bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopic study of ionizing and recombining plasma in a stationary plasma source
Authors: Namba S. , Goto M. , Tsuboi H. , Oda T. , Sato K.
Published: July 2002

A hot cathode, which is composed of La2O3–W needles, has been constructed and tested in a modified arc discharge source in order to produce a high-density plasma by large discharge current. No damage of needles was found after 100 h at a discharge current of 300 A. Characteristics of generated helium plasmas have been investigated by spectroscopic observations. The electron density has been found to be ≧2×1014 cm−3 at a discharge current 100 A with this cathode. Experimental results were analyzed with calculations based on a collisional radiative model including an effect of radiation trapping. As a result, generated plasmas were shown to be classified into two groups by the pressure of neutral helium, that is, an ionizing plasma and recombining plasma. In low temperature recombining plasma, population inversions between He+ ion n=2 and the higher excited levels have been obtained.
Ultrasonic attenuation in Zr41Ti14Cu12.5Ni10−xBe22.5Cx (x=0,1) bulk metallic glasses under high pressure
Authors: Wang Wei Hua , Wang R. J. , Li F. Y. , Pan M. X. , Qin Z. C. , Zhao D. Q. , Zhuang Y. X. , Zhang Y.
Published: July 2002

The pressure dependence of ultrasonic attenuation in Zr41Ti14Cu12.5Ni10−xBe22.5Cx (x=0,1) bulk metallic glasses has been studied up to 0.5 GPa by using a pulse echo overlap method. The effect of carbon addition on the attenuation is also investigated. Some unique characteristics of the ultrasonic attenuation are found and compared with those of other glasses. The origin of the anomalous attenuation behavior is discussed.
Moisture diffusion along the TiN/SiO2 interface and in plasma-enhanced chemical vapor deposited SiO2
Authors: Xu G. , Clarke D. R. , Ma Q. , Fujimoto H.
Published: July 2002

The diffusivity of moisture along the TiN/SiO2 interface has been determined by imaging the inward diffusion of O18 and D2 from isotopically labeled water using a secondary ion mass spectroscopy (SIMS) technique. The diffusivity, at room temperature, of the O18 and D2 labeled species along the interface are indistinguishable and have a value of 6.0±2.0×10−13 cm2/s, four orders of magnitude faster than bulk diffusivity of the same species in the plasma-enhanced chemical vapor deposition silica, also determined by SIMS. From 8 to 90 °C, the activation energies for interface and bulk diffusivities of the 2D labeled species are found to be 0.21 and 0.74 eV, respectively.
Enhanced and stable photoluminescence from partially oxidized porous Si coated with Si thin films
Authors: Siu G. G. , Wu X. L. , Gu Y. , Bao X. M.
Published: July 2002

Photoluminescence (PL) spectra of partially oxidized porous Si (POPS) coated with Si thin films were examined using the 488 nm line of Ar+ laser. The obtained PL is stable, peaks at 1.763 eV with a blueshift of ∼60 meV, and its maximal intensity is seven times larger than that of the POPS. Spectral analysis and the experimental results from infrared spectroscopy and electron spin resonance suggest that the enhanced and stable PL arises from optical transitions in the nonbridging oxygen hole centers (NBOHCs). Si coating mainly leads to introduction of the NBOHCs defects and thus makes the PL intensity enhanced. The blueshift of ∼60 meV is a result of the local equilibrium of NBOHCs defects under high temperature.
Local three-dimensional magnetostatic equilibria
Authors: Hegna C. C.
Published: July 2002

An analytic representation of a three-dimensional (3-D) magnetostatic equilibrium that is localized to a magnetic flux surface is introduced. This model, which generalizes prior work for axisymmetric systems, allows one to completely specify the structure of the equilibria through a finite number of 3-D shaping and profile variations. This model can be implemented to study the effects of the equilibrium variations on localized instabilities, such as ballooning modes or microinstabilities.
Electron saturation velocity of GaInP deduced in a GaInP/GaAs/GaInP double heterojunction bipolar transistor
Authors: Hsin Yue-Ming , Hsu Shih-Tzung , Fan Chen-Chung
Published: July 2002

GaInP/GaAs/GaInP double heterojunction bipolar transistors have been fabricated for the study of electron saturation velocity (vsat) in GaInP. The necessary composite design at the base–collector junction, which effectively reduces the conduction band spike and avoids the premature Kirk effect, enables us to use the Kirk effect to study vsat. The deduced electron saturation velocity in GaInP is ∼5×106 cm/sec.
dc and ac optical nulling bridges for sensitive transmittance measurements
Authors: Argueta-Dı́az V. , Trejo-Valdez M. , Garcı́a-Valenzuela A.
Published: July 2002

