bluuebunny/crossref_metadata_2025 huggingface dataset:
An investigation of density fluctuations and electron transport in the Madison Symmetric Torus reversed-field pinch
Authors: Lanier N. E. , Craig D. , Anderson J. K. , Biewer T. M. , Chapman B. E. , Den Hartog D. J. , Forest C. B. , Prager S. C. , Brower D. L. , Jiang Y.
Published: July 2002

The origin of large scale density fluctuations and their role in electron transport has been investigated in the Madison Symmetric Torus reversed-field pinch [R. N. Dexter et al., Fusion Tech. 19, 131 (1991)]. At medium frequencies (10–30 kHz), ñ is associated with core-resonant magnetic fluctuations having poloidal mode number m=1 and toroidal mode numbers n=6–10. These chord-averaged density fluctuations are shown to be coherent with large scale radial flow fluctuations, permitting fluctuation-induced particle flux of the form 〈ñṽr〉 to be studied. In the edge, although amplitudes are large, ñ and ṽr are out of phase, consistent with advection of the mean density gradient by perturbed magnetic field lines and no electron transport. During improved confinement discharges, produced by inductive current drive, both fluctuation amplitudes and the total radial electron flux are greatly reduced over the entire plasma cross section.
Picosecond Z-scan measurements on bulk GaN crystals
Authors: Pačebutas V. , Stalnionis A. , Krotkus A. , Suski T. , Perlin P. , Leszczynski M.
Published: July 2002

Bulk GaN crystals were characterized by using picosecond laser pulses at λ=0.527 μm and Z-scan techniques. The role of the free-carrier absorption was evaluated by a dynamical, pump-and-probe-type transmitivity measurement. The values of two-photon absorption coefficient (17–20 cm/GW) and refractive index changes at high optical irradiances due to bound (n2=−4×10−12 esu) and free (σr=−1.0×10−20 cm3) electrons in that material were determined.
A Linux cluster for between-pulse magnetic equilibrium reconstructions and other processor bound analyses
Authors: Peng Q. , Groebner R. J. , Lao L. L. , Schachter J. , Schissel D. P. , Wade M. R.
Published: July 2002

A 12-processor Linux PC cluster has been installed to perform between-pulse magnetic equilibrium reconstructions during tokamak operations using the EFIT code written in FORTRAN. The MPICH package implementing message passing interface is employed by EFIT for data distribution and communication. The new system calculates equilibria eight times faster than the previous system yielding a complete equilibrium time history on a 25 ms time scale 4 min after the pulse ends. A graphical interface is provided for users to control the time resolution and the type of EFITs. The next analysis to benefit from the cluster is CERQUICK written in IDL for ion temperature profile analysis. The plan is to expand the cluster so that a full profile analysis (Te, Ti, ne, Vr, Zeff) can be made available between pulses, which lays the ground work for Kinetic EFIT and/or ONETWO power balance analyses.
Modified Mach–Zender laser interferometer for probing bulk acoustic waves
Authors: Fattinger G. G. , Tikka P. T.
Published: July 2002

This letter reports the usage of a modified Mach–Zehnder interferometer for visualization of the mechanical vibrations of high-frequency, bulk-mode, thin-film resonators. The setup is capable of detecting frequencies from 30 kHz up to 12 GHz with an amplitude down to less than 10 pm, which means a dynamic range above 80 dB. The achieved lateral resolution of the measured field image is more than 300 nm. Due to a new phase stabilizing system phase sensitive measurements are possible, additionally bringing up the possibility of establishing a tremendous measurement speed of up to 5 million points/h. This setup can be applied to bulk acoustic wave development in order to provide information such as material parameters, to verify theoretical models and to learn about device behavior which cannot be done using only electrical measurement tools.
The conformational structures and dipole moments of ethyl sulfide in the gas phase
Authors: Plusquellic D. F. , Suenram R. D. , Maté B. , Jensen J. O. , Samuels A. C.
Published: July 2002

The pure rotational spectrum of ethyl sulfide has been measured from 12 to 21 GHz in a 1 K jet-cooled expansion using a Fourier-transform microwave (FTMW) spectrometer. Prominent features in the spectrum are assigned to transitions from three conformational isomers. Additional assignments of the 13C and 34S isotopomer spectra of these conformers effectively account for all of the remaining transitions in the spectrum. Accurate “heavy-atom” substitution structures are obtained via a Kraitchman analysis of 14 rotational parameter sets, permitting definitive identification of the molecular structures of the three conformers. Two of the structures designated as the gauche–gauche (GG) and trans–trans (TT) conformers have symmetric forms with C2 and C2v symmetries, respectively, and the third trans–gauche (TG) configuration is asymmetric. The components of the electric dipole moment along the principal inertial axes have been determined from Stark measurements and are consistent with these structural assignments. Detailed comparisons are made with the calculated geometries, dipole moments, and energy-level ordering at both the HF (Hartree–Fock)/6-31* and MP2 (second-order Møller–Plesset)/6-311** levels of theory. Significant discrepancies are found, which are mainly attributed to errors in the calculated dihedral angles that define the different conformations. A graphical-user-interface computer program has aided in the identification and assignment of entangled hybrid-band spectra from the different conformers and isotopomers in this study. The program includes features that enable real-time refinement of rotational constants and hybrid band intensities through visual comparisons of the experimental data with simulated spectra. Capacities also exist to rapidly assign quantum number labels for least-squares fitting purposes.
Evidence for C–H–O interaction of acetone and deuterium oxide probed by high-pressure
Authors: Chang Hai-Chou , Jiang Jyh-Chiang , Lin Sheng H. , Weng Nai-Hsin , Chao Ming-Chi
Published: July 2002

