bluuebunny/crossref_metadata_2025 huggingface dataset:
Terahertz radiation from n-type GaAs with Be-doped low-temperature-grown GaAs surface layers
Authors: Liu Kai , Krotkus Arunas , Bertulis K. , Xu Jingzhou , Zhang X.-C.
Published: August 2003

We investigate the performance of n-type GaAs plasmon emitters with surface layers grown by low-temperature molecular-beam epitaxy. The emitters have as-grown Be-doped low-temperature-grown GaAs layers on the top surface, which radiate terahertz transients with a power four times greater than reference emitters without low-temperature-grown (LTG) GaAs layers. The power generated by the structures with low-temperature-grown GaAs layers shows much weaker saturation at large excitation densities. The mechanism of the surface field screened by photoexcited holes has been discussed to explain the negative dip in the THz wave form when the emitter is coated by a LTG GaAs layer.
Temporal study of a plasma loaded helix, backward wave oscillator
Authors: Abu-elfadl Tamer M. , Nusinovich Gregory S. , Antonsen Thomas M. , Granatstein Victor L.
Published: August 2003

The operation of the helix plasma-assisted slow-wave oscillator (PASOTRON) backward-wave oscillator (BWO) is simulated using a time-dependent three-dimensional (3D) model. The model gives a self-consistent solution of the wave-electron interaction, given a certain end reflection of the device. Regions of single frequency and multiple frequencies (automodulation) operation are obtained. For the device without end reflection, it is shown that due to the transverse motion, no automodulation is observed, no matter how large the injected beam current. Through comparison with conventional 1D BWO, it is also shown that 3D BWO efficiency is more than twice the 1D BWO efficiency, when all the parameters are the same in both devices.
Fluid treatment of convective-transport threshold model of neoclassical tearing modes in tokamaks
Authors: Mikhailovskii A. B. , Shirokov M. S. , Tsypin V. S. , Konovalov S. V. , Ozeki T. , Takizuka T. , Galvão R. M. O. , Nascimento I. C.
Published: August 2003

A fluid treatment of convective-transport threshold model of neoclassical tearing modes (NTMs) in tokamaks is developed. A Grad-type system of moment equations of the drift kinetic equation with a model perpendicular transport is derived. The essence of this moment equation system is to allow for the parallel heat flux on an equal footing with pressure and temperature, what goes beyond the scope of the Braginskii approach. The suggested moment equation system is applied for analyzing the bootstrap current drive of NTMs. As a result, a threshold model of these modes is derived, which coincides qualitatively with the convective-transport threshold model initially formulated by means of intuitive considerations.
Crossover of the preferred growth orientation of AlN/Si(001) films during off-axis radio frequency sputter growth
Authors: Jang H. W. , Kang H. C. , Noh D. Y. , Yi M. S.
Published: August 2003

We found that the crystallographic orientation of AlN/Si(001) thin films crosses over from the substrate normal towards the direction of incident flux during off-axis radio frequency magnetron sputter growth. At high growth temperatures, the crystalline c-axis orientation is maintained along the substrate normal direction initially, but jumps discontinuously towards the direction of incident flux. In contrast, at low growth temperatures, the c-axis direction shifts continuously towards the incident flux direction and saturates in the middle agreeing with the tangential rule of oblique deposition, i.e., tan β=1/2 tan α, where α and β denote the angles of incident flux and column incline, respectively. Selected area transmission electron diffraction patterns are consistent with the crossover measured by in situ x-ray scattering experiments.
Influence of stoichiometry on the dielectric properties of sputtered strontium titanate thin films
Authors: Taylor T. R. , Hansen P. J. , Pervez N. , Acikel B. , York R. A. , Speck J. S.
Published: August 2003

The dielectric permittivity, dielectric quality factor (inverse dielectric loss), and lattice parameter of 140 nm sputtered SrTiO3 films were dependent on the oxygen partial pressure and total chamber pressure (O2+Ar) during film growth. Films were grown at 25 and 75 mTorr (mT) in an oxygen rich and oxygen deficient sputtering gas environment concurrently on (100) SrTiO3 and (111) Pt/(0001) Al2O3 substrates. Films were deposited on platinized sapphire for electrical characterization and the homoepitaxial films were used as a structural and chemical standard. High resolution triple axis x-ray diffraction results showed an increase in mismatch between the film and substrate (200) peak in homoepitaxial SrTiO3 films with higher total growth and lower oxygen pressures. Dielectric quality factors of the SrTiO3 films on platinized sapphire at 1 MHz for the 25 mT (50 sccm Ar/50 sccm O2), 25 mT (90 sccm Ar/10 sccm O2), 75 mT (50 sccm Ar/50 sccm O2), and 75 mT (90 sccm Ar/10 sccm O2) film growths were 320, 251, 209, and 102, respectively; likewise, the dielectric constants follow as 241, 230, 220, and 170, respectively. Improved film dielectric properties were observed for films closer to stoichiometric SrTiO3.
Simulation of thickness effect in thin ferroelectric films using Landau–Khalatnikov theory
Authors: Lo Veng Cheong
Published: August 2003

