bluuebunny/crossref_metadata_2025 huggingface dataset:
Fast curve fitting using neural networks
Authors: Bishop C. M. , Roach C. M.
Published: July 2002

Neural networks provide a new tool for the fast solution of repetitive nonlinear curve fitting problems. In this article we introduce the concept of a neural network, and we show how such networks can be used for fitting functional forms to experimental data. The neural network algorithm is typically much faster than conventional iterative approaches. In addition, further substantial improvements in speed can be obtained by using special purpose hardware implementations of the network, thus making the technique suitable for use in fast real-time applications. The basic concepts are illustrated using a simple example from fusion research, involving the determination of spectral line parameters from measurements of B iv impurity radiation in the COMPASS-C tokamak.
Effect of hydrogen on the surface-diffusion length of Ga adatoms during molecular-beam epitaxy
Authors: Morishita Y. , Nomura Y. , Goto S. , Katayama Y.
Published: July 2002

Systematic measurements were carried out on the surface-diffusion length of Ga adatoms during the molecular-beam epitaxy of GaAs in the presence of hydrogen atoms (H⋅) or hydrogen molecules (H2). The spatial variation of the growth rate on the (100) surface adjacent to the (111)A surface was measured from the period of the reflection high-energy electron diffraction (RHEED) intensity oscillations using in situ scanning microprobe RHEED. The surface-diffusion length of Ga adatoms, which was derived from the spatial variation of the growth rate, becomes larger along with an increase in the H⋅ or H2 pressure. It also increases as the substrate temperature is raised under H⋅ or H2 pressure. The diffusion length in the case of H⋅ introduction is larger than that in the case of H2 introduction.
Number of Cu atom(s) in the 1.014 eV photoluminescence copper center and the center’s model in silicon crystal
Authors: Nakamura M. , Ishiwari S. , Tanaka A.
Published: July 2002

Number of Cu atom(s) in the 1.014 eV copper center in silicon crystal was determined from an accurately measured relationship between the photoluminescence (PL) intensity of the center and Cu concentration in the crystal. For Cu concentrations lower than about 1×1014 atom/cm3, the PL intensity increased linearly with increasing Cu concentration. From this result and the law of mass action, it was concluded that the Cu center had only one Cu atom in it, contradicting the long-believed pair Cu model. Based on already published data and the present result, a structural model in which the Cu atom was bonded at the center of the Si–Si bond was presented for the Cu center.
High ε gate dielectrics Gd2O3 and Y2O3 for silicon
Authors: Kwo J. , Hong M. , Kortan A. R. , Queeney K. T. , Chabal Y. J. , Mannaerts J. P. , Boone T. , Krajewski J. J. , Sergent A. M. , Rosamilia J. M.
Published: July 2002

We report on growth and characterization of both epitaxial and amorphous films Gd2O3 of (ε=14) and Y2O3(ε=18) as the gate dielectrics for Si prepared by ultrahigh vacuum vapor deposition. The use of vicinal Si (100) substrates is key to the growth of (110) oriented, single-domain films in the Mn2O3 structure. Typical electrical leakage results are 10−3 A/cm2 at 1 V for single domain epitaxial Gd2O3 and Y2O3 films with an equivalent SiO2 thickness, teq of 15 Å, and 10−6 A/cm2 at 1 V for smooth amorphous Y2O3 films (ε=18) with a teq of only 10 Å. For all the Gd2O3 films, the absence of SiO2 segregation at the interface is established from infrared absorption measurements.
Convergence testing of the analytic representation of an ab initio dipole moment function for water: Improved fitting yields improved intensities
Authors: Schwenke David W. , Partridge Harry
Published: July 2002

In general, when computing intensities for polyatomics, one has to interpolate the dipole moment function obtained from ab initio calculations. For some high overtones of the water molecule, the computed intensities can be very sensitive to the way in which the interpolation is done. Our previous analytic representation [H. Partridge and D. W. Schwenke, J. Chem. Phys. 106, 4618 (1997)] was not adequate. We show that stable results can be obtained, and these results are in much improved agreement with experiment. We also test the importance of core electron correlation on intensities, and find the effect to be negligible. Of the existing water dipole moment functions in the literature, the present one is the most accurate.
Effect of surface morphology of Ni thin film on the growth of aligned carbon nanotubes by microwave plasma-enhanced chemical vapor deposition
Authors: Choi Young Chul , Shin Young Min , Lim Seong Chu , Bae Dong Jae , Lee Young Hee , Lee Byung Soo , Chung Dong-Chul
Published: July 2002

Aligned carbon nanotubes were synthesized on Ni-coated Si substrates using microwave plasma-enhanced chemical vapor deposition. The surface morphology of Ni thin films was varied with the rf power density during the rf magnetron sputtering process. It was found that the growth of carbon nanotubes was strongly influenced by the surface morphology of Ni thin film. Pure carbon nanotubes were synthesized on Ni thin film with uniformly distributed grain sizes, whereas large amounts of carbonaceous particles were produced in addition to the nanotubes, when the nanotubes were grown on Ni thin film with widely distributed grain sizes. With decreasing Ni-grain size, the diameter of nanotubes decreased and the length increased. High-resolution transmission electron microscope images clearly demonstrated the nanotubes to be multiwalled, and the graphitized structures were confirmed from the Raman spectra. Efficient field emission was observed from the diode structure with the nanotube tips.
Low-loss channel waveguides with two-dimensional photonic crystal boundaries
Authors: Smith C. J. M. , Benisty H. , Olivier S. , Rattier M. , Weisbuch C. , Krauss T. F. , De La Rue R. M. , Houdré R. , Oesterle U.
Published: July 2002

