bluuebunny/crossref_metadata_2025 huggingface dataset:
Effusive-flow characterization of arbitrary size and geometry target/vapor transport systems: RIB applications
Authors: Bilheux J.-C. , Alton G. D. , Liu Y. , Murray S. N. , Williams C. L.
Published: July 2002

The principal factors that limit intensities of short-lived radioactive ion beams produced by the isotope separator on-line technique are time delays due to diffusion of radioactive species from solid or liquid target materials and their effusive-flow transport to the ion source. Although diffusion times can be reduced by proper design of short diffusion length, highly refractory targets, effusive-flow times are more difficult to assess. After diffusion from the target material, the species must travel through the target material and vapor transport system to the ion source. The time required for effusive-flow transport to the ion source depends on the conduction path, chemical reactions between the species and target material and materials of construction, as well as the physical size and geometry of the transport system. We have developed a fast valve (0.1 ms closing time) for introducing gaseous or vapor-state species into the target/vapor transport/ion source/system that permits measurement of effusive-flow times for any gaseous or vaporous species (chemically active or chemically inactive) through any vapor transport system, independent of size and geometry. Characteristic times are determined from the exponential decay of the momentum analyzed ion beam intensity for the species during effusive flow through the vapor transport system under evaluation. This article describes the effusive-flow apparatus and presents characteristic time spectra and characteristic effusive-flow time data for noble gases flowing through a serial-flow target reservoir system.
Challenges and applications of high brightness beams (plenary)
Authors: Gardner I. S. K.
Published: July 2002

High intensity proton and H− beams are being used and proposed for a wide variety of applications such as spallation neutron sources, drivers for neutrino factories, and accelerator driven systems for waste transmutation and energy production. In cases involving storage rings or synchrotrons the designs are dependent on the production of intense, low emittance, chopped H− beams. Some of these applications are described and the effect of ion source performance on the designs is examined.
Concentrated colloidal dispersions: On the relation of rheology with small angle x-ray and neutron scattering
Authors: Versmold Heiner , Musa Sameer , Bierbaum Axel
Published: July 2002

Although rheology as well as neutron and x-ray scattering studies of ordered dispersions have been performed no coherent picture is discussable. In this paper we report our synchrotron x-ray and neutron scattering data of concentrated, charge stabilized, monodisperse polymer colloids and compare them with rheological data. We found that the scattering data can be understood in terms of viscoelastic flow. When ordered by flow-, solid- and liquid-like behavior can be separated with two independent experiments: (a) The solid-like character can be seen at perpendicular incidence as Bragg reflections, (b) the liquid-like character becomes apparent by sample rotation as the intensity distribution along Bragg rods.
Importance of Cs adsorption on the cold surface in H− ion source
Authors: Ogasawara M.
Published: July 2002

A model for the time variation of the Cs coverage on the plasma grid (PG) surface is made and confirmed theoretically. Final coverage is determined by the sum of gaseous and adsorbed Cs on the PG surface. The sum is equal to the Cs not yet adsorbed on the cold surface. The sum decreases on account of the adsorption on the cold surface with time of order of hours. The time of the decrease of the sum is related to the quantity of the seeded Cs. For the case of 600 mg Cs seeding, the sum relaxes to some nonzero value, which gives the optimum coverage on the PG surface. The optimum coverage lasts long. For less than 600 mg Cs seeding the sum relaxes to zero and it makes the coverage go to zero with time of order of hours. The mechanism is shown and an illustrative model calculation is given of the hysteresis in the negative ion current seen when PG temperature is increased and decreased.
Application of electron cyclotron resonance plasma in arched magnetic mirrors to deposition of TiO2 thin films
Authors: Honbo E. , Miyata M. , Kato Y. , Ishii S.
Published: July 2002

We have been able to produce electron cyclotron resonance plasmas for large-area processing in arched magnetic mirrors. Applying the plasmas to deposition of titanium dioxide thin films by reactive sputtering, we study the uniformity of deposition and crystal structures of the thin films by controlling the state of the plasma. Sputtering voltage and target current are varied separately by changing the power of microwave. The deposition rate of films is almost uniform within an area of magnets forming the mirrors. It is slightly higher at the center and lower at the edges of target. The crystal structure of thin films is changed by the sputtering conditions. At the slow deposition rates of films, the films are mostly in the rutile phase. At the high deposition rates, the anatase phase shows up together with the rutile one. We suppose that the crystal structure is related to the plasma profile.
Generation of intense beams of Pb4+–Pb10+ ions in a laser ion source (abstract)
Authors: Kondrashev S. , Mescheryakov N. , Sharkov B. , Shumshurov A. , Balabaev A. , Logkin A. , Konukov K. , Khomenko S. , Makarov K. , Nishchuk S. , Roerich V. , Satov Y. , Smakovskii Y. , Stepanov A.
Published: July 2002

