bluuebunny/crossref_metadata_2025 huggingface dataset:
High resolution analysis of the complex symmetric CF3 stretching chromophore absorption in CF3I
Authors: He Yabai , Hollenstein Hans , Quack Martin , Richard Erik , Snels Marcel , Bürger Hans
Published: July 2002

We report a detailed high resolution rovibrational analysis of the ν1 fundamental absorption band system of CF3I, which appears as a very complex resonance polyad. Pulsed slit jet diode laser spectra (instrumental bandwidth ⩽25 MHz) of CF3I (neat and seeded in He) at rotational temperatures of about 7, 30, and 70 K have been obtained. The low J, K part of the observed transitions can be fitted by a model including the anharmonically coupled levels ν1(ν̃0=1076.0551 cm−1), 2ν50(ν̃0=1079.6732 cm−1) and ν3+3ν6±3(ν̃0=1072.3884 cm−1), and the local Coriolis perturber ν3+3ν6+1(ν̃0=1074.1845 cm−1). The level ν3+3ν6+3 was observed and included for the first time, and an additional anharmonic coupling between the levels ν3+3ν6+3 and ν3+3ν6−3 proved relevant. In order to analyze the higher J, K regions, perturbations arising from 2ν5±2 turned out to be important. In certain regions of the spectra, effects due to nuclear quadrupole interaction are clearly visible. We report new sets of parameters for a low J, K model and for an extended model including the perturber 2ν5±2. The results are discussed in relation to infrared multiphoton excitation of CF3I, infrared absorption in the atmospheric window and intramolecular vibrational redistribution.
Relativistic self-focusing and its effect on stimulated Raman and stimulated Brillouin scattering in laser plasma interaction
Authors: Mahmoud Saleh T. , Sharma R. P.
Published: July 2002

This paper presents the effect of relativistic electron mass variation on a very high-power laser beam propagating in unmagnetized plasma. By choosing the laser beam and plasma parameters, the laser beam could be propagated without convergence or divergence (self-trapped mode). When the laser power exceeds the critical power, the laser beam can undergo periodic self-focusing due to relativistic nonlinearity. The stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) processes in the presence of this relativistic nonlinearity are discussed. The effect of finite laser beam size and that of scattered beam and relativistic self-focusing of the pump laser beam on SRS and SBS back-reflectivity have also been studied, including pump depletion.
Linear theory of gyro-traveling-wave-tubes with distributed losses
Authors: Nusinovich G. S. , Sinitsyn O. V. , Kesar A.
Published: July 2002

A small-signal theory describing two-stage gyro-traveling-wave tubes (gyro-TWTs) with the first stage having distributed losses is developed. In addition to the study of a small-signal gain in this device, the self-excitation conditions for parasitic backward waves are also analyzed. The theory is illustrated by using it for describing the performance of the gyro-TWT designed at the Naval Research Laboratory. The results show a very good agreement between the predictions of analytical theory and a thorough numerical analysis based on the use of well-developed codes.
Nonlinear conductivity of lower-hybrid-wave heated plasma in the presence of an electric field
Authors: Farshi E. , Sato K. N.
Published: July 2002

A new conductivity for lower-hybrid-wave heated plasmas is derived. This induced conductivity shows a nonlinear behavior in the presence of an electric field and can represent the plasma as an active element. The role of runaway electrons produced is important on this nonlinear conductivity. The new expression may be important for radio frequency current drive ramp-up and current drive efficiency by the lower-hybrid wave.
Stepwise addition reactions in ammonia synthesis: A first principles study
Authors: Zhang Changjun , Liu Zhi-Pan , Hu P.
Published: July 2002

Catalytic ammonia synthesis is believed to proceed via dissociation of N2 and H2 with subsequent stepwise addition reactions from an adsorbed nitrogen atom to NH3. The first step, N2 dissociation, has been thoroughly studied. However, little is known about the microscopic details of the stepwise addition reactions. To shed light on these stepwise addition reactions, density functional theory calculations with the generalized gradient approximation are employed to investigate NHx (x=1,3) formation on Ru(0001). Transition states and reaction barriers are determined in each elementary step. It is found that the reaction barriers for stepwise addition reactions are rather high, for example, the barrier for NH hydrogenation is calculated to be 1.28 eV, which is comparable with that of N2 dissociation. In addition, one of the stepwise addition reactions on a stepped surface is also considered. The reaction barrier is found to be much higher than that of N2 dissociation on the same stepped surface, which indicates the importance of stepwise addition reactions in ammonia synthesis.
A generalized compressible Reynolds lubrication equation with bounded contact pressure
Authors: Wu Lin , Bogy D. B.
Published: July 2002

A new modified Reynolds equation is derived based on physical principles for rarefied gas for compressible and extremely thin layer gas lubrication. For the one-dimensional problem, theoretical analysis and numerical simulation are employed to show that the new equation does not predict an unphysical unbounded pressure singularity in the limit of contact between the bearing surface and the moving surface. We also show the same is true for other existing models with higher than first order slippage correction, which introduce additional diffusion terms that are functions of the spacing with similar order to that of the convection terms. These developments remove the ambiguity of some previously published analyses and correct prior erroneous statements that all existing generalized Reynolds equation models predict nonintegrable singular pressure fields in the limit of contact. The asymptotic analysis also supplies a means for deriving the needed additional boundary condition at the boundary of a contact region. For the two-dimensional problem, we show by numerical analysis that there are also no unbounded contact pressure singularities for the new model and other models with corrections higher than first order, and that the singularity is weaker than in the one-dimensional case for these lower order correction models due to the cross diffusion effect introduced by the additional dimension.
Influence of surface chemistry on the transport of H atoms in a supersonic hydrogen plasma jet
Authors: Mazouffre S. , Vankan P. , Engeln R. , Schram D. C.
Published: July 2002