We have used transmission measurements to estimate the propagation loss of submicron channels defined in two-dimensional photonic crystals patterned into a Ga(Al)As waveguide. The measured propagation loss of the fundamental mode is indistinguishable from the material absorption, setting an upper limit of 50 cm−1 (2 dB per 100 μm). We also find that, provided the etching is deep enough, propagation losses of these photonic crystal waveguides are lower than those of ridge waveguides etched in the same run.
Gas breakdown in an atmospheric pressure radio-frequency capacitive plasma source
Authors: Park Jaeyoung , Henins I. , Herrmann H. W. , Selwyn G. S.
Published: July 2002

Gas breakdown is studied in an atmospheric pressure rf capacitive plasma source developed for materials applications. At a rf frequency of 13.56 MHz, breakdown voltage is largely a function of the product of the pressure and the discharge gap spacing, approximating the Paschen curve. However, breakdown voltage varies substantially with rf frequency due to a change in the electron loss mechanism. A large increase in breakdown voltage is observed when argon, oxygen, or nitrogen is added to helium despite their lower ionization potential. Discussion is given for optimal breakdown conditions at atmospheric pressure.
Pyroelectric properties of AlN
Authors: Fuflyigin V. , Salley E. , Osinsky A. , Norris P.
Published: July 2002

Pyroelectric properties of high-quality (0001)AlN films grown on (111) Si were studied. The pyroelectric coefficient p was measured using the dynamic method. The value was in the range of 6–8 μC/(m2 K), yielding a p/ε figure-of-merit of 0.8–0.95. The pyroelectric coefficient was independent of temperature and applied bias. Leakage current as low as 1–2×10−9 A/cm2 was measured at 5 V on large area devices. The obtained results indicate that AlN films can be used in pyroelectric thin-film devices.
Detection of stacking faults in 6H-SiC by Raman scattering
Authors: Nakashima S. , Nakatake Y. , Harima H. , Katsuno M. , Ohtani N.
Published: July 2002

Raman spectra of 6H-SiC crystals including stacking faults have been examined for the c face using backscattering geometry. The intensity of the transverse optical phonon band at 796 cm−1, which corresponds to the phonon mode at the Γ point in 3C-SiC, is sensitive to the stacking faults. We found that the intensity of this band depends on the stacking fault density. This is explained based on the bond polarizability model. The spatial distribution of the stacking faults is studied by Raman image measurement.
Monocrystalline silicon carbide nanoelectromechanical systems
Authors: Yang Y. T. , Ekinci K. L. , Huang X. M. H. , Schiavone L. M. , Roukes M. L. , Zorman C. A. , Mehregany M.
Published: July 2002

SiC is an extremely promising material for nanoelectromechanical systems given its large Young's modulus and robust surface properties. We have patterned nanometer scale electromechanical resonators from single-crystal 3C-SiC layers grown epitaxially upon Si substrates. A surface nanomachining process is described that involves electron beam lithography followed by dry anisotropic and selective electron cyclotron resonance plasma etching steps. Measurements on a representative family of the resulting devices demonstrate that, for a given geometry, nanometer-scale SiC resonators are capable of yielding substantially higher frequencies than GaAs and Si resonators.
Enhanced room-temperature magnetoresistance in La0.7Sr0.3MnO3-glass composites
Authors: Gupta S. , Ranjit R. , Mitra C. , Raychaudhuri P. , Pinto R.
Published: July 2002

In this letter, we report the improved magnetotransport properties of La0.7Sr0.3MnO3-borosilicate glass composite with different weight percents of glass. All the composites showed ferromagnetic nature at room temperature. The microstructure of these composites was seen using a scanning electron microscope. The microstructure was reconfirmed using spot energy dispersive x-ray analysis. We observe an enhancement of the low-field magnetoresistance (<200 Oe) at room temperature for the optimal composition of 25 wt % of glass. It is argued that glass layer separating the grain boundaries may be acting as barrier for spin-polarized tunneling, thereby enhancing the low-field magnetoresistance.
Wavelength dependence of laser-induced phase transformations in semiconductor quantum dots
Authors: Gajdardziska-Josifovska M. , Lazarov V. , Reynolds J. , Yakovlev V. V.
Published: July 2002

We study the effect of wavelength on the laser-induced phase transformation in semiconductor quantum dots. In our earlier report [Yakovlev et al., Appl. Phys. Lett. 76, 2050 (2000)], we discovered that a nanosecond pulse of 532 nm radiation results in a phase transformation of CdS nanocrystals from an orthorhombic to cubic phase. In this study, we find that irradiation with the pulse of a different wavelength (355 nm) results in a completely different transformation to a hexagonal wurtzite phase and a significant broadening of nanocrystal size distribution. The nanocrystal stoichiometry remained unchanged by the laser irradiation, as verified by energy dispersive x-ray spectroscopy.
Construction scheme for regularized diabatic states
Authors: Köppel H. , Gronki J. , Mahapatra S.
Published: July 2002