We present a formal comparison between dc and ac nulling optical bridges. We consider their performance in monitoring over long periods of time (several minutes), small and slow (in a time scale of seconds) changes in transmittance of an optical component. We consider two fundamentally different ac optical bridges (OB), an amplitude modulated optical bridge (AM-OB), and a switching optical bridge (Sw-OB). For each OB, we derive a general expression for the minimum detectable change in transmittance (|ΔT|min), taking into account all the important noises. It is found that under optimum conditions the dc- and AM-OB have similar detection limits imposed by the 1/f noise of the photodetectors. It is shown that the Sw-OB can in principle overcome the 1/f noise of the detector and approach the shot-noise limit; however, it is sensitive to switching device imperfections (to first order in smallness), which could easily prevent achieving a detection limit below the 1/f noise. It is also shown that the Sw-OB has intrinsic advantages over the dc- and AM-OB for its use in remote sensors. It can eliminate more efficiently noise induced along the propagation of light from the sensing point to the photodetector. We conclude that when using low power optics (P⩽1 mW) and considering a bench instrument, the dc- and AM-OB can be used for a target resolution down to |ΔT|min≈10−5–10−6. The Sw-OB optical bridges should be chosen if |ΔT|min≈10−6–10−8 is to be attempted. In any case, strict conditions are to be met before considering approaching the detection limits imposed by electronic noises. These conditions are discussed in detail. In particular, atmospheric isolation will be needed in general below |ΔT|min≈10−4.
Stability of disconnected free surfaces in a cylindrical container under zero gravity: Simple cases
Authors: Slobozhanin Lev A. , Alexander J. Iwan D.
Published: July 2002

The stability of equilibrium configurations of a capillary liquid in a circular cylindrical container with planar ends is investigated. The liquid is under zero gravity conditions, and its wetting angle is constant over the entire solid surface. Attention is focused on the case for which the free surface consists of two disconnected pieces (connectivity components) that bound the connected liquid domain. First we outline the method used to determine critical states with disconnected free surfaces when each connectivity component is axisymmetric. Then we examine the stability of disconnected surfaces for the simple cases that arise when each connectivity component represents a closed sphere or a part of a sphere. Ten configurations were considered that represent all possible combinations of the following connectivity components: A closed sphere (that bounds a gas bubble), a spherical cap in contact with the lateral wall of a cylinder; a spherical cap in contact with a cylinder endwall, and a portion of a sphere (that does not cross the cylinder’s axis of symmetry) bounded by a cylindrical wall and a flat endwall.
The anisotropy of the small scale structure in high Reynolds number (Rλ∼1000) turbulent shear flow
Authors: Shen X. , Warhaft Z.
Published: July 2002

The postulate of local isotropy (PLI) is tested in a wind tunnel uniform shear flow in which the Reynolds number is varied over the range 100⩽Rλ⩽1, 100(6.7×102⩽Rl⩽6.3×104). The high Rλ is achieved by using an active grid [Mydlarski and Warhaft, J. Fluid Mech. 320, 331 (1996)] in conjunction with a shear generator. We focus on the increments of the longitudinal velocity fluctuations in the direction of the mean shear. PLI requires that odd order moments of these quantities approach zero as Rλ→∞. Confirming the lower Reynolds number measurements of Garg and Warhaft [Phys. Fluids 10, 662 (1998)], we show that the skewness of ∂u/∂y decreases as Rλ−0.5 (with a value of 0.2 at Rλ∼1000). Although the decrease is slower than classical scaling arguments suggest, the result is consistent with PLI, indicating a negligible value at high Rλ. However, the normalized fifth moment, 〈(∂u/∂y)5〉/〈(∂u/∂y)2〉5/2, is of order 10, and shows no diminution with Reynolds number, while the normalized seventh moment increases with Rλ. These dissipation range results are inconsistent with PLI. Within the inertial subrange we show that all the odd order moments of the increments of Δu(y) are nonzero, exhibiting scaling ranges. Here, the skewness structure function has a value ∼0.5 indicating that in the inertial subrange significant anisotropy is evident even at the third moment level. Fifth- and seventh-order inertial subrange skewness structure functions are of order 10 and 100, respectively. The results show that PLI is untenable, both at dissipation and inertial scales, at least to Rλ∼1000, and suggest it is unlikely to be so even at higher Reynolds numbers.
Geometric models of (d+1)-dimensional relativistic rotating oscillators
Authors: Cotăescu Ion I. , Cotăescu Ion I.
Published: July 2002

Geometric models of quantum relativistic rotating oscillators in arbitrary dimensions are defined on backgrounds with deformed anti-de Sitter metrics. It is shown that these models are analytically solvable, deriving the formulas of the energy levels and corresponding normalized energy eigenfunctions. An important property is that all these models have the same nonrelativistic limit, namely the usual harmonic oscillator.
Surface tension measurement technique by differential phase detection of capillary waves in liquids
Authors: Marinozzi F.
Published: July 2002

A simple device was developed for measuring surface tension of liquids (typically water solutions) based on the mechanical generation of capillary waves on a sample surface. The detection of the phase shift among two laser beams reflected by the liquid surface excited with a sinusoidal mechanical perturbation allows the calculation of the sample surface tension. Preliminary experimentation using a prototype of the instrument with water and water plus surfactant has been successfully performed. Some theoretical considerations are also pointed out to show the main design issues for proper operation mode. The proposed prototype appears innovative and suitable for online or field operation.
Multiphonon hopping conduction in nonconventional chromium-doped Bi3Pb1Sr3Ca3Cu4−nCrnOx (n=0.025–0.2) glasses with nanocrystalline particles and clusters
Authors: Bhattacharya S. , Chaudhuri B. K. , Sakata H.
Published: July 2002

