bluuebunny/crossref_metadata_2025 huggingface dataset:
New low temperature multiphase ferroelectric films
Authors: Bescher Eric , Xu Yuhuan , Mackenzie J. D.
Published: July 2002

This article describes the low-temperature synthesis of new multiphase ferroelectrics containing an inorganic ferroelectric phase entrapped in amorphous silica or in an organically modified silicate (ormosil). Sol gel derived LiNbO3 and BaTiO3 crystals were grown in SiO2 and in RSiO1.5 glass where R contains a chromophore (TDP) insensitive to hydrolysis and condensation reactions. The LiNbO3–SiO2 and BaTiO3–SiO2 compositions as well as the TDP–LiNbO3–SiO2 and TDP–BaTiO3–SiO2 ormosils exhibit ferroelectric-like properties. This unusual characteristic is due to the presence of small, partially ordered crystallites of the ferroelectric, dispersed in the amorphous matrix. In addition to their ferroelectric properties, the ormosils also exhibit interesting optical characteristics: the TDP–BaTiO3–SiO2 materials are red, whereas the TDP–LiNbO3–SiO2 are yellow. The materials described in this article are representative of two new classes of weak ferroelectrics. In the first class, a ferroelectric is dispersed in an amorphous matrix. The second class may be called “organically-modified crystals”: small ferroelectric crystals embedded in an organically modified matrix. The fabrication of such materials is possible for inorganic crystalline phases forming at temperatures below the decomposition temperature of the organic (about 250 °C). This article also contains some theoretical considerations explaining why these materials, although amorphous by x-ray diffraction, exhibit ferroelectric-like properties.
Three-dimensionally controlled size-reduction of silicon nanopillars by photoelectrochemical etching
Authors: Juhasz Robert , Linnros Jan
Published: July 2002

Silicon nanopillars, fabricated by electron beam lithography and reactive ion etching, were size-reduced using laser-assisted electrochemical etching in a dilute hydrofluoric acid solution. The progressing size reduction was followed by scanning electron microscopy down to final diameters of ∼15 nm. By varying the voltage bias, it was found that etching could be directed primarily at the pillar top (7 V) or at the pillar base (−0.6 V) whereas in an intermediate regime, conformal etching could be obtained. From the rate of volume change during etching, it was concluded that holes, participating in the dissolution reaction, were primarily generated within the pillar volume. The corresponding effective dissolution valence was ∼5–9, indicating substantial recombination losses within the pillar.
Lattice location of hydrogen in Mg doped GaN
Authors: Wampler W. R. , Myers S. M. , Wright A. F. , Barbour J. C. , Seager C. H. , Han J.
Published: July 2002

We have used ion channeling to examine the lattice configuration of hydrogen in Mg doped wurtzite GaN grown by metal organic chemical vapor deposition. Hydrogen is introduced by exposure to hydrogen gas or electron cyclotron resonance plasmas and by ion implantation. A density functional approach including lattice relaxation was used to calculate total energies for various locations and charge states of hydrogen in the wurtzite Mg doped GaN lattice. Results of channeling measurements are compared with channeling simulations for hydrogen at lattice locations predicted by the density functional theory.
Optimal generalized internal vibrational coordinates and potential energy surface for the ground electronic state of SO2
Authors: Zúñiga José , Bastida Adolfo , Requena Alberto
Published: July 2002

An optimization of generalized internal vibrational coordinates for the electronic ground state of the SO2 molecule is carried out. These coordinates are given by the magnitudes and the angle formed by two vectors expressed as linear combinations of the internal valence vectors, and depend on two external parameters which can be optimized so as to reduce the vibrational coupling of the molecule as much as possible. The optimal values of the parameters are found by minimizing a set of unconverged vibrational energies which are computed variationally using a small basis function set. It is shown that the optimal internal coordinates obtained for SO2 are superior to both valence and Radau coordinates, as well as to a set of normal coordinates previously derived by proper rotation of the Jacobi S–O2 coordinates. These optimal internal coordinates are then applied to calculate the vibrational energies of the molecule using an ab initio force field expressed as a Morse-cosine expansion, and then to refine it by nonlinear least-squares fitting to the observed vibrational frequencies.
Sol–gel derived grain oriented barium strontium titanate thin films for phase shifter applications
Authors: Majumder S. B. , Jain M. , Martinez A. , Katiyar R. S. , Van Keuls F. W. , Miranda F. A.
Published: July 2002

In the present work process methodology was optimized to synthesize oriented barium strontium titanate (BST) (50/50) and (60/40) thin films on strontium titanate (100) and lanthanum aluminate (LAO) (100) substrates by using the chemical solution deposition technique. These films were characterized in terms of their phase formation behavior and structural growth characteristics using x-ray diffraction and atomic force microscopy. A tentative mechanism of the epitaxial growth has been proposed. Films were also characterized in terms of their dielectric properties. The high tunability and low dielectric loss of these films make them attractive for fabricating tunable dielectric devices. Accordingly, we have fabricated eight element coupled microstrip phase shifters and tested them in terms of their degree of phase shift and insertion loss characteristics. An insertion loss of 8.435 dB, phase shift in the order of 320° (2.6–14.5 V/μm) and κ factor (phase shift/dB of loss) of about 38.0°/dB was achieved in BST (60/40) films deposited on LAO (100) substrate which is comparable to the films grown by other film deposition techniques reported in the literature. The microstructure of the sol–gel derived films show surface porosity which may be responsible for the low dielectric strength of these films. Presently, we are studying the sintering mechanism and kinetics of these films in order to improve the density which is believed to further improve the phase shift and lower insertion loss to result in an improved tunability.
A self-calibrating programable mobility spectrometer for atmospheric ion measurements
Authors: Aplin K. L. , Harrison R. G.
Published: July 2002

