bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral function and character of the motion of a conduction electron in an orientationally disordered molecular cryocrystal
Authors: Loktev V. M. , Sharapov S. G. , Beck H.
Published: July 2002

The spectral function of an excess particle (electron or hole) moving in a molecular cryocrystal having no long-range orientational order is calculated. It is shown that the spectral function, on the assumption of exponential decay (in time and space) of the correlations between the angles specifying the direction of the axes of the molecules, changes sharply when disorder appears. The motion of the particle is transformed from coherent to incoherent (diffusive), in qualitative agreement with the results of recent μSR studies of the motion of electrons in β-N2.
Progress in internal transport barrier plasmas with lower hybrid current drive and heating in JET (Joint European Torus)
Authors: Mailloux J. , Alper B. , Baranov Y. , Becoulet A. , Cardinali A. , Castaldo C. , Cesario R. , Conway G. , Challis C. D. , Crisanti F. , de Baar M. , de Vries P. , Ekedahl A. , Erents K. , Gowers C. , Hawkes N. C. , Hogeweij G. M. D. , Imbeaux F. , Joffrin E. , Litaudon X. , Lomas P. , Matthews G. F. , Mazon D. , Pericoli V. , Prentice R. , Rimini F. , Sarazin Y. , Stratton B. C. , Tuccillo A. A. , Tala T. , Zastrow K.-D. ,   EFDA-JET workprogramme
Published: July 2002

In optimized shear plasmas in the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)], safety factor (q) profiles with negative magnetic shear are produced by applying lower hybrid (LH) waves during the plasma current ramp-up phase. These plasmas produce a barrier to the electron energy transport. The radius at which the barrier is located increases with the LH wave power. When heated with high power from ion cyclotron resonance heating and neutral beam injection, they can additionally produce transient internal transport barriers (ITBs) seen on the ion temperature, electron density, and toroidal rotation velocity profiles. Due to recent improvements in coupling, q profile control with LH current drive in ITB plasmas with strong combined heating can be explored. These new experiments have led to ITBs sustained for several seconds by the LH wave. Simulations show that the current driven by the LH waves peaks at the ITB location, indicating that it can act in the region of low magnetic shear.
Phase behavior and structure of star-polymer–colloid mixtures
Authors: Dzubiella J. , Likos C. N. , Löwen H.
Published: July 2002

We calculate the phase diagrams of mixtures between hard-sphere colloids and star-polymers of arm numbers f=2,6,32 for different star-polymer–colloid size ratios 0.2⩽q⩽0.6 using an effective one-component description for the colloids in the presence of the stars. We map the full two-component system onto an effective one-component system by inverting numerically the Ornstein–Zernike equation for binary mixtures, supplemented by the Rogers–Young closure, in the low-colloid density limit. The free energy for the fluid and crystalline phase is calculated by using both hard-sphere perturbation theory and thermodynamic integration of simulation data. We find stable fluid–fluid demixing transitions for low arm numbers f=2,6 above a critical value of the size ratio qc below preempted by a fcc-solid. For the linear polymer limit, f=2, the critical size ratio is found to be qc≈0.4, in agreement with other approaches to colloid-polymer mixtures. Increasing the arm number, the region of stability of the demixing transition with respect to crystallization of the colloids shrinks, and qc grows. A comparison between the one- and two-component descriptions that demonstrates the consistency between the two routes is also carried out.
Invariant model for the plasma transition layer
Authors: Ivonin Igor A. , Pavlenko Vladimir P. , Wahlberg Christer , Revenchuk Sergey M.
Published: July 2002

The transition layer between different plasmas is investigated both analytically and numerically using the example of the penetration of the Solar wind plasma into the Earth’s magnetosphere. It is shown that quick and deep penetration is possible in the form of the squeezing of frozen-in fields without reconnection, which may create spatial structures (layers, current sheets) for plasmas having different frozen-in field invariants. The steady, unique distribution of plasmas in the transition layer is found to be determined by the plasma invariants only. A simple, four-fluid hydrodynamics model is applied both qualitatively and quantitatively to study the coexistence of the plasmas at the final stage of the penetration in the down cusp region. The typical features of satellite observations, such as the large variations of plasma density and temperature, as well as the large scale potential well of the electric field, are explained and reproduced in numerical simulations. The model predicts and explains the appearance of stable nonlinear structures with a large length of coherence.
Optical metastability in undoped GaN grown on Ga-rich GaN buffer layers
Authors: Dhar Subhabrata , Ghosh Subhasis
Published: July 2002

Investigations on defect-related optical metastability in undoped GaN epilayers grown on GaN buffer layers are presented. The III/V ratio in the buffer layer was varied over a range such that the resistivity of the GaN epilayers traversed a semiconducting to semi-insulating transition. The high-resistive and semi-insulating GaN epilayers show photo induced metastability, which is revealed through a number of features: (i) the intensity of the blue luminescence band decreases as the intensity of the yellow luminescence band increases; (ii) quenching of photocurrent; and (iii) persistent photoconductivity, under the illumination of a low-power ultraviolet laser. It has been shown that these unwanted transient effects can be eliminated in the GaN epilayers by reducing the III/V ratio in the buffer layer. A qualitative discussion of these results suggests that the metastable defects, which are associated with both yellow and blue luminescence bands, may have important consequences for our understanding of defect-related luminescence in GaN.
Calculations of periodic trajectories for the Hénon–Heiles Hamiltonian using the monodromy method
Authors: Davies K. T. R. , Huston T. E. , Baranger M.
Published: July 2002

