bluuebunny/crossref_metadata_2025 huggingface dataset:
Heterogeneity in microcrystalline-transition state: Origin of Si-nucleation and microcrystallization at higher rf power from Ar-diluted SiH4 plasma
Authors: Das Debajyoti , Jana Madhusudan , Barua A. K.
Published: July 2002

Using very high Ar-dilution to the SiH4 plasma, good quality amorphous Si:H films could be obtained at very low rf power. The a-Si:H film, prepared at a very low deposition rate of ∼10 Å/min, exhibited a σPh∼1×10−4 S cm−1, σPh/σD∼105, a notably wide optical gap of 2.10 eV and a very good stability against thermal annealing effects with reasonable light induced degradation. At higher rf power undoped μc-Si:H films were prepared with a high σD∼1×10−4 S cm−1, at a deposition rate of 30 Å/min from <1 sccm of SiH4. Micrograins were identified with several well-defined crystallographic orientations. However, porosity in the grain boundary zone contributed a significant amount of adsorbed effects on the electrical properties. At very high powers, the growth of a columnar network structure was demonstrated. Long-range structural relaxation permitted by the non-rigid and heterogeneous network structure associated with the physical vapor deposition-like growth at the microcrystalline-transition state, has been identified as the origin of nucleation to the Si network and microcrystallization at higher power. It is proposed that Ar* in the Ar-diluted plasma provides the energy required for nucleation and grain growth during microcrystallization, and plays an analogous role as atomic H does during chemical annealing in H2-diluted plasma.
Determination of closure domain penetration in electrodeposited microtubes by combined magnetic force microscopy and giant magneto-impedance techniques
Authors: Garcia J. M. , Asenjo A. , Vázquez M. , Yakunin A. M. , Antonov A. S. , Sinnecker J. P.
Published: July 2002

The domain structure of electrodeposited Co90P10 microtubes exhibiting radial magnetic anisotropy and giant magneto-impedance effect has been characterized by combined magnetic force microscopy imaging and impedance measurements. It has been shown that the size of the closure domains increases with the CoP layer thickness. Furthermore, the depth of the closure domains has been quantitatively determined from the high frequency behavior.
Dual rotating-compensator multichannel ellipsometer: Instrument development for high-speed Mueller matrix spectroscopy of surfaces and thin films
Authors: Lee Joungchel , Koh Joohyun , Collins R. W.
Published: July 2002

A multichannel ellipsometer in the dual rotating-compensator configuration has been developed for applications in real time Mueller matrix spectroscopy of anisotropic surfaces and thin films. The sequence of optical elements for this instrument configuration can be denoted PC1r(5ω)SC2r(3ω)A, where P, S, and A represent the polarizer, sample, and analyzer. In this sequence, C1r and C2r represent the first and second compensators, rotating with angular frequencies of 5ω and 3ω, respectively, where ω=π/TC is the base angular frequency (corresponding to 2 Hz) and TC=0.25 s is the fundamental optical period. The instrument can provide 170 point spectra over the wavelength range from 235 nm (5.3 eV) to 735 nm (1.7 eV) in all 16 elements of the unnormalized Mueller matrix with minimum acquisition and repetition times of TC=0.25 s. In this study, instrumentation calibration procedures are demonstrated in the transmission geometry without a sample, including (i) alignment of the two MgF2 zero-order biplate compensators, (ii) determination of the retardance and phase spectra for the compensators, (iii) determination of the offset angles for the optical elements, and (iv) characterization of the spectral response function of the ellipsometer. Calibration procedure (iv) allows the (1,1) element of the transmission Mueller matrix to be determined; thus the unnormalized Mueller matrix can be obtained. The dual rotating-compensator multichannel ellipsometer is assessed with respect to its performance in transmission without a sample, and is then applied in the transmission geometry to study anisotropy, depolarization, and light scattering effects for a MgF2 helicoidally sculptured thin film on a glass substrate. Numerous future applications of this instrument are anticipated for real time analysis of complex surfaces and thin films in the reflection geometry as well.
Temperature dependence of polarization and charge dynamics on the BaTiO3(100) surface by scanning probe microscopy
Authors: Kalinin Sergei V. , Bonnell Dawn A.
Published: July 2002

Variable-temperature atomic force microscopy, piezoresponse force microscopy (PFM), and scanning surface potential microscopy were combined to determine the temperature response of polarization and screening charge on BaTiO3(100) surfaces. The ferroelectric-domain induced surface corrugations and piezoelectric response decrease with temperature and disappear at the Curie temperature. The temperature dependence of the PFM contrast is explained within the framework of the Ginzburg–Devonshire theory with the effect of a dielectric tip-surface gap taken into account. The temperature dependence of the surface potential contrast is ascribed to the interplay between the release of the screening charges with temperature and their slow relaxation. The results indicate that surface potential polarity is reversed relative to that expected from polarization orientation on BaTiO3 in ambient.
Impurity photovoltaic effect in indium-doped silicon solar cells
Authors: Karazhanov S. Zh.
Published: July 2002

