bluuebunny/crossref_metadata_2025 huggingface dataset:
Cluster size distribution of voids in a polymer melt
Authors: Rane Sagar S. , Gujrati P. D.
Published: July 2002

By extending a recently developed Bethe lattice theory, we calculate the cluster size distribution and average cluster size of voids in the presence of polymers. Because of the presence of interactions and because polymers have a size different from that of voids, the model we investigate is a correlated percolation model. The effects of interactions, the pressure P, the degree of polymerization (DP) M, the coordination number q, and the possibility of void percolation on the above properties are evaluated. It is found that small-sized clusters are in overwhelming majority and constitute a large fraction of the total free volume in cases of interest. Attractive monomer–monomer interactions favor the formation of larger clusters. As a function of the DP, the average cluster size shows very different behavior in two regions: with void percolation and without void percolation. The following results are valid at constant temperature and pressure. In the presence of percolation, the average cluster size increases with M, whereas in the absence of percolation it decreases with M. In the absence of void percolation, the average cluster size decreases with increasing q due to the decrease in the total free volume. We present and discuss the results and compare them with those from experiments, simulations and random percolation. We conclude that we are able to qualitatively explain experimental results if we assume that there is no void percolation.
Comment on “Dynamic behaviors of dust particles in the plasma–sheath boundary” [Phys. Plasmas 8, 1886 (2001)]
Authors: Resendes D. P. , Sorasio G. , Shukla P. K.
Published: July 2002

In a recent paper, Takamura et al. [Phys. Plasmas 8, 1886 (2001)] described a variety of dynamic behaviors of dust grains in a dusty plasma device under conditions of a weak friction with gas molecules, that is, at low neutral background gas pressures. When the gas pressure is decreased below a critical value, an instability leading to large amplitude dust oscillations is observed in a single horizontal layer of dust grains trapped near the plasma–sheath boundary above a negatively biased mesh electrode. The instability mechanism has been identified as delayed charging of a dust grain in a plasma sheath. Our objective here is to indicate that the delayed charging mechanism cannot account for stochastic dust oscillations, and to point out a new mechanism that can explain the observed features of large amplitude self-excited dust oscillations.
Impact of deformation potential scattering on free-carrier induced optical nonlinearities: An experimental study in GaAs
Authors: Kotov V. , Stiens J. , Shkerdin G. , Ranson W. , De Tandt C. , Borghs G. , Vounckx R.
Published: July 2002

We present results of the experimental determination of the optical nonlinearities induced by heating free electrons distributed in the multivalley conduction band of highly doped n-GaAs. We propose a very sensitive multilayer leaky waveguide structure for transverse magnetic polarized waves in order to drastically reduce the required optical intensities. We explain in depth the dependence of the optical nonlinearity on doping concentration and deformation potentials. For a doping concentration no of 7.6×1018 cm−3, we found a nonlinear refractive index value n2≈(1∓0.2)×10−6 cm2/W at λ=10.6 μm, by fitting nonlinear reflection measurements with an equivalent intervalley deformation potential value ΛLL=(1.0±0.30)×109 eV/cm.
Ammonia cluster anions and their relationship to ammoniated (solvated) electrons: The photoelectron spectra of (NH3)n=41–1100−
Authors: Sarkas H. W. , Arnold S. T. , Eaton J. G. , Lee G. H. , Bowen K. H.
Published: July 2002

We report the negative ion photoelectron spectra of (NH3)n=41–1100−, recorded using 2.540 eV photons. The largest cluster anion in this series has a diameter of approximately 4.3 nm. The vertical detachment energies (VDEs) of these cluster anions increase smoothly from 0.55 eV for n=41 to 1.05 eV for n=1100. The VDEs throughout this size range are linear with n−1/3 and extrapolate to a VDE (n=∞) value, which is very close to the measured photoelectric threshold energy of condensed phase ammoniated electrons. The linear extrapolation of this data to an analogous condensed phase property implies that these cluster anions are gas-phase counterparts to ammoniated electrons, i.e., they are embryonic forms of ammoniated electrons which will mature with increasing cluster size to become condensed phase-solvated electrons. The VDE data further implies that these embryonic ammoniated electrons were generated in solid ammonia environments, consistent with the source conditions under which they were produced.
Optimal design of switched reluctance motor using two-dimensional finite element method
Authors: Kim Youn-Hyun , Choi Jae-Hak , Jung Sung-In , Chun Yon-Do , Kim Sol , Lee Ju , Chu Min-Sik , Hong Kyung-Jin , Choi Dong-Hoon
Published: July 2002

Switched reluctance motor (SRM) has some advantages such as low cost, high torque density, etc., but SRM has essentially high torque ripple due to its salient structure. To apply SRM to the industrial field, we have to minimize torque ripple, which is the weak point of SRM. This article introduces optimal design process of SRM using a numerical method such as two-dimensional (2D) finite element method. The electrical and geometrical design parameters have been adopted as 2D design variables. From this work, we can obtain the optimal design, which minimizes the torque ripple. We also can obtain the optimal design, which maximizes the average torque. Finally, this article presents performance comparison of two optimal designs, the minimized torque ripple, and the maximized average torque.
Time-resolved photoluminescence of red mercuric iodide
Authors: Wen X. M. , Ohno N.
Published: July 2002