Experiments were carried out to optimize the yield of Pb4+ ions from a plasma produced by a 100 J CO2 laser. Laser power density on the target surface was varied in the range 1–9×1010 W/cm2 by changing the focal spot size. Data on the generation of Pb4+–Pb10+ ions are presented for 15 and 40 ns laser pulse durations. A 10 mA/80 μs pulse of Pb4+ ions (about 1012 ions per pulse) was obtained in an extraction aperture with a diameter of 3.4 cm for optimal irradiation conditions. This is comparable with parameters of the MEVVA ion source. The laser ion source can be especially attractive due to the absence of the typical for MEVVA ion source “noise” problem. The emittance of a lead ion beam extracted from the laser-produced plasma was measured at a smaller laser providing parameters of plasma (electron temperature, ion velocities, and charge states) close to that of plasma generated by a 100 J CO2 laser optimized for the yield of Pb4+ ion conditions. The emittance was measured for different extraction voltages with temporal resolution. The value of about 400 pi mm mrad was found for the lead ion beam with a total current of 10 mA and pulse length of 75 μs.
Tandem surface–plasma source: A new concept for a dc negative ion source
Authors: Dimov G. I.
Published: July 2002

The newly developed tandem surface–plasma source (SPS) concept is expected to break new ground in the production of high-current, high-brightness H− ion beams. The tandem SPS consists of three successive separate stages: the multipole plasma driver; the H− ion generator with a low-voltage closed converter, and the ion extraction and beam formation system. To date the plasma driver stage has been designed, and is anticipated to produce an intense plasma stream with high efficiency (low energy-per-ion expenditure). The atomic processes in the second stage have been analyzed, the anticipated result is a dense population of H− ions with a very low temperature. The tandem SPS source is expected to produce dc beams of H− ions with currents up to 50 mA and pulsed H− beams at large duty factors with currents up to 100 mA.
Ensemble fluctuations and the origin of quantum probabilistic rule
Authors: Khrennikov Andrei
Published: July 2002

We demonstrate that the origin of the so-called quantum probabilistic rule (which differs from the classical Bayes’ formula by the presence of cos θ-factor) might be explained in the framework of ensemble fluctuations which are induced by preparation procedures. In particular, quantum rule for probabilities (with nontrivial cos θ-factor) could be simulated for macroscopic physical systems via preparation procedures producing ensemble fluctuations of a special form. We discuss preparation and measurement procedures which may produce probabilistic rules which are neither classical nor quantum; in particular, hyperbolic “quantum theory.”
Infrared resonance-enhanced multiphoton ionization spectroscopy of magnesium oxide clusters
Authors: van Heijnsbergen D. , von Helden G. , Meijer G. , Duncan M. A.
Published: July 2002

Neutral (MgO)n clusters are produced in a molecular beam by laser vaporization in a pulsed-nozzle cluster source. These clusters are ionized via multiphoton absorption from either an ultraviolet excimer laser or a far-infrared free electron laser. While ultraviolet ionization produces mass spectra consistent with previous measurements, infrared ionization produces higher molecular weight ions from the same nascent source distribution. Ultraviolet ionization occurs by direct electronic excitation/ionization, while infrared ionization occurs by vibrational excitation followed by thermionic electron emission. In both cases, prominent masses are observed corresponding to cubic nanocrystals with near equal x:y:z dimensions. By tuning the IR wavelength while recording the mass-resolved ion yield, vibrational spectra are obtained revealing two resonances near 16 and 22 microns. Clusters up to 300 atoms in size are studied, and spectra exhibit a gradual variation with size, converging to positions near to, but not matching the bulk phonon frequencies. Structural implications of these vibrational spectra are investigated.
Superconducting single-electron push–pull amplifier stage
Authors: Heij C. P. , Hadley P.
Published: July 2002

Two superconducting single-electron transistors were used to build a push–pull amplifier stage with a voltage gain of 5.2 that dissipates only 10 fW of power. This amplification stage would be most useful for measuring the voltage across a high-impedance sample at low temperature.
A differentially pumped pressure cell for in situ low-energy ion scattering analysis of catalysts during reactions
Authors: Jansen W. P. A. , v. d. Gon A. W. Denier , Wijers G. M. , Rikers Y. G. M. , Brongersma H. H. , v. d. Hoogen P. W. , de Laat J. A. M. , Maas T. M. , Dekkers E. C. A. , Brinkgreve P.
Published: July 2002

A differentially pumped pressure cell has been developed to enable in situ low-energy ion scattering (LEIS) analysis of catalysts during chemical reactions. The cell is fully compatible with an electrostatic analyzer and is, therefore, very well suited to study rough, highly dispersed catalysts. The pressure cell is a continuous flow cell (space velocity 1.2×103 s−1) and allows observation of dynamic surface reactions with various, perfectly mixed gases at fully controlled partial pressures and temperatures up to 800 K. The design decreases the pressure gap by three orders of magnitude. This offers ample opportunities such as the determination of specific adsorption sites, surface coverages of different species during reactions, and information on growth processes, poisoning or chemically induced segregation. As an example, an in situ LEIS study of the CO oxidation over Pt is presented. By combining in situ LEIS and quadrupole mass spectroscopy, both the surface and gas composition could be monitored during this reaction.
A rf discharge cell for saturated absorption spectroscopy of metastable argon
Authors: Sukenik C. I. , Busch H. C.
Published: July 2002

We have produced a rf discharge in Ar40 and used saturated absorption spectroscopy to offset lock a Ti:Sapphire laser to the absorption peak of the 43P2→43D3 cooling transition at 811 nm. We describe the procedure for fabrication of the cell and production of the discharge.
Polarity effect on the temperature dependence of leakage current through HfO2/SiO2 gate dielectric stacks
Authors: Xu Zhen , Houssa Michel , De Gendt Stefan , Heyns Marc
Published: July 2002

