bluuebunny/crossref_metadata_2025 huggingface dataset:
A full-dimensional quantum approach to the vibrational predissociation of tetra-atomic complexes based on the partially-separable time-dependent self-consistent-field approximation
Authors: Garcı́a-Vela A.
Published: July 2002

A full-dimensional time-dependent quantum approach is proposed to study the vibrational predissociation (VP) dynamics of BC–Rg2 (BC=diatomic molecule, Rg=rare-gas atom) clusters. The method applies the partially-separable time-dependent self-consistent-field approximation to express the six-dimensional total wave function as a product of two wave functions, one describing the three stretching modes of the system, the other one describing the three bending modes. The method is tested by simulating the VP of Cl2–Ne2 for the initial Cl2 vibrational excitations v=7–13, and of I2(v=21)–Ne2. The Cl2–Ne2 results are compared to experimental data and earlier simulations. The method is very efficient as compared to previous reduced-dimensional quantum models where the bending modes were not explicitly considered in the dynamics. Good agreement with experiment is found for the resonance lifetimes and Cl2 vibrational distributions for v⩾9, where the bending/stretching couplings are not strong. The model underestimates rotational excitation of the Cl2 fragment, failing to reproduce the Cl2 rotational distributions. In the case of I2 (v=21)–Ne2, the time evolution of the vibrational populations is compared with previous multiconfiguration time-dependent Hartree calculations. The favorable comparison obtained supports the reliability of the method within certain validity conditions.
GaCl molecular dynamics on a GaAs surface during GaAs epitaxial growth
Authors: Ohashi Masafumi , Ozeki Masashi
Published: July 2002

To verify that a high quality layer was grown on a GaAs(001) substrate but not on a GaAs(110) substrate, we studied the dynamical behaviors of GaCl molecules on GaAs(001) 2×4 and GaAs(110) 1×1 surfaces. Trapped GaCl desorbed rapidly from the GaAs(110) 1×1 surface compared to the GaAs(001) 2×4 surface. We estimated the activation energy corresponding to GaCl trapping wells to be 92 kJ/mol on the GaAs(001) 2×4 surface and 54 kJ/mol on the GaAs(110) 1×1 surface. These results suggest that the interaction between GaCl molecules and GaAs surfaces deeply influences the quality of layers grown on GaAs substrates.
Intermediate neglect of differential overlap excited state calculations in periodic boundary conditions: The absorption spectrum of poly(para-phenylene vinylene)
Authors: Weibel Jason D. , Yaron David
Published: July 2002

Much of our current theoretical understanding of the electronic structure of conjugated polymers is based on two-band systems, such as the π-electron model of polyacetylene. But poly(para-phenylene vinylene) (PPV) and many other systems exhibit a number of bands, and this raises new and interesting questions that are beginning to be addressed. Such studies are complicated by the large number of model parameters, and the sensitivity of the predictions to the values used. While the intermediate neglect of differential overlap (INDO) method provides a systematic and tested approach for deriving these parameters from the chemical structure, previous tools could be applied only to oligomers, making it difficult to put the results in the context of other well-studied models. Here, we report INDO calculations on oligomers of PPV as well as long chains with periodic boundary conditions. The long-chain calculations are used to assign the spectral features to transitions between bands, and these assignments are transferred to oligomers by examining how the calculated oligomer spectra evolve with chain length. The effects of various electron–hole symmetry breaking mechanisms on the intensity of peak II (at 3.7 eV) relative to peak III (at 4.7 eV) in the optical absorption spectrum are examined. INDO theory on planar unsubstituted PPV breaks electron–hole symmetry due to inclusion of next-nearest neighbor transfer integrals, giving peak II about 4% of the intensity observed for peak III in the long-chain limit. Alkoxy substitution at the 2 and 5 positions of the phenyl ring is modeled using hydroxy groups (OH-PPV). (This approach is confirmed by replacing the hydroxy substituents with methoxy substituents.) For a planar OH-PPV structure, peak II has 8% of the intensity of peak III. AM1 calculations indicate that in OH-PPV, the vinylene rotates out of the plane of the phenyl rings by 20°, and this raises the intensity of peak II to 15% that of peak III. Finally, finite-chain effects cause oligomers of OH-PPV with between 4 and 8 phenylene units to have a peak II intensity that is 25% that of peak III. Oligomers exhibit additional transitions in the vicinity of peak II, but these have a strong dependence on the length of the oligomer and merge with the lowest-energy peak in the long-chain limit.
Electronic linear energy transfer dependent molecular structural growth in polyethylene terephthalate
Authors: Biswas A. , Lotha S. , Gupta R. , Avasthi D. K. , Paul S. N.
Published: July 2002

