bluuebunny/crossref_metadata_2025 huggingface dataset:
A simple alignment procedure for the precision positioning of the sample at arbitrary points of the intracavity space of a TE104 double rectangular cavity
Authors: Mazúr M. , Valko M. , Morris H.
Published: July 2002

A significant source of errors in quantitative electron paramagnetic resonance measurements is incorrect sample positioning in the microwave cavity. In this article a modification to our previously described sample alignment tool which allowed the precise and reproducible positioning of cylindrical samples along the vertical central axis of the rectangular cavity is described. These revisions allow cylindrical sample movement in the horizontal (y,z) plane which is perpendicular to the cavity x axis. Using this upgrade, the new sample setup procedure is now able to position samples precisely and reproducibly at arbitrary points in the intracavity space.
Low-energy stripping of Kr+, Xe+, and Pb+ beams in helium and nitrogen
Authors: Decrock P. , Kanter E. P. , Nolen J. A.
Published: July 2002

Stripping efficiencies for Kr+, Xe+, and Pb+ beams in helium and nitrogen gas targets have been measured at energies varying between 0.8 and 2.0 MeV. The stripping yields were determined for different target densities, ranging from single-collision conditions to equilibrium. Based on these results, approximate predictions of the equilibrium charge state distributions for low velocity (v/c=0.0040–0.0060) heavy ions with 28<Z<92 in collisions with helium atoms are made. Furthermore, measurements of the small-angle scattering of these heavy ion beams in collisions with dilute helium and nitrogen gases are reported. The presented results are important in view of a proposed postacceleration scheme for low-energy radioactive ion beams using a linac. These data show that helium generally produces higher yields of 2+ and 3+ ions and that the optimal choice of gas thickness for the proposed application is somewhat less than that required for equilibrium.
Analysis of the optical reconstruction of shearograms using oblique illumination
Authors: Ng T. W.
Published: July 2002

Oblique illumination has superseded the use of lenses in the optical reconstruction of shearograms. Nevertheless, the mechanism behind this operation has, hitherto, not been clearly elucidated. In this article, we describe this mechanism via the use of ray and wave optics principles. A comparative analysis of the performances using white-light and monochromatic illumination is also furnished.
Development of a double-charge exchange He− source
Authors: Sasao M. , Taniike A. , Nishiura M. , Wada M.
Published: July 2002

A dc-mode He− source of the double charge exchange type has been developed for the purpose of application to alpha-particle measurement. Because the optimum energy for the two-step two-electron capture process in a rubidium vapor was reported to be around 6 keV, the essential point of development is extraction of an intense He+ beam at relatively low energy. As the discharge current of the He+ ion source is increased, the focusing property deteriorates, and higher electrostatic field is required for extraction. A He− current of 0.07 mA is continuously obtained from a He+ beam extracted from a 6 mm diameter single hole. The conversion efficiency from He+ to He− is about 2%–3% in the energy region of 5–10 keV.
Drift-free integrators
Authors: Gavin Henri P. , Morales Rodrigo , Reilly Kathryn
Published: July 2002

Bias errors introduced by systems designed to measure low-frequency transients negate zero-mean assumptions on the measurement noise. On-line signal processing methods that require accurate low-frequency information can be adversely affected by bias errors. On-line integration of dynamic signals is a classical example of a process that is unstable in the presence of bias errors. Accurately integrated quantities (like velocity and displacement), from easily measured quantities (like acceleration), can inform control systems and reduce on-line computational burdens. This article introduces a feedback stabilization method for a hybrid digital-analog integrator. The analytical performance of this integrator is compared to a filtered analog integrator in the time and frequency domains. For wide-band random signals, the analog circuit performs well with respect to linearity and hysteresis, but does less well for long-period signals. A stabilized hybrid analog-digital integrator has exceptional accuracy when integrating long-period signals, but produces phase and bias errors when integrating wide-band signals. The integrators examined in this study are unconditionally stable and robust to bias on the input, internal bias currents in the operational amplifiers, and finite slew rates of the components.
Transputer based acquisition/reconstruction system as a development tool for positron emission tomography
Authors: Bertolucci E. , Conti M. , Curto C. A. , Pennino G. , Russo P.
Published: July 2002

We present an acquisition and reconstruction system for positron emission tomography (PET), to be used as a development tool for testing PET reconstruction algorithms and for detector characteristics studies. It is based on a transputer network and modular electronics: acquired data are stored in memories associated with parallel processors and offline data analysis is performed, allowing for fast comparison of performances of different reconstruction algorithms. Its simple architecture and low cost makes it an easy-to-use and convenient tool to test new detectors for PET applications. We introduce here some results obtained with two CdZnTe radiation detectors on a rotating ring, as a prototype example of a small-animal PET camera.
Low temperature thermometry in high magnetic fields. VII. Cernox™ sensors to 32 T
Authors: Brandt B. L. , Liu D. W. , Rubin L. G.
Published: July 2002

