bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of carbon nanotubes as electrodes in gas discharge tubes
Authors: Rosen R. , Simendinger W. , Debbault C. , Shimoda H. , Fleming L. , Stoner B. , Zhou O.
Published: July 2002

Gas-tube protector units are used in telecom network interface device boxes and central office switching gears to provide protection from lightning and alternating current power cross faults on the telecom network. Current gas discharge tube (GDT) protector units are unreliable from the standpoint of mean turn-on voltage and run-to-run variability. Molybdenum electrodes with various interlayer materials were coated with single-walled carbon nanotubes and analyzed for both electron field emission and discharge properties. A mean direct current breakdown voltage of 448.5 V and a standard deviation of 4.8 V over 100 surges were observed in nanotube-based GDTs with 1 mm gap spacing between the electrodes. The breakdown reliability is a factor of 4–20 better and the breakdown voltage is ∼30% lower than the two commercial products measured. The enhanced performance shows that nanotube-based GDTs are attractive overvoltage protection units in advanced telecom networks.
A promising pathway to make multiwalled carbon nanotubes
Authors: Setlur A. A. , Doherty S. P. , Dai J. Y. , Chang R. P. H.
Published: July 2002

Current theories for multiwalled nanotube growth (without metal catalysts) describe the growth of multiwalled nanotubes by the addition of carbon atoms, ions, or small molecules to an open ended nanotube nucleus. In this letter, we describe a method to make carbon nanotubes similar in quality to those found on the cathode deposit of the carbon arc by manipulating the heat treatment of various nongraphitizable carbon precursors. It is proposed that this method of making nanotubes via heat treatments is analogous to a graphitization process, where aromatic/graphitic fragments in disordered carbons assemble into three-dimensional graphitic structures. In addition, we have demonstrated that simple precursors, such as sucrose, can be used as starting materials. These experiments offer new opportunities to understand nanotube growth and could lead to scalable methods to make multiwalled nanotubes.
Simultaneous imaging of dielectric properties and topography in a PbTiO3 crystal by near-field scanning microwave microscopy
Authors: Wang Y. G. , Reeves M. E. , Rachford F. J.
Published: July 2002

We use a near-field scanning microwave microscope to simultaneously image the dielectric constant, loss tangent, and topography in a PbTiO3 crystal. By this method, we study the effects of the local dielectric constant and loss tangent in the geometry of periodic domains on the measured resonant frequency, and quality factor. We also carry out theoretical calculations and the results agree well with the experimental data and reveal the anisotropic nature of the dielectric constant.
The Meyer–Neldel rule in organic thin-film transistors
Authors: Meijer E. J. , Matters M. , Herwig P. T. , de Leeuw D. M. , Klapwijk T. M.
Published: July 2002

We have measured and analyzed the temperature and gate voltage dependencies of the field-effect mobility in organic thin-film transistors. We find that the mobility prefactor increases exponentially with the activation energy in agreement with the Meyer–Neldel rule. This behavior is demonstrated in the mobility data of solution-processed pentacene, poly(2,5-thienylene vinylene) and in mobility data reported in literature. Surprisingly, the characteristic Meyer–Neldel energy for all analyzed materials is close to 40 meV. Possible implications for the charge transport mechanism in these materials are discussed.
A formation mechanism of carbon nanotube films on SiC(0001)
Authors: Kusunoki M. , Suzuki T. , Hirayama T. , Shibata N. , Kaneko K.
Published: July 2002

We report a remarkable difference of decomposed structures on the Si(0001) and C(0001̄) faces of a SiC single crystal observed by using a cross-sectional high-resolution electron microscopy. An aligned carbon nanotube (CNT) film was fabricated on the C face perpendicular to the surface after heating at 1700 °C for half an hour in a vacuum. On the contrary, a very thin layer of graphite sheets parallel to the surface was formed on the Si face under the same condition. It is proposed that the growth of CNTs is determined by the generation of nanocaps at the initial stage, by comparing the difference of the decomposition mechanisms on the both faces.
The polarization of electron cyclotron emission spectra in the Large Helical Device
Authors: de Vries P. C. , Kawahata K. , Nagayama Y. , Sasao H. , Inagaki S. ,   LHD Group, Nagasaki K.
Published: July 2002

Electron cyclotron emission (ECE) can be used to determine the electron temperature profile in magnetized plasmas. The complex structure of the magnetic field configuration in the Large Helical Device (LHD) [M. Fujiwara et al., J. Fusion Energy 15, 7 (1996)] complicates the analysis of the ECE spectrum. The magnetic field is a nonmonotonous function of the radius and has a very large shear. The effects of the magnetic-field shear on the polarization of the ECE spectrum have been numerically and experimentally analyzed. For a high enough density (ne>1.0×1019 m−3) the polarization is found to rotate with the sheared magnetic field, yielding only a negligible mode conversion. Experiments on the polarization have been carried out at LHD. X-mode emission was detected under a specific angle, in agreement with the numerical predictions. The elliptically polarized ECE emission showed a very strong O-mode component (70% of X-mode), that can be attributed to the effect of wall reflections.
Ultrasonic investigation of Pd39Ni10Cu30P21 bulk metallic glass upon crystallization
Authors: Wang Li Min , Wang W. H. , Wang R. J. , Zhan Z. J. , Dai D. Y. , Sun L. L. , Wang W. K.
Published: July 2002

