bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct tunneling hole currents through ultrathin gate oxides in metal-oxide-semiconductor devices
Authors: Hou Y. T. , Li M. F. , Jin Y. , Lai W. H.
Published: July 2002

We present a physical model to calculate the direct tunneling hole current through ultrathin gate oxides from the inversion layer of metal–oxide–semiconductor field-effect transistors. A parametric self-consistency method utilizing the triangular well approximation is used for the electrostatics of the inversion layer. For hole quantization in the inversion layer, an improved one-band effective mass approximation, which is a good approximation to the rigorous six-band effective mass theory, is used to account for the band-mixing effect. The tunneling probability is calculated by a modified Wentzel–Kramers–Brilliouin (WKB) approximation, which takes the reflections near the Si/SiO2 interfaces into account. It is found that the parabolic dispersion in the SiO2 band gap used in the WKB approximation is only applicable for hole tunneling in oxides thinner than about 2 nm and for low gate voltage. A more reasonable Freeman–Dahlke hole dispersion form with significantly improved fitting to all experimental data for different oxide thickness and gate voltage range is adopted and discussed.
Charge transfer photodissociation of phenol on Ag(111)
Authors: Lee Junseok , Ryu Sunmin , Ku Jong Seok , Kim Seong Keun
Published: July 2002

The photochemistry of phenol on Ag(111) has been investigated by post-irradiation temperature programmed desorption (TPD). Ultraviolet (UV) irradiation at 355 and 266 nm was found to affect only the chemisorption layer in direct contact with the metal surface, while leaving the multilayer virtually intact. The main photoinduced reaction was found to be photodissociation of the O–H bond of phenol. Two new peaks were observed at the mass of phenol in the post-irradiation TPD spectrum at 335 K and 455 K. These peaks were assigned to the recombinative desorption of phenoxy with the hydrogen from O–H bond photodissociation and from thermal C–H bond fission, respectively. The photodissociation cross section was measured at different wavelengths and coverages. A charge transfer type photodissociation mechanism was proposed, where hot electrons generated in the substrate by UV photons attach to the affinity level of the adsorbed phenol. The transition to the transient anionic potential then leads to facile dissociation of O–H bond. The affinity level of phenol has been estimated to lie at 3.2–3.5 eV above the Fermi level for the 1 ML case.
Significant reduction of the microwave surface resistance of MgB2 films by surface ion milling
Authors: Lee Sang Young , Lee J. H. , Lee Jung Hun , Ryu J. S. , Lim J. , Moon S. H. , Lee H. N. , Kim H. G. , Oh B.
Published: July 2002

The microwave surface resistance RS of MgB2 films with the zero-resistance temperature of ∼39 K was measured at 8.0–8.5 GHz. The MgB2 films were prepared by deposition of boron films on c-cut sapphire, followed by annealing in a magnesium vapor environment. The RS appeared significantly reduced by ion milling of the as-grown MgB2 film surface, with the observed RS of ∼0.8 mΩ at 24 K for an ion-milled MgB2 film as small as 1/15 of the value for the corresponding as-grown MgB2 film. The reduced RS of the ion-milled MgB2 films is attributed to the effects of the Mg-rich metallic layer existing at the surfaces of the as-grown MgB2 films.
Spiked harmonic oscillators
Authors: Hall Richard L. , Saad Nasser , von Keviczky Attila B.
Published: July 2002

A complete variational treatment is provided for a family of spiked-harmonic oscillator Hamiltonians H=−d2/dx2+Bx2+λ/xα(B>0,λ>0), for arbitrary α>0. A compact topological proof is presented that the set S={ψn} of known exact solutions for α=2 constitutes an orthonormal basis of the Hilbert space L2(0,∞). Closed-form expressions are derived for the matrix elements of H with respect to S. These analytical results, and the inclusion of a further free parameter, facilitate optimized variational estimation of the eigenvalues of H to high accuracy.
Depth and lateral extension of ion milled pn junctions in CdxHg1−xTe from electron beam induced current measurements
Authors: Haakenaasen R. , Moen T. , Colin T. , Steen H. , Trosdahl-Iversen L.
Published: July 2002

Ion milling has been used to convert molecular beam epitaxy vacancy-doped CdxHg1−xTe from p- to n-type. Electron beam induced current and remote electron beam induced current (REBIC) measurements have been performed to study the pn junction depth and lateral extension dependence on the milling time, milling current, and vacancy concentration. The conversion depth is linear with the milling time and current and inversely proportional to the vacancy concentration in layers thinner than 10 μm. This shows that filling of Hg vacancies in this region during conversion is limited by the rate of supply of extra Hg from the milling. The lateral extension also increases linearly with the milling time, the ratio of the lateral extension to the depth being ∼0.5. One can therefore use REBIC on the top surface to determine the junction depth, which greatly simplifies the measurement and does not destroy the diodes.
Microwave liquid crystal wavelength selector
Authors: Yang Fuzi , Sambles J. R.
Published: July 2002

The combined use of a zero-order grating with liquid crystals to control microwaves is presented. A nematic liquid crystal is aligned in the 75 μm gaps of a grating comprised of 1-mm-thick aluminum slats. A set of resonant transmission frequencies is recorded for microwaves incident on this structure with their electric field perpendicular to the slats. These resonances are due to the excitation of coupled surface plasmons. A voltage applied between adjacent pairs of slats causes the liquid crystal to realign, allowing sensitive control of the transmitted microwave frequencies.
On analytic properties of higher analogs of the second Painlevé equation
Authors: Li Yezhou , He Yuzan
Published: July 2002