A strong polarity effect on the temperature dependence of the leakage current in TiN/HfO2/SiO2/Si capacitors is reported. A model is proposed to explain these experimental results that combines tunneling through the stack and Frenkel–Poole hopping in the HfO2 layer, depending on the value of the gate voltage. It is shown that the polarity effect most probably results from the anisotropy of the band diagram of the HfO2/SiO2 stack, as well as from the location of the shallow traps with respect to the conduction band of the HfO2 layer. Comparison of the model with the experimental results allows an estimate of the trap depth to be between 0.5 and 0.8 eV.
Highly ordered monocrystalline silver nanowire arrays
Authors: Sauer G. , Brehm G. , Schneider S. , Nielsch K. , Wehrspohn R. B. , Choi J. , Hofmeister H. , Gösele U.
Published: July 2002

Highly ordered silver nanowire arrays have been obtained by pulsed electrodeposition in self-ordered porous alumina templates. Homogeneous filling of all the pores of the alumina template is achieved. The interwire distance is about 110 nm corresponding to a density of silver nanowires of 61×109 in.−2 and the diameter can be varied between 30 and 70 nm. The silver wires are monocrystalline with some twin lamella defects and grow perpendicular to the 〈110〉 direction. The previously encountered difficulty to obtain 100% filling of the alumina pores is discussed in the framework of electrostatic instabilities taking into account the different potential contributions during electrodeposition. To obtain homogeneously filled pore membranes, a highly conductive metal containing electrolyte, a homogeneous aluminum oxide barrier layer, and pulsed electrodeposition are a prerequisite.
A frame-cooled plasma grid for long pulse operations in a cesium-seeded volume negative ion source
Authors: Fujiwara Yukio , Okumura Yoshikazu , Watanabe Kazuhiro , Trainham Rusty , Jacquot Claude
Published: July 2002

A frame-cooled plasma grid for the International Thermonuclear Experimental Reactor (ITER) was designed in view of temperature and stress distributions using three-dimensional numerical simulations. As a material of the plasma grid, copper–chrome–zirconium was examined, since it has high mechanical strength as well as high thermal conductivity. The numerical simulations indicated that the plasma grid would be kept at about 300 °C, which is optimum temperature for negative-ion production. They also indicated that thermal stress in the plasma grid would be less than yield stress. To demonstrate it, a frame-cooled plasma grid, which corresponds to a subsegment of the ITER plasma grid, was fabricated; an experiment was performed with a prototype of an ITER ion source named Kamaboko source. Experimental results showed that its surface temperature is continuously kept at about 300 °C; it was demonstrated that the frame-cooled plasma grid is applicable to long pulse operations, meeting the temperature requirement for the cesium effect.
A new experimental setup for the time resolved x-ray diffraction study of self-propagating high-temperature synthesis
Authors: Vrel D. , Girodon-Boulandet N. , Paris S. , Mazué J. F. , Couqueberg E. , Gailhanou M. , Thiaudière D. , Gaffet E. , Bernard F.
Published: July 2002

A new experimental setup for time resolved x-ray diffraction is described. Designed for the LURE H10 beamline and its 4 (+2) circles goniometer, it allows simultaneous recordings of x-ray patterns with a rate of 30 patterns per second, a maximum 2θ range of 120°, infrared thermography at the same rate, and thermocouples readings at a frequency of up to 3×104 Hz. Preliminary results obtained using this setup are presented, showing how it is possible to analyze a solid–solid or solid-liquid reaction. As an example, an in situ study of phase transformation and temperature evolution during the self-sustaining synthesis of an FeAl intermetallic compound starting from a mechanically activated mixture is investigated. The versatility of the setup was proved and could even be enhanced by the design of new sample holders, thus expanding its area of use at low cost.
Amorphous silicon thin-film transistors and arrays fabricated by jet printing
Authors: Wong W. S. , Ready S. , Matusiak R. , White S. D. , Lu J.-P. , Ho J. , Street R. A.
Published: July 2002

Phase-change wax-based printed masks, in place of conventional photolithography, were used to fabricate hydrogenated amorphous silicon thin-film transistors (TFTs). Wax-mask features with a minimum feature size of ∼20 μm were achieved using an acoustic-ink-printing process. Both discrete and matrix addressing structured bottom-gate TFTs with source–drain contacts overlapping the channel were created using a four-mask process. The TFTs had current–voltage characteristics comparable to photolithographically patterned devices, with mobility of 0.6–0.9 cm2/V s, threshold voltage of 2–3 V, and on/off ratios exceeding 107 for devices with channel lengths below 50 μm.
Report on the workshop on Advanced Diagnostics for Magnetic and Inertial Fusion Villa Monestero, Varenna, Italy, 3–7 September 2001
Authors: Wootton Alan J. , Stott Peter , Thomas Dan M.
Published: July 2002

This workshop was the eighth in a series of meetings on the theme of fusion diagnostics that has been held in Varenna at intervals between two and four years since 1975. Whereas previous meetings have concentrated on diagnostics for magnetic confinement fusion, this meeting had the specific purpose of bringing together diagnosticians from magnetic and inertial confinement fusion. The meeting concentrated on advanced diagnostics in these fields.
Enhancing the surface passivation of TiO2 coated silicon wafers
Authors: Richards B. S. , Cotter J. E. , Honsberg C. B.
Published: July 2002

