bluuebunny/crossref_metadata_2025 huggingface dataset:
Phenomenological propagation loss theory for magnetostatic waves in thin ferrite films
Authors: Stancil Daniel D.
Published: July 2002

The phenomenological propagation loss theory based on the Gilbert form of the equation of motion for the magnetization is clarified for surface wave modes and extended to include volume wave modes. For small numbers, the expression 76.4ΔH (dB/μs) is found to accurately give forward volume wave losses, but to underestimate both surface and backward volume wave losses at low frequencies. Here ΔH is the full ferromagnetic resonance linewidth. As an example, the surface and backward volume wave losses at 3 GHz in yttrium iron garnet are more accurately given by 99.3ΔH (dB/μs) for small wave numbers. For a fixed bias field, surface wave losses per unit time are shown to be constant with frequency across the passband, whereas volume wave losses per unit time have a quadratic frequency dependence. After a review of existing experimental results, it is concluded that further data is necessary for a definitive test of the theory.
Two-dimensional-like nucleation of GaAs on Si by room-temperature deposition
Authors: Castagné J. , Fontaine C. , Bedel E. , Munoz-Yague A.
Published: July 2002

Nucleation of GaAs on Si in molecular-beam epitaxy is studied by Auger electron spectrometry and reflection high-energy electron diffraction. It is shown that, if growth is initiated at room temperature and if a GaAs equivalent thickness of ∼15 Å is deposited on Si, an amorphous, nonstoichiometric layer is obtained which covers completely the substrate surface. Stoichiometry and monocrystallinity can be restored by thermal annealing at 350 °C by a solid-phase epitaxy mechanism. Under such conditions the initial stages of GaAs/Si growth can then proceed via two-dimensional nucleation, instead of the three-dimensional mode observed at higher growth temperatures.
Analysis of tilt in the high-strain-rate pressure-shear plate impact experiment
Authors: Klopp Richard W. , Clifton Rodney J.
Published: July 2002

An analysis is presented of the effect of tilt in the high-strain-rate pressure-shear plate impact experiment. Normal and shear tractions and the velocity difference across the specimen are obtained from free-surface particle velocity, tilt angle, closure direction, and impact velocity data. The effect of tilt in high-strain-rate pressure-shear plate impact is shown to be small unless the velocity difference across the specimen is itself small.
A study of intermetallic compound formation in a copper–tin bimetallic couple
Authors: Bandyopadhyay A. K. , Sen S. K.
Published: July 2002

The formation of intermetallics in copper–tin bimetallic couples has been studied from room temperature to 183 °C by measuring the evolution of contact resistance and composite electrical resistance with time and temperature in order to assess the kinetic behavior of the system. X-ray diffractogram (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies have also been performed on the samples. As regards bulk diffusion, copper diffuses interstitially into tin rapidly at room temperature with the formation of η′-Cu6Sn5 intermetallic compound. Further diffusion through this phase as evaluated by composite electrical resistivity measurements is given by 0.40 eV, assuming a model of defect-assisted diffusion into the grains. The grain-boundary diffusion is found to occur with an activation energy of 0.78 eV as estimated from contact resistivity measurements. SEM confirms the presence of grain-boundary diffusion of tin in copper, whereas XRD and TEM measurements indicate the growth of η′ intermetallics at room temperature which exists up to 90 °C when it transforms to the ε-Cu3Sn phase. The presence of tin whiskers on the tin surface has also been occasionally observed.
Synthesis of diamond powder in acetylene oxygen plasma
Authors: Howard W. , Huang D. , Yuan J. , Frenklach M. , Spear K. E. , Koba R. , Phelps A. W.
Published: July 2002

Diamond particles 10–500 nm in diameter were produced by microwave-assisted combustion of acetylene in oxygen. Both premixed and diffusion flame configurations were investigated. A mixture of cubic and hexagonal polytypes of diamond were identified. Larger particle sizes were observed at lower reactor pressure and higher C to O atomic ratios. C to O atomic ratios between 0.83 and 1.0 produced crystalline diamond powder while other ratios produced graphite, soot, and amorphous carbon phases. Diamond formation was not observed when reaction pressures were above 150 Torr.
Macroscopic variations of surface potentials of conductors
Authors: Camp J. B. , Darling T. W. , Brown Ronald E.
Published: July 2002

The surface potential variations of the following conductors have been measured over a 0.7 cm2 area: bronze, NiP, conducting polymer (polyaniline), Al, Cu, Au, and graphite. The measurements were performed with a Kelvin probe of spatial and potential resolution of 1.5 mm and 1 mV, respectively. Subjecting the surfaces to no cleaning procedure other than a low temperature ultrahigh vacuum bake, we find a wide spectrum of variation in surface potential uniformity, with alloys typically the least uniform and nonreactive surfaces such as gold and graphite the most uniform (no fluctuations above the 1 mV probe sensitivity). Application of these results to the Los Alamos antiproton gravity experiment is discussed.
Atomistic and computer modeling of metallization failure of integrated circuits by electromigration
Authors: Kirchheim R. , Kaeber U.
Published: July 2002