This article describes the behavior of Cernox™ zirconium oxynitride film temperature sensors from Lake Shore Cryotronics, Inc. in magnetic fields up to 32 T at temperatures between 2 and 286 K. Results from a number of sensors with different dimensionless temperature sensitivities and from different production batches are analyzed and compared with previous results on carbon-glass sensors that extend to 19 T. In that field range, the Cernox™ sensors appear to be a real alternative to carbon glass. Furthermore, the magnitude of their uncorrected error, ΔT/T, is smaller than other sensors at most temperatures in fields less than 20 T, and their temperature correctabilities appear to apply to off-the-shelf sensors with dimensionless temperature sensitivities in the range of −0.74 to −1.9. The sensors show a negligible (<0.05%) orientation dependence of their ΔR/R at 78 K; at 4.2 K, that dependence can be as high as ∼0.7% at 20 T.
Mass measurement under weightless conditions
Authors: Fujii Yusaku , Fujimoto Hiroyuki , Namioka Shuhei
Published: July 2002

A method for quickly and accurately measuring mass under weightless conditions is proposed using the principle of action and reaction, in other words, the law of conservation of momentum. In this method, the velocities of two different uniform motion states of the object and the impulse, that is, the time integration of the impact force, between them are measured, and then the mass of the object is determined. The velocity can be easily measured highly accurately using an optical interferometer. For the preparatory experiments on earth, an instrument with which linear motion with constant velocity is realized using an air cushion linear bearing and which detects impact using a force sensor, has been developed. The relative combined standard uncertainty in mass measurements from 2 to 11 kg in a single impact measurement is estimated to be about uc,r=0.6×10−2 (0.6%). For more accurate measurements, we propose an improved method in which the impulse is measured using the reference mass and its momentum change instead of the force sensor which is the biggest error source in the experiments.
Transparent organic light emitting devices
Authors: Gu G. , Bulović V. , Burrows P. E. , Forrest S. R. , Thompson M. E.
Published: July 2002

We report the demonstration of transparent organic light emitting devices (OLEDs) which are ∼70% transparent throughout the visible spectrum when switched off, and emit light from both sides with a total external quantum efficiency of ∼0.1% when turned on. The devices are Alq3-based single heterostructure OLEDs grown on an ITO-coated glass substrate with a top electrode composed of a very thin layer of Mg–Ag and an overlaying ITO film. The top electrode is both electron injecting and transparent. The transparent OLEDs are expected to be useful in high-resolution full-color displays, as well as for helmet-mounted, windshield-mounted, or other ‘‘head-up’’ display applications.
Optical patterning of GaN films
Authors: Kelly M. K. , Ambacher O. , Dahlheimer B. , Groos G. , Dimitrov R. , Angerer H. , Stutzmann M.
Published: July 2002

Patterned etching of GaN films was achieved with laser-induced thermal decomposition. High-energy laser pulses are used to locally heat the film above 900 °C, causing rapid nitrogen effusion. Excess gallium is then removed by conventional etching. At exposures of 0.4 J/cm2 with 355 nm light, etch rates of 50–70 nm per pulse were obtained. Illumination with an interference grating was used to produce trenches as narrow as 100 nm.
Critical thickness of GaN thin films on sapphire (0001)
Authors: Kim Chinkyo , Robinson I. K. , Myoung Jaemin , Shim Kyuhwan , Yoo Myung-Cheol , Kim Kyekyoon
Published: July 2002

Synchrotron x-ray diffraction was employed to measure the lattice constants a and c of GaN films grown with an AlN buffer layer on sapphire (0001) over a thickness range of 50 Å to 1 μm. We used multiple reflections and a least-squares fit method for high reliability. As the thickness increased, the lattice constant a increased from 3.133 Å to 3.196 Å and c decreased from 5.226 Å to 5.183 Å. The expected trend was fitted to an equilibrium theory, allowing the critical thickness of GaN on AlN to be estimated at 29 Å ± 4 Å in good agreement with a theoretical prediction.
Doped organic electroluminescent devices with improved stability
Authors: Shi Jianmin , Tang C. W.
Published: July 2002

Remarkable improvement in stability has been demonstrated in an organic electroluminescent (EL) device using a doped emitter consisting of 8-hydroxyquinoline aluminum (Alq) as the host and N,N-dimethylquinacridone (DMQA) as the emissive dopant. A luminance half-life on the order of about 7500 h has been achieved in the DMQA/Alq EL device, operating under a constant current of 20 mA/cm2 and starting at a high luminance of 1400 cd/m2. The improved stability was attributed to the elimination of intermolecular hydrogen bonding between the dopant molecules.
Lithographic positioning of self-assembled Ge islands on Si(001)
Authors: Kamins T. I. , Williams R. Stanley
Published: July 2002

Ge islands were deposited on Si(001) partially covered with patterned oxide. Selective Si was deposited on some wafers before Ge deposition to form raised Si(001) plateaus with well-defined sidewall facets. On narrow lines, the Ge islands locate preferentially at the edges of the raised Si(001) regions, and the preference is strongest on the narrowest patterns aligned along a 〈100〉 direction. For a 450 nm wide plateau aligned in this direction, all the islands are positioned along the edges of the pattern, with a 300 nm space near the center of the pattern free of Ge islands. The islands appear to be uniformly spaced along the pattern edges. On wider lines, several rows of islands are aligned near the edges of the pattern, with the order decreasing farther from the edge.
Temperature dependence of MeV heavy ion irradiation-induced viscous flow in SiO2
Authors: Brongersma M. L. , Snoeks E. , Polman A.
Published: July 2002