In the present paper, we investigate value distribution of the higher analogs of the second Painlevé equation (vP2). After getting the precise expression of the dominant term of (vP2), we obtain some value distribution properties for the transcendental meromorphic solutions of (vP2) in the general case which generalize the theorems in Gromak and He [Proc. Math. Inst. Belarus Natl. Acad. Sci. 4, 37–48 (2000)]. We further get other results for the (vP2) and (4P2).
Characterization of high dose Fe implantation into p-GaN
Authors: Theodoropoulou N. , Hebard A. F. , Chu S. N. G. , Overberg M. E. , Abernathy C. R. , Pearton S. J. , Wilson R. G. , Zavada J. M.
Published: July 2002

High concentrations (3–5 at. %) of Fe were incorporated into p-GaN by direct implantation at elevated substrate temperature (350 °C). Subsequent annealing at 700 °C produced apparent ferromagnetic behavior up to ∼250 K for the 3 at. % sample. Selected area diffraction patterns did not reveal the presence of any other phases in the Fe-implanted region. The direct implantation process appears promising for examining the properties of magnetic semiconductors with application to magnetotransport and magneto-optical devices.
Evaporation of liquid during cone-jet mode electrospraying
Authors: Grigoriev D. A. , Edirisinghe M. J.
Published: July 2002

Heating and evaporation in the liquid meniscus during the cone-jet mode of electrospraying have been investigated. An expression for the ratio between emitted and evaporated liquid volumes has been developed and verified by experiment. This work indicates how the emitted liquid yield can be increased.
Nonlinear magneto-optic measurement of flux propagation dynamics in thin Permalloy films
Authors: Silva T. J. , Pufall M. R. , Kabos Pavel
Published: July 2002

Time-resolved nonlinear optics are used to study the propagation of magnetic flux pulses in a 250 nm Permalloy film. The flux is generated in the film by coupling it to a coplanar waveguide structure driven with broadband voltage pulses. Flux pulses propagated in the film with a phase velocity of 4.2×105 m/s and a group velocity of 1.5×105 m/s. Both velocities are consistent with the predictions of Damon–Eshbach theory for magnetostatic surface waves with 200–300 μm wavelengths. Within 100 μm of the excitation source, flux pulses decayed monotonically but with no measurable delay.
Capture and thermal re-emission of carriers in long-wavelength InGaAs/GaAs quantum dots
Authors: De Giorgi M. , Lingk C. , von Plessen G. , Feldmann J. , De Rinaldis S. , Passaseo A. , De Vittorio M. , Cingolani R. , Lomascolo M.
Published: July 2002

We investigate the ultrafast carrier dynamics in metalorganic chemical vapor deposition-grown InGaAs/GaAs quantum dots emitting at 1.3 μm. Time-resolved photoluminescence upconversion measurements show that the carriers photoexcited in the barriers relax to the quantum-dot ground state within a few picoseconds. At low temperatures and high carrier densities, the relaxation dynamics is dominated by carrier–carrier scattering. In contrast, at room temperature, the dominant relaxation process for electrons is scattering between quantum-dot levels via multiple longitudinal optical (LO)-phonon emission. The reverse process, i.e., multiple LO-phonon absorption, governs the thermal re-emission of electrons from the quantum-dot ground state.
Copper doping of silicate glasses by the ion-exchange technique: A photoluminescence spectroscopy study
Authors: Borsella E. , Dal Vecchio A. , Garcı̀a M. A. , Sada C. , Gonella F. , Polloni R. , Quaranta A. , van Wilderen L. J. G. W.
Published: July 2002

Copper–alkali ion exchange is used for doping superficial layers of different silicate glasses (commercial soda-lime and BK7) with copper ions. Spectroscopic and time-resolved photoluminescence properties of the obtained systems are studied in the range of 80–294 K. Analysis indicates the presence of Cu+ ions located in distorted octahedral sites, and a different position of the triplet electronic levels for the two glass matrices. The luminescence decay-time signal is simulated by a biexponential behavior, interpreted on the basis of a four-level scheme.
Chemical intermixing at FeMn/Co interfaces
Authors: Luo G. M. , Jiang H. W. , Liu C. X. , Mai Z. H. , Lai W. Y. , Wang J. , Ding Y. F.
Published: July 2002

Elemental distributions of Fe, Mn, and Co in ultrathin FeMn/Co multilayers have been profiled by the x-ray anomalous scattering technique. Chemical intermixing is observed at the FeMn/Co interfaces with intermixing regions up to 14 Å. The chemical compositions in the intermixing regions are found to vary gradually from FeMn to FeMnCo2 and from FeMnCo2 to Co., respectively, but not from FeMn to Co. This result suggests that a reaction may occur and FeMnCo2 appears at the interface during the deposition.
Suppression of radiatively generated currents in infrared detectors
Authors: Gordon Neil T. , Maxey Chris D. , Jones Chris L. , Catchpole Rose , Hipwood Les
Published: July 2002

