bluuebunny/crossref_metadata_2025 huggingface dataset:
The Restitution Portrait:
Authors: KALB SOMA S. , DOBROVOLNY HANA M. , TOLKACHEVA ELENA G. , IDRISS SALIM F. , KRASSOWSKA WANDA , GAUTHIER DANIEL J.
Published: May 2004

Introduction: Electrical restitution, relating action potential duration (APD) to diastolic interval (DI), was believed to determine the stability of heart rhythm. However, recent studies demonstrate that stability also depends on long‐term APD changes caused by memory. This study presents a new method for investigation of rate‐ and memory‐dependent aspects of restitution and for assessment of mapping models of APD.

Methods and Results: Bullfrog ventricular myocardium was paced with a “perturbed downsweep protocol.” Starting from a basic cycle length (BCL) of 1,000 ms, the tissue was paced until steady state was achieved, followed by single beats of longer and shorter cycle lengths. BCL was decreased by 50 to 100 ms and the process repeated. All APDs were plotted as a function of the preceding DI, which allowed simultaneous observation of dynamic, S1‐S2, and two constant‐BCL restitution curves in a “restitution portrait.” Responses were classified as 1:1 (stimulus:response), transient 2:2, or persistent 2:2 (alternans) and were related to the slopes of the restitution curves. None of these slopes approached unity for the persistent 2:2 response, demonstrating that the traditional restitution condition does not predict alternans. The restitution portrait was used to evaluate three mapping models of APD. The models with no memory and with one‐beat memory did not produce restitution portraits similar to the experimental one. A model with two‐beat memory produced a qualitatively similar portrait.

Conclusion: The restitution portrait allows a more comprehensive assessment of cardiac dynamics than methods used to date. Further study of models with memory may result in a clinical criterion for electrical instability. (J Cardiovasc Electrophysiol, Vol. 15, pp. 1‐12, June 2004)
Detection of Inadvertent Catheter Movement into a Pulmonary Vein During Radiofrequency Catheter Ablation by Real‐Time Impedance Monitoring
Authors: CHEUNG PETER , HALL BURR , CHUGH AMAN , GOOD ERIC , LEMOLA KRISTINA , HAN JIHN , TAMIRISA KAMALA , PELOSI FRANK , MORADY FRED , ORAL HAKAN
Published: May 2004

Introduction: During radiofrequency ablation to encircle or isolate the pulmonary veins (PVs), applications of radiofrequency energy within a PV may result in stenosis. The aim of this study was to determine whether monitoring of real‐time impedance facilitates detection of inadvertent catheter movement into a PV.

Methods and Results: In 30 consecutive patients (mean age 53 ± 11 years) who underwent a left atrial ablation procedure, the three‐dimensional geometry of the left atrium, the PVs, and their ostia were reconstructed using an electroanatomic mapping system. The PV ostia were identified based on venography, changes in electrogram morphology, and manual and fluoroscopic feedback as the catheter was withdrawn from the PV into the left atrium. Real‐time impedance was measured at the ostium, inside the PV at approximately 1 and 3 cm from the ostium, in the left atrial appendage, and at the posterior left atrial wall. There was an impedance gradient from the distal PV (127 ± 30 Ω) to the proximal PV (108 ± 15 Ω) to the ostium (98 ± 11 Ω) in each PV (P < 0.01). There was no significant impedance difference between the ostial and left atrial sites. During applications of radiofrequency energy, movement of the ablation catheter into a PV was accurately detected in 80% of the cases (20) when there was an abrupt increase of ≥4 Ω in real‐time impedance.

Conclusion: There is a significant impedance gradient from the distal PV to the left atrium. Continuous monitoring of the real‐time impedance facilitates detection of inadvertent catheter movement into a PV during applications of radiofrequency energy. (J Cardiovasc Electrophysiol, Vol. 15, pp. 1‐5, June 2004)
Ablation of Superior Pulmonary Veins Compared to Ablation of All Four Pulmonary Veins:
Authors: KATRITSIS DEMOSTHENES G. , ELLENBOGEN KENNETH A. , PANAGIOTAKOS DEMOSTHENES B. , GIAZITZOGLOU ELEFTHERIOS , KARABINOS ILIAS , PAPADOPOULOS ANASTASIOS , ZAMBARTAS CONSTANTINOS , ANAGNOSTOPOULOS CONSTANTINE E.
Published: May 2004

Introduction: Isolation of all pulmonary veins (PV) is advocated for treatment of paroxysmal atrial fibrillation (PAF). However, the superior PVs are responsible for most AF triggers, whereas the inferior PVs carry the higher risk for ablation‐induced ostial stenosis. The aim of this study was to compare a superior PV isolation approach with isolation of all PVs for treatment of PAF.

Methods and Results: Fifty‐two patients with PAF were randomized to either left superior pulmonary vein (LSPV) isolation followed by additional isolation of the right superior pulmonary vein (RSPV) in case of AF recurrence (group A, n = 27) or isolation of all four PVs followed by a repeat procedure in case of recurrence (group B, n = 25). At 1‐year follow‐up, 11 patients (41%) in group A and 8 patients (32%) in group B had AF relapse (P = 0.55). No significant differences in AF relapse were detected between groups at 3 and 12 months (log rank = 0.36, P = 0.54) and by Cox proportional hazards model analysis (P = 0.62). Nonsignificant PV stenosis was detected in two patients from group B. Total radiofrequency energy delivery and fluoroscopy and procedure times were lower in group A: 8.9 ± 1.4 minutes vs 25.6 ± 3.7 minutes (P < 0.001), 22.2 ± 6.8 minutes vs 62 ± 10.3 minutes (P < 0.001), and 131.8 ± 26.5 minutes vs 222.2 ± 32.3 minutes (P < 0.001), respectively.