In-situ wafer curvature measurements were performed to determine the mechanical stress in thermally grown SiO2 films on Si during 4 MeV Xe ion irradiation at various temperatures in the range from 90 to 575 K. Radiation induced viscous flow is observed and the radiation induced viscosity is determined at various temperatures. It ranges from 2.9×1023 at Pa ion/cm2 90–300 K to 1.6×1023 Pa ion/cm2 at 500 K. Both its magnitude and temperature dependence can be explained in terms of a phenomenological model in which stress relaxation takes place in locally heated, mesoscopic regions of low viscosity, centered around individual ion tracks. According to this model, stress relaxation occurs in ∼10 ps and within ∼3 nm of the ion track.
The influence of atomic nitrogen flux on the composition of carbon nitride thin films
Authors: Mérel P. , Chaker M. , Tabbal M. , Moisan M.
Published: July 2002

Carbon nitride (CNx) thin films have been deposited using a hybrid system combining pulsed laser deposition of graphite with the surface-wave discharge atomic nitrogen source (3% N2 in Ar). Using this system, an experiment is designed to study the influence of the atomic nitrogen flux on the composition of the CNx thin films at various laser intensities. The nitrogen percentage in the thin films is positively correlated with the N atom flux impinging on the substrate surface but it is counter-productive to use excessively high values of laser intensities on the graphite target. For a laser intensity of 6×108 W/cm2, the nitrogen percentage increases with the N atom flux and saturates at only about 16 at. %. On the other hand, a maximum nitrogen percentage of 30 at. % is obtained at the much lower laser intensity of 5×107 W/cm2.
Electron transport in AlGaN–GaN heterostructures grown on 6H–SiC substrates
Authors: Gaska R. , Yang J. W. , Osinsky A. , Chen Q. , Khan M. Asif , Orlov A. O. , Snider G. L. , Shur M. S.
Published: July 2002

We investigated two-dimensional electron transport in doped AlGaN–GaN heterostructures (with the electron sheet concentration ns≈1013 cm−2) grown on conducting 6H–SiC substrates in the temperature range T=0.3–300 K. The electron mobility in AlGaN–GaN heterostructures grown on SiC was higher than in those on sapphire substrates, especially at cryogenic temperatures. The highest measured Hall mobility at room temperature was μH=2019 cm2/V s. At low temperatures, the electron mobility increased approximately five times and saturated below 10 K at μH=10250 cm2/V s. The experimental results are compared with the electron mobility calculations accounting for various electron scattering mechanisms.
Lasing characteristics of low threshold ZnSe-based blue/green laser diodes grown on conductive ZnSe substrates
Authors: Katayama K. , Yao H. , Nakanishi F. , Doi H. , Saegusa A. , Okuda N. , Yamada T. , Matsubara H. , Irikura M. , Matsuoka T. , Takebe T. , Nishine S. , Shirakawa T.
Published: July 2002

Room temperature continuous wave operation of ZnSe-based blue/green laser diodes grown homoepitaxially on conductive ZnSe substrates with threshold current densities as low as 176 A/cm2 has been demonstrated. This is the lowest reported threshold among all short wavelength lasers in the blue/green region. Lifetimes at room temperature of up to 2.1 h have been obtained for lasers with pre-existing defect densities lower than 3×104 cm−2.
Preparation and photoluminescence of alumina membranes with ordered pore arrays
Authors: Du Y. , Cai W. L. , Mo C. M. , Chen J. , Zhang L. D. , Zhu X. G.
Published: July 2002

Porous alumina membranes with ordered pore arrays were prepared electrochemically from Al metal in C2H2O4 or H2SO4. Photoluminescence (PL) measurements show that a blue PL band occurs in the wavelength range of 400–600 nm. This band originates from singly ionized oxygen vacancies (F+ centers) in porous alumina membranes. Tentative reasons for the changes in intensity and peak position of the PL band with increasing heat-treatment temperature are discussed.
Phonon enhanced inverse population in asymmetric double quantum wells
Authors: Stroscio Michael A. , Kisin Mikhail , Belenky Gregory , Luryi Serge
Published: July 2002

Interwell optical-phonon-assisted transitions are studied in an asymmetric double-quantum-well heterostructure comprising one narrow and one wide coupled quantum wells (QWs). We show that the depopulation rate of the lower subband states in the narrow QW can be significantly enhanced thus facilitating the intersubband inverse population, if the depopulated subband is aligned with the second subband of the wider QW, while the energy separation from the first subband is tuned to the highest energy optical-phonon mode.
Electron diffusivity in p-InGaAs determined from the pulse response of InP/InGaAs uni-traveling-carrier photodiodes
Authors: Shimizu Naofumi , Watanabe Noriyuki , Furuta Tomofumi , Ishibashi Tadao
Published: July 2002

The minority electron diffusivity of p-In0.53Ga0.47As measured through the photoresponse of uni-traveling-carrier photodiodes is reported. An electro-optic sampling technique was used to determine the parameter with good accuracy. The obtained diffusivities are 260, 130, and 86 cm2/s at the acceptor doping levels of 2.5×1017, 1.0×1018, and 2.5×1018 cm−3, respectively. These values are contrary to those obtained from the theoretical analysis, which predicts a 50% mobility reduction by coupled polar-phonon plasmon scattering in the doping range below mid-1018 cm−3.
Influence of oxygen on the activation of p-type GaN
Authors: Hull B. A. , Mohney S. E. , Venugopalan H. S. , Ramer J. C.
Published: July 2002