Transport properties of Cr containing multicomponent oxide glasses Bi3Pb1Sr3Ca3Cu4−nCrnOx (n=0.025, 0.05, 0.1, and 0.2) dispersed with nanocrystalline particles (5–20 nm depending on the values of n) have been reported in the temperature range of 250–450 K. Conductivity of this glass-nanocrystal composite system shows little decrease with increasing Cr content. Above θD/2 (θD is the Debye temperature), conductivity data can be analyzed with small polaron hopping models. Interestingly, unlike undoped Bi4Sr3Ca3Cu4Ox (or Bi-4334) glasses [showing nonadiabatic small polaron hopping (SPH) conduction at T>θD/2], the Cr doped glasses supports adiabatic SPH conduction mechanism above θD/2 indicating change of glass network structure due to partial substitution of Cu by Cr. But below this temperature Mott’s or Greaves’ variable range hopping models can be consistently used to fit the experimental conductivity data only with larger (compared to the usual transition metal oxide glasses) values of the density of states at the Fermi level N(EF). The most probable transport mechanism for the entire range of temperature and glass compositions is concluded to be due to multiphonon tunneling of large polarons between the nanoclusters present in the glasses which is also in sharp contrast to the behavior of the undoped (Bi-4334) glass. All the glass samples (except n⩾0.2) are found to become superconductors by annealing at higher temperatures.
Anharmonic frequency shift of long-wavelength phonons in As and Sb
Authors: Pizani P. S. , Campos C. E. M. , Puech P.
Published: July 2002

Raman scattering was used to study the temperature dependence of the phonon frequencies of Eg and A1g modes of crystalline arsenic and antimony. The partial derivative of the frequency versus temperature for both materials was determined from a linear fitting approximation. The real anharmonic contribution to the shift was obtained after correction had been made for the contribution of thermal expansion.
The power law relation of spiral waves in the Belousov–Zhabotinsky reaction
Authors: Li Yan , Bai Shufeng , Ouyang Qi
Published: July 2002

The relationship of the period Ts and the wavelength λs of spiral waves with the control parameters is systematically studied with the Belousov–Zhabotinksy (BZ) reaction in a spatially extended quasi-two-dimensional system. Our experiments indicate that Ts and λs not only rely on the concentrations of sulfuric acid and sodium bromate, as indicated in the previous work [A. L. Belmonte, Q. Ouyang, and J. M. Flesselles, J. Phys. II 7, 1425 (1997)] but also have strong relation with the concentration of malonic acid (MA). With the influence of the concentration of MA taken into consideration, a revised power law of spiral waves is suggested, which is qualitatively in agreement with early works of numerical simulations and theoretical analysis.
Theoretical studies on anomalous phases of photodoped systems in two-band model
Authors: Nagao Hidemi , Nishino Masamichi , Shigeta Yasuteru , Yoshioka Yasunori , Yamaguchi Kizashi
Published: July 2002

Expressions of the transition temperature for various anomalous phases are first derived in the framework of the two-band model for copper oxides. The effects of the photon field for the model are discussed under the Thomas–Fermi approximation. A phase diagram for copper oxides in the photon field is shown. Phase diagrams for possible other mechanisms are also investigated theoretically in relation to electron scattering processes.
Control of magnetic properties in LaCrO3–LaFeO3 artificial superlattices
Authors: Ueda Kenji , Tabata Hitoshi , Kawai Tomoji
Published: July 2002

The magnetic properties of LaFeO3–LaCrO3 superlattices have been artificially controlled by adjusting the atomic order of the B site ions via changes of the stacking direction of the 〈100〉, 〈110〉 and 〈111〉 planes and adjustment of the stacking periodicity. When Fe and Cr layers are atomically stacked along the 〈111〉 direction, ferromagnetism appears. In the case of superlattices growing along the 〈100〉 and 〈110〉 directions, antiferromagnetic properties appear, and the Néel temperature changes systematically as the stacking periodicity increases. Although the total number of Fe and Cr ions are the same in the (100), (110) and (111) stacking planes, different magnetic properties can be created in artificial superlattices.
Chemically accurate conformational energies for aziridine-2-carbonitrile
Authors: Tschumper Gregory S.
Published: July 2002

Extensive ab initio computations have been carried out to characterize the energy of trans-aziridine-2-carbonitrile relative to cis-aziridine-2-carbonitrile (also known as 2-cyanoaziridine). Correlation consistent basis sets of double-, triple-, quadruple-, and pentuple-ζ quality have been employed to approach the one-particle basis set limit for energies obtained with the restricted Hartree–Fock method and second-order Møller–Plesset perturbation theory. Contributions from higher-order excitations have been determined with the popular coupled-cluster technique which includes single and double excitations as well as a perturbative estimate of triple excitations [CCSD(T)] and with the Brueckner doubles method including a perturbative estimate of triple as well as quadruple excitations [BD(TQ)]. From a focal point analysis, the electronic energy separation is found to be 3.62 kJ mol−1, which is in excellent agreement with previous theoretical estimates, but is in disagreement with an experimentally estimated lower bound to the free energy difference of 11 kJ mol−1. The electronic energy of the transition structure connecting the two conformers was determined to be 77.15 kJ mol−1 higher than the cis-isomer.
Improved atomistic simulation of diffusion and sorption in metal oxides
Authors: Skouras E. D. , Burganos V. N. , Payatakes A. C.
Published: July 2002