Time-resolved photoluminescence (TRPL) spectrum of red mercuric iodide single crystal for two kinds of different grown methods was measured and compared at low temperatures. The different spectral structures were found for the different grown crystals. At the band-gap region two kinds of photoluminescence were similar. Sharp near band-gap luminescence due to the radiative annihilation of free and bound excitons was observed at 530 nm and a luminescent tail band that originated from the nonradiative annihilation of bound excitons appeared at 540 nm for two crystals. The 560 nm band was observed for the sublimation crystals and a broad luminescent band appeared at 630 nm for the acetone solution crystals, respectively. The TRPL experiment of the near band-gap lines revealed that luminescence comprised a fast and a slow decay component and showed several relaxation processes in free and bound exciton annihilation. The temporal behavior of the tail band revealed the same regularity for two kinds of crystals. The behavior was described with the rate equations and was obviously different from the band-gap luminescence. The 560 nm band had much slower decay than near band-gap decay. At the 630 nm band PL intensity was expressed as I(t)=t−n(r). The experiment of the red band showed clearly that the luminescence was ascribed to the radiative recombination of donor–acceptor pairs.
Examination of even–odd distinction in magnetic susceptibility of silver nanoclusters on silver bromide in light of the Kubo effect
Authors: Tani Tadaaki
Published: July 2002

Silver nanoclusters were formed on octahedral AgBr microcrystals by light and reduction, and characterized by examining their even–odd distinction in light of the Kubo effect. It was observed with an electron microscope that only one Ag cluster per microcrystal was formed by light, while many clusters per microcrystal were formed by reduction. It was found that the average magnetic susceptibility of Ag clusters formed by light was more than 3 orders of magnitude larger than that of Ag clusters formed by reduction when the average size and concentration were the same between them. According to the Kubo theory [R. Kubo, J. Phys. Soc. Jpn. 17, 740 (1962)], Ag nanoclusters should be electrically neutral, and the clusters composed of odd number of Ag atoms (i.e., odd clusters) should be paramagnetic, while even clusters should be diamagnetic. It is therefore considered that the clusters formed by reduction were even clusters, while the clusters formed by light contained many odd clusters. In accord with the Kubo theory that the electron-accepting level of an odd cluster should be lower than that of an even cluster with the same size, the activation energy of the rate of photographic development initiated by Ag clusters formed by light was smaller than that by Ag clusters formed by reduction. The above-stated results proved the hypothesis that the reduction of AgBr microcrystal directly formed Ag dimers, which were stabilized at surface kink sites to act as reduction sensitization centers and coagulated to form Ag nanoclusters when the sites were saturated with Ag dimers.
Determination of transition shape by transition mode noise analysis
Authors: Jin Zhen , Luo Peng , Bertram H. Neal , Zhang Kezhao , Lin G. Herbert
Published: July 2002

Transition noise in thin film magnetic media has been shown to be dominated by three primary modes: transition jitter, pulse width, and amplitude fluctuations. Here, mode analysis of measured noise is used to obtain both the transition parameter and the cross track correlation length. In contrast to spectra measurements, mode analysis clearly shows the effect of transition shape on the parameters. This method yields significant differences of the fitted cross-track correlation length between a hyperbolic tangent and an error function type transition. A hyperbolic tangent transition shape gives a reasonable cross-track correlation length in both high density and medium density system measurements. In the higher density system with smaller head dimensions, the amplitude fluctuation mode emerges as the second dominant mode relative to the jitter mode. This indicates that in systems for increased recording density, local disk nonuniformities play an important role in the total transition noise.
Orbital ordering induced by high impurity levels: Does long-range orbital ordering exist in manganites?
Authors: Maignan A. , Martin C. , Hervieu M. , Raveau B.
Published: July 2002

Substitutions at the Mn site by cations M with oxidation states higher than 4+, such as Mo6+ and Ru5+, are shown to stabilize SrMn1−xMxO3 cubic perovskite manganites. This effect is explained by the induced Mn valency shift, which corresponds to electron doping by Mn3+ in the Mn4+ matrix. Interestingly, these electron-doped manganites, characterized by large A-site cationic size, exhibit coupled structural, magnetic, and transport transitions close to or beyond 300 K for M=Ru5+ and M=Mo6+, respectively. These transitions are characteristic of the dz2 orbital ordering generally associated with C-type antiferromagnetism. This result contrasts with the inhomogeneous ferromagnetic metallic state induced by Ru in CaMnO3. The possibility to induce orbital/charge ordering in CaMn1−xMoxO3 or orbital ordering in SrMn1−xMxO3 (M=Ru, Mo) with M contents superior to 10% makes it more difficult to explain based on long-range cooperative Jahn–Teller phenomena in mixed-valency manganites.
Microstructural and electronic properties of Ni thin films grown on p-InP (100) substrates
Authors: Kim T. W. , Lee D. U. , Choo D. C. , Oh H. J. , Hyun J. W. , Kang S. O. , Yoo K. H. , Yoon Y. S.
Published: July 2002

