bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum mechanics of layers with a finite number of point perturbations
Authors: Exner P. , Němcová K.
Published: July 2002

We study spectral and scattering properties of a spinless quantum particle confined to an infinite planar layer with hard walls containing a finite number of point perturbations. A solvable character of the model follows from the explicit form of the Hamiltonian resolvent obtained by means of Krein’s formula. We prove the existence of bound states, demonstrate their properties, and find the on-shell scattering operator. Furthermore, we analyze the situation when the system is put into a homogeneous magnetic field perpendicular to the layer; in that case the point interactions generate eigenvalues of a finite multiplicity in the gaps of the free Hamiltonian essential spectrum.
Ion beam extraction and transport for high-field electron cyclotron resonance ion sources (abstract)
Authors: Leitner M. A. , Wutte D. , Lyneis C. M.
Published: July 2002

The next-generation, very high magnetic-field electron cyclotron resonance (ECR) ion sources, like VENUS, SERSE, or PHOENIX, strive for substantially (a factor of 10) higher extracted heavy ion beam intensities than currently achievable. Such high-intensity ion beams present significant challenges for the design and simulation of an ECR extraction and low-energy ion beam transport system. Extraction and beam formation take place in a strong (up to 3 T) axial magnetic field, which leads to significantly different focusing properties for the different ion masses and charge states of the extracted beam. Typically, beam simulations must take into account the contributions of up to 50 different charge states and ion masses. Space charge effects must be correctly included since the extraction and mass analyzing system have to be designed for a proton-equivalent current of ∼25 mA at 30 kV extraction voltage. The article discusses state of the art two-dimensional and three-dimensional simulation techniques for such ion beam extraction and transport systems. Furthermore, the main contributions to the ion beam emittance are discussed: (1) The induced beam rotation due to the strong axial magnetic field, (2) the concentration of high charge state ions at the source center axis, and (3) the ion beam temperature. A novel large-gap analyzing magnet design is described which allows efficient correction of higher-order aberrations for high-intensity heavy ion beams. Such a magnet limits emittance blow up, and is necessary if the analyzed beam has to be further transported or accelerated, e.g., in a radio frequency quadrupole.
An integrated effective fragment—polarizable continuum approach to solvation: Theory and application to glycine
Authors: Bandyopadhyay Pradipta , Gordon Mark S. , Mennucci Benedetta , Tomasi Jacopo
Published: July 2002

A new discrete/continuum solvation model has been developed by combining the effective fragment potential (EFP) for the discrete part and the polarizable continuum model (PCM) for the continuum part. The usefulness of this model is demonstrated by applying it to the calculation of the relative energies of the neutral and zwitterionic forms of glycine. These calculations were performed by treating glycine with ab initio wave functions. Water clusters were treated with both ab initio and EFP methods for comparison purposes, and the effect of the continuum was accounted for by the PCM model. The energy barrier connecting the zwitterionic and neutral three-water clusters was also examined. The computationally efficient EFP/PCM model gives results that are in close agreement with the much more expensive full ab initio/PCM calculation. The use of methods that account for electron correlation is necessary to obtain accurate relative energies for the isomers of glycine.
A wide-range electrometer voltmeter for atmospheric measurements in thunderstorms and disturbed meteorological conditions
Authors: Harrison R. G.
Published: July 2002

A wide-range voltmeter system with a semiconductor electrometer input stage is described, primarily for measurements in atmospheric electricity. It is based on the floating input amplifier principle, using optical isolation to control a high voltage power supply. The system responds to unipolar direct current input voltages of up to +10 kV, with a full-scale rise time of ∼0.1 s. The input resistance is estimated as >0.3 TΩ and the output voltage tracks 0–10 V for input voltage variations of 0–10 kV, with voltage errors of 1%. A low impedance guard voltage is available to minimize input losses.
Simple fabrication scheme for sub-10 nm electrode gaps using electron-beam lithography
Authors: Liu K. , Avouris Ph. , Bucchignano J. , Martel R. , Sun S. , Michl J.
Published: July 2002

An innovative and simple method, based on electron-beam (e-beam) overlapping and overexposure techniques, is developed to fabricate sub-10 nm electrode gaps with very good electrical properties. Gaps with 4 to 10 nm spacing can be fabricated using a proper e-beam dose and pattern-developing time. The fabrication yield is nearly 100% for 8–9 nm gaps, but significantly smaller for 3–4 nm gaps. The gap leakage resistance is around 1012–1013 Ω, implying very good isolation. As an example, we present a transport study on a single 8 nm Co particle junction using a 10 nm gap.
Dynamics of Coulomb explosion and kinetic energy release in clusters of heterocyclic compounds
Authors: Card D. A. , Wisniewski E. S. , Folmer D. E. , Castleman A. W.
Published: July 2002

The studies presented herein elucidate details of the Coulomb explosion event initiated through the interaction of heterocyclic clusters with an intense femtosecond laser beam (⩾1 PW/cm2). Clusters studied include 7-azaindole and pyridine. Covariance analysis verifies that the fragmentation channels are competitive. Kinetic-energy analyses, from experiment and simulation, suggest that Coulomb exploded fragments are created with varying amounts of energy and have a strong mass-to-charge relationship. Backward-ejected protons are found to impact the repeller and eject adsorbed protons from the surface. Moreover, delayed fragmentation is suggested by fast-Fourier transformation of a proton time-of-flight mass spectrum and confirmed by deconvoluting the aforementioned signal through intensity decrements. Voltage gradient, laser power, and microchannel plate detector studies yield insight into the solvation effect of clusters in the Coulomb explosion event. Conceptually, the dynamic charge resonance enhanced ionization (Dynamic CREI) model best explains these results of heterocyclic Coulomb explosion.
Giant magnetoimpedance in glass-covered amorphous microwires at microwave frequencies
Authors: da Silva R. B. , de Andrade A. M. H. , Severino A. M. , Schelp L. F. , Sommer R. L.
Published: July 2002

