bluuebunny/crossref_metadata_2025 huggingface dataset:
Gyrokinetic turbulence simulation of profile shear stabilization and broken gyroBohm scaling
Authors: Waltz R. E. , Candy J. M. , Rosenbluth M. N.
Published: July 2002

A newly developed continuum gyrokinetic code GYRO has been formulated on a radial grid to operate at finite relative gyroradius in a noncyclic radial annulus with profile variation. The code is used to simulate ion temperature gradient mode turbulence and to demonstrate that gyroBohm scaling can be obtained well above the instability threshold but sufficiently strong profile shear stabilization can break gyroBohm scaling down to Bohm scaling (or worse) near threshold. An adaptive source technique is used to maintain profiles. Clear evidence for nonlocal transport is also found in which the local diffusivity depends on the plasma gradients at some considerable radial distance.
Picosecond amplified spontaneous emission bursts from a molecularly doped organic semiconductor
Authors: Kallinger C. , Riechel S. , Holderer O. , Lemmer U. , Feldmann J. , Berleb S. , Mückl A. G. , Brütting W.
Published: July 2002

We report the dynamics of amplified spontaneous emission (ASE) in thin organic films of tris-(8-hydroxyquinoline)-aluminum (Alq3) doped with small amounts of the laser dye 4-dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). The energy transfer from the initially photoexcited Alq3 to the DCM molecules affects the high excitation density dynamics significantly. The time delay between pulsed photoexcitation and the ASE burst depends on the pumping level. For higher pumping levels the competition between depopulation by ASE and refilling of the DCM states via energy transfer leads to an oscillatory emission with a fundamental frequency of up to 0.2 THz.
Damage evolution and recovery on both Si and C sublattices in Al-implanted 4H–SiC studied by Rutherford backscattering spectroscopy and nuclear reaction analysis
Authors: Zhang Y. , Weber W. J. , Jiang W. , Hallén A. , Possnert G.
Published: July 2002

Damage evolution and subsequent recovery in 4H–SiC epitaxial layers irradiated with 1.1 MeV Al22+ molecular ions at 150 K to ion fluences from 1.5×1013 to 2.25×1014 Al cm−2 were studied by Rutherford backscattering spectroscopy (RBS) and C12(d,p)13C nuclear reaction analysis (NRA) using a 0.94 MeV deuterium (D+) beam in channeling geometry. Disorder on both the Si and C sublattices was measured simultaneously from the RBS scattering and NRA reaction yields. The relative disorder on both sublattices follows a nonlinear dependence on ion fluence that is consistent with a model based on simple defect accumulation and a direct-impact, defect-stimulated process for amorphization. At low ion fluences, the relative disorder on the C sublattice is higher than that on the Si sublattice. Isochronal annealing up to 870 K revealed the existence of three distinct recovery stages at ∼350, 520, and 650 K for low to intermediate damage levels. In highly damaged samples, where a buried amorphous layer is produced, the onset of a fourth recovery stage appears above 800 K. Similar recovery behaviors on both the Si and C sublattices suggests some coupling of recovery processes for Si and C defects.
A neural network model as a globally coupled map and applications based on chaos
Authors: Nozawa Hiroshi
Published: July 2002

First, a neural network model as the globally coupled map (GCM) is proposed. The model is obtained by modification of a Hopfield network model that has a negative self-feedback connection. Second, information processed by this model is interpreted in terms of the variety of the maps acting on the network elements, and a new, dynamic information processing model is described. The search for information using vague keywords, and solution of the traveling salesman problem (TSP) are introduced as applications.
Saturn’s Great White Spots
Authors: Sanchez-Lavega Agustin
Published: July 2002

The term, Great White Spot, is used for large and unusual atmospheric disturbances on the planet Saturn. The phenomenology has been recorded only in five occasions during the last century, and its evolution can be described in terms of four different phases: (i) Onset (first week), outburst and rapid growth of a very bright cloud up to a size of ∼20 000 km; (ii) planetary disturbance (<one month), zonal expansion of the bright cloud elements according to the local wind velocities, whereas the main initial spot remains detached from this latitude band of white clouds; (iii) mature stage (two to three months), attained when the zonal expansion has fully encircled the latitude band. The white band of clouds has a turbulent texture and wavy structures develop along the periphery. The storm nucleus still survives and new outbreaks of smaller-scale bright spots take place at distant longitudes from it; and (iv) evolved and decay stage (≳one to three years), dissipation of the storm nucleus and gradual homogenization of the cloud banding, but with transient localized brightenings. Afterward there are changes in the location and reflectivity of the belts within the active band. Of the five events classified as GWS, three appeared in the equator, one in temperate latitudes (36°N) and the fifth in the subpolar region (60°N), with an average periodicity of 28.5±0.4 yr. This interval is close to the Saturnian year of 29.46 yr, so the outbursts could be linked to the seasonal insolation cycle. We propose that the initial spot could be driven by moist convection from the lower water clouds, whereas the planetary disturbance is the result of a wave dynamical instability, as indicated by cloud morphology and wind measurements.
Spin polarized Auger spectroscopy (invited)
Authors: Landolt M. , Allenspach R. , Mauri D.
Published: July 2002