Ion beam-assisted deposition of Ni on p-InP (100) at room temperature was performed in order to produce Ni thin films with high quality and Ni/p-InP (100) heterostructures with abrupt heterointerfaces. An atomic force microscopy image showed that the root mean square of the average surface roughness of the Ni film was 21.3 Å, and x-ray diffraction and transmission electron microscopy (TEM) measurements show that Ni film layers grown on InP (100) substrates were polycrystalline. Auger electron spectroscopy and TEM measurements showed that Ni films grown on p-InP (100) substrates at room temperature had no significant interdiffusion problems. The work function of the Ni thin film was determined from the secondary electron emission coefficients obtained with a focused ion beam. These results provide important information on the microstructural and electronic properties for Ni thin films grown on p-InP (100) substrates at room temperature.
An ab initio study of the ionization potentials and f–f spectroscopy of europium atoms and ions
Authors: Naleway Conrad , Seth Michael , Shepard Ron , Wagner Albert F. , Tilson Jeffrey L. , Ermler Walter C. , Brozell Scott R.
Published: July 2002

The first three ionization potentials of europium and the f–f spectroscopy of the two lowest multiplets of Eu+3 have been calculated using ab initio spin–orbit configuration interaction techniques. To accomplish this, a new averaged relativistic effective core potential has been developed which leaves only the 5s, 5p, and 4f in the valence space. A series of configuration interaction calculations were carried out up through single and partial double excitations with a double-zeta quality basis set. The computed ionization values have an absolute error of about 0.1 eV from the experimental values. The computed f–f spectroscopy for the lowest F7 multiplet of Eu+3 has a RMS error with experiment of about 100 cm−1. The computed f–f spectroscopy for the first excited D5 multiplet has a higher RMS error of about 350 cm−1. The computed center of gravity separation between the D5–F7 multiplet is underestimated by 750 cm−1. Comparisons between non-spin–orbit and spin–orbit configuration interaction calculations for the separations of the centers of gravity of multiplets are very favorable up through single and double excitations with differences of a tenth of an eV or less. The spin–orbit configuration interaction calculations are among the largest ever performed for lanthanides, with expansion lengths in excess of 1.9 million double-group-adapted functions. The calculations were achieved by application of a new parallel spin–orbit configuration interaction component in the COLUMBUS Program System.
Acoustic energy confinement in randomly layered structures
Authors: Ye Zhen , Luan Pi-Gang
Published: July 2002

Radiation from acoustic sources located inside randomly layered structures is studied using the transfer matrix method. It is shown that in contrast to the periodically layered cases where the radiation can be either enhanced or inhibited depending on the frequency and the characteristics and the material composition of the structures, in the random structures the radiation is always inhibited. The degree of inhibition depends on the acoustic frequency, number of random layers, and the randomness and acoustic parameters of the structures. Both spherically and cylindrically random structures are considered. The results point to the possibility of designing sonic waveguide devices that will not suffer from the energy loss caused by radiation, thus allowing effective energy confinement or long-range energy propagation.
Energy, particle and impurity transport in quiescent double barrier discharges in DIII-D
Authors: West W. P. , Wade M. R. , Greenfield C. M. , Doyle E. J. , Burrell K. H. , Brooks N. H. , Gohil P. , Groebner R. J. , Jackson G. L. , Kinsey J. E. , Lasnier C. J. , Mandrekas J. , McKee G. R. , Rhodes T. L. , Staebler G. M. , Wang G. , Watkins J. G. , Zeng L.
Published: July 2002

Quiescent double barrier discharges (QDB) on DIII-D [Luxon et al., Fusion Technol. 8, Part 2A, 441 (1985)] exhibit near steady high performance (βNH∼7) with a quiescent H-mode edge, i.e., free of edge localized modes (ELMs), but with effective particle control and strongly peaked density profiles. These QDB discharges exhibit an internal transport barrier with low ion thermal transport despite incomplete turbulence suppression. Very short correlation lengths, which reduce the transport step size, however, characterize the residual turbulence. This observation is consistent with simulations using the GLF23 [Waltz et al., Phys. Plasmas 4, 2482 (1997)] model, which reproduce the core ion temperature profile even in the presence of finite turbulence. Increased retention of high-Z impurities is observed and core nickel concentrations (an intrinsic impurity in DIII-D) are as high as 0.3%. To quantify impurity transport, trace impurity injection has been performed in steady QDB discharges showing a fast influx followed by a slow pump out. The measured decay times of the core concentration of two nonrecycling impurities, F(Z=9) and Ca(Z=22), are 299 and 675 ms, respectively, while the energy confinement time is 111 ms. Time dependent analysis of neon influx yields both diffusivities and inward convection velocities significantly greater than neoclassical predictions in the same region of the plasma where measured ion thermal transport is near neoclassical predictions yet significant turbulence is observed. The boundary of these discharges is characterized by a saturated coherent magnetohydrodynamic mode, the edge harmonic oscillation, which takes the place of ELMs in facilitating particle control by allowing particle transport to the open field lines, where both wall- and cryopumping play a major role in particle exhaust. Hot (∼5 keV) ions observed in the outer scrape-off layer may enhance wall pumping.
A fast algorithm for nonlinear finite element analysis using equivalent magnetization current
Authors: Lee Joon-Ho , Park Il-Han , Kim Dong-Hun , Lee Ki-Sik
Published: July 2002

