bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of reactant and product melting on self-propagating reactions in multilayer foils
Authors: Besnoin Etienne , Cerutti Stefano , Knio Omar M. , Weihs Timothy P.
Published: October 2002

The evolution of self-propagating reactions along nanostructured multilayer foils is analyzed computationally. A simplified physical model is used that combines a two-dimensional diffusion equation for the atomic concentration with a quasi-one-dimensional form of the energy equation which accounts for the melting of the reactants and products. The model thus generalizes previous formulations which have ignored melting effects. The computations are used to predict the evolution of self-propagating fronts in Ni/Al foils, and analyze the dependence of these fronts on the foil parameters. In particular, the results indicate that melting substantially affects the properties of the unsteady reactions, and generally results in an appreciable reduction of the average front speed.
Hall coefficients and resistivities of Bi2+xSr2−xCuOy single crystals: The presence of a mobility edge in correlated two-dimensional electron system
Authors: Inagaki Katsuhiko , Tanda Satoshi
Published: October 2002

Hall coefficients and resistivities of a series of Bi2+xSr2−xCuOy single crystals were measured to investigate the relation between the carrier concentration n and the resistivity-minimum temperature Tmin. The Tmin was found to obey a simple linear relation: Tmin ∝ nc−n, where nc≃4.2×1021 cm−3 is a critical carrier concentration. A model of two-dimensional electron systems with a mobility edge was introduced to explain this linear relation, and the presence of a mobility edge in two-dimensional copper-oxide-based metals was suggested.
Low-energy electron transmission through organic monolayers: An estimation of the effective monolayer potential by an excess electron interference
Authors: Yamane Hiroyuki , Ito Kazuyuki , Kera Satoshi , Okudaira Koji K. , Ueno Nobuo
Published: October 2002

In low-energy-electron transmission spectra of monolayer films of various organic-semiconductor molecules deposited on MoS2 and graphite surfaces, we found that the energy positions of spectral minima are proportional to (n+1/2)2, where n is positive integer and 0, independent of molecules and substrates. Despite the complex structure of each molecule, the (n+1/2)2 rule can be simply explained by the interference of an excess electron passing through the potential of the monolayer on the substrate. Using these results, we estimated the effective potential, the potential width and depth, of the monolayer felt by the injected excess electron.
Texture of TiSi2 thin films on Si (001)
Authors: Özcan A. S. , Ludwig K. F. , Rebbi P. , Lavoie C. , Cabral C. , Harper J. M. E.
Published: October 2002

The texture of blanket C49 and C54 phase TiSi2 films on Si (001) substrates was examined with x-ray pole figure analysis. Textures were studied both in films with initial Ti deposited by sputtering and by low-temperature, plasma assisted chemical vapor deposition. Detailed differences were observed between silicide films grown by subsequently annealing films deposited with the two different methods. However, in both cases, the dominant C49 phase orientational relationships develop with respect to the substrate (001) planes. In contrast, we find that the dominant crystallographic relationships of the C54 phases in both cases are not with the surface (001) plane of the substrate, but instead with the Si (111) planes. Texturing in the case of the sputtered films is particularly complex with one prominent orientation having C54 (11¯0)‖(1¯11) Si with C54 (3̄3̄1̄)‖(011̄) Si and a second having C54 (1¯03¯)‖(1̄1̄1) Si with C54 (040)‖(11¯0) Si. These orientations, which suggest local epitaxy of the disilicide with the Si {111} planes, have not been previously identified for C54 TiSi2 growth on Si (001) substrates. This strong orientational relationship suggests that significant {111} faceting of the Si surface may occur during the silicide formation.
Continuum spin foam model for 3d gravity
Authors: Zapata José A.
Published: October 2002

An example illustrating a continuum spin foam framework is presented. This covariant framework induces the kinematics of canonical loop quantization, and its dynamics is generated by a renormalized sum over colored polyhedra. Physically the example corresponds to 3d gravity with cosmological constant. Starting from a kinematical structure that accommodates local degrees of freedom and does not involve the choice of any background structure (e.g., triangulation), the dynamics reduces the field theory to have only global degrees of freedom. The result is projectively equivalent to the Turaev–Viro model.
Beneficial effect of nonmagnetic Y on magnetic properties due to the enhancement of exchange coupling in nanocomposite (Nd,Y)2Fe14B/α–Fe magnets
Authors: Zhang M. , Zhang Z. D. , Sun X. K. , Liu W. , Geng D. Y. , Jin X. M. , You C. Y. , Zhao X. G.
Published: October 2002

We have studied the nanocomposite magnets with nominal compositions of Nd8−xYxFe88B4 (x=0–4.0) by mechanical milling techniques and subsequent heat treatment. We find that there exists the beneficial effect of nonmagnetic Y substitution for magnetic Nd on magnetic properties in nanocomposite magnets. The extension of the exchange coupling length, due to a decrease of the anisotropy of the hard-magnetic phase, results in that more soft-magnetic grains participate well in the exchange coupling, improving the magnetic properties of the nanocomposite magnets.
Temperature dependence of free excitons in GaN
Authors: Reynolds D. C. , Hoelscher J. , Litton C. W. , Collins T. C.
Published: October 2002