Impurity photovoltaic effect is investigated in two groups of indium-doped single-crystalline silicon solar cells with n-type and p-type dopants in the base layer. The continuity equation for minority carriers is solved numerically using the charge neutrality condition and current–voltage characteristics are found. It is shown that the improvement of short-circuit current due to carrier photogeneration from the deep defect level is negligible for both groups of the cells considered. Short-circuit current increases with increasing the trap concentration and open-circuit voltage abruptly decreases for trap concentrations close to compensation by n-type dopant. However, these dependencies occur due to the increase of lifetime, the decrease of the total equilibrium carrier density, and take place even in the absence of the absorption of subgap photons. It is shown that indium is not the proper impurity for efficiency improvements of silicon solar cells due to the impurity photovoltaic effect.
Anomalous temperature dependence of coercivity and reversal mechanism in bulk-hardened rare earth-cobalt magnets
Authors: Gabay A. M. , Tang W. , Zhang Y. , Hadjipanayis G. C.
Published: July 2002

The “anomalous” nonmonotonic temperature dependence of coercivity, reported in Sm–Zr–Co–Cu magnets, has also been observed in bulk-hardened Y–Zr–Co–Fe–Cu alloys with a similar microstructure. The phenomenon appears to be universal for all R–Co magnets (R=rare earth) having a microstructure consisting of R2Co17 cells surrounded by the RCo5 phase. The effect of R and Cu on the temperature dependence of coercivity cannot be simply explained by traditional domain-wall pinning model based on the difference in a domain wall energy. Possibility that the coercivity is controlled by nucleation of reversed domains in magnetically isolated R2Co17 cells is discussed.
Probing radicals in hot wire decomposition of silane using single photon ionization
Authors: Duan H. L. , Zaharias G. A. , Bent Stacey F.
Published: July 2002

Radicals produced by the hot wire-induced decomposition of silane have been identified using vacuum ultraviolet single photon ionization (SPI). This laser-based technique uses 118 nm photons (10.5 eV) to ionize gas phase species; the resulting photoions are detected using time-of-flight mass spectrometry. The major silicon-containing gas-phase species identified by SPI during hot-wire activation of silane gas are Si, SiH3, and Si2H6. These results demonstrate that single photon ionization can be a powerful probe for in situ, real-time detection of multiple species in hot wire chemical vapor deposition.
Preisach modeling of piezoelectric nonlinearity in ferroelectric ceramics
Authors: Robert G. , Damjanovic D. , Setter N. , Turik A. V.
Published: July 2002

Piezoelectric response nonlinearity is approached using the Preisach description of hysteretic systems as collection of distributed bistable units. The Preisach model and its recent physical interpretation in terms of moving domain wall in a stochastically described pinning field are reviewed. It is shown that such an approach can effectively render not only the piezoelectric coefficient field dependences but also the field-response hysteresis, especially in the well-known case of linear piezoelectric field dependence (i.e., Rayleigh’s law) where the bistable units are distributed homogeneously. New expressions for piezoelectric nonlinear behavior departing from the classical linear dependence are then derived using a more complex distribution and are qualitatively compared to experimental data for piezoelectric materials as varied as lead titanate, strontium bismuth titanate, and lead zirconate titanate. Finally, these expressions are shown to be adequate for the description of various piezoelectric coefficient behaviors such as: polynomial dependence on the applied field, dc field effect on nonlinear contributions, and threshold field for nonlinearity.
Vacuum electron acceleration by an intense laser
Authors: Wang P. X. , Ho Y. K. , Yuan X. Q. , Kong Q. , Cao N. , Sessler A. M. , Esarey E. , Nishida Y.
Published: July 2002

Using three dimensional test particle simulations, the characteristics and essential conditions under which an electron, in a vacuum laser beam, can undergo a capture and acceleration scenario (CAS) have been examined. When a0≳100 the electron can be captured and violently accelerated to energies ≳1 GeV, with an acceleration gradient ≳10 GeV/cm, where a0=eE0/meωc is the normalized laser field amplitude. The physical mechanism behind the CAS is that diffraction of the focused laser beam leads to a slowing down of the effective wave phase velocity along the captured electron trajectory, such that the electron can be trapped in the acceleration phase of the wave for a longer time and thus gain significant energy from the field.
Atomic-scale surface morphology of epitaxial ferromagnetic MnAs thin films grown on vicinal GaAs(111)B substrates
Authors: Sugahara Satoshi , Tanaka Masaaki
Published: July 2002

We have investigated the atomic-scale surface morphology and magnetic anisotropy of ferromagnetic MnAs films grown by molecular beam epitaxy on vicinal GaAs(111)B substrates misoriented 2° toward the [1̄1̄2] direction. The MnAs films were grown in a step-flow-like manner and atomically flat terraces appeared on the surface in the temperature range between 220 and 300 °C. The terrace width was elongated dramatically in comparison with the original terrace width on the GaAs buffer layer, which resulted from the macrostep formation induced by the step-bunching phenomenon during the growth of MnAs. The terrace width and the macrostep height were 70 and 2 nm, respectively, for the MnAs film grown at 250 °C and both values increased with increasing the growth temperature. The MnAs films grown on the vicinal surface were found to exhibit in-plane uniaxial magnetic anisotropy induced by the macrosteps, which was not observed in the MnAs films grown on the exact GaAs(111)B surface.
Effects of Ag underlayers on the microstructure and magnetic properties of epitaxial FePt thin films
Authors: Hsu Yu-Nu , Jeong Sangki , Laughlin David E. , Lambeth David N.
Published: July 2002