The presence of oxygen in the annealing environment can exhibit a strong influence on the activation of p-GaN, as demonstrated by experiments described in this letter. We activated p-GaN at 600–900 °C in four environments: ultrahigh purity (UHP) N2 gettered to remove residual O2, UHP N2 without gettering, 99.5% UHP N2/0.5% UHP O2, and 90% UHP N2/10% UHP O2. The resistivity of the p-GaN was lowest when O2 was intentionally introduced during activation and was highest when extra care was taken to getter residual O2 from the annealing gas. The experiments also demonstrate that unintentionally incorporated O2 can be at high enough levels to influence the activation process.
One-dimensional three-body problem via symbolic dynamics
Authors: Tanikawa Kiyotaka , Mikkola Seppo
Published: July 2002

Symbolic dynamics is applied to the one-dimensional three-body problem with equal masses. The sequence of binary collisions along an orbit is expressed as a symbol sequence of two symbols. Based on the time reversibility of the problem and numerical data, inadmissible (i.e., unrealizable) sequences of collisions are systematically found. A graph for the transitions among various regions in the Poincaré section is constructed. This graph is used to find an infinite number of periodic sequences, which implies an infinity of periodic orbits other than those accompanying a simple periodic orbit called the Schubart orbit. Finally, under reasonable assumptions on inadmissible sequences, we prove that the set of admissible symbol sequences forms a Cantor set.
Variance reduced Brownian simulation of a bead-spring chain under steady shear flow considering hydrodynamic interaction effects
Authors: Kröger M. , Alba-Pérez A. , Laso M. , Öttinger H. C.
Published: July 2002

To obtain numerical estimates for the properties of a model for polymers in dilute theta solutions in its long-chain limit we follow a stochastic approach to polymer kinetic theory. The model takes into account configuration-dependent hydrodynamic interaction (HI) and simplifies to the Zimm bead-spring chain model in the case of preaveraged HI, for which parameter-free “universal ratios” such as the ratio between radius of gyration and hydrodynamic radius are known. The Chebyshev polynomial method and a variance reduction simulation technique are used to implement an efficient Brownian dynamics simulation. We resolve the full dependence of several characteristic ratios versus both chain length and hydrodynamic interaction parameter, we extrapolate their values to determine universal behaviors, and compare with analytical and experimental results.
Trifilar torsion pendulum for measurement of dissipation caused by an electric field
Authors: Mitrofanov V. P. , Styazhkina N. A.
Published: July 2002

A trifilar all-fused silica pendulum with a Q factor exceeding 107 is described. This pendulum has been used to study the damping caused by an electric field applied between the suspended mass and a nearby electrode. Among the different loss mechanisms the most interesting one is associated with the surfaces of the electrodes. Application of a field of 106 V/m added an excess loss Qe−1 of at least 10−7. These results are important for laser interferometer gravitational wave detectors since electrostatic actuators are supposed to be used in order to precisely align the cavity mirrors.
Theoretical investigation on dynamics of photopolymerization-induced phase separation and morphology development in nematic liquid crystal/polymer mixtures
Authors: Nwabunma Domasius , Chiu Hao-Wen , Kyu Thein
Published: July 2002

A theoretical investigation of the dynamics of photopolymerization-induced phase separation (PIPS) and morphology development in a nematic liquid crystal (LC) polymer network mixture has been undertaken by incorporating photopolymerization kinetics into the coupled time-dependent Ginzburg-Landau (TDGL–Model C) equations. The simulation on the spatio-temporal evolution of the coupled LC concentration and orientation order parameters reveals that both morphological and scattering patterns for the orientation order parameter initially lag behind those of the concentration order parameter. However, the two fields evolve to the same spatial topologies with the progression of time. The PIPS dynamics is characterized only by the late stage of phase separation. We also observed a subtle change in the curvature of the growth curve associated with the onset of nematic ordering. The growth behavior and the simulated morphology consisting of LC droplets dispersed in a matrix of polymer appears the same for all compositions, except that the size gets somewhat larger with increasing LC concentration. Decreasing the rate of reaction increases the size of droplets due to the dominance of structural growth driven by thermal relaxation. Of particular interest is that our simulation captures the observed domain topologies.
Computer monitoring and control of the GEO 600 gravitational wave detector
Authors: Casey M. M. , Ward H. , Robertson D. I.
Published: July 2002

The GEO 600 gravitational wave detector is at an advanced stage of construction at Ruthe, near Hannover in Northern Germany. Successful long-term stable operation of long-baseline interferometers like GEO 600 will critically depend on continuous monitoring and control of the very large number of interdependent feedback systems involved. We present here a description of the control infrastructure developed for this task for GEO 600. Our solution is based on a combination of purpose-built interfacing hardware, standard local area network-connected personal computers, and control software written using LabVIEW. The software techniques developed allow monitoring of all detector systems and also provide the mechanisms for manual and automated control both locally and from remote internet-connected sites.
High-pressure photoacoustic calorimetry
Authors: Daffron John A. , Farrell Gerard J. , Burkey Theodore J.
Published: July 2002

A high-pressure photoacoustic calorimeter has been developed to operate up to 200 MPa. Photoacoustic calorimetry can be used to study the microsecond kinetics and thermodynamics of reactions in solution. Both thermal expansion and volume of reaction contribute to the generation of the photoacoustic signal, and the separation of these contributions has been achieved by the variation of pressure. The construction of a stainless-steel flow cell, bellows, and a high-pressure manifold are described. These components enable rapid sample change and sample flow at high pressure. As a test, the enthalpy and volume of reaction were determined for diphenylcyclopropenone decomposition to diphenylethyne and carbon monoxide.
Nondestructive evaluation of steel structures using a superconducting quantum interference device magnetometer and a neural network system
Authors: Barbosa C. Hall , Vellasco M. , Pacheco M. A. , Bruno A. C. , Camerini C. S.
Published: July 2002