Ion measurements in atmospheric air allow atmospheric ion-aerosol charge exchange to be investigated, which is thought to be important in cloud processes. Classical aspiration ion measurement techniques have been significantly improved by the use of electronic switching systems to permit the selection of different mobilities, with adjacent instruments using the combination of Voltage Decay and ion current techniques used for calibration. An integrated programable ion mobility spectrometer using the same collection electrodes in both modes is described here, with which high-resolution measurements (1 s sampling) of ion concentration fluctuations have been made. Mobility spectrum information has also been extracted by varying the bias voltage under full computer control. The integrated system can determine small ion concentrations to ±1%, to a minimum mobility μ∼0.1 cm2 V−1 s−1 (corresponding to a particle radius of ∼2 nm).
Noncanonical quantization of gravity. II. Constraints and the physical Hilbert space
Authors: Klauder John R.
Published: July 2002

The program of quantizing the gravitational field with the help of affine field variables is continued. For completeness, a review of the selection criteria that singles out the affine fields, the alternative treatment of constraints, and the choice of the initial (before imposition of the constraints) ultralocal representation of the field operators is initially presented. As analogous examples demonstrate, the introduction and enforcement of the gravitational constraints will cause sufficient changes in the operator representations so that all vestiges of the initial ultralocal field operator representation disappear. To achieve this introduction and enforcement of the constraints, a well characterized phase space functional integral representation for the reproducing kernel of a suitably regularized physical Hilbert space is developed and extensively analyzed.
An exact quantum Monte Carlo calculation of the helium–helium intermolecular potential. II
Authors: Anderson James B.
Published: July 2002

We report “exact” ab initio calculations with reduced statistical error for the potential energy of interaction of two helium atoms. For the equilibrium internuclear distance of 5.6 bohr, the calculated electronic energy is −5.807 483 53±0.000 000 06 hartrees and the corresponding well depth is (ε/k) 10.98±0.02 K.
Three-dimensional dynamic Monte Carlo simulations of driven polymer transport through a hole in a wall
Authors: Chern Shyh-Shi , Cárdenas Alfredo E. , Coalson Rob D.
Published: July 2002

Three-dimensional dynamic Monte Carlo simulations of polymer translocation through a cylindrical hole in a planar slab under the influence of an external driving force are performed. The driving force is intended to emulate the effect of a static electric field applied in an electrolytic solution containing charged monomer particles, as is relevant to the translocation of certain biopolymers through protein channel pores embedded in cell membranes. The time evolution of the probability distribution of the translocation coordinate (the number of monomers that have passed through the pore) is extracted from three-dimensional (3-D) simulations over a range of polymer chain lengths. These distributions are compared to the predictions of a 1-D Smoluchowski equation model of the translocation coordinate dynamics. Good agreement is found, with the effective diffusion constant for the 1-D Smoluchowski model being nearly independent of chain length.
Algebraic treatment of super-integrable potentials
Authors: Chetouani L. , Guechi L. , Hammann T. F.
Published: July 2002

The so(2,1) Lie algebra is applied to three classes of two- and three-dimensional Smorodinsky–Winternitz super-integrable potentials for which the path integral discussion has been recently presented in the literature. We have constructed the Green’s functions for two important super-integrable potentials in R2. Among the super-integrable potentials in R3, we have considered two examples, one is maximally super-integrable and another one minimally super-integrable. The discussion is made in various coordinate systems. The energy spectrum and the suitably normalized wave functions of bound and continuous states are then deduced.
Local magnetic shear and drift waves in stellarators
Authors: Nadeem M. , Rafiq T. , Persson M.
Published: July 2002

A study of the effect of local magnetic shear on the drift wave stability is presented. The eigenvalue problem for the drift wave equation is solved numerically in fully three-dimensional stellarator plasma using the ballooning mode formalism. It is found that negative local magnetic shear has a stabilizing effect on the drift wave instability and positive local shear is destabilizing. This is in agreement with the effect of negative global magnetic shear in tokamaks and also agrees with the simple estimates. As a consequence the highly unstable modes found on a specific magnetic surface are localized in the region of positive local magnetic shear.
Magnetocaloric effect in Tb5(SixGe1−x)4
Authors: Morellon L. , Magen C. , Algarabel P. A. , Ibarra M. R. , Ritter C.
Published: July 2002