The transport of ground-state hydrogen atoms in the expansion of a thermal hydrogen plasma created by a cascaded arc is studied by means of two-photon absorption laser induced fluorescence. The low-dissociation degree measured at the source exit implies that H atoms flow in a H2 environment. It is shown that the H atom expansion pattern is in disagreement with the neutral gas supersonic expansion theory. Indeed the transport of H atoms in the plasma jet is strongly influenced by surface-recombination processes. Because of the large density gradients between the core of the jet and its surroundings induced by the recombination of H atoms at the reactor walls, hydrogen atoms diffuse out of the plasma jet in the course of the expansion. When the surface loss probability is high, i.e., the combination of a large wall-recombination probability with a long residence time, the losses of radicals by diffusion cannot be avoided even when the mass of the carrier gas is close to the mass of the radical.
Properties of CO adsorbed in ZSM5 zeolite: Density functional theory study using the embedding scheme based on electron density partitioning
Authors: Wesołowski T. A. , Goursot A. , Weber J.
Published: July 2002

The CO molecule is frequently used as a probe in studies of zeolites where it adsorbs on metal cations. Compared with the free CO molecule, the stretching frequency of CO adsorbed in a zeolite is blue-shifted. The magnitude of the shift depends on the cation. The theoretical studies by Ferrari et al. [J. Chem. Phys., 105, 4129 (1996)] show that the isolated cation does not provide a good model of the zeolite because the calculated shifts are significantly overestimated. In this work, the effects of the interactions between the Me+CO (Me=Li, Na, or K) complex and the zeolite framework on the properties of CO adsorbed on the cation site are investigated. The properties of the investigated complexes are studied using the embedded molecule approach applying the orbital-free effective embedding potential derived within the subsystem formulation of density functional theory. In order to identify the major microsopic effects affecting the properties of the bound probe molecule, a hierarchy of cluster models is used to represent the zeolite framework. For the largest cluster model applied, the calculated frequency shifts agree within few cm−1 with experimental data.
Integrating sphere optical zero spectral correction
Authors: Soares Olivério D. D. , Costa José L. C.
Published: July 2002

A single direct correction procedure is proposed comprehending optical zero correction and integrating sphere errors (dark level error, gloss trap error, and sphere nonuniformity error). Linearization is avoided as it sacrifices measurement accuracy. The analysis of this approach is in the spectrophotometric domain. The method could serve to standardize the procedure to permit harmonization of concerned metrological scales.
Periodic Hartree–Fock study of the adsorption of molecular oxygen on a reduced TiO2 (110) surface
Authors: de Lara-Castells M. P. , Krause Jeffrey L.
Published: July 2002

We present a theoretical analysis of O2 adsorption on a reduced TiO2 (110) rutile surface, based on periodic ab initio Hartree–Fock calculations. Three different orientational approaches, three different spin symmetries, and two different adsorption sites are considered. We also consider the possibility that the surface can absorb more than one oxygen molecule. Positions of the surface ions, oxygen ions belonging to the third and fourth layers of the slab, and the bond lengths of the O2 and O2-substrate distances are optimized. Adsorption energies, admolecule-substrate bond lengths, spin densities and Mulliken charges are analyzed. The model is tested by comparing spin densities and relaxation parameters obtained for the reduced TiO2 (110) system to previous theoretical results. Finally, we discuss the relationship of our results to experimental observations of thermal desorption rates at low temperatures (100–600 K).
Ferroparaelectric transitions in relaxor materials studied by a photoacoustic technique
Authors: Flores J. Luis Pineda , Castañeda-Guzmán R. , Villagran-Muniz M. , Huanosta-Tera Alfonso
Published: July 2002

Photoacoustic experiments were carried out in two ceramic compounds to present a promising way to study relaxor materials. This technique has been shown to have some advantages in the investigation of phase transitions in a variety of materials. We use the temperature-dependent data of both dielectric and photoacoustic responses from the ceramic compound Bi4−xRxTi3O12, with Rx=Pr, x=0 and 1.6, to illustrate this work. We establish definitely that Bi4Ti3O12 is a relaxor.
Formulation of thermodynamics for the glassy state: Configurational energy as a modest source of energy
Authors: Nieuwenhuizen Th. M.
Published: July 2002

Glass is an under-cooled liquid that very slowly relaxes towards the equilibrium crystalline state. Its energy balance is ill understood, since it is widely believed that the glassy state cannot be described thermodynamically. However, the classical paradoxes involving the Ehrenfest relations and Prigogine–Defay ratio can be explained when the effective or fictive temperature of the slow modes is taken as an additional system parameter. Having straightened out the proper picture, it is interesting to reconsider glass from a thermodynamic viewpoint. A shard of glass, kept at fixed temperature and volume, relaxes towards lower energy. Heat is released, inducing apparent violations of all basic thermodynamic laws. The most interesting application is to use glassy or amorphous systems as a source of energy, by extracting the configurational energy in a process of melting followed by crystallization.
Statistical mechanics of quantum-classical systems
Authors: Nielsen Steve , Kapral Raymond , Ciccotti Giovanni
Published: July 2002