Conclusion. A staged superior PVs isolation approach confers equal success rates but with reduced radiofrequency energy delivery and fluoroscopy and procedure times compared to isolation of all PVs at the initial ablation attempt. (J Cardiovasc Electrophysiol, Vol. 15, pp. 1‐5, June 2004)
Efficient quantum three-dimensional modeling of fully depleted ballistic silicon-on-insulator metal-oxide-semiconductor field-effect-transistors
Authors: Gilbert M. J. , Ferry D. K.
Published: May 2004

We present an efficient, fully quantum mechanical approach to calculating ballistic transport in fully-depleted silicon-on-insulator metal-oxide semiconductor field effect transistor devices in three dimensions and apply the technique to the calculation of threshold voltages for realistic devices with narrow channels. We illustrate the fact that each dopant configuration becomes exceedingly important for quantum simulations.
Monte Carlo study of hot-carrier transport in bulk wurtzite GaN and modeling of a near-terahertz impact avalanche transit time diode
Authors: Reklaitis A. , Reggiani L.
Published: May 2004

By Monte Carlo simulations we calculate static and dynamic charge transport properties of bulk wurtzite GaN in the presence of high electric fields. The microscopic model is validated by comparison with available experiments. On this basis a double-drift impact avalanche transit time diode made of wurtzite GaN is investigated and optimized by performing ensemble Monte Carlo simulations of the diode self-consistently coupled with a load circuit. The large-signal ac characteristics show that avalanche diodes based on GaN can operate as microwave generators in a wide frequency range up to 0.7 THz. A maximum efficiency of about 18% is found at 0.45 THz.
Electrical conduction and band offsets in Si/HfxTi1−xO2/metal structures
Authors: Afanas’ev V. V. , Stesmans A. , Chen F. , Li M. , Campbell S. A.
Published: May 2004

The electron energy band alignment in the Si/HfxTi1−xO2/metal (Au,Al) structures is determined as a function of oxide composition using internal photoemission of electrons and photoconductivity measurements. For x⩽0.5 the electron excitations with thresholds corresponding to the band-gap width of amorphous TiO2 (4.4 eV) and HfO2 (5.6 and 5.9 eV) are observed at the same time, suggesting formation of TiO2- and HfO2-like subnetworks. With respect to the Fermi level of Au the conduction band of TiO2 appears to be 1.4 eV below the conduction band of HfO2 which indicates that the valence bands of the two oxides are nearly aligned. This significant downshift of the conduction band due to Ti incorporation leads to low barriers for electrons at the interfaces of HfxTi1−xO2 with Si and Al (∼1 eV or less) strongly impairing insulating properties of the oxide. Crystallization of TiO2 upon high-temperature annealing further enhances leakage currents because of a significantly lower band-gap width of crystallized TiO2 (3.1–3.4 eV).
Asymmetric injection of cathodic arc plasma into a macroparticle filter
Authors: Anders André , MacGill Robert A.
Published: May 2004

The cathodic arc plasmas produced by cathode spots usually include macroparticles, which is undesirable for many applications. A common way of removing macroparticles is to use curved solenoid filters that guide the plasma from the source to the substrate. In this work, an arc source with a relatively small cathode is used, limiting the possible locations of plasma production. The relative position of cathodic arc source and macroparticle filter was systematically varied and the filtered plasma current was recorded. It was found that axis-symmetric plasma injection lead to maximum throughput only if an anode aperture was used, which limited the plasma to near-axis flow by scraping off plasma at larger angles to the axis. When the anode aperture was removed, more plasma could enter the filter. In this case, maximum filtered ion current was achieved when the plasma was injected off-axis; namely, offset in the direction where the filter is curved. Such behavior was anticipated because the plasma column in the filter is known to be shifted by E×B and centrifugal drift as well as by non-axis-symmetric components of the magnetic field in the filter entrance and exit plane. The data have implications for plasma transport variations caused by different spot locations on cathodes that are not small compared to the filter cross section.
Structure and morphology of the As-rich and the stoichiometric GaAs(114)A surface
Authors: Márquez J. , Kratzer P. , Jacobi K.
Published: May 2004

GaAs(114)A surfaces were prepared using molecular beam epitaxy followed by annealing in As2 pressure as well as in ultrahigh vacuum. Based on low-energy electron diffraction, in situ scanning tunneling microscopy measurements, and ab initio first-principles electronic-structure calculations the surface reconstructions are determined. Under As-rich conditions GaAs(114)A develops a β2(2×1) and/or β2c(2×2) reconstruction which is analogous to the GaAs(001)β2(2×4) one. Although a low surface free energy (46 meV/Å2 for As-rich conditions) is found for GaAs(114)Aβ2(2×1), it is split up into mesoscopic (113) and (115) facets. Also typical steps between 150 Å wide terraces are observed along [11̄0]. The stoichiometric GaAs(114)A surface is mesoscopically very flat and develops a α2(2×1) reconstruction of a low surface free energy of 53 meV/Å2. The surface free energies for the α(2×1) and a newly introduced ω(2×1) variant are larger.
Diffraction microscopy for disordered tetrahedral networks
Authors: Cheng Ju-Yin , Treacy M. M. J. , Keblinski P. J. , Gibson J. M.
Published: May 2004

