bluuebunny/crossref_metadata_2025 huggingface dataset:
Negative ion formation from low energy (0–15 eV) electron impact to CF2Cl2 under different phase conditions
Authors: Langer Judith , Matt Sara , Meinke Martina , Tegeder Petra , Stamatovic Aleksandar , Illenberger Eugen
Published: July 2002

Negative ion formation following low energy (0–10 eV) electron attachment to free and bound CF2Cl2 molecules is studied in (1) a molecular beam experiment (single molecules, homogeneous clusters, and mixed CF2Cl2/NH3 clusters) and (2) a UHV surface experiment where desorption of negative ions from condensed CF2Cl2 is observed. From single gas phase CF2Cl2 molecules we observe Cl− and F− generated via dissociative electron attachment from a resonance near 0 eV and 3 eV, respectively, as the most abundant ions. From homogeneous clusters (CF2Cl2)n, we additionally detect undissociated complexes of the form (M)n−(M=CF2Cl2) including the stabilized monomer CF2Cl2− and also “solvated fragment ions” of the form Mn⋅X−(X=Cl, F). Their relative abundance vs size (n) of the final product varies in a significant different way between (M)n− and Mn⋅X− reflecting the different relaxation probabilities in the initial cluster. In the desorption spectra, the dominant low energy Cl− gas phase resonance is strongly suppressed in favor of a significant resonant feature appearing near 8 eV. These last results are discussed in light of previously reported giant enhancements of electron induced desorption of Cl− and F− from CF2Cl2 on Ru coadsorbed with water or ammonia ices under 250 eV electron impact [Q. B. Lu and T. E. Madey, Phys. Rev. Lett. 82, 4122 (1999); J. Chem. Phys. 111, 2861 (1999)].
Synthesis of thick and crystalline nanotube arrays by spray pyrolysis
Authors: Kamalakaran R. , Terrones M. , Seeger T. , Kohler-Redlich Ph. , Rühle M. , Kim Y. A. , Hayashi T. , Endo M.
Published: July 2002

Arrays of aligned nanotubes of large diameter (100–250 nm) are synthesized by pyrolyzing a jet (spray) solution of Fe(C5H5)2 and C6H6 in an Ar atmosphere at relatively low temperatures (850 °C). The tubular structures consist of highly crystalline nested graphene cylinders (<200 concentric tubes) with tips that are usually open. Raman studies confirm the high degree of perfection of these “thick” structures. Tube diameter, degree of alignment, and crystallinity can be controlled by varying the Ar flow rate and the Fe:C ratio within the precursor solution. Based on these observations a possible growth mechanism is suggested. This low cost route for the synthesis of carbon nanotubes is advantageous due to the absence of H2 as a carrier gas and the low pyrolytic temperature.
X-ray refractive planar lens with minimized absorption
Authors: Aristov V. , Grigoriev M. , Kuznetsov S. , Shabelnikov L. , Yunkin V. , Weitkamp T. , Rau C. , Snigireva I. , Snigirev A. , Hoffmann M. , Voges E.
Published: July 2002

Silicon refractive planar parabolic lenses with minimized absorption were fabricated by a combination of photolithography and dry-etching techniques. Focusing and spectral properties of the lenses were studied with synchrotron radiation in the energy range 8–25 keV at the European Synchrotron Radiation Facility. A focal spot of 1.8 μm with a gain of 18.5 and transmission of more then 80% was measured at 15.6 keV. The spectral characteristics were analyzed taking into account material dispersion and photon-energy attenuation in the hard x-ray range.
Study of ion desorption induced by carbon core excitation for poly-methylmethacrylate thin film using electron–ion coincidence spectroscopy
Authors: Ikenaga E. , Isari K. , Kudara K. , Yasui Y. , Sardar S. A. , Wada S. , Sekitani T. , Tanaka K. , Mase K. , Tanaka S.
Published: July 2002

We have developed a new electron–ion coincidence apparatus combined with synchrotron radiation in order to examine the various ion desorption mechanisms related to the Auger process induced by core excitation. Photon stimulated ion desorption (PSID) of a poly-methylmethacrylate (PMMA) thin film has been investigated by this apparatus. The PSID of PMMA induced by carbon core excitation has been examined using Auger electron yield, total ion yield, resonant Auger electron, and Auger electron–photoion coincidence (AEPICO) spectra. The spectrum of the total ion yield divided by the Auger electron yield shows that the desorption efficiency is largely increased at the resonant excitation of carbon 1s electron in the O–CH3 side chain to σ*(O–CH3) orbital. In AEPICO measurement, H+ and CHn+ (n=1–3) ions are observed at various resonant excitations. The AEPICO signal intensity depends on the Auger electron energy. Particularly, the CH3+ ion desorption in coincidence with Auger electron at 270 eV shows strong enhancement with σ*(O–CH3) resonant excitation. The results of the resonant Auger spectra and AEPICO yield spectra demonstrate the relation of the ion desorption mechanism to the bonding/antibonding character and localized character of the excited σ*(O–CH3) orbital and the Auger final state.
Differential cross sections for rotationally inelastic scattering of NO from He and D2
Authors: Westley M. S. , Lorenz K. T. , Chandler D. W. , Houston P. L.
Published: July 2002