In this work, results on the giant magnetoimpedance of glass-covered amorphous microwires with nominal composition Co70.4Fe4.6Si15B10 are presented. The impedance Z=R+iX has been investigated as function of frequency (1 MHz–1.8 GHz) and magnetic field (up to ±400 Oe), using a HP4396B impedance analyzer and an appropriate coaxial microwave cavity. The effects of the thermal treatments (Joule heating, from 1 up to 50 mA for 10 min) for anisotropy induction and/or to induce recrystallization have been investigated.
Magnetization reversal in lithographically patterned sub-200-nm Co particle arrays
Authors: Hao Y. , Castaño F. J. , Ross C. A. , Vögeli B. , Walsh M. E. , Smith Henry I.
Published: July 2002

A series of Co particle arrays with rectangular elements having a thickness of 10 nm, a width of 90 nm and aspect ratios of 1.3, 2.2, and 3.3, has been fabricated using interference lithography. The switching behavior of these arrays has been studied by measuring isothermal remanence measurement (IRM), dc demagnetization measurement (DCD), and hysteresis loops using magnetometry and magnetic force microscopy (MFM). The single domain structure is the only stable structure at remanence. Nonuniformity and redeposition debris from ion beam etching (IBE) cause a large reversible magnetization component. The comparison between IRM and DVD curves shows that the interactions between the dots are negligible. Both vibrating sample magnetometer (VSM) measurements and MFM images show that the dots switch over a large range of fields, which is believed due mainly to the crystallographic orientation distribution of the grams within each element.
Electron acceleration and trapping by an oblique shock wave
Authors: Bessho Naoki , Ohsawa Yukiharu
Published: July 2002

Electron acceleration to ultrarelativistic energies by an oblique magnetosonic shock wave is studied. First, the maximum electron energy is analytically obtained with new simple calculations. The physical mechanism is also discussed in detail. In the wave frame, electrons reflected near the end of the main pulse region, which will be trapped, gain energy from the electric potential and constant electric field Ey0 perpendicular to the external magnetic field. For certain plasma parameters, these electrons can move a long distance in the direction of Ey0 and obtain a great amount of energy from Ey0. It is argued that the trapped electrons can hardly escape from the shock wave. Next, one-dimensional, relativistic, particle simulations are carried out. Theoretical estimates such as the maximum electron energy are found to be in good agreement with the simulations. Simulations also show that the number of trapped electrons continually increases with time.
Structure and dynamics of ice Ih films upon HCl adsorption between 190 and 270 K. II. Molecular dynamics simulations
Authors: Toubin C. , Picaud S. , Hoang P. N. M. , Girardet C. , Demirdjian B. , Ferry D. , Suzanne J.
Published: July 2002

Classical molecular dynamics simulations are carried out between 190 and 250 K on an ultrathin ice film doped by HCl deposition with a coverage varying from 0.3 to 1.0 monolayer. These conditions are similar to those defined in the experiments described in the companion paper. Within the assumption that the hydracid molecule remains in its molecular form, the order parameters and the diffusion coefficients for the H2O molecules are determined in the HCl doped ice film, and compared to the experimental data. The residence times of HCl at the ice surface are also calculated. Below 200 K, the HCl molecules are found to remain localized at the ice surface, while above 200 K, the HCl diffusion inside the film is easy and leads to a strong disorder of the ice structure. Although the formation of hydrates cannot be interpreted by the present calculations, the lowering of the ice melting temperature by 15 K measured in neutron experiments for an HCl doped ice film is qualitatively explained by simulation results.
An experimental setup for nondestructive deposition of size-selected clusters
Authors: Klingeler R. , Bechthold P. S. , Neeb M. , Eberhardt W.
Published: July 2002

An experimental setup for the deposition of mass-selected clusters using a laser vaporization source and a magnetic field mass selector is presented. Nondestructive deposition and a coverage of 1% of a monolayer within 5 h are achieved for mass-selected metallofullerene clusters as demonstrated for deposited Ce@C60 on highly oriented pyrolytic graphite.
Influence of cobalt and nickel substitutions on populations, hyperfine fields, and hysteresis phenomenon in magnetite
Authors: Sorescu Monica , Grabias A. , Tarabasanu-Mihaila D. , Diamandescu L.
Published: July 2002

