bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupled InAs/GaAs quantum dots with well-defined electronic shells
Authors: Fafard S. , Spanner M. , McCaffrey J. P. , Wasilewski Z. R.
Published: July 2002

Artificial molecules are studied using coupled quantum-dot (QD) ensembles with well-defined electronic shells. The coupling strength between the zero-dimensional states is varied by changing the distance between two layers of stacked self-assembled InAs/GaAs QDs. For strongly coupled QDs grown with a 4 nm spacer, state-filling spectroscopy reveals a shift of the QD symmetric state to lower energies by ∼23 meV. The wetting layer states are also strongly coupled because of the shallow confinement, resulting in a redshift of its symmetric state by ∼26 meV.
Room-temperature minority-carrier injection-enhanced recovery of radiation-induced defects in p-InGaP and solar cells
Authors: Khan Aurangzeb , Yamaguchi Masafumi , Bourgoin Jacques C. , de Angelis N. , Takamoto Tatsuya
Published: July 2002

We present here the direct observation of minority-carrier injection-enhanced annealing of radiation-induced defects in metalorganic chemical vapor deposition grown p-InGaP at room temperature, and the consequent recovery of radiation damage in InGaP n+-p junction solar cells. Deep level transient spectroscopy analysis shows that the main defect H2 (Ev+0.55 eV) in p-InGaP exhibits minority-carrier injection-enhanced annealing characterized by an activation energy (ΔE=0.51 eV) close to the activation energy for the recovery (ΔE=0.54 eV) of the defect responsible for diffusion length degradation in n+-p solar cells. The marked recovery of radiation damage in InGaP solar cells induced by minority-carrier injection is found to be correlated with the annihilation of the H2 defect.
On three dimensional self-organization and optical properties of InAs quantum-dot multilayers
Authors: González J. C. , Matinaga F. M. , Rodrigues W. N. , Moreira M. V. B. , de Oliveira A. G. , da Silva M. I. N. , Vilela J. M. C. , Andrade M. S. , Ugarte D. , Silva P. C.
Published: July 2002

We report on experiments aimed at producing three-dimensional self-organization in InAs quantum-dot multilayers embedded in GaAs. These InAs/GaAs quantum-dot multilayers have been grown by molecular beam epitaxy. Employing atomic force microscopy, we have analyzed the island density in samples with different number of periods of InAs/GaAs bilayers The results reveals a decrease and a tendency to saturation of the island density with an increase in the number of periods, as a three-dimensional self-organization characteristic of these samples. Optical properties of the samples are examined via photoluminescence spectroscopy. The evolution of the quantum-dot photoluminescence peak position indicates an increment in the mean size of the buried islands and a relative homogenization in size of the quantum dots, as the number of periods increases. The results of the optical measurements agree with the morphological data, and characterize a spatial process of self-organization, related to the increment of the number of periods in the multilayers.
Erbium-implanted silica colloids with 80% luminescence quantum efficiency
Authors: Slooff L. H. , de Dood M. J. A. , van Blaaderen A. , Polman A.
Published: July 2002

Silica colloids with a diameter of 240–360 nm, grown by wet chemical synthesis using ethanol, ammonia, water, and tetraethoxysilane, were implanted with 350 keV Er ions, to peak concentrations of 0.2–1.1 at. % and put onto a silicon or glass substrate. After annealing at 700–900 °C the colloids show clear room-temperature photoluminescence at 1.53 μm, with lifetimes as high as 17 ms. By comparing data of different Er concentrations, the purely radiative lifetime is estimated to be 20–22 ms, indicating a high quantum efficiency of about 80%. This high quantum efficiency indicates that, after annealing, the silica colloids are almost free of OH impurities. Spinning a layer of polymethylmethacrylate over the silica spheres results in an optically transparent nanocomposite layer, that can be used as a planar optical waveguide amplifier at 1.5 μm that is fully compatible with polymer technology.
Surface enhancement in near-field Raman spectroscopy
Authors: Ayars E. J. , Hallen H. D.
Published: July 2002

The intensity and selection rules of Raman spectra change as a metal surface approaches the sample. We study the distance dependence of the new Raman modes with a near-field scanning optical microscope (NSOM). The metal-coated NSOM probe provides localized illumination of a metal surface with good distance control. Spectra are measured as the probe approaches the surface, and the changes elucidated with difference spectra. Comparisons to a theoretical model for Raman excitation by evanescent light near the probe tip indicate that while the general trends are well described, the data show oscillations about the model.
High-energy recoil implantation of boron into silicon
Authors: Shao Lin , Lu Xinming , Jin Jianyue , Li Qinmian , Liu Jiarui , van der Heide P. A. W. , Chu Wei-Kan
Published: July 2002

One approach to fabricate shallow junctions made of B-doped Si is to deposit B on Si, followed by knocking the B into the Si substrate with Si ions. Conventional belief is that the higher the implantation energy, the deeper the recoil profile. While this is true for low-energy incident ions, we show here that the situation is reversed for incident Si ions of higher energy due to the fact that recoil probability at a given angle is a strong function of the energy of the primary projectile. Our experiments show that 500 keV high-energy recoil implantation produces a shallower B profile than lower-energy implantation such as 10 and 50 keV. The secondary ion mass spectrometry analysis shows that the distribution of recoiled B atoms scattered by the energetic Si ions agrees with that calculated on the basis of interatomic potential suggested by W. D. Wilson, L. G. Haagmark, and J. P. Biersack [Phys. Rev. B 15, 2458 (1977)]. Sub-100 nm p+/n junctions have been realized with a 500 keV Si ion beam.
Electrical characterization of white SrS/ZnS multilayer thin-film electroluminescent devices
Authors: Neyts K. , Meuret Y. , Stuyven G. , De Visschere P. , Moehnke S.
Published: July 2002