State selective differential cross sections for rotationally inelastic scattering of NO (Ji=0.5, 1.5, F1→Jf=2.5–12.5, F1 and Jf=1.5–9.5, F2) from He and D2 measured by crossed molecular beam product imaging are reported. The differential cross sections were extracted from the data images using a new basis image iterative fitting technique. The images typically exhibit a single broad rotational rainbow maximum that shifts from the forward to the backward scattering direction with increasing ΔJ. The angle of the rainbow maximum was lower at a given ΔJ for D2 than for He as a collision partner. At a collision energy of ∼500 cm−1, primarily the repulsive part of the potential surface is probed, which can be modeled with a two-dimensional hard ellipse potential. This model for rotationally inelastic scattering is shown to qualitatively match the experimental differential cross sections. A more advanced correlated electron pair approximation potential energy surface for NO+He does not give substantially better agreement with the experiment. The differences between scattering of He and D2 are partially attributed to their differing structure and partially to a small difference in collision energy used in the two experiments.
An evaluation of the density functional approach in the zero order regular approximation for relativistic effects: Magnetic interactions in small metal compounds
Authors: Belanzoni Paola , van Lenthe Erik , Baerends Evert Jan
Published: July 2002

The performance of the density functional approach in the relativistic zero order regular approximation for the evaluation of electron spin resonance (ESR) parameters in small metal compounds has been evaluated critically by comparison with experimental data and available theoretical results for 22 linear molecules, characterized by a Σ2 electronic ground state. For most of the molecules studied the calculated magnetic parameters are in good (A tensors) or reasonable (g tensors) agreement with experiment. Effects of spin-orbit coupling and spin polarization on the calculated hyperfine interaction are investigated. These two effects can only be evaluated separately, since the present method does not allow us to take spin-polarization effects into account in spin-orbit coupled density functional calculations. However, while spin-polarization effects are important for all the molecules investigated, spin-orbit effects are non-negligible only for the molecules containing heavier metal atoms. The ESR parameters, evaluated using different “standard” exchange-correlation potentials, have only shown little dependence on the specific functional. Direct relativistic contributions to the hyperfine parameters are often large, especially for the heavier metals, but also “secondary” contributions to the ligand hyperfine parameters can be large if the ligand is bound to a heavy element.
Structures and spectra of iodide–water clusters I−(H2O)n=1–6: An ab initio study
Authors: Lee Han Myoung , Kim Kwang S.
Published: July 2002

To investigate the structures of I−(H2O)n=1–6, extensive ab initio calculations have been carried out. Owing to very flexible potential surfaces of the system (in particular for n=5 and 6), the lowest energy structures are characterized from various possible low-lying energy conformers. In contrast to some previously reported structures, we find a new lowest energy structure (followed by a few low-lying energy conformers) for n=5 and four nearly isoenergetic conformers for n=6. These conformers have surface and near-surface structures with the coordination number of 4. The present results provide the information of possible structures in recent profuse experiments of infrared spectra of I−(H2O)n=1–6 and charge transfer from the excited iodide ion to water molecules. Our predicted ionization potentials and OH stretching frequencies are in good agreement with the experimental data available, while only the cases of the OH frequencies for n=4 and the ionization potential for n=5 need consideration of conformational change by the temperature effect.
Infrared and millimeter wave spectra of the CH4–CO complex in the A internal rotation state
Authors: Xia Changhong , Walker Kaley A. , McKellar A. R. W.
Published: July 2002

The weakly bound van der Waals complex CH4–CO has been observed spectroscopically for the first time in the infrared (C–O stretching, ≈2143 cm−1) and millimeter wave (80–107 GHz) regions. The spectra analyzed here resemble quite closely those of the rare gas–carbon monoxide complexes, like Ne–CO and Ar–CO, and they almost certainly arise from CH4–CO complexes composed of CH4 in the lowest j=0 rotational state of A symmetry. The effective ground state intermolecular separation is 3.994 Å. Predictions are given here for the K=0 and 1 pure rotational microwave transitions of CH4–CO in the A state. The infrared spectrum shows numerous additional transitions which must be due to CH4–CO composed of methane in the F and E symmetry states, but these have not yet been assigned. Future microwave measurements on these F and E states will aid further progress on the infrared spectrum.
Photodissociation of acetaldehyde: The CH4+CO channel
Authors: Gherman Benjamin F. , Friesner Richard A. , Wong Teh-Hwa , Min Zhiyuan , Bersohn Richard
Published: July 2002

Ab initio quantum chemical calculations for the molecular dissociation channel of acetaldehyde are reported. The enthalpy change for the dissociation of acetaldehyde into methane and carbon monoxide was calculated to be exoergic by 1.7 kcal/mol. The transition state for this unimolecular dissociation, confirmed by normal mode analysis, was found to have an activation energy of 85.3 kcal/mol. Experimental measurements are reported for the vibrational and rotational state distribution of the CO product. No v=1 CO is found and the rotational temperature is 1300±90 K. The reaction coordinate at the transition state implies that the CO product is vibrationally cold and rotationally hot. This conclusion, which requires quantum dynamics calculations to confirm definitively, does agree with and aids in explaining the experimental results.
Thermal instability near planar defects in superconductors
Authors: Gurevich A.
Published: July 2002