Observing the spin polarization of Auger electrons is a newly developed method for the investigation of electronic excitations in a magnetic solid. Information can be gained on electron correlation effects, on intrashell exchange interactions, as well as on local magnetizations in composite systems. We present and compare Auger electron spin polarizations from amorphous Fe83B17 and single-crystalline Ni as first examples and discuss what in principle can be learned with respect to magnetism.
Breakdown model of a short plane-parallel gap
Authors: Novak J. P. , Bartnikas R.
Published: July 2002

A two-dimensional model of the high-voltage breakdown has been developed, consisting of the Poisson equation and the conservation equations for electrons, ions, and excited particles. The model is based on the assumption of a low degree of ionization, so that the transport coefficients of the gas are uniquely determined by the local electric field, i.e., the model is limited to the initial stages of the channel formation. It is applied to a short plane-parallel gap in He at atmospheric pressure. The discharge is started by releasing the cloud of electrons near the cathode, and its sequence is followed up to 1.11 μs. Cathode emission is taken to include that due to ion and metastable impact and the photoeffect. The discharge develops from the initial Townsend-type discharge governed by cathode emission and the direct and Penning volume ionization, and progresses to the space-charge dominated stage. At the moment when the calculations are terminated, the maximum densities have already attained values of 1.6×1010, 2.5×1011, and 1.8×1012 cm−3 for the electrons, ions, and excited particles, respectively. The maximum axial component of the space-charge electric field is approximately 3.3 kV cm−1.
Dielectric measurements on substrate materials at microwave frequencies using a cavity perturbation technique
Authors: Dube D. C. , Lanagan M. T. , Kim J. H. , Jang S. J.
Published: July 2002

A cavity perturbation resonance technique suitable for microwave measurements on substrate materials is discussed. The technique makes use of thin rectangular samples placed in a rectangular waveguide cavity (Q∼5000). The availability of advanced microwave measurement equipment makes it possible to record experimental data at several frequencies (five in this present case). The estimated accuracy of measurements is ±2% for dielectric constant and 3×10−4 for dielectric loss. Results are reported in the 8.2–12.4-GHz frequency range for alumina and specially prepared silica.
Photoacoustic studies of excimer laser induced ablation of polymethylmethacrylate
Authors: Ghosh A. P. , Hurst J. E.
Published: July 2002

The ablation of polymethylmethacrylate (PMMA) at both 248 and 193 nm has been studied using photoacoustic techniques. The acoustic responses of PMMA during the ablation process are resolved into a bipolar, thermal component, and a unipolar, ablative component. This latter component is correlated to the center-of-mass velocity of the ablated material. Fluence-dependent studies show that the fragment center-of-mass velocity increases with increasing fluence even after the etch rate has reached saturation. In addition, results are presented suggesting that an increase in the absorption of the laser radiation by the fragments occurs at fluences above ∼4 J/cm2 causing a change in the profile of the ablated area.
Ablative and acoustic response of pulsed UV laser-irradiated vascular tissue in a liquid environment
Authors: Cross F. W. , Al-Dhahir R. K. , Dyer P. E.
Published: July 2002

Pulsed laser studies of the attenuation properties and ablation characteristics of vascular tissue in a liquid environment are reported. Acoustic waves produced by the thermoelastic effect and high fluence ablation have been studied, together with ablative removal rates for diseased and healthy tissue samples using the KrF, XeCl, and frequency-doubled YAG lasers. Tissue attenuation coefficients, ablation thresholds, and time scales are reported and compared with those for samples in air.
Oxidation and annealing of GaAs (100) studied by photoreflectance
Authors: Seebauer E. G.
Published: July 2002

Photoreflectance (PR) has until now been a technique used primarily for characterization of bulk electronic band structure in semiconductors. However, since reflectance is an interfacial phenomenon, PR should in principle be sensitive to surface treatment as well. The present work appears to be the first quantitative use of PR to measure the effects of surface treatment. In particular, PR spectra of air-oxidized and of annealed GaAs(100) are shown to be substantially different from those of the clean surface. Spectra at the E0 (1.4 eV) and E1 (3.0 eV) transitions respond quite differently to these treatments, and the differences are explained in terms of an optical interference phenomenon due to excitons that occurs only at E0. Effects of slow surface states have been observed with PR for the first time. The effective barrier between these states and the bulk decreases from 21±2 to 16± 2 meV upon air oxidation of the clean surface.
Analysis of Bleustein–Gulyaev wave propagation under thin periodic metal electrodes
Authors: Bright Victor M. , Hunt William D.
Published: July 2002

Bleustein–Gulyaev (BG) piezoelectric surface-wave propagation under a periodic metal-strip grating on piezoelectric cubic crystals is investigated. A general theory that fits both the elastic and electric periodic boundary conditions has been developed and is applicable to a grating with an arbitrary electrical connection between adjacent metal strips. Numerical examples are given for aluminum- and gold-shorted gratings on a gallium arsenide substrate. It is found that the BG waves which penetrate deep into the substrate in the absence of the metal grating become trapped at the surface when the grating is present. It is noted that the penetration depth is heavily dependent on the metal thickness. The relevant physical attributes of the wave such as particle displacement, electric potential, and dispersion are also computed and analyzed.
Electron transport in a resonant-tunneling diode under the effect of a transverse magnetic field: A quantum theory in the Wigner formalism
Authors: Wu G. Y. , Wu K.-P.
Published: July 2002