Thin-film electroluminescent devices with double or triple phosphor layers are used to produce a bright white emission. With the blue emitting SrS:Cu, the blue and green emitting SrS:Ce, the green emitting ZnS:Tb, and the green and red emitting ZnS:Mn, several white emitting combinations can be obtained. The electric field and electron current in such a multilayer phosphor are often not homogeneous. Combined electrical and optical measurements show that the field at the cathodic side of the phosphor is normally larger than at the anodic side, due to positive space charge in the phosphor layer. At low applied voltages, electrons can be trapped in the multilayer before reaching the anodic insulator interface. A part of the phosphor layer is then not excited, and this disturbs the balance of colors emitted from the multilayer phosphor device.
Structural properties of the glass system As–Se–S studied by x-ray absorption spectroscopy
Authors: Wang Jinn-Lung , Tsai Jyi-Ching , Liu Chia-Tung , Nachimuthu Ponnusamy , Jang Ling-Yun , Liu Ru-Gan , Chen Jin-Ming
Published: July 2002

The structural properties of As–Se–S glass system were investigated by x-ray absorption spectroscopy (XANES and EXAFS) using synchrotron radiation. A preliminary characterization by x-ray powder diffraction, differential scanning calorimetric and thermogravimetric measurements was also carried out. The changes in glass transition temperature (Tg) and glass decomposition temperature (Td) associated with glass compositions indicate that sulfur contributes to the instability of the ternary As–Se–S glass system. The XANES and EXAFS results suggest that arsenic is in trivalent state with approximately threefold coordination and selenium is in approximately twofold coordination in all the glass compositions under study. The homopolar bonding (As–As and Se–Se) has a significant contribution in AsSe1.42 glass matrix unlike in AsS1.56 glass where homopolar bonding (As–As) is negligible. However, the homopolar bonding (As–As) is increasingly eliminated with increasing replacement of selenium by sulfur in ternary glasses. In addition, the contribution of sulfur to the coordination of arsenic is more dominant than selenium even when glass matrices contain a larger quantity of selenium. Thus the present XANES and EXAFS results demonstrate that the As–Se–S glass system is well represented by the chemically ordered network model.
Radiation enhanced diffusion of Ti in Al2O3
Authors: Weiss M. , Lu M. , van der Heide P. , Lee S. M. , Ada E. , Lee H. S. , Rabalais J. W.
Published: July 2002

Titanium bulk and dislocation diffusion coefficients at 1000 °C have been determined for Ti in single crystal α-Al2O3(0001) for three types of samples: (i) Ti evaporated onto Al2O3, (ii) Ti+48at 100 eV deposited onto Al2O3, and (iii) Ti+48 at 100 eV deposited onto radiation damaged Al2O3 (damage inflected by implantation of Ti+46 at 7 keV). Diffusion penetration profiles were obtained by using secondary ion mass spectrometry depth profiling techniques. For the deposited and implanted Ti+ samples, the bulk diffusion coefficients (D) are 5.4×10−21 and 5.6×10−17 m2 s−1, respectively, and dislocation diffusion coefficients are 3.6×10−17 and 1.4×10−12 m2 s−1, respectively. Comparing the D's for Ti in the undamaged and damaged Al2O3, the D's for the latter samples are higher by a factor of 104–105, reflecting the radiation enhanced diffusion due to the defect structure inflected by the implanted 7 keV Ti+46. Comparing the D's for Ti deposited onto Al2O3 (both the evaporated and 100 eV Ti+48 samples) with those for Cr deposited onto Al2O3, the Ti D's are larger by a factor of 10, reflecting the influence of the valence state of the cation. These results show that cationic diffusion coefficients in Al2O3 can be controlled by varying the level of defects in the crystal.
Investigation and modeling of the infrared optical properties of direct current sputtered SiC films on silicon
Authors: Lei Y. M. , Yu Y. H. , Cheng L. L. , Sundaraval B. , Luo E. Z. , Ren C. X. , Zou S. C. , Wong S. P. , Chen D. H. , Wilson I. H.
Published: July 2002

Silicon carbide films were reactively dc sputtered onto Si(111) substrates using a silicon target in a mixed CH4/Ar atmosphere. Non-Rutherford backscattering using a high energy incident He+ beam (4.3 MeV for carbon analysis) and Auger electron spectroscopy were employed to analyze the composition of the films. Structural investigations of the stoichiometric SiC films showed that they were composed of microcrystalline and amorphous SiC. The formation mechanism of the microcrystalline and amorphous SiC during our deposition process was discussed. The optical behavior of the SiC film was studied by infrared (IR) reflectance in the range of 400–4000 cm−1. The experimental IR reflectance in this range was fitted by calculating the complex dielectric function of the films based on effective medium theory, in which the SiC films were assumed to consist of homogeneously distributed SiC (amorphous and crystalline). The fitting of the experimental data using our model is quite satisfactory; thus the assumed model in our simulation is suitable for describing the IR optical properties of the sputtered SiC films.
Effects of a Ta interlayer on the phase transition of TiSi2 on Si(111)
Authors: Jeon Hyeongtag , Jung Bokhee , Kim Young Do , Yang Woochul , Nemanich R. J.
Published: July 2002