In this work the magnetic properties of magnetite powders doped with Co and Ni are investigated as a function of dopant concentration. Two sets of Fe3−xTxO4 powders with T=Co and Ni, x=0–1 were prepared using a hydrothermal method, with particle sizes of about 1 μm. The Mössbauer measurements revealed that both Co2+ and Ni2+ ions are located mostly on the octahedral sites, affecting the hyperfine fields and relative populations of both sites. In the case of Co-doped magnetite, the hyperfine magnetic fields increase almost linearly with increasing cobalt content. In the case of Ni-doped magnetite, the influence of annealing temperature during preparation was studied. For both subcritical and critical temperatures, the hyperfine fields of the tetrahedral and octahedral sites are larger than those corresponding to the magnetite powder. Bulk magnetic properties of these powders were studied by means of hysteresis loops recorded at 4.2 K in an applied field of 1.5 T. The results are compared with the pure magnetite powder obtained by the same hydrothermal method with similar particle sizes. It was found that Co addition significantly increases the coercive field of doped magnetite, while Ni causes its slight decrease.
Ultralow frequency perfect saw-tooth generator
Authors: Bayard J.
Published: July 2002

A method allowing the making of a perfect ultralow frequency saw-tooth generator is presented. A description, with experimental results obtained on a complete realization using only versatile components, is also given. This saw-tooth generator was used to feed a three-phase, voltage controlled sinusoidal oscillator.
Two-dimensional double simple waves in a pair plasma at relativistic temperatures
Authors: Tskhakaya D. D. , Eshraghi H.
Published: July 2002

The two-dimensional problem for double simple waves in an electron–positron plasma is analytically solved. The relations between the hydrodynamical quantities and two components of the wave velocity are found. For explicit description of spatial and time behavior of quantities, the initial distributions (for instance of the velocity) must be known. The initial distributions must satisfy a definite condition, which means that the double simple wave cannot be developed from every initial condition. As in the usual simple wave, the two-dimensional double simple wave can suffer discontinuities during its evolution. An equation for the characteristic surface (characterizing the propagation of disturbances under the double simple wave restriction) is constructed. It is also shown that in the two-dimensional case it is impossible to find any Riemann invariant.
Development of new steady-state, low-energy, and high-flux ion beam test device
Authors: Shimada Tomohisa , Ueda Yoshio , Sagara Akio , Nishikawa Masahiro
Published: July 2002

A new steady-state, low-energy, and high-flux ion beam test device has been developed to study erosion, re-deposition, and hydrogen retention mechanisms of plasma facing materials under high flux conditions. Source plasma was produced by electron cyclotron resonance discharge with 2.45 GHz microwave. Ion extraction was made by spherical multiaperture triode electrodes to obtain high ion flux at the focal point. Ion extraction aperture geometry was designed so that high current density can be obtained in the case of low energy (<500 eV) by applying high deceleration voltage. It was demonstrated that low-energy and high-flux ion beam can be produced by increasing deceleration voltage with the fixed acceleration voltage (beam energy). In the case of hydrogen beam energy of 300 eV, beam power density has rapidly increased with a deceleration voltage over accel–decel ratio 4, and maximum beam flux is 4.1×1020 H/m2 s. On the other hand, maximum beam flux of 3.6×1021 H/m2 s was obtained for 3 keV hydrogen beam. These fluxes are much higher than that of conventional mass analyzed ion beam (<1.0×1020 H/m2 s).
Optimal control in laser-induced population transfer for two- and three-level quantum systems
Authors: Boscain Ugo , Charlot Grégoire , Gauthier Jean-Paul , Guérin Stéphane , Jauslin Hans-Rudolf
Published: July 2002

We apply the techniques of control theory and of sub-Riemannian geometry to laser-induced population transfer in two- and three-level quantum systems. The aim is to induce complete population transfer by one or two laser pulses minimizing the pulse fluences. Sub-Riemannian geometry and singular-Riemannian geometry provide a natural framework for this minimization, where the optimal control is expressed in terms of geodesics. We first show that in two-level systems the well-known technique of “π-pulse transfer” in the rotating wave approximation emerges naturally from this minimization. In three-level systems driven by two resonant fields, we also find the counterpart of the “π-pulse transfer.” This geometrical picture also allows one to analyze the population transfer by adiabatic passage.
Synthesis of 3C–SiC nanowhiskers and emission of visible photoluminescence
Authors: Zhang Yafei , Nishitani-Gamo Mikka , Xiao Changyong , Ando Toshihiro
Published: July 2002

Single-crystal 3C–SiC nanowhiskers with [111] axial orientation have been synthesized directly on an Si substrate with a large area and high surface density. The nanowhiskers were grown into the Si substrate like a foundation pile by using an Fe film as a catalyst in a microwave plasma chemical vapor deposition system. Most of the whiskers were cylinder shaped with a circular cross section, while some of the whiskers had cross sections in the shapes of squares, rectangles, triangles, and hexagons. The nanowhiskers possessed visible photoluminescence at room temperature and blueshift effect depending on the whisker diameter. These results offer interesting prospects for the fabrication of an Si-based light emitter through the visible wavelength regions. The synthesis process should allow for the development of nanowhisker devices integrated into Si technology.
Isotropic Raman line shapes near gas–liquid critical points: The shift, width, and asymmetry of coupled and uncoupled states of fluid nitrogen
Authors: Musso Maurizio , Matthai Frank , Keutel Dietmar , Oehme Karl-Ludwig
Published: July 2002