The magnetocaloric effect has been investigated in a series of the R5(SixGe1−x)4 pseudobinary alloys with R=Tb and x=0.0, 0.5, and 1.0. In Tb5Si4 and Tb5Ge4, a maximum magnetic entropy change of 9.8 and 3.7 J/kg K, respectively, has been found at their ordering temperatures for a magnetic field change of 50 kOe. In the case of Tb5Ge4, we have observed the existence of a previously unreported magnetic transition below the Néel temperature. Tb5(Si0.5Ge0.5)4 has been successfully synthesized, and a large entropy change of 21.8 J/kg K (ΔH=50 kOe) is achieved at a first-order para- to ferromagnetic transition, TC≅105 K. This study demonstrates that Tb5(SixGe1−x)4 alloys are good candidates for magnetic refrigeration and that very exciting phenomenology in these systems awaits further investigation.
Influence of the growth conditions and postdeposition treatments upon the grain boundary barrier height of CdTe thin films deposited by close space vapor transport
Authors: Vigil-Galán Osvaldo , Vaillant Lı́dice , Mendoza-Pérez Rogelio , Contreras-Puente Gerardo , Vidal-Larramendi Julio , Morales-Acevedo Arturo
Published: July 2002

CdTe thin films deposited by close space vapor transport (CSVT) under variable growth conditions and postthermal and chemical treatments were studied by means of photoconductivity measurements in the temperature range of 90–300 K. The influences of the deposition temperature gradient, the oxygen content in the growth chamber, and the CdCl2 treatment and chemical etching upon the intergrain barrier height of CSVT polycrystalline CdTe thin films were determined. The grain boundary barrier height (Ebφ) as a function of the intensity of the incident light was analyzed, and values of Ebφ under illumination of 100 mW/cm2 (AM1) were obtained in each case. These results are important for improving CdS/CdTe solar cell performance in the near future.
Different contrast mechanisms induced by topography artifacts in near-field optical microscopy
Authors: Gucciardi P. G. , Colocci M.
Published: July 2002

Topography artifacts can affect the elastic scattering optical images in near-field optical microscopy. Moreover, such effects can completely mask the information on the optical properties of the samples, yielding unexpectedly high lateral resolutions. In this letter, we provide experimental evidence of how changes involving the collection geometry and the roughness of the sample can lead to radical modifications of the contrast induced by the artifacts to the optical map. An explanation of the different regimes is provided by means of approach curves in which the optical signal is detected as a function of the probe/sample distance.
Calculation of the chemical potential of chain molecules using the staged particle deletion scheme
Authors: Boulougouris Georgios C. , Economou Ioannis G. , Theodorou Doros N.
Published: July 2002

A formulation is presented for the calculation of the chemical potential of chain molecules from molecular simulation based on the staged deletion of a test molecule. This formulation is an extension of a formulation presented recently [Boulougouris et al., Mol. Phys. 96, 905 (1999)] for the case of spherical molecules. An analytical method for the calculation of the volume accessible to a hard-sphere chain molecule is used together with the excluded volume map sampling technique. The new method is applied to a fluid of tangent sphere dimers and to ethane under various conditions. NPT, NVT, and Gibbs ensemble Monte Carlo simulation results are presented. Extensive comparison is made against calculations with the Widom test particle insertion method. In all cases, the new method results in considerable savings in CPU time.
Phase modulated Lee–Goldburg magic angle spinning proton nuclear magnetic resonance experiments in the solid state: A bimodal Floquet theoretical treatment
Authors: Vinogradov Elena , Madhu P. K. , Vega Shimon
Published: July 2002

Interference phenomena between sample spinning and radio frequency (RF) irradiation in solid state high resolution proton nuclear magnetic resonance (NMR) spectroscopy are examined. A bimodal Floquet treatment is exploited in order to overcome the limitations of the average Hamiltonian theory (AHT) approach. Frequency switched Lee Goldburg (FSLG) and its variant, phase modulated Lee Goldburg (PMLG-n), homonuclear dipolar decoupling experiments on protons that are rotating at the magic angle are examined. Average Hamiltonian theory (AHT) is used for the synchronous application of FSLG and PMLG-n RF sequences with the sample spinning. A bimodal Floquet approach is introduced to treat both synchronous and nonsynchronous cases. The Floquet approach, providing a general theoretical framework for describing rotating spin systems exposed to periodically applied RF field, reveals several features of the interference between the sample spinning and the RF irradiation. These features can be characterized by mapping out resonance conditions in terms of the Floquet energy level crossings. Line broadening effects occurring when the RF sequences are applied synchronously with the sample spinning are discussed. The appearances of RF-rotor frequency lines in the decoupled spectra are explained. In addition PMLG-n magic angle spinning (MAS) experiments with a reduced number of phases, n⩾3, per RF cycle are explored. All theoretical predictions are verified by simulations of proton spectra and by PMLG-n-MAS experiments on uniformly labeled C13− tyrosine.
Charging effects in gold nanoclusters grown on octanedithiol layers
Authors: Ohgi T. , Sheng H.-Y. , Dong Z.-C. , Nejoh H. , Fujita D.
Published: July 2002

Strong interaction of gold with the terminal sulfur atoms of dithiol molecules on Au(111) effectively suppresses the penetration of deposited Au atoms through the dithiol layer and results in the formation of homogeneous Au nanoclusters. These nanoclusters, 10–15 Å (σ<2 Å) in height, spread over the surface with a density of ∼1.2×1013/cm2 for coverage between 0.25–2.5 monolayers. Decoupling of the clusters from Au(111) by the octanedithiol layer (∼12 Å in thickness) and the small self-capacitance of these nanoparticles (10−19–10−18 F) make it possible to observe both the Coulomb blockade in scanning tunneling spectroscopy and the Au 4f core level shifts in x-ray photoelectron spectroscopy at room temperature. Both phenomena can be attributed to a common physical origin—e2/2C—the Coulomb energy of charged particles.
Kinetic aspects of the coil-stretch transition of polymer chains in dilute solution under extensional flow
Authors: Hernández Cifre J. G. , Garcı́a de la Torre J.
Published: July 2002