The radiative contribution to the dark current in an infrared detector has been measured over the temperature range 200–300 K. Good agreement was obtained between the measured currents and a theory based on radiative coupling between neighboring elements in the array. Detectors have been made which are close to radiatively limited and which become background limited in f/2 for temperatures below 200 K. By reverse biasing neighboring elements it has been possible to switch off up to 25% of the radiatively generated component of the dark current, demonstrating that radiative generation need not be a fundamental limitation for infrared detector arrays.
Switching dynamics and write endurance of magnetic tunnel junctions
Authors: Bauer M. , Lopusnik R. , Fassbender J. , Hillebrands B. , Bangert J. , Wecker J.
Published: July 2002

The switching dynamics of magnetic tunnel junctions has been studied by means of time resolved magneto-optic Kerr magnetometry. Magnetic field pulses as short as 250 ps are found to be sufficiently long to switch the storage content of the element. In order to test the write endurance the magnetization of one single element has been reversed 1011 times. Shortly after the initialization of the hard magnetic layer the magnetization reversal process of the soft magnetic layer remains unchanged, indicating that no further degradation of the pinned layer comes into effect.
Delivery of femtosecond pulses by flexible hollow fibers
Authors: Matsuura Yuji , Miyagi Mitsunobu , Shihoyama Kazuhiko , Kawachi Masako
Published: July 2002

A flexible, hollow glass fiber is proposed as a delivery medium of femtosecond pulses. It was shown that ultrashort pulses with a pulse width of 196 fs, energy of 700 μJ, and repetition rate of 1 kHz are transmitted by the hollow fibers with no damage on the fiber. In the fiber with an inner diameter of 1 mm, the transmission loss is as low as 0.13 dB and the pulse broadening after transmission in 1 m long fiber is 17 fs. It is also shown that the numbers of transmitted modes in the fiber are estimated from the pulse broadening width.
Influences on ionization fraction in an inductively coupled ionized physical vapor deposition device plasma
Authors: Juliano Daniel R. , Ruzic David N. , Allain Monica M. C. , Hayden Douglas B.
Published: July 2002

A computer simulation was created to model the transport of sputtered atoms through an ionized physical vapor deposition (IPVD) system. The simulation combines Monte Carlo and fluid methods to track the metal atoms that are emitted from the target, interact with the IPVD plasma, and are eventually deposited somewhere in the system. Ground-state neutral, excited, and ionized metal atoms are tracked. The simulation requires plasma conditions to be specified by the user. Langmuir probe measurements were used to determine these parameters in an experimental system in order to compare simulation results with experiment. The primary product of the simulation is a prediction of the ionization fraction of the sputtered atom flux at the substrate under various conditions. This quantity was experimentally measured and the results compared to the simulation. Experiment and simulation differ significantly. It is hypothesized that heating of the background gas due to the intense sputtered atom flux at the target is primarily responsible for this difference. Heating of the background gas is not accounted for in the simulation. Difficulties in accurately measuring plasma parameters, especially electron temperature, are also significant.
Residual water modulates the dynamics of the protein and of the external matrix in “trehalose coated ” MbCO: An infrared and flash-photolysis study
Authors: Librizzi Fabio , Viappiani Cristiano , Abbruzzetti Stefania , Cordone Lorenzo
Published: July 2002

We report on dynamic properties of carbon monoxy myoglobin (MbCO) embedded in trehalose matrices of different water content, studied by FTIR spectroscopy and CO rebinding after flash-photolysis. FTIR spectroscopy was used to study the thermal behavior of the bound CO stretching and of the adjacent bands arising from trehalose and residual water, as a function of the sample water content. These measurements enabled us to get information on the relation between the interconversion among A substates (as evidenced by the thermal behavior of the CO stretching band) and the dynamics of the trehalose-water matrix. Under condition of drought, the protein internal dynamics is tightly coupled to the dynamics of the external matrix and is modulated by traces of residual water. Under such condition, substates interconversion is hindered due to extreme increase of energy barriers. At variance sizeable substates interconversion takes place following a small water uptake, obtained by exposure of the dry sample to a nondry surrounding atmosphere. FTIR results were in full agreement with flash-photolysis data. In particular, the heme pocket dynamics, which regulates the migration of the flashed off CO molecule within the protein matrix, can be well described on the basis of the information obtained by FTIR measurements. Under condition of drought, in which vanishing temperature dependence of the CO stretching band is observed, the rebinding kinetics is governed by quasi-static rate constant distributions. At variance, stretched exponentials describe geminate rebinding following slight water uptake by the sample, which, as shown by FTIR, brings about interconversion among conformational A substates.
Baxter T-Q equation for shape invariant potentials. The finite-gap potentials case
Authors: Lipan Ovidiu , Rasinariu Constantin
Published: July 2002

The Darboux transformation applied recurrently on a Schrödinger operator generates what is called a dressing chain, or from a different point of view, a set of supersymmetric shape invariant potentials. The finite-gap potential theory is a special case of the chain. For the finite-gap case, the equations of the chain can be expressed as a time evolution of a Hamiltonian system. We apply Sklyanin’s method of separation of variables to the chain. We show that the classical equation of the separation of variables is the Baxter T-Q relation after quantization.
Moisture-assisted subcritical debonding of a polymer/metal interface
Authors: Kook Seung-Yeop , Dauskardt Reinhold H.
Published: July 2002

