bluuebunny/crossref_metadata_2025 huggingface dataset:
Instrument for the measurement of hysteresis loops of magnetotactic bacteria and other systems containing submicron magnetic particles
Authors: de Waard Hendrik , Hilsinger James , Frankel Richard B.
Published: July 2002

An electronic control system for the measurement of hysteresis curves of microscopically observed magnetic structures such as chains of magnetosomes in magnetotactic bacteria suspended or swimming in water is described. Using continuous magnetic fields generated by four coils for guidance or orientation of the bacteria or other magnetic structures, and pulsed magnetic fields in two additional coils for changing the degree of magnetization in small steps, hysteresis curves can be traversed. The circuits described can be constructed with readily available components. The guiding- and pulsed-field coils can be fashioned in any standard machine shop. The typical sensitivity of the system is better than 10−12 ergs/G, which makes a wide range of bacteria accessible to quantitative measurement of their magnetosome chains. The electronic circuits as well as the coil systems are described in detail.
Uniaxial strain system to investigate strain rate regulation in vitro
Authors: Clark Craig B. , Burkholder Thomas J. , Frangos John A.
Published: July 2002

Cells are able to sense and respond to mechanical strain both in vivo and in vitro, and though the ability of strain to stimulate intracellular biochemical events is well established, the influence of the rate at which these strains are applied has not been extensively investigated. In order to study the role of strain as well as strain rate, an in vitro device has been developed and validated for applying cyclic uniaxial strains to cells cultured on a silicone sheet substrate. The stepper motor driven system provides strains up to 50% in increments as small as 12 nm (0.25 μstrain) at strain rates from μstrain/day to 300%/s. Computer control allows all displacement parameters to be easily modified and provides precise control, while the low profile design and planar culture surface allows the cells to be visualized during all phases of cell culture and strain application. Displacement parameters were verified using a linear variable displacement transformer to track linear motion, while strain analysis of the membrane deformation revealed a relatively homogeneous strain field across the culture wells. As a demonstration of both the device capability and the ability of strain rate to actively modulate the cellular response, the strain rate dependence of protein synthesis was investigated in C2C12 skeletal muscle myotube cultures subjected to cyclic uniaxial strain of 7% at strain rates ranging from 1.4% to 70% s−1. Protein synthesis rates displayed a biphasic response, with increased protein synthesis at strain rates up to 25% s−1, and a peak synthesis rate of 135% of control. The uniaxial strain system described provides a high performance and versatile platform with which to study the role of mechanical strain in a variety of cells and tissues.
Carrier thermalization versus phonon-assisted relaxation in quantum-cascade lasers: A Monte Carlo approach
Authors: Iotti Rita Claudia , Rossi Fausto
Published: July 2002

In this letter, we present a microscopic analysis of the hot-carrier dynamics governing intersubband light-emitting devices. In particular, a global Monte Carlo simulation scheme is proposed in order to directly access details of the three-dimensional carrier relaxation, without resorting to phenomenological parameters. The competition between intercarrier thermalization and phonon-assisted relaxation in quantum-cascade lasers is investigated and their relative importance on device performance is clearly identified.
Single pentagon in a hexagonal carbon lattice revealed by scanning tunneling microscopy
Authors: An B. , Fukuyama S. , Yokogawa K. , Yoshimura M. , Egashira M. , Korai Y. , Mochida I.
Published: July 2002

The electronic structure of a single pentagon in a hexagonal carbon lattice has been revealed on an atomic scale by scanning tunneling microscopy. The pentagon is located at the apex of the conical protuberance of the graphitic particle. The enhanced charge density localized at each carbon atom in the pentagon is identified, and the ringlike pattern of the (∛×∛)R30° superstructure of graphite is clearly observed around the pentagon.
Spectroscopic ellipsometry and Raman study of fluorinated nanocrystalline carbon thin films
Authors: Lee Hosun , Kim In-Young , Han S.-S. , Bae B.-S. , Choi M. K. , Yang In-Sang
Published: July 2002

Using spectroscopic ellipsometry and Raman spectroscopy, we measured the pseudodielectric function and the phonon frequencies of fluorinated nanocrystalline carbon (nc-C:F) thin films grown on silicon substrate at varying growth temperature and gas flux ratio of CH4 and CF4. Utilizing the Tauc–Lorentzian formula, we performed multilayer analysis to estimate the dielectric function of the fluorinated nanocrystalline carbon thin films. We also adopted Gaussian-like density-of-states model proposed by Demichelis et al. [Phys. Rev. B 45, 14364 (1992)] and estimated the amplitude A, the transition energy Eπ, and the broadening σπ of π→π* transitions. Based on this model, we explained the change of the optical gap and the refractive index in terms of the change of the amplitude A rather than the shift of transition energy Eπ of π→π* transitions. Raman and ellipsometric study suggested that the average size of nanocrystallites in the fluorinated carbon thin films was smaller than that of amorphous hydrogenated carbon films studied by Hong et al. [Thin Solid Films 352, 41 (1999)].
Enhanced brightness in organic light-emitting diodes using a carbon nanotube composite as an electron-transport layer
Authors: Fournet P. , Coleman J. N. , Lahr B. , Drury A. , Blau W. J. , O’Brien D. F. , Hörhold H.-H.
Published: July 2002