When linear polymer chains in dilute solution are subject to extensional flow, each chain in the sample experiences the coil-stretch transition at a different time. Using Brownian dynamics simulation, we have studied the distribution of transition times in terms of the extensional rate and the length of the chains. If instead of time one characterizes the effect of the flow by the accumulated strain, then the distribution and its moments seem to take general forms, independent of molecular weight and flow rate, containing some numerical, universal constants that have been evaluated from the dynamical simulation. The kinetics of the transition, expressed by the time-dependence of the fraction of remaining coils, has also been simulated, and the results for the kinetic rate constant has been rationalized in a manner similar to that used for the transition time. The molecular individualism, characterized in this work by the distribution of transition times, is related to the excess of the applied extensional rate over its critical value, which will determine the transition time and other features of the coil-stretch transition.
Structural transformations in confined lamellar phases in oil–water–surfactant mixtures
Authors: Tasinkevych M. , Ciach A.
Published: July 2002

Lamellar phases in oil–water–surfactant mixtures are studied in a slit geometry. Within a lattice vector model we obtain the following results in a mean-field approximation. (i) In the case of swollen phases the lamellae are parallel to the walls for all distances L between them and for all surface-field strengths; the system responds elastically to the external stress. (ii) For shorter periods of the bulk phase (surfactant volume fraction ρs≈1/3) the lamellae are parallel to hydrophilic, and perpendicular to neutral or very weakly hydrophilic surfaces. For a small range of the surface-fields changes of L lead to a switch from the parallel to the perpendicular orientation, as found previously for confined block copolymers. (iii) For still shorter periods (ρs⩾1/2) the perpendicular orientation is stable up to much higher hydrophilicity of the walls. Changes of L induce the switch between the two orientations for a large range of the surface fields. For ρs⩾1/2 the structure in the slit can no longer be described by one-dimensional density profiles even for the parallel orientation. Structural deformations similar to dislocations lead to an inelastic response to compression or expansion.
Weak ferromagnetism in the antiferromagnetic magnetoelectric crystal LiCoPO4
Authors: Kharchenko N. F. , Kharchenko Yu. N. , Szymczak R. , Baran M. , Schmid H.
Published: July 2002

A study of the magnetization of the antiferromagnetic magnetoelectric crystal LiCoPO4 as a function of temperature and the strength of a magnetic field oriented along the antiferromagnetic vector reveals features due to the presence of a weak ferromagnetic moment. The value of the magnetic moment along the b axis at 15 K is approximately 0.12 G. The existence of a ferromagnetic moment can account for the anomalous behavior of the magnetoelectric effect observed previously in this crystal.
Correlation between the dielectric constant and porosity of nanoporous silica thin films deposited by the gas evaporation technique
Authors: Si J. J. , Ono H. , Uchida K. , Nozaki S. , Morisaki H. , Itoh N.
Published: July 2002

Nanoporous silica thin films with low dielectric constants were deposited by gas evaporation of SiO2 nanoparticles in an argon atmosphere. With increasing gas pressure during the evaporation, the dielectric constant decreases, while the porosity increases. The correlation between the dielectric constant and porosity is well modeled by a serial connection of two capacitors, one with air and the other with SiO2 as the dielectric medium. This suggests that the dielectric constant of the nanoporous silica thin film using the gas evaporation technique is more effectively lowered by forming “uniformly” distributed voids of closed gaps than those of the nanoporous silica films with pores extending from the back to front surface. Therefore, the former nanoporous silica thin film requires less porosity to obtain a low dielectric constant and is regarded as an ideal low-k material.
Photoinduced electron transfer from aliphatic amines to coumarin dyes
Authors: Nad Sanjukta , Pal Haridas
Published: July 2002

Electron transfer (ET) interactions of a series of 7-aminocoumarin dyes with aliphatic amine donors have been investigated using steady-state (SS) and time-resolved (TR) fluorescence quenching, picosecond laser flash photolysis (LFP) and pulse radiolysis (PR) techniques. For different coumarin–amine pairs, the estimated quenching constants (kq) from SS and TR fluorescence measurements are found to be similar within the experimental error. That ET from amine donors to excited (S1) coumarin dyes takes place has been established from the LFP and the PR results. For different coumarin–amine pairs, the kq values are seen to correlate well with the free energy changes (ΔG0) for the ET reactions following Marcus’ outer-sphere ET theory. The total reorganization energy (λ) estimated from this correlation is seen to be just similar to the solvent reorganization energy (λs). The leveled-off kq value under diffusion-controlled condition (kqDC) appears to be much lower (∼2.5 times) for the present systems compared to the corresponding value obtained for the ET reactions in coumarin–aromatic amine systems. The large difference in the kqDC values with aliphatic and aromatic amines as the electron donors has been rationalized on the basis of the shapes of the highest occupied molecular orbitals (HOMO) of the amine donors. For aliphatic amines, since their HOMOs are largely localized on the amino group, a large orientation factor is involved in the encounter complexes for the ET reaction to take place. With π like HOMOs, such orientational restriction is just nominal with the aromatic amines as the electron donors. Present ET results under diffusive conditions have also been discussed with a comparison to the ET rates observed under nondiffusive conditions, where the amines are directly used as the solvent donors.
Modified spontaneous emission in erbium-doped SiO2 spherical colloids
Authors: de Dood M. J. A. , Slooff L. H. , Polman A. , Moroz A. , van Blaaderen A.
Published: July 2002