Thin films (13 μm) of polyethelene terephthalate (PET) are irradiated by different swift metallic heavy ions (180 MeV Ag14+ and 200 MeV Au15+) with the projectile linear energy transfer (LET) (∼10–14 keV/nm), respectively. LET dependence on the molecular structural changes in PFT irradiated at different ion fluences has been studied by the Fourier transform infrared spectroscopy. The study has revealed that beyond a critical LET entirely different pathways of amorphization beginning with partial recrystallization at lower ion fluence impact occurs in PET, contrary to the earlier established results. At considerably higher LET (∼14 keV/nm), the most characteristic crystalline stretching and bending vibration bands such as at 850 cm−1 (CH2 rocking), 972 cm−1 (C=O stretching), 1341 and 1471 cm−1 (CH2 bending) in PET have shown a significant rise in the respective infrared absorbance intensities upon lower ion fluence (∼1011 ions/cm2) impact. The absence of previously reported unsaturations such as alkynes at both the LET beam used are also observed. Interestingly, the aromatic system also appears to be unstable and participating in the modification process, particularly at the higher LET (∼14 KeV/nm). Possible interpretations are discussed.
Temporal decay measurement of condensate luminescence and its application to characterization of silicon-on-insulator wafers
Authors: Ibuka Shigeo , Tajima Michio
Published: July 2002

We have investigated the temporal decay of luminescence due to electron–hole droplet (EHD) in silicon-on-insulator (SOI) wafers. The formation of EHD is realized under ultraviolet light excitation because of its shallow penetration depth and the presence of the buried oxide which acts as a diffusion barrier to carriers. The decay mechanism of the EHD luminescence is discussed on the basis of its dependence on the excitation intensity, surface/interface condition, temperature, and thickness of the superficial Si film as well as of time-resolved spectroscopic analysis. The dependence can be explained by assuming that the decay is determined by the concentration of free excitons and the intrinsic lifetime of the spatially localized EHD. We demonstrate that the formation of EHD plays an important role in the luminescence decay observed in a superficial thin film. Comparison of the EHD luminescence decay among SOI wafers fabricated by various techniques shows the great potential of this method for characterization of SOI wafers.
Molecular engineering versus energy level alignment: Interface formation between oligothiophene derivatives and a metal substrate studied with photoemission spectroscopy
Authors: Mäkinen A. J. , Hill I. G. , Kinoshita M. , Noda T. , Shirota Y. , Kafafi Z. H.
Published: July 2002

Two series of thin films of oligothiophene derivatives grown on Ag substrates have been studied with photoelectron spectroscopy. The oligothiophenes were end-capped with either electron-deficient (dismesitylboryl) or electron-rich (diphenyltolylamine) moieties to create molecules with electron-accepting or -donating properties, respectively. The position of the highest occupied molecular orbital (HOMO) at the metal/organic interface is found to be strongly dependent on the effective π-conjugation length of the oligothiophenes capped with dimesitylboryl groups, whereas in the oligothiophenes capped with diphenyltolylamine, the position of the HOMO is independent of the molecular length. The difference in the observed HOMO characteristics is attributed to the different make-up of the frontier orbitals in the two molecular series. This will particularly affect the overall energy barrier for charge injection at the conductor/organic interface in a device structure, such as an organic light-emitting diode, utilizing the investigated molecules for carrier injection and transport.
Evolution of quasi-breakdown in thin gate oxides
Authors: Loh Wei Yip , Cho Byung Jin , Li Ming Fu
Published: July 2002

The mechanism for quasi-breakdown (QB) in thin gate oxides was studied using bipolar current stress and unipolar constant current stress. Continual bipolar current stressing on the gate oxide shows two distinct stages of QB—recoverable and unrecoverable QB. During the recoverable QB stage, the gate leakage current recovers to the stress-induced leakage current level upon application of a proper reverse bias. In contrast, no electrical recovery is observed within the unrecoverable QB stage. This stage is characterized by a higher gate leakage current than that of the recoverable QB stage and a very stable gate voltage during stressing. Carrier separation measurements further demonstrate that two different modes of conduction can occur during the recoverable QB stage. In the early stage, Fowler–Nordheim electron tunneling dominates the conduction mechanism although a small hole current is observed. With prolonged electrical stress, the hole direct tunneling current becomes dominant. Based on the aforementioned observations and the monitoring of the generation of oxide traps using the direct-current current–voltage technique, a QB model of positive hole trapping at the anode is proposed.
Metal
/
Alq
3
interactions in organic light emitting devices: The different roles of Mg, Al, and Li atoms
Authors: Zhang R. Q. , Lu W. C. , Lee C. S. , Hung L. S. , Lee S. T.
Published: July 2002

The geometrical and electronic structures of tris(8-hydroxy-quinoline)aluminum (Alq3) molecule interacting with low work function metals of Mg, Al, and Li used in organic light emitting devices have been studied by first-principle calculations using density functional theory. We found that energetically the most favorable complexation for the interacting systems is the metal atom inserting into the core of the Alq3 molecule with the metal atom bridging two oxygen atoms and being coplanar with one of the quinoline ligands. The related various core level Al(2p), O(1s), and N(1s) energy shifts and the characteristic vibrational modes determined consequently are in reasonable agreements with the available experimental data. The cohesion energies (Ec) of the metal-Alq3 complexes increase in the order of Ec(Mg)<Ec(Al)<Ec(Li), among which the Ec(Mg) is considerably smaller, indicating the distinctive feature of Mg–Alq3 interaction from those of Al and Li. The calculated electronic structures show that there are only slight changes in the Alq3 frontier orbitals for Mg–Alq3 and Li–Alq3 complexations, while for the Al insertion into Alq3 considerable electronic localizations are induced, indicating the significantly different roles they may play in metal–organic interface and thus in the device performance.
A vanishing nucleation barrier for the n-alkane rotator-to-crystal transformation
Authors: Herhold Amy B. , King H. E. , Sirota E. B.
Published: July 2002

