bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupling of defect pairs and generation of dynamical band gaps in the impurity bands of nonlinear photonic crystals for all-optical switching
Authors: Lan Sheng , Ishikawa Hiroshi
Published: July 2002

We investigate the coupling of two single defects in two-dimensional photonic crystals (PCs) with the same frequency but different field distributions. The defect pair like this is generally present in PCs as a combination of a reduced-size defect and an increased-size defect. In spite of the significant difference in field distribution, quasiflat impurity bands suitable for the transmission of ultrashort pulses can be achieved by properly choosing defect pairs. More importantly, the coupled cavity waveguide constructed with defect pairs offers an opportunity to establish a periodic modulation of defect modes with a control light. The dynamical band gap generated by the periodic modulation of defect modes suggests a high-efficiency all-optical switching operation in nonlinear PCs.
Surface relaxation effects on the properties of porous silicon
Authors: Vázquez E. , Tagüeña-Martı́nez J. , Sansores L. E. , Wang C.
Published: July 2002

In this article, surface relaxation and its effects on the electronic and structural properties of porous silicon are studied by using the total-energy pseudopotential formalism within the density-functional theory. Our model is based on a 32-atom supercell, where columns of atoms are removed and saturated with hydrogen atoms. Samples with 4.4%, 13.6%, 16.8%, 28.9%, and 41.3% porosity are analyzed in detail. The results show a clear expansion of the system along the pore direction as the porosity increases. Moreover, this expansion is very sensitive to the hydrogen-atom concentration and a linear dependence is observed. The dependence of the band gap and the effective mass on the porosity are also analyzed. Here, the hydrogen-atom number and pore shapes are observed to play a fundamental role.
Magnetoresistive effect in p-type semiconducting diamond films
Authors: Kong C. Y. , Wang W. L. , Liao K. J. , Wang S. X. , Fang L. , Ma Y.
Published: July 2002

Magnetoresistive effects were studied in p-type heteroepitaxial diamond films with a strip or Corbino disk structure in a magnetic field ranging from 0 to 5 T. The films were grown by microwave plasma chemical vapor deposition and boron doped by cold ion implantation and rapid thermal annealing. The experimental results show that the magnetoresistance (MGR) of p-type heteroepitaxial diamond films strongly depends on the geometric form of the samples and the magnetic field. Diamond films are assumed to be an isotropic isothermal solid in which conduction is by holes from light, heavy and split-off bands. Based on the Fuchs and Sondheimer thin-film theory, considering spherical energy surfaces and mixed scattering by lattice vibrations and ionized impurities and surface, a theoretical description of the magnetoresistive effect in diamond films is presented by solving the Boltzmann transport equation in the relaxation time approximation. A relationship between the MGR and the thickness of films, magnetic field, and mobility is shown.
Nanostructure formation by localized decomposition of Mo(CO)6 on Si(111)-(7×7) surfaces
Authors: Schöffel U. R. , Rauscher H. , Behm R. J.
Published: July 2002

We report on the fabrication of nanometer-sized structures by scanning tunneling microscope induced decomposition of Mo(CO)6 precursor molecules on Si(111)-(7×7). Based on the analysis of characteristic properties of these structures in the region of the deposition threshold the mechanism leading to Mo(CO)6 fragmentation under these conditions and subsequent formation of nanostructures can be identified. This mechanism consists of an electron induced excitation of the molecule to an antibonding state with subsequent spontaneous separation of a CO ligand.
Magnetic properties and microstructures of α-Fe/Nd2Fe14B nanocomposite microalloyed with Zr
Authors: Wu Y. Q. , Ping D. H. , Xiong X. Y. , Hono K.
Published: July 2002

Magnetic properties of α-Fe/Nd2Fe14B nanocomposite produced from Nd8Fe87−xB5Zrx (x=0–5) alloys and their microstructures are studied by three-dimensional atom probe (3DAP) and transmission electron microscopy techniques. Zr substitution is found to increase coercivity significantly from 356 kA/m for x=0 to 625.2 kA/m for x=5 alloys at the expense of remanence. Zr atoms are exclusively partitioned in hard phase and being slightly enriched at the interface, resulting in the refinement of microstructure.
Effects of ions on the dielectric permittivity and relaxation rate and the decoupling of ionic diffusion from dielectric relaxation in supercooled liquid and glassy 1-propanol
Authors: Power G. , Johari G. P. , Vij J. K.
Published: July 2002

The effects of intermolecular H bonds on the equilibrium permittivity, εs, the relaxation rate, fm, and changes in the dielectric relaxation spectra of 1-propanol were studied by adding a strongly dissociating electrolyte LiClO4. The spectra of both 1 mol % LiClO4 containing 1-propanol (1 molecule of LiClO4 per 99 molecules of 1-propanol), and of pure 1-propanol were measured over a temperature range from ∼90–160 K in the frequency range 1 mHz–1 MHz. An analysis of the spectra showed that the three relaxation processes (I, II, and III in ascending rapidity), that occur in the pure alcohol persist in the presence of ions, the equilibrium permittivity, εs, decreases, the distribution characteristics of the processes changes, and the rate of relaxation, fm, of processes I and III remains constant within analytical errors, while that of process II is reduced. The temperature dependencies of fm,I and fm,II are non-Arrhenius while that of fm,III is Arrhenius. The temperature dependence of the dc conductivity, σ0 is also non-Arrhenius, but the parameters of the fit differ from those of fm,I and are closer to those of fm,II. Its 1000-fold increase on adding 1 mol % LiClO4 is due to the increase in ion concentration and a decrease in the viscosity as a result of the breaking of H bonds. However, as fm,I and fm,III remain constant on the addition of LiClO4, and fm,II decreases slightly, an expected decrease in viscosity would seem to be unrelated to the relaxation rates of these processes. In that case, σ0 is decoupled from these dipolar relaxation modes.
Laser interference structuring of a-Ge films on GaAs
Authors: Santos P. V. , Zanatta A. R. , Jahn U. , Trampert A. , Dondeo F. , Chambouleyron I.
Published: July 2002