We have studied the effects of using a composite fabricated from carbon nanotubes and a host polymer, poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-vinylene) (PmPV), as an electron-transport layer in organic light-emitting diodes. Double layer devices using this composite as an electron-transport layer, triple layer devices with a composite electron-transport layer, and poly(9-vinylcarbazole) as a hole-transport layer, as well as poly(2,5-dimethoxy-1,4phenylene-vinylene-2-methoxy- 5(2′-ethylhexyloxy)-1,4-phenylene-vinylene (M3EH-PPV) single layer devices were prepared. Current–voltage–luminance and electroluminescent spectral measurements were performed using six different nanotube powder to polymer mass ratios (0%, 2%, 4%, 8%, 16%, and 32%) for all device structures studied. dc transport and photoluminescence behavior of the polymer-nanotube composite were also investigated. Although a potential barrier is introduced at the M3EH-PPV/composite interface, a significant increase in efficiency was observed using the composite. The best efficiency was obtained for those devices with an electron-transport layer of mass ratio 8%. In addition, on doping with nanotubes, electron conductivity in the composite increased by over 4 orders of magnitude with little quenching of photoluminescence.
Inelastic effects in electron tunneling through water layers
Authors: Galperin Michael , Nitzan Abraham
Published: July 2002

Calculations of tunneling matrix elements associated with electron transfer through molecular environments are usually done for given frozen nuclear configurations, with the underlying assumption being that nuclear motions are slow relative to the time scale of a tunneling event. This paper examines this issue for the case of electron tunneling through water. The motivation for this study is a recent calculation [Peskin et al., J. Chem. Phys. 111, 7558 (1999)] that indicates that electron tunneling through water may be enhanced by tunneling resonances in the range of ∼1 eV below the vacuum barrier, and finds that the lifetimes of such resonances are in the 10 fs range, same order as OH stretch periods. Our calculation is based on the absorbing-boundaryconditions-Green’s-function (ABCGF) method and proceeds in two steps. First we consider the effect of a single symmetric OH-stretch mode on electron tunneling in an otherwise frozen water environment, and establish that the inelastic tunneling probability is small enough to justify an approach based on perturbation theory limited to single phonon transitions. Next we note that on the short time scale of a tunneling event, even under resonance conditions, water nuclear dynamics may be represented in the instantaneous normal modes picture. We generalize the ABCGF method to take into account low order inelastic scattering from a continuum of such harmonic normal modes. We find that near resonance the total inelastic transmission probability is of the same order as the elastic one, and may lead to an additional ∼20–40% enhancement of the overall transmission in the range of up to 1 eV below the vacuum barrier. The absolute energy exchange is small, of the order of 1% of the incident electron energy. Surprisingly, we find that the main contribution to the inelastic transmission is associated with energy transfer into the rotational–librational range of the water instantaneous normal mode spectrum.
Diameter control of single-walled carbon nanotubes using argon–helium mixture gases
Authors: Farhat Samir , Lamy de La Chapelle Marc , Loiseau Annick , Scott Carl D. , Lefrant Serge , Journet Catherine , Bernier Patrick
Published: July 2002

A method is reported for controlling the diameter of single-walled carbon nanotubes (SWCNTs) during the electric-arc-discharge process. Using argon as inert atmosphere provides smaller diameters as compared with those when pure helium is used. Varying the gas mixture from argon to helium changes the diameter distribution to higher values. A linear fit of the average diameter shows a 0.2 Å diam decrease per 10% increase in the argon–helium ratio.
Determination of diffusion length in samples of diffusion-length size or smaller and with arbitrary top and back surface recombination velocities
Authors: Luke Keung L.
Published: July 2002

Higher-quality semiconductor materials and smaller devices present new challenges to the electron-beam-induced current technique as device sizes become less than or approximately equal to diffusion length. Oftentimes the regions of interest are bounded by surfaces with arbitrary surface recombination velocities. How can the diffusion length in such a region be determined? The aim of this article is to lay the theoretical foundation showing how this determination can be carried out and to provide useful results to guide the experimentalist in both making the measurements and extracting the diffusion length from the data. Practical device conditions considered are top surface recombination velocity of 1×103 and 1×104 cm/s; back surface recombination velocity of 1×104 and 1×106 cm/s; and diffusion length/sample thickness ratios of 0.5, 1.0, and 2.0, respectively.
Some exact nontrivial global solutions with values in unit sphere for two-dimensional Landau–Lifshitz equations
Authors: Guo B. , Yang G.
Published: July 2002

We present some exact global solutions with values in unit sphere for two-dimensional Landau–Lifshitz equations with initial-boundary conditions, and obtained a continuum which can be made from those solutions of a tuft of Landau–Lifshitz equations.
Coalescence kinetics of copper clusters on highly oriented pyrolytic graphite and Dow Cyclotene, as determined by x-ray photoelectron spectroscopy
Authors: Yang D.-Q. , Sacher E.
Published: July 2002

Copper clusters, deposited by evaporation onto highly oriented pyrolytic graphite (HOPG) and Dow Cyclotene, a low permittivity polymer, have been found to coalesce through surface diffusion at room temperature. We study this process by x-ray photoelectron spectroscopy under ultrahigh vacuum conditions, where cluster growth was measured through photoelectron emission intensity changes. Using a simple model, the coalescence (as opposed to diffusion) coefficients were determined for both untreated and Ar+-treated substrates. It was found that Ar+ treatment results in reductions in both coalescence coefficient and cluster size due to increased cluster/surface interaction. The atypical coalescence behavior of Cu on untreated HOPG is shown to be due to an absence of interfacial interaction.
To what extent do the classical equations of motion determine the quantization scheme?
Authors: Cislo J. , Lopuszański J.
Published: July 2002