The thickness effect in ferroelectric thin films has been theoretically investigated using the Landau–Khalatnikov theory. Ferroelectric properties such as the hysteresis loop, and its associated coercive field and the remanent polarization of various film thicknesses have been numerically simulated. In this simulation, the thin film was modeled by the stacking of layers, each of which has unique parameters for the Landau free energy. Due to the interfacial effects near the electrodes, the parameters for the surface layers are different from those for the bulk. The simulated result shows that the coercive field decreases while the remanent polarization increases with thickness. Both of these trends qualitatively agree with experiments.
Refractive index study of AlxGa1−xN films grown on sapphire substrates
Authors: Sanford N. A. , Robins L. H. , Davydov A. V. , Shapiro A. , Tsvetkov D. V. , Dmitriev A. V. , Keller S. , Mishra U. K. , DenBaars S. P.
Published: August 2003

A prism coupling method was used to measure the ordinary (no) and extraordinary (ne) refractive indices of AlxGa1−xN films, grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire, at several discrete wavelengths from 442 nm to 1064 nm. In addition, spectroscopic transmittance and reflectance, correlated with the prism coupling results, were used to measure no as a continuous function of wavelength between the band gap of each sample (255 nm to 364 nm, depending on Al fraction) and 2500 nm. The Al mole fractions (x), determined by energy dispersive x-ray spectroscopy (EDS), were x=0.144, 0.234, 0.279, 0.363, 0.593, and 0.657 for the HVPE-grown samples, and x=0.000, 0.419, 0.507, 0.618, 0.660, and 0.666 for the MOCVD-grown samples. The maximum standard uncertainty in the EDS-determined value of x was ±0.02. The maximum standard uncertainty in the refractive indices measured by prism coupling was ±0.005 and a one-Sellmeier-term equation was adequate to fit the wavelength dependence of ne from 442 nm to 1064 nm. Due to the spectral proximity of the absorption edge, the wavelength dependence of no measured by spectroscopic transmittance/reflectance (correlated with the prism-coupling results), from the band gap of each sample to 2500 nm, was fit with a two-Sellmeier-term equation.
Wigner–Yanase information on quantum state space: The geometric approach
Authors: Gibilisco Paolo , Isola Tommaso
Published: August 2003

In the search of appropriate Riemannian metrics on quantum state space, the concept of statistical monotonicity, or contraction under coarse graining, has been proposed by Chentsov. The metrics with this property have been classified by Petz. All the elements of this family of geometries can be seen as quantum analogs of Fisher information. Although there exists a number of general theorems shedding light on this subject, many natural questions, also stemming from applications, are still open. In this paper we discuss a particular member of the family, the Wigner–Yanase information. Using a well-known approach that mimics the classical pull-back approach to Fisher information, we are able to give explicit formulas for the geodesic distance, the geodesic path, the sectional and scalar curvatures associated to Wigner–Yanase information. Moreover, we show that this is the only monotone metric for which such an approach is possible.
Source of heavy molecular ions based on Taylor cones of ionic liquids operating in the pure ion evaporation regime
Authors: Romero-Sanz I. , Bocanegra R. , Fernandez de la Mora J. , Gamero-Castaño M.
Published: August 2003

The full spray emitted by Taylor cones of the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMI+BF4−) held in a vacuum is investigated at room temperature by time of flight mass spectrometry. The current is composed mainly of ions under most conditions studied, but contains a small component of nanometer drops that tends to dominate the emitted mass flow. Exceptionally, drop ejection vanishes close to the smallest flow rate at which the Taylor cone is steady. The present discovery of a stable strictly ionic regime in Taylor cones of substances other than liquid metals owes much to earlier observations with sulfuric acid, where most but not all the current was ionic. Most striking is the fact that this purely ionic regime is obtained at an electrical conductivity K of only 1.3 S/m, much smaller than that of sulfuric acid, and smaller than that at which formamide electrolytes with K>2 S/m do still emit substantial drop currents. The ion emission includes primarily the dimer (EMI–BF4)EMI+, accompanied by fair currents of the monomer EMI+ and the trimer ions. The modest spread of ion energies observed and considerable currents attained (0.6 μA) suggests the use of this and other ionic liquids as ion beam sources with a much wider range of mass/charge and chemical compositions than previously available. The present data provide a direct proof of the reality of the often doubted mechanism of ion evaporation from liquid surfaces.
Raman study of thin films of amorphous-to-microcrystalline silicon prepared by hot-wire chemical vapor deposition
Authors: Han Daxing , Lorentzen J. D. , Weinberg-Wolf J. , McNeil L. E. , Wang Qi
Published: August 2003

The structure changes of thin films of amorphous (a) to microcrystalline (μc) silicon are studied by Raman scattering in terms of three deposition parameters: the silane flow rate, the hydrogen flow rate, and the total gas pressure in hot-wire chemical vapor deposition. The Raman transverse optical (TO) mode is deconvoluted into two Gaussian functions for a-Si:H and intermediate components and one Lorenzian function for the c-Si component. We found that (a) in general, the change in structure is a function of the ratio of hydrogen to silane gas flow, R, but also depends on the SiH4 flow rate and total gas pressure; (b) there is a narrow structural transition region in which the short-range order of the a-Si:H network improves, i.e., the variation in bond angle of the a-Si network decreases from ∼10° to ∼8° once the c-Si grains start to grow; and (c) when the films were deposited using a high SiH4 flow rate of 22 sccm, the narrow TO mode with low peak frequency could be related to the column-like structures.
Deep level transient spectroscopic study of neutron-irradiated n-type 6H–SiC
Authors: Chen X. D. , Fung S. , Ling C. C. , Beling C. D. , Gong M.
Published: August 2003