This study examines the effects of a thin Ta interlayer on the formation of TiSi2 on Si(111) substrate. The Ta interlayer was introduced by depositing Ta and Ti films sequentially on an atomically clean Si(111) substrate in an ultrahigh vacuum (UHV) system. Samples of 100 Å Ti with 5 and 10 Å Ta interlayers were compared to similar structures without an interlayer. After deposition, the substrates were annealed for 10 min, in situ, at temperatures between 500 and 750 °C in 50 °C increments. The TiSi2 formation with and without the Ta interlayer was analyzed with an X-ray diffractometer, Auger electron spectroscopy (AES), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and a four-point probe. The AES analysis data showed a 1:2 ratio of Ti:Si in the Ti-silicide layer and indicated that the Ta layer remained at the interface between TiSi2 and the Si(111) substrate. The C 49–C 54 TiSi2 phase transition temperature was lowered by ∼200 °C. The C 49–C 54 TiSi2 phase transition temperature was 550 °C for the samples with a Ta interlayer and was 750 °C for the samples with no Ta interlayer. The sheet resistance of the Ta interlayered Ti silicide showed lower values of resistivity at low temperatures which indicated the change in phase transition temperature. The C 54 TiSi2 displayed different crystal orientation when the Ta interlayer was employed. The SEM and TEM micrographs showed that the TiSi2 with a Ta interlayer significantly suppressed the tendency to islanding and surface agglomeration.
Evolution of ion-acoustic solitary waves in an inhomogeneous discharge plasma
Authors: Das G. C. , Sarma Manoj Kumar
Published: July 2002

The physical processes in the generation of a solitary wave through a special nonlinear wave equation, called the Korteweg–deVries (K–dV) equation [D. J. Korteweg and G. deVries, Philos. Mag. 39, 422 (1895)], have been studied in an ideal inhomogeneous plasma incorporating the electric field and low current discharges. It is concluded that the usual procedure, because of the plasma inhomogeneity and ionization, is not suitable to exhibit soliton propagation unless an assumption for a mathematical simplification is made. Despite that, the soliton solution does not satisfactorily correspond to experiments. A new approach has been applied for finding the exact soliton propagation with a view to relating the observation to that expected exactly in an experiment. The results let one evaluate the formation of a tailing structure, which follows the main soliton with a growing nature as a function of the plasma configuration. Finally, the theoretical observations seem to significantly stimulate the observations in laboratory and space plasmas.
Nonlinear dynamo mode dynamics in reversed field pinches
Authors: Fitzpatrick Richard , Yu Edmund P.
Published: July 2002

The nonlinear dynamics of a typical dynamo mode in a reversed field pinch, under the action of the braking torque due to eddy currents excited in a resistive vacuum vessel and the locking torque due to a resonant error-field, is investigated. A simple set of phase evolution equations for the mode is derived: these equations represent an important extension of the well-known equations of Zohm et al. [Europhys. Lett. 11, 745 (1990)] which incorporate a self-consistent calculation of the radial extent of the region of the plasma which corotates with the mode; the width of this region being determined by plasma viscosity. Using these newly developed equations, a comprehensive theory of the influence of a resistive vacuum vessel on error-field locking and unlocking thresholds is developed. Under certain circumstances, a resistive vacuum vessel is found to strongly catalyze locked mode formation. Hopefully, the results obtained in this paper will allow experimentalists to achieve a full understanding of why the so-called “slinky mode” locks in some reversed field pinch devices, but not in others. The locking of the slinky mode is currently an issue of outstanding importance in reversed field pinch research.
Hole diffusion at the recombination junction of thin film tandem solar cells and its effect on the illuminated current–voltage characteristic
Authors: Palit N. , Dasgupta A. , Ray S. , Chatterjee P.
Published: July 2002

Computer simulation of experimental current density–voltage (J–V) and quantum efficiency characteristics of thin film p1-i1-n1-p2 structures and of double junction solar cells (p1-i1-n1-p2-i2-n2), has been used to understand the hole transport mechanisms near the np “tunnel” junction between two subcells of a multijunction structure. Two different types of p layers at the junction have been studied: (i) hydrogenated microcrystalline silicon (μc-Si:H) and (ii) hydrogenated amorphous silicon carbide (a-SiC:H). There is a striking difference between the experimental J–V characteristics for the p1-i1-n1-p2 structures, with case (i) having a fairly high fill factor (FF) and conversion efficiency (η), as against a very low FF and η in case (ii). Although the difference is much smaller for double junction cells employing these two types of materials as the p layer at the junction, the fill factor of the cell employing μc-Si:H is about 8% higher. Analysis of transport properties as a function of position by computer modeling reveals that the main difference in behavior between the two cases is due to the much higher free hole population in the p layer at the junction when it is microcrystalline; which in turn, is a direct consequence of the lower activation energy for this case. We also learn that not only tunneling and the electric field in the bottom subcell, but also diffusion, plays a major role in pushing the holes produced in it by the incident light towards the recombination layer at the junction; and thereby helps improve cell performance, especially its fill factor. We conclude that the p layer at the junction should have a high free hole density (low activation energy in the device), to attain an overall high fill factor and conversion efficiency. Another interesting inference is the fact that tunneling as transport mechanism for holes towards the junction is more important when the p layer at the junction is a-SiC:H than when it is microcrystalline, while diffusion plays a more prominent role in propelling holes towards the junction in the latter case.
Ti atomic bonding environment in Ti-containing hydrocarbon coatings
Authors: Meng W. J. , Tittsworth R. C. , Jiang J. C. , Feng B. , Cao D. M. , Winkler K. , Palshin V.
Published: July 2002

