bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanometer-scale metal precipitates in multicrystalline silicon solar cells
Authors: McHugo Scott A. , Thompson A. C. , Mohammed A. , Lamble G. , Périchaud I. , Martinuzzi S. , Werner M. , Rinio M. , Koch W. , Hoefs H.-U. , Haessler C.
Published: July 2002

In this study, we have utilized characterization methods to identify the nature of metal impurity precipitates in low performance regions of multicrystalline silicon solar cells. Specifically, we have utilized synchrotron-based x-ray fluorescence and x-ray absorption spectromicroscopy to study the elemental and chemical nature of these impurity precipitates, respectively. We have detected nanometer-scale precipitates of Fe, Cr, Ni, Cu, and Au in multicrystalline silicon materials from a variety of solar cell manufacturers. Additionally, we have obtained a direct correlation between the impurity precipitates and regions of low light-induced current, providing direct proof that metal impurities play a significant role in the performance of multicrystalline silicon solar cells. Furthermore, we have identified the chemical state of iron precipitates in the low-performance regions. These results indicate that the iron precipitates are in the form of oxide or silicate compound. These compounds are highly stable and cannot be removed with standard silicon processing, indicating remediation efforts via impurity removal need to be improved. Future improvements to multicrystalline silicon solar cell performance can be best obtained by inhibiting oxygen and metal impurity introduction as well as modifying thermal treatments during crystal growth to avoid oxide or silicate formation
Semiclassical description of diffraction and its quenching by the forward–backward version of the initial value representation
Authors: Gelabert Ricard , Giménez Xavier , Thoss Michael , Wang Haobin , Miller William H.
Published: July 2002

It is shown that the forward–backward (FB) version of the semiclassical (SC) initial value representation (IVR) is able to describe quantum interference/coherence (i.e., diffraction) of particles transmitted by a two-slit potential. (In contrast, the linearized approximation to the SC-IVR, which leads to the classical Wigner model, is unable to do so.) FB-IVR calculations are also used to describe the (partial) quenching of this interference structure (i.e., “de-coherence”) when the two-slit potential is coupled to a bath of harmonic oscillators.
Spatial distribution of Nd3+ dopant ions in vitreous silica: A pulsed electron paramagnetic resonance spectroscopic study
Authors: Sen S. , Orlinskii S. B. , Rakhmatullin R. M.
Published: July 2002

The structural aspects of clustering of Nd3+ ions in Nd2O3-doped SiO2 glasses and the effect of Al3+ codoping on these clusters have been investigated with pulsed electron paramagnetic resonance (EPR) techniques over a temperature range of 1.5–4 K. The Nd2O3 concentrations in these glasses range between 700 and 2400 ppm by weight. The Nd3+ echo-detected EPR (EDEPR) spectra of the Al-free glasses show indications of weak exchange coupling between Nd3+ ions due to clustering. The EDEPR spectra also suggest that the local coordination environment of Nd3+ ions is affected by codoping with Al. The electronic spin-lattice relaxation rates are found to be not sensitive to the spatial distribution of Nd3+ ions over the entire temperature range of measurements. On the other hand, the concentration dependence of phase relaxation rates show clear evidence of clustering of Nd3+ ions in Al-free glasses, even at the lowest Nd2O3 doping levels. These Nd3+ clusters are found to break up and homogenize with an increase in the average Nd–Nd distance on codoping with Al. Analyses of the hyperfine sublevel correlation spectrum of a Nd and Al codoped glass indicate that the homogenization of Nd3+ clusters is possibly a consequence of the formation of Nd–O–Al linkages.
Internal plasma potential profiles in a laboratory-model Hall thruster
Authors: Haas James M. , Gallimore Alec D.
Published: July 2002

The Plasmadynamics and Electric Propulsion Laboratory High-speed Axial Reciprocating Probe system is used in conjunction with a floating emissive probe to measure plasma potential in the discharge chamber of the P5 Hall thruster. Plasma potential measurements are made at a constant voltage, 300 V, at two different discharge current conditions: 5.4 and 10 A. The plasma potential contours for the 5.4 A case indicate that the acceleration region begins several millimeters upstream of the exit plane, extends several centimeters downstream, and is uniform across the width of the discharge chamber. The 10 A case is similar to the 5.4 A case with the exception that the acceleration region is shifted downstream on centerline. Axial electric field profiles, computed from the measured potential, show a double peak structure in the 5.4 A case, indicating a zone of ion deceleration. Perturbations to the discharge current are shown to correspond spatially with the location of the peak electric field indicating that thruster perturbations may result from a disturbance to the Hall current, as opposed to ablation of probe material. This conclusion is supported by the lack of any observable material ablation.
Efficient organic red electroluminescent device with narrow emission peak
Authors: Tao X. T. , Miyata S. , Sasabe H. , Zhang G. J. , Wada T. , Jiang M. H.
Published: July 2002