We have investigated the laser interference crystallization (LIC) of amorphous germanium films on (100)-oriented GaAs substrates using nanosecond laser pulses. We demonstrate that LIC can produce periodic arrays of epitaxially crystallized Ge lines on GaAs with submicrometer widths. The gratings display a surface undulation with faceted surfaces, which depends on laser fluency. The undulation is attributed to the lateral solidification process induced by the temperature gradients created during the LIC process.
A new analytic form of ab initio potential energy function: An application to H2O
Authors: Yagi Kiyoshi , Taketsugu Tetsuya , Hirao Kimihiko
Published: July 2002

The potential energy function (PEF) for H2O has been generated by the modified Shepard interpolation method using the fourth-order Taylor expansion at the MP2/aug-cc-pVTZ level. The reference points for the interpolation have been set at only three points, i.e., the equilibrium structure plus two symmetrically equivalent points in the coupling region of OH symmetric and antisymmetric stretching modes. The vibrational self-consistent field (VSCF) and virtual configuration interaction (VCI) methods have been applied to determine fundamentals, overtones, and combination bands of H2O. The mean absolute deviation over these energy levels from the corresponding ones determined by the direct VCI method is only 4.8 cm−1.
Microstructure and magnetic properties of multilayer [(Nd2Fe14B)x/M]n films (M=Nb or Cr)
Authors: Tsai Jai-Lin , Chin Tsung-Shune , Yao Yeong-Der , Kronmüller H.
Published: July 2002

Multilayer [(Nd2Fe14B)x/M]n films (x being thickness in nm; M=Nb or Cr, with a fixed thickness 5 nm; n being the number of layers) have been deposited by ion-beam sputtering. Spacer layer was found to have substantial effects on the multilayers to exhibit different magnetic properties and microstructure. The average grain size of the multilayer films can be well controlled by both annealing temperature and thickness of the Nd2Fe14B layer. From a cross-sectional HRTEM study, the average grain size of [(Nd2Fe14B)x/M]n films is 50 nm for x=40 nm, n=10, that is smaller than the grain size of single-layer Nd2Fe14B films with 200 nm thickness. The average grain size of single-layer film is 160 nm. From selected area diffraction patterns, the structure of Nb spacer is amorphous, while that of Cr is columnar, crystalline, and with some soft magnetic Fe–Cr phase in the broken layer structure around the hard magnetic Nd2Fe14B phase. Therefore, the multilayer films with Nb spacer exhibit good hard magnetic properties. While for those with Cr spacer, random exchange-coupling effect was observed in demagnetization curves.
Zero electron kinetic energy spectroscopy of the XeCl− anion
Authors: Lenzer Thomas , Yourshaw Ivan , Furlanetto Michael R. , Pivonka Nicholas L. , Neumark Daniel M.
Published: July 2002

Zero electron kinetic energy spectroscopy has been used to study the XeCl− anion and the X1/2 electronic state of neutral XeCl. The spectrum is vibrationally resolved, yielding anion and neutral vibrational frequencies. With the aid of earlier scattering measurements for the neutral state we construct a Morse–Morse-switching–van der Waals model potential function for the anion from our spectroscopic data, for which Rm=3.57±0.03 Å and ε=145.8±0.7 meV. This represents the first accurate experimental potential for the XeCl− anion.
Two-photon resonant ionization spectroscopy of the allyl-h5 and allyl-d5 radicals:  Rydberg states and ionization energies
Authors: Liang Chi-Wei , Chen Chun-Cing , Wei Chia-Yin , Chen Yit-Tsong
Published: July 2002

The vibronic spectra of allyl-h5 (CH2CHCH2) and allyl-d5 (CD2CDCD2) radicals at 6.0–8.2 eV have been observed using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The allyl-h5 and allyl-d5 radicals were produced in the nozzle of a supersonic jet expansion by the pyrolysis of CH2CHCH2I and CD2CDCD2Br, respectively. Together with our previous study, Rydberg states of ns (n=4–12) for allyl-h5 and (n=4–10) for allyl-d5 have been observed in the REMPI spectra. Prominent vibrational progressions with the spacings of ∼440 cm−1 in allyl-h5 and ∼360 cm−1 in allyl-d5 are observed in the s Rydberg series. The totally symmetric ∠CCC-bending mode is responsible for the vibrational progressions. Facilitated with ab initio calculated Franck–Condon factors, other vibrations (CH2 rocking and C3 stretching) observed in the s Rydberg series have also been identified. The adiabatic ionization energies, 65 594±9 cm−1 (8.133±0.001 eV) for allyl-h5 and 65 527±16 cm−1 (8.124±0.002 eV) for allyl-d5, are extrapolated from the convergence of the observed ns Rydberg series.
Three-dimensional modeling of perpendicular reading with a soft underlayer
Authors: Roscamp T. A. , Boerner E. D. , Parker G. J.
Published: July 2002