Acoustic velocities of Pd39Ni10Cu30P21 bulk metallic glass (BMG) are measured by an ultrasonic technique upon annealing. The elastic constants and the Debye temperature are obtained. A large softening of the transverse phonon is exhibited in the as-quenched BMG relative to its crystallized state. Upon crystallization, the shear modulus and the Debye temperature increase by ∼30% and ∼12%, respectively; however, the density increases by only ∼0.6%. Some anomalous acoustic and elastic behaviors are observed near the glass transition temperature and in the supercooled liquid region of the BMG. The anomalies are explained with regard to the structural changes.
Nonlinear evolution of double tearing modes in tokamaks
Authors: Fredrickson E. , Bell M. , Budny R. V. , Synakowski E.
Published: July 2002

The Δ′ formalism with neoclassical modifications has proven to be a useful tool in the study of tearing modes in high β, collisionless plasmas. In this paper the formalism developed for the inclusion of neoclassical effects on tearing modes in monotonic q-profile plasmas is extended to plasmas with hollow current profiles and double rational surfaces. First, the classical formalism of tearing modes in the Rutherford regime in low beta plasmas is extended to q profiles with two rational surfaces and it is shown that this formalism is readily extended to include neoclassical effects. The evolution of a double tearing mode on the Tokamak Fusion Test Reactor is then simulated. Reasonable agreement is found for the outer island, but the stabilizing effect of the neoclassical term is overly large for the inner island.
Break-junction experiments on the Kondo semiconductor CeNiSn: tunnelling versus direct conductance
Authors: Naidyuk Yu. G. , Gloos K. , Takabatake T.
Published: July 2002

The rare-earth Kondo semiconductor CeNiSn is investigated by point-contact and tunneling spectroscopy using mechanically controllable break junctions. I(V) characteristics and their derivatives are recorded for contacts from the metallic to the tunneling regime at temperatures between 0.1–8 K and in magnetic fields up to 8 T. It is found that CeNiSn behaves like a compound with typical metallic properties instead of exhibiting the expected semiconducting behavior. The main spectral feature is a pronounced zero-bias conductance minimum of about 10 meV width, which appears to be of magnetic nature. These break-junction experiments provide no clear-cut evidence for an energy (pseudo) gap in CeNiSn.
The influence of single magnetic impurities on the conductance of quantum microconstrictions
Authors: Namiranian A. , Kolesnichenko Yu. A. , Kolesnichenko Yu. A. , Kolesnichenko Yu. A. , Omelyanchouk A. N.
Published: July 2002

The nonlinear ballistic conductance of three-dimensional quantum microconstrictions containing magnetic impurities is investigated. The nonlinear part of the conductance, which is due to the interaction of electrons with magnetic impurities, is obtained. The analytical results are analyzed numerically. It is shown that the intensity of the Kondo anomaly in the conductance as a function of the applied voltage depends on the diameter of the constriction and the positions of the impurities.
Nuclear spin echo with allowance for the dynamic frequency shift
Authors: Buishvili T. L. , Chotorlishvili L. L. , Tsanava M. G.
Published: July 2002

The two-pulse nuclear spin echo is studied with allowance for the dynamic frequency shift due to the Suhl–Nakamura interaction. The time dependence of the echo signal amplitude is obtained for small angles of deviation of the magnetization.
X-ray diffraction and optical characterization of interdiffusion in self-assembled InAs/GaAs quantum-dot superlattices
Authors: Xu S. J. , Wang H. , Li Q. , Xie M. H. , Wang X. C. , Fan W. J. , Feng S. L.
Published: July 2002

We report on the characterization of thermally induced interdiffusion in InAs/GaAs quantum-dot superlattices with high-resolution x-ray diffraction and photoluminescence techniques. The dynamical theory is employed to simulate the measured x-ray diffraction rocking curves of the InAs/GaAs quantum-dot superlattices annealed at different temperatures. Excellent agreement between the experimental curves and the simulations is achieved when the composition, thickness, and stress variations caused by interdiffusion are taken in account. It is found that the significant In–Ga intermixing occurs even in the as-grown InAs/GaAs quantum dots. The diffusion coefficients at different temperatures are estimated.
Observation of diffraction-free propagation of surface acoustic waves around a homogeneous isotropic solid sphere
Authors: Tsukahara Yusuke , Nakaso Noritaka , Cho Hideo , Yamanaka Kazushi
Published: July 2002

This letter shows an unexpected phenomenon where a surface acoustic wave (Rayleigh wave) excited by a line source with a finite length on a solid sphere propagates along the great circle in a direction perpendicular to the line source without beam spreading due to diffraction. In experiments, a piezoelectric transducer, 1.5 mm in width and 20 mm in length, was glued on a surface of a glass ball, 80 mm in diameter, as a line source with a finite length. A beam of Rayleigh waves with frequencies centered at 1.1 MHz was excited in either direction perpendicular to the transducer length. A receiving transducer with a circular aperture, 2 mm in diameter, was used in direct contact with the surface to detect the distribution of vibration over the surface of the ball. It was observed that the excited Rayleigh wave propagated along a great circle of the ball for at least four roundtrips. The beam was confined within a narrow path around the ball, the width of which was no more than 20 mm.
Transition of electron heating mode in a planar microwave discharge at low pressures
Authors: Sugai H. , Ghanashev I. , Mizuno K.
Published: July 2002

