bluuebunny/crossref_metadata_2025 huggingface dataset:
A comprehensive photoabsorption, photoionization, and shake-up excitation study of the C 1s cross section of benzene
Authors: Rennie E. E. , Kempgens B. , Köppe H. M. , Hergenhahn U. , Feldhaus J. , Itchkawitz B. S. , Kilcoyne A. L. D. , Kivimäki A. , Maier K. , Piancastelli M. N. , Polcik M. , Rüdel A. , Bradshaw A. M.
Published: July 2002

The absolute photoabsorption cross section of benzene (C6H6), encompassing the C 1s−1 π*e2u resonance, the C 1s threshold, the satellite thresholds, and extending up to 800 eV, has been measured using synchrotron radiation. Measurements of the discrete absorption structure from below the C 1s ionization threshold have been performed at high resolution. In order to unambiguously assign all structure present in the photoabsorption cross section, C 1s photoelectron spectra were measured from the C 1s threshold region up to 350 eV along with satellite spectra. The C 1s−1 single-hole and the satellite cross sections have been derived in absolute units, and their angular distributions have been determined. Resonant and normal Auger spectra were taken on the main features of the photoabsorption and single-hole cross sections. From the best resolved photoelectron spectra the underlying structure in the asymmetric benzene photoelectron peak can be partly disentangled. The experimental data show that at least two vibrational modes play a role in the C 1s photoelectron spectrum. The behavior of the investigated shake-up structure closely resembles that of ethene and ethyne, where the satellite bands due to π→π* excitations gain intensity towards threshold, an observation which may be attributed to conjugate shake-up processes. These processes lead to a significant contribution of the satellite intensity to the production of the absorption features traditionally assigned to the carbon shape resonances in benzene. An EXAFS analysis of the wide range oscillations present on the photoabsorption cross section has been performed, and reveals the C–C nearest-neighbor distance.
High pressure-jump apparatus for kinetic studies of protein folding reactions using the small-angle synchrotron x-ray scattering technique
Authors: Woenckhaus J. , Köhling R. , Winter R. , Thiyagarajan P. , Finet S.
Published: July 2002

An apparatus suitable for pressure-jump experiments with variable pressure amplitude and a fast response time to facilitate time-resolved small-angle x-ray scattering at synchrotron facilities is described. The high pressure-jump apparatus is capable of performing bidirectional pressure jumps at a time resolution as high as 5 ms. The high pressure sample cell presented has flat diamond windows and is suited for pressures up to 0.7 GPa operating in the temperature range from −40 to 120 °C. The cell is designed for investigating biological and other soft condensed matter materials. Modifications on the window supports allow also simultaneous wide-angle x-ray scattering data to be taken. We have used the equipment to study the kinetics of protein folding reactions. The performance of the apparatus is demonstrated by presenting data on the pressure-induced un/refolding reaction of the water-soluble protein SNase WT.
Crucial role of metal surface in enhanced transmission through subwavelength apertures
Authors: Grupp D. E. , Lezec H. J. , Ebbesen T. W. , Pellerin K. M. , Thio Tineke
Published: July 2002

In order to understand the details of the extraordinary enhancement of the optical transmission through subwavelength holes in metal films, we have fabricated perforated free-standing metal films where the nature of the bulk and surfaces can be controlled independently. We find that the transmission enhancement depends only on the dielectric properties of the metal within a skin depth of the two in-plane surfaces and not on those of the film core, nor on the metal constituting the hole walls. This provides direct and conclusive evidence that the phenomenon is mediated by surface plasmons.
Water pair potential of near spectroscopic accuracy. I. Analysis of potential surface and virial coefficients
Authors: Mas Eric M. , Bukowski Robert , Szalewicz Krzysztof , Groenenboom Gerrit C. , Wormer Paul E. S. , van der Avoird Ad
Published: July 2002

A new ab initio pair potential for water was generated by fitting 2510 interaction energies computed by the use of symmetry-adapted perturbation theory (SAPT). The new site–site functional form, named SAPT-5s, is simple enough to be applied in molecular simulations of condensed phases and at the same time reproduces the computed points with accuracy exceeding that of the elaborate SAPT-pp functional form used earlier [J. Chem. Phys. 107, 4207 (1997)]. SAPT-5s has been shown to quantitatively predict the water dimer spectra, see the following paper (paper II). It also gives the second virial coefficient in excellent agreement with experiment. Features of the water dimer potential energy surface have been analyzed using SAPT-5s. Average values of powers of the intermolecular separation—obtained from the ground-state rovibrational wave function computed in the SAPT-5s potential—have been combined with measured values to obtain a new empirical estimate of the equilibrium O–O separation equal to 5.50±0.01 bohr, significantly shorter than the previously accepted value. The residual errors in the SAPT-5s potential have been estimated by comparison to recent large-scale extrapolated ab initio calculations for water dimer. This estimate—together with the dissociation energy D0 computed from SAPT-5s—leads to a new prediction of the limit value of D0 equal to 1165±54 cm−1, close to but significantly more accurate than the best empirical value.
Streaming and removal forces due to second-order sound field during megasonic cleaning of silicon wafers
Authors: Deymier P. A. , Vasseur J. O. , Khelif A. , Djafari-Rouhani B. , Dobrzynski L. , Raghavan S.
Published: July 2002