Spherical SiO2 colloids with two different diameters (175 nm, 340 nm) were doped with erbium at different concentrations. The spheres show sharply peaked photoluminescence centered at 1.535 μm, due to intra-4f transitions in Er3+. From measurements of the Er decay rate for different Er concentrations the decay rate of isolated Er ions (i.e., in absence of concentration quenching) was determined for the two colloid diameters. The data were compared to spontaneous emission rates derived from calculations of the local optical density of states in the colloids. The calculation predicts a large difference in the spontaneous emission rate for both colloid sizes (61 vs 40 s−1), in perfect agreement with the measured data.
Passive microring-resonator-coupled lasers
Authors: Liu Bin , Shakouri Ali , Bowers John E.
Published: July 2002

In this letter, a passive microring-resonator-coupled semiconductor laser structure is proposed. The weakly coupled high-Q microring resonator provides a strong mode-selection filter and could considerably extend the effective cavity length of a conventional Fabry–Perot laser. The side-mode suppression ratio, the linewidth and the frequency chirp of this laser are dramatically improved comparing to distributed feedback and distributed Bragg reflector lasers.
Modeling and analysis of hydrogen–methane plasma in electron cyclotron resonance chemical vapor deposition of diamond-like carbon
Authors: Yoon S. F. , Tan K. H. , Rusli  , Ahn J.
Published: July 2002

Diamond-like carbon films were deposited using the electron cyclotron resonance chemical vapor deposition (ECR-CVD) system. A model for the ECR plasma was formulated using deposition parameters, such as microwave power, pressure, and hydrogen/methane ratio as inputs. Using the model, electron energy, rate constant of electron impact reactions, and density of species in the plasma are calculated. The outputs of the model are analyzed as a function of deposition conditions, such as microwave power, pressure, and hydrogen/methane ratio and compared to experimental data measured using a Langmuir probe. The results show that ion density increases following the increase in microwave power and hydrogen/methane ratio, and decreases following the increase pressure. Results from the model are in agreement with experimental data, and show that the main neutral species are H2, CH4, H, CH3, CH, C2H5, CH2, and C2H6. The main ionic species are H2+, CH4+, CH3+, CH2+, H+, CH5+, C2H4+, C2H5+, and CH+.
Microstructure and optical properties of scandium oxide thin films prepared by metalorganic chemical-vapor deposition
Authors: Xu Z. , Daga A. , Chen Haydn
Published: July 2002

Dense, high-index, and reproducible scandium oxide (Sc2O3) thin films with high mechanical strength were grown on glass substrates by metalorganic chemical-vapor deposition. The influence of deposition temperature on the microstructure evolution and optical properties of Sc2O3 thin films was investigated by x-ray diffraction, scanning electron microscopy, atomic-force microscopy, transmission electron microscopy, and spectrophotometry. A close relationship between microstructure and optical properties was found for Sc2O3 thin films prepared at different deposition temperatures.
Synthesis and characterization of luminescent ZrO2:Mn, Cl powders
Authors: Garcı́a-Hipólito M. , Falcony C. , Aguilar-Frutis M. A. , Azorı́n- Nieto J.
Published: July 2002

ZrO 2 :Mn , Cl luminescent powders have been synthesized at temperatures ranging from 250 to 500 °C. X-ray diffraction measurements indicate changes in the crystallinity of the material as a function of the processing temperature. The photoluminescence spectra show bands associated with transitions from the 4T1 level to the 6A1 levels of the Mn2+ ions. An impurity concentration quenching is observed with the Mn contents. In addition, the presence of Cl in the synthesized material has been found to have an important role on the photoluminescence intensity emission. Preliminary results on cathodoluminescent emission are also reported.
Stresses and transport phenomena in sheared granular flows with different wall conditions
Authors: Hsiau Shu-San , Yang Wen-Lung
Published: July 2002

Experiments were performed in a shear cell device with four different wall conditions. The glass spheres with a mean diameter of 3 mm were used as granular materials. Three bidirectional stress gauges were installed to measure the normal and shear stresses along the upper wall. The motions of the granular materials were recorded by a high-speed camera. The image processing technology and particle tracking method were employed to measure the average and fluctuation velocities in the streamwise and the transverse directions. By tracking the movements of particles continually, the variation in the mean-square diffusive displacements with time was plotted and the self-diffusion coefficient was determined. The self-diffusion coefficients in the streamwise direction were much higher than those in the transverse direction. The self-diffusion coefficients were increased with the increase of the velocity fluctuations, the shear rates, and the stresses. The influences of the wall conditions (wall frictional coefficients) on these properties are discussed in the paper.
Thermal annealing study of 1 MeV electron-irradiation-induced defects in n+p InGaP diodes and solar cells
Authors: Khan Aurangzeb , Yamaguchi Masafumi , Bourgoin Jacques C. , Takamoto Tatsuya
Published: July 2002

