bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimization of apertures and collimators for multi-channel plasma diagnostics
Authors: Ingesson L. C. , Wilson D. J.
Published: September 2002

Aperture, pin-hole and collimator detection systems are often used in plasma diagnostics, for example, in soft x-ray detection and bolometer systems. In this article the simultaneous optimization of viewing-beam overlap and light yield is considered in multi-channel aperture and collimator systems for two-dimensional (2D) tomography. This article briefly highlights the relation between beamwidth overlap and spatial aliasing in tomography, and how aliasing can be avoided in theory and in practice. Three-dimensional (3D) single-channel aperture and collimator systems can be approximated by a combination of two planar systems if the aperture is rectangular. Three ways to optimize beamwidth overlap and light yield for planar aperture and collimator systems are considered in detail: overlap of the angular étendue at the full width at half maximum (FWHM), overlap of the geometric function at the FWHM a certain distance from the aperture, and arbitrary overlap for a given maximum beamwidth. The combination of 2D effects from all three optimization methods were used in the design of 3D apertures for a new multi-channel bolometer camera on the Joint European Torus tokamak. The resulting apertures are complex, but the new camera has several advantages over previous cameras.
Low lying electronic states of rare gas–oxide anions: Photoelectron spectroscopy of complexes of O− with Ar, Kr, Xe, and N2
Authors: de Clercq Helen L. , Hendricks Jay H. , Bowen Kit H.
Published: September 2002

The negative ion photoelectron spectra of the oxide anion complexes O−Rg, Rg=Ar, Kr, and Xe, and O−N2 have been recorded. In each spectrum, two partially resolved peaks were observed, their relative intensities varying with source conditions. These peaks were assigned to photodetachment transitions from the 2Σ ground state and unresolved 2Π3/2,1/2 low-lying excited states of the anion. From our data we find dissociation energies and bond lengths for the 2Σ and 2Π anion states. Periodic trends in the bond length and dissociation energy are examined and compared to those in the isoelectronic neutral halogen rare gas systems and the effect of anisotropy in the interatomic potential and relative interaction strength is examined. From our data we find that the dissociation energies in the anion system are much larger but that the 2Σ-2Π splitting is significantly lower. In addition to the diatomic clusters, we report the photoelectron spectra of the O−Krn=2–5 and O−Xen=2–3 clusters and tabulate the vertical detachment energies and peak widths. From a comparison of the energetics and peak broadening we are able to make a determination of the general structure of the n=2 and n=3 clusters.
Rovibrational energy levels of the LiOLi molecule from dispersed fluorescence and stimulated emission pumping studies
Authors: Bellert D. , Winn Darin K. , Breckenridge W. H.
Published: September 2002

Dispersed fluorescence (DF) and stimulated emission pumping (SEP) experiments have been carried out on the jet-cooled 7Li16O7Li molecule. Rotationally resolved SEP bands to υ2l(l=0,2) vibrational angular momentum levels show that LiOLi is a linear molecule. The DF experiments have resulted in the measurement of 54 (υ1,υ2l,0) levels of the X̃ 1Σg+ ground state, up to 6000 cm−1. The energy levels determined experimentally are amazingly close to those calculated very recently for a high-level ab initio LiOLi(X̃ 1Σg+) potential surface. The DF vibrational level energies are analyzed and discussed in terms of a global fit to an 11-term power series. From the SEP experiments, effective rotational constants Bυ1,υ2l,0 for several of these vibrational levels [including σg+(l=0) and δg(l=2) vibrational angular momentum states for (υ1,υ2l⩾2(even),υ3=0) levels] have also been determined. The bond distance R0 is estimated to be 1.611±0.003 Å from an extrapolation of Bυ1,0,0 values. Finally, the unusual “Li+O−2Li+” ionic bonding and the low ω2=112 cm−1 bending frequency for LiOLi(X̃ 1Σg+) are briefly discussed.
Direct visualization of phonon-polariton focusing and amplitude enhancement
Authors: Stoyanov Nikolay S. , Ward David W. , Feurer Thomas , Nelson Keith A.
Published: September 2002

We report a new method to generate focused phonon polaritons inside ferroelectric crystals. Using an excitation pulse in the form of a ring we generate a phonon-polariton wave packet that propagates inward toward a focal point at the center. This results in a sharp increase in the polariton intensity. It also permits direct visualization and quantitative measurements of the Gouy phase shift and of anisotropy in polariton propagation. Since vibrational and electromagnetic modes are coupled, coherent vibrational amplitudes can be increased substantially through exploitation of electromagnetic wave propagation.
Coulomb explosion of benzene induced by an intense laser field
Authors: Shimizu Seiji , Zhakhovskii Vasilii , Sato Fuminobu , Okihara Shinichiro , Sakabe Shuji , Nishihara Katsunobu , Izawa Yasakazu , Yatsuhashi Tomoyuki , Nakashima Nobuaki
Published: September 2002