A fast algorithm for iterative nonlinear finite element analysis is presented in this paper. The algorithm replaces updated permeability by an equivalent magnetization current and moves it to the source current term. Once the initial system matrix is decomposed in the LU form, the iterative procedure involves the trivial step of back-substitution from the LU form. Consequently, the computation time for the nonlinear analysis is greatly reduced. A numerical model of a cylindrical conductor enclosed with saturable iron is tested to validate the proposed algorithm. Numerical results are compared with those obtained using conventional Newton–Raphson algorithm in respect to accuracy and computational time.
Solvation effects on electronic excited states of methanol: A study of neat and mixed methanol/water clusters
Authors: Wisniewski E. S. , Hershberger M. A. , Castleman A. W.
Published: July 2002

Femtosecond pump-probe spectroscopic studies on mixed methanol/water clusters are presented. Two-photon absorption of light was employed to generate methanol molecules in the 3p Rydberg excited state, with transfer to the 3s Rydberg state during the ensuing dynamics. The experiments provide evidence that excited state hydrogen transfer occurs between a methanol molecule, which undergoes hydroxyl dissociation following electronic excitation, and a water molecule, leading to the involvement of a metastable H3O species.
Propagating density fluctuations in liquid mixtures: From hydrodynamic to molecular length scales
Authors: Anento N. , Padró J. A.
Published: July 2002

Longitudinal collective modes in liquid binary mixtures are investigated by molecular dynamics simulation. Li–Mg alloys at three concentrations and two isotopic liquid mixtures with mass ratios m2/m1=3 and m2/m1=6 have been simulated. The study is based on the calculation of both the dynamic structure factors and the longitudinal current spectra together with their corresponding dispersion curves. At low wave numbers the system shows a hydrodynamiclike behavior whereas at intermediate wave numbers the molecular nature of the system becomes apparent and the dynamic collective properties of light and heavy particles show a noticeable decoupling. Special attention is paid to the changes on the properties in the transition between the hydrodynamic and molecular regions. The existence of concentration modes in liquid mixtures is also discussed.
Progress of fast ignitor studies and Petawatt laser construction at Osaka University
Authors: Kitagawa Y. , Sentoku Y. , Akamatsu S. , Mori M. , Tohyama Y. , Kodama R. , Tanaka K. A. , Fujita H. , Yoshida H. , Matsuo S. , Jitsuno T. , Kawasaki T. , Sakabe S. , Nishimura H. , Izawa Y. , Mima K. , Yamanaka T.
Published: July 2002

100 TW light from the Petawatt Module (PWM) laser illuminated a preimploded spherical deuterated polystyrene(CD) shell target. The DD neutron yield increased from 2.5×105–106. Analysis indicates that hundreds of keV deuterons, generated around the critical density, collide with cold fuel deuterons and play the leading role in the enhancement of the neutron yield. A two-dimensional particle-in-cell (2D PIC) simulation predicted well the deuteron spectrum. A 60 TW laser was used for MeV proton emissions and megagauss magnetic fields generation on the rear surface of a Poly p-xylene(C8H8) plane target. The 2D PIC simulation explained well the results. The PWM laser was upgraded to one PW, making it the world biggest Petawatt laser (PW laser). An optically parametric chirped amplification was introduced in the front end. The pulse was synchronized to the GEKKO XII imploding beams to within 10 ps.
Multiphoton fluorescence microscopic imaging through double-layer turbid tissue media
Authors: Deng Xiaoyuan , Gan Xiaosong , Gu Min
Published: July 2002

Image formation in multiphoton fluorescence microscopy through double-layer turbid tissue media is investigated using Monte Carlo simulation. With the help of the concept of the effective point spread function, the relationship of image resolution and signal level to the thickness and scattering properties of the double-layer turbid media under single-, two-, and three-photon excitation is revealed. Results show that for a double-layer turbid medium of a given thickness, small particles in the top layer result in a quicker degradation of signal level than large particles in the top layer. This model is then applied to study the penetration depth of multiphoton fluorescence microscopy through human skin tissue which exhibits a layered structure. It is predicated that using 3p excitation leads to a signal level up to two orders of magnitude higher than that under 2p excitation, while diffraction-limited image resolution can be maintained for skin tissue of thickness up to 500 μm.
Nonlinear fluid closure: Three-mode slab ion temperature gradient problem with diffusion
Authors: Holod I. , Weiland J. , Zagorodny A.
Published: July 2002

The three-mode slab ion temperature gradient problem was considered. Starting from the drift kinetic equation with nonlinear term and diffusion, the hierarchy of fluid equations up to fourth moment was developed. As a closure, the nonlinear fluid closure by N. Mattor and S. Parker [Phys. Rev. Lett. 79, 3419 (1997)] was applied. Numerical solutions of the system of fluid equations have been obtained and analyzed. The time evolution of electrostatic potential shows that nonlinear fluid closure is able to capture particle trapping, which is important for fusion plasmas. Great attention was paid to studies of the role of diffusion. Diffusion here represents effects of background turbulence and can be described by a Fokker–Planck operator [A. Zagorodny and J. Weiland, Phys. Plasmas 6, 2359 (1999)]. The three wave system can be considered as a system of test waves in a turbulent background. This system can be used to study situations of varying partial coherence.
Spatially resolved photoconductive properties of profiled polycrystalline silicon thin films
Authors: Savenije Tom J. , van Veenendaal Patrick A. T. T. , de Haas Matthijs P. , Warman John M. , Schropp Ruud E. I.
Published: July 2002