A simple construction scheme for quasidiabatic electronic states implemented earlier for a Jahn–Teller situation [J. Chem. Phys. 110, 9371 (1999)] is extended to the case of a seam of symmetry-allowed conical intersections. It is based on the idea of removing only the singular part of the nonadiabatic coupling elements, leading to the notion of “regularized” diabatic states. Explicit working equations are given for the resulting potential energy matrix which require only information from the adiabatic potential energy surfaces alone. The approach is tested for the photodissociation of H2S and O3 and very good agreement with reference data from the literature is found.
Radiation-induced densification in amorphous silica: A computer simulation study
Authors: Wootton Adam , Thomas Bronwyn , Harrowell Peter
Published: July 2002

Using classical dynamics simulations, we model the long-lived structural effect of ultraviolet irradiation on amorphous silica. We find a significant increase in density of a model of amorphous silica following localized energy deposition, in agreement with experimental observation. We present evidence that this densification arises as a result of the rapid local cooling that follows irradiation. Similar high density forms of amorphous silica are found following fast quenches of bulk samples. In support of this proposal we demonstrate that very rapidly quenched silica undergoes dilation, rather than compaction, on irradiation.
Temperature dependence of the lasing wavelength of InGaAs quantum dot lasers
Authors: Thomson J. D. , Summers H. D. , Smowton P. M. , Herrmann E. , Blood P. , Hopkinson M.
Published: July 2002

We observe that the emission wavelength of edge-emitting InGaAs quantum dot lasers has a much weaker temperature dependence (0.6 Å K−1) than equivalent quantum well devices (3 Å K−1). Measured gain and absorption spectra show that the gain peak wavelength due to dot states is almost independent of temperature for a given value of peak gain whereas the absorption edge shifts at a rate of about 2 Å K−1. Above 100 K the occupancy of dot states can be described by Fermi functions and on this basis we find that the measured gain and absorption spectra are in excellent quantitative agreement. Although the band edge energy reduces with increasing temperature, this analysis shows that the energy distribution of dot states matches the evolution of the Fermi functions such as to leave the quasi Fermi level separation and the wavelength of the gain peak unchanged as a function of temperature for a given value of peak gain. This energy distribution is a consequence of the dot size distribution so the match to the Fermi functions is probably fortuitous.
From topology to dynamics in biochemical networks
Authors: Fox Jeffrey J. , Hill Colin C.
Published: July 2002

Abstract formulations of the regulation of gene expression as random Boolean switching networks have been studied extensively over the past three decades. These models have been developed to make statistical predictions of the types of dynamics observed in biological networks based on network topology and interaction bias, p. For values of mean connectivity chosen to correspond to real biological networks, these models predict disordered dynamics. However, chaotic dynamics seems to be absent from the functioning of a normal cell. While these models use a fixed number of inputs for each element in the network, recent experimental evidence suggests that several biological networks have distributions in connectivity. We therefore study randomly constructed Boolean networks with distributions in the number of inputs, K, to each element. We study three distributions: delta function, Poisson, and power law (scale free). We analytically show that the critical value of the interaction bias parameter, p, above which steady state behavior is observed, is independent of the distribution in the limit of the number of elements N→∞. We also study these networks numerically. Using three different measures (types of attractors, fraction of elements that are active, and length of period), we show that finite, scale-free networks are more ordered than either the Poisson or delta function networks below the critical point. Thus the topology of scale-free biochemical networks, characterized by a wide distribution in the number of inputs per element, may provide a source of order in living cells.
Investigation of In segregation in InAs/AlAs quantum-well structures
Authors: Schowalter M. , Rosenauer A. , Gerthsen D. , Arzberger M. , Bichler M. , Abstreiter G.
Published: July 2002

In this letter, we report the investigation of In segregation in InAs/AlAs heterostructures. InAs layers with different thicknesses were grown by molecular beam epitaxy on GaAs (001) substrates. The layers were investigated by transmission electron microscopy. Profiles of the chemical composition of the InAs layers in the [001] direction were deduced from high-resolution lattice fringe images using the composition evaluation by lattice fringe analysis method. The segregation efficiency was derived by fitting the measured In concentration profiles with the segregation model of Muraki et al. [K. Muraki et al., Appl. Phys. Lett. 61, 557 (1992)]. We obtain efficiency of R=0.77±0.03 for the segregation of In in AlAs/InAs at a temperature of 530 °C.
Direct and trap-assisted elastic tunneling through ultrathin gate oxides
Authors: Jiménez-Molinos F. , Gámiz F. , Palma A. , Cartujo P. , López-Villanueva J. A.
Published: July 2002

The direct and assisted-by-trap elastic tunnel current in metal–oxide–semiconductor capacitors with ultrathin gate oxide (1.5–3.6 nm) has been studied. Bardeen’s method has been adapted to obtain the assisted tunnel current, in addition to the direct tunnel current. The dependence of the assisted current on the trap distribution in energy has also been analyzed. This allows us to obtain the trap distribution in energy from experimental current curves. Finally, we have analyzed the role of the image force, the inclusion of which can avoid a barrier height dependence on the oxide thickness.
X-ray photoelectron microscopy of the C 1s core level of free-standing single-wall carbon nanotube bundles
Authors: Goldoni A. , Larciprete R. , Gregoratti L. , Kaulich B. , Kiskinova M. , Zhang Y. , Dai H. , Sangaletti L. , Parmigiani F.
Published: July 2002