Coulomb explosions of benzene induced by an intense femtosecond laser field were investigated by time-of-flight (TOF) mass spectrometry at a laser intensity of 8.0×1016 W cm−2, with a pulse width of 120 fs. The multiply charged ions of Cq+ (q=1–4) and H+ were detected, and their energies were found to be distributed in the range of 0–160 eV. The explosions were concluded to be anisotropic because the kinetic energies of multiply charged carbon ions are highly parallel to the laser electric field. Molecular dynamics simulations, including the effects of tunnel ionization, electron recombination, and the spatial configuration of benzene for laser electric fields, were performed to elucidate the kinetic-energy distributions and the dynamics of exploded benzene ions. The results of the simulations reproduced the essential points of observation concerning the energies and distributions of the ions in the TOF experiments. The simulations suggest that the charge-hopping processes enhance ionization, finally leading to an anisotropic explosion, which events are similar to those found in the case of C60 explosions. Furthermore, our findings have strongly suggested that time evolution of the hopping greatly depends on spatial configuration in the case of planar molecule benzene.
Second order nonlinear spatial stability analysis of compressible mixing layers
Authors: Ladeinde F. , Wu J.
Published: September 2002

Second order nonlinear spatial stability to three-dimensional perturbation waves is analyzed for compressible mixing layers by expanding the perturbations into amplitude-dependent harmonic waves and truncating the Landau equation to the second term. This leads to a system of nonlinear ordinary differential equations for the harmonics. The two constants in Landau equation are calculated, wherein the independent variable, time t, is replaced by the streamwise coordinate direction x. The basic procedure in this paper is similar to that by Liu for compressible laminar wakes [Phys. Fluids 12, 1763 (1969)]. However, unlike this reference, which does not provide any results for their analysis, the present paper obtained many interesting results. The linear results from the present work compare very favorably with those reported by Day, Reynolds, and Mansour [Phys. Fluids 10, 993 (1998)], who employed a different procedure and limited their analysis to the linear regime. In the present studies, both the linear and nonlinear problems were analyzed in exactly the same manner, with the implication that the nonlinear results are probably accurate. These results include the convergence of the amplitude to an equilibrium value that depends on the two constants in the amplitude equation from Landau’s procedure. The present analysis is restricted to exponentially decaying linear solutions at the boundaries and hence to region one in the phase speed-Mach number diagram. However, we have observed that nonlinear effects could introduce constant, decaying, or outgoing wave solutions at the boundaries, depending on the velocity and density ratios and the Mach number of the fast stream. Other effects of these parameters are reported.
A Brillouin scattering study of end-linked poly(dimethylsiloxane) networks
Authors: Sinha Moitreyee , Mark J. E. , Jackson Howard E. , Walton D.
Published: September 2002

We have studied inelastic light scattering from acoustic phonons in end-linked poly(dimethylsiloxane) networks using a Brillouin scattering technique. A series of nearly monodisperse networks with molecular weights between cross links ranging from 800 g/mol to 30 000 g/mol are probed to study the effects of cross-linking and deformation on the phonon speed and attenuation. The Brillouin spectra are obtained using a combination of a three-pass tandem Fabry–Perot interferometer in conjunction with a CCD area detector. Longitudinal acoustic phonons of frequency 2–6 GHz propagating in the networks are studied at an excitation wavelength of 514.5 nm. The phonon frequencies and linewidths are measured as a function of the phonon wave vector by employing different scattering geometries. For highly cross-linked systems the phonon speed is shown to depend on the molecular weight. With decreasing cross-link density, the phonon velocity is observed to approach an asymptotic value. The dispersion of the acoustic phonon is investigated over a limited frequency range to probe the acoustic nature of the phonon. For poly(dimethylsiloxane) networks it is verified that at room temperature there is no measurable dispersion in the phonon speed at GHz frequencies. The phonon velocity is also measured as a function of strain, employing different geometries in elongation and compression experiments. These results are discussed in terms of the change in the compressibility and the local viscosity of the network.
Determination of distances between aluminum and spin-1/2 nuclei using cross polarization with very weak radio-frequency fields
Authors: Mali Gregor , Kaučič Venčeslav
Published: September 2002

In this work the possibility of using cross-polarization (CP) experiment for the determination of distances between spin-5/2 and spin-1/2 nuclei in polycrystalline and amorphous materials was investigated. The properties of the method were experimentally studied in an isolated Al2H spin system within the as-synthesized AlPO4-31 and in “infinite” aluminophosphate networks within the calcined AlPO4-31 and the hydrated VPI-5. In all three cases time-dependent oscillations due to coherent polarization transfer between aluminum and spin-1/2 nuclei were detected. The crucial parameters that influenced the visibility of dipolar oscillations were rotating-frame spin-lattice relaxation times of both nuclear species involved in a CP process and the homogeneity of rf fields. For a successful measurement the relaxation times, which varied remarkably with amplitudes of applied rf fields, had to be comparable to or larger than the period of time-domain oscillations. Radio-frequency field inhomogeneity was minimized when amplitudes of rf fields were adjusted to the Hartmann–Hahn sideband matching condition 3ν1I+ν1S=νR, the nutation frequency of spin-5/2 nuclei was limited to νR/2<3ν1I<νR, and the nutation frequency of spin-1/2 nuclei was limited to 0<ν1S<νR/2. Such adjustment assured also an efficient spin locking of quadrupolar spins. Experiments in the as-synthesized and calcined AlPO4-31 showed that in small isolated spin systems the time evolution of CP can elucidate the underlying geometry of the system whereas in “infinite” spin networks the splitting of a Pake-type doublet in the Fourier transform of a CP signal can yield an overall strength of the dipolar coupling. Although the use of weak rf fields reduced the robustness of the experiment it also introduced the selectivity of polarization transfer, which, as indicated by numerical simulation, in some cases allows the extraction of pairwise dipolar couplings in multispin systems. The possibility of a selective polarization transfer and a selective determination of distances between octahedrally coordinated Al1 nuclei and tetrahedrally coordinated P2 and P3 nuclei in the hydrated VPI-5 was demonstrated experimentally.
A simple passive monitor for integrating measurements of indoor thoron concentrations
Authors: Zhuo Weihai , Tokonami Shinji , Yonehara Hidenori , Yamada Yuji
Published: September 2002