C–H–O interaction of acetone and deuterium oxide has been probed by high pressure. High-pressure study provides the first experimental evidence for the enhancement of hydrophobic hydration of acetone as its aqueous solution was compressed to high-pressure ices. Based on the results, we conclude that the C–H–O interaction may be a distinct possibility to understand the origin of the spectral feature located at ∼2950 cm−1, being sensitive to concentration and pressure dependence. Ab initio calculation results, forecasting the frequency red shift of the C–H stretching vibration as C–H–O is interacted via hydrogen bonding, are discussed. This study demonstrates that high pressure can be used as a valuable means of triggering and investigating C–H–O hydrogen-bonding interaction.
Electron vortex generation by strong, localized plasma heating
Authors: Abdalla T. M. , Kuvshinov B. N. , Schep T. J. , Westerhof E.
Published: July 2002

The generation of electron vortical structures by strong localized heating is investigated analytically and numerically. The structure formation and its dynamics are described by the model of electron magnetohydrodynamics, extended to include pressure effects. Magnetic structures are generated by the thermal electromotive force that arises due to nonalignment of pressure and density gradients. Their topology is determined by the heating power and the equilibrium density gradient. It is shown that strong localized heating of a nonuniform plasma generates vortical structures that are capable of confining and transporting heat across the plasma. The long term behavior of the structures agrees qualitatively with stationary equilibrium solutions. As these structures survive for rather long times, a filamentary profile of the temperature develops.
Photoluminescence properties of a GaN0.015As0.985/GaAs single quantum well under short pulse excitation
Authors: Luo X. D. , Xu Z. Y. , Ge W. K. , Pan Z. , Li L. H. , Lin Y. W.
Published: July 2002

Under short pulse laser excitation, we have observed an extra high-energy photoluminescence (PL) emission from GaNAs/GaAs single quantum wells (QWs). It dominates the PL spectra under high excitation and/or at high temperature. By measuring the PL dependence on both temperature and excitation power and by analyzing the time-resolved PL results, we have attributed the PL peak to the recombination of delocalized excitons in QWs. Furthermore, a competition process between localized and delocalized excitons is observed in the temperature-dependent PL spectra under the short pulse excitation. This competition is believed to be responsible for the temperature-induced S-shaped PL shift often observed in the disordered alloy semiconductor system under continuous-wave excitation.
Quantum dynamics for a system coupled to slow baths: On-the-fly filtered propagator method
Authors: Sim Eunji
Published: July 2002

An on-the-fly filtered propagator functional path integral scheme is introduced as an efficient way of calculating the iterative dynamics of complex condensed systems. Time evolution of the reduced density matrix of a dissipative quantum system is evaluated iteratively by filtering the negligible propagator elements at each propagation step. This on-the-fly filtering along with the finiteness of the bath memory dramatically reduces the configuration space to be integrated without losing numerical accuracy of the results. The required computational storage space for the configuration and the weight of the survived path segments increases linearly with the bath memory length. Up to the bath memory time, it is found that a strikingly small fraction of the configurations survives the on-the-fly filtering process and the number of surviving configurations increases algebraically with the propagation time. At times longer than the bath memory time, the number of surviving configurations required for numerically accurate results is essentially saturated and is less than 0.1% of the total number of the configurations. This new scheme is extremely useful for the problems of low-frequency solvents for which the bath memory spans many time steps of the propagation.
Vitrification and determination of the crystallization time scales of the bulk-metallic-glass-forming liquid Zr58.5Nb2.8Cu15.6Ni12.8Al10.3
Authors: Hays C. C. , Schroers J. , Johnson W. L. , Rathz T. J. , Hyers R. W. , Rogers J. R. , Robinson M. B.
Published: July 2002

The crystallization kinetics of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 were studied in an electrostatic levitation (ESL) apparatus. The measured critical cooling rate is 1.75 K/s. Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the first bulk-metallic-glass-forming liquid that does not contain beryllium to be vitrified by purely radiative cooling in the ESL. Furthermore, the sluggish crystallization kinetics enable the determination of the time-temperature-transformation (TTT) diagram between the liquidus and the glass transition temperatures. The shortest time to reach crystallization in an isothermal experiment; i.e., the nose of the TTT diagram is 32 s. The nose of the TTT diagram is at 900 K and positioned about 200 K below the liquidus temperature.
Thermal and pulsed laser induced surface reactions in Ti/Si(001) interfaces studied by spectromicroscopy with synchrotron radiation
Authors: Larciprete R. , Danailov M. , Barinov A. , Gregoratti L. , Kiskinova M.
Published: July 2002