Subcritical debonding of polymer/metal interfaces has important implications for the reliability of a range of modern device technologies containing thin layered structures, although the physics of the underlying crack-growth mechanisms are not well understood. This study investigates the effect of moisture on subcritical debonding at the interface between a silica-filled epoxy resin and a copper substrate electroplated with a nickel layer. Subcritical debond-growth rates in the range of 10−5 to 10−11 m/s were characterized as a function of the applied strain energy release rate, GA. Particular attention was given to the relationship between the debond tip strain energy release rate, Gtip, and the measured debond growth rates. The kinetics of interfacial debond growth were rationalized in terms of a stress-dependent chemical reaction and mass transport of the environmental species to the debond tip. The order of the chemical reaction with respect to water was determined and found to be surprisingly high (n∼5). In the transport-controlled region, the rate of debond growth was found to increase linearly with the partial pressure of water vapor. The effects of salient interface parameters including the peak stress in a cohesive zone used to model the fracture mechanism were explored.
Orientational dynamics of the glass forming liquid, dibutylphthalate: Time domain experiments and comparison to mode coupling theory
Authors: Brace David D. , Gottke S. D. , Cang H. , Fayer M. D.
Published: July 2002

Orientation dynamics of the glass forming liquid, dibutylphthalate (DBP), were studied using optical heterodyne detected optical Kerr effect (OHD-OKE) techniques. A combination of experimental methods permitted acquisition of data over 6 decades of time and 5 decades in signal amplitude. Data collected from several hundred fs to several hundred ns, cover the full range of orientational dynamics. The data are compared to the predictions of ideal mode-coupling theory (MCT). Two of the MCT scaling law predictions yield an average value of 227 K for the MCT critical temperature, Tc. Measurements were made at temperatures below Tc. The data were found to agree well with some of the predictions of MCT. The long time scale structural relaxation (α relaxation) obeys the MCT scaling law, and the slowest time scale power law (the von Schweidler power law) is also in accord with MCT predictions. However, the “master curves” predicted by MCT fit the data well only at higher temperatures. The master curves show increasingly significant deviations at shorter times (<100 ps) as the temperature is decreased, in contrast to previous OHD-OKE studies on salol [G. Hinze, D. D. Brace, S. D. Gottke, and M. D. Fayer, J. Chem. Phys. 113, 3723 (2000)] and ortho-terphenyl [S. D. Gottke, G. Hinze, D. D. Brace, and M. D. Fayer, J. Phys. Chem. B 105, 238 (2000)] in which the agreement with ideal MCT is far superior. The DBP data show no discontinuity in the trends as the temperature is reduced below Tc, but an increasingly prominent peak appears in the data at ∼2.5 ps. The peak may be related to the boson peak.
Structural relation between Si and SiC formed by carbon ion implantation
Authors: Eichhorn F. , Schell N. , Mücklich A. , Metzger H. , Matz W. , Kögler R.
Published: July 2002

The formation of crystalline SiC by implantation of C ions into silicon is not a single-step process. The implantation results in an elastic distortion of the Si matrix lattice and in the formation of crystalline SiC particles, depending on ion fluence and thermal conditions during implantation and postannealing. The growth of the SiC particles in the Si matrix was studied with various synchrotron x-ray scattering techniques and high-resolution transmission electron microscopy. Crystallites of the 3C–SiC polytype are formed in a buried layer. Three groups of crystallites with different orientation relative to the Si matrix are found: with a random orientation like in a powder material, with a fiber texture axis parallel to the surface normal, and completely aligned to the Si matrix lattice due to a partially coherent growth of SiC in the Si matrix. The thermal treatment favors the growth of highly oriented material: a higher implantation temperature is more efficient than a postimplantation treatment even at higher temperatures.
Effect of lattice strain and defects on the superconductivity of MgB2
Authors: Serquis A. , Zhu Y. T. , Peterson E. J. , Coulter J. Y. , Peterson D. E. , Mueller F. M.
Published: July 2002

The influence of lattice strain and Mg vacancies on the superconducting properties of MgB2 samples has been investigated. High quality samples with sharp superconducting transitions were synthesized. The variations in lattice strain and Mg vacancy concentrations were obtained by varying the synthesis conditions. It was found that high strain (∼1%) and the presence of Mg vacancies (∼5%) resulted in lowering the Tc by only 2 K.
Characterization of InGaSb by photoreflectance spectroscopy
Authors: Collins S. , Birdwell A. G. , Glosser R. , Bennett Brian R.
Published: July 2002

Photoreflectance (PR), a contactless form of electromodulation, is used in a study of the In1−xGaxSb alloy series. The samples consist of epilayers of In1−xGaxSb (0.780⩽x⩽0.900) grown by molecular beam epitaxy onto (100) GaSb substrates, resulting in percent mismatches between the substrate and layers ranging from 0.62% to 1.36%. These relatively small mismatches result in the epilayers being under a considerable amount of compressive stress. The effects of this strain field on the optical properties are seen through shifts and splittings of the PR spectra. These are exploited to determine the relative changes in the electronic band structure of the material. It is found that the shifting of the heavy-hole and spin orbit split off valence bands correspond well with the theory, while the light hole bands do not. A possible explanation for this is given. Results of a study of the compositional dependence of the In1−xGaxSb energy gaps are also presented.
Studies on ECR4 for the CERN ion program
Authors: Hill C. E. , Küchler D. , Scrivens R. , Wenander F.
Published: July 2002