The excitons involved in this study are the longitudinal and the Γ5 and Γ6 free excitons, as well as the donor bound exciton (D0,X). The temperature dependence of the energy positions of the Γ5, Γ6, and (D0,X) excitons are well accounted for by the Varshni equation (Y. P. Varshni, Physica (Amsterdam) 34, 149 (1967)). In the same temperature range, the energy positions of the longitudinal excitons depart from the predictions of the Varshni equation. The separation between the longitudinal- and transverse-mode free excitons has been previously reported. One component of this separation is the polarizability, which has a temperature dependence. The longitudinal exciton therefore has a band-gap temperature dependence, predicted by the Varshni equation, as well as an additional dependence due to polarizability. This temperature dependence has been accounted for by the Varshni equation, plus an additional linear and a quadratic temperature dependent term.
Output features of vacuum laser acceleration
Authors: Cao N. , Ho Y. K. , Wang P. X. , Pang J. , Kong Q. , Shao L. , Wang Q. S.
Published: October 2002

Electrons acceleration by the vacuum laser acceleration scheme CAS [see, e.g., P. X. Wang et al., Appl. Phys. Lett. 78, 2253 (2001)] (capture and acceleration scenario) is simulated. The general features of the outgoing electrons are examined at different laser intensities. Explanations based on the mechanism behind the CAS scheme of those output characteristics are presented. The results show it is hopeful that CAS becomes a useful scheme for laser accelerators.
Theoretical study of the Ti+(NO,O2)→TiO+(N,O) reactions
Authors: Kim Kyoung Hoon , Lee Yoon Sup , Moon Jung-Ho , Kim Yoonho , Jeung Gwang-Hi
Published: October 2002

Multiconfiguration self-consistent-field and configuration interaction calculations of the potential energy surfaces were performed for the mildly exoergic reaction, Ti+NO→TiO+N and the largely exoergic reaction, Ti+O2→TiO+O. The end-on approach appears to be most efficient for the Ti+NO reaction, while the side-on one is favored for the Ti+O2 reaction. Both reactions have activation barriers in the initial collision phase. Two stable forms of the intermediates were found for the first reaction in agreement with a recent density functional study, which can be represented as NTiO with an obtuse NTiO angle and Ti[NO] with an acute NTiO angle. The former isomer is calculated to be more stable than the latter. Similarly, two stable isomers were found for the second reaction, namely OTiO and Ti[O2], the former being more stable than the latter. We describe here the general shape of the potential energy surfaces involving these intermediates and their implication on reaction mechanism. The electron transfer from the metal atom to the ligand occurs at short intermolecular distances.
Interferometric measurements of plasma density in microcapillaries and laser sparks
Authors: Ping Y. , Geltner I. , Morozov A. , Suckewer S.
Published: October 2002

The spatial and temporal evolution of electron density in laser- and discharge-created plasmas in microcapillaries and laser sparks in gases, has been measured by interferometry. The results are compared and the features of these three types of plasmas are summarized from the point of view of x-ray lasers and Raman amplification of ultrashort pulses.
On the study of instability of ion-acoustic waves in relativistic plasma: Unbounded and bounded
Authors: Bhattacharya S. K. , Paul S. N. , Mondal K. K.
Published: October 2002

The instability of ion-acoustic waves has been theoretically studied in a multicomponent plasma consisting of positive ions, negative ions and two-temperature isothermal electrons, and also considering the streaming of ions through the derivation of dispersion relation in an unbounded and bounded system involving a comparative study between the results obtained in the two cases. Results for the phase velocity and the instability have been graphically illustrated for the plasmas (O−,Ar+) and (Cl−,H+).
Photoresponse of the conductivity in functionalized pentacene compounds
Authors: Tokumoto T. , Brooks J. S. , Clinite R. , Wei X. , Anthony J. E. , Eaton D. L. , Parkin S. R.
Published: October 2002

We report the investigation of the photoresponse of the conductivity of a recently synthesized class of organic semiconductors based on functionalized pentacene. These materials form high quality single crystals that exhibit a thermally activated resistivity. Unlike pure pentacene, the functionalized derivatives are readily soluble in acetone, and can be evaporated or spincast as thin films for potential device applications. The electrical conductivity of the single crystal materials is noticeably sensitive to ambient light changes. The purpose, therefore, of the present study, is to determine the nature of the photoresponse in terms of carrier activation versus heating effects, and also to measure the dependence of the photoresponse on photon energy. We describe a method, involving the temperature dependent photoresponse, which allows an unambiguous identification of the signature of heating effects in materials with a thermally activated conductivity. We find strong evidence that the photoresponse in the materials investigated is predominantly a highly localized heating mechanism. Wavelength dependent studies of the photoresponse reveal resonant features and cutoffs that indicate the photon energy absorption is related to the electronic structure of the material.
Size determination of field-induced water menisci in noncontact atomic force microscopy
Authors: Calleja Montserrat , Tello Marta , Garcı́a Ricardo
Published: October 2002

We have studied the dimensions of water capillaries formed by an applied electrical field between an atomic force microscope tip and a flat silicon surface. The lateral and vertical dimensions of the liquid meniscus are in the 5–30 nm range. The size depends on the duration and strength of the voltage pulse. It increases by increasing the voltage strength or the pulse duration. The meniscus size is deduced from the experimental measurement of the snap-off separation. These results are of special relevance to optimize local oxidation nanolithography.
On bound states for systems of weakly coupled Schrödinger equations in one space dimension
Authors: Melgaard Michael
Published: October 2002

