bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of the biased electrode on the plasma potential in ECRIS
Authors: Mironov V. , Stiebing K. E. , Hohn O. , Schmidt L. , Schmidt-Böcking H. , Runkel S. , Schempp A. , Shirkov G. , Biri S. , Kenéz L.
Published: July 2002

Dedicated experiments have been carried out at the Frankfurt 14 GHz electron cyclotron resonance ion source (ECRIS) by using a special double biased-electrode assembly, which consists of a conventional disk electrode and a separately biased ring electrode installed in front of it. In this assembly, the ring can be used to modulate the fluxes to the disk and allows a detailed study of the role of secondary electron fluxes in ECRIS operation. It was found that these fluxes contribute more than 50% to the total disk currents. However, blocking them did not result in a drop in the extracted ion currents. Instead, it was observed that, under certain operational conditions, the injection of secondary electrons results in a decrease in the extracted currents by up to 20%. Parallel to the double disk measurements, Langmuir probe measurements have been performed close to the position of Bmax. From the probe characteristics, plasma potentials were determined to be about +30 V at the conditions of the experiment. Applying a negative voltage to the double disk electrodes leads to a decrease of the plasma potential by approximately 5 V. Changes in the plasma shape were observed when the biased electrode voltage was changed. We conclude that the main effect of the biased electrode is a decrease of the plasma potential by reflecting a sufficient amount of electrons back to plasma, which otherwise would have been lost.
On a simulation of ion confinement in ECRIS plasmas
Authors: Mironov V. , Stiebing K. E.
Published: July 2002

A particle-in-cell code has been developed for modeling the charged particle three-dimensional dynamics in the magnetic field of an electron cyclotron resonance ion source (ECRIS). The code incorporates the leap-frog particle pusher and Takizuka–Abé’s method for simulating the small-angle Coulomb collisions between the ions. Ionization dynamics and electron–ion heating are also included. The code has been used to estimate ion confinement times in the ECRIS plasma due to ion–ion collisions. Good agreement has been obtained with results from the gas-dynamic trapping model. The charge state distributions (CSD) of extracted argon ions were obtained under different boundary conditions, and good agreement is achieved with experimentally observed CSD. It was shown that the geometry of atom fluxes inside the source chamber plays an important role in determining the electron cyclotron resonance plasma parameters. Generally, ion temperatures were obtained to be around 0.5 eV, and ion confinement times are in a range 0.2–1 ms for the typical parameters of ECRIS plasma.
Characteristics of a helicon ion source as a high current density ion source (abstract)
Authors: Jung H. D. , Hong I. S. , Hwang Y. S.
Published: July 2002

A novel helicon ion source has been developed as a high current density ion source. High plasma generation efficiency of helicon wave lends various high-density, high-temperature plasmas available for ion sources. High-density plasmas of 1012 cm−3 are generated to implement a high current density ion source with 1 kW, 13.56 MHz rf power source. An axial magnetic field of up to 900 G has been applied with a set of permanent magnets to make the helicon wave propagate. Various gaseous ion beams have been extracted with this source. Beam characteristics such as ion energy spread and ionic charge states will be discussed with the plasma parameters.
Radio-frequency antenna studies for high-current, high-duty-cycle, H− volume sources (abstract)
Authors: Stockli M. P. , Welton R. , Janney M. , Lauf R. , Gough R. , Keller R. , Leung K. N. , Reijonen J. , Schenkel T. , Staples J. , Thomae R.
Published: July 2002

The Spallation Neutron Source requires an ion source capable of delivering a high-current (∼50 mA) H− beam with a 6% duty cycle continuously for the three weeks between the scheduled maintenance periods. The cesium-enhanced, multicusp volume ion source under development on the integrated test facility at LBNL delivers H− ion currents up to 50 mA, increasing approximately linearly with the rf power. Initial experience using porcelain-coated copper antennas, however, indicates lifetimes will fall below the desired three-week period, mostly limited by antenna failures. In an effort to improve our understanding of the antenna limitations, we are in the process of developing an antenna test dome, which will allow us to visually observe and study the rf-initiated discharge at low-power levels. We hope to be able to test antennas for invisible defects by observing and measuring the onset of the discharge. In addition, we are planning to test different antennas. Results will be presented at the meeting.
Study by high resolution electron microscopy and electron diffraction of the La0.03Sr0.255Ba0.7Nb2−yTiyO6−y/2 ferroelectric ceramic
Authors: Martı́nez E. , Fundora A. , Amorı́n H. , Portelles J. , Siqueiros J. M.
Published: July 2002

The stoichiometric La0.03Sr0.255Ba0.7Nb2−yTiyO6−y/2 (LSBNT) ceramic system was obtained for three different concentrations of titanium ions (y=1%, 3%, and 5%) where the Nb5+ was substituted by Ti4+ according to the expression La0.03Sr0.255Ba0.7Nb2−yTiyO6−y/2. The incommensurate modulation in the LSBNT structure has been studied by high-resolution transmission electron microscopy (HRTEM) and electron diffraction. The location of the incommensurate superlattice spots in the tetragonal phase through the [110] zone in the electron diffraction patterns was measured to determine the incomensuration parameter δ. Structural modulation was also evident by HRTEM for the [001] zone, where a sinusoidal variation along [110] and [11̄0] was identified in coincidence with the electron diffraction patterns. Random occupation of the crystallographic sites that promotes structural variations may be the cause for structural incommensuration. It was possible to identify directly the tetragonal tungsten bronze structure and the presence of point defects that influence the dielectric properties.
Statistical modeling of the spectral performance of a two-dimensional array of gamma-ray spectrometers
Authors: Ifraimov M. , Ludwig A. , Nemirovsky Y.
Published: July 2002