A simple example of one particle moving in a (1+1) space–time is considered. As an example we take the harmonic oscillator. We confirm the statement that the classical equations of motion do not determine at all the quantization scheme. To this aim we use two inequivalent Lagrange functions, yielding Euler–Lagrange equations, having the same set of solutions. We present in detail the calculations of both cases to emphasize the differences occuring between them.
Operation of the SERSE superconducting electron cyclotron resonance ion source at 28 GHz
Authors: Gammino S. , Ciavola G. , Celona L. , Hitz D. , Girard A. , Melin G.
Published: July 2002

The SERSE source [P. Ludwig et al., Rev. Sci. Instrum. 69, 4082 (1998), and references therein] is a superconducting electron cyclotron resonance (ECR) ion source, operating at the Laboratori Nazionali del Sud in Catania since 1998; it is currently used as the main injector for the K-800 superconducting cyclotron. Its high magnetic field provides a high plasma confinement and large currents of highly charged ions, as compared to conventional sources. It can efficiently operate at the microwave frequency of 14 and 18 GHz [S. Gammino and G. Ciavola, Rev. Sci. Instrum. 71, 631 (2000); S. Gammino et al., ibid.70, 3577 (1999)] and has been used as a test bench for injection at 28 GHz. High-frequency operation is expected to create a higher plasma density, thus resulting in larger currents of multiply charged ions. In this article, we report the first operation of an ECR ion source at 28 GHz by using a gyrotron. The gyrotron itself and the waveguide line are described, along with the operational results (in xenon gas for the sake of simplicity). Given the limited amount of power (about 4 kW), which can be injected in the plasma chamber during dc-mode operation, the results are less outstanding than in the pulsed mode (up to 6.5 kW). However, in both cases the beam intensities are far better than the ones obtained by the other ECR ion sources operating at lower frequencies.
La@C
60
:
A metallic endohedral fullerene
Authors: Klingeler R. , Kann G. , Wirth I. , Eisebitt S. , Bechthold P. S. , Neeb M. , Eberhardt W.
Published: July 2002

We have produced an endohedrally doped fullerene that shows a metal-like density of states at the Fermi level. Individual La@C60 clusters deposited onto graphite exhibit a zero band gap as observed by scanning tunneling spectroscopy on single clusters at room temperature. Moreover, we find that an isolated La@C60 cluster on graphite shows a reversible opening of a band gap at a transition temperature of ∼28 K. The transition is associated with a freezing of the vibrational motion of the La atom inside the fullerene cage. The metallic behavior of La@C60 is attributed to the presence of a dynamical dipole in the single cluster.
Satellite and rocket-borne atomic oxygen sensor techniques
Authors: Osborne J. J. , Harris I. L. , Roberts G. T. , Chambers A. R.
Published: July 2002

Neutral atomic oxygen (AO)—the dominant atmospheric species at typical low Earth orbit altitudes—is responsible for the erosion, or other degradation, of many satellite materials. Therefore, AO has become an important consideration for spacecraft designers and manufacturers. The study of AO is also of interest to atmospheric physicists because it is involved in many of the chemical reactions occurring naturally in the mesosphere and lower thermosphere. Both these groups rely on atmospheric models for computer-based simulation and prediction of atomic oxygen concentrations. Such models require, or are enhanced by, empirical input data—that is, actual measurements of AO number densities. A review is presented of the different measurement techniques that, to date, have been used on satellites and sounding rockets to perform AO studies. Rather than reporting results from every sensor application, this article takes a more general view of the experimental methods, using example devices to highlight their advantages and disadvantages. New or promising equipment, or techniques that could be exploited for performing such measurements, are also described. We attempt some semiquantitative comparison of the techniques, although the most appropriate experimental method for any given flight opportunity depends heavily on the mission conditions and science goals. Our emphasis is on missions where the available mass and power are limited. In these situations the most suitable established device is probably that of the thin film actinometer. If more risk can be assumed then a more promising, but as yet unqualified, method is that of the fiber-optic reflectance sensor. However, since both these devices are nonreusable, it is shown that semiconducting sensors may be better for long duration, mass- and power-limited applications.
Phase transformation of germanium nitride (Ge3N4) under shock wave compression
Authors: He Hongliang , Sekine T. , Kobayashi T. , Kimoto K.
Published: July 2002

The phase transformation behavior of hexagonal germanium nitride, including both α- and β-Ge3N4, has been studied under shock wave compression. The shock compressed quenched samples indicate phase transformation from hexagonal into a cubic spinel structure (γ-Ge3N4). This transformation is completed with increasing shock pressure up to 40–46 GPa (temperature of 1300–1500 K). The lattice constant of γ-Ge3N4 is measured to be 0.820 63±0.000 19 nm, and the crystal density 6.581 g/cm3, by the powder x-ray diffraction. The stability of γ-Ge3N4 also has been investigated under shock wave compression. It is found that the spinel structure is very stable, and up to at least 63 GPa (temperature of ∼1700 K) there is no indication of the formation of a postspinel phase.
Hofstadter butterfly as quantum phase diagram
Authors: Osadchy D. , Avron J. E.
Published: July 2002