Spatial distributions of electron density ne, electron temperature Te, and wave field intensity in a planar microwave discharge at low pressures (10–100 mTorr) are measured with the discharge power as a parameter. Two different modes of electron heating are found: a bulk heating mode characterized with high Te(∼10 eV) in underdense plasma, and a surface heating mode with low Te(∼3 eV) in overdense plasma with a hot layer near the plasma-dielectric boundary. Abrupt transition between the two modes takes place near the cutoff density nc where the electron plasma frequency coincides with the discharge frequency (2.45 GHz). In the underdense case (nenc), the evanescent wave (surface wave) near the microwave window strongly heats the electrons in the boundary layer, which are transported to the bulk region and ionize the neutral particles to therein produce relatively cold electrons.
Surface effects and high quality factors in ultrathin single-crystal silicon cantilevers
Authors: Yang Jinling , Ono Takahito , Esashi Masayoshi
Published: July 2002

Surface effects in ultrathin single-crystal silicon cantilevers of 170 nm thickness, which are optically actuated mainly by the light pressure effect, are investigated under ultrahigh vacuum (UHV) condition. Annealing the cantilevers at 1000 °C for 30 s in UHV results in an over 1 order of magnitude increase of the quality factor (Q factor), up to about 2.5×105 for cantilevers of 30–90 μm in length. The improvement of Q factor was found to be associated with the deoxidization of the surface, as determined by x-ray photoelectron spectroscopy. These results suggest that the surface effects in the ultrathin cantilevers dominate their mechanical behavior. With the promising mechanical behavior, the cantilever can be easily actuated by a laser beam (beam size: about 300×100 μm2) with power down to less than 40 μW at a wavelength of 680 nm, corresponding to 480 nW, i.e., 1.64×1012 photons/s, irradiated on the cantilever surface (60×6 μm2). This provides a rather simple way to operate the ultrathin cantilevers dynamically in UHV. Atomic scale force resolution (4.8×10−17 N) at 300 K is also expected with these cantilevers.
Effect of a nonmagnetic impurity on the superconducting transition temperature in layered structures with a non-phonon-mediated pairing mechanism
Authors: Palistrant M. E. , Kochorbé F. G.
Published: July 2002

The effect of nonmagnetic impurities on the superconducting transition temperature Tc in superconductors with a non-phonon-mediated (nonphononic) mechanism of superconductivity is investigated. Two features inherent to high-Tc superconductors are taken into account: the overlap of the energy bands, and the variable density of charge carriers. The cases of s and dx2−y2 symmetry of the order parameter are considered. It is demonstrated that the Anderson theorem is violated in the case of s symmetry of the order parameters, both on account of interband scattering of electrons on impurities and on account of intraband scattering due to the electron–hole asymmetry in systems with a nonphononic mechanism of superconductivity. Analytical expressions are obtained for the value of Tc in the regions of small and large impurity concentrations, and numerical solutions of a self-consistent system of equations are found. The influences on Tc due to various mechanisms (the change in the chemical potential and in the intraband and interband relaxation times at an impurity) are analyzed for the types of symmetry indicated above. It is shown that a number of the features inherent to high-Tc superconductors can be explained, including, in particular, the possibility of complete suppression of superconductivity by a nonmagnetic impurity and the possibility of attaining high values of Tc upon doping of an insulator. It is also shown that as the interband scattering on the impurity potential increases, there is a change in the dependence of Tc on the impurity concentration. Qualitative agreement is obtained between the proposed theory and the experimental data on the dependence of Tc on the concentration of Zn, Al, and Ga impurities in yttrium and lanthanum ceramics.
Impedance of a thin metal film in the regime of strong magnetodynamic nonlinearity
Authors: Derev’anko S. A. , Tkachev G. B. , Yampol’skiı̆ V. A.
Published: July 2002

The nonlinear response of a metallic film to the self-magnetic field of an ac transport current is investigated theoretically. The nonlinearity is due to a magnetodynamic mechanism which derives from the influence of the magnetic field of the current on the dynamics of the conduction electrons and is most typical for pure metals at low temperatures. The nonlinearity leads to a dependence of the surface impedance on the amplitude of the ac current. It is shown that the real part of the surface impedance, which is responsible for the Joule losses, has a smooth maximum as a function of the amplitude. The imaginary part of the surface impedance is also a nonmonotonic function of the amplitude and has a minimum.
Absorption of electromagnetic field of the millimeter-wave band in perfect dielectric crystals
Authors: Ganapolskiı̆ E. M.
Published: July 2002

The dielectric losses in high-quality crystals of sapphire, ruby, fluorite, and quartz are measured at a frequency of 36 GHz in the temperature range 4.2–300 K by the spherical dielectric resonator method. The absorption coefficient of the electromagnetic field per unit wavelength Γ in the crystals is found to agree both in order of magnitude and in its temperature dependence with the Gurevich theory, which treats the absorption as being the result of an electrophonon interaction of the field with acoustical phonons as a result of the anharmonicity of the crystal lattice. It is found that the crystals studied have extremely low absorption in the millimeter-wave band even at moderately low temperatures (e.g., for sapphire Γ≈10−8 at T≈40 K). Measurements are made of the residual absorption of the electromagnetic field at low temperatures, which is due to a single-phonon process of field absorption at defects of the crystal structure. It is found that the temperature dependence of the absorption of an electromagnetic field in a crystal is correlated with that for the absorption of hypersound.
Reactivity of curved and planar carbon–nitride structures
Authors: Stafström Sven
Published: July 2002