We calculate the second-order streaming force in a fluid in the vicinity of the solid/fluid interface for two systems of importance in the technology of megasonic cleaning of silicon wafers. The first system consists of a single planar interface between a solid elastic medium representing silicon and a viscous fluid, namely water. The second system accounts for the finite thickness of silicon wafers. It is composed of one silicon slab (wafer) immersed in water. The components of the streaming force parallel and normal to the silicon/water interface are determined as functions of frequency and wave vector of the incident acoustic wave. The normal component of the streaming force is used to calculate the removal force defined as the net force perpendicular to the solid/fluid interface acting on a spherical contaminant particle adhering to the silicon surface. The removal force is too small to remove submicron particles. In contrast the streaming force parallel to the solid/fluid interface may remove particles by pushing or rolling them. The streaming force is shown to be very sensitive to the angle the incident acoustic wave makes with the silicon/water interface.
Development of a horizontal component seismometer using a magnetic spring
Authors: Otake Yuji
Published: July 2002

Discoveries in seismology depend on seismological-observational networks using highly sensitive/broadband seismometers, such as the STS-I. A pendulum with a long natural period is directly associated with the high sensitivity and wideband characteristics of these seismometers. The aim of this study on a vibration detector is to realize a seismometer with a sensitivity of less than several tens of microgal and a natural period of more than 100 s, which would surpass those of the STS-I. A new principle for a vibration detector, rather than a mechanical spring, which traditional seismometers use, is needed to avoid any spring-constant drift that depends on the temperature and the elastic deformation. The principle is a magnetic spring using the interaction force between a magnetic field and a permanent magnet. The magnet, which is inserted into the inner bore of a solenoid coil, behaves like a pendulum with a magnetic restoring force. The detector comprises a cylindrically shaped permanent magnet, a solenoid coil, a position detector, and a weight with a levitation system. This detector for horizontal components, which can achieve a natural period of more than several hundred seconds, has been theoretically shown by a calculation using parameters that would be possible with commercial parts. A detector using pneumatic levitation was recently tested. The results of the test showed that the natural period could reach more than 27 s without a position-feedback control system for the weight. The basic principle of the magnetic spring, that acts as a mechanical pendulum, was proved by this test. The detector showed the possibility of realizing a long natural period of more than 100 s with a position-feedback control system for the weight.
A basis set study for the calculation of electronic excitations using Monte Carlo configuration interaction
Authors: Larsson J. A. , Tong L. , Cheng T. , Nolan M. , Greer J. C.
Published: July 2002

A systematic study of basis sets and many-body correlations for the treatment of electronic excitations is presented. Particular emphasis is placed on the highly accurate treatment of transition energies within a computationally tractable scheme. All calculations have been performed using the Monte Carlo configuration interaction method and correlation-consistent basis sets augmented by diffuse functions constructed for the description of anions, and with the inclusion of additional Rydberg functions. The importance of a balanced description of the excited states and the ground state has been emphasized and the resulting electronic transitions have been compared with experimental values. We have found that the aug-cc-pVTZ basis set further augmented with Rydberg functions constitutes a good choice of basis set for which we report electronic excitations in excellent agreement with experiment.
Raman studies on GaAs1−xBix and InAs1−xBix
Authors: Verma Prabhat , Oe K. , Yamada M. , Harima H. , Herms M. , Irmer G.
Published: July 2002

The lattice vibrational properties of new semiconductor alloys, GaAs1−xBix and InAs1−xBix, are reported. These alloys, which were grown by metalorganic vapor phase epitaxy technique, contain a small amount (1.2%–3.8%) of Bi. A detail Raman scattering study of these new alloys, which exhibit weak temperature dependence of the band gap with increasing amount of Bi, is reported here. Good crystalline quality and spatial homogeneity was confirmed using micro-Raman technique. The alloys show ternary compound behavior, confirming substitutional incorporation of Bi into the lattice site. New vibrational modes observed were assigned to GaBi-like and InBi-like modes. In addition, phonon-plasmon coupled modes and vibrational modes corresponding to Bi and As materials were also observed. Results are discussed to characterize these new alloys in detail.
Evidence for surface initiated solidification in Ge films upon picosecond laser pulse irradiation
Authors: Siegel J. , Solis J. , Afonso C. N. , Vega F. , Bankmann J. , Martı́nez Sacristán O. , Sokolowski-Tinten K.
Published: July 2002