Nonstationary and stationary concentration profiles of vacancies in a grain boundary are calculated for the case of electromigration and various conditions of vacancy formation and annihilation. If supersaturation of vacancies is assumed for void formation, current densities necessary for voiding have to be larger than a critical value jcrit, which is related to the length l of the grain boundary by the relation ljcrit=const. An estimation of the value of the constant in the last equation is in agreement with experimental values. An approximate equation for the median time to failure (MTF) is derived from the model for grain boundaries conducting the flux of matter parallel or in series. The results explain many features of experimental studies. First values of MTF for a network of grain boundaries obtained by computer simulations are presented as well, and the effect of resistance heating and nonstationarity on the current exponent is discussed in some detail.
Breakdown characteristics in nonplanar geometries and hollow cathode pseudospark switches
Authors: Pak Hoyoung , Kushner Mark J.
Published: July 2002

Breakdown voltages of gases in parallel-plate geometries are well represented by Paschen’s law, whose scaling parameter is pd (gas pressure×electrode separation). In nonplanar geometries, Paschen’s law is not directly applicable due to the ambiguity in the distance between the electrodes and distortion of the electric field. A Monte Carlo computer model is used to investigate breakdown characteristics in nonplanar geometries and hollow cathode pseudospark switches in particular. The model tracks the trajectories of both electrons and ions, including ionizing collisions in the gas phase by electrons and ions, and secondary electron emission by ions on surfaces. It is found that under typical operating conditions in helium (0.1 to a few Torr, voltages of tens of kV, effective electrode separation of a few mm), approximately two-thirds of ionizing collisions are attributable to ion impact, of which half are due to ion impact in the gas phase.
Heteroepitaxial wurtzite and zinc-blende structure GaN grown by reactive-ion molecular-beam epitaxy: Growth kinetics, microstructure, and properties
Authors: Powell R. C. , Lee N.-E. , Kim Y.-W. , Greene J. E.
Published: July 2002

Reactive-ion molecular-beam epitaxy has been used to grow epitaxial hexagonal-structure α-GaN on Al2O3(0001) and Al2O3(011̄2) substrates and metastable zinc-blende-structure β-GaN on MgO(001) under the following conditions: growth temperature Ts=450–800 °C; incident N+2/Ga flux ratio JN+2/JGa=1–5; and N+2 kinetic energy EN+2=35–90 eV. The surface structure of the α-GaN films was (1×1), with an ≊3% contraction in the in-plane lattice constant for films grown on Al2O3(0001), while the β-GaN films exhibited a 90°-rotated two-domain (4×1) reconstruction. Using a combination of in situ reflection high-energy electron diffraction, double-crystal x-ray diffraction, and cross-sectional transmission electron microscopy, the film/substrate epitaxial relationships were determined to be: (0001)GaN∥ (0001)Al2O3 with [21̄1̄0]GaN∥[11̄00]Al2O3 and [11̄00]GaN∥[12̄10]Al2O3, (21̄1̄0)GaN∥(011̄2)Al2O3 with [0001]GaN∥[01̄11]Al2O3 and [01̄10]GaN∥[21̄1̄0]Al2O3, and (001)GaN∥(001)MgO with [001]GaN∥[001]MgO.

Films with the lowest extended defect number densities (nd≂1010 cm−2 threading dislocations with Burgers vector a0/3<112̄0≳) and the smallest x-ray-diffraction ω rocking curve widths (5 min) were obtained using Al2O3(0001) substrates, Ts≥650 °C, JN+2/JGa≥3.5, and EN+2=35 eV. Higher N+2 acceleration energies during deposition resulted in increased residual defect densities. In addition, EN+2 and JN+2/JGa were found to have a strong effect on film growth kinetics through a competition between collisionally induced dissociative chemisorption of N2 and stimulated desorption of Ga as described by a simple kinetic growth model. The room-temperature resistivity of as-deposited GaN films grown at Ts=600–700 °C with EN+2=35 eV increased by seven orders of magnitude, from 10−1 to 106 Ω cm, with an increase in JN+2/JGa from 1.7 to 5.0. Hall measurements on the more conductive samples yielded typical electron carrier concentrations of 2×1018 cm−3 with mobilities of 30–40 cm2 V−1 s−1. The room-temperature optical band gaps of α-GaN and β-GaN were 3.41 and 3.21 eV, respectively.
Magnetic properties of microemulsion synthesized cobalt fine particles
Authors: Chen J. P. , Lee K. M. , Sorensen C. M. , Klabunde K. J. , Hadjipanayis G. C.
Published: July 2002

Two microemulsion systems, the ternary system H2O/AOT/isooctane and the binary system DDAB/toluene, were used to prepare metallic cobalt particles by borohydride reduction of a cobalt salt. The particles prepared in the AOT system were extremely small and superparamagnetic with magnetic moment per particle of 11.5μB, a blocking temperature of 2 K, and σs=146 emu/g at 1.7 K. The magnetic moment and σs combined imply a particle size of about 5.4 Å. However, the particles made in the DDAB system were relatively large, ∼10 nm. They were ferromagnetic at room temperature but with no coercivity.
Exchange stiffness constant and wall energy density of amorphous GdTb-FeCo thin films
Authors: Raasch D. , Reck J. , Mathieu C. , Hillebrands B.
Published: July 2002