In this work Ag underlayers, with a slightly larger unit cell than FePt, were found not only to induce epitaxial growth of the FePt films but also to reduce the FePt ordering temperature. Without using the Ag underlayer, the FePt film deposited onto the Si substrate was fcc disordered. By the use of the Ag underlayer, it was observed that the FePt unit cells were expanded in the film plane. This has caused the shrinkage of the FePt unit cells along the film normal direction and resulted in the in situ ordering of the FePt thin film at reduced temperatures. The microstructural and magnetic properties of the FePt/Ag films at varied substrate temperature and FePt thickness were studied to investigate the L10 FePt ordering.
Molecular simulation and continuum mechanics investigation of viscoelastic properties of fluids confined to molecularly thin films
Authors: Khare Rajesh , de Pablo Juan , Yethiraj Arun
Published: July 2002

A combination of molecular dynamics simulations of oscillatory shear flow and continuum mechanics is used to investigate viscoelastic properties of materials confined to molecularly thin films. The atoms of the simple liquid interact via a repulsive Lennard-Jones potential. The chain molecules are modeled as strings of similar spheres connected via finite extensible nonlinear elastic springs. The fluid is confined between two surfaces composed of identical spheres that are moved to simulate oscillatory flow. In order to mimic experiments, the temperature is controlled by coupling the wall atoms to a heat bath, and the viscoelastic properties are obtained via an analysis using continuum mechanics. Both simple and polymeric fluids exhibit linear viscoelastic behavior under typical simulation conditions, although inertial effects play an important role in determining the flow behavior. Simple fluids display a smooth transition from liquidlike to solidlike behavior when confined to molecularly thin films, whereas linear chain polymers and gels display predominantly elastic shear response at all frequencies investigated. These results are in qualitative agreement with the surface forces apparatus experiments on similar systems.
Influence of magnetic field on plasma sheath and electron temperature
Authors: Singha Bornali , Sarma A. , Chutia Joyanti
Published: July 2002

Experimental observation has been carried out to see the effect of magnetic field and grid biasing voltage in controlling the sheath thickness in a magnetized plasma system. The experiment is carried out in a stainless steel chamber which is divided into two regions by a mesh grid, via the source region and the diffused region. The characteristic behavior of the ion rich sheath formed across the grid under various conditions of the applied magnetic field and grid biasing voltage has been investigated experimentally. It has been observed that at both conditions of increasing magnetic field and grid biasing voltage, sheath width expands in the source region, whereas in the diffused region, no such noticeable variation has been found. This study has been accompanied by the measurement of the electron temperature in both regions of the chamber via the source region and the diffused region with the help of the Langmuir probe. Plasma is produced in the source region and it penetrates into the diffused region through the grid. It has been found that the electron temperature decreases with increasing magnetic field in the source region while kept at a constant grid biasing voltage. However, in the diffused region the opposite variation has been observed. The variation of electron temperature with grid biasing voltage in both regions is not very significant.
Search for the ion temperature gradient mode in a tokamak plasma and comparison with theoretical predictions
Authors: Rettig C. L. , Rhodes T. L. , Leboeuf J. N. , Peebles W. A. , Doyle E. J. , Staebler G. M. , Burrell K. H. , Moyer R. A.
Published: July 2002

In recent experiments in DIII-D, comparison of specific turbulence characteristics with linear and nonlinear modeling has identified common features associated with the ion temperature gradient (ITG) mode. A low-frequency turbulence feature is observed in high-density saturated Ohmic confinement discharges, which is absent in low-density linear Ohmic confinement discharges. The feature is in a range of wavelength (k⊥ρs≈0.2–0.5) and the frequency expected for the ITG mode and onset of the feature is coincident with onset of confinement saturation. The density profile is significantly broader in the high-density discharge, a known destabilizing effect on the ITG mode. Gyrokinetic stability calculations of the growth rate of the most unstable drift ballooning mode show the ITG mode to be more unstable in the high-density discharges.
Calculation of rovibrational states of weakly bound complexes by transformation from an Eckart frame: Benzene-N2
Authors: Felker Peter M.
Published: July 2002

We apply a method for the construction of rotational-intermolecular vibrational states to the van der Waals molecule benzene-N2. The method, as described in Felker et al. [J. Chem. Phys. 114, 1233 (2001)] is based on the transformation of approximate rovibrational eigenfunctions from functions of coordinates referred to an Eckart body-fixed axis system to functions of coordinates referred to a more convenient body-fixed frame. This transformation allows the computation of approximate rovibrational eigenenergies, as well as other molecular properties, from one or a small set of J=0 eigenfunctions. By comparing rovibrational energies and matrix elements computed by this Eckart method with those computed variationally with a large basis, we show that the former yields excellent results for benzene-N2. The results are particularly significant because of the presence of essentially free internal rotation in the species, a large-amplitude motion that complicates the definition of an Eckart frame.
Control of magnetic fluctuations in the reversed field pinch with edge current drive
Authors: Craig D. , Fiksel G. , Prager S. C. , Sarff J. S.
Published: July 2002

The generation of auxiliary current in the extreme edge of the reversed field pinch is shown to affect edge and core resonant magnetic fluctuations, the recurrence time of relaxation oscillations (sawteeth), and the energy and particle confinement. Current is driven in the edge by electrostatic current sources. Although the injected current is expected to primarily affect edge resonant fluctuations, the coupling of edge and core modes enables changes in the extreme edge to have global consequences.
Monomode optical waveguide in lithium niobate formed by MeV Si+ ion implantation
Authors: Hu Hui , Lu Fei , Chen Feng , Shi Bo-Rong , Wang Ke-Ming , Shen Ding-Yu
Published: July 2002