The reactivity of different carbon–nitride structures has been studied using density functional theory calculations. The studies involve C59N and clusters of curved and planar CNx structures. Nitrogen is shown to lower the energy of pentagon defects in the graphite like structures, whereas heptagons are unlikely to be present. From this observation, it follows that nitrogen stimulates growth of fullerene like structures in CNx. The presence of nitrogen also increases the reactivity of the carbon atoms around the nitrogen. This leads to cross linking between basal planes which can explain the hardness and elasticity of CNx films.
Nature of critical current and coherent phenomena in granular MoNx thin films
Authors: Prokhorov V. G. , Kaminsky G. G. , Lee Y. P. , Kravchenko I. I.
Published: July 2002

Investigations of the critical current versus temperature and applied magnetic field are carried out for granular MoNx films. All samples display a two-stage superconducting transition and can be treated as a percolating network of SNS contacts with a Josephson coupling between grains. The temperature behavior of the critical current for the films studies is the same as the Ic(T) dependence for a SNS junction in the diffusive limit. The value of critical current in a magnetic field is governed by the pinning of Josephson vortices.
Anomalies of the magnon spectrum of a bounded antiferromagnet with a center of antisymmetry. I. Zero-exchange approximation
Authors: Tarasenko S. V.
Published: July 2002

It is shown that if the magnetic structure of an infinite antiferromagnet has a center of antisymmetry rather than a center of symmetry, then in a slab of such a crystal the formation of propagating nonexchange bulk spin waves of a previously unknown type can occur. The relationship between the Néel and Debye temperatures of the antiferromagnetic crystal is of fundamental importance for the structure of the spectrum of these magnons.
Mechanisms for the generation of thermopower carbon–graphite materials with different crystal structures
Authors: Matsuı̆ L. Yu. , Ovsienko I. V. , Vovchenko L. L.
Published: July 2002

The results of experimental and theoretical studies of the temperature dependence of the thermopower of carbon–graphite materials with different crystal structures are presented. It is shown that the mechanisms giving rise to the thermopower in these materials are different. In coarse-grained graphites the main contribution to the total thermopower at low temperatures derives from the phonon-drag effect on the charge carriers, while the contribution of the diffusion component of the thermopower at these temperatures is insignificant. In a fine-grained graphite the main contribution to the total thermopower in the entire temperature interval is from the diffusion component of the thermopower, while the contribution of the phonon component is small even at low temperatures.
Dependence of the shear modulus of a Bi ceramic on external influences in the temperature interval 65–130 K
Authors: Chirkina L. A. , Sergeeva G. G. , Starodubov Ya. D. , Lazareva M. B. , Okovit V. S.
Published: July 2002

The dynamic shear modulus of a two-phase bismuth ceramic (Bi-2212 and Bi-2223) is measured as a function of temperature, magnetic field, and deformation. The results attest to the multistage penetration of magnetic field into the sample. It is found that the level of the shear modulus is sensitive to the magnetic and defect structure of the ceramic. The conjecture that the observed defects of the shear modulus are due to phase separation in the CuO2 planes of the 2212 phase is discussed.
Mechanisms for very long electromigration lifetime in dual-damascene Cu interconnections
Authors: Hu C-K. , Gignac L. , Malhotra S. G. , Rosenberg R. , Boettcher S.
Published: July 2002

Electromigration in 0.27 μm wide Cu damascene interconnections has been investigated. The results show that the electromigration time to failure of Cu interconnections is greatly influenced by the thickness of the metal liner at the contact between the via and underlying line. A remarkably long lifetime was achieved when a 3 nm thick liner (at the via/metal line interface) was used, since the abrupt mass flux divergence at this interface normally seen is greatly diminished. Voids were found in the regions where there was no electric field and on the bamboo Cu grain structure. Void formation is explained by the effect of a vacancy wind.
Hydrodynamic fluctuations in a gas of quasiparticles
Authors: Aleksin V. F. , Khodusov V. D.
Published: July 2002

The hydrodynamic fluctuations in a slightly nonideal gas of Bose quasiparticles with nonconservation of quasiparticle number are considered. The fluctuation dissipation theorem is used to find the spectral densities of the correlations of the fluctuations for the basic quantities characterizing the quasiparticle gas, which are expressed in terms of the kinetic coefficients. In the existence region of secondary waves in the gas of quasiparticles these spectral densities have a Lorentzian shape. For certain pure single crystals of cubic symmetry the temperature dependence of the resonant spectral density of energy fluctuations of the phonon gas are given, from which one can determine the temperature interval in which second sound waves exist. For NaF this interval agrees with the temperature region in which second sound is observed in experiment.
A new computational scheme for the Dirac–Hartree–Fock method employing an efficient integral algorithm
Authors: Yanai Takeshi , Nakajima Takahito , Ishikawa Yasuyuki , Hirao Kimihiko
Published: July 2002