Alkanes exhibit intermediate plastic–crystalline rotator phases between the liquid and the low-temperature fully ordered crystal. Here we report measurements of the liquid-to-rotator and rotator-to-crystal homogeneous nucleation kinetics using x-ray scattering on emulsified samples. These are the first reported studies of rotator-to-crystal homogeneous nucleation, and they show an anomalous temperature dependence of nucleation. Through use of mixed alkanes, we probe this over a wide temperature range. For three compositions along the C19H40–C20H42 binary, we find that the liquid-to-rotator interfacial free energy, calculated from the nucleation barrier in classical nucleation theory, is the same for all samples. However, the rotator-to-crystal interfacial free energy is both considerably smaller and strongly dependent on composition and temperature. As the temperature range of the rotator phase widens due to chain mixing, both the thermodynamic transition temperature as well as the interfacial energy for the rotator-to-crystal transition decrease. The interfacial energy reaches half its initial value at the lowest point measured, and it extrapolates to zero at a temperature about 5 K lower. Surprisingly, this corresponds to a situation where the latent heat itself is still finite. Typically, first-order phase transitions exhibit hysteresis due to a kinetic barrier. Our data suggests the unusual case of zero hysteresis for a first-order transition due to the disappearance of the homogeneous nucleation barrier.
Ab initio simulations of oxygen atom insertion and substitutional doping of carbon nanotubes
Authors: Mann David J. , Halls Mathew D.
Published: July 2002

Ab initio molecular dynamics simulations have been used to study the subpicosecond chemistry and dynamics of hyperthermal O(3P) collisions with single-walled carbon nanotubes, with a particular focus on insertion and substitutional doping. The barrier for inserting an O(3P) atom through the center of a hexagonal carbon ring was determined to be 13.7 eV, compared to an estimated dynamic effective barrier of 15.9 eV and a threshold energy on the order of 15–20 eV. Under similar conditions, collisions with a nanotube carbon atom are observed to generate oxygen substitutionally doped nanotubes and heptagonal carbon ring defects. At incident energies above the insertion threshold, an oxygen atom that passes through the center of a hexagonal carbon ring has the tendency to become accelerated and pass completely through the nanotube. Under thermal conditions the O(3P) atom binds to the nanotube without a barrier, leading to one of two products: an epoxide, and an adatom oxygenated nanotube. Tube curvature effects result in an increase of the epoxide binding energy with a decrease in tube diameter. However, no noticeable effects of tube diameter on insertion were established from the simulations.
Electromagnetic ion temperature gradient modes of tearing mode parity in high β sheared slab plasmas
Authors: Gao Zhe , Dong J. Q. , Liu G. J. , Ying C. T.
Published: July 2002

The ion temperature gradient modes of tearing mode parity are investigated for arbitrary β (=plasma pressure/magnetic pressure) plasmas in a sheared slab. Under the low β limit, the results agree with Reynders’ conclusion that the lowest order (l=1) mode of tearing mode parity persists after the fundamental mode (l=0) of drift mode parity is completely stabilized by finite β [J. V. M. Reynders, Phys. Plasmas 1, 1953 (1994)]. However, when the effects of the magnetic gradient drift and the coupling to the compressional Alfvén waves are included, the l=0 mode is more difficult to stabilize than the l=1 mode. It is also shown that the l=1 mode is much easier to stabilize by the magnetic shear than the l=0 mode. Generally, the l=1 mode grows faster in the low magnetic shear and low β regime while the l=0 mode is the dominant eigenmode of ion temperature gradient instability in high β plasmas.
The singlet–triplet energy gap in organic and Pt-containing phenylene ethynylene polymers and monomers
Authors: Köhler A. , Wilson J. S. , Friend R. H. , Al-Suti M. K. , Khan M. S. , Gerhard A. , Bässler H.
Published: July 2002

We have studied the evolution of the T1 triplet excited state in an extensive series of phenylene ethynylene polymers and monomers with platinum atoms in the polymer backbone and in an analogous series of all-organic polymers with the platinum(II) tributylphosphonium complex replaced by phenylene. The inclusion of platinum increases spin–orbit coupling so T1 state emission (phosphorescence) is easier to detect. For both, the platinum-containing polymer series and for the all-organic polymers, we find the T1 state to be at a constant separation of 0.7±0.1 eV below the singlet S1 state. It is not possible to change this singlet–triplet splitting by altering the size or the charge-transfer character of the polymer repeat unit or by changing the electron delocalization along the polymer backbone. The S1–T1 gap can be increased by confining the S1 state in oligomers and monomers.
Quantum chemical study of the mechanism of aluminum oxide atomic layer deposition
Authors: Widjaja Yuniarto , Musgrave Charles B.
Published: July 2002

The atomic layer deposition of alumina using water and trimethylaluminum is investigated using the density functional theory. The atomistic mechanisms of the two deposition half-cycles on Al–CH3* and Al–OH* surface sites are investigated. Both half-cycle reactions proceed through the formation of an Al–O Lewis acid-base complex followed by CH4 formation. The Al–O complexes are relatively stable, with formation energies between 0.57 and 0.74 eV. The CH4 formation activation energies range from 0.52 to 0.91 eV and both half-cycle reactions are exothermic with overall enthalpies of reaction between 1.30 and 1.70 eV.
The dynamics of spatiotemporal modulations
Authors: Aubry Nadine , Lima Ricardo
Published: July 2002

