bluuebunny/crossref_metadata_2025 huggingface dataset:
Biphotonic holographic gratings in azobenzene polyesters: Surface relief phenomena and polarization effects
Authors: Sánchez C. , Alcalá R. , Hvilsted S. , Ramanujam P. S.
Published: July 2002

Biphotonic holographic gratings have been recorded in a side-chain azobenzene liquid crystalline polyester using a blue incoherent source and a He–Ne laser. Intensity gratings and the appearance of surface relief have been observed when two linearly polarized beams from a He–Ne laser are made to interfere on a film illuminated with blue light. Polarized holographic gratings are also created with two orthogonally circularly polarized He–Ne beams. All these gratings are stable in darkness but can be erased with blue light.
Optimization of reflective bistable twisted nematic displays with retardation compensation
Authors: Xie Z. L. , Kwok H. S.
Published: July 2002

The contrast ratios of reflective bistable twisted nematic liquid crystal displays with a rear quarter-wave film are optimized by means of a parameter space approach. By plotting the contrast ratio as a function of the twist angle and thickness-birefringence product (dΔn) of the display, the regions where the displays show the best contrast ratio are clearly shown. Both the absolute reflectance and the wavelength dispersion of two bistable twisted states are investigated. It is confirmed that superhigh contrast ratio (infinite in principle), reflectance about 95% with polarized-light input, a large dΔn value (0.63 μm), and low dispersion are possible for such reflective bistable liquid crystal display.
Onset of electroluminescence from bilayer light emitting diodes under space charge limited majority carrier injection
Authors: Nikitenko V. R. , Bässler H.
Published: July 2002

An analytic theory has been developed to analyze the rise of electroluminescence from bilayer light emitting diodes upon applying a rectangular voltage pulse if the current of majority carriers is space charge limited while minority carrier injection is electrode limited. The onset of electroluminescence is governed by the growth of the interfacial charge densities rather than by the transit time. This result suggests that, for appropriate interfacial energy barriers, balanced injection can be established even if only one of the electrodes is ohmic. A qualitative agreement between theory and experiment has been obtained.
Asymptotic layer-type model of high pressure direct current glow discharge in electronegative gases
Authors: Bektursunova Rimma M. , Graham William G.
Published: July 2002

Here a self-consistent one-dimensional continuum model is presented for a narrow gap plane-parallel dc glow discharge. The governing equations consist of continuity and momentum equations for positive and negative ions and electrons coupled with Poisson’s equation. A singular perturbation method is developed for the analysis of high pressure dc glow discharge. The kinetic processes of the ionization, electron attachment, and ion–ion recombination are included in the model. Explicit results are obtained for the asymptotic limits: δ=(rD/L)2→0, ω=(rS/L)2→0, where rD is the Debye radius, rS is recombination length, and L is the gap length. The discharge gap divides naturally into four layers with multiple space scales: anode fall region, positive column, transitional region, cathode fall region and diffusion layer adjacent to the cathode surface, its formation is discussed. The effects of the gas pressure, gap spacing and dc voltage on the electrical properties of the layers and its dimension are investigated.
Photoluminescence of mesoporous silica molecular sieves
Authors: Zhang Y. , Phillipp F. , Meng G. W. , Zhang L. D. , Ye C. H.
Published: July 2002

Mesoporous silica molecular sieves (MCM-41), which are a mixture of microtubules and hexagonal prisms, were prepared by a hydrothermal self-organizing method under acidic conditions. The nanostructure of silica was characterized by scanning electron microscope and transmission electron microscope. The composition of the sample was analyzed by Infrared spectra, Raman spectra, and energy-dispersed x-ray spectra. The photoluminescence spectrum shows that a broad emission band appears in the wavelength range from 400 to 800 nm, which consists of two broad emission subbands, one at 477 nm and the other at 512 nm. The visible luminescence appearing in calcined MCM-41 silica is ascribed to significant amounts of the twofold coordinated silicon centers (Si20).
Substrate/layer interface of amorphous-carbon hard coatings
Authors: Böhme O. , Cebollada A. , Yang S. , Teer D. G. , Albella J. M. , Román E.
Published: July 2002

A combined study of the crystalline structure, the chemical interaction, and diffusion processes of the substrate/layer interface of amorphous-carbon hard coatings is presented. The structure of the coatings and their gradient layer interface to a chromium buffer layer has been investigated on two substrates [Si(100) and tool steel] using x-ray diffraction (XRD). Chemical interaction and diffusion processes at the interfaces and within the layers were analyzed by Auger electron spectroscopy and x-ray photoemission spectroscopy depth profiles. The chromium buffer layer revealed similar textured structure on both substrates. The subsequent gradient layer was determined (within XRD limits) to be amorphous and composed of an amorphous-carbon and chromium-carbide composite. The chromium carbide maintains the same stoichiometry (Cr3C2), regardless of the gradually changing chromium content. No large-scale interdiffusion was measured, either between or within the layers.
Capacitance spectroscopy of InAs self-assembled quantum dots embedded in a GaAs/AlAs superlattice
Authors: Chiquito A. J. , Pusep Yu. A. , Mergulhão S. , Galzerani J. C. , Moshegov N. T. , Miller D. L.
Published: July 2002