We establish the Birman–Schwinger relation for a class of Schrödinger operators −d2/dx2⊗1H+V on L2(R,H), where ℋ is an auxiliary Hilbert space and V is an operator-valued potential. As an application we give an asymptotic formula for the bound states which may arise for a weakly coupled Schrödinger operator with a matrix potential (having one or more thresholds). In addition, for a two-channel system with eigenvalues embedded in the continuous spectrum we show that, under a small perturbation, such eigenvalues turn into resonances.
Zonal flow generation in ion temperature gradient mode turbulence
Authors: Anderson J. , Nordman H. , Singh R. , Weiland J.
Published: October 2002

In the present work the zonal flow (ZF) growth rate in toroidal ion-temperature-gradient (ITG) mode turbulence including the effects of elongation is studied analytically. The scaling of the ZF growth with plasma parameters is examined for typical tokamak parameter values. The physical model used for the toroidal ITG driven mode is based on the ion continuity and ion temperature equations whereas the ZF evolution is described by the vorticity equation. The results indicate that a large ZF growth is found close to marginal stability and for peaked density profiles and these effects may be enhanced by elongation.
Semiquantitative analysis of feedback systems for resistive wall modes
Authors: Garofalo A. M. , Jensen T. H. , Strait E. J.
Published: October 2002

Resistive wall modes (RWMs) are long wavelength plasma instabilities that would be stabilized if the surrounding walls were perfectly conducting. In DIII–D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159], RWMs limit lifetime and β achieved in advanced tokamak regimes, thus the feasibility of RWM stabilization through application of external magnetic fields from a feedback system is being investigated. To understand the behavior of different feedback algorithms and as a guide in the optimization of parameter settings, a “simple” feedback model is desirable. Here, a formulation of the Intelligent Shell problem which includes a realistic modeling of the electronics system involved is described. Analysis using this formulation is reduced to finding roots of a polynomial. Experimental parameter scans with present DIII–D RWM feedback system show quantitative agreement with the model.
La-doped effect on the ferroelectric properties of Bi4Ti3O12–SrBi4Ti4O15 thin film fabricated by pulsed laser deposition
Authors: Zhu J. S. , Su D. , Lu X. M. , Qin H. X. , Wang Y. N. , Wang D. Y. , Chan H. L. W. , Wong K. H. , Choy C. L.
Published: October 2002

Bi 4 Ti 3 O 12 – SrBi 4 Ti 4 O 15 (BT–SBTi) thin film was fabricated successfully on Pt/TiO2/SiO2/Si(110) substrates by the pulsed laser deposition technique. Films annealed at 650 °C by the rapid temperature process (RTP) have Pr=12 μC/cm2. But, the fatigue behavior has been observed although no obvious decrease in Pr up to 105 s retained time in the BT–SBTi capacitor. After being La doped, the Bi3.25La0.75Ti3O12–SrBi4Ti4O15 (BLT–SBTi) has fatigue free properties. Pr=13.5 μC/cm2 was measured in a BLT–SBTi film of 300 nm thickness. It did not show any significant fatigue up to 1010 switching cycles above the applied field of 250 kV/cm. It also has good retention properties. The field dependence of fatigue behavior and La-doped effect are discussed.
Development of preferred orientation in polycrystalline NaCl-structure δ-TaN layers grown by reactive magnetron sputtering: Role of low-energy ion surface interactions
Authors: Shin C.-S. , Gall D. , Kim Y.-W. , Hellgren N. , Petrov I. , Greene J. E.
Published: October 2002

We have investigated the effects of low-energy ion irradiation on texture evolution during growth of B1–NaCl-structure polycrystalline δ-TaN layers on SiO2 by ultra-high-vacuum reactive magnetron sputter deposition at 350 °C in mixed Ar+15%N2 discharges. In parallel sets of experiments, the ion-to-metal flux ratio Ji/JTa was varied from 1.3 to 10.7 while maintaining the incident ion energy Ei constant at 20 eV, and Ei was varied from 8.5 to 30 eV with Ji/JTa=10.7. All TaN layers, irrespective of Ji/JTa or Ei, were overstoichiometric with N/Ta=1.13±0.03. Layers grown with Ei=30 eV are multiphase consisting of hexagonal ε-TaN and δ-TaN, while those grown with Ei⩽20 eV are single-phase δ-TaN. With Ei=8.5 eV, the δ-TaN layers have a mixed 111/002/022/113 texture, even for film thicknesses t up to 500 nm. In contrast, δ-TaN layers deposited with Ei=20 eV initially exhibit competitive texture evolution until a single texture dominates at t≳200 nm. The preferred orientation of 500-nm-thick Ei=20 eV layers can be selectively and continuously varied from predominantly 111 to nearly complete 002 by increasing Ji/JTa from 1.3 to ⩾ 7.4. The change in texture is primarily due to an increased steady-state atomic N coverage, resulting from collisionally induced dissociative chemisorption of incident energetic N2+ ions, with increasing Ji/JTa. 111-oriented δ-TaN layers are underdense with both inter- and intracolumnar porosity and a tensile stress of ≃ 1.4 GPa, while 002-oriented layers are fully dense and have small compressive stresses, ≲ 0.7 GPa.
Single domain and vortex state in ferromagnetic circular nanodots
Authors: Hoffmann H. , Steinbauer F.
Published: October 2002