The modulational instability of traveling waves is often thought to be a crucial point in the mechanism of transition to space–time disorder and turbulence. The aim of this paper is to study the effect of spatiotemporal modulations on some dynamics u0(x,t), which may occur as an instability process when a control parameter varies, for instance. We analyze the properties of the modulated dynamics of the form g1(x)g2(t)u0(x,t) compared to those of the reference dynamics u0(x,t), using operator theory. We show that, if the reference dynamics is invariant under some space–time symmetry in the sense of Ref. [J. Nonlinear Sci. 2, 183 (1992)], the modulation has the effect of either deforming this symmetry or breaking it, depending on whether the corresponding operator remains unitary or not. We also demonstrate that the smallest Euclidean space containing the modulated dynamics has a dimension smaller than or equal to the smallest Euclidean space containing u0(x,t). The previous results are then applied to the case of modulated uniformly traveling waves. While the spatiotemporal translation invariance of the wave never persists in the presence of a modulation, the existence of a spatiotemporal symmetry depends on the resonance of the Fourier sidebands due to the modulation. In case of nonresonance, a spatiotemporal symmetry exists and is explicitly determined. In this situation, the modulated wave and the carrier wave have the same spectrum (up to a normalization factor), the same entropy, and the spatial (resp., temporal) two-point correlation is deformed only by the spatial (resp., temporal) modulation.
A method for visualization of ventricular fibrillation: Design of a cooled fiberoptically coupled image intensified CCD data acquisition system incorporating wavelet shrinkage based adaptive filtering
Authors: Witkowski Francis X. , Leon L. Joshua , Penkoske Patricia A. , Clark Robert B. , Spano Mark L. , Ditto William L. , Giles Wayne R.
Published: July 2002

The measurement of cardiac transmembrane potential changes with voltage sensitive dyes is in increasing use. Detection of these very small fluorescent alterations using large multiplexed arrays, such as charge coupled device (CCD) cameras at high sampling rates, has proven challenging and usually requires significant averaging to improve the signal-to-noise ratio. To minimize the damage of living tissue stained with voltage sensitive dyes, excitation photon exposure must be limited, with the inevitable consequence of diminishing the fluorescence that is generated. State-of-the-art high frame rate CCD cameras have read noise levels in the 5–10 e− rms range, which is at least two orders of magnitude above that required to detect voltage sensitive dye alterations at individual pixels corresponding to 1 mm2 heart regions illuminated with levels of 100 mW/cm2 at frame rates approaching 1000 frames/sec. Image intensification is thus required prior to photon quantification. We report here the development of such a data acquisition system using commercially available hardware. Additionally, in the past ten years, a mathematical theory of multiresolution has been developed, and new building blocks called wavelets, allow a signal to be observed at different resolutions. Wavelet analysis also makes possible a new method of extricating signals from noise. We have incorporated spatially adaptive filters based on wavelet denoising of individual pixels to significantly reduce the multiple noise sources present in the acquired data.
Separation of losses in low-alloy, nonoriented electrical steels
Authors: Stephenson E. T.
Published: July 2002

A recent study on low-alloy, nonoriented, semiprocessed electrical steels indicated that at 60 Hz and 15 and 17 kG the anomalous loss per cycle Wa increased linearly with t2/ρ, i.e., Wa=Wao+kt2/ρ, where t is sheet thickness, ρ is resistivity, and Wao and k are empirical constants. The present study explores the above relationship over inductions B of 5 to 17 kG and frequencies f of 20 to 200 Hz. A separation of hysteresis loss Wh into synchronous and asynchronous components is also considered. Six laboratory heats with Si+Al<1.1 wt. % were hot and cold rolled to 0.035, 0.07, and 0.11 cm. The sheets were annealed, critically strained, and decarburized. Longitudinal strips were tested with dc as well as ac current in a 25-cm Epstein frame. The above linear equation was generally valid. At each combination of B and f, the intercepts Wao were always positive, but the slopes k were negative at 5 and 10 kG and positive at 15 and 17 kG. Furthermore, a fifth of the Wa values were negative. The negative Wa and k were associated with lack of flux penetration. Wh, but not Wa, increased with increasing oxygen and decreasing grain size, and these effects resided in the B1.6 component of Wh.
Valence fluctuation and Kondo effect in noncubic Ce-transition metal compounds (invited)
Authors: Gignoux D. , Gomez-Sal J. C.
Published: July 2002

In the compounds between cerium and transition metals, with an almost filled d band, the electronegativity difference between the constituents can lead to a position of the Fermi level compared to the 4f state which strongly depends on the stoichiometry. The following states of cerium are then expected: magnetic 3+state, 3+state with Kondo effect, and intermediate valence state. The various noncubic compounds of the Ce-Ni and Ce-Pt systems, which are presented in this paper, are very interesting because all these Ce states are observed. The interest of these alloys is enhanced because of their low symmetry which allows one to observe the anisotropic effects resulting from the hybridization of the 4f state with the conduction band; these properties cannot be observed in the cubic compounds such as CeSn3, CePd3, CeAl2. Because of the large difference of behavior between the isomorphous equiatomic CeNi and CePt compounds, a continuous evolution from a Ce3+ magnetic state to an intermediate valence state is observed in the CeNixPt1−x pseudobinary compounds. These compounds are among the best examples of a ferromagnetic dense Kondo system in which the variation of the Curie temperature TC and of the Kondo temperature TK as a function of the Ni content are similar to those predicted in the Kondo lattice model when ‖Γn(EF)‖ increases (Γ is the negative coupling constant between the 4f shell and the conduction band).
Drift mobility, electron trapping, and diffusion-limited kinetics in sulfur-sensitized AgBr microcrystals
Authors: Deri R. J. , Spoonhower J. P.
Published: July 2002