In order to improve the experimental database about the additional nonrotational broadening of vibrational line shapes observable when a simple fluid approaches its gas–liquid critical point, we improved the pioneering measurements of Clouter and Kiefte [for their own review see Phys. Rev. A 33, 2749 (1986)] on the critical behavior of the polarized Raman line of fluid nitrogen by using the isotopic mixture (14N2).975−(14N15N).025, giving special attention to the fact that the isotropic line shape of liquid N2 (ν̃≈2327 cm−1) is affected by intermolecular vibrational resonance couplings. Using a highest-resolution double monochromator and modern CCD detection techniques, we were able to follow the temperature dependencies of the line shape parameters (i.e., shift, width, and asymmetry) of the coupled N142 and, depending on the S/N ratio available, of the uncoupled N1415N in the range 45 K≲T≲300 K with up to mK resolution (1) in the β-solid phase, (2) in the coexisting liquid and gas phases, and (3) along the critical isochore. Comparing the line shifts of both isotopic species, clear evidence was found that vibrational resonance couplings are present in all dense phases studied, the line position ν̃0 being more density than temperature dependent. Additionally, the existence of (negative) cross correlations between resonant and nonresonant dephasing mechanisms has been confirmed by the change in sign observed for the small but non-negligible difference in the linewidths between coupled N142 and uncoupled N1415N around 90 K. The λ-shaped dependencies of the width parameters, observed when moving along the coexistence line through the critical point, Tcrit=126.192 K, and along the critical isochore, is much more evident in the line asymmetry than in the usually considered linewidth. Clear proof was found that, in accordance with theoretical predictions, the linewidth converges to a constant maximum value regardless if the critical point is reached along the coexistence line or along the critical isochore, i.e., it does not diverge approaching the critical temperature up to our closest value |T/Tcrit−1|≈10−5.
Square-well fluids: The statistical and thermodynamic properties of short chains
Authors: Fridrikh Sergey V. , Lipson J. E. G.
Published: July 2002

The statistical and thermodynamic properties of tangent square-well trimers and tetramers have been studied using the Born–Green–Yvon integral equation method. We have calculated the temperature and concentration dependence of the site-site correlation functions over a range of temperatures, and use these results to predict the equation of state. We have made use of our compressibility results in order to map out an approximate spinodal diagram, and have obtained both a binodal and spinodal via the energy route. We discuss our results in the context of simulation data and the predictions of alternative theories.
Field-cooled static nonlinear response of relaxor ferroelectrics
Authors: Kutnjak Z. , Pirc R. , Blinc R.
Published: July 2002

The third-order static field-cooled nonlinear dielectric permittivity in relaxor ferroelectric PbMg1/3Nb2/3O3 has been measured as a function of temperature. It is shown that the observed behavior can be explained by a mechanism of nonlinear response within the framework of the spherical random bond-random field model.
Theab initiopotential energy surface and vibrational–rotational energy levels of dilithium monoxide, Li2O
Authors: Koput Jacek , Peterson Kirk A.
Published: July 2002

The equilibrium structure and potential energy surface of dilithium monoxide, Li2O, have been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with basis sets of double- through quintuple-zeta quality. The effects of core–electron correlation on the calculated molecular parameters were investigated. The vibrational–rotational energy levels of the LiO77Li and LiO76Li isotopic species were calculated by a variational approach. A comparison with results of recent experimental high-resolution studies is presented.
Evolution of Ge/Si(100) island morphology at high temperature
Authors: Zhang Yangting , Floyd Margaret , Driver K. P. , Drucker Jeff , Crozier P. A. , Smith David J.
Published: July 2002

Atomic force microscopy, transmission electron microscopy, and electron energy-loss spectroscopy have been used to study the size, structure, and composition of Ge/Si(100) islands grown by molecular beam epitaxy at 700 °C. It is found that the island evolution is qualitatively different than for growth at lower substrate temperatures. For growth at 1.4 ML/min, the composition is determined to be Si0.56Ge0.44 and appears to be independent of island size. A higher growth rate, 4.8 ML/min, kinetically stabilizes pure Ge pyramids prior to Si interdiffusion taking place. These pure Ge clusters are absent at the lower growth rate, demonstrating the influence of deposition rate on island evolution. This result indicates that deposition kinetics can control island composition and morphology without varying growth temperature and associated thermally activated processes.
Influence of GaN domain size on the electron mobility of two-dimensional electron gases in AlGaN/GaN heterostructures determined by x-ray reflectivity and diffraction
Authors: Zhong Zhenyang , Ambacher O. , Link A. , Holy V. , Stangl J. , Lechner R. T. , Roch T. , Bauer G.
Published: July 2002

X-ray reflectivity and diffraction measurements were performed on Ga-face AlGaN/GaN heterostructures to determine the influence of interface roughness scattering and GaN domain boundaries scattering on the electron mobility of polarization induced two-dimensional electron gases. From simulations of the specular reflectivity, the root-mean-square roughness of the AlGaN/GaN interfaces was obtained. In reciprocal space maps, laterally elongated streaks passing through the Bragg peaks have been observed, which are attributed to column-like domains in the GaN buffer layers. The relationship between electron mobility measured by Hall effect and the interface roughness on one hand, and the column domain size on the other hand, demonstrates that the interface roughness scattering is not limiting the electron mobility, whereas the transport properties of the two-dimensional electron gas degrade with decreasing size of columnar domains in the GaN layer.
Nanomechanical resonant structures as tunable passive modulators of light
Authors: Sekaric L. , Zalalutdinov M. , Turner S. W. , Zehnder A. T. , Parpia J. M. , Craighead H. G.
Published: July 2002