The exchange stiffness constant A of evaporated (Gd,Tb)-(Fe,Co) amorphous alloys with constant Curie temperature of 465±25 K was determined with two different methods for a large composition and temperature range. One is the excitation of standing spin waves using Brillouin light scattering. The other method is the deviation of A from measurements of the wall energy density σw and the uniaxial anisotropy Ku using the Bloch wall energy equation. σw could be determined from the applied magnetic field difference for collapsing and expanding of thermomagnetically written domains. The compositional and experimental limits of this method are described. The uniaxial anisotropy Ku was measured with a torque magnetometer. In GdTb-FeCo the resulting exchange stiffness constant of A=(2.6±0.6)×10−12 J/m was found to be independent of the Tb content. The results obtained from Brillouin light-scattering measurements on Gd21.2 Co78.8 and Gd13.5Tb6.2Fe80.3 are in good agreement.
Thermal stresses in tightly jacketed double-coated optical fibers at low temperature
Authors: Shiue Sham-Tsong
Published: July 2002

The thermal stresses in tightly jacketed double-coated optical fibers at low temperature have been analyzed by the thermoelastic approach. The lateral pressure and normal stresses in the glass fiber, primary coating, secondary coating, and jacket have been derived. The thermal stresses in the optical fibers are affected by the temperature difference, material properties of polymeric coatings, and their thicknesses. It is possible to select the suitable polymeric coatings to produce a minimum lateral pressure in the glass fiber. The microbending loss is dominated by the lateral pressure in the glass fiber. To minimize such a microbending loss, the Poisson’s ratio of the primary and secondary coatings, and the Young’s modulus of the jacket should be increased. On the other hand, the Young’s modulus of the primary coating should be decreased. However, the thickness of the primary coating, and the thickness and Young’s modulus of the secondary coating exist the optimum values. The glass transition temperature of the primary coating should be lower than the temperature ranges under consideration. The thermal stresses for plane strain and zero axial force conditions are also discussed.
Observations of electromigration induced void nucleation and growth in polycrystalline and near-bamboo passivated Al lines
Authors: Marieb T. , Flinn P. , Bravman J. C. , Gardner D. , Madden M.
Published: July 2002

Electromigration voiding in passivated pure Al lines was observed in situ using high voltage scanning electron microscopsy. Two different types of lines were investigated; one deposited under high purity conditions with a near-bamboo microstructure, and one deposited under conventional conditions with a polycrystalline microstructure with stress voids present. The samples were observed while being tested under accelerated electromigration conditions. Samples were then thinned and analyzed with transmission electron microscopy techniques to investigate the relationship between line microstructure and void nucleation. Electromigration voids and stress voids were seen to nucleate at very specific, unique, microstructural sites that require the intersection of a grain boundary with the line sidewall. Void movement after initiation is also dictated by microstructure, with voids only growing into and causing failure in grains oriented with a {111} plane near perpendicular to the line. The conditions for void nucleation suggest that heterogeneities at the line sidewall are necessary for void nucleation.
Performance of NbN lattice-cooled hot-electron bolometric mixers
Authors: Kawamura Jonathan , Blundell Raymond , Tong Cheuk-yu Edward , Gol’tsman Gregory , Gershenzon Eugene , Voronov Boris
Published: July 2002

The heterodyne performance of lattice-cooled hot-electron bolometric mixers is measured at 200 GHz. Superconducting thin-film niobium nitride strips with ∼5 nm thickness are used as waveguide mixer elements. A double-sideband receiver noise temperature of 750 K at 244 GHz is measured at an intermediate frequency centered at 1.5 GHz with 500 MHz bandwidth and with 4.2 K device temperature. The instantaneous bandwidth for this mixer is 1.6 GHz. The local oscillator power required by the mixer is about 0.5 μW. The mixer is linear to within 1 dB up to an input power level 6 dB below the local oscillator power. A receiver incorporating a hot-electron bolometric mixer was used to detect molecular line emission in a laboratory gascell. This experiment unambiguously confirms that the receiver noise temperature determined from Y-factor measurements reflects the true heterodyne sensitivity.
Concentration of atomic hydrogen in the ground state in a CH4-H2 microwave plasma
Authors: Barshilia H. C. , Vankar V. D.
Published: July 2002

Ground state atomic hydrogen (H:1s 2S1/2) concentrations in the CH4-H2 microwave plasma have been measured using actinometry. These measurements have been made over a wide range of plasma conditions including power inputs of 100–800 W and pressures of 0.5–60 Torr. A trace amount of argon was added to serve as an inert reference gas for concurrent optical emission measurements, in which optical emission intensities from the Hγ line (2p2P0–5d2D) at 434 nm and the Ar* line (4s′[1/2]°−4p′[1/2]) at 750 nm were recorded.
Low-voltage electrochromic device for photovoltaic-powered smart windows
Authors: Bechinger C. , Bullock J. N. , Zhang J.-G. , Tracy C. E. , Benson D. K. , Deb S. K. , Branz H. M.
Published: July 2002