We present a theoretical study of electron transport in the resonant-tunneling diode under the effects of an in-plane magnetic field. We work in the Wigner formalism. We generalize the formalism to include effects of an in-plane magnetic field. The transport equation is derived for the Wigner distribution function. We solve for the function, from which the charge distribution and the tunnel current are calculated. Discussions of magnetic effects on electron tunneling are presented.
Rate-dependent ductile failure model
Authors: Addessio F. L. , Johnson J. N.
Published: July 2002

A rate-dependent constitutive model for the dynamic deformation of ductile materials is developed. The model introduces a physical length scale into the equations governing the progressive failure of materials due to void growth. Consequently, mesh sensitivity or localization problems inherent to rate-independent models are precluded. The model is implemented into an explicit, finite-difference computer code. The insensitivity of the model to changes in the mesh size is demonstrated. Comparisons are provided between numerical simulations and data for uniaxial impact experiments. Excellent agreement is established between the final porosity levels and the width of the damage zone. Also, excellent agreement is provided for the stress histories, including the peak stress values and the spall signal.
Structural, optical, and dielectric properties of sol-gel derived SrTiO3 thin films
Authors: Kamalasanan M. N. , Deepak Kumar N. , Chandra Subhas
Published: July 2002

Transparent and crackfree SrTiO3 thin films were deposited on silicon wafers, fused silica, and stainless-steel substrates by sol-gel technique. Strontium ethyl hexanoate and titanium isopropoxide were used as starting materials. The surface topology of the films were studied by electron micrography and the structural properties by x-ray diffraction. The refractive index and band gap were measured by optical transmission and absorption spectroscopy. The films show very low leakage current and nearly temperature independent dielectric constant at high frequencies. The dielectric constant and loss factor at 1 kHz at room temperature were 131 and 0.022, respectively. The frequency dependent ac conductivity has been explained on the basis of potential barriers formed by the charge carriers trapped at intercrystalline regions.
Effect of Cl2 additions to an argon glow discharge
Authors: Bassett Nancy L. , Economou Demetre J.
Published: July 2002

A steady-state model has been developed to predict the important species densities and the self-sustaining electric field in a plasma. The effect of excited states and radical species produced in the plasma was taken into account in a self-consistent manner. The model was used to study the effect of attaching gas (Cl2) additions to a noble gas (argon) glow discharge. Specifically a 5% Cl2/95% Ar discharge was compared to a pristine argon discharge. There are dramatic differences between the two discharges. Most notably, the argon metastable density and the electron density are an order of magnitude lower, while the self-sustaining electric field is much higher for the mixture. The electron density increased with pressure in pure argon, but the inverse trend was predicted for the mixture. Results from this work are compared to available experimental data where possible, and reasonable agreement is obtained.
Mössbauer study of ion-beam mixing of Fe/Zr multilayers
Authors: Kopcewicz M. , Jagielski J. , Stobiecki T. , Stobiecki F. , Gawlik G.
Published: July 2002

The Ar-ion-beam mixing of Fe/Zr multilayers is studied in detail by conversion electron Mössbauer spectroscopy (CEMS) and x-ray diffraction (XRD). The dependence of the ion-beam induced amorphization and interfacial mixing on the sublayer thickness and ion dose (1×1013–2×1016 Ar/cm2) is studied systematically for samples with Fe to Zr thickness ratios dFe/dZr=1 and 0.5 and modulation wavelengths Λ=dFe+dZr of 5–80 nm and 7.5–90 nm, respectively. The CEMS results allowed the evaluation of the mixing efficiency from the thickness of the mixed layers. The experimentally determined mixing efficiency was compared with theoretical estimates based on the ballistic collision and thermal spike models, showing good agreement with the predictions of the modified ballistic collision model. For high degrees of amorphization the composition of the amorphous phase formed due to ion-beam mixing is close to the nominal composition of the sample, as revealed by CEMS measurements. These results were compared with those obtained for amorphous Fe-Zr alloys formed by vapor deposition. The XRD results fully agree with CEMS measurements and show that due to ion irradiation the amorphous Fe-Zr phase is formed. The XRD results show that a change of texture occurs from Zr(002) to Zr(100) in the samples with small Λ irradiated with high ion dose. XRD reveals in these samples the formation of the ZrO2 phase.
Resistivity of static and antistatic insulators from surface charge measurement
Authors: Liesegang J. , Senn B. C. , Smith E. R.
Published: July 2002

A method for determining surface charge decay [Q(t)], using a coaxial cylindrical capacitor arrangement and an electrometer interfaced to a PC, has been adapted so as to perform relatively straightforward measurement of resistivity in the surface region of insulators. A charge transport model is presented, based on Mott–Gurney diffusion, which allows interpretation of the data especially for the initial phase of surface charge decay. Resistivity measurements are presented for glass, mica, plexiglas, and polyethylene, covering the range 109–1018 Ω m, as an illustration of the useful range of the instrument for static and antistatic materials, particularly in film or sheet form. Values of surface charge diffusion constants have also been determined for the materials.
Ideal theory of quantum well solar cells
Authors: Anderson Neal G.
Published: July 2002