This work combines two state-of-the-art techniques in the area of magnetic nondestructive evaluation: the application of the superconducting quantum interference device (SQUID) as the magnetic field sensor; and the use of artificial neural networks as analysis tools applied to the detected magnetic signals. Pioneering measurements using the SQUID sensor have been made in steel samples containing various types of flaws, and a neural network system, based on the time-delay neural network and radial basis function algorithms, has been implemented to characterize the flaws. The neural network system aims to, based on the measured magnetic field, provide information about defect geometry, thus allowing the assessment of defect severity, as a basis for maintenance and repair procedures.
Sheath modification in the presence of dust particles
Authors: Arnas C. , Mikikian M. , Bachet G. , Doveil F.
Published: July 2002

Negatively charged dust particles are expected to modify the local sheath potential where they are in equilibrium. In the conditions of a hot cathode discharge, sheath profiles are deduced from the measurement of ion drift velocities, with dust particles in suspension and without. In the unperturbed potential profile, the surface potential of an isolated dust particle, its charge, and its potential energy can be estimated as a function of the position in the sheath. In the presence of dust particles, an average increase of the ion drift velocity is measured showing a modification of the local sheath profile. This experimental result suggests that the dust particle charge due to the plasma particle fluxes in the sheath, modifies in turn the local plasma particle distributions.
Asymptotic correction approach to improving approximate exchange–correlation potentials: Time-dependent density-functional theory calculations of molecular excitation spectra
Authors: Casida Mark E. , Salahub Dennis R.
Published: July 2002

The time-dependent density functional theory (TD-DFT) calculation of excitation spectra places certain demands on the DFT exchange–correlation potential, vxc, that are not met by the functionals normally used in molecular calculations. In particular, for high-lying excitations, it is crucial that the asymptotic behavior of vxc be correct. In a previous paper, we introduced a novel asymptotic-correction approach which we used with the local density approximation (LDA) to yield an asymptotically corrected LDA (AC-LDA) potential [Casida, Casida, and Salahub, Int. J. Quantum Chem. 70, 933 (1998)]. The present paper details the theory underlying this asymptotic correction approach, which involves a constant shift to incorporate the effect of the derivative discontinuity (DD) in the bulk region of finite systems, and a spliced asymptotic correction in the large r region. This is done without introducing any adjustable parameters. We emphasize that correcting the asymptotic behavior of vxc is not by itself sufficient to improve the overall form of the potential unless the effect of the derivative discontinuity is taken into account. The approach could be used to correct vxc from any of the commonly used gradient-corrected functionals. It is here applied to the LDA, using the asymptotically correct potential of van Leeuwen and Baerends (LB94) in the large r region. The performance of our AC-LDA vxc is assessed for the calculation of TD-DFT excitation energies for a large number of excitations, including both valence and Rydberg states, for each of four small molecules: N2, CO, CH2O, and C2H4. The results show a significant improvement over those from either the LB94 or the LDA functionals. This confirms that the DD is indeed an important element in the design of functionals. The quality of TDLDA/LB94 and TDLDA/AC-LDA oscillator strengths were also assessed in what we believe to be the first rigorous assessment of TD-DFT molecular oscillator strengths in comparison with high quality experimental and theoretical values. And a comparison has been given of TDLDA/AC-LDA excitation energies with other TD-DFT excitation energies taken from the literature, namely for the PBE0, HCTH(AC), and TDLDA/SAOP functionals. Insight into the working mechanism of TD-DFT excitation energy calculations is obtained by comparison with Hartree–Fock theory, highlighting the importance of orbital energy differences in TD-DFT.
Optimization of the surface morphologies and electron mobilities in GaN grown by plasma-assisted molecular beam epitaxy
Authors: Heying B. , Smorchkova I. , Poblenz C. , Elsass C. , Fini P. , Den Baars S. , Mishra U. , Speck J. S.
Published: July 2002

The morphology and electrical properties of homoepitaxial GaN layers grown by molecular beam epitaxy at 720 °C were investigated as a function of Ga/N ratio. GaN films grown with low Ga/N ratios (N-stable regime) are semi-insulating and have heavily pitted morphologies. GaN films grown with higher Ga/N ratios (intermediate regime) have fewer pits with areas of atomically flat surface. The room-temperature electron mobilities in samples grown in the intermediate regime are greater than 800 cm2/V s and increase with Ga/N ratio. At the highest Ga/N ratios (Ga-droplet regime), Ga droplets formed on the surface during growth. Although the surface morphology is free of pits and atomically flat for films grown within the Ga-droplet regime, the mobility decreases significantly compared to films grown in the intermediate regime. Room-temperature electron mobilities as high as 1191 cm2/V s were measured in a GaN film grown with the highest Ga/N ratio within the intermediate regime.
Vibrational corrections to electric properties of relativistic molecules: The coinage metal hydrides
Authors: Avramopoulos Aggelos , Ingamells Victoria E. , Papadopoulos Manthos G. , Sadlej Andrzej J.
Published: July 2002