A simple passive monitor was developed for integrating measurements of indoor thoron concentrations. The monitor was remodeled from a commercially available radon monitor with allyl diglycol carbonate (CR-39) detector. By adding four holes (φ=12 mm) and covering them with high permeability of filter paper, the air exchange rate of the monitor was largely enhanced. The technical characteristics of both the radon and thoron monitors were examined through calibration experiments. A high conversion factor of 1.32±0.14 tracks cm−2⋅(kBq m−3 h−1)−1 and the low lower detection limit for thoron measurements provide the essential conditions for measuring thoron more precisely and sensitively. Furthermore, the main physical advantages of the monitor are its simple construction, light weight, and compactness as well as its low cost, which are preferable for large-scale and long-term indoor surveys. Simultaneous measurements of both indoor thoron and radon are indispensable for accurate assessments of public exposure to radon.
Dynamics of star-burst dendrimers in solution in relation to their structural properties
Authors: Rathgeber S. , Monkenbusch M. , Kreitschmann M. , Urban V. , Brulet A.
Published: September 2002

We have measured both the static and dynamic structure factors of a single dendrimer with small-angle x-ray scattering (SAXS) and neutron spin-echo spectroscopy under good solvent conditions with the aim of finding a consistent correlation between the structural properties of dendrimers and their dynamic behavior. The samples under investigation were star-burst polyamidoamine dendrimers with generations g=0 to 8 in dilute methanol solutions. A model independent approach employing inverse Fourier transformation and square root deconvolution methods has been used to analyze the SAXS data to obtain the pair distance distribution function p(r) and the radial excess electron density profile Δρ(r). In addition, we formulated a model that takes both the colloidal (globular, compact shape with form polydispersity or fuzzy surface) as well as the loose, polymeric (self-avoiding random walk) character of dendrimers into account. With this model we were able to describe the spectra of all dendrimer generations consistently. Parameters discussed as a function of the dendrimer generation are, among others, the correlation length of the density fluctuations (blob radius) ξ, the radius of gyration Rg, the sphere radius Rs, the form polydispersity σs or analogously, the width of the fuzzy surface region 2σf. Both the model-independent approach and the model fits reveal that at least down to the third generation the dendrimers exhibit a rather compact, globular shape. These findings are in agreement with the dynamic results obtained by NSE spectroscopy which probes length scales both larger and much smaller than the dimension of a single dendrimer. The method reveals that the dynamics throughout is dominated by the center-of-mass diffusion—the internal dynamics is suppressed. The diffusion coefficients obtained are close to the values calculated from the Stokes–Einstein relation using the sphere radius Rs determined from the SAXS spectra. Dynamically, the dendrimers behave like “hard”, solid spheres.
Classical mapping for second-order quantized Hamiltonian dynamics
Authors: Prezhdo Oleg V.
Published: September 2002

Second-order quantized Hamiltonian dynamics (QHD-2) is mapped onto classical mechanics by doubling the dimensionality. The mapping establishes the classical canonical structure for QHD-2 and permits its application to problems showing zero-point energy and tunneling via a standard molecular dynamics simulation, without modifying the simulation algorithms, by introducing new potentials for the extra variables. The mapping is applied to the family of Gaussian approximations, including frozen and thawed Gaussians, which are special cases of QHD-2. The mapping simplifies numerous applications of Gaussians to simulations of spectral intensities and line shapes, nonadiabatic and other quantum phenomena. The analysis shows that frozen Gaussians conserve the total energy, while thawed Gaussians do not, unless an additional term is introduced to the equation of motion for the thawed Gaussian momentum. The classical mapping of QHD-2 is illustrated by tunneling and zero-point energy effects in the harmonic oscillator, cubic and double-well potential, and the Morse oscillator representing the O–H stretch of the SPC-F water model.
Jet formation in bubbles bursting at a free surface
Authors: Duchemin Laurent , Popinet Stéphane , Josserand Christophe , Zaleski Stéphane
Published: September 2002

We study numerically bubbles bursting at a free surface and the subsequent jet formation. The Navier–Stokes equations with a free surface and surface tension are solved using a marker-chain approach. Differentiation and boundary conditions near the free surface are satisfied using least-squares methods. Initial conditions involve a bubble connected to the outside atmosphere by a preexisting opening in a thin liquid layer. The evolution of the bubble is studied as a function of bubble radius. A jet forms with or without the formation of a tiny air bubble at the base of the jet. The radius of the droplet formed at the tip of the jet is found to be about one tenth of the initial bubble radius. A series of critical radii exist, for which a transition from a dynamics with or without bubbles exist. For some parameter values, the jet formation is close to a singular flow, with a conical cavity shape and a large curvature or cusp at the bottom. This is compared to similar singularities investigated in other contexts such as Faraday waves.
Water confined in cylindrical micropores
Authors: Giaya Arjan , Thompson Robert W.
Published: September 2002