The monodromy method, for calculating classical periodic trajectories, is applied to the famous Hénon–Heiles potential, which is invariant under the group D3. The monodromy method is computationally very efficient and is used to find many families of periodic trajectories, including a number of simple bifurcations from the main families of the Hénon–Heiles potential.
Film-edge-induced dislocation generation in silicon substrates. I. Theoretical model
Authors: Vanhellemont J. , Amelinckx S. , Claeys C.
Published: July 2002

A theoretical model is presented to predict the Burgers vector and the glide plane of the dislocations generated at thin-film edges on silicon substrates. The model is based on a theoretical analysis of the stress field at the film edge. It is shown that the external force acting upon a dislocation with a known Burgers vector is a linear combination of the stress-field components. In equilibrium the total external glide force is balanced by the counteracting line tension force and the critical glide force, both only depending on the length of the Burgers vector and not on its orientation. The (a/2)〈110〉-type dislocation on which the largest external climb and glide force is exerted will nucleate first and subsequently grow by gliding to its equilibrium shape and position.
Interaction of surface acoustic waves with a two-dimensional electron system in a LiNbO3-GaAs/AlGaAs sandwich structure
Authors: Wixforth A. , Scriba J. , Wassermeier M. , Kotthaus J. P. , Weimann G. , Schlapp W.
Published: July 2002

We describe a simple, contactless method to study the interaction of surface acoustic waves (SAW) with a two-dimensional electron system (2DES) in GaAs/AlGaAs heterostructures at low temperatures and in high magnetic fields. The heterostructure is part of a sandwich structure on a Y-cut Z-propagating LiNbO3-SAW-delay line. The interaction of the SAW with the 2DES leads to quantum oscillations of the SAW amplitude as a function of the applied magnetic field which can be used to characterize the sample and to investigate the transport properties of the 2DES.
The fractal nature of the cluster model dielectric response functions
Authors: Dissado L. A. , Hill R. M.
Published: July 2002

Calculable fractal circuit models are used to show that the cluster model response functions result from the combination of two types of self-similarity. The analysis is extended to the molecular scale where the cluster model is seen to be based on sequential relaxation processes. An outline is given of the physical origin for such behavior, and the self-similar processes are identified with the basic concepts of (i) an efficient (compact) exploration of a fractal lattice and (ii) self-similarity in the contacts between internally connected regions (clusters). The relationship of the cluster model parameters n and m to system dimensionalities are derived for a number of cases.
A new method to analyze multiexponential transients for deep-level transient spectroscopy
Authors: Hanak Thomas R. , Ahrenkiel Richard K. , Dunlavy Donald J. , Bakry Assem M. , Timmons Michael L.
Published: July 2002

A new technique is introduced to analyze digitally recorded capacitive transients in order to determine the properties of deep states. Using a nonlinear double exponential fitting routine, it is shown that a two-trap model can be applied to the transient data. We determine the individual trap concentrations and produce two Arrhenius plots. The latter yields the thermal activation energies and capture cross sections of closely spaced traps. The excellent agreement between the new technique and the standard rate window technique is shown via a simulation deep-level transient spectroscopy spectrum. The new method is applied to Se-doped AlxGa1−xAs (x=0.19 and 0.27) grown by metal-organic chemical vapor deposition. The measured results for all deep states including the DX centers agree well with the values published in the literature.
A comparison of deposition rates and temperature measurements for dc and rf diode sputtering
Authors: Maniv S.
Published: July 2002

A comparison between the deposition rates from a copper target in a diode sputtering system using either rf (13.56 MHz) or dc power shows that the deposition rates are the same for equal powers. In situ measurements of the temperature difference, ΔT, between the inlet and outlet target cooling water support this observation. These results contrast with those of magnetron sputtering, where the deposition rate is substantially different for rf and dc power sources.
Design and operation of a module of phase-locked relativistic magnetrons
Authors: Levine J. S. , Aiello N. , Benford J. , Harteneck B.
Published: July 2002

We have achieved phase-locked operation of a module consisting of up to seven relativistic magnetrons. The magnetrons are connected by waveguide in a peer/peer configuration, with no one magnetron acting as a master oscillator. The signals used for coupling are each a substantial fraction of a magnetron’s radiated power. Total extracted power, at 2.8 GHz, was 2.0 GW with four magnetrons and 2.9 GW with seven. Several interconnection geometries were explored. One particular geometry was found to produce qualitatively better phase-locked operation than any other. This was in agreement with the predictions of a numerical model of the magnetrons as coupled van der Pol oscillators.
Excimer laser induced ablation of polyetheretherketone, polyimide, and polytetrafluoroethylene
Authors: Babu S. V. , D’Couto G. C. , Egitto F. D.
Published: July 2002