The statistical mechanics of systems whose evolution is governed by mixed quantum-classical dynamics is investigated. The algebraic properties of the quantum-classical time evolution of operators and of the density matrix are examined and compared to those of full quantum mechanics. The equilibrium density matrix that appears in this formulation is stationary under the dynamics and a method for its calculation is presented. The response of a quantum-classical system to an external force which is applied from the distant past when the system is in equilibrium is determined. The structure of the resulting equilibrium time correlation function is examined and the quantum-classical limits of equivalent quantum time correlation functions are derived. The results provide a framework for the computation of equilibrium time correlation functions for mixed quantum-classical systems.
Origin of point defects in AgInS2/GaAs epilayer obtained from photoluminescence measurement
Authors: You S. H. , Hong K. J. , Youn C. J. , Jeong T. S. , Moon J. D. , Kim H. S. , Park J. S.
Published: July 2002

The AgInS2 epilayers with a chalcopyrite structure grown using a hot-wall epitaxy method have been confirmed to be a high quality crystal. From the optical absorption measurement, the temperature dependence of the energy band gap on AgInS2/GaAs was found to be Eg(T)=2.1365 eV−(9.89×10−3 eV)T2/(2930+T). After the as-grown AgInS2/GaAs was annealed in AgInS2/GaAs has been investigated by using the photoluminescence (PL) at 10 K. The native defects of VAg, VS, Agint, and Sint obtained from PL measurements were classified as a donors or acceptors type AgInS2/GaAs to an optical p type. Also, we confirmed that In in AgInS2/GaAs did not form the native defects because In in AgInS2 did exist in the form of stable bonds.
Phased array imaging on a 4.7T/33cm animal research system
Authors: Beck Barbara L. , Blackband Stephen J.
Published: July 2002

Although phased array technology has been standard on clinical magnetic resonance imaging (MRI) systems for many years, it is just now becoming available on small animal MRI systems. Bruker Instruments Ltd. has recently provided multiple rf channels on a 4.7T/33cm Avance, but further development was necessary to complete the phased array implementation. This work discusses the development of this other hardware, specifically the rf array coil, low impedance preamplifiers, excitation coil, and image combination algorithm. Comparison of the array coil to a quadrature coil indicates superior signal-to-noise ratio from the array. In vivo images of a cat spine acquired simultaneously from the individual channels of the array and a sum of squares combination are shown.
Shear-force distance control at megahertz frequencies for near-field scanning optical microscopy
Authors: Simon A. , Brunner R. , White J. O. , Hollricher O. , Marti O.
Published: July 2002

We show that the use of fiber overtone resonance modes up to 3 MHz for shear-force distance control has two advantages for near-field microscopy. The higher dither frequency allows a shorter feedback loop time delay, which in turn allows scanning speeds of 100 μm/s on a sample with, for example, a 150 nm height variation over a 1 μm distance. Experiments on a hard semiconductor device and on a soft polymer sample demonstrate a factor of 5 improvement in scanning speed when the dither frequency is increased by a factor of 20. The second advantage is a reduction of the minimum lateral force required for height regulation, which is important for soft samples. Modeling the piezoelectric detection system as a harmonic oscillator indicates a factor of 33 increase in lateral force sensitivity when using the third overtone resonance of a typical fiber tip. This result is confirmed experimentally by immersing the tip into water.
Fabrication and photoluminescence of ordered GaN nanowire arrays
Authors: Zhang J. , Zhang L. D. , Wang X. F. , Liang C. H. , Peng X. S. , Wang Y. W.
Published: July 2002

Large-scale of crystalline GaN nanowires (diameter∼50 nm) have been fabricated through chemical-vapor deposition in the nanochannels of the anodic alumina template. X-ray diffraction and selected area electron diffraction pattern investigations indicate that the nanowires are single crystal with hexagonal wurtzite structure. A typical scanning electron microscopy image and the energy dispersive x-ray spectroscopy results indicate that indium nanoparticles only act as catalyst in GaN nanowires growth. At room temperature, photoluminescence spectrum of the GaN nanowire arrays shows a visible broadband with three peaks, which are located at about 363, 442, and 544 nm. The light emission may be attributed to GaN band-edge emission, the existence of defects or surface states, and the interaction between the ordered GaN nanowires and anodic alumina membrane. The growth mechanism of crystalline GaN nanowires is discussed. The method makes it possible to synthesize other nitride nanowire arrays.
Reducing influence of ion current on measurements of electron energy distributions in collisional plasmas
Authors: Demidov V. I. , Ratynskaia S. V. , Rypdal K.
Published: July 2002

A method for reducing the influence of ion current on probe measurements of electron velocity distributions in plasmas has been developed and tested in noble gas afterglow plasmas. It is valid for diffusive transport of ions to the probe. Experiments have demonstrated high accuracy of the proposed method.
An ultrasound velocity and attenuation scanner for viewing the temporal evolution of a dispersed phase in fluids
Authors: Nelson Phillip V. , Povey Malcolm J. W. , Wang Yongtao
Published: July 2002