The Hofstadter butterfly is viewed as a quantum phase diagram with infinitely many phases, labeled by their (integer) Hall conductance, and a fractal structure. We describe various properties of this phase diagram: We establish Gibbs phase rules; count the number of components of each phase, and characterize the set of multiple phase coexistence.
Formation of C54 TiSi2: Effects of niobium additions on the apparent activation energy
Authors: d’Heurle F. M. , Zhang S.-L. , Lavoie C. , Gas P. , Cabral C. , Harper J. M. E.
Published: July 2002

The formation of C54 TiSi2 using Ti–Nb alloys deposited on polycrystalline Si substrates was studied by means of in situ x-ray diffraction and resistance measurements during temperature ramping. Alloys with Nb contents ranging from 0 to 13.6 at. % were used. The formation temperature of C54 TiSi2 was reduced in the presence of Nb. However, the addition of Nb in Ti did not cause fundamental changes in the evolution of resistance versus temperature. This latter observation suggests that the mechanism for the formation of C54 TiSi2 remained the same in spite of the enhancement effect. For alloys with up to 8 at. % of Nb, the C49 TiSi2 phase formed first, as with pure Ti. When annealing the alloy with 13.6 at. % Nb, neither C49 TiSi2 nor C54 were found in the usual temperature ranges, instead, C40 (Nb,Ti)Si2 was observed. This phase transformed to C54 (Nb,Ti)Si2 above 950 °C. The apparent activation energy associated with the formation of C54 TiSi2 was obtained by annealing the samples at four different ramp rates from 3 to 27 K/s; it decreased continuously from 3.8 to 2.5 eV with increasing Nb content from 0 to 8 at. %. The apparent activation energy for the formation of C40 (Nb,Ti)Si2 was found to be 2.6 eV. The possible physical meaning, or lack thereof, of the high activation energies derived from experimental measurements is extensively discussed. A qualitative model is proposed whereby nucleation would be rate controlling in pure TiSi2, and interface motion in samples with 8 at. % Nb.
Optical and electrical properties of Al-rich AlGaN alloys
Authors: Li J. , Nam K. B. , Lin J. Y. , Jiang H. X.
Published: July 2002

Al x Ga 1−x N alloys with x up to 0.7 were grown by metalorganic chemical vapor deposition and their optical properties were investigated by deep UV time-resolved photoluminescence (PL) spectroscopy. Our results revealed that both the activation energy of the PL emission intensity and the PL decay lifetime exhibit sharp increases at x of around 0.4. The results can be understood in terms of the sharp increase of the impurity binding energy or the carrier/exciton localization energy around x=0.4. A three orders of magnitude increase in resistivity of undoped AlGaN alloys at x of around 0.4 was also observed, which further corroborated the optical results.
Structure and properties of carbon onion layers deposited onto various substrates
Authors: Cabioc’h T. , Thune E. , Rivière J. P. , Camelio S. , Girard J. C. , Guérin P. , Jaouen M. , Henrard L. , Lambin Ph.
Published: July 2002

120 keV carbon ions implantations at high fluences (0.5–8×1017 ions cm−2) were performed at elevated temperature (⩾500 °C) in silver layers deposited on various substrates (Si (100), 304 L stainless steel, and pure fused silica). Spherical carbon onions (3–15 nm in diameter) were so produced in the silver layers. A pure carbon onion thin film deposited on the substrate was obtained after annealing in vacuum. Atomic force microscopy and high-resolution transmission electron microscopy experiments were performed to characterize the structure of the thin films. Optical transmittance spectra of carbon onion layers deposited onto silica substrates revealed two absorption peaks centered at 220–230 nm and at 265 nm that were attributed to the presence of carbon onions and residual disordered graphitic carbon, respectively. Tribological experiments performed on silver–carbon onions composite thin films revealed that the friction coefficient is close to that of a pure silver film (0.2) but with much better wear behavior.
High intensity metal ion beam production with ECR ion sources at the Lawrence Berkeley National Laboratory
Authors: Wutte D. , Abbott S. , Leitner M. A. , Lyneis C. M.
Published: July 2002

The large number of different experiments performed at the 88 Inch Cyclotron requires great variety and flexibility in the production of ion beams. This flexibility is provided by the two high performance electron cyclotron resonance (ECR) ion sources, the LBL ECR and the AECR-U, which can produce beams of ions as light as hydrogen and as heavy as uranium. With these two sources, up to six different metals can be preloaded using two types of ovens. The ovens are mounted radially on the ion sources and inject the metal vapor though the open sextupole structure into the plasma chamber. For the superconducting ECR ion source VENUS, which is under construction at Lawrence Berkely National Laboratory, the use of radial ovens is no longer possible, because the magnetic structure is closed radially. Therefore, we are developing two new axial oven types for low and high temperature applications. Metal ion beam production in ECR ion sources using the oven technique is discussed. The design of the axial oven is presented. Finally, the efficiency of the axial oven is compared with the radial oven for the production of Ca.
Entropy driven phase transitions in colloid–polymer suspensions: Tests of depletion theories
Authors: Ramakrishnan S. , Fuchs M. , Schweizer K. S. , Zukoski C. F.
Published: July 2002