Deep level transient spectroscopy has been employed to study the deep level defects introduced in n-type 6H–SiC after neutron irradiation. Deep levels situated at EC−0.23, EC−0.36/0.44, EC−0.50, and EC−0.62/0.68 eV have been detected in the temperature range of 100–450 K, which have been identified with the previously reported deep levels ED1, E1/E2, Ei, and Z1/Z2, respectively. Thermal annealing studies of these deep levels reveal that ED1 and Ei anneal at a temperature below 350 °C, the Z1/Z2 levels anneal out at 900 °C, while the intensity of the E1/E2 peaks is increased with annealing temperature, reaching a maximum at about 500–750 °C, and finally annealing out at 1400 °C. The possible nature of the deep levels ED1, E1/E2, Ei, and Z1/Z2 are discussed in the context of their annealing behavior. Upon further annealing at 1600 °C, four deep levels labeled NE1 at EC−0.44 eV, NE2 EC−0.53 eV, NE3 EC−0.64 eV, and NE4 EC−0.68 eV are produced. Evidence is given that these levels are different in their origin to E1/E2 and Z1/Z2.
Molecular-beam epitaxial growth and characterization of quaternary III–nitride compounds
Authors: Monroy E. , Gogneau N. , Enjalbert F. , Fossard F. , Jalabert D. , Bellet-Amalric E. , Dang Le Si , Daudin B.
Published: August 2003

We report on the controlled growth and characterization of quaternary AlGaInN compounds by plasma-assisted molecular beam epitaxy. Two-dimensional growth is achieved with a monolayer of In segregating at the growth front. In incorporation is hindered by increasing growth temperature and Al mole fraction, which is explained by the lower binding energy of InN compared to GaN and AlN. The mosaicity of the layers is determined by the substrate quality, whereas the alloy disorder increases with the Al content, independent of the In mole fraction. Room temperature photoluminescence is dominated by a narrow band-edge emission, whose Stokes shift and activation energy increase with the In content. This behavior is interpreted in terms of carrier localization in self-formed alloy inhomogeneities. An In-related band bowing parameter of 2.5 eV has been estimated.
Far-infrared spectroscopy in ordered and disordered BaMg1/3Nb2/3O3 microwave ceramics
Authors: Dias Anderson , Moreira Roberto Luiz
Published: August 2003

Ba ( Mg 1/3 Nb 2/3 ) O 3 ceramics with suitable microwave dielectric properties for application in wireless communications and information access technologies were studied by far-infrared spectroscopy. Samples with different B-site ordering degrees, obtained by hydrothermal synthesis followed by sintering at various temperatures, were employed in this investigation. The sixteen infrared modes predicted by factor-group analysis were observed and adjusted according to a four-parameter semiquantum model. The dispersion parameters were determined in order to calculate the real part of the dielectric permittivity and the quality factors associated with the dielectric losses in the microwave region. The materials exhibited increasing ε0 and Q values up to 1100 °C, increasing more substantially when the temperature attained 1300 °C. The B-site ordering played an important role on this behavior along with the microstructural evolution above 1100 °C (grain growth), which also increased the phonon lifetime and contributed to the Q improvement. Kramers–Kronig analyses were carried out in all experimental data and the contributions of the main optical polar modes to the dielectric and microwave properties were carefully analyzed in order to identify and attribute the Ba–BO3 external mode, the inner modes related to the O–Mg–O and O–Nb–O bending vibrations, and the stretching modes of each MgO6 and NbO6 octahedron.
Effect of material properties on the packing of fine particles
Authors: Yang R. Y. , Zou R. P. , Yu A. B.
Published: August 2003

The packing of fine particles differs from that of coarse particles because of the strong cohesive interparticle forces. Consequently, material properties of particles have a strong effect on the packing structure of fine particles. This article presents a study of this effect by means of discrete element method. Variables considered include sliding and rolling friction coefficients related to the surface forces, and Hamaker constant and particle density related to the body forces acting on a particle. The results are analyzed in terms of porosity, mean coordination number and radial distribution function. It is shown that porosity increases with sliding and rolling friction coefficients as well as the Hamaker constant, but decreases with particle density, and coordination number and radial distribution function vary with these variables in line with porosity. The results have also been used to link the macrostructural property such as porosity to microstructural properties such as coordination number and radial distribution function. It is demonstrated that porosity can be described as a function of the force ratio between the van der Waals force and gravity force on a particle but the relationship varies with the sliding and rolling friction coefficients.
Demonstration of a blueshift in type II asymmetric InP/InAsP/InGaAs multiple quantum wells
Authors: Haywood S. K. , Lim A. C. H. , Gupta R. , Emery S. , Hogg J. H. C. , Hewer V. , Stavrinou P. N. , Hopkinson M. , Hill G.
Published: August 2003