The characteristics of the InAs self-assembled quantum dots embedded both in a GaAs bulk matrix and in a GaAs/AlAs superlattice were investigated. Evidences of electrons confinement inside the InAs quantum dots were obtained using both capacitance–voltage measurements and Raman spectroscopy. A much stronger electron localization was detected for the quantum dots embedded in the superlattice in comparison with those embedded in bulk GaAs. As a consequence, the electrical characteristics of the structures with quantum dots grown in superlattices were found to be significantly thermo-stabilized. The origins of these effects are discussed in connection with the differences between the electronic features of the two kinds of structures.
Drift mobilities and Hall scattering factors of holes in ultrathin Si1−xGex layers (0.3<x<0.4) grown on Si
Authors: Lander R. J. P. , Ponomarev Y. V. , van Berkum J. G. M. , de Boer W. B. , Loo R. , Caymax M.
Published: July 2002

Sheet resistivity and Hall measurements have been performed on a series of p-type modulation-doped Si/Si1−xGex heterostructures. The structures were grown by a production-compatible atmospheric-pressure chemical-vapor deposition technique and all the epitaxial layers were lattice matched to the silicon substrates. A depleted-doping technique was used to supply the quantum well with holes, and this approach has enabled the transport properties of the SiGe layers to be characterized between 4.2 and 295 K. Measurements of the Hall scattering factor and drift mobility are reported for ultrathin, high-Ge content layers (0.3<x<0.4) with a range of hole densities up to 4×1012 cm−2. The drift mobilities are shown to be substantially and consistently higher than comparable mobilities reported for holes in Si/SiO2 inversion layers. A drift mobility of 460±20 cm2/V s was measured at 295 K for a 6 nm-thick Si0.65Ge0.35 layer. This is more than a factor of 2 greater than the equivalent Si inversion layer mobility and is comparable to the highest room-temperature mobilities reported for holes in fully strained Si/SiGe heterostructures. The Hall scattering factor is shown to be very dependent upon temperature and, at 295 K, very dependent upon the hole density. At 295 K the Hall scattering factor does not exhibit a strong dependence upon Ge content. The variation of the hole drift mobility with temperature has been shown to fit a power dependence μ∼Tγ in the range 120–300 K. The γ values are consistent with simple acoustic phonon scattering theory, although a variation in γ with hole density is also observed, suggesting that a more rigorous theoretical treatment is required.
Quasistatic capacitance-voltage characteristics of plane-parallel structures: Metal/semi-insulator/metal
Authors: Ždánský K.
Published: July 2002

Quasistatic capacitance-voltage characteristics of plane-parallel structures of the semi-insulating GaAs with two metal contacts are calculated numerically. The model of residual shallow acceptors compensated for by an excess of deep donors is used for the semi-insulating GaAs. Under consideration is the relaxation regime for the charge transport in the semi-insulator. We analyze metal/semi-insulator/metal structures for various parameters of the metal contacts and of the semi-insulator. It is shown that a negative capacitance can be obtained in some cases. A more detailed analysis is performed for the following structure: p-type like metal contact/n-type semi-insulator/n-type like metal contact.
Measurement and control of absolute nitrogen atom density in an electron-beam-excited plasma using vacuum ultraviolet absorption spectroscopy
Authors: Tada Shigekazu , Takashima Seigou , Ito Masafumi , Hori Masaru , Goto Toshio , Sakamoto Yuichi
Published: July 2002

The absolute nitrogen (N) atom density in an electron-beam-excited plasma (EBEP) operating at an ultralow pressure has been investigated by vacuum ultraviolet absorption spectroscopy, employing a microdischarge hollow-cathode lamp. The measured N atom density was estimated to be around 6×1011 cm−3, and the dissociation fraction was 4.9% at a N2 pressure of 0.05 Pa, an electron-beam current of 10 A, and an electron-beam acceleration voltage of 120 V. The EBEP potentially enables us to control the electron density and electron energy independently with the electron-beam current and electron-beam acceleration voltages, respectively. It was found that N atom densities increased with increasing electron-beam current and electron acceleration voltage under low-pressure conditions. The EBEP shows great promise as a N atom source operating at an ultralow pressure.
The mapping of the local contributions of Fermi and Coulomb correlation into intracule and extracule density distributions
Authors: Fradera Xavier , Duran Miquel , Mestres Jordi
Published: July 2002

The contributions of the correlated and uncorrelated components of the electron-pair density to atomic and molecular intracule I(r) and extracule E(R) densities and its Laplacian functions ∇2I(r) and ∇2E(R) are analyzed at the Hartree–Fock (HF) and configuration interaction (CI) levels of theory. The topologies of the uncorrelated components of these functions can be rationalized in terms of the corresponding one-electron densities. In contrast, by analyzing the correlated components of I(r) and E(R), namely, IC(r) and EC(R), the effect of electron Fermi and Coulomb correlation can be assessed at the HF and CI levels of theory. Moreover, the contribution of Coulomb correlation can be isolated by means of difference maps between IC(r) and EC(R) distributions calculated at the two levels of theory. As application examples, the He, Ne, and Ar atomic series, the C2−2, N2, O2+2 molecular series, and the C2H4 molecule have been investigated. For these atoms and molecules, it is found that Fermi correlation accounts for the main characteristics of IC(r) and EC(R), with Coulomb correlation increasing slightly the locality of these functions at the CI level of theory. Furthermore, IC(r), EC(R), and the associated Laplacian functions, reveal the short-ranged nature and high isotropy of Fermi and Coulomb correlation in atoms and molecules.
Focused nanoparticle-beam deposition of patterned microstructures
Authors: Di Fonzo F. , Gidwani A. , Fan M. H. , Neumann D. , Iordanoglou D. I. , Heberlein J. V. R. , McMurry P. H. , Girshick S. L. , Tymiak N. , Gerberich W. W. , Rao N. P.
Published: July 2002