The phase behavior of model athermal silica (radius R=50 nm)–polystyrene–toluene suspensions has been determined over nearly two orders of magnitude in polymer or colloid size asymmetry. Fluid–gel, fluid–crystal, and fluid–fluid transitions are observed as Rg, the polymer radius of gyration, increases. Based on the polymer concentration relative to the dilute–semidilute crossover density, cp/cp*, as the relevant measure of depletion attraction, we find that suspension miscibility monotonically improves as Rg increases for all colloid volume fractions. This trend is in contradiction to all classic depletion theories of which we are aware. However, the predictions of fluid–fluid spinodal phase separation by the microscopic polymer reference interaction site model integral equation theory of athermal polymer–colloid suspensions are in agreement with the experimental observations. Polymer–polymer interactions, chain fractal structure, and structural reorganizations are implicated as critical physical factors. A fluid–gel transition is observed in the one-phase region for Rg=0.026R. The recently proposed dynamic mode-coupling theory is found to provide a nearly quantitative prediction of the gel line. With increasing Rg/R, gelation is predicted to require larger values of cp/cp* such that the nonergodicity transition shifts into the metastable region of the phase diagram in agreement with experiment. Comparison of the gelation behavior predicted based on the assumption that it is coincident with the static percolation line is also examined, with mixed results.
Measurement of the laser plasma properties of the direct plasma injection method to the RFQ LINAC on the RIKEN laser ion source
Authors: Takeuchi T. , Katayama T. , Nakagawa T. , Okamura M. , Yano K. , Sakumi A. , Ozawa S. , Kondrashev S. , Mescheryakov N. , Sharkov B.
Published: July 2002

The laser plasma properties of the RIKEN laser ion source on the direct plasma injection system (DPIS) were measured for analysis of the DPIS beam dynamics. From the installed electrostatic analyzer and the measured calibration value from Ceratron to Faraday Cup, the current values and pulse time structure for each carbon charge state in the laser plasma were obtained.
Realization and properties of YBa2Cu3O7−δ Josephson junctions by metal masked ion damage technique
Authors: Kang D.-J. , Burnell G. , Lloyd S. J. , Speaks R. S. , Peng N. H. , Jeynes C. , Webb R. , Yun J. H. , Moon S. H. , Oh B. , Tarte E. J. , Moore D. F. , Blamire M. G.
Published: July 2002

We have developed a simple process to fabricate high-TC Josephson junctions by a combination of focused ion beam milling and 100 keV H2+ ion implantation. The resistively shunted junction-like current–voltage characteristics were observed in the temperature range of 48 to 4.2 K. The devices showed clear dc and ac Josephson effects. This technique is very promising in terms of simplicity and flexibility of fabrication and has potential for high-density integration.
High-performance micromachined unimorph actuators based on electrostrictive poly(vinylidene fluoride–trifluoroethylene) copolymer
Authors: Xu T.-B. , Cheng Z.-Y. , Zhang Q. M.
Published: July 2002

We report on the performance of micromachined unimorph actuators [(polymer micromachined actuator PMAT)] based on the electrostrictive poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] copolymer. Because of the high electrostrictive strain and high elastic energy density (∼1 J/cm3) of the active polymer [the electrostrictive P(VDF–TrFE)], the PMAT exhibits a very high stroke level with high load capability and high displacement voltage ratio. In addition, the experimental data also demonstrate that the PMAT is capable of operating over a broad frequency range (>100 kHz). The PMAT performance was modeled and the agreement between the modeling results and experimental data confirms that the response of the PMAT indeed follows the design parameters.
Difference in hydrogen sensitivity between Pt and Pd field-effect devices
Authors: Löfdahl Mikael , Eriksson Mats , Johansson Martin , Lundström Ingemar
Published: July 2002

An explanation is given for the large differences in the hydrogen sensitivity in air observed for gas sensitive field-effect devices with palladium and platinum gates, respectively. It is demonstrated that this difference is mainly due to a difference in the hydroxyl formation rate between the two metals. The water production rates are, however, almost the same for the two metals. The considerably smaller sensitivity of platinum devices in air is then due to the much lower steady-state hydrogen atom concentration on the platinum surface compared with the palladium surface. This leads to a smaller coverage of hydrogen atoms at the metal–oxide interface and thus a smaller response of the device at a given hydrogen concentration in air.
Effect of Pr–O hybridization on the anomalous magnetic properties of Pr1+xBa2−xCu3O7−y system
Authors: Ku H. C. , Lin B. N. , Lin Y. X. , Hsu Y. Y.
Published: July 2002

The Pr L3-edge x-ray absorption near edge structure spectra and antiferromagnetic TN(Pr) data for the parent system PrBa2Cu3O7−y (0⩽y<1) and the oxygenated system Pr1+xBa2−xCu3O7−y (0⩽x⩽1; −0.27⩽y⩽0.05) are reported. The formal Pr valence in the PrO8 cage decreases from 3.18 for PrBa2Cu3O6.95 [TN(Pr)=18.5 K], to 3.12 for Pr1.2Ba1.8Cu3O6.98 (15.5 K), 3.03 for Pr1.6Ba1.4Cu3O7.16 (8 K), and 3.00 for Pr2BaCu3O7.27 [TN(Pr) not observed]. The Pr valence depends both on the number of holes available in the PrO8 cage and the Pr 4f-O 2pπ hybridization strength. On the other hand, anomalous TN(Pr) is a direct result of exchange interaction though Pr–O–Pr hybridization. However, the relative contribution from PrIV–PrIV superexchange via strong Pr–O orbital hybridization and PrIII–PrIII/PrIII–PrIV superexchange paths is unclear.
Electrical and thermal properties of C60-filled single-wall carbon nanotubes
Authors: Vavro J. , Llaguno M. C. , Satishkumar B. C. , Luzzi D. E. , Fischer J. E.
Published: July 2002