The monomode enhanced-index LiNbO3 waveguide fabricated by low-dose ion implantation is reported. LiNbO3 crystals were implanted with 3 MeV Si+ ions to doses around 1014 ions/cm2. After annealing, the waveguides were formed by the extraordinary refractive index enhancement in the waveguide regions. The effective extraordinary refractive index of the waveguide increased with ion implantation dose. The loss was 0.64 dB/cm in the X-cut sample with an implantation dose of 3.3×1014 ions/cm2. The scattering loss in the Z-cut samples was even lower than that in the X-cut samples.
Gaussian basis sets for use in correlated molecular calculations. X. The atoms aluminum through argon revisited
Authors: Dunning Thom H. , Peterson Kirk A. , Wilson Angela K.
Published: July 2002

For molecules containing second row atoms, unacceptable errors have been found in extrapolating dissociation energies calculated with the standard correlation consistent basis sets to the complete basis set limit. By carefully comparing the convergence behavior of De(O2) and De(SO), we show that the cause of these errors is a result of two inter-related problems: near duplication of the exponents in two of the d sets and a lack of high-exponent functions in the early members of the sets. Similar problems exist for the f sets (and probably in higher angular momentum sets), but have only a minor effect on the calculated dissociation energies. A number of approaches to address the problems in the d sets were investigated. Well behaved convergence was obtained by augmenting the (1d) and (2d) sets with a high-exponent function and by replacing the (3d) set by the (4d) set and the (4d) set by the (5d) set and so on. To ensure satisfactory coverage of both the L and M shell regions, the exponents of the new d sets were re-optimized. Benchmark calculations on Si2, PN, SO, and AlCl with the new cc-pV(n+d)Z sets show greatly improved convergence behavior not only for De but for many other properties as well.
Ordinary and extraordinary dielectric functions of 4H– and 6H–SiC from 3.5 to 9.0 eV
Authors: Lindquist O. P. A. , Järrendahl K. , Peters S. , Zettler J. T. , Cobet C. , Esser N. , Aspnes D. E. , Henry A. , Edwards N. V.
Published: July 2002

We report ordinary (ε⊥c axis) and extraordinary (ε∥c axis) dielectric function data of 4H– and 6H–SiC from 3.5 to 9.0 eV. These data, which were obtained by with spectroscopic ellipsometry, are also compared to recently reported ab initio calculations. Critical point energies were found using real and reciprocal space analysis.
p-type conductivity in CuCr1−xMgxO2 films and powders
Authors: Nagarajan R. , Draeseke A. D. , Sleight A. W. , Tate J.
Published: July 2002

CuCr 1−x Mg x O 2 , a wide band gap semiconductor with the delafossite structure, has been synthesized in bulk and thin-film form. Bulk undoped CuCrO2 is almost black and has moderate conductivity with p-type carriers. Upon doping with 5% Mg, the conductivity increases by a factor of 1000. In films, the best p-type conductivity is 220 S cm−1 in CuCr0.95Mg0.05O2, a factor of 7 higher than previously reported for Cu-based p-type delafossites. Undoped films have a conductivity of order 1 S cm−1. Films are usually polycrystalline on amorphous substrates, but undoped films can be c-axis oriented if deposited at or above 650 °C. Optical and ultraviolet transmission data indicate a direct band gap of 3.1 eV.
Uniaxial stress dependence of the binding energy of shallow donor impurities in GaAs–(Ga,Al)As quantum dots
Authors: Oyoko H. O. , Duque C. A. , Porras-Montenegro N.
Published: July 2002

We have studied the effects of an uniaxial stress on the binding energy of a shallow donor impurity in a parallelepiped-shaped GaAs–(Ga,Al)As quantum dot. In the calculations we have used a variational technique within the effective-mass approximation. The stress was applied in the z direction and the donor impurity was located at various positions along the z axis. Our results show that the donor binding energy increases with increasing stress and for decreasing sizes of the quantum dot. Also, we have found that the binding energy for various values of the donor position along the z axis for constant quantum well box size increases with the proximity of the impurity to the center of the structure. Moreover, we obtain the shallow-donor binding energies as functions of uniaxial stress in the limit in which the quantum dot turns into either a quantum well or a quantum-well wire.
Scaling and modeling in the analysis of dispersive relaxation of ionic materials
Authors: Macdonald J. Ross
Published: July 2002

Problems with scaling of conductive-system experimental Mdat″(ω) and σdat′(ω) data are considered and resolved by dispersive-relaxation-model fitting and comparison. Scaling is attempted for both synthetic and experimental M″(ω) data sets. A crucial element in all experimental frequency-response data is the influence of the high-frequency-limiting dipolar-and-vibronic dielectric constant εD∞, often designated ε∞, and not related to ionic transport. It is shown that εD∞ precludes scaling of Mdat″(ω) for ionic materials when the mobile-charge concentration varies. When the effects of εD∞ are properly removed from the data, however, such scaling is viable. Only the σ′(ω) and ε″(ω) parts of immittance response are uninfluenced by εD∞. Thus, scaling is possible for experimental σ′(ω) data sets under concentration variation if the shape parameter of a well-fitting model remains constant and if any parts of the response not associated with bulk ionic transport are eliminated. Comparison between the predictions of the original-modulus-formalism (OMF) response model of 1972–1973 and a corrected version of it that takes proper account of εD∞, the corrected modulus formalism (CMF), demonstrates that the role played by εD∞ (or ε∞) in the OMF is incorrect. Detailed fitting of data for three different ionic glasses using a Kohlrausch–Williams–Watts response model, the KWW1, for OMF and CMF analysis clearly demonstrates that the OMF leads to inconsistent shape-parameter (β1) estimates and the CMF does not. The CMF KWW1 model is shown to subsume, correct, and generalize the recent disparate scaling/fitting approaches of Sidebottom, León, Roling, and Ngai.
Order on disorder: Copper phthalocyanine thin films on technical substrates
Authors: Peisert H. , Schwieger T. , Auerhammer J. M. , Knupfer M. , Golden M. S. , Fink J. , Bressler P. R. , Mast M.
Published: July 2002