We report on the optical parametric excitation of silicon nanomechanical resonant structures. The threshold laser power needed to set these structures into self-oscillation was estimated to be around 70 μW. We measured resonant frequencies of up to 38 MHz by optical excitation; this method should extend to much higher frequencies. Under optical amplification of motion, the spectral response at resonance narrowed to the equivalent of a mechanical quality factor >20 000 at room temperature. These structures act as frequency tunable passive modulators of light, requiring no additional drive.
Influence of sample size on ultrasonic phase velocity measurements in piezoelectric ceramics
Authors: Zhang Rui , Jiang Bei , Cao Wenwu
Published: July 2002

Phase velocities of ultrasound are used for characterizing elastic properties of solids. One of the basic requirements is that the sample area should be much larger than the size of the transducer so that plane wave approximation will hold. This geometric requirement may not be possible to realize for some materials that can only be made in very small size. Using poled and unpoled lead zirconate titanate (PZT-5H) ceramics as examples, we have analyzed experimentally the sample size influence to the ultrasonic measurements. The smallest dimension that is in contact with the transducer is only 14% of the diameter of the transducer. We found that the phase velocity increases when the contact area becomes smaller. The velocity increase is 1.4% and 0.9%, respectively, in the unpoled and poled PZT-5H for the smallest dimension sample compared to bulk values.
Submicron liquid crystal pixels on a nanopatterned indium tin oxide surface
Authors: Behdani M. , Rastegar A. , Keshmiri S. H. , Missat S. I. , Vlieg E. , Rasing Th.
Published: July 2002

We have prepared a grooved indium tin oxide (ITO) surface with groove widths of ∼40–90 nm and a variable groove separation up to 36 μm using atomic force microscopy nanolithography. Twisted nematic pixels with 4-n-pentyl-4-cyanobiphenly (5CB) liquid crystal were prepared using a rubbed polyimide counter plate. The 5CB molecules align along the direction of the grooves, but no pretilt angle was observed on the ITO surface. The surface anchoring energy appeared not to depend on the groove separation nor on the scan force. The lateral correlation length of 5CB on ITO was measured to be 2.2 μm. If a grooved surface is scanned in another direction, liquid crystal molecules follow the direction of the last scan.
Optical properties and quantum efficiency of a-Si1−xCx:H/a-Si:H solar cells
Authors: Schade H. , Smith Z E.
Published: July 2002

The optical properties of all components of heterojunction solar cells based on hydrogenated amorphous silicon (a-Si:H) have been measured individually. By combining these data with knowledge of the cell geometry, an envelope quantum efficiency curve is generated, being simply the fraction of incident photons absorbed in the i layer of the p-i-n cell. Resultant curves for typical cell dimensions are seen to agree remarkably well with observed values for cells with long minority-carrier collection lengths. The model is then used as a diagnostic tool, and finally is demonstrated to have predictive value in the quest for further optimization of solar-cell performance.
Electrical characteristics of zinc oxide varistors subjected to hydrostatic pressure
Authors: Dorlanne O. , Ai Bui , Destruel P. , Loubière A.
Published: July 2002

This paper presents the changes of electrical characteristics in zinc oxide varistors subjected to hydrostatic pressure up to 4 kbar. Electrical measurements have been carried out with pressurized samples. Beyond 1 kbar, the leakage current increases exponentially with pressure as described by the relationship IF=P7,4. This leads to a decrease, by a factor of 2, of the nonlinear coefficient. In spite of choosing a different experimental approach and different material compounds, our results are in very good agreement with those already published.
Initial phase formation and dissociation in the thin-film Ni/Al system
Authors: Colgan E. G. , Nastasi M. , Mayer J. W.
Published: July 2002

We have investigated the interactions of Al/Ni, Al/NiAl/Ni, Al/Ni3Al, and NiAl3/Ni thin films between 300 and 425 °C. The films were prepared by sequential evaporation and coevaporation, then vacuum annealed and analyzed by Rutherford backscattering and x-ray diffraction. The reaction always started at the interface in contact with the most Al-rich film and resulted in formation and growth of NiAl3, unless that was the most Al-rich layer. Subsequently the reaction proceeded to Ni2Al3. We conclude that the kinetics rather than the thermodynamics is responsible for determining the growth of these phases.
Reactively magnetron sputtered Hf-N films. II. Hardness and electrical resistivity
Authors: Johansson B. O. , Sundgren J.-E. , Helmersson U.
Published: July 2002

The microhardness and electrical resistivity have been measured on thin Hf-N films, covering the entire composition range from pure Hf to overstoichiometric HfN. Influence of substrate bias on the properties of stoichiometric HfN films has also been studied. All films have been prepared by high-rate reactive magnetron sputtering at a substrate temperature of 400 °C. Both hardness and resistivity increase as nitrogen is added to the α-Hf phase. For the cubic HfN phase the hardness has a maximum, ≊3500 HV, and the resistivity a minimum, 225 μΩ cm, at a composition close to stoichiometry. However, both values are considerably higher than those reported for bulk samples. This is explained in terms of nonequilibrium growth conditions, giving rise to high densities of dislocations and interstitially incorporated nitrogen atoms. For films with a nitrogen content above 50 at. % a very high-resistivity value is found, 2.0 Ω cm at maximum. By applying a low substrate bias voltage the resistivity of stoichiometric HfN was decreased with a factor of about 3, and simultaneously a grain growth and/or reduced defect concentration was observed. At higher bias voltages the resistivity increases and the grain sizes decrease.
Light ion beam transport in plasma channels
Authors: Ozaki T. , Miyamoto S. , Imasaki K. , Nakai S. , Yamanaka C.
Published: July 2002