Thin titanium silicide layers, produced by thermal or ultraviolet (UV) and visible pulsed laser annealing of Ti films deposited on Si substrates, have been studied by synchrotron radiation scanning photoemission spectroscopy (SR-SPEM) with lateral resolution of 0.12 μm. The evolution of the Ti 2p, Si 2p, and valence band spectra were used as fingerprints for the occurring morphological changes and interfacial reactions. For thermal processes the Ti films were deposited through a mask and by performing spectromicroscopy across the edge of the Ti patch the influence of the film thickness on the interface reaction was probed. The advancement of the interfacial reaction as a function of the annealing temperature was studied as well. The three components in the Si 2p spectra with chemical shift of −0.76, −0.50 and −0.18 eV, observed after thermal annealing at 650 and 850 °C were attributed to TiSi, C49 TiSi2, and C54 TiSi2, respectively. For the laser treated Ti/Si interfaces SPEM was successfully used to map the lateral distribution of these silicide phases formed within the laser irradiated region. In all cases the laser beams were focused and the photon density values were chosen to limit the temperature rise below the Si and Ti melting thresholds. We found that in the external region of the laser spots where the local temperature does not exceed 500 °C the dominating C49 TiSi2 phase coexists with some TiSi, whereas in the hottest central region the formation of C54 TiSi2 is favored. The similarity of the lateral distribution and the chemical phases formed within the laser spots obtained using UV and visible radiation confirmed that the local laser-induced temperature rise controls the interfacial processes, whereas the radiation wavelength plays a negligible role.
Sequential quantum measurements
Authors: Gudder Stan , Nagy Gabriel
Published: July 2002

A quantum effect is an operator A on a complex Hilbert space H that satisfies 0⩽A⩽I. We denote the set of quantum effects by E(H). The set of self-adjoint projection operators on H corresponds to sharp effects and is denoted by P(H). We define the sequential product of A,B∈E(H) by A∘B=A1/2BA1/2. The main purpose of this article is to study some of the algebraic properties of the sequential product. Many of our results show that algebraic conditions on A∘B imply that A and B commute for the usual operator product. For example, if A∘B satisfies certain distributive or associative laws, then AB=BA. Moreover, if A∘B∈P(H), then AB=BA and A∘B=B or B∘A=B if and only if AB=BA=B. A natural definition of stochastic independence is introduced and briefly studied.
Fast switching of magnetic fields in a magneto-optic trap
Authors: Dedman C. J. , Baldwin K. G. H. , Colla M.
Published: July 2002

Magneto-optic traps which employ current windings to generate pulsed magnetic fields require rapid switch-off times for many applications. Practical difficulties in attaining rapid switch-off of the magnetic field, including the generation of induced currents, are addressed. Several methods for minimizing the switch-off time are presented which do not require complex feedback mechanisms involving direct measurement of the magnetic field.
Activation of methane by size-selected iron cluster cations, Fen+ (n=2–15): Cluster-CHx (x=0–3) bond energies and reaction mechanisms
Authors: Liyanage Rohana , Zhang Xiao-Guang , Armentrout P. B.
Published: July 2002

The kinetic energy dependences of the reactions of Fen+ (n=2–15) with CD4 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0–10 eV. All reactions exhibit thresholds and two main products, FenD+ and FenCD2+, are formed. These primary products decompose at higher energies to form secondary and higher order products, FenCD+, FenC+, Fen−1D+, Fen−1CD2+, Fen−1CD+, and Fen−1C+. The cross-section magnitudes for the dehydrogenation products, FenCD2+, are observed to vary considerably as a function of cluster size; subsequent dehydrogenation to form FenC+ becomes more facile for larger clusters. Thresholds for the various primary and secondary reactions are analyzed and bond energies for iron cluster cation bonds to C, CD, CD2, and CD3 are determined. As a function of cluster size, these bond energies rapidly reach relatively constant values, which are argued to lie close to bulk phase values. The relative magnitudes in these bond energies are consistent with simple bond order considerations. On the basis of this thermochemistry, we find that there are barriers to the primary dehydrogenation reactions for all the clusters, except n=3 and 4. Evidence that this barrier for n⩾5 corresponds to the chemisorption step is discussed.
Intramolecular charge transfer of π-conjugated push–pull systems in terms of polarizability and electronegativity
Authors: Yong Lee Jin , Kim Kwang S. , Jin Mhin Byung
Published: July 2002

We have derived a simple expression to evaluate the amount of intramolecular charge transfer (ICT) of π-conjugate push–pull systems from the properties of electronegativity (χ) and polarizability (α) of the corresponding push and pull systems. This simple model is verified from ab initio calculations of disubstituted benzenes, stilbenes, and butadienes (push–pull systems) and their monosubstituted (push or pull) systems with various donors and acceptors. The bond length alternation (BLA) is often used as a good structural parameter to describe the amount of ICT; however, it is not a complete parameter because the amounts of ICT for the same sets of donor/acceptor pairs are different for different bridge systems. Here, we report a parameter composed of polarizability and electronegativity to give a consistent amount of ICT for different bridge systems. In particular, when a highly electropositive donor is used, the polarizability of an acceptor is the most determining factor for ICT. On the basis of this model, we find a very strong acceptor with large polarization effect, CH=C5H4, which gives a large nonlinear optical (NLO) response when a highly electropositive donor is used. The model would be a very useful utility to design various types of new functional molecular systems involving ICT optimization.
Strain-induced anisotropic low-field magnetoresistance of La–Sr–Mn–O thin films
Authors: Choi Kyung-Ku , Taniyama Tomoyasu , Yamazaki Yohtaro
Published: July 2002