X-ray and electron sources are extensively used to explore disordered structures. In the case of electron, small-angle diffraction can help to testify the argument about micro-crystallites in glassy states. Diffraction intensity has two types of variance in reciprocal space: radial and azimuthal. Previously, variance as a function of k was largely used to elucidate medium-range order in amorphous semiconductors. Here azimuthal variance is introduced. This variance reveals orientational order for possible crystallites. Furthermore, the oversampling method proposed here can change our view on amorphous structure. We find that a broad peak might not truly reflect one single crystallite. In fact, two reflections can be folded to yield one broad peak. In this paper, the issues are discussed with three examples: silica, silicon, and germanium.
Three-dimensional simulations of the complex dielectric properties of random composites by finite element method
Authors: Zhao Xuanhe , Wu Yugong , Fan Zhigang , Li Fei
Published: May 2004

Complex dielectric constants of binary-phase random composites are simulated for a three-dimensional structure consisting of cubic grains using the Monte Carlo-finite element method. Numerical results are fitted using Maxwell–Garnett, Bruggeman symmetrical, and general effective media formulas, and the fitting efficiencies of the formulas are quantitatively evaluated. The general effective media formula gives the best fitting to our simulation results and its accuracy is better than 3.7%. The effects of frequencies on the spatial distribution of electrostatic potentials in dielectric composites are discussed. The distribution of potential contours drawn in the low-frequency region and the high-frequency region show great variation, because of different lengths of time for charge to accumulate near the interphase boundaries. Dielectric spectra are drawn by varying volume fraction and lossy property of one phase in binary-phase composites. General properties of the dielectric spectra are discussed and the characteristics of the dielectric spectra caused by the differences in the lossy properties of the two phases at certain volume fractions are analyzed.
Tin whisker growth driven by electrical currents
Authors: Liu S. H. , Chen Chih , Liu P. C. , Chou T.
Published: May 2004

Tin whisker growth was investigated in pure Sn using Blech structure. Blech structure was used to investigate the electromigration behavior in pure tin, in which 5000 Å tin strips were on 700 Å titanium films. Tin whiskers grew on the anode side, and voids were observed on the cathode side after stressing at the current densities of 7.5×104 and 1.5×105 A/cm2 at room temperature. To investigate temperature effect, samples are stressed at room temperature and 50 °C. The whisker growth rates were estimated to be 3 and 7.7 Å/s at room temperature and at 50 °C, respectively, in the current density of 1.5×105 A/cm2. The whisker growth rate reduced to 0.4 Å/s at the current density of 7.5×104 A/cm2, which is still faster than that driven by mechanical stress. Transmission electron microscopy results showed that the whiskers are single crystalline and a thin Sn oxide formed on their surfaces. The mechanism of tin whisker growth driven by electrical force is proposed in this article.
Growth of exchange-biased MnPd/Fe bilayers
Authors: Blomqvist P. , Krishnan Kannan M. , McCready David E.
Published: May 2004

The growth of exchange-biased MnPd/Fe bilayers has been investigated using x-ray diffraction. The bilayers were deposited on MgO(001) substrates in an ultrahigh-vacuum ion-beam sputter system. It was found that the orientation of the MnPd unit cell and the crystalline quality could be controlled as a function of the deposition temperature. Twinned a-axis oriented MnPd films are obtained below 100 °C while single-crystalline c-axis films are obtained above 450 °C. Intermediate temperatures yield a mixture of both orientations with a poor crystalline quality. Moreover, the interface quality depends strongly on the deposition temperature and also the order in which MnPd and Fe are deposited. The results clearly show that interdiffusion is initiated at the Fe/MnPd interface at a lower temperature as compared to the MnPd/Fe interface. The close relationship between the structural and magnetic properties is also discussed.
Selective production of armchair-type carbon nanotubes
Authors: Matsunaga Yasushi , Kato Tomokazu , Hatori Tadatsugu
Published: May 2004

The controllability and promotion of the production of metallic nanotubes by imposing an external electric field are examined based on a numerical simulation. The electronic states of the π electrons are described by the Hückel method and their electric interactions are self-consistently taken into account through the Poisson equation. The frontier electron density at both ends of the nanotubes with open ends is evaluated. For armchair tubes, the frontier electron density at both ends becomes a maximum near the electric field of 1 V/nm. The ratio of the frontier density of the armchair tubes to zigzag tubes is 3:1 near this electric field. The frontier density for zigzag tubes shows a maximum in the absence of an external electric field, while, by imposing an electric field, the frontier density decreases. The optimal intensity of a direct current electric field to efficiently make metallic nanotubes is determined.
Axiotaxy of CoSi2 thin films on Si(100) substrates and the effects of Ti alloying
Authors: Özcan Ahmet S. , Ludwig Karl F. , Detavernier Christophe , Lavoie Christian , Jordan-Sweet Jean L.
Published: May 2004