New experimental data on the photoablation rate of polyetheretherketone and Kapton film by 25 ns pulses of XeCl laser radiation are presented. The fluences used were in the range of 5–70 J/cm2. The etching rate increases with fluence up to about 50 J/cm2 and then saturates for both the polymers. The data can be described very well by the model proposed earlier by Srinivasan, Smrtic, and Babu (SSB), which includes both thermal and photochemical contributions to polymer ablation. This model is compared with that of Sauerbrey and Petit for describing the etch rate of polyimide with 308 nm laser pulses of various durations. It is also shown that the polytetrafluoroethylene ablation rate data of Kuper and Stuke obtained with 300 fs 248 nm laser pulses can also be described by the SSB model by modifying it to include the effects of nonlinear absorption.
Deep-level transient charge spectroscopy of Sn donors in AlxGa1−xAs
Authors: Arora B. M. , Chakravarty S. , Subramanian S. , Polyakov V. I. , Ermakov M. G. , Ermakova O. N. , Perov P. I.
Published: July 2002

Deep-level transient charge spectroscopy (QDLTS) measurement is described. The technique is used to investigate energy levels of tin in AlxG1−xAs. The QLDTS spectra have multiple peaks which show the multilevel nature of the tin donors.
A study of residual stress and the stress-optic effect in mixed crystals of K(DxH1−x)2PO4
Authors: De Yoreo J. J. , Woods B. W.
Published: July 2002

In electro-optic applications of the mixed crystal K(DxH1−x)2PO4 (KDP-KD*P), strain-induced refractive index variations result in beam depolarization and transmitted wave-front distortion. Here the combined linear stress-optic and electro-optic effects in crystals of KDP-KD*P oriented perpendicular to the c axis are analyzed and it is shown that while the depolarization is caused by the induced birefringence, the wave-front distortion is due to average index shifts. Furthermore, the birefringence is determined by the shear stress in the xy plane of the crystal while the average index shift depends only on the normal stresses. For depolarization losses of 0.1%–1% and wave-front distortion of 0.1–1λ, the critical range of stress is 105–106 Pa. Measured depolarization loss and wave-front distortion profiles of 5, 16, and 27 cm K(DxH1−x)2PO4 crystals for 0≤x≤0.98 are also presented. Using the analysis described above it is shows that the maximum internal stresses in these crystals are within the critical range, but that the area-averaged stresses are substantially lower. It is found that crystals from different locations along the length of a boule can have similar strain birefringence and wave-front distortion profiles indicating that the growth conditions which generate the internal strain persist throughout a significant portion of the growth history of the boule. It is also observed that the most highly strained crystals come from near the seed cap. Finally, potential sources of strain in KD*P are discussed and thermodynamic and structural arguments are given which suggest that inhomogeneities in the H/D ratio are a potential source of the strain in KD*P.
Photoemission investigation of the electronic structure at polycrystalline CuInSe2 thin-film interfaces
Authors: Nelson Art J. , Swartzlander A. B. , Tuttle J. R. , Noufi R. , Patel R. , Höchst Hartmut
Published: July 2002

The surface versus bulk composition and electronic structure of polycrystalline CuInSe2 thin-film interfaces were studied by synchrotron radiation soft-x-ray photoemission spectroscopy. An n-type In2Se3/CuIn3Se5 surface layer forms on enhanced-grain polycrystalline thin-film p-type CuInSe2 during fabrication. Enhanced-grain CuInSe2 films were sputter etched (500 V Ar) and analyzed in situ to determine core-level binding energies and Fermi-level positions for the n-type surface and the p-type CuInSe2 bulk within ±0.1 eV. The transition between the n-type surface and the p-type bulk was experimentally observed by noting the change in the position of the valence-band maximum relative to the Fermi level EF. From these measurements, the valence-band offset ΔEv between the layers was determined to be 0.50 eV. Measurement of the work functions φ was also completed and reveals φ=4.75 eV for the In2Se3 (CuIn3Se5) surface layer and φ=4.04 eV for the bulk CuInSe2. Combining these results allows construction of a surface band diagram for this device configuration as well as determination of the relationship between composition, electronic structure, and device performance.
Cesium telluride photocathodes
Authors: Kong S. H. , Kinross-Wright J. , Nguyen D. C. , Sheffield R. L.
Published: July 2002

Cesium telluride (Cs2Te) photocathodes, with quantum efficiencies (QEs) of 15%–18% at 251 nm, were fabricated by vapor deposition of Te and Cs onto a Mo substrate and used as an electron source for the Los Alamos Advanced Free-Electron Laser. In the fabrication chamber, the spectral response from 251 to 578 nm was measured before and after a controlled exposure of several photocathodes to air. The 251-nm QE dropped by about a factor of 20 when exposed to 2×10−4 Torr of air for 1 h. Heating degraded photocathodes to 150–200 °C partially rejuvenated their QEs to about 60% of the value before air exposure. The performance of Cs2Te as a source of electrons for accelerators was evaluated in the photoinjector stage of the Advanced Free-Electron Laser. The response time, saturation level, and dark current of cesium telluride photocathodes and the emittance and energy spread of the resulting electron beam were determined to be sufficient for free electron laser applications.
High-flux low-energy (≂20 eV) N+2 ion irradiation during TiN deposition by reactive magnetron sputtering: Effects on microstructure and preferred orientation
Authors: Hultman L. , Sundgren J.-E. , Greene J. E. , Bergstrom D. B. , Petrov I.
Published: July 2002