Room temperature photocurrent measurements were carried out on two InAsxP1−x/In0.53Ga0.47 As asymmetric quantum wells with InP barriers, grown by molecular beam epitaxy. The lowest energy exciton transition in these samples was close to 1.5 μm. A significant blueshift of this transition was observed on application of an electric field. We attribute this blueshift to the type II band lineup that exists between InAsP and InGaAs for the samples studied. The resulting stepped well causes separation of the electron and hole wave functions at zero bias, which is then reduced by the applied field. X-ray diffraction spectra indicated layer thicknesses and compositions to be close to the intended values. However, to fit the observed transition energies, spectral shifts, and x-ray data simultaneously, it was necessary to assume low levels of arsenic contamination in the InP barriers and of gallium in the InAsP layers. From these results, we infer the maximum value of the conduction band offset (ΔEc) for InAsP/InP to be approximately 65% of the band-gap difference (ΔEg).
Structure and effects of double-positioning twin boundaries in CdTe
Authors: Yan Yanfa , Al-Jassim M. M. , Jones K. M.
Published: August 2003

Using the combination of high-resolution transmission electron microscopy, first-principles density-functional total-energy calculations, and image simulations, we studied the atomic structure and effects of double-positioning twin boundaries in CdTe. The structure for the double-positioning twin boundaries is found to contain more Te dangling bonds than Cd dangling bonds. The two sides of the boundaries have the same polarities. The structure produces energy states in the band gap that are detrimental to the electronic properties of CdTe.
Observation of radial propagation of electrostatic fluctuations in the Saskatchewan torus-modified tokamak
Authors: Singh A. K. , Morelli J. , Asai T. , Hirose A.
Published: August 2003

Radial propagation of electrostatic fluctuations in the edge region of the Saskatchewan torus-modified [W. Zhang et al., Phys. Fluids B 4, 3277 (1992)] tokamak has been measured using Langmuir probes. The propagation characteristics of the density fluctuations are analyzed by two point correlation techniques. A finite radial propagation speed is measured and is found to be a large fraction of the poloidal propagation speed. The speed of propagation is inversely proportional to the toroidal magnetic field, suggesting that the origin of these fluctuations could be drift wave type turbulence. The radial correlation length of turbulence is found to decrease with increasing magnetic field. The power averaged wave number shows an increase with increasing magnetic field, suggesting a decrease in the correlation lengths.
Absorption of terahertz radiation by plasmon modes in a grid-gated double-quantum-well field-effect transistor
Authors: Popov V. V. , Polischuk O. V. , Teperik T. V. , Peralta X. G. , Allen S. J. , Horing N. J. M. , Wanke M. C.
Published: August 2003

The terahertz absorption spectrum of plasmon modes in a grid-gated double-quantum-well (DQW) field-effect transistor structure is analyzed theoretically and numerically using a first principles electromagnetic approach and is shown to faithfully reproduce important physical features of recent experimental observations. We find that the essential character of the response—multiple resonances corresponding to spatial harmonics of standing plasmons under the metal grating—is caused by the static spatial modulation of electron density in the channel. Higher order plasmon modes become more optically active as the depth of the electron density modulation in the DQW tends towards unity. The maximum absorbance, at plasma resonance, is shown to be 1/2. Furthermore, the strongest absorption also occurs when the standing plasmon resonance coincides with the fundamental dipole mode of the ungated portion of the channel.
Brillouin light scattering study of Langmuir–Blodgett films: Elastic properties versus thickness
Authors: Bonello B. , Djemia Ph. , Ganot F. , Rouaı̈ L. , Louis G.
Published: August 2003

Brillouin light scattering was used to measure the elastic properties of a set of Langmuir–Blodgett films with varying thickness. Samples are constituted of stacks from 3 to 75 monolayers of a octabutoxyphthalocyanine copper derivative, deposited on Si substrates by the Langmuir–Blodgett (LB) technique. Due to the strong contrast in the elastic properties of the organic films and the substrate, it is shown that an elastic anomaly, if any, should be revealed by the Rayleigh mode and the first Sezawa mode which both strongly depend on the value of C44. Rayleigh and Sezawa dispersion curves are computed assuming that the elastic constants do not depend on the number of individual monolayers in the LB film. Within the accuracy of the experimental data, the theory well reproduces the trend of the effective velocities, showing so that the elastic properties of the material do not depend on the number of molecular monolayers in the stack.
“Massive” spin-2 field in de Sitter space
Authors: Garidi T. , Gazeau J-P. , Takook M. V.
Published: August 2003

In this paper we present a covariant quantization of the “massive” spin-2 field on de Sitter (dS) space. By “massive” we mean a field which carries a specific principal series representation of the dS group. The work is in the direct continuation of previous ones concerning the scalar, the spinor, and the vector cases. The quantization procedure, independent of the choice of the coordinate system, is based on the Wightman-Gärding axiomatic and on analyticity requirements for the two-point function in the complexified pseudo-Riemanian manifold. Such a construction is necessary in view of preparing and comparing with the dS conformal spin-2 massless case (dS linear quantum gravity) which will be considered in a forthcoming paper and for which specific quantization methods are needed.
Detecting ill posed boundary conditions in general relativity
Authors: Calabrese Gioel , Sarbach Olivier
Published: August 2003

