bluuebunny/crossref_metadata_2025 huggingface dataset:
Resolution and signal-to-background enhancement in gas-phase electron spectroscopy
Authors: Baltzer P. , Karlsson L. , Lundqvist M. , Wannberg B.
Published: July 2002

The most important factors contributing to line broadening and background intensity in electron spectra are identified and new design principles for high quality measurements are presented. It is shown that time-dependent potential gradients in the gas cell are responsible for a large part of the ‘‘normal’’ line broadening, while scattering processes inside the gas cell are responsible for a large part of the background intensity. By designing the experiment according to these results, gas-phase electron spectra can be recorded routinely at an instrument resolution level of better than 5 meV even at comparatively high gas cell pressures necessary to give high intensity for weak lines, and in principle unlimited counting times, at a much improved signal-to-background level. The resulting improvements in the spectral quality are demonstrated by spectra of Ar and HBr.
High-sensitivity computer-controlled infrared polariscope
Authors: Yamada Masayoshi
Published: July 2002

A high-sensitivity computer-controlled infrared polariscope has been developed to measure a small amount of phase retardation, and the principal axes of birefringence induced by residual strains in commercial III-V compound wafers with standard dimensions. In order to check the performance of the polariscope, semi-insulating LEC-grown GaAs (100) wafers, currently used for high-speed integrated circuits, were examined. The residual strain components of ‖Sr−St‖, ‖Syy−Szz‖, and ‖Syz‖ were evaluated from the measured values of the phase retardation and the principal axes of birefringence. It was found that the polariscope developed here was highly sensitive enough to characterize the residual strain components to the order of 10−7 in thin commercial GaAs wafers.
Highly sensitive wideband seismometer using a laser interferometer
Authors: Araya Akito , Kawabe Keita , Sato Toshikazu , Mio Norikatsu , Tsubono Kimio
Published: July 2002

A wideband (dc to 2 kHz) highly sensitive seismometer using a Michelson laser interferometer has been developed. The horizontal acceleration of the ground is sensed by a suspended mirror; its position is then detected as the fringe signal of the light in the interferometer. The minimum measurable displacement with this seismometer is 3×10−11 m/√Hz at 1 Hz (limited by the thermal fluctuation of air and the frequency noise of the laser) and 2×10−15 m/√Hz above 2 kHz (limited by the shot noise of the light). One application of this seismometer is to use it for very sensitive vibration measurements, such as in seismic noise investigations of possible sites for a large-scale laser-interferometric gravitational wave detector.
High-G accelerometer dynamic calibration by a laser differential Doppler technique
Authors: Wang Shengyou , Cao Caizhi , Fei Peiduan , Zhang Aihua
Published: July 2002

An operational system was developed for dynamic and absolute calibration of the high-G accelerometers. A test accelerometer, mounted on the end of an anvil which can be moved along a sliding guideway, experiences a high-G acceleration when a hammer impacts the anvil. Its half-sine acceleration pulse can be obtained by itself. At the same time, using the differential Doppler technique, the velocity change during the period of impacting can be independently measured. Comparing both, the calibrated sensitivity of the test accelerometer can be obtained. The working system and its principle are described. Experimental results are given, which verify the validity of the principle and the method. The maximum acceleration calibrated is now up to 100 000 G, limited by the test accelerometer, with a total uncertainty of ±5%.
Hybrid electron cyclotron emission imaging array system for Texas experimental tokamak upgrade
Authors: Hsia R. P. , Deng B. H. , Geck W. R. , Liang C. , Domier C. W. , Luhmann, Jr. N. C. , Brower D. L. , Cima G.
Published: July 2002

A novel wide bandwidth, low-cost, 20 channel hybrid Schottky diode mixer array-based imaging system has been developed, tested and implemented for electron cyclotron emission diagnostics on the Texas experimental tokamak upgrade (TEXT-U). The array has been successfully utilized to measure 1D and 2D electron temperature profiles, to study sawteeth and magnetohydrodynamic phenomena, and to make measurements of the poloidal/radial correlation lengths and wave-number spectra of electron temperature fluctuations. Fabrication and laboratory characterization results are presented, together with details and test results from the actual implementation on TEXT-U.
MDSplus data acquisition system
Authors: Stillerman J. A. , Fredian T. W. , Klare K.A. , Manduchi G.
Published: July 2002

MDSplus, a tree based, distributed data acquisition system, was developed in collaboration with the ZTH Group at Los Alamos National Lab and the RFX Group at CNR in Padua, Italy. It is currently in use at MIT, RFX in Padua, TCV at EPFL in Lausanne, and KBSI in South Korea. MDSplus is made up of a set of X/motif based tools for data acquisition and display, as well as diagnostic configuration and management. It is based on a hierarchical experiment description which completely describes the data acquisition and analysis tasks and contains the results from these operations. These tools were designed to operate in a distributed, client/server environment with multiple concurrent readers and writers to the data store. While usually used over a Local Area Network, these tools can be used over the Internet to provide access for remote diagnosticians and even machine operators. An interface to a relational database is provided for storage and management of processed data. IDL is used as the primary data analysis and visualization tool. IDL is a registered trademark of Research Systems Inc.
A high spatial resolution distributed optical fiber sensor for high-temperature measurements
Authors: Feced Ricardo , Farhadiroushan Mahmoud , Handerek Vincent A. , Rogers Alan J.
Published: July 2002