The effects of the incident ion/metal flux ratio (1≤Ji /JTi≤15), with the N+2 ion energy Ei constant at ≂20 eV (≂10 eV per incident accelerated N), on the microstructure, texture, and stoichiometry of polycrystalline TiN films grown by ultrahigh-vacuum reactive-magnetron sputtering have been investigated. The layers were deposited in pure N2 discharges on thermally oxidized Si(001) substrates at 350 °C. All films were slightly overstoichiometric with a N/Ti ratio of 1.02±0.03 and a lattice constant of 0.4240±0.0005 nm, equal to that of unstrained bulk TiN. Films deposited with Ji/JTi=1 initially exhibit a mixed texture—predominately (111), (002), and (022)—with competitive columnar growth which slowly evolves into a pure (111) texture containing a network of both inter- and intracolumn porosity with an average column size of ≂50 nm at t=1.6 μm. In contrast, films grown with Ji/JTi≥5 do not exhibit competitive growth. While still columnar, the layers are dense with an essentially complete (002) preferred orientation and an average column size of ≂55 nm from the earliest observable stages. The normalized x-ray diffraction (002) intensity ratio in thick layers increased from ≂0 to 1 as Ji/JTi was varied from 1 to ≥5. Both 111 and 001 interplanar spacings remained constant as a function of film thickness for all Ji/JTi. Thus, contrary to previous models, strain is not the dominant factor in controlling the development of preferred orientation in these films. Moreover, once film texture is fully evolved—whether it be (002) or (111)—during deposition, changing Ji/JTi has little effect as preferred orientation becomes controlled by pseudomorphic forces. Film porosity, however, can be abruptly and reversibly switched by increasing or decreasing Ji/JTi.
A simple model for delta-doped field-effect transistor electronic states
Authors: Gaggero-Sager L. M. , Pérez-Alvarez R.
Published: July 2002

We propose a simple potential model which reproduces the main properties of the electronic structure of a delta-doped field-effect transistor. On the basis of selfconsistent calculations we conclude that the energies, wavefunctions and other characteristic properties obtained with the proposed model are very close to the results of full selfconsistent calculations.
Epitaxial growth and characterization of Ge1−xCx alloys on Si(100)
Authors: Krishnamurthy M. , Drucker J. S. , Challa A.
Published: July 2002

The initial stages (<20 monolayers) of molecular beam epitaxial growth of Ge1−xCx on Si(100) has been studied using both in situ surface analytical techniques and ex situ electron microscopy. The Ge1−xCx films studied had nominal C concentrations of 0.2<x<0.8. In situ reflection high-energy electron diffraction indicates crystalline, layer-by-layer growth for room temperature deposition of lower C concentrations (x<0.4) films and amorphous growth for higher C concentrations. Subsequent high-temperature anneals between 350 and 600 °C caused the Ge and C to phase separate, with the Ge forming defective islands while the C precipitates out of the diamond lattice. Mechanisms leading to the C concentration-dependent island morphologies are suggested.
Relative sensitivity of photomodulated reflectance and photothermal infrared radiometry to thermal and carrier plasma waves in semiconductors
Authors: Salnick A. , Mandelis A. , Ruda H. , Jean C.
Published: July 2002

A quantitative theoretical comparison between two photothermal techniques—the photomodulated reflectance (PMR) and the photothermal infrared radiometry (PTR)—from the standpoint of their relative sensitivity to the thermal and carrier plasma waves in semiconductors is presented. The coefficients representing the relative contributions from the thermal and plasma waves to the total PMR and PTR signals arising as a result of the same temperature increase and photoinjected excess carrier concentration are calculated for three crystalline semiconductors: Si, Ge, and GaAs. The PTR signal is found to be extremely sensitive to the plasma-wave effects exhibiting up to five orders of magnitude higher carrier plasma-to-thermal contrast than that of the PMR method.
Lattice site location studies of ion implanted Li8 in GaN
Authors: Dalmer M. , Restle M. , Sebastian M. , Vetter U. , Hofsäss H. , Bremser M. D. , Ronning C. , Davis R. F. , Wahl U. , Bharuth-Ram K. ,   ISOLDE Collaboration
Published: July 2002

The lattice sites of ion implanted Li atoms in GaN were studied as a function of implantation temperature between room temperature and 770 K. We measured the channeling and blocking patterns of α-particles emitted in the radioactive decay of implanted Li8 ions to determine the Li lattice sites. Below 700 K Li atoms occupy mainly interstitial sites in the center of the c-axis hexagons at positions c/4 and 3c/4, where c is the lattice constant in c-axis direction. Around 700 K Li starts to diffuse and presumably interacts with vacancies created in the implantation process. This leads to the formation of substitutional Li above 700 K. An activation energy of about 1.7 eV for interstitial Li diffusion was determined.
Texture development mechanisms in ion beam assisted deposition
Authors: Dong L. , Srolovitz D. J.
Published: July 2002