A persistent challenge in numerical relativity is the correct specification of boundary conditions. In this work we consider a many-parameter family of symmetric hyperbolic initial-boundary value formulations for the linearized Einstein equations and analyze its well posedness using the Laplace–Fourier technique. By using this technique ill posed modes can be detected and thus a necessary condition for well posedness is provided. We focus on the following types of boundary conditions: (i) boundary conditions that have been shown to preserve the constraints, (ii) boundary conditions that result from setting the ingoing constraint characteristic fields to zero, and (iii) boundary conditions that result from considering the projection of Einstein’s equations along the normal to the boundary surface. While we show that in case (i) there are no ill posed modes, our analysis reveals that, unless the parameters in the formulation are chosen with care, there exist ill posed constraint violating modes in the remaining cases.
Calibration of heavy ion beam probe energy analyzer using mesh probe in the JFT-2M tokamak
Authors: Kamiya K. , Miura Y. , Ido T. , Hamada Y.
Published: August 2003

A direct calibration technique for a heavy ion beam probe (HIBP) parallel plate electrostatic energy analyzer using the mesh probe is established. When an ac bias voltage (±1.0 kV, 50 Hz) is applied to the inner mesh box and He gas is puffed into the vacuum vessel after setting the sample volume within the inner mesh box, it is confirmed that the normalized difference (ND) depends linearly on the applied bias voltage. The slope of the ND with respect to the bias voltage, d(ND)/d(Φbias), shows the clear dependence on the in-plane entrance angle to the analyzer slit that is expected from the ideal analyzer model. Although the mesh transparency of the beam is about 40%, good signal to noise ratio is obtained with a beam current of about 10 μA, and the uncertainty for an absolute calibration of the energy analyzer is less than 10%. Calibration results are in good agreement with previous results using a standard gas target method.
Detection of multiple fluorescent labels using superconducting tunnel junction detectors
Authors: Fraser G. W. , Heslop-Harrison J. S. , Schwarzacher T. , Holland A. D. , Verhoeve P. , Peacock A.
Published: August 2003

We show that cryogenically cooled, photon counting superconducting tunnel junctions (STJs) can be used to simultaneously record the optical spectra from multiple biological fluorochromes. Measurements with a single-pixel tantalum STJ with a wavelength resolving power R=λ/Δλ of about 10 confirm the expected sensitivity advantage with respect to the photomultiplier-based detectors commonly used to record signals from microarrays and other fluorescent biological systems.
A highly sensitive scanning far-infrared microscope with quantum Hall detectors
Authors: Ikushima Kenji , Sakuma Hisato , Komiyama Susumu
Published: August 2003

We develop a highly sensitive scanning far-infrared (FIR) microscope, which consists of a silicon solid immersion lens that probes FIR and a condenser lens that focuses the FIR onto a small quantum Hall detector (400 μm×400 μm). The solid immersion lens is in contact with the backside of a Hall bar sample, which is moved with a mechanical XY stage. The quantum Hall detector, which function as a narrow band FIR detector (bandwidth of about 2%), is a Hall bar with a large length-to-width ratio in integer quantum Hall effect regimes. The microscope is successfully applied to image extremely weak cyclotron emissions from quantum Hall devices with a spatial resolution of about 50 μm and a signal-to-noise ratio improved by a factor 18 compared to a previous system.
Oscillatory magnetic tweezers based on ferromagnetic beads and simple coaxial coils
Authors: Trepat Xavier , Grabulosa Mireia , Buscemi Lara , Rico Fèlix , Fabry Ben , Fredberg Jeffrey J. , Farré Ramon
Published: August 2003

We report the design and validation of simple magnetic tweezers for oscillating ferromagnetic beads in the piconewton and nanometer scales. The system is based on a single pair of coaxial coils operating in two sequential modes: permanent magnetization of the beads through a large and brief pulse of magnetic field and generation of magnetic gradients to produce uniaxial oscillatory forces. By using this two step method, the magnetic moment of the beads remains constant during measurements. Therefore, the applied force can be computed and varies linearly with the driving signal. No feedback control is required to produce well defined force oscillations over a wide bandwidth. The design of the coils was optimized to obtain high magnetic fields (280 mT) and gradients (2 T/m) with high homogeneity (5% variation) within the sample. The magnetic tweezers were implemented in an inverted optical microscope with a videomicroscopy-based multiparticle tracking system. The apparatus was validated with 4.5 μm magnetite beads obtaining forces up to ∼2 pN and subnanometer resolution. The applicability of the device includes microrheology of biopolymer and cell cytoplasm, molecular mechanics, and mechanotransduction in living cells.
Frequency dependence of capacitance standards
Authors: Wang Yicheng
Published: August 2003

The frequency dependence of a 10 pF transportable fused-silica capacitance standard was measured from 50 Hz to 20 kHz. Its capacitance shows a broad minimum from 1592 Hz to 4 kHz. Due to dielectric relaxation, the capacitance at 50 Hz is fractionally 1.2×10−6 over the minimum; due to residual inductance, the capacitance at 20 kHz is 0.68×10−6 over the minimum. The results have a relative standard uncertainty of 0.61×10−6, 0.32×10−6, 0.15×10−6, and 0.37×10−6 at 50 Hz, 100 Hz, 400 Hz, and 20 kHz, respectively. This will reduce the expanded standard uncertainties for the capacitance calibrations at NIST by a factor of three or four. For this work, we used two reference capacitors. The first reference was a 1 pF cross capacitor chosen for its negligible frequency dependence at low frequencies. The second reference was a 10 pF nitrogen-filled capacitor chosen because it has a very small inductance, thereby reducing problems at higher frequencies.
Molecular-beam/surface-science apparatus for state-resolved chemisorption studies using pulsed-laser preparation
Authors: Schmid Mathieu P. , Maroni Plinio , Beck Rainer D. , Rizzo Thomas R.
Published: August 2003