We report a red fluorescent material of 3-(dicyanomethylene)-5,5-dimethyl-1(4-dimethylamino-styryl)cyclohexene (DCDDC) and its use as emitter or dopant in tris-(8-hydroxyquinoline) aluminum based electrolumiescent (EL) devices. The DCDDC was synthesized by a simple “one-pot” reaction and shows bright red photoluminescence with a peak wavelength at 650 nm. EL properties of double-layer light-emitting diodes of indium tin oxide (ITO)/poly(N)-vinylcarbazole (PVK):N,N′-diphenyl-N,N-bis(3-methylphenyl)1,1–biphenyl-4,4′-diamine (TPD)/DCDDC/Mg:Ag and ITO/PVK:TPD/DCDDC(1%):Alq3/Mg:Ag using DCDDC as red emitters or dopants were characterized. The device with 1% DCDDC-doped Alq3 as emitting layer has a turn-on voltage of 5 V, and the highest luminance of 5600 cd/m2 is obtained at 15 V. The EL efficiency of the device at 100 cd/m2 is in the range of 1.1 l m/W, and the highest EL efficiency is 1.6 l m/W. The devices emit in the red region, and the full width at half maximum of the EL spectra is 70 nm.
Hydration of Li+ ion. An ab initio molecular dynamics simulation
Authors: Lyubartsev A. P. , Laasonen K. , Laaksonen A.
Published: July 2002

Ab initio Car–Parrinello molecular dynamics simulations of a Li+ ion in water have been carried out using the density-functional theory with Becke–Lee–Yang–Parr (BLYP) functional and ultrasoft Vanderbildt pseudopotentials. Both structural and dynamical properties of Li+ have been studied in detail and compared with available neutron scattering and spectroscopic data. Excellent agreement is obtained with the existing experimental data for the structure of the first hydration shell around the Li+ ion. Spectral features of bound water are compared to those of bulk water. Reasonable agreement is obtained with IR and Raman experiments. The ab initio simulation results have also been used to derive a Li+–water interaction potential. The best fit of the data gave a simple single-exponential potential function, which reproduces very well the liquid structure from the original ab initio simulations. This potential model, together with the simple point charge (SPC) water model, was applied to calculate the hydration free energy of Li+ using the Expanded Ensemble method.
Multiplication noise of AlxGa1−xAs avalanche photodiodes with high Al concentration and thin multiplication region
Authors: Zheng X. G. , Sun X. , Wang S. , Yuan P. , Kinsey G. S. , Holmes A. L. , Streetman B. G. , Campbell J. C.
Published: July 2002

We report that homojunction AlxGa1−xAs avalanche photodiodes (APDs) exhibit very low multiplication noise when the Al content is ⩾80%. It was also found that, due to nonlocal effects, the multiplication noise decreased as the ionization region thickness was reduced from 0.8 μm to ⩽0.2 μm for Al ratios (from 0 to 0.9). The excess noise factor of the thin (140 nm) Al0.9Ga0.1As APDs is the lowest reported to date for III–V compounds and is comparable to that of Si avalanche photodiodes.
Quadratic Poisson algebras of two-dimensional classical superintegrable systems and quadratic associative algebras of quantum superintegrable systems
Authors: Daskaloyannis C.
Published: July 2002

The integrals of motion of the classical two-dimensional superintegrable systems with quadratic integrals of motion close in a restrained quadratic Poisson algebra, whose the general form is investigated. Each classical superintegrable problem has a quantum counterpart, a quantum superintegrable system. The quadratic Poisson algebra is deformed into a quantum associative algebra, the finite-dimensional representations of this algebra are calculated by using a deformed parafermion oscillator technique. It is shown that the finite dimensional representations of the quadratic algebra are determined by the energy eigenvalues of the superintegrable system. The calculation of energy eigenvalues is reduced to the solution of algebraic equations, which are universal, that is for all two-dimensional superintegrable systems with quadratic integrals of motion.
Computer simulation studies of aqueous solutions at ambient and supercritical conditions using effective pair potential and polarizable potential models for water
Authors: Koneshan S. , Rasaiah Jayendran C. , Dang Liem X.
Published: July 2002

This paper discusses the computer simulation of the diffusion coefficients and structure of infinitely dilute aqueous ionic solutions at ambient (298 K, solvent density 0.997 g cm−3) and supercritical (683 K, solvent density 0.35 g cm−3) conditions using two different models for water. They are the extended simple point charge (SPC/E) and renormalized polarizability (RPOL) models in which the electronic polarizations of the water molecule are treated differently. The effect of polarizability is implicit in the SPC/E model and explicit in the RPOL model. The RPOL model shows slightly greater hydrogen bonding, at room temperature than the SPC/E model, but less hydrogen bonding at 683 K. It is concluded that the explicit neglect of electronic polarization of the solvent and ions in calculations based on the SPC/E model of water, has only a small effect on the diffusion coefficients of the ions. Both models predict diffusion coefficients of ions in supercritical water that are weakly dependent on their size in contrast to their behavior under ambient conditions discussed in previous work [S. Koneshan et al., J. Phys. Chem. 102, 4193 (1998)]. The simulations suggest that the mechanism of diffusion at the ambient and supercritical states of the solvent water is different.
An experiment to test centrifugal confinement for fusion
Authors: Ellis R. F. , Hassam A. B. , Messer S. , Osborn B. R.
Published: July 2002

