bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser ablation of solid substrates in a water-confined environment
Authors: Zhu S. , Lu Y. F. , Hong M. H.
Published: July 2002

Laser ablation of Si under a water surface has been investigated. The laser used is a KrF excimer laser, which has a wavelength of 248 nm and a pulse duration of 23 ns. It is found that the laser ablation rate of Si varies with the thickness of the water layer above the Si substrates. The laser ablation rate is the most highly enhanced with a water layer of 1.1 mm. It is assumed that the plasma generated in the water confinement regime with an optimal water layer thickness induces the strongest pressure. This high-pressure, high-temperature plasma results in the highest ablation rate. A wide-band microphone is used to detect the audible acoustic wave generated during the laser ablation. The amplitude of the acoustic wave is closely related to the ablation rate. It is found that the first peak-to-peak amplitude of the acoustic wave is the strongest when the water layer thickness is 1.1 mm above the substrate. Fast Fourier transform analysis of the wave forms shows that there are several frequency components included in the acoustic waves. The dominant frequency component decreases from 10.6 to 3.5 kHz as the water layer thickness varies from 1 to 2.2 mm. Diagnostics of the acoustic wave emission can be used to find the optimal water layer thickness to enhance the laser ablation rate. With proper calibration, acoustic-wave detection can be used as a real-time monitoring of the laser ablation.
Effect of purification of the electrical conductivity and complex permittivity of multiwall carbon nanotubes
Authors: Grimes C. A. , Dickey E. C. , Mungle C. , Ong K. G. , Qian D.
Published: July 2002

In this work we report on the complex permittivity spectra and electrical conductivity of both as-fabricated and graphitized multiwall carbon nanotubes (MWNTs). The high-temperature annealing removes the Fe3C catalyst particles present in the as-fabricated material, enabling the intrinsic MWNT properties to be measured. The permittivity spectra of 1 wt % MWNT-polystyrene composite films are measured from 75 to 1875 MHz. Comparison of measurements with an appropriate effective medium model shows that the residual catalyst inclusions in the core of the nanotube increase the average electrical conductivity by approximately a factor of 3.5.
Alternative configuration interaction expansions for transition metal ions with intermediate oxidation states in crystals: The structure and absorption spectrum of Cs2GeF6:Mn4+
Authors: Barandiarán Zoila , Seijo Luis
Published: July 2002

The vertical absorption spectrum of the MnF62− cluster embedded in the Cs2GeF6 host crystal was recently calculated using the averaged coupled pair functional method leading to very large discrepancies with accurate one- and two-photon spectra. The same multiconfigurational expansions had previously been successful in similar systems which involved transition metal impurities in lower oxidation states. In this paper we show that the ligand-to-metal charge transfer configurations become so important in this intermediate oxidation state impurity (and, possibly, non-negligible ligand–ligand weak bonding interactions) that none of the 18 molecular orbitals of F 2p character should be left inactive in the correlation treatment. This requirement can be satisfied in MnF62− because of the higher oxidation state of manganese, which enhances the ligand field splittings in the Mn 3d3 configuration manifold so much that one dominant Mn 3d3 configuration rather than the full Mn 3d3 active space can be used as a single reference for single and double excitations from all occupied ligand 2p orbitals. The results of this work, together with those of previous studies, outline two different alternative truncation schemes of the valence electron correlation which produce the same, necessary, high accuracy in structural and spectroscopic properties of transition metal ions doped in ionic crystals. Whether one or the other should be used depends, basically, on the formal oxidation state of the transition metal impurity.
An electron–positron beam–plasma instability
Authors: Gilbert S. J. , Dubin Daniel H. E. , Greaves R. G. , Surko C. M.
Published: July 2002

Using a new technique to generate cold electron beams, an electron-beam positron-plasma experiment was performed in a previously unexplored range of energies. An electron beam, formed from a thermalized room-temperature electron plasma, is transmitted through a positron plasma stored in a quadrupole Penning trap geometry. The transit-time instability, which is excited by the beam, was previously studied using a hot-cathode electron gun. The large beam energies produced by the cathode did not permit an investigation of the instability in the interesting range of energies near its onset. Using a new 0.1 eV energy width electron beam, we have reinvestigated the system. The experimental data are compared with the results of a theoretical model, also described in this paper. The theory employs a linearized cold fluid and Vlasov approach to model the plasma and beam dynamics, respectively. The data and predictions are in good agreement over the broad range of energies and beam currents studied.
Femtosecond mid-infrared spectroscopy of aqueous solvation shells
Authors: Kropman M. F. , Bakker H. J.
Published: July 2002

We use femtosecond two-color mid-infrared spectroscopy to study the dynamics of aqueous solutions of salts in HDO:D2O. We find that the lifetime of the O–H stretch vibration of HDO molecules in the solvation shell of the halogenic anions Cl−, Br−, and I− is much longer than the lifetime of the O–H stretch vibration of the HDO molecule in bulk D2O solution. This difference in lifetime allows for a clear separation of the response of the solvation shell from that of the bulk liquid. A detailed investigation of the spectral dynamics of the solvating HDO molecules reveals that the hydrogen-bond dynamics of these molecules are much slower than the hydrogen-bond dynamics of bulk liquid water.
Influence of charge exchange on the collection of the laser produced ions
Authors: Hasegawa Shuichi , Takei Manabu , Suzuki Atsuyuki , Kurosawa Hitoshi
Published: July 2002