Gas diffusion and sorption on the surface of metal oxides are investigated using atomistic simulations, that make use of two different force fields for the description of the intramolecular and intermolecular interactions. MD and MC computations are presented and estimates of the mean residence time, Henry’s constant, and the heat of adsorption are provided for various common gases (CO, CO2, O2, CH4, Xe), and semiconducting substrates that hold promise for gas sensor applications (SnO2, BaTiO3). Comparison is made between the performance of a simple, first generation force field (Universal) and a more detailed, second generation field (COMPASS) under the same conditions and the same assumptions regarding the generation of the working configurations. It is found that the two force fields yield qualitatively similar results in all cases examined here. However, direct comparison with experimental data reveals that the accuracy of the COMPASS-based computations is not only higher than that of the first generation force field but exceeds even that of published specialized methods, based on ab initio computations.
Finite conformal modules over N=2,3,4 superconformal algebras
Authors: Cheng Shun-Jen , Lam Ngau
Published: July 2002

In this paper we continue the study of representation theory of formal distribution Lie superalgebras initiated by Cheng and Kac [Asian J. Math. 1, 181–193 (1997); 2, 153–156 (1998) (erratum)]. We study finite Verma-type conformal modules over the N=2, N=3 and the two N=4 superconformal algebras and also find explicitly all singular vectors in these modules. From our analysis of these modules we obtain a complete list of finite irreducible conformal modules over the N=2, N=3 and the two N=4 superconformal algebras.
Anisotropic dielectric function spectra from single-crystal CuInSe2 with orientation domains
Authors: Kreuter A. , Wagner G. , Otte K. , Lippold G. , Schindler A. , Schubert M.
Published: July 2002

The anisotropic optical constants of slightly In-rich CuInSe2 are determined on the (112) surface of a naturally grown single-crystal sample with orientation domains using generalized spectroscopic ellipsometry for photon energies between 0.8 and 4.5 eV. Orientation domains within the sample, revealed by electron diffraction investigation, cause biaxial response of the sample surface. The CuInSe2 dielectric functions are extracted at each wavelength rigorously considering orientation domains and surface roughness effects, and improve previous results.
On solvable potentials related to SO(2,2)
Authors: Baran S. A. , Dalgic A. , Kerimov G. A.
Published: July 2002

Scattering problems in one dimension associated with the SO(2,2) group are studied. The S matrices for the systems under consideration are calculated by using the theory of intertwining operators for semisimple Lie groups. The wave functions are expressed in terms of matrix elements of principal series representation of the SO(2,1) group.
Plasmon excitation modes in nanowire arrays
Authors: Sander M. S. , Gronsky R. , Lin Y. M. , Dresselhaus M. S.
Published: July 2002

Electron energy loss spectrometry and energy-filtered transmission electron microscopy reveal characteristic plasmon excitations in both isolated Bi nanowires and an array of Bi nanowires within an Al2O3 matrix. As the average nanowire diameter decreases from 90 to 35 nm, both the volume plasmon energy and peak width increase. In addition, a lower-energy excitation is present in a very localized region at the Bi–Al2O3 interface. These results are discussed in the context of quantum confinement and the influence of interfaces on the electronic properties of nanocomposite materials.
Structural and magnetic properties of RFe9Si2Cx compounds (R=Ce, Pr, Nd, Sm; x=0.5, 1.0, 1.5)
Authors: Zhang Xiao-dong , Chang Hong , Yang Ying-chang
Published: July 2002

A systematic investigation of crystallographic and intrinsic magnetic properties of the compounds RFe9Si2Cx (R=Ce, Pr, Nd, Sm; x=0.5, 1.0, 1.5) has been carried out. The lattice parameters a, c and the unit cell volumes increase with increasing C contents, resulting in the increases of Curie temperatures by about 100–150 K between x=0.5 and x=1.5. The saturation magnetization of RFe9Si2Cx (R=Pr, Nd, Sm) at 1.5 K passes through a maximum at x=1.0, and then decreases slightly. Meanwhile, the average iron moment μFe at 1.5 K is 1.57μB for x=1.0 and 1.63μB for x=1.5 in CeFe9Si2Cx compounds. SmFe9Si2Cx compounds have uniaxial anisotropy with the anisotropy fields HA estimated around 132 kOe at 1.5 K and 50 kOe at 300 K (x=1.5), respectively. First-order magnetization processes are observed in the NdFe9Si2Cx compounds and the first-order magnetization processes critical field Hcr is found to increase with x. In addition, the critical temperature for the onset of the first-order magnetization processes transition is around 90 K in the case of x=1.0.
Thermal effects on charge transfer in atom-surface scattering
Authors: Goodman Frank O. , Sulston Kenneth W.
Published: July 2002

This paper studies the effect of varying substrate temperatures on the resonant charge-transfer process between a scattered particle and a solid surface. Two possible mechanisms for an effect are considered: (1) coupling of the transferring electron to the thermal motion of the target atom on the surface, and (2) changing occupancies of electron orbitals in the solid as temperature varies. Only the latter is found to give a noticeable effect, occurring when the electronic energy level on the projectile is favorably positioned relative to the Fermi level of the target solid. However, even though this effect is noticeable, both mechanisms we consider give very small thermal effects. All other mechanisms, for example those leading to the Kondo (or mixed-valent) effect, are neglected.
Stability of the high-spin ground state in the Peierls-extended Hubbard model
Authors: Huang Zhongbing , Lin Hai-Qing
Published: July 2002