An ideal model for quantum well solar cells is developed and is used to theoretically explore the dependence of terminal characteristics on the host cell and quantum well properties. The model, which explicitly treats carrier generation and recombination in the quantum wells, is described and compared with an analogous ideal model for bulk homojunction cells. Open-circuit voltages, short-circuit current densities, and conversion efficiencies are then calculated as functions of the well and barrier band gaps for ideal cells in the radiative limit, assuming air-mass-zero (AM0) solar illumination at a cell temperature of 300 K. Qualitative trends in these characteristics and regimes of operation are identified, the effects of non-radiative recombination are explored, and idealized approximations used in the model are assessed. Finally, published experimental data for quantum well solar cells are surveyed and discussed, and are found to exhibit strong qualitative consistencies with predictions from the ideal analysis. Results from this work provide the most comprehensive qualitative picture of quantum well solar cell operation yet proposed, and should serve as a useful guide for designing and interpreting the performance characteristics of quantum well cells fabricated from a wide variety of materials.
Tank circuit model applied to particles in a Penning trap
Authors: Feng X. , Charlton M. , Holzscheiter M. , Lewis R. A. , Yamazaki Y.
Published: July 2002

The equivalent circuit model is used to describe analytically the coupling process of the center of mass motion of a cloud of particles in a Penning trap. From the response of this coupled circuit to white noise a way of nondestructively diagnosing the number of trapped particles is given which is valid for all values of this quantity. Experimental results are presented and compared with this analysis.
Electron spectroscopic analysis of the SiO2/Si system and correlation with metal–oxide–semiconductor device characteristics
Authors: Iwata Seiichi , Ishizaka Akitoshi
Published: July 2002

ESCA (electron spectroscopy for chemical analysis) measurement results on thin SiO2/Si samples are examined comprehensively, critically, and in detail to show that it is possible to correlate these results with MOS (metal–oxide–semiconductor) device characteristics such as flatband (threshold) voltage, oxide breakdown field, mobile-ion density, hole and electron trap density, and hot-carrier lifetime. Up to now, much effort has been made to detect SiOx phases at SiO2/Si interfaces since they are thought to have a significant effect on MOS device characteristics. However, correlating the SiOx phases with device characteristics is difficult and involves overcoming two problems. First, the chemical state is difficult to determine exactly due to x-ray irradiation effects. Second, the amount of defects and impurities which influence device characteristics is usually below the ESCA detection limit (1012–1013 cm−2) in device-quality SiO2/Si samples. Investigation of the first problem led to the conclusion that it is possible to correct for these effects from the x-ray intensity or oxide thickness dependence of the chemical shift. However, accurate (better than ±0.2 eV) chemical state determination is not easy. It is therefore necessary to approach this detection problem from a different viewpoint. Our first attempt involves measuring the ESCA thickness, which decreases when oxide defects like unoxidized Si or uneven thickness (or pinholes) are present, resulting in breakdown field degradation. Our second attempt started while we were studying how to interpret the measured chemical shift. The photoelectron peaks of the SiO2 and the Si can be observed to shift due to small amounts of charged defects and impurities, although they cannot be detected as peaks. This method is considered to be especially useful for characterizing ultrathin (a few nm thick) SiO2/Si samples which are difficult to characterize using conventional C-V (capacitance–voltage) measurements because of tunneling currents. Accordingly, we discuss the data obtained in steady-state and transient peak position measurements of SiO2/Si samples containing 1010–1012 cm−2 of Na (sodium) ions, 1012–1013 cm−2 of hole and electron traps, and 1014–1021 cm−3 of impurities such as P (phosphorus) (in the Si). It is shown that a correlation with MOS characteristics is possible. A close scrutiny of various results concerning x-ray irradiation time, intensity, and oxide thickness dependence of the above peak positions indicates that electric charging during ESCA measurements is correlated to the trap-capturing process. As MOS characteristics are also related to this process, more studies in this direction are needed and will certainly yield more information on the defects influencing the MOS characteristics and the trap-capturing mechanism.
Sonochemical preparation of nanosized amorphous Fe-Ni alloys
Authors: Shafi K. V. P. M. , Gedanken A. , Goldfarb R. B. , Felner I.
Published: July 2002

Nanosized amorphous alloy powders of Fe20Ni80, Fe40Ni60, and Fe60Ni40 were prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO)5 and Ni(CO)4 in decalin, under an argon pressure of 100 to 150 kPa at 273 K. Magnetic susceptibility of Fe40Ni60 and Fe60Ni40 indicates blocking temperatures of 35 K and a magnetic particle size of about 6 nm. Thermogravimetric measurements of Fe20Ni80 give Curie temperatures of 322 °C for amorphous and 550 °C for crystallized forms. Differential scanning calorimetry exhibits an endothermic transition at 335 °C from a combination of the magnetic phase transition and alloy crystallization. The Mössbauer spectrum of crystallized Fe20Ni80 shows a sextet pattern with a hyperfine field of 25.04 T.
Nucleation of misfit and threading dislocations during epitaxial growth of GaSb on GaAs(001) substrates
Authors: Qian W. , Skowronski M. , Kaspi R. , De Graef M. , Dravid V. P.
Published: July 2002