A model characterizing water-like fluids confined in cylindrical micropores is presented. The equation of state was derived based on perturbation theory assuming that the reference system of hard spheres confined in the cylindrical pores is homogenous. The perturbed state accounts for fluid–fluid, fluid–wall, and hydrogen bonding interactions. Fluid–fluid and fluid–wall interactions are modeled as the pairwise sum of Lennard-Jones potentials. The hydrogen bonding model accounts for the open structure of liquid water, as well as for the fact that the hydrogen bonding capabilities of confined molecules are distorted. This model was used to analyze the dependence of the density of water inside the micropores on the density outside the pores, the pore radius, and the affinity of the pore walls for water molecules. For gas-phase adsorption, the model predicts that the density of water inside the pores depends on the fluid–wall interactions. The state of the adsorbed phase varies from the density of vapor outside the pores (for the hard sphere wall) to a bulk liquid-like density (for a hydrophilic sample). The predicted behavior of confined water in the presence of bulk liquid outside the pores was more interesting. The model predicts that for small pores of hydrophobic materials, the density of fluid inside the pores is much smaller than the bulk liquid density, i.e., vapor-like. However, as the radius and/or hydrophility are increased, the fluid density inside the pores approaches the bulk liquid density very rapidly.
Thermal diffusivity of porous cordierite ceramic burners
Authors: Garcı́a E. , Osendi M. I. , Miranzo P.
Published: September 2002

The applicability of the laser flash method for measuring the thermal diffusivity of highly porous cordierite materials is investigated. Due to the surface roughness of the samples, some indetermination in the sample thickness measurement is produced, which induces errors in the thermal diffusivity calculation. This problem was partially overcome by attaching two thin Cu layers to both surfaces of the samples. The thermal diffusivity and conductivity values of two porous cordierite materials (40 and 50 vol % of porosity) are reported using this procedure and results are discussed comparing with data for three-layer models.
Segregation of the Cu atom at the ferromagnetic/antiferromagnetic interlayer in spin-valve structures
Authors: Li M. H. , Yu G. H. , Jiang H. W. , Cai J. W. , Lai W. Y. , Zhu F. W.
Published: September 2002

Two sets of NiFe/FeMn films with Ta and Ta/Cu buffer layers were prepared by magnetron sputtering. Results show that the exchange bias field of NiFe/FeMn films with a Ta/Cu buffer is lower than that of the films with a Ta buffer. The crystalline texture, surface roughness, and element distribution of these two sets of samples were examined, and there is no apparent difference for the texture and roughness. However, the segregation of Cu atoms on the surface of NiFe in the trilayer of Ta/Cu/NiFe has been observed by using the angle-resolved x-ray photoelectron spectroscopy. The decrease of the exchange bias field for NiFe/FeMn films with a Ta/Cu buffer layers is mainly caused by the segragation of Cu atoms to the surface of the NiFe layer.
Identification of the ν1(σ) mode of linear GeC3Si
Authors: Robbins D. L. , Rittby C. M. L. , Graham W. R. M.
Published: September 2002

The first optical observation of a mixed germanium-silicon-carbon (GelSimCn) cluster has been made by trapping the products of the laser ablation of carbon/silicon/germanium rods in Ar at 10 K. The ν1(σ) C–C stretching mode of linear GeC3Si has been identified at 1939.0 cm−1. The assignment is confirmed by the very good agreement between Fourier transform infrared (FTIR) measurements of frequencies and isotopic shifts and predictions of our quantum theoretical calculations.
Crystallinity of In2O3(ZnO)5 films by epitaxial growth with a self-buffer-layer
Authors: Ohashi Naoki , Sakaguchi Isao , Hishita Shunichi , Adachi Yutaka , Hareda Hajme , Ogino Tsuyoshi
Published: September 2002

Naturally modulated structures, films of In2O3(ZnO)5, were epitaxially grown by rf magnetron sputtering on (0001) sapphire substrates. The crystallographic and morphological features of the films were characterized by using x-ray-diffraction and electron-diffraction patterns. The deposition of strongly c axis orientated films of In2Zn5O8 was achieved by epitaxy with a self-buffer layer (SBL). A film of In2Zn5O8 of approximately 10 nm in thickness was initially deposited as the SBL. This initial film was annealed in situ for 10–15 min, after which the bulk of the film of In2Zn5O8 was grown. Finally, well-crystallized In2O3(ZnO)5 films were obtained by annealing the resultant film at 800 °C. The electrical properties of the synthesized film are discussed in relation to the mechanism by which the modulated In2O3(ZnO)5 structure is formed.
On entanglement-assisted classical capacity
Authors: Holevo A. S.
Published: September 2002

We give a modified proof of the recent result of C. H. Bennett, P. W. Shor, J. A. Smolin, and A. V. Thapliyal concerning entanglement-assisted classical capacity of a quantum channel and discuss the relation between entanglement-assisted and unassisted classical capacities.
Leakage current characteristics of laser-ablated SrBi2Nb2O9 thin films
Authors: Das Rasmi R. , Bhattacharya P. , Pérez W. , Katiyar Ram S. , Bhalla A. S.
Published: September 2002

We have studied the leakage current conduction mechanism of pulsed-laser-deposited SrBi2Nb2O9 (SBN) thin films on platinized silicon substrates. The time-dependent dc leakage current densities of SBN thin films do not follow Curie–von Schweidler power law. Instead the contribution of conduction current is predominantly electronic. At lower fields, the leakage current follows the ohmic behavior, and it increases exponentially for higher fields. The leakage current density of the SBN thin films was studied at elevated temperatures, and the data were fitted with the Schottky emission model. The effective Richardson’s constant was calculated to be about 8.7×10−6 A/cm2 K2. The Schottky barrier height of the SBN thin films was estimated to be 1.37 eV.
Surface photovoltage measurements in μc-Si:H: Manifestation of the bottom space charge region
Authors: Švrček V. , Pelant I. , Fojtı́k P. , Kočka J. , Fejfar A. , Toušek J. , Kondo M. , Matsuda A.
Published: September 2002