One of the advantages of the use of optical fibers for sensing is their ability to perform distributed measurements. Several distributed sensors that measure temperature along optical fibers are already commercially available. All of them are based on Raman thermometry. However, the spatial resolution that they achieve is limited to 1 m, making them unsuitable for applications where higher spatial resolution is needed. Also, their temperature range is very dependent on the behavior of fibers at high temperature, a subject which still needs investigation. In this article, we present a distributed optical fiber sensor that addresses these two issues. On one hand, the combination of Raman thermometry with the time-correlated single-photon counting technique permits the achievement of high spatial resolutions (0.1 m). On the other hand, the use of specially coated fibers allows measurement of high temperatures. We have investigated the system temperature sensitivity and have evaluated the measurement errors introduced by the high-temperature behavior of carbon- and gold-coated fibers. We demonstrate that the system is very appropriate for high-temperature distributed measurements with high spatial resolution.
Hydrogen negative ion production in an electron cyclotron resonance driven plasma
Authors: Dougar Jabon V. D. , Chacon Velasco A. J. , Vivas F. A.
Published: July 2002

A 2.45 GHz microwave hydrogen discharge in a magnetic mirror trap created by disk poles is studied with respect to negative ion production. The distance between the permanent magnets was set so that the magnetic equipotentials corresponding to electron cyclotron resonance condition and the first three harmonics form concentric rings at the middle plain of the trap. The radii of azimuthally rotating rings could be varied by changing the distance between poles. It is found that the negative ion generation efficiency depends sharply on microwave power, gas pressure, and the distance between the plasma electrode of the extraction system and the nearest ring. The maximum current density of 67 mA/cm2 is observed at microwave power of 200 W and pressure of 0.2 mTorr when a ring corresponding to the second harmonic is in contact with the surface of the plasma extraction electrode. The enhancement of the negative hydrogen ion production when the plasma electrode touches the resonance heating rings is explained by a two step process: vibrational excitation of molecules in the discharge volume and the dissociation of these molecules in a layer of low energy electrons. Vibrational excited hydrogen molecules are formed in the discharge volume by plasma electrons. This process is followed by a dissociative attachment of low energetic, secondary electrons from the plasma electrode, which are created by the ring ion and electron bombardment of the surface.
Use of scattered radiation for absolute x-ray energy calibration
Authors: Cross Julie O. , Frenkel Anatoly I.
Published: July 2002

A small, simple, and effective x-ray energy monitor has been built for use in energy-sensitive experiments where normal transmission monitoring is not an option. The instrument uses x rays scattered elastically from air, thin polymer films, or other featureless scatterers as a secondary source for measuring the transmission versus energy through a calibrated x-ray spectroscopy standard. In general, this type of energy monitor can be made from many combinations of detectors and scatterers, including ultrahigh vacuum compatible components. We show experimental results from a miniature, p-i-n diode-based monitor, which fits unobtrusively anywhere along the optical path of the experiment, and can be made easily from inexpensive and readily available parts.
Constant current: A method for obtaining hysteresis loops in ferroelectric materials
Authors: Giacometti José A. , Wisniewski Célio , Moura Walterley A. , Ribeiro Paulo António
Published: July 2002

A new method is proposed to obtain hysteresis loops in ferroelectric materials in which the sample is submitted to a constant electric current. This technique was applied on poly(vinylidene fluoride), PVDF, biaxially stretched samples and the hysteresis loops of the electric displacement and electric polarization versus the electric field compared well with those obtained with the Sawyer–Tower method. The constant current method is advantageous in that no measuring amplifiers are needed and it is insensitive to prebreakdown noise.
Core correlation reflectometer at the JT-60U tokamak
Authors: Shinohara K. , Nazikian R. , Fujita T. , Yoshino R.
Published: July 2002

Core correlation reflectometer has been newly installed on the JT-60U tokamak. The target of this reflectometer is to measure the fluctuation of the electron density and its correlation in the core region, especially in the internal transport barrier, region of high-performance-reversed shear plasma. The reflectometer consists of four channels, two of which operate in fixed frequency and the other two channels are tunable. The tunable channels can step through five frequencies in each 60 ms. The fluctuation correlation is determined from the correlation between fixed and variable frequency channels and the radial profile of the correlations can be measured every 60 ms in a discharge. The polarization of the launched wave is the extraordinary mode, X mode, in order to obtain a high space resolution and to access the high-field side of the magnetic axis.
Light output measurements of the organic light-emitting devices
Authors: He Yi , Hattori Reiji , Kanicki Jerzy
Published: July 2002

In this article we describe a novel light output measurements method of the organic light-emitting devices (OLEDs). This method not only provides the electroluminescence (EL) spectral response of the OLEDs, but also gives the spectral distribution of the radiant power (W), luminance (cd/m2), luminous flux (lm), and photon emission of the measured light-emitting object. Also an accurate calculation method of the OLED EL external quantum efficiency is proposed.
Scanning nanoscale multiprobes for conductivity measurements
Authors: Bo/ggild P. , Hansen T. M. , Kuhn O. , Grey F. , Junno T. , Montelius L.
Published: July 2002