To study the mobility and lifetime of charge carriers in thin film polycrystalline silicon deposited by hot-wire chemical vapor deposition, time-resolved microwave conductivity measurements have been performed. Using this technique the change in conductivity in the polycrystalline silicon films after pulsed excitation can be monitored on a nanosecond time scale, without the use of electrodes. Due to the different penetration depths of the laser wavelengths used (320, 500, and 690 nm) combined with illumination from different sides, the photoconductivity in different regions within the sample can be measured. Four different samples of polycrystalline silicon deposited on Corning glass have been studied: Poly1 (highly defective), Poly2 (device quality) and profiled layers of Poly1 and Poly2. For front or back illumination, the conductivity transients for the Poly1 film are very similar and show that lifetimes of the charge carriers generated are less than 1 ns. For the Poly2 film the mobility in the interfacial substrate region (μ=0.17 cm2/V s) is more than 1 order of magnitude lower than in the top region (μ=3.8 cm2/V s). The formation of a thin Poly1 film on the surface of the Corning substrate, acting as a seed layer for the Poly2 layer, followed by the deposition of the Poly2 layer, results in only a relatively small increase in the mobility in the region close to the substrate as compared to the bare Poly2 layer, while the mobility in the top region remains approximately constant.
Calculation of the group-based pressure in molecular simulations. II. Numerical tests and application to liquid water
Authors: Oliva Baldomero , Hünenberger Philippe H.
Published: July 2002

In the preceding article [J. Chem. Phys. 116, 6880 (2002)] we developed a general formulation for the calculation of the isotropic or anisotropic group-based pressure in molecular simulations under periodic boundary conditions. Two sets of equations were derived for the calculation of the group-based virial, referred to as the “traditional” set and the “alternative” set. The new alternative set offers a number of advantages: (i) it requires no bookkeeping of group definitions in the inner loop of the nonbonded interaction calculation; (ii) the isotropic virial corresponding to each homogeneous pairwise interaction term can be computed directly from the corresponding interaction energy, without knowledge of the pairwise forces; (iii) its application to lattice-sum electrostatics is straightforward. In the present article, the validity and consistency of the different equations derived in this previous study are assessed through a series of numerical tests. In the case of particle-particle–particle-mesh electrostatics, a problem associated with the loss of accuracy of the force calculation due to volume fluctuations in constant-pressure simulations is also discussed. Finally, the new formalism is applied to constant-volume and constant-pressure simulations of systems containing 16 to 1024 simple-point-charge water molecules.
Study of the effect of compressional Alfvén modes on thermal transport in the National Spherical Torus Experiment
Authors: Fredrickson E. D. , Gorelenkov N. , Cheng C. Z. , Bell R. , Darrow D. , Gates D. , Johnson D. , Kaye S. , LeBlanc B. , McCune D. , Menard J. , Roquemore L. , Kubota S.
Published: July 2002

With the first injection of neutral beams into the National Spherical Torus Experiment (NSTX) [Ono et al., Nucl. Fusion 40, 557 (2000)], a broad spectrum of fluctuations consisting of nearly equally spaced peaks in the frequency range from about 0.2 to 1.2 times the ion cyclotron frequency was observed. The frequencies scale with toroidal field and plasma density consistently with Alfvén waves. From these and other observations, the modes have been identified as compressional Alfvén eigenmodes (CAE). It has also recently been found that the ratio of the measured ion and electron temperatures in NSTX during neutral beam heating is anomalously high [Bell, Bull. Am. Phys. Soc. 46, 206 (2001)]. To explain the anomaly in the ratio of ion to electron temperature, it has been suggested that the CAE, driven by the beam ions, stochastically heat the thermal ions [Gates et al., Phys. Rev. Lett. 87, 205003 (2001)]. In this paper it is shown through studies of the power balance that stochastic heating of the thermal ions by the observed CAE alone is not solely responsible for the anomaly in the ion to electron temperature ratio.
Phase-amplitude method for numerically exact solution of the differential equations of the two-center Coulomb problem
Authors: Fröman Per Olof , Larsson Kilian
Published: July 2002

A numerically exact phase-amplitude method that has been presented earlier by P. O. Fröman, Larsson, and Hökback [J. Math. Phys. 40, 1764–1779 (1999)] is particularized and modified in order to make it adapted to the solution of the differential equations of the two-center Coulomb problem.
Study on the quantum efficiency of resonant cavity enhanced GaAs far-infrared detectors
Authors: Zhang Y. H. , Luo H. T. , Shen W. Z.
Published: July 2002

We present a detailed theoretical analysis on the quantum efficiency of a resonant cavity enhanced (RCE) GaAs homojunction work function internal photoemission far-infrared (FIR) detector. The quantum efficiency under both resonant and nonresonant conditions has been calculated. All the detector parameters are optimized under the realistically nonresonant condition. The further investigation of the standing wave effect (SWE) shows that the SWE is important and cannot be neglected for the FIR detector. The resulting quantum efficiency is about two times higher than that in the normal GaAs homojunction FIR detector measured by experiment, showing a promising effect. In contrast to the case in the near-infrared region, the wavelength selectivity is not obvious in the FIR region. The theoretical analysis can be applied to other RCE homojunction FIR detectors.
Kolmogorov spectra of long wavelength ion-drift waves in dusty plasmas
Authors: Onishchenko O. G. , Pokhotelov O. A. , Sagdeev R. Z. , Pavlenko V. P. , Stenflo L. , Shukla P. K. , Zolotukhin V. V.
Published: July 2002