It is shown that the local Joule heating due to planar defects, such as grain boundaries, microcracks, etc., can cause thermal instabilities, which limit the current-carrying capability of YBa2Cu3O7-coated conductors. Explicit instability criteria are obtained for a planar defect in a film and for a grain boundary. Thermal instabilities can be triggered by low-angle grain boundaries or planar defects, which block only a small fraction of the sample cross section. Hot spots near small defects and overheating of grain boundaries are essential for interpretation of experimental data on ac losses and E–J curves of polycrystals.
Two-dimensional analysis of magnetoimpedance in magnetic/metallic multilayers
Authors: Panina L. V. , Makhnovskiy D. P. , Mapps D. J. , Zarechnyuk D. S.
Published: July 2002

This article concerns the effect of the in-plane film size on the magnetoimpedance (MI) characteristics in magnetic/metallic multilayers. The problem is approached by a two-dimensional solution of the Maxwell equations in a symmetrical three-layer film. If the edge effect is neglected, the magnetic flux generated by the current flowing through the film is confined within the outer magnetic layers. In a finite width sandwich, the flux leaks through the inner conductor. This process eventually results in a considerable drop in MI ratio if this width is smaller than the critical flux decay length depending on the effective transverse permeability, layer thickness, and frequency.
Structure of boron nitride nanotubules
Authors: Demczyk B. G. , Cumings J. , Zettl A. , Ritchie R. O.
Published: July 2002

We have used high-resolution transmission electron microscopy to resolve the [0001] projected basal plane structure and chirality relationships in boron nitride nanotubes. Evidence for tube growth along both 〈101̄0〉 and 〈112̄0〉 is found. Our results suggest that the 〈112̄0〉 (armchair) tubes form first. Subsequent growth can take the form of a 〈101̄0〉 (zig-zag) tube or additional armchair tubes. In both cases, the additional walls can be accommodated without the need for defect formation for circumferential tube closure. These results suggest that real boron nitride tubes may, in practice, realize their inherently high modulus and display less variation of mechanical properties than tubes comprised of carbon.
Wafer bonded 1.55 μm vertical-cavity lasers with continuous-wave operation up to 105 °C
Authors: Karim Adil , Abraham Patrick , Lofgreen Daniel , Chiu Yi-Jen , Piprek Joachim , Bowers John
Published: July 2002

We report 105 °C continuous-wave, electrically pumped operation of a 1526 nm vertical-cavity surface-emitting laser. An InP/InGaAsP active region was wafer bonded to GaAs/AlGaAs mirrors, with a superlattice barrier to reduce the number of nonradiative recombination centers in the bonded active region.
Stability and evolution of a dry spot
Authors: López P. G. , Miksis M. J. , Bankoff S. G.
Published: July 2002

The motion of a thin viscous layer of fluid on a horizontal solid surface bounded laterally by a dry spot and a vertical solid wall is considered. A lubrication model with contact line motion is studied. We find that for a container of fixed length the axisymmetric equilibrium solutions with small dry spots are unstable to axisymmetric disturbances. As the size of the dry spot increases, the equilibrium solutions become unstable to nonaxisymmetric disturbances. In addition, we present numerical solutions of the nonlinear evolution equations in the axisymmetric and nonaxisymmetric cases for different values of the parameters. The axisymmetric results show good agreement with existing experimental results.
Design and development of a rapid acquisition laser-based fluorometer with simultaneous spectral and temporal resolution
Authors: Pitts Jonathan D. , Mycek Mary-Ann
Published: July 2002

We report the design, development, and characterization of a sensitive, time-resolved fluorescence spectrometer capable of measuring fluorescence spectra and transient decays simultaneously, with data acquisition times less than 1 s. The spectrometer, a portable fluorescence lifetime spectrometer (FLS), was designed to be compatible with both laboratory and clinical research studies on biological systems, and was applied to the study of several biological fluorophores in vitro and human tissue in vivo. The instrument consisted of a nitrogen laser pumping a dye laser for excitation from 337.1 nm through the near infrared, a quartz fiber-optic probe for remote light delivery and collection, and amplified detectors for rapid spectral and temporal detection from 350 to 800 nm. The spectral resolution of the FLS was determined to be 3 nm, which is sufficient for accurately detecting the broad spectral bands associated with biological fluorophores. The FLS was able to detect 5×10−7 M fluorescein dye concentrations with spectral signal-to-noise ratios (SNRs) of 29. Time-resolved detection with the FLS had a dynamic range of approximately three decades with a SNR of 200. Using fluorescence lifetime standards, the FLS was determined to be capable of accurately resolving fluorophore lifetimes from hundreds of picoseconds to tens of nanoseconds in duration, with an ultimate temporal resolution of 360 ps.
Apparatus for the rapid measurement of electrical transport properties for both “needle-like” and bulk materials
Authors: Pope A. L. , Littleton R. T. , Tritt Terry M.
Published: July 2002