We report fabrication and measurements with two- and four-point probes with nanoscale dimensions, for high spatial resolution conductivity measurements on surfaces and thin films. By combination of conventional microfabrication and additive three-dimensional nanolithography, we have obtained electrode spacings down to 200 nm. At the tips of four silicon oxide microcantilevers, narrow carbon tips are grown in converging directions and subsequently coated with a conducting layer. The probe is placed in contact with a conducting surface, whereby the electrode resistance can be determined. The nanoelectrodes withstand considerable contact force before breaking. The probe offers a unique possibility to position the voltage sensors, as well as the source and drain electrodes in areas of nanoscale dimensions.
Sub-10 nm lithography with self-assembled monolayers
Authors: Lercel M. J. , Craighead H. G. , Parikh A. N. , Seshadri K. , Allara D. L.
Published: July 2002

Dots demonstrating critical resist dimensions of approximately 5 to 6 nm were formed in an octadecylsiloxane monolayer on silicon by electron beam exposure using a digital scanning electron microscope at 20 keV beam energy. The patterned dots were observed by imaging with an atomic force microscope (AFM). The electron beam size was measured to confirm that it is not the limiting factor in the exposure resolution. The limit that prevents the observation of smaller structures is either the small contrast in the AFM imaging for smaller dots or an intrinsic material limit caused by the secondary electron range.
Photorefractive hologram fixing by a 4 K cooldown to the phase transition in K1−xLixTa1−yNbyO3
Authors: Agranat Aharon J. , Razvag Meir , Balberg Michal
Published: July 2002

A fixing process of photorefractive holograms is reported. The process is based on the formation of an ionic space charge which becomes mobile at the phase transition temperature. A fixed grating was formed in paraelectric potassium lithium niobate tantalate by a 4 K cooldown to Tc, and a warmup back to the paraelectric phase. The diffraction efficiency of the fixed grating was 66% of the original photorefractive space charge grating.
Fatigue of piezoelectric properties in Pb(Zr,Ti)O3 films
Authors: Kholkin A. L. , Colla E. L. , Tagantsev A. K. , Taylor D. V. , Setter N.
Published: July 2002

Piezoelectric properties of Pb(Zr,Ti)O3 thin films are investigated as a function of the number of bipolar (switching) and unipolar (nonswitching) voltage pulses. The longitudinal piezoelectric coefficient d33 decreases with bipolar fatigue reflecting the decrease of switchable polarization. Simultaneously, a strong vertical shift of piezoelectric hysteresis loops is observed, which is considered as the buildup of fixed internal polarization due to the pinning of ferroelectric domains in a preferred orientation. Piezoelectric fatigue induced by unipolar (nonswitching) pulses is considerably smaller than the fatigue under bipolar conditions and can be described by the internal bias field which shifts piezoelectric hysteresis loops along the field axis.
An all-chromium single electron transistor: A possible new element of single electronics
Authors: Kuzmin L. S. , Pashkin Yu. A. , Tavkhelidze A. N. , Ahlers F.-J. , Weimann T. , Quenter D. , Niemeyer J.
Published: July 2002

The realization of an all-chromium single electron tunneling (SET) transistor is reported. Chromium was chosen as a normal metal with small grain structure forming the oxide layer with a low potential barrier and great chemical and thermal stability. The transistor showed classical I-V curves with an offset voltage of 450 μV and an amplitude of gate modulation of 160 μV. Fitting a tunnel current expression in the experimental I-V curve gave a height of the potential barrier φ=170 meV and a width of the barrier d=16 Å. The SET transistor showed a charge sensitivity of 7×10−4 e/Hz1/2 at 10 Hz.
Extended x-ray absorption fine structure of metastable bcc and fcc phases in mechanically alloyed Fe–Cu
Authors: Schilling P. J. , He J.-H. , Cheng J. , Ma E.
Published: July 2002

The extended x-ray absorption fine structure (EXAFS) technique has been employed to monitor the formation of thermodynamically unstable solid solutions induced by mechanical alloying in the positive heat of mixing Fe–Cu system. Local structural changes are clearly observed, with Cu taking on bcc coordination in the bcc (Fe80Cu20) solid solution, and Fe taking on fcc coordination in the fcc (Fe50Cu50) solid solution. Atomic level alloying is demonstrated through curve fitting to ab initio simulations. A difference is observed in the behavior of bcc and fcc ball milled samples, with a large reduction in first shell coordination number observed in bcc nanocrystalline structures (pure Fe and Fe80Cu20), but not in the fcc nanocrystalline structures (Fe50Cu50). This is believed to be due to the inability of the fcc structure to retain a high defect density.
Oriented growth of SrBi2Ta2O9 ferroelectric thin films
Authors: Desu Seshu B. , Vijay Dilip P. , Zhang X. , He BaoPing
Published: July 2002

We report on the ferroelectric properties of c-axis oriented ferroelectric SrBi2Ta2O9 thin films. Pt/SrBi2Ta2O 9/Pt capacitors were grown on single crystal MgO (and/or SrTiO3) substrates using pulsed laser ablation. These substrates provide the necessary template for (100) texture in platinum due to their close lattice matching. This in turn facilitates the c-axis orientation in the ferroelectric films. The degree of orientation in the layered structure ferroelectric film was systematically varied from highly c-axis oriented to random polycrystalline by varying the growth conditions of the bottom metal electrode. The polarization and coercive field values were found to decrease with an increasing degree of c-axis orientation; while the randomly oriented films exhibited a remnant polarization of 5 μC/cm 2, a coercive field of 70 kV/cm, and a dielectric constant of 320, the c-axis oriented films exhibited very low polarization (∼1 μC/cm2), coercivity (22 kV/cm), and dielectric constant (∼200) values.
Octahedral void defects observed in the bulk of Czochralski silicon
Authors: Ueki Takemi , Itsumi Manabu , Takeda Tadao
Published: July 2002