In this letter, we demonstrate good surface passivation of lightly diffused n-type solar cell emitters using titanium dioxide (TiO2) thin films treated with a furnace oxidation process. Transient-photoconductance decay, x-ray photoelectron spectroscopy, and scanning electron microscopy measurements indicate that the silicon dioxide layer formed at the TiO2:Si interface provides excellent surface passivation. Emitter dark saturation current densities of 4.7×10−14 A/cm2 are achieved by this method, demonstrating that TiO2 films are compatible with high-efficiency solar cell structures.
Integral equation theory for the structure of DNA solutions
Authors: Shew Chwen-Yang , Yethiraj Arun
Published: July 2002

The static structure of solutions of DNA fragments is investigated using integral equation theory. The solution is modeled as a four-component system with DNA molecules, bound counterions, free counterions, and coions, all of which are treated explicitly. Each DNA fragment is modeled as a shish-kebab chain with three kinds of sites, i.e., charged sites, neutralized (protonated) sites, and sites with bound counterions. The partial structure factors are obtained using a generalization of the polymer reference interaction model. The undetermined parameters in the model, namely the fraction of protonated and bound sites, are obtained by fitting theoretical predictions for the polymer–polymer and polymer–counterions structure factors to experimental data. It is found that a large majority of counterions is localized near the DNA molecules due to counterions binding and protonation. The bound counterions make a dominant contribution to the total scattering from counterion species. The best fit is obtained when each DNA molecule contains about 22% protonated sites and 53% counterion occupied sites, i.e., the effective DNA charge fraction is about 0.25. This DNA charge fraction is consistent with electrospray ionization and DNA titration experiments.
Microelectromechanical system microbridge deflection monitoring using integrated spin valve sensors and micromagnets
Authors: Li Haohua , Boucinha M. , Freitas P. P. , Gaspar J. , Chu V. , Conde J. P.
Published: July 2002

Spin valve sensors are used to measure on chip, the deflection of microelectromechanical system resonators (bridges or cantilevers), with nm resolution. A spin valve sensor (10×2 μm2, MR=6.5%) was placed 3.2 μm away and 2.8 μm below the central region of an a-Si:H bridge, with a 1.2 μm air gap. The bridge movement was controlled by an applied voltage on the gate. In the undeflected state, a Co78Pt22 micromagnet (0.2 μm×18 μm×15 μm, Mrt=12.5 memu/cm2) deposited on top of the bridge, and saturated in an external field, creates a fringe field in the sensor shifting the transfer curve. An external bias field was then used to linearize the sensor response. Final sensor output versus bridge gate voltage was obtained using a low frequency square-wave gate excitation voltage (1 Hz). Bridge deflections from a few nm to 0.23 μm were detected.
Correlation time of density fluctuation for supercritical ethylene studied by dynamic light scattering
Authors: Saitow Ken-ichi , Ochiai Hiroto , Kato Tadashi , Nishikawa Keiko
Published: July 2002

Time dependence of density fluctuation for neat supercritical ethylene (C2H4) is investigated by dynamic light scattering at various densities in isothermal conditions of reduced temperatures Tr=T/Tc=1.02, 1.04, and 1.06. Time correlations of the density fluctuation for all thermodynamic states decay as single exponential functions with the time constant of submicrosecond. Critical slowing down of diffusive motions is observed in the time domain, and the correlation time of the density fluctuation becomes maximum at the extension of the gas–liquid coexistence curve on the P–T phase diagram. It is revealed that the time dependence of the density fluctuation just corresponds to the magnitude of static density fluctuation obtained by small-angle x-ray scattering measurements. By measuring correlation times as a function of scattering angle, the critical slowing down is thermodynamically discussed. It is elucidated experimentally that the critical slowing down of diffusive motions considerably correlates to the increase of specific heat capacity, the decrease of the thermal diffusivity, and rotational relaxation time in the supercritical state of the neat fluid.
Rotational spectra, structures, and dynamics of small Arm–(H2O)n clusters: The Ar–(H2O)2 trimer
Authors: Arunan E. , Emilsson T. , Gutowsky H. S.
Published: July 2002

Rotational-tunneling spectra for Ar–(H2O)2 and Ar–(D2O)2 have been observed with the Balle–Flygare Fourier transform microwave spectrometer. The tunneling levels of the trimer appear to correlate with those of the water dimer. The “a” dipole transitions from the A1+ and E+ states of Ar–(H2O)2 and A1+, B1+, and E+ states of Ar–(D2O)2 could be fit to a semirigid rotor Watson Hamiltonian. However, only the E+ states give “b” dipole transitions near rigid rotor predictions. The “b” dipole transitions for A1+ and B1+ are rotational-tunneling spectra. For Ar–(D2O)2, these transitions were observed and the donor–acceptor interchange tunneling splitting is determined as 106.3 MHz, compared to about 1100 MHz in the free (D2O)2. From this splitting, the barrier for interchange tunneling is calculated to be 642 cm−1. This splitting for Ar–(H2O)2 is estimated as 4–5 GHz. This and the spin statistical weight of 0 for the B1+ state have made it difficult to observe the “b” dipole rotational tunneling spectra for Ar–(H2O)2. From the rotational constants for (H2 18O) containing trimers, the O–O distance in the trimer is estimated as 2.945 Å. This is significantly (0.035 Å) shorter than the O–O distance reported for water dimer. The Ar is located on the “b” axis of the water dimer. Assuming the water to be a structureless sphere in the trimer, leads to Ar-c.m.(H2O) distance of 3.637 Å, very close to the same value in the Ar–H2O dimer.
Dependence of the second-order scalar structure function on the Schmidt number
Authors: Antonia R. A. , Orlandi P.
Published: July 2002