Three-dimensional molecular dynamics simulations of ion beam assisted deposition (IBAD) are performed to determine the mechanisms of crystallographic texture selection during the IBAD of polycrystalline films. A face centered cubic bicrystal consisting of [111] and [110] oriented grains is grown while an ion beam bombards the growing film at normal incidence. As the film grows, the grain boundaries delimiting the [111] and [110] grains move towards each other, eventually pinching off the [111] grain such that the film texture changes from equal densities of [111] and [110] to purely [110]. Examination of single crystals grown in the presence of ion beams shows two important effects: ion beam induced atomic sputtering from the surface and ion beam induced damage are significantly reduced when the ion beam is oriented along channeling directions of the crystals. The first observation suggests that grains with channeling directions aligned parallel to the ion beam grow more quickly than those where they are not aligned. This leads to grain-to-grain variations in the film thickness that increase in magnitude during growth. Variations in thickness result in a shadowing effect that further slows the growth of the less thick (nonaligned) grains—eventually leading to pinch-off of these grains. The second observation suggests that the stored energies within the grains with channeling directions aligned parallel to the ion beam will be lower than that of the nonaligned grains. This difference in stored energy (in the form of crystal defects) is shown to lead to grain boundary migration—a process equivalent to primary recrystallization. Both of these effects can lead to changes in crystallographic texture during film growth and both are observed in the bicrystal simulations. Which mechanism will dominate under a prescribed set of conditions remains to be delineated.
Room-temperature anisotropic, thermoelectric, and electrical properties of n-type (Bi2Te3)90 (Sb2Te3)5 (Sb2Se3)5 and compensated p-type (Sb2Te3)72 (Bi2Te3)25 (Sb2Se3)3 semiconductor alloys
Authors: Taylor P. J. , Maddux J. R. , Jesser W. A. , Rosi F. D.
Published: July 2002

The anisotropy ratios (parallel to the c axis versus perpendicular to the c axis) for the electrical resistivity, Seebeck coefficient, and thermal conductivity of horizontal Bridgman-grown n-type (Bi2Te3)90 (Sb2Te3)5 (Sb2Se3)5 and p-type (Sb2Te3)72 (Bi2Te3)25 (Sb2Se3)3 were measured at 300 K. The orientation having the largest thermoelectric figure of merit was perpendicular to the zone axis of the c planes (along the natural growth direction). For the n-type alloy, the maximum thermoelectric figure of merit was determined to be 3.2×10−3/K. For the p-type alloy, the maximum thermoelectric figure of merit was determined to be 3.6×10−3/K when compensated with tellurium only, and 3.8×10−3/K when compensated with both tellurium and iodine. These values represent a significant increase over previously reported p-type thermoelectric figure of merit values. Hall coefficient data coupled with Seebeck coefficient measurements suggest a difference in carrier transport phenomena caused by an increased compensation of holes in the p-type alloy without the deleterious precipitation of the tellurium-rich second phase.
General model for mechanical stress evolution during electromigration
Authors: Sarychev M. E. , Zhitnikov Yu. V. , Borucki L. , Liu C.-L. , Makhviladze T. M.
Published: July 2002

A model is presented for the development of stress during electromigration. Formally similar to a thermal stress model, it provides a method of calculating all of the components of the stress tensor and clearly couples vacancy transport and stress evolution with the boundary conditions that apply to the metal. Analytic solutions are discussed for electromigration either normal or parallel to a plate. The solution parallel to a plate is used to reinterpret x-ray microdiffraction experiments from the literature. We find that the effective charge for vacancies in pure polycrystalline aluminum at 533 K is about 0.84. Using parameters that were either measured or calculated with the embedded atom method, our model displays good agreement with both transient electromigration data and drift data.
Anomalous resisto-acoustic effect
Authors: Zaitsev B. D. , Kuznetsova I. E. , Joshi S. G.
Published: July 2002

A new effect, the anomalous resisto-acoustic effect, is theoretically predicted. This effect deals with increase in the velocity of surface acoustic waves due to deposition of a thin conducting film on the piezoelectric substrate. The influence of a thin conducting layer on the propagation of surface acoustic waves in a piezoelectric medium is investigated theoretically. It is generally believed that wave velocity would decrease monotonically with increasing sheet conductance of the layer. Our calculations show, however, that in the case of weakly inhomogeneous surface waves, such as Bleustein–Gulyaev and Love waves, there exists a certain conductance interval in which the wave velocity increases with increasing layer conductance. This anomalous behavior is related to the penetration depth of the wave into the substrate. The anomalous effect is present if the penetration depth exceeds a certain critical value, and absent otherwise. The characteristics of surface acoustic waves in various structures containing layers of finite and/or in finite conductivity have been investigated. It is found that under certain conditions vm, the velocity for a metallized surface, may actually be greater than v0, the velocity for an unmetallized surface. The anomalous resisto-acoustic effect allows one to obtain value of Δv/v, the financial velocity change, that is greater than the normally predicted value of Δv/v=(v0−vm)/v0. For example, for Bleustein–Gulyaev waves propagating in Y-cut, X-propagation potassium niobate, values of Δv/v greater than 0.32 can be obtained, compared to the normally predicted value of 0.26. This has significant advantages in improving the performance of various sensing and signal processing devices.
Opto-dielectric effect on a nematic liquid crystal doped with a photoactive azo mesogen
Authors: Nair Geetha G. , Prasad S. Krishna , Yelamaggad C. V.
Published: July 2002