We evaluated influences of charge exchange on ion collection of laser isotope separation of uranium. We made a two-dimensional code based on fluid dynamics taking the charge exchange term into consideration. Parametric study was performed in terms of electric amplitude, ion density, and electron temperature. In addition to parallel electrode arrangement, calculations were performed for Π- and M-type arrangements. The ratio of charge exchanged ions is found to largely depend on the collection time.
Carbon nanotube field-effect inverters
Authors: Liu Xiaolei , Lee Chenglung , Zhou Chongwu , Han Jie
Published: July 2002

This letter presents p-type metal–oxide–semiconductor (PMOS) and complementary metal–oxide–semiconductor (CMOS) inverters based on single-walled carbon nanotube field-effect transistors. The device structures consist of carbon nanotubes grown via a chemical-vapor deposition method and contacted by two metallic source/drain electrodes. Electrical properties of both p-type (without doping) and n-type nanotube transistors with potassium doping have been measured. By utilizing a resistor as the load for a p-type nanotube field-effect transistor, a PMOS inverter is demonstrated. Furthermore, by connecting a p-type nanotube transistor and an n-type nanotube transistor, a CMOS inverter is demonstrated. Both types of inverters exhibit nice transfer characteristics at room temperature. Our work represents one step forward toward integrated circuits based on nanoelectronic devices.
Physical and electrical characterization of Hafnium oxide and Hafnium silicate sputtered films
Authors: Callegari A. , Cartier E. , Gribelyuk M. , Okorn-Schmidt H. F. , Zabel T.
Published: July 2002

Hafnium oxides and hafnium silicate films were investigated as a possible replacement for the SiO2 gate dielectric. Hafnium oxide films were formed by reactive sputtering from a single Hf oxide target in a predominantly Ar atmosphere containing small additions of oxygen. Hafnium silicates were made by adding a He-diluted silane gas for Si incorporation. By changing the silane gas flow, different Si atomic concentrations were incorporated into the Hf oxide films. Depositions were performed with the substrate held at temperatures of 22 °C and 500 °C. The chemical composition of the films was determined with nuclear techniques. Optical reflectivity was used to measure the optical band gap. The film morphology was investigated by transmission electron microscopy (TEM) and the electrical properties were measured with capacitance–voltage and current–voltage measurements using aluminum gate capacitors. TEM and electrical measurement showed that a SiO2 interfacial layer of about 3 nm formed at the Si interface due to the oxidizing sputter ambient. This precluded the growth of Hf based high-K films with small equivalent thickness. After correction for the interfacial oxide layer, the dielectric constant was found to decrease from about 21 for Hf oxide to about 4–5 for the Hafnium silicates with low Hf content (3 at. % Hf and 32 at. % Si). The optical band gap was found to increase from 5.8 eV for Hf oxide to about 7 eV for the silicate films. After annealing at 1000 °C followed by a 300 °C postmetallization anneal, negligible flat band voltage shift were measured on hafnium silicate films and good interface passivation was observed. However, leakage currents increased due to the high temperature processing.
Optical absorption and photoluminescence studies on CdS quantum dots in Nafion
Authors: Nandakumar P. , Vijayan C. , Murti Y. V. G. S.
Published: July 2002

Excitonic effects are observed in the optical absorption and photoluminescence of strongly confined CdS quantum dots embedded in the polymer Nafion. The three bands identified in the optical absorption spectra could be attributed to 1se-1sh, 1pe-1ph, and 2se-2sh transitions of the noninteracting particle model. Photoluminescence spectra show a strong emission band corresponding to electron–hole recombination and a weak band due to emission from defect states. The strength of electron–phonon coupling is small in the regime of strong confinement and decreases with decreasing particle size.
Helicon-wave-excited-plasma sputtering epitaxy of ZnO on sapphire (0001) substrates
Authors: Chichibu S. F. , Yoshida T. , Onuma T. , Nakanishi H.
Published: July 2002

A promising approach to obtain epitaxial films of oxide semiconductors was demonstrated, namely helicon-wave-excited-plasma sputtering epitaxy. Due to the surface-damage-free nature, completely a-axis-locked c(0001)-orientation ZnO epilayers were successfully grown on sapphire (0001) substrates having ultrasmooth surfaces with atomic steps. The ZnO epilayer exhibited a dominant near-band-edge photoluminescence (PL) peak at 300 K. Since the PL was considered to be due to the recombination of excitons bound to an impurity or a defect and certain tilting and twisting of the films were observed when Ar/O2 were used as sputtering gases, purification and optimization of the overall process are necessary to obtain improved epilayer qualities.
The distance fluctuation criterion for melting: Comparison of square-well and Morse potential models for clusters and homopolymers
Authors: Zhou Yaoqi , Karplus Martin , Ball Keith D. , Berry R. Stephen
Published: July 2002