The room-temperature photoelectron kinetics in 1.3- and 0.4-μm AgBr octahedral emulsion microcrystals have been investigated for time scales ranging from several nanoseconds to several microseconds after exposure. Two first-order decay processes were observed in 1.3-μm octahedra. The fast process is attributable to relatively shallow electron trapping that equilibrates with thermal detrapping after ∼60 nsec; the longer decay may involve ionic processes. Sulfur sensitization of the emulsion enhanced the fast electron decay rate (decay time=14 nsec). At higher levels of sulfur sensitization, the decay rate did not change with increasing sulfur levels; such behavior can indicate diffusion-limited surface trapping. A bulk-to-surface electron drift mobility of 0.8 cm2/V sec on nanosecond time scales has been deduced from these kinetics. Implications of the data to current theories of photographic image formation and sulfur sensitization are discussed.
Time-dependent excitation in high- and low-frequency chlorine plasmas
Authors: Flamm Daniel L. , Donnelly Vincent M.
Published: July 2002

Time-resolved voltage, current and optical emission from Cl, Cl+, Cl+2, and small amounts of added Ar were studied in 0.3 Torr chlorine discharges at 13.2 MHz and 220 kHz, above and below the ion transit frequency (ITF). Emissions, measured as a function of position, were deconvoluted to correct for the finite fluorescence lifetimes and extract the electron impact excitation rates. At 220 kHz, emission falls to zero at the voltage zero point crossings due to electron energy relaxation and attachment. Emission and excitation in a sheath peak sharply about 70 ns before electrode voltage reaches its positive maximum and are stimulated by electron current. Emission then falls to zero and reaches a second maximum ∼300 ns after the electrode voltage reaches its negative peak. This emission is excited by a secondary electron avalanche from ions crossing the sheath. Excitation maxima in the center of the discharge lag the voltage peaks by ∼25°. Analyses of time and spatially resolved ratios of emission from Cl and Ar show that actinometric techniques can give an erroneous measure of atom concentrations unless the viewing area and phase are taken into account. At 13.2 MHz, ions cannot respond to the time-varying fields, and excitation in the sheath peaks in phase with the anodic part of the cycle with minimal excitation during the cathodic phase. Emission from Cl+ was not observable. The lack of high-energy ion bombardment drastically reduces secondary emission and excitation in the cathodic sheath. In the center of the discharge, excitation peaks twice per cycle, but unlike the low-frequency case, it does not fall to zero. The electron energy relaxation frequency (νu) is estimated to be 64 MHz.
Diagnostic methods for investigation of collective ion acceleration
Authors: Markovits M. , Blaugrund A. E.
Published: July 2002

A method is described for investigating the spatial and temporal characteristics of the potential well held responsible for collective acceleration of ions by relativistic electron beams. The method is based on direct probing of the potential well with an externally generated ion beam probe, observing the deflection of this probe with a scintillator-image converter streak camera system. In addition, time-resolved magnetic spectrometer measurements of the collectively accelerated ions are described. Time resolution is achieved by using a plastic scintillator and a fast streaking camera as the spectrometer’s detector. Examples of measurements are presented and the potential and limitations of the methods are discussed.
Use of digital image analysis to estimate fluid permeability of porous materials: Application of two-point correlation functions
Authors: Berryman James G. , Blair Stephen C.
Published: July 2002

Scanning electron microscope images of cross sections of several porous specimens have been digitized and analyzed using image processing techniques. The porosity and specific surface area may be estimated directly from measured two-point spatial correlation functions. The measured values of porosity and image specific surface were combined with known values of electrical formation factors to estimate fluid permeability using one version of the Kozeny-Carman empirical relation. For glass bead samples with measured permeability values in the range of a few darcies, our estimates agree well (±10–20%) with the measurements. For samples of Ironton-Galesville sandstone with a permeability in the range of hundreds of millidarcies, our best results agree with the laboratory measurements again within about 20%. For Berea sandstone with still lower permeability (tens of millidarcies), our predictions from the images agree within 10–30%. Best results for the sandstones were obtained by using the porosities obtained at magnifications of about 100× (since less resolution and better statistics are required) and the image specific surface obtained at magnifications of about 500× (since greater resolution is required).
Partial epitaxial growth of HfSi2 films grown on silicon
Authors: Chang C. S. , Nieh C. W. , Chen L. J.
Published: July 2002