We describe a new apparatus that combines pulsed laser excitation in a molecular beam with surface-science methods for preparation of clean single-crystal surfaces and detection of adsorbates to enable state-selected studies of gas–surface reaction dynamics. Reactant molecules are prepared in specific vibrationally excited states via overtone pumping using tunable, narrow-band laser radiation. The collision-free environment of the molecular beam prevents relaxation of the prepared molecules before impact on the target surface and enables complete control over the collision energy and incidence angle. Chemisorption products are detected after a given deposition time by Auger electron spectroscopy. To achieve sufficient beam flux of state-selected reactant molecules for product detection by standard surface-science techniques, we use a high-intensity, short-pulse molecular-beam source matched to the low duty cycle of the pulsed lasers used in our experiments. We present the design and characterization of this new apparatus together with a scheme for generating infrared laser pulses of high spectral brightness for saturating weak vibrational overtone transitions within a significant volume of the molecular beam. The effectiveness of our approach is demonstrated by state-resolved sticking coefficient measurements for overtone-excited (2ν3) CH4 on Ni(100) for a range of impact energies.
Magnetic tweezers for intracellular applications
Authors: Hosu Basarab G. , Jakab Károly , Bánki Péter , Tóth Ferenc I. , Forgacs Gabor
Published: August 2003

We have designed and constructed a versatile magnetic tweezer primarily for intracellular investigations. The micromanipulator uses only two coils to simultaneously magnetize to saturation micron-size superparamagnetic particles and generate high magnitude constant field gradients over cellular dimensions. The apparatus resembles a miniaturized Faraday balance, an industrial device used to measure magnetic susceptibility. The device operates in both continuous and pulse modes. Due to its compact size, the tweezers can conveniently be mounted on the stage of an inverted microscope and used for intracellular manipulations. A built-in temperature control unit maintains the sample at physiological temperatures. The operation of the tweezers was tested by moving 1.28 μm diameter magnetic beads inside macrophages with forces near 500 pN.
Setup for in situ surface investigations of the liquid/glass transition with (coherent) x rays
Authors: Seydel Tilo , Madsen Anders , Sprung Michael , Tolan Metin , Grübel Gerhard , Press Werner
Published: August 2003

A dedicated setup comprising an efficient cryogenic device for the in situ preparation of large surface areas of prototypical organic glass formers in a wide temperature range (170–340 K) is presented. This setup provides the necessary temperature and vibrational stability for surface x-ray and neutron scattering experiments, including the extremely sensitive technique of x-ray photon correlation spectroscopy (XPCS). XPCS is an emerging method which is made possible by the high coherent photon flux produced by third-generation synchrotrons. We demonstrate that microscopic motion at the surface can be studied in a direct way in the liquid and supercooled state using XPCS. In addition, we have used a charge-coupled-device detector to record two-dimensional images of static speckle patterns forming on surfaces in the glassy state.
Signal-to-noise ratio improvement by sideband intermixing: Application to Doppler ultrasound vibrometry
Authors: Petculescu Andi G. , Sabatier James M.
Published: August 2003

A simple technique for improving the sensitivity of measuring devices based on angle modulation of a continuous-wave carrier is presented. The proposed approach involves the addition of a second carrier signal separated by twice the modulation frequency. In the Fourier domain, the upper sideband of the first carrier and the lower sideband of the second carrier intermix. By sweeping the relative phase between the two carriers, the magnitude of the resultant sideband can grow up to double the original value. In noisy environments, this effect can lead to an improvement of the signal-to-noise ratio. As a proof of concept, the technique is applied to a Doppler ultrasonic vibration sensor, with encouraging results.
High-resolution state-selected ion-molecule reaction studies using pulsed field ionization photoelectron-secondary ion coincidence method
Authors: Qian X.-M. , Zhang T. , Chang C. , Wang P. , Ng C. Y. , Chiu Yu-Hui , Levandier Dale J. , Miller J. Scott , Dressler Rainer A. , Baer Tomas , Peterka Darcy S.
Published: August 2003

We have developed an octopole-quadrupole photoionization apparatus at the Advanced Light Source for absolute integral cross-section measurements of rovibrational-state-selected ion-molecule reactions. This apparatus consists of a high-resolution photoionization ion source, a wired ion gate lens, a dual radio-frequency (rf) octopole ion guide reaction gas cell, and a quadrupole mass spectrometer for reactant and product ion detection. The unique feature of this apparatus is the implementation of the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, which has allowed the rotational-state selection of diatomic ions for ion-molecule reaction studies. The novel application of the wired ion gate lens for the rejection of false coincidence background ions is described. This application, along with the differential-ion-gate scheme, has made possible the measurements of rovibrational-state-selected absolute integral reaction cross sections for ion-molecule collisions using the PFI-PE-secondary ion coincidence (PFI-PESICO) method. The successful measurement of absolute state-selected cross sections for H2+(X,v+,N+)+Ar(Ne) with v+ up to 17 [the third to the last vibrational state of H2+(X)] demonstrates the high sensitivity of this differential-ion-gate PFI-PESICO method. In order to gain a detailed understanding and to obtain optimal performance of the wired ion gate lens for PFI-PESICO measurements, we have carried out ion trajectory calculations of reactant ions between the photoionization region and the rf-octopole ion guide. On the basis of these calculations, possible future improvements for the application of this differential-ion-gate PFI-PESICO scheme are discussed.
Magnetomechanical testing machine for ferromagnetic shape-memory alloys
Authors: Shield T. W.
Published: August 2003