A three-dimensional (3D) reciprocity based model of the perpendicular read process with a soft underlayer is presented. In 3D with finite track width, the read flux does not decay to zero because flux is always entering the reader from the field caused by a nonvanishing curl of the magnetization. As the reader is moved off track, the signal amplitude and resolution decrease, and at some off track position there is a reversal in the sign. A comparison of the reciprocity based reader model with a micromagnetic CPP reader model shows excellent agreement both on and off track.
Micromagnetic study of effect of media intergranular exchange interaction in perpendicular recording
Authors: Wang Lei , Li Shaoping , Giusti James H. , Fernandez-de-Castro Juan
Published: July 2002

A micromagnetic study is performed for the writing and reading processes in perpendicular recording at high areal densities. In particular, the effect of intergranular exchange interactions in media is examined. Calculations are done for three different values of intergranular exchange coupling. The calculated media hysteresis loops show that the saturation magnetic field increases when exchange becomes weaker. The writer deep gap field is chosen to be equal to the saturation field so that proper overwrite is assured. Modeling shows that the magnetic cluster domain size becomes larger and the readback signal exhibits larger variation at its peaks when exchange constant increases. The variation of readback signal at its peaks in square wave perpendicular recording is primarily due to the variation of bit cell length of written patterns. On the other hand, transition curvature increases when exchange constant decreases. Although on-track isolated pulse width of a differentiated signal is not affected much by transition curvature, the off-track isolated pulse width is larger when transition curvature is larger.
3D dynamic heat plasma magnetohydrodynamic calculation
Authors: Fujisaki Keisuke
Published: July 2002

This paper describes numerical magnetohydrodynamic (MHD) calculation of heat plasma used in heating and welding processes. Most of the calculation models have been two dimensional and static. Since physical phenomena are three dimensional and nonstatic, three dimensional, dynamic MHD calculation of heat plasma is developed in this paper. Since local heat balance is assumed, the heat plasma is considered as a conductive fluid dynamic material such as molten metal. However, heat plasma’s material constants such as density, heat transfer coefficient, specific heat, and electrical conductivity are strong functions of the temperature. Temperature is governed by heat plasma material transfer and heat transfer. Material transfer is driven by the electromagnetic force. Turbulent flow is modeled by the large eddy simulator including heat transfer. The electrical phenomenon is modeled using the Poisson equation in a dc magnetic field. It is assumed that the static electrical force may be ignored. Calculations show that the heat plasma concentrates at the center. The center area is of low density and high electrical conductivity. To verify the usefulness of this 3D dynamic MHD method, a conventional model, which is 2D and static is compared to a k−ε model. Comparison of velocity distributions shows good agreement.
Magnetic studies of amorphous CoZrGdDy films with perpendicular and in-plane uniaxial anisotropy
Authors: Shin D. H. , Suran G. , Chumakov D. , Schafer R. , Henry Y.
Published: July 2002

Amorphous Co73±x±yZr5Gd14±xDy8±y thin films with x≈2 and y≈1 were prepared by rf sputtering in the presence of a dc field applied along the film plane. The films exhibit a perpendicular Hp and an in-plane Hu anisotropy related to the clusters formed around Gd and Dy, respectively. Here Hp>4πMs, while the respective values of 4πMs and Hu, so the magnetic properties changed with composition. The behavior of the stripe domains is classical for 4πMs>Hu, but affected by Hu for 4πMs∼Hu. When Hu>4πMs perpendicular domains sensitive to high perpendicular fields and in-plane domains to low ones are observed.
Thermal magnetic characteristic for high frequency induction heating analysis
Authors: Arita Hideaki , Todaka Takashi , Enokizono Masato
Published: July 2002

This paper presents results of finite element analysis of induction heating problems considering temperature dependence of material characteristics. In this analysis, we have used the three dimensional finite element method in order to correctly express the induction heating coil’s shapes and to make clear its effects on temperature distributions. The heat conducting problem and the eddy current problem are coupled and solved by using step-by-step calculations.
Thermal and magnetic properties of CeGe2
Authors: Lin C. L. , Yuen Tan , Riseborough P. , Huang X.-Y. , Li Jing
Published: July 2002

The magnetization data measured at low applied fields show that CeGe2 undergoes an antiferromagnetic transition at TN=7 K and exhibits a second ferromagnetic transition at TC=4.3 K. As the applied field increases, TC increases, but TN is almost unchanged. These two magnetic transitions merge to become a single ferromagnetic transition at a field equal to or larger than 5 kG. This is the first report of an antiferromagnetic transition in CeGe2 and the data are significantly different from those previously reported. The high temperature magnetic susceptibility shows the Curie–Weiss behavior and the effective paramagnetic moment is 2.38 μB indicating that the Ce ions in CeGe2 are trivalent. The specific heat data also show anomalies at 4.3 and 7 K associated with two magnetic transitions, respectively. An enhanced electronic specific heat coefficient of γHT=120 mJ/mol K2 is deduced for temperatures above TN.
Hybridization and pressure effects in UTX compounds
Authors: Alsmadi A. M. , Sechovsky V. , Lacerda A. H. , Prokes K. , Kamarad J. , Brück E. , Chang S. , Jung M. H. , Nakotte H.
Published: July 2002