Amorphous Ge films on Si films have been melted by single 30 ps laser pulses at λ=583 nm and a solidification process has been followed by means of real time reflectivity (RTR) measurements with nanosecond time resolution. Evidence is provided for the occurrence of surface initiated solidification for films with thicknesses in the 80–130 nm range. This process occurs at high fluences following complete melting of the film and of a thin layer of the Si substrate which undergoes mixing with the liquid Ge. The release of the solidification enthalpy of the latter layer together with its lower solidification temperature favored by constitutional undercooling are proposed as the origin of the initial gradient inversion required for such a process. This scenario leads also to the formation of a secondary solidification front counterpropagating towards the film surface, as suggested by optical simulations of the experimental RTR transients. A transition from a surface initiated solidification process towards a bulk solidification process is shown to occur when increasing the film thickness beyond 130 nm. The results further show that, besides the film thickness, the pulse duration has a major influence on the type of solidification process induced.
Surface recombination velocity of phosphorus-diffused silicon solar cell emitters passivated with plasma enhanced chemical vapor deposited silicon nitride and thermal silicon oxide
Authors: Kerr M. J. , Schmidt J. , Cuevas A. , Bultman J. H.
Published: July 2002

The emitter saturation current density (JOe) and surface recombination velocity (Sp) of various high quality passivation schemes on phosphorus-diffused solar cell emitters have been determined and compared. The passivation schemes investigated were (i) stoichiometric plasma enhanced chemical vapor deposited (PECVD) silicon nitride (SiN), (ii) forming gas annealed thermally grown silicon oxide, and (iii) aluminum annealed (alnealed) thermal silicon oxide. Emitters with sheet resistances ranging from 30 to 430 and 50 to 380 Ω/□ were investigated for planar and random-pyramid textured silicon surfaces, which covers both industrial and laboratory emitters. The electronic surface passivation quality provided by PECVD SiN films was found to be good, with Sp values ranging from 1400 to 25 000 cm/s for planar emitters. Thin thermal silicon oxides were found to provide superior passivation to PECVD SiN, with the best passivation provided by an alnealed thin oxide (Sp values between 250 and 21 000 cm/s). The optimized PECVD SiN films are, nevertheless, sufficiently good for most silicon solar cell applications.
Compression and shear wave measurements to characterize the shocked state in silicon carbide
Authors: Yuan G. , Feng R. , Gupta Y. M.
Published: July 2002

Compression and shear wave experiments using plate impact loading were conducted on polycrystalline silicon carbide (SiC). The material was subjected to combined compression-shear loading to peak compressive stresses ranging from 3 to 18 GPa. The compression (shock) wave profiles and the propagation velocities of shear and longitudinal release waves in the shocked SiC were measured using in situ, electromagnetic velocity gauges. The Hugoniot elastic limit (HEL) of the material was found to be 11.5±0.4 GPa. The measured wave velocities were used to determine the elastic moduli of the material as functions of density compression in the shocked state. The data were further analyzed to obtain the mean stress response of the SiC under uniaxial-strain compression. The longitudinal and mean stress results completely characterize the material stress state. Numerical simulations were also carried out to verify the peak-state data analysis. Our results show that the Poisson’s ratio of the material increases with elastic shock compression from an ambient value of 0.161 to 0.192 at the HEL. Above the elastic limit, the maximum shear stress supported by the material increases from 4.5 to 6.4 GPa at a peak stress of 18 GPa. This finding verifies independently the results from lateral manganin gauge measurements in the same material [R. Feng et al., J. Appl. Phys. 83, 79 (1998)].
Broad-band fluorescence upconversion for femtosecond spectroscopy
Authors: Schanz R. , Kovalenko S. A. , Kharlanov V. , Ernsting N. P.
Published: July 2002

With near-infrared gating and improved light collection geometry, the entire fluorescence band can be upconverted in a broad range of 10 000 cm−1 without readjusting optical elements, thus allowing measurements with a single pump-gate scan. Monitoring of the pump-induced white light continuum provides for the time correction of the up-converted fluorescence spectra. The overall time resolution is then limited by the pump-gate cross correlation. The technique is illustrated with the femtosecond evolution of fluorescence from two molecular probes in solution.
Three-dimensional reconstruction of buried nanoparticles by element-sensitive tomography based on inelastically scattered electrons
Authors: Möbus G. , Inkson B. J.
Published: July 2002

Energy-filtered transmission electron microscopy is used to image the nanocomposite FeAl+Y2O3, an oxide-dispersion-strengthened intermetallic alloy, over a tilt range of ±60° using inelastically scattered electrons only. The properties of electron spectroscopic imaging are exploited to recover a projection relationship between the three-dimensional chemical concentration distribution and the micrographs. This allows recovery of the full information on volume shape, distribution, and homogeneity of the buried nanoparticles by backprojection. Restrictions to low atomic number, common in bio-objects, are here overcome at the expense of higher electron exposures.
Low-order models for the flow in a differentially heated cavity
Authors: Podvin Bérengère , Le Quéré Patrick
Published: July 2002

The proper orthogonal decomposition (P.O.D.) is applied to the flow in a differentially heated cavity. The fluid considered is air, and the aspect ratio of the cavity is 4. At a fixed Rayleigh number, P.O.D. empirical functions are extracted, and low-dimensional models are built and compared to the numerical simulation. Generally speaking, low-D models provide a coarse picture of the flow, which is also quick, cheap, and easy to understand. They can help pinpoint leading instability mechanisms. They are potentially key players in a number of applications such as optimization and control. Our goal in this study is to determine how well the flow can be represented by very low-dimensional models. Two moderately complex situations are examined. In the first case, at some distance from the bifurcation point, the dynamics can still be reduced down to two modes, although it is necessary to account for the effect of higher-order modes in the model. In the second case, farther away from the bifurcation, the flow is chaotic. A ten-dimensional model successfully captures the essential dynamics of the flow. The procedure was seen to be robust. It clearly illustrates the power of the P.O.D. as a reduction tool.
Bifocal spherical mirror for laser processing
Authors: Ardila D. Reyes , Barbosa L. B. , Andreeta J. P.
Published: July 2002