We found octahedral void defects in the bulk of silicon wafers by using infrared tomography. These voids are often twin type and their sizes are about 100 nm. A 2-nm-thick layer exists on the side walls of the void defects. Our analysis suggests that the 2-nm-thick layer is SiOx. It is believed that the void structure is formed as a result of agglomeration of vacancies during Si–ingot growth.
Silicon quantum well as passivation barrier on GaAs surface
Authors: Lu Z. H. , Baribeau J.-M.
Published: July 2002

Synchrotron radiation Si L-edge absorption spectroscopy is used to study the band structure of thin silicon passivation films on GaAs(100) surface. The measurements show that the Si L-edge, which measures the conduction band minimum (CBM), is shifted drastically to a higher energy as the Si film thickness is reduced to below 2.5 nm. This shift in the CBM is found to scale with the Si layer thickness L as 1/L1.7, indicating that the electronic states within the Si layer are quantum confined. Within a confined quantum well, the energy levels of all states including localized gap states will be shifted to higher energy. Quantum confinement effects in the thin Si layer is found responsible for its successful use for passivation of GaAs surfaces.
Optical switches based on magnesium lanthanide alloy hydrides
Authors: van der Sluis P. , Ouwerkerk M. , Duine P. A.
Published: July 2002

It is shown that thin layers of palladium coated magnesium lanthanide alloys reversibly go through an optical transition by variation of the hydrogen concentration, just as has recently been shown for pure lanthanides. With these layers optical switches can be constructed that exhibit three different optical states: a color-neutral transparent state at high hydrogen concentration, a nontransparent dark absorbing state at intermediate hydrogen pressures, and a highly reflective metallic state at low hydrogen pressures. The ratio in transmission between the transparent state and the reflecting state is more than 1000.
Influence of roughness and interdiffusion in zone plate structures on the diffraction efficiency described by coupled wave theory
Authors: Schneider G.
Published: July 2002

Diffractive transmission x-ray optics, e.g., zone plates and gratings, are well suited for x-ray microscopy and spectroscopy. X-ray optics for use at short wavelengths require high zone aspect ratios which can be obtained by alternately depositing two materials with different x-ray scattering properties on a microwire. The roughness and the diffusion inherent to this process influence the diffraction efficiency of these optics. Using coupled wave theory, it is shown that roughness and interdiffusion increase the structure height required for optimal diffraction efficiency compared to ideally smooth structure profiles. In addition, the maximum obtainable diffraction efficiency decreases with increasing root-mean-square roughness and interdiffusion.
Breakdown of equilibrium approximation for nanosecond laser-induced electron emission from silicon
Authors: Mao Samual S. , Mao Xianglei , Greif Ralph , Russo Richard E.
Published: July 2002

We demonstrate that nonequilibrium carrier dynamics play a significant role in nanosecond laser-induced electron emission from semiconductor surfaces. Surface emission current and electron yields due to thermionic and photoelectric effects are calculated for a 2 ns laser pulse irradiation, with fluences below the threshold for melting. The photoelectric effect is found to dominate electron emission only at low fluences, whereas thermionic emission from interband absorption is responsible for electron emission at high incident fluences. The results present a satisfactory interpretation of experimental observations for nanosecond laser-induced electron emission from silicon.
Magnetoresistance and planar Hall effects in submicron exchange-coupled NiO/Fe19Ni81 wires
Authors: Nemoto A. , Otani Y. , Kim S. G. , Fukamichi K. , Kitakami O. , Shimada Y.
Published: July 2002

Magnetization reversal processes of submicron NiO/Fe19Ni81 exchange-coupled Hall cross with a variable width in the range from 0.2 to 0.4 μm were studied by the magnetoresistivity and the planar Hall effect measurements. The magnetization reversal was found to take place via a coherent rotation in the Hall cross, suggesting that the size of the antiferromagnetic domain is regulated by the wire width. The magnitude of the exchange coupling field Hex varied in proportion to the inverse wire width.
Ferroelectric fatigue of Pb(Zr,Ti)O3 thin films measured in atmospheres of varying oxygen concentration
Authors: Brazier M. , Mansour S. , McElfresh M.
Published: July 2002

Ferroelectric fatigue in Pb(Zr,Ti)O3 films was found to be remarkably sensitive to the oxygen partial pressure of the atmosphere above the film during measurement. Films with Zr/Ti ratios of 55/45 and 75/25 were subjected to ferroelectric fatigue testing at room temperature in atmospheres of various oxygen partial pressure, or pO2. The number of switching cycles at which the onset of fatigue occurred was found to be sharply peaked with respect to the oxygen partial pressure surrounding the sample. Such a striking dependence on the oxygen concentration strongly supports current theories for fatigue that involve oxygen vacancy migration.
Germanium dioxide whiskers synthesized by laser ablation
Authors: Tang Y. H. , Zhang Y. F. , Wang N. , Bello I. , Lee C. S. , Lee S. T.
Published: July 2002