Ultrasound scanning using the group velocity of sound to determine the concentration of a liquid or solid phase dispersed in a fluid has been used for many years to characterize dispersions with regard to their long-term stability. The technique has the twin advantages of speed and operation in concentrated, optically opaque dispersions. In this work, the group velocity technique is combined in a single instrument with phase velocity and attenuation spectroscopy measurements to give valuable additional information about particle size and the microscopic particle distribution, related to important destabilization phenomena such as particle flocculation. This provides earlier evidence of the processes that finally lead to gravitational destabilization and reduced shelf life of fluid dispersions such as emulsions. A further advantage is the ability to compare the measured spatial and temporal variation with computer models. The technique works in optically opaque materials and in concentrated colloids, giving a quantitative picture of the macroscopic spatial distribution of the dispersed phase and a semiquantitative picture of microscopic particle aggregation processes. Since these microscopic particle rearrangements are often responsible for the ultimate gravitational destabilization of colloidal systems, the Acoustiscan, as we have called the ultrasonic scanner described herein, may indicate product instability long in advance of visual evidence. New data are presented for protein containing sunflower oil-in-water emulsions, destabilized with Tween 20, in order to exemplify the use of the Acoustiscan for the characterization of food emulsions. The Acoustiscan instrument provides quantitative information about the destabilization of emulsions, dispersions, and colloidal systems in a rapid and informative manner. It simultaneously measures changes in the dispersed phase and follows microscopic changes in the arrangement of particles. The instrument has many other uses, for example, for characterizing crude oil, pharmaceuticals, cosmetics, and agrochemicals. It can also be used to follow crystallization processes. It does all this in materials over an extremely wide concentration range, from a few percent up to the highest concentrations obtainable. Moreover, when data are compared with computer models, it is possible to infer the presence of gels whose yield stress is far lower than any measurable by contemporary rheological equipment. This makes the Acoustiscan ideal for the study of the new soft solid materials currently in development.
Monte Carlo simulations of peptide adsorption on solid surfaces (Monte Carlo simulations of peptide adsorption)
Authors: Song Da , Forciniti Daniel
Published: July 2002

Monte Carlo simulations (MC) were used to study the adsorption of a negatively charged peptide (ASP-ASP-ILE-ILE-ASP-ASP-ILE-ILE) dissolved in water onto charged surfaces and in vacuum onto neutral surfaces. When the peptide was placed between two charged surfaces, it always adsorbed sideways onto the positively charged surface even when it was initially positioned at the negatively charged one. The structure of water around the peptide significantly changed upon adsorption. The peptide adsorbed on a neutral surface heads-on. The subsequent addition of water does not seem to affect the location and orientation of the peptide at the surface. Our results indicate that dehydration of the peptide upon adsorption always happens to the residues that are closest to the surface independently of the surface’s charge.
Molecular dynamics simulations of the liquid–vapor interface of a molten salt. I. Influence of the interaction potential
Authors: Aguado Andrés , Wilson Mark , Madden Paul A.
Published: July 2002

Molecular dynamics simulations are used to calculate the surface tension γ and study the structural properties of the liquid–vapor interface of the simple molten salt KI. The focus of the present paper is the effect on the calculated surface tension of different terms in the interionic potential and of the way that long-ranged interactions are treated. Specifically, we analyze the dependence of γ on: (a) boundary conditions employed in the Ewald summations of Coulomb interactions; (b) truncation of dispersion interactions; (c) inclusion of polarization effects. Our results show that (a) the use of vacuum boundary conditions in the direction perpendicular to the interface helps to maintain the mechanical equilibrium of the interface; (b) an Ewald summation of dispersion interactions is necessary to avoid substantial truncation effects; (c) polarization tends to decrease γ by significant amounts, and improves the agreement with experiment. In all cases, a reduction of γ comes with a corresponding increase of the interfacial width. As a structural indicator, the tangential pair distribution functions at the interface are compared with those of the bulk liquid.
Theoretical derivation of the isothermal transient ionic current in an ion conductor: Migration, diffusion, and space-charge effects
Authors: Frenning G. , Strømme M.
Published: July 2002

The Green’s function technique was used to determine the isothermal transient ionic current (ITIC) caused by the application of a constant potential across an ion conductor which initially contained the ions under consideration. The analysis included migration, diffusion, and space-charge effects as contributions to the ionic motion. Furthermore, the relative importance of diffusion and space-charge effects as current limiters was investigated. It was found that ion concentrations as low as ∼1016–1017 cm−3 are sufficient to make space-charge effects the dominant contribution to the decay of the ITIC at low applied electric fields (∼104 V/cm). Hence such effects should be taken into consideration when the ITIC method is used to determine conduction parameters.
Optical elements based on the use of renewable liquid films with magneto-electrostatic control
Authors: Ryutov D. , Toor A.
Published: July 2002

A novel concept of capillary magneto-electrostatic (CAMEL) optics is presented. The concept is based on the use of thin liquid films pressed through a porous substrate and quickly pulled back after the optical pulse, thereby allowing one to create optical elements anew after every pulse. Possible optical elements include planar and focusing mirrors, reflecting diffraction gratings, and reflecting zone plates. A set of engineering equations required for design of the CAMEL optics is presented.
Anisotropy of superconductivity from MgB2 single crystals
Authors: Xu M. , Kitazawa H. , Takano Y. , Ye J. , Nishida K. , Abe H. , Matsushita A. , Tsujii N. , Kido G.
Published: July 2002