In this article, we present an analytical approximation for calculating the magnetic energy in circular nanostructures for the vortex phase (closed magnetic flux) and the single domain phase. Comparing these energies in remanent state lead to a magnetic phase boundary between single domain phase and vortex phase, given by a critical thickness, which depends on the radius of the circular nanostructure. Experimental data and micromagnetic simulations have verified the analytical results.
Far-infrared laser-induced hot-carrier impact ionization photovoltaic effect in silicon junctions: Lattice temperature dependence
Authors: Encinas-Sanz F. , Marı́n M. , Guerra J. M.
Published: October 2002

An experimental study has been carried out on the dependence of the hot-carrier highly nonlinear photovoltaic effect induced in silicon n+/p junctions by 10.6 μm laser pulses on the lattice temperature. A four orders of magnitude increase in the photovoltage was observed for a lattice temperature increase of 150 K. A model based on the hot-carrier plasma generated by the laser radiation fit the experimental results. Sensitivity can possibly be further improved by increasing the junction temperature.
A model of phase transition induced antiphase boundaries in perovskite and layered perovskite oxides
Authors: Ding Y. , Liang D. D.
Published: October 2002

A structural model of the antiphase boundaries (APBs) induced by phase transition in perovskite and layered perovskite oxides is put forward. Besides the distortion of oxygen octahedra, the existence of two types of ion with different sizes on A sites in perovskite blocks and their ordering in local areas might be the reasons for the occurrence of APBs. It could explain why APBs exist in SrBi2Ta2O9, Bi3.25La0.75Ti3O12, La2−xBaxCuO4, and La2/3Ca1/3MnO3, but not in Bi4Ti3O12 and Nd2NiO4 through their specific structure phase transitions. The formation of APBs might reflect the chemical inhomogeneity in local areas. Empirical results support this model.
Photoionization spectroscopy in AlGaN/GaN high electron mobility transistors
Authors: Klein P. B.
Published: October 2002

A model is developed to describe the light-induced restoration of the drain current from current collapse in AlGaN/GaN high electron mobility transistors. The model assumes that the collapse results from a transfer at large drain bias of hot carriers from the gate–drain region of the two-dimensional electron gas to deep traps in the high-resistivity GaN layer. Application of the model provides a means of determining the photoionization cross sections and the areal densities of the responsible traps. Where multiple trapping species are involved, it is shown that photoinduced transitions between trapping sites can significantly alter the response of the drain current to light illumination, and must therefore be taken into account.
Magnetization reversal in half-metallic epitaxial CrO2 films
Authors: Siu I. L. , Egelhoff W. F. , Yang David X. , Chopra Harsh Deep
Published: October 2002

We have investigated the magnetization reversal in thin films of the half-metallic ferromagnet chromium dioxide, CrO2, grown on single crystal TiO2 (100) and (110) substrates. X-ray diffraction analysis indicated epitaxial growth of CrO2 in the rutile crystal structure with the c axis (also the magnetization easy-axis), lying in the plane of the film. Direct, real-time observation of the magnetization reversal revealed that reversal occurs via nucleation of domain walls, and their subsequent motion perpendicular to themselves. The reversed domains are formed when the applied field is close to the coercive field Hc of the CrO2 film, and the reversal is complete within a small field range, indicating a small dispersion in the film switching field.
Tunneling magnetoresistance in granular composites
Authors: Ju Sheng , Li Zhen-Ya
Published: October 2002

The temperature dependence of magnetoresistance in insulating granular systems is studied. At low temperatures the magnetoresistance is enhanced drastically as a result of high-order tunneling. On the other hand, decay of spin polarization will lead to a decrease of magnetoresistance at high temperatures. The role of grain size distribution is investigated and the intergrain correlation is found very crucial in determining the magnitude of magnetoresistance. Our calculations agree with the experimental data well.
Texture formation in sputter-deposited (Nb0.7,Ti0.3)N thin films
Authors: Iosad N. N. , van der Pers N. M. , Grachev S. , Roddatis V. V. , Jackson B. D. , Polyakov S. N. , Dmitriev P. N. , Klapwijk T. M.
Published: October 2002

We studied the properties of (Nb0.7,Ti0.3)N films deposited by reactive magnetron sputtering in an atmosphere of argon and nitrogen at ambient substrate temperature, with a particular focus on the technological factors that determine film texture. The texture in the nitrides of transition metals determines many processes, including the wear resistance of tool coatings, diffusion in microelectronic devices, and the rate of chemical etching. Thus, since our goal is to use (Nb0.7,Ti0.3)N films in superconducting microelectronic devices, texture control is an essential element of our technology. We find that increasing the total gas pressure, while keeping the film chemical composition constant, results in a decrease in the ratio of the [200] and [111] x-ray diffraction (XRD) line intensities on Θ–2Θ Bragg–Brentano scans. Similar changes in XRD patterns are observed as the nitrogen injection increases for a constant sputtering pressure. In addition, XRD examination shows that some samples have in-plane texture developed due to self-shadowing during growth. Transmission electron microscopy reveals that all of the films consist of textured, elongated grains. Analyzing the experimental data, it is concluded that the thermalization of the sputtering yield determines the process of texture formation in the experiment with pressure variation, with an increase in adatom energy resulting in a change in texture from [111] to [100]. However, adatom energy is not the only determining factor—the nitrogen concentration in the sputtering gas also has a strong impact on the film texture. In particular, despite the fact that an increase in nitrogen injection results in an increase in adatom energy, the film texture is driven toward [111].
Spinodal decomposition in amorphous metal–silicate thin films: Phase diagram analysis and interface effects on kinetics
Authors: Kim H. , McIntyre P. C.
Published: October 2002