We report characterization of the average Ti atomic bonding environment in Ti-containing hydrocarbon (Ti–C:H) coatings by x-ray absorption near edge structure (XANES) spectroscopy, extended x-ray absorption fine structure (EXAFS) spectroscopy, and high-resolution transmission electron microscopy (TEM). Ti–C:H coatings have been synthesized in a hybrid chemical vapor deposition/physical vapor deposition deposition system, which combines inductively coupled plasma and sputter deposition. Combining x-ray absorption spectroscopy with high resolution TEM imaging, we have determined that the dissolution limit of Ti atoms in an amorphous hydrocarbon (a-C:H) matrix is between 0.9 and 2.5 atomic percent. At Ti compositions >2.5 at. %, XANES and EXAFS data indicate that the average Ti atomic bonding environment in Ti–C:H resembles that in cubic B1–TiC, consistent with direct TEM observation of the precipitation of TiC nanocrystallites in an a-C:H matrix. Beyond the Ti dissolution limit, Ti–C:H coatings are in fact TiC/a-C:H nanocomposites, in which the TiC nanoprecipitates are very much bulk like.
Enhancement of tensile stress near a hole in superconducting trapped-field magnets
Authors: Johansen T. H. , Wang C. , Chen Q. Y. , Chu W.-K.
Published: July 2002

The mechanical stress caused by flux pinning in a cylindrical superconductor with a concentric hole is investigated theoretically. Exact expressions for the radial and hoop stress are derived using the critical-state model. Stress profiles during a magnetization process often used to activate high-Tc superconductors as strong trapped-field magnets are presented and analyzed in detail. It is shown that due to the hole the tensile hoop stress is enhanced by a factor of 2 or more, depending on the hole diameter. The dramatic increase in cracking probability is emphasized.
Amount of hole conversion across AlxGa1−xN/GaN heterojunctions
Authors: Ekbote S. , Cahay M.
Published: July 2002

Starting with the 6×6 valence band Burt Hamiltonian for semiconductor heterojunctions with wurtzite symmetry that was recently derived [F. Mireles and S. E. Ulloa, Phys. Rev. B 60, 13659 (1999)], we study the tunneling properties of holes across an AlxGa1−xN/GaN heterojunction while including the effects of strain on the valence band edges. We analyze the amount of hole conversion between the heavy-, light-, and crystal-field split-off-valence bands during tunneling across the heterointerface. The results are compared with an approach based on a widely used symmetrized form of the Luttinger–Kohn effective-mass Hamiltonian and associated boundary conditions for wurtzite materials
Photo-induced reactions in mass-selected complexes Mg+(FCH3)n, n=1–4
Authors: Yang Xin , Liu Haichuan , Yang Shihe
Published: July 2002

Photo-induced reactions in the metal cation–molecule complexes Mg+(CH3F)n have been studied as a function of the number of solvent molecules. While a photoreaction of the singly solvated complex Mg+-FCH3 yields exclusively CH3+, excitation of larger complexes Mg+(FCH3)2–4 produces predominantly bare and solvated MgF+. Photo-induced evaporation of the larger complexes was also observed, although with much lower yields. Possible mechanisms are suggested to interpret the abrupt change in the photoreaction patterns with an increasing complex size. The action spectra of all the complexes are discussed based on the transitions 2P←2S centered on the Mg+ ion but perturbed by the presence of the FCH3 molecules. Quantum ab initio calculations were performed to obtain the structures and action spectra of the complexes, which are directly compared with the experimental results.
Flyer acceleration by a high-power KrF laser with a long pulse duration
Authors: Kadono T. , Yoshida M. , Takahashi E. , Matsushima I. , Owadano Y. , Ozaki N. , Fujita K. , Nakano M. , Tanaka K. A. , Takenaka H. , Kondo K.
Published: July 2002

The experiments of flyer acceleration by the irradiation of a high power laser are carried out using the ASHURA system at the Electrotechnical Laboratory, in which the laser has a short wavelength (∼249 nm) and a long pulse duration (∼30 ns). Three-layered targets (aluminum–polyimide–tantalum) are irradiated. The laser ablates the aluminum and polyimide layers and the rear layer (tantalum) is accelerated as a flyer. It is suggested that the tantalum flyer is in a condensed state for the duration of flight. The flyer velocity estimated from the acceleration profile is at least 8 km/s. One-dimensional numerical simulation indicates that the terminal flyer velocity becomes higher than 15 km/s. The energy conversion from laser energy to flyer kinetic energy is more efficient than that in the previous experiments using the three-layered targets with a longer wavelength and a shorter pulse duration (1–2 ns). Thus, it is found that a facility with a long pulse duration and a short wavelength is suitable for the realization of a fast flyer keeping its state in a condensed phase.
Study of x-ray emission from a table top plasma focus and its application as an x-ray backlighter
Authors: Beg F. N. , Ross I. , Lorenz A. , Worley J. F. , Dangor A. E. , Haines M. G.
Published: July 2002