The CERN heavy ion community, and some other high energy physics experiments, are starting to demand other ions, both heavy and light, in addition to the traditional lead ions. Studies of the behavior of the afterglow for different operation modes of the ECR4 at CERN have been continued to try to understand the differences between pulsed afterglow and continuous operation, and their effect on ion yield and beam reproducibility. The progress in adapting the source and ion beam characteristics to meet the new demands will be presented, as will new information on voltage holding problems in the extraction.
Kinetics and reaction mechanisms of laser-assisted chemical vapor deposition of polycrystalline silicon dots from silane
Authors: Pauleau Y. , Tonneau D.
Published: July 2002

Polycrystalline silicon dots have been deposited on silicon-coated SiO2 substrates from silane irradiated with a continuous wave (cw) argon ion laser and a cw CO2 laser. Various experimental configurations were adopted for laser-assisted chemical vapor deposition of Si dots. The cw argon ion laser was focused either directly on the surface of absorbent polycrystalline Si films acting as substrates (front side visible laser illumination) or penetrated through the transparent SiO2 plate and focused at the SiO2–Si interface (rear side visible laser illumination). The cw CO2 laser beam penetrated through the transparent polycrystalline Si film and was absorbed into the SiO2 plate (front side IR laser illumination). The deposition rate of polycrystalline Si dots was investigated as a function of the laser power (or surface temperature which was proportional to the laser power) and silane pressure. With the direct visible laser irradiation of Si/SiO2 substrates (front side laser illumination), at a surface temperature in the range 700–1000 °C, the growth kinetics of polycrystalline Si dots was found to be limited by surface reactions. The reaction mechanism proposed for chemical vapor deposition of Si films from pyrolysis of silane in conventional reactors can be invoked to explain the kinetic data, i.e., the rate-limiting step is the release of hydrogen from the Si surface. Visible photon interactions with reactive species were found to promote the release of hydrogen since this kinetic regime was not observed with other experimental configurations investigated. The deposition rate of polycrystalline Si dots produced at a surface temperature in the range 1100–1410 °C was proportional to the laser power or surface temperature and silane pressure. In addition, the deposition rate was independent of the experimental configuration used. In other words, only pyrolytic effects are involved in the laser-induced deposition of Si dots even with the IR laser light which is absorbed by silane molecules. With the gas phase in the fluid regime, the rate-limiting step was probably the diffusion of reactive species through the boundary layer developed in the vicinity of the surface of the deposition zone. Silylene radicals, SiH2, formed in the gas phase from inelastic collisions between excited silane molecules seem to be the major reactive species. A reaction mechanism of laser-assisted chemical vapor deposition of polycrystalline Si dots involving the formation and diffusion of silylene radicals through silane which acts as a buffer gas with respect to silylene radicals is proposed and discussed in this article.
Accurate impact ionization model which accounts for hot and cold carrier populations
Authors: Grasser T. , Kosina H. , Heitzinger C. , Selberherr S.
Published: July 2002

Conventional macroscopic impact ionization models which use the average carrier energy as a main parameter can not accurately describe the phenomenon in modern miniaturized devices. Here, we present a model which is based on an analytic expression for the distribution function. In particular, the distribution function model accounts explicitly for a hot and a cold carrier population in the drain region of metal–oxide–semiconductor transistors. The parameters are determined by three-even moments obtained from a solution of a six-moments transport model. Together with a nonparabolic description of the density of states, accurate closed form macroscopic impact ionization models can be derived based on familiar microscopic descriptions.
Tailoring the surface morphology of amorphous thin films by appropriately chosen deposition conditions
Authors: Mayr S. G. , Samwer K.
Published: July 2002

Amorphous thin film growth on a substrate by cocondensation is dominated by growth instabilities arising from self shadowing and surface diffusion, and these lead to pronounced three dimensional growth. It is possible to influence structure formation profoundly through systematic variation of the deposition characteristics of the particles, i.e., the deposition energy, the deposition angle, and the angle distribution. Experimentally, this can be achieved by varying the deposition technique from vapor deposition to sputtering, and changing the deposition angle with or without simultaneous rotation of the substrate. While roughening can be enhanced by oblique particle incidence, sputtered amorphous films have smooth surfaces. This behavior can be understood in terms of shadowing and energy transfer effects as illustrated by Monte Carlo and continuum growth models.
Graded mixed-layer organic light-emitting devices
Authors: Chwang Anna B. , Kwong Raymond C. , Brown Julie J.
Published: July 2002

We describe the performance of graded, mixed-layer organic light- emitting devices (OLEDs). The devices are step graded from a mostly hole transporting layer (HTL) to a mostly electron transporting layer (ETL) from anode side to cathode side, respectively. Luminous efficiencies of >4.5 lm/W and 10 cd/A are obtained at 1000 cd/m2 for green, electrofluorescent, graded mixed OLEDs. These efficiencies are significantly higher than those of a uniformly mixed device, i.e., a device in which the HTL and ETL are uniformly mixed, but lower than those of a conventional heterostructure device employing the same dopant material. The operating lifetime of the graded mixed OLEDs, however, is much improved over the heterostructure device. The results of our work suggest that the graded mixed OLED device structure represents a path to achieving extended lifetimes with sufficient efficiency for flat panel display applications in which this parameter is critical to market acceptance.
Collisional and radiative processes in high-pressure discharge plasmas
Authors: Becker Kurt H. , Kurunczi Peter F. , Schoenbach Karl H.
Published: July 2002