Based on the Peierls-extended Hubbard model and breaking the alternative symmetry, we examine the stability of the high-spin ground state on a π-conjugated hydrocarbon chain with respect to the intersite Coulomb interaction and dimerization of the backbone. We find that the ground state switches from a low-spin to a high-spin state only when the on-site Coulomb interaction approaches a critical value. When increasing the intersite Coulomb interaction or dimerization, the ground state switches from a high-spin to a low-spin state.
Vibrational density of states of selenium through the glass transition
Authors: Caprion D. , Schober H. R.
Published: July 2002

We present an investigation of the vibrational density of states (VDOS) of selenium calculated using two different methods. First we compute the VDOS from the velocity autocorrelation and study its evolution with the temperature. Secondly we compute it from the dynamical matrix. This latter method leads to imaginary frequencies for finite temperatures. In the literature these frequencies are called unstable and are related to the diffusion and/or the anharmonicity. We investigate the character of these frequencies, asking whether they are localized or extended, more acoustic or optic. We show that, below a temperature-dependent threshold frequency, they are similar to their real low-frequency counterparts. The temperature evolution of the number of these modes changes at the glass transition.
Phase transition of Zr41Ti14Cu12.5Ni10Be22.5 bulk amorphous below glass transition temperature under high pressure
Authors: Pan Ming Xiang , Wang Jing Guo , Yao Yu Shu , Zhao De Qian , Wang Wei Hua
Published: July 2002

The effect of high pressure annealing on phase transition of the Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass (BMG) is studied on compressed specimens by x-ray diffraction and differential scanning calorimetry (DSC) after annealing treatments at 573 K and under pressures up to 6 GPa. The results of DSC and transmission electron microscope show that the high pressure annealing cannot only promote structural relaxation, but also induce the occurrence of a phase transition at a temperature below the glass transition temperature in the BMG. The role of pressure on structural relaxation and phase transition is discussed.
Neoclassical current drive by waves with a symmetric spectrum
Authors: Helander P. , Catto Peter J.
Published: July 2002

It is shown that plasma waves need not have an asymmetric spectrum in order to produce an electric current in a plasma embedded in a curved magnetic field. For instance, in a toroidal plasma, up–down asymmetric electron–cyclotron heating drives a toroidal current even if there is no net wave–particle momentum transfer and the wave field does not interact preferentially with electrons travelling in any one direction. The resulting current drive efficiency is calculated and is found to be smaller than that of the conventional current drive mechanism in the banana regime, but not insignificant in the plateau regime.
Ultrahigh-speed optical communications
Authors: Hasegawa Akira
Published: July 2002

Fiber-based ultrahigh-speed optical communications have achieved a remarkable level of success reaching multiterabit/s error-free transmission over several thousand kilometers. In the near future the total capacity is expected to approach a petabit/s. In this article critical issues that are involved behind the success of these achievements are reviewed and the future direction of ultrahigh-speed optical communications is discussed. The idea of the use of optical solitons for an information carrier is discussed comparatively with alternative approaches.
Resonant cavity light emitting diodes with two single quantum wells of different widths
Authors: Kato Toshihiro , Mizuno Yoshiyuki , Hirotani Masumi , Saka Takashi , Horinaka Hiromichi
Published: July 2002

Resonant cavity light emitting diodes containing two single quantum wells of different well width were fabricated. The quantum wells, consisting of GaAs with AlGaAs barriers, were placed in separate antinodes of the optical emission mode in a Fabry–Perot cavity whose optical length is twice that of the resonant wavelength. For low forward bias currents, the emission from the wider quantum well is resonant with the cavity. For larger forward bias currents, the emission shifts to longer wavelengths due to an increase in the device temperature. The narrower single quantum well then becomes resonant with the cavity. Using the double quantum well structure, high output powers and high directivity are obtained for a wide range of forward bias current.
Neoclassical effects in the annular Penning trap
Authors: Robertson Scott , Espejo Joe , Kline John , Quraishi Qudsia , Triplett Matt , Walch Bob
Published: July 2002

Neoclassical transport has been investigated with a modified Malmberg–Penning trap which has conductors along the axis to create an azimuthal magnetic field. The axial bounce motion of the electrons is accompanied by a radial drift which changes sign at the ends of the device causing drift orbits of finite radial extent. Analysis and numerical simulations show that the transport is neoclassical with mobility and diffusion coefficients depending upon the axial magnetic field alone rather than the absolute value of the magnetic field. Experiments with added helium gas to create electron-neutral collisions show that the electron mobility from an applied radial electric field and the Ware drift from an azimuthal electric field both have neoclassical values over a wide range of magnetic fields and collision frequencies.
Kinetics of the reaction of O2+ with CH4 from 500 to 1400 K: A case for state specific chemistry
Authors: Dotan Itzhak , Viggiano A. A.
Published: July 2002