We report the properties of an all-solid-state electrochromic (EC) device that can be switched over a useful range of optical transmissions with voltages below 1 V. This switching voltage is smaller than required by other solid-state EC devices reported to date. We attribute the lower-than-normal switching voltage to the use of a thermally evaporated MgF2 thin film as the lithium ion conducting layer. Electrochemical impedance spectroscopy studies show that high lithium ion conductivity and low interfacial barriers for lithium exchange with the adjacent electrochromic and ion storage layers make MgF2 a good choice for the ion conductor in EC devices. This reduction in switching voltage is a first step toward powering an EC device by an integrated semitransparent single-junction photovoltaic (PV) cell. In a side-by-side bench test, where the EC device is connected to a semitransparent a-SiC:H PV cell having on open circuit voltage of 0.87 V, a relative transmission change in the EC device of 40% is achieved in less than 60 s.
Grain-boundary slit propagation in an electric field
Authors: Klinger L. M. , Chu X. , Mullins W. W. , Bauer C. L.
Published: July 2002

Advancement of a fine slit along a planar grain boundary in an electric field E0, applied parallel to the slit, is investigated by considering electromigration along both the grain boundary and the slit surface. Electrically induced flux in the grain boundary Igb (+ toward the slit tip) and both electrically and curvature-induced fluxes on the slit surfaces are considered assuming 2Is>Igb, where Is is the flux (+ away from the slit tip) on each of the parallel slit surfaces far removed from the tip. Steady-state solutions of the transport equations are classified according to the value of a parameter β=tan−1 (2Is/Igb) which, under reasonable assumptions, depends on material parameters only. For 5π/4≥β≥β2, unique steady-state solutions exist; for β2>β>β1, multiple steady-state solutions occur; below β1≥π/4, no steady-state solution is possible. Since β1<π/2, Igb>0 (flux exiting the grain boundary into the slit) for all cases in which no steady-state solution is possible. In the case of multiple solutions, those corresponding to smallest width (and hence largest velocity) are determined. For all steady-state solutions, slit width and tip velocity scale as E−1/20 and E3/20, respectively. Results also apply to the propagation of a slit within a grain or along a passivation layer. Generally, tip velocities can approach 1 nm/s (3.6 μm/h), thereby representing a likely failure mechanism in fine-line (near bamboo structure) interconnects.
The overshoot effect in transient electroluminescence from organic bilayer light emitting diodes: Experiment and theory
Authors: Nikitenko V. R. , Arkhipov V. I. , Tak Y.-H. , Pommerehne J. , Bässler H. , Hörhold H.-H.
Published: July 2002

Transient electroluminescence (EL) from organic bilayer light emitting diodes addressed by a rectangular voltage pulse often features an overshoot when the voltage is switched off. Experimental results are presented for a variety of hole transporting layers in contact with an oxadiazole layer simultaneously acting both as a blockade for holes and as an electron transport layer. The overshoot occurs in spin coated yet not in vapor deposited samples. A model is developed to rationalize charge recombination under the premise (i) of an interfacial layer of finite thickness between hole and electron transport layers in which both transport molecules coexist and (ii) of interfacial energy barriers impeding both hole and electron passage. It predicts the occurrence of an EL overshoot due to the recombination of stored electrons and holes under the action of their mutual space charge field when the external voltage is switched off. The temporal pattern of the predicted transient EL signal is in good agreement with experiment.
Measurement of laser-induced acoustic waves with a calibrated optical transducer
Authors: Paltauf G. , Schmidt-Kloiber H.
Published: July 2002

Irradiation of an absorbing material with a short laser pulse generates a thermoelastic stress wave, from which the distribution of absorbed energy can be derived. This method is ideal to measure the light penetration in biological tissue. Especially for in vivo applications, we developed an optical stress transducer that can be positioned directly in front of the irradiated surface, inside the laser beam, in order to avoid distortion of the stress wave due to acoustic diffraction. The detector is based on stress wave-induced changes of optical reflectance of a glass-water interface, probed with a continuous laser beam that is incident at an angle close to the critical angle of total internal reflection to achieve maximum sensitivity. In this study, we describe the theory for the calibration of the transducer and compare the measured with the theoretically predicted signals. In the experiments, an aqueous dye solution is irradiated with pulses from either a Q-switched, frequency-doubled Nd:yttrium aluminum garnet (YAG) laser or from an optical parametric oscillator with pulse durations of 8 and 6 ns, respectively. Good agreement between the measured and calculated waveforms as well as the possibility to obtain photoacoustic absorption spectra from the shape of the recorded signals is demonstrated. From our experimental and theoretical findings, it follows that the detector is characterized by a high temporal and spatial resolution and by an adjustable sensitivity, depending on the incident angle of the probe beam at the glass-water interface. Apart from the applications proposed in the present work, it seems to be possible to use this kind of transducer for the two-dimensional recording of stress waves.
Nitrogen and aluminum implantation in high resistivity silicon carbide
Authors: Dwight Deborah , Rao Mulpuri V. , Holland O. W. , Kelner G. , Chi P. H. , Kretchmer J. , Ghezzo M.
Published: July 2002