Discharge plasmas at high pressures (up to and exceeding atmospheric pressure), where single collision conditions no longer prevail, provide a fertile environment for the experimental study of collisions and radiative processes dominated by (i) step-wise processes, i.e., the excitation of an already excited atomic/molecular state and by (ii) three-body collisions leading, for instance, to the formation of excimers. The dominance of collisional and radiative processes beyond binary collisions involving ground-state atoms and molecules in such environments allows for many interesting applications of high-pressure plasmas such as high power lasers, opening switches, novel plasma processing applications and sputtering, absorbers and reflectors for electromagnetic waves, remediation of pollutants and waste streams, and excimer lamps and other noncoherent vacuum-ultraviolet light sources. Here recent progress is summarized in the use of hollow cathode discharge devices with hole dimensions in the range 0.1–0.5 mm for the generation of vacuum-ultraviolet light.
Dissipation of grid turbulence in helium II
Authors: Stalp Steven R. , Niemela J. J. , Vinen W. F. , Donnelly Russell J.
Published: July 2002

The results of a number of recent experiments on high Reynolds number grid turbulence in helium II suggest that its flow on large length scales resembles that of a classical fluid. It has been known for some time that the effective kinematic viscosity of this turbulent fluid, describing energy flow in the inertial range of wave numbers, is of the order of η/ρ where η is the normal fluid viscosity and ρ is the total density of the liquid. However, dissipation must be strongly influenced by quantum processes, and it cannot be associated simply with the normal-fluid viscosity. The importance of quantum processes arises because the dissipation occurs at small length scales, comparable with the spacing of the quantized vortex lines that allow turbulent motion in the superfluid component. We report an analysis of experimental data that allows us to deduce experimental values of the effective kinematic viscosity, which we call ν′(≠η/ρ), to which theories of the quantum dissipative processes can be compared.
Spontaneous thermocapillary interaction of drops: Effect of surface deformation at nonzero capillary number
Authors: Berejnov V. , Leshanksy A. M. , Lavrenteva O. M. , Nir A.
Published: July 2002

The axisymmetric spontaneous thermocapillary induced motion and deformation of two viscous droplets caused by interfacial mass transfer is considered. Convective transport and inertial effects are neglected. The case of slightly deformable drops at close proximity was analyzed making use of lubrication approximation and tangent sphere coordinates. The effects of the drop-to-medium viscosity ratio and drops size ratio on the motion is illustrated. For cases with significant deformations, solute concentration and velocity fields are computed using boundary-integral techniques for Laplace and Stokes equations, respectively. Numerical results on axisymmetric drop motion, deformations and temporal evolution of the separation between the drops are presented for equal viscosities of the drops and the ambient fluid.
Temperature dependence of ternary rate coefficients for the (CO)n−1++2CO⇌(CO)n++CO reaction, and the role of isomers for the growth of larger (CO)n+ clusters
Authors: Schlemmer S. , Luca A. , Glosik J. , Gerlich D.
Published: July 2002

Ternary rate coefficients for the title reaction have been measured using a rf 22-pole ion trap in the temperature range 65–300 K. For the formation of dimers, the ternary rate coefficient, k3, follows the power law 2.3×10−28 cm6/s×(300 K/T)m, with m=1.6±0.2 over the full temperature range of the experiment. This result is in good agreement with previous measurements at higher temperatures and also with theoretical predictions. The formation of trimers is represented by k3=1.3×10−29 cm6/s×(300 K/T)m, with m=3.2±0.3. Including into the evaluation also previously published data measured at higher temperatures, leads to an even steeper temperature dependence. These findings are tentatively explained by the fact that (CO)2+ is a nonlinear molecule with low lying vibrational modes. The formation of tetramers, (CO)4+, is very slow and shows a positive or near zero temperature dependence. This behavior is explained by the presence of two isomers. In order to pass this bottleneck and to reach (CO)5+ and larger clusters, up to (CO)9+, a short but intense gas pulse has been injected into the trap. In contrast to previous high pressure studies where (CO)6+ appears to be especially stable, (CO)7+ turns out to be a favored cluster in the 80 K trap experiment. Also this result can be explained by the existence of two isomers, in which four CO molecules are loosely arranged around a strongly bound (CO)2+ or (CO)3+ core.
Photodissociation dynamics of acetyl bromide at 234 nm
Authors: Lee Keon Woo , Jee Yun-Jung , Jung Kyung-Hoon
Published: July 2002

The photodissociation dynamics of acetyl bromide was investigated at 234 nm. A two-dimensional photofragment ion-imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme was utilized to obtain speed and angular distributions of the nascent Br(2P3/2) and Br*(2P1/2) atoms. The recoil anisotropies for the Br and Br* channels were measured to be β=1.47±0.05 for Br and 1.44±0.05 for Br*. The relative quantum yield for Br* was found to be ΦBr*=0.37±0.05. The probability of nonadiabatic transition between two A′ states was determined to be 0.36. Broad Gaussian distributions were observed for the total translational energy distributions corresponding to the Br and Br* channels. The soft radical limit of the impulsive model adequately modeled the related energy partitioning. It is proposed that the σ*←n transition localized on the C–Br chromophore mainly contributes to the initial transition of acetyl bromide at 234 nm.
Transition in multiple-scale-lengths turbulence in plasmas
Authors: Itoh S.-I. , Itoh K. , Yagi M. , Kawasaki M. , Kitazawa A.
Published: July 2002