The texture of CoSi2 films on Si(001) substrates has been examined using synchrotron based high-resolution pole figure analysis. We find that axiotaxy, a recently recognized type of texture which was observed in NiSi thin films on Si(001), is also occurring in CoSi2 layers. Typical of axiotaxy texturing, the CoSi2 pole figures show symmetrical patterns of lines. Calculations show that these features on the pole figures are created by three off-normal fiberlike texture components. These off-normal fiber orientations are a consequence of the alignment of CoSi2〈110〉 planes with Si〈110〉 planes because of very similar d spacings, which causes a periodic arrangement along Si〈100〉 directions in the plane of the interface. This one-dimensional periodicity may provide a lower interfacial energy; however, the periodic arrangement along the Si〈100〉 directions may also be indicative of growth anisotropy. The influence of Ti alloying has been investigated by using in situ x-ray diffraction measurements, as well as pole figure analysis. These results show that the nucleation of both the CoSi and CoSi2 phases is delayed to higher temperatures for the 5 at. % Ti alloyed films. The texture of the CoSi2 phase is also influenced by the addition of Ti.
Modeling of light-sensitive resonant-tunneling-diode devices
Authors: Coêlho I. J. S. , Martins-Filho J. F. , Figueiredo J. M. L. , Ironside C. N.
Published: May 2004

We present a method to include the effects of light excitation on two different models of resonant-tunneling-diode-based devices. Our approach takes into account both photoconductive and charge accumulation effects responsible for shifting the static I–V curve when the structure is under light excitation. Computational simulations led to good agreement between the model and experimental results.
Characterization of the plasma in a radio-frequency magnetron sputtering system
Authors: Palmero A. , van Hattum E. D. , Arnoldbik W. M. , Vredenberg A. M. , Habraken F. H. P. M.
Published: May 2004

In order to understand the fundamental mechanisms in a radio-frequency magnetron sputtering system, the main properties of the argon plasma used in the process have been measured. A complete three-dimensional map of the ion density, electron temperature, and plasma potential has been obtained using a Langmuir probe. The electron temperature as well as the ion density have been found to increase in the region of the so called race track at the cathode. Furthermore, from the plasma potential map, the time-averaged local electric field has been obtained, pointing out the race track as the region where the most intense ion bombardment takes place. Besides, only the ions produced near the race track are accelerated towards the cathode, whereas those produced in the remaining volume move towards the anode. Finally, the dependence of the plasma quantities on the incident radio-frequency power and deposition pressure has been studied. The plasma potential measured using the Langmuir probe has been found to agree with that determined using an energy resolved mass spectrometer in all studied conditions.
Determination of functional center local environment in copper-modified Pb[Zr0.54Ti0.46]O3 ceramics
Authors: Eichel Rüdiger-A. , Kungl Hans , Hoffmann Michael J.
Published: May 2004

Ferroelectric copper(II)-modified polycrystalline Pb[Zr0.54Ti0.46]O3 ceramics with a dopant concentration of 0.25 mol % were investigated by means of electron paramagnetic resonance and hyperfine sublevel correlation spectroscopy. Special emphasis was put on the resolution of the Pb207 hyperfine couplings, which were resolved in the first and second coordination sphere. The results prove that copper is incorporated at the octahedrally coordinated [Zr,Ti] site, acting as an acceptor and disfavor a directly coordinated oxygen vacancy to the copper center for charge compensation.
Local structure analysis of Ge-Sb-Te phase change materials using high-resolution electron microscopy and nanobeam diffraction
Authors: Naito Muneyuki , Ishimaru Manabu , Hirotsu Yoshihiko , Takashima Masaki
Published: May 2004

As-sputtered and melt-quenched amorphous structures together with the laser-induced crystallized structure of Ge-Sb-Te thin films were investigated using high-resolution electron microscopy (HREM) and nanobeam electron diffraction (NBED). Each of the Ge-Sb-Te thin films was embedded in a four-layered stack, which is the same as the layered structure of phase-change optical disks. Cross-sectional HREM revealed crystalline atomic clusters in the melt-quenched amorphous layer at a greater frequency than in the as-sputtered amorphous layer. Autocorrelation function analysis of the HREM images revealed similarity between the structures of atomic ordered regions in the amorphous phase and that of crystalline Sb. Atomic pair-distribution functions derived from halo NBED intensity analysis indicated that the atomic neighbor correlations developed more in the melt-quenched amorphous phase than in the as-sputtered phase. The development of locally ordered regions is considered to be closely related to the differences in optical properties and crystallization behaviors between these two amorphous phases.
Absence of island–island interaction during formation of isolated Ge islands in small windows
Authors: Stoica T. , Vescan L. , Sutter E.
Published: May 2004

Isolated Ge islands, i.e., islands not connected by a wetting layer can be obtained by selective epitaxial growth in voids of ultrathin oxides of thickness 1–2 nm. Voids of 30–600 nm size were created before epitaxy during a high temperature anneal of the ultrathin oxide. The formation of one island per window was investigated at 700 °C as a function of Ge thickness and void size. Islands nucleate mainly at the edge of the void and for this reason they have an anisotropic shape. In voids smaller than 300 nm only one island is nucleated. Islands form only in voids greater than a critical size (30–80 nm) which depends on the total amount of Ge deposited. We observe that height, width, and aspect ratio of isolated islands increase with void size for a given Ge thickness. A metastable state of Ge in small windows was observed. Moreover, the Si interdiffusion is strongly reduced with decreasing island size (i.e., with void size) reaching only ∼10% in comparison with ∼50% in islands on large areas.
Peculiar behavior of the initial magnetization in CoNi thin films
Authors: Altuncevahir Baki , Koymen Ali R.
Published: May 2004