Sputtered La0.71Sr0.29Mn1.01O3−δ (LSMO) thin films on (001) SrTiO3, polycrystalline yttria-stabilized zirconia (YSZ) and (112̄0) sapphire substrates demonstrate the distinctive low-field magnetoresistance (MR) correlated with the microstructure and the strain of the films. The epitaxial LSMO film on (001) SrTiO3 shows the in-plane magnetic anisotropy with [110] easy axis and the attendant anisotropic MR. The polycrystalline films on YSZ and sapphire substrates with grain sizes from 20 to 60 nm exhibit different anisotropic feature of transport: the isotropic MR of the film on YSZ and the large anisotropy on sapphire substrates. Moreover, in the (112̄0) film plane of sapphire substrate, the [1̄100]SAP magnetic easy axis appears due to a large tensile stress, and the longitudinal MR becomes pronounced along the [0001]SAP hard axis. This implies that the anisotropy of the low-field MR is attributed to the stress induced by the thermal expansion mismatch between film and substrate. These results emphasize that the low-field MR in the polycrystalline manganite can be advanced by the strain induced magnetic anisotropy.
Mechanism for low temperature activation of Mg-doped GaN with Ni catalysts
Authors: Waki I. , Fujioka H. , Oshima M. , Miki H. , Okuyama M.
Published: July 2002

The activation mechanism of Mg-doped GaN with Ni catalysts has been investigated by thermal desorption spectroscopy. It has been revealed that Ni deposited on Mg-doped GaN enhances the hydrogen recombination reaction at temperatures below 200 °C with the activation energy of 1.3 eV. The hydrogen desorbed at this temperature can be attributed to a part of the passivating hydrogen in GaN with a weak binding energy. The enhancement of the hydrogen recombination reaction on the GaN surface is essential to decreasing hydrogen concentration efficiently at low temperatures.
Iterative path integral formulation of equilibrium correlation functions for quantum dissipative systems
Authors: Shao Jiushu , Makri Nancy
Published: July 2002

We present an iterative path integral algorithm for computing multitime correlation functions of a quantum system coupled to a dissipative bath of harmonic oscillators. By splitting the Boltzmann operator into two parts and reordering the propagators in the expression for canonical correlation functions, we are able to transform the evolution time contour into a symmetric one so that a forward propagation and a backward one are specified. Because the memory induced by the bath through the Feynman–Vernon influence functional decays rapidly in the complex time plane, long-time correlations are negligible. Taking advantage of this fact, we show that the correlation function can be obtained via an iterative procedure. The method is used to calculate three-time correlation functions of a dissipative two-level system.
High-resolution ellipsometric study of an n-alkane film, dotriacontane, adsorbed on a SiO2 surface
Authors: Volkmann U. G. , Pino M. , Altamirano L. A. , Taub H. , Hansen F. Y.
Published: July 2002

Using high-resolution ellipsometry and stray light intensity measurements, we have investigated during successive heating-cooling cycles the optical thickness and surface roughness of thin dotriacontane (n-C32H66) films adsorbed from a heptane (n-C7H16) solution onto SiO2-coated Si(100) single-crystal substrates. Our results suggest a model of a solid dotriacontane film that has a phase closest to the SiO2 surface in which the long-axis of the molecules is oriented parallel to the interface. Above this “parallel film” phase, a solid monolayer adsorbs in which the molecules are oriented perpendicular to the interface. At still higher coverages and at temperatures below the bulk melting point at Tb=341 K, solid bulk particles coexist on top of the “perpendicular film.” For higher temperatures in the range Tb<TTs, a uniformly thick fluid film wets to the parallel film phase. This structure of the alkane/SiO2 interfacial region differs qualitatively from that which occurs in the surface freezing effect at the bulk alkane fluid/vapor interface. In that case, there is again a perpendicular film phase adjacent to the air interface but no parallel film phase intervenes between it and the bulk alkane fluid. Similarities and differences between our model of the alkane/SiO2 interface and one proposed recently will be discussed. Our ellipsometric measurements also show evidence of a crystalline-to-plastic transition in the perpendicular film phase similar to that occurring in the solid bulk particles present at higher coverages. In addition, we have performed high-resolution ellipsometry and stray-light measurements on dotriacontane films deposited from solution onto highly oriented pyrolytic graphite substrates. After film deposition, these substrates proved to be less stable in air than SiO2.
Engineering of Si surfaces by electrochemical grafting of p-nitrobenzene molecules
Authors: Hartig P. , Rappich J. , Dittrich Th.
Published: July 2002

The changes of the band bending and of the nonradiative (nr) surface recombination are investigated by use of photovoltage and photoluminescence techniques during the electrochemical deposition of p-nitrobenzene molecules on atomically flat and rough hydrogenated as well as on chemically oxidized Si(111) surfaces. A simple and well-reproducible procedure has been developed for electrochemical grafting of organic molecules on hydrogenated Si surfaces in aqueous electrolytes. The grafting of a monolayer of p-nitrobenzene molecules on atomically flat p-Si(111):H surfaces induces a change of the band bending of about 0.1 eV and the amount of nr surface defects, Ns, is only slightly increased by a factor of about 3 (Ns<1011 cm−2) with respect to the hydrogenated Si surface. The role of the formation of radicals for the engineering of Si surfaces is discussed.
NASA’s Deep Space 1 ion engine (plenary)
Authors: Brophy John R.
Published: July 2002