The basic idea of centrifugal confinement is to use centrifugal forces from supersonic rotation to augment conventional magnetic confinement. Optimizing this “knob” results in a fusion device that features four advantages: steady state, no disruptions, superior cross-field confinement, and a simpler coil configuration. The idea rests on two prongs: first, centrifugal forces can confine plasmas to desired regions of shaped magnetic fields; second, the accompanying large velocity shear can stabilize even magnetohydrodynamic (MHD) instabilities. A third feature is that the velocity shear also viscously heats the plasma; no auxiliary heating is necessary to reach fusion temperatures. Regarding transport, the velocity shear can also quell microturbulence, leading to fully classical confinement, as there are no neoclassical effects. Classical parallel electron transport then sets the confinement time. These losses are minimized by a large Pastukhov factor resulting from the deep centrifugal potential well: at Mach 4–5, the Lawson criterion is accessible. One key issue is whether velocity shear will be sufficient by itself to stabilize MHD interchanges. Numerical simulations indicate that laminar equilibria can be obtained at Mach numbers of 4–5 but that the progression toward laminarity with increasing Mach number is accompanied by residual convection from the interchanges. The central goal of the Maryland Centrifugal Torus (MCT) [R. F. Ellis et al., Bull. Am. Phys. Soc. 44, 48 (1998)] is to obtain MHD stability from velocity shear. As an assist to accessing laminarity, MCT will incorporate two unique features: plasma elongation and toroidal magnetic field. The former raises velocity shear efficiency, and modest magnetic shear should suppress residual convection.
Coarse-graining Landau–Lifshitz damping
Authors: Feng Xuebing , Visscher P. B.
Published: July 2002

High speed switching in magnetic materials is usually studied with the Landau–Lifshitz (LL) equation, which describes damping through a phenomenological coefficient. The results of micromagnetic calculations based on the LL equation have been observed to depend strongly on the cell size. We take a coarse-graining or renormalization-group approach to this cell size dependence: from a simulation using cell size L, we look at the dynamics of a cell of size 2L and determine an effective damping coefficient that describes the larger-scale dynamics. This can be thought of as a Green–Kubo calculation of the effective damping coefficient. In principle, this makes it possible to coarse grain from the atomic scale to determine the micromagnetic damping coefficient.
Radio frequency energy coupling to high-pressure optically pumped nonequilibrium plasmas
Authors: Plönjes Elke , Palm Peter , Lee Wonchul , Lempert Walter R. , Adamovich Igor V.
Published: July 2002

This article presents an experimental demonstration of a high-pressure unconditionally stable nonequilibrium molecular plasma sustained by a combination of a continuous wave CO laser and a sub-breakdown radio frequency (rf) electric field. The plasma is sustained in a CO/N2 mixture containing trace amounts of NO or O2 at pressures of P=0.4–1.2 atm. The initial ionization of the gases is produced by an associative ionization mechanism in collisions of two CO molecules excited to high vibrational levels by resonance absorption of the CO laser radiation with subsequent vibration-vibration (V-V) pumping. Further vibrational excitation of both CO and N2 is produced by free electrons heated by the applied rf field, which in turn produces additional ionization of these species by the associative ionization mechanism. In the present experiments, the reduced electric field, E/N, is sufficiently low to preclude field-induced electron impact ionization. Unconditional stability of the resultant cold molecular plasma is enabled by the negative feedback between gas heating and the associative ionization rate. Trace amounts of nitric oxide or oxygen added to the baseline CO/N2 gas mixture considerably reduce the electron–ion dissociative recombination rate and thereby significantly increase the initial electron density. This allows triggering of the rf power coupling to the vibrational energy modes of the gas mixture. Vibrational level populations of CO and N2 are monitored by infrared emission spectroscopy and spontaneous Raman spectroscopy. The experiments demonstrate that the use of a sub-breakdown rf field in addition to the CO laser allows an increase of the plasma volume by about an order of magnitude. Also, CO infrared emission spectra show that with the rf voltage turned on the number of vibrationally excited CO molecules along the line of sight increase by a factor of 3–7. Finally, spontaneous Raman spectra of N2 show that with the rf voltage the vibrational temperature of nitrogen increases by up to 30%. This novel energy efficient approach allows sustaining large-volume high-pressure molecular plasmas without the use of a high-power CO laser. This opens a possibility of using the present technique for high-yield plasma chemical synthesis and plasma material processing.
Cu(In,Ga)Se
2
solar cells with controlled conduction band offset of window/Cu(In,Ga)Se2 layers
Authors: Minemoto Takashi , Hashimoto Yasuhiro , Satoh Takuya , Negami Takayuki , Takakura Hideyuki , Hamakawa Yoshihiro
Published: July 2002

Our group studied the effects of conduction band offset of window/Cu(In,Ga)Se2 (CIGS) layers on CIGS-based solar cell performance. To control the conduction band offset, we considered the use of a window layer of Zn1−xMgxO thin film with a controllable band gap as an alternative to the conventional window layer using CdS film. From the measurement of valence band offset between Zn1−xMgxO/CIGS layers and the band gap of each layer, we confirmed that the conduction band offset of Zn1−xMgxO/CIGS layers could be controlled by changing the Mg content of the Zn1−xMgxO film. The CIGS-based solar cells prepared for this study consisted of an ITO/Zn1−xMgxO/CIGS/Mo/soda-lime glass structure. When the conduction band minimum of Zn1−xMgxO was higher than that of CIGS, the performance of CIGS-based solar cells with a Zn1−xMgxO window layer was equivalent to that of CIGS-based solar cells with CdS window layers. We confirmed that the control of the conduction band offset of the window/CIGS layers decreases the majority carrier recombination via the Zn1−xMgxO/CIGS interface defects.
Low frequency noise in GaN metal semiconductor and metal oxide semiconductor field effect transistors
Authors: Rumyantsev S. L. , Pala N. , Shur M. S. , Gaska R. , Levinshtein M. E. , Khan M. Asif , Simin G. , Hu X. , Yang J.
Published: July 2002