Core level photoemission spectra from a free-standing bundle of single-wall carbon nanotubes have been measured using a high-flux soft x-ray spectromicroscope. The good signal-to-noise ratio for the C 1s emission provides information on fundamental quantities such as the core-hole lifetime and binding energy, free from uncontrolled interactions between the nanotubes and the substrate or between the nanotubes and contaminants. We show that it is possible to distinguish chemically different nanotubes from the binding energy and line shape of the C 1s core level. This finding opens unique opportunities to probe in situ the response of the nanotube electronic properties and chemical activity to mechanical actions, doping, and functionalization.
Growth of multiwall carbon nanotubes in an inductively coupled plasma reactor
Authors: Delzeit Lance , McAninch Ian , Cruden Brett A. , Hash David , Chen Bin , Han Jie , Meyyappan M.
Published: July 2002

A high density plasma from a methane–hydrogen mixture is generated in an inductively coupled plasma reactor, and multiwalled carbon nanotubes (MWNTs) are grown on silicon substrates with multilayered Al/Fe catalysts. The nanotubes are vertically aligned, and the alignment is better than the orientation commonly seen in thermally grown samples. A detailed parametric study varying inductive power, pressure, temperature, gas composition, catalyst thickness, and power to the substrate is undertaken. Transmission electron microscopy and Raman spectroscopy are used to characterize the nanotubes. Emission spectroscopy and a global model are used to characterize the plasma. The power in the lower electrode holding the substrate influences the morphology and results in a transition from MWNTs to nanofibers as the power is increased.
Color-selective semiconductor nanocrystal laser
Authors: Eisler Hans-Jürgen , Sundar Vikram C. , Bawendi Moungi G. , Walsh Michael , Smith Henry I. , Klimov Victor
Published: July 2002

Theoretical predictions of the benefits of three-dimensional quantum confinement have provided motivation for the development of quantum-dot lasers. Such lasers, developed in the case of self-assembled quantum dots, have not been successfully demonstrated with quantum-confined colloidal nanocrystals (NCs). Here, using recently developed NC-titania chemistry, we report the successful development of an optically pumped, NC-based distributed feedback laser, in which the narrow gain profiles of these nanoparticles have been matched with the feedback of a second-order distributed feedback laser. This laser, whose output color can be selected by choosing appropriately sized nanocrystals, operates at 80 K and at room temperature.
Epitaxial growth and characterization of CaF2 on Si
Authors: Schowalter L. J. , Fathauer R. W. , Goehner R. P. , Turner L. G. , DeBlois R. W. , Hashimoto S. , Peng J.-L. , Gibson W. M. , Krusius J. P.
Published: July 2002

CaF2 films have been grown epitaxially on (100) and (111) Si substrates by molecular beam epitaxy. These films have been characterized by electron microscopy, reflection high-energy electron diffraction, Rutherford backscattering ion channeling, and back-reflection Laue x-ray diffraction. In addition, chemical etching has been used to reveal dislocations and to delineate cracks. Film cracking appears to be related to crystalline perfection through misfit dislocation mobility. It is possible to grow high quality, (xmin=3.0%) single-crystal films on (111) Si which are free of cracks and atomically flat. However, the high free energy of the (100) surface in an ionic fluorite crystal prevents the growth of comparable CaF2 films on the (100) Si surface.
The relation between positive charge and breakdown in metal-oxide-silicon structures
Authors: Weinberg Z. A. , Nguyen T. N.
Published: July 2002

The relation between positive-charge accumulation at the Si-SiO2 interface and the occurrence of high-field breakdown in metal-oxide-silicon structures has been investigated. Oxides having different hole-trapping properties were prepared with the addition of short rapid thermal anneals in O2. Experiments testing hole trapping, high-field stressing, the initial current transients at constant gate voltage, and breakdown statistics were performed on these oxides to examine the correlation between positive charge and breakdown. The conclusion is that positive-charge generation is only one of the processes occurring during high-field stress but is not the main cause for breakdown. Large current increases were observed for oxides that have large hole-trapping efficiencies, but the current increase is followed by fast current decay. The mechanism causing the current decay was investigated and was found to be an intrinsic mechanism which is related to the neutralization of the positive charge. These processes always accompany the formation of positive charge and explain why the effectiveness of the positive charge in causing current runaway is inhibited.
Deep levels of copper in silicon
Authors: Brotherton S. D. , Ayres J. R. , Gill A. , van Kesteren H.W. , Greidanus F. J. A. M.
Published: July 2002

Defect impurity levels have been examined in copper-diffused p-and n-type silicon using deep level transient spectroscopy. Levels at Ev+0.09, Ev+0.23, and Ev+0.42 eV have been observed in both types of material, although the deeper levels were only oberved in n-type material after post-diffusion annealing at 200 °C. Associated with the appearance of these levels in n-type material was another level at Ec−0.16 eV. This may be a further charge state of the center responsible for the Ev+0.23 eV and Ev+0.42 eV levels or the two centers may be decomposition products of a thermally unstable complex. Luminescence measurements have revealed the previously reported Cu-Cu spectrum in all the copper-diffused samples. The occurrence of this signal could not be correlated with the presence of the levels at Ev+0.23, Ev+0.42, or Ec−0.16 eV; this leaves the center at Ev+0.09 eV as the likely origin of the signal.
Perpendicular magnetic anisotropy in Pd/Co and Pt/Co thin-film layered structures
Authors: Carcia P. F.
Published: July 2002

rf sputtered Pd/Co and Pt/Co thin-film layered structures (LS) have perpendicular magnetic anisotropy, when the Co layer is ultrathin (<8 Å in Pd/Co and <14 Å in Pt/Co). The Co thickness (T) dependence of the anisotropy energy (Ku∼T) and the effective anisotropy field (HK∼1/T) in Pd/Co LS support an interfacial anisotropy as the source of the perpendicular magnetic easy axis. In contrast, the anisotropy is independent of Co thickness for thin Co layers in Pt/Co LS, and thus the mechanism for the perpendicular easy axis is thought to be different. We describe magnetization, resistivity, and x-ray diffraction data that consistently support sharper interfaces in Pd/Co LS than in Pt/Co LS.
Diffusion of oxygen in YBa2Cu3O7−y
Authors: Ikuma Yasuro , Akiyoshi Susumu
Published: July 2002