The UTX intermetallic compounds (T=transition metal and X=p-electron element) were found to crystallize mainly in two large groups, the orthorhombic TiNiSi and the hexagonal ZrNiAl structure. For both groups, magnetic-ordering phenomena depend sensitively on the 5f-ligand hybridization. The 5f-ligand hybridization is very sensitive to the interatomic distances, which can be effectively controlled by external pressure. Here, we have summarized existing and new pressure studies on various single-crystalline UTX compounds (T=Co, Ni, Ir;X=Al, Ga, Ge). We performed magnetoresistance studies in magnetic fields up to 18 T under external hydrostatic pressure up to 10 kbar. The observed dependencies of the ordering temperatures and the critical fields are compared with the different contributions of the 5f-ligand hybridization, which were deduced from tight-binding calculations. We find relatively weak pressure dependence in compounds, where 5f electrons are more localized, while substantially large pressure effects are found in the more itinerant systems.
Spectroscopic determination of the two-dimensional vibrational potential energy surfaces for the ring-puckering and ring-flapping modes of indan in its S and S1(π,π*) electronic states
Authors: Arp Zane , Meinander Niklas , Choo Jaebum , Laane Jaan
Published: July 2002

The vapor-phase far-infrared, mid-infrared, ultraviolet, Raman, and laser-induced fluorescence spectra of indan have been recorded and analyzed. The far-infrared spectra, which are very similar to those previously reported, together with the Raman and dispersed fluorescence (SVLF) spectra of the jet-cooled molecules were used to reassign the ring-puckering and ring-flapping energy levels for the S0 ground state. These were then utilized to calculate a two-dimensional vibrational potential energy surface (PES) which nicely fits all of the assigned puckering and flapping levels. The PES has a barrier of 488 cm−1 as compared to a previously reported value of 1979 cm−1, which was based on a one-dimensional analysis and earlier assignments. The dihedral angle of puckering is ±30°. Fluorescence excitation spectra of jet-cooled indan together with ultraviolet absorption spectra were used to assign the flapping and puckering levels in the S1(π,π*) electronic excited state. The PES for this state has a barrier of 441 cm−1 and the energy minima correspond to puckering angles of ±39°. The flapping frequency and the stiffness of the PES along the flapping coordinate both decrease substantially in the excited state. The barriers to planarity for both states are higher than those for analogous molecules due to the two –CH2–CH2– torsional interactions. Ab initio calculations do a fairly good job of predicting the experimental barriers for indan and related molecules in their S0 and S1 states.
C
70
Ph
8
and C70Ph10: A computational and solid solution spectroscopic study
Authors: Fanti Marianna , Zerbetto Francesco , Galaup Jean-Pierre , Rice James H. , Birkett Paul R. , Wachter Norbert , Taylor Roger
Published: July 2002

Mapping of the potential energy surface of the torsions of the phenyl groups of C70Ph8 and C70Ph10 by molecular mechanics reveals the existence of two conformers for each molecule. Energetically, both pairs are quasi- degenerate and therefore they could be experimentally detectable despite the low activation barriers for the inter-conversion (2.4 kcal mol−1 for C70Ph8 and 3.9 kcal mol−1 for C70Ph10, respectively). In an effort to pin down the existence of the conformers, the laser induced luminescence spectra of C70Ph8 and C70Ph10 were recorded in several solid solutions. The spectral features for C70Ph8 are rather independent of the nature of the environment and closely resemble those observed before in liquid solution with two main bands separated by more than 2000 cm−1. For C70Ph10, two new low-energy bands appear which were previously undetected in the liquid phase studies and which depend on the matrix. It is proposed that the doublet bands are fingerprints of the presence of the conformers.
Lattice models of ionic systems
Authors: Kobelev Vladimir , Kolomeisky Anatoly B. , Fisher Michael E.
Published: July 2002

A theoretical analysis of Coulomb systems on lattices in general dimensions is presented. The thermodynamics is developed using Debye–Hückel theory with ion-pairing and dipole–ion solvation, specific calculations being performed for three-dimensional lattices. As for continuum electrolytes, low-density results for simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) lattices indicate the existence of gas–liquid phase separation. The predicted critical densities have values comparable to those of continuum ionic systems, while the critical temperatures are 60%–70% higher. However, when the possibility of sublattice ordering as well as Debye screening is taken into account systematically, order–disorder transitions and a tricritical point are found on sc and bcc lattices, and gas–liquid coexistence is suppressed. Our results agree with recent Monte Carlo simulations of lattice electrolytes.
Probing molecular geometry of solids by nuclear magnetic resonance spin exchange at the n=0 rotational-resonance condition
Authors: Tekely Piotr , Gardiennet Carole , Potrzebowski Marek J. , Sebald Angelika , Reichert Detlef , Luz Zeev
Published: July 2002