The dipole moment, parallel dipole polarizability, and the parallel first hyperpolarizability of the coinage metal hydrides (CuH, AgH, AuH) are studied in the context of their vibrational contributions. The partition of the property values into pure electronic, vibrational averaging, and pure vibrational terms is discussed, and the corresponding contributions are calculated by means of the field-dependent Numerov–Cooley integration technique. The potential energy and property surfaces, required for the evaluation of these contributions, follow from different high-level electronic structure calculations. Parallel to the nonrelativistic studies, a relativistic treatment of the investigated properties has been carried out. It has been found that relativistic effects significantly reduce the magnitude of the vibrational corrections to the studied properties. A particularly large relativistic reduction has been found for the vibrational contribution to the first hyperpolarizability. The relativistic effect is enhanced with increasing nuclear charge of the heavy atom, and can be interpreted in terms of the relativistic effect on the potential energy and electric property curves.
Theoretical consideration of x-ray transmission through cylinder capillaries
Authors: Chen Baozhen
Published: July 2002

A new model for calculating the transmission efficiency of x rays through a cylinder capillary is proposed. Based on some reasonable physical and mathematical assumptions the model can give a simple, straightforward procedure for the calculation of transmission efficiency. The calculation results given by the model can get a better fit to the experimental results than the results given by the previous model with taking some factors into account which affect the transmission of x rays through a capillary.
Ultraviolet-emitting ZnO nanowires synthesized by a physical vapor deposition approach
Authors: Kong Y. C. , Yu D. P. , Zhang B. , Fang W. , Feng S. Q.
Published: July 2002

ZnO nanowires were mass produced using a physical vapor deposition approach. The ZnO nanowire monocrystallites have an average diameter around 60 nm and length up to a few micrometers. The unidirectional growth of the ZnO nanowires was controlled by the conventional vapor-liquid-solid mechanism. Intensive UV light emission peaked around 3.27 eV was observed at room temperature, which was assigned to emission from free exciton under low excitation intensity. The observed room temperature UV emission was ascribed to the decrease in structure defects as compared to bulk ZnO materials, and in particularly to the size effect in the ZnO wires.
Stress effect on the kinetics of silicon thermal oxidation
Authors: Yen Jui-Yuan , Hwu Jenn-Gwo
Published: July 2002

Oxidation of silicon wafers under external mechanical stress was studied in this work. From the oxide thickness profile measured by an automatic-scanning ellipsometer, it was found that the oxidation kinetics of silicon were significantly affected by mechanical stress. There are two distinct features of oxide thickness distribution corresponding to short and long times. By comparing the kinetic constants taken from experiments and the simulated stress distribution on the silicon wafer, we can possibly explain the two features of oxide thickness distribution: the initial rate constant is deformation dependent and the parabolic rate constant is stress dependent. The observed stress-dependent oxidation rates are important in the study of thin gate oxide reliability.
Evolution process of the mode wall-locking and phase-locking in a reversed-field pinch plasma
Authors: Yagi Y. , Koguchi H. , Sakakita H. , Sekine S. , Brunsell P. R. , Malmberg J-A.
Published: July 2002

Wall-locking and phase-locking modes are studied in detail in a reversed-field pinch device, TPE-RX [Y. Yagi et al., Fusion Eng. Design 45, 409 (1999)]. These mode-locking phenomena arise from tearing instabilities. Wall locking means the stopping of mode rotations, and phase locking means the locking of the phases of multiple modes. Phase locking induces a toroidally localized enhanced magnetic amplitude. There are two types of mode-locking states in TPE-RX. One of them exhibits a clear phase-locked structure, while the other exhibits a weak toroidal localization. Both types show finite toroidal rotation during the current-rising phase of the discharge, and are eventually wall locked during the current flat-top phase. However, the rotation speeds are clearly different between the two types. Confinement properties are compared between the two types of mode-locking states. It is shown that the threshold for the mode amplitude necessary to wall lock the toroidal rotation, as well as the bifurcation of phase-locked structures, agrees with theoretical predictions [R. Fitzpatrick et al., Phys. Plasmas 6, 1168, 3878 (1999)].
High-quality aluminum oxide gate dielectrics by ultra-high-vacuum reactive atomic-beam deposition
Authors: Guha S. , Cartier E. , Bojarczuk N. A. , Bruley J. , Gignac L. , Karasinski J.
Published: July 2002

We demonstrate the potential for ultrathin aluminum-oxide films as alternate gate dielectrics for Si complementary metal–oxide–semiconductor technology. Films are deposited in ultrahigh vacuum utilizing atomic beams of aluminum and oxygen on Si(100) surfaces. We show device-quality Si(100)/Al2O3 interfaces with interfacial trap densities in the 1010 cm−2 eV−1 range, and with leakage current densities five orders of magnitude lower than what is observed in SiO2 insulators at the same equivalent electrical thickness. As-grown films possess an amorphous-to-microcrystalline structure, depending upon the deposition temperature, and any interfacial layers between the Si(100) and Al2O3 layer are <∼0.5 nm.
Variational nonequilibrium thermodynamics of reaction-diffusion systems. III. Progress variables and dissipation of energy and information
Authors: Gaveau Bernard , Moreau Michel , Toth Janos
Published: July 2002