We report the detailed temperature variation of the dielectric constant across the nematic-isotropic transition in both the normal and UV-irradiated states of a commercially available room temperature nematic mixture (E7 from E-Merck) doped with a small quantity (4.5 mol %) of a liquid crystalline azo compound. The photoirradiation was done using a very low power (0.1 mW/cm2) UV source. Data of time-resolved measurements of the dielectric constant through both the UV-activated “trans-cis” transformation and the thermal back relaxation process are also presented and compared with time-dependent absorbance data. Even for this low concentration of the photoactive material, UV irradiation of as small a magnitude as 0.1 mW/cm2 is observed to lower the nematic-isotropic transition temperature by 15 °C.
Superfocusing of surface polaritons in the conical structure
Authors: Babadjanyan A. J. , Margaryan N. L. , Nerkararyan Kh. V.
Published: July 2002

It is shown that at the propagation of a surface polariton through a pointed cone its wavelength decreases to zero as it approaches the edge of the cone. As a result, the polariton is focused in a very small region and the strengths of the wave fields anomalously increase. The phenomenon could be used for creation of a scanning near-field optical microscope to investigate the nanometer-scale surface areas.
Measurement of thermal diffusivity based on the photothermal displacement technique using the minimum phase method
Authors: Lee E. H. , Lee K. J. , Jeon P. S. , Yoo J.
Published: July 2002

A method of measuring the thermal diffusivity of solid material at room temperature using photothermal displacement is proposed. The influence of the parameters, such as the radius and modulation frequency of the pump beam and the sample thickness, was studied. From the minimum position of phase of measured deflection with respect to the pump beam, the thermal diffusivity of the materials can be obtained. The position where phase has the minimum value is determined using multiparameter least-square regression fitting. The experimental values for different samples obtained by applying the method are in good agreement with the literature values.
On the thermodynamic foundations of viscoelasticity
Authors: Lebon G. , Perez-Garcia C. , Casas-Vazquez J.
Published: July 2002

It is shown that extended irreversible thermodynamics provides a natural scheme for describing viscoelastic bodies. This is achieved by introducing the inelastic stress tensor as variable in complement of the standard variables. The Poynting–Thomson, Maxwell, and Kelvin–Voigt models are recovered as particular cases of the formalism. Nonlinear and more complicated models, like Jeffreys’ model, are also suggested. Propagation of plane harmonic waves and the consequences of the application of external sinusoidal solicitations are investigated. Finally, a comparison with some other theories is made.
A molecular beam scattering investigation of the oxidation of CO on Rh(111). II. Angular and velocity distributions of the CO2 product
Authors: Brown L. S. , Sibener S. J.
Published: July 2002

Molecular beam and time of flight methods have been used to examine the angular distributions and velocity distributions of the CO2 product molecules formed in the catalytic oxidation of CO on a Rh(111) single crystal in the surface temperature range 700–1000 K. The angular distribution was sharply peaked about the surface normal, and cannot be described by a simple cosn θ expression. No temperature dependence was observed in the angular distribution over the range of temperatures studied here. Observed velocity distributions were clearly non-Maxwellian and had average translational energies in excess of those expected at the surface temperatures. Furthermore, the average velocity depended strongly on the desorption angle. Molecules desorbing along the surface normal had an average translational energy of ∼8 kcal/mol. The average energy decreased with increasing angle, reaching a value of ∼4 kcal/mol at an angle of 60°. All of the observed velocity distributions were narrower than Maxwellian distributions with the same average energies. Product velocity distributions did not appear to vary with surface temperature. The observed excess energies are believed to arise from the crossing of the activation barrier to reaction, with a fraction of the reaction energy being carried away from the surface by the product molecules.
Molecular dynamics algorithm for multiple time scales: Systems with long range forces
Authors: Tuckerman Mark E. , Berne Bruce J. , Martyna Glenn J.
Published: July 2002

A frequently encountered problem in molecular dynamics is how to treat the long times that are required to simulate condensed systems consisting of particles interacting through long range forces. Standard methods require the calculation of the forces at every time step. Because each particle interacts with all particles within the interaction range of the potential the longer the range of the potential the larger the number forces that must be calculated at each time step. In this note we present a variant of the RESPA (reference system propagator algorithm), which we developed for handling systems with multiple time scales like disparate mass mixtures. This version of RESPA greatly reduces the number of forces that must be computed at each time step and thereby leads to a dramatic acceleration of such simulations. The RESPA method uses ideas similar to NAPA, an algorithm we invented to treat high frequency oscillators interacting with low frequency bath. The method is based on a choice of a reference system in which the particles interact through short range forces. The reference system is numerically integrated for n time steps δt and the error incurred by using short range forces is corrected by solving a rigorous set of equations once every Δt=nδt. This method reduces the cpu time dramatically. It is shown that this approach and suitable generalizations should be very useful for future simulations of quantum and classical condensed matter systems.
Herringbone structure in two-dimensional single crystals of cyanine dyes. II. Optical properties
Authors: Kirstein S. , Möhwald H.
Published: July 2002