Exploration of the molecular geometry in rotating powder solids on the basis of magnetization exchange between spins with identical isotropic chemical shifts but differing chemical shielding tensor orientations is demonstrated experimentally. For this we take advantage of the potential of the ODESSA (one-dimensional exchange spectroscopy by sidebands alternation) experiment for the accurate measurement of spin exchange rate constants. We also report the observation of oscillatory behavior of the rotor-driven magnetization exchange at this so-called n=0 rotational-resonance condition which, in contrast to n=1,2,3,… rotational-resonance conditions, takes place at nearly arbitrary magic-angle spinning frequencies. The sensitivity of the longitudinal exchange decays to the relevant physical parameters of the spin system under conditions of rotor-driven and proton-driven magnetization exchange is discussed theoretically and demonstrated experimentally. Several C13 and P31 spin-exchange measurements have been performed on a series of model compounds covering a broad range of internuclear distances between carboxyl carbon atoms, and on a series of phosphorylated amino acids with different internuclear distances between phosphorus sites. The capacity of the ODESSA experiment for an unambiguous recognition of distinct internuclear distances is demonstrated. Potential applications of such measurements involve the exploration of intermolecular distances and the determination of the mutual orientation of neighboring molecular fragments in polycrystalline and noncrystalline solids.
A compact, high-resolution Paul ion trap mass spectrometer with electron-impact ionization
Authors: Orient O. J. , Chutjian A.
Published: July 2002

A Paul ion trap has been developed for use as a high-resolution mass spectrometer. It is of small size (r0=10 mm), having a resolution of m/Δm=324, which is limited by the machining accuracy of the trap. It has a demonstrated mass range of 1–300 u, and a sensitivity of 2×1014 counts/Torr s, or to 500 parts per trillion detection sensitivity in a typical vacuum of 10−5 Torr. Ionization of the room-temperature gas within the trap is carried out with an electron beam traversing the trapping volume. The trap operates in a radio frequency only mode, and no dc is required. Trapping is accomplished within the well depth of the dynamic radio frequency potential, and no cooling gas is required, such as helium. This combination of factors makes the trap potentially of use for autonomous operation in harsh environments requiring low power, low weight, and low volume, such as undersea, on the surface of a planet or asteroid, or in a spacecraft.
Computationally useful bridge diagram series. III. Lennard-Jones mixtures
Authors: Dyer Kippi , Perkyns John , Pettitt B. Montgomery
Published: July 2002

The first two orders of bridge diagrams for the f-bond expansion and the h-bond expansion are calculated for a binary mixture of Lennard-Jones spheres. The method used follows the Legendre polynomial integration methods outlined in the first two papers of this series. As for the pure fluid cases, the thermodynamic results which follow from these methods are found to be in reasonable agreement with the simulation result. Analysis of the thermodynamic and structural results in comparison to the best current bridge function approximations indicate that accurate descriptions of higher order mixtures will require methods beyond the current mean field treatments which are of utility in simple fluids. The methods given are unfortunately not computationally convenient at highest order; however, the lower order diagrams are both accessible and give reasonable numerical results.
Formation of ordered mesoscopic polymer arrays by dewetting
Authors: Karthaus Olaf , Gråsjö Lars , Maruyama Norihiko , Shimomura Masatsugu
Published: July 2002

It could be shown that by a simple casting process from solution two-dimensionally ordered arrays of mesoscopic (i.e., in the range of submicrometer to micrometer) polymer aggregates on solid substrates can be formed. Patterns were investigated by optical microscopy and atomic force microscopy. The pattern formation was observed in situ by optical and fluorescence microscopy and it was found that a “fingering instability” at the three-phase-line of a solution droplet is the crucial process for pattern formation.
Multidimensional modeling of transverse avalanche laser discharges: Applications to the HgBr laser
Authors: Kushner M. J. , Pindroh A. L. , Fisher C. H. , Znotins T. A. , Ewing J. J.
Published: July 2002

Geometrical considerations are important with respect to the stability and efficiency of avalanche electric discharge lasers. Parameters such as the electrode contours and the distribution of preionization electrons affect excitation rates through the relative values of the local electric field, local depletion of initial species, and through the response of the discharge circuitry to spatially dependent conditions within the plasma. Constriction of the discharge and subsequent impedance mismatch of the discharge to the pulse forming line result from these spatial nonuniformities. In this paper geometrical effects in the mercury bromide electric discharge laser are examined by comparing the results from a multidimensional discharge and kinetics model with experimental observations. The code models electron and heavy particle kinetics and laser intensity in time and one spatial dimension: parallel to the electrodes and perpendicular to the optical axis. Quantities whose spatial dependence is perpendicular to this dimension, such as the contours of the electrodes, are accounted for through their impact on the local electric field. HgBr laser efficiency and spatial characteristics are examined as a function of stored electrical energy, the impedance of the pulse-forming circuitry, electrode contours, and profile of the preionization electron density. Parasitic discharges within the gas, but exterior to the optical cavity, are examined as a mechanism through which the magnitude of the preionization density can effect laser efficiency.
Long pulse operation and premature termination of a high-power discharge-pumped XeCl laser
Authors: Osborne M. R. , Hutchinson M. H. R.
Published: July 2002

This paper describes an x-ray preionized avalanche discharge-pumped XeCl laser capable of producing output pulses of greater than 250-ns FWHM duration and energies of ∼1 J. This represents the longest pulse duration yet reported from such a high-power laser. Nevertheless, the laser output pulse is shown to terminate while the laser is still apparently under strong electrical excitation. The severity of this premature laser termination has been found to be dependent on both the partial pressure of the halogen donor molecule, HCl, in the laser gas mixture, and on the magnitude of the electrical excitation rate. Investigations of the discharge homogeneity, using time-integrated and time-resolved photographic observation of the discharge, scattering of a short duration dye laser probe beam, and time-integrated and time-resolved fluorescence spectra of species within the laser gas mixture have led to the conclusion that it is the formation of small, discrete current channels in the body of the discharge which is the prime cause of the premature termination. These scatter the laser radiation and also lead to nonuniform and inefficient formation of the excimers.
Nordheim dependence in the electrical resistivity of surface-contaminated thin metal films
Authors: Lessie Douglas L. , Crosson Eric R.
Published: July 2002