We develop a statistical model for the calculation of the spectral performance of two-dimensional (2D) array of gamma-ray spectrometers. We consider a nonuniform field in the device, determined by the geometrical design of the anodes. The induced charge is calculated by solving the Poisson’s equation with the appropriate boundary conditions. Furthermore, the presented model simultaneously considers a random point of photon absorption and a random drift length for each carrier, taking into account the effect of multiple trapping and detrapping. The model developed in this article enables the optimal geometrical design of the 2D array of anodes according to the electrical properties of the given semiconductor material, in order to achieve optimal results for the spectral performances. The model consists of three parts. The first is the calculation of the potential at each point of a three-dimensional (3D) map of the device. The second is the calculation of the charge induced on the anode by a point charge positioned in the device (for each point of a 3D map). The third is the statistical calculation of the pulse height spectrum. Finally, the pulse height spectrum is calculated as a function of the geometrical design, photon energy, electron and hole mobility, lifetime, mean detrapping time and applied voltage as well as the shaping time.
Step-induced magnetic-hysteresis anisotropy in ferromagnetic thin films
Authors: Zhao D. , Liu Feng , Huber D. L. , Lagally M. G.
Published: July 2002

We investigate the quasistatic magnetic hysteresis of ferromagnetic thin films grown on a vicinal substrate, using Monte Carlo simulations within a two-dimensional XY model. Intrinsic in-plane anisotropy is assigned to surface sites according to their local symmetry. The simulated hysteresis loops show a strong anisotropy: the coercive field is the largest when the external field is along the step direction and vanishes when the external field is perpendicular to the step direction. In general, the coercivity increases with increasing step density, but displays a more complex dependence on film thickness. The simulations also suggest that the mechanism for the magnetization reversal is coherent rotation. These results are in good agreement with experiments.
Influence of exchange on signal-to-noise ratio in longitudinal recording media
Authors: Chubykalo O. , Lengsfield B. , Kaufman J. , Jones B.
Published: July 2002

In this work a systematic study of the influence of exchange on media signal-to-noise ratio, (SNR) was undertaken. Log-normal grain distributions were utilized and four models of the intergranular exchange interaction were studied. In addition, three models of media SNR were examined. In all of our studies media SNR was found to plateau and then exhibit a small maximum as exchange is increased. This maximum occurs because remanent magnetization at the center of the bit increases faster than the noise grows at the bit transition in this low coupling regime. For larger values of exchange, the signal-to-noise ratio decreases due to the rapid increase of transition noise. These results suggest that this plateau region should be exploited in studies of the effect of exchange on thermal stability.
Two-dimensional periodic alignment of self-assembled Ge islands on patterned Si(001) surfaces
Authors: Kitajima Takeshi , Liu Bing , Leone Stephen R.
Published: July 2002

Two-dimensional alignment of Ge islands is obtained by molecular beam epitaxy of Ge on lithographically patterned Si(001) surfaces composed of periodic arrays of square Si mesas. When the period of the Si mesa arrays is reduced to 140 nm, a “one island on one mesa” relationship is achieved. The Ge islands have an average base width of 85 nm and take on the shape of a truncated pyramid with four {114} facets and a (001) top. The patterning also serves to improve the island size uniformity. The dependencies of the island morphology on the sizes of the Si mesas and Ge coverages are examined to clarify the mechanism of preferential nucleation of Ge islands on the tops of Si mesas.
Modeling thermal cross talk and overwrite jitter in growth dominant phase change optical recording media at high data rates
Authors: Sheila Aparna C. , Schlesinger T. E.
Published: July 2002

Two methods for the reduction of thermal cross talk (cross erase) and overwrite jitter in growth dominant phase change materials are addressed through simulations. An additional Au layer and a modified multipulse technique are considered. A model is described that simulates mark formation in growth dominant media. The disk with an additional Au layer is shown to reduce the temperature in adjacent tracks. The modified multipulse scheme, utilizes write power more efficiently and reduces the cross erase significantly. In growth dominant media, the main cause of overwrite jitter is shown to be the reduction in erasability with large marks at high velocities. Jitter is shown to be almost insensitive to parameters such as erase power, bias power at the cooling width region, and the length of the cooling width itself at high write powers. At lower write powers, clear minima in jitter are found for specific values of these parameters. At high write powers, jitter is substantially reduced using a modified multipulse scheme.
Relaxed SiGe-on-insulator substrates without thick SiGe buffer layers
Authors: Mizuno Tomohisa , Sugiyama Naoharu , Tezuka Tsutomu , Takagi Shin-ichi
Published: July 2002