Magnesium diboride (MgB2) single crystals, with a maximum size of 0.5×0.5×0.02 mm3, were grown by the vapor transport method in a sealed molybdenum crucible. A superconducting transition with the onset temperature of 38.6 K was confirmed by both transport and magnetization measurements. The upper critical field anisotropy ratio, Hc2∥ab(0)/Hc2∥c(0), was estimated to be 2.6 from the magnetic field-temperature phase diagram for MgB2 single crystals.
On plasma rotation induced by traveling fast Alfvén waves
Authors: Perkins F. W. , White R. B. , Chan V. S.
Published: July 2002

Absorption of fast Alfvén waves by the minority fundamental ion–cyclotron resonance, coupled with finite banana width physics, generates torque distributions, and ultimately rotational shear layers in the bulk plasma, even when the toroidal wave number kφ=n/R of the fast wave vanishes (n=0) and cyclotron absorption introduces no angular momentum nor canonical angular momentum [F. W. Perkins, R. B. White, P. T. Bonoli, and V. S. Chan, Phys. Plasmas 8, 2181 (2001)]. The present work extends these results to traveling waves with nonzero n where heating directly introduces angular momentum. Since tokamak fast-wave antennas have approximately one wavelength per toroidal field coil, the toroidal mode number n lies in the range n=10–30, independent of machine size. A zero-dimensional analysis shows that the rotation rate arising from direct torque is comparable to that of the rotational shear layer and has the same scaling. Nondimensional rotation profiles for n=(−10, 10) show modest changes from the n=0 case in the expected direction. For a balanced antenna spectrum, the nondimensional rotational profile (averaged over n=−10, 10) lies quite close to the n=0 profile.
Calculation of the spectral dependence of the radiance temperature of the high-stability tungsten strip lamp
Authors: Ma C. K. , Bedford R. E. , Gaertner A. A.
Published: July 2002

Tungsten strip lamps are calibrated at specific wavelengths. The calibration at one wavelength can be corrected to make it valid at another wavelength λ for calibrating a radiation thermometer that operates at λ. The correction can be obtained by a calculation based on either Wien’s or Planck’s radiation law with the knowledge of the spectral emissivity ε of the lamp filament and the spectral transmittance τ of the lamp window. Using published measurements of ε for tungsten and our measurement of τ for the window of a high-stability tungsten strip lamp, we develop analytical expressions for ε as a function of wavelength and temperature and for τ as a function of wavelength. These allow accurate calculations of the spectral dependence of the radiance temperature of such a lamp, from which the appropriate corrections can be made. Results are presented.
Dielectric property and conduction mechanism of ultrathin zirconium oxide films
Authors: Chang J. P. , Lin Y.-S.
Published: July 2002

Stoichiometric, uniform, amorphous ZrO2 films with an equivalent oxide thickness of ∼1.5 nm and a dielectric constant of ∼18 were deposited by an atomic layer controlled deposition process on silicon for potential applications in metal–oxide–semiconductor (MOS) devices. The conduction mechanism is identified as Schottky emission at low electric fields and as Poole–Frenkel emission at high electric fields. The MOS devices showed low leakage current, small hysteresis (<50 mV), and low interface state density (∼2×1011 cm−2 eV−1). Microdiffraction and high-resolution transmission electron microscopy showed a localized monoclinic phase of α-ZrO2 and an amorphous interfacial ZrSixOy layer which has a corresponding dielectric constant of 11.
Velocity auto-correlation functions in three-dimensional vibro-fluidized granular beds
Authors: Wildman R. D. , Hansen J.-P. , Parker D. J.
Published: July 2002

The velocity autocorrelation functions of grains moving in highly fluidized three-dimensional granular beds have been determined for the first time. Positron emission particle tracking was used to determine the location of a tracer particle. The velocity auto- and cross-correlation functions were then calculated following estimation of the velocities of the grains. The cross-correlations were found to be about zero for all times and the velocity autocorrelation functions differed in the x and y directions. A numerical study was performed to model the velocity auto-correlation function in the y-direction and this analysis was found to capture many of the observed experimental features.
Characteristics of δ-doped InP heterostructures using In0.34Al0.66As0.85Sb0.15 Schottky layer
Authors: Hsu Wei-Chou , Lee Ching-Sung , Lin Yu-Shyan
Published: July 2002

Single and coupled δ-doped InP channel heterostructure field-effect transistors using high barrier height (>0.73 eV) and wide energy band gap (∼1.8 eV) In0.34Al0.66As0.85Sb0.15 Schottky layer have been successfully grown by low-pressure metalorganic chemical vapor deposition. Comprehensive material characterizations have been conducted and investigated for various growth specifications, including the Hall measurement, secondary ion mass spectrometry, double crystal x-ray diffraction technique, and photoluminescence measurement. Excellent gate-source breakdown characteristics (>40 V), low leakage current (111 μA/mm), improved gate voltage swing, improved carrier mobility, good saturation and pinch-off properties, and low output conductance (0.5 mS/mm) have been achieved by using the high barrier height In0.34Al0.66As0.85Sb0.15 Schottky layer and InP channels. The proposed structure provides good potential for high-power circuit applications.
Paramagnetic defects in ultrafine silicon particles
Authors: Dohi Minoru , Yamatani Hiroshi , Fujita Tetsuo
Published: July 2002