The Nordheim, or parabolic, dependence of thin metal film resistivity on surface contamination is theoretically established by incorporating a more complete specular probability distribution function into the Lucas equation than has been used heretofore. This specular probability is angle dependent and obtained by adapting an average T-matrix approximation to the case of statistical scattering of conduction electrons by a smooth surface that is randomly contaminated by highly localized atomic impurities. Multiple scattering, interference effects, and diffraction of the conduction electron waves are all considered in forming the specular probability function. Although the Lucas equation cannot directly account for diffraction of conduction electrons by a contaminated surface, its approximate validity is demonstrated in the case we consider of low-index surfaces without superlattice formations. It is shown that, while a Born approximation of surface scattering leads erroneously to a monotonically increasing dependence of film resistivity on surface contamination, a full treatment of the higher-order multiple scattering series yields the desired explanation of the Nordheim effect.
Optical pulse narrowing by backward, transient stimulated Brillouin scattering
Authors: Ghosh Roy D. N. , Rao D. V. G. L. N.
Published: July 2002

An experiment demonstrating optical pulse narrowing of a 6943-Å, 20-ns ruby laser pulse to a few ns in the isophase of the liquid-crystal cholesteryl oleate is reported. An analytical expression is derived for the backscattered intensity at the entrance to the scattering cell. Also, rough estimates of the pulse widths, time to reach intensity maximum, and asymmetry of the pulse shape are presented.
X-ray diffraction under ultrahigh pressure generated with sintered diamond anvils
Authors: Utsumi W. , Toyama N. , Endo S. , Fujita F. E. , Shimomura O.
Published: July 2002

High pressures generated with sintered diamond as the second stage anvils in the ‘‘6-2 type’’ double-stage apparatus have been calibrated by x-ray diffraction with synchrotron radiation. Sintered diamond has permitted a fair amount of x-ray transparency although the chemical analysis shows the existence of 14-wt. % Co binder. Anvils with a top face of 0.7 mm diam performed the generation of 60 GPa, which was confirmed on the lattice parameters of B1 and B2 phases of NaCl. No appreciable plastic deformation of the used anvils has certified the higher pressure generation.
Mass spectrometric transient study of dc plasma etching of Si in SF6/O2 mixtures
Authors: Brandt Werner W. , Honda Takuya
Published: July 2002

A thin wafer of polycrystalline Si was mounted on the cathode of a small cylindrical plasma cell which consisted of two flat stainless steel electrodes and a piece of alumina tubing. A discharge was supported by a slow flow of Ar, and short pulses of SF6 and O2 were injected simultaneously into the Ar stream. The contents of the plasma cell were sampled continuously, and analyzed by mass spectrometer. The concentration transients of various species resulting from the SF6 and O2 pulses were measured as a function of time. The maxima of the SiF+3 transients and thus the etching rates were found to depend on oxygen concentrations in the mixtures, but far less so than the atomic fluorine concentrations, as reported by others. By contrast, the maxima of several S–F and S–O–F species concentrations depended very strongly on O2 concentrations. It is concluded that neither atomic fluorine, nor any one of the other observed species can be the sole important reactive species in the etching reaction studied. The experimental results also bear on the fate of SiF2 initially formed and thus illustrate the potential of the pulse injection method used in mechanistic studies.
Effective properties of fiber-reinforced composites: Effects of polydispersity in fiber diameter
Authors: Joslin C. G. , Stell G.
Published: July 2002

We investigate the effect of polydispersity in fiber cross section on the effective properties of a fiber-reinforced composite. For a model microstructure in which the fibrous inclusions are fully penetrable cylinders with varying diameters, we compute the microstructural parameters ζ′1 and η1 which determine Milton’s upper and lower bounds on transport coefficients and elastic moduli of the composite. From our results we infer that polydispersity in fiber size has very little influence on these properties.
Conformational and crystallographic changes occurring in polyvinylidene fluoride during the production of D-E hysteresis loops
Authors: Guy I. L. , Unsworth J.
Published: July 2002

When polarization hysteresis loops are produced for biaxially oriented polyvinylidene fluoride, significant changes between successive loops are found to occur. These changes are most noticeable during the first few cycles and are shown, with the aid of infrared spectroscopy, to be the result of changes in chain conformation and consequent crystal structure in the material. Movement of free charge appears to play a negligible role in the changes observed.
Microfields in stroboscopic voltage measurements via electron emisson. II. Effects on electron dynamics
Authors: Clauberg Rolf
Published: July 2002

In this article, the transit-time and cross-talk effects occurring in voltage measurement via electron emission are analyzed as a function of geometry, extraction fields, electron start energies, and rise times of the actual input signals. The investigation was performed to obtain information about the maximum achievable time resolution and the disturbance of measured signals by cross-talk effects in corresponding contactless measurement techniques like photoemissive sampling and electron-beam probing. The field distribution above the sample was assumed as being two-dimensional, and the influence of space-charge effects neglected. A strong correspondence to the purely geometry-dependent response function of the potential energy, analyzed in Part I [J. Appl. Phys. 62, 1553 (1987)], is established.
Microfields in stroboscopic voltage measurements via electron emission. I. Response function of the potential energy
Authors: Clauberg Rolf
Published: July 2002