Described in this article is an apparatus in which thermopower and resistivity can be measured almost simultaneously from 10 to 300 K in a closed cycle refrigerator system. A distinction of this system is that the samples are mounted on integrated circuit (IC) chips for rapid introduction of the sample into the measurement apparatus. One advantage of this system is that the sample is mounted on the IC chip and then all the electrical contacts to the sample can be checked prior to insertion into the sample measurement apparatus. This system is applicable for measurement of either bulk samples (approximately 2×2×8 mm3) or “needle-like” samples (approximately 0.1×0.05×2–3 mm3).
Direct and indirect methods for studying the energetics and dynamics of the Auger Doppler effect in femtosecond ultra-fast dissociation
Authors: Björneholm O.
Published: July 2002

Molecules may fragment within a few femtoseconds after core-excitation, a phenomenon known as ultra-fast dissociation. With the aim of providing an understanding of the fundamental phenomenology of the Auger Doppler effect, two methods are presented to study the energetics and dynamics, i.e., the kinetic energy release and the fragment velocities in such processes. The first, direct, method is based on the shifts in kinetic energy of the Auger electrons due to the velocity acquired by the fragment in the ultra-fast dissociation process, i.e., the Auger Doppler effect. The second, indirect, method is based on total-energy arguments in a Born–Haber cycle for excitation, dissociation, and ionization. A combination of the two methods is shown to be able to reproduce experimental spectra well. Based on this, predictions are made for other, yet unstudied, molecular systems. It is also shown that the Auger Doppler effect is not static, but will exhibit dynamic photon energy dependence. The complete energetics of the three-body dissociation of a molecule into an electron, an ion, and a neutral fragment on a time-scale of a few femtoseconds can thus be accounted for.
Overcritical density plasma diagnosis inside laser-produced craters
Authors: Renner O. , Krouský E. , Rosmej F. B. , Sondhauss P. , Uschmann I. , Förster E. , Kalachnikov M. P. , Nickles P. V.
Published: July 2002

Spatially resolved x-ray spectra of the Al Lyman α group emitted from the surface of the laser- irradiated targets display plasma regions where the resonance line is suppressed and the dielectronic satellites dominate. Calculations based on multilevel collisional-radiative code and two-dimensional hydrodynamic simulations relate this spectral distribution to a presence of the cold overcritical density plasma inside the crater at the target surface.
Application of a multichannel detection system to the high-resolution fast electron energy loss spectrometer
Authors: Liu Xiao-Jing , Zhu Lin-Fan , Jiang Xi-Man , Yuan Zhen-Sheng , Cai Bei , Chen Xiang-Jun , Xu Ke-Zun
Published: July 2002

A microchannel-plate based resistive-anode position sensitive detection system is applied to a high-resolution fast electron energy loss spectrometer. The biasing voltage divider network for the detector is investigated. The measurement efficiency is increased by about 20 times, and to some extent, the energy resolution is also improved. The static mode and the scanning mode of the spectrometer are discussed in detail. In the scanning mode, a spectrum with higher energy resolution can be obtained through a coarse scanning. As a testing experiment, the optical oscillator strength spectrum for the Rydberg series of helium is measured and the results are compared.
Stability conditions, nonlinear dynamics, and thermal runaway in microbolometers
Authors: Brandão G. B. , de Almeida L. A. L. , Deep G. S. , Lima A. M. N. , Neff H.
Published: July 2002

The nonlinear dynamic behavior of microbolometers, operating at room temperature (300 K) under conditions of positive electrothermal feedback is investigated. An improved device model, based on the heat balance equation is developed. It takes into account the temperature dependence of the thermophysical parameters, such as thermal coupling coefficient between the sensor and its surroundings, and sensor heat capacity and its thermal resistance coefficient. Operational considerations for thermoresistive microbolometer with positive and negative temperature coefficient of resistance are discussed for both, constant current and constant voltage modes of operation. Analytical expressions are derived for predicting stable and unstable operation. Safety factors L0, establishing the biasing conditions for stable device operation are proposed for the positive temperature coefficient of resistance and negative temperature coefficient of resistance type sensors. Limits for fast catastrophic destruction are provided, and the dynamic characteristics of the associated thermal runaway phenomenon is illustrated. This effect, as predicted by analysis and numerical simulation, was observed experimentally, confirming the validity of the proposed modeling approach for the microbolometer.
Monte Carlo analysis of the spectral photon emission and extraction efficiency of organic light-emitting devices
Authors: Badano Aldo , Kanicki Jerzy
Published: July 2002

We report on a Monte Carlo method for modeling light transport phenomena in multilayer organic polymer light-emitting devices on plastic flexible substrates. The method allows modeling of Cartesian geometrical structures describing the fate of photons through multiple scattering events determined by the wavelength-dependent material optical properties. We apply the method to analyze the wavelength distribution of emitted light spectra. We find that for all organic polymers considered, the light emission is slightly shifted toward the longer wavelengths, and that this shift is maximum for light emissions with peaks around 530 nm. The photon extraction efficiency is higher (0.430) for organic polymers emitting in the longer wavelengths, while the photon absorbed fraction is higher (0.676) for spectra with a maximum in the short wavelengths.
Spiral electron beam interaction with whistler waves at cyclotron resonances
Authors: Volokitin A. S. , Krafft C.
Published: July 2002