Two defects in the surface oxide layer of ultrafine Si particles, temporarily named an EXL center and an EXH center, were investigated by electron spin resonance (ESR). The Si particles were prepared by a conventional gas evaporation method. The specimens were exposed to air at room temperature to oxidize the surfaces. After the heat treatment in vacuum at 600 and 1000 °C, the ESR signal of the EXL and EXH center appeared, respectively, and after successive annealing in O2 gas at that temperature, the signal disappeared completely. The ESR signals of the defects have different microwave power saturabilities. The EXL center and the EXH center are different defects though they have the same g value. The relationship between ESR signals and infrared spectra was studied. The EXL center was found to be a defect related to the Si–OH structure.
Spatial-domain cavity ringdown from a high-finesse plane Fabry–Perot cavity
Authors: Lee Jae Yong , Kim Jae Wan , Yoo Yong Shim , Hahn Jae Won , Lee Hai-Woong
Published: July 2002

We investigate the optical transmission of a tilted plane Fabry–Perot cavity leading to spatial cavity ringdown, the exponentially decaying intensity output present along the transverse spatial coordinate. Primary features of the spatial cavity ringdown are theoretically predicted from the spectral and spatial cavity transfer function which is derived analytically on the combined basis of ray optics and diffraction theory applied to an ideal diffraction lossless cavity of one transverse dimension. Spatial frequency filtration by a narrow Lorentzian-shaped cavity resonance is shown to play key roles on the spatial aspects of transmitted beam profiles. Our theoretical formulation is further extended to the case of wedged plane Fabry–Perot cavities. The experimental observation of spatial cavity ringdown signals exhibits an excellent agreement with the theoretical prediction.
Preparation and thermoelectric properties of CeFe4As12
Authors: Watcharapasorn A. , Feigelson R. S. , Caillat T. , Borshchevsky A. , Snyder G. J. , Fleurial J.-P.
Published: July 2002

The thermoelectric properties of CeFe4As12 have been measured. The compound was synthesized by reacting a powder mixture of CeAs2 and FeAs2 with As metal. The sample was characterized by both x-ray diffractometry and electron microprobe analysis. The Seebeck coefficient, electrical resistivity, Hall effect, and the thermal conductivity were measured between room temperature and about 900 K. The results indicate that CeFe4As12 possesses semimetallic behavior similar to that of CeFe4Sb12 with a room temperature electrical resistivity of 0.49 mΩ cm and a Seebeck coefficient of about 40 μV/K. The thermal conductivity of CeFe4As12 lies between that of CeFe4P12 and CeFe4Sb12 as may be expected from considerations of anionic mass and rattling effect of Ce atoms inside the skutterudite unit cell. A maximum thermoelectric figure of merit value of about 0.4 was obtained at 850 K for this compound.
Atomic hydrogen cleaning of InP(100): Electron yield and surface morphology of negative electron affinity activated surfaces
Authors: Hafez M. A. , Elsayed-Ali H. E.
Published: July 2002

Atomic hydrogen cleaning of the InP(100) surface has been investigated using quantitative reflection high-energy electron diffraction. The quantum efficiency of the surface when activated to negative electron affinity was correlated with surface morphology. The electron diffraction patterns showed that hydrogen cleaning is effective in removing surface contaminants, leaving a clean, ordered, and (2×4)-reconstructed surface. After activation to negative electron affinity, a quantum efficiency of ∼6% was produced in response to photoactivation at 632 nm. Secondary electron emission from the hydrogen-cleaned InP(100)-(2×4) surface was measured and correlated to the quantum efficiency. The morphology of the vicinal InP(100) surface was investigated using electron diffraction. The average terrace width and adatom-vacancy density were measured from the (00) specular beam at the out-of-phase condition. With hydrogen cleaning time, there was some reduction in the average terrace width. The surface quality was improved with hydrogen cleaning, as indicated by the increased (00) spot intensity-to-background ratio at the out-of-phase condition, and improved quantum efficiency after activation to negative electron affinity.
Nanocrystal formation in SiC by Ge ion implantation and subsequent thermal annealing
Authors: Schubert Ch. , Kaiser U. , Hedler A. , Wesch W. , Gorelik T. , Glatzel U. , Kräußlich J. , Wunderlich B. , Heß G. , Goetz K.
Published: July 2002

Ge nanocrystals were produced in 4H–SiC by implantation of 250 keV Ge+ ions with a dose of 1×1016 cm−2 and subsequent rapid thermal annealing at 1400–1600 °C. Radiation damage and Ge distribution after implantation and annealing were analyzed by Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy methods, and x-ray diffraction. After ion implantation a significant amount of Ge is incorporated into the SiC lattice and Ge nanocrystallites were not found. Thermal annealing leads to a local Ge redistribution and both Ge-rich and Ge nanocrystals are detected after annealing. The size of the nanocrystals varies between 2 and 10 nm.
GaN metal–oxide–semiconductor structures using Ga-oxide dielectrics formed by photoelectrochemical oxidation
Authors: Fu D. J. , Kwon Y. H. , Kang T. W. , Park C. J. , Baek K. H. , Cho H. Y. , Shin D. H. , Lee C. H. , Chung K. S.
Published: July 2002

GaN metal–oxide–semiconductor (MOS) capacitors were fabricated by using Ga oxide formed by photoelectrochemical oxidation of GaN. The electrical properties of the MOS structures as characterized by capacitance–voltage measurement were found to be dependent on the oxidation time and posttreatment. Positive flatband voltage was observed in devices with thin oxide layers indicating the existence of negative oxide charge. Very thin oxide exhibits high capacitance and reverse leakage, which can be reduced by rapid thermal annealing (RTA). Passivation of the interface by RTA is partially responsible for the improvement. Thicker oxide layers exhibit improved electrical properties. Low density of interface states (∼1011 eV−1 cm−2) was obtained in the Ga-oxide/GaN structure grown under optimized conditions.
Annihilation rate in positronic systems by quantum Monte Carlo: e+LiH as test case
Authors: Mella Massimo , Chiesa Simone , Morosi Gabriele
Published: July 2002