We have developed a technology for relaxing top SiGe layers with low dislocation density on Si substrates, without using thick SiGe buffer structures. By introducing a thin strained SiGe layer and the internal-oxidation (ITOX) process following the separation-by-implanted-oxygen (SIMOX) process, we have experimentally demonstrated relaxed SiGe-on-insulator (SGOI) substrates with the Ge content of 20%, and it has been realized that their dislocation density is a factor of 20 lower than that of SGOI with SiGe buffer layer.
A theoretical determination of the methyl and aldehydic torsion far-infrared spectrum of propanal-d with the vibrational zero point correction
Authors: Smeyers Y. G. , Villa M.
Published: July 2002

The methyl and aldehydic torsions far-infrared (FIR) spectrum of standard propanal is determined theoretically taking into account the vibrational zero point energy (ZPE) of the remaining small amplitude vibration modes in an anharmonic approach. For this purpose, the potential energy function obtained in a previous calculation at the 6-311(3df,p) restricted Hartree–Fock/second-order Møller–Plesset level [J. Chem. Phys. 109, 2279 (1998)] is modified for the anharmonic ZPE corrections. With this corrected potential for the ZPE, the kinetic parameters, as well as the electric dipole moment variations, the FIR frequencies, and intensities for the methyl and aldehyde torsions of propanal were determined theoretically using a two-dimensional approach. The calculated torsional band structures of propanal determined without and with the ZPE corrections were compared between themselves and with the available experimental data. It is found that the theoretical calculations permit one to reproduce and assign 24 of the 25 observed bands. In the same way, the introduction of the ZPE corrections is seen to yield more satisfactory frequency values for the aldehyde torsion, double quanta, and methyl–aldehyde combination bands.
The frequency detuning correction and the asymmetry of line shapes: The far wings of H2O–H2O
Authors: Ma Q. , Tipping R. H.
Published: July 2002

A far-wing line shape theory that satisfies the detailed balance principle is applied to the H2O–H2O system. Within this formalism, two line shapes are introduced, corresponding to band averages over the positive and negative resonance lines, respectively. Using the coordinate representation, the two line shapes can be obtained by evaluating 11-dimensional integrations whose integrands are a product of two factors. One depends on the interaction between the two molecules and is easy to evaluate. The other contains the density matrix of the system and is expressed as a product of two three-dimensional distributions associated with the density matrices of the absorber and the perturber molecule, respectively. If most of the populated states are included in the averaging process, to obtain these distributions requires extensive computer CPU time, but only have to be computed once for a given temperature. The 11-dimensional integrations are evaluated using the Monte Carlo method, and in order to reduce the variance, the integration variables are chosen such that the sensitivity of the integrands on them is clearly distinguished. Numerical tests show that by taking into account about 107 random selections, one is able to obtained converged results. We find that it is necessary to consider frequency detuning, because this makes significant and opposite contributions in the two band-averaging processes and causes the lines to be asymmetric. Otherwise, the two line shapes become symmetric, are the same, and equal to the mean of the two shapes obtained including the frequency detuning effects. For the pure rotational band, we find that the magnitude of the line shape obtained from the positive line average is larger than that obtained from the negative line average for ω>0 and vice versa for ω<0, and their relative gap increases as the frequency displacement from the line center increases. By adopting a realistic potential model and optimizing its parameters, one is able to obtain these two line shapes and calculate the corresponding absorption coefficients that are in good agreement with laboratory data. Also, this same potential yields good theoretical values for other physical properties of the dilute H2O gas.
An experimental method to dynamically test pressure sensors using a rupture disk
Authors: Liu Hanying , Miller Don W. , Talnagi Joseph W.
Published: July 2002

The response time of a pressure sensor is required when it is used in control systems and in some measurement applications. It is often difficult to measure the response time of a pressure sensor since it is difficult to obtain changes in fluid pressure sufficient to characterize the sensor dynamic response. In this article we describe a relatively simple system for measuring or validating the response time of pressure sensors with fast dynamic response. The system consists of two chambers isolated by a graphite rupture disk, a device that fully and rapidly opens at a known rupture or break pressure. A pressure transient in the second chamber is initiated by slowly increasing the pressure in the first chamber until reaching the nominal break pressure of the rupture disk. Performance of the system was validated by comparing the rise time predicted by a theoretical model with the rise time of the pressure transient measured by a piezoelectric pressure transducer. The method was evaluated by comparing the response to the pressure transient of an optical based pressure transducer with the response of the reference piezoelectric pressure transducer. The time constant of the tested fiber optic pressure sensor was found using the method presented in this article to be 0.488 ms, which is close to the time constant of 0.455 ms measured by a comparison method.
Spin injection in ferromagnet-semiconductor heterostructures at room temperature (invited)
Authors: Ploog Klaus H.
Published: July 2002

In this article we summarize our recent work on room-temperature spin injection in Fe/GaAs and MnAs/GaAs heterostructures. The most critical issue for injection of spin polarized electrons (holes) from the ferromagnet (FM) into the semiconductor (SC) is the control of the atomic arrangement at the FM/SC interface during molecular beam epitaxial growth of these rather dissimilar materials. For many years the formation of a magnetically dead layer at the Fe/GaAs interface has prevented spin injection. In addition to the accurate control of the FM/SC interface, the formation of a Schottky barrier between FM and SC for efficient spin injection via tunneling is the second critical issue for successful experiments. We describe in detail our approaches to solve these problems.
Pressure effect on transport properties of electron-doped Sm0.2Ca0.8Mn1−xRuxO3(x=0; 0.04) manganites
Authors: Markovich V. , Rozenberg E. , Gorodetsky G. , Martin C. , Maignan A. , Hervieu M. , Raveau B.
Published: July 2002