Among several metal silicate candidates for high permittivity gate dielectric applications, the mixing thermodynamics of the ZrO2–SiO2 system were analyzed, based on previously published experimental phase diagrams. The driving force for spinodal decomposition was investigated in an amorphous silicate that was treated as a supercooled liquid solution. A subregular model was used for the excess free energy of mixing of the liquid, and measured invariant points were adopted for the calculations. The resulting simulated ZrO2–SiO2 phase diagram matched the experimental results reasonably well and indicated that a driving force exists for amorphous Zr–silicate compositions between ∼40 mol % and ∼90 mol % SiO2 to decompose into a ZrO2-rich phase (∼20 mol % SiO2) and SiO2-rich phase (>98 mol % SiO2) through diffusional phase separation at a temperature of 900 °C. These predictions are consistent with recent experimental reports of phase separation in amorphous Zr–silicate thin films. Other metal–silicate systems were also investigated and composition ranges for phase separation in amorphous Hf, La, and Y silicates were identified from the published bulk phase diagrams. The kinetics of one-dimensional spinodal decomposition normal to the plane of the film were simulated for an initially homogeneous Zr–silicate dielectric layer. We examined the effects that local stresses and the capillary driving force for component segregation to the interface have on the rate of spinodal decomposition in amorphous metal–silicate thin films.
Analysis of an eddy-current brake considering finite radius and induced magnetic flux
Authors: Lee Kapjin , Park Kyihwan
Published: October 2002

Since the eddy-current problem usually depends on the geometry of the moving conductive sheet and the pole shape, there is no general method to find an analytical solution. The analysis of the eddy currents in a rotating disk with an electromagnet is performed in the case of time-invariant field with the consideration of the boundary conditions of the rotating disk and induced magnetic flux. First, the concept of Coulomb’s law and the method of images are introduced with the consideration of the boundary conditions. Second, the induced magnetic flux density is calculated by using Ampere’s law. Third, the net magnetic flux density is introduced by defining the magnetic Reynolds number Rm as the ratio of the induced magnetic flux density to the applied magnetic flux density. Finally, the braking torque is calculated by applying the Lorentz force law and the computed results are compared with experimental ones.
Single asperity tribochemical wear of silicon nitride studied by atomic force microscopy
Authors: Maw W. , Stevens F. , Langford S. C. , Dickinson J. T.
Published: October 2002

Nanometer scale single asperity tribochemical wear of silicon nitride was examined by measuring the wear of atomic force microscope tips translated against a variety of substrates in aqueous solutions. We show that the chemical nature of the substrate plays an important role: significant wear was observed only when the substrate surface is populated with appropriate metal-hydroxide bonds. Mica and calcite substrates, whose water-exposed cleavage surfaces lack these bonds, produced little if any tip wear. As a function of contact force FN and scan duration t, the length of the tips in this work decreases approximately as (FNt)0.5. We propose that pressure-induced intermediate states involving hydroxyl groups form on both the tip and the substrate; chemical reactions subsequently form transient bridging chemical bonds that are responsible for tip wear.
Optical and electrical properties of GaMnN films grown by molecular-beam epitaxy
Authors: Polyakov A. Y. , Govorkov A. V. , Smirnov N. B. , Pashkova N. Y. , Thaler G. T. , Overberg M. E. , Frazier R. , Abernathy C. R. , Pearton S. J. , Kim Jihyun , Ren F.
Published: October 2002

Optical absorption spectra, microcathodoluminescence (MCL) spectra, and electrical properties of GaMnN films grown by molecular-beam epitaxy with Mn concentration in the range of 3 to 10 at. % were studied. Optical absorption and MCL spectra show the presence of strong bands corresponding to the transition from the Mn acceptors near Ec−2 eV to the conduction band. The other strong band observed in MCL measurements was the blue band peaked near 2.9 eV and associated with the transition from the valence band to deep donors with a level near Ec−0.5 eV. All GaMnN samples were shown to be lightly n-type which suggests close self-compensation of the Mn acceptors by some native defect donors. A plausible scenario is that such compensating donors could be due to nitrogen vacancies and that the Ec−0.5 eV donor defects are complexes between the Mn acceptors and the nitrogen vacancy donors.
Diagnostics and two-dimensional simulation of low-frequency inductively coupled plasmas with neutral gas heating and electron heat fluxes
Authors: Ostrikov K. N. , Denysenko I. B. , Tsakadze E. L. , Xu S. , Storer R. G.
Published: October 2002