We explore the distance fluctuation criterion (“Lindemann criterion”) for melting transitions. Distances from average positions in accord with Lindemann, or interparticle distances, in accord with Jellinek and Berry or Etters and Kaelberer, are examined. The primary goal is to determine which of these offers the more useful criterion. The choice of origin can sometimes effect the significance of the index. We study three systems with two kinds of potentials. They are all composed of 64 particles: (a) and (b), a homopolymer and a cluster that consist of beads interacting pairwise through square-well potentials, and (c) a cluster of particles interacting pairwise through Morse potentials. For each of the noncrystalline structures, in contrast to the crystals originally studied by Lindemann, the fluctuation parameter based on interparticle distances gives a clearer separability of liquid and solid phases than that based on fluctuations from average positions. The solid-like forms of the Morse cluster, the square-well cluster, and the square-well homopolymer have similar behavior, indicating that a broad class of systems can be evaluated with this index. In these systems, relative fluctuation parameters provide a suitable criterion for the melting transition. The critical values for the interparticle distance criterion, which are in the range of 0.03–0.05, are smaller than those for the Lindemann criterion (0.1–0.15).
Nitrogen bonding structure in carbon nitride thin films studied by soft x-ray spectroscopy
Authors: Hellgren Niklas , Guo Jinghua , Såthe Conny , Agui Akane , Nordgren Joseph , Luo Yi , Ågren Hans , Sundgren Jan-Eric
Published: July 2002

Soft x-ray absorption (SXAS) and emission (SXES) spectroscopies were applied to study the nitrogen bonding structure in magnetron sputtered CNx thin films. By comparing with calculated spectra of N in different model systems, N in three main bonding environments can be identified: (i) C≡N bonds, with a sharp SXAS peak at 399.5 eV, (ii) pyridine-like N (i.e., N bonded to two C atoms), with an x-ray absorption resonance at ∼398.5 eV, and (iii) N substituted in graphite, possibly with one sp3 carbon as a neighbor (SXAS energy ∼401 eV). These bondings are present in all CNx films analyzed; however, as shown earlier, the relative intensities between the peaks may vary with the growth conditions. Differences in the coordination of the nearest or second nearest C neighbors only cause slight changes in the peak positions and spectrum shape.
Localized exciton dynamics in strained cubic In0.1Ga0.9N/GaN multiple quantum wells
Authors: Chichibu S. F. , Sugiyama M. , Onuma T. , Kitamura T. , Nakanishi H. , Kuroda T. , Tackeuchi A. , Sota T. , Ishida Y. , Okumura H.
Published: July 2002

Radiative and nonradiative recombination dynamics in strained cubic (c-) In0.1Ga0.9N/c-GaN multiple quantum wells were studied using temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy. In contrast to hexagonal InGaN quantum wells, low-excitation photoluminescence peak energy increased moderately with decreasing well thickness L and the PL lifetime did not strongly depend on L. The results clearly indicated that the piezoelectric field was not acting on the transition process. The TRPL signal was well fitted as a stretched exponential decay from 10 to 300 K, showing that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures such as In clusters. The localized states were considered to have two-dimensional density of states at 300 K (quantum disk size), since the radiative lifetime increased with increasing temperature above 150 K.
Concentrated aqueous urea solutions: A molecular dynamics study of different models
Authors: Sokolić Franjo , Idrissi Abdenacer , Perera Aurélien
Published: July 2002

Molecular dynamics studies of concentrated aqueous urea solutions, with urea mole fraction ranging from 0.04 to 0.25, have been performed, essentially focussed on the study of the tendency of urea to self-aggregate. Four of the existing urea models have been studied herein, with number of particles of N=256 and N=864. It was generally found that unusually long MD runs were required in order to reach proper thermodynamic equilibrium, and particularly so for the N=256 systems. This slow convergence of the trajectories is related here to the urea self-aggregation tendency that appears in our calculation not to be limited to pairing processes only. Moreover, no evidence for urea dimerization has been found in the present calculations. Evidence for urea self-aggregation is the most visible in the structural properties, particularly in the way density–density correlations decay to their unit asymptotic limit. Densities, energies, diffusion constants, and structural data have been calculated for all models, and compared with corresponding experimental data. This comparison allows a classification of the different models in terms of their relative agreements with experimental data. Urea aggregation appears to be quite different for different models.
Continuum elastic model of fullerenes and the sphericity of the carbon onion shells
Authors: Ahmad Shoaib
Published: July 2002

A continuum elastic model of fullerenes is presented by utilizing the analogy between the closed carbon cages and elastic shells. We derive expressions for the curvature related strain energies Ep of the pentagonal protrusions. We propose to explain the observed sphericity of the carbon onions shells as opposed to the predicted protrusions around the pentagonal defects on the basis of our continuum elastic model of fullerenes. In our model the energy inherent in the pentagonal protrusions Ep is due to the stretching and bending of the shell and shown to be a function of the structural parameters. It also defines the upper limit on the size of the free-standing fullerenes. Using Ep and the topological arguments, we show that the pentagonal protrusions will be smoothed out, resulting in spherical shells of the carbon onions denoted as C60@C240@C540@C960@C1500,… .
Time dependence of forces between mica surfaces in water and its relation to the release of surface ions
Authors: Raviv Uri , Laurat Pierre , Klein Jacob
Published: July 2002