We discuss results of surface photovoltage (U) measurements for d=10 μm thick layers of undoped hydrogenated microcrystalline silicon (μc-Si:H). By applying excitation with low energetic photons (down to 1.1 eV), i.e., with small absorption coefficient α and a large penetration depth α−1, a photovoltage peak appears on a curve U=U(α). This peak is located at α≳d−1 and its occurrence depends critically on the substrate material. The peak is present in a μc-Si:H film grown on p+ crystalline silicon (c-Si), on the other hand it is missing in the μc-Si:H samples grown on n+ c-Si or ZnO film. We present a mathematical model that enables us to link this peak to photocharge separation in the bottom space charge region at the interface μc-Si:H/substrate. Besides the magnitude of the ambipolar carrier diffusion length L, a parameter particularly critical for the occurrence of the peak turns out to be the ratio n of reverse saturation current densities of the two diodes representing surface and bottom space charge regions. The peak can be observed only when |n| is below a certain threshold value |nth|. In the simplified case when d/L≫1 and when the thicknesses of the upper and lower depletion layers can be neglected, we have found |nth|≈0.135 or 1, depending on the orientation of the top and bottom barriers to each other. However, the magnitude of the peak increases exponentially with further lowering of |n|. Therefore, the surface photovoltage method is suitable not only for evaluating the minority carrier diffusion length L, but also for detecting the occurrence and properties of the bottom space charge region in thin film solar cells.
The uniqueness theorem for entanglement measures
Authors: Donald Matthew J. , Horodecki Michał , Rudolph Oliver
Published: September 2002

We explore and develop the mathematics of the theory of entanglement measures. After a careful review and analysis of definitions, of preliminary results, and of connections between conditions on entanglement measures, we prove a sharpened version of a uniqueness theorem which gives necessary and sufficient conditions for an entanglement measure to coincide with the reduced von Neumann entropy on pure states. We also prove several versions of a theorem on extreme entanglement measures in the case of mixed states. We analyze properties of the asymptotic regularization of entanglement measures proving, for example, convexity for the entanglement cost and for the regularized relative entropy of entangle ment.
Carrier generation in highly oriented WO3 films by proton or helium implantation
Authors: Miyakawa M. , Ueda K. , Hosono H.
Published: September 2002

Implantation of H+ or He+ ions into highly oriented WO3 films, which were deposited on sapphire R-plane substrates by the pulsed laser deposition method, was carried out to generate carrier electrons. X-ray diffraction measurements revealed that the films consisted of only the two types of grains that were epitaxially grown with their c axes perpendicular to the surface of the substrate. After implantation to a dose of 1×1017 cm−2, a remarkable increase in conductivity at 300 K was observed to 200 Scm−1 for H+ implantation or to 3 Scm−1 for He+ implantation. The efficiency of carrier generation was ∼60% for H+ implantation or ∼10−3% for He+ implantation. This striking difference demonstrates that the chemical effect of bronze formation is much more effective for carrier generation in WO3 than is the physical effect of oxygen vacancy formation by nuclear collision processes. The resulting conductivity of the oriented films was higher by 1–3 orders of magnitude than that of the corresponding polycrystalline film prepared by the rf sputtering method. Hall mobility in the oriented films (∼1 cm2 V−1 s−1) was as high as that (1–2 cm2 V−1 s−1) in single-crystal Na0.40WO3 or WO2.99 and was higher by several orders of magnitude than that in polycrystalline films. The results of Hall voltage and x-ray diffraction measurements led to the conclusion that the higher conductivity in the highly oriented films mainly originates from an increase in carrier mobility, which results from decreases in grain boundary effects as scattering centers.
Rb-doped potassium titanyl phosphate for periodic ferroelectric domain inversion
Authors: Jiang Q. , Thomas P. A. , Hutton K. B. , Ward R. C. C.
Published: September 2002

This article reports the dielectric, conductive, and ferroelectric properties of Rb-doped potassium titanyl phosphate (RKTP) crystals, of formula type RbxK1-xTiOPO4, where x=0.0065 or 0.013, both of which are shown to undergo ferroelectric switching at room temperature with coercive fields varying from 2.3 to 2.7 kV/mm. The measured spontaneous polarizations are 0.18(±0.01) and 0.19(±0.01) Cm-2 for RbxK1−xTiOPO4 with x=0.0065 and x=0.013, respectively. The dependence of the conductivity of Rb:KTP on electric-field amplitude at high fields leads to distorted ferroelectric hysteresis loops. As a result of thermal effects and the redistribution of vacancies, the switching times and switching current peaks vary throughout a series of switching processes at a frequency of 20 Hz. The measured switching times range from 2 to 6 ms and are inversely proportional to the peak current densities. The dielectric relaxation of Rb:KTP is found to be further from the Debye response than that of KTP. A correlation effect is seen to play an important role in reducing the ac conductivity of Rb:KTP at low frequencies relative to that of KTP. The results are discussed in the context of obtaining a KTP derivative, which is suitable for periodic patterning by electric field poling at room temperature.
Self-limiting behavior of the grain growth in lead zirconate titanate thin films
Authors: Lee Jang-Sik , Joo Seung-Ki
Published: September 2002