We have studied the molecular orientation of the commonly used organic semiconductor copper phthalocyanine (CuPC) grown as thin films on the technically relevant substrates indium tin oxide, oxidized Si, and polycrystalline gold using polarization-dependent x-ray absorption spectroscopy, and compare the results with those obtained from single crystalline substrates [Au(110) and GeS(001)]. Surprisingly, the 20–50 nm thick CuPC films on the technical substrates are as highly ordered as on the single crystals. Importantly, however, the molecular orientation in the two cases is radically different: the CuPC molecules stand on the technical substrates and lie on the single crystalline substrates. The reasons for this and its consequences for our understanding of the behavior of CuPC films in devices are discussed.
Ion-acoustic shocks in space and laboratory dusty plasmas: Two-dimensional and non-traveling-wave observable effects
Authors: Gao Yi-Tian , Tian Bo
Published: July 2002

Some recent observations of the ion-acoustic shocks in dusty plasmas have been explained via the one-dimensional Korteweg–de Vries–Burgers equation (1-D-KdVB). To help study the space and laboratory plasma systems, the investigation on a two-dimensional generalization of the 1-D-KdVB is performed with computerized symbolic computation. An auto-Bäcklund transformation and some monotonic-shock-wave-like, exact analytic solutions are found. Then, several observable effects are predicted, which the future space and laboratory plasma experiments might discover, beyond the existing one-dimensional and traveling-wave considerations.
Structure and properties of the electrostatic fluctuations in the far scrape-off layer region of Alcator C-Mod
Authors: Carreras B. A. , Lynch V. E. , LaBombard B.
Published: July 2002

The temporal statistics of density and potential fluctuations measured in both the main chamber and divertor far scrape-off layer (SOL) regions (>5 mm outside the magnetic separatrix) of the Alcator C-Mod tokamak [I. M. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are found to change markedly with plasma density. As the plasma density is increased, the fluctuations become correlated over long and short time scales and exhibit a higher level of persistence and intermittency. Correspondingly, fluctuation-induced cross-field particle fluxes are found to increase by a factor of ∼5 when the plasma density is doubled. These observations are consistent with results from SOL particle balance, which indicate an increased level of cross-field transport as the density is raised. These observations also support the idea that SOL transport dynamics behave as a self-organized system, particularly at high plasma densities.
Coercivity and squareness enhancement in ball-milled hard magnetic–antiferromagnetic composites
Authors: Sort J. , Nogués J. , Suriñach S. , Muñoz J. S. , Baró M. D. , Chappel E. , Dupont F. , Chouteau G.
Published: July 2002

The room-temperature coercivity, HC, and squareness, MR/MS (remanence/saturation magnetizations), of permanent magnet, SmCo5 powders have been enhanced by ball milling with antiferromagnetic NiO (with Néel temperature, TN=590 K). This enhancement is observed in the as-milled state. However, when the milling of SmCo5 is carried out with an antiferromagnet with TN below room temperature (e.g., for CoO, TN=290 K), the coercivity enhancement is only observed at low temperatures after field cooling through TN. The ferromagnetic–antiferromagnetic exchange coupling induced either by local heating during milling (SmCo5+NiO) or field cooling (SmCo5+CoO) is shown to be the origin of the HC increase.
Effect of gamma-ray irradiation on the characteristics of 6H silicon carbide metal-oxide-semiconductor field effect transistor with hydrogen-annealed gate oxide
Authors: Ohshima Takeshi , Itoh Hisayoshi , Yoshikawa Masahito
Published: July 2002

The effects of gamma-ray (γ-ray) irradiation on the channel mobility of enhancement-type n-channel 6H silicon carbide (SiC) metal-oxide-semiconductor field effect transistors (MOSFETs) with gate oxides annealed in a hydrogen atmosphere after pyrogenic oxidation were studied. Irradiation induced interface traps produced in the MOSFETs were evaluated using the associated change in the subthreshold-current curve. It was found that the MOSFET channel mobility depended on the net number of radiation-induced interface traps. By a detailed comparison with the radiation response of silicon MOSFETs, 6H–SiC MOSFETs are shown to exhibit a channel mobility with a much higher radiation resistance.
Single-gap pseudospark discharge experiments
Authors: Yin H. , He W. , Cross A. W. , Phelps A. D. R. , Ronald K.
Published: July 2002