Ion propulsion is now a legitimate propulsion option for future deep space missions. The long journey required to get from the first laboratory test of an ion engine in 1960 to the first successful flight of an ion propulsion system on NASA’s Deep Space 1 mission in 1998 is briefly summarized herein. An overview of the operation of the Deep Space 1 ion engine is provided along with a description of the complete ion propulsion system on the spacecraft. Engine performance measured in space compares well with that based on ground test data. Future deep space missions desire improved engine performance in the form of longer engine life (greater total impulse) and greater specific impulse. Derivatives of the NSTAR ion engine are being evaluated to assess their capability to meet these future needs.
Critical parameters of the restricted primitive model
Authors: Panagiotopoulos Athanassios Z.
Published: July 2002

The critical parameters for the restricted primitive model of electrolyte solutions were determined from extensive grand canonical Monte Carlo simulations combined with mixed-field finite-size scaling. The fine-lattice discretization method was used for the calculations, with Ewald summation of the long-range coulombic forces. Ising criticality and no pressure mixing were assumed in the finite-size scaling analysis. The critical parameters were obtained as a function of boundary conditions at infinite distance (ε∞), system size L, and lattice discretization parameter ζ. They were found to be sensitive to L for vacuum boundary conditions (ε∞=1), but much less so for “tin-foil” boundary conditions (ε∞=∞). The critical temperature and density decrease with increasing ζ. These calculations are compared to previous estimates of the critical parameters for this model. Extrapolation of our results to the thermodynamic limit in continuous space (L→∞ and ζ→∞) yields Tc*=0.0489±0.0003, ρc*=0.076±0.003.
Formulation and implementation of the full coupled-cluster method through pentuple excitations
Authors: Musiał M. , Kucharski S. A. , Bartlett R. J.
Published: July 2002

Using the quasilinearized formulation of CC theory in terms of recursively computed intermediates, we present the detailed equations and implementation of coupled-cluster theory with single, double, triple, quadruple, and pentuple excitations, CCSDTQP. We illustrate its results by comparison with several full CI results in double zeta, polarized basis sets (DZP), at different geometries. The maximum error compared to full CI occurs for H2O at twice Re which is 0.026 mH. For all other cases, HF, SiH2, and CH2 in its singlet state, the largest errors are 0.001 mH. The magnitude of the connected T5 contribution is as large as 0.35 mH, but usually less than 0.1 mH for these examples.
Characterization of UV detectors at SURF III (invited)
Authors: Shaw Ping-Shine , Larason Thomas C. , Gupta Rajeev , Lykke Keith R.
Published: July 2002

The Synchrotron Ultraviolet Radiation Facility (SURF III) at the National Institute of Standards and Technology provides a unique research opportunity in precision measurements with its continuous and calculable radiation stretching from the soft x ray to the far infrared. In response to the rapid development of ultraviolet photodetectors for applications ranging from industrial photolithography to astrophysics, we have developed measurement capabilities to characterize photodetectors with high accuracy. The absolute measurements at SURF III are based on a high-accuracy liquid-helium cooled cryogenic radiometer for measuring the power of the dispersed radiation from SURF III through a monochromator. Typical detector calibration uncertainties achieved at SURF III using cryogenic radiometer is better than 0.5% in the ultraviolet. Equipped with such a tool, we were able to study a variety of ultraviolet detectors and determine their spectral responsivities, surface reflectivities, and the effects of radiation damage. Due to the accuracy of these measurements, the internal quantum efficiencies of the photodetectors can be derived and theoretically modeled to provide information on the mechanism of photo detection.
Piezo-XAFS–time-resolved x-ray absorption spectroscopy
Authors: Richwin Matthias , Zaeper Ralf , Lützenkirchen-Hecht Dirk , Frahm Ronald
Published: July 2002

The piezo-x-ray absorption spectroscopy technique is a novel tool for time-resolved x-ray absorption spectroscopy in the hard x-ray range. It makes use of piezo tilt tables mounted below the crystals in a double crystal or channel cut crystal monochromator. Repetitive energy scans are performed by applying an oscillatory high voltage to the piezo translators of the tilt tables. Currently, this allows one to scan an energy range of several hundred eV in the hard x-ray range with repetition frequencies of typically 10 Hz. The capability to record full extended x-ray absorption fine structure spectra on a subsecond time scale is demonstrated.
Elastic and piezoelectric fields around a quantum dot: Fully coupled or semicoupled model?
Authors: Pan E.
Published: July 2002