Weakly turbulent Kolmogorov spectra of ion-drift waves in dusty plasmas with an arbitrary ratio between the ion-drift and the Shukla–Varma frequencies are investigated. It is shown that in the long wavelength limit, when the contribution to the wave dispersion associated with the inhomogeneity of the dust component is larger than that related to the plasma inhomogeneity, the wave dispersion and the matrix interaction element coincide with those for the Rossby or the electron-drift waves described by the Charney or Hasegawa–Mima equations with an accuracy of unessential numerical coefficients. It is found that the weakly turbulent spectra related to the conservation of the wave energy are local and thus the energy flux is directed towards smaller spatial scales.
Interaction force induced by composition fluctuations in confined polymer blends close to the consolute point
Authors: Hachem E. K. , Benhamou M. , Daoud M.
Published: July 2002

We consider a critical binary mixture made of two incompatible polymers A and B, confined between two parallel plates 1 and 2, which are at a finite distance L from each other. We assume that the latter adsorb strongly both polymers at a temperature above the consolute point Tc. The strong fluctuations of composition generate an effective force between the two plates. Our aim is precisely the computation of such a force as a function of the separation L. Use is made of the standard ψ4–theory, where ψ is the composition fluctuation or order parameter. We show that the presence of surfaces can be taken into account by imposing two boundary conditions, which depend on the energy necessary to adsorb one monomer on the plates and the surface chemical potential change. For simplification, we choose two special boundary conditions: symmetric and asymmetric plates, depending on whether the plates have the same or opposite preferences for polymers A and B. We demonstrate that, in the strong adsorption limit and close the consolute point (T∼Tc), the resulting effective force becomes universal. For both symmetric and asymmetric plates, we find that the (attractive and repulsive) forces (per unit area) decrease with distance L according to a negative power law, Πa,r∼∓L−4, and we compute exactly the corresponding universal amplitudes.
Generalized symmetries in mechanics and field theories
Authors: Fatibene L. , Ferraris M. , Francaviglia M. , McLenaghan R. G.
Published: July 2002

Generalized symmetries are introduced in a geometrical and global formalism. Such a framework applies naturally to field theories and specializes to mechanics. Generalized symmetries are characterized in a Lagrangian context by means of the transformation rules of the Poincaré–Cartan form and the (generalized) Nöther theorem is applied to obtain conserved quantities (first integrals in mechanics). In the particular case of mechanics it is shown how to use generalized symmetries to study the separation of variables of Hamilton–Jacobi equations recovering standard results by means of this new method. Supersymmetries (Wess–Zumino model) are considered as an intriguing example in field theory.
Laser spectroscopic study of the SiAr van der Waals complex
Authors: Tao Chong , Teslja Alexey , Dagdigian Paul J. , Atahan Sule , Alexander Millard H.
Published: July 2002

Laser fluorescence excitation spectra of the SiAr van der Waals complex, in the vicinity of the Si D°3←3P atomic resonance transition near 220.7 nm are reported. At low resolution, a single excited-state (v′,0) progression of bands terminating in a dissociation continuum is observed. Several weaker bands associated with many of these strong bands are found in scans at higher resolution. A transition to an excited Σ−3 state which correlates with the excited Si(3D°)+Ar asymptote was assigned, and a rotational and vibrational analysis of the observed bands was carried out. The dissociation energies of the Ω=0+ components of the ground X 3Σ− and excited Σ−3 states were determined [D0″=178.8±0.4 and D0′=122.5±0.4 cm−1]. Ab initio calculations of SiAr X 3Σ− and A 3Π electronic states correlating with the ground-state Si(3s23p2 3P)+Ar asymptote were also carried out. The potential energy curves of the definite-Ω states were computed and used to estimate the dissociation energy, rotational constant, and phenomenological spin–spin interaction in the X 3Σ− state. These parameters were found to be in reasonable agreement with the experimental determinations.
Numerical approaches for computing nonadiabatic electron transfer rate constants
Authors: Liao J. L. , Voth Gregory A.
Published: July 2002

A major difficulty in computing nonadiabatic electron transfer (ET) reaction rate constants is that they, in principle, involve the computation of real-time quantum dynamics. The saddle point approximation and the maximum entropy method are in this paper explored with the goal of circumventing this problem. The real-time correlation function in the ET rate formalism must first be analytically continued into the imaginary time domain before these approaches can be employed. Both methods are then applied to a model problem of a two-state ET system coupled to a dissipative bath. The resulting numerical studies are primarily focused on the effects of the anharmonicity of the intramolecular vibrational modes and the coupling of the motion of these modes to the electronic tunneling in ET reactions. These calculations show how the anharmonicity and the electronic-vibrational coupling can significantly effect the value of an ET rate constant.
Magnetoelastic behavior of Ni–Fe films. The Procopiu effect
Authors: Ercuta Aurel , Mihalca Ioan , Schlett Zeno
Published: July 2002