We report measurements of electrical resistivity, thermopower, and thermal conductivity of highly C60-filled single-wall carbon nanotubes and unfilled controls, from 1.5 to 300 K. The data suggest that the C60 chains provide additional conductive paths for charge carriers, increase the rate of phonon scattering, and block interior sites from sorbing other gas molecules.
Adjustable boron carbonitride nanotubes
Authors: Zhi C. Y. , Guo J. D. , Bai X. D. , Wang E. G.
Published: July 2002

The adjustable photoluminescence (PL) and field electron emission (FEE) properties of boron carbonitride (B–C–N) nanotubes grown under well-controlled conditions are studied systematically. Large-scale highly aligned B–C–N nanotubes are synthesized directly on Ni substrates by the bias-assisted hot filament chemical vapor deposition method. Single-walled B–C–N nanotubes and nanometric B–C–N heterojunctions are obtained by the pulsed-arc-discharge technique and pause-reactivation two-stage process, respectively. It is found that the microstructures, orientations, and chemical compositions of the nanotubes can be controlled by varying growth parameters. The mechanism of the controllable growth is also investigated. Intense and stable PL from the nanotubes is observed in both blue-violet (photon energies 3.14–2.55 eV) and yellow-green bands (photon energies 2.13–2.34 eV) and the emission bands are adjusted by varying the compositions of the nanotubes. FEE properties are also studied and optimized by varying the B or N atomic concentrations in the nanotubes. All these results verify the controllability of the electronic band structure of the B–C–N nanotubes.
High intensity focusing of laser pulses using a short plasma channel lens
Authors: Hubbard R. F. , Hafizi B. , Ting A. , Kaganovich D. , Sprangle P. , Zigler A.
Published: July 2002

Plasma channels have been used to guide intense laser pulses over distances of many Rayleigh lengths. This paper investigates the possibility of using a short plasma channel to provide focusing or control of the spot size of a laser pulse at intensities far above the usual damage limits of conventional optical elements. Analytical models for the focal length and focused spot size of a single plasma channel lens and a nonconverging laser pulse are presented, and results are compared with the two-dimensional simulation code LEM [J. Krall et al., Phys. Rev. E 48, 2157 (1993)]. Several advanced thin lens configurations, including multiple lens transport systems, and both focusing and defocusing lenses for externally focused converging laser pulses are also analyzed. Experimental techniques for producing appropriate plasma profiles are reviewed, and evidence for plasma channel focusing in a capillary discharge guiding experiment is analyzed. Thick “overmoded” lenses offer a possible alternative if there are experimental difficulties in producing sufficiently thin plasma channels. A variety of potential applications exist for the various proposed lens configurations.
Microstructure and magnetic properties of uncoupled Sm2Co17–Cu nanocomposites
Authors: Zhang J. X. , Bessais L. , Djéga-Mariadassou C. , Leroy E. , Percheron-Guégan A. , Champion Y.
Published: July 2002

Nanocomposite Sm2Co17–Cu particles have been fabricated by low energy comilling of mechanically alloyed Sm2Co17 particles with Cu nanoparticles. The x-ray diffraction analyses show that the diffraction crystallite size (DCS) of Sm2Co17 decreases with increasing comilling time. Scanning and transmission electron microscopy observation demonstrates that the particle size is in the range of the DCS and the Sm2Co17 particles are separated by Cu particles. The coercivity as well as the remanence ratio decreases with increasing milling time due to the grain size reduction and grain separation. The nanocomposite Sm2Co17–Cu exhibits suitable magnetic and microstructure properties for high-density recording.
Thermodynamic control and dynamical regimes in protein folding
Authors: Faisca P. F. N. , Ball R. C.
Published: July 2002

Monte Carlo simulations of a simple lattice model of protein folding show two distinct regimes depending on the chain length. The first regime well describes the folding of small protein sequences and its kinetic counterpart appears to be single exponential in nature, while the second regime is typical of sequences longer than 80 amino acids and the folding performance achievable is sensitive to target conformation. The extent to which stability, as measured by the energy of a sequence in the target, is an essential requirement and affects the folding dynamics of protein molecules in the first regime is investigated. The folding dynamics of sequences whose design stage was restricted to a certain fraction of randomly selected amino acids shows that while some degree of stability is a necessary and sufficient condition for successful folding, designing sequences that provide the lowest energy in the target seems to be a superfluous constraint. By studying the dynamics of under annealed but otherwise freely designed sequences we explore the relation between stability and kinetic accessibility. We find that there is no one-to-one correspondence between having low energy and folding quickly to the target, as only a small fraction of the most stable sequences were also found to fold relatively quickly.
Effect of substitutional atoms in the tip on field-emission properties of capped carbon nanotubes
Authors: Zhang Gang , Duan Wenhui , Gu Binglin
Published: July 2002

By use of first-principles calculations, we investigate the effect of introducing substitutional boron or nitrogen atom in the tip on field-emission properties of capped single-wall carbon nanotubes. We find that the substitution in the tip can significantly enhance the localized density of states at the Fermi level, which is important for field emission. The substitution of nitrogen introduces unsaturated dangling bond states at the tip, yielding a reduction of the work function of the tip. Our results suggest that the capped tube with substitutional nitrogen or boron atom in the tip could have much larger field-emission current than the tube without substitution.
Spiral waves and formation of low work function areas in catalytic NO reduction with hydrogen on a Rh(111) surface
Authors: Schaak A. , Imbihl R.
Published: July 2002