A method was developed for fabricating nanocrystalline microstructures. This method involves synthesizing nanoparticles in a thermal plasma expanded through a nozzle, and then focusing the nanoparticles to a collimated beam by means of aerodynamic lenses. High-aspect-ratio structures of silicon carbide and titanium were deposited on stationary substrates, and lines and two-dimensional patterns were deposited on translated substrates. Linewidths equalled approximately 50 μm. This approach allows the use of much larger nozzles than in previously developed micronozzle methods, and also allows size selection of the particles that are deposited.
Quantum calculations of the effect of bend excitation in methane on the HCl rotational distribution in the reaction CH4+Cl→CH3+HCl
Authors: Skokov Sergei , Bowman Joel M.
Published: July 2002

We report novel, reduced dimensionality quantum calculations of the effect of bend excitation in methane on the rotational distribution of HCl in the reaction CH4+Cl→CH3+HCl. We use a modification of a semiempirical potential developed by Yu and Nyman in these calculations. Reaction probabilities for zero total angular momentum are obtained using an L2 wave packet method for selected HCl rotational states (in the ground vibrational state) and summed over final CH bend/stretch states in CH4. The calculations are done in two ranges of the total energy. In the low energy range only the ground bend/stretch state of CH4 is open, and in the higher energy range that state and the second excited bend state are energetically open. With just the ground state of CH4 open we find a cold, unimodal rotational distribution of HCl, in qualitative agreement with experiment. With the opening of the excited bend state we find a multimodal HCl rotational distribution. We rationalize these results using a simple Franck–Condon argument.
A multiconfiguration self-consistent field/molecular dynamics study of the (n→π*)1 transition of carbonyl compounds in liquid water
Authors: Martı́n M. E. , Sánchez M. L. , Olivares del Valle F. J. , Aguilar M. A.
Published: July 2002

A model is presented for the electrostatic component of solvatochromic shifts in vertical electronic excitation energies. The model, which makes use of the mean-field approximation, combines quantum mechanics (QM) in the description of the solute molecule and molecular mechanics (MM) in the description of the solvent. The method is implemented at the multiconfigurational self-consistent field level. We present illustrative applications to the (n→π*)1 transitions of formaldehyde, acetaldehyde, and acetone in liquid water. The solvent shifts obtained compare well with other ab initio QM/MM calculations and when the electron correlation components are included with the experimental solvent shift, but differ from the results obtained with semiempirical QM/MM and continuum models.
Electrostatic Parameters of Ionic Crystals
Authors: Mestechkin M. M.
Published: July 2002

More than 200 Madelung constants (MCs), site potentials, and electric field gradient components for ionic crystals of different nature calculated by means of the modified Madelung–Born method are presented. The same technique has helped in finding superposition rules interconnecting different classic crystals, which are useful for checking the accuracy of calculated MCs, the local site potentials, and electric field gradient tensors. The definition of Madelung interaction potentials (MIPs) is introduced, and these purely geometric quantities, independent of particular charge distribution, are found the most suitable for tabulation of crystal electric field parameters. MIPs are calculated between a number of characteristic points of fcc, bcc, hcp, and some other cells. MIPs depend strongly on the choice of cell geometric parameters but allow easy calculation of the local potentials for arbitrary point charge distribution, which are independent of this choice. The site potentials determined for yttrium ceramics, fullerides, and superfullerides make it possible to examine some regularities useful for interpretation of the observed phenomena already at the electrostatic level. The comparison of the most recent results with the calculations of MC of known crystals by means of the Evald and other techniques reveals complete agreement. The advantages of the present approach are manifested in calculations of the surface electrostatic parameters, which are found for a number of crystal planes, surfaces, and sets of layers including those composed of different crystals and those containing charged crystal planes. This technique can be applied for computation of MCs, site potentials, and electric field gradients for a crystal body restricted by two parallel planes, oriented arbitrarily to the crystal axes, in a layer situated at any depth from the surface.
Ab initio investigations of Li−+nH2→LiH2−(H2)n−1,n=1–3
Authors: Sharp Stephanie B. , Gellene Gregory I.
Published: July 2002