An accurate method to compute the annihilation rate in positronic systems by means of quantum Monte Carlo simulations is tested and compared with previously proposed methods using simple model systems. This method can be applied within all the quantum Monte Carlo techniques, just requiring the accumulation of the positron–electron distribution function. The annihilation rate of e+LiH as a function of the internuclear distance is studied using a model potential approach to eliminate the core electrons of Li, and explicitly correlated wave functions to deal with all the remaining particles. These results allow us to compute vibrationally averaged annihilation rates, and to understand the effect of the Li+ electric field on positron and electron distributions.
Tunneling systems for field programmable spin-logic gates
Authors: Richter R. , Bär L. , Wecker J. , Reiss G.
Published: July 2002

Field programmable spin-logic gates can be realized with a circuit of spin dependent tunneling elements. We investigate the possibility to use a magnetically semihard reference layer in the tunneling system which is switchable (programmable) by a current on chip, which would lead to an increase of the programmable logic functions per logic gate. As the programmable reference layer system we chose Ru/CoFe and compare the magnetic stability of the tunneling system to CoFe/Ru/CoFe and to IrMn/CoFe/Ru/CoFe reference layer based tunneling systems. We found, that the magnetic stability is a problem for the programmable reference layer system.
Effect of annealing on torsion giant impedance of Co-rich amorphous wires with vanishing magnetostriction
Authors: Blanco J. M. , Zhukov A. , Prida V. M. , Gonzalez J.
Published: July 2002

Torsion giant impedance (TGI) ratio, (ΔZ/Z)ξ, has been measured in as-cast and annealed (Co0.94Fe0.06)72.5B15Si12.5 amorphous wire. Two-step current annealing has been employed: pre-annealing (550 mA, 2 min) plus current annealing under torsion stress (33 π rad/m, 450 mA, up to tann=105 min). Pre-annealing results in a drastic increase of (ΔZ/Z)ξ (from 155% to 260%) and decrease of the asymmetry. The consequent annealing under torsion of pre-annealed sample gives rise to the appearance of significant asymmetry of the TGI. In this case (ΔZ/Z)ξ has a tendency to achieve rather asymmetric shape with a sharp maximum at certain torsion ξm. The value of ξm depends on the time of annealing under torsion and on the applied torsion during the annealing. The asymmetric shape of the (ΔZ/Z)ξ in as-cast state and after torsion annealing could be ascribed to the spontaneous or induced helical anisotropy, which can be compensated by the application of certain torsion stress. Evolution of (ΔZ/Z)ξ(ξ) and ξm (ξ) as a function of annealing time reflects the kinetic of the induced magnetic anisotropy.
Ultrahigh-resolution x-ray monochromators for elastic and inelastic x-ray scattering studies (abstract) (invited)
Authors: Toellner T. S. , Hu M. Y. , Sturhahn W. , Alp E. E. , Zhao J.
Published: July 2002

High-resolution spectroscopies that use hard x rays like nuclear resonant scattering and inelastic x-ray scattering require monochromatization to the meV level. Currently, tunable x-ray monochromators using silicon allow one to achieve sub-meV energy bandwidths in the 10–30 keV energy range. Attempts to improve the energy resolution beyond 108 encounter issues such as stability, efficiency, and crystal-quality-limited resolution. Cryogenically cooling the silicon can potentially mitigate the problems related to thermal stability, as well as improve the efficiency. Cooling high-resolution monochromators, while maintaining the requisite 10 nrad angular control over the crystals, poses a challenging technical problem. Recent attempts to improve the limits of energy resolution and to improve efficiency and stability of high-resolution monochromators with cryogenic cooling will be discussed.
Study of angular ferromagnetic resonance spectra of nanocrystalline ribbons
Authors: Suran G. , Ouahmane H. , Shin D. H.
Published: July 2002

An angular ferromagnetic resonance experiment is reported on FINEMET nanocrystalline ribbons, which shows that complex resonance spectra are detected for 0<°,θ<20°, where θ is the angle between the applied field and the normal to the ribbon. The spectra comprise the mode related to the residual amorphous phase (UM) and modes related to the nanocrystallites (PM). When θ>20° only UM is detected. The variations with θ of PM are due to the nanocrystallites, which exhibit a magnetic phase transition between a superparamagnetic state and a ferromagnetic one. The interfacial exchange-coupling model is used to explain the results.
Depth profiles of magnetic anisotropy in annealed Co-based amorphous ribbon
Authors: Gan’shina E. A. , Perov N. S. , Kochneva M. Yu. , Sheverdyaeva P. M. , Kim C. G. , Kim C. O.
Published: July 2002

For Co–Fe–Si–B amorphous ribbons the influence of weak-field annealing on transformation of magnetic field and spectral transverse Kerr effect (TKE) dependencies has been studied. The TKE spectra and magnetization curves were found to exhibit anisotropic behavior. The results indicate that the magnetic structure is inhomogeneous relative to thickness and confirm an increase of anisotropy in the near surface layer. It is suggested that the anisotropy is brought about by the shape effect of magnetic microcrystallites in the surface layer, the shape and size of which both change during annealing.
Transitions of natural convection in a vertical fluid layer
Authors: Mizushima Jiro , Tanaka Hiroki
Published: July 2002