Transport properties of electron rich Sm0.2Ca0.8Mn1−xRuxO3(x=0; 0.04) were measured under hydrostatic pressures up to 1 GPa and applied magnetic fields up to 1.5 T at the temperature interval 77<T<300 K including critical points of these manganites. The 4% Ru doping in Mn site converts low temperature robust orbitally ordered (OO) antiferromagnetic (AF) phase observed in the undoped compound to a two-phase [coexisting ferromagnetic metallic (FMM) and OO/AF phases] state. As a result of the appearance of the phase separated state, colossal magnetoresistance and slow relaxation of resistivity with logarithmic time dependence that are enhanced by pressure, are observed in Sm0.2Ca0.8Mn0.96Ru0.04O3. In contrast, negative magnetoresistance, originated from some minute FM interaction that was observed in Sm0.2Ca0.8MnO3, is depressed and practically vanished by external pressure. Except for this, the relaxation of resistivity is also absent in Sm0.2Ca0.8MnO3.
Hydrogenated amorphous and microcrystalline GaAs films prepared by radio-frequency magnetron sputtering
Authors: Ouyang L. H. , Rode D. L. , Zulkifli T. , Abraham-Shrauner Barbara , Lewis N. , Freeman M. R.
Published: July 2002

We report on the optical absorption, adhesion, and microstructure of radio-frequency magnetron sputtered films of hydrogenated amorphous and microcrystalline GaAs films for the 1 to 25 μm infrared wavelength range. Sputtering parameters which were varied include sputtering power, temperature and pressure, and hydrogen sputtering-gas concentration. Transmission electron microscopy results show a sharp transition from purely amorphous GaAs to a mixture of microcrystalline GaAs in an amorphous matrix at a substrate-holder temperature of 34±2 °C. By optimizing the sputtering parameters, the optical-absorption coefficient can be decreased below 100 cm−1 for wavelengths greater than about 1.25 μm. These results represent the lowest reported values of optical absorption for sputtered films of GaAs directly measured by spectrophotometry for the near-infrared wavelength region.
Temperature fields generated by impulsive interfacial heat sources
Authors: Shendeleva Margarita L.
Published: July 2002

Theoretical study of temperature fields generated by an instantaneous line heat source located at the interface between two different media is presented. Green’s functions for the temperature fields in both media are derived using the Cagniard–de Hoop method. It is shown that in a more conductive medium the temperature field has the character of a wave originating at the point source, while in the less conductive medium the temperature field can be decomposed into the “spherical” wave emanating from the two-dimensional point source and the “head” wave arising due to the heat exchange through the interface. The derived analytical expressions are accompanied by the diagrams of the wave surfaces of temperature fields and numerical examples.
Thermodynamics of a soft disk glass: The role of configurational constraints
Authors: Padilla F. G. , Harrowell Peter
Published: July 2002

Monte Carlo calculations of the specific heat of a model glass have been carried out to determine the thermodynamic consequences of the configurational constraints required to define a single glass state. Three different configurational constraints were employed. Using a cell model, we find a steady decrease in the heat capacity CV with increasing temperature indicating a significant anharmonic behavior in the soft disk glass. If, instead, we constrain the particle fluctuations to uncoupled instantaneous normal modes (INM’s), we find that the heat capacity is almost independent of temperature and equal to the harmonic value, even when the full anharmonic potential is used. To resolve this contradiction, we employ a tether constraint and find reasonable agreement with the cell model results. We conclude (i) that the harmonic behavior under the INM constraint is an artifact arising from the arbitrary restrictions on particle motions, and (ii) that the soft disk glass is an anharmonic solid with a heat capacity that decreases on heating along an isobar.
Photonic-band-gap properties of two-dimensional lattices of Si nanopillars
Authors: Poborchii Vladimir V. , Tada Tetsuya , Kanayama Toshihiko
Published: July 2002

We studied photonic-band-gap properties of two-dimensional lattices of Si nanopillars by theoretical calculation and measurement of reflection and transmission spectra. We focused on advantages of these photonic crystals compared to other Si photonic crystals, which usually operate in the range of transparency of bulk Si (wavelengths longer than ∼1.1 μm). We showed that the available spectral range for the photonic crystals of Si nanopillars can be extended to the submicron wavelengths, light absorption by Si nanopillars being insignificant. Another important advantage of Si nanopillar lattices is the ability to incorporate luminescent materials into the huge free space of this photonic crystal. We demonstrate the inhibition of spontaneous emission of dye incorporated into the nanopillar lattice.
Low resistance spin-dependent tunnel junctions with ZrAlOx barriers
Authors: Wang Jianguo , Freitas P. P. , Snoeck E. , Battle X. , Cuadra J.
Published: July 2002