Ab initio investigations at the coupled-cluster single double (triple) [CCSD(T)] and MRCISD level with augmented triple and quadruple zeta basis sets have identified various stationary points on the Li−/(H2)n,n=1–3, hypersurfaces. The electrostatic complexes, Li−(H2)n, are very weakly bound (De<0.25 kcal/mol with respect to H2 loss) and H2/H2 interactions play a contributing role in determining the equilibrium structures within the electrostatic constraint of a linear or near-linear Li−–H–H orientation. The covalent molecular ion, LiH2−, is found to have a linear centrosymmetric structure and to be bound with respect to Li−+H2 in agreement with previous calculations. The interaction of LiH2− with additional H2 is purely electrostatic but with a De larger than those of the Li−(H2)n complexes. LiH2−(H2) is found to have a linear equilibrium structure and LiH2−(H2)2 is found to have two almost isoenergetic structures: linear with an H2 on either end of the LiH2−, and C2v with both H2 on the same end of the LiH2−. Of particular interest is the dramatic change in the nature of the transition state for LiH2− production depending on the number of H2 molecules present. For n=1, the reaction proceeds through a conical intersection between the lowest energy B21 and A11 electronic surfaces in C2v symmetry. For n=2, the reaction occurs on a single surface in a pericyclic mechanism through a transition state consisting of a planar five-member ring where simultaneously two H2 bonds are broken while two LiH bonds and one new H2 bond are formed. For n=3, the reaction proceeds by direct insertion of Li− into one of the H2 molecules with the two additional H2 molecules providing substantial stabilization of the transition state by taking on part of the negative charge in a weakly covalent interaction. The results are discussed in comparison to the isoelectronic B+/(H2)n systems where significant sigma bond activation through a cooperative interaction mechanism has been identified recently.
Anomalous infrared transmission of adsorbates on ultrathin metal films: Fano effect near the percolation threshold
Authors: Krauth Oliver , Fahsold Gerhard , Magg Norbert , Pucci Annemarie
Published: July 2002

IR absorption lines of CO on ultrathin epitaxial iron films are both enhanced and asymmetric. In this letter we show new experimental results which demonstrate a correlation of the asymmetry of the CO-stretching line to electronic properties of the underlying metal film. The new finding indicates the important role of metal film morphology for nonadiabatic effects. Such effects are strongest slightly above the percolation threshold as our results show.
Controlling molecular alignment rephasing through interference of Raman-induced rotational coherence
Authors: Hertz E. , Faucher O. , Lavorel B. , Chaux R.
Published: July 2002

Quantum control over molecular alignment rephasing is experimentally investigated in gaseous CO2. The control process is achieved by illuminating the medium with a pair of pump-pulses separated in time by approximately an integer value of T0=1/8B0, where B0 is the rotational constant. Through a Raman-type process, each pulse alone produces rotational coherence leading to a periodic orientational anisotropy. It is the combination of the two pulses that yields to quantum interference, resulting in a modification of this anisotropy probed by a third delayed pulse. The effect is accurately analyzed for different time delays between the two pulses. A theoretical analysis supplies a clear understanding of the role played by the different rotational motions involved in the overall process. The relative orientation of the electric field vector for the two pulses is discussed in terms of an additional control parameter.
Single molecule vibrational spectroscopy and microscopy: Cu(II) etioporphyrin-I on Cu(001)
Authors: Wallis T. M. , Chen X. , Ho W.
Published: July 2002

A scanning tunneling microscope was used to perform inelastic electron tunneling spectroscopy on single Cu(II) etioporphyrin-I molecules. The resulting vibrational spectra showed an increase in ac tunneling conductance at 359 mV. This change was attributed to the vibrational excitation of the methene bridge C–H stretch mode. Vibrational microscopy was used to image the spatial distribution of the inelastic tunneling channels involved in this excitation.
Selective control of self-organized In0.5Ga0.5As/GaAs quantum dot properties: Quantum dot intermixing
Authors: Bhattacharyya D. , Helmy A. Saher , Bryce A. C. , Avrutin E. A. , Marsh J. H.
Published: July 2002

Selective postgrowth control of the photoluminescence (PL) wavelength has been demonstrated for a single layer self-organized In0.5Ga0.5As/GaAs quantum dot (QD) structure. This was achieved by rapid thermal processing of dots using different dielectric caps. Selective band gap shifts of over 100 meV were obtained between samples capped with sputtered and plasma enhanced silica deposition, with the band gap shift under regions covered with plasma enhanced chemical vapor deposition SiO2 less than 70 meV. The effects of different caps on the PL linewidth were also observed. The differential band gap shift will enable the integration of passive and active devices in QD systems.
Spectroscopy of self-assembled CdS quantum dots in ZnSe
Authors: Zhou Hailong , Nurmikko A. V. , Nakamura S. , Kitamura K. , Umeya H. , Jia A. , Kobayashi M. , Yoshikawa A. , Shimotomai M. , Kato Y.
Published: July 2002

Self-assembled nanocrystalline dots of CdS grown within a ZnSe host have been studied by steady state and transient optical spectroscopies. This material system features an unusually low density of the dots, into which the excitation transfer of excitons from the host has been identified.
Elastic constants of cubic boron nitride films
Authors: Pastorelli R. , Ossi P. M. , Bottani C. E. , Checchetto R. , Miotello A.
Published: July 2002

The elastic constants of a cubic boron nitride film, with a fraction of tetrahedral bonding of 96% were measured by surface Brillouin spectroscopy. Precise values for both shear and Young modulus were obtained. A comparison with nanoindentation results was made and a critical discussion of the discrepancies is offered.
Acetylenic C–H and methyl C–D bond fission in photodissociation of vibrationally excited propyne-d3
Authors: Chen X. , Ganot Y. , Bar I. , Rosenwaks S.
Published: July 2002