This article presents the results on the diagnostics and numerical modeling of low-frequency (∼460 KHz) inductively coupled plasmas generated in a cylindrical metal chamber by an external flat spiral coil. Experimental data on the electron number densities and temperatures, electron energy distribution functions, and optical emission intensities of the abundant plasma species in low/intermediate pressure argon discharges are included. The spatial profiles of the plasma density, electron temperature, and excited argon species are computed, for different rf powers and working gas pressures, using the two-dimensional fluid approach. The model allows one to achieve a reasonable agreement between the computed and experimental data. The effect of the neutral gas temperature on the plasma parameters is also investigated. It is shown that neutral gas heating (at rf powers⩾0.55 kW) is one of the key factors that control the electron number density and temperature. The dependence of the average rf power loss, per electron–ion pair created, on the working gas pressure shows that the electron heat flux to the walls appears to be a critical factor in the total power loss in the discharge.
Study of a low β classical tearing mode in DIII-D
Authors: Chu M. S. , La Haye R. J. , Austin M. E. , Lao L. L. , Lazarus E. A. , Pletzer A. , Ren C. , Strait E. J. , Taylor T. S. , Waelbroeck F. L.
Published: October 2002

The tearing mode stability of low β plasmas is studied experimentally in the DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] tokamak. The linear and nonlinear characteristics of the plasma are measured and compared with theoretical predictions. In contrast to the neoclassical tearing mode, which occurs at high β and is dominated by the bootstrap current effect, in the low β regime the neoclassical bootstrap current effect is minimal. Thus the stability properties are more amenable to comparison with presently available theoretical codes. In the linear phase, the onset of the instability has been found to be predictable from equilibrium reconstruction to agree with the availability of the tearing mode free energy. In the nonlinear phase, the temperature and its fluctuation amplitude and phase have been fitted to the prediction of achieving a perturbed three-dimensional equilibrium with an assumed perturbation eigenfunction.
Equilibrium and low-frequency stability of a uniform density, collisionless, spherical Vlasov system
Authors: Barnes D. C. , Chacón L. , Finn J. M.
Published: October 2002

Equilibrium and stability of a collisionless, spherical Vlasov system with uniform density are considered. Such an electron system is useful for the Periodically Oscillating Plasma Sphere (POPS) fusion system. In POPS the space charge of a uniform-density spherical electron cloud provides a harmonic well for an under-dense thermal ion population. Previous special solutions [D. C. Barnes, Phys. Plasmas 6, 4472 (1999)] are extended to arbitrary energy dependence. These equilibrium distribution functions and their first derivatives may be made nonsingular, in contrast to the previous solutions. Linear stability of general spherical equilibria is considered, and reduced to a one-dimensional calculation by the introduction of a spherical harmonic decomposition. All azimuthal mode numbers are degenerate. Using this formalism, the low-frequency stability of a collisionless, spherical Vlasov electron system coupled to a minority ion cloud is studied for the class of uniform-density electron equilibria found. In the low-frequency (adiabatic) limit, the general kinetic stability formalism can be integrated to find a closed form for the response of electron number density. The adiabatic response operator is shown to be self-adjoint. Computation of its eigenvalues proves the constant-density electrons/thermal ions system in POPS to be mostly stable to ion-electron electrostatic modes. Unstable modes are avoided unless central electrons have an extremely small energy spread. These results may also be useful for the consideration of gravitational and beam systems.
Ultrafast nonlinear response of high density carriers at silicon surface detected by simultaneous measurements of transient reflecting first and second order diffractions
Authors: Katayama K. , Inagaki Y. , Sawada T.
Published: October 2002

The simultaneous detections of transient reflectivity (TR), transient reflecting first and second order diffraction signals, at a silicon surface revealed that each signal reflected different physical processes of carrier dynamics under a high pump power of 5 mJ/cm2. It was shown that the second order diffraction could detect a refractive index change which was not linearly dependent on the excited carrier density, and it was suggested that the nonlinearity was caused by many-body interactions among carriers at the band-edge states. The dynamics observed with the second order diffraction corresponded to the recombination of the band-edge carriers. Analysis of the first and second order diffractions in combination with the recently developed spectroscopic detection provided selective information on the ultrafast carrier and heat dynamics for a silicon surface, that is, carrier-phonon scattering, recombination of carriers, heat generation, and diffusion. Additionally, it was shown the TR might allow observation of mixed physical processes detected by the first and second order diffractions and it was suggested that deducing exact physical processes only from the TR signal, especially under high pump power conditions, was difficult.
Reflection high-energy electron diffraction analysis of polycrystalline films with grain size and orientation distributions
Authors: Hartman J. W. , Brewer R. T. , Atwater Harry A.
Published: October 2002

We report a computationally efficient algorithm to calculate reflection high-energy electron diffraction (RHEED) intensities from well-textured, small-grained polycrystalline films in the kinematic limit. We also show how the intensity maps of the spots in a RHEED pattern from such a film can be quantitatively analyzed to determine the film’s average grain size, as well as its in-plane orientation and texture distributions. We find that the in-plane orientation and texture distribution widths of these films can be determined to within 1 degree and that the average lateral grain size can be measured to within a fraction of a nanometer after suitable calibration of our technique.
Validity of mobility universality for scaled metal–oxide–semiconductor field-effect transistors down to 100 nm gate length
Authors: Matsumoto S. , Hisamitsu K. , Tanaka M. , Ueno H. , Miura-Mattausch M. , Mattausch H. J. , Kumashiro S. , Yamaguchi T. , Odanaka S. , Nakayama N.
Published: October 2002