The initial magnetization curve of ac demagnetized CoNi thin films were outside the hyteresis loop boundary before saturation. Using −dc demagnetization, the initial magnetization curve was brought into the hysteresis loop boundary. Henkel plot shows negative deviation in ac demagnetized CoNi thin film, and both positive and negative deviation, in −dc demagnetized CoNi film. Bitter domain imaging technique showed that the initial magnetization effects are due to the corresponding domain configurations obtained in ac and in −dc demagnetization.
Microstructure and magnetic properties of a Co/Pd multilayer on a controlled Pd/Si seed layer for double-layered perpendicular magnetic recording media
Authors: Kawaji J. , Asahi T. , Hashimoto H. , Hokkyo J. , Osaka T. , Matsunuma S. , Sáfrán G. , Ariake J. , Ouchi K.
Published: May 2004

A 10 nm Pd/Si dual thin film was developed as a seed layer for a Co/Pd multilayer in double-layered perpendicular magnetic recording media. The Pd/Si seed layer, sputter deposited under Ar sputtering gas containing N2 and postannealed at 400 °C, markedly reduced intergranular exchange coupling of the Co/Pd multilayered film, resulting in a decrease in both the slope parameter, defined as 4π(dM/dH)H=Hc, and the magnetic cluster size. Consequently, medium noise was essentially reduced, thereby improving the signal-to-noise ratio of the Co/Pd recording media on a CoZrNb soft magnetic underlayer. The addition of N2 gas effectively decreased the grain size of Pd in the seed layer. A Pd/Si seed layer prepared with both processes exhibited a granular structure of fine Pd-rich grains surrounded by a Si-rich amorphous region, which provided nucleation sites for the growth of well-separated Co/Pd multilayered grains.
Electric field in hard superconductors with arbitrary cross section and general critical current law
Authors: Badı́a-Majós A. , López C.
Published: May 2004

The induced electric field E(x) during magnetic flux entry in superconductors with arbitrary cross section Ω and general critical current law, has been evaluated by integration along the vortex penetration paths. Nontrivial flux motion streamlines are obtained from a variational statement of the critical state, which takes the form of an optimization problem on the finite element discretization of Ω. The generality of the theory allows one to deal with physical conditions not considered before. In particular, it is shown that the boundary condition to be used for determining E is the knowledge of the locus E=0 within the sample. This is emphasized for anisotropic materials in which the electric field is not parallel to the surface. Both numerical and analytical evaluations are presented for homogeneous materials with different geometries: convex and concave contours, samples with holes, variable curvature contours, and for anisotropic samples. In the isotropic case, discontinuities in the electric current paths are shown to be related to changing curvature of the sample’s surface. Anisotropic samples display the same kind of discontinuities, even for constant surface curvature.
Electron field-emission mechanism in nanostructured carbon films: A quest
Authors: Gupta S. , Morell G. , Weiner B. R.
Published: May 2004

An open question to the community about the general consensus on the field-emission mechanism in carbon-based materials led to this study. By applying the Fowler–Nordheim (FN) model for carbon-based films, despite the fact that the microstructure and the resulting physical properties of the films can be tuned by scanning various process parameters, providing, in turn, from almost insulating (less defective) to semiconducting (highly defective) films and even a mixture of the two, the material can be categorized as electrically heterogeneous nanostructured carbon. The electrical heterogeneity arises from the different carbon hybridizations (sp2- versus sp3-bonded carbon). In an attempt to tackle these issues, we have performed a comprehensive analysis of I–V data obtained from filament-assisted chemical-vapor-deposition-grown sulfur-incorporated nanocomposite carbon thin films with different microstructures. Studies of the augmentation of the field-emission properties in this material indicated various roles of sulfur in modifying the film properties [Gupta et al., Appl. Phys. Lett. 80, 3446 (2002)]. The I–V data were fitted to various mathematical forms: I=AV2 exp(−B/V) [FN model], I=C exp(aV1/2/kT) [Schottky model], and I=Vn (n>1, for high fields) [space-charge-limited current (SCLC) model]. The goodness of fit along with the theoretical justification(s) on the electron field-emission results were taken into consideration to provide favorable indications for accepting or discarding any particular model. These findings suggest that there is an apparent crossover from SCLC to FN behavior as a function of film microstructure occurring due to the impurity incorporation as the microstructure transits smoothly from microcrystalline to nanocrystalline carbon. Other evidence in support of the aforementioned suggestion is based on the concept of percolation occurring in this nanocomposite carbon (a mix of conducting–insulating/semiconducting) material, whereby the electrons are allowed to tunnel from one conductive cluster to another separated by an insulating matrix, which is demonstrated through electrical conductivity measurements.
Two-color photorefractive properties in near-stoichiometric lithium tantalate crystals
Authors: Liu Youwen , Kitamura Kenji , Takekawa Shunji , Nakamura Masaru , Furukawa Yasunori , Hatano Hideki
Published: May 2004