Vibrational preexcitation of a state containing three quanta of C–H stretch (3ν1) results in C–H acetylenic and C–D methyl bond rupture in the ∼243.1 nm photolysis of CD3C≡CH, in contrast to previous observations of the almost isoenergetic 193 nm photodissociation of propynes. The C–D bond fission is the dominant pathway with a D/H branching ratio of 2.0±0.5 at a combined energy of ∼50 830 cm−1. The average translational energies of D and H atoms are nearly identical, although the C–H acetylenic and C–D methyl bond energies differ quite extensively, pointing to different dynamics on the involved potential energy surfaces.
Ro-vibrational levels for HF2+: The effect of rotation on vibrational splittings for proton transfer
Authors: Hamilton I. P. , Wu Xudong T. , Carrington Tucker
Published: July 2002

Ro-vibrational energy levels of HF2+ are computed to study the effect of rotation on tunneling splittings for proton transfer. The effect of rotation is small but systematic. Numerically exact results are tabulated and trends are discussed in terms of a model that neglects Coriolis coupling.
Luminescence quantum efficiency and local optical density of states in thin film ruby made by ion implantation
Authors: Hensen T. M. , de Dood M. J. A. , Polman A.
Published: July 2002

Single crystal (0001) oriented, sapphire samples were implanted with 150 keV Cr ions at fluences between 6.0×1014 and 4.0×1015 Cr/cm2. The peak concentrations ranged from 0.04 to 0.28 at %. Characteristic photoluminescence of the R lines at 694.3 and 692.9 nm was observed. Annealing at 1450 °C for 2 h increased the luminescence intensity by a factor of 45, due to the increasing fraction of substitutional Cr ions as confirmed by Rutherford backscattering spectrometry. The Cr luminescence decay rate in an annealed sample implanted with 3.0×1015 at/cm2 at 300 K is 299 s−1. Decay rates were also measured for samples covered with a range of transparent liquids (refractive index n=1.33–1.57), showing a clear increase with increasing refractive index of the liquid. This effect is explained by the increase of the local optical density of states in the Cr-implanted region. By comparing the measured data with the calculated optical density of states the radiative decay rate is found to be 164±10 s−1 and the internal quantum efficiency ∼50%. The quantum efficiency decreases slightly for increasing Cr concentration.
Cyclotron amplification of whistler waves by nonstationary electron beams in an inhomogeneous magnetic field
Authors: Demekhov A. G. , Trakhtengerts V. Y. , Hobara Y. , Hayakawa M.
Published: July 2002

The linear amplification of whistler waves with varying frequency by beams of energetic electrons in the inhomogeneous magnetic field is studied. Rigorous treatment developed by Trakhtengerts et al. [Phys. Plasmas 6, 692 (1999)] for the distribution function with a steplike deformation over parallel velocities is applied here for the δ-like distribution which is appropriate for triggered emissions. The formalism developed enables one to determine the one-hop amplification of a wavelet with variable frequency and the “optimal” spatio-temporal variation of the frequency corresponding to the maximum amplification; we show this variation to coincide with the so-called second-order cyclotron resonance condition, similarly to the case of the steplike distribution. The results allow one to compare the dependence of the maximum amplification on characteristic parameters of a plasma in the magnetic trap for smooth, steplike, and beamlike distributions.
Integral encounter theories of multistage reactions. II. Reversible inter-molecular energy transfer
Authors: Ivanov K. L. , Lukzen N. N. , Doktorov A. B. , Burshtein A. I.
Published: July 2002

The matrix Modified Encounter Theory (MET), developed in Part I of this work, is applied here to reversible inter-molecular energy transfer in liquid solutions. For fluorescence quantum yield at contact transfer the Stern–Volmer law is confirmed, but the concentration corrections to its constant are diffusion-dependent unlike those obtained earlier with Superposition Approximation. In the particular case of irreversible energy transfer, when the exact solution is available, the latter is used to discriminate between all competing approaches and establishes MET superiority. In the case of reversible energy transfer producing the long-lived or even stable products, the energy is stored there and dissipates due to backward energy transfer in re-encounters. The kinetics of this process, resulting in a delayed fluorescence, is shown to be qualitatively different in cases of short and long encounter times as compared to the excitation lifetime.
Improvement on epitaxial grown of InN by migration enhanced epitaxy
Authors: Lu Hai , Schaff William J. , Hwang Jeonghyun , Wu Hong , Yeo Wesley , Pharkya Amit , Eastman Lester F.
Published: July 2002

Epitaxial growth of InN on (0001) sapphire with an AlN buffer layer was studied by migration-enhanced epitaxy, which is composed of an alternative supply of pure In atoms and N2 plasma. A series of samples were prepared with different substrate temperatures ranging from 360 to 590 °C. As-grown films were characterized by x-ray diffraction (XRD), reflective high-energy electron diffraction, atomic-force microscopy (AFM), and Hall measurements. Both XRD θ–2θ and ω scans show that the full width at half maximum of the (0002) peak nearly continuously decrease with increasing growth temperature, while InN grown at 590 °C shows the poorest surface morphology from AFM. It is suggested that three-dimensional characterization is necessary for an accurate evaluation of the quality of the InN epilayer. Hall mobility as high as 542 cm2/V s was achieved on film grown at ∼500 °C with an electron concentration of 3×1018 cm−3 at room temperature. These results argue against the common view that nitrogen vacancies are responsible for the high background n-type conductivity of InN. To illuminate the relationship between Hall mobility and carrier concentration, the electrical properties of all InN films grown recently were summarized.
Simulation of nanoscale density fluctuations
Authors: Reiss Howard , Bowles Richard K.
Published: July 2002