Spin-dependent tunnel junctions with ZrAlOx barriers were fabricated with low resistance×area product 4 Ω×μm2, and tunnel magnetoresistance of 15.2%. Barrier fabrication was done by natural oxidation (5 min, at oxidation pressures ranging from 0.5 to 10 Torr). The junctions were deposited on top of 600 Å thick, ion beam smoothed, low resistance, Al electrodes. X-ray photoelectron spectroscopy analysis indicates the presence of AlOx, ZrO2, some remnant metallic Zr, but no metallic Al in the as-deposited barriers. High resolution transmission electron microscopy indicates that ZrAlOx forms an amorphous barrier that is smoother than pure crystalline ZrOx or pure amorphous AlOx barriers. These low resistance tunnel junctions are attractive for read head applications above 100 Gbit/in2 where competitive signal to noise ratios imply resistance×area product below a few Ω×μm2, and tunneling magnetoresonance signals near or above 20%.
Magnetoresistance study in Co–Al–Co and Al–Co–Al double tunneling junctions
Authors: Chen C. D. , Yao Y. D. , Lee S. F. , Shyu J. H.
Published: July 2002

Magnetoresistance (MR) in Co–Al–Co and Al–Co–Al double tunneling junctions has been studied at temperatures between 65 and 750 mK and in magnetic fields up to 3 T. The electrical resistance decreases with increasing temperature. In the low magnetic field region, the electrical resistance hysteresis behavior is attributed to a typical TMR for ferromagnetic Co and insulator Al2O3 elements. In the high magnetic field region, a sharp resistance variation roughly near 2.0 T for the Co–Al–Co system and 1.75 T for the Al–Co–Al system has been observed and is attributed to the electron tunneling effect. From the current–voltage and dI/dV characteristics, the superconductivity energy gap is roughly 0.5 meV in the Co–Al–Co system and roughly 0.3 meV in the Al–Co–Al system; and magnetic tunneling energy is roughly 0.01 meV in the Co–Al–Co system and roughly 0.06 meV in the Al–Co–Al system.
Magnetic field dependent noise in magnetic tunnel junction
Authors: Kim K. S. , Shim H. J. , Hwang I. J. , Cho B. K. , Seok J. H. , Kim Jin-Tae
Published: July 2002

The measurements of voltage fluctuations at room temperature are performed for magnetic tunnel junction, patterned by photolithography to have 20×10 μm2 in junction area. The observed noise power follows 1/f behavior for the whole applied magnetic field (H) range (−30 Oe⩽H⩽30 Oe) and shows strong field dependence. The sharp increase in noise power spectral density is observed near a transition region from antiparallel to parallel magnetic state in two electrodes during field ramping down. In addition, the reduction of noise power density is accompanied by the reduction of magnetoresistance ratio after thermal annealing at 300 °C for 700 s in the same junction. It is conjectured that the observed reduction of the noise level in our junction is due mainly to magnetic origin, such as thermally activated domain wall motion, rather than electrical origin.
Metastable conductivity in low-doped manganites
Authors: Yuzhelevski Y. , Markovich V. , Rozenberg E. , Gorodetsky G. , Jung G. , Shulyatev D. A. , Mukovskii Ya. M.
Published: July 2002

Experimental manifestations of current enforced metastable conductivity in La0.82Ca0.18MnO3 single crystals are discussed. Metastable conductivity manifests itself by complex current dependence, a pronounced bias dependent structure, resistance hysteresis, enhanced noise with non-Gaussian components, and long relaxation times. Spin polarized tunneling through intrinsic tunnel junctions associated with a network of twin boundaries has been tentatively associated with the phenomena observed.
Extraordinary Hall effect in CoxPt100−x films
Authors: Zheng R. K. , Wen G. H. , Zhang X. X.
Published: July 2002

Co x Pt 100−x alloy films, 8.7⩽x⩽40.6, have been fabricated by the magnetron co-sputtering technique. The structure of the films has been studied by x-ray diffraction and transmission electron microscopy. The results indicate the existence of CoPt3 phase in these films. Electric transport and magnetotransport measurements have been performed in the temperature range of 5–300 K and field range of −5–5 T, in which the longitudinal resistivity (ρxx) and extraordinary Hall resistivity (ρs) have been obtained. A power law dependence of extraordinary Hall resistivity on the longitudinal resistivity, ρs∝ρxxv, is observed, however the value of the exponent (v) changes from 1 to 1.6 as the temperature changes from 5 to 100 K and above. Different v values suggest different mechanisms of the extraordinary Hall effect. The dependence of the Hall resistivity on the Co concentration, x, has also been studied.
Diffraction response of a low-temperature-grown photorefractive multiple quantum well modulator
Authors: Abeeluck A. K. , Garmire E.
Published: July 2002

The transient diffraction response of a low-temperature-grown (LTG) photorefractive multiple quantum well (MQW) spatial light modulator (SLM) is studied experimentally as a function of the frequency and the on/off ratio of a single-sided applied voltage when a grating is written using a continuous-wave sinusoidal illumination. LTG MQW modulators have high spatial resolution, but their diffraction efficiency and speed are reduced. By varying the on/off ratio of the applied voltage, it is shown that the frequency response of the SLM can be optimized while maintaining its high spatial resolution. The flatness of the frequency response with respect to the peak diffracted signal is thus shown to increase by an order of magnitude. By temporally gating the diffracted signal, low and bandpass filtering capabilities of the SLM are experimentally demonstrated for applications such as speckle elimination and novelty filtering. When the diffracted signal is measured over an adjustable time window, it is shown that high frame rate operations in integrated optoelectronic systems can be achieved. A numerical model that includes the effect of high trap densities on the space-charge dynamics is developed. It is shown that the space-charge field grows nonuniformly along the MQW growth direction with the largest modulation in lateral field occurring only near the entrance face of the incident light, thus accounting partially for the low diffraction efficiency of LTG materials.
The ultraviolet photodissociation of jet-cooled ClO and BrO radicals
Authors: Zou Peng , Kim Hahkjoon , North Simon W.
Published: July 2002