The two-color photorefractive properties in undoped as-grown near-stoichiometric lithium tantalate crystals were investigated, where a near-infrared laser and a cw ultraviolet beam were used for writing and gating, respectively. The key parameters in characterizing two-color photorefractive effect, light-induced absorption change, two-color sensitivity, refractive index change, readout characteristics, and dark decay were measured by changing intensities of gating and writing beams, wavelengths of gating and writing beams for the crystals with different near-stoichiometric crystal compositions, and proton concentrations. The results showed that there exists an optimal crystal composition of around 49.65% for both sensitivity and refractive index change together with moderate lifetime of small polarons. The achieved refractive index change was on the order of 10−4, and the obtained maximum sensitivity was 0.18 cm/J. The extrapolated lifetime of holograms at room temperature in the crystals without observable OH− absorption was longer than 50 yr. The measurements of UV-induced absorption change at room temperature and low temperature of 77.3 K suggested that the unintentional impurity of Fe and intrinsic defects were responsible for two-color photorefractive effect. The excellent two-color photorefractive properties of undoped as-grown near-stoichiometric lithium tantalate crystals were discussed based on this mechanism and the physical properties of lithium tantalate.
Ionic and electronic impedance imaging using atomic force microscopy
Authors: O’Hayre Ryan , Lee Minhwan , Prinz Fritz B.
Published: May 2004

Localized alternating current (ac) impedance measurements are acquired directly through a conductive atomic force microscope (AFM) tip. Both a spectroscopy mode (where full impedance spectra are obtained at fixed locations on a sample surface) and an imaging mode (where single frequency impedance maps are acquired across a sample) are used to characterize Au/Si3N4 test structures, ZnO varistors, and Nafion membrane (an ion conductor). Both modulus and phase information are acquired simultaneously. The use of an ac technique permits the study of electrochemical systems and ion conductors in addition to electronic systems. The capabilities and limitations of the AFM impedance imaging technique are discussed in detail.
A numerical study on the intrinsic error involved in the use of a shear cell to measure liquid diffusion coefficients
Authors: Yang B. J. , Huang W. D. , Smith R. W.
Published: May 2004

The shear cell technique has some merits not possessed by the long capillary method for the measurement of diffusion coefficients. However, it also possesses the possibility of an intrinsic error arising from any liquid mixing associated with its shearing operation. A dynamic numerical simulation, with a grid sliding technique, was developed and was used to explore the solute exchange between the two liquid charges in a diffusion couple, the concentration distribution along the liquid charges, and to investigate the error of the shear cell method. This was shown to be associated with the shear rate and annealing time involved. It was found that the intrinsic error was caused mainly by the convection resulting from the shear and strongly depends on the mass Peclet number and the Reynold’s number. The intrinsic error arising from the shearing action in the shear cell required to create the diffusion couple deceases with increasing time of the diffusion anneal. The variation of the maximum intrinsic error with the elapsed diffusion time was calculated for a capillary diameter of 1.5 mm similar to that which was used in diffusion coefficient measurements.
Effects of finite crystal size in the quartz crystal microbalance with dissipation measurement system: Implications for data analysis
Authors: Reviakine Ilya , Morozov Alexander N. , Rossetti Fernanda F.
Published: May 2004

A quartz crystal microbalance with dissipation, or QCM-D, allows the properties of a loaded quartz oscillator—resonance frequency and dissipation [1/(the quality factor)]—to be monitored on several overtones. The frequency and the dissipation factor of the loaded oscillator are functions of the physical properties (such as thickness, density, viscosity, elasticity, roughness) of the media loading the resonator (e.g., a liquid, a thin polymer film, a phospholipid bilayer, etc.) and the frequency at which the measurement was performed. In this contribution, it is shown that the frequencies measured with the QCM-D instrument on the first, third, fifth, and seventh overtones do not agree with those expected of an ideal, infinite crystal. Therefore, the results of QCM-D measurements cannot be directly compared with theoretical models that require the resonance frequency of an unloaded crystal to be used as one of the input parameters. A phenomenological procedure for dealing with this problem is proposed.
Theory of dielectrophoresis in colloidal suspensions
Authors: Dong L. , Huang J. P. , Yu K. W.
Published: May 2004

A theoretical study of the dielectrophoretic (DEP) spectrum of a pair of touching colloidal particles or biological cells in a host fluid under the application of a nonuniform alternating current (ac) electric field is presented. The main objective of this work is to investigate the correlation effects due to the presence of mutual polarization on the DEP spectrum. In particular, we employ the multiple image method to account for the effect of multiple images, and obtain an analytic expression for the DEP force. It is found that, at low frequencies, the DEP force can be enhanced (reduced) significantly for the longitudinal (transverse) field case due to the presence of multiple images. The numerical results can be well understood in the spectral representation theory. To first order in the dipole interaction, our results reduce to the established formula derived by an alternative method. Our approach demonstrates the importance of the correlation effects in ac electrokinetic phenomena of colloidal suspensions.
Characterization of extreme ultraviolet light-emitting plasmas from a laser-excited fluorine containing liquid polymer jet target
Authors: Abel B. , Assmann J. , Faubel M. , Gäbel K. , Kranzusch S. , Lugovoj E. , Mann K. , Missalla T. , Peth Ch.
Published: May 2004

The operation of a liquid polymer jet laser-plasma target and the characterization of the absolute x-ray emission in the extreme ultraviolet wavelength window from 9–19 nm is reported. The target is a liquid polymer (perfluoro-polyether) that is exposed to pulsed and focused laser light at 532 nm in the form of a thin, liquid microjet (d=40 to 160 μm) in vacuum. The spectral brightness of the source in the 13 nm range is relatively high because a large fraction of radiative energy is emitted in one single line only, which is assigned to be the 2p–3d FVII doublet at 12.8 nm, with a laser energy conversion efficiency of 0.45% (2π sr, 2% bandwidth) in our initial experiment. A further increase of the relative emission has been found in the wavelength range between 7 and 17 nm when the jet diameter was increased from 40 to 160 μm. The two-dimensional spatial profile of the source plasma (d=40 to 50 μm) has been analyzed with a pinhole camera.
Vibration analysis of an elastic-sphere oscillator contacting semi-infinite viscoelastic solids in resonant ultrasound microscopy
Authors: Tian Jiayong , Ogi Hirotsugu , Hirao Masahiko
Published: May 2004