A study of a 2 kJ, 200 kA, table top plasma focus device as an intense x-ray source is reported. The x-ray yield from a number of gases, (deuterium, nitrogen, neon, argon, and xenon) is measured as a function of filling pressure and in neon as a function of anode length. In gases with Z<18, the plasma implodes to form a uniform cylindrical column, whereas for Z⩾18, the plasma consists of a number of hot spots. A maximum x-ray yield of 16.6 J and pulse length of 10–15 ns was obtained in neon. The x-ray emission was established to be due to H- and He-like line radiation. The temperature estimated from spectroscopic observations was about 300–400 eV at an electron density of (3–5)×1020 cm−3 in neon. At low pressures in neon, hard x-ray radiation, presumably due to electron beams was dominant. Mesh images of different wire materials were recorded at the optimum pressure in neon as a proof of principle for x-ray backlighting.
Efficacy of high-frequency, low-voltage plasma immersion ion implantation of a bar-shaped target
Authors: Tian Xiubo , Zeng Zhaoming , Zeng Xuchu , Tang Baoyin , Chu Paul K.
Published: July 2002

Elevated-temperature plasma immersion ion implantation (PIII) increases the surface hardness and thickness of the modified layer and is traditionally performed at a high energy (typically above 5 keV) and low current density. In this article, we report the benefits of a different approach by high-frequency, low-voltage plasma immersion ion implantation (HLPIII). Experiments and a two-dimensional theoretical simulation are conducted to demonstrate the advantages of the process on a bar-shaped sample in terms of ion dose, dose uniformity, and modified layer thickness. Simulation of the sheath dynamics illustrates that the thinner plasma sheath in HLPIII is geometrically more conformal to the target surface, and the incident ion flux is more uniform along the exposed surface when compared to the traditional high-voltage PIII process. The higher ion dose and thicker modified layer can be attributed to the higher ion current density. HLPIII is thus the preferred technique to enhance the surface properties of large and complex-shaped specimens such as a metal track.
Magnetic properties and granular structure of CoPt/B films
Authors: Karanasos V. , Panagiotopoulos I. , Niarchos D. , Okumura H. , Hadjipanayis G. C.
Published: July 2002

Granular CoPt/B films, consisting of high anisotropy face-centered-tetragonal (fct) CoPt particles embedded in a boron matrix have been prepared by heat treating cosputtered films. Transmission electron microscopy and x-ray diffraction studies show that the as-deposited films consist of a mixture of an amorphous-like phase and face-centered-cubic CoPt nanocrystallites. After a heat treatment at 700 °C for 5 min a microstructure consisting of interconnected particles of partially ordered fct CoPt with sizes ranging from 20 to 200 nm is formed. These samples have coercivities of 6 kOe and high reduced remanence (Mr/MS=0.9) due to the interparticle interactions. After prolonged annealing the particles become spherical and their size increases to around 400 nm. The coercivities of these overaged samples are higher but the hysteresis loops are constricted and the remanence is reduced. Measurements of irreversible susceptibility and viscosity as a function of temperature and applied field were used to determine the fluctuation field and activation volume in the optimally annealed sample.
Scanning near-field optical microscopy based on the heterodyne phase-controlled oscillator method
Authors: Shubeita G. T. , Sekatskii S. K. , Riedo B. , Dietler G. , Dürig U.
Published: July 2002

The heterodyne phase-controlled oscillator method to monitor the resonance frequency and quality factor of the tip oscillations was used to control the scanning near-field optical microscope (SNOM) and to study the nature of the shear-force interaction routinely used in SNOM. Both optical and nonoptical (tuning fork-based) detection schemes of the shear force have been investigated using the same electronic unit, which enables a direct comparison of the results. It is shown that the possibility to record simultaneously the topography and dissipative interaction (Q-factor) channels gives additional information about the sample and helps to interpret the data in a manner analogous to that of a usual dynamic force microscope. The peculiarities of the recorded approach curves (increase of the resonance frequency and Q factor when the tip approaches the sample) are consistent with the “repetitive bumping” mechanism of tip–sample interaction for the shear force. Evidence for the transition from the bumping to the permanent sliding mechanism has been obtained for the case of larger vibration amplitudes of the tip.
On the triplet structure of binary liquids
Authors: Jorge S. , Kahl G. , Lomba E. , Abascal J. L. F.
Published: July 2002

An approach to calculate the triplet structure of a simple liquid, that was proposed some years ago by Barrat, Hansen, and Pastore [Phys. Rev. Lett. 58, 2075 (1987)] has been tested in the binary case. This approach is based on a factorization ansatz for the triplet direct correlation function c(3); the unknown factor function is determined via the sum rule relating c(3) and the pair direct correlation function which is the only input information of the system that is required in this formalism. We present an efficient and stable numerical algorithm which solves the six (partly coupled) integral equations for the unknown factor functions. Results are given for the case of a binary hard-sphere mixture and complemented by computer simulation data.
Photoconductivity and excited state absorption in Cr4+:Yttrium  aluminum  garnet
Authors: Brickeen B. K. , Ching W. Y.
Published: July 2002

Photoconductivity measurements were conducted on Ca2+ charge compensated Cr4+: Yttrium aluminum garnet crystals in an attempt to validate a theoretical model for saturable absorption with excited state absorption that requires transitions to the conduction band. Photocurrent was induced by absorption of 1.06 μm laser light, accompanied by a thermally induced current of smaller magnitude. Increased photocurrent, larger by three orders of magnitude, was observed in one of the four samples suggesting the crystal exhibits increased excited state absorption. Passive Q-switch tests were performed using the four samples, with results supporting a direct correlation between photoconductivity measurements and excited state absorption.
Excitonic structure in a quantum well under the tilted magnetic field
Authors: Kasapoglu E. , Sari H. , Sökmen I.
Published: July 2002