An experimental investigation of a single-gap pseudospark was conducted using a flexible discharge chamber. The voltage breakdown characteristics were studied against a wide range of parameters such as gas pressure, gap separation, cathode cavity depth, cathode aperture size, external capacitance and applied voltage. An empirical formula, VB=(0.20±0.05) p−4.02±0.18 d−1.77±0.01, was obtained for the breakdown voltage VB in kV, given the gas pressure p in Torr and the gap separation d in mm. The electron beam extracted from this single gap was also studied and a current of up to 100 A was measured at 10 kV.
Electron transfer reactions at metal electrodes: Influence of work function on free energy of activation and exchange current density
Authors: Harinipriya S. , Sangaranarayanan M. V.
Published: July 2002

The dependence of the free energy of activation on the work function of electrodes, solvation energies, and surface potentials of the reactant species pertaining to electron transfer reactions at metal/solution interfaces is derived using thermodynamic considerations. The standard exchange current density is calculated for Fe3++e↔Fe2+ at different metal electrodes and compared with experimental data as well as molecular dynamics simulations.
Linear and nonlinear piezoelectric response of charged cellular polypropylene
Authors: Kressmann Reiner
Published: July 2002

Piezoelectricity in a charged cellular polypropylene, called EMFi, is investigated with respect to nonlinearities to explain the strong differences in longitudinal piezoelectric constants published in the literature and ranging from 90 to 250 pC/N. The inverse constant was measured interferometrically to be 90 pm/V. Quasistatic and dynamic measurements with small loads yielded the same value for the direct constant. The direct constant was also investigated with respect to large-signal behavior becoming noticeable at static and dynamic loads higher than 10 kPa. Both the quasistatic and the dynamic constant increase up to 130 pC/N at such loads. Furthermore, an additional resonance appears under strong loading in the range of about 10 Hz shifting down with increasing load. In addition, the piezoelectric constant increases also with increasing dynamic load under constant static load. The nonlinearity also results in the generation of harmonics. Finally, boundary effects can be detected if just a small area of the sample is loaded. This effect appearing mainly at frequencies below 20 Hz is attributed to airflow between the air bubbles. A load-dependent Young’s modulus, mainly responsible for the nonlinear behavior, is calculated from the experiments. It diminishes from 2 to 1.5 MPa at a load of 60 kPa.
In-plane thermal diffusivity evaluation by infrared thermography
Authors: Cernuschi F. , Russo A. , Lorenzoni L. , Figari A.
Published: July 2002

A thermographic technique for measuring the in-plane thermal diffusivity of slabs is presented; it consists of heating the front surface of a platelike sample by a circular Gaussian source. The in-plane thermal diffusivity is obtained by monitoring the time evolution of the spatial distribution of the rear surface temperature by an infrared camera. Specifically, from the temperature distribution taken at different times along a line crossing the heating image of the circular spot center on the rear surface, it is possible to obtain the time evolution of the radius of the Gaussian. In order to get the thermal diffusivity value from the widening of this radius, a method for the reduction of the experimental data is presented; the resulting in-plane thermal diffusivity value is then compared with the value found in the literature and with values furnished by the laser flash and the thermal wave interferometry experiments carried out on the samples extracted by the plate. Satisfactory agreement between all the values was noted.
Magnetic phase competing in MnAu systems
Authors: Wang Jian-Tao , Wang Ding-Sheng , Kawazoe Yoshiyuki
Published: July 2002

First-principles total energy calculations on tetragonal MnAu superlattices are performed by means of the self-consistent full-potential linearized augmented-plane-wave method under the generalized gradient approximation. It is shown that the stability of the spin configuration strongly depends on the lattice distortion due to its competitive nature. Using Monte Carlo simulations with exchange parameters deduced from ab initio results, finite-temperature magnetism is also studied and a magnetic phase diagram connected with lattice distortion is obtained.
Dispersion-related assessments of temperature dependences for the fundamental band gap of hexagonal GaN
Authors: Pässler R.
Published: July 2002

We have analyzed a series of data sets available from published literature for the temperature dependence of A and B exciton peak positions associated with the fundamental band gap of hexagonal GaN layers grown on sapphire. In this article, in contrast to preceding ones, we use the dispersion-related three-parameter formula Eg(T)=Eg(0)−(αΘ/2)[(1+(π2/6)(2T/Θ)2+(2T/Θ)4)1/4−1], which is a very good approximation in particular for the transition region between the regimes of moderate and large dispersion. This formula is shown here to be well adapted to the dispersion regime frequently found in hexagonal GaN layers. By means of least-mean-square fittings we have estimated the limiting magnitudes of the slopes, S(T)≡−dEg(T)/dT, of the Eg(T) curves published by various experimental groups to be of order α≡S(∞)≈(5.8±1.0)×10−4 eV/K. The effective phonon temperature has been found to be of order Θ≈(590±110) K, which corresponds to an ensemble-averaged magnitude of about 50 meV for the average phonon energy. The location of the latter within the energy gap between the low- and high-energy subsections of the phonon energy spectrum of h-GaN suggests that the weights of contributions made by both subbands to the limiting slope α are nearly the same. This explains the order of Δ≈0.5–0.6 as being typical for the dispersion coefficient of the h-GaN layers under study. The inadequacies of both the Bose–Einstein model (corresponding to the limiting regime of vanishing dispersion Δ→0) and Varshni’s ad hoc formula (corresponding to a physically unrealistic regime of excessively large dispersion Δ≈1) are discussed. Unwarranted applications of these conventional models to numerical fittings, especially of unduly restricted data sets (T⩽300 K), are identified as the main cause of the excessively large scatter of parameters quoted for h-GaN in various recent articles.
Production of calcium phosphate coatings on Ti6Al4V obtained by Nd:yttrium–aluminum–garnet laser cladding
Authors: Lusquiños F. , Pou J. , Arias J. L. , Boutinguiza M. , León B. , Pérez-Amor M. , Driessens F. C. M. , Merry J. C. , Gibson I. , Best S. , Bonfield W.
Published: July 2002