The statistical theory of strong turbulence in inhomogeneous plasmas is developed for the cases where fluctuations with different scale lengths coexist. Statistical nonlinear interactions between semimicro and micro modes are first kept in the analysis as the drag, noise and drive. The nonlinear dynamics determines both the fluctuation levels and the cross field turbulent transport for the fixed global parameters. A quenching or suppressing effect is induced by their nonlinear interplay, even if both modes are unstable when analyzed independently. Influence of the inhomogeneous global radial electric field is discussed. A new insight is given for the physics of the internal transport barrier. The thermal fluctuation of the scale length of λD is assumed to be statistically independent. The hierarchical structure is constructed according to the scale lengths. Transitions in turbulence are found and phase diagrams with cusp type catastrophe are obtained. Dynamics is followed. Statistical properties of the subcritical excitation are discussed. The probability density function (PDF) and transition probability are obtained. Power laws are obtained in the PDF as well as in the transition probability. Generalization for the case where turbulence is composed of three classes of modes is also developed. A new catastrophe of turbulent states is obtained.
Low frequency responses, waves and instabilities in dusty plasmas
Authors: Tsytovich V. N. , de Angelis U. , Bingham R.
Published: July 2002

The charging collisions of electrons and ions with dust particles and the collisions of all particles with neutral atoms are self-consistently taken into account to study low-frequency modes and instabilities in dusty plasmas. These were previously treated with a multicomponent approach for fixed grain charge or taking into account the dust charge variations but not the effect of the electron/ion–dust collisions on the plasma distributions and response. A new general expression for the dusty plasma response is found and examples of waves and instabilities in dusty plasmas are considered for a broad range of frequencies lower than both the charging frequency and the frequency of plasma particle interactions with dust. Suggestions for future experiments are given.
Nonlocal k-space interaction and the rare existence of coherent structures in the tokamak edge plasma
Authors: Antar G. Y. , Devynck P. , Fenzi C.
Published: July 2002

The relation between turbulent density fluctuations occurring at two wave numbers is investigated at the plasma edge of the Tore Supra tokamak [G. Antar et al., Plasma Phys. Controlled Fusion 41, 947 (1998)]. To achieve this goal, light scattering is used with two laser beams performing a Fourier transform about two given wave vectors. Consequently a band-pass filter is applied to the spatial turbulent density fluctuations about each wave vector. When the separation is small between the analyzing wave numbers, it is found that the cross correlation between the two channels is dominated by the autocorrelation caused by the diagnostic resolution in the wave number space. For large separations, the instrumental resolution contribution is negligible but a cross correlation amplitude of about 1% to 2% is still detected. It reflects nonlocal coupling in the k space caused by eddy organization rarely taking place. In agreement with this behavior, a rather small deviation of the probability distribution from a Gaussian is reported.
Exchange coupling effects in nanocrystalline alloys studied by Monte Carlo simulation
Authors: Crisan O. , Labaye Y. , Berger L. , Coey J. M. D. , Greneche J. M.
Published: July 2002

The magnetic behavior of nanocrystalline alloys has been modeled using atomic Monte Carlo simulation. The model consists of a cubic lattice composed of a ferromagnetic nanograin in a ferromagnetic matrix. The magnetizations of nanograin core, surface and interface regions and matrix were studied as a function of the exchange coupling between the nanograin and the matrix, as well as of the nanograin/matrix volume ratio, equivalent to the crystalline fraction in the nanocrystalline alloys. The mechanism of polarization of the matrix by fields penetrating from the nanograin is discussed and correlated with the matrix–nanograin exchange coupling. Competition between interface anisotropy and magnetocrystalline anisotropy produces spin-glass-like magnetic order of the interfacial regions.
Amplification of small perturbations in a Hartmann layer
Authors: Gerard-Varet D.
Published: July 2002

We investigate in this paper the stability of the laminar Hartmann layer to small perturbations. Previous works have shown that the critical Reynolds number for linear stability is Rc≈50 000. It is much higher than the experimental threshold values for laminarization of magnetohydrodynamic flows (150<R<250), highlighting a possible subcritical bifurcation. In the following, we discuss the validity of Rc as a good indicator of stability to small disturbances. We show that, far below Rc, there exist some very small perturbations which do not support the linear approximation. The main reason is the phenomenon of transient growth, due to the non-normality of the governing linear operator. Through numerical computations we determine the optimal amplification of three-dimensional small perturbations, in the absence of nonlinear effects. Then, we examine how nonlinear mechanisms modify the fluid velocity. In this way, we show that some very small perturbations evolve very differently from their linear approximation, and that, in such cases, the critical number Rc loses its physical meaning.
Coulomb scattering in a strong magnetic field
Authors: Hu B. , Horton W. , Chiu C. , Petrosky T.
Published: July 2002

The presence of a strong magnetic field intrinsically changes the orbit of electrons in their Coulomb interactions with ions for a range of parameters. The characteristic scale for the orbit modification is r0=(Zme/4πε0B2)1/3 where Ze is the ion charge, me the electron mass, and B the magnetic field strength. The scale length is comparable to the interparticle spacing when ne1/3r0=(ωpe/ωce)2/3(Z/4π)1/3∼1 where ωpe/ωce are the electron plasma and cyclotron frequencies, respectively. For large angle scattering events we show complex chaotic scattering interaction events for low-energy electrons. The scattering angle has a fractal dependence on the impact parameter in the chaotic scattering intervals. The process is thought to be important in strongly magnetized, low-temperature plasmas, but the overall macroscopic effects remain to be determined. Test particle simulations are presented that show probability distributions are required to describe the outgoing states of the events with fixed impact parameter and energy which specify a unique Rutherford scattering angle.
Atomization energies and enthalpies of formation of the SnBin (n=1–3) gaseous molecules by Knudsen cell mass spectrometry
Authors: Meloni G. , Gingerich K. A.
Published: July 2002