Optical spectra of two-dimensional single crystals of cyanine dye molecules are studied experimentally and theoretically. Crystallites of three carbocyanine dye molecules, which are chemically different, but exhibit nearly identical optical properties, are prepared by adsorption from aqueous solution at a charged ammonium lipid monolayer. By means of electron diffraction it was found that the crystal structure of the aggregates consists of a rectangular unit cell containing two molecules of different orientation. Measurements of polarized absorption, reflectance, and fluorescence spectra of single crystallites reveal that this structure leads to Davydov splitting of the electronic transition into two exciton bands. The band structure of these exciton bands is calculated with the help of Kuhn’s extended dipole model. It is demonstrated, how the exciton energy depends on the relative orientation of the two molecules within the unit cell. Calculated splitting energies are in reasonable good agreement with experimentally observed values.
Prediction of the liquid–vapor equilibrium pressure using the quasi-Gaussian entropy theory
Authors: Amadei A. , Roccatano D. , Apol M. E. F. , Berendsen H. J. C. , Di Nola A.
Published: July 2002

We derived a method to evaluate the liquid–vapor equilibrium pressure, with high accuracy over a large range of temperature, using the quasi-Gaussian entropy theory. The final expression that we obtain for the equilibrium pressure as a function of the temperature can be considered as a very accurate approximate solution of the Clausius–Clapeyron equation. The method was applied to water, methanol and mercury, and was compared to two usual approximations of the Clausius–Clapeyron equation.
Raman dispersion spectroscopy on the highly saddled nickel(II)-octaethyltetraphenylporphyrin reveals the symmetry of nonplanar distortions and the vibronic coupling strength of normal modes
Authors: Schweitzer-Stenner Reinhard , Stichternath Andreas , Dreybrodt Wolfgang , Jentzen Walter , Song Xing-Zhi , Shelnutt John A. , Nielsen Ole Faurskov , Medforth Craig J. , Smith Kevin M.
Published: July 2002

We have measured the polarized Raman cross sections and depolarization ratios of 16 fundamental modes of nickel octaethyltetraphenylporphyrin in a CS2 solution for 16 fundamental modes, i.e., the A1g-type vibrations ν1, ν2, ν3, ν4, ν5, and φ8, the B1g vibrations ν11 and ν14, the B2g vibrations ν28, ν29, and ν30 and the antisymmetric A2g modes ν19, ν20, ν22, and ν23 as function of the excitation wavelength. The data cover the entire resonant regions of the Q- and B-bands. They were analyzed by use of a theory which describes intra- and intermolecular coupling in terms of a time-independent nonadiabatic perturbation theory [E. Unger, U. Bobinger, W. Dreybrodt, and R. Schweitzer-Stenner, J. Phys. Chem. 97, 9956 (1993)]. This approach explicitly accounts in a self-consistent way for multimode mixing with all Raman modes investigated. The vibronic coupling parameters obtained from this procedure were then used to successfully fit the vibronic side bands of the absorption spectrum and to calculate the resonance excitation profiles in absolute units. Our results show that the porphyrin macrocycle is subject to B2u-(saddling) and B1u-(ruffling) distortions which lower its symmetry to S4. Thus, evidence is provided that the porphyrin molecule maintains the nonplanar structure of its crystal phase in an organic solvent. The vibronic coupling parameters indicate a breakdown of the four-orbital model. This notion is corroborated by (ZINDO/S) calculations which reveal that significant configurational interaction occurs between the electronic transitions into |Q〉- and |1B〉-states and various porphyrin→porphyrin, metal→porphyrin, and porphyrin→metal transitions. The intrastate coupling parameters are used to estimate the excited electronic states’ displacements along the normal coordinates with respect to the ground state and their contributions to the reorganization energy. It turns out that the |B〉-state is predominantly affected by symmetric A1g-displacements, whereas the |Q〉-state is subject to A2g, B1g, and B2g displacements of its equilibrium configuration. While the former is induced by the combined effect of ruffling and saddling, the latter arises from Jahn–Teller coupling within the degenerate states.
Investigations of surface forces between gypsum crystals in electrolytic solutions using microcantilevers
Authors: Finot Eric , Lesniewska Eric , Mutin Jean-Claude , Goudonnet Jean-Pierre
Published: July 2002

This paper introduces a new approach to the study of the interactions between gypsum faces in electrolytic solutions of calcium sulfate. A systematic study with respect to the orientation of crystals, the concentration of the solution, the type of electrolyte, and the duration time of the contact leads to an improved understanding of both the DLVO theory and the mechanism involved in the development of the ionic correlation force. Using an atomic force microscope, direct local measurement of forces between crystals defines the ideal conditions favorable to the adhesion. The most important factor in crystal coagulation was found to be the effective surface charge of each face.
Overall rotation of a many-body system in Dixon’s theory
Authors: San Miguel A. , Vicente F.
Published: July 2002

Using Dixon’s theory, an expression for the angular momentum of an N-body system, whose constituent bodies are monopoles that do not interact gravitationally, is obtained. In this expression the contributions to the angular momentum due to the rotational and internal motions of the material system are separated. It is also shown how the existence of a frame with respect to which the total angular momentum of an N-body system is reduced to an expression formally similar to that corresponding to a rigid body instantaneously equivalent to the material system considered. Finally, in space–times of small curvature, a separation of the energy into a part due to the rotational motion and other due to the internal motion, is investigated.
Phase flow of an axially symmetrical gyrostat with one constant rotor
Authors: Elipe A. , Lanchares V.
Published: July 2002