The initial stages of molecular beam epitaxy of GaSb on highly mismatched GaAs(001) substrates were investigated. Transmission electron microscopy was used to analyze the defect structure in GaSb islands and at their interfaces with GaAs(001) at different stages of growth. Based on experimental observations, we propose that the semiperiodic net of 90° misfit dislocations at the GaSb/GaAs(001) interface nucleate homogeneously at the leading edges of advancing {111} planes. After nucleation, they glide inwards along the interface plane to reach their equilibrium position. Threading dislocations in GaSb layers were directly correlated with the misfit dislocation net. We demonstrate that there are no threading defects in GaSb islands when their interfaces consist solely of 90° misfit dislocations, and that threading dislocations in the GaSb epilayer are all associated with minority 60° misfit dislocations nucleated in growing islands. The number of threading dislocations per unit area of the GaSb film is found to be independent of GaSb coverage, indicating that island coalescence does not substantially increase the number of 60° dislocations.
Transmission of stress induced electric signals in dielectric media
Authors: Varotsos P. , Sarlis N. , Lazaridou M. , Kapiris P.
Published: July 2002

The conditions under which pressure (stress) variations on solids, containing charged defects, can lead to the emission of transient electric signals, are discussed. The resulting electric field E varies as 1/d3 (where d denotes the distance from the emitting source), in the simple case when the surrounding medium is homogeneous and isotropic. We show that this behavior changes to 1/d when studying the electric field within a cylindrical channel of radius R and infinite length having conductivity appreciably larger than that of the host medium; this holds up to a certain (reduced) distance d/R, which increases versus the conductivity ratio. We also investigate the variation of the electric field, versus the distance, inside a layer of width w and infinite extent having conductivity appreciably larger than that of the host medium; we then find that the electric field decreases as 1/d2, in a wide range of distances up to a certain value of d/w, which is controlled by the conductivity ratio. In both conductive paths, i.e., cylinder and layer, the electric field approaches the 1/d3 behavior, but only at very large values of d/R and d/w, respectively, reaching the value that would be measured if the host medium was solely present. This implies a high current density inside the path. The case, when the highly conductive path terminates within the host medium, is also discussed; it is found that the “edge effects” play a prominent role in electric field values measured within the host medium, but close to the outcrop of the path. It is shown that a simplified calculation could erroneously obtain electric field values that are several orders of magnitude smaller than those calculated in this article. As an example, the transmission of low frequency electric signals in the earth is discussed. It is concluded that charged-defect generation mechanisms lead to electric field values that are measurable at large distances.
Two-dimensional Z scan for arbitrary beam shape and sample thickness
Authors: Chen P. , Oulianov D. A. , Tomov I. V. , Rentzepis P. M.
Published: July 2002

A two-dimensional Z-scan technique has been developed using a two-dimensional charge-coupled device (CCD) detector to study the nonlinear optical properties and beam profile evolution within optical limiting devices. Using the split step beam propagation method, the far field pattern at each Z position can be calculated precisely for any arbitrary beam shape and sample thickness. A two-dimensional far field pattern is recorded by the CCD detector, and the evolution of the beam distribution inside the nonlinear optical medium can be obtained directly. This technique has been applied to study the nonlinear optical parameters and laser beam profiles in nonlinear liquids. We have used thin and thick samples and Gaussian, top-hat and split Gaussian beam distributions, and the experimental results agreed very well with the calculation. This technique also offers a simple and accurate means for optimizing the design of optical limiting devices.
Mechanism of electrical conduction of Mn-doped β-FeSi2
Authors: Tani Jun-ichi , Kido Hiroyasu
Published: July 2002

In order to clarify the mechanism of electrical conduction of Mn-doped β-FeSi2 (Fe1−xMnxSi2), the Hall effect and electrical resistivity of Fe1−xMnxSi2 (0.01≦x≦0.10) have been measured in the temperature range between 80 and 300 K. The hole concentration of Fe1−xMnxSi2 at 300 K ranges from 8.9×1017 cm−3 for x=0.01 to 1.1×1019 cm−3 for x=0.10, which is almost proportional to the Mn doping concentration (x). The temperature dependence of electrical resistivity of Fe1−xMnxSi2 shows an S-like decay curve at high temperatures. It is concluded that this shape results from the drastic decrease of the mobility with increasing temperature. We present experimental evidence that explains the electrical conduction of Fe1−xMnxSi2 at high temperatures by the impurity scattering mechanism.
Simulation studies of persistent photoconductivity and filamentary conduction in opposed contact semi-insulating GaAs high power switches
Authors: Joshi R. P. , Kayasit P. , Islam N. , Schamiloglu E. , Fleddermann C. B. , Schoenberg J.
Published: July 2002