Recently, a new class of active materials, known as ferromagnetic shape-memory alloys, has shown promise as actuator and sensor materials. These alloys, such as Ni2MnGa and Fe3Pd, are ferromagnetic and undergo a reversible diffusionless structural phase transformation giving rise to shape-memory behavior. In order to explore the behavior of these materials it is necessary to combine both mechanical materials testing, magnetic field application, and magnetization measurements. To accomplish this, an apparatus called a magnetomechanical testing machine (MMTM) has been constructed. The MMTM combines a screw driven mechanical load frame with a dual-dipole electromagnet to load specimens both mechanically and magnetically. The pair of magnets allows the magnetic field to point in any direction in the plane containing the uniaxial loading axis of the mechanical test frame. In addition, the specimen temperature is controlled and the surface is observed microscopically. This apparatus is described and some preliminary measurements are reported. Saturation magnetization measurements for Fe3Pd made with the MMTM are found to be within 2% of previously reported values.
Quantum mechanics of damped systems
Authors: Chruściński Dariusz
Published: August 2003

We show that the quantization of a simple damped system leads to a self-adjoint Hamiltonian with a family of complex generalized eigenvalues. It turns out that they correspond to the poles of energy eigenvectors when continued to the complex energy plane. Therefore, the corresponding generalized eigenvectors may be interpreted as resonant states. We show that resonant states are responsible for the irreversible quantum dynamics of our simple model.
Nonlinear instability of thin current sheets in antiparallel and guided magnetic fields
Authors: Silin I. , Büchner J.
Published: August 2003

The influence of a current-aligned guide magnetic field on the nonlinear resonant instability of thin current sheets is investigated by means of three-dimensional Vlasov-code simulations. Similarly to the zero-guide field case, the pressure gradient excites lower-hybrid-drift (LHD) waves at the current sheet edges. However, since the LHD waves are excited perpendicular to the local magnetic field they propagate obliquely to the current direction. As a result, the number of resonant particles, i.e., the drift-resonance efficiency, decreases with increasing guide field strength. Hence, the driving of global current sheet kink/sausage instabilities becomes less efficient.
Equilibrium and resistive steady state of an axisymmetric co-axial helicity injection plasma
Authors: Tang X. Z. , Boozer A. H.
Published: August 2003

Steady state (∂B/∂t=∂v/∂t=0) coaxial helicity injection (CHI) has a purely electrostatic E =−∇Φ(ψ,θ) field. The electrostatic potential implies an E×B flow that scales with the externally imposed voltage and is negligibly affected by plasma dissipation. Both the electrostatic potential and the magnetic configuration in the resistive steady state description can be significantly different from those in a Grad-Shafranov open field line equilibrium for CHI discharges. The B×∇Φ flow in a resistive steady state induces localized perpendicular current for force balance with the plasma inertia. The perpendicular current can drive far stronger parallel current that is localized poloidally on a flux surface. This effect is pronounced for CHI discharges because the desirable CHI hollow current profile has interior high q and there is a magnetic X point in the neighborhood of the absorber, where a large ψ gradient of q and poloidal field strength exists.
Generation of nonlinear Alfvén and magnetosonic waves by beam–plasma interaction
Authors: Wang X. Y. , Lin Y.
Published: August 2003

One-dimensional (1-D) and two-dimensional (2-D) hybrid simulations are carried out to study the interaction between a background plasma and an ion beam, whose velocity is parallel to the ambient magnetic field B0. It is found that the beam–plasma interaction and the associated wave evolution can be divided into four phases. The simulation results in phase 1 in the early stage of wave evolution are consistent with the linear theory. Right-hand nonresonant instabilities are present and dominant in cases with a relatively strong ion beam (e.g., the ratio of beam ion density to background ion density >0.06 for beam velocity =10VA, where VA is the Alfvén speed), while right-hand resonant instabilities are present in the weak beam cases. During phases 2 and 3, the waves grow to form nonlinear structure, and are then saturated. A detailed analysis shows that the wave evolution in these phases is through secondary instabilities associated with parametric decay or the wave modulation. In addition, it is shown for the first time from the self-consistent simulation that in the final phase, nonlinear shear Alfvén waves with right-hand polarization in the magnetic field are generated. The magnetohydrodynamic (MHD) wave conditions of the Alfvén mode are satisfied. These Alfvén waves propagate mainly with k⋅B0>0, and the dispersion relation ω=kVA cos α is satisfied, where α is the angle between the wave vector k and B0. On the other hand, fast magnetosonic/whistler waves and slow mode waves are formed in the final phase of weak beam cases. In the 2-D simulations, field-aligned filaments (with k∼k⊥) in the density and magnetic field can be present due to the 2-D effects, in addition to the Alfvén, fast, and slow modes. The heating rate of background ions and its dependence on the wave propagation direction are also examined.
Dependence of interface roughness and diffuseness of Cu–Co electrodeposited multilayers on electrochemical additives
Authors: Merkourakis Spyridon , Hÿtch Martin J. , Chassaing Elisabeth , Walls Michael G. , Leprince-Wang Yamin
Published: August 2003