It is shown that the response of the electrostatic potential on voltage changes at a conducting line in a microscopic structure is fully described by a pure geometry function. This function is determined by the special microscopic character of the conducting line and by the screening due to neighboring conductors. It already allows a qualitative description of transit-time and cross-talk effects in voltage measurements via electron emission or electro-optical sampling, and provides a basis for the quantitative calculation of these effects. The geometry dependence of this function will be analyzed in detail.
Photoemission sampling measurements of a dispersing voltage pulse traveling on a transmission line
Authors: Blacha A. , Clauberg R. , Seitz H. K.
Published: July 2002

The newly developed method of photoemission sampling, based on the single-photon process, has been applied to measure the dispersion of a picosecond voltage pulse traveling on a transmission line. The dispersion of the propagating pulse is clearly revealed in the spectra which show signal rise times of about 17 ps at 2 mm away from the pulse generating switch, and change into rise times of about 30 ps at 1.5 mm further away. The time resolution of the measurements is better than 7 ps. Therefore, photoemission sampling with picosecond laser pulses permits the measurement of changes in signal waveforms at arbitrary points of a transmission line and for arbitrary substrate materials with sufficient time resolution.
Crystallographic and electrical properties of Fe/Cr and Cu/Ni superlattices
Authors: Shiroishi Y. , Sellers C. , Hilliard J. E. , Ketterson J. B.
Published: July 2002

Crystallographic and electrical properties of Fe/Cr and Cu/Ni superlattices synthesized by e-beam evaporation have been analyzed. The films of Fe/Cr and Cu/Ni have strong (110) and (111) textures, respectively. X-ray linewidths of both systems were studied and found to be broadest at a layer period of about 10 atomic planes. The in-plane electrical resistivities were found to closely correlate with the x-ray linewidths. The behavior of both properties is accounted for by considering the roughness of the interface induced by misfit dislocations arising from the relaxation of coherency strain. Our analysis uses a modified paracrystalline theory for x-ray line broadening and a modified Soffer theory for the electrical resistivity.
Low-field characterization of YBa2Cu3O7−δ single crystals
Authors: Claus H. , Crabtree G. W. , Liu J. Z. , Kwok W. K. , Umezawa A.
Published: July 2002

Low-field magnetization measurements in flelds from 0 to 100 G at temperatures from 4.2 to 100 K on single crystals of YBa2 Cu3 O7−δ are presented. The crystals have sharp inductive transitions at 91.5 K, occasionally with additional structure suggesting a second phase with a lower Tc . For all crystals in all directions the Meissner effect is a small fraction of the shielding effect. Low-field measurements of Hc2 within 1 K of Tc show a linear temperature dependence with a slope of 80 G/K, much smaller than observed at higher field.
Molecular-field coefficients of MnFe2O4 and NiFe2O4 spinel ferrite systems
Authors: Dionne Gerald F.
Published: July 2002

Calculated thermomagnetization curves for MnFe2 O4 and NiFe2 O4 have been matched to reported experimental results. Since both Mn2+ and Ni2+ ions are also strongly exchange coupled to the Fe3+ ions, separate sets of molecular-field coefficients were determined for each of these interactions and then averaged into the basic two-sublattice solution. For the usual case with 80% of the manganese in tetrahedral (A) sites, the ratios of coefficients determined from the fit to experiment agree closely with the values found from high-temperature susceptibility measurements, with ‖NAA /NAB‖ = 0.840 and ‖NBB /NAB‖ =0.295. For Ni ferrite, these ratios exceed those determined from magnetic susceptibilty measurements, suggesting an intrasublattice exchange for nickel that is stronger than previously considered. Calculations of magnetic dilution effects in both the manganese and nickel families show good accuracy for a practical range of Zn2+ substitutions in A sites.
Hollow-anode discharge with axial-magnetic field as an electron beam source
Authors: Miljević Vujo I.
Published: July 2002

The hollow-anode discharge, with a hollow-anode aperture 0.5-mm i.d., on which the axial-magnetic field has been applied, is studied. From the hollow anode, outside the discharge tube, an intensive, narrow, and well-focused beam of fast electrons, more than 30 cm long, is obtained.
Crack nucleation at the surface of stressed fibers
Authors: Bouten P. C. P. , de With G.
Published: July 2002

A model is proposed for crack initiation on a pristine surface. In this model failure starts from a flat surface and the crack develops during the strength or lifetime experiment. Both energy and kinetic arguments are used. The model is applied to lifetime calculations of pristine optical fibers. Surface energy and bulk elastic energy are calculated for a fiber with a flat and a slightly distorted surface. When a small amount of elastic energy is removed locally from the fiber, e.g., by dissolution, the distorted state is more stable. Like cracks in brittle materials, the stable distortions for pristine optical fibers have local radii of curvature at atomic dimensions. In this model the lifetime of a pristine fiber is mainly determined by the time needed to pass from a flat surface to a relatively small surface distortion. From the derived expression for the lifetime, the exponent n in the empirical power law expression for subcritical crack growth can be interpreted. It is shown that n is stress dependent. The discrepancy in n value observed for silica glass fibers (n≂20) and bulk fused silica (n≂40) is removed.
D
X-center-like traps and persistent photoconductivity in Te-doped AlxGa1−xSb on GaSb
Authors: Zhu Yu , Takeda Yoshikazu , Sasaki Akio
Published: July 2002