By using an appropriate coordinate transform we have calculated variationally the ground state exciton binding energy in the single square well under the tilted magnetic field. The dependence of the binding energy to the magnetic field strength, well width, and the direction of the field is discussed. We conclude that for small well widths the exciton binding energy is very sensitive to the tilt angle.
High-resolution transmission electron microscopy of cubic Si3N4
Authors: Zhang Ming , He Hongliang , Xu F. F. , Sekine T. , Kobayashi T. , Bando Y.
Published: July 2002

A cubic Si3N4 phase/nanostructure has been characterized by means of high-resolution analytical electron microscopy. The specimen prepared from β-Si3N4 powders at a high pressure and temperature by shock wave compression contained nanometer-sized Si3N4 crystallites. The results of nano-beam electron diffraction analysis and high-resolution lattice images as well as computer simulations revealed that the Si3N4 crystallites had a cubic symmetry with spinel structure. The electron energy loss spectrum suggested that the chemical compositions of these nanostructures were close to Si3N4.
Dilution effect on magnetic properties of Co3O4 nanocrystals
Authors: Sato Michi , Kohiki Shigemi , Hayakawa Yuko , Sonda Yuko , Babasaki Tomohiro , Deguchi Hiroyuki , Mitome Masanori
Published: July 2002

We have prepared diluted systems of Co3O4 nanocrystals dispersed in an amorphous silicate by calcination of molecular sieve soaked in precursor solutions with concentrations of 0.01 and 0.1 mol/l. For both systems we have observed the disappearance of the antiferromagnetic phase transition at 33 K of bulk Co3O4, a difference in the dc susceptibility between field cooling and zero field cooling, and the frequency dependence of the ac susceptibility. Nonlinear susceptibility measurements demonstrated that the systems from the solution of 0.01 and 0.1 mol/l were in superparamagnetic and spin glass phases, respectively. The 0.01 mol/l solution system showed no peaks at any temperature, although the peak temperature for the 0.1 mol/l solution system was 18–22 K. The interparticle interactions were independent and collective for the systems from 0.01 and 0.1 mol/l solutions, respectively.
Relaxational polarization in polar dielectric barium magnesium niobate
Authors: Gupta Surya M. , Furman E. , Colla E. , Xu Z. , Viehland Dwight
Published: July 2002

Relaxor-like dielectric behavior, analogous to that found in lead magnesium niobate, has been induced in the polar dielectric barium magnesium niobate (BMN). In BMN, the dielectric constant was increased and the temperature of the maximum dielectric constant was shifted to a lower temperature with a decrease in measurement frequency with increasing A-site vacancy concentrations. The frequency dispersion of the permittivity maximum exhibited good agreement with the Arrhenius relationship. An activation energy of 0.15 eV and a preexponential factor 1014 s−1 were determined. No frequency dispersion in the imaginary part of the permittivity was found below the temperature of the dielectric maximum. A linear polarization-electric field dependence was observed at room temperature. A polarization of 0.3 μC/cm2 was found under a field of 75 kV/cm at 25 °C. Phase analysis revealed a single phase perovskite structure with a hexagonal unit cell with a=5.77 Å and c=7.08 Å. 〈110〉 selected area electron diffraction patterns revealed superlattice reflections along the 〈111〉. High resolution Z-contrast imaging was used to study the local ordering and the origin of the relaxational polarization.
Line tension in Langmuir monolayers probed by point forces
Authors: Wurlitzer S. , Steffen P. , Wurlitzer M. , Khattari Z. , Fischer Th. M.
Published: July 2002

A Langmuir monolayer of methyl octadecanoate in the phase coexistence region liquid expanded/liquid condensed is observed with fluorescence microscopy and mechanically manipulated using optical tweezers. A circular liquid condensed droplet is locally fixed with the tweezers and deformed by hydrodynamic flow of the surroundings. The stationary shape is determined by the competition of the bare line tension λ, dipole and hydrodynamic forces. The dipole contributions to the shape can be accounted for by introducing an effective line tension λeff. At the front of the droplet a wedge angle θ is formed, from which λeff can be deduced. The electrostatic contribution λeff-λ is calculated for the experimental shape in the limit of weak deformations. The range of deformation where this approach holds is determined for a triangular shape.
Mass analyzed threshold ionization of the Cl35and Cl37 isotopomers of p-chloroaniline
Authors: Lin J. L. , Tzeng W. B.
Published: July 2002

One-color, two-color resonant two-photon ionization (R2PI), and mass analyzed threshold ionization (MATI) spectroscopic methods have been used to study the electronic transition and the threshold ionization of the Cl35and Cl37 isotopomers of p-chloroaniline. The band origins of the S1←S0 electronic transitions of both species are found to be 32 573±1 cm−1. The ionization energies (IEs) of both isotopomers of p-chloroaniline are determined to be 62 409±7 cm−1 by the two-color R2PI spectroscopy and 62 410±4 cm−1 by the MATI spectroscopy. Analyses on the spectral features show that most of the active modes are related to the in-plane ring vibrations in the S1 state and cationic ground state. Isotope effect on the ring deformation vibrations 1, 6a, and 12 gives rise to a frequency shift of 1–3 cm−1 in the S1 state and 3–9 cm−1 in the ion state. The experimental results are well supported by the computed ones on the basis of ab initio and density functional theory calculations.
Dependence of the leakage current on the film quality in polycrystalline silicon thin-film transistors
Authors: Dimitriadis C. A. , Farmakis F. V. , Brini J. , Kamarinos G.
Published: July 2002