The control of the thermal gradient in single crystal grown by the laser-heated pedestal growth method is demonstrated through focalization of the laser beam into two different regions of the starting feed stock. To create these conditions, an optical component consisting of two incomplete spherical mirrors with the same focal distances was designed and built in copper and coated with aluminum. The performance of this component was tested in the crystal growth of lithium niobate rods with a radius larger than the critical radius calculated from the theoretical model for single focalization of the laser beam. Thermal gradient and temperature profiles along the crystal axis were measured with single and double focalization of the laser beam during the crystal growth process. The measured data indicate that the main differences in the thermal gradients of both configurations are found in the first two millimeters away from the melt–crystal interface. Single crystal rods of lithium niobate grown along the c axis with radius larger than the critical one are demonstrated with the use of this optical component.
Focused ion beam modification of atomic force microscopy tips for near-field scanning optical microscopy
Authors: Krogmeier Jeffrey R. , Dunn Robert C.
Published: July 2002

A probe for near-field scanning optical microscopy is demonstrated based on a high index glass sphere attached to the end of a conventional atomic force microscopy tip. The sphere is machined into a pyramid geometry using a focused ion beam (FIB) instrument, coated with aluminum to confine the excitation light, and milled further with the FIB to open an aperture at the end of the tip. Near-field fluorescence images of 50 nm fluorescent latex spheres reveal subdiffraction limit spatial resolution, illustrating the utility of these probes for near-field scanning optical microscopy.
Improving diamond–metal adhesion with graded TiCN interlayers
Authors: Sun Chang Q. , Fu Y. Q. , Yan B. B. , Hsieh J. H. , Lau S. P. , Sun X. W. , Tay B. K.
Published: July 2002

An approach improving diamond–metal adhesion has been developed based on modeling predictions and experimental verifications on the interfacial stresses modified by catalytic reaction. It is found that N-plasma irradiating onto Ti and tungsten-carbide substrates generates tensile surface stresses while C-plasma irradiation creates strongly compressive stress at the surfaces, both of which deteriorate the diamond–metal adhesion. It is also found that surface oxidation prevents diamond nucleation. Therefore, we applied a graded TiCN interlayer with carefully adjusting the ratio of C and N in the gas mixture to neutralize the interfacial stress and, hence, we have improved the diamond–metal adhesion substantially.
Microstructure and phase composition evolution of nano-crystalline carbon films: Dependence on deposition temperature
Authors: Hoffman A. , Heiman A. , Strunk H. P. , Christiansen S. H.
Published: July 2002

Nano-crystalline carbon films possessing a prevailing diamond or a graphite character, depending solely on substrate temperature and deposition time, can be deposited from a methane–hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. In this study we investigate the evolution of nano-crystalline carbon films deposited in the 800–950 °C temperature range onto silicon substrates aiming to enlight the physicochemical processes leading to the formation of nano-diamond films. While at a deposition temperature of ∼880 °C the formation of a thin precursor graphitic film is followed by deposition of a film of diamond character, at higher and lower temperatures the films maintain their graphitic character. The morphology of the films and their growth rate vary with deposition temperature: slower growth rates and higher film roughness are obtained at lower temperatures suggesting the importance of kinetic effects during the growth process. For deposition times longer than ∼60 min, similar morphologies are obtained irrespectively of the deposition temperature. A preferred spatial alignment of the basal planes of the graphitic film at the interface with the silicon substrate was determined. The alignment was found to differ with deposition temperature: at 800 and 880 °C the alignment occurs along the graphitic â axis perpendicular to the silicon substrate, while at 950 °C the ĉ axis is aligned perpendicular to the silicon substrate. However, it was determined that for films a few hundred nm thick close to the evolving surface the films display a preferred alignment of the basal planes vertical to the surface, irrespectively of their orientation at the interface. The reason for this alignment is suggested to be associated with a stress relaxation mechanism in the graphitic films. It was determined that film growth is accompanied by the evolution of large local stresses which obtain a maximum value for the films deposited at 880 °C. The relaxation of these stresses is suggested to lead to the transformation of the graphitic material into the diamond phase. The narrow range of temperatures (880+/−10 °C) which enables the formation of the diamond phase indicates the importance of hydrogen adsorption/desorption processes in the nucleation and growth of the nano-crystalline diamond films. The morphological evolution of the films was analyzed by atomic force microscope. By electron diffraction and high-resolution transmission electron microscopy the phase composition of the films and their microstructure were examined. The alignment of the graphitic films within the near-surface region of the evolving films as a function of the deposition time and temperature was investigated by angle-resolved near edge x-ray absorption fine structure measurements. Raman spectroscopy was applied to determine the presence of stresses within the films and their phase composition.
Anisotropic elastic-stiffness coefficients of an amorphous Ni–P film
Authors: Ogi Hirotsugu , Shimoike Goh , Hirao Masahiko , Takashima Kazuki , Higo Yakichi
Published: July 2002