During the lateral grain-growth process in lead zirconate titanate (PZT) thin films by selectively nucleated lateral crystallization using PZT seed, it was found that the lateral growth was saturated with the annealing temperature. The saturation of lateral growth was analyzed by the interface energy, which appeared during the crystallization process. The factors affecting the saturation of lateral growth were found to be the interface energy between the perovskite phase and the pyrochlore phase, and the one between the PZT thin film and the bottom Pt electrode. When the ion damage was introduced to the growth-saturated PZT thin films, further lateral growth was observed. Pt bottom electrode thickness was changed to control the interface energy between the PZT thin film and the Pt bottom electrode. PZT grain size increased according to the Pt thickness, because the lattice parameter of Pt films increased with the thickness of the Pt films. The incubation time for nucleation in the PZT thin films increased with the amount of the ion damage on the Pt films.
State-to-state photodissociation of OCS (ν2=0,1|JlM). I. The angular recoil distribution of CO (X 1Σ+;v=0|J)
Authors: van den Brom Alrik J. , Rakitzis T. Peter , van Heyst Jeroen , Kitsopoulos Theofanis N. , Jezowski Sebastian R. , Janssen Maurice H. M.
Published: September 2002

State-to-state photodissociation experiments of OCS at 230 nm are reported using hexapole state selection of the parent molecule and velocity map ion imaging of the angular recoil of the CO photofragment. The role of the initial rovibrational state (ν2=0,1|JlM) of OCS on the angular recoil distribution is investigated. The CO (X 1Σ+;v=0|J) rotational distribution as well as the angular recoil anisotropy parameter β of the CO photofragment are reported for dissociation of single rovibrational (ν2=0,1|JlM) quantum states of OCS. A strong dependence of the anisotropy parameter β on the initial bending state, ν2=0 or 1, of OCS is observed. The effects of the initial bending state of OCS are rationalized in terms of the strong angular dependence of the transition dipole moment function of OCS for the 1 1Σ−(1 1A″) and 1 1Δ(2 1A′) excited state surfaces involved in the dissociation at 230 nm. The state-to-state imaging experiment provides a revised and improved determination of the binding energy of OCS (ν1,ν2,ν3=0,0,0|J=0)→CO (X 1Σ+;v=0|J=0)+S (1D2), D0=(4.284±0.009) eV.
Quantitative spectral analysis of HCl and DCl in 120–220 nm: Effects of singlet–triplet mixing
Authors: Cheng Bing-Ming , Chung Chao-Yu , Bahou Mohammed , Lee Yuan-Pern , Lee L. C.
Published: September 2002

Photoabsorption cross sections of HCl and DCl in the spectral region 120–220 nm were measured using synchrotron radiation as a light source. The absorption contour of the A 1Π←X 1Σ+ band of HCl in the 134–195 nm region is slightly asymmetric, whereas that of DCl is symmetric, with wavelength of absorption maximum shifted from ∼153.9 nm for HCl to ∼155.8 nm. The asymmetry in HCl and the atypical direction of shift in peak wavelengths upon deuteration are attributed to singlet–triplet mixing in its ground X 1Σ+ state of HCl; this triplet component contributes to the oscillator strengths of the A–X band of HCl via transition to the repulsive triplet state 1 3Σ+. The small triplet character in HCl is further supported by an observation that the oscillator strength of the (0,0) band for the b 3Π–X 1Σ+ transition of DCl is ∼17% smaller than that of HCl. This triplet component may account for greater values of spin–orbit branching ratios for products, Cl*(2P1/2)/Cl(2P3/2), observed upon photolysis of HCl in the 143–167 nm region. For both HCl and DCl, ratios of observed oscillator strengths to calculated Franck–Condon factors of the C 1Π state decrease with vibrational levels, whereas those of the b 3Π state increases with increasing vibrational levels. Such a variation was attributed to perturbation between the b and C states, for which quantitative data are established in this study.
Large fully nonlinear internal solitary waves: The effect of background current
Authors: Stastna Marek , Lamb Kevin G.
Published: September 2002

In this paper we consider what effect the presence of a nonconstant background current has on the properties of large, fully nonlinear solitary internal waves in a shallow, stratified ocean. In particular, we discuss how the amplitude of the largest nonbreaking wave that it is possible to calculate depends on the background current as well as the nature of the upper bound. We find that the maximum wave amplitude is given by one of three possibilities: The onset of wave breaking, the conjugate flow amplitude or a failure of the wave calculating algorithm to converge (associated with shear instability). We also discuss how wave properties such as propagation speed, half-width, etc. vary with background current amplitude.
Measurement of high electron temperatures in single atom metal point contacts by light emission
Authors: Downes A. , Dumas Ph. , Welland M. E.
Published: September 2002

Light emission with a blackbody-like spectrum was observed during current flow through atomic-size metallic contacts formed in the scanning tunneling microscope. Within the contact, the electron temperature rises above the lattice temperature as electron–phonon energy transfer vanishes. Electron temperatures of up to 9000 K were deduced from optical spectroscopy of stable contacts. An important consequence of greatly reduced electron energy losses is that these atomic-size metal contacts have maximum current densities of ∼1015 A m−2, several orders of magnitude greater than for macroscopic wires.
From incipient ferroelectricity in CaTiO3 to real ferroelectricity in Ca1−xPbxTiO3 solid solutions
Authors: Lemanov V. V. , Sotnikov A. V. , Smirnova E. P. , Weihnacht M.
Published: September 2002