In the study of elastic and piezoelectric fields in semiconductors due to buried quantum dots (QDs), the semicoupled piezoelectric model is commonly adopted. However, its accuracy and suitability have never been studied. In this article, by developing a fully coupled piezoelectric model and deriving the analytical elastic and piezoelectric fields based on this and the semicoupled models, we are able to verify that when the piezoelectric coupling is weak, like GaAs with the electromechanical coupling factor g=0.04, the semicoupled model predicts very accurate results as compared to those based on the fully coupled model. However, if the piezoelectric coupling is relatively strong, like AlN with g=0.32, we have shown that the semicoupled model gives very serious errors or even totally wrong results. Applying these two models to a uniformly strained AlN layer grown along the polar axis has also confirmed our observation. Therefore, for semiconductors like AlN, the fully coupled model presented in this article must be employed in order to give a reliable and accurate prediction for the elastic and piezoelectric fields. Also presented in this article is the distribution of the piezoelectric field on the surface of a half-space GaAs due to a buried QD located at 2 nm below the surface with a volume 4π/3 (nm)3. It is observed that the horizontal electric field on the traction-free and insulating surface shows some special features and its maximum magnitude can be as high as 3.5×107 V/m when the uniform mismatch eigenstrain is 0.07. Furthermore, the piezoelectric field on the traction-free and conducting surface exhibits different characters as compared to the traction-free and insulating case.
Imaging of neutron incoherent scattering from hydrogen in metals
Authors: Chen-Mayer H. H. , Mildner D. F. R. , Lamaze G. P. , Lindstrom R. M.
Published: July 2002

Neutron incoherent scattering is a technique that may be used to determine the concentration of hydrogen within various metallic systems. By measuring the scattered neutrons using a position-sensitive detector, we can determine the amount of hydrogen as well as its location in the matrix. Using a slit or aperture in the scattering geometry, we have demonstrated the imaging of hydrogen in layers of polypropylene sandwiched between sets of titanium plates, and also in titanium standards containing known amounts of hydrogen. We have shown the ability to image hydrogen in titanium at the 100 μg/g level as a function of location. Analysis of the images shows that the scattering from the hydrogen increases linearly with its mass fraction within the titanium. We have also investigated the effects on the images of attenuation of the incident beam prior to scattering.
Thickness effects on magnetic properties and ferromagnetic resonance in Co–Ni–Fe–N soft magnetic thin films
Authors: Kim Y. M. , Han S. H. , Kim H. J. , Choi D. , Kim K. H. , Kim J.
Published: July 2002

In this study, the magnetic properties and microstructures have been investigated in Co–Ni–Fe–N soft magnetic thin films with various film thicknesses (0.01–1.0 μm). As the film thickness decreases, the coercivity, electrical resistivity, and magnetic anisotropy field of these films increases from 1.1 to 7.9 Oe, 53 to 188 μΩ cm, and 20 to 70 Oe, respectively. In addition, excellent high frequency characteristics were achieved in Co–Ni–Fe–N films with thicknesses less than 0.3 μm, where an effective permeability of, ∼1000 was maintained out to 700 MHz. Analysis results from XRD, TEM, and FMR show that a 0.1-μm-thick film is composed of an amorphous phase. This amorphous phase gradually changes to a crystalline phase with increasing film thickness.
Temperature dependence of the sticking coefficient of methyl radicals at hydrocarbon film surfaces
Authors: Meier Matthias , von Keudell Achim
Published: July 2002

The temperature dependence of the interaction of methyl radicals with the surface of a hard, amorphous hydrogenated carbon film is investigated using in situ real-time ellipsometry and infrared spectroscopy. This interaction is considered as an important process during plasma deposition of polymer-like hydrocarbon films or formation of polycrystalline diamond in methane-containing discharges. At room temperature CH3 adsorbs at sp2-coordinated CC bonds at the physical surface of the hard C:H film and forms a completely sp3-hybridized C:H adsorbate with a thickness of ∼0.17 nm. In the following, steady-state film growth is observed with a sticking coefficient of s(CH3)=10−4. At a substrate temperature of T=570 K, incident CH3 causes net erosion with an etching yield of Y(CH3)=10−4. At temperatures above 650 K the sticking coefficient of CH3 becomes positive again, leading to a graphite-like C:H adsorbate. CH3 adsorption is described by a reaction scheme based on the creation of dangling bonds at the film surface via abstraction of surface-bonded hydrogen by incoming CH3 radicals. These dangling bonds act as CH3 adsorption sites at room temperature or as a precursor for chemical erosion at elevated temperature.
Characterization of a gamma-ray source based on a laser-plasma accelerator with applications to radiography
Authors: Edwards R. D. , Sinclair M. A. , Goldsack T. J. , Krushelnick K. , Beg F. N. , Clark E. L. , Dangor A. E. , Najmudin Z. , Tatarakis M. , Walton B. , Zepf M. , Ledingham K. W. D. , Spencer I. , Norreys P. A. , Clarke R. J. , Kodama R. , Toyama Y. , Tampo M.
Published: July 2002

The application of high intensity laser-produced gamma rays is discussed with regard to picosecond resolution deep-penetration radiography. The spectrum and angular distribution of these gamma rays is measured using an array of thermoluminescent detectors for both an underdense (gas) target and an overdense (solid) target. It is found that the use of an underdense target in a laser plasma accelerator configuration produces a much more intense and directional source. The peak dose is also increased significantly. Radiography is demonstrated in these experiments and the source size is also estimated.
Organic light-emitting devices for illumination quality white light
Authors: Duggal Anil R. , Shiang J. J. , Heller Christian M. , Foust Donald F.
Published: July 2002