In this Brief Communication we consider the dependence of 〈(δθ)2〉, the second-order moment of the passive scalar increment, on the Schmidt number Sc, using direct numerical simulation data for decaying homogeneous turbulence over the range 0.07⩽Sc⩽7. Independently of Sc, Batchelor-normalized distributions of 〈(δθ)2〉 converge when the separation r approaches the Batchelor length scale. As Sc increases above 1, the shape of 〈(δθ)2〉 becomes inflectional, but the range of r for which 〈(δθ)2〉 may be considered to approach a plateau is not readily identifiable with the convective–diffusive range predicted by Batchelor [J. Fluid Mech. 5, 113 (1959)].
Stepwise morphological change of porous amorphous ice films observed through adsorption of methane
Authors: Horimoto Noriko , Kato Hiroyuki S. , Kawai Maki
Published: July 2002

Morphological change of amorphous ice films of D2O has been studied through adsorption of methane using thermal desorption spectroscopy (TDS) and infrared reflection absorption spectroscopy under ultrahigh vacuum. The investigated ice films were prepared under several different conditions; first, water (D2O) molecules are evaporated onto a Ru substrate at 25 K, and then subjected to an annealing process at various temperatures prior to methane deposition. On ice annealed at low temperatures, two desorption species of methane were observed in TDS: one was derived from methane adsorbed near the ice surface and the other was attributed to the desorption of methane encapsulated in ice during heating. Only the former species was observed when the annealing temperature exceeded 60 K. This indicates that reconstruction of ice occurs below 60 K, which inhibits the encapsulation of methane molecules from the amorphous ice. On the other hand, infrared spectra of ice covered with methane show that the micropores in ice start to collapse at ∼80 K, and ice becomes pore free at 120 K. We have found that the morphological change of amorphous ice induced by thermal relaxation takes place in a stepwise manner.
Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory
Authors: Nishimura Naoki , Hirai Tadahiko , Koganei Akio , Ikeda Takashi , Okano Kazuhisa , Sekiguchi Yoshinobu , Osada Yoshiyuki
Published: July 2002

We present here a magnetic tunnel junction device using perpendicular magnetization films designed for magnetic random access memory (MRAM). In order to achieve high-density MRAM, magnetic tunnel junction devices with a small area of low aspect ratio (length/width) is required. However, all MRAMs reported so far consist of in-plane magnetization films, which require an aspect ratio of 2 or more in order to reduce magnetization curling at the edge. Meanwhile, a perpendicular magnetic tunnel junction (pMTJ) can achieve an aspect ratio=1 because the low saturation magnetization does not cause magnetization curling. Magnetic-force microscope shows that stable and uniform magnetization states were observed in 0.3 μm×0.3 μm perpendicular magnetization film fabricated by focused-ion beam. In contrast, in-plane magnetization films clearly show the presence of magnetization vortices at 0.5 μm×0.5 μm, which show the impossibility of information storage. The PMTJ shows a magnetoresistive (MR) ratio larger than 50% with a squareness ratio of 1 and no degradation of MR ratio at 103 Ω μm2 ordered junction resistance.
Critical length and time scales during the initial stages of nucleation in polymer blends
Authors: Lefebvre A. A. , Lee J. H. , Balsara N. P. , Hammouda B.
Published: July 2002

The initial stages of nucleation during liquid–liquid phase separation in polymer mixtures were studied by time-resolved small angle neutron scattering. The time required for nucleation vanishes exponentially as the stability limit (spinodal) is approached. The critical nucleus size decreases monotonically with increasing quench depth and remains finite at the spinodal. Our data differ qualitatively from theoretical predictions.
Long time behavior of relativistic ions incessantly accelerated by an oblique shock wave
Authors: Usami Shunsuke , Ohsawa Yukiharu
Published: July 2002

Incessant acceleration of nonthermal fast ions by an oblique magnetosonic shock wave is studied with hybrid simulations. First, magnetic and electric field profiles of an oblique shock wave are obtained by means of a one-dimensional, relativistic, electromagnetic, particle simulation. Test particle trajectories of fast ions are then calculated using these wave profiles. Some fast ions are trapped by the shock wave owing to the relativistic effect that velocity is limited by the speed of light while the momentum can grow indefinitely. In these simulations, some fast ions are found to suffer energy jumps several tens of times; as a result, their final maximum Lorentz factors far exceeded 100.
Deformation potentials of the E1(TO) mode in AlN
Authors: Darakchieva V. , Paskov P. P. , Paskova T. , Birch J. , Tungasmita S. , Monemar B.
Published: July 2002

The deformation potentials of the E1(TO) mode in AlN are experimentally determined by combining infrared reflection spectroscopy and x-ray diffraction measurements and using a reported value of the Raman-stress factor for hydrostatically stressed bulk AlN. The deformation potentials are found to strongly depend on published stiffness constants of AlN. A comparison with earlier theoretically calculated values of the deformation potentials is made.
Theory of stimulated Raman adiabatic passage in a degenerated reaction system: Application to control of molecular handedness
Authors: Ohta Y. , Hoki K. , Fujimura Y.
Published: July 2002