A surface force balance has been used to investigate the time-dependent behavior of the normal forces between two smooth mica surfaces immersed in salt-free (conductivity) water. A long-ranged repulsion on initial approach to adhesive contact indicates an equilibrium surface-charge density σ=σ0 due to the loss of K+ ions from the mica surface into the water. Subsequent force measurements at times τ following separation of the surfaces reveal that σ drops markedly following contact and separation, but then relaxes back to σ0 with a characteristic time τr=11±2 min. This behavior is attributed to condensation of the counterions into the surfaces when they come into contact, and to their subsequent slow release into the water. The energy barrier associated with the counterion release was estimated from the value of τr to be 33±4 kBT.
Atomistic mechanism of the initial oxidation of the clean Si(100)-(2×1) surface by O2 and SiO2 decomposition
Authors: Widjaja Yuniarto , Musgrave Charles B.
Published: July 2002

Density functional theory simulations are used to investigate the reaction mechanism of oxidation of the bare Si(100)-(2×1) surface by molecular oxygen. O2 adsorbs molecularly on the “up” surface Si atom with no activation barrier and an adsorption energy of 35 kcal/mol. Adsorbed O2 is found to be negatively charged. O2(a) then transforms into the peroxide bridge structure with a barrier of 10 kcal/mol and exothermicity of 33 kcal/mol. The bridged peroxide O2 then dissociates by first inserting one oxygen atom into the Si–Si dimer bond followed by insertion of the remaining oxygen atom into a Si–Si backbond. The activation barriers are 36 kcal/mol and 13 kcal/mol for the first and second oxygen insertions, respectively. We have also calculated the activation barriers for SiO2 film decomposition, which becomes prevalent at high temperatures, in which SiO(g) desorbs from SiO2 films. The SiO desorption barriers are found to be in the range of 65–67 kcal/mol.
Lateral displacement of a Gaussian beam reflected from a grounded slab with negative permittivity and permeability
Authors: Kong Jin Au , Wu Bae-Ian , Zhang Yan
Published: July 2002

A unique negative lateral shift is demonstrated in this letter for a Gaussian beam reflected from a grounded slab with both negative permittivity and permeability, which is distinctly different from a shift caused by a regular grounded slab. The incident beam is modeled as a tapered wave with a Gaussian spectrum. The waves inside and outside the slab are solved analytically from Maxwell’s equations by matching the boundary conditions at the interfaces. It is shown that the field values in all regions can be unambiguously determined. Numerical simulations are presented and the field values as well as the power densities are computed for all regions, and a dramatic negative lateral shift is observed when both ε and μ are negative.
Submillimeter-wave Fourier transform spectroscopy of biological macromolecules
Authors: Globus T. R. , Woolard D. L. , Samuels A. C. , Gelmont B. L. , Hesler J. , Crowe T. W. , Bykhovskaia M.
Published: July 2002

In this article we report experimental results on Fourier-transform infrared spectroscopy of deoxyribonucleic acid (DNA) macromolecules and related biological materials in the submillimeter range (i.e., ∼10–500 cm−1). Film samples made from commercial DNA fibers, polyadenylic acid potassium salt, and cellular agents such as the spore form of Bacillus subtillis have been prepared and measured. A broad series of measurements carried out in the low frequency region (10–50 cm−1) with a higher resolution of 0.2 cm−1 revealed fine features—multiple dielectric resonances in the submillimeter-wave spectra obtained from DNA samples. These long-wave absorption features are shown to be intrinsic properties of biological materials determined by phonon modes. The emphasis is on reproducibility of experimental spectra and on receiving reliable results. The effects of differences in sample preparation, including sample geometry, orientation, and aging are studied and separated from the phonon effects that determine the fine structure of transmission spectra. A direct comparison of spectra between different DNA samples reveals a large number of modes and a reasonable level of sequence-specific uniqueness. A theoretical study of two double helical DNA fragments has applied a normal mode analysis to predict spectra in the far infrared. Most of the modes determined by long-distance interactions are at frequencies below 220 cm−1, with the density higher than one mode per cm−1, which is approximately what was observed experimentally.
Two dimensional particle-in-cell simulations of the lunar wake
Authors: Birch Paul C. , Chapman Sandra C.
Published: July 2002

The solar wind plasma from the Sun interacts with the Moon, generating a wake structure behind it, since the Moon is to a good approximation an insulator, has no intrinsic magnetic field, and a very thin atmosphere. Following earlier one and one-half dimensional (112D) simulations, 212D simulations have been studied for the first time. 212D simulations in the solar wind rest frame with an initial circular void reveal structures similar to those found in the earlier simulations, indicating that the 112D simulations are a good approximation to the physics. The infilling of the wake was found to be anisotropic, with the infilling occurring predominantly parallel to the magnetic field. The 212D simulations that are stationary with respect to the Moon, allow the effect of solar wind magnetic fields oblique to the solar wind flow to be studied, and these reveal additional assymetries.
Growth mechanism of carbon nanotube forests by chemical vapor deposition
Authors: Louchev Oleg A. , Sato Yoichiro , Kanda Hisao
Published: July 2002