A highly efficient computational scheme for four-component relativistic ab initio molecular orbital (MO) calculations over generally contracted spherical harmonic Gaussian-type spinors (GTSs) is presented. Benchmark calculations for the ground states of the group IB hydrides, MH, and dimers, M2 (M=Cu, Ag, and Au), by the Dirac–Hartree–Fock (DHF) method were performed with a new four-component relativistic ab initio MO program package oriented toward contracted GTSs. The relativistic electron repulsion integrals (ERIs), the major bottleneck in routine DHF calculations, are calculated efficiently employing the fast ERI routine SPHERICA, exploiting the general contraction scheme, and the accompanying coordinate expansion method developed by Ishida. Illustrative calculations clearly show the efficiency of our computational scheme.
Interlayer coupling field in spin valves with CoFe/Ru/CoFe/FeMn synthetic antiferromagnets (invited)
Authors: Kim K. Y. , Jang S. H. , Shin K. H. , Kim H. J. , Kang T.
Published: July 2002

Top synthetic spin valves with structure Ta/NiFe/CoFe/Cu/CoFe(P1)/Ru/CoFe(P2)/FeMn/Ta on Si (100) substrate with natural oxide were prepared by dc magnetron sputtering system. We have changed only the thickness of the free layer and the thickness difference (P1−P2) in the two ferromagnetic layers separated by Ru, and investigated the effect of magnetic film thickness on the interlayer coupling field in a spin valve with a synthetic antiferromagnet. As the free layer thickness decreased from 70 to 20 Å, the interlayer coupling field was increased due to the magnetostatic coupling (orange peel coupling). In the case of the thickness difference in the pinned layers, the interlayer coupling field agreed with the modified Néel model suggested in the top synthetic spin valve structures. However, in the case of tP1=tP2, and tP1=tP2+5 Å, it was found that the interlayer coupling field could not be explained by the modified Néel model. The deviation of the modified Néel model at the dip zone could be due to the large canting of the pinned layers, which depend on applied field and different thickness in synthetic antiferromagnetic structure. The dependence of Cu thickness on the interlayer coupling field was investigated and 10 Oe of the interlayer coupling field was obtained when the Cu thickness is 32 Å.
Fully integrated probe for proton nuclear magnetic resonance magnetometry
Authors: Boero G. , Frounchi J. , Furrer B. , Besse P.-A. , Popovic R. S.
Published: July 2002

In this article, we present the first fully integrated nuclear magnetic resonance probe ever realized. Planar spiral coils, a radio-frequency preamplifier, a mixer, and an audio-frequency amplifier are integrated using a standard complementary metal–oxide–semiconductor technology on a single silicon chip of 10 mm2. A 1 mm3 solid sample of cis-polyisoprene, placed over one of the integrated coils, is used as resonating material. The realized integrated probe allows measurements of static magnetic fields from 0.7 to 7 T with a magnetic-field resolution better than 1 ppm/Hz1/2, an absolute accuracy better than 3 ppm, and a spatial resolution of about 1 mm.
Temperature and gate voltage dependent transport across a single organic semiconductor grain boundary
Authors: Chwang Anna B. , Frisbie C. Daniel
Published: July 2002

Temperature and gate voltage dependent transport measurements on single grain boundaries in the organic semiconductor sexithiophene (6T) are described. Isolated grain boundaries are formed by vacuum deposition of pairs of 6T grains between Au electrodes 1.5–2.0 μm apart on SiO2/Si substrates; grain boundary formation is monitored using atomic force microscopy. The Si substrate serves as the gate electrode. We show from the activation energy, threshold voltage, and field effect resistance of the grain boundary junction that carrier transport is limited by the grain boundary. The activation energy of room temperature transport is of the order of 100 meV at carrier densities of ∼1018 cm−3 and decreases with increasing carrier concentration (gate voltage). We also observe that longer grain boundaries with smaller misorientation angles result in larger currents through a grain boundary. We relate our data to two models, one that assumes acceptor-like traps localized at a grain boundary and another that assumes localized donor-like traps. Using the activation energy as a measure of the potential well (acceptor model) or barrier (donor model) at a grain boundary, we calculate trap densities of the order of 1012 cm−2 assuming discrete trap energies of 0.015 and 0.34 eV relative to the valence band in the acceptor and donor models, respectively. We note that the calculated values of the trap density are sensitive to the estimated value of the trap level.
Dipole–dipole interaction in superparamagnetic nanocrystalline Fe63.5Cr10Si13.5B9Cu1Nb3
Authors: Franco V. , Conde C. F. , Conde A. , Kiss L. F. , Kaptás D. , Kemény T. , Vincze I.
Published: July 2002

Cr-substituted Finemet-type nanocrystalline alloy (Fe63.5Cr10Si13.5B9Cu1Nb3) has been studied by differential scanning calorimetry, x-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. The Curie temperature of the remaining amorphous phase decreases as the crystalline volume fraction increases, reaching values below room temperature. This feature makes the alloy adequate for studying the magnetic decoupling of the (Fe,Si) nanocrystals at moderated temperatures and, in particular, the superparamagnetic relaxation in broad temperature and crystalline fraction ranges. It was shown that the anomalous dependence of the coercive field on the annealing temperature can be satisfactorily explained assuming a dipolar-type interaction between the crystallites.
Effect of multiplicative noise on the nonequilibrium properties of vortices in layered superconductors
Authors: Shaposhnikova T. S.
Published: July 2002

The conditions for the appearance of multiplicative noise in the vortex system of superconductors with pronounced layering are considered. A stochastic differential equation of motion is constructed for a pinned 2D vortex, and its stationary solution is found. The region of parameters of this equation in which a stochastic phase transition can occur in the vortex system is determined. An experimental scheme is proposed for observing such a transition by the method of power absorption from an alternating magnetic field.
Influence of pressure, temperature, and magnetic field on the resistivity and magnetoresistive effect of lanthanum manganite ceramics and films with the composition La0.7Mn1.3O3±δ
Authors: Kucherenko S. S. , Pashchenko V. P. , Polyakov P. I. , Shtaba V. A. , Shemyakov A. A.
Published: July 2002