The low frequency noise in GaN field effect transistors has been studied as function of drain and gate biases. The noise dependence on the gate bias points out to the bulk origin of the low frequency noise. The Hooge parameter is found to be around 2×10−3 to 3×10−3. Temperature dependence of the noise reveals a weak contribution of generation–recombination noise at elevated temperatures.
Integration of self-assembled diblock copolymers for semiconductor capacitor fabrication
Authors: Black C. T. , Guarini K. W. , Milkove K. R. , Baker S. M. , Russell T. P. , Tuominen M. T.
Published: July 2002

We combine a self-organizing diblock copolymer system with semiconductor processing to produce silicon capacitors with increased charge storage capacity over planar structures. Our process uses a diblock copolymer thin film as a mask for dry etching to roughen a silicon surface on a 30 nm length scale, which is well below photolithographic resolution limits. Electron microscopy correlates measured capacitance values with silicon etch depth, and the data agree well with a geometric estimate. This block copolymer nanotemplating process is compatible with standard semiconductor processing techniques and is scalable to large wafer dimensions.
Ab initio study of O2 precursor states on the Pd(111) surface
Authors: Honkala Karoliina , Laasonen Kari
Published: July 2002

Interactions of O and O2 with the Pd(111) surface are studied using spin-density-functional theory with gradient corrections. The investigation of potential energy surface of O atom on Pd(111) shows the face centered cubic site adsorption to be the most favorable. The diffusion barrier to an adjacent hexagonal closed pack site is 0.56 eV. Several adsorption trajectories are calculated for O2 on Pd(111). We find that the molecule dissociation is either direct but strongly activated or precursor mediated with considerably lower barrier. Three precursor states are found and identified according to geometry, energy, and vibrational frequency. Two precursor states are energetically degenerate with only slightly different geometries. Density difference analysis reveals that the electronic structure of both the molecule and the nearest Pd atoms is modified during the adsorption. However, according to density difference analysis the molecules in the precursor states are neutral and the interaction between O2 and the substrate resembles the donation–backdonation scheme.
Formation of ordered CoAl alloy clusters by the plasma-gas condensation technique
Authors: Konno Toyohiko J. , Yamamuro Saeki , Sumiyama Kenji
Published: July 2002

Co x Al 1−x alloy clusters were synthesized from a mixture of Co and Al metal vapors generated by the sputtering of pure metal targets. We observed that the produced alloy clusters were uniform in size, ranging from approximately 20 nm for Al-rich clusters to 10 nm for Co-rich clusters. For a wide average composition range (x≈0.4–0.7), the alloy clusters have the ordered B2 (CsCl-type) structure. In the Co-rich cluster aggregates (x=0.76), the clusters are composed of face-centered-cubic (fcc) Co and minor CoAl(B2) clusters. In the Al-rich aggregates (x=0.23), the clusters are mainly composed of the fcc-Al phase, although clusters occasionally possess a “core-shell structure” with the CoAl(B2) phase surrounded by an Al-rich amorphous phase. These observations are in general agreement with our prediction based on the equilibrium phase diagram. We also noticed that the average composition depends not only on the relative amount of Co and Al vapors, but also on their absolute amount, and even on the Ar gas flow rate, which promotes mixing and cooling the two vapors. These findings show that the formation of alloy clusters in vapor phase is strongly influenced by the kinetics of cluster formation, and is a competing process between the approach to equilibrium and the quenching of the whole system.
Quantitative modulated excitation Fourier transform infrared spectroscopy
Authors: Baurecht Dieter , Fringeli Urs Peter
Published: July 2002

The detection of weak absorption changes induced by an external excitation is often hindered by intense background absorptions as well as by noise. Modulation spectroscopy is an adequate tool to be applied in such a case, provided the system may be periodically stimulated, leading to a periodic reversible or pseudoreversible response. In modulated excitation (ME) Fourier transform infrared spectroscopy the phase sensitive detection (PSD) used for the demodulation of the periodic system response is generally performed during data acquisition, i.e., applied to the intensity of the interferogram. This leads to a number of problems in quantitative analysis and the requirement of optional equipment. In this article, a method is presented to perform an off-line vector PSD of conventional time-resolved spectra after data acquisition. A detailed mathematical analysis of PSD applied to the spectral intensity, the interferogram intensity, and to time-resolved spectra is presented. It is shown, that vector PSD applied to a set of time-resolved spectra is straightforward and avoids any additional mathematical corrections. Furthermore, it will be shown how ME spectroscopy can be used for experimental separation of overlapping bands and a detailed description for the determination of absolute modulation amplitudes and phase lags is given.
Origin of photoluminescence around 2.6–2.9 eV in silicon oxynitride
Authors: Noma Takashi , Seol Kwang Soo , Kato Hiromitsu , Fujimaki Makoto , Ohki Yoshimichi
Published: July 2002