Axial magnetization changes activated by ac circular magnetic fields in ferromagnetic wires produce transverse hysteresis loops of various shapes, depending on both applied dc longitudinal magnetic field and static torsion. Symmetric magnetization reversals occur when the sample is twisted in zero longitudinal field. The results observed in a partially stress-relieved thin cylindrical Permalloy sample are discussed in terms of coherent rotations of the magnetization within large domains of effective biaxial anisotropy. Considering anisotropic magnetostriction and elastic moduli, a two-dimensional energetic model was used to generate numerically transverse hysteresis loops and time dependence of the axial magnetization and its rate of change, in good qualitative agreement with the experimental data. A quasilinear axial magnetization swing versus applied torsion was observed in circular fields of amplitude around 200 Am−1 if the strain did not exceed 10−4 rad, providing basis for simple and stable small angle transducers.
Dipole-exchange theory of hybrid electromagnetic-spin waves in layered film structures
Authors: Demidov V. E. , Kalinikos B. A. , Edenhofer P.
Published: July 2002

A theory has been developed for normal mode electromagnetic-spin waves propagating in metal–dielectric-ferromagnetic-dielectric-metal film structures. Dipole and exchange interactions are taken into account. An arbitrary direction of the internal bias magnetic field is assumed. A dispersion equation for hybrid waves is derived. Effects of varying the dielectric constants of the dielectric layers and the geometry of the layered structure are analyzed for the dispersion characteristics of hybrid waves. The obtained results are applied to layered structures containing ferromagnetic and ferroelectric layers that could be used as waveguiding structures for dual electrically or/and magnetically tunable microwave devices.
Role of As precipitates on ultrafast electron trapping in low-temperature-grown GaAs and AlGaAs alloys
Authors: Loukakos P. A. , Kalpouzos C. , Perakis I. E. , Hatzopoulos Z. , Sfendourakis M. , Kostantinidis G. , Fotakis C.
Published: July 2002

The role and interplay of basic structure parameters of arsenic precipitates on the ultrafast trapping of conduction band electrons have been studied in a series of low-temperature-grown GaAs and AlGaAs epilayers grown at various temperatures ranging from 170 up to 325 °C and annealed at 600 °C. Cross-sectional electron-transmission microscopy was used to determine the density and size of the precipitated arsenic clusters as a function of the growth temperature. The dependence of the electron trapping times (τ) on the spacing (R) and radius (α) of the arsenic precipitates has been systematically investigated by time-resolved pump–probe transient transmission spectroscopy. The electron trapping times follow a τ∝R3/α law which holds for low-temperature-grown GaAs and is applicable for low-temperature-grown AlGaAs as well. This trend generalizes the electron trapping mechanism for low-temperature-grown and annealed GaAs and related semiconductor alloys.
Second order self-organized pattern of terbium–scandium–aluminum garnet and terbium–scandium perovskite eutectic
Authors: Pawlak D. A. , Lerondel G. , Dmytruk I. , Kagamitani Y. , Durbin S. , Royer P. , Fukuda T.
Published: July 2002

Tb 3 Sc 2 Al 3 O 12 has been grown as a crystal showing high Faraday effect. A self-organized pattern, of directionally formed eutectic, has been observed at the edge of the Tb3Sc2Al3O12 microcrystals grown by the micro-pulling down method. Binary (Tb3Sc2Al3O12/TbScO3) and ternary eutectic (depending on the pulling rate and the composition) are formed at the edges showing a highly packed pseudo-hexagonal spot pattern (over large areas) in the cross section and a rod pattern (length up to 200 μm) in the longitudinal section. The size of the spot pattern can be controlled by the pulling rate. The observed arrays can be suitable for various photonic devices.
Molecular-beam epitaxy of GaN/AlxGa1−xN multiple quantum wells on R-plane (101̄2) sapphire substrates
Authors: Ng H. M.
Published: July 2002

GaN / Al 0.15 Ga 0.85 N multiple quantum wells (MQWs) have been grown by plasma-assisted molecular-beam epitaxy on R-plane (101̄2) sapphire substrates. The orientation relationship was found to be (112̄0) (Al)GaN∥(10 1̄2) Al2O3, resulting in nonpolar GaN/AlGaN heterostructures. Room-temperature photoluminescence studies were performed to compare the optical properties of the MQWs grown on (0001) and (101̄2) Al2O3 substrates. The peak transition energy, as a function of well width for the (112̄0) MQWs, followed the trend for rectangular potential profiles indicating the absence of built-in electrostatic fields. In comparison, the peak transition energies for the (0001) MQWs showed a significant redshift due to the quantum-confined Stark effect, consistent with a built-in field value of 750 kV/cm. In addition, the photoluminescence intensity was 20 to 30 times higher for the (112̄0) MQWs compared to the (0001) MQWs.
The width of the exponentially narrow stochastic layers
Authors: Zaslavsky George M.
Published: July 2002

Exponentially small splitting of the separatrix has been calculated for a high frequency large amplitude perturbation and the correspondent correction to the width of the stochastic layer is obtained. The result can be applied to the large amplitude perturbation.
Secondary homoclinic bifurcation theorems
Authors: Rom-Kedar Vered
Published: July 2002