We have developed a new type of stimulated Raman adiabatic passage (STIRAP) that is applicable to a degenerated reaction system. The direction of the photon polarization vector is the adiabatic parameter in the STIRAP. The molecular handedness of H2POSH, a preoriented phosphinotioic acid that has two stable configurations, L and R enantiomers, is used as a model system. The control of molecular handedness in both pure and mixed state cases are considered. In the case of a pure state, a STIRAP with a linearly polarized single laser allows an almost complete transfer from an L (R) enantiomer to the other by adiabatically changing its polarization direction. The adiabatic criterion for changing the polarization direction is clarified. In the case of a mixed state, a STIRAP with two linearly polarized laser pulses allows a selective preparation of pure enantiomers from its racemic mixture. In the low temperature limit, a five-level model reduces a three-level model by setting the direction of the polarization of the pump and Stokes pulses in such a way that only the forward transfer is allowed, while the reverse is forbidden. Furthermore, in the case of mixed state, relaxation effects originating from vibrational mode couplings are taken into account, and the influence of the population decay from intermediate states on the STIRAP is compared with that by a π-pulse approach.
Selective correlation scheme within diffusion quantum Monte Carlo
Authors: Schautz Friedemann , Flad Heinz-Jürgen
Published: July 2002

We present a selective correlation scheme allowing us to correlate only subsets of electrons, which can be assigned to arbitrary groups of orbitals, within diffusion quantum Monte Carlo calculations. The set of occupied orbitals, obtained from an all-electron mean-field calculation, is divided into two parts: frozen orbitals and explicitly considered orbitals. Electrons residing in frozen orbitals are excluded from the correlation treatment and handled within mean-field theory. The effects of such electrons on the remaining correlated electrons are represented by a model potential consisting of Coulomb and exchange parts, combined with a projectionlike operator to ensure orthogonality between the two sets of orbitals. Applying a localization procedure, similar to that used in connection with atomic semilocal pseudopotentials, to the exchange and projectionlike operators, local many-particle representations of these operators are obtained, which are suitable for use within quantum Monte Carlo calculations. While localizing the exchange part is rather straightforward, special care has to be taken to localize the projectionlike operator properly. As an illustrating example we consider the nitrogen dimer with the triple bond being correlated, while the nonbonding orbitals are kept frozen. By comparison with coupled cluster calculations, we demonstrate that with properly localized operators, the correlation energy of the triple bond can be quantitatively recovered.
Controlled polymer dewetting by physical confinement
Authors: Suh K. Y. , Park Joonhyung , Lee Hong H.
Published: July 2002

We report ordering of polymer drops that takes place when a thin polystyrene film confined by polydimethylsiloxane walls dewets on a silicon substrate. When annealed above the glass transition temperature, the thin polymer film dewets, resulting in the formation of a regular structure inside and outside the confinement. It is found that the ordering becomes strongly suppressed as the film thickness increases and the pattern size decreases due to the physical confinement of the mold. As a result, the ordering can take place only when the wavelength of the capillary wave is smaller than the characteristic length of the physical confinement.
Edge plasma density convection during ion cyclotron resonance heating on Tore Supra
Authors: Bécoulet M. , Colas L. , Pécoul S. , Gunn J. , Ghendrih Ph. , Bécoulet A. , Heuraux S.
Published: July 2002

The two-dimensional (2D) edge plasma density distribution in the scrape off layer magnetically connected to the ion cyclotron resonance heating (ICRH) antennae is studied experimentally and numerically in the tokamak Tore Supra. A local density decrease in front of the loaded ICRH antenna (“pump-out” effect) is demonstrated by Langmuir probe measurements in a low recycling regime. An up–down asymmetry in the heat-flux and in the antenna erosion is also observed, and is associated with poloidal variations of the local density. These density redistributions are ascribed to an E×B convection process linked with rf sheaths. To assess this interpretation, the 2D transport code CELLS for modeling the density distribution near an ICRH antenna was developed and described in this paper. The code takes into account perpendicular diffusion, parallel transport and convection in rf-sheath-driven potentials. The strong density differences obtained in simulations reproduce up–down asymmetries of the heat fluxes.
On the Hamiltonian replica exchange method for efficient sampling of biomolecular systems: Application to protein structure prediction
Authors: Fukunishi Hiroaki , Watanabe Osamu , Takada Shoji
Published: July 2002