The transport system of intense light ion beams through plasma channels is considered in the reactor system. Channel parameters for the beam transport are determined by diode brightness, pellet requirements, and channel constraints. We considered the desirable reactor parameters and the transportable power window. We have studied most of the single-channel formation by the laser initiation, the beam transport in this channel, and the multiple-channel formation within this window. Uniform and stable plasma channels were formed in 10–40 Torr of ethylene gas by the guidance of the CO2 laser with up to 1-m length. Ion beams were injected to this channel from a pinch-reflex ion diode on the Reiden IV generator. High total transport efficiencies of up to 70% were achieved. Beam orbits, estimated from the post-transported beam pattern on the detector film with energy resolution, were in good agreement with the calculation in the center current mode. We proposed the uniform irradiation reactor channel system using the dodecahedral arrangement so as to satisfy the transportable window constraints. A triple plasma channel was also formed as a simulational model of an overlap channel system. A desirable channel system is expected from experiments and calculations when channels and return paths are alternately arranged.
The effect of an interfacial oxide layer on the Schottky barrier height of Er-Si contact
Authors: Wu C. S. , Scott D. M. , Lau S. S.
Published: July 2002

We have studied the Er and ErSi2 contacts on p-Si. A correlation is found between the barrier height and the presence of an interfacial oxide layer. Samples with a thin interfacial oxide (∼ 3 Å) exhibit a barrier height of 0.68 eV. In contrast, samples without any detectable interfacial oxide show a higher barrier height of ∼0.8 eV. The barrier heights were determined by current-voltage measurements and the thicknesses of the interfacial oxide were measured by the 16O(d,α)14 N nuclear reaction technique.
Doublet state of resonantly coupled AlGaAs/GaAs quantum wells grown by metalorganic chemical vapor deposition
Authors: Kawai H. , Kaneko J. , Watanabe N.
Published: July 2002

Quantum-mechanically coupled well systems consisting of two GaAs wells 30 Å thick separated by an Al0.5Ga0.5As barrier whose thickness was varied from 12 to 40 Å have been grown by metalorganic chemical vapor deposition. The photoluminescence spectra of these systems indicated the splitting of a degenerate single well state into a doublet state, a symmetrical state, and an antisymmetric state. The location of the spectrum peak and shoulders agreed well with the calculated energies using Dingle’s connection rule which assumes 85% conduction band offset and the continuous first derivative of the wave function across the AlGaAs/GaAs heterojunction. Two other connection rules were tried, but the agreement was worse.
Strain and surface damage induced by proton exchange in Y-cut LiNbO3
Authors: Campari A. , Ferrari C. , Mazzi G. , Summonte C. , Al-Shukri S. M. , Dawar A. , De La Rue R. M. , Nutt A. C. G.
Published: July 2002

When Y-cut LiNbO3 substrates are proton exchanged in pure benzoic acid to fabricate optical waveguides, they suffer surface damage, and a consequent degradation in optical properties. This effect is mainly produced by a remarkably large strain in the exchanged layer in a direction normal to the surface. This strain leads to a large number of cracks and to the peeling off of the exchanged layer itself. This paper presents a probable explanation of the mechanism involved.
Intrinsic stress and hydrogen bonding in glow-discharge amorphous silicon films
Authors: Kakinuma H. , Nishikawa S. , Watanabe T. , Nihei K.
Published: July 2002

We have found a strong correlation between intrinsic stress and hydrogen bonding in glow-discharge amorphous silicon (a-Si:H) films. The stress of high-deposition-rate (HDR) films continuously changes from tensile to compressive with increasing rf power. In contrast, low-deposition-rate (LDR) films indicated compressive stress in all the rf power range which was varied. However, adding a small amount of Ar to SiH4 in the LDR film deposition changes its stress from compressive to tensile as in the HDR films. IR absorption spectroscopies showed that films with compressive stress always indicated a smaller ratio of absorptions at 2070 and 2000 cm−1 α(2070)/α(2000) than the value Rc(=0.9–1.2), whereas those with tensile stress showed a larger ratio. Consequently, zero or very weak stress is reached at the ratio approximately equal to Rc, regardless of varying deposition parameters. It is also shown that annealing alters the stress of all the HDR films to strong tensile, and that α(2070)/α(2000) decreases, differing from the correlation where the rf power and Ar mixing ratio are varied.
Contactless measurement of bulk free-carrier lifetime in cast polycrystalline silicon ingots
Authors: Johnson S. M. , Johnson L. G.
Published: July 2002

The spatial dependence of the bulk free-carrier lifetime in cast polycrystalline silicon ingots was determined from contactless measurements of optically modulated free-carrier infrared absorption. Using a cw Nd: yttrium aluminum garnet (Nd:YAG) laser (λ=1.319 μm) for a probe and a novel pulsed tunable infrared dye laser (λ=1.10–1.13 μm), with photon energies near the Si band gap, for a pump, the free-carrier lifetime was determined from transient absorption measurements to a maximum depth of 3.0 cm from the surface of polycrystalline silicon ingots. Unmodulated infrared probe transmission measurements were used to monitor relative changes in the bulk precipitate density in ingots due to infrared scattering by the precipitates. Free-carrier lifetime profiles measured along the growth axis of polycrystalline silicon ingots had the same qualitative features as the minority carrier lifetime profile measured from n+p junction solar cells fabricated on the wafered ingots. Differences between the ingot-measured free-carrier lifetimes and the minority carrier lifetime, apparently due to high injection and possibly carrier trapping effects, are discussed.
Bounds on the properties of fiber-reinforced composites
Authors: Joslin C. G. , Stell G.
Published: July 2002