We develop criteria for detecting secondary intersections and tangencies of the stable and unstable manifolds of hyperbolic periodic orbits appearing in time-periodically perturbed one degree of freedom Hamiltonian systems. A function, called the ‘‘Secondary Melnikov Function’’ (SMF) is constructed, and it is proved that simple (resp. degenerate) zeros of this function correspond to transverse (resp. tangent) intersections of the manifolds. The theory identifies and predicts the rotary number of the intersection (the number of ‘‘humps’’ of the homoclinic orbit), the transition number of the homoclinic points (the number of periods between humps), the existence of tangencies, and the scaling of the intersection angles near tangent bifurcations perturbationally. The theory predicts the minimal transition number of the homoclinic points of a homoclinic tangle. This number determines the relevant time scale, the minimal stretching rate (which is related to the topological entropy) and the transport mechanism as described by the TAM, a transport theory for two-dimensional area-preserving chaotic maps. The implications of this theory on the study of dissipative systems have yet to be explored.
Exponential asymptotic expansions and approximations of the unstable and stable manifolds of singularly perturbed systems with the Hénon map as an example
Authors: Tovbis Alexander , Tsuchiya Masa , Jaffé Charles
Published: July 2002

The subject of this paper is the construction of the exponential asymptotic expansions of the unstable and stable manifolds of the area-preserving Hénon map. The approach that is taken enables one to capture the exponentially small effects that result from what is known as the Stokes phenomenon in the analytic theory of equations with irregular singular points. The exponential asymptotic expansions were then used to obtain explicit functional approximations for the stable and unstable manifolds. These approximations are compared with numerical simulations and the agreement is excellent. Several of the main results of the paper have been previously announced in A. Tovbis, M. Tsuchiya, and C. Jaffé [“Chaos-integrability transition in nonlinear dynamical systems: exponential asymptotic approach,” Differential Equations and Applications to Biology and to Industry, edited by M. Martelli, K. Cooke, E. Cumberbatch, B. Tang, and H. Thieme (World Scientific, Singapore, 1996), pp. 495–507, and A. Tovbis, M. Tsuchiya, and C. Jaffé, “Exponential asymptotic expansions and approximations of the unstable and stable manifolds of the Hénon map,” preprint, 1994].
A tutorial on the Rayleigh–Marangoni–Bénard problem with multiple layers and side wall effects
Authors: Johnson Duane , Narayanan R.
Published: July 2002

A brief review in the form of a tutorial is presented on convective instabilities that arise from thermocapillary and buoyancy effects. This tutorial primarily focuses on the effect of multiple layers and side walls on the nature of the convective flows and associated patterns. A comprehensive explanation of the physics of this type of convection is followed by a discussion of the mathematical features of bifurcation associated with the problem and some of the recent experimental studies.
Fitting curves by simulated annealing
Authors: Penna T. J. P.
Published: July 2002

Simulated annealing is shown to be a powerful method to fitting curves. This method is particularly appropriate to fit curves with discontinuous derivatives where the traditional methods fail. Recent generalizations of the simulated annealing method enable easy and efficient implementation on microcomputers. © 1995 American Institute of Physics.
Mechanical properties of composition modulated Cu-Ni foils
Authors: Baral D. , Ketterson J. B. , Hilliard J. E.
Published: July 2002

Measurements have been made of the Young’s, flexural, and torsional moduli of compositionally modulated Cu-Ni foils. The average composition of the foils was 50-at. % Ni and the wavelength of the modulation ranged from 8 to 80 Å. At a wavelength of 20 Å, the flexural modulus exhibited a maximum and had a value two times greater than that measured at shorter or longer wavelengths or that calculated from the bulk single-crystal elastic constants of a homogeneous alloy. The Young’s and torsion moduli exhibited two maxima at 12 and 28 Å. These results are consistent with earlier measurements of the biaxial modulus using a bulge tester. It was also observed that at a wavelength of 20 Å there was a threefold increase in the breaking stress of the foils as compared with homogeneous bulk alloys.
Physical properties of a〈100〉 dislocations in magnesium oxide
Authors: Narayan J.
Published: July 2002

Glide and climb phenomena of a〈100〉 loops and dislocations in magnesium oxide have been investigated using electron microscope techniques. It has been clearly demonstrated that a〈100〉 dislocations could glide along 〈100〉 directions and from these observations the lattice friction stress was estimated to be 1.0±0.1×109 dyn cm−2. From the observed shrinkage rates of loops as a function of temperature, a value of 4.5±0.3 eV for the activation energy for self-diffusion was obtained. This study had also shown that a〈100〉 dislocations can split into, or be formed from, the more commonly observed a/2〈110〉 dislocations in magnesium oxide.
Silicon epitaxy at 650–800 °C using low-pressure chemical vapor deposition both with and without plasma enhancement
Authors: Donahue T. J. , Reif R.
Published: July 2002

A system and a procedure using chemical vapor deposition of silane at very low pressures (<10−2 Torr) have been developed for depositing uniform, specular silicon epitaxial films both with and without plasma enhancement at temperatures as low as 650 °C. In situ cleaning of the substrate surface that overlaps into the deposition is the most critical aspect of the procedure. Undoped films deposited on substrates heavily doped with antimony or boron have abrupt doping profiles. Preliminary measurements indicate that the hole mobility of epitaxial films obtained with this process is 90% of that in bulk silicon. Films oxidized and decorated with a Secco etch show twice as many defects as a similarly treated substrate. Nonplasma growth kinetics are sensitive to surface conditions such as crystallographic orientation, and surface diffusion of adsorbed species appears to be the rate-limiting step for depositing epitaxial films above 700 °C. Around 650 °C, the growth mechanism appears to change, possibly due to the increased presence of hydrogen on the surface. Finally, plasma enhances the growth rate, and plasma kinetics do not seem to be sensitive to surface conditions.