The effects of magnetic fields H=0–8 kOe and high hydrostatic pressures P=0–2.2 GPa on the resistivity ρ, magnetoresistive effect ΔR/R0, metal–semiconductor phase transition temperature Tms, and peak temperature Tp of the magnetoresistive effect are investigated over a wide range of temperatures T=77–350 K in a ceramic target and a laser film of the same cationic compound La0.7Mn1.3O3±δ. Increasing H and P leads to a decrease of the resistivity. The observed difference in ρ, Tms, and Tp between the ceramics and film samples is explained by the difference of their oxygen nonstoichiometry. The magnetoresistive effect decreases with increasing H and decreases with increasing P. The temperatures Tms and Tp of the ceramics and film increase with increasing P, but the effect is stronger in the film. The observation of two phase transitions in the ceramics (the main transition at Tms=250 K and an additional transition at Tms′=210 K) is explained by its mesoscopic inhomogeneity of the cluster type.
Dimensional effects on field emission properties of the body for single-walled carbon nanotube
Authors: Zhou Gang , Duan Wenhui , Gu Binglin
Published: July 2002

In this letter, by use of the first-principles calculations, field emission properties of the body are investigated for a series of dimension-variational single-walled carbon nanotubes (SWNTs). We find that in the course of increasing tube diameter, at low emission currents, field emission properties worsen, chemical stability enhances and binding energy increases, which indicates that optimized field emitter could be achieved; while at high applied voltages, the highest emission current density may appear for the SWNT with some peculiar dimension. We predict that the body of the SWNT could behave as a peculiar field emitter with high threshold voltage and high current density.
An ab initio procedure for deriving atomic polarizability tensors in molecules
Authors: Maple Jon R. , Ewig Carl S.
Published: July 2002

A new method for deriving anisotropic atomic dipole polarizability tensors for predicting molecular polarizabilities is presented. The procedure uses the relationships between atomic and molecular multipoles to rigorously derive the atomic polarizability tensors from quantum mechanically computed molecular dipole–quadrupole polarizabilities and their derivatives with respect to molecular deformations. The resulting atomic polarizabilities have been tested for the water molecule at several ab initio levels of approximation by comparing the summed atomic polarizabilities with the conventional ab initio molecular results and with experiment. Good agreement was found between molecular polarizabilities found using the two methods. The model is also shown to give interaction energies between a water molecule and a point charge that accurately reproduce ab initio results. A similar comparison of the atomic polarizability tensors is shown for the N-methylacetamide molecule. For most atom types, the atomic polarizability tensors were found to be significantly anisotropic, indicating that this method can significantly improve on the isotropic approximation for predicting molecular properties that depend strongly on polarization effects.
A direct method for integrable couplings of TD hierarchy
Authors: Zhang Yufeng , Zhang Hongqing
Published: July 2002

A direct method for establishing integrable couplings is proposed in this paper. As an example illustration, integrable couplings of TD hierarchy are obtained by constructing a suitable transformation of Lax pairs and a new Lie algebra.
Modeling of diamagnetic stabilization of ideal magnetohydrodynamic instabilities associated with the transport barrier
Authors: Huysmans G. T. A. , Sharapov S. E. , Mikhailovskii A. B. , Kerner W.
Published: July 2002

A new code, MISHKA-D (Drift MHD), has been developed as an extension of the ideal magnetohydrodynamics (MHD) code MISHKA-1 in order to investigate the finite gyroradius stabilizing effect of ion diamagnetic drift frequency, ω*i, on linear ideal MHD eigenmodes in tokamaks in general toroidal geometry. The MISHKA-D code gives a self-consistent computation of both stable and unstable eigenmodes with eigenvalues |γ|≅ω*i in plasmas with strong radial variation in the ion diamagnetic frequency. Test results of the MISHKA-D code show good agreement with the analytically obtained ω*i spectrum and stability limits of the internal kink mode, n/m=1/1, used as a benchmark case. Finite-n ballooning and low-n kink (peeling) modes in the edge transport barrier just inside the separatrix are studied for high confinement mode (H-mode) plasmas with the ω*i effect included. The ion diamagnetic stabilization of the ballooning modes is found to be most effective for narrow edge pedestals. For low enough plasma density the ω*i stabilization can lead to a second zone of ballooning stability, in which all the ballooning modes are stable for any value of the pressure gradient. For internal transport barriers typical of the Joint European Torus [JET, P. H. Rebut et al., Proceedings of the 10th International Conference, Plasma Physics and Controlled Nuclear Fusion, London (International Atomic Energy Agency, Vienna, 1985), Vol. I, p. 11] optimized shear discharges, the stabilizing influence of ion diamagnetic frequency on the n=1 global pressure driven disruptive mode is studied. A strong radial variation of ω*i is found to significantly decrease the stabilizing ω*i effect on the n=1 mode, in comparison with the case of constant ω*i estimated at the foot of the internal transport barrier.
Unusual vibrational dynamics of the acetic acid dimer
Authors: Lim Manho , Hochstrasser Robin M.
Published: July 2002