Problems associated with the simulation of density fluctuations of limited breadth in a small cell are exposed and studied. The fluctuations are viewed as “physical clusters” of the type that might appear in nucleation processes and related phenomena. One of the most important features of the study stems from the fact that the simulation of a small heterogeneity in a macroscopic system presents problems that do not occur in the simulation of a bulk homogeneous property of the system. For example, once having simulated the probability of appearance of the fluctuation in a small cell, how is that result to be “mapped” onto the macrosystem in order to specify the equilibrium number of such fluctuations in that system? This problem is closely associated with the proper separation of the translational and internal degrees of freedom of the system, and has arisen in a number of fields, including the theory of nucleation. There are other problems associated with exponential dependence of cluster probability on the work of formation of the cluster, and also with rareness of some important clusters. In the latter case, simulative “umbrella sampling” does not always solve the entire problem. The present study is confined to clusters that appear in rarefied gases. Such systems are important in a number of scenarios, including nucleation processes. Several cluster models are considered including those consisting of molecules confined to a “container” of fixed volume and those constructed on the center of mass of the cluster. Connections between them are derived and rigorous solutions to the mapping problem are derived. Quantitative measures for the accuracy of approximate solutions, applied to cases in which the cluster is compact, are provided and exact solutions are provided even for the noncompact case. Some surprising results emerge from the study, among which is the fact that a cluster whose location is determined by one of its molecules, does not always have a probability of location that is uniform throughout a volume even if the cluster molecules do not see the boundaries of that volume. Also, the translational degrees of freedom of such a cluster cannot always be fully “activated” by freeing the “locational” molecule to move throughout the system.
Oblique stimulated Raman scattering of a short laser pulse in a plasma channel
Authors: Turano E. J. , McKinstrie C. J.
Published: July 2002

The spatiotemporal evolution of parametric instabilities such as stimulated Raman scattering is studied analytically in time and two spatial dimensions. Initial and boundary conditions are chosen to represent the entrance, propagation, and exit of a laser pulse of finite extent as it progresses through a homogeneous collisional plasma channel. For most scattering angles daughter wave growth is enhanced by lateral reflections within the channel. At late times the two-dimensional interaction approaches a one-dimensional damped mode in which the dissipative loss from lateral transmission of the Stokes wave through the channel boundaries is equivalent to an overall damping of the Stokes amplitudes within the channel.
Pseudospark-based electron beam and Cherenkov maser experiments
Authors: Yin H. , Robb G. R. M. , He W. , Phelps A. D. R. , Cross A. W. , Ronald K.
Published: July 2002

Detailed experimental results from the first free-electron maser experiment to use a pseudospark-based electron beam are presented in this paper. These include the design and realization of a pseudospark-based electron beam source and Cherenkov maser experiment. A pulsed, 70–80 kV, 10 A electron beam was obtained from the hollow cathode discharge phase of an 8-gap pseudospark (PS) discharge. The beam was used to produce coherent microwave radiation via a Cherenkov interaction between the electron beam and the TM01 mode of a 60-cm long alumina-lined waveguide. A gain of 29±3 dB was measured and an output power of 2±0.2 kW in the frequency range 25.5–28.6 GHz. Results from numerical simulations of the Cherenkov amplification are also presented and found to be consistent with the experimental results.
Limit of temperature anisotropy relaxation by ion cyclotron waves: A statistical theory
Authors: Nakamura Tadas K.
Published: July 2002

The ion velocity distribution resulting from ion anisotropy reduction by the growth of ion cyclotron waves has been investigated by the use of a Maximum Entropy Principle. The anisotropy is reduced due to waves induced by an electromagnetic instability, but the system saturates before a completely isotropic Maxwellian distribution is realized, and a nonzero level of anisotropy remains in the final state. This is because the scattering process has a restriction; waves do not alter particle energy in the wave rest frame. Taking this restriction into account as an additional constraint, the final state is calculated by maximizing the entropy. The results show that the upper bound of the plasma β and anisotropy are inversely correlated, which is quantitatively in agreement with recent studies on satellite observations, numerical simulations, and laboratory experiments.
Evaluation of small-angle scattering data of charged particles using the generalized indirect Fourier transformation technique
Authors: Fritz G. , Bergmann A. , Glatter O.
Published: July 2002