Mobility universality, confirmed for long-channel metal–oxide–semiconductor field-effect transistors (MOSFETs), is demonstrated to be preserved for scaled MOSFET technologies down to 100 nm gate length, although phenomena such as quantum-mechanical and poly-silicon depletion effects play important roles. This result was obtained by applying a compact model based on the drift-diffusion approximation and relying only on Ids–Vgs measurements instead of using a conventional method with supplemental Cgate–Vgs measurements. It is confirmed that the carrier mobility is still governed by the electric field applied, and that the drift-diffusion approximation remains valid down to channel length of 100 nm. Consequently, the carrier behavior of such scaled small-size MOSFETs can be precisely described by simple analytical equations, which is important for the development of efficient circuit-simulation models.
Lie algebroid structures on a class of affine bundles
Authors: Sarlet W. , Mestdag T. , Martı́nez E.
Published: October 2002

We introduce the notion of a Lie algebroid structure on an affine bundle whose base manifold is fibered over ↛. It is argued that this is the framework which one needs for coming to a time-dependent generalization of the theory of Lagrangian systems on Lie algebroids. An extensive discussion is given of a way one can think of forms acting on sections of the affine bundle. It is further shown that the affine Lie algebroid structure gives rise to a coboundary operator on such forms. The concept of admissible curves and dynamical systems whose integral curves are admissible brings an associated affine bundle into the picture, on which one can define in a natural way a prolongation of the original affine Lie algebroid structure.
Scale invariant Euclidean field theory in any dimension
Authors: Haba Z.
Published: October 2002

We discuss a D-dimensional scalar field interacting with a metric field. The metric depends on d-dimensional coordinates (where d<D). We choose formal Gaussian scale invariant correlation functions for the metric field. By a projection to a lower dimensional subspace we obtain a scale invariant non-Gaussian model of Euclidean quantum field theory in D−d or d dimensions.
Analog of secondary invariants for holomorphic vector bundles
Authors: Biswas Indranil
Published: October 2002

Given a pair of holomorphic structures on a complex vector bundle over a complex manifold, Dolbeault cohomological invariants are constructed. The construction is based on the construction of Chern–Simons secondary classes.
Collisionless fast electron losses in magnetic toroidal ripples during lower hybrid current drive in Tore Supra
Authors: Ju M. , Peysson Y. , Basiuk V.
Published: October 2002

A consistent estimation of the losses of collisionless fast electrons driven by the lower hybrid wave and trapped in magnetic ripples in the Tokamak Tore Supra [Equipe TORE SUPRA, in Proceedings of the 15th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville (International Atomic Energy Agency, Vienna, 1995), Vol. 1, IAEA-CN-60/A1-5, p. 105], is carried out using a two-dimensional relativistic bounce-averaged Fokker–Planck solver. A reasonable agreement is found between the simulations and the experimentally observed results obtained by the diagnostic named DRIPPLE (Diagnostic-Ripple) dedicated to magnetic ripple loss measurements. The analysis shows that the radial profile of the ripple loss current is dominated by the shape of the supertrapped domain in momentum space, and is therefore weakly dependent on the lower hybrid wave power absorption.
Ex situ spectroscopic ellipsometry and Raman spectroscopy investigations of chemical vapor deposited sulfur incorporated nanocrystalline carbon thin films
Authors: Gupta S. , Weiner B. R. , Morell G.
Published: October 2002

Sulfur incorporated nanocrystalline carbon (n-C:S) thin films were grown on molybdenum substrates by a hot-filament chemical vapor deposition technique using gas mixtures of methane, hydrogen, and a range of hydrogen sulfide (H2S) concentrations (100–500 ppm with an interval of 100 ppm) at a fixed substrate temperature of 900 °C. They were optically characterized using Raman spectroscopy (RS) and ex situ spectroscopic phase modulated ellipsometry from near-infrared to near UV (1.5–5.0 eV) obtaining their vibrational frequencies and pseudodielectric function, respectively, as a function of [H2S]. The ellipsometry data [〈εr(E)〉,〈εi(E)〉] were modeled using Bruggeman effective-medium theory and dispersion relations for the amorphous semiconductors: Forouhi and Bloomer (FB) parameterization model. A simplified two-layer model consisting of a top layer comprising an aggregate mixture of sp3C+sp2C+void and a bulk layer (L2), defined as a dense amorphized FB-modeled material, was found to simulate the data reasonably well. Through these simulations, it was possible to estimate the dielectric function of our n-C:S material, along with the optical band gap (Eg), film thickness (d), void fraction (fv), and roughness layer (σ) as a function of H2S concentration. The physical interpretation of the five modeling parameters obtained in the amorphous dispersion model applied to the case of n-C:S thin films is discussed. The Raman and ellipsometry results indicate that the average size of nanocrystallites in the sulfur-incorporated carbon thin films becomes smaller with increasing H2S concentration, consistent with atomic force microscopy measurements where the distribution of grain size yielded a gamma around 20 nm. The band gap was found to decrease systematically with increasing H2S concentration, indicating an enhancement of π-bonded carbon (sp2C), in agreement with RS results. These results are compared to those obtained for films grown without sulfur (n-C), in order to study the influence of sulfur addition on film microstructure. These analyses led to a correlation between the film microstructure and its electronic properties.
Charge transport due to photoelectric interface activation in pure nematic liquid-crystal cells
Authors: Pagliusi P. , Cipparrone G.
Published: October 2002