Epitaxial HfSi2 has been grown locally on (001)Si. In samples annealed in one step at 1100 °C or in two steps at 450–1100 °C, islands of HfSi2 of 0.8 μm in average grain size were found to cover about 40% of the surface area. About 70% of the disilicide in areal fraction was found to be epitaxial silicide, 0.6–1.2 μm in size. The orientation relationships between epitaxial HfSi2 and (001)Si substrate were analyzed to be [010]HfSi2∥[001]Si and (002)HfSi2∥(220)Si (with about 1° misorientation). Interfacial dislocations, 75 Å in average spacing, were identified to be of edge type with 1/2〈110〉 Burgers vectors. No HfSi2 epitaxy was found to form on (111)Si.
Charge transport in viscous vortex flows
Authors: Lean Meng H. , Domoto Gerald A.
Published: July 2002

The physics of charge transport in a bent channel is analyzed using a vorticity transport model for fluid flow to better represent recirculation conditions at corners. The electrostatics is governed by the coupled nonlinear set comprised of the Poisson and current continuity equations. Electrostatic body forces are assumed to have a negligible effect on the flow field. The boundary element method (BEM) is used to solve the vorticity-stream function integral equation formulation for the flow field. Two advantages of this method are no requirement for discretization of the interior, and no need for specification of vorticity boundary conditions. The Poisson equation is also solved using the BEM. Influence coefficient matrices are used to facilitate both reanalysis and rapid calculations of field parameters on a fixed grid. This technique allows desired quantities to be computed by matrix multiplication rather than time-consuming repetitive integrations. The current continuity equation is solved using the method of characteristics. The solution algorithm alternates between the methods for field and source solution. Sample results for full ON and full OFF of the ion current are discussed for both inviscid potential and viscous flows.
Beam deflection phenomena in a Pulselac post-acceleration gap
Authors: Johnson D. J. , Lockner T. R.
Published: July 2002

The post-acceleration of a 400-keV, 10-kA proton beam by a 200-kV magnetically insulated gap is investigated. The defections from self- and applied E and B fields are measured and compared to calculated values. Several important principles of post-acceleration gap operation are observed for the first time that are important for multigap operation. First, the beam is inadequately space-charge neutralized without gas puffs or preformed plasma in regions of transverse applied-B field to allow efficient transport. The beam is also noncurrent neutralized in these regions. Second, the applied-B field defines equipotential surfaces in the gap allowing the voltage on the gap to steer and focus the beam, and it has an axial field component that acts like a pair of solenoidal lenses to focus the beam. It is also pointed out how azimuthal asymmetries in the beam current density and cathode plasma cause beam self-field asymmetries that lead to growth of beam emittance. Finally, we discuss a model which can be used to extrapolate the experimental results to multigap operation.
Fourier transform analysis of light-beam-induced current profiles
Authors: Mittiga A. , Capizzi M.
Published: July 2002

The observation that the Fourier transform of light-beam-induced current profiles can be expressed in a closed form leads to a new method for their analysis. This method takes full advantage of the knowledge of the profile line shape and does not depend critically on the details of the current asymptotic behavior. The Fourier transform can be quickly calculated, making possible the use of standard minimization procedures on a personal computer. The influence of the experimental errors on the estimate of the diffusion length and of the recombination velocity is evaluated.
Electrical conduction in clay bearing sandstones at low and high salinities
Authors: Sen Pabitra N. , Goode Peter A. , Sibbit Alan
Published: July 2002

We find the formula σ=φm[σω+AQv/ (1+CQv/σw)] that embodies the fact that the rock conductivity σ is a nonlinear function of water conductivity σw, can fit data on 140 cores rather well with m≊2, A=1.93×m (mho/m)(l/mol) and CQv=0.7 (mho/m). The observed curvature at low salinity is due to an interplay of tortuosity and water conductivity. Empirical correlation shows that m increases with the clay content, as the tortuosity increases with the clay content. Thus, the conductivity of a fully water saturated clay bearing (shaly) sand is completely determined from porosity φ, charge density Qv, and water conductivity σw.
X-ray diffraction determination of stress in magnetron-sputtered Permalloy films
Authors: England C. D. , Falco C. M. , Aboaf J. A.
Published: July 2002

The stresses in magnetron-sputtered films depend on the deposition parameters. Magnetron-sputtered Permalloy (Ni 81%, Fe 19%) films were deposited onto fused quartz substrates using varying deposition parameters, including argon pressure, substrate bias, deposition rate, and film thickness. A Bragg–Brentano diffractometer was used to find the average stress in the films. A Seemann–Bohlin diffractometer was used to measure the strain either parallel or perpendicular to the magnetic easy axis in the Permalloy film. Kroner x-ray elastic constants were used to calculate the stresses from the strains determined by x-ray diffraction techniques. The average stress in the magnetron-sputtered Permalloy films was highly tensile but could be reduced by the appropriate combination of deposition parameters. The strain difference could also be modified by changing the deposition parameters.
The use of circular birefringent material as an interstage element of a linear birefringent network
Authors: Chakraborty Biswajit
Published: July 2002

In a previous communication the transmission characteristics of both narrow bandpass filters and birefringent band suppression filters (BBSF) have been investigated where a retarder-rotator combination is used at each stage. The tuning procedures of these fixed-type filters have also been studied where the thickness of each retarder plate was the same and the thickness of each rotator plate was the same. In the present paper we first study the variation of intensity transmittance with the variation of number of stages of the previously proposed filter. A simple way of synthesizing a BBSF system by using a number of equal thickness linear retarder plates is also indicated. Lastly we study the transmission characteristics and the tuning procedures of both narrow bandpass filters and birefringent band suppression filters where the retarder and the rotator plates of unequal thicknesses are used for their construction.
High-Q magnetostatic surface wave planar yttrium iron garnet resonator
Authors: De D. K.
Published: July 2002