The photodissociation dynamics of jet-cooled ClO and BrO radicals have been studied using resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry. Four asymptotic electronic channels are energetically allowed for ClO photodissociation at 235 nm, although the dominant products observed at 235 nm are Cl(2P3/2) and O(1D2). Polarization-dependent time-of-flight profiles indicate that this channel has an anisotropy parameter of β=1.2±0.2. Evidence for O(3PJ) products was detected in coincidence with Cl(2P1/2) fragments, and we estimate that this channel constitutes <3% at this wavelength. In the photodissociation of BrO at 355 nm we observe two asymptotic electronic state channels, Br(2P3/2)+O(3P2) and Br(2P1/2)+O(3P2) in a 0.75:0.25 ratio. Both channels have an anisotropy parameter of β=1.5±0.1. Based on the measured asymptotic velocities of both channels, we have directly determined the Br–O bond dissociation energy to be 55.8±1.0 kcal/mol, providing a heat of formation for the BrO radical at 298 K of 29.7±1.0 kcal/mol in good agreement with previous spectroscopic determinations.
Thermal effects in dynamics of interfaces
Authors: Umantsev A.
Published: July 2002

Dynamical Ginzburg–Landau theory is applied to the study of thermal effects of motion of interfaces that appear after different phase transitions. These effects stem from the existence of the surface thermodynamic properties and temperature gradients in the interfacial transition region. Thermal effects may be explained by the introduction of a new thermodynamic force exerted on the interface, called here Gibbs–Duhem force, and the internal energy density flux through the interface. The evolution equations for the interfacial motion are derived. For the experimental verification of the thermal effects during continuous ordering the expression is derived for the amplitude of temperature waves.
Quantum phase interference in nanomagnets with tetragonal symmetry
Authors: Kim Gwang-Hee
Published: July 2002

Employing a spin-coherent-state path integral method, we study the spin-parity dependence of the tunnel splitting in the uniaxial system with tetragonal symmetry. The tunnel splitting is found to vanish for magnetic particles with half integer or odd number spin. Applying an external magnetic field along the hard axis, we find that topological effect results in oscillation of the tunnel splitting, and present the relation between the quenching period and the ratio of two anisotropy constants of the nanomaget with any total spin including molecular magnets.
The Stark effect in the v+=1 autoionizing Rydberg states of NO
Authors: Goodgame A. L. , Dickinson H. , Mackenzie S. R. , Softley T. P.
Published: July 2002

The Stark effect in autoionizing Rydberg states of NO, n=10–20,v+=1, is studied by a combination of experimental and theoretical methods. Double resonant excitation via selected intermediate rotation levels of the A 2Σ+, v′=1 state is carried out in the presence of fields 0–1000 V/cm. The spectra are simulated using both matrix diagonalization and full multichannel quantum defeat theory (MQDT) approaches, providing a test of quantum defect parameters and transition dipoles for Rydberg series from l=0 to l=4. The use of the same input parameters in these two types of calculation allows comparison of the validity and utility of these methods.
Coherently suppressed ringing of the magnetization in microscopic giant magnetoresistive devices
Authors: Schumacher H. W. , Chappert C. , Crozat P. , Sousa R. C. , Freitas P. P.
Published: July 2002

We study the magnetotransport response of 1 μm×4 μm size spin valve elements to ultrashort (pulse length 0.2–4 ns) easy axis magnetic field pulses. When applying an additional static hard axis field, pronounced magnetic precession is observed in the magnetoresistance. Damped precession occurs after the setting of the pulse and after pulse termination (“ringing”). The strength of the ringing can be controlled by adjusting the ratio of the precession period to the effective pulse length. When the pulse length matches the precession period, the ringing after pulse termination is effectively suppressed.
Magnetic force microscopy observations of the magnetic behavior in Co–C nanodot arrays
Authors: Gao L. , Liou S. H. , Zheng M. , Skomski R. , Yan M. L. , Sellmyer D. J. , Polushkin N. I.
Published: July 2002

The nanomagnetic behavior of Co–C nanodot arrays was investigated by magnetic force microscopy (MFM) and an alternative gradient force magnetometer. The direction of the easy axis can be observed directly with MFM by comparing the saturated magnetization state and the remanent magnetization state. Interaction of the domain wall with local defects was observed by field dependent MFM measurements. Some types of defects that can pin domain wall movement were identified.
Magnetization patterns of slave media duplicated by using patterned master media
Authors: Sugita R. , Saito O. , Muranoi T. , Nishikawa M. , Nagao M.
Published: July 2002

It is possible that a servo signal can be written on hard and floppy disks at high speed and low cost by using a magnetic contact duplication technique with patterned master media. The duplicated magnetization patterns of the slave media is elucidated. The duplicated magnetization is not only reversed at transitions, but also changes between transitions for bit lengths more than several μm, while the magnetization is reversed at transitions, and is almost uniform between transitions for bit lengths less than several μm. Therefore, when the above-mentioned duplication technique is applied to hard and floppy disks, it is expected that the magnetization changes only at transitions.
Magnetohydrodynamic mode identification from magnetic probe signals via a matched filter method
Authors: Edgell Dana H. , Kim Jin-Soo , Bogatu Ioan N. , Humphreys David A. , Turnbull Alan D.
Published: July 2002