We examine the effect of two organic additives, sds and saccharin, and also the effect of the solution pH on the interface properties of Cu/Co nanolayers, produced by pulsed electrodeposition from a single aqueous bath. Quantitative Fresnel fringe transmission electron microscopy is applied to cross-sectional samples of the layers. The widths of their respective interfaces as well as the widths of individual Cu and Co layers are determined via comparison with computer simulations. These initial results are further numerically treated to yield information about the separate contributions of interdiffusion and roughness to total interface widths. Conclusions on the behavior of these organic additives are considered in the light of the giant magnetoresistance properties of the multilayers, as reported in previous work.
Influence of duty cycle on the structure and secondary electron emission properties of MgO films deposited by pulsed mid-frequency magnetron sputtering
Authors: Cheng Y. H. , Kupfer H. , Richter F.
Published: August 2003

MgO films were deposited by using a pulsed mid-frequency magnetron sputtering technique. X-ray diffraction, atomic force microscopy, Rutherford backscattering, and a diode discharge device were used to characterize crystalline structure, surface morphology, oxygen content, and secondary electron emission (γ) coefficient of the films, respectively. The influence of the duty cycle on the structure and secondary electron emission properties of the films was systematically studied. No remarkable change in the oxygen content in the films with duty cycle can be observed, but crystalline structure, surface morphology, and secondary electron emission properties of the films are significantly affected by the duty cycle. All films exhibit a strong (220) preferred orientation growth, but the increase of the duty cycle results in a continuous decrease in grain size, surface roughness, and γ coefficient, as well as an increase in the defect density. The increase of the defect density with increasing duty cycle is the main reason for the reduction of the γ coefficient.
Absorption of surface acoustic waves by a two-dimensional electron gas in the presence of spin-orbit interaction
Authors: Xu W. , Lin L. B. , Tang C. S.
Published: August 2003

A theoretical study is presented for interactions between surface acoustic waves (SAWs) and a two-dimensional electron gas (2DEG) in the presence of spin-orbit (SO) interaction (SOI) induced by the Rashba effect. It is found that the presence of the SOI in a 2DEG can open up new channels for electronic transitions. As a result, an enhanced absorption of the SAWs by a 2DEG can be achieved through intra- and inter-SO electronic transition around the Fermi level. These results indicate that spintronic systems can be the candidate of the SAW devices.
Adaptive ferroelectric states in systems with low domain wall energy: Tetragonal microdomains
Authors: Jin Y. M. , Wang Y. U. , Khachaturyan A. G. , Li J. F. , Viehland D.
Published: August 2003

Ferroelectric and ferroelastic phases with very low domain wall energies have been shown to form miniaturized microdomain structures. A theory of an adaptive ferroelectric phase has been developed to predict the microdomain-averaged crystal lattice parameters of this structurally inhomogeneous state. The theory is an extension of conventional martensite theory, applied to ferroelectric systems with very low domain wall energies. The case of ferroelectric microdomains of tetragonal symmetry is considered. It is shown for such a case that a nanoscale coherent mixture of microdomains can be interpreted as an adaptive ferroelectric phase, whose microdomain-averaged crystal lattice is monoclinic. The crystal lattice parameters of this monoclinic phase are self-adjusting parameters, which minimize the transformation stress. Self-adjustment is achieved by application of the invariant plane strain to the parent cubic lattice, and the value of the self-adjusted parameters is a linear superposition of the lattice constants of the parent and product phases. Experimental investigations of Pb(Mg1/3Nb2/3)O3–PbTiO3 and Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals confirm many of the predictions of this theory.
On the origin of “intermittency” in the scrape-off layer of linear magnetic confinement devices
Authors: Antar G. Y.
Published: August 2003

The goal of this article is to show that the “intermittent” convective transport in the scrape-off layer (SOL) of linear magnetic confinement plasmas actually takes place periodically. This is reflected in several features that are unveiled in this article. It is found that the distribution of the time between high-amplitude bursts, selected above a given threshold, has a central frequency in the SOL. Moreover, this distribution is found similar to the one taken inside the main plasma column as the threshold is reduced, suggesting links between the SOL and the main plasma column. The power spectrum of the radial flux is determined, indicating the presence of a coherent peak that accounts for about 50% of the transport. Using two probes that are poloidally and radially separated, it is shown that a burst in the SOL is correlated to a wave packet inside the main plasma column. The relation between the two distant regions is thus assured by avaloids that are nonlocal structures in nature as they have velocities much higher than the background.
The influence of the arc current on the cold electrode erosion
Authors: Essiptchouk Alexei M. , Marotta A. , Sharakhovsky Leonid I.
Published: August 2003

A study of copper cathode erosion in a magnetically driven arc setup, as a function of arc current, is reported. The experiments showed the presence of two different erosion regimes. In the low current micro-erosion regime, erosion slowly rises with arc current. Then, at a certain critical current, macro-erosion begins, and has a strong dependence of erosion on current. It was shown that the critical current for transition from micro- to macro-erosion is a linearly decreasing function of the applied magnetic field. The erosion versus current behavior is shown to be explained by a recently published thermal model.