We analyze the attitude dynamics of an axially symmetric gyrostat under no external forces and one constant internal spin. We introduce coordinates to represent the orbits of constant angular momentum as a flow on a sphere. With these coordinates, we realize that the problem belongs to a general class of Hamiltonian systems, namely the problem here considered is the one parameter Hamiltonian that is a polynomial of at most degree two in a base of the Lie algebra so (3). The parametric bifurcations are found for both cases, when the rotor is spinning about the axis of symmetry of the gyrostat, and when it is spinning about another axis of inertia. The general solution for the global general flow is expressed in terms of the Jacobian elliptic functions.
Spin coefficients as Lanczos scalars: Underlying spinor relations
Authors: Andersson F. , Edgar S. B.
Published: July 2002

It has been conjectured by Lopez-Bonilla and co-workers that there is some linear relationship between the NP spin coefficients and the Lanczos scalars, and examples have been given for a number of different classes of space–times. We show that in each of those examples a Lanczos potential can be defined in a very simple way directly from the spinor dyad. Although some of these examples seem to have no deeper geometric meaning, we emphasize that there are structural links between Lanczos potential and spin coefficients which we highlight in some other examples. In particular we show that the direct identification of Lanczos potentials with spin coefficients is possible for some important classes of space–times while the direct identification of Lanczos potentials with the properly weighted spin coefficients is also possible for several important classes of space–times. In both of these cases we obtain the necessary and sufficient conditions on the spin coefficients for such identifications to be possible, which enables us to test space–times directly.
On the energy levels of a finite square-well potential
Authors: Paul Prabasaj , Nkemzi Daniel
Published: July 2002

The theory of Cauchy integrals and the Riemann problem is used to derive an explicit formula for the bound-state energy levels of the finite square-well potential problem and is used to obtain a simple asymptotic expression for them.
Low-temperature plasticity of dilute solid solutions of Ne in n-H2
Authors: Alekseeva L. A. , Strzhemechny M. A. , Butenko Yu. V.
Published: July 2002

Samples of strongly dilute solid solutions of Ne in n-H2 are tested under step-wise axial extension at temperatures 1.8–4.2 K. A clearly manifested plasticization effect of neon impurities has been observed: smaller loads are required for crystals with a higher neon concentration to obtain the same strain at a fixed temperature. The ultimate strength of neon-doped crystals is noticeably lower than for pure samples of normal hydrogen. The creep of such solid solutions has also been studied. The decrease in the strength of n-H2 can be due to a specific mechanism of overcoming by dislocations of the barrier formed near impurities which probably form Van der Waals complexes with the crystal environment.
Stirring properties of vortex rings
Authors: Auerbach David
Published: July 2002

Ring vortex evolution, from the initial roll-up phase through to the final turbulent phase, was experimentally studied to see the dependence of its stirring properties on both the initial (accelerating, constant, decelerating, slow, fast) piston motion as well as on the boundary (tube/hole geometry) conditions. Stirring between fluid initially upstream and that initially downstream of the nozzle plane is done more by convective entrainment at the beginning (roll-up and contraction phases), by diffusive entrainment during the laminar and wavy phases, and by mixed entrainment and ejection during the transition to turbulence and the turbulent phase itself. During vortex roll-up, it was found that tubes eject shorter streaklines than do holes, and that there is less Re dependence for this for tubes than for holes. During the contraction phase, entrainment ends, save for minimal entrainment due to axial inflow into the ring from along the cores of Goertler-type vortices. Generally, the rate of fluid ejected is largest during the transition from the wavy to the turbulent state. As far as the stability of the vortices is concerned, rings generated at holes are less stable than those generated at tubes. During the final turbulent phase, rings not only entrain fluid but eject it periodically into the wake: Between two and four hairpin vortices are generated and laid off in the wake during each ejection. The frequency at which such ejections takes place scales as a Strouhal number that takes on values of between 2 and 4.
Ion-acoustic waves in a plasma with negative ions
Authors: Song B. , D’Angelo N. , Merlino R. L.
Published: July 2002

Propagation and damping of ion-acoustic waves have been investigated in a Q-machine plasma consisting of K+ positive ions, SF−6 negative ions, and electrons. The phase velocity of the ion-acoustic ‘‘fast’’ mode increases with increasing ε, the concentration of negative ions. The wave damping decreases with increasing ε, and nearly disappears, for the highest wave frequencies investigated, when ε≳0.9. Both results are in agreement with predictions from Vlasov theory.
Edge poloidal rotation profiles of H-mode plasmas in the JFT-2M tokamak
Authors: Ida K. , Hidekuma S. , Kojima M. , Miura Y. , Tsuji S. , Hoshino K. , Mori M. , Suzuki N. , Yamauchi T. ,   the JFT−2M Group
Published: July 2002

Parameter dependence of the radial structure of edge poloidal rotation is studied with spectroscopic measurements for L- and H-mode plasmas in the JFT-2M tokamak [Y. Miura et al., in Plasma Physics and Controlled Nuclear Fusion Research, Proceedings of the 13th International Conference, Washington 1990 (IAEA, Vienna, 1991), Vol. 1, p. 325]. The poloidal flow is in the electron diamagnetic direction and appears suddenly near the plasma edge at the H-mode transition. The poloidal rotation velocity profile in the H mode has a peak at the separatrix. No critical normalized ion collisionality, ν*i, for the transition of the L to the H mode is observed. The size of the poloidal flow in the H mode has no dependence on the poloidal gyroradius.