Transitions of natural convection in a vertical fluid layer contained in a tall cavity with differentially heated side walls are investigated numerically and theoretically. Steady-state solutions are calculated by solving the steady-state equations numerically and their stability is investigated by applying the linear stability theory. It is found that the bifurcation structure of the steady-state solution consists of smooth transition and saddle-node branches and that the change in flow pattern occurs without any instabilities along the smooth transition branch, whereas transitions from the saddle-node branch to the smooth transition branch may happen which accompany the change of flow pattern.
Adsorption and reaction of sulfur dioxide with Cu(110) and Cu(110)-p(2×1)-O
Authors: Alemozafar Ali R. , Guo Xing-Cai , Madix Robert J.
Published: July 2002

On Cu(110)-p(2×1)-O at 300 K SO2(g) reacts stoichiometrically with O(a) to form a surface covered with both c(4×2)-SO3 and p(2×2)-SO3 structures. With heating SO2(g) evolves from the surface in distinct reaction-limited states at 384 K, 425 K, and 470 K, and the surface reverts to its initially oxidized state. On Cu(110), SO2(g) adsorbs molecularly below 300 K; upon annealing to 300 K, the sulfur dioxide disproportionates according to 3SO2(a)→S(a)+2SO3(a) with concomitant desorption of excess SO2(a). The surface formed in this manner exhibits large c(2×2)-S domains which encompass scattered c(4×2)-SO3 and p(2×2)-SO3 structures in a 1:2 coverage ratio. After being annealed to 400 K, the surface exhibits large p(2×2)-SO3 domains surrounding smaller c(4×2)-SO3 and c(2×2)-S islands. Continued heating past 400 K results in decomposition of sulfite according to SO3(a)→SO2(g)+O(a), evolving sulfur dioxide at 470 K and leaving the surface covered with atomic sulfur and oxygen. Real-time STM images show the mobility of oxygen at island boundaries and the mobility of sulfite amid the p(2×1)-O structures. STM measurements suggest that the sulfite occupy four-fold hollow sites.
Correlation among the structural and magnetic properties of CoCu granular alloys
Authors: Prieto A. Garcı́a , Fdez-Gubieda M. L. , Meneghini C. , Garcı́a-Arribas A.
Published: July 2002

We have analyzed the magnetic and structural properties of CoCu melt-spun granular alloys annealed at increasing temperatures by means of their room-temperature hysteresis loops and high resolution x-ray diffraction (XRD) experiments. The magnetic analysis has been performed taking into account two contributions to the total magnetization: one from superparamagnetic nanometer Co clusters responsible also of the giant magnetoresistance (GMR) and another ferromagnetic; originated by a Co-rich phase observed by XRD. We have also detected a percentage of Co that remains diluted into the Cu matrix at high annealing temperatures (Tann⩾500 °C) due to the mixcibility of Co and Cu at those temperatures, and this is in the origin of the drop of the GMR.
Self-consistent theory of orientational order and fluid–solid equilibria in weakly anisotropic fluids
Authors: Woo Hyung-June , Song Xueyu
Published: July 2002

A theoretical method of studying the effect of weak shape anisotropy on the freezing properties of classical fluids is discussed. A choice of an appropriate reference isotropic potential for a given general anisotropic model leads to the separation of the free energy into the part due to lattice formation, and the orientational correction. The reference free energy is calculated by applying the density functional theory. The anisotropic contribution to the free energy is treated by a self-consistent theory of orientational order. As an application, fluid–solid equilibria in the hard dumbbell model are considered. For the plastic crystal and the orientationally ordered phases of the hard dumbbell model, appropriate choices are made for the isotropic reference potential, density functional method is applied, and the resulting translational distribution of the molecular centers are utilized in the self-consistent calculation of the orientational ordering in the solid. The results obtained for the hard dumbbell fluids with various anisotropies are compared with the existing simulation data.
On the magnetic and superconducting properties of Ru1−xSr2RECu2+xO8−d, RE=Gd, Eu, compounds
Authors: Klamut P. W. , Dabrowski B. , Mini S. M. , Kolesnik S. , Maxwell M. , Shengelaya A. , Khasanov R. , Keller H. , Savic I. , Sulkowski C. , Matusiak M. , Wisniewski A. , Puzniak R. , Fita I.
Published: July 2002

The temperature of the superconducting transition of Ru1−xSr2RECu2+xO8−d (RE=Gd, Eu) compounds was raised up to 72 K (for x=0.4; compare to TC=45 K observed for RuSr2GdCu2O8) by changing the ratio between Ru and Cu. Magnetization experiments performed under external pressure for RE=Gd: x=0.1, 0.2, 0.3, and 0.4, and RE=Eu: x=0.4 samples reveal that TC increases at a rate of 5–6 K/GPa. The hole doping achieved along the series has been probed in Hall effect, thermopower, and XANES experiments and is found to increase with x. Muon spin rotation spectroscopy experiments reveal the presence of a low temperature magnetic transition for Ru0.6Sr2EuCu2.4O8−d (Tm≈5 K), similar to that recently observed in the Ru1−xSr2GdCu2+xO8−d system [P. W. Klamut, et al., Physica C 364, 313 (2001)]. Low temperature magnetization data suggest the constrained dimensionality of the superconducting phase in both RE=Gd and Eu based compounds.