In this article, the results on N and Al implantations into undoped high-resistance and vanadium doped semi-insulating bulk 6H-SiC are reported for the first time. The N implants were performed at 700 °C and the Al implants at 800 °C to create n- and p-type layers, respectively. For comparison, implants were performed into epitaxial layers at the above temperatures and, for N, also at room temperature. The implanted/annealed material was characterized by van der Pauw Hall, secondary ion mass spectrometry, and Rutherford backscattering (RBS) measurements. After annealing, the room temperature N implantation gave similar electrical and RBS results as the 700 °C implantation for a total implant dose of 8×1014 cm−2 which corresponds to a volume concentration of 2×1019 cm−3. The Al implant redistributed in the bulk crystals during annealing, resulting in a shoulder formation at the tail of the implant profile. Lower implant activation was obtained in V-doped material compared to the undoped bulk and epitaxial layers, but the results were promising enough to use implantation technology for making planar high frequency devices in the bulk V-doped substrates, especially as the quality of the substrates continue to improve.
Enhanced silicon oxide film growth on Si (100) using electron impact
Authors: Xu Jiazhan , Choyke W. J. , Yates John T.
Published: July 2002

The effect of electron beam impact on the oxidation of Si (100) by oxygen has been studied using x-ray photoelectron spectroscopy and Auger electron spectroscopy. It was found that electron beam impact can enhance the oxidation of Si (100) by oxygen at low temperatures, resulting in silicon dioxide formation. Furthermore, electron energy-dependent film growth experiments were carried out on O2(a) and an electron attachment resonance energy of ∼10.1 eV was found. A possible electron-induced oxidation mechanism is proposed which involves dissociative electron attachment for adsorbed O2 species and the formation of O and O− species from adsorbed O2−.
Influence of ionic composition of water on the corona discharge around water drops
Authors: Škarja M. , Berden M. , Jerman I.
Published: July 2002

We performed a series of experiments to investigate the influence of different salts dissolved in water on the corona discharge patterns around water drops and on subsequent image formation. Considering our research needs, we developed a special method of corona discharge electrography of water drops with computer analysis of images. Our method differs from the usual corona discharge photography (known also as Kirlian photography) in the mechanisms of image formation. It relies on chemical reactions and deposition of substances transported in the discharge area rather than on the usual photoreactions. Higher reproducibility of results is achieved through simultaneous imaging and comparing two drops from different samples. In the presented experiments we compare different salt solutions with distilled water and between themselves. Results show distinct behavior of corona discharge patterns as regards ionic composition of salts, both radially and angularly. The present investigation may also shed some light on the problem of the influence of a different ionic composition in using this type of method for diagnostic purposes.
Numerical study of electrical transport in inhomogeneous Schottky diodes
Authors: Osvald J.
Published: July 2002

The Poisson equation together with the drift-diffusion equations have been used to simulate both forward and reverse I–V and C–V characteristics of inhomogeneous Schottky diodes. The barrier height distribution has been modeled by a single Gaussian. It is shown that the I–V and C–V curves and consequently extracted apparent Schottky diode parameters depend only slightly, if at all, on a lateral correlation between the single barrier patches in the structure for larger dimension of patches. The apparent barrier height of ordered structures differ only in several thousandths of volt from that of uncorrelated barrier patches. Very small differences were also found between the currents through the diodes with large inhomogeneities and nanosize inhomogeneities. The numerical results show that there is almost no dependence of the current on a pinch-off effect of electric potential. The diminishing of a drift part of the total current in the area of pinched-off potential is probably compensated by a greater diffusion current in the region. Consequently, very small and not unambiguous differences were found between the so called interacting and noninteracting diodes and this division is, according to the above results, questionable.
Magnetic anisotropy of asbestos fibers
Authors: Ulanowski Z. , Kaye P. H.
Published: July 2002

The anisotropy of magnetic susceptibility of single asbestos fibers is measured. The alignment of both chrysotile and crocidolite fibers in magnetic fields is found to be due to the anisotropy. The average measured anisotropy of volume susceptibility is 0.40×10−6 for chrysotile and 83×10−6 for crocidolite. Fiber shape effects are estimated to contribute, on average, about 10% and 6%, respectively, to the total anisotropy of the two types of fiber. There is no evidence of significant permanent magnetic moments. The magnitude of the observed alignment makes the effect potentially useful in real-time detection of airborne asbestos fibers. The experimental technique developed in the study can be used for measuring the anisotropy of small particles of well-defined shape. High sensitivity of the technique permits the measurement of torques lower than 10−21 Nm on particles down to picogram mass.
A finite-element study of strain fields in vertically aligned InAs islands in GaAs
Authors: Benabbas T. , Androussi Y. , Lefebvre A.
Published: July 2002

Finite-element calculations are used to study strain fields in vertically aligned InAs islands in GaAs. Such strain fields are found to be quite different from those of uncovered islands and nearly insensitive to the position of the island in the stacking. The driving force for vertically self-organized growth is known to be the interacting strain fields induced by the islands. The calculation of strain fields by the finite-element method makes it possible to model the correlations between adjacent InAs layers. A kinetic approach based on the effect of strain on surface diffusion is first proposed. A thermodynamic model is then analyzed to predict local island nucleation probabilities. Pairing probabilities of correlation between stacked islands, first calculated in the case of the InAs/GaAs system, are extended to the case of III-V semiconductors with a cubic crystalline structure. They are shown to be essentially dependent both on the ratio between the spacer layer thickness and the island height and on the lattice mismatch between islands and spacer layers.
Electric currents and fields induced in cells in the human brain by radiation from hand-held cellular telephones
Authors: King Ronold W. P.
Published: July 2002