The catalytic NO+H2 reaction on Rh(111) has been investigated in the 10−6–10−5 mbar range using photoelectron emission microscopy as spatially resolving method. Without extended oxygen pretreatment the system is excitable, i.e., one observes spiral waves and target patterns. In addition, irregular islands form. The existence range of the patterns has been mapped out for pNO=1×10−6 mbar. After extended oxygen treatments (pO2=2×10−4, T=770 K, tox>24 h) low work function (WF) areas develop upon the collision of pulses which are assigned to subsurface oxygen (sub-O). A WF decrease up to 0.8 eV below the level of the clean surface is found. The long range interaction of pulses leading to sub-O formation below a distance of ≈100–150 μm is attributed to diffusing atomic hydrogen.
Dislocation jumping over the sound barrier in tungsten
Authors: Li Qikai , Shi San-Qiang
Published: July 2002

It is commonly believed that dislocations cannot surmount the sound barrier at the shear wave velocity. This letter presents results of molecular dynamics simulation showing the contrary, namely that a stationary dislocation can be accelerated above the sound barrier provided that the theoretical shear strength is reached at the dislocation core and the total energy of a subsonic perfect dislocation is sufficiently high to cause its dissociation into transonic partial dislocations.
Self-assembly of Si nanoclusters on 6H–SiC(0001)-(3×3) reconstructed surface
Authors: Ong W. J. , Tok E. S. , Xu H. , Wee A. T. S.
Published: July 2002

Scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) were used to observe the formation of metastable (6×6)-Si nanoclusters (diameter ∼16.5±0.1 A) on 6H–SiC(0001)-(3×3) surface. STM and XPS data suggest that these clusters are derived from the ejection of the Si-tetracluster unit of the initial (3×3) reconstruction at elevated temperatures and occur in a less Si-rich environment than the initial surface. The observed surface restructuring is related to the reconfiguration of coplanar Si bonds within the (3×3) unit cell. The occurrence of these regularly sized “magic” clusters demonstrates the potential of nanostructure formation of Si on SiC.
Tungstate fluorophosphate glasses as optical limiters
Authors: Poirier Gaël , de Araújo Cid B. , Messaddeq Younes , Ribeiro Sidney J. L. , Poulain Marcel
Published: July 2002

Nonlinear absorption measurements were performed on fluorophosphate glasses with high concentrations of tungsten oxide. Large two-photon absorption coefficients, α2, were determined at 660 nm using nanosecond laser pulses. It was observed that α2 increases for increasing tungsten oxide concentrations and, hence, the optical limiting performance of this glass composition can be controlled.
Nonlinear macroscopic polarization in GaN/AlxGa1−xN quantum wells
Authors: Vaschenko G. , Patel D. , Menoni C. S. , Ng H. M. , Cho A. Y.
Published: July 2002

We present experimental evidence of the nonlinear behavior of the macroscopic polarization in GaN/AlxGa1−xN quantum wells. This behavior is revealed by determining the barrier-well polarization difference as a function of applied hydrostatic pressure. The polarization difference and corresponding built-in electric field in the wells increase with applied pressure at a much higher rate than expected from the linear model of polarization. This result, universally observed in the quantum well structures with different AlN mole fraction in the barriers, is explained by the nonlinear dependence of the piezoelectric polarization in GaN and AlN on the strain generated by pressure.
The importance of interstitial gas expansion in infrared photoacoustic spectroscopy of powders
Authors: McGovern Stephen J. , Royce Barrie S. H. , Benziger Jay B.
Published: July 2002

Model calculations and experiments have been performed to explore the modes of photoacoustic signal generation from highly porous powders. These indicate that two modes of signal generation are important: (i) a thermal mode resulting from heating of the sample and conduction to the transducer gas above the sample, and (ii) a pressure mode resulting from the volumetric expansion of the interstitial gas in the porous sample. For highly porous materials, such as silica, the pressure mode can dominate the photoacoustic signal. Under conditions where the pressure mode is dominant, the frequency dependence of the acoustic magnitude and phase is similar to that of a homogeneous sample in photoacoustic saturation, however, the photoacoustic signal will still scale with absorption coefficient in this regime. The pressure mode of signal generation can be attenuated by compaction of the powdered sample to reduce the porosity. Unambiguous determination of photoacoustic saturation is shown to require measurements as functions of both frequency and porosity. The experimental results show that for highly porous silica even the strongest absorption bands are not in photoacoustic saturation. The results also suggest that infrared light scattering by powders of submicron-sized particles is not of major significance. This suggests that photoacoustic spectroscopy is extremely well suited to the study of highly porous materials.
Analysis of dc SQUID readout for a gravity wave detector using an inductive transducer
Authors: Xu Bu-Xing , Hamilton W. O.
Published: July 2002

We show that it is possible to construct a quantum-limited gravitational wave antenna if it is operated at 0.05 K. We assume a dc SQUID readout of a Paik transducer. We take account of the correlations between the SQUID voltage and current noise and the SQUID dynamic impedance. We derive expressions for the back action of the SQUID on the antenna, for the noise properties of the system, and the sensitivity and matching conditions.
Studies of SiC formation on Si (100) by chemical vapor deposition
Authors: Bozso F. , Yates J. T. , Choyke W. J. , Muehlhoff L.
Published: July 2002