This study presents a complete set of effective elastic-stiffness coefficients of a Ni80P20 amorphous-alloy thin film deposited on an aluminum-alloy substrate by electroless plating. The film thickness was 12 μm. The electromagnetic-acoustic-resonance method detected resonance frequencies of the triple-layered specimens (film/substrate/film), which enabled us to determine all five independent elastic-stiffness coefficients of the film using known substrate elastic properties. The resulting coefficients were those of a transverse isotropic material. There was strong anisotropy between the in-plane and normal directions; the in-plane Young’s modulus is larger than the normal Young’s modulus by 34%, for example. The anisotropic coefficients can be interpreted by considering a micromechanics model for local incomplete cohesion (thin ellipsoidal voids) aligned parallel to the film surface.
Plasma induced microstructural, compositional, and resistivity changes in ultrathin chemical vapor deposited titanium nitride films
Authors: Kröger R. , Eizenberg M. , Marcadal C. , Chen L.
Published: July 2002

Extremely thin titanium nitride (TiN) barrier layers for Cu based interconnects were deposited using metal organic chemical vapor deposition. The effect of the subsequently performed nitrogen/hydrogen plasma treatment on the microstructure, composition, and electrical properties of these films is studied using conventional and high resolution transmission electron microscopy, Auger electron spectroscopy, and four point probe resistivity measurements. In the studied system the crystallization of the TiN film starts from an amorphous matrix and a polycrystalline morphology is developed upon the H2/N2 plasma treatment. After a short plasma treatment, most of the film is already crystalline and consists of grains of a few nanometers in diameter. Continued plasma treatment leads to grain growth and a significant reduction of contaminants such as oxygen and carbon. The resistivity of the films drops with plasma treatment time, and a correlation between resistivity and oxygen content is found, which suggests that oxygen in the grain boundaries plays a decisive role for the resistivity of the films. It is shown that the oxygen in the grain boundaries leads to an electron reflectance of 0.9. Thus, the oxygen accumulation in the grain boundaries is the limiting factor for the reduction of film resistivity by plasma treatment.
Effects of the number of periods on strain in superlattices
Authors: Nishida K. , Hagiwara S. , Teranishi M. , Sirakawa O. , Hirakai A. , Katoda T.
Published: July 2002

The effect of periods on the accumulation and release of stress in GaAs/AlInAs superlattices structure is reported here. It is observed that in GaAs/AlInAs superlattices, when the Indium (In) content is greater than 10%, stress accumulates monotonically as the number of period increases. In GaAs/AlInAs superlattices with an In content of 5%, the accumulated stress is larger when the number of periods is less than 10. However when the number of periods exceeds 10, it was observed that suddenly there is a significant increase in defects and stress release. However, with any further increase in period number, there is once again an accumulation of stress.
Solving the Poisson equation for solute–solvent systems using fast Fourier transforms
Authors: Peter Christine , van Gunsteren Wilfred F. , Hünenberger Philippe H.
Published: July 2002

An iterative algorithm based on fast Fourier transforms is proposed to solve the Poisson equation for systems of heterogeneous permittivity (e.g., solute cavity in a solvent) under periodic boundary conditions. The method makes explicit use of the dipole–dipole interaction tensor, and is thus easily generalizable to arbitrary forms of electrostatic interactions (e.g., Coulomb’s law with straight or smooth cutoff truncation). The convergence properties of the algorithm and the influence of various model parameters are investigated in detail, and a set of appropriate values for these parameters is determined. The algorithm is further tested by application to three types of systems (a single spherical ion, two spherical ions, and small biomolecules), and comparison with analytical results (single ion) and with results obtained using a finite-difference solver under periodic boundary conditions. The proposed algorithm performs very well in terms of accuracy and convergence properties, with an overall speed comparable in the current implementation to that of a typical finite-difference solver. Future developments and applications of the algorithm will include: (i) the assessment of periodicity- and cutoff-induced artifacts in explicit-solvent simulations; (ii) the design of new electrostatic schemes for explicit-solvent simulations mimicking more accurately bulk solution; (iii) a faster evaluation of solvation free energies based on continuum electrostatics in cases where periodicity-induced artifacts can be neglected.
Chaotic dynamics of sea clutter
Authors: Haykin Simon , Puthusserypady Sadasivan
Published: July 2002