Experimental results on the magnetostatic surface wave resonance characteristics of planar yttrium iron garnet resonators are given. Single-mode resonance with wide tunability range and high Q have been achieved. Q as high as 3500 and insertion loss as low as 13 dB with off-resonance rejection as high as 15 dB have been achieved at 6 GHz. A microwave oscillator built with the resonators and a laboratory-made hybrid amplifier provided oscillations in the range 3–5.3 GHz with phase noise of −105 dBc/Hz at 10 KHz offset from the carrier frequency.
Initial solid-state reactions between crystalline Sb and amorphous Si thin films
Authors: Gong S. F. , Hentzell H. T. G. , Robertsson A. E.
Published: July 2002

Bilayers of Sb and Si thin films were deposited at room temperature on a thin (20–30 nm) Si3N4 film using electron-beam evaporation. The solid-state reactions in the bilayers were investigated using transmission electron microscope (TEM) during in situ annealing and Auger electron spectroscopy (AES). The reactions resulted in either an amorphous Sb-Si (a-Sb-Si) alloy or caused crystallization of amorphous silicon (a-Si) at low temperatures, depending on the film thickness of an a-Si layer as well as the heating rate. As predicted from the phase diagram, no compounds between Sb and Si were observed. The initial intermixing of Sb and a-Si was found to be anomalously fast.
Use of the longitudinal magneto-optical Kerr effect to study nonmagnetic/magnetic bilayers
Authors: Dillon J. F. , Gyorgy E. M. , Hellman F. , Walker L. R. , Fulton R. C.
Published: July 2002

We report the results of longitudinal magneto-optical Kerr effect (LMKE) measurements on bilayers consisting of a thick permalloy film covered by varying thicknesses of Nb. The LMKE pertains to situations in which light with a finite angle of incidence is reflected from a plane surface, and M in the sample is parallel both to the plane of incidence and the surface. Either s- or p-linearly polarized 6328-Å light is incident on the sample at 60° to the normal. The two components of the complex Kerr rotation are measured with a modulation technique utilizing a λ/4 plate and a Faraday rotator driven by dc and 2000-Hz signals. With cycling of an in-plane magnetic field, square magneto-optical hysteresis loops are observed in both rotation and ellipticity. There is a change in sign, as well as a decrease in the magnitude of the switching discontinuity in these quantities with increasing overlayer thickness. These results are very similar to those of the corresponding idealized boundary value problem. The technique gives experimental information on the magnetic properties of films buried below a nonmagnetic material or of thin films bounded on both sides by a nonmagnetic material. Further, it gives a probe of the phase and intensity of optical frequency electric fields below a reflecting surface.
Transverse spin freezing in a-Fe92Zr8 (invited)
Authors: Ryan D. H. , Strom-Olsen J. O. , Provencher R. , Townsend M.
Published: July 2002

High field magnetization measurements combined with Mössbauer spectroscopy with and without an external magnetic field have been used to study the evolution of magnetic order from wandering axis ferromagnet to asperomagnet at the transverse spin freezing transition (Txy=76±3 K) in amorphous Fe92Zr8. Below Txy, the total iron moment derived from the Mössbauer spectra is larger than its z component, determined by magnetization measurements. Mössbauer spectra measured with an external field of 3 T parallel to the γ beam show absorption due to the ΔmI=0 transitions only below Txy and confirm ordering of transverse spin components. Unlike the irreversibility in the dc susceptibility, the transition is not suppressed by the large measuring field, and therefore the fundamental nature of the transition is being probed rather than a sympton or side effect. The transition may be broadly interpreted in terms of a mean-field mixed exchange model, although the detailed behavior is somewhat different. A phase diagram describing the effect of exchange frustration on magnetic ordering is presented, and the observed properties of the phases are outlined.
Magnetic structure of Tb-Fe films with an artificially layered structure
Authors: Yamauchi Kazushi , Habu Kazutaka , Sato Noboru
Published: July 2002

The magnetic structure of Tb-Fe films with an artificially layered structure has been investigated by measuring the temperature dependence of the magnetization of the films. Ferrimagnetic coupling between Tb and Fe through the interface was explicitly observed up to about 9-Å Tb and 10-Å Fe layers. Films with thinner Tb and Fe layers than these thicknesses are composed of only ferrimagnetically coupled Tb-Fe regions. Films with thicker layers of Tb and Fe are composed of ferrimagnetically coupled Tb-Fe, ferromagnetic Fe, ferromagnetic Tb, and/or magnetically compensated Tb regions. The Tb-Fe films exhibit various temperature dependencies of the magnetization corresponding to these magnetic structures.
Phenomenology and interpretation of the Barkhausen effect in ferromagnetic materials (invited)
Authors: Alessandro B. , Beatrice C. , Bertotti G. , Montorsi A.
Published: July 2002