A broad photoluminescence (PL) around 2.6–2.9 eV is known to appear in hydrogenated silicon oxynitride. Although its origin was reported to be Si–N bonds, it is not so clear since the material contains hydrogen. In the present research, we have confirmed that the same PL appears in silicon oxynitride grown by nitriding of silicon dioxide. The depth profile of the PL intensity agrees with that of the nitrogen concentration. Furthermore, the emission spectrum, excitation spectrum, and decay constant of this PL agree with those of the PL observed in silicon nitride. Based on these results and theoretical discussion, the origin of the 2.6–2.9 eV PL is estimated to be Si–N bonds.
Intrinsic stress, island coalescence, and surface roughness during the growth of polycrystalline films
Authors: Sheldon Brian W. , Lau K. H. A. , Rajamani Ashok
Published: July 2002

During film growth by a variety of techniques, intrinsic tensile stresses can be created by the coalescence of neighboring islands. Experimental results with diamond films produced by chemical vapor deposition are compared with a relatively simple model to demonstrate that a realistic interpretation of these coalescence stresses must account for effects that are associated with surface roughness. First, the interpretation of curvature measurements during the early stages of film growth must account for this surface roughness. Also, the experiments show that tensile stresses are induced by grain boundary formation during continuing growth after the initial island coalescence event. This understanding differs from the traditional interpretation that continuing intrinsic stress is produced by “templated” growth onto an already strained crystalline lattice. A kinetic model of stress evolution during postcoalescence growth is also presented.
Stacking of metamorphic InAlAs/InGaAs heterostructures on GaAs substrate
Authors: Cordier Y. , Zaknoune M. , Trassaert S. , Chauveau J. M.
Published: July 2002

In this work, two lattice mismatched heterostructures called stacked metamorphic high-electron mobility transistors have been grown showing the feasibility of the metamorphic concept for the vertical integration of structures having different lattice parameters. Molecular-beam epitaxy of linear graded buffer layers allows the relaxation of the mismatch strain by generation of misfit dislocations. X-ray reciprocal space mapping and cross sectional transmission electron microscopy confirm the interest of growing inverse step buffer layers to achieve high relaxation rates with good confinement of misfit dislocations. The quality of these stacked structures was confirmed by photoluminescence, Hall effect measurements, and the dc characteristics of the transistors.
Selective photoetching and transmission electron microscopy studies of defects in heteroepitaxial GaN
Authors: Weyher J. L. , Tichelaar F. D. , Zandbergen H. W. , Macht L. , Hageman P. R.
Published: July 2002

Photoelectrochemical (PEC) etching has been used to study defects in heteroepitaxial GaN layers. In Ga-polar layers PEC etching reveals only dislocations in the form of filamentary etch features (whiskers). Transmission electron microscopy (TEM) confirmed a one-to-one correspondence between the whiskers and straight threading dislocations, which are mainly of edge and mixed type. In N-polar layers, apart from dislocations, inversion domains (IDs) also give rise to the formation of more complex etch features that also have been confirmed by TEM. IDs of nanometer diameter result in formation of whiskers similar to the dislocation-related ones. However, when the diameter of IDs exceeds a critical size (about 100 nm), crater-like deep etch features are formed during PEC etching. Based on the mechanism of PEC etching of GaN in aqueous KOH solutions, it is argued that inversion domain boundaries are electrically active defects.
An interatomic pair potential for cadmium selenide
Authors: Rabani Eran
Published: July 2002

We have developed a set of interatomic pair potentials for cadmium selenide based on a form similar to the Born–Mayer model. We show that this simple form of the pair potential, which has been used to describe the properties of alkali halides in the sixfold-coordinate structure, provides a realistic description of the properties of cadmium selenide in all three crystal structures: wurtzite, zinc blende, and rocksalt. Using the new pair potential we have studied the pressure-induced phase transition from the fourfold-coordinate wurtzite structure to the sixfold-coordinate rocksalt structure. The pressure transformation and the equation of state are in good agreement with experimental observations. Using the dispersion term in our pair potential we have also calculated the Hamaker constant for cadmium selenide within the framework of the original microscopic approach due to Hamaker. The results indicate that for ionic materials many-body terms that are included in the Lifshitz theory are well captured by the simple pair potential.
Experimental study of the instability of the viscous flow past a flexible surface
Authors: Muralikrishnan R. , Kumaran V.
Published: July 2002

The viscous instability in the flow past a soft material is experimentally studied. The experiment is carried out using the parallel plate geometry of a rheometer, and a sheet of polyacrylamide gel of thickness about 4.5 mm is placed on the bottom plate. The fluid, silicone oil, is placed on the surface of the gel and the top plate is lowered till a preset gap of thickness between 300 and 1000 μm is attained. The rheometer is operated in the stress controlled mode, where the stress is increased at a constant rate, and the strain rate and apparent viscosity (assuming the flow in the gap is laminar) are recorded. Care is taken to ensure the Reynolds number is less than 1, so that inertial effects are negligible. The experimental results show that there is an anomalous increase in the apparent viscosity, determined assuming the flow is laminar, at a certain strain rate. This indicates that the flow becomes unstable and undergoes a transition from a laminar flow to a more complicated flow profile. This transition is repeatable if the experiment is stopped before there is irreversible damage to the gel surface. The experimental results are compared with theoretical predictions, and quantitative agreement is found with no adjustable parameters for a range of gap thicknesses and gel moduli.
S
1
/S
2
exciton splitting in the (2-pyridone)2 dimer
Authors: Müller Andreas , Talbot Francis , Leutwyler Samuel
Published: July 2002