After a review of recent work on the interaction of electromagnetic fields from cellular telephones with the human head, the structural and radiating properties of two common types of transceivers are determined. These include the impedance and current amplitude distribution of the antennas. The tangential electric field maintained by the antennas on the adjacent surface of the head is next determined. From this, the electric field propagating through the skull into the brain is analyzed and, from it, the electric field in spherical and long cylindrical cells is determined. It ranges from 27 to 13.5 V/m in the first 3 cm inside the skull. Of interest is the fact that the induced field in the interior of all cells, regardless of their shape, is the same as the incident field in the brain. It is hoped that biomedical scientists will review these results and determine possible biological effects.
Effect of fill-pulse parameters on deep-level transient spectroscopy peaks in highly doped p-type InP
Authors: Darwich R. , Massarani B.
Published: July 2002

The effect of different fill-pulse parameters on the characteristics of deep-level transient spectroscopy (DLTS) peaks has been studied in the example of the hole traps H4F and H5 in electron-irradiated highly doped p-type InP. It is shown that the saturation peak height, the temperature of the peak maximum and its full width at half maximum depend on the applied reverse bias, the pulse amplitude, its frequency and duration. Our results show that the origin of this dependence is the electric field present in the space charge region (SCR). The experimental results are analyzed in terms of the effect of the electric field on the refilled traps in the SCR. The appropriate experimental conditions for the correct extraction of information from the DLTS spectrum are defined.
Consideration of the spontaneous polarization of water at the solid/liquid interface
Authors: Luzar A. , Svetina S. , Z̆eks̆ B.
Published: July 2002

The orientation of water molecules close to a hard wall is studied. A simple model of water near interfaces is proposed in which only the hydrogen bond interactions among molecules and steric restrictions imposed by the wall are taken into account. The average number of hydrogen bonds per molecule is determined as a function of the distance from the interface. It is shown that a water molecule exhibits a preferred orientation at the surface because of its asymmetric character. Angular distribution of water molecules at the interface is studied by considering the effect of steric restrictions and orientational dependence on hydrogen bond probability. The magnitude of the resulting average polarization is estimated. The corresponding surface electrostatic potential is evaluated.
Structural and dielectric properties of dipolar hard sphere mixtures
Authors: Lee Pil H. , Ladanyi Branka M.
Published: July 2002

Reference linearized hypernetted chain (RLHNC) equations for mixtures of dipolar molecules with a spherically symmetrical reference potential are presented and solved for binary mixtures of hard spheres. The dependence of the dielectric constant, the dipolar contribution to the internal energy, and the orientation-dependent components of the pair distribution functions on composition, molecular sizes, and dipole moments is studied. The RLHNC approximation results for the composition dependence of the dipolar internal energy are found to be in excellent agreement with Monte Carlo simulation data. The RLHNC theory is found to overestimate the dielectric constant at high dipolar strengths. Values of the dielectric constant in much better agreement with Monte Carlo data are obtained by using the perturbation theory.
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o relativistic effective potentials with spin–orbit operators. III. Rb through Xe
Authors: LaJohn L. A. , Christiansen P. A. , Ross R. B. , Atashroo T. , Ermler W. C.
Published: July 2002

A refined version of the ‘‘shape consistent’’ effective potential procedure of Christiansen, Lee, and Pitzer was used to compute averaged relativistic effective potentials (AREP) and spin–orbit operators for the elements Rb through Xe. Particular attention was given to the partitioning of the core and valence space and, where appropriate, more than one set of potentials is provided. These are tabulated in analytic form. Gaussian basis sets with contraction coefficients for the lowest energy state of each atom are given. The reliability of the transition metal AREPs was examined by comparing computed atomic excitation energies with accurate all-electron relativistic values. The spin–orbit operators were tested in calculations on selected atoms.
Molecular applications of multireference coupled-cluster methods using an incomplete model space: Direct calculation of excitation energies
Authors: Pal Sourav , Rittby Magnus , Bartlett Rodney J. , Sinha Dhiman , Mukherjee Debashis
Published: July 2002

A multireference coupled-cluster (MRCC) formulation for the direct calculation of excitation energies and ionization potentials is presented. The reference space connects a set of p–h excited determinants built from all the set of active particles and holes in the model space. This model space is incomplete, requiring a Fock-space approach and the postulate of a ‘‘universal’’ wave operator to arrive at a linked diagram expression for the effective Hamiltonian Heff, whose eigenvalues are the excitation energies for the problem. Use of a normal-ordered exponential cluster ansatz allows one to construct, hierarchically, the CC equations for the p–h model space starting from the ground state. We present an extension of an earlier formulation for excitation energies that allows us to have both active and inactive particles and holes in our method. Numerical applications are reported for the prototypical small molecules CO and N2.
The calculation of the thermal rate coefficient by a method combining classical and quantum mechanics
Authors: Wahnström Göran , Carmeli Benny , Metiu Horia
Published: July 2002

We propose and test a method for computing flux–flux correlation functions (and thermal rate coefficients) which divides the degrees of freedom in two groups, one treated classically and the other quantum mechanically. The method is tested by applying it to a simple model for which we can also obtain exact results. The approximate method gives good results if the mass associated with the classical degrees of freedom exceeds 16 a.u.
Predictions of the rotational and vibrational spectra of SiF+, PO+, and NS+ by Mo/ller–Plesset perturbation theory
Authors: Peterson Kirk A. , Woods R. Claude
Published: July 2002