We continue, in this article, to develop the formalism of nonequilibrium thermodynamics in variational form. We prove that in the framework of progress variables, the Hamilton–Jacobi equation has always a simple solution, and we prove that this solution becomes a state function if and only if there is a thermodynamic equilibrium for the system. We study an inequality between the dissipation of energy and of information, and we discuss the notion of relative entropy. Finally we also study in detail the case of a system with one chemical species, where all the previous quantities can be calculated explicitly.
Carbon at pressures in the range 0.1–1 TPa (10 Mbar)
Authors: Nellis W. J. , Mitchell A. C. , McMahan A. K.
Published: July 2002

Single-shock (Hugoniot) equation-of-state data of shock-compressed C (graphite) are reported at pressures of 480 and 760 GPa (7.6 Mbar). Graphite is shock-compressed completely into a diamond-like phase at pressures below 80 GPa. At pressures of 80–800 GPa comparison of an ensemble of experimental Hugoniot data for shock-compressed graphite and diamond, and theoretical calculations of the Hugoniots of graphite and diamond, and the 0 K isotherm of diamond suggest diamond melts at ∼300 GPa on the Hugoniot of graphite and that the diamond phase is the ground-state structure of C up to at least 600 GPa.
Weakly two-dimensional dust acoustic waves
Authors: Duan Wen-shan
Published: July 2002

For dust acoustic waves in unmagnetized plasma comprising cold dust grains and both isothermal electrons and ions, small but finite amplitude nonlinear waves are governed by the Korteweg–de Vries equation. The weakly transverse long wavelength perturbations for nonlinear dust acoustic waves are investigated in this paper. It has been shown that the governing equation for this process is the Kadomtsev–Petviashvili equation. It suggests that the transverse perturbations for the nonlinear acoustic solitary waves in a dusty plasma are stable.
A second-order perturbative correction to the coupled-cluster singles and doubles method: CCSD(2)
Authors: Gwaltney Steven R. , Head-Gordon Martin
Published: July 2002

Recently, we introduced a new ansatz for developing perturbative corrections to methods based on coupled-cluster theory. In this article we apply this ansatz to the coupled-cluster singles and doubles (CCSD) method, generating the CCSD(2) method. We use the CCSD(2) method to study the double dissociation of water and to calculate spectroscopic constants of first row diatomic molecules. As long as Hartree–Fock is a reasonable approximation, CCSD(2) works very well.
A high-pressure scanning tunneling microscope
Authors: Laegsgaard E. , Österlund L. , Thostrup P. , Rasmussen P. B. , Stensgaard I. , Besenbacher F.
Published: July 2002

We present the design and performance of a high-pressure scanning tunneling microscope (HP–STM), which allows atom-resolved imaging of metal surfaces at pressures ranging from ultrahigh vacuum (UHV) to atmospheric pressures (1×10−10–1000 mbar) on a routine basis. The HP–STM is integrated in a gold-plated high-pressure cell with a volume of only ∼0.5 l, which is attached directly to an UHV preparation/analysis chamber. The latter facilitates quick sample transfer between the UHV chamber and the high-pressure cell, and allows for in situ chemical and structural analysis by a number of analytical UHV techniques incorporated in the UHV chamber. Reactant gases are admitted to the high-pressure cell via a dedicated gas handling system, which includes several stages of gas purification. The use of ultrapure gasses is essential when working at high pressures in order to achieve well-defined experimental conditions. The latter is demonstrated in the case of H/Cu(110) at atmospheric H2 pressures where impurity-related structures were observed.
Strain effects on exciton resonance energies of ZnO epitaxial layers
Authors: Makino T. , Yasuda T. , Segawa Y. , Ohtomo A. , Tamura K. , Kawasaki M. , Koinuma H.
Published: July 2002

Magnitudes of strain in ZnO epitaxial layers grown on sapphire(0001) substrates under various growth conditions were experimentally determined by x-ray diffraction. We discuss the strain-induced energy shift on the exciton resonances, the results of which were analyzed theoretically using the Hamiltonian for the valence bands under in-plain biaxial strain. Comparative studies with GaN evidenced the advantages of ZnO in terms of sensitivity of the strain-induced energy shift and of piezoelectric effect in heterostructures.
Momentum transfer of a Boltzmann-lattice fluid with boundaries
Authors: Bouzidi M’hamed , Firdaouss Mouaouia , Lallemand Pierre
Published: July 2002

We study the velocity boundary condition for curved boundaries in the lattice Boltzmann equation (LBE). We propose a LBE boundary condition for moving boundaries by combination of the “bounce-back” scheme and spatial interpolations of first or second order. The proposed boundary condition is a simple, robust, efficient, and accurate scheme. Second-order accuracy of the boundary condition is demonstrated for two cases: (1) time-dependent two-dimensional circular Couette flow and (2) two-dimensional steady flow past a periodic array of circular cylinders (flow through the porous media of cylinders). For the former case, the lattice Boltzmann solution is compared with the analytic solution of the Navier–Stokes equation. For the latter case, the lattice Boltzmann solution is compared with a finite-element solution of the Navier–Stokes equation. The lattice Boltzmann solutions for both flows agree very well with the solutions of the Navier–Stokes equations. We also analyze the torque due to the momentum transfer between the fluid and the boundary for two initial conditions: (a) impulsively started cylinder and the fluid at rest, and (b) uniformly rotating fluid and the cylinder at rest.
Barrier recrossing in the vinylidene–acetylene isomerization reaction: A five-dimensional ab initio quantum dynamical investigation
Authors: Schork Rainer , Köppel Horst
Published: July 2002