The microstructure of a fiber-reinforced composite material can be characterized in terms of a set of n-point matrix probability functions Sn which give the probability of finding n points all in the matrix phase. From a knowledge of these functions one can place rigorous upper and lower bounds on effective thermal, electrical, and mechanical properties of the composite. Third- and fourth-order bounds derived by Milton involve threefold integrals over the three-point matrix function S3. Exact analytic determination of S3 is impossible except in idealized cases, and the highly oscillatory nature of the integrands makes numerical integration costly and time consuming. In this paper we discuss an approximation to S3 in terms of the two-point function S2, which is often quite easy to determine. We show that in this approximation the bounds can be expressed in terms of twofold integrals for which the integrands are mostly of one sign, and which can be evaluated very cheaply to a high degree of precision. For a model microstructure in which the inclusions consist of fully penetrable cylinders, we compare our approximate results with the exact results. Agreement is excellent.
A model of radial variations in the low-pressure mercury–argon positive column
Authors: Dakin James T.
Published: July 2002

The low-pressure Hg–Ar positive column is studied with a model which accounts for radial transport mechanisms. The model calculates, not assumes, the radial density distribution for each of the species and the radial temperature distributions for the electron and the neutral gases. Solutions to the discharge equations are found for specified values of the wall (Hg reservoir) temperature, electric current, Ar pressure, and tube diameter. These solutions compare favorably with available parametric data for 3.6- and 1.4-cm-diam. discharge tubes. The model is used to investigate effects such as radial cataphoresis, the effect of radiation trapping on radial distributions, and deviations from a radially uniform electron temperature. The work supports some of the simplifying assumptions often made in modeling the discharge at the standard fluorescent lamp operating condition, such as a radially invariant electron temperature, the use of a constant escape factor to describe radiation trapping, and negligible radial cataphoresis. Under conditions of high Ar pressure, the assumption of a radially invariant electron temperature is found to be less valid.
Emission spectroscopy of long pulse relativistic electron-beam-produced argon plasmas
Authors: Brake M. L. , Repetti T. , Pearce K. , Lucey R.
Published: July 2002

Visible emission spectroscopy of long pulse, relativistic, electron-beam (300 ns, 1 kA, 300 keV) -produced argon plasmas has been performed over a wide pressure range (1–750 Torr). The emission spectra were observed between 350 and 600 nm with a spectrograph coupled to an optical multichannel analyzer, which was gated with pulses of 50 and 500 ns. Singly ionized argon lines were observed at all pressures and all times during the beam current. The relative line intensities for the (4s–4p) transitions fit those described by an equilibrium distribution at approximately 2 eV. The emission lines originating from the higher excited states of (4s′–4p′) and (4s″–4p″) were much larger than expected in a 2-eV plasma, and this enhancement can be attributed to the presence of the beam electrons.
Defect structure and formation mechanism of drawing-induced absorption at 630 nm in silica optical fibers
Authors: Hibino Yoshinori , Hanafusa Hiroaki
Published: July 2002

This paper presents a study on drawing condition dependences of optical absorption at 630 nm and nonbridging oxygen hole centers (NBOHCs) in optical fibers drawn from low-OH-content synthetic silica glass. The intensity of the 630-nm absorption is proportional to the NBOHC concentration, and the value of the oscillator strength for the NBOHC is 4.2×10−4, which nearly equals the theoretical values. These results confirm that the 630-nm absorption is due to the NBOHCs in low-OH-content silica glass. Furthermore, the 630-nm absorption increases with increasing drawing tension. This drawing tension dependence is explained by applying kinetics of the stress-induced crack growth in metal to the formation of the NBOHCs in the drawing process.
Electron beam pumped lasing in ZnSe/ZnSSe superlattice structuresn grown by molecular-beam epitaxy
Authors: Cammack D. A. , Dalby R. J. , Cornelissen H. J. , Khurgin J.
Published: July 2002

We report the first operation of molecular-beam epitaxy (MBE) grown ZnSe/ZnSxSe1−x superlattice electron beam pumped lasers in the temperature range 100–300 K and the first room-temperature operation of MBE grown ZnSe on GaAs transverse geometry electron beam pumped lasers. Threshold current densities of 2.5 A/cm2 at 100 K were achieved by both devices. At room temperature, threshold current was 5 A/cm2 for the ZnSe devices and 12 A/cm2 for the superlattice.
rf characterization of thermally diffused superconducting niobium nitride
Authors: Tu M. Pham , Mbaye K. , Wartski L. , Halbritter J.
Published: July 2002

We have prepared niobium nitride by a thermal diffusion technique of nitrogen into bulk niobium at 800 °C. The resulting NbN layer has been characterized at both low and high frequencies. We obtained transition temperature Tc=11.5 K, ‘‘penetration depth’’ λ0=85 nm at 9 GHz, and surface resistance Rs evaluated at 5.5 K and 9 GHz of 2.8×10−5 Ω. Moreover, this high-frequency Rs measured for thermally diffused NbN is better than the Rs of Nb, and is comparable to the Rs of sputtered NbN. Our diffusion technique is well adapted to all kinds of shapes. In addition, improved diffusion conditions allow Tc as high as 16 K.
Transverse flow cw atomic iodine laser at 1.315 μm
Authors: Schlie L. A. , Rathge R. D.
Published: July 2002