A matched filter analysis has been developed to identify the amplitude and phase of magnetohydrodynamic modes in DIII-D tokamak plasmas using magnetic probe signals (δBp). As opposed to conventional Fourier spatial analysis of toroidally spaced probes, this analysis includes data from both toroidally and poloidally spaced magnetic probe arrays. Using additional probes both improves the statistics of the analysis and more importantly incorporates poloidal information into the mode analysis. The matched filter is a numeric filter that matches signals from the magnetic probes with numerically predicted signals for the mode. The numerical predictions are developed using EFIT equilibrium reconstruction data as input to the stability code GATO and the vacuum field code VACUUM. Changes is the plasma equilibrium that occur on the same time scale as the mode are taken into account by modeling simple matched filter vectors corresponding to changes in total plasma current, plus vertical and horizontal plasma shifts. The matched filter method works well when there is good understanding of a mode and good modeling of its structure. Matched filter analysis results for a fast growing ideal kink mode, where equilibrium change effects are minimal, show the effectiveness of this method. A slow growing resistive-wall mode (RWM) is also analyzed using the matched filter method. The method gives good results for identifying the amplitude and phase of the RWM but the simple equilibrium vectors are insufficient for complete elimination of equilibrium changes on this time scale. An analysis of the computational requirements of the scheme indicates that real-time application of the matched filter for RWM identification will be possible.
Apparatus to measure wafer curvature for multilayer systems in a vacuum furnace
Authors: Mann A. B. , Tapson J. , van Heerden D. , Lewis A. C. , Josell D. , Weihs T. P.
Published: July 2002

A laser-based technique for measuring the curvature of a multilayer/substrate couple is described. Unlike most wafer curvature systems, the instrument described measures the local curvature of the multilayer/substrate couple, correcting for the local topography of the substrate, rather than measuring changes in the average curvature of the multilayer/substrate couple. The apparatus has been designed specifically to perform biaxial zero-creep measurements at elevated temperatures in vacuum. It can also be used to examine the development of biaxial stresses during thermal cycling of thin films deposited on substrates.
An improved light-emitting element driver
Authors: Liao Tai-Shan
Published: July 2002

An improved light element driver employing two transistors, one capacitor and one grounded resistor, is presented. It also has the fewest number of possible components and can drive a red light-emitting diode with a supplied voltage as low as 0.8 V. Its power efficiency is about 70% to deliver to a light-emitting element at 1 V supply voltage.
Optimization of a magnetic disk drive actuator with small skew actuation
Authors: He Zhimin , Ong Eng Hong , Guo Guoxiao
Published: July 2002

Currently the utilization of the voice-coil motor for actuating read/write head elements in magnetic hard disk drives results in a skewed actuation, which necessitates an involved microjogging process and thus a complicated servo system. Furthermore, in perpendicular recording systems, a small skew actuation will relax the requirement on pole trimming. This article presents a magnetic hard disk drive actuator and suspension assembly with small skew actuation. In the present study, the distance from the actuator pivot to the read/write head is chosen so that the skew angle variation is minimized. After that, the suspension head is assembled to the actuator arm at a slant angle with respect to the actuator longitudinal direction to achieve an absolute small skew actuation. Finite element modeling and experimental measurements reveal that there are no significant changes of the actuator assembly dynamic performance with and without the slant angle.
Pulsed laser deposition of iron oxide films
Authors: Tepper T. , Ross C. A.
Published: July 2002

Iron oxide films have been grown onto MgO and oxidized silicon substrates using pulsed laser deposition with a KrF laser. Films deposited in vacuum consisted of the stoichiometric ferrimagnetic γ-Fe2O3 phase, with a saturation magnetization of 371±27 kA/m. However, films deposited in an O2 atmosphere were iron deficient and consisted of the antiferromagnetic α-Fe2O3 phase, with a saturation magnetization of 40±10 kA/m. Films predominantly consisting of the metastable γ-Fe2O3 phase at least 680 nm in thickness could be grown on MgO (001) substrates, which is thicker than films reported elsewhere. The effects of stoichiometry, thickness, and annealing on the magnetic properties of the films are discussed.
Comparative study of the d-band filling effect on the magnetic behavior of CoxNi1−x and FexNi1−x ultrathin films on Cu(100)
Authors: Kuo C. C. , Chuang S. F. , Pan W. , Lin W. C. , Lin Minn-Tsong
Published: July 2002