The notion that a deterministic nonlinear dynamical system (with relatively few degrees of freedom) can display aperiodic behavior has a strong bearing on sea clutter characterization: random-looking sea clutter may be the outcome of a chaotic process. This new approach envisages deterministic rules for the underlying sea clutter dynamics, in contrast to the stochastic approach where sea clutter is viewed as a random process with a large number of degrees of freedom. In this paper, we demonstrate, convincingly for the first time, the chaotic dynamics of sea clutter. We say so on the basis of results obtained using radar data collected from a series of extensive and thorough experiments, which have been carried out with ground-truthed sea clutter data sets at three different sites. The study includes correlation dimension analysis (based on the maximum likelihood principle) and Lyapunov spectrum analysis. The Lyapunov (Kaplan-Yorke) dimension, which is a byproduct of Lyapunov spectrum analysis, shows that it is indeed a good estimator of the correlation dimension. The Lyapunov spectrum also reveals that sea clutter is produced by a coupled system of nonlinear differential equations of order five or six.
Ion and electron mobilities in cryogenic liquids: Argon, nitrogen, methane, and ethane
Authors: Gee Norman , Floriano M. Antonio , Wada Toshinori , Huang Sam S.-S. , Freeman Gordon R.
Published: July 2002

Ion mobilities at low fields have been measured over the entire liquid range in argon, nitrogen, and methane. Gas-phase values for argon and nitrogen are also given. Comparison of the ratio of electron to ion mobilities confirmed that the electron remains nearly free in argon and methane liquids but is an anion in liquid nitrogen. At T≲116 K, the product of viscosity and either cation or anion mobility is constant in liquid nitrogen under its vapor pressure. At T>118 K, an abrupt destabilization of the anion in the low density liquid occurs; the breakdown strength of nitrogen should sharply decrease. The utility of breakdown studies in liquid ethane and propane in bridging studies of cryogenic inorganic liquids to studies of normal liquid hydrocarbons is discussed.
Manganese absorption in CaF2:Mn; I
Authors: McKeever S. W. S. , Jassemnejad B. , Landreth J. F. , Brown M. D.
Published: July 2002

We report detailed optical absorption spectra for Mn2+ in CaF2. The spectra from unirradiated samples, reported here for the first time, enable the 495-nm luminescence emission to be clearly assigned to a transition from the first excited 4T1g (4G) level to the 6A1g (6S) ground state. On the basis of the absorption curves energy level assignments are made for all of the 4G, 4D, 4P, and 4F levels. Racah parameters are evaluated and the reduction of these from their free-ion values, due to covalency effects, is estimated. A value of Dq=420 cm−1 is calculated and an Orgel diagram for this system is constructed. Finally, oscillator strengths for each of the transitions are estimated from the absorption spectra. The optical absorption spectra from gamma irradiated samples show a complex array of overlapping bands, most of which can be shown to be related to the presence of Mn. The thermal annealing of the bands is associated with the production of thermoluminescence at an emission wavelength of 495 nm. Although no definite models can be offered for the defects involved in these processes, they are believed to be complexes of Mn and radiation-induced defects (e.g., Mn/F centers).
Proposal for strained type II superlattice infrared detectors
Authors: Smith D. L. , Mailhiot C.
Published: July 2002

We show that strained type II superlattices made of InAs-Ga1−xInxSb x∼0.4 have favorable optical properties for infrared detection. By adjusting the layer thicknesses and the alloy composition, a wide range of wavelengths can be reached. Optical absorption calculations for a case where λc∼10 μm show that near threshold the absorption is as good as for the HgCdTe alloy with the same band gap. The electron effective mass is nearly isotropic and equal to 0.04 m. This effective mass should give favorable electrical properties, such as small diode tunneling currents and good mobilities and diffusion lengths.
Theoretical analysis of the CO2 molecule decomposition and contaminants yield in transversely excited atmospheric CO2 laser discharge
Authors: Hokazono Hirokazu , Fujimoto Haruo
Published: July 2002

Theoretical studies on the phenomena of a CO2 molecule decomposition and contaminants yield in transversely excited atmospheric CO2 laser discharge plasma have been conducted by our comprehensive plasma kinetic model. In addition to the reliable 175 plasma kinetic rate equations, the excitation circuit and the steady-state Boltzmann equations were included in the theoretical model in order to exactly simulate the time-dependent discharge condition. When the total capacitance and the charging voltage of the main discharge capacitor were varied over a wide range, the amount of the CO2 molecule decomposed per discharge pulse was found to be almost in proportion to only the deposited energy density: the energy deposited in the unit laser gas volume. It was also found that the amounts of CO, O2, O3, N2O, NO2, and NO yielded per discharge pulse increased almost proportionally with the increase of deposited energy density. The amount that the CO2 molecule decomposition and contaminants yielded against the unit deposited energy density are summarized in this article. Moreover, the effect of the addition of CO, H2, and H2O to the laser gas mixture on the suppression of the CO2 molecule decomposition are also studied.
Elastic wave propagation in noncentrosymmetric, isotropic media: Dispersion and field equations
Authors: Lakhtakia Akhlesh , Varadan Vasundara V. , Varadan Vijay K.
Published: July 2002