We report a study of the crucial role of liquid-crystal–polymer interface on photoinduced transport and redistribution of charges in pure nematic liquid-crystal cells that exhibit a photorefractivelike effect. A stationary photocurrent that is 30% of the dark current has been measured for very low power illumination (few mW) and low applied dc electric field (about 0.1 V/μm). The experimental results indicate a clear dependence of the effect on the light wavelength. The absence of photocurrent in cells with only one component, liquid-crystal, or polymer, suggests that both are not intrinsically photoconductive, rules out light-induced charge injection by the electrodes, and indicates the polymer–liquid-crystal interface as the photoactive element in the effect. The photocurrent dynamics indicate the presence of various mechanisms. We suppose that the effect is due to photoinduced carriers injection through the liquid-crystal–polymer interface and recombination process with the counterions present on the opposite side. Different hypotheses are made and discussed.
Macroscopic modeling and simulations of supercoiled DNA with bound proteins
Authors: Huang Jing , Schlick Tamar
Published: October 2002

General methods are presented for modeling and simulating DNA molecules with bound proteins on the macromolecular level. These new approaches are motivated by the need for accurate and affordable methods to simulate slow processes (on the millisecond time scale) in DNA/protein systems, such as the large-scale motions involved in the Hin-mediated inversion process. Our approaches, based on the wormlike chain model of long DNA molecules, introduce inhomogeneous potentials for DNA/protein complexes based on available atomic-level structures. Electrostatically, treat those DNA/protein complexes as sets of effective charges, optimized by our discrete surface charge optimization package, in which the charges are distributed on an excluded-volume surface that represents the macromolecular complex. We also introduce directional bending potentials as well as non-identical bead hydrodynamics algorithm to further mimic the inhomogeneous effects caused by protein binding. These models thus account for basic elements of protein binding effects on DNA local structure but remain computational tractable. To validate these models and methods, we reproduce various properties measured by both Monte Carlo methods and experiments. We then apply the developed models to study the Hin-mediated inversion system in long DNA. By simulating supercoiled, circular DNA with or without bound proteins, we observe significant effects of protein binding on global conformations and long-time dynamics of the DNA on the kilo basepair length.
Spontaneous decrease in the heat capacity of a glass
Authors: Tombari E. , Presto S. , Salvetti G. , Johari G. P.
Published: October 2002

The real and imaginary components, Cp′ and Cp″, respectively, of the complex heat capacity, Cp*=Cp′−iCp″, of a molecular liquid have been measured in the temperature range of its vitrification and in the glassy state, and the effect of spontaneous structural relaxation has been determined in real time. Cp′ of the glassy state is found to decrease with time. Analysis shows that this is mainly due to the decrease of configurational entropy as the characteristic time of the Cp′ spectra increases and consequently the contribution from the unfrozen, faster modes of the α-relaxation process decreases. There may also be a significant decrease in the vibrational and anharmonic force contributions as the glass densifies. Interpretations in terms of the potential energy landscape model suggest that for each state of lower energy attained with time, the number of minima in the potential energy surface decreases, and the minima become deeper.
Computer generation of dense polydisperse sphere packings
Authors: Kansal Anuraag R. , Torquato Salvatore , Stillinger Frank H.
Published: October 2002

We present an extension of the Lubachevsky and Stillinger [J. Stat. Phys. 60, 561 (1990)] packing algorithm to generate packings of polydisperse spheres. The original Lubachevsky–Stillinger algorithm is a nonequilibrium protocol that allows a set of monodisperse spheres to grow slowly over time eventually reaching an asymptotic maximum packing fraction. We use this protocol to pack polydisperse spheres in three dimensions by making the growth rate of a sphere proportional to its initial diameter. This allows us to specify a size distribution of spheres, which is then preserved throughout the growth process (except the mean diameter increases). We use this method to study the packing of bidisperse sphere systems in detail. The packing fractions of the configurations generated with our method are consistent with both previously generated experimental and simulated packings over a large range of volume ratios. Our modified Lubachevsky–Stillinger protocol, however, extends the range of sphere volume ratios well beyond that which has been previously considered using simulation. In doing so, it allows both small volume ratios and large volume ratios to be studied within a single framework. We also show that the modified Lubachevsky–Stillinger algorithm is appreciably more efficient than a recursive packing method.
Effects of nonlinear frequency shifts on certain induced scattering processes
Authors: Yoon Peter H. , Gaelzer Rudi
Published: October 2002

The present paper is a followup to an earlier paper (accepted in Physics of Plasmas, 2002) in which the nonlinear frequency shifts of electrostatic plasma eigenmodes in an unmagnetized plasma were investigated, and in which a promise was made that the methodology employed in such a study will be employed to deal with certain induced scattering terms which contain apparent singularities. This paper implements the analytical technique developed in the first paper, and demonstrates how these singular terms can be regularized.