Rate constants and branching ratios have been measured for the reaction of O2+ with CH4 over the temperature range from 500 to 1400 K. The rate constants increase dramatically over this temperature range, in good agreement with a previous study from our laboratory. A complex chemistry was found. The main product at low temperature, CH2O2H+, disappears almost completely at high temperature, in part due to thermal dissociation. The main products at high temperature are CH4+ and HCO+; the latter ion has not been observed previously. Also prominent at high temperature are H3O+ and CH3+. A small amount of CH3O+ is observed at all temperatures. H2O+ is observed in small quantities at high temperature. Branching ratios for the reaction of O2+ with CD4 are also reported, confirming the ion assignments. Comparison to drift tube and beam results indicates that vibrational excitation of the CH4 promotes several of these channels more than other forms of energy. Channels which proceed through the t-CH3OOH+ A″2 intermediate (described in the accompanying paper) are probably not strongly dependent on the form of energy, although uncertainties prevent a definitive conclusion. The charge-transfer channel and the channels requiring substantial rearrangement are greatly enhanced by vibrational excitation. Thus, specific channels do appear to be governed by vibrational excitation.
Fabrication of tungsten micro-punch for micro-nozzles
Authors: Lim Hyung-Jun , Lim Young-Mo , Kim Soo Hyun
Published: July 2002

A very thin electrode with a tapered shape and a cylindrical tip, named ultrathin punch, is fabricated by electrochemical etching and by controlling the vertical position of the electrode during etching. We used electrochemical etching by which we obtain punches quickly and simply. The fabrication procedure and its results are presented in this article. With this punch, we can get holes or nozzles for ink-jet printers, textile machines, and other uses. Various angles of the taper can be obtained by adjusting the immersion depth. The tip diameter also can be controlled by adjusting current density and etching time.
Flux lattice imaging of a patterned niobium thin film
Authors: Roseman M. , Grütter P. , Badı́a A. , Metlushko V.
Published: July 2002

Using our cryogenic magnetic force microscope, we have investigated a superconducting Nb thin film, 100 nm in thickness with Tc∼6.5 K. The film is patterned with a square array (1 μm×1 μm) of antidots, which serve as artificial pinning centers for magnetic flux. We have observed flux lattice matching as a function of applied magnetic field and temperature, for field strengths up to the third matching field, with evidence of flux dragging by the tip around the antidots. Force gradient distance curves acquired at temperatures about Tc clearly demonstrate an observable Meissner force between tip and sample, and allow for an estimation of the magnetic screening penetration depth.
Monte Carlo study of the finite-size effects on the magnetization of maghemite small particles
Authors: Iglesias Òscar , Labarta Amı́lcar , Ritort Fèlix
Published: July 2002

Monte Carlo simulations of a model for γ-Fe2O3 (maghemite) single particle of spherical shape are presented aiming at the elucidation of the specific role played by the finite size and the surface on the anomalous magnetic behavior observed in small particle systems at low temperature. The influence of the finite-size effects on the equilibrium properties of extensive magnitudes, field coolings, and hysteresis loops is studied and compared to the results for periodic boundaries. It is shown that for the smallest sizes the thermal demagnetization of the surface completely dominates the magnetization while the behavior of the core is similar to that of the periodic boundary case, independently of D. The change in shape of the hysteresis loops with D demonstrates that the reversal mode is strongly influenced by the presence of broken links and disorder at the surface.
Heavy fermion behavior in Ce(NixPd1−x)2Si2
Authors: Huo D. , Sakurai J. , Kuwai T. , Mizushima T. , Isikawa Y.
Published: July 2002

Measurements of the magnetic susceptibility χ, specific heat C, and thermoelectric power S were carried out for the solid solution Ce(NixPd1−x)2Si2 (0⩽x⩽1). With increasing x, the ground state changes from an antiferromagnetic Kondo state (x<0.2) to an intermediate valence state (x>0.3). The heavy fermion state was found to evolve at the crossover concentration at x=0.2, where the long-range magnetic order seems to disappear.
Magnetic phase diagram of layered manganites in the highly doped regime
Authors: Mitchell J. F. , Ling C. D. , Millburn J. E. , Argyriou D. N. , Berger A. , Medarde M.
Published: July 2002

The naturally layered colossal magnetoresistive manganites La2−2xSr1+2xMn2O7 exhibit an extremely varied range of magnetic and electronic behavior over a very narrow composition range between x=0.3 and x=0.5. The successful synthesis in our laboratories of compounds with nominally greater than 50%Mn4+ concentration has now allowed the study of this region of the phase diagram. Here we present detailed neutron diffraction measurements of these compounds with doping levels 0.5<x<1.0. As predicted by simple theories, the type-A layered antiferromagnetic (AF) structure is found at x∼0.5 and the type-G “rocksalt” AF structure at x=1.0. Between these two extremes is found a C-type structure (ferromagnetic rods parallel to b coupled antiferromagnetically to all neighboring rods) stabilized by orbital ordering of y2 states. Also in this Mn4+-rich regime is found a region in which no long-range magnetic order is observed. We discuss how semiempirical models can explain the variety of magnetic structures and how structural trends as a function of doping corroborate the unifying notion of a shift from in-plane to axial orbital occupation as the Mn4+ concentration is decreased.
Effect of layering and doping on the magnetic anisotropy of the layered manganites La2−2xSr1+2xMn2O7 (x=0.3–0.4)
Authors: Welp U. , Berger A. , Vlasko-Vlasov V. K. , You H. , Gray K. E. , Mitchell J. F.
Published: July 2002