A self-consistent, two-dimensional, time-dependent, drift-diffusion model is developed to simulate the response of high power photoconductive switches. Effects of spatial inhomogeneities associated with the contact barrier potential are incorporated and shown to foster filamentation. Results of the dark current match the available experiment data. Persistent photoconductivity is shown to arise at a high bias even under the conditions of spatial uniformity. Filamentary currents require an inherent spatial inhomogeneity, and are more likely to occur for low optical excitation. Under strong uniform illumination, the spatial nonuniformities were quenched as a result of a polarization-induced collapse in the internal fields. However, strong electric fields resulting at the contacts create a bipolar plasma, and hence, a virtual “double injection.”
Polarized off-axis Raman spectroscopy: A technique for measuring stress tensors in semiconductors
Authors: Loechelt G. H. , Cave N. G. , Menéndez J.
Published: July 2002

A characterization technique was developed for measuring the complete tensor nature of stress fields in semiconductors. By combining incident light tilted away from the normal axis with polarization of the incident and scattered beams, any Raman-active optical phonon mode can be selectively studied. Once the frequencies and intensities of these phonons are known, the complete stress tensor can be determined. Using this concept, a general, systematic theory and methodology for implementing polarized off-axis Raman spectroscopy was developed that took into account realistic effects which would be encountered in an actual experiment. This methodology was applied to mechanically deformed silicon wafers. By applying loads in different configurations across the wafer, various types of stress were created including tension, compression, and shear. The polarized off-axis Raman technique was validated by comparing its results to both analytic calculations based upon the theory of elasticity and to direct measurements of the wafer curvature using a laser deflection method.
Temperature dependence of Raman scattering in hexagonal gallium nitride films
Authors: Li W. S. , Shen Z. X. , Feng Z. C. , Chua S. J.
Published: July 2002

First-order Raman scattering of hexagonal GaN single crystal films deposited on sapphire substrate by low pressure metal organic chemical vapor deposition is studied between 78 and 870 K. The temperature dependence of the five GaN Raman modes is obtained. Both the linewidth and Raman shift exhibit a quadratic dependence on temperature in our measured temperature range. Excellent agreement was found between the experiment data and calculated results based on a model involving three- and four-phonon coupling. Our results indicate it is necessary to include the contributions of both the thermal expansion and four-phonon terms in the four-phonon anharmonic processes to explain the change of Raman shift and linewidth with temperature. In addition, a decrease in the splitting between the longitudinal optical and transverse optical phonons with increasing temperature was also observed. From these data a weak nonlinear decrease of the transverse effective charge with increasing temperature is derived. The comparison of the transverse effective charge eT* at room temperature was made between experimental data and theoretical calculations by a pseudopotential expression and bond orbital model. Good agreement between theory and experiment is achieved.
Low temperature phase transition of Li0.12Na0.88NbO3 studied by Raman scattering
Authors: Juang Y. D. , Dai S. B. , Wang Y. C. , Hwang J. S. , Hu M. L. , Tse W. S.
Published: July 2002

In this article, we will report the low temperature Raman studies of Li0.12Na0.88NbO3 ceramics at the ferroelectric–ferroelectric phase transitions. The spectral pattern indicates the phase transition occurs at 200 K in the cooling process and at 280 K on heating. The observed spectra are discussed as they relate to the internal stretching modes of the NbO6 octahedron and cation translation modes. The temperature-dependent spectra show that the phase transitions may involve the rearrangement of Na+ cation, the relative orientation of the NbO6 octahedral, and the displacement of Nb ions.
Electronic spectra of NeXe, ArXe, and KrXe using resonantly enhanced multiphoton ionization
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

Two photon resonant, three photon ionization spectra of the heteronuclear rare gas dimers NeXe, ArXe, and KrXe were measured in the energy region of the Xe* 5d and 6p levels. A number of new bands corresponding to two photon transitions from the electronic ground state to bound vibrational levels of the resonant intermediate states were observed. These data provide new information on the dissociation energies and potential curves of the low-lying excited states of these heteronuclear rare gas dimers.
Electron–molecular vibration (e–mv) coupling in charge-transfer compounds and its consequences on the optical spectra: A theoretical framework
Authors: Painelli A. , Girlando A.
Published: July 2002

A general framework for the theory of electron–intramolecular vibration (e–mv) interaction in charge-transfer (CT) compounds is presented, particular attention being devoted to the effect of this interaction on the optical spectra of quasi-one-dimensional organic semiconductors. The common physical basis of different theoretical models of e–mv coupling is evidenced by taking into consideration an isolated CT pair. In particular, it is shown that a complete and substantially equivalent description of the phenomena is provided by the approaches based on the linear response theory or on the Herzberg–Teller coupling scheme, the latter method being developed here to its full extent. The comparison with experimental data shows that a collection of isolated CT pairs can satisfactorily model the consequences of e–mv interaction on the optical spectra of dimerized stack organic semiconductors, either of segregated or mixed type. In addition, we develop here for the first time a model also able to take into account the effect of a nonzero interdimer CT integral. Finally, the case of regular stack CT crystals is analyzed, in such a way, a complete picture of the e–mv (vibronic) perturbation on the optical spectra is obtained, putting in evidence how the spectra can be used to obtain detailed information on the structure and properties of quasi-one-dimensional organic semiconductors. The consequences of the coupling between molecular vibrations and lattice modes are also briefly investigated.
On the dynamics of polaron formation in a deformable medium
Authors: Brown David W. , Lindenberg Katja , West Bruce J.
Published: July 2002