Analysis of kinetics processes involved in carbon nanotube (NT) forest growth during chemical vapor deposition suggests that: (i) carbon species are unable to penetrate to the forest bottom whenever the mean free path in gas is much larger than the typical distance between NTs; instead they collide with NT surfaces, chemisorbing within the top few microns, diffuse along the surface, and feed the growth at nanotube tips, (ii) wherever a catalyst nanoparticle is present, at the substrate or on the nanotube tip, in the postnucleation stage its role in feeding NT growth by C dissolution and bulk diffusion is negligibly small in comparison with the surface diffusion of C species adsorbing on the lateral surface of nanotubes, and (iii) bulk diffusion of C through the catalyst nanoparticle, defining the characteristic times of C penetration to nanoparticle base and surface saturation with C, is shown to play a major role in selection of the initial mode of nanotube nucleation and growth.
Electronic properties of ferroelectric SrBi2Ta2O9, SrBi2Nb2O9, and PbBi2Nb2O9 with optimized structures
Authors: Miura K.
Published: July 2002

Electronic properties of ferroelectric SrBi2Ta2O9 (SBT) SrBi2Nb2O9 (SBN) and PbBi2Nb2O9 (PBN) have been investigated using a first-principles calculational method with optimized structures. In optimized SBT, the displacement of Ta in TaO6 octahedra is small, while the displacement of Nb in NbO6 octahedra is larger in optimized SBN. In optimized PBN, on the other hand, due to the hybridization between Pb 6s and O 2p, the displacement of NbO6 octahedra is larger than that in SBN.
Channel-facilitated membrane transport: Transit probability and interaction with the channel
Authors: Berezhkovskii Alexander M. , Pustovoit Mark A. , Bezrukov Sergey M.
Published: July 2002

Transport of metabolites between cells and between subcellular compartments is facilitated by special protein channels that form aqueous pores traversing biological membranes. Accumulating evidence demonstrates that solute-specific channels display pronounced binding to the corresponding solutes. In this paper we rationalize this observation by showing that a wide and deep potential well inside the channel is able to greatly increase the transit probability of the particle through the channel. Using a one-dimensional diffusion model with radiation boundary conditions, we give exact analytical expressions for the particle translocation probabilities. We also run Brownian dynamics simulations to verify the model and the quantitative predictions of our theory.
Crack-free thick AlGaN grown on sapphire using AlN/AlGaN superlattices for strain management
Authors: Zhang J. P. , Wang H. M. , Gaevski M. E. , Chen C. Q. , Fareed Q. , Yang J. W. , Simin G. , Khan M. Asif
Published: July 2002

We report on an AlN/AlGaN superlattice approach to grow high-Al-content thick n+-AlGaN layers over c-plane sapphire substrates. Insertion of a set of AlN/AlGaN superlattices is shown to significantly reduce the biaxial tensile strain, thereby resulting in 3-μm-thick, crack-free Al0.2Ga0.8N layers. These high-quality, low-sheet-resistive layers are of key importance to avoid current crowding in quaternary AlInGaN multiple-quantum-well deep-ultraviolet light-emitting diodes over sapphire substrates.
Depth dependence of the tetragonal distortion of a GaN layer on Si(111) studied by Rutherford backscattering/channeling
Authors: Wu M. F. , Chen ChangChun , Zhu DeZhang , Zhou Shengqiang , Vantomme A. , Langouche G. , Zhang B. S. , Yang Hui
Published: July 2002

Rutherford backscattering and channeling have been used to characterize the structure of a GaN layer grown on a Si(111) substrate. The results show that a 1.26 μm GaN epitaxial layer with a rather abrupt interface and a good crystalline quality (χmin=3.4%) can be grown on a Si(111) substrate. Using the channeling angular scan around an off-normal 〈12̄13〉 axis in the {101̄0} plane of the GaN layer, the tetragonal distortion eT, which is caused by the elastic strain in the epilayer, can be determined. Moreover, the depth dependence of the eT can be obtained using this technique. A fully relaxed (eT=0) GaN layer for a thickness <2.8 μm is expected.
Channeling of low energy heavy ions: Er in Si〈111〉
Authors: Hogg S. M. , Pipeleers B. , Vantomme A. , Swart M.
Published: July 2002

Channeled implantation of 80 keV Er ions into Si(111) has been investigated as a function of the angle between the ion beam and the 〈111〉 normal axis. The angular dependence of both the projected range and the crystalline quality of the resulting silicide shows excellent agreement with theoretical predictions and simulations. Simulations indicate that the critical angle for channeling is largely independent of substrate temperature. The postimplantation strain in the silicide layer is determined by irradiation-induced damage and therefore increases with the angle between the crystalline axis and the ion beam direction.
Computerized mapping of fibrillation in normal ventricular myocardium
Authors: Chen Peng-Sheng , Garfinkel Alan , Weiss James N. , Karagueuzian Hrayr S.
Published: July 2002