Magnetization experiments were performed on a series of single crystals of the bilayer manganites La2−2xSr1+2xMn2O7. The magnetic anisotropy constants were determined by fitting expressions based on the tetragonal anisotropy energy to the magnetization curves. It is shown that there is a significant dipolar contribution to the first order anisotropy constant arising from the layered crystal structure. This contribution determines the magnetization direction in the doping range x=0.32–0.33 where the anisotropy due to the electronic structure is small. With increasing doping the magnetic anisotropy changes from strong uniaxial anong the c axis (K≈2.5×106 erg/cm3) at x=0.3 to strong easy plane (K≈−3.7×106 erg/cm3) at x=0.4 in an almost linear fashion. This evolution is explained through the change of the orbital nature of the eg electrons from predominantly d3z2−r2 to predominantly dx2−y2. On the samples displaying easy-plane anisotropy a small anisotropy in the basal plane of about −7×104 erg/cm3 was determined with (110) being the easy axis. An estimate of the spin-wave gap based on these results is in reasonable agreement with experimental determinations.
Thermal magnetization reversal in arrays of nanoparticles
Authors: Brown Gregory , Novotny M. A. , Rikvold Per Arne
Published: July 2002

The results of large-scale simulations investigating the dynamics of magnetization reversal in arrays of single-domain nanomagnets after a rapid reversal of the applied field at nonzero temperature are presented. The numerical micromagnetic approach uses the Landau–Lifshitz–Gilbert equation including contributions from thermal fluctuations and long-range dipole–dipole demagnetizing effects implemented using a fast-multipole expansion. The individual model nanomagnets are 9 nm×9 nm×150 nm iron pillars similar to those fabricated on a surface with scanning tunneling microscope assisted chemical vapor deposition [S. Wirth et al., J. Appl. Phys. 85, 5249 (1999)]. Nanomagnets oriented perpendicular to the surface and spaced 300 nm apart in linear arrays are considered. The applied field is always oriented perpendicular to the surface. When the magnitude of the applied field is less than the coercive value, about 2000 Oe for an individual nanomagnet, magnetization reversal in the nanomagnets can only occur by thermally activated processes. Even though the interaction from the dipole moment of neighboring magnets in this geometry is only about 1 Oe, less than 1% of the coercive field, it can have a large impact on the switching dynamics. What determines the height of the free-energy barrier is the difference between the coercive and applied fields, and 1 Oe can be a significant fraction of that. The magnetic orientations of the neighbors are seen to change the behavior of the nanomagnets in the array significantly.
Characteristics of superconducting quantum interference devices using multi-barrier superconducting junctions
Authors: Nakayama Akiyoshi , Sugio Takuma , Manabe Koji , Okabe Yoichi
Published: July 2002

We have calculated the dc Josephson current through multi-barrier superconducting junctions (MBSJs). The current-phase relationship of the MBSJs has been obtained numerically from the probability amplitude of the process in which an electron-like quasiparticle is reflected as a hole-like quasiparticle (Andreev reflection) in the junctions. Moreover, using the obtained current-phase relationship of MBSJs, the characteristics of superconducting quantum interference devices containing MBSJs have also been analyzed.
Rapid dissipation of magnetic field energy driven by plasma flows in force-free collisionless pair plasmas
Authors: Haruki T. , Sakai J. I.
Published: July 2002

It is shown by using a two-dimensional fully electromagnetic and relativistic particle-in-cell code that magnetic field energy can be strongly dissipated when external plasma flow interacts with the force-free magnetic field configuration in pair plasmas. During the early stage of the interaction, the streaming instability occurs, which induces the electromagnetic perturbations associated with the generation of a quasistatic magnetic field. In the nonlinear stage, the force-free magnetic field becomes unstable against the firehose instability, and then magnetic islands are formed through magnetic reconnection. The dissipated magnetic field energy is converted to plasma heating, as well as high-energy particle production. The energy spectrum in the high-energy region shows a law of the exponential type. When the plasma flow velocity becomes relativistic (0.9c), the effective energy conversion from the initial magnetic field energy is observed, with a conversion rate of about 90%. The interaction process between the force-free collisionless plasmas and the relativistic plasma flows may play an important role for effective magnetic field energy conversion, formation of filament structures, and high-energy particle production in astrophysical plasmas.
Experimental study of Rayleigh–Taylor instability: Low Atwood number liquid systems with single-mode initial perturbations
Authors: Waddell J. T. , Niederhaus C. E. , Jacobs J. W.
Published: July 2002

Single-mode Rayleigh–Taylor instability is experimentally studied in low Atwood number fluid systems. The fluids are contained in a tank that travels vertically on a linear rail system. A single-mode initial perturbation is given to the initially stably stratified interface by gently oscillating the tank in the horizontal direction to form standing internal waves. A weight and pulley system is used to accelerate the fluids downward in excess of the earth’s gravitational acceleration. Weight ranging from 90 to 450 kg produces body forces acting upward on the fluid system equivalent to those produced by a gravitational force of 0.33–1.35 times the earth’s gravity. Two fluid combinations are investigated: A miscible system consisting of a salt water solution and a water–alcohol solution; and an immiscible system consisting of a salt solution and heptane to which surfactant has been added to reduce the interfacial tension. The instability is visualized using planar laser-induced fluorescence and is recorded using a video camera that travels with the fluid system. The growth in amplitude of the instability is determined from the digital images and the body forces on the fluid system are measured using accelerometers mounted on the tank. Measurements of the initial growth rate are found to agree well with linear stability theory. The average of the late-time bubble and spike velocities is observed to be constant and described by Uave=0.22(πAG/k(1+A)+πAG/k(1−A)), where A is the Atwood number, k is the wave number, and G is the apparent gravity of the fluid system (i.e., the fluid system acceleration minus the earth’s gravity).
A theory of the helical ripple-induced stochastic behavior of fast toroidal bananas in torsatrons and heliotrons
Authors: Smirnova M. S.
Published: July 2002