We consider the standard model for exciton transport in which the energy of interaction between the exciton and the lattice is linear in the lattice coordinates and local in the exciton coordinates. We focus on the dynamics of polaron formation by restricting our analysis to the immobile exciton limit in which the model is exactly solvable. We study the evolution of the many-body system from initial states representing a bare exciton in a quiescent bare lattice and a polaron in a quiescent dressed lattice. The total energy, the dynamical energy content of the bare lattice, the deformation of the bare lattice, and many-body Green’s functions are calculated exactly. The reduced density matrix appropriate to each initial condition is obtained exactly for any temperature.
Time-dependent coupled cluster approach: Excitation energy calculation using an orthogonally spin-adapted formalism
Authors: Takahashi M. , Paldus J.
Published: July 2002

In the time-dependent coupled cluster approach the excitation energies can be obtained as eigenvalues of a generally non-Hermitian matrix characterizing the homogeneous part of a linear response equation. This matrix consists of the Hermitian configuration interaction part as well as of a non-Hermitian part which describes the ‘‘renormalization’’ due to ground state correlations. The relationship between a singular behavior of the ground state coupled cluster equations and the existence of vanishingly small excitation energies in the linear response formalism is pointed out. The explicit form of the required matrix elements in the orthogonally spin-adapted version of the theory is given for configuration spaces restricted to single and double excitations. A numerical test of the formalism for π-electron models of trans-butadiene and benzene is carried out and the relationship of the time-dependent coupled cluster approach with other many body approaches to excitation energies calculation is discussed.
Molecular dynamics of the A+BC reaction in rare gas solution
Authors: Bergsma John P. , Reimers Jeffrey R. , Wilson Kent R. , Hynes James T.
Published: July 2002

Molecular dynamics are computed for model atom transfers A+BC→AB+C in rare gas solvents at liquid densities. We find that the reaction dynamics can be understood in terms of a simple picture which consists of three stages: (1) activation of reactants, (2) barrier crossing, and (3) deactivation of products. The effects seen in stages (1) and (3) can be largely interpreted in terms of existing models of energy and phase decay in solution, while the effects seen in stage (2) can be largely interpreted in terms of gas phase A+BC barrier crossing dynamics. We find that transition state theory is in perfect agreement with the simulations for the 20 and 10 kcal/mol barrier reactions and is a very good description for a 5 kcal/mol reaction barrier. At low barrier curvature, dynamical effects due to the solvent are shown to induce some recrossings of the transition state barrier, thus causing rate constants calculated by simple transition state theory to be slightly too high. The Grote–Hynes modification of transition state theory, which considers the effect of the time dependent friction of the solvent on the dynamics at the transition state, predicts corrections to the rate constants in good agreement with the results from the simulations.
Optimized virtual orbital space for high-level correlated calculations
Authors: Adamowicz Ludwik , Bartlett Rodney J.
Published: July 2002

The second order Hylleraas functional and a Newton–Raphson orbital optimization technique have been used to generate an active, optimized virtual orbital space (OVOS) of substantially reduced dimension for correlated calculations. Numerical examples for CH2(1A1), C6H6, and potential curves for B2H6 and H2O2 using MBPT and coupled-cluster theory demonstrate that most of the correlation energy can be obtained with a much smaller number of optimized virtual orbitals, and effectively ∼100% of the correlation energy if the OVOS result is combined with the exact second-order energy that is evaluated as a byproduct of the OVOS generation. This suggests a potentially wide applicability of the OVOS method in high accuracy quantum mechanical calculations.
Adsorption of a polyelectrolyte chain to a charged surface
Authors: Muthukumar M.
Published: July 2002

The criteria for the adsorption of a polyelectrolyte chain in a salt solution on a uniformly charged planar membrane are derived using mean field arguments. Explicit formulas are obtained to describe the adsorption characteristics for varying surface charge density, charge on the polymer, Debye screening length κ−1, chain length L, and temperature T. The adsorption can be tuned using any one of these parameters. When T is the tuning variable, for example, the chain is adsorbed at T<Tc, where Tc is proportional to κ−3L−1 and κ−11/5L−1/5 in the weak and strong Coulombic screening limits, respectively. The thickness of the adsorbed layer is derived to vary linearly with κ−1 ln(Tc /T) for T<Tc.
Polaron formation in the acoustic chain
Authors: Brown David W. , Lindenberg Katja , West Bruce J. , Cina Jeffrey A. , Silbey Robert
Published: July 2002

Exact calculations are presented which detail the process of polaron formation in the one-dimensional acoustic chain. Exact solution is possible because the limit considered is the transportless limit in which the Hamiltonian matrix elements responsible for the motion of an excitation among site states have been set to zero. The polaron formation process is found to be decomposable into two subprocesses having distinct time scales. A disparity of time scales is possible in the case of long wavelength excitations. The consistency of our conclusions is demonstrated through the consensus of results obtained for the discrete acoustic chain, the Debye model, and the elastic continuum.
Quantum mechanical optimal control of physical observables in microsystems
Authors: Shi Shenghua , Rabitz Herschel
Published: July 2002