A transverse flow cw atomic iodine laser at 1.315 μm excited by the photolytic pumping of C3F7I and demonstrating virtually an unlimited operating time is reported. The use of transverse flow establishes a much more constant integrated gain along the optical axis than that previously reported for the longitudinal flowing cw atomic iodine laser. Consequently, a cw atomic iodine laser is achieved having a greatly improved amplitude stability and easier operation in only the fundamental TEM00 transverse mode. Such an operation makes this type of cw atomic iodine lasr much more useful as a diagnostic probe laser at 1.315 μm than the previously reported longitudinal device.
Wall-induced inhomogeneities in the low-pressure Hg-Ar positive column
Authors: Dakin James T. , Bigio Laurence
Published: July 2002

Radial variations of the Hg(63P0,1,2) densities are measured in a low-pressure Hg-Ar discharge and the results are compared with a model. The 34-mm-diam discharge tube is operated in the vicinity of 400-mA current, 6.4-mTorr Hg pressure, and 2.81-Torr Ar pressure. A tunable dye laser beam passes axially through the discharge tube, and densities are measured using both a saturated absorption technique and an interference technique. The model incorporates continuity equations for Hg excited and ionic states, and a two-temperature approach to parametrizing the electron energy distribution function. The shapes of the radial density distributions of the various species are predicted rather than assumed. Considerable attention is given to the role of tail electrons above the energy threshold for Hg inelastic scattering. Theoretical considerations and the measured variations of the 63P0 and 63P2 metastable densities suggest that the tail electron temperature is lower near the wall than at the center of the discharge.
Lasers pumped by ion beams
Authors: Ulrich A. , Hammer J. W. , Biermayer W.
Published: July 2002

Eleven rare-gas laser lines in the near infrared and the 1.45-μm line in carbon have been pumped with an intense 3.6-MeV He+ dc beam from the Stuttgart Dynamitron accelerator. In one experimental run a 40 Ar beam was also successfully tested. Laser action was observed in gas mixtures up to several hundred mbar total pressure using a differentially pumped gas target. A maximum output power of 170 mW was observed from He-Ar laser lines. This shows that ion beam accelerators which are widely used in nuclear physics can also be used as a versatile pumping source for lasers. The maximum power which can presently be provided by the Stuttgart accelerator is about 1 kW (dc). The observed laser lines in the infrared region between 1.15 and 3.07 μm are known from low-pressure electrical discharge lasers. The excitation scheme at the relatively high pressures used had been investigated in connection with nuclear pumped lasers by other authors. Ion beam pumping is found to be an interesting pumping method in itself and at present a suitable way to optimize the conditions relevant for nuclear-pumped lasers as the measurements do not have to be performed in the vicinity of a high flux nuclear reactor.
Stress-induced changes in the magnetic properties of some nickel-copper and nickel-cobalt alloys
Authors: Jiles D. C. , Chang T. T. , Hougen D. R. , Ranjan R.
Published: July 2002

This paper presents results on the magnetic properties of NiCu and NiCo alloys with compositions in the range 70%–100% Ni. Measurements include coercivity initial susceptibility, maximum differential susceptibility, magnetostriction, and Barkhausen effect emissions. Results revealed a surprising increase in the magnetostriction as a function of magnetic induction B as the copper content increased. The saturation magnetostriction λs was nevertheless found to decrease with copper content. The addition of cobalt reduced the magnitude of the magnetostriction resulting in a change of sign from negative to positive at between 20% and 30% cobalt. Coercivity was found to be little affected by elastic stress, but strongly affected by plastic deformation such as occurs, for example, in cold working. A similar result was true for initial susceptibility. Maximum differential susceptibility was, however, found to be strongly dependent on elastic stress. Results were interpreted in terms of stress induced uniaxial anisotropy. As the tensile stress increased the magnetization became more reversible, but the susceptibility decreased.
Ion-beam-induced epitaxial crystallization kinetics in ion implanted GaAs
Authors: Johnson S. T. , Williams J. S. , Nygren E. , Elliman R. G.
Published: July 2002

Thin amorphous GaAs layers on (100)-oriented substrates, generated by Si+ ion bombardment at 77 K, have been observed to recrystallize epitaxially during 1.5-MeV Ne+ bombardment in the temperature range 75–135 °C. Crystallization proceeds linearly with increasing ion fluence, except in the near-surface region, and the process is characterized by an activation energy of 0.16 eV, which is an order of magnitude smaller than that obtained for conventional thermal annealing at much higher temperatures.
Radial variations of Hg(63P
J) in a narrow-diameter low-pressure Hg-Ar discharge
Authors: Bigio Laurence , Dakin James T.
Published: July 2002

Two laser techniques are used to measure densities of mercury atoms in the three 63PJ states populated in the positive column of a narrow (13.7 mm diameter) low-pressure Hg-Ar dc discharge. Relative densities are obtained as a function of radial position using a saturated laser absorption technique, while on-axis absolute densities are measured using the hook method (an interferometric technique). Results are shown for two mercury vapor pressures and are compared with computer model predictions.