The S1↔S0 and S2↔S0 vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)2 and its 13C-, d1-, and d2-isotopomers were investigated by two-color resonant two-photon ionization and fluorescence spectroscopies. For the C2h symmetric (2PY)2–h2 and (2PY)2–d2 complexes, the 2PY moieties are equivalent and the S1↔S0 (1Ag↔1Ag) transition is forbidden. A single H/D or 12C/13C isotopic substitution reduces the symmetry to Cs, so that (2PY)2–13C and (2PY)2–d1 both exhibit S1↔S0 and S2↔S0 transitions. The S1/S2 state exciton splittings are 43.6 cm−1 and 52.4 cm−1, respectively. These are analyzed in terms of a Frenkel model and compared to calculated splittings based on ab initio monomer transition dipole moments. For (2PY)2–d1, whose 2PY subunits are different, an excitation transfer time of 318 fs is calculated from the exciton splitting. The S1↔S0 and S2↔S0 spectra are analyzed and assigned. Several bu intermolecular vibrations of S1 appear via vibronic coupling to the S2(Bu) state. Combination of the fluorescence data from excitation of the S1 and S2 origins and vibrational excitations of (2PY)2–h2, (2PY)2–d1, and (2PY)2–d2 allows the determination of the six S0 state intermolecular vibrational frequencies.
Fluorescence imaging of surface plasmon fields
Authors: Ditlbacher H. , Krenn J. R. , Felidj N. , Lamprecht B. , Schider G. , Salerno M. , Leitner A. , Aussenegg F. R.
Published: July 2002

We demonstrate that surface plasmon fields can be imaged in real time by detecting the fluorescence of a molecular film close to the plasmon carrying metal surface. We use this method to image the field profile of surface plasmons launched at lithographically designed nanoscopic defects.
Sulfur 1s excitation of S2 and S8: Core–valence- and valence–valence–exchange interaction and geometry-specific transitions
Authors: Rühl E. , Flesch R. , Tappe W. , Novikov D. , Kosugi N.
Published: July 2002

Sulfur 1s excitation of S2 and S8 is reported. The experiments made use of monochromatic synchrotron radiation from the BW-1 beam line at the HASYLAB (Hamburg, Germany), where total cation yields and photoionization mass spectra were investigated as a function of the source temperature that was used for evaporation of α-sulfur. The near-edge structure undergoes characteristic changes as a function of the source temperature, reflecting significant size- and geometry-dependent changes in electronic structure of the sulfur clusters. The detailed analysis of the near-edge features is obtained from ab initio (MR-SDCI) calculations that focus on S8 and S2. The present results are discussed in comparison to previous work on inner-shell excited sulfur clusters as well as condensed sulfur. The near-edge structure of S2 is compared to that of O2 and Se2, yielding characteristic differences in core–valence and valence–valence–exchange interaction.
Spatial distribution of a depletion potential between a big solute of arbitrary geometry and a big sphere immersed in small spheres
Authors: Kinoshita Masahiro
Published: July 2002

The hypernetted-chain integral equations are solved on a three-dimensional cubic grid to calculate the spatial distribution of the depletion potential between a big solute of arbitrary geometry and a big sphere immersed in small spheres forming the solvent. By analyzing the potential along a specific trajectory of the big sphere, effects due to the geometric feature of the big solute (step edges, trenches, corners, changing curvature, etc.) can be examined in detail. As an illustration, effects of the step edge on the lateral depletion potential along a wall surface are analyzed. Along the trajectory considered, the big sphere moves at constant height, starting on the center of the wall surface and moving horizontally past the edge. The big sphere is repelled from the edge into the wall surface, and to escape to the bulk it must overcome a significantly high free-energy barrier. As another illustration, simple model calculations are performed for the lock and key steric interaction between macromolecules. The potential at contact (i.e., the stabilization free energy) for the key that exactly fits the lock is far larger than for smaller and larger keys and considerably in excess of the value predicted by the Asakura–Oosawa theory.
Calculating electron transport in a tight binding model of a field-driven molecular wire: Floquet theory approach
Authors: Tikhonov Alexander , Coalson Rob D. , Dahnovsky Yuri
Published: July 2002

This paper considers electron transport through a molecular bridge coupled to two metal electrodes in the presence of a monochromatic radiation field. Current flow through the wire is calculated within a nondissipative one-electron tight binding model of the quantum dynamics. Using Floquet theory, the field-driven molecular wire is mapped to an effective time-independent quantum system characterized by a tight-binding Hamiltonian with the same essential structure as the nondriven analog. Thus, Green’s Function methods for computing current flow through the wire, which have been profitably applied to the molecular wire problem in the absence of driving, can also be used to analyze the corresponding field-driven system. Illustrative numerical calculations on a simple model system are presented.
Second harmonic generation investigations of charge transfer at chemically-modified semiconductor interfaces
Authors: Fomenko Vasiliy , Hurth Cédric , Ye Tao , Borguet Eric
Published: July 2002