A quantum mechanical formalism for the optimal control of physical observables of microsystems is developed. The computational procedure for numerical implementation is presented. Three illustrative examples with a model Morse oscillator show that optimal pumping fields can be found to reach the physical objectives: selective excitations, steering a system to a specified state, and breaking a bond.
Fourier transform infrared study of tricarbon hydride radicals trapped in Ar at 10 K
Authors: Huang J. W. , Graham W. R. M.
Published: July 2002

The vibrational spectra of tricarbon hydride radicals formed by trapping the products of the vacuum ultraviolet photolysis of methylacetylene and various deuterated isotopomers in argon at 10 K have been investigated by Fourier transform spectroscopy in the range 200–3400 cm−1. Supplementary studies have examined the photolysis products of allene as well as C3H6 and C3D6, cyclopropane. The results reveal new information on the vibrations of C3H; five C3H2 isomers, cyclopropenylidene, vinylidenecarbene, triplet propargylene, trans- and cis-propenediylidene; two C3H3 isomers, propargyl and cyclopropenyl; and C3H5, allyl.
The definition of reaction coordinates for reaction-path dynamics
Authors: Natanson Gregory A. , Garrett Bruce C. , Truong Thanh N. , Joseph Tomi , Truhlar Donald G.
Published: July 2002

We present equations for generalized-normal-mode vibrational frequencies in reaction-path calculations based on various sets of coordinates for describing the internal motions of the system in the vicinity of a reaction path. We consider two special cases in detail as examples, in particular three-dimensional atom–diatom collisions with collinear steepest descent paths and reactions of the form CX3+YZ→CX3 Y+Z with reaction paths having C3v symmetry. We then present numerical comparisons of the differences in harmonic reaction-path frequencies for various coordinate choices for three such systems, namely, H+H2→H2+H, O+H2→OH+H, and CH3+H2→CH4+H. We test the importance of the differences in the harmonic frequencies for dynamics calculations by using them to compute thermal rate constants using variational transition state theory with semiclassical ground-state tunneling corrections. We present a new coordinate system for the reaction CH3+H2 that should allow for more accurate calculations than the Cartesian system used for previous reaction-path calculations on this and other polyatomic systems.
Controlling bimolecular reactions: Mode and bond selected reaction of water with hydrogen atoms
Authors: Sinha Amitabha , Hsiao Mark C. , Crim F. Fleming
Published: July 2002

Vibrational overtone excitation prepares water molecules in the ‖13〉−, ‖04〉−, ‖12〉−, ‖02〉−‖2〉, and ‖03〉− local mode states for a study of the influence of reagent vibration on the endothermic bimolecular reaction H+H2O→OH+H2. The reaction of water molecules excited to the ‖04〉− vibrational state predominantly produces OH(v=0) while reaction from the ‖13〉− state forms mostly OH(v=1). These results support a spectator model for reaction in which the vibrational excitation of the products directly reflects the nodal pattern of the vibrational wave function in the energized molecule. Relative rate measurements for the three vibrational states ‖03〉−, ‖02〉−‖2〉, and ‖12〉−, which have similar total energies but correspond to very different distributions of vibrational energy, demonstrate the control that initially selected vibrations exert on reaction rates. The local mode stretching state ‖03〉− promotes the H+H2O reaction much more efficiently than either the state having part of its energy in bending excitation (‖02〉−‖2〉) or the stretching state with the excitation shared between the two O–H oscillators (‖12〉−). The localized character of the vibrational overtone excitation in water has permitted the first observation of a bond selected bimolecular reaction using this approach. The reaction of hydrogen atoms with HOD molecules excited in the region of the third overtone of the O–H stretching vibration, 4νOH, forms at least a 100-fold excess of OD over OH, reflecting the preferential cleavage of the vibrationally excited bond.
Fluorescence excitation and resolved emission spectra of supersonically cooled Al2O
Authors: Cai Mingfang , Carter Christopher C. , Miller Terry A. , Bondybey V. E.
Published: July 2002

The triatomic oxide Al2O was prepared by oxidation of laser vaporized aluminum, or by direct vaporization of aluminum oxide, and detected by laser induced fluorescence. The fluorescence excitation and resolved emission spectra of a transition located near 38 249 cm−1 are consistent with a linear, centrosymmetric Al–O–Al structure. Analysis of the spectrum yields an AlO bond length of 0.164 nm and values of 525 (Σ+g), 99 (Πu), and 992 cm−1 (Σ+g) for the ground state fundamental vibrations, in good agreement with theoretical predictions.
Electron-impact dissociation of nitrogen
Authors: Cosby P. C.
Published: July 2002

The electron-impact dissociation of N2 to form two nitrogen atoms is observed in a crossed beam experiment at electron energies between 18.5 and 148.5 eV. Detection of the correlated dissociation fragments with a time and position sensitive detector permits detection of both ground and excited state fragments, but excludes interference from dissociative ionization products. The observed translational energy releases in the N2 dissociation are consistent with predissociation to N(2D)+N(4S) fragments as the primary dissociation mechanism. Absolute cross sections for the electron impact dissociation are measured and compared with previous measurements. Recommended values of this cross section are given for electron-impact energies between 10 and 200 eV.