Small-angle x-ray and neutron scattering are widely used techniques to study the structure of colloidal particles in the size range up to 100 nm. The indirect Fourier transformation technique is well established to obtain model free real space information, but the interpretation of the results is limited to cases where particle interaction can be neglected. The extended generalized indirect Fourier transform (GIFT) allows one to separate inter- and intraparticle effects, but needs models for the particle interaction. We present the application of three different models for the calculation of interaction effects of charged particles, represented by the structure factor. With this extension, useful real space information can be obtained by the GIFT method for solutions with volume fractions up to about 0.3 without any assumption for the shape of the particles. Only the interaction effects need a model assumption, and the parameters determined from this model can give some additional information. Simulations show that it is impossible to determine charge and ionic strength simultaneously. There exists another ambiguity between the parameter sets for charge, radius, and volume fraction, but we show how this problem can be overcome in most cases. The practical applicability of the method is demonstrated by means of the micellar system CTAB in different concentrations from 1% up to 20% and with varying amounts of added salt to screen the charges and change the particle shape.
Comparative study of pulsed laser ablated plasma plumes from single crystal graphite and amorphous carbon targets. Part I. Optical emission spectroscopy
Authors: Yamagata Y. , Sharma A. , Narayan J. , Mayo R. M. , Newman J. W. , Ebihara K.
Published: July 2002

A comparative study of ablation plasma plumes originated from single crystal graphite (SCG) and amorphous carbon (a-C) targets during the preparation of diamond-like carbon (DLC) films by KrF excimer pulsed laser deposition (PLD) has been carried out by means of a monochromator equipped with an intensified optical multichannel analyzer. In high vacuum, the emission lines of carbon neutral C and ions of C+, C2+, and C3+ can be observed from both the SCG and a-C plasma plumes. The emission intensity from C atoms increases with laser energy density (EL) increase for both cases. The C2 emission intensity from the SCG plasma plume changes drastically with EL, while that from the a-C plasma plume is almost constant. The C2/C emission intensity ratio for the a-C case decreases with EL increase. As for the SCG case, the C2/C ratio decreases with EL increase up to 3.0 J/cm2, and increases slightly with further EL increase. Nanohardness of the deposited films decreases with the increase of the C2/C emission intensity ratio. It is suggested that for both the SCG and a-C target cases, the C2 molecule in the ablated plasma plume may not play an important role in producing high quality DLC films. It is further proposed that the threshold of laser fluence for the formation of diamond-like character film using KrF excimer PLD is 2.1 J/cm2(0.84×108 W/cm2) for the a-C target and 3.0 J/cm2(1.2×108 W/cm2) for the SCG target. The C2 vibrational temperature of the SCG and the a-C plasma plumes show different features on both the laser energy density and nitrogen pressure dependencies. Through optical emission spectroscopy and Langmuir probe measurements in vacuum and nitrogen background, it is concluded that there are many particles with higher mass in the SCG plasma plume, especially at relatively lower laser energy density below 3.0 J/cm2.
Shear-flow driven current filamentation: Two-dimensional magnetohydrodynamic-simulations
Authors: Konz C. , Wiechen H. , Lesch H.
Published: July 2002

The process of current filamentation in permanently externally driven, initially globally ideal plasmas is investigated by means of two-dimensional magnetohydrodynamic-simulations. This situation is typical for astrophysical systems like jets, and the interstellar and intergalactic medium where the dynamics is dominated by external forces. Two different cases are studied. In one case, the system is ideal permanently and dissipative processes are excluded. In the second case, a system with a current density dependent resistivity is considered. This resistivity is switched on self-consistently in current filaments and allows for local dissipation due to magnetic reconnection. Thus, one finds tearing of current filaments and, besides, merging of filaments due to coalescence instabilities. Energy input and dissipation finally balance each other and the system reaches a state of constant magnetic energy in time.
Application of sheet shaped plasma supplemented with radio frequency plasma source for production of thin films
Authors: Shibata Kanetoshi , Ito Hiroaki , Yugami Noboru , Nishida Yasushi , Miyazaki Tadaomi
Published: July 2002

Production technique of thin films is described as an application of a sheet shaped electron cyclotron resonance heating plasma supplemented with a radio frequency plasma source for controlling the plasma parameters. The deposition rate of thin films onto the substrate can be varied from 6 to 102 nm/min depending on the experimental conditions under control. The thin film is made uniformly in space over a wide range of plasma parameters. The present plasma source also has typical characteristics of sharp density and temperature gradient at the edge of the sheet plasma to make a uniform, low temperature (Te⩽1 eV) plasma in the outer peripheral region. The present experimental technique could be applicable to the plasma source for material processing such as thin film formation, semiconductor devices such as solar batteries or flat panel display, and so on.
Symmetrized double quantum stochastic product integrals
Authors: Hudson R. L. , Pulmannová S.
Published: July 2002

A theory is developed of product integrals of the form ∏a<s<b ∏c<t<d(1+g[h] (ds,dt)). Here [a,b[ and [c,d[ are disjoint finite subintervals of R+, and g[h] is a formal power series in the indeterminate h whose constant term is zero and whose coefficients are elements of L⊗L, where ℒ is the space of basic differentials of a multidimensional quantum stochastic calculus. The product integrals are themselves formal power series in h whose coefficients are finite sums of iterated stochastic integrals against the elements of ℒ. They are symmetrized in such a way that ∏a<s<b ∏c<t<d(1+g[h](ds,dt)) is the image, obtained by applying the representation J[a,b[⊗J[c,d[ to the coefficients, where J[a,b[ is the representation canonically associated with the interval [a,b[, of a formal power series ∏ ∏(1+dg[h]) whose coefficients lie in U⊗U, where 𝒰 is the universal enveloping algebra of the Lie algebra ℒ. It is shown that the naturally conjectured multiplication rule, analogous to the multiplication rule for simple product integrals, holds in the commutative case.
Integrability of the Cn and BCn Ruijsenaars–Schneider models
Authors: Chen Kai , Hou Bo-yu , Yang Wen-Li
Published: July 2002