Charge transfer and accumulation at semiconductor devices can lead to device degradation. Understanding and controlling such a process is therefore important. Second harmonic generation has been shown to be a sensitive probe of charging of semiconductor interfaces, with the added advantages of high spatial and temporal resolution. We have investigated the use of self assembled monolayers (SAMs) as a means to control charging. Our results suggest that octadecylsiloxane SAMs, bound to the native oxide, significantly reduce charge accumulation at oxide interfaces.
Variational principles for describing chemical reactions: Condensed reactivity indices
Authors: Ayers Paul W. , Morrison Robert C. , Roy Ram K.
Published: July 2002

Two recent papers [P. W. Ayers and R. G. Parr, J. Am. Chem. Soc. 122, 2010 (2000); 123, 2007 (2001)] have shown how variational principles for the energy may be used to derive and elucidate the significance of the chemical reactivity indices of density-functional theory. Here, similar ideas are applied, yielding a systematic, mathematically rigorous, and physically sound approach to condensed reactivity indices. First, we use the variational principle for the energy to derive an expression for the condensed Fukui function index in terms of the condensed hardness kernel. Next, we address an important open problem pertaining to condensed reactivity indices: when (if ever) is the condensed Fukui function for an atom in a molecule negative? In particular, our analysis confirms the observation, hitherto based only on computational evidence, that the Hirshfeld partitioning is optimal for obtaining non-negative Fukui functions. We also hypothesize that the strong diagonal dominance of the condensed hardness kernel is sufficient for the non-negativity of the Fukui function. Errors in the partitioning of molecules into atoms and inadequate treatment of correlation are pinpointed as the most likely causes of negative condensed Fukui functions. We conclude by noting that the condensed Fukui functions are, in some respects, more appropriate indicators of a molecular site’s reactivity than the Fukui function itself.
Adhesive bonding in microelectronics and photonics
Authors: Yacobi B. G. , Martin S. , Davis K. , Hudson A. , Hubert M.
Published: July 2002

The use of adhesive bonding in joining of materials with different characteristics is of major importance in a variety of microelectronic and photonic applications. The curing of such adhesives is also of great consequence, with the use of optical radiation for adhesive curing becoming the method of choice in various applications, especially bonding of components in microelectronics and fiber–optic assembly. This article reviews recent advances in the development of adhesives, their applications, and their curing methods using optical radiation; it also includes a brief overview of the adhesion mechanisms.
Field emission characteristics of iridium oxide tips
Authors: Chalamala Babu R. , Reuss Robert H. , Dean Kenneth A. , Sosa Edward , Golden David E.
Published: July 2002

An important issue in field emission vacuum microelectronics is the stability of the field emitters with the residual ambient gas. Particularly important is that the field emitter tips made of refractory metals like molybdenum, niobium and tungsten are susceptible to oxidation. The corresponding metal oxides are insulating and adversely affect the emission current characteristic by increasing the width of the effective tunneling barrier. With this perspective, we studied iridium oxide field emitters to evaluate the characteristics of conductive oxide tips. We studied the field emission characteristics of iridium and thermally prepared iridium oxide field emitters using field emission microscopy and current–voltage measurements. We found that, upon oxidation, the voltage required to achieve the desired emission current desire dropped significantly. In addition, oxidation led to a decrease of emission current fluctuations. The development of stable conductive oxide field emitters should improve the performance of field emitter tips, especially under adverse vacuum conditions.
Structure factor of dilute ring polymers
Authors: Calabrese Pasquale , Pelissetto Andrea , Vicari Ettore
Published: July 2002

We consider ring polymers in good solvents in the dilute limit. We determine the structure factor and the monomer–monomer distribution function. We compute accurately the asymptotic behavior of these functions for small and large momenta and distances by using field-theoretical methods. Phenomenological expressions with the correct asymptotic behaviors are also given.
A monolithic polycapillary focusing optic for polychromatic neutron diffraction applications
Authors: Mildner D. F. R. , Chen-Mayer H. H. , Gibson W. M. , Gnäupel-Herold T. , Miller M. E. , Prask H. J. , Schultz A. J. , Vitt R. , Youngman R.
Published: July 2002

We have conducted measurements at five different thermal neutron wavelengths to determine the transmission characteristics of a tapered monolithic focusing lens with a focal length of 100 mm, suitable for time-of-flight diffraction. Both the width of the focused beam and the intensity gain of the optic increase as a function of wavelength. We have performed similar measurements on a polychromatic beam on a pulsed neutron source, where the results are subject to background from short wavelength neutrons. The use of a beryllium filter shows the increased effective gain for the longer wavelengths at the expense of an increased focused beam width by a factor of 2.
Synchronization of cellular automaton pairs
Authors: Urı́as Jesús , Salazar Gelasio , Ugalde Edgardo
Published: July 2002

The phenomenon of synchronization in pairs of cellular automata coupled in a driver–replica mode is studied. Necessary and sufficient conditions for synchronization in linear cellular automaton pairs are given. The couplings that make a pair synchronize are determined for all linear elementary cellular automata.
Hydraulic theory for a debris flow supported on a collisional shear layer
Authors: Jenkins J. T. , Askari E.
Published: July 2002