A method of generating white light by combining a blue organic light-emitting diode with a down-conversion phosphor system is presented. It is demonstrated that the use of the down-conversion phosphor system actually leads to an overall power efficiency increase, an effect attributed to the high quantum efficiency of phosphor materials and the presence of light scattering in the phosphor layers. It is also shown that this approach permits the generation of illumination quality white light over the full range of color temperatures required for lighting applications. For the model device demonstrated in this work, an overall electrical to optical power conversion efficiency of 1.3% was achieved at a brightness of 1080 cd/m2.
A novel preterm respiratory mechanics active simulator to test the performances of neonatal pulmonary ventilators
Authors: Cappa Paolo , Sciuto Salvatore Andrea , Silvestri Sergio
Published: July 2002

A patient active simulator is proposed which is capable of reproducing values of the parameters of pulmonary mechanics of healthy newborns and preterm pathological infants. The implemented prototype is able to: (a) let the operator choose the respiratory pattern, times of apnea, episodes of cough, sobs, etc., (b) continuously regulate and control the parameters characterizing the pulmonary system; and, finally, (c) reproduce the attempt of breathing of a preterm infant. Taking into account both the limitation due to the chosen application field and the preliminary autocalibration phase automatically carried out by the proposed device, accuracy and reliability on the order of 1% is estimated. The previously indicated value has to be considered satisfactory in light of the field of application and the small values of the simulated parameters. Finally, the achieved metrological characteristics allow the described neonatal simulator to be adopted as a reference device to test performances of neonatal ventilators and, more specifically, to measure the time elapsed between the occurrence of a potentially dangerous condition to the patient and the activation of the corresponding alarm of the tested ventilator.
Role of substrate in the pseudomorphic stabilization of rocksalt-type AlN phase in AlN/TiN superlattices
Authors: Pankov V. , Evstigneev M. , Prince R. H.
Published: July 2002

The nonequilibrium B1 (rocksalt-type) AlN phase was successfully stabilized in stress-free AlN/TiN superlattices prepared by room-temperature pulsed laser deposition on thin film amorphous carbon substrates. The coatings displayed the same properties as reported for multilayers grown on thick crystalline substrates; namely, the existence of a critical thickness of AlN (about 1 nm for the present study) below which AlN crystallizes in the B1–AlN phase. Exceeding this value lead to AlN crystallization in its equilibrium hexagonal wurtzite-type structure, accompanied by considerable degradation of coating crystallinity due to the loss of coherent epitaxial growth of alternating TiN and AlN layers in the coating. It is concluded that neither an orientational effect of the substrate nor its stress-sustaining ability is required to provide a pseudomorphic stabilization of the nonequilibrium B1–AlN phase in AlN/TiN multilayered coatings.
A study of subbands in AlGaN/GaN high-electron-mobility transistor structures using low-temperature photoluminescence spectroscopy
Authors: Fang C. Y. , Lin C. F. , Chang Edward Yi , Feng M. S.
Published: July 2002

Al 0.15 Ga 0.85 N/GaN high-electron-mobility transistor (HEMT) structures with various δ-doping concentrations and spacer thicknesses grown on sapphire by metalorganic chemical-vapor deposition are investigated. The Hall mobility is as high as 1333 cm2/V s at room temperature and 6330 cm2/V s at 77 K. Two-dimensional electron gas (2DEG) phenomena, which have not been clearly resolved in the literature, are observed by photoluminescence (PL) spectra at low temperature in this study. The PL spectra peaks of the interband transitions from 2DEG subbands to the valence band are in the range from 3.486 to 3.312 eV. The effects of the strain caused by different Al fractions of the top layer, and that of the spacer thickness on the 2DEG phenomena are discussed. Redshifts due to temperature variations for various HEMT structures are observed in 2DEG subbands and in the band-edge emission, which is believed to be evidence of interband transitions from 2DEG subbands to valence bands.
Turning point properties as a method for the characterization of the ergodic dynamics of one-dimensional iterative maps
Authors: Diakonos F. K. , Schmelcher P.
Published: July 2002

Dynamical as well as statistical properties of the ergodic and fully developed chaotic dynamics of iterative maps are investigated by means of a turning point analysis. The turning points of a trajectory are hereby defined as the local maxima and minima of the trajectory. An examination of the turning point density directly provides us with the information of the position of the fixed point for the corresponding dynamical system. Dividing the ergodic dynamics into phases consisting of turning points and nonturning points, respectively, elucidates the understanding of the organization of the chaotic dynamics for maps. The turning point map contains information on any iteration of the dynamical law and is shown to possess an asymptotic scaling behaviour which is responsible for the assignment of dynamical structures to the environment of the two fixed points of the map. Universal statistical turning point properties are derived for doubly symmetric maps. Possible applications of the observed turning point properties for the analysis of time series are discussed in some detail.
Using chaos to generate variations on movement sequences
Authors: Bradley Elizabeth , Stuart Joshua
Published: July 2002

We describe a method for introducing variations into predefined motion sequences using a chaotic symbol-sequence reordering technique. A progression of symbols representing the body positions in a dance piece, martial arts form, or other motion sequence is mapped onto a chaotic trajectory, establishing a symbolic dynamics that links the movement sequence and the attractor structure. A variation on the original piece is created by generating a trajectory with slightly different initial conditions, inverting the mapping, and using special corpus-based graph-theoretic interpolation schemes to smooth any abrupt transitions. Sensitive dependence guarantees that the variation is different from the original; the attractor structure and the symbolic dynamics guarantee that the two resemble one another in both aesthetic and mathematical senses.
Laser-initiated microchemistry in thin films: Development of new types of periodic structure
Authors: Fisanick G. J. , Gross M. E. , Hopkins J. B. , Fennell M. D. , Schnoes K. J. , Katzir A.
Published: July 2002