Motivated by the protein structure prediction problem, we develop two variants of the Hamiltonian replica exchange methods (REMs) for efficient configuration sampling, (1) the scaled hydrophobicity REM and (2) the phantom chain REM, and compare their performance with the ordinary REM. We first point out that the ordinary REM has a shortage for the application to large systems such as biomolecules and that the Hamiltonian REM, an alternative formulation of the REM, can give a remedy for it. We then propose two examples of the Hamiltonian REM that are suitable for a coarse-grained protein model. (1) The scaled hydrophobicity REM prepares replicas that are characterized by various strengths of hydrophobic interaction. The strongest interaction that mimics aqueous solution environment makes proteins folding, while weakened hydrophobicity unfolds proteins as in organic solvent. Exchange between these environments enables proteins to escape from misfolded traps and accelerate conformational search. This resembles the roles of molecular chaperone that assist proteins to fold in vivo. (2) The phantom chain REM uses replicas that allow various degrees of atomic overlaps. By allowing atomic overlap in some of replicas, the peptide chain can cross over itself, which can accelerate conformation sampling. Using a coarse-gained model we developed, we compute equilibrium probability distributions for poly-alanine 16-mer and for a small protein by these REMs and compare the accuracy of the results. We see that the scaled hydrophobicity REM is the most efficient method among the three REMs studied.
Reaction cross sections for the H+D2(ν=1)→HD+D and D+H2(ν=1)→DH+H systems. A multiconfiguration time-dependent Hartree (MCTDH) wave packet propagation study
Authors: Sukiasyan S. , Meyer H.-D.
Published: July 2002

Cumulative initial-state-selected reaction cross sections of the H+D2 and D+H2 systems are computed for collision energies up to 1.6 eV and initial vibrational and rotational quantum numbers ν0=1 and j0=0–4. The Boothroyd–Keogh–Martin–Peterson (BKMP2) potential energy surface is taken as interaction potential. For comparison the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential energy surface is also considered, however, for j0=0 only. The cross sections are computed by propagating wave packets employing the multiconfiguration time-dependent Hartree scheme. The reactive flux, which determines the integral cross section, is evaluated through the interaction of the wave packet with a complex absorbing potential. A new approach of J-interpolation for the reaction probabilities has been developed. This approach allows to skip about every second of the individual propagations. The presence of weak oscillations appearing in the total integral cross sections has been observed. As in our previous calculations [J. Phys. Chem. 105, 2604 (2001)] on ν0=0, we attribute them to transition state resonances associated with excitations of the bending motion. Some of the present results are compared with previous results obtained by using the coupled states approximation.
Thermoelectric properties of the Bi- and Na-substituted Ca3Co4O9 system
Authors: Xu Gaojie , Funahashi Ryoji , Shikano Masahiro , Matsubara Ichiro , Zhou Yuqin
Published: July 2002

Bi- and Na-substituted Ca3Co4O9 polycrystalline samples have been prepared using a hot-pressing technique and their thermoelectric properties were carefully studied in air from room temperature to 1000 K. The substitutions of Bi3+ and Na+ for Ca2+, as well as Bi3+ and Na+ double substitution, cause both the electrical conductivity (σ) and thermoelectric power (S) to increase simultaneously. The double substitution has also been demonstrated to be effective to decrease the thermal conductivity (κ). The dimensionless figure of merit ZT (=S2σT/κ) reaches 0.32 at 1000 K in the double-substituted sample.
Advanced interferometric phase and amplitude noise measurements
Authors: Rubiola Enrico , Giordano Vincent
Published: July 2002

The measurement of the close-to-the-carrier noise of two-port radio frequency and microwave devices is a relevant issue in time and frequency metrology and in some fields of electronics, physics, and optics. While phase noise is the main concern, amplitude noise is often of interest. Presently the highest sensitivity is achieved with the interferometric method, that consists of the amplification and synchronous detection of the noise sidebands after suppressing the carrier by vector subtraction of an equal signal. A substantial progress in understanding the flicker noise mechanism of the interferometer results in new schemes that improve by 20–30 dB the sensitivity at low Fourier frequencies. These schemes, based on two or three nested interferometers and vector detection of noise, also feature closed-loop carrier suppression control, simplified calibration, and intrinsically high immunity to mechanical vibrations. This article provides the complete theory and detailed design criteria, and reports on the implementation of a prototype working at the carrier frequency of 100 MHz. In real-time measurements, a background noise of −175 to −180 dBrad2/Hz has been obtained at f=1 Hz off the carrier; the white noise floor is limited by the thermal energy kBT0 referred to the carrier power P0 and by the noise figure of an amplifier. Exploiting correlation and averaging in similar conditions, the sensitivity exceeds −185 dBrad2/Hz at f=1 Hz; the white noise floor is limited by thermal uniformity rather than by the absolute temperature. A residual noise of −203 dBrad2/Hz at f=250 Hz off the carrier has been obtained, while the ultimate noise floor is still limited by the averaging capability of the correlator. This is equivalent to a signal-to-noise (S/N) ratio of 2×1020 with a frequency spacing of 2.5×10−6. All these results have been obtained in a relatively unclean electromagnetic environment, and without using a shielded chamber. Implementation and experiments at that sensitivity level require skill and tricks, for which a great effort is spent in the article. Applications include the measurement of the properties of materials and the observation of weak flicker-type physical phenomena, out of reach for other instruments. As an example, we measured the flicker noise of a by-step attenuator (−171 dBrad2/Hz at f=1 Hz) and of the ferrite noise of a reactive power divider (−173.7 dBrad2/Hz at f=1 Hz) without need of correlation. In addition, the real-time measurements can be exploited for the dynamical noise correction of ultrastable oscillators.
High-density and size-controlled GaN self-assembled quantum dots grown by metalorganic chemical vapor deposition
Authors: Miyamura M. , Tachibana K. , Arakawa Y.
Published: July 2002