Resonant-ultrasound microscopy evaluates local Young’s modulus of materials by the resonant-frequency shift of a vibrating oscillator. This study presents a dynamic-contact model to analyze free vibrations of an isotropic elastic-sphere oscillator contacting two semi-infinite viscoelastic solids, which sandwich the sphere. Assuming frictionless contacts and smaller vibrational amplitude, dynamic-contact pressure distributions are obtained with the linearized maximum contact pressure and contact radius. Combining the sphere oscillation and the solid motions through contact-displacement boundary conditions, resonant frequencies of the elastic sphere are obtained. Unlike the quasistatic model, this dynamic model agrees well with the measurements.
Numerical method for calculating the apparent eddy current conductivity loss on randomly rough surfaces
Authors: Yu Feng , Nagy Peter B.
Published: May 2004

Because of their frequency-dependent penetration depth, eddy current measurements are capable of mapping the near-surface depth profile of the electrical conductivity. This technique is used to nondestructively characterize the subsurface residual stress and cold work distributions in shot-peened metal components. Unfortunately, the spurious surface roughness produced by the shot peening process causes an apparent loss of eddy current conductivity, thereby decreasing the accuracy of the measurements, especially in thermally relaxed specimens where the primary material effects are significantly reduced. In this paper, a numerical method is introduced based on the Rayleigh approximation for calculating the apparent eddy current conductivity loss exhibited by 1D randomly rough surfaces. The relevant boundary conditions are satisfied using the so-called point-by-point technique, and the results are first compared to the previously developed Rayleigh–Fourier technique for a 1D sinusoidal corrugation. Pseudorandom surface profiles of different autocorrelation functions are considered. It is found that the Gaussian model lends itself the best to numerical simulations, but it significantly underestimates the apparent eddy current conductivity loss expected on real shot-peened surfaces, which exhibit essentially exponential correlation function. It is also demonstrated that the Lorentzian model is numerically less stable, but physically closer to the exponential one. The latter could not be simulated reliably by the present numerical technique because of its slowly decaying high-frequency spectral components.
Templated wide band-gap nanostructures
Authors: Alizadeh A. , Sharma P. , Ganti S. , LeBoeuf S. F. , Tsakalakos L.
Published: May 2004

In this two-pronged work we report (a) a study of defect nucleation in three-dimensional confined nanoislands and (b) a surface-elasticity induced size effect in the optoelectronic properties of embedded and templated semiconducting nanostructures. Several key features in the design of nanostructure templates are analyzed and dislocation free contour maps are presented for combination of various lattice mismatches, substrates, and geometrical dimensions. Unlike the case for thin epitaxial films, it is found that for nanostructures, below a certain critical lateral dimension, dislocation free structures of any thickness can be grown. With regards to the optoelectronic properties of nanostructures, while size dependency due to quantum confinement and electrostatic interactions are well known, we show that an additional size-dependent strain is caused by the distinct elastic behavior of surfaces and interfaces at the nanoscopic scale compared to the macroscopic scale. This is in contrast to the usual way strain is linked to optoelectronic properties, i.e., via classical elasticity, which ignores surface energies and is intrinsically size independent. Surface strains appear to be only influential in the nanometer regime due to appreciable surface-to-volume ratios. Among our major conclusions are that errors as large as 100 meV in band-gap prediction can incur if this size-dependent surface effect is ignored.
Geometrical effect in magneto-Peltier cooling of single crystal Bi
Authors: Yamashita Osamu , Satou Kouji , Tomiyoshi Shoichi
Published: May 2004

The cooling temperatures of rectangular parallelepiped Bi single crystals with various widths W and thickness t were measured at 293 K as a function of electric current in the magnetic field B up to 2.17 T. The magnetic field was aligned along the thickness of a sample and the current flows along its length L through the copper leads soldered to both end surfaces of cross section (W×t), where W, t, and L are parallel to the binary, bisector, and trigonal axes of Bi single crystal, respectively. The thermoelement was not in contact with a heat sink. The cooling temperature at the cooled surface increased with increasing the magnetic field, and it depended strongly on the thickness rather than the width of the crystal in high magnetic fields. The largest maximum cooling temperature was achieved when a thermoelement has optimum dimensions so that no heat energy is generated at the cold side. The cooling temperature of Bi single crystal with optimum dimensions of L=15 mm, W=4 mm, and t=2 mm increased from 4.1 K in B=0 T to 8.5 K in B=+2.17 T, so that it exceeded maximum cooling temperatures of 5.7 K obtained for a typical Bi2Te3 and 5.2 K measured previously for a polycrystalline Bi in B=+2.17 T.
Work function of the mixed-valent manganese perovskites
Authors: Reagor D. W. , Lee S. Y. , Li Y. , Jia Q. X.
Published: May 2004