It is well known that the ability to fibrillate is intrinsic to a normal ventricle that exceeds a critical mass. The questions we address are how is ventricular fibrillation (VF) initiated and perpetuated in normal myocardium, and why is VF not seen more often in the general population if all ventricles have the ability to fibrillate. To study the mechanisms of VF, we used computerized mapping techniques with up to 512 channels of simultaneous multisite recordings for data acquisition. The data were then processed for dynamic display of the activation patterns and for mathematical analyses of the activation intervals. The results show that in normal ventricles, VF can be initiated by a single strong premature stimulus given during the vulnerable period of the cardiac cycle. The initial activations form a figure-eight pattern. Afterward, VF will perpetuate itself without any outside help. The self-perpetuation itself is due to at least two factors. One is that single wave fronts spontaneously break up into two or more wavelets. The second is that when two wavelets intersect perpendicular to each other, the second wavelet is broken by the residual refractoriness left over from the first wavelet. Mathematical analyses of the patterns of activation during VF revealed that VF is a form of chaos, and that transition from ventricular tachycardia (VT) to VF occurs via the quasiperiodic route. In separate experiments, we found that we can convert VF to VT by tissue size reduction. The physiological mechanism associated with the latter transition appears to be the reduction of the number of reentrant wave fronts and wandering wavelets. Based on these findings, we propose that the reentrant wave fronts and the wandering wavelets serve as the physiological equivalent of coupled oscillators. A minimal number of oscillators is needed for VF to perpetuate itself, and to generate chaotic dynamics; hence a critical mass is required to perpetuate VF. We conclude that VF in normal myocardium is a form of reentrant cardiac arrhythmia. A strong electrical stimulus initiates single or dual reentrant wave fronts that break up into multiple wavelets. Sometimes short-lived reentry is also generated during the course of VF. These organized reentrant and broken wavelets serve as coupled oscillators that perpetuate VF and maintain chaos. Although the ability to support these oscillators exists in a normal ventricle, the triggers required to generate them are nonexistent in the normal heart. Therefore, VF and sudden death do not happen to most people with normal ventricular myocardium.
Implementation of the Lanczos algorithm for the Hubbard model on the Connection Machine system
Authors: Leung P. W. , Oppenheimer Paul E.
Published: July 2002

An implementation of the Lanczos algorithm for the exact diagonalization of the two dimensional Hubbard model on a 4×4 square lattice on the Connection Machine CM-2 system is described. The CM-2 is a massively parallel machine with distributed memory. The program is written in C/PARIS. This implementation minimizes memory usage by generating the matrix elements as needed instead of storing them. The Lanczos vectors are stored across the local memory of the processors. Using translational symmetry only, the dimension of the Hilbert space at half filling is more than 10 million. A speed of about 2.4 min per iteration is achieved on a 64K CM-2. This implementation is scalable. Running it on a bigger machine with more processors speeds up the process. The performance analysis of this implementation is shown and discuss its advantages and disadvantages are discussed.
Populations in the metastable and the resonance levels of argon and stepwise ionization effects in a low-pressure argon positive column
Authors: Ferreira C. M. , Loureiro J. , Ricard A.
Published: July 2002

The contribution of the ionization from the two metastable and the two resonance levels of argon to the total ionization rate in a low-pressure argon positive column is investigated theoretically and experimentally. A simple yet self-consistent discharge model is developed by coupling the balance between the electron total production and loss rates to the steady-state rate balance equations for the excited states. The predictions of this model, i.e., the maintenance field for the positive column and the populations in the individual excited states as a function of the gas pressure and the discharge current are compared with the experiment (0.05<p<1 Torr; 0.2<I<20 mA; φ=2.6-cm-i.d. discharge tube). It is shown that stepwise ionization becomes predominant for pressures larger than a few tenths of a Torr even for the lowest currents of this experiment. The predicted values of the maintenance field are considerably lower than those that would be required to sustain the discharge by ground-state ionization and compare favorably with the measurements. The calculated populations in the metastable states also compare reasonably well with the experiment. The discrepancies are, however, quite large in the case of the resonance levels owing, presumably, to the lack of accuracy of the theoretical escape factors for the resonance radiation.
Determination of silicon-silicon dioxide interface state properties from admittance measurements under illumination
Authors: Kar S. , Varma S.
Published: July 2002

For the present investigation n-Si/SiO2/In2O3 transparent gate structures were fabricated by electron-beam evaporation of a tin-doped indium oxide layer on thermally oxidized silicon. Admittance measurements were carried out in dark and under three different levels of illumination. A comprehensive technique has been presented for obtaining interface state density distribution from illuminated capacitance data as well as from illuminated conductance data, and hole and electron capture cross sections from the latter. This technique includes a procedure for identification of the dominant quasi-Fermi level (imref) under illumination for determination of the state energy level, three independent methods for evaluating the imref separation from measured data, and a procedure for ascertaining whether electron or hole exchange is the source of a set of Gp/ω vs V or ω characteristics. The interface state density distribution of unannealed structures exhibited two peaked profiles. One peak was located about 0.36–0.41 eV above the valence-band edge, while the other was located about 0.30–0.17 eV below the conduction-band edge. The peak magnitude was found to increase with the illumination intensity and the peak position moved towards the respective band edge. Electron capture cross sections of states in the upper band gap half were found to be in the range of 10−18–10−20 cm2, and hole capture cross sections of states in the lower band gap half in the range of 10−15–10−16 cm2. Both σe and σh were found to vary with band-gap energy and illumination level.
Growth and characterization of flash-evaporated ferroelectric antimony sulphoiodide thin films
Authors: Mansingh A. , Rao T. Sudersena
Published: July 2002