We obtained germanium dioxide (GeO2) whiskers in bulk quantity by ablating a germanium target at 820 °C with a pulsed KrF excimer laser in an argon atmosphere. Most of the GeO2 whiskers were smooth and straight with hexagonal or triangular, or quadrilateral cross sections while some of them had a bamboo-shoot-shaped form. Results of scanning electron microscopy, transmission electron microscopy, and x-ray diffraction showed that the whiskers are hexagonal crystalline GeO2.
Synchrotron x-ray microdiffraction diagnostics of multilayer optoelectronic devices
Authors: Cai Z.-H. , Rodrigues W. , Ilinski P. , Legnini D. , Lai B. , Yun W. , Isaacs E. D. , Lutterodt K. E. , Grenko J. , Glew R. , Sputz S. , Vandenberg J. , People R. , Alam M. A. , Hybertsen M. , Ketelsen L. J. P.
Published: July 2002

Synchrotron-based x-ray microbeam techniques have been used to map crystallographic strain and multilayer thickness in micro-optoelectronic devices produced with the selective area growth technique. Our main results show that growth enhancements in InGaAsP multilayer device material are different for well and barrier material. Comparison with a vapor-phase model for selective area growth suggests that this difference is due to different vapor-phase incorporation rates for the group III metals.
Radiation damage to fullerite (C60) in the transmission electron microscope
Authors: Egerton R. F. , Takeuchi Manabu
Published: July 2002

Electron energy-loss spectroscopy was used to monitor structural damage to solid C60 as a function of electron exposure. The characteristic dose was found to be in the range 300–700 C/cm2 for incident energies in the range 100–200 keV and specimen temperatures between 100 and 300 K. The absolute value of this dose, and its energy and temperature dependence, suggest that the damage mechanism is predominantly electronic rather than knock-on displacement.
The irreversibility behavior of NdBaCuO fabricated by top-seeded melt processing
Authors: Hari Babu N. , Lo W. , Cardwell D. A. , Campbell A. M.
Published: July 2002

Large, single grain NdBa2Cu3O7−δ (Nd123)–Nd4Ba2Cu2O10 (Nd422) composites have been fabricated up to 2 cm in diameter in a controlled 1% O2 in N2 atmosphere using a top-seeded melt growth technique. The irreversibility field as a function of temperature has been obtained from magnetic hysteresis loops for magnetic fields applied parallel and perpendicular to the c axis of small specimens (≈2×2×3 mm3) cut from individual grains. These specimens are observed to exhibit a pronounced peak effect for current flowing in the a–b plane and a five-fold anisotropy in the irreversibility line over a wide temperature range for fields applied along the major crystallographic axes. An exceptionally high irreversibility field (>12 T at 89 K) is observed in this material for field applied perpendicular to the c axis, which extrapolates to well over 40 T at 77 K.
Effects of nitrogen on the band structure of GaNxP1−x alloys
Authors: Xin H. P. , Tu C. W. , Zhang Yong , Mascarenhas A.
Published: July 2002

We report that the incorporation of N in GaNxP1−x alloys (x⩾0.43%) leads to a direct band-gap behavior of GaNP. For N concentration lower than 0.43%, a series of sharp emission lines from the various N pair centers are observed for GaNP bulk layers. With increasing N concentration higher than 0.43%, a strong photoluminescence (PL) emission from GaNP bulk layers is observed at room temperature. While the PL peak redshifts with increasing N concentration to 3.1%, the PL intensity remains as intense. Absorption measurements show a direct band-gap behavior of GaNP alloys.
Paths, Demazure crystals, and symmetric functions
Authors: Kuniba Atsuo , Misra Kailash C. , Okado Masato , Takagi Taichiro , Uchiyama Jun
Published: July 2002

The path realization of Demazure crystals is reviewed and Demazure characters in the light of symmetric functions are discussed.
Holographic gratings in the optically isotropic state of polymer azobenzene liquid-crystal films
Authors: Yamamoto Takahiro , Yoneyama Satoshi , Tsutsumi Osamu , Kanazawa Akihiko , Shiono Takeshi , Ikeda Tomiki
Published: July 2002

We attempted formation of holographic gratings in an optically isotropic state of polymer azobenzene liquid-crystal films. On irradiation of writing beams at 488 nm, first-order diffraction beams of a probe beam at 633 nm were immediately observed even though the films exhibited the optically isotropic state. The diffraction beams quickly disappeared by turning off the writing beams. The first-order diffraction efficiency was affected by polarization of the writing beams. When unpolarized light was employed as the writing beams, diffraction beams were hardly observed. On the other hand, the highest diffraction efficiency was obtained with the linearly polarized writing beams. It is speculated that photoinduced alignment of azobenzene moieties in bright fringes of the interference pattern would be responsible for the grating formation. Furthermore, it was found that rubbing treatment slightly enhanced the diffraction efficiency. By turning on and off the writing beams, dynamic control of the diffraction beams could be achieved.
Modifications to the edge current profile with auxiliary edge current drive and improved confinement in a reversed-field pinch
Authors: Chapman B. E. , Biewer T. M. , Chattopadhyay P. K. , Chiang C.-S. , Craig D. J. , Crocker N. A. , Den Hartog D. J. , Fiksel G. , Fontana P. W. , Prager S. C. , Sarff J. S.
Published: July 2002

