too high, crystalline intermetallic phases are formed. On the other hand, the powder needs an extended milling time to become completely amorphous if the milling intensity is too low. Conclusions on the actual temperature of the individual particle during mechanical alloying and on the glass-forming process are drawn from these results.
Nondestructive characterization of oxygen-ion-implanted silicon- on-insulator using multiple-angle ellipsometry
Authors: Dutta Pradip , Candela George A. , Chandler-Horowitz Deane , Marchiando J. F. , Peckerar Martin C.
Published: July 2002

Silicon-on-insulator formed by high-dose and high-energy oxygen ion implantation in silicon, called SIMOX (separation by implanted oxygen), has been characterized nondestructively by multiple-angle ellipsometry using a He–Ne laser at 632.8 nm. A multilayered model exhibiting two interlayers, one between the top silicon layer and the buried oxide layer and the other between the buried oxide and the substrate silicon, offers a simple representation of SIMOX. The difference between low-temperature furnace anneal (1150 °C) and an additional high-temperature rapid thermal anneal (1150+1350 °C) on as-implanted wafers is shown by the better agreement between the theoretical model and the experimental results for the high-temperature annealed SIMOX sample.
Geometrical models for the high-frequency dielectric properties of brine saturated sandstones
Authors: Tyč Stéphane , Schwartz Lawrence M. , Sen Pabitra N. , Wong Po-zen
Published: July 2002

In the MHz–GHz range, geometrical effects associated with platy grains (e.g., clay particles in sandstones) are especially important in determining the complex dielectric function, ε(ω), of rocks. We consider two classes of geometrical models based on rather different pictures for the spatial arrangement of the clay particles. In the bimodal model, spherical and platy grains (with a single aspect ratio) are randomly dispersed throughout the system while in the coated grain consolidation model the platy grains are confined to a thin layer at the pore grain interface. Calculations based on these models are compared with experimental data on five sandstones and the coated grain solidation model is generally found to be in better agreement. In sandstones exhibiting the strongest frequency dependence in ε(ω), we find that a continuous distribution of platy grain aspect ratios is required to obtain a satisfactory fit to the experimental data. Such distributions are easily treated within the coated grain consolidation framework.