GaN self-assembled quantum dots (QDs) with high quality and high density have been grown by low-pressure metalorganic chemical vapor deposition under very low V/III ratios. In depositing over a critical thickness of four monolayer GaN, we observed a transition from two-dimensional to three-dimensional growth mode. The density of the QDs could be changed between 109 and 1010 cm−2. The typical diameter and height of the QDs were 20 and 2 nm, respectively. The size of the QDs was controlled to a considerable extent by changing the growth temperature and V/III ratio. Moreover, we observed two photoluminescence peaks from both the QDs and the wetting layer at room temperature. This result clearly demonstrates that the GaN QDs were formed with the Stranski–Krastanow growth mode.
Determination of the phonon dispersion of zinc blende (3C) silicon carbide by inelastic x-ray scattering
Authors: Serrano J. , Strempfer J. , Cardona M. , Schwoerer-Böhning M. , Requardt H. , Lorenzen M. , Stojetz B. , Pavone P. , Choyke W. J.
Published: July 2002

We present an experimental and theoretical investigation of the phonon dispersion relations in zinc blende (3C) SiC. The experimental data were obtained for the entire Brillouin zone by inelastic x-ray scattering (IXS) using a synchrotron radiation source. Eigenvector analysis is performed with the aid of state-of-the-art linear response first principles calculations based on density functional theory. The theoretical predictions reproduce the experimental phonon dispersion remarkably well. These results are compared with data obtained previously for the 〈111〉 direction by Raman spectroscopy using several SiC polytypes and the backfolding technique. IXS data for 4H modification along the c axis are also reported.
The coherent structures of shallow-water turbulence: Deformation-radius effects, cyclone/anticyclone asymmetry and gravity-wave generation
Authors: Polvani L. M. , McWilliams J. C. , Spall M. A. , Ford R.
Published: July 2002

Over a large range of Rossby and Froude numbers, we investigate the dynamics of initially balanced decaying turbulence in a shallow rotating fluid layer. As in the case of incompressible two-dimensional decaying turbulence, coherent vortex structures spontaneously emerge from the initially random flow. However, owing to the presence of a free surface, a wealth of new phenomena appear in the shallow-water system. The upscale energy cascade, common to strongly rotating flows, is arrested by the presence of a finite Rossby deformation radius. Moreover, in contrast to near-geostrophic dynamics, a strong asymmetry is observed to develop as the Froude number is increased, leading to a clear dominance of anticyclonic vortices over cyclonic ones, even though no β effect is present in the system. Finally, we observe gravity waves to be generated around the vortex structures, and, in the strongest cases, they appear in the form of shocks. We briefly discuss the relevance of this study to the vortices observed in Jupiter’s atmosphere.
An analytical construction of the SRB measures for Baker-type maps
Authors: Tasaki S. , Gilbert Thomas , Dorfman J. R.
Published: July 2002

For a class of dynamical systems, called the axiom-A systems, Sinai, Ruelle and Bowen showed the existence of an invariant measure (SRB measure) weakly attracting the temporal average of any initial distribution that is absolutely continuous with respect to the Lebesgue measure. Recently, the SRB measures were found to be related to the nonequilibrium stationary state distribution functions for thermostated or open systems. Inspite of the importance of these SRB measures, it is difficult to handle them analytically because they are often singular functions. In this article, for three kinds of Baker-type maps, the SRB measures are analytically constructed with the aid of a functional equation, which was proposed by de Rham in order to deal with a class of singular functions. We first briefly review the properties of singular functions including those of de Rham. Then, the Baker-type maps are described, one of which is nonconservative but time reversible, the second has a Cantor-like invariant set, and the third is a model of a simple chemical reaction R↔I↔P. For the second example, the cases with and without escape are considered. For the last example, we consider the reaction processes in a closed system and in an open system under a flux boundary condition. In all cases, we show that the evolution equation of the distribution functions partially integrated over the unstable direction is very similar to de Rham’s functional equation and, employing this analogy, we explicitly construct the SRB measures.
Identification of low order manifolds: Validating the algorithm of Maas and Pope
Authors: Rhodes Carl , Morari Manfred , Wiggins Stephen
Published: July 2002

The algorithm of Reference 1992 is presented as a method for identification of invariant reduced-order manifolds for stable systems which exhibit dynamics with a time-scale separation. While this method has been published previously in the literature, theoretical justification for the algorithm was not presented in the original work. Here, it will be shown rigorously that the algorithm correctly identifies the slow manifold. Before the theoretical results are presented, a brief background on the behavior of singularly perturbed systems is presented. The algorithm of Reference 1992 is then introduced. This method will be applied to two different examples, a distillation column and a two-phase chemical reactor. For each of these examples, the resulting reduced-order description will be compared to other standard methods of producing reduced-order models. In addition, some preliminary thoughts on how this method can be used to form reduced-order models are presented.
Noise and O(1) amplitude effects on heteroclinic cycles
Authors: Stone Emily , Armbruster Dieter
Published: July 2002

The dynamics of structurally stable heteroclinic cycles connecting fixed points with one-dimensional unstable manifolds under the influence of noise is analyzed. Fokker-Planck equations for the evolution of the probability distribution of trajectories near heteroclinic cycles are solved. The influence of the magnitude of the stable and unstable eigenvalues at the fixed points and of the amplitude of the added noise on the location and shape of the probability distribution is determined. As a consequence, the jumping of solution trajectories in and out of invariant subspaces of the deterministic system can be explained.