The interaction of a radially bounded electron beam spiraling in a magnetoplasma with a quasi-monochromatic whistler at the Cherenkov and Doppler-shifted resonance conditions is investigated. For the linear stage of the interaction, the instability growth rate as well as the radial flux of wave energy emitted by the thin spiral beam are calculated. Compared to previous works considering a cylindrical beam or electrostatic lower hybrid waves, the present study gives the possibility to extend the investigation of the whistler-beam interaction to wider ranges of physical parameters, and, in particular, to situations where the beam is injected obliquely with respect to the ambient magnetic field. The main physical process occurring during the dissipative beam-wave interaction—that is, which takes into account the energy leakage out of the bounded beam volume—is the nonlinear self-organization of part of the beam electrons which leads to the formation of dynamically stable bunches that are continuously decelerated or accelerated while keeping resonance with the emitted wave. In a long time evolution, bunches are the main cause which supports the wave emission whereas the nonresonant beam electrons practically do not exchange energy with the wave. It is shown that the efficiency of the electromagnetic wave emission by a thin spiral beam is less at the cyclotron resonances than at the Cherenkov resonance.
Electrochemical machining of stainless steel microelements with ultrashort voltage pulses
Authors: Kirchner Viola , Cagnon Laurent , Schuster Rolf , Ertl Gerhard
Published: July 2002

An electrochemical pulse technique enables the fabrication of three-dimensional microelements from stainless steel. The method is based on the application of ultrashort (nanosecond) voltage pulses, whereupon electrochemical reactions are locally confined with submicrometer precision. Employing properly shaped tool electrodes enables the machining of freestanding cantilevers or microstructures directly to a metal sheet. Due to gentle removal of the material, the grain structure of the material is revealed without any chemical or mechanical modifications. This is demonstrated by measuring the vibration frequency of a cantilever, and agrees well with the value derived from the bulk material properties.
Conversion of infrared radiation into red emission in YVO4:Yb,Ho
Authors: Ryba-Romanowski W. , Golab S. , Dominiak-Dzik G. , Solarz P. , Lukasiewicz T.
Published: July 2002

Upon continuous wave excitation around 1 μm, a YVO4 crystal codoped with ytterbium and holmium exhibits intense red emission originating in the F55 level and considerably weaker green emission originating in the S52 level of Ho3+. The ratio of the red to green emission intensities is 17:1 at 300 K. The dependence of the intensity of both emissions on the pump power is nearly the same but the mechanisms determined on the basis of short pulse excitation are found to be different. It is concluded that the green emission is excited by two consecutive energy transfers from Yb3+ to Ho3+, whereas excited state absorption is involved in the excitation of red emission.
New infrared spectroscopic beamline at BESSY II
Authors: Schade U. , Röseler A. , Korte E. H. , Bartl F. , Hofmann K. P. , Noll T. , Peatman W. B.
Published: July 2002

At BESSY a multipurpose infrared (IR) beamline is being built for biological and materials science investigations. It will provide useful IR intensities over the energy range from about 10 000 down to 50 cm−1 and lower. After commissioning the first beamline section in the spring of this year first quantitative measurements on the performance have been made in the near and mid-infrared wavelength region.
Longitudinal carrier density measurement of high power broad area laser diodes
Authors: Rinner F. , Rogg J. , Friedmann P. , Mikulla M. , Weimann G. , Poprawe R.
Published: July 2002

The longitudinal carrier density distribution of an InGaAlAs high-power broad-area semiconductor laser has been measured using spontaneous emission from the side of the device. The laser shows continuously increasing carrier densities on the facet with the high reflectivity coating (reverse facet). This has a major impact on the efficiency and the lifetime of the laser. This behavior is in good agreement with one-dimensional calculations for the longitudinal carrier distribution.
Dilute suspension of high inertia particles in the turbulent flow near the wall
Authors: Sergeev Y. A. , Johnson R. S. , Swailes D. C.
Published: July 2002

A steady flow of a very dilute suspension of solid particles in the turbulent flow near the wall is considered. The particle phase is described by the system of mean field mass, momentum, and turbulent Reynolds stress conservation equations incorporating the closure approximation for the third-order correlation of particle fluctuating velocity; this closure approximation, as well as the mean field equations, were derived earlier from the PDF (probability density function) kinetic equation governing the particle transport in the turbulent flow. In the turbulent flow near the wall, particles can be characterized as “high inertia” or “low inertia” by small or large values, respectively, of a single nondimensional parameter: the normalized particle relaxation time combining the wall friction velocity and physical parameters of the fluid and particle phase. Making use of the technique of matching asymptotic expansions, the distributions are found of the normal and shear turbulent stress, volume fraction, and mean velocity in the particle phase of the suspension of high inertia particles.
Effect of natural oxidation conditions on low resistance spin tunnel junctions
Authors: Zhang Z. G. , Freitas P. P. , Ramos A. R. , Barradas N. P. , Soares J. C.
Published: July 2002