It has been recently demonstrated by us that acoustic waves in solids can discriminate between a chiral scatterer and its mirror image. Thus, it is possible to construct an acoustically chiral composite medium by embedding chiral microstructures in a host medium. The microstructure size should be large enough compared to the shear wavelength in the matrix medium so that an incident wave can sense its handedness; at the same time, the microstructure size should be small enough that, at least in some frequency range, the composite structure should appear to be effectively chiral. Isotropic composite media with chiral microstructure can be modeled as hemitropic micropolar elastic solids, which have been the subject of some recent investigations. The simplest possible constitutive equations have been obtained, and the dispersion equations have been derived and studied. Approximate solutions of the inhomogeneous field equations have also been derived using dyadic algebra.
Thermoelectric properties of lanthanum sulfide with Sm, Eu, and Yb additives
Authors: Nakahara J. F. , Takeshita T. , Tschetter M. J. , Beaudry B. J. , Gschneidner K. A.
Published: July 2002

The Seebeck coefficient and electrical resistivity have been measured for the La3−x Mx S4 alloys with the Th3 P4 -type structure for M=Sm, Eu, Yb, and x=0.1 to 0.9 in order to determine their potential as high-temperature thermoelectric energy conversion materials. An analysis of the lattice parameters of these alloys suggests that Sm has a valence of 2.2 over the entire composition range. Furthermore, the variation of the Seebeck coefficient and electrical resistivity as a function of electron concentration is different for the Sm-doped alloys than for the Eu- and Yb-doped alloys, which are all different from that of the pure binary LaSy alloys. A maximum in the electrical power factor at 1000 °C is found to occur at x ≂ 0.3, but when a reasonable estimate is made of the thermal conductivity the maximum in the figure-of-merit at 1000 °C shifts to larger x values (∼0.7) in all alloy systems. The long term stability of the Sm and Eu ternaries is also reported.
Formulation of silver loaded manganite based thermistor paste and application of percolation theory for sudden transition in conductance
Authors: Vittal Nitya , Aiyer R. C. , Aiyer C. R. , Setty M. S. , Phadke R. N. , Karekar S. D.
Published: July 2002

A low cost thermistor paste using manganite (Mn1.5CoN0.5O4) as functional material, prepared by sintering manganese, cobalt, and nickel oxides, is formulated for use as planar temperature sensors. The paste contains glass frit as binder and silver metal (0% to 60% by weight) as conducting phase instead of RuO32. Thermistors are fabricated by screen printing of these pastes on alumina substrates and effect of addition of various percentages of silver on electrical properties like sheet resistivity and temperature coefficient of resistance (TCR) are measured in the temperature range of 25–200 °C. Up to 50% addition of silver the pastes show a negative TCR with thermistor constant in the range 3000–5000 K, and sheet resistivity of the order of 106 Ω/⧠, whereas the pastes with 55% and 60% silver show a drastic reduction in the value of sheet resistivity (20 Ω/⧠) and have positive TCR with thermistor constant of the order of 200–300 K. NTC thermistors have high sensitivity of 5%–7%/K and PTC thermistors have sensitivity of 1%–2%/K. This interesting result of NTC-PTC transformation in relation to silver concentration, along with the sudden change in the conductivity (of about 6 orders of magnitude) at a critical concentration of silver, is discussed in the paper. This has been explained on the basis of an effective medium theory (Springett model) and formation of some metallic silver complex.
Determination of spall strength from surface motion studies
Authors: Gathers G. Roger
Published: July 2002

An expression in the literature has been used to determine the spall strength of materials from measurements of the surface velocity on reflection of a nonsteady shock wave. It uses initial density, the sound speed, and the difference between the initial surface jump-off velocity and the minimum in the velocity seen when the spall shock reaches the surface. Workers using the expression have found it necessary to choose the sound speed carefully in order to get reasonable results. It is shown that this is due to the inadequacy of the acoustic approximation used to represent the release isentrope. Approximation of the release isentrope by the Hugoniot makes it unnecessary to use special velocities and gives better agreement with experiment.
Effect of indium on the properties ofDXcenters in Si-doped Iny(Ga0.3Al0.7)1−yAs
Authors: Pann L. S. , Tischler M. A. , Mooney P. M. , Neumark G. F.
Published: July 2002

The addition of In to Si-doped Al0.3Ga0.7As has been investigated to determine its effect on DX centers. As expected, the persistent photoconductivity of the material is reduced as the band gap decreases with increasing In concentration. In addition, a new deep level transient spectroscopy peak is observed for the first time, which we attribute to DX centers having near In neighbors. This is clear evidence that the DX levels are highly localized states associated with donor impurities, whose properties are very sensitive to the local atomic configuration near the donor atom. This work supports previously published work on the effects of alloy disorder on DX centers, which is the strongest evidence to date for the microscopic configuration of the DX level.
Large nonlinear elastoresistivity of epitaxial thin silver films
Authors: Gaballa H. E. , Juretschke H. J.
Published: July 2002

Unexpectedly large increases in longitudinal elastoresistance γ=(1/Δe)(ΔR/R0) with decrease in temperature have been reported in the past for thin films of silver (thickness/carrier mean free path ≥1) on mica substrates. Here we subject such samples to electrically induced strain modulation, in the presence of large static strains. We trace the observed increases in γ to an anomalously large nonlinear response of the film resistivity to strain. Isolated from other experimental contributions, this nonlinearity by itself is temperature insensitive. Auxiliary experiments indicate that the anomaly persists for arbitrary combinations of applied planar strains. The data suggest that the large effect is caused by the nonlinear response under large strains of the scattering of carriers by structural defects such as grain boundaries.
The effect of grain size on fracture: Dislocation-free zone in the front of the finite crack tip
Authors: Shiue Sham-Tsong , Lee Sanboh
Published: July 2002