Ferroelectric thin films of antimony sulphoiodide (SbSI) have been grown by flash evaporation in the thickness range of 0.1–1.5 μm. Annealing the films in a sulphur atmosphere at 100 °C for 10–12 min improved their stoichiometry and crystallinity. Structural studies revealed that films deposited normal to the substrate have a tendency to grow with the c axis oriented at 6° to the substrate. The refractive index and band gap estimated from the optical transmission curves for 1.0-μm-thick films were found to be 4 and 1.97 eV, respectively. The measured relative dielectric constant (ε′) for a 1.0-μm thick film at 100 KHz, 300 K was ∼73. Reproducible ferroelectric-to-paraelectric phase transitions were observed for films thicker than 0.2 μm.
Optical properties of semiconducting iron disilicide thin films
Authors: Bost M. C. , Mahan J. E.
Published: July 2002

Iron disilicide thin films were prepared by furnace reaction of ion beam sputtered iron layers with single-crystal silicon wafers and with low-pressure chemical vapor deposition (LPCVD) polycrystalline silicon thin films. X-ray diffraction indicates the films are single-phase, orthorhombic, β-FeSi2. Impurity levels are below the detection limit of Auger spectroscopy. Normal incidence spectral transmittance and reflectance data indicate a minimum, direct energy gap of 0.87 eV. The apparent thermal activation energy of the resistivity in the intrinsic regime is about half of this minimum optical gap. With such a direct band gap, the material may be suitable for the development of both light-sensitive and light-emitting thin-film devices within the silicon microelectronics technology.
Magnetic anisotropy of MnAl and MnAlC permanent magnet materials
Authors: Pareti L. , Bolzoni F. , Leccabue F. , Ermakov A. E.
Published: July 2002

The magnetocrystalline anisotropy of MnAl and MnAlC alloys, in the range of existence of the ferromagnetic τ phase, has been studied by using the singular point detection technique. The anisotropy field value at 293 K varies from 39 kOe for low manganese and high carbon content to 55 kOe for high manganese and carbon-free samples. The anisotropy of the MnAl τ phase is well described by using only the K1 anisotropy constant. The temperature dependence of the calculated anisotropy constant indicates that a single ion mechanism is the origin of the magnetocrystalline anisotropy in these alloys.
Laser-induced morphology changes in thin copper films
Authors: Bloch J. , Zeiri Y.
Published: July 2002

The interaction of Ar+ laser pulses produced by an acousto-optic modulator with copper thin films deposited on glass substrates were studied in the range of 400–5000-Å film thickness and up to 1-W beam power using reflection-transmission measurements. Two consequential processes could be distinguished, attributed to film melting and hole formation. During melting, the reflected beam changes to form scattered ring patterns. Hole formation is indicated by transmission increase. From the behavior of the reflection-transmission intensities during irradiation, the mechanism of the processes is deduced. The dependence of the processes’ kinetics on beam power and film thickness is determined, and an empirical model calculation interpreting the results is presented.
NMR studies in orthorhombic Fe3B1−xCx (0.1≤x≤0.4)
Authors: Zhang Y. D. , Budnick J. I. , Sanchez F. H. , Hines W. A. , Yang D. P. , Livingston J. D.
Published: July 2002

In order to study the local atomic environments of both B and Fe in Fe3B1−xCx (0.1≤x≤0.4), NMR measurements of the hyperfine field distributions for 10B, 11B, and 57Fe have been performed at 4.2 K and from 10 to 60 MHz. It was found that the 11B and 57Fe NMR frequencies do not change with C concentration. From these results, the 11B NMR frequency characteristic of orthorhombic Fe3B was unambiguously determined to be 36.3 MHz, which is in good agreement with the value obtained for the orthorhombic Fe3B phase prepared by annealing amorphous Fe80B20. Our results show that the substitution of C atoms for B atoms has a small effect on the hyperfine field at both the B and Fe nuclei as well as on the magnetic moment of the Fe atoms.
Experimental study of a ZnO modulator using guided wave resonance
Authors: Persegol D. , Pic E. , Plantier J.
Published: July 2002

A novel type of electro-optic (EO) modulator using guided wave resonance in a planar zinc-oxide waveguide is presented. The device is studied as a bulk EO modulator. We report experimental results concerning the determination of the coefficients of the EO tensor of the ZnO layer.
Dislocation density reduction through annihilation in lattice-mismatched semiconductors grown by molecular-beam epitaxy
Authors: Sheldon P. , Jones K. M. , Al-Jassim M. M. , Yacobi B. G.
Published: July 2002

Epitaxial InAs/GaAs, GaAs/Ge/Si, GaAs/InP, and InAs/InP heterostructures are grown by molecular-beam epitaxy. Transmission electron microscopy studies reveal that, for these heteroepitaxial systems, the threading dislocation density is inversely proportional to the epilayer thickness. At a given thickness, the threading dislocation density is relatively insensitive to lattice mismatch (3.2%<‖Δa‖/a<7.2%), to differences in thermal expansion coefficients (6.9×10−7<‖Δα‖<3.4×10−6 K−1), to interfacial surface chemistry, and to epilayer morphology. Epitaxial layers incorporating growth interrupts produce lower overall defect densities, yet they maintain defect-reduction profiles similar to those observed in layers without the growth interrupt.
Resonant tunneling in heterostructures: Numerical simulation and qualitative analysis of the current density
Authors: Collins S. , Lowe David , Barker J. R.
Published: July 2002