Among the various techniques that have been investigated to produce Hydroxyapatite (HA) coatings in order to promote fixation and osteointegration of cementless prosthesis, the plasma spray (PS) technique is the most popular method commercially in use. PS presents some disadvantages such as the poor coating-to-substrate adhesion, low mechanical strength, and brittleness of the coating. In order to overcome the drawbacks of plasma spraying, an approach on how to bind HA to the Ti alloy will be introduced in this work, using a well-known technique in the metallurgical field: laser surface cladding. Different techniques were applied to characterize the coated samples, including x-ray diffraction, scanning electron microscopy, and energy dispersive x-ray analysis.
Velocities of speckles in a smoothed laser beam propagating in a plasma
Authors: Walraet François , Bonnaud Guy , Riazuelo Gilles
Published: July 2002

By means of three-dimensional simulations, the paraxial propagation of a laser beam in a nonlinear plasma modeled by ion-acoustic waves is examined. The beam speckles are smoothed by a one-dimensional transverse spectral dispersion technique. In the case of weak smoothing and when the incident speckle velocity is subsonic, the propagation in the plasma is found to exhibit an asymptotic speckle velocity and to enhance filamentation. With contrary conditions, it is found to reduce simply the speckle velocity.
A unified scheme for ab initio molecular orbital theory and path integral molecular dynamics
Authors: Shiga Motoyuki , Tachikawa Masanori , Miura Shinichi
Published: July 2002

We present a general approach for accurate calculation of chemical substances which treats both nuclei and electrons quantum mechanically, adopting ab initio molecular orbital theory for the electronic structure and path integral molecular dynamics for the nuclei. The present approach enables the evaluation of physical quantities dependent on the nuclear configuration as well as the electronic structure, within the framework of Born–Oppenheimer adiabatic approximation. As an application, we give the path integral formulation of electric response properties—dipole moment and polarizability, which characterize the changes both in electronic structure and nuclear configuration at a given temperature when uniform electrostatic field is present. We also demonstrate the calculation of a water molecule using the present approach and the result of temperature and isotope effects is discussed.
Vertical cavity violet light emitting diode incorporating an aluminum gallium nitride distributed Bragg mirror and a tunnel junction
Authors: Diagne M. , He Y. , Zhou H. , Makarona E. , Nurmikko A. V. , Han J. , Waldrip K. E. , Figiel J. J. , Takeuchi T. , Krames M.
Published: July 2002

We have designed and implemented a vertical cavity violet light emitting diode which features an optical resonator composed of an in situ grown GaN/AlGaN DBR and a high reflectivity dielectric mirror. The active InGaN MQW medium is grown directly atop the AlGaN DBR and the structure includes an intracavity lateral current spreading layer based on a p++/n++ InGaN/GaN tunnel junction. Electroluminescence shows directional emission, with modal linewidths as narrow as 0.6 nm.
Signature of the Meyer–Neldel rule on the correlated barrier-hopping model
Authors: Abdel-Wahab Fouad
Published: July 2002

In this study, a modified model for bipolaron correlated barrier hopping (CBH) is proposed, since the usual bipolaron CBH model has been found to be inconsistent with experimental data for ac conductivity for chalcogenide glasses at high temperatures. The present model is based on the existence of a focal point when the experimental data are extrapolated to high temperatures on the σ(ω)–1/T plot. The correct analysis must introduce the concept of the Meyer–Neldel rule in the expression of the relaxation time. The results of the present model are in quantitative agreement with reported ac conductivity measurements.
Effect of rotation on H-mode transport in DIII–D via changes in the E×B velocity shear
Authors: Petty C. C. , Wade M. R. , Kinsey J. E. , Baker D. R. , Luce T. C.
Published: July 2002

The effect of rotation on the heat and particle transport is measured in the DIII–D tokamak [Fusion Technol. 8, 441 (1985)] for high-confinement mode (H-mode) plasmas with edge localized modes. In a novel experiment, transport is compared for nearly identical scans of the relative gyroradius in co- and counter-rotating plasmas. Since the plasma profiles are the same, the difference in the transport scaling can be attributed to changes in the sheared E×B flow caused by the shift in the toroidal plasma velocity. The ion heat and particle transport are found to be sensitive to the change in the rotation direction and magnitude whereas the electron heat transport is not. Simulations using a gyroLandau-fluid drift wave transport model show that the variation in the ion heat transport for co/counter rotation is due to changes in the E×B shear stabilization, but the electrons appear to be governed by a different transport mechanism.
Electric field effects on fluorescence quenching due to electron transfer
Authors: Hilczer Maria , Traytak Sergey , Tachiya M.
Published: July 2002