Spin dependent tunnel junctions with AlOx barrier were fabricated by in situ natural oxidation of a 7 Å (or 8 Å) thick Al film. Junctions were deposited on top of preannealed, ion-beam smoothed, 600 Å thick Al bottom electrodes. By changing the oxidation time from 10 min to 10 h with the oxygen pressure of 10 and 100 Torr, the top-pinned junctions have resistances varying from 31 to 406 Ω μm2 and the tunneling magnetoresistance (TMR) varies from 8.5% to 19.6% for the as deposited state. For the bottom-pinned junctions, the oxygen pressure was varied from 0.5 to 100 Torr, and oxidation time ranged from 5 min to 2 h. Bottom-pinned junction resistances as low as 10–12 Ω μm2 were obtained with TMR ranging from 14% to 17% for the junctions oxidized at the lower pressure (0.5 Torr). Rutherford backscattering analysis (RBS) indicates an O/Al ratio of 1.29±0.34 denoting incomplete oxidation of the Al for junctions oxidized at the lower pressures and short times. Junctions oxidized at higher pressures (⩾10 Torr) can reach 25%–30% TMR, with resistances ranging from 30 to 70 Ω μm2. RBS shows near stoichiometric Al2O3 oxide composition (O/Al=1.51±0.43) in these barriers.
Kinetic theory for identical, frictional, nearly elastic spheres
Authors: Jenkins James T. , Zhang Chao
Published: July 2002

We derive a simple kinetic theory for collisional flows of identical, slightly frictional, nearly elastic spheres that is based on a physically realistic model for a frictional collision between two spheres. When the coefficient of friction is small, the equations of balance for rotational momentum and energy can be solved in approximation. This permits the rotational temperature to be related to the translation temperature and the introduction of an effective coefficient of restitution in the rate of dissipation of translation fluctuation energy. With this incorporation of the additional loss of translational energy to friction and the rotational degrees of freedom, the structure of the resulting theory is the same as for frictionless spheres.
Cobalt single-molecule magnet
Authors: Yang En-Che , Hendrickson David N. , Wernsdorfer Wolfgang , Nakano Motohiro , Zakharov Lev N. , Sommer Roger D. , Rheingold Arnold L. , Ledezma-Gairaud Marisol , Christou George
Published: July 2002

A cobalt molecule that functions as a single-molecule magnet, [Co4(hmp)4(MeOH)4Cl4], where hmp− is the anion of hydroxymethylpyridine, is reported. The core of the molecule consists of four Co(II) cations and four hmp− oxygen atom ions at the corners of a cube. Variable-field and variable-temperature magnetization data have been analyzed to establish that the molecule has a S=6 ground state with considerable negative magnetoanisotropy. Single-ion zero-field interactions (DSz2) at each cobalt ion are the origin of the negative magnetoanisotropy. A single crystal of the compound was studied by means of a micro-superconducting quantum interference device magnetometer in the range of 0.040–1.0 K. Hysteresis was found in the magnetization versus magnetic field response of this single crystal.
Kinetic inflation of stimulated Raman backscatter in regimes of high linear Landau damping
Authors: Vu H. X. , DuBois D. F. , Bezzerides B.
Published: July 2002

Kinetic simulations and analysis show that backward stimulated Raman scattering (BSRS), in regimes of large linear Landau damping of the primary Langmuir wave, attains levels greatly exceeding the predictions of models based on fixed damping. These regimes are encountered in plasma conditions expected for target designs to be fielded at the National Ignition Facility [J. D. Lindl, Inertial Confinement Fusion (Springer-Verlag, New York, 1998)]. Trapped electrons in the Langmuir wave have the dual effect of reducing its damping, thereby enhancing the BSRS response, and saturating this response by phase detuning, a consequence of the trapping-induced, time-dependent, frequency shift. BSRS, then, occurs as a train of sub-picosecond pulses, arising from the competition between phase detuning and parametric regeneration. A simple three wave parametric model, including the effect of the nonlinear frequency shift and residual nonlinear damping, reproduces these essential features. A similar scenario applies to backward stimulated Brillouin scattering (BSBS). BSRS activity many orders of magnitude above noise level is found for intense laser speckles even when the primary Langmuir wave number times the Debye length is as high as 0.55. The simulation model consistently accounts for the competition of other instabilities, including BSBS, forward stimulated Raman scattering, and the Langmuir decay instability with cavitation.
Lattice distortions and the transmuted-Ge related luminescence in neutron-transmutation-doped GaN
Authors: Kuriyama K. , Tokumasu T. , Takahashi Jun , Kondo H. , Okada M.
Published: July 2002

The lattice distortion and the transmuted-Ge related luminescence in neutron-transmutation-doped (NTD) GaN are studied by combining Rutherford backscattering spectroscopy/channeling, Raman scattering, and photoluminescence methods. The lattice displacement of Ga atoms of ∼0.12 Å from the 〈0001〉 row is estimated from the normalized angular yield profiles, preserving the single crystallinity in as-irradiated GaN with a minimum yield (χmin) of 7%. A 2.84 eV emission band observed in 600 °C annealed NTD-GaN is associated with the Ga interstitial, supporting the lattice distortion. Two emission bands at 2.90 eV and 2.25 eV observed in 1000 °C annealed NTD-GaN are assigned to a negatively charged DX-like center of Ge at Ga site and a complex defect attributed to Ge at Ga site and Ga vacancy, respectively.
A class of boron-rich solid-state neutron detectors
Authors: Robertson B. W. , Adenwalla S. , Harken A. , Welsch P. , Brand J. I. , Dowben P. A. , Claassen J. P.
Published: July 2002