The oxygen lattice diffusion in YBa2Cu3O7−y was studied with 18O as a tracer. For the composition of y=0.07, the oxygen lattice diffusion coefficient is represented by Dl,O =2.87×10−1 exp[−173(kJ/mol)/RT] cm2/s in the temperature range 250–400 °C. The oxygen lattice diffusion coefficient decreases with the increasing value of y.
A model for silicon-oxide breakdown under high field and current stress
Authors: Avni E. , Shappir J.
Published: July 2002

A recently developed self-consistent model for gate-oxide degradation due to charge injection, described in a companion paper, is expanded to include electrical ‘‘wear out’’ breakdown. In the present work, gate-oxide breakdown is defined to occur when the density of generated neutral trapping sites reaches a critical threshold value at the anode. Breakdown experimental results obtained under constant tunneling current are treated and simulated. The new model deals successfully with oxide breakdown dependence on: injection history, gate-oxide thickness, charge-injection current density, injection polarity reversal, gate electrode material, and oxide anneal temperatures.
Classical versus semiclassical predictions for harmonic generation in laser-free electron scattering under experimentally realizable conditions
Authors: Puntajer Anna K. , Leubner C.
Published: July 2002

It is shown that for currently attainable experimental parameters, classical electrodynamics is perfectly adequate for the prediction of all features of harmonic generation in intense laser-free electron scattering. While the customary semiclassical approach is characterized by lengthy calculations and unwieldy expressions, the classical framework arrives at the same results in only a few lines, is valid for arbitrary electron injection energies, and provides at the same time an intuitive understanding of the predicted features. This is confirmed by graphical comparisons of the various available cross sections, which also show some of the published results to be incorrect.
Magnetic refrigeration in the low-temperature range
Authors: Tishin A. M.
Published: July 2002

Rare-earth metals and their alloys are considered for effective use in magnetic refrigerators operating over a variety of temperature ranges. The analysis is based on the notion of refrigerant capacity. Specific refrigerant capacity of dysprosium is shown to reach a maximum value (ΔSM⋅ΔTcyc)max/B=20 J/mol T at B∼1.1 T. The effective use of rare earths and their alloys in magnetic refrigerators has been established to be determined to a considerable extent by the region of antiferromagnetic structure available and by the value of the field in which it is destroyed.
Characterization of diamond films synthesized in the microwave plasmas of CO/H2 and CO/O2/H2 systems at low temperatures (403–1023 K)
Authors: Muranaka Yasushi , Yamashita Hisao , Miyadera Hiroshi
Published: July 2002

Diamond films grown in (A)CO/H2 and (B)CO/O2/H2 systems at substrate temperatures (Ts) between 403 and 1023 K were characterized by x-ray diffraction, Raman spectroscopy, cathodoluminescence, and scanning electron microscopy. A large amount of polyacetylene inclusion occurred in the (A)CO/H2 system on reducing Ts, resulting in worsening of the diamond crystallinity (FWHM of the diamond Raman peak broadened from 6.4 to 19.5 cm−1 when Ts was decreased from 1023 to 403 K). On the contrary, polyacetylene inclusion was significantly suppressed in the (B)CO/O2/H2 system, and high quality diamond films (FWHM=4.0–4.1 cm−1) close to natural diamond (FWHM=2.6–3.0 cm−1) were obtained between 684 and 1023 K. Though there was a little deterioration of crystallinity at 403 K, the obtained film still had good crystallinity (FWHM=10.2 cm−1) compatible with conventional chemical vapor deposition diamond films. The presence of a large amount of atomic hydrogen, atomic oxygen, O2, and OH contributed to suppression of polyacetylene formation on a growth surface and promoted cleaning of deposited amorphous phases. These species provided the best condition for selective growth of pure diamond of good crystallinity in the (B)CO/O2/H2 system even at low temperature (∼403 K), where impurities are likely to be involved. Films grown in the (B)CO/O2/H2 system were characterized as large and well-defined crystallites of octahedral forms emitting intensive blue CL at 440 nm. The actual activation energy (7.0 kcal/mol) for homoepitaxial diamond growth was obtained using the (B)CO/O2/H2 system, and was in good agreement with previous quantum chemical calculations (6.33 kcal/mol) based on the methyl precursor model. Finally, the (B)CO/O2/H2 system was suggested to be one of the most promising gas combinations for low temperature growth of high quality diamond.
Equation of state of Al, Cu, Mo, and Pb at shock pressures up to 2.4 TPa (24 Mbar)
Authors: Mitchell A. C. , Nellis W. J. , Moriarty J. A. , Heinle R. A. , Holmes N. C. , Tipton R. E. , Repp G. W.
Published: July 2002

Equation-of-state data and corresponding first-principles theory for the metals Al, Cu, Mo, and Pb are reported over the shock pressure range 0.4–2.4 TPa (4–24 Mbar). Strong shock waves were generated by nuclear explosions and a two-stage light-gas gun. The experimental data occur in the hot liquid-metal regime, where condensed-matter theory applies but with unusually large thermal components to the equation of state.
Numerical studies of streamer to arc transition
Authors: Ganesh S. , Rajabooshanam A. , Dhali S. K.
Published: July 2002