Dielectric spectra of ceramic samples of Ca1−xPbxTiO3 (0⩽x⩽0.4) solid solutions were studied at frequencies between 10 Hz and 1 MHz in a temperature range of 4.2–300 K. A ferroelectric phase transition was observed at x=0.3 with transformation to a relaxor type behavior at x>0.3. The transition temperature Tc follows a linear dependence Tc∝(x−xc) with xc=0.28 contrary to a square-root dependence Tc∝(x−xc)1/2 predicted by the theory for incipient ferroelectrics. The critical concentration essentially exceeds the xc values for SrTiO3 and KTaO3 incipient ferroelectrics.
Role of trace impurities in large-volume noble gas atmospheric-pressure glow discharges
Authors: Yuan Xiaohui , Raja Laxminarayan L.
Published: September 2002

A computational study of capacitively coupled atmospheric-pressure glow discharges in high-purity helium is presented, and the role of trace impurities in such noble gas plasmas is established. Trace impurities result in generation of significant amounts of charged species through Penning ionization and charge exchange reactions. The altered charged species balance in the plasma causes a large change in the discharge impedance when compared to a completely pure noble gas discharge. Peak electron temperatures of the order of 50 000 K and electron densities of order 1011 cm−3 are predicted. Significant electron generation is observed inside the sheath region due to the highly collisional nature of the plasma. This study underscores the importance of modeling the effect of even trace amounts of impurities in noble gas atmospheric-pressure glow discharges.
Reduced pressure chemical vapor deposition of Si/Si1−yCy heterostructures for n-type metal–oxide–semiconductor transistors
Authors: Hartmann J. M. , Ernst T. , Loup V. , Ducroquet F. , Rolland G. , Lafond D. , Holliger P. , Laugier F. , Séméria M. N. , Deleonibus S.
Published: September 2002

We have grown by reduced pressure chemical vapor deposition Si/Si1−yCy/Si heterostructures for electrical purposes. The incorporation of substitutional carbon atoms into Si creates a carrier confinement in the channel region of metal–oxide–semiconductor (MOS) transistors. Indeed, tensile strain Si1−yCy layers present a type II band alignment with Si, with a conduction band offset of the order of 60 meV per at. % of substitutional carbon atoms. For small SiH3CH3 flows, all the incoming carbon atoms are incorporated into substitutional sites. At 600 °C, when the SiH3CH3 flow increases, the substitutional carbon concentration saturates at 1.12%. Meanwhile, the total carbon concentration CT still increases, following a simple law: CT/(1−CT)=0.88  [F(*SiH3CH3)/F(SiH4)]. This is a sign that a growing number of C atoms incorporates into interstitial sites. The hydrogenated chemistry adopted does not enable one to achieve selectivity over SiO2-masked wafers, but does not however generate any adverse loading effect. We have integrated Si/Si1−yCy/Si stacks (which have been shown to be stable versus conventional gate oxidations and electrical activation anneals) into the channel region of ultrashort gate length (50 nm) nMOS transistors. Secondary ions mass spectrometry profiling has shown that C atoms segregate from the Si1−yCy layer into the Si cap and the SiO2 gate, but also that they block the diffusion paths of B coming from the antipunch through layer towards the gate, generating very retrograde doping profiles. The addition of C leads to a degradation of the electron mobility which seems to be linked to the high amount of C atoms into interstitial sites.
Ab initio calculations on electronic states of CaOH
Authors: Theodorakopoulos Giannoula , Petsalakis Ioannis D. , Liebermann Heinz-Peter , Buenker Robert J. , Koput Jacek
Published: September 2002

Ab initio configuration interaction calculations have been carried out on the potential energy surfaces of the ground and the doublet excited electronic states of CaOH. The calculated transition energies are in good agreement with previous theoretical values and with the available experimental data. The calculated Ca–O stretching potentials at different values of the bond angle show typical Rydberg minima at short internuclear distances but are complicated for large RCa–O. The calculated bending potentials are rather shallow for deviations of 5°–10° from linearity and, depending on the values of the Ca–O and O–H bond lengths, off-linear geometries have low energies. There is no evidence for the existence of an electronic state with bent equilibrium geometry, which would correspond to the reported state correlating with F̃ 2Π of CaOH. The 7 2A′ state (which does correlate with F̃ 2Π) is found to be quasi-linear with a barrier to linearity of only 92 cm−1, and similarly the 4 2A″ state (correlating with C̃ 2Δ) is found to be quasilinear with a barrier to linearity of 84 cm−1.
Ellipsometric study of the polysilicon/thin oxide/single-crystalline silicon structure and its change upon annealing
Authors: Petrik P. , Lohner T. , Fried M. , Gyulai J. , Boell U. , Berger R. , Lehnert W.
Published: September 2002