The study presents detailed isothermal and isochronal annealing recovery of photovoltaic parameters in n+/p InGaP solar cells after 1 MeV electron irradiation. Correlation of the solar cells characteristics with changes in the deep level transient spectroscopy data observed in irradiated and annealed n+/p InGaP diodes and solar cells shows that the H2 (Ev+0.50 eV) and H3 (Ev+0.76 eV) defects have a dominant role in governing the minority-carrier lifetime as well as carrier removal. However, capacitance–voltage measurements indicate that other defects must also play a role in the carrier removal process. In addition, the concentration of the H2 defect is found to decay significantly as a result of room temperature storage for 40 days, suggesting that InGaP-based solar cells will display superior radiation tolerance in space. Finally, the deep donor-like-defect H2 is tentatively identified as a phosphorus Frenkel pair.
Surface-modulation-controlled three-dimensional colloidal crystals
Authors: Yi Dong Kee , Seo Eun-Mi , Kim Dong-Yu
Published: July 2002

Significant effects of the modulation depth (h) on a three-dimensional (3D) colloidal self-assembly with symmetric and asymmetric two-dimensional (2D) templates are demonstrated. When the ratio “h/D” of the surface modulation (h) to the diameter of the bead (D) exceeded 0.35, then 3D square array patterns were obtained, and when the ratio h/D was from 0.28 to 0.35, pseudo-{110} structure resulted. When the ratio h/D was less than 0.28, the 3D crystallization of colloids was not observed but regular tetragonal or hexagonal clusters were formed in a 2D pattern.
Effects of proton irradiation on n+p InGaP solar cells
Authors: Dharmarasu Nethaji , Khan A. , Yamaguchi Masafumi , Takamoto Tatsuya , Ohshima Takeshi , Itoh Hisayoshi , Imaizumi Mitsuru , Matsuda Sumio
Published: July 2002

3 MeV proton irradiation effects on In0.5Ga0.5P single junction and In0.5Ga0.5P/GaAs tandem solar cells have been investigated for the fluence range from 1×1011 to 1×1013 cm−2. The overall radiation degradation of In0.5Ga0.5P/GaAs tandem cells was higher than In0.5Ga0.5P single junction cells. It was observed that the spectral response of the GaAs bottom cell degrades more than the InGaP top cell. Proton irradiation decreases the longer wavelength spectral response more significantly than the shorter wavelength in both In0.5Ga0.5P and In0.5Ga0.5P/GaAs cells. The difference in the degradation properties of n+p and p+n polarity InGaP solar cells is discussed. The radiation response of a tandem n+p InGaP/GaAs cell is very nearly that which is predicted from the information of these two cells independently. The minority-carrier diffusion length in the base layer was determined from the spectral response data. The minority-carrier diffusion length damage coefficient KL was analyzed for In0.5Ga0.5P and GaAs cells. The minority-carrier injection-enhanced annealing of radiation-induced defects in In0.5Ga0.5P and In0.5Ga0.5P/GaAs cells were also observed.
Soft magnetic properties of Co–Fe–Zr–B–Al–O films
Authors: Chen L. H. , Chen H. K. , Hsieh C. T. , Shih Y. H. , Chen I. G. , Chen S. Y. , Liu H. P.
Published: July 2002

We have investigated magnetic and electrical properties of amorphous Co–Fe–Zr–B alloy films with Al and O additions, deposited on rotating Si substrates by dc magnetron sputtering. Microstructural characterization carried out on these films shows the amorphous nature of films. The Co–Fe–Zr–B–Al–O films do not exhibit any uniaxial anisotropy in magnetization behavior in the as-deposited condition. However, a strong uniaxial anisotropy is introduced in the film plane by annealing at 200 °C for 2 h and in an applied field of ∼150 Oe. The observed in-plane uniaxial anisotropy field (Hk) is as high as ∼38 Oe and the easy-axis coercivity is less than 1.5 Oe for the Co–Fe–Zr–B–Al–O film with 7 at. % Al. The ferromagnetic resonance frequency reaches a high value of ∼2.05 GHz, as a result of this large anisotropy field. It is also found that the soft magnetic properties of amorphous films have a strong dependence on the film composition. This is attributed to the change in the microstructure of films, which is closely related to the film composition.
Acoustic phonon scattering of two-dimensional electrons in GaN/AlGaN heterostructures
Authors: Knap W. , Borovitskaya E. , Shur M. S. , Hsu L. , Walukiewicz W. , Frayssinet E. , Lorenzini P. , Grandjean N. , Skierbiszewski C. , Prystawko P. , Leszczynski M. , Grzegory I.
Published: July 2002

We have measured the temperature dependence of the mobility of the two-dimensional electron gas in AlGaN/GaN heterostructures grown on bulk GaN substrates. The linear dependence of the inverse mobility on temperature at temperatures below 50 K indicates the importance of acoustic phonon scattering in these high mobility heterostructures. Using the temperature dependence of the mobility at a range of carrier densities, we determined the GaN conduction band deformation potential to be ac=9.1±0.7 eV. This result provides a crucial parameter for accurate calculations of intrinsic mobility limits in AlGaN/GaN heterostructures.
Intrachain correlation functions and shapes of homopolymers with different architectures in dilute solution
Authors: Timoshenko Edward G. , Kuznetsov Yuri A. , Connolly Ronan
Published: July 2002