The numerical calculations of the transition phase of a streamer to an arc in nitrogen is reported. The proposed model includes an extensive set of collisional processes leading to gas heating. Radial variations of the species concentration and temperature are included in the calculation. Starting with the streamer parameters as an initial condition, the calculations are carried out until ‘‘quasisteady’’ state arc conditions are reached. The time evolution of the species concentration, the discharge voltage, and the current are discussed. The steady-state arc temperatures, radii, currents, and voltages are compared with available experimental data.
Interface characterization of chemically vapor deposited diamond on titanium and Ti-6Al-4V
Authors: Perry Scott S. , Ager Joel W. , Somorjai Gabor A. , McClelland Robert J. , Drory Michael D.
Published: July 2002

Continuous 1-μm-thick diamond films have been grown by chemical vapor deposition (CVD) at ∼900 °C on pure titanium and on a Ti alloy, Ti-6Al-4V. The diamond film exhibits good adhesion to the substrates in spite of the presence of ∼7 GPa of in-plane residual stress which arises from the large differences in thermal expansion coefficients between diamond and titanium. The interface between the CVD diamond film and the substrate was exposed by deforming the substrate, thereby removing parts of the diamond film, under both ultrahigh vacuum and ambient conditions. After fracture, both the substrate and diamond film sides of the interface were characterized by a combination of x-ray photoelectron spectroscopy (XPS), scanning Auger microscopy, secondary electron microscopy, and Raman microprobe spectroscopy. The substrate fracture surface is inhomogeneous, containing some areas of diamond and amorphous carbon. XPS analysis revealed that carbon and oxygen are present on the substrate fracture surface. Micron-size areas of Ti were also found on the diamond fracture surface. Raman spectroscopy of the substrate fracture surfaces found evidence for the presence of amorphous, nonstoichiometric titanium oxides; no evidence of crystalline TiC or stoichiometric TiO2 was seen. Analysis of the XPS core level structure of the Ti and C spectra confirmed the presence of titanium carbide; little evidence of metallic titanium was seen in the interfacial region. Differences in the structure of the substrate fracture surface between titanium and the Ti alloy were also seen. The interface at the diamond/Ti-6Al-6V alloy was more heavily oxidized than the diamond/titanium interface. Depth profiling studies also revealed a thicker oxygen-containing surface layer on the alloy fracture surface. The presence of diamond and Ti compounds on both sides of the exposed interfaces indicates that the fracture did not occur discretely at the diamond/reaction layer interface. From these findings we propose a model of the failure region of the highly adherent diamond/titanium system.
Surface kinetics and plasma equipment model for Si etching by fluorocarbon plasmas
Authors: Zhang Da , Kushner Mark J.
Published: July 2002

Plasma-surface interactions during plasma etching are important in that, in addition to determining the rate and quality of the etch, they can also influence the properties of the bulk plasma. To address this coupling of bulk and surface processes the surface kinetics model (SKM) was developed as a module in the two-dimensional hybrid plasma equipment model (HPEM) with the goal of combining plasma chemistry and surface chemistry in a self-consistent fashion. The SKM obtains reactive fluxes to the surface from the HPEM, and generates the surface species coverages and the returning fluxes to the plasma by implementing a user defined surface reaction mechanism. Although the SKM is basically a surface-site-balance model, extensions to those algorithms have been made to include an overlying passivation layer through which reactants and products diffuse. Etching of Si in an inductively coupled plasma sustained in Ar/C2F6 was investigated using the SKM. Results from parametric studies are used to demonstrate the sensitivity of etching rates and polymer thickness to the sticking coefficient of fluorocarbon radicals on the reactor walls, polymer erosion rates and F atom diffusion through the polymer layer.
Microstructure of two-phase random media. V. The n-point matrix probability functions for impenetrable spheres
Authors: Torquato S. , Stell George
Published: July 2002

We examine the two- and three-point matrix probability functions for a two-phase random and homogeneous system of impenetrable spheres. For such a system, we give an exact analytical expression for the two-point matrix function S2 through second order in the number density of particles. Moreover, the two-point matrix function is evaluated, for the first time, for a very wide range of densities. We also discuss the evaluation of the three-point matrix function S3 for an impenetrable-sphere system and provide new expressions that may be used to estimate it.
The wobbling-in-a-cone analysis of internal motion in macromolecules
Authors: Fujiwara Toshimichi , Nagayama Kuniaki
Published: July 2002

The autocorrelation functions of second order generalized harmonics for the restricted librational motion of a spheroid were calculated using a wobbling-in-a-cone model. The model represents a motion intuitively abstracted from the multiple internal rotations of a side chain or the anisotropic libration of backbone segments in macromolecules. The exact correlation functions expressed by multiexponential decay were found well approximated by a single exponential decay where integrated areas of the exact and approximated functions were taken to be the same. The results were used to express the relaxation of nuclear spins caused by the fluctuation of the dipole–dipole interaction. The multiexponential nature of exact correlation functions was related to the concept of the relaxation spectrum.
Transition metal–ligand bonding. II
Authors: Bauschlicher Charles W.
Published: July 2002