The influence of the material quality and thickness on the leakage current of polycrystalline silicon thin-film transistors is investigated. Improvement of the polycrystalline silicon layer (i.e., increase of the average grain size or decrease of the intragrain defect density) reduces only the leakage current at low electric fields in the drain region. At high electric fields, the leakage current is independent of the film quality and thickness due to the fundamental nature of the leakage current mechanisms. The experimental data indicate that Poole–Frenkel enhanced emission from traps at low electric fields and band-to-band tunneling at high electric fields are the dominant conduction mechanisms of the leakage current.
Analyses of edge effects on residual stresses in film strip/substrate systems
Authors: Hsueh Chun-Hway
Published: July 2002

The residual stress distribution in a thin-film strip overlaid on a substrate is influenced by the edges of the strip. An analytical model is developed to derive a closed-form solution for the stress distribution along the film width. Because the film is much thinner than the substrate, the stress variation through the film thickness is ignored; however, the stress variation through the substrate thickness is considered in the analysis. Compared to the existing analytical models, the present model is more rigorous and the analytical results agree better with both finite element results and experimental measurements.
The shock Hugoniot of the intermetallic alloy Ti–46.5Al–2Nb–2Cr
Authors: Millett Jeremy , Gray George T. (Rusty) , Bourne Neil
Published: July 2002

Plate impact experiments were conducted on a γ-titanium aluminide (TiAl) based ordered intermetallic alloy. Stress measurements were recorded using manganin stress gauges supported on the back of TiAl targets using polymethylmethacrylate windows. The Hugoniot in stress-particle velocity space for this TiAl alloy was deduced using impedance matching techniques. The results in this study are compared to the known Hugoniot data of the common alpha-beta engineering Ti-based alloy Ti-6Al–4V. The results of the current study on the intermetallic alloy TiAl support that TiAl possesses a significantly higher stress for a given particle velocity than the two-phase Ti–6Al–4V alloy.
Experiment on the plasma-loaded backward-wave oscillator using a gas-loaded foil-less diode
Authors: Qian Bao-Liang , Li Chuan-Lu , Liu Yong-Gui , Zhang Jian-De , Tan Qi-Mei , Liu Jin-Liang , Liu Cun-Hua , Chen Dong-Qun , Li Xiang-Sheng , Liu Che-Bo
Published: July 2002

High-power microwave radiation of 9.5–13.4 GHz was generated in a plasma-loaded backward-wave oscillator employing a relativistic electron beam of 400–500 keV and 1–3 kA. This experiment was to re-examine and confirm the previous works that had been done in other various institutions. The relativistic electron beam, which was produced from a helium gas-loaded foil-less diode, was injected into the helium gas-loaded rippled-wall waveguide of the backward-wave oscillator, generating a plasma in the waveguide and then the high-power microwave radiation. A sharp increase in the output microwave power has been observed in a narrow range of the helium gas pressure, and two dips in the microwave emission have also been found in a certain range of the axial magnetic field. Additionally, at certain values of the helium gas pressure and guide magnetic field, the total microwave emission of the plasma-loaded backward-wave oscillator was found to be seven times as large as that of the vacuum case. The highest interaction efficiency was estimated to be about 29% for the present experiment.
The absolute continuity of the spectrum of Maxwell operator in a periodic media
Authors: Morame Abderemane
Published: July 2002

We prove that the spectrum of Maxwell operator, (B,E)⟼(−rot(E),a(x)rot(b(x)B)), with non-negative and periodic functions a(x) and b(x), is absolutely continuous.
Surface acoustic waves on a dusty plasma sheet
Authors: Yaroshenko Victoria V. , Verheest Frank
Published: July 2002

The propagation of linear and nonlinear surface acoustic waves in a dusty plasma sheet is considered. Spatial restrictions of a dusty plasma lead to an increase in the dispersion in the long wavelength limit, to substantial modifications of the longitudinal modulational instability, and to the possible formation of nonlinear Schrödinger type solitons.
A computer simulation study of tilted smectic mesophases
Authors: Withers Ian M. , Care Christopher M. , Cleaver Douglas J.
Published: July 2002

We present comprehensive results from constant NVT and constant NPT Monte Carlo simulations of particles interacting via a biaxial variant of the Gay–Berne potential which we term the Internally Rotated Gay–Berne (IRGB) potential. The IRGB potential may be considered to be a single-site approximation to the interaction between two zig–zag shaped molecules, the extent of this molecular biaxiality being characterized by an internal rotation angle δ. We find that increasing the value of δ frustrates the formation of orientationally ordered phases, all phase transitions being shifted to lower temperatures and higher densities. Additionally, for δ⩾30 degrees, the smectic B phase is replaced by the tilted smectic J phase. The smectic A phase, in contrast, is destabilized completely for sufficiently large δ, with neither smectic A nor its tilted equivalent, smectic C, being observed. This suggests that models for smectic C-formation which are based on biaxial intermolecular attractions may not offer the best route to obtaining this phase.
Theoretical study of the dynamics, stereodynamics, and microscopic mechanism of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction
Authors: González Miguel , Hernando Jordi , Puyuelo Marı́a P. , Sayós R.
Published: July 2002