The vibrational relaxation of the C=O stretching mode of the CH3CO2H cyclic dimer, the CH3CO2D cyclic dimer, and CH3CO2CH3 were measured in CCl4 solution at room temperature. The population relaxation of the v=1 state of the C=O mode is nonexponential, modeled with a biexponential decay having a fast time constant in the subpicosecond regime and a slow time constant of a few picoseconds. For the cyclic dimers of the acetic acids, the fast component dominates the population decay, whereas the slow component dominates the decay of the CH3CO2CH3, the model compound for the monomeric acetic acid. Deuteration of the dimer increases the relaxation time constant. The non-hydrogen-bonding monomer methyl acetate also has a subpicosecond decay constant. The pump–probe anisotropy decay reveals that the orientational dynamics of these molecules also occurs on the subpicosecond time scale and is reasonably well described by rotational diffusion in the slip hydrodynamic limit. Stimulated infrared photon echo decay experiments reveal that the correlation function of the frequency fluctuations of the cyclic acid dimer has a motionally narrowed process described by a 4 ps pure dephasing time and process with a 2.1 ps correlation time, comparable to a solvent response time. The dephasing dynamics is dominated by the population relaxation. In analyzing the photon echo data, the contribution from the rotational diffusion is incorporated by approximating the cyclic acid dimer as a symmetric top diffuser with its transition dipole located in the molecular plane but not parallel to any of the principal axes. General formulas, which will be useful in other applications, for incorporation of the diffusive dynamics of the symmetric top into the third order response functions are obtained. Nonexponential fast vibrational relaxation of C–CO2–X moiety is not adequately described by the anharmonic coupling with the nearby combination and overtone bands. In the regime where the rotational, vibrational, and dephasing times are all comparable, the solvent memory effects may play a role in vibrational dynamics, causing unusually rapid nonexponential population decay.
Finite-correlation-time effects in the kinematic dynamo problem
Authors: Schekochihin Alexander A. , Kulsrud Russell M.
Published: July 2002

Most of the theoretical results on the kinematic amplification of small-scale magnetic fluctuations by turbulence have been confined to the model of white-noise-like (δ-correlated in time) advecting turbulent velocity field. In this work, the statistics of the passive magnetic field in the diffusion-free regime are considered for the case when the advecting flow is finite-time correlated. A new method is developed that allows one to systematically construct the correlation-time expansion for statistical characteristics of the field such as its probability density function or the complete set of its moments. The expansion is valid provided the velocity correlation time is smaller than the characteristic growth time of the magnetic fluctuations. This expansion is carried out up to first order in the general case of a d-dimensional arbitrarily compressible advecting flow. The growth rates for all moments of the magnetic-field strength are derived. The effect of the first-order corrections due to the finite correlation time is to reduce these growth rates. It is shown that introducing a finite correlation time leads to the loss of the small-scale statistical universality, which was present in the limit of the δ-correlated velocity field. Namely, the shape of the velocity time-correlation profile and the large-scale spatial structure of the flow become important. The latter is a new effect, that implies, in particular, that the approximation of a locally-linear shear flow does not fully capture the effect of nonvanishing correlation time. Physical applications of this theory include the small-scale kinematic dynamo in the interstellar medium and protogalactic plasmas.
Measurement of spurious impurity concentrations in ASDEX Upgrade by x-ray spectroscopy
Authors: Bolshukhin D. , Neu R. , Schlögl D. , Dux R. ,   ASDEX Upgrade Team
Published: July 2002

A two channel multicrystal spectrometer of Bragg geometry was constructed for measurements of absolute photon fluxes of ASDEX Upgrade plasmas in the wavelength range from 0.5 up to 26 Å. The calibration of the spectrometer was done by an ab initio calculation using the absolute reflection data from literature of four crystals. The calibration was validated by the direct comparison with other absolute calibrated diagnostics at the ASDEX Upgrade as well as by using the impurity transport code STRAHL. The absolute impurity concentrations of elements from N up to Cu have been extracted. O and F appear to be main impurity components after C, which is routinely measured by an independent diagnostic. Despite the significantly lower concentration of Ar, it contributes measurably to Zeff and can account for main plasma impurities as well. Other impurities either have never been seen or appear only sporadically (like Fe and Cu) and normally do not contribute significantly to the plasma radiation in the soft x-ray range.
Electron energy state dependence on the shape and size of semiconductor quantum dots
Authors: Li Yiming , Voskoboynikov O. , Lee C. P. , Sze S. M. , Tretyak O.
Published: July 2002

In this article we present a unified model for studying the effect of the sizes and shapes of small semiconductor quantum dots on the electron and hole energy states. We solved the three-dimensional effective one band Schrödinger equation for semiconductor quantum dots with disk, lenticular, and conical shapes. For small InAs/GaAs quantum dots we found a substantial difference in the ground state and first excited state electron energies for dots with the same volume but different shapes. Electron energy dependence on volume is found to be quite different from the commonly quoted V−2/3. The exponent can vary over a wide range and depends on the dot shapes.
Analysis of large-scale fluctuation structures in the scrape-off layer of the Wendelstein 7-AS stellarator
Authors: Grulke O. , Klinger T. , Endler M. , Piel A. ,   W7-AS Team
Published: July 2002