We consider a heap of grains driven by gravity down an incline. We assume that the heap is supported at its base on a relatively thin carpet of intensely sheared, highly agitated grains that interact through collisions. We adopt the balance laws, constitutive relations, and boundary conditions of a kinetic theory for dense granular flows and determine the relationship between the shear stress, normal stress, and relative velocity of the boundaries in the shear layer in an analysis of a steady shearing flow between identical bumpy boundaries. This relationship permits us to close the hydraulic equations governing the evolution of the shape of the heap and the velocity distribution at its base. We integrate the resulting equations numerically for typical values of the parameters for glass spheres.
Effective-medium theories for two-phase dielectric media
Authors: Norris A. N. , Sheng P. , Callegari A. J.
Published: July 2002

Two different effective-medium theories for two-phase dielectric composites are considered. They are the effective medium approximation (EMA) and the differential effective medium approximation (DEM). Both theories correspond to realizable microgeometries in which the composite is built up incrementally through a process of homogenization. The grains are assumed to be similar ellipsoids randomly oriented, for which the microgeometry of EMA is symmetric. The microgeometry of DEM is always unsymmetric in that one phase acts as a backbone. It is shown that both EMA and DEM give effective dielectric constants that satisfy the Hashin–Shtrikman bounds. A new realization of the Hashin–Shtrikman bounds is presented in terms of DEM. The general solution to the DEM equation is obtained and the percolation properties of both theories are considered. EMA always has a percolation threshold, unless the grains are needle shaped. In contrast, DEM with the conductor as backbone always percolates. However, the threshold in EMA can be avoided by allowing the grain shape to vary with volume fraction. The grains must become needlelike as the conducting phase vanishes in order to maintain a finite conductivity. Specifically, the grain-shape history for which EMA reproduces DEM is found. The grain shapes are oblate for low-volume fractions of insulator. As the volume fraction increases, the shape does not vary much, until at some critical volume fraction there is a discontinuous transition in grain shape from oblate to prolate. In general, it is not always possible to map DEM onto an equivalent EMA, and even when it is, the mapping is not preserved under the interchange of the two phases. This is because DEM is inherently unsymmetric between the two phases.
Electroluminescence emission from indium oxide and indium-tin-oxide
Authors: Falcony C. , Kirtley J. R. , DiMaria D. J. , Ma T. P. , Chen T. C.
Published: July 2002

Electroluminescence emission from indium-tin-oxide (ITO) and indium oxide films incorporated in a Si-rich SiO2-SiO2-ITO (In2O3) multiple-layer structure is reported. The light emitted has a peak at approximately 3.3 eV for ITO and at 2.6 eV for In2O3. The intensity of the light is found to depend on the applied electric field.
Doping dependence of the Schottky-barrier height of Ti-Pt contacts to n-gallium arsenide
Authors: Broom R. F. , Meier H. P. , Walter W.
Published: July 2002

The barrier height and ideality factor of Ti–Pt contacts on n-type GaAs have been measured in the doping range Nd =3.3×1016 to 3×1018 cm−3. The flat-band barrier height, determined from capacitance-voltage measurements, is found to be independent of Nd whereas the effective barrier height for current transport, defined by the relation for thermionic emission, decreases rapidly at Nd >1×1018 cm−3. The results agree quite well with thermionic field-emission theory.
Elastic moduli of polycrystalline, intermetallic compounds of titanium
Authors: Fleischer R. L. , Gilmore R. S. , Zabala R. J.
Published: July 2002

Ultrasonic measurements have been done of the elastic constants of a number of Ti-containing intermetallic compounds and two-phase alloys that melt above 1400 °C. The data give directly the stiffness and specific stiffness of these materials and are expected to be indicators of their strength and specific strength. The data include Ti compounds with Cr, Ge, Re, Ru, and Sn and two-phase systems consisting of Ti plus an intermetallic compound.
The wavelength dependence of Faraday rotation in magnetic fluids
Authors: Yusuf Nihad A. , Rousan Akram A. , El-Ghanem Hassan M.
Published: July 2002

Measurements of Faraday rotation over the wavelength range 450–633 nm in a dilute Fe3O4 particle magnetic fluid are reported. The results, in agreement with the theory, show an enhancement of Faraday rotation near the wavelength λ≊500 nm.
New Fe-based soft magnetic alloys composed of ultrafine grain structure
Authors: Yoshizawa Y. , Oguma S. , Yamauchi K.
Published: July 2002

The magnetic properties of Fe-Si-B-M (M: additives) alloys prepared by annealing amorphous alloys made by the single roller method over their crystallization temperature have been investigated for development of new Fe-based soft magnetic alloys. Excellent soft magnetic properties were obtained by adding the two elements Cu and Nb to Fe-Si-B alloys. It was found that these new alloys, called ‘‘FINEMET,’’ have an ultrafine grain structure composed of bcc Fe solid solution. They are suitable for many kinds of magnetic components such as saturable reactors, choke coils, and transformers, because they have superior soft magnetic properties and a high saturation flux density, and because different types of B-H hysteresis loops are obtained by magnetic field annealing.