The nature of the bonding of CO, H2O, and NH3 to transition metal atoms is analyzed using the constrained-space-orbital-variation (CSOV) technique. The cooperative effects for Ni(CO)2 are found to be different than those for Ni(H2O)2. The bonding between neutral systems and the positive ions is found to be quite different; NiCO+ has little π bonding, while NiCO has strong π bonding. The positive ion of NiH2O is far more strongly bound than the neutral, while for NiCO the positive ion and neutral are bound by about the same energy.
Stability of rare gas cluster ions
Authors: Hiraoka Kenzo , Mori Toshiharu
Published: July 2002

Thermochemical values, ΔH0n−1,n and ΔS0n−1,n, for clustering reactions, Rg+n−1+2Rg=Rg+n+Rg (Rg=He, Ne, Kr, and Xe), were measured with a pulsed electron-beam mass spectrometer. The −ΔH0n−1,n values show a sudden decrease between n=3 and 4, for all rare gases suggesting that the core ion in Rg+n is Rg+3. The values −ΔH0n−1,n with n=3 are found to be in the order Ne<He<Ar<Kr<Xe, and those with n≥4 in the order of atomic radii.
Quinoid vs aromatic structure of polyisothianaphthene
Authors: Kürti J. , Surján P. R.
Published: July 2002

We show that the optimum structure of polyisothianaphthene strongly depends on the chain length. Short oligomers have an aromatic structure, while chains consisting of more than 9 monomers exhibit an aromatic to quinoidal transition along the chain.
A maximum-entropy analysis of the problem of the rotameric distribution for substituted biphenyls studied by 1H nuclear magnetic resonance spectroscopy in nematic liquid crystals
Authors: Catalano Donata , Di Bari Lorenzo , Veracini Carlo Alberto , Shilstone Garth N. , Zannoni Claudio
Published: July 2002

The statistical principle of maximum entropy is applied to the analysis of dipolar couplings from 1H NMR of nonrigid molecules dissolved in liquid-crystalline phases. A distribution function for the orientational and inter-ring angles is so obtained. The most probable internal angle φ is determined for 4,4′-dichlorobiphenyl (φ=34°) in the nematic phase of I52, 4-pentyl-4′-cyanobiphenyl (φ=32°) and 4′-Br-4-Cl-2,6-difluorobiphenyl (φ=42°) in EBBA. The physical reliability of the distributions determined is discussed. The maximum-entropy treatment seems to indicate a limit for the information on the internal motion obtainable from the experimental data.
Computer simulation study of free energy barriers in crystal nucleation
Authors: van Duijneveldt J. S. , Frenkel D.
Published: July 2002

We show how relatively standard Monte Carlo techniques can be used to probe the free-energy barrier that separates the crystalline phase from the supercooled liquid. As an illustration, we apply our approach to a system of soft, repulsive spheres [v(r)=ε(σ/r)12]. This system is known to have a stable face-centered-cubic (fcc) crystal structure up to the melting temperature. However, in our simulations, we find that there is a surprisingly low free-energy barrier for the formation of body-centered-cubic (bcc) crystallites from the melt. In contrast, there appears to be no ‘easy’ path from the melt to the (stable) fcc phase. These observations shed new light on the results of previous simulations that studied the dynamics of crystal nucleation in the r−12 system. We argue that the techniques developed in this paper can be used to gain insight in the process of homogeneous nucleation under conditions where direct, dynamical simulations are inconclusive or prohibitively expensive.
Dielectric relaxation of the Kohlrausch-type in aqueous polymer solution
Authors: Miura Nobuhiro , Shinyashiki Naoki , Mashimo Satoru
Published: July 2002

Micro-Brownian motion of polymer chain in aqueous solution was clearly observed at first for poly(vinyl pyrrolidone) as a dielectric relaxation by a time domain reflectometry method which covers an extremely wide frequency region from 100 kHz–10 GHz. The dielectric response function observed is of the Kohlrausch-type as expected for the chain motion, φ(t)=exp[−(t/τl)β l]. Concentration dependence of the relaxation time τl at 25 °C is given by log τl=−9.77−1.35/(cd−2.07), where cd is the polymer concentration (g/cc), and that of the coupling parameter βl is given by βl=−0.182cd+0.662. These dependences are characteristic features of the relaxation caused by the chain motion.
An adaptive umbrella sampling procedure in conformational analysis using molecular dynamics and its application to glycol
Authors: Hooft Rob W. W. , van Eijck Bouke P. , Kroon Jan
Published: July 2002

A novel way has been developed for using the umbrella potential method of calculating the potential of mean force using molecular dynamics. During a simulation, the umbrella potential is improved periodically, taking into account all results obtained previously. The method was tested by the determination of the potential of mean force for the central dihedral angle in glycol (as a molecule in vacuo and in solutions in water and in CCl4). Advantages over other methods are that a relatively small amount of manual intervention is necessary and that an intermediate result after a small simulation time is already a complete, although crude estimate of the potential of mean force. A robust method to estimate the error in obtained energy differences between conformations and in the parameters of the potential of mean force (PMF) is also supplied.
Rotational, fine, and hyperfine analyses of the (0,0) band of the D 3Π–X 3Δ system of vanadium mononitride
Authors: Balfour Walter J. , Merer Anthony J. , Niki Hideaki , Simard Benoit , Hackett Peter A.
Published: July 2002