The effect of an external electric field on fluorescence quenching due to electron transfer from a photoexcited electron donor to an acceptor has been analyzed theoretically. The model predicts that at weak fields the variation ΔI(c,F)/I(c,0) in the steady-state monomer fluorescence intensity induced by an external electric field is proportional to the square of the field strength F and to the concentration of acceptors c. Similar relations have been reported for the fluorescence intensity of ethylcarbazole doped in poly-methyl-methacrylate films in the presence of dimethyl terephtathalate and an external electric field with a strength up to 0.01 V/Å. The effect of the free energy change of the electron transfer reaction on the c and F dependencies of ΔI(c,F)/I(c,0) has been discussed within the framework of the present model.
General solution for the thermal wave scattering in fiber composites
Authors: Terrón J. M. , Salazar A. , Sánchez-Lavega A.
Published: July 2002

In this article we present a general solution for the ac temperature field generated from a modulated heating in an opaque material containing a series of aligned subsurface cylinders. The model includes the multiple scattering effects suffered by the plane thermal wave, generated by a planar heat source, when interacting with the cylinders and with the sample surfaces. A series solution to the heat diffusion equation and a matrix formulation to solve the scattering coefficients are used. There are no restrictions on the cylinders number, on their size and position inside the material, nor on the thermal properties of cylinders and sample. For completeness the case of coated cylinders is also investigated. Numerical calculations for the case of nonconducting cylinders illustrate the effect of the multiple scattering of the incident thermal wave on the sample surface temperature. A specific case of interest of the model is its application to the thermal characterization and nondestructive evaluation of fiber reinforced composites.
Creation of nanodiamonds by single impacts of highly charged ions upon graphite
Authors: Meguro T. , Hida A. , Suzuki M. , Koguchi Y. , Takai H. , Yamamoto Y. , Maeda K. , Aoyagi Y.
Published: July 2002

The local modification of the electronic states of highly oriented pyrolytic graphite (HOPG) surfaces using highly charged ion (HCI) irradiation has been demonstrated as a promising technique in the design of nanoscale materials. The high potential energy of HCI and subsequent surface treatment by either electron injection from a scanning tunneling microscopy (STM) tip or by He–Cd laser irradiation provide a localized transition from sp2 to sp3 hybridization in HOPG, resulting in the formation of nanoscale diamond-like structures (nanodiamond). It is of interest that a single impact of HCI creates one nanodiamond structure without inducing any defects in the area surrounding the impact region, suggesting potential applications of HCI in nanoscale material processing.
Dynamics of O(3Pj)+Rg collisions on ab initio and scattering potentials
Authors: Krems Roman V. , Buchachenko A. A. , Szczȩśniak M. M. , Kłos J. , Chałasiński G.
Published: July 2002

Interaction potentials of the Π3 and Σ-3 electronic states of the Rg–O(3P) systems (Rg=He–Kr) are computed at the coupled cluster single, double (triple) level of ab initio theory using extended basis sets augmented by bond functions. The ab initio potentials agree well with the scattering potentials determined from experiments in molecular beams [Aquilanti et al., J. Chem. Phys. 89, 6157 (1988)]. Both sets of the interaction potentials are employed for accurate close-coupling calculations of cross sections and rate constants for intramultiplet transitions in collisions of O(3Pj) with Rg atoms and analytical approximations for temperature dependencies of rate constants over temperature interval 50–3500 K are proposed. The sensitivity of the dynamical results to the nature of Rg atoms and interaction potentials is analyzed and the dynamics of intramultiplet mixing in O(3Pj) is investigated in both high- and low-energy limits.
Automorphisms of the fine grading of sl(n,C) associated with the generalized Pauli matrices
Authors: Havlı́ček Miloslav , Patera Jiřı́ , Pelantová Edita , Tolar Jiřı́
Published: July 2002

We consider the grading of sl(n,C) by the group Πn of generalized Pauli matrices. The grading decomposes the Lie algebra into n2−1 one-dimensional subspaces. In the article we demonstrate that the normalizer of grading decomposition of sl(n,C) in Πn is the group SL(2,Zn), where Zn is the cyclic group of order n. As an example we consider sl(3,C) graded by Π3 and all contractions preserving that grading. We show that the set of 48 quadratic equations for grading parameters splits into just two orbits of the normalizer of the grading in Π3.
Ion source with closed drift anode layer plasma acceleration
Authors: Dudnikov V. , Westner A.
Published: July 2002

Several versions of ion sources with closed drift anode layer plasma acceleration (ALPA sources) were built and tested. Robust, “all iron” design and water cooling permeate an unlimited operation with oxygen and with other high reactive gases. Long time operation of discharges in oxygen, nitrogen, argon, water vapor, propane, acetone, ethyl alcohol vapors, and in different gas cocktails has been tested. Beam parameters, generated in discharges with a voltage of Vd=0.3 to 3 kV will be presented. A simple design, only one simple power supply, and the operation without special electron source are advantages of discussed ALPA sources very suitable for many applications.
Characterization of the magnetic modification of Co/Pt multilayer films by He+, Ar+, and Ga+ ion irradiation
Authors: Rettner C. T. , Anders S. , Baglin J. E. E. , Thomson T. , Terris B. D.
Published: July 2002

Co/Pt multilayers can exhibit large perpendicular anisotropies and coercivities that are very sensitive to the quality of the Co/Pt interfaces. We have characterized the dependence of coercivity of Co/Pt multilayers on irradiation with various ion species (He+, Ar+, and Ga+), energies (20 keV–2 MeV), and doses (1011–1017 ions/cm2), in order to understand better the nature of the structural changes responsible for the magnetic changes. We find that, in general, the system is much more sensitive to irradiation than expected on the basis of a nearest-neighbor coupling model and simple ballistic ion-beam mixing.