Floating potential and ion saturation current fluctuations in the scrape-off layer of the Wendelstein 7-AS stellarator are investigated by using conditional averaging. If floating potential fluctuations are taken as the reference signal for the conditional averaging procedure, large-scale single-vortex structures are predominately found. If ion saturation current fluctuations are taken as the reference signal, double-vortex structures are detected. The phase shift between the two different structures is π/2. The double-vortices are interpreted as a superposition of counter rotating single-vortex potential structures. Both lead to ion saturation current fluctuations of the same sign due to E×B convection of plasma in parallel to the background plasma pressure gradient. The conditional averaging approach is directly compared to cross-correlation analysis. Both methods lead to similar physical conclusions. Lifetime and extent of structures are systematically overestimated by correlation analysis. It is demonstrated that only fluctuations with amplitudes larger than 0.5σ contribute to the detected large-scale structures.
Femtosecond laser triggering of a sub-100 picosecond jitter high-voltage spark gap
Authors: Luther B. M. , Furfaro L. , Klix A. , Rocca J. J.
Published: July 2002

We have demonstrated sub-100 ps jitter operation of a pressurized high-voltage air spark gap triggered by a femtosecond Ti:Sapphire laser. Time delay statistical fluctuations with a standard deviation as low as +−σ=0.037 ns were obtained.
Operation of a gated field emitter using an individual carbon nanofiber cathode
Authors: Guillorn M. A. , Melechko A. V. , Merkulov V. I. , Ellis E. D. , Britton C. L. , Simpson M. L. , Lowndes D. H. , Baylor L. R.
Published: July 2002

We report on the operation of an integrated gated cathode device using a single vertically aligned carbon nanofiber as the field emission element. This device is capable of operation in a moderate vacuum for extended periods of time without experiencing a degradation of performance. Less than 1% of the total emitted current is collected by the gate electrode, indicating that the emitted electron beam is highly collimated. As a consequence, this device is ideal for applications that require well-focused electron emission from a microscale structure.
Reaction pathways in remote plasma nitridation of ultrathin SiO2 films
Authors: Niimi Hiro , Khandelwal Amit , Lamb H. Henry , Lucovsky Gerald
Published: July 2002

Low-temperature nitridation of 3 nm SiO2 films using He/N2 and N2 remote radio frequency (rf) plasmas was investigated. On-line Auger electron spectroscopy and angle-resolved x-ray photoelectron spectroscopy (ARXPS) were employed to determine the concentration, spatial distribution, and local chemical bonding of nitrogen in the resultant films. Experiments were performed using a substrate temperature of 300 °C and 30 W rf power. Nitridation using an upstream He/N2 remote plasma at 0.1 Torr incorporates nitrogen at the top surface of the SiO2 film. In contrast, a lower concentration of nitrogen distributed throughout the film is obtained when the process pressure is increased to 0.3 Torr. ARXPS indicates a N–Si3 local bonding configuration, irrespective of the spatial distribution of N atoms. Slightly more nitrogen is incorporated using a downstream He/N2 plasma at each process pressure. By comparison, nitridation of SiO2 films using a N2 remote plasma at 0.1 Torr is very slow. Optical emission spectroscopy indicates that He dilution enhances the generation of N2+(B 2Σu+) species by altering the plasma electron energy distribution and by providing an additional kinetic pathway (Penning ionization). Changing the He/N2 remote plasma configuration from upstream to downstream (at 0.1 and 0.3 Torr) also enhances N2+(B 2Σu+) generation. For upstream He/N2 remote plasmas, the intensity of N2 first positive emission from N2(B 3Πg) states increases with pressure, whereas the N2+ first negative emission from N2+(B 2Σu+) states decreases. We infer from these observations that N2+ species are primarily responsible for top surface nitridation at 0.1 Torr, and that neutral species [N2(A 3Σu+) metastables and N atoms] are associated with sub-surface nitrogen incorporation.
Effect of low temperatures on deformation localization in supersaturated Al–Li alloys
Authors: Grigorova T. V. , Isaev N. V. , Natsik V. D. , Shumilin S. É.
Published: July 2002

Active loading at temperatures 295, 77, and 0.5 K is used to study the parameters of the stretching curve and the plastic deformation distribution in quenched and aged Al–Li alloys with 7.0 and 10.4 at.% lithium. It is shown that for aged alloys, in contrast to quenched alloys, the strength is characteristically high, the plasticity reserves are low, and the deformation distribution (macroscopic sections of deformation localization) is nonuniform, if loading is conducted at room and low temperatures (295 and 77 K). The deformation of quenched and aged alloys under deep cooling (at temperature 0.5 K) is distributed more homogeneously through the samples. The results are attributed to the influence of low temperatures on the rate of thermally activated processes, controlling the motion of dislocations through impurity barriers and the deformation-diffusion dissolution of δ′ precipitates.
Parametric fluorescence in oxidized aluminum gallium arsenide waveguides
Authors: De Rossi A. , Berger V. , Calligaro M. , Leo G. , Ortiz V. , Marcadet X.
Published: July 2002

Parametric fluorescence in low-loss oxidized aluminum gallium arsenide heterostructure waveguides is quantitatively analyzed. A parametric fluorescence efficiency as high as 6×10−7 W/W has been measured in a 3.2-mm-long waveguide. This corresponds to a normalized conversion efficiency, scaled with the waveguide length, of about 1000% cm−2 W−1, eight times higher than with LiNbO3 waveguides. This opens the perspective of a microoptical parametric oscillation threshold below 100 mW.