The equilibria involving the gaseous species SnBi, SnBi2, and SnBi3 above the condensed system Bi–Sn contained in a graphite cell have been investigated by the Knudsen effusion technique combined with mass spectrometry. Third law enthalpies for the reactions SnBin(g)=Sn(cond)+nBi(g), n=1–3, were evaluated. By combining the experimental reaction enthalpies with the appropriate thermodynamic data taken from literature, the following atomization energies, ΔaH0o, and enthalpies of formation, ΔfH298.15o, in kJ mol−1, have been derived: SnBi, 191.1±12.0 and 317.5±12.0; SnBi2, 415.2±15.0 and 303.0±15.0; SnBi3, 603.4±18.0 and 323.0±18.0.
Effect of nitrogen and temperature on the electronic band structure of GaAs1−xNx alloys
Authors: Chtourou R. , Bousbih F. , Bouzid S. Ben , Charfi F. F. , Harmand J. C. , Ungaro G. , Largeau L.
Published: July 2002

We have studied the band-gap reduction of GaAs1−xNx alloys with N contents between 0.1% and 1.5% using the absorption measurements in a series of samples grown by molecular-beam epitaxy. At room temperature, we observed a redshift of the band edge of about 205 meV for 1% of nitrogen. To interpret this effect, we assume that the incorporation of nitrogen in GaAs breaks the symmetry of the system and introduces localized N states that are weakly coupled to the extended states of the semiconductor. We have also studied the temperature dependence of the band gap of GaAsN alloys. The band-gap energy difference between 15 and 300 K decreases from 110 meV for GaAs, to 70 meV for GaAs0.985N0.015. All these experimental results can be explained by the band anticrossing model [W. Shan et al., Phys. Status Solidi B 223, 75 (2001)] with a coupling constant CNM of 2.7 eV, taking into account the contribution of tensile strain to the band-gap reduction.
Experimental and theoretical investigations of the O(1D) reaction with cyclopropane
Authors: Wang Chia C. , Shu Jinian , Lin Jim J. , Lee Yuan T. , Yang Xueming , Nguyen Thanh Lam , Mebel Alexander M.
Published: July 2002

In this report, the O(1D)+c-C3H6 reaction has been investigated using the universal crossed molecular beam technique. Angular resolved time-of-flight spectra have been measured for the observed reaction products in a single set of experiments. Four reaction channels have been observed clearly for this reaction. In addition to the OH+C3H5 channel, the two microchannels for C2H4 formation and the H-atom formation channel are also present. Different product angular distributions were measured for the observed product channels, indicating that each reaction channel occurs with distinctive dynamics. Branching ratios for all observed reaction channels have also been estimated. High-level theoretical calculations have been performed on the energetics of this reaction, indicating two major insertion pathways are likely responsible for most of the reaction channels.
Ensemble representable densities for atoms and molecules. III. Analysis of polarized neutron diffraction experiments when several Zeeman levels are populated
Authors: Cassam-Chenaï P.
Published: July 2002

Polarized neutron diffraction experiments conducted at 4.2 K on Cs3CoCl5 crystals are analyzed by using a 4-dimensional model Hilbert space made of ab initio n-electron wave functions of the CoCl42− molecular ion. The magnetic structure factors given by the best ensemble density operator that is representable in our model space are fitted to the experimental ones by optimizing two spin-orbit mixing coefficients of the embedded CoCl42− ions and several configuration interaction coefficients. The optimized density operator can in turn be used to calculate any observable. Here we present density maps of the spin density, and the orbital current density. The method, which is general, gives a goodness of fit, χ2, less than 1 with fewer parameters optimized than other methods employed so far. It provides a new way of gaining information about spin-orbit coupling and the relative contributions of spin and orbital motion to the magnetic properties of an atomic or molecular system. Another interesting finding is that a χ2 less than 1, can be obtained with a spin density of the same sign everywhere in space, leading to the conclusion that spin polarization is within the experimental error. The so-called “collinear approximation” has been avoided in this work although it has been found to be justified for this system. A direct comparison is made between calculated and experimental flipping ratios.
Foldability and the funnel of HP-36 protein sequence: Use of hydropathy scale in protein folding
Authors: Srinivas Goundla , Bagchi Biman
Published: July 2002

Brownian dynamics simulation study of the folding of a model thermostable chicken villin head piece subdomain, a 36-residue protein (HP-36), is carried out using the hydropathy scale of amino acids. The diverse interactions among the amino acid residues are categorized into three classes by introducing a simplified hydrophobic scale. The simulations incorporate all the six different inter- and intraamino acid interactions. The model protein reproduces some of the qualitative features of the complex protein folding, including the funnel-like energy landscape. Although there are several states near the minimum of the folding funnel, we could identify a stable native configuration. In addition, the study reveals a correlation between the contact order, topology, and the stability.