In order to predict accurately the spectroscopic constants of SiF+, PO+, and NS+, calculations at the MP4SDQ level have been carried out for the following series of 22 electron diatomics: AlF, SiF+, SiO, PO+, PN, NS+, CS, CCl+, and BCl. Two basis sets, one containing 66 contracted Gaussian-type orbitals and the other with 93, have been employed for each of the nine molecules to calculate potential energy functions and spectroscopic constants. The ab initio re and ωe values for the three ions whose high resolution spectra were not previously observed were corrected by reference to deviations between theory and experiment for the other six species, whose spectra were already well known. The dipole moments and electric field gradients of each species at the equilibrium bond distance were calculated at the CI-SD level, and their dissociation energies were found to be predicted well by MP2 and very well by MP4SDTQ calculations. In order to understand better the differences between the spectral predictions of the two basis sets, re and ωe of the well known first row analogs BF, CF+, CO, NO+, and N2 were computed at the MP4SDQ level with a series of five basis sets of increasing flexibility. Using the current prediction for SiF+ we have recently located its rotational spectrum, extensive measurements of which will be published separately.
Shape resonance behavior in 1πg photoionization of O2
Authors: Braunstein M. , McKoy V.
Published: July 2002

We report calculations of vibrationally resolved cross sections and photoelectron angular distributions for photoionization of O2 leading to the X 2 Πg (ν+ =0–4) states of O+2 using Hartree–Fock continuum photoelectron orbitals. These studies were motivated by recent results which show that a σu shape resonance plays a dominant role in producing non-Franck–Condon vibrational distributions in resonant multiphoton ionization of O2 via the C 3Πg (1πg3sσg) Rydberg state. In the present study, we investigate how this shape resonance influences photoionization dynamics in single-photon ionization. Below 21 eV photon energy, we find significant non-Franck–Condon effects in the vibrational branching ratios as well as in the vibrationally resolved photoelectron angular distributions. Substantial autoionization hinders a direct comparison between theory and experiment.
The contribution of hydrogen bonds to bulk and surface thermodynamic properties of dimethylsulfoxide–water mixtures
Authors: Luzar A.
Published: July 2002

A simple model for a hydrogen-bonded mixture treated in mean field approximation is used to study the excess bulk and interfacial thermodynamic properties of DMSO/water solutions at various compositions. According to the model, the system is treated as an ensemble of hydrogen bonding pairs involving water and DMSO molecules. In the interfacial layer of thickness equal to the length of the hydrogen bond, the number of hydrogen bonding pairs is reduced. Considering the difference in the hydrogen bonding strength for the water–water and water–DMSO hydrogen pairs, as well as orientational restrictions at the liquid/vapor interface, the number densities of water–water and water–DMSO hydrogen bonds are calculated as functions of the distance from the interface. The concentration profile is determined by minimizing the free energy function of the system. The model describes reasonably well the experimental data for excess enthalpies and free energies of mixing, as well as the excess surface free energy over the entire composition range. The resulting excess surface free energy is considered as a contribution of the hydrogen bond interaction among the two species to the total surface tension.
Theory and simulations of homonuclear spin pair systems in rotating solids
Authors: Levitt M. H. , Raleigh D. P. , Creuzet F. , Griffin R. G.
Published: July 2002

The theory of nuclear magnetic resonance (NMR) on a solid sample containing pairs of coupled homonuclear spins-1/2, rotating in a large magnetic field, is presented. The time dependence introduced by the sample rotation, in conjunction with the spin–spin coupling, makes it appear that each of the central two levels in the four-level system split into a pair of ‘‘virtual states.’’ Each of the eight possible single-quantum coherences between the virtual states and the two outer levels in general contribute to the spectrum, although four of these contributions are forbidden unless a rotational resonance occurs (matching of an integer multiple of the spinning speed with the difference in isotropic shifts). Analytical line shapes for the case of vanishing shift anisotropy are given and techniques for numerical simulation in the general case demonstrated. The theory of Zeeman magnetization exchange in the presence of zero-quantum dephasing is presented.
Configuration interaction potential energy and dipole moment functions for thirteen 22 electron diatomics
Authors: Peterson Kirk A. , Woods R. Claude
Published: July 2002

Singles and doubles CI with a large Gaussian basis set has been used to calculate potential energy and dipole moment functions for the following series of 22 electron diatomics: AlF, SiF+, AlO−, SiO, PO+, SiN−, PN, NS+, CP−, CS, CCl+, BS−, and BCl. A size consistency correction was applied to the CI-SD potentials to obtain results designated CI-SD(s). The calculations on the neutral species were used to correct the ab initio values of re and ωe for the ions, resulting in agreement with experiment to within 0.002 Å for re and 10 cm−1 for ωe in those cases where high resolution spectroscopic results were available (CCl+, SiF+, and PO+), and predictions for the spectra of the other five ions are expected to be of similar accuracy. The CI-SD(s) corrected values for CCl+, SiF+, and PO+ are somewhat closer to experiment than our earlier corrected MP4SDQ values, and the problems with RHF→UHF instability encountered in the MP4SDQ calculations of vibrational constants of certain species do not appear to affect the present work. Dipole moment matrix elements have been calculated from the CI-SD dipole moment functions and CI-SD(s) potential functions.
Viscosity of dilute polyelectrolyte solutions: Temperature dependence
Authors: Cohen J. , Priel Z.
Published: July 2002