After reviewing the main aspects of Barkhausen effect (BE) phenomenology and the principal theoretical models proposed for its interpretation, a new experimental and theoretical approach is presented. It is shown that definite progress in the comprehension of BE phenomenology is achieved by restricting BE experiments to the central part of the hysteresis loop, where BE behaves as a stationary stochastic process and domain wall motion is the dominant magnetization process. From the theoretical point of view, a critical reconsideration of literature results on domain wall dynamics is proposed, which leads to a Langevin equation for BE, whose solution provides definite analytic expressions for BE power spectrum and amplitude probability distribution. A comparison between the obtained theoretical predictions and typical experimental results is presented and discussed.
The doping concentration dependence of zinc and tin in InGaAs
Authors: Su Y. K. , Wu M. C. , Huang C. H. , Chiu B. S.
Published: July 2002

In0.53Ga0.47As epitaxial layers doped with Zn and Sn were grown on InP (100) substrates by liquid-phase epitaxy with a 5 °C supersaturation temperature. The physical properties of the doped layers have been investigated. The peak energy of (D0,A0) pair band emission in Zn-doped InGaAs layers decreases with a linear slope of 0.7×10−8 eV cm as the cube root of net hole concentration is increased. In Sn-doped layers, the photoluminescent peak wavelength decreases with increasing electron concentration due to the Burstein–Moss shift and band-tailing effects. The relative peak intensity deteriorates in highly doped layers due to the formation of complex precipitates.
Ionization and current growth in N2 at very high electric field to gas density ratios
Authors: Gylys V. T. , Jelenković B. M. , Phelps A. V.
Published: July 2002

Measurements and analyses have been made of electron impact ionization and of current growth in pulsed, low-current, prebreakdown discharges in parallel-plane geometry in N2 at very high electric field to gas density ratios E/n and low products of the gas density n and electrode separation d. The E/n range and nd ranges were 1<E/n<52 kTd, where 1 Td=10−21 V m2 , and 6×1018 <nd<3×1020 m−2 (or 0.02<pd<1 Torr cm, where p is pressure) and were below breakdown values. Measurements were made of the transported charge on the time scales of electron transit, ion transit, and metastable decay. Measurements were also made of the growth of steady-state discharge currents as a function of discharge voltage. The contributions of avalanches resulting from ion- and metastable-induced secondary electrons were determined from the ratio of electron-excited N+2 391.4-nm emission integrated over all avalanches to the integrated emission during the laser-initiated electron pulse. Calculations based on ionization by electrons only show good agreement with the measured charge transported during the electron avalanche and with the current multiplication. Analysis of 391.4-nm emission data and of charge transported at E/n≥10 kTd and voltages near breakdown using the assumption of electron impact ionization leads to large apparent secondary electron yields at the cathode which increase significantly with nd. At E/n≥10 kTd, and for the voltages of our experiments, fast N2 produced in charge transfer collisions of N+2 with N2 appears to provide most of the secondary electrons. The apparently small contribution of ionization of N2 by N2, N+2 , N, and N+ is consistent with our assumption that each of these ionization cross sections is equal to that for N2-N2 collisions and so are much smaller than for Ar+ -Ar or Ar-Ar collisions.
Role of deep levels and interface states in the capacitance characteristics of all-sputtered CuInSe2/CdS solar cell heterojunctions
Authors: Santamaria J. , Diaz G. Gonzalez , Iborra E , Martil I. , Sanchez-Quesada F.
Published: July 2002

All-sputtered CuInSe2/CdS solar cell heterojunctions have been analyzed by means of capacitance-frequency (C-F) and capacitance-bias voltage (C-V) measurements. Depending on the CuInSe2 layer composition, two kinds of heterojunctions were analyzed: type 1 heterojunctions (based on stoichiometric or slightly In-rich CuInSe2 layers) and type 2 heterojunctions (based on Cu-rich CuInSe2 layers). In type 1 heterojunctions, a 80-meV donor level has been found. Densities of interface states in the range 1010–1011 cm2 eV−1 (type 1) and in the range 1012–1013 cm−2 eV−1 (type 2) have been deduced. On the other hand, doping concentrations of 1.6×1016 cm−3 for stoichiometric CuInSe2 (type 1 heterojunction) and 8×1017 cm−3 for the CdS (type 2 heterojunction) have been deduced from C-V measurements.
Optical waveguide preform fabrication: Silica formation and growth in a high-temperature aerosol reactor
Authors: Biswas Pratim , Li Xiaoming , Pratsinis Sotiris E.
Published: July 2002

Silica particle formation and growth in a high-temperature aerosol reactor has been studied. A novel sampling probe has been used to obtain the evolution of the silica particle size distribution in the reactor. A simple model approximating the particle size distribution by a unimodal lognormal function was used to describe the aerosol behavior in the reactor. Good agreement of the integral properties of the aerosol size distribution was obtained between theory and experiment. Reasonable agreement was also obtained with the prediction of more sophisticated models for aerosol dynamics.
Full range analytic approximations for Fermi energy and Fermi–Dirac integral ℱ−1/2 in terms of ℱ1/2
Authors: Chang T. Y. , Izabelle A.
Published: July 2002

New full range analytic approximation formulas for efficient calculation of the reduced Fermi energy η and the Fermi–Dirac integral ℱ−1/2 in terms of u=ℱ1/2, which is simply related to the free-carrier concentration and temperature, are presented. The maximum fractional error is 0.033% for η and 0.05% for ℱ−1/2, while the maximum absolute error for η is 0.000 41. These formulas facilitate analyses of carrier transport and device characteristics in structures involving smaller band-gap semiconductors.