Decomposition of Au metallopolymer thin films induced by a scanning cw Ar+ laser leads to metallic features that, under a broad range of experimental conditions, exhibit marked periodic structure. Preliminary mechanistic studies point to a thermal decomposition pathway in which the heat released in the exothermic reaction drives the reaction front ahead of the laser-irradiated area. The spatial period is determined by laser power, scan speed, and film thickness. We argue that this mechanism is analogous to laser-initiated explosive phenomena observed in the recrystallization of amorphous semiconductors.
An electro-optical technique for intense microwave measurements
Authors: Chang J. , Vittitoe C. N. , Neyer B. T. , Ballard W. P.
Published: July 2002

An electro-optical technique has been developed to measure high-frequency electric fields in free space. Electrically induced birefringence in an electro-optical crystal is used to modulate a linearly polarized continuous-wave laser beam. The modulation impressed on the laser beam contains both frequency and field intensity information. A way to use this technique as both a frequency and power meter is discussed. A proof-of-principle experiment has been carried out with a 3.1-GHz magnetron source.
A method to eliminate wetting during the homogenization of HgCdTe
Authors: Su Ching-Hua , Lehoczky S. L. , Szofran F. R.
Published: July 2002

Adhesion of HgCdTe samples to fused silica ampoule walls, or ‘‘wetting,’’ during the homogenization process was eliminated by adopting a slower heating rate. The idea is to decrease Cd activity in the sample so as to reduce the rate of reaction between Cd and the silica wall.
Anisotropic self-diffraction in Mg-doped LiNbO3
Authors: Arizmendi Luis , Powell Richard C.
Published: July 2002

The characteristics of anisotropic self-diffraction were studied for undoped LiNbO3 crystals and LiNbO3:Mg at doping levels of a few mole percent. From the absolute magnitude and the time dependence of the signal, it was possible to determine the induced refractive-index change and the conductivity of the charges involved in the photorefractive effect for each sample. For these excitation conditions, only charge migration perpendicular to the c axis contributes to laser-induced grating, and thus the conductivity is associated with charge diffusion. The results show that the total refractive-index change causing the self-diffraction is only slightly smaller for the Mg-doped samples than for undoped LiNbO3 while the conductivity is 1 order of magnitude greater for the Mg-doped samples.
Laser direct-write metallization in thin palladium acetate films
Authors: Gross M. E. , Appelbaum A. , Gallagher P. K.
Published: July 2002

Micron-scale palladium lines have been produced by a photothermal laser direct-write process in thin palladium acetate films. The range of observed structures relates to the complex thermal profiles generated by coupling of the incident laser radiation with the exothermic heat of reaction. Surprisingly, the chemical composition of these features does not vary significantly as a function of laser power and scan speed. Rather, deviation of the electrical resistivities of these features from that of pure palladium results from porosity in the lines.
Computation of conductivity and dielectric constant of periodic porous media
Authors: Shen L. C. , Liu C. , Korringa J. , Dunn K. J.
Published: July 2002

The static conductivity and dielectric constant of two-component periodic composite material are calculated using the Fourier expansion technique. The composite material consists of spheres that are arranged in simple, face-centered, or body-centered cubic lattices. The spheres may be isolated to yield high porosity or pore space, or they may intercept each other, leaving small pore space in between. The effective static conductivity and dielectric constant of such structures are computed theoretically when the pore space is filled with a material that has a conductivity or dielectric constant which differs from that of the matrix of the structure.
The role of grain size in the magnetization process of soft magnetic materials
Authors: Bertotti G. , Fiorillo F. , Montorsi A.
Published: July 2002

We investigate the role of grain size in the magnetization process of nonoriented 1.7% Al 1.9% Si Fe steels by measuring the power spectrum and the amplitude probability distribution of the Barkhausen noise. Experiments refer to single strips of variable grain size (0.03 mm<s<0.27 mm) and are made under controlled values of magnetization rate and permeability. We also study the propagation along the specimen of individual Barkhausen reversals at very low magnetization rates (3×10−4 T s−1). The data are interpreted in the frame of a theory recently proposed by the authors, which connects the Barkhausen effect to the statistical properties of the random local coercive field experienced by a moving domain wall.
The resonant circular array of electrically short elements
Authors: Fikioris George , King Ronold W. P. , Wu Tai Tsun
Published: July 2002

Different resonant distributions of current in circular arrays with N=60, 72, and 90 elements are investigated theoretically with the method of symmetrical components and a two-term representation of current in each element. The arrays studied have only one driven element. The remarkable results—which are displayed in a sequence of graphs—indicate that the relative currents in the elements can be distributed in triplets, triplets alternating with singlets, two independent sets of singlets, or one set of singlets. The driving-point impedance of this last distribution is a pure resistance, the Q of the 90-element array is over 30 000, and the half-power beamwidth of each of the 90 sharp peaks in the power pattern is 2°.