Real-time solid-state neutron detectors have been fabricated from semiconducting boron–carbon alloys, deposited by plasma-enhanced chemical vapor deposition. Single neutrons were detected and signals induced by gamma rays were determined to be insignificant. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron–carbon alloys with only the natural isotopic abundance of B10. Devices made of thicker boron–carbon alloy layers enriched in B10 could lead to increased detection efficiency and active diodes could use the inherent micron scale spatial resolution, increasing the range of possible applications.
Flat panel imagers with pixel level amplifiers based on polycrystalline silicon thin-film transistor technology
Authors: Lu J. P. , Van Schuylenbergh K. , Ho J. , Wang Y. , Boyce J. B. , Street R. A.
Published: July 2002

We report here the realization of a large-area compatible, flat panel imager with pixel level amplifiers. The imager is based on excimer-laser crystallized, polycrystalline silicon (poly-Si) thin-film transistors. By incorporating pixel level amplification, flat panel imagers are expected to be able to achieve unprecedented noise performance, with the hope of achieving single photon detection. We have demonstrated good noise performance of 1300 erms, exceeding the commonly accepted industry standard of 2000 erms. We also briefly discuss the source of the extra noise, as well as the possibility of further reducing the noise level.
Teaching special relativity with a computer
Authors: Horwitz Paul , Taylor Edwin F. , Barowy William
Published: July 2002

RelLab, a computer-based Relativity Laboratory designed to lead high school or introductory college physics students to a qualitative understanding of Special Relativity is described. RelLab is an open-ended ‘‘construction kit’’ for building a wide variety of gedanken experiments. Students are posed problems by the teacher, usually in the form of paradoxes. They break into small groups to discuss the paradox, construct a gedanken experiment to try to understand and solve it, and then come together as a class to discuss their results. The effectiveness of the software and the pedagogical approach are evaluated by giving high school students a test problem drawn from the physics education literature. The students demonstrated a level of qualitative understanding of relativity superior to that of the graduate students used in a previous study.
Parallel decomposition of the tight-binding fictitious Lagrangian algorithm for molecular dynamics simulations of semiconductors
Authors: Yeh Mei-Ling , Kim Jeongnim , Khan Furrukh S.
Published: July 2002

We present a parallel decomposition of the tight-binding fictitious Lagrangian algorithm for the Intel iPSC/860 and the Intel Paragon parallel computers. We show that it is possible to perform long simulations, of the order of 10 000 time steps, on semiconducting clusters consisting of as many as 512 atoms, on a time scale of the order of 20 h or less. We have made a very careful timing analysis of all parts of our code, and have identified the bottlenecks. We have also derived formulas which can predict the timing of our code, based on the number of processors, message passing bandwidth, floating point performance of each node, and the set up time for message passing, appropriate to the machine being used. The time of the simulation scales as the square of the number of particles, if the number of processors is made to scale linearly with the number of particles. We show that for a system as large as 512 atoms, the main bottleneck of the computation is the orthogonalization of the wave functions, which consumes about 90% of the total time of the simulation. © 1995 American Institute of Physics.
Computer code long path method for long path differential-absorption predictions using CO2 laser lines
Authors: Zuev V. V. , Mitsel’ A. A. , Kataev M. Yu. , Ptashnik I. V. , Firsov K. M.
Published: July 2002

A computer program LPM (Long Path Method) has been developed for imitative modeling of the concentration at gases (H2O, CO2, O3, NH3, C2H4) in the atmosphere using a long-path double-wavelength laser system equipped with two tunable CO2 lasers. The model is designed for four different lasing isotopes of CO2 (12C16O2, 13C16O2, 12C18O2, 13C18O2). The program determines optimal pairs of CO2 laser wavelengths, and the gas concentration retrieval errors from sounding data caused both by detector noise and systematic inaccuracy. The program was written in MS FORTRAN and Visual Basic languages for Windows 3.1 and an IBM-compatible PC. © 1995 American Institute of Physics.
Simple scheme for the numerical evaluation of nearly singular integrals
Authors: John George C. , Hasbun J. E. , Singh Vijay A.
Published: July 2002

Nearly singular integrals are encountered in the evaluation of the Green’s function for a variety of physical problems. One example is the evaluation of the electronic density of states in condensed matter physics. We propose a simple scheme for the accurate evaluation of a class of commonly encountered nearly singular integrals using panels of equal width. This scheme is an extension of a method introduced by Roth [Phys. Rev. B 7, 4321 (1973)]. We demonstrate the usefulness of the scheme by employing it to evaluate the electronic density of states of a one- and of a three-dimensional ordered solid within a nearest neighbor tight binding model. A Fortran version of the scheme is also presented. © 1997 American Institute of Physics.
Pulsed laser-induced transient thermoelectric effects in silicon crystals
Authors: Sasaki Minoru , Negishi Hiroshi , Inoue Masasi
Published: July 2002

A new technique is described in which a transient thermoelectric (TTE) voltage of a semiconductor is measured after a pulsed laser irradiation—a modification of photodiffusion or Dember effect. This technique has been successfully applied to test samples of n- and p-Si over the wide time range from 20 ns to 2 s and the temperature 120–284 K. The decay curves of the TTE voltage consist of three stages with the characteristic relaxation or decay times. Stage 1 with the relaxation time τ1, is associated with the carrier generation and recombination and explained by the Dember effect. Stage 2 characterized by the relaxation time τ2, is due to the diffusion of majority carriers from a higher to a lower temperature region; for p-Si, a double-relaxation process is observed arising from the difference in the drift mobilities of light and heavy holes. At stage 3, the TTE voltage vanishes completely, which can be reasonably interpreted by the diffusion of thermal flux or phonons along a temperature gradient produced by the laser pulse. Analytical expressions for each stage are presented to discuss these experimental results.
Low-temperature magnetic susceptibility of Fe-B amorphous alloys (abstract)
Authors: Puźniak R. , Dmowski W.
Published: July 2002