Polysilicon/thin oxide/single-crystalline silicon structures used as emitters of bipolar transistors were measured using spectroscopic ellipsometry. The thin SiOx layer was deposited on the substrate in a rapid thermal processing chamber, then polysilicon was deposited, implanted with As, and annealed. During annealing the SiOx layer dissolves to islands retarding the diffusion of As, which results in a shallow p-n junction. The process—and as a consequence, the device performance—depends sensitively on the thickness of the oxide layer. We developed optical models to measure the thickness of the SiOx layer at each process step, i.e., after SiOx deposition, after polysilicon deposition, and after annealing. The structure, the surface quality, and the homogeneity of the polysilicon layers were obtained from the same optical model. The thickness of the initially 0.76–0.86 nm SiOx layer decreased to the detection limit of about 0.2 nm during annealing, together with a significant crystallization of the deposited amorphous Si resulting in a perfectly smooth surface with a native oxide layer. The uncertainty of the measurement of the SiOx layer after annealing was typically 0.2 nm even below the polysilicon layer of about 150 nm. Our results show that ellipsometry is a proper tool for the measurement of the thickness of the buried SiOx boundary layer, which can be applied as an in line or in situ measurement as well.
Angle-dependent differential-photovoltage spectroscopy for the characterization of a GaAs/GaAlAs based vertical-cavity surface-emitting laser structure
Authors: Wang S. D. , Liang J. S. , Huang Y. S. , Tien C. W. , Chang Y. M. , Chen C. W. , Li N. Y. , Tiong K. K. , Pollak Fred H.
Published: September 2002

An angle-dependent wavelength-modulated differential-photovoltage spectroscopy (DPVS) investigation has been performed at room temperature on a bare as-grown wafer of GaAs/GaAlAs-based vertical-cavity surface-emitting laser (VCSEL) structure, designed for emitting at a wavelength near 850 nm. The differential-photovoltage (DPV) spectra exhibit both the fundamental conduction to heavy-hole excitonic transition and cavity mode plus an interference pattern related to the mirror stacks. By changing the angle of incidence in the DPV measurements the energy positions of the cavity mode and distributed Bragg reflector features show a blueshift while the excitonic transition remains unchanged. The energies of the excitonic transition and cavity mode are accurately determined from the DPV spectra. The advantages of DPVS in relation to other methods of characterizing VCSEL structures, such as surface photovoltage, photoreflectance, photocurrent, and differential photocurrent spectroscopy, are discussed. The results demonstrate considerable potential of DPVS for the nondestructive characterization of these structures at room temperature.
Electron drift mobility in polystyrene doped with bispyrazolopyridine derivatives
Authors: Tameev A. R. , He Z. , Milburn G. H. W. , Kozlov A. A. , Vannikov A. V. , Puchala A. , Rasala D.
Published: September 2002

Electron transport properties of bispyrazolopyridine derivatives have been studied using a conventional time-of-flight method. A field dependent electron drift mobility, in the range of 4×10−7–2×10−5 cm2/(V s) at an electric field of (0.6–7.0)×105 V/cm, has been found from the bispyrazolopyridine doped polystyrene specimen. The correlation between chemical structure and measured mobility is found to fit the correlated disorder model reasonably well when the charge transport is represented in terms of the one-electron transfer between frontier molecular orbitals. Optimization of molecule configuration and calculation of the orbital energy and wave-function decay constant have been computed by the semiempirical PM3 and AM1 methods.
Clusters containing open-shell molecules. II. Equilibrium structures of ArnOH Van der Waals clusters (X2Π, n=1 to 15)
Authors: Xu Minzhong , Bačić Zlatko , Hutson Jeremy M.
Published: September 2002

The equilibrium and low-lying isomeric structures of ArnOH (X2Π) clusters for n=1 to 15 are investigated by simulated annealing calculations. Potential energy surfaces are obtained by a pairwise-additive approach, taking into account the open-shell nature of OH X2Π and including spin-orbit coupling. It is found that the spin-orbit coupling suppresses the Jahn–Teller effect, and many of the clusters have high-symmetry structures (Cnν with n>2) which would be forbidden in the absence of spin-orbit coupling. The structures are generally similar to those previously found for the closed-shell systems ArnHF and ArnHCl, but different from those for the open-shell systems ArnNO and ArnCH. This is because Ar–OH (X2Π), like Ar–HF and Ar–HCl but unlike Ar–NO and Ar–CH, has a near-linear equilibrium structure. ArnOH clusters for n up to 6 have all Ar atoms in a single shell around OH. In the clusters with n=7 to 9, OH is under a pentagonal pyramid formed by six Ar atoms, while the others bind to its exterior, away from OH. For n=10 to 12, the minimum-energy structures have OH inside an Arn cage, which is essentially icosahedral for n=12 but has vacancies for n=10 and 11. For n>12, the extra Ar atoms begin to form a second solvation shell. The global minimum of ArnOH may be constructed from the minimum-energy structure of Arn+1 by replacing one Ar atom with OH.
Structural properties of aluminum–nitrogen films prepared at low temperature
Authors: Ribeiro C. T. M. , Alvarez F. , Zanatta A. R.
Published: September 2002

Aluminum–nitrogen thin films have been obtained under well-controlled conditions by ion-beam-assisted deposition (IBAD). The films were deposited on crystalline silicon and sapphire substrates at relative low temperature (∼325 K). Taking advantage of the ion energy control provided by the IBAD technique, the films were prepared with N2+ ions with energies ranging from 100 to 300 eV. After deposition, the films were investigated by in situ x-ray photoelectron spectroscopy, ex situ by optical spectroscopy, and x-ray diffraction. Detailed Raman scattering measurements in the 100–2500 cm−1 wave number range were also performed revealing interesting features related to the atomic composition and structure of the films. The Raman data suggest that a misidentification of some vibration modes can lead to incorrect interpretations of the crystalline quality of aluminum–nitrogen films. Finally, the results indicate the suitability of IBAD to produce crystalline AlN films at considerably lower temperatures.