Auxiliary edge current drive is routinely applied in the Madison Symmetric Torus [R. N. Dexter, D. W. Kerst, T. W. Lovell et al., Fusion Technol. 19, 131 (1991)] with the goal of modifying the parallel current profile to reduce current-driven magnetic fluctuations and the associated particle and energy transport. Provided by an inductive electric field, the current drive successfully reduces fluctuations and transport. First-time measurements of the modified edge current profile reveal that, relative to discharges without auxiliary current drive, the edge current density decreases. This decrease is explicable in terms of newly measured reductions in the dynamo (fluctuation-based) electric field and the electrical conductivity. Induced by the current drive, these two changes to the edge plasma play as much of a role in determining the resultant edge current profile as does the current drive itself.
Determination of thickness and optical constants of amorphous silicon films from transmittance data
Authors: Mulato M. , Chambouleyron I. , Birgin E. G. , Martı́nez J. M.
Published: July 2002

This work presents the application of a recently developed numerical method to determine the thickness and the optical constants of thin films using experimental transmittance data only. This method may be applied to films not displaying a fringe pattern and is shown to work for a-Si:H (hydrogenated amorphous silicon) layers as thin as 100 nm. The performance and limitations of the method are discussed on the basis of experiments performed on a series of six a-Si:H samples grown under identical conditions, but with thickness varying from 98 nm to 1.2 μm.
High order finite difference algorithms for solving the Schrödinger equation in molecular dynamics. II. Periodic variables
Authors: Guantes Raul , Farantos Stavros C.
Published: July 2002

Variable high order finite difference methods are applied to calculate the action of molecular Hamiltonians on the wave function using centered equi-spaced stencils, mixed centered and one-sided stencils, and periodic Chebyshev and Legendre grids for the angular variables. Results from one-dimensional model Hamiltonians and the three-dimensional spectroscopic potential of SO2 demonstrate that as the order of finite difference approximations of the derivatives increases the accuracy of pseudospectral methods is approached in a regular manner. The high order limit of finite differences to Fourier and general orthogonal polynomial discrete variable representation methods is analytically and numerically investigated.
Gyrokinetic study of ion temperature gradient instability in vicinity of flux surfaces with reversed magnetic shear
Authors: Dong J. Q. , Horton W. , Kishimoto Y.
Published: July 2002

Ion temperature gradient (ITG) driven instability is investigated in the vicinity of a flux surface where the magnetic shear reverses. The generic properties of the profile of the magnetic shear are taken into account with gyrokinetic stability theory in the local sheared slab geometry integral equation. The stability analysis shows that there are four distinct unstable ITG branches with significantly different eigenvalues and mode structures existing simultaneously in the vicinity of the minimum q layer. The variation of eigenmode structures with magnetic shear is investigated in detail. The mixing length estimation of the induced plasma transport is performed. Detailed numerical results are presented and general correlations with simulations and experiments are noted.
Chemical ordering in AlGaN alloys grown by molecular beam epitaxy
Authors: Iliopoulos E. , Ludwig K. F. , Moustakas T. D. , Chu S. N. G.
Published: July 2002

Aluminum gallium nitride alloys were grown by molecular beam epitaxy and their film composition, structure, and microstructure were investigated by Rutherford backscattering spectroscopy, atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It was found that the ratio of group-III to group-V fluxes influences the relative incorporation of gallium and aluminum in the films. The transmission electron microscopy and x-ray diffraction studies revealed the existence of three types of spontaneously formed superlattice structures with periodicities of 2, 7, and 12 ML. While the 2 ML ordering is preferred under group-V rich conditions of growth, the 7 and 12 ML orderings were observed under group-III rich conditions of growth.
Ferroelectric and microstructural characteristics of SrBi2Ta2O9 thin films crystallized by the rapid thermal annealing process
Authors: Zanetti S. M. , Bueno P. R. , Leite E. , Longo E. , Varela J. A.
Published: July 2002

Ferroelectric SrBi2Ta2O9 thin films on Pt/Ti/SiO2/Si were successfully synthesized by the modified polymeric precursor method. The films were deposited by spin coating and crystallized by rapid thermal annealing in a halogen lamp furnace, followed by postannealing at temperatures ranging from 700 °C to 800 °C in an oxygen atmosphere. Microstructural and phase evaluations were followed by x-ray diffraction and atomic force microscopy. The films displayed spherical grain structures with a superficial roughness of approximately 3–6 nm. The dielectric constant values were 121 and 248 for films treated at 700 °C and 800 °C, respectively. The P–E curve showed a voltage shift toward the positive side, which was attributed to crystallization under the halogen illumination. The remanent polarization (2Pr) and coercive field (Ec) were 7.1 μC/cm2 and 113 kV/cm, and 18.8 μC/cm2 and 93 kV/cm for the films treated at 700 °C and 800 °C, respectively.
Thickness dependence of ferroelectric polarization switching in poly(vinylidene fluoride–trifluoroethylene) spin cast films
Authors: Xia Feng , Xu Haisheng , Fang Fei , Razavi B. , Cheng Z.-Y. , Lu Yu , Xu Baomin , Zhang Q. M.
Published: July 2002

In poly(vinylidene fluoride–trifluoroethylene) copolymer spin cast films, it has been observed that the polarization switching time increases as the film thickness is reduced to below 1 μm. We will show that this change with film thickness can be divided into two thickness regimes, i.e., those above 120–150 nm and those below that thickness. For films thicker than 120–150 nm, the change in the switching behavior is due to interface effects that can be modeled by an effective interface layer with lower dielectric constant that is in series with the film. For films below 120–150 nm thickness, there is an additional and very large increase of the switching time with reduced film thickness. This additional effect is caused by the precipitous drop of the crystallinity in films at this thickness range.
Thickness-dependent dynamic hysteresis scaling behavior in epitaxial Fe/GaAs(001) and Fe/InAs(001) ultrathin films
Authors: Moore T. A. , Rothman J. , Xu Y. B. , Bland J. A. C.
Published: July 2002