The reduced specific viscosity of fully sulfonated sodium neutralized polystyrene under ‘‘free salt’’ conditions was investigated as a function of temperature. It was found that the position of the maxima is strongly dependent on temperature. Small temperature changes (ΔT=20 K) introduce shifts of two orders of magnitude in the polyelectrolyte concentration at which the maximum appear. At a given temperature Cpmax is linearly dependent on molecular weight, the slope of the linear plot is temperature dependent increasing with temperature. The value of Cp at the maximum increases linearly with concentrations of externally added salt at all temperatures and molecular weights. At a given molecular weight, the logarithm of Cpmax is inversely dependent on temperature. The activation energy was calculated and found to be independent of the molecular weight of the polyelectrolyte. The dependence of the reduced specific viscosity on normalized polyelectrolyte concentration (Cp/Cpmax) resulted in one ‘‘master curve’’ for all temperatures at a given molecular weight. Below the maximum, at lower polyelectrolyte concentration, a linear dependence of ηsp/Cp on Cp was obtained even for salt-free solutions. The apparent intrinsic viscosity and the Huggins coefficient were calculated, as it is done for noncharged polymers in the linear regime. High values of apparent intrinsic viscosity and the Huggins coefficient were obtained. The high measured values cannot be explained by hydrodynamic contribution of fully stretched molecules, indicating that even at extremely high dilutions the main contribution is the one of long range interactions (Culombic or others). The dependence of the apparent intrinsic viscosity on molecular weight was established. These measurements could be performed thanks to the availability of the apparatus developed by us which makes possible accurate measurements of the shear viscosity of low ionic strength, dilute polyelectrolyte solutions, down to polymer concentrations below one part per million.
The microwave spectrum of PO+: Comparison to SiF+
Authors: Petrmichl Rudolph H. , Peterson Kirk A. , Woods R. Claude
Published: July 2002

The microwave spectrum of PO+ has been detected in discharges in mixtures of PF3, O2, and Ar. Precise frequencies were obtained for 48 rotational transitions in the range 140–470 GHz, including all vibrational states v=0–11. Eight of the lines measured were transitions of P18O+, obtained using 18O2. To fit all of these to a mass independent Dunham expansion, a Watson ΔO01 parameter was required to describe the oxygen isotope shift. From the combined analysis a set of eight mass independent Dunham parameters Ukl and the Dunham potential constants a1–a5 were extracted. Harmonic and anharmonic vibrational constants are obtained from the microwave analysis and compare very well to published results from low resolution emission spectroscopy; the final microwave values of re and ωe are 1.424 992 7(4) Å and 1411.5(3) cm−1. These and the a1–a5 coefficients are in very good agreement with the results of several ab initio calculations we have done on PO+ (and other 22 electron diatomics), including those that were used to define the limits of our initial search for the microwave spectrum of PO+. The vibrational temperature of PO+ in our magnetically enhanced negative glow discharge was found to be very high, near 5500 (500) K. Some additional observations of the microwave spectrum of SiF+, used as a probe of the dynamical and chemical behavior of SiF+ in discharges, are also reported.
Temperature, kinetic energy, and internal energy dependences of the rate constant and branching fraction for the reaction of O+ (4S) with CO2
Authors: Viggiano A. A. , Morris Robert A. , Van Doren Jane M. , Paulson John F.
Published: July 2002

Rate constants and branching fractions were measured for the reaction of O+ (4S) with CO2 as a function of average ion-neutral center-of-mass kinetic energy (〈KEcm〉) at several temperatures. The reaction produces both O+2 and CO+2 as product ions. The total rate constants were found to be nearly collisional at low 〈KEcm〉 and to decrease slightly at higher 〈KEcm〉. No temperature dependence of the rate constants was found at any given 〈KEcm〉, indicating that neither CO2 rotational excitation nor CO2 bending mode excitation has much effect on the overall reactivity. In contrast, the branching fraction for the endothermic charge transfer channel leading to CO+2 was found to increase with increasing 〈KEcm〉 and also to increase with temperature at fixed 〈KEcm〉. Analysis of the data indicates that the branching fraction depends on the total energy in the system (electronic, kinetic, rotational, and vibrational).
Study on dynamical structure in water and heavy water by low-frequency Raman spectroscopy
Authors: Mizoguchi K. , Hori Y. , Tominaga Y.
Published: July 2002

Depolarized Raman spectra below 250 cm−1 in water and heavy water were measured and analyzed from 373 K to the supercooled region. The spectral feature of the central component below 20 cm−1 is stressed in the present work. The spectra below 250 cm−1 in water and heavy water are interpreted as a superposition of one Debye-type relaxation mode and two damped harmonic oscillators. The damped harmonic oscillators (broadbands around 60 cm−1 and 190 cm−1) are interpreted as the restricted translational modes. Analyzing the temperature dependence of the relaxation mode, the reciprocal relaxation time τ−1 in heavy water changes linearly with τ−1 ∝ (T − 240 K) in the whole temperature range. On the other hand, the temperature dependence of the reciprocal relaxation time in water above 303 K deviates from a straight line which holds below 298 K as τ−1 ∝ (T − 225 K).