The ultrathin FexNi1−x/Cu(100) and CoxNi1−x/Cu(100) alloy films with x⩽10% were prepared for the study of spin-reorientation transition with precise variation of composition and coverage of the films. The spin-reorientation transition with the variation of the film coverage was observed in CoxNi1−x/Cu(100) with x<10% as well as FexNi1−x/Cu(100) with x<6%. This transition was found to be strongly affected by the alloy composition of the alloy films. The critical thickness for the spin-reorientation transition changes from 7.5 ML with x=0 to 17.5 ML with x=8% for CoxNi1−x/Cu(100). A more sensitive composition-driven effect has been found in FexNi1−x/Cu(100) by varying the Fe concentration for the critical thickness to 16 ML with x=5%. The more drastic influence of the concentration of Fe than Co on the critical thickness of the spin-reorientation transition can be attributed to the greater deviation of the difference in 3d electron number of majority and minority bands between Fe and Ni than that between Co and Ni.
Anisotropic electrical spin injection in ferromagnetic semiconductor heterostructures
Authors: Young D. K. , Johnston-Halperin E. , Awschalom D. D. , Ohno Y. , Ohno H.
Published: July 2002

A fourteen-fold anisotropy in the spin transport efficiency parallel and perpendicular to the charge transport is observed in a vertically biased (Ga, Mn)As-based spin-polarized light emitting diode. The spin polarization is determined by measuring the polarization of electroluminescence from an (In, Ga)As quantum well placed a distance d (20–420 nm) below the p-type ferromagnetic (Ga, Mn)As contact. In addition, a monotonic increase (from 0.5% to 7%) in the polarization is measured as d decreases for collection parallel to the growth direction, while the in-plane polarization from the perpendicular direction (∼0.5%) remains unchanged.
Stepped piston modification in the piston manometer of mixed operational principle (simple-piston and re-entrant cylinder system)
Authors: Urbański Michał K. , Wiśniewski Roland
Published: July 2002

The optimization of mixed-principle operation pressure balance consisting of two parts: simple-piston and re-entrant cylinder units is reported. The maximum high pressure corrections for low pressure effective area of the system can be reduced by 50%. The solution was obtained by application of a small reduction (step) of the piston diameter. The modified design is characterized by the performance of the metrological piston of 10 mm2 nominal cross-sectional area with a (1–3) μm inward circumferential step located between the upper (simple piston) and lower (re-entrant) parts of the cylinder in the area where existing intermediate pressure is constant.
Impurity density profile measurements with a fast filter spectrometer and a modulated diagnostic beam on Tore Supra by charge exchange recombination spectroscopy
Authors: Hess W. R. , Farjon J. L. , Guirlet R. , Druetta M.
Published: July 2002

A simple interference filter spectrometer, together with a modulated neutral diagnostic beam is used on the Tore Supra tokamak for the measurement of carbon impurity profiles by charge exchange spectroscopy. A temporal resolution of 8 ms with a good signal to noise ratio is obtained with a neutral hydrogen beam of about 1 A equivalent neutral current at the plasma edge at 45 kV. Experimental results with ion cyclotron heating show that fast changes of the heating power are followed by transient modifications of the carbon impurity profile. The evolution of the C6+ profile from peaked to hollow and back to normal can be followed step by step and a typical diffusion time of about 30 ms is measured.
Cationic molecular wave packets: “Settin’ the pace”
Authors: Gräfe S. , Erdmann M. , Engel V.
Published: July 2002

We discuss an excitation scheme, where the interaction of two ultrashort laser pulses prepares an ensemble of ionized molecules with an oscillating average bond length. By adjusting the delay-time between the pulses it is possible to vary the oscillation period continuously within the subpicosecond regime. Possible applications in producing electron beams, modulated on a femtosecond time scale, are outlined.
Structural and morphologic evolution of Pt/Ba0.7Sr0.3TiO3/Pt capacitors with annealing processes
Authors: Qin Y. L. , Jia C. L. , Urban K. , Liedtke R. , Waser R.
Published: July 2002

The microstructure and chemistry of the as-grown, the postannealed and the forming-gas-atmosphere-treated Pt/Ba0.7Sr0.3TiO3/Pt capacitors are studied by means of high-resolution transmission electron microscopy and energy-disperse x-ray spectroscopy. It is found that the annealed Ba0.7Sr0.3TiO3 films have larger grain size and more smooth top film-electrode interfaces. High-resolution images reveal the presence of disordered or amorphous regions at the interfaces in the Ba0.7Sr0.3TiO3 film heated in the forming-gas atmosphere. These regions show a higher Ti/(Ba+Sr) ratio than the grain matrix. The effects of these amorphous regions on the electrical properties of Ba0.7Sr0.3TiO3 films are discussed.
Simulations of internal transport barrier formation in tokamak discharges using the GLF23 transport model
Authors: Kinsey J. E. , Staebler G. M. , Waltz R. E.
Published: July 2002

Results are presented for simulations of tokamak discharges exhibiting internal transport barriers (ITBs) with significant reductions in the core thermal transport using a comprehensive theory-based model for drift-wave transport. The predicted temperature and toroidal velocity profiles from the GLF23 model are compared against the experimental data for twenty-two L- and high-confinement mode (H-mode) ITB discharges from three large tokamaks including DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], Tokamak Fusion Test Reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)], and Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)]. The combined effects of E×B shear and Shafranov shift stabilization of the turbulent transport are essential in reproducing the barriers in the plasma core. Shafranov shift or α-stabilization is found to be an essential ingredient in suppressing the thermal transport due to ion and electron temperature gradient and trapped electron modes that can result in simultaneous electron and ion barriers. Another consequence of α-stabilization is that the power threshold for ITB formation is predicted to decrease for strongly reversed magnetic shear cases in comparison with weakly reversed shear cases.