We present results of Monte Carlo study of the monomer–monomer correlation functions, static structure factor, and asphericity characteristics of a single homopolymer in the coil and globular states for three distinct architectures of the chain: ring, open, and star. To rationalize the results we introduce the dimensionless correlation functions rescaled via the corresponding mean-squared distances between monomers. For flexible chains with some architectures these functions exhibit a large degree of universality by falling onto a single or several distinct master curves. In the repulsive regime, where a stretched exponential times a power law form (de Cloizeaux scaling) can be applied, the corresponding exponents δ and θ have been obtained. The exponent δ=1/ν is found to be universal for flexible strongly repulsive coils and in agreement with the theoretical prediction from improved higher-order Borel-resummed renormalization group calculations. The short-distance exponents θv of an open flexible chain are in a good agreement with the theoretical predictions in the strongly repulsive regime also. However, increasing the Kuhn length in relation to the monomer size leads to their fast crossover towards the Gaussian behavior. Likewise, a strong sensitivity of various exponents θij on the stiffness of the chain, or on the number of arms in star polymers, is observed. The correlation functions in the globular state are found to have a more complicated oscillating behavior and their degree of universality has been reviewed. Average shapes of the polymers in terms of the asphericity characteristics, as well as the universal behavior in the static structure factors, have been also investigated.
Branchy alumina nanotubes
Authors: Zou Jianping , Pu Lin , Bao Ximao , Feng Duan
Published: July 2002

Branchy alumina nanotubes (bANTs) have been shown to exist in aluminum oxide. Electron-beam evaporated 400 nm Al film on Si substrate is stepwise anodized in dilute sulfuric acid under the constant dc voltage 40 V at 10.0 °C. This electrochemical-anodizing route resulted in the formation of individual bANTs. Transmission electron microscopy showed that the length of the bANTs was around 450 nm, and the inner diameter was around 10–20 nm. We deduced that the bANTs, the completely detached multibranchy cells of anodic porous alumina (APA) film, should be evolved from the stagnant cells of the APA mother film. The bANTs may be used as templates in fabrication of individual branchy nanoscale cables, jacks, and heterojunctions. The proposed formation mechanisms of the bANTs and the stagnant cells should give some insights into the long-standing problem of APA film, i.e., the self-ordering mechanism of the cells arrangement in porous anodization of aluminum.
Temperature evolution of the low-frequency optical Kerr effect spectra of liquid benzene in quasicrystalline approach
Authors: Ratajska-Gadomska Bozena
Published: July 2002

Theoretical calculations of low-frequency spectra, due to intermolecular collective modes in liquid benzene, are presented. The molecules in a liquid are assumed to be organized in instantaneous, quasicrystalline, short-range structures of the lifetime of order of 10−13 s, which perform rotational and translational vibrations. Those vibrations are responsible for the intermolecular part of the femtosecond optical Kerr effect response. The spectra, obtained for five different temperatures, show a good agreement with the experimental results of other authors. We prove that the low-frequency feature of the spectrum, which is evidently augmented at low temperatures, is associated with collective translational intermolecular vibrations.
Revival structures in picosecond laser-induced alignment of I2 molecules. I. Experimental results
Authors: Rosca-Pruna F. , Vrakking M. J. J.
Published: July 2002

We report experiments on the formation of wave packets consisting of coherently excited ground electronic state rotational levels of I2, using excitation with an intense nonresonant picosecond laser. As measured in a velocity-map-imaging experiment using Coulomb explosion for the determination of the angular distribution of the molecules, these wave packets display alignment of the internuclear axis along the laser polarization axis, both during the laser interaction and at well-defined time delays following the laser interaction, which correspond to rotational revival times of the molecule. The alignment is studied as a function of the intensity and the pulse duration of the pump laser, the rotational temperature of the molecular beam, the polarization geometry of the pump and probe lasers, and the fragment-ion-charge state used to probe the alignment. We observe experimentally that the alignment at revival times is maximal for intermediate pulse durations of a few picoseconds, where the laser–molecule interaction is neither diabatic nor adiabatic. The alignment increases with intensity, but reaches saturation once the intensity is raised sufficiently high. At this point the degree of alignment is limited by the initial rotational temperature of the molecular beam. Our conclusions are corroborated by model calculations, which are presented in detail in the following paper.
Open-system density matrix description of femtosecond laser desorption of electronically and vibrationally relaxing adsorbates: Single- and two-pulse scenarios
Authors: Nest Mathias , Saalfrank Peter
Published: July 2002

The femtosecond laser pulse induced desorption of a molecule (NO) from a metal surface (Pt) is investigated by means of open-system density matrix theory. Single- and two-pulse scenarios in the so-called DIMET limit (Desorption Induced by Multiple Electronic Transitions) are considered. Special emphasis is given (a) to a realistic modeling of the temperature profiles of substrate degrees of freedom in response to a laser pulse, (b) to the role of vibrational relaxation of the adsorbate-surface bond, and (c) to a critical comparison of our anharmonic “excitation–deexcitation” approach to the simpler Truncated Harmonic Oscillator model with electronic friction. For the single-pulse case, nonlinear scaling of the desorption yield with laser fluence is demonstrated and found to be in good agreement with experiment. For the two-pulse case, time-resolved correlation diagrams are calculated and interpreted.
Tunable Fermi resonance in a C2F6 monolayer on graphite
Authors: Hess G. B.
Published: July 2002