We study the Cn and BCn Ruijsenaars–Schneider models with interaction potential of trigonometric and rational types. The Lax pairs for these models are constructed and the involutive Hamiltonians are also given. Taking a nonrelativistic limit, we also obtain the Lax pairs for the corresponding Calogero–Moser systems.
Non-Debye screening in ionic solutions: Generalization of the modified mean spherical approximation
Authors: Varela L. M. , Ruso J. M. , Garcia M. , Mosquera V.
Published: July 2002

In this paper screening phenomena in general binary ionic solutions are analyzed from an Ornstein–Zernike point of view. The modified mean spherical approximation (MMSA) closure relation for the direct correlation function [Varela et al., J. Chem. Phys. 109, 1930 (1998)] is generalized in order to allow the presence of several species in solution. This generalized approximation (GMMSA) explicitly accounts for specific interactions between ionic species and is seen to allow the prediction of the hypernetted chain calculated decay length of 1:z(z=1,…,4) electrolyte solutions, in terms of a mean interaction parameter. Particularly, for 1:1 electrolyte solutions the results of the GMMSA for the effective decay length are seen to recover the original MMSA ones, and they are compared to all the theoretical results which exist in literature.
Surface recombination of hydrogen atoms studied by a pulsed plasma excitation technique
Authors: Rousseau A. , Cartry G. , Duten X.
Published: July 2002

The H atom lifetime in a low pressure hydrogen microwave plasma was measured using a pulse induced fluorescence technique. This technique is compared to results obtained by a laser spectroscopy technique. We first demonstrate the validity of the method and then deduce H atom lifetime pressure dependence. The H atom surface loss probability on fused silica was also deduced from our measurements. We show that this coefficient is not constant in the time afterglow but decreases almost by one order of magnitude (from 2.3×10−3 to 2.1×10−4) during the first milliseconds. These results are explained using recent experimental and theoretical works concerning atom-surface interaction in low temperature plasmas.
A pulsed supersonic entrainment reactor for the rational preparation of cold ionic complexes
Authors: Robertson W. H. , Kelley J. A. , Johnson M. A.
Published: July 2002

We describe an ion source for the efficient preparation of cold ion-molecule complexes, X−⋅M. The method relies on condensation of solvent molecules, M, onto argon-solvated ions, X−⋅Arm, where the X−⋅Arm species are formed in a primary expansion and the molecular partner, M, is interfaced to this flow in the hydrodynamic region by supersonic entrainment. This hybrid “supersonic afterglow” reactor provides a clean synthetic approach for both bare and argon-solvated complexes, where the latter are particularly useful since their structures can be characterized by “nanomatrix” infrared predissociation spectroscopy.
Spectral decomposition and Bloch equation of the operators represented by fixed-centroid path integrals
Authors: López-Ciudad Telesforo , Ramírez Rafael
Published: July 2002

Interesting approaches to study statical and dynamic properties of quantum systems, e.g., the quantum transition state theory and the centroid molecular dynamics, have been previously derived using fixed centroid path integrals. We show that these constrained propagators can be alternatively defined using an operator formalism. An interesting result is the finding of the differential equations that determine the temperature dependence of these propagators. One equation applies to path integrals with fixed-centroid position (i.e., those used in quantum transition state theory), and the other one to path integrals with fixed-centroid position and momentum (i.e., those used in centroid molecular dynamics). Both equations are solved for a harmonic oscillator, so that the spectral decomposition of the operators represented by fixed-centroid path integrals is derived for this particular case. Their eigenvalues build an alternating geometric series, showing explicitly the impossibility of considering such operators as true density operators, i.e., some eigenfunctions are associated to “negative probabilities.” The eigenfunctions are shown to be a generalization of the coherent and squeezed states of the harmonic oscillator. The physical meaning of centroid molecular dynamics, an approximation to study the time evolution of these mixed states, is clarified by considering the time evolution of the corresponding eigenfunctions. The mixed states constructed with “negative probabilities” display vanishing small position and momentum dispersion in the high temperature limit.
Arrangement of atoms and strain distribution in CdSe/ZnSe strained single quantum well on vicinal GaAs substrate
Authors: Nabetani Y. , Kato T. , Matsumoto T.
Published: July 2002

Strain distribution in a ZnSe/CdSe/ZnSe strained single quantum well structure grown on a vicinal GaAs substrate is investigated by calculating the positions of all the atoms in the structure. The strain distribution around atomic step edges is inhomogeneous, while that in terrace regions is the same as that of quantum wells grown on just-oriented substrates. The inhomogeneity extends for only 2 or 3 ML along the vicinal direction from the step edge. Microscopic lattice-planetilting of CdSe and ZnSe epitaxial layers becomes the largest at the step edge. CdSe expands and ZnSe shrinks in the growth plane at the lateral interface of CdSe and ZnSe to minimize the total strain energy. Cd atoms at the step edge have the largest strain energy due to shear deformation. Conduction and valence band potential profiles calculated by using the strain distribution and deformation potentials are found to enhance carrier confinement into the terrace region of the quantum well, although the confinement effect is not enough to explain the experimentally observed blueshift in photoluminescence.