Magnetic properties of amorphous iron based alloys have been extensively studied, however, the mechanism of thermal demagnetization is not well understood and temperature dependence of the static magnetic susceptibility is not well known. In order to address some of these problems we performed an accurate magnetization study of Fe100−xBx metallic glasses (x=20,18,16,14) at low temperatures (4.2–160 K) and in several magnetic fields up to 15 kOe. Magnetization measurements were carried out using VSM with an accuracy of 10−4. At a fixed temperature, the magnetization was measured as a function of an external magnetic field. The demagnetization field was subtracted and the saturation magnetization was determined by linear extrapolation to zero internal field. The saturation susceptibility was obtained from the slope of this curve. The temperature dependence of the saturation magnetization is best described by a T3/2 term, however, the saturation susceptibility is a nonmonotonous function of temperature and cannot be described by spin wave contribution. It exhibits a minimum at low temperature suggesting that some atoms are in zero effective internal fields induced by the mixture of ferro- and antiferromagnetic interactions. The temperature of the susceptibility minimum is lower for higher iron concentration in the alloys. The influence of magnetovolume effects on experimental data is addressed and the results are discussed in terms of inhomogeneous magnetic order.
Raman spectroscopy of submicron KNO3 films. II. Fatigue and space-charge effects
Authors: Scott J. F. , Pouligny B.
Published: July 2002

Raman spectroscopy of KNO3 thin-film ferroelectric memories has been performed as a function of film thickness, temperature, annealing procedures, and read-write cycles (fatigue). The results show that conversion to phase II is not the cause of failure in these memory devices, in agreement with the x-ray results of Schaffer and Mikkola. Waiting time effects are in accord with the space-charge theory of Takahashi.
The dense radiating deuterium Z-pinch plasma
Authors: Thornhill W. , Giuliani J. L. , Davis J.
Published: July 2002

The radiative emission from a frozen deuterium Z-pinch driven by a slowly rising current (∼3 kA/ns) is investigated. This approach is in marked contrast to the standard radiating Z-pinch plasma which is driven by a rapid-current rise leading to a fast implosion. The slow pinch is modeled as a quasistatic contraction subject to Bennett-pressure equilibrium and driven by a constant generator voltage of 200 kV. Electron degeneracy is approximately accounted for in the pressure, however, the resistivity is taken as purely classical. The bremsstrahlung spectrum transitions from thin to nearly thick during the collapse phase. The predictions from this simple model suggests that this approach may have interesting applications as a radiation source in the kilovolt regime. In particular, we find that a deuterium plasma of 1018 cm−1 line density yields ∼60 kJ of radiation above 3 keV while undergoing a radiative collapse.
First- and second-order longitudinal piezoresistive coefficients of n-type metal-oxide-semiconductor field-effect transistors
Authors: Schörner R.
Published: July 2002

This paper reports experimental results on the longitudinal piezoresistive effect of n-type metal-oxide-semiconductor field-effect transistors (MOSFETs) with the particular aim of obtaining data appropriate for the computer simulation of silicon sensors. The measurements were performed on MOSFETs of different geometries including short channel devices with a channel length of 2 μm. The MOSFETs were integrated on (100)-oriented silicon with current flow parallel to the crystallographic 〈110〉 direction. The influence of the piezoresistive effect on the total output characteristic of these devices was studied. The corresponding first- and second-order piezoresistive coefficients were determined in the linear regime of the output characteristic as a function of the transistor geometry as well as of the gate voltage.
Thickness measurement of thin films by x-ray absorption
Authors: Chaudhuri J. , Shah S.
Published: July 2002

An x-ray diffraction method for determining thicknesses of thin films grown on single-crystal substrates is presented. The equations, based on the kinematical theory of x-ray diffraction and the mosaic crystal model, were developed. The thickness of the thin film was computed from the absorption of the integrated diffracted x-ray intensity from the single-crystal substrate. Since the diffracted intensity from the film is not required, the film does not have to be single crystal in nature. Thus, thicknesses of less ordered, polycrystalline, or even amorphous films can be measured with high precision by this technique.
Computer-aided simulation for oxygen precipitation in silicon
Authors: Isomae Seiichi
Published: July 2002

This paper describes a computer simulation for oxygen precipitation in silicon crystals subjected to thermal treatments. In the simulation based on the classical nucleation theory, it is assumed that an oxygen precipitate is spherical and its nucleus is a carbon atom. In addition, to explain oxygen precipitation at high temperatures, the simulation takes into consideration coarsening of the precipitate, which is inferred from obtained three-step annealing results. Simulated results are compared with experimental results obtained from a complementary metal-oxide-silicon transistor thermal process in addition to the three-step annealings results. As a result, it is proven that the proposed simulation can predict the precipitated oxygen concentration in wafers subjected to various thermal treatments.