The infrared absorption spectrum of C2F6 physisorbed on graphite in the commensurate 2×2 phase has two strong, narrow peaks associated with the ν5 molecular vibration. They are interpreted as a Fermi resonance between ν5 and the ν8+ν11 combination band, which are shifted into near-coincidence by dynamic dipole coupling between the adsorbate molecules. The splitting and relative strengths have been measured as the coupling is reduced in a lower-density, tilted, incommensurate phase and are consistent with theory. It is shown that two alternative descriptions, as Fermi resonance between exciton modes of the layer or as dynamic dipole coupling between molecules with two vibration modes, are essentially equivalent. To fit the experimental frequency shifts and absolute absorption strengths, it is necessary to use a value for the ν5 vibrational polarizability of the adsorbed molecule that is somewhat smaller than the value for the free molecule.
Raman mapping, photoluminescence investigations, and finite element analysis of epitaxial lateral overgrown GaN on silicon substrates
Authors: Benyoucef M. , Kuball M. , Beaumont B. , Gibart P.
Published: July 2002

Epitaxial lateral overgrown (ELO) GaN grown by metalorganic vapor phase epitaxy on Si substrates was characterized using Raman mapping, photoluminescence (PL) experiments and finite element (FE) analysis. Stress in the structures was determined from the E2 phonon frequency and compared to FE results. Low temperature PL spectra are dominated by donor bound exciton (DBE) emission at (3.457–3.459) eV. PL spectra reveal a peak at ∼3.404 eV in window regions attributed to structural defects in the GaN. Differences in crystalline quality between window and overgrown regions of ELO GaN were investigated.
Photoelectron imaging spectroscopy of molecular and cluster anions: CS2− and OCS−(H2O)1,2
Authors: Surber Eric , Sanov Andrei
Published: July 2002

We report the photoelectron imaging study of molecular and cluster anions CS2− and OCS−(H2O)1,2 at 800, 529, and 400 nm, comparing the results for the hydrated OCS− cluster ions to CS2−. The photoelectron angular distributions are interpreted qualitatively using group theory in the framework of the one-electron, electric-dipole approximations. The energetics of OCS−(H2O)1,2 are compared to the theoretical predictions. The vertical detachment energies of OCS−⋅H2O and OCS−(H2O)2 are determined to be 2.07±0.07 and 2.53±0.07 eV, respectively. An indirect estimate of the adiabatic electron affinity of OCS yields a value of −0.04 eV.
Nematic ordering-induced volume phase transitions of liquid crystalline gels
Authors: Matsuyama Akihiko , Kato Tadaya
Published: July 2002

We present a mean field theory to describe swelling behaviors of a nematic gel immersed in a low molecular weight liquid crystal solvent under an external field such as magnetic and electric fields. We calculate the equilibrium concentration of the gel, orientational order parameters, and elongation of the gel. As a result of a nematic ordering between the gel and solvent inside the gel, the shape of the gel is discontinuously (or continuously) elongated with increasing the strength of the external field. We examine the condition for a nematic ordering-induced first- and second-order volume phase transition of the gel.
Theoretical and experimental study of impact of electric field on the atomic layer epitaxy of ZnO on α–Al2O3 surface
Authors: Liu C. H. , Chang R. P. H.
Published: July 2002

High quality ZnO films on α–Al2O3 (sapphire) have a great important application for optoelectronic devices in the future. This study investigated the influence of electric field on the atomic layer epitaxy of ZnO on the (0001) sapphire substrates. Theoretical analyses indicated that the polarization orientation and the polarization forces were the two effects that might influence the ZnO epitaxial growth. The experimental results indicated, for the first time, that electric field could make remarkable growth difference for the ZnO films with this method. Higher quality of epitaxial films has been obtained with a proper magnitude and gradient of the electric field in the geometry of this study. Comprehensive analyses revealed that electric field-induced increase of the hydroxyl packing density on the substrate surface accounted for the results. This study provides a new resort to grow high quality films by chemical vapor deposition (CVD) methods.
Control of evaporatively driven instabilities of thin liquid films
Authors: Grigoriev Roman O.
Published: July 2002

In the process of drying, thin volatile liquid films often undergo a long-wavelength instability leading to nonuniformities or formation of dry spots, with the strength of the instability increasing with the volatility and temperature of the liquid. Perhaps counterintuitively, this evaporative instability can be actively suppressed by an appropriate heating procedure. We use linear stability analysis of the lubrication approximation to show that spatially nonuniform time-dependent radiative heating can indeed have a stabilizing effect. Evaporation is shown to introduce several fundamentally new aspects into the control problem for heated liquid films, compared to the relatively well-studied case of thermal convection. The control problem becomes especially interesting and nontrivial for mixtures and solutions with negative Marangoni numbers due to a peculiar cancellation effect rendering the system insensitive to temperature control at a certain wavelength. It is shown that taking the time dependence of the mean thickness of the film into account is necessary to circumvent this insensitivity.