We have performed resistivity and work function measurements on a series of samples with Ln0.7D0.3MnO3 composition, where Ln is a lanthanide (La, Pr, or Nd) and D is a nominally divalent ion (Sr, Ca, or Pb). The resistivity measurements are, within some small variation, in agreement with those previously reported for polycrystalline samples of similar composition. The measurement of a work function in air is often difficult to reproduce, mainly owing to the fact that it is a surface probe and can be modified significantly by attachment of contaminants from the atmosphere. Here we have obtained reproducible results that are slightly more stable than the reference metals used in the measurements. This required the introduction of a higher order correction to a common Kelvin probe experimental procedure. Another factor contributing to the high quality of the results is the low carrier density of the conducting oxides. This allows greater penetration of quasistatic electric fields. The work function results here are in overall agreement with previous photoemission work that determined an approximate electron binding energy. We find the best correlation of increased work function is to strain introduced by larger divalent ions and smaller Lanthanide ions. We conclude that the work function is dominated by electrons near the divalent site, and this is interpreted in terms of strain modifying the coulomb energy of the electronic states.
Characterization of nonlinear elasticity and elastic instability in single-walled carbon nanotubes
Authors: Xiao Tan , Xu Xiaojing , Liao Kin
Published: May 2004

Nonlinear elastic properties and elastic instability of single-walled carbon nanotubes (SWNTs) under large-scale axial compression were investigated by molecular simulations using the second-generation Brenner potential. It was found that the energy changes of the nanotube can be closely fitted by a cubic function of applied strains. Therefore the in-plane stiffness C of the nanotube is linearly dependent on the strain. It shows that SWNTs harden under compression but soften in tension. At large strain, C is also sensitive to chirality and diameters of nanotubes when these are small. The critical strains of compressed nanotubes are inversely proportional to their diameters on the condition that local buckling occurs in simulations, which can be properly predicted by continuum elasticity theory if the effective thickness is known.
Optical properties of an all-plastic WO3⋅H2O-based infrared modulator
Authors: Bessière A. , Beluze L. , Morcrette M. , Viana B. , Frigerio J.-M. , Andraud C. , Lucas V.
Published: May 2004

WO 3 ⋅ H 2 O powder embedded in a plastic matrix has demonstrated attractive properties as a flexible emissivity modulator driven by the electrochemical insertion of lithium. The near infrared optical properties of LixWO3⋅H2O are investigated as a function of the lithium intercalation level x for 0⩽x⩽2.65. The reflectivity at 2.5 μm, measured through a glass window, was found to vary between 0.15 and 0.4 over the α intercalation phase (0⩽x<0.3), i.e., demonstrating an emissivity contrast of ∼0.25. For x⩾0.1 the LixWO3⋅H2O-based plastic device exhibits a Drude behavior comparable to that of monoclinic tungsten oxide (m-WO3) thin films. For larger intercalation levels, the performances are affected by some irreversible structural modifications of the LixWO3⋅H2O lattice.
Light emission in a laser-initiated transient glow discharge in argon and air with butane
Authors: Hoshi Y. , Yokota A. , Yoshida H. , Akedo J.
Published: May 2004

By observing light emissions from flares, we systematically investigated transient phenomena from glow to arc discharges under designated durations. To describe properly the discharge flare behavior, we introduced a parameter νa defined as (electrode gap)/(flare traveling time). The main findings are as follows. The parameter νa is proportional to the initial electrode voltage. The maximum value of νa is obtained at a certain gas pressure and it falls on the Paschen curve. The flare behavior follows Townsend’s theory at each instant, therefore, it is governed by the state of the electrolytic dissociation of the gas.
Be–Se double-phonon behavior in Zn1−x−yMgyBexSe alloy
Authors: Pagès O. , Ajjoun M. , Bormann D. , Tournié E.
Published: May 2004

The longitudinal optical (LO) and transverse optical (TO) phonons of zincblende Zn1−x−yMgyBexSe with x,y∼0.15 are identified by Raman scattering. The Be–Se region exhibits a two-mode behavior similar to that observed with the random Zn1−xBex(Se,Te) systems. This indicates that the percolation picture set for the latter mixed crystals with contrasted bond stiffness extends to Be-based multinaries. Accordingly the low- and high-frequency modes refer to Be–Se bonds within the Be-rich hard-like region and the (Zn,Mg)-rich soft-like one, respectively. Besides, we derive the (x,y) values together with the fraction p of Be atoms in the hard-like region from combined contour modeling of the TO and LO Raman line shapes. We find p∼x, as in the ZnBe(Se,Te) references. We deduce that the Be incorporation is random in ZnMgBeSe.
Optical properties and energy levels of Ce3+ in lutetium pyrosilicate scintillator crystal
Authors: Pidol Ludivine , Viana Bruno , Kahn-Harari Andrée , Galtayries Anouk , Bessière Aurélie , Dorenbos Pieter
Published: May 2004

Cerium doped lutetium pyro-silicate Ce3+:Lu2Si2O7 (LPS) is an inorganic scintillator that displays particularly promising performance. The spectroscopic properties of pure and Ce3+-doped Lu2Si2O7 are investigated. X-ray-excited luminescence, time-resolved excitation and emission spectroscopy, as well as decay time spectra are presented. For better understanding of the scintillation mechanism, several approaches were undertaken to locate the Ce3+ energy levels relative to levels of LPS host. To estimate the energy position of Ce3+ 4f levels with respect to the valence band, x-ray photoemission spectroscopy (XPS) was used. Scintillation properties of LPS: Ce are explained in terms of the relative positions of Ce3+-5d levels and the conduction band.