The dynamic hysteresis scaling behavior in epitaxial Fe/GaAs(001) and Fe/InAs(001) thin films (thickness range 7.3–150 Å) has been investigated as a function of Fe film thickness in the field sweep rate range 0.005–1000 kOe/s using the magneto-optic Kerr effect. The hysteresis loop area A follows the scaling relation A∝(dH/dt)α. We find two distinct dynamic regimes: the low dynamic regime in the sweep rate range 0.005–250 kOe/s, and the high dynamic regime beyond 250 kOe/s. There is a marked increase in α between the low and high dynamic regimes which we attribute to the dominant reversal mechanism changing from domain wall motion to nucleation. In the low dynamic regime α is a decreasing function of Fe film thickness, and this behavior is attributed to the effect of interface-induced pinning.
Static electrical response properties of F−, Ne, and HF using explicitly correlated R12 coupled cluster approach
Authors: Franke Robert , Müller Hendrik , Noga Jozef
Published: July 2002

The usefulness of the application of explicitly correlated coupled cluster R12 approach (CC-R12), to calculate electrical properties of many electron systems is examined. Results close to the basis set limit are presented for the dipole moment of HF, static dipole polarizability, and higher dipole hyperpolarizabilities of F−, Ne, and HF. It is shown that one can substantially decrease the uncertainty for the prediction in the given property, at least as far as the one-electron basis saturation is concerned. Relativistic effects were not negligible in the case of neon. Recommended values with error bars derived from differences between high precision CC approaches are given.
Becchi–Rouet–Stora–Tyutin formalism and zero locus reduction
Authors: Grigoriev M. A. , Semikhatov A. M. , Tipunin I. Yu.
Published: July 2002

In the Becchi–Rouet–Stora–Tyutin (BRST) quantization of gauge theories, the zero locus ZQ of the BRST differential Q carries an (anti)bracket whose parity is opposite to that of the fundamental bracket. Observables of the BRST theory are in a 1:1 correspondence with Casimir functions of the bracket on ZQ. For any constrained dynamical system with the phase space N0 and the constraint surface Σ, we prove its equivalence to the constrained system on the BFV-extended phase space with the constraint surface given by ZQ. Reduction to the zero locus of the differential gives rise to relations between bracket operations and differentials arising in different complexes (the Gerstenhaber, Schouten, Berezin–Kirillov, and Sklyanin brackets); the equation ensuring the existence of a nilpotent vector field on the reduced manifold can be the classical Yang–Baxter equation. We also generalize our constructions to the bi-QP manifolds which from the BRST theory viewpoint correspond to the BRST–anti-BRST-symmetric quantization.
Effect of a microstructure on the formation of self-assembled laser cavities in polycrystalline ZnO
Authors: Ong H. C. , Dai J. Y. , Li A. S. K. , Du G. T. , Chang R. P. H. , Ho S. T.
Published: July 2002

The optical properties of polycrystalline ZnO have been studied to elucidate the occurrence of random laser action. The spatially-resolved refractive index has been mapped out by using the scanning electron energy loss spectroscopy across the grain boundary. It is observed that the refractive index decreases gradually when the probe beam is approaching to the grain boundary. A thin reflective layer of ∼10 nm is found to form in the vicinity of the grain boundary, which assists the optical scattering. The photon scattering factor of the reflective layer has been determined and is shown to correlate well with the results of the coherent backscattering method. Together with the cathodoluminescence studies, it is suggested that the overall structure, which includes the grain and grain boundary, determines the laser action in ZnO.
Determination of structural parameters characterizing thin films by optical methods: A comparison between scanning angle reflectometry and optical waveguide lightmode spectroscopy
Authors: Picart C. , Ladam G. , Senger B. , Voegel J.-C. , Schaaf P. , Cuisinier F. J. G. , Gergely C.
Published: July 2002

We present a comparative study of the structural parameters characterizing thin macromolecular adsorbed films that are obtained from two optical techniques: optical waveguide lightmode spectroscopy (OWLS) and scanning angle reflectometry (SAR). We use polyelectrolyte multilayers and polyelectrolyte multilayers/protein films to perform this study. The comparison between the information obtained with the two methods is possible because the buildup of the polyelectrolyte multilayers is known to become substrate independent after the deposition of the first few polyelectrolyte layers. The analysis of the optical data requires usually to postulate a refractive index profile for the interface. Two profiles have been used: the homogeneous and isotropic monolayer and the bilayer profiles. When the refractive index profile of an adsorbed film is well approximated by a homogeneous and isotropic monolayer, as shown by using an analysis of the deposited films in terms of optical invariants, the two optical techniques lead to similar values for the film thickness and the optical mass. The situation is more complex in the case of the multilayers/protein films for which the calculated parameters can strongly depend upon the refractive index profile that is postulated to analyze the optical data. Whereas the optical mass and, to a lesser extent, the thickness seem fairly model independent for OWLS, they appear to be extremely sensitive to the model for SAR. For proteins deposited on top of the polyelectrolyte film, optical mass and protein thickness were found to be comparable when determined by OWLS and by SAR using the bilayer model. The data analysis of the SAR curves with the monolayer model leads to much larger and even physically unreasonable film thicknesses and optical masses. This was particularly noticeable for proteins having a large size (human serum albumin and fibrinogen), whereas both models lead to similar results for small sized proteins. By means of the different refractive index profiles, we show that great care must be taken in the physicochemical interpretation of the structural parameters determined by these optical techniques.
Preparation and monitoring of high-ground-state vibrational wavepackets by femtosecond coherent anti-Stokes Raman scattering
Authors: Pinkas Iddo , Knopp G. , Prior Yehiam
Published: July 2002