The spectroscopy and dynamics of the vinylidene–acetylene isomerization reaction are studied theoretically. Based on a new ab initio potential energy surface, the nuclear dynamics is followed by grid methods and wave packet propagation techniques. All five planar degrees of freedom are included in the calculation, for all three different isotopomers. The experimental photoelectron spectra by Lineberger and co-workers are very well reproduced; upon a small adjustment of the calculated anionic equilibrium geometry the agreement becomes excellent. The vinylidene survival probability for broadband photodetachment exhibits three different time regimes, the longest of which points towards an unusual stability of this reactive intermediate. The latter finding is corroborated by the calculated state-specific lifetimes which exceed previous estimates in the literature by ∼3 orders of magnitude. These findings are found to be reconfirmed when taking the discrete level structure of vibrationally highly excited acetylene into account. They amount to heavy barrier recrossing effects in this isomerization reaction and lend strong support to the interpretation of CEI experiments on vinylidene by Levin et al. where this species has been identified ∼3 μs after its formation.
Effect of unequal cylinder spacing on vortex streets behind three side-by-side cylinders
Authors: Zhang H. J. , Zhou Y.
Published: July 2002

The turbulent near-wake of three side-by-side circular cylinders with equal or unequal spacing has been experimentally investigated using various techniques, including the hot wire, laser Doppler anemometer, and flow visualization. The work aims to understand the effect of unequal cylinder spacing on the vortex streets behind the three cylinders. When the cylinder center-to-center spacing is identical, i.e., T1/d=T2/d=1.5, the flow is symmetric about the centerline, with one wide wake behind the central cylinder and one narrow wake on each side of the wide wake. The dominant frequency in the narrow wakes is about 5.4 times that in the wide wake. The observation is consistent with previous reports, thus lending credence to the present measurement. As T2/d is slightly increased to 1.6, a remarkable change occurs in the flow. A comparison is made between the cases of equally and unequally spaced cylinders in terms of the pressure around the cylinders, drag, lift, dominant frequencies, and vortex formation mechanisms. The flow topology (vortex patterns) and downstream evolution are also discussed in detail.
Molecular-dynamics simulation of homogeneous nucleation in the vapor phase
Authors: Toxvaerd S.
Published: July 2002

Ten independent quenches of a gas of 40 000 Lennard-Jones particles are followed until the systems exhibit droplet growth. The cluster distributions and the kinetics are determined for the quenched quasi-equilibrium state, at the onset of nucleation and at droplet growth. All the distributions are isomorphic with the particle distribution in the equilibrium gas state and asymptotically given by simple exponentials. The kinetics show detailed balance of particles and clusters which join and which leave the successful critical nuclei. The systems exhibit chaoticlike behavior with respect to the onset of nucleation, so that only marginal changes in a system will change the onset of nucleation.
Measurement of optical properties of highly doped silicon by terahertz time domain reflection spectroscopy
Authors: Nashima S. , Morikawa O. , Takata K. , Hangyo M.
Published: July 2002

Optical properties of doped silicon wafers have been measured by means of terahertz time domain reflection spectroscopy. A method is proposed to obtain the relative phase by reflection accurately. By using this method, the relative phase is obtained within an error of less than 10 mrad at 1 THz. The experimentally obtained complex conductivity of relatively high-doped silicon (ρ=0.136 Ω cm) in the terahertz region agrees with the simple Drude model.
A vitrifying structure transition in the Dy/Mo(112) adsorption system
Authors: Fedorus A. , Koval V. , Naumovets A. , Pfnür H.
Published: July 2002

Annealing-driven irreversible structural transitions are studied by low-energy electron diffraction in submonolayer Dy films adsorbed on the Mo(112) surface. In a wide coverage range θ>0.07, Dy overlayers deposited at low temperatures (T≈100 K) are ordered and keep their structure upon annealing up to 350–600 K. Near θ=0.68, the overlayers are stable to high-temperature annealing (Ta=1000 K) as well, whereas the denser films are metastable and transform to more stable ordered structures. An unusual annealing effect is found for θ<0.58: the initially ordered metastable phases are replaced by phases having no extended order, which are assumed to be two-dimensional glasses.
Single-mode solid-state single photon source based on isolated quantum dots in pillar microcavities
Authors: Moreau E. , Robert I. , Gérard J. M. , Abram I. , Manin L. , Thierry-Mieg V.
Published: July 2002

We report the fabrication of a single-mode solid-state single photon source, based on an isolated InAs quantum dot (QD) on resonance with the fundamental mode of a pillar microcavity. Photon correlation experiments under pulsed excitation reveal a clear antibunching behavior. We show that a preparation of the single photons in a given quantum state (same spatial mode, same polarization) can be obtained by placing a QD in resonance with the nondegenerate fundamental mode of an elliptical micropillar.
Excellent charge offset stability in a Si-based single-electron tunneling transistor
Authors: Zimmerman Neil M. , Huber William H. , Fujiwara Akira , Takahashi Yasuo
Published: July 2002

We have measured the long-term drift and the short-term 1/f noise in the charge offset Q0(t) in two Si-based single-electron tunneling transistors (SETTs). In contrast to metal-based SETTs, these devices show excellent charge stability, drifting by less than 0.01e over weeks. The short-term 1/f noise magnitude is similar to the metal-based devices, demonstrating that different mechanisms are responsible for the short-term noise versus the long-term drift. Finally, we show that, in addition to the excellent stability over time, it may be possible to make the devices more robust with respect to voltage-induced instability as well.