Dislocation pileups between a finite crack tip and a grain boundary with a dislocation-free zone is investigated by the method of continuous dislocation modeling. Analytic solutions of both the dislocation distributions inside the crack and plastic zone and the stress field in the space are obtained. The stress intensity factor at the crack tip and the stress concentration at the grain boundary are derived. When the applied stress exceeds τΛ0 [cos−1(c/a),k], the dislocations emitted from the crack tip pile up on the grain boundary. Note that Λ0 is the Heuman lambda function and τ is the lattice friction stress. a and c are the grain size and surface crack length, respectively. K is defined as (c/b)(a2−b2)/(a2−c2), where (b−c) is the dislocation-free zone. The stress concentration at the grain boundary increases with increasing grain size but decreases with increasing dislocation-free zone. In contrast, if the applied stress is less than or equal to τΛ0 [cos−1(c/a),k], no stress concentration exists at the grain boundary. The stress intensity factor at the crack tip increases with increasing size of dislocation-free zone but decreases with increasing grain size. Our results are reduced to several special cases.
Textured aluminum-doped zinc oxide thin films from atmospheric pressure chemical-vapor deposition
Authors: Hu Jianhua , Gordon Roy G.
Published: July 2002

Aluminum-doped zinc oxide films have been deposited on soda lime glass substrates from diethyl zinc, triethyl aluminum, and ethanol by atmospheric pressure chemical-vapor deposition in the temperature range 367–444 °C. Film roughness was controlled by the deposition temperature and the dopant concentration. The films have resistivities as low as 3.0 × 10−4 Ω cm, infrared reflectances close to 90%, visible transmissions of 85%, and visible absorptions of 5.0% for a sheet resistance of 4.0 Ω/⧠. The aluminum concentration within doped films measured by electron microprobe is between 0.3 and 1.2 at. %. The electron concentration determined from Hall coefficient measurements is between 2.0 × 1020 and 8.0 × 1020 cm−3, which is in agreement with the estimates from the plasma wavelength. The Hall mobility, obtained from the measured Hall coefficient and dc resistivity, is between 10.0 and 35.0 cm2/V s. Over 90% of the aluminum atoms in the film are electrically active as electron donors. Scanning electron microscopy and x-ray diffraction show that the films are crystalline with disklike structures of diameter 100–1000 nm and height 30–60 nm. The films have the desired electrical and optical properties for applications in solar cell technology and energy efficient windows.
The frequency dependence of eddy current losses in Terfenol-D
Authors: Kendall D. , Piercy A. R.
Published: July 2002

The frequency response of the magnitude and phase of the dynamic relative permeability of Terfenol-D (Tb0.27Dy0.73Fe2) has been measured for low drive fields (a few hundred A m−1 rms) and the energy loss per cycle determined. The data shows that when eddy current shielding is not significant, the frequency dependence of the magnitude of this energy loss follows that calculated from classical eddy current theory, suggesting that there is negligible anomalous loss. The permeability, along with the dynamic strain coefficient, is observed to possess a flat response with frequency up to the value at which eddy current shielding of the sample core is expected to be significant. However, the ratio of the dynamic strain coefficient to the dynamic susceptibility (d/χ) remains independent of drive frequency, suggesting that the magnetization processes do not change. Measurements using high drive fields show that at 5 kA m−1 rms the frequency dependence of the loss is still in good agreement with classical eddy current theory. This data also shows that the loss angle is identical to that measured at low drive fields, indicating that the eddy current losses scale as the square of the drive field.
Microstructure of photodeposited Fe in porous Vycor glass
Authors: Sunil D. , Sokolov J. , Rafailovich M. H. , Kotyuzhanskii B. , Gafney H. D. , Wilkens B. J. , Hanson A. L.
Published: July 2002

The products derived from the UV photolysis of Fe(CO)5 physisorbed onto porous Vycor glass have been examined by x-ray microprobe analysis, Mossbauer spectroscopy, and extended x-ray absorption fine structure spectroscopy (EXAFS). Although the photolysis and subsequent heatings are carried out in air, spectroscopic data reveal two surprisingly different forms of iron. Isomer shift, pre-edge, and EXAFS data indicate that one product is similar to α-Fe2O3 and consists of an Fe3+ ion octahedrally surrounded by six oxygen atoms at a distance of 1.8 Å. Isomer shift, EXAFS, and pre-edge feature analysis show that the second compound, which comprises ∼50% of the reaction product, is mainly elemental iron, in which a central Fe atom is surrounded by approximately eight other Fe atoms at a distance of 2 Å. X-ray microprobe analysis shows that aggregation occurs during photolysis due to the diffusion of Fe(CO)5 from the interior into exterior photodepleted volumes of glass. Heating has little effect on product ratio or distribution, but consolidating the glass at 1200 °C leads to further aggregation and formation of magnetically ordered particles that exhibit magnetic hyperfine fields of 370 and 425 kG.