Femtosecond time-delayed coherent anti-Stokes Raman scattering is presented not only as a tool for monitoring but also as a viable method for the preparation of vibrational wavepackets with very high quantum numbers in the ground electronic state of molecules. We experimentally demonstrate a particularly useful approach of using two separate time delays between the pulses for preparing vibrational wavepackets as high as v″=38 [ΔEv=7000 cm−1] in bulk gas-phase molecular iodine. By means of an ultrashort laser pulse, we prepare a wavepacket in an electronic excited state, optimize the frequency and timing of a second pulse to efficiently generate the targeted ground-state vibrational wavepacket, and monitor the wavepacket by coherent scattering from a third pulse. The method is further used to probe interference effects in femtosecond four-wave-mixing signals generated by molecular wavepackets.
Nonlinear three-dimensional self-consistent simulations of negative central shear discharges in the DIII-D tokamak
Authors: Popov A. M. , Chan V. S. , Chu M. S. , Liu Y. Q. , Rice B. W. , Turnbull A. D.
Published: July 2002

Nonlinear simulations of experimentally observed magnetohydrodynamic (MHD) bursts in DIII-D [J. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] L-mode negative central magnetic shear (NCS) discharges were performed with a full three-dimensional nonlinear MHD code. The effects of plasma rotation in the presence of resistivity and viscosity are included and an effectively implicit numerical scheme allows the transport profile to evolve self-consistently with the nonlinear MHD instabilities and externally applied sources and sinks. The simulations follow the MHD bursts and disruptions through the linear and nonlinear phases and identify the connections between the early MHD bursts and the ultimate disruption phase. Specific predictions of the growth and saturation of the modes are directly compared with experimental diagnostic measurements in DIII-D. The simulations show that the bursts observed in experiments are triggered by MHD instability of a resistive interchange mode and a resistive kink mode that are excited for critical plasma profiles. The critical profiles are determined by the balance between inductive and noninductive sources of current density.
Nonlinear gyroviscous force in a collisionless plasma
Authors: Belova E. V.
Published: July 2002

Nonlinear gyroviscous forces in a collisionless plasma with temperature variations are calculated from the gyrofluid moments of the gyrokinetic Vlasov equation. The low-frequency gyrokinetic ordering and electrostatic perturbations are assumed, and an additional finite Larmor radius (FLR) expansion in a parameter ε⊥≡(k⊥ρ)2<1 is performed. This approach leads naturally to an expression for the gyroviscous force, (∇⋅πg), in terms of the gyrocenter distribution function, thus including all resonant effects, and represents a systematic FLR expansion in a general form (no assumption of any closure is made). The expression of (∇⋅πg) is also calculated in terms of the particle-fluid moments by making the transformation from the gyrocenter to particle coordinates. The calculated (∇⋅πg) represents a modification of the Braginskii gyroviscosity for a collisionless plasma with ∇T≠0. It is compared with previous calculations based on the traditional fluid approach. As a byproduct of the gyroviscosity calculations, we derive a set of nonlinear reduced gyrofluid (and a corresponding set of particle-fluid) moment equations with FLR corrections, which exhibit a generalized form of the “gyroviscous cancellation.”
Direct-to-indirect band gap crossover for the BexZn1−xTe alloy
Authors: Maksimov O. , Tamargo M. C.
Published: July 2002

We have investigated the growth and optical properties of a set of BexZn1−xTe epitaxial layers having different composition, with x ranging from 0–0.7. Comparison of the reflectivity and the photoluminescence spectra allowed us to locate the direct-to-indirect band gap crossover for this alloy at x≈0.28. The Γ→Γ direct band gap exhibits a linear dependence on composition over the entire compositional range and can be fitted to the equation EgΓ(x)=2.26*(1−x)+4.1*x. It increases linearly with BeTe content at a rate of 18 meV for a change of 1% in BeTe content. The Γ→X indirect band gap for BexZn1−xTe can be fitted to the equation EgX(x)=3.05*(1−x)+2.8*x−0.5*x*(1−x), suggesting that the energy of the indirect Γ→X transition for ZnTe is about 3.05 eV.
Dark conductivity, photoconductivity, and light-induced absorption in photorefractive sillenite crystals
Authors: Mosquera L. , de Oliveira I. , Frejlich J. , Hernandes A. C. , Lanfredi S. , Carvalho J. F.
Published: July 2002

We measure the bulk and light-induced absorption as well as the dark- and photoconductivity in doped and undoped photorefractive Bi12TiO20 crystals and compare the results obtained using different techniques. From these data we compute the quantum efficiency for the photogeneration of charge carriers and the carriers mobility–lifetime product, and characterize the suitability of these crystals for holographic recording. A two-center model theory is shown to adequately describe absorption and photoconduction in these materials.