A previously reported potential energy surface (PES) and a new barrierless PES (both based on ab initio data and describing the CH3 group as a pseudoatom) were used to study the O(1D)+CH4→OH+CH3 reaction with the quasiclassical trajectory (QCT) method. The new PES accurately reproduces the experimental rate constant values, in contrast to the previous PES. The QCT study was mainly performed at the relative translational energy (ET) resulting from the photodissociation of N2O at 193 nm (〈ET〉=0.403 eV), although the collision energy obtained from the photodissociation of O3 at 248 nm (〈ET〉=0.212 eV) was also considered. Good agreement between theory and experiment was obtained for the OH vibrational populations and for the OH rotational populations for the v′⩾2 vibrational levels, while the rotational distributions for v′=0–1 are more excited than in the experiment. The QCT results at ET=0.403 eV satisfactorily reproduce the experimental kk′ angular distribution of the state-specific channel OH(v′=4, N′=8) and the corresponding ET′ distribution. For OH(v′=0, N′=5) the reproduction of these properties is poorer, especially for the ET′ distribution. At 0.403 eV the contribution of the abstraction mechanism to the reaction mode is negligible and two insertion like mechanisms (with fast or slow elimination) are found to be predominant, as suggested experimentally. The discrepancies observed between the QCT and experimental results can be explained on the basis of the defective description of the insertion/slow elimination mechanism provided by the model.
Spontaneous compactification of D=5 two-form gauge fields and the obtainment of Maxwell and Yang–Mills theories
Authors: Barcelos-Neto J.
Published: July 2002

We show that the spontaneous compactification of the Abelian and non-Abelian two-form gauge field theories from D=4+1 to D=3+1 leads to the same theories plus the Maxwell and Yang–Mills ones, respectively. The vector potential comes from the zero mode of the fifth component of the tensor gauge field in D=5. Concerning the non-Abelian case, it is necessary to make a more refined definition of the three-form stress tensor in order to be compatible, after the compactification, with the two-form stress tensor of the Yang–Mills theory.
K-shell photorecombinations in strongly coupled plasmas
Authors: Jung Young-Dae
Published: July 2002

Plasma screening effects on the K-shell photorecombination of the free electron with the ion in strongly coupled plasmas are investigated. The recombination cross section is obtained by the principle of detailed balance with the photoionization cross section of the hydrogenic ion. It is found that the plasma screening effects on the recombination cross section are found to be less than 20% when the ion-sphere radius (RZ) is greater than two times of the Bohr radius (aZ) of the hydrogenic ion with nuclear charge Z.
A remarkable sequence derived from Euler products
Authors: Morgan Dennis R.
Published: July 2002

An infinite product of the form a0=Πm=1∞[1/(1−αm)], 0<α<1 was introduced by Euler in a famous theorem of number theory. A generalized form of this infinite product is used to define a sequence an≡Πm=0m≠n∞[1/(1−αm−n)], which is shown to have many interesting properties. The an coefficients are in fact the partial-fraction expansion coefficients associated with the characteristic function of a first-order Markov process driven by an uncorrelated sequence of random variates with exponential density. The an coefficients are recursively calculated from a0, and monotonically converge to zero O(αn2). The sum of the sequence is equal to 1, and the alternating sum is equal to a02(α)/a0(α2). A more remarkable property is that the an sequence is orthogonal to all exponentially increasing sequences of the form α−kn, where k is a positive integer. Various other expressions are also derived for the moments of anα−kn, k⩾0. The z-transform of the an sequence is shown to be characterized by an infinite set of zeros αm on the real axis and an essential singularity at the origin.
Generalized low-energy expansion formula for Green’s function of the Fokker–Planck equation
Authors: Miyazawa Toru
Published: July 2002

The frequency expansion formula for the one-dimensional Fokker–Planck equation derived in a previous paper is generalized to an expansion formula for arbitrary powers of the Green’s function. Based on this expansion, the small-frequency behavior of the Green’s function is studied for the cases where the potential V(x) tends to infinity at both x→+∞ and x→−∞.
Composition and its impact on shape evolution in dislocated Ge(Si)/Si islands
Authors: Liao X. Z. , Zou J. , Cockayne D. J. H. , Jiang Z. M. , Wang X. , Leon R.
Published: July 2002

The composition distribution of Ge(Si)/Si (001) islands grown at 700 °C by molecular beam epitaxy is investigated using high-spatial resolution x-ray energy dispersive spectrometry in a scanning transmission electron microscope. Island shapes are investigated using cross-section transmission electron microscopy. Results show nonuniformity of the composition distribution in the islands, which affects the evolution of the aspect ratios of height-to-base diameter of dislocated islands.
Bifurcations in elliptical, asymmetric non-neutral plasmas
Authors: Fajans J. , Gilson E. , Backhaus E. Yu.
Published: July 2002

A pure electron plasma held in a Malmberg–Penning trap deforms into an ellipse when subjected to a stationary, l=2 voltage perturbation on the trap wall. At first, the plasma’s ellipticity is proportional to the strength of the perturbation, but once the perturbation increases beyond a critical value, the plasma equilibrium bifurcates into two stable off-axis equilibria and an unstable saddle. At the bifurcation point, the l=1 diocotron frequency dips to near zero. The diocotron orbits become very elliptical just below the bifurcation, and, after the bifurcation, split into three classes delimited by a separatrix: two classes surrounding the individual new equilibria, and one class surrounding both equilibria. The mode frequencies slow near the separatrix, and the trajectories themselves slow near the saddle at the origin. Interaction with the elliptical mode causes the diocotron mode to spontaneously and reversibly jump across the separatrix.