The donor-related deep electron traps in Te-doped Alx Ga1−x Sb on GaSb substrate were investigated by deep level transient spectroscopy, capacitance-voltage, photocapacitance, and Hall-effect measurements. Deep electron traps were not detected in the Al composition range 0≤x<0.2, but were detected in the higher range of x. The concentration of the deep electron traps increases steeply with x and then saturates. The concentration also increases linearly with donor concentration for the same Al composition. In the temperature-dependent Hall-effect measurement, both shallow donor and deep donor levels were observed. The deep donor is dominant for x≥0.4, and the thermal activation energy E0 increases dramatically from 6 to 110 meV in the range of 0.2<x≤0.5. Persistent photoconductivity was observed for x≥0.3 at temperatures below 100 K. All the experimental results indicate that the deep electron traps in Te-doped Alx Ga1−x Sb are quite similar to the DX center in Alx Ga1−x As.
Tensile plasticity and ductile fracture
Authors: Johnson J. N. , Addessio F. L.
Published: July 2002

A mathematical model of tensile plasticity and void growth based on the Gurson flow surface and associated flow law is developed and applied to the problem of ductile fracture under general tensile loading conditions. The flow surface defines the plastic strain components in the tensile region; conditions of fracture are defined in terms of the plastic deformational strain, porosity, and the ratio of mean stress to shear stress, p/τ. This model reduces to the Carroll and Holt [J. Appl. Phys. 43, 759 (1972)] tensile threshold pressure for void growth, and to the Rice and Tracey [J. Mech. Phys. Solids 17, 201 (1969)] expression relating the fractional change in void radius to the incremental plastic deformational strain and p/τ in a triaxial tensile stress field. The model has sufficient generality to represent plastic flow and fracture in notched and smooth tensile bars as well as in uniaxial-strain spallation tests. One- and two-dimensional finite-difference calculations demonstrate this capability.
A two-dimensional computer simulation for dry etching using Monte Carlo techniques
Authors: Ulacia F. J. Ignacio , McVittie James P.
Published: July 2002

This work introduces a novel two-dimensional dry-etching simulation technique and presents experimental verifications of etching profiles based on an algorithm that calculates the transport of charged particles across a plasma sheath, the etch rate at each node, and the overall time evolution of the simulated structure. Monte Carlo techniques are applied to solve for the transport of ions across the plasma sheath to obtain the angular ion-distribution functions at the wafer surface. The etch rates are calculated from the particle fluxes and energies on the wafer during each time step. These fluxes depend on the relative position of the node with respect to the general geometry of the structure. Different etching mechanisms are decoupled into a chemical component proportional to the reactive neutral concentration in the gas and an ion-enhancement component proportional to the energy flux deposited on the wafer. The chemical component is assumed to be uniform and perpendicular to the surface being etched; the ion-enhancement component is calculated from the angular ion-distribution function. These mechanisms are incorporated into a two-dimensional computer-simulation program that computes their interaction as a function of time. The simulation program allows the addition of secondary effects as input options, such as energy and flux thresholds, glancing ions, and inhibitor layers. The predictions based on the simulations agree well with experimental results obtained on extreme cases of chemical and ion-enhanced etching.
Effect of surface composition observed by Auger electron spectroscopy on magnetization and magnetostriction of NiFe and NiFeRh thin films
Authors: Ounadjela K. , Lefakis H. , Speriousu V. S. , Hwang C. , Alexopoulos P. S.
Published: July 2002

The saturation magnetization, 4πMs, and the magnetostriction constant, λs, of Ni81Fe19, Ni81Fe19/Ta, and Ni72Fe17Rh11/Ta thin films were studied as a function of film thickness before and after annealing at 250 °C in vacuum for 2 h. For films of thickness t<200 Å, 4πMs and λs were found to be strongly dependent on film thickness with an even larger variation after annealing. For all films, at t=60Å, 4πMs decreases by 20%–30% compared to bulk value. The value of λs is in the range of 10−6 for all films and shows a complex behavior, including a change of sign, depending on film composition as well as Ta capping. Auger depth profiling of uncapped films has shown the presence of an Fe-rich surface oxide layer whose composition changed and thickness increased after annealing. The composition and thickness of the Ta-film interface layer in the Ta-capped films did not change with annealing. The presence of such layers, at the film surface, Ta-film interface, and film-substrate interface, of different composition and magnetic properties from the film bulk, can explain the observed behavior of 4πMs and λs.
The emittance characteristics of a gallium liquid-metal ion source
Authors: Alton G. D. , Read P. M.
Published: July 2002

First measurements have been made of the emittances of momentum-analyzed 69Ga+ ion beams extracted from a liquid-metal ion source. Comparisons of the emittances of beams extracted and postaccelerated with conical and planar geometry electrode systems indicate that the planar geometry system is less aberrant. Space-charge effects and other sources of energy spread lead to increases in the emittances in the energy-dispersion plane of the momentum-analysis system (x direction) over their y-direction counterparts. The energy spread increases rapidly with beam intensity. Brief descriptions are provided of the experimental apparatus and procedures used during these investigations, and emittance and brightness data measured for ion beams accelerated by the two electrode systems are presented.