The reaction of Si (100) with C2H4 from a molecular beam source has been studied using x-ray photoelectron spectroscopy, electron-energy-loss spectroscopy, and Auger spectroscopy. Using these methods, we have studied the kinetics of SiC formation under conditions where no gas-phase excitation processes can contribute. At Si (100) temperatures below 940 K, a ‘‘Si-C alloy’’ forms on the surface; annealing to higher temperatures produces SiC exhibiting electron spectroscopic properties identical to SiC (0001). By studies of the characteristic bulk- and surface-plasmon-loss features in the SiC thin film, it has been shown that surface aggregation of bulk Si on top of the growing SiC film occurs at T≥940 K. Under optimum SiC growth conditions, C2H4 yields about 2×10−3 SiC units per C2H4 surface collision on Si (100).
Refractive index of Bi-substituted gadolinium iron garnet films grown by liquid-phase epitaxy
Authors: Hibiya Taketoshi , Okada Osamu , Masumoto Toshiaki
Published: July 2002

Refractive indices of (GdBi)3Fe5O12 and (GdBi)3(FeAlGa)5O12 films on Nd3Ga5O12 substrates were measured at 0.633  μm by a waveguide technique. The dispersion of refractive index was determined from the interference spectra over the wavelength region of 0.5–2.0  μm. Wavelength dispersion was analyzed by the Sellmeier oscillator formulation: n2=1+E01 Ed1/(E201−E2) +E02 Ed2/(E202−E2). Increase in refractive index with increase in Bi3+ concentration is attributed to a decrease in oscillator energy E01 and an increase in dispersion energy Ed1 associated with oxygen–2p–iron-3d charge transfer transitions enhanced by hybridization of bismuth-6s2 and oxygen-2p electrons.
A virtual-cathode reflex triode for high-power microwave generation
Authors: Burkhart S. C. , Scarpetti R. D. , Lundberg R. L.
Published: July 2002

Microwave generation by a relativistic electron beam injected through a screen or foil into a cylindrical waveguide is investigated. The electron beam is generated by a field-emission diode with an anode-cathode gap of ∼1 cm. The electron density exceeds the space-charge limit for the beam, which forms a virtual cathode inside the waveguide. The instability of the virtual cathode causes oscillations which couple to bunched reflexing electrons trapped in the potential well. Frequency locking between the oscillating virtual cathode and the reflexing electrons is found to generate a much purer, narrowband microwave output. Using diode mismatch to increase the beam impedance is shown numerically to be a useful technique to improve the microwave generation efficiency and spectral purity. The peak conversion efficiency is calculated to be 2.2% at 7 GHz. The numerical results are compared to experimental results, and the agreement is within 8% in frequency and 25% in peak power. Experimentally, we describe the qualitative effects of different cathode materials and dimensions. The anode materials used were screens and foils of various transparencies and thicknesses. Microwave generation efficiency dropped as the mean scattering angle increased with the thicker foils. Also, the increased transparency of fine screens caused an upward frequency shift due to the increased electron density.
Surfaces of polymer blends: X-ray photoelectron spectroscopy studies of polystyrene/poly(vinyl methyl ether) blends
Authors: Pan D.H.-K. , Prest W. M.
Published: July 2002

The surface compositions of polystyrene (PS)/poly(vinyl methyl ether) (PVME) blends have been investigated using x-ray photoelectron spectroscopy (XPS). The XPS results demonstrate that there are significant differences between the composition of the surfaces (e.g., outmost ∼6 nm) and the bulk of both miscible and immiscible blends. The surfaces of PS/PVME films dip-coated from either toluene or trichloroethylene solutions are found to be enriched in PVME. An additional enrichment in the PVME content of the surface is also observed when miscible PS/PVME blends are phase separated by heating them above the lower critical solution temperature. The average composition profile of a miscible 50/50(w) blend has been obtained by angular-dependent XPS measurements. This experiment suggests that the enrichment is the result of a concentration gradient rather than a monolayer coating. These results are discussed in terms of the surface free energies of PS and PVME and the degree of mixing in the surface layer.
Dielectric flocculation of nondiffusive particles in a nonconducting fluid and a uniform electric field
Authors: Shapiro Michael , Shalom Allen L. , Lin Israel J.
Published: July 2002

A model for dielectric flocculation of nondiffusive particles in a uniform electric field is proposed. Flocculation time is determined as a function of inertial and dielectrophoretic force parameters. The flocculation time for inertialess particles is calculated analytically while for inertial particles, a numerical solution is given. A diagram is shown which allows to determine flocculation occurrence for various particle parameters and operating conditions.
Fabrication of high-purity chalcogenide glasses by chemical vapor deposition
Authors: Katsuyama Toshio , Satoh Shin , Matsumura Hiroyoshi
Published: July 2002

High-purity Ge–Se chalcogenide glasses were fabricated using chemical vapor deposition. Gas-phase GeCl4 and SeCl2 were used together with H2 for the reaction. High transparency was observed over a wide IR region of 2–10 μm. In addition, it is reported that heat treatment under NH3 and CO gas flow has been found to be effective for eliminating oxygen impurities. Using this technique, the oxygen content was reduced to below 1 wt. ppm. These glass compounds are applicable for infrared-transmitting windows as well as infrared optical fibers.