The VN molecule has been produced in a molecular beam apparatus using a laser vaporization source and its D 3Π–X 3Δ(0,0) band has been studied by laser-induced fluorescence at low (∼0.1 cm−1) and sub-Doppler resolution (∼0.004 cm−1). Lifetimes of single rotational levels of the D 3Π0 component have been measured and interpreted. Rotational, fine, and hyperfine structures in six of the nine subbands possible for a 3Π ← 3Δ transition have been recorded. Both states exhibit a rapid transition from case (a) → case (b) coupling cases, manifested by reversals in the Landé patterns of the hyperfine structure. The data have been reduced to a set of 35 molecular constants using a modified case (aβ) effective Hamiltonian in which two additional magnetic hyperfine parameters are required for each state. The distortions in the hyperfine structure are due almost entirely to second-order spin–orbit interaction between states arising from the same configuration. Analysis of the derived parameters indicates that the X 3Δ state is well represented by the single electron configuration ...8σ2 3π4 9σ1 1δ1, in which the 9σ molecular orbital (MO) is a V 4s–4p hybrid (88% V 4s) and the 1δ MO is a pure V 3d orbital; the dominant configuration for the D 3Π state is ...8σ2 3π4 1δ1 4π1, in which the 4π MO is an antibonding orbital composed of at most 82% V 3dπ. The isoconfigurational a 1Δ and e 1Π states are calculated to lie 3390 and 2200 cm−1 above their respective high spin companions. The lambda doubling in the D 3Π0 component has been interpreted in terms of spin–orbit interactions with the B 3Σ− and d 1Σ+ states, both states arising from the ...8σ2 3π4 1δ2 configuration; the d 1Σ+ state is known [Simard, Masoni, and Hackett, J. Mol. Spectrosc. 136, 44 (1989)] to lie 102 cm−1 above D 3Π0, while the B 3Σ− state probably lies about 8000 cm−1 below.
General approach to barrier crossing in multicomponent nucleation
Authors: Wu David T.
Published: July 2002

Multicomponent nucleation analysis is reformulated to include all paths of nucleation. Starting with simple measurement theory, one is led to define a global nucleation rate that corresponds to what is observable experimentally. This rate can be calculated at steady state and estimated for arbitrary times, provided the rate matrix and the free energy obey certain constraints. When applied to the special case of a quadratic barrier, the global nucleation rate is smaller than the standard result based on the rate of saddle-point nucleation. Cases with more general barriers can also be treated by the same method.
Calculation of low-energy elastic cross sections for electron-CF4 scattering
Authors: Gianturco F. A. , Lucchese R. R. , Sanna N.
Published: July 2002

A new computational approach has been used to evaluate the rotationally summed, vibronically elastic integral cross sections from the scattering of slow electrons (energy ranging from 1.0 eV up to 40.0 eV) by tetrafluoromethane molecules in the gas phase. The various symmetry components have been analyzed using the exact static exchange approximation and also by including a nonempirical, model polarization potential employed before by our group. A comparison with earlier calculations and with existing experiments allows us to assign the symmetries of the shape resonances in the 5–30 eV energy region which are seen by experiments and are also shown by the present calculations.
Spectroscopy of TiH: Rotational analysis of the 4Γ→X 4Φ (0,0) band at 530 nm
Authors: Launila O. , Lindgren B.
Published: July 2002

Emission bands of the TiH radical in the region 18 000–20 000 cm−1 have been recorded both with grating spectrograph and Fourier transform techniques. The rotational structure of the 530 nm band, which is the strongest one of the three observed bands, has been analyzed and molecular parameters have been derived for the electronic states involved. The electronic assignment of this band is confirmed as 4Γ→X 4Φ, with a heavily perturbed upper state. The principal parameters (cm−1) of the analyzed states are X 4Φ (v=0): A=33.083(29), B=5.362 06(19), and γ=0.1823(27). 4Γ (v=0): T=18 878.10(47), A=41.54(10), B=5.7408(22), γ=−4.33(12), and λ=0.77(14). The derived first order spin–orbit parameter values of X 4Φ and 4Γ are shown to be consistent with the electronic configurations δπσ and δππ′, respectively. The local perturbations in the 4Γ state are discussed and arguments are given to suggest that the dominating perturber is of 4Φ symmetry.
Relativistic and correlation effects on molecular properties. I. The dihalogens F2, Cl2, Br2, I2, and At2
Authors: Visscher L. , Dyall K. G.
Published: July 2002

A benchmark study of a number of relativistic correlation methods is presented. Bond lengths, harmonic frequencies, and dissociation energies of the molecules F2, Cl2, Br2, I2, and At2 are calculated at various levels of theory, using both the Schrödinger and the Dirac–Coulomb–(Gaunt) Hamiltonian.
Magic numbers in transition metal (Fe, Ti, Zr, Nb, and Ta) clusters observed by time-of-flight mass spectrometry
Authors: Sakurai Masaki , Watanabe Koji , Sumiyama Kenji , Suzuki Kenji
Published: July 2002

We have measured time-of-flight (TOF) mass spectra of transition metal free clusters, TMn (TM=Fe, Ti, Zr, Nb, and Ta and n is the number of atoms per cluster), produced by a laser vaporization source. The size resolved TOF intensities at n=7, 13, 15 are much higher than those at the neighboring n values for all TMn. Such specific n values are assigned to the magic numbers of these transition metal clusters and can be related to pentagonal bipyramid, icosahedron, and bcc structure units. The other magic numbers are observed for larger TMn: n=19 and 23 for Fe, n=19 and 25 for Ti, being attributable to the polyicosahedron. The TOF spectra of Nb and Ta clusters are similar to each other and display the common magic number of n=22.