A theory of the helical ripple-induced stochastic behavior of fast toroidal bananas in torsatrons and heliotrons [K. Uo, J. Phys. Soc. Jpn. 16, 1380 (1961)] is developed. It is supplemented by an analysis of the structure of the secondary magnetic wells along field lines. Conditions, under which these wells are suppressed in torsatrons–heliotrons by poloidally modulated helical field ripple, are found. It is shown that inside the secondary magnetic well-free region, favorable conditions exist for a transition of fast toroidal bananas to stochastic trajectories. The analytical estimation for the value of an additional radial jump of a banana particle near its turning point, induced by the helical field ripple effect, is derived. It is found to be similar to the corresponding banana radial jump in a tokamak with the toroidal field ripple. Critical values of the helical field ripple dangerous from the viewpoint of a banana transition to stochastic behavior are estimated.
High-efficiency nickel phase zone plates with 20 nm minimum outermost zone width
Authors: Peuker M.
Published: July 2002

Fresnel zone plates for high-resolution imaging in the soft x-ray regime were fabricated in nickel by a trilevel process, which makes use of electron-beam lithography, reactive-ion etching, and electrodeposition. In order to improve the zone plate’s resolution, which is determined by its outermost zone width, ZEP-7000 electron-beam resist was employed. Residues, which arose during pattern transfer by reactive-ion etching, and which hindered etching in small structures, were suppressed by a cleaning procedure. For improved electrodeposition process control, a nickel electroplating bath was optimized. Zone plates with minimum outermost zone widths of 20, 25 and 30 nm were fabricated, yielding 9.2%, 16.2% and 18.0% first-order diffraction efficiency, respectively, at λ=2.4 nm.
Energy deposition at front and rear surfaces during picosecond laser interaction with fused silica
Authors: Salleo A. , Génin F. Y. , Feit M. D. , Rubenchik A. M. , Sands T. , Mao S. S. , Russo R. E.
Published: July 2002

The difference between front-surface and rear-surface energy deposition of a 35 ps laser pulse (λ=1064 nm) in fused silica was investigated using shadowgraphic and laser-deflection techniques. Shock waves were observed in the glass and in air. Shock waves generated in air at the front surface are stronger than at the rear surface. Less than 35% of the energy incident on the surface drives the air shock waves at the rear surface. Up to 90% of the incident energy drives the air shock waves at the front surface. Laser-plasma interaction is responsible for this difference and for limiting the amount of energy deposited inside the sample during front-surface ablation. Energy deposition at the rear surface is mostly limited by self-focusing inside the material.
Measurements of edge-localized-mode induced electron cyclotron emission bursts in DIII-D
Authors: Fuchs Ch. , Austin M. E.
Published: July 2002

Intense bursts of nonthermal electron cyclotron emission (ECE) have been observed in H-mode (high confinement) discharges in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], measured with heterodyne radiometers and a broadband far infrared (FIR) detector. These bursts are triggered by edge-localized modes (ELMs) and consist of single or several spikes, most lasting 5–150 μs. Radiation temperatures reach up to several MeV. Observed spectral bandwidths are typically less than 4 GHz. Bandwidths as low as 500 MHz, which is the limit of the instrumental filter spacing, have been found. Statistical analysis shows a steady decrease of the number of spikes with rising spike energy. Burst frequencies seem to depend linearly on the magnetic field. It is speculated that the bursts are coherent second harmonic ECE.
Adaptive mesh generation for edge-element finite element method
Authors: Tsuboi Hajime , Gyimothy Szabolcs
Published: July 2002

An adaptive mesh generation method for two- and three-dimensional finite element methods using edge elements is proposed. Since the tangential component continuity is preserved when using edge elements, the strategy of creating new nodes is based on evaluation of the normal component of the magnetic vector potential across element interfaces. The evaluation is performed at the middle point of edge of a triangular element for two-dimensional problems or at the gravity center of triangular surface of a tetrahedral element for three-dimensional problems. At the boundary of two elements, the error estimator is the ratio of the normal component discontinuity to the maximum value of the potential in the same material. One or more nodes are set at the middle points of the edges according to the value of the estimator as well as the subdivision of elements where new nodes have been created. A final mesh will be obtained after several iterations. Some computation results of two- and three-dimensional problems using the proposed method are shown.
Investigation of phonon emission processes in an AlGaN/GaN heterostructure at low temperatures
Authors: Lee K. J. , Harris J. J. , Kent A. J. , Wang T. , Sakai S. , Maude D. K. , Portal J.-C.
Published: July 2002

We have measured the temperature- and field-dependent magnetoresistance in an undoped AlGaN/GaN two-dimensional electron gas sample at temperatures below 10 K. Well-resolved Shubnikov-de Haas oscillations have been observed, as well as spin splitting at magnetic fields above 8 T. The amplitude of the oscillations has been used as a thermometer for the electron temperature, Te, studies as a function of driving current have shown that under high applied electric fields the power input per electron follows a Te4.4 dependence. Comparison with numerical modeling indicates that in this heterostructure, electron-acoustic phonon scattering via the screened piezoelectric interaction is the dominant energy-loss mechanism at low temperatures.