In this paper it is demonstrated that a modified coventional current density expression can be employed to model tunnel currents and to numerically fit observed characteristics. A derivation of the current density expression is presented along with numerical simulations. A detailed account is given of the numerical techniques used and an explicit comparison with previously published experimental characteristics presented. We point out that part of the experimental situation is not accounted for within the model. This gives rise to the only fitting parameter used, which physically represents a resistance in series with the resonant structure. The series resistance has important effects upon the observed characteristics, explaining the different types of published characteristics and the disappointing peak-to-valley ratios. The effect upon the peak-to-valley ratio of the series resistance means that extensive numerical calculations would be premature; therefore, a qualitative analysis is undertaken to identify the desired properties of a resonant tunneling potential in order to assist in the design of future resonant tunneling systems. Finally, the hot electron spectrometer is considered. The work indicates that this spectrometer will only yield meaningful results if extreme care is taken in its operation and interpreting the results obtained.
A method for preparing superconducting single crystals of Ba2GdCu3O7−y
Authors: Wan Rong-xing , Zhao Guo-meng , Tang Xiao-ming , Li Wen-zhou , Hu Shi-zhao , Yao Hon-nian
Published: July 2002

A method has been developed for growing single crystals of Ba2GdCu3O7−y of much larger size (∼2×3×6 mm3). The crystals as grown were semiconductors and became superconductors with a zero resistance temperature of 92 K after they were annealed in flowing oxygen. Good x-ray diffraction and Laue patterns were obtained from a superconducting crystal (∼2×3×4 mm3) that was cleaved in (001) plane, and lattice constants were determined to be a=3.8401 Å, b=3.9007 Å, and c=11.7104 Å.
A modified Norde function for the measurement of the series resistance and the voltage-dependent barrier height of triangular barrier diodes
Authors: Chilana G. S. , Gupta R. S.
Published: July 2002

The Norde function [J. Appl. Phys. 50, 5052 (1979)] used for measuring the barrier height, series resistance, and ideality factor of a Schottky barrier diode has been modified to obtain the same parameters for a triangular barrier diode (TBD). Unlike Schottky barrier diodes, the barrier height of a TBD depends upon the applied voltage and changes linearly with it. In order to calculate the additive term in the TBD barrier height, the modified function is therefore plotted for both reverse and forward bias cases. It is shown later that the modified Norde function enhances the accuracy of the results.
Tube diameter and wave frequency limitations when using the electro magnetic surface wave in the m=1 (dipolar) mode to sustain a plasma column
Authors: Margot-Chaker J. , Moisan M. , Chaker M. , Glaude V. M. M. , Lauque P. , Paraszczak J. , Sauvé G.
Published: July 2002

An exhaustive experimental investigation of the conditions required to sustain a plasma column through the propagation of the m=1 mode surface wave has been conducted. It reveals that, given a discharge tube radius a, there corresponds a minimum frequency value fm below which the discharge cannot be achieved; conversely, for a given operating frequency f, the tube radius must exceed some minimum value am for the plasma to be sustained. These minimum conditions required to obtain the discharge are observed to obey a scaling law of the form (fa)m≂const., where the constant is independent of the gas nature and pressure. Theoretically, the dispersion equation of the m=1 mode wave shows no low-frequency cutoff. However, it is found that the specific dependence of the wave attenuation coefficient on the frequency and on the tube diameter can ultimately account for the observed limitations when the wave is used to sustain a plasma. A discharge stability criterion is proposed that recovers the observed scaling law determining the minimum tube radius and wave frequency values.
Excitation of ultrasonic Lamb waves in piezoelectric plates
Authors: Joshi S. G. , Jin Y.
Published: July 2002

The excitation of ultrasonic Lamb waves by an interdigital transducer (IDT) deposited on a piezoelectric plate is analyzed using the Green’s-function method. The amplitudes of the generated Lamb waves are obtained in terms of the charge density on transducer electrodes. An electrostatic analysis that neglects piezoelectric coupling is used to relate this charge density to the voltage applied across the IDT. This is then used to calculate the radiation conductance of the transducer. Experimentally measured characteristics of interdigital transducers deposited on 128° Y-X and Y-Z lithium niobate plates are found to be in fair agreement with theoretical calculations. The analysis can also be used to calculate the electromechanical coupling between the IDT and various Lamb-wave modes. It is found that when the plate is more than a few acoustic wavelengths thick, the lowest-order symmetric and antisymmetric Lamb modes are almost equally generated. The slight difference in the velocities of these modes gives rise to the beating effect, whereby energy launched by the IDT on one surface transfers periodically back and forth between the two surfaces as a sinusoidal function of the distance traveled. A 65-MHz Lamb-wave delay line fabricated on a 0.25-mm-thick Y-Z lithium niobate plate shows the beating effect, with a beat wavelength of 14.7 mm, in good agreement with theoretical calculations.
New scaling relation for ion extraction by external electric field on a barium plasma produced between parallel-plate electrodes
Authors: Yamada Kimio , Tetsuka Tsutomu , Deguchi Yoshihiro
Published: July 2002

Parameter dependencies of ion extraction time by an external electric field have been investigated by using barium plasmas suddenly produced between parallel-plate electrodes under a variety of experimental conditions. It is concluded that the ion extraction time can be expressed as the product of exponents of the three controllable parameters, applied voltage, plasma width, and ion density. This scaling relation can estimate the ion extraction time with an accuracy of 15% for a given controllable parameter set, supporting its universality.